A synthetic, electrochemical and kinetic study of polysiloxane-bound rhodium carbonyl complexes

English: Two ferrocene-containing β-diketonato ligands, FcCOCH2COR, as well as their dicarbonyl rhodium(I) complexes, [Rh(FcCOCHCOR)(CO)2], were synthesized. Chloro/bromo functionalized polysiloxanes of the type [(OSiMe2)2OSiMe{(CH2)mX}]n with X = Cl and Br as well as iodo functionalized polysiloxanes, [(OSiMe2)2OSiMe{(CH2)mI}]n, were synthesized. The latter were converted to phosphine-containing polysiloxanes, [(OSiMe2)2OSiMe{(CH2)mPPh2}]n and used as a starting material to synthesize six new rhodium(I)-containing polysiloxane complexes of the type [(OSiMe2)2OSiMe{(CH2)mPPh2(CO)(FcCOCHCOR)Rh}]n, with Fc = ferrocenyl, R = CH3 and CF3, m = 3, 4, 5. Characterization techiniques included 1H and 31P NMR, electrochemistry, UV/vis and FTIR. 1H and 31P NMR data showed the existence of two isomers for each of the rhodium(I) polysiloxane complexes in solution. Elemental analysis confirmed the elemental composition of the polymers and from the XPS, the binding energies of elements were determined. The inherent viscosity for all polymers was found to be in the range of 0.011-0.28 g/dL. The new rhodium(I) polysiloxane complexes as well as the free rhodium(I) compexes were studied by cyclic voltammetry in CH2Cl2/[NnBu4][B(C6F5)4]. Two isomers were identified for the rhodium(I) polysiloxane complexes. In a one broad oxidation peak, Rh(1) and Rh(2) was electrochemically irreversibly oxidized first in a one-electron transfer followed by one-electron electrochemical reversible oxidations of the two ferrocenyl groups, Fc(1) and Fc(2). A pseudo-first order kinetic study of methyl iodide oxidative addition to rhodium polymer [(OSiMe2)2OSiMe{(CH2)3PPh2(CO)(FcCOCHCOCF3)Rh}]n was monitored by FTIR and UV/vis techiques. A two stage mechanism was identified wherein kinetically favoured Rh(III)alkyl1 and Rh(III)acyl1 formed first at the same reaction rate in stage 1. The kinetically favoured products converted in stage 2 to the thermodynamically favoured Rh(III)alkyl2 and Rh(III)acyl2 species. Stage 1 was dependent on the CH3I concentration and stage 2 was independent thereof. The FTIR results also showed possible existence of a rhodium(I) polysiloxane square pyramidal complex. The UV/vis-determined second-order rate constant for the first stage (k1 = 0.1853 M-1.min-1) at 25 °C was found to be in agreement with the FTIR-determined rate constant (k1 = 0.1499 M-1.min-1). The influence of temperature on the reaction was studied at 15, 35 and 45 °C and the activation entropy for stage 1 was found to be -97(6) J.mol-1.K-1 while for stage 2 it was 6(3) J.mol-1.K-1. Stage 1 reacted in an associative mechanism, while stage 2 involved geometrical isomerizations. Keywords: Rhodium(I), polysiloxane, silane, phosphine, polymerization, ferrocene, β-diketone, oxidative addition.