Organometallic μ-Nitridodiiron Complexes in Oxidation States Ranging from (III/III) to (IV/IV)

Abstract

Iron complexes with nitrido ligands are of interest as molecular analogues of key intermediates during N2-to-NH3 conversion in industrial or enzymatic processes. Dinuclear iron complexes with a bridging nitrido unit are mostly known in relatively high oxidation states (III/IV or IV/IV), originating from the decomposition of azidoiron precursors via high-valent FeN intermediates. The use of a tetra-NHC macrocyclic scaffold ligand (NHC = N-heterocyclic carbene) has now allowed for the isolation of a series of organometallic μ-nitridodiiron complexes ranging from the mid-valent FeIII–N–FeIII (1) via mixed-valent FeIII–N–FeIV (type 4) to the high-valent FeIV–N–FeIV (type 5) species that are interconverted at moderate potentials, accompanied by axial ligand binding at the FeIV sites. Magnetic measurements and electron paramagnetic resonance spectroscopy showed the homovalent complexes to be diamagnetic and the mixed-valent system to feature an S = 1/2 ground state due to very strong antiferromagnetic coupling. The bonding in the Fe–N–Fe moiety has been further probed by crystallographic structure determination, 57Fe Mössbauer and UV–vis spectroscopies, as well as density functional theory computations, which revealed high covalency and nearly identical Fe–N distances across this redox series. The latter has been rationalized in terms of the nonbonding nature of the combination of Fe d z 2 atomic orbitals from which electrons are successively removed upon oxidation, and these redox processes are best described as being metal-centered. The tetra-NHC-ligated μ-nitridodiiron series complements a set of related complexes with single-atom μ-oxido and μ-phosphido bridges, but the Fe–N–Fe core exhibits a comparatively high stability over several oxidation states. This promises interesting applications in view of the manifold catalytic uses of μ-nitridodiiron complexes based on macrocyclic N-donor porphinato­(2−) or phthalocyaninato(2−) ligands.

Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

“…This leads to a reduced electron density at these positions and diamagnetic deshielding (-downfield shift). Indeed, the 13 83 The rather small difference in 13 C NHC signals in the series of the neutral iron tetracarbene 27 (Scheme 6) does not lead to noticeable changes in the backbone 13 C signals.…”

“…For instance, a higher oxidation state in an Fe-N-Fe complex led to an upfield shift of the carbene signal from 187.5 ppm for Fe III -N-Fe III (55, Scheme 22) to 163.7 ppm for Fe IV -N-Fe IV complex (58, Scheme 22) (see Section 4.5). 83 This can be explained by the higher Lewis acidity of the iron center at higher oxidation states leading to a larger upfield shift of the 13 C NHC signal due to a lower paramagnetic shielding term, and lower Fe -NHC backbonding. 83,85,86,89 The significant shift of the 13 C NHC signal upon exchange of the axial ligands of one iron tetracarbene framework (Fig.…”

“…83 This can be explained by the higher Lewis acidity of the iron center at higher oxidation states leading to a larger upfield shift of the 13 C NHC signal due to a lower paramagnetic shielding term, and lower Fe -NHC backbonding. 83,85,86,89 The significant shift of the 13 C NHC signal upon exchange of the axial ligands of one iron tetracarbene framework (Fig. 4 and Table 2) can also be explained with the Lewis acidity of iron.…”

“…In fact, this observation is likely due to the removal of an electron from a molecular orbital involved in Fe -NHC backbonding, resulting in diminished backbonding and weaker Fe-NHC interaction and consequently a longer Fe-NHC bond. 83,104,105 A holistic analysis can however become quite complex. 104 A weakening of the MeCN-Fe bond in dependence to the s-donation strength of the tetracarbene, as one might expect based on HEP, 86 cannot be found.…”

“…Chemical redox agents include [(4-BrC 6 H 4 ) 3 N]SbCl 6 (E 0 = +0.67 V in MeCN 127,128 ), AgOTf (E 0 = +0.65 V in DCM 128,129 ), AgNO 3 (E 0 = +0.04 V in MeCN 128,129 ), NOBF 4 (E 0 = +0.87 V in MeCN 128,130 ) or thianthrenyl hexafluorophosphate (ThPF 6 , E 0 = +0.86 in MeCN 128,131 ) and with exception of the first all have been applied in the oxidation to iron(IV) centers (vide infra). 54,61,73,82,83,132 The choice of oxidant depends on e.g. the solubility of the complex, the required oxidant strength or the counterion of the iron tetracarbene.…”