Nonheme iron enzymes utilize S = 2 iron(IV)-oxo intermediates as oxidants in biological oxygenations. In contrast, corresponding synthetic nonheme Fe^IV═O complexes characterized to date favor the S = 1 ground state that generally shows much poorer oxidative reactivity than their S = 2 counterparts. However, one intriguing exception found by Nam a decade ago is the S = 1 [Fe^IV(O)(Me₃NTB)]²⁺ complex (Me₃NTB = [tris((N-methyl-benzimidazol-2-yl)methyl)amine], 1O) with a hydrogen atom transfer (HAT) reactivity that is 70% that of the S = 2 [Fe^IV(O)(TQA)]²⁺ complex (TQA = tris(2-quinolylmethyl)amine, 3O). In our efforts to further explore this direction, we have unexpectedly uncovered a family of new S = 1 complexes with HAT reaction rates beyond the currently reported limits in the tripodal ligand family, surpassing oxidation rates found for the S = 2 [Fe^IV(O)(TQA)]²⁺ complex by as much as an order of magnitude. This is achieved simply by replacing the secondary sphere methyl groups of the Me₃NTB ligand with larger cycloalkyl-CH₂ (R groups in 2O^R) moieties ranging from c-propylmethyl to c-hexylmethyl. These 2O^R complexes show Mössbauer data at 4 K and ¹H NMR spectra at 193 and 233 K that reveal S = 1 ground states, in line with DFT calculations. Nevertheless, they give rise to the most reactive synthetic nonheme oxoiron(IV) complexes found to date within the tripodal ligand family. Our DFT study indicates transition state stabilization through entropy effects, similar to enzymatic catalysis.

中文翻译：

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通过仲球环烷基取代降低反应势垒，一系列 S = 1 FeIV═O 络合物的氢原子转移反应性在 S = 2 [（TQA）FeIV═O]2+ 络合物上增加 10 倍


非血红素铁酶在生物氧合中利用 S = 2 铁 （IV）-氧代中间体作为氧化剂。相比之下，迄今为止表征的相应合成非血红素 Fe^IV═O 配合物倾向于 S = 1 基态，其氧化反应性通常比 S = 2 对应物差得多。然而，Nam 十年前发现的一个有趣的例外是 S = 1 [Fe^IV（O）（Me₃NTB）]²⁺ 络合物（Me₃NTB = [三（（N-甲基-苯并咪唑-2-基）甲基）胺]，1O），其氢原子转移 （HAT） 反应性是 S = 2 [Fe^IV（O）（TQA）]²⁺ 络合物（TQA = 三（2-喹啉甲基）胺， 在我们进一步探索这一方向的努力中，我们出乎意料地发现了一个新的 S = 1 配合物家族，其 HAT 反应速率超出了目前报道的三足配体家族中的极限，超过了 S = 2 [Fe^IV（O）（TQA）]²⁺ 配合物的氧化速率多达一个数量级。这是通过用较大的环烷基-CH₂（2O^R 中的 R 基团）基团（从 c-丙基甲基到 c-己基甲基）部分替换 Me₃NTB 配体的次级球甲基即可实现的。这些 2O^R 复合物显示了 4 K 和 193 K 和 ²³³H NMR 波谱的穆斯堡尔数据，揭示了 S = 1 基态，与 DFT 计算一致。然而，它们产生了迄今为止在三足配体家族中发现的最具反应性的合成非血红素氧铁 （IV） 复合物。 我们的 DFT 研究表明，过渡态通过熵效应稳定，类似于酶催化。