Synthesis of novel molybdaboranes from (η5-C5R5)MoCln precursors (R=H,Me; n=1,2,4)

Reaction of Cp*MoCl4 (1), or (Cp*MoCl2)(2) (2), Cp*=eta(5)-C5Me5, with BH3 . THF ultimately generates the Mo(II) cluster (Cp*Mo)(2)B5H9 (7), together with the Mo(III) species (Cp*MoCl)(2)B4H10, 4. Prereduction of 2 before reaction with BH3 . THF yields only 7. The structure of 4 consists of two Cp*Mo units bridged by two chlorides and a [B2H5(B2H5)](2-) ligand in which the two diboron moieties are connected by a B-B-B three center bond. Closer inspection of the reaction by B-11 and H-1 NMR reveals the existence of three intermediate species (Cp*MoCl)(2)B2H6 (3), (Cp*MoCl)(2)B3H7 (5), and (Cp*Mo)(2)(B2H6)(2) (6). Each of these species has been characterized spectroscopically, and crystal structures have been obtained for 3 and 5. Compound 3 features molybdenum centers bridged by two chlorides and an ethane-like [B2H6](2-) ligand such that the B-B bond is perpendicular to the Mo-Mo bond. Replacing one terminal H by [B2H5] generates 4. The structure of 5 is based on a trigonal bipyramidal Mo2B3 core, and the molecule is electronically unsaturated although the Mo-Mo distance (3.096 Angstrom) precludes the existence of multiple bonding between the metal centers. 5 exists as a relatively stable molecule despite having too few electrons and too few atoms to adopt a capped structure based on a polyhedron with fewer vertexes. Comparison of MO descriptions of the electronic structure of 5 with that of the later transition metal species (Cp*Co)(2)B3H7 (8) shows that this stabilization is derived from the appropriate energy match between Cp*Mo and borane based orbitals which elevates the energy of the Mo-B antibonding LUMO, a cluster orbital which would normally be filled, into the region of unoccupied orbitals. The concentration vs time behavior for the final products 4 and 7, for the intermediates 3, 5, and 6, for the monoboron species BH3 . THF and BH2Cl, and selected non-boron containing species is used to define a pathway for the molybdaborane cluster condensation. With 1, use of LiBH4 as the monoboron source yields 6 as the primary product via 3 as an intermediate, whereas prereduction of 2 with [Et3BH](-) results in the formation of 7. The varied cluster building abilities of BH3 . THF vs LiBH4 originate in the differing reduction and coordination properties of the two monoboranes. Investigation of the analogous Cp=eta(5)-C5H5 system reveals similar chemistry albeit simpler and on a shorter time scale.