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.