TY - JOUR
T1 - Novel Synthesis of Reduced Mixed-Valence Molybdenum and Tungsten Complexes by Electrochemical Oxidation of the Seven-Coordinate Complexes MX(S2)(R2dtc)2 (M = Mo, X = O; M = W, X = O, S; R = Alkyl; dtc = Dithiocarbamate)
AU - Bond, A. M.
AU - Broomhead, J. A.
AU - Hollenkamp, A. F.
PY - 1988/3/1
Y1 - 1988/3/1
N2 - The electrochemical oxidation and reduction of a series of seven-coordinate complexes MVIX(S2)(R2dtc)2 (M= Mo, X = O; M = W, X = O, S; R = alkyl; dtc = dithiocarbamate) have been studied at both platinum and mercury electrodes in a range of organic solvents. At platinum electrodes a single irreversible oxidation process is observed at around 1.0 V vs Ag/AgCl. The potential is largely independent of the metal and the dithiocarbamate R group and, in the case of tungsten, whether X is oxygen or sulfur, which is consistent with a ligand-based oxidation process initially involving the disulfide (S22-) group. Data at mercury electrodes support this conclusion although the oxidation process occurs at a potential different from that observed at platinum electrodes. Surprisingly, the ultimate metal-containing product of oxidation (controlled-potential electrolysis (1.5 ± 0.1 faraday mol‥-1) or chemical oxidation with NOPF6 is a mixed-valence molybdenum(V)-molybdenum(IV) dimer in which the metal has been reduced. For MoO(S2)(R2dtc)2(R = Me, Et), oxidation yields isolable yellow (R = Me) or orange (R = Et) [MoIVMovO2(R2dtc)4]PF6, which has been characterized by elemental analysis, conductivity measurements, and IR and EPR spectroscopy. This compound can be reduced to form the molybdenum(IV) species, MoO(R2dtc)2, which can in turn be oxidized back to form the mixed-valence dimer. Reduction of MX(S2)(R2dtc)2 at platinum electrodes is an irreversible two-electron process at room temperature with slow scan rates (100 mV s-1) but becomes a chemically reversible one-electron step at -70°C or with fast scan rate cyclic voltammetry (50 V s-1) at 20 °C. The initially formed [MvX(S2)(R2dtc)2]- anion is inherently unstable and decreases its coordination number while undergoing further reduction. The reduction potentials, in contrast to the oxidation potentials, follow established trends for metal-based processes with WO(S2)(R2dtc)2 being more difficult to reduce than MoO- (S2)(R2dtc)2 while replacement of oxygen with sulfur in the former facilitates reduction. Reduction at mercury electrodes is again substantially different from that at platinum electrodes. Isolation of the products of reduction proved difficult except for WS(S2)(R2dtc)2 in which case reduction with cobaltocene, CoCp2, yields deep red [CoCp2] [WIVS(S2)R2dtc].
AB - The electrochemical oxidation and reduction of a series of seven-coordinate complexes MVIX(S2)(R2dtc)2 (M= Mo, X = O; M = W, X = O, S; R = alkyl; dtc = dithiocarbamate) have been studied at both platinum and mercury electrodes in a range of organic solvents. At platinum electrodes a single irreversible oxidation process is observed at around 1.0 V vs Ag/AgCl. The potential is largely independent of the metal and the dithiocarbamate R group and, in the case of tungsten, whether X is oxygen or sulfur, which is consistent with a ligand-based oxidation process initially involving the disulfide (S22-) group. Data at mercury electrodes support this conclusion although the oxidation process occurs at a potential different from that observed at platinum electrodes. Surprisingly, the ultimate metal-containing product of oxidation (controlled-potential electrolysis (1.5 ± 0.1 faraday mol‥-1) or chemical oxidation with NOPF6 is a mixed-valence molybdenum(V)-molybdenum(IV) dimer in which the metal has been reduced. For MoO(S2)(R2dtc)2(R = Me, Et), oxidation yields isolable yellow (R = Me) or orange (R = Et) [MoIVMovO2(R2dtc)4]PF6, which has been characterized by elemental analysis, conductivity measurements, and IR and EPR spectroscopy. This compound can be reduced to form the molybdenum(IV) species, MoO(R2dtc)2, which can in turn be oxidized back to form the mixed-valence dimer. Reduction of MX(S2)(R2dtc)2 at platinum electrodes is an irreversible two-electron process at room temperature with slow scan rates (100 mV s-1) but becomes a chemically reversible one-electron step at -70°C or with fast scan rate cyclic voltammetry (50 V s-1) at 20 °C. The initially formed [MvX(S2)(R2dtc)2]- anion is inherently unstable and decreases its coordination number while undergoing further reduction. The reduction potentials, in contrast to the oxidation potentials, follow established trends for metal-based processes with WO(S2)(R2dtc)2 being more difficult to reduce than MoO- (S2)(R2dtc)2 while replacement of oxygen with sulfur in the former facilitates reduction. Reduction at mercury electrodes is again substantially different from that at platinum electrodes. Isolation of the products of reduction proved difficult except for WS(S2)(R2dtc)2 in which case reduction with cobaltocene, CoCp2, yields deep red [CoCp2] [WIVS(S2)R2dtc].
UR - http://www.scopus.com/inward/record.url?scp=24844438864&partnerID=8YFLogxK
U2 - 10.1021/ic00279a008
DO - 10.1021/ic00279a008
M3 - Article
AN - SCOPUS:24844438864
SN - 0020-1669
VL - 27
SP - 978
EP - 985
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 6
ER -