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.