New transition metal carbene complexes for application in homogeneous catalysis
Date
2005-12
Authors
Julius, Gerrit R.
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
This study comprises the preparation and full characterisation of new carbene complexes of group
10 metals (Ni, Pd or Pt), the group 9 metal, rhodium, as well as group 6 metals (Cr and/or W).
N-heterocyclic carbene (NHC) complexes of nickel and palladium were prepared via oxidative
addition of the corresponding carbene precursors imidazolium-, imidazolinium-, pyridinium- and
quinolinium chloride salts, to M(PPh3)4 (M = Ni or Pd). Three types of carbene complexes, namely
the standard five-membered two-N carbene complexes, new six-membered NHC complexes and
novel six-membered rNHC complexes received attention. In the rNHC complexes the heteroatom
(N) is removed from the carbene carbon. These new square planar carbene complexes of the general
formula trans-[(PPh3)2MCl(L)]X (M = Ni or Pd; X = BF4 or PF6) L = 1,3-dimethyl-2,3-dihydro-1Himidazol-
2-ylidene, 1,3-dimethyl-2,3,4,5-tetrahydro-1H-imidazol-2-ylidene, 1-methyl-1,2-dihydropyridin-
2-ylidene, 1-methyl-1,2-dihydro-quinolin-2-ylidene, 1,4-dimethyl-1,2-dihydro-quinolin-2-
ylidene, 2-methoxy-1-methyl-1,4-dihydro-quinolin-4-ylidene, 1-methyl-1,4-dihydro-pyridin-4-
ylidene) have been isolated and characterised. The preparation of the corresponding carbene
complexes of platinum was complicated by the formation of [PtCl(PPh3)3]BF4 and the desired
carbene complexes could not be isolated in pure form. The investigation of rNHC complexes was
extended to include the synthesis of (CO)5M{CSC(CNCMe2CH2O)CHCH} (M = Cr and W).
The molecular and crystal structures of thirteen of the new carbene complexes including the
structures of both cis- (only formed below –20°C) and trans-chloro(1-methyl-1,2-dihydro-quinolin-
2-ylidene)bis(triphenylphosphine)palladium(II) tetrafluoroborate were determined. The metalcarbene
bond distances in both the palladium and nickel carbene families do not differ significantly.
The carbene ligands can be arranged in a series of increasing trans-influence, using the metalchloride
bond distance as a guideline, as follows: 1,3-dimethyl-2,3-dihydro-1H-imidazol-2-ylidene
and 1,3-dimethyl-2,3,4,5-tetrahydro-1H-imidazol-2-ylidene < 1-methyl-1,2-dihydro-pyridin-2-
ylidene < 2-methoxy-1-methyl-1,4-dihydro-quinolin-4-ylidene, 1-methyl-1,4-dihydro-pyridin-4-
ylidene. The crystal and molecular structures of two platinum compounds, cis-chloro(2-methoxy-1-
methyl-1,4-dihydro-quinolin-4-ylidene)bis(triphenylphosphine)platinum(II) tetrafluoroborate and
the byproduct [PtCl(PPh3)3]BF4 were also determined. Trans-chloro(2-methoxy-1-methyl-1,4-dihydro-quinolin-4-ylidene)bis(triphenylphosphine)
palladium(II) tetrafluoroborate was found to be a very active catalyst, compared to simpler
palladium carbene and phosphine complexes, in the Mizoroki-Heck and Suzuki-Miyaura coupling
reactions. Quantum mechanical calculations indicated that the rNHC ligand in this complex is
bound stronger to the palladium than a standard imidazole-derived NHC ligand. Further
calculations suggested that the remote heteroatom carbene (rNHC) complexes of nickel(II) are
significantly more stable when compared to the normal carbene (NHC) complexes. Energy
decomposition analysis suggested that the rNHC ligands are strong s-donors and weak -acceptors.
Unsymmetrical imidazolium-derived bis(carbene) complexes, [Rh(NHC)2COD]Br, bromomono(
carbene) complexes, Rh(Br)COD(NHC), and chloro-mono(carbene) complexes,
Rh(Cl)COD(NHC) where NHC = 1-R-3-methyl-2,3-dihydro-1H-imidazol-2-ylidene (R = ethyl,
propyl or butyl), were formed in each reaction of the corresponding free carbene ligand with
[Rh(Cl)COD]2. [Rh(Br)COD(NHC)] formed as a result of substitution of a chloride ligand by a Br--
anion. The carbonyl complexes, cis-[Rh(CO)2X(NHC)] (X = Br or Cl; NHC = 1-ethyl-3-methyl-
2,3-dihydro-1H-imidazol-2-ylidene) were isolated after the substitution of the COD ligand in
Rh(X)COD(NHC) (X = Br or Cl) with CO. Isomerisation of these cis-carbonyl complexes to the
trans isomers was observed.
Cis-[(h4-1,5-cyclooctadiene)bis(1-butyl-3-methyl-2,3-dihydro-1H-imidazol-2-ylidene)rhodium(I)]
bromide, bromo(h4-1,5-cyclooctadiene)(1-methyl-3-propyl-2,3-dihydro-1H-imidazol-2-ylidene)
rhodium(I) and cis-[(h4-1,5-cyclooctadiene)bis(1-butyl-3-methyl-2,3-dihydro-1H-imidazol-2-
ylidene)rhodium(I)]bromide were also characterised by single crystal X-ray diffraction.
The synthesis and structural characterisation of a series of acyclic and heterometallacyclic Fischertype
carbene complexes in which a soft donor atom (P) attached to the carbene side chain is either
uncoordinated, (CO)5M=C(NMe2)CH2PPh2 (M = Cr or W), bonded to the original central metals (Cr
or W) in four-membered chelates, (CO)4M=C(NMe2)CH2PPh2, or coordinated to a second M(CO)5
unit (only isolated for chromium) (CO)5Cr=C(NMe2)CH2P(Ph2)Cr(CO)5, were carried out. These
compounds were produced by the reaction of the anionic Fischer-type aminocarbene complexes,
[(CO)5Cr=C(NMe2)CH2]Li (M = Cr or W), with ClPPh2. The formation of the four-membered
chelates, via cyclisation, occurs much faster for Cr than for W. The aminocarbene-phosphine chelates represent the first examples of structurally characterised, four-membered C,P-chelate
carbene complexes.
Description
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch,2005.
Keywords
Dissertations -- Chemistry, Theses -- Chemistry, Carbenes (Methylene compounds), Transition metal complexes