Axial Ligand Lability and Coordination Induced Spin State Variations of Tetracarbene Iron(II) Thiolato Complexes

Abstract

Starting from the six-coordinate tetracarbene iron(II) bis(acetonitrile) complexes, a series of structurally characterized iron(II) thiolato complexes [(MTC)Fe^II(SR)(NCMe)]OTf (R=^tBu, Ada) was synthesized based on the equatorial macrocyclic tetracarbene (MTC) ligands ^18HL (3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohex-acosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene) and its octamethylated derivative ^18MeL. Those thiolato complexes can be viewed as bioinspired organometallic analogues of the iron(II) state of heme enzymes with axial cysteine coordination like cytochrome P450. Depending on the equatorial and axial ligands, the spin states of the complexes can change from low-spin (S=0) to intermediate-spin (S=1), but the strong donation by the MTC prevents the formation of any high-spin species. The electronic structure was analyzed using ⁵⁷Fe Mössbauer spectroscopy and magnetic susceptibility measurements. In addition, both axial ligands – acetonitrile and the thiolate – were found to be labile, leading to scrambling in solution as observed via NMR spectroscopy and ESI-MS. A temperature-dependent equilibrium was found for all complexes due to dissociation of the axial MeCN forming a 5-coordinate species in solution. This 5-coordinate species is significantly preferred for the ^18MeL being in line with the higher donating ability of the equatorial ligand and thus the weakening of the axial ligand bonds, in line with cyclic voltammetry data.