Engineering 2D→2D parallel interpenetration using long alkoxy-chain substituents

Solvent Induced Conversion of a Self-Assembled Gyrobifastigium to a Barrel and Encapsulation of Zinc-Phthalocyanine within the Barrel for Enhanced Photodynamic Therapy

A rare gyrobifastigium architecture (GB) was constructed by self-assembly of a tetradentate donor (L) with Pd^II acceptor in DMSO. The GB was converted to its isomeric tetragonal barrel (MB) upon treatment with water. The hydrophobic cavity of MB has been explored for the encapsulation of zinc-phthalocyanine (ZnPc), which is an excellent photosensitizer for photodynamic therapy (PDT). However, the poor water-solubility and aggregation tendency are the main reasons for the suboptimal PDT performance of free ZnPc in the aqueous medium. Effective solubilization of ZnPc in an aqueous medium was achieved by encapsulating it in the cavity of MB. The inclusion complex (ZnPc⊂MB) showed enhanced singlet oxygen generation in water. Higher cellular uptake and anticancer activity of the ZnPc⊂MB compared to free ZnPc on HeLa cells indicate that encapsulation of ZnPc in an aqueous host is a potential strategy for enhancement of its PDT activity in water.

A Tail Does Not Always Make a Difference: Assembly of cds Nets from Co(NCS)2 and 1,4-bis(n-Alkyloxy)-2,5-bis(3,2':6',3″-terpyridin-4'-yl)benzene Ligands

The consistent assembly of a (6⁵.8) cds net is observed in reactions of cobalt(II) thiocyanate with 1,4-bis(n-alkyloxy)-2,5-bis(3,2′:6′,3″-terpyridin-4′-yl)benzene ligands in which the n-alkyloxy substituents are n-propyl (ligand 3), n-butyl (4), n-pentyl (5), n-hexyl (6), n-heptyl (7), and n-octyl (8). Crystals were grown by layering a methanol solution of Co(NCS)₂ over a 1,2-dichlorobenzene solution of each ligand. The choice of crystallization solvents is critical in directing the assembly of the cds net. Single-crystal structures of [Co(NCS)₂(3)]_n^.3.5nC₆H₄Cl₂, [Co(NCS)₂(4)]_n^.5.5nC₆H₄Cl₂, [Co(NCS)₂(5)]_n^.4nC₆H₄Cl₂, [Co(NCS)₂(6)]_n^.3.8nC₆H₄Cl₂, [Co(NCS)₂(7)]_n^.3.1nC₆H₄Cl₂, and [Co(NCS)₂(8)]_n^.1.6nC₆H₄Cl₂^.2nMeOH (C₆H₄Cl₂ = 1,2-dichlorobenzene) are presented and compared. The n-alkyloxy chains exhibit close to extended conformations and are accommodated in cavities in the lattice without perturbation of the coordination framework.

Isomers of Terpyridine as Ligands in Coordination Polymers and Networks Containing Zinc(II) and Cadmium(II)

The use of divergent 4,2′:6′,4″- and 3,2′:6′,3″-terpyridine ligands as linkers and/or nodes in extended coordination assemblies has gained in popularity over the last decade. However, there is also a range of coordination polymers which feature 2,2′:6′,2″-terpyridine metal-binding domains. Of the remaining 45 isomers of terpyridine, few have been utilized in extended coordination arrays. Here, we provide an overview of coordination polymers and networks containing isomers of terpyridine and either zinc(II) and cadmium(II). Although the motivation for investigations of many of these systems is their luminescent behavior, we have chosen to focus mainly on structural details, and we assess to what extent assemblies are reproducible. We also consider cases where there is structural evidence for competitive product formation. A point that emerges is the lack of systematic investigations.

The terpyridine isomer game: from chelate to coordination network building block

The first 4,2':6',4''-terpyridine (4,2':6',4''-tpy) containing coordination polymer was reported over 20 years ago and in the last decade, there has been increased interest in the use of ditopic 4,2':6',4''-tpy ligands as linkers in coordination polymers and 2D-networks. Functionalization in the 4'-position of 4,2':6',4''-tpy is synthetically straightforward, giving access to a large suite of building blocks. Less well explored is the coordination chemistry of 3,2':6',3''-tpy ligands which exhibit greater conformational flexibility than 4,2':6',4''-tpy. One approach to making the transition from 2D- to 3D-networks is to utilize tetratopic bis(4,2':6',4''-tpy) and bis(3,2':6',3''-tpy) ligands which act as 4-connecting nodes. In this highlight, we survey recent progress towards a better understanding of the design principles associated with the use of ditopic and tetratopic 4,2':6',4''-tpy and 3,2':6',3''-tpy containing ligands and their roles both as linkers and nodes in coordination assemblies.

Straight Versus Branched Chain Substituents in 4'-(Butoxyphenyl)-3,2':6',3″-terpyridines: Effects on (4,4) Coordination Network Assemblies

The preparation and characterization of the isomers rac-4'-(4-butan-2-yloxyphenyl)-3,2':6',3″-terpyridine (rac-2), 4'-(2-methylpropoxyphenyl)-3,2':6',3″-terpyridine (3) and 4'-(tert-butoxyphenyl)-3,2':6',3″-terpyridine (4) are reported. The compounds react with Co(NCS)2 under conditions of crystal growth at room temperature to give single crystals of [{Co(rac-2)2(NCS)2}·CHCl3]n, [Co(3)2(NCS)2]n and [{Co(4)2(NCS)2}·CHCl3]n which possess (4,4) networks, with the Co centers acting as 4-connecting nodes. Powder X-ray diffraction (PXRD) was used to confirm that the crystals chosen for single crystal X-ray diffraction were representative of the bulk samples. The detailed structures of the three networks have been compared with that of the previously reported [{Co(1)2(NCS)2}·4CHCl3]n in which 1 is 4'-(butoxyphenyl)-3,2':6',3″-terpyridine. Whereas the switch from 1 with the straight-chain butoxy substituent to rac-2, 3 and 4 with branched chains causes significant structural perturbation, changes in the spatial properties of the branched substituents are accommodated with subtle conformational changes in the 3,2':6',3″-tpy domain.

Directing 2D-Coordination Networks: Combined Effects of a Conformationally Flexible 3,2':6',3″-Terpyridine and Chain Length Variation in 4'-(4-n-Alkyloxyphenyl) Substituents

The synthesis and characterization of 4'-(4-n-propoxyphenyl)-3,2':6',3″-terpyridine is described. Five 2D-coordination networks have been isolated by crystal growth at room temperature from reactions of Co(NCS)2 with 4'-(4-n-alkyloxyphenyl)-3,2':6',3″-terpyridines in which the n-alkyl group is ethyl, n-propyl, n-butyl, n-pentyl and n-hexyl in ligands 2-6, respectively. The single-crystal structures of [{Co(2)2(NCS)2}.0.6CHCl3]n, [{Co(3)2(NCS)2}.4CHCl3.0.25H2O]n, [{Co(4)2(NCS)2}.4CHCl3]n, [Co2(5)4(NCS)4]n and [Co(6)2(NCS)2]n have been determined, and powder X-ray diffraction has demonstrated that the single-crystal structures are representative of the bulk materials. Each compound possesses a (4,4) net with Co centres as 4-connecting nodes. For the assemblies containing 2, 3 and 4, the (4,4) net comprises two geometrically different rhombuses, and the nets pack in an ABAB... arrangement with cone-like arrangements of n-alkyloxyphenyl groups being accommodated in a similar unit in an adjacent net. An increase in the n-alkyloxy chain length has two consequences: there is a change in the conformation of the 3,2':6',3″-tpy metal-binding domain, and the (4,4) net comprises identical rhombuses. Similarities and differences between the assemblies with ligands 2-6 and the previously reported [{Co(1)2(NCS)2}.3MeOH]n and [{Co(1)2(NCS)2}.2.2CHCl3]n in which 1 is 4'-(4-methoxyphenyl)-3,2':6',3″-terpyridine are discussed. The results demonstrate the effects of combining a variable chain length in the 4'-(4-n-alkyloxyphenyl) substituents of 3,2':6',3″-tpy and a conformationally flexible 3,2':6',3″-tpy metal-binding domain.

Sierpiński Pyramids by Molecular Entanglement

Planar, terpyridine-based metal complexes with the Sierpiński triangular motif and alkylated corners undergo a second self-assembly event to give megastructural Sierpiński pyramids; assembly is driven by the facile lipophilic-lipophilic association of the alkyl moieties and complementary perfect fit of the triangular building blocks. Confirmation of the 3D, pyramidal structures was verified and supported by a combination of TEM, AFM, and multiscale simulation techniques.

Trinodal Self-Penetrating Nets from Reactions of 1,4-Bis(alkoxy)-2,5-bis(3,2’:6’,3’’-terpyridin-4’-yl)benzene Ligands with Cobalt(II) Thiocyanate

The tetratopic ligands 1,4-bis(2-ethylbutoxy)-2,5-bis(3,2’:6’,3’’-terpyridin-4’-yl)benzene (1) and 1,4-bis(3-methylbutoxy)-2,5-bis(3,2’:6’,3’’-terpyridin-4’-yl)benzene (2) have been prepared and characterized by 1H and 13C{1H} NMR, IR, and absorption spectroscopies and mass spectrometry. Reactions of 1 and 2 with cobalt(II) thiocyanate under conditions of crystal growth at room temperature result in the formation of [{Co(1)(NCS)2}·MeOH·3CHCl3]n and [{Co(2)(NCS)2}·0.8MeOH·1.8CHCl3]n. Single-crystal X-ray diffraction reveals that each crystal lattice consists of a trinodal self-penetrating (62.84)(64.82)(65.8)2 net. The nodes are defined by two independent cobalt centres and the centroids of two crystallographically independent ligands which are topologically equivalent.

Sometimes the Same, Sometimes Different: Understanding Self-Assembly Algorithms in Coordination Networks

The syntheses and characterizations of three new ligands containing two 4,2':6',4″-tpy or two 3,2':6',3″-tpy metal-binding domains are reported. The ligands possess different alkyloxy functionalities attached to the central phenylene spacer: n-pentyloxy in 3, 4-phenyl-n-butoxy in 4, benzyloxy in 5. Crystal growth under ambient conditions has led to the formation of {[Co(NCS)₂(3)]·0.8C₆H₄Cl₂}n and {[Co(NCS)₂(4)]·1.6H₂O·1.2C₆H₄Cl₂}n, with structures confirmed by single crystal X-ray diffraction. Both the cobalt(II) center and ligand 3 or 4 act as 4-connecting nodes and both {[Co(NCS)₂(3)]·0.8C₆H₄Cl₂}n and {[Co(NCS)₂(4)]·1.6H₂O·1.2C₆H₄Cl₂}n possess a 3D cds net despite the fact that 3 and 4 contain two 4,2':6',4″-tpy and two 3,2':6',3″-tpy units, respectively. Taken in conjunction with previously reported data, the results indicate that the role of the alkyloxy substituent is more significant than the choice of 4,2':6',4″- or 3,2':6',3″-tpy isomer in determining the assembly of a particular 3D net. The combination of Co(NCS)₂ with 5 resulted in the formation of the discrete molecular complex [Co(NCS)₂(MeOH)₂(5)₂]·2CHCl₃·2MeOH in which 5 acts as a monodentate ligand. The pendant phenyls and both coordinated and non-coordinated 4,2':6',4″-tpy units are involved in efficient π-stacking interactions.

Terpyridine-based metallosupramolecular constructs: tailored monomers to precise 2D-motifs and 3D-metallocages

Comprehensive summary of the recent developments in the growing field of terpyridine-based, discrete metallosupramolecular architectures.

4,2':6',4"- and 3,2':6',3"-Terpyridines: The Conflict between Well-Defined Vectorial Properties and Serendipity in the Assembly of 1D-, 2D- and 3D-Architectures

A comparative investigation of the coordination assemblies formed between Co(NCS)₂ and two monotopic 4,2':6',4''-terpyridine (4,2':6',4"-tpy) ligands or two related ditopic ligands is reported. Crystals were grown by layering MeOH solutions of Co(NCS)₂ over a CHCl₃ or 1,2-C₆H₄Cl₂ solution of the respective ligand at room temperature. With 4'-(2-methylpyrimidin-5-yl)-4,2':6',4"-terpyridine (6), the 1D-coordination polymer {[Co₂(NCS)₄(MeOH)₄(6)₂]∙2MeOH∙8H₂O}n assembles with 6 coordinating only through the outer N-donors of the 4,2':6',4"-tpy unit; coordination by the MeOH solvent blocks two cobalt coordination sites preventing propagation in a higher-dimensional network. A combination of Co(NCS)₂ and 1-(4,2':6',4"-terpyridin-4'-yl)ferrocene (7) leads to {[Co(NCS)₂(7)₂]∙4CHCl₃}n which contains a (4,4) net; the 2D-sheets associate through π-stacking interactions between ferrocenyl and pyridyl units. A 3D-framework is achieved through use of the ditopic ligand 1,4-bis(npropoxy)-2,5-bis(4,2':6',4"-terpyridin-4'-yl)benzene (8) which acts as a 4-connecting node in {[Co(NCS)₂(8)₂].2C₆H₄Cl₂}n; the combination of metal and ligand planar 4-connecting nodes results in a {6⁵.8} cds net. For a comparison with the coordinating abilities of the previously reported 1,4-bis(noctoxy)-2,5-bis(4,2':6',4"-terpyridin-4'-yl)benzene (3), a more flexible analogue 9 was prepared. {[Co(NCS)₂(9)]∙2CHCl₃}n contains a (4,4) net defined by both metal and ligand planar 4-connecting nodes. The noctoxy tails of 9 protrude from each side of the (4,4) net and thread through adjacent sheets; the arene-attached noctoxy chains associate through a combination of van der Waals and C-H...π interactions.

Constructing chiral MOFs by functionalizing 4,2′:6′,4′′-terpyridine with long-chain alkoxy domains: rare examples of neb nets

Three chiral 3D MOFs with uncommon neb topologies assemble from Co(NCS)₂ and 4′-(4-ⁿalkyloxyphenyl)-4,2′:6′,4′′-terpyridines (alkyl = hexyl or nonyl).

Divergent 4,2′:6′,4′′- and 3,2′:6′,3′′-terpyridines as linkers in 2- and 3-dimensional architectures

This Highlight illustrates the strategies applied to encourage the formation of 2- and 3-dimensional metal–organic architectures as opposed to 1-dimensional chains using 4,2′:6′,4′′- and 3,2′:6′,3′′-terpyridines as organic linkers.