Optimized Octahedral Geometry and Hidden Rigidity of a New Tridentate N-Heterocyclic Carbene Ligand Scaffold Enhances Luminescence of an Fe(III) Complex

Fe(III) N-heterocyclic carbene (NHC) complexes are emerging photoactive first-row transition metal complexes. While optimizing ligand bite angles is a widely employed strategy to extend photoactive excited states, the lack of tridentate NHC ligands prevents exploration of better coordination geometries in the first-row transition metal complexes. Here, we demonstrate that a newly designed methylene-bridged mer-tridentate NHC ligand provides an improved coordination bite angle. Continuous Shape Measure (CShM) analysis reveals that the Fe(III) complex exhibits a significantly improved octahedral geometry (0.35) compared to existing phenylene-bridged mer-tridentate ligands (3.15). This optimized geometry results in a prolonged 2LMCT lifetime (τ = 670 ps), which is longer compared to the Fe(III) complexes with reported bidentate or mer-tridentate ligands. The complex retains a better octahedral geometry even in the distorted quartet metal-centered (4MC) state, indicating enhanced structural rigidity of the coordination environment. The methylene-bridges seem inherently flexible; however, we revealed that the geometric constraints of the mer-tridentate arrangement and methylene moieties provide counterintuitive hidden structural rigidity. This work establishes the new mer-tridentate NHC ligand scaffold as a promising ligand platform for developing photoactive first-row transition metal complexes.