A series of two-coordinate complexes of iron(ii) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe 3)(Dipp)]2 (1), Fe[C(SiMe3)3] 2 (2), Fe[N(H)Ar′]2 (3), Fe[N(H)Ar*] 2 (4), and Fe(OAr′)2 (5) feature a linear geometry at the FeII center, while the sixth compound, Fe[N(H)Ar #]2 (6), is bent with an N-Fe-N angle of 140.9(2)° (Dipp = C6H3-2,6-Pri2; Ar′ = C6H3-2,6-(C6H3-2,6-Pr i2)2; Ar* = C6H 3-2,6-(C6H2-2,4,6-Pri 2)2; Ar# = C6H3-2,6- (C6H2-2,4,6-Me3)2). Ac magnetic susceptibility data for all compounds revealed slow magnetic relaxation under an applied dc field, with the magnetic relaxation times following a general trend of 1 > 2 > 3 > 4 > 5 ≫ 6. Arrhenius plots created for the linear complexes were fit by employing a sum of tunneling, direct, Raman, and Orbach relaxation processes, resulting in spin reversal barriers of Ueff = 181, 146, 109, 104, and 43 cm-1 for 1-5, respectively. CASSCF/NEVPT2 calculations on the crystal structures were performed to explore the influence of deviations from rigorous D∞h geometry on the d-orbital splittings and the electronic state energies. Asymmetry in the ligand fields quenches the orbital angular momentum of 1-6, but ultimately spin-orbit coupling is strong enough to compensate and regenerate the orbital moment. The lack of simple Arrhenius behavior in 1-5 can be attributed to a combination of the asymmetric ligand field and the influence of vibronic coupling, with the latter possibility being suggested by thermal ellipsoid models to the diffraction data.
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