Coordination Structure Conversion of Hydrazone-Palladium(II) Complexes in the Solid State and in Solution

We prepared hydrazone-palladium(II) complexes of [PdCl₂(HLⁿ)] and [PdCl(Lⁿ)] (n = 1-3) by the reaction of [PdCl₂(cod)] or [PdCl₂(PhCN)₂] and the hydrazone ligands of HLⁿ {N′-(pyridin-2-ylmethylene)picolinohydrazide (HL¹), N′-[1-(pyridin-2-yl)ethylidene]picolinohydrazide (HL²), and N′-[(6-methylpyridin-2-yl)methylene]picolinohydrazide (HL³)}. The structures of the complexes were determined by X-ray analysis. The hydrazone ligands had κN(py1),κN(imine) and κN(amidate),κN(py2) bidentate coordination modes in [PdCl₂(HLⁿ)] (1, n = 1; 2, n = 2) and in [PdCl₂(HL³)] (3), respectively. In contrast, tridentate coordination modes of κN(py1),κN(imine),κN(py2) and κN(py1),κN(amidate),κN(py2) were observed in [PdCl(Lⁿ)] (4, n = 1; 5, n = 2) and in [PdCl(Lⁿ)] (6, n = 1; 7, n = 2; 8, n = 3). Thermal conversion of complexes 1-3 to complexes 6-8 proceeded in acetonitrile. Complexes 4 and 5 were obtained from complexes 1 and 2, respectively, in a basic acetonitrile solution under dark conditions. Complex 4 reverted immediately to complex 1 in an acidic acetonitrile solution that included hydrochloric acid. However, under room light, in the basic acetonitrile solution that included trimethylamine, complex 4 converted photochemically to complex 6. The thermochromic or vapochromic structure conversion of these complexes also occurred in the solid state. On heating at 180 °C, the color of the crystals of complexes 1, 2, and 3 changed from yellow to orange in the solid state. ¹H NMR and/or UV-vis absorption spectroscopy confirmed that the orange complexes 6-8 were produced. The reddish-orange crystals of complexes 4 and 5 were exposed to hydrogen chloride vapor to yield the yellow products of complexes 1 and 2, respectively. (Chemical Equation Presented).