Synthesis, spectroscopic studies and structure of gold (III) chloride compounds of heterocyclic N-oxide diadducts revealing short hydrogen bond

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Saud A.Aziz Al-Hamoud
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Saudi Digital Library
The present study deals with the synthesis and structure elucidation of several noval gold(III) chloride compounds having general formula [H(R-pyNO)₂ ] [AuC1₄ ] where R = 2-Me, 4-Me 2,6-(CH₃ )₂ ' p-NO₂ and m-COOH. All compounds were characterized by the usual analytical and spectroscopic techniques, such as elemental analysis, ultraviolet, visible, infrared and proton-N.M.R. spectra. The three-dimensional single crystal x-ray structure analysis was completed on three compounds, namely, [H(2-picNO)₂ ][AuC1₄], [H(3-picNO)₂][AuC1₄] and [H(4-picNO)₂] [Auc1₄]. In all compounds, the short hydrogen bonded dimeric cation was stabilized by the [AuC1₄]⁻anion. The O-H-O bond in the diadducts is among the shortest so far reported literature. The short hydrogen bond in the [H(2-picNO)₂][AuC1₄ ] is unique in that the bridged proton has no symmetry rectrictions usually known to exist in similar hydronium ion diadducts. The energetics of the O-H-O linkage were explaiend in terms of the chemical potentials and the O…O separation. The presence of a strong and broad absorption band around 1050 cm⁻¹ in IR spectra has been established as a diagnostic observation for he existence of a short hydrogen bond with O…O separation less than 2.5A. Similarly, a δ-shift of 10.0 to 18.0 p.p.m. is usually observed for the resonance of the bridged proton in these intramolecular short hydrogen bonded species. Efforts were made to explain the spectroscopic and crystallographic results in terms of inductive effects of the substituents on the aromatic N-oxides. Substituted pyridine N-oxides are potentially useful reagents for extraction and photometric determination of gold because of the quantitative separation and the high extinction coefficients of the products formed. The present study will aid in developing an extractive-photometric procedure for estimation of gold. The present work is a part of the ongoing research programme dealing with the chemistry of compounds having short (or strong) hydrogen bond.