Pedro Palanca

Model compounds for the active sites of oxo-transfer molybdoenzymes. Synthesis, structural characterization, and electrochemical properties of [NH4]2[MoO2{O2CC(S)Ph2}2]

[NH4]2[MoO2{O2CC(S)Ph2}2]middot;2H2O has been prepared and its crystal structure solved, providing the first example of a co-ordinatively saturated molybdenum complex which reacts readily and reversibly with organophosphines at room temperature in water or methanol; its variable-temperature 13C{1H} NMR spectra and electrochemical properties are presented.

Polymer-supported molybdenyl thioglycolate as oxygen atom transfer reagent

Abstract Oxo-transfer reactions of a variety of substrates in DMF or methanol using polymer-supported molybdenyl thioglycolate (PSMT) have been investigated. The clean oxidation of Me 2 PhP, n -butanethiol or benzoin to yield Me 2 PhPO, disulfide or benzil, respectively, occurs in high yield. In the presence of air or pyridine N-oxide, a catalytic cycle is accomplished which goes on until the completion of the substrate.

Molybdenum(VI)-dioxo complexes with sterically bulky thiocarboxylate ligands. Reactions with aliphatic thiols and electrochemical properties

Abstract Under acid conditions, the reaction of (Bu n 4 N)2[Mo VI O 2 (O 2 CC(S)Ph 2 ) 2 ] with aliphatic thiols yields the monomeric [Mo V O(O 2 CC(S)Ph 2 ) 2 ] − as unique complex product. The experimental pseudo-first order rate constant with respect to the Mo(VI) complex was found to be K =6.1 × 10 −5 s −1 . At neutral pH, however, an unstable Mo(IV) species was formed which was also electrochemically detected in a reversible Mo(V,IV) couple. [Mo V O(O 2 CC(S)Ph 2 ) 2 ] − appears to be obtained by the reaction of [Mo IV O(O 2 CC(S)Ph 2 ) 2 ] 2− with unreacted [Mo VI O 2 (O 2 CC(S)Ph 2 ) 2 ] 2− . Steric features on the ligand (gemdiphenyl groups) explain that the latter reaction does not lead to the expected formation of μ-oxo Mo(V) dimers. [Mo V O(O 2 CC(S)Ph 2 ) 2 ]− undergoes a one-electron reversible reduction on the cyclic voltammogram time scale, being also oxidized by nitrate ions to the starting Mo(VI) complex. The relation of these results to enzymatic systems is briefly discussed.

Structure and electrochemical properties of the new triply bridged molybdenum(V) complex [NBun4]2[Mo2O2(µ-O)(SCH2CO2)2(µ-SCH2CO2)]

The reaction of molybdate ions with an excess of thioglycolic acid has been studied in aqueous solution at room temperature by UV/VIS and NMR spectroscopy. As previously observed, spectroscopic data suggest initial formation of the complex [MoO2(SCH2CO2)2]2–1, which is subsequently reduced by the excess of acid. Two molybdenum(V) complexes are formed in equilibrium in this redox reaction. The main product has been isolated as an orange-red solid with the formula [NBun4]2[Mo2O3(SCH2CO2)3]·2H2O 2. It crystallizes in the monoclinic space group P21/a with a= 20.761 (8), b= 17.441(7), c= 14.587(6)A, β= 101.08(6)°, and Z= 4. The structure contains two cofacial MoVO2(SCH2CO2)2 distorted octahedra sharing one oxygen atom and one thioglycolate ligand on a pseudo-symmetry plane (Ot–Mo 1.677, Mo–Mo 2.643, Mo–S1,b 2.489, Mo–Ol,b 2.337, Mo–Ob 1.929 A, Mo–Sl,b–Mo 64.1, Mo–Ol,b–Mo 68.9, Mo–Ob–Mo 86.5°; t =terminal, b = bridging). Cyclic voltammetry shows that complexes 1 and 2 undergo a two-electron irreversible reduction at –1.27 and –1.38 V vs. the saturated calomel electrode, respectively, in methanol. In both reduction processes the monooxomolybdenum(IV) species [MoO(SCH2CO2)2]2– and [MoO(SCH2CO2)(solv)2] are generated. A reaction mechanism for the oxidation of thioglycolic acid by molybdate ions is proposed from the combined analysis of spectrophotometric, NMR, structural, and voltammetric data. Dimeric molybdenum(V) products are generated from reaction of the above monooxomolybdenum(IV) species with the starting dioxomolybdenum(VI) complex 1. A particularly remarkable supporting feature is the formation of Me2S when the reaction is carried out in the presence of Me2SO.

Synthesis, structure and spectroscopic characterization of sodium tetrabutylammonium tris(thiobenzilato)-molybdate(IV)

The synthesis, structure and spectroscopic properties of the mixed salt [NBun4]Na[Mo{O2CC(S)Ph2}3]·H2O·MeOH are reported. It crystallizes in the space group Pbca, with a= 19.369(6), b= 25.175(6), c= 24.273(6)A, and Z= 8. The molybdenum atom is six-co-ordinated and bonded to both the thiolate sulfur and carboxylate oxygen atoms of each of the three co-ordinated thiobenzilate ligands. The average Mo–S and Mo–O distances are 2.291(4) and 2.076(9)A, respectively, the former being the shortest observed for any tris(chelate)molybdenum complex. Contrary to the result predicted from previous molecular orbital calculations, the co-ordination polyhedron around the molybdenum atom is essentially trigonal prismatic. The two triangular faces, formed by three sulfur and three oxygen atoms from the thiolate and carboxylate groups, respectively, are nearly parallel and twisted by ca. 4.7° with respect to a regular prism. Sodium cations are intercalated between Mo(S3O3) units and are also subject to a trigonal-prismatic co-ordination by three carboxylate oxygen atoms, two water and one methanol molecule. Cyclic voltammetry indicates that [Mo{O2CC(S)Ph2}3]2– undergoes a reversible one-electron oxidation to [Mo{O2CC(S)Ph2}3]– at –0.26 V (ESR parameters 〈g〉= 1.968, 〈A〉= 36 × 10–4 cm–1). The latter species can also be generated in solution by chemical oxidation.

Synthesis and characterization of molybdenum(VI)-dioxo complexes containing both coordinated thiolate and carboxylate groups. Reactions with their own free ligands

The synthesis, characterization and spectroscopic properties of a group of Mo(VI) complexes having thiocarboxylate ligands of type [MoVIO2(O2CC(S)MePh-X)2]2 have been reported (X = H, p-Me, p-Cl). The peak potential for the Mo(VI) reduction increasing according to the electron-donor ability of X (Me > H > Cl). Reaction of these Mo(VI) complexes with their own free ligands has been studied by ESR and UV-Vis spectroscopy, yielding the monomeric [MoVO(O2CC(S)MePh-X)2]− as unique complex products. The kinetic study of this oxidation reaction has also been investigated.

Modeling for the active site nitrate reductase. Oxidation of the complex [MovO(O2CC(S) CH3Ph)2]− by nitrate and nitrite in methanol

Abstract Under acid conditions the [MoVIO2(O2CC(S)CH1Ph)2]2 reacts with thiols to yield the monomeric [MoVO(O2CC(S)CH3Ph)2] and disulfide. The reduced complex [MoVO(O2CC(S)CH3Ph)2]− can react with NO3− and NO2− in a one-electron step yeilding respectively NO2 and NO and the original molybdenum (VI)-dioxo complex. The experimental pseudo-first-order rate constant with respect to the Mo(V) complex at 25°C was found to be kobs=2.3×10−4s−1 for NO3− and kobs=1.0×10−2 for NO2−. Oxo transfers to and from the substrate have been coupled to produce a catalytic system which turns over the reaction RSH+(No3− or NO2−)+H+a 1 2 [ RS ] 2 +( NO ] 2 or NO )+ H 2 O , in which thiols, NO1− and NO2− serve as a model substrates and molybdenum complex as a catalyst. This result has permitted us to consider the system as a possible model for the nitrate reductase ‘Chlorella’ (S.P. Cramer et al., J. Am. Chem. Soc., 106 (1984) 1467; G.N. Geoige et al., Biochem. J., 259 (1989) 693).

[MoO2(SCPh2CO2)2]2− and [MoO(SCPh2CO2)2]− anion complexes. A theoretical structure characterization

Abstract Geometry optimization of [MoO 2 (SCH 2 CO 2 ) 2 ] 2− and [MoO(SCH 2 CO 2 ) 2 ] − systems as models of [MoO 2 (SCPh 2 CO 2 ) 2 ] 2− and [MoO(SCPh 2 CO 2 ) 2 ] − anion complexes have been carried out at STO-3G, 3-21G, LANL1MB and LANL2DZ basis set levels. A comparison of the theoretical results and X-ray experimental data has been performed. STO-3G minimal basis set produces the best geometrical agreement, in particular the distances and orientations of the different ligands linked to molybdenum transition metal. A large structural overlap with STO-3G optimized geometry and X-ray data has been found for the [MoO 2 (SCPh 2 CO 2 ) 2 ] 2− and [MoO(SCPh 2 CO 2 ) 2 ] − anion complexes.