Attempted synthesis of axial–equatorial pyrazine-bridged copper(II) complexes toward homometallic ferromagnetic compounds

Two temperature-induced 1D CuII chain enantiomeric pairs showing different magnetic properties and nonlinear optical responses

At different reaction temperatures, using Cu(NO₃)₂·3H₂O to react with enantiomerically pure N-donor ligands (L_S/L_R), respectively, two pairs of chiral one-dimensional (1D) Cu^II chain enantiomers formulated as [Cu(μ₂-NO₃)(NO₃)(L_S)]_n/[Cu(μ₂-NO₃)(NO₃)(L_R)]_n (S-1-Cu/R-1-Cu, formed at 40 °C with an NO₃^- group as a sole bridging ligand) and [Cu(μ₂-L_S)(NO₃)₂]_n/[Cu(μ₂-L_R)(NO₃)₂]_n (S-2-Cu/R-2-Cu, formed at 25 °C with L_S or L_R as a bridging ligand) were prepared, where L_S/L_R = (+)/(-)-4,5-pinenepyridyl-2-pyrazine. Interestingly, such a disparity in bridging ligands leads not only to their distinct structural features but also to their completely different magnetic couplings together with a large difference in their nonlinear optical responses. S-1-Cu with a 1D helical structure shows weak ferromagnetic coupling between Cu^II ions, while S-2-Cu with a 1D stairway-like structure presents weak antiferromagnetic coupling. In particular, they simultaneously possess both second- and third-harmonic generation (SHG and THG) responses in one molecule with large strength differences. More remarkably, S-1-Cu exhibits a very large THG response (162 × α-SiO₂), which is 22.5 times that of S-2-Cu, and the SHG strength of S-1-Cu is more than 3 times that of S-2-Cu. This work demonstrates that reaction temperature has a great impact on the self-assembled structures of coordination polymers and subsequently results in their large performance differences.

First one-dimensional homochiral stairway-like Cu(II) chains: crystal structures, circular dichroism (CD) spectra, ferroelectricity and antiferromagnetic properties

The reactions of enantiopure chiral ligands (+)/(−)-4,5-pinenepyridyl-2-pyrazine (LS/LR) with CuCl2·2H2O in CH3OH/CH2Cl2 solution led to the formations of one-dimensional homochiral enantiomeric pairs with the formula [Cu(LR/S)Cl2]n·2H2O (R-1 and S-1, the isomers containing the LR and LS ligands, respectively). The circular dichroism (CD) spectra verified their chiroptical activities and enantiomeric natures. Their structures have been determined by X-ray single-crystal analyses, showing stairway-like and mirror-symmetric features, which represent the first examples of homochiral metal complexes with stairway-like structures. The ferroelectric property measurement indicated that R-1 exhibits ferroelectricity with the remnant polarization (Pr) value of 0.02 μC cm−2 under an applied electric field of 6.1 kV cm−1 at room temperature. The magnetic investigation of R-1 showed a weak intrachain antiferromagnetic coupling between Cu(II)–Cu(II) ions mediated by pyrazine, which can be interpreted by a spin-polarization mechanism. All these results suggested that R-1 and S-1 are potential multifunctional molecule-based materials combining optical activity, ferroelectricity and magnetism within one molecule.