title: Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles. Chemistry and excited-state behavior of 1,4-C2-cyclam complexes.
abstract: The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,4-C(2)-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-C(2)-cyclam)Cl(2)]PF(6) is presented. The constraint imposed by the additional C(2) linkage distorts the bond angles significantly away from the ideal values of 90 and 180 degrees. The effect of the distortion is to enhance the aquation rate of trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) (k(obs) for trans-[Cr(1,4-C(2)-cyclam)(H(2)O)(2)](3+) formation = 6.5 x 10(-)(2) s(-)(1), 0.01M HNO(3), 25 degrees C) by over 5 orders of magnitude relative to trans-[Cr(cyclam)Cl(2)](+). The complexes trans-[Cr(1,4-C(2)-cyclam)Cl(2)](+) and trans-[Cr(1,4-C(2)-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-C(2)-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 micros). Extended photolysis (350 nm, 15 h) of trans-[Cr(1,4-C(2)-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 micros.
authors.affiliation: Department of Chemistry, San Jose State University, San Jose, California 95192, USA.
authors.forename: Kimberley
authors.initials: K
authors.lastname: Wright-Garcia
authors.forename: Jillian
authors.initials: J
authors.lastname: Basinger
authors.forename: Sarah
authors.initials: S
authors.lastname: Williams
authors.forename: Chuanjiang
authors.initials: C
authors.lastname: Hu
authors.forename: Paul S
authors.initials: PS
authors.lastname: Wagenknecht
authors.forename: Lawrence C
authors.initials: LC
authors.lastname: Nathan
mesh_terms: D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000138:chemical synthesis / Q000737:chemistry*; D017608:Chromium Compounds / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007202:Indicators and Reagents; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010777:Photochemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: C482064:1,4-C(2)-cyclam; D019086:Bridged Bicyclo Compounds, Heterocyclic; D017608:Chromium Compounds; D007202:Indicators and Reagents; D008024:Ligands; D009942:Organometallic Compounds; C482065:dichloro(1,4-C(2)-cyclam)chromium(III)