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Preparation of trimetallic alkoxide complexes exploiting the isomorphous substitution approach.
Polyhedron 21 (2002) 2317 /2322 www.elsevier.com/locate/poly
Preparation of trimetallic alkoxide complexes exploiting the isomorphous substitution approach. Synthesis, X-ray single crystal and mass-spectrometric study of NbTa(OMe)8(ReO4)2 and Nb2Ta2O2(OMe)14(ReO4)2 Pavel Shcheglov a, Gulaim A. Seisenbaeva b, Suresh Gohil b, Dmitrii V. Drobot a, Vadim G. Kessler b,* b
a Moscow State Academy of Fine Chemical Technology, Pr. Vernadskogo 86, 117571 Moscow, Russia Department of Chemistry, Swedish University of Agricultural Sciences, Box 7015, 75007 Uppsala, Sweden
Received 22 May 2002; accepted 30 July 2002
Abstract Alkoxylation of the rhenium heptoxide, Re2O7, with the bimetallic niobium-tantalum methoxide, NbTa(OMe)10, provided, dependent on the reaction conditions, either (Nb,Ta)2(OMe)8(ReO4)2 (I) or (Nb,Ta)4O2(OMe)14(ReO4)2 (II). Niobium and tantalum are distributed very uniformly between the correspondent positions in the molecular structure of I (only one symmetry independent position present for MV atoms). In the structure of II, the metal atom position connected to the perrhenate ligand has higher niobium content, while that connected to alkoxide groups and the oxoligand has higher tantalum content. Possible reasons for this difference and its influence on the decomposition behavior of I and II on vacuum sublimation are discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Trimetallic alkoxides; Molecular structure; X-ray study; Mass-spectrometric study; Rhenium methoxides; Niobium methoxide; Tantalum methoxide
1. Introduction Trimetallic alkoxide complexes have recently attracted attention of researchers as possible single-source precursors of complex materials [1]. Among the large number of compounds of this class described [2] only a few have unequivocally proved to exist and have been characterized by X-ray single crystal studies, among i them [CdMIIMIV [3], LiMo4Ta3O14(Oi 2 (O Pr)12]2 Pr)9(OC2H4OMe)3 [4] and NaPb2Ti2O(Oi Pr)10Cl [2]. The synthetic approaches to the trimetallic species developed so far were based predominantly on the interaction of a metal halide or alkoxide halide with a bimetallic alkoxide, derived from some other metal and an alkali metal [2,3].
* Corresponding author. Tel.: /46-18-67-1541/1502; fax: /46-1867-3476/3477
We have developed recently a new synthetic approach to heterometallic oxoalkoxides of rhenium based on the alkoxylation of rhenium heptoxide, Re2O7, with an alkoxide of another metal [5,6]. In the present work we report the synthesis and structural characterization of the trimetallic derivatives obtained by using the bimetallic niobium /tantalum methoxide, NbTa(OMe)10 [7], as reactant in this process. The trimetallic rhenium / niobium /tantalum alkoxide complexes are of interest as precursors for the preparation of fine powders of related alloys applying the soft chemistry approaches [8].
2. Experimental All of the preparative procedures were carried out in dry nitrogen atmosphere using a dry box. Methanol (Merck, p.a. ) was dehydrated by refluxing over magne-
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 1 8 0 - 4
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sium methoxide with subsequent distillation. Toluene (Merck, p.a. ) was distilled after refluxing with LiAlH4. IR spectra of Nujol mulls were obtained with a Perkin/Elmer FT-IR 1720 X spectrometer. Metal ratio in the samples was determined at the Arrhenius Laboratory, Stockholm University using a JEOL JSM-820 scanning microscope (SEM), supplied with a Link AN10000 energy dispersive spectrometer (EDS). Mass spectra (electron beam ionization, direct probe introduction) were recorded using JEOL JMS-SX/SX-102A mass-spectrometer. The determination of carbon and hydrogen contents was carried out by Mikrokemi AB, Uppsala, Sweden, using the conventional organic microanalysis technique. The alkoxides, Nb2(OMe)10 and Ta2(OMe)10, used in this work as starting materials for the synthesis of bimetallic complexes, were prepared by anodic oxidation of the corresponding metals in methanol and purified according to conventional techniques [9]. Rhenium(VII) oxide (99.9/%) was purchased from Aldrich. 2.1. Synthesis of (Nb,Ta)2(OMe)8(ReO4)2 (I) Solutions of Nb(OMe)5 (0.131 g, 0.529 mmol) in 3.5 ml of C6H5CH3 and Ta(OMe)5 (0.182, 0.543 mmol) in 2.5 ml of C6H5CH3 were mixed. Re2O7 (0.273 g, 0.564 mmol) was added to the obtained solution upon stirring. No color changes at the initial stage were observed. Slow dissolution of Re2O7 occurred and the solution gradually became brownish-pink. Slight brownish coloration of Re2O7 particles and the formation of a small amount of nearly colorless crystals were observed. At the end of the process, after dissolution of almost all the Re2O7 (40 min), the mixture was heated to 60 8C for 10 min. The solution was cooled to room temperature (r.t.), then to /30 8C for crystallization. After 3 days the solution was decanted and the crystalline product (light pink prismatic platelets) was dried in vacuum. Yield: 0.411 g (76%). IR, cm 1: 1138 s br, 998 m, 945 sh, 935 s (n-Re /O), 896 sh, 818 s br (n-Re /O Nb(Ta), compare [6,8]), 723 w, 618 w, 569 m, 514 s. MS, m /z (I,%), interpretation: 641(1.0), Ta2(OMe)9, 595(1.0), Ta2O(OMe)7 , 553(1.0), TaNb(OMe)9, 522(0.5), TaNb(OMe)8, 507(1.1), TaNbO(OMe)7, 476(0.5), TaNbO(OMe)6, 461(0.6), TaNbO2(OMe)5, 305(19.3), Ta(OMe)4 , 281(1.7), ReO2(OMe)2, 274(3.6), Ta(OMe)3, 266(61.4), ReO3(OMe) , 265(51.5), ReO3(OCH2), 251(2.7), ReO4, 249(2.1), Re(OMe)2, 248(1.6), Re(OCH2)(OMe) and Nb(OMe)5 , 247(1.8), Nb(OMe)4(OCH2) , 238(6.5), Re(OH)3, 237(7.5), ReO(OH)2, 236(100), ReO2(OH) , 235(32.9), ReO3, 234(59.3), ReO(OMe) , 233(17.3), ReO(OCH2) , 231(1.9), TaO(OH)2, 220(17.0), ReO(OH), 219(61.6), ReO2, 217(50.8), Nb(OMe)4, 204(4.8), Re(OH) , 203(22.0), ReO and Nb(OMe)3(OH) , 187(10.5), Re, 186(1.8),
Nb(OMe)3, 172(10.1), Nb(OMe)2(OH), NbO(OMe)2, 157(2.9), NbO(OH)(OMe) , Nb(OMe)2, 141(1.8), Nb(OMe)(OH) , NbO(OMe) , 139(2.5), NbO(OCH2) , NbO2.
171(8.4), 155(2.4), 140(1.5), 125(2.6),
2.2. Synthesis of (Nb,Ta)4O2(OMe)14(ReO4)2 (II) Solutions of Nb(OMe)5 (0.140 g, 0.565 mmol) in 3.5 ml of C6H5CH3 and Ta(OMe)5 (0.194, 0.577 mmol) in 2.5 ml of C6H5CH3 were mixed. Re2O7 (0.276 g, 0.570 mmol) was added to the obtained solution upon stirring. No color changes at the initial stage were observed. Upon slow dissolution of Re2O7, the solution became light pink. After 10 min of stirring, the mixture was heated. The solution became reddish-brown and Re2O7 changed color to black. The mixture was refluxed for 15 min. The formation of a black non-crystalline residue as a by-product was observed (EDS analysis showed the by-product to comprise of Re as the major component, and Nb and Ta in minor quantities, 5/7% atom). The solution was cooled to r.t., then to /30 8C for crystallization. After 3 days the solution was decanted and a crystalline product (thick needle shaped crystals brownish-pink in color) was dried in vacuum. Yield: 0.306 g (71% on M(OMe)5 and 35% on Re2O7). IR, cm 1: 1124 s br, 998 m w, 978 w, 931 s (n-Re /O), 893 w, 808 s (nRe /O Nb(Ta), compare [5]), 735 sh, 722 m, 569 m, 512 s. MS, m /z (I,%), interpretation: 641(2.8), Ta2(OMe)9, 595(4.2), Ta2O(OMe)7, 553(1.7), TaNb(OMe)9, 549(1.4), Ta2O2(OMe)5, 522(0.9), TaNb(OMe)8, 517(0.4), Ta2(OMe)5, 508(0.5), TaNb(OH)(OMe)7, 507(4.4), TaNbO(OMe)7, 503(0.5), Ta2(OMe)4(OH) , 476(2.0), TaNbO(OMe)6, 461(2.3), TaNbO2(OMe)5 , 446(0.5), TaNb(OH)(OMe)5, 429(0.7), TaNb(OMe)5, 419(0.9), Nb2O(OMe)7, 415(0.7), TaNb(OH)(OMe)4, 399(0.7), TaNbO2(OMe)3, 385(0.6), TaNbO2(OH)(OMe)2, 373(0.6), Nb2O2(OMe)5, 305(19.0), Ta(OMe)4, 281(5.3), ReO2(OMe)2, 275(4.6), TaO(OH)(OMe)(OCH2) , 274(1.1), Ta(OMe)3, 267(3.2), ReO2(OH)(OMe) , 266(92.1), ReO3(OMe) , 265(86.8), ReO3(OCH2), 259(2.4), TaO(OMe)2, 251(6.0), ReO4, 249(4.6), Re(OMe)2, 248(3.0), Re(OCH2)(OMe) and Nb(OMe)5, 247(3.0), Nb(OMe)4(OCH2) , 238(9.4), Re(OH)3, 237(12.4), ReO(OH)2, , 236(100), ReO2(OH), 235(51.8), ReO3, 234(96.8), ReO(OMe) , 233(27.3), ReO(OCH2) , 231(2.4), TaO(OH)2, 221(3.2), Re(OH)2, 220(26.8), ReO(OH) , 219(95.7), ReO2, 217(72.4), Nb(OMe)4, 204(8.0), Re(OH) , 203(32.3), ReO and Nb(OMe)3(OH), 187(15.3), Re , 186(2.3), Nb(OMe)3, 172(22.9), Nb(OMe)2(OH) , 171(15.1), NbO(OMe)2, 157(5.5), NbO(OH)(OMe), 155(4.1), Nb(OMe)2, 141(3.3), Nb(OMe)(OH) , 140(2.9), NbO(OMe) , 139(4.4), NbO(OCH2) , 125(5.7), NbO2.
P. Shcheglov et al. / Polyhedron 21 (2002) 2317 /2322
2.3. Alternative technique for the synthesis of (Nb,Ta)4O2(OMe)14(ReO4)2 (II) Solutions of Nb(OMe)5 (0.132 g, 0.532 mmol) in 3.5 ml of C6H5CH3 and Ta(OMe)5 (0.178, 0.531 mmol) in 2.5 ml of C6H5CH3 were mixed. Re2O7 (0.139 g, 0.288 mmol) was added to the obtained solution upon stirring. No color changes at the initial stage were observed. Upon slow dissolution of Re2O7, a brownish-violet color of the latter and precipitation of a small amount of lightly colored pink fine crystals were observed. After approximately 20 min of stirring, the solution was light pink in color. After 40 min of stirring most of the Re2O7 was dissolved and further formation of fine crystalline precipitate occurred. After 70 min the color of the solution was light orange /pink and almost all of the Re2O7 was dissolved. After 80 min the mixture was heated and the solution became brownish/pink. The mixture was refluxed for 15 min. In the course of the heating the dissolution of the initially formed crystals occurred. At the end of refluxing the solution became dark reddish-brown. The solution was cooled to r.t., then to /30 8C for crystallization. After 3 days the solution was decanted and the crystalline product (thick needle shaped crystals brownish-pink in color) was dried in vacuum. Yield: 0.258 g (64%). IR and MS data were identical to those reported above. 2.4. Crystallography The crystal data and experimental conditions are shown in Table 1. The data were collected using a SMART CCD 1K diffractometer at 22 8C. All calculations were performed on an IBM PC using the SHELXTLNT [10] program package. The absorption correction has Table 1 Crystal data and details of diffraction experiments for compounds I and II
Chemical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z m (mm 1) Independent reflections Observed reflections R1 wR2
I
II
C8H24NbO16Re2Ta 1022.53 monoclinic P 21/c 8.564(2) 12.982(3) 10.597(2) 90 97.279(4) 90 1168.7(5) 2 15.532 794 [R (int) 0.1204] 586 [I 2s (I )] 0.0677 0.1765
C14H42Nb2O24Re2Ta2 1514.60 monoclinic P 21/c 9.745(9) 15.882(14) 12.204(10) 90 100.98(2) 90 1854(3) 2 13.054 2402 [R (int) 0.0977] 998 [I 2s (I )] 0.0591 0.1134
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been introduced for both experiments using Bruker SADABS program. Both structures were solved by direct methods. The positions of metal atoms were obtained from the initial solution and refined first in isotropic and then anisotropic approximation. The occupancy factors for all the metal atom positions in the structures of compounds I and II were refined in supposition that they can be occupied with equal opportunity by both niobium and tantalum atoms; the total occupancy for each position being 1.0. The values obtained are within experimental errors in agreement with those obtained by EDS-analysis. The positions of all other non-hydrogen atoms were obtained from difference Fourier syntheses and refined first in isotropic and then anisotropic approximation. The positions of hydrogen atoms were calculated geometrically and included into the final refinement in isotropic approximation; the thermal parameters for H-atoms were taken as Uiso / 1.50Ueq(C), where Ueq(C) is the equivalent parameter for the carbon atom to which the hydrogen atom is attached.
3. Results and discussion 3.1. Reaction pathways The reaction of the alkoxylation of rhenium heptoxide, Re2O7, has recently been proved to provide a facile access to bimetallic oxoalkoxide derivatives of rhenium [5,6] and it appeared attractive to investigate its potential for the preparation of trimetallic compounds, applying a bimetallic alkoxide as reactant. We have chosen in this purpose the bimetallic methoxide of niobium and tantalum formed easily on mixing the two homometallic species in hydrocarbon media [7] and existing due to the extraordinary likeness of the coordination geometries of the two metals, which permits to consider it as a product of isomorphous substitution of one metal in the structure with the other. Carrying out of this reaction at ambient temperature was found to lead practically exclusively to compound I, independently of the ratio of the reactants: Re2 O7 NbTa(OMe)10 0 (Nb; Ta)2 (OMe)8 (ReO4 )2 (I). . . It may be supposed that the reaction pathway includes in this case, first, some partial oxidation of the methoxide ligands producing water that leads to formation of perrhenic acid, which produces compound I via the chemical modification of the bimetallic alkoxide: Re2 O7 H2 O 0 2HReO4 2HReO4 NbTa(OMe)10 0 (Nb; Ta)2 (OMe)8 (ReO4 )2 2MeOH
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In favor of this pathway speaks the quite high, but far not quantitative, yield of compound I, and the absence of the intensive coloration, indicating otherwise the formation of low-valent rhenium alkoxides in the course of the alkoxylation [5,6]. The processes occurring on heating follow evidently the mechanism proposed earlier [5,6] for the alkoxylation of rhenium heptoxide, but the elevated temperature leads to deeper condensation of the products resulting in formation of a more oxo-substituted tetranuclear core of compound II. It is to be noted also that compound I formed in the reaction mixture at low temperature can on further heating undergo an almost quantitative thermal condensation in solution with an excess of NbTa(OMe)10 and provide II with a significantly increased yield in relation to Re2O7. The condensation occurs supposedly via partial oxidation of the methoxide ligand by the perrhenate ligand present, which provides the water necessary for this process. It is important to note that the metal atom content in the product corresponds perfectly to that in the reaction media and the Nb:Ta /1:1 ratio is conserved in the trimetallic products obtained opening the prospect of the application of these precursors in the preparation of trimetallic oxide and alloy materials. 3.2. Molecular structures Unfortunately, both I and II are almost insoluble in common organic solvents at room temperature. On heating, they remain unchanged in hydrocarbons (toluene, hexane), while in methanol the crystals turn slowly dark, supposedly because of the reduction of rhenium(VII) by the alcohol, but still no transfer into solution of the metal-containing species could be observed. This precluded the application of NMR for the estimation of the possible molecular structures in solution (in statu nascendi) and those discussed below are the solid state ones obtained by the X-ray single crystal studies. The structure of I (Fig. 1) turned out to be very close to the structures of its bimetallic analogues, M2(OMe)8(ReO4)2, M/Nb, Ta [6]. Its centrosymmetric molecules are pairs of octahedra sharing a common (mOR)2 edge, the perrhenate substituents being situated in the axial positions. The oxo-bridging bond to the ˚ as that rhenium atom is as long M(1) /O(2) 2.12(2) A in the structure of the tantalum-only derivative (Ta(1) / ˚ ) and has evidently also a decreased O(5) 2.115(16) A multiplicity. The terminal alkoxide ligand situated in the trans -position to it has quite logically a significantly shortened M(1) /O(7) 1.78(3) distance (compared to ˚ in the tantalum-only analogue) (Ta(1) /O(4) 1.814(19) A with the multiplicity increased due to the pp /dp interaction. The bridging M/O distances are noticeably asymmetric and significantly longer than the terminal
Fig. 1. Molecular structure of (Nb,Ta)2(OMe)8(ReO4)2 (I).
˚ M /O bonds (M(1)/O(1) 2.13(2), M(1) /O(1)# 2.02(2) A in I compared to Ta(1) /O(1) 2.08(2), Ta(1) /O(1)# ˚ in Ta2(OMe)8(ReO4)2 [6]. The structure of I 2.10(2) A contains only one symmetrically independent position for the quinquevalent metal present, and its unit occupancy is achieved by 50% of niobium and by 50% by tantalum atoms within the standard deviations for this determination (Tables 2 and 3). The structure of compound II (see Fig. 2) is built up of centrosymmetric molecules that contain a planar tetranuclear M4(m-O)2(m-OMe)4 core, where M /Nb or Ta, coordinating 10 methoxo and 2 perrhenate ligands. The M /O(Me) distances fall into the range usually observed and are very close to those observed in the structures of the bimetallic analogues based on niobiumor tantalum-only cores [5]. The most interesting feature of this structure is the non-uniform distribution of niobium and tantalum between the two symmetrically Table 2 Selected bond lengths and angles in the structure of I Bond lengths Re(1) O(11) Re(1) O(8) Re(1) O(9) Re(1) O(2) M(1) O(6)
1.63(3) 1.65(3) 1.75(3) 1.74(2) 1.76(2)
M(1) O(7) M(1) O(14) M(1) O(1)#1 M(1) O(1) M(1) O(2)
1.78(3) 1.88(3) 2.02(2) 2.13(2) 2.12(2)
Bond angles O(11) Re(1) O(8) O(11) Re(1) O(9) O(8) Re(1) O(9) O(11) Re(1) O(2) O(8) Re(1) O(2) O(9) Re(1) O(2) O(6) M(1) O(7) O(6) M(1) O(14) O(7) M(1) O(14) O(6) M(1) O(1)#1
122.0(2) 102.0(2) 104.0(2) 110.5(14) 108.9(17) 109.5(17) 96.0(12) 104.2(10) 96.0(10) 92.3(10)
O(14) M(1) O(1)#1 O(6) M(1) O(1) O(7) M(1) O(1) O(14) M(1) O(1) O(1)#1 M(1) O(1) O(6) M(1) O(2) O(7) M(1) O(2) O(14) M(1) O(2) O(1)#1 M(1) O(2) O(1) M(1) O(2)
157.6(9) 161.1(10) 89.8(9) 93.0(9) 69.1(9) 88.0(11) 175.7(10) 84.6(9) 80.9(9) 85.9(9)
M(1) (0.4690.05)Nb(0.5490.05)Ta. Symmetry transformations used to generate equivalent atoms: #1 x1, y , z .
P. Shcheglov et al. / Polyhedron 21 (2002) 2317 /2322 Table 3 Selected bond lengths and angles in the structure of II Bond lengths M(1) O(7) M(1) O(9) M(1) O(1) M(1) O(3) M(1) O(6) M(1) O(4) M(2) O(3) M(2) O(8)
1.772(16) 1.878(14) 1.888(14) 2.043(11) 2.056(13) 2.098(13) 1.791(11) 1.830(15)
M(2) O(2) M(2) O(4)#1 M(2) O(6)#1 M(2) O(5) Re(3) O(13) Re(3) O(11) Re(3) O(12) Re(3) O(5)
1.855(14) 2.066(12) 2.080(11) 2.197(13) 1.53(2) 1.656(19) 1.67(3) 1.727(13)
Bond angles O(7) M(1) O(9) O(7) M(1) O(1) O(9) M(1) O(1) O(7) M(1) O(3) O(9) M(1) O(3) O(1) M(1) O(3) O(7) M(1) O(6) O(6) M(1) O(4) O(3) M(1) O(6) O(13) Re(3) O(11) O(13) Re(3) O(12) O(11) Re(3) O(12) M(2)#1 O(4) M(1) M(1) O(6) M(2)#1
101.0(7) 95.5(7) 96.9(6) 87.4(6) 88.6(5) 173.1(6) 163.3(6) 69.5(5) 85.4(5) 109.4(11) 105.3(14) 108.0(13) 109.6(6) 110.7(5)
O(3) M(2) O(8) O(3) M(2) O(2) O(8) M(2) O(2) O(3) M(2) O(4)#1 O(8) M(2) O(4)#1 O(2) M(2) O(6)#1 O(4)#1 M(2) O(6)#1 O(3) M(2) O(5) O(2) M(2) O(5) O(13) Re(3) O(5) O(11) Re(3) O(5) O(12) Re(3) O(5) M(2) O(3) M(1) Re(3) O(5) M(2)
98.8(6) 97.7(5) 101.4(6) 93.7(5) 160.2(5) 157.1(6) 69.6(5) 175.7(6) 83.4(6) 110.4(11) 111.5(9) 112.1(11) 173.2(7) 145.4(9)
M(1) (0.2990.05)Nb(0.7190.05)Ta, M(2) (0.7090.05)Nb(0.3090.05)Ta. Symmetry transformations used to generate equivalent atoms: #1 x1, y , z .
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of predominantly s-character */even the bond to the oxoligand is a strongly elongated one (M(1)/O(3) ˚ ), which is thermodynamically preferential 2.043(11) A for the heavier tantalum atom. The metal atom in the M(2) position has a very much shorter distance to the ˚ ), which bridging oxo-ligand (M(2)/O(3) 1.791(11) A indicates a considerable p-input in its nature that can easier be made by a light niobium atom. The difference in composition and molecular structures between compounds I and II is reflected also in the differences between their mass-spectra. Both compounds undergo a complete decomposition on evaporation. The preparative investigation of the thermal properties of I and II in vacuum showed the deposition of two different products on the cool parts of the glass vessel at the initial stage (135 8C, 102 mm Hg) of the sublimation process, namely the white film of a less volatile product and the reddish purple one of a more volatile product. The latter became dark on further heating and resembled the sublimation product of the Re4O6(OMe)12 [5]. The formation of two different sublimation products should have occurred through the separation of rhenium- and niobium and tantalumcontaining volatile methoxide species upon the decomposition of the complex compound. The mass-spectral investigation confirmed this observation as no heterometallic species containing rhenium together with niobium or tantalum could be observed in the spectra. The ‘volatility’ (total of intensities of different ions in the spectra) appears generally to be higher for II, containing a smaller number of oxidizing perrhenate ligands. It is also to be noted that the intensity of the tantalumcontaining ions is relatively slightly but noticeably higher for II compared to I, reflecting thus the placement of predominantly tantalum atoms in the position not connected directly to the perrhenate ligands hindering volatility.
4. Supplementary material
Fig. 2. Molecular structure of (Nb,Ta)4O2(OMe)14(ReO4)2 (II).
independent positions: the M(1) position, surrounded by five alkoxide and one bridging oxoligand, turns out to be occupied by tantalum to 70% and niobium by 30% within the accuracy of the experiment, while the M(2) one, connected to four alkoxide, one bridging oxo- and one terminal perrhenate ligand, is in contrast richer in niobium (70%) and poorer in tantalum (30%). This difference can be explained by the difference in the chemical bonding for the atoms occupying these positions: the metal atom in M(1) is involved in the bonding
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 185852 and 185853 for compounds I and II, respectively. Copies of this information may be obtained free of charge from The Director, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
Acknowledgements The authors would like to express their gratitude for financial support of this work to the Swedish Council for the Scientific Research (Vetenskapsra˚det), the Swed-
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ish Royal Academy of Sciences (KVA), the Russian Foundation for Basic Research (project No. 0-03-32527) and the Ministry of Education of Russian Federation (project No. 1 ‘‘V’’-27-871).
References [1] R.C. Mehrotra, J. Sol-Gel Sci. Tech. 2 (1994) 1. [2] N.Ya. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, The Chemistry of Metal Alkoxides, Kluwer Academic Publishers, London, 2002, p. 104.
[3] M. Veith, S. Mathur, C. Mathur, Polyhedron 17 (1998) 1005. [4] A. Johansson, V.G. Kessler, Polyhedron 19 (2000) 1791. [5] G.A. Seisenbaeva, A.V. Shevelkov, L. Kloo, S. Gohil, J. Tegenfeldt, V.G. Kessler, Dalton Trans. (2001) 2762. [6] G.A. Seisenbaeva, V.G. Kessler, Inorg. Chem. Commun. 4 (2001) 534. [7] L.G. Hubert-Pfalzgraf, A.A. Pinkerton, J.G. Riess, Inorg. Chem. 17 (1978) 663. [8] P.A. Shcheglov, D.V. Drobot, G.A. Seisenbaeva, S. Gohil, V.G. Kessler, Chem. Mater. 14 (2002) 2378. [9] E.P. Turevskaya, N.Ya. Turova, A.V. Korolev, A.I. Yanovsky, Yu.T. Struchkov, Polyhedron 14 (1995) 1531. [10] SHELXTL-NT Reference Manual, Bruker AXS, 1998.
Preparation of trimetallic alkoxide complexes exploiting the isomorphous substitution approach. Synthesis, X-ray single crystal and mass-spectrometric study of NbTa(OMe)8(ReO4)2 and Nb2Ta2O2(OMe)14(ReO4)2 Pavel Shcheglov a, Gulaim A. Seisenbaeva b, Suresh Gohil b, Dmitrii V. Drobot a, Vadim G. Kessler b,* b
a Moscow State Academy of Fine Chemical Technology, Pr. Vernadskogo 86, 117571 Moscow, Russia Department of Chemistry, Swedish University of Agricultural Sciences, Box 7015, 75007 Uppsala, Sweden
Received 22 May 2002; accepted 30 July 2002
Abstract Alkoxylation of the rhenium heptoxide, Re2O7, with the bimetallic niobium-tantalum methoxide, NbTa(OMe)10, provided, dependent on the reaction conditions, either (Nb,Ta)2(OMe)8(ReO4)2 (I) or (Nb,Ta)4O2(OMe)14(ReO4)2 (II). Niobium and tantalum are distributed very uniformly between the correspondent positions in the molecular structure of I (only one symmetry independent position present for MV atoms). In the structure of II, the metal atom position connected to the perrhenate ligand has higher niobium content, while that connected to alkoxide groups and the oxoligand has higher tantalum content. Possible reasons for this difference and its influence on the decomposition behavior of I and II on vacuum sublimation are discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Trimetallic alkoxides; Molecular structure; X-ray study; Mass-spectrometric study; Rhenium methoxides; Niobium methoxide; Tantalum methoxide
1. Introduction Trimetallic alkoxide complexes have recently attracted attention of researchers as possible single-source precursors of complex materials [1]. Among the large number of compounds of this class described [2] only a few have unequivocally proved to exist and have been characterized by X-ray single crystal studies, among i them [CdMIIMIV [3], LiMo4Ta3O14(Oi 2 (O Pr)12]2 Pr)9(OC2H4OMe)3 [4] and NaPb2Ti2O(Oi Pr)10Cl [2]. The synthetic approaches to the trimetallic species developed so far were based predominantly on the interaction of a metal halide or alkoxide halide with a bimetallic alkoxide, derived from some other metal and an alkali metal [2,3].
* Corresponding author. Tel.: /46-18-67-1541/1502; fax: /46-1867-3476/3477
We have developed recently a new synthetic approach to heterometallic oxoalkoxides of rhenium based on the alkoxylation of rhenium heptoxide, Re2O7, with an alkoxide of another metal [5,6]. In the present work we report the synthesis and structural characterization of the trimetallic derivatives obtained by using the bimetallic niobium /tantalum methoxide, NbTa(OMe)10 [7], as reactant in this process. The trimetallic rhenium / niobium /tantalum alkoxide complexes are of interest as precursors for the preparation of fine powders of related alloys applying the soft chemistry approaches [8].
2. Experimental All of the preparative procedures were carried out in dry nitrogen atmosphere using a dry box. Methanol (Merck, p.a. ) was dehydrated by refluxing over magne-
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 1 8 0 - 4
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sium methoxide with subsequent distillation. Toluene (Merck, p.a. ) was distilled after refluxing with LiAlH4. IR spectra of Nujol mulls were obtained with a Perkin/Elmer FT-IR 1720 X spectrometer. Metal ratio in the samples was determined at the Arrhenius Laboratory, Stockholm University using a JEOL JSM-820 scanning microscope (SEM), supplied with a Link AN10000 energy dispersive spectrometer (EDS). Mass spectra (electron beam ionization, direct probe introduction) were recorded using JEOL JMS-SX/SX-102A mass-spectrometer. The determination of carbon and hydrogen contents was carried out by Mikrokemi AB, Uppsala, Sweden, using the conventional organic microanalysis technique. The alkoxides, Nb2(OMe)10 and Ta2(OMe)10, used in this work as starting materials for the synthesis of bimetallic complexes, were prepared by anodic oxidation of the corresponding metals in methanol and purified according to conventional techniques [9]. Rhenium(VII) oxide (99.9/%) was purchased from Aldrich. 2.1. Synthesis of (Nb,Ta)2(OMe)8(ReO4)2 (I) Solutions of Nb(OMe)5 (0.131 g, 0.529 mmol) in 3.5 ml of C6H5CH3 and Ta(OMe)5 (0.182, 0.543 mmol) in 2.5 ml of C6H5CH3 were mixed. Re2O7 (0.273 g, 0.564 mmol) was added to the obtained solution upon stirring. No color changes at the initial stage were observed. Slow dissolution of Re2O7 occurred and the solution gradually became brownish-pink. Slight brownish coloration of Re2O7 particles and the formation of a small amount of nearly colorless crystals were observed. At the end of the process, after dissolution of almost all the Re2O7 (40 min), the mixture was heated to 60 8C for 10 min. The solution was cooled to room temperature (r.t.), then to /30 8C for crystallization. After 3 days the solution was decanted and the crystalline product (light pink prismatic platelets) was dried in vacuum. Yield: 0.411 g (76%). IR, cm 1: 1138 s br, 998 m, 945 sh, 935 s (n-Re /O), 896 sh, 818 s br (n-Re /O Nb(Ta), compare [6,8]), 723 w, 618 w, 569 m, 514 s. MS, m /z (I,%), interpretation: 641(1.0), Ta2(OMe)9, 595(1.0), Ta2O(OMe)7 , 553(1.0), TaNb(OMe)9, 522(0.5), TaNb(OMe)8, 507(1.1), TaNbO(OMe)7, 476(0.5), TaNbO(OMe)6, 461(0.6), TaNbO2(OMe)5, 305(19.3), Ta(OMe)4 , 281(1.7), ReO2(OMe)2, 274(3.6), Ta(OMe)3, 266(61.4), ReO3(OMe) , 265(51.5), ReO3(OCH2), 251(2.7), ReO4, 249(2.1), Re(OMe)2, 248(1.6), Re(OCH2)(OMe) and Nb(OMe)5 , 247(1.8), Nb(OMe)4(OCH2) , 238(6.5), Re(OH)3, 237(7.5), ReO(OH)2, 236(100), ReO2(OH) , 235(32.9), ReO3, 234(59.3), ReO(OMe) , 233(17.3), ReO(OCH2) , 231(1.9), TaO(OH)2, 220(17.0), ReO(OH), 219(61.6), ReO2, 217(50.8), Nb(OMe)4, 204(4.8), Re(OH) , 203(22.0), ReO and Nb(OMe)3(OH) , 187(10.5), Re, 186(1.8),
Nb(OMe)3, 172(10.1), Nb(OMe)2(OH), NbO(OMe)2, 157(2.9), NbO(OH)(OMe) , Nb(OMe)2, 141(1.8), Nb(OMe)(OH) , NbO(OMe) , 139(2.5), NbO(OCH2) , NbO2.
171(8.4), 155(2.4), 140(1.5), 125(2.6),
2.2. Synthesis of (Nb,Ta)4O2(OMe)14(ReO4)2 (II) Solutions of Nb(OMe)5 (0.140 g, 0.565 mmol) in 3.5 ml of C6H5CH3 and Ta(OMe)5 (0.194, 0.577 mmol) in 2.5 ml of C6H5CH3 were mixed. Re2O7 (0.276 g, 0.570 mmol) was added to the obtained solution upon stirring. No color changes at the initial stage were observed. Upon slow dissolution of Re2O7, the solution became light pink. After 10 min of stirring, the mixture was heated. The solution became reddish-brown and Re2O7 changed color to black. The mixture was refluxed for 15 min. The formation of a black non-crystalline residue as a by-product was observed (EDS analysis showed the by-product to comprise of Re as the major component, and Nb and Ta in minor quantities, 5/7% atom). The solution was cooled to r.t., then to /30 8C for crystallization. After 3 days the solution was decanted and a crystalline product (thick needle shaped crystals brownish-pink in color) was dried in vacuum. Yield: 0.306 g (71% on M(OMe)5 and 35% on Re2O7). IR, cm 1: 1124 s br, 998 m w, 978 w, 931 s (n-Re /O), 893 w, 808 s (nRe /O Nb(Ta), compare [5]), 735 sh, 722 m, 569 m, 512 s. MS, m /z (I,%), interpretation: 641(2.8), Ta2(OMe)9, 595(4.2), Ta2O(OMe)7, 553(1.7), TaNb(OMe)9, 549(1.4), Ta2O2(OMe)5, 522(0.9), TaNb(OMe)8, 517(0.4), Ta2(OMe)5, 508(0.5), TaNb(OH)(OMe)7, 507(4.4), TaNbO(OMe)7, 503(0.5), Ta2(OMe)4(OH) , 476(2.0), TaNbO(OMe)6, 461(2.3), TaNbO2(OMe)5 , 446(0.5), TaNb(OH)(OMe)5, 429(0.7), TaNb(OMe)5, 419(0.9), Nb2O(OMe)7, 415(0.7), TaNb(OH)(OMe)4, 399(0.7), TaNbO2(OMe)3, 385(0.6), TaNbO2(OH)(OMe)2, 373(0.6), Nb2O2(OMe)5, 305(19.0), Ta(OMe)4, 281(5.3), ReO2(OMe)2, 275(4.6), TaO(OH)(OMe)(OCH2) , 274(1.1), Ta(OMe)3, 267(3.2), ReO2(OH)(OMe) , 266(92.1), ReO3(OMe) , 265(86.8), ReO3(OCH2), 259(2.4), TaO(OMe)2, 251(6.0), ReO4, 249(4.6), Re(OMe)2, 248(3.0), Re(OCH2)(OMe) and Nb(OMe)5, 247(3.0), Nb(OMe)4(OCH2) , 238(9.4), Re(OH)3, 237(12.4), ReO(OH)2, , 236(100), ReO2(OH), 235(51.8), ReO3, 234(96.8), ReO(OMe) , 233(27.3), ReO(OCH2) , 231(2.4), TaO(OH)2, 221(3.2), Re(OH)2, 220(26.8), ReO(OH) , 219(95.7), ReO2, 217(72.4), Nb(OMe)4, 204(8.0), Re(OH) , 203(32.3), ReO and Nb(OMe)3(OH), 187(15.3), Re , 186(2.3), Nb(OMe)3, 172(22.9), Nb(OMe)2(OH) , 171(15.1), NbO(OMe)2, 157(5.5), NbO(OH)(OMe), 155(4.1), Nb(OMe)2, 141(3.3), Nb(OMe)(OH) , 140(2.9), NbO(OMe) , 139(4.4), NbO(OCH2) , 125(5.7), NbO2.
P. Shcheglov et al. / Polyhedron 21 (2002) 2317 /2322
2.3. Alternative technique for the synthesis of (Nb,Ta)4O2(OMe)14(ReO4)2 (II) Solutions of Nb(OMe)5 (0.132 g, 0.532 mmol) in 3.5 ml of C6H5CH3 and Ta(OMe)5 (0.178, 0.531 mmol) in 2.5 ml of C6H5CH3 were mixed. Re2O7 (0.139 g, 0.288 mmol) was added to the obtained solution upon stirring. No color changes at the initial stage were observed. Upon slow dissolution of Re2O7, a brownish-violet color of the latter and precipitation of a small amount of lightly colored pink fine crystals were observed. After approximately 20 min of stirring, the solution was light pink in color. After 40 min of stirring most of the Re2O7 was dissolved and further formation of fine crystalline precipitate occurred. After 70 min the color of the solution was light orange /pink and almost all of the Re2O7 was dissolved. After 80 min the mixture was heated and the solution became brownish/pink. The mixture was refluxed for 15 min. In the course of the heating the dissolution of the initially formed crystals occurred. At the end of refluxing the solution became dark reddish-brown. The solution was cooled to r.t., then to /30 8C for crystallization. After 3 days the solution was decanted and the crystalline product (thick needle shaped crystals brownish-pink in color) was dried in vacuum. Yield: 0.258 g (64%). IR and MS data were identical to those reported above. 2.4. Crystallography The crystal data and experimental conditions are shown in Table 1. The data were collected using a SMART CCD 1K diffractometer at 22 8C. All calculations were performed on an IBM PC using the SHELXTLNT [10] program package. The absorption correction has Table 1 Crystal data and details of diffraction experiments for compounds I and II
Chemical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A Z m (mm 1) Independent reflections Observed reflections R1 wR2
I
II
C8H24NbO16Re2Ta 1022.53 monoclinic P 21/c 8.564(2) 12.982(3) 10.597(2) 90 97.279(4) 90 1168.7(5) 2 15.532 794 [R (int) 0.1204] 586 [I 2s (I )] 0.0677 0.1765
C14H42Nb2O24Re2Ta2 1514.60 monoclinic P 21/c 9.745(9) 15.882(14) 12.204(10) 90 100.98(2) 90 1854(3) 2 13.054 2402 [R (int) 0.0977] 998 [I 2s (I )] 0.0591 0.1134
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been introduced for both experiments using Bruker SADABS program. Both structures were solved by direct methods. The positions of metal atoms were obtained from the initial solution and refined first in isotropic and then anisotropic approximation. The occupancy factors for all the metal atom positions in the structures of compounds I and II were refined in supposition that they can be occupied with equal opportunity by both niobium and tantalum atoms; the total occupancy for each position being 1.0. The values obtained are within experimental errors in agreement with those obtained by EDS-analysis. The positions of all other non-hydrogen atoms were obtained from difference Fourier syntheses and refined first in isotropic and then anisotropic approximation. The positions of hydrogen atoms were calculated geometrically and included into the final refinement in isotropic approximation; the thermal parameters for H-atoms were taken as Uiso / 1.50Ueq(C), where Ueq(C) is the equivalent parameter for the carbon atom to which the hydrogen atom is attached.
3. Results and discussion 3.1. Reaction pathways The reaction of the alkoxylation of rhenium heptoxide, Re2O7, has recently been proved to provide a facile access to bimetallic oxoalkoxide derivatives of rhenium [5,6] and it appeared attractive to investigate its potential for the preparation of trimetallic compounds, applying a bimetallic alkoxide as reactant. We have chosen in this purpose the bimetallic methoxide of niobium and tantalum formed easily on mixing the two homometallic species in hydrocarbon media [7] and existing due to the extraordinary likeness of the coordination geometries of the two metals, which permits to consider it as a product of isomorphous substitution of one metal in the structure with the other. Carrying out of this reaction at ambient temperature was found to lead practically exclusively to compound I, independently of the ratio of the reactants: Re2 O7 NbTa(OMe)10 0 (Nb; Ta)2 (OMe)8 (ReO4 )2 (I). . . It may be supposed that the reaction pathway includes in this case, first, some partial oxidation of the methoxide ligands producing water that leads to formation of perrhenic acid, which produces compound I via the chemical modification of the bimetallic alkoxide: Re2 O7 H2 O 0 2HReO4 2HReO4 NbTa(OMe)10 0 (Nb; Ta)2 (OMe)8 (ReO4 )2 2MeOH
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In favor of this pathway speaks the quite high, but far not quantitative, yield of compound I, and the absence of the intensive coloration, indicating otherwise the formation of low-valent rhenium alkoxides in the course of the alkoxylation [5,6]. The processes occurring on heating follow evidently the mechanism proposed earlier [5,6] for the alkoxylation of rhenium heptoxide, but the elevated temperature leads to deeper condensation of the products resulting in formation of a more oxo-substituted tetranuclear core of compound II. It is to be noted also that compound I formed in the reaction mixture at low temperature can on further heating undergo an almost quantitative thermal condensation in solution with an excess of NbTa(OMe)10 and provide II with a significantly increased yield in relation to Re2O7. The condensation occurs supposedly via partial oxidation of the methoxide ligand by the perrhenate ligand present, which provides the water necessary for this process. It is important to note that the metal atom content in the product corresponds perfectly to that in the reaction media and the Nb:Ta /1:1 ratio is conserved in the trimetallic products obtained opening the prospect of the application of these precursors in the preparation of trimetallic oxide and alloy materials. 3.2. Molecular structures Unfortunately, both I and II are almost insoluble in common organic solvents at room temperature. On heating, they remain unchanged in hydrocarbons (toluene, hexane), while in methanol the crystals turn slowly dark, supposedly because of the reduction of rhenium(VII) by the alcohol, but still no transfer into solution of the metal-containing species could be observed. This precluded the application of NMR for the estimation of the possible molecular structures in solution (in statu nascendi) and those discussed below are the solid state ones obtained by the X-ray single crystal studies. The structure of I (Fig. 1) turned out to be very close to the structures of its bimetallic analogues, M2(OMe)8(ReO4)2, M/Nb, Ta [6]. Its centrosymmetric molecules are pairs of octahedra sharing a common (mOR)2 edge, the perrhenate substituents being situated in the axial positions. The oxo-bridging bond to the ˚ as that rhenium atom is as long M(1) /O(2) 2.12(2) A in the structure of the tantalum-only derivative (Ta(1) / ˚ ) and has evidently also a decreased O(5) 2.115(16) A multiplicity. The terminal alkoxide ligand situated in the trans -position to it has quite logically a significantly shortened M(1) /O(7) 1.78(3) distance (compared to ˚ in the tantalum-only analogue) (Ta(1) /O(4) 1.814(19) A with the multiplicity increased due to the pp /dp interaction. The bridging M/O distances are noticeably asymmetric and significantly longer than the terminal
Fig. 1. Molecular structure of (Nb,Ta)2(OMe)8(ReO4)2 (I).
˚ M /O bonds (M(1)/O(1) 2.13(2), M(1) /O(1)# 2.02(2) A in I compared to Ta(1) /O(1) 2.08(2), Ta(1) /O(1)# ˚ in Ta2(OMe)8(ReO4)2 [6]. The structure of I 2.10(2) A contains only one symmetrically independent position for the quinquevalent metal present, and its unit occupancy is achieved by 50% of niobium and by 50% by tantalum atoms within the standard deviations for this determination (Tables 2 and 3). The structure of compound II (see Fig. 2) is built up of centrosymmetric molecules that contain a planar tetranuclear M4(m-O)2(m-OMe)4 core, where M /Nb or Ta, coordinating 10 methoxo and 2 perrhenate ligands. The M /O(Me) distances fall into the range usually observed and are very close to those observed in the structures of the bimetallic analogues based on niobiumor tantalum-only cores [5]. The most interesting feature of this structure is the non-uniform distribution of niobium and tantalum between the two symmetrically Table 2 Selected bond lengths and angles in the structure of I Bond lengths Re(1) O(11) Re(1) O(8) Re(1) O(9) Re(1) O(2) M(1) O(6)
1.63(3) 1.65(3) 1.75(3) 1.74(2) 1.76(2)
M(1) O(7) M(1) O(14) M(1) O(1)#1 M(1) O(1) M(1) O(2)
1.78(3) 1.88(3) 2.02(2) 2.13(2) 2.12(2)
Bond angles O(11) Re(1) O(8) O(11) Re(1) O(9) O(8) Re(1) O(9) O(11) Re(1) O(2) O(8) Re(1) O(2) O(9) Re(1) O(2) O(6) M(1) O(7) O(6) M(1) O(14) O(7) M(1) O(14) O(6) M(1) O(1)#1
122.0(2) 102.0(2) 104.0(2) 110.5(14) 108.9(17) 109.5(17) 96.0(12) 104.2(10) 96.0(10) 92.3(10)
O(14) M(1) O(1)#1 O(6) M(1) O(1) O(7) M(1) O(1) O(14) M(1) O(1) O(1)#1 M(1) O(1) O(6) M(1) O(2) O(7) M(1) O(2) O(14) M(1) O(2) O(1)#1 M(1) O(2) O(1) M(1) O(2)
157.6(9) 161.1(10) 89.8(9) 93.0(9) 69.1(9) 88.0(11) 175.7(10) 84.6(9) 80.9(9) 85.9(9)
M(1) (0.4690.05)Nb(0.5490.05)Ta. Symmetry transformations used to generate equivalent atoms: #1 x1, y , z .
P. Shcheglov et al. / Polyhedron 21 (2002) 2317 /2322 Table 3 Selected bond lengths and angles in the structure of II Bond lengths M(1) O(7) M(1) O(9) M(1) O(1) M(1) O(3) M(1) O(6) M(1) O(4) M(2) O(3) M(2) O(8)
1.772(16) 1.878(14) 1.888(14) 2.043(11) 2.056(13) 2.098(13) 1.791(11) 1.830(15)
M(2) O(2) M(2) O(4)#1 M(2) O(6)#1 M(2) O(5) Re(3) O(13) Re(3) O(11) Re(3) O(12) Re(3) O(5)
1.855(14) 2.066(12) 2.080(11) 2.197(13) 1.53(2) 1.656(19) 1.67(3) 1.727(13)
Bond angles O(7) M(1) O(9) O(7) M(1) O(1) O(9) M(1) O(1) O(7) M(1) O(3) O(9) M(1) O(3) O(1) M(1) O(3) O(7) M(1) O(6) O(6) M(1) O(4) O(3) M(1) O(6) O(13) Re(3) O(11) O(13) Re(3) O(12) O(11) Re(3) O(12) M(2)#1 O(4) M(1) M(1) O(6) M(2)#1
101.0(7) 95.5(7) 96.9(6) 87.4(6) 88.6(5) 173.1(6) 163.3(6) 69.5(5) 85.4(5) 109.4(11) 105.3(14) 108.0(13) 109.6(6) 110.7(5)
O(3) M(2) O(8) O(3) M(2) O(2) O(8) M(2) O(2) O(3) M(2) O(4)#1 O(8) M(2) O(4)#1 O(2) M(2) O(6)#1 O(4)#1 M(2) O(6)#1 O(3) M(2) O(5) O(2) M(2) O(5) O(13) Re(3) O(5) O(11) Re(3) O(5) O(12) Re(3) O(5) M(2) O(3) M(1) Re(3) O(5) M(2)
98.8(6) 97.7(5) 101.4(6) 93.7(5) 160.2(5) 157.1(6) 69.6(5) 175.7(6) 83.4(6) 110.4(11) 111.5(9) 112.1(11) 173.2(7) 145.4(9)
M(1) (0.2990.05)Nb(0.7190.05)Ta, M(2) (0.7090.05)Nb(0.3090.05)Ta. Symmetry transformations used to generate equivalent atoms: #1 x1, y , z .
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of predominantly s-character */even the bond to the oxoligand is a strongly elongated one (M(1)/O(3) ˚ ), which is thermodynamically preferential 2.043(11) A for the heavier tantalum atom. The metal atom in the M(2) position has a very much shorter distance to the ˚ ), which bridging oxo-ligand (M(2)/O(3) 1.791(11) A indicates a considerable p-input in its nature that can easier be made by a light niobium atom. The difference in composition and molecular structures between compounds I and II is reflected also in the differences between their mass-spectra. Both compounds undergo a complete decomposition on evaporation. The preparative investigation of the thermal properties of I and II in vacuum showed the deposition of two different products on the cool parts of the glass vessel at the initial stage (135 8C, 102 mm Hg) of the sublimation process, namely the white film of a less volatile product and the reddish purple one of a more volatile product. The latter became dark on further heating and resembled the sublimation product of the Re4O6(OMe)12 [5]. The formation of two different sublimation products should have occurred through the separation of rhenium- and niobium and tantalumcontaining volatile methoxide species upon the decomposition of the complex compound. The mass-spectral investigation confirmed this observation as no heterometallic species containing rhenium together with niobium or tantalum could be observed in the spectra. The ‘volatility’ (total of intensities of different ions in the spectra) appears generally to be higher for II, containing a smaller number of oxidizing perrhenate ligands. It is also to be noted that the intensity of the tantalumcontaining ions is relatively slightly but noticeably higher for II compared to I, reflecting thus the placement of predominantly tantalum atoms in the position not connected directly to the perrhenate ligands hindering volatility.
4. Supplementary material
Fig. 2. Molecular structure of (Nb,Ta)4O2(OMe)14(ReO4)2 (II).
independent positions: the M(1) position, surrounded by five alkoxide and one bridging oxoligand, turns out to be occupied by tantalum to 70% and niobium by 30% within the accuracy of the experiment, while the M(2) one, connected to four alkoxide, one bridging oxo- and one terminal perrhenate ligand, is in contrast richer in niobium (70%) and poorer in tantalum (30%). This difference can be explained by the difference in the chemical bonding for the atoms occupying these positions: the metal atom in M(1) is involved in the bonding
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC Nos. 185852 and 185853 for compounds I and II, respectively. Copies of this information may be obtained free of charge from The Director, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
Acknowledgements The authors would like to express their gratitude for financial support of this work to the Swedish Council for the Scientific Research (Vetenskapsra˚det), the Swed-
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ish Royal Academy of Sciences (KVA), the Russian Foundation for Basic Research (project No. 0-03-32527) and the Ministry of Education of Russian Federation (project No. 1 ‘‘V’’-27-871).
References [1] R.C. Mehrotra, J. Sol-Gel Sci. Tech. 2 (1994) 1. [2] N.Ya. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, The Chemistry of Metal Alkoxides, Kluwer Academic Publishers, London, 2002, p. 104.
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