Metal Complexes of Bridging Neutral Radical Ligands: pymDTDA and pymDSDA

Origin of Weak Magnetic Coupling in a Dimanganese(II) Complex Bridged by the Tetrathiafulvalene-Tetrathiolate Radical

Magnetic exchange coupling (J) between different spin centers plays a crucial role in molecule-based magnetic materials. Direct exchange coupling between an organic radical and a metal is frequently stronger than superexchange through diamagnetic ligands, and the strategy of using organic radicals to engender desirable magnetic properties has been an area of active investigation. Despite significant advances and exciting bulk properties, the magnitude of J for radical linkers bridging paramagnetic centers is still difficult to rationally predict. It is thus important to elucidate the features of organic radicals that govern this parameter. Here, we measure J for the tetrathiafulvalene-tetrathiolate radical (TTFtt3-•) in a dinuclear Mn(II) complex. Magnetometry studies show that the antiferromagnetic coupling in this complex is much weaker than that in related Mn(II)-radical compounds, in contrast to what might be expected for the S-based chelating donor atoms of TTFtt. Experimental and computational analyses suggest that this small J coupling may be attributed to poor overlap between Mn- and TTFtt-based magnetic orbitals coupled with insignificant spin density on the coordinating S-atoms. These factors override any expected increase in J from the comparatively strong S-donors. This work elucidates the magnetic coupling properties of the TTFtt3-• radical for the first time and also demonstrates how multiple competing factors must be considered in rationally designing organic radical ligands for molecular-based magnetic compounds.

Syntheses and magnetic properties of a bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine and its metal complexes

A novel bis-bidentate nitronyl nitroxide radical based on triazolopyrimidine, NIT-2-TrzPm (NIT-2-TrzPm = (2-(2'-triazolopyrimidine)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide)) and six new transition metal complexes of this ligand, namely [M(hfac)₂(NIT-2-TrzPm)]·CH₂Cl₂ (M = Mn (1Mn) and Co (2Co)), [M(hfac)₂]₂(NIT-2-TrzPm) (M = Mn (3Mn) and Co (4Co)), [Mn(NIT-2-TrzPm)₂(MeOH)₂](ClO₄)₂·MeOH (5Mn), and [Co(NIT-2-TrzPm)₂(MeOH)₂]₂(ClO₄)₄·4MeOH (6Co) were prepared and characterized structurally and magnetically. These complexes can be selectively synthesized by controlling the reaction ratio of M(hfac)₂·2H₂O to the radical ligand (for 1Mn to 4Co) or using metal perchlorates as the starting materials (for 5Mn and 6Co). Single crystal X-ray crystallographic analyses confirmed that 1Mn and 2Co are isostructural 3d-2p M^II-radical complexes, in which the NIT-2-TrzPm radical acts as a terminal bidentate ligand chelating to one 3d ion, while 3Mn and 4Co are isostructural 3d-2p-3d M^II-radical-M^II complexes with the NIT-2-TrzPm radical acting as a bridging ligand between two 3d ions. For complexes 5Mn and 6Co, two NIT-2-TrzPm ligands from the equatorial positions coordinate with the metal center to form the 2p-3d-2p structures with the axial positions occupied by two methanol molecules. Magnetic analysis on the Mn^II complexes revealed the existence of a strong antiferromagnetic interaction between the Mn^II and the NIT radical spin, while weak ferromagnetic coupling for Mn⋯Mn and Rad⋯Rad in the Mn-NIT-Mn and Rad-Mn-Rad spins was confirmed. Interestingly, although the NIT-bridged complexes 3Mn and 4Co possess significantly different magnetic anisotropy, field-induced slow magnetic relaxation can be observed in both complexes, which was assigned to the phonon bottleneck effect for 3Mn and field-induced SMM behavior for 4Co. To the best of our knowledge, 3Mn is the first example of the NIT-bridged binuclear Mn^II complex undergoing slow magnetic relaxation.

Heisenberg Spin Chains via Chalcogen Bonding: Noncovalent S···O Contacts Enable Long-Range Magnetic Order

The new radical ligand 5,8-dimethyl-1,4-dioxonaphtho[2,3-d][1,2,3]dithiazolyl (1) is reported. Two crystal polymorphs, 1α and 1β, differing in their pancake-bonded dimerization motif and S···O contact network, are identified. The self-assembly of Mn(II) metal ions with 1 leads to the formation of [Mn(hfac)₂]₃(1)₂ that exhibits a Mn(II)-radical-Mn(II)-radical-Mn(II) linear arrangement of three Mn(hfac)₂ units bridged by two radical ligands (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-). Characterization by single-crystal X-ray diffraction of this Mn(II) complex packing structure reveals close noncovalent S···O contacts between the [Mn(hfac)₂]₃(1)₂ units in one dimension along the b-c direction. The magnetic properties of the coordination complex are characterized by dc and ac susceptibility measurements on a microcrystalline solid. The magnetic data down to 4.8 K indicate the presence of effective ferromagnetic interactions (J/k_B = +0.16 K) between the molecular S_T = 13/2 units along the supramolecular chain involving noncovalent S···O contacts. Below 2.9 K, a non-zero out-of-phase component appears in the ac susceptibility, indicating the presence of a three-dimensional magnetic phase transition.

Synthesis, crystal structures, and magnetic properties of nitroxide radical-coordinated manganese(II) and cobalt(II) complexes

Coordination complexes constructed from the nitronyl nitroxide radical NIT-Pyra-3-Isobu and MII(hfac)2(H2O)2 as building blocks (NIT-Pyra-3-Isobu = 2-(3-isobutyl-pyrazole)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hfac = hexafluoroacetylacetonato, and M = Mn or Co) are successfully synthesized. The crystal structures of the coordination compounds reveal the usefulness of the functionalized radical in providing discrete or extended architectures. Single-crystal X-ray diffraction analyses indicated that the ligand is coordinated through the oxygen atom of NO• and the N atom of the pyrazole linkage to the metal, and the metals adopt an octahedral geometry leading to one-dimensional zig-zag chain systems. The magnetic behavior of the manganese and cobalt shows overall antiferromagnetic interactions between the metal center and the organic radical.

Automated oxidation-state assignment for metal sites in coordination complexes in the Cambridge Structural Database

The Cambridge Structural Database (CSD) currently contains over 400 000 transition-metal-containing entries, however many entries still lack curated oxidation-state assignments. Surveying and editing the remaining entries would be far too resource- and time-intensive to be carried out manually. Here, a highly reliable automated workflow for oxidation-state assignment in transition-metal coordination complexes via CSD Python API (application programming interface) scripts is presented. The strengths and limitations of the bond-valence sum (BVS) method are discussed and the use of complementary methods for improved assignment confidence is explored. In total, four complementary techniques have been implemented in this study. The resulting workflow overcomes the limitations of the BVS approach, widening the applicability of an automated procedure to more CSD entries. Assignments are successful for 99% of the cases where a high consensus between different methodologies is observed. Out of a total number of 54 999 unique metal atoms in a test dataset, the procedure yielded the correct oxidation state in 47 072 (86%) of cases.

High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)2Br2] (L = tert-Butyl 5-Phenyl-2-pyridyl Nitroxide)

Complexation of nickel(II) bromide with tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) gave two morphs of doubly chelated [Ni(phpyNO)₂Br₂] as a 2p-3d-2p heterospin triad. The α phase crystallizes in the orthorhombic space group Pbcn. An asymmetric unit involves a half-molecule. The torsion angle around Ni-O-N-C_2py is as small as 6.5(3)° at 100 K and 7.0(6)° at 400 K, guaranteeing an orthogonal arrangement between the magnetic radical π* and metal 3d_x²-y² and 3d_z² orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2J/k_B = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The β phase crystallizes in the monoclinic space group P2₁/n. The whole molecule is an independent unit. The Ni-O-N-C_2py torsion angles are 24.2(6) and 37.2(5)° at 100 K and 10.4(7) and 25.9(6)° at 400 K. A magnetic study revealed a very gradual and nonhysteretic spin transition. An analysis based on the van't Hoff equation gave a successful fit with the spin-crossover temperature of 134(1) K, although the susceptibility did not reach the theoretical high-spin value at 400 K. Density functional theory calculation on the β phase showed ground S_total = 0 in the low-temperature structure while S_total = 2 in the high-temperature structure, supporting the synchronized exchange coupling switch on both sides. Consequently, the β phase can be recognized as an "incomplete spin crossover" material, as a result of conflicting thermal depopulation effects in a high-temperature region.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex

The radical bridged compound [(Ni(TPMA))₂-μ-bmtz^•-](BF₄)₃·3CH₃CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 Ni^II centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm^-1 (-2J_Ni-radS_NiS_rad). DFT calculations support the existence of strong ferromagnetic exchange.

Syntheses, structures and magnetic properties of the lanthanide complexes of the pyrimidyl-substituted nitronyl nitroxide radical

Four 2p-4f Ln^III-radical complexes, [(NIT-2-Pm)Ln(hfac)₃]·0.5C₇H₁₆ (NIT-2-Pm = 2-pyrimidyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide, hfac = hexafluoroacetylacetonato, Ln = Tb (1), Dy (2), Ho (3) and Er (4)), and four 4f-2p-4f Ln^III-radical-Ln^III complexes, [(μ-NIT-2-Pm)Ln₂(hfac)₆(H₂O)₂]·0.5C₇H₁₆ (Ln = Tb (5), Dy (6), Ho (7) and Er (8)) have been synthesized and characterized structurally and magnetically. These compounds can be selectively obtained by controlling the reaction ratio of Ln(hfac)₃·2H₂O to the radical ligand NIT-2-Pm. The crystal structures show that in the former four complexes 1-4, the NIT-2-Pm radical acts as a terminal bidentate ligand chelating to one Ln^III ion, while in 5-8, the NIT-2-Pm acts as a bridging ligand linking two Ln^III ions to form a binuclear three-spin system. Magnetic studies revealed that complexes 1-4 and 6 show frequency-dependent ac magnetic susceptibilities, suggesting a possible single-molecule magnet behavior. To the best of our knowledge, complexes 3 and 4 are the first Ho-NIT and Er-NIT compounds showing slow magnetic relaxation. Compounds 5-8 represent a rare family of compounds showing the NIT bridged 4f-2p-4f three-spin motif, while complex 6 is a rare NIT bridged multinuclear lanthanide compound possessing SMM-like behaviour. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these compounds suggests weak antiferromagnetic coupling between the Ln^III and NIT radical in 1-8 and weak ferromagnetic Ln^III-Ln^III interactions in 5-8.

Reversible crystal-to-crystal chiral resolution: making/breaking non-bonding S⋯O interactions

Unconventional non-covalent interactions are the key to reversible chiral resolution in polycrystalline samples of a small, achiral molecule.

A designed room-temperature triplet ligand from pyridine-2,6-diyl bis(tert-butyl nitroxide)

The magnetic study on newly developed 4-mesitylpyridine-2,6-diyl bis(tert-butyl nitroxide) shows that almost the whole population has a ground triplet state at room temperature, and the ability of complex formation as a tridentate triplet ligand was proven with a diamagnetic yttrium(iii) ion.

Syntheses and magnetic properties of a pyrimidyl-substituted nitronyl nitroxide radical and its cobalt(ii) complexes

A new bis-bidentate pyrimidyl-substituted nitronyl nitroxide radical and two cobalt(ii) complexes of this ligand were synthesized and characterized.

Strong Direct Magnetic Coupling in a Dinuclear Co(II) Tetrazine Radical Single-Molecule Magnet

The ligand-centered radical complex [(CoTPMA)2 -μ-bmtz(.-) ](O3 SCF3 )3 ⋅CH3 CN (bmtz=3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA=tris-(2-pyridylmethyl)amine) has been synthesized from the neutral bmtz precursor. Single-crystal X-ray diffraction studies have confirmed the presence of the ligand-centered radical. The Co(II) complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal-radical coupling combined with positive axial and strong rhombic anisotropic contributions from the Co(II) ions.