5-甲基-4-咪唑甲醛的希夫碱缩合,5Me4ImCHO,和氨基酸的阴离子,H2N-CH(R)CO2-(R=-CH3,-CH(CH3)2和-CH2CH(CH3)2),给出了醛亚胺互变异构体,Im-CH=N-CH(R)CO2-,而5-甲基咪唑-4-甲胺,5MeIm-4-CH2NH2,带有2-氧代羧酸根阴离子,R-C(O)-CO2-,给出了异构的酮亚胺互变异构体,Im-CH2-N=C(R)CO2-。全部分离为中性镍(II)配合物,NiL2,并通过单晶结构测定,IR,和正离子ESIMS。在4个取代的咪唑的情况下,5MeIm-4-CHO或5MeIm-4-CH2NH2,醛亚胺和酮亚胺复合物都被干净地分离,在实验条件下没有证据表明两种互变异构体之间存在平衡。醛亚胺是蓝色的,而互变异构酮亚胺是绿色的。相比之下,对于2-取代的咪唑,与Im-2-CHO或Im-2-CH2NH2,从希夫碱缩合分离的产物是酮亚胺,在固体中是绿色的,正如所观察到的4-异构体。这些结果表明,对于2-取代的咪唑,在醛亚胺和酮亚胺互变异构体之间有一个容易的平衡,并且酮亚胺形式是热力学上有利的互变异构体。4-取代咪唑的醛亚胺互变异构体具有三个立体中心,镍(Δ或η)和两个α碳原子(R或S)。观察到的对映体是RR/ΔSS对映体对,这表明该对的能量比其他对的低,并且这通常是这些复合物中优选的手性相关性。
The Schiff base condensation of 5-methyl-4-imidazole carboxaldehyde, 5Me4ImCHO, and the anion of an amino acid, H2N-CH(R)CO2- (R = -CH3, -CH(CH3)2 and -CH2CH(CH3)2), gives the aldimine tautomer, Im-CH=N-CH(R)CO2-, while that of 5-methylimidazole-4-methanamine, 5MeIm-4-CH2NH2, with a 2-oxocarboxylate anion, R-C(O)-CO2-, gives the isomeric ketimine tautomer, Im-CH2-N=C(R)CO2-. All are isolated as the neutral nickel(II) complexes, NiL2, and are characterized by single crystal structure determination, IR, and positive ion ESI MS. In the cases of the 4 substituted imidazoles, either 5MeIm-4-CHO or 5MeIm-4-CH2NH2, both the aldimine and ketimine complexes are isolated cleanly with no evidence of an equilibrium between the two tautomers under the experimental conditions. The aldimines are blue while the tautomeric ketimines are green. In contrast, for the 2-substituted imidazoles, with either Im-2-CHO or Im-2-CH2NH2, the isolated product from the Schiff base condensation is the ketimine, which in the solid is green, as observed for the 4-isomer. These results suggest that for the 2-substituted imidazoles, there is a facile equilibrium between the aldimine and ketimine tautomers, and that the ketimine form is the thermodynamically favored tautomer. The aldimine tautomers of the 4-substituted imidazoles have three stereogenic centers, the nickel (Δ or Ʌ) and the two alpha carbon atoms (R or S). The observed pair of enantiomers is the ɅRR/ΔSS enantiomeric pair, suggesting that this pair is lower in energy than the others and that this is in general the preferred chiral correlation in these complexes.