Enhancing the Magnetization Blocking Energy of Biradical-Metal System by Merging Discrete Complexes into One-Dimensional Chains.

The reaction of nitronyl nitroxide biradical NITPhMeImbis [5-(2-methylimidazole)-1,3-bis(1-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)-benzene] with Ln(hfac)3 ⋅ 2H2 O and Cu(hfac)2 (hfac=hexafluoroacetylacetonate), led to two series of 2p-3d-4f complexes, namely, nona-spin clusters, [Ln2 Cu3 (hfac)12 (NITPhMeImbis)2 ] (Ln=Gd 1, Dy 2), or one-dimensional chains [LnCu2 (hfac)7 (NITPhMeImbis)] (Ln=Y 3, Dy 4, Tb 5) depending on the temperature of the reaction. All five complexes contain a biradical-Ln unit in which the biradical chelates the LnIII ion by the means of one aminoxyl (i. e. NO) group of each NIT unit. For the discrete complexes, a Cu(hfac)2 links two biradical-Ln units via one of the remaining NO groups, while for the chain compounds, the two remaining NO groups of the biradical-Ln moiety are each coordinated to a Cu(hfac)2 unit to form a 1D coordination polymer. Moreover, a terminal Cu(hfac)2 unit is coordinated to the imidazole-N atom of the NITPhMeImbis ligand. Spin dynamics investigations evidenced the onset of slow relaxation of the magnetization for 2, whereas 4 and 5 exhibit a typical single-chain magnet behavior. This highlights the vital role of the 1D spin correlation in the blocking of the magnetization. These results illustrate that from the same basic building blocks, magnetic relaxation can be carefully modulated by structural adjustments.

Cyclic [Cu-biRadical]2 Secondary Building Unit in 2p-3d and 2p-3d-4f Complexes: Crystal Structure and Magnetic Properties.

Employing the new nitronyl nitroxide biradical ligand biNIT-3Py-5-Ph (2-(5-phenyl-3-pyridyl)-bis(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)), a 16-spin Cu-radical complex, [Cu8(biNIT-3Py-5-Ph)4(hfac)16] 1, and three 2p-3d-4f chain complexes, {[Ln(hfac)3][Cu(hfac)2]2(biNIT-3Py-5-Ph)2}n (LnⅢ= Gd 2, Tb 3, Dy 4; hfac = hexafluoroacetylacetonate), have been prepared and characterized. X-ray crystallographic analysis revealed in all derivatives a common cyclic [Cu-biNIT]2 secondary building unit in which two bi-NIT-3Py-5-Ph biradical ligands and two CuII ions are associated via the pyridine N atoms and NO units. For complex 1, two such units assemble with four additional CuII ions to form a discrete complex involving 16 S = 1/2 spin centers. For complexes 2-4, the [Cu-biNIT]2 units are linked by LnIII ions via NO groups in a 1D coordination polymer. Magnetic studies show that the coordination of the aminoxyl groups with Cu or Ln ions results in behaviors combining ferromagnetic and antiferromagnetic interactions. No slow magnetic relaxation behavior was observed for Tb and Dy derivatives.

Influence of the Coordinated Transition Metal Ion on Magnetic Relaxation of Lanthanide Based Complexes with Imino Nitroxide Biradical Ligands.

In spite of achievement of a lot of Ln-radical SMMs, how to improve magnetic behavior of Ln-radical system remains challenging. Here, two series of Ln-radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln2 (hfac)6 (ImPhPyobis)2 ] (LnIII =Gd 1, Tb 2, Dy 3) and [Ln2 Cu2 (hfac)10 (ImPhPyobis)2 ] (LnIII =Gd 4, Dy 5; hfac=hexafluoroacetylacetonate and ImPhPyobis=5-(4-oxypyridinium-1-yl)-1,3-bis(1'-oxyl-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). For these biradical-metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4-oxypyridinium units to produce a binuclear {Ln2 O2 } unit. Those imino nitroxide groups are free for complexes 1-3, however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5. The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(Ueff ) of 74.0 K while complex 5 exhibits dual relaxation processes with Ueff values for the fast- and slow-relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation.

Designing Multicoordinating Nitronyl Nitroxide Radical Toward Multinuclear Lanthanide Aggregates.

Profiting from a multicoordinating nitronyl nitroxide radical, i.e. a functionalized nitronyl nitroxide biradical ligand, a family of novel tetranuclear lanthanide complexes, formulated as [RE4(hfac)12(NITPhO-3Pybis)2] (RE = Gd 1, Tb 2, Dy 3, Ho 4, and Y 5; NITPhO-3Pybis = 5-(3-pyridinyloxy)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene; hfac = hexafluoroacetylacetonate) were successfully constructed and characterized. In these complexes, the designed functionalized nitronyl nitroxide biradical ligand functions as the chelating and/or bridging ligand to bind the lanthanide ions, resulting in tetranuclear octa-spin lanthanide complexes with a circle-shaped arrangement. Direct-current magnetic data show that antiferromagnetic interaction dominates in the Gd derivative, while ferromagnetic coupling plays a leading role in complex Y, in which the relevant magnetic exchange parameters were obtained through applicable magnetic models. Dynamic magnetic studies of Tb and Dy analogues exhibit apparent frequency-dependent out-of-phase signals, which are typical features for slow magnetic relaxation behavior. The values of Ueff and τ0 were obtained as follows: Ueff = 10.5 K and τ0 = 6.6 × 10-7 s for the Tb complex and Ueff = 5.2 K and τ0 = 2.5 × 10-6 s for the Dy compound. Intriguingly, the butterfly shaped hysteresis loop is found for the Tb analogue. Guided by fluorescence spectra, the representative peaks are identified for the Tb derivative.

Lanthanide-Nitronyl Nitroxide Chains Derived from Multidentate Nitronyl Nitroxides.

Unprecedented lanthanide (Ln)-radical loop-chain coordination polymers were achieved using multidentate pyridyl- or triazole- substituted nitronyl nitroxide ligands. Their magnetic units consist of ferromagnetic [Ln2Radical] moieties, leading for the dysprosium(III) derivatives to slow relaxation of magnetization, which was found to be dependent on the heterocyclic ligands.

Functionalized Nitronyl Nitroxide Biradicals for the Construction of 3d-4f Heterometallic Compounds.

Functionalized nitronyl nitroxide biradical ligands incorporating pyridine groups hold CoII and LnIII ions together, creating biradical-based 3d-4f tetranuclear complexes [Ln2Co2(hfac)10(NITPhPybis)2] [LnIII = Gd (1), Tb (2), Dy (3), and Ho (4); NITPhPybis = 5-(4-pyridyl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1 H-imidazol-2-yl)benzene; hfac = hexafluoroacetylacetonate]. These complexes have a centrosymmetric cyclic molecular structure in which two biradicals perform as tetradentate ligands to bind two CoII and two LnIII ions, resulting in a rare octaspin system. Direct-current (dc) magnetic susceptibility studies reveal that the strong antiferromagnetic CoII-NO magnetic exchange dominates the present magnetic system, while magnetic coupling of Gd-ON is ferromagnetic. Analysis of the magnetic data of the Gd complex allows us to determine the magnetic parameters through the appropriate magnetic model. Alternating-current (ac) magnetic susceptibility investigations indicate that 2 displays frequency-dependent out-of-phase signals under a zero dc field, while ac magnetic susceptibilities of 3 show field-induced frequency dependence, which is a typical feature of slow relaxation of the magnetization. Complexes 1-4 represent the first nitronyl nitroxide biradical-based 3d-4f compounds.

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.

Thermal Magnetic Hysteresis in a Copper-Gadolinium-Radical Chain Compound.

Magnetic bistability spanning over a temperature domain of 40 K can result from a small structural deformation of the gadolinium aminoxyl coordination. This is illustrated for a nitronyl nitroxide 3d-4f chain, [Ln(hfac)3Cu(hfac)2(NIT-Pyrim)2] (Ln(III) = Gd, Dy), which is the first example of a bistable lanthanide-based complex.

Tuning Magnetic Relaxation in a Tb-Nitronyl Nitroxide Complex by Using Cocrystalline Paramagnetic Complex.

New 2p-4f and 2p-3d-4f compounds [Tb(hfac)3(NIT-PhNO2)2]· 0.5C7H16 (1) and [Ln(hfac)3(NIT-PhNO2)2]2[Cu(hfac)2(NIT-PhNO2)2] (Ln(III) = Gd 2, Tb 3; hfac = hexafluoroacetylacetonate; NIT-PhNO2 = 2-(p-nitrophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been obtained. Complex 1 consists of mononuclear trispin [Tb(hfac)3(NIT-PhNO2)2] units in which two radical ligands are ligated to the Tb(III) ion as monodentate ligands through the NO groups, while complexes 2 and 3 contain two kinds of trispin moieties, namely, [Ln(hfac)3(NIT-PhNO2)2] and [Cu(hfac)2(NIT-PhNO2)2]. In the [Cu(hfac)2(NIT-PhNO2)2] moiety, the radicals are bonded to the copper(II) ion in the axial positions via the nitroxides. For three compounds, 1D supramolecular chains are formed via the π-π stacking interactions involving the radical ligands. Magnetic investigations show that both Tb complexes exhibit slow relaxation of magnetization at low temperature; strikingly, complex 3 displays a higher energy barrier than that of 1. It represents the first example to use the paramagnetic complex to tune magnetic relaxation of 4f-based compounds.

[(Cu-radical)2-Ln]: structure and magnetic properties of a hetero-tri-spin chain of rings (Ln = Y(III), Gd(III), Tb(III), Dy(III)).

Novel hetero-tri-spin coordination polymers formed of ring-shaped Cu-nitronyl nitroxide spin clusters and Ln(III) linkers are reported. These mixed 2p-3d-4f compounds of formula {[Ln(hfac)3][Cu(hfac)2(NIT-3Py)]2·C6H14}n [Ln(III) = Y (1), Gd (2), Tb (3), and Dy (4); NIT-3Py = 2-(3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; hfac = hexa-fluoroacetylacetonate], exhibit a 1D chain structure consisting of [Cu(NIT-3Py)]2 rings linked by Ln(hfac)3 units. Their magnetic behavior is characteristic for ferromagnetic interactions between the metal centers and the coordinated radical units. The Tb derivative was found to exhibit slow relaxation of its magnetization.

Hetero-tri-spin [2p-3d-4f] chain compounds based on nitronyl nitroxide lanthanide metallo-ligands: synthesis, structure, and magnetic properties.

Employing nitronyl nitroxide lanthanide(III) complexes as metallo-ligands allowed the efficient and highly selective preparation of three series of unprecedented hetero-tri-spin (Cu-Ln-radical) one-dimensional compounds. These 2p-3d-4f spin systems, namely [Ln3Cu(hfac)11(NitPhOAll)4] (Ln(III) = Gd 1Gd, Tb 1Tb, Dy 1Dy; NitPhOAll = 2-(4'-allyloxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), [Ln3Cu(hfac)11(NitPhOPr)4] (Ln(III) = Gd 2Gd, Tb 2Tb, Dy 2Dy, Ho 2Ho, Yb 2Yb; NitPhOPr = 2-(4'-propoxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and [Ln3Cu(hfac)11(NitPhOBz)4] (Ln(III) = Gd 3Gd, Tb 3Tb, Dy 3Dy; NitPhOBz=2-(4'-benzyloxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) involve O-bound nitronyl nitroxide radicals as bridging ligands in chain structures with a [Cu-Nit-Ln-Nit-Ln-Nit-Ln-Nit] repeating unit. The dc magnetic studies show that ferromagnetic metal-radical interactions take place in these hetero-tri-spin chain complexes, these and the next-neighbor interactions have been quantified for the Gd derivatives. Complexes 1Tb and 2Tb exhibit frequency dependence of ac magnetic susceptibilities, indicating single-chain magnet behavior.

Three-dimensional frameworks based on dodecanuclear Dy-hydroxo wheel cluster with slow relaxation of magnetization.

Two new heterometallic coordination polymers, [Na4Ln12(stp)8(OH)16(H2O)12]·10H2O [Ln = Dy (1) and Ho (2)], have been prepared from monosodium 2-sulfoterephthalate (NaH2stp), dysprosium acetate, or holmium acetate. They are isostructural, possessing a [Ln12(µ3-OH)16](20+) wheel-cluster core based on four vertex-sharing cubane-like [Ln4(µ3-OH)4](8+) units. The Ln12 cores are linked by stp ligands into a three-dimensional (3D) architecture. Magnetic studies indicate that complex 1 exhibits slow relaxation of magnetization, and it can be regarded as the first 3D coordination assembly of a Dy12 cluster single-molecule magnet.

Tuning spin dynamics of binuclear Dy complexes using different nitroxide biradical derivatives.

By employing nitronyl/imino nitroxide biradicals, three Ln-Zn complexes, namely, [Ln2Zn2(hfac)10(ImPhPyobis)2] (LnIII = Gd 1, Dy 2; hfac = hexafluoroacetylacetonate; ImPhPyobis = 5-(4-oxypyridinium-1-yl)-1,3-bis(1'-oxyl-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene) and [Dy2Zn2(hfac)10(NITPhPyobis)2] 3 (NITPhPyobis = 5-(4-oxypyridinium-1-yl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene), have been successfully prepared. The three complexes possess {Ln2O2} cores bridged by the oxygen atoms of the 4-oxypyridinium rings of the biradical ligands and one of the imino/nitronyl nitroxide groups of the biradical is coordinated to a ZnII ion, then producing a centrosymmetric tetranuclear six-spin structure. The studies of spin dynamics indicate that complexes 2 and 3 exhibit distinct magnetic relaxation behaviors at zero dc field: complex 2 presents single relaxation with an effective energy barrier (Ueff) of 69.8 K, while complex 3 exhibits double relaxation processes with Ueff values for the fast and slow relaxation being 15.8 K and 50.9 K, respectively. The observed different magnetic relaxation behaviors for the two Dy complexes could be mainly ascribed to the influence of the distinct nitroxide biradical derivatives.

Unprecedented nitronyl nitroxide bridged 3d-4f complexes: structure and magnetic properties.

Two novel 2p-3d-4f compounds, {Ln(hfac)3[Cu(hfac)2]3(NITPhPyrim)2} [Ln = Gd (1), Dy (2)], have been obtained by reacting phenyl pyrimidyl nitronyl nitroxide with Cu(hfac)2 and Ln(hfac)3. These two compounds are the first examples of two-dimensional 3d-4f complexes bridged by nitronyl nitroxide radicals. Overall ferromagnetic behaviors were observed in both compounds.

Syntheses, structures, and magnetic and luminescence properties of a new Dy(III)-based single-ion magnet.

Three new Ln(III) complexes based on 2,2'-bipyridine [Ln(hfac)3(bpy)] (Ln = Dy (1), Tb (2), or Ho (3); hfac = hexafluoroacetylacetonate; and bpy = 2,2'-bipyridine) have been synthesized and characterized structurally and magnetically. Single-crystal X-ray analysis shows that all these complexes contain one [Ln(hfac)3(bpy)] unit in which a center Ln(III) ion is surrounded with a slightly distorted square-antiprismatic LnO6N2 coordination sphere formed by three bischelate hfac anions and one bpy ligand. Both static and dynamic magnetic properties were studied for complex 1, which is proved to be a new single-ion magnet. The luminescence characterizations of complexes 1 and 2 are also studied in this paper.

Magnetic relaxation in unique nitronyl nitroxide biradical-Ln-Cu chains with Ln-bis(NIT)-Cu-bis(NIT)-Ln units.

Utilizing a nitronyl nitroxide biradical NITPhPybis [5-(4-pyridyl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)-benzene], a new family of isomorphic 2p-3d-4f chains {[LnCu(hfac)5(NITPhPybis)]·CHCl3}n (hfac: hexafluoroacetylacetonate; LnIII: Gd 1; Dy 2; Ho 3; Tb 4) have been successfully produced. In complexes 1-4, the NITPhPybis biradical chelates one LnIII ion through its bis(NIT) moiety while the N donor of pyridine and another uncoordinated NO group of the biradical, respectively, bind one CuII ion, yielding a biradical-Ln-Cu 1D zigzag chain with a unique [Ln-bis(NIT)-Cu-bis(NIT)-Ln] structural motif. DC magnetic studies reveal that ferromagnetic exchanges dominate in these Cu-Ln-biradical chains, originating from the ferromagnetic Ln-NO and NOaxial-Cu exchanges. Non-zero χ'' signals were observed for Dy/Tb-Cu derivatives implying slow magnetic relaxation behavior. The obtained effective energy barrier is Ueff = 18.0 K and τ0 = 2.0 × 10-8 s for the DyCu derivative.

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.

Slow magnetic relaxation in a Dy3 triangle and a bistriangular Dy6 cluster.

Two lanthanide single-molecule magnets (SMMs) [Dy3(µ3-OH)(HL-1)3(H2O)3](NO3)2·3H3O (1, H3L-1 = (E)-3-(((8-hydroxyquinolin-2-yl)methylene)amino)propane-1,2-diol) and [Dy6(µ3-OH)4(H2L-2)4(HL-2)2(L-2)2] (2, H3L-2 = (E)-2-hydroxy-N'-(2-hydroxy-3-methoxybenzylidene)benzohydrazide) were synthesized and characterized structurally and magnetically. Complex 1 contains a triangular Dy3 core in which the three Dy3+ ions share a µ3-OH- anion and the deprotonated ligands of (HL-1)2- serve both capping and bridging functions, while 2 displays a centrosymmetric hexanuclear DyIII structure with two similar Dy3 triangular cores ligated by two fully deprotonated (L-2)3- ligands, each of which shares two µ3-OH- anions. All the DyIII ions are eight-coordinated with quasi D2d or C2v symmetry. Magnetic studies reveal that 1 exhibited two-step magnetic relaxation under an applied dc field of 800 Oe, with effective energy barriers of 40.1 and 31.0 K for the slow relaxation (SR) and fast relaxation regimes (FR), respectively. Meanwhile, 2 only showed a tail of slow magnetic relaxation at above 2 K. Ab initio calculations have been carried out to show the nature of their different magnetic properties.

Slow magnetic relaxation in novel Dy4 and Dy8 compounds.

Two novel dysprosium(III) clusters have been prepared and structurally characterized. One has a tetranuclear core with a rare zigzag arrangement, and the other is an unprecedented octanuclear cluster with six triangular Dy(3) units sharing vertices. Both dysprosium(III) clusters possess frequency-dependent on alternating-current magnetic susceptibilities, indicating possible single-molecule magnet behavior.

Modulating the magnetization dynamics in Ln-Cu-Rad hetero-tri-spin complexes through cis/trans coordination of nitronyl nitroxide radicals around the metal center.

Self-assembling the novel nitronyl nitroxide radical NIT-3Py-5-Ph (2-(5-phenyl-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) with Ln(hfac)3·2H2O and Cu(hfac)2 (hfac = hexafluoroacetylacetonate) resulted in two heterometallic complexes with formula [LnCu(hfac)5(NIT-3Py-5-Ph)2] (Ln = Gd 1, Dy 2), in which two NIT-3Py-5-Ph radicals are coordinated with the LnIII ion via their nitroxide units in the cis-arrangement manner and the CuII ion is ligated by the pyridyl N donors of the radicals. Interestingly, when the phenyl group of NIT-3Py-5-Ph was replaced with a p-pyridyl group, a new family of 2D networks, namely, {[Ln(hfac)3][Cu(hfac)2]2(NIT-3Py-5-4Py)2}n (Ln = Gd 3, Tb 4, Dy 5; NIT-3Py-5-4Py = 2-(5-(4-pyridyl)-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was obtained. In the 2D sheet, each NIT-3Py-5-4Py ligand serves as a µ3-bridge to bind one LnIII center by the aminoxyl moiety and two CuII ions through two pyridine groups to form a 2D structure. The LnIII ion is coordinated by two NO units of two radicals in a trans configuration. DC magnetic measurements indicate that ferromagnetic LnIII-NO exchange occurs in 1-5. AC studies reveal that 2 displays slow relaxation of the magnetization while no such magnetic relaxation is found in complex 5. The observed different magnetic relaxation behaviors of two Dy analogues could be attributed to the different coordination modes of NO groups of the radicals, and the coordination geometry of the Dy center is from C2v in 2 to D2d in 5.