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Mononuclear copper(i) complexes demonstrate T1 → S0 phosphorescence in the near-infrared region associated with ligand-to-metal charge transfer. The most effective intersystem crossing channels are S6 → T7, S6 → T8 and S10 → T12.

Two pyrimidine-based ligands, 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(morpholino)pyrimidine (L2) and 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-phenoxypyrimidine (L3), and a series of mixed-valence copper(I,II) halide complexes, [Cu(L2)2Br]2[Cu2Br4] (1), [Cu(L2)2Cl][CuCl2] (2), and [Cu2L3Br3]n (3), have been synthesized. The complex [Cu(L2)2Br]2[Cu2Br4] was prepared by the reaction of L2 with CuBr2 in a 1 : 1 molar ratio in MeCN. Its chlorido-analogue, the complex [Cu(L2)2Cl][CuCl2], was synthesized by the reaction between L2, CuCl2 and CuCl in a 2 : 1 : 1 molar ratio in MeCN. The ligand L2 acts as a chelating one. In the structures of the complexes [Cu(L2)2Br]2[Cu2Br4] and [Cu(L2)2Cl][CuCl2] the Cu2+ ion is in the cationic part of the complex whereas the Cu+ ion is located in the anionic part. The best way to synthesize the mixed-valence 1D coordination polymer [Cu2L3Br3]n is to react CuBr2 with L3 in a 2 : 1 molar ratio in the MeCN/CHCl3 mixture on heating. In the structure of [Cu2L3Br3]n the ligand L3 shows chelating/bridging tridentate coordination. This is the first example of the tridentate coordination of 4-(1H-pyrazol-1-yl)-6-R-pyrimidines. The striking difference between the coordination behavior of L2 and L3 (chelating bidentate vs. chelating/bridging coordination) is related with the possibility of rotation of the 6-phenoxy group around the C–O bond which makes the N1 pyrimidine atom less sterically hindered, enabling it to participate in metal ion binding. Importantly, all copper ions in [Cu2L3Br3]n show similar tetrahedral environments, CuNBr3 and CuN2Br2, which is extremely rare for mixed-valence copper(I,II) compounds. The ligands L2 and L3 show blue emission which is quenched upon their coordination to copper ions. The 1D coordination polymer [Cu2L3Br3]n shows high thermal stability and unusual solvent-occlusion properties. The role of the substituents favoring the formation of the mixed-valence copper(I,II) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines is discussed.

A mononuclear complex, [FeL2](BF4)2·xH2O (1LS·xH2O), where L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine, can be converted into several phases showing different spin crossover regimes. In the first heating 1LS·xH2O loses water molecules and converts into a mixture of two high spin phases, 1A/HS and 1C/HS. Further cycling produces the low spin phase 1A/LS. The transition 1A/LS ↔ 1A/HS is accompanied by a 130 K wide hysteresis loop (Tc↑ = 490 K, Tc↓ = 360 K). Annealing the complex 1LS·xH2O at 500 K yields a high spin phase 1C/HS. The phase 1C/HS undergoes spin conversion to the corresponding low spin phase 1C/LS on cooling, T1/2 ≈ 320 K. Dehydration of 1LS·xH2O at 370 or 400 K yields a certain low spin phase, 1X/LS, which irreversibly transforms into a high spin phase 1B/HS, which, in turn, reversibly transforms to the low spin phase 1B/LS on cooling, T1/2 ≈ 320 K.