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Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 4: Comparison of the trans-difluoro-complexes of tet a, 1,4-C2-cyclam, and 1,11-C3-cyclam |
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| Authors: | Michael T. Vagnini Chuanjiang Hu Paul S. Wagenknecht |
| Affiliation: | a Department of Chemistry, Furman University, Greenville, SC, 29613, United States b Department of Chemistry, San José State University, San José, CA 95192, United States c Department of Chemistry, Clemson University, Clemson, SC 29634, United States |
| Abstract: | The synthesis and characterization of the trans-difluorochromium(III) complexes of the constrained macrocyclic ligands 1,4-C2-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane and 1,11-C3-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-C2-cyclam)F2]ClO4 and trans-[Cr(1,11-C3-cyclam)F2]ClO4 are presented. The photochemical and photophysical behavior of the 1,4-C2-cyclam and 1,11-C3-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-C2-cyclam and 1,11-C3-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(N4) plane. This reduction in symmetry correlates with a monotonic increase in the extinction coefficients in going from the tet a to the 1,11-C3-cyclam, to the 1,4-C2-cyclam complex. These three complexes also exhibit large variations in their aqueous room-temperature excited state behavior; namely, the lifetimes of the 2T1g (Oh) excited states are 30, 60, and 1.0 μs for the tet a, 1,11-C3-cyclam, and 1,4-C2-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 °C suggest that the 1,4-C2-cyclam complex accesses a temperature-dependent relaxation mechanism at significantly lower temperatures than do the tet a or 1,11-C3-cyclam complexes. The emission behavior of these complexes is also solvent-dependent, consistent with established theories that the degree of splitting of the emitting 2T1g (Oh) state varies with the hydroxylic nature of the solvent. |
| Keywords: | Photochemistry Photophysics Chromium(III) complexes Thermally activated relaxation Constrained macrocycles Quadrate field strength |
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