Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2.: Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-CY cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane)is reported. Only trans complexes are formed, and the structure of trans-[Cr(1,1 1-C-3-cyclam)Cl-2]PF6 is presented. The chemical and photophysical behavior of the 1,11-C-3-cyclam complexes are compared with those of the corresponding cyclarn (1,4,8,11 tetraazacyclotetradecane) and 1,4-C(2-)cyclam (1,4,8,11-tetraazabicyclo[10.2.2)hexadecane) complexes. The aquation rate of trans-[Cr(1,11-C-3-cyclam)Cl-2](+) is similar to that of the corresponding 1,4-C-2-cyClam complex and is more than 5 orders of magnitude faster than the cyclam counterpart. A monotonic increase in the extinction coefficient is observed on going from the cyclam complexes to the 1,11-C(3-)cyclam complexes to the 1,4-C-2-cyclam complexes, and this is related to the degree of centrosymmetry in each complex. The trans- [Cr(1, 11-C-3-cyclam) (CN)(2)](+) complex is a weak emitter in aqueous solution with a room-temperature emission maximum at 724 nm (tau = 23 mu s). Like the corresponding 1,4-C-2-cyclam complex (tau = 0.24 mu s), the 1,11-C-3-cyclam complex shows no deuterium-isotope effect in room-temperature solution. This is in marked contrast to the corresponding cyclam complex which has an emission lifetime of 335 mu s and a significant deuterium isotope effect in room-temperature solution. Low temperature (77K) data are also presented in an attempt to understand the differences in photophysical behavior.