Structure and coordinate bonding nature of the manganese–σ–borane complexes

Stretched σ-borane complexes of rhodium: A theoretical study

“…The optimized structure of 1Rh which contains almost similar HBpin ligands, differs from the optimized geometry of [(g 5 tions (1.19 Å ) and those obtained by theoretical study for the free borane ligands (1.190 Å in HBpin). The B1-Rh-H1 bond angles 55.6°and 52.6°in 1Rh and 2Rh, respectively and B2-Rh-H2 bond angle in 1Rh are larger than the B-M-H bond angles in r-borane complexes (37-41°) [9,22,23] and are consistent with preserving some B-H bond character ( Fig. 2A).…”

“…Mechanistic studies have revealed that r-borane complexes can be intermediates in catalytic hydroboration and borylation reactions [13,[19][20][21]. Theoretical calculations have been performed for r-borane complexes of titanium [2,20], manganese [22], rhenium [23] and ruthenium [9].…”

“…Hartwig relative stabilities of r-borane and hydride boryl complexes such as behavior of the ancillary ligands, substitutions on the boron atom and the replacement of one metal center by other. The Lewis acidity of the borane ligand affects its bonding nature to the metal center [9,22,23]. The question arises as to whether there may be stable structures, which are intermediate between r-borane and hydride boryl.…”

Mechanism‐Based Enantiodivergence in Manganese Reduction Catalysis: A Chiral Pincer Complex for the Highly Enantioselective Hydroboration of Ketones

“…Elimination of the boronic ester finally regenerates the catalyst (step d). Thetendencyof manganese borane adducts to undergo bridging is welldocumented for carbonyl and cyclopentadienyl complexes, [21] as is the analogous affinity of the low-coordinate highly thermally unstable p-acceptor free manganese hydrides to undergo dimerization. [22] Therefore,w ee xpect the boranemanganese adduct to be ak ey intermediate in the hydroboration cycle.T his pathway is significantly faster than the corresponding direct migration of the hydrogen from the manganese hydride (Scheme 5, right), thus allowing for excellent enantiocontrol in the hydroboration at sufficiently high pinacol borane loadings and in the absence of strongly coordinating co-ligands.H owever,o ur results also indicate that the direct hydride transfer is favored for manganesecatalyzed hydrosilylations,t hus following ac atalytic cycle similar to the iron-catalyzed process.…”