Coordination Driven Self-Assembly in Co (II) Coordination Polymers

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Coordination Driven Self-Assembly in Co(II) Coordination Polymers Displaying Unprecedented Topology, Water Cluster, Chirality, and Spin Canted Magnetic Behavior Sumi Ganguly, and Raju Mondal Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg5018592 • Publication Date (Web): 13 Mar 2015 Downloaded from http://pubs.acs.org on March 15, 2015

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Crystal Growth & Design

Coordination Driven Self-Assembly in Co(II) Coordination Polymers Displaying Unprecedented Topology, Water Cluster, Chirality, and Spin Canted Magnetic Behavior Sumi Ganguly† and Raju Mondal‡* †

Department of Organic Chemistry, Indian Association for the Cultivation of Science, Raja S. C. Mullick Road, Kolkata-700032, India. ‡

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Raja S. C. Mullick Road, Kolkata-700032, India. Supporting Information Placeholder Keywords: Coordination Polymers ∙Topology∙ Water Cluster∙ Chirality∙Circular Dichroism∙ Magnetic Property ABSTRACT:The present work reports facile synthesis, full characterization and architectural diversity of seven Co(II) coordination polymers namely [{Co(H2MBP)2(12-CDA)}.CH3OH]∞ (1), [{Co(H2MBP)(12-CDA)(H2O)}]∞ (2), [{Co0.5(H2MBP)(14CDA)0.5}.(H2O)2.5]∞(3), [{Co(H2MBP)(14-CDA)(H2O)2}]∞ (4), [{Co(H2MBP)(13-ADA)0.5}]∞ (5), [{Co(H2MBP)2(1352 CTA)(H2O)}(H2O)3.5]∞(6) and [{Co2(μ O)(H2MBP)3(1245-CHA)}(H2O)12]∞ (7) derived from 4,4΄-methylene bispyrazole (H2MBP)ligand and various flexible dicarboxylatecoligands. The type of alicyclic flexible carboxylate coligand used in the synthesis and their coordination to the metal center plays a key role in controlling the dimensionality and topology of the frameworks. Topological analysis of the simplified nets reveals that 1 and 3 possesses 4-connected sql topology, 2 features 3-connected uninodalhcb topology, 5 contains 3D cds topology and7possessesan unprecedented new3D topology. Interestingly, well resolved 1D water chain is found within 3 and presence of heptanuclear water cluster assisted via free carboxylate group is observed in6.Chirality associated with 5 has been supported by solid state CD spectra and variable temperature magnetic studies indicate that7exhibits canted magnetic coupling between the carboxylate bridged Co(II) ions.

INTRODUCTION In the past few decades, rational design and fabrication of Coordination Polymers (CPs) has flourished as an emerging area of research because of their promising application in guest molecule inclusion, gas and vapour storage, chemical 1-7 sensing, magnetism, heterogeneous catalysis etc. One of the fundamental aspects of tuning the properties of CPs is to control their structural architectures which is directly influenced by the judicious choice and self-assembly process involving metal-ions and organic ligands. Presently, coordination driven self-assemblies of metal−ligand ensembles have occupied an important place in the domain of coordination as well as supramolecular chemistry due to their easy and 8-14 spontaneous synthesis and novel applications. Several synthetic parameters such as temperature, pH of the medium, choice of metal precursors, ligand to metal ratios are found to have profound effect in controlling this self15-18 assembly process. Moreover, literature reports have demonstrated that these self-assembled CPs often exhibit 19-25 fascinating structure dependent physical properties. One such very important property is chirality which is intrinsically related to the topology of the framework obtained from the self-assembly process. In principle, chirality can be spon-

taneously induced into the network of CPs even if it is built from achiral building blocks and such chiral CPs are important in realizing various applications such as in heteroge26-32 neous catalysis, sensing, enantiomorph separation etc. The self-assembly process also leads to the occurrence of different interesting structural topology which is another vital aspect of CPs since it significantly regulates their properties and applications. Generation of these architectural topologies often require a system devoid of small organic or inorganic counter ions, since counter ions are often coordinated to the metal ion resulting in a rather simpler framework. In this regard, a very promising approach is the use of mixed ligands such as N donor heterocyclic ligands together with polycarboxylate organic co-linkers in the same system. N donor heterocyclic ligands particularly pyrazole based flexible or rigid linker are acclaimed for their high thermal sta33-38 bility and interesting structural topologies. Aromaticpolycarboxylates are used extensively as organic co-linkers since they often produce inherently rigid structures which have 39-41 potential application in gas adsorption and storage. But, flexible cyclohexane carboxylate linkers are less explored as co-linkers in designing CPs although they are capable of inducing interesting structural features in the framework. The cyclohexane ring of such polycarboxylic acids can adopt various conformations including

ACS Paragon Plus Environment


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Scheme 1. Schematic representation of synthesis of complexes 1-7.

boat, chair, twist boat etc. depending on the coordination requirement of the metal ion, thus can lead to unexpected and topologically unusual polymeric architectures. Literature study reveals that various conformationalskeletons of cyclohexane poly carboxylic acids (CPAs) with lack of rigidity in the structure lead to considerable complexity in the resulting 42-48 frameworks. On the other hand, 1H-pyrazole moiety has theability to form inter-molecular H-bondsas well as N-H···π interactions resulting in higher dimensional supramolecular networks and imparting stability to the frameworks. Presence of flexible carboxylate coligandscan also facilitate such supramolecularinteractions by creating suitable hydrophilic environment for encapsulation of guest solvents such as H2O.Notably,inclusion of H2Omolecules inside porous CPs is unique because H2Oplays an important role in various biological and chemical processesby forming different types of aggregates like discrete H2Ocluster, 1D water chain , 2D wa49-55 ter layer and 3D water structures. Among various water clusters, the presence of 1D helical water channel inside porous CPs is of utmost importance since it resembles the helical water channel associated with the biological water transport phenomenon mediated by the membrane bound 56-60 protein Aquaporin. Inspired by these fundamentally important features of the self-assembled CPs obtained by mixed ligand strategy we extend our research endeavours to synthesize pyrazole based functional CPs using H2MBP (H2MBP = 4,4΄methylenebispyrazole) and various cyclohexane polycarboxylic acids(CPA) as flexible organic linkers. So far, wide range of CPs have been reported using H2MBP and various aro61-64 matic polycarboxylate ligands, however,cyclohexane polycarboxylatesare yet to be exploited in such mixed ligand system. Keeping this in mind, in this contribution we report the syntheses and characterization of seven Co (II) CPs using H2MBP and five cyclohexane polycarboxylates as co-ligands. The compounds are formulated as [{Co(H2MBP)2(12CDA)}.CH3OH]∞ (1), [{Co(H2MBP)(12-CDA)(H2O)}]∞ (2), [{Co0.5(H2MBP)(14-CDA)0.5}.(H2O)2.5]∞(3), [{Co(H2MBP)(14-

CDA)(H2O)2}]∞ (4), [{Co(H2MBP)(13-ADA)0.5}]∞ (5), [{Co(H2MBP)2(135-CTA)(H2O)}(H2O)3.5]∞(6) and 2 [{Co2(μ O)(H2MBP)3(1245-CHA)}(H2O)12]∞ (7). The structural insights of the resulting CPs along with the topological features have been thoroughly investigated by analysing their single crystal structures. CP 7 displays an unprecedented 3D topology. In addition, we also observe 1D helical water chains in the crystal structure of3. Interestingly, CP 5crystallizesin chiral space groupalthough none of the ligands are chiral. The presence of chirality in 5isalso supported by CD spectra. Furthermore, magnetic properties of 7 have been investigated in detail which revealdominant antiferromagnetic interactions at high temperature region and spin canting behaviour at the lower temperature regime, arising mainly due to the canted orientation of the spins on the adjacent Co(II) centers.

Results and Discussion: The ligand H2MBP is synthesized according to a report65 edprocedure and all the coordination polymers are synthesized via solvothermalmethods taking the reactants in 1:1:1 molar ratio and are found to be air stable. The phase purity of the compounds isconfirmed by PXRD analyses(figure S1-S7 in SI) and thermal stability is measured by thermogravimetric analysis (TGA) (figure S8-S14 in SI). Various flexible alicycliccyclohexane polycarboxylic acids (CPAs) namely 1,2cyclohexane dicarboxylic acid (12-CDA), 1,3-adamentane dicarboxylic acid (13-ADA) , 1,4-cyclohexanedicarboxylic acid (14-CDA), 1,3,5-cyclohexanetricarboxylic acid acid (135-CTA) and 1,2,4,5-cyclohexanetetracarboxylic acid (1245-CHA) are used as co-ligands. Conformational flexibility of the CPAs playsdecisive role in determining the overall structural patterns of the coordination networks. The insertion of flexible CPAs not only modifies the coordination environment of the metal ions by extending or blocking the polymerization, but also dramatically affects the crystal packing through various weak interactions.Conformational mobility of CPAs coupled with strong supramolecular interactions (H-bonding, metal organic interactions etc.) may also produce crystalline


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(b)

(a)

(d)

(c)

Figure 1. Structural fragments of compound 1: (a) coordination environment of Co(II) atom in the crystal structure (30 % thermal ellipsoids), dark grey atoms indicate carbon atoms; (b) molecular representation of 4 connected net;(c)topological representation of the sql net; (d) formation of left and right handed helix in the famework.

frameworks in which individual molecules may exist in energetically unfavourable or unusual conformation as observed in compound 7.However, when there are more carboxylic groups on cyclohexane ring, there will be more possible conformations, consequently it is more difficult to control their orientation in the final coordination networks. In this present contribution we have systematically investigated the coordination chemistry of five conformationally flexible CPAs. It has been also noticed that solvothermal reaction conditions also controls the spatial orientation of carboxylate groups attached with CPAs. In case of compounds 1-4, direct influence of solvothermal reaction temperature on the crystal structure of resulting CPs is observed.

Structural Description: Crystal Structures CDA)}.(CH3OH)]∞ (1) CDA)(H2O)}]∞(2).

of and

[{Co(H2MBP)2(12[{Co(H2MBP)(12-

Block shapedcrystals (pink) of 1 are isolated from solvothermal reaction of H2MBP with 12-CDA in presence of Co(OAc)2

at 70°C temperature. The same reaction, when carried out at 120°C, purple crystals of2are obtained.1 crystallizes in the monoclinic P21/n space group with two crystallographically independent H2MBP molecules, one 12-CDA, one Co(II) atom and one non coordinated methanol molecule in the asymmetric unit. The cyclohexane ring of 12-CDA molecule in the crystal structure of compound 1is found to exhibit some disordered effect which presumably resulted due to the flipping of chair conformation of cyclohexane. In the crystal structure Co(II) atom attains an octahedral geometry (Figure 1a) where basal sites are occupied by four nitrogen atoms from two symmetry independent H2MBP molecules and other two axial sites areoccupied by oxygen atoms fromthe carboxylate ligand (corresponding bond angles and lengths are summarized in table S1 in SI).In the crystal structure, the N-donor ligand forms a 1D looped-chain topology by extended coordination with the adjacent Co(II) metal centers and such 1D chains are further bridged by the dicarboxylate ligand resulting in a 2D network (Figure 1b). Such 2D sheets are further packed inparallel ABABfashion and the interstitial space is occupied by the lattice occluded MeOH molecules further sustained by H-bonding with the N-H of H2MBP and carboxylate O atoms (table S2 in SI). Due



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(b)

(a)

Figure 2. Structural fragments of compound 2: (a) coordination environment of Co(II) atom in the crystal structure (30 % thermal ellipsoids); (b) molecular representation of 6 connected net. ture as depicted in figure 2b.As observed in compound 1, the cyclohexane ring of 12-CDA molecule in the crystal structure of compound 2 suffers from similar kind of disorder effect. Unlike 1, crystal structure of 2 lacks any helical character. Topological analysis of the crystal structure reveals it as a 3coordinated uninodalhcb(honeycomb) net with point symbol 3 {6 } where M2L2 type loop motif has been utilized as vertex of the net (Figure 3). Each 2D hcb layers are further stacked in ABCABC fashion through weak interactionbetween parallel layers that is further reinforced by H-bonding interactions(table S4 in SI)involving carboxylate oxygen atoms and pyrazole N-H to form overall 3D structure.

Figure 3. Topological hcb(honeycomb)net.

representation

of

the

to the (e,e) conformation of 12-CDA molecule in the crystal structure, it typically acts as a V-shaped linker. Mutual interplay of bent N-donor heterocyclic moiety and V-shaped carboxylate linker coordinated to the adjacent metal center in an alternating fashion result in helical network of both handednesses(Figure 1d)which is also reflected on its achiral space group (P21/n). For the sake of topological classification of the coordination networks we have carried out topological analy66 sis using TOPOS 4.0 software following the concept of sim67-70 plifying underlying nets. Topological investigations reveal that it is a uninodal 4-connected sql(square lattice)(Figure 4 2 1c)net with point symbol {4 .6 }. The presence of helicity in 71-72 sql type nets is rarely observed. CP 2 belongs to monoclinic C2/c space group with one Co(II) atom, one H2MBP molecule and one 12-CDA molecule and one coordinated water molecule in the asymmetric unit. Co(II) adopts a distorted octahedral geometry with basal sites occupied by carboxylate oxygen atoms coming from two different 12-CDA molecule and one nitrogen atom from H2MBP molecule; the axial sites are occupied by one watermolecule and one nitrogen atom from H2MBP molecule (Figure 2a, table S3 in SI). Here, H2MBP links two adjacent metal centers forming M2L2 type discrete loops. Such loops are further bridged by 12-CDA linker forming 2D sheet struc-

Crystal Structure of [{Co0.5(H2MBP)(14CDA)0.5}.(H2O)2.5]∞(3) and[{Co(H2MBP)(14CDA)(H2O)2}]∞(4).When diverging 14-CDA ligand has been used as a coligand instead of 12-CDA, another two coordination polymers3and 4 are isolated once again attwo different temperatures. Solvothermal reaction of H2MBP and Co(OAc)2 in presence of 14-CDA at 120°C results in the formation of 3 whereasrepeating the same reaction at 70°C, pink crystals of 4 are isolated. The asymmetric unit of3 consists of one Co(II) atom (situated on an inversion centre), one H2MBP molecule, one 14-CDA molecule situated on a special position (inversion center)(Figure 4)and three lattice occluded water molecules. A pair ofcarboxylate oxygen atoms coming from two adjacent 12-CDA coligands binds axially with Co(II) atom whereas four N-atoms coming from H2MBP molecules constitute the equatorial plane of the octahedron (bond angles and bond length table S5 in SI).

Figure 4.Coordination environment of Co(II) atom in the crystal structure of compound 3(30 % thermal ellipsoids).


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Crystal Growth & Design (a)

(b)

Figure 5.(a) Topological representation of the 4 connected sql topology in compound 3; (b) infinite chain of H-bonded water molecules.

The auxiliary linker 14-CDA adopts a trans-e,e conformation in the crystal structure which connects the 1D looped chain constituted by H2MBP molecules resulting in a 2D coordination network with 4 connected sql topology (Figure 5a). Such 2D sheets are further packed in parallel fashion and 1D hydrogen bonded water clusters are located between the 2D layers sustained by various H-bonding interactions with the carboxylate oxygen atoms and pyrazole N atoms (Figure 5b). The geometrical parameters pertaining to the water chain are shown in ESI(Table S6). One of the lattice occluded solvent O3 is situated around a crystallographic inversion center with occupancy 0.5. The oxygen atoms associated with water chain are not coplanar and rather form an unprecedented wave pattern extending along crystallographic a axis. The O···O distance in between the oxygen atoms in water chain ranges from 2.74-2.95 Å which is consistent with the O···O distance found in other 1D helical water channels stabilised 73-75 in porous MOFs. Interaction of pyrazole N atoms and carboxylate O atoms with the lattice solvent molecules further strengthens the stabilization of water chain. TGA of 3(Figure S10 in SI) is performed to check the thermal stability ofthe CP and also that of the 1D water channels. The TG profile showsthat initial weight loss started at an onset temperature 30°C with 15.10 % weight loss which corresponds to the loss of lattice occluded water molecules. DSC data (navy blue graph in figure S10 in SI) shows that the water loss process is endothermic in nature. The compound remains stable upto 200°C and starts dissociating after that.

cules. The central Co(II) atom assumes anoctahedral geometry as depicted in figure 6a with equatorial coordination from two N atoms of symmetry related H2MBP ligands and two O atoms of 14-CDA molecules, the axial sites are occupied by water molecules (bond angle and bond length table S7 in SI). Unlike CP3 here orientation of 14-CDA molecule is such that it provides hydrophobic character to the framework and thus repels the polar solvent molecules. In the crystal structure N donor heterocyclic ligand and the carboxylate linker mutually participate in forming helical 1D framework. Such helical chains propagate in 2D via H-bonding interaction sustained by metal bound water molecules and further supported by carboxylate oxygen and pyrazole nitrogen atoms as depicted in figure 6b (table S8 in SI).

Since we have utilized commercially available 14-CDA which is found as a mixture of cisand trans isomers in a molar ratio of 3:2 as starting material, a new coordination polymer4 is isolated with cis conformer of 14-CDA. It has been well documented that mixing of isomers in the starting materials results in the formation of products with different conformations of 14-CDA depending upon the external condi42 tions. Difference in the spatial orientation of carboxyl group in two different isomers of 14-CDA brings subtle differencesinthe resulting structures. In the crystal structure of CP 4, the carboxylate ligand 14-CDA assumes a cis-a,e conformation which resemblesa L-shaped bent linker.The asymmetric unit contains one H2MBP molecule, one 14-CDA ligand, one Co(II) atom and two coordinated water mole-

Crystal Structure of [{Co(H2MBP)(13-ADA)0.5}]∞(5).

Interestingly, the compound exhibit chirality with a noncentrosymmetric space group P212121 although none of the starting materials are chiral. The circular dichroism (CD) spectra (Figure S18 in SI) of the compound confirm the chirality of the molecule. The chirality arises in this case presumably because of helical type framework formed by mutual participation of H2MBP and 12-CDA linker. The solid state CDspectrum is recorded using diffuse reluctance technique by taking very small amount of crystalline sample in KBr matrix. 5 exhibits a positive Cotton effect with a dichoric signal at 238 nm. Repeating the experiment with several batches of crystals shows similar positive Cotton effect which supports the formation of enantiomerically pure product.

When 13-ADA is used in place of 14-CDA, and the synthesis condition was changed into mixed solution of DMF, MeOH and H2O at 80°C, CP 5 is obtained.From the X-ray crystal structure analysis it is revealedthat the asymmetric unit consists of a half Co(II) atom situated on a glide plane, one H2MBP ligand and one half of 13-ADA molecule situated on a two fold axis. The central Co(II) atom exhibits an octahedral geometry (Figure 7a) with four pyrazole N atoms at the basal positions whereas the axial sites are occupied by carboxylate O atoms coming from 13-ADA molecules (bond angles, bond length table and H-bonding table are provided table S9 and S10 in SI).



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(a)

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(b)

Figure 6.Structural fragments of compound 4: (a) coordination environment of Co(II) atom in the crystal structure (30 % thermal ellipsoids); (b) Topological representation of the helical net topology in compound 4. (b) (a)

Figure 7.Structural fragments of compound 5: (a) coordination environment of Co(II) atom in the crystal structure (30 % thermal ellipsoids); (b) Topological representation of the cds net topology in compound 5.

The fundamental building block of this 3D framework is 1D looped chain topology in which metal centers are coordinated with N donor heterocyclic moiety. Such 1D looped chains oriented almost orthogonally are further bridged by 13-ADA coligandresulting in 3D framework (figure S19 in SI). A better insight of this intriguing framework can be achieved by employing a topological approach which reduces the complicated multidimensional nets into rather simpler framework. Considering all Co(II) as four connected node and connecting the nodes according to the connectivitydefined by H2MBP and 13-ADA the crystal structure can be simplified as a 3D cadmium sulphate (cds) topology (Figure 7b) with point 5 symbol {6 .8}. Crystal Structure of [{Co(H2MBP)2(135CTA)(H2O)}(H2O)3.5]∞ (6). Structural investigation reveals that compound 6 crystallizes in the monoclinic P21/c space group. The asymmetric unit comprises of one Co(II) atom, two H2MBP molecules, one 135-CTA molecule and four lattice occluded water molecules. One of the lattice occluded water molecules O13 is situated around an inversion centre and its occupancy becomes 0.5. The central Co(II) atom attains an octahedral geometry with four N atoms coming from two different H2MBP molecule at the basal plane whereas

one of the axial site is occupied by a free water molecule and other site is ligated by one carboxylate oxygen atom of 135CTA(Figure 8a) (bond angles and bond length table S11 in SI). The auxiliary ligand 135-CTA adopts its most stable cise,e,econformation in the crystal structure. As observed in earlier cases, H2MBP forms a 1D looped chain topology by coordinating with adjacent metal centers(Figure 8b)in which 135-CTA utilizes only one carboxylate group to bind with the metal keeping the other two carboxylates free. Such coordination of the coligand create suitable environment for solvent inclusion. Interestingly, a seven membered water clusterH-bonded to the carboxylic moieties free from coordination has been identified within the crystal structure (Figure 8c). The O···O distance found in the water cluster ranges from 2.69-3.00 Å and the O···O···O angels span from 76.2681.59° which deviates considerably from the value of 109.3° found in ice. In addition not all the oxygen atoms present in the crystal structure exhibits four coordination. This reflects strong influence of host crystalline framework on the conformation exhibited by water cluster.The water cluster fits itself inside the cavity of the framework by maximizing the interactions with the MOF (H-bonding table S12 in SI). The TGA profile of 6(Figure S13 in SI) also suggests considerably strong MOF-water cluster interaction. Initial weight


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(c)

(a)

(b)

Figure 8.Structural fragments of compound 6: (a) coordination environment of Co(II) atom in the crystal structure (30 % thermal ellipsoids); (b)1D coordination network resulted from M2L2 type metallocyclic motifs.(c)heptameric water cluster sustained by free carboxylate group in the framework. (a)

(b)

Figure 9.Structural fragments of compound 7: (a) packing diagram of complex 7 along a axis; (b) unprecedented topological framework with large voids.

loss starts at an onset temperature of 30°C after which the compound remains stable up to 320°C.The overall framework is a 3D supramolecular architecture in which each 1D chains are further packed in two other directions by coordination through lattice bound water molecules further sustained by carboxylate oxygen atoms and pyrazole N-H atoms from H2MBP. 2

Crystal Structure of [{Co2(μ O)(H2MBP)3(1245-CTA) }(H2O)12]∞ (7): 1245-CHA is a versatile organic linker that can exist in four possible conformational isomers namely I(e,a,a,e), II (e,a,e,e), III(e,a,e,a) and IV(e,e,e,e). As suggested by the free energy values conformer II is the most stable and conformer I is the least stable form both in gas and solution 42 phases. Coordination with the metal ion plays a vital role in controlling the orientation of carboxylate group on cyclohexane ring in such sterically crowded system. In compound 7under hydrothermal conditions the ligand 1245-CHA remain in conformation I presumably because the energy disadvantage is compensated via metal coordination and several supramolecular interactions such as hydrogen bonding. Crystal structure analysis reveals that this compound crystallizes

in the P21/n space group with three crystallographically independent H2MBP molecules, one 1245-CTA, two Co(II) atoms, 2 one μ bridging O atom and two non-coordinated water molecules in the asymmetric unit. Two crystallographically independent Co(II) atoms acquire two completely different sets of environment in the crystal structure. The octahedral geometry of Co (1) is composed of four N atoms coming from two different H2MBP molecules and two O atoms from 1245CHA linker (figure S20 in SI). Co (2) possesses a distorted octahedral coordination in which the square base is offered by one N atom from H2MBP molecule, two carboxylate oxygen atoms and one bridging O atom. Axial coordination comes from one N atom from H2MBP molecule and one bridging O atom (respective bond lengths and bond distances are given intable S13 in SI). Two Co2 centers are bridged by two carboxyl group from 1245-CHA with a Co···Co distance of 3.10 Å forming a system which resembles paddle wheel. Versatile coordination of 1245-CHA produces complicated 3D framework depicted in figure 9a. Better insight into the complicated 3D architecture can be achieved from topological analysis. Since the structure contains polynuclear groups, cluster method has been utilized which revealed 3



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Table 1. Crystallographic data and refinement parameters for 1-7.

CCDC Number

1 1026877

Empirical formu- C23H30CoN8 O5 la

2 1026874

3 1026873

4 1026879

5 1026875

6 1026876

7 1026878

C15H20CoN4 O5 C22H36CoN8O C15H18CoN4 O6 9

C28H34CoN8O C46H70Co2N16 C31H56Co2N12 O21 O21 4

557.4749

395.280

615.5019

413.296

605.5623

1301.02278

1050.7266

0.30×0.20

0.30×0.20

0.25×0.14

0.14×0.09

0.40×0.34

0.10×0.10

0.30×0.25

×0.10 Monoclinic

×0.07 Monoclinic

×0.08 Triclinic

×0.05 ×0.18 Orthorhom- Monoclinic bic

×0.08 Monoclinic

×0.11 Monoclinic

Space group

P21/n

C2/c

P-1

P212121

C2/c

P21/c

P21/n

a/Å

8.3729(15)

24.019(4)

8.8072(10)

5.418(3)

15.5219(3)

11.0261(11)

11.6164(11)

b/Å

17.325(3)

8.4244(13)

8.9158(11)

17.007(8)

9.1986(2)

31.406(3)

16.3416(16)

c/Å

17.548(3)

17.676(3)

11.1345(13)

18.413(8)

19.0046(4)

8.4970(8)

24.198(2)

α/°

90.00

90.00

67.889(5)

90.00

90.00

90.00

90.00

β/°

101.549(4)

113.821(4)

69.643(5)

90.00

95.1370(10)

99.010(3)

99.020(3)

90.00

90.00

62.584(5)

90.00

90.00

90.00

90.00

2493.9(8)

3271.9(9)

702.66(14)

1696.7(13)

2702.57(10)

2906.1(5)

4536.6(8)

Density

1.4845

1.604

1.459

1.618

1.488

1.4865

1.538

Z

4

8

1

4

4

2

4

1164

1640

323

848

1268

1360

2208

0.202

0.202

Mo Kαradiation Temperature/K

λ=0.71073Å 293(2)

λ=0.71073Å 293(2)

0.202λ=0.710 0.202λ=0.710 0.202λ=0.710 0.202λ=0.710 0.202 73Å 73Å 73Å 73Å λ=0.71073Å 293(2) 293(2) 293(2) 293(2) 293(2)

Rint

0.1163

0.0364

0.0658

0.1665

0.0221

-11/11,-22/19,

-32/34,-11/11,

-11/11,-11/11,

-7/6,-19/21,

-20/20,-12/12, -14/14,-40/41, -15/15,-21/21,

-23/23

-23,25

-14/11

-19/23

-25/24

-9/11

-30/29

θmin/max/°

1.67/28.26

1.85/ 30.95

2.02/28.31

1.63/27.49

2.15/28.40

1.30/28.30

1.510/28.315

Reflections

34918/6064

23589/5061

11120/3435

14182/3516

21513/3395

40892/7190

63192/10935

collect/3056 ed/unique/observ ed[I>2σ(I)]

/3899

/2331

/2033

/3107

/2954

/6416

Data/restraints/

6064/0/333

5061/0/227

3435/9/205

3516/0/235

3395/0/188

7190/12/408

10921/0/572

1.034

1.087

0.995

0.946

1.030

0.899

1.079

Final

R1=0.0675

R1=0.0485

R1=0.0501

R1=0.07o1

R1=0.0267

R1=0.0770

R1=0.0770

Rindices[I>2σ(I)]

wR2= 0.1701

wR2=0.1463

wR2= 0.1107

wR2= 0.1318

wR2= 0.0723 wR2= 0.1453

wR2= 0.1990

R1=0.1541

R1=0.0651

R1=0.0870

R1=0.1459

R1=0.0294

R1=0.1353

wR2= 0.2094

wR2= 0.1560

wR2= 0.1234

wR2= 0.1565

wR2= 0.0742 wR2= 0.1827

Formula weight Crystal size/mm Crystal system

γ/° Volume/Å

3

F(000) µMoKα/mm

-1

Range of h,k,l

0.2399

0.1094

parameters Goodnessoffit 2

on F

Rindices(all data)

nodal net (Figure 9b) with stoichiometry (3-c)2(4-c)(4-c)2. 5 2 2 3 The point symbol for the net is {10 .12}{3.10 }2{3 .10 .11}2.

R1=0.2225

Structural Comparison of the CPs


wR2= 0.2323

Page 9 of 14

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Structural comparison of the CPs 1-7 shows a stepwise increase in complexity of the resultant frameworks as either the flexibility or the substituents of the carboxylate colinkers are increased.Depending on the coordination requirement of the central metal ion, H2MBP molecule adopts either cis or trans conformations (Scheme 2). Various parameters associated with the conformational variation of H2MBP moleculearelistedin table 2 (figure S21 in SI). There are some striking resemblance in the structural features of the seven CPsalthough they give rise various topological architectures.

(a)

cis

trans

(b)

Scheme 2. (a)Representation of cis and trans conformation of H2MBP, (b) pyrazole-carboxylate H-bonding. Table 2. Conformational parameters of H2MBP molecules in CPs (1-7).

Structures

θ(°)

φ(°)

d(Å)

conformation

1

93.69

56.03

8.373

trans

2

97.96

-43.69

8.731

trans

3

100.03

-54.90

8.807

trans

4

107.50

3.61

9.237

cis

5

103.94

9.32

9.021

cis

6

94.77

-53.01

8.497

trans

7

98.56

-3.97

8.594

cis

carboxylate cluster formation while the other remain in monodented coordination.Furthermore, reaction temperatures, solvents, ligand to metal ratios are found to have crucial influence on the ultimate structures. Magnetic Property Studies Magnetic measurements are carried out on a well-powdered crystalline sample using a SQUID VSM Magnetometer (Quantum Design, USA). Temperature dependence of the molar magnetic susceptibility (χM) of complex 7 is measured in the temperature range of 2 – 300 K at 0.1 T and is shown as χMTvs. T and χMvs. T plots in Figure 10. At room temperature 3 -1 a χMT value of 4.82 cm mol K is observed which is greater 3 -1 than the expected spin-only value of 3.75 cm mol K for two high-spin octahedral Co(II) ions with S = 3/2 and g = 2.0. Here, only dinuclear unit is considered for magnetic data interpretation because due toappreciable distance (6.37–8.59 Å) between the dinuclear Co(II) unit and the isolated Co(II) centers there is lack of suitable magnetic exchange pathways between them. The higher χMTvalue at room temperature is due to unquenched orbital–moment as a consequence of 4 spin–orbit coupling typical of the T1g ground state of octa76-81 hedral high-spin Co(II) ion. As the temperature decreases from room temperature, χMT declines very slowly up to about 105 K, after which the value decreases rapidly until a 3 -1 value of 4.39 cm mol K is reached at ~25 K. This is followed by a sharp increment in χMT value at low temperatures to a 3 -1 value of 4.71 cm mol K at 6 K. Below this temperature again an abrupt drop in the curve is noticed. The decline in the χMT value from 300 K is due to an antiferromagnetic (AFM) interaction between the neighbouring Co(II) centres. For an antiferromagnetic system, the exhibition of such a feature in the susceptibility curve at the lower temperature region can 82-94 be ascribed to spin – canting behaviour which appears to arise from canted arrangements of spins on the adjacent Co(II) centres. The spin-canting mechanism which is known 95-99 to originate through an anti-symmetric interaction, requires the absence of inversion centre between the canted spins. Since in compound7 no inversion center is actually present so this structural feature might beresponsible for the spin canting behaviour.

Here θ represents the angle between two coordinating nitrogen atoms of H2MBP, φ represents dihedral angle between two pyrazole moiety of H2MBP molecule and d represents metal -metal distance. It has been observed that in most of the CPs (except 2 and 4) the V-shaped H2MBP ligand connects the adjacent metal centersforming 1D looped chain topology. Another important feature of 1H-pyrazole is their ability to restrict metal carboxylate cluster formation. Among two available nitrogens of 1H-pyrazoles, one coordinates to the metal whereas the other NH remains free which invariably forms hydrogen bonds with carboxylate oxygen atoms. Thus carboxylate groups are forced to adopt monodentate coordination in the resulting CPs.This trend is observed for almost all the coordination polymer described in this study with a partial deviation for 7.In case of 7, 1245-CHA is the auxiliary linker which contains four carboxylate groups. Due to the steric bulk of this molecule, it orient itself in the resulting framework in such a way that one of the carboxylate groups participate in metal-

Figure 10.χM vs. T andχMT vs. T plots for compound 7 measured at 0.1 T.


9


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The sudden drop in the χMT value at even lower temperatures (6 K – 2 K) is attributed to a zero-field splitting (ZFS) 100-104 effect. The temperature dependence of the reciprocal susceptibility (1/χM) is fitted according to the Curie – Weiss law, [χMT = C/(T − θ)], affording a Curie constant C = 4.84 3 -1 cm mol K and Weiss temperature θ = −1.08 K (Figure S15in SI). The negative θ value can be attributed to both AFM exchange coupling and spin – orbit coupling effects and therefore, the exchange parameters could not be estimated. The isothermal magnetization curve (M/NµBvs.H curve, Figure S16a in SI) shows an almost saturated value of 3.62 NµB at 2 K and 5 T, which isless than the expected value of 6 NμB for two Co(II) ions, also suggesting the spin-canting state at low temperature range. Additionally, from M/NµBvs.H/T plot (Figure S16b in SI) it is observed that the isothermal magnetization curves do not superimpose which indicate anisotropy and so the AC magnetization dynamics were also studied as a function of temperature and frequency, in zero and higher DC fields and 3.5 Oe AC field (Figure S17a-d). But no ac signals were obtained which is likely due to small energy barrier or quantum tunnelling of magnetization (QTM) and this rule out the possibility of single molecular magnetic (SMM) behaviour of complex 7. Experimental Section: Materials and Methods. All reagents and chemicals are purchased from commercial sources and are used without further purification. FT-IR spectra are obtained on a Nicolate MAGANA-IR 750 spectrometer with samples prepared as KBr pellets. TGA measurements are carried out with a TA instrument SDT Q600. Powder X-ray diffraction patterns are collected on a Bruker D8 AVANCE instrument. C, H and N microanalyses are done with a 2400 Series-II CHN Analyzer, Magnetic measurements are performed using a Quantum Design VSM SQUID magnetometer. The measured values are corrected for the experimentally measured contribution of the sample holder, while the derived susceptibilities are corrected for the diamagnetism of the samples, estimated from 105 Pascal’s tables. Synthesis of [{Co(H2MBP)2(12-CDA)}.(CH3OH)]∞(1). To a mixture of Co(OAc)2. 4H2O (24.908 mg), H2MBP (14.8 mg) and trans-12-CDA (17.21 mg), 2 mL water and 2 mL methanol are added. The mixtureis heated at 70°C for 72 h. Upon slow cooling to room temperature pink coloured block shaped crystals were obtained (52% yield based on H2MBP) which was then filtered and washed with MeOH and dried in air. -1

IR (400-4000 cm ): 3346 (s), 3130 (w), 2941 (s), 2858 (m), 1722 (s), 1572 (s), 1416 (s), 1388 (s), 1259 (w), 1201 (w), 1124 (w), 1064 (m), 960 (w), 748 (w),611 (w). Elemental Analysis: C23H30CoN8O5 (557.47): Calcd. C, 49.55; H,5.42; N, 20.10; found C,49.53, H 5.38, N 20.14. Synthesis of [{Co(H2MBP)(12-CDA)(H2O)}]∞ (2). 2 was synthesized following a similar procedure as that of1 except that, here the reaction is carried out in Teflon-lined reactor and heated at 120°C for 72 h. Upon slow cooling to room temperature block shaped purple crystals were obtained (65% yield based on H2PBPC) which were filtered and washed with MeOH and dried in air.

Page 10 of 14 -1

IR (400-4000 cm ): 3369 (brs), 3221 (s), 2929 (m), 1655 (w), 1550 (s), 1473 (s), 1439 (s), 1400 (s), 1359 (w), 1290 (w), 1196 (w), 1100 (w), 950 (w), 773 (w), 615 (w). Elemental Analysis: C15H20CoN4O5 (395.28): Calcd. C, 45.57; H,5.09; N, 14.17; found C, 45.56, H 5.10, N 14.27.

Synthesis of [{Co0.5(H2MBP)(14-CDA)0.5}.(H2O)2.5]∞(3). To a mixture of Co(OAc)2. 4H2O (24.908 mg), H2MBP (14.8 mg) and 14-CDA (mixture of cis and trans) (17.21 mg), 5 mL water was added. The mixture was heated at 120°C for 72 hours in Teflon-lined reactor. Upon slow cooling to room temperature pink coloured plate shaped crystals were obtained (42% yield based on H2MBP) which was then filtered and washed with MeOH and dried in air. IR (400-4000 cm-1): 3445 (s), 3425 (s), 2920 (m), 1720 (s), 1570 (m), 1420 (w), 1381 (m), 1226 (w), 1164 (w), 1008 (w), 874 (w), 753 (w), 629 (w). Elemental Analysis. C22H36CoN8O9 (615.50): Calcd. C, 42.93; H, 5.89; N, 18.20; found C 42.91, H 5.93, N 18.23. Synthesis of [{Co(H2MBP)(14-CDA)(H2O)2}]∞ (4).4 was synthesized following a similar procedure as that of3 except that, here the reaction is carried out in a sealed container at 70°C for 72 h. On slow cooling to room temperature pink coloured plate shaped crystals were isolated (63% yield based on H2MBP) which was then filtered and washed

with MeOH and dried in air. -1

IR (400-4000 cm ): 3242 (s), 2964 (w), 1537 (s), 1450 (w), 1342 (w), 1301 (w), 1209 (w), 1180 (w), 1066 (m), 1004 (m), 927 (w), 885 (w), 756(m), 711 (w).

Elemental Analysis. C15H22CoN4O6 (413.29): Calcd. C, 43.55; H, 5.32; N, 13.54; found C 43.61, H 5.37, N 13.51. Synthesis of [{Co(H2MBP)(13-ADA)0.5}]∞(5). Aqueous solution of Co(OAC)2.4H2O (24.908 mg) was added to the mixed solution of H2MBP (14.8 mg) and 13-ADA (25.23 mg) in 1:1 methanol DMF mixture The reaction was heated to 80°C in a sealed container for 72 h. Upon slow cooling to room temperature block shaped pink crystals isolated (65% yield based on H2MBP) this was then filtered and washed with MeOH and dried in air. -1

IR (400-4000 cm ): 3365 (w), 3089 (w), 2910 (s), 285o (m), 1537 (s), 1480 (m), 1396 (s), 1372 (m), 1303 (m), 1153 (w), 1064 (w), 956 (m), 860 (w), 750(m),655 (w).

Elemental Analysis. C28H34CoN8O4 (605.56): Calcd. C, 55.53; H, 5.65; N, 18.50; found C 55.49, H 5.58, N 18.52. Synthesis of [{Co(H2MBP)2(135-CTA)(H2O)}(H2O)3.5]∞ (6).To a mixture of Co(OAc)2. 4H2O (24.908 mg), H2MBP (14.8 mg) and Cis-Cis-135-CTA (21.619 mg), 5 mL water was added. The mixture was heated at 70°C for 72 hours in a sealed container. Upon slow cooling to room temperature pink coloured thin plate like crystals were obtained


10

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(48% yield based on H2MBP) which was then filtered and washed with MeOH and dried in air. IR (400-4000 cm-1): 3426 (s), 3230 (s), 2930 (w), 2851 (w), 1730 (s), 1560 (s), 1421 (m), 1386 (s), 1359 (m), 1290 (w), 1145 (w), 1066 (w), 999 (w), 744 (w), 597 (w). Elemental Analysis. C46H70Co2N16O21 (1301.02): Calcd. C, 42.46; H, 5.42; N, 17.22; found C 42.43, H 5.37, N 17.18. Synthesis of [{Co2(μ2O)(H2MBP)3(1245-CHA)}(H2O)12]∞. To a mixture of Co(OAc)2. 4H2O (24.908 mg), H2MBP (14.8 mg) and 1245-CHA (mixture of Cis and trans) (26.02 mg), 5 mL water was added. The mixture was heated at 100°C for 72 hours in a sealed container. Upon slow cooling to room temperature purple coloured block shaped crystals were obtained (62% yield based on H2MBP) which was then filtered and washed with MeOH and dried in air. Elemental Analysis. C31H56Co2N12O21 (1050.726): Calcd. C, 35.43; H, 5.37; N, 15.99; found C 35.68, H 5.29, N 16.09. -1

IR (400-4000 cm ): 3492 (s), 3330 (s), 3020 (m), 2958 (w), 1722 (s), 1521 (s), 1453 (m), 1312 (w), 1229 (w), 1131 (w), 1104 (w), 964 (m), 886 (w), 728 (w), 671 (w). X-ray Crystallography. Single-crystal X-ray diffraction data were collected using Mo Kα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with a CCD area detector. Data collection, data reduction, and structure solution refinement were carried out using APEX II. All the structures 1-7 were solved by direct methods and refined in a routine manner. In all the cases, the non hydrogen atoms are treated anisotropically except for disordered atoms.Whenever possible, the hydrogen atoms were located on a difference Fourier map and refined. In other cases, the hydrogen atoms were geometrically fixed. In case of compound 7 the disordered water molecules are refined by constraints using the PART command, with a total occupancy of 1. Crystallographic data are summarized in table 1, and CIF files for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC). CCDC-1026873-1026879 contains the supplementary crystallographic data for the paper.Copies of the data can be obtained, free of charge on application to the CCDC, 12 Union Road, Cambridge, CB2 1EZ UK [Fax: 44 (1233) 336 033 email: [email protected]].

CONCLUSIONS In conclusion, seven Co(II) coordination polymers have been successfully constructed via solvothermal methods and thoroughly characterized by various physicochemical techniques including SXRD. Angular orientation of the N donor heterocyclic moiety along with versatile coordination mode of various carboxylate coligandsused contributes an important role in the manifestation of various structural topologies ranging from 1D chain to unprecedented 3D framework. In addition to interesting structural features, because of the H-bonding backbone of H2MBP, various water clusters are found to be encapsu-

lated inside the resulting frameworks. One of the coordination polymers namely5 is found to exhibit chirality which further corroborated by solid state CD spectra. From the magnetic experiments, we observe some interesting magnetic behaviors for 7 which reveal the canted magnetic behavior arising due to canted orientation of spins between two carboxylate bridged Co(II) centers.

ASSOCIATED CONTENT Supporting Information Include details of TGA, PXRD, plots of magnetic analysis, CD spcetra and selected bond angles and bond lengths table for compound 1-7.This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENTS SG gratefully thanks Prof. ParthasarathiDastidar for helping throughout this work. SG gratefully thanks Prof. Davide M. Proserpio for helping to find out the new topology by TOPOS. SG thanks Dr. Ahmad Husain for useful crystallographic suggestions. SG thanks Indian Association for the Cultivation of Science, Kolkata for the research fellowship. SCXRD data were collected in the DBT-funded X-ray diffraction facility under the CEIB program in the Department of Organic Chemistry, IACS, Kolkata.This work was financially supported by Science and Engineering Research Board (SERB), India (Project No. SR/S1/IC-65/2012).

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For Table of Contents Use Only

Coordination Driven Self-Assembly in Co(II) Coordination Polymers Displaying Unprecedented Topology, Water Cluster, Chirality, and Spin Canted Magnetic Behavior Sumi Ganguly and Raju Mondal*

Seven Co(II) coordination polymers are constructed from angular 4, 4΄-methylenebispyrazole (H2MBP) ligand with various flexible carboxylate colinkers. The resulting coordination polymer exhibit unprecedented topology, water cluster, chirality and spin canted magnetic behavior.


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Coordination Driven Self-Assembly in Co (II) Coordination Polymers

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