title: 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.
abstract: The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-C3-cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane) is reported. Only trans complexes are formed, and the structure of trans-[Cr(1,11-C3-cyclam)Cl2]PF6 is presented. The chemical and photophysical behavior of the 1,11-C3-cyclam complexes are compared with those of the corresponding cyclam (1,4,8,11 tetraazacyclotetradecane) and 1,4-C2-cyclam (1,4,8,11-tetraazabicyclo[10.2.2]hexadecane) complexes. The aquation rate of trans-[Cr(1,11-C3-cyclam)Cl2]+ is similar to that of the corresponding 1,4-C2-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-C3-cyclam complexes to the 1,4-C2-cyclam complexes, and this is related to the degree of centrosymmetry in each complex. The trans-[Cr(1,11-C3-cyclam)(CN)2]+ complex is a weak emitter in aqueous solution with a room-temperature emission maximum at 724 nm (tau=23 micros). Like the corresponding 1,4-C2-cyclam complex (tau=0.24 micros), the 1,11-C3-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 micros 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.
authors.affiliation: Department of Chemistry, Furman University, Greenville, South Carolina 29613, USA. paul.wagenknecht@furman.edu
authors.forename: Paul S
authors.initials: PS
authors.lastname: Wagenknecht
authors.forename: Chuanjiang
authors.initials: C
authors.lastname: Hu
authors.forename: Denise
authors.initials: D
authors.lastname: Ferguson
authors.forename: Lawrence C
authors.initials: LC
authors.lastname: Nathan
authors.forename: Robert D
authors.initials: RD
authors.lastname: Hancock
authors.forename: Jason R
authors.initials: JR
authors.lastname: Whitehead
authors.forename: Kimberley
authors.initials: K
authors.lastname: Wright-Garcia
authors.forename: Michael T
authors.initials: MT
authors.lastname: Vagnini
mesh_terms: D001372:Aza Compounds / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007700:Kinetics; D047029:Lactams, Macrocyclic / Q000138:chemical synthesis / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010777:Photochemistry; D012680:Sensitivity and Specificity; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013237:Stereoisomerism
chemical_list: D001372:Aza Compounds; D047029:Lactams, Macrocyclic; D008024:Ligands; D009942:Organometallic Compounds; D002857:Chromium