Evaluation of Cobalt Complexes Bearing Tridentate Pincer Ligands for Catalytic C–H Borylation

Cobalt­(II)
dichloride complexes supported by a variety of neutral,
tridentate pincer ligands have been prepared and, following in situ
activation with NaBEt3H, evaluated for the catalytic borylation
of 2-methylfuran, 2,6-lutidine, and benzene using both HBPin and B2Pin2 (Pin = pinacolate) as boron sources. Preparation
of well-defined organometallic compounds in combination with stoichiometric
experiments with HBPin and B2Pin2 provided insight
into the nature and kinetic stability of the catalytically relevant
species. In cases where sufficiently electron donating pincers are
present, such as with bis­(phosphino)­pyridine chelates, Co­(III) resting
states are preferred and catalytic C–H borylation is efficient.
Introduction of a redox-active subunit into the pincer reduces its
donating ability and, as a consequence, the accessibility of a Co­(III)
resting state. In these cases, unusual mixed-valent μ-hydride
cobalt complexes have been crystallographically and spectroscopically
characterized. These studies have also shed light on the active species
formed during in situ activated cobalt alkene hydroboration catalysis
and provide important design criteria in base metal catalyzed C–B
bond forming reactions.