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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 (T1 and T2) 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 T1 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 T2 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 T1 and T2 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. 

Achieving control of phase memory relaxation times ( T m ) in metal ions is an important goal of molecular spintronics. Herein we provide the first evidence that nuclear-spin patterning in the ligand shell is an important handle to modulate T m in metal ions. We synthesized and studied a series of five V( iv ) complexes with brominated catecholate ligands, [V(C 6 H 4−n Br n O 2 ) 3 ] 2− ( n = 0, 1, 2, and 4), where the 79/81 Br and 1 H nuclear spins are arranged in different substitutional patterns. High-field, high-frequency (120 GHz) pulsed electron paramagnetic resonance spectroscopic analysis of this series reveals a pattern-dependent variation in T m for the V( iv ) ion. Notably, we show that it is possible for two molecules to have starkly different (by 50%) T m values despite the same chemical composition. Nuclear magnetic resonance analyses of the protons on the ligand shell suggest that relative chemical shift ( δ ), controlled by the patterning of nuclear spins, is an important underlying design principle. Here, having multiple ligand-based protons with nearly identical chemical shift values in the ligand shell will, ultimately, engender a short T m for the bound metal ion. 

Using transition metal ions for spin-based applications, such as electron paramagnetic resonance imaging (EPRI) or quantum computation, requires a clear understanding of how local chemistry influences spin properties. Herein we report a series of four ionic complexes to provide the first systematic study of one aspect of local chemistry on the V( iv ) spin – the counterion. To do so, the four complexes (Et 3 NH) 2 [V(C 6 H 4 O 2 ) 3 ] ( 1 ), ( n -Bu 3 NH) 2 [V(C 6 H 4 O 2 ) 3 ] ( 2 ), ( n -Hex 3 NH) 2 [V(C 6 H 4 O 2 ) 3 ] ( 3 ), and ( n -Oct 3 NH) 2 [V(C 6 H 4 O 2 ) 3 ] ( 4 ) were probed by EPR spectroscopy in solid state and solution. Room temperature, solution X-band ( ca. 9.8 GHz) continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy revealed an increasing linewidth with larger cations, likely a counterion-controlled tumbling in solution via ion pairing. In the solid state, variable-temperature (5–180 K) X-band ( ca. 9.4 GHz) pulsed EPR studies of 1–4 in o -terphenyl glass demonstrated no effect on spin–lattice relaxation times ( T 1 ), indicating little role for the counterion on this parameter. However, the phase memory time ( T m ) of 1 below 100 K is markedly smaller than those of 2–4 . This result is counterintuitive, as 2–4 are relatively richer in 1 H nuclear spin, hence, expected to have shorter T m . Thus, these data suggest an important role for counterion methyl groups on T m , and moreover provide the first instance of a lengthening T m with increasing nuclear spin quantity on a molecule.