Nitrosyliron complexes of macrocyclic [N4 2−] and open-chain [N2O2 2−] chelate ligands: influence of the equatorial ligand on the NO binding mode

Several new nitrosyliron complexes FeLNO (L2−: macrocyclic [N4] coordinated or open-chain [N2O2] coordinated Schiff base ligand) were synthesised and compared with respect to their chemical behaviour (reactivity with dioxygen; redox potentials), their T-dependent magnetic susceptibility and their molecule structure. These properties reflect clearly the electronic influence of the equatorial chelate ligand (if present, also of an additional axial ligand) on the electron density at the central atom and, as a consequence, on the NO-binding mode. The macrocyclic complexes with the strongest equatorial donor ligands give pentacoordinated low spin (S=1/2) derivatives, best to interpret as [FeIIIL+NO−]—similar as the halides or pseudo-halides FeLX. These complexes show no tendency to add additional axial ligands and are insensitive to air in non-coordinating solvents. In presence of a donor ligand D (pyridine, MeOH) the reaction with dioxygen results in an octahedral nitro derivative FeL(NO2)×D that can also be prepared by reaction of the iron(III) complex [FeL×2D]+ with sodium nitrite. This could be proved especially for the most electron-rich complex Fe1NO which can also be oxidised electrochemically in MeCN to give [Fe1NO]+ and [Fe1NO]2+ in two reversible steps. The [N2O2] coordinated complexes with weaker donor ligands in the equatorial plane show a more differentiated behaviour depending on electronic effects of the peripheral ligand substituents. Octahedral adducts FeLNO×D could be isolated from several of such complexes under anaerobic conditions. Their reaction with dioxygen gives the μ-oxo derivative (FeL)2O as the main product. Decreasing electron density at the central atom results in a trend towards decreasing transition temperature between a low-temperature S=1/2 and a high-temperature S=3/2 state. The atom distances within the first coordination sphere of the complex Fe5NO×MeOH with the most electron withdrawing substitution of the equatorial ligand are significantly longer than those of the other complexes and show a surprising similarity to those of octahedral high-spin di-adducts FeL×2D of the iron(II) complexes. This fact suggests an interpretation as [FeIILNOradical dot×MeOH] with a coupling of the iron(II) S=4/2 state with the unpaired electron of the NO radical. The electronic influence of the equatorial ligands is also reflected in a significant correlation between the angle FeNO (varying from 140 to 167°) and the NO stretching frequencies (varying from 1629 to 1812 cm−1).