Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Reversible Double Nucleophilic Substitution Reaction inside SingleCrystal MOF Tuned Remarkable Magnetic Behavior Hai-Yun Ren and Xian-Ming Zhang* School of Chemistry & Material Science, Shanxi Normal University, Linfen, Shanxi 041004, China S Supporting Information *
combination with sufficient void space for the migration of guest molecules and flexible coordination geometries and anisotropic magnetic behavior of Co ions is beneficial to observing advanced reactivity accompanied by modification of electronic spin topology. In this Communication, we present an octahedral cobalt−oxygen chain-based three-dimensional sqc3868 topological coordination network encapsulating DMF molecules formulated as [Co2(μ2−OH2 )2(tptc)(H2O) 2]· 1.5DMF (1; tptc = terphenyl-4,2″,5″,4′-tetracarboxyate), which undergoes a reversible double nucleophilic substitution inside single crystals that involved concerted and spatially ordered migration and coordination of DMF molecules and carboxylate oxygen atoms at the reactive sites and then transforms into the f rl topological network [Co2(μ2-OH2)(DMF)(tptc)]·0.5DMF (1@343K), accompanied by tuning magnetism from antiferromagnetism to ferromagnetism. Compound 1 was solvothermally obtained by treatment of a mixture of Co(NO3)2, H4tptc, DMF, and H2O at 80 °C. Singlecrystal X-ray analysis reveals that 1 features an octahedral cobalt−oxygen chain-based 3D coordination framework with encapsulated DMF molecules. It crystallizes in the triclinic space group P1̅, and all three distinct Co atoms display distorted octahedral geometries (Figure 1a). The Co(1) and Co(3) localize at inversion centers with a site occupancy of 0.5, coordinated by four carboxylate oxygen atoms in the equatorial plane and two bridging aqua ligands at the axial positions with Co−O distances of 2.022(3)−2.263(3) Å. The Co(2) atom is coordinated to two carboxylate oxygen atoms, two μ2-OH2
ABSTRACT: Chemical reactions inside single crystals are highly selective but quite challenging. We present herein an octahedral cobalt−oxygen chain-based 3D coordination network with sqc3868 topology, which underwent a reversible double nucleophilic substitution inside a single crystal involving encapsulated DMF molecules and was converted into a topologically highly related f rl network, accompanied by magnetic tuning from antiferromagnetism to ferromagnetism. Combined UV−vis, XPS, EPR, and XANES showed most of the Co centers keep a divalent state with less remarkable electronic structure change during the substitution reaction, indicating magnetic tunability mainly comes from a minor change of local geometry of cobalt atoms with large anisotropy.
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hemical reactions generally proceed in liquid and gas phase because reactants can easily move and collide, which will accelerate reactions. In contrast, chemical reactions inside single crystals are very uncommon but highly selective,1 since single crystallinity of reactants is commonly lost during the breaking and forming of bonds of chemical reactions. Only cooperative movement of atoms or molecules inside single crystals during whole chemical reactions could keep single crystal form. As such, studies on chemical reactivity inside single crystals are fundamentally important not only to direct observation of reactions at the solid/gas and solid/liquid interface in real space but also to develop various stimuliresponding smart materials.2 In this aspect, metal organic coordination networks provide a chance to study chemical reactivity inside single crystals because they show unusual and suitable flexibility including structural and functional response to various stimuli such as guest molecule, pressure, heat and light, etc.,3−5 which are very important for advanced applications in selective separation of specific molecules, switches, and sensor.6−8 To date, only limited reactions including hydration, metathesis, oxidation, addition, and substitution reactions inside single crystals have been observed, 9−11 and a dehydration-induced double nucleophilic substitution reaction involving a captured molecule accompanied by a remarkable change of physical properties has never been revealed. We have been continuing studies on cobalt−oxygen cluster and chain-based coordination frameworks and found guest molecules tuned a change of physical properties.12,13 Moreover, suitable flexibility of cobalt−oxygen cluster and chain SBUs and overall networks were also revealed in the field, which in © XXXX American Chemical Society
Figure 1. (a) Views of the coordination environments of Co(II) atoms and (b) rod-like {Co2(μ2-OH2)2} SBUs in 1 (pink, Co; red, O; gray, C; blue, N; green, H). Received: April 18, 2018
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DOI: 10.1021/acs.inorgchem.8b00945 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
angles are changed to 109.90(7)° (Figure S5a). It is noteworthy that 1@343K displays a binodal f rl “Férey ladder” topology, which carries the same short Schläf li symbol of {42.64}4{64.82} as that of sqc3868 but a slightly different extended point symbol of [6.6.62.62.82.82] [4.6.4.6.6.83]. It should be noted that SBUs are zigzag {Co2(μ2-OH2)(DMF)} ladders in 1@343K but trapezoidal {Co2(μ2-OH2)2} in 1 (Figure S6). Interestingly, the double nucleophilic substitution event is reversible. After exposure to air for 2 weeks, 1@343K can reuptake atmospheric water to give a rehydrated phase 1@recovered. N2 sorption isotherms of 1 and 1@343K under various activation conditions show negligible adsorption, indicating a lack of permanent pore (Figure S7). More interestingly, double nucleophilic substitution inside single crystals induced remarkable magnetic behavior from antiferromagnetic exchange of 1 to ferromagnetic exchange of 1@343K. Variable-temperature dc magnetic susceptibility measurements were performed (Figure 3), and the χmT value
molecules, and two terminal water molecules. The long and weak Co−O bonds involve bridging aqua ligands. Four Co(II) atoms in the Co(1), Co(2), Co(3), and Co(2) sequence are alternately bridged by μ2-OH2 molecules to give a zigzag {Co2(μ2-OH2)2} chain (Figure S1a). Within the chain, Co(1) was coordinated by four monodentate carboxylates, while Co(2) and Co(3) were bridged by bidentate carboxylates with Co···Co distances of 4.0569(6) and 3.5432(6) Å (Figure 1b). The Co−O−Co angles are 134.30(14)° and 109.20(18)°. The overall structure is a rod-like {Co2(μ2-OH2)2} SBU-based 3D sqc3868 topological nework with DMF molecules encapsulated. The short Schläf li symbol is {42.64}4{64.82}, and the extended point symbol is [6.6.62.62.82.82] [4.6.4.6.6.83]. In situ variable-temperature powder XRD of 1 upon heating showed the disappearance of diffraction peaks (2θ = 6.62 and 12.86) and appearance of additional peaks (2θ = 13.54), indicating the possibility of solid-state reactivity (Figure S2). When single crystals of 1 were in situ heated to 343 K for 3 h, dehydrated single crystals 1@343K were obtained accompanied by color change from pink to dark pink (Figure 2). X-ray
Figure 3. (a) Temperature dependence of susceptibility data for 1 (a) and 1@343K (b) (inset: the fit of the curve within 300−8.5 K). Solid line represents the fitting results according to model discussed in the text.
Figure 2. Contrastive view of nucleophilic substitution involving groups showing migration of encapsulated DMF and uncoordinated carboxylate inside single crystal 1 (left) to 1@343K (right).
per Co unit for 1 and 1@343K at 300 K is 2.86 and 3.05 cm3 mol−1K, respectively, higher than the spin-only value of 1.87 cm3 mol−1 K for S = 3/2 owing to a significant orbital contribution of high-spin Co(II) in an octahedral surrounding. For 1, the χmT value displays a continuous decrease upon cooling, characteristic of dominant antiferromagnetic exchange. Compared to 1, the χmT value of 1@343K slowly decreases upon cooling to a minimum value at 30 K and then takes a sharp upturn to reach a maximum at 3.25 K and finally decreases again down to 2 K, different from the antiferromagnetic interaction in analogue [Co2L(μ2-H2O)(μ2-DMA)]· DMF.14 The “noncritical scaling” theory was used to acquire an assessment of the strength of the exchange interaction.15 The fitted values for 1 (Figure 3a) are E2/k = 1.67 K and J = −3.34K (= −2.32 cm−1), according to the Ising chain approximation χT ∝ exp(J/2kT), which indicates weak antiferromagnetic interaction. For 1@343K, the fitted parameters give E2/k = −6.46 K. The negative activation energy (E2) demonstrates ferromagnetic interactions within a chain (Figure 3b). To calculate the magnetic exchange interaction between Co(II) ions for 1@343K, the spin−orbit coupling effect was subtracted from experimental data,16 and then an infinite chain model based on the spin Hamiltonian (H = −J∑iSiSi+1) was carried out to simulate the resulting data.17 The obtained values are g = 2.71 and J = 3.76 cm−1 (Figure S9), indicating a moderate ferromagnetic interaction within the {Co2(μ2-OH2)(DMF)} chain. No magnetic hysteresis loop was observed (Figure S10). The field dependence of magnetization at 2 K of 1@343K clearly displays a drastic rise in M vs H at the beginning and
diffraction reveals the occurrence of an unusual double nucleophilic substitution reaction inside single crystals. One is substitution of spatially correlated and incoming carboxylates for terminal water molecules, and the other comes from partial substitution of DMF molecules for bridging aqua molecules. Xray refinement indicated that around 50% of bridging aqua molecules were replaced by DMF, giving transformation of the {Co2(μ2-OH2)2} chain in 1 into a {Co2(μ2-OH2)(DMF)} chain in 1@343K, which well matches with the weight loss of 7.82% (Anal. Calcd 7.82%) in the range of 42−75 °C in the TGA curve (Figure S3). For the occurrence of a double nucleophilic substitution reaction, synergetic movement between the structurally relaxed Co−O chain, tptc, and DMF is necessary. As can be seen from the structure of 1, the distances between uncoordinated carboxylate oxygen atoms to Co(2) centers are ca. 4.015−4.122 Å, and thus a distance of ca. 2.0 Å must be moved for pending carboxylate to substitute for terminal water molecules. The Co2···ODMF and Co3···ODMF distances are ca. 6.8 Å, which indicate a longer migration distance (ca. 4.8 Å) is needed to replace bridging μ2-OH2 by DMF. The double substitution reaction transforms the space group from triclinic P1̅ to monoclinic C2/c. The entering carboxylates are bound to Co sites to result in uniform bidentate modes of carboxylates in 1@343K (Figure S4). Co atoms keep octahedral geometries, but the average Co−O length in 1@ 343K is contracted by 0.021 Å. The changes in Co−O−Co angles are limited to 0.58°. The Co···Co distances are shortened to 3.6083(5) Å, and the Co(2)−O(5)−Co(1) B
DOI: 10.1021/acs.inorgchem.8b00945 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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then reaches a value of 1.92 Nβ at the highest field, much higher than 1.14 Nβ of 1 and close to the expected saturation value of 2.17 Nβ (Figure S11). Dehydration upon heating may possibly induce an elusive redox reaction (i.e., partial valence state change of Co atom and conversion of water into hydroxide), which could not be directly revealed by single crystal structure. To reveal a possible change of electronic structure of Co atoms, combined UV−vis, EPR, XPS, and X-ray absorption near-edge spectroscopy (XANES) have been performed. Solid UV−vis spectra of 1 and 1@343K show one adsorption band at 527 nm ascribed to the 4T1g(F) → 4T1g(P) transition of octahedrally coordinated Co(II).18−20 Compared with 1, the 4T1g(F) → 4T1g(P) band in 1@343K is slightly broader and red-shifted by 24 nm (Figure S12). The EPR spectra of 1 and 1@343K are similar and related to octahedral Co(II) ions21−23 (Figure S13). The Co 2p XPS in 1 displays 2p3/2 peak at 781.44 eV and a 2p1/2 peak at 797.34 eV (Figure 4). It is generally known that the very
Communication
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00945. Crystal structural data for all compounds, experimental details, and additional figures and tables (PDF) Accession Codes
CCDC 1550892−1550893 and 1550895 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xian-Ming Zhang: 0000-0002-8809-3402 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from 973 Program (2012CB821701) and 10000 Talents Plan is greatly appreciated.
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Figure 4. Photoemission features of Co 2p for compound 1 (a) and 1@343K (b).
REFERENCES
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intense satellite structure is specific to Co(II) ions. The 2p3/2 and 2p1/2 peak distance of 15.9 eV and the 0.67 intensity ratio of satellite to main peak are very close to those of CoO(100),24 15.9 eV and 0.66. For 1@343K, the relative intensity ratio of satellites has been significantly reduced to 0.48. Meanwhile, the distance of 2p3/2−2p1/2 peaks is decreased to 15.5 eV, close to that in Pt/Co3O4,25 which implies possible dehydration oxidation and the presence of Co(III) ions in 1@343K. XANES at the Co K-edge (Figure S14) for both 1 and 1@343K shows similar weak pre-edge 1s → 3d transition at ∼7709.1 eV, consistent with Co(II) with a centrosymmetric local environment.26−28 The combined UV−vis, EPR, XPS, and XANES indicates that most of the Co centers remain in a divalent state in 1@343K, but the presence of very minor Co(III) could not be completely eliminated. In summary, an octahedral cobalt−oxygen chain-based coordination network with sqc3868 topology undergoes double nucleophilic substitution inside a single crystal and converts into a topologically highly related f rl network. This course induced remarkable magnetic change from antiferromagnetism to ferromagnetism. Various measurements indicated that during the substitution reaction, most of the Co centers remain Co(II), but the presence of minor Co(III) could not be completely eliminated. The work might provide a promising and efficient pathway to develop selective reaction and to tune physical properties inside single crystals. C
DOI: 10.1021/acs.inorgchem.8b00945 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.8b00945 Inorg. Chem. XXXX, XXX, XXX−XXX
