Trimetallic μ3-Borylene Complexes of Group 8 Metals.

Trimetallic μ3-borylene complexes comprising first row transition metals have been synthesized and structurally characterized. Thermolysis of [Cp*Os(PPh3)2Br] (1; Cp* = η5-C5Me5) with [Fe3(CO)12] in the presence of [BH3·THF] yielded [{Fe(CO)3}3(μ-H)2(μ-CO)(μ3-BH)] (2) and [Cp*Os(PPh3)2H] (3). Complex 2 is a tetrahedral trimetallic iron complex comprising a triply bridging borylene ligand that is a part of the tetrahedral core. In a different synthetic route, we have carried out the metathesis of [Cp*OsBr2]2 (4) with [LiBH4·THF] followed by thermolysis with [Fe3(CO)12] that led to the formation of [(Cp*Os)2{Fe(CO)3}(μ-CO)3(μ3-BH)] (5) and [(Cp*Os)2{Fe(CO)3}B5H5] (6). Complex 5 is a tetrahedral heterotrimetallic species analogous to that of 2 involving a μ3-borylene component. A similar reaction of 4 using [Co2(CO)8] yielded [(Cp*Os)2{Co(CO)2}(μ-CO)B4H7] (7). Complexes 6 and 7 are metal-rich metallaboranes having closo bicapped octahedral and nido monocapped pentagonal pyramidal cores, respectively. All the synthesized molecules were characterized using multinuclear NMR and IR spectroscopy, mass spectrometry and single crystal X-ray diffraction analyses. Theoretical calculations were carried out to gain insight into the electronic structures and bonding of complexes 2 and 5-7.