Grup de Química Bioinorgànica i Supramolecular (QBIS), Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus Montilivi, E17071 Girona, Catalonia, Spain.
Inorg Chem. 2013 Aug 19;52(16):9229-44. doi: 10.1021/ic4004033. Epub 2013 Jul 31.
A family of iron complexes with the general formula Fe(II)((R,R)'Pytacn)(X)2 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, (1)H 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 Fe(II)((H,R)'Pytacn)(CD3CN)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.
描述了一类具有通式Fe(II)((R,R)'Pytacn)(X)2的铁配合物，其中(R,R)'Pytacn 是四齿配体 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 指吡啶的α-位基团，R' 对应于γ-位基团，X 表示 CH3CN 或 CF3SO3。在这里，我们通过 X 射线衍射、(1)H NMR 和紫外可见光谱以及磁化率测量等结构和光谱表征的结合，研究了吡啶取代基 R 和 R'对配位铁中心电子性质的影响。吡啶环γ-位取代基（R'）的电子性质调节配体场的强度，这可以通过在 CD3CN 溶液中的磁化率测量来证明，这直接表明了顺磁性高自旋 S = 2 亚铁态的配体占有率。实际上，一系列配合物Fe(II)((H,R)'Pytacn)(CD3CN)2以高自旋（S = 2）和低自旋（S = 0）配合物的混合物形式存在，其有效磁矩直接与 R'的电子供体能力相关。另一方面，吡啶的α-位上氢原子被甲基、氯或氟取代有利于高自旋态。整个配合物系列已在过氧化物 H2O2 存在下的 C-H 和 C═C 氧化反应中进行了测试。这些催化剂在烷烃的立体特异性羟化和烯烃的氧化中表现出优异的效率。值得注意的是，R'-取代基对配合物催化活性的效率和化学选择性影响不大，但对于 R = Me、F 或 Cl 的配合物，烯烃顺式二羟基化的选择性得到增强。在环氧化和顺式二羟基化反应中的同位素标记研究表明，R 在决定转移到环氧化反应中的氧原子的来源方面起着决定性的作用。
