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(CO) ( = 1, = 3; = 2, = 6, 5; = 3, = 9, 8) have been investigated by density functional theory. The lowest-energy (hericene)Fe(CO) ( = 1, 2, 3) structures have the cavities of the hericene ligand filled with a single Fe(CO) moiety bonded to a 1,3-diolefin unit. Such species have been synthesized by the reaction of Fe(CO) with hericene. For the (hericene)Fe(CO) system, the lowest energy structures are singlet structures with an Fe(CO) unit bonded to a 1,3-diene unit in one hericene cavity and an Fe(CO) unit in another hericene cavity bonded to three C=C double bonds from two 1,3-diene units. Higher energy (hericene)Fe(CO) structures include a structure in which a single hericene cavity contains a Fe(CO)(µ-CO) moiety with each iron atom bonded to a 1,3-diene unit. In addition, both singlet and triplet (hericene)Fe(CO) structures are found in which an Fe(CO) moiety and an Fe(CO) moiety located in separate hericene cavities are each bonded to a 1,3-diene unit. The lowest-energy (hericene)Fe(CO) structures have two hericene cavities containing Fe(CO) moieties fully bonded to 1,3-diene units and a third hericene cavity containing an Fe(CO) moiety fully bonded to a 1,3-diene unit.