Crystal structure and magnetic properties of two nitronyl nitroxide biradicals and of their copper(II) complexes

Supramolecular heptanuclear Ln-Cu complexes involving nitronyl nitroxide biradicals: structure and magnetic behavior

Four novel heptanuclear Ln-Cu complexes with the formula [Ln₂Cu(hfac)₈(NITPhTzbis)₂][LnCu(hfac)₅(NITPhTzbis)]₂ (LnCu = YCu 1, TbCu 2, DyCu 3 and HoCu 4; hfac = hexafluoroacetylacetonate) were successfully constructed by employing the triazole functionalized nitronyl nitroxide biradical ligand NITPh-Tzbis (NITPh-Tzbis = 5-(1,2,4-triazolyl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). These hetero-tri-spin complexes are composed of two biradical-bridged dinuclear [(LnCu(hfac)₅(NITPhTzbis)] units and one trinuclear [Ln₂Cu(hfac)₈(NITPhTzbis)₂] unit which form a heptanuclear supramolecular structure through π-π interactions. Magnetic susceptibility investigations indicate that ferromagnetic exchange interactions dominate at low temperature for this supramolecular system which can be attributed to the Ln-nitroxide exchange and intramolecular NIT⋯NIT coupling mediated by the m-phenylene moiety. The DyCu derivative was found to exhibit a slow magnetic relaxation behavior.

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.

Synthesis, crystal structure, and magnetic properties of a nitronyl nitroxide biradical-coordinated copper(II) complex

A coordination compound constructed from a nitronyl nitroxide biradical NITPh(3-NIT) and CuII(hfac)2(H2O)2 building blocks [NITPh(3-NIT) = 1,3-bis(1′-oxyl-3′-oxido-4′,4′, 5′, 5′-tetramethyl-4,5-dihydro-1 H-imidazol-2-y1)-benzene, hfac = hexafluoroacetylacetonato] is successfully synthesized. The crystal structure is determined by X-ray single-crystal diffraction. The asymmetric complex {[(NITPh(3-NIT)]Cu(hfac)2} consists of one Cu(II) ion and two >N–O• groups and adopts a distorted triangular bipyramid with a penta-coordinated central copper(II) atom and three hfac oxygen atoms at the base and a >N–O• oxygen atom and one hfac oxygen atom at the apices. Intramolecular O. . .O bonding and π–π stacking interactions are observed between molecules. A magnetic susceptibility study of the coordination compound shows antiferromagnetic interactions between Cu(II) ions and >N–O• groups and very weak ferromagnetic interactions between Cu(II) ions and the free >N–O• group through O. . .O bonding between the nitroxide group oxygen atom and the oxygen atom of hfac.

Nitronyl Nitroxide Biradical-Based Binuclear Lanthanide Complexes: Structure and Magnetic Properties

Employing a new nitronyl nitroxide biradical NITPhPzbis(NITPhPzbis = 5-(1-pyrazolyl)-1,3-bis(1’-oxyl-3’-oxido-4’,4’,5’,5’-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene), a series of 2p-4f complexes [Ln2(hfac)6(H2O)(NITPhPzbis)] (LnIII = Gd1, Tb2, Dy3; hfac = hexafluoroacetylacetonate) were successfully synthesized. In complexes 1–3, the designed biradical NITPhPzbis coordinates with two LnIII ions in chelating and bridging modes to form a four-spin binuclear structure. Direct-current magnetic study of Gd analogue indicates that ferromagnetic exchange exists between the Gd ion and the radical while antiferromagnetic coupling dominates between two mono-radicals. Dynamic magnetic data show that the χ” signals of complex 3 exhibit frequency dependence under zero field, demonstrating slow magnetic relaxation behavior in complex 3. And the estimated values of Ueff and τ0 are about 8.4 K and 9.1 × 10−8 s, respectively.

Molecule Isomerism Modulates the Diradical Properties of Stable Singlet Diradicaloids

Inclusion of quinoidal cores in conjugated hydrocarbons is a common strategy to modulate the properties of diradicaloids formed by aromaticity recovery within the quinoidal unit. Here we describe an alternative approach of tuning of diradical properties in indenoindenodibenzothiophenes upon anti → syn isomerism of the benzothiophene motif. This alters the relationship of the S atom with the radical center from linear to cross conjugation yet retains the same 2,6-naphtho conjugation pattern of the rearomatized core. We conduct a full comparison between the anti and syn derivatives based on structural, spectroscopic, theoretical, and magnetic measurements, showing that these systems are stable open-shell singlet diradicaloids that only access their triplet state at elevated temperatures.

Slow relaxation of magnetization for a Tb derivative in a biradical-based lanthanide chain

Four novel lanthanide-biradical chains have been obtained and the Tb derivative reveals field-induced slow magnetic relaxation behavior with an anisotropy barrier Δ/k_B = 22.08 K.

Exchange interactions in photoinduced magnetostructural states of copper(ii)–nitroxide spin dyads

We investigate intra- and intercluster exchange couplings in photoinduced states of copper(ii)–nitroxide based molecular magnets (“breathing crystals”) using variable-temperature EPR spectroscopy.

Heterometallic Ln-Cu complexes derived from a phenyl pyrimidyl substituted nitronyl nitroxide biradical

Utilizing a novel nitronyl nitroxide biradical bisNITPhPyrim involving a pyrimidine group ([5-(5-pyrimidyl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)]benzene), three heterometallic Ln-Cu complexes with formulas [Ln(hfac)₃Cu(hfac)₂(bisNITPhPyrim)] (Ln^III = Gd 1, Tb 2, and Dy 3; hfac = hexafluoroacetylacetonate) have been achieved. In these complexes, the Cu^II ions are linked by N atoms of pyrimidine rings of bisNITPhPyrim radicals to form a one-dimensional chain structure, whereas each Ln^III ion is chelated with two neighboring NO groups of two NIT moieties of the radical. Magnetic studies reveal the presence of ferromagnetic interactions between the Gd^III ion and coordinated NO groups, and between the pyrimidine-bridged copper(ii) spins, which are quantified by a magnetic model, giving J₁ = 4.20 cm^-1 and J₃ = 0.50 cm^-1 (J₁ and J₃ are magnetic exchanges for Gd^III-ON and Cu^II-Cu^II, respectively). Interestingly, ac magnetic measurements show that complex 3 exhibits slow relaxation of the magnetization.

Pressure-Controlled Migration of Paramagnetic Centers in a Heterospin Crystal

A study of the single-crystal-to-single-crystal transformation induced by temperature variation for the chain polymer Cu(II) complex with nitronyl nitroxide showed that an increase in the hydrostatic pressure of up to ∼0.07 GPa completely changes the intracrystalline displacements of molecules relative to one another. This, in turn, significantly affects the interaction energy of the unpaired electrons of the paramagnetic centers and hence the form of the temperature dependence of the magnetic susceptibility χT. The cooling of crystals under normal conditions causes a rearrangement of the intrachain exchange clusters {>N-•O-Cu(II)-O•-N<} accompanied by a shortening of the distances between the paramagnetic centers. This changes the character of exchange interactions and generates multistage spin transitions. An increase in the hydrostatic pressure leads to a drastic change in the O···O distances between the nitroxyl fragments of adjacent chains, an increase in the antiferromagnetic exchange between them, and complete suppression of spin transitions.

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.

Carbene derived diradicaloids - building blocks for singlet fission?

Organic singlet diradicaloids promise application in non-linear optics, electronic devices and singlet fission. The stabilization of carbon allotropes/cumulenes (C1, C2, C4) by carbenes has been equally an area of high activity. Combining these fields, we showed recently that carbene scaffolds allow as well for the design of diradicaloids. Herein, we report a comprehensive computational investigation (CASSCF/NEVPT2; fractional occupation DFT) on the electronic properties of carbene-bridge-carbene type diradicaloids. We delineate how to adjust the properties of these ensembles through the choice of carbene and bridge and show that already a short C2 bridge results in remarkable diradicaloid character. The choice of the carbene separately tunes the energies of the S1 and T1 excited states, whereas the bridge adjusts the overall energy level of the excited states. Accordingly, we develop guidelines on how to tailor the electronic properties of these molecules. Of particular note, fractional occupation DFT is an excellent tool to predict singlet-triplet gaps.

A family of lanthanide compounds with reduced nitronyl nitroxide diradical: syntheses, structures and magnetic properties

A new diradical based on the pyrazine ring and a series of Ln₂ compounds with its reduced form have been synthesized and characterized structurally and magnetically.

The role of the multiconfigurational character of nitronyl-nitroxide in the singlet–triplet energy gap of its diradicals

CAS(2,2) reference may not be sufficient for the computation of singlet–triplet energy gap by DDCI.

Functionalized nitronyl nitroxide biradical bridged one-dimensional lanthanide chains: slow magnetic relaxation in the Tb and Dy analogues

The first examples of Ln-nitronyl nitroxide 1D chains based on the functionalized nitronyl nitroxide biradical have been obtained, and complexes Dy and Tb exhibit slow magnetic relaxation behavior.

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.

C(sp(2))-coupled nitronyl nitroxide and iminonitroxide diradicals

Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide (DR1) and with two iminonitroxide (DR2) fragments linked through the C(sp2) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR1 and DR2 have singlet ground states. The singlet–triplet energy splitting (2J) is low (J/k=−7.4 for DR1 and −6.0 K for DR2), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac)2] with DR1 gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac)2}2(DR1)(H2O)] complex, sufficiently strong ferromagnetic interactions (J1/k=42.7 and J2/k=14.1 K) between two coordinating CuII ions and DR1 were revealed. In [{Cu(hfac)2}2(DR1)(H2O)][Cu(hfac)2(H2O)], the very strong and antiferromagnetic (J/k=−416.1 K) exchange interaction between one of the coordinating CuII ions and DR1 is caused by the very short equatorial CuO bond length (1.962 Å).

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.

A New Quinoxalinyl-Substituted Nitronyl Nitroxide Radical and its Five-Spin CuII and Four-Spin MnII Complexes: Syntheses, Crystal Structures, and Magnetic Properties

A new radical of QNXL-2NIT (1) (QNXL-2NIT = 2-(2-quinoxalinyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and its complexes [Cu(hfac)2]3(QNXL-2NIT)2·2CHCl3 (2) and [Mn(hfac)2]2(QNXL-2NIT)2 (3) (hfac = 1,1,1,5,5,5-hexfluoroacetylacetone) were first reasonably designed and synthesised in this paper. The X-ray structure determination revealed that by introducing the quinoxalinyl ring group to the nitronyl nitroxide radical, QNXL-2NIT affords more coordination sites for metal ions. We successfully obtained its copper complex with linear tri-nuclear five-spin structure. However, when we substituted MnII as the central metal, complex 3 exhibits a rectangle-like four-spin framework containing two MnII ions and two radicals in one molecule. The magnetic study of the two complexes shows that there are strong antiferromagnetic interactions between the radical and the terminal CuII ions with g = 2.25, J = –291.67 cm–1 and zJ′ = –0.25 cm–1 and the dominating interactions between the MnII ion and the radical in complex 3 are also antiferromagnetic with the parameters of g = 1.99, J 1 = –77.00 cm–1 and J 2 = 0.34 cm–1.

A Polymorphic Crystal Structure and Magnetic Properties of a Nitronyl Nitroxide Radical: PhNIT

Nitroxides radicals are attractive tools for study in biochemistry and materials science. A polymorphic crystal of a nitroxide radical, 4,4,5,5-tetramethyl-2-phenylimidazoline-1-oxyl 1-oxide was prepared and structurally characterised. The compound crystallised in the monoclinic, space group C2/c with a = 11.170(6) Å, b = 10.417(5) Å, c = 10.995(5) Å, β = 108.059(9)°, V = 1216.3(10) Å, C13H17N2O2, Mr = 233.29, Dc = 1.274 g cm−3, μ (MoKα) = 0.087 mm−1, Z = 4 and F(000) = 500. The structure was solved by direct methods and refined by least-squares techniques to the final R1 = 0.0626 and wR2 = 0.1679 for 1070 observed reflections (I > 2σ(I)). The magnetic susceptibility was measured.