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ESIPT-capable 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)pyrimidines exhibit dual emission associated with interplay of phosphorescence and fluorescence.

DFT and TD-DFT calculations at the B3LYP/LANL2DZ level of theory are performed in order to explain the origins of dual fluorescence and uncover the relaxation pathways of 2-amino-4-methylpyrimidine (L). The computations are in good agreement with experimental data. A high-energy emission band in the photoluminescence spectrum of 2-amino-4-methylpyrimidine (λmax = 350 nm) is related to the fluorescence of monomer L. Comparison of calculated and experimental IR and absorption spectra reveals that in the solid state L molecules form supramolecular {N–H···N}n hydrogen bonded associates. The formation of such associates favors the excited state double proton transfer (ESDPT). Calculated 2D potential energy curves for the ground and first singlet excited states of dimeric associates L2 indicate the presence of two stable forms, L2-amino and L2-imino. The low-energy band in the emission spectrum of 2-amino-4-methylpyrimidine (λmax = 445 nm) is probably associated with the fluorescence of the L2-imino form. Both fluorescence bands have a mixed π–π*, n–π* character as is evidenced by frontier orbitals involved in S1–S0 transitions.

Silver(I) complexes with 2-amino-4-phenyl-6-methylpyrimidine (L), [Ag3L2(H2O)0.55(NO3)3]n (1) and [Ag2L2(PPh3)2(NO3)2] (2), have been synthesized and structurally characterized. In the solid state the free ligand L demonstrates dual fluorescence: a high energy band at 375 nm is the emission of the monomer L while a low-energy band at 450 nm can be attributed to the emission of the tautomeric dimer, L2-imino, formed in the excited state double proton transfer (ESDPT) process. In the solid state the complexes demonstrate excitation wavelength dependent photoluminescence originating from three emission mechanisms (hereafter abbreviated as A, B and C for complex 1; A′, B′ and C′ for complex 2). The spectroscopic and computational results reveal that these emission mechanisms can be assigned to metal + nitrate anion to ligand L charge transfer phosphorescence (mechanisms A and A′), excimer 1π–π* fluorescence (mechanism B), fluorescence of L2-imino dimeric units formed due to ESDPT (mechanism B′) and ligand-centered 1π–π*/1n–π* fluorescence (mechanisms C and C′). The luminescence of complex 1 at λex = 340 nm shows thermochromic behavior; the emission color dramatically changes from blue-green at 77 K to yellowish green at 175 K and white at 300 K.

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.

A polymeric manganese(II) complex, [MnL2Cl2]n (L = 2-amino-4-methylpyrimidine) has been synthesized by the reaction between MnCl2·4H2O and L in the 1:1 and 1:2 metal-to-ligand molar ratios. The Mn2+ ions adopt distorted octahedral environment formed by four Cl− ligands and two N1 atoms of the pyrimidine rings of two L molecules. The chlorido-ligands adopt a μ2-bridging coordination mode linking manganese atoms into the polymeric chain. The free ligand L shows excitation-wavelength-dependent emission originating from the π-π* and n-π* excited states with nanosecond lifetimes in the blue region of the spectrum. The complex [MnL2Cl2]n demonstrates photoluminescence with the maximum in the near-infrared region (λmax = 740 nm, φ = 1.7%) with microsecond lifetimes (τ = 124 μs).

Two mononuclear iron(ii) complexes with isomeric N,N,N-tridentate pyrimidine-based ligands were synthesized. Both complexes show reproducible hysteretic spin crossover. Low spin state to high spin state switching is cooperative due to autocatalysis.

A mononuclear manganese(II) complex with a chelating 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine ligand (L), [MnL2Cl2]·H2O, shows intriguing excitation wavelength-dependent emission. Depending on the excitation wavelength, the complex demonstrates three emission bands with the maxima at 380 nm, 440 nm and 495 nm. The 380 nm and 440 nm emissions originate from the π → π* and n → π* ligand-centered transitions. The long-wave 495 nm emission with microsecond lifetimes is related to the d–d transitions and/or metal-to-ligand and halogen-to-ligand charge transfer. The emission behavior of this complex is strongly temperature-dependent: upon cooling from 300 K down to 77 K, the intensity of emission considerably increases. The enhancement of the luminescence upon cooling is accompanied by the appearance of the vibrational structure. This complex is the first example of manganese(II) complexes demonstrating excitation wavelength-dependent emission.

Thermodynamic and kinetic parameters of the tautomeric tetrazole-azide rearrangement for a series of 2,6-disubstituted 4-azidopyrimidines are determined by NOESY/EXSY and DNMR: ΔH = 15—28 kJ mol–1; ΔS = 47—65 J mol–1 К–1; Е a = 93—117 kJ mol–1; lgA = 15.1—18.9. They are dependent on electronic properties of the substituents and the polarity of solvent.

Iron(II) hexafluorophosphate complexes with N,N,N-tridentate ligands, 2-(pyridin-2-yl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (L(H,H)) and 2-(pyridin-2-yl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L(Me,Me)) were synthesized by reacting FeCl2·4H2O, (NBu4)PF6 and L(H,H) or L(Me,Me) in alcohol media. Supramolecular 3D structure of [FeL(H,H)2](PF6)2·nH2O is organized by CH…π interactions, π…π-stacking and CH…F hydrogen bonds. In the stricture of [FeL(Me,Me)2](PF6)2·nH2O the cations and anions are assembled into a 3D network via CH…F hydrogen bonds and lone pair…π interactions. Both complexes demonstrate high thermal stability. Under vacuum, the complex [FeL(Me,Me)2](PF6)2·nH2O is predominantly in the low spin state below 400 K, but at higher temperatures it shows gradual spin state switching which remains incomplete even at 495 K. Under vacuum or in sealed ampoule in the first heating run the complex [FeL(H,H)2](PF6)2·nH2O shows abrupt spin state switching related to dehydration near 400 K. On further thermal cycling under vacuum the complex demonstrates gradual spin crossover curve centered at ca. 250 K, whereas in sealed ampoules this compound demonstrates a very abrupt transition near 250 K.

11-Acetyl-8-bromo-2-methyl-5,6-dihydro-2H-2,6-methano-1,3,5-benzoxadiazocin-4(3H)-one and -4(3H)-thione are prepared via the three-component Biginelli condensation as the only diastereomers. For the first time we observed that these compounds in DMSO solution undergo a slow isomerization with formation of mixtures with the respective 4-(5-bromo-2-hydroxyphenyl)-3,4-dihydropyrimidin-2(1Н)-one(thione). The same methanobenzoxadiazocines underwent deacetylation in boiling ethanol in the presence of HCl.

The Biginelli reaction between 5-R-salicylic aldehyde (R = H, Me, and Br), α-nitroacetophenone, and urea affords 8-R-11-nitro-2-phenyl-5,6-dihydro-2H-2,6-methanobenzo-[g][1,3,5]oxadiazocin-4(3Н)-ones as mixtures of two diastereomers. The ratio of diastereomers depends on the catalyst (HCl) concentration in the reaction medium. In DMSO and DMF, the resulting compounds undergo oxadiazocine ring opening with establishment of a three-component equilibrium between 4-(2-hydroxy-5-R-phenyl)-5-nitro-6-phenyl-3,4-dihydropyrimidin-2(1Н)-one as the major component and diastereomeric methanobenzoxadiazocines as two minor components. Dilution of these solutions with water favors the oxa-Michael reaction resulting in the reverse transformation of dihydropyrimidinones into the corresponding starting diastereomers.

The system [FeL2](BF4)2 (1)-EtOH-H2O (L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine) shows a complicated balance between the relative stabilities of solvatomorphs and polymorphs of the complex [FeL2](BF4)2. New solvatomorphs, 1(LS)·EtOH·H2O and β-1(LS)·xH2O, were isolated in this system. They were converted into four daughter phases, 1(A/LS), 1(D/LS), 1(E/LS)·yEtOH·zH2O and 1(F/LS). On thermal cycling in sealed ampoules, the phases 1(LS)·EtOH·H2O and β-1(LS)·xH2O transform into the anhydrous phase 1(A/LS). The hysteresis loop width for the (A/LS) ↔ (A/HS) spin transition depends on the water and ethanol contents in the ampoule and varies from ca. 30 K up to 145 K. The reproducible hysteresis loop of 145 K is the widest ever reported one for a spin crossover complex. The phase 1(A/LS) combines the outstanding spin crossover properties with thermal robustness allowing for multiple cycling in sealed ampoules without degradation. The kinetics of the 1(A/LS) → 1(A/HS) transition is sigmoidal which is indicative of strong cooperative interactions. The cooperativity of the 1(A/LS) → 1(A/HS) transition is related to the formation of a 2D supramolecular structure of the phase 1(A/LS). The activation energy for the spin transition is very high (hundreds of kJ mol(-1)). The kinetics of the 1(A/HS) → 1(A/LS) transition can either be sigmoidal or exponential depending on the water and ethanol contents in the ampoule. The phases 1(D/LS) and 1(F/LS) show gradual crossover, whereas the phase 1(E/LS)·yEtOH·yH2O shows a reversible hysteretic transition associated with the solvent molecule release and uptake.

Cadmium(II) complexes, [CdLCl 2 ] n ( 1 ), α-{[CdLCl 2 ]·H 2 O} n ( 2 ), β-{[CdLCl 2 ]·H 2 O} n ( 3 ), and [Cd 4 L 2 (H 2 O) 2 Cl 8 ]·2H 2 O ( 4 ), with 3,5-bis(pyrimidin-2-yl)-4 H -1,2,4-triazol-4-amine (L), have been synthesized. They are the first examples of structurally characterized metal complexes with L. In these structures, the ligand L adopts either bidentate (N′ pyrimidine ,N 1 triazole ) or bis -bidentate (N′ pyrimidine ,N 1 triazole ,N 2 triazole ,N″ pyrimidine ) coordination. The complexes 1 – 3 are linear coordination polymers, the compounds 2 and 3 are polymorphs. The linear chains are assembled into 2D supramolecular structure due to π–π stacking interactions between practically planar L molecules. The structure of 4 consists of [Cd 4 L 2 (H 2 O) 2 Cl 8 ] complex molecules with μ 2 - and μ 3 -chlorido-bridged quasi-cubane tetracadmium core and non-coordinated water molecules. In the structure of 4 , the intermolecular lone pair–π interactions were observed between Cl atom and π-deficient triazole ring. The photoluminescent properties of L, 3 , and 4 were studied in the solid state. These compounds were found to display bright blue luminescence. The relative intensity of emission increases in the order L 4 L 2 (OH 2 ) 2 Cl 8 ]·2H 2 O 2 ]·H 2 O} n .

Non-isothermal magnetic studies can be used to estimate the activation energy of cooperative spin transition. Model-free isoconversional integral methods give reliable estimates of the activation barrier height for highly cooperative spin crossover systems with wide thermal hysteresis.

A Biginelli reaction of 5-R-salicylic aldehydes (R = H, Me, Br) with nitroacetone and urea in each case leads to a predominant formation of 2R*,6S*,11S* diastereomers of 8-R-2-methyl-11-nitro-5,6-dihydro-2H-2,6-methanobenzo[g][1,3,5]oxadiazocin-4(3H)-one. In solutions in DMF and DMSO, these diastereomers undergo the oxadiazocine ring opening with setting a three-component equilibrium between predominant 4-(2-hydroxy-5-R-phenyl)-6-methyl5-nitro-3,4-dihydropyrimidin-2(1H)-ones and 2R*,6S*,11S* and 2R*,6S*,11R* diastereomers of methanobenzoxadiazocine as two minor components.

A series of mononuclear heteroleptic copper(I) halide complexes, [CuL(PPh3)X] (X = Cl, Br, I), based on 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) and triphenylphosphine, have been synthesized by reaction between CuX (X = Cl, Br, I), L and PPh3 in a molar ratio of 1/1/1 in MeCN solutions. The copper atom, showing the distorted tetrahedral environment, is bound by the N,N-chelating ligand L, triphenylphosphine and a halide ion. The complexes [CuL(PPh3)Cl] and [CuL(PPh3)Br] are isostructural. In CH2Cl2 solutions, L and the complexes [CuL(PPh3)X] (X = Cl, Br, I) display a luminescence band with λ(max) = 377 nm and a lifetime of 1.9 ns (ligand-based luminescence (LL*)). However, the complex [CuL(PPh3)I] has an additional weak luminescence band with λ(max) = 681 nm and a lifetime of 96 ns of (3)MLCT origin. In the solid state, L shows the splitting of the luminescence band to λ(max) = 365 and 384 nm and a slight increase of the lifetime to 2.66 ns. Solid samples of the complexes [CuL(PPh3)X] demonstrate (3)MLCT luminescence bands at 620 nm (X = Cl), 605 nm (X = Br) and 559 nm (X = I) with lifetimes in the range 3.6-11.2 μs, whereas the LL* band (377 nm) is absent. Quantum yields and rate constants of radiative and nonradiative processes were determined in CH2Cl2 solutions and in the solid state for all complexes. The luminescence quantum yield and lifetimes for the solid samples increase in the order [CuL(PPh3)Cl] < [CuL(PPh3)Br] < [CuL(PPh3)I]. This is due to the increase of radiative decay and simultaneous suppression of nonradiative decay. The complex [CuL(PPh3)I] shows a high quantum yield of 29.4% and an excited state lifetime of 11.2 μs.

A series of Cu(II), Cu(I,II) and Cu(I) halide complexes, [CuL2Br]2[Cu2Br6], [CuL2Br2], [Cu2L2Br3]·EtOH, [Cu2L2Br3]·CHCl3, [Cu2L2Cl3]·MeCN, [Cu2L2Cl3]·CHCl3, [CuLCl], [CuLBr] and [Cu2L2I2] with 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) have been synthesized. Copper(II) compounds, [CuL2Br]2[Cu2Br6] and [CuL2Br2], were obtained by the reactions of CuBr2 with L in the Cu:L = 1:1 and 1:2 molar ratios, respectively, in EtOH solutions at room temperature. In the structure of [CuL2Br2], the double intermolecular lone pair– interactions were observed between N(piperidine) atom and π-deficient pyrimidine ring. Dinuclear mixed-valence copper(I,II) bromide compounds, [Cu2L2Br3]·EtOH and [Cu2L2Br3]·CHCl3, were prepared (i) by the reaction of CuBr2 and L in a 1:1 molar ratio upon heating in EtOH either CHCl3/MeCN solutions, and (ii) by recrystallizing [CuL2Br]2[Cu2Br6] from CHCl3/MeCN mixture. Their analogs, mixed-valence copper(I,II) chloride compounds, [Cu2L2Cl3]·MeCN and [Cu2L2Cl3]·CHCl3, were prepared by reacting the complex [CuL2Cl2] with CuCl in MeCN/CHCl3 mixtures (various v/v ratios of the solvents). The frozen solutions of copper(II) and copper(I,II) complexes in CHCl3 were studied by EPR spectroscopy at 77 K. These data revealed that mixed valence compounds are type I systems. In the structures of [Cu2L2Br3]·CHCl3 and [Cu2L2Cl3]·CHCl3, lone pair–π interactions were observed between Cl atom of chloroform solvent molecule and the phenyl group. The Cu(I) complexes, [CuLCl], [CuLBr] and [Cu2L2I2], were obtained by the reactions of CuHal (Hal = Cl, Br, I) and L in a 1:1 molar ratio in MeCN solutions. The complex [Cu2L2I2] has dinuclear structure with rather short Cu–Cu distance, 2.59 A, within {Cu(μ-I)2Cu} central unit, while its chloride and bromide analogs seem to be mononuclear compounds. The complexes [CuLCl], [CuLBr] and [Cu2L2I2] were found to show bright yellow luminescence in the solid state at room temperature. The relative intensity of emission increases across the series: [CuLCl]

A series of mono- and oligonuclear copper(II) complexes with new hybrid NNN-tridentate 4-(1 H -pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine ligands, 4-(1 H -pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine (L 1 ) and 4-(3,5-dimethyl-1 H -pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine (L 2 ), have been synthesized. The reaction of CuCl 2 with L 1 in EtOH in the 1:1 metal-to-ligand molar ratio affords the mononuclear complex [CuL 1 Cl 2 ]·0.5H 2 O ( 1 ). An increase of the metal-to-ligand molar ratio up to 4:1 as well as the use of less polar mixed solvent, EtOH/CHCl 3 , leads to mononuclear complex [CuL 1 Cl 2 ] ( 2 ). Attempts to crystallize di- or oligonuclear copper(II) complex with L 1 (the 8:1 metal-to-ligand molar ratio, EtOH/CHCl 3 ) resulted in the formation of ionic halocuprate compound, [Cu 7 L 1 4 Cl 12 ][Cu 2 Cl 6 ] ( 3 ), containing unique heptanuclear cation, [Cu 7 L 1 4 Cl 12 ] 2+ , and dinuclear anion [Cu 2 Cl 6 ] 2− . The reaction of CuCl 2 with L 2 in EtOH in the 1:1 metal-to-ligand molar ratio affords mononuclear complex [CuL 2 Cl 2 ] ( 4 ). Di- and tetranuclear complexes, [Cu 2 L 2 Cl 4 ] ( 5 ) and [Cu 4 L 2 2 Cl 8 ] ( 6 ), can be crystallized from ethanolic solution when the 2:1 metal-to-ligand molar ratio is used. The former is the product of kinetic control, while the latter is that of thermodynamic one. Being coordinated, the molecules of L 1 and L 2 adopt tridentate chelating coordination mode binding copper atoms through pyrazolyl N 2 , pyrimidine N 3 and pyridinyl N atoms and forming thus two five-membered chelate rings (CuN 3 C and CuN 2 C 2 ). Depending on the metal-to-ligand molar ratio in the solution, electrospray ionization mass spectrometry (ESI-MS) evidenced the presence of mono- and oligonuclear copper species in the CuCl 2 –L 1 and CuCl 2 –L 2 solutions. The isolation of oligonuclear species in the solid state seems to be related with the absence of hydrogen-bond donor groups in the molecules of 4-(1 H -pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine ligands.

The complex [ FeL 2 1 ](ClO4)2·H2O (1a) (L1 = 4-methyl-2,6-bis(1H-pyrazol-1-yl)pyrimidine) was synthesized by the reaction of Fe(ClO4)2·6H2O with new NNN-tridentate pyrazolylpyrimidine ligand L1 in MeOH/EtOH mixture. It is the first example of iron(II) spin crossover complexes with 2,4(6)-di(1H-pyrazol-1-yl)pyrimidines and manifests gradual spin transition centered around 150 K on the χMT versus T curve without hysteresis. A broad heat capacity anomaly with the maximum at 150 K was observed on the Cp(T) dependence, ΔtrsH = 5.50 kJ/mol and ΔtrsS = 42.0 J/(mol K). Surprisingly, the syntheses of iron(II) perchlorate compound with L1 performed in EtOH and iPrOH afford another modification of the complex, [ FeL 2 1 ](ClO4)2·H2O (1b), being high spin in the 78–300 K temperature range. The complexes with 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L2), [ FeL 2 2 ](ClO4)2·EtOH (2), [ FeL 2 2 ](CF3SO3)2 (3), [FeL2Cl2] 0.5H2O (4) and [FeL2(NCS)2] (5) were synthesized in EtOH solutions. They are in the high spin state within 78–300 K temperature range.

Copper(II) and copper(I) complexes, [Cu II L 2 (BF 4 ) 2 ]·0.5H 2 O, [Cu I L 2 ](BF 4 )·2MeCN and [Cu I L 2 ](BF 4 )·H 2 O, with 4-(3,5-diphenyl-1 H -pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) have been synthesized. The compound [Cu II L 2 (BF 4 ) 2 ]·0.5H 2 O was obtained in the M:L = 1:1 and 1:2 molar ratios in EtOH solutions. It represents an example of a complex containing coordinated BF 4 − anion. The Cu–F(BF 3 ) distances, 1.964(2) and 2.000(2) A, are the shortest for copper tetrafluoroborate complexes. The compounds [Cu I L 2 ](BF 4 )·2MeCN and [Cu I L 2 ](BF 4 )·H 2 O were obtained in the M:L = 1:1 and 1:2 molar ratios in MeCN solutions. According to X-ray data, the molecules of L in the structures of [Cu II L 2 (BF 4 ) 2 ]·0.5H 2 O and [Cu I L 2 ](BF 4 )·2MeCN are coordinated to the copper atoms through N 2 (pyrazole) and N 3 (pyrimidine) atoms. The complex [Cu I L 2 ](BF 4 )·2MeCN is unstable in the ambient air and transforms into [Cu I L 2 (BF 4 )]·H 2 O. The complex [Cu I L 2 ](BF 4 )·H 2 O was found to display orange photoluminescence.

A copper(II) complex with 6-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidin-4-amine (L), [CuLCl2], has been synthesized. This compound is formed irrespective of the Cu : L molar ratio (Cu : L = 1 : 1, 2 : 1, and 20 : 1) in the MeOH/H2O/DMF mixture as a single product. ESI-MS data demonstrate that the additional amount of CuCl2 above the Cu : L = 1 : 1 molar ratio, is effectively solvated, and high-nuclearity species are formed in trace amounts in the solution. The complex adopts a distorted square-pyramidal geometry with two chlorides and three nitrogen atoms from L. The electronic spectrum of the complex contains a broad band with a maximum at 12,820 cm−1 within the region characteristic for square-pyramidal chromophores CuA5 (A = Cl, N). Due to Cu ··· Cl contacts, the molecules of [CuLCl2] form the dinuclear [CuLCl2]2 unit. Surprisingly, the NH2-group participates in the formation of NH ··· Cl hydrogen bonds instead of the formation of (NH ··· N3(pyrimidine))2 synthon, which is common for N-het...

The copper(II) complexes CuLCl2 (1) and CuLBr2 (2), with the chelating pyrazolylpyrimidine ligand 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-methyl-6-phenylpyrimidine (L), have been synthesized. A single crystal X-ray diffraction study revealed that 1 and 2 have molecular mononuclear structures. The molecules of 1 and 2 form chains in the crystal structures of these compounds due to the formation of π–π-stacking interactions between the pyrimidine and the phenyl rings. The complexes, in combination with the co-catalyst methylaluminoxane (MAO), reveal catalytic activity in ethylene polymerization, while the free ligand L is inactive.

A series of zinc(II) and cadmium(II) complexes with 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L), ZnLCl2, CdLCl2, ZnL2Cl2, CdL2Cl2, CdL2Cl2·0.5Me2CO·1.5H2O and CdL2Cl2·0.5CHCl3·0.5H2O, have been synthesized. The compounds ZnLCl2 and CdLCl2 were obtained in a M:L = 1:1 molar ratio in EtOH solutions, while ZnL2Cl2 and CdL2Cl2 were isolated in a M:L = 1:3 molar ratio in EtOH/Me2CO mixtures. Surprisingly, attempts to crystallize CdLCl2 from EtOH/Me2CO mixture afforded single crystals of a compound, having 1:2 metal-to-ligand stoichiometry, CdL2Cl2·0.5Me2CO·1.5H2O, instead of a complex with 1:1 stoichiometry. At the same time, crystallization of CdL2Cl2 from Me2CO/CHCl3 mixture afforded CdL2Cl2·0.5CHCl3·0.5H2O single crystals. According to X-ray crystal structure data, ZnLCl2, ZnL2Cl2, CdL2Cl2·0.5Me2CO·1.5H2O and CdL2Cl2·0.5CHCl3·0.5H2O complexes have molecular mononuclear structures. The molecules of L adopt bidentate chelating binding mode being coordinated to the metal ions through N2 atom of the pyrazole and N3 atom of the pyrimidine rings. The coordination core of zinc atom in ZnLCl2 complex is a distorted ZnN2Cl2 tetrahedron. The coordination cores of metal atoms in the structures of ZnL2Cl2, CdL2Cl2·0.5Me2CO·1.5H2O and CdL2Cl2·0.5CHCl3·0.5H2O are the distorted cis-MN4Cl2 (M = Zn, Cd) octahedra. In the structure of ZnL2Cl2 double lone pair(N(piperidine))–π(pyrimidine) interactions were observed. The photoluminescent properties of L, ZnLCl2, CdLCl2, ZnL2Cl2 and CdL2Cl2 were studied in the solid state under the same experimental conditions. These compounds were found to display bright blue luminescence. Highest relative intensity of emission was detected for ZnL2Cl2.

Zinc(II) and cadmium(II) complexes, ZnL2 ⋅ 0.5H2O, ZnL2 ⋅ 2MeOH and CdL2 ⋅ 1.5H2O, based on a new hybrid compound, 2-(3,5-dimethyl-1 H-pyrazol-1-yl)-6-(4-methoxyphenyl)pyrimidine-4-carboxylic acid (HL), have been synthesized. They are the first examples of metal complexes with pyrazolylpyrimidinecarboxylates. The complex ZnL2 ⋅ 2MeOH was structurally characterized. It has mononuclear structure; zinc atom is in the distorted octahedral coordination polyhedron 4N + 2O formed by donor atoms of two coordinated L− ions. The compounds HL, ZnL2 ⋅ 0.5H2O and CdL2 ⋅ 1.5H2O manifest bright blue photoluminescence.

A series of copper(II) complexes, i.e. Cu2LCl4, CuLCl2·H2O and [Cu2L2Cl4]·2MeCN (8), based on a new potentially polytopic ligand, 3,5-bis(4,6-dimethylpyrimidin-2-yl)-4H-1,2,4-triazol-4-amine (3b, L), have been synthesized. The crystal structures of L and [Cu2L2Cl4]·2MeCN were studied by X-ray single crystal analysis. The dinuclear compound [Cu2L2Cl4]·2MeCN represents the first example of structurally characterized metal complexes with 3,5-di(pyrimidin-2-yl)-4H-1,2,4-triazol-4-amines. Both copper atoms have distorted tetragonal–pyramidal 3N + 2Cl environment. Surprisingly, in contrast to the complexes based on 3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-amine (pyridinyl analog of L), the compound [Cu2L2Cl4]·2MeCN adopts a dinuclear trans-(N′,N1,N2)2 double bridging binding mode which is due to tridentate coordination of two L molecules linking two copper atoms through N1,N2-triazole and N′-pyrimidine atoms. It seems to be reasonable that it is methyl groups in pyrimidinyl moiety that obstruct the expected dinuclear (N′,N1,N2,N″)2 double bridging coordination being one of the most common for 4-substituted 3,5-di(pyridin-2-yl)-4H-1,2,4-triazoles and 3,5-di(pyridin-2-yl)-1,2,4-triazolates. Due to π–π stacking interactions, molecules of Cu2L2Cl4 in the structure of [Cu2L2Cl4]·2MeCN form 1D chains.

The reactions of pyrimidine-2-carbonitrile and 4,6-dimethylpyrimidine-2-carbonitrile with hydrazine hydrate were investigated. Intermediates in the route of successive transformations of pyrimidine-2-carbonitrile (pyrimidine-2-carbamidrazone and 1,2-bis[amino(pyrimidin-2-yl)methylidene]hydrazine) into trinuclear heterocyclic compounds, viz., symmetrical di(pyrimidin-2-yl)-1,4-dihydro-1,2,4,5-tetrazines and di(pyrimidin-2-yl)-4H-1,2,4-triazol-4-amines (potential polydentate ligands), were isolated. The oxidative dehydrogenation of di(pyrimidin-2-yl)-1,4-dihydro-1,2,4,5-tetrazines afforded the corresponding 3,6-di(pyrimidin-2-yl)-1,2,4,5-tetrazines.

Complex [FeL2(NCS)2]·H2O (L = 3,5-di(pyrimidin-2-yl)-4H-1,2,4-triazol-4-amine) was prepared as the first example of complex compounds of transition metals with 3,5-di(pyrimidin-2-yl)-4H-1,2,4-triazol-4-amines. The coordination core of the iron atom is a distorted octahedron FeN6. In the range 80–300 K the complex is high-spin, μeff ∼ 5.3 MB. The parameter of the crystal field splitting, 10Dq, for [FeL2(NCS)2]·H2O is ∼10800 cm−1.

Three-component acid-catalyzed cyclocondensation of 3,5-di-tret-butyl-4-hydroxybenzaldehyde with urea and ethyl acetoacetate or α-nitroacetophenone (Biginelli reaction) under homogeneous conditions gave the corresponding 5-substituted 3,4-dihydropyrimidin-2(1H)-ones having in position 4 of the heteroring an aryl substituent with sterically shielded hydroxy group. The condensation catalyzed by inorganic salts (Fe3+, Co2+, Zn2+, Li+) was successful only with ethyl acetoacetate as initial methylene-active component. Under analogous conditions, acetophenone and 4-fluoroacetophenone gave rise to 4,6-diarylpyrimidin-2(1H)-ones which are capable of undergoing phenol-quinonemethide tautomerism.

Copper(II) and cobalt(II) complexes with 4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methyl-2-phenylpyrimidine (L) of the general formula MLX2 (M = Cu(II), X = Cl and Br; M = Co(II), X = Cl, Br, and I) were obtained. According to X-ray diffraction data, CuLBr2 and CoLX2 (X = Cl, Br, and I) are mononuclear molecular complexes. The ligand L is coordinated to the metal atom in a chelating bidentate fashion through the N atoms of the pyrimidine and pyrazole rings. The coordination polyhedron of the metal atom is extended to a distorted tetrahedron by two halide ions. In solution, CuLBr2 undergoes slow transformation into CuL(1−x)L′xBr2 and the binuclear (X-ray diffraction data) Cu(I) complex [CuL(1−x)L′xBr]2 (L′ is 4-(4-bromo-3,5-dimethyl-1H-pyrazol-1-yl)-6-methyl-2-phenylpyrimidine). The complexes MLX2 show weak antiferromagnetic interactions between the M2+ ions.

The Cu(II) complexes with 4,6-bis(3,5-dimethyl-1H-pyrazole-1-yl)pyrimidine (L1) and 4-(3,5-dimethyl-1H-pyrazole-1-yl)-6-(3,5-diphenyl-1H-pyrazole-1-yl)pyrimidine (L2) of the composition Cu2L1Br4 and Cu2L2A4 (A = Cl, Br), respectively, were synthesized and studied by IR and magnetochemical methods. The molecular structure of the complexes is likely to be binuclear. In the presence of cocatalysts methylaluminoxane and triisobutylaluminium, the title complexes exhibit catalytic activity in the ethylene polymerization reaction.

The three-component condensation of aromatic aldehydes, methylurea, and α-nitroacetophenone affords both N(1)-and N(3)-methyl-substituted 4,6-diaryl-5-nitro-3,4-dihydropyrimidin-2(1H)-ones depending on the structure of aldehyde. Intermediate 4,6-diaryl-4-hydroxy-3-methyl-5-nitrohexahydropyrimidin-2-ones and trisubstituted urea, which is the transformation product of the 4-hydroxy-3-methyl derivative in an acidic medium (retro-Henry reaction), were identified in the reaction mixtures.

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.

The Cu(II) and Cu(I) complexes with 2-(3,5-diphenyl-1H-pyrazole-1-yl)-4,6-diphenylpyrimidine (L) of the composition CuLX2 (X = Cl, Br) and CuL(MeCN)Br are synthesized. According to X-ray diffraction data, the complexes have molecular structures. The molecules L are coordinated to the copper atom in bidentate-cyclic mode, i.e., through the N2 atom of pyrazole and N1 atom of pyrimidine rings. The coordination polyhedron of the Cu2+ ion in CuLX2 compounds is completed to a distorted tetrahedron with halide ions, that of the Cu+ ion in CuL(MeCN)Br compounds, with the bromide ion and the nitrogen atom of acetonitrile molecule. The CuLX2 complexes (X = Cl, Br) in combination with cocatalysts (methylaluminoxane and triisobutylaluminium) exhibit catalytic activity in ethylene polymerization.

Condensation of salicylaldehyde with nitroacetone and urea yields one diastereomer of the title compound. In DMF or DMSO solutions, this compound undergoes oxadiazocine ring opening leading to the equilibrium between its 2R*,6S*,11S*- and 2R*,6S*,11R*-diastereomers and 4-(2-hydroxyphenyl)-6-methyl-5-nitro-3,4-dihydropyrimidin-2(1H)-one.

Long-term exposure of nifedipine to daylight in ethanol gives 2,2"-bis[3,5-bis(methoxycarbonyl)-2,6-dimethylpyridin-4-yl]azoxybenzene and 6-hydroxy-1-methoxycarbonyl-2,4-dimethyl-5-oxo-5,6-dihydrobenzo[c][2,7]naphthyridine as the major products.

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.

Condensation of salicylaldehyde with nitroacetone and urea yields one diastereomer of the title compound. In DMF or DMSO solutions, this compound undergoes oxadiazocine ring opening leading to the equilibrium between its 2R*,6S*,11S*- and 2R*,6S*,11R*-diastereomers and 4-(2-hydroxyphenyl)-6-methyl-5-nitro-3,4-dihydropyrimidin-2(1H)-one.

The intramolecular hydrogen bond between the phenolic hydroxyl and the pyrimidine nitrogen atom in the title compounds exerts a destabilizing effect on the tetrazole ring and shifts the azide-tetrazole equilibrium toward the azide form, especially in the case of tetrazolo[c]pyrimidine and -[c]quinazoline. It has been found that the introduction of a methoxy group into theortho-position of the phenyl fragment stabilizes the tetrazole tautomer more efficiently than introduction of this group into thepara-position.

The reaction of 2- and 4-hydroxypyrimidines containingortho- andpara-hydroxyphenyl substituents with Vilsmeier-Haack reagents generatedin situ from DMF and SOCl2 or POCl3 results in the chemoselective replacement of the heterocyclic hydroxyl group by chlorine and formylation of the phenolic hydroxyl group. Aryl formates are hydrolyzed under the conditions of their isolation to give the corresponding phenols, especially if the pyrimidyl fragment isortho to the formyloxy group.

Three-component acid-catalyzed cyclocondensation of 3,5-di-tret-butyl-4-hydroxybenzaldehyde with urea and ethyl acetoacetate or α-nitroacetophenone (Biginelli reaction) under homogeneous conditions gave the corresponding 5-substituted 3,4-dihydropyrimidin-2(1H)-ones having in position 4 of the heteroring an aryl substituent with sterically shielded hydroxy group. The condensation catalyzed by inorganic salts (Fe3+, Co2+, Zn2+, Li+) was successful only with ethyl acetoacetate as initial methylene-active component. Under analogous conditions, acetophenone and 4-fluoroacetophenone gave rise to 4,6-diarylpyrimidin-2(1H)-ones which are capable of undergoing phenol-quinonemethide tautomerism.

The three-component condensation of aromatic aldehydes, methylurea, and α-nitroacetophenone affords both N(1)-and N(3)-methyl-substituted 4,6-diaryl-5-nitro-3,4-dihydropyrimidin-2(1H)-ones depending on the structure of aldehyde. Intermediate 4,6-diaryl-4-hydroxy-3-methyl-5-nitrohexahydropyrimidin-2-ones and trisubstituted urea, which is the transformation product of the 4-hydroxy-3-methyl derivative in an acidic medium (retro-Henry reaction), were identified in the reaction mixtures.

Three-component condensation of α-nitroacetophenone with aromatic aldehydes and urea in the presence of iron(III), cobalt(II), nickel(II), and copper(II) salts as catalyst led to the formation of 4,6-diaryl-5-nitro-3,4-dihydropyrimidin-2(1H)-ones and N-benzoyl-N′-(1-aryl-2-nitroethyl)ureas. The latter were formed as a result of retro-nitroaldol (retro-Henry) reaction of intermediate 4,6-diaryl-6-hydroxy-5-nitro-3,4,5,6-tetra-hydropyrimidin-2(1H)-ones.

It has been discovered with the aid of IR and NMR spectroscopy that 5-azidotetrazolo [1,5-a]quinazoline exists in solutions in a tautomeric equilibrium with ditetrazolo [1,5-a:1′,5′-c]quinazoline, and the thermodynamic and kinetic characteristics of the equilibrium have been determined. Azide-tetrazole tautomerism is not observed under these conditions for ditetrazolo[1,5-a:5′,1′-c]pyrazine, 6-azidotetrazolo [1,5-b]pyridazine, and their benzo analogs.

The diazotization of 2-aminopyrimidines and their N-oxides in solutions with various acidities was studied. It is shown that the amino group in pyrimidine N-oxides is not diazotized in strongly acidic media and that bromination in the 5 position of the pyrimidine ring is observed in concentrated hydrobromic acid. When the reaction was carried out in moderately acidic media, it was possible to synthesize the difficult-to-obtain 2-halopyrimidine N-oxides.

The proton-coupled 13C NMR and 1H NMR spectra of azidobenzene, tetrazolo[1,5-a]pyridine, 2-azidopyrimidine and tetrazolo[1,5-a]pyrimidine in different solvents have been analysed, and criteria for using the NMR spectral parameters (especially the 13C-1H coupling constants) for tautomeric structure determination have been elaborated. The efficiency of these criteria has been demonstrated in studies of azide--tetrazole equilibria in 2,4- and 4,6-diazidopyrimidines. The ditetrazolo tautomeric form of 2,4-diazidopyrimidine has been found in polar solvents.