Malme Justin T, Clendening Reese A, Ash Ryan, Curry Taylor, Ren Tong, Vura-Weis Josh
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States.
J Am Chem Soc. 2023 Mar 22;145(11):6029-6034. doi: 10.1021/jacs.2c13532. Epub 2023 Mar 13.
Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal Fe complex, Fe(HMTI)(CN) (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(C═N), which enhances Δ via stabilizing t orbitals. The inflexible geometry of the macrocycle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an Fe macrocyclic species.
具有长寿命（≥1纳秒）电荷转移态的铁配合物的例子仅限于具有强σ供体螯合物的伪八面体几何结构。基于改变配位模式和配体给电子能力的替代策略非常可取。本文报道了一种空气稳定的四方铁配合物Fe(HMTI)(CN)（HMTI = 5,5,7,12,12,14-六甲基-1,4,8,11-四氮杂环十四碳-1,3,8,10-四烯），其金属到配体电荷转移（MLCT）寿命为1.25纳秒。已确定其结构，并在多种溶剂中研究了其光物理性质。由于低能π*(C═N)，HMTI配体具有高度的π酸性，通过稳定t轨道增强了Δ。大环的刚性几何结构导致Fe-N键短，密度泛函理论计算表明这种刚性导致了一组不寻常的嵌套势能面。此外，MLCT态的寿命和能量强烈依赖于溶剂环境。这种依赖性是由溶剂与氰基配体之间的路易斯酸碱相互作用对轴向配体场强度的调节引起的。这项工作代表了铁大环物种中长寿命电荷转移态的第一个例子。
