Azacrown ether–copper(II)–hexanoate complexes. From monomer to 1-Dmetal organic polymer†
Kamil Wojciechowski,*a,b Anna Bitner,a Gerald Bernardinellic and Marcin Bryndad
Received 4th July 2008, Accepted 28th October 2008First published as an Advance Article on the web 6th January 2009DOI: 10.1039/b811374b
The synthesis and characterisation of two new Cu(II)-hexanoate-azacrown ether species are described.The first monomeric complex is a “classical” macrocyclic Cu(II) complex with two monodentate axiallycoordinated carboxylates serving as counterions. The second is an adduct of the former complex withthe copper(II) hexanoate paddlewheel dimer, Cu2(hexanoate)4, forming an infinite 1-D chain ofalternating monomer–dimer units. The electronic and magnetic properties of both species are discussedbased on UV-vis, IR, X-ray and EPR studies combined with the DFT calculations. The UV-vis titrationresults prove that complex 1 and the polymeric species 2 are in equilibrium in toluene solution.
Introduction
The most often encountered dinuclear Cu(II) species are paddle-wheel dimeric copper alkanoates–Cu2(alkanoate)4, which can existboth in solid state1–4 and in non-polar organic solutions.5–9 Insuch a dimer, two copper(II) ions are bridged by four carboxylategroups (Fig. 1). In the paddlewheel copper(II) carboxylate, thecoordination sphere around copper atoms is square pyramidal10
and the dimer bears D4h symmetry.11 The two neighbouring copperions in such dimeric species are most often characterised bya strong magnetic coupling between the two metal centres via
Fig. 1 Structures of paddlewheel copper(II) carboxylate and di-aza-18-crown-6 ether (ACE).
aDepartment of Analytical Chemistry, Warsaw University of Technology,Noakowskiego 3, 00-664 Warsaw, PolandbAnalytical and Biophysical Environmental Chemistry (CABE) Departmentof Analytical, Inorganic and Applied Chemistry, Sciences II, 30 quaiE. Ansermet, CH-1211 Geneve 4, SuissecLaboratory of X-Ray Crystallography, University of Geneva. 24, quaiE. Ansermet, CH-1211 Geneva 4, SwitzerlanddDepartment of Chemistry, University of Calfornia, Davis. One ShieldsAvenue, 95616 Davis, USA† Electronic supplementary information (ESI) available: Details of the QMcomputations on complex 1 and its simplified model S1. The relative coor-dinates and isotropic displacement parameters for 2, selected geometricalparameters for 1, the complete diagram of the frontier Kohn–Sham orbitalsobtained from DFT for 1 and the calculated Mayer bond orders for 1.CCDC reference number 689064 (1). For ESI and crystallographic data inCIF or other electronic format see DOI: 10.1039/b811374b
superexchange pathway.12 Non-ionised carboxylic acid, solventmolecules or other ligands can occupy the axial positions in thedimer. The common axial ligands for copper(II) dimers mostoften reported in the literature are amines. In many cases, axialligation of the paddlewheel dimer prevents formation of inter-dimer bridges and subsequent precipitation of the polymericspecies.2
Since both diaza-18-crown-6 ether and fatty acid are able tobind copper(II), the simultaneous presence of both molecules inthe mixture results in a competing complexation process towardsthe Cu(II) ion. This is especially interesting in the context of Cu(II)transport through permeation liquid membranes (PLMs) contain-ing a mixture of N,N-alkylated azacrown ether and dodecanoicacid as a carrier. The synergistic effects of the presence of bothcarrier molecules in Cu(II) transport through such systems wereindeed observed.14 The PLM-transport results strongly suggestthat some mixed Cu(II)-azacrown ether-carboxylate complexes,involving paddlewheel dimers, can be formed in the membraneduring the Cu(II) transport.15
In the course of this study, we have investigated the interac-tion between diaza-18-crown-6 ether (ACE) and a paddlewheelcopper(II) hexanoate Cu2(hexanoate)4 in order to gain moreinsights into the mechanism of the PLM used for studying thetransport and speciation of Cu(II). Two types of species aredescribed. A “classical” ACE-Cu-(hexanoate)2 monomer and anew type of polymeric species containing a trimeric Cu(II) core asrecurring unit, described as Cu3-ACE-(hexanoate)6. In the latter,the axial positions of the paddlewheel dimer Cu2(hexanoate)4 areoccupied by the monomeric ACE-Cu-(hexanoate)2 units. In thispaper, we present the spectroscopic (UV-vis, IR, EPR, 1H NMR)and X-ray diffraction results, showing a coexistence of both typesof species in the solid state and in toluene solution. The electronicstructure of the monomeric and dimeric species is elucidated withhelp of the computational (DFT) methods.
The addition of ACE to a toluene solution of Cu2(hexanoate)4
resulted in change of colour from green to green-blue. Uponsolvent evaporation, two different types of crystals were obtained:blue (1) and green (2). The two products were separated byrecrystallisation from isopropyl ether. The blue needle-like crystalsof complex 1 suitable for the X-ray diffraction were obtained inone day after solvent evaporation. The polymeric species 2 wasobtained as very small green crystals of the quality not suitablefor full resolution of X-ray diffraction. After separation andpurification, both types of crystals were consecutively analysedby IR, EPR, X-ray diffraction (solid) and by UV-vis, 1H NMRand EPR (solution).
The two solids (1 and 2) obtained from a mixture of ACE andCu2(hexanoate)4 are clearly two different species, as evidenced bytheir corresponding IR and UV-vis spectra (Fig. 2 and 3). TheN–H band in the IR spectrum of pure ACE seen at 3334 cm-1,
Fig. 2 UV-vis spectra of Cu2(hexanoate)4 ( ), complex 1 (---) andpolymeric species 2 ( ◊ ◊ ◊ ) in toluene at room temperature.
Fig. 3 The ATR IR spectra of ACE and complex 1 and polymeric species2 (all solids).
in both species shifts to lower wavenumbers (3107 and 3148 cm-1
for 1 and 2, respectively, Fig. 3), which is typical for coordinatedamines. This confirms the presence of ACE in both species, furtherevidenced by its spectral signature involving multiple peaks of theazacrown ether bands n(C–N–C) and n(C–O–C) in the spectra of1 and 2. In both spectra, these peaks are better separated thanin pure ACE (1010–1145 cm-1 for 1 and 2, as compared to 1068–1119 cm-1 for ACE).
Monomeric complex 1. Complex 1 was obtained as blueneedle-like crystals from cold isopropyl ether by recrystallisationof the reaction product of ACE with Cu2(hexanoate)4. The X-raystructure of 1 is depicted in Fig. 4. Complex 1 is monomeric withcopper(II) ion sitting inside the ACE cavity. The two nitrogenatoms of ACE and two carboxylate oxygen atoms constitutethe basal plane, with the distances of 2.037(3) and 1.975(3) A,respectively. The two oxygen atoms of ACE that coordinate weaklyto copper atom, are located 2.620(2) A, and the two other, whichdo not coordinate to Cu(II) are located 3.692(2) A from it. Based onthe X-ray results, the coordination geometry of complex 1 can thusbe described as elongated octahedral (4 + 2) with weak interactionbetween the central Cu(II) and two oxygen atoms of ACE. Thecarboxylate oxygen atoms, which do not participate in Cu(II)coordination are hydrogen-bonded to the nitrogen atoms (N1–H01 = 0.85(4), H01 ◊ ◊ ◊ O2a = 1.98(4), N1 ◊ ◊ ◊ O2a = 2.787(5) A,N1–H01 ◊ ◊ ◊ O2a = 157(4)◦), forming a 6-membered chelate ring,which additionally stabilises the complex.
Fig. 4 ORTEP view of 1 with atoms numbering scheme. Ellipsoidsare represented at 40% probability level. The central Cu atom is lo-cated in a centre of inversion. Selected bond distances [A] and angles[◦]: Cu–Ohexanoate = 1.975(3), Cu–OACE = 2.620(2), Cu–N = 2.037(3),O–Cu–O = N–Cu–N = 180.0, Ohexanoate–Cu–N = 92.2(1), OACE–Cu–N =101.46(9), OACE–Cu–OACE = 84.37(9). Symmetry operation for the primedatoms: -x, -y, 1 - z.
The positions of asymmetric (nas) and symmetric (nsym) stretch-ing vibrations of the carboxylate group in the IR spectra canbe used to distinguish between different types of coordinationof copper(II) by the carboxylate group (monodentate, chelating,
bridging). Deacon and Phillips16 proposed a difference of thecorresponding wavenumbers for the two vibrations (D = nas
COO- -nsym
COO-) as the quantitative criterion. A value of D in the range150–170 cm-1 is an indication of the bridging bidentate Cu(II)coordination, while D > 200 cm-1 is typical for complexes withmonodentate carboxylic groups. In the IR spectrum of 1, nas
COO- =1595 cm-1 and nsym
COO- = 1384 cm-1, giving the value of D =211 cm-1 (Fig. 3), which is in full agreement with the X-raystructure of the complex (Fig. 4), in which hexanoate groups areindeed coordinated in a monodentate fashion to the Cu(II) ion.The presence of a single N–H band (shifted to lower wavenumberswith respect to ACE) confirms that both nitrogen atoms of ACEin 1 are involved in symmetric coordination with Cu(II). This alsoagrees well with the X-ray structure, in which Cu(II) is locatedinside the cavity of ACE, at equal distance from symmetricallyspaced nitrogen atoms of the azacrown ring.
The toluene solution of 1 shows a broad absorption band inthe visible region of the UV-vis spectrum at lmax = 598 nm (e =33 M-1 cm-1) with a shoulder at l ª 680 nm (Fig. 2). The band canbe resolved into two Gaussian components with maxima at 564and 666 nm. Formation of monomeric complex 1, which requiresbreaking up of the paddlewheel dimer is also accompanied by adecrease of the intensity of the band at l = 375 nm, which is oftenassociated with the presence of the dimeric Cu(II) species.
EPR is a powerful spectroscopic tool that proved especiallyuseful in elucidation of the structural and electronic features ofcopper(II) complexes. In our case, even though on the basis ofthe X-ray diffraction data, the coordination geometry of 1 is bestdescribed as 4 + 2, in the subsequent EPR/DFT analysis, theeffect of the distant weakly coordinated oxygen atoms of ACE ishardly noticeable. The X-band CW-EPR powder spectrum of 1(not shown) is typical of square-planar Cu2+ complexes with theground state corresponding to the unpaired electron in the 3d(x2 -y2) orbital of copper and contains three broad peaks at lower fields,with the fourth peak overlapping with central transition at aroundg = 2.03. A parallel component of the 63/65Cu hyperfine coupling,A// = 165 G. The frozen solution EPR spectrum of 1 in tolueneat 77 K is very similar to the powder pattern obtained from thecrystalline form of the compound. The spectrum (Fig. 5a) containsthe same lower field pattern of three well-resolved peaks (the fourthtransition overlaps with the g^ peak) with a central transition ataround g = 2.03 and the same 63Cu hyperfine coupling (165 G), asalready observed in the powder sample. The pseudo-modulated Q-band electron spin-echo detected spectrum (Fig. 5b) exhibits thesame transitions with slightly better resolved g values (g// = 2.271and g^ = 2.047) and an A// value of 164 G. The higher g-resolutionat this frequency (34 GHz) allows for spreading the quartet centredat g//. The g values obtained from experimental EPR spectralpatterns are only moderately larger than those of typical square-planar Cu(II) complexes (e.g., for D4h CuCl4 molecule, g// = 2.233and g^ = 2.049).17 The experimental g values give also an estimateof the ground state (x2 - y2) Cu character of the metal centre ofabout 60%, and this covalency is similar to that of D4h CuCl4,which has 63% Cu character in the ground state.
DFT calculations using BP86 approximation to the exchange–correlation functional and a triple-z quality Slater type orbitalall-electron basis set (TZP) performed on the monomeric complex1 (see ESI for details of the QM computations†) yielded anelectronic structure involving the presence of the unpaired electron
Fig. 5 (a) X-Band CW-EPR spectra of frozen toluene solutions (77 K) of1 (blue curve) and 2 (red curve); non-normalized spectra, the experimentalintensity ratio kept for the same concentrations of 1 and 2, (b) Q-Bandpseudo-modulated ESE-detected EPR spectrum of complex 1 (blue curve)and polymeric species 2 (red curve) (7 K).
predominantly in the singly occupied MO corresponding tothe d(x2 - y2) orbital of copper, which is consistent with ourspectroscopic results. As expected, the calculated spin densities(see Table 1) show unambiguously that most of the electronspin density remains localized on the central copper atom, andonly a small amount of the unpaired electron migrates onto twonitrogen atoms of ACE and two oxygen atoms of the carboxylateligands. No noticeable spin density is found on the four oxygenatoms of ACE. In very good agreement with the X-ray diffractionresults, the analysis of calculated Mayer bond orders (MBO)show, that only nitrogen atoms of ACE (MBO’s = 0.395, 0.418)and carboxylate oxygen atoms (one of each carboxylate group,MBO’s = 0.544, 0.548) participate significantly in coordinationof the central copper atom (see ESI†). The corresponding MBO’sfor the interaction of Cu with the two remaining oxygen atomsoccupying axial position in the ACE ring are very small (<0.2),which is consistent with the long Cu–Oaxial distance observed inthe X-ray structure.
The corresponding values of the computed 63Cu hyperfineinteraction (Table 1) are overestimated, most probably due tothe unreliable calculation of the isotropic part of the HF tensor,
Table 1 Charges and spin densities from DFT computations with the corresponding isotropic and anisotropic parts of the hyperfine interaction (A) forselected nuclei in 1 (see Fig. 4 for the numbering scheme) obtained at BP86/TZP level of theory
Nucleus Spin density Charge/e Axx/G Ayy/G Azz/G Aiso/G
a Experimental values obtained from the simulations of the X and Q band spectra of the monomeric species.
but remain in a reasonable agreement with the correspondingexperimental EPR parameters. Although the local environment ofthe copper ion in monomeric complex 1 exhibits low symmetry andit is substantially more complicated than for simple square-planarcomplexes of this type, the comparison of its electronic structurewith that obtained for the well known square-planar CuCl4
-2 yieldsa very similar picture. For the complex 1, as schematically depictedin Fig. 6, the unpaired electron resides similarly in a SOMO orbital,of which the major component is the d(x2 - y2) type atomic orbitalof the central copper atom. The complete diagram of the frontierKohn–Sham orbitals obtained from DFT for 1 can be found inthe ESI.†
Fig. 6 Schematic diagram of the frontier atomic orbitals with importantmetal character of the complex 1, with the HOMO composed of thehalf-occupied d(x2 - y2) orbital of Cu containing the unpaired electron.Energies in eV, HOMO–LUMO gap not to scale on the picture, ligandbased only orbitals are omitted for clarity (for the complete representationof the frontier KS orbitals see ESI).†
The DFT calculations of the g-factor performed at non-relativistic and scalar-relativistic level (ZORA approximation)yield similar values with a close-to-axial g matrix (Table 2). Thesecalculations follow the same trends as earlier computations forsquare-planar Cu complexes containing Cl- and/or ammonialigands.18–21 It is however clear that for our monomeric species 1 theparallel g-tensor component is underestimated, gzz showing a pro-
Table 2 Experimental and calculated g-matrices for complex 1
a For the details of the g-matrix computations see the DFT calculationssection.
nounced disagreement (0.097–0.142) with the experimental values.To check whether this error is intrinsic to our system, or rather ageneral trend in the computed g-matrices for similar compounds,we performed additional calculations on structurally relatedand experimentally well-characterized square-planar CuCl4
2- andCu(NH3)4 complexes, as well as on the compressed octahedralCuCl4
2-(NH3)2 species (note that in this case g// < g^). Whereasthe gxx and gyy values (corresponding to the g^ in the square-planar form) remain very close to the experimental g^ values, thecomputations using both ZORA and the non relativistic approachresult in much less agreement for gzz. For example, the gzz elementis lower (as compared to the experimental g//) by ca. 0.129 for thesquare-planar CuCl4
2-. Thus, if the value of 0.129 is subtractedfrom the gzz for our monomeric species 1, the computed g-matrixelements are in reasonable agreement with the experimentalg values for complex 1 (Table 2).
A characteristic feature of the computed g-matrix is its orien-tation, which even in the low-symmetry of 1 is similar to the oneobtained for perfectly square-planar (D4h) symmetry.21 The twosmaller elements of g-matrix (gxx, gyy corresponding to g^) lie, asexpected, approximately along x and y axes (see Fig. 7) connectingthe central copper to carboxylate oxygen atoms and nitrogenatoms of ACE, respectively. The highest value of the g matrixgzz (g//) lies approximately along the z axis, perpendicular to theN–N–Ocarboylate–Ocarboxylate moiety, which exhibits a pseudo square-planar arrangement. The similarity between this gzz matrix elementin 1 and in CuCl4
2- (for both, experimental and computed values)indicate again that contrarily to octahedral symmetry provided bythe presence of additional ligands along the z axis, as for examplein the case of the above mentioned CuCl4
2-(NH3)2, the presence ofthe distant oxygen atoms of the ACE ring, perpendicular to theN–N–Ocarboylate–Ocarboxylate fragment, does not contribute signifi-cantly to the ligand field of the pseudo square-planar core.
Except for the simplest cases of high symmetry and with allligands being equivalent, it is very difficult to determine whetherthe complex belongs to a square-planar or distorted-octahedral
Fig. 7 Schematic representation of the principal g-values orientation in1, with the highlighted pseudo square-planar (dotted line) arrangementof the Cu–N–N–Ocarboylate–Ocarboxylate motif perpendicular to z axis. Colourcoding: light blue (Cu), dark blue (N), red (O) and green (C).
category. In presence of several ligands with different characteris-tics, different local electric properties and low symmetry, one canhardly draw the exact line between different types of the ligand fieldclassifications. Even though, on the basis of the X-ray diffractiondata, the coordination geometry of 1 is best described as 4 + 2,the EPR results alone do not allow distinguishing between theoctahedral with weakly coordinated apical ligands and square-planar geometry. On the other hand, DFT analysis indicates thatthe effect of the distant weakly coordinated oxygen atoms of ACEis small. Since in 1 the geometrical features (Cu–ligand distancesand Mayer bond orders) are indicative of a distorted square planargeometry with very weak bonding to apical ACE oxygen atoms,we found it interesting to investigate to what extent the two oxygenatoms of the ACE ring could be involved in a local ligand fieldresulting in a distorted octahedral geometry. For this purpose,we have performed QM calculations on model complex M1. Thesimplest way to assess the role of these oxygen atoms is to monitorthe changes in the g-matrix upon elongation of the OACE–Cu–OACE
fragment. We have computed such changes, as well as the intrinsiccontribution of the six atoms (two acetate oxygen atoms, twoACE nitrogen atoms and two ACE oxygen atoms), involved inthe ligand field sensed by the central Cu atom. The results arepresented and discussed in details in the ESI.† Briefly, at theexperimental Cu–OACE distances (~2.6 A) the paramagnetic spin–orbit contribution of the axial oxygen atoms to the g-matrix is verysmall, as compared to two other oxygen atoms and the nitrogenatoms (difference exceeding two orders of magnitude), and theg-matrix is not significantly affected upon variation of the Cu–OACE
distance in the range 2.6–3.6 A. However, even though the d(x2 -y2) ground state is apparent from the EPR and QM computationsand it is typical of square-planar complexes, the complexity ofthe ligand and the presence of a weak interaction with the distantapical ACE oxygen atoms, prompts us to describe the complex asdistorted-octahedral with weakly interacting apical oxygen atoms.
Polymeric species 2. Polymeric species 2 was obtained as agreen powder from hot isopropyl ether by recrystallisation of thereaction product of ACE with Cu2(hexanoate)4 after separation of
complex 1. The crystal structure of 2 shows the presence of twodistinct chains parallel to the (100). One chain (Fig. 8) is located ingeneral position and the other contains centres of inversion, one inthe centre of the dimeric (Cu2(C6H11N2O4)4) unit and the second inthe centre of the azacrown ether (CuC12H26N2O4), which is affectedby some disorders. Both chains are parallel and present a similararrangement of their constituents. In the polymeric species 2, thepaddlewheel Cu2(hexanoate)4 units are linked via the complex1 units through one of the carboxylate groups of the latter. Oneoxygen atom of the bridging carboxylate is coordinated to theCu(II) ion inside the ACE cavity, and the second one takes the api-cal position of the neighbouring Cu(II) ion of the Cu2(hexanoate)4
paddlewheel unit. The coordination geometry of the dimericpaddlewheel copper(II) units can be described as square pyramidal,with four carboxylate oxygen atoms forming a basal square plane,similar to that of paddlewheel Cu2(hexanoate)4. The averagedistance between Cu atoms is 2.617(4) A, similar to that of theparent Cu2(hexanoate)4 (2.5791(5)).2
Fig. 8 View of 2 showing the polymeric chain located in general position,parallel to the (100) direction, showing the sequence of Cu2(hexanoate)4
and Cu(ACE) complexes (1) bridged by hexanoate fragments.
The asymmetric COO- band (nasCOO-) in the IR spectrum of 2
(Fig. 3) is split into two peaks at 1630 and 1552 cm-1, suggestingthe existence of two types of carboxylic groups in the molecule.The 1630 cm-1 peak could also be assigned to water coordinatedin axial positions of the dimer. In such a case, however, a broad
n(OH) band around 3500 cm-1 should also be present.22 However,the only band in this region (3148 cm-1) can be assigned to theN–H stretching shifted to lower wavenumbers, due to involvementof the lone electron pair of nitrogen in the coordination with Cu(II)ion. Consequently, (as equally evidenced by the X-ray results) nowater is present in 2, and the peak at 1630 cm-1 can indeed beassigned to nas
COO-. Interestingly, the band corresponding to thefree N–H group (clearly seen in the spectrum of ACE, Fig. 3) is notpresent in the spectrum of 2, suggesting that both nitrogen atomsof ACE are involved in coordination with Cu(II). This implies asymmetric (but different than in complex 1) coordination of Cu(II)by ACE in this molecule.
Splitting of the asymmetric stretching carboxylate bands in 2(1552 cm-1 and 1630 cm-1) suggests a coexistence of two differ-ent modes of carboxylate ion coordination. The correspondingsymmetric stretching band is located at nsym
COO- = 1404 cm-1.Thus, the Deacon and Philips¢ D parameter (nas
COO- - nsymCOO-)
for the two pairs of nasCOO-, D = 148 and 226 cm-1, typical for the
bridging bidentate and monodentate coordination, respectively.The assignment of these two bands is possible in combinationwith the analysis of the X-ray structure of 2. One of the bandscould be associated with a structure similar to that in originalCu2(hexanoate)4 paddlewheel dimer. The second peak wouldoriginate from the carboxylate group of complex 1, connecting apaddlewheel dimer unit in polymeric species 2. The X-ray structureof 2 (Fig. 8) indeed shows that the latter is an adduct of complex 1and paddlewheel Cu2(hexanoate)4, with two different carboxylategroups present in one molecule. As such, 2 should exhibit mixedproperties of 1 and paddlewheel Cu2(hexanoate)4.
The toluene solution of 2 has a very similar UV-vis spectrum tothat of Cu2(hexanoate)4 (Fig. 2). The maximum absorbance of 2 isred-shifted by only 10 nm and the intensity is slightly lower thanthat of the bare paddlewheel dimer (lmax = 680, e = 165 M-1 cm-1
for 2 vs. lmax = 690, e = 190 M-1 cm-1 for the dimer). The wholeband is also broadened with respect to that of the dimer. Thesmall red shift of 2 with respect to Cu2(hexanoate)4 suggests thatno drastic changes of the structure take place when 2 is formed,and that the original square pyramidal geometry of the dimer isprobably maintained.
The observed d–d bands in the UV-vis spectra of Cu(II) D4h
complexes originate from the transitions from filled d levels (z2,xy, xz/yz) into half empty d(x2 - y2) level, where the unpairedelectron resides.23–25 Ross et al. have analysed in detail the solidstate UV-vis spectrum of a pyrazine-coordinated copper(II) acetatepaddlewheel dimer.11 They have assigned the band around 700–800 nm as originating from d(xz)/d(yz) → d(x2 - y2) andd(xy) → d(x2 - y2) transitions. Therefore, in agreement withour observations, in the square pyramidal ligand field of theCu2(hexanoate)4 unit of 2, the changes in the axial positions ofthe dimer would have only little influence on this band. On theother hand, the d(z2) orbitals in dimeric Cu(II) complexes of D4h
symmetry are located along the z axis, connecting two copperatoms. Hence, in dimeric copper(II) alkanoate species, the lowestenergy d(z2) → d(x2 - y2) transition should be strongly dependenton changes in the dimer’s apical positions, which are located alongthe z-axis. In polymeric species 2 these positions are occupied bythe monomeric 1 units (Fig. 8). The d(z2) → d(x2 - y2) transitionhas been assigned by Ross et al. to the near-IR band at 1000 nm.Indeed, the absorbance of a solution of polymeric species 2 above
900 nm is higher than that of the parent Cu2(hexanoate)4 (Fig. 2).This may suggest that the polymeric species 2 in solution retainsthe original trimeric moiety, with axial positions of paddlewheeldimers bridged with the monomeric complex 1 units.
The powder EPR spectrum of 2 is very similar to that of 1,with the sole difference in the intensity (data not shown). Thefrozen toluene solution EPR spectrum of 2 reveals again thesame features as that of complex 1 but with less intensity for thesame spectrometer acquisition conditions (Fig. 5a). The Q-bandpseudo-modulated ESE-detected spectrum of 2 (Fig. 5b) showsa clear shift of the g// towards lower field as compared to thespectrum of 1 and a very small increase of the correspondinghyperfine values (A// = 169 G). Even though at this frequencythe possible small disparities in the g^ matrix elements aredifficult to detect, the high-field EPR techniques should helpto clearly distinguish between the two species (monomers andpolymeric chains) by providing the resolved differences betweenthe overlapping g^ components with increased g resolution. Thecomparison of integrated EPR signals of both samples fromFig. 5a shows that the signal of 2 is ca. 50% of that of 1 under thesame conditions. This result can be easily understood assumingweak magnetic interactions between the dimeric (paddlewheel)and monomeric units in 2. Given the lack of EPR signal ofthe antiferromagnetically coupled paddlewheel unit, the onlycontribution to the four-peak EPR signal in the spectrum of2 would be that from the monomeric units (complex 1), whereCu(II) is in the square planar environment. Thus, the ratio ofthe integrated signals would simply reflect the molar fractionof monomeric Cu(II) units (1) in 2, which equals 0.49. Similar,monomer-like EPR spectra have recently been obtained for sometrimeric Cu(II) complexes, in which two of the three copper centreswere coupled antiferromagnetically.26,27 As a result, a doublet(S = 1
2) ground state of the complex was observed.
DFT calculations on a simplified model S2 (a trimer comportingone Cu2(acetate)4 and one ACE–Cu–(acetate)2 unit) clearly showan almost complete lack of interaction between monomeric anddimeric units, as observed experimentally in the EPR spectrumof 2. From Fig. 9 it is clear that the unpaired electron is mainlylocalized in d(x2 - y2) copper orbitals on the monomeric ACE–Cu–(acetate)2 fragment (see Fig. 9), which shows the same spindensity pattern as in the isolated complex 1 (Fig. 6). Moreover,an unpaired electron spin difference between the two stronglyantiferromagnetically coupled copper atoms on the dimeric ac-etate unit is clearly evidenced by the DFT calculations. Furthertheoretical investigations of the electronic structure of 2 usingextended models, including the computation of the J couplingsbetween copper ions in monomeric-dimeric subunits, are currentlyon the way.
UV-vis solution studies (monomer–dimer equilibrium in solution)
Even though in the solid state copper(II) alkanoates are dimeric,it is not always the case in solution. In this respect, the polarityof the solvent is the critical factor: the less polar the solvent, themore a monomer–dimer equilibrium is shifted toward dimer.3,5,6,8
For example, copper(II) is extracted by fatty acids to a non-polartoluene solely in dimeric form.28 Such findings are also supportedby our IR spectroscopic results. Therefore in this study, toluenewas used as a solvent for all the UV-vis and NMR titrations to
Fig. 9 The unpaired electron density of S2, a simplified trimeric modelfor polymeric species 2.
avoid any additional equilibria, which would be difficult to accountfor.
The 1H NMR spectra of both isolated 1 and 2 in toluene arefeatureless. They both show very broad and weak peaks in theregion where the azacrown protons can be observed in pure ACEsolution, despite an intense colour (blue and green for 1 and 2,respectively). As expected for non-diamagnetic copper(II) species,the proton peaks in 1H NMR spectrum of the azacrown etherdisappear completely upon titration of ACE with Cu2(hexanoate)4
in deuterated toluene. No peaks due to the free hexanoic acid wereobserved in the spectra neither, suggesting that all hexanoate ionspresent in original Cu2(hexanoate)4 participate in the formationof 1 and 2.
The addition of ACE to the Cu2(hexanoate)4 solution in tolueneresults in a decrease of the intensity of the d–d band at 670 nm(e = 190 M-1 cm-1) and rise of the less intense band around 590 nm(e = 34.5 M-1 cm-1). Interestingly, analogous titration with the 18-crown-6 ether (CE), which possesses only oxygen heteroatoms inthe cavity, results in only minor changes in the UV-vis spectra (notshown). The reason for this is a more hard character of oxygenatoms in CE as compared to the nitrogen atoms in ACE, whichlessens the affinity to the intermediate hardness Cu(II) ions. Infact, in the X-ray structure of the CE-copper(II) acetate adduct,free CE, and not its copper(II) complex, is coordinated to the axialpositions of the paddlewheel dimer.22
The titration curves of Cu2(hexanoate)4 with ACE (Fig. 10)could be reasonably fitted using a simple 1 : 1 (copper(II)hexanoate : ACE) complexation model to give log(b/M-1) = 5.1.Only an analysis of the UV-vis spectra of both separated species(Fig. 2) combined with the structural information described above,and a thorough fitting of the curves at all wavelengths allow forproper interpretation of changes in the UV-vis spectra duringtitration. The factor analysis of the spectra from the titration ofCu2(hexanoate)4 with ACE using Specfit/32TM software suggestedformation of a 3 : 6 : 1 (copper(II)–hexanoate–ACE) adduct.
Fig. 10 UV-vis titration of Cu2(hexanoate)4 with ACE in toluene at roomtemperature.
The equilibrium in the toluene solution of Cu2(hexanoate)4 andACE can thus be described as follows:
2 2 5 11 2 2 11ACE Cu C H COO Cu ACE C H(paddlewheel dimer)
5+ ( )ÈÎ ˘ ¨ Ææ ( ) CCOO
Cu ACE C H COO Cu C H COO5 11
( )
+ ( )( ) - ( )ÈÎ ˘
2
2 5 11 2 2
(1)
(2)
(1)
or alternatively as two consecutive reactions:
2 ACE Cu C H COO
2 Cu ACE C H
(paddlewheel dimer)
5
+ ( )ÈÎ ˘
¨ Ææ ( )
5 11 2 2
1K 111COO( )2
(1)
(2)
Cu ACE C H COO Cu C H COO5 11
(paddlewheel dimer
( )( ) + ( )ÈÎ ˘2 5 11 2 2
(1) ))
5 11 5Cu ACE C H COO Cu C H COOK2 2 11 2 2
¨ Æææ ( )( ) - ( )ÈÎ ˘(2)
(3)
The non-linear least-squares fitting of the spectrophotometrictitration data was performed using the Dynafit 4.0 program.29
A global fit of the data at five different wavelengths resulted inthe following values of the association constants: K1 = (3.98 ±1.65) ¥ 106 M-1, and K2 = 70.1 ± 7.7 M-1. On the basisof the Le Chatelier–Brown principle considerations, one wouldexpect that the polymeric species 2 should be favoured at higherconcentrations, while lower concentrations should favour complex1 (eqn (3)). Indeed, upon 100 times dilution of solution containing2, some 40 nm blue shift of the d–d band was observed. Takinginto account that the monomeric form (1) has lmax blue-shiftedby ca. 100 nm with respect to 2, this result clearly confirms thatthe dimer-monomer equilibrium was shifted toward the monomerupon dilution.
ether, THF and dioxane (all puriss p.a.) were purchasedfrom Fluka. Cu(NO3)2·3H2O (puriss p.a.) was obtained fromMerck.
Physical measurements
FTIR measurements in the range 400–4000 cm-1 were performedusing Perkin Elmer Spectrum One equipped with the Golden Gatediamond ATR from Specac, UK. The spectra were correctedfor the wavelength-dependence of the penetration depth in theATR mode. UV-vis spectra were recorded on Perkin ElmerLambda 35 in the range 350–800 nm using 10 mm quartzcuvettes.
X-Band CW-EPR spectra were collected at room temperatureor at 77K (except for variable temperature experiments, where thespectra were recorded between 7–300 K), using the Bruker ECS106 X-band CW-EPR spectrometer at frequency of 9.76 GHz. Thespectrometer was equipped with an Oxford ESR900 liquid heliumcryostat and an ITC503 temperature controller. Low microwavepower (10 mW) was used to prevent saturation of the signal. TheQ-Band field sweep spin-echo detected spectra were recorded onthe ELEXSYS E580 equipped with a pulsed Q-Band probe usinga 2-pulse Hahn-echo sequence (p/2 = 60, p = 120 ns) at 7 K andfrequency of 35.6 GHz. The CW-EPR simulations were performedwith SIMFONIA program from Bruker Corporation and withthe EasySpin simulation package by S. Stoll.30,31 1H NMR spectrawere recorded in the range -20–120 ppm on Bruker 500 MHzspectrometer.
DFT calculations
The DFT calculations were performed at both scalar relativisticand non-relativistic level of theory. The relativistic effects wereincluded using the zero order relativistic approximation (ZORA)relativistic Hamiltonian,32 which treats the core electrons explicitly,as implemented in ADF program.33 In this case, the ZORAHamiltonian was combined with the Kohn–Sham formalismusing BP86 functional and Slater type of orbitals of triple-zquality with one polarisation function (TZP). The non-relativisticcalculations were carried out using Gaussian 03 program34 withthe Ahlrichs all electron basis set of double-z quality augmentedwith polarisation functions combined with B3LYP functional.The coordinates were extracted from the X-ray crystallographicfiles. The g tensor calculations were performed with either ZORAapproximation using ADF (BP86 functional and all-electron basisset of triple-z quality TZP for all atoms) or at non-relativistic levelusing coupled-perturbed Kohn–Sham equations (CP-KS) with all-electron augmented CP(PPP) basis set for Cu plus all-electronaugmented SV(P) basis set for all other atoms as implementedin Neese’s ORCA program.35 In addition, the g-tensor was cal-culated with the gauge-independent GIAO method using hybridB3LYP functional combined with a double-z quality basis setaugmented with polarisation functions (DZVp) as implemented inGaussian 03.
Charges and spin densities were calculated from Mulliken pop-ulation analysis using B3LYP functional combined with CP(PPP)basis set augmented with TZV(P) for Cu and polarised SV basisset for remaining atoms. Hyperfine couplings were computed atthe same level of theory.
Preparation of complex 1 and polymeric species 2
Anhydrous Cu2(hexanoate)4 was obtained by dissolving 10 g ofCu(NO3)2·5H2O (0.04 mol) and 1.7 g of LiOH (0.04 mol) in 10 mLof hexanoic acid (0.04 mol). After 1 h heating, the green productcrystallized upon cooling. The product was rinsed with tolueneand then recrystallised from hot toluene as green needles. Yield80%. Anal. Calcd for Cu2C24H44O8 (587.4 as dimer): C 49.03,H 7.55, Cu 21.64%. Found: C 48.47, H 7.45, Cu 24.38%.
Species 1 (monomeric) and 2 (monomeric + dimeric) wereobtained from the same mixture of anhydrous Cu2(hexanoate)4
(137 mg, 0.46 mmol) and N,N¢-diaza-18-crown-6 ether (ACE)(80 mg, 0.30 mmol) in toluene. After the solvent evaporated,the resulting green-blue powder was dissolved in cold isopropylether and filtered. From the supernatant solution, blue needle-like crystals were obtained by slow evaporation during 1 d (1).The filtrate was rinsed with isopropyl ether and recrystallisedfrom hot isopropyl ether (2). Anal. for 1, yield 30%. Calcd forCuC24H48O8N2 (555.9): C 51.81, Cu 11.43, H 8.70, N 5.04%.Found: C 51.83, Cu 11.07, H 8.66, N 4.98%. For 2, yield 34%.Calcd for Cu3C48H92O16N2 (1143.4 repeating unit): C 50.38, Cu16.67, H 8.11, N 2.45%. Found: C 50.56, Cu 15.47, H 8.14, N2.68%.
Crystal data for 1. [(Cu(C12H26N2O4) (C6H11O2)2], M = 556.3,monoclinic, space group P21/c, a = 10.5343(9), b = 16.7311(9),c = 8.3453(5) A, b = 91.272(9)◦, V = 1470.5(2) A3, Z = 2, T =200 K, m = 0.79 mm-1, Dc = 1.256 g cm-3, l(MoKa) = 0.71073 A.19 393 measured reflections, 3529 unique (Rint = 0.066) from which2026 with |F o| > 4s (F o), final R = wR = 0.045. wR2(all) = 0.084.CCDC-689064. Selected interatomic distances and angles for 1 arelisted in the ESI (Table S1).†
Crystal data for 2. {[(Cu3(C12H26N2O4)(C6H11O2)6]1.5}• crys-tallises with unobserved solvent molecules in a triclinic sys-tem, space group P1, a = 13.2193(7), b = 17.5953(14), c =22.5887(15) A, a = 106.279(9), b = 91.889(7), g = 106.310(8)◦,V = 4805.0(4) A3, Z = 2, T = 200 K, l(MoKa) = 0.71073 A.The best crystal obtained for this species shows a poor diffraction(about 27% of observed (|F o| > 4s (F o)) reflections). Althoughthe hexanoates show large displacement parameters and possibledisorders, the observed topology of the obtained model is unam-biguous. Refinement of this model leads to R and wR values ofabout 0.10.
Conclusions
Two new Cu(II) hexanoate species with azacrown ether (ACE)were synthesized starting from paddlewheel Cu2(hexanoate)4 andACE. Both were analysed by spectroscopic (IR, UV-vis, EPR,NMR), X-ray diffraction, and computational (DFT) methods.The first complex consists of Cu(II) coordinated inside the cavity ofthe azacrown ether with two hexanoate counterions in elongatedoctahedral geometry (complex 1), hexanoate oxygen atoms andACE nitrogen atoms constituting the distorted square-planar coreof the complex. The non-zero interaction of the two apical oxygenatoms from the ACE cavity with the central Cu(II) ion suggestedby the computed Mayer indexes is, however, too weak to be clearlyevidenced by EPR. The second, polymeric species 2, consists ofinfinite chains of alternate units of paddlewheel Cu2(hexanoate)4
and complex 1. In toluene solution, equilibrium exists betweenthe two complexes, the polymeric species 2 being favoured in moreconcentrated solutions. The synergistic complexation of Cu(II) byboth hexanoate and azacrown ether clarifies the origin of thesynergistic effect of fatty acid and azacrown ether in the Cu(II)transport through the permeation liquid membranes. In the latter,probably both components of the carrier, azacrown ether and fattyacid, participate in transport of Cu(II) across the membrane.
Extensive studies of the magnetic properties as well as thedetailed electronic structure of 2 using very-high field EPR andadvanced QM computations are currently under way and will bepublished in a separate report.
Acknowledgements
This work was financially supported by the Polish Ministryof Science and Higher Education (grant no. N N204 236934).Dr Petr Kuzmic is acknowledged for his help in performingthe spectrophotometric data fitting. KW wishes to express hisgratitude to Prof. Jacques Buffle for his encouragement anddiscussions.
References
1 R. E. Del Sesto, L. Deakin and J. S. Miller, Synth. Met., 2001, 122, 543.2 A. Doyle, J. Felcman, M. T. d. P. Gambardella, C. N. Verani and
M. L. B. Tristao, Polyhedron, 2000, 19, 2621.3 M. Kato, H. B. Jonassen and J. C. Fanning, Chem. Rev., 1964, 64, 99.4 P. Sharrock and M. Melnik, Can. J. Chem., 1985, 63, 52.5 Y. Fujii, K. Jimbo, H. Yamada and M. Mizuta, Polyhedron, 1985, 4,
491.6 Y. Fujii, N. Nakasuka, M. Tanaka, H. Yamada and M. Mizuta, Inorg.
Chem., 1989, 28, 3600.7 M. Tanaka and T. Niinomi, J. Inorg. Nucl. Chem, 1965, 27, 431.8 H. Yamada, K. Yajima and H. Wada, Anal. Sci., 1995, 11, 715.9 H. Yamada, K. Takahashi, Y. Fujii and M. Mizuta, Bull. Chem. Soc.
Jpn., 1984, 57, 2847.10 A. Harada, M. Tsuchimoto, S. Ohba, K. Iwasawa and T. Tokii, Acta
Crystallogr., Sect. B: Struct. Sci., 1997, 53, 654.11 P. K. Ross, M. D. Allendorf and E. I. Solomon, J. Am. Chem. Soc.,
1989, 111, 4009.12 M. Kato and Y. Muto, Coord. Chem. Rev., 1988, 92, 45.13 R. M. Izatt, K. Pawlak, J. S. Bradshaw and R. L. Bruening, Chem. Rev.,
1991, 91, 1721.
14 J. Buffle, N. Parthasarathy, N. K. Djane and L. Matthiasson, IUPACSeries on Analytical and Physical Chemistry of Environmental Systems,2000, 6, 407.
15 K. Wojciechowski, M. Kucharek and J. Buffle, J. Membr. Sci., 2008,314, 152.
16 G. B. Deacon and R. J. Phillips, Coord. Chem. Rev., 1980, 33, 227.17 C. Chow, K. Chang and R. D. Willett, J. Chem. Phys., 1973, 59, 2629.18 N. J. Trappeniers, G. De Brouckere and C. A. Ten Seldam, Chem. Phys.
Lett., 1971, 8, 327.19 P. A. Narayana and K. V. L. N. Sastry, J. Chem. Phys., 1973, 58, 4381.20 G. De Brouckere, N. J. Trappeniers and C. A. Ten Seldam, Physica
B + C, 1976, 84 B + C, 295.21 S. Vancoillie and K. Pierloot, J. Phys. Chem. A, 2008, 112, 4011.22 J. W. Steed, B. J. McCool and P. C. Junk, J. Chem. Soc., Dalton Trans.,
1998, 3417.23 E. I. Solomon, M. J. Baldwin and M. D. Lowery, Chem. Rev., 1992, 92,
521.24 F. Tuczek and E. I. Solomon, Coord. Chem. Rev., 2001, 219–221,
1075.25 E. I. Solomon, R. K. Szilagyi, S. De Beer George and L. Basumallick,
Chem. Rev., 2004, 104, 419.26 G. Mezei, R. G. Raptis and J. Telser, Inorg. Chem., 2006, 45, 8841.27 L. Spiccia, B. Graham, M. T. W. Hearn, G. Lazarev, B. Moubaraki,
K. S. Murray and E. R. T. Tiekink, J. Chem. Soc., Dalton Trans., 1997,4089.
28 S. Jimenez Gomez, J. Perez Sarmentero and A. Molina, Anales deQuimica (1968–1979), 1978, 74, 1080.
29 P. Kuzmic, Anal. Biochem., 1996, 237, 260.30 S. Stoll and A. Schweiger, Biol. Magn. Reson., 2007, 27, 299.31 S. Stoll and A. Schweiger, J. Magn. Reson., 2006, 178, 42.32 E. van Lenthe, E. J. Baerends and J. G. Snijders, J. Chem. Phys., 1994,
101, 9783.33 E. J. Baerends, Amsterdam density functional, Theoretical Chemistry,
Vrije Universiteit, Amsterdam (URL http://www.scm.com).34 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E.Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M.Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,P. Salvador, J. J. Dannenberg, D. K. Malick, A. D. Rabuck, K.Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul,B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. G.Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle and J. A. Pople, GAUSSIAN 98 (RevisionA.10), Gaussian, Inc., Pittsburgh, PA, 2001.
35 F. Neese, ORCA, version 2.6, Institute for Physical and TheoreticalChemistry at the University of Bonn, Germany, 2008.
