Comparative calculation of EPR spectral parameters in [Mo(V)OX4]-, [Mo(V)OX5]2-, and [Mo(V)OX4(H2O)]- complexes

Insights into periplasmic nitrate reductase function under single turnover

Nitrate reductases play pivotal roles in nitrogen metabolism by leveraging the molybdopterin cofactor to facilitate the reduction of nitrate to nitrite. Periplasmic nitrate reductases (NapA) utilize nitrate as a terminal electron acceptor when oxygen is limiting, helping to drive anaerobic metabolism in bacteria. Despite extensive research into NapA homologs, open questions about the mechanism remain especially at the molecular level. More broadly, little is understood of how the molybdopterin cofactor is tuned for catalysis in these enzymes enabling broad substrate scope and reactivity observed in molybdenum-containing enzymes. Here, we have prepared NapA from Campylobacter jejuni under single turnover conditions to generate a singly reduced enzyme that can be further examined by electron paramagnetic resonance (EPR) spectroscopy. Our results provide new context into the known spectra and related structures of NapA and related enzymes. These insights open new avenues for understanding nitrate reductase mechanisms, molybdenum coordination dynamics, and the role of pyranopterin ligands in catalysis.

Influence of the ligand-field on EPR parameters of cis- and trans-isomers in MoV systems relevant to molybdenum enzymes: Experimental and density functional theory study

The electron paramagnetic resonance (EPR) investigation of mononuclear cis- and trans-(L1O)MoOCl2 complexes [L1OH = bis(3,5-dimethylpyrazolyl)-3-tert-butyl-2-hydroxy-5-methylphenyl)methane] reveals a significant difference in their spin Hamiltonian parameters which reflect different equatorial and axial ligand fields created by the heteroscorpionate donor atoms. Density functional theory (DFT) was used to calculate the values of principal components and relative orientations of the g and A tensors, and the molecular framework in four pairs of isomeric mononuclear oxo‑molybdenum(V) complexes (cis- and trans-(L1O)MoOCl2, cis,cis- and cis,trans-(L-N2S2)MoOCl [L-N2S2H2 = N,N'-dimethyl-N,N'-bis(mercaptophenyl)ethylenediamine], cis,cis- and cis,trans-(L-N2S2)MoO(SCN), and cis- and trans-[(dt)2MoO(OMe)]2- [dtH2 = 2,3-dimercapto-2-butene]). Scalar relativistic DFT calculations were conducted using three different exchange-correlation functionals. It was found that the use of hybrid exchange-correlation functional with 25% of the Hartree-Fock exchange leads to the best quantitative agreement between theory and experiment. A simplified ligand-field approach was used to analyze the influence of the ligand fields in all cis- and trans-isomers on energies and contributions of molybdenum d-orbital manifold to g and A tensors and relative orientations. Specifically, contributions that originated from the spin-orbit coupling of the dxz, dyz, and dx2-y2 orbitals into the ground state have been discussed. The new findings are discussed in the context of the experimental data of mononuclear molybdoenzyme, DMSO reductase.

Lewis Acid Behavior of MoF5 and MoOF4: Syntheses and Characterization of MoF5(NCCH3), MoF5(NC5H5)n, and MoOF4(NC5H5)n (n = 1, 2)

The Lewis acid-base adducts MoF₅(NC₅H₅)_n and MoOF₄(NC₅H₅)_n (n = 1, 2) were synthesized from the reactions of MoF₅ and MoOF₄ with C₅H₅N and structurally characterized by X-ray crystallography. Whereas the crystal structures of MoF₅(NC₅H₅)₂ and MoOF₄(NC₅H₅)₂ are isomorphous containing pentagonal-bipyramidal molecules, the fluorido-bridged, heptacoordinate [MoF₅(NC₅H₅)]₂ dimer differs starkly from monomeric, hexacoordinate MoOF₄(NC₅H₅). For the weaker Lewis base CH₃CN, only the 1:1 adduct, MoF₅(NCCH₃), could be isolated. All adducts were characterized by Raman spectroscopy in conjunction with vibrational frequency calculations. Multinuclear NMR spectroscopy revealed an unprecedented isomerism of MoOF₄(NC₅H₅)₂ in solution, with the pyridyl ligands occupying adjacent or nonadjacent positions in the equatorial plane of the pentagonal bipyramid. Paramagnetic MoF₅(NC₅H₅)₂ was characterized by electron paramagnetic resonance (EPR) spectroscopy as a dispersion in solid adamantane as well as in a diamagnetic host lattice of MoOF₄(NC₅H₅)₂; EPR parameters were computed using ZORA with the BPW91 functional using relativistic all-electron wave functions for Mo and simulated using EasySpin. Density functional theory calculations (B3LYP) and natural bond orbital analyses were conducted to elucidate the distinctive bonding and structural properties of all adducts reported herein and explore fundamental differences observed in the Lewis acid behavior of MoF₅ and MoOF₄.

The Taming of Redox-Labile Phosphidotitanocene Cations

Tame d⁰ phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d¹ precursors with [Cp₂ Fe][BPh₄ ]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d⁰ complexes, whereas dπ-pπ repulsion prevents such interactions in the d¹ complexes. In addition, CH-π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d⁰ complexes were found to be light sensitive, and decompose through Ti-P bond homolysis to give Ti^III species. A naked d⁰ phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.

Planar-Chiral 1,1'-Diboryl Metallocenes: Diastereoselective Synthesis from Boryl Cyclopentadienides and Spin Density Analysis of a Diborylcobaltocene

The reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes₂ (Mes = 2,4,6-trimethylphenyl) and ClBCy₂ as electrophiles. The boryl group is selectively introduced in the remote position to minimize steric congestion. The new boryl dienes are obtained as mixtures of isomers, and subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. Direct assembling of tert-butylated boryl cyclopentadienides with MCl₂ (M = Fe, Co) selectively leads to 1,1'-planar chiral ferrocenes 6a,b and cobaltocene 7. To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface was scrutinized so as to identify and compare its diastereoisomers and conformers. This stereoselectivity is attributed to minimized steric repulsions between the tert-butyl and the BMes₂ groups in the eclipsed conformation of the racemic diastereoisomers. The X-ray structures of boryl diene 2a and diboryl ferrocene 6a are reported. The electronic structure of cobaltocene 7 was analyzed by EPR and DFT calculations. The spin density of this unique open-shell complex is mainly localized on the Co center, but significant spin density is also found on the boron atoms, indicating substantial delocalization of the unpaired electron over the Lewis acid moieties. Consistently, the singly occupied molecular orbital is a combination of a Co-centered 3d orbital with π(BC) orbitals on each CpBMes₂ rings. There is only weak, if any, direct M···B interaction in 6 and 7.

d(1) Oxosulfido-Mo(V) Compounds: First Isolation and Unambiguous Characterization of an Extended Series

Reaction of Tp(iPr)Mo(VI)OS(OAr) with cobaltocene in toluene results in the precipitation of brown, microcrystalline oxosulfido-Mo(V) compounds, [CoCp2][Tp(iPr)Mo(V)OS(OAr)] (Cp(-) = η(5)-C5H5(-), Tp(iPr)(-) = hydrotris(3-isopropylpyrazol-1-yl)borate, OAr(-) = phenolate or 2-(s)Bu, 2-(t)Bu, 3-(t)Bu, 4-(s)Bu, 4-Ph, 3,5-(s)Bu2, 2-CO2Me, 2-CO2Et or 2-CO2Ph derivative thereof). The compounds are air- and water-sensitive and display ν(Mo═O) and ν(Mo[Formula: see text]S) IR absorption bands at ca. 890 and 435 cm(-1), respectively, 20-40 cm(-1) lower in energy than the corresponding bands in Tp(iPr)MoOS(OAr). They are electrochemically active and exhibit three reversible cyclovoltammetric waves (E(Mo(VI)/Mo(V)) = -0.40 to -0.66 V, E([CoCp2](+)/CoCp2) = -0.94 V and E(CoCp2/[CoCp2](-)) = -1.88 V vs SCE). Structural characterization of [CoCp2][Tp(iPr)MoOS(OC6H4CO2Et-2)]·2CH2Cl2 revealed a distorted octahedral Mo(V) anion with Mo═O and Mo[Formula: see text]S distances of 1.761(5) and 2.215(2) Å, respectively, longer than corresponding distances in related Tp(iPr)MoOS(OAr) compounds. The observation of strong S(1s) → (S(3p) + Mo(4d)) S K-preedge transitions indicative of a d(1) sulfido-Mo(V) moiety and the presence of short Mo═O (ca. 1.72 Å) and Mo[Formula: see text]S (ca. 2.25 Å) backscattering contributions in the Mo K-edge EXAFS further support the oxosulfido-Mo(V) formulation. The compounds are EPR-active, exhibiting highly anisotropic (Δg 0.124-0.150), rhombic, frozen-glass spectra with g1 close to the value observed for the free electron (ge = 2.0023). Spectroscopic studies are consistent with the presence of a highly covalent Mo[Formula: see text]S π* singly occupied molecular orbital. The compounds are highly reactive, with reactions localized at the terminal sulfido ligand. For example, the compounds react with cyanide and PPh3 to produce thiocyanate and SPPh3, respectively, and various (depending on solvent) oxo-Mo(V) species. Reactions with copper reagents also generally lead to desulfurization and the formation of oxo-Mo(V) or -Mo(IV) complexes.

Infrared multiple photon dissociation spectroscopy of a gas-phase oxo-molybdenum complex with 1,2-dithiolene ligands

Electrospray ionization (ESI) in the negative ion mode was used to create anionic, gas-phase oxo-molybdenum complexes with dithiolene ligands. By varying ESI and ion transfer conditions, both doubly and singly charged forms of the complex, with identical formulas, could be observed. Collision-induced dissociation (CID) of the dianion generated exclusively the monoanion, while fragmentation of the monoanion involved decomposition of the dithiolene ligands. The intrinsic structure of the monoanion and the dianion were determined by using wavelength-selective infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The IRMPD spectrum for the dianion exhibits absorptions that can be assigned to (ligand) C=C, C–S, C—C≡N, and Mo=O stretches. Comparison of the IRMPD spectrum to spectra predicted for various possible conformations allows assignment of a pseudo square pyramidal structure with C_2v symmetry, equatorial coordination of MoO²⁺ by the S atoms of the dithiolene ligands, and a singlet spin state. A single absorption was observed for the oxidized complex. When the same scaling factor employed for the dianion is used for the oxidized version, theoretical spectra suggest that the absorption is the Mo=O stretch for a distorted square pyramidal structure and doublet spin state. A predicted change in conformation upon oxidation of the dianion is consistent with a proposed bonding scheme for the bent-metallocene dithiolene compounds [Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc.1976, 98, 1729−1742], where a large folding of the dithiolene moiety along the S···S vector is dependent on the occupancy of the in-plane metal d-orbital.

Dithiolopyranthione Synthesis, Spectroscopy and an Unusual Reactivity with DDQ

The bicyclic pyran thiolone tetrahydro-3αH-[1,3]dithiolo[4,5-β]pyran-2-thione (3a) engages in a highly unusual fragmentation in the presence of DDQ. The pyran thiolone, 3a, was synthesized by chlorination of 3,4-dihydro-2H-pyran (1), followed by condensing with CS2 and NaSH. Reaction of 3a with DDQ generates the isomerized pyran thiolone tetrahydro-3αH-[1,3]dithiolo[4,5-β]pyran-2-thione (3b) and 4-benzyl-5-(3-hydroxypropyl)-1,3-dithiole-2-thione (4) via a deep-seated rearrangement. The identity of 3b was confirmed by single crystal X-ray analysis: P21/c, a=5.807(9) Å, b = 12.99(2) Å, c = 11.445(15), β=113.23(6)°. Mechanistic experiments and computational insight is used to explain the likely sequence of events in the highly unusual formation of 4. Collectively, these results establish fundamental reactivity patterns for further research in this area.

Structural studies of the molybdenum center of mitochondrial amidoxime reducing component (mARC) by pulsed EPR spectroscopy and 17O-labeling

Mitochondrial amidoxime reducing components (mARC-1 and mARC-2) represent a novel group of Mo-containing enzymes in eukaryotes. These proteins form the catalytic part of a three-component enzyme complex known to be responsible for the reductive activation of several N-hydroxylated prodrugs. No X-ray crystal structures are available for these enzymes as yet. A previous biochemical investigation [Wahl, B., et al. (2010) J. Biol. Chem., 285, 37847-37859 ] has revealed that two of the Mo coordination positions are occupied by sulfur atoms from a pyranopterindithiolate (molybdopterin, MPT) cofactor. In this work, we have used continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy and density functional theoretical (DFT) calculations to determine the nature of remaining ligands in the Mo(V) state of the active site of mARC-2. Experiments with samples in D(2)O have identified the exchangeable equatorial ligand as a hydroxyl group. Experiments on samples in H(2)(17)O-enriched buffer have shown the presence of a slowly exchangeable axial oxo ligand. Comparison of the experimental (1)H and (17)O hyperfine interactions with those calculated using DFT has shown that the remaining nonexchangeable equatorial ligand is, most likely, protein-derived and that the possibility of an equatorial oxo ligand can be excluded.