Aurophilicity

When the ligand on the left is reacted with 3 equivalents of a gold(I) halide (with each phosphine group coordinating a separate gold center), the aurophilic interaction between gold atoms hinders free rotation around single bonds. The temperature required to restore free rotation on the NMR timescale is a measure of the strength of the aurophilic interaction.^[1]

In chemistry, aurophilicity refers to the tendency of gold complexes to aggregate via formation of weak gold-gold bonds.^[1][2]

Overview

The phenomenon of aurophilicity is most commonly observed crystallographically for Au(I) compounds. The aurophilic bond has a length of about 3.0 Å and a strength of about 7-12 kcal/mol,^[1] which is comparable to the strength of a hydrogen bond. The aurophilic interaction is thought to result from electron correlation of the closed-shell components, which is unusual in light of the fact that closed-shell atoms generally have negligible interaction with one another at distances on the scale of the Au-Au bond. This is somewhat similar to van der Waals interactions, but is unusually strong due to relativistic effects. Observations and theory show that, on average, 28% of the binding energy in aurophilic interaction can be attributed to relativistic expansion of the gold d orbitals.^[3]

Another important feature of aurophilicity is the propensity of gold atoms to aggregate around nucleation sites—specifically, though not limited to, ligands that bind through phosphorus, nitrogen, and sulfur centers. While both intra- and inter-molecular aurophilic interactions exist, only intramolecular aggregation has been observed at such nucleation sites.^[4]

Applications

The similarity in strength between hydrogen bonding and aurophilic interaction has proven to be a convenient tool in the field of polymer chemistry. There has been much research into self-assembling supermolecular structures, both those that aggregate by aurophilicity alone and those that contain both aurophilic and hydrogen-bonding interactions.^[5] An important and exploitable property of aurophilic interactions relevant to their supermolecular chemistry is that while both inter- and intramolecular interactions are possible, intermolecular aurophilic linkages are comparatively weak and easily broken by solvation; most complexes that exhibit intramolecular aurophilic interactions retain such moieties in solution.^[1]

Gold(I) complexes can polymerize by intermolecular aurophilic interaction. Nanoparticles that form from this polymerization often give rise to intense luminescence in the visible region of the spectrum. Strength of particular intermolecular aurophilic interactions can be gauged by solvating the nanoparticles and observing the extent to which luminescence diminishes.^[1]

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Other Phenomena

Similar metallophilic interactions exist for a few other heavy metals, such as mercury, and can also be observed between atoms of different elements. Some documented examples include Hg(II)-Au(I), Hg(II)-Pt(II), and Hg(II)-Pd(II).^[6] In accordance with theoretical calculations, which predict a local maximum for relevant relativistic effects for gold atoms, none of these other interactions are as strong as aurophilicity.^[1][7]

References

1. ^ ^{a b c d e f} Hubert Schmidbaur (2000). "The Aurophilicity Phenomenon: A Decade of Experimental Findings, Theoretical Concepts and Emerging Application". Gold Bulletin 33 (1): 3–10. doi:10.1007/BF03215477. http://www.goldbulletin.org/index.php?option=com_remository&Itemid=26&func=download&filecatid=43. 
2. ^ Hubert Schmidbaur (1995). "Ludwig Mond Lecture. High-carat gold compounds". Chem. Soc. Rev. 24 (6): 391–400. doi:10.1039/CS9952400391. 
3. ^ Nino Runeberg, Martin Schütz, and Hans-Joachim Werner (1999). "The aurophilic attraction as interpreted by local correlation methods". J. Chem. Phys. 110 (15): 7210–7215. doi:10.1063/1.478665. 
4. ^ Hubert Schmidbaur, Stephanie Cronje, Bratislav Djordjevic, and Oliver Schuster (2005). "Understanding gold chemistry through relativity". J. Chem. Phys. 311: 151–161. doi:10.1016/j.chemphys.2004.09.023. 
5. ^ William J. Hunks, Michael C. Jennings, and Richard J. Puddephatt (2002). "Supramolecular Gold(I) Thiobarbiturate Chemistry: Combining Aurophilicity and Hydrogen Bonding to Make Polymers, Sheets, and Networks". Inorg. Chem. 41 (17): 4590–4598. doi:10.1021/ic020178h. 
6. ^ Kim Mieock, Taylor Thomas J., Gabbai François P. (2008). "Hg(II)···Pd(II) Metallophilic Interactions". J. Am. Chem. Soc. 130 (20): 6332–6333. doi:10.1021/ja801626c. PMID 18433123. 
7. ^ Behnam Assadollahzadeh and Peter Schwerdtfege (2008). "A comparison of metallophilic interactions in group 11[X–M–PH3]n (n = 2–3) complex halides (M = Cu, Ag, Au; X = Cl, Br, I) from density functional theory". Chemical Physics Letters 462 (4–6): 222–228. doi:10.1016/j.cplett.2008.07.096.