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

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posted on 2016-03-09, 15:59 authored by Nabhendu Pal, Amit Majumdar
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 3a3c were studied in comparison with the complex, [Fe2(N-Et-HPTB)­(μ-O2CMe)]­(BF4)2 (5). Reaction of (Cp2Fe)­(BF4) with 1b yielded [FeII2(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 {FeIIFeIII} 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 1a1c and 4. Prospects of the complexes of the type 3a3c 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.