Cyclopalladated compounds derived from [C,N,O] terdentate ligands: synthesis, characterization and reactivity

Abstract

Treatment of the Schiff base ligands 2-ClC 6H 4C(H)NCH 2(C 4H 7O) ( a) and 3,4-(MeO) 2C 6H 3C(H)NCH 2(C 4H 7O) ( b) with palladium(II) acetate in toluene or glacial acetic acid gave the dinuclear cyclometallated complexes [Pd{2-ClC 6H 3C(H)NCH 2(C 4H 7O)}(μ-OAc)] 2 ( 1a) and [Pd{3,4-(MeO) 2C 6H 2C(H)NCH 2(C 4H 7O)}(μ-OAc)] 2 ( 1b) with the ligand bonded to the palladium atom through the imine nitrogen and the C6 carbon atom. The 1H and 13C–{ 1H} NMR spectra of the complexes show the occurrence in solution of three pairs of enantiomers. The acetato-bridged cyclometallated complex [Pd{3,4-(MeO) 2C 6H 2C(H)NCH 2(C 4H 7O)}(μ-OAc)] 2 ( 1c) was prepared from the enantiopure ligand ( R)-3,4-(MeO) 2C 6H 3(H)NCH 2(C 4H 7O) ( c). The NMR spectra of 1c showed the existence of only two diastereomers in the solution. The reaction of 1a and 1b with aqueous sodium chloride gave the chloro-bridged complexes [Pd{2-ClC 6H 3C(H)NCH 2(C 4H 7O)}(μ-Cl)] 2 ( 2a) and [Pd{3,4-(MeO) 2C 6H 2C(H)NCH 2(C 4H 7O)}(μ-Cl)] 2 ( 2b) after a metathesis reaction. The reaction of 2a with PPh 3 in acetone gave the mononuclear cyclometallated complex [Pd{2-ClC 6H 3C(H)NCH 2(C 4H 7O)}(Cl)(PPh 3)] ( 3a) in a bridge-splitting reaction. Treatment of 3a with silver triflate gave the cyclometallated complex [Pd{2-ClC 6H 3C(H)NCH 2(C 4H 7O)}(PPh 3)][CF 3SO 3] ( 4a) in which the palladium atom is bonded to four different atoms C, N, O and P. When complex 2a was reacted with silver triflate in acetone complex [Pd{2-ClC 6H 3C(H)NCH 2(C 4H 7O)}((CH 3) 2CO)][CF 3SO 3] ( 5a) was obtained with the ligand as [C,N,O] coordinated. The reaction of complexes 2a and 2b with the diphosphine dppp in a complex–phosphine 1:1 molar ratio gave the dinuclear cyclometallated complexes [{Pd[2-ClC 6H 3C(H)NCH 2(C 4H 7O)](Cl)} 2(μ-Ph 2P(CH 2) 3PPh 2)] ( 8a) and [{Pd[3,4-(MeO) 2C 6H 2C(H)NCH 2(C 4H 7O)](Cl)} 2(μ-Ph 2P(CH 2) 3PPh 2)] ( 5b) with the diphosphine bridging the two palladium atoms. Reaction of complexes 2a and 2b with the phosphines dppp and cis-dppe in a 1:2 molar ratio gave the mononuclear complexes [Pd{2-ClC 6H 3C(H)NCH 2(C 4H 7O)}{Ph 2P(CH 2) 3Ph 2- P, P}][ClO 4] ( 6a), [Pd{3,4-(MeO) 2C 6H 2C(H)NCH 2(C 4H 7O)}{Ph 2P(CH 2) 3Ph 2– P, P}][ClO 4] ( 3b) and [Pd{2-ClC 6H 2C(H)NCH 2(C 4H 7O)}( cis-Ph 2PCHCHPh 2– P, P)][Cl] ( 4b). The treatment of 2a with the triphosphine bis(2-diphenylphosphinoethyl)phenylphosphine in 1:2 molar ratio, followed by treatment with sodium perchlorate gave [Pd{3,4-(MeO) 2ClC 6H 3C(H)NCH 2(C 4H 7O)}{(PPh 2CH 2CH 2) 2PPh- P, P, P}][ClO 4] ( 7a) in which the palladium atom is bonded to the triphosphine through the three phosphorus atoms and to the Schiff base ligand through one carbon atom and the imine nitrogen.