Synthesis and crystal structure of propane-1,2,3-triamine trihydrochloride monohydrate. Reactions of propane-1,2,3-triamine with hydrogen ion. Thermodynamic functions and molecular mechanics calculations

Abstract

The crystal and molecular structure of propane-1,2,3-triamine trihydrochloride monohydrate (1·3HCl·H₂O) is reported. 1·3HCl·H₂O crystallizes in the triclinic system, space group P(no. 2), a= 11.078(1), b= 11.935(2), c= 7.862(1)Å, α= 102.88(2), β= 100.79(1), γ= 81.07(1)°, Z= 4. The structure refinement converged to R= 0.036 and R_w= 0.038. Two 1·H₃³⁺ cations, six Cl^– anions and two water molecules are contained in the asymmetric unit. The two triammonium ions have almost the same conformation. The four C–C bond lengths average 1.521 (4)Å while the six C–N bond lengths average 1.485(4)Å. A network of strong hydrogen-bond interactions involve all of the chloride ions, the triprotonated nitrogen atoms as well as the water molecules. These forces are responsible for the non-equivalence in the two C–C–N(terminal) bond angles for the two 1·H₃³⁺ cations.

The protonation equilibria of propane-1,2,3-triamine (1) in aqueous solution (0.15 mol dm^–3 NaCl, 298 K) have been studied with potentiometric and calorimetric techniques. The logarithms of the protonation constants are 9.642 ± 0.002, 7.981 ± 0.001 and 3.715 ± 0.002 for 1, 1·H⁺ and 1·H₂²⁺, respectively. A comparison with the log K values previously determined for similar triamines containing three equivalent –CH₂NH₂ groups, such as 2-methyl-2-aminomethylpropane-1,3-diamine (2) and 2-ethyl-2-aminomethylpropane-1,3-diamine (3), shows that 1 is the least basic ligand. The decrease in the basicity is particularly evident for the third step. The enthalpy changes for 1 are – 10.97 ± 0.04, –11.07 ± 0.07 and –8.52 ± 0.08 kcal mol^–1‡ in the first, second and third protonation step, respectively.

A molecular-mechanics analysis was carried out on the 1·H₃³⁺ cation to obtain a proper force field for this kind of triamine. Atomic charges for all the atoms were calculated by means of the semiempirical MNDO method. The energy-minimized conformation for the folded 1·H₃^{3 +} cation is in good agreement with the molecular structure found in the solid state for 1·3HCl·H₂O. However, the two calculated C–C–N (terminal) bond angles are equal to each other showing that intermolecular interactions are responsible for the distortion found in the solid-state structure. In the most stable conformation of the 1·H₂²⁺ species, the protonated amino groups are the terminal ones. The cation shows an elongated chain similar to that previously found for the protonated linear polyamine spermine when co-crystallized with nucleic acid molecules.