The nature of some rhodium(I) complexes derived from polymer-supported ligands

INORG. NUCL. CHEM. LETTERS Vol. 13, pp. 577-580, 1977. Pergamon Press. Printed in Great Britain.

THE NATURE OF SOME RHODIUM(I) COMPLEXES DERIVED FROM POLY~R-SUPPORTED LIGANDS William R. Cullen, Brian R. James, Aubrey D. Jenkins~ Giorgio Strukul~ + and Yoshihiro Sugi Chemistry Department, University of British Columbia Vancouver, B.C., Canada V6T IW5

(Received 15 July 1977) We have previously reported (i) that the stochiometric polyphosphinite, poly-OP, obtained from atactic poly(methallyl alcohol) according to equation I, reacts with [Rh(CO)2C£]2 in THF to give an insoluble complex of formula (poly-OP)2Rh(CO)C£ and

I base --(-CH2-C(CH3)CH2OH)n + C£PPh 2 ÷

(I)

a soluble one of the same composition.

I --{-CH2-C(CH3)CH2OPPh~n

The insoluble complex in toluene suspension

proved to be a catalyst for olefin hydrogenation after a conditioning period which required both hydrogen and olefin.

Because of the current interest in "heteroge-

nized homogeneous catalysts" (2) and the unusual nature of this reaction (the model compound RhC£CO (PPh2OBui)2 is not a homogeneous hydrogenation catalyst) we have carried out further studies which indicate that the properties of complexes derived from poly-OP are very dependent on the nature of the polymer itself. We now report that different batches of (poly-OP)2Rh(CO)C£, prepared from atactic poly(methyl methacrylate), (i) showed complete irreproducibility with respect to the rates of olefin hydrogenation.

Moreover, the time necessary for

activation of the complexes was very variable, and was accompanied by a darkening of the catalyst. metal

These observations strongly suggest that decomposition to rhodium

occurs during the activation period. Indeed the stability of some of the P,h(CO)C£ derivatives make them attrac-

tive candidates for the type of solid state ketone syntheses recently described by Pittman and Hanes (S). These workers used RhC~(CO) derivatives of phenylphosphinated styrene-divinylbenzene resins. In order to ascertain the factors causing the destabilization of the polymeranchored complex, the preparation procedure was slightly modified.

When poly-OP

(P% = 11.63, corresponding to 90% substitution) was reacted with [RHC£(CO)212 imme-

+School of Molecular Sciences, U n i v e r s i t y of Sussex, England. ++

On leave of absence from Istituto di Chimica Generale, Universit~ di Yenezia, Italy. 577

578

Rhodium (I) Complexes

d i a t e p r e c i p i t a t i o n o f a compound ~ occurs, which r a p i d l y r e d i s s o l v e s completely

SCHEME 1 4 poly-OP + [RhC£(CO)2] 2

benzene ~(insoluble) + ~(soluble) imediate ~ I

~(soluble) + C(insoluble)

24 h

complete redissolution

and gives after 24 h another insoluble compound ~. region are reported in Table i. A is (poly-OP)2P.h(CO)C£,

rapid

Infrared data in the carbonyl

The single CO stretching band indicates compound

whereas ~ and ~ are most probably mixtures of Rh carbonyl

complexes containing less than 2 phosphorus atoms coordinated per metal (5), together with non coordinated phosphinite.

In fact the microanalytical data for each

of these complexes indicate an average P/Rh ratio of 2.

A reasonable explanation

for this behaviour can be found by considering how compound ~ is formed.

The start-

ing phosphinite is a linear polymer, and upon coordination, the metal center probably acts as a cross-linking agent between different chains therefore causing precipitation (the monomeric counterpart RhC£CO(PPh2OBui)2

is highly soluble in benzene).

On the other hand, such a high Rh density (Rh% = 12.50) results in a high degree

TABLE 1 Summary o f Carbonyl S t r e t c h i n g Frequencies (nujol mull) v(CO)(cm -1)

Compound

RhC~CO(PPh2OBui ) 2

1980 (vs) 1985 (vs)

C ev

198S(vs), 2060 (m) 198S(m), 2010(m), 2060(vs), 2065(vs)

D

1985(s), 2010(s), 2055(s), 2060(s)

n#

~,

1985 (vs)

o f " s t r a i n " on the polymer c h a i n , t h e whole system i s u n s t a b l e and spontaneously r e a r r a n g e s , p r o b a b l y d i s s o c i a t i n g one p h o s p h i n i t e ; c r o s s - l i n k i n g i s l o s t and the precipitate redissolves.

In f a c t , i ~ compound ~ i s r a p i d l y f i l t e r e d and d r i e d i t

i n v a r i a b l y decomposes t o metal under hydrogenation c o n d i t i o n s . The concept o f "polymer s t r a i n " e l u c i d a t e d i n t h i s example has a l r e a d y been invoked (4) to e x p l a i n decomposition to Rh metal during prolonged hydrogenation r e a c t i o n s by [Rh(NBD)P2 ]+ complexes anchored on s t y r e n e - d i v i n y l b e n z e n e copolymers. (i)

Two d i f f e r e n t approaches have been used to s t a b i l i z e (poly-OP)2Rh(CO)C£: the use o f more d i l u t e d p h o s p h i n i t e s (P% = 8.49 and 4.89 corresponding to

Rhodium (I) Complexes

40% and 16% substitution respectively).

579

In this case no precipitation dur-

ing complex preparation occurred (the degree of cross-linking is too low) and only a soluble mixture D was obtained. (ii)

an excess of phosphorus (4/1 = P/Rh) was used employing the highly substituted polyphosphinite.

In this latter case the precipitate E obtained is

a stable pure (poly-OP)2Rh(CO)C£ compound containing an excess of non coordinated phosphinite (P% = 9.85; Rh% = 8.16).

As expected compound ~ is

stable under hydrogenation conditions and is not a hydrogenation catalyst. Similar complexes were obtained starting from isotactic and syndiotactic poly(methallyl alcohol) and these, apart from one sample which apparently decomposed to rhodium metal after one week of exposure to hydrogen and olefin, did not catalyze the hydrogenation of octene. In order to study the effect of changing the ligand from a phosphinite to a phosphine, we have prepared the polymeric phosphine poly(methallyl diphenyl phosphine), poly-P. with KPPh 2

This was obtained in 90% yield by refluxing the polytosylate

in THF for 5 days (LiPPh 2

does not react).

The crude product was

purified by dissolving in CHC£ S or THF and precipitating with petroleum ether. The 31p n.m.r, spectra of syndiotactic, atactic, and isotactic poly-P are identical (singlet 26.6 ppm downfield from HSP04). Rather surprisingly all three polymeric poly-P reacted with [Rh(C0)2C£]2 to give precipitates which analyzed for (poly-P)2Rh(CO)C£ and have v(CO) 1955 cm

-I

The complexes did not show any tendency to redissolve as described above for (poly-OP)2Rh(CO)C£ and it is difficult to account for this difference. complexes are obtained when the P/Rh ratio is changed to 5.)

(Similar

Perhaps it is due

to the fact that the phosphorus donor is situated one atom closer to the side chain in the polyphosphine, although it seems more likely that electronic effects are overriding geometrical ones and that the initially precipitated complexes simply are more stable.

This is certainly evidenced in their behaviour with respect

to hydrogen andolefin exposure since none of the (poly-P)2Rh(CO)C£ complexes decompose to metal and act as hydrogenation catalysts.

This research was supported by the National Research Council of Canada in the form of operating grants (W.R.C. and B.R.J.) and NATO research grant No. 7S5 (W.R.C. and A.D.J.).

We thank Dr. Angela J. Chapman for preparing some of the

stereoregular polymers.

References i.

W.R. CULLEN, D.J. PATMORE, A.J. CHAPMAN, and A.D. JENKINS, J. Organometal. Chem. 102, C12 (1975).

2.

For reviews see, for example, J.C. BAILAR, Catalysis Rev., i0, 17 (1974); C.U. PITTMAN, Chem. Technol., 560 (1973); N. TAKAISHI, H. I~I, C.A. BERTELO, and J.K. STILLE, J. Am. Chem. Soc., 98, 5400 (1976).

3.

J. GALLAY, D. De MANTAUZON, and R. POILBLANC, J. Organometal. Chem., 38, 179

(1972).

580

Rhodium (I) Complexes

4.

G. STRUKUL, P. D'OLIMPI0, M. BONIVENTO, F. PINNA, and M. GRAZIANI, J. Molec ular Catal., 2, 179 (1977).

5.

C.U. PITTMAN and R.M. HANES, J. Org. Chem., 4~2, 1194 (1977).