Communication pubs.acs.org/JACS
Core−Shell {Mn7⊂(Mn,Cd)12} Assembled from Core {Mn7} Disc Ling-Yu Guo,†,⊥ Hai-Feng Su,‡,⊥ Mohamedally Kurmoo,§ Chen-Ho Tung,† Di Sun,*,†,‡ and Lan-Sun Zheng‡ †
Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China ‡ State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China § Institut de Chimie de Strasbourg, Université de Strasbourg, CNRS-UMR 7177, 4 rue Blaise Pascal, 67008 Strasbourg Cedex, France S Supporting Information *
the outer shell is progressively decorated through Mn10, Mn12 and Mn14 before reaching the final fully occupied outer ring compound Mn19. This encouraging result suggests that it may be possible to decorate the small Brucite disc Mn7 with a peripheral ring of different metals. Therefore, it prompted us to search for methods of making the first two-dimensional core− shell compounds, Mn7⊂M′12 (Scheme 1).
ABSTRACT: Postsynthetic decoration of the Mn7, {MnIII⊂MnII6}, core with CdII in the outer shell to form the next generation Mn13Cd6, {MnIII⊂MnIII3MnII3⊂ MnII6CdII6}, core−shell disc was achieved and confirmed by single-crystal X-ray diffraction. The formation of Mn13Cd6 has only been successful with CdII and if the Cd salt is added within the first half hour window when the inner Mn7 has formed. EDX and ICP-AES gave the accurate content and confirm the average found by X-ray diffraction. HR-ESI-MS was even more precise by revealing three prominent molecular species, Mn13Cd6, Mn14Cd5 and Mn15Cd4, having a distribution of metals. The presence of nonmagnetic metal on the periphery reduces the exchange between these clusters as well as the low magnetic moment decreases the dipolar interaction resulting in a paramagnet compared to the ferrimagnetism found for the parent Mn19, {MnIII⊂MnIII3MnII3⊂MnII12}, disc. This study opens the way for the syntheses of heterometallic core−shell clusters in a controllable fashion.
Scheme 1. Proposed Progressive Growing of a TwoDimensional Brucite-Disc from the Monomer {Mn} via the Core {Mn⊂Mn6} to the Core−Shell {Mn⊂Mn6⊂Cd12} Cluster
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ntermetallic materials are often thought to have better performance than those made up of single metals.1 For bulk metals, the melting point can be lowered, the electrical properties enhanced and the magnetic properties can show major differences when doping them with other metals.2 On the nanoscale, major change in physical properties have been reported for both solid solutions and for core−shell particles.3 For molecular materials, such as polynuclear coordination clusters,4 catalytic properties can be tuned to optimize the yield and the speed of the reactions,5 optical properties can be altered,6 and magnetic moment can oscillate because of intermetallic electron transfer.7 With so many advantages that exist for intermetallic compounds, there are major interests in designing materials with specific distribution and location of the metal ions. In particular, designing core−shell molecular systems is quite rare as are two-dimensional materials.8 In a recent study of the Brucite-disc compound [MnII15MnIII4(L)18(OH)12(N3)6](ClO4)6·12CH3CN (Mn19) where HL = 1-(hydroxymethyl)-3,5-dimethylpyrazole), we employed high-resolution electrospray ionization mass spectrometry (HRESI-MS) to understand its formation in solution.9 This study reveals that following the formation of the Mn7 core, © 2017 American Chemical Society
Using the knowledge of the previous study that the Mn7 core is formed within the first 30 min, we adopt a synthetic protocol of adding the second metal salt to the reaction after that time window. This procedure worked well. First, we isolated the core as a crystalline solid, [MnII6MnIII(L)12](ClO4)3·1.5H2O (Mn7), as confirmation in the absence of the second metal ions. Second, we followed through with the addition of the second metal ions and isolated a novel core−shell compound, [Cd II 6 Mn II 9 Mn III 4 (L) 18 (OH) 12 (N 3 ) 6 ](ClO 4 ) 6 ·12CH 3 CN (Mn13Cd6). It turns out that the outer shell ring contains a random mixture averaging six Cd and six Mn. Here, we detail the syntheses, structures, mass spectrometry and magnetic properties of these compounds. Our protocol for the syntheses of Mn7 and Mn13Cd6 involved a slight tweaking of that used for Mn19. Ultrasound treatment of a mixture of Mn(ClO4)2·6H2O, NaN3, Et3N and HL in CH3CN gave a turbid solution, which was placed in a fridge to produce crystals of Mn7. Otherwise, if a CH3CN Received: August 15, 2017 Published: September 19, 2017 14033
DOI: 10.1021/jacs.7b08679 J. Am. Chem. Soc. 2017, 139, 14033−14036
Communication
Journal of the American Chemical Society solution of Cd(ClO4)2 was added in the first 30 min during the ultrasound treatment the core−shell compound Mn13Cd6 is obtained following slow evaporation of CH3CN at ambient temperature. The timing for the addition of Cd(ClO4)2 is very crucial for the core−shell formation; if Cd(ClO4)2 is added later than half an hour the Mn19 disc (Mn19) will always crystallize together without the mixed-metal compound. Elemental analyses using ICP-AES on these crystals found a Cd:Mn atomic ratio of 6.0:12.9, compared to an expected ratio of 6:13 found by crystallography (see later). It has been a curiosity to isolate the core Mn7 intermediate and determine its structure as it is quite an elusive species in solution. Although it is ubiquitous during the assembly of Mn19 cluster and seen as a very important intermediate in the planar epitaxial growth mechanism for Mn19 cluster, we have not had access to its structure until now. In this work, we fortunately isolated Mn7 cluster as compound Mn7 by freezing the filtrate after reaction. Mn7 crystallizes in the trigonal R3̅c space group. X-ray diffraction analysis gave its formula as [MnII6MnIII(L)12](ClO4)3·1.5H2O. Its asymmetric unit contains one and onesixth Mn atoms, two L− and two ClO4− anions (1/3 and 1/6 occupancies). After symmetry generating the molecule, Mn7 comprises 7 Mn atoms and 12 L− ligands, but no N3− or OH− are present. The seven Mn atoms are almost located in one plane and the central Mn atom is +3 whereas the other six around it are +2. It is a new valence configuration compared to those known.10 Twelve L− ligands alternately arranged above and below the plane defined by the Mn7 core (Figure 1). It is
Figure 2. (a) Photograph of crystals of Mn13Cd6 taken under the optical microscope. (b) X-ray crystal structure of cationic [CdII6MnII9MnIII4(L)18(OH)12(N3)6]6+. Thermal contours are drawn at the 50% probability level. Color codes: purple, Mn; green, Mn/Cd; red, O; blue, N; gray, C. Hydrogen atoms are omitted for clarity. (c) EDX elemental mapping (Mn and Cd) results on an SEM image of single particle of Mn13Cd6.
connectivity of the metal centers, organic ligand and coordinated anions are the same as that for {MnII15MnIII4} Brucite disc (Mn19), thus they are isostructural (Figure 2b). The inner Mn7 unit is preserved as that in {MnII15MnIII4} disc including the oxidation state of Mn atoms, whereas the 12 metal sites around the Mn7 unit in the outer circle look abnormal no matter whether they are assigned to Cd or Mn in the refinement, indicating the Cd/Mn site occupancy disorder (Figure S1). The details about the disorder refinement are shown in SI. The EDX element mapping also shows the successful incorporation of Cd into the cluster as well as the satisfactory Cd:Mn atomic ratio of 6.0:12.7 (Calcd 6:13; Figure 2c). ESI-MS of a dissolved crystal of Mn13Cd6 in CH3CN found a sequence of 17 envelopes in the m/z range of 1875−2125 (Figure 3). Although each envelope typically contains more than one species and their peaks are severely overlapped within
Figure 1. Two orthogonal views of the ORTEP plot of Mn7 with thermal ellipsoids shown at the 50% probability level. The H atoms and ClO4− were omitted for clarity.
therefore a reduced-size version of the calix like Mn19 of Mn19. The isolation and structural identification of Mn7 not only provides a solid proof to the planar epitaxial growth mechanism of Mn19 cluster but also suggests the addition of competitive CdII influences the growth of Mn7 to Mn19 to some extent, leaving enough time to stabilize Mn7 as final crystalline product. One important difference compared to Mn19 is the valence states of the metal centers. While in Mn19, four of the core Mn atoms are trivalent and assumed to be the central and those in a triangle by symmetry and the rest are divalent, for Mn7 only the central Mn is trivalent and the ring Mn are all divalent. With several valence configurations known for the disc compounds, it appears that the facile electron transfer may be difficult to control. Single-crystal X-ray structure determination revealed that the main backbone of the core−shell Mn13Cd6 can be formulated as [CdII6MnII9MnIII4(L)18(OH)12(N3)6]6+ with perchlorate anions, which crystallizes in a trigonal space group of R3̅ as yellow polyhedral crystals (Figure 2a). Both Cd and Mn atoms are in octahedral coordination environments. The overall
Figure 3. HR-ESI-MS of Mn13Cd6 dissolved in acetonitrile. Charge states are indicated as 2+. Inset: Enlarged m/z range of 1866−2180 with different heterometallic backbones colored individually. 14034
DOI: 10.1021/jacs.7b08679 J. Am. Chem. Soc. 2017, 139, 14033−14036
Communication
Journal of the American Chemical Society
expected assuming g = 2 for all centers. The large negative Weiss constant suggests dominant antiferromagnetic interactions. The measured susceptibility deviates from the Curie− Weiss fit at low temperature and its excess suggests weak ferromagnetic exchange which leads to short-range ferrimagnetism. The isothermal magnetization at 2 K has a very slight curvature and reaches only 16 NμB in 50 kOe field. This behavior is anticipated for such a large antiferromagnetic coupling. One possible ground state will then be the six outer MnII interact with each other by weak ferromagnetic coupling and the MnII−MnIII is strongly antiferromagnetic. Such a model with antiferromagnetic coupling of MnII−MnIII and ferromagnetic MnII−MnII should result in a ground state total spin ST of 13 (Figure S3). However, the presence of one anisotropic center and six isotropic ones does not lead to single-molecule magnetism behavior above 2 K. Similar analysis of the data for Mn13Cd6 over the whole temperature region gives a Curie constant of 55.47(6) cm3 K/ mol and Weiss constant θ = −5.5(2) K. The Curie constant is within that expected for 4 MnIII (S = 2) and 9 MnII (S = 5/2) assuming g-value of the former to be greater than 2 due to anisotropy and 2 for latter. The average mean-field exchange is rather weak. The isothermal magnetization exhibits a smooth increase with field at all temperature with maximum value of 34 NμB at 2 K and 50 kOe. This value is lower than the expected 61 NμB for parallel alignment of all the moments, suggesting again the presence of antiferromagnetic coupling between near neighbors. Using similar arguments, antiferromagnetic coupled MnII−MnIII and MnII−MnII and ferromagnetic MnIII−MnIII, the ground state ST is a mere 1/2 (Figure S3). In contrast to Mn19, there is no long-range magnetic ordering (LRO) in the present compounds. One reason for Mn7 is the molecules are not exactly face-to-face, which may favor a moderate dipolar interaction. For Mn19, two reasons for such difference are possible. One is the presence of nonmagnetic CdII in the outer ring of the Brucite disc diminishes the exchange interaction between neighboring clusters. The other is that the effective dipolar interaction needed for LRO is weakened due to the low moment per cluster brought about by the integration of nonmagnetic CdII. The mixed-valent Mn19 and Mn13Cd6 clusters bear some structural similarities to reported singlemolecule-magnets of FeIII19 clusters.11 In conclusion, a synthetic protocol has been tested based on previously acquired information and found to be successful in the design of core−shell structure for two-dimensional Brucite disc. By choosing metal of appropriate size and especially the timing in the initial formation sequencing of the cluster as determined in the tracking experiments of ESI-MS, it was possible to synthesize disc with inner Mn core and mixed Cd and Mn in the outer shell. Given that the nonmagnetic Cd atoms are located on the outer ring, the exchange interaction between clusters is considerably reduced as well as the dipolar interaction because of the lower moment per cluster due to the low spin ground state of 1/2, resulting in a paramagnet but for the independent core cluster the low dipolar is due to the lack of face-to-face stacking.
one envelope, we were able to identify a total of 27 species in these 17 envelopes by matching the experimental and simulated isotope distributions (Figure S2), and their formulas are given in Table S3. According to the assigned formulas, we can clearly see the occurrence of three kinds of 19-metallic disc in the solution: Cd6Mn13, Cd5Mn14 and Cd4Mn15. The overlapped species in each envelope are mainly caused by the ligand exchange among L−, N3− and OH−. Although there are complicated exchanges between Cd and Mn, and anionic ligands, the 19-metal backbone is rather stable as the observation made for Mn19 disc in Mn19. The successful assembly of Mn13Cd6 is largely benefited from the acquired knowledge during the study of Mn19 cluster (Mn19). The real-time tracking HR-ESI-MS of the reaction solution identified the intermediate Mn7 is formed in rather short time (∼1 min), whereas larger species such as Mn10, Mn12 and Mn14 appeared after 20 min, and formation of Mn19 needed longer, about 120 min. Hence we speculated that the formation of Mn7 is a dynamic process, whereas the growth of it to larger discs should be a thermodynamic process. These observations motivated us to add the secondary metal ions following the growth period of Mn7 and the above results are consistent with our expectations. We also tried to increase the proportion of Cd(ClO4)2 with respect to Mn(ClO4)2 in order to obtain Cd 12 Mn 7 or Cd 19 discs, but these experiments were unsuccessful. This result relates two information: (a) the peripheral shell is likely controlled by the fitting of the ionic radii and coordination geometry preference of the two metals; thus an optimum 6Cd and 6Mn is favored even in different concentration of ions and (b) the rapidly formed Mn7 disc has to be stable before the shell including CdII can be formed. The magnetic susceptibilities of the two compounds have continuous temperature dependence apart from a very small anomaly due to the effect of minor surface oxidation for Mn7 (Figure 4). The ZFC-FC magnetization data and acsusceptibilities of both are also without any anomaly. Curie− Weiss fit of the data between 100 and 300 K for Mn13Cd6 gives C = 26.7 ± 2 cm3 K/mol and Weiss constant θ = −37(2) K. The Curie constant is slightly less than the 29.25 cm3 K/mol
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ASSOCIATED CONTENT
S Supporting Information *
Figure 4. Magnetic properties: Temperature dependence of χ and χT [Curie−Weiss fit are shown as black lines] for Mn7 (a) and Mn13Cd6 (c) and isothermal magnetization for Mn7 (b) and Mn13Cd6 (d).
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08679. 14035
DOI: 10.1021/jacs.7b08679 J. Am. Chem. Soc. 2017, 139, 14033−14036
Communication
Journal of the American Chemical Society
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Experimental details, detailed crystallographic structure and data and more mass spectra (PDF) Data in cif format (CIF)
AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Mohamedally Kurmoo: 0000-0002-5205-8410 Chen-Ho Tung: 0000-0001-9999-9755 Di Sun: 0000-0001-5966-1207 Author Contributions ⊥
L. Y. Guo and H. F. Su contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NSFC (Grant Nos. 21571115, 21701133, 21227001), the Natural Science Foundation of Shandong Province (No. ZR2014BM027), Young Scholars Program of Shandong University (2015WLJH24) and the Fundamental Research Funds of Shandong University (104.205.2.5 and 2015JC045). M.K. is funded by the CNRSFrance.
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DOI: 10.1021/jacs.7b08679 J. Am. Chem. Soc. 2017, 139, 14033−14036
