A HIGH-VALENT NONHEME IRON INTERMEDIATE - STRUCTURE AND PROPERTIES OF [FE-2(MU-O)(2)(5-ME-TPA)(2)](CLO4)(3)
A HIGH-VALENT NONHEME IRON INTERMEDIATE - STRUCTURE AND PROPERTIES OF [FE-2(MU-O)(2)(5-ME-TPA)(2)](CLO4)(3)
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DOI：
10.1021/ja00115a013
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发表时间：
1995-03-15
影响因子：
15
通讯作者：
MUNCK, E
中科院分区：
文献类型：
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作者：
DONG, YH;FUJII, H;MUNCK, E
在我们对甲烷单加氧酶（MMO）和核糖核苷酸还原酶（RNR）的R2蛋白的氧活化化学进行建模的努力中，我们在H2 O2与（μ-氧代）二铁（III）TPA络合物（TPA =三（2-吡啶基甲基）胺）的反应中发现了一种短暂的绿色物质（3）。研究表明，[Fe_2 O（TPA）（2）（OH）（H_2 O）]（ClO_4）（3）（2a）是3的前体，它可以通过[Fe_2 O（TPA）（2）（H_2 O）（ClO_4）]-（ClO_4）（3）（1）与一当量碱反应得到。晶体学研究表明，1具有近线性（μ-氧代）二铁（III）核心，末端为水和高氯酸根配体（角度Fe-（μ-O）-Fe = 174.1（4）度），而2a的5-Et-TPA类似物2c具有由H3 O2-桥支撑的弯曲（μ-氧代）二铁（III）核，在后者中H3 O2-桥的存在由短的O-O分离（2.464（9）埃）、3.346（9）埃的Fe-Fe距离和136.3（3）度的Fe-（μ-O）-Fe角指示。因此，用当量的碱处理1导致结合的高氯酸盐被氢氧化物取代，并且Fe-O-Fe单元弯曲以形成2。弯曲的Fe-O-Fe核心在溶液中持续存在，这由其UV-vis特征和NMR光谱指示，其反映了关于单个铁位点的不同TPA配位模式。绿色中间体3通过2，[Fe 2 O（L）（2）（OH）（H2O）]（ClO 4）（3）（L = TPA、5-Me-TPA和5-Et-TPA）与H2 O2在CH 3CN中在-40 ℃下反应生成;当5-Me-TPA用作三脚架配体时，3b可以在-40 ℃下静置过夜后分离为固体。配合物3b在366（λ = 7900 M（-1）cm（-1））和616 nm（λ = 5200 M（-1）cm（-1））处具有电子吸收特征，并且具有S = 3/2 EPR光谱，g值为4.45、3.90和2.01。在100 K时，它具有一个尖锐的穆斯堡尔双峰，Δ E（Q）= 0.49 mm/s，Δ = 0.12 mm/s，占固体中铁的90%。元素分析和电喷雾电离质谱表明3b是一个双核配合物，最佳公式为[Fe-2（O）（2）（5-Me-TPA）（2）]（ClO 4）（3）。磁化率测量证实了这种双核公式，表明3 B具有3.9 μ（B）/2 Fe的高温磁矩，对应于EPR观察到的δ = 3/2中心。3 B的分子式表明了两个独特的性质：（a）它有一个Fe-2（μ-O）（2）核，（B）它的形式是（FeFe IV）-Fe-III。3b中Fe-2（μ-O）（2）核的存在由其EXAFS谱表示，其需要在2.89埃处包含Fe散射体以获得满意的拟合。这进一步得到了在676和656 cm（-1）处观察到的共振增强拉曼特征的支持（这两个特征都随着（H2O）-O-18的加入而移动到634 cm（-1）），这与Mn 2 O2络合物中观察到的那些类似，与Fe 2 O2呼吸模式相关。3b的高价性质被ca证实。通过化学（二茂铁滴定）和循环伏安法（E（1/2）= 0.96 V vs NHE），3b在-40 ℃还原为二铁（III）态。配合物3b具有不寻常的电子结构。EPR、磁化强度和穆斯堡尔研究表明，3b具有S = 3/2基态，具有大的和接近轴向的零场分裂，D = 35 +/- 15 cm（-1）和E/D = 0.04。穆斯堡尔谱数据表明3含有两个等价的铁位，它们具有异常小的磁超精细相互作用，A =（-7.8，-7.9，-6.5）MHz.各种交换耦合模型被认为是描述的电子性质的3b，这些包括（FeFeIII）-Fe-III网站耦合到配体自由基和价离域（FeFeIV）-Fe-III中心。在所考虑的模型中，唯一可能解释所观察到的格位等价、同质异能素位移和其它性质的模型是由价离域的低自旋（S = 1/2）Fe-III-低自旋（S = 1）Fe-IV对通过Heisenberg耦合以及双交换组成的;然而，在作出这种分配之前，需要对涉及低自旋铁位置的双交换相互作用进行详细的理论研究。无论它的电子结构，3b是唯一的特点是高价nonheme铁物种，是来自H2 O2和（μ-氧代）diiron（III）复合物的反应。因此，它与MMO和RNR R2的氧化化学中观察到的瞬态物质有关，并提供了如何在非血红素环境中获得高价态的合成实例。
In our efforts to model the oxygen activation chemistry of methane monooxygenase (MMO) and the R2 protein of ribonucleotide reductase (RNR), we have discovered a transient green species (3) in the reaction of H2O2 with a (mu-oxo)diiron(III) TPA complex (TPA = tris(2-pyridylmethyl)amine). Our studies show that the precursor to 3 is [Fe2O(TPA)(2)(OH)(H2O)](ClO4)(3) (2a), which can be obtained by the treatment of [Fe2O(TPA)(2)(H2O)(ClO4)]-(ClO4)(3) (1) With an equivalent of base. Crystallographic studies show that 1 has a nearly linear (mu-oxo)diiron(III) core with terminal aqua and perchlorato ligands (angle Fe-(mu-O)-Fe = 174.1(4)degrees), while 2c, the 5-Et-TPA analogue of 2a, has a bent (mu-oxo)diiron(III) core that is supported by an H3O2- bridge, The presence of an H3O2- bridge in the latter is indicated by the short O-O separation (2.464(9) Angstrom), the Fe-Fe distance of 3.346(9) Angstrom, and the Fe-(mu-O)-Fe angle of 136.3(3)degrees. Thus treatment of 1 with an equivalent of base results in the replacement of the bound perchlorate with hydroxide and the bending of the Fe-O-Fe unit to form 2, That the bent Fe-O-Fe core persists in solution is indicated by its UV-vis features and NMR spectra that reflect distinct TPA coordination modes about the individual iron sites. The green intermediate 3 is generated by the reaction of 2, [Fe2O(L)(2)(OH)(H2O)](ClO4)(3) (L = TPA, 5-Me-TPA, and 5-Et-TPA), with H2O2 in CH3CN at -40 degrees C; when 5-Me-TPA is used as the tripodal ligand, 3b can be isolated as a solid upon standing overnight at -40 degrees C. Complex 3b exhibits electronic absorption features at 366 (epsilon = 7900 M(-1) cm(-1)) and 616 nm (epsilon = 5200 M(-1) cm(-1)) and an S = 3/2 EPR spectrum with g values at 4.45, 3.90, and 2.01. It exhibits one sharp Mossbauer doublet with Delta E(Q) = 0.49 mm/s and delta = 0.12 mm/s at 100 K, which accounts for 90% of the iron in the solid. Elemental analysis and electrospray ionization mass spectrometry show that 3b is a dinuclear complex best formulated as [Fe-2(O)(2)(5-Me-TPA)(2)](ClO4)(3). This dinuclear formulation is corroborated by magnetic susceptibility measurements showing that 3b has a high-temperature moment of 3.9 mu(B)/2Fe, corresponding to the 5 = 3/2 center observed by EPR. The formula for 3b suggests two unique properties: (a) that it has an Fe-2(mu-O)(2) core, and (b) that it is formally (FeFeIV)-Fe-III. The presence of an Fe-2(mu-O)(2) core in 3b is indicated by its EXAFS spectrum, which requires the inclusion of an Fe scatterer at 2.89 Angstrom for a satisfactory fit. It is further supported by the observation of resonance-enhanced Raman features at 676 and 656 cm(-1) (both of which shift to 634 cm(-1) with added (H2O)-O-18), which are associated with an Fe2O2 breathing mode by analogy to those observed for Mn2O2 complexes. The high-valent nature of 3b is corroborated by the ca. 3 eV upshift of its higher X-ray absorption K-edge relative to that of 2b and the reduction of 3b to the diiron(III) state at -40 degrees C by chemical (ferrocene titration) and cyclic voltammetric (E(1/2) = 0.96 V vs NHE) methods.Thus, 3b represents a bis(mu-oxo)-diiron complex with a formally (FeFeIV)-Fe-III valence state. Complex 3b has an unusual electronic structure. EPR, magnetization, and Mossbauer studies show that 3b has an S = 3/2 ground state with a large and nearly axial zero-field splitting, D = 35 +/- 15 cm(-1) and E/D = 0.04. The Mossbauer data show that 3 contains two equivalent iron sites which have unusually small magnetic hyperfine interactions, A = (-7.8, -7.9, -6.5) MHz. A variety of exchange coupling models are considered to describe the electronic properties of 3b; these include (FeFeIII)-Fe-III sites coupled to a ligand radical and valence-delocalized (FeFeIV)-Fe-III centers. Among the models considered, the only one that could possibly explain the observed site equivalence, isomer shift, and other properties consists of a valence-delocalized low-spin (S = 1/2) Fe-III-low-spin (S = 1) Fe-IV pair coupled by Heisenberg as well as double exchange; however, detailed theoretical studies of double exchange interactions involving low-spin iron sites are required before such an assignment can be made. Whatever its electronic structure, 3b is the only well-characterized high-valent nonheme iron species that is derived from the reaction of H2O2 and a (mu-oxo)diiron(III) complex. As such, it is relevant to the transient species observed in the oxidation chemistry of MMO and RNR R2, and provides a synthetic example of how a high-valent state can be attained in a nonheme environment.