The rates of replacements of MeOH by pyridine (py) and
imidazole (imH) type ligands L (=4CN-py, 3CN-py,
3Cl-py, 4Cl-py, py, 4Et-py, 4Me-py, 4NH2-py,
4(Me)2N-py; 4,5(Cl)2-imH,
4,5(CN)2-imH, Bz-imH, imH, 1Et-im, 1Me-im, 2Me-imH, 2Et-imH) in
[(CoIIIP(MeO)(MeOH)] (P = protoporphyrin IX
dimethyl ester) were measured
by stopped-flow techniques. The methoxide ligand is firmly held
while the MeOH ligand is labile and is replaced
by L in a dissociative (D) mechanism. The MeO-
“orienting” ligand, an excellent electron donor, favors
entry
of the least basic L, not the most basic as usually observed. The
plot of ln k
obs vs pK
a of
pyridine and its derivatives
exhibits a minimum rate at pK
a of about 5.
The “V” diagram is explained as being due to a change in
the
electronic structure of the transition state, from predominantly π
bonding (descending branch) to the predominantly
σ bonding (ascending branch). Imidazoles show similar trends,
but their rates level off for pK
a values above
7.
The free energy of activation for the reaction between
[CoIIIP(MeO)] and L is more sensitive to the change
in the
strength of the π bond than to that of the σ bond.