Crystal Structure and NQR Studies of Compounds (RH)[ZnBr₃(R)], (RH)₂[ZnBr₄] and [ZnBr₂(R)₂] (R = Py, n-MePy: n = 2, 3, 4); on the Dominant Stability of the Monoanionic Complexes over the Dianionic and Neutral Species

Abstract

The monoanionic complexes (C5H5NH)[ZnBr3(C5H5N)] (1) and (n-CH3C5H4NH)[ZnBr3(n- CH3C5H4N)] (n = 2 (2), 3 (3), 4 (4)) were prepared by crystallization from ethanol solutions through redistribution reactions between the corresponding dianionic complexes (C5H5NH)2[ZnBr4] (5) and (n-CH3C5H4NH)2[ZnBr4] (n = 2, 3 (6), 4 (7)) and the neutral complexes [ZnBr2(C5H5N)2] (10) and [ZnBr2(n-CH3C5H4N)2] (n = 2 (8), 3, 4 (9)). The crystal structures of 1, 4, 9, and 10 were determined; 1: triclinic, P1, a = 7.6957(5), b = 7.7975(4), c = 12.4768(8) Å , α = 90.857(5), β = 95.917(5), γ = 107.899(6)°, Z = 2, 150 K; 4: monoclinic, P21/c, a = 14.8369(6), b = 13.9504(5), c = 8.0041(3) Å , β = 96.318(4)°, Z = 4, 299 K; 9: monoclinic, P21/c, a = 14.2883(5), b = 8.0269(3), c = 13.6031(5) Å , β = 100.581(4)°, Z = 4, 150 K; 10: monoclinic, P21/c, a = 8.7388(5), b = 17.9730(10), c = 8.5452(5) Å , β = 100.024(6)°, Z = 4, 300 K. The cation and anion are paired up via bifurcated hydrogen bonds in the structure of 1 and via a normal N-H· · ·Br hydrogen bond in the structure of 4. 81Br NQR resonance lines coinciding in number with the Br atoms in the chemical formulae were observed for the compounds 1 - 5 and 7 - 9 throughout the temperature range from 77 to ca. 320 K. The comparison between the net charges on the Br atoms obtained by the Townes-Daily analysis or by the Mulliken population analysis seems to indicate that the formation of the intermolecular N-H· · ·Br hydrogen bonds and the π-π and the CH3-π interactions in the crystal structures of the monoanionic complexes are the driving forces to the redistribution reactions