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)][BArF4] are described, where PNP-Np is the neopentyl-substituted pincer ligand 2,6-(Np2PCH2)2C5H3N (Np = CH2tBu), biph = 2,2′-biphenyl, and ArF = 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)][BArF4], 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).