N

The impact of isomerism of pyrimidine-based ligands and their rhodium(III) complexes with regard to their structures and properties was investigated. Two isomeric ligands, 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2,5-diphenylpyrimidine (HL2,5) and 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2,6-diphenylpyrimidine (HL2,6), were synthesized. The ligands differ by the degree of steric bulk: the molecular structure of HL2,5 is more distorted due to presence of pyrazolyl and phenyl groups in the neighbouring positions 4 and 5 of the pyrimidine ring. The complexation of HL2,5 and HL2,6 with RhCl3 leads to the sp2 C-H bond activation, resulting in the isolation of two complexes, [RhL2,5(Solv)Cl2]·nEtOH and [RhL2,6(Solv)Cl2]·nEtOH (Solv = H2O, EtOH), with the deprotonated forms of the pyrazolylpyrimidine molecules which coordinate the Rh3+ ion as N^N^C-tridentate ligands. According to DFT modelling, the mechanism of the deprotonation involves (i) the C-H bond breaking in the 2-phenyl group followed by the coordination of the C atom to the Rh atom, (ii) the protonation of coordinated chlorido ligand, (iii) the ejection of the HCl molecule and (iv) the coordination of the H2O molecule. The ligand isomerism has an impact on emission properties and cytotoxicity of the complexes. Although the excited states of the complexes effectively deactivate through S0/T1 and S0/S1 crossings associated with the cleavage of the weak H2O ligands upon excitation, the [RhL2,5(Solv)Cl2]·nEtOH complex appeared to be emissive in the solid state, while [RhL2,6(Solv)Cl2]·nEtOH is non-emissive at all. The complexes show significant cytotoxic activity against cancerous HepG2 and Hep2 cell lines, with the [RhL2,6(Solv)Cl2]·nEtOH complex being more active than its isomer [RhL2,5(Solv)Cl2]·nEtOH. On the other hand, noticeable cytotoxicity of the latter against HepG2 is supplemented by its non-toxicity against non-cancerous MRC-5 cells.

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms.

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HLH), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn2LH2Cl2] complex and an ionic (H2LH)4[ZnCl4]2·3H2O solid. Another ionic solid, (H2LH)Br, was obtained from the solution of HLH acidified with HBr. In both ionic solids, the H+ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HLH, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HLH molecules and ionic (LH)- and (H2LH)+ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HLH on the photoluminescence response and on altering the emission mechanism. The neutral HLH compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T1 → S0 phosphorescence of the enol form and (ii) anti-Kasha S2 → S0 fluorescence of the keto form, which if feasible due to the large S2-S1 energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HLH is low. Protonation or deprotonation of the HLH ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [Zn2LH2Cl2] and (H2LH)4[ZnCl4]2·3H2O. Despite both (H2LH)4[ZnCl4]2·3H2O and (H2LH)Br have the same (H2LH)+ cation in the structures, their emission properties differ significantly, whereas (H2LH)Br shows dual emission associated with two radiative decay channels: (i) S1 → S0 fluorescence and (ii) T1 → S0 phosphorescence, (H2LH)4[ZnCl4]2·3H2O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H2LH)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H2LH)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).

Luminescence of ESIPT-capable zinc(II) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety.

The rational design of ESIPT-capable metal complexes (ESIPT - Excited State Intramolecular Proton Transfer) requires two sites, namely, an ESIPT site and a metal binding site, to be spatially separated into the ligand core. Ligands featuring such sites are able to bind metal ions without being deprotonated upon their coordination. The use of ESIPT-capable ligands for the synthesis of metal complexes paves the way toward the exploration of ESIPT in the field of coordination chemistry. In this study, we present a new ESIPT-capable ligand on the base of 1-hydroxy-1H-imidazole, 1-hydroxy-5-methyl-4-[(2,2'-bipyridin)-6-yl]-2-(pyridin-2-yl)-1H-imidazole (HLb), and a series of ESIPT-capable zinc(II) halido complexes, [Zn(HLb)X2] (X = Cl, Br, I). Due to the incorporation of a (2,2'-bipyridin)-6-yl group at position 4 of the imidazole cycle, HLb acts as an N,N,N-chelating ligand. In the solid state, HLb and [Zn(HLb)X2] emit in the yellow region of the spectrum with excited state lifetimes in the nanosecond domain. Chelation-induced emission enhancement (CHEF) effect in zinc(II) complexes leads to an increase in the photoluminescence quantum yield (PLQY) for these compounds in comparison with free HLb ligand. The ESIPT process in HLb and [Zn(HLb)X2] is barrierless. The emission of [Zn(HLb)X2] is associated with the S1T → S0 transition in the tautomeric form (T-form). In contrast, due to (i) the dark nature of the S1 state and the bright nature of the S2 state and (ii) the large S1-S2 energy gap, HLb shows weak S2T → S0 fluorescence, in violation of Kasha's rule. Finally, the analysis of atomic charges in a series of ESIPT-capable 1-hydroxy-1H-imidazoles and their zinc(II) complexes allowed us to reveal the influence of expanding π-conjugation in the proton-donating and proton-accepting moieties on the stabilization/destabilization of the T-form and on the position of the emission band.

Efficient emission of Zn(II) and Cd(II) complexes with nopinane-annelated 4,5-diazafluorene and 4,5-diazafluoren-9-one ligands: how slight structural modification alters fluorescence mechanism.

Zinc(II) and cadmium(II) chlorido complexes with an N,N-chelating nopinane-annelated 4,5-diazafluoren-9-one ligand (LO) were synthesized. While the zinc(II) complex is mononuclear and adopts a tetrahedral ZnN2Cl2 coordination geometry, its cadmium(II) analogue features a 1D polymeric structure due to the bridging coordination of chlorido ligands with Cd2+ ions having an octahedral CdN2Cl4 coordination geometry. The photophysical properties of the oxygen-containing LO ligand and its zinc(II) and cadmium(II) complexes were studied in solution and in the solid state and matched against the properties of its oxygen-free 4,5-diazafluorene congener L and its complexes of the same metal ions. Comprehensive experimental and theoretical studies revealed the impact of the oxygen atom in the ligand core on the luminescence of the ligands and the complexes. For the oxygen-free L ligand and L-based complexes, the structural differences between the S0 and S1 geometries are small, which leads to fluorescence with extraordinarily small Stokes shifts. The emission of these compounds is of locally excited character for L and of mixed locally excited + ligand-to-halide charge transfer character for the L-based complexes. The introduction of the oxygen atom in the ligand core results in a drastic red-shift of the emission band due to short-range charge transfer. The differences between the S0 and S1 geometries are much more pronounced for LO and LO-based compounds than those of their oxygen-free analogues, leading to an order of magnitude larger Stokes shifts. On going from solution to the solid state, LO and its complexes exhibit aggregation-induced emission (AIE) behaviour with photoluminescence quantum yields (PLQYs) reaching tens of percent.

ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence.

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LOH,OH) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of LOH,OH with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LH,OH), a monohydroxy congener of LOH,OH. Both LOH,OH and LH,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for LH,OH and two for LOH,OH) strongly affects the luminescence mechanism and color of the emission: LH,OH emits in the light green region, whereas LOH,OH luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T1 → S0 phosphorescence and not related to ESIPT. At the same time, LOH,OH also exhibits S1 → S0 fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for LH,OH due to the absence of the hydroxy group in the imidazole moiety.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands.

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal2 with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λmax = 600-650 nm). The coordination of HLq by Zn2+ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S1 state, leading to the only possible fluorescence channel in the tautomeric form (T), S1T → S0T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH2Cl2 solutions, the compounds demonstrate dual emission in the UV (λmax = 358 nm) and green (λmax = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S1N → S0N and S1T → S0T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S0T, resulting in the possibility of the S0T → S1T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand.

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn2+ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λex = 300 nm and 27% at λex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S1 state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL2][Zn(OAc)2]2·2H2O and [ZnL2][ZnCl2]2·4H2O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

Near-infrared emitting copper(I) complexes with a pyrazolylpyrimidine ligand: exploring relaxation pathways.

Mononuclear copper(I) complexes [CuL2]I (1), [CuL2]2[Cu2I4]·2MeCN (2) and [CuL2]PF6 (3) with a new chelating pyrazolylpyrimidine ligand, 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4,6-diphenylpyrimidine (L), were synthesized. In the structures of complex cations [CuL2]+, Cu+ ions coordinate two L molecules (N,N-chelating coordination). Extended π-systems of the L molecules in [CuL2]+ favor the formation of paired π-π stacking intramolecular interactions between the pyrimidine and phenyl rings leading to significant distortions of tetrahedral coordination cores, CuN4. The free ligand L demonstrates dual excitation wavelength dependent luminescence in the UV and violet regions, which is attributed to S1 → S0 fluorescence and T1 → S0 phosphorescence with intraligand charge transfer character. The complexes 1-3 demonstrate T1 → S0 phosphorescence in the near-infrared region. Theoretical investigations point to its ligand-to-metal charge transfer (LMCT) origin. Large Stokes shifts of emission (ca. 200 nm) are the result of notable planarizations of CuN4 cores in the T1 state as compared to the S0 state. Spin-orbit coupling computations revealed that the most effective intersystem crossing channels for [CuL2]+ appear in high-lying excited states, while the S1 → T1 transition is unfavourable according to El-Sayed's rule and the energy gap law. Electron-vibration coupling calculations showed that the C-C and C-N stretching vibrations of the pyrimidine and phenyl moieties, the asymmetric Cu-N stretching vibrations and the wagging motions of phenyl rings contribute the most to the non-radiative deactivation of L and [CuL2]+.

A 1-Hydroxy-1H-imidazole ESIPT Emitter Demonstrating anti-Kasha Fluorescence and Direct Excitation of a Tautomeric Form.

The ability of 1-hydroxy-1H-imidazoles to exist in the form of two prototropic tautomers, the N-hydroxy and the N-oxide forms, can be utilized in the design of new types of ESIPT-fluorophores (ESIPT=excited state intramolecular proton transfer). Here we report the first example of 1-hydroxy-1H-imidazole-based ESIPT-fluorophores, 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL), featuring a short intramolecular hydrogen bond O-H⋅⋅⋅N (O⋅⋅⋅N 2.56 Å) as a pre-requisite for ESIPT. The emission of HL originates from the anti-Kasha S2 →S0 fluorescence in the N-oxide form as a result of a large S2 -S1 energy gap slowing down the S2 →S1 internal conversion. Due to an energy barrier between the N-hydroxy and N-oxide forms in the ground state, the HL molecules can be trapped and photoexcited in the N-oxide form leading to the Stokes shift of ca. 60 nm which is the smallest among known ESIPT-fluorophores.

Coordination-induced emission enhancement in copper(I) iodide coordination polymers supported by 2-(alkylsulfanyl)pyrimidines.

First examples of copper(i) complexes with 2-(alkylsulfanyl)pyrimidine ligands have been synthesized. Reactions of copper(i) iodide with 2-(methylsulfanyl)pyrimidine (L1) in various metal-to-ligand molar ratios in MeCN afford a ladder-type coordination polymer [Cu2L1I2]n with polymeric chains built from double-stranded (Cu2I2)n ribbons supported on both sides by µ2-N,S-L1 molecules. Although the second ligand, 2-(ethylsulfanyl)pyrimidine (L2), differs from L1 only by a methylene group, its reactions with copper(i) iodide in MeCN yield not only a congenerous coordination polymer, [Cu2L2I2]n, but also [CuL2I]n, in which a similar (Cu2I2)n ribbon is decorated by N-monodentate L2 molecules. Absorption spectra of all compounds represent an interplay of metal + iodine-to-ligand charge transfer (XMLCT) and ligand-centered (LC) and cluster-centered (CC) transitions, while the emission occurs from the excited states of XMLCT nature. The luminescence of [Cu2L1I2]n and [Cu2L2I2]n is blue-shifted and greatly enhanced in comparison with that of [CuL2I]n (quantum yields: 89% and 68% for [Cu2L1I2]n and [Cu2L2I2]nvs. 23% for [CuL2I]n at 77 K), which can be associated with a more rigid µ2-N,S coordination of 2-(alkylsulfanyl)pyrimidine ligands in [Cu2L1I2]n and [Cu2L2I2]n leading to a less distorted T1 state.

Luminescent Zn(ii) and Cd(ii) complexes with chiral 2,2'-bipyridine ligands bearing natural monoterpene groups: synthesis, speciation in solution and photophysics.

Mononuclear zinc(ii) and cadmium(ii) complexes, ZnLCl2 (1), CdLCl2 (2), ZnL1Cl2·2H2O (3), and CdL1Cl2·2H2O (4), with chiral ligands containing a 2,2'-bipyridine moiety and natural terpene (+)-limonene (L) or (+)-3-carene (L1) moieties were synthesized. In these complexes the L and L1 ligands are shown to coordinate Zn2+ and Cd2+ ions through the 2,2'-bipyridine moiety. The acetamide group of the ligands interacts with M2+ ions by forming NM2+ and C[double bond, length as m-dash]OM2+ contacts and N-HCl hydrogen bonds with coordinated Cl- ions. In solutions the complexes have several conformers differing by the degree of the turn of the acetamide moiety relative to the ligand core and the type of its interaction with the coordination core. The ligands and complexes exhibit luminescence with the quantum yield increasing in the order: ligand < cadmium(ii) complex < zinc(ii) complex. The complexes 3 and 4 demostrate excitation wavelength independent single-channel fluorescence. As opposed to 3 and 4, the complexes 1 and 2 demonstrate excitation wavelength dependent emission with nanosecond and microsecond lifetimes of the excited states. According to our TD-DFT calculations, an interplay of ligand centered and halide to ligand transitions facilitates two deactivation channels in 1 and 2: S1-S0 and T1-S0.

Hysteretic spin crossover in isomeric iron(ii) complexes.

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.

Excitation-Wavelength-Dependent Emission and Delayed Fluorescence in a Proton-Transfer System.

Manipulating the relaxation pathways of excited states and understanding mechanisms of photochemical reactions present important challenges in chemistry. Here we report a unique zinc(II) complex exhibiting unprecedented interplay between the excitation-wavelength-dependent emission, thermally activated delayed fluorescence (TADF) and excited state intramolecular proton transfer (ESIPT). The ESIPT process in the complex is favoured by a short intramolecular OH⋅⋅⋅N hydrogen bond. Synergy between the excitation-wavelength-dependent emission and ESIPT arises due to heavy zinc atom favouring intersystem crossing (isc). Reverse intersystem crossing (risc) and TADF are favoured by a narrow singlet-triplet gap, ΔEST ≈10 kJ mol-1 . These results provide the first insight into how a proton-transfer system can be modified to show a synergy between the excitation-wavelength-dependent emission, ESIPT and TADF. This strategy offers new perspectives for designing ESIPT and TADF emitters exhibiting tunable excitation-wavelength-dependent luminescence.

Kinetics of spin crossover with thermal hysteresis.

Non-isothermal kinetic studies of spin crossover allow establishing a reaction model of hysteretic spin state switching.

Temperature- and excitation wavelength-dependent emission in a manganese(ii) complex.

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.

Non-isothermal kinetics of spin crossover.

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.

Prototypical iron(ii) complex with 4-amino-1,2,4-triazole reinvestigated: an unexpected impact of water on spin transition.

The magnetic and thermodynamic properties of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole) were reinvestigated to gain an insight into the impact of water molecules on the spin transition. Variations in the outerspheric water molecule content in the complex induce drastic and unpredictable changes in its spin crossover regimes. Under vacuum the complex loses water molecules and shows a wide (ca. 30 K) and reproducible hysteresis loop, Tc↑ = 337-345 K and Tc↓ = 316-313 K. In sealed ampoules the complex Fe(ATrz)3(NO3)2·H2O shows a narrow hysteresis (ca. 1-4 K), Tc↑ = 326-329 K and Tc↓ = 326-324 K. After adsorption of water the complex Fe(ATrz)3(NO3)2·nH2O (n = 1.25-1.6) demonstrates a narrow two-step spin transition. In all these cases the kinetics of the LS → HS and HS → LS transitions has decelerating non-cooperative character. For the system Fe(ATrz)3(NO3)2·nH2O (n = 3.6-16.6) wide hysteresis (ca. 5-20 K) re-appears near room temperature (Tc↑ = 319-321 K and Tc↓ = 300-315 K). Surprisingly, the kinetics of the HS → LS spin transition for the systems with high water content switches from decelerating to sigmoidal (cooperative). The activation energy of the LS → HS transition was estimated for the first time for iron(ii) spin crossover complexes with 1,2,4-triazoles (ca. 1000-2000 kJ mol-1). The systems Fe(ATrz)3(NO3)2 and Fe(ATrz)3(NO3)2·nH2O show compensation effects (ΔH - ΔS, ln A - Ea). A correlation between the Tonset↑, the ΔH values and the water content in the complex is observed: the highest ΔH values (27-29 kJ mol-1) and the lowest Tonset↑ values (317-320 K) correspond to the samples with high water content, whereas the lowest ΔH values (19-23 kJ mol-1) and the highest Tonset↑ values (337-345 K) correspond to water-free samples, Fe(ATrz)3(NO3)2. Our results provide the first experimental evidence that the presence of water (and even air humidity) produces dramatic changes in the spin crossover behavior of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole).

Compensation effects and relation between the activation energy of spin transition and the hysteresis loop width for an iron(ii) complex.

The enthalpy-entropy compensation was observed for the cooperative → spin transition (the phase is a mononuclear complex [FeL2](BF4)2, L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine). The physical origin of this effect is the fact that the → spin transition is the first order phase transition accompanied by noticeable variations in the Tonset↑, ΔH and ΔS values. Higher ΔH and ΔS values are correlated with higher Tonset↑ values. The higher the enthalpy and entropy of the spin transition, the wider the hysteresis loop. The kinetic compensation effect, i.e. a linear relationship between ln A and Ea, was observed for the → spin transition. Moreover, an isokinetic relationship was detected in this system: the Arrhenius lines (ln k vs. 1/T) obtained from magnetochemical data for different samples of the phase undergoing the → transition show a common point of intersection (Tiso = 490 ± 2 K, ln kiso = -6.0 ± 0.2). The validity of this conclusion was confirmed by the Exner-Linert statistical method. This means that the isokinetic relationship and the kinetic compensation effect (ln A vs. Ea) in this system are true ones. The existence of a true kinetic compensation effect is supported independently by the fact that the hysteresis loop width for the cooperative spin transition ↔ increases with increasing activation barrier height. Estimating the energy of excitations for the phase with Tiso ∼ 490 K yields wavenumbers of ca. 340 cm(-1) corresponding to the frequencies of the stretching vibrations of the Fe(LS)-N bonds, i.e. the bonds directly involved in the mechanism of the spin transition. This is the first observation of the kinetic compensation effect (ln A vs. Ea) and the isokinetic relationship for a cooperative spin crossover system showing thermal hysteresis. Our results provide the first experimental evidence that the higher the activation barrier for the spin transition, the wider the hysteresis loop for a series of related spin crossover systems.

Mixed-valence copper(I,II) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines: from ionic structures to coordination polymers.

Two pyrimidine-based ligands, 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(morpholino)pyrimidine () and 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-phenoxypyrimidine (), and a series of mixed-valence copper(i,ii) halide complexes, [Cu(L(2))2Br]2[Cu2Br4] (), [Cu(L(2))2Cl][CuCl2] (), and [Cu2L(3)Br3]n (), have been synthesized. The complex [Cu(L(2))2Br]2[Cu2Br4] was prepared by the reaction of with CuBr2 in a 1 : 1 molar ratio in MeCN. Its chlorido-analogue, the complex [Cu(L(2))2Cl][CuCl2], was synthesized by the reaction between , CuCl2 and CuCl in a 2 : 1 : 1 molar ratio in MeCN. The ligand acts as a chelating one. In the structures of the complexes [Cu(L(2))2Br]2[Cu2Br4] and [Cu(L(2))2Cl][CuCl2] the Cu(2+) 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 [Cu2L(3)Br3]n is to react CuBr2 with in a 2 : 1 molar ratio in the MeCN/CHCl3 mixture on heating. In the structure of [Cu2L(3)Br3]n the ligand 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 and (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 N(1) pyrimidine atom less sterically hindered, enabling it to participate in metal ion binding. Importantly, all copper ions in [Cu2L(3)Br3]n show similar tetrahedral environments, CuNBr3 and CuN2Br2, which is extremely rare for mixed-valence copper(i,ii) compounds. The ligands and show blue emission which is quenched upon their coordination to copper ions. The 1D coordination polymer [Cu2L(3)Br3]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 iron(II) complex: cooperativity, kinetics and activation energy of the solvent-dependent spin transition.

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 ß-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 ß-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.