Conformational Change of an Azamacrocycle Containing Nitrophenol Side Arms by Proton Coordination. Crystal Structures, Heat of reaction and Molecular Mechanics Calculations

Abstract

The crystal structures of the azamacrocycle 1,7–bis(2–hydroxy-5–nitrobenzyl)-4,10–dimethyl-1, 4,7,10–tetraazadodecane (L) (zwitterionic form) (L-0.5CH₃OH-3.5H₂O) and the monocharged derivative (HL⁺) (HL-Br-2C₂H₅OH) are reported. The crystals of L-0.5CH₃ OH-3.5H₂O are rhombohedral (hexagonal axes), space group R3, a = b = 42.472(7), c = 9.415(9) A, Z = 18, R = 0,034, whereas the crystals of HL-Br-2C₂H₅OH are monoclinic, space group C2/c, a = 20.418(5), b = 13.814 (4), c = 11.785(3) A, β = 99.00(2)°, Z = 4, R = 0.067. The differences in their conformation are discussed. Molecular mechanics calculations were carried out and indicate that the energy difference between the two species is very small. The stepwise enthalpies of protonation in solution were determined by direct microcalorimetry: ΔH° = −23.8(4) kJ mol⁻¹ for H₋₁L⁻ + H⁺ = L; ΔH°≅O for reactions L + H₊ = HL⁺ and HL⁺ + H₊ = H₂L²⁺. The addition of the third proton is also athermic and fully entropy-controlled.

Key Words:

• Crystal structures
• molecular mechanics
• aza-macrocycles
• protonations