Isolated versus Condensed Anion Structure IV: An NQR Study and X-ray Structure Analysis of [H3N(CH2)3NH3]CdI4 * 2H2O, [H3CNH2(CH2)3NH3]CdBr4, [(CH3)4N]2CdBr 4, and [(CH3)3S]2CdBr4*

Abstract

The phase I of [H3N(CH2)3NH3]CdI4 • 2H2O (1) crystallizes with isolated [CdI4]2-tetrahedra; monoclinic, C2/c, Z = 8, a = 1702.6(3), b= 1459.3(3), c= 1555.5(3) pm, and ß= 120.32(3)° at 299 K. (1) shows a first-order phase transition at T1↔II = 245 K. The eight 127I(v1) NQR lines in phase II change discontinuously into four lines in phase I. The transition entropy from DSC measurements, ∆S = 5.0 J K1- mol-1 , shows that this transition is probably due to order-disorder of cations. [H3CNH2(CH2)3NH3]CdBr4 (2) crystallizes with isolated [CdBr4]2- tetrahedra; orthorhombic P212121, Z = 4,a= 1447.8(5), b = 1280.3 (4), c = 709.7(3) pm at 299 K. (2) shows four 81Br NQR lines between II and around 325 K, above which temperature the lines disappear. [(CH3)4N]2CdBr4 (3) shows a second-order phase transition at T1↔II = 271 K. Three of four 81Br NQR lines in phase II disappear below this transition point, the other line can be observed up to 315 K. The transition entropy, ∆S = 9.01 J KT-1 mol-1 , indicates that the transition is an order-disorder type of the cations. [(CH3)3S]2CdBr4 (4) shows a first-order type phase transition at T1↔II = 304 K. The four lines spectrum of 81Br NQR is observed in phase II and disappears above the transition point. The transition entropy, ∆S = 46.8 J K- 1 mol-1 is abnormally large. The role of the hydrogen bond and the bridging power between the halogen and cadmium atoms upon the formation of the condensed anion structure is discussed.