Journal of the American Chemical Society, Vol.119, No.17, 4050-4058, 1997
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+) Complexes
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+). H-1-exchange rate constants for the various NH protons are reported (k(H)/M(-1)s(-1), 25 degrees C, D2O) and are correlated with the derived isomerization rate constants (k(N)/M(-1)s(-1), 25 degrees C, H2O). At equilibrium (25 degrees C, I similar to 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 k(2) values for lone pair inversion being between 5 x 10(6) and 2 x 10(8)s(-1) at 25 degrees 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.