Synthesis, characterisation of two hexa-iron clusters with {Fe 2S 2(CO) x} ( x = 5 or 6) fragments and investigation into their inter-conversion

Abstract

Reaction of a trithiol ligand, 2-(mercaptomethyl)-2-methylpropane-1,3-dithiol ( H 3L ), with tri-iron dodecacarbonyl in toluene produces two hexa-iron clusters ( 1 and 2). The two clusters are characterised crystallographically and spectroscopically. NMR spectroscopy reveals that the cluster 2 exists in two conformations in equilibrium 2 anti ⇔ 2 syn and the equilibrium constant K eq = 0.55 under CO atmosphere. In the cluster 2, the central {Fe 2S 2(CO) 6} sub-unit is flanked by two identical {Fe 2S 2(CO) 6} satellite sub-units through thiolate linkages whereas one of the thiolate linkages can further form Fe–S bond with the proximal Fe atom in one of the two satellite sub-units to produce the cluster 1 by expelling one CO. This conversion can be entirely reversed by continuously purging CO through the solution of the cluster 1. As suggested by DFT calculations, the conversion features a key step, the rotation of the Fe prox(CO) 3 to expose a vacant site for exogenic ligand binding (the S atom from the central sub-unit in this case) with concomitant switch for one of the three CO ligands in the unit of Fe prox(CO) 3 from terminal to bridging orientation. The conversion from the clusters 1– 2 involving one CO uptake is much faster than its reverse process since the latter is an endergonic process characterised by large reaction barriers, as revealed by the DFT calculations.