Co-ordination chemistry of bis(ferrocenylcarbaldimine) Schiff bases

Abstract

The complex chemistries of 1,2-bis(ferrocen-1-ylmethyleneamino)ethane (L¹), 1,2-bis(ferrocen-1-ylmethyleneamino)benzene (L²) and its 4-methyl (L³), 4-chloro (L⁴) and 4-nitro (L⁵) derivatives have been reexamined. Complexation of L¹ by ZnCl₂ afforded [ZnCl₂(L¹)], whose single crystal structure reveals a distorted tetrahedral zinc(II) centre with Zn· · ·Fe 4.730(2) and 4.803(2) Å. The complexes [ZnCl₂(L)] (L = L²–L⁵), [ZnBr₂(L)] (L = L²–L⁵) and [CoBr₂(L³)] are accessible by MX₂-templated condensation of 2 equivalents of ferrocenecarbaldehyde (fcCHO) with the appropriate 1,2-diaminobenzene. Treatment of [Cu(NCMe)₄]X (X^– = BF₄^– or PF₆^–) with L¹, or fcCHO and the corresponding 1,2-diaminobenzene, yielded [Cu(L)₂]X (L = L¹–L³). The single crystal structure of [Cu(L³)₂]PF₆·1.7CH₂Cl₂ shows a tetrahedral copper(I) centre, the chelate ligands being substantially distorted from planarity. Compounds [ZnCl₂(L)], [ZnBr₂(L)] (L = L²–L⁵) and [CoBr₂(L³)] exhibit weak electronic communication between the two ferrocenyl centres, showing by cyclic voltammetry two chemically reversible Fe^II–Fe^III oxidations separated by 50–60 mV in CH₂Cl₂–0.5 M NBuⁿ₄PF₆ at 293 K; [ZnCl₂(L¹)] and [Cu(L)₂]X (L = L¹–L³) exhibit a single Fe^II–Fe^III couple under these conditions. Attempted template syntheses of L²–L⁵ employing other MX₂ (M = Mn, Co, Ni or Cu; X^– = Cl^–, Br^–, NO₃^–, BF₄^– or ClO₄^–) salts yielded primarily 2-ferrocenylbenzimidazole intramolecular cyclisation derivatives; the crystal structure of one such product was determined.