Palladium(II) Complexes of Ambidentate and Potentially Cyclometalating 5-Aryl-3-(2 '-pyridyl)-1,2,4-triazine Ligands

The 5-aryl-3-(2'-pyridiyl)-1,2,4-triazine ligands under study [5-phenyl-(PyTZPh) L-1; 5-(3-methoxyphenyl)-(PyTZ3Me-OPh) L-2; 5-(4-methoxyphenyl)-(PyTZ4MeOPh) L-3; 5-(4-trifluoromethylphenyl)-(PyTZ4CF(3)Ph) L-4; 5-(4-fluorophenyl)-(PyTZ4FPh) L-5; and tris-3,5,6-(2'-pyridyl)-1,2,4-triazine (Py(3)TZ) L-6] react with [(COD) PdCl2] (COD = 1,5-cyclooctadiene) to form complexes [(L1-6) PdCl2] with N,N bidentate binding ligands, also including the potentially N,N,N tridentate ligand L-6. This was concluded from an in-depth NMR spectroscopic study of the new complexes and from comparison with Pd-terpy complexes [(R' terpy)PdCl]Cl [R'terpy = 4'-R'-2,2': 6',2 ''-terpyridine; R' = H or SMe], showing definite tridentate N,N,N coordination, and with the Pd-bpy complexes [(bpy)Pd(Mes)Cl] and [(bpy)PdCl2], showing definite bidentate N,N binding. The new ligands and complexes were fully characterised by multinuclear NMR spectroscopy, IR spectroscopy and mass spectrometry. No evidence for the parent triazine complexes is observed in EI-MS; instead, cyclometalated complexes (HCl elimination) were detected in all cases. TDA/TG experiments support this assumption. Attempts to prepare the cyclometalated derivatives as substances failed, in line with the unfavourable binding mode. Detailed electrochemical measurements reveal ligand-centred reductions at very moderate potentials, in line with UV/Vis absorption spectroscopy and DFT calculations, revealing very low-lying triazine-centred LUMOs. Results from cyclic voltammetry also support the composition of [(Py(3)TZ)PdCl2]