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

Abstract

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.)

Afrikaans: Die doel van hierdie studie was om die meganisme vir die reaksie tussen jodometaan en komplekse van die tipe [Rh(N,O-BID)(CO)(XR3)] te bepaal; N,O-BID = mono anioniese bidentate ligande van die algemene formule: (i) dimetielaminovinielketoon; dmavk, (ii) metieltrifluoroaminovinielketoon; tavk; X = As of P, R = feniel en gefunksionaliseerde feniele, PPh3, AsPh3, P(p-CI-Ph)3 of P(p-OMe-Ph)3. Die bepaling van die meganisme is gedoen deur gebruik te maak van X-straal kristallografie, reaksiekinetika en 31p_KMR. Die volgende uitgangstowwe is kristallografies gekarakteriseer: [Rh(dmavk)(CO)(PPh3)] kristalliseer in 'n ortorombiese kristalstelsel met ruimtegroep Pca21 en finale R-waarde van 2.04 %. [Rh(dmavk)(CO)(AsPh3)] en [Rh(dmavk)(I)(CH3)(CO)(PPh3)] kristalliseer in trikliniense kristalstelsels met ruimtegroep PI. Die finale R-waarde van bogenoemde komplekse was onderskeidelik 3.88 en 4.77 %. [Rh(dmavk)(I)(COCH3)(PPh3)] kristalliseer in 'n monokliniese kristalstelsel met ruimtegroep P21/c en 'n finale R-waarde van 6.72 %. In die geval van monokarbonielkomplekse, soos bv. [Rh(dmavk)(CO)(PPh3)] en [Rh(dmavk)(CO)(AsPh3)], koordineer die PIAs atoom trans t.o.v. die stikstofatoom van die N,O-BID ligand. Die Rh-As bindingslengte is aansienlik langer as dié van Rh-P ( 2.3834(6) en 2.2751(13) A onderskeidelik). Die suksesvolle isolasie en X-straalkristallografiese karakterisering van die uitgangstof, [Rh(dmavk)(CO)(PPh3)], en sy oksidatiewe addisie produkte vir die reaksie tussen jodometaan, t.w., [Rh(dmavk)(I)(CH3)(CO)(pPh3)] en Rh(dmavk)(I)(COCH3)(PPh3)], is vir die eerste keer in hierdie laboratorium gedoen. In beide die uitgangstof en die oksidatiewe addisieprodukte word die trans-(N-Rh-P) konfigurasie behou . . ' 31p_KNIR studies van die [Rh(L,L-BID)(CO)(PPh3)] komplekse: 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 het aangetoon dat 'n redelike korrelasie tussen IJ(PRh) en die Rh-P bindingsafstand, met In variasie in IJ(PRh) bestaan. Die bereide [Rh(N,O-BID)(CO)(XR3)] komplekse ondergaan oksidatiewe addisie van jodometaan wat tot die vorming van die Rh(III)-alkielspesie lei (via 'n ewewigstap), gevolg deur die vormmg van die Rh(III)-asiel kompleks, volgens die volgende reaksi eskema. [Rh(L,L'-BID)(CO)(pR.)] + CH31 [Rh(L,L'-BID)(I)(CI!)CCO)(pPh3)] J alkiel 1{2 [Rh(L,L'-BID)(I)(COCH3)(PR:!)] asiel 'Ii Beduidende oplosmiddelafhanklikheid ten opsigte van die oksidatiewe addisie reaksietempo tussen jodometaan en [Rh(dmavk)(CO)(PPh3)] is waargeneem. Die reaksie verloop 8 keer vinniger in die mees polêre oplosmiddel asetonitriel (k, = 89(6)xl02 M-Is- 1) in vergelyking met die mins polêre oplosmiddel chloroform (k, = 11.4(4)xl02 M-IS-I) by 25°C. Konvensionele aktiveringsparameters van k, (ÓH# en ÓS#) is vanaf die temperatuurafhanklikheid in asetoon bepaal. Groot negatiewe ÓS#-waardes (ÓS# = _ 139(40) J Kl mol") en tipiese ÓH#-waarde (ÓH# = 35(4) kj mol") is verkry. 'n Lineêre, polêre oorgangstoestand, gevolg deur die vorming van 'n ioonpaar-intermediêr, is op grond van bogenoemde resultate as intieme meganisme vir die oksidatiewe addisie reaksie gepostuleer. Manipulasie van die tempokonstante vir die oksidatiewe addisiestap is vermag deur beide elektroniese en steriese variasie. Die elektroniese manipulasie is eerstens gedoen deur die implimentering van elektron donerende substituente (CH3 i.p.v. CF3) op die bidentate ligand, wat tot 'n viervoudtoename in grootte van die tempo van .' oksidatiewe addisie gelei het; en tweedens deur trifenielfosfien te vervang met sy afgeleides [PPh3 vs. P(p-Cl-Ph)3 vs. P(p-OMe-Ph)3J. Steriese manipulasie is gedoen deur PPh3 te vervang met AsPh3, en dit het gelei tot 'n toename in die tempo van oksidatiewe addisie en 'n afname in die tempo van reduktiewe eliminasie, wat 'n toename in ewewigskonstante (Kj) tot gevolg gehad het. 'n Betekenisvolle toename in CO-insersie is waargeneem vir PPh3 in vergelyking met AsPh3 (12.0(6)xl0-4 en 1.32(2)xlO·4 S·l onderskeidelik). Die implimentering van hierdie N,O-BID ligande in die [Rh(N,O-BID)(CO)(PPh3)]- komplekse bring ten minste 'n viervoud grootte aktivering van die die Rh(I) sentra, soos waargeneem in die oksidatiewe addisietempo, in vergelyking met bekende ligandsisteme (d.i. acac, ox, ens.) mee.