Synthesis, characterisation of two hexa-iron clusters with {Fe2S2(CO)x} (x = 5 or 6) fragments and investigation into their inter-conversion

Reaction of a trithiol ligand, 2-(mercaptomethyl)-2-methylpropane-1,3-dithiol (H3L), 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 2anti ⇔ 2syn and the equilibrium constant Keq = 0.55 under CO atmosphere. In the cluster 2, the central {Fe2S2(CO)6} sub-unit is flanked by two identical {Fe2S2(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 Feprox(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 Feprox(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.