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.The metathesis 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.The reaction 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.The structure of compounds 2a, 1c and 1d has been determined by X-ray diffraction analysis.

Graphical abstractThe reactions of Pd(AcO)2 and [Pd2(dba)3] with terdentate [C,N,N] Schiff base ligands gives mononuclear cyclometallated complexes via C–H or C–Br activation, respectively. However, the reaction of Pd(AcO)2 and [C,N,O] ligands gives cyclometallated complexes with tetranuclear structure via C–H activation.The reactivity of the complexes with triphenylphosphine is also reported, putting forward the influence of the second donor atom at the metal, nitrogen or oxygen, which gives way to the different final species.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Cyclometallation of [C,N,N] terdentate ligands gives mononuclear complexes. ► Cyclometallation of [C,N,O] ligands under similar conditions gives tetranuclear complexes. ► The Cyclometallation proceeds via C–H activation. ► Cyclometallation at the lees activated C–Br carbon proceeds via oxidative addition reaction. ► Reactivity towards triphenylphosphine is determined by the nuclearity of the complex.