Journal of Molecular Structure 1067 (2014) 64–73

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Fabrication of two supramolecular self-assemblies ofMn(II)-dicarboxylates with trans-4,40-azobispyridine: Analysisof H-bonding interactions with Hirshfeld surfaces and DFT calculations

http://dx.doi.org/10.1016/j.molstruc.2014.02.0590022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 9433230515; fax: +91 3324146223.E-mail address: dghoshal@chemistry.jdvu.ac.in (D. Ghoshal).

Rajdip Dey, Biswajit Bhattacharya, Pallab Mondal, Rajarshi Mondal, Debajyoti Ghoshal ⇑Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India

h i g h l i g h t s

� Qualitative and quantitativedetermination of hydrogen bonding.� The hydrogen bonding interaction

recognizes two differentdicarboxylates.� The supramolecular interactions

directs the formation of coordinationstructure.

g r a p h i c a l a b s t r a c t

Two new supramolecular self assembly of Mn(II) have been designed with an aliphatic as well as anaromatic dicarboxylate in combination with an N,N0 donor ligand. The variation in the use of differentdicarboxylates, creates a huge variation in the co-ordination modes of the metal ion as well as in theirsupramolecular structure.

a r t i c l e i n f o

Article history:Received 7 October 2013Received in revised form 6 February 2014Accepted 20 February 2014Available online 17 March 2014

Keywords:X-ray structureH-bondingDFT calculationSelf assemblyMn(II)

a b s t r a c t

Reactions with an aliphatic dicarboxylate (oxalate) as well as an aromatic dicarboxylate (terepthalate) incombination with an N,N0 donor ligand (trans-4,40-azobispyridine) results two new coordination polymerof divalent manganese, namely {[Mn(azbpy)(H2O)4]�(bdc)�(H2O)2}n, (1) and {[Mn(ox)(H2O)2]�(azbpy)�(H2O)2}n,(2). Both the coordination polymers have one dimensional structure and extended totwo-dimension by means of H-bonding. Interestingly, in solid state structure, the hydrogen bondinginteraction recognizes the dicarboxylate (bdc) in case of 1 and N donor ligands (azbpy) in 2. The variationin the use of dicarboxylates, creates here a huge variation in the co-ordination modes of the metal ion aswell as in the supramolecular structure within the crystal of 1 and 2. The contribution of various types ofnon-covalent forces are quantitatively explained in the light of Hrishfield surface analysis, which justifiesthe role of hydrogen bonding in the recognition of the above organic linkers. A DFT calculation of NBOalso gives a quantitative understanding of the formation of the solid state structure by H-bonding. Athermogravimetric analysis, solid state fluorescence spectra and EPR spectroscopic study of thecomplexes have also performed which also nicely corroborated their crystal structures.

� 2014 Elsevier B.V. All rights reserved.

Introduction

In last two decades, chemists have made significant progress onunderstanding the fundamental rules of self-assembling processes

R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73 65

[1–3] involving non-covalent interactions [4–7]. Owing to thedirectionality and specificity, hydrogen bonds are found very use-ful for the design of self-assembly from smaller molecular compo-nents to generate supramolecular ensembles with predefinedstructural features [8–10]. The correct choice of subunits and link-ers appear to be the most important criteria for the success of con-structing these supramolecular self assemblies [11,12]. In thedesign of metal organic frameworks, the combinational use ofnegative and neutral ligand has become the most popular tech-nique, where both the ligands are coordinated to the metal centre[13–16]. The use of such mixed ligand system can also be useful toproduce excellent supramolecular self-assemblies by exploitingthe non-covalent forces [17,18]. In this context, the interplay ofcovalent and non-covalent interaction for the generation of suchassemblies will be an attractive area to explore by both the syn-thetic as well as the theoretical chemists.

Here we have synthesized two Mn(II) complexes, using an ali-phatic dicarboxylate (oxalate) as well as an aromatic dicarboxylate(terepthalate) in combination with an N,N0 donor ligand [17–19]trans-4,40-azobispyridine (azbpy) which gives a nice understandingon the interplay of covalent and non-covalent interactions. Here inone case, the H-bonding forces interacted with the dicarboxylates(bdc) and in another case, with the N, N donor ligands (azbpy).The oxalate can be more easily coordinated with the metal centercompared to terepthalate, due to the absence of any electron with-drawing aromatic ring, which is present in case of terepthalate. Onthe other hand, the electronegativity of oxygen is greater than thatof nitrogen, and hence the binding capability of dicarboxylatesshould much higher than that of azbpy. Following these aspects,keeping the metal ion and N,N0 donor ligand [17–19] (azbpy) fixed,we have changed the dicarboxylate ligands (terepthalate and oxa-late) which results a significant difference in the metal–ligand co-ordination as well as in the solid-state structure (Scheme 1).

In complex 1, {[Mn(azbpy)(H2O)4]�(bdc)�(H2O)2}n, azbpy is di-rectly coordinated to the metal centre to create a one dimensionalchain and the terepthalate anions, present in the lattice. The coor-dinated water molecules present in the chain and the lattice watermolecule are linked to each other with the lattice terepthalate an-ion by intra-molecular H-bonding to create a supramolecular 2Dsheet. Whereas in complex 2, {[Mn(ox)(H2O)2]�(azbpy)�(H2O)2}n

the oxalate is directly co-ordinated to the metal centre and the azb-py ligands are not involved in the bonding, but remain in the crys-tal lattice. There are intra-chain H-bonding between the oxygenatom of oxalate and the coordinated water molecules. The chainsare further linked to each other with the lattice azbpy and the lat-tice water molecule by intra-molecular H-bonding to create asupramolecular 2D arrangement. The formations of solid-statestructures are also explained with the help of theoretical calcula-tions in a quantitative manner. A thermogravimetric study of the

Scheme

complexes, the solid state fluorescence spectra and EPR study alsocorroborate their structures.

Experimental

Materials

Trans 4,40-azobispyridine (azbpy) was synthesized following aslightly modified procedure reported earlier, by oxidative couplingof 4-aminopyridine [20]. High purity Manganese (II) chloride tetra-hydrate was purchased from the Aldrich Chemical Company Inc.and used as received. All other chemicals were of AR grade andwere used as received.

Physical measurements

Elemental analyses (carbon, hydrogen and nitrogen) wereperformed using a Heraeus CHNS analyzer. Infrared spectra(4000–400 cm�1) were taken on KBr pellets, using a PerkinElmerSpectrum BX-II IR spectrometer. TGA was carried out on aShimadzu DT-30 thermal analyzer under dinitrogen (flow rate:30 cm3 min�1). Emission spectra were recorded on a HORIBA JobinYvon (Fluoromax-3) fluorescence spectrophotometer. EPR spectraof powder samples have collected in JEOL JES-FA 200 Q-band ESRspectrophotometer. X-ray powder diffraction (PXRD) pattern ofthe powder sample of both the complexes were recorded on aPANalytical X’Pert PRO XRD instrument using Cu Ka radiation.

Synthesis of complex {[Mn(azbpy)(H2O)4]�(bdc)�(H2O)2}n (1)An aqueous solution (4 ml) of manganese chloride tetrahydrate

(0.0247 g, 0.125 mmol) was dissolved and it was taken in a layertube of 20 ml capacity. Then 5 ml of buffer mixture (1:1 of waterand MeOH) was added. After that, a methanolic solution (3 ml) oftrans-4,40-azobispyridine (0.023 g, 0.125 mmol) was added to anaqueous solution (3 ml) of disodium terepthalate (0.0263 g,0.125 mmol) and the mixture was stirred for 20 min to mix it well.The ligand mixture was then slowly layered on the Mn(II) solution.After four days, an orange compound was formed at the bottom ofthe tube. Shiny orange single crystals suitable for X-ray diffractionanalysis were obtained at the wall of the tube after one week (yield69%). Anal. Calc. (%) for C18H24N4O10Mn: C, 42.27; H, 4.73; N, 10.95.Found: C, 42.24; H, 4.72; N, 10.93. IR spectra (in cm�1): m(OAH),3435–3270; m(C@C), 1600–1420; m(CAO), 1320–1210; m(C@O),1655; m(CHAAr), 3100–2900 and m(N@N) 1420.

Synthesis of complex {[Mn(ox)(H2O)2]�(azbpy)�(H2O)2}n(2)This has been synthesized using the same procedure as that of 1

using disodium oxalate (0.0167 g, 0.125 mmol) instead ofdisodium terepthalate. After ten days, an orange compound was

1.

66 R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73

formed at the bottom of the tube. Shiny orange single crystals suit-able for X-ray diffraction analysis were obtained at the wall of thetube after two weeks (yield 57%). Anal. Calc. (%) for C12H16N4O8Mn:C, 36.10; H, 4.03; N, 14.03. Found: C, 36.07; H, 4.02; N, 14.01. IRspectra (in cm�1): m(OAH), 3435–3270; m(C@C), 1600–1420;m(CAO), 1320–1210; m(C@O), 1653; and m(N@N) 1415.

The bulk microcrystalline powder of both the compounds havealso been synthesized by direct mixing of the correspondingligands mixtures with aqueous solution of Mn(II) followed byovernight stirring. The purity of the compounds was checked andconfirmed from similarity of XRPD patterns of the bulk phase withthe simulated pattern from single crystal X-ray data. Thecompounds were also characterized by IR spectra and elementalanalyses which also found in accordance with the data obtainedfor the single crystals.

Crystallographic data collection and refinement

Suitable orange colored single crystals of 1 and 2 were mountedon the tips of the glass fibers coated with fomblin oil. X-ray singlecrystal data collection for both crystals were performed at roomtemperature using Bruker APEX II diffractometer, equipped witha normal focus, sealed tube X-ray source with graphite monochro-mator Mo Ka radiation (k = 0.71073 Å). The data were integratedusing the SAINT [21] program and the absorption corrections weremade with SADABS. Both the structures were solved by SHELXS-97[22] using the Patterson method, followed by successive Fourierand difference Fourier synthesis. Full matrix least-squaresrefinements were performed on F2 using SHELXL-97 [23] withanisotropic displacement parameters for all non-hydrogenatoms. All calculations were carried out using SHELXS-97 [22],SHELXL-97 [23], PLATON V1.15 [24], ORTEP-3V2 [25] and WINGXsystem Ver-1.80 [26]. Data collection, structure refinementparameters and crystallographic data for both the complexesare given in Table 1 and selected bond length and angles arereported in Supplementary Tables S1 and S4 for 1 and 2respectively.

Table 1Crystallographic and structural refinement parameters for 1 and 2.

Complex 1 Complex 2

Formula C18H24N4O9Mn C12H16N4 O7MnF.W. 511.35 399.23Crystal system Triclinic TriclinicSpace group P-1 P-1a/Å 6.3158(4) 5.1160(1)b/Å 6.9554(5) 5.6944(1)c/Å 12.4897(8) 14.8728(4)a/� 82.785(2) 100.814(1)b/� 84.992(2) 95.293(2)c/� 85.845(2) 101.042(1)V/Å3 541.19(6) 414.000(16)Z 1 1Dcalc (g/cm3) 1.569 1.601l (mm�1) 0.674 0.847F (000) 265 205hrange/� 1.6–27.6 1.4–27.5Refl collected 8261 6724Unique refls 2457 1890Rint 0.027 0.017No. of refls (I > 2.0 R(I)) 2224 1793Goodness-of-fit 1.05 1.12R1(I > 2R(I))a 0.0433 0.0290wR2

a 0.1217 0.0817Dq max/min/e Å3 �0.35, 0.70 �0.28, 0.40

a R1 = R||Fo|–|Fc||/R|Fo|, wR2 = [Rw (Fo2 � Fc2)2/Rw (Fo2)2]1/2.

Hirshfeld surface analysis

Hirshfeld surfaces [27,28] and the associated 2D-fingerprint[29–31] plots for both the structure were calculated using CrystalExplorer [32], taking single X-ray crystal structure as input. Bondlengths to hydrogen atoms were set to standard values. For eachpoint on the Hirshfeld iso-surface, two distances de, the distancefrom the point to the nearest nucleus external to the surface anddi, the distance to the nearest nucleus internal to the surface, areconsidered. The normalized contact distance (dnorm) based on de

and di is given by,

dnorm ¼di � rvdW

i

rvdWi

þ de � rvdWe

rvdWe

where rvdWi and rvdW

e are the van der Waals radii of the atoms. Thevalue of dnorm may be positive or negative depending on intermo-lecular contacts being either longer or shorter than the van derWaals separations. The parameter dnorm displays a surface with apopularly used red–white–blue color scheme, where bright redspots highlight shorter contacts, white areas represent contactsaround the van der Waals separation, and blue regions are devoidof close contacts [33].

DFT study and computational details

The energy calculation for the H-bonded assembly in 1 and 2were performed in gas phase in their high spin ground state(S = 5/2) by the DFT [34] method with B3LYP exchange correlationfunctional [35] approach along with HF and MP2 methods. Thegeometry used for the energy calculation is based on their crystalstructures. In all the calculations, a ‘‘double-n’’ quality basis setLANL2DZ was adopted as the basis set for Mn(II). For H atomswe have used 6-31(g) basis set whereas for C, N and O atoms wehave employed 6-31+g basis set. All the calculations were per-formed with the Gaussian 03 software package [36]. Natural bondorbital (NBO) calculations were performed with the NBO code [37]included in Gaussian 03.

Results and discussions

Crystal structures of 1

Complex 1 crystallizes in triclinic P-1 space group and the struc-ture determination reveals that the formation of a two dimensionalsupramolecular architecture consists of a one dimensional chain ofMn(II)-azbpy and lattice terepthalate molecule. Here in coordi-nated 1D chains each Mn(II) centers are located in a special posi-tion (½,0,½) and exhibits a distorted octahedral geometry withMnO4N2 chromophore (Fig. 1a, Supplementary Table S1). The twoaxial co-ordination sites of each hexacoordinated Mn(II) aresatisfied by the two symmetry related N (N1, N1a) atoms [Mn1AN1distance 2.2556(18) Å; N1AMn1AN1a angle 180�] of bridgingbidented azbpy ligand. The remaining four sites which createsthe equatorial plane of the octahedron are occupied by two setsof symmetry related O atoms of coordinated water molecules[O2W, O2Wa, O3W and O3Wa; Mn1–O2W distance 2.181(2) Å,Mn1-O3W distance 2.204(2) Å] (Supplementary Table S1).

In the crystal packing, the chains of Mn(II)-azbpy running alongc-axis and the lattice terepthalate dianions are arranged in parallelfashion between the chains. Due to this arrangement, the coordi-nated water molecules present in the chain are linked to the ter-epthalate anion and water molecules present in the lattice, byinter-molecular H-bonding (Table 2). The H-bonding here createsthree ring motifs with two R4

2 (10) and one R22 (6) Etters graph nota-

tion [38,39] to give the supramolecular 2D structure (Fig. 1b). It is

Fig. 1. (a) ORTEP drawing (40% probability ellipsoid) of 1 showing atom labeling diagram of a part of the 1D chain along with the lattice terepthalate and (b) 3D arrangementin 1 constructed by H-bonding and p–p interaction.

Table 2Hydrogen bonding interactions (Å, �) of 1 and 2.

DAH� � �A DAH H� � �A D� � �A <DAH� � �A

Complex 1 O3WAH3WB� � �O1Wa 0.78(3) 2.02(3) 2.788(3) 173(3)O2WAH2WA� � �O3b 0.77(4) 2.05(4) 2.805(3) 168(4)O3WAH3WA� � �O4c 0.83(4) 1.86(4) 2.683(3) 171(4)O2WAH2WB� � �O1Wd 0.79(4) 1.97(4) 2.761(3) 173(4)O1WAH1WB� � �O3 0.78(4) 1.97(4) 2.749(3) 173(4)O1WAH1WA� � �O4e 0.83(4) 1.93(4) 2.746(3) 167(3)

Complex 2 O1WAH1WA� � �N1 0.85 1.9500 2.799(3) 176O1WAH1WB� � �O1f 0.85 1.9700 2.8129(19) 175O2wAH2WA� � �O1W 0.81(2) 1.87 (2) 2.676(2) 174(3)

Symmetry code: a = 1 + x, 1 + y, z; b = 1�x, �1�y, 1�z; c = 1�x, �y, 1�z; d = 1 + x, y, z; e = �1 + x, y, z; f = �x, �y, 1�z.

R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73 67

interesting to note that the O atom of lattice water molecules hastaken part in bifurcating H-bonding during the construction of thesolid-state structure. The aromatic rings of lattice terepthalate arealso involved in face to face p–p interaction (SupplementaryTable S2) with the adjacent aromatic rings of bridging azbpy(Fig. 1b). Thus the H-bonded 2D structure is further stabilized byp–p interaction to give overall supramolecular structure. Thereare some structures also reported of that type with transition me-tal where terepthalate is uncoordinated and water molecules arecoordinated to metal [40–45]. There are either the shapes of waterclusters have been highlighted along with orientations of carboxyl-ate groups of terepthalate dianions [40], or they are represented asa mixed ligand coordination polymers [41], but in all the cases therole of H-bonding is notably important.

Crystal structures of 2

Complex 2 also crystallizes in the triclinic P-1 space group andthe structure determination reveals that the formation of a twodimensional supramolecular architecture consists of a one dimen-sional chain of Mn(II)-oxalate and lattice azbpy ligand. Like 1, herealso Mn(II) centers are located in a special position (½,0,½) andshows a distorted octahedral geometry (Fig. 2a) with MnO6 chro-mophore. Two pairs of symmetry related O [O1, O1a, O2 and O2a;MnAO1 distance 2.1909(13) Å; Mn1AO2 distance 2.204(2) Å]atoms from the two different chelating oxalate ligand creates the

equatorial plane of the octahedron (Supplementary Table S3).The axial sites are occupied by two symmetry related O atoms ofcoordinated water molecules [O2w and O2wa; MnAO2w distance2.1680(12) Å; O2wAMn1AO2aw angle 180�] (SupplementaryTable S3). Here the azbpy ligands are not involved in the bondingto the metal center and remain in the lattice along with latticewater molecules.

These lattice azbpy and the lattice water molecules play animportant role in the organization of solid-state structure in 2. Inthe crystal packing, the chains of Mn(II)-oxalate runs along crystal-lographic b-axis and these chains are stitched by the intra-molec-ular H-bonding mediated by the lattice waters and lattice azbpyligand arranged perpendicular to the chains; forming a supramo-lecular 2D sheet along ab plane (Fig. 2b, Table 2). There are alsoan intra-chain H-bonding between the O atoms of oxalate andthe coordinated water molecules (Table 2). The intra-molecularH-bonding creates a chain motif and a ring motif with Etters graphnotation [38,39] of C1

1(2) and R22(6) respectively, whereas the inter-

chain H-bonding consist of ring motif with R11(4) graph set. It is also

interesting to note that the lattice water molecules act as a connec-tor between the chains and the lattice azbpy ligand, during the for-mation of the supramolecular architecture. In the solid-statestructure the lattice azbpy ligand, are also arranged face-to-facein crystal lattice, creating a strong p–p interaction within it’s thearomatic rings which gives the further stabilization of the 2Dsupramolecular assembly (Fig. 2c, Supplementary Table S2).

Fig. 2. (a) ORTEP drawing (40% probability ellipsoid) of 2 showing atom labeling diagram of a part of the 1D chain along with the lattice azbpy, (b) formation ofsupramolecular 2D architecture through H-bonding interactions in 2 (inset: zoom view of H bonding) and (c) supramolecular 3D structure in 2 constructed by H-bonding andp–p interaction.

68 R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73

TGA study

Thermogravimetric analyses of the crystalline powder samplesof 1 and 2 have been performed in the temperature range of25–400 �C under the nitrogen environment (Fig. 3). The TGA studyof 1 shows a weight loss of �20.4% at 90–130 �C; which corre-sponds to the loss of the lattice water as well as the coordinatedwater molecules. After that it decomposed in two consecutivesteps to an unidentified product. The TGA analysis of 2 shows�19.01% weight loss around 40–110 �C, which corresponds to theloss of both lattice water and co-ordinate water molecules in 2.After that it decomposed in three consecutive steps to an uniden-tified product. In both the cases the two types of water moleculeshave been lost in same step which indicates the strong H-bonding

Fig. 3. TGA of Complex 1 and Complex 2 under nitrogen atmosphere.

association of the lattice water molecule and coordinatedmolecules.

Fluorescence spectral analysis

In the solid state, the fluorescence emission spectra of theligand (azbpy) shows emission band at �361 nm (very strong)and �467 nm (weak) when it is excited at 280 nm. This strongemission is due to the inter ligand charge-transfer transitionthrough the delocalized azo linkage present in the ligand, whereasthe weak one is due to the n—p� transition of the pyridyl N atom.The fluorescence spectra for both the complexes are recorded also

Fig. 4. Photoluminescences of free azbpy ligand and complexes 1 and 2 in the solidstate at room temperature.

R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73 69

after exciting the samples at same wave length as that of ligand(280 nm) (Fig. 4). The emission spectra of 1 shows a moderatelystrong emission at �470 nm which is red shifted from the emissionband of free ligand (�361 nm), which also corroborates the metalazpy bond formation in 1. The weak emission band (467 nm,n—p� transition) is absent, which further corresponds the involve-ment of lone pair of pyridyl N atom in coordinate bond formationwith Mn(II). In the emission spectra of 2, there is a moderatelystrong band at �401 nm and weak band at 465 nm which nearlycorrespond to the emission band that found in case of free ligand.Thus the emission spectra also support the presence of uncoordi-nated lattice azpy in 2.

EPR study

The solid-state EPR spectra of both complex 1 and 2 at 298 K(Supplementary Fig. 1) are also in accordance with the solid-statestructure of both the complexes. Complex 1 shows a broad spec-trum with anisotropy having g|| value 2.48 and g? value 2.02.The EPR spectrum of 2 shows broad isotropic pattern with giso

value 2.01. Complex 2 have the same coordination environmentaround the octahedral Mn(II) ion (MnO6) but 1 have two N atomin axial position and four O atom in equatorial position [46,47].This difference in coordination environment makes gz – gy � gz incase of 1 which has been reflected in the EPR spectrum in termsof the abovementioned anisotropy [48].

Hirshfeld surface analysis

Although with the observation of crystallographic study the solidstate structure determination has been established but to elucidatethe unique intermolecular interactions present in the solid-statestructure of the complexes, in a visual manner; we have performedthe Hirshfeld surface analysis. For a given crystal structure and set ofspherical atomic electron densities, the Hirshfeld surface is unique

Fig. 5a. Hirshfeld surfaces mapped with (a) dnorm; (b) shape-index and (c)curvedness for 1.

[33] and it is this property that suggests the possibility of gainingadditional insight into the intermolecular interaction of molecularcrystals. The Hirshfeld surfaces of both the crystal structures havebeen mapped over dnorm (�0.5 to 1.5 Å), shape index and curvedness(Figs. 5a and 5b). The surfaces are shown as transparent to allow thevisualization of the molecular moiety, in a similar orientation forboth structures, around which they were calculated. The asymmet-ric unit of Mn(II)-oxalate and lattice azbpy ligand were considered toanalyze and visualize the subtle packing features. The dominantO� � �H interactions are viewed by the bright red area of dnorm surface(Figs. 5a and 5b). Light red spots are due to CAH� � �O interactions. Theshape index and curvedness surfaces have been shown to give theinformation about each donor–acceptor pair and to measure howmuch shape effectively divides the surfaces into set of patchesrespectively. In both the cases, the fingerprint plots are quite asym-metric, because the interactions occur between two chemically andcrystallographically distinct molecules. In 1 the CAO� � �C intermo-lecular interactions appear as two distinct spikes in the fingerprintplots (Fig. 6a). Due to the presence of an aromatic terepthalatesystem; the electron withdrawing property lowers down the C� � �Ointeractions in 1 compared to that of in 2, which has been reflectedin the percentage of C� � �O/O� � �C interactions (0.9 and 7.9 of the totalHirshfeld surface of 1 and 2 respectively). The OAH� � �O intermolec-ular interactions appear as two distinct spikes in the fingerprintplots (Figs. 6a and 6b). Complementary regions are visible in the

Fig. 5b. Hirshfeld surfaces mapped with (a) dnorm; (b) shape-index and (c)curvedness for 2.

Fig. 6a. Fingerprint plots of 1: full (upper left) and resolved into different intermolecular interactions showing the percentages of contacts contributed to the total Hirshfeldsurface area of molecules.

70 R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73

fingerprint plots where one molecule acts as donor (de > di) and theother as an acceptor (de < di). Unlike 1 the electron withdrawingaromatic ring is absent in oxalate system; which has used in 2, theelectron density over the O� will moderately increases, conse-quently the donor properties of O� will also increases. As a resultthe oxalate can easily coordinated with the Mn(II) center decreasingthe O� � �H interactions capability with the water molecules. Thisobservation also supports by the Hirshfeld analysis; which showsthat the percentage of O� � �H/H� � �O interactions are 29.0 and 24.9of the total Hirshfeld surface of 1 and 2 respectively. The Hirshfeldsurface analysis do not show a similar proportion of O� � �H interac-tions for each molecule (15.7% in 1 and 13.6% in 2), whereas theproportion of H� � �O interactions have quite less variation than itsO� � �H counterparts (13.3% in 1 and 11.3% in 2). In 2, The O� � �H inter-actions are represented by the spike (di 1.087 Å, de 0.736 Å), whichindicates that the solvent molecules are interacting with the Natoms of the azbpy ligand responsible for the formation of supramo-lecular self-assembly. In the case of 1, the Hirshfeld surface contribu-tions for the CAH interactions and H� � �H contacts are 12.2% and35.1% respectively but in 2, these contributions are less (8.7% and29% respectively). The proportion of the contribution of N� � �H/H� � �N interactions are very close in two systems (6.2% and 6.3% for1 and 2). The contributions of variety of contacts exhibited by bothcomplexes have been depicted in Supplementary Fig. 2, whichclearly shows that the other interactions are minimal in 1 (16.6%)compared to that in 2 (23.2%). The proportion of N� � �N interactionscomprising 2.7% and 4.5% of the total Hirshfeld surface of 1 and 2respectively, which indicates the coordination of pyridyl N atom tothe metal in 1.

Electronic structure, charge distribution and NBO analysis

The energy calculation for complexes 1 and 2 were performedin gas phase in their high spin ground state (S = 5/2). The geometryused for the energy calculation is based on their crystal structure.The optimized structures of 1 and 2 were given in SupplementaryFigs. 3 and 4, respectively. The partial molecular orbital diagramwith some isodensity frontier molecular orbitals of 1 and 2 wereshown in Fig. 7. In the ground state in complex 1 the electron den-sity in HOMO (H), H�1 and H�2 orbitals mainly reside on metal dorbital and p orbital of the oxalate ligand while the same in LUMO(L) and L+2 orbitals primarily reside on p� orbital of the oxalateligand. The energy difference between H and L is 1.817 eV. In caseof complex 2 the electron density in H�1 and H�2 orbitals mainlyreside on terephthalate moiety while HOMO is composed of metald orbital and p (azbpy) orbital. Both LUMO and L+1 orbitals origi-nate from metal contribution. The HOMO and LUMO energy gapis greatly reduced to 1.048 eV in complex 2 compared to that in1. The DFT calculation for the energy of complexes 1 and 2 wereperformed in gas phase considering high spin ground state (S = 5/2). The geometry used for the energy calculation is based on theirsingle crystal X-ray crystal structure. The atomic charges from theNatural Population Analysis (NPA) calculated at B3LYP levels for 1and 2 respectively are represented in Tables 3 and 4 while thesame calculation carried out by HF and MP2 method are given inESI (Supplementary Tables S4 and S5 for complex 1 and Supple-mentary Tables S6 and S7 for complex 2). In both the supramolec-ular structure hydrogen atoms act as H-bond donors and hencelooses electron while nitrogen and oxygen atoms being the H-bond

Fig. 6b. Fingerprint plots of 2: full (upper left) and resolved into different intermolecular interactions showing the percentages of contacts contributed to the total Hirshfeldsurface area of molecules.

−−2.0

−3.0

−6.0

E (eV)

−7.0

H−2

H−1H

L

L+2

L+1

1.817

Complex 1

H−2

H−1

H

L

L+1

L+2

1.048

Complex 2

Fig. 7. Partial molecular orbital diagram with some isodensity frontier molecular orbitals for complexes 1 and 2.

R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73 71

Table 3Atomic charges from the natural population analysis (NPA) for 1 calculated at B3LYPlevels.

Atom Atomic charge Atom Atomic charge

Mn1 2.89573 O4 �0.37013N1 �0.25533 H1WA 0.22044N2 �0.08268 H1WB 0.21723O1W �0.44078 H2WA 0.21714O2W �0.38514 H2WB 0.23695O3W �0.37784 H3WA 0.22211O3 �0.40941 H3WB 0.22587

Table 4Atomic charge from the natural population analysis (NPA) for 2 calculated at B3LYPlevels.

Atom Atomic charge Atom Atomic charge

Mn1 2.7848 O1W �0.5082N1 �0.2361 H1WA +0.2567N2 �0.0939 H1WB +0.2503O1 �0.3343 H3WA +0.2364O2 �0.3300 H3WB +0.2266O3 �0.3860

72 R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73

acceptors acquire electron. Now it is evident from the Table S7,that for 2 the atomic charge on azo nitrogen atom (N2) is greaterthan that of the pyridyl nitrogen atom (N1) confirming their partic-ipation in H-bonding through the pyridyl nitrogen atom. In both 1and 2 the calculated charge on the oxygen atoms are significantlylow indicating their participation in H-bonding.The second-orderperturbative estimates for ‘‘donor–acceptor’’ (bond–antibond)interactions in NBO basis calculated at B3LYP levels, for both thestructure 1 and 2, respectively are summarized in Tables 5 and 6.Here also similar type of NBO analysis are also carried out by HFand MP2 methods and the corresponding data are given in ESI(Supplementary Tables S8 and S9 for complex 1 and Supplemen-tary Tables S10 and S11 for complex 2). It was carried out byexamining all possible interactions between ‘‘filled’’ (donor)Lewis-type NBOs and ‘‘empty’’ (acceptor) non-Lewis NBOs, and

Table 5Donor–acceptor NBO interaction in 1 and second-order perturbation stabilization energie

Donor NBO (i) Composition of donor NBO (i) Acceptor N

BD(1) O2WAH2WB 0.5127(s)H + 0.8586(sp)O BD�(1) O2BD(1) O3WAH3WA 0.5270(s)H + 0.8499(sp)O BD�(1) O3BD(1) O3WAH3WB 0.5232(s)H + 0.8522(sp)O BD�(1) O3LP(1) O4 BD�(1) O1LP(1) O4 BD�(1) O1LP(2) O4 BD�(1) O1LP(2) O4 BD�(1) O1

a BD represents bonding orbital, BD� represents antibonding orbital, LP represents loneLP: (1) and (2) represents the first and the second lone pair electron, respectively.

Table 6Donor–acceptor NBO interaction in 2 and second-order perturbation stabilization energie

Donor NBO (i) Composition of donor NBO (i) Acceptoro

BD(1) O3AH3WA 0.5048(s)H + 0.8632(sp)O BD�(1) O3BD(1) O3AH3WB 0.5225(s)H + 0.8526(sp)O BD�(1) O3BD(1) O1WAH1WA 0.4809(s)H + 0.8768(sp)O BD�(1) O3BD(1) O1WAH1WA 0.4986(s)H + 0.8668(sp)O BD�(1) O3LP(1) N1 BD�(1) O1LP(2) O1W BD�(1) O3

a BD represents bonding orbital, BD� represents antibonding orbital, LP represents loneLP: (1) and (2) represents the first and the second lone pair electron, respectively.

estimating their stabilization energy by second-order perturbationtheory [26]. In typical stable complex, the stabilization energy E isdefined as the interaction between orbital i of electronic donor andorbital j of electronic acceptor. The larger E indicates strongerinteraction between i and j, i.e., higher electronic transfer tendencyfrom i to j. The stabilization energies E are proportional to NBOinteraction intensities and the intermolecular NBO interaction thatreveal the origin of intermolecular interactions. The analysis ofNBO data obtained from DFT (B3LYP), HF and MP2 methods showsthat in 1, all the lone pair of electrons on O4 transfer to r�

antibonding orbitals of O1WAH1WA and O1WAH1WB with stabil-ization energy of ranging from 0.125–8.200 kJ mol�1. In 2 which infact possesses a strong hydrogen bond, and their second-orderperturbation energies for interaction are in range of 0.1251–29.999 kJ mol�1. Also, the lone pair electrons on N1 transfer to r�

antibonding-orbital of O1WAH1WA with stabilization energyranging from 26.652–29.999 kJ mol�1 while the lone pair electronson O1W transfer to r� antibonding-orbital of O3AH3WB with sta-bilization energy of 0.125 kJ mol�1.Thus it is evident from theabove discussion, that the NPA and NBO analysis also corroboratesthe formation of different types of H-bonding in the solid-statestructure of 1 and 2.

Conclusion

It is evident from the crystal structure, that two different mor-phologies (Complex 1 and Complex 2) were formed under thesame reaction condition as well as using the same reactant ratiosof the very similar reactants. The two syntheses are different onlyin terms of one co-ligand. The judicial use of the aliphatic dicarbox-ylate (oxalate) and the aromatic dicarboxylate (terepthalate) hasbecome the vital factor in directing the coordination structure aswell as the supramolecular structure, in both the cases. In complex1, azbpy is directly co-ordinated to the metal centre and the dicar-boxylate (bdc) is present in the lattice taking part in intermolecularH-bonding. But when oxalate dianion is used in the fabrication of 2the dicarboxylate (ox) is directly co-ordinated to the metal centreand azbpy is outside of the co-ordination sphere. This is probablydue to the absence of any electron withdrawing aromatic ring in

s E.a calculated at B3LYP levels.

BO (j) Composition of acceptor NBO (j) E/(kJ mol�1)

WAH2WA 0.5315(sp)O–0.8471(s)H 1.54808WAH3WB 0.5232(sp)O–0.8522(s)H 1.54808WAH3WA 0.5270(sp)O–0.8499(s)H 1.58992WAH1WA 0.5251(sp)O–0.8510(s)H 7.15464WAH1WB 0.5314(sp)O–0.8471(s)H 0.16736WAH1WA 0.5251(sp)O–0.8510(s)H 2.4267WAH1WB 0.5314(sp)O–0.8471(s)H 0.12552

-air. For BD and BD�: (1) and (2) represents R-orbital and p-orbital, respectively. For

s E.a calculated at B3LYP levels.

r NBO (j) Composition of acceptor NBO (j) E/(kJ mol�1)

AH3WB 0.5225(sp)O � 0.8526(s)H 1.5481AH3WA 0.5048(sp)O � 0.8632(s)H 1.0878AH3WA 0.5048(sp)O � 0.8632(s)H 0.25104AH3WA 0.5048(sp)O � 0.8632(s)H 0.92048WAH1WA 0.8668(sp)O � 0.4986(s)H 26.652AH3WB 0.5225(sp)O � 0.8526(s)H 0.12552

-air. For BD and BD�: (1) and (2) represents R-orbital and p-orbital, respectively. For

R. Dey et al. / Journal of Molecular Structure 1067 (2014) 64–73 73

the oxalate system; which allow easy coordination with the metalcenter compared to the terepthalate. Thus it has been demon-strated that how the differences in supramolecular structure oftwo similar systems can be possible, by simply tuning the placeof a same bridging ligand within or out of the coordination sphere.In summary, this work is a nice example where the tuning ofsupramolecular structures of a coordination polymer can beachieved by tuning the coordination sphere of that.

Acknowledgements

Authors gratefully acknowledge the financial support fromUGC. Authors are also thankful to Dr. S. Malik, IACS, Kolkata; forthe fluorescence spectral measurement, Dr. K.K. Rajak, JadavpurUniversity, for valuable discussion during the preparation of themanuscript and Shibashis Halder, Masters project student of JU;for his help. RD acknowledges CSIR for the senior research fellow-ship. The X-ray diffractometer facility of Dept. of Chemistry, JU;under the DST-FIST program is also gratefully acknowledged.

Appendix A. Supplementary material

Electronic supplementary information (ESI) available: EPR spec-tra, pi diagram of Hirshfeld surface, Optimized structure of bothcomplexes calculated at B3LYP methods, bond length, bond anglesand other related tables of both complexes and HF, MP2 methodswith their tabular results for both the complexes are available assupplementary informations. Crystallographic data of the com-plexes are available in CIF format with CCDC reference numbers927426 and 927427. Copies of the data can be obtained free ofcharge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK; fax: +44 1223033; email: deposit@ccdc.cam.ac.uk,http://www.ccdc.cam.ac.uk. Supplementary data associated withthis article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.02.059.

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