Tris(triphenylphosphine)rhodium carbonyl hydride
| Tris(triphenylphosphine)rhodium carbonyl hydride | |
|---|---|
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| Identifiers | |
| CAS number | 17185-29-4 |
| Properties | |
| Molecular formula | C55H46OP3Rh |
| Molar mass | 918.78 g mol−1 |
| Appearance | yellow solid |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) | |
| Infobox references | |
Tris(triphenylphosphine)rhodium carbonyl hydride is an organorhodium compound with the formula HRh(CO)(PPh3)3 (Ph = C6H5). It is a yellow, benzene-soluble solid which is used industrially for hydroformylation.[1]
Preparation
HRh(CO)(PPh3)3 was first prepared by the reduction of RhCl(CO)(PPh3)2 and hydrazine in an ethanolic suspension:
- RhCl(CO)[PPh3]2 + NaBH4 + PPh3 → RhH(CO)(PPh3)3 + NaCl + BH3
Alternative syntheses involve the reaction of the same rhodium precursor with sodium tetrahydroborate, or triethylamine and hydrogen, in ethanol in the presence of excess triphenylphosphine. It can also be prepared from rhodium trichloride and triphenylphosphine in basic alcoholic media.[2]
Structure
The complex adopts a trigonal bipyramidal geometry with trans CO and hydride groups, 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
In comparison with other metals, and particularly cobalt (which is also a group 9 element), hydroformylation catalysts made with rhodium are more active and operate under milder conditions.
The development of this catalyst grew from attempts use tris(triphenylphosphine)rhodium chloride as a hydroformylation catalyst. It was found that the complex would quickly carbonylate and that its catalytic activity could be enhanced by a variety of additives but inhibited by halides. This inhibition did not occur in the presence of base, suggesting that the hydride-complex represented the catalytic form of the complex.[5]
Mechanistic considerations
HRh(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
- ^ a b J. F. Hartwig; Organotransition metal chemistry - from bonding to catalysis. University Science Books. 2009. 753, 757-578. ISBN: 978-1-891-38953-5.
- ^ N. Ahmad, J. J. Levison, S. D. Robinson, M. F. Uttley "Hydrido Phosphine Complexes of Rhodium(I)" Inorg. Syntheses 1990, volume 28, p. 81–83. doi:10.1002/9780470132593.ch19.
- ^ 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.
- ^ S. J. la Placa, J. A. Ibers "Crystal and Molecular Structure of Tristriphenylphosphine Rhodium Carbonyl Hydride" Acta Cryst. 1965, p. 511. doi:10.1107/S0365110X65001093
- ^ D. Evans, J. A. Osborn, G. Wilkinson "Hydroformylation of Alkenes by Use of Rhodium Complex Catalysts" J. Chem. Soc. 1968. 3133-3134. doi: 10.1039/J19680003133
- ^ 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.
