A dynamic combinatorial library of metal ion Schiff-base complexes have been studied for the extraction of Zn(II) or Cd(II) from aqueous solution into chloroform. Library components consist of different aminophenols and 2-pyridinecarboxaldehyde. Extraction of both Zn(II) and Cd(II) into chloroform was 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 buffer at pH 8.5. Extraction was dependent on pH but not on counterions including Cl-, Br-, or NO3-. Studies showed that equilibrium was attained between the Schiff-base complexes across the two-phase chloroform-water system after 24 h of stirring. Analysis of the extracted species by use of 1H NMR spectroscopy and mass spectrometry as well as solubility studies on characterized complexes suggested that the major extracted species is the neutral bis-Schiff-base metal ion complex. In libraries containing mixtures of two different aminophenols and 2-pyridinecarboxaldehyde, an enhanced extent of extraction of Zn(II) into chloroform is observed. Studies suggest that a Zn(II) complex, which is likely the mixed Schiff-base complex, has superior extraction properties compared to simple libraries with a single aminophenol component. The structures of two bis-Schiff-base complexes of Zn(II) and one of Cd(II) have been determined by X-ray crystallography. The geometries of the two Zn(II) complexes, which differ only by a methyl substituent on the Schiff-base ligand, are markedly different, supporting the use of combinatorial methods in coordination chemistry. Zn(SB14)2 crystallized as the sesquihydrate (C24H18N4O2Zn.1.5 H2O) in the space group C2/c, with cell dimensions a = 23.219(15) A, b = 11.299(7) A, c = 16.822(11) A, beta = 102.91(5) degrees, V = 4302(5) A3, and Z = 8. Zn(SB15)2 crystallized as a 1:1 methanol solvate (C26H22N4O2Zn.CH3OH) in the space group P2(1)/c with cell dimensions a = 13.981(5) A, b = 7.978(3) A, c = 22.568(8) A, beta = 104.53(3) degrees, V = 2436.8(15) A3, and Z = 4. Cd(SB14)2 crystallized as a 1:1 ethanol solvate (C24H18N4O2Cd.CH3CH2OH) in the space group R3 with unit cell dimensions of a = 36.423(2) A, c = 9.2930(10) A, V = 10678(2) A3, and Z = 18.
Metal ion coordination chemistry is well suited to dynamic combinatorial chemistry approaches. [1±3] Diverse metal ion complex libraries can be created simply by mixing a labile metal ion with different ligands. [4] If ligand exchange is rapid, these metal ion complexes form the equilibrating components of a dynamic library. The equilibrium between complexes may be shifted upon addition of molecular targets. For example, small molecule or biopolymer targets perturb the equilibrium between metal ion complexes to increase the concentration of the metal ion complex that best binds the target. [5±7] We previously reported on dynamic combinatorial libraries of metal ion Schiff-base complexes of limited diversity. [8] Our selection protocol utilizes extraction of metal ion complex libraries from aqueous into organic solvent. In this method, the organic solvent is the ultimate target and complexes with the greatest stability and solubility in organic solvent versus aqueous solution are selected. [9] Here we present studies using acylhydrazone ligand libraries and show that these libraries lead to improvements in efficiency and selectivity of metal ion extraction in comparison to single ligand systems.Acylhydrazone ligands (Scheme 1) were chosen to explore the possibility of forming a double-orthogonal library [4] by exchange at metal±ligand (MÀL) and C¼N bonds. [10±13] Initial studies were carried out to establish the feasibility of using these exchange processes. Zn II extracts into chloroform from a buffered aqueous solution in the presence of two equivalents of acylhydrazone ligand 1 (Table 1, Scheme 1). The predominant complex that extracts into chloroform is the neutral complex [Zn(1 À ) 2 ] as determined by comparison of 1 H NMR and mass spectral data to that of an authentic sample. After two hours, the extent of extraction of Zn II from a water/chloroform mixture containing 0.0500 mm [Zn(1 À ) 2 ] is identical within experimental error to a second experiment containing 0.0500 mm Zn(NO 3 ) 2 and 0.100 mm 1, confirming that our system is at equilibrium. Addition of a different acylhydrazone ligand (13) to either of the solutions above followed by stirring for 2 h increases the amount COMMUNICATIONS 4096
A robust macrocyclic tetraamide complex of Th(IV) binds phosphate diesters and promotes cleavage of RNA and phosphate diesters at 37 °C in the pH range 5.00-7.90.
Metal ion coordination chemistry is well suited to dynamic combinatorial chemistry approaches. [1±3] Diverse metal ion complex libraries can be created simply by mixing a labile metal ion with different ligands. [4] If ligand exchange is rapid, these metal ion complexes form the equilibrating components of a dynamic library. The equilibrium between complexes may be shifted upon addition of molecular targets. For example, small molecule or biopolymer targets perturb the equilibrium between metal ion complexes to increase the concentration of the metal ion complex that best binds the target. [5±7] We previously reported on dynamic combinatorial libraries of metal ion Schiff-base complexes of limited diversity. [8] Our selection protocol utilizes extraction of metal ion complex libraries from aqueous into organic solvent. In this method, the organic solvent is the ultimate target and complexes with the greatest stability and solubility in organic solvent versus aqueous solution are selected. [9] Here we present studies using acylhydrazone ligand libraries and show that these libraries lead to improvements in efficiency and selectivity of metal ion extraction in comparison to single ligand systems.Acylhydrazone ligands (Scheme 1) were chosen to explore the possibility of forming a double-orthogonal library [4] by exchange at metal±ligand (MÀL) and C¼N bonds. [10±13] Initial studies were carried out to establish the feasibility of using these exchange processes. Zn II extracts into chloroform from a buffered aqueous solution in the presence of two equivalents of acylhydrazone ligand 1 (Table 1, Scheme 1). The predominant complex that extracts into chloroform is the neutral complex [Zn(1 À ) 2 ] as determined by comparison of 1 H NMR and mass spectral data to that of an authentic sample. After two hours, the extent of extraction of Zn II from a water/chloroform mixture containing 0.0500 mm [Zn(1 À ) 2 ] is identical within experimental error to a second experiment containing 0.0500 mm Zn(NO 3 ) 2 and 0.100 mm 1, confirming that our system is at equilibrium. Addition of a different acylhydrazone ligand (13) to either of the solutions above followed by stirring for 2 h increases the amount ZUSCHRIFTEN 4270
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