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

Abstract

Complexes [L 2 Fe][Li(DME)] 2 , 1(DME), {[L 2 Fe][Na 2 (DME) 3 ]} 1 , 2(DME) and [L 2 Fe][K(DME) 2 ] 2 , 3(DME) were synthesized by deprotonation of LH 2 (LH 2 = O(SiPh 2 OH) 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 τ-values. The distortions are also reflected in the Mössbauer 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 À ligand, with a significantly weakened NO bond.

Low‐Coordinated Iron(II) Siloxide Complexes – Structural Diversity and Reactivity Towards O2 and Oxygen Atom Transfer Reagents

“…However, reactions of 2 with these OAT reagents occur rather rapidly and are instantly finished at r.t., as indicated by a color change of the solution from blueish‐green to yellow‐brown, and no intermediate could be observed, even at temperatures as low as −125 °C using 2‐methyltetrahydrofuran as a solvent. The extreme sensitivity of 2 towards OAT reagents is at first sight surprising, as in our previous studies on a high‐spin square planar iron(II) siloxide complex, namely [Li(THF) 2 ] 2 [Fe(O(SiPh 2 O) 2 ) 2 ], I , no reactivity, even at temperatures up to 80 °C, was observed [11] . This may be attributed to the fact that 2 is lacking one ligand in comparison, so that the sterics are more favorable and furthermore it has a lower charge.…”

“…Compared to bidentate disilanolate ligands such as [O(SiPh 2 O) 2 ] 2− the monodentate silanolate ligand Ph 3 SiO − thus enables the selective synthesis of complexes varying with regards to coordination spheres around the iron(II) ions as well the nuclearity, solely based on the ratio of ligand to iron precursor. Complex 1 crystallizes in form of reddish‐brown needles, that is, with a color clearly different from the one displayed by the abovementioned mono‐ and dinuclear iron(II) siloxides with high‐spin square planar FeO 4 centers, which are colored in shades of blue [9–11] . This may be the result of the unusual combination of the two iron centers with different connectivities in 1 (see discussion below).…”

“…Complex 1 crystallizes in form of reddish-brown needles, that is, with a color clearly different from the one displayed by the abovementioned mono-and dinuclear iron(II) siloxides with high-spin square planar FeO 4 centers, which are colored in shades of blue. [9][10][11] This may be the result of the unusual combination of the two iron centers with different connectivities in 1 (see discussion below). Complex 2 forms pale blueish-green needles.…”

“…This is also reflected in the NO reactivity: both 2 and the DME analogue of I , I‐DME , immediately react with NO resulting in purple solutions. In the case of I‐DME a NO complex could be isolated by crystallization, [11] while the purple solution of 2 with NO turned brown in ∼30 min and it was not possible to isolate a NO complex.…”

“…[2] With this background we have developed an interest in the chemistry of iron siloxide chemistry and in recent years accessed various high-spin square planar representatives, which served as structural and spectroscopic models for the abovementioned α-Fe sites. [8][9][10][11] Investigating their reactivity, they were found to behave inert toward oxygen atom transfer reagents but extremely reactive towards O 2 .…”