W-band ELDOR-detected NMR (EDNMR) spectroscopy as a versatile technique for the characterisation of transition metal-ligand interactions

ELDOR-detected NMR (EDNMR) spectra for a series of hydrated transition metal complexes: Mn^II(H₂O)₆, Cu ^II(H₂O)₆ and V^IVO(H ₂O)₅ are reported. All EDNMR experiments were performed at W-band (94 GHz) employing two independent microwave frequencies. A purpose-built broadband microwave resonator (spectral range 300 MHz) was used, sufficient to detect all single quantum nuclear transitions of the three model systems. The EDNMR spectral lineshape observed is essentially the same as in conventional ENDOR (Electron-Nuclear Double Resonance). EDNMR presents two technical advantages over ENDOR for transition metal complexes: (i) enhanced sensitivity, reducing acquisition times by at least one order of magnitude; and (ii) simultaneous detection of transitions from all magnetic nuclei. This includes ligand (¹H, ²H, ¹⁷O) and metal centred hyperfine couplings. For the latter, both isotropic couplings in the case of the ⁵⁵Mn complex and highly anisotropic couplings in the case of ⁵¹V and ^63,65Cu complexes could be resolved. By monitoring the intensity of the EDNMR lines as function of the amplitude of the pumping microwave pulse, transitions from non-equivalent nuclei can be differentiated. Double quantum transitions are also readily identified. In case of the Mn ^II(H₂¹⁷O)₆ complex, spectral lines involving the simultaneous pumping of both the ⁵⁵Mn and ¹⁷O nuclear transitions are observed.