Ligand Effects on Migratory Insertion of Carbon Monoxide into Nitridorhenium(V) PNP Alkyl Complexes

The migratory insertion of carbon monoxide into Re–R (R = methyl, Me; benzyl, Bn) bonds of nitridorhenium(V) PNP alkyl complexes bearing tBu 2-PyNHO, (where tBu 2-PyNHO = (Z)-6-((di tert -butylphosphaneyl)methyl)-2-((di tert -butylphosphaneyl)methylene)-1,2-dihydropyridine) and tBu 2,6-Pip, ( tBu 2,6-Pip = 2,6-bis((di tert -butylphosphaneyl)methyl)piperidine) ligands has been systematically investigated. The reaction proceeds to completion under 30 psi CO in toluene- d 8 at 50–100 °C affording Re-acyl products cleanly and quantitatively. Insertion into benzyl complexes is intrinsically faster than methyl analogues, with the tBu 2-PyNHO benzyl ligand accelerating the reaction 23 times relative to its methyl congener and seven times relative to the tBu 2,6-Pip benzyl complex. Variable-temperature 31 P NMR kinetic studies yield highly negative entropies of activation (Δ S ‡ = −42.6 to −49.4 cal mol –1 K –1 ), supporting a bimolecular, ordered transition state. DFT calculations (M06-D3) corroborate a concerted insertion mechanism without the formation of a stable CO adduct. NBO analysis shows enhanced charge polarization involving the phosphorus donors in the tBu 2-PyNHO benzyl complex. Further EDA analyses revealed a more robust CDA mixing stabilizing tBu 2,6-Pip complex presenting a hard to perturb geometry toward acyl formation. This work provides clear evidence that ligand architecture modulates electronics around the metal affecting the CO insertion barrier toward acyl formation.