Slow magnetization dynamics in a series of two-coordinate iron(II) complexes

A series of two-coordinate complexes of iron(II) were prepared and studied for single-molecule magnet behavior. Five of the compounds, Fe[N(SiMe3)(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 Fe-II center, while the sixth compound, Fe [N(H)Ar-#](2) (6), is bent with an N-Fe-N angle of 140.9(2)degrees (Dipp = C6H3-2,6-Pr-2(i); Ar' = C6H3-2,6-(C6H3-2,6-Pr-2(i))(2); Ar* = C6H3-2,6-(C6H2-2,4,6-Pr-2(i))(2); Ar-# = C6H3-2,6-(C6H2-2,4,6-Me-3)(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 U-eff = 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-infinity 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.