Molybdenum

The complexes MoX4(Multiimine)2, where X = Cl, Br, I and Multiimine = dimethyl-bipyridine, bipyridine, phenanthroline, bipyrazine, bipyridazine and bipyrimidine, have been prepared. The product complexes apparently contain non-bridged quadruple molybdenum-molybdenum bonds. Each molybdenum is coordinated to a bidentate diimine and two halogen atoms. An electronic absorption study reveals an important trend that the intensity of the delta-->delta* transition increases with decreasing energy. This shows the energy of this band is determined by mixing of this transition with a metal-to-ligand charge transfer transition. An EEC type mechanism is proposed for the redox behavior of these compounds on the basis of an electrochemical study and some consistent results are obtained by correlating the oxidation potentials with the delta-->delta* transition energies. Also, fairly good correlations of both the delta-->delta* transition energies and the oxidation potentials with pk(a) of L are obtained.

Mo2o3 (Et2dtc) 2 (THF) 2I2, readily reduces various oxides. The Mo vio2+2 product of this reaction oxidizes TPP to triphenylphosphine oxide. The transient Mo(iv) species formed in the later reaction rapidly and irreversibly reacts with excess Mo vio2+2 to form the original Mo2 o3 4+ complex. These reactions can be also be coupled to provide catalytic oxygen transfer from PNO to TPP. This catalytic cycle can be monitored using a reverse phase high pressure liquid chromatography method that will also be discussed. The oxides chosen ranged from pyridine-N-oxide to the biological substrates: diphenylsufoxide, DMSO, nicotinamide-N-oxide, and biotin-S-oxide. Since Mo2o3 (Et2dtc) 2 (THF) 2I2 has the ability to abstract oxygen from these biologically significant substrates, it may result in the reconsideration of the role of Mo(V) complexes in catalytic cycles.

A series of eight-coordinate complexes of molybdenum with 1, 1-dithio
ligands are prepared and characterized by electrochemical and spectroscopic
methods. Cyclic voltammetry in acetonitrile and methylene
chloride solvents reveals reversible metal-based Mo (VI)/Mo(V) and
Mo(V)/Mo(IV) electron transfers. Half-wave potentials are linear
functions of electron withdrawing and donating parameters of the ligand
substituent groups, but do not correlate with charge transfer, vibrational,
or electron paramagnetic resonance spectral data. A linear
relationship does exist between electrochemical half-wave potentials
and Mo 3d and S 2p x-ray photoelectron binding energies. The results
suggest that ligands control half-wave potentials in these complexes
by inductive charge donation to the metal center through bonds not
directly involved in the redox process.

The binuclear , di-mju-oxo-bridged molybdenum(V)-cysteine complex,
Na2Mo2O4(cys)2, is frequently cited as a chemical model for nitrogenase
enzyme. Coulometric reduction of Na2Mo2O4(cys)2 at the potential of
Mo(V)--> Mo(III) reduction (-1. 35 to -1.50 V vs. SCE) in the presence o£
the nitrogenase substrate, acetylene, produces ethylene and ethane. Reduction of C2H2 does not occur in the absence of the molybdenumcysteine
complex. The quantities of C2H4 and C2H6 produced are greater
than the stoichiometric molybdenum content, hence the reduction is
catalytic. Acetylene reduction is accompanied by catalytic hydrogen
evolution at the surface of the mercury electrode. The rate of acetylene
reduction and product yields vary with pH, applied electrode potential,
concentration of Na2Mo2O4(cys)2, and the buffer salt. A mechanism for
the catalytic process is proposed, and its relationship to chemical
models for nitrogenase enzyme is discussed.

The electrochemical reduction of Mo 2O4 (EDTA)^2- is studied at a mercury electrode in pH 7-10 borate and phosphate buffers utilizing the techniques
of cyclic voltammetry, chronoamperometry, and controlled potential coulometry.
The electrochemical reduction of Mo 204(EDTA)^2- is a four-electron, diffusion
controlled process in which the initial step is an irreversible two-electron
transfer. The product is a binuclear dioxo-bridged complex of Mo (III)
with EDTA. Dependence of the electrode reaction mechanism on pH, buffer
composition, and sodium ion concentration is studied . These results indicate
that a chemical protonation step occurs prior to electrochemical reduction.
A treatment of the effect of changes in double layer structure
on the protonation reaction is presented.