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

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The synthesis and characterization of the trans-difluorochromium(III) complexes of the constrained macrocyclic ligands 1,4-C 2-cyclam = 1,4,8,11-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]ClO 4 and trans-[Cr(1,11-C 3-cyclam)F 2]ClO 4 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° and 180° 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 2T 1g ( O h ) excited states are 30, 60, and 1.0 μs 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 H 2O/dimethyl sulfoxide over the temperature range between −30 and +95 °C suggest that the 1,4-C 2-cyclam 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 2T 1g ( O h ) state varies with the hydroxylic nature of the solvent.