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Robust hydrogen bonding synthon in one-dimensional and two-dimensionalcoordination polymers of pyridine-appended reverse amides and amides†

Lalit Rajput and Kumar Biradha*

Received 30th March 2009, Accepted 15th April 2009

First published as an Advance Article on the web 11th May 2009

DOI: 10.1039/b906285h

Two dimensional iso-structurality and control was achieved in

coordination polymers by carefully selecting the ligands that contain

two types of bis-amido pyridines which have different connectivities.

These networks were found to include a variety of large aromatic

guest molecules.

Table 1 Geometrical parameters for N–H/O hydrogen bonding in thecrystal structures of 3–5

Intra-network Inter-network

N/O/A N–H/O/� N/O/A N–H/O/�

3a 2.950(9) 159 2.939(9) 1563b 2.936(7) 162 2.899(7) 1563c 3.153(3) 163 2.884(3) 1463d 3.056(3) 169 2.848(3) 1474a 3.022(6) 163 2.969(6) 1694b 3.000(4) 164 2.953(4) 1635 2.911(3) 156 — —

3.063(3) 143 — —

Establishing vigorous connections between molecular and supra-

molecular structures via non-covalent interactions is of importance

for synthesizing new materials with predefined properties.1,2 The

perfect solution for this exercise is the identification of a robust

synthon which is transferable from one crystal structure to another.

In this regard we have recently explored homologous series of

compounds containing amide and pyridine functionalities (1, amides

and 2, reverse amides) which have the same combination of func-

tional groups at the molecular level but behave very differently at the

supramolecular level.3 The pyridine group showed interference in

amide-to-amide hydrogen bonds and disrupted N–H/O hydrogen

bonding in the analogues of 2 while no such interference was

observed in analogues of 1. However, we felt that the coordination

networks of 1 and 2 should be iso-structural as there exists no free

pyridine moieties for interference due to their coordination with the

metal. To prove this hypothesis we have prepared coordination

polymers of 1 and 2 with Cu(II) under similar conditions. In this

communication we would like to present how the weak interactions

between the spacers provide the iso-structurality between coordina-

tion polymers of 1 and 2 and also the two-dimensional iso-structur-

ality of N–H/O hydrogen bonds between one-dimensional and

two-dimensional coordination networks.

Department of Chemistry, Indian Institute of Technology, Kharagpur,721302, India. E-mail: kbiradha@yahoo.com; Fax: +91 (0)3222282252; Tel: +91 3222 282252

† Electronic supplementary information (ESI) available: Details of thesynthesis of the coordination polymers, TGA, elemental analysis andcrystallographic parameters. CCDC reference numbers 720551–720557.For ESI and crystallographic data in CIF or other electronic formatsee DOI: 10.1039/b906285h

1220 | CrystEngComm, 2009, 11, 1220–1222

The single crystals of complex 3 {Cu(2a)2(SCN)2]$2G} were

obtained by the addition of a methanolic solution of Cu(NO3)2 and

NaSCN to a DMF solution of the ligand (2a) and the corresponding

guest molecule. The guest molecules used were 1,4-diiodobenzene

(3a), 1,4-dibromobenzene (3b), pyrene (3c) or phenanthrene (3d).

In all four cases, the Cu(II) centers have octahedral coordination

with two pyridyl ligands and two SCN anions in the equatorial plane

and the remaining two pyridyl groups in the axial positions.‡ They all

exhibit 1D-coordination networks which contain intra-network b-

sheet hydrogen bonding between the ligands (Table 1). These

networks further associate via inter-network b-sheets to form a 2D-

layer (Fig. 1).

The 2D-layers pack in two different modes depending on the

presence of guest molecules (Fig. 2). In the cases of 3a and 3b (1,4-

dihalobenzene as guests) the layers exhibit perfect flatness. Adjacent

layers are related by translational symmetry and overlapped over

Fig. 1 A two-dimensional layer formed by assembling 1D-chains via N–

H/O hydrogen bonds in the crystal structures of 3a–d. Notice the inter-

and intra-network hydrogen bonds.

This journal is ª The Royal Society of Chemistry 2009

Fig. 2 Packing of the two-dimensional layers in the crystal structures of a) 3b (viewed along the channels) and b) 3c (side view of the channels); notice

the differences in packing. Interactions between the guest molecules c) in 3b and d) in 3c.

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each other with interlayer separations of 8 A and 8.3 A in 3a and 3b

respectively. The guest molecules form columns via C–H/I (H/I

3.22 A) or C–H/Br (H/Br 3.17 A) interactions and exist between

the layers. The same halogen atom which is involved in C–H/halogen interactions also engages in halogen bonding with SCN

anions (I/S 3.506(4) A & Br/S 3.492(2)A).4 However, in the cases

of 3c and 3d (hydrocarbons as guests) the layers are corrugated and

packed in a slipped fashion. Interestingly, the adjacent layers are

related by a screw axis and are separated by 7.49 A and 7.42 A in 3c

and 3d respectively. The guest molecules are sandwiched by the

pyridine moiety of one layer and the amide moieties of the adjacent

layers and they occupy channels which exist along the diagonals of

the layer. The edges of the guest molecules interact with each other via

C–H/p interactions. They contribute 35%, 33%, 42% and 41% to

the crystal volumes of 3a, 3b, 3c and 3d respectively.5 It is interesting

to note that both the packing patterns observed in these structures

have almost identical packing indexes of �68%.

In order to test the iso-structurality, the single crystals of complex 4

{[Cu(1a)2(SCN)2]$2G} were grown in similar conditions in the

presence of 1,4-dibromobenzene (4a) or phenanthrene (4b). The

crystal structures reveal complete iso-structurality between coordi-

nation networks of amides and reverse amides (Fig. 3). The guest

Fig. 3 A two-dimensional layer assembled via N–H/O hydrogen bonds

in the crystal structure of 4. Compare with Fig. 1.

This journal is ª The Royal Society of Chemistry 2009

molecules have similar influences on the packing modes of the layers

and have two types of packing as discussed above. The dibromo-

benzene forms a halogen bond with SCN (Br/S 3.427(3) A) similar

to 3a and 3b. It is worth noting that the N/O distances of inter-

network N–H/O hydrogen bonds were found to be shorter than

those of intra-network interactions.

Here it is important to state that we have already explored the

coordination polymers of amides (1) when the spacer X ¼ –(CH2)2–

or X ¼ –(CH2)4– and they have shown several network variations

even in identical reaction conditions.6 These spacers exhibited gauche

conformations to give versatile network geometries. The results

obtained here indicate that the hexyl spacer had shown more

consistency in exhibiting linearity, with anti geometries throughout

the alkyl chains, than the –(CH2)2– and –(CH2)4– spacers. The

increased hydrophobic interactions between –(CH2)6– spacers might

have helped the consistent formation of hydrogen bonding layers.

Therefore, we felt that the iso-structurality between amides and

reverse amides can also be observed with the phenyl spacer as they

provide good aromatic interactions within the b-sheets. Earlier we

have shown that the ligand 1b (amide) forms a two-dimensional

coordination network when reacted with Cu(SCN)2.7 To realize our

above mentioned hypothesis we have prepared single crystals of 5

{[Cu(2b)2(SCN)2]$2benzonitrile} by complexing the ligand 2b

(reverse amide) with Cu(SCN)2 in the presence of benzonitrile.

The crystal structure of 5 reveals the formation of a two-dimen-

sional coordination network which is similar to the one observed in

complex 6 {Cu(1b)2(SCN)2} with amide ligand 1b (Fig. 4). Within the

coordination layer the ligands interact via b-sheet hydrogen bonds.

The only difference between the coordination polymers of 1b and 2b

is the inclusion of guest molecules between the layers in the case of the

present structure (5). Here it is important to note that although the

dimensionality with respect to the coordination differs, the layers are

iso-structural in all the crystal structures of 3–6 with respect to the N–

H/O hydrogen bonds. Further, the packing of the layers in 5 is

similar to that of the complexes 3a, 3b and 4a. There exists channels

between the layers which are filled by the columns of benzonitriles. In

the columns the benzonitriles interact via edge-to-face C–H/p

interactions and the N-atom of nitrile engages in C–H/N hydrogen

bonds with the pyridine C–H group.

CrystEngComm, 2009, 11, 1220–1222 | 1221

Fig. 4 Two-dimensional network involving N–H/O hydrogen bonds in the crystal structure of a) 5 and b) 6. Compare the layers with those in Fig. 1

and 3. Illustrations for guest inclusion in 5: c) columns of benzonitrile molecules between the layers and d) interactions between the benzonitrile

molecules in a column.

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We note here that the similar iso-structurality between the coor-

dination polymers of amides and reverse amides were not observed

when the spacers were –(CH2)2– and –(CH2)4–. Therefore, the results

observed here indicate that the longer spacers as well as the blocking

of pyridine groups by coordination are important to obtain iso-

structural coordination networks. Our efforts to grow the coordina-

tion polymers with mixed amides using 1a and 2a or 1b and 2b were

unsuccessful and always resulted in the crystals of 3 or 5 respectively.

Notes and references

‡ The single crystal data was collected on a Bruker APEX-2 CCD X-raydiffractometer that uses graphite monochromated Mo Ka radiation (l ¼0.71073 A) by the hemisphere method. The structures were solved bydirect methods and refined by least square methods on F2 using SHELX-97.8 Non-hydrogen atoms were refined anisotropically and hydrogenatoms were fixed at calculated positions and refined using a riding model.

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