A New Nitronyl Nitroxide Radical as Building Blocks for a Rare S = 13

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A New Nitronyl Nitroxide Radical as Building Blocks for a Rare S = 13/2 High Spin Ground State 2p-3d Complex and a 2p-3d-4f Chain Binling Yao, Zhilin Guo, Xuan Zhang, Yue Ma, Zhenhao Yang, Qing-Lun Wang, Li-Cun Li, and Peng Cheng Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01276 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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A New Nitronyl Nitroxide Radical as Building Blocks for a Rare S= 13/2 High Spin Ground State 2p-3d Complex and a 2p-3d-4f Chain Binling Yao†, ZhilinGuo†, Xuan Zhang‡, Yue Ma†*, Zhenhao Yang†, Qinglun Wang†, Licun Li† and Peng Cheng† †

Department of Chemistry and Key Laboratory of Advanced Energy Materials Chemistry (MOE) and TKL of

Metal and Molecule Based Material Chemistry, Nankai University, Tianjin 300071, China.

‡

Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States.

Keywords: High spin ground state / 2p-3d complex/ 2p-3d-4f chain complex / Crystal structure / Magnetic properties

ABSTRACT

A

new

nitronyl

nitroxide

radical

L

2-(4-(5-methyl-carbonyl-3-pyriyl)benzoxo)-4,4,5,5-tetramethylimidazoline

(L

=

-1-oxyl-3-oxide)

containing N-O groups and the pyridyl nitrogen group was designed and synthesized as a multidentate ligand to get compounds with interesting structures and magnetic properties from 3d or

3d-4f

precursors.

The

reaction

of

Cu(hfac)2

and/or

Gd(hfac)3·2H2O

(hfac

=

hexafluoroacetylacetonate) with L resulted in a rare S= 13/2 high spin ground state CuII complex [(Cu(hfac)2)7(L)6] (1) and a CuII - GdIII chain complex [Gd(hfac)3Cu(hfac)2(L)2]n·0.5CH2Cl2 (2). Single crystal X-ray diffraction studies indicate that the N-O groups of the L radicals are all axially bound to CuII ions in complex 1, which result in the ferromagnetic exchange between CuII and radicals and an S= 13/2 high spin ground state. While adding Gd(hfac)3 units to the system of


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Cu(hfac)2 and L radical, a one dimension chain structure is obtained, and there are ferromagnetic GdIII-radical interactions and antiferromagnetic radical-radical coupling in the chain.

INTRODUCTION

The study on metal-organic radical complexes has attracted more and more attention in the past few decades due to the intriguing magnetic interactions between metal ions and the organic spin carriers.1-3 For example, Long and coworkers reported their work on radical bridged lanthanide single molecular magnets with high spin-reversal barrier and record TB.4,5 Among the commonly used organic radicals, nitronyl nitroxide radicals (NITR) are more attractive because they are stable and easy to synthesize.6-9 In addition, modification of the substituted R groups of NITR with functional coordination groups can generate various metal-radical complexes with diverse magnetic properties.10,11 For example, Cu(II)-nitronyl nitroxide radicals complexes with different R group can form mono- and polynuclear complexes, as well as one dimensional and higher dimensional archetechtures.12-23 To pursue polynuclear curpic adducts with high spin ground state, a

novel

bridging

tridentate

nitronyl

nitroxide

radical

L

(L

=

2-(4-(5-methyl-carbonyl-3-pyriyl)benzoxo)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)

with

N-O groups and pyridyl ring functional coordination groups, which can provide valuable radical building blocks for the design and synthesis of 3d or 3d-4f complexes of high nuclearity. Herein, a high spin ground state (S= 13/2) is realized in a heptanuclear L radical-based Cu(II) complex. Additionally, 2p-3d-4f compounds containing three different spin carriers are not widely investigated because of the difficulty of synthesis.3, 24-28 According to the Pearson classification, lanthanide ions are considered to be hard acids and prefer to coordinate with oxygen atoms, while



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transition metal ions prefer N atoms.29-31 With this in mind, L radical links Gd(III) ion and Cu(II) ion through the N-O group and the pyridyl nitrogen respectively, forming a one-dimensional 2p-3d-4f compound.

Scheme.1 Chemical structure of nitronyl nitroxide radical L.

EXPERIMENTAL SECTION

Materials and Physical techniques

All commercially available chemicals and solvents were used as received without further purification. The salts Cu(hfac)2, Gd(hfac)3·2H2O and the organic nitroxide radical L were synthesized according to similar methods as reported in the literature.6-9,28 All of IR spectra were obtained with a Bruker TENOR 27 spectrophotometer in the range of 4000-400 cm-1. Elemental analyses were carried out using Perking-Elmer elemental analyzer model 240. Magnetic susceptibility measurements (with a field of 1000 Oe) in the temperature range of 2-300 K were carried out using a SQUID MPMS XL-7 magnetometer. The diamagnetic corrections of the samples were estimated using Pascal’s constants.32-34

Synthesis of complex [(Cu(hfac)2)7(L)6] (1) Cu(hfac)2 (0.02 mmol, 0.009 g) was dissolved in 20 ml of boiling n-heptane. Then the


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solution was cooled to 65 ºC, to which a CH2Cl2 solution containing 0.02 mmol (0.008 g) of L was added with stirring for 15 min. The mixture was cooled, filtered and stored at room temperature. After several days, dark-green crystals were obtained for single crystal X-ray analysis. Anal. calcd (1): C190H146Cu7F84N18O58: C, 40.39; H, 2.60; N, 4.46%. Found (1): C, 40.40; H, 2.59; N, 4.47. IR (KBr, ν/cm−1): 1732(w), 1643(s), 1458(s), 1360(s), 1253(s), 1198(vs), 1139(vs), 996(m), 796(w), 764(w), 670(w), 588(m).

Synthesis of complex [Gd(hfac)3Cu(hfac)2(L)2]n·0.5CH2Cl2 (2) Gd(hfac)3·2H2O (0.02 mmol, 0.016g) was dissolved in 20 ml of boiling n-heptane. Then the solution was cooled to 65 ºC, to which a CH2Cl2 solution containing 0.02 mmol (0.016 g) of L was added with stirring for 30 min. Then anhydrous Cu(hfac)2 (0.02 mmol, 0.009 g) was added under stirring for another 15 min. The mixture was cooled, filtered and stored at room temperature. After several days, dark-green crystals were obtained for single crystal X-ray analysis. Anal. calcd (2): C131H100Cl2Cu2F60Gd2N12O40: C, 38.03; H, 2.44; N, 4.06%. Found (2): C, 38.05; H, 2.44; N, 4.07 IR (KBr, ν/cm−1): 1725(w), 1650(s), 1488(w), 1309(s), 1253(s), 1197(vs), 1142(vs), 1003(w), 797(m), 660(s), 584(w).

X-ray crystal structure determination

The diffraction intensity data of L, 1 and 2 was collected on a Rigaku Saturn CCD diffractometer using the standard procedure of the Mo Kα radiation at 113 K for L, 1 and 2. The structures of all non-hydrogen atoms were solved by direct methods and refined by the full-matrix least-squares methods on F2 anisotropically using SHELXL-97.35, 36 The H atoms were added as the difference electron density of geometry and refined with an isotropic



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thermal parameter as the riding groups.37,

38

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The crystal data and selected structural

parameters for compound L, 1 and 2 are provided in the ESI, Table 1 and S1-S3. Crystallographic data for the structures of this paper provided have been deposited, CCDC 1477620, 1477584 and 1477361, respectively.

Table 1 Crystal data and structure parameters for compounds L, 1 and 2.

Empirical formula Formula weight Temperature / K Crystal system Space group a [Å] b [Å] c [Å] α [deg] β[deg] γ [deg] Volume [Å3] Z ρcalc [g cm-3] µ [mm-1] F(000) Reflections collected Unique / parameters R(int) Theta/Completeness Max. / min. transmission Goodness-of-fit on F2 R1,wR2 [I>2σ(I)] R1,wR2 (all data)

L C20H22N3O5 384.41 113 monoclinic P21/n 7.0466(11) 17.605(3) 15.244(2) 90 101.379(4) 90 1854.0(5) 4 1.377 0.100 812.0 15916 3254/341 0.0209 25.000/99.5% 1.000/0.97 1.080 0.0294, 0.0754 0.0323, 0.0773

1 C190H146Cu7F84N18O58 5650.03 113 triclinic P-1 10.4986(10) 18.9418(14) 28.946(2) 77.898(4) 80.006(4) 85.532(5) 5537.8(8) 1 1.694 0.814 2835.0 60983 19370/1624 0.0260 25.009/99.0 % 1.000/ 0.873 1.010 0.0440, 0.1033 0.0545,0.1104

2 C65.5H50ClCuF30GdN6O20 2067.36 113 triclinic P-1 12.8049(10) 16.2402(10) 21.2093(13) 69.755(4) 79.564(4) 82.558(5) 4058.9(5) 2 1.692 1.244 2050.0 44666 14146/1155 0.0310 25.010/99.0 % 1.000/ 0.822 1.047 0.0288, 0.0681 0.0344,0.0707

RESULTS AND DISCUSSION

Description of the crystal structures

The radical L is a bridging tridentate ligand with variable coordination groups of two N-O groups and pyridyl ring, so it could possibly link metal ions by “head and head” through two N-O groups or “head and tail” through N-O and pyridine.10, 39 The crystal structure and


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packing arrangement of the radical are shown in Figure1 and Figure S1. L crystallizes in the monoclinic space group P21/n. The typical N-O distances and dihedral angle between the planes CN2O2 of nitronyl nitroxides are 1.2848(12) Å (O4-N2), 1.2768(12) Å (O5-N3) and 10.619 °, respectively, which is in good agreement with those reported in other NITR radicals.40, 41 The molecules form a zigzag dimerized structure via π-π interaction between the pyridyl rings from different molecules, with a centroid-centroid distance of 3.907(1) Å. The shortest intermolecular N-O distance is 3.488(1) Å.

Figure 1 The crystal structure of radical L, H atoms are omitted. Crystal structure of complex 1[(Cu(hfac)2)7(L)6]. In Figure 3, complex 1 has an “inversion butterfly-like” centrosymmetric hepta-nuclear structure crystalizing in the triclinic space group P-1, consisting of seven Cu(hfac)2 units linked by six L radicals to form a spin S= 13/2 complex (seeing from magnetic properties). In this complex, seven CuII ions are all six coordinated by L radicals and hfac ligands in an octahedron geometry. And Cu4 is in the centre of the complex, which is coordinated to two radicals through N-O group in the axial position (Cu4-O27 2.344(2) Å). Additionally, Cu1, Cu2 and Cu3 have a similar elongated octahedron geometry formed by hfac, radical N-O and pyridine groups. The O atom of the radical occupied axial coordinate sites in the



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adduct of Cu(hfac)2, as a result, the apical Cu(Cu1, Cu2, and Cu3)-O(radical) and Cu-O(hfac) bond lengths are 2.453(2)-2.697(2) Å and 2.221(3)-2.264(2) Å, while the equatorial Cu-O(hfac)/N(pyridine) are much shorter than the apical bonds(Table S2). Cu1 links to Cu3 by radical L through “head and head” mode, Cu2 links to Cu1 or Cu3 through “head and tail” mode, and links to Cu3 by “head to tail” mode, which make the centrosymmetric hepta-nuclear copper complexes. The whole molecular and packing arrangement of 2 is shown in ESI, Figure S2.

Figure 2 The symmetric unit of complex 1, H and F atoms are omitted. Crystal structure of complex 2 [Gd(hfac)3Cu(hfac)2(L)2]n·0.5CH2Cl2. When combining three different spins (Gd III, CuII ions and radical) in one system, the structure of comoplex 2 changes to a one dimension chain (Figure 4 and Figure S3) according to the Pearson classification29-31. In this principle, O atoms prefer to coordinate to Gd(III) ions which is a hard acid, while the N atoms coordinate to Cu(II) ions. As a result, GdIII ions are linked to CuII ions through L radical by “head and tail” mode. And complex 2 crystallizes in the triclinic space group P-1 and the repeating unit contains one Cu(hfac)2, one Gd(hfac)3, two L radicals, and half of a dichloromethane solvent molecule also exists between the


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adjacent chains. Gd(III) ion has an eight-coordinated D2d triangular dodecahedral geometry (as shown in Figure S4 and calculated by the SHAPE software42-44), formed by six oxygen from three hfac and other two O atoms from the N-O groups of L radicals. The Gd-O(rad) bond lengths are 2.340(18) Å and 2.288(18) Å and the nearest intrachain and interchain GdIII···GdIII distances are 19.335 (13) Å and 12.805(10) Å. The distance of GdIII···CuII is 9.8475(7) Å. An elongated octahedral geometry of CuII ion is constructed by two bidenate hfac ions and two L radical through pyridine N. In the equatorial plane of octahedral geometry, the Cu-O and Cu-N distances fall in the range of 1.964(18)-2.026(2) Å, while the apical Cu-O bond lengths associated with the hfac groups are 2.295(19) Å and 2.232(19) Å. The packing arrangement of the chains of complex 2 is shown in Figure S3. There are no inter-/intra-molecular π-π interactions and hydrogen bonding interactions between the molecules.

Figure 3 Crystal structure of complex 2 (H and F atoms are omitted for clarity).

Magnetic susceptibility studies.

The temperature-dependence of χMT and χM plots for complex 1 in the range of 2-300 K under 1000 Oe field are displayed in Figure 4. The χMT of room-temperature is 5.35 cm3 K



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mol-1, which is in good agreement with those reported for other CuII complexes,16,17,20,41,45 and is higher than the expected values χMT (4.88 cm3 K mol-1) of non-interacting six radicals (S= 1/2，g= 2, χMT= 0.375 cm3 K mol-1) plus seven cupric ions (S= 1/2, g> 2,

χMT= 0.375 cm3 K mol-1). Upon cooling, the plot of χMT increases steadily to 8.21 cm3 K mol-1 at 2 K, indicating the dominating ferromagnetic interactions in the complex.

From the crystal structure point of view, there are two kinds of magnetic interactions (also can be seen in scheme 2), namely, (i) Cu(II) coupling with radical through N-O group, J1 and J3, (ii) Cu(II) coupling with radical through nitrogen atom of pyridine ring, J2. To evaluate the magnetic interactions in the spin 13/2 system, the magnetic susceptibility data can be analyzed by using the Magpack program46 based on the following Hamiltonian.

H = −2 J1 ( S Rad 1SCu 2 +S Rad 2 SCu 3 + S Rad 3 SCu 4 ) − 2 J 2 ( S Rad 1SCu1 + S Rad 2 SCu 2 + S Rad 3 SCu 3 ) −2 J 3 ( S Rad 2 SCu1 )

The best fitting parameters were obtained as g= 2.05 and the exchange coupling values J1,

J2, and J3 (which are shown to be the interactions between different paramagnetic center in scheme 2) are 18.72, 0.06 and 16.73 cm-1, respectively. As is known, the magnetic coupling between Cu(II) ions and radicals through N-O groups is always much stronger than that through the nitrogen atom of pyridine.47,48 And the value differences of J1 and J3 is possibly due to the various CuII-Orad bond distances.13,41 From magneto-structure point of view, the N-O groups of the radicals L are all axially bound to CuII ions in complex 1, which makes the π* orbital of NO group orthogonal to dx2−y2 orbital of Cu2+. This results in parallel spin


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orientations of copper(II) ion and nitronyl nitroxide and hence the ferromagnetic exchange between CuII and radicals with an S= 13/2high spin ground state. The M versus H curve at 2.0 K for complex 1 is shown in Figure S5 and the magnetization value 13.30 Nβ at 7 T is slightly higher than the expected saturation value of 13.00 Nβ, as predicted by the Brillouin function for 13 uncoupled spins (g= 2, T= 2 K), which supports the presence of ferromagnetic interaction in this complex. (Figure S5)

Figure 4 The experimental temperature-dependence magnetic susceptibility of complex 1 (○ for χMT, □ for χM and solid lines for the theoretical fits). Scheme 2 Magnetic interactions in complex 1.

The variable-temperature magnetic susceptibilities for 2 were measured between 2-300 K under 1000 Oe field as shown in Figure 5. The χMT value at 300 K is 18.09 cm3 K mol-1, which is close to the expected values of 17.26 cm3 K mol-1 for one uncoupled GdIII ion (S= 7/2，g= 2, χMT= 7.88 cm3 K mol-1), two L (S= 1/2，g= 2, χMT= 0.375 cm3 K mol-1) and one Cu II ion (S= 1/2, g> 2, χMT= 0.375 cm3 K mol-1). Monotonic increase of the χMT value is



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observed in Figure 6 from 17.26 cm3 K mol-1at 300 K to 29.51 cm3 K mol-1 at 2 K. As reported that the exchange interaction between the Cu(II) ion and nitroxide radical via the pyridine nitrogen is always very weak, the system can be simplified as a three spin unit of RadO-N-GdIII-Rad O-N plus an independent Cu(II) ion. Thus, the magnetic interaction in this system can be well fitted by Equation (1) - (2), using an isotropic exchange Hamiltonian

H = −2 J1 ( SRad1SGd1 + SGd1SRad 2 ) − 2 J 2 SRad1SRad 2 to analyze the RadO-N-GdIII-RadO-N three spin unit. The J1 and J2 in the Hamiltonian can be described as the exchange coupling between Rad-GdIII and the exchange interaction of two L radicals through the rare earth ion. The possible magnetic couplings between three spin unit and independent Cu(II) are introduced as the zJ’ term in Equation (2).

χ +

m

=

N g 2β 4 kT

N g C2 u β 3kT

χ to ta l =

2

1 6 5 + 8 4 ex p ( − 9 J 1 / kT ) + 8 4 ex p [(− 7 J 1 − 2 J 2 ) / kT ] + 3 5 ex p (− 1 6 J 1 / kT ) 5 + 4 e x p ( − 9 J 1 / kT ) + 4 ex p [(− 7 J 1 − 2 J 2 ) / kT ] + 3 ex p (− 1 6 J 1 / kT )

2

S C u ( S C u +1 )

(1)

χm '

1 − ( zJ χ m / N g 2 β 2 )

(2)

The best fitting yields g = 2.02, J1 = 0.30, J2 = -2.35, zJ’ = 0.025 and gCu = 2.08. The positive J1 indicates ferromagnetic interactions between Gd(III) and the radical. The negative J2 can be considered as the antiferromagnetic coupling between the radicals through GdIII ion which is consistent with what has been reported in literature.49, 50


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Figure 5 Temperature dependence of the magnetic χMT and χM products of 3(○ for χMT, □ for χM and solid lines for the theoretical fits). CONCLUSIONS

A new multidentate radical containing variable functional coordination groups of two N-O groups and pyridyl ring has been reported to provide a possibility to link metal ions by “head and head” though two N-O groups or “head and tail” though N-O and pyridine. The radical L coordinates with Cu(hfac)2 and/or Gd(hfac)3·2H2O to produce a rare high spin ground state S= 13/2 CuII complex and a CuII-GdIII chain compound. The positive exchange interaction in complex 1 are consistent with the point in which an octahedral Cu(II) ion axially coordinated with the N-O groups of the radical ligand leads to ferromagnetically coupling. After adding isotropic Gd(III) ions, the structure changes from an inversion butterfly complex to one dimensional chains. The change of structure and central metal resulted in a different magnetic behavior. As a result, the magnetic coupling of Gd(III) ions and L radical is ferromagnetic and there is antiferromagnetic coupling between the radicals through GdIII ion. This investigation may open up new opportunities to develop high spin ground 2f-3d complexes and novel 2p-3d-4f magnetic complexes.



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ASSOCIATED CONTENT

Electronic supplementary information (ESI) available. X-ray crystallographic data for L，1， 2 in CIF format. Selected bond lengths and angles, Figures of crystal structures, and magnetic measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Funding Sources This work was supported by the National Natural Science Foundation of China 21471084, 21471083, 21371104 and 21101096. 100 Projects of Creative Research for the Undergraduates of Nankai University in China (Grant No.BX14210)

Notes The authors declare no competing financial interest.

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For Table of Contents Use Only

A New Nitronyl Nitroxide Radical as Building Blocks for a Rare S = 13/2 High Spin Ground State 2p-3d Complex and a 2p-3d-4f Chain Binling Yao†, Zhilin Guo† , Xuan Zhang‡, Yue Ma†*, Zhenhao Yang†, Qinglun Wang†, Licun Li† and Peng Cheng†

Using New nitronxyl nitroxides containing variable functional coordination cites as two N-O groups and pyridyl ring to coordinate Cu(hfac)2 and/or Gd(hfac)3·2H2O achieved a rare high spin ground state S = 13/2 CuII complex and a CuII -GdIII chain complex. The CuII-L complex exhibits an “inversion” butterfly but the GdIII-CuII-L is a one dimensional chain, which arise from the Pearson classification.


A New Nitronyl Nitroxide Radical as Building Blocks for a Rare S = 13

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