Supramolecular reactions of metallo-architectures: Ag2-double-helicate/Zn4-grid, Pb4-grid/Zn4-grid interconversions, and Ag2-double-helicate fusion† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c5sc04403k

A Ag+ dinuclear double-helicate was converted into a Zn2+ tetranuclear grid, and a Pb2+ grid, into a Zn2+ grid; a Ag+ heterostranded double helicate was obtained from two homostranded helicates.


Introduction
Like covalent molecules, supramolecular 1 assemblies may participate in various reactions. The understanding of supramolecular reactions is of much interest because they are involved in many areas such as complex chemical systems and networks, 2 adaptive 3 and stimuli-responsive 4 chemical systems, fabrication of nanodevices and materials, 4 biomolecular processes. Thus, in the complexity and diversity of supramolecular chemistry, the reactivity of supramolecules plays a crucial role. It includes the processes: (a) of (self)assembly (i.e. formation of supramolecular architectures through assembly, but also their participation, as subunits, in more complex assemblies), and correlatively, partial or total disassembly; (b) of partial or total reorganization or exchange (at the supramolecular and, additionally and possibly, at the covalent level), that involves the breaking of several or all of the initial supramolecular connections and formation of new ones; (c) without breaking or formation of new supramolecular connections (e.g. covalent modications aer self-assembly 5 ).
Amongst supramolecular architectures, double helices and helicates, 6 as well as grids 7 arouse much interest and work. For example, DNA 8 and the ion channel generated by gramicidine 9 have a double helical structure, and there are double helical complexes that act as molecular machines 10 or catalysts. 11 Gridlike complexes have been studied for their electrochemical and magnetic properties, 7 for their capacity to encapsulate ions 12 or as starting materials for building more complex architectures (e.g. a Solomon link 13 ), amongst other things. However, supramolecular interconversions of grids and helicates have not, except several examples, 14,15 been much explored.
With these ideas in mindand using principles such as the displacement of an equilibrium through precipitation, and the preference of Ag + for tetrahedral and of Zn 2+ for octahedral coordinationwe designed, as reported herein, three supramolecular reactions 16  that produce the supramolecular complexes, as well as through the nature of complexes, and occur due to the dynamic character of the present metal-ligand connections. These reactions ( Fig. 1) can be seen as: (i) a change of the nature of the supramolecular architecture, from a Ag + dinuclear double helicate (DH) into a Zn 2+ tetranuclear grid (G), induced by replacement of Ag + by Zn 2+ (Fig. 3). In this reaction, not only the nature of the complex and that of the metal ion change, but also the conformation of the ligand (helical / unfolded), the charge (2 + / 8 + ) and the nuclearity of the complex (2 / 4) and the number of ligands per complex (2 / 4). In regard to this last change, this process can be compared with the conversion or the equilibrium between supramolecular dimer and tetramer of bioactive proteins, 19 or between other homo-oligomers 20 with inuence on the protein functions.
While in case (ii) the equilibrium is shied towards the Zn 2+ grid through the precipitation of Pb 2+ as its halides (chloride and bromide), in cases (i) and (iii), the conversions can be done without precipitation.

Results and discussion
(i) The conversion Ag 2 L 2 -DH / Zn 4 L 4 -G (L ¼ 1, 2) through transmetallation is a dramatic reorganization of the nature of the metallo-supramolecular architecture induced by the replacement of Ag + by Zn 2+ (Fig. 3a and c): 2 Ag 2 L 2 -DH + 4 Zn 2+ / Zn 4 L 4 -G + 4 Ag + . Ag + prefers a tetrahedral coordination geometry which is, in the case of ligands 1 and 2, achieved from 2 two-Nsp 2 -atom bidentate pyridine-hydrazone sites. In this    way, Ag + induces the formation of double helicates with ligands 1 and 2. Zn 2+ prefers an octahedral coordination environment that results from 2 three-Nsp 2 -atom tridentate sites of type pyridine-hydrazone-pyrimidine, thus generating a grid.
Reaction of 1 equiv. of Ag 2 L 2 -DH 15c with 2 equiv. of Zn(OTf) 2 (OTf À ¼ CF 3 SO 3À ) produceswithout the need to precipitate Ag + as a halidethe corresponding grid Zn 4 L 4 -G 15b,22a (solvent: CD 3 NO 2 with 6-14% CD 3 CN; ESI, pp. S9-S11 †). Where ZnCl 2 is used in the reaction with Ag 2 1 2 -DH, two equivalents of AgOTf per equiv. of DH are required according to the equation (ESI p. S8 †): 2Ag 2 1 2 -DH + 4ZnCl 2 + 4Ag + / Zn 4 1 4 -G + 8AgCl On treatment of the double helicate Ag 2 2 2 -DH in CD 3 NO 2 with 2 equiv. of Zn(OTf) 2added as a solution in a small volume of CD 3 CN, or as a solidthe grid Zn 4 2 4 -G was obtained. When the double helicate Ag 2 1 2 -DH in CD 3 NO 2 was treated with 2 equiv. of Zn(OTf) 2 , added as a solution in a small volume of CD 3 CN (about 6-14% of the CD 3 NO 2 volume), the grid Zn 4 1 4 -G was obtained. When Zn(OTf) 2 was added as a solid, without CD 3 CN, was obtained a mixture without the Zn 4 1 4 -G grid; addition of a small volume of CH 3 CN (about 6-14% of the CD 3 NO 2 volume) to this mixture produced the expected grid Zn 4 1 4 -G. A possible explanation could be that, in the case of the reaction Ag 2 1 2 -DH / Zn 4 1 4 -G, the CH 3 CN acts as a coordinating species for the Ag + ions and so contributes to the displacement of the equilibrium from the double helix towards the grid. The grid Zn 4 2 4 -G should bedue to the p-stacking aromatic interaction between a phenyl ring and the two ligands between which that phenyl is located within the gridmore stable than the grid Zn 4 1 4 -G. This stability may be sufficient to make possible the formation of the grid Zn 4 2 4 -G from the corresponding double helicate without, unlike in the case of the grid Zn 4 1 4 -G, the assistance of CH 3 CN.
DOSY NMR was also used to study the conversion Ag 2 L 2 -DH / Zn 4 L 4 -G (L ¼ 1, 2). As expected, the volume of the grid species obtained from double helicates on treatment with Zn(OTf) 2 was found in agreement with that of the grid prepared from the free ligands L and Zn(OTf) 2 .
The reverse conversion Zn 4 L 4 -G/ Ag 2 L 2 -DH can be done as follows: aer treatment of the grid with KOH, the solvent (CD 3 CN or CD 3 NO 2 ) is removed, and the ligand is extracted with CDCl 3 and separated from the solid residue (by centrifugation or ltration); aer removal of CDCl 3 , CD 3 NO 2 is added, then AgOTf is added to form the helicate. In order to simplify the procedure, we used ligand 2 and a mixture of CDCl 3 and CD 3 NO 2 where ligand 2, as well as the corresponding grid and double helicate were soluble. Aer precipitation of Zn 2+ with KOH, the mixture was centrifuged (the ligand 2 being soluble in the mixture of solvents), and to the recovered liquid phase AgOTf was added to produce the Ag 2 2 2 -DH (ESI, p. S13 †).
In a pH-dependent system (Fig. 3b), the interconversion between Ag 2 1 2 -DH and Zn 4 1 4 -G was achieved as follows (ESI, p. S10 †): the grid was generated from the double helicate by reaction with Zn 2+ ; then, Zn 2+ was complexed with hexacyclen, and the double helicate was regenerated; partial protonation of hexacyclen with TfOH caused release of Zn 2+ and formation of the grid (incomplete yield); nally, addition of triethylamine reactivated the hexacyclen that again encapsulated Zn 2+ and resulted in the reformation of the double helicate.
(ii) The Pb 4 1 4 -G / Zn 4 1 4 -G conversion (Fig. 4a) can formally be seen as a substitution of Pb 2+ by Zn 2+ ions, although the real mechanism, involving breaking and formation of supramolecular bonds, must be more complex. Reaction of Pb 4 1 4 -G 15b with 4 equiv. of Zn(OTf) 2 produces a mixture which no longer contains the grid-like species Pb 4 1 4 -G or Zn 4 1 4 -G (ESI p. S2 †). This suggests that the affinity of Zn 2+ for the ligand, as well as its preference for octahedral coordination are not sufficient to displace the equilibrium towards Zn 4 1 4 -G. We considered that the involvement of Pb 2+ ions in a weakly dissociating or sparingly soluble compound should displace the equilibrium. Indeed, addition of Br À (as Bu 4 P + Br À ) to the above mixture, or treatment of Pb 4 1 4 -G with four equivalents of ZnBr 2 or ZnCl 2 producedalong with the formation of PbX 2 (X ¼ Br, Cl) which precipitates and, doing so, shis the equilibriumthe expected Zn 4 1 4 -G grid (solvent: CD 3 CN; ESI pp. S3-S4 †): Pb 4 1 4 -G + 4 ZnX 2 / Zn 4 1 4 -G + 4 PbX 2 .
The reverse conversion Zn 4 1 4 -G / Pb 4 1 4 -G grid was achieved in several steps (Fig. 4b). Treatment of Zn 4 1 4 -G (in CD 3 CN) with KOH led to the precipitation of Zn 2+ (as Zn(OH) 2 or K 2 [Zn(OH) 4 ]), as well as of the free ligand 1. Aer removal of CD 3 CN, the free ligand 1 was extracted with CDCl 3 and used further for the preparation of Pb 4 1 4 -G (see ESI, p. S5 †).
Thus, in addition to its self-assembly from Zn 2+ and a ligand, the same Zn 2+ grid, Zn 4 1 4 -G, can be obtained, in reactions (i) and (ii), from a Ag + dinuclear double helicate or from a Pb 2+ tetranuclear grid (exchange of metal ions and reorganization of the architectures).

Experimental
For experimental details, see the ESI. †

Conclusions
To summarize, three supramolecular reactions were investigated: (i) a Ag 2 L 2 -DH double-helicate into Zn 4 L 4 -G grid conversion, where the exchange of metal ions changes the nature of the metallo-supramolecular architecture, (ii) a Zn 4 1 4 -G grid into Pb 4 1 4 -G grid conversion driven by a halide-induced precipitation and where the nature of the metallo-supramolecular architecture is conserved, and (iii) a double exchange of ligands during the fusion of two double helicates.
The grid/grid and double-helicate/grid conversions were made reversible by precipitation of Zn 2+ with KOH and subsequent reaction of the free ligand with Ag + or Pb 2+ , or, for one DH/G interconversion, in a pH-dependent way.
In perspective, such ligands could be introduced in larger and more complex, suitably decorated, architectures where such supramolecular reactions can act as actuators of various properties (charge, volume, multivalency).