TY - JOUR T1 - Postsynthetic Treatment of ZIF-67 with 5-Methyltetrazole: Evolution from Pseudo-Td to Pseudo-Oh Symmetry and Collapse of Magnetic Ordering JF - Inorganic Chemistry Y1 - 2022 A1 - Daniel J. SantaLucia A1 - Wenhui Hu A1 - Denan Wang A1 - Jeir Huang A1 - John Berry KW - Group theory KW - Ions KW - Ligand exchange KW - Ligands KW - Mathematical methods AB -

Reaction of Co(II) nitrate with 2-methylimidazole (2mIm) yields ZIF-67, the structure of which features Co(II) ions in pseudo-tetrahedral coordination geometry. Strong antiferromagnetic interactions between Co(II) ions mediated by the 2mIm ligands lead to antiferromagnetic ordering at 22 K. Postsynthetic treatment of Co(II) ZIF-67 with 5-methyltetrazole (5mT) results in the loss of crystallinity and magnetic order. The local structure of the Co(II) ions was probed by a combination of diffuse-reflectance electronic absorption spectroscopy and Co K-edge X-ray absorption spectroscopy (in the XANES and EXAFS regions). Upon reaction with 5mT, the 4A2(F)–4T1(F) and 4A2(F)–4T1(P) transitions at 1140 and 585 nm, respectively, of the pseudo-tetrahedral Co(II) center in ZIF-67 become less prominent and are replaced by transitions at 990 and 475 nm attributable to the 4T1g(F)–4T2g(F) and 4T1g(F)–4T1g(P) transitions of a pseudo-octahedral Co(II) center, respectively. Furthermore, the 1s–3d pre-edge absorption feature in the Co K-edge XANES spectrum loses intensity during this reaction, and the edge feature becomes more sharp, consistent with a change from pseudo-Td to pseudo-Oh geometry. EXAFS analysis further supports the proposed change in geometry: EXAFS data for ZIF-67 are well fitted to four Co–N scatterers at 1.99 Å, whereas the data for the 5mT-substituted compound are best fitted with 6 Co–N scatterers at 2.14 Å. Our results support the conclusion that a six-coordinate, pseudo-Oh geometry is adopted upon ligand substitution. The increase in coordination number directly increases the Co–N bond distances, which in turn weakens magnetic exchange interactions. No magnetic ordering is found in the 5mT-substituted materials.

VL - 61 UR - https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c00066 IS - 16 ER -