A novel hexanuclear silver(I) cluster containing a regular Ag6 ring with short Ag-Ag distances and an argentophilic interaction

The hexanuclear complex [HQ][Ag(p-mpspa)] (H-2-p-mpspa = 3-(4-methoxyphenyl)-2-sulfanylpropenoic acid) was prepared by reacting the precursor [Ag(H-p-mpspa)] with diisopropylamine (Q). The complex was characterized by spectroscopic techniques and the structure was solved by a single crystal X-ray diffraction study. The crystal contains hydrogen-bonded diisopropylammonium cations and [Ag-6(p-mpspa)(6)](6-) anions that are based on a regular Ag6 ring with each S-donor atom of the sulfanylcarboxylate ligand bridging two Ag atoms. The Ag-Ag bond distances, 2.8036(6) angstrom, are very short and suggest a closed shell d(10)center dot center dot center dot d(10) argentophilic interaction. To analyze the relative role of this interaction and that of the S-bridging atom the anionic [Ag-6(p-mpspa)(6)](6-) moiety has been studied theoretically at the HartreeFock (HF) and 2nd order Moller-Plesset perturbation theory (MP2) levels on a very simple [Ag-6(SH)(6)] A model system. A large model system [Ag-6(p-mpspa)(6)](6-) B has also been studied by applying the ONIOM (QM/MM) approach using HF/UFF and MP2/UFF combinations as levels of theory. The six experimentally observed Ag(I). Ag(I) supported interactions are reproduced when dispersion-type interactions are considered in the theory levels MP2 and ONIOM MP2/UFF for models A and B, respectively. The use of HF and ONIOM HF/UFF levels led to a similar hexanuclear structure but displayed a large hexagonal disposition without argentophilic contacts for both models A and B. The steric hindrance exerted by the ligands did not preclude the formation of argentophilic interactions, as observed experimentally.