New Crystalline Polymers of Ag(TCNQ) and Ag(TCNQF4): Structures and Magnetic Properties

Abstract

Reaction of Ag(BF4) with [n-Bu4N][TCNQ] in CH3CN yields a new phase of Ag(TCNQ) referred to as phase I (1). A second polymorph of Ag(TCNQ), denoted phase II (2), which had earlier been prepared by electrocrystallization methods, can be accessed directly from silver powder and neutral TCNQ. The products were characterized by a combination of spectroscopic and physical methods. A powder X-ray diffraction pattern of (1) was indexed as a tetragonal cell with a=b=12.1422 Å, c=17.0498 Å, and V=2513.70 Å3. Compound (2) crystallizes in the orthorhombic system with a=7.2892 Å, b=16.5790 Å, c=17.4448 Å, and V=2108 Å3, which correlates well with the lattice parameters a=6.975 Å, b=16.686 Å, c=17.455 Å, and V=2031.5 Å3 reported for Ag(TCNQ) phase II. Ag(TCNQF4) (5), prepared by electrochemical reduction of TCNQF4 at an Ag electrode, crystallizes in the monoclinic space group C2/c, a=13.429(3) Å, b=6.9331(14) Å, c=25.735(5) Å, β=116.66(3)°. Ag(TCNQF4) consists of coordinated TCNQF4 radicals arranged in a parallel stacking arrangement along the b axis. The nearly “eclipsed” type stacking of the TCNQF4 radicals leads to a short intradimer distance of 3.119(2) Å and an interdimer separation of 3.330(2) Å. Single crystal X-ray data for the salts [Bu4N][TCNQ] (3) and [n-Bu4N][TCNQF4] (4a) and (4b) are also included for comparison to the Ag(I) materials. [n-Bu4N][TCNQ] (3) crystallizes in the monoclinic space group P21/n with a=9.3028(7) Å, b=19.3520(14) Å, c=15.3880(11) Å, β=100.979 (2)°, V=2719.6(3) Å3. [n-Bu4N] [TCNQF4] crystallizes in two forms; in a monoclinic space group P21/n (4a), a=7.4244(15) Å, b=20.212(4) Å, c=19.758(4) Å, β=97.32(3)°, V=2940.8(10) Å3, and in the triclinic space group P1 (4b), a=9.6335(3)Å, b=14.9574(3) Å, c=21.3615(5) Å, α=107.440(1)°, β=93.437(2)°, γ=104.785(2)° and V=2808.07(12) Å3. Ring–edge “slipped” stacking arrangements are present in both (4a) and (4b), in which a strongly paired set of TCNQF4 anions interacts with another set. The shortest distance within a dimer pair TCNQF4 in (4a) is 3.09 Å. In contrast, there are two independent types of TCNQF4 interactions in (4b), one that involves strongly interacting anions and one that comprises only weakly dimerized anions. The presence of the latter type in (4b) leads to magnetic properties indicative of thermal population of the triplet excited state at higher temperatures.