Gold(I) as a Nucleophile - a Base.
Ahmed A. Mohamed, Alfredo Burini, Rossana Galassi, Mohammad A. Omary, Manal A. Rawashdeh-Omary, Lisa M. Thomson,
The ionization potentials of gaseous Au(I), 20.5 eV, Ag(I), 21.48 eV, and Cu(I), 20.29 eV, are similar and relatively large. The ionization potentials of the coinage elements, Au(0), 9.22 eV, Ag(0), 7.57 eV, and Cu(0), 7.72 eV, are much greater than the IP's of the alkali metals, cesium, 3.89 eV, being the easiest element of all to oxidize. It is therefore not surprising that oxidation of the coinage elements does not readily occur. Indeed, Au(0) is noble and only readily oxidized in the presence of strongly coordinating ligands such as thiolates or cyanide. Single electron oxidation of Au(I) compounds generally gives Au(III), not Au(II), species. However, dinuclear Au(I) compounds readily oxidize to form metal-metal bonded Au(II) compounds which, with strongly coordinating S or C ligands, often are readily isolable (Mohamed, 1999). Many have been structurally characterized (Grohmann, 1995). Clearly the ease with which electrons are lost from closed shell M(I) species increases with dinuclear species (anti-bonding M-M interactions) which form M-M bonded oxidized products. This was superbly demonstrated by Cotton (Cotton, 2002) when his group reported the isolation and characterization of some dinuclear closed shell species more readily oxidized than even cesium.
Trinuclear Au(I) pyrazolates, carbeniates and benzylimidazolates are well known and undergo electron loss through oxidative addition forming first Au(I, III) mixed valence species and ultimately (with the carbeniates, TR(carb), and the benzylimidazolates, TR(bzim)) trinuclear Au(III) species (Fackler, 2002). Surprisingly, these Au(I) carbeniates and benzylimidazolates also are excellent bases for the metals cations Tl(I) and Ag(I). The acidic, neutral [Hg(C6F4)3]3 also interacts with TR(carb) and TR(bzim) as seen by solid state and solution measurements. Recently we have shown that the neutral pi acids C6F6 and TCNQ also form stacked pi-acid, pi-base solid state products. In work to be described here, the structure of the octafluoronaphthalene acid-base product will be presented. It displays a luminescence not observed with the starting materials. It is photoluminescent with a bright yellow emission apparently coming from a triplet state associated with the naphthalene. The 3.5 ms lifetime is very long and reminiscent of the long lifetimes observed by Balch (Vickery, 1997) for the stacked methyl,methoxy Au-TR(carb). It should be noted that C6F6 intercalation quenches the luminescence of the p-tolyl, ethoxy TR(carb) we have studied, which is a dimer in the solid state.
It is interesting that pi-acid, pi-base stacking can involve either a ABAB pattern or an ABBABBA pattern of molecules, where B is the basic TR(carb) or TR(bzim). The B units are aurophilically bonded to each other by two Au...Au linkages. DFT calculations demonstrate that the basicity of the BB dimer is increased relative to the molecular B species as a result of these aurophilic interactions.
Planar tetranuclear Au(I) compounds have been synthesized to further explore the basicity, oxidation and photophysical properties of these arrays. A rare example of gold(I) atoms in a square is the tetranuclear Au(I) pyrazolate complex, [(dppm)2Au4(3,5-Ph2Pz)2](NO3)2●H2O which has been synthesized and structurally characterized. It is the first tetranuclear pyrazolate of Au(I) to have been found (Mohamed, 2003), although the trinuclear pyrazolates of Au(I) are well known. This complex exhibits luminescence at 77K when excited at 333 nm with an emission maximum at 454 nm. The emission has been assigned to ligand to metal charge transfer, LMCT, based upon the vibronic structure that is observed. The average of Au...Au distances is ~3.1 Å. A second tetranuclear Au(I) cluster also has been studied, this one with the [PhNCHNPh]- (formamidinate)ligand. It had produced dimers with very short M-M,(M = Cu, Ag) interactions in Cotton’s group. With Au(I), only the planar tetranuclear species could be isolated. This material, containing ligands having only C, N, and H, has provided a small cluster which turns out to be very successful for the catalytic oxidation of CO at room temperature.
The oxidation of the formamidinate cluster, [Au4(form)4], which has short, ~2.91Å, Au...Au distances, has been studied electrochemically in 0.1 M Bu4NPF6/CH2Cl2 at a Pt working electrode with different scan rates. Three waves were obtained at 0.75, 0.95, and 1.09 V vs Ag/AgCl with a scan rate of 500 mV/s, the first two being reversible. The potentials are independent of the scan rate in the range 50 mV/s to 3 V/s. The current at the third wave is larger than those at the first two waves, possibly suggesting a two electron process. Chemical oxidation of the cluster species is underway in an attempt to isolate the product electrochemically oxidized products.
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(Mohamed, 2003) A. A. Mohamed, J. M. López-de-Luzuria, J. P. Fackler, Jr.,J. Cluster Sci., in press.
tetranuclear gold, benzylimidazolate, formamidinate, DFT calculations, catalysis, luminescence, carbeniate, pyrazolate, trinuclear gold, Oxidation