Supramolecular Networks of Aurophilic and Hydrogen Bonds
Since the identification of sub-van der waals gold-gold contacts in a wide range of gold(I) complexes, there have been many attempts to exploit this unusual bonding to construct novel supramolecular motifs in the solid state. The energy associated with 'aurophilic' bonds is similar to that found in hydrogen bonding (15-30 kJ/mol). The work outlined here has attempted to incorporate both of these bonding interactions working in a complementary manner to create extended networks.
As has been mentioned in the section on Gold(I) Thiolate Chemistry, sulfur is an exceptionally good donor for monovalent gold. Therefore, thiol ligands displaying hydrogen-bonding functionality (carboxyl, amino groups) appeared a good starting point to explore the ways in which gold-gold contacts and hydrogen bonding could be introduced into the same system.
The obvious choice was 4-mercaptobenzoic acid which reacted in the presence of base with gold(I) complexes of the form [(R3P)AuCl] (R = Me, Et, Ph) to yield the species [(R3P)Au(SC6H4CO2H)]. The supramolecular structure of the diphenylpyridylphosphine variant is shown below. It can be seen that gold-gold and gold-sulfur interactions are present in addition to the hydrogen bonding associated with the carboxyl groups. The result is an infinite chain held together by these three bonding interactions:
However, a quite different structure is observed for the triphenylphosphine complex [(Ph3P)Au(SC6H4CO2H)] which forms a tetrameric unit through cooperative use of hydrogen and aurophilic bonding. This demonstrates the subtle interplay of steric and electronic factors (in this case a change of phosphine ligand from PPh2Py to PPh3) that can cause a radically different structure to occur:
Another promising ligand identified with which to explore gold(I) hydrogen-bonded networks was 4-amino-2-pyrimidinethiol. Reaction of the deprotonated ligand with [(dppm)(AuCl)2] resulted in the complex shown below containing two aminopyrimidinethiolate units:
The network formed through hydrogen bonding and gold-gold contacts can be seen below in which the outer layer of bis(phosphine)digold units gives way to a core held together exclusively by hydrogen bonds:
Further details of this work can be found in the following articles:
J. D. E. T. Wilton-Ely, A. Schier, N. W. Mitzel and H. Schmidbaur. Structural diversity in gold(I) complexes of 4-sulfanylbenzoic acid. J. Chem., Soc., Dalton Trans., 2001, 1058.
J. D. E. T. Wilton-Ely, H. Ehlich, A. Schier and H. Schmidbaur. The effect of hard and soft donors on structural motifs in (isocyanide)gold(I) complexes. Helvetica Chimica Acta, 2001, 84, 3216.
J. D. E. T. Wilton-Ely, A. Schier, N. W. Mitzel S. Nogai and H. Schmidbaur. Hydrogen-bonded networks: (Phosphine)gold(I)4-amino-2-pyrimidine-thiolates. J. Organomet. Chem., 2002, 643, 313.
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