Trinuclear iron carbonyl complexes of the non-benzenoid tricyclic hydrocarbon aceheptylene: Relationship to heptalene metal complexes

Abstract

Aceheptylene as its dimethyl derivative is the most readily available tricyclic non-benzenoid aromatic hydrocarbon. This dimethyl derivative has been used to synthesize the trinuclear iron carbonyl complex (C14H8Me2)Fe3(CO)8. Theoretical studies on the unsubstituted complex C14H10Fe3(CO)8 show the experimental structure to be the lowest energy structure. Related structures having the aceheptylene ligand partitioned into an azulene subunit bonded to an Fe2(CO)5 moiety and a cis 1,3-diene subunit bonded to an Fe(CO)3 moiety are the lowest energy C14H10Fe3(CO)8 structures. Higher energy C14H10Fe3(CO)8 structures with the aceheptylene ligand partitioned into two equivalent outer cis 1,3-diene subunits bonded to Fe(CO)3 moieties and a central fulvene subunit bonded to an Fe(CO)2 moiety are also found. Low energy structures of the C14H10Fe3(CO)9 precursor to C14H10Fe3(CO)8 are found with three separate Fe(CO)3 moieties bonded to separate cis 1,3-diene subunits of the aceheptylene ligand leaving an uncomplexed CC double bond. Deleting the five-membered ring from aceheptylene gives heptalene, which is predicted to form viable C12H10Fe3(CO)n (n=9, 8) complexes. The lowest energy C12H10Fe3(CO)9 structure has one Fe(CO)3 moiety bonded to a cis 1,3-diene subunit of one seven-membered ring and two Fe(CO)3 moieties bonded to overlapping cis 1,3-diene subunits on opposite sides of the other seven-membered ring sharing two carbon atoms thus leaving an uncomplexed CC bond. The lowest energy C12H10Fe3(CO)8 structure is derived from such a C12H10Fe3(CO)9 structure by CO loss accompanied by complexation of the uncomplexed CC double bond.