Attempted synthesis of axial-equatorial pyrazine-bridged copper(II) complexes toward homometallic ferromagnetic compounds

The introduction of a relatively bulky alkyl group at 2-position of pyrazine afforded unique μ-pyz complexes, namely, dinuclear [{Cu(hfac)₂}₂(μ-2-isobutylpyrazine)], trinuclear [{Cu(hfac)₂}₃(μ-2-isopropylpyrazine)₂], and polymeric [Cu(hfac)₂(μ-ibupyz)]_n (Hhfac stands for 1,1,1,5,5,5-hexafluoropentane-2,4-dione). These compounds have an axial-equatorial dinuclear moiety, which is expected to show ferromagnetic interaction. However, they exhibited practically negligible or very weak antiferromagnetic interaction. This may be due to violation of orthogonality because of the steric hindrance of the isobutyl or isopropyl groups. We analyzed the relationship between the structural deformation parameter defined as the N(eq)?N(ax)-Cu angle and the exchange coupling parameter. A critical angle, at which the sign of the magnetic coupling changes from positive to negative, is estimated to be 165(2)°. The present results are reasonably explained in this relation.     

Dinuclear copper complexes with cyanamide derivatives as bridging ligands

Three mixed-valence copper complexes [{Cu(phen)₂}₂(μ-L)](PF₆)₂ (where phen = 1,10-phenanthroline, L = 1,4-dicyanamidobenzene (dicyd)), 1,4-dicyanamido-2,5-dimethylbenzene (Me₂dicyd) and 1,4-dicyanamido-2,5-dichlorobenzene (Cl₂dicyd), and one dinuclear Cu(II) complex [{Cu(phen)₂}₂(μ-apc)](PF₆)₃ (where apc = monoanion of 4-azo(phenylcyanamido)benzene) have been prepared and characterized by elemental analysis, IR and electronic absorption spectroscopies and cyclic voltammetry. [{Cu(phen)₂}₂(μ-apc)](PF₆)₃ · 2CH₃COCH₃ crystallized in the triclinic system and both five-coordinate Cu(II) ions in the dinuclear unit are linked through a bridging 4-azo(phenylcyanamido)benzene (apc) ligand. The cyanamide group (NCN) of the bridging ligand is coordinated to Cu(II) ions through the cyano-nitrogen and amido-nitrogen. The bond length between Cu(1) and cyano-nitrogen is slightly larger than that formed by Cu(2) and amido-nitrogen. The angular structural index parameters, τ, for Cu(1) and Cu(2) are 0.9 and 0.5, respectively. The copper(II) atoms display a different geometry with a N₅ chromophore group. The intra Cu?Cu separation is 5.156(1) Å. All of the dicyd dinuclear copper complexes show radical anion absorption.     

Mono- and Binuclear Copper(II) and Nickel(II) Complexes with the 3,6-Bis(picolylamino)-1,2,4,5-Tetrazine Ligand

Four new compounds of formulas [Cu(hfac)₂(L)] (1), [Ni(hfac)₂(L)] (2), [{Cu(hfac)₂}₂(µ-L)]·2CH₃OH (3) and [{Ni(hfac)₂}₂(µ-L)]·2CH₃CN (4) [Hhfac = hexafluoroacetylacetone and L = 3,6-bis(picolylamino)-1,2,4,5-tetrazine] have been prepared and their structures determined by X-ray diffraction on single crystals. Compounds 1 and 2 are isostructural mononuclear complexes where the metal ions [copper(II) (1) and nickel(II) (2)] are six-coordinated in distorted octahedral MN₂O₄ surroundings which are built by two bidentate hfac ligands plus another bidentate L molecule. This last ligand coordinates to the metal ions through the nitrogen atoms of the picolylamine fragment. Compounds 3 and 4 are centrosymmetric homodinuclear compounds where two bidentate hfac units are the bidentate capping ligands at each metal center and a bis-bidentate L molecule acts as a bridge. The values of the intramolecular metal···metal separation are 7.97 (3) and 7.82 Å (4). Static (dc) magnetic susceptibility measurements were carried out for polycrystalline samples 1–4 in the temperature range 1.9–300 K. Curie law behaviors were observed for 1 and 2, the downturn of χ_MT in the low temperature region for 2 being due to the zero-field splitting of the nickel(II) ion. Very weak [J = −0.247(2) cm⁻¹] and relatively weak intramolecular antiferromagnetic interactions [J = −4.86(2) cm⁻¹] occurred in 3 and 4, respectively (the spin Hamiltonian being defined as H = −JS₁·S₂). Simple symmetry considerations about the overlap between the magnetic orbitals across the extended bis-bidentate L bridge in 3 and 4 account for their magnetic properties.     

Synthesis,structure, spectroscopic,magnetic and electrochemical studies of a rare syn–anti acetate-bridged copper(II) complex

The structure, spectroscopy and electrochemical properties of a novel dinuclear copper(II) complex, [{Cu(phen)₂}₂(μ-CH₃COO)][PF₆]₃ where phen = 1,10-phenanthroline, is reported. The crystal structure contains two independent Cu(II) ions, with different geometry around each copper center, which are bridged by an acetate anion. The acetate-bridged ligand shows a syn–anti coordination mode with a trigonal bipyramidal geometry for the Cu(1) center and a distorted square-based pyramidal geometry for the Cu(2) center. The angular structural index parameter τ for Cu(1) and Cu(2) is 0.9 and 0.33, respectively. The copper(II) atoms display a different geometry with a N₄O chromophore group and with Cu–O distances of 1.993(5)–1.996(5) Å and Cu–N distances which vary from 1.980(5) to 2.161(6) Å. The intra Cu…Cu separation is 4.9904(5) Å. The effective magnetic moment (μ_eff) of the complex was measured by the Evans method. The cyclic voltammogram of [{Cu(phen)₂}₂(μ-CH₃COO)][PF₆]₃ shows two waves at positive potential which are assigned to the two Cu(II/I) reduction couples.     

(Azulene‐1,3‐diyl)‐bis(nitronyl nitroxide) and (Azulene‐1,3‐diyl)‐bis(iminonitroxide) and Their Copper Complexes

In contrast to diradicals connected by alternant hydrocarbons, only a few studies on those connected by nonalternant hydrocarbons have been reported. The syntheses, structures, and magnetic properties of azulene‐1,3‐diyl linked bis(nitronyl nitroxide) (NN₂Az) and bis(iminonitroxide) (IN₂Az) diradicals and their Cu(hfac)₂ (hfac=hexafluoroacetylacetonate) complexes were investigated. NN₂Az was shown to have an intramolecular ferromagnetic interaction with J _obs/k _B=+10.0 K (H =−2J S ₁ ⋅S ₂) between (nitronyl nitroxide) spins, whereas IN₂Az was estimated to have a much weaker intramolecular magnetic interaction. The reactions of NN₂Az and IN₂Az with Cu(hfac)₂ gave a 1:2 [{Cu(hfac)₂}₂(NN₂Az)] complex and a 1:1 [Cu(hfac)₂(IN₂Az)] ⋅ C₆H₁₂ complex, respectively. [{Cu(hfac)₂}₂(NN₂Az)] showed strong intramolecular antiferromagnetic interactions (J _1‐Cu‐R/k _B≈−800 K, J _2‐Cu‐R/k _B≈−500 K) between the Cu^II spins and the coordinating NN spins, whereas [Cu(hfac)₂(IN₂Az)] exhibited a ferromagnetic exchange interaction (J _obs‐Cu‐R/k _B=+114 K) between the Cu^II spin and the imino‐coordinated iminonitroxide spin.     

Structures and magnetic properties of copper(II) complexes containing tetrakis(2-pyridyl)methane

We designed spiro-fused dinculear complexes using tetrakis(2-pyridyl)methane (py₄C) for the development of ground high-spin molecules. We attempted to prepare a dinuclear copper(II) complex [{Cu(hfac)₂}₂(py₄C)], where hfac stands for 1,1,1,5,5,5-hexafluoropentane-2,4-dionate, but we obtained [Cu(hfac)₂(py₄C)] and [Cu(hfac)(py₄C) · Cu(hfac)₃]. These molecular structures were determined by the X-ray crystal structure analysis.     

A Spin-Frustrated System Composed of Organic Radicals and Magnetic Metal Ions

Despite antiferromagnetic coupling between any two neighboring spins, parallel alignment of the 3d spins of the Mn^II ions and the 2p spins of the organic ligand is observed for the (degenerate) ground state of [{Mn(hfac)₂}₂(bnn)] (shown on the right). The “butterfly”-shaped arrangement of the spins contributes to this noteworthy spin configuration. hfac=hexafluoroacetylacetonate, bnn=2,2′-bis(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolinyl).     

Crystal structure and magnetic properties of [{Cu(cyclam)}3{Fe(CN)6}2] · 6H2O,a cyano-bridged assembly with a rope-ladder chain structure

The synthesis, crystal structure and magnetic properties of the cyano-bridged complex [{Cu(cyclam)}₃{Fe(CN)₆}₂] · 6H₂O are reported. Its structure is made up of centrosymmetric S-shaped pentanuclear [{Cu(cyclam}₃{Fe(CN)₆)}₂] units, in which three [Cu(cyclam)]²⁺ units are alternatively bridged by two trans-CN groups of [Fe(CN)₆]³⁻ anions and water molecules. The pentanuclear Fe₂Cu₃ units are held together by two complementary and very weak Fe–CN?Cu1 bonds, forming a rope-ladder chain along the c axis. The compound exhibits a ferromagnetic interaction between the Cu(II) and Fe(III) ions as a consequence of the orthogonality of their magnetic orbitals of σ and π nature, respectively. The magnetic data were fitted to the calculated magnetic susceptibility equation for a pentanuclear model, leading to the following magnetic parameters: J₁ = 9.0(3) cm⁻¹, J₂ = 3.8(4) cm⁻¹, g = 2.2, θ = −1.2 K. These results show that the interactions through the long Cu–N axial bonds are not so weak as is usually assumed.     

C(sp2)‐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 Cu^II 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 Cu^II ions and DR¹ is caused by the very short equatorial Cu?O bond length (1.962 Å).     

Ferro- and antiferromagnetic interactions in polymeric and molecular complexes of Cu(hfac)2 with 1-oxoazin-2-yl-substituted nitronyl nitroxides

Solid heterospin compounds based on Cu(hfac)₂ complexes with a new group of nitronyl nitroxides bearing different azine-N-oxide substituents at position 2 of the 2-imidazoline ring (Lⁿ) were studied. The major factor responsible for the change in the magnetic characteristics of the [Cu(hfac)₂L¹] complex with triazine nitronyl nitroxide with temperature was shown to be the specific pairwise packing of heterospin molecules with the dominant antiferromagnetic exchange between the radical fragments of adjacent molecules. For complexes of Cu(hfac)₂ with 1-oxoazin-2-yl-substituted nitronyl nitroxides L² and L⁴, 7-membered metallocycles were obtained, although they form rarely. It was shown that polymer chains formed in the solid complex with spin-labeled pyrazine-N-oxide [(Cu(hfac)₂)₃(L³)₂] due to the cross-linking of {(Cu(hfac)₂)₂(L³)₂} binuclear fragments via the bridging [Cu(hfac)₂].     

Exchange couplings in one-dimensionally arrayed 4f-3d heterometallic [Ln2Cu2]n compounds: A chemical trend of the coupling parameter

Heterometallic polymeric coordination compounds [{Ln(hfac)₂(CH₃OH)}₂{Cu(dmg)(Hdmg)}₂]_n (abbreviated as [Ln₂Cu₂]_n) involving light lanthanoid ions, Pr, Nd, Sm, and Eu, were synthesized, where H₂dmg and Hhfac stand for dimethylglyoxime and 1,1,1,5,5,5-hexafluoropentane-2,4-dione, respectively. The X-ray crystallographic analysis shows that their structures are isomorphous to those of the known heavy lanthanoid analogs. The exchange couplings were evaluated by high-frequency electron paramagnetic resonance and pulsed-field magnetization studies, giving ferromagnetic exchange parameters: J_Pr-Cu/k_B = 0.039(1) K, J_Nd-Cu/k_B = 0.38(1) K, J_Sm-Cu/k_B = 0.95(2) K, and no interaction between Eu and Cu ions. A significant trend is found in the order of the periodic table. The ferro- and antiferromagnetic 4f-3d couplings were characterized for the light and heavy lanthanoid derivatives, respectively. The magnitude of J_Ln-Cu decreases with a decrease of the 4f-spin portion, i.e., the number of unpaired electrons, whether the coupling is ferro- or antiferromagnetic.     

Magnetic and catalytic properties of the 2D copper(II) functionalized VPO hybrid system [{Cu(bpy)}2(VO)3(PO4)2(HPO4)2]·2H2O

The hybrid 2D compound [{Cu(bpy)}₂(VO)₃(PO₄)₂(HPO₄)₂]·2H₂O (1), has been investigated due to its interesting magnetic and catalytic properties. Compound (1) acts as an efficient catalyst in the epoxidation of cyclohexene and styrene. The chemoselectivity towards the epoxidation of cyclohexene is notoriously higher than the one observed towards styrene. The bulk antiferromagnetic behaviour of [{Cu(bpy)}₂(VO)₃(PO₄)₂(HPO₄)₂]·2H₂O (1) can be well described with a pentanuclear model, using five J values. Both antiferromagnetic and ferromagnetic interactions mediated by phosphate bridges are found to be present in this hybrid copper(II)–vanadium(IV) material.     

A Pentanuclear Cobalt Complex with two [CoII(CH3O)3]– Units Wrapping a Triangular [CoIII3O]7+ Core: Synthesis,Structure, and Magnetic Properties

The reaction of Hppko (Hppko = phenyl 2‐pyridyl ketone oxime) and CoCl₂ · 6H₂O in the CH₃OH solvent with the presence of triethylamine (NEt₃) at room temperature and the exposure to air resulted in the formation of a new pentanuclear, mixed‐valence cobalt complex with the molecular formula [{Co^II(CH₃O)₃}₂{Co^III₃(μ₃‐O)(ppko)₃}Cl₂]. X‐ray single crystal analysis displays a trigonal bipyramid configuration with the terminal two Co^II ions wrapping an triangle [Co^III₃O]⁷⁺ core. The intermolecular C–H ··· O and C–H ··· Cl interactions form a 2D network framework. The analysis of magnetic susceptibility revealed the dominant antiferromagnetic interactions and strong orbital contribution of Co^II ions.     

Solid-state anion interactions in the diastereoisomers of dinuclear ruthenium complexes based on 2,2′-bipyrimidine

The cations in the solid-state structures of meso-(ΛΔ)-[{Ru(bpy)₂}₂(μ-bpm)](PF₆)₄, meso-(ΛΔ)-[{Ru(Me₂bpy)₂}₂(μ-bpm)](tos)₄ · 2CH₃OH · 4H₂O and meso-(ΛΔ)-[{Ru(Me₄bpy)₂}₂(μ-bpm)](tos)₄ · 26H₂O (bpm = 2,2′-bipyrimidine; bpy = 2,2′-bipyridine; Me₂bpy = 4,4′-dimethyl-2,2′-bipyridine; Me₄bpy = 4,4′,5,5′-tetramethyl-2,2′-bipyridine; tos⁻ = toluene-4-sulfonate anion) exhibit similar features including comparable bond lengths and angles, and metal–metal separations of 5.56–5.59 Å. The counter-ions present in the structures reside in the clefts above and below the plane of the bridging ligand, but show considerable variation in location compared with their known occupancy in solution.     

Synthesis and study of Cu<Superscript>II</Superscript> complex with nitroxide,a jumping crystal analog

We synthesized 1-ethylimidazolyl-substituted nitronyl nitroxides, i.e., 2-(1-ethylimidazol-4-yl)- (L^4Et) and 2-(1-ethylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L^5Et). The stable radical L^5Et is an ethyl analog of 2-(1-methylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L^5Me) described earlier, the reaction of which with Cu(hfac)₂ (hfac is 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) leads to the formation of the [Cu(hfac)₂(L^5Me)₂] jumping crystals. The reaction of Cu(hfac)₂ with L^5Et with reagent ratios 1: 2 and 1: 1 yields heterospin complexes [Cu(hfac)₂(L^5Et)₂] and [Cu(hfac)₂L^5Et]₂, respectively. X-ray diffraction study of the mononuclear complex [Cu(hfac)₂(L^5Et)₂] determined that the compound has a packing similar to that of jumping crystals studied earlier, with the only difference being that the O...O contacts between neigh- boring nitroxide groups were found to be 0.3—0.5 Å longer than in [Cu(hfac)₂(L^5Me)₂]. As a result of the lengthening of these contacts, [Cu(hfac)₂(L^5Et)₂] crystals lack chemomechanical activi- ty. We found that when cooling crystals of binuclear complex [Cu(hfac)₂L^5Et]₂ below 50 K, the antiferromagnetic exchange between unpaired electrons of the >N—?O groups of neighboring molecules leads to the full spin-pairing of the nitroxides, with only the Cu²⁺ ions contributing to the residual paramagnetism of the compound.     

Structure,stacking interactions and magnetism of compounds with oxalate-bridged dinuclear CuCu and CuNb units

Two novel compounds of the formulae [{Cu(phen)₂}₂(μ-C₂O₄)][Cu(phen)₂(μ-C₂O₄)NbO(C₂O₄)₂]₂ · 8H₂O (1) and [{Cu(bpy)₂}₂(μ-C₂O₄)][Cu(bpy)₂(μ-C₂O₄)NbO(C₂O₄)₂]₂ · 0.5bpy · 7H₂O (2) (phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine) have been prepared and characterized by single crystal X-ray diffraction, IR spectroscopy and magnetic susceptibility measurements. The molecular structure of both 1 and 2 consists of a discrete Cu^IICu^II oxalate-bridged dinuclear [{Cu(L)₂}₂(μ-C₂O₄)]²⁺ cation (A unit) and two Cu^IINb^V oxalate-bridged dinuclear [Cu(L)₂(μ-C₂O₄)NbO(C₂O₄)₂]⁻ anions (B units) (L = phen, bpy). In 1 a crystallographic inversion centre is located in the middle of the oxalate bridge of the A unit, whereas in 2 an analogous inversion centre is missing. In the A units the copper(II) atoms adopt a tetragonally elongated octahedral coordination with the equatorial planes being perpendicular to the mean planes of the oxalate bridge and parallel to each other. In both structures, similar one-dimensional motifs are generated through the ligand stacking interactions, with a difference that one free bipyridine molecule, present in 2, intercalates into one of the motifs. It is shown that the phenanthroline ligand, due to its ability of stacking through the central aromatic ring, causes longer intermolecular Cu?Cu distances than the bipyridine ligand. The magnetic susceptibility measurements (1.8–290 K) show the ferromagnetic exchange interaction between the copper(II) atoms in the A units of both compounds, with J = +5.9 cm⁻¹ and +7.9 cm⁻¹ for 1 and 2, respectively (J – the exchange parameter in the isotropic spin Hamiltonian H_INT = −JS₁ · S₂).     

Structures and magnetic properties of one-dimensional copper(II) complexes bridged with diazaaromatic rings

Seven kinds of polynuclear complexes of [Cu(hfac)₂] (Hhfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) with diazaaromatic rings have been prepared. The crystal structures of [{Cu(hfac)₂(μ-L)}_n] (L = 2,5- and 2,6-dimethylpyrazines, propylpyrazine (prpyz), quinoxaline, phenazine, 4,6-dimethylpyrimidine, and 1,6-naphthyridine) have been determined. These complexes consist of a one-dimensional chain structure, and the geometry around the copper ion is approximately an octahedral structure. The relations between the magnetic properties and coordination structure were discussed from the magnetic measurements. In the μ-prpyz complex, one nitrogen atom is coordinated to a copper ion at an axial position, and at the same time the other coordinated at an equatorial site of a neighboring copper ion. This complex showed antiferromagnetic interaction with J/k_B = −0.086(3) K estimated from the Bonner–Fisher model. Weak magnetic interaction is caused by the somewhat long Cu–N distances due to the steric effect from the bridging ligands.     

Structure and magnetic properties of 3d–4f heterometallic complexes containing di-2-pyridyl ketoximate: An approach to single-molecule magnets

[{Ln(hfac)₃}₂{Ni(dpk)₂(phen)}] (1Ln) and [{Ln(hfac)₃}₂{Ni(dpk)₂(py)₂}] (2Ln) were synthesized and characterized, where dpk = di-2-pyridyl ketoxmate and Ln = La, Tb, Dy, Ho, Er. The N–O groups from dpk bridged the central nickel(II) ion and terminal lanthanide(III) ions, giving a linear trinuclear array. Dc magnetic susceptibility measurements revealed that they did not possess appreciable intramolecular ferromagnetic or ferrimagnetic interaction. Ac magnetic susceptibility measurements clarified that frequency dependence of out-of-phase ac susceptibility was observed only for Dy derivatives 1Dy and 2Dy, which is regarded as an indication of single-molecule magnets.     

Spirocyclic derivatives of nitronyl nitroxides in the design of heterospin CuII complexes manifesting spin transitions

A method was developed for the synthesis of a nitronyl nitroxide containing cyclopentane substituents in positions 4 and 5 of the imidazoline ring, viz., 2-(3-pyridyl)-4,5-bis(spiropentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L^CP). The reaction of Cu^II hexafluoroacetylacetonate with L^CP affords different products depending on the reaction conditions: mononuclear [Cu(hfac)₂(L^CP)₂], binuclear [Cu(hfac)₂L^CP]₂, tetranuclear {[Cu(hfac)₂]₄(L^CP)₂}, or chain polymer {[Cu(hfac)₂]₃(L^CP)₂} _n . Temperature changes induce structural transformations accompanied by a change in the spin state in exchange clusters in the solid [Cu(hfac)₂L^CP]₂ and {[Cu(hfac)₂]₄(L^CP)₂}.     

Structural and Magnetic Properties of 2p‐3d‐4f Hetero‐Tri‐Spin Chains Comprising [{Cu(hfac)2‐Radical}2] Dimers and Ln(hfac)3 (hfac=hexafluoroacetylacetonate)

A new family of 2p‐3d‐4f hetero‐tri‐spin complexes [Ln(hfac)₃{Cu(hfac)₂(NIT‐3 PyPh)}₂] (Ln=Gd ( 1 ), Tb ( 2 ), Dy ( 3 ), Ho ( 4 ); NIT‐3 PyPh=2‐[4‐(3‐pyridinylmethoxy)phenyl]‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide; hfac=hexafluoroacetylacetonate) have been synthesized. Four complexes possess a 1D chain structure in which two radical ligands join two Cu(hfac)₂ molecules to form a [{Cu(hfac)₂‐rad)}₂] dimer cycle and the dimer rings are linked by Ln(hfac)₃ units. Magnetic studies show that ferromagnetic exchange couplings exist between the coordinated NO groups of radical ligands and metal ions. Field‐induced slow relaxation of the magnetization was observed in the Tb and Dy compounds.