DOI: 10.1021/cg901027c
[Mn(acac)2(HOCH3)2]3[Ce(NO3)6];Icosahedral Anion Builds a Cubic Network
2010, Vol. 10 426–429
A. R. Biju and M. V. Rajasekharan* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India Received August 25, 2009; Revised Manuscript Received October 14, 2009
ABSTRACT: Oxidation of Mn(II) in aqueous solution by ammonium ceric nitrate in the presence of acetylacetone (acacH) followed by subsequent crystallization from methanol leads to [Mn(acac)2(HOCH3)2]3[Ce(NO3)6] (1). The icosahedrally coordinated Ce(NO3)63- ion with six octahedrally disposed hydrogen bond acceptor sites links the cations into a hydrogen bonded R-polonium-like network with a 2-fold parallel interpenetration.
Table 1. Crystallographic Data for 1
Introduction
*Corresponding author: Phone: þ91-40-23134857. Fax: þ91-40-23012460. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 11/12/2009
Mn3C36H66 μ (mm-1) 1.505 CeN6O36 formula weight 1463.89 F(000) 2235 T (K) 100(2) reflection collected 16956 2362 [0.0265] crystal system trigonal unique reflect., [Rint] goodness of fit on F2 1.118 space group R3 a 0.0238 a (A˚) 16.5842(3) R1 [I > 2σ(I)] 0.0580 c (A˚) 18.3249(8) wR2 4364.8(2) R indices (all data) V (A˚3) 0.0248 Z 3 R1 1.671 wR2 0.0585 Dcalc (g cm-3) P P P a 2 2 2 P R1 = Fo| - |Fc / |Fo|. wR2 = [ w(Fo - Fc ) / w(Fo2)2]1/2. )
formula
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The interest in crystalline solids with a designed structure and predictable properties continues to motivate research in coordination networks.1 Such networks consist of metal complex molecules (or ions) at the nodes and inorganic and organic molecules (or ions) which act as connectors. The connections may be made via coordinate bonds or other noncovalent interactions such as hydrogen bonding. Complexity may be built into the network through the use of connectors having dissimilar characteristics and through the process of interpenetration. Several recent reviews deal with the network analysis of interpenetrating2 and noninterpenetrating3 networks. The cubic network based on an octahedral (six-connected) node is among the simplest coordination network. Recent examples include rare earth based materials showing enhanced photoluminescence,4a,4b enantioselectivity in network formation,4c and ferromagnetic interactions.4d Use of unequal connectors to generate distorted cubic (cuboid) structures has been demonstrated recently.5 The present study describes a new distorted cubic network in which both the nodes and connectors are metal complex ions linked via O-H...O hydrogen bonding. The hexanitrato cerous ion, Ce(NO3)63-, often gets incorporated in crystals as a counterion when ammonium ceric nitrate is used as an oxidizing agent for the preparation of coordination compounds.6 Sometimes this ion is used to crystallize bulky cationic complexes.7 The Ce(III) complex ion has chelating nitrate ions, leading to (distorted) icosahedral coordination. There are six uncoordinated oxygen atoms arranged in a nearly octahedral mode on the periphery of the icosahedron. Hydrogen bonding with these sites has the potential to generate a cubic network if suitable donor sites are available on the complex cation. However, to the best of our knowledge, this has not yet been realized in practice in crystals containing Ce(NO3)63- or any other hexanitrato complex ion. Such a network, which may be categorized as a R-polonium-like arrangement based on Hbonds,1 is now seen in [Mn(acac)2(HOCH3)2]3[Ce(NO3)6] (1).
Experimental Section Materials and Procedures. All chemicals were purchased from Ranbaxy chemicals and used without further purification. IR spectra were measured using a Shimadzu FT-IR 8000 spectrometer. Elemental analysis was performed using a FLASH EA 1112 SERIES CHNS analyzer. Magnetic susceptibility for a polycrystalline sample was measured at room temperature using a Sherwood Scientific magnetic susceptibility balance. Synthesis of [Mn(acac)2(HOCH3)2]3[Ce(NO3)6]. Manganese(II) acetate (1.00 g, 4.08 mmol) and acetylacetone (0.800 g, 8.29 mmol) were dissolved in 2.5 mL of water. To this solution, solid ceric ammonium nitrate (3.30 g 6.02 mmol) was added slowly while stirring. The green precipitate formed was filtered off and dried (1.30 g). The entire precipitate was dissolved in methanol, and the volume of the solution was reduced considerably by heating on a water bath and maintained for crystallization at room temperature. Green colored block shaped crystals formed within 24 h. Yield 1.26 g (0.861 mmol, 64.6%). Anal. cald for Mn3C36H66N6O36 (MW 1463.9): C, 29.54; H, 4.54; N, 5.74. Found: C, 29.60; H, 4.50; N, 5.80. Important IR absorptions (KBr disk, cm-1): 3410, 2471, 2357, 1757, 1628, 1521, 1385, 1032, 939, 823, 688, and 490. The crystals are stable in air and melt at 137-138° followed by decomposition. X-ray Crystallography. X-ray data were collected for a dark green crystal of 1, having dimensions 0.24 0.20 0.18 mm3, on a Bruker SMART APEX CCD X-ray diffractometer using graphite monochromated Mo KR radiation. The data were reduced using SAINTPLUS,8 and multiscan absorption correction using SADABS9 was applied. The structure was solved using SHELXS-97 and refined using SHELXL-97.10 Drawings were made using Mercury.11 Crystallographic data are summarized in Table 1. Important bond distances and angles are in Table S1. Theoretical Calculations. DFT calculations were done for the isolated Mn(III) complex cation by using the B3LYP exchange correlation functional 12 as implemented in Gaussian-03. 13 The r 2009 American Chemical Society
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Figure 1. Thermal ellipsoid plot of the coordination environment of the complex molecules 1: Atoms are represented as 50% probability ellipsoids. Hydrogen atoms have been omitted from the drawing. Symmetry code: (i) -x, -y, -z; (ii) x - y, -y, z; (iii) -x þ y, -x; z; (iv) -x þ 1 /3, -y þ 2/3, -z þ 2/3; (v) y þ 1/3, -x þ y þ 2/3, -z þ 2/3; (vi) x - y þ 1/3, x þ 2/3, -z þ 2/3.
Figure 2. View of hydrogen bonded distorted R-polonium networks of 1. spin-unrestricted version was employed for open shell ions. For both types of calculations, triple-ζ basis sets 14 were used for Mn atom, and double-ζ basis sets 15 were used for all other atoms.
Results and Discussion A molecular diagram of compound 1 is shown in Figure 1. The asymmetric unit contains 1/2 [Mn(acac)2(HOCH3)2]þ cation and 1/6 [Ce(NO3)6]3-. Ce(III) ion, which is located on a 3 site, is coordinated to six chelating nitrate anions. The nitrate ion shows a slight asymmetry in chelation: Ce-O(5), 2.601(1) A˚; Ce-O(4), 2.665(1) A˚; O-Ce-O, 48.43(4)°. The manganese atom situated on a crystallographic inversion center is coordinated to two bidentate acetylacetone ligands in the equatorial plane with two methanol molecules occupying axial positions. The average equatorial Mn-O distance is 1.909(1) A˚, and the axial Mn-O distance is 2.220(1) A˚.
Each Ce(NO3)63- is hydrogen bonded to six methanol molecules belonging to different [Mn(acac)2(HOCH3)2]þ ions (O3-H31...O6, 2.22(3) A˚, 164(3)°) forming a three-dimensional R-polonium 46 network with rectangular voids (Figure 2). The Ce centered icosahedra, due to the presence of six free nitrate oxygen atoms, act as six-connected nodes of the cubic network. The voids, however, are blocked by 2-fold parallel interpenetration of identical networks (Figure 3). Additional drawings are in the Supporting Information (Figures S1 and S2). The acetylacetonate groups in complex 1 show deviation from the Mn-O4 equatorial coordination plane, which is inclined by 14.2° to the plane containing the sp2 carbon atoms of the acetylacetonate groups. Such a deviation (15.6°) was also observed in [Mn(acac)2(H2O)2]ClO4 3 2H2O.16 However, the deviation was much less (1.6°) in the Mn(acac)2(N3), which is a one-dimensional coordination polymer.17 In order to check the stable form in the “gas phase”, geometry optimization of the cationic part of 1 was done without any constraints. The optimized structure of the isolated cation is superimposed on a crystallographic structure in Figure 4. The bond distances in the optimized geometry are within 0.06 A˚ of the crystallographic values. In the optimized geometry, the acetyl acetonato groups are exactly coplanar with the Mn-O(4) equatorial plane, suggesting that the deviations from planarity, if any, arise upon lattice formation. Though both the cation (spin quantum number, S = 2) and anion (S = 1/2) in compound 1 are paramagnetic, it is expected to be essentially magnetically dilute due to the weakness of the coupling between MnIII(3d4) and CeIII(4f1) via hydrogen bonding. Assuming a spin-only value of 4.9 μB for the cation, and 2.40 μB for the anion,18 which has unquenched orbital angular momentum, the expected paramagnetic susceptibility at 300 K is 0.0324 emu mol,-1 which is comparable to the experimentally measured value of 0.0355 emu mol,-1.
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Figure 3. Representation of 2-fold interpenetration of distorted R-polonium nets in compound 1. The spheres represent Ce coordination icosahedra, and the rods are [Mn(acac)2(HOCH3)2]þ ions that are connected by hydrogen bonds. An icosahedron with its six hydrogen bond acceptor sites (red spheres) is also shown.
Figure 4. Optimized structure of the cation (coplanar chelation in the equatorial plane, blue) superimposed on the crystallographic structure (chair distortion of the equatorial plane, red).
Conclusion In summary, the icosahedral ion Ce(NO3)63- is found to generate a cubic network when crystallized with the complex cation [Mn(acac)2(HOCH3)2]þ having two hydrogen bond donor sites located on one of its coordination axes. It is not yet clear whether other networks could be made with the icosahedral anion, which, in effect, is an octahedral secondary building unit (SBU) for hydrogen bonded networks. Crystallization of the aquo analogue of 1 is hampered by the instability of Mn(III) in aqueous medium. A search of the Cambridge Crystallographic Database shows 70 crystal structures
incorporating M(NO3)6n- anions, where M is a 4f, 5f, 6s, or 6p metal and n varies from 2 to 4, with organic as well as metalorganic cations. However, in none of these crystals is a hydrogen bonded cubic network found. The preparation of the present compound points to the possibility of generating such networks by choosing cations in which hydrogen bond donor sites are properly disposed. The actual network formed will be controlled by the stoichiometry, which is dependent on the charge (n) and the charge on the cation. It might even be possible to prepare cocrystals with neutral hydrogen bonding donors, using alkali metal cations to neutralize charge. These aspects of this new SBU and possible ways of controlling
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interpenetration19 are to be explored. Though in the present case interpenetration has destroyed the porosity, this need not be the case always.20 Acknowledgment. This work was supported by CSIR, India. A.R.B. thanks UGC for a research fellowship. Infrastructure support from UGC (UPE program) and DST (X-ray diffractometer facility) is also acknowledged. DFT calculations were performed using the CMSD facility, University of Hyderabad. Supporting Information Available: X-ray crystallographic file in CIF format, tables of bond lengths and angles (Table S1), and two figures (Figures S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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