2D NMR Studies of Paramagnetic Diiron Complexes

Diiron complexes of dinucleating ligands have been demonstrated to be good structural and spectroscopic models for the diiron-oxo proteins. Although such complexes show relatively sharp isotropically shifted ₁;H NMR resonances due to their fast electronic relaxation rates, detailed assignment of their isotropically shifted ₁H NMR resonances has thus far not been achieved due to the difficulty in establishing bond connectivities on paramagnetic complexes. In this report, we present 2D COSY, TOCSY, and EXSY studies of the complex [Fe₂(BPMP)(O₂P(OPh)₂)₂]^π+(BPMP = 2, 6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol; n = 1, 2) in both its diferrous and mixed-valence states. The resonances due to the two distinct pyridine rings can be unambiguously assigned from the observation of bond-correlated cross signals in the 2D spectra due to the meta₁-para-meta₂pyridyl proton connectivities. The 2D data used in connection with relaxation time measurements and the known crystal structure of the reduced complex allow the isotropically shifted proton resonances of the reduced complex to be conclusively assigned. The resonances of the diferrous and mixed-valence complexes can also be correlated by EXSY and saturation transfer experiments. This report represents the first detailed NMR study of model complexes for nonheme iron proteins by the use of 2D NMR techniques, demonstrating the versatility of NMR spectroscopy for the study of paramagnetic metal complexes.