σ-Borane Complexes of Iridium: Synthesis and Structural Characterization

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