Assessing the Impact of Electronic and Steric Tuning of the Ligand in the Spin State and Catalytic Oxidation Ability of the FeII(Pytacn) Family of Complexes

A family of iron complexes with the general formula [FeII(R,R′Pytacn)­(X)2]n+ is described, where R,R′Pytacn is the tetradentate ligand 1-[(4-R′-6-R-2-pyridyl)­methyl]-4,7-dimethyl-1,4,7-triaza­cyclo­nonane, R refers to the group at the α-position of the pyridine, R′ corresponds to the group at the γ-position, and X denotes CH3CN or CF3SO3. Herein, we study the influence of the pyridine substituents R and R′ on the electronic properties of the coordinated iron center by a combination of structural and spectroscopic characterization using X-ray diffraction, 1H NMR and UV–vis spectroscopies, and magnetic susceptibility measurements. The electronic properties of the substituent in the γ-position of the pyridine ring (R′) modulate the strength of the ligand field, as shown by magnetic susceptibility measurements in CD3CN solution, which provide a direct indication of the population of the magnetically active high-spin S = 2 ferrous state. Indeed, a series of complexes [FeII(H,R′Pytacn)­(CD3CN)2]2+ exist as mixtures of high-spin (S = 2) and low-spin (S = 0) complexes, and their effective magnetic moment directly correlates with the electron-releasing ability of R′. On the other hand, the substitution of the hydrogen atom in the α-position of the pyridine by a methyl, chlorine, or fluorine group favors the high-spin state. The whole family of complexes has been assayed in catalytic C–H and CC oxidation reactions with H2O2. These catalysts exhibit excellent efficiency in the stereospecific hydroxylation of alkanes and in the oxidation of olefins. Remarkably, R′-substituents have little influence on the efficiency and chemoselectivity of the catalytic activity of the complexes, but the selectivity toward olefin cis-dihydroxylation is enhanced for complexes with R = Me, F, or Cl. Isotopic labeling studies in the epoxidation and cis-dihydroxylation reactions show that R has a definitive role in dictating the origin of the oxygen atom that is transferred in the epoxidation reaction.本文报道了一系列通式为[Fe(II)(R,R′Pytacn)(X)₂]ⁿ⁺的铁配合物，其中R,R′Pytacn为四齿配体(tetradentate ligand)1-[(4-R′-6-R-2-吡啶基)甲基]-4,7-二甲基-1,4,7-三氮杂环壬烷；R指代吡啶环α位的取代基，R′对应吡啶环γ位的取代基，X则代表乙腈(CH₃CN)或三氟甲磺酸根(CF₃SO₃⁻)。本文通过X射线衍射(X-ray diffraction)、氢核磁共振(¹H NMR)、紫外-可见(UV–vis)光谱学以及磁化率测量(magnetic susceptibility measurements)等结构与光谱表征手段，探究了吡啶环上取代基R与R′对配位铁中心电子性质的影响。吡啶环γ位取代基(R′)的电子性质可调控配体场强度，这一点可通过CD₃CN溶液中的磁化率测量得以验证，该测量可直接反映具有磁活性的高自旋(high-spin) S=2亚铁态(ferrous state)的占据情况。具体而言，一系列通式为[Fe(II)(H,R′Pytacn)(CD₃CN)₂]²⁺的配合物以高自旋(S=2)与低自旋(low-spin) (S=0)的混合形式存在，其有效磁矩与R′的给电子能力直接相关。另一方面，将吡啶α位的氢原子替换为甲基、氯原子或氟原子，有利于体系稳定在高自旋态。本系列全部配合物均以过氧化氢(H₂O₂)为氧化剂，被用于催化C-H键与碳碳双键的氧化反应的性能测试，该类催化剂在烷烃(alkanes)的立体特异性羟基化(stereospecific hydroxylation)反应以及烯烃(olefins)的氧化反应中展现出优异的催化效率。值得注意的是，R′取代基对配合物催化活性的效率与化学选择性几乎无影响，但当R为甲基、氟或氯时，配合物对烯烃顺式二羟基化(cis-dihydroxylation)的选择性会得到提升。在环氧化反应(epoxidation)与顺式二羟基化反应中开展的同位素标记研究(isotopic labeling studies)表明，取代基R在决定环氧化反应中转移氧原子的来源方面起着决定性作用。