B–H Bond Cleavage via Metal–Ligand Cooperation by Dearomatized Ruthenium Pincer Complexes

Organic derivatives of boronic acid
are widely used reagents useful in various synthetic applications.
A fundamental understanding and the exploration of new reaction pathways
of boronic reagents with organometallic systems hold promise for useful
advancement in chemical catalysis. Herein we present the reactions
of simple boranes with dearomatized ruthenium pincer complexes based
on PNP (2,6-bis­(di-tert-butylphosphinomethyl)­pyridine)
or PNN (2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)­pyridine)
ligands. NMR studies revealed dehydrogenative addition of the borane
B–H bond across the metal center and the ligand. Remarkably,
new complexes were observed, which contain the boryl moiety at the
benzylic carbon of the pincer ligand arm. X-ray crystal structures
of new dearomatized boryl pincer complexes were obtained, and DFT
calculations revealed mechanistic details of the adduct formation
process through a dehydrogenative pathway. In addition, catalytic
aryl–boron coupling reactions were explored. The new boryl
pincer systems may possibly be useful in future postmodification techniques
for ruthenium pincer complexes, as well as in catalytic B–B
and B–C coupling reactions.