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 (kH/M-1 s-1, 25 °C, D2O) and are correlated with the derived isomerization rate constants (kN/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 kH/kN 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.