Coordination Cluster Nuclearity Decreases with Decreasing Rare

Mar 10, 2015 - Our systematic studies to date have allowed us to demonstrate how certain self-assembled motifs tend to persist across the rare-earth s...
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Coordination Cluster Nuclearity Decreases with Decreasing Rare Earth Ionic Radius in 1:1 Cr/Ln N‑Butyldiethanolamine Compounds: A III III Journey across the Lanthanide Series from CrIII 4 La4−Cr4 Tb4 via Cr3 Dy3 III III and CrIII 3 Ho3 to Cr2 Er2−Cr2 Lu2 Julia Rinck,† Yanhua Lan,† Christopher E. Anson,† and Annie K. Powell*,†,‡ †

Institute of Inorganic Chemistry, Karlsruhe Institute of Technology, Engesserstrasse 15, 76131 Karlsruhe, Germany Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

‡

S Supporting Information *

ABSTRACT: Reactions of the N-substituted diethanolamine ligand N-n-butyldiethanolamine with chromium(II) and lanthanide(III)/rare earth salts (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) in the presence of coligands give access to three series of isostructural 1:1 3d(CrIII)/4f(LnIII) coordination cluster compounds that can be designated in terms of octanuclear “square-in-square” (Ln = La−Tb), hexanuclear “triangle-in-triangle” (Ln = Dy, Ho, Y) and tetranuclear “butterfly” or defect dicubane core (Ln = Er−Lu) topologies as revealed by single-crystal X-ray crystallographic analysis. The bulk magnetic properties were also measured. The influences of the various components in the reaction system on the final topology and the role of the ionic radius are discussed.

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INTRODUCTION The synthesis and magnetic characterization of 3d/4f coordination clusters is currently an area of great interest in modern coordination chemistry.1−6 One aspect of interest is the possibility of producing new types of single-molecule magnets (SMMs), which are usually coordination clusters displaying a purely molecular-based hysteresis of magnetization resulting from a large uniaxial magnetic anisotropy combined with a nonzero spin state. In the past, work tended to focus on 3d/4f systems incorporating the later 3d transition metals often mixed with the GdIII ion, which have the highest spins available for any single ions of elements in the periodic table, but subsequent work revealed the “magic of dysprosium” in the search for new SMMs utilizing 3d/4f blends, mostly in conjunction with the Jahn−Teller distorted high-spin d4 MnIII as 3d ion.1 Although DyIII ions can contribute relatively high spins, it is the huge uniaxial anisotropy resulting from the spin− orbit contributions that plays a key role in inducing large energy barriers to magnetization reversal for SMMs. As a result, many synthetic groups have focused attention on the synthesis, structures, and properties of 3d/4f coordination clusters. Patterns in terms of structural features are now beginning to emerge. In this contribution we share our insights into 3d/4f coordination cluster chemistry using a systematic approach that utilizes the similarity of groups of 4f ions in terms of their coordination preferences in conjunction with chromium as the 3d transition metal ion and adjustable ligand/coligand/ counterion blends. Azide is a versatile bridging ligand that © XXXX American Chemical Society

can often replace bridging hydroxides in the synthesis of coordination clusters. This is attractive since it can mediate ferromagnetic interactions,7,8 which is important when aiming to stabilize high-spin ground states, potentially enhancing SMM properties.9−12 Indeed, the first example of a chromium/ lanthanide complex {Cr4Dy4} showing SMM properties, which we reported earlier,13 has azide ligands. In this work we will show that selection of certain variants of all of these can lead us toward control over the resulting coordination cluster arising from discovering and understanding the correct interplay of all of the synthetic components. In terms of the quest for new molecular-based magnets and SMM 3d/4f systems, attention has turned to the synthesis, structures and investigation of the magnetic properties for Cr/ Ln systems containing chromates,14−17 carboxylates,18 cyanides,19−25 and oxalates.26−29 On the other hand, we have shown that since it is possible to use a systematic self-assembly approach to certain 3d/4f motifs it is also possible to gauge the influence of the contributions of 3d and 4f components in given systems. One such system is that of the {M4Ln4} core in which a square of 4f (or rare earth) ions is inscribed within a butterfly of transition metal ions (M) and which we designated as a “square-in-square” topology for the MnIII congener,30 even though the transition metal ions do not all lie in one plane. Subsequently with the help of ab initio calculations we were able to demonstrate that combining isotropic 3d ions such as Received: August 19, 2014

A

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filtered. Slow admission of air with evaporation of the solvents then gave the CrIII/4f complexes as magenta crystals. Full synthetic details are given in the Experimental Section. Previous studies on 3d/4f aggregates showed that DyIII with its potentially high single-ion anisotropy and being a Kramers ion is often the best lanthanide to choose for the synthesis of coordination clusters with enhanced SMM behavior.1 We initiated the present study based on our previously reported results obtained when reacting CrII salts with Dy(NO3)3·6H2O in combination with N-methyldiethanolamine (H2mdea) using pivalate and azide as coligands, giving the octanuclear 3d/4f complex [Cr4IIIDyIII 4 (mdea)4(μ3-OH)4(μ-N3)4(piv)8]·3CH2Cl2, which was the first example of an SMM with CrIII and 4f ions. 13 Replacing the ligand N-methyldiethanolamine (H2mdea) by H2bdea in the reaction with Dy(NO3)3·6H2O III does not result in an analogous octanuclear {CrIII 4 Dy4 (bdea)4} III complex but rather a {CrIII Dy (bdea) } compound with a 3 3 3 hexanuclear structural motif. This synthetic route using the H2bdea ligand could be successfully applied to all other available lanthanide salts. This made it possible to perform a systematic study across the whole lanthanide series to gauge the relative contributions the size of the rare earths can make to the overall properties such as magnetism. The yttrium ion has a radius between those of holmium and erbium and was therefore also used. The complexes obtained all have a Cr/Ln ratio of 1:1 and can be formulated in terms of three structural types: [Cr4Ln4(bdea)4(μ3-OH)4(μ-X)4(piv)8] (X = N3, piv, NO3), [Cr3Ln3(bdea)3(μ3-OH)3(μ-X)(piv)6(L)3] (X = OH, N3; L = Hpiv, H2O), or [Cr2Ln2(bdea)2(μ3-OH)2(piv)4(NO3)4]. These three types can essentially be regarded as formally tetrameric, trimeric, or dimeric structures, respectively, resulting from aggregations of a dinuclear {CrLn(bdea)(μ-OH)(piv)2} monomeric units to which a small number of additional ligands may then be added. These structures can also be described in terms of being built from four, three, or two {CrLn 2(bdea)(μ3-OH)(piv) 2} triangular units with common lanthanide centers. In this motif, the deprotonated diethanolamine ligand both chelates the CrIII ion and also bridges through its oxygen atoms to the two LnIII ions. The hydroxo ligand forms a triple-bridge joining all three metal ions, and the two pivalates form syn−syn bridges along the Cr−Ln edges of the triangle. The three different resulting motifs result. First, four of these units sharing lanthanide vertices combine to give a “square-in-square” topology. Here each edge of a Ln4 square is bridged to a CrIII cation such that the four chromium ions form a larger square, which is never completely planar and thus is better described as a butterfly with a LnIII ion at the midpoint of each Cr··Cr edge. Second, three CrLn2 triangles can share lanthanide vertices to give a “triangle-in-triangle” topology. Finally, two triangles can share a Ln··Ln edge to give a rhomboid butterfly or “defect-dicubane” topology. These topologies are shown in Figure 1. We show below that the choice between tetramer (square-insquare), trimer (triangle-in-triangle), or dimer (butterfly) topology is primarily dependent on the ionic radius of the lanthanide cation. Using N-n-butyldiethanolamine yielded complexes with three square-in-square topologies similar to the previously published [Cr 4IIIDy 4III(mdea) 4(μ3-OH)4 (μIII N3)4(piv)8]·3CH2Cl2.13 These [CrIII 4 Ln4 (bdea)4(μ3-N3)x(μ32 OH)4−x(μ-NO3)2(μ,η -piv)2(piv)8]·solv complexes were obtained for the light to middleweight lanthanides Ln = La, x =

CrIII is a more effective means of optimizing SMM properties than trying to blend disparate sets of uniaxially anisotropic ions such as MnIII and DyIII. This we explored with our results on the first example of a Cr/4f complex {CrIII 4 Dy4} showing SMM behavior,13,31 and subsequently further examples of Cr/Dy systems have been reported.32−36 Our systematic studies to date have allowed us to demonstrate how certain self-assembled motifs tend to persist across the rare-earth series for 3d/4f or pure 4f coordination clusters, probably largely dictated by the chemical similarity of 4f ions and their congeners, with the most important structuredirecting parameter being the significant decrease (ca. 20%) in ionic radius as the series is traversed from left to right. Examples include a dinuclear pure 4f system, which forms an isostructural series across virtually the whole lanthanide series allowing for the magnitude of magnetic coupling between similarly bridged 4f ions to be established.37 In addition, the two series of compounds Mn5Ln838 and Fe5Ln839 where the 3d/4f cores are isostructural although the N-substituted diethanolamine ligands we use in stabilizing these coordination clusters differ very slightly in that for the Fe series they have nbutyl substitution, whereas for the Mn series it is t-butyl. Finally there is the series of Mn4Ln5 SMM compounds we reported,40 which allowed us to explore in detail the contributions of the 3d and 4f metal ions to the magnetic behavior in four isostructural compounds. Overall this previous work has indicated the often subtle effects that substitution on N-substituted diethanolamine ligands, sometimes in conjunction with the nature of the substituents on the carboxylate coligands we often use, can have on the self-assembly process for these coordination clusters. Added to this are the effects of the choice of 3d ion to incorporate into the assembly. Hence we find that 3d/4f clusters are much easier to obtain using our synthetic approach41 when manganese starting materials (sometimes preformed clusters) are used rather than when other open-shell 3d ions are provided. Finally, there is the possible influence that the radius of the LnIII ion has on the optimum stoichiometry of the resulting coordination cluster. Whereas we have sometimes observed a dramatic structural change, usually with the crossover point occurring in the vicinity of GdIII and resulting in seemingly completely unrelated core types to give two distinct sets of isostrutural compounds from the “left and right wings” of the lanthanide series, here we will report on our results with 1:1 or N/N stoichiometry 3d (CrIII)/4f compounds where the overall nuclearity changes from octanuclear 4:4 {Cr4Ln4} through hexanuclear 3:3 {Cr3Ln3} to tetranuclear 2:2 {Cr2Ln2} as the ionic radius of the 4f ions decreases.31

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RESULTS Synthesis and Structures. Coordination clusters incorporating chromium(III) ions are usually difficult to obtain in good yields largely as a result of the inertness of this ion.42 Thus, to target CrIII/4f coordination clusters we opted for the convenient and flexible synthetic route starting from the kinetically more reactive lower oxidation state CrII. Reaction of CrCl2, NaN3, and N-n-butyldiethanolamine (H2bdea) in acetonitrile solution under an inert atmosphere at ambient temperature for 50 min was followed by addition of the corresponding lanthanide nitrate hexahydrate and pivalic acid, and the mixture was stirred for 10 min. After addition of dichloromethane, the solution was stirred overnight and B

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with the hydroxo/azide disorder, which also shows the same site occupancies. Compounds (2)−(4) (crystallizing in P1̅ with Z = 2) and (5)−(7) (P21/n with Z = 4) form two short isomorphous series. The compound containing TbIII could only be obtained as rather weakly diffracting crystals, but it was possible to see that a {Cr4Tb4} compound with a similar structure to that seen for (1)−(7) was present. However, the structure could not be refined to an acceptable standard, and this compound will not be discussed further. The reason for the poor crystal quality can be explained as {Cr4Tb4} is the last example of the 4/4-structure within the lanthanide row before the changeover to the following 3/3-structures occurs. Thus, the reaction of H2bdea with DyIII results in a new structural motif, namely, a hexanuclear {Cr3Dy3(bdea)3} compound, [Cr III 3 Dy III 3 (μ 3 -N 3 ) 0.36 (μ 3 -OH) 3.64 (bdea) 3 (μpiv)6(η1-piv)2(OH2)]·H2O (8), which can be described as a triangle-in-triangle with a disordered mixture of hydroxide and azide in the central μ3-bridging position. It crystallizes in P1̅ with Z = 2 and has a central ligand that is a disordered superposition of hydroxo and azide bridging (Figure 3). The reaction with HoIII also results in a Cr3Ln3 complex with a triangle-in-triangle topology. However, the structure is not isostructural to (8). Thus, although [Cr III 3 Ho III 3 (μ 3 OH)4(bdea)3(μ-piv)6(OH2)3]Cl2·2.5H2O·1.5MeCN (9) has a similar topology to that of (8), the compound crystallizes in the acentric space group P1 with Z = 2, and the structure of one of the independent molecules is shown in Figure 3. The difference in the structures of these two compounds results from the charge-balancing pivalate moieties being coordinating in (8), whereas the charge-balancing chloride ions in (9) appear as counterions in the crystal structure. This leads to a more symmetrical core for the {Cr3Dy3}-compound (8) compared with that of the {Cr3Ho3}-compound (9). Thus, the three monodentate ligands on each of the three Ho centers in (9) are all water molecules, whereas in (8) these three ligands are provided by two pivalates and one water molecule. Furthermore, in (8) intramolecular hydrogen bonding constrains these three ligands to be on the same side of the {Cr3Dy3} core. In (9), on the other hand, two of the water ligands are on one face of the core, and the third lies over the

Figure 1. Cr4Ln4 square-in-square, Cr3Ln3 triangle-in-triangle, and Cr2Ln2 butterfly (or defect-dicubane) topologies shown by the clusters in this work (Cr green, Ln maroon, O red, N blue).

0.9, (1); Ln = Ce, x = 1, (2); Ln = Pr, (3); Ln = Nd, x = 0.9, (4); Ln = Sm, x = 0.7, (5); Ln = Eu, x = 0.6, (6); Ln = Gd, (7). The four LnIII cations form a perfect square with each Ln··Ln edge bridged by two chelating-bridging pivalates and by two symmetrically bridging (μ2,η1-NO3) ligands. The two lanthanides forming an Ln··Ln edge are linked via a μ3-OH bridge to a CrIII cation. These four hydroxo ligands are alternately above and below the Dy4 plane. Each CrIII is chelated by a doubly deprotonated diethanolamine ligand, the oxygen atoms of which bridge to the adjacent DyIII cations. Peripheral ligation is provided by the eight syn,syn-bridging pivalate ligands, which link adjacent pairs of CrIII and DyIII cations. The CrIII centers have slightly distorted octahedral O5N coordination environments, which is the case for all the compounds presented here, while the DyIII cations are eight-coordinate. There is a slight disorder of the four (μ3-OH) ligands in (1)−(7) with partial replacement by azide up to a maximum of one azide per III complex in the {CrIII 4 Ce4 } complex (2), and the molecular structure of this complex is shown in Figure 2. The {Cr4La4} complex (1) crystallizes in the orthorhombic space group Pbcn with Z = 4, the molecule lies on a twofold axis passing through two of the LaIII cations. One of the two CrIII ions in the asymmetric unit of (1) showed twofold disorder with relative site occupancies that refined to 0.55:0.45. Attempts to refine the structure in the lower symmetry space groups Pca21 or P21/n with the twofold axis obeying a twin law did not lead to an ordered structure. The Cr disorder was therefore considered to be real since this seems to correlate

Figure 2. Molecular structure of [Cr4Ce4(μ3-OH)3(μ3-N3)(bdea)4(piv)10(NO3)2] (2). Disorder and H atoms of the organic ligands omitted for clarity; in the cluster core (right), one of the two possible arrangements of the disordered triply bridging azide and hydroxo ligands is shown using smaller, paler atoms. C

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III 1 Figure 3. Molecular structures of the {Cr3Ln3(bdea)3} compounds in [CrIII 3 Dy3 (μ3-N3)0.36(μ3-OH)3.64(bdea)3(μ-piv)6(η -piv)2(OH2)]·H2O (8) III III (left) and [Cr3 Ho3 (μ3-OH)4(bdea)3(μ-piv)6(OH2)3]Cl2·2.5H2O·1.5MeCN (9) with core for (8) and (9) showing how the intra- or intermolecular hydrogen bonding results in different arrangements of the monodentate ligands on the core.

other face. This means that two of the μ3-OH− ligands bridging the Ho2Cr triangles are on the opposite face of the {Cr3Ho3} core to the central hydroxide. As can be seen in Figure 3, this leads to a very different looking core structure compared with that of (8). Specifically, the pseudo-threefold symmetry of the core is reduced to approximate mirror symmetry for the core of (9). The supramolecular structure is also very different in that, unlike all the other compounds presented here where hydrogen bonds are either intramolecular or to the nitrogen atom of a lattice MeCN molecule, in (9) there is a network of hydrogen bonds between the hydroxo and aqua ligands of the cluster molecules, the chloride counterions, and the lattice waters. The reaction with the diamagnetic rare earth metal ion YIII results in III 1 a [CrIII 3 Y3 (μ3-OH)4(bdea)3(μ-piv)6(η -piv)2(OH2)]·H2O complex (10) isostructural to (8). We note here that compounds 9 and 10, as well as compounds 11−14 described below, do not incorporate the azide used in the synthetic method. Since azide (as a weak base) can also help regulate the pH of the blend thus assisting in crystallization, it is not always possible to identify its role in the synthetic reaction in precise detail. Indeed, it was not necessary to add azide to obtain samples of compounds 9 and 11−13. The series of compounds is completed with Cr 2 Ln 2 coordination clusters, which are obtained for the heaviest lanthanides. [Cr2IIILn2(μ3-OH)2(bdea)2(μ-piv)4(NO3)2]·(2−

x)MeCN·xCH2Cl2, Ln = Er, x = 0.3, (11); Ln = Tm, x = 0.5, (12); Ln = Yb, x = 0.5, (13); Ln = Lu, x = 0.22, (14) crystallize isomorphously in the monoclinic space group P21/n with Z = 2 with the molecules having crystallographic inversion symmetry. The molecular structure of the Cr2Er2 complex (11) is shown in Figure 4. The central core consists of a Cr2Ln2 rhombus with the CrIII ions at the acute corners and the LnIII at the obtuse corners. The two edge-sharing CrLn2 triangles are each bridged by a (μ3-OH) ligand. As seen previously, the two deprotonated diethanolamine ligands chelate their respective CrIII cations and form bridges to each of the LnIII cations. Four syn,syn-pivalates bridge each Cr··Ln edge, and the LnIII coordination spheres are completed by a chelating nitrate ligand. The complex thus has a Cr2Ln2 defect-dicubane or butterfly topology. Comparing the bond lengths and distances within the isostructural 4:4, 3:3, and 2:2 compounds the effects of the lanthanide contraction can be seen clearly. In all compounds the structural motif of the lanthanide ions is inscribed within the analogue Cr motif of the structure allowing the chromium ions to move closer to each other as the lanthanide radius decreases (Figure 5 and Supporting Information, Table S2). This trend is the most pronounced within each structural motif and also continues as the nuclearity decreases. As the Cr atoms D

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Figure 4. Molecular structure OH)2(bdea)2(piv)4(NO3)2] (11).

of

and Cr−O bond length to the chelating deprotonated diethanolamine oxygen for all the structural motifs. The bond lengths from a given lanthanide ion to oxygen decrease within the periodic table from left to right as expected given the sharp decrease in radius across the lanthanide series. The Cr−O distances, however, do not differ significantly (Figure 5). Figure 6 displays the average change of the Ln−Cr−Ln angle within a structure along with the average change in the angles between adjacent lanthanides and the bridging (μ3-OH) or (μ3N3) ligand. Within the 4:4 structures the Ln−Cr−Ln and Ln− O(H)/N−Ln angle increases as the radius of the lanthanide ion decreases regardless of slight changes within this structural motif. The lanthanide contraction is clearly visible and has a much stronger effect on the structure than any change in the central hydroxo and/or azide bridging ligand. For the 2:2 compounds, which have a linear arrangement for the respective metal ions, the effect of the decrease in radius of the lanthanide ion is almost negligible, and for the 3:3 structures it must be kept in mind that not only the bridging ligand varies, but for compounds (8) and (9) there are different arrangements of ligands and different orientations of the triangular units. In the {Cr3Dy3} (8) the Cr3 plane sits parallel to and slightly above the Dy3 plane, while in the {Cr3Ho3} (9) two chromium atoms lie above and the third below the Dy3 plane.

[Cr2Er2(μ3-

move closer together the Ln/Cr distances become smaller. The Ln/Ln distances are less affected but show the same trend. In the 4:4 and 3:3 structures the lanthanides are (μ3-OH) and (μ3-N3) bridged through the center and also the Ln−O/N bond lengths within these compounds decrease accordingly. Keeping in mind that all compounds can be described as aggregations of dinuclear {CrLn(bdea)(μ-OH)(piv)2} monomeric units, we can usefully compare the Cr−O and Ln−O bond lengths of the (μ3-OH) bridging ligand between two lanthanide ions and the adjacent chromium ion and the Ln−O

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MAGNETISM

Table 1 provides a comparison of the measured and expected χT values.43 The magnetization at 2 K and 7 T is presented along with the dominant magnetic interactions observed.

Figure 5. Average Ln−Ln, Cr−Cr, and Ln−Cr distances and Ln−O(H), Ln/Cr−O/N, and Ln/Cr−O bond lengths. E

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Figure 6. Average Ln−Cr−Ln and Ln−O(H)/N−Ln angles.

Table 1. Magnetic Data from the Direct Current Measurements compounds Cr4La4 (1) Cr4Ce4 (2) Cr4Pr4 (3) Cr4Nd4 (4) Cr4Sm4 (5) Cr4Eu4 (6) Cr4Gd4 (7) Cr3Dy3 (8) Cr3Ho3 (9) Cr3Er3 (11) Cr2Tm2 (12) Cr2Yb2 (13) a

Curie constant for each Ln [cm3 K mol−1]

χT expected for noninteracting ions per complexa [cm3 K mol−1]

χT measured at 300 K per complex (cm3 K mol−1)

χT measured at 1.8 K per complex [cm3 K mol−1]

magnetization at 2 K and 7 T [μB]

dominant magnetic interactionsb

0 0.8 1.6 1.6 0.1

7.5 10.7 13.9 14.1 7.9

7.4 10.5 13.6 14.0 8.5

7.2 11.1 9.8 18.3 14.9

12.1 15.2 15.5 17.2 12.4

AF F AF F F

0 7.9 14.2 14.1

7.5 39.0 48.1 47.8

12.9 39.2 47.1 47.4

7.4 33.1 8.9 13.8

12.3 24.8 21.7 23.0

AF AF AF AF

11.5

40.1

37.5

12.1

23.5

AF

7.2

18.1

18.2

6.2

11.4

AF

2.6

8.9

7.8

2.5

5.4

AF

Using a g factor of 2.0 for CrIII. bF = ferromagnetic; AF = antiferromagnetic.

temperature for {Cr4Pr4(bdea)4} (3) slowly decreases to 80 K before dropping more rapidly, which indicates either antiferromagnetic exchange interactions among the spin carriers or thermal depopulation of the mJ sublevels, or a combination of the two. For compound (1), which contains the diamagnetic LaIII, the χT product drops below 5 K, suggesting the presence of antiferromagnetic interactions. However, above 5 K, the χT product remains constant indicating that only very weak antiferromagnetic exchange interactions are present between the CrIII ions. It is worth bearing in mind that the electronic “Aufbau” of diamagnetic shells in diamagnetic metal centers such as LaIII has been recognized as providing a possible means

For the earlier Cr/Ln-4:4 structures {Cr4Ce4(bdea)4} (2), Cr4Nd4(bdea)4} (4), and {Cr4Sm4(bdea)4} (5) ferromagnetic behavior is observed. On lowering the temperature the χT product decreases slowly to 10 K after which the product rapidly increases to reach the maximum value at 1.8 K. The value of the χT product is in good agreement with the expected values (Ln = Ce: S = 1/2, C = 0.80; Ln = Nd: S = 3/2, C = 1.64 cm3 K mol−1; Ln = Sm: S = 5/2, C = 0.09 cm3 K mol−1) (Figure 7). One exception to this behavior for the 4:4 complexes formed with the earlier lanthanides is found for the compound {Cr4Pr4(bdea)4} (3), which shows antiferromagnetic rather than ferromagnetic behavior. Thus, the χT product at room F

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In the case of the {Cr4Eu4} compound (6) the χT product at room temperature is much higher (12.9 cm3 K mol−1) than the expected value of 7.5 cm3 K mol−1 (Ln = Eu: S = 6/2, C = 0 cm3 K mol−1) indicating that the low-lying excited states of EuIII ions are magnetically active and thermally populated even if the ground state of the ion is diamagnetic. On lowering the temperature, the χT product decreases steadily to a minimum value of 7.4 cm3 K mol−1 at 1.8 K, which is a result of the depopulation of the excited states and that perfectly matches the theoretical value for four noninteracting Cr(III) centers. (Figure 10). For {Cr4Gd4} (7) dominant antiferromagnetic coupling leads to a ferrimagnetic spin arrangement as seen in the temperaturedependent behavior of the χT. The χT product at room temperature (39.2 cm3 K mol−1) is in good agreement with the expected value of 39.0 cm3 K mol−1 for the uncoupled ions. On decreasing the temperature, the χT product drops isotonically to 80 K, below which it drops rapidly to a value of 25.73 cm3 K mol−1 at 5 K before rapidly increasing. This is the signature of dominant antiferromagnetic interactions, which are not completely compensated among the spin carriers in the complex (Figure 8) and can be described as a molecular version of a ferrimagnetic spin arrangement.44 The field dependence of the magnetization for (7) at low temperatures increases rapidly below 1 T and then steadily increases without saturation to 7 T, suggesting the presence of magnetic anisotropy and/or the population of low-lying excited states, which is supported by the nonsuperposed reduced magnetization curves. Furthermore, it is noticeable that a crossover of magnetization at different temperatures was observed at ∼5 T, indicating that the population of the excited states in this system is progressive below and above 5 T suggesting a possible spin flip (Figure 8). This interesting observation clearly

Figure 7. Temperature dependence of χT under a 0.1 T applied field for {Cr4Ce4} (2), {Cr4Nd4} (4), and {Cr4Sm4} (5).

to promote or block magnetic interactions between paramagnetic centers in some 3d/4f systems.44 In contrast to the square-in-square compounds of the earlier lanthanides, the later square-in square and also the triangle-intriangle compounds tend to show the same antiferromagnetic behavior that was observed for the previously published squarein-square compound [Cr 4I I I Dy 4II I (mdea) 4 (μ 3 -OH) 4 (μN3)4(piv)8]·3CH2Cl2 and for which the magnetic properties and SMM behavior have been analyzed in detail.13 On lowering the temperature, the χT products of all compounds steadily decrease to 80 K and then drop rapidly to reach a minimum value at 1.8 K indicating the presence of intramolecular antiferromagnetic interactions among the spin carriers and/or thermal depopulation of the mJ sublevels.

Figure 8. Temperature dependence of χT under a 0.1 T applied field (upper), the field-dependence of the magnetization (left), and reduced magnetization M vs H/T at the indicated temperatures (right) for {Cr4Gd4} (7). G

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Figure 9. Field dependence of magnetization at different temperatures for (8) (left) and the reduced magnetization M vs H/T at the indicated temperatures (right).

demands further study and will be the subject of a separate paper. The field dependence of the magnetization at low temperatures for {Cr3Dy3} (8) shows a rapid increase for fields to 2 T and then a slower and essentially linear increase to 7 T, where it reaches 21.7 μB (Figure 9). This linearity at B > 2 T may be a peculiarity of the powder magnetization as discussed before for {Cr4Dy4}.13 The reduced magnetization curves at different temperatures are nonsuperposed suggesting the presence of magnetic anisotropy and/or the population of low-lying excited states. Therefore, the relaxation of the magnetization was investigated using alternating current (ac) susceptibility measurements as a function of temperature at different frequencies (Supporting Information, Figure S4). No out-ofphase signal above 1.8 K was observed. A similar behavior has been seen for the previously published {Cr4Dy4},13 but in the case of this compound, with the bdea ligand in place of the mdea ligand used previously, no SMM behavior can be observed within the applied temperature and field range. The {Cr3Dy3} (8) and {Cr3Ho3} (9) show qualitatively similar behavior (Supporting Information, Figure S5), and again, no out of phase signal for (9) in the measured temperature and field range was observed. The χT products for the dimeric compounds {Cr2Tm2mdea2} (12) and {Cr2Yb2mdea2} (13) are in good agreement with the expected values (Ln = Tm: C = 7.15 cm3 K mol−1; Ln = Yb: C = 2.57 cm3 K mol−1). On lowering the temperature, the χT products decrease steadily before dropping rapidly to reach minimum values at 1.8 K, indicating the presence of intramolecular antiferromagnetic interactions among the spin carriers (Figure 10).

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Figure 10. Temperature dependence of χT under a 0.1 T applied field for {Cr4La4} (1), {Cr4Pr4} (3), {Cr4Eu4} (6), {Cr4Tb4}, {Cr3Dy3} (8), {Cr3Ho3} (9), {Cr2Tm2} (12), and {Cr2Yb2} (13).

resulting core correlating to the ionic radius of the lanthanide or rare earth ion. The {Cr4Ln4} square-in-square compounds could be obtained for the largest number of lanthanide cations, and from Ln = La to Tb, {Cr4Ln4} square-in-square complexes were exclusively obtained. For the smaller radius ions DyIII, YIII, HoIII, and ErIII, {Cr3Ln3} triangle-in-triangle structures were obtained. For the smallest lanthanides TmIII and YbIII {Cr2Ln2} butterfly compounds were formed. Overall, the n-butyl ligand appears to be robust in terms of allowing the lanthanide ionic radius to determine which core topology self-assembles in solution and crystallizes out. Thus, we can suggest how the coordination clusters form in solution. It is probable that most of the changes to the coordination sphere of the chromium take place while it is still in the CrII oxidation state, since this is much less inert than the resulting CrIII ion. This will include chelation of the CrII by the diethanolamine ligand and ligation by pivalate ligands. The deprotonation of the diethanolamine alcohol groups is more likely to be induced by CrIII cations since these have higher

DISCUSSION

We presented here the syntheses and structures of selfassembled CrnLnn complexes formed with the N-substituted diethanolamine ligand H2bdea. We monitored the effects on the resulting structural topology on moving across the lanthanide series as well including the rare earth ions Y(III) and La(III) in the survey. In all cases, compounds form with a Cr/Ln ratio of 1:1, and the structures of the compounds can be grouped into three basic topologies, which we have termed {Cr4Ln4} square-in-square (4:4), {Cr3Ln3} triangle-in-triangle (3:3), and {Cr2Ln2} butterfly or defect dicubane (2:2). For this particular H2bdea ligand, the observed structures are wellseparated into these three groupings with the size of the H

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Article

Inorganic Chemistry Scheme 1. Overview of the Structural Motifs Formed with N-n-Butyldiethanolamine and Their Magnetic Behaviora

a

W = weak interactions, F = ferromagnetic interactions, AF = antiferromagnetic interactions. Blue represents the 4:4, green the 3:3, and yellow the 2:2 structures.

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CONCLUSIONS In contrast to what has been described as the “serendipitous” nature of producing coordination clusters based on 3d metal ions45 incorporating 4f ions into such clusters can offer a programmed approach to target families of structural motifs. The decreasing nuclearity and resulting topology of these compounds follows the decrease in ionic radius as the lanthanide series is traversed. This gives an indication of how the self-assembly process of such compounds can be steered. Having established some of these ground rules opens the door to exploring more subtle effects on the resulting structural core topologies as well as their electronic structures.

Lewis acidity to deprotonate such groups. A hypothetical {CrIII(R-dea)(piv)2(OH2)}− intermediate, which stabilizes unidentate carboxylate groups by hydrogen bonding from the aqua ligand to the noncoordinated carboxylate oxygens, is a possibility. Thus, two possible mechanisms for the formation of the n/n coordination clusters can be proposed. The first scenario is that the lanthanide cations first assemble into tetranuclear (square), trinuclear (triangle), or dinuclear systems, and subsequently the Ln··Ln edges of these are bridged by mononuclear CrIII units. Alternatively, it can be envisaged that dinuclear {CrLn(Rdea)(μ-OH)(piv)2(solv)x}+ complexes form in solution and subsequently self-assemble through the formation of ligand bridges between their lanthanide centers. Since the stoichiometry of the compounds in this work always involves a Cr/Ln ratio of 1:1, the latter scenario seems to us to be more likely, although an investigation of the solution reaction chemistry would be needed to establish this. Naturally, such a study would demand some very challenging experiments and interpretation on systems that are paramagnetic and mostly highly anisotropic. Nevertheless, the results of the structural studies offer insights into the preferred self-assembled motifs for these 1:1 Cr/4f systems. The magnetic behavior changes along the lanthanide row with the later lanthanides tending to show solely antiferromagnetic behavior irrespective of their favored structure (square, triangular, dimeric). The Nd and Sm compounds show ferromagnetic behavior as does the {Cr4Ce4} complex. For {Cr4Gd4} dominant antiferromagnetic interactions lead to a ferrimagnetic ground state. Antiferromagnetic interactions are observed for all compounds with the later lanthanides and the {Cr4Pr4}, irrespective of their structure (4:4, 3:3, 2:2). An overview of the structural motifs and their magnetic properties is given in Scheme 1.

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EXPERIMENTAL SECTION

Materials and Methods. Chemicals were used as received. The solvents used were dried prior to use. Acetonitrile was dried using calcium hydride and phosphorus pentoxide then distilled and stored over molecular sieves (3 Å). Dichloromethane was dried over phosphorus pentoxide, distilled, and stored over molecular sieves (4 Å). Methanol was refluxed with magnesia and iodine for 2 h and then distilled. All procedures up to the final crystallization were carried out under an atmosphere of dry O2-free nitrogen or argon using standard Schlenk techniques or a glovebox. After the final filtration, the solution was transferred into a small beaker and sealed with a plastic lid with 10 small holes under inert gas conditions; this was then allowed to stand under aerobic conditions until crystallization took place, thus allowing slow evaporation of the solvent and slow diffusion of oxygen into the reaction mixture. Synthetic Procedures. The same general synthetic method was used for all compounds. A mixture of CrCl2 (0.124 g, 1.00 mmol), NaN3 (0.110 g, 1.69 mmol), and H2bdea (541 μL, 3.55 mmol) in acetonitrile (15 mL) was stirred at room temperature under inert atmosphere for 50 min. After addition of the corresponding Ln(NO3)3·6H2O (0.44 mmol) and pivalic acid (400 mg, 3.91 mmol), the mixture was stirred for another 10 min before addition of dichloromethane (10 mL). The mixture was then stirred at room temperature overnight (13 h), still under inert atmosphere. The solution was filtered under argon. Slow evaporation of the solvent with I

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Inorganic Chemistry

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admission of air gave magenta crystals of the CrnLnn product, typically in 30−50% yield, depending on when the crystals were harvested. Satisfactory elemental analyses were obtained. No azide was added in the synthesis of compounds 9 and 11−13. Supporting Information, Table S4 summarizes the exact yields as well as the obtained elemental analysis for each compound. Crystallography. Data were measured on a Bruker SMART Apex CCD diffractometer or on Stoe IPDS I and IPDS II image plate diffractometers using graphite-monochromated Mo Kα radiation and were corrected semiempirically for absorption.46 Structure solution by direct methods and full-matrix least-squares refinement against F2 (all data) were carried out using the SHELXTL package.46 All ordered non-H atoms were refined anisotropically. Organic H atoms were placed in calculated positions; coordinates of H atoms bonded to O or N were refined when possible. In most cases the hydrogen atoms on the μ3-hydroxide bridges could be located and refined. Bond-valence sum calculations for the oxygen atoms gave values of the range 1.0−1.2 consistent with -OH−, and monoanionic bridges give the correct charge balance. Many butyl groups within the organic ligands, some triply bridging hydroxo/azide ligands, and many lattice solvent molecules were disordered and were refined using partial occupancy atoms with isotropic temperature factors. Similarity restraints were applied to the geometries and temperature factors as appropriate. In some structures, some or all of the lattice solvent could not be refined satisfactorily and was treated using the SQUEEZE option within the PLATON software.36 Details of data collection and structure refinement (for fully refined structures) and unit cell data are summarized in Supporting Information, Table S1. III The crystal structure of [CrIII 4 Dy4 (mdea)4(μ3-OH)4(μ-N3)4(piv)8]· 3CH2Cl2 has been previously reported and deposited as CCDC 749296. Additional crystallographic information is available in the Supporting Information. Magnetic Measurements. Magnetic data were obtained using a Quantum Design MPMS-XL SQUID susceptometer equipped with a magnet of 7 T. The ac susceptibility measurements were measured using an applied oscillating ac field of 3 Oe and frequencies ranging from 1 to 1500 Hz. Measurements were performed on ground polycrystalline samples with compounds containing highly anisotropic ions such as DyIII dispersed in Apiezon grease to avoid alignment problems. The magnetic data were corrected for the sample holder, and the diamagnetic component of the sample and the diamagnetic contributions for the samples were calculated using Pascal’s constants.47−50

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ACKNOWLEDGMENTS J.R. would like to thank Lena Friedrich for her help and support in the repeated chemical syntheses. This work was supported by funding from the DFG Center for Functional Nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

Magnetic data including field dependence of magnetization at different temperatures and temperature dependence of ac susceptibility, crystallographic information including bond lengths, angles, and CIF files, powder XRD patterns, chemical (elemental) analysis data (calculated and experimental). This material is available free of charge via the Internet at http:// pubs.acs.org. Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 826531−826548. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK: https://summary.ccdc.cam.ac. uk/structure-summary-form, e-mail: [email protected]. ac.uk, or fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. J

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