Carbonato-bridged NiII2LnIII2(LnIII = GdIII, TbIII, DyIII) complexes generated by atmospheric CO2 fixation and their single-molecule-magnet behavior: [(4-CO3)2{NiII(3-MeOsaltn)(MeOH or H₂O)LnIII(NO3)}2solvent [3-MeOsaltn = N, N -Bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato]

Atmospheric CO₂ fixation of [Ni^II(3-MeOsaltn)(H ₂O)₂]·2.5H₂O [3-MeOsaltn = N,N′-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato], Ln ^III(NO₃)₃·6H₂O, and triethylamine occurred in methanol/acetone, giving a first series of carbonato-bridged Ni^II₂Ln^III₂ complexes [(μ₄-CO₃)₂{Ni^II(3- MeOsaltn)(MeOH)Ln^III(NO₃)}₂] (1Gd, 1Tb, and 1Dy). When the reaction was carried out in acetonitrile/water, it gave a second series of complexes [(μ₄-CO₃)₂{Ni ^II(3-MeOsaltn)(H₂O)Ln^III(NO₃)} ₂]·2CH₃CN·2H₂O (2Gd, 2Tb, and 2Dy). For both series, each Ni^II₂Ln^III ₂ structure can be described as two di-μ-phenoxo-bridged Ni ^IILn^III binuclear units bridged by two carbonato CO ₃^2- units to form a carbonato-bridged (μ₄- CO₃)₂{Ni^II₂Ln^III ₂} structure. The high-spin Ni^II ion has octahedral coordination geometry, and the Ln^III ion is coordinated by O ₉ donor atoms from Ni^II(3-MeOsaltn), bidentate NO ₃^-, and one and two oxygen atoms of two CO ₃^2- ions. The NO₃^- ion for the first series roughly lie on Ln-O(methoxy) bonds and are tilted toward the outside, while for the second series, the two oxygen atoms roughly lie on one of the Ln-O(phenoxy) bonds due to the intramolecular hydrogen bond. The temperature-dependent magnetic susceptibilities indicated a ferromagnetic interaction between the Ni^II and Ln^III ions (Ln ^III = Gd^III, Tb^III, Dy^III) for all of the complexes, with a distinctly different magnetic behavior between the two series in the lowest-temperature region due to the Ln^III-Ln ^III magnetic interaction and/or different magnetic anisotropies of the Tb^III or Dy^III ion. Alternating-current susceptibility measurements under the 0 and 1000 Oe direct-current (dc) bias fields showed no magnetic relaxation for the Ni^II₂Gd^III ₂ complexes but exhibited an out-of-phase signal for Ni ^II₂Tb^III₂ and Ni^II ₂Dy^III₂, indicative of slow relaxation of magnetization. The energy barriers, Δ/k_B, for the spin flipping were estimated from the Arrhenius plot to be 12.2(7) and 6.1(3) K for 1Tb and 2Tb, respectively, and 18.1(6) and 14.5(4) K for 1Dy and 2Dy, respectively, under a dc bias field of 1000 Oe. Compound 1Dy showed relatively slow relaxation of magnetization reorientation even at zero dc applied field with Δ/k_B = 6.6(4) K.