Direct observation of low spin – high spin electronic ground states and cross-over exchange in manganocene derivatives, (η5-C5H4R)2Mn, R = H, CH3, C2H5 by paramagnetic nuclear magnetic resonance

The paramagnetic 1 H nmr spectra for manganocene (1) and 1.1′-dimethylmanganocene (2), and the 13 C nmr spectra for 1,1′-diethylmanganocene (3) have been recorded in toluene solvent over a −90 to 90 °C temperature range. 1 shows a low field and a high field ring proton resonance in its spectrum near −59 °C. At higher temperatures the low field resonance is prevalent and becomes gradually averaged due to a fast spin exchange process that dominates the spectrum at 90 °C. For the ring substituted derivatives 2 and 3, resonances due to only one paramagnetic species were detected in the low temperature range. Above ambient temperature a new spectrum due to rapid spin exchange averaging is observed for these complexes. Results are readily interpreted in terms of ground state molecular cross-over exchanges between the 2 E 2g , 6 A 1g , and 2 A 1g spin states of the complexes.