Silver macrocyclic complexes: synthesis, crystal structures and redox properties of [Ag([18]aneN2S4)]PF6 and [Ag(Me2[18]aneN2S4)]BPh4([18]aneN2S4= 1,4,10,13-tetrathia-7,16-diazacyclooctadecane, Me2[18]aneN2S4= 7,16-dimethyl-1,4,10,13-tetrathia-7,16-diazacyclooctadecane)

Abstract

Reaction of AgNO₃ with one molar equivalent of [18]aneN₂S₄(1,4,10,13-tetrathia-7,16-diazacyclooctadecane) or Me₂[18]aneN₂S₄(7,16-dimethyl analogue) in refluxing MeOH–H₂O affords the complex cations [Ag([18]aneN₂S₄)]⁺ and [Ag(Me₂[18]aneN₂S₄)]⁺ respectively. [Ag([18]aneN₂S₄)]PF₆ crystallises in the orthorhombic space group Pcab with a= 16.7327(25), b= 17.5023(11), c= 14.7031(6)Å and Z= 8. The metal ion is bound to a highly distorted octahedral arrangement of the six macrocyclic donor atoms [Ag–S 2.630(4), 2.664(4), 2.719(4), 2.774(4)Å; Ag–N 2.533(10), 2.817(15)Å]. The complex cation adopts a rac configuration in which the two S–N–S linkages are bound meridionally to the metal ion. [Ag(Me₂[18]aneN₂S₄)]BPh₄ crystallises in the monoclinic space group P2₁/n with a= 10.7529(15), b= 19.0571(28), c= 18.9360(22)Å, β= 106.035(10)° and Z= 4. The cation is disordered and shows two different configurations. The major [70.8(8)%] component shows an unusual kite-based pyramidal geometry with Ag^I bound via a periplanar arrangement of four S-donor atoms [Ag–S(1) 2.583(4), Ag–S(4), Ag–S(4) 2.819(3), Ag–S(10) 2.663(4), Ag–S(13) 2.673(4)Å] and one apical N-donor atom [Ag–N(7) 2.517(11)Å]. The second N-donor atom does not interact with the metal centre [Ag ⋯ N(16) 3.684(11)Å]. In view of the asymmetric bond length distribution, the overall stereochemistry may alternatively be described as tetrahedral through S(1), N(7), S(10) and S(13), with S(4) interacting more weakly at long range. Although this complex is severely distorted, it is clear that unlike in [Ag([18]aneN₂S₄)]⁺, the S(4)–N(7)–S(10) linkage in [Ag(Me₂[18]aneN₂S₄)]⁺ binds facilally to the Ag^I centre to give a distorted meso isomer. The minor [29.2(8)%] component differs only in the orientation of the S(1), N(16′) and S(13) linkage. Although the C–C and C–S bond lengths in this region were constrained, it is clear that N(16′) is directed towards, and is co-ordinated to the Ag¹ centre [Ag–N(16′) 2.778(10)Å], giving an octahedral complex in a genuine meso configuration. [Ag([18]aneN₂S₄)]⁺ shows a reversible Ag^I–Ag^II couple at E_½=+0.65 V and a quasi-reversible Ag^I–Ag⁰ couple at E_½=–0.74 V while [Ag(Me₂[18]aneN₂S₄)]⁺ shows an irreversible Ag^I–Ag^II couple at E_pa=+0.78 V and an irreversible Ag^I–Ag⁰ couple at E_pc=–0.78 V (all potentials vs. ferrocene–ferrocenium).