Phase Transitions and Inclusion of Alcohols and Amines in the Layer Compounds of Nickel(II) Cyanide with Alkylamines

Abstract

The layer compounds of nickel cyanide with alkylamines, Ni(CN)₄Ni(C_nH_2n+1NH₂)₂, where n=12, 14, 16, and 18, exhibit two phase transitions respectively above room temperature. Although the endothermic peaks recorded on a differential scanning calorimeter are broad, the basal spacing rapidly increases upon the transition (see Fig.1). The packing coefficients of the amine molecules are estimated to be 0.66-0.68 in the room temperature phase, 0.59-0.62 in the intermediate temperature phase, and 0.56-0.58 in the high temperature phase. The second moment of the broad-line'H-NMR spectrum is about 12 to 14 G just below the lower transition temperature, about 3 G in the intermediate temperature phase, and in the order of 0.1 G2 above the higher transition temperature, indicating liquid-like characteristics of the alkyl chains in the last phase (see Figs.3 and 4). Two more step-wise changes in the second moment are noted in the range from 100 to 0°C. The addition of the alcohol with the same chain length as that of the amine or an excess of the amine results in the disappearance of one of the phase transitions. On the basis of the basal spacing, the room temperature phase is found to include up to about 0.4 moles of the alcohol or the amine per the above-mentioned formula regardless of the number of carbon atoms in the alkylamine molecule (see Figs.5 and 6). Above the transition temperature, the amount of the alcohol taken by the layer compound is dependent on the chain length; about 1.1 moles when n=14, about 0.9 moles when n =16, and about 0.7 moles when n=18. On the other hand, saturation is not observed up to two moles per formula when the alcohol is replaced with amine. From the viewpoint of motional behavior of the alkyl chains, the phase lost by the addition of alcohols or amines corresponds to the intermediate temperature phase of the parent layer compound.