Possibilities with 2-pyrimidinecarbonitrile to design MOFs

Magnetic and Luminescent Properties of Isostructural 2D Coordination Polymers Based on 2-Pyrimidinecarboxylate and Lanthanide Ions

A couple of isostructural coordination polymers with the general formula [Ln4(pymca)4(AcO)8]n have been obtained from reactions between pyrimidine-2-carboxylate (pymca) ligand and rare-earth ions (Ln = Dy (1), Nd (2)). These two-dimensional compounds have been characterized and the crystal structures have been solved by single-crystal X-ray diffraction technique, resulting in layers along the bc plane based on pymca and acetate anions that act as bridging ligands between metal atoms. Given that pymca and acetate anions possess carboxylate and hetero-nitrogen groups, it is possible to build a coordination polymer whose metal centers have a nine coordination. Furthermore, static and dynamic magnetic measurements of compound 1 reveal the lack of single molecule-magnet (SMM) behavior in this system due to the following two effects: (i) the ligand field does not stabilize magnetic ground states well separated from excited states, and (ii) anisotropy axes are not collinear, according to results with Magellan software. On another level, luminescent properties of compounds 1 and 2 are attributed to singlet π-π* transitions centered on pymca ligand as corroborated by time-dependent density functional theory (TD-DFT) calculations.

Designing Single-Ion Magnets and Phosphorescent Materials with 1-Methylimidazole-5-carboxylate and Transition-Metal Ions

Detailed structural, magnetic, and photoluminescence (PL) characterization of four new compounds based on 1-methylimidazole-5-carboxylate (mimc) ligand and transition metal ions, namely [Ni(mimc)₂(H₂O)₄] (1), [Co(μ-mimc)₂]_n (2), {[Cu₂(μ-mimc)₄(H₂O)]·2H₂O}_n (3), and [Cd(μ-mimc)₂(H₂O)]_n (4) is reported. The structural diversity found in the family of compounds derives from the coordination versatility of the ligand, which coordinates as a terminal ligand to give a supramolecular network of monomeric entities in 1 or acts as a bridging linker to build isoreticular 2D coordination polymers (CPs) in 2-4. Magnetic direct-current (dc) susceptibility data have been measured for compounds 1-3 to analyze the exchange interactions among paramagnetic centers, which have been indeed supported by calculations based on broken symmetry (BS) and density functional theory (DFT) methodology. The temperature dependence of susceptibility and magnetization data of 2 are indicative of easy-plane anisotropy (D = +12.9 cm^-1, E = +0.5 cm^-1) that involves a bistable M_s = ±1/2 ground state. Alternating-current (ac) susceptibility curves exhibit field-induced single-ion magnet (SIM) behavior that occurs below 14 K, which is characterized by two spin relaxation processes of distinct nature: fast relaxation of single ions proceeding through multiple mechanisms (U_eff = 26 K) and a slow relaxation attributed to interactions along the polymeric crystal building. Exhaustive PL analysis of compound 4 in the solid state confirms low-temperature phosphorescent green emission consisting of radiative lifetimes in the range of 0.25-0.43 s, which explains the afterglow observed during about 1 s after the removal of the UV source. Time-dependent DFT and computational calculations to estimate phosphorescent vertical transitions have been also employed to provide an accurate description of the PL performance of this long-lasting phosphor.

Co-ligand tuned pyrimidine-2-carboxylate Mn(ii) complexes from a 2D 63 layer to an interpenetrated srs-net

A 6³ 2D layer complex [Mn₃Cl₃(L₃)]_n·H₂O (1) (L = pyrimidine-2-carboxylate) was obtained by assembling 2-cyanopyrimidine and manganese chloride, in which the L ligands were generated in situ. In 1 six-membered Mn rings were constructed from Mn^II ions and L ligands, which were connected to each other by double chloride anions affording a 2D layer. When the chloride anions in 1 were substituted partly by formate, [Mn₄Cl₃L₄(HCO₂)]_n (2) was obtained. In 2, the L ligands bridge Mn^II to give a 1D chain, which was further connected by the double chloride anions and Cl/formate bridges to form a two-fold interpenetrated srs-net. Interestingly, 2 exhibits an obvious SHG response of approximately 0.8 times that of KDP. Furthermore, 2 is an antiferromagnet with a field induced spin flop transition.