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

Proligands based on bis(8-quinolinyl)amine (LI) 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 Jr 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 pi-extension on the electronic properties of the metal complexes. Unlike what is typically observed for benzannulated ligand metal complexes, extending the pi-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