Mononuclear Molybdenum Complexes Research Articles

DFT/TDDFT Investigation on the Electronic Structure and Spectroscopic Properties of Cis-Dioxomolybdenum (VI) Complexes

In this contribution, two mononuclear molybdenum complexes with H2L tridentate (ONO) Schiff base ligand are characterized using computational techniques. Density functional theory (DFT) and its time extension (TD-DFT) calculations are performed to study the electronic structure in ground and excited state and to interpret the electronic absorption spectra in gas and aqueous phases. TDDFT calculations affirm that the LMCT charge transfer dominates for both complexes and a hypochromic effect on absorption properties is observed according to solvent substitution. All theoretical results compare nicely with available experimental data.

Mechanistic Consequences of Chelate Ligand Stabilization on Nitrogen Fixation by Yandulov–Schrock-Type Complexes

The Yandulov–Schrock catalyst, a mononuclear molybdenum complex with a tetra-coordinate triamidoamine chelate ligand with hexa-iso-propyl-terphenyl groups at the amide nitrogen atoms, catalyzes the reduction of dinitrogen to ammonia. Its turnover number is very low, which may be attributed to the (partial) loss of the chelate ligand. Protonation of an amide nitrogen atom of the ligand and subsequent reduction leads to the formation of a labile amine ligand. We find that this equatorial amine group can detach from the molybdenum center of the Yandulov–Schrock complex with a comparatively small barrier. This decomposition reaction is in direct competition with reactions producing intermediates of the Chatt–Schrock cycle. Clamping the substituents on the amide nitrogen atoms by a calix[6]arene unit (replacing the hexa-iso-propyl-terphenyl groups) successfully suppresses the detachment of a generated equatorial amine group from the molybdenum center. We discuss dinitrogen reduction according to the Chatt–Schr...

Synthesis and Structures of Mono‐ and Dinuclear Molybdenum Complexes with Reduced α‐Diimine Ligands

The mononuclear molybdenum complex [Mo(HL)2] (1), Mo–Mo single‐bond compound [HLClMo(µ‐Cl)3MoHLCl] [2; Mo–Mo 2.832(7) Å], and Mo≡Mo triple‐bond compounds [Mo2Cl2(µ‐HL)2] [3; Mo–Mo 2.194(6) Å] and [Mo2Cl2(µ‐Cl)(µ‐HL)Cl2Na(THF)3] [4; Mo–Mo 2.167(2) Å] have been synthesized by the reaction of HL {HL = [(2,6‐iPr2C6H3)NCH]2} with 0.5, 1, and 2 equiv. of MoCl5 and different amounts of Na metal. In the single‐bond compound 2, the α‐diimine ligand HL displays the normal chelating coordination mode. In contrast, the triple‐bond compounds 3 and 4 feature the less common bridging mode of α‐diimine ligands. Complexes 1–4 have been characterized by X‐ray diffraction, 1H and 13C NMR, IR spectroscopy, elemental analysis, and their electronic structures were studied by EPR and DFT computations.

Reactivity of [CpMo(CO)2]2 towards heterocyclic thiols: Synthesis, structure, and bonding in the sulfido-ligated cluster Cp3Mo3(μ-CO)2(μ-κ2-C7H4NS)(μ-S)(μ3-S)

The electronically unsaturated dimolybdenum complex [CpMo(CO)2]2 (1) reacts with the π-excessive heterocycle 2-mercapto-1-methylimidazole at 110°C to afford the mononuclear molybdenum complex CpMo(CO)2(κ2-C4H5N2S) (2) and the trimolybdenum cluster Cp3Mo3(μ-CO)2(μ-κ2-C4H5N2)(μ-S)(μ3-S) (3) in 22% and 20% yields, respectively. A similar reaction between 1 and the benzoannulated heterocycle 2-mercaptobenzothiazole furnishes CpMo(CO)2(κ2-C7H4NS2) (4) and Cp3Mo3(μ-CO)2(μ-κ2-C7H4NS)(μ-S)(μ3-S) (5) in 17% and 20% yields, respectively. Compounds 2 and 4 consist of a single molybdenum atom with two carbonyl groups, a cyclopentadienyl ligand, and a chelating heterocyclic thiolato ligand. The trimolybdenum clusters 3 and 5 consist of three cyclopentadienyl ligands, a metalated heterocyclic ligand, edge-bridging and face-capping sulfido ligands, and a pair of semi-bridging carbonyl ligands. All new compounds have been fully characterized in solution by IR and NMR spectroscopy, and the solid-state structures of 3 and 4 have been determined by single-crystal X-ray diffraction analyses. The bonding in cluster 3 has been computationally investigated by Density Functional Theory (DFT), and the data support a cluster that is electronically saturated with 48e and where both sulfido ligands function as 4e donor groups.

Molybdenum Complex as Potential Photosensitiser for Direct Water Splitting

Mononuclearmolybdenum complex with the general formula [MoTp*(NO)I(X)], where Tp* =tris(3,5-dimethylpyrazolyl)hidroborate and X =1,4-bis(pyridin-4-ylethynyl)benzene) was found to exhibit the required chemical and physical properties as a photosensitiser in photoelectrochemical cell. The complex was structurally characterised by infrared, UV-Vis and electrochemical analysis. TiO2 electrode films were prepared by paste technique and characterised by SEM and XRD. Photoelectrochemical properties of [MoTp*(NO)I(X)]-sensitisedTiO2 photoelectrode were carried out in 1.0 M NaOH electrolyte with a SCE reference electrode and a platinum counter electrode. The [MoTp*(NO)I(X)]-sensitised TiO2 photoelectrode showed positive photoinduced current compared to unsensitised TiO2 photoelectrode.

Solvent‐Free Epoxidation of Olefins Catalyzed by “[MoO2(SAP)]”: A New Mode of tert‐Butylhydroperoxide Activation

AbstractThe mononuclear molybdenum complexes [MoO2(acac)2] (1, acac=acetylacetonate), [MoO2(SAP)(MeOH)] (2), and dinuclear oxomolybdic complexes [MoO2L]2 [L=salicylideneaminophenolato (SAP, 5), salicylideneaminoethanolato (SAE, 6), salicylideneaminomethylpropanolato (SAMP, 7)] have been investigated as (pre)catalysts for the epoxidation of olefins under solvent‐free conditions, using tert‐butylhydroperoxide (TBHP, 70 % in water) as an oxidant. Complexes 6 and 7, although active, are limited by ligand hydrolysis during the catalytic process, whereas complexes 2 and 5 are not altered under catalytic conditions and yield essentially the same selectivity and activity, which is not suppressed by excess MeOH. Although these catalysts are less active than 1, their selectivity is higher (97–98 %). DFT calculations are consistent with the active form of the catalyst being the 5‐coordinate “[MoO2(SAP)]”. The oxidant is activated by forming a weak adduct stabilized by a very loose Mo⋅⋅⋅O interaction and a hydrogen bond, predisposing it to the oxygen transfer to external olefin by a mechanism closely related to Bartlett’s epoxidation with peroxyacids.

Molybdenum-Catalyzed Transformation of Molecular Dinitrogen into Silylamine: Experimental and DFT Study on the Remarkable Role of Ferrocenyldiphosphine Ligands

A molybdenum-dinitrogen complex bearing two ancillary ferrocenyldiphosphine ligands, trans-[Mo(N(2))(2)(depf)(2)] (depf = 1,1'-bis(diethylphosphino)ferrocene), catalyzes the conversion of molecular dinitrogen (N(2)) into silylamine (N(SiMe(3))(3)), which can be readily converted into NH(3) by acid treatment. The conversion has been achieved in the presence of Me(3)SiCl and Na at room temperature with a turnover number (TON) of 226 for the N(SiMe(3))(3) generation for 200 h. This TON is significantly improved relative to those ever reported by Hidai's group for mononuclear molybdenum complexes having monophosphine coligands [J. Am. Chem. Soc.1989, 111, 1939]. Density functional theory (DFT) calculations have been performed to figure out the mechanism of the catalytic N(2) conversion. On the basis of some pieces of experimental information, SiMe(3) radical is assumed to serve as an active species in the catalytic cycle. Calculated results also support that SiMe(3) radical is capable of working as an active species. The formation of five-coordinate intermediates, in which one of the N(2) ligands or one of the Mo-P bonds is dissociated, is essential in an early stage of the N(2) conversion. The SiMe(3) addition to a "hydrazido(2-)" intermediate having the NN(SiMe(3))(2) group will give a "hydrazido(1-)" intermediate having the (Me(3)Si)NN(SiMe(3))(2) group rather than a pair of a nitrido (≡N) intermediate and N(SiMe(3))(3). The N(SiMe(3))(3) generation would not occur at the Mo center but proceed after the (Me(3)Si)NN(SiMe(3))(2) group is released from the Mo center. The flexibility of the Mo-P bond between Mo and depf would play a vital role in the high catalysis of the Mo-Fe complex.

Investigations of Pyridine-2-thiol as a Ligand: Synthesis and X-ray Structures of the Mixed Mo−Mn Dinuclear Complex CpMoMn(CO)3(μ-CO)(μ-η2-pyS)(μ-η1-pyS), the Electron-Deficient Trimolybdenum Cluster Cp3Mo3(μ-CO)2(μ-S)(μ3-S)(μ-η2-NC5H4), and the Mononuclear CpMo(CO)2(μ-η2-pyS)

The reaction of Mn2(CO)6(pyS)2 (1) with (CpMo(CO)3)2 (2) at 110 °C gives the heterodinuclear complex CpMoMn(CO)3(μ-CO)(μ-η2-pyS))(μ-η1-pyS) (3), the mononuclear molybdenum complex CpMo(CO)2(μ-η2-pyS) (4), and the 46-electron trimolybdenum cluster Cp3Mo3(μ-CO)2(μ-S)(μ3-S)(μ-η2-NC5H4) (5). Compound 4 could also be prepared in high yield from the reaction of 2 with pyridine-2-thiol at 110 °C. Compound 3 represents a rare example of a Mo−Mn heterodinuclear complex containing a μ-η2-pyridine-2-thiolato, a μ-η1-pyridine-2-thiolato, and a cyclopentadienyl ligand. The 46-electron compound 5 represents a unique example of a trimolybdenum compound containing three cyclopentadienyl ligands, an ortho-metalated pyridyl ligand, a capping sulfido, a bridging sulfido, and two semibridging CO ligands. Curiously, although this complex is formally electron deficient, it is unreactive toward two-electron donors. The structures of all the compounds have been unambiguously established by single-crystal X-ray crystallography.

First heterobimetallic complexes of phosphocavitands

The synthesis and characterisation were described for tetranuclear phosphocavitand complexes 4, where each phosphorus atom is coordinated to the silver atom, and tetranuclear heterobimetallic phosphocavitand complexes 6, in which only three phosphorus atoms are coordinated to silver and the fourth phosphorus atom is coordinated to molybdenum. Compounds 4 were obtained by the reaction of AgBr with amidophosphitocavitands 1. Compounds 6 were synthesised by the reaction of AgBr with mononuclear molybdenum complexes of amidophosphocavitands 3. It was found that, in the presence of diethylammonium cation, 4a forms anionic complex [(C 2H 5) 2NH 2][cavitand·Ag 4(μ-Br) 4(μ 4-Br)] ( 5), whose structure was supported by X-ray diffraction analysis.

Structure of oxobis(phenolato)[tris(3,5-dimethyl-1-pyrazolyl)-hydroborato]molybdenum (V).

C27H32BMoN6O3, Mr = 595.34, monoclinic, P2(1)/n, a = 16.376 (6), b = 10.438 (5), c = 17.016 (8) A, beta = 107.25 (3) degrees, V = 2777.8 A3, Z = 4, Dm = 1.37, Dx = 1.43 g cm-3, Mo K alpha,lambda = 0.71073 A, mu = 5.02 cm-1, F(000) = 1228, T = 296 K, R = 0.038, wR = 0.045 for 2457 reflections. This molecule is the first structurally characterized mononuclear molybdenum complex containing a terminal aryl oxide ligand. The central molybdenum atom adopts a distorted octahedral coordination geometry with one face of the octahedron occupied by the tridentate pyrazolylborate ligand and the opposite face by the oxo and the two phenolato ligands. The structure is compared with that of the analogous benzenethiolate complex.

Tungsten and molybdenum phosphine hexahydrides: The first examples of nine coordination among mononuclear molybdenum complexes

An improved preparation and some reactions of WH 6(PMe 2Ph) 3 are described. This method has been extended to prepare the novel MoH 6 (PR 3) 3 compounds, the first mononuclear nine-coordinate molybdenum complexes.