Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

We report herein the first example of an oxoiron­(IV) complex of an ethylene-bridged dialkylcyclam ligand, [FeIV(O)­(Me2EBC)­(NCMe)]2+ (2; Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]­hexadecane). Complex 2 has been characterized by UV–vis, 1H NMR, resonance Raman, Mössbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [FeIV(O)­(TMC)­(NCMe)]2+ (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron­(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC is forced to deviate from the trans-I configuration typically found for FeIV(O)­(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordinate cis to the FeIVO unit in 2 rather than in the trans arrangement found in 3. However, switching from the trans geometry of 3 to the cis geometry of 2 did not significantly affect their ground-state electronic structures, although a decrease in ν­(FeO) was observed for 2. Remarkably, despite having comparable FeIV/III reduction potentials, 2 was found to be significantly more reactive than 3 in both oxygen-atom-transfer (OAT) and hydrogen-atom-transfer (HAT) reactions. A careful analysis of density functional theory calculations on the HAT reactivity of 2 and 3 revealed the root cause to be the higher oxyl character of 2, leading to a stronger O---H bond specifically in the quintet transition state.本文报道了首例亚乙基桥联二烷基环胺配体的氧合铁(IV)配合物，即[FeIV(O)(Me2EBC)(NCMe)]2+（2；Me2EBC = 4,11-二甲基-1,4,8,11-四氮杂双环[6.6.2]十六烷）。配合物2已通过紫外-可见光谱（UV–vis）、核磁共振氢谱（1H NMR）、共振拉曼光谱（resonance Raman）、穆斯堡尔谱（Mössbauer）、X射线吸收光谱以及电喷雾电离质谱进行了表征，并将其性质与结构密切相关的[FeIV(O)(TMC)(NCMe)]2+（3；TMC = 1,4,8,11-四甲基-1,4,8,11-四氮杂环十四烷）进行了对比——后者是被广泛研究的大环四甲基环胺配体类典型氧合铁(IV)配合物。Me2EBC拥有与TMC近乎一致的四氮供体配位位点，但以亚乙基桥取代了TMC的1位和8位甲基。因此，Me2EBC无法维持FeIV(O)(TMC)类配合物常见的反式-I构型，转而采取折叠型顺式-V立体化学结构，这使得配合物2中的乙腈（MeCN）配体需与FeIV=O单元顺式配位，而非配合物3中所采用的反式配位方式。不过，从配合物3的反式几何结构切换至配合物2的顺式几何结构，并未对二者的基态电子结构造成显著影响，尽管配合物2的ν(Fe=O)振动频率有所降低。值得注意的是，尽管二者的FeIV/III还原电位相当，但配合物2在氧原子转移（OAT）和氢原子转移（HAT）两类反应中的活性均显著高于配合物3。对配合物2和3的氢原子转移反应活性开展的密度泛函理论计算的细致分析表明，其根源在于配合物2具有更高的氧自由基特征，这使得其五重态过渡态中形成的O---H键更强。