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

Abstract

The hexanuclear complex [HQ][Ag(p-mpspa)] (H₂-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 [Ag₆(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) Å, are very short and suggest a closed shell d¹⁰⋯d¹⁰ argentophilic interaction. To analyze the relative role of this interaction and that of the S-bridging atom the anionic [Ag₆(p-mpspa)₆]⁶⁻ moiety has been studied theoretically at the Hartree–Fock (HF) and 2^nd order Møller–Plesset perturbation theory (MP2) levels on a very simple [Ag₆(SH)₆] A model system. A large model system [Ag₆(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.