A [4Fe−4S] cluster dimer bridged by Bis(2,2′:6′,2″-terpyridine-4′-thiolato)iron(II)

The use of 2,2′:6′,2″-terpyridine-4′-thiol (tpySH) was explored as a bridging ligand for the formation of stable assemblies containing both [4Fe−4S] clusters and single metal ions. Reaction of tpySH (2 equiv) with (NH4)2Fe(SO4)2·6H2O generated the homoleptic complex [Fe(tpySH)2]2+, which was isolated as its PF6− salt. The compound could be fully deprotonated to yield neutral [Fe(tpyS)2], and the absorption spectrum is highly dependent on the protonation state. Reaction of [Fe(tpySH)2](PF6)2 with the new 3:1 site-differentiated cluster (n-Bu4N)2[Fe4S4(TriS)(SEt)] yielded the first metal-bridged [4Fe−4S] cluster dimer, (n-Bu4N)2[{Fe4S4(TriS)(μ-Stpy)}2Fe]. Electrochemical studies indicate that the [4Fe−4S] clusters in the dimer act as independent redox units, while UV–vis spectroscopy provides strong evidence for a thioquinonoid electron distribution in the bridging tpyS− ligand. TpySH thus acts as a directional bridging ligand between [4Fe−4S] clusters and single metal ions, thereby opening the way to the synthesis of larger, more complex assemblies.