Unique Anionic Eight-Connected Net with 36418536 TopologyDerived from a Rare Co6(µ3-OH)2(µ-H2O)(CO2)12 Building Block

Feng Luo,† Ji-min Zheng,*,† and Gary J. Long*,‡

Department of Chemistry, Nankai UniVersity, Tianjin, 300071, China, and Department of Chemistry,Missouri UniVersity of Science and Technology, UniVersity of Missouri, Rolla, Missouri 65409-0010

ReceiVed October 16, 2008; ReVised Manuscript ReceiVed December 25, 2008

ABSTRACT: [(CH3)2NH2]2[Co6(µ3-OH)2(µ-H2O)(ip)6] ·2.5H2O (1), where ip2- is the 1,3-benzenedicarboxylate dianion, has been preparedby the solvothermal self-assembly of CoCl2 and H2ip in N,N′-dimethylformamide. Complex 1 contains a rare Co6(µ3-OH)2(µ-H2O)(CO2)12

building block with six crystallographically distinct cobalt(II) ions, one with a trigonal bipyramidal, two with a pseudotetrahedral, and threewith a pseudooctahedral coordination environment; the bridging ip2- dianions serve as spacers to yield a unique anionic eight-connectednet with 36418536 topology. The magnetic properties of 1 indicate that the six cobalt(II) ions are paramagnetic with, at most, very weakintramolecular antiferromagnetic exchange coupling.

Coordination complexes with metal-organic frameworks mayhave a large variety of interesting structural networks with differingtopologies; many of these complexes have applications as functionalmaterials.1 A large number of such complexes with low connectivitymineral topologies, that is, NbO, quartz, pyrite, rutile, sodalite,CdSO4, and halite have been reported.2 In contrast, the body-centered cubic and fluorite networks, that have higher connectivitiesof 8-12, are rather rare because the construction of such networksis severely hampered both by the number of available coordinationsites and by the sterically demanding nature of many ligands.

An eight-connected network in this context is one in which eachvertex or node is connected to eight adjacent vertices or nodes. Atan n-connected vertex there will be n(n - 1)/2 angles subtendingthe n adjacent vertices, or 28 such angles for an eight-connectednetwork.3 For the compound under study herein there are six suchvertices or nodes in the equatorial plane and two in the axialpositions, and the point symbol or Schlafli symbol is 36418536, oralternatively, 36.418.53.6, where AaBbCc..., with A < B < C..., arethe shortest cycles about a single angle, a, b, c,..., of the numberof shortest cycles bridged by the angle at the vertex and a + b +c +... ) n(n - 1)/2 ) 28.

There are two general methods for making complexes with eight-connected nodes and their corresponding eight-connected networks.The first, often effective, approach is to use highly coordinated rare-earth ions.4 The second approach is to use polynuclear buildingblocks with eight-connected nodes, an approach that has utilizedCu2,

5 Zn3,6 Co3,

7 Cd3,8 Cu3,

5 Cd4,9 Co4,

10 Cu4,11 Zn5,

12 Pb8,13

Cd11,14 and Cu4V4-based15 building blocks. From a topological point

of view, most of these building blocks have a body-centered cubicnet with the 42464 topology. However, because of recent advancesin crystal engineering, a variety of new eight-connected networkswith the 42068, 36414576, 334155862, and 354115864 topologies, as wellas the self-penetrating 424563 topology, have been reported.5-15

Herein, we report a new polynuclear compound, [(CH3)2NH2]2-[Co6(µ3-OH)2(µ-H2O)(ip)6] ·2.5H2O, 1, which contains a Co6(µ3-OH)2(µ-H2O)(CO2)12 building block that contains an eight-con-nected network with the 36418536 topology. To the best of ourknowledge, this building block topology is unique and unprecedented.

Polymer 1 has been prepared by solvothermal self-assembly at200 °C of CoCl2 ·6H2O and H2ip in N,N′-dimethylformamide (seeSupporting Information). The [(CH3)2NH2]+ cations were generatedduring the synthesis by the hydrolysis of N,N′-dimethylformamide.16

Once formed, crystals of 1 are air-stable and insoluble in common

solvents, such as water, CH3OH, C2H5OH, N,N′-dimethylforma-mide, and CH3CN. Additional studies indicate that the [(CH3)2-NH2]+ cations cannot be replaced by Na+ or K+ ions.

The single crystal X-ray diffraction results (see SupportingInformation) indicate that the asymmetric unit17 of 1 contains sixdistinct cobalt(II) ions (Figure 1). Co1 is pseudooctahedrallycoordinated by four equatorial ip2- oxygens, a µ3-OH oxygen, O2,and a terminal water oxygen, O1; the latter oxygens are trans toeach other. Similarly, both Co3 and Co4 are pseudooctahedrallycoordinated by five ip2- oxygens and one µ3-OH oxygen, O2. Incontrast, both Co5 and Co6 are pseudotetrahedrally coordinatedby three ip2- oxygens and one µ3-OH oxygen, O2; these two sitesalso have weak Co5-O26 and Co6-O13 bonds of 2.419(17) and2.437(12) Å, respectively. Finally, Co2 has a trigonal bipyramidalcoordination environment18 with τ ) 0.58 consisting of four ip2-

oxygens and a µ3-OH oxygen, O2. To the best of our knowledge,the simultaneous presence of three different cobalt(II) coordinationenvironments in 1 is rather rare.19 The Co-O bond lengths in 1are in the range of 1.934 to 2.299 Å, a range that is comparablewith that observed in other cobalt(II)-containing compounds.18

The six cobalt(II) ions in 1 are linked by ip2- carboxylate oxygensand µ3-OH oxygens, to yield Co6-based building blocks that containboth a Co3(µ3-OH)(µ-H2O)(CO2)6 and a Co3(µ3-OH)(CO2)6 moiety(Figure 1). In 1 Co1-Co2, Co1-Co3, Co2-Co4, Co3-Co5, andCo4-Co6 are bridged by two carboxylate ligands, Co2-Co3 andCo4-Co5 are bridged by a single carboxylate ligand, whereasCo3-Co4 is linked by two carboxylate oxygens of a single ip2-

ligand. The Co-Co distances and Co-O-Co angles are sum-marized in Table S2, Supporting Information.

The Co3(µ3-OH)(µ-H2O)(CO2)6 and Co3(µ3-OH)(CO2)6 constitu-ents of the Co6(µ3-OH)2(µ-H2O)(CO2)12 building blocks found in1 are obviously different from the earlier Cr3(µ3-OH)(CO2)6, Al3(µ3-OH)(CO2)6, and Co3(µ3-OH)(CO2)6 building blocks in which eachpair of metal ions is bridged by two carboxyl groups.19 In 1 the1,3-benzenedicarboxylate dianionic ligands have the µ3:η2:η2, µ4:η2:η2, µ4:η3:η2, and µ5:η3:η2 coordination environments.

As is illustrated in Figures 2 and 3, each Co6-based buildingblock is linked to eight adjacent Co6 blocks through 12 ip2- bridgingligands. In four directions the blocks are connected by two ip2-

spacers, shown as blue lines in Figure 2, whereas in the other fourdirections they are linked by a single ip2- spacer, shown as redlines. As a result, based on charge balance, an eight-connectedanionic net is formed; a schematic description of this network isshown in Figure 3.

A topologic analysis20 of 1 with the Topos40 program yields a36418536 topology for this anionic net. In the literature, we havenoticed two cases also featuring the same 36418536 topology,6,7

* To whom correspondence should be addressed. E-mail: jmzheng@nankai.edu.cn (J.-m.Z.), glong@mst.edu (G.J.L.).

† Nankai University.‡ Missouri University of Science and Technology.

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10.1021/cg801164f CCC: $40.75 2009 American Chemical SocietyPublished on Web 02/03/2009

where similar Co3 or Zn3 building blocks act as eight-connectednodes, and both dicarboxylate- and bipyridine-like ligands play theorganic connector role, thus leading to the neutral eight-connectednet. In contrast, complex 1 consists of an anionic net. Thereby, itis believed that polymer 1 presents a unique anionic eight-connected

36418536 net. Further, in addition to the volume occupied by the[(CH3)2NH2]2

+ cations, the solvent-accessible void volume, asdetermined with the Platon program, is 1044.6 Å3, a volume thatis 16.4% of the unit cell volume.21 The large void volume indicatesthe large, potentially useful, microporous framework of 1.

As discussed above, to date, there are reports of six differenteight-connected nets featuring the 42464, 424563, 42068, 36414576,334155862, or 354115864 topologies. Obviously 1, with the 36418536topology, the so-called hex net, is different from the previouslyreported nets, but the net in 1 appears to be close to the rare 36414576net, namely, a hex-like net,2a and both of them contain the featureof the intersection of 36 and 44 nets. A further detailed comparisonof these nets is discussed below. They both have the same six three-membered rings, apparent in all the horizontal “layer” planes; onesuch ring is shown in red in Figure 4. More specifically, 1 has 18four-membered rings, six in each of the layers shown in the left ofFigure 4 where one of these rings is shown in green. In contrast,the 36414576 net has only 14 such rings, six in the central plane butonly four each in the upper and lower planes; see the right portionof Figure 4. However, in addition to the six four-membered rings,each node in the 36418536 net is also shared by 12 four-memberedrings one of which is shown in blue between the planes, in the leftportion of Figure 4. In contrast, each node in the 36414576 net isshared by only eight four-membered rings, in conjunction with fourfive-membered rings, one of which is shown in purple in the rightportion of Figure 4. Further, it should be noted that in the 36414576net (see the right portion of Figure 4) one-half of the pillars adopta vertical orientation, whereas the remaining pillars adopt a slanted

Figure 1. Molecular, left, and polyhedral, right, views of the Co6(µ3-OH)2(µ-H2O)(CO2)12 building block of 1.

Figure 2. A view of the eight-connected Co6(µ3-OH)2(µ-H2O)(CO2)12 building blocks and the ip2- ligands, of 1, left, and diagrams indicating thelocation of the 12 ip2- spacer ligands, the red and blue lines, connecting one building block, in pink, to its eight neighbors, in brown, center, andthe eight connections, right.

Table 1. Comparison Between the Topologies of the 36418536 and 36414576 Nets

Schlaflisymbol

long or vertexsymbol coordination sequence coordination volume TD10

36418536 3.3.3.3.3.3.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4.4

8, 22, 34, 35, 21, 4 64 99 120 124 124 124 124 124 124

36414576 3.3.3.3.3.3.4.4.4.4.4.4.4.4.4.4.4.4.4.4.52.52.52.52

8, 30, 63, 85, 82, 75, 51, 27, 9, 1 101 186 268 343 394 421 430 431 431

Figure 3. Schematic diagram of the eight-connected 36418536 net foundin 1. The red and green circles indicate the [(CH3)2NH2]+ cations andthe anionic net, respectively.

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orientation relative to the two-dimensional layered sheets. Incontrast, in the 36418536 net found in 1, all of the linking pillarshave a vertical orientation relative to the two-dimensional layeredsheets. In other words, the difference between them appears tolargely rest in the stacking pattern of the layer components: AA in1 and AB in the 36414576 net.

The differences between the eight-connected 36418536 net of 1and the 36414576 net are also apparent in Table 1, obtained throughthe use of the TOPOS40 program.20 In this table the coordinationsequence3,22,23 is the sequence of numbers indicating the numberof vertices in sequential shells of 1 centered on the central point ofthe Co6(µ3-OH)2(µ-H2O)(CO2)12 building block, and TD10 is thesum of these values for the first 10 shells.

Thermogravimetric analysis of 1 (see Figure 5) indicates the lossof the free and coordinated water molecules between 25 and 150°C; the calculated loss is 4.1%, and the observed loss is 3.9%. Thereis no further weight loss below 230 °C. However, between 230and 370 °C 1 decomposes as the result of the loss of the[(CH3)2NH2]+ cations; the calculated loss is 9.7%, and the observedloss is 9.6%. The loss of these cations results in the chemicaldecomposition of 1. The residue may be Co2O3 and the calculatedresidue is 32.9% and the observed residue is 33.4%.

The magnetic properties of 1 have been measured, after zero-field cooling, from 2 to 300 K in a 0.05 T applied field. The inversemolar magnetic susceptibility of 1 exhibits approximate Curie-Weisslaw behavior (see the inset to Figure 6) with a Weiss temperatureof -25 K and a Curie constant of 2.54 emu K/mol Co. This Curieconstant corresponds to an effective magnetic moment, µeff, of 4.50µB, a value that is reasonable for paramagnetic cobalt(II) with S )3/2 and a substantial orbital contribution to its moment. Thus it

appears that the cobalt(II) ions in 1 are close to paramagnetic. Amore detailed analysis of the magnetic properties of 1 is difficultbecause of the presence of six crystallographically distinct cobalt(II)ions in three different coordination environments. However, as isshown in Figure 6, the average µeff for these six ions is ratherconsistent with S ) 3/2, J ) -1 to -1.5 cm-1, g ) 2.22 to 2.35,and D ) -100 cm-1, average values that are not unreasonable24

for the six cobalt(II) ions present in 1.

The magnetization of 1 has been measured at 2 K in an appliedfield of 0 to 5 T; see Figure 7. The mass magnetization increasesuniformly to reach 7.5 A m2/kg at 5 T, but the magnetization isclearly not saturated at this field. The slope of the field dependenceof the magnetization reaches a maximum at ca. 0.2 T and thendecreases; the corresponding molar magnetic susceptibility is shownin the inset to Figure 7. This decrease in susceptibility is consistent

Figure 4. A comparison between the 36418536 net found in 1, left, andthe 36414576 net, right.

Figure 5. Thermogravimetric analysis of 1.

Figure 6. The temperature dependence of the effective magneticmoment of 1. All the fits correspond to S ) 3/2 and D ) -100 cm-1.For the red line, J ) -1 cm-1 and g ) 2.35, for the black line, J )-1.5 cm-1 and g ) 2.3, and for the blue line, J ) -1 cm-1 and g )2.22. Inset: A Curie-Weiss law fit of the inverse molar magneticsusceptibility of 1.

Figure 7. The field dependence of the mass magnetization of 1 obtainedat 2 K. Inset: The low field dependence of the molar magneticsusceptibility of 1 obtained at 2 K.

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with the presence of the very weak antiferromagnetic couplingobserved in the temperature dependence of the magnetic suscep-tibility of 1.

The solvothermal preparation of 1 has led to a new buildingblock, Co6(µ3-OH)2(µ-H2O)(CO2)12, that contains six crystallo-graphically distinct cobalt(II) ions, in three different coordinationenvironments. The six cobalt(II) ions in the building block are, onaverage, paramagnetic with very weak intramolecular antiferro-magnetic exchange coupling. The long-range coupling together ofthese building blocks through the bridging ip2- dianions yields anew, to date unobserved, anionic eight-connected net with 36418536topology symbol. This topology is similar to and distinctly differentfrom the very rare eight-connected 36414576 net.

Acknowledgment. The authors thank Prof. F. Grandjean forhelpful discussions during the course of this work. This work wassupported by the National Natural Science Foundation of China(50572040).

Supporting Information Available: The crystallographic resultsfor 1, in the cif format, experiments, and the calculated and observedpowder X-ray diffraction pattern of 1. This material is available freeof charge via the Internet at http://pubs.acs.org.

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