Mechanism and Driving Force of NO Transfer from S-Nitrosothiol to Cobalt(II) Porphyrin:  A Detailed Thermodynamic and Kinetic Study

The thermodynamics and kinetics of NO transfer from S-nitrosotriphenylmethanethiol (Ph₃CSNO) to a series of α,β,γ,δ-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCo^II], generating the nitrosyl cobalt atom center adducts [T(G)PPCo^IINO], in benzonitrile were investigated using titration calorimetry and stopped-flow UV-vis spectrophotometry, respectively. The estimation of the energy change for each elementary step in the possible NO transfer pathways suggests that the most likely route is a concerted process of the homolytic S−NO bond dissociation and the formation of the Co−NO bond. The kinetic investigation on the NO transfer shows that the second-order rate constants at room temperature cover the range from 0.76 × 10⁴ to 4.58 × 10⁴ M^-1 s^-1, and the reaction rate was mainly governed by activation enthalpy. Hammett-type linear free-energy analysis indicates that the NO moiety in Ph₃CSNO is a Lewis acid and the T(G)PPCo^II is a Lewis base; the main driving force for the NO transfer is electrostatic charge attraction rather than the spin−spin coupling interaction. The effective charge distribution on the cobalt atom in the cobalt porphyrin at the various stages, the reactant [T(G)PPCo^II], the transition-state, and the product [T(G)PPCo^IINO], was estimated to show that the cobalt atom carries relative effective positive charges of 2.000 in the reactant [T(G)PPCo^II], 2.350 in the transition state, and 2.503 in the product [T(G)PPCo^IINO], which indicates that the concerted NO transfer from Ph₃CSNO to T(G)PPCo^II with the release of the Ph₃CS^• radical was actually performed by the initial negative charge (−0.350) transfer from T(G)PPCo^II to Ph₃CSNO to form the transition state and was followed by homolytic S−NO bond dissociation of Ph₃CSNO with a further negative charge (−0.153) transfer from T(G)PPCo^II to the NO group to form the final product T(G)PPCo^IINO. It is evident that these important thermodynamic and kinetic results would be helpful in understanding the nature of the interaction between RSNO and metal porphyrins in both chemical and biochemical systems.