Proton Exchange and Inversion at Coordinated <i>sec</i>-Amine Centers. Particularly Rapid Inversion at Sterically Strained “Planar” N in Some [Co(cyclen)(S-AlaO)]<sup>2+</sup> and [Co(Mecyclen)(S-AlaO)]<sup>2+</sup> Complexes<sup>1</sup> David A. Buckingham Charles R. Clark Andrew J. Rogers 10.1021/ja963631v.s001 https://acs.figshare.com/articles/journal_contribution/Proton_Exchange_and_Inversion_at_Coordinated_i_sec_i_-Amine_Centers_Particularly_Rapid_Inversion_at_Sterically_Strained_Planar_N_in_Some_Co_cyclen_S-AlaO_sup_2_sup_and_Co_Mecyclen_S-AlaO_sup_2_sup_Complexes_sup_1_sup_/3663531 It is shown that the <i>syn</i>(N),<i>syn</i>(O)- isomer (<b>3</b>) is the intermediate on the reaction path for interconversion between the <i>syn</i>(N),<i>anti</i>(O)- (<b>1</b>) and <i>syn</i>(O),<i>anti</i>(N)- (<b>2</b>) diastereomers of <i>syn</i>(Me)-[Co(Mecyclen)(S-AlaO)]<sup>2+</sup> and [Co(cyclen)(S-AlaO)]<sup>2+</sup>. <sup>1</sup>H-exchange rate constants for the various N<i>H</i> protons are reported (<i>k</i><sub>H</sub>/M<sup>-1</sup> s<sup>-1</sup>, 25 °C, D<sub>2</sub>O) and are correlated with the derived isomerization rate constants (<i>k</i><sub>N</sub>/M<sup>-1</sup> s<sup>-1</sup>, 25 °C, H<sub>2</sub>O). At equilibrium (25 °C, <i>I</i> ∼ 0.1 M) the [<b>1</b>]:[<b>2</b>]:[<b>3</b>] ratios are 73:21:6 and 63:32:5, respectively, for the two complexes. Ratios of <i>k</i><sub>H</sub>/<i>k</i><sub>N</sub> show that the two “planar” <i>sec</i>-N centers of the coordinated cyclen ligand are particularly susceptible to inversion, with estimated <i>k</i><sub>2</sub> values for lone pair inversion being between 5 × 10<sup>6</sup> and 2 × 10<sup>8</sup> s<sup>-1</sup> at 25 °C. The effects of long-range electronegative substituents (<i>trans</i> carboxylate-O, <i>trans</i> amine-N) and CNC bond angle strain on inversion at metal-coordinated amines is discussed. 1997-04-30 00:00:00 N centers tran sec N H protons syn isomerization rate constants D 2 O Proton Exchange CNC bond angle strain cyclen ligand H 2 O pair inversion Rapid Inversion Co electronegative substituents k 2 values Amine Centers reaction path