Photoelectron spectroscopy and electronic structure calculations of d1 vanadocene compounds with chelated dithiolate ligands: implications for pyranopterin Mo/W enzymes

Isolation and characterization of a bis(dithiolene)-supported tungsten-acetylenic complex as a model for acetylene hydratase

Acetylene hydratase is currently the only known mononuclear tungstoenzyme that does not catalyze a net redox reaction. The conversion of acetylene to acetaldehyde is proposed to occur at a W(IV) active site through first-sphere coordination of the acetylene substrate. To date, a handful of tungsten complexes have been shown to bind acetylene, but many lack the bis(dithiolene) motif of the native enzyme. The model compound, [W(O)(mnt)2]2-, where mnt2- is 1,2-dicyano-1,2-dithiolate, was previously reported to bind an electrophilic acetylene substrate, dimethyl acetylenedicarboxylate, and characterized by FT-IR, UV-vis, potentiometry, and mass spectrometry (Yadav, J; Das, S. K.; Sarkar, S., J. Am. Chem. Soc., 1997, 119, 4316-4317). By slightly changing the electrophilic acetylene substrate, an acetylenic-bis(dithiolene)‑tungsten(IV) complex has been isolated and characterized by FT-IR, UV-vis, NMR, X-ray diffraction, and X-ray absorption spectroscopy. Activation parameters for complex formation were also determined and suggest coordination-sphere reorganization is a limiting factor in the model complex reactivity.

Interaction of metallocene dichlorides with apo-human transferrin: A spectroscopic study and cytotoxic activity against human cancer cell lines

Metallocene dichlorides (Cp₂M(IV)Cl₂) are the first class of small and hydrophobic organometallic compounds classified as anticancer agents against numerous cancer cell lines and tumors. In this study, the antiproliferative activities of Cp₂VCl₂,Cp₂NbCl₂, Cp₂HfCl₂ and Cp₂ZrCl₂were assessed on two human cancer cell lines (HT-29 and MCF-7) using MTT assay. Spectroscopic studies were also conducted using these and other known metallocene dichlorides on apo-human transferrin (apo-hTf) at pH 7.4. UV-Vis and CD showed that their interaction with apo-hTf could induce conformational changes of its secondary structure during binding process. In fluorescence, a decrease in intensity of the emission peak was observed when the apo-hTf:Cp₂M(IV)Cl₂ complex is being formed, probably due to changes in the microenvironment of its tyrosine and tryptophan residues. Among all metallocene dichlorides studied, Cp₂VCl₂ has the strong ability to quench the intrinsic fluorescence of apo-hTf through a static quenching mechanism. The association constants for each protein-compound complex were also determined at different temperatures (296 K, 303 K, 310 K, and 317 K) based on fluorescence quenching results. Positive enthalpy changes (ΔH) and entropy changes (ΔS) as well as negative free energies (ΔG) suggest that hydrophobic interactions are the main intermolecular forces involved in the binding process, probably via an endothermic and spontaneous reaction mechanism. The distance, r, between donor (apo-hTf) and acceptor (Cp₂M(IV)Cl₂) obtained according to Forster's theory of non-radiation energy transfer suggest that the energy transfer from apo-hTf to Cp₂M(IV)Cl₂ occurs with high probability and distances obtained by FRET with high accuracy.

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

A multitechnique spectroscopic and theoretical study of the Cp₂M(benzenedithiolato) (M = Ti, V, Mo; Cp = η⁵-C₅H₅) 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 d² M = Mo complex and the resonance Raman spectra of the d¹ M = V complex reveals the power of this combined spectroscopic approach. Here, we observe that the UPS spectrum of Cp₂Mo(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.

Consensus structures of the Mo(v) sites of sulfite-oxidizing enzymes derived from variable frequency pulsed EPR spectroscopy, isotopic labelling and DFT calculations

Sulfite-oxidizing enzymes from eukaryotes and prokaryotes have five-coordinate distorted square-pyramidal coordination about the molybdenum atom. The paramagnetic Mo(v) state is easily generated, and over the years four distinct CW EPR spectra have been identified, depending upon enzyme source and the reaction conditions, namely high and low pH (hpH and lpH), phosphate inhibited (P_i) and sulfite (or blocked). Extensive studies of these paramagnetic forms of sulfite-oxidizing enzymes using variable frequency pulsed electron spin echo (ESE) spectroscopy, isotopic labeling and density functional theory (DFT) calculations have led to the consensus structures that are described here. Errors in some of the previously proposed structures are corrected.

Pulsed electron paramagnetic resonance spectroscopy of (33)S-labeled molybdenum cofactor in catalytically active bioengineered sulfite oxidase

Molybdenum enzymes contain at least one pyranopterin dithiolate (molybdopterin, MPT) moiety that coordinates Mo through two dithiolate (dithiolene) sulfur atoms. For sulfite oxidase (SO), hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of magnetic nuclei (I ≠ 0) near the Mo(V) (d(1)) center have been measured using high-resolution pulsed electron paramagnetic resonance (EPR) methods and interpreted with the help of density functional theory (DFT) calculations. These have provided important insights about the active site structure and the reaction mechanism of the enzyme. However, it has not been possible to use EPR to probe the dithiolene sulfurs directly since naturally abundant (32)S has no nuclear spin (I = 0). Here we describe direct incorporation of (33)S (I = 3/2), the only stable magnetic sulfur isotope, into MPT using controlled in vitro synthesis with purified proteins. The electron spin echo envelope modulation (ESEEM) spectra from (33)S-labeled MPT in this catalytically active SO variant are dominated by the "interdoublet" transition arising from the strong nuclear quadrupole interaction, as also occurs for the (33)S-labeled exchangeable equatorial sulfite ligand [ Klein, E. L., et al. Inorg. Chem. 2012 , 51 , 1408 - 1418 ]. The estimated experimental hfi and nqi parameters for (33)S (aiso = 3 MHz and e(2)Qq/h = 25 MHz) are in good agreement with those predicted by DFT. In addition, the DFT calculations show that the two (33)S atoms are indistinguishable by EPR and reveal a strong intermixing between their out-of-plane pz orbitals and the dxy orbital of Mo(V).

Metal-sulfur valence orbital interaction energies in metal-dithiolene complexes: determination of charge and overlap interaction energies by comparison of core and valence ionization energy shifts

The electronic interactions between metals and dithiolenes are important in the biological processes of many metalloenzymes as well as in diverse chemical and material applications. Of special note is the ability of the dithiolene ligand to support metal centers in multiple coordination environments and oxidation states. To better understand the nature of metal-dithiolene electronic interactions, new capabilities in gas-phase core photoelectron spectroscopy for molecules with high sublimation temperatures have been developed and applied to a series of molecules of the type Cp(2)M(bdt) (Cp = η(5)-cyclopentadienyl, M = Ti, V, Mo, and bdt = benzenedithiolato). Comparison of the gas-phase core and valence ionization energy shifts provides a unique quantitative energy measure of valence orbital overlap interactions between the metal and the sulfur orbitals that is separated from the effects of charge redistribution. The results explain the large amount of sulfur character in the redox-active orbitals and the 'leveling' of oxidation state energies in metal-dithiolene systems. The experimentally determined orbital interaction energies reveal a previously unidentified overlap interaction of the predominantly sulfur HOMO of the bdt ligand with filled π orbitals of the Cp ligands, suggesting that direct dithiolene interactions with other ligands bound to the metal could be significant for other metal-dithiolene systems in chemistry and biology.

Structural distortions upon oxidation in heteroleptic [Cp(2)W(dmit)] tungsten dithiolene complex: combined structural, spectroscopic, and magnetic studies

Four different cation radical salts are obtained upon electrocrystallization of [Cp(2)W(dmit)] (dmit = 1,3-dithiole-2-thione-4,5-dithiolato) in the presence of the BF(4)(-), PF(6)(-), Br(-), and [Au(CN)(2)](-) anions. In these formally d(1) cations, the WS(2)C(2) metallacycle is folded along the S···S hinge to different extents in the four salts, an illustration of the noninnocent character of the dithiolate ligand. Structural characteristics and the charge distribution on atoms, for neutral and ionized complexes with various folding angles, were calculated using DFT methods, together with the normal vibrational modes and theoretical Raman spectra. Raman spectra of neutral complex [Cp(2)W(dmit)] and its salts formed with BF(4)(-), AsF(6)(-), PF(6)(-), Br(-), and [Au(CN)(2)](-) anions were measured using the red excitation (λ = 632.8 nm). A correlation between the folding angle of the metallacycle and the Raman spectroscopic properties is analyzed. The bands attributed to the C═C and C-S stretching modes shift toward higher and lower frequencies by about 0.3-0.4 cm(-1) deg(-1), respectively. The solid state structural and magnetic properties of the three salts are analyzed and compared with those of the corresponding molybdenum complexes. Temperature dependence of the magnetic susceptibility shows the presence of one-dimensional antiferromagnetic interactions in the BF(4)(-), PF(6)(-), and [Au(CN)(2)](-) salts, while an antiferromagnetic ground state is identified in the Br(-) salt below T(Néel) = 7 K. Interactions are systematically weaker in the tungsten salts than in the isostructural molybdenum analogs, a consequence of the decreased spin density on the dithiolene ligand in the tungsten complexes.

On the molecular and electronic structures of AsP3 and P4

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.

Electronic Structure of the d Bent-metallocene Cp(2)VCl(2): A Photoelectron and Density Functional Study

The Cp(2)VCl(2) molecule is a prototype for bent metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp(2)VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp(2)VCl from the Cp(2)VCl(2)/Cp(2)VCl mixed spectrum yields the Cp(2)VCl(2) spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d(1) electron and the energies of the higher positive ion states of Cp(2)VCl(2). The calculations also account well for the trends to the other Group V bent metallocene dichlorides Cp(2)NbCl(2) and Cp(2)TaCl(2). The first ionization energy of Cp(2)VCl(2) is considerably greater than the first ionization energies of the second- and third-row transition metal analogues.