Mononuclear and tetranuclear palladacycles with terdentate [C,N,N] and [C,N,O] Schiff base ligands. C–H versus C–Br activation reactions

Reaction of the Schiff base ligands 2-Br-4,5-(OCH2O)C6H2C(H)NCH2CH2NMe2 (a) and 4,5-(OCH2CH2)C6H3C(H)NCH2CH2NMe2 (b) with Pd(OAc)2 or K2[PdCl4] leads to the mononuclear cyclometallated compounds [Pd{2-Br-4,5-(OCH2O)C6HC(H)NCH2CH2NMe2–C6,N,N}(OCOMe)] (1a) and [Pd{4,5-(OCH2CH2)C6H2C(H)NCH2CH2NMe2–C6,N,N}(Cl)] (1b), derived from C–H activation at the C6 carbon. Treatment of a with Pd2(dba)3 gave [Pd{4-5-(OCH2O)C6H2C(H)NCH2CH2NMe2–C2,N,N}(Br)] (2a), via C–Br activation. tathesis reaction of 1a with aqueous sodium chloride gave [Pd{2-Br-4,5-(OCH2O)C6HC(H)NCH2CH2NMe2–C6,N,N}(Cl)] (3a), with exchange of the acetate group by a chloride ligand. Treatment of the cyclometallated monomers 1a–3a with PPh3 in a 1:1 molar ratio yielded the mononuclear complexes [Pd{2-Br-4,5-(OCH2O)C6HC(H)NCH2CH2NMe2–C6,N}(L)(PPh3)] (L: OAc, 4a; Cl, 5a) and [Pd{4-5-(OCH2O)C6H2C(H)NCH2CH2NMe2–C2,N}(Br)(PPh3)] (6a), with Pd–NMe2 bond cleavage. However, treatment of a solution of 3a or 2a with silver trifluoromethanesulfonate, followed by reaction with PPh3 in acetone yielded the cyclometallated complexes [Pd{2-Br-4,5-(OCH2O)C6HC(H)NCH2CH2NMe2–C6,N,N}(PPh3)][CF3SO3] (7a) and [Pd{4-5-(OCH2O)C6H2C(H)NCH2CH2NMe2–C2,N,N}(PPh3)][CF3SO3] (8a), respectively, where the Pd–NMe2 bond was retained. action of the ligands 2-Br-4,5-(OCH2O)C6H2C(H)N(2′-OH-5′-tBuC6H3) (c) and 4,5-(OCH2CH2)C6H3C(H)N(2′-OH-5′-tBuC6H3) (d) with Pd(OAc)2 gave the tetranuclear complexes [Pd{2-Br-4,5-(OCH2O)C6HC(H)N(2′-O-5′-tBuC6H3)–C6,N,O}]4 (1c) and [Pd{4,5-(OCH2CH2)C6H2C(H)N(2′-O-5′-tBuC6H3)-C6,N,O}]4 (1d), respectively. Treatment of 1c with PPh3 in 1:4 molar ratio, gave the mononuclear species [Pd{2-Br-4,5-(OCH2O)C6HC(H)N(2′-(O)-5′-tBuC6H3)–C6,N,O}(PPh3)] (2c) with opening of the polynuclear structure after P–Obridging bond cleavage. ructure of compounds 2a, 1c and 1d has been determined by X-ray diffraction analysis.