Metal-Organic Frameworks from Single-Molecule Magnetsusers.uoa.gr/~gspapaef/Publications/Encyclopedia of... · chemistry—molecular magnetism and supramolecular chemistry, is an

Metal-Organic Frameworks from Single-MoleculeMagnets

Athanassios D. Katsenis

National and Kapodistrian University of Athens, Athens, Greece

Euan K. Brechin

The University of Edinburgh, Edinburgh, UK

and

Giannis S. Papaefstathiou

National and Kapodistrian University of Athens, Athens, Greece

1 Introduction 2452 2D MOFs from SMMs 2473 3D MOFs from SMMs 2534 Conclusion 2555 Related Articles 2566 Abbreviations and Acronyms 2567 References 257

1 INTRODUCTION

During the last three decades, the field of molec-ular materials science has emerged from the bases set byclassical molecular chemistry.1–3 It is not only the easeof synthesis that makes molecular materials desirable, butthe opportunity that they offer to finely tune propertiesby selecting the correct ingredients that these species aremade of. Since most molecular materials are crystallinesolids, the way that they are distributed in the solid statelargely affects their properties. To this end, the principles setby supramolecular chemistry and crystal engineering pro-vide the tools to control the synergistic effects between themolecules and finally tune the properties of the bulk molec-ular material.4–6

Metal–organic frameworks (MOFs) have emergedwithin the realm of the molecular world as leading materi-als for a series of industrial applications.7 MOFs are crys-talline hybrid organic–inorganic solids built from organicmolecules and inorganic building blocks (metal ions ormetal clusters) connected in space through coordination

Metal-Organic Framework Materials. Edited by Leonard R. MacGillivray and Charles M. Lukehart.© 2014 John Wiley & Sons, Ltd. ISBN 978-1-119-95289-3

bonds to create extended polymeric frameworks. In otherwords, MOFs are polymeric coordination complexes oftencalled coordination polymers. Contrary to the purely inor-ganic polymers (i.e., zeolites), MOFs are made under mildconditions (e.g., solution chemistry, solvothermal synthe-sis at low temperatures <300 ∘C) which are in general highyielding and scalable, features that make MOFs attractivetargets for technological applications. The major advantageof MOFs over other traditional polymeric materials, suchas zeolites, is the greater scope for tailoring these materi-als for specific applications, which is feasible because oftheir modular synthesis.4 Their inherent modularity (i.e.,the ability to modify the organic and/or inorganic com-ponents) imparts unique properties for a wide range ofpotential applications including gas storage, separation,catalysis, and sensing, placing them among the most inter-esting classes of solid-state materials.8

The easiest way to make a coordination polymer isto extend the coordination geometry of a metal ion usinga linear organic bridge.9 The coordination preferences of

246 METAL-ORGANIC FRAMEWORK MATERIALS

the metal ion and the steric requirements of the organicbridge dictate the way that the inorganic and the organiccomponents will assemble to form the extended frame-work. At the early stages of this chemistry, the construc-tion of topologically interesting frameworks of increasingdimensionality and complexity dominated.10,11 While thefield of coordination polymers was growing, pioneeringwork at the beginning of the century gave a new boost and anew outlook in this chemistry. Polynuclear metal complexes(i.e., metal clusters), in the form of oligonuclear metal car-boxylates, were employed in MOF synthesis, instead of sin-gle metal ions.12 In contrast to single metal ions, the metalclusters provided a means to fix the coordination environ-ment around the nodes while the clusters imparted rigidityon the structures of the frameworks, giving rise to highlystable porous solids.

Polynuclear metal complexes or metal clustersexisted before MOFs and constitute a distinctive fieldwithin the context of coordination chemistry.13 Metalclusters are discrete entities consisting of at least threemetal ions that are bridged by organic ligands. Over theyears, several synthetic strategies have been developed forthe isolation of such species,14 with the “serendipitous self-assembly” of suitable bridging and terminal ligands withmetal ions remaining the most prominent and productiveapproach.13 Clusters based on paramagnetic metal ionsare of special interest as they provide a means to study themagnetic properties originating from the intramolecularexchange coupling of the individual spin carriers (i.e.,metal ions), which are in close proximity within the clus-ter. In 1993, the first polynuclear metal complex capableof storing magnetic information at the molecular levelwas discovered, and since then, many such complexeshave been shown to act in a similar manner.15,16 Theliterature pertaining to clusters built from paramagneticmetal ions can often be rather difficult to navigate forthe nonspecialist, as the terminology employed (a blighton much modern chemistry!) can be a little confusing.Most commonly, these species are referred to as single-molecule magnets (SMMs) and molecular nanomagnets(MNMs).17 The latter is a generic term that refers to anymolecule possessing any interesting magnetic properties,whereas the former specifically refers to molecules that arebistable, exhibiting slow relaxation of the magnetizationand hysteresis loops in magnetization versus field sweeps,below their blocking temperature (TB), that are purely ofmolecular origin. In transition metal-based SMMs, thisbistability originates from a (relatively) large spin groundstate (S) that is split in zero field such that its largest±Ms levels lie lowest in energy and its smallest ±Ms stateshighest in energy (a negative axial zero-field splitting, D).The resultant energy barrier (U) to reorient “spin-up”to “spin-down” is given by S2|D| and (S2−1/4)|D| forinteger and noninteger spins, respectively. Relaxationproceeds via a thermally activated process (“up and over”

the energy barrier) and via tunneling through the barrier(magnetic quantum tunneling, MQT).18 Experimentally,this relaxation process is most easily determined throughobservation of temperature and frequency-dependentout-of-phase (𝜒 ′′) AC susceptibility signals, as hysteresisis often not observed above 2 K, the temperature limit ofmost commercial SQUID magnetometers. An Arrheniusanalysis of the AC data provides access to the effectivebarrier to magnetization relaxation (Ueff), which is nor-mally a little smaller than the theoretical upper limit (U)due to the presence of MQT, a shortcut to magnetizationreversal.

Although the magnetic properties of SMMs areof molecular origin, in the solid state, crystal-packingeffects, which include weak interactions (e.g., hydrogenbonding and 𝜋· · ·𝜋 interactions), may influence their mag-netic response, especially at low temperatures. A decadeago, the tetranuclear SMM [Mn4O3Cl4(O2CEt)3(py)3]was reported to form a supramolecular dimer in thesolid state held together by six C–H· · ·Cl bonds and oneCl· · ·Cl interaction.19 Although both the C–H· · ·Cl andthe Cl· · ·Cl interactions are very weak, they induce anti-ferromagnetic coupling between the two SMMs resultingin different quantum behavior from that of the individualSMMs. Each [Mn4] within the dimer acts as a field biason its neighbor, shifting the MQT resonances to newpositions relative to the isolated [Mn4] molecules. Morerecently, similar behavior was also observed in a family of[MnIII

3O(R-sao)3(X)(L)3–4] (saoH2 = salicylaldoxime; R= Et, Ph; X = RCO2

– , ClO4– ; L = solvent) SMMs.20 The

[Mn3] molecules self-assemble to form one-dimensional(1D), two-dimensional (2D), and three-dimensional (3D)hydrogen-bonded frameworks through multiple O–H· · ·Oand/or C–H· · ·𝜋 interactions. Single crystal hysteresisloop measurements (Figure 1) demonstrated that theseinteractions are strong enough to cause a clear field bias,but too weak to transform the spin networks into classicalantiferromagnets. The above results of exchange-coupledSMMs demonstrate that MQT can be controlled usingweak exchange interactions, suggesting that supramolec-ular chemistry can be exploited to modulate the quantumphysics of SMMs.

It is becoming self-evident that linking SMMsmay alter their magnetic response providing access tomaterials with fascinating physical properties. In addition,the perspective of constructing MOFs from SMMs mayalso lead to dual- or multifunctional materials combing theinherent magnetic properties of the cluster-SMMs with theporosity of the framework, opening new frontiers in molec-ular magnetism and molecular recognition by offering theability to study the interplay between magnetism andencapsulation of species within such frameworks. The con-trolled aggregation and organization of magnetic buildingblocks into designed 0–3D architectures, combining twofundamentally important and exciting areas of modern

MOFs FROM SINGLE-MOLECULE MAGNETS 247

𝜇0H (T)

–0.5 0 0.5 1–1

𝜇0H (T)

–0.5 0 0.5 1–1

M/M

s

–1

–0.5

0

0.5

2 mT/s 0.017 T/s

1.8 K

1 K

2 K1.8 K

1.7 K

1.6 K1.5 K

1.4 K 1.3 K

1 K

0.8 K0.6 K

0.04 K

0.9 K

0.8 K

0.04 – 0.6 K

1.5 K

1.4 K

1.3 K1.2 K

1.1 K

1

M/M

s

–1

–0.5

0

0.5

1

Figure 1 Magnetization versus field hysteresis loops, measured along the easy axis of magnetization, for a single crystal of [MnIII3O(Et-

sao)3(ClO4)(MeOH)3] (left) and [Mn3O(Et-sao)3(μ2-O2Ph(CF3)2)(CH3CH2OH)(H2O)3] (right) at the indicated temperatures and fieldsweep rates. M is normalized to its saturation value

chemistry—molecular magnetism and supramolecularchemistry, is an extremely important facet of moderninorganic chemistry as such materials have long-termpotential in areas that transcend chemistry, physics, andbiology. Applications for molecule-based magnetic mate-rials can be envisioned in low-temperature refrigeration,information storage, spintronics, molecular recognition,sensing and separations, biocompatible agents for useas contrast and polarizing agents in magnetic resonanceimaging (MRI), and as agents for magnetic hyperthermictreatments. However, in order to make such applicationsfeasible and to exploit the inherently interesting physicsdemonstrated by molecular magnets, they need to be easilyaddressable, that is, they should be organized in boththe solution and the solid state. This is nontrivial. Onepotential way to circumvent this problem is to preorganizethe magnets in the solid state by converting them into1–3D arrays. The added bonus of this approach is thatthe scientist can choose to make the linker units innocentor noninnocent, the former maintaining the molecule-only properties, the latter offering adding functionalitythrough intermolecular interactions that can be switchedon and off.

While several 2D frameworks and many more1D systems have been reported, literature examples of3D SMM-MOFs are few and far between.21 The pur-pose of this account is not to be comprehensive in listingall such examples, but rather to highlight some impor-tant/interesting examples possessing 2D and 3D networkswhile, when appropriate, introducing some pertinent 1Dsystems. We analyze the topological features of thesesystems providing details on some synthetic aspects andhighlighting their basic magnetic properties.

2 2D MOFS FROM SMMS

2.1 MOFs based on [MnIII3] and [MnIII

6] SMMs

Among a plethora of manganese clustersknown today, the families of hexanuclear and trinu-clear MnIII SMMs of general formulae [MnIII

6O2(R-sao)6(O2CR)2(L)4–6]22 ([Mn6]) and [MnIII

3O(R-sao)3(X)(L)3–4]23 ([Mn3]) (saoH2 = salicylaldoxime; R = H,Me, Et etc; X = RCO2

– , ClO4– ; L = solvent) hold a

prominent position as they contain more than 30 memberseach, allowing detailed magnetostructural correlationsto be performed. The disklike triangular clusters consistof an [MnIII

3(μ3-O)]7+ core with three R-sao2− ligandsbridging the MnIII ions along the edges of the triangle,whereas the anions and the solvent molecules are placedon either side of the disk, occupying the Jahn–Teller (JT)axes of the MnIII ions. The hexanuclear clusters can beconsidered as two fused [Mn3] disks with the carboxylateanions and the solvent molecules occupying the two sidesof the [MnIII

6(μ3-O)2(R-sao)6]2+ core. Therefore, each[Mn3]/[Mn6] cluster carries six potential open coordi-nation sites, three on each side, which are available forcoordination to exo-dentate ligands capable of bridgingthe clusters. It has also been demonstrated that it is pos-sible to significantly increase the ground spin state fromS= 4 to S= 12 in [Mn6] and from S= 2 to S= 6 in [Mn3],enhancing the effective energy barrier for magnetizationreversal (Ueff) to record levels. The origin of the antifer-romagnetic to ferromagnetic (AF→F) switch between theMnIII ions arises from an intracluster structural distortionof the [Mn3]/[Mn6] molecule induced by the twisting ofthe (–Mn–N–O–) ring, as evidenced by the significantincreases in the Mn–N–O–Mn torsion angles observedwhen derivatized salicylaldoximes were employed.


Figure 2 The structure of (1) (top), phase and side views of thelayer in (2) and the supramolecular entanglement of the layers in(2) (bottom). All hydrogen atoms as well as many carbon atomsand solvent molecules have been removed for clarity. Color code:C, gray; O, red; N, blue; Mn, purple

Employment of the exo-dentate bridgingligands 4,4′-bipyridine (4,4′-bpy) and trans-1,2-bis(4-pyridyl)ethylene (4,4′-bpe) in [Mn3]/[Mn6] chemistryafforded a 1D polymer and a 2D framework, respectively(Figure 2).24,25 The [Mn3] triangles and the 4,4′-bpy lig-ands in {[MnIII

3O(Et-sao)3(4,4′-bpy)2(MeOH)]ClO4}n(1) have assembled to create a ladderlike {-[Mn3]-(4,4′-bpy)2-[Mn3]-}n 1D polymer with two 4,4′-bpy molecules

bridging between the [Mn3] disks. In {[MnIII3O(sao)3(4,4′-

bpe)1.5]ClO4}n (2), one 4,4′-bpe molecule is attached toeach MnIII ion, with all three being oriented on the sameside of the [Mn3] disk, giving rise to the formation ofan approximately 12.6 Å thick 2D framework possessingconical cavities within its body. Each cavity is big enoughto host a [Mn3] unit from neighboring layers resultingin a unique interlocking of the layers that increases thedimensionality of the material from 2D to 3D. Thissupramolecular interlocking is based on host–guest andhydrogen bonding interactions and does not belong to theknown topological and Euclidean entanglements.26

Although the [Mn3] core proved to be stableenough to maintain its structural integrity on reactionwith pyridyl-type ligands, leading to polymeric materials,it achieved it by altering its local symmetry, which proveddisadvantageous for the construction of magnetic mate-rials, as the changes in the magnetic core dramaticallyreduced the Mn–N–O–Mn torsion angles resulting inthe [Mn3] moieties within the materials possessing smallor even diamagnetic spin ground states. In other words,the use of long pyridyl-type ligands flattened the trian-gular building blocks, promoting dominant AF exchangethrough the oximate bridges.

On the contrary, the employment of bis-carboxylate ligands, such as disodium iso-phthalateand disodium succinate, resulted in two poly-meric species where the [Mn6] clusters retainedtheir SMM behavior.27 Complexes [MnIII

6O2(Et-sao)6(EtOH)x(H2O)y(O2C–Ph–CO2)]n (3) and[MnIII

6O2(Et-sao)6(EtOH)4(H2O)2(O2C–(CH2)2 –CO2)]n(4) (x+y = 6, HO2C–Ph–CO2H = isophthalic acid andHO2C–(CH2)2 –CO2H = succinic acid) are composedof [Mn6] SMMs spaced by the dicarboxylate ligands tocreate 1D coordination polymers. Following this approach,more bis-carboxylate-bridged 1D [Mn6] polymers werereported.28–30 Since the incorporation of bis-carboxylateligands resulted in polymeric species where the [Mn6] clus-ters retained their SMM behavior, higher dimensionalitycoordination polymers (i.e., 2D) were intentionally builtfrom [Mn6] SMMs.31 For this purpose, the tris-carboxylateligand 1,3,5-benzene-tricarboxylic acid (trimesic acid,tmaH3) was incorporated into blends of manganese/saoH2reaction mixtures. This reaction system was found to besensitive to reaction conditions and, depending on theorder that the starting materials were mixed, a 1D coor-dination polymer and a 2D framework were isolated. Thestructures of [Mn6O2(sao)6(tmaH)(MeOH)8.5(H2O)0.5]n(5) and [Mn6O2(sao)6(tma)0.66(MeOH)3.33(H2O)1.33]n (6)are shown in Figure 3. Complex 5 consists of two typesof [Mn6] clusters bridged by the tmaH2− ligands to form1D chains. These chains are arranged in parallel, with thefree carboxylic acids of the tmaH2− ligands participat-ing in hydrogen bonding with neighboring chains, thusbridging them to create a 2D nonregular network with


Figure 3 The structures of (3), (5), and (6) (from top to bottom).All hydrogen atoms as well as many carbon atoms and solventmolecules have been removed for clarity. Color code is same as inFigure 2

3,4L13 topology and point symbol (4.62)2(42.62.82). In thisarrangement, the [Mn6] units serve either as bridges or as4-connected nodes, and the tmaH2− ligands as 3-connectednodes. Complex (6) consists of three crystallographicallyindependent [Mn6] clusters and tma3– ligands that haveassembled to create a regular (6,3) 2D framework withan hcb topology and point symbol 63, commonly knownas honeycomb. In this inverted metal–organic framework(IMOF),9 the tma3− ligands serve as 3-connected nodesand the [Mn6] clusters as the spacers.

Alternating-current (AC) susceptibility mea-surements on (3), (4), (5), and (6) at low temperatureswith the field oscillating at various frequencies were per-formed. A cusp in the real component 𝜒 ′ was found, whichwas accompanied by a nonzero imaginary component𝜒 ′′ (Figure 4) at ∼3.5, 4.3, 2.7, and 2.7 K for (3), (4),(5), and (6), respectively. The maxima for all complexes

were strongly frequency-dependent, suggesting super-paramagnetic blocking of the magnetization below theblocking temperatures (TB), indicative of SMM behavior.Arrhenius plots constructed from the 𝜒 ′′ data afforded𝜏0 = 1.6× 10−10 s and Ueff = 48 K for (3), 𝜏0 = 1.8× 10−10 sand Ueff = 60 K for (4), 𝜏0 = 3.3× 10−9 s and Ueff = 32.84 Kfor (5), and 𝜏0 = 5.6× 10−8 s and Ueff = 24.54 K for (6). Thespin dynamics at low temperatures were investigated by cal-culating the frequency shift (k) of TB. The presence of sig-nificant inter-[Mn6] interactions would be expected to slowdown the spin dynamics at low temperatures and this wouldbe manifested in a small frequency shift. Using the averageTB values of 3.5, 4.3, 2.7, and 2.69 K for (3), (4), (5), and (6),respectively, the frequency shift of TB was calculated fromk=ΔTB × (TBΔlog f)–1, where ΔTB is the change in TB forthe given change in frequency Δlog f, where Δlog f= 2.4 for(3) and (4) and 1.30 for (5) and (6). This provided values of0.08 for (3) and (4), 0.19 for (5) and 0.24 for (6), which arewithin the range expected for ideal noninteracting super-paramagnets (0.1≤ k≤ 1) and close to those reported forthe magnetically isolated [Mn6] complexes.22 This suggeststhat the relaxation is in accordance with SMM behavior,and is not attributed to long-range interactions mediatedthrough the polycarboxylate ligands.

2.2 MOFs based on [MnIII2MnII

2] SMMs

Another interesting family of SMMs are thosepossessing the tetranuclear [MnIII

2MnII2(μ3-O)2(μ-O)4]

core.32 In this core, the four manganese ions are locatedat the corners of a defective double cubane (two cubanessharing one face and each missing one vertex) and areeither planar or very close to planarity. An interestingfeature of this type of cluster is that the two MnII ionspossess potentially open coordination sites (sites thatterminal counter anions or solvent molecules are bound)enabling the clusters to be used as building blocks for thesynthesis of extended frameworks. The exploitation ofthis type of cluster as building blocks has afforded four2D frameworks, namely {[Mn4(hmp)6(dcn)2](ClO4)2}n(7),33 {[Mn4(hmp)4(Hpdm)2(dcn)2](ClO4)2}n (8),33

[Mn4(hmp)4Br2(MeO)2(dcn)2]n (9),33 and [Mn4(hmp)6{Cu(pic)2(ClO4)2}2]n (10)34 (Hhmp = 2-hydroxymethyl-pyridine, H2pdm= pyridine-2,6-dimethanol, dcn– = dicya-nimide, Hpic= picolinic acid). Complexes (9) and (10) weresynthesised by the reaction of a preisolated [MnIII

2MnII2]

SMM with sodium dicyanamide (NaN(CN)2 or Nadcn)and copper picolinate [Cu(pic)2]⋅2H2O, respectively,whereas complexes (7) and (8) were isolated by a one-potreaction from the starting materials in the presence ofNaN(CN)2. All complexes consist of a [MnIII

2MnII2(μ3-

O)2(μ-O)4] core sitting on a center of inversion, whichenforces planarity to the four manganese ions. The[MnIII

2MnII2] units have assembled by either N(CN)2

–


0.03

2

1

0

0.2

1

0.0

T(K)

2 4 6 8 10

T(K)

2 43 65 7 8

0.2

0.4

0.6

40 Hz90 Hz330 Hz930 Hz1730 Hz3670 Hz6470 Hz9300 Hz

40 Hz90 Hz170 Hz330 Hz770 Hz1270 Hz2130 Hz3370 Hz5730 Hz9300 Hz



0.8

1.0

1.2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

χ′′ M

(cm

3 m

ol–1

)χ′′ M

(cm

3 m

ol–1

)

(3)

(4) (6)

(5)

Figure 4 Plots of 𝜒 ′′M versus T for complexes (3)–(6) at the indicated temperature and frequency ranges

(dcn– ) or [Cu(pic)2(ClO4)2]2– anions to form 2D frame-works with an sql topology and point symbol 44.62,commonly known as square grid (Figure 5). In thisarrangement, the [MnIII

2MnII2] units serve as four con-

nected nodes with only the MnII ions acting as connectionpoints while the dcn– or [Cu(pic)2(ClO4)2]2– anions serveas linear bridges. Although the underlying networks ofthese 2D frameworks are the same, the [MnIII

2MnII2]

planes are oriented differently within the layers, a featurethat largely affects the orientation of the JT axes of theMnIII ions and subsequently the magnetic response of thesematerials. From the magnetic point of view, complexes (8)and (10) are best described as consisting of noninter-acting SMMs (with 𝜏0 = 2.23× 10−7 s and Ueff = 12.8 Kfor (8) and 𝜏0 = 5.6× 10−10 s and Ueff = 14 K for (10)),whereas long-range magnetic order (canted antiferromag-netism) was observed for (7) below 4.6 K, and an interplaybetween SMM behavior and long-range magnetic order(canted antiferromagnetism) observed for complex (9) (seeFunctional Magnetic Materials Based on Metal FormateFrameworks).


6] SMMs

A rather unusual SMM comprising eleven MnIII

and six MnII ions was isolated serendipitously from the

reaction of Mn(O2CCH3)2⋅2H2O, Mn(ClO4)2⋅6H2O,NaN3 and 1,3-propanediol (H2pd) in a mixture ofCH3CN/pyridine (py).35 This discrete entity was foundto possess dominant ferromagnetic nearest-neighborinteractions between the metal centers giving rise to

(7) (9)

(8) (10)

Figure 5 The structures of (7)–(10) emphasizing the[MnIII

2MnII2] planes (blue rhombs) and the bridging anions

(see text for details)


Figure 6 The structures of (11) (top) and (12) (bottom). Onlythe [MnIII

11MnII6] core and the bridging anions are shown. Color

code: C, gray; O, red; N, blue; MnII, yellow; MnIII, purple

a large spin ground state (of approximately S≈ 37).Single-crystal magnetization versus applied field studiesrevealed hysteresis loops below 0.7 K characteristic ofSMM behavior; DC magnetization decay data afford-ing Ueff = 13 K and 𝜏0 = 1.0× 10−13 s. By changingthe reaction conditions, the 1D coordination polymer[Mn17O8(N3)5(O2CMe)(pd)10(py)6]n (11) was obtained,whereas replacement of N3

– by OCN– resulted in the2D MOF [Mn17O8(OCN)7(O2CMe)2(pd)10(py)4]n (12)(Figure 6). Both polymeric species comprise the same corefound in the discrete cluster. The [MnIII

11MnII6] units in

(11) are singly bridged by an end-on (EO) azide (N3– )

to create a linear chain, whereas in (12), they are singlybridged by EO OCN– anions to one dimension and doublybridged by two EO OCN– anions to the second dimensionto form a 2D framework with an sql topology. None ofthe polymeric species exhibited any frequency dependenceof their out-of-phase (𝜒 ′′) AC susceptibility signals downto 1.8 K, suggestive of intercluster antiferromagneticinteractions.

2.4 MOFs based on [3d–4f] SMMs

Given that all paramagnetic lanthanide ions (Ln,4f), except the f7 and f14 congeners, possess anisotropicelectron configurations, their incorporation within a 3dmetal cluster may introduce enhanced anisotropy into themagnetic ground state of the molecule. Since anisotropyis vital in forming SMMs, the synthesis of such 3d–4fclusters is potentially exciting.36 In line with these obser-vations, two similar 2D MOFs comprising a [CoII

4LnIII2]

Figure 7 The doubly bridged nearly planar [CoII4LnIII

2] cores (inblue) in (13) and (14). Color code: Co, purple; Ln, green

core [Ln = Dy (13) or Ho (14)] were synthesized.37

To this end, the bifunctional ligand 4′-(4-carboxyphenyl)-2,2′:6′,2′′-terpyridine (HL), comprising a tridentate chelateterpyridine claw and a pendant carboxylic acid, was syn-thesized. Eight L– anions wrap around the nearly planar[CoII

4LnIII2(CO3)4]6+ core with the carboxylate anions

coordinated to the strongly Lewis acidic LnIII ions andthe terpyridine claws to CoII ions. In this arrangement, a2D framework with an sql topology forms from doublybridged [CoII

4LnIII2] units in both directions (Figure 7).

AC susceptibility measurements on (13) and (14) at lowtemperatures exhibited very weak frequency-dependent𝜒 ′′ signals. When the AC susceptibility of 13 was repeatedin the presence of an applied DC field, the 𝜒 ′′ maximashifted to higher temperatures permitting calculation ofUeff = 7.6 K with 𝜏0 = 1.9× 10−6 s.

2.5 MOFs based on [DyIII2] SMMs

Over the last decade, there has been a reigni-tion of interest in Ln-based SMMs.38,39 The large andunquenched orbital contribution to the magnetic momentand their numerous unpaired electrons potentially makeparamagnetic Ln ions good candidates for construct-ing SMMs. However, as the 4f orbitals are contracted,the intramolecular exchange coupling in polynuclear Lncomplexes is very weak and in most cases their magneticproperties are dominated by single-ion effects. A bifunc-tional ligand that promotes ferromagnetic interactionsbetween DyIII ions and leads to the isolation of a discrete[DyIII

2] SMM and a 2D framework composed of [DyIII2]

SMMs40 is (2-hydroxy-3-methoxyphenyl)methylene (ison-icotino)hydrazine (H2hmi), a compartmental ligand able


Figure 8 The structure of (16). All hydrogen atoms as well asmany carbon atoms and solvent molecules have been removed forclarity. Color code: C, gray; O, red; N, blue; Dy, purple

to host two DyIII ions, and possessing a pendant pyridylgroup capable of bridging the dinuclear units. The reac-tion of H2hmi with Dy(NO3)3⋅5H2O was found to besensitive to reaction conditions, small changes leadingto the discrete [Dy2(hmi)2(NO3)2(MeOH)2] (15) versusthe 2D framework [Dy2(hmi)2(NO3)2(MeOH)2]n (16).The two hmi2− ligands in (16) wrap around the two DyIII

ions, being arranged in a head-to-tail manner with thepyridyl groups spanning across the Dy· · ·Dy vector. In thisarrangement, each [Dy2] unit serves as a linear bridge whileit is bound to two pyridyl groups from neighboring dimerscreating an sql network (Figure 8). Both DC magneticsusceptibility and magnetization measurements suggestthe presence of intramolecular ferromagnetic interactionsbetween the DyIII ions. AC susceptibility measurementson (15) and (16) resulted in the calculation of Ueff and𝜏0 values of 56 K and 3× 10−7 s for (15) and 71 K and7× 10−8 s for (16).

2.6 MOFs based on [CoII4] SMMs

Although octahedral CoII ions can display largesingle-ion anisotropy and possess up to three unpairedelectrons, CoII SMMs are vastly outnumbered comparedto those of MnIII.17 Two isomorphous 2D anionic frame-works based on [CoII

4] moieties were recently reported.41

The {Cs4[μ-{Co(glycol)(H2O)2}{Co4(cit)4}]}n (17) and{Rb4[μ-{Co(glycol)(H2O)2}{Co4(cit)4}]}n (18) SMMswere prepared by treating a CoII citrate (cit4−) precursorwith CsOH and RbOH, respectively, in the presence ofethylene glycol (glycol). The four CoII ions and the four

Figure 9 The augmented sql network in (17) and (18)

hydroxyl groups of the cit4− anions within the [Co4(cit)4]8−

core are arranged at alternate corners of a cube. Allthree carboxylate groups of the four cit4− ligands pro-vide peripheral ligation to the cubane core, whereas onecarboxylate from each cit4− ligand is further bonded toa [Co(glycol)(H2O)2]2+ unit comprising an octahedralCoII ion. In this arrangement, each [Co4(cit)4]8− unitis bound to four [Co(glycol)(H2O)2]2+ while the lattersimply bridges two [Co4(cit)4]8− units creating a 2Dframework with an sql topology (Figure 9). The Cs+,Rb+, ethylene glycol, and H2O molecules are differentlydisordered within the square cavities of (17) and (18),respectively, whereas the interlayer separation is 0.2 Ålarger in (17) as compared to the respective interlayerseparation in (18). The magnetic behaviors of (17) and(18) have been investigated from 300 K down to 90 mK.In each case, the ferromagnetically coupled [Co4] unitswithin the 2D frameworks display SMM behavior mani-fested by frequency-dependent 𝜒 ′′ signals in out-of-phaseAC susceptibility studies at approximately 4 K, with Ueffvalues of approximately 13 K. This is entirely analogousto that observed for molecular (0D) CoII cubanes that aremagnetically isolated and show little or no intermolec-ular interactions in the solid state. However, at lowertemperatures, magnetic exchange between the cubaneunits and the single CoII ions enforce a transition to amagnetically ordered phase between 0.20 and 0.25 K (seeFunctional Magnetic Materials Based on Metal FormateFrameworks), the small differences in TC being attributedto the difference in the nature of the s-block metal ion(Rb+ or Cs+) and the disordered solvent in the crystallattice.


3 3D MOFs FROM SMMs

3.1 MOFs based on [CoII4] and [CoII

5] SMMs

Further exploitation of the CoII citrate chem-istry led to the isolation of an anionic 3D MOF, whichis based on [CoII

4] cubanes and octahedral CoII ions.{Na4(H2O)15[μ3-{Co(H2O)3}2{Co4(cit)4}]}n (19)42 wasobtained through the one-pot reaction of CoSO4⋅7H2O,citric acid (H4cit), and (CH3)4NOH in H2O. As in theprevious 2D frameworks [complexes (17) and (18) inSection 2.6], (19) consists of [Co4(cit)4]8− cubanes and[Co(H2O)3]2+ units comprising octahedral CoII ions.Each [Co4(cit)4]8− is bound to six [Co(H2O)3]2+, with thelatter bridging three cubanes. In this arrangement, a 3Dframework is constructed with an underlying topologyof ant (anatase) and point symbol (42.6)2(44.62.88.10)(Figure 10). The DC magnetic susceptibility of (19) above120 K is analogous to that observed for the magneticallyisolated and structurally analogous [CoII

4] SMMs, whereasbelow 20 K, ferromagnetic exchange between the [CoII

4]unit and the single octahedral CoII ions is observed.Frequency-dependent 𝜒 ′′ signals in out-of-phase ACsusceptibility studies were observed below 6 K with anunusual crossover in the region ∼3–4.5 K for AC oscillat-ing fields in the range of 2–10 kHz, suggesting the presenceof two relaxation processes. Magnetization versus appliedfield hysteresis loops, obtained on single crystals of (19)were observed below 0.6 K and were both temperature-and sweep-rate-dependent—having features different tothose observed for discrete [CoII

4] SMMs, suggesting thepresence of intermolecular interactions to be an importantcontributory factor (see Functional Magnetic MaterialsBased on Metal Formate Frameworks).

A similar {K4(H2O)8[{Co}2{Co4(Hcit)2(cit)2}]}n(20)43 anionic 3D framework with ant topol-ogy was obtained by the hydrothermal reac-tion of Co(O2CCH3)2⋅4H2O and citric acid in aH2O/CH3CH2OH mixture in the presence of KOH. In thisframework, the [Co4(Hcit)2(cit)2]6− cubanes are connectedto six tetrahedral CoII ions, with the latter bridging three[CoII

4] units. Magnetic measurements revealed that thisframework is a canted antiferromagnet exhibiting anti-ferromagnetic interactions between the [CoII

4] cubanesand the tetrahedral CoII ions (see Functional MagneticMaterials Based on Metal Formate Frameworks). Gassorption isotherms for this 3D MOF revealed permanentmicroporosity with a Langmuir surface area of 939 m2 g–1

and pore volume of 0.31 cm3 g–1.A 3D framework based on coplanar [CoII

5] unitswas isolated under hydrothermal conditions from the reac-tion of Co(NO3)2⋅6H2O, benzophenone-2,4′-dicarboxylicacid (H2bpdc), 1,4-diazabicyclo[2,2,2] octane (dabco),and KOH in H2O.44 [Co5(μ3-OH)2(bpdc)4(dabco)(H2O)2]

Figure 10 The augmented ant network found in (19) and (20)

(21) is built around a [Co5(μ3-OH)2]8+ cluster com-prising both octahedral and tetrahedral CoII ions.The [CoII

5] units are bridged by four bpdc2− dicar-boxylates to form a square grid (sql) that is pillaredby the neutral dabco ligands to create a pcu (primi-tive cubic, commonly known as 𝛼-Po) network withpoint symbol 412.63 (Figure 11). The magnetic proper-ties of the [Co5(μ3-OH)2]8+ cluster are dominated bycompeting intramolecular antiferromagnetic exchangeinteractions. AC susceptibility measurements revealeda Ueff of 56.6 K with 𝜏0 = 8.74× 10−9 s, consistent withsuperparamagneticlike behavior. Nevertheless, furthermagnetic studies including zero-field-cooled (ZFC) andfield-cooled (FC) magnetization measurements and hys-teresis loop studies indicated the possible occurrence oflong-range order consistent with a spin canted antiferro-magnet.


2] SMMs

A 3D MOF based on the already-described[MnIII

2MnII2] core (Section 2.2) was obtained via the

one-pot reaction between Mn(ClO4)2⋅6H2O with Hhmpin CH3CN in the presence of NaN(CN)2 (Nadcn) and(CH3CH2)4NOH.45 [Mn4(hmp)4(OH)2Mn(dcn)6] (22)consists of [MnIII

2MnII2] units and [Mn(dcn)6]4− anions.

As in the 2D frameworks (7)–(10), each of the two


Figure 11 The pillared framework with a pcu topology found in (21)

MnII ions in the [MnIII2MnII

2] moiety is bound to threedcn− anions from three different [Mn(dcn)6]4− units.In this arrangement, each [Mn4(hmp)4(OH)2]4+ is sur-rounded by six [Mn(dcn)6]4− units with the latter alsobeing surrounded by six [Mn4(hmp)4(OH)2]4+. Therefore,a six-connected pcu network forms (Figure 12), whichis based on two different six-connected nodes (i.e., the[Mn4(hmp)4(OH)2]4+ and [Mn(dcn)6]4− units). DC mag-netic susceptibility measurements down to 30 K revealedthe presence of ferromagnetic interactions within the[Mn4(hmp)4(OH)2]4+ core. AC magnetic susceptibilitymeasurements revealed a frequency-independent peak inthe in-phase (𝜒 ′) and out-of-phase (𝜒 ′′) signals at 4 and4.1 K, respectively, indicating the presence of 3D magneticordering. Magnetization measurements at low appliedfields below TC = 4.1 K are consistent with an orderedferrimagnetic phase in which the S= 9 [MnIII

2MnII2]

units are antiferromagnetically coupled with the S= 5/2[MnII(dcn)6]4− units.

3.3 MOFs based on [MnIIMnIII3]2 SMMs

The new 3D framework {[MnIIMnIII3O(Me-

sao)3(N3)3(dpp)1.5]2}n (23)46 (dpp = 1,3-di-4-pyridyl-propane, saoH2 = salicylaldoxime) was obtained from theone-pot reaction of Mn(ClO4)2⋅6H2O with Me-saoH2 ina mixture of dimethylformamide/methanol in the presenceof NaN3. The [MnIIMnIII

3] unit comprises the already-described disklike [MnIII

3(μ3-O)(R-sao)3]2+ core (Section2.1), which is capped by one MnII ion. The latter is con-nected to all three MnIII ions through three EO N3

−. Twosuch [MnIIMnIII

3] units are loosely connected through theoximic oxygen atoms of the Me-sao2− ligands (MnIII· · ·O

Figure 12 The pcu network found in (22). Light blue and pur-ple spheres represent the six-connected [Mn4(hmp)4(OH)2]4+ and[Mn(dcn)6]4−, respectively

distances of ∼2.736 Å) to form a [MnIIMnIII3]2 cluster.

Each MnII ion is further connected to three Npyridyl atomsfrom three different dpp ligands. In this arrangement, apcu network forms, akin to those in Figures 11 and 12,with the [MnIIMnIII

3]2 clusters serving as six-connectednodes and the dpp ligands as linear bridges. Two suchpcu networks are grown within the empty cubic cavitiesof each other resulting in a 2-fold interpenetrated 3D


Figure 13 The two interpenetrated pcu networks in (23). Color code: C, gray; O, red; N, blue; Na, yellow; Mn, purple. Blue and redoctahedra represent the [MnIIMnIII

3]2 SMMs

framework (Figure 13) (see Interpenetration and Entangle-ment in Coordination Polymers). Frequency-dependent 𝜒 ′′

signals were observed below ∼4 K indicating superpara-magneticlike behavior, with a temperature shift, k= 0.13(the magnetic behavior is dominated by the moleculeswith little or no intermolecular interactions). Magnetiza-tion versus applied field hysteresis loops were observedbelow 1.8 K, which show stepwise magnetization andlarge coercivities, indicating significant quantum tunnelingeffects. Using both DC and AC decay data, the Ueff and𝜏0 values of 37 K and 4.3× 10−10 s were calculated, whichare comparable to those calculated for complexes (3)–(6)(Section 2.1).


7] SMMs

Two similar 3D frameworks comprising a[MnIII

12MnII7] core were synthesized by the reaction of

the trinuclear precursor [Mn3O(O2CCH3)6(py)3]⋅py (py =pyridine) with either 1,3-propanediol (H2pd) or 2-methyl-1,3-propanediol (H2mpd) in CH3CN.47 [Mn19Na(μ4-O)9(μ3-O)(μ3-OH)3(O2CMe)9(pd)9(H2O)3][OH] (24) and[Mn19Na(μ4-O)9(μ3-O)(μ3-OH)3(O2CMe)9(mpd)9(H2O)3][OH] (25) have strikingly similar structures and magneticproperties. The Mn ions are arranged in a triangular-pyramidal-frustum manner, each being connected to three[Na(O2CCH3)6]5− units, with the latter bridging three[MnIII

12MnII7] clusters. In this arrangement, a 3D frame-

work with an srs topology and point symbol 103 forms

with both [MnIII12MnII

7] and [Na(O2CCH3)6]5− servingas three-connected nodes (Figure 14). Magnetization databelow 10 K at various applied magnetic fields lead tothe calculation of a large spin ground state S≈ 23/2 anda small but negative axial zero-field splitting parame-ter D≈−0.09 cm−1. Single-crystal magnetization studiesrevealed hysteresis loops below 1.1 K with their coercivityincreasing with decreasing temperature and increasingfield sweep rate as expected for isolated SMMs.

4 CONCLUSION

In this account, we have reviewed those MOFsthat are based on SMMs. Although both fields (i.e., MOFsand SMMs) have grown rapidly over the last two decades,SMM-based MOFs remain extremely rare with thosepossessing a 3D framework limited to just a handfulof examples. MOFs are robust porous molecular mate-rials based on polynuclear metal complexes with openframework structures enabling them to host a variety ofmolecules, gases, liquids, and solids that have been pre-viously dissolved in an appropriate solvent. SMMs aremost commonly based on polynuclear transition metalcomplexes possessing relatively large spin ground states,split in zero field to produce a barrier to magnetizationreversal, that display fascinating spin relaxation dynamics


Figure 14 The srs network found in (24) and (25). Color code: C, gray; O, red; Na, yellow; Mn, purple. Blue and red triangles representthe [MnIII

12MnII7] and the [Na(O2CCH3)6]5− three-connected nodes

and have the ability to store magnetic information at themolecular level. Although the physics of these SMMs isintrinsically derived from intramolecular properties, thesecan be modulated by crystal-packing effects. The contin-ued combination of these two molecular materials shouldtherefore act to boost future efforts in both fields, as SMMscan transmit their magnetic properties to a MOF, whereastheir incorporation within a framework, or the inclusion ofadditional guests within the pores of an SMM-MOF, mayalter/modulate magnetic behavior. In other words, this classof molecular materials offers the opportunity to examinethe chemistry and physics of intriguing molecule-basedcomplexes in a somewhat more controlled manner thanhas hitherto been possible. Most SMMs are easily obtainedin large yields employing standard benchtop coordinationchemistry using solution methods. They are thereforeideal candidates to serve as building blocks for the con-struction of MOFs. This also suggests that an alternativeway to obtain an SMM-based MOF would be to employexo-dentate bridging ligands, including metalloligands, toa reaction blend known to result in an SMM.

The magnetic properties of previously reportedSMM-based MOFs have shown behaviors varying fromzero intermolecular interactions through to LRMO. This

scale is truly enormous and the potential to discover manyfascinating new molecule-based materials within it is ripefor exploration and exploitation (see Functional MagneticMaterials Based on Metal Formate Frameworks). Whentruly porous SMM-MOFs are synthesized, the ability tostudy the interplay between magnetism and structure andthe encapsulation of species (e.g., small radicals) within thepores of such frameworks will be realized. We close thisaccount with the hope that this will inspire chemists tomake such materials.

5 RELATED ARTICLES

Functional Magnetic Materials Based on MetalFormate Frameworks; Interpenetration and Entanglementin Coordination Polymers.

6 ABBREVIATIONS AND ACRONYMS

FC = field-cooled; LRMO = long-range mag-netic order; MNM = molecular nanomagnet; MOF =


Metal–organic framework; MQT = magnetic quantumtunneling; MRI = magnetic resonance imaging; SMM= single-molecule magnet; ZFC = zero-field-cooled;1D = one-dimensional; 2D = two-dimensional; 3D =three-dimensional.

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