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

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 dz2 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.

Functional Models for the Mono- and Dinitrosyl Intermediates of FNORs: Semireduction versus Superreduction of NO.

The reduction of NO to N2O by flavodiiron nitric oxide reductases (FNORs) is related to the disruption of the defense mechanism in mammals against invading pathogens. The proposed mechanism for this catalytic reaction involves both nonheme mono- and dinitrosyl diiron(II) species as the key intermediates. Recently, we reported an initial account for NO reduction activity of an unprecedented mononitrosyl diiron(II) complex, [Fe2(N-Et-HPTB)(NO)(DMF)3](BF4)3 (1) (N-Et-HPTB is the anion of N,N,N',N'-tetrakis(2-(l-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane; DMF = dimethylformamide) with [FeII{FeNO}7] formulation [Jana et al. J. Am. Chem. Soc. 2017, 139, 14380]. Here we report the full account for the selective synthesis, characterization, and reactivity of FNOR model complexes, which include a dinitrosyl diiron(II) complex, [Fe2(N-Et-HPTB)(NO)2(DMF)2](BF4)3 (2) with [{FeNO}7]2 formulation and a related, mixed-valent diiron(II, III) complex, [Fe2(N-Et-HPTB)(OH)(DMF)3](BF4)3 (3). Importantly, whereas complex 2 is able to produce 89% of N2O via a semireduced mechanism (1 equiv of CoCp2 per dimer = 50% of NO reduced), complex 1, under the same conditions (0.5 equiv of CoCp2 per dimer = 50% of NO reduced), generates only ∼50% of N2O. The mononitrosyl complex therefore requires superreduction for quantitative N2O generation, which constitutes an interesting dichotomy between 1 and 2. Reaction products obtained after N2O generation by 2 using 1 and 2 equiv of reductant were characterized by molecular structure determination and electron paramagnetic resonance spectroscopy. Despite several available literature reports on N2O generation by diiron complexes, this is the first case where the end products from these reactions could be characterized unambiguously, which clarifies a number of tantalizing observations about the nature of these products in the literature.

Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles.

Starting from their six-coordinate iron(II) precursor complexes [L8R Fe(MeCN)]2+ , a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L8H and its new octamethylated derivative L8Me , both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl- , OTf- , MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and 57 Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and µ-oxo diiron(III) complexes were also studied.

Disproportionation Equilibrium of a µ-Oxodiiron(III) Complex Giving Rise to C-H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct.

µ-Oxodiiron(III) species are air-stable and unreactive products of autoxidation processes of monomeric heme and non-heme iron(II) complexes. Now, the organometallic [(LNHC )FeIII -(µ-O)-FeIII (LNHC )]4+ complex 1 (LNHC is a macrocyclic tetracarbene) is shown to be reactive in C-H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) (2) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2. Kinetic analysis gave an equilibrium constant for the disproportionation of 1, which is very small (Keq =7.5±2.5×10-8 m). Increasing Keq might by a useful strategy for circumventing the formation of dead-end µ-oxodiiron(III) products during Fe-based homogeneous oxidation catalysis.