N^N^C-Cyclometalated rhodium(III) complexes with isomeric pyrimidine-based ligands: unveiling the impact of isomerism on structural motifs, luminescence and cytotoxicity

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 S₁^T → 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 S₂^T → 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.

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 (L^OH,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 L^OH,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 (L^H,OH), a monohydroxy congener of L^OH,OH. Both L^OH,OH and L^H,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for L^H,OH and two for L^OH,OH) strongly affects the luminescence mechanism and color of the emission: L^H,OH emits in the light green region, whereas L^OH,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, L^OH,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 L^H,OH due to the absence of the hydroxy group in the imidazole moiety.

Synthesis of esters and amides of 2-aryl-1-hydroxy-4-methyl-1H-imidazole-5-carboxylic acids and study of their antiviral activity against orthopoxviruses

Smallpox was eradicated >40 years ago but it is not a reason to forget forever about orthopoxviruses pathogenic to humans. Though in 1980 the decision of WHO to cease vaccination against smallpox had seemed logical, it led to the decrease of cross immunity against other infections caused by orthopoxviruses. As a result, in 2022 the multi-country monkeypox outbreak becomes a topic of great concern. In spite of existing FDA-approved drugs for the treatment of such diseases, the search for new small-molecule orthopoxvirus inhibitors continues. In the course of this search a series of novel 2-aryl-1-hydroxyimidazole derivatives containing ester or carboxamide moieties in position 5 of heterocycle has been synthesized and tested for activity against Vaccinia virus in Vero cell culture. Some of the compounds under consideration revealed a selectivity index higher than that of the reference drug Cidofovir. The highest selectivity index SI = 919 was exhibited by ethyl 1-hydroxy-4-methyl-2-[4-(trifluoromethyl)phenyl]-1H-imidazole-5-carboxylate 1f. The most active compound also demonstrated inhibitory activity against the cowpox virus (SI = 20) and the ectromelia virus (SI = 46).

Dual emission of ESIPT-capable 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)pyrimidines: interplay of fluorescence and phosphorescence

ESIPT-capable pyrimidine-based compounds featuring short O–H···N intramolecular hydrogen bonds, 2-(2-hydroxyphenyl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (HL1) and 2-(2-hydroxyphenyl)-4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-methylpyrimidine (HL2), were synthesized by the condensation of 4-hydrazinyl-2-(2-hydroxyphenyl)-6-methylpyrimidine with acetylacetone and dibenzoylmethane. In solution, HL1 and HL2...

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 (λ_max = 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), S₁^T → S₀^T. 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 (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, 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, S₀^T, resulting in the possibility of the S₀^T → S₁^T 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 λ_ex = 300 nm and 27% at λ_ex = 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.

First 1-hydroxy-1H-imidazole-based ESIPT emitter with an O–H···O intramolecular hydrogen bond: ESIPT-triggered TICT and speciation in solution

Search for new types of ESIPT-capable molecules is crucial for understanding factors controlling the ESIPT photoreaction. In this paper we present combined experimental and theoretical studies of the proton transfer...

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...

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.

Excitation wavelength dependent emission of silver(i) complexes with a pyrimidine ligand

Interplay of three emission mechanisms for silver(i) complexes leads to luminescence thermochromism and color tunable emission.

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.

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.

A mononuclear iron(ii) complex: cooperativity, kinetics and activation energy of the solvent-dependent spin transition

A mononuclear iron(ii) pyrimidine-based complex shows remarkable spin crossover properties and unprecedented thermal robustness.

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

All copper ions in a copper(i,ii) mixed-valence 1D-polymer show tetrahedral coordination cores, CuNBr₃ and CuN₂Br₂, which is extremely rare for mixed-valence copper(i,ii) compounds.

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

Wide thermal hysteresis loops for iron(ii) spin crossover complexes are associated with high activation barriers: the higher the activation barrier, the wider the hysteresis loop for a series of related spin crossover systems.

[Photogenerator of trichloroacetic acid--a perspective detritylation agent for microchip oligonucleotide synthesis]

For the purpose of new detritylation agents search for microarray oligonucleotide synthesis we investigated applicability of [4-(4-methoxyphenyl)-2,6-dinitro-phenyl](phenyl)methyl 2,2,2-trichloroacetate for 4,4'-dimethoxytrityl group detritylation during oligonucleotide syntheses generating trichloroacetic acid at radiation by light with a length of wave of 405 nanometers. [4-(4-Methoxyphenyl)-2,6-dinitro-phenyl](phenyl)methyl 2,2,2-trichloroacetate has been successfully used for solid-phase oligonucleotide synthesis of desired oligonucleotides.