Bulky Terphenyl Phosphines Stabilize Otherwise Highly Reactive Iridium Fragments, Key in C-H Activation Reactions

The photogenerated Ir(I) compound [Cp*Ir(PMe3)] and its Ir(III) derivative [Cp*IrMe(PMe3)(solvent)]+ [Cp* = η5-(C5Me5)] have tamed the field of C-H bond activation chemistry; however, analogues using bulkier phosphines have been barely investigated. We report the synthesis and characterization of their congested versions using sterically demanding terphenyl (C6H3-2,6-Ar2) phosphine ligands. Two Ir(I) species, [Cp*Ir(PMe2ArDtbp2)] (3, ArDtbp2 = C6H3-2,6-(C6H3-3,5-tBu2)2) and [Cp*Ir(PMe2ArDipp2)] (3', ArDipp2 = C6H3-2,6-(C6H3-2,6-iPr2)2), were isolated and structurally authenticated, revealing strong metal-arene interactions that confer remarkable stability to these otherwise highly unsaturated species. Despite their apparent similarity, these complexes exhibit sharply contrasting reactivity toward methyl triflate: only the PMe2ArDtbp2 derivative undergoes clean formal oxidative addition to form the targeted cationic methyl complex [Cp*IrMe(PMe2ArDtbp2)]+ (4). Computational studies rationalize this divergent behavior in terms of steric effects beyond conventional metrics. Besides, compound 4 displays rich chemistry, including intramolecular activation of a tert-butyl group and selective intermolecular functionalization of 1,2-difluorobenzene, as well as reactions with H2 and phenylsilane. These findings evince the role of terphenyl phosphines in stabilizing reactive iridium fragments and highlight their potential for selective C-H activation under sterically congested environments.