Chemistry of Molybdaboranes: Synthesis, Structures, and Characterization of a New Class of Open-Cage Dimolybdaheteroborane Clusters

Reaction of [Cp*MoCl4], 1 (Cp* = η5-C5Me5), with [LiBH4·thf] in toluene at −70 °C, followed by pyrolysis with excess dichalcogenides RE-ER (R = Ph, CH2Ph, 2,6-(tBu)2-C6H2OH, (CH3)3C = tBu); E = S, Se) yielded a new class of hybrid clusters, 3−8: (3, [(Cp*Mo)2(μ-η1-SPh)2(μ3-S)(H2BSPh)]; 4, [(Cp*Mo)2B5H8(SPh)]; 5, [(Cp*Mo)2B5H8(SePh)]; 6, [(Cp*Mo)2B2S2H2(μ-η1-S)]; 7, [(Cp*Mo)2B2H5(BSR)2(μ-η1-SR)], (R = 2,6-(tBu)2-C6H2OH); and 8, [(Cp*Mo)2B2H5(BSePh)2(μ-η1-SePh)]. Compounds 3−8 have been isolated in modest yields as green or brown crystalline solids. In parallel with 3−8, [(Cp*Mo)2B5H9] was isolated as a major product in all cases. The isolation and structural characterization of compounds 3 and 6−8 provided the first direct evidence of the existence of [(Cp*Mo)2B4H8], 2, an intermediate in the formation of [(Cp*Mo)2B5H9]. These new compounds have been characterized in solution by mass spectrometry, 1H, 11B, 13C NMR, and IR spectroscopy, and elemental analysis. The structural types were unequivocally established by X-ray crystallographic analysis of compounds 3−8.