Cadmium cyclam complexes: interconversion of cis and trans configurations and fixation of CO(2).

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))(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.