Transition metal complex catalysts

Substrates are oxidized by means of two-oxidative addition reactions of a dimeric, dinuclear transition metal complex oxidant containing four binucleating diisocyanide bridge ligands. The complex is reoxidized by means of a secondary oxidant which is a stronger oxidizing agent than the complex which in turn is oxidized by molecular oxygen. Though the direct oxidation of the complex by oxygen involves a large energy barrier and is relatively slow, the kinetics of the two stage oxidation of the complex by the secondary oxidant and of the oxidant by oxygen permit regeneration of the oxidant at reasonable rate. Substrates, such as the olefins, ethylene or propylene, have been continuously oxidized by bubbling oxygen and the olefin through a solution of a dimeric, dirhodium complex containing four 1,3-diisocyanopropane bridge ligands and a secondary oxidant such as a cerium salt.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to metal-organic complex oxidation catalysts 
and, more particularly, to a combination of a dinuclear transition 
metal-tetra-diisocyanide complex with a stonger oxidant. 
2. Description of the Prior Art 
Dinuclear transition metals complexed with four binucleating diisocyanides 
bridge ligands have previously been reported. The dirhodium 
tetra-diisocyanopropane dimers undergo two-center oxidative addition 
reactions with several substrates. The orbital interactions between the 
directly coupled metal centers give rise to striking electronic absorption 
properties, the most prominent being a low-lying system attributable to 
the .sup.1 A.sub.1g .fwdarw..sup.1 A.sub.2u (1a.sub.2u .fwdarw.2a.sub.1g) 
excitation. 
The dimeric complex are very interesting oxidizing agents in multielectron 
redox processes since each metal center can furnish or remove one or more 
electrons from a substrate. Though analogous monomeric metal complexes are 
not good oxidation catalysts, the dimers have shown good oxidation 
properties with numerous substrates, probably due to the capability of the 
binucleating ligands to maintain or reduce metal-metal spacing on 
oxidation. However, the redox reactions are stoichiometric requiring 
reoxidation of the complex. The complex has been shown to be readily 
oxidized by halogens such as bromine, chlorine or iodine. Oxidation by 
molecular oxygen or air would be desirable for industrial processes due to 
formation of a peroxide intermediate at low cost. However, the direct 
oxidation by oxygen is relatively slow due to the large energy barrier for 
this reaction. Peroxidic intermediates are also of value due to the 
selectivity in certain reactions such as in the oxidation of olefins such 
as propylene to propylene oxide under mild conditions while minimizing 
formation of by-products. 
Present commercial processes for producing propylene oxide suffer from one 
or more major drawbacks. The chlorohydrin process produces chlorine 
compounds that pose pollution problems. The oxirane process produces about 
twice as much styrene or t-butanol co-product as propylene oxide. Other 
processes under consideration such as peracid or hydrogen peroxide also 
produce co-product and/or require hydrogen peroxide, an expensive reagent. 
SUMMARY OF THE INVENTION 
Substrates are oxidized at effective rate in accordance with the invention 
by oxygenation of a dimeric dinuclear metal complex containing four 
binucleating biisocyanides in the presence of a secondary oxidant which is 
a stronger oxidizing agent than the complex. Though the direct oxidation 
of the complex by oxygen involves a large energy barrier and is relatively 
slow, the kinetics of the two stage oxidation of the complex by the 
secondary oxidant and of the oxidant by oxygen permit regeneration of the 
oxidant at reasonable rate. Substrates, such as the olefins, ethylene or 
propylene, have been oxidized by bubbling the olefin through a solution of 
a dimeric, dirhodium complex containing four 1,3-diisocyanopropane bridge 
ligands and a secondary oxidant such as a cerium salt. 
These and many other attendant advantages of the invention will become 
readily apparent as the invention becomes better understood by reference 
to the following detailed description when considered in conjunction with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Substrates are oxidized according to the following general reactions: 
EQU O.sub.2 +Oxidant.fwdarw.Oxidant.sup.-2e +O.sub.2.sup.+2 
EQU Oxidant.sup.-2e +[M.sub.2 (Bridge).sub.4 ].sup.+2 .fwdarw.[M.sub.2 
(Bridge).sub.4 ].sup.+4 
EQU [M.sub.2 (Bridge).sub.4 ].sup.+4 +Substrate.fwdarw.[M.sub.2 (Bridge).sub.4 
].sup.+2 +Substrate.sup.+2 
where M is a transition metal and Bridge is a binucleating biisocyanide. 
The transition metal can be selected from metals in The platinum group such 
as rhodium, cobalt, iridium, platinum, palladium, nickel, osmium, 
ruthenium or iron. The anion is selectively depending on whether the 
complex is to be utilized in aqueous or organic media. Suitable anions are 
halides, boron tetrafluoride, tetraphenyl borate or PF.sub.6.sup.--. 
The Bridge ligand can be any binucleating biisocyanide particularly 
aliphatic biisocyanides containing 2 to 20 carbon atoms such as 
1,3-diisocyanopropane, 1,4-diisocyanobutane (4-Bridge), 
2,5-dimethyl-2,5-diisocyanohexane (TM-4 Bridge) and 
cis-1-isocyano-4(2-isocyanopropyl)cyclohexane (Cyclo-5-Bridge). 
The structure and names of other binucleating biisocyanides are illustrated 
in Tables 1 and 2 which follow: 
TABLE 1 
______________________________________ 
##STR1## 
##STR2## 
##STR3## 
##STR4## 
##STR5## 
##STR6## 
##STR7## 
##STR8## 
##STR9## 
##STR10## 
##STR11## 
##STR12## 
______________________________________ 
TABLE 2 
______________________________________ 
NAMES OF LIGANDS 
______________________________________ 
1. 2,4 diisocyano pentane 
2. 2-substituted-1,3-diisocyanopropane 
3. 1-isocyano benzylisocyanide 
4. 1,2-diisocyanobenzene 
5. 1,4-diisocyanocyclohexane 
6. 1,8-diisocyanonaphthalene 
7. 1,8-diisocyanofluorene 
8. m-diisocyanophenyl sulfone 
9. 1,3-diisocyano 2,2,4,4 tetramethylcycolbutane 
10. 1,3-diisocyano diisocyano 2 hydroxy propane 
11. 3,5-diisocyano butanesulfonic acid 
12. .alpha., .alpha.' diisocyano-oxylene 
______________________________________ 
Salts of the reduced complexes are prepared by addition of the biisocyanide 
bridge ligand to a stoichiometric amount of [Rh(COD)Cl].sub.2 in solvent 
where COD is cyclooctadiene. The latter compound was synthesized by a 
standard method: J. Chatt and L. M. Venanzi, J. Chem. Soc., 4735 (1957). 
Experiments follow: 
EXAMPLE 1 
1,3-diisocyanopropane(bridge) 
To a 3 liter, 3-necked flask equipped with overhead stirrer and two Claisen 
condensors was added 600 ml of a 50% aqueous solution of NaOH (prepared by 
mixing excess solid NaOH and water and allowing the phases to come to 
equilibrium at 25.degree. C. over several days) and 170 ml (2.1 moles) of 
chloroform. 500 ml of dichloromethane was added as solvent, followed by 84 
ml (1 mole) of 1-3-diaminopropane (Aldrich Chemical Company) and finally, 
2.1 g of the phase-transfer catalyst, benzyl-triethylammonium chloride. 
The mixture was then rapidly stirred until refluxing of the 
dichloromethane was observed. When the rate of reflux becomes excessive, 
the stirring rate is decreased to slow the reaction; caution is advised, 
as pressure explosions of solvent gushing out the condensors may occur if 
the reaction rate is too rapid. The flask temperature should be maintained 
at about 40.degree. C. for 30 minutes, after which the stirring may be 
accelerated once again. The reaction mixture is stirred for about three 
additional hours, and the solution will have darkened slightly due to 
formation of polymeric side product. The layers are separated, and the 
organic phase is washed four times with 500 ml portions of water. The 
solvent is then removed, and the ligand is purified by vacuum 
distillation. The bridge distills at 55.degree. C. at 1 mm Hg as a clear 
liquid. Use extreme care while distilling the product. The infrared 
spectrum of the ligand shows a very strong and narrow .nu. (C.tbd.N) 
stretch at 2149 cm.sup.-1, with other prominent peaks at 2930 (m), 1660 
(m), and 1490 (s) cm.sup.-1. The 60 MHz PMR spectrum of bridge exhibits 
two multiplets integrated in the ratio of 2:1, the first is a triplet of 
triplets at 3.48 .delta. (terminal CH.sub.2), and the second is a complex 
multiplet centered at 1.76 .delta. (central CH.sub.2). With small amounts 
of the material, an alternative, safer purification is elution of crude 
product with toluene over alumina, with pure bridge being the first 
fraction off the column. 
EXAMPLE 2 
Rh.sub.2 (L).sub.4 PF.sub.6.sup.-, L=TM 4-Bridge or Cyclo 5-Bridge were 
prepared as follows: 0.630 g AgPF.sub.6 (2.5) mmol) was added to 25 ml of 
a stirred acetonitrile solution containing 0.616 g (1.25 mmol) 
[Rh(COD)Cl].sub.2. The AgCl precipitate was filtered and then either 0.822 
g of Tm 4-Bridge or 0.95 g Cyclo 5-Bridge was added to the light yellow 
filtrate. Diethyl ether was added and the resulting precipitate was 
recrystallized from acetonitrile/ether and air dried. 
Yields were about 80%. Rh.sub.2 (TM 4-Bridge).sub.4 (PF.sub.6).sub.2 -Anal. 
Calcd: C, 41.68; H, 5.60; N, 9.72. Found: C, 41.38; H, 5.47; N, 10.08; 
.nu.(CN) 2152 cm.sup.-1 Ch.sub.2 Cl.sub.2. Rh.sub.2 (Cyclo 5-Bridge).sub.4 
(PF.sub.6).sub.2 -Anal. Calcd: C, 45.87; H, 5.77; N, 8.92. Found: C, 
46.01; H, 5.60; N, 9.09; .nu.(CN) 2160 cm.sup.-1 CH.sub.2 Cl.sub.2 
solution. 
4-Bridge TM 4-Bridge, and Cyclo 5-Bridge were prepared from the 
corresponding amine by the method of W. P. Weber, et al., Angew. Chem. 
Internat. Ed., 11, 530 (1972). 4-Bridge was purified by vacuum 
distillation; TM 4-Bridge and Cyclo 5-Bridge were purified by 
recrystallization from CH.sub.2 Cl.sub.2. The infrared spectra and NMR 
spectra are as follows: bridge'; IR, 2145 cm.sup.-1 .nu.(CN), neat; TM 
4-Bridge, IR, 2126 cm.sup.-1 .nu.(CN) CH.sub.2 Cl.sub.2 ; NMR. 
EXAMPLE 3 
Co.sub.2 (TM 4-Bridge).sub.4 (CoCl.sub.4).sub.2.4 H.sub.2 O 
To a stirred solution of 0.30 g (1.26 mmoles) of CoCl.sub.2.6 H.sub.2 O in 
50 ml of anhydrous ethanol was added 0.25 g (1.52 mmoles) of TM 4-Bridge. 
The resulting green solution was refrigerated for three days affording 
green crystal of Co.sub.2 (TM 4-Bridge).sub.4 (CoCl.sub.4).sub.2.4 H.sub.2 
O. 
Calcd: C, 38.49; H, 5.30%; N, 8,89%. Found: C, 38.49%; H, 5.813%, 8.98%. 
EXAMPLE 4 
Rh.sub.2 (Bridge).sub.4 (BF.sub.4).sub.2 
To a solution of 1.23 g of (Rh(COD)Cl).sub.2 in 20 ml of acetonitrile was 
added 0.97 g of silver tetrafluoroborate in 10 ml of acetonitrile. The 
solution was stirred and filtered by gravity to remove the silver chloride 
formed. Then 0.94 g of 1,3-diisocyanopropane bridge in 10 ml of 
acetonitrile was added dropwise with stirring to the rhodium solution. The 
purple powder was filtered, washed with diethyl ether, and dried in vacuo. 
This salt is soluble in acetonitrile, DMF, and DMSO. 
Calcd: C, 31.78; H, 3.20; N, 14.82; F, 20.11. Found: C, 31.62; H, 3.37; N, 
14.66; F, 19.82. 
EXAMPLE 5 
Rh.sub.2 (Bridge).sub.4 Cl.sub.2 
This compound was obtained by adding a stoichiometric amount of 
1,3-diisocyanopropane bridge to a chloroform solution of (Rh(COD)Cl).sub.2 
and filtering the blue precipitate, washing with diethyl ether, and drying 
in vacuo. Soluble in methanol, water, DMSO and DMF. 
EXAMPLE 6 
1.1 g 4-bridge was dissolved in 100 ml CHCl.sub.3. 10 ml of this solution 
was added to 50 ml of CH.sub.3 CN in an erlenmeyer flask. The resulting 
solution was purged with nitrogen for 5 minutes. Rh.sub.2 (CO).sub.4 
Cl.sub.2 (0.10 g) in 5 ml CHCl.sub.3 was then added dropwise to the 
4-bridge while maintaining vigorous stirring and a nitrogen blanket. After 
complete addition, the solution was deep red purple and some precipitation 
had occurred. Stirring was maintained for 5 minutes more and an equal 
volume of diethyl ether was added to precipitate all solids. The resulting 
dull blue gray powder was washed with CHCl.sub.3 and ether and dried under 
a stream of N.sub.2. Once dry, the product was worked up in air. The blue 
gray powder was extracted with methanol several times. To the filtrate was 
added solid NaBPH.sub.4 (excess). A navy blue product precipitated 
immediately. This was isolated by filtration, washed with water, methanol 
and ether. The product was air dried. Yields are in general poor (less 
than 30% based on Rh.sub.2 (CO).sub.4 Cl.sub.2) and variable. The complex 
obtained in this fashion can be purified by reprecipitation from CH.sub.3 
CN/ether mixtures and gives reproducible UV/VIS and IR spectra consistent 
with Rh.sub.2 (4-bridge).sub.4 (BPh.sub.4).sub.2. 
Elemental analysis (RH.sub.2 (4-bridge).sub.4 (BPh.sub.4).sub.2. Anal. 
Calcd: C, 67.73; H, 5.68; N, 8.78; .nu.(CN)2170 cm.sup.-1 KBr pellet. 
Found: 66.75; H, 5.65; N, 8.99. 
The bridged dirhodium complex with 1,3-diisocyanopropane of Example 5, 
Rh.sub.2 (1,3-diisocyanopropane).sub.4.sup.2+ system has been investigated 
in more detail. This cation is called rhodium bridge because of the nature 
of its molecular structure. A view of this cation based on X-ray crystal 
structure analysis is shown in FIG. 1. The binuclear complex has near 
D.sub.4h symmetry, with a Rh-Rh distance of 3.26 A. The occupied d.sub.Z 2 
oribals on each d.sup.8 planar Rh(I) center interact, yielding two MO's of 
symmetries a.sub.1g and a.sub.2u ; and the lowest unoccupied monomer 
orbitals (of a.sub.2u symmetry) also interact and split into a.sub.1g and 
a.sub.2u levels in the binuclear complex. The orbitals of interest in 
discussing the low-lying absorption and emission bands, and the 
photochemistry, are, in order of increasing energy, 1n.sub.1g &lt;1a.sub.2u 
&lt;2a.sub.1g &lt;2a.sub.2u. The ground state of Rh.sub.2 (bridge).sub.4.sup.2+ 
is .sup.1 A.sub.1g (1a.sub.1g.sup.2 1a.sub.2u.sup.2). 
The intense absorption band in the spectrum of Rh.sub.2 
(bridge).sub.4.sup.2+ at 553 nm (.epsilon. 14,500) in acetonitrile 
solution is attributed to .sup.1 A.sub.1g -.sup.1 A.sub.2u (1a.sub.2u 
-2a.sub.1g), which is an allowed transition. The band falls well to the 
red of the analogous .sup.1 A.sub.1g -.sup.1 A.sub.2u (d.sub.z.sbsp.2 
-a.sub.2u) transition in a reference monomeric complex (e.g., this band in 
the spectrum of Rh(CNEt).sub.4.sup.+ peaks at 380 nm), which illustrates 
the importance of the axial orbital interactions (d.sub.z.spsb.2 
-d.sub.z.spsb.2 and a.sub.2u -a.sub.2u) in the rhodium bridge binuclear 
case. 
The complex-oxidant-oxygen catalyst system can be used to oxidize any 
substrate wheter inorganic such as water or carbon monoxide or organic 
such as olefins such as alkylene containing 2 to 20 carbon atoms, 
acetylene, alcohols, aldehydes, ketones, etc. The secondary oxidant can be 
any oxidizing agent stronger than the complex such as salts of cerium, 
molybdenum, iron.sup.+3 or cupric metals, quinones, permanganates, 
(IrCl.sub.6).sup.-2, dichromates, and the like. The secondary oxidants are 
present in at least stoichiometric-amount to the rhodium metal and 
preferably in excess. A source of protons such as hydrochloric or sulfonic 
acid should also be present in the solution. 
Gaseous substrates can be bubbled through the solution while liquid 
substrates can be added incrementally or at the start of the reaction. The 
reaction can be conducted at temperatures from 0.degree. C. to 150.degree. 
C. preferably from 60.degree. C. to 100.degree. C. and at pressures from 
below atmospheric to 20 atmospheres or more preferably from 1 to 5 
atmospheres. The complex concentration can be from 10 to 200 ppm generally 
from 50 to 150 ppm. 
Referring now to FIG. 2, the oxidation system 10 includes a reactor 12 
having an inlet 14 for feeding in the catalyst solution comprising the 
metal bridge complex and an excess of secondary oxidant dissolved in 
solvent such as water. After the solution 16 is charged into the reactor, 
substrates can be continuously oxidized by bubbling the substrate and air 
through the solution 16 from inlet 18, 20 respectively. The oxidized 
substrate is recovered as product through outlet 22. 
EXAMPLE 7 
The purple BF.sub.4 salt (O-IM) of Example 4 was dissolved in methane 
sulfuric acid. 1 mole of an acidic solution ceric sulfate Ce.sup.+4 
(SO.sub.4).sub.2 was added (pH 1). The complex was oxidized to a light 
yellow form. 
EXAMPLE 8 
The purple BF.sup.4 salt of Example 4 was dissolved in concentrated HCI and 
air was bubbled through the solution at room temperature. The complex was 
slowly converted to a yellow oxidized form. 
It is to be realized that only preferred embodiments of the invention have 
been described and that numerous substitutions, modifications and 
alterations are permissible without departing from the spirit and scope of 
the invention as defined in the following claims.