The famous α-Fe active sites in Fe-zeolites have recently been revealed to correspond to mononuclear high-spin iron(ii) centres in square planar coordination environments. Here we report a first iron siloxide complex which represents a faithful structural and spectroscopic model of such sites. Notably, also an allogon with a distorted structure exists and could be crystallised.
A frustrated Lewis pair composed of an acidic aluminum function (AlR 2 ) and a basic phosphine entity, linked by a xanthene spacer, is capable of cleaving THF. The rate of the ringopening reaction is higher by a factor of 10 for R = C 6 F 5 than that for R = Mes. Structural and theoretical investigations revealed for the case of C 6 F 5 aromatic interactions in the secondary coordination sphere whichin case they also exist in the transition statecould aid ring-opening through a preorientation of the reacting functions. However, in detailed computational studies these were found to occur without contributing significantly to the lowering of the activation barrier.
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.
Frustrated Lewis pairs (FLPs) composed of acidic alane and basic phosphane functions, separated by a xanthene linker, can be prepared through the corresponding Me3Sn derivative and methyl aluminum compounds with elimination of Me4Sn. This way MeClAl‐, Cl2Al‐ and (C6F5)2Al‐ moieties could be introduced and the resulting FLPs are stabilized by a further equivalent of the alane precursors. In contact with the FLPs CO2 is bound via the C atom at the phosphane functions and the two O atoms at the Al centers. The residues at the latter determine the binding strength. Hence, in case of MeClAl CO2 capture occurs at higher pressure and under ambient conditions CO2 is released again, while for Cl2Al and (C6F5)2Al CO2 binding becomes irreversible. The results of DFT calculations rationalize these findings by the high thermodynamic stabilization in case of more electronegative residues, which concomitantly lead to higher barriers, and in case of (C6F5)2Al further stabilization is achieved through a low reorganization energy.
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