# Interplay between ligand field strength and the nephelauxetic effect in chromium(iii) complexes with anionic amido ligands

## TL;DR

This paper explores how to tune the properties of chromium(III) complexes to achieve near-infrared light absorption by adjusting ligand design.

## Contribution

A new strategy is introduced to balance ligand field strength and the nephelauxetic effect in CrIII complexes using carbazolide ligands with variable σ-donor side arms.

## Key findings

- Substituting pyridine with stronger σ-donors increased ligand field strength from 17,500 to 24,400 cm−1 with minimal change in B.
- Carbazolide ligands enable near-infrared spin-flip transitions but face challenges like low-lying charge-transfer states and geometric distortions.
- An approach for estimating dark spin-flip state energies via photoinduced electron transfer and Rehm–Weller analysis is demonstrated.

## Abstract

Incorporation of the nephelauxetic effect into ligand design enabled red-shifting of spin-flip transitions of CrIII and MnIV 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(Lpy)2]+ ([Lpy]− = 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(LNHC)2]+ or [Cr(LMIC)2]+ ([LNHC]− = 3,6-di-tert-butyl-1,8-bis(imidazolin-2-yliden-1-yl)carbazolide and [LMIC]− = 3,6-di-tert-butyl-1,8-bis(4,5,6,7-tetrahydro-2H-[1,2,3]triazolo[1,5-a]pyridin-2-yl)-carbazol-9-ide) increased the ligand field strength from 17 500 to 24 400 cm−1, with only a modest rise in B from 550 to 600 cm−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 CrIII systems. Additionally, we demonstrate an approach for estimating energies of dark, low-energy spin-flip states in CrIII 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.

Carbazolide-based pincer ligands bearing NHCs/MICs enhance ligand field strength in CrIII complexes with minimal impact on the Racah parameter B, maintaining NIR-active spin-flip excited states and extending their lifetimes, establishing design principles in first-row transition metal photophysics.