Structures and thermal behavior in the series of two-dimensional molecular composites NH3-(CH2)4-NH3 MCl4 related to the nature of the metal M. Part 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 MCl4 complexes with M = Cu and Pd. Both complexes are structurally very similar at room temperature (monoclinic P21/c: a = 0.9270 (3) nm, b = 0.7600 (3) nm, c = 0.7592 (3) nm, β = 103.14 (4)° for M = Cu; a = 0.9087 (2) nm, b = 0.7699 (2) nm, c = 0.7792 (2) nm, β = 103.82 (2) ° 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 [MCl4]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 P21/c: a = 1.0420 (3) nm, b = 0.7442 (1) nm, c = 0.7225 (5) nm, β = 93.46 (4)°.) 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.