Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 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(NImR)3 that can bind [Fe4S4]2+ or [Fe6S6]3+ clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NImR)3 ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NImMe)3 (tris(1,3,5-(3-(N,N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph4P]2[Fe4S4Cl4] and 3 equiv of NaBPh4 furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph4P]2[Fe4S4Cl4] provides (L(NImMe)3)Fe6S6Cl6 in high yield. Formation of an [Fe6S6]3+ cluster using L(NImTol)3 (tris(1,3,5-(3-(N,N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe4S4]2+ cluster [(L(NImTol)3)(Fe4S4C...