Electron paramagnetic resonance and electron nuclear double resonance investigation of the diradical...

The neutral complex Zn(L*)(2) and its monocationic analogue [ZnL(L*).THF](1+) have been previously reported to contain two and one monoanionic alpha-iminopyridinate((1-)) pi radical ligands, respectively [Lu, C.C.; Bill, E.; Weyhermüller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc. 2008, 130, 3181-3197] (L represents the neutral alpha-iminopyridine form and THF is tetrahydrofuran). The electronic structures of these complexes have now been studied by Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) spectroscopy in conjunction with density-functional theory (DFT) calculations. The ENDOR spectra of the triplet Zn(L*)(2) are characteristic of a localized diradical: these two ligand radicals exhibit dipolar exchange interactions, but no superexchange mechanism is operative, which would be consistent with its nearly tetrahedral coordination geometry. The monocationic species [ZnL(L*).THF](1+) (S = 1/2) has also been investigated by pulse EPR spectroscopy using large interpulse separations. It is shown that no radical hopping takes places on the time scale of the EPR experiment. The results obtained here, in particular the lack of asymmetry in the charge distribution between the two ligands in the triplet state, may be relevant for a better understanding of the electronic structure of naturally occurring diradicals and triplet states.