σ- and π-Bonding Modes of Pyridine and Imidazole Type Ligands in the Transition States of Their Reactions with [Co^III(protoporphyrin IX dimethyl ester)(MeO)(MeOH)] in Methanol

Abstract

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.

Synthesis, Characterization, and Preliminary Oxygenation Studies of Benzyl- and Ethyl-Substituted Pyridine Ligands of Carboxylate-Rich Diiron(II) Complexes

“…The chloro substituent of the benzylpyridine in 2-(4-ClBn)py is far enough removed from the diiron center not to affect the steric properties at the dimetallic center, but it will withdraw electron density from the benzylic position. The p K a values of pyridinium and 4-chloropyridinium are 5.25 and 3.84, respectively . This difference in donating ability of H versus Cl may be responsible for the diminished amount of substrate that is oxidized in 2 compared to than in 1 .…”

Chelate ring closing and opening behavior in cyclopentadienyl cobalt(III) complexes with pendant nitrogen functional group

“…As for 2c, the nitrogen atom is in an aromatic ring system. The moderate p-acceptor property of the picolyl moiety in the pendant side chain facilitates the formation of a partial double bond between the metal and nitrogen when the pyridine plane is perpendicular to the Cp plane and bisects the CpCoI 2 moiety as is found in 2c [28][29][30]. This effect combined with less steric hindrance as well as shorter chain in the functionalized arm of 2c leads to the fastest rate of chelation observed among the three reaction intermediates [31].…”

Role of Imidazole and Chelate Ring Size in Copper Oxidation Catalysts: An Experimental and Theoretical Study

“…The energy values obtained for the σand π-donating MOs of the ligands shown in Figure 4 support the fact that imidazole-containing ligands (dipa, mimp) are better σand πdonors than sole pyridine-containing ligands. 41,42 Additionally, the energy values of MOs of the ligands that are capable of a six-membered ring chelate (Figure 4B) miep and dpma are better σand π-donors than their mimp and dpba analogues, respectively.…”