Mo Bis Research Articles

Tuning the redox properties of a [4Fe-4S] center to modulate the activity of Mo-bisPGD periplasmic nitrate reductase

Molybdoenzymes are ubiquitous in living organisms and catalyze, for most of them, oxidation-reduction reactions using a large range of substrates. Periplasmic nitrate reductase (NapAB) from Rhodobacter sphaeroides catalyzes the 2-electron reduction of nitrate into nitrite. Its active site is a Mo bis-(pyranopterin guanine dinucleotide), or Mo-bisPGD, found in most prokaryotic molybdoenzymes. A [4Fe-4S] cluster and two c-type hemes form an intramolecular electron transfer chain that deliver electrons to the active site. Lysine 56 is a highly conserved amino acid which connects, through hydrogen-bonds, the [4Fe-4S] center to one of the pyranopterin ligands of the Mo-cofactor. This residue was proposed to be involved in the intramolecular electron transfer, either defining an electron transfer pathway between the two redox cofactors, and/or modulating their redox properties.In this work, we investigated the role of this lysine by combining site-directed mutagenesis, activity assays, redox titrations, EPR and HYSCORE spectroscopies. Removal of a positively-charged residue at position 56 strongly decreased the redox potential of the [4Fe-4S] cluster at pH 8 by 230 mV to 400 mV in the K56H and K56M mutants, respectively, thus affecting the kinetics of electron transfer from the hemes to the [4Fe-4S] center up to 5 orders of magnitude. This effect was partly reversed at acidic pH in the K56H mutant likely due to protonation of the imidazole ring of the histidine. Overall, our study demonstrates the critical role of a charged residue from the second coordination sphere in tuning the reduction potential of the [4Fe-4S] cluster in RsNapAB and related molybdoenzymes.

Synthesis and characterization of cyano and isocyano complexes of bis(dithiolato) molybdenum using Me3SiCN: a route to a cyanide-bridged multimer to a monomer

The Mo(iv) bis (dithiolate) complex showed diverse reactions with Me3SiCN in the presence and absence of a coligand and under protic and aprotic media.

Catalytic Production of Isothiocyanates via a Mo(II)/Mo(IV) Cycle for the “Soft” Sulfur Oxidation of Isonitriles

In the presence of excess amounts of elemental sulfur, the dimolybdenum dinitrogen complex {Cp*Mo[N(iPr)C(Ph)N(iPr)]}2(μ-N2) (4; Cp* = η5-C5Me5) serves as a precatalyst for the production of isothiocyanates from isonitriles via highly efficient and atom-economical metal-mediated sulfur atom transfer (SAT) under mild conditions. Mechanistic and structural studies support a catalytic cycle for SAT involving initial formation of a Mo(II) bis(isonitrile) complex that then undergoes sulfination to generate a formal “side-bound” Mo(IV) κ2-(C,S)-isothiocyanate as the key intermediate. This metal-catalyzed SAT process has further been employed for the “on-demand” production of isothiocyanates that are trapped in situ by benzhydrazides to provide thiosemicarbazides, which are useful precursors to biologically active thiadiazoles.

ChemInform Abstract: Catalyst‐Controlled Stereoselective Olefin Metathesis as a Principal Strategy in Multistep Synthesis Design: A Concise Route to (+)‐Neopeltolide.

Molybdenum-, tungsten-, and ruthenium-based complexes that control the stereochemical outcome of olefin metathesis reactions have been recently introduced. However, the complementary nature of these systems through their combined use in multistep complex molecule synthesis has not been illustrated. A concise diastereo- and enantioselective route that furnishes the anti-proliferative natural product neopeltolide is now disclosed. Catalytic transformations are employed to address every stereochemical issue. Among the featured processes are an enantioselective ring-opening/cross-metathesis promoted by a Mo monoaryloxide pyrrolide (MAP) complex and a macrocyclic ring-closing metathesis that affords a trisubstituted alkene and is catalyzed by a Mo bis(aryloxide) species. Furthermore, Z-selective cross-metathesis reactions, facilitated by Mo and Ru complexes, have been employed in the stereoselective synthesis of the acyclic dienyl moiety of the target molecule.

Catalyst‐Controlled Stereoselective Olefin Metathesis as a Principal Strategy in Multistep Synthesis Design: A Concise Route to (+)‐Neopeltolide

AbstractMolybdenum‐, tungsten‐, and ruthenium‐based complexes that control the stereochemical outcome of olefin metathesis reactions have been recently introduced. However, the complementary nature of these systems through their combined use in multistep complex molecule synthesis has not been illustrated. A concise diastereo‐ and enantioselective route that furnishes the anti‐proliferative natural product neopeltolide is now disclosed. Catalytic transformations are employed to address every stereochemical issue. Among the featured processes are an enantioselective ring‐opening/cross‐metathesis promoted by a Mo monoaryloxide pyrrolide (MAP) complex and a macrocyclic ring‐closing metathesis that affords a trisubstituted alkene and is catalyzed by a Mo bis(aryloxide) species. Furthermore, Z‐selective cross‐metathesis reactions, facilitated by Mo and Ru complexes, have been employed in the stereoselective synthesis of the acyclic dienyl moiety of the target molecule.

Catalyst-controlled stereoselective olefin metathesis as a principal strategy in multistep synthesis design: a concise route to (+)-neopeltolide.

Molybdenum-, tungsten-, and ruthenium-based complexes that control the stereochemical outcome of olefin metathesis reactions have been recently introduced. However, the complementary nature of these systems through their combined use in multistep complex molecule synthesis has not been illustrated. A concise diastereo- and enantioselective route that furnishes the anti-proliferative natural product neopeltolide is now disclosed. Catalytic transformations are employed to address every stereochemical issue. Among the featured processes are an enantioselective ring-opening/cross-metathesis promoted by a Mo monoaryloxide pyrrolide (MAP) complex and a macrocyclic ring-closing metathesis that affords a trisubstituted alkene and is catalyzed by a Mo bis(aryloxide) species. Furthermore, Z-selective cross-metathesis reactions, facilitated by Mo and Ru complexes, have been employed in the stereoselective synthesis of the acyclic dienyl moiety of the target molecule.

Molybdenum catalyzed ammonia borane dehydrogenation: oxidation state specific mechanisms.

Though numerous catalysts for the dehydrogenation of ammonia borane (AB) are known, those that release >2 equiv of H2 are uncommon. Herein, we report the synthesis of Mo complexes supported by a para-terphenyl diphosphine ligand, 1, displaying metal–arene interactions. Both a Mo0 N2 complex, 5, and a MoII bis(acetonitrile) complex, 4, exhibit high levels of AB dehydrogenation, releasing over 2.0 equiv of H2. The reaction rate, extent of dehydrogenation, and reaction mechanism vary as a function of the precatalyst oxidation state. Several Mo hydrides (MoII(H)2, [MoII(H)]+, and [MoIV(H)3]+) relevant to AB chemistry were characterized.

High-Resolution Electron Spectroscopy and Rotational Conformers of Group 6 Metal (Cr, Mo, and W) Bis(mesitylene) Sandwich Complexes

Group 6 metal-bis(mesitylene) sandwich complexes are produced by interactions between the laser-vaporized metal atoms and mesitylene vapor in a pulsed molecular beam source, identified by photoionization time-of-flight mass spectrometry, and studied by pulsed-field ionization zero-electron kinetic energy spectroscopy and density functional theory calculations. Although transition metal-bis(arene) sandwich complexes may adopt eclipsed and staggered conformations, the group 6 metal-bis(mesitylene) complexes are determined to be in the eclipsed form. In this form, rotational conformers with methyl group dihedral angles of 0 and 60° are identified for the Cr complex, whereas the 0° rotamer is observed for the Mo and W species. The 0° rotamer is in a C(2v) symmetry with the neutral ground state of (1)A1 and the singly positive charged ion state of (2)A1. The 60° rotamer is in a C(i) symmetry with the neutral ground state of (1)A(g) and the ion state of (2)A(g). Partial conversion of the 60 to 0° rotamer is observed from He to He/Ar supersonic expansion for Cr-bis(mesitylene). The unsuccessful observation of the 60° rotamer for the Mo and W complexes is the result of its complete conversion to the 0° rotamer in both He and He/Ar expansions. The adiabatic ionization energies of the 0° rotamers of the three complexes are in the order of Cr-bis(mesitylene) < W-bis(mesitylene) < Mo-bis(mesitylene), which is different from that of the metal atoms. These metal-bis(mesitylene) complexes have lower ionization energies than the corresponding metal-bis(benzene) and -bis(toluene) species.

Efficient and Selective Formation of Macrocyclic Disubstituted Z Alkenes by Ring‐Closing Metathesis (RCM) Reactions Catalyzed by Mo‐ or W‐Based Monoaryloxide Pyrrolide (MAP) Complexes: Applications to Total Syntheses of Epilachnene, Yuzu Lactone, Ambrettolide, Epothilone C, and Nakadomarin A

The first broadly applicable set of protocols for efficient Z-selective formation of macrocyclic disubstituted alkenes through catalytic ring-closing metathesis (RCM) is described. Cyclizations are performed with 1.2-7.5 mol% of a Mo- or W-based monoaryloxide pyrrolide (MAP) complex at 22 °C and proceed to complete conversion typically within two hours. Utility is demonstrated by synthesis of representative macrocyclic alkenes, such as natural products yuzu lactone (13-membered ring: 73% Z) epilachnene (15-membered ring: 91% Z), ambrettolide (17-membered ring: 91% Z), an advanced precursor to epothilones C and A (16-membered ring: up to 97% Z), and nakadomarin A (15-membered ring: up to 97% Z). We show that catalytic Z-selective cyclizations can be performed efficiently on gram-scale with complex molecule starting materials and catalysts that can be handled in air. We elucidate several critical principles of the catalytic protocol: 1) The complementary nature of the Mo catalysts, which deliver high activity but can be more prone towards engendering post-RCM stereoisomerization, versus W variants, which furnish lower activity but are less inclined to cause loss of kinetic Z selectivity. 2) Reaction time is critical to retaining kinetic Z selectivity not only with MAP species but with the widely used Mo bis(hexafluoro-tert-butoxide) complex as well. 3) Polycyclic structures can be accessed without significant isomerization at the existing Z alkenes within the molecule.

Necessity of fine tuning in Mo(iv) bis(dithiolene) complexes to warrant nitrate reduction

Four desoxo Mo(iv) bis(dithiolene) complexes, [Et(4)N][Mo(IV)(PPh(3))(SC(6)H(4)-p-Me)(mnt)(2)] () (mnt = maleonitrile dithiolate), [PNP][Mo(IV)(PPh(3))(SC(6)H(4)-o-COOH)(mnt)(2)].CH(3)CN.(i)PrOH () (PNP is [Ph(3)PNPPh(3)](+)), [Et(4)N][Mo(IV)(PPh(3))(NCS)(mnt)(2)] () and [Et(4)N](2)[Mo(IV)(NCS)(2)(mnt)(2)] () are synthesized and characterized structurally. Complexes and are found to release the pentacoordinated species, [Mo(IV)(SR)(mnt)(2)](1-) (R = -C(6)H(4)-o-COOH, -C(6)H(4)-p-Me) due to the dissociation of coordinated PPh(3) in solution and this pentacoordinated species can then reduce nitrate to nitrite. But unlike the model complex of nitrate reductase, [Et(4)N][Mo(IV)(PPh(3))(SPh)(mnt)(2)] () (A. Majumdar, K. Pal and S. Sarkar, J. Am. Chem. Soc. 2006, 128, 4196), these reactions remain incomplete. Such inefficiency of complexes and to mediate a complete reduction of nitrate is attributed to the steric bulk exerted by the para-methyl and ortho-carboxylic acid functionalities thereby retarding the formation of the nitrate bound Mo(iv) complex necessary for nitrate reduction. Complex does not release a pentacoordinated species in solution and hence is unable to reduce nitrate. Although dissociates one coordinated NCS ligand in solution to release a penta-coordinated species {[Mo(IV)(NCS)(mnt)(2)]}(1-), which remains inactive towards nitrate reduction indicating the necessity of thiolate coordination and hence the necessity of fine electronic tuning to effect nitrate reduction. Complexes , and can be converted to or depending on the amount of thiocyanate employed. Complexes and undergo a facile interconversion among themselves. Substitution of essential thiolate coordination rather than replacing the dissociable PPh(3) in and by thiocyanate resulted in inactivation (formation of ) towards nitrate reduction which is somewhat similar to the dead-end type inhibition often encountered in native systems. These results followed by theoretical calculations at DFT level establish the necessity of fine stereoelectronic tuning at the axial position of desosxo molybdenum bis(dithiolene) complexes to warrant nitrate reduction.

Preparation of a bis(hydrosulfido) complex of Mo having a tetraphosphine co-ligand and its transformation into MoRh2 and MoIr2 mixed-metal sulfido clusters

The treatment of a Mo(0) complex with a tetraphosphine co-ligand [Mo(kappa(4)-P4)(Ph(2)PCH(2)CH(2)PPh(2))] (P4 = meso-o-C(6)H(4)(PPhCH(2)CH(2)PPh(2))(2)) with H(2)S gas in toluene at room temperature afforded [Mo(SH)(2)(kappa(4)-P4)] (1), which demonstrates the first Mo(II) bis(hydrosulfido) complex. Its trigonal-prismatic structure rarely observed for Mo(II) complexes has been determined by X-ray analysis. The reactions of 1 with [RhCl(CO)(2)](2) and [IrCl(CO)(2)(p-toluidine)] in the presence of NEt(3) resulted in the formation of the sulfido-bridged trinuclear clusters [Mo(kappa(4)-P4)(mu(3)-S)(2){Rh(CO)(2)}(2)] (4) and [Mo(kappa(4)-P4)(mu(3)-S)(2){Ir(CO)(2)}(2)] (5), respectively. The X-ray diffraction study has disclosed that the MoRh(2) cluster 4 in the solid state exists in two isomeric forms arising from the different orientation of P4 to the trigonal-prismatic Mo center, whereas the crystals of the Ir analogue 5 contains the molecules corresponding to only one of the two isomers observed for 4. The fluxional features of 1, 4, and 5 in solution have been confirmed by VT NMR studies.

EPR and redox properties of periplasmic nitrate reductase from Desulfovibrio desulfuricans ATCC 27774

Nitrate reductases are enzymes that catalyze the conversion of nitrate to nitrite. We report here electron paramagnetic resonance (EPR) studies in the periplasmic nitrate reductase isolated from the sulfate-reducing bacteria Desulfovibrio desulfuricans ATCC 27774. This protein, belonging to the dimethyl sulfoxide reductase family of mononuclear Mo-containing enzymes, comprises a single 80-kDa subunit and contains a Mo bis(molybdopterin guanosine dinucleotide) cofactor and a [4Fe-4S] cluster. EPR-monitored redox titrations, carried out with and without nitrate in the potential range from 200 to -500 mV, and EPR studies of the enzyme, in both catalytic and inhibited conditions, reveal distinct types of Mo(V) EPR-active species, which indicates that the Mo site presents high coordination flexibility. These studies show that nitrate modulates the redox properties of the Mo active site, but not those of the [4Fe-4S] center. The possible structures and the role in catalysis of the distinct Mo(V) species detected by EPR are discussed.

Preparation and Properties of the Corner-Shared Double Cube [Mo(6)BiS(8)(H(2)O)(18)](8+) as a Derivative of [Mo(3)S(4)(H(2)O)(9)](4+) in Aqueous Acidic Solutions.

The reaction of [Mo(3)S(4)(H(2)O)(9)](4+) with Bi(III) in the presence of BH(4)(-) (rapid), or with Bi metal shot (3-4 days), gives a heterometallic cluster product. The latter has been characterized as the corner-shared double cube [Mo(6)BiS(8)(H(2)O)(18)](8+) by the following procedures. Analyses by ICP-AES confirm the Mo:Bi:S ratio as 6:1:8. Elution from a cation-exchange column by 4 M Hpts (Hpts = p-toluenesulfonic acid), but not 2 M Hpts (or 4 M HClO(4)), is consistent with a high charge. The latter is confirmed as 8+ from the 3:1 stoichiometries observed for the oxidations with [Co(dipic)(2)](-) or [Fe(H(2)O)(6)](3+) yielding [Mo(3)S(4)(H(2)O)(9)](4+) and Bi(III) as products. Heterometallic clusters [Mo(6)MS(8)(H(2)O)(18)](8+) are now known for M = Hg, In, Tl, Sn, Pb, Sb, and Bi and are a feature of the P-block main group metals. The color of [Mo(6)BiS(8)(H(2)O)(18)](8+) in 2.0 M Hpts (turquoise) is different from that in 2.0 M HCl (green-blue). Kinetic studies (25 degrees C) for uptake of a single chloride k(f) = 0.80 M(-)(1) s(-)(1), I = 2.0 M (Hpts), and the high affinity for Cl(-) (K > 40 M(-)(1)) exceeds that observed for complexing at Mo. A specific heterometal interaction of the Cl(-) not observed in the case of other double cubes is indicated. The Cl(-) can be removed by cation-exchange chromatography with retention of the double-cube structure. Kinetic studies with [Co(dipic)(2)](-) and hexaaqua-Fe(III) as oxidants form part of a survey of redox properties of this and other clusters. The Cl(-) adduct is more readily oxidized by [Co(dipic)(2)](-) (factor of approximately 10) and is also more air sensitive.

Comparison between Mo BIS and Mo K-Edge Absorption Spectrum

The extended X-ray bremsstrahlung isochromat fine structure of bcc Mo for the photon energy 5415 eV is studied and compared with the extended X-ray absorption fine structure at the Mo K-edge from the literature. In both spectra a minimum of oscillation amplitude is observed at the momentum 4.2 and 5.3 Α-1 for extended X-ray bremsstrahlung isochromat fine structure and extended X-ray absorption fine structure respectively, which is caused by the Ramsauer—Townsend effect at 5.5 A -1 and by beating effect at 3.7 A -1 from the first and second atomic shells. The Fourier transform of Mo extended X-ray bremsstrahlung isochromat fine structure was found to be in good agreement with the radial structure function of bcc Mo only in the case when the integration was performed in momentum range below 4.2 Α -1 .