Systematic Tuning of Zn(II) Frameworks with Furan, Thiophene, and

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Systematic Tuning of Zn(II) frameworks with Furan, Thiophene and Selenophene Dipyridyl and Dicarboxylate Ligands Carol Hua, and Deanna M. D'Alessandro Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00940 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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Crystal Growth & Design

Systematic Tuning of Zn(II) frameworks with Furan, Thiophene and Selenophene Dipyridyl and Dicarboxylate Ligands Carol Hua and Deanna M. D’Alessandro*

School of Chemistry, The University of Sydney, New South Wales 2006, Australia. Fax: +61 (2) 9351 3329; Tel: +61 (2) 9351 3777; E-mail: [email protected]

Abstract Twelve Zn(II) frameworks with furan, thiophene and selenophene dicarboxylate and dipyridyl ligands have been synthesised by solvothermal methods in both the presence and absence of water to investigate the subtle differences in structure and properties of the framework with changes in the linearity of the ligands. The large change in the ligand bend angle (10°) of the furan ring when compared to either thiophene or selenophene results in the formation of a number of frameworks with unexpected topologies. By using only ligands containing thiophene and selenophene, which differ in their ligand bend angle by only 2°, a series of four isostructural Zn(II) paddlewheel frameworks were obtained and their subtle differences investigated using UV/Vis/NIR spectroscopy and gas adsorption experiments with N2, H2 and CO2.

Introduction Porous coordination polymers (PCPs)1 and Metal-Organic Frameworks (MOFs)2 have been extensively investigated for application in a wide variety of areas ranging from gas storage and separation to use as sensors and in biomedical applications.3-4 Their versatility stems from the high degree of tunability available in the synthesis and design of MOFs which allows the properties of the material to be systematically optimised for a specific application. The linker used in the formation of MOFs plays a crucial role in controlling the porosity, chirality and interpenetration of the resulting network.5 Towards the development of smart functional materials,6 the incorporation of light and redox-active ligands has resulted in the formation of MOFs sensitive to light and electrochemical stimuli, such that multiple properties are able to be obtained within the same material.7-8 Specific properties within a series of reticular MOFs can be fine-tuned to a high degree by variation of the ligand used;5 changes in the length of a linear linker results in a corresponding change in pore size as demonstrated with the IRMOF series.9 While the variation of a linear linker can afford the synthesis of an isostructural series, modifying the linearity of a linker can result in significant changes

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in the topology of the resulting material such that unpredictable and unique networks are obtained.10 This can be attributed to the ability of small deviations of the ligand from linearity to be propagated and extended in supramolecular structures.11 In an elegant example, Fujita and co-workers demonstrated that the magnitude of deviation from linearity (defined as the “ligand bend angle”)12 of furan, pyrrole, thiophene and selenophene ligands has a significant impact upon the size of the nanoball formed.13-15 Additionally, changing the rigidity of the 2,5-dipyridyl heterocyclic ligand resulted in selective tuning of the size of the nanoball.13 We are therefore interested in investigating the changes in topology that occur upon variation of the ligand bend angle in a series of heterocyclic ligands. In the present work, a series of twelve Zn(II) structures have been obtained by combinations of 2,5-dipyridyl and 2,5-dicarboxylate furan, thiophene and selenophenes. Further, the effects of linearity of the ligand upon the resulting topology are explored in the presence and absence of water. The subtle changes in the porosity and spectral properties of a series of four isostructural frameworks using exclusively thiophene and selenophene containing ligands are correlated to both the ligand bend angle and the heteroatom present. Results and Discussion The series of twelve Zn(II) frameworks were synthesized by the reaction of the dicarboxylate ligand; 2,5-furan dicarboxylate (fdc), 2,5-thiophene dicarboxylate (tdc) or 2,5-selenophene dicarboxylate (SeDC) with a dipyridyl ligand; 2,5-dipyridyl furan (fdp), 2,5-dipyridyl thiophene (tdp) or 2,5dipyridyl selenophene (SeDP) in the presence of Zn(NO)2·6H2O at 80 °C in DMF or a mixture of DMF and H2O to yield the frameworks as colourless or yellow block crystals. A crystal structure of the SeDP ligand was obtained from the slow evaporation of a solution of the ligand in chloroform to yield large prismatic yellow crystals which crystallised in the tetragonal space group P42212 (ESI). Both of the pyridyl rings are slightly offset from being planar to the central selenophene ring; this rotation is also observed in Zn(II) frameworks containing the ligand (vide infra). The ligand bend angle, defined as the deviation of the ligand from linearity, in SeDP is 167.64°. There is about a ~2° variation in the ligand bend angle between analogous thiophene and selenophene ligands, but a significantly larger ~10° variation between furan and selenophene ligands (Table 2). The large ligand bend angle difference results in the formation of frameworks with unpredictable and significantly different topologies. Furan Containing Frameworks The furan containing frameworks (1, 1W, 2, 2W, 3W, 4W, 7) all exhibited different topologies and dimensionalities ranging from 1D chains to 3D structures. The furan containing frameworks were synthesised in either pure DMF or as a 1:1 mixture of DMF and water. All frameworks contained a


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1:1:1 ratio of the dipyridyl to dicarboxylate ligand to Zn(NO3)2·6H2O regardless of whether excess dicarboxylate ligand or Zn(NO3)2·6H2O was used in the synthesis. The [Zn3(fdc)4]n (1) framework was obtained by the reaction of fdc and fdp with Zn(NO3)·6H2O where the fdp ligand was excluded in the crystallisation. The structure of 1 was solved and refined in the monoclinic space group P21/c with unit cell parameters of a = 9.26680(10), b = 15.20770(10), c = 17.8002(2) Å, β = 96.1230(10)°. 1 displays a non-interpenetrated net with pcu topology with a Zn trimer centre serving as the 6-c node. All three Zn atoms in the Zn trimer display an octahedral coordination sphere with O-donors from the fdc ligand. Four fdc ligands form links between four different Zn trimers to yield a distorted 2D square grid (sql) structure which are then coordinated to πstacked fdc dimers that serve as the struts joining the 2D layers to form the 3D pcu structure. A very similar structure has previously been reported18 in the same space group but with subtle differences in the unit cell, a = 9.426(5), b = 15.424 (5), c = 17.359 (5) Å and β = 92.474 (5). Particularly pronounced is the 4° difference for the β angle; 96.1230(10)° for 1 and 92.474(5)° for the literature structure. The change in the β angle manifests due to the different orientation of carboxylates in the fdc ligand. In the literature structure, the oxygen atoms in the carboxylate group are orientated into the void and away from the central Zn of the Zn trimer, such that it results in two of the carboxylates in fdc binding in a monodentate fashion to the Zn centres. This contrasts with 1 where the oxygen atom from the carboxylate is orientated towards the central Zn in the Zn trimer. The different orientation of the carboxylate oxygen in 1 contributes to the formation of two adjacent O-Zn dative bonds in addition to the monodentate coordination with a bond length of 2.186 Å. A significant void space of 47.8% is found in 1 with channels down the b and c axes.

a)

b)

Figure 1. View of [Zn3(fdc)4]n (1) displaying the (a) Zn trimer node and (b) view down the b axis. When heating the fdc and fdp ligands together with Zn(NO3)2·6H2O in a 1:1 mixture of water and DMF, the fdp ligand was also excluded from the crystallization to yield [Zn(fdc)(H2O)3]n (1W). 1W



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was solved and refined in the same monoclinic space group P21/c as 1 with unit cell parameters of a = 22.09850(10), b = 7.26040(4), c = 22.1240(2) Å and β = 93.5560°. The asymmetric unit contained four crystallographically distinct Zn(II) atoms each containing a trigonal bipyramidal coordination sphere. Two O-donors from distinct fdc ligands are bound in the equatorial plane, with a water molecule taking the third donor site. Coordinated to the axial positions of the Zn(II) were a further two water molecules such that each Zn(II) centre serves as a 2-c node, which is contrast to the 6-c node in 1. The carboxylate groups in the fdc ligands are bound in a monodentate fashion with each of the two O-donors bound to different Zn(II) centres. In contrast to significant void space of 1, 1W consists of a series of 2D nets that are densely stacked upon each other (Figure 2b) with consequently, negligible accessible pore space. The closely packed structure of the framework can also be observed down the a axis (Figure 2c). A very similar structure has previously been reported in the monoclinic space group Pn with unit cell parameters of a = 22.1775(5), b = 7.3566(2), c = 22.1781(5) Å, β = 94.089(1)°.19 The subtle difference between the two structures arises predominantly from the presence of an additional network and 8 crystallographically distinct Zn sites in the literature structure within the unit cell. In one of these networks, the mode of coordination of the fdc ligand to Zn1 and Zn2 differs from that of 1W where Zn1 has a longer distance of 2.588(1) Å to O18. In the literature structure, the carboxylate is bound to Zn1 in a bidentate manner with a bond length of 2.559(7) Å. A similar trend is observed with coordination of Zn2 to O18 in the opposite manner with bidentate coordination of fdc to Zn2, resulting in a bond distance of 2.564(1) Å, whilst in the literature structure, O18 has a longer distance of 2.582(9) Å to Zn2.


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a)

c)

b)

Figure 2. View of [Zn(fdc)(H2O)3]n (1W) displaying the (a) asymmetric unit, (b) view down the a axis and (c) view down the b axis. The [Zn2(fdp)2(tdc)2]n (2) framework was solved and refined in the monoclinic C2/c space group with unit cell parameters of a = 15.947(3), b = 13.343(3), c = 21.423(4) Å and β = 92.60°. The asymmetric unit of 2 consists of one tdc and one fdp ligand along with the Zn(II) centre (Figure 3a). The Zn(II) centre exhibits a trigonal pyramidal coordination sphere with three oxygen donors from three tdc ligands bound in a monodentate fashion in the equatorial plane, whilst two nitrogen donors from two fdp ligands occupy the axial positions (Figure 3d). The 3D non-interpenetrated network is made up of a 2D network containing the Zn(II) centres linked in a metallocycle with two fdc ligands in a monodentate manner which is then extended into a 3D network through coordination of the fdp ligand to the metal nodes (Figure 3b). Framework 2 exhibits cds topology with a point symbol of {65.8} due to the 4-c node of the Zn dimer formed by the bridging O-donors of the tdc ligand. Only one of the oxygen donors on each of the fdc ligands is coordinated to the Zn(II), with the other oxygen donor remaining uncoordinated. The accessible channels through the structure as viewed down the c axis result in a 23% accessible void space.



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b)

a)

c)

d)

Figure 3. View of [Zn2(fdp)2(tdc)2]n (2) displaying the (a) asymmetric unit, (b) view down the b axis, (c) view down the c axis and (d) coordination around the Zn metal centre.

[Zn2(fdp)2(tdc)2·H2O]n (2W), synthesised in 1:1 mixture of DMF and water was solved and refined in the orthorhombic Pbca space group with unit cell parameters of a = 14.62700(10), b = 13.14910(10), c = 19.65630(10). The asymmetric unit features one fdp and one tdc ligand coordinated to the Zn(II) centre with one associated water molecule. The Zn(II) atom exhibits a distorted trigonal bipyramidal coordination with one tdc bound in a bidentate manner, another tdc bound in a monodentate manner in addition to two N-donors from distinct fdp ligands. The fdp ligand in the framework features a slight rotation of the pyridyl rings with respect to the central heterocyclic ring. The framework consists of 2D sheets that appear as zig-zag undulating layers when viewed down the b axis. Each of the discrete nets are stacked upon each other and consist of a 4-c net with sql topology. In contrast to 2, there is only a 2% accessible void space available in the structure, which is due to the dense packing of the 2D


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layers. The addition of water to the reaction mixture of 2W resulted in the synthesis of a denser structure with lower dimensionality when compared to 2.

a)

b)

c)

Figure 4. View of the [Zn2(fdp)2(tdc)2·H2O]n (2W) framework where showing the (a) asymmetric unit, (b) view down the a axis and (c) view down the b axis. [Zn2(fdp)2(SeDC)2]n (3W) was synthesised by heating a 1:1 ratio of the dipyridyl and dicarboxylate ligands with Zn(NO3)2·6H2O in a 1:1 mixture of DMF and water. The structure was solved and refined in the orthorhombic Pbca space group with unit cell parameters of a = 14.7042(2), b = 13.1647(2), c = 19.7355(3). The asymmetric unit features one fdp and one SeDC ligand coordinated to the Zn(II) centre with one water molecule. The dipyridyl ligands feature a slight rotation of the pyridyl rings with respect to the central heterocyclic ring.



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The 3W framework consists of 2D sheets that appear as zig-zag undulating layers when viewed down the b axis. Each of the nets are stacked upon each other and consist of binodal (4,4)-c nets with two dicarboxylate and two dipyridyl ligands coordinated to 4 Zn(II) centres. A topos and systre analysis indicates that this is a new topology with 8 nodes and 16 edges and a point symbol of {54.68}{55.6}. The space group for the net is the orthorhombic P2mm which differs from the crystallographic space group of triclinic P1 due to the stacking of the 2D net. Channels through the framework can be viewed down the a axis, resulting in an accessible pore volume of 23%.

a)

b)

c)

d)

Figure 5. View of [Zn2(fdp)2(SeDC)2]n (3W) displaying the (a) asymmetric unit, (b) view down the a axis, (c) 2D network and (d) schematic of the network viewed down the b axis. The [Zn(tdp)(fdc)]n (4W) framework was synthesised by heating a 1:1 ratio of the tdp and fdc ligands with Zn(NO3)2·6H2O in a 1:1 mixture of DMF and water to yield yellow platelets. The structure was solved and refined in the monoclinic C2/c space group with unit cell parameters of a = 38.0359(7), b = 5.76550(10), c = 20.2957(5) Å and β = 103.455(2)°. The asymmetric unit of 4W contains one tdp and fdc ligand each coordinated to one tetrahedral Zn(II) centre. The framework consists of 1D ladders with 3-c nodes and a point symbol of {42.6} due to the coordination of a water molecule to the


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Zn(II) centre. The 1D ladders π-stack with each other when viewed down the a axis to yield a dense structure with 27% accessible pore space.

b)

a)

c)

Figure 6. View of the [Zn(tdp)(fdc)]n (4W) framework where showing the (a) asymmetric unit, (b) view down the b axis and (c) view down the a axis. [Zn2(SeDP)2(fdc)2]n (7) crystallised in the orthorhombic space group, Pnma with unit cell parameters of a = 17.124(3), b = 30.169(6) and c = 14.903(3) Å. The asymmetric unit of 7 consists of one SeDP and one fdc ligand along with one Zn(II) centre (Figure 7a). The Zn(II) centre features a distorted octahedral coordination sphere with two nitrogen donors from two distinct SeDP ligands and four oxygen donors from three fdc ligands. Two of the oxygen donors are bound in a monodentate fashion (from two distinct fdc ligands), whilst the third fdc ligand is bound in a bidentate fashion, resulting in the distortion from ideal angles about the octahedral Zn(II) (O1-Zn1-O2 = 115.94°, O1-Zn1-O4 = 92.90°, O2-Zn1-O3 = 92.34° and O3-Zn1-O4 = 58.76°). Significantly, in contrast to the isostructural Zn paddlewheel frameworks 5, 6, 8, 9 (vide infra), the [Zn2(SeDP)2(fdc)2]n (7) framework contains a single non-interpenetrated pcu 3D network. The 3D framework consists of a 2D layer with the Zn(II) centres linked by the fdc ligands as viewed down the b axis (Figure 7c). The 2D layer features a metallocycle with two Zn(II) centres linked by fdc ligands where the carboxylate moiety is bound in a monodentate fashion to each Zn(II) centre. The 3D network is formed by the SeDP ligand acting as pillars between the 2D networks, with π-stacking observed between the phenyl and selenophene rings



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in adjacent SeDP ligands which can be viewed down the c axis (Figure 7d). In accordance with the 3D non-interpenetrated network of 7, a high pore accessible volume of 56.5% was obtained. The framework exhibits a slow mass loss in the TGA, likely due to the evaporation of solvent from the pores, before decomposing at ~250 °C (Figure S5b, ESI).


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a)

b)

d) c)

Figure 7. View of the [Zn2(SeDP)2(fdc)2]n (7) framework where showing the (a) asymmetric unit, (b) view down the a axis, (c) view down the b axis and (d) view down the c axis.



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Effect of Adding Water to the Reaction Mixture The effect of the presence of water in the reaction mixture resulted in overall lower coordination geometries for the Zn(II) ion in the furan containing frameworks 1W and 2W when compared to the frameworks synthesised purely in DMF (1 and 2, respectively). It is well known that heating DMF and H2O results hydrolysis of the DMF to yield formate and dimethylamine.16 Formate has been commonly used in the synthesis of framework materials such as UiO-66 as a modulator to slow down the rate of crystallisation, yield materials with higher crystallinity and control the growth of defects.17 It is likely that the competitive coordination of the formate to the Zn(II) node plays a direct role in the formation of alternate structure topologies in the presence of water. The presence of water appears to inhibit the formation of metal dimers and trimers as the nodes in these furan containing networks, where distinct metal ions are more commonly observed. Notably, the addition of water to the reaction mixture of the thiophene containing ligands, tdp and tdc with Zn(NO3)2·6H2O resulted in the formation of [Zn(tdp)(tdc)·H2O]n (5W) which did not follow the trends observed for the addition of water to the furan containing frameworks (Figure 8). Framework 5W was crystallised in the orthorhombic space group Pnna with unit cell parameters of a = 17.8908(3), b = 18.2373(3) and c = 29.3112(7) Å. There are two types of Zn nodes present; one is a Zn dimer which forms a 8-c node, whilst the other is a single Zn atom that forms the 4-c node to yield the (4,8)-c flu topology. The 8-c and 4-c nodes alternate along the struts of the framework. Framework 5W is twofold interpenetrated with π-stacking occurring between the aromatic π systems to yield offset interpenetration in the two networks.


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b)

a)

c)

d)

Figure 8. View of [Zn2(tdp)2(tdc)2]n (5W) where showing the (a) asymmetric unit, (b) 8-c Zn node (c) view down the a axis and (d) view down the b axis.



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Table 1. Topology and Properties of the frameworks in this study Formula

Space group

Topology

Node

Point symbol

% Void Space

Dimensionality

Interpenetration

1

[Zn3(fdc)4]n

P21/c

pcu

6-c

412.63

47.8

3D

No

2

[Zn2(fdp)2(tdc)2]n

C2/c

cds

4-c

65.8

23.0

3D

No

5

[Zn2(tdp)(tdc)2]n

C2/c

pcu

6-c

412.63

28.4

3D

Yes – 2 fold

6

[Zn2(tdp)(SeDC)2]n

C2/c

pcu

6-c

412.63

31.4

3D

Yes – 2 fold

7

[Zn2(SeDP)2(fdc)2]n

Pnma

pcu

6-c

412.63

56.5

3D

No

8

[Zn2(SeDP)(tdc)2]n

C2/c

pcu

6-c

412.63

27.1

3D

Yes – 2 fold

9

[Zn2(SeDP)(SeDC)2]n

C2/c

pcu

6-c

412.63

32.6

3D

Yes – 2 fold

1W

[Zn(fdc)(H2O)3]n

P21/c

-

-

-

0

3D

No

2W

[Zn2(fdp)2(tdc)2·H2O]n

Pbca

sql

4-c

44.62

2.0

2D

No

3W

[Zn2(fdp)2(SeDC)2]n

Pbca

new

(4,4)-c

{54.68}{55.6}

23.0

2D

No

4W

[Zn2(fdc)2(tdp)2]n

C2/c

new

3-c

42.6

27.0

1D

No

5W

[Zn2(tdp)2(tdc)2]n

Pnna

flu

(4,8)-c

{412.612.84}{46}2

22.2

3D

Yes – 2 fold


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Table 2. Schematic of frameworks obtained in this study.

(164.35°)

(154.26°)

(166.44°)

(1) Non-interpentrating 3D

(2) Non-interpenetrating 3D

(3W) Non-interpenetrating 3D

uninodal 6-c net (pcu)

uninodal 4-c net (cds)

binodal (4,4)-c net (new topology)

(1W) 1D chain with Zn

(2W) Layers (0 0 1) with Zn,

(153.11°)

uninodal 4-c net (sql) (4W) Chains (0 0 1) with Zn,

(5) Two 3D interpenetrating nets,


uninodal 3-c net (new topology)



(5W) Two 3D interpenetrating nets,

(165.13°)

binodal (4,8)-c net (flu) (7) Non-interpenetrating 3D



unimodal 6-c net (pcu)



(167.64°)



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Isostructural Frameworks with Thiophene and Selenophene In contrast to the many different topologies obtained using furan containing ligands, a series of four isostructural frameworks with thiophene (tdc, tdp) and selenophene (SeDC, SeDP) were obtained. This observation can be attributed to the small ~2° variation in the ligand bend angle between the thiophene and selenophene ligands when compared to the significantly larger 10° variation in the ligand bend angle for the furan ligands. The four isostructural frameworks [Zn2(tdp)(tdc)2]n (5), [Zn2(tdp)(SeDC)2]n (6), [Zn2(SeDP)(tdc)2]n (8) and [Zn2(SeDP)(SeDC)2]n (9) were synthesised by heating the respective ligands with Zn(NO3)2·6H2O in DMF at 80 °C to yield yellow block crystals. The asymmetric unit consists of two dicarboxylate ligands and one dipyridyl ligand, where two Zn(II) centres are coordinated in a bridging monodentate manner to Zn atoms from each dicarboxylate ligand (Figure 9). Disorder due to the rotation of the heterocycles or the pyridyl ring was observed in several of the framework structures. The four isostructural Zn paddlewheel frameworks all crystallise in the monoclinic space group C2/c with pcu topology and are two-fold interpenetrated with a 6-c Zn paddlewheel node. A 2D sql layer consisting of Zn(II) and dicarboxylate ligands is linked by the dipyridyl ligands acting as pillars to yield the 3D pcu structure (Figure 9b). Due to the non-linear nature of the ligands used the layers appear slanted, which is evident down the c axis (Figure 9c). Each of the frameworks consists of two interpenetrating networks (Figure 9d). The void accessible volume was calculated to be 28.4% for 5, 31.4% for 6, 27.1% for 8 and 32.6% for 9. The ligand bend angle of the carboxylate ligand has a larger effect on the amount of accessible pore space than the bond angle of the dipyridyl ligand. Comparing frameworks with the same dicarboxylate ligand, 5 (28.4%) and 8 (27.1%), the difference in the pore accessible volume is minimal (1.2 to 1.3%). When pairs of framework containing the same dipyridyl ligand but different dicarboxylate ligands are compared, for example, 5 (28.4%) and 6 (31.4%), there is a significantly larger 3-5.5% difference in the accessible pore space. This difference in accessible pore space between the four frameworks is reflected in the %weight loss in the thermal gravimetric analysis (TGA). At 200 °C, the two frameworks with a higher accessible pore volume, 6 and 9 exhibit mass losses of ~16% while the two frameworks with a lower accessible pore volume, 5 and 8 exhibit a mass loss of ~13%.


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a)

b)

c)

d)

Figure 9. View of [Zn2(SeDP)(SeDC)2]n (9) isostructural with [Zn2(SeDP)(tdc)2]n (8), [Zn2(tdp)(SeDC)2]n (6) and [Zn2(tdp)(tdc)2]n (5) showing the (a) asymmetric unit, (b) view down the b axis, (c) view down the c axis and (d) the two interpenetrating networks.

Spectral Properties The solid-state UV/Vis/NIR spectrum of the frameworks were obtained over the range 500040000 cm-1. Variation in the dipyridyl ligand from furan to thiophene to selenophene (with a common dicarboxylate ligand, tdc) resulted in the lowest energy band exhibiting a red shift from 26000 cm-1 (fdp, 7) to 23600 cm-1 (tdp, 8) and finally to 22500 cm-1 (SeDP, 9) (Figure 10a). The shift in the lowest energy band can be attributed to the greater orbital delocalisation present on the heteroatom from oxygen to sulfur to selenium. A similar trend is observed when the dicarboxylate ligand is varied from fdc to tdc to SeDC with a common dipyridyl ligand (SeDP) (Figure 10b).



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Comparison of the UV/Vis/NIR spectra of the four isostructural frameworks 5, 6, 8 and 9 revealed a larger red shift in the low energy band at ~18000 cm-1 with increasing the amounts of selenophene in the framework (Figure 10c). Changing the pyridyl ligand from tdp to SeDP had a greater impact upon the extent of the red shift than a change in the dicarboxylate ligand which is presumably due to the greater extent of orbital overlap of the heterocycle to an aromatic pyridyl group resulting in a lower energy transition. The bands >22500 cm-1 are assigned predominantly to the π to π* transitions of the aromatic rings in the framework.


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a)

b)

c)

Figure 10. Solid state UV/Vis/NIR of Zn(II) paddlewheel frameworks with (a) the tdc ligand, (b) SeDP ligand and (c) the isostructural frameworks 5, 6, 8, 9 over the range 5000 - 40000 cm-1.



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Gas sorption The N2 isotherms at 77 K of all the isostructural frameworks 5, 6, 8 and 9 exhibited a type I isotherm. The step in the isotherm upon desorption at 540 mbar (0.52 p/p0) may be indicative of movement of the two interpenetrated nets relative to each other (Figure 11a). Each of the frameworks exhibit pore sizes of between 5-6 Å and 6.5-8 Å (ESI). The small differences in accessible pore volume (calculated from the structure) and solvent loss in the TGA resulted in marked changes in the gas uptake of the frameworks. [Zn2(tdp)(tdc)2]n (5) and [Zn2(SeDP)(tdc)2]n (8) had the highest accessible pore volumes and correlate to the two highest BET surface areas obtained for this series, 540 and 620 m2/g respectively (Table 3). The two frameworks with lower accessible pore volumes, [Zn2(tdp)(SeDC)2]n (7) and [Zn2(SeDP)(SeDC)2]n (9) resulted in significantly lower BET surface areas of 170 and 220 m2/g respectively. Changing the dipyridyl ligand in the frameworks with a common dicarboxylate ligand from thiophene to selenophene results in an increase in the BET surface area (e.g. 5 vs. 8 and 6 vs. 9), whilst changing the dicarboxylate ligand in the frameworks with a common dipyridyl ligand results in a relatively larger lowering of the BET surface area (e.g. 5 vs. 6 and 8 vs. 9). The frameworks 5, 6, 8, and 9 can be considered to contain two pore shapes, formed by the bend of the heterocyclic ligands. The smaller cavity is formed by the dipyridyl ligands curving inwards, whilst the larger cavity is formed by the dipyridyl ligands curving outwards (Figure 11d). Changing the dicarboxylate ligand results in a larger change in the diameters of the pores in the network due to the shorter length of the ligand when compared to the dipyridyl ligand. A shorter ligand length means that a minor change in the ligand bend angle will propagate to yield a larger change overall throughout the network. Replacing tdc with SeDC in the frameworks with a common dipyridyl ligand results in an increase in the width of the large cavity, presumably allowing greater freedom of movement of the interpenetrated networks relative to each other resulting in a reduction of the BET surface area. Replacing tdp with SeDP in the frameworks with a common dicarboxylate ligand results in a much smaller change in the size of both the small and larger cavities, resulting in only a slight opening of the cavities, which is presumably not enough to allow free movement of the interpenetrated networks. Correspondingly, only a small increase in the BET surface area is observed. The uptake of H2 at 77 K for 5, 6, 8, 9 exhibited a similar trend to the uptake of N2, where [Zn2(SeDP)(tdc)2]n (8) exhibited the highest uptake (5.92 mmol/g at 1200 mbar), followed by [Zn2(tdp)(tdc)2]n (5) (5.40 mmol/g), [Zn2(SeDP)(SeDC)2]n (9) (3.59 mmol/g) and [Zn2(tdp)(SeDC)2]n (2.10 mmol/g) (6) (Figure 11b). The uptake in H2 appears to be dependent firstly upon the surface area of the framework and secondly, upon the quantity of selenium present. The surface areas of [Zn2(SeDP)(SeDC)2]n and [Zn2(tdp)(SeDC)2]n as determined from the N2 isotherm are comparable, however the amount of H2 taken up by these frameworks exhibit a significant variation (3.59 mmol/g for [Zn2(SeDP)(SeDC)2]n and 2.10 mmol/g for [Zn2(tdp)(SeDC)2]n). This implies that there may be a


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favourable interaction between selenium and H2, possibility due to the more diffuse orbitals of the selenium atom. The four isostructural frameworks exhibit a moderate uptake of between 1.5 and 2.5 mmol/g (at 1200 mbar) of CO2 at 298 K with a similar trend to that for N2 and H2 (Figure 11c). In all cases, significant hysteresis was observed, which may be indicative of slow adsorption kinetics and/or movement of the two interpenetrating networks relative to each other. Table 3. BET Surface area, crystallographic density and void accessible volume for the isostructural frameworks, 5, 6, 8 and 9. Framework

BET Surface 2

Area (m /g)

Crystallographic 3

Density (cm /g)

Void Accessible Volume (%)

[Zn2(tdp)(tdc)2]n (5)

540

1.288

28.4

[Zn2(tdp)(SeDC)2]n (6)

170

1.421

31.4

[Zn2(SeDP)(tdc)2]n (8) c) [Zn2(SeDP)(SeDC)2]n (9)

620

1.364

27.1

220

1.481

32.6



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a)

c)

b)

d)

Figure 11. Gas sorption of the four isostructural frameworks, [Zn2(tdp)(tdc)2]n (5), [Zn2(SeDP)(tdc)2]n (8), [Zn2(tdp)(SeDC)2]n (6), [Zn2(SeDP)(SeDC)2]n (9) with (a) N2 at 77 K, (b) H2 at 77 K and (c) CO2 at 298 K where the filled symbols indicate adsorption and the hollow symbols indicate desorption, (d) schematic of the two different pore shapes found in frameworks 5, 6, 8 and 9, where the dipyridyl ligands are represented in blue and the dicarboxylate ligands in red.


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Conclusions Twelve Zn(II) coordination networks with pyridyl and carboxylate furan, thiophene and selenophene ligands have been synthesised at elevated temperatures in DMF in the presence and absence of water. The addition of water to the reaction mixture yielded generally lower connectivity nodes and lower dimensionality frameworks. The similar linear bond angles of thiophene and selenophene afforded the synthesis of a series of isostructural Zn paddlewheel frameworks, whilst the frameworks containing furan, which was significantly more bent, yielded unpredictable topologies and structures. Through gas adsorption measurements on the series of isostructural frameworks, it was found that even small changes in the pore accessible space resulted in markedly different porosities. This study of materials containing furan, thiophene and selenophene highlights the critical role that the linearity of a ligand plays in dictating the topology and porosity of the resulting framework.



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Experimental All chemicals and solvents were used as obtained without further purification unless stated. DMF was dried over activated CaSO4. Pd(PPh3)420 was synthesised according to literature procedures. Ligand syntheses for SeDP, tdp, fdp and SeDC are given in the ESI. Solution state 1H and

13

C{1H} NMR

spectra were recorded on either a Bruker AVANCE200 or Bruker AVANCE500 operating at 200, 500 MHz for 1H and 50, 125 MHz for 13C respectively. 1H and 13C NMR chemical shifts were referenced internally to residual solvent resonances. Spectra were recorded at 298 K and chemical shifts (δ), with uncertainties of ± 0.01 Hz for 1H and ± 0.05 Hz for 13C are quoted in ppm. Coupling constants (J) are quoted in Hz and have uncertainties of ± 0.05 Hz for 1H-1H. Deuterated solvents were obtained from Cambridge Stable Isotopes and used as received. Mass spectrometry was carried out at the Mass spectrometry analysis facility at the University of Sydney on a Bruker amaZon SL mass spectrometer. Microanalyses were carried out at the Chemical Analysis Facility – Elemental Analysis Service in the Department of Chemistry and Biomolecular Science at Macquarie University, Australia. Framework syntheses Synthesis of Frameworks in DMF (1, 2, 5-9). The dicarboxylic acid (0.02 mmol), dipyridyl ligand (0.01 mmol) and Zn(NO3)2·6H2O (6.0 mg, 0.02 mmol) were dissolved in DMF (2.0 mL) then heated at 0.1 °C/min to 90 °C for 48 hours then cooled at 0.1 °C/min to room temperature to yield the formation of yellow prismatic crystals. Synthesis of Frameworks in DMF/H2O (1W, 2W, 3W, 4W and 5W). The dicarboxylic acid (0.01 mmol), dipyridyl ligand (0.01 mmol) and Zn(NO3)2·6H2O (3.0 mg, 0.01 mmol) were dissolved in a mixture of DMF (0.5 mL) and water (0.5 mL) then heated at 0.1 °C/min to 90 °C for 48 hours then cooled at 0.1 °C/min to room temperature to yield the formation of yellow prismatic crystals. Physical Characterisation and Instrumentation Crystallography. In house source: The crystal was mounted on a SuperNova, Dual Atlas diffractometer employing mirror monochromated CuKα radiation generated from a sealed X-ray tube. Data was collected at 150(2) Kelvin with ω scans and subsequent computations carried out with the WinGX21 graphical user interface. Synchrotron: The crystal was mounted on a ADSC Quantum 315r diffractometer employing Silicon Double Crystal monochromated Synchrotron radiation generated from the MX1 Beamline at the Australian Synchrotron in a stream of nitrogen at 100 K where the Blu-Ice graphical interface was used.22 The structures were solved by direct methods using SHELXT,23 (1, 1W, 2, 2W, 3W, 4W, 7) or SIR9724 (frameworks 5, 6, 8, 9) and extended and refined with SHELXL-2014/7.25 The non-hydrogen atoms in the asymmetric unit were modelled with anisotropic displacement parameters. A riding atom model with group displacement parameters was used for the hydrogen atoms.


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Thermal Gravimetric Analysis (TGA). TGA was performed under a flow of nitrogen (0.1 L/min) on a TA Instruments Discovery Thermogravimetric Analyser from 40-600 °C at 2 °C/min. Solid state UV/Vis/NIR spectroscopy. UV/Vis/NIR spectra were obtained on the samples at room temperature using a CARY5000 Spectrophotometer equipped with a Harrick Praying Mantis accessory over the wavenumber range 5000-40000 cm-1. BaSO4 was used for the baseline. Spectra are reported as the Kubelka-Munk transform, where F(R) = (1−R)2/2R (R is the diffuse reflectance of the sample as compared to BaSO4). Powder X-ray Diffraction (PXRD). PXRD data were obtained using a PANanalytical X’Pert PRO Multi-Purpose Diffractometer producing CuKα (λ = 1.5406 Å) radiation, equipped with a solid-state PIXcel detector. Samples were collected at a rate of 0.028° min-1 over the interval 5 ≤ 2θ ≤ 60° with a step size of 0.013°. Powder pattern simulations from SCXRD data were generated using the program Mercury (version 3.3).26 Gas sorption. Gas sorption measurements over the 0-1.0 bar range were conducted using a Micromeritics 3-Flex instrument. The DMF in the pores of the [Zn2(tdp)(tdc)2]n and [Zn2(SeDP)(tdc)2]n frameworks were removed by solvent exchange with MeOH, while a supercritical CO2 wash was conducted for the [Zn2(tdp)(SeDC)2]n and [Zn2(SeDP)(SeDC)2]n frameworks. Approximately 50-100 mg of the solvent exchanged or supercritical CO2 washed samples were degassed under vacuum at 50 °C (supercritical CO2) or 80 °C (MeOH) for approximately 20 hours. Nitrogen adsorption isotherms were measured at 77 K via the incremental dosing of nitrogen from 0-1 bar, and surface areas determined via the BET method using the 3-Flex Microactive software. CO2 isotherms at 298 were measured on the same instrument, with the temperature controlled by a Julabo F25 Circulating Heating and Cooling bath. An estimation of the porosity distribution in the frameworks was obtained using the Tarazona Non-Local Density Functional Theory (with cylindrical pores) implemented within the Micromeritics software (Version 4.0). Supplementary Information The Supporting Information is available free of charge on the ACS Publications website. Ligand syntheses, crystal data and refinement information, powder X-ray diffraction (PXRD) and thermal gravimetric analysis (TGA) data. Accession Codes CCDC 1556850-1556862 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.



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Acknowledgements We gratefully acknowledge support from the Australian Research Council and the Australian Institute of Nanoscale Science and Technology (AINST) at the University of Sydney. Structures [Zn2(fdp)2(tdc)2]n (2) and [Zn2(SeDP)2(fdc)2]n (7) were collected on the MX1 beamline at the Australian Synchrotron, Victoria, Australia. Abbreviations fdp, 2,5-Dipyridylfuran; tdp, 2,5-dipyridylfuran; SeDP, 2,5-dipyridylselenophene, fdc, 2,5 furan dicarboxylic acid; tdc, 2,5-thiophene dicarboxylic acid; SeDC, 2,5-selenophene dicarboxylic acid. References 1. Batten, S. R.; Neville, S. M.; Turner, D. R., Coordination Polymers: Design, Analysis and Application. Royal Society of Chemistry: 2009. 2. MacGillivray, L. R.; Lukehart, C. M., Metal-Organic Framework Materials. John Wiley & Sons: 2014; p 592. 3. Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M., Science 2013, 341 (6149). 4. Pettinari, C.; Marchetti, F.; Mosca, N.; Tosi, G.; Drozdov, A., Polym. Int. 2017, 66, 731-744. 5. Lu, W.; Wei, Z.; Gu, Z.-Y.; Liu, T.-F.; Park, J.; Park, J.; Tian, J.; Zhang, M.; Zhang, Q.; Gentle Iii, T.; Bosch, M.; Zhou, H.-C., Chem. Soc. Rev. 2014, 43, 5561-5593. 6. Coudert, F.-X., Chem. Mater. 2015, 27, 1905-1916. 7. D'Alessandro, D. M., Chem. Commun. 2016, 52, 8957-8971. 8. Nagarkar, S. S.; Desai, A. V.; Ghosh, S. K., Chem.–Asian J. 2014, 9, 2358-2376. 9. Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi, O. M., Science 2002, 295, 469-472. 10. Guillerm, V.; Kim, D.; Eubank, J. F.; Luebke, R.; Liu, X.; Adil, K.; Lah, M. S.; Eddaoudi, M., Chem. Soc. Rev. 2014, 43, 6141-6172. 11. Smulders, M. M. J.; Riddell, I. A.; Browne, C.; Nitschke, J. R., Chem. Soc. Rev. 2013, 42, 1728-1754. 12. Harris, K.; Fujita, D.; Fujita, M., Chem. Commun. 2013, 49, 6703-6712. 13. Fujita, D.; Ueda, Y.; Sato, S.; Yokoyama, H.; Mizuno, N.; Kumasaka, T.; Fujita, M., Chem 2016, 1, 91-101. 14. Tominaga, M.; Suzuki, K.; Kawano, M.; Kusukawa, T.; Ozeki, T.; Sakamoto, S.; Yamaguchi, K.; Fujita, M., Angew. Chem. Int. Ed. 2004, 43, 5621-5625. 15. Bunzen, J.; Iwasa, J.; Bonakdarzadeh, P.; Numata, E.; Rissanen, K.; Sato, S.; Fujita, M., Angew. Chem. Int. Ed. 2012, 51, 3161-3163. 16. Buncel, E.; Symons, E. A., J. Chem. Soc. D: Chem. Commun. 1970, 164-165. 17. Gutov, O. V.; Hevia, M. G.; Escudero-Adán, E. C.; Shafir, A., Inorg. Chem. 2015, 54, 83968400. 18. Nagarkar, S. S.; Chaudhari, A. K.; Ghosh, S. K., Cryst. Growth Des. 2012, 12, 572-576. 19. Sen, R.; Mal, D.; Brandao, P.; Rogez, G.; Lin, Z., CrystEngComm 2013, 15, 2113-2119. 20. Coulson, D. R.; Satek, L. C.; Grim, S. O., Tetrakis (Triphenylphosphine) Palladium (0). In Inorg. Synth., John Wiley & Sons, Inc.: 2007; pp 121-124. 21. Farrugia, L., J. Appl. Crystallogr. 1999, 32, 837-838. 22. McPhillips, T. M.; McPhillips, S. E.; Chiu, H.-J.; Cohen, A. E.; Deacon, A. M.; Ellis, P. J.; Garman, E.; Gonzalez, A.; Sauter, N. K.; Phizackerley, R. P.; Soltis, S. M.; Kuhn, P., J. Synchrotron Radiat. 2002, 9, 401-406. 23. Sheldrick, G., Acta. Crystallogr. Sect. A 2015, 71, 3-8. 24. Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M., J. Appl. Crystallogr. 1994, 27, 435-435.


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25. Sheldrick, G. M., Acta Crystallogr., Sect. A: Found. Crystallogr. 2015, A71, 3-8. 26. Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.; Taylor, R.; Towler, M.; van de Streek, J., J. Appl. Crystallogr. 2006, 39, 453-457.



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For Table of Contents Use Only Systematic Tuning of Zn(II) frameworks with Furan, Thiophene and Selenophene Dipyridyl and Dicarboxylate Ligands Carol Hua and Deanna M. D’Alessandro*

TOC Graphic

Synopsis Twelve Zn(II) frameworks with furan, thiophene and selenophene dicarboxylate and dipyridyl ligands have been synthesised by solvothermal methods in both the presence and absence of water to investigate the subtle differences in structure and properties of the framework with variation in the ligand bend angle.


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Systematic Tuning of Zn(II) Frameworks with Furan, Thiophene, and

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