Controlling the Reactivity of Bifunctional Ligands: Carboxylate-Bridged Nonheme Diiron(II) Complexes Bearing Free Thiol Groups

Carboxylate-bridged nonheme diiron­(II) complexes, bearing free functional groups in general, and free thiol groups in particular, were sought. While the addition of sodium γ-hydroxybutyrate into a mixture of Fe­(BF₄)₂·6H₂O, HN-Et-HPTB, and Et₃N afforded the complex [Fe₂(N-Et-HPTB)­(μ-O₂C–(CH₂)₃–OH)]­(BF₄)₂ (2) (where N-Et-HPTB is the anion of N,N,N′,N′-tetrakis­(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane), a similar, straightforward process could not be used for the synthesis of diiron­(II) complexes with free thiol groups. In order to circumvent this problem, a new class of thiolate bridged diiron­(II) complexes, [Fe₂(N-Et-HPTB)­(μ-SR¹)]­(BF₄)₂ (R¹ = Me (1a), Et (1b), ^tBu (1c), Ph (1d)) was synthesized. Selective proton exchange reactions of one representative compound, 1b, with reagents of the type HS–R²–COOH yielded the target compounds, [Fe₂(N-Et-HPTB)­(μ-O₂C–R²–SH)]­(BF₄)₂ (R² = C₆H₄ (3a), CH₂CH₂ (3b), CH₂(CH₂)₅CH₂ (3c)). Redox properties of the complexes 3a–3c were studied in comparison with the complex, [Fe₂(N-Et-HPTB)­(μ-O₂CMe)]­(BF₄)₂ (5). Reaction of (Cp₂Fe)­(BF₄) with 1b yielded [Fe^II₂(N-Et-HPTB)­(DMF)₃]­(BF₄)₃·DMF (4) (when crystallized from DMF/diethyl ether), which might indicate the formation of a transient ethanethiolate bridged {Fe^IIFe^III} species, followed by a rapid internal redox reaction to generate diethyldisulfide and the solvent coordinated diiron­(II) complex, 4. This possibility was supported by a comparative cyclic voltammetric study of 1a–1c and 4. Prospects of the complexes of the type 3a–3c as potential building blocks for the synthesis of nonheme diiron­(II) complexes covalently attached with other redox active metals has been substantiated by the synthesis of the complexes, [Fe₂(N-EtHPTB)­(μ-O₂C–R²–S)­Cu­(Me₃TACN)]­(BF₄)₂ (R = p-C₆H₄ (7a), CH₂CH₂ (7b)). All the compounds were characterized by a combination of single-crystal X-ray structure determinations and/or elemental analysis.