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

摘要

Reaction of the Schiff base ligands 2-Br-4,5-(OCH_2O)C _6H_2C(H)NCH_2CH_2NMe_2 (a) and 4,5-(OCH_2CH_2)C_6H_3C(H)NCH _2CH_2NMe_2 (b) with Pd(OAc)_2 or K _2[PdCl_4] leads to the mononuclear cyclometallated compounds [Pd{2-Br-4,5-(OCH_2O)C_6HC(H)NCH _2CH_2NMe_2-C6,N,N}(OCOMe)] (1a) and [Pd{4,5-(OCH_2CH_2)C_6H_2C(H)NCH _2CH_2NMe_2-C6,N,N}(Cl)] (1b), derived from C-H activation at the C6 carbon. Treatment of a with Pd_2(dba)_3 gave [Pd{4-5-(OCH_2O)C_6H_2C(H)NCH _2CH_2NMe_2-C2,N,N}(Br)] (2a), via C-Br activation. The metathesis reaction of 1a with aqueous sodium chloride gave [Pd{2-Br-4,5-(OCH_2O)C_6HC(H)NCH_2CH _2NMe_2-C6,N,N}(Cl)] (3a), with exchange of the acetate group by a chloride ligand. Treatment of the cyclometallated monomers 1a-3a with PPh_3 in a 1:1 molar ratio yielded the mononuclear complexes [Pd{2-Br-4,5-(OCH_2O)C_6HC(H)NCH_2CH _2NMe_2-C6,N}(L)(PPh_3)] (L: OAc, 4a; Cl, 5a) and [Pd{4-5-(OCH2O)C6H2C(H)NCH2CH 2NMe2-C2,N}(Br)(PPh3)] (6a), with Pd-NMe _2 bond cleavage. However, treatment of a solution of 3a or 2a with silver trifluoromethanesulfonate, followed by reaction with PPh_3 in acetone yielded the cyclometallated complexes [Pd{2-Br-4,5-(OCH _2O)C6HC(H)NCH_2CH_2NMe _2-C6,N,N}(PPh_3)][CF_3SO_3] (7a) and [Pd{4-5-(OCH_2O)C_6H_2C(H)NCH_2CH _2NMe_2-C_2,N,N}(PPh_3)][CF_3SO _3] (8a), respectively, where the Pd-NMe2 bond was retained. The reaction of the ligands 2-Br-4,5-(OCH_2O)C 6H_2C(H)N(2′-OH-5′-~tBuC _6H_3) (c) and 4,5-(OCH_2CH_2)C 6H_3C(H)N(2′-OH-5′-~tBuC _6H_3) (d) with Pd(OAc)_2 gave the tetranuclear complexes [Pd{2-Br-4,5-(OCH_2O)C_6HC(H)N(2′-O- 5′-tBuC_6H_3)-C6,N,O}]_4 (1c) and [Pd{4,5-(OCH_2CH_2)C_6H_2C(H) N(2′-O-5′-~tBuC6H3)-C_6,N,O}] 4 (1d), respectively. Treatment of 1c with PPh_3 in 1:4 molar ratio, gave the mononuclear species [Pd{2-Br-4,5-(OCH_2O)C _6HC(H)N(2′-(O)-5′-~tBuC_6H _3)-C6,N,O}(PPh_3)] (2c) with opening of the polynuclear structure after P-O_(bridging) bond cleavage. The structure of compounds 2a, 1c and 1d has been determined by X-ray diffraction analysis.