Waters Tom, Wang Xue-Bin, Yang Xin, Zhang Lianyi, O'Hair Richard A J, Wang Lai-Sheng, Wedd Anthony G
School of Chemistry, University of Melbourne, Victoria, Australia, 3010.
J Am Chem Soc. 2004 Apr 28;126(16):5119-29. doi: 10.1021/ja039652o.
Photodetachment photoelectron spectroscopy was used to investigate the electronic structure of the doubly charged complexes [MIVO(mnt)2]2- (M = Mo, W; mnt = 1,2-dicyanoethenedithiolato). These dianions are stable in the gas phase and are minimal models for the active sites of the dimethyl sulfoxide reductase family of molybdenum enzymes and of related tungsten enzymes. Adiabatic and vertical electron binding energies for both species were measured, providing detailed information about molecular orbital energy levels of the parent dianions as well as the ground and excited states of the product anions [MVO(mnt)2]-. Density functional theory calculations were used to assist assignment of the detachment features. Differences in energy between these features provided the energies of ligand-to-metal charge-transfer transitions from S(pi) and S(sigma) molecular orbitals to the singly occupied metal-based orbital of the products [MVO(mnt)2]-. These unique data for the M(V) species were obtained at the C(2)(v)() geometry of the parent M(IV) dianions. However, theoretical calculations and available condensed phase data suggested that a geometry featuring differentially folded dithiolene ligands (Cs point symmetry) was slightly lower in energy. The driving force for ligand folding is a favorable covalent interaction between the singly occupied metal-based molecular orbital (a1 in C2v) point symmetry; highest occupied molecular orbital (HOMO)) and the least stable of the occupied sulfur-based molecular orbitals (b1 in C2v point symmetry, HOMO-1) that is only possible upon reduction to the lower symmetry. This ligand folding induces a large increase in the intensity predicted for the a' S(pi) --> a' dx2 - y2 charge-transfer transition originating from the HOMO-2 of MVO(mnt)2 under Cs point symmetry. Electronic absorption spectra are available for the related species [MoVO(bdt)2]- (bdt = 1,2-benzenedithiolato) and for the oxidized form of dimethyl sulfoxide reductase. The intense absorptions at approximately 1.7 eV have been assigned previously to S(sigma) --> Mo transitions, assuming C2v geometry. The present work indicates that the alternative a' S(pi) --> a' dx2 - y2 of Cs geometry must be considered. Overall, this study confirms that the electronic structure of the M-dithiolene units are exquisitely sensitive to dithiolene ligand folding, reinforcing the proposal that these units are tunable conduits for electron transfer in enzyme systems.
采用光解离光电子能谱研究了双电荷配合物[MIVO(mnt)2]2-（M = Mo、W；mnt = 1,2 - 二氰基乙烯二硫醇盐）的电子结构。这些二价阴离子在气相中稳定，是钼酶和相关钨酶的二甲基亚砜还原酶家族活性位点的最小模型。测量了这两种物质的绝热和垂直电子结合能，提供了有关母体二价阴离子的分子轨道能级以及产物阴离子[MVO(mnt)2]-的基态和激发态的详细信息。利用密度泛函理论计算辅助确定解离特征。这些特征之间的能量差异提供了从S(π)和S(σ)分子轨道到产物[MVO(mnt)2]-的单占据金属基轨道的配体 - 金属电荷转移跃迁的能量。这些关于M(V)物种的独特数据是在母体M(IV)二价阴离子的C2(v)几何构型下获得的。然而，理论计算和现有的凝聚相数据表明，具有不同折叠二硫烯配体（Cs点群对称性）的几何构型能量略低。配体折叠的驱动力是单占据金属基分子轨道（C2v点群对称性下的a1；最高占据分子轨道（HOMO））与占据的硫基分子轨道中最不稳定的轨道（C2v点群对称性下的b1，HOMO - 1）之间有利的共价相互作用，这种相互作用只有在还原到较低对称性时才可能发生。这种配体折叠导致在Cs点群对称性下，源自[MVO(mnt)2]-的HOMO - 2的a' S(π)→a' dx2 - y2电荷转移跃迁的预测强度大幅增加。对于相关物种[MoVO(bdt)2]-（bdt = 1,2 - 苯二硫醇盐）和二甲基亚砜还原酶的氧化形式，有电子吸收光谱可用。先前已将约1.7 eV处的强吸收归属于S(σ)→Mo跃迁，假设为C2v几何构型。目前的工作表明必须考虑Cs几何构型的替代a' S(π)→a' dx2 - y2跃迁。总体而言，这项研究证实了M - 二硫烯单元的电子结构对二硫烯配体折叠极其敏感，强化了这些单元是酶系统中电子转移的可调通道的提议。
