Comparing the Acidity of (R₃P)₂BH-Based Donor Groups in Iridium Pincer Complexes

In the current manuscript, we describe the reactivity of a series of iridium(III) pincer complexes with the general formulae [(PEP)IrCl(CO)(H)]ⁿ (n = +1, +2) towards base, where PEP is a pincer-type ligand with different central donor groups, and E is the ligating atom of this group (E = B, C, N). The donor groups encompass a secondary amine, a phosphine-stabilised borylene and a protonated carbodiphosphorane. As all ligating atoms E exhibit an E−H bond, we addressed the question of wether the coordinated donor group can be deprotonated in competition to the reductive elimination of HCl from the iridium(III) centre. Based on experimental and quantum chemical investigations, it is shown that the ability for deprotonation of the coordinated ligand decreases in the order of (R₃P)₂CH⁺ > R₂NH > (R₃P)₂BH. The initial product of the reductive elimination of HCl from [(PBP)IrCl(CO)(H)]ⁿ (1c), the square planar iridium(I) complex, [(PBP)Ir(CO)]⁺ (3c), was found to be unstable and further reacts to [(PBP)Ir(CO)₂]⁺ (5c). Comparing the C−O stretching vibrations of the latter with those of related complexes, it is demonstrated that neutral ligands based on tricoordinate boron are very strong donors.