Rhodium(III) and Iridium(III) Complexes of a Neopentyl-Substituted PNP Pincer Ligand that Feature Agostic Interactions

The synthesis and characterization of five-coordinate rhodium(III) and iridium(III) complexes of the form [M(PNP-Np)(biph)][BAr^F₄] are described, where PNP-Np is the neopentyl-substituted pincer ligand 2,6-(Np₂PCH₂)₂C₅H₃N (Np = CH₂tBu), biph = 2,2′-biphenyl, and Ar^F = 3,5-(CF₃)₂C₆H₃. These complexes are notable for the adoption of δ-agostic interactions in the solid state, as evidenced by X-ray crystallography (50–150 K) and ATR-IR spectroscopy, but are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the ¹H NMR time scale (185–308 K). The strength of the agostic interactions is discussed with reference to the known tert-butyl-substituted analogues [M(PNP-tBu)(biph)][BAr^F₄], probed by reaction with carbon monoxide, and quantified computationally through NBO analysis, from which the conclusion is that 3-center–2-electron bonding increases in the order M = Ir > Rh (cf. 1.5× greater perturbation energy) and pincer ligand = PNP-Np > PNP-tBu (cf. 3.3× greater perturbation energy).