Metal Alkoxides: Model for Metal Oxides. 16. Synthesis and Characterization of a New Class of Homoleptic 12-Electron Molybdenum and Tungsten Alkoxides Clusters of Formula M4(OR)12: Structural Characterization of Mo4(OCH2-c-Bu)12*(HOCH2-c-Bu)

Article abstract of DOI:10.1021/ja00196a040

Addition of primary alcohols, RCH2OH (>6 equiv), to hydrocarbon solutions of M2(O-t-Bu)6 compounds has led to the isolation of the first homoleptic series of cluster alkoxides M4(OCH2R)12 for both M=Mo and W where R=i-Pr, Cy (Cy=cyclohexyl), Cp (Cp=cyclopentyl), and c-Bu (c-Bu=cyclobutyl).The NMR data ((1)H and (13)C) indicate a coommon structural unit for this class of compounds containing a molecular mirror plane.For each M4(OCH2R)12 compound there are eight types of alkoxide ligands in the integral intensity 2:2:2:2:1:1:1:1.Those having integral intensity twopossess diastereotopic methylene protons, indicating that they do not lie on the mirror plane, whereas the other four alkoxides (1:1:1:1) are contained within the mirror plane.Though these compounds yield crystals from 1,2-dimethoxyethane, they failed to diffract beyond 10 deg indicating a molecular disorder within hexagonal packing of M4 units.However, the cyclobutylmethoxide derivatives crystallize with an additional molecule of alcohol, M4(OCH2-c-Bu)12(HOCH2-c-Bu), and the molecular structure of the molybdenum compound was determined by X-ray study.Crystal data for Mo4(OCH2-c-Bu)12(HOCH2-c-Bu) at -156 deg C: a=b=19.952 (7) Angstroem, c=34.755 (16) Angstroem, Z=8, d_calcd=1.43 g cm^-3, and space group P4₁.The unit cell contains two crystallographically independent molecules that are essentially equivallent with respect to M-M and M-O distances and angles.In each there is a central Mo4 butterfly with a dihedral angle between the Mo3 triangles of 132.5 deg.One wingtip metal atom is octahedrally coordinated by oxygen atoms of which three are terminal, two edge bridging, and one triply bridging.The backbone Mo atoms areevidently strongly bonded, Mo-Mo 2.47 Angstroem, and are coordinated to two edge bridging alkoxides and one terminal alkoxide.The other wingtip Mo atom is similarly coordinated, and the unique molecule of alcohol is bound to one of the backbone Mo atoms, Mo-O (HOCH2-c-Bu) = 2.34 (1) Angstroem.In solution (benzene-d₆ and toluene-d₈) the NMR spectra are similar to those obtained for the other members of this class of compounds, M4(OCH2R)12, with additional signals due to free HOCH2R.This implies that the cyclobutylmethanol is weakly bonded and readily dissociates in solution.The present findings are compared with other 12-electron alkoxide supported clusters of molybdenum and tungsten.