Tris(triphenylphosphine)rhodium carbonyl hydride

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.[1]

Tris(triphenylphosphine)rhodium carbonyl hydride
Identifiers
ECHA InfoCard 100.037.467
UNII
Properties
C55H46OP3Rh
Molar mass 918.78
Appearance yellow solid
Melting point 172–174 °C (342–345 °F; 445–447 K) sealed capillary
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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 + BH3

It can also be prepared from an aldehyde, rhodium trichloride and triphenylphosphine in basic alcoholic media.[2]

Structure

The complex adopts a trigonal bipyramidal geometry with trans CO and hydrido ligands, resulting in pseudo-C3v symmetry. The Rh-P, Rh-C, and Rh-H distances are 2.32, 1.83, and 1.60 Å, respectively.[3][4] 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.[5]

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.[1] 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. J. F. Hartwig; Organotransition metal chemistry - from bonding to catalysis. University Science Books. 2009. 753, 757-578. ISBN 978-1-891-38953-5.
  2. Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. (1990). "Hydrido Phosphine Complexes of Rhodium(I)". Inorganic Syntheses. 28: 81–83. doi:10.1002/9780470132593.ch19.
  3. 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. doi:10.1007/BF01185550.
  4. S. J. la Placa, J. A. Ibers "Crystal and Molecular Structure of Tristriphenylphosphine Rhodium Carbonyl Hydride" Acta Crystallogr. 1965, p. 511. doi:10.1107/S0365110X65001093
  5. D. Evans, J. A. Osborn, G. Wilkinson "Hydroformylation of Alkenes by Use of Rhodium Complex Catalysts" J. Chem. Soc. 1968, pp. 3133-3142. doi:10.1039/J19680003133
  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. doi:10.1021/ba-1982-0196.ch003.
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