Abnormal-NHC Palladium(II) Complexes: Rational Synthesis, Structural Elucidation, and Catalytic Activity

Rottschäfer D, Schuermann CJ, Lamm J-H, Paesch AN, Neumann B, Ghadwal R. Abnormal-NHC Palladium(II) Complexes: Rational Synthesis, Structural Elucidation, and Catalytic Activity. ORGANOMETALLICS. 2016;35(19):3421-3429.Reaction of a C2-arylated imidazolium iodide (IPrPh)I (1) (IPrPh = 1,3-bis(2,6-diisopropylpheny1)-2-phenyl-imidazolium) with PdCl2 in the presence of Ag2O affords abnormal N-heterocyclic carbene (aNHC) palladium complexes (aIPr(Ph))PdCl2 (2) and (aIPr(Ph))(2)PdCl2 (3) (aIPr(Ph) = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene). Treatment of 2 with a pyridine gives Pd-PEPPSI-type complexes (aIPr(Ph))-PdCl2(L) (L = pyridine (py), 5; L = 3-chloropyridine (3Cl-py), 6). Compounds 5 and 6 are also accessible by a one -pot reaction of 1, PdCl2, and Ag2O in a pyridine solvent. While the use of a conventional base K2CO3 leads to the formation of mixed halide complexes (aIPr(Ph))Pd(Cl)I(L) (7, L = py; 8, L = 3Cl-py), iodide derivatives (aIPr(Ph))PdI2(L) (9, L = py; 10, L = 3Cl-py) can be selectively prepared with addition of an excess of KI to the reaction mixture. Albeit in a low yield, a putative transmetalation agent {(aIPr(Ph))(2)Ag}AgI2 characterized. Compounds 2-10 are air stable crystalline solids and have been characterized (4) has been isolated and by elemental analysis, mass spectrometry, and NMR spectroscopic studies. Molecular structures of 2-10 have been established by single crystal X-ray diffraction analyses. Catalytic activity of three representative compounds 2, 5, and 6 has been tested for the Suzuki-Miyaura cross coupling reactions