Temperature-Driven Axial-Equatorial Isomerism and Magnetic Relaxation on Low-Spin Co(II) Borohydride Complexes

Abstract

Crystalline materials with diastereomerism serve as ideal prototypes for investigating the influence of coordination environments on chemophysical properties. In this study, we synthesized a pair of axial-equatorial isomers [Co(II)(HB(tim ^tBu) ₃)( cis-dppen)](BF ₄)· solv ([HB(tim ^tBu) ₃] ^- = hydrotris(3-tertbutyl-2-thioxoimidazol-1-yl)borate; cis-dppen = cis-1, 2-bis(diphenylphosphino)ethene; solv = 0.5THF·2H ₂O and 2H ₂O for ax-CoHS ₂P ₂ and eq-CoHS ₂P ₂, respectively), by varying the crystallization temperatures. Despite both diastereoisomers adopting distorted square pyramidal geometries, the bidentate cis-dppen ligand chelates to the central Co(II) either in an axial-equatorial or equatorial-equatorial manner, with a boron-hydrogen binding to the metal center. Magnetic studies reveal differences in g-values between these axial-equatorial isomers. X-band electron paramagnetic resonance spectra suggest rapid equilibrium and potential conformational interconversion in response to temperature changes in solution. Magnetic measurements indicate field-induced slow relaxation of magnetization in this low-spin S = 1/2 system, with spin-lattice relaxations dominated by Raman and quantum tunneling of magnetization mechanisms due to the absence of thermally populated excited states.

Keywords

Metal borohydride complexes / Axial-equatorial isomerism / Low spin / Magnetism / Stereochemistry / Crystal structure / Isomers

RIGHTS & PERMISSIONS

2024 SIOC, CAS, Shanghai, & WILEY-VCH GmbH.