Organo-palladium(II) complexes bearing unsymmetrical N,N,N-pincer ligands: synthesis, structures and oxidatively induced coupling reactions

摘要

The 2-(2'-aniline)-6-imine-pyridines, 2-(C6H4-2'-NH2)-6-(CMe=NAr)C5H3N (Ar = 4-i-PrC6H4 (HL1a), 2,6-i-Pr2C6H3 (HL1b)), have been synthesised via sequential Stille cross-coupling, deprotection and condensation steps from 6-tributylstannyl-2-(2-methyl-1,3-dioxolan-2-yl)pyridine and 2-bromonitrobenzene. The palladium(II) acetate N,N,N-pincer complexes, [{2-(C6H4-2'-NH)-6-(CMe=NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)), can be prepared by reacting HL1 with Pd(OAc)(2) or, in the case of 1a, more conveniently by the template reaction of ketone 2-(C6H4-2'-NH2)-6-(CMe=O)-C5H3N, Pd(OAc)(2) and 4-isopropylaniline; ready conversion of 1 to their chloride analogues, [{2-(C6H4-2'-NH)-6-(CMe=NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C(6)H(3) (2b)), has been demonstrated. The phenyl-containing complexes, [{2-(C6H4-2'-NH)-6-(CMe=NAr)C5H3N}PdPh] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)), can be obtained by treating HL1 with (PPh3)(2)PdPh(Br) in the presence of NaH or with regard to , by the salt elimination reaction of with phenyllithium. Reaction of with silver tetrafluoroborate or triflate in the presence of acetonitrile allows access to cationic [{2-(C6H4-2'-NH)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NCMe)][X] (X = BF4 (4), X = O3SCF3 (5)), respectively; the pyridine analogue of 5, [{2-(C6H4-2'-NH)-6-(CMe=N(4-i-PrC6H4)C5H3N}Pd(NC5H5)][O3SCF3] (5'), is also reported. Oxidation of phenyl-containing 3a with one equivalent of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor (TM)) in acetonitrile at low temperature leads to a new palladium species that slowly decomposes to give 4 and biphenyl; biphenyl formation is also observed upon reaction of 3a with XeF2. However, no such oxidatively induced coupling occurs when using 3b. Single crystal X-ray diffraction studies have been performed on HL1b, 1a, 1b, 2a, 2b, 3a, 3b, and 5'.