Structures and thermal behavior in the series of two-dimensional molecular composites NH3-(CH2)(4)-NH3 MCl(4) related to the nature of the metal M .1. Crystal structures and phase transitions in the case M=Cu and Pd

X-ray diffraction and differential scanning calorimetry studies have been undertaken on the layered NH3-(CH2)(4)-NH3 MCl(4) complexes with M = Cu and Pd. Both complexes are structurally very similar at room temperature (monoclinic P2(1)/c : a = 0.9270(3) nm, b = 0.7600(3) nm, c = 0.7592(3) nm, beta = 103.14(4)degrees for M = Cu; a = 0.9087(2) nm, b = 0.7699(2) nm, c = 0.7792(2) nm, beta = 103.82(2)degrees for M = Pd). The organic layers are composed of [NH3-(CH2)(4)-NH3](2+) cations with a left-handed conformation at both ends; the mineral layers are composed of [MCl(4)](2-) square planar anions. The structure cohesion is achieved via N-H---Cl hydrogen bonds. The copper complex exhibits a structural phase transition at T = 328 K, characterized by an increase of the interlayer distance (+0.137 nm) from powder diffraction results and which can be interpreted as due to a change in the molecular conformations, from left-handed to all-trans. This assumption is confirmed by single crystal structure determination. (High temperature phase M = Cu, monoclinic P2(1)/c : a = 1.0420(3) nm, b = 0.7442(1) nm, c = 0.7225(5) nm, beta = 93.46(4)degrees.) The palladium complex is stable up to its decomposition temperature (490 K). However, a detailed thermal expansion analysis shows that a virtual structural phase transition is expected above 490 K. The correlation between the nature of the metal, the strength of the hydrogen bonds and the occurrence of left-handed conformations and related phase transitions, is discussed.