Effects of bridge redox state levels on the electron transfer and optical properties of intervalence compounds with hydrazine charge-bearing units
Effects of bridge redox state levels on the electron transfer and optical properties of intervalence compounds with hydrazine charge-bearing units
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DOI：
10.1021/ja972760o
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发表时间：
1998-07
影响因子：
15
通讯作者：
S. Nelsen;R. Ismagilov;D. Powell
中科院分区：
文献类型：
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作者：
S. Nelsen;R. Ismagilov;D. Powell
本工作关注零驱动力的热电子转移（ET）反应内的电荷局域对称间隔（SIV）的化合物。SIV化合物具有不同的电荷，在其他方面相同的电荷承载单元上通过桥连接，并且大多数研究的例子都具有过渡金属电荷承载单元。1 SIV化合物具有电荷转移（CT）带，根据Marcus-Hush理论，可以得到表征ET的两个基本参数：垂直重组能（λ）和电子耦合矩阵元（V）。[2]我们用下标SE（表示超交换）表示电荷局域最小值之间对应于ET的CT带，以及与该带相关的λ和V。[3]在Marcus-Hush模型中（见图1a），非绝热SE能面是在ET坐标X上分别以0和1为中心的抛物线。通过桥的电子耦合，由2 × 2久期行列式中的非对角矩阵元（VSE）测量，产生基态双阱绝热能面和单个最小激发态表面。它们在基态最小值处的能量间隔为λ SE，这是CT带在其最大值处的跃迁能量（h ν max）。Hush从CTSE谱带推导出一个简单的VSE评价方程。2e从CTSE带获得的ET参数的最直接测试是将分子内ET的计算速率常数与实验测量的速率常数（ket）进行比较，但该测试尚未应用于金属中心的例子，因为计算的ket值太大而无法测量。4肼类化合物的内振动重组能（λ v）远大于金属配合物，因此它们的λ SE值也高得多。这使得即使VSE相当大，ket也在可测量范围内，5使CT带足够强，易于观察。用动态电子自旋共振（ESR）谱测定了均四甲苯桥联化合物1+在-8 ℃乙腈溶液中的ket = 2.6 × 108s~（-1）。[6]它具有足够大的VSE，使ket落入绝热状态，在绝热状态下，它只对Δ G * 非常敏感。2对CTSE谱带的Marcus-Hush分析稍作修改，就可以从1+、它的类似物（少两个甲基）、6b和三个饱和桥的双（肼）得到的λ SE和VSE精确计算ket; 5因此，Hush VSE方程2e是一个相当好的近似。[7]本文主要讨论2+，我们期望它的ket值不大于1+，因为NAr孤对，芳基π系统的扭曲角φ，因此λ SE应该几乎相同。结晶1+的φ值在中性肼单元处平均为50.5，在阳离子肼单元处平均为66.2。6b中性3（φ）53.2）和3+（φ）66.7）的X射线数据提供了2+肼单元扭曲的模型（它们的结构在支持信息中报道）。VSE取决于连接电荷承载单元和桥的键的重叠，因此它取决于cos φ和所涉及原子的轨道系数。2+的桥的较大π系统应该相对于1+降低其VSE，因此2+的ket可能比1+小。然而，2+的ket远大于1+。2+的ESR谱是在所有可及温度下在CH3CN和CH2Cl2中快速交换的肼中心物种的ESR谱，但在丙酮中观察到动态交替线宽效应，允许在-105、-100和-95 ℃下测定ket分别为1.6、2.1和2.9 × 108 s-1。1+的173，148和135在这些温度下，对应于Δ Gq...
This work concerns zero driving force thermal electron transfer (ET) reactions within charge-localized symmetrical intervalence (SIV) compounds. SIV compounds have different charges on otherwise identical charge-bearing units that are connected by a bridge, and most examples studied have had transition-metal charge-bearing units. 1 SIV compounds show charge-transfer (CT) bands from which two fundamental parameters for ET, the vertical reorganization energy (λ) and the electronic coupling matrix element (V), may be obtained using Marcus-Hush theory. 2 We designate the CT band corresponding to ET between the chargelocalized minima, as well as the λ and V associated with this band, with subscript SE (for superexchange). 3 In the Marcus-Hush model (see Figure 1a), the diabatic SE energy surfaces are parabolas centered at 0 and 1, respectively, on an ET coordinate X. Electronic coupling through the bridge, measured by the offdiagonal matrix element (VSE) in a 2× 2 secular determinant produces a ground-state double-well adiabatic energy surface and a single minimum excited-state surface. Their energy separation at the ground-state minimum is λSE, which is the transition energy of the CT band at its maximum (hνmax). Hush derived a simple equation for evaluation of VSE from the CTSE band. 2e The most direct test of ET parameters obtained from a CTSE band is comparing the calculated rate constant for intramolecular ET with that measured experimentally (ket), but this test has not been applied to metal-centered examples because the ket values calculated are too large to measure. 4 Hydrazines have far larger internal vibrational reorganization energies (λv) than metal complexes, so their λSE values are much higher. This allows ket to be in the measurable range even when VSE is rather large, 5 making the CT band intense enough to observe easily. The durene-bridged compound 1+ has ket) 2.6× 108 s-1 at-8 C in CH3CN, determined by dynamic electron spin resonance (ESR) spectroscopy. 6 It has a large enough VSE to make ket fall in the adiabatic regime, where it is very sensitive only to ΔG*. 2 A slight modification of the Marcus-Hush analysis of CTSE bands allows accurate calculation of ket from the λSE and VSE obtained for 1+, its analogue with two fewer methyl groups, 6b and three bis-(hydrazines) with saturated bridges; 5 therefore, the Hush VSE equation2e is a rather good approximation. 7 This paper principally concerns 2+, which we expected to have a ket value no larger than that of 1+ because the NAr lone pair, aryl π system twist angle φ, and, hence, λSE should be nearly the same. The φ values of crystalline 1+ average 50.5 at the neutral hydrazine unit and 66.2 at the cationic hydrazine unit. 6b X-ray data for neutral 3 (φ) 53.2) and 3+(φ) 66.7) provide models for the twist of the hydrazine units of 2+(their structures are reported in the Supporting Information). VSE depends on overlap at the bonds connecting the charge-bearing units to the bridge, so it depends on cos φ and the orbital coefficients at the atoms involved. The larger π system of the bridge for 2+ should lower its VSE relative to that for 1+, so smaller ket for 2+ than for 1+ might be expected. However, ket of 2+ is far larger than that of 1+. The ESR spectrum of 2+ is that of a rapidly exchanging hydrazine-centered species at all accessible temperatures in CH3CN and CH2Cl2, but a dynamic alternating line width effect was observed in acetone, allowing determination of ket at-105,-100, and-95 C as 1.6, 2.1, and 2.9× 108 s-1, respectively.ESR data for 1+ in CH3CN give rate ratios for 2+: 1+ of 173, 148, and 135 at these temperatures, corresponding to a ΔGq …