Gallagher Audrey T, Lee Jung Yoon, Kathiresan Venkatesan, Anderson John S, Hoffman Brian M, Harris T David
Department of Chemistry , Northwestern University , 2145 Sheridan Road , Evanston , IL 60208-3113 , USA . Email:
Chem Sci. 2017 Dec 14;9(6):1596-1603. doi: 10.1039/c7sc03739b. eCollection 2018 Feb 14.
The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese(iv) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal-organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese(ii) porphyrin complex and examine its reactivity with O using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese(iv) porphyrin species, which exhibits a side-on, η binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese(iv) electronic structure, and show that O binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O gas adsorption measurements are employed to quantify the enthalpy of O binding as = -49.6(8) kJ mol. This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the M redox couple.
过氧金属物种作为众多生物过程中的反应中间体，激发了人们对类似分子模型配合物的合成与研究。过氧锰（IV）卟啉配合物因其通过可逆氧结合形成的潜在能力而备受关注，但以分子形式分离和表征却极其困难。另一方面，将金属卟啉位点固定在金属有机框架（MOF）中，可以研究低配位金属中心与气态底物之间的相互作用，而不受双分子反应和溶剂分子轴向配位的干扰。在此，我们采用这种方法分离出首个严格意义上的四配位锰（II）卟啉配合物，并利用红外光谱、单晶X射线衍射、电子顺磁共振光谱和氧吸附分析研究其与氧的反应性。X射线衍射实验首次揭示了一种过氧锰（IV）卟啉物种，其呈现侧基η配位模式。红外光谱和电子顺磁共振光谱数据证实了过氧锰（IV）电子结构的形成，并表明在环境温度下氧结合是可逆的，这与分子形式中观察到的情况相反。最后，利用氧气吸附测量来量化氧结合的焓为 = -49.6(8) kJ mol。该焓值显著高于相应的铁基和钴基MOF中的焓值，并且发现随着M氧化还原对还原能力的增加而增加。
