Coupling of trifluoromethyl isocyanide ligands in binuclear iron carbonyl complexes

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 ∼11 kcal/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 1 kcal/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 29 kcal/mol.