Platinum Metals Rev., 1976, 20, (3), 96
The Platinum Metals in Catalysis
Papers at The Annual Chemical Congress
Among the papers presented at the Dalton Symposium held during the Annual Congress of the Chemical Society in Glasgow were contributions concerned with the use of the platinum group metals as heterogeneous and homogeneous catalysts and with recent developments concerning the molecular structure of some osmium and ruthenium complexes.
The use of the platinum metals as heterogeneous hydrogenation catalysts was reviewed by Professor S. J. Thomson and Dr G. Webb of the University of Glasgow. These authors demonstrated that, in spite of the vast number of papers which have been published concerning hydrogenation reactions on metal catalysts, the subject still has no basic unifying theory. The controversy over reaction mechanisms and the underlying properties of metals which account for their behaviour as catalysts was demonstrated by considering typical results relating to the basic features of many aspects of hydrogenation. Examining each of these aspects in turn, the problems and inconsistencies of interpretations were emphasised. These included the inability of kinetics to yield unequivocal results; the importance of catalyst pretreatments to determinations of activity; the insensitivity of activation energy to the metal and of reaction rates, on an atom for atom basis, to the nature of the surface; the insensitivity of hydrogenation to catalyst structure, when compared with the “demanding” reactions of skeletal isomerisation and hydrocarbon cracking; the occurrence of self-hydrogenation and the retention of “carbidic” residues on the catalyst surface and the lack of correlation between the electronic, magnetic and geometrical factors with activity. This last point is well demonstrated in alloy catalysis, where neither the rates of hydrogenation nor the activation energy shows the expected change at the point corresponding to the filling of the d-band of the alloy.
From these considerations it was concluded that the metal was only of secondary importance in hydrogenation. It was suggested that all of the various aspects, outlined here, can be satisfactorily interpreted by considering a general mechanism in which hydrogenation involves a hydrogen transfer between an adsorbed carbonaceous residue M–C2Hx and adsorbed olefin, rather than by direct addition of adsorbed hydrogen to adsorbed olefin. Hydrogenation thus becomes an extension of self-hydrogenation; the latter reaction is self-poisoning, but in the presence of added hydrogen (hydrogenation) becomes a continuous process.
Dr G. W. Parshall of E.I. du Pont de Nemours, U.S.A., outlined the role of transition metal complexes to some catalytic and synthetic processes. Among the examples cited was the rhodium(I)-catalysed dimerisation of ethylene to but-1-ene (see Scheme I) and the commercial synthesis of hexa-1 : 4-diene (see Scheme II). In the latter example one of the major advantages of using rhodium as catalyst, compared with iron or cobalt, is its ability to catalyse the isomerisation of the thermodynamically more favoured anti-1-methyl-π-allyl complex to the syn complex and hence to produce the desired trans-hexa-1:4-diene product. The stereochemical aspects of the mechanism of the co-dimerisation of ethylene and vinyl chloride in the presence of rhodium(I) as catalyst were discussed:
The major reaction pathway postulated is:
Dr Parshall concluded by describing some recent work on the reactions of carbon dioxide with some iridium(I) complexes. [Ir(dmpe)2]+ in the presence of CO2 yields two complexes as shown in Fig. 1, while the action of CO2 on the complex IrCl(C8H14)(PMe3)3 produces the novel complex [A] shown in Fig. 2, which has been characterised crystallographically. Slow heating of [A] results in the reversible loss of CO2 whereas rapid heating leads to the carbonyl complex and .
Dr A. C. Skapski of Imperial College, London, described recent developments relating to the structures of some osmium and ruthenium complexes in the crystalline state. Whereas derivations of OsO4, formed in cis - hydroxylation of olefins and in biological staining, are usually considered to have a structure [B], a more likely structure is that found with [OsO2(O2C2H4)2]2, structure [C], involving 5-coordinate osmium. A similar structure [D] has been found for the monomeric complex (C2H4O2)2OsO, showing that osmium can bridge two C–C linkages in both monomer and dimer. It was suggested that OsO4 could also attack –SH and –NH2 groups in proteins; a complex of hexamethylene tetramine with OsO4 has the structure [E]. The Os–N bond length is very large (2.42Å) explaining why the OsO4 can be readily vaporised from the complex.
Using the large cation [Ph4As]+ to minimise packing effects on the anion, it has been found that in the tetraphenyl arsine complexes, the anions [OsNCl4]−, [OsNI4]− and [RuNCl4]− are isostructural having C4v symmetry and a short M≡N and large M-halogen bond lengths.
In contrast to this, in the complex Ru(NPEt2Ph)Cl3(PEt2Ph)2 (structure [F]) the Ru-N bond is longer than expected for a single bond while the P–N bond is somewhat shorter than in a P=N double bond.
Finally Dr Skapski referred to the novel “flying-bedstead” structure of the tetrameric [Pt(OAc)2]4 involving a square of strong Pt–Pt bonds (Fig. 3, [G]).