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
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- 作者：Suman Mukhopadhyay ; Debdas Mandal ; Dipesh Ghosh ; Israel Goldberg ; and Muktimoy Chaudhury
- 刊名：Inorganic Chemistry
- 出版年：2003
- 出版时间：December 15, 2003
- 年：2003
- 卷：42
- 期：25
- 页码：8439 - 8445
- 全文大小：133K
- 年卷期：v.42,no.25(December 15, 2003)
- ISSN：1520-510X
文摘
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, and3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformationsthat allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedralcomplexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in thecomplexes have profound influences on the coordination geometry of the isolated products. With the parentligand L1-OH, the product obtained is either a planar red compound [Ni(L1-O)]+, isolated as tetraphenylboratesalt (1a), or an octahedral green compound [Ni(L1-NH)(H2O)3]2+, isolated with sulfate anion (1b); both havebeen 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 usingthe protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L1-O)]+ + 3H2O + H+ 
[Ni(L1-NH)(H2O)3]2+ (
H
= -46 ± 0.2 kJ mol-1 and S = -133 ± 5 J K-1 mol-1) reflects formation of threenew Ni-OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, thesole 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 beenstructurally characterized by X-ray diffraction analysis.
[Ni(L1-NH)(H2O)3]2+ (H = -46 ± 0.2 kJ mol-1 and S = -133 ± 5 J K-1 mol-1) reflects formation of threenew Ni-OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, thesole 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 beenstructurally characterized by X-ray diffraction analysis.