Structures and Bonding of Early Transition Metallaborane Clusters

Structures and bonding of various homo- and heterometallic metallaborane clusters are described, which are stabilized in the coordination sphere of early transition metals. For example, the bimetallic triborane species [{Cp*Mo(CO)}2{μ–η3:η3-B3H7}], 2, has been synthesized from the pyrolysis of an in situ generated intermediate, produced from the reaction of [Cp*Mo(CO)3Me], 1, and [LiBH4·THF] with an excess amount of [BH3·THF]. Superficially cluster 2 is isostructural but not isoelectronic with [(Cp*MoCl)2B3H7] (I). Both clusters 2 and I contain a bimetallic template bridged by {B3H7} moiety. Cluster 2 is denoted as a unique saturated and unsubstituted closo bimetallic B3H7 cluster. Theoretical output implies that saturation in such clusters brings thermodynamic stability. Further, we have developed a new strategy for the synthesis of various heterometallic metal-rich metallaborane clusters. For example, pyrolysis of an intermediate, obtained from the reaction of [Cp*MoCl4], 4, and [LiBH4·THF] with cobalt and iron carbonyl compounds yielded triply bridging hydrido(hydroborylene), [{Cp*Mo(CO)2}2Co(CO)3BH(μ-H)], 5, tetrametallic μ4-boride, [{Cp*Mo(CO)2}2(μ4-B)(μ-H){Co2(CO)5}], 6, triple decker complex, [(Cp*Mo)2{μ–η6:η6-B4H4Co2(CO)5}(μ-H)2], 7, metal-rich dimolybdaborane cluster, [(Cp*Mo)2Co2 (CO)3(μ-CO)3B3H3(μ-H)2], 8, and mixed-metal cluster, [(Cp*Mo)2B4H8Fe(CO)3], 9. All the molecules have been characterized by 1H, 11B{1H}, and 13C{1H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray diffraction studies for clusters 2 and 5–9. The electron counting rules and density functional theory (DFT) calculations provided further insight into the bonding and electronic structures of these clusters.