Relaxing amid Three: Slow Magnetization Relaxation in Three-Coordinate Chromium(II) N-Heterocyclic Carbene Complexes

Three-coordinate, paramagnetic Cr^II complexes of type [Cr(amido)_nBn_m(NHC)], NHC = N, N'-bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene (IMes); N, N'-bis-(2,6-di-isopropylphenyl)-imidazol(in)-2-ylidene, (S)IDiPP; N, N'-bis-(2,6-di-isopropylphenyl)-imidazol-4-ylidene, (aIDiPP); amido = N(SiMe₃)₂, NH(DiPP); Bn = benzyl, n = 2, m = 0; n = 1, m = 1, were prepared by substitution or aminolysis and thermolysis methods from [Cr{N(SiMe₃)₂}₂(THF)₂] or [CrBn₂((S)IDiPP)], respectively. Depending on the nature of the NHC and the amido ligands, different geometries at Cr^II (ranging from distorted trigonal planar, to extended Y-, compressed Y-, and distorted T-shaped) and conformations, were observed. HFEPR spectroscopy was employed to accurately determine spin Hamiltonian parameters, consisting of zero-field splitting (axial D and rhombic E components) and g-values, of five S = 2 complexes exhibiting D and E/D in the range from -2.98 to -1.63 cm^-1 and 0.026 - 0.069, respectively. AC magnetometry established slow magnetization relaxation in three complexes, operating by Raman or combined Raman-Orbach processes. Ab initio calculations provided computed zfs values, in good agreement with those obtained by HFEPR. Magnetostructural comparisons are made within this three-coordinate Cr^II family, as well as with previously studied two- or four- coordinate Cr^II complexes.