Structural variations in Ni(II) complexes of salen type di-Schiff base ligands

Three di-Schiff-base ligands, N,N′-bis(salicylidene)-1,3-propanediamine (H2Salpn), N,N′-bis(salicylidene)-1,3-pentanediamine (H2Salpen) and N,N′-bis(salicylidine)-ethylenediamine (H2Salen) react with Ni(SCN)2 · 4H2O in 2:3 molar ratios to form the complexes; mononuclear [Ni(HSalpn)(NCS)(H2O)] · H2O (1a), trinuclear [{Ni(Salpen)}2Ni(NCS)2] (2b) and trinuclear [{Ni(Salen)}2Ni(NCS)2] (3) respectively. All the complexes have been characterized by elemental analyses, IR and UV–VIS spectra, and room temperature magnetic susceptibility measurements. The structures of 1a and 2b have been confirmed by X-ray single crystal analysis. In complex 1a, the Ni(II) atom is coordinated equatorially by the tetradentate, mononegative Schiff-base, HSalpn. Axial coordination of isothiocyanate group and a water molecule completes its octahedral geometry. The hydrogen atom attached to one of the oxygen atoms of the Schiff base is involved in a very strong hydrogen bond with a neighboring unit to form a centrosymmetric dimer. In 2b, two square planar [Ni(Salpen)] units act as bidentate oxygen donor ligands to a central Ni(II) which is also coordinated by two mutually cis N-bonded thiocyanate ligands to complete its distorted octahedral geometry. Complex 3 possesses a similar structure to that of 2b. A dehydrated form of 1a and a hydrated form of 2b have been obtained and characterized. The importance of electronic and steric factors in the variation of the structures is discussed.

The different ligating behavior of three very similar Schiff-base ligands, N,N′- bis(salicylidene)-1,3-propanediamine, N,N′-bis(salicylidene)-1,3-pentanediamine and N,N′-bis(salicylidine)-ethylenediamine with Ni(SCN)2 · 4H2O results in the formation of mononuclear [Ni(HSalpn)(NCS)(H2O)] · H2O (1a) and trinuclear [{Ni(Salpen)}2Ni(NCS)2] (2b) and trinuclear [Ni(Salen)]2[Ni(NCS)2] (3) complexes respectively. In 1, one of the oxygen atoms of the Schiff base is protonated and it is involved in a very strong hydrogen bond with a neighboring unit to form a centrosymmetric dimmer.Figure optionsDownload as PowerPoint slide