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

Abstract

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+) (DeltaH degrees=-46 +/- 0.2 kJ mol(-1) and DeltaS degrees=-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.

On the Effect of Coordination and Protonation Preferences in the Amphiphilic Behavior of Metallosurfactants with Asymmetric Headgroups

“…The pseudo-octahedral nickel(II) species 2, 6, and 7 do not display this band and are characterized by less intense signals around 605-620 nm, (ε ≈ 20-30) and 950-965 nm (ε ≈ 5-15), respectively attributed to 3 A 2g Ǟ 3 T 1g (P) and 3 A 2g Ǟ 3 T 2g spin-allowed d-d transitions. [35] This is in good agreement with the observation that lower wavelength absorption bands are associated with low-spin square planar electronic configurations, while higher wavelength bands are found for highspin octahedral configurations. [36] The positions of the observed absorptions in 1, 2, 5-7 indicate that in solution, 1 and 5 remain low-spin and 2, 6, and 7 remain high-spin.…”

Unprecedented Tetranuclear Complexes with 20‐Electron NiII Centers: The Role of Pressure and Temperature on Their Solid‐State and Solution Fragmentation

“…This equilibrium explains why rapid precipitation of a solution of 1 b or 2 b affords 1 a or 2 a, respectively, since precipitation shifts the equilibrium towards the less soluble species. Equilibria between square-planar and octahedral Ni complexes in solution are not uncommon and may involve solvation [11] and hemilability. [12] Other examples include tetrahedral to square-planar isomerism.…”

Dichlorido(4-methoxy-2-{[2-(piperazin-4-ium-1-yl)ethyl]iminomethyl}phenolate)cadmium

“…For similar structures, see: Mukhopadhyay et al (2003); Xu et al (2008);Saleh Salga et al (2010). For a description of the geometry of complexes with five-coordinated metal ions, see: Addison et al (1984).…”

“…The ligands containing a piperazine ring have been reported to possess ambidentate character, capable of binding metal centers through one or both piperazine N atoms (Mukhopadhyay et al, 2003;Xu et al, 2008;Saleh Salga et al, 2010).…”