Temperature‐Driven Axial‐Equatorial Isomerism and Magnetic Relaxation on Low‐Spin Co(<scp>II</scp>) Borohydride Complexes

Comprehensive Summary 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 t Bu ) 3 )( cis ‐dppen)](BF 4 )· solv ([HB(tim t Bu ) 3 ] − = hydrotris(3‐tertbutyl‐2‐thioxoimidazol‐1‐yl)borate; cis ‐dppen = cis ‐1,2‐bis(diphenylphosphino)ethene; solv = 0.5THF·2H 2 O and 2H 2 O for ax ‐CoHS 2 P 2 and eq ‐CoHS 2 P 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.