Cobalt(III) Complexes of [35]Adamanzane, 1,5,9,13-Tetraazabicyclo[7.7.3]nonadecane. Report of an Inert, Chelate Hydrogen Carbonate Ion

Three cobalt(III) complexes of the macrocyclic tetraamine [35]adamanzane (1,5,9,13-tetraazabicyclo[7.7.3]nonadecane) were isolated as salts. The X-ray crystal structures were solved for the compounds [Co([35]adz)(CO3)]AsF6 (1b), [Co([35]adz)(HCO3)]ZnBr4·H2O (2a), and [Co([35]adz)(SO4)]AsF6·H2O (3a). The coordination geometry around the cobalt(III) ion is a distorted octahedron with the inorganic ligands at cis-positions. Complex 2 is the second example of a cobalt(III) complex for which the X-ray structure shows a chelate binding mode of the hydrogen carbonate entity. The pK a value of the [Co([35]adz)(HCO3)]2+ ion (2) was determined spectrophotometrically to be 0.27 (25 °C, I = 5.0 M). The protonation appears to occur at the noncoordinated carbonyl oxygen atom of the carbonate group, with hydrogen bonding to the crystal water molecule. Evidence is presented for this oxygen atom as the site of protonation in solution as well. In 5.0 M CF3SO3H a slow reaction of the carbonato complex, quantitatively yielding the [Co([35]adz)(H2O)2]3+ ion, was observed. k obs = 7.9(1) × 10-6 s-1 at 25 °C.