Gold(I) Complexes Of Ethylene And Alkynes
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Several varieties of nitrogen based weakly coordinating tris(pyrazolyl)borate and triazapentadienyl ligands have been synthesized. In the tris(pyrazolyl)borate family, the focus has been the B-phenyl and B-tert-butyl substituted tris(pyrazolyl)borates. Unlike the traditional systems with hydrogen on the boron, these bulkier and electron richer substituents on boron are expected to provide greater protection to metal and show structural variations in the metal adducts as well as different reactivity of the isolated coinage metal complexes. The thallium complexes of these tris(pyrazolyl)borates indeed show differences in coordination modes.The boron-alkylated and -arylated tris(pyrazolyl)borates and hydrotris(pyrazolyl)borates were utilized to isolate coinage metal(I) (Cu, Ag, Au) complexes with ethylene. Structural and spectroscopic investigations indicate higher ð-back-donation in Au(I) ethylene complexes followed by the Cu(I) and Ag(I). This experimental result is in good agreement with the computational studies of bond dissociation energy of coinage metals with C2H4 in the order of Au(I) > Cu(I) > Ag(I).The experimental success of synthesis of coinage metal ethylene complexes, especially with the elusive Au(I) adducts, prompted us to investigate equally rare Au(I) alkyne complexes. Interest at this study resides in the wide applications of gold meditated organic synthesis with alkyne as substrate. We were able to isolate several Au(I) alkyne complexes including a gold(I) chloride adduct of hexyne and characterized by means of a number of analytical techniques like X-ray crystallography, 1H and 13C NMR, IR and Raman spectroscopy. Fluorinated 1,3,5-triazapentadienyls were used to obtain unusually stable gold-alkyne complexes. Related silver and copper analogs were also synthesized for comparative studies. Our experimental and DFT calculations agree on the point that the combined effect of the ó and ð-back-bonding in these complexes is greatest for Au, followed by Cu and Ag.