Conformational change of an azamacrocycle containing nitrophenol side armsby proton coordination. Crystal structures, heat of reaction and molecularmechanics calculations

The crystal structures of the azamacrocycle 1,7-bis(2-hydroxy-5-nitrobenzyl )-4,10-dimethyl-l, 4,7,10-tetraazadodecane (L) (zwitterionic form) (L . 0.5 CH(3)OH . 3.5H(2)O) and the monocharged derivative (HL+) (HL . Br . 2C(2)H( 5)OH) are reported. The crystals of L . 0.5CH(3) OH . 3.5H(2)O are rhombohe dral (hexagonal axes), space group R3, a = b = 42.472(7), c=9.415(9) Angstr om, Z = 18 ,R = 0,084, whereas the crystals of HL . Br . 2C(2)H(5)OH are mo noclinic, space group C2/c, a = 20.418(5), b = 13.814 (4), c = 11.785(3) An gstrom, beta = 99.00(2)degrees, Z = 4, R = 0.067. The differences in their conformation are discussed. Molecular mechanics calculations were carried o ut and indicate that the energy difference between the two species is very small. The stepwise enthalpies of protonation in solution were determined b y direct microcalorimetry: Delta H degrees=-23.8(4) kJ mol(-1) for H-1L- H+ = L; Delta H degrees congruent to 0 for reactions L + H+ = HL+ and HL+ H+ =H2L2+. The addition of the third proton is also athermic and fully ent ropy-controlled.