摘要/Abstract

由于具有特殊的电子结构和独特的反应性，低价锕系配合物已得到广泛关注.目前，实验中得到的可分离、晶体结构确定的+2锕系元素配合物少之又少.本工作通过相对论密度泛函理论探索了低价[AnL]^z（An=Ac~Pu；L=[（^Me，MeArOH）₃Ar]^3-；z=0和-1）的结构和氧化还原性质.计算发现，在[AnL]→[AnL]^-还原过程中An-C_Ar和An-Ar_cent距离缩短，这是由于还原电子进入到An和Ar共享区域，增强了δ（An-Ar）成键作用所致.还原时，配合物[AcL]和[ThL]的Ar基团捕获了大部分还原电子，而Pa~Pu配合物的则更多地定域在金属周围.因此，还原产物中Ac和Th仍为+3，而Pa~Pu则为+2、具有5fⁿ电子组态.得到的还原电势值随着Ac~Pu整体呈上升趋势，在U和Np处出现相对Pa和Pu的低点.这一趋势与电子亲和能和An-C_Ar/Ar_cent距离变化有很好的相关性.
关键词: δ(An-Ar)型低价锕系配合物, 三(芳氧基)芳烃配体, 电子结构, 氧化还原性质, 相对论密度泛函理论
It is of great significance to identify new oxidation state of actinide, which will enrich actinide coordination chemistry and advance its exploration of chemical bond and reactivity. So far, uranium with +3~+6 oxidation states has been widely recognized in complexes. Comparatively, isolated, crystallographically identified U(II) complexes remain rare. Inspired by the pioneering work of Evans and co-workers that Y·[U^II(Cp')₃] (Y=[K(2.2.2-cryptand)]⁺, Cp'=[C₅H₄SiMe₃]^-) was structurally characterized, several uranium(II) complexes such as Y·[UL^E] (L^E=[(^Ad,MeArO)₃ mesitylene]^3-, Ad=adamantyl),[U(NHAr^iPr6)₂] (Ar^iPr6=2,6-(2,4,6-ⁱPr₃C₆H₂)₂C₆H₃), Y·[U{N(SiMe₃)₂}₃] and[U(η⁵-C₅ⁱPr₅)₂] were synthetically accessible. Inspection finds that all these U(II) complexes were prepared in the same route, i.e., utilizing potassium graphite or potassium sphere to reduce respective U(III) parent at low temperature. Cyclopentadiene (Cp) and arene (Ar)-based ligands are involved. They are key to determine U(II) electron configuration, leading to 5f³6d¹ and 5f⁴, respectively. Moreover, δ(U-Ar) bonds play a significant role in stabilizing arene-ligated complexes. With the supporting of Cp-derived ligands, actinide(II) complexes were extended to Th, Np and Pu. Unfortunately, it is not the case for the arene ligands, even with massive efforts. Given the prevailing route that actinide(II) complex was synthesized by reducing its trivalent parent, the exploration of redox property will help to guide the synthesis of more novel U(II) and even other actinide(II) complexes. In this respect, theoretical computation based on accurate methodology is greatly appealing. Herein, relativistic density functional theory was exploited to investigate structural and redox properties of[AnL]^z (An=Ac~Pu; L=[(^Me,MeArOH)₃Ar]^3-; z=0 and -1), where analogues of uranium complexes were experimentally known. It is found that the central arene moiety is redox-active for Ac and Th complexes in the reduction reaction, while the metal center is reduced for other complexes. So Ac and Th in reduced products still remain +3 oxidation states, whereas metals in others turn +2. The 5fⁿ electronic configuration is unraveled for actinide of[AnL]^- (An=Pa~Pu), having 3~6 electrons, respectively. Calculated redox potential (E⁰) increases from Ac to Pu in general, where U and Np show lower values than adjacent elements. A good correlation has been built between E⁰ and Δ(An-C_Ar/An-Ar_cent)/electron affinity. In brief, the study is expected to provide theoretical support for the synthesis of novel arene-based actinide(II) complexes.
Key words: δ(An-Ar)-type low-valent actinide complexes, tris(aryloxide)arene, electronic structure, redox reaction, relativistic density functional theory


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