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)2Fe4S4(TriS)(SEt)] yielded the first metal-bridged 4Fe-4S] cluster dimer, (n-Bu4N)2{Fe4S4(TriS)(mu-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.