Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 4: Comparison of the trans-difluoro-complexes of tet a, 1,4-C-2-cyclam, and 1,11-C-3-cyclam

摘要

The synthesis and characterization of the trans-difluorochromium(III) complexes of the constrained macrocyclic ligands 1,4-C-2-cyclam = 1,4,8,1 1-tetraazabicyclo[10.2.2]hexadecane and 1,11-C-3-cyclam = 1,4,8,11-tetraazabicyclo[9.3.3]heptadecane is reported. Only trans complexes are formed, and the structures of both trans-[Cr(1,4-C-2-cyclam)F-2]ClO4 and trans-[Cr(1,11-C-3-cyclam)F-2]CIO4 are presented. The photochemical and photophysical behavior of the 1,4-C-2-cyclam and 1, 11-C-3-cyclam complexes is compared with that of the corresponding tet a (C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) complex. The constraint imposed by the additional bridging groups of 1,4-C-2-cyclam and 1, 11-C-3-cyclam distorts the complexes away from both octahedral symmetry and centrosymmetry, as evidenced by the bond angles that deviate from ideal 90 degrees and 180 degrees values and by a significant distortion of the Cr(N-4) plane. This reduction in symmetry correlates with a monotonic increase in the extinction coefficients in going from the tet a to the 1, 11-C-3-cyclam, to the 1,4-C-2-cyclam complex. These three complexes also exhibit large variations in their aqueous room-temperature excited state behavior; namely, the lifetimes of the T-2(1g) (Oh) excited states are 30,60, and 1.0 mu s for the tet a, 1,11-C-3-cyclam, and 1,4-C-2-cyclam complexes, respectively. Studies of the excited-state lifetime of these complexes in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C suggest that the 1,4-C-2-cyclarn complex accesses a temperature-dependent relaxation mechanism at significantly lower temperatures than do the tet a or 1, 11-C-3-cyclam complexes. The emission behavior of these complexes is also solvent-dependent, consistent with established theories that the degree of splitting of the emitting T-2(1g) (Oh) state varies with the hydroxylic nature of the solvent. (c) 2006 Elsevier B.V. All rights reserved.