Reversible ring opening and closure reactions of the triazine ligands derived from 1-phenylazo-2-naphthylamine and pyridine-2-aldehyde or quinoline-2-aldehyde – structure and reactivity of the palladium (II) complexes

New ligands containing a heterocyclic ring, L1 (1-anilino-2-(2-pyridyl)-naphth[1,2-d]imidazol-1-io-3-ide), L2 (2-phenyl-3-(2-pyridyl)-3,4-dihydro-naphtho[2,1-e][1,2,4]triazin-1-io-4-ide), and L3 (1-anilino-2-(2-quinolyl)-naphth[1,2-d]imidazol-1-io-3-ide), and their palladium (II) complexes have been prepared. Structures of the ligands and the complexes were determined by X-ray crystallography. The mononuclear square-planar complexes of [PdCl2(Ln)] (n = 1 (1), n = 2 (2) and n = 3 (3)) had didentate Ln (n = 1–3) ligands. The Ln (n = 1–3) ligands were stable and their absorption spectra did not change in dichloromethane and methanol. On the other hand, the absorption spectrum of [PdCl2(L2)] (2) in dichloromethane changed rapidly when methanol was added to the solution, and [PdCl(L4b)] (5) (L4b = N-[methoxy(2-pyridyl)methyl]-1-(phenylazo)-2-naphthylamide) was obtained from the concentrated reaction mixture. In this reaction, the dihydrotriazine ring of the didentate L2 ligand in complex 2 opened and the resulting tridentate L4b ligand coordinated to the Pd atom in complex 5. When an excess amount of (nBu)4NCl was added to complex 5 in dichloromethane, the absorption spectrum reverted to that of complex 2. Thus, the reversible ring opening and closure reactions of the coordinating dihydrotriazine ligand were observed. We also prepared [PdCl2(L5)] (9) (L5 = 1-(phenylazo)-N-[1-(2-pyridyl)ethylidene]-2-naphthylamine) and determined the structure. It is noted that neither the ring closure reaction nor the coordination of the azo nitrogen atom of the L5 ligand occurred in complex 9.