Leoní A. Barrios,a Ivana Borilovic,a Jorge Salinas Uber,a David Aguilà,a
Olivier Roubeaub and Guillem Aromí*a
In the presence of Na+ ions, the reaction of the ligand 2-hydroxy-1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-
propionyl)-benzene, H5L4, with M(AcO)2 salts (M = Mn, Co) gathers the conditions for the assembly of
coordination complexes (NBu4)3[M2Na2(H2L4)3] (M = Mn(II) 1 and Co(II) 2), which exhibit a new paddle-
wheel structure. The features of this new category of compounds are discussed as well as their magnetic
properties and solution behaviour.
1 Introduction
Paddle-wheel complexes are dimetallic coordination com-pounds bridged by (almost1 exclusively) four ligands posses-sing a negatively charged allylic type XYZ binding moiety(Scheme 1A), with atoms X and Z attached to one metal atomeach.2 Thereby, the central axis is made of both metal ions,while the flat five-membered dimetallacycles around it consti-tute the four paddles of the wheel (Scheme 1A). They are anentire category of compounds the study of which has made adeep impact on the understanding of many fundamentalaspects of chemistry, such as the nature of the intramolecularmagnetic exchange,3,4 the electron transfer5 or the metal–metal bond.6 The archetypal paddle-wheel complex derivesfrom the dimeric form of copper(II) acetate, Cu2(AcO)4·2H2O,which features four acetate groups around the metallic axis.Since its structure was first determined,7 paddle-wheels withthis coordinating fragment (OCO) have been obtained, in adiscrete form or as part of coordination polymers, involvingalmost all transition metals (TMs), adding up to more thantwo thousand registered structures. Another important cat-egory of paddle-wheels is formed by NCN type ligands such asformamidinates,8,9 pyridylamines or fused heterocycles.10
Paddle-wheels exhibiting the mixed donor sequence NCO arealso very numerous, made primarily of amidates11 or deriva-tives of pyridones,12 pyrrolidinones13 or similar fragments.14,15
Paddle-wheels with only three symmetrically distributedligands are extremely rare.16
The above type of complexes may be extended to paddle-wheels with more than two metal atoms at the axis if ligandswith more than one adjacent coordinating unit are used. A par-ticularly rich family of such extended linear complexes hasbeen created following the design and synthesis of a collectionof oligo-pyridylamine ligands (Scheme 1B).17,18
This category of extended paddle-wheels (Scheme 2A) withnitrogen donors is not mirrored by any counterpart of onlyoxygen donor ligands. For some time we have been engaged inthe synthesis of ligands exhibiting rows of equidistant oxygendonors that could be used for this purpose.19 The strategyemployed is by alternating the β-diketone groups together withphenolyl groups (Scheme 1C). The structure and stereo-chemical properties of this set of donors, however, favor theformation of bidentate chelates around each metal ion,
Scheme 1 (A) Generic representation of the bridging coordination moietygiving paddle-wheel complexes and representation of one paddle of the wheel,(B) scheme of oligo pyridylamine ligands, and (C) scheme of oligo phenolyl-β-di-ketone ligands.
Scheme 2 (A) Generic representation of extended paddle-wheel complexesmade of oligo-pyridylamine ligands (only relevant atoms of the heterocycles areshown), (B) generic representation of extended paddle-wheel complexes madeof ligands of the poly-(phenolyl-1,3-diketone) type, and (C) representation of apaddle made of two fused chelate rings.
†Electronic supplementary information (ESI) available: 1H NMR spectra andcyclic voltammograms. CCDC 923301 and 923302. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/c3dt51232k
aDepartament de Química Inorgànica, Universitat de Barcelona, Diagonal 645,
08028 Barcelona, Spain. E-mail: guillem.aromi.qi.ub.es; Tel: +34 934039760bInstituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de
Zaragoza, Plaza San Francisco s/n, 50009 Zaragoza, Spain
thereby forcing the presence of monoatomic bridges betweenthem (Scheme 2B). Thus, the concept of the paddle-wheelcomplex could be extended, if necessary, to the case where thepaddles are made of two fused six-membered chelate rings(Scheme 2C) instead of the dimetallic five membered rings ofthe well established definition. In fact, a few dinuclear com-plexes in the literature can be said to fall within thiscategory.20–22 We and others have prepared ligands (Scheme 3)that follow strictly the definition in Scheme 1C, or are veryclosely related.23–26 However, the preparation of paddle-wheeltype complexes with them has remained very elusive. Forexample, linear complexes with [MMMM]27 or [MM⋯MM]28
topologies have been prepared with some of these ligands fea-turing the expected coordination mode. However, the presenceof other co-ligands (e.g. solvent molecules) or the stereo-chemical preferences of the metal centers involved have alwaysprevented the observation of a paddle-wheel arrangement.Nonetheless, the structure of two such complexes, [Mn3(HL2)3]and [Co3(HL2)3],
29,30 was found to be very reminiscent of thenew paddle-wheels proposed here. They possess a central met-allic axis of three metals surrounded by three ligands, approxi-mately as in a paddle-wheel with three paddles. However, bothcompounds show one metal vacancy inside the chain andtherefore the ligands exhibit a discontinuity of the fused che-lating rings that are supposed to form the paddles. As a resultof this absence, each of these molecules is chiral and for thisreason the compounds were portrayed as “irregular helicates”.
We present now two new complexes of the ligand H5L4(2-hydroxy-1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-benzene),with the formula (NBu4)3[M2Na2(H2L4)3] (M = Mn(II) 1 andCo(II) 2). Both molecules exhibit a central Na–M–M–Nametallic axis and three H2L3
3− ligands around them as thethree paddles of a paddle-wheel in the extended definitionmade above. Their structure and properties are described here,
as well as some parameters, proposed to quantify the deviationof these new types of topologies from the corresponding idealpaddle-wheel.
2 Experimental2.1 Synthesis
The ligand 2-hydroxy-1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-prop-ionyl)-benzene (H5L4) was prepared according to a procedurepublished by us.26 Solvents and reagents were used as receivedwithout purification.
(NBu4)3[Mn2Na2(H2L4)3] (1). A red solution of H5L4 (30 mg,0.07 mmol) and TBAOH (0.08 mL of a 1 M solution in metha-nol, 0.08 mmol) in pyridine (10 mL) was added dropwise to asuspension of Mn(AcO)2·4H2O (12 mg, 0.05 mmol) and NaAcO(4 mg, 0.05 mmol) in pyridine (10 mL). The resulting brown-reddish suspension was stirred at room temperature for 24 h.A small amount of brown solid was removed by filtration andthe dark red filtrate was layered with Et2O. Dark yellow crystalssuitable for X-ray crystallography formed after a week. Theyield was 35% (18 mg). EA (%); Calcd (Found for 1·H2O):C 67.12 (67.24), H 7.28 (7.40), N 1.96 (2.04).
(NBu4)3[Co2Na2(H2L4)3] (2). This compound was preparedfollowing the same procedure as 1 using Co(AcO)2·4H2O(12 mg, 0.05 mmol) instead of the Mn salt. The resulting deepred suspension was stirred at room temperature for 24 h. Thedark red solution was filtered and the filtrate was layered withEt2O. Orange/red crystals suitable for X-ray crystallographyformed after 48 h. The yield was 38% (19 mg). EA (%); Calcd(Found for 2·H2O): C 66.87 (66.85), H 7.25 (7.32), N 1.95 (1.96).
2.2 X-ray crystallography
Data for compound 1 were collected on a yellow rod at 100 Kwith a Bruker APEX II CCD diffractometer on the AdvancedLight Source beamline 11.3.1 at the Lawrence BerkeleyNational Laboratory, from a silicon 111 monochromator (λ =0.7749 Å). Data reduction and absorption corrections were per-formed with SAINT and SADABS.31 Data for 2 were obtainedusing Mo Kα radiation (λ = 0.7107 Å) on an Oxford DiffractionSuperNova diffractometer at 100 K. Data reduction and absorp-tion corrections were performed with CrysalisPro.32 The struc-tures of 1 and 2 were solved with SHELXS33 and SIR9734
respectively. Both structures were refined on F2 using theSHELXTL suite.31,33 Crystallographic and refinement para-meters are summarized in Table 1. Selected bond distancesand angles are given in Table 2. All details can be found in thesupplementary crystallographic data for this paper in cifformat with CCDC numbers 923301 and 923302.
2.3 Physical measurements
Variable-temperature magnetic susceptibility data wereobtained with a Quantum Design MPMS5 SQUID magneto-meter. Pascal’s constants were used to estimate diamagnetic cor-rections to the molar paramagnetic susceptibility. The elementalanalysis was performed with an Elemental Microanalyzer (A5),
Scheme 3 Representation of some reported ligands that combine β-diketonegroups together with phenolyl groups.
model Flash 1112, at the Servei de Microanàlisi of CSIC, Barce-lona, Spain. IR spectra were recorded as KBr pellet samples ona Nicolet AVATAR 330 FTIR spectrometer. Voltammetry experi-ments were performed on an SP-150 (BioLogic) potentiostatmodulated by EC-Lab V 9.9 software. All measurements werecarried out in DMF at ambient temperature, with a convention-al three-electrode configuration consisting of two platinumwires as the auxiliary and reference electrodes, and a workingglassy carbon (GC) electrode (3 mm diameter). TBA·PF6 wasused as a supporting electrolyte and the scanning rate was100 mV s−1. All potentials are referred to the ferrocenium/ferrocene (fc+/fc) couple. 1H NMR spectra were carried out onan Innova 300 MHz spectrometer. The protio-solvent signalswere used as internal references.
3 Results and discussion3.1 Synthesis
Previously reported work with ligand H5L4 showed that itsreactions with Co(AcO)2 and Mn(AcO)2 in pyridine producecomplexes that are very different structurally in the form ofdinuclear or zigzag tetranuclear aggregates.26,27 In the lattercompound, the Mn ions complete some of their coordinationsites with acetate and pyridine ligands and the core displays azigzag conformation. With cobalt, the same reaction yields thedinuclear complex [Co2(H3L4)2(py)4], which contains twoμ-H3L4
2− groups binding the metal ions only through theβ-diketonate groups while axial pyridine ligands complete thecoordination.27 Interestingly, we show here that if the abovereactions are made in the presence of NaAcO and an organichydroxide (NBu4OH), the product is completely new, and thesame for both metals: (NBu4)3[M2Na2(H2L4)3] (M = Mn, 1; Co,2), with a paddle-wheel structure. The key factor to reach thisstructure is the concurrence of Na+ ions. These metal centers fitwell as the ends of the Na–M–M–Na axis of the paddle-wheelbecause they are capable of establishing long enough inter-actions with the donors from H2L4
3− thus supporting the struc-tural requirements for the latter to act as a paddle (see below).As suggested by the chemical eqn (1), the role of the saltNBu4OH is that of providing the counter ion of the complex saltrather than deprotonating the external phenol groups of H5L4.
The above reactivity suggests that other cations with ionicradii similar to that of Na+ and variable charges could serve tomake other members of this type of paddle-wheels.
3.2 Description of structures
Complexes 1 and 2 crystallize in the hexagonal group P63/m,and their crystallographic data and selected structural para-meters are in Tables 1 and 2, respectively. Both compoundsare isostructural, the precise identity of the central metal onlycausing very slight changes to the various structural para-meters. Therefore, only the structure of 1 will be described indetail. It contains two complex anions and six NBu4
+ cationsin the unit cell while the asymmetric unit is composed ofone sixth of the complex and half cation. The anion[Mn2Na2(H2L4)3]
3− (Fig. 1) consists of a straight axial chain ofmetal ions in the sequence Na+–Mn2+–Mn2+–Na+, surroundedby three H2L4
3− ligands running parallel to this axis with theiridealized planes disposed 120° from each other, the ensemblefeaturing exact C3h symmetry (Fig. 2). The H2L4
3− moietiesmaintain their external phenolyl groups protonated whileusing their central phenoxide unit and both β-diketonate func-tionalities for chelating and bridging the metal centers. Thuseach ligand coordinates to the four cations of the axis throughfour fused six membered chelate rings (Fig. 1 and 2), therebyalso linking pairs of adjacent metal atoms via monoatomicbridges. Each pair of adjacent metals in the complex is
Table 1 Crystal data and structure refinement for compounds 1 and 2
therefore surrounded by three pairs of fused chelate rings,which constitute three paddles around the dimetallic axisaccording to the definition provided in the introduction(Scheme 2C). In this sense, the complex constitutes anextended paddle-wheel, where each (extended) paddle is com-posed of four fused chelate rings from each ligand (Fig. 2,right).
This configuration is enabled by the electronic properties ofthe metals, which allow coordination geometries that arecloser to a trigonal prism than to an octahedron (Fig. 3), facili-tating an approximately planar disposition of each ligandalong the central axis (see below). The Mn–O bond distancesare 0.07 Å shorter to the β-diketonate oxygen (O3) donors thanto those of central phenolates (O4), (2.107(2) vs. 2.178(2) Å).Likewise, the Na–O bond distances to the peripheral β-diketo-nate oxygen atoms (O2) are 0.07 Å longer (2.506(4) Å) than tothe three other oxygen donors (O3), (2.432(3) Å (see Table 2 forfull data on complexes 1 and 2). The coordination geometryaround both, Mn2+ and Na+, lies closer to a trigonal prismthan to an octahedron, as gauged by “Bailar twist” distortionsof 26.13° and 25.19°, respectively.35 However, for the latter ion,the pseudo-prism is flattened on one side by a bending of
three Na–O bonds towards the opposite triangular face (Fig. 3),certainly imposed by the structure of the ligand.
The proximity of these coordination polyhedra to an idealtrigonal prism and octahedron, respectively, was evaluated bymeans of continuous shape measures (CShMs).36 The resultsare given in Table 3, confirming the higher similarity to the tri-gonal prism in both cases, less pronounced for Na+ as a resultof the additional distortion (see above).
The inter-metallic distances in complexes 1 and 2 are:d(Na⋯Mn) = 3.107(3) Å, d(Mn⋯Mn) = 2.8724(15) Å, d(Na⋯Co) =3.128(3) Å, d(Co⋯Co) = 2.8272(15) Å. The central M(II)⋯M(II)vectors are in fact exceptionally short although not corres-ponding to M–M bonds (see below). A search of the CCDC(V5.33, update 4) reveals that the moiety M–(μ-O)3–M is notmassively abundant: 58 examples for Mn and 49 cases for Co.Of the very few examples (6) with Mn⋯Mn vectors shorter thanin 1, one is an organometallic alkoxide bridged Mn(I) moleculeexhibiting a metal–metal bond,37 four cases are triply oxide/hydroxide bridged Mn(IV) complexes where the presence ofsuch bonds is speculated38–41 and one consists of a mixedvalence Mn(II)/Mn(III) pair inscribed within a macrocycliccomplex.42 Therefore, complex 1 exhibits the shortest Mn(II)⋯Mn(II) vector with three monoatomic bridges made so far.The reason for this unusual proximity are the structural
Fig. 1 Representation of the molecular structure of the complex anion of 1,[Mn2Na2(H2L4)3]
3−. Only unique heteroatoms are labeled. Color code: orange,Mn; purple, Na; red, O; grey, C; yellow, H. Only H atoms bound to oxygen areshown. Complex 2 is isomorphous.
Fig. 2 (Left) View of the complex anion of 1, [Mn2Na2(H2L4)3]3−, down the
molecular C3 symmetry axis. (Right) Representation of the central moiety of theanion, emphasizing its paddle-wheel configuration. Each paddle is in a differentcolor. Complex 2 is isomorphous.
Fig. 3 Representation of the coordination geometry of Na+ (top) and Mn2+
(bottom) in the anion of 1, [Mn2Na2(H2L4)3]3−. (Left) View of the polyhedra per-
pendicular to the molecular axis. (Middle) Polyhedra along the molecular axis.(Right) View along the axis emphasizing the bond directions and the distortiontowards the octahedron via the “Bailar twist” (twist angle represented by θ,formed by the projections of pairs of bonds in the plane perpendicular to the C3axis).
Table 3 Continuous Shape Measures (CShMs) for the coordination geometryof metals in compounds 1 and 2. Both metal ions are compared with the idealoctahedron (OC-6) and trigonal prisms (TPR-6). Smaller distances indicate highersimilarity to the ideal polyhedron in a normalized scale from 0 to 10036
Metal (complex) Distance to OC-6 Distance to TPR-6
constraints imposed by the assembly, which encapsulates theMn(II) cations and forces them to be closer than expected. TheCo(II) atoms in complex 2 likely suffer from the same constric-tion. For this reason, most of the dozen compounds featuringCo–(μ-O)3–Co vectors shorter than in 2 involve Co(III), with onlytwo examples of Co(II)⋯Co(II) systems.43,44
3.2.1 Distortion of paddle-wheels. The first coordinationsphere of the metal centers in the classical (tetragonal)paddle-wheel complexes exhibits a perfect or a very approxi-mate D4h symmetry. Deviations from this symmetry by twistingthe equatorial bonds around the dimetallic axis generate heli-city and thus are at the origin of chirality.45 The distortion canbe quantified by measuring the torsion angle, α, formed by theedges of the paddle that are perpendicular to the metal axis(X–M–M–Z torsion angle in Scheme 1A). In extended paddle-wheel complexes, the distortion normally propagates along theaxis. If the helicity is conserved, the whole complex constitutesa helicate with an absolute chirality.46 However, the distortionof the paddle may invert its helicity, which may lead to meso-cates.47 The structure of the new type of paddle-wheels pro-posed here is more complex; thus the distortion is moredifficult to quantify. It can be done using two essential para-meters (Fig. 4): (i) the angle β between the idealized planes ofadjacent chelate rings, and (ii) the O–M–M–O torsion angle, γ,involving the outermost oxygen atoms bound to two adjacentmetal atoms (thick bonds in Scheme 2C, see also Fig. 4). Theextent and mutual relation between the “Bailar twist” of twoadjacent metals determine the magnitude of β, whereas γ willdictate the overall helicity. Because of their high symmetry, thepaddle-wheels of 1 and 2 exhibit only two different β angleseach: βM–M and βM–Na. These angles are (in the 1/2 format)15.12°/14.18° and 29.98°/28.92°, respectively. The C3h sym-metry of these objects causes all the γM–M torsion angles to be0° in both complexes and the two distinct γM–Na angles of eachcomplex to differ only in their sign. Thus, for 1 and 2, γM–Na
are 0.94° (and −0.94°) and 1.86° (and −1.86°), respectively.These complexes constitute perfect mesocates, the only impor-tant source of distortion being the successive “Bailar twist” dis-tortions of the metals, causing significant deviations of the β
angles from zero. However, since these angles alternate insign, they correct each other, and lead to very small (or nil)torsion angles, γ.
3.3 Bulk magnetization properties
The extent of the magnetic interaction through the triple phen-oxide bridge between the M(II) metal ions of 1 and 2 was exam-ined through variable temperature magnetic susceptibilitymeasurements under constant magnetic fields. The results arerepresented as χMT vs. T plots in Fig. 5 (χM is the molar para-magnetic susceptibility). For the case of (NBu4)3[Mn2-
Na2(H2L4)3] (1), χMT at 250 K is 6.58 cm3 K mol−1, below theexpected (8.75 cm3 K mol−1) value for two non-interacting Mn(II)ions (S = 5/2). The curve declines upon cooling down to0.10 cm3 K mol−1 at 2 K. Both observations reflect the fact thatthe Mn(II) ions of the molecule experience a weak antiferro-magnetic interaction, which was quantified by fitting the datato the χM = f(T) expression derived from the Van Vleckequation,48 using the Heisenberg spin Hamiltonian H =−2JS1S2 (S1 = S2 = 5/2). This fitting produced as the best para-meters J = −7.61 cm−1 and g = 2.00. There are about a dozencompounds in the literature for which the magnetic couplingwithin a Mn(II)–(μ-O)3–Mn(II) has been examined.20,29,49–57 Thevalues of J observed range between −9.1 and +1.25 cm−1.20,50 Ithas been suggested that for Mn–O–Mn angles narrower than90°, ferromagnetic interactions are to be expected.50,52 In fact,however, most systems exhibit antiferromagnetic exchangeand exhibit acute bridging angles. There seems to be no corre-lation either with this angle or with the Mn⋯Mn distance.This probably reflects on the fact that the exchange is affectedby many structural and electronic parameters simultaneously,perhaps including a certain degree of direct overlap between dorbitals. The value of χMT for the complex (NBu4)3[Co2-Na2(H2L4)3] (2) at 250 K is 6.68 cm3 K mol−1, which is signifi-cantly higher than the expected spin-only value for twoisolated Co(II) centers (S = 3/2), and then decreases monotoni-cally upon cooling, mostly below 50 K, down to 0.38 cm3 Kmol−1 at 2 K. This is because this ion is often affected by theinfluence of the orbital angular momentum,58 while thesystem experiences the depopulation of excited states withhigher magnetic moments as the temperature decreases. The
Fig. 4 Representation of a dimetallic fragment of a paddle-wheel formed byfused chelating rings, emphasizing the two main parameters of distortion: di-hedral and torsion angles β and γ, respectively (see text).
Fig. 5 Plots of χMT vs. T for complexes 1 (empty squares) and 2 (empty circles).The full lines are fits of the experimental data, see text for details. The inset is anisothermal plot of M (reduced magnetization) vs. H at 2 K for complex 2, linesare guides to the eye.
presence of spin–orbit coupling renders the task of simulatingthe susceptibility data a difficult one. Nevertheless, the lowestvalue of χMT suggests that its decline with decreasing temp-erature is also caused by the antiferromagnetic interactionbetween both Co(II) ions of the molecule. The step featured bythe isothermally reduced magnetization vs. field plot at 2 K(inset of Fig. 5) is also indicative of this coupling; a diamag-netic state is almost fully populated at this temperature whenthe magnetic field is low, while at higher magnetic fields acrossover to a magnetic ground state occurs, thus effectivelyovercoming the weak antiferromagnetic interaction.
An estimate of this antiferromagnetic coupling wasobtained by fitting the χMT vs. T data to a spin only modelinvolving two S = 3/2 ions, in the same manner as done forcomplex 1. This fitting produced J = −1.41 cm−1 and g = 2.70as parameters. The high value of g could be anticipated fromthe χMT vs. T plot and is a reflection of the influence of theorbital angular momentum on the susceptibility in this com-pound. Using the same approximate model, the value of Jderived from the field corresponding to the inflection of theS-shaped M vs. T curve (ca. 1.9 T) is approximately −1.15 cm−1,as obtained by equalizing exchange and Zeeman energies atthat point, i.e. by making gμBHCS = 2|J|S2 (for g = 2.70; see thedetailed explanation in the ESI†). Both calculated values arethus in good agreement. Compounds where the magneticexchange within a Co(II)–(μ-O)3–Co(II) moiety have been studiedare very scarce in the literature.30,43,51,59 As was the case of Mn(II),the coupling can be ferro- or antiferromagnetic, with couplingconstants ranging from −9.8 to +1.7 cm−1. In most cases,however, the interpretation is further complicated by theeffects of spin–orbit coupling. From the fact that in both, com-plexes 1 and 2, the coupling is relatively weak, it can bededuced safely that in this kind of paddle-wheels there is notany appreciable metal–metal interaction, which would cer-tainly cause a much stronger coupling.
3.4 Studies in solution
The integrity of the clusters in 1 and 2 in solution could beestablished by 1H NMR. Paramagnetic centers cause a deepimpact on the 1H NMR of the molecules where they reside; themagnetic moment of these centers cause very large chemicalshifts while the fast relaxation of the unpaired electrons are atthe origin of large line broadenings.60 Sometimes these effectsprevent the detection of signals, but in many instances, veryinformative spectra may be obtained.61,62 Molecules contain-ing high spin Co(II) are often amenable to 1H NMR studiesbecause the relaxation rate of this metal ion is sufficiently fast,thus furnishing relatively sharp signals. The spectrum ofcomplex 2 in d6-DMSO is shown in Fig. 6. In addition to thesignals from the solvent and H2O, it reveals seven equallyintensive signals and one with a half integration value(Fig. S1†). This is in perfect agreement with the patternexpected from the crystallographic symmetry of this species.The signals from the NBu4
+ cations are also present in thecorrect intensity. This result indicates that the solid state
structure of the anion [Co2Na2(H2L4)3]3− is preserved in a
DMSO solution.The case of complex 1 is very different. The electronic spin
of Mn(II) relaxes about three orders of magnitude slower thanthat of Co(II), which causes the protons to relax much fasterand, therefore, leads to very broad 1H NMR signals, oftenbeyond detection. Thus, the spectrum of 1 in d7-DMF exhibitsonly three to four very broad signals (Fig. S2†). This does notdisprove the supposition that the complex anion maintains itsmolecular structure in solution, which may be assumed byextrapolating the observations made on 1.
Compounds 1 and 2 were studied using cyclic voltammetryas DMF solutions. Both display two irreversible oxidationwaves (Fig. S3 and S4†), very broad in the case of complex 1,which could be metal based. The possibility of obtaining oxi-dized versions of complexes 1 and 2 using bulk electrolysis isbeing investigated. Within the window of the solvent, onlyan irreversible reduction was apparent in the manganesecomplex, most probably centered on the ligand.
4 Conclusions
The ligand 2-hydroxy-1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-propio-nyl)-benzene, H5L4, designed to assemble oxygen-only paddle-wheel complexes, does not produce such a type of compoundsin reactions with only Mn(AcO)2 or Co(AcO)2. However, thepresence of Na+ in the above reactions allows the formation ofa new type of paddle-wheel ensembles, in the form of com-pounds (NBu4)3[M2Na2(H2L4)3] (M = Mn(II) 1 and Co(II) 2).These species exhibit pairs of unusually close Mn(II) and Co(II)ions, respectively. This proximity does not have any particularconsequence for the magnetic exchange between them. Theintegrity of the [M2Na2(H2L4)3]
3− anions in solution is sup-ported by 1H NMR spectroscopy, which, in the case of 2 (Co),
Fig. 6 300 MHz 1H NMR in d6-DMSO of complex 2. The scheme emphasizesthe eight unique signals from the complex anion integrating for six protons(1 to 7; ●) or three protons (8; ▲). The circled figure is an expansion emphasiz-ing the signals of the NBu4
reproduces perfectly the crystallographic symmetry. The expan-sion of this family to other suitable combinations of metals isunder investigation.
Acknowledgements
GA thanks the Generalitat de Catalunya for the prize ICREA Aca-demia 2008 for excellence in research and the ERC for a StartingGrant (258060 FuncMolQIP). The authors thank the SpanishMICINN for funding through CTQ2009-06959 (GA, LAB, DA)and MAT2011-24284 (OR) and the ERC for a Predoctoral Fellow-ship under Grant 258060 FuncMolQIP (JSU and IB). Theauthors thank Dr Carolina Sañudo and Dr José Sánchez Costafor collecting single crystal X-ray diffraction data of 2. Data for 1were collected through access to ALS beamline 11.3.1. TheAdvanced Light Source is supported by the Director, Office ofScience, Office of Basic Energy Sciences of the U.S. Departmentof Energy under contract no. DE-AC02-05CH11231.
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