Synthesis and characterization of new gold and silver complexes based on P/N and P/S ligands : P/S ligand-stabilized gold nanoparticles.

Abstract

This dissertation comprises three distinct studies, two of which concerns molecular inorganic chemistry, and one concerns inorganic materials science. Firstly, the synthesis and characterization of new dithiophosphonate ligands of the type (NH4)[S2P(4-C6H4OMe)(OCH2C6H5)] and (NH4)[S2P(4-C6H4OMe)(OCH2CH2CH2CH3)] followed by their reaction with [AuCl(tht)] (tht = tetrahydrothiophene) and [Ag(NO3)] (molar ratio 1:1). This yielded two new dinuclear gold(I) complexes, [Au{S2P(4-C6H4OMe)(OCH2C6H5})]2, 1 and [Au{S2P(4-C6H4OMe)(OCH2CH2CH2CH3})]2, 2, and one new silver complex [Ag{S2P(4-C6H4OMe)(OCH2CH2CH2CH3})]2, 3. Complex 1 and 2 displayed intramolecular Au···Au interactions of 3.0923(5) and 3.0613(7) Å, respectively, and revealed no intermolecular Au···Au interactions. Complex 1 additionally showed intermolecular hydrogen bonding interactions between the sulfur atom and an aromatic hydrogen atom with a contact distance of ca. 2.895 Å. The reaction of 1 with bis(diphenylphosphino)methane (dppm) (molar ratio 1:1) yielded a metallacyclic mixed-ligand complex tentatively assigned as [Au2(dppm)2{S2P(4-C6H4OMe)(OCH2C6H5)}2] 4, (based on partial X-ray refinement) which is nevertheless a complex type derived from bis-phosphine reactions that has never been observed. Complex 3 followed the same predictable 1H, 13C, 31P and IR patterns as with 1 and 2. Secondly, the synthesis and characterization of a phosphinamide ligand, [(C6H5)2PO(NH-t-C4H9)], 5, has been described. The ligand showed solid state structures with a remarkable number of structural features which include hydrogen bond interactions as well as unique packing properties. Compound 5 contains two molecules in a single asymmetric unit linked via hydrogen bonds with a contact measuring ca. 2.03 Å. The dimeric asymmetric units in turn are held together by a network of weak hydrogen interactions measuring ca. 2.28 Å. The phosphinamide ligand, 5, was further reacted with silver triflate which led to the formation of a novel dinuclear silver(I) complex [Ag(CF3SO3)(OPPh2N(H)CMe3)2}{Ag(OPPh2N(H)CMe3)2}]SO3CF3, 6, that adopts two unique geometries around the silver nuclei, namely a trigonal planar and linear geometry. The two silver atoms show a strong intramolecular argentophillic interaction of 2.8979(3) Å. From the above complexes, 1, 2, 5 and 6 were fully characterized by single crystal X-ray crystallography, 1H, 13C and 31P solution NMR, elemental analysis, and IR. Complex 3 and 4 were characterized using 1H, 13C and 31P solution NMR, elemental analysis, and IR. The final study comprised the synthesis and characterization of gold nanoparticle/silica or AuNP/SiO2-composites. The nanocomposites were prepared by the in-situ thermal reduction of tetrachloroauric acid, H[AuCl4].3H2O. Two variable parameters were considered namely temperature (50 °C, 100 °C, 200 °C and 300 °C) and concentration of the gold acid (0.01 M, 0.5 M and 1.25 M). The gold solution was pipetted onto the silica and allowed to soak for 2 hours and was then reduced in a furnace for one hour. The AuNP/SiO2 composites were characterized by four methods, namely Brunauer Emmet and Teller (BET) surface area analysis, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). SEM revealed the gold particles was deposited on the surface of the silica. TEM results showed the AuNPs were formed with the smallest diameter around 3.6 nm and the largest 54 nm. Not surprisingly, the distribution of AuNPs increased with increase in concentration of gold solution. BET surface area results showed that surface area decreases with increase in concentration of gold solution. In addition to the synthesis of the composites, it was also used in the preparation of self-assembled monolayers (SAMs). SEM results showed post work-up images and analyses of P and S elements derived from a dithiophosphonate-cholesterol compound on the gold surface.