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

Abstract

The 2-(2′-aniline)-6-imine-pyridines, 2-(C₆H₄-2′-NH₂)-6-(CMeNAr)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-(CMeNAr)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-(CMeO)C₅H₃N, Pd(OAc)₂ and 4-isopropylaniline; ready conversion of 1 to their chloride analogues, [{2-(C₆H₄-2′-NH)-6-(CMeNAr)C₅H₃N}PdCl] (Ar = 4-i-PrC₆H₄ (2a), 2,6-i-Pr₂C₆H₃ (2b)), has been demonstrated. The phenyl-containing complexes, [{2-(C₆H₄-2′-NH)-6-(CMeNAr)C₅H₃N}PdPh] (Ar = 4-i-PrC₆H₄ (3a), 2,6-i-Pr₂C₆H₃ (3b)), can be obtained by treating HL1 with (PPh₃)₂PdPh(Br) in the presence of NaH or with regard to 3a, by the salt elimination reaction of 2a with phenyllithium. Reaction of 2a with silver tetrafluoroborate or triflate in the presence of acetonitrile allows access to cationic [{2-(C₆H₄-2′-NH)-6-(CMeN(4-i-PrC₆H₄)C₅H₃N}Pd(NCMe)][X] (X = BF₄ (4), X = O₃SCF₃ (5)), respectively; the pyridine analogue of 5, [{2-(C₆H₄-2′-NH)-6-(CMeN(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™) 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′.