Title:
ELECTROCHEMICAL GENERATION OF SOLUBLE NICKEL(O) CATALYSTS
United States Patent 3668086


Abstract:
A method of generating soluble zero-valent nickel catalyst consisting of electrochemically reducing suitable nickel(II)-ligand complexes wherein said electrochemical reduction can be achieved with or without the presence of an electrolyte.



Inventors:
HUGHES WILLIAM B
Application Number:
05/059018
Publication Date:
06/06/1972
Filing Date:
07/28/1970
Assignee:
PHILLIPS PETROLEUM CO.
Primary Class:
Other Classes:
205/234, 502/162, 556/18, 556/21, 556/23, 556/146
International Classes:
B01J37/34; C07C2/46; (IPC1-7): B01K3/00
Field of Search:
204/59,48,14N,112,49 260
View Patent Images:
US Patent References:
3551467PROCESS FOR PREPARING DIOLEFIN NICKEL HALIDE COMPLEXES1970-12-29Arakawa et al.
3501332METAL PLATING OF PLASTICS1970-03-17Buckman
3490745NICKEL COMPLEX CATALYST COMPOSITIONS1970-01-20Chappell et al.
3475293ELECTRODEPOSITION OF METALS1969-10-28Haynes et al.
3327015Olefin dimerization by nickel compounds1967-06-20Spitzer
3131134Electroplating from an organic electrolytic solution1964-04-28Micillo



Primary Examiner:
Mack, John H.
Assistant Examiner:
Kaplan, Neil A.
Claims:
1. A method for generating a soluble zero-valent nickel catalyst comprising:

2. A method according to claim 1 wherein the soluble zero-valent nickel catalyst is carried by an organic solvent selected from the group consisting of acetonitrile, sulfolane, propionitrile,

3. A method according to claim 1 wherein the soluble zero-valent nickel catalyst is electrochemically reduced in the presence of an electrolyte selected from the group consisting of tetraalkylammonium halides and

4. A method according to claim 1 wherein the electrochemical reduction of the nickel(II) compound occurs under a constant voltage in the range of from about -1.7 to about -3.6 volts.

Description:
This invention relates to the electrochemical reduction of nickel(II)-ligand complexes.

This invention further relates to a method for the synthesis of soluble nickel(0) catalyst.

I have found a method for the generation of zero-valent nickel based on the electrochemical reduction of nickel(II) complexes which, without isolation or purification, are capable of acting as homogeneous catalysts for the oligomerization of dienes. As a method of catalyst generation, the electrochemical reduction method of my invention has a number of advantages. It can eliminate catalyst purification as well as the use of highly reactive, difficult to handle chemical reducing agents, for example, organoaluminum compounds.

The nickel(II)-ligand complexes reduced by the method of my invention comprise a group which are readily soluble in organic solvents, for example, nickel(II) chloride-tertiary phosphine complexes. The phosphine complexes can be formed either before or after charging the reactants to the electrolytic cell. Prior preparation of the complex is accomplished by direct combination of the nickel(II) salt with the phosphine. Alternatively, the complex may be formed from the combination of the nickel(II) salt with the phosphine, said combination being made within the electrolytic cell prior to the reduction. Certain of the nickel salts will form a complex with certain of the solvents, such as the 1:1 nickel(II) chloride dimethoxyethane complex, and if desired these nickel salt complexes can be combined with the phosphine or other suitable ligand. The reduction method of my invention is carried out in the presence of a ligand capable of stabilizing nickel(0). As indicated by Run 3 of the following TABLE I, metallic nickel is produced when nickel chloride-dimethoxyethane in the absence of such a ligand is subjected to electrolysis. Triaryl- and trialkylphosphines are effective in stabilizing the reduced nickel as illustrated by Runs 2 and 4 of the following TABLE I.

The use of an added electrolyte in the method of my invention is optional. Several electrolytes may be used with a choice of supporting electrolytes representing a noncritical element of the method of my invention. Although some higher conversions were obtained with added electrolytes, notice must be taken of the results of Run 7 of TABLE I in which the nickel complex acted as its own electrolyte. Basically, and as seen by contrasting Runs 2 and 7 of TABLE I, the presence of an added electrolyte appears to have little effect on the catalytic activity of the resulting nickel(0) catalyst prepared ky the method of my invention. However, when an electrolyte is added to the method of my invention, concentrations normally range from 0 to 1 molar. Tetraalkylammonium halides or nitrates or perchlorates can be employed as added electrolytes, for example, tetrabutylammonium perchlorate, tetrabutylammonium bromide, and tetrapropylammonium nitrate.

Polar organic compounds that are nonreducible and nonreactive under conditions met in this system are applicable as solvents, for example, acetonitrile, sulfolane, propionitrile, N,N-dimethylformamide, 1,2-dimethoxyethane and dioxane. Acetonitrile and N,N-dimethylformamide are the solvents preferred for the method of my invention since high current densities can be achieved in these solvents.

A variety of nickel(II) compounds can be used as starting materials, for example, nickel chloride, nickel bromide, nickel nitrate, nickel acetate, and the like. These nickel compounds can be employed in a concentration range of 0.001 to 0.1 molar. Suitable ligands are aryl, alkyl and mixed aryl alkyl phosphines; for example, triphenylphosphine, triethylphosphine, dimethylphenylphosphine, and chelating ligands such as 1,2-diphenylphosphinoethane. Additional quantities of ligands of the type specified above may be added beyond that complexed by the nickel atom in the initial nickel complex in quantities ranging up to 10 moles of ligand per mole of nickel complex. The individual reductions are carried out, preferably, at specified and constant voltages. However, some variation in voltage is permissible. The voltage must be sufficient to cause reduction of the nickel species but must be less than the reduction potential of the solvent. For example, voltages applicable for reductions in acetonitrile can be as low as -1.7 and as high as -2.5 with -2.0 volts being preferred. In 1,2-dimethoxyethane, the voltage can be as high as -3.6. Current is passed through the electrolytic cell until the desired reduction is achieved. Depending on the cell construction, solvent employed, nature of the species to be reduced and other variables, the initial current can vary from 10 to 500 mA. As the reduction proceeds the current decreases and approaches a value of zero as the reduction approaches completion. In general practice the reduction will be continued until the current decreases to 5 mA or less.

The reductions and subsequent reactions are carried out under an inert atmosphere. Reductions are carried out in any suitable equipment, such as two-compartment, U-shaped cell utilizing three electrodes (test, counter, and reference). The test and counter electrodes consisted of stirred mercury pools separated by fritted-glass disks. A suitable reference electrode consists of two parts: a salt bridge and a sealed glass tube containing silver and silver perchlorate (10-3 M solution) separated by a fritted glass disk from the slat bridge. The reduction potentials, maintained between the test and the reference electrodes during the reduction, can be referred to the silver-silver perchlorate couple. EXAMPLE I, Run No. 2, as follows, represents a typical example run of the nine runs as disclosed in the following TABLE I.

EXAMPLE I

A mixture of the nickel chloride-1,2-dimethoxyethane complex (0.22 g., 0.001 "mole" of the 1:1 nickel chloride-1,2-dimethoxyethane complex), triphenylphosphine (0.52 g., 0.002 mole), and tetrabutylammonium perchlorate (0.86 g., 0.0025 mole) was dissolved in sufficient acetonitrile to make a 25 ml solution under an inert atmosphere of 96 volume percent nitrogen and 4 volume percent hydrogen. This atmosphere was circulated over copper at 400° C. to remove any adventiously present oxygen and this hydrogen was included to properly maintain this copper in the active, reduced state. This hydrogen did not enter into the electrochemical reduction. Maintaining this atmosphere, the solution was transferred to the cathode compartment of the electrolysis cell and reduction was carried out at -2.0 volts. Reduction was terminated after 40 minutes by which time the current value had declined to 2 mA.

Continuing to maintain the 96% nitrogen-4% hydrogen atmosphere, the electrolysis product mixture (now containing the reduced nickel species) was transferred to a Fisher-Porter aerosol compatability bottle and the bottle sealed. Butadiene (11.9 gram, 0.22 mol) was added at room temperature affording a total pressure of 25 psig at 25° C. The reduction mixture was stirred and heated under conditions shown in the table. The product mixture was cooled to room temperature and analyzed. The polymer content of the product mixture was determined as being the material which would not distill into a Dry Ice-cooled receiver at 100° C./0.075 mm. The volatile portion of the product mixture was analyzed by gas/liquid partition chromatography (glpc). The percentage conversion of butadiene to the respective products was: 4-vinylcyclohexene, 25; 1,5-cyclooctadiene, 36; cyclododecatrienes, 5. The total percentage conversion of butadiene was about 77 percent.

The following TABLE I shows 9 runs that are representative of my invention.