Apical Oxo Ligand Research Articles

Syntheses of Molybdenum Oxo Benzylidene Complexes.

The reaction between Mo(O)(CHAro)(ORF6)2(PMe3) (Aro = ortho-methoxyphenyl, ORF6 = OCMe(CF3)2) and 2 equiv of LiOHMT (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) leads to Mo(O)(CHAro)(OHMT)2, an X-ray structure of which shows it to be a trigonal bipyramidal anti benzylidene complex in which the o-methoxy oxygen is coordinated to the metal trans to the apical oxo ligand. Addition of 1 equiv of water (in THF) to the benzylidyne complex, Mo(CArp)(OR)3(THF)2 (Arp = para-methoxyphenyl, OR = ORF6 or OC(CF3)3 (ORF9)) leads to formation of {Mo(CArp)(OR)2(μ-OH)(THF)}2(μ-THF) complexes. Addition of 1 equiv of a phosphine (L) to Mo(CArp)(ORF9)3(THF)2 in THF, followed by addition of 1 equiv of water, all at room temperature, yields Mo(O)(CHArp)(ORF9)2(L) complexes in good yields for several phosphines (e.g., PMe2Ph (69% by NMR), PMePh2 (59%), PEt3 (69%), or P( i-Pr)3 (65%)). The reaction between Mo(O)(CHArp)(ORF9)2(PEt3) and 2 equiv of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo(O)(CHArp)(OHMT)2, a four-coordinate syn alkylidene complex. Mo(O)(CHArp)(OHMT)2 reacts with ethylene (1 atm in C6D6) to give (in solution) a mixture of Mo(O)(CHArp)(OHMT)2, Mo(O)(CH2)(OHMT)2, and an unsubstituted square pyramidal metallacyclobutane complex, Mo(O)(CH2CH2CH2)(OHMT)2, along with ethylene and ArpCH═CH2. Mo(O)(CHArp)(OHMT)2 also reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the "first-insertion" products that contain a cis C═C bond.

C–S Bond Cleavage during Reactions between N,N‐Diethylthiocarbamoyliminothianes and [MOCl4]– Complexes (M = Re, Tc)

Abstract[MOCl4]– complexes (M = Re, Tc) react with thianes derived from N‐[(diethylamino)(thiocarbonyl)]benzamidoyl chloride and 2‐mercaptophenol (HL1) and 2‐mercaptoaniline (HL2) under C–S bond cleavage and formation of oxorhenium(V) and oxotechnetium(V) complexes with the resulting decomposition products 2‐mercaptophenol and N‐[(dialkylamino)thiocarbonyl]‐N′‐2‐mercaptophenyl)benzamidine (H2L2a), respectively. The products, (NBu4)[ReO(S,O‐C6H4)2], [ReOCl(L2a)] and [TcO(L2a)] were obtained in good yields and isolated in crystalline form. Single‐crystal X‐ray studies reveal square‐pyramidal coordination environments with apical oxo ligands for all three product complexes under study.

Transition metal complexes with thiosemicarbazide-based ligands, Part 58. Synthesis, spectral and structural characterization of dioxovanadium(V) complexes with salicylaldehyde thiosemicarbazone

The first two complexes of dioxovanadium(V) with salicylaldehyde thiosemicarbazone (SALTSC), of the coordination formulas [VO2(SALTSC-H)]?H2O (1) and NH4[VO2(SALTSC-2H] (2), were synthesized and characterized by elemental analysis, conductometric measurements, IR and UV-vis spectroscopy and X-ray analysis. The complexes were obtained in the reaction of an aqueous ammoniacal solution of NH4VO3 and SALTSC. The results of the characterization showed that SALTSC was coordinated in the usual ONS tridentate mode as monoanion in complex 1 and dianion in complex 2. In both complexes, the vanadium atom is in a deformed square-pyramidal environment and is slightly shifted towards the apical oxo-ligand (? 0.52 ?).

Synthesis, characterization and structural analysis of isostructural dinuclear molybdenum and tungsten oxo-bis-μ-sulfido-benzenedithiolene complexes

Reactions of the molybdenum and tungsten precursors [MO 2S 2] 2− with equimolar amounts of benzenedithiol in acetonitrile give the title compounds [M 2O 2(μ-S) 2(bdt) 2] 2− with M = Mo, W and bdt = benzene-dithiolate. In case a tungsten to ligand ratio of 1:2 is used the dimer forms as well but only as a minor species whereas the monomer [WO(bdt) 2] 2− is the main product. In both dimeric compounds the syn-isomers are formed referring to the position of the apical oxo ligands with respect to the M 2S 2 plane. For the molybdenum compound this contrasts with a published crystal structure of the anti-isomer. Both complexes give highly symmetric isomorphous crystals but still show subtle differences in their bond lengths and angles around the central metal. The X-ray crystal structures of both are analyzed in detail and compared with each other and with the isomeric molybdenum compound. Differences and similarities between tungsten and both isomers of molybdenum complexes are shown to be more influenced by the conformation than by the central metal and a reason for the formation of syn- and anti-isomers based on the respective synthetic procedures is proposed.

Phthalocyanine as a Chemically Inert, Redox-Active Ligand: Structural and Electronic Properties of a Nb(IV)-Oxo Complex Incorporating a Highly Reduced Phthalocyanine(4−) Anion

This report describes the reduction of a niobium(V) phthalocyanine complex and investigation of the electronic structure of the resulting products. The reduction of PcNbCl(3) (Pc = phthalocyanine dianion) with 5.5 equiv of potassium graphite in 1,2-dimethoxyethane (DME) resulted in the isolation of K(2)PcNbO.5DME (1a). Addition of 18-crown-6 to 1a gave [K(18-crown-6)](2)(mu-DME)PcNbO (1b). Both 1a and 1b were structurally characterized by single-crystal X-ray diffraction analysis. In both complexes, the niobium center adopts a square pyramidal geometry and is coordinated by four basal Pc nitrogen atoms and an apical oxo ligand. Notably, the Pc ligand in 1a is saddle-shaped, with significant bond length alternation, rather than flat with delocalized bonding. The production of ethylene during the reduction of PcNbCl(3), detected by gas chromatography/mass spectrometry (GC/MS), suggests that the oxo ligand likely results from double C-O bond activation of DME solvent. A combination of spectroscopic techniques and density functional theory (DFT) calculations were used to establish the electronic structure of 1a. The close correspondence of the electronic absorption spectrum of 1a to that of [PcZn](2-) with a di-reduced Pc(4-) ligand, indicates a similar electronic structure for the two complexes. Evaluation of the electronic transitions for 1a and [PcZn](2-) by time-dependent DFT calculations further suggests a similar electronic structure for both complexes, indicating that differences in symmetry between 1a and [PcZn](2-) do not significantly affect the nature of the electronic transitions. Electron paramagnetic resonance (EPR) spectroscopy of 1a in solution at room temperature gave a 10-line spectrum, while frozen-solution X- and Q-band EPR spectra are consistent with powder-pattern spectra defined by uniaxial g and (93)Nb hyperfine tensors: these imply the presence of a d(1) Nb(IV) metal center. EPR and electron nuclear double resonance spectroscopy suggests that the spin density in 1a is centered almost completely on the niobium, in agreement with the DFT calculations. These results illustrate the value of Pc as a chemically inert, redox-active ligand for stabilizing reactive metal centers.

Synthesis of Structural Analogues of the Oxidized Sites in the Xanthine Oxidoreductase Enzyme Family

The oxidized active site of members of the xanthine oxidoreductase enzyme family consists of a pyranopterindithiolene ligand chelated to square pyramidal (SP) Mo(VI) with an apical oxo ligand and basal sulfido and oxo or hydroxo ligands. The first structural analogues of this sites have been synthesized and structurally characterized as [WVIO2S (bdt)]2- for the unprotonated site and [WVIOS(OSiPri3)(bdt)]1- for the protonated site in which the silyloxide simulates protonation. Complexes were isolated as Et4N+ salts. The bdt and sulfido ligands occupy basal positions. In addition, (Et4N)2[WVIOS2(S2C2Me2)] was prepared. Density functional theory calculations predict small structural preferences (<1 kcal/mol) for SP structures with basal versus apical sulfido ligands and for SP versus trigonal bipyramidal (TBP) structures (<2 kcal/mol). In the crystalline state, [WVIOS2(S2C2Me2)]2- occurs in both (distorted) SP and TBP configurations, presumably stabilized by weak lattice forces (bdt = benzene-1,2-dithiolate(...

Monodithiolene molybdenum(V, VI) complexes: a structural analogue of the oxidized active site of the sulfite oxidase enzyme family.

The active sites of the xanthine oxidase and sulfite oxidase enzyme families contain one pterin-dithiolene cofactor ligand bound to a molybdenum atom. Consequently, monodithiolene molybdenum complexes have been sought by exploratory synthesis for structural and reactivity studies. Reaction of [MoO(S(2)C(2)Me(2))(2)](1-) or [MoO(bdt)(2)](1-) with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl(2)(S(2)C(2)Me(2))](1-) (1) or [MoOCl(2)(bdt)](1-) (2), which undergoes ligand substitution reactions to form other monodithiolene complexes [MoO(2-AdS)(2)(S(2)C(2)Me(2))](1-) (3), [MoO(SR)(2)(bdt)](1-) (R = 2-Ad (4), 2,4,6-Pr(i)(3)C(6)H(2) (5)), and [MoOCl(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (6) (Ad = 2-adamantyl, bdt = benzene-1,2-dithiolate). These complexes have square pyramidal structures with apical oxo ligands, exhibit rhombic EPR spectra, and 3-5 are electrochemically reducible to Mo(IV)O species. Complexes 1-6 constitute the first examples of five-coordinate monodithiolene Mo(V)O complexes; 6 approaches the proposed structure of the high-pH form of sulfite oxidase. Treatment of [MoO(2)(OSiPh(3))(2)] with Li(2)(bdt) in THF affords [MoO(2)(OSiPh(3))(bdt)](1-) (8). Reaction of 8 with 2,4,6-Pr(i)(3)C(6)H(2)SH in acetonitrile gives [MoO(2)(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (9, 55%). Complexes 8 and 9 are square pyramidal with apical and basal oxo ligands. With one dithiolene and one thiolate ligand of a square pyramidal Mo(VI)O(2)S(3) coordination unit, 9 closely resembles the oxidized sites in sulfite oxidase and assimilatory nitrate reductase as deduced from crystallography (sulfite oxidase) and Mo EXAFS. The complex is the first structural analogue of the active sites in fully oxidized members of the sulfite oxidase family. This work provides a starting point for the development of both structural and reactivity analogues of members of this family.

Basicity of uranyl oxo ligands upon coordination of alkoxides.

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide ligands. Imposition of steric congestion at the metal center prevents oxo-alkoxide ligand exchange in the reactions using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3. Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions, and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group. These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4, single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of aggregation of compounds 2-4 in various solvents. Vibrational spectroscopic measurements of 2-4 imply that, in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl U=O bonds are weakened. Crystal data are as follows. 1: monoclinic space group C2/m, a = 10.6192(8) A, b = 18.36(1) A, c = 10.6151(8) A, beta = 109.637(1) degrees, V = 1949.1(3) A3, Z = 2, dcalc = 1.297 g cm-3. Refinement of 2065 reflections gave R1 = 0.045. 2: monoclinic space group P2(1)/c, a = 6.1796(4) A, b = 15.669(1) A, c = 16.169(1) A, beta = 95.380(1) degrees, V = 1558.7(2) A3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections gave R1 = 0.036. 4: tetragonal space group I4, a = 17.8570(6) A, b = 17.8570(6) A, c = 11.4489(6) A, V = 3650.7(3) A3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.

Vanadium complexes as models for vanadium species of marine organisms. Synthesis, X-ray structure of oxo( O,O-sulfate)(2,2′:6′,2″-terpyridine)vanadium(IV) hydrate, and density functional geometry optimization analysis of vanadyl complexes

The addition of an alcoholic solution of 2,2′:6′,2″-terpyridine (terpy) to a vanadyl sulfate (VOSO 4·3H 2O) aqueous solution (molar ratio 1:1) produced a dark green solution from which brown single crystals suitable for X-ray diffraction studies grew within 12 h. The crystals contain neutral complex molecules [VO(terpy)( O,O-SO 4)] and water molecules (at a ratio 1:0.5). The complex molecules have a hexacoordinate vanadium(IV) center linked to the three nitrogen atoms from terpy, to the apical oxo ligand and to two oxygen atoms from sulfate. The analysis of the geometrical parameters relevant to the VSO 4 group shows some significant distortion of the SO 4 2− ligand upon chelation to the V(IV)O core. The two S–O(d, donor) bonds (lengths: 1.495(3) trans to oxo; 1.546(3) Å cis to oxo) are stretched differently when compared to the S–O(t, terminal) bonds (1.447(3) Å). The effect is not due to packing forces but comes from electronic effects. This is shown by a DFT geometry optimization analysis carried out at the density functional B3LYP/LANL2DZ level on model complexes of the type [VO(H 2O)] 2+, [VOL] 2+ (L, NHCH–CH N–C(CH 2)–CH NH), [VO(H 2O) 2L] 2+ and [VOL( O,O-SO 4)]. This analysis shows also that the [VOL] 2+ moiety has a very large affinity for the sulfate anion; the formation energy at the gas phase for [VOL( O,O-SO 4)] from [VOL] 2+ and SO 4 2− is −448.11 kcal mol −1, much higher than the corresponding value for [VO(H 2O) 2L] 2+ (−91.47). This can be a rationale for the role vanadyl–sulfate interactions play in blood cells of Ascidians.

N,N'-Bis(5-methoxysalicylidene)-1,2-diphenyl-1,2-ethenediamine]oxovanadium(IV)

The title complex, [VO(C 30 H 24 N 2 O 4 )], adopts a monomeric structure in the crystals, which are red due to the charge-transfer band from the conjugated π-system of the tetradentate Schiff base ligand. The geometry around the V IV atom is distorted square pyramidal. The V=O distance is 1.588 (3) A, and the V atom is displaced by 0.57 (1) A from the N 2 O 2 coordination plane towards the apical oxo ligand.

Synthesis and Reactions of Tungsten Oxo Vinylalkylidene Complexes: Reactions of WCl2(O)(PX3) (X = OMe, R) Precursors with 3,3-Diphenylcyclopropene

Complexes of the type WCl2(O)[PX3]3 (PX3 = P(OMe)3 (1); PMePh2 (2)) were synthesized according to procedures previously described in the literature. Upon treatment with 3,3-diphenylcyclopropene, complexes 1 and 2 afforded the corresponding η2-cyclopropene complexes. Spectroscopic data of W(η2-diphenylcyclopropene)Cl2(O)[PX3]2 (PX3 = P(OMe)3 (4); PMePh2 (5)) were consistent with an octahedral geometry in which the two mutually trans PX3 ligands and the cyclopropene ligand occupy equatorial positions cis to the apical oxo ligand. The thermal rearrangement of the η2-cyclopropene complexes leads to the dimerization product of 3,3-diphenylcyclopropene: 1,1,6,6-tetraphenyl-1,3,5-hexatriene. The presence of carbene species before the dimerization occurs has been demonstrated. Upon treatment with 2 equiv of lithium hexafluoro-tert-butoxide, complexes 4 and 5 give the vinyl alkylidene complexes W(CHCHCPh2)(O)[OC(CH3)(CF3)2]2[PX3] (PX3 = P(OMe)3 (10); PMePh2 (7)). The spectroscopic data of 10 and 7 indicated a dis...