Charles G. Young

Mixed-valence compounds of the early transition metals

Caracterisation structurale par diffraction RX et etude spectrometrique de complexes de metaux de transition polynucleaires pontes avec des coordinats organiques

Catalytic decarbonylation, hydroacylation, and resolution of racemic pent-4-enals using chiral bis(di-tertiary-phosphine) complexes of rhodium(I)

Abstract Attempts to decarbonylate racemic aldehydes catalytically using rhodium(I) complexes containing chiral di-tertiary-phosphine ligands are described. Incorporation of an alkenic moiety into the aldehyde, for subsequent probing of induced asymmetry by chiral shift reagents, leads instead to formation of optically active hydroacylated products via kinetic resolution of the precursor racemic aldehyde. For example, ( RS )-2-methyl-2-phenylpent-4-enal ( 1a ) yields, on treatment with [Rh( S,S -chiraphos) 2 ]Cl, 2-methyl-2-phenylcyclopentanone with up to 69% e.e. of the (−)-(S) optical isomer and remaining unreacted aldehyde which is possibly the enantiomerically pure (−)-(R) form. Extension of this cyclization reaction to a 3,3-disubstituted pent-4-enal similarly provides a synthesis for an optically active 3,3-disubstituted cyclopentanone. Decarbonylation by-products are also observed; those from 1a appear as E - and Z -2-phenylpent-2-ene. The cyclization of 1a is catalyzed also by Rh(chiraphos)(solvent) 2 + but with lower e.e.

Carbonyl-molybdenum complexes of the hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl) borate ligand

The reaction of KBH4 with 3 equiv. of 3,5-dimethyl-1,2,4-triazole at ca. 230°C yielded potassium hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl) borate, K{HB(Me2tz)3}. The reaction of Mo(CO)6 and K{HB(Me2tz)3} in N,N-dimethylformamide at 110°C gave [{HB(Me2tz)3}Mo(CO)3]−, which was isolated as the salt NEt4[{HB(Me2tz)3}Mo(CO)3] (1). Crystal characterisation of 1 revealed a six-coordinate, distorted octahedral anion composed of a facial, tridentate HB(Me2tz)3− ligand and three mutually cis (facial) carbonyl ligands. Spectroscopic data were consistent with greater electron-withdrawal by the triazolyborate ligand than by the related hydrotris(3,5-dimethylpyrazol-1-yl)borate ligand. Reaction of 1 with iodine gave {HB(Me2tz)3}Mo(CO)3I, which was spectroscopi characterised as a seven-coordinate 3:3:1 face-capped octahedral complex.

Models for the molybdenum hydroxylases

Abstract This commentary reviews structural, spectroscopic, and chemical models for the molybdenum hydroxylases. It briefly describes the current state of modeling and identifies areas where model chemistry may play a future role in understanding these enzymes.

Influence of the oxygen atom acceptor on the reaction coordinate and mechanism of oxygen atom transfer from the dioxo-Mo(VI) complex, Tp(iPr)MoO(2)(OPh), to tertiary phosphines.

The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp(iPr)MoO(2)(OPh) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe(3), PMe(2)Ph, PEt(3), PBu(n)(3), PEt(2)Ph, PEtPh(2), and PMePh(2)) has been investigated. The first step in all the reactions follows a second-order rate law indicative of an associative transition state, consistent with nucleophilic attack by the phosphine on an oxo ligand, namely, Tp(iPr)MoO(2)(OPh) + PR(3) --> Tp(iPr)MoO(OPh)(OPR(3)). The calculated free energy of activation for the formation of the OPMe(3) intermediate (Chem. Eur. J. 2006, 12, 7501) is in excellent agreement with the experimental DeltaG() value reported here. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide by acetonitrile, Tp(iPr)MoO(OPh)(OPR(3)) + MeCN --> Tp(iPr)MoO(OPh)(MeCN) + OPR(3), is first-order in starting complex in acetonitrile. The reaction occurs via a dissociative interchange (I(d)) or associative interchange (I(a)) mechanism, depending on the nature of the phosphine oxide. The activation parameters for the solvolysis of Tp(iPr)MoO(OPh)(OPMe(3)) (DeltaH(++) = 56.3 kJ mol(-1); DeltaS(++) = -125.9 J mol(-1) K(-1); DeltaG(++) = 93.8 kJ mol(-1)) and Tp(iPr)MoO(OPh)(OPEtPh(2)) (DeltaH(++) = 66.5 kJ mol(-1); DeltaS(++) = -67.6 J mol(-1) K(-1); DeltaG(++) = 86.7 kJ mol(-1)) by acetonitrile are indicative of I(a) mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp(iPr)MoO(OPh)(OPEt(3)) (DeltaH(++) = 95.8 kJ mol(-1); DeltaS(++) = 26.0 J mol(-1) K(-1); DeltaG(++) = 88.1 kJ mol(-1)) and the remaining complexes by the same solvent are indicative of an I(d) mechanism. The equilibrium constant for the solvolysis of the oxo-Mo(V) phosphoryl complex, [Tp(iPr)MoO(OPh)(OPMe(3))](+), by acetonitrile was calculated to be 1.9 x 10(-6). The oxo-Mo(V) phosphoryl complex is more stable than the acetonitrile analogue, whereas the oxo-Mo(IV) acetonitrile complex is more stable than the phosphoryl analogue. The higher stability of the Mo(V) phosphoryl complex may explain the phosphate inhibition of sulfite oxidase.

Toward Multifunctional Mo(VI-IV) Complexes: cis-Dioxomolybdenum(VI) Complexes Containing Hydrogen-Bond Acceptors or Donors

The complexes cis-TpiPrMoVIO2(OAr-R) (TpiPr=hydrotris(3-isopropylpyrazol-1-yl)borate, -OAr-R=hydrogen-bonding phenolate derivative) are formed upon reaction of TpiPrMoO2Cl, HOAr-R, and NEt3 in dichloromethane. The orange, diamagnetic, dioxo-Mo(VI) complexes exhibit strong nu(MoO2) IR bands at ca. 935 and 900 cm(-1) and NMR spectra indicative of Cs symmetry. They undergo electrochemically reversible, one-electron reductions at potentials in the range -0.836 to -0.598 V vs SCE; the only exception is the 2-CO2Ph derivative, which exhibits an irreversible reduction at -0.924 V. The complexes display distorted octahedral geometries, with a cis arrangement of terminal oxo ligands and with d(Mo=O)av=1.695 A and angle(MoO2)av=103.2 degrees. The R groups of the 2-CHO and 2-NHCOMe derivatives are directed away from the oxo groups and into a cleft in the TpiPr ligand; these derivatives are characterized by Mo-O-Cipso angles of ca. 131 degrees (conformation 1). The R group(s) in the 2-CO2Me and 2,3-(OMe)2 derivatives lie above the face of the three O-donor atoms (directed away from the TpiPr ligand) and the complexes display Mo-O-Cipso angles of 153.1(2) and 149.7(2) degrees, respectively (conformation 2). Conformations 1 and 2 are both observed in the positionally disordered 2-COMe and 2-COEt derivatives, the two conformers having Mo-O-Cipso angles of 130-140 and >150 degrees, respectively. The 3-COMe and 3-NEt2 derivatives have substituents that project away from the TpiPr ligand and Mo-O-Cipso angles of 134.2(2) and 147.7(2) degrees, respectively. Many of the complexes exhibit fluxional behavior on the NMR time scale, consistent with the rapid interconversion of two conformers in solution.

Applications of 95Mo NMR to inorganic and bioinorganic chemistry

Abstract A variety of dinuclear Mo(V), Mo(IV), Mo(III) and Mo(I) complexes, and tri- and tetranuclear homo- and heterometallic complexes of molybdenum have been studied by 95 Mo NMR. The Mo(V) complexes, Cp′ 2 Mo 2 Y 2 (μ-X) 2 (Y = O or S, X = S or Se, Cp′ = η 5 -C 5 Me 5 ), exhibit resonances in the −93 to 478 ppm region, the chemical shifts being sensitive to changes in the oxygen, sulfur and selenium content of the coordination spheres. The Mo(IV) complexes, Cp′ 2 Mo 2 (μ-X) 2 (μ-X 2 ) (X = S or Se), and their derivatives exhibit resonances in the 382–790 ppm region: isomers of Cp′ 2 Mo 2 (μ-S) 2 (μ-SH) 2 which differ in the arrangement of the bridging ligands were also detected. Dinuclear Mo(III) complexes of the form Mo 2 L 6 (L = amido, alkoxy or alkyl ligand) exhibit very deshielded resonances (2430–3624 ppm). The complexes Cp 2 Mo 2 {μ-S 2 C 2 (CF 3 ) 2 } 2 and Cp 2 Mo 2 (CO) 2 {μ-S 2 C 2 (CF 3 ) 2 } 2 (Cp = η 5 -C 5 H 5 ) exhibit resonances consistent with their formulation as distinct Mo(III) and mixed-valence Mo(IV-II) complexes, respectively. The Mo(I) complexes exhibit resonances which are very sensitive to the bond order of the MoMo bond: the resonances of the L 2 Mo 2 (CO) 6 complexes (L = Cp, −1856 ppm; L = Cp′, −1701 ppm) are more than 1800 ppm more shielded than the triply metalmetal bonded complexes, L 2 Mo 2 (CO) 4 (L = Cp, 182 ppm; L = Cp′, 133 ppm). The molybdenum containing homo- and heterometallic complexes exhibit resonances in the −133 to −1619 ppm region. The Fe 2 Mo 2 cubane complex, Cp′ 2 Mo 2 Fe 2 (μ 3 -S) 4 (CO) 4 , exhibits a resonance at −506 ppm. The ease of observation of the sulfurized complexes suggests that 95 Mo NMR may be a valuable technique for the study of hydro-desulfurization processes.

Methylierung eines schwefelreichen dimolybdänkomplexes: darstellung, trennung und charakterisierung von (C5Me5)2Mo2(μ-SCH3)2(μ-S)2 in zwei isomeren formen

Abstract The methylation reaction of (C 5 Me 5 ) 2 Mo 2 (μ-S 2 )(μ-S) 2 (Ia) leads to two isomers of composition (C 5 Me 5 ) 2 Mo 2 (SMe) 2 S 2 (IV and IVb) which were separated by column chromatography. As an intermediate product of this reaction the iodine-containing adduct (C 5 Me 5 ) 2 Mo 2 S 4 I 2 is formed. All compounds were investigated by means of 1 H and 95 Mo NMR spectroscopy as well as by an X-ray structure analysis in the case of IVa. Thus, the SCH 3 ligands in IV are found to be arranged in a trans position. As a further result a cleavage of the η 2 -disulfur bridge, originally present in Ia, has been found to occur in the second step of the reaction sequence.

cis-Dioxo- and cis-(hydroxo)oxo-Mo(V) complexes stabilized by intramolecular hydrogen-bonding.

The reactions of Tp(iPr)Mo(VI)O(2)Cl with salicylanilides and NEt(3) produce cis-Tp(iPr)Mo(VI)O(2)(2-OC(6)H(4)CONHR) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate, R = Ph, 4-C(6)H(4)Cl, 4-C(6)H(4)OMe). The N-methyl complex, Tp(iPr)MoO(2){2-OC(6)H(4)CON(Me)Ph}, is similarly prepared. Reduction of the amido complexes by cobaltocene produces green, EPR-active compounds, [CoCp(2)][Tp(iPr)Mo(V)O(2)(2-OC(6)H(4)CONHR)], that exhibit strong, low energy, ν(MoO(2)) IR bands at ∼ 895 and 790 cm(-1) (cf. ∼ 935 and 900 cm(-1) for the Mo(VI) analogues). The X-ray structures of all seven complexes have been determined. In each case, the Mo center exhibits a distorted octahedral coordination geometry defined by mutually cis oxo and phenolate ligands and a tridentate fac-Tp(iPr) ligand. The Mo(V) anions exhibit greater Mo═O distances (av. 1.738 Å vs 1.695 Å) and O═Mo═O angles (av. 112.4° vs 102.9°) than their Mo(VI) counterparts, indicative of the presence of a three-center (MoO(2)), π* semioccupied molecular orbital in these d(1) complexes. The amido Mo(VI) and Mo(V) complexes exhibit an intramolecular hydrogen-bond between the NH and O(phenolate) atoms. Protonation of [CoCp(2)][Tp(iPr)Mo(V)O(2)(2-OC(6)H(4)CONHR)] by lutidinium tetrafluoroborate is quantitative and produces EPR-active, cis-(hydroxo)oxo-Mo(V) complexes, Tp(iPr)Mo(V)O(OH)(2-OC(6)H(4)CONHR), related to the low pH Mo(V) forms of sulfite oxidase.

Molybdenum X-ray absorption edges from 200 to 20,000eV: the benefits of soft X-ray spectroscopy for chemical speciation.

We have surveyed the chemical utility of the near-edge structure of molybdenum X-ray absorption edges from the hard X-ray K-edge at 20,000eV down to the soft X-ray M(4,5)-edges at approximately 230eV. We compared, for each edge, the spectra of two tetrahedral anions, MoO(4)(2-) and MoS(4)(2-). We used three criteria for assessing near-edge structure of each edge: (i) the ratio of the observed chemical shift between MoO(4)(2-) and MoS(4)(2-) and the linewidth, (ii) the chemical information from analysis of the near-edge structure and (iii) the ease of measurement using fluorescence detection. Not surprisingly, the K-edge was by far the easiest to measure, but it contained the least information. The L(2,3)-edges, although harder to measure, had benefits with regard to selection rules and chemical speciation in that they had both a greater chemical shift as well as detailed lineshapes which could be theoretically analyzed in terms of Mo ligand field, symmetry, and covalency. The soft X-ray M(2,3)-edges were perhaps the least useful, in that they were difficult to measure using fluorescence detection and had very similar information content to the corresponding L(2,3)-edges. Interestingly, the soft X-ray, low energy ( approximately 230eV) M(4,5)-edges had greatest potential chemical sensitivity and using our high-resolution superconducting tunnel junction (STJ) fluorescence detector they appear to be straightforward to measure. The spectra were amenable to analysis using both the TT-multiplet approach and FEFF. The results using FEFF indicate that the sharp near-edge peaks arise from 3d-->5p transitions, while the broad edge structure has predominately 3d-->4f character. A proper understanding of the dependence of these soft X-ray spectra on ligand field and site geometry is necessary before a complete assessment of the utility of the Mo M(4,5)-edges can be made. This work includes crystallographic characterization of sodium tetrathiomolybdate.