Molecular self-recognition : a chiral [ Mn ( II ) 6 ] wheel via donor – acceptor

Molecular recognition phenomena are of paramount importance in biochemistry, with one of the most useful expressions found in their double helix of DNA. Synthetic chemists have been able to recreate these processes by exploiting the intermolecular interactions expected between judiciously designed molecules, especially in the arena of organic chemistry. However, a particularly relevant area of supramolecular chemistry involves metal-containing molecules. In this context, helical coordination complexes have proven to bind non-covalently to the major groove of DNA and also to the heart of the Y junctions featured occasionally by this macromolecule. These interactions have been proven to dramatically influence the normal development of crucial DNA transactions. Such precedents suggest that the design and development of coordination assemblies incorporating specific functions for molecular recognition represent a promising avenue to influence or perturb biological processes through selective interaction with macro biomolecules. Here, we have employed the new ligand 2,6-bis-(5-(naphth-2yl)-pyrazol-3-yl)pyridine (H2L, Scheme 1, details have been reported elsewhere), exhibiting several coordinating atoms for binding to metals. In addition, H2L incorporates potential donor and acceptor moieties for the establishment of hydrogen bonds, as well as electron rich and electron poor aromatic fragments. Upon reacting with Mn(AcO)2, this ligand facilitates the formation of the chiral trinuclear cluster [Mn3(AcO)4(HL)2] (1), which possesses all the ingredients for molecular recognition. In fact, pairs of molecules with the same parity interact with each other by means of a series of complementary intermolecular contacts leading to metallo-supramolecular dimers of a helical nature. The structural details of this assembly, resulting from a selfrecognition process, together with its physico-chemical properties, are presented here. The H2L ligand is analogous to other 2,6-bis-(pyrazol-3yl)pyridine ligand derivatives, exhibiting aromatic extensions at both ends of the central core, which have proven successful in fostering intermolecular interactions in the context of spin crossover molecular materials. The ligand could be easily formed in a high yield by reacting the bis-b-diketone precursor, which we have made using a modified form of the Claisen condensation, with hydrazine. It reacts with Mn(II) from a mixture of Mn(AcO)2 and NBu4MnO4, originally intended to contain also Mn(III), as a way to emulate the rich chemistry demonstrated by the 2-hydroxyphenyl counterpart of the naphthyl ligand H2L. The resulting coordination product, [Mn3(AcO)4(HL)2] (1), only contains Mn(II). However, it was not possible to generate this compound in the absence of the permanganate salt. While the crucial role of NBu4MnO4 in this process is not clear, it could be related to the necessity of a good base to partially deprotonate H2L. However, attempts to form 1 from its constituents in basic conditions were unsuccessful and in fact, they only led to the crystallization of the composite ensemble [H2L (Bu4N)(AcO) H2O] (Fig. S1 and Tables S1 and S2, ESI†). Compound 1, in turn, crystallizes in the monoclinic Scheme 1 The structure of H2L.

The same procedure was repeated using acetone as a solvent and produced essentially analogous results.

Physical Measurements
Variable-temperature magnetic susceptibility data were obtained with a Quantum Design MPMS5 SQUID magnetometer.Pascal's constants were used to estimate diamagnetic corrections to the molar paramagnetic susceptibility.The elemental analysis was performed with an Elemental Microanalizer (A5), model Flash 1112 at the Servei de Microanàlisi of CSIC, Barcelona, Spain.IR spectra were recorded as KBr pellet samples on a Nicolet AVATAR 330 FTIR spectrometer.Fluorescence emission spectra were carried out in DMF (c = 1x10 -6 M) using Horiba Jobin-Yvon SPEX Nanolog-TM and Cary Eclipse spectrofluorimeters.

Data for the crystal with composition [H 2 L•(Bu 4 N)(AcO)•H 2 O]
were collected on a colorless parallelepiped at 100 K on a Bruker APEX II CCD diffractometer on Advanced Light Source beam-line 11.3.1 at Lawrence Berkeley National Laboratory, from a silicon 111 monochromator (λ = 0.7749 Å).Data reduction and absorption corrections were performed with SAINT and SADABS [4] , respectively.The structure was solved and refined on F 2 with SHELXTL-2013 suite [5,6] .Hydrogens on the lattice water molecule O1W as well as those on the pyrazole nitrogens N2 and N4 were found in a difference Fourier map and refined freely with their Ueq 1.5 times that of their carrier Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2015 atom.The rest of hydrogens were placed on their carrier atom geometrically and refined with a riding model.Data for compound 1 were collected on a colorless lozenge plate at 100 K on a Bruker APEX II CCD diffractometer on Advanced Light Source beam-line 11.3.1 at Lawrence Berkeley National Laboratory, from a silicon 111 monochromator (λ = 0.7749 Å).Data reduction and absorption corrections were performed with SAINT and SADABS [4] , respectively.The structure was solved and refined on F 2 with SHELXTL-2013 suite [5,6] .Systematically on several crystals, significant diffraction was only observed for resolutions better than ca.0.85 Å, likely due to the presence of diffuse solvent areas in the structure.The present data were cut at 0.85 Å for this reason.One of the naphthyl groups was refined disordered over two positions of the external ring, with relative occupancies of 0.53:0.47.The rest of naphthyl groups also showed some thermal disorder but splitting them did not converge.All were refined with displacement parameters restraints, as well as the lattice THF molecules.Hydrogens were placed on their carrier atom geometrically and refined with a riding model.At the end of the refinement there remained areas in the cell with only weak electron density peaks that could not be modeled as lattice solvent molecules.This was analyzed and taken into account with SQUEEZE as implemented in the PLATON package. [7,8]A total of 86 electrons per cell were recovered over mostly 2 large voids of 725 cubic angstroms and 4 voids of 165 cubic angstroms.At the most these figures would agree with two diffuse lattice THF molecules per cell, e.g.half a THF molecule per formula unit or less.It was thus not included in the formula.

All details can be found in CCDC-1022213 (1) and -1022214 ([H 2 L•(Bu 4 N)(AcO)•H 2 O])
that contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystallographic and refinement parameters are summarized in Table S1.Selected bond lengths and angles are given in Tables S2, S3 and S4.S4 for inter-centroid distances).Oxygen is in red, nitrogen is purple, carbon is grey and hydrogen atoms are not shown.

Figure S3
. Representation of the intermolecular interactions established between each [Mn 3 ] 2 dimers of 1 and its four first-neighbors, showing that each dimeric assembly interacts with two such entities of the same chirality on one side and with two of opposite chirality on the opposite side.This view shows that the interaction between species of the same chirality is more efficient than the contacts between systems of opposite chirality.

Figure S4
Plot of χT vs T for per mole of 1.The solid line is a fit to the experimental data using the Van Vleck equation (see text).The data have been previously corrected for diamagnetic contributions and temperature independent paramagnetism.

Figure S1 .
Figure S1.Labelled representation of the AcO -, H 2 O and H 2 L components of the compound [H 2 L•(Bu 4 N)(AcO)•H 2 O], emphasizing the H-bonding interactions as dashed lines.Hydrogen atoms are not shown.

Table S1 .
Crystal data for compounds 1 and [

Table S2 .
Hydrogen bonding in the structure of compound [

Table S4 .
Hydrogen bonding between [Mn 3 ] 2 pairs in the structure of 1

Table S5 .
Inter-centroid distances of the eight main donor/acceptor π•••π interactions within the [Mn 3 ] 2 pairs in the structure of 1 together with the closest interatomic distances between the rings of interest.See Figure S2 for labelled centroids.(Cd,Cd')