Iron Carbonyl Complexes of [2.2.2]Hericene as a Rigid Tris(1,3-diene) Ligand

Hericene is an unusual hexaolefin consisting of three 1,3-diene units located on a rigid bicyclo [2.2.2]octane framework that restricts the geometrical relationships of metal atoms bonded to these olefinic units. In order to explore possible effects of this rigidity limiting metal-metal interaction in polynuclear derivatives possibly stabilizing coordinatively unsaturated species, the structures and energetics of the hericene iron carbonyl complexes (hericene)Fe<i>_m</i>(CO)<i>_n</i> (<i>m</i> = 1, <i>n</i> = 3; <i>m</i> = 2, <i>n</i> = 6, 5; <i>m</i> = 3, <i>n</i> = 9, 8) have been investigated by density functional theory. The lowest-energy (hericene)Fe<i>_m</i>(CO)_3<i>m</i> (<i>m</i> = 1, 2, 3) structures have the cavities of the hericene ligand filled with a single Fe(CO)₃ moiety bonded to a 1,3-diolefin unit. Such species have been synthesized by the reaction of Fe₂(CO)₉ with hericene. For the (hericene)Fe₂(CO)₅ system, the lowest energy structures are singlet structures with an Fe(CO)₃ unit bonded to a 1,3-diene unit in one hericene cavity and an Fe(CO)₂ unit in another hericene cavity bonded to three C=C double bonds from two 1,3-diene units. Higher energy (hericene)Fe₂(CO)₅ structures include a structure in which a single hericene cavity contains a Fe₂(CO)₄(µ-CO) moiety with each iron atom bonded to a 1,3-diene unit. In addition, both singlet and triplet (hericene)Fe₂(CO)₅ structures are found in which an Fe(CO)₃ moiety and an Fe(CO)₂ moiety located in separate hericene cavities are each bonded to a 1,3-diene unit. The lowest-energy (hericene)Fe₃(CO)₈ structures have two hericene cavities containing Fe(CO)₃ moieties fully bonded to 1,3-diene units and a third hericene cavity containing an Fe(CO)₂ moiety fully bonded to a 1,3-diene unit.