First-Row Transition-Metal-Diborane and -Borylene Complexes

A combined experimental and quantum chemical study of Group 7 borane, trimetallic triply bridged borylene and boride complexes has been undertaken. Treatment of [{Cp∗CoCl}2] (Cp∗=1,2,3,4,5-pentamethylcyclopentadienyl) with LiBH4·thf at -78°C, followed by room-temperature reaction with three equivalents of [Mn2(CO)10] yielded a manganese hexahydridodiborate compound [{(OC)4Mn}(η6-B2H6){Mn(CO)3}2(μ-H)] (1) and a triply bridged borylene complex [(μ3-BH)(Cp∗Co)2(μ-CO)(μ-H)2MnH(CO)3] (2). In a similar fashion, [Re2(CO)10] generated [(μ3-BH)(Cp∗Co)2(μ-CO)(μ-H)2ReH(CO)3] (3) and [(μ3-BH)(Cp∗Co)2(μ-CO)2(μ-H)Co(CO)3] (4) in modest yields. In contrast, [Ru3(CO)12] under similar reaction conditions yielded a heterometallic semi-interstitial boride cluster [(Cp∗Co)(μ-H)3Ru3(CO)9B] (5). The solid-state X-ray structure of compound 1 shows a significantly shorter boron-boron bond length. The detailed spectroscopic data of 1 and the unusual structural and bonding features have been described. All the complexes have been characterized by using 1H, 11B, 13C NMR spectroscopy, mass spectrometry, and X-ray diffraction analysis. The DFT computations were used to shed light on the bonding and electronic structures of these new compounds. The study reveals a dominant B-H-Mn, a weak B-B-Mn interaction, and an enhanced B-B bonding in 1.