A 3-dimensional {4²·8⁴} lvt net built from a ditopic bis(3,2′:6′,3″-terpyridine) tecton bearing long alkyl tails

Abstract

Divergent bis(terpyridine) tectons are versatile ligands for the assembly of coordination networks; we demonstrate the assembly of a 3-dimensional {4²·8⁴} lvt net (still relatively sparse among 4-connected nets in metal–organic frameworks) from the reaction of 1,4-bis(n-octoxy)-2,5-bis(3,2′:6′,3″-terpyridin-4′-yl)benzene and Co(NCS)₂.

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Oligopyridines¹ remain as one of the most widespread building blocks in the toolbox of a coordination chemist. The bis(chelate) 2,2′:6′,2″-terpyridine (2,2′:6′,2″-tpy) is especially popular and multitopic ligands containing peripheral 2,2′:6′,2″-tpy domains² have been used in a wide variety of architectures. 4,2′:6′,4″-Terpyridine (4,2′:6′,4″-tpy, Scheme 1) and 3,2′:6′,3″-terpyridine (3,2′:6′,3″-tpy, Scheme 1) are less familiar isomers of terpyridine, but in the last decade, the use of 4,2′:6′,4″-tpy as a building block of coordination polymers has grown significantly.³ In metal complexes of 4,2′:6′,4″-tpy, the central pyridine ring is non-coordinated and the remaining N,N′-donor set presents a divergent domain, with vectorial properties that are unaffected by inter-ring bond rotation (red bonds in Scheme 1). In contrast, rotation about the interannular bonds in 3,2′:6',3″-tpy alters its divergent coordination mode.⁴

Scheme 1 Structures of divergent isomers of terpyridine and the ditopic ligand 1. See text for significance of the bonds marked in red.

The beauty of terpyridine metal-binding domains is the ease with which substituents can be introduced into the 4′-position, for example by using Kröhnke's⁵ or Wang and Hanan's⁶ strategies. With coordinatively innocent 4′-substituents, reactions between 4,2′:6′,4″-terpyridines and metal ions yield metallomacrocycles, 1-dimensional chains or 2-dimensional nets.³ Extension to 3-dimensions is achieved by introducing non-innocent domains such as diphenylphosphino,⁷ carboxylato,⁸ or pyridyl⁹ functionalities, or by using co-ligands.¹⁰ An alternative approach to increase dimensionality is through the coordination capacity of multitopic 4,2′:6′,4″-tpy ligands, although such compounds have received scant attention.^11–14 We have demonstrated that 1,4-bis(n-octoxy)-2,5-bis(4,2′:6′,4″-terpyridin-4′-yl)benzene reacts with ZnCl₂ to give a network consisting of (4,4)-sheets engaging in 2D → 2D parallel interpenetration,¹⁴ and a report on a triply interpenetrating network formed between cobalt(II) and 1,3-di((4,2′:6′,4″-terpyridin)-4′-yl)benzene has appeared.¹² We now describe the synthesis of the bis(3,2′:6′,3″-tpy) ligand 1 (Scheme 1) and its reaction with Co(NCS)₂ to give a 3-dimensional {4²·8⁴} lvt net.¹⁵ The inclusion of the long alkoxy chains in 1 enhances the solubility of the ligand with respect to analogues with simple phenylene spacers,¹⁴ and also has a stabilizing influence on an infinite architecture.

Ligand 1 was synthesized† using the one-pot method of Wang and Hanan⁶ starting from 2,5-bis(octoxy)benzene-1,4-dicarbaldehyde and 4.7 equivalents of 3-acetylpyridine in EtOH in the presence of NH₃. Compound 1 was isolated in 41% yield. The electrospray mass spectrum exhibited a base peak at m/z 797.8 arising from [M+H]⁺, and ¹H (Fig. S1†) and ¹³C NMR spectra (assigned using COSY, DEPT, HMQC and HMBC techniques) were consistent with the symmetrical structure shown in Scheme 1. The absorption spectrum of 1 has broad and intense, high energy bands arising from π* ← n and π* ← π transitions which extend into the visible region (Fig. S2†).

Layering of MeOH and CHCl₃ solutions of Co(NCS)₂ and 1, respectively, resulted in the formation of pink crystals of [Co(NCS)₂(1)·4CHCl₃]_n within 2–4 weeks in 14% yield. The crystal selected for single crystal X-ray diffraction was solved and refined in the non-centrosymmetric space group Pna2₁. The Flack parameter of 0.480(10) suggested that it was a twin by inversion¹⁶ as every attempt to either solve or refine the structure in Pnma failed and ADDSYMM¹⁷ could not identify an alternative space group. The asymmetric unit contains one molecule of 1 and a Co(NCS)₂ unit; the octoxy chain containing atom C19 was refined isotropically and three of the C–C distances were restrained to chemically reasonable values. The cobalt ion is octahedrally coordinated, and 1 binds through the outer N-donors to four cobalt centres (Fig. 1), leaving the central nitrogen atoms N2 and N6 uncoordinated. Atom Co1 coordinates to two [NCS]⁻ ligands in a trans-arrangement and to four different 1 ligands in the equatorial plane. The two independent octoxy chains are in non-extended conformation, and each is folded over a 3,2′:6′,3″-tpy unit with close C–H⋯π contacts (H⋯centroid = 3.19 and 3.23 Å for the pyridine rings containing N1 and N4).

Fig. 1 The repeat unit (with symmetry generated Co atoms) in [Co(NCS)₂(1)·4CHCl₃]_n (H atoms and solvent molecules are omitted). Symmetry codes: i = 1/2 + x, 1/2 − y, z; ii = −x, 1 − y, −1/2 + z; iii = 1/2 − x, 1/2 + y, 1/2 − z; iv = 1/2 – x, −1/2 + y, 1/2 + z; v = −1/2 + x, 1/2 – y, z; vi = −x, 1 − y, 1/2 + z. Selected bond parameters: Co1–N1 = 2.185(8), Co1–N3i = 2.212(7), Co1–N4ii = 2.204(8), Co1–N5iii = 2.181(8), Co1–N7 = 2.096(8), Co1–N8 = 2.079(8) Å; Co1–N7–C53 = 161.0(8), Co1–N8–C54 = 151.2(8), N1–Co1–N5iii = 176.2(3), N3i–Co1–N4ii = 178.6(3)°.

Pairs of cobalt atoms are either bridged by a single 4,2′:6′,4″-tpy (e.g. Co1 and Co1^v in Fig. 1) or by two N-donors from each of the two 4,2′:6′,4″-tpy domains of 1 (e.g. Co1 and Co1^iv in Fig. 1). The latter coordination mode extends to the formation of [2 + 2] metallomacrocycles (Fig. 2a) which are interlinked through the cobalt centres, as shown in Fig. 2b. The structure propagates into a 2-nodal {4²·8⁴} lvt net.¹⁴ The two 4-connected nodes are Co1 and the centroid of the arene spacer in 1, which are planar and approximately tetrahedral, respectively. Although the local {Co(N_tpy)₄} domain in [Co(NCS)₂(1)·4CHCl₃]_n is square planar (N_tpy–Co–N_tpy angles = 90.9(3), 89.1(3), 92.9(3) and 87.1(3)°), the cobalt node is distorted in the topological description of the net (centroid–Co–centroid angles = 96.6, 64.6, 132.2 and 67.4°) while remaining planar. Topological representations of the framework in [Co(NCS)₂(1)·4CHCl₃]_n are shown in Fig. 3. The view down the a-axis in Fig. 3a is directly comparable with the structure in Fig. 2b.

Fig. 2 (a) A [2 + 2] metallomacrocycle formed from two Co1 atoms and two half-ligands (in orange). (b) Interconnection of metallomacrocycles with the unit cell viewed down the a-axis.

Fig. 3 TOPOS¹⁸ representations of the {4²·8⁴} lvt net in [Co(NCS)₂(1)·4CHCl₃]_n: (a) view down the a-axis for comparison with Fig. 2b, and (b) showing the 4- and 8-membered metallomacrocycles. Metal nodes, purple; ligand–centroid nodes, green.

The voids in the net are occupied by the octoxy chains and CHCl₃ molecules. Fig. 4 illustrates that the chains lie in the ac-plane, a consequence of the close CH⋯π contacts between the terminal CH₂CH₃ units of each chain and pyridine rings (see above).

Fig. 4 Superimposition of the topological representation of the lvt net in [Co(NCS)₂(1)·4CHCl₃]_n and the structure (H atoms and CHCl₃ molecules are omitted); the octoxy chains of one ligand are shown in the space-filling representation: views down the (a) a-axis and (b) c-axis.

In conclusion, we have shown that, by adopting a conformation in which the two 3,2′:6′,3″-tpy metal-binding domains are orthogonal, ligand 1 can combine with a planar 4-connecting metal node to produce a {4²·8⁴} lvt net. Among metal–organic frameworks, the lvt topology is scarce in comparison with other 4-connected nets.¹⁹ The conformational flexibility of ditopic bis(4,2′:6′,4″-tpy) and bis(3,2′:6′,3″-tpy) ligands allows the two tpy units to lie on a path between coplanar¹³ and orthogonal (as in the current work), making these isomeric ligands attractive tectons. We are currently developing the coordination chemistry of multitopic 4,2′:6′,4″-tpy and 3,2′:6′,3″-tpy ligands to investigate which building blocks favour the assembly of 2- versus 3-dimensional networks.

Acknowledgements

We thank the Swiss National Science Foundation (grant 200020_149067), the European Research Council (Advanced grant 267816 LiLo) and the University of Basel for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic and crystallographic details; Fig. S1–S2 solution ¹H NMR and absorption spectra of 1. CCDC 1035825. See DOI: 10.1039/c4ce02347a

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