Mononuclear trigonal planar trichloroargentate(I) and tribromoargentate(I) ions isolated as their dibenzo-18-crown-6 potassium and rubidium salts

Abstract

Mononuclear trichloroargentate(I) and tribromoargentate(I) ions have been isolated with dibenzo-18-crown-6 potassium and rubidium cations and characterised by means of crystal structure determination. The four compounds prepared [ML]₃[AgX₃]X [L = dibenzo-18-crown-6 (6,7,9,10,17,18,20,21-octahydrodibenzo[b,k][1,4,7,10,13,16]hexaoxacyclooctadecine); X = Cl, M = K 1 or Rb 2; X = Br, MK 3 or Rb 4], crystallise in the hexagonal space group P6₃/m, with Z= 2, 1 and 3, and 2 and 4 being isostructural, respectively [1, a= 14.012(2) and c= 18.958(2)Å; 2, a= 13.786(2) and c= 19.507(2)Å; 3, a= 14.154(2) and c= 19.045(4)Å; and 4, a= 14.000(1) and c= 19.591(3)Å]. The anions exhibit perfect D_3h symmetry with Ag–Cl 2.463(2)Å in 1, 2.447(2)Å in 2 and Ag–Br 2.561(2)Å in 3, 2.550(1)Å in 4. In compounds 2 and 4, the rubidium ions appear to be somewhat too large for the crown cavity and are displaced towards the anion, such that each Rb⁺ is involved in two short Rb⁺⋯ X contacts [3.272(2) and 3.487(2)Å in 2, 3.420(2) and 3.597(2)Å in 4]. In the potassium compounds, 1 and 3, however, K⁺ is accommodated in the cavity of the crown and the cations are rotated relative to the anions so that there is only one short K⁺⋯ X contact per potassium ion [3.019(3)Å in 1 and 3.199(5)Å in 3]. The stronger outer-sphere co-ordination in the potassium compounds thus results in a slight lengthening of the Ag–X bond.