Stibine and phosphite mixed ligand rhodium vaska-type complexes

English: The aim of this study was to synthesise simple rhodium stibine complexes and to react them with a range of phosphite ligands in order to determine the rate constants and reaction mechanism for the substitution reactions. The phosphites were selected in order to provide a range of sterically demanding incoming ligand systems, as determined by their Tolman cone angles. Spectroscopic investigation revealed there were two different reaction mechanisms evident for the reaction of the stibine system, trans-[Rh(Cl)(CO)(SbPh3)2] with the larger and smaller cone angle phosphites. Low temperature 31P NMR indicated that the reaction of trans-[Rh(Cl)(CO)(SbPh3)2] with small cone angle phosphites resulted in a series of addition and elimination reactions to form a range of four and five coordinate mixed stibine and phosphite intermediate species. These reactions appeared to be in equilibrium and were terminated by the formation of a phosphite analogue of Wilkinson’s catalyst, [Rh(Cl){P(OR)3}3]. The bulky phosphites, however, reacted by two consecutive substitution reactions to form firstly a mono-stibine mono-phosphite intermediate, trans- [Rh(Cl)(CO)(SbPh3){P(OR)3}] followed by a bis-phosphite complex, trans- [Rh(Cl)(CO){P(OR)3}2]. While attempting to characterise the mixed stibine/phosphite complexes crystallographically, a single crystal was obtained. This was subsequently solved as the Rh(III) complex, trans-mer-[Rh(Cl)2(Ph)(SbPh3)3].2CH2Cl2. This system appears to form through oxidative addition and phenyl migration of triphenylstibine onto rhodium(I). This Rh(III) complex was reacted with triphenylphosphine and single crystals of [Rh(Cl)2(Ph)(PPh3)2] were collected. The six coordinate stibine system crystallised from dichloromethane in the triclinic space group, Pi with Z = 2, while the five coordinate phosphine complex crystallized in the monoclinic space group, C2/c with Z = 4. Both complexes contain a rhodium center with two trans chloride atoms and a metal bound phenyl ring. The stibine system contains two trans triphenylstibine molecules, with a third stibine trans to the phenyl. The phosphine system contains two triphenylphosphine groups bound to the metal. A kinetic study was conducted to investigate the reaction of trans-[Rh(Cl)(CO)(SbPh3)2] with the bulky phosphite, tris(2,4-di-tbutylphenyl)phosphite (2,4-TBPP). Stopped-Flow spectrophotometry showed two consecutive reactions at 310nm, a fast first reaction followed by a slower second reaction. The kinetic investigation was conducted in two different solvents, namely, dichloromethane and ethyl acetate, to determine the effect of solvent polarity and donicity on the reaction rates. It soon became evident that the first reaction was too fast to follow under standard first order conditions and excess tripehenylstibine was added to the system to introduce the five coordinate tris-stibine complex, trans-[Rh(Cl)(CO)(SbPh3)3]. This had the desired effect of slowing down the reaction and the kinetic data for the first reaction could be calculated from the derived rate law. The first order rate constants, k12, for the reaction to form trans- [Rh(Cl)(CO)(SbPh3)(2,6-TBPP)] from trans-[Rh(Cl)(CO)(SbPh3)2] are 5.2(1) M-1.s-1 and 4.2(3) M-1.s-1 for DCM and ethyl acetate, respectively. While the first order rate constants, k13, forming trans-[Rh(Cl)(CO)(SbPh3)(2,6-TBPP)] from trans- [Rh(Cl)(CO)(SbPh3)3] are 3.3(9) M-1.s-1 and 4(8) M-1.s-1 for DCM and ethyl acetate, respectively. The second reaction step to form [Rh(Cl)(CO)(2,4-TBPP)2] from [Rh(Cl)(CO)(SbPh3)(2,4-TBPP)] was investigated in order to determine the thermodynamic data for the reaction step. The first order rate constants, k2 at 298 K, are 33.0(8) M-1.s-1 and 719(16) M-1.s-1 for the reaction in DCM and ethyl acetate respectively. The corresponding activation parameters are DH† = 22.6(6) kJ.mol-1 and DS† = -214(2) J.mol-1.K-1 for DCM and DH† = 27.8(5) kJ.mol-1 and DS† = -171(2) J.mol-1.K-1 for ethyl acetate. The significantly negative entropy calculated indicates an associative pathway forming the transition state, as has been found for many stibine systems that readily form five coordinate complexes. Scheme 1 gives the predicted reaction mechanism. See Scheme in full text.