Coupling of trifluoromethyl isocyanide ligands in binuclear iron carbonyl complexes

Abstract

The trifluoromethyl isocyanide ligand, CF3NC is of interest in being a strong back-bonding ligand similar to carbon monoxide. Furthermore, Lentz and coworkers have shown that free CF3NC is stable enough in the free state at low temperatures to use as a reagent for the synthesis of metal complexes. In this connection the binuclear (CF3NC)2Fe2(CO)n (n=7, 6, 5) and mononuclear (CF3NC)Fe(CO)n (n=4, 3) have been studied by density functional theory. Coupling of CF3NC ligands to form a bridging η2,η2-μ-CF3NCCNCF3 ligand is predicted to occur in the lowest energy binuclear (CF3NC)2Fe2(CO)n (n=6, 5) structures. A similarly coupled Fe2(CO)7(η2,η2-μ-CF3NCCNCF3) structure is also found for (CF3NC)2Fe2(CO)7. However, this structure lies ∼11kcal/mol in energy above the lowest energy Fe2(CO)6(μ-CO)(μ-CF3CN)2 isomer having separate bridging CF3NC ligands analogous to the well-known triply bridged Fe2(CO)9 structure. The mononuclear derivative (CF3NC)Fe(CO)4 is shown to be similar to Fe(CO)4(CS) in having equatorial and axially substituted trigonal bipyramidal isomers of nearly identical energies within 1kcal/mol. The binuclear (CF3NC)2Fe2(CO)n (n=7, 6) derivatives are predicted to be viable toward dissociation into the mononuclear fragments (CF3NC)Fe(CO)m (m=4 and/or 3) by substantial energies exceeding 29kcal/mol.