Coordination polymer particles with ligand-centred pH-responses and spin transition

A bis-catechol ligand connected through an imine bond is used to fabricate switchable coordination polymer particles with pH-tuned spin transition responses.

Longer exposition times show the formation of additional side-products, most likely tetrahydroisoquinoline (THIQ), associated to the presence of additional Pictet-Spengler cyclization reaction (see Supplementary Material Fig. S1 and S2). 13 On the contrary, exposition of L 1 to pH~7 indicates that it was stable with less than 20% of the initial aldehydic species recovered after 5 hours (see Fig. S3).
CPPs were fabricated afterwards by interfacial polymerization by mixing an aqueous solution of cobalt acetate with an organic solution (EtOH/DMSO) of 1,4-bis(imidazole-1-ylmethyl)benzene (bix) and L 1 .The precipitate formed after 3 days was collected, washed several times with water and EtOH, and dried under vacuum resulting in particles (CPP 1 ) with average diameters between 0.2-1.6 m, as shown by scanning electron microscopy (SEM) (see Fig. 2b).Smaller nanoparticles (average diameter size around 40 nm) can also be obtained by using magnetic stirring technique (see Supplementary Material S3.2 and Fig. S4).Infrared analysis revealed the introduction of ligand L 1 within the structure of the new nanoparticles with the presence of bands in the 1275-1289 cm -1 range attributed to the C-O stretching of the catecholate mode (see Supplementary Material Fig. S5).Moreover, the lack of bands in the 3460-3240 cm -1 region assigned to the O-H stretching as well as the band at 1357 cm -1 associated to the O-H bending, confirmed the coordination state of L 1 .The presence of bix was also verified by the appearance of the typical bands around 3136, 1232 and 1105 cm -1 resultant of stretching and bending mode of C-H present in the imidazole and aromatic rings, respectively.Moreover, UV-Vis spectroscopy of CPP 1 showed clearly the presence of L 1 ligand through the absorption band centred in 392nm (see Fig S6).X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray (EDX) confirmed finally the presence of the cobalt ion (see Supplementary Material Fig. S7 and S8).Worth to mention, elemental analysis on different nanoparticle batches slightly differ from the expected values for a 1:1:1 (L 1 :bix:cobalt) ratio (see Fig. 2a and Supplementary Material S3.1), fact that has been tentatively attributed to the encapsulation of free ligand molecules or solvent molecules within the particles along its formation process, as already reported. 14he pH-response of CPP 1 was studied in two different buffer solutions at pH~5 (CBS) and pH~7 (PBS) under magnetic stirring.After fixed periods of time the resulting samples were centrifuged and washed several times with water and EtOH.The SEM images of the dispersions at pH~5 at 3 hours already show a remarkable loss of their spherical shape while inducing agglomeration (see Figure 3b).On the contrary, the same particles retain their characteristic spherical shape upon exposition at pH~7 even for 14hs (see Supplementary Material Fig. S9).pH-induced morphological modifications were associated to the disruption of the imine bond, as confirmed by chemical means.Infrared analysis of CPP 1 did not show any difference before and after exposition at pH~7 for 14 hours whereas significant changes were observed at pH~5 (see Fig. S10 and S11).As expected, the characteristic band at 1645 cm -1 of the C=N of imine as well as the bands at 1486 cm -1 ( C=C) and 1099 cm -1 ( C-H, bix) disappear with the time.Interestingly, the imine rupture does not affect the coordination bonds as confirmed by magnetization measurements.
Variable-temperature magnetic characterization of CPP 1 was done on the 15-370 K temperature range operating at a magnetic field strength of 0.1 T (Figure 3a, black squares).At high temperatures, the T value of 2.07 emu•K•mol -1 is close to the expected value for the high-spin [Co II (DBSQ) 2 ].On cooling, we observe a decrease of T down to a value of 1.79 emu•K•mol -1 at 300 K that is associated with the interconversion from the high-spin [Co II (Semiquinone)] to the low-spin [Co III (Catecholate)] isomer.Below 200 K, the T value monotonically decreases down to a value of 1.20 emu•K•mol -1 at 15 K.Such decrease is typical of magnetically isolated Co(II) centres, caused by the spin-orbit coupling, 15 and, in less degree, to the valence tautomeric interconversion.Though, when the same magnetic measurements are done on the pH sensitive nanoparticles after exposure at pH~5 for 14h, the characteristic VT behaviour of most VT samples obtained upon solvent evaporation is found.10b As can be seen in Figure 3a, the resulting sample exhibits a T value essentially independent of temperature and close to the value of 0.5 emu.K.mol -1 .Such value confirms that this sample remains mostly on the low-spin [Co III (Catecholate)] form along the whole temperature range.Considering that the coordination sphere around the cobalt ion remains unaltered, the different VT behaviour between the amorphous CPP 1 and the product resulting upon imine dissociation can be associated to the matrix modification upon dissociation process.
Finally, to fully confirm that dissociation of CPP 1 takes place because the presence of the pH-sensitive imine bond, novel nanoparticles were fabricated using a related commercial ligand L 2 (norhydroguaiaretic acid), where the imino group is replaced by an alkyl group (see Fig. 1a).Initially, the stability of L 2 under acid condition (pH~5) was tested by 1 H NMR, proving to be stable for long time (see Supplementary Material Fig. S12).Afterwards, amorphous nanoparticles (CPP 2 ) were also obtained by interfacial polymerization and fully characterized by SEM, FT-IR, EDX and XPS (see Supplementary Material S6).As expected, the pH-response of CPP 2 colloidal suspensions did not reflect any remarkable difference after 24 hours at pH~7 (PBS) none at pH~5 (CBS) under magnetic stirring, as monitored by SEM, FT-IR, UV-Vis and magnetization measurements (see Suplementary Material S7).

Conclusions
pH-sensitive amorphous CPP particles with ligand centered responses have been reported for the first time.For this, a flexible bis-catecholate bridging ligand that: I) can induced polymerization, II) assures strong coordination capabilities at different pHs and III) its redox properties allow to monitor the particle dissociation with time, is used.pH-triggered response of the CPP particles resulting from this ligand is therefore ensured by introducing a sensitive imine bridge; in this way, CPP 1 particles turn out to dissociate after a few hours at pH 5 while remain stable at pH 7 for many more hours.Moreover, similar CPP particles without the pH sensitive ligand do not exhibit any response to pH.These results open the door for the use of CPPs with improved performances in well-developed fields such as sensing, drug delivery or molecular electronics, among many others.

Fig. 1
Fig. 1 (a) Chemical structures of ligands L1 and L2.(b) Synthesis of L1 ligand by condensation reaction between dopamine hydrochloride and 3,4dihydroxybenzaldehyde.(c) Time-dependence 1 H NMR of L1 at pH~5 at the times thereby indicated.

Fig. 2
Fig. 2 a) Schematic illustration describing the coordination between ligands bearing several anchor groups (L1 and bix) and Co(II) to form nanoscale CPPs.b) SEM image and size distribution histogram of CPP1.

Fig. 3
Fig. 3 (a) T values as a function of temperature for the pH sensitive CPP1 before (■) and after acidic treatment for 14h (•).(b) SEM images of CPP1 before and after decomposition at pH 5 (CBS buffer) at the times thereby indicated.Scale bars are 4 m.