From Isolated 1H -Pyrazole Cryptand Anion Receptors to Hybrid Inorganic-Organic 1D Helical Polymeric Anion Receptors.

hybrid

1D-Coordination polymers (1D-CPs) have been widely explored in the last years. 1owever, although several examples of coordination polymers based on macrocycles have been reported, 1,2 as far as we know 1D-CPs built up connecting cryptands have not yet been described.On the other hand, as it is known that cryptand receptors when protonated can tightly confine in its interior anionic guest by means of electrostatic attraction and hydrogen bonding interactions, 3 the idea of constructing CPs that can serve as multi-anion receptors is very appealing.Here we report on an 1D-CP that fulfils these characteristics.
To achieve this goal we have made use of cryptand 1 consisting of two tren polyamine subunits connected to three 1H-pyrazole macrocycles through methylene spacers.We had previously proved that this receptor forms above pH 5 binuclear or hexanuclear dimer Cu 2+ complexes depending on whether the metal:cryptand ratio is 2 or 3 (see structures 2 and 3 in Scheme 1). 4 In such complexes pyrazole was deprotonated behaving as a bis(monodentate) or exobidentate ligand (Scheme 1).However to interconnect cryptands we need to move from this endo coordination, in which the metal ions are located inside the cavity, to an exo coordination with the metal ions placed outside the cavity acting as connectors between different macrocycles. 1,2This change would in principle require of an appropriate acidic pH range so that the amine groups are protonated while the pyrazole groups are in the neutral form and therefore bind the Cu 2+ ions in a monodentate mode (Scheme 1), 5 In this way a polymeric strand of charged cryptands ready to include anions in a multivalent fashion might be fabricated.In addition to OW1 and OW2, the Cl1 is hydrogen bonded to two of the ammonium groups of its macrocyclic side (N2 and N10, Figure 1 right).Moreover, the distance to the third ammonium group of this side (3.349(3)Å) suggests also an additional weak hydrogen bond contact.On the other hand, the water molecules OW1 and OW2 accept hydrogen bonds from the ammonium groups N5, N14 and N7, respectively, all these at the macrocyclic side not occupied by the chloride anion (Figure 1 right).The OW1 and OW2 complete their tetrahedral hydrogen bond arrangement with one exo chloride anion and a water molecule, respectively, both of them placed outside the cage.A list of relevant hydrogen bond contacts is reported in Table S2.Interestingly, the nitrogen atoms at the edges of the 1H-pyrazole moieties point outwards from the macrocyclic cavity establishing two hydrogen bonds to an exo chloride anion and a water molecule while the third one hydrogen bonds to a water molecule and a pyrazole sp 2 nitrogen of an adjacent cryptand.
To evaluate the stability of the included chloride we have first performed potentiometric titrations of the aqueous solutions of the cryptand using sodium trifluoromethanesulfonate (NaTFMS) as background electrolyte since the large size of the TFMS anion prevents its inclusion in the cage cavity. 7From these measurements the protonation constants of 1 were derived.Then, from titrations of the macrocycle in the presence of chloride we obtained the stability constants of the anion complexes formed (Table S3 and Figure S3).At pH 2 the effective binding constant 8 is in the order of 10 5 and such high values prevented their derivation from 1 H NMR titrations (Figure S5).The JOB plots obtained from the 1 H NMR measurements supported the 1:1 stoichiometry of the complexes formed inferred from the potentiometric titrations (Figure S6).On the other hand, the stability of the chloride complex at pH 2 is higher than that of the fluoride complex as confirmed by 19 F NMR studies.The 19 F NMR spectrum of a solution containing fluoride and the macrocycle at pH=2 in 1:1 molar ratio displays peaks at -118 ppm, -140 ppm and -155 ppm that can be ascribed to F -, HF and F-H-F interacting with the macrocycle (Figure S7-S8). 9 5) suitable for Xray analysis (Table S1).The crystal structure can be defined as a racemic mixture of left-and right-handed helical 1Dcoordination polymers (1D-CPs) in which every hexaprotonated macrocycle is connected to its two neighbours through monodentate pyrazole units by alternating sequence of one CuCl 3 -(Cu1) and two CuCl 3 -(Cu2, Cu3) triangular moieties (Figure 2a).In every macrocycle all the secondary amino groups are protonated as in the case of 4.
The positive charge of the 1-D polymer is compensated by a chloride anion included in each one of the macrocycles and by chloride counter-anions.The coordination geometry around each connecting Cu 2+ is trigonal bipyramidal with three chloride ligands occupying the equatorial plane and two pyrazole nitrogens at the shortened axial positions.Each helical turn is repeated every five macrocycles (Figure 3).MALDI-TOF spectra show a peak at m/z 1773.32 attributable to [K(Cu 2 (H 9 1) 2 )Cl 13 ] + which corresponds to two macrocycles interconnected by two Cu 2+ ions.A peak at m/z 1512.55 corresponds, on the other hand, to two macrocycles connected by a single Cu 2+ ion (Figures S10-S14).These are the two basic building blocks of the 1D-chains.Interestingly enough the same crystal structure is obtained when the binuclear Cu 2+ complex 2 is brought to pH 1.5 with HCl.Therefore, the acidification of the solutions leads to detachment of the metal from the amino groups and to re-organisation of the pyrazole units which change from a bis-monodentate (anionic pyrazolate) to a monodentate coordination mode (neutral pyrazole) (Scheme 1).There are two different protonated macrocyclic subunits in the helix; one of them includes a chloride anion and two water molecules while the other one encapsulates one chloride anion and three water molecules (Figure 2b).Each chloride anion is accepting hydrogen bonds from the neighbouring water molecules.In turn, each water molecule accepts hydrogen bonds from two secondary ammonium groups of the cryptand.A intricate hydrogen bonding bond network is thus formed between the chloride anions and the water molecules inside of the macrocyclic cavity.In summary, pH modulation permits to switch from an endo to an exo coordination of Cu 2+ that, in this way, interconnects charged cryptand receptors forming a 1D-helical CP able to behave as a multi-anion receptor.

Figure 2 .
Figure 2.a) View of the two different monomers of Cu 2+ cryptand complexes (hydrogen bonds are represented as red dotted lines).b) View of the two different cryptand subunits.

Figure 3 .
Figure 3. a) Stick view of cryptand Cu(II) helicate 5 along the b axis.b) Spacefill view.Green spheres stand for chloride anions.