Approaching the gas-phase structures of [AgS_8]~+ and [AgS_(16)]~+ in the solid state

摘要

Upon treating elemental sulfur with [AgSbF_6], [AgAl(hfip)_4], [AgAl(pftb)_4] (hfip = OCH(CF_3)_2, pftb = OC(CF_3)_3 the compounds [Ag(S_8)_2][SbF_6) (1), [AgS_8][Al(hfip)_4] (2), and [Ag(S_8)_2]~+ [[Al(pftb)_4]~- (3) formed in SO_2 (1), CS_2 (2) or CH_2Cl_2 (3). Compounds 1-3 were characterized by single-crystal X-ray structure determinations: 1 by Raman spectroscopy, 2 and 3 by solution NMR spectroscopy and elemental analyses. Single crystals of [Ag(S_8)_2]~+[Sb(OTeF_5)_6]~- 4 were obtained from a disproportionation reaction and only characterized by X-ray crystal structure analysis. The Ag~+ ion in 1 coordinates two monodentate SbF_6~- anions and two bidentate S_8 rings in the 1,3-position. Compound 2 contains an almost C_(4v)-symmetric {AgS_8}~+ moiety: this is the first example of an eta~4-coordinated S_8 ring (d(Ag-S)= 2.84-3.00 A). Compounds 3 and 4, with the least basic anions, contain undistorted, approximately centrosymmetric Ag(eta~4-S_8)_2~+ cations with less symetric eta~4-coordinated S_8 rings (d(Ag-S)=2.68 - 3.35 A). The thermochemical radius and volume of the undistorted Ag(S_8)_2~+ cation was deduced as r_(therm)(Ag(S_8)_2~+) = 3.378+0.076/-0.120 A and V_(therm)(Ag(S_8)_2~+)= 417 +-4/-6 A. AgS_8~+ and several isomers of the Ag(S_8)_2~+ cation were optimized at the BP86, B3LYP, and MP2 levels by using the SVP and TZVPP basis sets. An analysis of the calculated geometries showed the MP2/TZVPP level to give geometries closes to the experimental data. Neither BP86 nor B3LYP reproduced the longer weak dispersive Ag-S interactions in Ag-(eta~4-S_8)_2~+ but led to Ag(eta~3-S_8)_2~+ geometries. With the most accurate MP2/TZVPP level, the enthalpies of formation of the gaseous [AgS_8]~+ and [Ag(S_8)_2]~+ cations were established as DELTA_fH~(298)([Ag(S_8)_2])~+, g) = 856 kJ mol~(-1) and DELTA_fH~(298)([AgS_8])~+, g) = 902 kJ mol~(-1). It is shown that the {AgS_8}~+ moiety in 2 and the {AgS_8}_2~+ cations in 3 and 4 are the best approximation of these ions, which were earlier observed by MS methods. Both cations reside in shallow potential-energy wells where larger structural changes only lead to small increases in the overall energy. It is shown that the covalent Ag-S bonding contributions in both cations may be described by two components: i) the interaction of the spherical empty Ag 5s~0 acceptor orbital with the filled S 3p~2 lone-pair donor orbitals and ii) the interaction of the empty Ag 5p~0 acceptor orbitals with the filled S 3p~2 lone-pair donor orbitals. This latter contribution is responsible for the observedlow symmetry of the centrosymmetric Ag(eta~4-S_8)_2~+ cation. The positive charge transferred from the Ag~+ ion in 1-4 to the coordinated sulfur atoms is delocalized over all the atoms in the S_8 ring by multipe 3p~2 -> 3sigma interactions that results in a smal long-short-long-short S-S bond-length alternation starting from S1 with the shortest Ag-S length. The driving force for all these weak bonding interactions is positive charge delocalization from the formally fully localized charge of the Ag~+ ion.