Cobalt-59 Nuclear Magnetic Relaxation Studies of Aqueous Octahedral Cobalt(III) Complexes

Abstract

Variable-temperature 59 Co nuclear magnetic longitudinal relaxation and line width measurements have been performed on dilute aqueous solutions of 12 octahedral cobalt(III) complexes spanning a large range of isotropic chemical shifts (-1-12 570 ppm) at two fields (4.7 and 11.7 T). A variable-concentration 59 Co longitudinal relaxation study of Na[Co(ethylenediaminetetraacetate)] was performed, and the results of this study indicated a need for the use of dilute solutions. The quadrupolar mechanism of relaxation was found to be dominant for 59 Co nuclei in diamagnetic octahedral complexes in aqueous solution, and values of the quadrupolar coupling constants in all 12 complexes have been determined. At temperatures above 323 K, the spin-rotation relaxation mechanism contributes to the overall rate of relaxation for 59 Co in cobalt(III) complexes with high local symmetry or large moments of inertia. Previous claims that the chemical shielding anisotropy mechanism of relaxation is also a major contributor to the total 59 Co relaxation in cobalt(III) complexes are apparently unjustified. We believe that the previous incorrect conclusions were due to concentration-dependent relaxation behavior that we and others have recently observed. An approximate absolute chemical shielding scale for cobalt is also presented.

Influence of ligand encapsulation on cobalt-59 chemical-shift thermometry

“…In contrast, the species of highest encapsulation, 4 and 5 , have the shortest T 1 values (346(1) and 323(1) μs, respectively). For 1 and 2 , these values match previously reported results 40–42. Quadrupolar relaxation is also enhanced in systems with a higher 59 Co quadrupole coupling constant, and this constant is smaller for high-symmetry complexes 9.…”

“…This symmetry difference is likely an important contributor to the T 1 of 1 versus 2–5 , but quadrupolar couplings in this latter set of compounds are similar (when known) 42,43. Moreover, solution-phase rotational rates for the series extrema, 1 and 5 , are similar,42 suggesting rotational correlation is also not driving the difference in T 1 across the series. Together, these points suggest that considerations beyond symmetry/rotation define T 1 for these compounds.…”

Ligand Control of 59Co Nuclear Spin Relaxation Thermometry

“…Between the minimum and maximum values of 1 and 6 , absolute changes in Δ T 1 for 2 – 5 are 17(1) ms, 11.2(1) ms, 4.8(2) ms, and 511(7) μs, respectively. The general magnitudes of these values are consistent with previous 59 Co relaxation data on structurally similar cobalt systems [ 30 – 33 ].…”

“…For these complexes, quadrupolar relaxation is expected due to the interaction between the electric quadrupolar moment and the lower-symmetry electric field gradient at the 59 Co nucleus (relative to O h 1 and 2 ). However, the non-linear relaxation behaviors of 1 and 2 suggest different operative relaxation processes of the central 59 Co nucleus [ 30 , 34 ]. For these complexes, curvature in the plots of ln( T 1 /s) vs. T (°C) ( Figure 2b ) show a gradual decline with increasing temperature, indicative of another contributing relaxation mechanism.…”

“…For these complexes, curvature in the plots of ln( T 1 /s) vs. T (°C) ( Figure 2b ) show a gradual decline with increasing temperature, indicative of another contributing relaxation mechanism. The spin–rotation relaxation mechanism is known to contribute to relaxation in similar O h 59 Co complexes, [ 30 , 31 ] thus is the likely origin of the non-linear temperature dependence in 1 and 2 .…”