Phosphaethynolate Dimerization and Carbonyl Migration in Cyclopentadienyliron Carbonyl Systems: A Theoretical Study

Successive decarbonylation and cyclodimerization of the mono- and binuclear FeCp­(PCO)­(CO)2, Fe2Cp2(PCO)2(CO)4, and Fe2Cp2(CO)2(P2) complexes have been investigated using density functional theory calculations at the M06L/DZP level. For the mononuclear complexes, the lowest energy FeCp­(PCO)­(CO)m (m = 2, 1) structures always prefer the η-P­(CO) bent configuration relative to other models. However, the lowest energy structure for the species of stoichiometry FeCp­(PCO) is actually FeCp­(P)­(CO) with a formal FeP triple bond. The binuclear complexes are much more complicated. The Fe2Cp2(PCO)2(CO)4 structure, highly entropy controlled, has a P2C2 ring originating from cyclodimerization of two PCO groups from two FeCp­(PCO)­(CO)2 monomers. The lowest energy Fe2Cp2(PCO)2(CO)3 structure has a μ-P­(CO) group donating two lone-pair electrons to each iron atom in a nonbonded Fe···Fe unit. The most favorable structure for the species of stoichiometry Fe2Cp2(PCO)2(CO)2 has an end-to-end diphosphene P2 bridge connecting two FeCp­(CO) fragments in a trans arrangement. The lowest energy Fe2Cp2(PCO)2(CO) structure has μ-P­(P) and μ-C­(O) groups bridging an Fe–Fe single bond. The lowest energy structures for the species of stoichiometries Fe2Cp2(P2)­(CO)2 and Fe2Cp2(P2)­(CO) have a central P2Fe2 tetrahedron as well as two or one CO group(s) bridging Fe–Fe or FeFe bonds. The lowest energy carbonyl-free Fe2Cp2P2 structure has a rhombic Fe2P2 core with no direct P–P bond.