posted on 2023-10-18, 16:40authored byR. Joseph Lastowski, Jonathan T. Yarranton, Lingyang Zhu, Konstantinos D. Vogiatzis, Gregory S. Girolami
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.