Chloroform-soluble Schiff-base Zn(II) or Cd(II) complexes from a dynamic combinatorial library

A dynamic combinatorial library of metal ion Schiff-base complexes have bee n studied for the extraction of Zn(II) or Cd(II) from aqueous solution into chloroform. Library components consist of different aminophenols and 2-pyr idinecarboxaldehyde. Extraction of both Zn(II) and Cd(II) into chloroform w as observed from aqueous solutions containing 0.0500 mM M(NO3)(2). 0.100 M aminophenol, 0.100 M 2-pyridinecarboxaldehyde, 0.100 M NaCl, and 5.00 mM bu ffer at pH 8.5. Extraction was dependent on pH but not on counterions inclu ding Cl-, Br-, or NO3-. Studies showed that equilibrium was attained betwee n the Schiff-base complexes across the two-phase chloroform-water system af ter 24 h of stirring. Analysis of the extracted species by use of H-1 NMR s pectroscopy and mass spectrometry as well as solubility studies on characte rized complexes suggested that the major extracted species is the neutral b is-Schiff-base metal ion complex. In libraries containing mixtures of two d ifferent aminophenols and 2-pyridinecarboxaldehyde, an enhanced extent of e xtraction of Zn(II) into chloroform is observed. Studies suggest that a Zn( II) complex, which is likely the mixed Schiff-base complex, has superior ex traction properties compared to simple libraries with a single aminophenol component. The structures of two bis-Schiff-base complexes of Zn(II) and on e of Cd(II) have been determined by X-ray crystallography. The geometries o f the two Zn(II) complexes, which differ only by a methyl substituent on th e Schiff-base ligand, are markedly different. supporting the use of combina torial methods in coordination chemistry. Zn(SB14)(2) crystallized as the s esquihydrate (C24H18N4O2Zn .1.5 H2O) in the space group C2/c, with cell dim ensions a = 23.219(15) Angstrom, b = 11.299(7) Angstrom, c 16.822(11) Angst rom, beta = 102.91(5)degrees, V = 4302(5) Angstrom (3), and Z = 8. Zn(SB15) (2) crystallized as a 1.1 methanol solvate (C26H22N4O2Zn CH3OH) in the spac e group P2(1)/c with cell dimensions a 13.981(5) Angstrom, b = 7.978(3) Ang strom. c = 22.568(8) Angstrom. beta = 104.53(3)degrees, V = 2436.8(15) Angs trom (3), and Z = 4. Cd(SB14)2 crystallized as a 1:1 ethanol solvate (C24H1 8N4O2Cd . CH3CH2OH) In the space group R (3) over bar with unit cell dimens ions of a = 36.423(2)