New transition metal carbene complexes for application in homogeneous catalysis

[摘要] This study comprises the preparation and full characterisation of new carbene complexes of group10 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 oxidativeaddition of the corresponding carbene precursors imidazolium-, imidazolinium-, pyridinium- andquinolinium chloride salts, to M(PPh3)4 (M = Ni or Pd). Three types of carbene complexes, namelythe standard five-membered two-N carbene complexes, new six-membered NHC complexes andnovel 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 generalformula 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 carbenecomplexes of platinum was complicated by the formation of [PtCl(PPh3)3]BF4 and the desiredcarbene complexes could not be isolated in pure form. The investigation of rNHC complexes wasextended 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 thestructures 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 metalcarbenebond 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 metalchloridebond distance as a guideline, as follows: 1,3-dimethyl-2,3-dihydro-1H-imidazol-2-ylideneand 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 andthe 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 simplerpalladium carbene and phosphine complexes, in the Mizoroki-Heck and Suzuki-Miyaura couplingreactions. Quantum mechanical calculations indicated that the rNHC ligand in this complex isbound stronger to the palladium than a standard imidazole-derived NHC ligand. Furthercalculations suggested that the remote heteroatom carbene (rNHC) complexes of nickel(II) aresignificantly more stable when compared to the normal carbene (NHC) complexes. Energydecomposition 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 inRh(X)COD(NHC) (X = Br or Cl) with CO. Isomerisation of these cis-carbonyl complexes to thetrans 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 Fischertypecarbene complexes in which a soft donor atom (P) attached to the carbene side chain is eitheruncoordinated, (CO)5M=C(NMe2)CH2PPh2 (M = Cr or W), bonded to the original central metals (Cror W) in four-membered chelates, (CO)4M=C(NMe2)CH2PPh2, or coordinated to a second M(CO)5unit (only isolated for chromium) (CO)5Cr=C(NMe2)CH2P(Ph2)Cr(CO)5, were carried out. Thesecompounds 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-memberedchelates, 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-chelatecarbene complexes.

[发布机构] Stellenbosch University