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 [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 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.