Actinide Redox-Active Ligand Complexes: Reversible Intramolecular Electron-Transfer in U(dpp-BIAN)₂/U(dpp-BIAN)₂(THF)

Actinide complexes of the redox-active ligand (dpp-BIAN)2− (dpp-BIAN = 1,2-bis(2,6-diisopropylphenylimino)acenaphthylene), U(dpp-BIAN)2 (1), U(dpp-BIAN)2(THF) (1-THF), and Th(dpp-BIAN)2(THF) (2-THF), have been prepared. Solid-state magnetic and single-crystal X-ray data for complex 1 indicate a ground-state UIV−π*4 configuration, whereas a (dpp-BIAN)2−-to-uranium electron transfer occurs for 1-THF, resulting in a UIII−π*3 ground configuration. The solid-state magnetic data also indicate that interconversion between the two forms of the complex is possible, limited only by the ability of tetrahydrofuran (THF) vapor to penetrate the solid upon cooling of the sample. In contrast to those in the solid state, spectroscopic data acquired in THF indicate only the presence of the UIV−π*4 form for 1-THF in solution, evidenced by electronic absorption spectra and by measurement of the solution magnetic moment in THF-d8 using the Evans method. Also reported is the electrochemistry of the complexes collected in CH2Cl2, CF3C6H5, and THF. As expected from the solution spectroscopic data, only small differences are observed in half-wave potentials of ligand-based processes in the presence of THF, consistent with the solution UIV−π*4 configuration of the complexes in all cases. Density functional theory calculations were undertaken for complexes 1 and 1-THF to determine if intrinsic energetic or structural factors underlie the observed charge-transfer process. While the calculated optimized geometries agree well with experimental results, it was not possible to arrive at a convergent solution for 1-THF in the UIII−π*3 configuration. However, perturbations in the orbital energies in 1 versus 1-THF for the UIV−π*4 configuration do point to a diminished highest occupied molecular orbital−lowest unoccupied molecular orbital energy gap in 1-THF, consistent with the solid-state magnetic data. These results represent the first example of a stable and well-defined, reversible intramolecular electron transfer in an actinide complex with redox-active ligands.本工作合成了氧化还原活性配体(redox-active ligand) (dpp-BIAN)²⁻（dpp-BIAN = 1,2-双(2,6-二异丙基苯亚氨基)苊烯）对应的锕系配合物(actinide complexes)，包括U(dpp-BIAN)₂（配合物1）、U(dpp-BIAN)₂(四氢呋喃(THF))（配合物1-THF）以及Th(dpp-BIAN)₂(THF)（配合物2-THF）。配合物1的固态磁学与单晶X射线衍射数据表明，其基态为U⁴⁺−π*⁴构型；而配合物1-THF则发生了(dpp-BIAN)²⁻向铀的电子转移，最终形成U³⁺−π*³基态构型。固态磁学数据同时表明，该配合物的两种构型之间可发生相互转化，其转化限制仅取决于样品冷却时四氢呋喃蒸气渗入固体的能力。与固态情况不同，在四氢呋喃溶剂中测得的光谱数据显示，溶液态的配合物1-THF仅以U⁴⁺−π*⁴构型存在，该结论可通过电子吸收光谱，以及采用埃文斯法(Evans method)在氘代四氢呋喃(THF-d8)中测定溶液磁矩得到验证。本研究同时报道了配合物在二氯甲烷(dichloromethane, CH₂Cl₂)、三氟甲苯(trifluorotoluene, CF₃C₆H₅)以及四氢呋喃中的电化学行为。正如溶液光谱数据所预期的那样，在四氢呋喃溶剂中，基于配体的氧化还原过程的半波电位仅存在微小差异，这与所有测试条件下配合物均以U⁴⁺−π*⁴构型存在于溶液中的结论相符。研究人员针对配合物1与1-THF开展了密度泛函理论(Density Functional Theory, DFT)计算，以探究观测到的电荷转移过程是否由内在的能量或结构因素所主导。尽管计算得到的优化几何构型与实验结果吻合良好，但针对U³⁺−π*³构型的配合物1-THF，无法得到收敛的计算解。不过，针对U⁴⁺−π*⁴构型，配合物1与1-THF的轨道能级存在偏移，这表明配合物1-THF的最高占据分子轨道(Highest Occupied Molecular Orbital, HOMO)-最低未占据分子轨道(Lowest Unoccupied Molecular Orbital, LUMO)能级差有所减小，与固态磁学数据一致。该研究成果为首例带有氧化还原活性配体的锕系配合物中稳定且结构明确的可逆分子内电子转移案例。