Protonation and Non-Innocent Ligand Behavior in Pyranopterin Dithiolene Molybdenum Complexes.
Protonation and Non-Innocent Ligand Behavior in Pyranopterin Dithiolene Molybdenum Complexes.
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DOI：
10.1021/acs.inorgchem.2c01234
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发表时间：
2022-09-05
影响因子：
4.6
通讯作者：
Burgmayer, Sharon J. Nieter
中科院分区：
文献类型：
--
作者：
Gates, Cassandra;Varnum, Haley;Getty, Catherine;Loui, Natalie;Chen, Ju;Kirk, Martin L.;Yang, Jing;Burgmayer, Sharon J. Nieter
配合物[TEA][Tp*MoIV(O)(S2BMOPP)](1)(TEA=四乙基铵，Tp*=三(3，5-二甲基吡唑基)氢硼酸酯，BMOPP=6-(3-丁炔基-2-甲基-2-醇)-2-新戊酰基蝶呤)是所有吡喃蝶呤钼酶通用的钼辅助因子的结构类似物，因为它具有吡喃喋呤-烯-1，2-二硫酸盐配体(S2BMOPP)，主要以硫代硫酸酯和硫代硫酸酯的共振杂化形式存在于闭环吡喃结构中。用电化学、电子吸收和EPR谱相结合的方法研究了质子化形式[TP*MoIV(O)(S2BMOPP-H)](1-H)和单电子被氧化的[TP*MoV(O)(S2BMOPP)](1-Mo(5+))物种。对这些分子性质的更多洞察来自于电子结构计算。二硫代烯C-S键长的差异与烯二硫酸酯和硫酮硫醇共振结构的相对贡献有关。当1质子化为1-H时，观察到了较大的光谱变化，跃迁分别为Mo(Xy)→吡喃喋呤金属-配体电荷转移和二硫杂菲→吡喃喋呤离子电荷转移，这突显了1-H之间电子结构的显著变化。1的质子化时发生的电子结构变化也反映在1-H的Mo(V/IV)氧化还原电势增加300 mV，这是由于更大的硫酮-硫醇共振贡献和减少的电荷给予，相对于单电子氧化，稳定了1-H中的Mo(IV)状态。单电子氧化的1-Mo(5+)和未环化的[TP*MoV(O)(S2BDMPP)](3-Mo(5+))(BDMPP=6-(3-丁炔基-2，2-二甲基)-2-新戊酰基蝶呤)的EPR自旋哈密顿参数彼此非常相似，并且与[TP*MoVo(Bdt)](Bdt=1，2-二硫代烯)非常相似。这表明配体的二硫代形式在Mo(V)水平上占主导地位，这与要求更大的S→Mo电荷给予以及随着金属氧化态的增加而相应地增加Mo-S共价性的要求是一致的。1的质子化代表了一个简单的反应，该反应模拟了质子如何从邻近的酸性氨基酸残基转移到吡喃喋呤钼酶中PDT氮的Mo辅因子，从而影响Mo-PDT单元的电子结构。这项工作还说明了吡喃环链互变异构化如何驱动二硫杂环烯螯合物的共振贡献的变化，并可能调节钼离子的还原电位。对模拟钼辅助因子的吡喃蝶呤二硫杂环钼(IV)络合物的研究揭示了蝶呤质子化如何影响氧化还原反应性，并说明了蝶呤可以作为质子依赖的氧化还原过程的质子继电器。钼-吡喃喋呤二硫杂多烯结构产生了一个质子敏感的电子开关，其中质子化通过增加硫酮-硫醇共振贡献来获得二硫杂多烯的氧化还原能力，并导致Mo(V/IV)氧化还原电位增加300 mV。
The complex [TEA][Tp*MoIV(O)(S2BMOPP)] (1) (TEA = tetraethylammonium, Tp* = tris(3,5-dimethylpyrazolyl)hydroborate, BMOPP = 6-(3-butynyl-2-methyl-2-ol)-2-pivaloyl pterin) is a structural analog of the molybdenum cofactor common to all pyranopterin molybdenum enzymes since it possesses a pyranopterin-ene-1,2-dithiolate ligand (S2BMOPP) that exists primarily in the ring-closed pyrano structure as a resonance hybrid of ene-dithiolate and thione-thiolate forms. The protonated form [Tp*MoIV(O)(S2BMOPP-H)] (1-H), and the one-electron oxidized [Tp*MoV(O)(S2BMOPP)] (1-Mo(5+)) species have been studied using a combination of electrochemistry, electronic absorption and EPR spectroscopies. Additional insight into the nature of these molecules has been derived from electronic structure computations. Differences in dithiolene C-S bond lengths correlate with relative contributions from both ene-dithiolate and thione-thiolate resonance structures. Upon protonation of 1 to form 1-H, large spectroscopic changes are observed with transitions assigned as Mo(xy) → pyranopterin metal-to-ligand charge transfer (MLCT) and dithiolene → pyranopterin ILCT, respectively, and this underscores a dramatic change in electronic structure between 1 and 1-H. The electronic structure changes that occur upon protonation of 1 are also reflected in a large > 300 mV increase in the Mo(V/IV) redox potential for 1-H, resulting from the greater thione-thiolate resonance contribution and decreased charge donation that stabilize the Mo(IV) state in 1-H with respect to one-electron oxidation. EPR spin-Hamiltonian parameters for one-electron oxidized 1-Mo(5+) and uncyclized [Tp*MoV(O)(S2BDMPP)] (3-Mo(5+)) (BDMPP = 6-(3-butynyl-2,2-dimethyl)-2-pivaloyl pterin) are very similar to each other and to [Tp*MoVO(bdt)] (bdt = 1,2-ene-dithiolate). This indicates that the dithiolate form of the ligand dominates at the Mo(V) level, consistent with the demand for a greater S → Mo charge donation and a corresponding increase in Mo-S covalency as the oxidation state of the metal is increased. Protonation of 1 represents a simple reaction that models how proton transfer from neighboring acidic amino acid residues to the Mo cofactor at a PDT nitrogen in pyranopterin molybdenum enzymes can impact the electronic structure of the Mo-PDT unit. This work also illustrates how pyran ring-chain tautomerization drives changes in resonance contributions to the dithiolene chelate and may adjust the reduction potential of the Mo ion. An investigation of pyranopterin dithiolene-Mo(IV) complexes that model the molybdenum cofactor reveals how pterin protonation impacts redox reactivity and illustrates that the pterin can serve as a proton relay for proton dependent redox processes. The Mo-pyranopterin dithiolene structure creates a proton sensitive electronic switch where protonation accesses the redox capability of the dithiolene by increasing the thione-thiolate resonance contribution and leads to a 300 mV increase in the Mo(V/IV) redox potential.
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