STEREOCHEMICAL ALTERATIONS OF 2E CHROMIUM(III) EXCITED-STATE BEHAVIOR IN DICYANOTETRAAZACYCLOTETRADECANE COMPLEXES - GROUND-STATE X-RAY CRYSTAL-STRUCTURES, PHOTOPHYSICAL BEHAVIOR, AND MOLECULAR MECHANICS SIMULATIONS OF STEREOCHEMICAL EFFECTS

Comparisons have been made of the stereochemistries of several CrIII(N4)X2 complexes and their photophysical behaviors, where N4 is a macrocyclic tetraamine and X is CN- or NH3. The trans complexes tend to have (2E)-Cr(III) excited-state lifetimes which are about 2 orders of magnitude longer than those of their cis analogs under ambient conditions. This trans:cis lifetime ratio increased about 10-fold when the coordinated amines were perdeuterated, and some of the cyano-tetraamine-d4 complexes had 2-4-ms lifetimes in D2O solutions at 25-degrees-C. Stereochemical perturbations have been introduced by means of complexes containing methyl group substituents (C-CH3), in the tetraamine ligand which assume positions near the Cr-X coordination sites. Ground-state geometries of the methylated complexes have been determined by X-ray crystallography. The influence of these stereochemical perturbations on potential associative or dissociative relaxation coordinates has been modeled using MM2 approaches. The results of the MM2 calculations have been compared to the experimental results for (2E)Cr(III) relaxation and found to support the hypothesis that the more facile relaxation of the cis-CrIII(N4)X2 complexes is largely a stereochemical effect. The effects of these simple stereochemical perturbations on (2E)Cr(III) lifetimes suggest that the predominant relaxation pathway could have either associative or dissociative components and that the nature of the predominant components changed from dissociative to associative when X was changed from NH3 to CN- in trans-Cr(N4)X2 complexes. Overall, simple, limiting back-intersystem-crossing or doublet-state reaction models do not account for the (2E)Cr(III) excited-state relaxation behavior found for these am(m)ine or cyanoam(m)ine complexes. Rather it is proposed that the thermally activated decay is the result of nuclear distortions which relax the electronic constraints on excited-state decay. [trans-Cr(5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) (OH2)OH](ClO4)2-H2O, CrN4Cl6H41O11Cl2, crystallized in the triclinic space group P1BAR with Z = 2, a = 10.217 (1) angstrom, b = 11.071 (1) angstrom, c = 12.824 (1) angstrom, alpha = 99.49 (1)-degrees, beta = 95.73 (1)-degrees, gamma = 112.86 (1)-degrees. The structure refined to R = 0.048 and R(w) = 0.045 using 4472 observed reflections. [cis-Cr(5,12-rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)(CN)2]Cl, CrCl8H36N6Cl, crystallized in the orthorhombic space group Fd2d (nonstandard setting of Fdd2, No. 43) with Z = 8, a = 9.362 (1) angstrom, b = 12.866 (2) angstrom, and c = 34.742 (7) angstrom. The structure refined to R = 0.033 and R(w) = 0.030 using 1922 observed reflections.