Fundamental aspects of selected rhonduim complexes in homogeneous catalytic acetic acid production.

[摘要] English: The aim of this study was to determine the mechanism for the reaction betweeniodomethane and complexes of the type [Rh(N,O-BID)(CO)(XR3)]; N,O-BID = monoanionic bidentate ligands of the general formula: (i) dimethylaminovinylketone; dmavk(ii) methyltrifluoroaminovinylketone; tavk; X= As or P, R = phenyl and substitutedphenyls, PPh3, AsPh3, P(p-CI-Ph)3 or P(p-OMe-Ph)3. Determination of the mechanismwas achieved utilizing X-ray crystallography, reaction kinetics and 31p_NMR.[Rh(dmavk)(CO)(PPh3)] crystallizes in the orthorombic crystal system with space groupPca21 and final R value of 2.04 %. [Rh(dmavk)(CO)(AsPh3)] and[Rh(dmavk)(I)(CH3)(CO)(PPh3)] crystallize in the triclinic crystal system with spacegroup PI. The final R value for each was 3.88 and 4.77 % respectively.[Rh(dmavk)(I)(COCH3)(PPh3)] crystallizes in the monoclinic crystal system with spacegroup P21/c and a final R value of6.72. In the case of the mono carbonyl complexes, i.e.,for [Rh(dmavk)(CO)(PPh3)] and [Rh(dmavk)(CO)(AsPh3)], the PIAs atom is trans to thenitrogen atom of the N,O-BID ligand. The Rh-As bond is significantly longer than theRh-P bond ( 2.3834(6) and 2.2751(13) A respectively). The successful isolation and Xraycrystallographic characterization of the starting complex, [Rh(dmavk)(CO)(PPh3)],and its oxidative addition products for the reaction between and iodomethane, i.e.,[Rh(dmavk)(I)(CH3)(CO)(PPh3)] and [Rh(dmavk)(I)(COCH3)(pPh3)] was for the firstoxidative addition products is also retained from the same configuration present in thestarting complex.31p_NMR studies showed that for the [Rh(L,L-BID)(CO)(PPh3)] complexes: L,L'-BID =O,O-BID: tfaa, trop, cupf, acac, tta; O,S-BID: pbtu, hpt, anmeth, sacac; N,S-BID: cacsm,hacsm; N,O-BID: dmavk, ox, pie a fair correlation between 1J(PRh) and the Rh-P bond distance exists In these complexes; a decrease in Rh-P bond distance results In anincrease in IJ(PRh).The [Rh(N,O-BID)(CO)(XR3)] complexes undergo oxidative addition by iodomethane,forming the Rh(III)-alkyl species via an equilibrium step, followed by the formation ofthe Rh(lII)-acyl species according to the following reaction:A significant solvent effect was observed for the oxidative addition reaction betweeniodomethane and [Rh(dmavk)(CO)(PPh3)]. At 25°C, this reaction proceeds 8 timesfaster in the highly polar solvent acetonitrile (k, = 89(6)xl02 MISI)compared to the leastpolar solvent chloroform (k, = 11.4(4)xl02 MISI). The activation parameters (L1H# andL1S#) were determined from the temperature dependence of k, in acetone. Large negativeL1S# values (L1S# = -139(40) J KI mol) and typical L1H# values (L1H# = 35(4) kj mol)were obtained. Considering these experimental results, the formation of a linear, polartransition state with subsequent formation of an ion-pair intermediate is postulated. Therate constant of the oxidative addition was increased by both electronic and stericmanipulation. The electronic manipulation was achieved by firstly the introduction ofelectron/donating substituents (CH3 in place ofCF3) on the bidentate ligand, resulting in afour fold increase in magnitude for the rate of oxidative addition; and secondly by theinterchanging triphenyl phosphine and its derivatives [PPh3 vs. P(p-CI-Ph)3 vs. P(p-OMe-Ph)3]. The' formation rate of the Rh(III)-acyl species was found to be relativeindependent of the variation in nucleophilic character of the metal center. Stericmanipulation was achieved by interchanging PPh3 with AsPh3. Replacing the PPh3ligand by AsPh3 leads to an increase in the rate of oxidative addition and a decrease in therate of reductive elimination, resulting in an increase in the equilibrium constant for thisstep. A significant (ca. one order of magnitude) decrease in CO-insertion was observedfrom PPh3 to AsPh3 (l2.0(6)xl0-4 compared to 1.32(2)xlO-4 sol).An increase in the K, values (i.e. thermodynamic stability of the Rh(III)-alkyl species)were observed by increasing the nucleophilic character on the Rh center and bydecreasing the steric demand on the meta! center.The introduction of these N,O-BID ligands to the Rh(I) center in these complexesresulted in at least a ea. 4 fold activation in the oxidative addition rate as compared to theknown O,O-BID ligand systems (i.e. acac, ox, etc.)

[发布机构] University of the Free State