Zeeman Level Anti-Crossing and Slow Magnetic Relaxation in an S-Functionalized Abnormal NHC-Supported Linear CoII3 Single-Molecule Magnet.

A novel trinuclear cobalt(II) complex, [(S-MIC)2Co(II)3I2(μ2-Cl)4(THF)2]·4THF (1), supported by a zwitterionic ylide-type sulfur-functionalized abnormal N-heterocyclic carbene (S-MIC) ligand, has been synthesized and characterized as the first S-functionalized aNHC-supported cobalt(II) single-molecule magnet (SMM). X-ray diffraction reveals a linear trinuclear CoII3 core in which the central Co(II) adopts a distorted octahedral geometry, while each terminal Co(II) is distorted tetrahedral, coordinated by two μ2-chlorides, one terminal iodide, and one sulfur donor from S-MIC. The χT value of 10.3 cm3 K mol-1 at 300 K indicates significant orbital angular momentum contributions. Ab initio SA-CASSCF/NEVPT2 calculations with POLY_ANISO analysis reveal weak antiferromagnetic coupling (J = -3.39 cm-1; J' = -1.15 cm-1) and axial anisotropy (D values of -66.81 cm-1 for the central and -20.46 cm-1 for the terminal Co(II)). This weak coupling causes level anti-crossing of low-lying Zeeman levels, yielding open hysteresis and a marked magnetization-slope change near 35 kOe. Isothermal DC magnetization at 35 kOe confirms slow relaxation, with a τ of 451(3) s at 1.8 K. AC susceptibility measurements at 20 DC fields yield an effective barrier (Ueff = 25(20) K) under a QTM+Orbach model, discussed alongside ab initio calculations, underscoring the potential of S-functionalized carbene ligands for tuning coordination geometries and constructing multinuclear Co-based SMMs.