High-spin square planar iron(II) alkali metal siloxide complexes - influence of the alkali metal and reactivity towards O2 and NO

Complexes [L2Fe][Li(DME)](2), 1(DME), {[L2Fe][Na-2(DME)(3)]}(infinity), 2(DME) and [L2Fe][K(DME)(2)](2), 3(DME) were synthesized by deprotonation of LH2 (LH2=O(SiPh2OH)(2)) with the respective alkali metal tert-butoxides followed by recrystallization from DME. It turned out that upon crossing over from Li+ via Na+ to K+ counterions the structures of the high-spin iron(II) complexes are increasingly distorted from a square planar towards a tetrahedral structure so that 3(DME) represents a borderline case, as indicated by the tau-values. The distortions are also reflected in the Mossbauer spectra through the quadrupole splittings. The compounds behave inert in contact with O atom transfer reagents but react rapidly with dioxygen. The reaction rates are too high to be determined even by stopped-flow measurements quantitatively, but qualitatively it emerged that the rates increase from Li to Na to K. Using NO as an O-2 surrogate an NO adduct with an S = 3/2 ground state was isolated where NO is coordinated in an end-on binding mode, formally as a NO- ligand, with a significantly weakened NO bond.