Carbene ligand and complex design directed towards application in synthesis and homogeneous catalysis

[摘要] Alkylated acetonitrile that forms during the synthesis of the sulfonium salt, [(Me3)2(MeS)S][BF4], isinvolved 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 productobtained 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 andquantum 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 transformedinto 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 thesofter 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 bondformation on a pyridine ring to furnish various ligand types i.e. C2 for nN1HC6, C3 for aN1HC6 orC4 for rN1HC6 received attention. The ligands were arranged in increasing order of carbenecharacter, aNHC < nNHC < rNHC and trans influence, nN2HC5 ~ aN1HC6 ~ nN1HC6 < rN1HC6.In competitive situations, oxidative substitution occurred selectively at C4 of the pyridine ringrather than at C2 and on the aromatic ring containing the heteroatom (C4), rather than on anannealed aromatic ring (C7). Crystal and molecular structure determinations confirmed thepreferred 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 ofthe 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 nitrogenatoms. New azole complexes of palladium and nickel with remote heteroatoms were also preparedfrom N-methyl-4',4'-dimethyl-2'-thiophen-3-chloro-2-yl-4,5-dihydro-oxazole. Employing thecompound 1,5-dichloroanthraquinone, the product of a double oxidative substitution on two Pdcentra could be isolated but not alkylated.The fact that the chemical shift of the metal bonded carbon in the 13C NMR spectrum can not beused as absolute measure of carbene character, was emphasised in a compound where theheteroatom 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 thiolatocomplexes with N1HC6 ligands were unexpectedly found in the attempt to substitute the halogen onchosen square planar carbene complexes of palladium, widening the application possibilities ofN1HC6 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 byoxidative substitution of the respective 2-, 4- and 5-bromothiazolium salts with M(PPh3)4 (M = Pdor Ni), and unequivocally characterised. In a preliminary catalytic investigation, all the thiazoliniumand simple pyridinium derived palladium complexes showed activity in the Suzuki-Miyauracoupling reaction. Little variation in activity in the order a (N next to carbon donor) > n > a (S nextto carbon donor) was found for the former series, whereas decreased activity was exhibited in thesequence r > a > n of the latter group. The pyridinium derived complexes showed superior activityto the thiazolinium ones. The rNHC complex, trans-chloro(N-methyl-1,2,4-trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate, showed similarSuzuki-Miyaura activity to the standard N2HC5 carbene complex precatalyst, trans-chloro[(1,3-dimethyl-imidazol-2-ylidene)triphenylphosphine]palladium(II) triflate.

[发布机构] Stellenbosch University