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

Abstract

Several complexes of Co(II) or Fe(II) with 1,4,7,10-tetraazacyclododecane (CYCLEN) appended with 1,7-(6-methyl)2-picolyl groups are studied as ¹H 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 5.0–5.9μ_BM and 5.3–5.8μ_BM for Co(II) and Fe(II) complexes, respectively. The ¹H 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 °C⁻¹ and 0.49 ppm °C⁻¹, respectively. Fe(II) complexes have less shifted methyl proton resonances and smaller temperature coefficients. The ¹H resonances of [Fe(2MPC)]²⁺ appear at 105 ppm and −46 ppm with corresponding temperature coefficients (CT) of −0.29 ppm °C⁻¹ and 0.22 ppm °C⁻¹, respectively. The relatively narrow linewidths of [Fe(2MPC)]²⁺, however, produce superior CT/FWHM values of 0.44 and 0.31 °C⁻¹ for the N-methyl and picolyl proton resonances where FWHM is the full width at half maximum of the ¹H resonance. The crystal structure of [Co(BMPC)]Cl₂ 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.