Kira E. Vostrikova

Coordination of organic biradicals to transition metals is a promising route to multifunctional materials, but controlling the metal oxidation state during complexation remains challenging. Here we attempted to prepare a ruthenium(III) complex with the 2,6-bis(1-oxyl-4,4,5,5-tetramethyl-4,5-dihydro-1-H-imidazol-2-yl)pyridine (bisimpy), tridentate imino-nitroxide, from [RuCl3(PhCN)3]. Instead, redox processes between the ligand and the ruthenium precursor in solution yielded the ruthenium(II) complex [Ru(bisimpy)(PhCN)Cl2]. The product was characterized by single-crystal X-ray diffraction and UV-vis absorption in solution supported by TD-DFT and BS-DFT calculations. Structural analysis confirms the tridentate coordination of bisimpy to a centrosymmetric Ru(II) ion, rendering the two iminonitroxide fragments equivalent. UV-vis spectra show intense ligand-centered bands in the UV region and MLCT transitions in the near-IR region absent in the free biradical. TD-DFT computation reproduces the experimental spectra assigning the features to metal-to-ligand, and mixed ligand–ligand/metal–ligand transitions. Notably, BS-DFT computation predicts a dramatic change in the intramolecular exchange between the two radical centers: whereas the free biradical is weakly ferromagnetic, coordination to the diamagnetic Ru(II) center switches the coupling to strongly antiferromagnetic through a metal-mediated superexchange pathway. These results show that the imino-nitroxide biradical directs the reaction toward Ru(II) rather than the targeted Ru(III) species and demonstrate that a formally diamagnetic d⁶ metal can actively modulate radical–radical magnetic communication. In the Ru(II) complex, the diamagnetic low-spin 4d⁶ center (S = 0) does not participate in direct magnetic exchange with the radicals, but it mediates a strong antiferromagnetic coupling between the two radical centers through superexchange involving the 4d metal orbitals.

The development of lanthanide-based molecular magnetic materials incorporating organic radicals requires a fundamental understanding of how ligand modification influences magnetic exchange interactions while preserving the coordination environment around the metal center. In this predictive computational study, the electronic structures and magnetic properties of two hypothetical Gd³⁺ complexes with donor–acceptor semiquinone–nitronylnitroxide biradicals, Tp₂Gd(SQNN) (1) and Tp₂Gd(SQPhNN) (2), were investigated using ab initio calculations and compared with the previously characterized monoradical complex Tp₂Gd(SQ); Tp is hydro-tris(1-pyrazolyl)borate. For the first time, the related zinc complex Tp^CumZn(SQNN) (3) was also computed using its experimental geometry, providing a reference point for the reliability of the computational protocol. The inclusion of dynamic correlation at the NEVPT2 (N-Electron Valence State Perturbation Theory, 2nd order) level was essential for correctly predicting the antiferromagnetic nature of the Gd³⁺–biradical interaction, while the SA-CASSCF (State-Average Complete Active Space Self-Consistent Field) alone yielded a ferromagnetic ground state. The computed exchange interaction in the –2J formalism between Gd³⁺ and the biradical in complexes 1 and 2 (J ≈ –2.1 cm⁻¹) is more than two times weaker than that in Tp₂Gd(SQ) (J = –5.4 cm⁻¹). The intraligand exchange parameters J_SQ–NN = 690 cm⁻¹ and J_SQ–PhNN = 122 cm⁻¹ reflect strong ferromagnetic coupling within the biradical moieties. Application of the valence bond configuration interaction model yielded electronic coupling matrix elements H_DA of 15,283 cm⁻¹ and 6546 cm⁻¹ for the yttrium analogues of 1 and 2, respectively. The calculated magnetic susceptibility data for complexes 1 and 2 closely resemble the behavior of Tp₂Gd(SQ), although the characteristic temperatures differ due to the unequal exchange coupling strengths. This study demonstrates that modern computational methods enable consistent prediction of the key magnetic characteristics of heterospin systems based on their possible molecular structures.

The development of lanthanide-based molecular magnetic materials incorporating organic radicals requires a fundamental understanding of how ligand modification influences magnetic exchange interactions while preserving the coordination environment around the metal center. In this predictive computational study, the electronic structures and magnetic properties of two hypothetical gadolinium(III) complexes with donor–acceptor semiquinone–nitronylnitroxide biradicals, Tp₂Gd(SQNN) (1) and Tp₂Gd(SQPhNN) (2), were investigated using SA-CASSCF/NEVPT2 calculations and compared with the previously characterized monoradical complex Tp₂Gd(SQ). The inclusion of dynamic correlation at the NEVPT2 level was essential for correctly predicting the antiferromagnetic nature of the Gd³⁺–biradical interaction, whereas SA-CASSCF alone yielded a ferromagnetic ground state. The computed exchange interaction between Gd³⁺ and the biradical in complexes 1 and 2 (J ≈ –2.6 cm⁻¹) is approximately two times weaker than that in Tp₂Gd(SQ) (J = –5.4 cm⁻¹). The intraligand exchange parameters JSQ–NN = 679.1 cm⁻¹ and JSQ–PhNN = 71.0 cm⁻¹ reflect strong ferromagnetic coupling within the biradical moieties. Application of the valence bond configuration interaction model yielded electronic coupling matrix elements HDA of 15283 cm⁻¹ and 6546 cm⁻¹ for the yttrium analogues of 1 and 2, respectively. The calculated magnetic susceptibility data for complexes 1 and 2 closely resemble the behavior of Tp₂Gd(SQ), although the characteristic temperatures differ due to the unequal exchange coupling strengths. This study demonstrates that modern computational methods enable reliable prediction of the key magnetic characteristics of heterospin systems based on their possible molecular structures.

Phenylene-based bis-oxamate polydentate ligands offer a unique opportunity for creating a large variety of coordination compounds, in which paramagnetic metal ions are strongly magnetically coupled. The employment of imino nitroxyl (IN) radicals as supplementary ligands confers numerous benefits, including the strong ferromagnetic interaction between Ni and IN. Furthermore, the chelating IN can act as a capping ligand, thereby impeding the formation of coordination polymers. In this study, we present the molecular and crystal structure and experimental and theoretical magnetic behavior of an exceptional neutral trinuclear complex [Ni(L³⁻)₂(IN)₃]∙5CH₃OH (1) (L is N,N′-1,3-phenylenebis-oxamic acid; IN is [4,4,5,5-tetramethyl-2-(6-methylpyridin-2-yl)-4,5-dihydro-1H-imidazol-1-yl]oxidanyl radical) with a cyclic triangular arrangement. Moreover, in this compound three Ni²⁺ ions are linked by the two bis-oxamate ligands playing a rare tritopic function due to an unprecedented triple deprotonation of the related meta-phenylene-bis(oxamic acid). The main evidence of such a deprotonation of the ligand is the neutrality of the cluster, since there are no anions or cations compensating for its charge in the crystals of the compound. Despite the presence of six possible magnetic couplings in the trinuclear cluster 1, its behavior was reproduced with a high degree of accuracy using a three-J model and ZFS, under the assumption that the three different Ni-IN interactions are equal to each other, whereas only two equivalent-in-value Ni-Ni interactions were taken into account, with the third one being equated to zero. Our study indicates the presence of two opposite-in-nature types of magnetic interactions within the triangular core. DFT and CASSCF/NEVPT2 calculations were completed to support the experimental magnetic data simulation.

Phenylene-based bis-oxamate polydentate ligands offer unique opportunities for creating a large variety of coordination compounds in which paramagnetic metal ions are strongly magnetically coupled. The employment of iminonitroxyl (IN) radicals as supplementary ligands confers numerous benefits, including the strong ferromagnetic interaction between Ni and IN. Furthermore, the chelating IN can function as a blocking ligand, thereby impeding the formation of coordination polymers. In this study, we present the molecular, crystal structure, experimental, and theoretical magnetic behavior of an exceptional neutral trinuclear complex [Ni(L3−)2(IN)3]∙5MeOH (1) with a cyclic triangular arrangement. Moreover, in this compound three Ni2+ ions are linked by the two bis-oxamate ligands playing rare tritopic function due to an unprecedented triple deprotonation of the related meta-phenylene-bis(oxamic acid). Despite the presence of six possible magnetic couplings in the three-nuclear cluster 1, its behavior was reproduced well using a three-J model and ZFS, under the assumption that the three different Ni-IN interactions are equal to each other, whereas only two equivalent in value Ni-Ni interactions are taken into account, and the third one was equated to zero. These studies indicate the presence of two opposite in nature type of magnetic interactions within triangular core. DFT and CASSCF/NEVPT2 calculations were completed to support the experimental magnetic data simulation.

In this paper, we investigate the magnetic exchange interaction and magnetization dynamics of two new members of the [LnRad(NO₃)₃] family, where Rad is a tripodal nitroxide, and Ln is Er(III) or Yb(III), having the prolate type electron density. Single OK crystal and powder X-ray diffraction studies showed that these complexes are isostructural with their previously investigated Y, Gd, Dy, Tm, Tb, Eu, and Lu congeners. A magnetometric investigation, supported by ab initio calculations, showed the presence of antiferromagnetic coupling between the lanthanide ion and the radical in both compounds with estimated J values of ≈7 and ≈20 cm⁻¹ for Er and Yb, respectively (+J S_eff∙ S formalism).

A combined EPR and magnetic study of [LnRad(NO₃)₃], supported by calculations, unveiled an unprecedentedly large antiferromagnetic coupling between Eu³⁺ and radical spins with slow magnetization dynamics below 20 K discovered for Eu and Lu species.

Small bifunctional molecules are attractive for use as models in different areas of knowledge. How can their functional groups interact in solids? This is important to know for the prediction of the physical and chemical properties of the materials based on them. In this study, two new hydrogen-bonded organic frameworks (HOFs) based on sterically demanding molecular compounds, bis(1-hydroxy-2-methylpropane-2-aminium) sulfate (1) and 2-methyl-4-oxopentan-2-aminium hydrogen ethanedioate hydrate (2), were synthesized and fully characterized by means of FTIR and NMR spectroscopies, as well as by X-ray powder diffraction and thermogravimetric analyses. Their molecular and crystal structures were established through single-crystal X-ray diffraction analysis. It was shown that both compounds have a layered structure due to the formation of a 2D hydrogen-bonding network, the layers being linked by systematically arranged Van der Waals contacts between the methyl groups of organic cations. To unveil some dependencies between the chemical nature of bifunctional molecules and their solid structure, Hirschfeld surface (HS) analysis was carried out for HOFs 1, 2, and their known congeners 1-hydroxy-2-methylpropan-2-aminium hemicarbonate (3) and 1-hydroxy-2-methylpropan-2-aminium (1-hydroxy-2-methylpropan-2-yl) carbamate (4). HS was performed to quantify and visualize the close intermolecular atomic contacts in the crystal structures. It is clearly seen that H–H contacts make the highest contributions to the amino alcohol based compounds 1, 3 and 4, with a maximal value of 65.2% for compound 3 having CO₃²⁻ as a counterion. A slightly lower contribution of H–H contacts (64.4%) was found for compound 4, in which the anionic part is represented by 1-hydroxy-2-methylpropan-2-yl carbamate. The significant contribution of the H–H contacts in the bifunctional moieties is due to the presence of a quaternary carbon atom with a short three-carbon chain.

Three Eu³⁺ complexes containing a neutral tripodal ligand possessing a predictable coordination mode have been obtained and studied. The trispyrazolylmethane complexes have an aqua ligand in the coordination sphere, forming both the mononuclear species [Eu(HCPz₃)H₂O(NO₃)₃] and the dimer [Eu(HCPz₃)H₂O(CF₃SO₃)₃]₂, having a Chinese lantern structure, whereas the use of the methylated tripod leads to the water-free complex, [Eu(HC(Pz^Me₂)₃)(NO₃)₃]. A qualitative analysis of the magnetic susceptibility of polycrystalline samples demonstrated that the magnetic properties can be described using a simple Van Vleck formula with spin–orbit coupling parameter (λ = 383 ÷ 406 cm⁻¹) close to the values for free Eu³⁺ ions. The stereochemical analysis of the coordination environment of [Ln(HC(Pz^Me₂)₃)(NO₃)₃] has shown that these complexes can be used as diamagnetic model systems to obtain information on the crystal field effects in the paramagnetic monoradical complexes, [LnRad(NO₃)₃], since both types of compounds have the same type of coordination polyhedron (symmetry point group D_3h) and very close Ln–donor atom distances.