Field- and Temperature-Dependent Paramagnetic Relaxation Enhancements in Co(II) Trispyrazolylmethanes.

A comprehensive field- and temperature-dependent examination of nuclear magnetic resonance paramagnetic relaxation enhancements (PREs) for the constitutive protons of [Co(Tpm) 2 ][BF 4 ] 2 is presented. Data for an apically substituted derivative clearly establish that bis-Tpm complexes of Co(II) undergo Jahn-Teller dynamics about the molecular threefold axis. PREs from the parent Tpm complex were used to numerically extract the electron relaxation times ( T 1e ). The Tpm complex showed field-dependent behavior, with an approximately 40% higher activation barrier than the related trispyrazolylborate (Tp) complex, based on fits to the T 1e vs T , B 0 data. Analysis of the field-dependent line widths revealed a surprisingly large contribution from susceptibility (Curie) relaxation (20-35% at the highest field), and a molecular radius (9.5 Å) that is consistent with a tightly associated counterion slowing rotation in solution. Density functional theory showed a shared vibration that is consistent with the Jahn-Teller and appears proportionately higher in energy in [Co(Tpm) 2 ] 2+ . Complete active-space self-consistent field calculations support ascribing electron relaxation to enhanced mixing of the two E g orbital sets that accompanies the tetragonal distortion and the differences in electron correlation times to the higher Jahn-Teller activation barrier in [Co(Tpm) 2 ] 2+ .