NMR spectroscopy of paramagnetic complexes. Part 39—natural abundance ²H NMR of paramagnetic sandwich compounds

The methylated metallocenes (MeCp)₂M with M = V, Cr, Mn, Co, Ni (1–5) were investigated by ¹H and ²H NMR spectroscopy at natural abundance. The ²H NMR signals are narrower by a factor of up to 30 compared with the corresponding ¹H NMR signals, thus establishing an inexpensive method and a general improvement of the NMR spectra of paramagnetic π‐complexes. This has allowed the resolution of the signal splitting of Cp deuterons of 1 and 5 which could not be observed earlier in the ¹H NMR spectra. The origin of the small (1 and 5) and large (2–4) signal splittings is discussed and related to an extended Hückel calculation. The relative magnitude of the signal splitting is reproduced, but the signal assignment had to be deduced from ¹³C NMR results. Primary isotope shifts of up to 4.0 ppm were found for 1, 2, 4 and 5. The much higher values for 3 (up to 16.6 ppm) reflect the combined influence of the intrinsic isotope shift and the isotope effect on the spin crossover.