Reversible Borylene Formation from Ring Opening of Pinacolborane and Other Intermediates Generated from Five-Coordinate Tris-Boryl Complexes: Implications for Catalytic C–H Borylation

Catalytic C–H borylation using
the five-coordinate tris-boryl
complex (dippe)­Ir­(Bpin)3 (5a, dippe = 1,2-bis­(diisopropylphosphino)­ethane)
has been examined using 31P­{1H} and 1H NMR spectroscopy. Compound 5a was shown to react rapidly
and reversibly with HBpin to generate a six-coordinate borylene complex,
(dippe)­Ir­(H)­(Bpin)2(BOCMe2CMe2OBpin)
(6), whose structure was confirmed by X-ray crystallography.
Under catalytic conditions, the H2 generated from C–H
borylation converted compound 6 to a series of intermediates.
The first is tentatively assigned from 31P­{1H} and 1H NMR spectra as (dippe)­Ir­(H2B3pin3) (7), which is the product of
formal H2 addition to compound 5a. As catalysis
progressed, compound 7 was converted to a new species
with the formula (dippe)­Ir­(H3B2pin2) (8), which arose from H2 addition to compound 7 with loss of HBpin. Compound 8 was characterized
by 31P­{1H} and 1H NMR spectroscopy,
and its structure was confirmed by X-ray crystallography, where two
molecules with different ligand orientations were found in the unit
cell. DFT calculations support the formulation of compound 8 as an IrIII agostic borane complex, (dippe)­IrH2(Bpin)­(η2-HBpin). Compound 8 was gradually
converted to (dippe)­Ir­(H4Bpin) (9), which
was characterized by 31P­{1H} and 1H NMR spectroscopy and X-ray crystallography. DFT calculations favor
its formulation as an agostic borane complex of IrIII with
the formula (dippe)­IrH3(η2-HBpin). Compound 9 reacted further with H2 to afford the dimeric
structure [(dippe)­IrH2(μ2-H)]2 (10), which was characterized by 1H NMR
and X-ray crystallography. Compounds 7–10 are in equilibrium when H2 and HBpin are present.