Relationship between the Thermally Induced Reorientations of Aromatic Solvate Molecules in Cu(hfac)2–Nitroxide Breathing Crystals and the Character of the Magnetic Anomaly

N-Alkylimidazol-5-yl-substituted Nitronyl Nitroxides and Their Mononuclear Cu(II) Complexes: Synthesis, Structure and Magnetic Properties

A novel synthetic approach has been employed to synthesize a series of new nitronyl nitroxides: 2-(1-propyl-1H-imidazol-5-yl)- (Ln-Pr ), 2-(1-isopropyl-1H-imidazol-5-yl)- (Li-Pr ) and 2-(1-butyl-1H-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (Ln-Bu ). The reaction of Cu(hfac)2 with LR in a 1 : 2 ratio yields mononuclear heterospin complexes [Cu(hfac)2 (LR )2 ] (LR =Ln-Pr , Li-Pr , Ln-Bu ), which have a similar crystal structure to the "jumping" crystals [Cu(hfac)2 (LMe )2 ] that exhibit chemomechanical activity. It was shown that an increase in the alkyl substituent R leads to changes in the crystal packing of the molecules and the absence of chemomechanical activity. Furthermore, it was found that two polymorph modifications of the heterospin complex [Cu(hfac)2 (Ln-Pr )2 ] can be obtained, and magnetic properties of [Cu(hfac)2 (Ln-Pr )2 ] strongly depend on the angle between the planes of the paramagnetic fragment O•-N-C=N→O and the imidazole ring in Ln-Pr .

New Ruthenium Nitrosyl Complexes Combining Potentially Photoactive Nitrosyl Group with the Magnetic Nitroxide Radicals as Ligands

Two ruthenium nitrosyl complexes of Na[RuNOCl4L] with nitronyl nitroxide radicals coordinated to ruthenium with N-donor pyridine rings were prepared and described. The crystal structure of both complexes is 1D or 2D polymeric, due to the additional coordination of sodium cation by bridging the chloride ligands or oxygen atoms of nitroxides. Partially, the oligomeric forms remain in the solutions of the complexes in acetonitrile. The magnetic measurements in the solid state demonstrate the presence of antiferromagnetic interactions through the exchange channels, with the distance between paramagnetic centers equal to 3.1-3.9 Å. The electrochemical behavior of the prepared complexes was investigated in acetonitrile solutions.

Three-Dimensional Supramolecular Architectures with MnII Ions Assembled from Hydrogen Bonding Interactions: Crystal Structures and Antiferromagnetic Properties

Two novel cocrystal Mn^II compounds were successfully synthesized. The composition of two kinds crystals correspond to [Mn(hfac)₂L^a ₂·Mn(hfac)₂L^a(H₂O)·Mn(hfac)₂(H₂O)₂] (1) and [Mn(hfac)₂L^b ₂·Mn(hfac)₂(H₂O)₂·0.5(C₆H₁₄)] (2) [L^a = 1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-dihydro-1H-imidazol-2-y1)benzene; L^b = 1-(1'-oxyl-4',4',5',5'-tetramethylimidazolin-2-yl)-3-(1'-oxyl-3'-oxo-4',4',5',5'-tetramethylimidazolin-2-yl)benzene; hfac = hexafluoroacetylacetonato). Surprisingly, the compounds were not polymeric or clusters but, more interestingly, different ratio biradical-metal coordination compound cocrystals. The extensive intramolecular H-bonds are the cause of formation of the cocrystal structures by assembly in the two manganese(II) derivatives; and another factor is the halogen bonds between CF₃ of hfac groups. Furthermore, three-dimensional supramolecular architectures were formed. The magnetic susceptibility of both compounds showed strong antiferromagnetic interactions involving the coordinated radical unit and the metal and lesser contribution from ferromagnetic interactions between the radical units. For compound 1, a good fit was obtained for g _Mn = 2.08, g _rad = 2.00 (fixed), J ₁ = -294.3 cm^-1, J ₂ = 6.2 cm^-1 and J ₃ = 10.8 cm^-1. A reasonable fit for compound 2 was obtained for g _Mn = 2.04, g _rad = 2.00 (fixed), J ₁' = -273.4 cm^-1 and J ₂' = 8.6 cm^-1.

One-dimensional magnetic chain composed of CuII and polychlorinated dipyridylphenylmethyl radical: temperature-dependent Jahn-Teller distortion correlated to π-conjugation and magnetic properties

The one-dimensional coordination polymer composed of Cu^II and an organic radical bisPyTM displayed temperature dependent Jahn-Teller distortion, which is correlated to the π-conjugation mode at the ligand as well as the magnetic properties. The ferromangetic bisPyTM-Cu^II interaction was enhanced by the reorientation of the Cu^II d_x²-y² orbital at low temperature.

Copper–nitroxide based breathing crystals: a unified mechanism of gradual magnetostructural transition supported by quantum chemistry calculations

A unified mechanism, based on the thermal average of two limit phases, governs the gradual magnetic transitions of Cu(hfac)₂L^R complexes.

Light-Induced Spin Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations

The electronic structure and magnetic interactions of three members of the breathing crystal Cu(hfac)₂ L^R family (hfac=hexafluoroacetylacetonato, L^R =pyrazole-substituted nitronyl nitroxides with R=Me, Et, Pr, iPr, Bu ), mainly Cu(hfac)₂ L^Pr (1), Cu(hfac)₂ L^Bu ⋅0.5 C₈ H₁₈ (2) and Cu(hfac)₂ L^Bu ⋅0.5 C₈ H₁₀ (3), have been analyzed by means of periodic plane-wave based DFT+U calculations. These Cu^II -nitroxide based molecular magnets display thermally and optically induced switchable behavior and light-induced excited spin state trapping phenomena. The calculations confirm the presence of temperature-dependent exchange interaction within the spin triads formed by the nitroxide-copper(II)-nitroxide units, in line with the changes observed in the effective magnetic moment. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighboring polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature, and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low-lying conduction states potentially involved in the UV/Vis transitions are located on the nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group. This suggests the possibility of light-induced spin switching for other members of this family. The 500 nm band of the low-temperature phase can be ascribed to a ligand-to-metal charge transfer transition between the nitroxide and Cu bands.

Mechanism of Magnetostructural Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from ab Initio Quantum Chemistry Calculations

The gradual magnetostructural transition in breathing crystals based on copper(II) and pyrazolyl-substituted nitronyl nitroxides has been analyzed by means of DDCI quantum chemistry calculations. The magnetic coupling constants (J) within the spin triads of Cu(hfac)2L(Bu)·0.5C8H18 have been evaluated for the X-ray structures reported at different temperatures. The coupling is strongly antiferromagnetic at low temperature and becomes ferromagnetic when the temperature increases. The intercluster magnetic coupling (J') is antiferromagnetic and shows a marked dependence on temperature. The magnetostructural transition can be reproduced using the calculated J values for each structure in the simulation of the magnetic susceptibility. However, the μ(T) curve can be improved nicely by considering the coexistence of two phases in the transition region, whose ratio varies with temperature corresponding to both the weakly and strongly coupled spin states. These results complement a recent VT-FTIR study on the parent Cu(hfac)2L(Pr) compound with a gradual magnetostructural transition.

Spatial distribution of phases during gradual magnetostructural transitions in copper(ii)–nitroxide based molecular magnets

A thorough analysis of EPR and XRD data led to conclusions on the spatial distribution of phases in copper–nitroxide molecular magnets.

Synthesis, crystal structures and magnetic behaviour of four coordination compounds constructed with a phosphinic amide-TEMPO radical and [M(hfac)2] (M = Cu(II), Co(II) and Mn(II))

In the present work we describe the synthesis, crystal structures and magnetic properties of four coordination compounds obtained by assembling a new phosphinic amide containing the TEMPO moiety, 1-piperidinyloxy-4-[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl radical (dppnTEMPO), with [M(hfac)2] building blocks (M = Cu(II), Co(II), Mn(II)). The crystal structures of the coordination compounds revealed the usefulness of the functionalized radical to provide discrete or extended architectures. In the copper compound () the ligand is coordinated through the oxygen atom of the NP[double bond, length as m-dash]O linkage to the metal, which exists in a square pyramidal or octahedral geometry. For the cobalt and manganese complexes (), both the phosphinic amide and the nitroxide oxygen atoms are involved in the coordination to the metal leading to one dimensional systems. In the cobalt complex () an interesting spin topology in the zig-zag chain was obtained due to the oxygen atom of the phosphinic amide group being μ2 coordinated to two cobalt(ii) ions. The magnetic behaviour of the coordination compounds shows overall antiferromagnetic interactions involving the metal ion and the organic radical. DFT calculations were performed in order to assign the main path for the magnetic interactions.

Cooperative spin-crossover-like transitions in the inhomogeneous chain of exchange clusters

A statistical theory of temperature-induced spin-crossover-like transitions has been developed for spin chains with a few exchange clusters in a unit cell. Using static magnetic susceptibility measurements, a general expression for the effective magnetic moment μ(eff) of exchange clusters is obtained for an arbitrary structure of cluster terms. For heterospin Cu(hfac)2L(R) complexes ("breathing crystals") with a "head-to-head" chain motif, a two-point approximation for a partition function of the Jahn-Teller Cu(+2) paramagnetic center (spin 1/2) is suggested. Theoretical results are compared with available experimental data on thermal spin-crossover-like transitions in the "breathing crystal" compounds. It is shown that the model developed is able to successfully describe both of the cases of smooth and abrupt (cooperative) spin transitions for chains of three-spin exchange clusters. Spin-crossover-like transitions of a new type with step-wise changes in a positive exchange integral are predicted.

Magnetic Mn and Co complexes with a large polycyclic aromatic substituted nitronylnitroxide

2-(1'-Pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (PyrNN) was reacted with M(hfac)(2) (M = Mn(II) and Co(II), hfac = hexafluoracetylacetonate) to give two isostructural ML(2) stoichiometry M(hfac)(2)(PyrNN)(2) complexes and a ML stoichiometry one-dimensional (1-D) polymer chain complex [Mn(hfac)(2)(PyrNN)]. The ML(2) complexes have similar crystal structures with monoclinic unit cells, in which one NO unit from each PyrNN ligand is bonded to the transition metal on cis vertices of a distorted octahedron. The major magnetic interactions are intracomplex metal-to-radical exchange (J), and intermolecular exchange across a close contact between the uncoordinated NO units (J'). For M = Mn(II) an approximate chain model fit gives g = 2.0, J = (-)125 cm(-1) and J' = (-)49 cm(-1); for M = Co(II), g = 2.4, J = (-)180 cm(-1), and J' = (-)70 cm(-1). Hybrid density functional theory (DFT) computations modeling the intermolecular exchange by using only the radical units across the close contact are in good accord with the estimated values of J'. The chain type complex structure shows solvent incorporation for overall structure [Mn(hfac)(2)(PyrNN)](n)·0.5(CHCl(3))·0.5(C(7)H(16)). Both NO groups of the PyrNN ligand are complexed to form helical chains, with very strong metal to radical antiferromagnetic exchange that gives overall ferrimagnetic behavior.