Iron and cobalt ion complexes utilizing the TACN (1,4,7-triazacyclononane) azamacrocycle framework that is appended with N-heterocyclic donor groups, such as 1H-pyrazol-3-methylene and 1-methylimidazol-2-methylene, are redox-active. These Co(III)/Co(II) and Fe(III)/Fe(II) complexes demonstrate reversible CV waves with the redox potentials spanning from -120 to 380 mV vs. NHE. The Fe(L1) complex (Figure 1) is an example of redox switch between two paramagnetic high-spin (HS) Fe(II) (3d6, S = 2) and low-spin (LS) Fe(III) (3d5, S = 1/2) states in aqueous solution. These complexes are characterized by the effective magnetic moment measurements in solution, as well as by EPR studies of the LS Fe(III) species. The hyperfine-shifted proton resonances of [Fe(L1)]2+ and [Fe(L1)]3+ in deuterium oxide (Figure 1) demonstrate promise for 1H NMR chemical shift spectroscopy (paraSHIFT) applications. The exchangeable protons of 1H-pyrazol-3-methylene pendents are utilized in redox-responsive MR imaging using the Chemical Exchange Saturation Transfer (CEST) technique. The CEST signal is turned on for HS Co(II) and LS Fe(III) states depending on the redox conditions. Kinetic studies of the Fe(III)/Fe(II) and Co(III)/Co(II) redox reactions were done in the presence of reducing (Na2S2O4, L-Cys) and oxidizing (H2O2, O2) agents. Moreover, the complexes are kinetically inert over a wide range of pH values, and in the presence of biologically relevant cations (Cu2+, Zn2+) and anions (PO43-, CO32- and Cl-). These redox-active complexes represent a new family of the spin state switches promising as MR contrast agents for biological redox imaging.

Redox-triggered Fe(III)/Fe(II) and Co(III)/Co(II) spin state switches for MRI applications

252nd National Meeting (2016)

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