From Metallaborane to Borylene Complexes: Syntheses and Structures of Triply Bridged Ruthenium and Tantalum Borylene Complexes

Reaction of [1,2-(Cp*RuH)(2)B3H7] (1; Cp*=eta(5)-C5Me5) with [Mo(CO)(3)(CH3CN)(3)] yielded arachno-[(Cp*RuCO)(2)B2H6] (2), which exhibits a butterfly structure, reminiscent of 7 sep B4H10. Compound 2 was found to be a very good precursor for the generation of bridged borylene species. Mild pyrolysis of 2 with [Fe-2(CO)(9)] yielded a triply bridged heterotrinuclear borylene complex [(mu(3)-BH)(Cp*RuCO)(2)(mu-CO){Fe(CO)(3)}] (3) and bis-borylene complexes [{(mu(3)-BH)-(Cp*Ru)(mu-CO)}(2)Fe-2(CO)(5)] (4) and [{(mu(3)-BH)(Cp*Ru)Fe(CO)(3)}(2)(mu-CO)] (5). In a similar fashion, pyrolysis of 2 with [Mn-2(CO)(10)] permits the isolation of mu(3)-borylene complex [(mu(3)-BH)-(Cp*RuCO)(2)(mu-H)(mu-CO){Mn(CO)(3)}] (6). Both compounds 3 and 6 have a trigonal-pyramidal geometry with the mu(3)-BH ligand occupying the apical vertex, whereas 4 and 5 can be viewed as bicapped tetrahedra, with two mu(3)-borylene ligands occupying the capping position. The synthesis of tantalum borylene complex [(mu(3)-BH)(Cp*TaCO)(2)(mu-CO){Fe(CO}(3)] (7) was achieved by the reaction of [(Cp*Ta)(2)B4H8(mu-BH4)] at ambient temperature with [Fe-2(CO)(9)]. Compounds 2-7 have been isolated in modest yield as yellow to red crystalline solids. All the new compounds have been characterized in solution by mass spectrometry; IR spectroscopy; and H-1, B-11, and C-13 NMR spectroscopy and the structural types were unequivocally established by crystallographic analysis of 2-6.