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1. Programmable Nuclear-Spin Dynamics in Ti(IV) Coordination Complexes

   https://doi.org/10.1021/acs.inorgchem.0c00244  

   Johnson, Spencer H.; Jackson, Cassidy E.; Zadrozny, Joseph M. (April 2020, Inorganic Chemistry)

   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. 

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2. Counterion influence on dynamic spin properties in a V( iv ) complex

   https://doi.org/10.1039/C8SC04122A  

   Lin, Chun-Yi; Ngendahimana, Thacien; Eaton, Gareth R.; Eaton, Sandra S.; Zadrozny, Joseph M. (January 2019, Chemical Science)

   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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3. Ligand-Nuclei Effects on Spin Relaxation in V(IV) Complexes

   https://doi.org/10.1021/acs.inorgchem.5c01930  

   Martinez, Roxanna; Üngör, Ökten; Jackson, Cassidy E; Khan, Firoz_S T; van_Tol, Johan; Zadrozny, Joseph M (July 2025, Inorganic Chemistry)

   Understanding how magnetic nuclei affect spin relaxation is vital for designing robust spin coherence in magnetic materials and molecules. A key question is the extent that magnetic nuclei close to a spin (e.g., in the ligand shell of a metal complex) influence relaxation and how it varies over different classes of nuclei. Herein, we apply high-field EPR, X-band EPR, and ac magnetic susceptibility techniques to a family of five V(IV) complexes of the type [V(C6X4O2)3]2–, featuring five different sets of 12 nuclear spins on the ligand shell: X = 1H (1), 2H (2), 19F (3), 35/37Cl (4), and 79/81Br (5). We found several unanticipated results in these studies. For example, at high-field/-frequency, we found that compound 1, with the highest-magnetic-moment ligand nuclear spins, exhibits the longest phase memory relaxation times of the series. Furthermore, at lower fields, we found that the spin–lattice relaxation time and its field dependence were ligand-dependent, despite no obvious change in electronic structure across the five species. Based on this data, structural comparisons, and Raman spectroscopic data, we tentatively conclude that the spin–lattice relaxation properties of 1–5 stem from fine-tuning of the local magnetic environment with changing identity of the X atoms. 

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4. Metal‐Templated, Tight Loop Conformation of a Cys‐X‐Cys Biomimetic Assembles a Dimanganese Complex

   https://doi.org/10.1002/anie.201913259  

   Le, Trung; Nguyen, Hao; Perez, Lisa M.; Darensbourg, Donald J.; Darensbourg, Marcetta Y. (January 2020, Angewandte Chemie International Edition)

   Abstract With the goal of generating anionic analogues to MN₂S₂⋅Mn(CO)₃Br we introduced metallodithiolate ligands, MN₂S₂²⁻prepared from the Cys‐X‐Cys biomimetic, ema⁴⁻ligand (ema=N,N′‐ethylenebis(mercaptoacetamide); M=Ni^II, [V^IV≡O]²⁺and Fe^III) to Mn(CO)₅Br. An unexpected, remarkably stable dimanganese product, (H₂N₂(CH₂C=O(μ‐S))₂)[Mn(CO)₃]₂resulted from loss of M originally residing in the N₂S₂⁴⁻pocket, replaced by protonation at the amido nitrogens, generating H₂ema²⁻. Accordingly, the ema ligand has switched its coordination mode from an N₂S₂⁴⁻cavity holding a single metal, to a binucleating H₂ema²⁻with bridging sulfurs and carboxamide oxygens within Mn‐μ‐S‐CH₂‐C‐O, 5‐membered rings. In situ metal‐templating by zinc ions gives quantitative yields of the Mn₂product. By computational studies we compared the conformations of “linear” ema⁴⁻to ema⁴⁻frozen in the “tight‐loop” around single metals, and to the “looser” fold possible for H₂ema²⁻that is the optimal arrangement for binucleation. XRD molecular structures show extensive H‐bonding at the amido‐nitrogen protons in the solid state. 

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5. Cerium( iv ) complexes with guanidinate ligands: intense colors and anomalous electronic structures

   https://doi.org/10.1039/D0SC05193D  

   Qiao, Yusen; Yin, Haolin; Moreau, Liane M.; Feng, Rulin; Higgins, Robert F.; Manor, Brian C.; Carroll, Patrick J.; Booth, Corwin H.; Autschbach, Jochen; Schelter, Eric J. (March 2021, Chemical Science)

   null (Ed.)

   A series of cerium( iv ) mixed-ligand guanidinate–amide complexes, {[(Me 3 Si) 2 NC(N i Pr) 2 ] x Ce IV [N(SiMe 3 ) 2 ] 3−x } + ( x = 0–3), was prepared by chemical oxidation of the corresponding cerium( iii ) complexes, where x = 1 and 2 represent novel complexes. The Ce( iv ) complexes exhibited a range of intense colors, including red, black, cyan, and green. Notably, increasing the number of the guanidinate ligands from zero to three resulted in significant redshift of the absorption bands from 503 nm (2.48 eV) to 785 nm (1.58 eV) in THF. X-ray absorption near edge structure (XANES) spectra indicated increasing f occupancy ( n f ) with more guanidinate ligands, and revealed the multiconfigurational ground states for all Ce( iv ) complexes. Cyclic voltammetry experiments demonstrated less stabilization of the Ce( iv ) oxidation state with more guanidinate ligands. Moreover, the Ce( iv ) tris(guanidinate) complex exhibited temperature independent paramagnetism (TIP) arising from the small energy gap between the ground- and excited states with considerable magnetic moments. Computational analysis suggested that the origin of the low energy absorption bands was a charge transfer between guanidinate π orbitals that were close in energy to the unoccupied Ce 4f orbitals. However, the incorporation of sterically hindered guanidinate ligands inhibited optimal overlaps between Ce 5d and ligand N 2p orbitals. As a result, there was an overall decrease of ligand-to-metal donation and a less stabilized Ce( iv ) oxidation state, while at the same time, more of the donated electron density ended up in the 4f shell. The results indicate that incorporating guanidinate ligands into Ce( iv ) complexes gives rise to intense charge transfer bands and noteworthy electronic structures, providing insights into the stabilization of tetravalent lanthanide oxidation states. 

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