Postsynthetic Treatment of ZIF-67 with 5-Methyltetrazole: Evolution from Pseudo-Td to Pseudo-Oh Symmetry and Collapse of Magnetic Ordering

Reaction of Co(II) nitrate with 2-methylimidazole(2mIm) yields ZIF-67, the structure of which features Co(II) ionsin pseudo-tetrahedral coordination geometry. Strong antiferromag-netic interactions between Co(II) ions mediated by the 2mImligands lead to antiferromagnetic ordering at 22 K. Postsynthetictreatment of Co(II) ZIF-67 with 5-methyltetrazole (5mT) results inthe loss of crystallinity and magnetic order. The local structure ofthe Co(II) ions was probed by a combination of diffuse-reflectanceelectronic absorption spectroscopy and Co K-edge X-ray absorptionspectroscopy (in the XANES and EXAFS regions). Upon reactionwith 5mT, the4A2(F)-4T1(F) and4A2(F)-4T1(P) transitions at1140 and 585 nm, respectively, of the pseudo-tetrahedral Co(II)center in ZIF-67 become less prominent and are replaced bytransitions at 990 and 475 nm attributable to the4T1g(F)-4T2g(F)and4T1g(F)-4T1g(P) transitions of a pseudo-octahedral Co(II) center, respectively. Furthermore, the 1s-3d pre-edge absorptionfeature in the Co K-edge XANES spectrum loses intensity during this reaction, and the edge feature becomes more sharp, consistentwith a change from pseudo-Tdto pseudo-Ohgeometry. EXAFS analysis further supports the proposed change in geometry: EXAFSdata for ZIF-67 are wellfitted to four Co-N scatterers at 1.99 A, whereas the data for the 5mT-substituted compound are bestfittedwith 6 Co-N scatterers at 2.14 A. Our results support the conclusion that a six-coordinate, pseudo-Ohgeometry is adopted uponligand substitution. The increase in coordination number directly increases the Co-N bond distances, which in turn weakensmagnetic exchange interactions. No magnetic ordering is found in the 5mT-substituted materials

Automated summary

Reaction of Co(II) nitrate with 2-methylimidazole(2mIm) leads to the formation of ZIF-67 with a pseudo-tetrahedral coordination geometry for Co(II) ions. Strong antiferromagnetic interactions between Co(II) ions mediated by the 2mIm ligands result in antiferromagnetic ordering at 22 K. The postsynthetic treatment of Co(II) ZIF-67 with 5-methyltetrazole (5mT) leads to the loss of crystallinity and magnetic order. The change in local structure and geometry of the Co(II) ions upon substitution is confirmed by various spectroscopic techniques. The increase in coordination number weakens the magnetic exchange interactions.