New selective nickel(II)-N3 nitrogen bond in adenine: Synthesis and structure of [(tren) (adenine) (monochloro)nickel(II)] chloride and [(tren)(imidazole)(monoaqua)nickel(II)] dichloride

With the aim to obtain further insight into the nature of the Ni(II)-nitrogen bond in complexes with nucleobases, we have synthesized two novel Ni(II) ternary complexes constituted by Ni(II), tris(2-aminoethyl)amine (tren) and neutral adenine (AdH) or imidazole (ImH). The reaction of NiCl₂·6H₂O with tren and imidazole yields the ternary Ni(tren)(ImH)(H₂O)]Cl₂·H₂O complex and the same reaction with adenine instead of imidazole gives the ternary Ni(tren)(AdH)Cl]Cl complex. The complexes have been studied by spectrophotometric and spectroscopic measurements and by X-ray diffraction. The ternary complex of adenine is monoclinic, space groupC2/c, and exhibits a pseudo-octahedral geometry, being Ni(II) coordinated to the four nitrogen atoms of the tetradentate tren ligand, to a Cl^– ion and to the pyrimidine N3 site of a neutral adenine. Such an exclusive Ni-N3 bonding 2.081(4)Å] is now reported for the first time. The ternary complex of imidazole is monoclinic, space groupP2₁/n. Its pseudo-octahedral geometry is similar to that found in the above adenine complex although a water molecule instead of a Cl^– ion is now present in the coordination sphere. In fact, Ni(II) is coordinated to tetradentate tren, to the imidazole N1 nitrogen and to a water molecule. Electronic and¹H NMR spectra in solution indicate that the octahedral structures found in the solid state are substantially maintained in solution. Furthermore, the present investigation suggests that the adenine Ni-N3 and imidazole Ni-N1 bonds have the same chemical nature, involving mainly - and only partially-bonding. The Ni-N3 bonding is discussed in connection with biological implications and possible applications.