Proton Exchange and Inversion at Coordinated sec-Amine Centers. Particularly Rapid Inversion at Sterically Strained “Planar” N in Some [Co(cyclen)(S-AlaO)]²⁺ and [Co(Mecyclen)(S-AlaO)]²⁺ Complexes¹

It is shown that the syn(N),syn(O)- isomer (3) is the intermediate on the reaction path for interconversion between the syn(N),anti(O)- (1) and syn(O),anti(N)- (2) diastereomers of syn(Me)-[Co(Mecyclen)(S-AlaO)]²⁺ and [Co(cyclen)(S-AlaO)]²⁺. ¹H-exchange rate constants for the various NH protons are reported (k_H/M^-1 s^-1, 25 °C, D₂O) and are correlated with the derived isomerization rate constants (k_N/M^-1 s^-1, 25 °C, H₂O). At equilibrium (25 °C, I ∼ 0.1 M) the [1]:[2]:[3] ratios are 73:21:6 and 63:32:5, respectively, for the two complexes. Ratios of k_H/k_N show that the two “planar” sec-N centers of the coordinated cyclen ligand are particularly susceptible to inversion, with estimated k₂ values for lone pair inversion being between 5 × 10⁶ and 2 × 10⁸ s^-1 at 25 °C. The effects of long-range electronegative substituents (trans carboxylate-O, trans amine-N) and CNC bond angle strain on inversion at metal-coordinated amines is discussed.