Highly Active Iridium Catalysts for Alkane Dehydrogenation. Synthesis and Properties of Iridium Bis(phosphine) Pincer Complexes Based on Ferrocene and Ruthenocene

Novel bis(phosphine) pincer complexes of iridium based on ferrocene and ruthenocene have been
synthesized. The reaction of 1,3-bis((di-tert-butylphosphino)methyl)metallocenes [t-BuP,CH,PM] (M =
Fe, 10; M = Ru, 13) with [Ir(COE)2Cl]2 in refluxing toluene leads to the corresponding chloro-hydrido
pincer complexes as a mixture of H-endo- (M = Fe, 11; M = Ru, 14) and H-exo-IrH(Cl)[t-BuP,C,PM] (M
= Fe, 12; M = Ru, 15) isomers. Treatment of compounds 11, 12 and 14, 15 with NaH in hot cyclooctane
generates corresponding dihydrido complexes IrH2[t-BuP,C,PM] (M = Fe, 6; M = Ru, 7). Reactivity of
the chelated iridium atom in hydrido-iridium pincer complexes and oxidation centered at the iron atom
of ferrocene-based pincer complexes were studied. Complexes H-exo-IrH(Cl)(CO)[t-BuP,C,PFe] (20),
Ir(CO)[t-BuP,C,PFe] (21), and Ir(CO)[2,6-(tBu2PO)2C6H3] (30) were characterized by X-ray crystallography.
On the basis of νCO values for carbonyl iridium derivatives, electronic characteristics of the corresponding
fragments {Ir[t-BuP,C,PM]}n (n = 0, +1) were elucidated. Complexes 6 and 7 reveal unprecedented catalytic
activity in cyclooctane dehydrogenation in the presence of tert-butylethylene as a hydrogen acceptor: 
turnover numbers 3300, 2571, and 1843 were obtained for 6, 7, and the known complex IrH2[2,6-(tBu2PO)2C6H3] (4c), respectively, at 180 °C for 8 h. The high catalytic activity for 6 and 7 and the difference
in the catalytic activity of these “isostructural” complexes are discussed in terms of steric and electronic
effects.