Synthesis of Iridaboratranes Bearing Phosphine-Tethered Borane: Reversible CO/PR₃ (R = Me, OMe, OEt) Substitution Reactions Induced by a σ-Electron-Acceptor Borane Ligand

The iridaboratrane [{o-(Ph2P)­C6H4}3B]­IrH­(CO) (1-Ir), bearing phosphine-tethered borane, was synthesized via phosphine ligand exchange between the tris­(triphenylphosphine) carbonyl hydride IrH­(CO)­(PPh3)3 (2-Ir) and the tris­(phosphine)­borane {o-(Ph2P)­C6H4}3B (3). 1-Ir was fully characterized on the basis of its 1H, 11B, and 31P NMR spectra, X-ray diffraction analysis, and elemental analysis. Density functional theory calculations revealed the important properties of the σ-acceptor borane ligand that led to its unique electron distribution in 1-Ir. The borane ligand extracts a significant amount of electron density from the iridium center, but the iridium center maintains an electron density similar to that of the boron-free compound 2-Ir by decreasing π back-donation from Ir to CO and strengthening the donation from the phosphorus atom (or by weakening the dmetal–σ*P–R interaction). The properties of the borane ligand can promote the reversible CO/PR3 (R = Me, OMe, OEt) substitution reaction.