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Abstract The first consistent series of mononuclear 17‐electron complexes of three Group 7 elements has been isolated in crystalline form and studied by X‐ray diffraction and spectroscopic methods. The paramagnetic compounds have a composition of [M⁰(CO)(CNp‐F‐Ar^DArF2)₄] (M=Mn, Tc, Re; Ar^DArF2=2,6‐(3,5‐(CF₃)₂C₆H₃)₂C₆H₂F) and are stabilized by four sterically encumbering isocyanides, which prevent the metalloradicals from dimerization. They have a square pyramidal structure with the carbonyl ligands as apexes. The frozen‐solution EPR spectra of the rhenium and technetium compounds are clearly anisotropic with large⁹⁹Tc and^185,187Re hyperfine interactions for one component. High‐field EPR (Q band and W band) has been applied for the elucidation of the EPR parameters of the manganese(0) complex. 

Abstract Dynamic nuclear polarization (DNP) is a powerful tool to enhance the NMR signals of molecules by transferring polarization from unpaired electron spins to nuclei through microwave irradiation. The resulting signal enhancements can enable the analysis of samples that have previously been intractable by NMR spectroscopy, including proteins, nucleic acids, and metabolites in cells. To carry out DNP, the sample is doped with a polarization agent, a biradical containing two nitroxide moieties. DNP applications in cells, however, present significant challenges as nitroxides are often susceptible to the reducing cellular environment. Here, we introduce a novel polarization agent, POPAPOL, that exhibits increased lifetimes under reducing conditions. We also compare its bioresistance and DNP performance with three popular, commercially available polarization agents. Our work indicates that pyrrolidine‐based nitroxides can outperform piperidine‐based nitroxides in cellular environments, and that future polarization agent designs must carefully balance DNP performance and stability for cellular applications.