Interplay Between Ligand Field Strength and the Nephelauxetic Effect in Chromium(III) Complexes with Anionic Amido Ligands

Abstract

Incorporation of the nephelauxetic effect into ligand design enabled red-shifting of spin-flip transitions of Cr^III and Mn^IV complexes into the near-infrared region. Using carbazolide complexes as a model, we present a strategy for tuning the ratio of ligand field strength to the Racah parameter B by combining a covalent carbazolide core with variable σ-donor ligand “side arms.” Substitution of pyridine, as in [Cr(L^py)₂]⁺ ([L^py]⁻ = 3,6-di-tert-butyl-1,8-di(pyridin-2-yl)carbazol-9-ide), with stronger σ-donors such as N-heterocyclic or mesoionic carbenes in [Cr(L^NHC)₂]⁺ or [Cr(L^MIC)₂]⁺ ([L^NHC]^– = 3,6-di-tert-butyl-1,8-bis(imidazolin-2-yliden-1-yl)carbazolide and [L^MIC]⁻ = 3,6-di-tert-butyl-1,8-bis(4,5,6,7-tetrahydro-2H-[1,2,3]triazolo[1,5-a]pyridin-2-yl)-carba-zol-9-ide) increased the ligand field strength from 17,500 to 24,400cm⁻¹, with only a modest rise in B from 550 to 600cm⁻¹. This balance favors near-infrared spin-flip transitions while extending their excited-state lifetimes. Despite these advances, carbazolide-based ligands exhibit also drawbacks, including low-lying charge-transfer states and geometric distortions, which limit lifetimes and prevent emission, contrasting with other near-infrared-emissive Cr^III systems. Additionally, we demonstrate an approach for estimating energies of dark, low-energy spin-flip states in Cr^III complexes via photoinduced electron transfer and Rehm-Weller analysis. Our results offer guidance on balancing ligand field strength and metal–ligand bond covalency to optimize the photophysical and photochemical properties of first-row transition metal complexes.