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 tBu2-PyNHO, (where tBu2-PyNHO = (Z)-6-((ditert-butylphosphaneyl)methyl)-2-((ditert-butylphosphaneyl)methylene)-1,2-dihydropyridine) and tBu2,6-Pip, (tBu2,6-Pip = 2,6-bis((ditert-butylphosphaneyl)methyl)piperidine) ligands has been systematically
investigated. The reaction proceeds to completion under 30 psi CO
in toluene-d8 at 50–100 °C
affording Re-acyl products cleanly and quantitatively. Insertion into
benzyl complexes is intrinsically faster than methyl analogues, with
the tBu2-PyNHO benzyl ligand accelerating the reaction
23 times relative to its methyl congener and seven times relative
to the tBu2,6-Pip benzyl complex. Variable-temperature 31P NMR kinetic studies yield highly negative entropies of
activation (ΔS‡ = −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 tBu2-PyNHO benzyl
complex. Further EDA analyses revealed a more robust CDA mixing stabilizing tBu2,6-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.