Zhu Xiao-Qing, Zhang Jian-Yu, Cheng Jin-Pei
Department of Chemistry and the State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China.
Inorg Chem. 2007 Jan 22;46(2):592-600. doi: 10.1021/ic061427v.
The thermodynamics and kinetics of NO transfer from S-nitrosotriphenylmethanethiol (Ph(3)CSNO) to a series of alpha,beta,gamma,delta-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCoII], generating the nitrosyl cobalt atom center adducts [T(G)PPCoIINO], 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 x 10(4) to 4.58 x 10(4) 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 Ph(3)CSNO is a Lewis acid and the T(G)PPCoII 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)PPCoII], the transition-state, and the product [T(G)PPCoIINO], was estimated to show that the cobalt atom carries relative effective positive charges of 2.000 in the reactant [T(G)PPCoII], 2.350 in the transition state, and 2.503 in the product [T(G)PPCoIINO], which indicates that the concerted NO transfer from Ph(3)CSNO to T(G)PPCoII with the release of the Ph(3)CS* radical was actually performed by the initial negative charge (-0.350) transfer from T(G)PPCoII to Ph(3)CSNO to form the transition state and was followed by homolytic S-NO bond dissociation of Ph(3)CSNO with a further negative charge (-0.153) transfer from T(G)PPCoII to the NO group to form the final product T(G)PPCoIINO. 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.
分别采用滴定量热法和停流紫外可见分光光度法，研究了在苯甲腈中，亚硝基三苯基甲硫醇（Ph(3)CSNO）向一系列α,β,γ,δ-四苯基卟啉钴（II）衍生物[T(G)PPCoII]转移NO的热力学和动力学，生成亚硝酰钴原子中心加合物[T(G)PPCoIINO]。对可能的NO转移途径中每个基元步骤的能量变化估计表明，最可能的途径是S-NO均裂键解离和Co-NO键形成的协同过程。对NO转移的动力学研究表明，室温下的二级速率常数范围为0.76×10(4)至4.58×10(4) M(-1) s(-1)，反应速率主要受活化焓控制。哈米特型线性自由能分析表明，Ph(3)CSNO中的NO部分是路易斯酸，T(G)PPCoII是路易斯碱；NO转移的主要驱动力是静电荷吸引，而非自旋-自旋耦合相互作用。估计了钴卟啉中钴原子在不同阶段（反应物[T(G)PPCoII]、过渡态和产物[T(G)PPCoIINO]）的有效电荷分布，结果表明，钴原子在反应物[T(G)PPCoII]中的相对有效正电荷为2.000，在过渡态为2.350，在产物[T(G)PPCoIINO]中为2.503，这表明从Ph(3)CSNO向T(G)PPCoII协同转移NO并释放Ph(3)CS*自由基，实际上是通过首先从T(G)PPCoII向Ph(3)CSNO转移负电荷（-0.350）形成过渡态，随后Ph(3)CSNO的S-NO均裂键解离，并伴随进一步从T(G)PPCoII向NO基团转移负电荷（-0.153）形成最终产物T(G)PPCoIINO。显然，这些重要的热力学和动力学结果将有助于理解化学和生化系统中RSNO与金属卟啉之间相互作用的本质。
