The Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States.
J Am Chem Soc. 2021 Apr 28;143(16):6104-6113. doi: 10.1021/jacs.0c13001. Epub 2021 Apr 14.
The work described herein demonstrates the exquisite control that the inner coordination sphere of metalloenzymes and transition-metal complexes can have on reactivity. We report one of few crystallographically characterized Mn-peroxo complexes and show that the tight correlations between metrical and spectroscopic parameters, established previously by our group for thiolate-ligated RS-Mn(III)-OOR complexes, can be extended to include an alkoxide-ligated RO-Mn(III)-OOR complex. We show that the alkoxide-ligated RO-Mn(III)-OOR complex is an order of magnitude more stable ( = 6730 s, = 1.03 × 10 s) than its thiolate-ligated RS-Mn(III)-OOR derivative ( = 249 s, = 2.78 × 10 s). Electronic structure calculations provide insight regarding these differences in stability. The highest occupied orbital of the thiolate-ligated derivative possesses significant sulfur character and π-backdonation from the thiolate competes with π-backdonation from the peroxo π(O-O). DFT-calculated Mulliken charges show that the Mn ion Lewis acidity of alkoxide-ligated RO-Mn(III)-OOR (+0.451) is greater than that of thiolate-ligated RS-Mn(III)-OOR (+0.306), thereby facilitating π-backdonation from the antibonding peroxo π*(O-O) orbital and increasing its stability. This helps to explain why the photosynthetic oxygen-evolving Mn complex, which catalyzes O-O bond formation as opposed to cleavage, incorporates O- and/or N-ligands as opposed to S-ligands.
本文介绍了内配位球对金属酶和过渡金属配合物反应性的精确控制。我们报道了少数几个结晶学上表征的 Mn-过氧配合物之一，并表明我们小组先前为硫醇配体 RS-Mn(III)-OOR 配合物建立的度量和光谱参数之间的紧密相关性可以扩展到包括烷氧基配体 RO-Mn(III)-OOR 配合物。我们表明，烷氧基配体 RO-Mn(III)-OOR 配合物比其硫醇配体 RS-Mn(III)-OOR 衍生物稳定一个数量级（ = 6730 s， = 1.03×10 s）。电子结构计算提供了对这些稳定性差异的深入了解。硫醇配体衍生物的最高占据轨道具有显著的硫原子特征，并且硫醇的 π 反馈与过氧 π(O-O)的 π 反馈竞争。DFT 计算的 Mulliken 电荷表明，烷氧基配体 RO-Mn(III)-OOR 的 Mn 离子路易斯酸度（+0.451）大于硫醇配体 RS-Mn(III)-OOR 的路易斯酸度（+0.306），从而促进了来自反键过氧 π*(O-O)轨道的 π 反馈，并增加了其稳定性。这有助于解释为什么光合作用产氧 Mn 配合物催化 O-O 键形成而不是裂解，它会结合 O-和/或 N-配体而不是 S-配体。
