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 31 P{1 H} and 1 H 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 (BOCMe2 CMe2 OBpin) ( 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 31 P{1 H} and 1 H NMR spectra as (dippe)Ir(H2 B3 pin3 ) ( 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(H3 B2 pin2 ) ( 8 ), which arose from H2 addition to compound 7 with loss of HBpin. Compound 8 was characterized by 31 P{1 H} and 1 H 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(H4 Bpin) ( 9 ), which was characterized by 31 P{1 H} and 1 H 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 1 H NMR and X-ray crystallography. Compounds 7-10 are in equilibrium when H2 and HBpin are present.