This report details how the design of specific nuclear-spin patterns on ligands modulates spin-relaxation times in a set of open-shell vanadium(iv) complexes.
Studying the correlation between temperature-driven molecular structure and nuclear spin dynamics is essential to understanding fundamental design principles for thermometric nuclear magnetic resonance spin-based probes. Herein, we study the impact of progressively encapsulating ligands on temperature-dependent 59Co T1 (spin–lattice) and T2 (spin–spin) relaxation times in a set of Co(III) complexes: K3[Co(CN)6] (1); [Co(NH3)6]Cl3 (2); [Co(en)3]Cl3 (3), en = ethylenediamine); [Co(tn)3]Cl3 (4), tn = trimethylenediamine); [Co(tame)2]Cl3 (5), tame = triaminomethylethane); and [Co(dinosar)]Cl3 (6), dinosar = dinitrosarcophagine). Measurements indicate that 59Co T1 and T2 increase with temperature for 1–6 between 10 and 60 °C, with the greatest ΔT1/ΔT and ΔT2/ΔT temperature sensitivities found for 4 and 3, 5.3(3)%T1/°C and 6(1)%T2/°C, respectively. Temperature-dependent T2* (dephasing time) analyses were also made, revealing the highest ΔT2*/ΔT sensitivities in structures of greatest encapsulation, as high as 4.64%T2*/°C for 6. Calculations of the temperature-dependent quadrupolar coupling parameter, Δe2qQ/ΔT, enable insight into the origins of the relative ΔT1/ΔT values. These results suggest tunable quadrupolar coupling interactions as novel design principles for enhancing temperature sensitivity in nuclear spin-based probes.
Interstitial patterning
of nuclear spins is a nascent design principle for controlling electron
spin superposition lifetimes in open-shell complexes and solid-state
defects. Herein we report the first test of the impact of the patterning
principle on ligand-based nuclear spin dynamics. We test how substitutional
patterning of 1H and 79/81Br nuclear spins on
ligands modulates proton nuclear spin dynamics in the ligand shell
of metal complexes. To do so, we studied the 1H nuclear
magnetic resonance relaxation times (T
1 and T
2) of a series of eight polybrominated
catechol ligands and six complexes formed by coordination of the ligands
to a Ti(IV) ion. These studies reveal that 1H T
1 values can be enhanced in the individual ligands by
a factor of 4 (from 10.8(3) to 43(5) s) as a function of substitution
pattern, reaching the maximum value for 3,4,6-tribromocatechol. The T
2 for 1H is also enhanced by a factor
of 4, varying by ∼14 s across the series. When complexed, the
impact of the patterning design strategy on nuclear spin dynamics
is amplified and 1H T
1 and T
2 values vary by over an order of magnitude.
Importantly, the general trends observed in the ligands also match
those when complexed. Hence, these results demonstrate a new design
principle to control 1H spin dynamics in metal complexes
through pattern-based design strategies in the ligand shell.
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