Influence of Rare-Earth Ion Radius on Metal–Metal Charge Transfer in Trinuclear Mixed-Valent Complexes

We report the synthesis and characterization of a highly
conjugated
bisferrocenyl pyrrolediimine ligand, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions(Fc2PyrDI)M(N(TMS)2)2 (2-M,
M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance
(NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis–NIR)
data are presented. The latter are in good agreement with DFT calculations,
illuminating the impact of the rare earth ionic radius on NIR charge
transfer excitations. For [2-Sc]+, the charge
transfer is at 11,500 cm–1, while for [2-Y]+, only a d–d transition at 8000 cm–1 is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu]+ complex exhibits both transitions. From
time-dependent density functional theory (TDDFT) analysis, we assign
the 11,500 cm–1 transition as a mixture of metal-to-ligand
charge transfer (MLCT) and metal-to-metal charge transfer (MMCT),
rather than pure metal-to-metal CT because it has significant ligand
character. Typically, the ferrocenes moieties have high rotational
freedom in bis-ferrocenyl mixed valent complexes. However, in the
present (Fc2PyrDI)M(N(TMS)2)2 complexes,
ligand–ligand repulsions lock the rotational freedom so that
rare-earth ionic radius-dependent geometric differences increasingly
influence orbital overlap as the ionic radius falls. The Marcus–Hush
coupling constant HAB trends as [2-Sc]+ > [2-Lu]+ > [2-Y]+.

本研究报道了一种高共轭双二茂铁基吡咯二亚胺配体（bisferrocenyl pyrrolediimine ligand）Fc₂PyrDIH（1）的合成与表征，及其与稀土离子（rare earth ions）形成的三核配合物（trinuclear complexes）(Fc₂PyrDI)M(N(TMS)₂)₂（2-M，M=Sc、Y、Lu、La）。本文给出了该系列配合物的晶体结构、核磁共振（Nuclear Magnetic Resonance, NMR）光谱以及紫外-可见-近红外（Ultraviolet/Visible/Near-Infrared, UV/vis–NIR）测试数据。上述光谱数据与密度泛函理论（Density Functional Theory, DFT）计算结果高度吻合，阐明了稀土离子半径对近红外电荷转移激发过程的影响。对于[2-Sc]⁺，电荷转移激发的波数为11500 cm⁻¹；而对于[2-Y]⁺，仅观测到位于8000 cm⁻¹的d-d跃迁。Lu的离子半径介于Sc和Y之间，[2-Lu]⁺配合物则同时呈现这两种跃迁。通过含时密度泛函理论（Time-Dependent Density Functional Theory, TDDFT）分析，我们将11500 cm⁻¹处的跃迁归因为金属到配体电荷转移（Metal-to-Ligand Charge Transfer, MLCT）与金属到金属电荷转移（Metal-to-Metal Charge Transfer, MMCT）的混合态，而非纯金属到金属电荷转移，因为该跃迁具有显著的配体特征。通常而言，双二茂铁基混合价态配合物中的二茂铁基团具有较高的旋转自由度。但在本研究中的(Fc₂PyrDI)M(N(TMS)₂)₂配合物中，配体间的排斥作用锁定了旋转自由度，使得随着离子半径减小，稀土离子半径依赖的几何差异对轨道重叠的影响逐渐增强。马库斯-赫什（Marcus–Hush）耦合常数H_AB的变化趋势为[2-Sc]⁺ > [2-Lu]⁺ > [2-Y]⁺。