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 Suman Mukhopadhyay, † Debdas Mandal, † Dipesh Ghosh, †,§ Israel Goldberg, ‡ and Muktimoy Chaudhury* ,† Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Kolkata 700 032, India, and School of Chemistry, Tel AViV UniVersity, Tel AViV 69978, Israel Received June 4, 2003 Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L 1 -OH-L 3 -OH) based on 1-(2-aminoethyl)- piperazine (X ) H, L 1 -OH; X ) NO 2 ,L 2 -OH; and X ) Br, L 3 -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 L 1 -OH, the product obtained is either a planar red compound [Ni(L 1 -O)] + , isolated as tetraphenylborate salt (1a), or an octahedral green compound [Ni(L 1 -NH)(H 2 O) 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 1 H NMR technique using the protocol of Evans’s method. The large exothermicity of the equilibrium process [Ni(L 1 -O)] + + 3H 2 O + H + h [Ni(L 1 -NH)(H 2 O) 3 ] 2+ (∆H° ) -46 ± 0.2 kJ mol -1 and ∆S° ) -133 ± 5JK -1 mol -1 ) reflects formation of three new Ni-OH 2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L 2 -OH, the sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L 3 -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. Introduction The possibility of the existence of an equilibrium in solution between the octahedral and planar forms of nickel- (II) complexes was reported for the first time with some sterically constrained ligands, viz. stilbenediamine (stien) by Lifschitz et al. 1,2 The proposed equilibrium [Ni(stein)X 2 ] h [Ni(stien) 2+ ] + 2X - (X - ) various anions) was latter established by magnetic, spectroscopic, and X-ray structure analyses. 3,4 Over the years, several other such equilibria have been reported using macrocyclic 5-12 and open chain * Author to whom correspondence should be addressed. E-mail: icmc@ mahendra.iacs.res.in. † Indian Association for the Cultivation of Science. ‡ Tel Aviv University. § Present address: Department of Chemistry, University of Missouris Kansas City, Kansas City, MO 64110. (1) Lifschitz, I.; Bos, J. G.; Dijkema, K. M. Z. Anorg. Allg. Chem. 1939, 24, 97. (2) Lifschitz, I.; Bos, J. G. Recl. TraV. Chim. 1940, 5, 407. (3) Higginson, W. C. E.; Nyburg, S. C.; Wood, J. S. Inorg. Chem. 1964, 3, 463. (4) Nyburg, S. C.; Wood, J. S. Inorg. Chem. 1964, 3, 468. (5) Fabbrizzi, L.; Paoletti, P.; Clay, R. M. Inorg. Chem. 1978, 17, 1042. (6) Sabatini, L.; Fabbrizzi, L. Inorg. Chem. 1979, 18, 438. (7) Swisher, R. G.; Dayhuff, J. P.; Stuehr, D. J.; Blinn, E. L. Inorg. Chem. 1980, 19, 1336. (8) Hay, R. W.; Bembi, R.; Sommerville, W. Inorg. Chim. Acta 1982, 59, 147. (9) Steenland, M. W. A.; Dierck, I.; Herman, G. G.; Devreese, B.; Lippens, W.; Van Beeumen, J.; Goeminne, A. M. J. Chem. Soc., Dalton Trans. 1997, 3637. (10) Sakata, K.; Wada, S.; Sato, N.; Kurisu, M.; Hashimoto, M.; Kato, Y. Inorg. Chim. Acta 1986, 119, 111. (11) Bembi, R.; Bhardwarj, V. K.; Singh, R.; Singh, R.; Taneja, K.; Aftab, S. Inorg. Chem. 1984, 23, 4153. (12) Hay, R. W.; Jeragh, B.; Ferguson, G.; Kaitner, B.; Ruhl, B. L. J. Chem. Soc., Dalton Trans. 1982, 1531. Inorg. Chem. 2003, 42, 8439-8445 10.1021/ic0346174 CCC: $25.00 © 2003 American Chemical Society Inorganic Chemistry, Vol. 42, No. 25, 2003 8439 Published on Web 11/11/2003