Equilibrium Studies in Solution Involving Nickel(II) Complexes of Flexidentate Schiff Base Ligands: Isolation and Structural Characterization of the Planar Red and Octahedral Green Species Involved in the Equilibrium

Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L1-OH−L3-OH) based on 1-(2-aminoethyl)piperazine (X = H, L1-OH; X = NO2, L2-OH; and X = Br, L3-OH) and their nickel(II) complexes (1a, 1b, 2, and
3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations
that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral
complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the
complexes have profound influences on the coordination geometry of the isolated products. With the parent
ligand L1-OH, the product obtained is either a planar red compound [Ni(L1-O)]+, isolated as tetraphenylborate
salt (1a), or an octahedral green compound [Ni(L1-NH)(H2O)3]2+, isolated with sulfate anion (1b); both have
been crystallographically characterized. In aqueous solution, both these planar (S = 0) and octahedral (S = 1)
forms are in equilibrium that has been followed in the temperature range 298−338 K by 1H NMR technique using
the protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L1-O)]+ + 3H2O + H+ ⇌
[Ni(L1-NH)(H2O)3]2+ (ΔH° = −46 ± 0.2 kJ mol-1 and ΔS° = −133 ± 5 J K-1 mol-1) reflects formation of three
new Ni−OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, the
sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L3-OH,
the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been
structurally characterized by X-ray diffraction analysis.