Selective Synthesis of Site-Differentiated Fe 4 S 4 and Fe 6 S 6 Clusters

Obtaining rational control over the structure and nuclearity of metalloclusters is an ongoing challenge in synthetic Fe-S cluster chemistry. We report a new family of tridentate imidazolin-2-imine ligands L(NIm ) that can bind [Fe S ] or [Fe S ] clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm ) ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NIm ) (tris(1,3,5-(3-( N, N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph P] [Fe S Cl ] and 3 equiv of NaBPh furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph P] [Fe S Cl ] provides (L(NIm ) )Fe S Cl in high yield. Formation of an [Fe S ] cluster using L(NIm ) (tris(1,3,5-(3-( N, N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe S ] cluster [(L(NIm ) )(Fe S Cl)][BPh ] is cleanly generated when 1 equiv of [Ph P] [Fe S Cl ] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe S ] clusters but not [Fe S ] clusters. Thus, the structure and nuclearity of L(NIm ) -bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.