4f-Metal Cluster Chemistry

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New dioximes as bridging ligands in 3d/4f-metal cluster chemistry: 1-D chains of ferromagnetically-coupled {Cu6Ln2} clusters bearing acenaphthenequinone dioxime and exhibiting magnetocaloric properties Paul Richardson, Kevin J Gagnon, Simon J. Teat, Giulia Lorusso, Marco Evangelisti, Jinkui Tang, and Theocharis C. Stamatatos Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00011 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 18, 2017

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Crystal Growth & Design

New dioximes as bridging ligands in 3d/4f-‐metal cluster chemistry: 1-‐D chains of ferromagnetically-‐coupled {Cu6Ln2} clusters bearing acenaphthenequinone dioxime and exhibiting magnetocaloric prop-‐ erties Paul Richardson,† Kevin J. Gagnon,# Simon J. Teat,# Giulia Lorusso,§ Marco Evangelisti,§ Jinkui Tang,*,‡ and Theocharis C. Stamatatos*,† †

Department of Chemistry, 1812 Sir Isaac Brock Way, Brock University, L2S 3A1 St. Catharines, Ontario, Canada

‡

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China #

Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA

§

Instituto de Ciencia de Materiales de Aragón (ICMA) and Departamento de Física de la Materia Condensada, CSIC-‐Universidad de Zaragoza, 50009 Zaragoza, Spain Supporting Information ABSTRACT: The employment of the tetradentate ligand acenaphthenequinone dioxime (acndH2) for a first time in heterome-‐ II

III

tallic Cu /Ln (Ln = Gd and Dy) chemistry has afforded the 1-‐D coordination polymers [Cu6Gd2(acnd)6(acndH)6(MeOH)6]n (1) and [Cu6Dy2(acnd)6(acndH)6(MeOH)2]n (2), which consist of repeating {Cu6Ln2} clusters that are intermolecularly linked to each 2 1 1 2-‐ 4+ other through the oximate groups of two η :η :η :μ3 acnd ligands. The [Cu6Ln2(μ3-‐NO)6(μ-‐NO)8] core is unprecedented in heterometallic cluster chemistry and comprises two symmetry-‐related {Cu3Ln} subunits, each with a distorted trigonal pyrami-‐ dal topology. Magnetic susceptibility studies revealed the presence of predominant ferromagnetic exchange interactions within the {Cu3Ln} subunits and weak antiferromagnetic interactions between them. As a result, the magnetic and magnetocaloric properties of the {Cu6Gd2}n compound could be rationalized in terms of two weakly-‐coupled S = 5 spins that yield a magnetic -‐1 -‐1 entropy change of -‐ΔSm = 11.8 Jkg K at T = 1.6 K for µ0ΔH = 7 T.

of adopting geometries associated with large coordination numbers (i.e., from 7 up to 12).

INTRODUCTION II

The rich coordination chemistry of Cu has been further developed over the last decade mainly due to its combination with a variety of highly paramagnetic 4f-‐metal ions, such as III III III 1-‐8 Gd , Dy and Tb . With the necessary assistance of thor-‐ oughly chosen organic chelating/bridging ligands, the II III Cu /Ln heterometallic “blend” has yielded many beautiful coordination complexes with large nuclearities (coordination 9-‐14 clusters) and occasionally nanosized dimensions. There-‐ fore, from a structural viewpoint, there is no doubt that this chemistry is very promising in delivering compounds with aesthetically pleasing motifs and topologies not previously seen in homometallic 3d-‐ and 4f-‐metal cluster chemistry II III alone. The structural diversity of Cu /Ln complexes mainly II stems from the ability of Cu ions to coordinate with many different donor atoms (i.e., N and O donors) and adopt a wide variety of geometries, from square planar to trigonal bipyramidal and distorted octahedral. In addition, 4f-‐metal ions are known to primarily bind to O-‐donor ligands, as a result of their pronounced oxophilicity, and they are capable

II

III

Apart from the structural interest in Cu /Ln cluster chemistry, there are also important prospects of this diverse research field in various areas of molecular magnetism, such 15-‐19 as single-‐molecule magnetism and magnetic refrigera-‐ 20-‐25 tion. Single-‐molecule magnets (SMMs) are molecular species that show superparamagnet-‐like properties and ex-‐ 15-‐19,26-‐28 hibit relaxation of their magnetization. Experimental-‐ ly, SMMs exhibit frequency-‐dependent out-‐of-‐phase alternat-‐ ing-‐current (ac) magnetic susceptibility signals and hystere-‐ 29 sis loops, the diagnostic property of a magnet. On the other hand, magnetic refrigeration is based on the magnetocaloric effect (MCE), i.e., the change of magnetic entropy (ΔSm) and adiabatic temperature (ΔTad) following a change of the ap-‐ plied magnetic field (ΔH), and can be used for cooling pur-‐ 20-‐25 poses via adiabatic demagnetization. II

III

Heterometallic Cu /Gd complexes have already proved 9-‐14,30-‐33 their ability to act as molecular magnetic refrigerants. II III The Cu ·∙·∙·∙Gd magnetic interactions are usually weak due to

1

ACS Paragon Plus Environment

Crystal Growth & Design III

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III

the very efficient shielding of the Gd 4f orbitals by the fully 9-‐14,30-‐39 occupied 5s and 5p orbitals. This generates multiple low-‐lying excited states, which enhance the dependence of the magnetocaloric effect on the applied field. Furthermore, II III both Cu and Gd metal ions possess small to negligible magnetic anisotropy. Magnetic anisotropy serves to remove 2,40 the degeneracy of the ms or mJ microstates of a molecule. While having a well-‐isolated spin ground state is desirable for an efficient SMM, in a molecular magnetic refrigerant, it is preferable to have all of the ms or mJ states degenerate to increase the degrees of freedom (i.e. the entropy) present at 20-‐25 the spin ground state. Furthermore, recall that magnetic anisotropy describes the directional dependence of the mag-‐ 15-‐25 netization. The larger the magnetic anisotropy is, the less sensitive the spin polarization to the external magnetic field. Therefore, relatively larger external magnetic fields would be needed to rotate the spins, thus decreasing the maximum of the MCE in the molecule.

OH

OH

46

EXPERIMENTAL SECTION Syntheses. All manipulations were performed under aero-‐ bic conditions using chemicals and solvents as received. Acenaphthenequinone dioxime (acndH2) was prepared and characterized according to a literature method described 45-‐47 elsewhere. The lanthanide(III) acetylacetonate precur-‐ sors, Ln(acac)3·H2O (Ln = Gd, Dy), were synthesized as pre-‐ 48 viously reported. [Cu6Gd2(acnd)6(acndH)6(MeOH)6]n (1). A tan solid of the ligand acndH2 (0.04 g, 0.2 mmol) was added to a yellow-‐ green solution of CuCl2·∙2H2O (0.02 g, 0.1 mmol) in DMF (15 mL). The resulting dark green solution was stirred for 5 min, during which time NEt3 (14 μL, 0.1 mmol) was added, result-‐ ing in a color change of the solution from dark green to brown. Addition of solid Gd(acac)3·H2O (0.10 g, 0.2 mmol) and further stirring for 20 min led to a brown suspension which was filtered to remove the insoluble materials. The resulting brown filtrate was mixed with MeOH (15 mL) and left undisturbed at ambient temperature to afford orange plate-‐like crystals of 1·0.5MeOH after ~14 days. The crystals were collected by filtration, washed with cold MeOH (2 x 2 mL) and dried in air. The yield was 20 %. Elemental analysis (%) calcd for the lattice solvent-‐free 1: C 52.74, H 3.01, N 9.84; found: C 52.59, H 2.95, N 9.92. Selected IR data (ATR): ν = 1653 (m), 1592 (m), 1527 (w), 1485 (m), 1378 (m), 1324 (m), 1276 (m), 1199 (m), 1134 (m), 1004 (vs), 972 (s), 894 (m), 821 (m), 768 (s), 734 (m), 685 (m), 664 (m), 541 (m), 506 (m), 474 (m), 438 (m). [Cu6Dy2(acnd)6(acndH)6(MeOH)2]n (2). This complex was prepared in the exact same manner as complex 1, but using Dy(acac)3·H2O (0.10 g, 0.2 mmol) in place of Gd(acac)3·H2O. After 14 days, X-‐ray quality orange crystals of 2·2.6MeOH·4DMF were collected by filtration, washed with cold MeOH (2 x 2 mL) and dried in air. The yield was 35 %. Elemental analysis (%) calcd for 2·4DMF: C 52.85, H 3.20, N 10.92; found: C 52.64, H 2.96, N 10.96. Selected IR data (ATR): ν = 1700 (m), 1649 (m), 1591 (m), 1532 (w), 1486 (m), 1381 (m), 1320 (m), 1258 (m), 1196 (m), 1133 (m), 1024 (vs), 1001 (s), 967 (s), 894 (m), 821 (m), 769 (s), 661 (mb), 543 (m), 510 (m), 472 (m), 435 (m).

Scheme 1. Structural Formula and Abbreviation of the Organic Chelating/Bridging Ligand Used in this Work. The Arrows Indicate the Potential Donor Atoms.

N

II/III

antiferromagnetically-‐coupled Mn and Mn clusters. We herein report the synthesis, structures and detailed mag-‐ II III netic studies of a new family of {Cu 6Ln 2} clusters which extend their structures into covalently-‐linked 1-‐D chains; the compounds are predominantly ferromagnetically-‐coupled and the {Cu6Gd2}n analogue shows magnetocaloric proper-‐ ties.

One of the key synthetic variables to the preparation of II III structurally novel and magnetically interesting Cu /Ln complexes is the choice of the organic chelate. The latter should be able to satisfy the coordination needs of both metal ions by comprising the preferable donor atoms and concurrently showing a bridging capacity which would allow for the aggregation of many metal centers into a polymetallic motif. Thus, there is a continuous need for the employment of new organic chelates with negligible previous use in metal cluster chemistry as a means of obtaining unprecedented compounds with multiple physical properties. Hence, we decided to synthesize, characterize and use a new dioxime II III ligand in Cu /Ln (Ln = Gd, Dy) chemistry as an extension of our previous interest in the use of pyridyl dioximes in 3d-‐, 41-‐44 4f-‐ and 3d/4f-‐metal cluster chemistry. The resulting mol-‐ ecule was acenaphthenequinone dioxime (acndH2, Scheme 1), which comprises four oximate-‐based donor atoms (2N and 2O atoms) and includes the bulky acenaphthene functionali-‐ ty, which could enhance the solubility and crystallinity of coordination compounds in a variety of polar and nonpolar solvent media.

N

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X-‐ray Crystallography. Diffraction data for both com-‐ plexes 1 and 2 were collected at the Advanced Light Source, Lawrence Berkeley National Lab on beamline 11.3.1, using synchrotron radiation monochromated (silicon(111) to a wavelength of 0.7749(1) Å). Samples were mounted on MiTeGen® kapton loops and placed in a 100(2) K nitrogen cold stream provided by an Oxford Cryostream 700 Plus low temperature apparatus on the goniometer head of a Bruker D8 diffractometer equipped with a PHOTON 100 CMOS detector operating in shutterless mode. An approximate full-‐ sphere of data was collected using a combination of phi and

acenaphthenequinone dioxime acndH2 The ligand acndH2 has only been used in homometallic 3d-‐ metal cluster chemistry for the synthesis of a family of {Zn3} 45 linear clusters with luminescent properties and a series of

2


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Crystal Growth & Design Crystal type

omega scans with scan speeds of 1 second per 4 degrees for the phi scans, and 5 seconds per degree for the omega scans at 2θ = 0 and -‐45°, respectively.

-‐1

Fw / g mol

C73H43Cu3DyN12O13

1708.20

1649.37

Triclinic

P-‐1

P-‐1

a / Å

14.2405(6)

14.2319(9)

b / Å

16.3599(7)

17.6762(11)

c / Å

17.0630(6)

17.9066(11)

α / °

111.529(2)

61.904(3)

β / °

105.305(2)

67.059(4)

γ / °

102.670(3)

70.679(4)

3337.9(2)

3598.9(4)

2

2

100(2)

100(2)

1.708

1.695

2.520

2.491

θ range (°)

2.343-‐29.061

2.431-‐29.032

Index ranges

-‐17 ≤ h ≤ 16 -‐20 ≤ k ≤ 19 0 ≤ l ≤ 21 13788

-‐17 ≤ h ≤ 17 -‐22 ≤ k ≤ 22 -‐22 ≤ l ≤ 22 49636

3

Z T / K -‐3

Dc / g cm -‐1

μ / mm

Reflections collected Independent reflec-‐ tions Final R indices a,b [I>2σ(I)] Final R indices (all data) -‐3 (Δρ)max,min / e Å

10389 (Rint = 14744 (Rint = 0.0866) 0.0570) R1 = 0.0487 R1 = 0.0470 wR2 = 0.1026 wR2 = 0.0913 R1 = 0.0851 R1 = 0.0851 wR2 = 0.1234 wR2 = 0.1222 1.882 and 1.038 and -‐1.685 -‐1.108 a b 2 2 2 2 2 1/2 R1 = Σ(||Fo| – |Fc||)/Σ|Fo|. wR2 = [Σ[w(Fo -‐ Fc ) ]/ Σ[w(Fo ) ]] , w = 2 2 2 2 2 1/[σ (Fo ) + [(ap) +bp], where p = [max(Fo , 0) + 2Fc ]/3.

Physical Measurements. Infrared spectra were recorded in the solid state on a Bruker’s FT-‐IR spectrometer (ALPHA’s -‐1 Platinum ATR single reflection) in the 4000-‐400 cm range. Elemental analyses (C, H, and N) were performed on a Per-‐ kin-‐Elmer 2400 Series II Analyzer. Variable-‐temperature direct and alternating current (dc and ac, respectively) mag-‐ netic susceptibility studies were conducted at the tempera-‐ ture range 1.9-‐300 K using a Quantum Design MPMS XL-‐7 SQUID magnetometer equipped with a 7 T magnet. Pascal’s constants were used to estimate the diamagnetic correction, which was subtracted from the experimental susceptibility to 56 give the molar paramagnetic susceptibility (χΜ). Heat ca-‐ pacity data were collected for temperatures down to 0.3 K by 3 using a Quantum Design PPMS, equipped with a He cryo-‐ stat. The experiments were performed on a thin pressed pellet (ca. 1 mg) of a polycrystalline sample, thermalized by ca. 0.2 mg of Apiezon N grease, whose contribution was subtracted by using a phenomenological expression.

Table 1. Crystallographic Data for Complexes 1 and 2 C75H51Cu3GdN12O15

Triclinic

V / Å

Formula

0.04×0.02×0.01

Space group

Additional information about crystallographic data collec-‐ tion and structure refinement details are summarized in Table 1. Crystallographic data for the reported structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication numbers: CCDC-‐1524445 and 1524446 for complexes 1 and 2, respecti-‐ vely. Copies of these data can be obtained free of charge via https://summary.ccdc.cam.ac.uk/structure-‐summary-‐form.

2

0.05×0.03×0.01

Crystal system

The images obtained during the data collection for com-‐ 50 plex 2 were processed with the software SAINT, and the absorption effects were corrected by the multi-‐scan method 52 implemented in SADABS. All the non-‐hydrogen atoms were successfully refined using anisotropic displacement parame-‐ ters. Hydrogen atoms were placed geometrically on the car-‐ bon atoms and refined with a riding model. Hydrogen atoms on the oxygen atoms were found in the Fourier difference map, their distances were fixed and allowed to refine with a riding model. The structures were solved using the algorithm 53,54 implemented in SHELXT, and refined by successive full-‐ 2 matrix least-‐squares cycles on F using the latest SHELXL-‐ 53,55 v.2014. One DMF molecule in 2 is disordered over multi-‐ ple positions. Two positions were modelled and SAME/SIMU/DELU were utilized appropriately.

1

Orange block

Crystal size / mm

Several crystals of 1 were tried and the reported data are from the best crystal. The diffraction pattern showed “twin-‐ 49 ning”. Using cell_now, two orientation matrices were de-‐ termined; the relationship between these components was found to be 3.2 degrees about real axis -‐0.888 1.000 -‐0.202. 50 The data was integrated using the two matrices in SAINT. TWINABS was used to produce a merged HKLF4 file for structure solution and initial refinement, and HKLF5 file for 51 final structure refinement. The HKLF5 file contained the merged reflections first component and those that over-‐ lapped with this component, which were split into two re-‐ flections. TWINABS indicated the twin fraction to be 60:40. The structure was solved using the HKLF4 file, but the best refinement was given by the HKLF5 file. The final refinement gave a ratio of 59:41. All non-‐hydrogen atoms were refined anisotropically, except from the partial occupied methanol solvent molecule, which was left isotropic. Hydrogen atoms were placed geometrically on the carbon atoms of the lig-‐ ands. Hydrogen atoms on the oxygen atoms were placed to give the best hydrogen bonds, constrained and refined using a riding model. On the coordinated methanol molecules, the hydrogen atoms on the carbons were placed to give the ideal staggered conformation. The OH-‐hydrogen atoms could neither be found nor placed and the same was the case for the partial solvent molecule; thus, they were both omitted from the refinement but not from the chemical formula.

Parameter

Orange plate 3

RESULTS AND DISCUSSION Synthetic Comments. The general reaction between CuCl2·∙2H2O, Ln(acac)3·H2O, acndH2 and NEt3 in a 1:2:2:1 molar ratio, in a solvent mixture comprising DMF and

3



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MeOH, has afforded orange crystals of a new family of 1-‐D coordination polymers in yields of 20-‐35% depending on the 4f-‐metal ion. The [Cu6Ln2(acnd)6(acndH)6(MeOH)x]n (Ln = Gd, 1; x = 6 and Ln = Dy, 2; x = 2) polymeric compounds are built up by repeating {Cu6Ln2} cluster units, and their general formation is summarized in stoichiometric eq. 1.

and 2 are listed in Tables 2 and 3, respectively. The crystallo-‐ 2-‐ graphically established coordination modes of the acnd and -‐ acndH ligands present in complex 1 are shown in Scheme 2. The repeating unit of the 1-‐D polymeric complex 1 consists of {Cu6Gd2} clusters that are intermolecularly linked to each 2 1 1 2-‐ other through the oximate groups of two η :η :η :μ3 acnd ligands to give an overall zigzag chain along the c axis (Fig-‐ ure 1). The connection between adjacent {Cu6Gd2} clusters within 1 is achieved through {Cu-‐(μ-‐NO)2-‐Cu} units. The oximate bridges in the latter units are not planar, as previ-‐ 2+ 57-‐66 ously seen in many dinuclear {Cu2(μ-‐NO)2} complexes, but instead very ‘twisted’, with the Cu-‐N-‐O-‐Cu torsion angles being 83.3°. The centrosymmetric [Cu6Gd2(acnd)6(acndH)6(MeOH)6] repeating unit of 1 com-‐ prises two symmetry-‐related {Cu3Gd} subunits, each with a distorted trigonal pyramidal topology (Figure 2, top). The II three Cu ions form the equatorial triangular plane (Cu1···Cu2···Cu3 = 60.1°, Cu2···Cu3···Cu1 = 57.8° and Cu3···Cu1···Cu2 = 62.1°) while the apical position is occupied III by the Gd ion which is displaced by 1.663 Å out of the Cu3 best-‐mean-‐plane (Cu1···Gd1···Cu2 = 97.3°, Cu2···Gd1···Cu3 = II 102.6° and Cu3···Gd1···Cu1 = 99.8°). Each Cu ion is linked to III -‐ the Gd apex by two monoatomic -‐NO oximate bridges 2 1 1 2-‐ (O1/O3, O5/O7 and O9/O11) of three η :η :η :μ3 acnd and 2 1 -‐ II III three η :η :μ acndH ligands (Scheme 2). The Cu -‐O-‐Gd II angles span the range 107.4(2)-‐110.3(2)°. The three Cu cen-‐ ters within the triangular plane are bridged by a diatomic -‐ -‐ 2 1 1 2-‐ NO oximate group from three different η :η :η :μ3 acnd ligands. The Cu-‐N distances within the three Cu-‐N-‐O-‐Cu linkages are noticeably large (Cu1-‐N11 = 2.825(6) Å, Cu2-‐N3 = 2.682(5) Å and Cu3-‐N7 = 2.951(5) Å) and can be considered as very weakly bonding, whereas the respective Cu-‐N-‐O-‐Cu torsion angles span the range 173.5-‐178.0°. Therefore, the II coordination geometry of all Cu ions is well described as Jahn-‐Teller distorted octahedral (Figure 4) with four short bonds formed by two oximate N and two oximate O atoms, and a remaining axial coordination site occupied by an oxi-‐ mate O atom from a neighbouring {Cu6Gd2}n chain. These Cu-‐O distances (Cu1-‐O2# = 2.610(5) Å, Cu2-‐O6# = 2.765(4) Å and Cu3-‐O12# = 2.855(5) Å, where “#” is the general symbol used for the atoms in adjacent chains) are significantly long-‐ er than the equatorial ones and they serve to connect adja-‐ cent chains into weakly coupled 2-‐D sheets (Figure 3).

DMF

6n Cu2+ + 2n Ln3+ + 12n acndH2 + 18n NEt3 + xn MeOH

MeOH

[Cu6Ln2(acnd)6(acndH)6(MeOH)x]n + 18n NHEt3+

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(1)

Ln = Gd (1), x = 6; Dy (2), x = 2

Complexes 1 and 2 are very stable under the prevailing basic conditions, and their identities are not dependent on either the nature of the base (i.e., NEt3, NMe3, NPr3 and Me4NOH) or the molar ratio of the reagents. However, their crystallinity and yields are heavily affected by the appropriate II III solvent mixture (DMF/MeOH) and the Cu :Ln molar ratio. In particular, the same reactions in only DMF or MeOH gave microcrystalline solids which were identified as {Cu6Ln2} from IR spectroscopic studies and elemental analyses. Fur-‐ II III thermore, similar reactions but in 1:1, 2:1, 3:1 and 1:3 Cu :Ln molar ratios gave crystals of 1-‐2 in 2-‐8% yields. Although the -‐ -‐ II III Cl and acac anions of the Cu and Ln starting materials, respectively, do not appear in the crystal structures of 1 and 2 (vide infra), the analogous reactions with a variety of differ-‐ ent metal precursors (i.e., CuBr2, Cu(NO3)2, Cu(ClO4)2 and LnCl3, Ln(NO3)3, Ln(ClO4)3, Ln(O2CMe)3) have afforded dark red insoluble precipitates which we were unable to crystallize and eventually determine their crystal structures. To date, we have been unable to obtain single-‐crystals of the {Cu6Tb2}n analogue suitable for X-‐ray diffraction studies. Further syn-‐ thetic attempts are currently undergoing, utilizing different metal precursors and adjusting the reaction conditions dif-‐ ferently. Description of Structures. Complexes 1 and 2 are very similar to each other and differ only in the lanthanide ion present, the number of terminally bound MeOH groups, and the nature of the lattice solvate molecules (Figures 1 and S1). Complex 1 will be described in detail as a representative example but comparisons between the coordination numbers and geometries of the metal ions present in 1-‐2 will be made. Selected interatomic distances and angles for compounds 1

Figure 1. A small portion of the 1-‐D zigzag chain of 1 as extended along the c axis. All H atoms are omitted for clarity. The purple thick bonds highlight the {Cu-‐(μ-‐NO)2-‐Cu} units which contribute to the polymerization of adjacent {Cu6Gd2} clusters. Color II III scheme: Cu cyan, Gd yellow, O red, N green, C dark gray.

4


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Scheme 2. Crystallographically Established Coordina-‐ 2-‐ -‐ tion Modes of acnd and acndH Ligands Present in Complex 1 Cu2

Gd1 Cu N O

N Cu

Cu

O

N

η2:η1:η1:µ3

N O

Cu

Gd

Gd

OH

N

Cu3′

Cu1 Cu1′

Cu3 N

O

O(H)

Gd1′

Cu

Gd

η2:η1:η1:µ3

Cu2′

η2:η1:µ

The two {Cu3Gd} subunits are intramolecularly connected -‐ 2 1 1 through two μ-‐NO bridges from two different η :η :η :μ3 2-‐ acnd ligands (Scheme 2); the two Cu1-‐N2-‐O2-‐Cu1′ torsion angles are 94.1°. Thus, the complete core of the repeating 4+ unit of 1 is [Cu6Gd2(μ3-‐NO)6(μ-‐NO)8] (Figure 2, bottom). All dioximate ligands in complexes 1-‐2 act as bidentate che-‐ II lates to a Cu ion, utilizing the O and N atoms from different oximate moieties to form stable six-‐member chelating rings, while the deprotonated O atoms act as linkers to additional III II Ln and Cu centers.

Cu2 Cu3′ N3

O5 O7

N7

O2′

O3 Gd1

O9

Cu1

Gd1′ Cu1′

O1

O11 N11

N2′

N2

O2

Cu3

Cu2′

Figure 2. (top) Partially labeled representation of the centro-‐ symmetric {Cu6Gd2} repeating unit of 1, and (bottom) its 4+ complete [Cu6Gd2(μ3-‐NO)6(μ-‐NO)8] core; the dashed lines indicate the weak Cu-‐N bonding distances. Inset: The trigo-‐ nal pyramidal topology of the {Cu3Gd} asymmetric unit. Color scheme as in Figure 1. Symmetry code: ′ = 1-‐x, 1-‐y, 2-‐z.

Cu3# Cu3

II

Figure 3. A small portion of the 2-‐D sheets formed by the weak interactions (dashed line) of the Cu centers in 1 with the oxi-‐ mate O atoms from adjacent chains. The lanthanide ions in complexes 1 and 2 are nine-‐ and seven-‐coordinate, respectively, with different number of terminal MeOH molecules completing the coordination spheres. To estimate the closer coordination polyhedra

defined by the donor atoms around Gd1 and Dy1 in the asymmetric units of 1 and 2, respectively, a comparison of the experimental structural data with the theoretical data for the most common polyhedral structures with different number

5


Crystal Growth & Design 67

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

of vertices was performed by means of the program SHAPE. 68 Following the proposal by Avnir and co-‐workers to consid-‐ er symmetry and polyhedral shape as continuous properties that can be quantified from structural data, Alvarez and co-‐ workers have applied these concepts and the associated methodology to the stereochemical analysis of very large sets of molecular structures, including systems with 7 to 9 vertex 69 polyhedra. The so-‐called Continuous Shape Measures (CShM) approach allows one to numerically evaluate by how 70 much a particular structure deviates from an ideal shape. The best fit was obtained for the spherical tricapped trigonal prism (Gd1; Figure 4, left) and capped octahedron (Dy1, Fig-‐ ure 4, right) with CShM values of 0.77 and 1.18, respectively. Values of CShM between 0.1 and 3 usually correspond to a not negligible, but still small, distortion from ideal geome-‐ 71 try. Finally, the Ln ions present in 1 and 2 are bound exclu-‐ sively to O atoms from the dioximate and MeOH groups, further highlighting the oxophilicity of 4f-‐metal ions. The Ln-‐O bond distances in 1-‐2 fall into the expected range for similar compounds and take shorter values as we move from 1 to 2, in agreement with the well-‐known lanthanide contrac-‐ tion effect.

a

O7

O13

O1

O7

Dy1

Gd1 O13

O3 O9

O1 O5 O15

O14

O11

O6#

O2#

Cu(3)-‐N(12) Cu(3)-‐N(10) Cu(3)-‐N(7) Cu(3)-‐O(12#) Gd(1)-‐O(7) Gd(1)-‐O(9) Gd(1)-‐O(5) Gd(1)-‐O(3) Gd(1)-‐O(11) Gd(1)-‐O(1) Gd(1)-‐O(15) Gd(1)-‐O(13) Gd(1)-‐O(14) Cu(2)-‐O(7)-‐Gd(1) Cu(3)-‐O(9)-‐Gd(1) Cu(3)-‐O(11)-‐Gd(1)

1.954(5) 1.977(6) 2.951(5) 2.855(5) 2.415(4) 2.426(4) 2.427(4) 2.432(4) 2.438(4) 2.446(4) 2.466(4) 2.480(4) 2.513(4) 108.1(2) 110.3(2) 108.8(2)

Symmetry code: (#) = 1-‐x, 1-‐y, 1-‐z.

Cu(1)-‐O(1) Cu(1)-‐O(3) Cu(1)-‐N(4) Cu(1)-‐N(2) Cu(1)-‐O(2#) Cu(1)-‐N(11) Cu(2)-‐O(5) Cu(2)-‐N(8) Cu(2)-‐O(7) Cu(2)-‐N(6) Cu(2)-‐O(6#) Cu(2)-‐N(3) Cu(1)-‐O(1)-‐Dy(1) Cu(1)-‐O(3)-‐Dy(1) Cu(2)-‐O(5)-‐Dy(1)

O5

O3

O9

1.932(4) 1.950(5) 1.953(4) 1.982(5) 2.825(6) 2.610(5) 1.942(4) 1.944(4) 1.961(5) 1.979(5) 2.682(5) 2.765(4) 1.925(4) 1.954(4) 107.6(2) 107.4(2) 107.7(2)

Table 3. Selected Interatomic Distances (Å) and Angles a (°) for Complex 2

Complexes 1 and 2 join a handful of previously reported 72-‐74 {Cu6Ln2} clusters. Furthermore, the [Cu6Ln2(μ3-‐NO)6(μ-‐ 4+ NO)8] core is reported for a first time in heterometallic cluster chemistry and this urged us to investigate in detail the magnetic and magnetocaloric properties of 1 and 2. O11

Cu(1)-‐O(1) Cu(1)-‐N(4) Cu(1)-‐O(3) Cu(1)-‐N(2) Cu(1)-‐N(11) Cu(1)-‐O(2#) Cu(2)-‐O(5) Cu(2)-‐O(7) Cu(2)-‐N(8) Cu(2)-‐N(6) Cu(2)-‐N(3) Cu(2)-‐O(6#) Cu(3)-‐O(9) Cu(3)-‐O(11) Cu(1)-‐O(1)-‐Gd(1) Cu(1)-‐O(3)-‐Gd(1) Cu(2)-‐O(5)-‐Gd(1)

Page 6 of 13

O12#

a

1.942(3) 1.943(3) 1.943(4) 1.956(4) 2.704(6) 2.770(4) 1.929(3) 1.940(4) 1.951(3) 1.964(4) 2.817(4) 2.730(4) 106.5(2) 103.8(1) 107.0(2)

Cu(3)-‐O(9) Cu(3)-‐N(10) Cu(3)-‐N(12) Cu(3)-‐O(11) Cu(3)-‐O(10#) Cu(3)-‐N(7) Dy(1)-‐O(1) Dy(1)-‐O(9) Dy(1)-‐O(5) Dy(1)-‐O(13) Dy(1)-‐O(7) Dy(1)-‐O(11) Dy(1)-‐O(3) Cu(2)-‐O(7)-‐Dy(1) Cu(3)-‐O(9)-‐Dy(1) Cu(3)-‐O(11)-‐Dy(1)

1.931(3) 1.960(4) 1.946(4) 1.944(3) 2.709(7) 2.809(7) 2.283(3) 2.296(3) 2.296(3) 2.308(4) 2.342(3) 2.343(3) 2.354(3) 104.6(1) 107.1(1) 104.9(1)

Symmetry code: (#) = 1-‐x, 1-‐y, 2-‐z.

N4 Cu1 N2

O3

O1

N11

N8

Cu2

O7 O11

N6 O5

N3

Solid-‐State Magnetic Susceptibility Studies. The cova-‐ lently linked {Cu3Ln} units of complexes 1-‐2 are expanded firstly into {Cu6Ln2} clusters and subsequently to 1-‐D poly-‐ meric chains. As a result, the interpretation of their magnetic properties in terms of fitting the magnetic susceptibility data and rationalizing the ground state spin values using an over-‐ all ‘spin-‐up’/’spin-‐down’ vector scheme was not straightfor-‐ ward and when attempted (in the case of isotropic complex 1) it led us to unreasonable results. Unfortunately, we have not been able to isolate crystals of the Y-‐ or La-‐analogues of 1 and 2, and therefore the nature and magnitude of the mag-‐ II netic exchange interactions between the Cu ions remain II II unclear. However, it is well known that the Cu ·∙·∙·∙Cu mag-‐ netic exchange interactions, promoted exclusively by diatom-‐ ic NO-‐oximate bridges, are antiferromagnetic and the strength of the interactions depends on the Cu-‐N-‐O-‐Cu 57-‐66 torsion angles.

O9

Cu3

N10

N12

N7

Figure 4. (top) Spherical tricapped trigonal prismatic (left) and capped octahedral (right) coordination geometries of Gd1 and Dy1 atoms in the structures of 1 and 2, respectively. Points connected by the black thin lines define the vertices of the ideal polyhedra. (bottom) Jahn-‐Teller distorted octa-‐ II hedral geometry for the three Cu atoms in 1. The dashed lines indicate the weak Cu-‐N bonding interactions. Table 2. Selected Interatomic Distances (Å) and Angles a (°) for Complex 1

6


Page 7 of 13

Solid-‐state direct current (dc) magnetic susceptibility (χM) data on dried and analytically pure and crystalline samples of 1 and 2·4DMF were collected in the temperature range 2.0-‐ 300 K in an applied field of 0.1 T, and are plotted as χMT ver-‐ sus T plots in Figure 5. The experimental χMT values at 300 K for both complexes are in excellent agreement with the theo-‐ 3 -‐1 3 -‐1 retical ones (19.54 cm Kmol for 1; 30.59 cm Kmol for 2) for II III 6 Cu and 2 Ln non-‐interacting ions. For complex 1, the χΜT product steadily increases with decreasing temperature to reach the value of 29.88 at 5.5 K. The magnetic behavior of 1 is consistent with the presence of predominant ferromagnet-‐ ic exchange interactions between the metal centers and a relatively strong coupling between them, as indicated by the sharp increase of the χΜT product in the 300-‐5.5 K region. For complex 2, the magnetic response is slightly different than that of 1; the χMT product remains almost constant at a value 3 -‐1 of ~35.2 cm Kmol from 300 K to ~35 K, and then steadily 3 -‐1 decreases to a value of 32.72 cm Kmol at 20 K. This behav-‐ iour indicates either the presence of predominant antiferro-‐ magnetic exchange interactions between the metal ions or III 15-‐19,75-‐79 depopulation of the excited MJ states of the Dy ions, or both. Below 20 K though, the χMT product of 2·4DMF starts increasing with decreasing temperature to reach a 3 -‐1 maximum value of 33.55 cm Kmol at 6 K. This increase may be attributed to some weak ferromagnetic exchange interac-‐ II III tions between the Cu and Dy ions in 2. The low tempera-‐ ture (T < 6-‐7 K) decrease of the χMT products of 1-‐2 is likely due to the presence of weak antiferromagnetic exchange interactions and magnetic anisotropy. Thus, the magnetic susceptibility data of complexes 1-‐2 are suggestive of ferro-‐ magnetically-‐coupled systems albeit the presence of some antiferromagnetic contribution from the Cu-‐N-‐O-‐Cu path-‐ ways should not be ruled out. Anticipating the analysis of the heat capacity data for complex 1, we interpret its magnetic properties as a result of the ferromagnetic coupling between III II each Gd (SGd = 7/2) ion and the peripheral Cu (SCu = 1/2) ions, as such to form two S = 5 units per molecule, which are very weakly intercoupled antiferromagnetically. Assuming g = 2.0, the susceptibility of two S = 5 units with negligible 3 -‐1 interaction amounts to 30 cm Kmol , which nicely agrees with the low-‐T value reached by the experimental χΜT prod-‐ uct. II

40

35

χ T (cm3 mol-1 K) M

{Cu6Gd2}n (1)

30

{Cu6Dy2}n (2)

25

20

15 0

50

100

150

200

250

300

Temperature (K)

Figure 5. χΜT versus T plots for complexes 1 and 2·4DMF. Magnetization (M) versus field (H) studies were also per-‐ formed for complexes 1 and 2·4DMF (Figure 6) at different low temperatures and magnetic fields. The data for the {Cu6Gd2} compound shows a very fast increase of magnetiza-‐ tion with increasing field, resulting in a fast saturation of M at a value of ~20 NμB, which is indicative of the presence of dominant ferromagnetic exchange interactions between the metal ions. The magnetization at saturation agrees nicely with two weakly coupled {Cu3Gd} units, each carrying an S = 5 net spin state, as anticipated. The M versus H plot for 2·4DMF between 1.9 and 5 K are not superimposed on a single master curve, thus indicating the presence of magnetic anisotropy and/or the population of low-‐lying excited states, as well as the effect from some weak antiferromagnetic com-‐ 30-‐39,80-‐87,95-‐102 ponents between the metal centers. 2K

M / N µB

20

10K

15

20K

10

5

III

The magnetic coupling between Cu and Gd ions is fre-‐ II quently found to be ferromagnetic for a wide range of Cu -‐ III 30-‐39,80-‐87 O-‐Gd angles. In addition, the sign and magnitude of the Jij coupling constants depend on various structural and physical parameters, such as the degree of planarity of the 88-‐90 91-‐94 bridging core, the hinge angle, the orthogonality of 30-‐33 the d-‐ and f-‐orbitals, and the efficient electron transfer from the singly occupied 3d copper(II) orbital to an empty 5d 30-‐33,88-‐90 gadolinium(III) orbital. Ferromagnetic coupling II III between Cu and Dy ions is also of precedence in 3d/4f-‐ metal cluster chemistry but this is more rarely observed due to the concurrent presence of first-‐order angular momen-‐ 95-‐102 tum.

0 0

1

2

3

4

5

µ0H (T)

20

15

M / Nµ B

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


10

5 5K 3K 1.9 K

0 0

1

2

3

4

µ0H (T)

7


5

6

7

Crystal Growth & Design Figure 6. M versus μ0H plots for complexes 1 (top) and 2·4DMF (bottom) carried out for several constant tempera-‐ tures, as labeled. The solid lines are guides for the eye.

capacity or magnetization, respectively, are in nice agree-‐ ment to each other. Note that the estimation of the non-‐ magnetic lattice contribution to the heat capacity is irrele-‐ vant since it cancels out in dealing with differences in total entropies.

Alternating current (ac) magnetic susceptibility studies were also conducted in order to investigate the magnetiza-‐ tion dynamics of complex 2. Unfortunately, complex 2 did not show any χ′′M signals (Figure S2) either in the absence or presence of external dc field. This is indicative of the pres-‐ ence of a very fast relaxation of magnetization, presumably derived from the non-‐ideal coordination environment (lig-‐ III 15-‐19,95-‐102 and field) around the Dy atoms.

-‐1 -‐1

For µ0ΔH = 1 T, -‐ΔSm(T,1T) reaches 7.9 Jkg K at T = 0.5 K, whereas ΔTad(T,1T) reaches 3.2 K at T = 1.0 K. The field de-‐ pendence of the MCE is not linear on increasing ΔH. As can be seen in Figure 8, for the maximum applied field change, -‐1 -‐1 i.e., µ0ΔH = 7 T, -‐ΔSm(T,7T) reaches 11.8 Jkg K at T = 1.6 K, whereas ΔTad(T,7T) reaches 7.9 K at T = 0.8 K. These values turned out to be not significantly large when compared with those obtained from other molecular complexes comprising II III 14,103-‐104 Cu and Gd spins. One of the reasons is that complex 1 has a relatively large molecular mass (or a small met-‐ al/ligand mass ratio), thus not favouring a large MCE since 22 the nonmagnetic ligands contribute passively. An addition-‐ al reason is that, as already noted, fields as large as 7 T are II III not sufficient to decouple the Cu and Gd spins, which stabilize two net S = 5 spins per molecule at low tempera-‐ tures. Therefore, -‐ΔSm has to be limited by the available en-‐ -‐1 -‐1 tropy, which reduces to 2×Rln(2×5 + 1) = 4.8R = 11.8 Jkg K . This value is reached experimentally for µ0ΔH = 7 T, thus corroborating that the antiferromagnetic coupling between the S = 5 spins has to be very weak, and so is their magnetic anisotropy.

Magnetocaloric Studies. Figure 7 shows the experi-‐ mental heat capacity data, C/R, where R is the gas constant, for complex 1, collected from 30 K down to 0.3 K for several applied fields. A non-‐magnetic contribution appears to dom-‐ inate the heat capacity above 5 K. We attributed this contri-‐ bution to the lattice heat capacity, Clatt, which can be de-‐ scribed by the Debye model (dotted line) and simplifies to a 3 Clatt/R = aT dependence at the lowest temperatures, where a −2 −3 = 2.5 × 10 K . At low temperatures, the heat capacity strongly depends on the applied fields. The magnetic contri-‐ bution to the heat capacity, Cm, consists of a Schottky-‐like anomaly that shifts to higher temperatures on increasing the applied field. We associated the Schottky-‐like anomaly to the splitting of two S = 5 net spin-‐multiplets per molecule, which III resulted from the ferromagnetic coupling between each Gd II ion and the three peripheral Cu ions. Note that the height, Cmax, of the Schottky anomaly is very sensitive to the value of the spins involved. For instance, a fully ferromagnetic net spin S = 10 per molecule should give Cmax/R = 1.0, whereas the II sum of six non-‐interacting Cu spins and two non-‐ III interacting Gd spins per molecule should provide Cmax/R = 4.0, both in clear disagreement with the experimental data. In the case of two non-‐interacting S = 5 spin-‐multiplets, Cmax/R = 1.9 and the calculated Schottky anomalies (solid lines) nicely describe the magnetic contribution to the exper-‐ imental heat capacity for µ0H ≥ 1 T. The fact that a field of 1 T is already sufficient for fully decoupling the S = 5 spin-‐ multiplets, implies a weak coupling between these units, which likely is antiferromagnetic, in agreement with the susceptibility data. Next, from the heat capacity data, we calculated the entropy S(T,H) = ∫C(T,H)/TdT that we plot in Figure 7, together with the lattice entropy, calculated from Clatt. Since no experimental C data available for the tempera-‐ ture range between absolute zero and 0.3 K, the so-‐obtained zero-‐field entropy data were further scaled as to match all other data for H ≠ 0 at the high temperature region. As ex-‐ pected, the resulting magnetic contribution of the zero-‐field entropy tends to a value corresponding to two uncorrelated S = 5 spin units per mole, i.e., 2×Rln(2×5 + 1) = 4.8R (Figure 8).

µ0H = 0 10

µ0H = 1T µ0H = 3T

C/R

µ0H = 7T 1

0.1

1

10

Temperature (K) 10

µ0H = 0 8

µ0H = 1T µ0H = 3T µ0H = 7T

S/R

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

Page 8 of 13

6

4

Finally, we evaluated the magnetocaloric effect for com-‐ plex 1. From the entropy curves, we obtain the magnetic entropy change, ΔSm(T,ΔH), and adiabatic temperature change, ΔTad(T,ΔH), that follow from the magnetic field change ΔH = Hf − Hi (Figure 8). Likewise, the magnetic en-‐ tropy change is also obtained from the magnetization data (Figure 6), by using the Maxwell relation: ΔSm(T,ΔH) = ∫[∂M(T,H)/∂T]HdH. The two sets of data for the magnetic entropy change, independently derived from either heat

2

0 1

10

Temperature (K) Figure 7. (top) Molar heat capacity for 1 as a function of temperature for the indicated applied fields. The solid lines

8


Page 9 of 13

are Schottky anomalies calculated for two non-‐interacting S = 5. Dotted line is the lattice contribution, Clatt. (bottom) Temperature dependence of the total entropy for several applied fields, as obtained from integration of heat capacity data. Dotted line is the lattice entropy, calculated from Clatt. 5

AUTHOR INFORMATION

M 9

µ0ΔH = (3 − 0) T

* E-‐mail: [email protected]

-1

µ0ΔH = (5 − 0) T

3

Corresponding Author

-1

µ0ΔH = (1 − 0) T

−ΔSm (J kg K )

4

−ΔSm / R

Supporting Information Crystallographic data of complexes 1 and 2 in CIF formats, and various structural and magnetism figures. This material is available free of charge via the Internet at http://pubs.acs.org.

12 From: C

µ0ΔH = (7 − 0) T

6

2 3

1

0

ACKNOWLEDGMENTS This work was supported by Brock University (Chancellor’s Chair for Research Excellence, to Th.C.S), NSERC-‐DG and ERA (to Th.C.S), MINECO (MAT2015-‐68204-‐R to M.E and postdoctoral contract to G.L), and the National Natural Sci-‐ ence Foundation of China (grants 21371166, 21331003 and 21221061 to J.T). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Scienc-‐ es, of the U.S. Department of Energy under Contract No. DE-‐ AC02-‐05CH11231.

0 0

5

10

15

20

25

30

8

µ0ΔH = (1 − 0) T

ΔTad (K)

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


µ0ΔH = (3 − 0) T

6

REFERENCES

µ0ΔH = (7 − 0) T

[1] Feltham, H. L. C.; Brooker, S. Review of purely 4f and mixed-‐ metal nd-‐4f single-‐molecule magnets containing only one lanthanide ion. Coord. Chem. Rev. 2014, 276, 1-‐33. [2] Piquer, L. R.; Sañudo, E. C. Heterometallic 3d–4f single-‐molecule magnets. Dalton Trans. 2015, 44, 8771–8780. [3] Winpenny, R. E. P. The structures and magnetic properties of complexes containing 3d-‐ and 4f-‐metals. Chem. Soc. Rev. 1998, 27, 447-‐452. [4] Sharples, J. W.; Collison, D. The coordination chemistry and magnetism of some 3d–4f and 4f amino-‐polyalcohol compounds. Coord. Chem. Rev. 2014, 260, 1-‐20. [5] Langley, S. K.; Le, C.; Ungur, L.; Moubaraki, B.; Abrahams, B. F.; Chibotaru, L. F.; Murray, K. S. Heterometallic 3d−4f Single-‐Molecule Magnets: Ligand and Metal Ion Influences on the Magnetic Relaxa-‐ tion. Inorg. Chem. 2015, 54, 3631–3642. [6] Polyzou, C. D.; Efthymiou, C. G.; Escuer, A.; Cunha-‐Silva, L.; Papatriantafyllopoulou, C.; Perlepes, S. P. In search of 3d/4f-‐metal single-‐molecule magnets: Nickel(II)/lanthanide(III) coordination clusters. Pure Appl. Chem. 2013, 85, 315–327. [7] Sessoli, R.; Powell, A. K. Strategies towards single molecule mag-‐ nets based on lanthanide ions. Coord. Chem. Rev. 2009, 253, 2328-‐ 2341. [8] Chow, C. Y.; Trivedi, E. R.; Pecoraro, V.; Zaleski, C. M. Heterome-‐ tallic Mixed 3d-‐4f Metallacrowns: Structural Versatility, Lumines-‐ cence, and Molecular Magnetism. Comments on Inorg. Chem. 2015, 35, 214–253. [9] Liu, J.-‐ L.; Lin, W.-‐ Q.; Chen, Y.-‐ C.; Gómez-‐Coca, S.; Aravena, D.; II III Ruiz, E.; Leng, J.-‐ D.; Tong, M.-‐ L. Cu – Gd Cryogenic Magnetic Refrigerants and Cu8Dy9 Single-‐Molecule Magnet Generated by In Situ Reactions of Picolinaldehyde and Acetylpyridine: Experimental and Theoretical Study. Chem. Eur. J. 2013, 19, 17567–17577. [10] Liu, J.-‐ L.; Chen, Y.-‐ C.; Lin, W.-‐ Q.; Gómez-‐Coca, S.; Aravena, D.; Ruiz, E.; Leng, J.-‐ D.; Tong, M.-‐ L. Two 3d–4f nanomagnets formed via a two-‐step in situ reaction of picolinaldehyde. Chem. Commun. 2013, 49, 6549-‐6551. [11] Zhang, J.-‐ J.; Hu, S.-‐ M.; Xiang, S.-‐ C.; Sheng, T.; Wu, X.-‐ T.; Li, Y.-‐ M. Syntheses, Structures, and Properties of High-‐Nuclear 3d−4f Clusters with Amino Acid as Ligand: {Gd6Cu24}, {Tb6Cu26}, and {(Ln6Cu24)2Cu} (Ln= Sm, Gd) Inorg. Chem. 2006, 45, 7173-‐7181. [12] Xiang, S.; Hu, S.; Sheng, T.; Fu, R.; Wu, X.; Zhang, X. A Fan-‐ Shaped Polynuclear Gd6Cu12 Amino Acid Cluster:   A “Hollow” and

4

2

0 0

5

10

15

20

25

30

Temperature (K) Figure 8. (top) Magnetic entropy change of 1, as obtained from heat capacity and magnetization data, and (bottom) adiabatic temperature change of 1, as obtained from heat capacity data, for the indicated applied field changes.

CONCLUSIONS In conclusion, we have reported two new 1-‐D coordination polymers consisting of repeating {Cu6Ln2} cluster units that were assembled through the oximato arms of the ligand acenaphthenequinone dioxime (acndH2). The tetradentate -‐ 2-‐ chelating ligands acndH /acnd have demonstrated their bridging capacity and their ability to coordinate to both transition metal ions and lanthanides. Each {Cu3Ln} subunit of the {Cu6Ln2} cluster is ferromagnetically-‐coupled and weakly coupled to its neighbouring {Cu3Ln}. Therefore, the magnetic properties of the {Cu6Gd2} analogue were ascribed to the weak antiferromagnetic coupling between two S = 5 spin units, resulting in a magnetic entropy change of -‐ -‐1 -‐1 ΔSm(T,7T) = 11.8 Jkg K , in excellent agreement with the theoretically expected value. We are currently trying to op-‐ timize the synthetic conditions and isolate all possible ana-‐ logues of this family of {Cu6Ln2}n polymers-‐of-‐clusters. We are also seeking ways to expand this chemistry to other 3d/4f-‐systems, such as Mn/Ln and Ni/Ln.

ASSOCIATED CONTENT

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For Table of Contents Use Only New dioximes as bridging ligands in 3d/4f-‐metal cluster chemistry: 1-‐D chains of ferromagnetically-‐coupled {Cu6Ln2} clusters bearing acenaphthenequinone dioxime and exhibiting magnetocaloric properties

Paul Richardson,† Kevin J. Gagnon,# Simon J. Teat,# Giulia Lorusso,§ Marco Evangelisti,§ Jinkui Tang,*,‡ and Theocharis C. Stamatatos*,†

CuII

N

N

OH

OH

LnIII

The employment of the tetradentate ligand acenaphthenequinone dioxime (acndH2) for a first time in heterometallic CuII/LnIII chemistry has afforded the 1-D coordination polymers [Cu6Ln2(acnd)6(acndH)6(MeOH)x]n (Ln = Gd, Dy) which exhibit interesting magnetic and magnetocaloric properties.

13 ACS Paragon Plus Environment

4f-Metal Cluster Chemistry

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