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Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. Chemistry and Excited-State Behavior of 1,4-C2-Cyclam Complexes

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posted on 16.07.2003, 00:00 by Kimberley Wright-Garcia, Jillian Basinger, Sarah Williams, Chuanjiang Hu, Paul S. Wagenknecht, Lawrence C. Nathan
The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C2-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane is reported. The ligand appears to form only trans complexes, and the structure of trans-[Cr(1,4-C2-cyclam)Cl2]PF6 is presented. The constraint imposed by the additional C2 linkage distorts the bond angles significantly away from the ideal values of 90 and 180°. The effect of the distortion is to enhance the aquation rate of trans-[Cr(1,4-C2-cyclam)Cl2]+ (kobs for trans-[Cr(1,4-C2-cyclam)(H2O)2]3+ formation = 6.5 × 10-2 s-1, 0.01M HNO3, 25 °C) by over 5 orders of magnitude relative to trans-[Cr(cyclam)Cl2]+. The complexes trans-[Cr(1,4-C2-cyclam)Cl2]+ and trans-[Cr(1,4-C2-cyclam)(CN)2]+ are found to have extinction coefficients four to five times higher than their cyclam analogues, owed to the lack of centrosymmetry caused by the steric constraint. The trans-[Cr(1,4-C2-cyclam)(CN)2]+ complex is a very weak emitter in aqueous solution with a broad room-temperature emission centered at 735 nm (τ = 0.24 μs). Extended photolysis (350 nm, 15 h) of trans-[Cr(1,4-C2-cyclam)(CN)2]+ in aqueous solution results in CN- ligand loss. This is in stark contrast to its unconstrained cyclam analogue, which is photoinert and has a room-temperature emission lifetime of 335 μs.