Tris(triphenylphosphine)rhodium carbonyl hydride explained

Carbonyl hydrido tris(triphenylphosphine)rhodium(I) [Carbonyl(hydrido)tris(triphenylphosphane)rhodium(I)] is an organorhodium compound with the formula [RhH(CO)(PPh₃)₃] (Ph = C₆H₅). It is a yellow, benzene-soluble solid, which is used industrially for hydroformylation.

Preparation

[RhH(CO)(PPh₃)₃] was first prepared by the reduction of [RhCl(CO)(PPh₃)₂], e.g. with sodium tetrahydroborate, or triethylamine and hydrogen, in ethanol in the presence of excess triphenylphosphine:

[RhCl(CO)(PPh₃)₂] + NaBH₄ + PPh₃ → [RhH(CO)(PPh₃)₃] + NaCl + BH₃It can also be prepared from an aldehyde, rhodium trichloride and triphenylphosphine in basic alcoholic media.^[1]

Structure

The complex adopts a trigonal bipyramidal geometry with trans CO and hydrido ligands, resulting in pseudo-C_3v symmetry. The Rh-P, Rh-C, and Rh-H distances are 2.32, 1.83, and 1.60 Å, respectively.^{[2] [3]} This complex is one of a small number of stable pentacoordinate rhodium hydrides.

Use in hydroformylation

This precatalyst was uncovered in attempts to use tris(triphenylphosphine)rhodium chloride as a hydroformylation catalyst. It was found that the complex would quickly carbonylate and that the catalytic activity of the resulting material was enhanced by a variety of additives but inhibited by halides. This inhibition did not occur in the presence of base, suggesting that the hydrido-complex represented the catalytic form of the complex.^[4]

Mechanistic considerations

[RhH(CO)(PPh₃)₃] is a catalyst for the selective hydroformylation of 1-olefins to produce aldehydes at low pressures and mild temperatures. The selectivity for n-aldehydes increases in the presence of excess PPh₃ and at low CO partial pressures.^[5] The first step in the hydroformylation process is the dissociative substitution of an alkene for a PPh₃. The migratory insertion of this 18-electron complex can result in either a primary or secondary rhodium alkyl. This step sets the regiochemistry of the product, however it is rapidly reversible. The 16-electron alkyl complex undergoes migratory insertion of a CO to form the coordinately unsaturated acyl. This species once again gives an 18-electron acyl complex.^[6] The last step involves β-H elimination via hydrogenolysis which results in the cleavage of the aldehyde product and regeneration of the rhodium catalyst.

References

1. Book: 10.1002/9780470132593.ch19. Hydrido Phosphine Complexes of Rhodium(I). 1990. Ahmad. 28. N.. Levison. J. J.. Robinson. S. D.. Uttley. M. F.. Inorganic Syntheses. 81–83. 9780470132593.
2. I. S. Babra, L. S. Morley, S. C. Nyburg, A. W. Parkins "The crystal and molecular structure of a new polymorph of a carbonlyhydridotris(triphenylphosphine)rhodium(I) having a Rh-H stretching absorption at 2013 cm⁻¹" Journal of Crystallographic and Spectroscopic Research 23. 1993. 999. .
3. S. J. la Placa, J. A. Ibers "Crystal and Molecular Structure of Tristriphenylphosphine Rhodium Carbonyl Hydride" Acta Crystallogr. 1965, p. 511.
4. D. Evans, J. A. Osborn, G. Wilkinson "Hydroformylation of Alkenes by Use of Rhodium Complex Catalysts" J. Chem. Soc. 1968, pp. 3133-3142.
5. J. F. Hartwig; Organotransition metal chemistry - from bonding to catalysis. University Science Books. 2009. 753, 757-578. .
6. R. V. Kastrup, J. S. Merola, A. A. Oswald; P-31 NMR Studies of Equilibria and Ligand Exchange in Triphenylphosphine Rhodium Complex and Related Chelated Bisphosphine Rhodium Complex Hydroformylation Catalyst Systems. J. Am. Chem. Soc. 1982.44-16. .