Red Light Absorption of [ReI(CO)3(α-diimine)Cl] Complexes through Extension of the 4,4′-Bipyrimidine Ligand’s π-System

Abstract

Rhenium(I) complexes of type [Re(CO)3(NN)Cl] (NN = α-diimine) with MLCT absorption in the orange-red region of the visible spectrum have been synthesized and fully characterized, including single crystal X-ray diffraction on two complexes. The strong bathochromic shift of MLCT absorption was achieved through extension of the π-system of the electron-poor bidiazine ligand 4,4′-bipyrimidine by the addition of fused phenyl rings, resulting in 4,4′-biquinazoline. Furthermore, upon anionic cyclization of the twisted bidiazine, a new 4N-doped perylene ligand, namely, 1,3,10,12-tetraazaperylene, was obtained. Electrochemical characterization revealed a significant stabilization of the LUMO in this series, with the first reduction of the azaperylene found at E1/2(0/−) = −1.131 V vs. Fc+/Fc, which is the most anodic half-wave potential observed for N-doped perylene derivatives so far. The low LUMO energies were directly correlated to the photophysical properties of the respective complexes, resulting in a strongly red-shifted MLCT absorption band in chloroform with a λmax = 586 nm and high extinction coefficients (ε586nm > 5000 M−1 cm−1) ranging above 700 nm in the case of the tetraazaperylene complex. Such low-energy MLCT absorption is highly unusual for Re(I) α-diimine complexes, for which these bands are typically found in the near UV. The reported 1,3,10,12-tetraazaperylene complex displayed the [Re(CO)3(α-diimine)Cl] complex with the strongest MLCT red shift ever reported. UV–Vis NIR spectroelectrochemical investigations gave further insights into the nature and stability of the reduced states. The electron-poor ligands explored herein open up a new path for designing metal complexes with strongly red-shifted absorption, thus enabling photocatalysis and photomedical applications with low-energy, tissue-penetrating red light in future.

Extending the π‐System in MnI Diimine Tricarbonyl Complexes: Impacts on Photochemistry, Electrochemistry, and CO2 Catalytic Reduction Activity

“…Since the molecular LUMO is an α‐diimine π* centered orbital, extending the π‐system is expected to lower the energy of that orbital, leading to a red‐shift in the MLCT band, and a more positive first reduction peak potential. These trends are recorded in Table 1 and are consistent with previous work in our lab and with rhenium tricarbonyl complexes [9,21] . The LUMO energies were calculated using DFT and decrease as the π‐system increases.…”

“…These trends are recorded in Table 1 and are consistent with previous work in our lab and with rhenium tricarbonyl complexes. [9,21] The LUMO energies were calculated using DFT and decrease as the πsystem increases.…”