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

English: The aim of this study was to determine the mechanism for the reaction between iodomethane and complexes of the type [Rh(N,O-BID)(CO)(XR3)]; N,O-BID = mono anionic bidentate ligands of the general formula: (i) dimethylaminovinylketone; dmavk (ii) methyltrifluoroaminovinylketone; tavk; X= As or P, R = phenyl and substituted phenyls, PPh3, AsPh3, P(p-CI-Ph)3 or P(p-OMe-Ph)3. Determination of the mechanism was achieved utilizing X-ray crystallography, reaction kinetics and 31p_NMR. [Rh(dmavk)(CO)(PPh3)] crystallizes in the orthorombic crystal system with space group Pca21 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 space group 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 space group 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 the nitrogen atom of the N,O-BID ligand. The Rh-As bond is significantly longer than the Rh-P bond ( 2.3834(6) and 2.2751(13) A respectively). The successful isolation and Xray crystallographic 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 first oxidative addition products is also retained from the same configuration present in the starting 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 an increase 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 of the Rh(lII)-acyl species according to the following reaction: A significant solvent effect was observed for the oxidative addition reaction between iodomethane and [Rh(dmavk)(CO)(PPh3)]. At 25°C, this reaction proceeds 8 times faster in the highly polar solvent acetonitrile (k, = 89(6)xl02 M"IS"I)compared to the least polar solvent chloroform (k, = 11.4(4)xl02 M"IS"I). The activation parameters (L1H# and L1S#) were determined from the temperature dependence of k, in acetone. Large negative L1S# 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, polar transition state with subsequent formation of an ion-pair intermediate is postulated. The rate constant of the oxidative addition was increased by both electronic and steric manipulation. The electronic manipulation was achieved by firstly the introduction of electron/donating substituents (CH3 in place ofCF3) on the bidentate ligand, resulting in a four fold increase in magnitude for the rate of oxidative addition; and secondly by the interchanging 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 relative independent of the variation in nucleophilic character of the metal center. Steric manipulation was achieved by interchanging PPh3 with AsPh3. Replacing the PPh3 ligand by AsPh3 leads to an increase in the rate of oxidative addition and a decrease in the rate of reductive elimination, resulting in an increase in the equilibrium constant for this step. A significant (ca. one order of magnitude) decrease in CO-insertion was observed from 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 by decreasing the steric demand on the meta! center. The introduction of these N,O-BID ligands to the Rh(I) center in these complexes resulted in at least a ea. 4 fold activation in the oxidative addition rate as compared to the known O,O-BID ligand systems (i.e. acac, ox, etc.)