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.