Dynamic pseudo Jahn–Teller distortion in a compressed octahedral CuO6 complex

Polyhedron 25 (2006) 2824–2828 www.elsevier.com/locate/poly

Dynamic pseudo Jahn–Teller distortion in a compressed octahedral CuO6 complex Bojan Kozlevcˇar a

a,*

, Amalija Golobicˇ a, Peter Strauch

b

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Asˇkercˇeva 5, P.O. Box 537, SI-1001 Ljubljana, Slovenia b Institute of Chemistry, University of Potsdam, P.O. Box 601553, D-14415 Potsdam, Germany Received 27 February 2006; accepted 6 April 2006 Available online 27 April 2006

Abstract The crystal structure of cis-[Cu(C8H7O3)2(H2O)2] (115 K data) reveals bidentate vanillinate ions coordinated via methoxy and deprotonated hydroxy oxygen atoms and water molecules in a distorted octahedral CuO6 chromophore. A cis orientation of the ligands ˚ ), and the third enables two non-identical O(methoxy)ACuAO(water) coordination axes (2.354(1) + 2.163(1); 2.151(1) + 2.020(1) A ˚ ). This 115 K coordination sphere differs importantly to the one shortest O(hydroxy)ACuAO(hydroxy) axis (1.919(1) + 1.914(1) A obtained from the 293 K data of the same compound, where two long O(methoxy)ACuAO(water) axes are of the same length, and only minor changes at the short O(hydroxy)ACuAO(hydroxy) axis are noticed. An axial symmetry of the complex with an inverse g1;2 ðg? Þ > g3 ðgk Þ pattern is observed in the temperature range from 298 to 180 K. A further decrease of temperature reveals gradual changes from axial to rhombic symmetry (g1 > g2 > g3) that is reversible. A mean-square displacement amplitude (MDSA) analysis reveals a disorder in the CuAO(methoxy) bonds, but not in the other metalAligand CuAO(hydroxy) and CuAO(water) bonds at 293 and 115 K. The disorder is significantly weaker in the 115 K structure. The MSDA analysis and the structural-EPR agreement show vibrational disorder in two coordination axes, due to the cis conformation of the complex with two O(methoxy)ACuAO(water) axes.  2006 Elsevier Ltd. All rights reserved. Keywords: Copper; Vanillin; Jahn–Teller distortion; MSDA; cis; EPR

1. Introduction Copper(II) attracts inorganic chemists as well as spectroscopists because of its user-friendly complexes, usually giving easy information with typical spectroscopic signatures. The biological and technological relevance of copper compounds has led to a great number of well studied Cu complexes. In an octahedral coordination sphere, copper(II) with a d9 configuration is Jahn–Teller active and the odd d-electron occupies one of the d-orbitals, giving rise to structural flexibility. During our research on fungicides for wood preservation [1–3], studies on copper complexes with lignin model compounds, to mimic copper and lignin interactions, have been extended [4–6]. One of the reported *

Corresponding author. Tel.: +386 1 241 91 27; fax: +386 1 241 92 20. E-mail address: [email protected] (B. Kozlevcˇar).

0277-5387/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.04.009

vanillinate complexes is a cis isomer with a compressed axial CuO6 coordination sphere (293 K), showing significant differences of the EPR spectra measured at two temperatures [5]. Such a phenomenon is described in the literature as one of the proofs for a dynamic vibrational disorder [7–9]. Namely, if the coordination bond distances and the EPR spectra do not change with temperature, the disorder is static, but if they do, the disorder is dynamic. It is especially important for determination of the ground state, if the 1 room temperature data suggest a fdz2 g . Nevertheless, an additional explanation was introduced for temperaturedependent Jahn–Teller distortion by Halcrow et al. [7,10], namely the electronic ground state is different at two temperatures. It should be possible with the close presence of an inductively withdrawing distal ligand substituent. The main goal of this work has been a detailed EPR and structural investigation of cis-[Cu(C8H7O3)2(H2O)2] with a

B. Kozlevcˇar et al. / Polyhedron 25 (2006) 2824–2828

CuO6 chromophore. It has focused on the analysis of a disorder, noticed in large thermal ellipsoids of the coordinated atoms [5]. An impact of this disorder was correlated to changes in the ground state of the complex. 2. Experimental All starting compounds and solvents were used as purchased, without any further purification. The title compound, cis-[Cu(C8H7O3)2(H2O)2], was synthesized as described previously [4]. Diffraction data were collected on a Nonius Kappa CCD diffractometer with graphite monochromated Mo Ka radiation. The data were processed using the DENZO program [11]. The structure was solved by direct methods using SIR97 [12]. We employed full-matrix least-squares refinements on F magnitudes with anisotropic displacement factors for all non-hydrogen atoms using XTAL3.6 [13]. The hydrogen atoms were located from the difference Fourier maps. Their positions together with their isotropic displacement factors were refined using restraints on OAH and CAH distances. Crystal data: C16H18CuO8, Mr = ˚ , b = 10.4922(2) A ˚, 401.86, monoclinic, a = 7.7401(1) A ˚ , b = 95.6277(7), V = 1716.58(5) A ˚ 3, c = 21.2398(3) A T = 115(2) K, space group P21/c (No. 14), Z = 4, l = 1.313 mm1, crystal size 0.30 · 0.25 · 0.10 mm3 (plate), 19 814 reflections measured (integrated), hmax 27.48, 2 2 3921 unique (RP int = 0.031), 3244 P observed [F > 2r(F )], R = 0.031 (R = P(||Fo|  |Fc||)/ |Fo|), wR = 0.026 (wR = P (w||Fo|  |Fc||)/ (w|Fo|)), 298 refined parameters, 3676 reflections. Spectra of the powdered samples were recorded by a Bruker ESP-300 spectrometer, operating at X-band (9.59 GHz) at variable temperatures. 3. Results and discussion X-ray data of the title compound cis-diaqua-bis(4-formyl-2-methoxyphenolato-O:O 0 )copper(II) were collected at 115 K and an analysis reveals two bidentate vanillinate anions (4-formyl-2-methoxyphenolate) and two water molecules in a cis orientation around the central copper(II) ion (Fig. 1). The vanillinates are coordinated through methoxy ˚ ) and deprotonated (CuAO1a 2.151(1), CuAO1b 2.354(1) A ˚ ) oxygen hydroxy (CuAO2a 1.919(1), CuAO2b 1.914(1) A

Fig. 1. An ORTEP molecular structure of cis-[Cu(C8H7O3)2(H2O)2], 115 K [14,15]. Thermal ellipsoids are drawn at the 50% probability level.

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atoms, while water molecules complete the octahedral coordination sphere (CuAO1w 2.163(1), CuAO2w ˚ ). The methoxy oxygen atoms and the water 2.020(1) A molecules are in trans positions with respect to the copper ion, thus forming two non-symmetrical axes O1aACuA O2w and O1bACuAO1w. The hydroxy oxygen atoms describe the third axis O2aACuAO2b. The cis arrangement of the ligands is found also in the room temperature structure of the title compound. The whole conformation of the complex molecules is very similar in both structures, however at room temperature the complex posses a twofold symmetry axis, which is lost at 115 K (Table 1, Scheme 1) [5]. This is related with two O(methoxy)ACuAO(water) coordination axes of the same length, and the third shorter O(hydroxy)ACuAO(hydroxy) axis of the axial coordination sphere in the 293 K structure. Consequently, in spite of a similarity of the packing arrangement and the unit cell parameters, the symmetry of the space group C2/c at room temperature is lowered to its sub-group P21/n at 115 K. The unit cell parameters for the 293 and 115 K data, respectively, are: a = ˚ ; b = 10.6891(2), 10.4922(2) A ˚; 22.0422(3), 23.3084(3) A ˚ ; b = 104.8463(8), 114.9252(7); c = 7.6668(1), 7.7401(1) A ˚ 3 (The later set of parameV = 1746.08(5), 1716.58(5) A ters is transformed from that written in Section 2, using the 3 · 3 matrix (1 0 1 0 1 0 1 0 0) and corresponds to the P21/n space group, enabling a more adequate comparison of both structures. Initially, the P21/c space group was chosen, since the corresponding b angle 95.6277(7) is closer to 90 and thus favourable for computations reasons). The unit cell parameters show an anisotropic change, similarly as noticed for the coordination bond distances. It is interesting that the colour of the compound changes reversibly from orange at room temperature to yellow-green at 115 K. Fig. 2 shows an ORTEP plot of the central coordination sphere of the title complex, determined from the 293 K dataset [5]. It can easily be seen that the thermal ellipsoids of the donor atoms mainly, as well as that of the central copper(II) atom, are large and significantly elongated. In the structure of the same complex measured at 115 K, an electron density distribution of these atoms is much more located (see Fig. 1). Since the thermal ellipsoids of the donor atoms are more perpendicular to the corresponding CuAO bonds, a static distortion would in theory be suggested (for the dynamic disorder, the ellipsoids should be parallel to the CuAO bond) [8,9]. However, such a disorder along a metalAligand bond is not always evident in the visible shape of the thermal ellipsoids of the atoms concerned. Therefore, a more sensitive MSDA (mean-square displacement amplitude) analysis was applied [7,8,14,15]. Certain disordered bonds may show high Æd2æ values from the high temperature structure thermal motion data. If they are accompanied with significant reduction of Æd2æ for the low temperature data, it should be a proof for the dynamic disorder. Indeed, much higher Æd2æ values and their temperature dependence along

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Table 1 ˚ ), angles () and hydrogen-bonding geometry for cis-[Cu(C8H7O3)2(H2O)2] at two temperatures Selected bond distances (A 293 K [5] CuAO1 CuAO2 CuAO1w O1ACuAO1w O2ACuAO2 0 O1ACuAO1 0 O1wACuAO1w 0 O1ACuAO2 O2ACuAO1 0 O1 0 ACuAO1w 0 O2ACuAO1w O2ACuAO1w 0

2.2603(12) 1.9086(11) 2.0869(13) 169.97(5) 169.14(7) 91.09(7) 93.15(9) 77.31(4) 95.00(5) 88.74(6) 92.71(6) 94.75(6)

115 K CuAO1a CuAO2a CuAO1w O1aACuAO2w O2aACuAO2b O1aACuAO1b O1wACuAO2w O1aACuAO2a O1aACuAO2b O1aACuAO1w O2bACuAO2w O1wACuAO2b

2.1513(8) 1.9192(8) 2.1629(8) 172.08(3) 168.43(3) 93.62(3) 91.11(3) 79.32(3) 93.01(3) 88.35(3) 94.91(4) 92.94(3)

CuAO1b CuAO2b CuAO2w O1bACuAO1w

2.3541(8) 1.9140(8) 2.0199(8) 168.42(3)

O1bACuAO2b O1bACuAO2a O1bACuAO2w O1wACuAO2a O2aACuAO2w

75.57(3) 96.12(3) 88.49(3) 95.45(4) 92.87(3)

DAH  A

D  A

DAH  A

DAH  A

D  A

DAH  A

115 K O1wAH1w1  O3bc O2wAH1w2  O3ad O1wAH2w1  O2be O2wAH2w2  O2aa

2.780(2) 2.727(2) 2.767(2) 2.713(2)

175(2) 174(2) 166(2) 167(2)

293 K O1wAH11  O2a

2.759(2)

166(3)

O1wAH12  Ob

2.760(2)

168(2)

a b c d e

x, 1  y, 1  z. 1/2 + x, 1/2  y, 1/2 + z. x, 3/2  y, 1/2 + z. x, 3/2  y, 1/2 + z. 1  x, 1  y, 1  z.

the CuAO(methoxy) than for the other coordination bonds CuAO(hydroxy) and CuAO(water) were found (Scheme 1). A comparison with cis-diaqua-bis(methoxyacetato)zinc(II) of related geometry (295 K), does not show the same ellipsoid elongation effect of the related atoms [9]. This is in agreement with a Zn(II) d10 configuration, where the Jahn–Teller phenomenon is not expected. Altogether this indicates more towards a higher pseudo-Jahn–Teller fluxionality in the title Cu(II)-complex above 180 K, which is gradually frozen out by decreasing temperature (see Fig. 3). Thus, a librational dynamic disorder of the Jahn–Teller distorted axes leads to an equalisation of the determined bond lengths, imitating a higher symmetry in the structure at higher temperature. The herein described disorder may be induced with one type of a ligand (high Æd2æ only for CuAO(methoxy)), but the consequences are certainly evident in both O(methoxy)A CuAO(water) coordination axes, as a whole.

Fig. 2. An ORTEP-plot of the central coordination sphere of the title complex (293 K), showing 50% atomic displacement ellipsoids [5,14,15]. Other atoms are omitted for clarity.

EPR spectra of the cis-vanillinate title complex were measured in the temperature range from 298 to 115 K (Fig. 3). The room temperature spectrum shows an inverse axial symmetry pattern with g? ðg1  g2 Þ > gk ðg3 Þ ¼ 2:005ð3Þ, while the 115 K spectrum shows rhombic symmetry with

˚ ) in cis-[Cu(C8H7O3)2(H2O)2] (h – hydroxy, Scheme 1. A temperature induced reversible change of the CuAO coordination bond distances (A ˚ 2) are in square brackets. m – methoxy, w – water). CuAO Æd2æ values (·104 A

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Scheme 2. An orbital splitting diagram of a Jahn–Teller compressed octahedron along the z-axis.

Fig. 3. Splitting of the g1,2 signal to g1 and g2 in the EPR spectra of cis[Cu(C8H7O3)2(H2O)2], while cooling from 298 to 115 K. An asterisk marks a weak signal between dominant g1 and g2 noticed in the low temperature spectra.

g1 > g2 > g3 = 2.010(3) [5]. The g12 (g^) signal is unchanged in the temperature range from 298 to 180 K, and further decrease of temperature shows its gradual splitting to g1 and g2 that probably continues below 115 K. This change between the low and the high temperature spectra is reversible (Scheme 1). For a more detailed discussion of the complex electronic situation, an assignment of the axes orientation is necessary. In our case, we would orient the coordination system as follows: x-axis O1w 0 ACuAO1(m), O1wACuAO1b(m); y-axis O1wACuAO1 0 (m), O2wACuAO1a(m); z-axis O2(h)ACuAO2 0 (h), O2a(h)ACuAO2b(h) (m – methoxy, h – hydroxy) for the high and the low temperature situation, respectively. Therefore, we have for the two compressed octahedral cases, a tetragonal and a rhombic stereochemistry with short z-axis and more or less elongated x- and y-axes. The resulting orbital splitting diagrams are given in Scheme 2. Due to the non-equivalence of the six donor sites in the complex, a rhombic distortion of the coordination sphere is expected and the d-energy levels would not posses any degeneracy. However, at room temperature, the dynamic librational disorder of the molecule is fast enough to simulate degeneracy along the x and y-axes (see Scheme 2). This librational dynamics is fast in the X-ray and EPR timescales showing averaged bond lengths along these axes, and an ‘inverse’ axial (tetragonal) symmetry found in the X-ray structure analysis and EPR spectra g1;2 ðg? Þ > g3 ðgk ; gZ Þ  ge (pseudo Jahn–Teller distortion). Due to the orbital splitting (see Scheme 2), the high temper1 ature EPR spectrum and the low g3 value (2.005), a fdz2 g ground state can be assumed (see Table 2).

By decreasing temperature, the dynamic process will be frozen, thus adopting the true rhombic symmetry of the coordination sphere and splitting all degeneracy of the energy levels. As in the X-ray structure analysis (Scheme 1), this is also clearly observable in the EPR spectra, which are of rhombic symmetry below 178 K, with g1(gx) > g2(gy) > g3(gz) (see Fig. 3). The g3(gz) value is more or less unchanged at different temperatures according to the relatively small changes of the bond distances along the z-axis, whereas g^ splits into g1(gx) > g2(gy), due to more static situation in the structure with the expected rhombic symmetry 1 (D2h). Therefore, a fdx2 y 2 g ground state of the title complex at low temperature might be suggested, but the unchanged signal for g3(gz) indicates that the ground states at 293 and 115 K have a similar character. Furthermore, a mixing of the d x2 y 2 and the dz2 orbital cannot be excluded for this symmetry. This can easily lead to miss-assignments of the ground state, especially if the structure is solved only from the room temperature data and no other complementary methods (EPR, UV–Vis, EXAFS, MSDA) with temperature variation are used [7]. Cu(II) complexes with a 1 compressed octahedral symmetry and a fdz2 g ground state are therefore very rare [7,16–20]. Since the theory with a disorder in such cases is not fully clear-cut, an alternative explanation for related complexes was introduced. Namely, a switch of the ground state

Table 2 Temperature dependence of selected EPR parameters of the vanillinate complex with a CuO6 chromophore H1 (mT) H2 (mT) H3 (mT) g1 g2 g3

298 K

115 K

297.4 297.4 341.5 2.302 2.302 2.005

286.0 309.2 340.6 2.393 2.214 2.010

The estimated errors are H ± 0.5 mT, g ± 0.003.

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from a fdz2 g1 at 295 K to a fdx2 y 2 g1 at 5 K for [Cu(2,6bis(hydrazonomethyl)pyridine)2](ClO4)2 and [Cu(2,6-bis(oximomethyl)pyridine)2](ClO4)2 was reported [7,10]. It was suggested that a temperature-dependent switching between two different ground states would be possible, if the distal CuAligand bond is inductively weakened, possibly by an electron withdrawing substituent. Through this perspective, the rigid vanillinate ion in cis-[Cu(C8H7O3)2(H2O)2], due to its strong hydroxy coordination that would not allow the methoxy group to coordinate closer, was expected to be interesting for a possible switch. A small signal between g1 and g2 (see the low temperature spectra in Fig. 3), observed also in the spectra of the above mentioned 1 two fdz2 g species [7,10] that was tentatively assigned as a small fraction of fluctional fdx2 y 2 g1 Cu(II) spins, could be in favour of the last explanation. Additionally, it should not be forgotten that the title compound was isolated in a pure form only under specific conditions by quick and careful melting of vanillin, followed by its immediate dissolution in hot acidified Cu2+(aq), since otherwise another isomer (trans) predominates [4]. Therefore, a high temperature was crucial during the synthesis, possibly enabling a sterical advantage of the compressed cis isomer against the elongated trans isomer. Maybe the complex was already formed in the fdz2 g1 state, which requires a specific geometry around the Cu(II) ion, later preserved after cooling to room temperature, due to specific conditions with a fixed cis coordination enabled by the rigid bidentate ligand with a withdrawing group. The preservation of a very similar conformation and packing of the molecules with a temperature change is seen also in only minor differences in the coordination sphere angles and H-bonding network (Table 1, and as well from a great similarity of the powder diffraction patterns). This would agree with the ligand imposed distortion or packing in the crystal. 4. Concluding remarks X-ray structure analysis of cis-[Cu(C8H7O3)2(H2O)2] at 115 K reveals a cis coordination of vanillinate ions and water molecules, with three rhombic coordination axes. The distances and the symmetry of the coordination sphere differ importantly from the room temperature molecular structure, where compressed axial axes were determined. The MSDA thermal ellipsoid analysis confirms the vibrational disorder in the structure at high temperature, and its gradual freezing by decreasing temperature. These results are fully in agreement with the EPR spectra, showing also the reversibility of this change. Based on the EPR 1 data, a fdz2 g ground state is proposed, although the room 1 temperature spectrum suggests a ‘normal’ fdx2 y 2 g ground state, but with a small g3 value (close to ge). A careful analysis of the data in such cases is recommended.

Acknowledgements The financial support of the MVZT P-6209, P1-0175 and X-2000, Republic of Slovenia, is gratefully acknowledged. We thank Dr. M. Sˇentjurc, EPR center, ‘Jozˇef Stefan’ Institute, Ljubljana, for EPR spectra. Appendix A. Supplementary data All atom parameters and other crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary material with the deposition number CCDC 296921. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or http:// www.ccdc.cam.ac.uk/conts/retrieving.html). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2006.04.009. References [1] M. Petricˇ, F. Pohleven, I. Turel, P. Sˇegedin, A.J.P. White, D.J. Williams, Polyhedron 17 (1998) 255. [2] M. Petricˇ, M. Pavlicˇ, F. Pohleven, P. Sˇegedin, B. Kozlevcˇar, S. Polanc, B. Sˇtefane, R. Lenarsˇicˇ, Internat. Res. Group Wood Preserv., IRG/WP 99-30198, 1999. [3] B. Kozlevcˇar, N. Lah, I. Leban, I. Turel, P. Sˇegedin, M. Petricˇ, F. Pohleven, A.J.P. White, D.J. Williams, G. Giester, Croat. Chem. Acta 72 (1999) 427. [4] B. Kozlevcˇar, B. Musˇicˇ, N. Lah, I. Leban, P. Sˇegedin, Acta Chim. Slov. 52 (2005) 40. [5] B. Kozlevcˇar, M. Humar, P. Strauch, I. Leban, Z. Naturforsch. 60b (2005) 1273. [6] B. Kozlevcˇar, D. Odlazek, A. Golobicˇ, A. Pevec, P. Strauch, P. Sˇegedin, Polyhedron 25 (2006) 1161. [7] M.A. Halcrow, J. Chem. Soc., Dalton Trans. (2003) 4375. [8] L.R. Falvello, J. Chem. Soc., Dalton Trans. (1997) 4463. [9] K. Prout, A. Edwards, V. Mtetwa, J. Murray, J.F. Saunders, F.J.C. Rossotti, Inorg. Chem. 36 (1997) 2820. [10] M.A. Halcrow, C.A. Kilner, J. Wolowska, E.J.L. McInnes, A.J. Bridgeman, New J. Chem. 28 (2004) 228. [11] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1997) 307. [12] A. Altomare, M.C. Burla, M. Camalli, G. Cascarano, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst. 32 (1999) 115. [13] S.R. Hall, D.J. du Boulay, R. Olthof-Hazekamp (Eds.), XTAL3.6 System, University of Western Australia, 1999. [14] A.L. Spek, PLATON, University of Utrecht, The Netherlands, 2003. [15] L.J. Farrugia, J. Appl. Cryst. 30 (1997) 565. [16] G. Wingefeld, R. Hoppe, Z. Anorg. Allg. Chem. 516 (1984) 223. [17] J.M. Holland, X. Liu, J.P. Zhao, F.E. Mabbs, C.A. Kilner, M. Thornton-Pett, M.A. Halcrow, J. Chem. Soc., Dalton Trans. (2000) 3316. [18] Z. Mazej, I. Arcˇon, P. Benkicˇ, A. Kodre, A. Tressaud, Chem. Eur. J. 10 (2004) 5052. [19] N.K. Solanki, E.J.L. McInnes, F.E. Mabbs, S. Radojevic´, M. McPartlin, N. Feeder, J.E. Davies, M.A. Halcrow, Angew. Chem. Int. Ed. 37 (1998) 2221. [20] I. Persson, P. Persson, M. Sandstro¨m, A.-S. Ullstro¨m, J. Chem. Soc., Dalton Trans. (2002) 1256.