Unprecedented Nitronyl Nitroxide Bridged 3d–4f Complexes: Structure and Magnetic Properties

Multifunctional properties of {CuLn} systems involving nitrogen-rich nitronyl nitroxide: single-molecule magnet behavior, luminescence, magnetocaloric effects and heat capacity

A series of nitrogen-rich nitronyl nitroxide radical PPNIT (1)-based (PPNIT = 2-(1-(pyrazin-2-yl)-1H-pyrazole)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) 3d-4f ring-shaped tetranuclear clusters [Ln2Cu2(hfac)10(PPNIT)2(H2O)2]·CHCl3 (LnIII = Gd 2, Tb 3, Dy 4; hfac = hexafiuoroacetylacetonate) with multifunctional properties were isolated. The magnetic behavior, luminescence and heat capacity of the 3d-4f complexes were investigated, displaying interesting multiple properties of the molecular materials. The Gd derivative shows a magnetocaloric effect with the maximum entropy change (-ΔSm) of 15.3 J kg-1 K-1 at 2 K for ΔH = 70 kOe. The Tb cluster exhibits spin glass behavior and the characteristic fluorescence emission of the TbIII ion, while the Dy cluster exhibits SMM behaviour, and the heat capacity has been investigated. Notably, in nitronyl nitroxide radical-metal systems, the investigation of diverse properties is still scarce so far. This work can pave the way towards the synthesis of multifunctional materials that combine SMM behavior, and optical or/and thermodynamic properties.

A nitronyl nitroxide and its two 1D chain Cu–Tb complexes: synthesis, structures, and magnetic properties

A new nitronyl nitroxide, namely NIT-Ph-p-OCH₂trz (1), and two types of one-chain Cu–Tb complexes, namely [TbCu(hfac)₅NIT-Ph-p-OCH₂trz·0.5C₆H₁₄]_n (2a) and [LnCu(hfac)₅NIT-Ph-p-OCH₂trz]_n (2b) (NIT-Ph-p-OCH₂trz = 2-(4-((1H-1,2,4-triazol-1-yl)methoxy)phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate), have been successfully synthesized simultaneously through reacting nitronyl nitroxide radical NIT-Ph-p-OCH₂trz with Cu(hfac)₂ and Ln(hfac)₃, and the molecular structures have been elucidated via single-crystal X-ray structural analysis. 2a features a ‘ladder-like’ chain structure, while 2b displays a linear chain structure built up by Ln(hfac)₃ units bridged with the NO groups of radical ligands. The additional Cu(ii) ions are coordinated to the nitrogen atoms of the triazole units. Nonzero out-of-phase signals are observed for the Tb derivatives (2a and 2b) and they exhibit frequency-dependent out-of-phase signals indicating a single-chain magnet behavior.

A new nitronyl nitroxide and two types of one-chain Cu–Tb complexes have been successfully synthesized, and the molecular structures have been elucidated via single-crystal X-ray structural analysis.

Synthesis of a series of hetero-multi-spin Ln2Cu3 complexes based on a methyl-pyrazole nitronyl nitroxide radical with slow magnetic relaxation behaviors

Multinuclear hetero-tri-spin complexes based on a methyl-pyrazole nitronyl nitroxide radical, namely, [Ln2Cu3(hfac)12(4-NIT-MePyz)4] (Ln = Gd(1), Tb(2), Dy(3); hfac = hexafluoroacetylacetone; 4-NIT-MePyz = 2-{4-(1-methyl)-pyrazolyl}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been successfully obtained through a one-pot reaction of the radical ligand (4-NIT-MePyz) with Cu(hfac)2 and Ln(hfac)3. These 2p-3d-4f complexes exhibit five-nuclear structures with the sequence [Cu-Rad-Ln-Rad-Cu-Rad-Ln-Rad-Cu], in which each 4-NIT-MePyz radical acting as a bidentate bridging ligand is coordinated to one Ln(hfac)3 unit through one oxygen atom of the NO groups and to one Cu(hfac)2 unit with one nitrogen atom from the pyrazole ring. For complex 1, based on the spin Hamiltonian calculations and MAGPACK program, it is concluded that there exist ferromagnetic couplings between GdIII and NIT radicals, as well as between CuII and free radicals with J1 = 6.8(1) and J3 = 1.3(2) cm-1, respectively, and antiferromagnetic interactions between radical and radical with J2 = -2.8(5) cm-1. Complex 2 shows frequency-dependent out-of-phase signals under a zero or 2000 Oe dc field indicating single-molecule magnetic behavior.

A loop chain and a three-dimensional network assembled from a multi-dentate nitronyl nitroxide radical and M(hfac)2 (M = CoII, CuII)

A multi-dentate nitronyl nitroxide radical Nit-Ph-3,5-bIm (Nit-Ph-3,5-bIm = 2-[3,5-bis(1-imidazole)-phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) reacted with Co(hfac)2·2H2O or Cu(hfac)2 to yield two novel 3d-radical complexes {[Co(hfac)2]2(Nit-Ph-3,5-bIm)}n1 and {[Cu(hfac)2]7(Nit-Ph-3,5-bIm)3}n2 (hfac- = hexafluoroacetylacetonate). In both compounds, the Nit-Ph-3,5-bIm radical ligand behaves as a tetradentate ligand to link four M(hfac)2 units in a μ4-η1:η1:η1:η1-coordination mode via its two NO groups and two N atoms of imidazole, generating a unique loop chain for 1 and a three-dimensional framework for 2. The magnetic studies revealed a strong antiferromagnetic Co-ON exchange interaction in 1 while a ferromagnetic Cu-ON interaction is observed in 2. The dc magnetic behaviors of two complexes are analyzed by means of appropriate magnetic models. Furthermore, ac magnetic susceptibilities of the Co complex reveal slow relaxation of magnetization.

Two-dimensional Co-Ln networks bridged by phenyl pyrimidyl substituted nitronyl nitroxides: structural and magnetic properties

Two-dimensional hetero-tri-spin 2p-3d-4f coordination polymers involving Co(ii) and Ln(iii) ions assembled with a polydentate paramagnetic nitronyl nitroxide ligand are reported. [Ln(hfac)3{Co(hfac)2}2(NITPhPyrim)2] (LnIII = Gd 1, Tb 2; hfac = hexafluoroacetylacetonate, NITPhPyrim = 2-[4-(5-pyrimidyl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) consists of a two-dimensional network in which the NITPhPyrim ligand is coordinated to the metal ions by means of its pyrimidine and aminoxyl (NO) groups. In the layer, the Tb center interacts with two NO groups from two different radical ligands, and each pyrimidine connects two Co(hfac)2 units through its two nitrogen atoms to form one-dimensional Co(ii) arrays. The magnetic behavior of the two compounds results from concomitant ferro- and antiferromagnetic interactions between the spin carriers and the spin-orbit effect from Co(ii) as revealed by modeling performed for the Gd(iii) derivative.

A functional nitroxide ligand builds up two 2p–3d complexes with different spin ground states and a 2p–3d–4f chain of rings

Three new 2p–3d and 2p–3d–4f complexes derived from a functional nitroxide radical with different spin ground states and spin topologies have been synthesized.

Slow Magnetic Relaxation in Ladder-Type and Single-Strand 2p-3d-4f Heterotrispin Chains

Ladder-type and chain 2p-3d-4f complexes based on a bridging nitronyl nitroxide radical, namely, [LnCu(hfac)₅(NIT-Ph-p-OCH₂trz)]·0.5C₆H₁₄ [Ln = Y (1a), Dy (1b)] and [LnCu(hfac)₅(NIT-Ph-p-OCH₂trz)] [Ln = Y (2a), Dy (2b); NIT-Ph-p-OCH₂trz = 2-[4-[(1H-1,2,4-triazol-1-yl)methoxy]phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate) have been successfully achieved through a one-pot reaction of the NIT-Ph-p-OCH₂trz radical with Cu(hfac)₂ and Ln(hfac)₃·2H₂O. Complexes 1a and 1b feature a ladder-like structure, where the rails are made of Ln(III) and Cu(II) ions alternatively bridged by nitronyl nitroxide and the triazole units while the NIT-Ph-p-OCH₂trz moieties act as the rungs of the ladder. Complexes 2a and 2b consist of one-dimensional nitronyl nitroxide bridged Ln coordination polymers with dangly Cu(II) units connected to the triazole moieties. All of compounds exhibit ferromagnetic NIT-Dy and/or NIT-Cu interactions. Both Dy derivatives (1b and 2b) show frequency-dependent out-of-phase magnetic susceptibility signals in a zero field indicating slow magnetic relaxation behavior.

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.

A chimeric design of heterospin 2p–3d, 2p–4f, and 2p–3d–4f complexes using a novel family of paramagnetic dissymmetric compartmental ligands

Original end-off bicompartmental ligands bearing a nitronyl–nitroxide arm have been designed for synthesizing various heterospin molecular systems.

LnIII–CoII heterometallic chains based on pyridine substituted nitronyl nitroxides

Unprecedented nitronyl nitroxide bridged Ln^III–Co^II compounds have been obtained. Different magnetic relaxation behaviors were observed for two Dy complexes.

2p–3d–4f hetero-tri-spin molecule-based magnetic compounds

The most up-to-date advances in the design and synthesis of molecular magnetic materials by means of a 2p–3d–4f strategy are reviewed.

Nitronyl nitroxide based 2p-3d-4f chains with the magnetocaloric effect and slow magnetic relaxation

Four new nitronyl nitroxide radical based hetero-tri-spin one-dimensional compounds, namely [{Ln(hfac)3}3{Cu(hfac)2}{NIT-Ph(OMe)2}4]n (Ln = Gd (1), Tb (2), Dy (3), Er (4); hfac = hexafluoroacetylacetonate; NIT-Ph(OMe)2 = 2-(2',4'-dimethoxyphenyl)-4,4,5,5-tetramethyl-imidazolyl-1-oxyl-3-oxide) have been successfully prepared. Single crystal X-ray crystallographic analysis reveals that complexes 1-4 possess a 1D chain structure with a repeating [Cu-Rad-Ln-Rad-Ln-Rad-Ln-Rad] moiety in which Ln(hfac)3 and Cu(hfac)2 units are bridged by nitronyl nitroxide radicals through the NO groups. DC magnetic studies found that ferromagnetic interactions between metals and the coordinated NO groups are active in all four compounds. The Tb derivative displays frequency dependent ac magnetic susceptibilities, indicating slow magnetic relaxation behavior. The Gd complex shows an important cryogenic magnetocaloric effect with the entropy change (-ΔSm) of 13.5 J kg(-1) K(-1) at 2 K and a magnetic field of 7 T, representing the first example of Gd-radical molecular species exhibiting the magnetocaloric effect.

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.

Tetrazolate–azido–copper(ii) coordination polymers: tuned synthesis, structure, and magnetic properties

Two antiferrimagnetic tetrazolate–azido–copper(ii) coordination polymers, [Cu(tz)(N₃)]_n (1) and [Cu(tz)(N₃)(NH₃)₂]_n (2) with distinct topological structures were synthesized under different reaction conditions and structurally characterized.

Exploiting verdazyl radicals to assemble 2p–3d–4f one-dimensional chains

Exploiting the coordination abilities of an isopropyl substituted verdazyl radical to transition metal and lanthanide(iii) ions represents an efficient approach to produce a system incorporating 2p, 3d and 4f spin carriers.

Slow magnetic relaxation in two-dimensional 3d–4f complexes based on phenyl pyrimidyl substituted nitronyl nitroxide radicals

Three isostructural two-dimensional nitronyl nitroxide bridged 3d–4f complexes have been achieved. The Tb complex shows field-induced two-step magnetic relaxation behavior.

Magnetic exchange in {GdIII–radical} complexes: method assessment, mechanism of coupling and magneto-structural correlations

Which functional is best suit to calculate Js in lanthanide–radical pairs?

Mn(ii) coordination polymers assembled from 8 or 9-connected trinuclear secondary building units: topology analysis and research of magnetic properties

Two highly-connected trinuclear Mn(ii)-cluster polymers were successfully fabricated. The FC and ZFC data of 1 show a divergence over a temperature range of 40–3.5 K. 2 features a ferrimagnetic chain with a (5/2, 10/2) spin topology.