Six-coordinate Iron(II) and Cobalt(II) paraSHIFT Agents for Measuring Temperature by Magnetic Resonance Spectroscopy

Paramagnetic Fe(II) and Co(II) complexes are utilized as the first transition metal examples of (1)H NMR shift agents (paraSHIFT) for thermometry applications using Magnetic Resonance Spectroscopy (MRS). The coordinating ligands consist of TACN (1,4,7-triazacyclononane) and CYCLEN (1,4,7,10-tetraazacyclododecane) azamacrocycles appended with 6-methyl-2-picolyl groups, denoted as MPT and TMPC, respectively. (1)H NMR spectra of the MPT- and TMPC-based Fe(II) and Co(II) complexes demonstrate narrow and highly shifted resonances that are dispersed as broadly as 440 ppm. The six-coordinate complex cations, [M(MPT)](2+) and [M(TMPC)](2+), vary from distorted octahedral to distorted trigonal prismatic geometries, respectively, and also demonstrate that 6-methyl-2-picolyl pendents control the rigidity of these complexes. Analyses of the (1)H NMR chemical shifts, integrated intensities, line widths, the distances obtained from X-ray diffraction measurements, and longitudinal relaxation time (T1) values allow for the partial assignment of proton resonances of the [M(MPT)](2+) complexes. Nine and six equivalent methyl protons of [M(MPT)](2+) and [M(TMPC)](2+), respectively, produce 3-fold higher (1)H NMR intensities compared to other paramagnetically shifted proton resonances. Among all four complexes, the methyl proton resonances of [Fe(TMPC)](2+) and [Co(TMPC)](2+) at -49.3 ppm and -113.7 ppm (37 °C) demonstrate the greatest temperature dependent coefficients (CT) of 0.23 ppm/°C and 0.52 ppm/°C, respectively. The methyl groups of these two complexes both produce normalized values of |CT|/fwhm = 0.30 °C(-1), where fwhm is full width at half-maximum (Hz) of proton resonances. The T1 values of the highly shifted methyl protons are in the range of 0.37-2.4 ms, allowing rapid acquisition of spectroscopic data. These complexes are kinetically inert over a wide range of pH values (5.6-8.6), as well as in the presence of serum albumin and biologically relevant cations and anions. The combination of large hyperfine shifts, large temperature sensitivity, increased signal-to-noise ratio, and short T1 values suggests that these complexes, in particular the TMPC-based complexes, show promise as paraSHIFT agents for thermometry.