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Structural and thermal study of asymmetric α-dioxime complexes of Co(III) with Cl and methyl-pyridines
Polyhedron 29 (2010) 2185–2189
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Structural and thermal study of asymmetric a-dioxime complexes of Co(III) with Cl and methyl-pyridines Imre Miklós Szilágyi a,*, Andrea Deák b, Csaba Várhelyi Jr. c, János Madarász d, György Pokol d, Ágnes Gömöry b, Csaba Várhelyi e a Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry, H-1111 Budapest, Szt. Gellért tér 4, Hungary b Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri út 59-67, Hungary c Chemical Faculty of the Babesß-Bolyai University, 400028-Cluj-Napoca, Arany János Str. 11, Romania d Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Szt. Gellért tér 4, Hungary e Transylvanian Museum Society, 400009-Cluj-Napoca, Napoca Str. 2, Romania
a r t i c l e
i n f o
Article history: Received 4 January 2010 Accepted 13 April 2010 Available online 27 April 2010 Keywords: Methyl-ethyl-dioxime Co(III) complex TG/DTA-MS XRD FTIR ESI-MS
a b s t r a c t Two new cobalt(III)-chelates, trans-bis(methyl-ethyl-dioximato)-chloro-b-picoline-cobalt (III) (1), and trans-bis(methyl-ethyl-dioximato)-chloro-3,4-lutidine-cobalt (III) (2) were obtained by oxidizing a mixture of CoCl2, methyl-ethyl-dioxime and amines: b-picoline (3-methyl-pyridine) for (1) and 3,4-lutidine (3,4-dimethyl-pyridine) for (2). The crystal structure of (1) was determined by single crystal XRD (monoclinic, space group P21/c (No. 14) with a = 8.391(3) Å, b = 14.421(5) Å, c = 18.383(8) Å, b = 114.57(2)°, R = 0.0499), while both (1) and (2) were studied by middle and far FTIR spectroscopy, electrospray ionization (ESI) MS, powder XRD and thermal analysis (TG/DTA-MS). Melting of the related complexes 1 and 2 at 219 and 184 °C, respectively, results in an immediate chemical degradation of their whole structure and tarring of ligands. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Transition metal dioxime complexes have been extensively studied for various purposes in organic, analytical, inorganic, bio, medical and industrial chemistry [1–3]. As a new potential application, recently, bis(dimethyl-dioximato)-chloro-pyridine-cobalt (III) type complexes, very similar to the compounds prepared in our present study, have been used successfully for visible light driven hydrogen production from water [4]. Hence, there has been continuous effort to synthesize newer members of these compounds to optimize their composition and structure for to the various applications. A lot of octahedral Co(III) a-dioxime (DioxH)2 derivatives [Co(DioxH)2X2] , [Co(DioxH)2XL]0, [Co(DioxH)2L2]+, (X = Cl , Br , I , NO2 , NCS ; L = H2O, NH3, amines, phosphines) can be prepared if Co(II)-salts are reacted with symmetric (R1 = R2) or asymmetric (R1 – R2) a-dioximes – (DioxH)2: R1AC(@NOH)–C(@NOH)–R2 – in the presence of oxidizing agents (O2, O3, H2O2, etc.) [5–7]. If asymmetric dioximes are used in the synthesis, the obtained Co(III) complexes can have cis and trans isomers [8].
* Corresponding author. Tel.: +36 1 463 4047; fax: +36 1 463 3408. E-mail address: [email protected] (I.M. Szilágyi). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.04.014
The increased interest about various cobaloxime compounds is showed by that single crystal XRD data of more than 700 such complexes have been already published. However, the vast majority of the studies used only symmetric dioximes. Asymmetric dioximes were used only in nine papers out of the 700. It is interesting that in these cases practically only the trans isomers of Co(III) complexes were obtained [9–16], while to the best of our knowledge there is only one paper, which describes the formation of also the cis-isomer besides the trans one [8]. As only a small number of Co(III) complexes with asymmetric dioximes had been prepared previously, our aim was to prepare new members of such compounds. Due to the new potential application of using bis(dimethyl-dioximato)-chloro-pyridine-cobalt (III) type complexes in visible light driven hydrogen production from water [4], we intended to obtain similar compounds by using asymmetric dioximes, as well as halogen and new amine ligands, and characterize the obtained products thoroughly. The fact, that there were no halogen containing complexes in any of the nine studies, which described cis or trans isomers of bis(dioximato) Co(III) complexes, encouraged us further to prepare these new compounds. From the more than 700 bis(dioximato) Co(III) complexes already published, the most similar ones compared to the complexes of our interest with halogenids (where L = various ligands) were:
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[Co(DioxH)2ClL] [17–33], [Co(DioxH)2BrL] [19,20], [Co(DioxH)2IL] [34–37], [Co(DioxH)2L2]F [38–44,15,45], [Co(DioxH)2L2]Cl [46– 48,21,49–56], [Co(DioxH)2L2]Br [57,58]. Besides XRD characterization, only a few number of these complexes were studied by other methods, i.e. by IR [6,34,44], UV–Vis [20,25,39], NMR [20,24,25, 28,39,21], elemental analysis [24,39,44], mass spectroscopy [39], cyclic voltammetry [25], however, the thermal stability of these compounds has not been investigated at all. Hence, in this paper the synthesis of two new asymmetric trans[Co(methyl-ethyl-DioxH)2Cl(amine)] complexes is described, where b-picoline (3-methyl-pyridine) and 3,4-lutidine (3,4-dimethyl-pyridine) have been used as amines. The composition and structure of the complexes have been characterized by chemical analysis, middle and far FTIR, single crystal and powder XRD, electrospray ionization (ESI) MS, and thermal analysis (TG/DTAMS).
2. Experimental 2.1. Synthesis of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)] (2) About 0.04 mol methyl-ethyl-dioxime (Me-Et-DioxH2) was dissolved in 150 ml 80% ethanol together with 0.02 mol CoCl2, and the mixture was oxidized by air bubbling (1 h) at room temperature. The dark brown solution was treated with 10 g ammonium-acetate (buffer solution, pH 4.5–5) and 0.02 mol amine (either b-picoline – 3-methyl-pyridine – or 3,4-lutidine – 3,4-dimethyl-pyridine). The air bubbling was continued for another ½ h. The brown solution with some crystalline product was filtered off after 2–3 days standing, washed with ice cold water and dried it on air. Trans-bis (methyl-ethyl-dioximato)-chloro-b-picoline-cobalt (III), hereinafter trans[Co(Me-Et-DioxH) 2 Cl(b-picoline)] (1), and trans-bis (methyl-ethyl-dioximato)-chloro-3,4-lutidine-cobalt (III), hereinafter trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)], (2) were obtained as products. Yield: 50–60%.
2.2. Chemical analysis 1 and 2 were characterized by C, H, N-analysis and their Co content was determined volumetrically with 0.01 m EDTA using murexide indicator (Table 1).
Table 1 Composition of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(Me-EtDioxH)2Cl(3,4-lutidine)] (2) No.
Compound and mass formula
Mol. mass Calc.
Analysis (%) Calc.
Found
1
trans[Co(Me-EtDioxH)2Cl(b-picoline)] CoC16H25N5O4Cl
445.79
C: 43.11 H: 5.65 N: 15.71 Co: 13.2 C: 44.41 H: 5.92 N: 15.23 Co: 12.8
42.93
2
trans[Co(Me-EtDioxH)2Cl(3,4lutidine)] CoC17H27N5O4Cl
459.82
Microscopic aspect quadratic brown plates
2.3. X-ray crystallography Crystal data for trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1): C16H25Cl1Co1N5O4, Mr = 445.79, monoclinic, space group P21/c (No. 14) with a = 8.391(3) Å, b = 14.421(5) Å, c = 18.383(8) Å, b = 114.57(2)°, V = 2023.1(2) Å3, Z = 4, qcalc = 1.464 Mg/m3, F(0 0 0) = 928, k = 0.71073 Å, T = 295(2) K, l(Mo Ka) = 1.011 mm 1, crystal size 0.19 0.30 0.34 mm. Intensity data of 14 053 reflections were measured on a Rigaku R-AXIS RAPID image plate diffractometer of which 2904 were independent (Rint = 0.070). Several scans in the u and x direction were made to increase the number of redundant reflections, which were averaged over the refinement cycles. The structures were solved by direct method (SIR92) [59] and refined by full-matrix least-squares (SHELXL-97) [60]. All calculations were carried out using the WINGX package of crystallographic programs [61]. All non-hydrogen atoms were refined anisotropically in F2 mode. The C3 and C4 atoms were modelled as disordered over two orientations, giving occupancies of 0.45(1) and 0.55(1), respectively. Hydrogen atomic positions were generated from assumed geometries. The riding and rotating model was applied for the methyl hydrogen atoms. The C5 and C10 methyl hydrogen atoms were modelled as disordered over six sites. The final R values were R1 = 0.0499 (I > 2r(I)) and wR2 = 0.1196 (all data); the goodness of fit was S = 1.22; min/max residual electron density – 0.24/0.29 e Å 3. 2.4. Powder X-ray diffraction (XRD) Powder XRD patterns of 1 and its decomposition product at 800 °C in air, as well as of 2 were recorded by a PANalytical X’pert Pro MPD X-ray diffractometer equipped with an X’Celerator detector using Cu Ka radiation and Ni filter. 2.5. FTIR spectroscopy The FTIR spectra of 1 and 2 were obtained from KBr and polyethylene pellets in the mid-IR (4000–450 cm 1) and far-IR (650– 150 cm 1) range, respectively. The measurements were performed at room temperature on a Perkin Elmer System 2000 FTIR spectrometer operating with MCT detector in the mid-IR (16 scans) and DTGS detector in the far-IR (64 scans) range. The resolution was 4 cm 1. 2.6. Electrospray ionization (ESI) MS Mass spectrometric (MS) measurements of 1 and 2 were carried out by a PE Sciex API 2000 triple quadruple mass spectrometer using electrospray ionization (ESI) in the 400–1200 m/z region. By ESI-MS, ionization takes place at milder conditions as compared with classical mass spectrometry, therefore it is more sensible for the detection of single and associated dimer molecule ions and their fragment ions. 2.7. Thermal analysis
5.52 15.6 13.3 44.42 5.91 15.08 12.7
rhombohedral brown prisms
The thermal decomposition of the complexes 1 and 2 was studied by a TG/DTA-MS apparatus, which consisted of an STD 2960 Simultaneous DTA/TGA (TA Instruments Inc.) thermal analyzer and a Thermostar GSD 200 (Balzers Instruments) quadrupole mass spectrometer. On-line coupling between the two parts was provided through a heated (T = 200 °C) 1 m 100% methyl deactivated fused silica capillary tube with inner diameter of 0.15 mm. A mass range between m/z = 1–200 was monitored by scan mode. During the measurements an open platinum crucible, a heating rate of 10 °C min 1, sample sizes of 5–6 mg and flowing air (130 ml min 1) were used.
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3. Results and discussion 3.1. Description of the crystal structure The molecular structure trans[Co(Me-Et-DioxH)2Cl(b-picoline)], (1) is shown in Fig. 1. The X-ray diffraction analysis established that the Co(III) ion showed distorted octahedral coordination with the N atoms of the two Me-Et-dioximato ligands in the equatorial sites in trans position, and the N atom of b-picoline and the Cl atom in the axial sites. The axial Cl(1)–Co(1)–N(5) bond angle is 178.3(2)°. The monodeprotonated methyl-ethyl-dioximato (MeEt-DioxH) ligands are bound to the cobalt(III) center in a bidentate chelating fashion leading to five-membered CoC2N2 chelate rings. In these CoC2N2 chelate rings, the N–Co–N bite angles are 81.2(2)° and 81.0(2)°, respectively. Moreover, between equatorially disposed Me-Et-DioxH ligands six-membered rings are also formed as a result of intramolecular O(2)–H(2)O(3) [O(2)O(3): 2.503(6) Å, H(2)O(3): 1.74 Å, O(2)–H(2)O(3): 155.0°] and O(4)–H(4)O(1) [O(4)O(1): 2.502(6) Å, H(4)O(1): 1.72 Å, O(2)–H(2)O(3): 159.0°] hydrogen-bonds. The lengths of the equatorial Co–N bond are nearly equal in both Me-Et-DioxH ligands. The axial Co–N bond length of 1.953(4) Å is slightly longer than the average value of the equatorial Co–N bond length of 1.891(4) Å.
3.2. Comparison of powder XRD patterns of the single crystal and the bulk of 1
Fig. 2. (a) Simulated powder XRD pattern of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) calculated from its as-obtained single crystal XRD data; (b) Measured powder XRD pattern of the bulk of as-prepared trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) sample
3.3. FTIR spectroscopy
Fig. 2 reveals that the simulated powder XRD pattern of 1, which was calculated from the as-obtained single crystal XRD data, and the measured powder XRD pattern of the bulk of 1 were practically the same. This showed that the whole bulk of 1 was made up only by the trans isomer. This corroborates with previous data [8–16], which showed that in vast majority only the trans isomer of cobalt dioxime complexes precipitated as crystals. It is possible that the cis isomer was present in the solution, but as it could not be obtained in pure form, we have not investigated it further.
Though no single crystal XRD data could be collected for trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)] (2), FTIR study supported its expected structure. The FTIR spectra of 1 and 2 (Fig. 3) were very similar, especially the bands in their H-bonding system between 1700 and 4000 cm 1. From this it can be deduced that the coordination structure of 1 and 2 were also similar, i.e. 2 had also a structure with trans coordination. The minor differences between the two spectra were the result of the difference in the amines of the two complexes. The characteristic vibrations of oxime and the N-heterocyclic ligands appeared in the middle FTIR spectra of 1 and 2 [62,63]. Various Co–N valence and deformation vibrations could be observed
Fig. 1. Molecular structure of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1). Only the major component (C3A–C4A) of the disordered ethyl group is shown.
Fig. 3. FTIR spectra of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(MeEt-DioxH)2Cl(3,4-lutidine)] (2).
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Table 2 Selected FTIR data of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(Me-EtDioxH)2Cl(3,4-lutidine)] (2) with relative intensities of the vibrations. Vibration
trans[Co(Me-Et-DioxH)2Cl(bpicoline)] (1) (cm 1)
trans[Co(Me-Et-DioxH)2Cl(3,4lutidine)] (2) (cm 1)
mC–H
3160 m 3110 m 3040 m 1720–1760 w 1560 vs 1440 s 1370 m 1240 vs 1094 vs 985 m
3150 m 3080 m 2914 m 1710–1750 w 1555 vs 1445 s 1380 m 1230 vs 1110 vs 990 m
770 vs 513 vs, 464 s 450 m 400 s 169 m 120w
760 513 450 390 183 115
dO–HO
mC=N dasCH3 dsCH3
mN–O dasCH3,
cC–CH3 cC–H mCo–Nox sPy mCo–Cl dN–Co–N dN–Co–Cl
vs vs, 464 s m s m m
vs = very strong, s = strong, m = medium, w = weak.
in the far FTIR-region (Table 2) [62]. The dO–HO showed intramolecular hydrogen bond [64]. The mC=N was shifted toward lower frequency values, as compared with those of the free dioxime (1620– 1640 cm 1) [65]. The mN–O vibration of the free dioxime (960– 1000 cm 1) was split and shifted in the case of 1 and 2 (1240, 1100 cm 1) [66]. The mCo–N, mCo–X, dN–Co–X, etc. appeared as sharp, well defined frequency values [67]. The FTIR data prove the strong covalent character of the bonds (Co–N, Co–Cl) [68].
3.4. Electrospray ionization (ESI) MS With ESI-MS, the molecule ions of 1 (446+) and 2 (460+) were successfully detected, which also supported their expected structure. In the MS spectra, addition products (i.e. dimers) of 1 and 2 were also observed, which is characteristic to ESI-MS.
Fig. 4. Comparison of simultaneous TG/DTA curves of trans[Co(Me-Et-DioxH)2Cl(bpicoline)] (1) and trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)] (2) measured in flowing air (130 ml min 1, 10 °C min 1, open Pt crucible, sample sizes of 6.13 and 6.10 mg, respectively). The decomposition details have been shown only for (1).
3.5. Thermal stability of the complexes When (1) was annealed in air (Fig. 4), up to 200 °C no significant process took place, only adsorbed water was released in a slightly endothermic reaction (75 °C). At 203 °C the sample melted, but at the same time it started to chemically degrade and tarring also. No separate release-steps of individual amine ligands of 1 could be observed, and due to this the evolved gas analytical curves are not presented. Although, the ions corresponding to both b-picoline (93+) and Me-Et-DioxH (129+) were already identified right after the melting. Major fragments of the ligands were also observed (112+, 111+, 110+, 107+, 106+, 104+, 98+, 84+, 78+), and below 78+ several smaller fragments were detected. An analysis of the fragments suggested that in the melt the degradation products of 1 could have reacted with each other. Due to this such fragments were also observed that were hard to derive directly as coming from the ligands of 1. The released organic decomposition and degradation products, probably with exception of the amine, were burnt very soon and thus later only the evolution of H2O and CO2 was detected. Due to the melting, at first an endothermic DTA peak was observable (219 °C), but as the evolved decomposition and degradation products were burnt in air or reacted with the oxygen content of the samples, the DTA curve changed into exothermic (230 °C). After this the complete degradation of 1 involved probably several overlapping reactions. Based on powder XRD patterns, the final
product of the thermal decomposition of 1 was Co3O4 (ICDD 421467). Complex 2 started to melt and decompose at 20 °C lower temperature (184 °C), than 1. However, since the composition of 2 did not differ a great deal from that of 1, its thermal decomposition later was also very similar (Fig. 4). Similarly as by 1, in case of 2 the ions corresponding to both 3,4-lutidine (107+) and Me-Et-DioxH (129+) were already identified right after the melting.
4. Conclusion We managed to achieve our goal as we could prepare new asymmetric cobalt dioxime complexes with trans coordination structure by using asymmetric dioximes as starting materials. In this study, novel trans-bis[Co(Me-Et-DioxH)2Cl(b-picoline)]0 (1), and trans-bis[Co(Me-Et-DioxH)2Cl(3,4-lutidine)]0 (2) nonelectrolytic complex compounds were obtained by oxidizing a mixture of CoCl2, methyl-ethyl-dioxime and amines – b-picoline (3methyl-pyridine) for 1 and 3,4-lutidine (3,4-dimethyl-pyridine) for 2. The molecular structure of 1 was determined by single crystal XRD, and it was revealed that it had a structure with trans coordination. Powder XRD patterns showed that the whole bulk of one consisted of 1 with trans coordination. Though single crystal XRD
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data could not be collected for 2, FTIR study confirmed that it had also trans coordination structure, similar to 1. With electrospray ionization (ESI) MS, the molecule ions of 1 and 2 were successfully detected, which also supported their expected structure. ESI-MS revealed that various addition products of 1 and 2 formed during ionization. TG/DTA-MS study showed that 1 and 2 were stable to 200–220 °C, where they melted and simultaneously started to decompose and chemically degrade. The thermal decomposition and oxidation of 1 and 2 involved several steps, and various mass fragments of the released gases were detected by MS. 5. Supplementary data CCDC 724310 contains the supplementary crystallographic data for trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. Acknowledgement Cs. Várhelyi Jr. acknowledges a scholarship from ‘‘Domus Hungarica” Foundation, Hungary. A. Kovács (Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Hungary) is acknowledged for recording the FTIR spectra and helpful discussions. References [1] S.T. Mailnovskii, O.A. Bologa, É.B. Coropceanu, M. Gdaniec, N.V. Gerbelu, J. Struct. Chem. 48 (2007) 690. [2] H. Kantekin, A. Bakaray, Z. Biyiklioglu, M.B. Kilicaslan, Trans. Met. Chem. 33 (2008) 161. [3] M. Kurtoglu, F. Purtas, S. Toroglu, Trans. Met. Chem. 33 (2008) 705. [4] D. Pingwu, J. Schneider, G. Luo, W.W. Brennessel, R. Eisenberg, Inorg. Chem. 48 (2009) 4952. [5] H.J. Houben, T. Weyl, Methoden der organischen Chemie, Band VII/1, 455, 475, Band X/4, Georg Thieme Verlag, Stuttgart, 1968, p. 7. [6] B.D. Gupta, K. Quamungo, R. Yamuna, A. Pandy, T.U. Ashutosk, V. Vijaikanth, V. Singh, T. Barday, V. Cordes, J. Organomet. Chem. 608 (2000) 106. [7] A. Adkhis, O. Benali-Boutich, M.A. Khan, G. Bouet, Synth. React. Inorg. Met.-Org. Chem. 30 (2000) 1849. [8] R. Dreos, P. Siega, S. Scagliola, L. Randaccio, G. Nardin, C. Tavagnacco, M. Bevilacqua, Eur. J. Inorg. Chem. (2005) 3936. [9] M.M. Botoshanskii, Yu.A. Simonov, O.A. Bologa, Zh.Yu. Vaisbein, Koord. Khim. (Russ.) (Coord. Chem.) 8 (1982) 1527. [10] M.E. Rusanovskii, I.D. Samus, V.E. Zavodnik, N.M. Samus, G.I. Shpakov, N.I. Sorokina, Koord. Khim. (Russ.) (Coord. Chem.) 12 (1986) 1703. [11] S. Ozturk, M. Akkurt, M. Macit, S. Isik, H.K. Fun, Molecules 10 (2005) 767. [12] Y. Yanase, H. Yoshimura, S. Kinoshita, T. Yamaguchi, H. Wakita, Acta Crystallogr. C 46 (1990) 36. [13] A.S. Abusamleh, P.J. Chmielewski, P.R. Warburton, L. Morales, N.A. Stephenson, D.H. Busch, J. Coord. Chem. 23 (1991) 91. [14] A. Uchida, Y. Ohashi, Y. Sasada, Acta Crystallogr. C 41 (1985) 25. [15] S.T. Malinovskii, O.A. Bologa, E.B. Koropceanu, R. Luboradzki, N.V. Gerbeleu, Koord. Khim. (Russ.) (Coord. Chem.) 30 (2004) 363. [16] I.D. Samus, M.E. Rusanovskii, N.M. Samus, Eur. Cryst. Meet. 9 (1985) 246. [17] A.T.H. Lenstra, J.F.J. van Loock, S.K. Tyrlik, H. Stapowska, Bull. Soc. Chim. Belg. 91 (1982) 917. [18] A.T.H. Lenstra, H.J. Geise, S.K. Tyrlik, Acta Crystallogr. C 40 (1984) 749. [19] M. Font-Altaba, X. Solans, M. Aguilo, C. Lopez, S. Alvarez, Eur. Cryst. Meet. 9 (1985) 199. [20] C. Lopez, S. Alvarez, X. Solans, M. Font-Altaba, Inorg. Chem. 25 (1986) 2962. [21] S.K. Tyrlik, A.T.H. Lenstra, J.F.J. Van Loock, H.J. Geise, R.A. Dommisse, Acta Crystallogr. C 42 (1986) 553. [22] S. Geremia, R. Dreos, L. Randaccio, G. Tauzher, L. Antolini, Inorg. Chim. Acta 216 (1994) 125.
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Structural and thermal study of asymmetric a-dioxime complexes of Co(III) with Cl and methyl-pyridines Imre Miklós Szilágyi a,*, Andrea Deák b, Csaba Várhelyi Jr. c, János Madarász d, György Pokol d, Ágnes Gömöry b, Csaba Várhelyi e a Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Department of Inorganic and Analytical Chemistry, H-1111 Budapest, Szt. Gellért tér 4, Hungary b Institute of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri út 59-67, Hungary c Chemical Faculty of the Babesß-Bolyai University, 400028-Cluj-Napoca, Arany János Str. 11, Romania d Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Szt. Gellért tér 4, Hungary e Transylvanian Museum Society, 400009-Cluj-Napoca, Napoca Str. 2, Romania
a r t i c l e
i n f o
Article history: Received 4 January 2010 Accepted 13 April 2010 Available online 27 April 2010 Keywords: Methyl-ethyl-dioxime Co(III) complex TG/DTA-MS XRD FTIR ESI-MS
a b s t r a c t Two new cobalt(III)-chelates, trans-bis(methyl-ethyl-dioximato)-chloro-b-picoline-cobalt (III) (1), and trans-bis(methyl-ethyl-dioximato)-chloro-3,4-lutidine-cobalt (III) (2) were obtained by oxidizing a mixture of CoCl2, methyl-ethyl-dioxime and amines: b-picoline (3-methyl-pyridine) for (1) and 3,4-lutidine (3,4-dimethyl-pyridine) for (2). The crystal structure of (1) was determined by single crystal XRD (monoclinic, space group P21/c (No. 14) with a = 8.391(3) Å, b = 14.421(5) Å, c = 18.383(8) Å, b = 114.57(2)°, R = 0.0499), while both (1) and (2) were studied by middle and far FTIR spectroscopy, electrospray ionization (ESI) MS, powder XRD and thermal analysis (TG/DTA-MS). Melting of the related complexes 1 and 2 at 219 and 184 °C, respectively, results in an immediate chemical degradation of their whole structure and tarring of ligands. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Transition metal dioxime complexes have been extensively studied for various purposes in organic, analytical, inorganic, bio, medical and industrial chemistry [1–3]. As a new potential application, recently, bis(dimethyl-dioximato)-chloro-pyridine-cobalt (III) type complexes, very similar to the compounds prepared in our present study, have been used successfully for visible light driven hydrogen production from water [4]. Hence, there has been continuous effort to synthesize newer members of these compounds to optimize their composition and structure for to the various applications. A lot of octahedral Co(III) a-dioxime (DioxH)2 derivatives [Co(DioxH)2X2] , [Co(DioxH)2XL]0, [Co(DioxH)2L2]+, (X = Cl , Br , I , NO2 , NCS ; L = H2O, NH3, amines, phosphines) can be prepared if Co(II)-salts are reacted with symmetric (R1 = R2) or asymmetric (R1 – R2) a-dioximes – (DioxH)2: R1AC(@NOH)–C(@NOH)–R2 – in the presence of oxidizing agents (O2, O3, H2O2, etc.) [5–7]. If asymmetric dioximes are used in the synthesis, the obtained Co(III) complexes can have cis and trans isomers [8].
* Corresponding author. Tel.: +36 1 463 4047; fax: +36 1 463 3408. E-mail address: [email protected] (I.M. Szilágyi). 0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2010.04.014
The increased interest about various cobaloxime compounds is showed by that single crystal XRD data of more than 700 such complexes have been already published. However, the vast majority of the studies used only symmetric dioximes. Asymmetric dioximes were used only in nine papers out of the 700. It is interesting that in these cases practically only the trans isomers of Co(III) complexes were obtained [9–16], while to the best of our knowledge there is only one paper, which describes the formation of also the cis-isomer besides the trans one [8]. As only a small number of Co(III) complexes with asymmetric dioximes had been prepared previously, our aim was to prepare new members of such compounds. Due to the new potential application of using bis(dimethyl-dioximato)-chloro-pyridine-cobalt (III) type complexes in visible light driven hydrogen production from water [4], we intended to obtain similar compounds by using asymmetric dioximes, as well as halogen and new amine ligands, and characterize the obtained products thoroughly. The fact, that there were no halogen containing complexes in any of the nine studies, which described cis or trans isomers of bis(dioximato) Co(III) complexes, encouraged us further to prepare these new compounds. From the more than 700 bis(dioximato) Co(III) complexes already published, the most similar ones compared to the complexes of our interest with halogenids (where L = various ligands) were:
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[Co(DioxH)2ClL] [17–33], [Co(DioxH)2BrL] [19,20], [Co(DioxH)2IL] [34–37], [Co(DioxH)2L2]F [38–44,15,45], [Co(DioxH)2L2]Cl [46– 48,21,49–56], [Co(DioxH)2L2]Br [57,58]. Besides XRD characterization, only a few number of these complexes were studied by other methods, i.e. by IR [6,34,44], UV–Vis [20,25,39], NMR [20,24,25, 28,39,21], elemental analysis [24,39,44], mass spectroscopy [39], cyclic voltammetry [25], however, the thermal stability of these compounds has not been investigated at all. Hence, in this paper the synthesis of two new asymmetric trans[Co(methyl-ethyl-DioxH)2Cl(amine)] complexes is described, where b-picoline (3-methyl-pyridine) and 3,4-lutidine (3,4-dimethyl-pyridine) have been used as amines. The composition and structure of the complexes have been characterized by chemical analysis, middle and far FTIR, single crystal and powder XRD, electrospray ionization (ESI) MS, and thermal analysis (TG/DTAMS).
2. Experimental 2.1. Synthesis of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)] (2) About 0.04 mol methyl-ethyl-dioxime (Me-Et-DioxH2) was dissolved in 150 ml 80% ethanol together with 0.02 mol CoCl2, and the mixture was oxidized by air bubbling (1 h) at room temperature. The dark brown solution was treated with 10 g ammonium-acetate (buffer solution, pH 4.5–5) and 0.02 mol amine (either b-picoline – 3-methyl-pyridine – or 3,4-lutidine – 3,4-dimethyl-pyridine). The air bubbling was continued for another ½ h. The brown solution with some crystalline product was filtered off after 2–3 days standing, washed with ice cold water and dried it on air. Trans-bis (methyl-ethyl-dioximato)-chloro-b-picoline-cobalt (III), hereinafter trans[Co(Me-Et-DioxH) 2 Cl(b-picoline)] (1), and trans-bis (methyl-ethyl-dioximato)-chloro-3,4-lutidine-cobalt (III), hereinafter trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)], (2) were obtained as products. Yield: 50–60%.
2.2. Chemical analysis 1 and 2 were characterized by C, H, N-analysis and their Co content was determined volumetrically with 0.01 m EDTA using murexide indicator (Table 1).
Table 1 Composition of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(Me-EtDioxH)2Cl(3,4-lutidine)] (2) No.
Compound and mass formula
Mol. mass Calc.
Analysis (%) Calc.
Found
1
trans[Co(Me-EtDioxH)2Cl(b-picoline)] CoC16H25N5O4Cl
445.79
C: 43.11 H: 5.65 N: 15.71 Co: 13.2 C: 44.41 H: 5.92 N: 15.23 Co: 12.8
42.93
2
trans[Co(Me-EtDioxH)2Cl(3,4lutidine)] CoC17H27N5O4Cl
459.82
Microscopic aspect quadratic brown plates
2.3. X-ray crystallography Crystal data for trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1): C16H25Cl1Co1N5O4, Mr = 445.79, monoclinic, space group P21/c (No. 14) with a = 8.391(3) Å, b = 14.421(5) Å, c = 18.383(8) Å, b = 114.57(2)°, V = 2023.1(2) Å3, Z = 4, qcalc = 1.464 Mg/m3, F(0 0 0) = 928, k = 0.71073 Å, T = 295(2) K, l(Mo Ka) = 1.011 mm 1, crystal size 0.19 0.30 0.34 mm. Intensity data of 14 053 reflections were measured on a Rigaku R-AXIS RAPID image plate diffractometer of which 2904 were independent (Rint = 0.070). Several scans in the u and x direction were made to increase the number of redundant reflections, which were averaged over the refinement cycles. The structures were solved by direct method (SIR92) [59] and refined by full-matrix least-squares (SHELXL-97) [60]. All calculations were carried out using the WINGX package of crystallographic programs [61]. All non-hydrogen atoms were refined anisotropically in F2 mode. The C3 and C4 atoms were modelled as disordered over two orientations, giving occupancies of 0.45(1) and 0.55(1), respectively. Hydrogen atomic positions were generated from assumed geometries. The riding and rotating model was applied for the methyl hydrogen atoms. The C5 and C10 methyl hydrogen atoms were modelled as disordered over six sites. The final R values were R1 = 0.0499 (I > 2r(I)) and wR2 = 0.1196 (all data); the goodness of fit was S = 1.22; min/max residual electron density – 0.24/0.29 e Å 3. 2.4. Powder X-ray diffraction (XRD) Powder XRD patterns of 1 and its decomposition product at 800 °C in air, as well as of 2 were recorded by a PANalytical X’pert Pro MPD X-ray diffractometer equipped with an X’Celerator detector using Cu Ka radiation and Ni filter. 2.5. FTIR spectroscopy The FTIR spectra of 1 and 2 were obtained from KBr and polyethylene pellets in the mid-IR (4000–450 cm 1) and far-IR (650– 150 cm 1) range, respectively. The measurements were performed at room temperature on a Perkin Elmer System 2000 FTIR spectrometer operating with MCT detector in the mid-IR (16 scans) and DTGS detector in the far-IR (64 scans) range. The resolution was 4 cm 1. 2.6. Electrospray ionization (ESI) MS Mass spectrometric (MS) measurements of 1 and 2 were carried out by a PE Sciex API 2000 triple quadruple mass spectrometer using electrospray ionization (ESI) in the 400–1200 m/z region. By ESI-MS, ionization takes place at milder conditions as compared with classical mass spectrometry, therefore it is more sensible for the detection of single and associated dimer molecule ions and their fragment ions. 2.7. Thermal analysis
5.52 15.6 13.3 44.42 5.91 15.08 12.7
rhombohedral brown prisms
The thermal decomposition of the complexes 1 and 2 was studied by a TG/DTA-MS apparatus, which consisted of an STD 2960 Simultaneous DTA/TGA (TA Instruments Inc.) thermal analyzer and a Thermostar GSD 200 (Balzers Instruments) quadrupole mass spectrometer. On-line coupling between the two parts was provided through a heated (T = 200 °C) 1 m 100% methyl deactivated fused silica capillary tube with inner diameter of 0.15 mm. A mass range between m/z = 1–200 was monitored by scan mode. During the measurements an open platinum crucible, a heating rate of 10 °C min 1, sample sizes of 5–6 mg and flowing air (130 ml min 1) were used.
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3. Results and discussion 3.1. Description of the crystal structure The molecular structure trans[Co(Me-Et-DioxH)2Cl(b-picoline)], (1) is shown in Fig. 1. The X-ray diffraction analysis established that the Co(III) ion showed distorted octahedral coordination with the N atoms of the two Me-Et-dioximato ligands in the equatorial sites in trans position, and the N atom of b-picoline and the Cl atom in the axial sites. The axial Cl(1)–Co(1)–N(5) bond angle is 178.3(2)°. The monodeprotonated methyl-ethyl-dioximato (MeEt-DioxH) ligands are bound to the cobalt(III) center in a bidentate chelating fashion leading to five-membered CoC2N2 chelate rings. In these CoC2N2 chelate rings, the N–Co–N bite angles are 81.2(2)° and 81.0(2)°, respectively. Moreover, between equatorially disposed Me-Et-DioxH ligands six-membered rings are also formed as a result of intramolecular O(2)–H(2)O(3) [O(2)O(3): 2.503(6) Å, H(2)O(3): 1.74 Å, O(2)–H(2)O(3): 155.0°] and O(4)–H(4)O(1) [O(4)O(1): 2.502(6) Å, H(4)O(1): 1.72 Å, O(2)–H(2)O(3): 159.0°] hydrogen-bonds. The lengths of the equatorial Co–N bond are nearly equal in both Me-Et-DioxH ligands. The axial Co–N bond length of 1.953(4) Å is slightly longer than the average value of the equatorial Co–N bond length of 1.891(4) Å.
3.2. Comparison of powder XRD patterns of the single crystal and the bulk of 1
Fig. 2. (a) Simulated powder XRD pattern of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) calculated from its as-obtained single crystal XRD data; (b) Measured powder XRD pattern of the bulk of as-prepared trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) sample
3.3. FTIR spectroscopy
Fig. 2 reveals that the simulated powder XRD pattern of 1, which was calculated from the as-obtained single crystal XRD data, and the measured powder XRD pattern of the bulk of 1 were practically the same. This showed that the whole bulk of 1 was made up only by the trans isomer. This corroborates with previous data [8–16], which showed that in vast majority only the trans isomer of cobalt dioxime complexes precipitated as crystals. It is possible that the cis isomer was present in the solution, but as it could not be obtained in pure form, we have not investigated it further.
Though no single crystal XRD data could be collected for trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)] (2), FTIR study supported its expected structure. The FTIR spectra of 1 and 2 (Fig. 3) were very similar, especially the bands in their H-bonding system between 1700 and 4000 cm 1. From this it can be deduced that the coordination structure of 1 and 2 were also similar, i.e. 2 had also a structure with trans coordination. The minor differences between the two spectra were the result of the difference in the amines of the two complexes. The characteristic vibrations of oxime and the N-heterocyclic ligands appeared in the middle FTIR spectra of 1 and 2 [62,63]. Various Co–N valence and deformation vibrations could be observed
Fig. 1. Molecular structure of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1). Only the major component (C3A–C4A) of the disordered ethyl group is shown.
Fig. 3. FTIR spectra of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(MeEt-DioxH)2Cl(3,4-lutidine)] (2).
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Table 2 Selected FTIR data of trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1) and trans[Co(Me-EtDioxH)2Cl(3,4-lutidine)] (2) with relative intensities of the vibrations. Vibration
trans[Co(Me-Et-DioxH)2Cl(bpicoline)] (1) (cm 1)
trans[Co(Me-Et-DioxH)2Cl(3,4lutidine)] (2) (cm 1)
mC–H
3160 m 3110 m 3040 m 1720–1760 w 1560 vs 1440 s 1370 m 1240 vs 1094 vs 985 m
3150 m 3080 m 2914 m 1710–1750 w 1555 vs 1445 s 1380 m 1230 vs 1110 vs 990 m
770 vs 513 vs, 464 s 450 m 400 s 169 m 120w
760 513 450 390 183 115
dO–HO
mC=N dasCH3 dsCH3
mN–O dasCH3,
cC–CH3 cC–H mCo–Nox sPy mCo–Cl dN–Co–N dN–Co–Cl
vs vs, 464 s m s m m
vs = very strong, s = strong, m = medium, w = weak.
in the far FTIR-region (Table 2) [62]. The dO–HO showed intramolecular hydrogen bond [64]. The mC=N was shifted toward lower frequency values, as compared with those of the free dioxime (1620– 1640 cm 1) [65]. The mN–O vibration of the free dioxime (960– 1000 cm 1) was split and shifted in the case of 1 and 2 (1240, 1100 cm 1) [66]. The mCo–N, mCo–X, dN–Co–X, etc. appeared as sharp, well defined frequency values [67]. The FTIR data prove the strong covalent character of the bonds (Co–N, Co–Cl) [68].
3.4. Electrospray ionization (ESI) MS With ESI-MS, the molecule ions of 1 (446+) and 2 (460+) were successfully detected, which also supported their expected structure. In the MS spectra, addition products (i.e. dimers) of 1 and 2 were also observed, which is characteristic to ESI-MS.
Fig. 4. Comparison of simultaneous TG/DTA curves of trans[Co(Me-Et-DioxH)2Cl(bpicoline)] (1) and trans[Co(Me-Et-DioxH)2Cl(3,4-lutidine)] (2) measured in flowing air (130 ml min 1, 10 °C min 1, open Pt crucible, sample sizes of 6.13 and 6.10 mg, respectively). The decomposition details have been shown only for (1).
3.5. Thermal stability of the complexes When (1) was annealed in air (Fig. 4), up to 200 °C no significant process took place, only adsorbed water was released in a slightly endothermic reaction (75 °C). At 203 °C the sample melted, but at the same time it started to chemically degrade and tarring also. No separate release-steps of individual amine ligands of 1 could be observed, and due to this the evolved gas analytical curves are not presented. Although, the ions corresponding to both b-picoline (93+) and Me-Et-DioxH (129+) were already identified right after the melting. Major fragments of the ligands were also observed (112+, 111+, 110+, 107+, 106+, 104+, 98+, 84+, 78+), and below 78+ several smaller fragments were detected. An analysis of the fragments suggested that in the melt the degradation products of 1 could have reacted with each other. Due to this such fragments were also observed that were hard to derive directly as coming from the ligands of 1. The released organic decomposition and degradation products, probably with exception of the amine, were burnt very soon and thus later only the evolution of H2O and CO2 was detected. Due to the melting, at first an endothermic DTA peak was observable (219 °C), but as the evolved decomposition and degradation products were burnt in air or reacted with the oxygen content of the samples, the DTA curve changed into exothermic (230 °C). After this the complete degradation of 1 involved probably several overlapping reactions. Based on powder XRD patterns, the final
product of the thermal decomposition of 1 was Co3O4 (ICDD 421467). Complex 2 started to melt and decompose at 20 °C lower temperature (184 °C), than 1. However, since the composition of 2 did not differ a great deal from that of 1, its thermal decomposition later was also very similar (Fig. 4). Similarly as by 1, in case of 2 the ions corresponding to both 3,4-lutidine (107+) and Me-Et-DioxH (129+) were already identified right after the melting.
4. Conclusion We managed to achieve our goal as we could prepare new asymmetric cobalt dioxime complexes with trans coordination structure by using asymmetric dioximes as starting materials. In this study, novel trans-bis[Co(Me-Et-DioxH)2Cl(b-picoline)]0 (1), and trans-bis[Co(Me-Et-DioxH)2Cl(3,4-lutidine)]0 (2) nonelectrolytic complex compounds were obtained by oxidizing a mixture of CoCl2, methyl-ethyl-dioxime and amines – b-picoline (3methyl-pyridine) for 1 and 3,4-lutidine (3,4-dimethyl-pyridine) for 2. The molecular structure of 1 was determined by single crystal XRD, and it was revealed that it had a structure with trans coordination. Powder XRD patterns showed that the whole bulk of one consisted of 1 with trans coordination. Though single crystal XRD
I.M. Szilágyi et al. / Polyhedron 29 (2010) 2185–2189
data could not be collected for 2, FTIR study confirmed that it had also trans coordination structure, similar to 1. With electrospray ionization (ESI) MS, the molecule ions of 1 and 2 were successfully detected, which also supported their expected structure. ESI-MS revealed that various addition products of 1 and 2 formed during ionization. TG/DTA-MS study showed that 1 and 2 were stable to 200–220 °C, where they melted and simultaneously started to decompose and chemically degrade. The thermal decomposition and oxidation of 1 and 2 involved several steps, and various mass fragments of the released gases were detected by MS. 5. Supplementary data CCDC 724310 contains the supplementary crystallographic data for trans[Co(Me-Et-DioxH)2Cl(b-picoline)] (1). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. Acknowledgement Cs. Várhelyi Jr. acknowledges a scholarship from ‘‘Domus Hungarica” Foundation, Hungary. A. Kovács (Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Hungary) is acknowledged for recording the FTIR spectra and helpful discussions. References [1] S.T. Mailnovskii, O.A. Bologa, É.B. Coropceanu, M. Gdaniec, N.V. Gerbelu, J. Struct. Chem. 48 (2007) 690. [2] H. Kantekin, A. Bakaray, Z. Biyiklioglu, M.B. Kilicaslan, Trans. Met. Chem. 33 (2008) 161. [3] M. Kurtoglu, F. Purtas, S. Toroglu, Trans. Met. Chem. 33 (2008) 705. [4] D. Pingwu, J. Schneider, G. Luo, W.W. Brennessel, R. Eisenberg, Inorg. Chem. 48 (2009) 4952. [5] H.J. Houben, T. Weyl, Methoden der organischen Chemie, Band VII/1, 455, 475, Band X/4, Georg Thieme Verlag, Stuttgart, 1968, p. 7. [6] B.D. Gupta, K. Quamungo, R. Yamuna, A. Pandy, T.U. Ashutosk, V. Vijaikanth, V. Singh, T. Barday, V. Cordes, J. Organomet. Chem. 608 (2000) 106. [7] A. Adkhis, O. Benali-Boutich, M.A. Khan, G. Bouet, Synth. React. Inorg. Met.-Org. Chem. 30 (2000) 1849. [8] R. Dreos, P. Siega, S. Scagliola, L. Randaccio, G. Nardin, C. Tavagnacco, M. Bevilacqua, Eur. J. Inorg. Chem. (2005) 3936. [9] M.M. Botoshanskii, Yu.A. Simonov, O.A. Bologa, Zh.Yu. Vaisbein, Koord. Khim. (Russ.) (Coord. Chem.) 8 (1982) 1527. [10] M.E. Rusanovskii, I.D. Samus, V.E. Zavodnik, N.M. Samus, G.I. Shpakov, N.I. Sorokina, Koord. Khim. (Russ.) (Coord. Chem.) 12 (1986) 1703. [11] S. Ozturk, M. Akkurt, M. Macit, S. Isik, H.K. Fun, Molecules 10 (2005) 767. [12] Y. Yanase, H. Yoshimura, S. Kinoshita, T. Yamaguchi, H. Wakita, Acta Crystallogr. C 46 (1990) 36. [13] A.S. Abusamleh, P.J. Chmielewski, P.R. Warburton, L. Morales, N.A. Stephenson, D.H. Busch, J. Coord. Chem. 23 (1991) 91. [14] A. Uchida, Y. Ohashi, Y. Sasada, Acta Crystallogr. C 41 (1985) 25. [15] S.T. Malinovskii, O.A. Bologa, E.B. Koropceanu, R. Luboradzki, N.V. Gerbeleu, Koord. Khim. (Russ.) (Coord. Chem.) 30 (2004) 363. [16] I.D. Samus, M.E. Rusanovskii, N.M. Samus, Eur. Cryst. Meet. 9 (1985) 246. [17] A.T.H. Lenstra, J.F.J. van Loock, S.K. Tyrlik, H. Stapowska, Bull. Soc. Chim. Belg. 91 (1982) 917. [18] A.T.H. Lenstra, H.J. Geise, S.K. Tyrlik, Acta Crystallogr. C 40 (1984) 749. [19] M. Font-Altaba, X. Solans, M. Aguilo, C. Lopez, S. Alvarez, Eur. Cryst. Meet. 9 (1985) 199. [20] C. Lopez, S. Alvarez, X. Solans, M. Font-Altaba, Inorg. Chem. 25 (1986) 2962. [21] S.K. Tyrlik, A.T.H. Lenstra, J.F.J. Van Loock, H.J. Geise, R.A. Dommisse, Acta Crystallogr. C 42 (1986) 553. [22] S. Geremia, R. Dreos, L. Randaccio, G. Tauzher, L. Antolini, Inorg. Chim. Acta 216 (1994) 125.
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