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MOF-like supramolecular network of Mn 3 single-molecule magnets
formed by extensive p–p stacking
Tu N. Nguyen, Khalil A. Abboud, George Christou
Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, United States
a r t i c l e i n f o
Article history: Received 14 August 2015 Accepted 19 September 2015 Available online 28 September 2015
Dedicated to Malcolm H. Chisholm on the occasion of his 70th birthday; an outstanding scientist and friend.
Keywords: Manganese Mn/O triangular cluster Crystal structure Single-molecule magnet p–p stacking
a b s t r a c t
[Mn 3 O(dhb) 3 (mpko) 3 ](ClO 4 ) ( 4 ) has been obtained from the carboxylate substitution reaction of triangu- lar [Mn 3 O(O 2 CMe) 3 (mpko) 3 ](ClO 4 ) (mpkoH = methyl(2-pyridyl)ketone oxime) with 3,5-dihydroxyben- zoic acid (dhbH). Complex 4 possesses an equilateral triangle [Mn 3 O] 7+^ core with the dhb^ and mpko ligands on opposite sides of the Mn 3 plane. All dhb^ and mpko^ groups are involved in p–p stacking interactions with those on neighboring Mn 3 molecules to give a 3-D supramolecular network resembling a metal–organic framework (MOF), with periodic voids resulting from a repeating unit comprising an [Mn 3 ] 8 rhombohedron. Variable-temperature, solid-state magnetic susceptibility studies in the 5.0– 300 K temperature range reveal the predominance of antiferromagnetic inter-Mn 3 exchange couplings. Alternating current magnetic susceptibility studies on 4 show that the single-molecule magnet (SMM) behavior characteristic of [Mn 3 O(O 2 CR) 3 (mpko) 3 ]+^ complexes has been lost. This work demonstrates that constructing MOF-like networks from SMMs by weak interactions is feasible but that even these can introduce significant inter-SMM exchange interactions that weaken or destroy the SMM properties. Ó 2015 Elsevier Ltd. All rights reserved.
- Introduction
Metal–organic frameworks (MOFs) are polymeric coordination compounds that are formed by linking together inorganic building blocks, i.e., metal ions or metal clusters, using suitable chosen organic linkers. Different from synthetic inorganic polymers such as silica gel or zeolite, whose structures do not include organic car- bon units, MOFs offer high degrees of tunability allowing for tailor- ing of structural and physicochemical properties. Thus they have emerged as promising materials for a variety of applications such as gas separation and storage, heterogeneous catalysis, drug deliv- ery, biological imaging, battery components, and others [1–6]. Recently, there has been a growing interest in applying MOF chem- istry to molecular magnetism to take advantage of the high poros- ity of these materials and possible host–guest chemistry [7]. The amalgamation of molecular magnetism and supramolecular chem- istry presents an important facet of modern inorganic chemistry because such materials have potential in applications that span chemistry, physics, and biology [8]. Several studies have shown that constructing MOFs from magnetic building blocks together with controlling the encapsulation of the species within the frameworks lead to dual- or multifunctional materials [9–13]. Such
studies are providing great encouragement and inspiration to chemists to explore the various fundamental aspects of magnetic MOF development, e.g., to understand the factors that affect the degree of similarity of the magnetic properties of an obtained MOF with those of the monomeric building block, and the sensitiv- ity of the magnetic properties to the identity of the MOF linker group employed, allowing factors to be identified that should be prevented when constructing magnetic MOFs. In this work, we will face such questions in a particular case when a MOF-like network is assembled, using non-covalent interactions, from Mn 3 building blocks that are single-molecule magnets (SMMs). Single-molecule magnets are individual molecules that function as single-domain nanoscale magnetic particles below their blocking temperature, TB [14–18]. For Mn and most 3d transition metal SMMs, this behavior arises from the combination of a large ground-state spin (S) and Ising-type magnetoanisotropy (negative zero-field splitting parameter, D), which leads to a large (versus kT) barrier to magnetization relaxation. SMMs exhibit frequency- dependent out-of-phase ac magnetic susceptibility signals, and hysteresis in a plot of magnetization vs applied dc magnetic field. SMMs have been shown to also display interesting quantum phenomena such as quantum tunneling of magnetization (QTM) [19,20], quantum phase interference [21–23], spin–spin cross relaxation [24], and quantum entanglement [25–27]. Conse- quently, they have been proposed as qubits for quantum
http://dx.doi.org/10.1016/j.poly.2015.09. 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ (^) Corresponding author. Tel.: +1 352 392 8314; fax: +1 352 392 8757. E-mail address: [email protected] (G. Christou).
Polyhedron 103 (2016) 150–
Contents lists available at ScienceDirect
Polyhedron
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o l y
computation [28–31] and as components in molecular spintronics devices [32,33], which would exploit their quantum tunneling properties. Linking SMMs into 1-D chains or 2-D sheets is well-known [34,35] but only a few examples have been reported of 3-D MOFs constructed from SMM building-blocks, most of them by employ- ing covalent organic linkers [36–40]. A diamagnetic 3-D mesoporous MOF as a platform to incorporate Mn 12 Ac SMMs was also reported recently [41]. On the other hand, supramolecular net-
works connected with weak bonds such as hydrogen-bonds or p–p
stacking are very rare but would be interesting to study further because despite the molecular origin of the magnetic properties of SMMs, in the solid-state even weak exchange couplings can have significant influence on their magnetic properties. The report of supramolecular C–H Cl hydrogen-bonded pairs of [Mn 4 O 3 Cl 4 (O 2 CEt) 3 (py) 3 ] (S = 9 / 2 ) demonstrated such inter-SMM coupling for the first time, leading to identification of exchange- biased QTM steps, quantum superposition states, and quantum entanglement of the two SMMs [25,27,42]. We recently also
reported a p–p stacked 2-D array of Mn 4 SMMs [43], and to extend
our work in this area further, we have now turned our attention to exploring the [Mn 3 O(O 2 CR) 3 (mpko) 3 ] +^ (R = Me ( 1 ), Et ( 2 ), Ph ( 3 ); mpkoH = methyl(2-pyridyl)ketone oxime) family of SMMs [44] with spin S = 6 as starting points for network
mpkoH formation. Herein, we describe the synthesis, structural characteri- zation, and magnetic properties of a MOF-like supramolecular network formed by the new 3,5-dihydroxybenzoate member of the family.
- Experimental
2.1. Synthesis
All preparations were performed under aerobic conditions using materials and solvents as received unless otherwise stated.
Table 1 Crystallographic data for complex 4 4MeCN.
Formula a^ C 48 H 45 ClMn 3 N 9 O (^20) FW (g/mol) a^ 1268. Crystal system Rhombohedral Space group R 3 c a = b (Å) 18.6647(12) c (Å) 60.960(6) c (°) 120 V (Å^3 ) 18 392(2) Z 12 T (K) 100(2) k (Å)b^ 0. qcalc (mg/m 3 ) 1. l (mm^1 ) 0. R 1 c,d^ 0. wR 2 e^ 0. a (^) Including solvent molecules. b (^) Graphite monochromator. c (^) I > 2r(I). d (^) R 1 = R(||Fo| |Fc||)/R|Fo|. e (^) wR 2 = [R[w(Fo 2 Fc (^2) ) (^2) ]/R[w(Fo 2 ) 2 ]1/2 (^) where w = 1/[r (^2) (Fo 2 )+(m p) 2 + n p], p = [max(Fo 2 , 0) + 2Fc^2 ]/3, m and n are constants.
Fig. 1. Structure of the cation of 4 (top) and its core (bottom); H atoms have been omitted for clarity. The Mn III^ JT elongation axes are shown as green bonds. Color code: Mn III^ green, O red, N blue, C grey. (Color online.)
Table 2 Selected bond distances (Å) and angles (°) for complex 4. Mn1–Mn1^0 a^ 3.2057(10) Mn1–O1 1.8747(10) Mn1–O2 1.925(3) Mn1–N2 2.009(3) Mn1–N1 2.022(3) Mn1–O3 2.185(3) Mn1–O4 2.197(3) Mn1–O1–Mn1^0 117.52(8) Mn1–N2–O4–Mn1^0 b^ 15.10(4) a (^) Prime indicates crystallographic symmetry. b (^) Torsion angle.
Table 3 Bond valence sums (BVS) a^ for the Mn atoms and selected O atoms of 4. Atom Mn II^ Mn III^ Mn IV^ Assignment Mn1 3.30 3.08 3.16 Mn III O1 2.18 O 2 a (^) For Mn, the bold value is the one closest to the Mn charge for which it was calculated; the oxidation state is the nearest integer to the bold value. For O, it is non-protonated if its BVS is 1.8–2.2, mono-protonated if 1.0–1.4, and doubly- protonated if 0.2–0.4.
- Results and discussion
3.1. Synthesis
The synthesis of complex 4 was carried out by substitution of the acetate groups on complex 1 with dhbH. Carboxylate substitution is a commonly employed way for the carboxylate liga- tion to be modified as desired, and it has been especially crucial in allowing targeted modification of SMMs such as the Mn 4 and Mn 12 families [46,47]. The only necessary criterion for successful substi- tution is that the M/O core is robust and stable to carboxylic acids so that no side-reaction occurs. In the case of 1 , the presence of the firmly bound tridentate pyridyloximate groups of three mpko ligands imparted rigidity and robustness to the molecule during the reaction. The formation of 4 is summarized in Eq. (1-1).
½Mn 3 OðO 2 CMeÞ 3 ðmpkoÞ 3 þ^ þ 3Hdhb
! ½Mn 3 OðdhbÞ 3 ðmpkoÞ 3 þ þ 3MeCO 2 H ð1-1Þ
The substitution is an equilibrium whose position depends on the relative acidity of the incoming and outgoing carboxylic acids. Since dhbH (pKa = 4.04) is a stronger acid than acetic acid (pKa = 4.76), the equilibrium favors the product. Nevertheless, to
ensure clean product, toluene was added to allow removal of acetic acid under vacuum as its toluene azeotrope and thus drive the equilibrium to 100% product. The stoichiometric ratio of [Mn 3 ]: dhbH = 1:3 was used. The low yield (18%) was a consequence of the significant solubility of 4 in MeCN/EtOH, but the product was obtained pure and in a highly crystalline form; we were thus happy to settle for a low yield of such nice material and did not attempt to increase it by concentration or addition of other solvents.
3.2. Structural description of [Mn 3 O(dhb) 3 (mpko) 3 ](ClO 4 ) ( 4 )
The labeled structure of complex 4 and its core are shown in Fig. 1. Selected interatomic distances and angles are listed in Table 2. Complex 4 crystallizes in rhombohedral space group R 3 c with the Mn 3 molecule having crystallographic C 3 symmetry. Its core contains three MnIII^ ions in an equilateral triangle bridged
by a central l 3 -O^2 ^ ion. Each edge of the Mn 3 triangle is bridged
by a g^1 :g^1 :l-dhb^ and a g^1 :g^1 :g^1 :l-mpko^ group, with the pyridyl
and oximate nitrogen atoms chelating a MnIII^ , forming a five-
membered chelate ring. The central l 3 -O^2 ^ lies slightly above the
Mn 3 plane (d 0.30 Å). Overall, the core structure of the cation of 4 is essentially identical to those of the [Mn 3 O(O 2 CR) 3 (mpko) 3 ] + cations of the R = Me ( 1 ), Pr ( 2 ), Ph ( 3 ) analogues. The Mn oxidation states were determined from bond valence sum (BVS) calculations (Table 3), charge balance considerations, and the presence of Jahn–Teller (JT) axial elongations on the MnIII^ atoms (Fig. 1, right).
Inspection of the packing in 4 reveals extensive p–p stacking
interactions (3.4 Å, well below the p–p stacking upper-limit dis-
tance of 3.7 Å [48]) between neighboring Mn 3 cations. Each
cation p–p stacks with three neighboring Mn 3 cations on either
side of the Mn 3 plane via the dhb^ (Fig. 2a) and mpko^ ligands (Fig. 2b), respectively. Thus, a total of six Mn 3 cations interact with
each Mn 3 molecule (Fig. 3). In addition, each p–p stacked pair of
dhb^ rings on adjacent cations is also connected by strong H-bonds between dhb^ OH groups and ClO 4 -^ anion O atoms (O O 2.7 Å); this connects two sets of three Mn 3 cations into a trigonal antipris-
matic [Mn 3 ] 6 aggregate (Fig. 4). As a result of the p–p interactions,
a 3-D MOF-like network structure is formed (Fig. 5, bottom) with the repeating unit being a [Mn 3 ] 8 rhombohedron involving Mn (^3)
Fig. 3. Overall interactions between a Mn 3 cation and six neighboring Mn 3 cations viewed along the C 3 axis (top) and from a side-view (bottom). Color code: orange = interactions via dhb^ groups; purple = interactions via mpko^ groups. (Color online.)
Fig. 4. The [Mn 3 ] 6 ring resulting from aggregation of six Mn 3 cations by p–p stacking and H-bonds with the ClO 4 ^ anions. Color code: MnIII^ green, O red, N dark blue, C grey, Cl yellow. (Color online.)
cations lying at the eight vertices (Fig 5a, right). Each rhombic face of the rhombohedron comprises four Mn 3 cations linked together
by a total of four p–p interactions, two of which involve
dhb^ ligands and two involve mpko^ ligands (Fig 5a, left). The complete 3-D network is then formed by face-fused rhombohedra (Fig. 5b). The [Mn 3 ] 8 rhombohedron encloses a void with a solvent accessible volume of 640 Å^3 (0.64 nm^3 ) (probe radius 1.6 Å, grid spacing 0.5 Å). Of interest with all MOF and MOF-like frameworks containing sizeable voids is what guest molecules might be present in those voids. In 4 4MeCN, the rhombohedron is large enough for encapsulation of eight MeCN guest molecules, with this [MeCN] 8 collection packing in an interesting way to give a triangular-face- bicapped trigonal antiprismatic assembly of S 6 crystallographic symmetry (Fig 6); each [MeCN] 3 triangle has the MeCN molecules arranged like the blades of a propeller. This suggests that a variety of other small organic or inorganic molecules could also be accom- modated as guests. This host–guest chemistry provides another motivation for ongoing work since many examples of MOFs
encapsulating polar guest molecules such as ethanol, methanol or water have been found to exhibit ferroelectric responses [9,49–51].
3.3. Magnetochemistry
3.3.1. DC Magnetic susceptibility studies The Mn 3 cation structure of 4 is very similar to those of [Mn 3 O (O 2 CR) 3 (mpko) 3 ]+^ (R = Me ( 1 ), Pr ( 2 ), Ph ( 3 )) complexes. The latter have been established as having ferromagnetic (F) exchange couplings between the MnIII^ atoms as a result of core distortions caused by binding of the mpko^ chelates, such as the non-zero oxi- mate Mn–N–O–Mn torsion angles in the range 9.6–15.4°, leading to spin S = 6 ground states for the complexes [44]. Complex 4 , with a similar structure to 1 – 3 including torsion angles of 15.1° (Table 2), is therefore also expected to exhibit F coupling and have an S = 6 ground state. Variable-temperature, direct current (dc) magnetic susceptibil-
ity (vM) measurements were performed on dried polycrystalline
Fig. 5. (a) One [Mn 3 ] 4 rhombic face of the [Mn 3 ] 8 rhombohedron emphasizing the two types of p–p stacking (left), and the complete [Mn 3 ] 8 rhombohedron (right). (b) A section of the complete 3-D network formed from face-fused [Mn 3 ] 8 rhombohedra.
rings. These extensive interactions lead to the formation of a 3-D MOF-like network with rhombohedral repeating units comprising eight Mn 3 units at the vertices. It is interesting to note that the [Mn 3 O(O 2 CPh) 3 (mpko) 3 ] +^ cation of 3 was previously found not to
exhibit this inter-Mn 3 p–p stacking even though it differs from 4
only in the benzoate substituents, pointing to the importance of the two 3,5-dihydroxy substituents of dhbH to the solid-state structure. As a consequence of the 3-D network, there are many inter-Mn 3 AF interactions, which greatly decrease or destroy com- pletely the SMM properties of 4 versus previous members of this family even though the intra-Mn 3 coupling is still F. The present
work thus demonstrates that even weak inter-SMM p–p stacking
interactions can seriously damage SMM properties if there are enough of them, and thus great care is needed to minimize all sources of significant inter-SMM exchange interactions in attempt- ing to construct MOF or MOF-like 3-D networks of SMMs to take advantage in potential magnetic applications of the resulting voids that their structure forms.
Acknowledgments
This work was supported by the National Science Foundation (CHE-1410394). T.N.N. thanks the Vietnam Education Foundation for a fellowship.
Appendix A
CCDC 1417784 contains the supplementary crystallographic data for 4 4MeCN. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1 EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].
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