Inactivity of iridium–triphenylphosphine complexes for catalytic imine hydrogenation: formation of neutral, dihydrido-ortho-metallated species

Abstract

Reactions of cis,trans,cis-[Ir(H)2(PPh3)2(MeOH)2]PF6 in MeOH with the imines Ph(R′)C=NR under 1 atm H2 precipitate the neutral, Ir(III)-dihydrido complexes [Ir(H)2{RN=C(R′)(o-C6H4)}(PPh3)2], where R = alkyl or benzyl, and R′ = Ph, H, or Me; the dihydrides are well characterized through elemental analysis, NMR and IR data and, in one case, with the imine Ph2C=NCH2Ph, an X-ray structure. These products are formed via intermediate ortho-metallated species, exemplified by [Ir(H){PhCH2N=CPh(o-C6H4)}(PPh3)2(MeOH)]PF6 in the case of the Ph2C=NCH2Ph reaction; the intermediate then reacts with H2 (via net heterolytic cleavage: H2 → H− + H+) to give the neutral dihydride and HPF6 as co-product. The ‘unique’ imine PhCH=NPh, under the same conditions, does not form a dihydride and instead is readily catalytically hydrogenated to PhCH2N(H)Ph; remarkably, we have shown previously that this imine is ‘unique’ within the corresponding Rh system in that no catalytic hydrogenation occurs because the amine product ‘poisons’ the Rh centre by coordination through the N-phenyl ring.