Synthesis, X-ray Structure Determination, and Spectroscopy of the Silver(I) Polyselenides [(Ph₄P)Ag(Se₄)]_n, [(Me₄N)Ag(Se₅)]_n, [(Et₄N)Ag(Se₄)]₄, and (Pr₄N)₂[Ag₄(Se₄)₃]. Extreme Structure Dependence on Counterion Size

The reaction of sodium pentaselenide with silver nitrate in dimethylformamide (DMF) was investigated. The addition of various counterions, Ph₄P⁺, Me₄N⁺, Et₄N⁺, and Pr₄N⁺, to this common reaction solution resulted in the isolation of four new soluble silver(I) polyselenide complexes, [(Ph₄P)Ag(Se₄)]., (I), [(Me₄N)Ag(Se₅)]_n (II), [(Et₄N)Ag(Se₄)]₄ (III), and (Pr₄N)₂[Ag₄(Se₄)₃] (IV), in high yield. Compound I (85% in yield) crystallizes in the monoclinic space group P2₁/c with unit cell dimensions [formula omitted], and V = 2408 ų. Compound II (93% in yield) crystallizes in the monoclinic space group Cc with unit cell dimensions a = 11.350 (2) Å, b = 18.764 (3) Å, c = 7.434 (1) Å, β = 124.59 (1)°, and V = 1303 ų. Compounds III and IV (96% and 76% in yield, respectively) belong to the monoclinic space group P2_l/n with unit cell dimensions a = 16.229 (3) Å, b = 11.480 (5) Å, c = 17.171 (3) Å, β = 106.06 (I)°, and V = 3074 ų and a = 10.493 (2) Å,.b = 24.573 (3) Å, c = 17.499 (1) Å, β = 93.84 (1)°, and V = 4502 ų, respectively. All structures were solved and refined by direct methods and Fourier techniques. All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were calculated and included but not refined. Refinement by full-matrix least-squares methods on these structures gave final R values of 7.5% for I, 5.2% for II, 2.8% for III, and 3.3% for IV. The four compounds are divided into two different structural categories: one-dimensional chains, [(Ph₄P)Ag(Se₄)]_n and [(Me₄N)Ag(Se₅)]_n, and discrete molecular clusters, [(Et₄N)Ag(Se₄)]₄ and (Pr₄N)₂[Ag₄(Se₄)₃]. The structures of these four silver(I) polyselenide compounds feature two types of coordination spheres for the Ag atoms, trigonal planar and tetrahedral. Compound I consists of infinite macroanionic chains separated by the cation Ph₄P⁺, forming a unique one-dimensional structure containing five-membered AgSe₄ rings. These rings are generated from each other by a 2-fold screw axis operation. Each [formula omitted] chain contains trigonal-planar Ag⁺ atoms. Similarly, compound II is a one-dimensional polymer consisting of AgSe_s as repeat units in which AgSe₄ forms a five-membered ring. The [formula omitted] chain can be generated by a 2-fold screw axis along the chain direction. The Ag⁺ atoms in this structure adopt a tetrahedral coordination geometry. Compound III is molecular. The anion [formula omitted] possesses a crystallographic inversion center. Four silver atoms are symmetrically disposed around the inversion center, forming a planar rhombus. The structure features two different kinds of Ag coordination, Ag(1) and Ag’(I) are tetrahedrally coordinated, while the other two Ag(2) and Ag’(2) possess a trigonal-planar coordination. The [formula omitted] anion features a tetrahedral array of Ag atoms “glued” together by three Se₄^2- ligands, forming a Ag4Se₆ central adamantane-like core in which all Ag⁺ ions assume a trigonal-planar coordination. We found a correlation of counterion size and Ag coordination number in these compounds. The smaller the counterions, the higher the silver coordination number. Variable-temperature ⁷⁷Se NMR spectra of I-IV are reported. The IR spectra of the compounds exhibit two absorptions at ~ 265 and ~195 cm⁻¹. The former is assigned tentatively to the v_se-se vibration. I—III have similar UV/vis spectra in DMF with two absorptions at ~450 and ~630 nm, while IV has a featureless spectrum. Thermal gravimetric analysis data are reported.