Monomeric Molybdenum Research Articles

Structures and reactivity of monomeric MoO[formula omitted] moieties supported on ZrO[formula omitted] (111) slab: A DFT study

Supported molybdenum oxides have been widely employed as catalysts for variety of reactions including upgrading of bio-oil via hydrodeoxygenation. Herein, we perform spin-polarized density functional theory calculations to investigate the structures and reactivity of monomeric MoOx species supported on ZrO2 in the absence and presence of hydroxyl(s). Monomeric molybdenum moieties were found to have a mono-oxo (one molybdenyl), di-oxo (two molybdenyls) or a pero-oxo (one molybdenyl and Mo coordinated to a peroxide) configuration. Based on the formation energies, we find that mono-oxo moieties are the most stable among the three. This is also true for the moieties in the presence of hydroxyls on the support. Further, the computed vibrational frequencies of a mono-oxo moiety on a hydroxylated support is a closer match to the peaks observed in experiments. A thermodynamic analysis of the molybdenum oxide moieties on a hydroxylated support indicated that the dehydroxylation is favoured at high temperatures and low partial pressures of water. On adsorption of hydrogen on the oxygen of the support, hydrogen loses its electron and reduces Mo of the monomeric moiety without making an oxygen vacancy. We studied the activity of this reduced site for deoxygenation of anisole to benzene. The overall barriers for this reaction are comparable to previously computed barriers for deoxygenation of bio-oil model compounds on an oxygen-vacant molybdenum oxide surfaces, suggesting that this may be an important mechanistic pathway during a hydro-deoxygenation process.

Nature of Monomeric Molybdenum(VI) Cations in Acid Solutions Using Theoretical Calculations and Raman Spectroscopy.

The composition and structures of the two protonated species formed from uncharged molybdic acid, MoO2(OH)2(OH2)20, in strongly acidic solutions have been investigated using a combination of density functional theory calculations, first-principles molecular dynamics simulations, and Raman spectroscopy. The calculations show that both protonated species maintain the original octahedral structure of molybdic acid. Computed p Ka values indicated that the ═O moieties are the proton acceptor sites and, therefore, that MoO(OH)3(OH2)2+ and Mo(OH)4(OH2)22+ are the probable protonated forms of Mo(VI) in strong acid solutions, rather than the previously accepted MoO2(OH)2- x(OH2)2+ x x+ ( x = 1, 2) species. This finding is shown to be broadly consistent with the observed Raman spectra. Structural details of MoO(OH)3(OH2)2+ and Mo(OH)4(OH2)22+ are reported.

Monomeric and Dimeric Oxidomolybdenum(V and VI) Complexes, Cytotoxicity, and DNA Interaction Studies: Molybdenum Assisted C═N Bond Cleavage of Salophen Ligands.

Four novel dimeric bis-μ-imido bridged metal-metal bonded oxidomolybdenum(V) complexes [MoV2O2L'21-4] (1-4) (where L'1-4 are rearranged ligands formed in situ from H2L1-4) and a new mononuclear dioxidomolybdenum(VI) complex [MoVIO2L5] (5) synthesized from salen type N2O2 ligands are reported. This rare series of imido-bridged complexes (1-4) have been furnished from rearranged H3L'1-4 ligands, containing an aromatic diimine (o-phenylenediamine) "linker", where Mo assisted hydrolysis followed by -C═N bond cleavage of one of the arms of the ligand H2L1-4 took place. A monomeric molybdenum(V) intermediate species [MoVO(HL'1-4)(OEt)] (Id1-4) was generated in situ. The concomitant deprotonation and dimerization of two molybdenum(V) intermediate species (Id1-4) ultimately resulted in the formation of a bis-μ-imido bridge between the two molybdenum centers of [MoV2O2L'21-4] (1-4). The mechanism of formation of 1-4 has been discussed, and one of the rare intermediate monomeric molybdenum(V) species Id4 has been isolated in the solid state and characterized. The monomeric dioxidomolybdenum(VI) complex [MoVIO2L5] (5) was prepared from the ligand H2L5 where the aromatic "linker" was replaced by an aliphatic diimine (1,2-diaminopropane). All the ligands and complexes have been characterized by elemental analysis, IR, UV-vis spectroscopy, NMR, ESI-MS, and cyclic voltammetry, and the structural features of 1, 2, 4, and 5 have been solved by X-ray crystallography. The DNA binding and cleavage activity of 1-5 have been explored. The complexes interact with CT-DNA by the groove binding mode, and the binding constants range between 103 and 104 M-1. Fairly good photoinduced cleavage of pUC19 supercoiled plasmid DNA was exhibited by all the complexes, with 4 showing the most promising photoinduced DNA cleavage activity of ∼93%. Moreover, in vitro cytotoxic activity of all the complexes was evaluated by MTT assay, which reveals that the complexes induce cell death in MCF-7 (human breast adenocarcinoma) and HCT-15 (colon cancer) cell lines.

Selective Autooxidation of Ethanol over Titania-Supported Molybdenum Oxide Catalysts: Structure and Reactivity.

We study the selective catalytic oxidation of ethanol with air as a sustainable alternative route to acetaldehyde. The reaction is catalysed by molybdenum oxide supported on titania, in a flow reactor under ambient pressure. High selectivity to acetaldehyde (70%–89%, depending on the Mo loading) is obtained at 150 °C. Subsequently, we investigate the structure/performance relationship for various molybdenum oxide species using a combination of techniques including diffuse reflectance UV-visible, infrared, X-ray photoelectron spectroscopies, X-ray diffraction and temperature programmed reduction. As their surface density increases, the monomeric molybdenum oxide species undergo two-dimensional and three-dimensional oligomerisation. This results in polymolybdates and molybdenum oxide crystallites. Importantly, the ethanol oxidation rate depends not only on the overall molybdenum loading and dispersion, but also on the type of molybdenum oxide species prevalent at each surface density and on the domain size. As the molybdenum oxide oligomerisation increases, electron delocalisation becomes easier. This lowers the absorption edge energy and increases the reaction rate.

Structure of Isolated Molybdenum(VI) and Molybdenum(IV) Oxide Species on Silica: Periodic and Cluster DFT Studies

The structure of monomeric molybdenum oxide species on silica is still a subject under debate. In this work, a large number of advanced silica models are used to study molybdena–silica system with density functional theory. The calculated relative energies of the monooxo and dioxo Mo(VI) species depend on the location of the Mo center on the surface and on the structure of the model. Periodic and cluster calculations employing comparable models of silica give similar results. It is shown that the monooxo Mo(VI) species can be more stable than the dioxo species under dehydrated conditions, provided that the local structure of silica enables preferable 4-fold bonding to the surface. As most locations are favorable for the 2-fold bonded dioxo Mo(VI) species, they should be dominant in the molybdena–silica system, whereas the monooxo Mo(VI) species are predicted to be in minority. The calculated frequencies of the Mo═O stretching mode for the monooxo Mo(VI) species are generally higher than the frequencies of the symmetric O═Mo═O stretch for the dioxo species, corresponding to the strongest band observed experimentally. The relative energies of the reduced Mo(IV) species on silica are close to the relative energies of the corresponding Mo(VI) precursors.

The molybdenum species of MoO 3/SiO 2 and their catalytic activities for the epoxidation of propylene with cumene hydroperoxide

The MoO 3/SiO 2 catalysts containing different surface molybdenum species were prepared by a sol–gel method, and the effects of the preparation condition and MoO 3 loading on the surface molybdenum species and property of MoO 3/SiO 2 were studied. The XRD, FT-IR, UV–vis and Raman spectroscopies were used to characterize the surface molybdenum species, and temperature-programmed desorption of NH 3 adsorbed on a catalyst was used to detect the surface acidic properties. The results show that, there were the dispersed polymolybdate, α-MoO 3, β-MoO 3, monomeric molybdenum species and silicomolybdic acid on the MoO 3/SiO 2 catalyst, and their distributions and subsistence states were affected by the preparation condition and MoO 3 loading. Different molybdenum species exhibit different catalytic activities for the epoxidation of propylene with cumene hydroperoxide. In the 15 wt% MoO 3/SiO 2 catalyst synthesized at pH 9.1 and dried appropriately, there are the small size β-MoO 3 and monomeric molybdenum species that they are mainly effective catalyst components for the epoxidation of propylene. Using this catalyst, the ∼100% conversion of cumene hydroperoxide and ∼100% selectivity to propylene oxide can be obtained in the tert-butyl alcohol solvent at 2.6 MPa and 80 °C for 4 h.

Metal complexes of an amine bisphenol with a thiophene side-arm

Dioxomolybdenum(VI) and oxotungsten(VI) complexes with a new amine bisphenol ligand (H2L) are reported. The ligand which carries a neutral nitrogen atom, two phenolic oxygen atoms and a thiophene side-arm was synthesized by a simple one-pot Mannich reaction. Further reaction with [MoO2(acac)2] yielded a monomeric molybdenum complex [MoO2(L)(MeOH)] (2a) or a dimeric complex [Mo2O2(μ-O)2(L)2] (2b), depending on the reaction conditions. The reaction with a tungsten trisglycolate [W(eg)3] led to the formation of a monomeric compound [WO(eg)(L)] (3). In these complexes, the potentially tetradentate amine bisphenolate dianion coordinates as a tridentate O,N,O donor while the sulfur side-arm donor remains intact. The solid-state structure of 2a was investigated by X-ray crystallography.

A chiral diamine bis-phenolate complex of dioxomolybdenum(VI)

A new chiral tetradentate ligand reacts with [MoO 2 (acac) 2 ] to form a monomeric complex. The syntheses and X-ray crystal structures of the ligand and the complex are reported. A new dioxomolybdenum(VI) complex with a chiral tetradentate ligand is reported. The tripodal ligand containing two nitrogen atoms and two phenolic oxygen atoms was synthesized starting from a chiral diamine precursor. Further reaction with [MoO 2 (acac) 2 ] yielded a monomeric molybdenum complex as a bright yellow solid. The structures of the molybdenum complex and the free diamine bis-phenol ligand were determined by X-ray diffraction.

Effects of preparation procedure in sol–gel method on performance of MoO3/SiO2 catalyst for liquid phase epoxidation of propylene with cumene hydroperoxide

The 15% MoO3/SiO2 catalysts were synthesized by a modified base-catalyzed sol–gel method and characterized by Raman, XRD, FT-IR, UV–vis, NH3-TPD and ICP. The catalytic performance of MoO3/SiO2 for the liquid phase epoxidation of propylene with cumene hydroperoxide (CHP) and the effects of preparation procedure on its physicochemical and catalytic properties were studied. The results show that, in the processes of preparing catalyst, the solvent of ammonium heptamolybdate and the sequence of feeding would affect the existing states of molybdenum species on silica support significantly. When 1,2-propanediol was used as the solvent and an appropriate sequence of feeding was adopted, the β-MoO3 species and monomeric molybdenum species can form on silica to make for the promotion of MoO3/SiO2 catalyst. Without any depressor in the reaction system, at 80°C and 2.2MPa for 4h, the conversion of CHP is above 99% and the selectivity to PO reaches 85.3%.

N-heterocycle chelated oxomolybdenum(VI and V) complexes with bidentate citrate

A 1,10-phenanthroline (phen) chelated molybdenum(VI) citrate, [(MoO2)2O(H2cit)(phen)(H2O)2] x H2O (1) (H4cit = citric acid), is isolated from the reaction of citric acid, ammonium molybdate and phen in acidic media (pH 0.5-1.0). A citrato oxomolybdenum(V) complex, [(MoO)2O(H2cit)2(bpy)2] x 4H2O (2), is synthesized by the reduction of citrato molybdate with hydrazine hydrochloride in the presence of 2,2'-bipyridine (bpy), and a monomeric molybdenum(VI) citrate [MoO2(H2cit)(bpy)] x H2O (6) is also isolated and characterized structurally. The citrate ligand in the three neutral compounds uses the alpha-alkoxy and alpha-carboxy groups to chelate as a bidentate leaving the two beta-carboxylic acid groups free, that is different from the tridentate chelated mode in the citrato molybdate(VI and V) complexes. 1 and in solution show obvious dissociation based on 13C NMR studies.

Molybdenum Oxo and Imido Complexes of β-Diketiminate Ligands: Synthesis and Structural Aspects

Treatment of [MoO2(eta2-Pz)2] (Pz = 3,5-di-tert-butylpyrazolate) with the diketiminate ligand NacNacH (NacNac = CH[C(Me)NAr]2-, Ar = 2,6-Me2C6H3) at 55 degrees C leads under reduction of the metal to the formation of the dimeric molybdenum(V) compound [{MoO2(NacNac)}2] (1). The compound was characterized by spectroscopic means and by X-ray crystal structure analysis. The dimer consists of a [Mo2O4]2+ core with a short Mo-Mo bond (2.5591(5) A) and one coordinated diketiminate ligand on each metal atom. The reaction of [MoO2(eta2-Pz)2] with NacNacH in benzene at room temperature leads to a mixture of 1 and the monomeric molybdenum(VI) compound [MoO2(NacNac)(eta2-Pz)] (2). From such solutions, yellow crystals of 2 suitable for X-ray structural analysis were obtained revealing the coordination of one bidentate NacNac and one eta2-coordinate Pz ligand. This renders the two oxo groups inequivalent. Further high oxidation state molybdenum compounds containing the NacNac ligand were obtained by the reaction of [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H3) and [Mo(N-t-Bu)2Cl2(dme)] (dme = dimethoxyethane) with 1 equiv of the potassium salt NacNacK forming [Mo(NAr)2Cl(NacNac)] (3) and [Mo(N-t-Bu)2Cl(NacNac)] (4), respectively, in good yields. The X-ray structure analysis of 3 revealed a penta-coordinate compound where the geometry is best described as trigonal-bipyramidal.

Synthesis and properties of mononuclear and binuclear molybdenum complexes derived from bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone

The monomer molybdenum(VI) complex [MoO 2(napoxlhH 2)]·2H 2O (1) has been synthesized from the reaction of MoO 2(acac) 2 with bis(2-hydroxy-1-naphthaldehyde)oxaloyldihydrazone (napoxlhH 4) in 1:1 molar ratio in ethanol under reflux. This complex on reaction with pyridine/3-picoline/4-picoline yielded the dimer molybdenum(VI) complexes [Mo 2O 4(napoxlhH 2) 2(A) 2]·2H 2O (A = py (2), 3-pic (3), 4-pic (4)), whereas reaction with isonicotinoylhydrazine (inhH 3) and salicyloylhydrazine (sylshH 3) lead to the reduction of the metal centre yielding monomeric molybdenum(V) complexes [Mo(napoxlhH 2)(hzid)]·2H 2O (where hzidH 3 = inhH 3 (5) and sylshH 3 (6)). The complexes have been characterized by elemental analyses, molecular weight determinations, molar conductance data, magnetic moment data, electronic, IR, ESR and 1H NMR spectroscopic studies. The complexes (5) and (6) are paramagnetic to the extent of one unpaired electron. The electronic spectra of the complexes are dominated by strong charge transfer bands. In all of the complexes, the principal dihydrazone ligand has been suggested to coordinate to the metal centres in the anti- cis-configuration. The complexes (1), (5) and (6) are suggested to have six-coordinate octahedral stereochemistry around molybdenum(VI) and molybdenum(V) metal centres, respectively, while the complexes (2)–(4) are suggested to have eight coordinate dodecahedral stereochemistry around molybdenum(VI) metal centre.

Mechanism of White Phosphorus Activation by Three-Coordinate Molybdenum(III) Complexes: A Thermochemical, Kinetic, and Quantum Chemical Investigation

White phosphorus (P(4)) reacts with three-coordinate molybdenum(III) trisamides or molybdaziridine hydride complexes to produce either bridging or terminal phosphide (P(3)(-)) species, depending upon the ancillary ligand steric demands. Thermochemical measurements have been made that place the MoP triple bond dissociation enthalpy at 92.2 kcal.mol(-)(1). Thermochemical measurements together with computational analysis rule out simple P-atom abstraction from P(4) as a step in the phosphorus activation mechanism. Kinetic measurements made by the stopped-flow method show that the reaction between the monomeric molybdenum complexes and P(4) is first-order both in metal complex and in P(4). Cyclo-P(3) complexes can be obtained when ancillary ligand steric demands are small, but kinetic measurements rule them out as monometallic intermediates in the P(4) activation mechanism. Also studied by calorimetric, kinetic, and in one case variable-temperature NMR methods is the process of mu-phosphide bridge formation. Post-rate-determining steps of the P(4) activation process were examined in a search for minima on the reaction's potential energy surface, leading to the proposal of two plausible, parallel, bimetallic reaction channels.

A homoleptic molybdenum(IV) enolate complex: synthesis, molecular and electronic structure, and NCN group transfer to form a terminal cyanoimide of molybdenum(VI).

A monomeric molybdenum(IV) tetrakis enolate complex Mo(OC[Ad]Mes)(4), 1, where Ad = 2-adamantylidene and Mes = 2,4,6-Me(3)C(6)H(2), has been synthesized and characterized structurally by X-ray diffraction, chemically through NCN group-transfer reactivity, and computationally to investigate the origins of the observed structure that is intermediate between tetrahedral and square planar. No prior examples of Mo(OR)(4) have been structurally characterized despite having been the subject of both experimental and theoretical interest. Complex 1 has a singlet ground state and thus a metal-based lone pair of electrons. The latter has been visualized with the aid of the electron localization function (ELF) and appears as a two-bladed propeller with D(2)(d)() symmetry. Complex 1 makes a simple 1:1 adduct with t-BuNC that is trigonal bipyramidal with an axial isocyanide as demonstrated by X-ray crystallography. This trigonal bipyramidal 1:1 adduct has a triplet ground state and provides a model for the way in which 1 interacts with NCN group donor dbabhCN prior to NCN group transfer to form the terminal cyanoimide complex 1-NCN. The calculated Mo-N bond dissociation enthalpy for 1-NCN is 104 kcal mol(-1), 30 kcal mol(-1) greater than that for the corresponding dissociation of NCN from cyanophosphiniminato NCNPMe(3).

Stereospecific Ring-Opening Metathesis Polymerization of Cycloolefins Using Novel Molybdenum and Tungsten Complexes Having Biphenolate Ligands. Development of Crystalline Hydrogenated Poly(endo-dicyclopentadiene) and Poly(norbornene)

Novel molybdenum- and tungsten-based catalysts induced stereospecific ring-opening metathesis polymerization (ROMP) of cycloolefins to produce a new class of crystalline polymers. Various monomeric molybdenum(VI) and tungsten(VI) complexes of the general formula M(=O)(O−Ar)4 (M = Mo or W; (O−Ar)4 is two biphenolate or four phenolate ligands) were prepared. These catalysts exhibited moderate ROMP activity in the presence of cocatalyst such as n-BuLi and Et3Al. The molybdenum and tungsten complexes bearing substituted biphenolate ligands such as oxomolybdenum(VI) bis(racemic-5,5‘,6,6‘-tertamethyl-3,3‘-di-tert-butyl-1,1‘-biphenyl-2,2‘-diolate) promoted cis and isospecific ROMP of endo-dicyclopentadiene (cis > 90% and meso > 95%). The novel ternary and quarternary catalysts such as MoOCl4−biphenolate−n-BuLi (1:2:2) were developed as a new useful methodology to control the stereoselectivity of the ROMP. Hydrogenation of the cis-isotactic poly(endo-dicyclopentadiene)s and poly(norbornene)s provided novel crystalline polymers with high melting points (295 °C, ΔH = 50 J/g; 175 °C, ΔH = 60 J/g, respectively), which can be regarded as a new class of crystalline polymers.

Reductive cleavage and related reactions leading to molybdenum–element multiple bonds: new pathways offered by three-coordinate molybdenum(III)

Three-coordinate molybdenum(III) complexes comprise a new class of reactive-yet-isolable substances. Related to a well known class of metal–metal triple bonded dimers, the monomeric molybdenum(III) complexes supported by sterically-demanding N-tert-alkylanilide ligands offer previously unavailable synthetic routes to molybdenum–element multiple bonds. Terminal nitride, phosphide and carbide functionalities have been prepared from reactions with dinitrogen, white phosphorus and carbon monoxide, respectively.

Synthesis and characterization of the monomeric molybdenum trithiolate Mo(η7-dmpt)(η1-dmpt)2 (dmpt = 2,6-dimesitylphenylthiolate)

Treatment of MoCl3(THF)3 with 3 equiv. of LiSC6H3Mes2-2,6 (Mes = C6H2Me3-2,4,6) (Lidmpt) affords the monomeric molybdenum trithiolate Mo(η7-dmpt)(η1-dmpt)2, 1, which is spectroscopically and structurally characterized; the MoS3 array has distorted trigonal pyramidal geometry which is caused by strong π-interactions between molybdenum and an ortho-mesityl group from one of the thiolate substituents.

Synthesis and characterization of molybdenum(VI) oxo-species on the surface of fumed alumina and silica

Synthesis of molybdenum(VI) oxo-species on the surface of alumina and silica supports was carried out by their chemical modification with a dichloromethane solution of MoOCl 4 followed by hydrolysis of the anchored groups ( ▪ -O) n MoOCl 4− n and subsequent thermal treatment. In the case of alumina, the formation of monomeric molybdenum(VI) oxo-species which strongly interact with the support surface is observed. Mo 6+ ions in these species are found to be tetrahedrally coordinated. On the silica surface, at least two types of molybdenum(VI) oxo-species are detected. Aggregated molybdenum(VI) oxo-species in which Mo 6+ ions have an octahedral coordination appear to be predominant.

Adsorption and Impregnation of Alumina with Molybdenum or Tungsten Solutions

The equilibrium parameters of molybdenum and tungsten adsorption on alumina were compared. Adsorptions were performed from aqueous solutions of monomeric or polymeric ions of these elements. For all solutions tested, molybdenum showed a greater affinity for alumina than tungsten. Assuming a pore-filling impregnation of alumina spheres, a model that interprets this process was used for the calculation of theoretical profiles which showed a smooth decrease in concentration along the sphere radius. In turn, the experimental profiles were similar to those predicted for impregnations with solutions of heptameric molybdate, monomeric molybdate and monomeric tungstate. For polymeric tungstate, however, the experimental tungsten concentration rapidly fell to zero near the sphere surface. Using diffuse reflectance spectroscopy, it was observed that the molybdenum was in tetrahedral and octahedral coordination in spheres impregnated with polymeric or monomeric molybdenum solutions; in addition, the ratio of the concentrations of these species was approximately the same. The species observed in spheres impregnated with tungsten solutions was monomeric tungstate, indicating a distortion in tetrahedral symmetry as a consequence of interaction with the alumina.

Chemistry of molybdenum. Part 8. Syntheses, spectroscopy and electrochemical properties of molybdenum complexes containing dithioacid ligands

Complexes cis-[MoVIO2(aacda)2][aacda = 2-(alkylamino)cyclopent-1-ene-1-carbodithioate; alkyl = propyl (pacda) or butyl (bacda)] react with acids HX (X = BF4 or PF6) to form species [MoVIO(aacda)3]X 1a–1d, which contain an MoVIO core. Reactivity of the terminal oxo ligand of this core towards electron-donor reagents has been investigated. Non-oxo monomeric molybdenum(V) compounds [MoV(acda)(aacda)3]BF42a, 2b and [MoV(abt)(aacda)3]BF42c, 2d can be obtained when 2-aminocyclopent-1-ene-1-carbodithioic acid (Hacda) and o-aminobenzenethiol (Habt) are used for oxygen abstraction. With triphenylphosphine, however, the products are non-oxo molybdenum(IV) compounds [MoIV(PPh3)(aacda)3]BF43a, 3b. In frozen solution [dimethylformamide–MeCN (1 : 10 v/v), 140 K], compounds 2a–2d show ESR spectra with axial symmetry. The g and A tensor parameters (g∥≈ 1.977, g⊥≈ 1.975, A∥≈ 51.5 × 10–4 cm–1 and A⊥≈ 19.5 × 10–4 cm–1) for this series of molybdenum(V) compounds are very similar to each other indicating that the unpaired electron is located in a metal-centred orbital of essentially identical composition. Complexes 2a and 2b display two consecutive reversible one-electron waves in their cyclic voltammograms at E½=–0.24 and –0.45 V vs. saturated calomel electrode (SCE), which correspond to MoV–MoIV and MoIV–MoIII redox couples respectively. The molybdenum(IV) compounds 3a and 3b are seven-co-ordinate with a spin-triplet ground state (µeff, 2.48 and 2.53) and show two distinct redox waves due to the interconversion of the three [Mo(PPh3)(aacda)3]z(z=+1, 0 or –1) species. Half-wave potentials, E1/2, for these processes are at –0.36 (MoIV–MoIII) and –0.90 V (MoIII–MoII)vs. SCE. The presence of the strongly π-acceptor phosphine ligand facilitates the electrochemical generation of relatively scarce molybdenum(II) species.