Migration Tendencies of Group 14 Element Ligands in the Coordination Sphere of Cationic Phosphenium Iron Complexes

Abstract

Structural and bonding features of the model iron−phosphenium cation complexes of the type (E = group 14 element; R = H and CH3) as well as the migration of the EH2R group from iron to phosphorus (1,2-migration) and that of a CH3 group from E to phosphorus (1,3-migration) have been studied using the hybrid DFT-B3LYP method. The remarkable stability of these complexes is due to the conjugation around P, which can be partitioned into N−P+−N lone pair π-conjugation and the Fe−P+ π bond. Both together contribute 61.9 kcal/mol to its stability. The calculations suggest that all the 1,2-migrations are feasible reactions and the CH2R migration has the highest tendency because of its small activation barrier (1.3 kcal/mol) and the exothermicity. However, the charge distribution on the structures involved in these reactions points out that external factors such as solvent and the counteranion can greatly influence the courses of the reactions. Compared to the 1,2-migrations, the 1,3-migrations of a methyl group from C, Si, and Sn need much higher activation energies (51.7, 42.7, and 32.7 kcal/mol, respectively), and all are endothermic. A rationalization for the reaction energies and the activation barriers is obtained by bond strength analysis. The bond strengths obtained in this study for C−C, C−Si, C−Sn, P−C, P−Si, P−Sn, Fe−C, Fe−Si, Fe−Sn, FeC, FeSi, and FeSn are 90.4, 86.6, 69.6, 93.7, 81.4, 72.1, 31.8, 41.7, 36.3, 77.4, 82.3, and 69.9 kcal/mol, respectively. Further, in the presence of a water molecule as a model base the 1,3-methyl migrations from Si to P and Sn to P become exothermic, and their activation barriers were lowered to a significant amount. An explanation for this behavior is given based on the Fisher-type and Schrock-type metal−ligand double-bond formation in these complexes. When E = Sn and R = CH3, a realistic base molecule could favor the 1,3-migration over the 1,2-migration, consistent with the experimental facts.