Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 Clusters

Abstract

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 R ) 3 that can bind [Fe 4 S 4 ] 2+ or [Fe 6 S 6 ] 3+ clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm R ) 3 ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NIm Me ) 3 (tris(1,3,5-(3-(N,N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph 4 P] 2 [Fe 4 S 4 Cl 4 ] and 3 equiv of NaBPh 4 furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph 4 P] 2 [Fe 4 S 4 Cl 4 ] provides (L(NIm Me ) 3 )Fe 6 S 6 Cl 6 in high yield. Formation of an [Fe 6 S 6 ] 3+ cluster using L(NIm Tol ) 3 (tris(1,3,5-(3-(N,N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe 4 S 4 ] 2+ cluster [(L(NIm Tol ) 3 )(Fe 4 S 4 Cl)][BPh 4 ] is cleanly generated when 1 equiv of [Ph 4 P] 2 [Fe 4 S 4 Cl 4 ] 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 6 S 6 ] 3+ clusters but not [Fe 4 S 4 ] 2+ clusters. Thus, the structure and nuclearity of L(NIm R ) 3 -bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.