Phosphonate Based High Nuclearity Magnetic Cages - American

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Phosphonate Based High Nuclearity Magnetic Cages Javeed Ahmad Sheikh,†,‡ Himanshu Sekhar Jena,† Abraham Clearfield,*,‡ and Sanjit Konar*,† †

Department of Chemistry, IISER Bhopal, Bhopal 462066, India Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States

‡

CONSPECTUS: Transition metal based high nuclearity molecular magnetic cages are a very important class of compounds owing to their potential applications in fabricating new generation molecular magnets such as single molecular magnets, magnetic refrigerants, etc. Most of the reported polynuclear cages contain carboxylates or alkoxides as ligands. However, the binding ability of phosphonates with transition metal ions is stronger than the carboxylates or alkoxides. The presence of three oxygen donor sites enables phosphonates to bridge up to nine metal centers simultaneously. But very few phosphonate based transition metal cages were reported in the literature until recently, mainly because of synthetic difficulties, propensity to result in layered compounds, and also their poor crystalline properties. Accordingly, various synthetic strategies have been followed by several groups in order to overcome such synthetic difficulties. These strategies mainly include use of small preformed metal precursors, proper choice of coligands along with the phosphonate ligands, and use of sterically hindered bulky phosphonate ligands. Currently, the phosphonate system offers a library of high nuclearity transition metal and mixed metal (3d−4f) cages with aesthetically pleasing structures and interesting magnetic properties. This Account is in the form of a research landscape on our efforts to synthesize and characterize new types of phosphonate based high nuclearity paramagnetic transition metal cages. We quite often experienced synthetic difficulties with such versatile systems in assembling high nuclearity metal cages. Few methods have been emphasized for the self-assembly of phosphonate systems with suitable transition metal ions in achieving high nuclearity. We highlighted our journey from 2005 until today for phosphonate based high nuclearity transition metal cages with VIV/V, MnII/III, FeIII, CoII, NiII, and CuII metal ions and their magnetic properties. We observed that slight changes in stoichiometry, reaction conditions, and presence or absence of coligand played crucial roles in determining the final structure of these complexes. Most of the complexes included are regular in geometry with a dense arrangement of the above-mentioned metal centers in a confined space, and a few of them also resemble regular polygonal solids (Archimedean and Platonic). Since there needs to be a historical approach for a comparative study, significant research output reported by other groups is also compared in brief to ensure the potential of phosphonate ligands in synthesizing high nuclearity magnetic cages.

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INTRODUCTION Exploring high nuclearity cages containing transition metal ions continues to be an active area of research.1−4 These molecules can be prepared either by designed synthesis5,6 or via serendipitous assembly.7 Both these synthetic strategies are worthy to be investigated. However, in design synthesis, one might lead to undesired products because of incorrect predictions. In the field of molecular magnetism, it has been observed quite often that a large number of complexes with most interesting magnetic properties were obtained by serendipity. The serendipity approach can also be rationalized and favorably employed. Other than carboxylates, alkoxy functional groups, and polyazaheteroaromatic ligands (such as triazole, triazolate, tetrazolate, and related compounds), the role of phosphonates in fabricating molecular magnets is significant because of their diverse bridging modes.8,9 Besides, some metal phosphonates exhibit aesthetic self-assembled structures that resemble Roman solids (Platonic and Archimedean solids).10 In terms of magnetic properties, the magnetic exchange through oxygen atoms of © XXXX American Chemical Society

phosphonates proves to be rather weak; however, significant superexchange can be expected due to the presence of coligands (oxide, hydroxide, alkoxides). Quite often, polynuclear transition metal clusters with certain geometry exhibit significant magnetic anisotropy and also sometimes behave as single molecular magnets (SMMs).11,12 Our interest toward the synthesis and characterization of phosphonate based molecular cages arose because of (a) the diverse bridging modes of phosphonates and (b) their applications in molecular magnetism. Besides, a regular arrangement of metal centers in the final cage is important because it is less painful to model a smaller symmetry-related unit. In this Account, we highlight our journey from 2005 until today in quest of phosphonate based high nuclearity transition metal cages and their properties.13−21 Before 2005, Winpenny et al. reported a few polynuclear 3d phosphonate complexes, which were isolated either from small preformed clusters or by Received: December 4, 2015

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DOI: 10.1021/acs.accounts.5b00531 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research Chart 1. Various Possible Bridging Modes of Phosphonate Ligand with Harris Notations

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using pyridonate as coligand.22−24 It was found that some of them have a high symmetry related to Platonic solids. Chandrasekhar et al. also reported a dodecanuclear copper phosphonate pyrazolate cage in 2000.25 The same groups have also reported several 3d and 3d−4f multinuclear phosphonate clusters during the past few years.26−42 A few of them proved to be good magnetic refrigerants.

GENERAL SYNTHETIC STRATEGY FOR PHOSPHONATE BASED MOLECULAR CAGES In order to overcome the synthetic difficulty and solubility, various groups have followed different synthetic strategies as follows: (i) use of small clusters as precursors,8 (ii) use of ancillary ligands or coligands,9 and (iii) use of a sterically hindered lipophilic phosphonic acid or bulky phosphonate ligand.41 The role of the coligands is to coordinate to a few vacant sites and reduce the number of free coordination sites for the phosphonic acid and also to lessen the probability of layered networks. Small clusters containing replaceable ligands enlarge after reacting with the phosphonate ligands. Appropriate substituents on the phosphonate or coligand can help in improving the solubility of the resulting complexes. In this Account, the importance of phosphonates as ligands for the syntheses of magnetically potent phosphonate based high nuclearity transition metal cages using VIV/V, MnII/MnIII, FeIII, CoII, NiII, and CuII metal ions will be highlighted. The phosphonate ligands explored in this Account are shown in Chart 2. Most of the complexes included here have been isolated by using preformed small clusters and few of them upon employment of coligands such as pyrazole (pz) and 6-chloro-2hydroxypyridine (chpH).

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GENERAL SYNTHETIC STRATEGY FOR PHOSPHONATES AND THEIR COORDINATION CHEMISTRY The phosphonate group is a strongly anionic moiety, which tends to form strong bonds with metal ions. The phosphonic acid moiety, R-PO3H2, and its two deprotonated forms, R-PO3H−and R-PO32−, exhibit interesting pH-dependent behavior. Usually the first proton is very acidic, while the second is less acidic. If one compares the coordination adaptability of carboxylates and phosphonates, the latter are more compatible with the metal ions than the former. However, carboxylate ligands are well explored in coordination chemistry and also in practical applications compared with the phosphonates.43−46 Many synthetic methods are known in the literature for the preparation of phosphonate ligands.47 In comparison to carboxylates, the coordination chemistry of phosphonates is less predictable owing to various coordination modes. Its three oxygen donors bond up to nine metals in a variety of modes (Chart 1). The coordination modes of such ligands can easily be described by Harris notation48 such as X.Y1Y2Y3 (X = sum of coordinated metal ions to phosphonate oxygen atoms; Y1, Y2, and Y3 = number of metal ions bonded to each oxygen atom). The complexation of phosphonate ligands with various metal ions depends upon several factors including the nature of phosphonates, metal ions, substituents on the ligand strand, synthetic conditions (room temperature or solvothermal), and molar ratio.

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VANADIUM CAGES Oxo vanadium organophosphonate (V/O/RPO32−) based extended structures have been well explored mainly by Zubeita et al.49,50 However, vanadium cages containing phosphonates are comparatively less, particularly low valent (VIII) and mixed valent (VIII/IV) complexes being much rarer.26 VIII containing complexes can be particularly interesting from a magnetic point of view due to the large magnetic anisotropy of the VIII ion. Therefore, new synthetic routes using soluble vanadium salts and various phosphonates might lead to the formation of aesthetically pleasing vanadium cages. B


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Accounts of Chemical Research Chart 2. Schematic Representations of the Phosphonic Acids Used for the Study

V12 ([VIV8VV4] and [VV12])

among the synthetic methodologies adopted for the formation of cage complexes.

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Two V12 cages, (HNEt3)2[(V12O20)(H2O)12(Ph2CHPO3)8](H2O)7(CH3CN)4Cl2 (1) and (PyH)5[V12O24(Ph2CHPO3)6]Cl5(H2O)10(Py)6(CH3CN)3 (2), were isolated.13 The mixed valent cage 1 consists of two cationic [V4VO8]4+ fragments, coordinated equally via four Ph2CHPO32− ligands to [VIVO(H2O)3]2+ groups in the middle (Figure 1a). Each tetranuclear

MANGANESE CAGES Mn and mixed valent (MnII/III) based complexes form the largest group of SMMs among the 3d transition metals due to the large number of unpaired electrons, thus leading to a high spin ground state. Although a significant number of manganese cages with interesting structural and functional properties were reported, most of them contain alkoxo, oximato, and carboxylate ligands, and phosphonate based molecular cages are relatively rare. To our knowledge, a few manganese phosphonate cages synthesized by Winpenny et al. show SMM behavior. For example, a cage with a {MnIII18MnII2} core has an energy barrier of 43 K.27 At present, quite an appreciable number of research articles have been reported on manganese phosphonate cages and clusters.22,51−54 III

Mn16 and Mn19

[Mn16] and [Mn19] cages having formulas [Mn16(Ph2CHPO3)8(μ4-O)6(μ3-O)2(OMe)4(HOMe)2(Me3CCO2)6(OH2)(Py)2] (3) and [Mn19(μ4-O)6(μ3-O)2(O2CtBu)10(OCH3)16(O3PPh(Me)2)6] (4), respectively, contain mixed valent manganese centers confirmed by BVS calculation.14 Structural analysis showed a complete irregular core structure for 3 having two subunits (Figure 3a), an isovalent Mn3−O triangle bonded to

Figure 1. (a) Core structure of 1 and (b) view of one of the [V4O8(PO3)] caps in 1. Color code: dark cyan (V), orange (P), red (O), green (Cl).

fragment forms a VV containing octahedron bridged by oxo groups (Figure 1b). Complex 2 obtained from 1 features two open “bowl”-shaped neutral [VV6O12(H2O)3(Ph2CHPO3)3] cages and encapsulates five chloride ions inside (Figure 2). The bowls are mirror images

Figure 2. Core structures of 2; dashed lines represent H-bonding. Color code: dark cyan (V), orange (P), red (O), green (Cl).

Figure 3. Core structure of (a) 3 and (b) 4. Color code: pink (MnIII), olive (MnII), orange (P), red (O), gray (C), blue (N).

to each other and are held “face to face” by strong hydrogen bonding between the chloride ions and water molecules. However, in 2 all the vanadium centers are in +5 oxidation state, which was confirmed by bond valence sum (BVS) calculation and further supported by its diamagnetic nature. The title complexes are structurally unique over other reported V-phosphonate cages and encapsulate chloride ions inside the molecular cages. Further, the use of one V12 cage as starting precursor for the synthesis of another V12 cage is a rare method

a larger Mn13O7 unit through three phosphonates and one carboxylate group. The core of [Mn19] (Figure 3b) comprises six MnIII ions arranged in hexagonal manner and bridged to the central MnII ions by six μ4-O groups. The central unit is linked to two Mn−O triangles arranged one above the other. Furthermore, the Mn7 core is sandwiched between them (Figure 4). Magnetic investigation revealed weak antiferromagnetic interactions between manganese centers for both complexes, which was proven C


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Figure 6. Core structure of (a) 5 and (b) 7. Color code: maroon (Fe), orange (P), red (O), gray (C), blue (N).

[Fe6] cage, [Fe6O2(O)2(O2CtBu)8(C10P)2(H2O)2]·4(CH3CN) (7). It contains two FeIII triangles bridged by phosphonates, carboxylates, and a peroxide, resembling a “butterfly”-like [Fe6(O2)(O)2] core (Figure 6b). The [Fe6] cage looks more regular because of mirror plane arrangement of P atoms and peroxide ion.

Figure 4. (a) Illustration of Mn7 core in 4 and (b) Bridging pattern of Mn7 core (apex triangles are hidden for better representation). Color codes same as Figure 3.

Fe9

Employing different phosphonic acids, four [Fe9] cages, namely, [Fe9(O)4(O2CtBu)13(C10P)3] (8), [Fe9(O)2(OH)(O2CPh)10(C10P)6(H2O)2]7(CH3CN) (9), ([Fe9(μ3-O)4(O3PPh(Me)2)3(O2CCMe3) 13] (10), and [Fe9 (O) 3(OH) 3(O3PCHPh2) 6(O2CCMe3)6(H2O)9] (11), were isolated. Complexes 8 and 10 exhibit the same core structure comprising two subunits; one Fe6O3 core linked with another Fe3−μ3-O triangle through three phosphonate groups (Figure 7a). Figure 5. Temperature dependencies of χMT measured at 0.1 T for 3 and 4.

from the steady decrease of χMT value upon lowering the temperature (Figure 5). The role of solvent (methanol) is worth mentioning in the isolation of these two cages, because we were unable to obtain them in any other organic solvent after several attempts. This is further supported by the presence of bridging methoxide and coordinating methanol molecules.

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IRON CAGES The oxo bridged iron cages generally contain multinuclear high spin FeIII centers (3d5, S = 5/2) that quite often show strong antiferromagnetic exchange interactions. In order to understand the magnetic interactions, several iron phosphonate cages with different nuclearities ([Fe4], [Fe6], [Fe9], [Fe12]) were synthesized15−17 by employing FeCl3, FeCl3·6H2O, and iron triangles [FeIII3(O)(RCO2)6(OH2)3]X (R = phenyl− or tBu−, X = chloride ion). All the noted complexes are well characterized and are discussed in ascending nuclearity order in the following.

Figure 7. Core structures of (a) 8 and (b) 9. Color code: maroon (Fe), orange (P), red (O), gray (C), blue (N).

Complex 9 exhibits a “twisted basket” like core where two oxo centered iron triangles are connected to Fe−OH−Fe unit. Further, each middle FeIII is bridged to three phosphonates from either side (Figure 7b). The molecular structure of 11 contains three iron triangles, of which two are symmetrically linked to the other one by three phosphonates (Figure 8a).

Fe4

Two [Fe4] cages, namely, [Fe4(O)(O2CCMe3)4(C10P)3(Py)4]· 3CH3CN (5) and [Fe4(O)(O2CPh)4(C10P)3(Py)4]3(Py)· 2CH3CN (6), having the same core structures (Figure 6a) were isolated and characterized. In both complexes, three phosphonate ligands simply replaced the coordinated carboxylate ligand from the iron triangle.

Fe12

The molecular structure of the two obtained Fe12 cages, namely, [Fe12(μ2-O)4(μ3-O)4(O2CCHPh2)14(p-tBuPhPO3H)6] (12) and [Fe12(μ2-O)4(μ3-O)4(O2CPh)14(C10H17PO3H)6] (13), show that they exhibit a double butterfly like core (Figure 9a). Two symmetry related Fe6 units are linked to each other forming the Fe12 core where each Fe6 resembles a butterfly shape (Figure 9b). Magnetic study of the complexes (10, 11, and 12) reveals that the iron centers are antiferromagnetically coupled

Fe6

An attempt to synthesize a peroxo-bridged iron phosphonate cage using hydrogen peroxide facilitates the formation of an D


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were synthesized systematically using oxo-centered iron triangle and are highly organized and geometrically regular. Besides, the peroxo bridged Fe6 cage15 was one of our outstanding contributions to molecular phosphonates.

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COBALT AND NICKEL CAGES Of the 3d metal ions, cobalt and nickel are particularly attractive with regard to high-nuclearity cages or clusters because of their enormous flexibility in adopting different coordination environments, in terms of both coordination number and geometry. In terms of magnetic properties, there is a considerable orbital contribution because of the intrinsic orbital angular momentum in the octahedral ground state of CoII [4T1g(F)]. Similarly, zero field splitting is observed for NiII because of spin−orbit coupling and a low symmetry ligand field. Similar to other transition metal cages, several cobalt and a few nickel based phosphonate cages were reported. However, most of them were synthesized from the corresponding metal salts.24,31,42 With the expectation of isolating high nuclearity cages similar to previous manganese and iron cages, small cobalt and nickel dimers ([MII2(μ-OH2)(O2CtBu)4(HO2CtBu)4], where M = Co, Ni) were employed in the presence of different phosphonates. Accordingly, a library of cobalt and nickel phosphonate cages was obtained under either solvothermal conditions or ambient conditions, and they are summarized below.19,20

Figure 8. (a) Core structure of 11 and (b) repeated μ3-oxo centered triangles found in 11. Color code: maroon (Fe), orange (P), red (O), gray (C), blue (N).

Ni8

The molecular structure of Ni8 having molecular formula, [Ni 8 (μ 3 -OH) 4 (OMe) 2 (O 3 PC 1 0 H 1 7 PO 3 H) 2 (O 2 C t Bu) 6 (HO2CtBu)8] (14)20 consists of two Ni4 subunits resembling butterfly shape, held together by phosphonate groups (Figure 11) This compound exhibits similar core structure as reported earlier.30

Figure 9. Illustration of the core unit found in 12 and 13 showing (a) the overlapping of two butterfly units and (b) face to face sharing of two units by phosphonate and carboxylate ligands.

Figure 11. Butterfly shaped molecular core of 14. Color code: green (Ni), orange (P), red (O), gray (C), blue (N). Reproduced with permission from ref 20. Copyright 2014 American Chemical Society.

The magnetic properties of 14 were quite similar to the phosphonate cage reported earlier (Figure 12).30 The magnetic behavior was modeled with the Hamiltonian given in eq 1 (Figure 13) giving a nice fit with J1 = 7.6 cm−1, J2 = −22.4 cm−1, and g = 2.42. It was found that Ni−O−Ni angle in the range of 93−99° and 111−135° shows ferro- and antiferromagnetic interactions, respectively. Figure 10. Plot of χMT vs T (2−300 K) for complexes 10−12.

H Ni8 = −J1(S1S2 + S3S7 + S4S5 + S6S8) − J2 (S1S3 + S1S4 + S2S3 + S2S4 + S5S6 + S6S7

and χMT versus T plots show a steady decrease of χMT upon decreasing the temperature (Figure 10). Although a few more high nuclearity FeIII phosphonate cages were reported by various groups,23,28,55 the highlighted FeIII cages

8

+ S5S8 + S7S8) − gμB H ∑ Si i=1

E

(1)


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Figure 14. Core structure of 15. Color code: purple (Co), orange (P), red (O), gray (C), blue (N). Compounds 16 and 17 have exactly the same core. Reproduced with permission from ref 20. Copyright 2014 American Chemical Society.

Figure 12. Plot of χMT vs T (2−300 K) for complex 14; red line represents the best fit, and inset shows the magnetization curves.

Figure 15. Illustration of “ε-Keggin” like view (left) and “Stick” type view of the core (right) of 15 (all atoms except metal and phosphorus are omitted for clarity). Color code, purple, CoII, orange, P. Reproduced with permission from ref 20. Copyright 2014 American Chemical Society.

Figure 13. Schematic representation of the model used for the data fitting of 14 where green balls and black lines represent the metals and their connectivity, respectively. Adapted with permission from ref 20. Copyright 2014 American Chemical Society.

Co15

The M/(NμB) vs H plot (inset of Figure 12) shows a steady increase without any saturation, which can be explained by assuming strong magnetic anisotropy of the ions.

Two more quasi-isostructural pentadecanuclear cobalt phosphonate cages having general formulas [Co15(chp)8(chpH)(O3PR3)8(O2CtBul)6] {where R = m-ClPhCH2 (19)} and [Co15(chp)8(chpH)(O3PR)8(O2CtBu)6(CH3CN)3]·(CH3CN) {where R = p-NO2PhCH2 (20)} were obtained by incorporating 6-chloro-2-hydroxypyridine in the synthesis along with the phosphonate ligands.19,20 Their core structure represents a distorted cube (Figure 16a). Further, a significant perception was noticed by removing all the atoms except cobalt and phosphorus and connecting them with imaginary lines (Figure 16b). The polyhedral view is shown in Figure 17. The room temperature χMT values for all the cobalt containing complexes (15, 16, 18−20) are larger in comparison to their spin only values, which can be justified by the orbital contribution of CoII (Figures 18−20), whereas for the Ni12 (17), room temperature χMT value nearly equals the expected one (Figure 18). The gradual decrease signifies the intramolecular antiferromagnetic interaction between NiII centers in 17. The symmetry equivalent unit (Figure 21) was used for fitting the magnetic interactions for 17 with the Hamiltonian given in eq 2, which matched perfectly with the experimental results with values of g = 2.14, J1 = −3.1 cm−1, and J2 = 2.5 cm−1.30

M12 {M = Co (15,16); Ni (17)}

A series of quasi-isostructural dodecanuclear cobalt and nickel phosphonate cages having general formula [M12(μ3-OH)4(O 3 PR) 4 (O 2 C t Bu) 6 (HO 2 C t Bu) 6 (HCO 3 ) 6 ] {where R = p-tBuPhCH2 (15), p-tBuPh (16, 17)} were obtained using different phosphonic acids. (Figure 14).20 The core of the molecules after removal of all the atoms except metal and phosphorus centers can be better described as an ε-Keggin (Figure 15a). Each phosphorus atom is covered by six cobalt ions forming four hexagonal and trigonal faces with cobalt ions occupying all vertices (Figure 15b). The core structures of complexes 15−17 resemble the NiII phosphonate cage reported by Winpenny et al.30 Co12 (with Coligand)

Using coligand 6-chloro-2-hydroxypyridine (chpH)) along with phosphonic acid afforded another Co12 cage, [Co12{μ3(OH)4}(chp)2(p-NO2PhCH2PO3)4(O2CCMe3)8(HO2CtBu)4(HCO3)4]·CH3CN (18).19 Despite the presence of coligand, the core structure of 18 is similar to that of the earlier shown Co12 cages (15−17). F


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Figure 16. (a) Illustration of distorted cubic core found in 19, and its stick-type view (b) (purple (Co), orange (P), red (O), M−M, purple; P−P, light green). Complex 20 has exactly the same core. Reproduced with permission from ref 20. Copyright 2014 American Chemical Society.

Figure 19. Temperature dependencies of χMT at 0.1 T for complex 18 (inset represents the magnetization plot).

Figure 17. Polyhedral view representation of 19. Reproduced with permission from ref 20. Copyright 2014 American Chemical Society. Figure 20. Temperature dependencies of χMT measured at 0.1 T for complexes 19 and 20.

Figure 21. Schematic representation of the model used for data fitting of 17 (Green balls and black lines denote metal centers and their connectivity, respectively). Reproduced with permission from ref 20. Copyright 2014 American Chemical Society.

Figure 18. Temperature dependencies of χMT measured at 0.1 T for 15−17. The best fit obtained for 17 is shown by the solid red line.

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COPPER CAGES Generally, copper phosphonate cages are supported by ancillary coligands (mainly pyrazoles and substituted pyrazoles. Chandrasekhar et al. have reported several copper phosphonates using these coligands including a few high nuclearity copper phosphonate cages such as Cu12,25 Cu16,40 and Cu26.41 However, the mechanistic investigation of their self-assembly is still underdeveloped. So, to understand the mechanism of formation

H = − J1(S1S2 + S2S3 + S3S1 + S4S5 + S4S6 + S5S6) 6

− J2 (S3S4) − gμB H ∑ Si i=6

(2)

The M/(NμB) vs H plot for 18 (shown as a representative example for one of the cobalt complexes) from 3 to 7 K shows a steady increase without any saturation (Figure 19, inset).56 G


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Accounts of Chemical Research and explore the magnetic properties, an unprecedented octadecanuclear Cu-phosphonate pyrazolate nanocage was isolated and characterized.21 Cu18

An octadecanuclear copper cage (Cu18), [Cu18(RPO3)8(Pz)12(μ2-OH)6(μ3-OH)2(OH2)2(Py)4]·2(CH3CN)·2(H2O) (21) (R = p-MePh, Pz = pyrazolyl, Py = pyridine) was obtained under ambient conditions (Figure 22).21 The isolation of [Cu3(Pz)3(μ3-OH)(Py)3(NO3)2] (Figure 23a) in the

Figure 24. Temperature dependencies of χMT measured at 0.1 T for complex 21 (inset shows magnetization graph) measured at 2−10 K. Reproduced with permission from ref 21. Copyright 2013 American Chemical Society.

Figure 22. Core structure of 21. Color code: cyan (Cu), orange (P), red (O), gray (C), blue (N). Reproduced with permission from ref 21. Copyright 2013 American Chemical Society.

Figure 25. Schematic representation of the model used for the data fitting of complex 21. Adapted with permission from ref 21. Copyright 2013 American Chemical Society.

HCu18 = −J1(S1S2 + S2S3 + S1S3) − J2 (S4S8 + S5S7 + S6S9) − J3(S1S5 + S2S8 + S3S6) − J4 (S1S4 + S2S9 + S3S7) 9

− J5(S4S5 + S6S7 + S8S9) − gμB H ∑ Si i=1

(3)

A good fit of the experimental data at low temperature was not obtained, which might be attributed to the complex molecular structure. The inset of Figure 24 shows the magnetization plot, which is unsaturated even at higher field.

Figure 23. (a) Illustration of Cu trimer found in 21, (b) its tortoise view, (c) bowl view, and (d) tetrahedral arrangement of P atoms of phosphonates. Adapted with permission from ref 21. Copyright 2013 American Chemical Society.

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CONCLUDING REMARKS The nature and bridging modes of the phosphonate ligands were essential in fabricating aesthetically pleasing transition metal cages with diverse molecular properties (Table 1). In addition, the incorporation of appropriate substituents on the phosphonate ligands and the use of additional coligands probably favored the solubility properties of the complexes. Reaction conditions were also found to be equally important because different nuclearities were obtained by varying the reaction conditions with the same set of reactants. This Account has compiled all of our published results on phosphonate based molecular cages with an emphasis on synthesis, structural aspects, and magnetic properties. In this endeavor, we initially discussed the coordination chemistry of phosphonates and emphasized their reactivity toward different metal ions forming different cages of varied nuclearity.

absence of phosphonate ligand enabled us to propose its stepwise self-assembly. The asymmetric unit of 21 resembles a tortoise and contains nine copper centers (Figure 23b). Connecting the metal centers through imaginary lines results in a bowl like shape (Figure 23c). Similarly phosphonates are arranged in a tetrahedral like arrangement (Figure 23d). The room temperature χMT value for 21 is lower in comparison to the spin only value for 18 CuII centers due to significant antiferromagnetic interactions between the copper centers (Figure 24). The data was fitted with the Hamiltonian in eq 3, which gives g = 2.05, J1= −6.04 cm−1, J2 = −135.23 cm−1, J3 = 13.19 cm−1, J4 = −20.04 cm−1, and J5 = −1.85 cm−1 (Figure 25). Thus, both antiferro- and ferromagnetic interactions are present. H


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Table 1. A Synthetic Summary of the High Nuclearity Transition Metal Cages Discussed in This Account and Their Magnetic Properties

Many more phosphonate ligands with varying functionalities remain to be explored and examined for their potential of making high nuclearity cages, predicting that this area of phosphonate based high nuclearity molecular cages will continue to flourish.

Subsequently, we discussed in brief the structural aspects and magnetic properties of the synthesized complexes. Combined and dedicated efforts of a very few research groups around the scientific world over a rather short period of time has revealed a vast and beautiful array of molecular phosphonates. I


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: clearfi[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Dr. Javeed Ahmad Sheikh graduated from IISER, Bhopal, in 2015 under the supervision of Dr. Sanjit Konar. He is currently a postdoctoral fellow at Texas A&M University. His research interests are central to the design and development of high nuclearity cages with interesting magnetic properties like slow relaxation of magnetization and magnetic refrigeration. Dr. Himanshu Sekhar Jena received his Ph.D. in Chemistry from IIT Guwahati in 2013 under the supervision of Prof. V. Manivannan. After a postdoctoral stay with Dr. Sanjit Konar (March 2013−October 2014) at IISER, Bhopal, he moved to Osaka University as a JSPS Postdoctoral fellow with Prof. Kazushi Mashima (November 2014 to the present). Prof. Abraham Clearfield received his Ph.D. at Rutgers University in 1954. Before joining the faculty of Ohio University in 1963, he worked in industry for some period. He then moved to Texas A&M University, where he is currently a distinguished professor. During his academic tenure, he has mentored over 50 Ph.D. students and 48 postdoctoral fellows in addition to numerous undergraduates. He has comprehensively studied phosphonates, phosphates, and other layered materials. His initial effort on the syntheses and crystal structures of zirconium phosphates put the establishment for a global research effort that lasts to this day. Prof. Sanjit Konar obtained his Ph.D. in 2004 from IACS, Kolkata, under the supervision of Prof. N. Ray Chaudhuri. After two postdoctoral stays at University of Notre Dame (April 2004−March 2005) and University of Arkansas (April 2005−August 2005), he moved to Texas A&M University as Research Associate (September 2005−March 2008). In 2008, he was selected as an AVH fellow at Universität Bielefeld, Fakultätfür Chemie, Germany. He has been at IISER, Bhopal, since October 2009, where he is currently an Associate Professor. His current research interests are in the areas of molecular magnetism and metal−organic frameworks.

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ACKNOWLEDGMENTS J.A.S. thanks CSIR for a SRF fellowship. H.S.J. thanks IISER Bhopal for a postdoctoral fellowship. S.K. thanks DST, Government of India (Project No.SR/FT/CS-016/2010) and IISER Bhopal for generous financial and infrastructural support. J.A.S. and A.C. thank the R.A. Welch Foundation, Grant No. A0673, and the Natural Science Foundation, Grant No. DMR0332453, for financial support.

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Phosphonate Based High Nuclearity Magnetic Cages - American

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