1 Introduction

Transition metal telluride complexes bearing niobocene fragments are useful reagents for the synthesis of heterometallic telluride complexes and metal telluride clusters when reacting with binary transition metal carbonyls [1]. The main strategy in this chemistry uses a ligand cross-transfer (CO versus Te) from one metal center to the other. Finally, neutral or anionic telluride clusters are formed depending on the nature of the substituents at the metal centers [2–4].

Another strategy for the synthesis of metal telluride clusters employs the oxidative decarbonylation of metal carbonyls by polytelluride anions [5–9]. In this context we were interested to cleave the Te–Te bond in Cp′₂Nb(η²-Te₂)H (Cp′ = tBuC₅H₄) (1) [10] by reductive means. In a first attempt reaction of 1 with CH₃Li gave a red–violet, very reactive product of still unknown nature [11]. This product reacts with Co₂(CO)₈ to give a complex mixture, from which some compounds derived from the cubic body-centered Co₉Te₆ polyhedron (Scheme 1a) were obtained [11].

In this work we report on the attempt to alkylate the ionic compounds in this mixture by subsequent addition of CH₃I. As a result we found the surprising formation of the neutral cluster [Co₁₁Te₇(CO)₁₀], associated with and probably stabilized by the salt [Cp′₄Nb₂(CH₃Te)Te]I both in solution and in the solid state.

2 Results and discussion

Reaction of a dark orange THF solution of 1 with 2 equiv. of CH₃Li at room temperature gave an immediate color change to redviolet. Subsequent reaction of the solution with 2 equiv. of Co₂(CO)₈ caused vigorous gas evolution. The resulting dark mixture was refluxed for 18 h and then treated with 3 equiv. of CH₃I at room temperature. Chromatographic work-up gave traces of the redbrown neutral complex 2 along with a mixture of dark brown salts containing the [Co₉Te₆(CO)₈]^– anion and 5 (Scheme 1b). Complex 2 is identical in its spectroscopic data with known {Cp′₂Nb(CO)}₂[Co₉Te₆(CO)₈], which forms in much better yield in the reaction described in Scheme 1a [11].

The mixture of dark products was separated by chromatography and elution with CH₂Cl₂ and acetone into 3 and dark brown 5. The salt 3 may be transferred by metathesis reaction with PPh₄Cl into [PPh₄][Co₉Te₆(CO)₈] [12], whereas crystallization from THF gave a few needles of [Na(THF)₆][Co₉Te₆(CO)₈] (4). The composition of 4 was proven by comparison of X-ray crystallographic data with those of an authentic sample [11]. The existence of 4 may only be explained by the presence of impurities of Na in the employed CH₃Li.

The identification of 5 is based on electrospray ionization mass spectrometry (ESI-MS) and X-ray crystallography, the found composition is confirmed by elemental analysis. The PI-ESI-MS (solvent CH₃CN, T_cap = 150 °C) exhibits one peak with center of gravity at 941.0 mu which may be assigned to the dinuclear cation [Cp′₄Nb₂Te₂CH₃]⁺. The negative ion (NI) ESI-MS of 5 reveals several peaks with intensities between 2% and 11% centered at 1628.3, 1598.3, 1570.2 and 1542.2 mu apart from the base peak at m/z = 1382.2. The relatively weak peaks may be assigned to the respective loss of seven, eight, nine or 10 CO ligands from the intact cluster ion [Co₁₁Te₇(CO)₁₀]^–. The observation of the peak at 1542 mu means that even the naked Co₁₁Te₇ cage exhibits a certain stability in the gas phase. However, the base peak may be derived from [Co₉Te₆(CO)₈]^– by loss of five CO ligands. As there is no proof in the crystal structure for the cubic polyhedron one may assume a transformation of the pentagonal-prismatic Co₁₁Te₇ cluster into the cubic Co₉Te₆ framework under mass spectroscopic conditions. A related rearrangement of the cluster framework as a consequence of CO loss during mass spectroscopic investigations was investigated quantitatively for [Pt₃Ru₁₀C₂(CO)₃₂]^2– [13].

The IR spectrum of 4 exhibits a very strong absorption at 1973 cm⁻¹ along with absorptions at 1478 and 1385 cm⁻¹, which is in agreement with the ν(C–O–C) frequencies of coordinated THF ligands [11]. The IR spectrum of 5 reveals a strong and relatively broad absorption for terminal CO ligands at 1941 cm⁻¹. Additional absorptions in the region of 2960 and 1365 cm⁻¹ may be assigned to the tBuC₅H₄ ligands of the niobocene fragments. The ¹H NMR spectrum of 5 is not clear-cut because of the low proton concentration of the sample, caused by the high molar mass of 5 and its low solubility. Resonances at δ = 1.46 ppm (tBu) and between 4.33 and 6.30 (multiplets of aromatic hydrogens) resemble those of [Cp′₄Nb₂Te₂CH₃]I, a compound which forms on methylation of Cp′₄Nb₂Te₂ [10].

The crystal structure of 5 consists of two molecular units in the triclinic cell and each molecular unit contains one [Co₁₁Te₇(CO)₁₀] polyhedron, one [Cp′₄Nb₂Te₂CH₃]⁺ ion, two I^– ions (50% disordered each) and one CH₂Cl₂ molecule (Fig. 1). The polyhedra are stacked in parallel columns with respect to the a axis. These columnar structures are embedded in between two layers formed by the cations and the inversion centers of these cations lie in the ac plane (Fig. 1). The inversion centers of the disordered iodide ions are in the ac plane, too, but shifted by b/2. As the unit cell contains cations and anions in a 1:1 ratio the charge of the [Co₁₁Te₇(CO)₁₀] cluster must be zero.

The structure of the [Cp′₄Nb₂Te₂CH₃]⁺ cation contains a planar rhombic four-membered Nb₂Te₂ ring with four attached Cp′ ligands, which are arranged in the achiral meso form (Fig. 2). This arrangement is similar to that in Cp′₄Nb₂Te₂ [10,14] and [Cp′₄Nb₂(CH₃Te)₂]I₂ [10] and similarities are also observed for the metric parameters (Table 1). Two methyl groups (C29) are fixed at the Te bridges in a trans position, but with an occupancy of 50% for each of the methyl groups. Accordingly, the molecule contains an inversion center. The composition is confirmed by the PI-ESI mass spectrum, which contains only the peak of the monomethylated species. It may be noted that mass spectrometry is an excellent tool to determine the methylation degree (one or two) of the parent dimer Cp′₄Nb₂Te₂ after reaction with CH₃I [10,14].

The structure of the [Co₁₁Te₇(CO)₁₀] cluster is in a close analogy to that of other clusters with the Co₁₁Te₇ framework [2,15,16]: a body-centered pentagonal prism of ten Co atoms is capped by five μ₄-Te ligands and two μ₅-Te ligands and each of the Co vertices bears a CO ligand (Fig. 3). A comparison of the bond parameters (Table 1) of the Co₁₁Te₇ skeleton of 5 with those of [Co₁₁Te₇(CO)₅(PMe₂Ph)₅] [17] [Co₁₁Te₇(CO)₁₀]^– [2] and [Co₁₁Te₇(CO)₁₀]^2– [18] shows that deviations are at the limit of accuracy of the crystal structure determinations (Table 2).

The uncharged [Co₁₁Te₇(CO)₁₀] polyhedron possesses 147 valence electrons. Thus far it has been observed in electrochemical experiments after oxidation of solutions containing the [Co₁₁Te₇(CO)₁₀]^– anion (Scheme 2a) [2]. The violet salt [Cp′₄Nb₂Te₂CH₃]I can be prepared from Cp′₄Nb₂Te₂ and 1 equiv. of CH₃I (Scheme 2b). It is insoluble in THF, soluble in CH₂Cl₂ and does not migrate on silica gel. For these reasons one may discuss for the formation of 5 besides the possibility of a mere cocrystallization of both building blocks weak associative forces in solution, which prevents separation of one of the two compounds during work-up.

The organization of the distinct components in the unit cell may be promoted by attractive C−H···O contacts between hydrogen atoms of the tBu groups in the cation and oxygen atoms of terminal CO ligands of the cluster in the range between 2.50 and 2.89 Å (Fig. 1). The importance of hydrogen-bonding acceptor CO ligands for the crystal packing has already been discussed [15,16], and in our case it may be increased by electrostatic forces.

The role of large organometallic cations for the stabilization of anionic metal telluride clusters has already been mentioned [1]. The formation of the [Cp′₄Nb₂Te₂CH₃]⁺ cation cannot yet be explained and is therefore the subject of further investigations.

3 Experimental

Acknowledgements

We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft.

Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center. The deposit number is CCDC 248624 for compound 5. Copies of this information may be obtained free of charge from the Director, Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.uk.