STEREOCHEMICAL PERTURBATIONS OF THE RELAXATION BEHAVIOR OF (2E)CR(III) - GROUND-STATE X-RAY CRYSTAL-STRUCTURE, PHOTOPHYSICS, AND MOLECULAR MECHANICS SIMULATIONS OF THE QUASI-CAGE COMPLEX [4,4',4''-ETHYLIDYNETRIS(3-AZABUTAN-1-AMINE)]CHROMIUM TRIBROMIDE

The X-ray crystal structure of Cr(sen)3+ (sen = 4,4',4"-etbylidynetris(3-azabutan-1-amine)) demonstrates that the coordination environments are very similar in this complex and Cr(en)3(3+). However, the photophysical behaviors of these complexes contrast markedly, with Cr(sen)3+ having a much shorter 2E excited-state lifetime (by a factor of at least 10(4)) in ambient fluid solutions. The bond angles of the neopentyl cap (N-C-C and CH2-C-CH2) of Cr(sen)3+ are 5-10-degrees larger than normal, and this is attributed to trigonal strain in the ligand induced by coordination to Cr(III). MM2 calculations indicate that coordination geometry preferred by the ligand would be very nearly halfway between the ideal prismatic and antiprismatic microsymmetries. In contrast to the fluid solution behavior, (2E)Cr(sen)3+ doped into [Rh(sen)](ClO4)3 was found to have a 13-mu-s lifetime at 298 K. The facile, thermally activated fluid solution excited-state relaxation channel is attributed to a large amplitude ligand-promoted trigonal twist of the electronically excited complex. [Cr(sen)]Br3, CrN6C11H30Br3, crystallizes in the monoclinic space group P2(1)/c with Z = 4 and a = 13.964 (4) angstrom, b = 10.777 (2) angstrom, c = 13.027 (3) angstrom, and beta = 98.34 (2)-degrees. The structure refined to R = 0.055 and R(W) = 0.050 with 1612 observed reflections.