C-H activation at the 3-position of pentane chains to form [N-C(sp3)-N]- complexes incorporating six-membered pallada(II)cyclic rings and pyridine, pyrazole and N-methylimidazole donor groups. Structural studies and comparison with [N-C(sp2)-N]- complexes

Abstract

Alkylpalladium complexes bearing the [N-C(sp3)-N]- donor motif and two six-membered palladacycles are generated on activation of C(sp3)-H bonds by palladium(II) acetate. Cyclopalladation of the new reagents 1,5-bis(pyridin-2-yl)pentane [CH2(CH2CH2py)2] (1), 1,5-bis(pyrazol-1-yl)pentane [CH2(CH2CH2pz)2] (2) and 1,5-bis(N-methylimidazol-2-yl)pentane [CH2(CH2CH2mim)2] (3), followed by reaction with lithium chloride afford the palladium(II) complexes Pd{CH(CH2CH2py)2- N,C,N'}Cl (5), Pd{CH(CH2CH2pz)2-N,C,N'}Cl (6) and Pd{CH(CH2CH2mim)2-N,C,N'}Cl (7), respectively. Abstraction of chloride from 6 with AgBF4 in acetone generates the cationic acetone complex [Pd{CH(CH2CH2pz)2-N,C,N'}(OCMe2)][BF4] (8). X-ray crystal structures of 5, 6 and 8 reveal tridentate [N-C(sp3)-N']- intramolecular coordination of the ligands. These structures are compared with that of a closely related [N-C(sp2)-N]- system in [Pd{2,6-(pzCH2)2C6H3-N,C,N'}(OH2)][BF4] (10), obtained on cyclopalladation of 1,3-bis(pyrazol-1-ylmethyl)benzene followed by derivatisation to form the aqua complex.