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)](BF₄)·solv ([HB(tim^tBu)₃]⁻ = hydrotris(3-tertbutyl-2-thioxoimidazol-1-yl)borate; cis-dppen = cis-1,2-bis(diphenylphosphino)ethene; solv = 0.5THF·2H₂O and 2H₂O for ax-CoHS₂P₂ and eq-CoHS₂P₂, 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.