The present invention provides an improved method for preparing compounds of the formula Mo.sub.4 S.sub.4 L.sub.6 comprising: PA1 contacting a compound having the formula Mo.sub.2 S.sub.4 L.sub.2, wherein L is a 1,1-dithioacid ligand, with a reducing agent having a reduction potential sufficient to reduce Mo(V) to lower oxidation states, especially to Mo(III) and Mo(IV), at a temperature and for a time sufficient to form the Mo.sub.4 S.sub.4 L.sub.6 compound. Preferably, the Mo.sub.2 S.sub.4 L.sub.2 compound is dissolved in an organic solvent along with the reducing agent and the solution is heated at temperatures above 25.degree. C., up to the boiling point of the solvent and, more preferably, at temperatures in the range of from about 50.degree. C. to about 250.degree. C.

FIELD OF THE INVENTION 
This invention relates to improvements in the synthesis of Mo.sub.4 S.sub.4 
L.sub.6 compounds. 
BACKGROUND OF THE INVENTION 
Molybdenum compounds having a thiocubane structure are produced by a 
variety of methods. For example, T. Shibahara et al, J. Am. Chem. Soc., 
Vol. 106, pp. 789-791 (1984) discusses a method for making the [Mo.sub.4 
S.sub.4 (edta).sub.2 ]3--ion containing species by reacting a water 
soluble Mo(V) dimer in HCl. P. Kathirgamanathan et al, J. Chem. Soc., 
Chem. Commun., pp. 953-954 (1985), describes electrochemically reducing a 
Na.sub.2 [Mo(V).sub.2 S.sub.2 O.sub.2 (cysteine).sub.2 ].multidot.3H.sub.2 
O in HCl to form (Me.sub.4 N).sub.5 [Mo.sub.3 S.sub.4 (NCS).sub.9 ] and 
the tetramer (Me.sub.4 N).sub.7 [Mo.sub.4 S.sub.4 (NCS).sub.12 ]. P. 
Kathirgamanathan et al, J. Chem. Soc., Chem. Commun., pp. 1437-1439 
(1985), describes preparing mixtures of (Me.sub.4 N).sub.5 [Mo.sub.3 
X.sub.4 (NCS).sub.9 ] and (Me.sub.4 N).sub.7 [Mo.sub.4 X.sub.4 
(NCS).sub.12 ] compounds, where X is sulfur or oxygen. More recently, in 
U.S. Pat. No. 4,990,271 there is described a method for making thiocubane 
Mo compounds having the formula Mo.sub.4 S.sub.4 (ROCS.sub.2).sub.6 by 
reacting molybdenum hexacarbonyl, Mo(CO).sub.6, with a xanthogen 
disulfide. 
Notwithstanding the plethora of methods for preparing molybdenum containing 
thiocubane type compounds, there remains a need for a preparative method 
that is more simple and less expensive. 
It is, therefore, an object of the present invention to provide an improved 
method for forming thiocubane Mo compounds of the general formula Mo.sub.4 
S.sub.4 L.sub.6, where L is a dithioacid ligand. 
SUMMARY OF THE INVENTION 
Accordingly, there is provided an improved method for preparing compounds 
of the formula Mo.sub.4 S.sub.4 L.sub.6 comprising: 
contacting a compound having the formula Mo.sub.2 S.sub.4 L.sub.2, wherein 
L is a 1,1-dithioacid ligand with a reducing agent having a reduction 
potential sufficient to reduce Mo(V) to lower oxidation states, especially 
to Mo(III) and Mo(IV), at a temperature and for a time sufficient to form 
the Mo.sub.4 S.sub.4 L.sub.6 compound. Preferably, the Mo.sub.2 S.sub.4 
L.sub.2 compound is dissolved in an organic solvent along with the 
reducing agent and the solution is heated at temperatures above 25.degree. 
C., up to the boiling point of the solvent and, more preferably, at 
temperatures in the range of from about 50.degree. C. to about 250.degree. 
C. 
DETAILED DESCRIPTION 
In a preferred method of the present invention, a compound having the 
formula Mo.sub.2 S.sub.4 L.sub.2, wherein L is a 1,1-dithioacid ligand, is 
added to a sufficient amount of an organic solvent to form a solution. 
In general, any dithioacid ligand may be used. Thus, L may be a 
dithiocarbamate, xanthate, thioxanthate, dithiophosphate, 
dithiophosphinate, or other similar dithioacids and mixtures thereof. 
Preferably, the ligands, L, will have organo groups having from about 1 to 
30 carbon atoms. For example, when L is a dithiocarbamate, (S.sub.2 
CNR.sub.2 -), or a xanthate, (S.sub.2 COR-) the organo group R preferably 
will have from 1 to 30 carbon atoms. 
The Mo.sub.2 S.sub.4 L.sub.2 compound can be prepared by generally known 
techniques. 
Any organic solvent capable of dissolving the Mo.sub.2 S.sub.4 L.sub.2 
compound may be used in the method of this invention. Preferably, the 
organic solvent chosen will also be capable of dissolving the reducing 
agent used in the process. In general, hydrocarbons, ethers and formamides 
are useful. Especially useful are organic solvents that have boiling 
points above about 50.degree. and in the range, for example, of from about 
50.degree. C. to about 250.degree. C. Indeed, aromatic hydrocarbons, such 
as toluene or xylene, or other solvents, such as tetrahydrofuran, 
dimethylformamide and mixtures thereof are most preferred organic solvents 
for use in this invention. 
Thus, a solution of the Mo.sub.2 S.sub.4 L.sub.2 compound and the reducing 
agent is prepared. The reducing agent may be any compound with a reduction 
potential sufficient to reduce the Mo(V) in the Mo.sub.2 S.sub.4 L.sub.2 
compound to a lower oxidation state, such as Mo(III) and Mo(IV). Suitable 
reducing agents include Zn, Mg, NaBH.sub.4, dithionite salts, LiR.sub.3 
BH, R.sub.4 NBH.sub.4, where R is an alkyl group containing 1 to about 30 
carbon atoms, and mixtures thereof. 
The mole ratio of reducing agent to Mo.sub.2 S.sub.4 L.sub.2 compound 
employed will range generally from about 0 10 to about 100 and, 
preferably, from about 0.25 to about 10. 
Optionally but preferably, a disulfide of a 1,1-dithioacid is added to the 
solution of the Mo.sub.2 S.sub.4 L.sub.2 compound and reducing agent. In 
general, from about 0.1 to about 10 moles of disulfide per mole of dimer 
is added and, preferably, about 2 moles of disulfide per mole of Mo.sub.2 
S.sub.4 L.sub.2 compound. The additional disulfide improves the yield 
obtained in converting the Mo.sub.2 S.sub.4 L.sub.2 compound to the 
Mo.sub.4 S.sub.4 L.sub.6 compound. Also, the disulfide of the dithioacid 
added to the solution preferably is a disulfide of xanthates, 
dithiocarbamates, dithiophosphates, dithiophosphinates and the like. 
In yet another embodiment of the invention, a salt of any of the 
1,1-dithioacids, L, previously mentioned can be added to the solution of 
the Mo.sub.2 S.sub.4 L.sub.2 compound and reducing agent. Typical salts 
include alkali metal, alkaline earth metal, ammonium and alkylammonium 
salts. In general, these will be added in amounts ranging from about 0.1 
to 10 moles of salt per mole of Mo.sub.2 S.sub.4 L.sub.2 compound and, 
preferably, about 0.5 to about 2 moles per mole of Mo.sub.2 S.sub.4 
L.sub.2 compound. 
After forming the solution, as outlined above, in some instances a reaction 
may occur at ambient temperature and heating of the reactants will be 
unnecessary. In general, however, the solution is heated at a temperature 
and for a time sufficient to form the Mo.sub.4 S.sub.4 L.sub.6 compound. 
Typically, the solution will be heated at a temperature above room 
temperature up to the boiling point of the solvent. More typically, the 
solution will be heated at temperatures in the range of from about 
50.degree. C. to about 250.degree. C. The time of heating will depend upon 
a number of factors, such as the reducing agent, the solvent and the 
temperature employed. In general, however, the solution will be heated for 
times ranging between about 0.5 hours to about 24 hours or more. 
The Mo.sub.4 S.sub.4 L.sub.6 product can be isolated readily from the 
heated solution by any number of well known techniques. In some instances, 
the product will precipitate from solution and, hence, can be separated 
and recovered by filtration. In other instances, the solvent can be 
removed, for example, in vacuo, and the crude product will remain. The 
crude product can, of course, be purified by recrystallization, column 
chromatography or the like. 
The following examples will serve to illustrate specific procedures used in 
accordance with the claimed invention.