Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG, Utrecht The Netherlands.
Inorg Chem. 2011 Oct 3;50(19):9243-55. doi: 10.1021/ic200332y. Epub 2011 Sep 8.
The newly synthesized dinuclear complex Fe(III)(2)(μ-OH)(2)(bik)(4)(4) (1) (bik, bis(1-methylimidazol-2-yl)ketone) shows rather short Fe···Fe (3.0723(6) Å) and Fe-O distances (1.941(2)/1.949(2) Å) compared to other unsupported Fe(III)(2)(μ-OH)(2) complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The (57)Fe isomer shift (δ = 0.45 mm s(-1)) and quadrupole splitting (ΔE(Q) = 0.26 mm s(-1)) obtained from Mössbauer spectroscopy are consistent with the presence of two identical high-spin iron(III) sites. Variable-temperature magnetic susceptibility studies revealed antiferromagnetic exchange (J = 35.9 cm(-1) and H = JS(1)·S(2)) of the metal ions. The optimized DFT geometry of the cation of 1 in the gas phase agrees well with the crystal structure, but both the Fe···Fe and Fe-OH distances are overestimated (3.281 and 2.034 Å, respectively). The agreement in these parameters improves dramatically (3.074 and 1.966 Å) when the hydrogen-bonded nitrate groups are included, reducing the value calculated for J by 35%. Spontaneous reduction of 1 was observed in methanol, yielding a blue Fe(II)(bik)(3) species. Variable-temperature magnetic susceptibility measurements of Fe(II)(bik)(3)(2) (2) revealed spin-crossover behavior. Thermal hysteresis was observed with 2, due to a loss of cocrystallized solvent molecules, as monitored by thermogravimetric analysis. The hysteresis disappears once the solvent is fully depleted by thermal cycling. Fe(II)(bik)(3)(2) (2) catalyzes the oxidation of alkanes with t-BuOOH. High selectivity for tertiary C-H bond oxidation was observed with adamantane (3°/2° value of 29.6); low alcohol/ketone ratios in cyclohexane and ethylbenzene oxidation, a strong dependence of total turnover number on the presence of O(2), and a low retention of configuration in cis-1,2-dimethylcyclohexane oxidation were observed. Stereoselective oxidation of olefins with dihydrogen peroxide yielding epoxides was observed under both limiting oxidant and substrate conditions.
新合成的双核配合物Fe(III)(2)(μ-OH)(2)(bik)(4)(4)(1)(bik，双(1-甲基咪唑-2-基)酮)与其他未支撑的Fe(III)(2)(μ-OH)(2)配合物相比，Fe···Fe(3.0723(6)Å)和Fe-O距离(1.941(2)/1.949(2)Å)都相当短。1 中的桥接氢氧根与硝酸根阴离子强烈氢键合。穆斯堡尔光谱得到的(57)Fe 同位素位移(δ=0.45mm s(-1))和四极分裂(ΔE(Q)=0.26mm s(-1))与两个相同的高自旋铁(III)位一致。变温磁化率研究表明，金属离子之间存在反铁磁交换(J=35.9cm(-1)和H=JS(1)·S(2))。1 的阳离子在气相中的优化 DFT 几何形状与晶体结构吻合良好，但 Fe···Fe 和 Fe-OH 距离都被高估(分别为 3.281 和 2.034Å)。当包括氢键合的硝酸根基团时，这些参数的一致性显著提高(分别为 3.074 和 1.966Å)，从而将 J 的计算值降低了 35%。在甲醇中观察到 1 的自发还原，生成蓝色Fe(II)(bik)(3)物种。Fe(II)(bik)(3)(2)(2)的变温磁化率测量显示出自旋交叉行为。由于热重分析监测到共结晶溶剂分子的损失，2 中观察到热滞后。一旦溶剂通过热循环完全耗尽，滞后就会消失。Fe(II)(bik)(3)(2)(2)催化叔 C-H 键氧化的烷烃与 t-BuOOH。用金刚烷(3°/2°值为 29.6)观察到对叔 C-H 键氧化的高选择性；环己烷和乙苯氧化中醇/酮比低，总周转率强烈依赖于 O(2)的存在，以及顺-1,2-二甲基环己烷氧化中构型的保留低。在限制氧化剂和底物条件下，用过氧化氢对烯烃进行立体选择性氧化生成环氧化物。
