Two‐dimensional ¹H NMR studies of paramagnetic bimetallic mixed‐metal complexes

Synthetic dinuclear mixed‐metal complexes have proved useful as models for metal‐substituted dinuclear iron‐oxo proteins. However, a detailed assignment of their paramagnetically shifted proton NMR resonances has thus far been difficult to achieve. In the light of the significant role of two‐dimensional (2D) NMR techniques in establishing the bond connectivities in diamagnetic molecules, COSY studies were performed on a series of complexes, [Fe(II)M(II)BPMP(O₂CCH₂CH₃)₂]⁺ {BPMP = 2,6‐bis [bis(2‐pyridylmethyl)aminomethyl]‐4‐methylphenol, M  Mn, Co and Zn}, taking advantage of the short electronic relaxation time of Fe(II). The resonances from the four distinct pyridine rings of each of the three complexes were unambiguously assigned based on the bond‐correlated cross signals in the 2D spectra due to the (α‐)β‐γ‐β′ pyridyl proton connectivities. The resonances from the two bridging ligands of these complexes were also unambiguously assigned based on the same approach. Together with longitudinal relaxation time (T₁) measurements, the 2D data allow the assignment of all the paramagnetically shifted proton resonances of all the three bimetallic complexes.