Rational Preparation of Well-Defined Multinuclear Iridium–Aluminum Polyhydride Clusters and Comparative Reactivity

Abstract

We report an original alkane elimination approach, entailing the protonolysis of triisobutylaluminum by the acidic hydrides from Cp*IrH₄. This strategy allows access to a series of well-defined tri- and tetranuclear iridium aluminum polyhydride clusters, depending on the stoichiometry: [Cp*IrH₃Al(<i>i</i>Bu)₂]₂ (<b>1</b>), [Cp*IrH₂Al(<i>i</i>Bu)]₂ (<b>2</b>), [(Cp*IrH₃)₂Al(<i>i</i>Bu)] (<b>3</b>), and [(Cp*IrH₃)₃Al] (<b>4</b>). Contrary to most transition-metal aluminohydride complexes, which can be considered as [AlH_<i>x</i>+3]^x- aluminates and LnM⁺ moieties, the situation here is reversed: These complexes have original structures that are best described as [Cp*IrH_<i>x</i>]^<i>n</i>- iridate units surrounding cationic Al(III) fragments. This is corroborated by reactivity studies, which show that the hydrides are always retained at the iridium sites and that the [Cp*IrH₃]^- moieties are labile and can be transmetalated to yield potassium ([KIrCp*H₃], <b>8</b>) or silver (([AgIrCp*H₃]_<i>n</i>, <b>10</b>) derivatives of potential synthetic interest. DFT calculations show that the bonding situation can vary in these systems, from 3-center 2-electron hydride-bridged Lewis adducts of the form Ir-H⇀Al to direct polarized metal-metal interaction from donation of <i>d</i>-electrons of Ir to the Al metal, and both types of interactions take place to some extent in each of these clusters.