Phenanthridine-Containing Pincer-like Amido Complexes of Nickel, Palladium, and Platinum

Proligands based on bis­(8-quinolinyl)­amine (L1) were prepared containing one (L2) and two (L3) benzo-fused N-heterocyclic phenanthridinyl (3,4-benzoquinolinyl) units. Taken as a series, L1–L3 provides a ligand template for exploring systematic π-extension in the context of tridentate pincer-like amido complexes of group 10 metals (1-M, 2-M, and 3-M; M = Ni, Pd, Pt). Inclusion of phenanthridinyl units was enabled by development of a cross-coupling/condensation route to 6-unsubstituted, 4-substituted phenanthridines (4-Br, 4-NO2, 4-NH2) suitable for elaboration into the target ligand frameworks. Complexes 1-M, 2-M, and 3-M are redox-active; electrochemistry and UV–vis absorption spectroscopy were used to investigate the impact of π-extension on the electronic properties of the metal complexes. Unlike what is typically observed for benzannulated ligand–metal complexes, extending the π-system in metal complexes 1-M to 2-M to 3-M led to only a moderate red shift in the relative highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap as estimated by electrochemistry and similarly subtle changes to the onset of the lowest-energy absorption observed by UV–vis spectroscopy. Time-dependent density functional theory calculations revealed that benzannulation significantly impacts the atomic contributions to the LUMO and LUMO+1 orbitals, altering the orbital contributions to the lowest-energy transition but leaving the energy of this transition essentially unchanged.以双(8-喹啉基)胺(L1)为基础合成的前配体，分别引入1个（L2）和2个苯并稠合N杂环菲啶基（3,4-苯并喹啉基）单元。作为系列配体，L1至L3可作为配体模板，用于探究第10族金属的三齿钳形酰胺配合物（1-M、2-M、3-M；M=镍(Ni)、钯(Pd)、铂(Pt)）体系中系统性π延伸的构效关系。通过开发一条适用于衍生目标配体骨架的交叉偶联/缩合合成路线，成功得到了6位未取代、4位取代的菲啶类化合物（4-Br、4-NO2、4-NH2），从而实现了菲啶基单元的引入。配合物1-M、2-M及3-M均具有氧化还原活性；研究人员采用电化学法与紫外-可见吸收光谱法，探究了π延伸对该类金属配合物电子性质的影响。与苯并稠合配体-金属配合物的常规观测结果不同，将金属配合物1-M的π体系依次延伸至2-M、3-M后，通过电化学估算得到的最高占据分子轨道(HOMO)与最低未占据分子轨道(LUMO)的能隙仅发生了适度红移，紫外-可见吸收光谱观测到的最低能量吸收起始边也同样出现了细微变化。含时密度泛函理论(TDDFT)计算结果表明，苯并稠合作用显著影响了LUMO及LUMO+1轨道的原子轨道贡献，改变了最低能量跃迁的轨道贡献来源，但该跃迁的能量基本保持不变。