Cadmium Cyclam Complexes:  Interconversion of Cis and Trans Configurations and Fixation of CO₂

There is current interest in the antiviral activity of metal, especially zinc, cyclam (1,4,8,11-tetraazacyclotetradecane) complexes. Their biological activity appears to be dependent on recognition of membrane proteins (viral coreceptors) and therefore on their configurations. Here, we use Cd(II) as a probe for Zn(II) on account of its useful NMR properties. We have prepared and characterized Cd(II) complexes of cyclam, Cd(cyclam)(ClO(4))(2) (1), Cd(cyclam)Cl(2) (2), and [Cd(3)(cyclam)(3)(CO(3))](ClO(4))(4).3H(2)O (3), and have identified key markers for various configurations adopted by these complexes under a variety of solution conditions using 1D and 2D (1)H, (13)C, (15)N, and (111)Cd NMR spectroscopy, including Karplus-type analyses of (1)H, (1)H and (1)H, (111)Cd coupling constants. These complexes were stable at high pH (>8.2) but dissociated completely on lowering the pH to 5.3. Two major configurations of both 1 and 2 exist in aqueous solution: trans-I (R,S,R,S at nitrogen) and cis-V (R,R,R,R). (3)J((111)Cd, (1)H) coupling constants showed that the five-membered rings of the trans-I configuration adopt the eclipsed conformation, and the six-membered rings adopt chair conformations. The X-ray crystal structure of 3 shows that the cation adopts the unusual folded cis-I configuration in which all of the N-H bonds are oriented up (or down) in a novel tri-cadmium cluster. This complex contains triply bridged carbonate fixed from atmospheric CO(2). Each Cd(II) is bound by two cis oxygen atoms from CO(3)(2-) (Cd-O bond lengths 2.373 and 2.412 A) and four nitrogen atoms from cyclam (C-N bond lengths 2.270-2.323 A). The geometry can be described as trigonal bipyramidal with the two donor oxygen atoms occupying one of the apices of the in-plane triangle. In acetonitrile solution, complex 3 gives rise to only one configuration, trans-I, with eclipsed five-membered rings, and six-membered rings with chair conformations.