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

Reaction of a C2-arylated
imidazolium iodide (IPrPh)I (1) (IPrPh = 1,3-bis­(2,6-diisopropylphenyl)-2-phenyl-imidazolium)
with
PdCl2 in the presence of Ag2O affords abnormal
N-heterocyclic carbene (aNHC) palladium complexes (aIPrPh)­PdCl2 (2) and (aIPrPh)2PdCl2 (3) (aIPrPh = 1,3-bis­(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene).
Treatment of 2 with a pyridine gives Pd-PEPPSI-type complexes
(aIPrPh)­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 (aIPrPh)­Pd­(Cl)­I­(L) (7, L = py; 8, L =
3Cl-py), iodide derivatives (aIPrPh)­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 {(aIPrPh)2Ag}­AgI2 (4) has been
isolated and characterized. Compounds 2–10 are air stable crystalline solids and have been characterized
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.