Characterization, redox properties and structures of the iron nitridocarbonyl clusters [Fe4N(CO)11{PPh(C5H4FeC5H5)2}]–, [Fe6N(CO)15]3– and [Fe6H(N)(CO)15]2–

Abstract

The redox condensation of [Fe₂(CO)₈]^2– with [Fe₄N(CO)₁₂]^– yielded the cluster [Fe₆N(CO)₁₅]^3–. Single-crystal X-ray analysis showed it to possess an octahedral metal cage, with an interstitial nitride ligand. Under the D₃ idealized symmetry, all iron vertices are equivalent, being bound to one edge-bridging and two terminal carbonyls. The ion [Fe₆N(CO)₁₅]^3– can be oxidized to [Fe₅N(CO)₁₄]^– or protonated to the hydridic dianion [Fe₆H(N)(CO)₁₅]^2–. The molecular structure of the latter was determined, and is strikingly similar to that of the parent trianion. Small deformations of the ligand shell or elongations of the Fe–Fe distances are not sufficient to determine the location of the hydride. Electrochemical experiments were consistent with the chemical findings, showing that [Fe₆N(CO)₁₅]^3– undergoes three irreversible one-electron oxidation steps, ultimately generating [Fe₅N(CO)₁₄]^–. A lifetime of about 15 s was evaluated for the transient radical [Fe₆N(CO)₁₅]^2–. Thermal activation induces substitution of one carbonyl ligand of [Fe₄N(CO)₁₂]^– by PPh(C₅H₄FeC₅H₅)₂, yielding [Fe₄N(CO)₁₁{PPh(C₅H₄FeC₅H₅)₂}]^–, the molecular structure of which was also determined. The cluster adopts a butterfly arrangement of iron atoms, having an exposed µ₄-N atom and the phosphine ligand at a wingtip position. Cyclic voltammetry showed that communication between the two ferrocenyl units of the ligand PPh(C₅H₄FeC₅H₅)₂ is rather low in the free state, and is notably improved by co-ordination to the tetrairon cluster. The ¹⁵N-labelled [Fe₆N(CO)₁₅]^3– and [Fe₆H(N)(CO)₁₅]^2– complexes were synthesized, and the NMR chemical shifts and IR bands of the interstitial µ₆-N ligands measured.