Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. Chemistry and Excited-State Behavior of 1,4-C2Cyclam Complexes

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C2-cyclam = 1,4,8,11tetraazabicyclo[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 ((tau)= 0.24 (mu)s). 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 roomtemperature emission lifetime of 335 (mu)s.