Fac vs Mer Selection in Octahedral Complexes of the N‑Benzyl- Substituted Triazolylpyridine Diad with Labile Metal Ions (Zinc(II), Iron(II), and Nickel(II))

Abstract

Pseudo-octahedral complexes built on chelating ligands offer valuable isomeric properties such as the formation of fac vs mer isomers, and lambda vs delta enantiomers. This article discusses factors guiding the fac vs mer selection of complexes built from the triazolyl-pyridine platform bearing a benzyl substituent, in conjunction with labile metal ions such as zinc(II), iron(II) and nickel(II). Critical parameters that are explored include the geometrical flexibility of the individual metal ion, and the nature of the counter-anion. In essence, it was observed that ions which can withstand significant distortions in their first coordination sphere, such as zinc(II), generally prefer forming the mer isomer irrespective of the counter-anion, whereas ions with more restricted geometry, such as iron(II), are directed either to a fac arrangement by small, tetrahedral anions such as tetrafluoroborate or perchlorate, or to a mer organization by counter-anions of lower symmetry, such as triflate. The molecular interactions operating the selection are discussed. Interestingly, such iron(II) complexes are prime candidates for spin cross-over behavior; hence understanding their isomeric selection may assist in guiding the design of solid-state magnetic materials.