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

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]⁺ (k_obs 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.