Structures of square-pyramidal acyl and octahedral alkyl intermediates in the decarbonylation of acid halides by chlorotris(triphenylphosphine)rhodium(I)

Abstract

Oxidative addition of acetyl chloride to the compound [RhCI(PPh,),] leads to the formation of two acetyl-and one methyl-rhodium species, all isomeric, before reductive elimination of methyl chloride occurs to give [ RhCI(C0) -(PPh,),].The structures of all the compounds have been determined using i.r. and both l H and 31P n.rn.r. spectroscopy. The effects of changing the nature of the acid halide and the tertiary phosphine have been determined as have thermodynamic parameters for the ' carbonyl-insertion ' reactions involved. The X-ray crystal structure of the compound [RhCI,(COCH,CH,Ph) (PPh,),] demonstrates essentially a square pyramid with trans-basal chlorides, trans-basal phosphines, and an apical acyl group.REVERSIBLE ' insertion ' of carbon monoxide into is generally accepted that the mechanism involves a (carbonylation of) a metal-carbon CT bond to form an 1,2 migration of the alkyl group between a co-ordinated acylmetal compound is a very well known reaction in carbonyl group and a cis co-ordination site on the metal organotransition-metal chemistry, and is undoubtedly a [equation (l)]. This mechanism is consistent with a key step in hydroformylation-type syntheses.lY2 The group, have therefore been studied in great detail, and it reaction and it-reverse, decarbonylation of an acylmetal A. Wojcickip oyganometazzic 1973J 11, 87.

Five-coordinate dicarbonyl complexes of rhodium(I): [RhX(CO)2(PPh3)2] (X = Cl, Br, I)

“…Elimination of CO from the dicarbonyl complexes will be facile. and square-pyramidal structures occurs for [RhC12(COR)-(PPh,)2] (R = CH3, CH2CH2Ph) (15). Nevertheless, the nmr data for 2 indicate that the trigonal-bipyramidal is the most probable structure, based on a comparison of coupling constants between complexes 1 and 2.…”

“…The existence of complexes 2 has implications for various reactions catalysed by phosphine complexes of rhodium, especially carbonylation and decarbonylation reactions. Under mild conditions [RhCI(P.Ph,),] is an effective reagent for the stoichiometric decarbonylation of aldehydes and aryl or aroyl halides, but complex l a (which is formed) is not (5,7,15,18). Under more rigorous conditions each material is an effective catalyst for such decarbonylation reactions, and intermediates are formed which contain alkyl and hydrido or halogen ligands (15)(16)(17)(18)(19)(20)(21).…”

“…Under mild conditions [RhCI(P.Ph,),] is an effective reagent for the stoichiometric decarbonylation of aldehydes and aryl or aroyl halides, but complex l a (which is formed) is not (5,7,15,18). Under more rigorous conditions each material is an effective catalyst for such decarbonylation reactions, and intermediates are formed which contain alkyl and hydrido or halogen ligands (15)(16)(17)(18)(19)(20)(21). Although the mechanism of the stoichiometric reaction is well understood, the subsequent reactions of l a in the catalytic process, especially elimination of CO, have remained subject to speculation.…”

Reactivity of cationic methyl rhodium(III) complexes cis-[Rh(β-diket)(PPh3)2(CH3)(CH3CN)][BPh4] toward ligands of different character: pyridine, carbon monoxide, and triphenylphosphine

“…By analogy, we assign the signals at d 2.47 (3H) and 1.85 ppm (3H) in the spectrum of complex 3 to methyl groups of acetyl and benzoylacetonate ligands, respectively. It is easy to verify that a trigonal bipyramid with trans-phosphines and the acetyl group in the equatorial position fits all the experimental data: Acyl complexes of rhodium(III) usually exist in a pentacoordinate form, and their typical solid-state geometry is square pyramid (sp) with apical acyl ligand [4,9,10,[18][19][20][21][22][23]. The same geometry was evidenced for these complexes in solutions.…”

“…On the basis of NMR spectra, we assign to the cation of 1 the following structure, akin to the structure of cation cis-[Rh(Acac)(PPh 3 ) 2 (CH 3 )(NH 3 )] + , which has been determined by NMR spectra and X-ray analysis [6]. The IR spectra of complexes 2 and 3 display intense bands with maxima at 1726 cm À1 (2) and 1728 cm À1 (3), which correspond to the m(CO) stretching vibration in the -C(@O)Me group of acetyl rhodium(III) complexes [3,5,[8][9][10][11][12][13][14][15][16][17].…”