Three-Center M–H–B Bonds Are Strong Field Interactions. Synthesis and Characterization of M(CH2NMe2BH3)3 Complexes of Titanium, Chromium, and Cobalt

We describe new compounds of stoichiometry M(CH2NMe2BH3)3 (M = Ti, Cr, and Co),
each of
which contains three chelating boranatodimethylaminomethyl (BDAM)
ligands. In all three compounds, the BDAM anion, which is isoelectronic
and isostructural with the neopentyl group, is bound to the metal
center at one end by a metal–carbon σ bond and at the
other by one three-center M–H–B interaction. The crystal
structures show that the d1 titanium(III) compound is trigonal
prismatic (or eight-coordinate, if two longer-ranged M···H
interactions with the BH3 groups are included), whereas
the d3 chromium(III) compound and the d6 cobalt(III)
compounds are both fac-octahedral. The Cr and Co
compounds exhibit two rapid dynamic processes in solution: exchange
between the Δ and Λ enantiomers and exchange of the terminal
and bridging hydrogen atoms on boron. For the Co complex, the barrier
for Δ/Λ exchange (ΔG⧧298 = 10.1 kcal mol–1) is significantly
smaller than those seen in other octahedral cobalt(III) compounds;
DFT calculations suggest that Bailar twist and dissociative pathways
for Δ/Λ exchange are both possible mechanisms. The UV–vis
absorption spectra of the cobalt(III) and chromium(III) species show
that the ligand field splittings Δo caused by the
M–H–B interactions are unexpectedly large, thus placing
them high on the spectrochemical series (near ammonia and alkyl groups);
their nephelauxetic effect is also large. The DFT calculations suggest
that these properties of M–H–B interactions are in part
a consequence of their three-center nature, which delocalizes electron
density away from the metal center and reduces electron–electron
repulsions.

本描述了一种新的化合物系列，其化学式为M(CH2NMe2BH3)3（M分别为Ti、Cr和Co），每个化合物均含有三个螯合硼基二甲基氨基甲基（BDAM）配体。在这三种化合物中，BDAM阴离子，其电子构型和结构均与新戊基组相同，一端通过金属-碳σ键与金属中心结合，另一端则通过一个三中心M-H-B相互作用与之相连。晶体结构表明，d1钛(III)化合物呈三角棱柱形（若考虑与BH3基团的两个较长范围的M···H相互作用，则为八配位），而d3铬(III)化合物和d6钴(III)化合物均为正八面体结构。铬和钴化合物在溶液中表现出两种快速动态过程：Δ和Λ外消旋体的交换，以及硼上端位和桥位氢原子的交换。对于钴配合物，Δ/Λ交换的能垒（ΔG⧧298 = 10.1 kcal mol–1）明显低于其他八面体钴(III)化合物；DFT计算表明，Bailar扭曲和解离途径都是可能的交换机制。钴(III)和铬(III)物种的紫外-可见吸收光谱显示，由M-H-B相互作用引起的配位场分裂Δo出人意料地大，从而将它们置于光谱化学系列的高位（接近氨和烷基）；它们的去溶剂化效应亦显著。DFT计算表明，这些M-H-B相互作用的特性部分归因于其三中心特性，这种特性将电子密度从金属中心分散出去，并减少了电子-电子排斥。