Tricarbonylchlororhenium(I) carboxaldimine derivatives: Synthesis, structure, and NMR characterization of Z and E isomers

The ligand N-(2-pyridylmethyl)anthracene-9-carboxaldimine (1) has been synthesized and characterized by NMR spectroscopic techniques. The experimental procedure leads selectively to the E isomer, which has been studied by a combined theoretical (Density Functional Theory) and 2D NOESY NMR spectroscopic approach. The trifluoroacetic acid promotes the E to Z isomerization of 1. The process has been investigated by NMR spectroscopy and computationally for the neutral ligand 1. DFT methods calculate that the E isomer is 2.2 kcal/mol more stable than the Z isomer, whereas the reverse situation is observed for the protonated species, where the Z isomer is 4.5 kcal/mol more stable than the E isomer. Transition state calculations for the interconversion of the neutral and protonated E species into their corresponding Z isomers show that the energy barriers are 27.6 and 20.3 kcal/mol for the neutral and protonated species, respectively. The reaction with pentacarbonylchlororhenium leads, in the absence and in the presence of trifluoroacetic acid, to the synthesis of the corresponding tricarbonylchlororhenium isomer complexes Re-E-1 and Re-Z-1, respectively. Compounds Re-E-1 and Re-Z-1 were characterized by 1D and 2D NMR spectroscopy in solution, and their crystal structures were determined. Their photophysical and electrochemical properties are also reported.