Synthesis, characterization, luminescence, and electrochemistry of the tetranuclear gold(I) amidinate clusters, precursors to CO oxidation catalysts:: Au4[(ArNC(H)NAr)]4, Au4[(PhNC(Ph)NPh)]4 and Au4[PhNC(CH3)NPh)4]

A summary of the chemistry of the tetranuclear Au(I) amidinate complexes is presented. Tetranuclear Au(l) amidinate clusters are produced by the reaction of the sodium salt of a amidine ligand with the gold precursor Au(THT)Cl in a (1:1) stoichiometry. The structures of the tetranuclear Au-4[ArNC(H)NAr](4), Ar = C6H4-4-OMe, C6H3-3,5-Cl, C6H4-4-Me, C6H4-3-CF3, C6F5, C10H7 and the tetranuclear Au-4[(PhNC(Ph)NPh](4) and AU(4)[PhNC(CH3)NPh](4) have been characterized by X-ray crystallography. The average Au...Au distance between adjacent Au(I) atoms is similar to 3.0 angstrom, typical of compounds having an aurophilic interaction. The four gold atoms are located at the corner of a rhomboid with the amidinate ligands bridged above and below the near plane of the four Au(I) atoms. The angles at Au...Au...Au in the cyclic units are between 700 and 116 degrees. The tetranuclear gold(I) amidinate clusters each show different luminescence behavior. The tetranuclear clusters Au-4[(ArNC(H)NAr](4), Ar = C6H4-4-OMe, Ar = C6H4-3-CF3, Ar = C6H4-4-Me and Ar = C6H4-3,5-Cl are the first tetranuclear gold(I) cluster species from group I I elements that show fluorescence at room temperature. The tetranuclear naphthyl derivative Ar = C10H7 is luminescent only at 77 K. The pentafluorophenyl derivative Ar = C6F5 does not show any photoluminescence in the solid state nor in the solution. The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes. Electrochemical and chemical oxidation studies of the tetranuclear Au(l) amidinate clusters are presented. The tetranuclear complexes Au-4[ArNC(H)NAr](4), Ar = C6H4-4-OMe, Ar = C6H4-4-Me, and Ar = C6H3-3,5-Cl, show three reversible waves at 0.75, 0.95, 1.09 V vs. Ag/AgCl at a scan rate of 5 00 mV/s in 0. 1 M Bu4NPF6/CH2Cl2 at a Pt working electrode in CH2Cl2. Three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s are also observed for the tetranuclear complex AU(4)[PhNC(Ph)NPh](4) in CH2Cl2. The pentafluorophenyl amidinate derivative, AU(4)[ArNC(H)NAr](4), Ar = C6F5 shows no oxidation wave below 1.8 V. Recently it has been shown that Au-4[ArNC(H)NAr](4) is a very effective catalyst precursor for room temperature CO oxidation.