j ourna l h om epage: www.elsev ier .com/ locate /ccr
eview
pin state switching in 2,6-bis(pyrazol-3-yl)pyridine (3-bpp) basede(II) complexes
avin A. Craiga,∗, Olivier Roubeaub, Guillem Aromía,∗
Departament de Química Inorgànica, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, SpainInstituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza, Spain
rticle history:eceived 18 November 2013ccepted 10 February 2014vailable online 20 February 2014
eywords:pin crossover
a b s t r a c t
The area of spin crossover (SCO) attracts interest both on a fundamental level and in terms of potentialapplications for compounds displaying this phenomenon. A few families of Fe(II) complexes have becomeparamount for the advance of this topic, for example, compounds based on bis-pyrazolylpyridine (bpp)ligands. Here, we describe the versatile and rich SCO behaviour shown by the group of SCO compoundsbased on the related ligand 2,6-bis(pyrazol-3-yl)pyridine (L1) and its recently developed derivatives. Theuse of derivatives of L1 represents an advance, as prior to 2011, no functionalised L1-type ligands had
been employed to obtain SCO systems. These compounds are highly sensitive to the anion and degree ofsolvation within their lattices, many of which have been observed through single crystal X-ray diffractionstudies. The structural data that have been published in recent years has permitted a magneto-structuralcorrelation to be described, which proves to nicely complement the properties shown by the family basedon L1’s regio-isomer, 2,6-bis(pyrazol-1-yl)pyridine.
he d4–d7 ions may be distributed in two different configurations1]. One corresponds to the maximum possible spin pairing thatesults from a strong ligand field, �, and is therefore referred to ashe low spin (LS) state. Weaker ligand fields lead to the maximum
ultiplicity for the given ion in accordance to Hund’s Rule, lead-ng to the so-called high spin (HS) state. Occasionally, the energyalance between these two states is so delicate that the modu-
ation of an external parameter is sufficient to reversibly switchetween them in a phenomenon known as Spin Crossover (SCO)2]. While it has been observed in Mn(III) [3–5], Fe(III) [6–11],nd Co(II) [12–14], the vast majority of SCO systems contain Fe(II)15].
SCO may be observed when the splitting � induced by a givenigand set lies close to the discontinuity observed in the correspond-ng Tanabe Sugano diagram for the metal ion. The most commonpproach is to surround an Fe(II) ion with six nitrogen donor atoms16–18]. When the difference in the zero-point energies, �E◦
HL,etween the potential energy wells for the two configurations, HSr LS, is of the order of kBT, then thermal SCO may be observed [19].t low temperatures, there will be a majority population of the LState, with the HS state being increasingly formed at higher tem-eratures, since the latter is the state with larger entropy [20,21].hus, temperature variation has been the most common methodo switch between the spin states, although it can also be achievedhrough irradiation with light [22–25], the application of pressure26–28], or the application of an external magnetic field [29]. A fur-her possibility is to induce a switch of the spin state of the activeentre through the insertion of a guest molecule into the lattice30–33].
A spin transition may be represented as the fraction of Fe(II)ons that are in the HS state, �HS, vs. a dependent variable, suchs the temperature. The shapes of these spin transition curves leado classifications of the SCO behaviour observed, where the tem-erature at which half of the Fe(II) centres are in either state isenoted T1/2 (Fig. 1 displays some of the more common types) [34].he first, a gradual transition, refers to the case where the switcho the LS state takes place over a wide temperature range (tensf Kelvin). Extremely gradual transitions are typically observed inolution [35–38], where essentially an equilibrium Boltzmann pop-lation of both states is registered at all temperatures. In the second,n abrupt transition, the SCO process occurs in a much narrowerange of temperature, usually less than 10 K, as a result of the phe-omenon of cooperativity (see below). The third illustrated curve,ith increased cooperativity, presents hysteresis. In this case, there
re two values of T1/2, one at a lower temperature for the coolingode and another one for the heating mode, leading to a bi-stable
egion in which the state of the system depends on its thermal his-ory. The prospects of exploiting this phenomenon in the contextf molecular based switchable materials [39–41] renders this lastcenario the most desirable one because the co-existence of two
Fig. 1. Representations of some different type
try Reviews 269 (2014) 13–31
spin states at the same temperature is directly analogous to havingan “on” and an “off” state within the system [40].
Thus, the decisive factor behind the transitions shown in Fig. 1is the concept of cooperativity. The origin of cooperativity liesin the propagation of the volume change that occurs around themetal ion when it changes spin state, which is mostly conductedthrough long- and short-range elastic interactions arising fromintermolecular contacts [42–44]. The higher the cooperativity in aSCO system, the more abrupt the transition will be and a spin tran-sition with hysteresis occurs when it is particularly high. Therefore,three strategies have been outlined to increase cooperativity [45]within SCO compounds, represented in Fig. 2: (i) link the SCO cen-tres through covalent bonds, as coordination polymers; (ii) inducegreater degrees of hydrogen bonding via the choice of appropriateligands; and (iii) favour the formation of �· · ·� interactions broughtabout by the shape of the ligands or spin-active entities. Theseconsiderations guide the design of the ligands used to synthesisepotential SCO systems.
1.2. Scope of this review
A fruitful branch of SCO research has used terimine ligands asits basis, chelating two molecules around one Fe(II) centre to pro-vide the metal ion with an FeN6 first coordination sphere [17],and including aromatic rings and positions that may be function-alised within the ligand skeleton, allowing strategies (ii) and (iii) inFig. 2 to be pursued. Within this subset lie the bis-pyrazolyl ligands2,6-bis(pyrazol-x-yl)pyridine (x = 1, or 3, either 1-bpp or 3-bpp,respectively) [47]. This review intends to cover the use of 3-bpptype ligands in spin crossover research. Despite being well-knownand its great potential [47,48], the ligand 3-bpp (herein referred toas L1; see Fig. 3) has been less used in SCO than its regio-isomer1-bpp, however there has been a recent surge in works employ-ing this bis-pyrazolyl ligand and its derivatives. In this paper, allof the functionalised L1-type ligands that have been used in SCOwill be first compiled, together with a look at the different strate-gies that are used in the synthesis of its Fe(II) salts, which dependon whether a purely SCO system or a hybrid system that incorpo-rates more than one functionality is sought. Then, the general SCOproperties of Fe(II) compounds of these ligands will be discussed,before a closer look is taken at the various mechanisms that areavailable to switch the spin state in these compounds. Advantageis taken of the increasing amount of structural data that are avail-able associated with these systems to describe how many displaya similar crystal packing motif – the terpyridine embrace – whilethere are also many which possess interesting crystal lattices, often
as a consequence of attempts to confer multi-functionality uponthe compounds. Finally, some magneto-structural correlations aredrawn that link structural distortion to spin state, and complementprevious findings for SCO systems containing the ligand 1-bpp.
s of SCO behaviour, shown as �HS vs. T.
G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31 15
F centrep el of �
2L
tawbaatbri
datme
ig. 2. Strategies to increase the cooperativity of a spin transition: (i) linking Fe(II)ublished in Ref. [46]); (ii) increasing the level of H-bonding; (iii) increasing the lev
. Derivatives of the ligand 2,6-bis(pyrazol-3-yl)pyridine,1, applied in SCO
In their 2011 review, Olguín and Brooker observed that (athat time) there were “no examples in the literature of an SCO-ctive iron(II) complex of a substituted L1”, and suggested that thisas possibly due to “synthetic issues” [16]. Since then, there has
een a significant development in this regard, with eight new lig-nds based on L1 having been published in studies geared towardsttaining SCO phenomena in Fe(II) systems (Fig. 3). All of the deriva-ives of L1 that have been prepared in the last 9 years have recentlyeen covered by Halcrow, together with the possible syntheticoutes that lead to their formation [49], however here the focuss only on those which have been used in SCO.
The syntheses of all of these ligands use symmetric 2,6-isubstituted pyridine species as their starting point. Both L1
nd 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine (L2) are obtainedhrough firstly, the reflux of 2,6-diacetylpyridine with dimethylfor-
amide dimethyl acetal (L1) or dimethylformamide dimethylac-tamide (L2), before reflux with hydrazine monohydrate [50,51].
Fig. 3. 2,6-Bis(pyrazol-3-yl)pyridine (L1) and the de
s into polymers (figure prepared from the structure of [Fe(NH2trz)3](NO3)2·2H2O,· · ·� interactions.
The ligand L1 was used as the basis of the derivatives 2,6-bis(1-methylpyrazol-3-yl)pyridine (L6), 2,6-bis(1-isopropylpyrazol-3-yl)pyridine (L7), 2,6-bis(1-allylpyrazol-3-yl)pyridine (L8), and2,6-bis(1-benzylpyrazol-3-yl)pyridine (L9). The ligand L6 wasprepared from L1 by deprotonation of the pyrazolyl ringwith Na2CO3 and reaction with iodomethane [52]. The syn-theses of the ligands L7, L8, and L9 used the stronger baseLiH to deprotonate L1 prior to reaction with 2-iodopropane(L7), allyl bromide (L8), or benzyl bromide (L9) [53,54]. Theligands 2,6-bis(5-(2-hydroxyphenyl)-pyrazol-3-yl)pyridine (L3)and 2,6-bis(5-(2-methoxyphenyl)-pyrazol-3-yl)pyridine (L5) aresynthesised through a Claisen condensation involving ethyl-2,6-pyridinedicarboxylate and 2-hydroxyacetophenone (L3) or2-methoxyacetophenone (L5) with NaH as base, leading to the for-mation of bis-ˇ-diketone ligands [55], prior to the ring closingreaction with hydrazine monohydrate to yield the final products
[56]. The ligand L4 is observed as the product of fluoroboration ofthe ligand L3 upon coordination to the Fe(II) centre in the presenceof BF4
− [57]. Using the Claisen condensation approach, a variety ofderivatives should be accessible, simply through variation of the
rivatives that have been used in SCO research.
1 hemis
iasic
3
potFawfolotatfwa[Ns(ttraa[pioagtmbgnwoL
imaope(Tatphy2ot
6 G.A. Craig et al. / Coordination C
dentity and/or position of the functional group involved in thecetophenone reactant. While a derivative of L1 that has been sub-tituted at the para position of the pyridyl ring has been describedn the literature [58], none as yet has been used in SCO, in sharpontrast with the regio-isomer 1-bpp [48].
. Synthesis of [Fe(L1)2]2+ type salts
The first syntheses reported for these systems (Table 1 com-iles the known compounds of L1 and its derivatives) were carriedut under N2, and involved two methods. One was the direct reac-ion of the ligand L1 with the relevant Fe(II) salt (Fe(ClO4)2 ore(BF4)2) in hot EtOH/H2O mixtures before use of diethyl ethers a precipitating agent, while the other consisted of mixing L1ith FeCl2 prior to a NaX (X = Br−, I−, NO3
−, PF6−) salt being added
or anion exchange [59,60]. Generally, this gives powders that arerange/brown when they contain more molecules of H2O, andighten in colour towards yellow as the water molecules are drivenff on heating under N2. The first crystals suitable for single crys-al X-ray diffraction studies were obtained for [Fe(L1)2]I2·4H2Ond [Fe(L1)2](BF4)2·3H2O by growing them from water that con-ained a few drops of acetone or from acetone that contained aew drops of water, respectively [59]. The triflate salt of [Fe(L1)2]2+
as obtained by adding an aqueous solution of Li(CF3SO3) to hot EtOH/H2O solution of FeCl2 and L1 [61,62]. The saltsFe(L1)2](NCS)2·2H2O and [Fe(L1)2](NCSe)2 were obtained under2 by adding aqueous (NH4)SCN and KSeCN, respectively [63]. The
ynthesis of the compound [(Fe(L1)(NCS)2)2(4,4′-bipy)]·2MeOH4,4′-bipy = 4,4′-bipyridine) also had to be performed under N2,ogether with the layering of the reaction to produce the crys-als for single crystal X-ray diffraction studies [64]. The aerobiceaction of Fe(II) salts of more weakly coordinating anions suchs perchlorate or triflate with L3 in the presence of ascorbic acidllowed the successful isolation of various [Fe(L3)2]2+-based salts57,65,66]. An exception was provided by the synthesis of the com-ound [Fe(L3)(L4)](BF4)·5C3H6O·2H2O, where the anion BF4
− is notnert in the reaction conditions, leading to the fluoroboration [67]f one of the L3 ligands coordinated to the metal, via the addition of
“BF2” fragment between the O and the N atoms of one phenolroup and its adjacent pyrazolyl ring, together with the deprotona-ion of the latter two groups. Consequently, the complex cation was
onovalent, since the L4 ligand is anionic. No such issues were toe encountered for the ligand L5, due to the presence of a methoxyroup, rather than the hydroxyphenyl group, since the former can-ot be deprotonated. Similarly, aerobic conditions combined witheakly coordinating anions were sufficient to synthesise a family
f mononuclear compounds based on the L1 derivatives L2, L6, L7,8, and L9 [50,53,68].
Attempts to synthesise multifunctional SCO-based systemsnvolving L1 have tended to employ more exotic anions, frequently
etallates, to compensate the charge of the [Fe(L1)2]2+ cationsnd to confer additional functionalities upon the system. A linef research in the group of Coronado looked at combining theotential SCO properties of the [Fe(L1)2]2+ cation within a lay-red magnet by using oxalate based salts such as K3[Cr(ox)3]where ox = oxalate) to yield [Fe(L1)2]2[Cr(ox)3]ClO4·5H2O [69].his work was further developed to use the bimetallic 2Dnionic network [MnCr(ox)3]∞−, which had to be preparedhrough a metathesis reaction, in the synthesis of the com-ound [Fe(L1)2][MnCr(ox)3]2·L1·MeOH [70]. This approach toybrid systems also used a copper diselenolate complex to
ield the system [Fe(L1)2][Cu(pds)2]2·xS (where pds = pyrazine-,3-diselenolate, and xS = 3CH3CN or 2.5MeOH), with the aimf obtaining a lattice with SCO and high electrical conduc-ivity [71]. Anions of the type [Cr(Q)(ox)2]− (where Q = 1,
try Reviews 269 (2014) 13–31
10-phenanthroline (phen) or 2,2′-bipyridine (bpy)) were used toconstruct 3D lattices that were robust to dehydration/rehydrationprocesses with a view to achieving solvent sensing SCO systems[72–74].
A recent approach by Halcrow and co-workers used cyanomet-allate and thiocyanometallate salts, motivated by the possibilitythat the fluorescence properties of the resulting salts could be tunedby switching the spin state [75]. The systems [Fe(L1)2][M(CN)2]2(M = Ag, Au) and [Fe(L1)2][Au(SCN)2]2·CF3CH2OH were synthesisedthrough addition of K[M(CN)2] (M = Ag, Au) or K[Au(SCN)2] to anaqueous solution of [Fe(L1)2]Cl2.
An article by Murray and co-workers described their attemptto combine host/guest chemistry with SCO, by using N,N′-bis(4-pyridyl-methyl)diaza-18-crown-6 (bpmdc) in a reaction withFe(ClO4)2, KSeCN, and L1 [76]. The initial idea was that the crownether would bridge two Fe(II) centres, to yield a dinuclear sys-tem analogous to [(Fe(L1)(NCS)2)2(4,4′-bipy)]·2MeOH, with cationexchange within the crown ether providing a means of varying theguest and magnetic properties of the system. As described in Sec-tion 5, this combination of reaction components led to an unusualsystem [Fe(L1)2][K ⊂ bpmdc](SeCN)1.7(ClO4)1.3·MeOH·H2O whichpossessed an interesting crystal lattice.
4. Spin switching in [Fe(L1)2]2+ type salts
4.1. General comment
Salts based on L1 and its derivatives are notable for theirwide range of spin transition properties. They may be purelyHS systems, which is most frequently the case with the lig-ands that incorporate bulky substituents, as in the compounds[Fe(L3)2](ClO4)2·2C3H7OH [57], [Fe(L5)2](ClO4)2·0.5(CH3)2CO [85],or [Fe(L9)2](BF4)2 [53]. Many salts are also observed to be LScompounds as a consequence of their degree of hydration, withconversion to the HS state induced through heating of the sys-tem to drive off the lattice water molecules. Within those systemsthat display SCO, there also exists a wide variety of types oftransition. These encompass gradual transitions, for example thecompound [Fe(L1)2][BPh4]2·0.5H2O [75]; incomplete transitions,as in the case of [Fe(L1)2](ClO4)2·1.75C3H6O·1.5C4H10O [57]; andstructured transitions (transitions which take place in more thanone step), as found for the compound [Fe(L2)2](ClO4)2·2H2O [68].Most interesting are those transitions that take place with hys-teresis, and this family of compounds features notably broadbi-stable loops, particularly in the case of [Fe(L1)2](CF3SO3)2·H2O(140 K) (see below) [62], but includes recent examples such as[Fe(L3)2](ClO4)2·H2O·2(CH3)2CO (40 K) [66], and [Fe(L2)2](BF4)2(40 K) [50]. The magnetic properties of these systems depend ontheir degree of hydration/solvation, hence the strict distinctionin Table 1 of compounds depending on their solvent content. Inthe attempts to synthesise multifunctional SCO systems based onL1 and its derivatives, there has been a limited degree of suc-cess. In the case of the recent work by Halcrow and co-workersto obtain fluorescent thermometers by combining potentiallyemissive anions, SCO was observed (although not with hystere-sis) but not luminescence at room temperature. Inversely, theattempt by Coronado and Galán-Mascarós to attain magneticordering of the bimetallic anion [MnCr(ox)3]− and SCO in theaccompanying [Fe(L1)2]2+ cations was successful in the former,but not in the latter, with the cations not displaying any clearswitch in their spin state. The wide variety of SCO behaviour
observed in L1-based systems is also complemented by the dif-ferent methods through which the spin state of the systemsmay be controlled, which will now be discussed with key exam-ples.
G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31 17
Table 1All of the known Fe(II) compounds containing L1 or its derivatives, together with the relevant literature reference.
18 G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31
Fig. 4. Spin transition curve of the compound [Fe(L1)2](CF3SO3)2·H2O. Taken andaws
4
itashiItpaebiifp
sof[cstmiahwtiantapsHw
dapted from Ref. [62]. The squares are derived from Mossbauer measurements,hile the downward and upward triangles represent magnetic susceptibility mea-
urements in the cooling and heating mode, respectively.
.2. Thermal switching
Temperature variation is the most common method of seek-ng SCO in a given system. This mechanism [86] exploits the facthat the increased entropy resulting from the longer Fe-donortom bonds and the increased magnetic degeneracy in the HState favours the population of the corresponding quintet state atigher temperatures, while at low temperatures the singlet state
s mostly populated as it exhibits then the lowest free energy [19].n the family of L1 systems, it is important to distinguish betweenemperature variation as the cause of SCO in a compound, and tem-erature variation bringing about a change in the composition of
compound with consequences for the magnetic properties. Thexamples that are highlighted in Section 4.2 have been selectedecause temperature variation was seen to give rise to particularly
nteresting spin transition curves in the solid state, as well as includ-ng a study by Halcrow and co-workers which marked a departurerom the usual studies of L1 systems, by looking at the magneticroperties in solution.
Perhaps the most impressive thermally induced spin tran-ition encountered to date for L1 based systems is thatf [Fe(L1)2](CF3SO3)2·H2O [62]. This compound is obtainedrom a reaction which initially yields the LS trihydrate saltFe(L1)2](CF3SO3)2·3H2O [61]. Heating this compound to 355 Kauses the release of two water molecules, with a concomitantwitch to the high spin state. A combination of magnetic suscep-ibility measurements and Mössbauer spectroscopy showed this
onohydrate compound to remain in the HS state in the cool-ng mode down to 150 K (Fig. 4). Here, the compound underwent
very abrupt transition, with the process complete at 140 K. Oneating, the compound remained largely in the LS state until 160 K,here there was a partial, less abrupt transition to a HS popula-
ion of 33% at 190 K. At around 260 K, there was a sharp increasen the HS fraction, returning the system fully to the quintet statet room temperature. At its widest point, the hysteresis loop origi-ating from the divergence of the magnetic response measured inhe heating and cooling modes spans 140 K: the largest observed forny molecular SCO system. This highly cooperative system could be
hoto-excited or thermally quenched to yield meta-stable high spintates. The relaxation kinetics after both methods of meta-stableS state trapping deviated from simple exponential behaviour, andere also different to each other (see Section 4.3). Unfortunately,
Fig. 5. Spin transition curves for the compound [Fe(L1)2](NCS)2·2H2O.Taken and adapted from Ref. [95].
only the crystal structure of the LS trihydrate salt has so far been elu-cidated, which showed the direct interaction of the solvent watermolecules with three of the four pyrazolyl rings. There would begreat interest in obtaining that of the monohydrate, with the lightit could possibly shed on how intermolecular interactions may con-tribute to such a broad bi-stable domain [87–91].
The two-step transition observed in the heating mode of[Fe(L1)2](CF3SO3)2·H2O is rare in L1 systems. In fact, there areonly three other examples of such structured transitions. One wasdescribed in the case of the system [Fe(L1)2][NCS]2·2H2O [63]. Theas-synthesised compound was brown, indicative of a largely LSpopulation at room temperature, but this gave way to an orange-yellow colour on spontaneous loss of water molecules to yield thedihydrate. The initial study of the compound concluded that a ther-mal cycle of this stable dihydrate presented a two-step transitionwith thermal hysteresis associated with each step. The high tem-perature transition was centred at 251 K, with a hysteresis loopspanning 9 K, while the low temperature spin switch was over abroader region: 24 K, centred at 205 K. Unlike in the case of othertwo-step transitions [92,93], here there was no definitive proof ofan intermediate phase, and the steps did not correspond to 50% ofthe Fe(II) switching spin state.
The complex [Fe(L1)2][NCS]2·2H2O was further studied by Bhat-tacharjee and co-workers. They described an unusual property ofthe system, which was that on immediately repeating the ther-mal cycle the compound changed its behaviour from a two-steptransition to a one step, hysteretic transition with a bi-stableregion measuring only 2 K, centred around 232 K (Fig. 5) [94].Even more remarkable was that if this cycled sample (whichthey called “used”) was left for 1 day at 300 K, then the orig-inal two-step behaviour was recovered. A cycling effect on themagnetic properties has also recently been observed in the com-pound [Fe(L3)2](ClO4)2·2THF·H2O, although in that case, the effectswere irreversible, and could be ascribed to the exchange of lat-tice THF molecules for water molecules from the air [65]. This wasnot the case for [Fe(L1)2][NCS]2·2H2O. The compound possessestwo different phases, phase 1 and phase 2, where the fresh com-pound is in phase 1 at room temperature [95,96]. The cooling modeinduces a structural transition to phase 2 at 289 K, prior to the two-step SCO. A variable temperature differential scanning calorimetry
(DSC) study displayed three peaks in the cooling mode, and onlytwo in the heating mode. Two of the peaks in the cooling modewere the partner peaks for the electronic transitions in the heatingmode, arising from the two-step thermal hysteresis. The remaining
G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31 19
Fig. 6. Spin transition curves of the compound [Fe(L2)2](ClO4)2·2H2O (whitec[
T
attttPsstttwtXXiAisftbt
otaieeftmactHivscf
Fig. 7. Variable temperature magnetic susceptibility data for [Fe(L1)2](BF4)2 in
ircles). Also shown are the magnetic properties of the isostructural saltFe(L2)2](BF4)2·2H2O (full circles).
aken and adapted from Ref. [68].
nomaly from the cooling mode, at 289 K, was ascribed to a struc-ural phase transition. It was proposed therefore, that its partnerransition in the heating mode would occur above room tempera-ure. As a result, completion of the thermal cycle, and the returno room temperature, yields a mixture of phase 1 and phase 2.hase 1 was observed to be that which was thermodynamicallytable at room temperature, and the mixture is a result of thelow conversion of phase 2 to phase 1. A cycle of this mix leadso the one step spin crossover, and the sample is fully convertedo phase 2. These effects were attributed to the reorientation ofhe thiocyanate anions and rupture of the hydrogen bonding net-ork that was present in the system, brought about by the phase
ransition. While this phenomenon was elucidated with powder-ray diffraction (PXRD) studies, there is a lack of single crystal-ray data subsequent to the thermal cycle, which would allow
dentification of how the hydrogen bonding network is affected. combination of these phenomena, together with those observed
n the compound [Fe(L3)2](ClO4)2·2THF·H2O, was reported in theystem [Fe(L1)2][Pt(ox)2]·H2O [97]. Cycling the latter yielded dif-erent two-step transitions, associated with gradual dehydration ofhe sample, but leaving the compound to stand allowed the originalehaviour to be recovered, due to re-entry of water molecules tohe sample.
An even more highly structured LS → HS thermal transition wasbserved in the compound [Fe(L2)2](ClO4)2·2H2O [68]. At roomemperature, the compound was mostly in the HS state, based on
�T value of 3.1 cm3 mol−1 K (Fig. 6). SCO was induced on cool-ng the compound, with an abrupt step at 265 K, followed by anxtended gradual transition, and finally with a more highly coop-rative step at 120 K, which switched the remaining HS centresully to the LS state. The heating mode revealed a conversion backo the HS state that contained discontinuous steps at, approxi-
ately, 200, 230, and 265 K. The hysteresis loop measured 65 Kt its widest point. This very unusual sequence of spin transitionsontrasted sharply with the magnetic properties of the isostruc-ural salt [Fe(L2)2](BF4)2·2H2O, which exists in a fairly constant 1:1S:LS ratio (Fig. 6) [50]. Both systems displayed dramatic changes
n their magnetic properties on desolvation (see Section 4.5). A
ariable temperature PXRD study of the dihydrate perchlorate salthowed that between 300 K and 250 K, the compound underwent arystallographic phase change. Unfortunately, the results obtainedrom the single crystal X-ray diffraction study could not be directly
compared to the PXRD data, due to difficulties in unequivocallyassigning the space group of the compound at low temperatures. Ittherefore remains unclear what structural change lies behind theabrupt step at 120 K in the cooling mode.
In solution, the variation of temperature of SCO compoundsleads to essentially a Boltzmann-type population of the HS state asthe temperature is increased [35,37,98,99]. Barrett and co-workerstook advantage of the favourable solubility of the tetrafluoroboratesalt of [Fe(L1)2]2+ to investigate the effect of hydrogen bonding onthe magnetic properties of different solutions of these salts [36]. Themain conclusion of that work was that hydrogen bonding to watersignificantly stabilises the low spin state of [Fe(L1)2]2+ cations,manifested as a shift of T1/2 to higher values as the degree of inter-action with H2O molecules was increased. This is in Fig. 7: a plot ofthe variable temperature magnetic susceptibility of [Fe(L1)2](BF4)2in various mixtures of (CD3)2CO:D2O. The value of T1/2 was 247 Kin pure deuterated acetone, with a dramatic change in the criticaltemperature as the mix was tuned towards pure D2O.
4.3. LIESST and thermal trapping
One of the major developments in the field of SCO was thedemonstration that the spin state of some highly cooperative com-pounds could be switched through photo-excitation. The discoverywas made in the solid state by Decurtins and co-workers, who irra-diated the compound [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) atcryogenic temperatures and described how the relaxation back tothe ground LS state slowed sufficiently to enable trapping of thesample in a meta-stable HS state [100,101]. This phenomenon wascalled Light Induced Excited Spin State Trapping (LIESST). Here, thelarge difference in Fe-donor atom bond lengths between the HSand LS states together with the small �E◦
HL creates a barrier torelaxation, and so the system remains trapped [102]. The samegroup then achieved the reverse-LIESST effect – re-population ofthe LS state by irradiating the meta-stable HS state – by chang-ing the wavelength of light used to irradiate the same compound[103,104]. An alternative trapping method is by rapid or flash cool-ing [105]. For this technique, the SCO system is quickly inserted into
a given apparatus which has been pre-cooled to very low tempera-tures. This trapping, Thermally Induced Excited Spin State Trapping(TIESST) essentially freezes in the HS phase that was stable at hightemperature. The thermal limit of these meta-stable domains at low
20 G.A. Craig et al. / Coordination Chemis
Fig. 8. Spin transition curves and LIESST properties of the compound[Fe(L1)2](BF4)2·xH2O: x = 3 (white circles: LIESST experiment and subsequentheating; full circles: cooling mode) and x = 0 (white squares: LIESST experimentap
T
tttcasH
coptettotahoWproHwovatm
tvitmtcmcs
nd subsequent heating mode; full squares: cooling and heating mode prior tohoto-excitation).
aken and adapted from Ref. [80].
emperature is defined by T(LIESST) (or T(TIESST), analogously forhe thermal trapping experiment) [106]. This value is obtained byrapping the system and warming it at a given rate established byonvention [106] until it gradually enters the region where relax-tion to the ground state is thermally activated, leading to thewitch back to the LS state. The minimum of the derivative of theS → LS curve yields the value of T(LIESST).
The most essential work on spin state trapping in the L1 series ofompounds was performed by Marcén et al. in 2002 [80]. The aimsf the work were (i) to rigorously determine the SCO and LIESSTroperties of a series of [Fe(L1)2]2+ salts, thus, without modifyinghe first coordination sphere of the complex; and (ii) to analyse theffect of the degree of hydration and the charge-balancing anion onhe T(LIESST) values in this family. This can be illustrated throughhe example of the compound [Fe(L1)2](BF4)2·xH2O, where x = 3r 0 [59]. The trihydrate complex undergoes a largely completehermal spin transition, with a value of T1/2 of 288 K (Fig. 8). Thenhydrous salt displays a highly cooperative spin transition, with aysteresis loop measuring 16 K that is centred at 176 K. The effectf dehydration on the system’s LIESST properties is equally drastic.hile both forms may be almost fully excited to their room tem-
erature values of �T, the trihydrate displays a far more gradualelaxation to the LS ground state, associated with a T(LIESST) valuef 70 K, while the anhydrous salt is maintained in the meta-stableS state over a larger range of temperature, undergoing relaxationith a T(LIESST) value of 110 K. Therefore, not only does the degree
f hydration of [Fe(L1)2](BF4)2·xH2O determine its SCO properties,arying from a gradual, reversible transition, to a highly cooper-tive spin switch with hysteresis, but it also affects the range ofemperatures over which a light-induced meta-stable HS state may
aintained.In principle, at sufficiently low temperatures, the lifetime of
hese meta-stable states tends towards infinity. However, as thealue of T(TIESST) is approached, these lifetimes become increas-ngly short, and the activation energy associated with the relaxationo the LS state may be determined through isothermal relaxation
easurements. The shape of the curves then gives an indica-ion of the cooperativity of the relaxation process. High levels of
ooperativity cause self-accelerated behaviour, manifested as sig-oidal character in the relaxation curves [107]. Less cooperative
ases are observed to yield more purely exponential curves, andtretched exponential curves may result from self-deceleration
try Reviews 269 (2014) 13–31
caused by inhomogeneity in the lattice or disorder, which canlead to a distribution of activation energies for the relaxation[108–110]. Particularly interesting situations may arise when thereare discrepancies between the relaxation curves seen subsequentto photo-excitation and thermal quenching experiments. In thissense, [Fe(L1)2](BF4)2 may be grouped together with two othercompounds in the L1 family: [Fe(L3)2](ClO4)2·2(CH3)2CO·H2O [66]and [Fe(L1)2](CF3SO3)2·H2O [61,62].
Meta-stable HS states could be obtained in [Fe(L1)2](BF4)2through LIESST experiments as described above, and also via ther-mal quenching. The quenching experiments allowed freezing in ofthe high temperature crystal structure, and the relaxation fromthe HS to LS state was therefore concluded to be determined bythe rate of the crystallographic phase transition that accompaniedthe spin switch [77]. However, despite application of several phe-nomenological models for phase transitions, the authors of thestudy were unable to achieve a satisfactory reproduction of thekinetics observed. After LIESST experiments, the compound wasinterpreted to undergo a two-step relaxation. The first step caused5% of the HS Fe(II) ions to relax to the LS state. This adjustment inthe HS/LS ratio was posited as triggering a phase transition, withthe resulting new phase stabilising the meta-stable HS state, lead-ing to very long lifetimes. The second step, which completed thetransformation to the ground state, was observed on heating thecompound to the 90–100 K temperature range.
For the compound [Fe(L1)2](CF3SO3)2·H2O, the relaxation fromthe photo-excited phase was faster than the relaxation from thethermally trapped phase, for any given temperature [62]. Addi-tionally, the difference was clear between the sigmoidal natureof the relaxation after thermal quenching and the more exponen-tial behaviour after LIESST experiments (Fig. 9). This dissimilarityis evidence of a difference in the relaxation mechanisms of thiscompound, dependent on how the meta-stable HS state has beengenerated. As in the case of [Fe(L1)2](BF4)2, the authors concludedthat the quenching experiment froze in the high temperature HSphase, with the consequence that, prior to HS → LS relaxation, thereis an additional structural phase transition. This was not the caseafter LIESST experiments, where the relaxation was a purely elec-tronic effect.
However, in both of the above-described examples, there was(and still is) a crucial lack of structural data that would clarify whatthe structural phase transitions might entail. A recent paper con-cerning the system [Fe(L3)2](ClO4)2·2(CH3)2CO·H2O combined notonly the relevant magnetic measurements after LIESST and thermaltrapping experiments, but also was able to compare single crystalX-ray structures corresponding to both meta-stable phases [84].This compound was first demonstrated to undergo a highly coop-erative SCO with a hysteresis loop measuring 40 K [66]. The highvalue of T(TIESST) encountered for the compound, 106 K, allowed athermal trapping experiment to yield the crystal structure associ-ated with the meta-stable HS state. It was shown to possess disorderin half of the perchlorate anions and in all of the acetone moleculesthat were present in the crystal lattice. The crystal structures elu-cidated for the LS state at 100 K, and also within the hysteresis loopat 150 K, showed the singlet state to be free of structural disorder.Therefore, the thermal HS → LS spin switch in the compound wascoupled to a disorder/order transition. The follow up LIESST exper-iments showed that the thermally activated relaxation regime wasreached at lower temperatures after photo-excitation than afterthermal trapping, with T(LIESST) = 92 K. This discrepancy was thenreinforced by the kinetics measurements, for which the relaxationat 90 K after both methods of spin trapping were performed can
be seen in Fig. 10. As in the compound [Fe(L1)2](CF3SO3)2·H2O,the thermally trapped phase relaxed more slowly than the photo-induced phase, and the relaxation curve was more sigmoidal incharacter.
G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31 21
Fc
T
ror(bsa
Fm˛[
Fig. 11. (Top) Superposition of the cations in [Fe(L3)2](ClO4)2·2(CH3)2CO·H2O after
ig. 9. Relaxation curves after LIESST (top) and thermal quenching (bottom) for theompound [Fe(L1)2](CF3SO3)2·H2O.
aken and adapted from Ref. [62].
Importantly, it was possible to resolve the crystal structure cor-esponding to the photo-excited phase at 30 K [84]. The analysisf this system showed that there was no disorder in the perchlo-ate anions, and substantially less in the acetone solvent molecules
Fig. 11). The unit cell of the photo-induced phase also resem-les more closely that of the LS ground state, than that of the HState. DSC of the thermally quenched phase showed that the relax-tion to the LS state was associated with two processes, deduced
ig. 10. Relaxation curves at 90 K subsequent to LIESST (white squares) and ther-al quenching (white circles), together with variation of the crystallographic
parameter as measured at 93 K after thermal trapping of the compoundFe(L3)2](ClO4)2·2(CH3)2CO·H2O [84].
thermal trapping (TTHS) and photo-excitation (PIHS). (Bottom) The contents of theasymmetric unit cell, with the difference in the level of crystallographic disorderhighlighted (grey/black for acetone and yellow/red for perchlorate) [84].
from the presence of two thermodynamic anomalies in the heat-ing mode. It was therefore proposed that thermal trapping wouldlead to a phase that would undergo a disorder → order phase tran-sition, before relaxation of the [Fe(L3)2]2+ cations from the HS to theLS state. On the other hand, the photo-induced phase, with its farlesser degree of disorder, would relax directly from the HS to the LSstate. The model that was then applied to the kinetics showed thatonce the thermally trapped phase had undergone the structuralphase transition, then the Arrhenius law governing its relaxationto the LS ground state was the same as that of the photo-inducedHS state, in support of the idea that there were two processes at playin the TIESST relaxation. This work represented the first time thatthese suppositions could be founded on crystallographic data. Italso unveiled the potential of using LIESST experiments to decoupledisorder → order phase transitions from spin state transitions.
Compounds based on L1 have also provided the platform forstudying the consequences of T(LIESST) lying close to the value ofT1/2 in the cooling mode. According to the inverse energy-gap law[111,112], the stronger the ligand field experienced by the Fe(II)centre, the larger the zero-point energy gap �E◦
HL between thelowest vibrational levels of the meta-stable 5T2 HS and ground 1A1LS states, and so the shorter the lifetime of the meta-stable HS state.The immediate consequence of this is that high values of T1/2(↓) areassociated with low values of T(LIESST), and vice versa. Létard andco-workers thereby developed a strategy to rationally attain veryclose proximity of the T(LIESST) and T1/2 values [113,114]. This con-sisted of selecting a candidate system with values of T(LIESST) andT1/2(↓) that were already relatively close, opting for the compound[Fe(L1)2](BF4)2, which has values of 110 K and 168 K, respectively[115,116]. The next step was to dope the lattice with Mn(II) ions,with the aim of decreasing the temperature at which the thermalspin transition occurred, bringing it into closer proximity with theT(LIESST) limit. The presence of the Mn(II) ion, which is larger than
either the HS or LS Fe(II) ion, causes a negative lattice pressure.This lattice pressure induces a lengthening of the Fe N bonds anda weakening of the ligand field, resulting in a stabilisation of theHS state, with an increasing stabilisation observed with increasing
22 G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31
T
Mluts1
aHoksoofalwlccmltwchoto
4
L[cwtrtatbs
Fig. 13. (Top) The approaching of the values of T(LIESST) and T1/2 (both of the cool-ing and warming branches) observed on increasing the fraction of Mn(II) dopedinto the lattice of [FexMn1−x(L1)2](BF4)2 The shaded regions de-limit 5–95% of thethermal SCO and 95–5% of the LIESST relaxation. (Bottom) Isothermal relaxationmeasurements used to derive the quasi-static hysteresis loop (solid line) which is
Fig. 12. Spin transition curves for the doped system [FexMn1−x(L1)2](BF4)2.aken and adapted from Ref. [116].
n(II) content. At the same time, metal dilution had been shown toower the temperature of a thermal transition without impactingpon the stability of the photo-induced phase [2,113]. The effect ofhis doping is in Fig. 12. It may be observed that the strategy wasuccessful, shifting T1/2(↓) for the 100% Fe(II) sample from 175 K to31 K for the sample which is split 84.7:15.3 Fe(II):Mn(II).
The overlap that is produced of the T(LIESST) relaxation regimend that of the thermal SCO T1/2(↓) is shown in Fig. 13. The residualS fraction observed on lowering the temperature was interpretedn the basis that a certain proportion of the HS centres remaininetically blocked. The conclusion drawn was that therefore thehape of the hysteresis loop would strongly depend on the speedf the measurement. Close to the T(LIESST) regime, the relaxationf these trapped ions should be thermally activated, driving theormation of the thermodynamically stable LS state. This relax-tion could be tracked at a series of temperatures in the hysteresisoop, allowing a quasi-static hysteresis loop [22] to be determined
hich is well-separated from the thermally apparent hysteresisoop initially measured (Fig. 13, bottom). The difference was espe-ially pronounced in the cooling mode, while the heating modeould almost be superimposed on the result of the relaxationeasurements, demonstrating that the effect on heating is neg-
igible. A similar phenomenon has also been recently described inhe system [Fe(L3)2](ClO4)2·2(CH3)2CO·H2O, in the neat compoundithout the need for doping [124]. In that compound, the signifi-
ant self-decelerating character of the relaxation curves within theysteresis loop was attributed to the coupling of a crystallographicrdering process which was simultaneous with the relaxation fromhe quintet to the singlet state, as well as the varying morphologyf the samples that were used to perform the measurements.
.4. Application of pressure
Another method of inducing the switch between HS andS states is through the application of hydrostatic pressure27,28,92,117,118]. The application of external pressure to a spinrossover system increases the zero point energy difference �E◦
HL,hich corresponds to the vertical separation of the potential wells
hat represent the high and low spin states, by destabilising (i.e.aising the HS potential well in energy with respect to the LS poten-ial well) the HS state [119]. This decreases the activation energy
ssociated with the HS → LS transition, and drives the HS stateowards the denser LS state. This favouring of the LS ground staterings about an increase in the value of T1/2 with increasing pres-ure and, often, a flattening of the crossover [28]. However, it must
well separated from the initially measured cycle (dots and lines).
Taken and adapted from Ref. [116].
also be mentioned that a recent example showed the applicationof pressure to be antagonistic towards SCO, due to the occurrenceof strong negative linear compression resulting from the shape ofthe molecule under study [27].
To date, the most complete work describing the applicationof hydrostatic pressure to a L1 system is that of Shepherd et al.,performed on the compound [(Fe(L1)(NCS)2)2(4,4′-bipy)]·2MeOH,where 4,4′-bipy = 4,4′-bipyridine (Fig. 14). The compound wasoriginally described in 2008 [64], and consists of two pseudo-octahedral Fe(II) centres that are equatorially coordinated by one L1ligand each, and are each axially coordinated by two NCS− anions,leaving the remaining equatorial position to be occupied by the4,4′-bipy ligand, which bridges both Fe(II) ions. The compoundundergoes a partial transition at around 118 K, entering a plateau at�T = 4.7 cm3 mol−1 K at around 80 K. The dinuclear nature of the sys-tem meant that two possibilities were available: case A, in whichone of the metal centres in all of the molecules undergoes SCO,leading to LS–HS pairs; or case B, where both of the metal centresin half of the molecules undergo SCO, leading to an equal mixture ofLS–LS and HS–HS complexes. Unless the LS sites are randomly dis-tributed, then X-ray crystallography allows clarification of which
case is observed. This approach was used to reveal that the com-pound belonged to case A [83]. At low temperatures, the LIESSTeffect was observed, allowing for the LS centres to be excited tothe HS state, with a value of T(LIESST) for the system of 72 K. The
G.A. Craig et al. / Coordination Chemis
FF(
cwpiw
voLonpgopwstStstFt
[
F[
T
ig. 14. Molecular structure of the compound [(Fe(L1)(NCS)2)2(4,4′-bipy)]·2MeOH.igure made from the .cif available through the Cambridge Structural DatabaseCSD).
ompound undergoes a crystallographic phase transition at 161 K,ell above the temperature of the SCO (118 K). The effect of thishase transition was to remove the equivalency of the Fe(II) sites
n such a way that above 161 K there was a unique Fe(II) centre,hile below 161 K there were four.
The pressure-induced transition in this compound was followedia Raman spectroscopy and X-ray crystallography [26]. The changef vibrational modes within the lattice on switching from HS toS can be tracked effectively with Raman spectroscopy, which isften seen as a displacement in the Raman shifts of a given sig-al [20,21,120–123]. A quantitative assignment of the spin stateopulations is then possible based on the relative intensities of theiven signals. For [(Fe(L1)(NCS)2)2(4,4′-bipy)]·2MeOH, the resultsf this approach are shown in Fig. 15 [26]. Above 7 kbar of appliedressure, the LS marker peaks, which lie at higher Raman shiftsith respect to their HS counterparts, begin to appear. There is a
ignificant range of applied pressures over which both the HS andhe LS marker peaks co-exist, indicating that the pressure inducedCO is gradual in nature. The piezo-crystallographic study under-aken revealed that the application of pressure to the compounduppressed the crystallographic phase transition seen on varyinghe temperature. The result was that there was only one, uniquee(II) centre observed over the range of pressures measured, and
his resulted in a more complete, and more gradual, transition.
More recently, the compound [Fe(L3)2](ClO4)2·2(CH3)2CO·H2O66] was also switched from the HS to the LS state through the
ig. 15. Normalised Raman intensities of marker peaks in the compound(Fe(L1)(NCS)2)2(4,4′-bipy)]·2MeOH on increasing pressure.
aken and adapted from Ref. [26].
try Reviews 269 (2014) 13–31 23
application of hydrostatic pressure. In that study, there was seen tobe less overlap of the HS and LS marker peaks, suggesting a moreabrupt spin switch [124]. However, that work was not accompaniedby SQUID measurements, nor by a crystallographic study. In fact,the need for more piezo-crystallographic studies in the field of SCOhas recently been highlighted by Guionneau and Collet [125].
4.5. Absorption/desorption of guest molecules
The change in optical properties that accompanies a spin switchmay be exploited to make a sensor if the transition is caused by theinsertion/removal of guest molecules into/from the lattice. Often,the best way to ensure that a lattice is robust and stable towards thistype of exchange is to form an extended network, often of the MetalOrganic Frameworks/Porous Coordination Polymer (MOFs/PCPs)type [33,126–139]. In spite of this, there are a growing number ofworks where reversible exchange of solvents has been employed tocontrol the spin state of materials made of discrete molecules. Theremoval of one molecule of diethyl ether from the lattice of the1-bpp based system [Fe(dppFc)2](BF4)2·2H2O (where dppFc = 1-ferrocenyl-2-(2,6-bis(pyrazolyl)pyridyl)ethylene) [140] modulatesthe SCO properties of the system, although this effect was onlyobserved on lowering the temperature. A wider range of exchangephenomena were described for the compound [Fe(tpa)(NCS)2](where tpa = tris(2-pyridylmethyl)amine) on absorption of MeOHvapours [141]. At room temperature, the entry of the guest solventwas enough to switch half of the Fe(II) centres from the HS to the LSstate. A similar effect was observed on using EtOH, and both alco-hol molecules were employed to displace various organic solventsfrom the lattice of [Fe(tpa)(NCS)2] molecules [32].
While the dependence of the magnetic properties of L1-basedsystems on the degree of hydration of the sample has been illus-trated, few works have used this as a controlled means of switchingthe spin state through absorption/desorption processes. One suchinvestigation used the compounds [Fe(L1)2][Cr(bpy)(ox)2]2·2H2Oand [Fe(L1)2][Cr(phen)(ox)2]2·0.5H2O·0.5MeOH as the startingpoint for reversible hydration and dehydration of a compound asmethod of switching the spin state [72–74]. The as-crystalliseddihydrate [Fe(L1)2][Cr(bpy)(ox)2]2·2H2O possesses two crystallo-graphically independent Fe(II) ions at room temperature, each onein a different of the either high or low spin states, respectively. Heat-ing the compound to 400 K provoked an initial decrease followedby a sharp increase in the �T product to a fully HS state (Fig. 16).Thermogravimetric (TGA) and DSC analysis clarified that the ini-tial decrease was due to desorption of the water molecules, andthe subsequent increase was a result of SCO to the fully HS state. Asubsequent thermal cycle showed that the dehydrated compoundunderwent a complete, cooperative SCO with a hysteresis loop thatspanned 16 K, centred at 361 K. Therefore, at room temperature, thesystem was either in the 50% HS state, or in the 100% LS state, as afunction of the amount of water molecules present in the lattice.
This system could in fact be reversibly dehydrated and rehy-drated, however, the magnetic properties on rehydration wererather different to those of the original compound. Rehydrationbrought about the formation of a compound which was fully HSat room temperature, rather than the 50/50 mixture previouslyobserved. On heating, the same dehydration followed by hystereticSCO behaviour was registered. A single crystal X-ray diffractionstudy showed that the effect of this rehydration was to producea lattice with only one independent Fe(II) site, rather than thetwo independent sites that were first observed. The system wasthen exploited as a method of solvent sensing: rather than simply
reversibly dehydrating and rehydrating the sample, it was exposedto MeOH vapours, and was demonstrated to absorb the small alco-hol molecule. This solvation process was also associated with achange in the magnetic properties of the sample, inducing a spin
24 G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31
Fig. 16. (Left) Magnetic properties of the compound [Fe(L1)2][Cr(bpy)(ox)2]2·2H2O.(Right) Magnetic properties of [Fe(L1)2][Cr(bpy)(ox)2]2 after exposure to MeOHvb
T
satss
dtottcFaH3sDcc
FOc[
T
Fig. 18. Illustration of the terpyridine embrace, as present in [Fe(L1)2]Cl2·6.5H2O
apours. Arrows 2 and 3 correspond to measurements on the dehydrated sample inoth figures.
aken and adapted from Ref. [73].
witch from the 100% LS state at RT of the dehydrated sample, to 33% HS sample for the solvated sample (Fig. 16, right). Desorp-ion of alcohol upon heating led to the recovery of the dehydratedample, with its intrinsic SCO behaviour. However, in the case ofolvation, the crystal structure could not be resolved.
Similarly, in the system [Fe(L2)2](BF4)2·2H2O, the initial dihy-rate is approximately in a 1:1 HS:LS ratio, which may be convertedo a fully HS system by heating the compound in vacuo to driveff the water molecules, leading to the anhydrous phase B [50]. Ahermal cycle of this anhydrous phase revealed a hysteresis loophat was initially 65 K wide, but was reduced to 37 K on repeatedycling, with both centred around approximately 207 K (Fig. 17).urther investigation of the anhydrous compound showed it to bessociated with five different crystallographic phases. Starting fromS B, cooling induced a phase transition to another HS phase, C, at03 K, around 100 K above the onset of the thermal HS → LS tran-
ition. Further cooling leads to the formation of another HS phase,, at 268 K, and it is this phase which then undergoes the highlyooperative SCO process to the fully LS species E. This study wasonducted through PXRD measurements. Recrystallisation of the
ig. 17. Magnetic properties of the compound [Fe(L2)2](BF4)2·2H2O (white circles).n heating the sample is dehydrated (indicated) and the black and grey circlesorrespond to the first cooling and heating mode measured for the compoundFe(L2)2](BF4)2.
aken and adapted from Ref. [50].
[82]. Hydrogen atoms have been omitted for clarity. The double-headed blue arrowscorrespond to �· · ·� contacts, while the single-headed red arrows correspond toC H· · ·� interactions.
compound produces a fifth phase, A, that undergoes also a SCOwithout any crystallographic phase change, as revealed by singleX-ray diffraction crystal measurements in the high and low spinstates. Compound A is highly hygroscopic, preventing collection ofmagnetic data.
5. Structural aspects of [Fe(L1)2]2+ type salts
A description of the structure of these salts may be conducted byfocusing on the two most salient features that characterise it. Oneis the so called terpyridine embrace, which organises the [Fe(L1)2]2+
cations as sheets through a network of �· · ·� and complementaryC H· · ·� intermolecular interactions (see below). Thus, compoundsmay exhibit one undistorted such interaction, usually when “sim-ple” anions, of the type Cl−, BF4
−, and so on are involved. Otherwise,systems may deviate from this packing motif, a situation that canbe brought about accidentally through interactions with solvents inthe lattice, or deliberately through the use of “exotic” anions, suchas [Au(SCN)2]2
−, [Cr(L)(ox)2]−, amongst others. The other prismthat can be used to analyse the structure focuses on the shape anddistortions that may occur in the [Fe(L1)2]2+ cations themselves,and their bearing on spin state and magnetic properties [145].
5.1. Terpyridine embrace
The favoured packing of these systems is based on the “terpyri-dine embrace”. Scudder and co-workers demonstrated this motifto be predominant in compounds of the general formula [M(L)2]where L is an aromatic tridentate ligand and the complex displaysmeridional octahedral stereochemistry [142,143]. They detailedthis packing in the [Co(terpy)2]2+ (terpy = terpyridine) complex,and hence the arrangement is called the terpyridine embrace. Thepacking consists of intermolecular interactions between the cross-shaped [Fe(L1)2]2+ cations leading to the formation of co-planararrays (Fig. 18). Each cation interacts with four neighbours throughface-to-face �· · ·� interactions (double headed blue arrows inFig. 18); and through C H· · ·� interactions, either as donor oracceptor (red arrows in Fig. 18) [47,63,82,142,144]. As a result of
this ordering, the potential participants in hydrogen bonding (N Hgroups in the case of L1 or, for example, additional O H groupsin the case of L3) point outwards with respect to the layers. Theinteraction between these is then mediated by hydrogen bonding
G.A. Craig et al. / Coordination Chemis
Fig. 19. Interruption of the terpyridine embrace as observed in the compound[t
imiitt
tNtetotaSitwtpcgiapot
i
Ft
Fe(L3)2](ClO4)2·4C2H5OH. The hydrogen atoms have been omitted for clarity, andhe ethanol molecules are shown in the space filling mode [145].
nteractions between the cations in them and the anions or solventolecules that are found in the lattice. There may also be �· · ·�
nteractions between the pyridyl rings of cations from neighbour-ng layers, although it is frequently the case that the presence ofhe solvent molecules and anions causes mutual displacement ofhe cations, preventing the establishment of such contact.
This arrangement is most frequently seen in compounds con-aining the ligand L1 because the coordination of the three central-atoms to the Fe(II) provides a natural limit on the flexibility of
he ligand when participating in intermolecular interactions. How-ver, in some derivatives of L1, the functional groups appended tohe pyrazolyl rings can affect the terpyridine embrace. The authorsf the study of the compound [Fe(L2)2](BF4)2 highlighted thathe presence of the methyl groups on the pyrazolyl ring led to
less efficient packing in the compound (although the observedCO was still very highly cooperative) [50]. The hydrogen bondingnteractions together with the steric effects caused by the func-ionalisation of L1 with a hydroxyphenyl ring in the case of L3ere presented and discussed in depth [57]. While the packing of
he systems described in that work was largely based on the ter-yridine embrace (the only exception, ironically, being that of theompound [Fe(L1)2](ClO4)2·1.75(CH3)2CO·(C2H5)2O), the hydro-en bonding interactions with solvent molecules do occasionallynterfere, causing these to rupture the face-to-face �· · ·� inter-ctions that otherwise could have been established between theyrazolyl and the hydroxyphenyl rings. Fig. 19 shows an example
f this phenomenon, where ethanol molecules intercalate betweenhe cations in the system [Fe(L3)2](ClO4)2·4C2H5OH.
What the terpyridine embrace offers, therefore, is a platform tonvestigate how spin-inactive lattice components (anions, solvents)
ig. 20. Fingerprint plots of the compounds [Fe(L3)2](ClO4)2·2THF·H2O (left); [Fe(L3)2](Clhe corresponding .cifs available in the CSD.
try Reviews 269 (2014) 13–31 25
can affect the magnetic properties of [Fe(L1)2]2+ cations. This wasparticularly marked in the case of [Fe(L3)2](ClO4)2, where merelyby changing the solvents in the lattice a large variety of magneticresponses could be observed even though the crystal packing waslargely the same. This included, (i) a highly cooperative SCO with a40 K hysteresis loop [66], (ii) an incomplete transition with a nar-row hysteresis loop (10 K) [65], and (iii) or the complete absence ofthermal spin switching [57].
A means of assessing the intermolecular interactions at playin the crystal structures of SCO systems recently became avail-able in the form of the program CrystalExplorer 3.0 [146]. Thisprogram calculates the Hirshfeld surfaces associated with latticeentities, where the surface surrounds that volume in space withinwhich the electron density of the molecule of interest (the pro-molecule) predominates over the total contribution from the rest ofthe crystal components (the procrystal) [147]. These surfaces thengive a visual representation of the strength of the intermolecularcontacts in which the promolecule participates via a colour scale:red for the shortest contacts (strongest interactions), blue for thelongest (weakest), passing through white. From this surface, a 2-dimensional “fingerprint” plot may be generated [148]. Each pointon the surface is characterised by a pair of indices, di and de, whichcorrespond to the distances to nearest atom inside the surface andto the nearest atom outside the surface, respectively. The finger-print plot represents the density of surface points present for eachof these possible (di, de) pairs. However, the use of this techniquein SCO research has so far been scarce [26,46]. It has been appliedto the case of some systems based on the [Fe(L3)2](ClO4)2 salts,with the resulting fingerprint plots shown in Fig. 20 [65]. While theanalysis was effective in confirming the presence and extent of theterpyridine embrace in the compounds, as well as several of thehydrogen bonding motifs present, any kind of definitive magneto-structural correlation was beyond the possibilities of that work, dueto the lack of other studies.
5.2. Deviations from the terpyridine embrace
The crystal packing in [Fe(L1)2]2+ salts is frequently seen to devi-ate from the terpyridine embrace when bulky anions are used, astrategy often employed to seek multifunctional SCO systems (seeSection 3). In one such work, cyanometallate or thiocyanometallateanions were combined with the [Fe(L1)2]2+ cations, as an attemptto observe simultaneously fluorescence with SCO properties [75].The crystal packing for one of the yielded isostructural salts, of thetype [Fe(L1)2]2[M(CN)2][M3(CN)6]·2H2O (M = Ag, Au), is shown inFig. 21 (for M = Ag) [75]. An extended hydrogen bonding network
links the cations to the anions and water molecules into chains (3chains each running from top to bottom in the figure), and thesechains are linked through disordered anion sites hydrogen bondedeither directly to the cation or to lattice water molecules (4 chains
O4)2·2(CH3)2CO (middle); and [Fe(L3)2](ClO4)2·2C3H7OH (right). Plots derived from
26 G.A. Craig et al. / Coordination Chemis
Fig. 21. (Top) the 2D hydrogen bonding array observed in[Fe(L1)2]2[Ag(CN)2][Ag3(CN)6]·2H2O. (Bottom) Schematic view of the H-bondingbetween cations (cyan) and anions (green).
Taken and adapted from Ref. [75].
Fig. 22. Crystal packing in the compound [Fe(L1)2][bpmdcK](SeCN)1.7(ClO4)1.3·MeOH·H2
bonding between the pyrazolyl rings and the arms of the functionalised crown molecule.
Adapted from Ref. [76].
try Reviews 269 (2014) 13–31
running left to right in the figure). Direct contact between thecations occurred only through one weak �· · ·� interaction andvan der Waals contacts. This relatively low level of connectivitybetween the spin active components of the lattice was proposedas the cause of the extremely gradual spin transitions that wereobserved in each case.
An unusual structural topology was discovered by Murray et al.,when they attempted to combine the potential host–guest chem-istry of N,N′-bis(4-pyridyl-methyl)diaza-18-crown-6 (bpmdc) with[Fe(L1)2]2+ cations [76]. By making the perchlorate salt ofFe(II) react with L1, KNCSe, and bpmdc in a mixture ofethanol, methanol and dichloromethane they were able to obtainthe compound [Fe(L1)2][K ⊂ bpmdc](SeCN)1.7(ClO4)1.3·MeOH·H2O.The diaza-crown coordinates the K+ ions in the equatorial positionsof the metal, and the axial positions are occupied by perchlorateanions. Each perchlorate then links to an adjacent [K ⊂ bpmdc]+
moiety, extending in one dimension to form a chain (Fig. 22). The4-pyridyl-methyl branches of the ring then reach out, with the ter-minal nitrogen atom forming a hydrogen bond to the pyrazolylgroup of the [Fe(L1)2]2+ cations. At room temperature, this com-pound is mostly in the LS state, with a small residual HS fraction.Heating to 400 K brings about the loss of the solvent moleculesin the lattice, and the anhydrous form of the compound shows agradual spin crossover centred at T1/2 = 303 K in the cooling mode.
Coronado et al. looked to use a polycarboxylate anion, cyclohex-anetricarboxylate (chtc), to build a nanoporous structure to exploitthe type of reversible hydration/dehydration effects that theyhad observed in the compounds [Fe(L1)2][Cr(bpy)(ox)2]2·2H2Oand [Fe(L1)2][Cr(phen)(ox)2]2·0.5H2O·0.5MeOH. In the compound[Fe(L1)2](chtc)·5.5H2O, the chtc anions are singly protonated,leading to the formation of hydrogen bonds between two chtcanions [81]. These pairs of chtc anions then interact with two[Fe(L1)2]2+ cations each through hydrogen bonds to the pyra-zolyl rings (Fig. 23). The pentahydrated system was HS, but couldbe dehydrated at 324 K, leading to an anhydrous salt that dis-played SCO with a hysteresis loop of 17 K that was centred around321 K.
5.3. Magneto-structural correlations
The combination of the ligand structure and packing forces
determine the local geometry around the Fe(II) centre in SCOsystems. This has been especially well studied for the class ofcompounds with the general formula [FeLn(NCS)2] [149], whereL = a given nitrogen donor chelating ligand, and for the systems
O. Hydrogen atoms are omitted for clarity, and the black lines represent hydrogen
G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31 27
F tc)·5.5c CSD.
chh�pcpNrpbpTabab9wt
Fo
ig. 23. The H-bonded nanoporous framework seen in the compound [Fe(L1)2](chhtc anions are dark blue coloured. Figure made using the .cif obtained through the
ontaining the regio-isomer of L1, 1-bpp [48,150]. Four parametersave been developed to measure the distortion from the ideal octa-edron, seen within the cations of these SCO complexes: �, ˚, ˙,. The first two parameters characterise the distortion when two
lanar tri-dentate ligands coordinate in a mer- fashion to a metalentre, with � measuring the angle formed by the least squareslanes of the chelating ligands, and corresponding to the trans
Fe N angle formed by the coordination of the central pyridylings to the Fe(II) ion [151,152]. As such, the ideal values for thesearameters are 90◦ and 180◦, respectively. These parameters haveeen explained to give a measure of the shape of the molecule, andacking forces are the main responsible for their distortions [48].he parameters and � focus on the first coordination sphereround the Fe(II) ion (Fig. 24) [153,154]. The bite angle causedy the geometry of the ligand, together with the distortions on �nd ˚, induce deformations around the metal centre which can
e measured through as the sum of the deviation away from0◦ of the twelve possible cis-N Fe N bite angles, or through �,hich measures the twist away from perfect octahedral symme-
ry, Oh, towards a trigonal prismatic symmetry, D3h. These values
ig. 24. (Top) An illustration of the bite angles that are used in the sum ˙. (Bottom)
ctahedron is distorted towards a trigonal prismatic symmetry.
H2O. Hydrogen atoms and water molecules have been omitted for clarity, and the
are derived from the following formulae, where the angles and ˇcorrespond to those shown in Fig. 24 [155]:
∑=
12∑
i=1
|190 − ˛i|
� =24∑
j=1
|60 − ˇj|
Previous analyses have demonstrated that complexes in the HSstate are associated with greater flexibility, allowing them to reachhigher degrees of distortion than LS systems. In fact, in one setof systems, extreme distortion of the HS state was suggested totrap the compound in that state, causing a barrier to its possible
transition to the singlet state [152].
A preliminary evaluation of these parameters for the familyof L1 based complexes has been made before [57], but here it iscompleted to include the most recent additions, and the relevant
A view of the angle ˇ, which is used to evaluate �, and how it varies as the ideal
28
G.A
. Craig
et al.
/ Coordination
Chemistry
Review
s 269
(2014) 13–31
Table 2Distortion parameters for the mononuclear Iron compounds containing L1 derivatives in the CSD.
Formula REFCODE Cation spin (Temp/K) �/◦ ˚/◦ ˙/◦ �/◦
G.A. Craig et al. / Coordination Chemistry Reviews 269 (2014) 13–31 29
FCc
pvcc5pwoLa�tictbt
apiaso˙aov
FCcH
Fig. 27. Plot of � vs. � for the L1 family of compounds. The key displays the REF-CODES from the CSD of the SCO-active compounds, and the data for all of the
ig. 25. Plot of vs. � for the L1 family of compounds. The key displays the REF-ODES from the CSD of the SCO-active compounds, and the data for all of theompounds are given in Table 2.
arameters are compiled in Table 2. Fig. 25 displays a graph of ˚s. �. The distribution of compounds that remain in the LS state islearly narrower than that of HS systems, with the majority of LSations within 2◦ of the ideal value of 90◦ for �, and for within◦ of the ideal value of 180◦. Values of below 170◦ appear torohibit thermal SCO, although � appears to present no such limit,ith SCO-active compounds observed over the entire range of �
bserved. As could be expected, spin transitions from the HS to theS state see a relief in the distortion of the shape of the molecule,nd the SCO-active compounds move towards higher values of both
and ˚. The compounds involving the ligands L3 and L5 can leado greater degrees of distortion in ˚, as a result of crystal pack-ng effects. They provide hydrogen bonding donors/acceptors thatan have the effect of attracting lattice alcohol molecules or anionsowards the first coordination sphere, and subsequent interactionetween the lattice moieties and the cation’s ligands can deformhe cation.
The high level of distortion seen in the compounds containing L3nd L5 is also reflected in the parameters and � (Fig. 26). Thesearameters in fact show a marked correlation, in line with a sim-
lar trend seen in systems containing 1-bpp [150]. The correlationllows the delineation of ranges of parameters according to the spintate. In the quintet state, the Fe(II) are observed to possess valuesf > 140◦ and � > 450◦, with the LS state at much lower values:
< 105◦ and � < 330◦. There is therefore a range where the cationsre “mixed spin”. This situation arises when both HS and LS cationsccupy the same site within the lattice in a random fashion, pre-enting the observation of two crystallographically independent
ig. 26. Plot of � vs. for the L1 family of compounds. The key displays the REF-ODES from the CSD of the SCO-active compounds, and the data for all of theompounds are given in Table 2. The dashed lines mark proposed delimiters forS, LS and mixed spin systems.
compounds are given in Table 2. The dashed lines mark proposed delimiters forHS, LS and mixed spin systems.
cations. In the case of KALXAY [50], the Fe(II) salt was highly hygro-scopic, which meant that a bulk magnetic measurement could notbe performed reliably. The compound was described as undergo-ing a HS to LS transition, although the correlation presented heresuggests that the spin switch had not proceeded to completion.
The most consistent feature of these correlations is the tendencyof LS to display more octahedral symmetries when compared toHS systems. The plot of � vs. � (Fig. 27) shows the approximatedivision of the Fe(II) centres into HS, LS, and mixed spin systemsthat lie between the two extreme cases. However, as observed inFig. 27, there is very little correlation between � and other structuralparameters. On undergoing a spin transition, the values of � are seento shift closer towards the ideal value of 90◦.
6. Conclusions and perspectives
This review has attempted to highlight the recent developmentsin the SCO research centred on ligands of the L1-type (3-bpp).Compared to the regio-isomer 1-bpp, compounds based on L1 arerelatively under-exploited; however, as shown above, this situationis being redressed. Firstly, a slew of new derivatives of L1 have beensynthesised, aimed at their subsequent use in SCO research. Theyhave been designed using different approaches, whether based onthe Claisen condensation to append aromatic functional groups tothe wings of L1; adjustments to the synthesis that was employedto obtain L1; or through the use of L1 as the starting point fordeprotonation and derivatisation of the NH group of the pyrazolylrings. Secondly, these new derivatives have been successfully usedto synthesise an increasing collection of SCO-active compoundsthat show remarkable magnetic properties that justify the inter-est invested in this family of systems. On the one hand, this takesin broad hysteresis loops, structured spin transitions, and a wealthof new crystal structures to help partially understand their physicalproperties; and on the other hand they allow for a range of tools tobe employed to switch their spin state, such as the variation of tem-perature, the application of pressure, irradiation, or the controlledinsertion or removal of guest molecules within the lattice.
Acknowledgements
GA thanks the Generalitat de Catalunya for the prize ICREA
Academia 2008 and the ERC for a Starting Grant (258060 Func-MolQIP). The authors thank the Spanish MICINN for supportthrough CTQ2009-06959 (GAC, GA) and the Spanish MINECO for
3 hemis
Ma
R
0 G.A. Craig et al. / Coordination C
AT2011-24284 (OR). Dr Helena J. Shepherd and Dr Gabor Molnarre thanked for providing the Raman data presented in Fig. 15.
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