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2p-3d-4f Hetero-tri-spin Chains and Ring-chains Bridged by a Nitronyl Nitroxide Ligand: Structure and Magnetic Properties Juan Sun, Kang Wang, Pei Jing, Jiao Lu, and Licun Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00489 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Crystal Growth & Design

2p-3d-4f Hetero-tri-spin Chains and Ring-chains Bridged by a Nitronyl Nitroxide Ligand: Structure and Magnetic Properties Juan Sun, Kang Wang, Pei Jing, Jiao Lu, Licun Li* Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China

ABSTRACT Two types of the radical-3d-4f chains {[Cu(hfac)2][(NIT-4PyPh)]2[Ln(hfac)3]}n (Ln = Gd 1, Tb 2, Dy 3) and {[Cu(hfac)2(NIT-4PyPh)]2Ln(hfac)3}n (Ln = Gd 4, Tb 5, Dy

6;

NIT-4PyPh

=

2-[4-(4-pyridinylmethoxy)phenyl]-4,4,5,5-tetramethyl-

imidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate) have been successfully achieved upon adjusting the experimental conditions. Compounds 1-3 display 1D chain structure with NIT-4PyPh radical ligand bridging LnIII and CuII ions in “headto-head” mode through pyridine-N atom and nitroxide unit of the radical. Complexes 4-6 possess an interesting 1D topological structure consisting of cyclic [Cu-Radical]2 dimers connected by Ln ions. Magnetic investigations indicate that ferromagnetic the copper(II)/lanthanide(III)-NO exchange occurs in two series of compounds. Furthermore, complexes 3 and 5 were found to exhibit frequency dependence of ac susceptibilities, evidencing slow magnetic relaxation.

INTRODUCTION Combined 3d-4f heteronuclear compounds are well known for representing a promising way in the design of molecular nanomagnets.1-6 The first example of 3d-4f heterometallic SMM was obtained in 2004.7 Since then, the investigation of 3d-4f heterometallic complexes e.g. Mn-Ln,8-11 Fe-Ln,12-14 Co-Ln,15-18 Ni-Ln19-21 and CuLn22-25 have been spurred. A typical example, [Mn18Dy], which has been proven successful in enhancing the magnetic anisotropy through introducing the strong anisotropic dysprosium(III) ion into a Mn19 complex, and displays slow magnetic relaxation behavior.26,27 Recently, MII2DyIII (M=FeII, ZnII, CoII) system reported by Tong et al., achieves a record anisotropy barrier of 600 K (M =CoII) among all 3d-4f 1

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

complexes.28-30 In spite of these achievements, it is still a challenging task to enhance the JM-Ln of 3d-4f complexes.31,32 In view of this, the introduction of paramagnetic radical ligands would be an appealing strategy for 3d-4f heterometallic complexes. According to “hard and soft” concept, the functionalized nitronyl nitroxide radical containing N group is an excellent candidate to combine 4f and 3d ions for constructing 3d-4f system. However, the N-O groups of nitronyl nitroxide radicals have poor ability to coordinate, thus they only ligate to metal centers with electronwith-drawing groups such as hexafluoroacetylacetonate (hfac). To date, few 3d-4f multinuclear compounds involving nitronyl nitroxide radical ligands have been obtained by using hfac as coligand.33-36 More recently, we concentrate our study on hetero-tri-spin (Ln-radical-M) 1D systems using functionalized nitronyl nitroxide radicals, and a few d-f complexes linked by nitronyl nitroxides (i.e. hetero-tri-spin) with interesting topology structures or magnetic behaviors were obatined.37-39 In the present work, using nitronyl nitroxide radical ligand NIT-4PyPh ( NIT4PyPh=2-[4-(4-pyridinylmethoxy)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3oxide), we successfully prepared two types of hetero-tri-spin complexes. One is a family of the radical ligand bridging Ln and Cu centers (2p-3d-4f) chains by “head and head” mode, namely, {[Cu(hfac)2][(NIT-4PyPh)]2[Ln(hfac)3]}n (Ln = Gd 1, Tb 2, Dy 3). Changing the experimental conditions, the other series of interesting radical bridged Ln-Cu ring-chains {[Cu(hfac)2(NIT-4PyPh)]2Ln(hfac)3}n (Ln = Gd 4, Tb 5, Dy 6) were achieved, in which [Cu(hfac)2(NIT-4PyPh)]2 cyclic dimers are bridged by the LnIII ions. Ferromagnetic copper(II)/lanthanide(III)-NO interactions are active in these 1D chains. Slow magnetic relaxation behaviors are found in complexes 3 and 5.

EXPERIMENTAL SECTION Materials NIT-4PyPh radical was prepared according to the previous report.40 The used other chemicals including organic solvents were bought from Aldrich and Alfa Aesar and directly used. Physical measurements Chemical analyses including carbon hydrogen and nitrogen were tested with a Perkin−Elmer 240 elemental analyzer. PXRD data were collected with a Rigaku Ultima IV diffractometer for all six compounds. IR spectra (KBr pellets) were implemented on a Bruker Tensor 27 Spectrometer. Dc and ac magnetic data were 2


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collected on a Quantum Design SQUID-VSM magnetometer. Pascal’s constants41 were used for diamagnetic corrections. Synthesis of {[Cu(hfac)2][(NIT-4PyPh)]2[Ln(hfac)3]}n (Ln= Gd 1; Tb 2; Dy 3 ) 0.01 mmol Ln(hfac)32H2O in 15 mL C7H16 was under reflux for three hours. Cool the mixture to 80°C, 5 mL of CH2Cl2 solution containing NIT-4PyPh radical (0.0068g, 0.02 mmol) was introduced. The reaction was maintained at 80°C for 30 minutes under stirring, then 0.0048g (0.01 mmol) of solid Cu(hfac)2 was introduced. The blue block-shaped crystals were obtained after one day by evaporating the filtrate at room temperature. For 1, yield: 43% (based on Gd ion). Chemical analysis (%) found(calcd): C, 38.98(39.07); H, 2.68(2.55); N, 4.31(4.34). IR (KBr): 2360(m), 1655(s), 1504(m), 1259(s), 1205(s), 1148(s), 800(m), 665(m) cm−1. For 2, yield: 47% (based on Tb ion). Chemical analysis (%) found(calcd): C, 38.93(39.03); H, 2.72(2.55); N, 4.43(4.34). IR (KBr): 2360(m), 1654(s), 1504(m), 1260(s), 1205(s), 1148(s), 800(m), 665(m) cm−1. For 3, yield: 52% (based on Dy ion). Chemical analysis (%) found(calcd): C, 38.86(38.96); H, 2.74(2.54); N, 4.26(4.33). IR (KBr): 2360(m), 1655(s), 1506(m), 1259(s), 1205(s), 1149(s), 801(m), 665(m) cm−1. Synthesis of {[Cu(hfac)2(NIT-4PyPh)]2Ln(hfac)3}n (Ln = Gd, 4; Tb, 5; Dy, 6 ) A heptane solution (15 mL) containing 0.0096g (0.02 mmol) of Cu(hfac)2 and 0.01 mmol Ln(hfac)32H2O was stired under reflux for three hours, then 5 mL CH2Cl2 containing 0.0068g NIT-4PyPh radical (0.02 mmol) was added. After refluxing for 30 minutes, the reaction solution was cooled to RT. The blue filtrate was left at RT for one day, yielding blue rhombus crystals. For 4, yield: 51% (based on Gd ion). Chemical analysis (%) found(calcd): C, 36.23(36.31); H, 2.39(2.13); N, 3.48(3.48). IR (KBr): 2360(m), 1655(s), 1350(m), 1255(s), 1208(s), 1145(s), 802(m), 668(m), 586(m) cm−1. For 5, yield: 58% (based on Tb ion). Chemical analysis (%) found(calcd): C, 36.33(36.29); H, 2.32(2.13); N, 3.53(3.48). IR (KBr): 2361(m), 1656(s), 1350(m), 1254(s), 1214(s), 1148(s), 802(m), 669(m), 587(m) cm−1. For 6, yield: 63% (based on Dy ion). Chemical analysis (%) found(calcd): C, 36.39(36.23); H, 2.27(2.12); N, 3.59(3.47). IR (KBr): 2361(m), 1652(s), 1349(m), 1258(s), 1207(s), 1145(s), 802(m), 669(m), 587(m) cm−1. 3



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X-Ray Crystallography Crystallographic data of all six compounds were acquired on a Rigaku Saturn diffractometer using graphite-monochromated Mo K radiation ( = 0.71073 Å) at 113 K. Direct methods were employed to solve the structures using SHELXS-201442 and the refinements on full-matrix least-squares based on F2 were implemented using SHELXL-2014 programs.43 For non-H atoms, the anisotropic thermal parameters were empolyed. The H atoms of the liangds were positioned geometrically. Crystallographic refinement details for 1-6 are provided in Table 1. Significant bond parameters for all compounds are given in Tables S7-S8. Table 1. Crystallographic data for 1-6. 1 Formula Formula weight Crystal system Space group T, K a /Å b /Å c /Å , deg , deg , deg V /Å3 Z Dcalcd /g cm-3 F(000) min, max deg Reflections collected Unique reflns/ Rint GOF (F2) R1/wR2 [I > 2σ(I)]a R1/wR2 (all

data)a

2 3 C63H49CuF30LnN6O16 1936.89 1938.57 1942.14 Monoclinic Monoclinic Monoclinic P21/c P21/n P21/n 113 113 113 20.134(18) 20.106(2) 20.0670(15) 16.550(15) 16.5242(16) 16.5312(12) 28.159(17) 24.609(3) 24.6290(18) 90 90 90 121.48(4) 102.757(3) 102.688(2) 90 90 90 8002(12) 7974.2(15) 7970.7(10) 4 4 4 1.608 1.615 1.618 3840 3844 3848 2.990/25.010 3.043/27.475 1.712/27.908

4

5 6 C73H51Cu2F42LnN6O20 2414.55 2416.23 2419.80 Monoclinic Monoclinic Monoclinic C2/c C2/c C2/c 113 113 113 37.300(2) 37.287(4) 37.188(14) 12.3530(5) 12.3718(11) 12.339(4) 20.2349(12) 20.263(2) 20.224(7) 90 90 90 105.858(2) 105.705(4) 105.824(2) 90 90 90 8970.3(8) 8991.7(15) 8934(5) 4 4 4 1.788 1.785 1.799 4764 4768 4772 3.201/27.520 1.746/28.742 3.202/28.319

49660

78969

103278

41972

56418

42818

13973/0.1881 1.052

17937/0.0743 0.978

18908/0.0706 1.016

11076/0.0366 1.053

10307/0.0167 1.049

11384/0.0417 1.030

0.1189/0.2844

0.0574/0.1393

0.0531/0.1253

0.0357/0.0774

0.0265/0.0683

0.0355/0.0802

0.1821/0.3523

0.1019/0.1624

0.0899/0.1425

0.0447/0.0834

0.0285/0.0696

0.0472/0.0863

RESULTS AND DISCUSSION Synthesis To achieve 2p-3d-4f complexes, the nitronyl nitroxide radical with additional N atom was used. Based on the soft-hard rule, the NO unit of NIT-4PyPh can coordinate to the Ln(III) ions while the ring with N donor could ligate to the transition metal ion. Using the ratio of 1:2:1 of CuII/NIT-4PyPh/LnIII, one series of 1D chains have been 4


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synthesized, where the LnIII and CuII centers are linked by the nitroxides unit and pyridyl N atoms of NIT-4PyPh radicals. Varying the ratio of CuII/NIT-4PyPh/LnIII as 2:2:1, the other series of radical bridged Ln-Cu compounds exhibiting interesting chain of ring structure have been obtained, where [Cu-radical]2 cyclic dimers are connected by LnIII ions. It is the molar ratio of CuII, LnIII and the NIT-4PyPh radical that determines the structure of the product. Description of the crystal structure

Figure 1. (left) 1D chain of 2. (right) The ligation environment of Tb(III) ion. {[Cu(hfac)2][(NIT-4PyPh)]2[Ln(hfac)3]}n (Ln = Gd, 1; Tb, 2; Dy, 3 ). Complexes 1-3 are isostructural and display one-dimensional structure. Compound 1 belongs to the monoclinic P21/c space group, whereas complexes 2 and 3 crystallize in the monoclinic P21/n space group. The 1D chain structure is formed by the NIT-4PyPh radical ligands bridging CuII and LnIII ions via their nitroxides and pyridyl-N atoms (Figures 1, S1 and S2). The Ln ion is eight-coordinated by tri-hfac O-donor anions (Ln-Ohfac: 2.352(10)-2.415(9) Å for 1, 2.338(4)-2.398(4) Å for 2 and 2.330(3)-2.386(3) Å for 3) and two nitroxides of two NIT-4PyPh radicals (Ln-Orad: 2.365(9) and 2.388(9) Å for 1, 2.352(4) and 2.369(4) Å for 2, 2.335(3) and 2.357(3) Å for 3). SHAPE analysis44,45 reveals that all of the LnIII centers are located in triangular dodecahedron (D2d) coordination geometries (Table S9). The angle formed by two N-O units and the Ln(III) (Orad−Ln−Orad) is 137.3(3)° for 1, 137.51(13)° for 2 and 137.59(11)° for 3. The copper(II) center is defined by two nitrogen atoms and four oxygen atoms coming from pyridine rings of radicals and β-diketonate coligands, respectively. The equatorial Cu-Ohfac/N bond lengths are 1.958(13)-2.013(13) Å for 1, 1.985(5)2.004(4) Å for 2 and 1.981(4)-2.014(4) Å for 3. The apical sites are filled with two O atom of two β-diketonate coligands (Cu-Ohfac bond distances: 2.273(14) and 2.328(13) Å for 1, 2.261(5) and 2.325(5) Å for 2, 2.269(4) and 2.329(4) Å for 3). The greater 5



axial bond distances may be attributed to the effect of Jahn-Teller.46-48 In the chain, the Ln⋯Cu distances are 12.2785(94) Å for 1, 12.2750(17) Å for 2 and 12.5059(13) Å for 3. Chains packing of 1-3 is given in Figures S5-S7, the nearest interchain Ln⋯Ln and Ln⋯Cu and Cu⋯Cu contacts are 10.7564(67) Å and 9.0281(60) Å and 10.5628(72) Å for 1, 10.7383(10) Å and 9.0356(11) Å and 10.5472(14) Å for 2, 10.7433(8) Å and 9.0305(9) Å and 10.5541(11) Å for 3.

Figure 2. (left)Ring-chain structure of 5. (right) The ligation polyhedron of Tb(III) ion. {[Cu(hfac)2(NIT-4PyPh)]2Ln(hfac)3}n (Ln = Gd, 4; Tb, 5; Dy, 6). Complexes 4-6 belong to the monoclinic system and crystallize in space group C2/c. Complexes 4-6 are isomorphic and consist of 1D chain structure assembled by LnIII ions and cyclic [Cu-radical]2 moieties (Figures 2, S3 and S4). In three ring-chains, every NIT-4PyPh ligand serves as a bridge to connect two CuII centers via its one nitroxide and one pyridine ring to form centro-symmetric dimeric cycle [Cu-(NIT-4PyPh)]2. Dimers are linked by Ln centers via the uncoordinated N-O groups, thus developing a 1D ringchain structure. The copper atom possesses a octahedral ligation environment with equatorial one pyridyl N atom and three Ohfac atoms (Cu-Ohfac: 1.9577(19)-1.9651(18) Å for 4, 1.9616(15)-1.9686(15) Å for 5 and 1.957(2)-1.9668(19) Å for 6; Cu-N: 1.998(2) Å for 4, 2.0007(16) Å for 5 and 1.998(2) Å for 6). The apical sites are held by one nitroxide group from the other NIT-4PyPh and one O atom of hfac anion (CuOhfac: 2.2085(19) Å for 4, 2.2109(15) Å for 5 and 2.203(2) Å for 6; Cu-Orad: 2.5266(22) Å for 4, 2.5295(18) Å for 5 and 2.5200(23) Å for 6). The Ln center is ligated by two nitroxide units of the NIT-4PyPh radicals (Ln-Orad: 2.3606(18) Å for 4, 2.3553(14) Å for 5 and 2.3353(19) Å for 6) and three hfac anions (Ln-Ohfac: 2.3570(18)-2.3890(18) Å for 4, 2.3494(14)-2.3712(14) Å for 5 and 2.3323(19)2.3557(19) Å for 6). All eight-coordinate LnIII ions are located in triangular dodecahedron (D2d) coordination spheres (Table S9, SHAPE analysis: CShM =0.2526


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0.258).44,45 The O(nitroxide)−Ln−O(nitroxide) angles are 137.49(9)° for 4, 137.06(7)° for 5 and 137.16(9)° for 6. For the dimeric rings, the Cu⋯Cu separations are 13.1141(10) Å for 4, 13.1306(14) Å for 5 and 13.1125(46) Å for 6. The Ln⋯Cu distances bridged by the NIT moieties are 8.8760(8) Å for 4, 8.8735(11) Å for 5 and 8.8450(29) Å for 6. Chains packing of 4-6 are presented in Figures S8-S10, the closest interchain Ln⋯Ln and Ln⋯Cu and Cu⋯Cu contacts are 12.3530(6) Å and 11.1453(6) Å and 7.2391(6) Å for 4, 12.3718(12) Å and 11.1527(8) Å and 7.2487(7) Å for 5, 12.339(4) Å and 11.1443(24) Å and 7.2338(19) Å for 6. Magnetic properties

Figure 3. Temperature-dependent MT plots for 1 and 3 (left), 2 (right). The red line is theoretical curve based on the magnetic model for 1. The dc magnetic data of all six complexes over the 2-300 K were collected under 1000 Oe external field (Figures 3 and 4). The phase purity of 1-6 has been checked using PXRD patterns which well match with the simulated ones (Figures S11 and S12). As illustrated in Figure 3, the MT products at 300K are 9.84 cm3Kmol1 for 1, 13.75 cm3Kmol1 for 2 and 15.95 cm3Kmol1 for 3, close to the anticipated values (9.01 cm3Kmol1 for 1, 12.95 cm3K mol1 for 2 and 15.30 cm3Kmol1 for 3) for one uncoupled system including two S = 1/2 radicals, one CuII ion (S=1/2) and one lanthanide ion. For complex 1, as temperature is decreased, the χMT values gradually increase to a maximum of 11.02 cm3Kmol−1 at 4 K. Below this temperature the values of χMT decrease to 10.90 cm3K mol−1 at 2 K. This observed magnetic profile means the ferromagnetic Gd-nitroxide exchange.49 Based on the crystal structure of 1, these species of magnetic exchange couplings should be taken place: (i) the Gd-NO interaction (J1); (ii) the NO-ON magnetic 7



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exchange through the Gd(III) center (J2); (iii) the copper−nitroxide interaction via the benzene and pyridine rings (J3), which should be very weak and can be ignored. Hence, the magnetic data of 1 may be modeled as a tri-spin unit of NIT-Gd-NIT plus one independent Cu(II) ion; the spin Hamiltonian of the tri-spin NIT-Gd-NIT





unit can be described as H  2 J1 SˆGd SˆRad 1  SˆGd SˆRad 2  2 J 2 SˆRad 1SˆRad 2 . Other possible weak interactions between the NIT-Gd-NIT moiety and independent CuII are included by using the molecular mean-field approximation ( zJ) (eq 2). M  

Ng 2  2 165  84 exp(9 J1 / kT )  84 exp[(7 J1  2 J 2 ) / kT ]  35exp(16 J1 / kT ) { } 4kT 5  4 exp(9 J1 / kT )  4 exp[(7 J1  2 J 2 ) / kT ]  3exp(16 J1 / kT ) (1)

Ng 2  2 SCu ( SCu  1) 3kT

 total   M 1   zJ '  M Ng 2  2  

(2)

The simulation of experimental data produced the following magnetic parameters: g = 2.08, J1 = 1.02cm-1, J2 = -3.43 cm-1 and zJ '= -0.02 cm-1. The determined values of J1 and J2 match well with those of previous reports.50,51 For 2, the MT value steadily decreases from 300 to 36 K to attain a value of 13.35 cm3Kmol−1 and then lifts to 13.51 cm3Kmol−1 at 3 K and then declines to 13.46 cm3Kmol−1 at 2.0 K. While the MT product declines continuously to the value of 12.30 cm3Kmol−1 at 2 K for complexes 3,. The observed behaviors of 2 and 3 might be attributed to Tb/Dy-radical interactions as well as the effects of crystal-field of the DyIII or TbIII ions.

Figure 4. Thermal dependences of MT for 4-6. The red line is the curve calculated using Magpack program for 4. 8


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The room temperature MT values are 10.23 cm3Kmol1 for 4, 13.90 cm3Kmol1 for 5 and 16.53 cm3Kmol1 for 6 (Figure 4), which are close to the anticipated values (9.38 cm3Kmol1 for 4, 13.32 cm3K mol1 for 5 and 15.67 cm3Kmol1 for 6) for noninteracting rare earth ion, the radicals and copper(II) ions. For compounds 4 and 5, a continuous increase of the MT value with reducing the temperature is found and the maximums of 18.24 cm3Kmol-1 and 22.31 cm3Kmol-1 are obtained at 2 K, suggesting the dominant ferromagnetic couplings in the system. For compound 6, the MT value maintains almost unchanged until 40 K, then reduces to 14.14 cm3Kmol-1 at 2 K. For compound 4, the possible magnetic exchanges are as follows: (i) the copper(II)-nitroxide magnetic coupling through the whole radical ligand, that can be neglected due to the long distance; (ii) the copper(II)-nitroxide direct exchange (J1); (iii) the NO-ON coupling through the GdIII ion (J2); (iv) the Gd-nitroxide interaction (J3). Thus, the magnetic behavior of 4 could be modeled using the linear [Cu-NIT-GdNIT-Cu] motif (Scheme 1). To reproduce the magnetic data, MAGPACK software52 with the following spin Hamiltonian was employed.







H  2 J1 SˆCu1SˆRad 1  SˆRad 2 SˆCu 2  2 J 2 SˆRad 1SˆRad 2  2 J 3 SˆRad 1SˆGd  SˆGd SˆRad 2



Scheme 1. The scheme of magnetic exchanges in 4. J2 J1 Cu1

J3 NIT1

J3 Gd

J1 NIT2

Cu2

The fit to the magnetic data yields: g = 2.07, J1 = 23.92 cm−1, J2 = -6.98 cm−1 and J3 = 2.83 cm−1. The positive J1 demonstrates the ferromagnetic copper(II)nitroxide (axial) exchange, attributing to the orthogonal magnetic orbitals (the * (radical unit) and the dx2−y2 (Cu(II) ion) orbitals).46,53 The magnitude of the J1 is well compared with the exchange interactions found in other CuII-NIT complexes.54-56 The negative J2 and positive J3 are consistent with the antiferromagnetic radicalradical coupling via through the gadolinium(III) ion and the weak ferromagnetic gadolinium(III)-nitroxide interaction, respectively, as found in other reported Gd-NIT complexes.50,51

9



Figure 5. Field-dependent magnetization curve for 1 (2 K). The blue line is the theoretical behavior described in text. At 2.0K, the M vs. H curves of all six complexes were determined from 0 to 7 T (Figures 5, 6 and S13-S16). For complex 1, as the magnetic field strengthens, the M value attains 10.57 N at 7T, corresponding to the anticipated saturation value (10N) (Figure 5). In addition, the experimental M value is above corresponding theoretical data obtained from Brillouin equation for the system composed of independent three S=1/2 and one S=7/2 spins using T = 2.0K and g =2.0, clearly supporting ferromagnetic GdIII-nitroxide coupling. For 2 and 3, the M values reach 8.39 and 7.27 N at 70 kOe (Figures S13 and S14) and no saturations are observed because of the magnetic anisotropy originating from the 4f ions.

Figure 6. Field-dependent magnetization curve for 4 (2 K). The blue line is the theoretical behavior described in text. For 4, at low fields, the curve of M vs H displays a sharp rise and attains 11.43 Nβ at 7 KOe, reaching to the theoretical saturation value (11 Nβ) (Figure 6). The 10


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experimental M values are higher than those calculated by the Brillouin equation for the isolated spins (four S = 1/2 and one S = 7/2) using g = 2.0 and T = 2.0 K, further demonstrating the presence of ferromagnetic exchanges in 4. The M values exhibit a rapid increase with the magnetic field and attain 9.92 Nβ for 5 and 9.81 Nβ for 6 at 70 kOe (Figure S15 and S16). It can be noticed that M value does not reach saturation for 5 and 6, owing to the anisotropic 4f ions.

Figure 7. Thermal dependence of ac signals under zero (left) and 3000 Oe (right) dc field for 5.

Figure 8. Thermal-dependent ac signals for 3 in 3000 Oe dc field. To explore spin dynamic behaviors of Tb and Dy compounds, the ac susceptibilities were measured utilizing an oscillating 3.0 Oe field from 2K to 10 K at different frequencies. 5 exhibits clear frequency-dependent ′′ signals in zero dc field, which could be due to slow magnetic relaxation behavior (Figure 7). However, no 11



peak maxima of  was observed in the available window above 2 K for 5. For 2, 3 and 6, no non-zero  ac susceptibility was found in zero dc field (Figures S17-S19). Then the ac data were collected in 3 kOe dc field for all compounds to reduce or suppress the possible quantum tunneling process (QTM). 3 and 5 displayed frequency-dependent peaks in  susceptibility curves, yet no visible  peaks (Figures 7 and 8). Thus, the activation barrier (Ueff) and relaxation time (0) can't be obtained from the fit of Arrhenius law. If there is only one relaxation process in 3 and 5, the equation ln(/) = ln(0) + Ea/kBT could be used to roughly estimate the Ueff and 0 values.57,58 The best fitting ac data of 3 and 5 yield the energy barrier (Ea/kB) 8.86 K and 8.85 K with the relaxation time (0) 2.9210-7 s and 1.410-6 s, respectively (Figure 9). For 2 and 6, no χ″ components were observed even in a 3000 Oe dc field (Figures S20-S21).

Figure 9. Natural logarithm of / vs 1/T plots for 3 (left) and 5 (right), the lines are the linear fit to the equation given in the text.

CONCLUSIONS Two types of 2p-3d-4f chains bridged by the NIT-4PyPh radicals have been successfully achieved via varying the molar ratio of the starting materials. Complexes 1-3 display 1D chain structure with radical ligand bridging Ln and Cu ions via “head and head or tail and tail” mode. Complexes 3-6 exhibit interesting spin topologies assembled by LnIII units and [Cu-Radical]2 cyclic dimers. Magnetic investigations indicate that 3 and 5 exhibit frequency-dependencent χ″ signals, suggesting the slow magnetic dynamics. The present work not only demonstrates that functionalized nitronyl nitroxides are appealing blocks for constructing f-d heterometallic compounds exhibiting fascinating magnetic behaviors, but also provides the important information for the rational design of heterometallic chains by using paramagnetic 12


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nitronyl nitroxides. ASSOCIATED CONTENT Supporting Information Significant bond parameters, analysis of rare earth geometry, packing diagrams, PXRD patterns and ac and dc data. The CIF files of six complexes with CCDC 1892090-1892095 can be achieved from Cambridge Crystallographic Data Centre. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Licun Li: 0000-0001-8380-2946 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was found by NSFC (Nos. 21773122 and 21471083). REFERENCES (1) Mori, F.; Nyui, T.; Ishida, T.; Nogami, T.; Choi, K.-Y.; Nojiri, H. Oximatebridged trinuclear Dy-Cu-Dy complex behaving as a single-molecule magnet and its mechanistic investigation. J. Am. Chem. Soc. 2006, 128, 1440-1441. (2) Novitchi, G.; Wernsdorfer, W.; Chibotaru, L. F.; Costes, J. P.; Anson, C. E.; Powell, A. K. Supramolecular “Double-Propeller” Dimers of Hexanuclear CuII/LnIII Complexes: A {Cu3Dy3}2 Single-Molecule Magnet. Angew. Chem., Int. Ed. 2009, 48, 1614-1619. (3) Huang, Y. G.; Jiang, F. L.; Hong, M. C. Magnetic lanthanide–transitionmetal organic-inorganic hybrid materials: From discrete clusters to extended frameworks. Coord. Chem. Rev. 2009, 253, 2814-2834. (4) Rinck, J.; Novitchi, G.; Van den Heuvel, W.; Ungur, L.; Lan, Y.; Wernsdorfer, W.; Anson, C. E.; Chibotaru, L. F.; Powell, A. K. An Octanuclear

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For Table of Contents Use Only

2p-3d-4f Hetero-tri-spin Chains and Ring-chains Bridged by a Nitronyl Nitroxide Ligand: Structure and Magnetic Properties Juan Sun, Kang Wang, Pei Jing, Jiao Lu, Licun Li*

Synopsis: Two series of new 2p-3d-4f hetero-tri-spin compounds have been achieved via changing the experimental conditions. One is 2p-3d-4f 1D chain while the other possesses the interesting 1D ring-chain topological structure. Furthermore, Dy (chain) and Tb (ring-chain) derivatives exhibit slow relaxation of the magnetization.

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Structure and Magnetic Properties

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