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Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 Clusters

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journal contribution
posted on 12.11.2018, 17:35 by Alex McSkimming, Daniel L. M. Suess
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)­(Fe4S4Cl)]­[BPh4] is cleanly generated when 1 equiv of [Ph4P]2[Fe4S4Cl4] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe6S6]3+ clusters but not [Fe4S4]2+ clusters. Thus, the structure and nuclearity of L­(NImR)3-bound Fe–S clusters may be selectively controlled through rational modification the ligand’s substituents.