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

We report an original alkane elimination approach, entailing the protonolysis of triisobutylaluminum by the acidic hydrides from Cp*IrH ₄ . This strategy allows access to a series of well-defined tri- and tetranuclear iridium aluminum polyhydride clusters, depending on the stoichiometry: [Cp*IrH ₃ Al( i Bu) ₂ ] ₂ ( 1 ), [Cp*IrH ₂ Al( i Bu)] ₂ ( 2 ), [(Cp*IrH ₃ ) ₂ Al( i Bu)] ( 3 ), and [(Cp*IrH ₃ ) ₃ Al] ( 4 ). Contrary to most transition-metal aluminohydride complexes, which can be considered as [AlH _{x +3} ] ^x- aluminates and LnM ⁺ moieties, the situation here is reversed: These complexes have original structures that are best described as [Cp*IrH _x ] ^{n -} 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*IrH ₃ ] ^- moieties are labile and can be transmetalated to yield potassium ([KIrCp*H ₃ ], 8 ) or silver (([AgIrCp*H ₃ ] _n , 10 ) 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 d -electrons of Ir to the Al metal, and both types of interactions take place to some extent in each of these clusters.