Gold complexes, attractive interactions

Treatment of 77 with one equivalent of [AuCl(tht)] (tht = tetrahydrothiophene) leads to the type III AuCl bridged pincer complex 79 in high yield (counterion is Cl ) [149]. The unique complex 79 is the only heterodimetallic compound with two metal atoms attached at a carbodiphosphorane carbon(O) atom. The environment at the central carbon atom is planar in 77 and pyramidal in 78 and 79. P-C bond lengths and P-C-P angles in the dication 78 are closely related to those in the neutral Pt complex 36 (Fig. 25). The two metals of 79 attached to the carbon atom are coimected via a very short d -d ° pseudo closed shell interaction with a Pd-Au bond length of 2.8900(3) A a similar bonding situation (aurophilic attraction) is found in the gold complex 29 shown in Fig. 23. 

This type of cycloaurated complexes display short transannular Au- Au interactions, as observed in the crystal structures of Au(bonding interaction between the metal centers. This attractive interaction is probably the reason for the distortion of the linear environment of the gold atoms. The metallacycle has a twist conformation, probably as a consequence of the steric requirements of the fairly bulky phenyl substituents. 

Scheme 1 Some attractive interactions in gold(I) complexes.

It is well known that small cations form crown-ether-like associations with PEG polymers (77). The cations interact with pseudocrown ether structures of the polymers formed by coiling of the macromolecules. Furthermore, it has been shown that AuCU" ions become coordinated to the pseudocrown ether structures via ion-pairs (76). The Au(IU) complexes are attracted through electrostatic interactions to cations that are bound to the oxyethylene groups of the polymers. In our systems Na+ ions can bind to the pseudocrown ether structures, and form ion-pairs with the gold complexes. The bound complexes are reduced via oxidation of the oxyethylene groups by the metal center ... 

As described above, among the several closed-shell metal ions that form luminescent supramolecular entities with gold, thallium(I) forms the most numerous examples. While aurophilic attractions can be considered the upper extreme of the metallophilic interactions (with values up to 46 kJ mol-1), intermetallic contacts involving T1(I) centers appear as the weakest ones (even <20 kJ mol-1),46 which is explained by the enhancement of the Au---Au interactions and the weakening of the Van der Waals attractions between the s2 metal atoms produced by the relativistic effects.47 Nevertheless, the complexes in which this interaction appears are surprisingly stable, with additional electrostatic, packing forces, or the ligand architecture, responsible for this fact. 

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