Spontaneous Formation of an Fe/Mn Diamond Core: Models for the Fe/Mn Sites in Class 1c Ribonucleotide Reductases.

A handful of oxygen-activating enzymes has recently been found to contain Fe/Mn active sites, like Class 1c ribonucleotide reductases and R2-like ligand-binding oxidase, which are closely related to their better characterized diiron cousins. These enzymes are proposed to form high-valent intermediates with Fe-O-Mn cores. Herein, we report the first examples of synthetic Fe/Mn complexes that mimic doubly bridged intermediates proposed for enzymatic oxygen activation. Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis has been used to characterize the structures of each of these compounds. Linear compounds accurately model the Fe···Mn distances found in Fe/Mn proteins in their resting states, and doubly bridged diamond core compounds accurately model the distances found in high-valent biological intermediates. Unlike their diiron analogues, the paramagnetic nature of Fe/Mn compounds can be analyzed by EPR, revealing S = 1/2 signals that reflect antiferromagnetic coupling between the high-spin Fe(III) and Mn(III) units of heterobimetallic centers. These compounds undergo electron transfer with various ferrocenes, linear compounds being capable of oxidizing diacetyl ferrocene, a weak reductant, and diamond core compounds being capable of oxidizing acetyl ferrocene. Diamond core compounds can also perform HAT reactions from substrates with X-H bonds with bond dissociation free energies (BDFEs) up to 75 kcal/mol and are capable of oxidizing TEMPO-H at rates of 0.32-0.37 M-1 s-1, which are comparable to those reported for some mononuclear FeIII-OH and MnIII-OH compounds. However, such reactivity is not observed for the corresponding diiron compounds, a difference that Nature may have taken advantage of in evolving enzymes with heterobimetallic active sites.

Sc3+-Promoted O-O Bond Cleavage of a (µ-1,2-Peroxo)diiron(III) Species Formed from an Iron(II) Precursor and O2 to Generate a Complex with an FeIV2(µ-O)2 Core.

Soluble methane monooxygenase (sMMO) carries out methane oxidation at 4 °C and under ambient pressure in a catalytic cycle involving the formation of a peroxodiiron(III) intermediate (P) from the oxygenation of the diiron(II) enzyme and its subsequent conversion to Q, the diiron(IV) oxidant that hydroxylates methane. Synthetic diiron(IV) complexes that can serve as models for Q are rare and have not been generated by a reaction sequence analogous to that of sMMO. In this work, we show that [FeII(Me3NTB)(CH3CN)](CF3SO3)2 (Me3NTB = tris((1-methyl-1H-benzo[d]imidazol-2-yl)methyl)amine) (1) reacts with O2 in the presence of base, generating a (µ-1,2-peroxo)diiron(III) adduct with a low O-O stretching frequency of 825 cm-1 and a short Fe···Fe distance of 3.07 Å. Even more interesting is the observation that the peroxodiiron(III) complex undergoes O-O bond cleavage upon treatment with the Lewis acid Sc3+ and transforms into a bis(µ-oxo)diiron(IV) complex, thus providing a synthetic precedent for the analogous conversion of P to Q in the catalytic cycle of sMMO.

Oxoiron(IV) complexes as synthons for the assembly of heterobimetallic centers such as the Fe/Mn active site of Class Ic ribonucleotide reductases.

Nonheme oxoiron(IV) complexes can serve as synthons for generating heterobimetallic oxo-bridged dimetal complexes by reaction with divalent metal complexes. The formation of FeIII-O-CrIII and FeIII-O-MnIII complexes is described herein. The latter complexes may serve as models for the FeIII-X-MnIII active sites of an emerging class of Fe/Mn enzymes represented by the Class 1c ribonucleotide reductase from Chlamydia trachomatis and the R2-like ligand-binding oxidase (R2lox) found in Mycobacterium tuberculosis. These synthetic complexes have been characterized by UV-Vis, resonance Raman, and X-ray absorption spectroscopy, as well as electrospray mass spectrometry. The FeIII-O-CrIII complexes exhibit a three-band UV-Vis pattern that differs from the simpler features associated with FeIII-O-FeIII complexes. The positions of these features are modulated by the nature of the supporting polydentate ligand on the iron center, and their bands intensify dramatically in two examples upon the binding of an axial cyanate or thiocyanate ligand trans to the oxo bridge. In contrast, the FeIII-O-MnIII complexes resemble FeIII-O-FeIII complexes more closely. Resonance Raman characterization of the FeIII-O-MIII complexes reveals an 18O-sensitive vibration in the range of 760-890 cm-1. This feature has been assigned to the asymmetric FeIII-O-MIII stretching mode and correlates reasonably with the Fe-O bond distance determined by EXAFS analysis. The likely binding of an acetate as a bridging ligand to the FeIII-O-MnIII complex 12 lays the foundation for further efforts to model the heterobimetallic active sites of Fe/Mn enzymes.

Hydrogen evolution catalyzed by a cobalt complex containing an asymmetric Schiff-base ligand.

A cobalt(iii) complex containing an asymmetric Schiff-base ligand has been found to be active for proton reduction. Catalysis occurs at -1.2 V vs. Fc(+)/Fc (0.56 V vs. NHE), resulting in an overpotential of 350 mV. Additionally, the complex is active with a turnover frequency of 420 s(-1). An enhancement in activity is observed upon addition of water.

A nickel complex of a conjugated bis-dithiocarbazate Schiff base for the photocatalytic production of hydrogen.

We report a nickel complex containing a conjugated bis-dithiocarbazate ligand that is an active catalyst for the reduction of protons into hydrogen gas. Light-driven hydrogen generation is observed from a system containing this molecular nickel catalyst coupled with a fluorescein photosensitizer and triethylamine sacrificial donor. The photocatalytic system is stable for over 70 hours, achieving 3300 turnovers with respect to catalyst. The complex is also an active electrocatalyst for proton reduction with catalysis occurring at -1.7 V vs. Fc(+)/Fc. The nickel bis-dithiocarbazate complex represents a highly active and stable catalyst for hydrogen generation.