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

Abstract Reaction of the Schiff base ligands 2-Br-4,5-(OCH 2 O)C 6 H 2 C(H) NCH 2 CH 2 NMe 2 ( a ) and 4,5-(OCH 2 CH 2 )C 6 H 3 C(H) NCH 2 CH 2 NMe 2 ( b ) with Pd(OAc) 2 or K 2 [PdCl 4 ] leads to the mononuclear cyclometallated compounds [Pd{2-Br-4,5-(OCH 2 O)C 6 HC(H) NCH 2 CH 2 NMe 2 – C6 , N , N }(OCOMe)] ( 1a ) and [Pd{4,5-(OCH 2 CH 2 )C 6 H 2 C(H) NCH 2 CH 2 NMe 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 2 O)C 6 H 2 C(H) NCH 2 CH 2 NMe 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 2 O)C 6 HC(H) NCH 2 CH 2 NMe 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 2 O)C 6 HC(H) NCH 2 CH 2 NMe 2 – C6 , N }(L)(PPh 3 )] (L: OAc, 4a ; Cl, 5a ) and [Pd{4-5-(OCH 2 O)C 6 H 2 C(H) NCH 2 CH 2 NMe 2 – C2 , N }(Br)(PPh 3 )] ( 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 2 O)C 6 HC(H) NCH 2 CH 2 NMe 2 – C6 , N , N }(PPh 3 )][CF 3 SO 3 ] ( 7a ) and [Pd{4-5-(OCH 2 O)C 6 H 2 C(H) NCH 2 CH 2 NMe 2 – C2 , N , N }(PPh 3 )][CF 3 SO 3 ] ( 8a ), respectively, where the Pd–NMe 2 bond was retained. The reaction of the ligands 2-Br-4,5-(OCH 2 O)C 6 H 2 C(H) N(2′-OH-5′- t BuC 6 H 3 ) ( c ) and 4,5-(OCH 2 CH 2 )C 6 H 3 C(H) N(2′-OH-5′- t BuC 6 H 3 ) ( d ) with Pd(OAc) 2 gave the tetranuclear complexes [Pd{2-Br-4,5-(OCH 2 O)C 6 HC(H) N(2′-O-5′- t BuC 6 H 3 )– C6 , N , O }] 4 ( 1c ) and [Pd{4,5-(OCH 2 CH 2 )C 6 H 2 C(H) N(2′-O-5′- t BuC 6 H 3 )- C6 , 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 2 O)C 6 HC(H) N(2′-(O)-5′- t BuC 6 H 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.