Nickel(II) complexes with tetradentate N4 ligands: Synthesis, structure, electrochemistry and thermochromism in solution

Complexes of nickel(II) chloride with homologous tetradentate aminopyridine ligands 1,8-bis(2-pyridyl)-3,6-dimethyl-3,6-diazaoctane (pdao) and 1,6-bis(2-pyridyl)-2,5-dimethyl-2,5-diazahexane (bpmen) were prepared in the crystalline state and characterized by elemental analysis and X-ray diffraction as [Ni(pdao)(H2O)2]Cl2·H2O (1) and [Ni(bpmen)Cl2]·H2O (2). In the solid state, both complexes are blue-green and contain octahedral Ni(II) with a cis-α-coordinated tetramine. In aqueous solution, both complexes are strong electrolytes as follows from the measurements of electrical conductivity: complex 2 is hydrated and forms purple [Ni(bpmen)(H2O)2]2+(aq). Complex 1 displays thermochromic behavior in solution (it is blue-green at low temperatures and yellow at high temperatures) due to a temperature sensitive equilibrium [Ni(pdao)(H2O)2]2+(aq) ⇄⇄ [Ni(pdao)]2+(aq) + 2H2O(l), ΔH° = +30(1) kJ/mol and ΔS° = +80(3) J/mol K in 0.1 M NaClO4(aq). Complex 1, but not 2, can be reversibly reduced to the Ni(I) species (E1/2 = −0.9 V versus SHE in aqueous solution and −1.37 V versus Fc+/Fc in acetonitrile). The relative stabilization of nickel(I) by ligand pdao can be attributed to an optimal size of the ligand bite and to the presence of the pyridine and tertiary amine N-donors.

Graphical abstractComplexes of nickel(II) chloride with homologous tetradentate aminopyridine ligands coordinate two additional monodentate ligands (water molecules or chloride anions) in the solid state, but may adopt square–planar geometry in solution if a relatively flexible ethylpyridine arms are present in the ligand; hydration–dehydration equilibrium results in thermochromism in aqueous solutions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Ni(II) forms both square–planar and octahedral complexes with 6,5,6-chelating N4 ligands. ► Shorter aminopyridine ligand with 5,5,5-chelation forms only octahedral Ni complexes. ► Ni(II) 6,5,6-polychelate complexes display thermochromism in aqueous solutions. ► Flexible 6,5,6-ligand with two tertiary amine and two pyridine donors stabilizes Ni(I).