Cite this: DOI: 10.1039/c3ce00058c

Analysis of the contribution of the p-acidity of thes-tetrazine ring in the crystal packing of coordinationpolymers3

Received 10th January 2013,Accepted 12th February 2013

DOI: 10.1039/c3ce00058c

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Piya Seth,a Antonio Bauza,b Antonio Frontera,*b Chiara Massera,c Patrick Gamez*de

and Ashutosh Ghosh*e

Reaction of manganese(II) with the electron-deficient ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (pbptz)

leads to distinct coordination networks whose topologies are influenced by the nature of the anions used.

As anticipated, the linear ditopic ligand pbptz is involved in various types of supramolecular p interactions,

i.e. p…p, lone pair…p and C–H…p interactions, which clearly play a role in the formation of the different

solid-state architectures obtained, as shown by DFT calculations.

Introduction

Crystal engineering is based on the rational utilization ofintermolecular forces to construct particular molecular archi-tectures with tailored physical properties.1,2 Most syntheticstrategies exploit hydrogen or/and coordination bonds.3

However, a number of other supramolecular bonding contactsinvolving aromatic moieties, i.e. C–H…p,4 p…p (stacking),5,6

cation…p,7 but also anion…p8 and lone pair…p9 interactionsmay be employed as well to build solid-state networks.10,11

Tetrazine rings are particular aromatic units; indeed, theseare electron-deficient with respect to phenyl rings.12–14 As aresult, tetrazines have been used to intentionally generate p…p

interactions with electron-rich aromatic systems (as suchinteractions are favored14–16), with the objective to engineercrystal structures.17,18 Since tetrazines exhibit electron defi-ciency,19 they have also been employed to interact with anions(anion…p interactions20); for instance, the ligand 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz21) has allowed to prepare

remarkable supramolecular frameworks with short anion…p

contacts.12,22,23

In the present study, the ditopic ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (Scheme 1; pbptz17) has been used to producemanganese(II) coordination polymers. The aptitude of thep-acidic tetrazine ring to achieve p interactions with electron-rich entities, i.e. p-basic arenes (such as a phenyl group), hasbeen examined, and the crystal-packing effect of thesesupramolecular contacts has been evaluated (with theoreticalcalculations as well). For this purpose, reactions of pbptz withdifferent manganese(II) salts have been carried out. Aninorganic anion, namely thiocyanate SCN2, was first used toobserve the interactions taking place between the tetrazine-based ligands in the solid-state structure of the consequentMn compound. Subsequently, a purely organic anion, i.e.benzoate PhCOO2, was selected to explore the potentialoccurrence of p…p stacking interactions between the tetrazineand the phenyl rings (which are electron-rich aromaticmoieties). Lastly, the effect of the incorporation of a methylenegroup between the phenyl ring and the carboxylate function,namely using the phenylacetate anion (which can also beinvolved in p…p interactions) instead of benzoate, on thecrystal packing, was analysed by comparison of the solid-statestructure of the respective compound. The network topologies

aDepartment of Chemistry, University College of Science, University of Calcutta, 92 A.

P. C. Road, Kolkata 700009, IndiabDepartment of Chemistry, Universitat de les Illes Balears, Crta. De Valldemossa km

7.5, 07122 Palma de Mallorca (Baleares), Spain. E-mail: toni.frontera@uib.escDipartimento di Chimica, University of Parma, Viale delle Scienze 17/A, 43124

Parma, ItalydDepartament de Quımica Inorganica, Universitat de Barcelona, Martı i Franques

1-11, 08028 Barcelona, Spain. E-mail: patrick.gamez@qi.ub.es

www.bio-inorganic-chemistry-icrea-ub.comeCatalan Institution for Research and Advanced Studies (ICREA), Passeig Lluıs

Companys 23, 08010 Barcelona, Spain. E-mail: ghosh_59@yahoo.com

3 Electronic supplementary information (ESI) available: Crystallographic data inCIF format; Fig. S1–S5 illustrating specific crystal-packing features forcompounds 1–3 and Fig. S6–S9 showing the IR spectra for the free ligand pbptzand 1–3, respectively. CCDC 918798–918800. For ESI and crystallographic data inCIF or other electronic format see DOI: 10.1039/c3ce00058c

Scheme 1 Representation of the ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine(pbptz).

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of all three coordination polymers have been probed andcompared.

Experimental

Materials and general methods

All reagents except Mn(C6H5COO)2?4H2O andMn(C6H5CH2COO)2?H2O, which were prepared applying apublished procedure,24 were commercially available and usedas received. Elemental analyses (carbon, hydrogen andnitrogen) were performed using a Perkin-Elmer 240C elemen-tal analyser. FT-IR spectra (in KBr) were recorded using aPerkin-Elmer RXI FT-IR spectrophotometer (4000–500 cm21).

Caution! Perchlorate salts of metal coordination compoundswith organic ligands are potentially explosive. Only smallamounts of material should be prepared and handled withgreat care.

Preparation of pbptz

The ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (pbptz) wassynthesized as a purple solid, following a literature methodusing 4-cyanopyridine and hydrazine monohydrate as startingreagents.25

Synthesis of [Mn(NCS)2(pbptz)2]n (1). A solution ofMn(ClO4)2?6H2O (0.722 g, 2 mmol) in MeOH (5 mL) wasadded to a solution of pbptz (0.944 g, 4 mmol) in DCM (10mL). NH4SCN (0.152 g, 4 mmol) in 10 mL of H2O–MeOH (1 : 1)was added and the resulting reaction mixture was stirred forabout half an hour and filtered, subsequently. The filtrate waskept unperturbed at room temperature for the slow evapora-tion of the solvent. Deep-red single crystals suitable for X-raydiffraction studies were obtained after a few days. Yield = 78%(0.501 g). Elemental analysis for C26H16MnN14S2, calcd. (%): C,48.52; H, 2.51; N, 30.47. Found (%): C, 48.43; H, 2.62; N, 30.31.IR data (KBr pellet, cm21): 1391 n(NLN), 2076 n(NCS).

Syntheses of [Mn(C6H5CH2COO)2(pbptz)(H2O)2]n (2) and[Mn3(C6H5COO)6(pbptz)2]n (3). A methanolic solution (10mL) of Mn(C6H5CH2COO)2?H2O (0.686 g, 2 mmol) was addedto a solution of pbptz (0.944 g, 4 mmol) in 10 mL of DCM. Theresulting reaction mixture was stirred for about half an hour.Next, the solution was filtered, and the subsequent filtrate waskept unperturbed for the slow evaporation of the solvent. Red-coloured single crystals of 2 were collected after a few days.Compound 3 was obtained applying the same procedure,using Mn(C6H5COO)2?4H2O (0.738 g, 2 mmol) instead ofMn(C6H5CH2COO)2?H2O.

Compound 2: yield = 74% (0.441 g). Elemental analysis forC28H26N6O6Mn, calcd. (%): C, 56.29; H, 4.39; N, 14.07. Found(%): C, 56.44; H, 4.47; N, 13.95. IR data (KBr pellet, cm21): 1364n(NLN), 1577 n(COO), 3432 n(H2O).

Compound 3: yield = 70% (0.318 g). Elemental analysis forC66H46N12O12Mn3, calcd. (%): C, 58.12; H, 3.40; N, 12.32.Found (%): C, 58.04; H, 3.32; N, 12.40. IR data (KBr pellet,cm21): 1400 n(NLN), 1558 ns(COO), 1604 nas(COO).

The IR spectra of compounds 1–3 are shown in the ESI.3

Single-crystal X-ray crystallography

Suitable single crystals of compounds 1–3 were mounted on aBruker SMART diffractometer equipped with a graphitemonochromator and Mo Ka (l = 0.71073 Å) radiation. Thecrystallographic data were collected at room temperature. Thecrystals were positioned at 60 mm from the CCD 360 frameswere measured with a counting time of 10 s. The structureswere solved by Patterson methods using SHELXS-97.Subsequent difference Fourier synthesis and least-squarerefinement revealed the positions of the remaining non-hydrogen atoms. Non-hydrogen atoms were refined withindependent anisotropic displacement parameters. Hydrogenatoms were placed in idealized positions and their displace-ment parameters were fixed to be 1.2 times larger than thoseof the attached non-hydrogen atom. Successful convergencewas indicated by the maximum shift/error of 0.001 for the lastcycle of the least squares refinement. Absorption correctionswere carried out using the SADABS program.26 All calculationswere carried out using SHELXS-97,27 SHELXL-97,28 PLATON99,29 ORTEP-32,30 and WinGX systemVer-1.64.30 Data collec-tion, structure refinement parameters and crystallographicdata for the three complexes are given in Table 1.Crystallographic data for the structures reported in this paperhave been deposited in the Cambridge Crystallographic DataCentre with CCDC reference numbers 918798–918800 forcomplexes 1–3, respectively.

Theoretical methods

The energies of compounds 1 and 2 described in the presentstudy were computed at the BP86-D3/def2-TZVPD level oftheory using the crystallographic coordinates within theprogram TURBOMOLE version 5.10.31 The interaction energieswere calculated with correction for the basis set superpositionerror (BSSE) by using the Boys–Bernardi counterpoise techni-que.32 The calculation of the wavefunction to perform the‘‘atoms-in-molecules’’ (AIM)33 analysis was computed at thesame level of theory. The calculation of AIM properties wasperformed with the AIM2000 program.34

Results and discussion

Structure description

Reaction of manganese(II) perchlorate with pbptz in thepresence of ammonium thiocyanate yields red single crystalsof [Mn(NCS)2(pbptz)2]n (1). Compound 1 crystallizes in thecentrosymmetric space group Cmca. Crystallographic data for1 are given in Table 1. The coordination environment of 1 isshown in Fig. 1 together with its atomic-numbering scheme.Selected bond distances and angles are listed in Table 2. TheMn(II) ion lies on a crystallographic inversion centre, in aslightly distorted octahedral environment (the angles varyingfrom 86.04(8) to 93.96(8)u). The axial positions are occupied bytwo thiocyanate nitrogen atoms with Mn–NSCN distances of2.158(3) Å. The equatorial plane of the octahedron consists offour nitrogen atoms (N2, N2a, N2b and N2c) belonging to fourdifferent bridging pbptz ligands, with Mn–Npbptz bond lengthsof 2.309(2) Å, conferring an axially-compressed octahedral

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geometry about the metal ion. The four bridging tetrazineligands connect the manganese ion to four adjacent metals,generating a 2D (4,4) square-grid framework, with netdimensions of 15.694 6 15.694 Å2 (Fig. 2).

These large voids inevitably result in interpenetration, (seeFig. S1, ESI3), as will be illustrated in more details in thetopological section (see below). This intricate structuralarchitecture is stabilized by p…p interactions involving botharomatic groups of the ligand pbptz, namely the pyridine andtetrazine rings, with centroid-to-centroid contact distances of3.881(2) Å (Cg1…Cg2; Table 3 and Fig. S2A, ESI3), and lonepair…p interactions between thiocyanate sulfur atoms and

coordinated pyridine units (S1…Cg1 = 3.730(2) Å; Table 3 andFig. S2B, ESI3), which are more p-acidic than free pyridines.35

Compound 2, i.e. [Mn(C6H5CH2COO)2(pbptz)(H2O)2]n, isobtained by reaction of manganese(II) phenylacetate withpbptz. X-ray diffraction studies on a red single crystal of 2

Table 1 Crystal data and structure refinement for 1–3

1 2 3

Empirical formula C26H16MnN14S2 C28H26MnN6O6 C66H46Mn3N12O12

Fw (g mol21) 643.61 597.49 1363.97Crystal system Orthorhombic Monoclinic TriclinicSpace group Cmca P21/c P1Temperature (K) 293(2) 293(2) 293(2)Crystal size (mm3) 0.03 6 0.05 6 0.07 0.05 6 0.06 6 0.07 0.05 6 0.06 6 0.07a (Å) 21.1177(16) 15.637(5) 11.336(5)b (Å) 14.9308(11) 5.627(5) 12.249(5)c (Å) 8.8929(7) 16.468(5) 13.784(5)a (u) 90 90 85.864(5)b (u) 90 116.561(5) 67.811(5)c (u) 90 90 74.415(5)V (Å3) 2804.0(4) 1296.1(13) 1706.2(12)Z 4 2 1rcalcd 1.525 1.531 1.327m (mm21) 0.666 0.566 0.614F(000) 1308 618 697h for data collection (u) 1.9–24.77 1.46–28.88 1.60–22.67Reflections collected/unique 14 626/1246 14 727/3407 6255/4141Completeness to theta 100 99.6 90.5Data/restraints/parameters 1246/0/104 3407/0/203 4141/0/421Goodness-of-fit on F2 1.037 1.048 0.903Final R indices [I . 2s(I)] R1 = 0.0379, wR2 = 0.0920 R1 = 0.0329, wR2 = 0.0824 R1 = 0.0581, wR2 = 0.1360R indices (all data) R1 = 0.0595, wR2 = 0.1029 R1 = 0.0429, wR2 = 0.0875 R1 = 0.1196, wR2 = 0.1639Largest diff. peak and hole (e Å3) 0.297 and 20.419 0.377 and 20.215 0.359 and 20.459

Fig. 1 Representation of the single-crystal X-ray structure of 1. Hydrogen atomshave been omitted for clarity. Symmetry operations: a = x, 2y, 1 2 z; b = 1 2 x,2y, 1 2 z; c = 1 2 x, 2y, 2 2 z.

Table 2 Selected bond distances (Å) and angles (u) for 1–3a

1

Mn1–N1 2.158(3) N2–Mn1–N2a 86.04(8)Mn1–N2 2.309(2) N2a–Mn1–N2b 93.96(8)

N1–Mn1–N1a 180Mn1…Mn1inter

b 8.689(3)

2Mn1–O1 2.159(2) O1–Mn1–O3 89.92(4)Mn1–O3 2.191(2) O1–Mn1–O3b 90.08(4)Mn1–N1 2.302(3) N1–Mn1–N1b 180Mn1…Mn1inter

b 5.627(3)

3Mn1–O1 2.070(5) O1–Mn1–O3 108.5(2)Mn1–O4 2.246(5) O3–Mn1–O4 58.5(2)Mn1–O5 2.092(5) O4–Mn1–N1 94.9(2)Mn1–O3 2.274(5) N1–Mn1–O1 95.1(2)Mn1–N1 2.234(6) N2–Mn1–O5 172.5(3)Mn1–N2 2.412(6)

Mn2–O2 2.211(5) O2–Mn2–O6 89.02(19)Mn2–O3 2.242(5) O2c–Mn2–O6 90.98(19)Mn2–O6 2.159(6) O3–Mn2–O3c 180

Mn1…Mn2 3.593(2) Mn1…Mn1interc 6.067(6)

a Symmetry operations: a = x, 2y, 1 2 z; b = 1 2 x, 2y, 1 2 z; c = 12 x, 2y, 2 2 z. b Closest inter-trinuclear Mn…Mn distance. c Closestinter-mononuclear Mn…Mn distance.

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reveal that it crystallizes in the centrosymmetric space groupP21/c. Crystallographic data are given in Table 1 and selectedbond distances and angles are listed in Table 2. Thecoordination environment of 2 exhibits one crystallographi-cally unique manganese(II) atom lying on an inversion centre,one pbptz ligand, two phenylacetate anions and two watermolecules (see Fig. 3, in which two pbptz ligands are shown tocomplete the coordination around the metal). Hence, eachmetal centre is six-coordinated to two oxygen atoms (O1 andO1b) belonging to two phenylacetato ligands, two watermolecules (O3 and O3b) and two nitrogen atoms from distinctpbptz ligands (N1 and N1b). The resulting regular octahedron(the equatorial angles being very close to the ideal value of 90u;Table 2) is characterized by equatorial bond distances of2.159(2) Å (Mn–O1; Table 2) and 2.191(2) Å (Mn–O3; Table 2),and by slightly elongated axial Mn–N1 bond lengths of 2.302(3)Å (Table 2). The bridging, ditopic pbptz ligands connectmanganese(II) ions to generate a 1D coordination polymeralong the crystallographic a axis (Fig. 4), which exhibits strongintramolecular hydrogen-bonding interactions between coor-dinated phenylacetato ligands and water molecules (O3–H40…O2; Fig. 4 and Table 3). In addition, the water molecules(O3) are involved in a second hydrogen-bonding network with

coordinated oxygen atoms (O2) from phenylacetato ligands ofadjacent 1D chains (Fig. S3A, ESI3).

These hydrogen bonds (O3–H41…O1; Table 3) thus connectthe coordination polymers to each other along the crystal-lographic b axis, producing a 2D supramolecular sheet in theab plane (Fig. S3B, ESI3). As anticipated, the electron-richphenyl units (Cg3) of the organic anions are stacking with theelectron-deficient tetrazine rings (Cg2) of neighbouring 2Dsheets, in the crystallographic c direction, generating a 3Dframework (Fig. S4, ESI3). These strong parallel-displaced p

stacking interactions36 are characterized by a centroid-to-centroid distance Cg2…Cg3 of 3.581(3) Å, with short C1…C9and C11…N3 contact distances of respectively 3.366(4) and3.249(4) Å (see Table 3 and Fig. S4, ESI3), which are in therange of the corresponding sum of the van der Waals radii of Cand C (3.40 Å), and C and N (3.25 Å).

Comparison of the solid-state structures of 1 and 2 disclosessome important features. First, most likely as the result of thesteric hindrance of the phenylacetate anions (in contrast to thelinear thiocyanate ions), only two pbptz ligands coordinate tothe manganese(II) ion in 2 while four of them are bound to themetal centre in 1. Consequently, the coordination sphere of 2contains two water molecules as small ligands, impeding theformation of a 2D square-grid network. However, these watermolecules are involved in hydrogen-bonding interactions,giving rise to a 2D supramolecular framework (while it is a2D coordination framework for 1). Second, as expected, thephenyl group of the organic anion strongly interacts with thetetrazine rings; these supramolecular bonding interactionsactually drive the formation of a non-interpenetrated (incontrast to 1) 3D architecture (thus resulting from thep-stacking induced self-assembly of the hydrogen-bonded 2Dsheets).

Fig. 2 2D (4,4) square grid in the structure of 1 with net dimensions. Hydrogenatoms have been omitted for clarity.

Table 3 Supramolecular bond distances (Å) and angles (u) for 1–3

1Cg1…Cg2 3.881(2) Cg1…S1 3.730(2)

2O3–H40…O2 2.658(3) O3–H40–O2 162(3)O3–H41…O1 2.772(3) O3–H41–O1 167(3)Cg2…Cg3 3.581(3)C1…C9 3.366(4) C11…N3 3.249(4)O3–H40…O2 2.658(3) O3–H40–O2 162(3)

3Cg9…Cg13 3.669(5) C16…C66 3.377(12)

Fig. 3 Representation of the single-crystal X-ray structure of 2. Only the waterhydrogen atoms are shown for clarity. Symmetry operations: b = 1 2 x, 2y, 1 2

z.

Fig. 4 Formation of a 1D coordination polymer along the crystallographic a axis.The green dotted lines symbolize the intramolecular strong hydrogen bondsinvolving the coordinated phenylacetate ligands (O2) and water molecules (O3).O3–H40…O2 = 2.658(3) Å and /O3–H40–O2 = 162(3)u.

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Compound 3, i.e. [Mn3(C6H5COO)6(pbptz)2]n, is prepared byreaction of manganese(II) benzoate with the ligand pbptz.X-ray diffraction studies on a red single crystal of 3 reveal thatthe coordination compound crystallizes in the triclinic spacegroup P1. Crystallographic data for 3 are given in Table 1 andselected bond lengths and angles are listed in Table 2. Arepresentation of the coordination environment of 3 isdepicted in Fig. 5. 3 exhibits two types of manganese(II) ions,namely Mn1 and Mn2.

Mn1 is characterized by a strongly distorted octahedralcoordination geometry, which is apparently due to the verysmall bite angle, i.e. O3–Mn1–O4 = 58.5(2)u (Table 3), of them-O,O9,O9 benzoato ligand [O3, O4]. The equatorial plane isconstituted of two oxygen atoms belonging to the m-O,O9,O9

benzoato ligand (O3 and O4), one oxygen atom from a m-O,O9

benzoato ligand (O1), and one nitrogen atom from a pbptzligand (N1). The distorted octahedron is completed by thenitrogen atom of a second pbptz ligand (N2) and one oxygenatom belonging to a third benzoato ligand (O5), at the axialpositions. The Mn–O and Mn–N bond distances can beconsidered as normal for this type of MnN2O4 coordinationenvironment (Table 2).

Mn2 lies on an inversion centre and its coordinationenvironment is an almost perfect octahedron, as illustratedby the angles of 89.02(19) and 90.98(19)u (close to the idealvalue of 90u). At the equatorial plane, Mn2 is coordinated byfour oxygen atoms (O2, O3, O2c and O3c; Fig. 5) from fourdifferent benzoato ligands. The axial positions are occupied bythe two oxygen atoms O6 and O6c belonging to two m-O,O9

benzoato ligands. Hence, Mn2 is bridged to two symmetry-related Mn1 ions via four m-O,O9 benzoato ligands and twom-O,O9,O9 benzoato ligands, generating a linear trinuclear[Mn1, Mn2, Mn1d] cluster (see Fig. 5).

The external manganese(II) ions Mn1 and Mn1d exhibit twoditopic pbptz ligands that are perpendicularly coordinated (theN1–Mn1–N2 angle amounts to 88.0(2)u). Accordingly, eachtrimanganese unit is connected to four adjacent trinuclearclusters, producing a 2D rhombohedral grid structure, asdepicted in Fig. 6. This coordination framework is character-ized by net dimensions of 17.767 6 20.084 Å2. In contrast to 1,interpenetration is not observed; actually, the 2D layersassemble in an alternated fashion (see Fig. S5A and S5B,ESI3) by means of strong p…p interactions between phenyl(Cg13) and tetrazine (Cg9) rings (Cg9…Cg13 = 3.669(5) Å;

Table 3), as evidenced by the C16(tetrazine)…C66(phenyl)contact distance of 3.377(12) Å, producing an interdigitatedpacking structure (Fig. S5C, ESI3).

In a first instance, the difference between the phenylacetateand benzoate anions appears insignificant. However, the solid-state structures of 2 and 3 present several differences. Thebinding mode of the two anionic ligands is not the same; whilephenylacetate acts as a monodentate ligand, benzoate func-tions as a bridging bidentate or tridentate ligand. In fact, thebenzoate anion has a clear tendency to behave as a bridgingligand. Indeed, a search at the Cambridge Structural Database(CSD v 5.33 including three updates (May 2012) usingConQuest v 1.14) reveals that out of the 1,490 solid-statestructures containing the benzoato ligand, 945 includebridging benzoato ligands. Actually, C6H5COO2 is often usedfor its bridging aptitudes to prepare metal clusters.37,38 For thephenylacetate, only 72 structures are found in the CSD, ofwhich 40 have this anion as bridging ligand. The formation ofa linear trinuclear cluster in 3 allows the bonding of two pbptzligands to the external manganese(II) ions, which permits theformation of a 2D coordination framework that resembles thatof 1. Similarly to 1, compound 3 does not exhibit coordinatedwater molecules (unlike 2); hence, no hydrogen-bondingnetwork is observed. However, since 3 contains an organicanion with an aryl group, p…p interactions take place with theelectron-deficient tetrazine ring that connect the 2D rhombo-hedral grids to produce a 3D architecture, as in compound 2.In summary, while the crystal structures of 1 and 2 aredrastically distinct, the 2D framework of 3 is related to that of1, and its 3D architecture is driven by pphenyl

…ptetrazine

interactions, like 2.

Network topologies

The topology of the three coordination compounds has beenexamined in more details using the TOPOS software.39,40 Asmentioned earlier, the structure of 1 consists of a 2D (4,4)square-grid framework, in which the metal is the node of thenet while the pbptz ligand behaves as 2-coordinated con-nector. Applying to this structure the simplification process

Fig. 5 Representation of the single-crystal X-ray structure of 3. Hydrogen atomshave been omitted for clarity. Symmetry operations: c = 1 2 x, 2y, 2 2 z; d = 2x,1 2 y, 1 2 z.

Fig. 6 2D rhombohedral grid in the structure of 3 with net dimensions.Hydrogen atoms have been omitted for clarity.

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which removes the 2-c units, the resulting framework can bedescribed as a sql/Shubnikov tetragonal plane net {44.62} withvertex symbol [4.4.4.4.*.*].41

A further analysis shows that the 2D sheets are related toeach other through polycatenation, giving rise to a parallel/parallel inclined interpenetration, which occurs when eachgrid of a sheet contains rods passing through them from othersheets (Fig. 7A and S1, ESI3).42,43 More specifically, theresulting 3D network is formed by two sets of parallel planes,namely (0, 2, 1) and (0, 2, 21) with a dihedral angle of 80u; theplanes cross through the line [1, 0, 0]. From a more detailedanalysis it is also possible to classify the types of 4-cyclesinvolved in the 3D net and to show how they are related toeach other. Two cases are present: i) pairs of 4-cycles belongingto two different sets in which only one type of crossing exists.Each cycle crosses another cycle one time forming one link(knot) of the Hopf type, and on the whole each cycle is crossedby four other cycles (Fig. 7B, the cycles belonging to differentsets are coloured in blue and red). ii) Pairs of 4-cycles involvedinto two crossings which are not links (the cycles are notentangled); each cycle is crossed by two other cycles (Fig. 7C).

In the case of compound 2, a meaningful topologicalanalysis should include the noncovalent interactions presentin the crystal structure. In fact, as previously shown in Fig. S3,ESI,3 the 1D coordination polymers are organized into non-interpenetrating, parallel layers via hydrogen bonds betweencoordinated water molecules and phenylacetato ligands. Ifthese H-bonds are formally treated as valence bonds, and asimplification is carried out on the structure to remove 0-, 1- or2-connected units, each of these layers can be again describedas a sql/Shubnikov tetragonal plane net (Fig. 8).

Finally, the topology of compound 3 can be best describedusing the cluster representation,44 in which the pbptz ligands(2-c linkers) connect [Mn3(C6H5COO)6] units, considered ascluster nodes. Following the simplification of the organicligand, the structure results again in a series of parallel, non-interpenetrating sheets (Fig. 6 and 8) of the topological typesql, as for compound 2 (even if, in contrast to 2, the 2Dframework is based on coordinative interactions and not onhydrogen bonding).

Computational studies

The noncovalent interactions attributable to the differentanions (namely thiocyanate and phenylacetate) used toprepare compounds 1 and 2 have been analysed in moredetail by theoretical calculations. More conventional interac-tions such as hydrogen bonding and coordination bonds arevery important as elements of cohesion within the networksstudied herein. However, we have focused our attention to therole played by lp…p, C–H/p and p–p interactions, which arecomparatively less studied noncovalent forces that are alsofundamental for the final architecture of the networks.

A remarkable supramolecular feature observed in thetridimensional architecture of 1 is represented by the(bifurcated) lone-pair…p interactions45 that occur betweenthe sulfur atom of the SCN2 ligand and the pyridine ring ofthe pbptz ligand (Fig. 9 and S2B, ESI3). In fact, thecoordination of the pyridine moiety to the manganese(II) ionincreases the p-acidity of the aromatic ring, thus favouring thelp…p bonding contact. This issue has been examinedtheoretically. In the theoretical model used to evaluate thisinteraction, the coordination sphere of the metal centre hasbeen saturated with ammonia molecules to simulate the pbptzligands (Fig. 9). We have used this small molecule as ligandmodel instead of other possible larger models like pyridine inorder to keep the size of the systems computationally feasible.Indeed, due to the large number of unpaired electrons, SCFconvergence problems are commonly encountered with

Fig. 7 A) View of the 2D+2D inclined interpenetration network. The twodifferent sets of parallel planes are shown in blue and red. B) View of the 4-4-Hopf link. C) View of the 4-4 non-Hopf link. D) Combined view of the possiblelinks. The blue 4-cycle on the left forms a Hopf link with the red one, while theone on the right just penetrates the window of the red cycle with one angle.

Fig. 8 View of the parallel 2D sheets describing the structures of 2 and 3. Theblue spheres are the Mn atoms acting as nodes of the net.

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calculations carried out on such complexes, which areminimized by reducing the size of the system. However, inthe simpler systems studied and discussed below we haveexamined the influence of the sp2/sp3 nature of the modelligand on the interaction energy and we have not foundsignificant differences.

The interaction energy of this system where two bifurcatedlp–p interactions are established is DE1 = 220.2 kcal mol21

(Fig. 9A). Therefore, each lp…p contact contributes toapproximately 25 kcal mol21, suggesting a more favourablebonding interaction than that calculated for lp…p interactionsinvolving hexafluorobenzene (ca. 23 kcal mol21).46 Thisenergy increase is probably due to an alteration of theelectronic nature of the pyridine ring upon coordination tothe manganese(II) ion that gives rise to an enhancement of theinteraction. To verify this hypothesis, the interaction energy ofa model for which one Mn ion has been removed and onepbptz ligand has been maintained at the position found in thecrystal structure (see Fig. 9B) has been computed. Using thissystem, the interaction energy is lessened to 22.7 kcal mol21

(instead of 25 kcal mol21; see above). This result clearlydemonstrates that the presence of the coordinated metalcentre significantly augments the lp…p interaction. In addi-tion, this also explains why the lp…p interaction is establishedwith the pyridine ring instead of the more p-acidic s-tetrazinering, which would be the situation expected without coordi-nated metal ions. Finally, we have also evaluated the relevantintermolecular C–H…S hydrogen bonds formed between thesulfur atom of the SCN2 ligand and the pyridine ring of thepbptz ligand (Fig. 9C). The interaction energy of this systemwhere two bifurcated hydrogen bonds are established is DE3 =213.4 kcal mol21. This energy is higher than that expected for

a bifurcated hydrogen bond involving aromatic protons.47 Thisis due to an enhancement of the acidity of the hydrogen atomsof the pyridine ring upon coordination to the manganese(II)ions.

Next, the p…p stacking interactions noticed between thecentral tetrazine ring of the bidentate ditopic ligand and thephenyl group of the organic anion (Fig. S4, ESI3) in the crystalpacking of 2 have been examined. Two aspects have beentaken into consideration. Possible cooperativity effects havebeen first evaluated since each tetrazine ring is establishingtwo concurrent p…p stacking interactions with two phenylunits (Fig. 10 and S4, ESI3). Subsequently, the influence of thecoordination of the manganese ion on the interaction energyhas been investigated (Fig. 11). These calculations have beencarried out using the BP86 functional with the latest availablecorrection for dispersion (D3) in order to properly assess thep-stacking interactions. Moreover, we have computed theinteraction energies DE4 and DE5 using the ab initio MP2method and the same basis set. We have included the MP2energies in italics in Fig. 10. It can be observed that theinteraction energies are comparable, giving reliability to theBP86-D3/def2-TZVPD level of theory.

The two theoretical models used to analyse the stackinginteractions between the pbptz ligand and phenyl group(s) inthe absence of manganese are depicted in Fig. 10. In both thebinary and ternary complexes, the phenyl–tetrazine interac-tion(s) involve(s) two different types of supramolecularinteractions, namely p…p stacking contact(s) (with a cen-troid-to-centroid distance of 3.58 Å and a centroid to meanplane distance of 3.35 Å), and C–H/p interaction(s) between themethylene unit of the organic anion and the ligand pyridinering (with a hydrogen atom-to-centroid distance of 2.99 Å). Thearomatic ring of the phenylacetate is located above thes-tetrazine ring and the methylene group over the pyridinering (Fig. 10). The interaction energy for the binary complex is29.7 kcal mol21 (DE4), which is stronger than that expected forweak noncovalent p…p and C–H/p interactions. This surpris-ingly high value is most likely due to particularly favoured p…p

interactions ascribed to the electronic nature of the interactingrings (i.e. donor–acceptor interactions are taking placebetween the interacting partners). To estimate the contribu-tion of each interaction, the interaction energy of the p…p

Fig. 9 A) Model of a fragment of the structure of 1 used to evaluatetheoretically the bifurcated lp…p interactions; B) theoretical model used toassess the role played by the Mn ions in the lp…p interactions; C) model of afragment of the structure of 1 used to evaluate theoretically the S…Hinteractions.

Fig. 10 Models used to compute the interaction energies of the binary (DE4 andDE4

BEN) and ternary (DE5) complexes observed in the solid-state structure ofcompound 2. Values in italics correspond to energies computed using the MP2method.

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stacking complex between the pbptz ligand and benzene(instead of phenylacetate) located exactly at the same position(denoted as DE4

BEN) has been computed (see Fig. 10, bottom).DE4

BEN amounts to 27.9 kcal mol21; hence, the energydifference with DE4 can be used to appraise the contributionof C–H/p contacts, which is 21.8 kcal mol21. This estimationagrees very well with values reported for a number of C–H/psystems, i.e. in the range 1.5–2.5 kcal mol21.48 Comparison ofthe interaction energies DE4 and DE5 clearly indicates lack ofcooperativity in the ternary complex. In fact, the interactionenergy is additive since the value for the ternary complex istwice that of the binary complex (219.5 versus 29.7 kcalmol21).

A theoretical analysis has been also performed with thepbptz ligand coordinated to two manganese(II) ions. The otherligands bound to the metal centres (found in the crystalstructure) have been replaced by smaller molecules to keep thesize of the model system feasible computationally. Hence, thephenylacetate ligands have been changed to formates and thetwo external pbptz ligands have been replaced by eitherammonia or pyridine molecules (Fig. 11). Pyridine wasemployed in these particular systems to validate the use ofammonia; indeed, the real structure includes a p-ligand thatmay have some electronic influence on the metal. Actually, theinteraction energies are comparable (see bottom of Fig. 11) forboth ligands, i.e. the difference is ,0.6 kcal mol21; thusvalidating the utilization of the smaller molecule as atheoretical model. It is interesting to compare the interactionenergies gathered in Fig. 10 (DE4 and DE5; withoutmanganese(II) ions) with those shown in Fig. 11 where exactlythe same interactions are studied in the presence of the metalcentre. The interaction (DE6 = 213.0 kcal mol21) of the binarycomplex is clearly more favourable when the ligand iscoordinated to two metal ions. In fact, the coordination ofthe manganese(II) ions increases the p-acidity of the s-tetrazinering, therefore strengthening the p…p stacking interactions. Asignificant intensification of the interaction is also observedfor the ternary complex, since the DE7 interaction energy valueof 226.4 kcal mol21 is considerably more negative than theDE5 one (Fig. 10), computed for the ternary complex in theabsence of manganese(II) ions.

Finally, the Bader’s theory of ‘‘atoms in molecules’’ (AIM),33

which provides an unambiguous definition of chemicalbonding, has been used to further describe the noncovalentinteractions described above for compounds 1 and 2. The AIMtheory has been successfully applied to characterize and betterunderstand a great variety of interactions including thoseinvestigated herein. The representation of critical points andbond paths computed for the present assemblies are depictedin Fig. 12. For compound 1, each bifurcated lp–p interaction ischaracterized by the presence of two bond critical points thatconnect the sulfur atom of the SCN2 ligand to two carbonatoms of two pyridine rings (Fig. 12A). For 2, each p…p

interaction involving the central tetrazine ring of the pbptzligand and the phenylacetate is characterized by the presenceof two bond critical points connecting one carbon and onenitrogen atoms of the tetrazine ring to two carbon atoms of thephenyl ring. This interaction is further described by thepresence of two ring- and one cage-critical points. In addition,each C–H/p interaction is characterized by the presence of onebond-critical point that connects the hydrogen atom of themethylene group to one carbon atom of the pyridine ring(Fig. 12B).

The presence of the aforementioned critical points confirmthe presence of the noncovalent interactions in the solid-statestructures of compounds 1 and 2, which have been energeti-cally evaluated by DFT calculations; these supramolecularbonding interactions certainly influence the solid-state archi-tecture of the two coordination polymers.

Conclusions

In the present study, a s-tetrazine-based linear and ditopicligand has been used to generate manganese-containingcoordination networks. Hence, different anions, one inorganic(i.e. thiocyanate) and two organic (i.e. phenylacetate andbenzoate), have been used to investigate their potentialnoncovalent interaction with the ligand 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (pbptz) in the respective solid-state structuresobtained. As anticipated pbptz, which includes the electron-deficient tetrazine ring and two pyridine groups that becomeelectron-poor upon coordination, is involved in a number of pinteractions. For compound 1 with the NCS2 ion, the organictetrazine unit undergoes p…p interactions and the coordinatedpyridine rings are implicated in lone pair…p bonding contacts

Fig. 12 Distribution of the critical points in crystal fragment of A) compound 1and B) compound 2. Bond-, ring- and cage-critical points are represented by red,yellow and blue spheres, respectively. The bond paths are also indicated.

Fig. 11 Models used to compute the interaction energies of the binary (DE6)and ternary (DE7) complexes observed in the solid state of compound 2,including two coordinated manganese(II) ions. Two different model ligandshave been used to evaluate the interactions.

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with the inorganic anion. As anticipated, the organic anionscontaining phenyl moieties (namely phenyl acetate or benzo-ate), undergo p…p stacking interactions with the tetrazinering. Hence, the size of the anion (for instance NCS2 versusPHCOO2) obviously has an important role in the formation ofthe solid-state structures presented herein, but their nature,namely whether or not they contain an electron-rich phenylgroup, certainly has an influence on the network topology ofcoordination polymers built from the electron-deficient ligandpbptz and manganese(II) ions.

Acknowledgements

PS is thankful to UGC for Junior Research Fellowship (UGC/20/Jr. Fellow (Sc) dated 10.01.2011). PG acknowledges theInstitucio Catalana de Recerca I Estudis Avançats (ICREA).PG and AF acknowledge the Spanish Ministerio de Economıa yCompetitividad (MINECO) for funding (Projects CTQ2011-27929-C02-01 and CTQ2011-27512/BQU, and CONSOLIDERINGENIO 2010, CSD2010-00065), FEDER funds and theDireccio General de Recerca i Innovacio del Govern Balear(project 23/2011, FEDER funds). X-ray diffraction studies wereperformed at the DST-FIST, India-funded Single CrystalDiffractometer Facility at the Department of Chemistry,University of Calcutta.

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