Selective Synthesis of Site-Differentiated FeS and FeS 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 [FeS] or [FeS] 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 [PhP][FeSCl] and 3 equiv of NaBPh furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [PhP][FeSCl] provides (L(NIm))FeSCl in high yield. Formation of an [FeS] 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 [FeS] cluster [(L(NIm))(FeSCl)][BPh] is cleanly generated when 1 equiv of [PhP][FeSCl] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [FeS] clusters but not [FeS] clusters. Thus, the structure and nuclearity of L(NIm)-bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.