Heterometallic Coordination Polymer Gels Supported by 2,4,6-Tris(pyrazol-1-yl)-1,3,5-triazine

Complexes of type [M(tpt)2]X2 (M2+ = Fe2+, Co2+, Ni2+; tpt = 2,4,6-tri{pyrazol-1-yl}-1,3,5-triazine; X– = BF4– or ClO4–) crystallize in a cubic lattice, with the metal ion and ligand conformation showing unusual symmetry-imposed disorder. Addition of 1 equiv AgX to the corresponding preformed [M(tpt)2]X2 salt in concentrated MeNO2 solution affords thixotropic gels. Gelation was not observed in analogous reactions using [Mn(tpt)2][ClO4]2, or from reactions in other, more donating solvents. Scanning electron microscopy (SEM) images from dilute solutions of the reagents confirmed the fibrous microstructure of the gels and their homogeneous elemental composition. However, energy-dispersive X-ray data show a reduced Fe/Ag ratio compared to the Co/Ag and Ni/Ag gels, where a 1:1 ratio of metals is evident. More concentrated gels decomposed to silver nanoparticles during SEM sample preparation. Mass spectrometry and 1H NMR indicate that silver induces partial ligand displacement reactions in [Fe(tpt)2]2+ and [Co(tpt)2]2+, but not in [Ni(tpt)2]2+. Hence, the strength of the gels, which follows the order M = Mn (no gel) < Fe < Co < Ni, correlates with the stability of octahedral [M(tpt)2]2+ under gelation conditions. Iron(II) complexes of the related ligands 2,4,6-tri{pyrazol-1-yl}pyridine (tpp) and 2,4,6-tri{pyrazol-1-yl}pyrimidine (tpym) did not undergo gelation with silver salts under the above conditions. The unique properties of tpt as a gelator in this work may reflect the crystallographically observed ability of metal-coordinated tpt to chelate to exogenous silver ions, through its pendant pyrazolyl group and triazinyl N donors. In contrast, the pendant azolyl substituents in silver complexes of the nongelators tpp and tpym only bind exogenous silver in monodentate fashion.


■ INTRODUCTION
Unravelling soft matter systems is a rich field of scientific investigation among chemists, physicists, and engineers. Over the last two decades, supramolecular gels have gained attention because of their potential applications in biomaterials, catalysis, displays, sensors, surface science, tissue engineering, and pollutant removal. 1−3 Supramolecular gels are usually formed by low-molecular-weight gelator molecules assembled in a 3D network, which traps a bulk amount of solvent via noncovalent interactions. The reversible and dynamic nature of the supramolecular interactions provides a mechanism for sensing or physical transformations in response to external stimuli. 2,3 More recently, coordination polymer gels (CPGs) or metallogels have also been widely reported, where metal ions play a crucial role in the assembly of the 3D network. 4−8 Inclusion of transition metals in the gel assembly brings tunability to the coordination strength, as well as new redox, optic, electronic, and magnetic properties which are intrinsic to the metal ion. These afford additional possibilities for applications in catalysis, luminescence, and adhesives as well as new types of sensing functionality. 7,8 Silver(I)-containing metallogels of pyridyl-containing gelators are a particularly common class of CPG, 9−20 which can often template the formation of silver nanoparticles under mild heating or reduction. 13−20 CPGs of first-row transition ions are also often supported by pyridyl gelators, 9,11,21−33 but can also be prepared from more diverse organic scaffolds based on other heterocyclic N-donors, carboxylates, or other donor groups. 5−8,34−44 A handful of f-element CPGs have also been prepared, with emissive or self-healing properties. 45−47 In some cases, the organic components have been found to be selective gelators for particular metal ions or salts, 31,35 whereas rare examples of heterometallic CPGs have also been reported. 21, 48 2,4,6-Tri(pyrazolyl)-1,3,5-triazine (tpt, Chart 1) and its derivatives are well-known ligands for transition ions, 49−57 and we have recently investigated two aspects of the chemistry of tpt. The first is the spin state of [Fe(tpt) 2 ] 2+ (Chart 1, M = Fe), which is related to a well-known family of iron(II) spincrossover (SCO) complexes. 56 The second are silver(I) complexes of tpt, which crystallize as 1D-coordination polymers of silver ions bridged by ditopic tpt ligands. 57 We therefore proposed [Fe(tpt) 2 ] 2+ centers might be linked into larger assemblies by coordination to exogenous silver ions. We now report a family of heterometallic CPGs derived by mixing preformed [M(tpt) 2 ]X 2 (M = Fe or another 3d metal ion; X − = BF 4 − or ClO 4 − ) with the corresponding silver(I) salt. To aid the interpretation of these results, complexes of two other, related ligands were also prepared and screened for silver-induced gelation behavior. Homometallic CPGs supported by a different trisubstituted 1,3,5-triazine scaffold have been described in a previous study. 58 ■ RESULTS AND DISCUSSION  Figure 1 and Table S2). 56 The complex adopts the cubic space-group Ia3̅ d, with one-sixth of a six-coordinate complex molecule in its asymmetric unit and one unique C 3 -symmetric ligand environment. The metal ion is distributed equally around the three N-donors of the unique triazinyl ring, with a concomitant twofold disorder of the coordinated pyrazolyl group. This symmetry-induced disorder yields a cubic lattice of tpt ligands, linked by a random array of nickel ions such that each tpt ligand coordinates only one metal atom (Figures S1 and S2). The triazinyl rings in the complex are sandwiched between two symmetry-equivalent BF 4 − ions, forming a typical anion···π interaction with a C···F distance of 2.756(8) Å ( Figure S3). 55,59 Crystals of the other Mn, Co, Ni, and Cu complex salts diffracted more weakly, but are isostructural with [Ni(tpt) 2 ]-[BF 4 ] 2 by X-ray powder diffraction ( Figure 2 and Table S3). Samples of [Cu(tpt) 2 ][BF 4 ] 2 were often contaminated by other crystals, however, including [Cu 2 (tpt) 3 (μ-bptO)(PzH)]-[BF 4 ] 3 , which was crystallographically characterized ( Figure S4 and Table S4). The 2,4-dipyrazolyl-6-hydroxy-1,3,5-triazine (bptOH, Chart 1) and pyrazole (PzH) ligands in this complex are derived from hydrolysis of tpt during the crystallization process (Scheme S1), and their presence in the compound was confirmed by mass spectrometry ( Figure S17). Although [Cu(tpt) 2 ][BF 4 ] 2 has previously been isolated in analytical purity, 55 copper(II) is known to promote hydrolysis of tpt under a variety of conditions owing to its high Lewis acidity. 53,54 Therefore, the following heterometallic gelation studies focus on the manganese, iron, cobalt, and nickel tpt complexes.
Mononuclear [M(tpt) 2 ]X 2 was reacted with 1 equiv of the appropriate silver salt AgBF 4 or AgClO 4 , in nitromethane. No reaction was observed when [Mn(tpt) 2 ][ClO 4 ] 2 was treated with AgClO 4 . However, when M = Fe, Co, or Ni, thixotropic CPGs assembled within the time of mixing ( Figure 3). Whereas the iron and cobalt gels show irreversible thixotropy, reverting to fluid solution upon mild shaking, the nickelcontaining gels are more robust and remain viscous after shaking. The metal-dependence of gel stability qualitatively The atoms in the asymmetric unit are shown with 50% displacement ellipsoids, whereas their symmetry-equivalent atom sites are de-emphasized with paler coloration. Bottom: complete [Ni(tpt) 2 ] 2+ complex dication. Only one orientation of the disordered pendant pyrazolyl substituents is shown, and H atoms were omitted for clarity. Symmetry codes: Color code: C, white or dark gray; Fe, pale or dark green; N, pale or dark blue.

ACS Omega
Article follows the order M = Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ , which corresponds to the Irving−Williams series for the strength of metal−ligand interactions. 60 The gels retain their viscosity when stored for a period of months at room temperature, in closed vials ( Figure S5).
The gelation procedure was also attempted in the different solvents water, acetone, methanol, acetonitrile, and dimethylformamide. Although the mononuclear complexes are soluble in those solvents, the CPGs only assembled in nitromethane and at concentrations of at least 16 mg·cm −3 of [M(tpt) 2 ]X 2 and 12 mg·cm −3 AgX. The Ag/[M(tpt) 2 ] 2+ stoichiometry is also critical for gelation. When more than 1 equiv silver salt is used, the viscosity of the gel is reduced, and when 3 equiv silver(I) salt was added the gel did not assemble at all ( Figure  S6). Lastly, the gelation process is also affected by temperature, with gelation being enhanced if the components are mixed at 273 K and inhibited in reactions above 298 K. Once formed, however, the gels are thermostable up to ca. 330 K.
Scanning electron microscopy (SEM) images of the Fe/Ag, Co/Ag, and Ni/Ag CPGs were investigated. When dilute solutions of [M(tpt) 2 ]X 2 and AgX were evaporated to dryness under ambient conditions, networks of gel-like fibers of submicron thickness were observed (Figures 4 and S7). In contrast, if preassembled gel was evaporated to dryness, the SEM showed an amorphous material homogeneously distributed with silver nanoparticles ( Figure S8). Hence, at higher concentrations the CPGs template the formation of silver nanoparticles when the solvent is removed. This is a common property of silver-containing CPGs. 13−20 Energy-dispersive X-ray (EDX) mapping experiments proved the homogeneous distribution of the elements in the intact gels ( Figures 5, S10, and S11

ACS Omega
Article gels also show ions containing the hydrolyzed tpt ligand fragment [bptO] − (Chart 1), which are not present in the ClO 4 − -containing samples. That could reflect participation of F − , produced by hydrolysis of BF 4 − inside the spectrometer, as a base or nucleophile in the hydrolysis reaction. 61 1 H NMR data from similar mixtures in CD 3 NO 2 contain one paramagnetic tpt ligand environment. For M = Fe and Co, addition of silver ions cleanly lowers the NMR symmetry of the tpt ligand from C 2 to C 1 , which clearly indicates formation of a heterometallic M/Ag/tpt species. Additional peaks in the diamagnetic region also indicate the presence of metal-free tpt in these silver-containing solutions. Interestingly, for M = Ni (which forms the strongest gels) addition of silver has little effect on the paramagnetic or diamagnetic parts of the NMR spectrum ( Figure 6). Hence, silver ions displace tpt ligands from [Fe(tpt) 2 ] 2+ and [Co(tpt) 2 ] 2+ in CD 3 NO 2 solution to form a new paramagnetic species, which is probably a heterometallic complex, but [Ni(tpt) 2 ] 2+ retains its integrity in the presence of silver ions under these conditions. That further supports the suggestion that the chemical structures of the [M(tpt) 2 ]X 2 /AgX CPGs depend on which "M" metal ion is present.
Our previous work demonstrated that [Fe(tpt) 2 ] 2+ is highspin at room temperature, and remains so on cooling. 56 With the aim of producing a new form of gel with SCO switching properties, 62,63 iron complexes of tpt-analogue ligands based on di(pyrazol-1-yl)pyridine and di(pyrazol-1-yl)pyrimidine scaffolds were investigated (Chart 2). These ligand types are well-known to afford SCO iron(II) complexes. 64 67 were newly synthesized for this study. These salts of [Fe(tpym) 2 ] 2+ do not form isostructural crystals from nitromethane/diethyl ether, but both adopt the expected sixcoordinate geometry with two pendant pyrazolyl groups per complex molecule (Figures S17, S18, and Table S4). The complexes were crystallographically high-spin at 120 K, which was confirmed by magnetic measurements showing them to be fully high-spin between 5 and 300 K ( Figure S21). [Fe-(tpym) 2 ][BF 4 ] 2 is also high-spin in CD 3 CN solution, over the liquid range of that solvent ( Figure S22). Hence, in contrast to a closely related compound, 65 tpym does not support SCO when bound to iron(II), which may reflect the inductive properties of its pendant pyrazolyl substituent on the tridentate ligand core. 66 Coordination of tpym to silver(I) yields dimeric or pentanuclear Ag/tpym molecular assemblies, containing μ,κ 1 :κ 3 -or μ 3 ,κ 1 :κ 2 :κ 2 -tpym ligands. 57 Neither [Fe(tpp) 2 ]X 2 nor [Fe(tpym) 2 ]X 2 (X − = BF 4 − and ClO 4 − ) formed gels when treated with 1 equiv AgX, under the conditions used to form the [M(tpt) 2 ] 2+ -containing CPGs. Hence, silver-induced gelation appears to be a unique property of [M(tpt) 2 ] 2+ in this work.

■ DISCUSSION
Among the ligands in this work, only the [M(tpt) 2 ] 2+ scaffold supports gelation upon addition of silver. Moreover, the strength of the [M(tpt) 2 ]X 2 /AgX gels depends markedly on the metal "M", in the order Mn (no gel) < Fe < Co < Ni, and on the solvent present. The mass spectra imply ligand exchange reactions occur in the M/Ag/tpt mixtures, to form mixed-metal multinuclear species with a lower M/tpt stoichiometry. NMR data confirm that conclusion for M = Fe and Co, but not for M = Ni whose NMR spectrum is unchanged upon addition of silver. Hence, the robustness of the [Ni(tpt) 2 ]X 2 /AgX gels probably reflects the increased

ACS Omega
Article stability of the [Ni(tpt) 2 ] 2+ center, which is consistent with the Irving−Williams series. 60 In that case, gelation of [Ni(tpt) 2 ]-X 2 /AgX occurs at concentrations high enough to promote weak coordination of silver ions to [Ni(tpt) 2 ] 2+ , yielding [Ag n {Ni(tpt) 2 } n ] 3n+ oligomers with reasonably regular structures. Formation of the [Fe(tpt) 2 ]X 2 /AgX and [Co(tpt) 2 ]X 2 / AgX gels involves more complicated chemistry and, although these gels have similar morphologies and compositions to the Ni gels by SEM, their chemical structures may be more complex.
The reluctance of [Fe(tpp) 2 ]X 2 and [Fe(tpym) 2 ]X 2 to undergo silver-induced gelation might be understood from the structures of the homoleptic silver complexes of those ligands. Crystals of [Ag(tpt)]X (X − = BF 4 − or ClO 4 − ) have been obtained as 1D coordination polymers with helical or linear connectivities, with κ 2 ,κ 3 :μ-tpt ligands ( Figure S23). 57 That is, the tpt ligands in these structures chelate to both silver ions that are coordinated to them, through their pyrazolyl and triazinyl N donors. Several other 1,3,5-triazine derivatives can also bridge between silver ions in a similar fashion, at least in the solid state. 68−72 In contrast, [Ag(tpp)]X can only assemble into larger aggregates by monodentate binding through its pendant pyrazolyl substituent ( Figure S24). 57 Moreover, although chelation of a second silver ion by tpym through the pyrimidinyl N1 atom and C6-pyrazolyl substituent is feasible in principle, this has not yet been observed in practice. 57 That may reflect a preferred transoid orientation of those N-donors, which avoids an intramolecular steric clash between the pyrimidinyl C4 and pyrazolyl C5 C−H groups (highlighted in red in Scheme 1). The transoid conformation is indeed observed crystallographically in [Fe(tpym) 2 ][ClO 4 ] 2 · nMeNO 2 ( Figure S17; the pendant pyrazolyl conformation in the BF 4 − salt of this complex is uncertain because of symmetryimposed crystallographic disorder).
Hence, of the ligands considered in this work, only tpt has a proven ability to chelate two silver ions simultaneously, which will afford more stable mixed-metal assemblies in solutions of [M(tpt) 2 ]X 2 and AgX. That might explain the unique gelation properties of the [M(tpt) 2 ]X 2 /AgX system. Thus, the strongest and most thermally stable gels were obtained for M = Ni. SEM images showing the expected fibrous microstructures were obtained from dilute solutions of the gel components, with element mapping demonstrating their chemical homogeneity. However, EDX analyses imply that the M/Ag ratio in the gels, n, is smaller for M = Fe than for M = Co or Ni where approximately 1:1 ratios of these metals were observed. 1 H NMR also demonstrates that solutions of [M(tpt) 2 ]X 2 and AgX contain different species when M = Fe or Co, than for M = Ni (where the [Ni(tpt) 2 ] 2+ cation retains its integrity in the presence of silver ions). Hence, the chemical structure of the gels seems to vary depending on which "M" metal ion is present. The related complexes [Fe(tpp) 2 ] 2+ and [Fe(tpym) 2 ] 2+ do not form CPGs when combined with silver salts, which we attribute to their reduced ability to bind exogenous silver ions in a chelating (as opposed to monodentate) fashion. The current work aims to modify the gelator ligand structure further, to produce new thermochromic CPGs from SCO-active iron complex precursors. 73 4 ] 2 in nitromethane (12.5 mg, 0.016 mmol in 0.7 cm 3 ; 17.9 mg·cm −3 ). After brief stirring at room temperature, the CPG was formed. All the other CPG combinations were prepared in an analogous manner, using equimolar quantities of the appropriate complex and silver salt precursors.
Crystallography. All the crystals characterized in this study were obtained by slow diffusion of diethyl ether vapor into nitromethane solutions of the compounds. Crystallographic data were measured with an Agilent Supernova dualsource diffractometer, using monochromated Cu Kα (λ = 1.5418 Å) radiation. The diffractometer was fitted with an Oxford Cryostream low-temperature device. Experimental data (Table S1) and refinement procedures for the structure determinations are given in the Supporting Information. The structures were solved by direct methods (SHELXS97 78 ) and developed by full least-squares refinement on F 2 (SHELXL97 78 ). Crystallographic figures were prepared using XSEED, 79 and coordination volumes (V Oh , Tables S2 and S4) were calculated using Olex2. 80 Other Measurements. Electrospray mass spectra were obtained on a Bruker MicroTOF spectrometer, from nitromethane solution. Sodium cations and formate anions in the molecular ion assignments originate from calibrants in the spectrometer feed solutions. Elemental microanalyses were performed by the University of Leeds School of Chemistry microanalytical service or the London Metropolitan University microanalytical service. X-ray powder diffraction patterns were measured using a Bruker D2 Phaser diffractometer, with Cu Kα radiation (λ = 1.5418 Å). SEM images were obtained using an FEI Nova NanoSEM 450 environmental microscope, operating at 3 kV. A silicon wafer supporting the gel was mounted on an SEM stub using an adhesive copper film, then coated with iridium before imaging.
Magnetic susceptibility measurements were obtained using a Quantum Design SQUID magnetometer in an applied field of 5000 G. Diamagnetic corrections were estimated from Pascal's constants, 81 and a diamagnetic correction for the sample holder was also applied. Susceptibility measurements in solution were obtained by the Evans method using a Bruker DRX500 spectrometer operating at 500.13 MHz. 82,83 A diamagnetic correction for the sample 81 and a correction for the variation of the density of the CD 3 CN solvent with temperature 84 were applied to these data.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10