Kinetics and Mechanism of Cobalt(II) Incorporation into meso-Tetra(4-pyridyl)porphine in Acetic Acid

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Abstract
The kinetics of reactions of meso-tetra(4-pyridyl)porphine (H2TPyP) with cobalt(II) acetate and cobalt(II) nitrate was studied in acetic acid spectrophotometrically. The rate of cobalt incorporation into H2TPyP is first-order in H2TPyP. The conditional first-order rate constants are expressed as follows: for the cobalt(II) acetate system k0(Co)=K[Co(OAc)2]0(1+K[Co(OAc)2]0)−1(k1+k2[Co(OAc)2]0), where at 25 °C K=(5.5±2.0)×104 kg mol−1 (ΔH=−36±15 kJ mol−1, ΔS=−30±30 J mol−1 K−1), k1=(6.3±0.5)×10−5 s−1 (ΔH\eweq=89±8 kJ mol−1, ΔS\eweq=−27±15 J mol−1 K−1), and k2=(1.17±0.05)×10−1 kg mol−1 s−1 (ΔH\eweq=75±3 kJ mol−1, ΔS\eweq=−11±8 J mol−1 K−1); for the cobalt nitrate system k0(Co)=K1[Co(NO3)2]0(1+K1[Co(NO3)2]0)−1(k1+k2[Co(NO3)2]0+k3K2[Co(NO3)2]02), where at 25 °C k1=(5.1±0.5)×10−5 s−1, k2=(2.0±0.1)×10−1 kg mol−1 s−1, K1=(7.2±2.0)×104 kg mol−1, and k3K2=(1.6±0.15)×102 kg2 mol−2 s−1. Formation constants of sitting-atop complexes were determined dynamically. Mechanisms are proposed for metal ion incorporation and some discussions are made on the difference in the reactivities of acetate and nitrate of cobalt(II) in acetic acid.