Multiwavelength spectrophotometric determination of acid dissociation constants Part IV. Water-insoluble pyridine derivatives

A multiwavelength spectrophotometric (WApH) titration method for determination of acid dissociation constants (pK(a) values) of ionizable compounds developed previously was applied in the case of pyridine derivatives of pharmaceutical interest. Specifically, UV absorption spectra of the drug solution are acquired in the course of a pH-metric titration using an optical device based on a fibre optics dip probe, a light source and a diode array detector. Target factor analysis was applied to deduce the pK(a) values from the spectral data recorded at different pH. Using this technique, the pK(a) values of six pyridine derivatives were determined successfully. It was demonstrated that the WApH technique in this case outperforms conventional pH-metric methods with respect to the measurement of pK(a) values of the sparingly water soluble samples reported in this study.     

Theoretical and experimental studies on formation of diethylzinc-triphenylphosphine complex

This work is mainly focused on understanding the complex-forming behavior of diethylzinc with neutral phosphine ligands such as triphenylphosphine (PPh₃) to yield [Zn(PPh₃)₂Et₂]. The complex formation in solution is observed in the presence of a large excess of diethylzinc, but leaving an insoluble solid on aging. The product formed in solution was analyzed by spectroscopic data. ³¹P-NMR was also used as a tool to observe this behavior, i.e., the disappearance of the chemical shift of PPh₃ (δ -5.45) requires 14-fold excess of ZnEt₂ in solution. The alkyl chains reduce the Lewis acidity on Zn and thereby the formation of phosphine adducts is restricted. Results obtained from orbital analyses calculations reveal that the LUMO appears to be asymmetrically distributed, and localized on one of the PPh₃ ligands. The length of alkyl chains also influence the stability of [Zn(PPh₃)₂R₂] and the longer chains on Zn impart less stability.