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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.
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Counterion influence on dynamic spin properties in a V( iv ) complex
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.
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- Award ID(s):
- 1836537
- PAR ID:
- 10078613
- Date Published:
- Journal Name:
- Chemical Science
- ISSN:
- 2041-6520
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8SC04122A
