σ-Borane Complexes of Iridium: Synthesis and Structural Characterization

Reaction of NaBH₄ with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH₂(BH₃) (1) (tBuPOCOP = κ³-C₆H₃-1,3-[OP(tBu)₂]₂). The structure of 1 determined from neutron diffraction data contains a B−H σ-bond to iridium with an elongated B−H bond distance of 1.45(5) Å. Compound 1 crystallizes in the space group P1̅ (Z = 2) with a = 8.262 (5) Å, b = 12.264 (5) Å, c = 13.394 (4) Å, and V = 1256.2 (1) ų (30 K). Complex 1 can also be prepared by reaction of BH₃·THF with (tBuPOCOP)IrH₂. Reaction of (tBuPOCOP)IrH₂ with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H₂ at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a σ-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) Å, b = 11.1233 (2) Å, c = 14.6122 (3) Å, and V = 6700.21 (19) ų (130 K). Reaction of (tBuPOCOP)IrH₂ with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays ¹H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH₂ in THF solutions.