Lim Mark D, Lorkovic Ivan M, Wedeking Katrin, Zanella Andrew W, Works Carmen F, Massick Steve M, Ford Peter C
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA.
J Am Chem Soc. 2002 Aug 21;124(33):9737-43. doi: 10.1021/ja020653a.
Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)2 (2), and this is seen only at low temperatures (K(1) < 3 M(-1) at ambient temperature). However, 1 does readily react reversibly with N2O3 in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO2) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO2, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO3) (5). Owing to such reactivity of Fe(TPP)(NO2), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO2)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO2 and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO2), respectively. The rate constant for the reaction of 1 with N2O3 to generate Fe(TPP)(NO2)(NO) was determined to be 1.8 x 10(6) M(-1) s(-1), and that for the NO reaction with 4 was similarly determined to be 4.2 x 10(5) M(-1) s(-1). Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s(-1). The rapid dilution experiments also demonstrated that Fe(TPP)(NO2) readily undergoes further oxidation to give Fe(TPP)(NO3). The mechanistic implications of these observations are discussed, and it is suggested that NO2 liberated spontaneously from Fe(P)(NO2) may play a role in an important oxidative process involving this elusive species.
本文描述了旨在阐明一氧化氮与铁（II）卟啉配合物Fe（TPP）（NO）（1，TPP = 中-四苯基卟啉二价阴离子）的溶液相反应相关的有争议化学过程的研究。与纯净的NO发生的唯一可观察到的反应是生成抗磁性的二亚硝酰基物种Fe（TPP）（NO）₂（2），并且这仅在低温下才能观察到（室温下K(1) < 3 M⁻¹）。然而，在过量NO存在下，1确实能与N₂O₃可逆地反应生成硝基亚硝酰基配合物Fe（TPP）（NO₂）（NO）（3），这表明先前关于1促进NO歧化生成3的说法可能受到了一氧化氮源中微量空气的影响。还需注意的是，3会可逆地失去NO生成难以捉摸的硝基物种Fe（TPP）（NO₂）（4），该物种在与各种底物的反应中被认为是一种强大的氧原子转移剂。此外，在过量NO₂存在下，后者会被氧化为稳定的硝酸根类似物Fe（TPP）（NO₃）（5）。由于Fe（TPP）（NO₂）具有这样的反应活性，对于准确测量298 K甲苯溶液中Fe（TPP）（NO₂）（NO）的解离平衡，采用闪光光解和停流动力学方法而非静态技术是必要的。对3进行闪光光解会导致竞争性的NO₂和NO解离，分别生成Fe（TPP）（NO）和Fe（TPP）（NO₂）。1与N₂O₃反应生成Fe（TPP）（NO₂）（NO）的速率常数被测定为1.8×10⁶ M⁻¹ s⁻¹，同样地，NO与4反应的速率常数被测定为4.2×10⁵ M⁻¹ s⁻¹。采用停流快速稀释技术测定了3中NO解离的速率常数为2.6 s⁻¹。快速稀释实验还表明，Fe（TPP）（NO₂）很容易进一步被氧化生成Fe（TPP）（NO₃）。讨论了这些观察结果的机理意义，并提出从Fe（P）（NO₂）自发释放的NO₂可能在涉及这种难以捉摸的物种的重要氧化过程中发挥作用。
