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