Carbene ligand and complex design directed towards application in synthesis and homogeneous catalysis
Date
2008-12
Authors
Stander-Grobler, Elzet
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
Alkylated acetonitrile that forms during the synthesis of the sulfonium salt, [(Me3)2(MeS)S][BF4], is
involved in the formation of new , -unsaturated Fischer-type carbene complexes from
(CO)5M=C(OMe)CH2Li (M = Cr, W). Metal migration observed when the substitution product
obtained from the reaction of the anionic carbene complexes (CO)5M=C(NMe2)CºC¯ (M = Cr, W)
with Ph3PAu+ was left in solution, was also kinetically and theoretically investigated. 1H NMR and
quantum mechanical (at the B3LYP level of theory) data indicated a complicated mechanism.
The a,b-unsaturated Fischer-type carbene complex, (CO)5Cr=C(OMe)CH=C(Me)NH(Me),
obtained from the reaction of (CO)5M=C(OMe)CH2¯ with alkylated acetonitrile, was transformed
into the new remote one-N, six-membered, carbene ligand (rN1HC6) complex,
(CO)5Cr=C(CH=C(Me)N(Me)CH=C(nBu). The carbene ligand unprecedentedly preferred the
softer Rh(CO)2Cl moiety to the Cr(CO)5 metal fragment and transferred readily.
A new series of remote and abnormal square planar compounds [r/a(NHC)(PPh3)2MCl]CF3SO3 (M
= Pd or Ni) was prepared by oxidative substitution. The various positions for metal-carbon bond
formation on a pyridine ring to furnish various ligand types i.e. C2 for nN1HC6, C3 for aN1HC6 or
C4 for rN1HC6 received attention. The ligands were arranged in increasing order of carbene
character, aNHC < nNHC < rNHC and trans influence, nN2HC5 ~ aN1HC6 ~ nN1HC6 < rN1HC6.
In competitive situations, oxidative substitution occurred selectively at C4 of the pyridine ring
rather than at C2 and on the aromatic ring containing the heteroatom (C4), rather than on an
annealed aromatic ring (C7). Crystal and molecular structure determinations confirmed the
preferred coordination sites. Quantum mechanical calculations (at the RI-BP86/SV level of theory)
indicated that the chosen carbene ligand has a much larger influence than the metal on the BDE of
the M-Ccarbene bond; the farther away the N-atom is from the carbene carbon, the stronger the bond.
In complexes that also contain additional external nitrogen atoms, e.g. trans-chloro(N-methyl-1,2,4-
trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate and transchloro(
N-methyl-1,2,4-trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)nickel(II)
triflate, stabilisation originates from both the nitrogens. 2-Chloro-1-methyl-1H-pyrid-4-ylidenephenylammonium triflate afforded complexes with both remote as well as normal nitrogen
atoms. New azole complexes of palladium and nickel with remote heteroatoms were also prepared
from N-methyl-4',4'-dimethyl-2'-thiophen-3-chloro-2-yl-4,5-dihydro-oxazole. Employing the
compound 1,5-dichloroanthraquinone, the product of a double oxidative substitution on two Pd
centra could be isolated but not alkylated.
The fact that the chemical shift of the metal bonded carbon in the 13C NMR spectrum can not be
used as absolute measure of carbene character, was emphasised in a compound where the
heteroatom was situated seven bonds away from the carbon donor.
In efforts to synthesise a sulphur-bridged complex that contains carbene ligands, crystals of transdi-
iodobis(1,3-dimethyl-imidazoline-2-ylidene)palladium were obtained. Bridged thiolato
complexes with N1HC6 ligands were unexpectedly found in the attempt to substitute the halogen on
chosen square planar carbene complexes of palladium, widening the application possibilities of
N1HC6 ligands in organometallic chemistry beyond that of catalysis. A trinuclear cluster,
[(PdPPh3)3(μ-SMe)3]BF4 was isolated as a by-product of these reactions.
A series normal and abnormal thiazolylidene complexes of nickel and palladium were prepared by
oxidative substitution of the respective 2-, 4- and 5-bromothiazolium salts with M(PPh3)4 (M = Pd
or Ni), and unequivocally characterised. In a preliminary catalytic investigation, all the thiazolinium
and simple pyridinium derived palladium complexes showed activity in the Suzuki-Miyaura
coupling reaction. Little variation in activity in the order a (N next to carbon donor) > n > a (S next
to carbon donor) was found for the former series, whereas decreased activity was exhibited in the
sequence r > a > n of the latter group. The pyridinium derived complexes showed superior activity
to the thiazolinium ones. The rNHC complex, trans-chloro(N-methyl-1,2,4-trihydro-2-
dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate, showed similar
Suzuki-Miyaura activity to the standard N2HC5 carbene complex precatalyst, trans-chloro[(1,3-
dimethyl-imidazol-2-ylidene)triphenylphosphine]palladium(II) triflate.
Description
Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008.
Keywords
Carbene complexes, N-heterocylic carbene, Fischer-type carbene, Catalysis, Carbenes (Methylene compounds), Complex compounds, Transition metal complexes, Heterocyclic compounds, Dissertations -- Chemistry, Theses -- Chemistry