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.