Goldsmith Christian R, Stack T Daniel P
Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
Inorg Chem. 2006 Jul 24;45(15):6048-55. doi: 10.1021/ic060621e.
The lipoxygenase mimic Fe(III)(PY5)(OH)2 is synthesized from the reaction of Fe(II)(PY5)(MeCN)2 with iodosobenzene, with low-temperature studies suggesting the possible intermediacy of an Fe(IV) oxo species. The Fe(III)-OH complex is isolated and identified by a combination of solution and solid-state methods, including EPR and IR spectroscopy. Fe(III)(PY5)(OH) reacts with weak X-H bonds in a manner consistent with hydrogen-atom abstraction. The composition of this complex allows meaningful comparisons to be made with previously reported Mn(III)-OH and Fe(III)-OMe lipoxygenase mimics. The bond dissociation energy (BDE) of the O-H bond formed upon reduction to [Fe(II)(PY5)(H2O)]2+ is estimated to be 80 kcal mol(-1), 2 kcal mol(-1) lower than that in the structurally analogous [Mn(II)(PY5)(H2O)]2+ complex, supporting the generally accepted idea that Mn(III) is the thermodynamically superior oxidant at parity of coordination sphere. The identity of the metal has a large influence on the entropy of activation for the reaction with 9,10-dihydroanthracene; [Mn(III)(PY5)(OH)]2+ has a 10 eu more negative DeltaS++ value than either [Fe(III)(PY5)(OH)]2+ or [Fe(III)(PY5)(OMe)]2+, presumably because of the increased structural reorganization that occurs upon reduction to [Mn(II)(PY5)(H2O)]2+. The greater enthalpic driving force for the reduction of Mn(III) correlates with [Mn(III)(PY5)(OH)]2+ reacting more quickly than [Fe(III)(PY5)(OH)]2+. Curiously, [Fe(III)(PY5)(OMe)]2+ reacts with substrates only about twice as fast as [Fe(III)(PY5)(OH)]2+, despite a 4 kcal mol(-1) greater enthalpic driving force for the methoxide complex.
脂氧合酶模拟物Fe(III)(PY5)(OH)2由Fe(II)(PY5)(MeCN)2与亚碘酰苯反应合成，低温研究表明可能存在Fe(IV)氧代物种中间体。通过溶液和固态方法相结合，包括电子顺磁共振（EPR）和红外光谱，分离并鉴定了Fe(III)-OH配合物。Fe(III)(PY5)(OH)以与氢原子夺取一致的方式与弱X-H键反应。该配合物的组成使得能够与先前报道的Mn(III)-OH和Fe(III)-OMe脂氧合酶模拟物进行有意义的比较。还原为[Fe(II)(PY5)(H2O)]2+时形成的O-H键的键解离能（BDE）估计为80 kcal mol(-1)，比结构类似的[Mn(II)(PY5)(H2O)]2+配合物低2 kcal mol(-1)，支持了普遍接受的观点，即在配位球相同的情况下，Mn(III)是热力学上更优越的氧化剂。金属的身份对与9,10-二氢蒽反应的活化熵有很大影响；[Mn(III)(PY5)(OH)]2+的ΔS++值比[Fe(III)(PY5)(OH)]2+或[Fe(III)(PY5)(OMe)]2+的更负10 eu，这可能是因为还原为[Mn(II)(PY5)(H2O)]2+时发生了更大的结构重组。Mn(III)还原的更大焓驱动力与[Mn(III)(PY5)(OH)]2+比[Fe(III)(PY5)(OH)]反应更快相关。奇怪的是，[Fe(III)(PY5)(OMe)]2+与底物反应的速度仅比[Fe(III)(PY5)(OH)]2+快约两倍，尽管甲醇盐配合物的焓驱动力大4 kcal mol(-1)。
