Vibrational Control of Covalency Effects Related to the Active Sites of Molybdenum Enzymes

Abstract

A multitechnique spectroscopic and theoretical study of the CpM(benzenedithiolato) (M = Ti, V, Mo; Cp = η-CH) series provides deep insight into dithiolene electronic structure contributions to electron transfer reactivity and reduction potential modulation in pyranopterin molybdenum enzymes. This work explains the magnitude of the dithiolene folding distortion and the concomitant changes in metal-ligand covalency that are sensitive to electronic structure changes as a function of d-electron occupancy in the redox orbital. It is shown that the large fold angle differences correlate with covalency, and the fold angle distortion is due to a pseudo-Jahn-Teller (PJT) effect. The PJT effect in these and related transition metal dithiolene systems arises from the small energy differences between metal and sulfur valence molecular orbitals, which uniquely poise these systems for dramatic geometric and electronic structure changes as the oxidation state changes. Herein, we have used a combination of resonance Raman, magnetic circular dichroism, electron paramagnetic resonance, and UV photoelectron spectroscopies to explore the electronic states involved in the vibronic coupling mechanism. Comparison between the UV photoelectron spectroscopy (UPS) of the d M = Mo complex and the resonance Raman spectra of the d M = V complex reveals the power of this combined spectroscopic approach. Here, we observe that the UPS spectrum of CpMo(bdt) contains an intriguing vibronic progession that is dominated by a "missing-mode" that is composed of PJT-active distortions. We discuss the relationship of the PJT distortions to facile electron transfer in molybdenum enzymes.

Engineering Fe–N–C sites onto Fe nanoparticles for synergistically boosting Cr(vi) reduction: performance, mechanism, and applications

“…The lower Φ of Fe NPs @Fe SAs NC than that of Fe NPs /CN is caused by the enriched electron density on the Fe–N–C interface, confirming a lower energy barrier for electron transfer from the internal zero-valent iron particles. 40 Moreover, as compared to Fe NPs /CN, Fe NPs @Fe SAs NC demonstrated a smaller semicircle in the Nyquist plot than that of Fe NPs /CN (Fig. 3(e)), indicating a lower electrochemical impedance and higher electron-transfer kinetics in the redox reactions.…”

Metal–Dithiolene Bonding Contributions to Pyranopterin Molybdenum Enzyme Reactivity

“…Investigations of Mo dithiolene compounds by electronic absorption, resonance Raman, and EPR spectroscopies showed that S → Mo charge transfer bands dominate the visible spectrum and that there is substantial delocalization of spin density onto the S atoms of the dithiolene. Recent comprehensive studies of bent metallocene-dithiolene compounds have shown that low-energy ligand fold distortions arise from a pseudo-Jahn-Teller (PJT) effect, which involves vibronic coupling of the electronic ground state with electronic excited states to control metal-ligand covalency [ 60 ] ( Section 5 ). This vibronic coupling process may play critical roles in the catalytic cycles of pyranopterin Mo and W enzymes by dynamic and/or static modulation of redox potentials and providing a superexchange pathway for electron transfer through the PDT framework.…”

“…A combination of MCD, electronic absorption, electron paramagnetic resonance, resonance Raman, and photoelectron spectroscopies has been used in conjunction with theory to reveal vibrational control of metal-ligand covalency in a series of Cp 2 M(bdt) complexes (M = Ti, V, Mo; Cp = η 5 -C 5 H 5 ) [ 60 ] ( Figure 4 ). The work is important because it has allowed for a detailed understanding of how redox orbital electron occupancy (Ti(IV) = d 0 , V(IV) = d 1 , Mo(IV) = d 2 ,) affects the nature of the M-dithiolene bonding scheme at parity of the ligand set and at parity of charge.…”

Spectroscopic Studies of Mononuclear Molybdenum Enzyme Centers

“…MCD spectroscopy has been used to probe the electronic structure of chicken sulfite oxidase that was poised in the catalytically relevant [Mo(V):Fe(II)] state [ 145 , 147 ] ( Figure 6 , middle). No charge transfer transitions were observed at energies below ~17,000 cm −1 and this was interpreted to result from a reduction in S ip –S op orbital mixing and S op –Mo d(xy) covalency, in addition to a small dithiolene fold angle [ 130 , 145 , 147 , 148 , 149 , 150 , 151 ]. A band at 22,250 cm −1 was observed as a positive C-term and assigned as a S σ (cysteine)→Mo(xy) LMCT transition.…”