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

Abstract

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)A] 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 σ- and only partiallyπ-bonding. The Ni-N3 bonding is discussed in connection with biological implications and possible applications.