Fe(II) and Co(II) N-methylated CYCLEN complexes as paraSHIFT agents with large temperature dependent shifts

Several complexes of Co(II) or Fe(II) with 1,4,7,10-tetraazacyclododecane (CYCLEN) appended with 1,7-(6methyl) 2-picolyl groups are studied as H-1 NMR paraSHIFT agents (paramagnetic shift agents) for the registration of temperature. Two of the complexes, [Co(BMPC)](2+) and [Fe(BMPC)](2+), contain methyl groups only on the methyl picolyl pendents. Two other complexes, [Co(2MPC)](2+) and [Fe(2MPC)](2+), contain picolyl groups and also methyl groups on the macrocyclic amines. All macrocyclic complexes are in high spin form as shown by solution magnetic moments in the range of 5.0-5.9 mu BM and 5.3-5.8 mu BM for Co(II) and Fe(II) complexes, respectively. The H-1 NMR spectra of both of the Fe(II) complexes and one of the Co(II) complexes are consistent with a predominant diastereomeric form in deuterium oxide solutions. The highly shifted methyl proton resonances for [Co(2MPC)](2+) appear at 164 and -113 ppm for macrocycle and pendent picolyl methyls and show temperature coefficients of -0.58 ppm degrees C-1 and 0.49 ppm degrees C-1, respectively. Fe(II) complexes have less shifted methyl proton resonances and smaller temperature coefficients. The H-1 resonances of [Fe(2MPC)](2+) appear at 105 ppm and -46 ppm with corresponding temperature coefficients (CT) of -0.29 ppm degrees C-1 and 0.22 ppm degrees C-1, respectively. The relatively narrow linewidths of [Fe(2MPC)](2+), however, produce superior CT/FWHM values of 0.44 and 0.31 degrees C-1 for the N-methyl and picolyl proton resonances where FWHM is the full width at half maximum of the H-1 resonance. The crystal structure of [Co(BMPC)]Cl-2 shows a six-coordinate Co(II) bound to the macrocyclic amines and two pendent picolyl groups. The distorted trigonal prismatic geometry of the complex resembles that of an analogous complex containing four 6-methyl-2-picolyl groups, in which only two picolyl pendents are coordinated.