Theoretical study of the CO migratory insertion reactions of Pt(Me)(OMe)(dppe) and Ni(Me)(OR)(bpy) (R = Me, O-p-C₆H₄CN): Comparison of group 10 metal-Alkyl, -alkoxide, and -aryloxide bonds

We report the results of theoretical mechanistic studies on the alternative migratory insertion reactions of CO with the metal-oxygen and metal-carbon bonds of Pt(Me)(OMe)(dhpe) (dhpe = H₂PCH₂CH₂PH₂) and Ni(Me)(OR)(α-diimine) (R = Me, Ph, α-diimine = NH=CHCH=NH) as models for Pt(Me)(OMe)(dppe) (dppe = Ph₂PCH₂CH₂PPh₂) and Ni(Me)(O-p-C₆H₄CN)(bpy) (bpy = 2,2′-bipyridyl), respectively. With Pt(Me)(OMe)(dhpe) the methoxycarbonyl product, Pt(Me)(CO₂Me)(dhpe), is favored over the acyl alternative, Pt{C(O)Me}(OMe)(dhpe), by 13 kcal.mol. Two alternative pathways for methoxycarbonyl formation were located, both of which are initiated via displacement of a chelate arm to form two isomers of Pt(Me)(OMe)(CO)(η¹-H₂PCH₂CH₂ PH₂) (2a, CO trans to OMe; 2b, CO trans to Me). Subsequent CO migratory insertion into the Pt-OMe bond of 2b yields the methoxycarbonyl product directly. Alternatively, isomerization of 2a to a third isomer, 2c (CO trans to phosphine), can occur, from which CO migratory insertion again produces the methoxycarbonyl species. This latter isomerization/migratory insertion process represents the lowest energy pathway. Alternative CO migratory insertion reactions involving the Pt-Me bonds of 2a,c suffer from very high activation barriers. The 2a to 2c isomerization is unusual, as it involves transfer of OMe to phosphine to give a metallophosphorane intermediate, followed by OMe transfer back to the metal. The net result is a swapping of the positions of the OMe and phosphine ligands. The computed kinetic and thermodynamic preference for reaction with the Pt-OMe bond is consistent with the observed reactivity of Pt(Me)(OMe)(dppe). With the Ni(Me)(OR)(α-diimine) systems CO migratory insertion proceeds via five-coordinate CO adducts. When R = Me, insertion into the Ni-OMe bond is more accessible kinetically but the acyl product is slightly more stable by 3.5 kcal/mol. Introduction of the Ph substituent dramatically lowers the reactivity of the Ni-OR bond, with the acyl becoming the kinetically more accessible species and being 18.4 kcal/mol more stable than the phenoxycarbonyl alternative. The lower reactivity of the Ni-OPh bond arises primarily from the weak C-O bond in the phenoxycarbonyl product and accounts for the experimental preference for acyl formation in the reaction of Ni(Me)(O-p-C₆H₄CN)(bpy) with CO.