95Mo NMR Research Articles

From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis.

Molybdenum alkylidynes endowed with tripodal silanolate ligands belong to the most active and selective catalysts for alkyne metathesis known to date. This paper describes a new generation that is distinguished by an unprecedented level of stability and practicality without sacrificing the chemical virtues of their predecessors. Specifically, pyridine adducts of type 16 are easy to make on gram scale, can be routinely weighed and handled in air, and stay intact for many months outside the glovebox. When dissolved in toluene, however, spontaneous dissociation of the stabilizing pyridine ligand releases an active species of excellent performance and functional group tolerance. Specifically, a host of polar and apolar groups, various protic sites, and numerous basic functionalities proved compatible. The catalysts are characterized by crystallographic and spectroscopic means, including 95Mo NMR; their activity and stability are benchmarked in detail, and the enabling properties are illustrated by advanced applications to natural product synthesis. For the favorable overall application profile and ease of handling, complexes of this new series are expected to replace earlier catalyst generations and help encourage a more regular use of alkyne metathesis in general.

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM‐5 Zeolite Identified by 2D 1H−95Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy

AbstractMethane dehydroaromatization (MDA) over Mo‐modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D 95Mo NMR and 2D 1H‐95Mo heteronuclear correlation solid‐state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM‐5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoOxCy species, with the more carbidic one (MoOxCy‐II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM‐5 pores are related to the MDA activity.

Active Ensembles in Methane Dehydroaromatization over Molybdenum/ZSM-5 Zeolite Identified by 2D 1 H-95 Mo Magic Angle Spinning Nuclear Magnetic Resonance Correlation Spectroscopy.

Methane dehydroaromatization (MDA) over Mo-modified zeolite is a potential catalytic route for converting natural gas into valuable aromatics. However, the active species in this reaction are highly complex, involving diverse Mo species, acidic sites of zeolite, and organic molecules. Herein, we apply 1D 95 Mo NMR and 2D 1 H-95 Mo heteronuclear correlation solid-state NMR spectroscopy to directly observe the active ensembles in the confined channels of Mo/ZSM-5 zeolite during the MDA reaction. We monitor the evolution of the spatial correlations of Mo species with the Brønsted acid sites and organic products (olefins and aromatics) in the zeolite channels. We identified two kinds of MoOx Cy species, with the more carbidic one (MoOx Cy -II) exhibiting higher activity for methane activation and benzene formation. The strong spatial interactions between the active Mo species and the organic species in the Mo/ZSM-5 pores are related to the MDA activity.

Active Site Descriptors from 95Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts.

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5-15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5-2.0. Notably, solid-state 95Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded 95Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that 95Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO-Mo(O)2-OSi) angles and low coordination numbers at Mo lead to highly deshielded 95Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the 95Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The 95Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites.

Proton Affinity in the Chemistry of Beta-Octamolybdate: HPLC-ICP-AES, NMR and Structural Studies.

The affinity of [β-Mo8O26]4- toward different proton sources has been studied in various conditions. The proposed sites for proton coordination were highlighted with single crystal X-ray diffraction (SCXRD) analysis of (Bu4N)3[β-{Ag(py-NH2)Mo8O26]}] (1) and from analysis of reported structures. Structural rearrangement of [β-Mo8O26]4- as a direct response to protonation was studied in solution with 95Mo NMR and HPLC-ICP-AES techniques. A new type of proton transfer reaction between (Bu4N)4[β-Mo8O26] and (Bu4N)4H2[V10O28] in DMSO results in both polyoxometalates transformation into [V2Mo4O19]4-, which was confirmed by the 95Mo, 51V NMR and HPLC-ICP-AES techniques. The same type of reaction with [H4SiW12O40] in DMSO leads to metal redistribution with formation of [W2Mo4O19]2-.

Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from 95Mo NMR Chemical Shift Descriptors.

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3Mo≡CR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the context of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of 95Mo chemical shift tensors extracted from solid-state 95Mo NMR spectra shows a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2 + 2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo-O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity.

The Ascent of Alkyne Metathesis to Strategy-Level Status.

For numerous enabling features and strategic virtues, contemporary alkyne metathesis is increasingly recognized as a formidable synthetic tool. Central to this development was the remarkable evolution of the catalysts during the past decades. Molybdenum alkylidynes carrying (tripodal) silanolate ligands currently set the standards; their functional group compatibility is exceptional, even though they comprise an early transition metal in its highest oxidation state. Their performance is manifested in case studies in the realm of dynamic covalent chemistry, advanced applications to solid-phase synthesis, a revival of transannular reactions, and the assembly of complex target molecules at sites, which one may not intuitively trace back to an acetylenic ancestor. In parallel with these innovations in material science and organic synthesis, new insights into the mode of action of the most advanced catalysts were gained by computational means and the use of unconventional analytical tools such as 95Mo and 183W NMR spectroscopy. The remaining shortcomings, gaps, and desiderata in the field are also critically assessed.

Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap.

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework (“canopy catalysts”) are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo≡CR entities in a stoichiometric triple‐bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex, 8, with a Mo≡Mo core. In addition to the regular analytical techniques, 95Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo‐tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris‐alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo‐oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single‐crystal X‐ray diffraction.

Exciting Opportunities for Solid-State 95Mo NMR Studies of MoS2 Nano-structures in Materials Research from Low to Ultra-high Magnetic Field (35.2 T).

Solid-state, natural-abundance 95Mo NMR experiments of four different MoS2 materials have been performed on a magnet B 0 = 19.6 T and on a new Series Connected Hybrid (SCH) magnet at 35.2 T. Employing two different 2H-MoS2 (2H phase) materials, a "pseudo-amorphous" MoS2 nano-material, and a MoS2 layer on the Al2O3 support of a hydrodesulphurization (HDS) catalyst have enabled introduction of solid-state 95Mo NMR as an important analytical tool in studies of MoS2 nano-materials. 95Mo spin-lattice relaxation time (T 1) studies of 160- and 4-layer 2H-MoS2 samples at 19.6 and 35.2 T show their relaxation rates (1/T 1) increase in proportion to B 0 2. This is in accord with chemical shift anisotropy (CSA) relaxation being the dominant T 1(95Mo) mechanism, with a large 95Mo CSA = 1025 ppm determined for all four MoS2 nano-materials. The dominant CSA mechanism suggests the MoS2 band-gap electrons are delocalized throughout the lattice-layer structures, thereby acting as a fast modulation source (ω oτc << 1) for 95Mo CSA in 2H-MoS2. A decrease in T 1(95Mo) is observed for an increase in B 0 field and for a decrease in the number of 2H-MoS2 layers. All four nano-materials exhibit identical 95Mo electric field gradient (EFG) parameters. The T 1 results account for the several failures to retrieve 95Mo spectral EFG and CSA parameters for multilayer 2H-MoS2 samples in the pioneering solid-state 95Mo NMR studies performed during the past two decades (1990-2010), because of the extremely long T 1(95Mo) = ~200-250 s observed at low B 0 (~9.4 T) used at that time. Much shorter T 1(95Mo) values are observed even at 19.6 T for the "pseudo-amorphous" and the HDS catalyst (MoS2-Al2O3 support) MoS2 nano-materials. These allowed useful solid-state 95Mo NMR spectra for these two samples to be obtained at 19.6 T in a few to < 24 h. Most importantly, this research led to observation of an impressive 95Mo MAS spectrum for an average of 1-4 thick MoS2-layers on a Al2O3 support, i.e., the first MAS NMR spectrum of a low natural-abundance, low-γ quadrupole-nucleus species layered on a catalyst support. While a huge gain in NMR sensitivity, factor ~ 60, is observed for the 95Mo MAS spectrum of the 160-layer sample at 35.2 T compared to 14.1 T, the MAS spectrum for the 4-layer sample is almost completely wiped out at 35.2 T. This unusual observation for the 4-layer sample (crumpled, rose-like and defective Mo-edge structures) is due to an increased distribution of the isotropic 95Mo shifts in the 95Mo MAS spectra at B 0 up to 35.2 T upon reduction of the number of sample layers.

Detoxification of mustard gas, nerve agents and simulants by peroxomolybdate in aqueous H2O2 solution: Reactive oxygen species and mechanisms

To overcome the insufficient understanding of the detoxification mechanisms of Chemical Warfare Agents (CWAs) and simulants by a molybdate-activated hydrogen peroxide solution (MoO42−-H2O2), detoxification performances toward CWAs and simulants were tested. The detoxification efficacy toward mustard gas (HD), nerve agent GD and VX by using MoO42−-H2O2 was higher than that by using H2O2 activated/modified by sodium hydroxide, ammonia, bicarbonate, and borates, but the vesicant mustard sulfone (HDO2) was only formed when using MoO42−-H2O2 to detoxify HD. Although Reactive Oxygen Species (ROS) such as singlet oxygen (1O2), superoxide anion (O2−), H2O2 and peroxide anion (OOH−) were detected by electron spin resonance (ESR) and chemiluminescence (CL) in the 0.1 M MoO42−-1.0 M H2O2 solution, 95Mo NMR indicated that Mo(O2)42− was the dominant oxidant for the oxidation of sulfide to sulfoxide and then to sulfone. Oxidation of HD by Mo(O2)42− occurred on the HD-drop/water interface through a direct oxygen transfer mechanism without the participation of H2O molecules, and further oxidation of HD to HDO2 was inevitable in an aqueous MoO42−-H2O2 solution. 18O tracer experiments confirmed that OOH−-based perhydrolysis dominated the detoxification reaction of nerve agents in an alkaline H2O2 solution. Considering the balance of oxidation/nucleophilic reactivity for a practical decontaminant, pH 10.5–11 range was recommended as the ideal for the MoO42−-H2O2 system to detoxify CWAs.

"Canopy Catalysts" for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a Tripodal Ligand Framework.

A new family of structurally well-defined molybdenum alkylidyne catalysts for alkyne metathesis, which is distinguished by a tripodal trisilanolate ligand architecture, is presented. Complexes of type 1 combine the virtues of previous generations of silanolate-based catalysts with a significantly improved functional group tolerance. They are easy to prepare on scale; the modularity of the ligand synthesis allows the steric and electronic properties to be fine-tuned and hence the application profile of the catalysts to be optimized. This opportunity is manifested in the development of catalyst 1f, which is as reactive as the best ancestors but exhibits an unrivaled scope. The new catalysts work well in the presence of unprotected alcohols and various other protic groups. The chelate effect entails even a certain stability toward water, which marks a big leap forward in metal alkylidyne chemistry in general. At the same time, they tolerate many donor sites, including basic nitrogen and numerous heterocycles. This aspect is substantiated by applications to polyfunctional (natural) products. A combined spectroscopic, crystallographic, and computational study provides insights into structure and electronic character of complexes of type 1. Particularly informative are a density functional theory (DFT)-based chemical shift tensor analysis of the alkylidyne carbon atom and 95Mo NMR spectroscopy; this analytical tool had been rarely used in organometallic chemistry before but turns out to be a sensitive probe that deserves more attention. The data show that the podand ligands render a Mo-alkylidyne a priori more electrophilic than analogous monodentate triarylsilanols; proper ligand tuning, however, allows the Lewis acidity as well as the steric demand about the central atom to be adjusted to the point that excellent performance of the catalyst is ensured.

Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts

AbstractA new type of molybdenum alkylidyne catalysts for alkyne metathesis is described, which is distinguished by an unconventional podand topology. These structurally well‐defined complexes are easy to make on scale and proved to be tolerant toward numerous functional groups; even certain protic substituents were found to be compatible. The new catalysts were characterized by X‐ray crystallography and by spectroscopic means, including 95Mo NMR.

Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts.

A new type of molybdenum alkylidyne catalysts for alkyne metathesis is described, which is distinguished by an unconventional podand topology. These structurally well‐defined complexes are easy to make on scale and proved to be tolerant toward numerous functional groups; even certain protic substituents were found to be compatible. The new catalysts were characterized by X‐ray crystallography and by spectroscopic means, including 95Mo NMR.

Zero-Magnetic-Field Splitting in the Excited Triplet States of Octahedral Hexanuclear Molybdenum(II) Clusters: [{Mo6X8}Y6]2- (X, Y = Cl, Br, I).

The temperature ( T) dependences of the emissions from the tetra- n-butylammonium salts of [{Mo6X8}Y6]2- (X, Y = Cl, Br, and I) in optically transparent polyethylene glycol dimethacrylate matrixes were studied in the T range of 3-300 K. [{Mo6Cl8}Y6]2-, [{Mo6Br8}Y6]2-, and [{Mo6I8}I6]2- showed the T-dependent emission characteristics similar to those of other hexanuclear Mo(II), Re(III), and W(II) clusters reported previously, while [{Mo6I8}Br6]2- and [{Mo6I8}Cl6]2- exhibited the emission properties different from those of other [{Mo6X8}Y6]2- clusters. The photophysical behavior of these clusters was explained by the excited triplet state spin-sublevel ( Φn, n = 1-4) model irrespective of the nature of X and Y. The zero-magnetic-field splitting energies between the lowest energy (Φ1) and the higher energy spin sublevels (Φ4 or Φ3) caused by the first- or second-order spin-orbit coupling, Δ E14 or Δ E13, were evaluated to be 620-870 or 50-99 cm-1, respectively. We found the linear correlation between the Δ E14 or Δ E13 value and the fourth power of the atomic number ( Z) of the inner halide X: Δ E14 or Δ E13 vs { Z(X)}4 (correlation coefficient: cc = ∼ 0.999). Furthermore, we also found the correlation between Δ E14 or Δ E13 and the 95Mo NMR chemical shift of the cluster. These findings gave very important insight into the spin-orbit coupling and zero-magnetic-field splitting in the excited triplet states of transition metal complexes.

Structural characterization, hydrolytic stability and dynamics of cis-MoVIO22+ hydroquinonate/phenolate complexes

Although, the investigation of the interactions of MoVI ion with the “non-innocent” phenols/hydroquinones is essential for the understanding of the role of molybdenum in the biological systems, the MoVI complexes of phenolate/hydroquinonate and their functionalized variants remain mainly unexplored. Reaction of Na2MoVIO4 with tripod iminodiacetate phenol/hydroquinone ligands in aqueous solutions resulted in the synthesis of mononuclear phenolate/hydroquinonate and dinuclear bridged-hydroquinonate cis-MoVIO22+ complexes. The new complexes were isolated and characterized in solid state by X-ray crystallography. The 1H, 13C and 95Mo NMR spectra of the aqueous solutions of the complexes show that they retain their structure at the pD range 2–4, with the dinuclear bridged-hydroquinonate cis-MoVIO22+ to form two isomers which are in dynamic equilibrium. At pDs 4–7 the MoVI-Ophenolate bond of the cis-MoVIO22+ compounds hydrolyzes to form MoVIO3-iminodiacetate species. Variable temperature (VT) and 2D {1H} EXSY NMR spectroscopies where employed for the determination of the thermodynamic and activation parameters of the isomerization reaction between the dinuclear bridged-hydroquinonate cis-MoVIO22+ isomers. The exchange mechanism between cis-MoVIO22+ and the MoVIO3 species were also investigated by 2D {1H} EXSY NMR spectroscopy. Despite of the higher strength of the MoVI-Ophenolate bond, it is more labile than the MoVI-Ocarboxylate bond. The rate determining step of the above exchange reactions was found to be the dissociation of the phenolate/hydroquinonate oxygen atom from MoVI ion.

Complexes of molybdenum(VI) oxide tetrafluoride and molybdenum(VI) dioxide difluoride with neutral N- and O-donor ligands

[MoOF4(MeCN)], prepared from reaction of MoF6 with (Me3Si)2O in anhydrous MeCN solution, reacts with the neutral O-donor ligands, thf, Ph3PO, Me3PO, dmf and dmso, (L) in a 1:1 molar ratio under rigorously anhydrous conditions to form six-coordinate [MoOF4(L)], which have been characterised by microanalysis, IR, 1H, 19F{1H} and 95Mo NMR spectroscopy. In the presence of moisture the major products are [MoO2F2(L)2], which can be made directly by reaction of [MoOF4(L)] with a further equivalent of L and (Me3Si)2O. [MoOF4(MeCN)] and 2,2′-bipyridyl produce the insoluble [MoOF4(bipy)], which is probably 7-coordinate. Ph3AsO is quantitatively converted to Ph3AsF2 by [MoOF4(MeCN)], and soft ligands, including Me2S, Me3P and Me3As, reduce the oxide fluoride on contact. Unstable [MoO2F2(MeCN)2] has also been prepared and the X-ray structure of [MoO2F2(MeCN)2]·MeCN is reported. X-ray crystal structures are reported for [MoOF4(Ph3PO)], [MoO2F2(Ph3PO)2], [MoO2F2(Me3PO)(H2O)] and [Mo2O4F2(μ-F)2(Me3PO)2]. Comparisons with the corresponding chemistries of WOF4 and WO2F2 are described.

Multinuclear solid-state magnetic resonance study of oxo-bridged diniobium and quadruply-bonded dimolybdenum carboxylate clusters

Carboxylate paddlewheels and their oxo-bridged analogues constitute ideal building blocks for the assembly of two- and three-dimensional framework materials. Here, we present a multinuclear (1H, 13C, 93Nb, 95Mo) magnetic resonance study of solid samples of Nb2OCl6(O2Ph)2 (1), Mo2(O2CMe)4 (2), and Mo2(O2CCHF2)4 (3). High-resolution proton and 13C CP/MAS NMR spectra provide valuable information on structure and crystal symmetry and on cocrystallized solvent. 93Nb solid-state NMR spectra of 1 provide quadrupolar coupling constants and chemical shift tensors which are characteristic of the axially asymmetric Nb-O-Nb bridging environment. 95Mo solid-state NMR spectra of 2 and 3 provide quadrupolar coupling constants and chemical shift tensors which are directly characteristic of the molybdenum-molybdenum quadruple bonds in these compounds. The quadruple bonds are characterized by particularly large 95Mo chemical shift tensor spans on the order of 5500ppm. Density functional theoretical computations provide good agreement with the 93Nb and 95Mo experimental data, with some exceptions noted. This work demonstrates possible NMR approaches to characterize more complex framework materials and provides key insight into the Mo-Mo quadruple bond.

Peroxomolybdate supported on water soluble polymers as efficient catalysts for green and selective sulfoxidation in aqueous medium

Peroxomolybdenum(VI) species immobilized on linear water soluble polymers formulated as, [MoO(O2)2(sulfonate)]–PS [PS=poly(sodium vinyl sulfonate)] (PSMo) and[Mo2O2(O2)4(carboxylate)]–PA [PA=poly(sodium acrylate)](PAMo) served as efficient and recyclable catalysts for selective oxidation of organic sulfides to sulfoxide in water with 30% H2O2. The new polymer anchored complex, PSMo has been prepared by reacting H2MoO4 and 30% H2O2 with macromolecular ligand, poly(sodium vinyl sulfonate) and characterized by elemental analysis, spectral studies (FTIR, Raman, 13C NMR, 95Mo NMR, UV–vis), SEM, EDX and TGA-DTG analysis. The catalytic protocol effects clean and facile oxidation of a wide range of sulfides to afford pure sulfoxides with high TOF and excellent chemoselectivity at ambient temperature,with an easy work-up procedure. The catalysts can be regenerated in-situ and recycled at least up to ten consecutive reaction cycles without significant loss of activity and are amenable for ready scalability. The catalytic procedure thus provides an ecologically sustainable alternative as it involves water as solvent, H2O2 as green oxidant, a recyclable non-polluting catalyst, and is absolutely free from halide, organic co-solvent or co-catalyst. This is the first catalytic protocol for selective aqueous oxidation of sulfides using a catalytic system based on peroxometal species supported on water soluble polymers.

95Mo NMR study of the effect of structure on complexation of molybdate with alpha and beta hydroxy carboxylic acid ligands

The utility of the quadrupolar, spin 5/2 95Mo nucleus in NMR studies of complexation of molybdate have been demonstrated using model alpha hydroxy carboxylic acids. The 95Mo NMR spectra show a down field shift of the resonance upon complexation, presumably due to the smaller 1/ΔE paramagnetic shift term for the octahedral molybdate complexes relative to the tetrahedral molybdate ion. The 95Mo resonances are sufficiently distinct to establish three solution complexes for the reaction of lactic acid with molybdate. The effect of the structure of hydroxy carboxylic acids on complexation to molybdate (MoO42−) was studied with 13C and 95Mo NMR spectroscopy. The use of the NMR spectra of both nuclei allows monitoring of the Lewis acidic molybdate site and the Lewis basic ligands. The beta hydroxy carboxylic acids, 3-hydroxypropanoic acid and salicylic acid, did not form complexes with molybdate, suggesting a preference for 5-membered chelated rings. The Lewis acidity of the negatively charged molybdate ion is rationalized with two schemes for the reaction with the chelating bidentate alpha hydroxy carboxylic acids.

Comparing spectroscopic and electrochemical properties of complexes of type Cp’M(η3-C3H5)(CO)2 (Cp’ = Cp, Ind, Flu): A complementary experimental and DFT study

A series of allyl complexes of the general formula [Cp'Mo(η3-C3H5)(CO)2], where Cp' = Cp, CpMe, Cp*, Ind, IndMe, IndMe2, Flu, and three tungsten analogues, has been prepared and characterized by 1H, 13C, and 95Mo NMR, cyclic voltammetry, and the structure of [IndMo(η3-C3H5)(CO)2] was determined by X-ray single crystal analysis. Two conformers, corresponding to the two extreme orientations of the allyl ligand (endo and exo), have been identified in solution by 1H and 95Mo NMR for all the complexes, except for [FluMo(η3-C3H5)(CO)2], which only presents an exo conformation in solution.A 95Mo NMR investigation also shows the influence of electron donor capability of the Cp' ligands on the 95Mo chemical shifts of the Mo center. He I and He II photoelectron spectra probe the electron richness of the metal center and the electronic structure of the complexes. Cyclic voltammetry studies show one oxidation process, which is only reversible for the Cp derivatives. DFT calculations provided a reliable way to determine ionization energies, and reflected very well the trends of the other properties, from the 95Mo NMR chemical shifts, to vibration frequencies, and oxidation potentials.