Volume profiles for the reversible binding of dioxygen to cobalt(II) complexes. Evidence for a substitution-controlled process

The rate constants for the binding and release of dioxygen from two cobalt(II) macrocycles have been determined in aqueous solution as a function of pressure at 25.0[degrees]C. The products of these reactions are L[sup 1]CoOO[sup 2+] and L[sup 2]CoOO[sup 2+], where L[sup 1] = cyclam and L[sup 2] = hexamethylcyclam. The pressure dependence of the equilibrium constant for the L[sup 2] system was also evaluated, giving [delta]V[degrees] = [minus]22.- [+-] 0.4 mL mol[sup [minus]1]. The binding rate constants have [delta]V* = +0.4 [+-] 0.5 mL mol[sup [minus]1] (L[sup 1]) and [minus]4.7 [+-] 0.3 mL mol[sup [minus]1] (L[sup 2]). The reverse reaction for L[sup 2]CoOO[sup 2+] has [delta]V* = +17.9 [+-] 0.5 mL mol[sup [minus]1]. The volume profile is presented for the L[sup 2] system, from which an interchange mechanism for substitution at Co(II) is proposed. Electron transfer is not significantly advanced at the transition state. The reverse reaction, released of dioxygen from the cobalt(III)-superoxo complex, has a large positive value of [delta]V*, consistent with the electron transfer preceding the partial dissociation of O[sub 2].