Enhancing the Blocking Temperature in Single‐Molecule Magnets by Incorporating 3d–5d Exchange Interactions

We report the first single‐molecule magnet (SMM) to incorporate the [Os(CN) 6 ] 3− moiety. The compound ( 1 ) has a trimeric, cyanide‐bridged Mn III ‐Os III ‐Mn III skeleton in which Mn III designates a [Mn(5‐Brsalen)(MeOH)] + unit (5‐Brsalen= N , N ′‐ethylenebis(5‐bromosalicylideneiminato)). X‐ray crystallographic experiments reveal that 1 is isostructural with the Mn III ‐Fe III ‐Mn III analogue ( 2 ). Both compounds exhibit a frequency‐dependent out‐of‐phase χ ′′( T ) alternating current (ac) susceptibility signal that is suggestive of SMM behaviour. From the Arrhenius expression, the effective barrier for 1 is found to be Δ eff / k B =19 K ( τ 0 =5.0×10 −7  s; k B =Boltzmann constant), whereas only the onset (1.5 kHz, 1.8 K) of χ ′′( T ) is observed for 2 , thus indicating a higher blocking temperature for 1 . The strong spin–orbit coupling present in Os III isolates the E′ 1g(1/2) ( O h *) Kramers doublet that exhibits orbital contributions to the single‐ion anisotropy. Magnetic susceptibility and inelastic neutron‐scattering measurements reveal that substitution of [Fe(CN) 6 ] 3− by the [Os(CN) 6 ] 3− anion results in larger ferromagnetic, anisotropic exchange interactions going from quasi‐Ising exchange interactions in 2 to pure Ising exchange for 1 with ${J{{{\rm MnOs}\hfill \atop \parallel \hfill}}}$ =−30.6 cm −1 . The combination of diffuse magnetic orbitals and the Ising‐type exchange interaction effectively contributes to a higher blocking temperature. This result is in accordance with theoretical predictions and paves the way for the design of a new generation of SMMs with enhanced SMM properties.