Cobalt(II) Ions Connecting [Co^II₄] Helicates into a 2‑D Coordination Polymer Showing Slow Relaxation of the Magnetization

The reactions of cobalt­(II) perchlorate with a diazine tetratopic helicand, H₄L, in the presence of sodium carbonate afford two coordination polymers constructed from tetranuclear anionic helicates as building blocks: _∞³[Co₄L₃Na₄(H₂O)₄]·4H₂O (<b>1</b>) and _∞²[Co₅L₃Na₂(H₂O)₉]·2.7H₂O·DMF (<b>2</b>). The tetranuclear triple-stranded helicates, {Co^II₄L₃}^4–, are connected in <b>1</b> by sodium­(I) ions and in <b>2</b> by sodium­(I) and cobalt­(II) ions (H₄L results from the condensation reaction between 3-formylsalicylic acid and hydrazine). The crystal structures of the two compounds have been solved. In both compounds the anionic helicates interact with the assembling cations through the carboxylato oxygen atoms. Compound <b>2</b> features chains resulting from connecting the tetranuclear helicates through cobalt­(II) ions. The analysis of the magnetic properties of compounds <b>1</b> and <b>2</b> evidenced a dominant antiferromagnetic coupling for <b>1</b>, resulting in a diamagnetic ground state. In contrast, the magnetic behavior of <b>2</b> is dominated at low temperature by the Co^II ion which connects the antiferromagnetically coupled {Co^II₄} helical moieties. The ac magnetic measurements for <b>2</b> reveal the occurrence of slow relaxation of the magnetization that is due to the single, uncorrelated cobalt­(II) ions, which are diluted in an essentially diamagnetic matrix of {Co^II₄} moieties (Δ<i><i>E</i></i>_eff = 26.7 ± 0.3 cm^–1 with τ₀ = (2.3 ± 0.2) × 10^–6 s)

Magnetic and Luminescent Binuclear Double-Stranded Helicates

Three new binuclear helicates, [M₂L₂]·3DMF (M = Co­(II), <b>1</b>, Zn­(II), <b>3</b>) and [Cu₂L₂]·DMF·0.4H₂O (<b>2</b>), have been assembled using the helicand H₂L that results from the 2:1 condensation reaction between <i>o</i>-vanillin and 4,4′-diaminodiphenyl ether. The metal ions within the binuclear helicates are tetracoordinated with a distorted tetrahedral geometry. Direct current magnetic characterization and EPR spectroscopy of the Co­(II) derivative point to an easy axis type anisotropy for both Co­(II) centers, with a separation of at least 55 K between the two doublets. Dynamic susceptibility measurements evidence slow relaxation of the magnetization in an applied dc field. Since the distance between the cobalt ions is quite large (11.59 Å), this is attributed in a first instance to the intrinsic properties of each Co­(II) center (single-ion magnet behavior). However, the temperature dependence of the relaxation rate and the absence of slow dynamics in the Zn­(II)-doped sample suggest that neither the simple Orbach mechanism nor Raman or direct processes can account for the relaxation, and collective phenomena have to be invoked for the observed behavior. Finally, due to the rigidization of the two organic ligands upon coordination, the pure zinc derivative exhibits fluorescence emission in solution, which was analyzed in terms of fluorescence quantum yields and lifetimes

Magnetic and Luminescent Binuclear Double-Stranded Helicates

Three new binuclear helicates, [M₂L₂]·3DMF (M = Co­(II), <b>1</b>, Zn­(II), <b>3</b>) and [Cu₂L₂]·DMF·0.4H₂O (<b>2</b>), have been assembled using the helicand H₂L that results from the 2:1 condensation reaction between <i>o</i>-vanillin and 4,4′-diaminodiphenyl ether. The metal ions within the binuclear helicates are tetracoordinated with a distorted tetrahedral geometry. Direct current magnetic characterization and EPR spectroscopy of the Co­(II) derivative point to an easy axis type anisotropy for both Co­(II) centers, with a separation of at least 55 K between the two doublets. Dynamic susceptibility measurements evidence slow relaxation of the magnetization in an applied dc field. Since the distance between the cobalt ions is quite large (11.59 Å), this is attributed in a first instance to the intrinsic properties of each Co­(II) center (single-ion magnet behavior). However, the temperature dependence of the relaxation rate and the absence of slow dynamics in the Zn­(II)-doped sample suggest that neither the simple Orbach mechanism nor Raman or direct processes can account for the relaxation, and collective phenomena have to be invoked for the observed behavior. Finally, due to the rigidization of the two organic ligands upon coordination, the pure zinc derivative exhibits fluorescence emission in solution, which was analyzed in terms of fluorescence quantum yields and lifetimes

Magnetic and Luminescent Binuclear Double-Stranded Helicates

Three new binuclear helicates, [M₂L₂]·3DMF (M = Co­(II), <b>1</b>, Zn­(II), <b>3</b>) and [Cu₂L₂]·DMF·0.4H₂O (<b>2</b>), have been assembled using the helicand H₂L that results from the 2:1 condensation reaction between <i>o</i>-vanillin and 4,4′-diaminodiphenyl ether. The metal ions within the binuclear helicates are tetracoordinated with a distorted tetrahedral geometry. Direct current magnetic characterization and EPR spectroscopy of the Co­(II) derivative point to an easy axis type anisotropy for both Co­(II) centers, with a separation of at least 55 K between the two doublets. Dynamic susceptibility measurements evidence slow relaxation of the magnetization in an applied dc field. Since the distance between the cobalt ions is quite large (11.59 Å), this is attributed in a first instance to the intrinsic properties of each Co­(II) center (single-ion magnet behavior). However, the temperature dependence of the relaxation rate and the absence of slow dynamics in the Zn­(II)-doped sample suggest that neither the simple Orbach mechanism nor Raman or direct processes can account for the relaxation, and collective phenomena have to be invoked for the observed behavior. Finally, due to the rigidization of the two organic ligands upon coordination, the pure zinc derivative exhibits fluorescence emission in solution, which was analyzed in terms of fluorescence quantum yields and lifetimes