Photophysics and photochemistry of 1,2,3-triazole-based complexes

Photochemistry of Ru(II) Triazole Complexes with 6-Membered Chelate Ligands: Detection and Reactivity of Ligand-Loss Intermediates

Photochemical ligand release from metal complexes may be exploited in the development of novel photoactivated chemotherapy agents for the treatment of cancer and other diseases. Highly intriguing photochemical behavior is reported for two ruthenium(II) complexes bearing conformationally flexible 1,2,3-triazole-based ligands incorporating a methylene spacer to form 6-membered chelate rings. [Ru(bpy)2(pictz)]2+ (1) and [Ru(bpy)2(btzm)]2+ (2) (bpy = 2,2'-bipyridyl; pictz = 1-(picolyl)-4-phenyl-1,2,3-triazole; btzm = bis(4-phenyl-1,2,3-triazol-4-yl)methane) exhibit coordination by the triazole ring through the less basic N2 atom as a consequence of chelation and readily undergo photochemical release of the pictz and btzm ligands (ϕ = 0.079 and 0.091, respectively) in acetonitrile solution to form cis-[Ru(bpy)2(NCMe)2]2+ (3) in both cases. Ligand-loss intermediates of the form [Ru(bpy)2(κ1-pictz or κ1-btzm)(NCCD3)]2+ are detected by 1H NMR spectroscopy and mass spectrometry. Photolysis of 1 yields three ligand-loss intermediates with monodentate pictz ligands, two of which form through simple decoordination of either the pyridine or triazole donor with subsequent solvent coordination (4-tz(N2) and 4-py, respectively). The third intermediate, shown to be able to form photochemically directly from 1, arises through linkage isomerism in which the monodentate pictz ligand is coordinated by the triazole N3 atom (4-tz(N3)) with a comparable ligand-loss intermediate with an N3-bound κ1-btzm ligand also observed for 2.

Pyrene Appendant Triazole-based Chemosensors for Sensing Applications

Over the last two decades, the design and development of fluorescent chemosensors for the targeted detection of Heavy Transition-metal (HTM) ions, anions, and biological analytes, have drawn much interest. Since the introduction of click chemistry in 2001, triazole moieties have become an increasingly prominent theme in chemosensors. Triazoles generated via click reactions are crucial for sensing various ions and biological analytes. Recently, the number of studies in the field of pyrene appendant triazole moieties has risen dramatically, with more sophisticated and reliable triazole-containing chemosensors for various analytes of interest described. This tutorial review provides a general overview of pyrene appendant-triazole-based chemosensors that can detect a variety of metal cations, anions, and neutral analytes by using modular click-derived triazoles.

An ab initio study on the electronic excited states and photodissociation mechanism of bromocarbene molecule

We used the internally contracted explicitly correlated multireference configuration interaction (icMRCI-F12) method combined with Davidson correction to conduct a high-precision ab initio study of CHBr. The spin-orbit coupling (SOC) is incorporated into the calculation. The 21 spin-free states split into 53 spin-coupled states of CHBr. The vertical transition energies and oscillator strengths are obtained of these states. The SOC effect on the equilibrium structures and the harmonic vibrational frequencies of the ground state X¹A', the lowest triplet state a³A'' and the first excited singlet state A¹A'' is investigated. The results reveal a significant effect of the SOC on the bond angle and the frequency of the bending mode of a³A''. The potential energy curves of electronic states of CHBr as functions of the H-C-Br bond angle, C-H bond length, and C-Br bond length, respectively, are also investigated. Based on the calculated results, the interactions between electronic states and photodissociation mechanism involved in CHBr in the ultraviolet region are explored. Our theoretical studies will shed light on the complicated interactions and dynamics of the electronic states of bromocarbenes.

Ligand Rigidity Steers the Selectivity and Efficiency of the Photosubstitution Reaction of Strained Ruthenium Polypyridyl Complexes

While photosubstitution reactions in metal complexes are usually thought of as dissociative processes poorly dependent on the environment, they are, in fact, very sensitive to solvent effects. Therefore, it is crucial to explicitly consider solvent molecules in theoretical models of these reactions. Here, we experimentally and computationally investigated the selectivity of the photosubstitution of diimine chelates in a series of sterically strained ruthenium(II) polypyridyl complexes in water and acetonitrile. The complexes differ essentially by the rigidity of the chelates, which strongly influenced the observed selectivity of the photosubstitution. As the ratio between the different photoproducts was also influenced by the solvent, we developed a full density functional theory modeling of the reaction mechanism that included explicit solvent molecules. Three reaction pathways leading to photodissociation were identified on the triplet hypersurface, each characterized by either one or two energy barriers. Photodissociation in water was promoted by a proton transfer in the triplet state, which was facilitated by the dissociated pyridine ring acting as a pendent base. We show that the temperature variation of the photosubstitution quantum yield is an excellent tool to compare theory with experiments. An unusual phenomenon was observed for one of the compounds in acetonitrile, for which an increase in temperature led to a surprising decrease in the photosubstitution reaction rate. We interpret this experimental observation based on complete mapping of the triplet hypersurface of this complex, revealing thermal deactivation to the singlet ground state through intersystem crossing.

Direct Determination of the Rate of Intersystem Crossing in a Near-IR Luminescent Cr(III) Triazolyl Complex

A detailed understanding of the dynamics of photoinduced processes occurring in the electronic excited state is essential in informing the rational design of photoactive transition-metal complexes. Here, the rate of intersystem crossing in a Cr(III)-centered spin-flip emitter is directly determined through the use of ultrafast broadband fluorescence upconversion spectroscopy (FLUPS). In this contribution, we combine 1,2,3-triazole-based ligands with a Cr(III) center and report the solution-stable complex [Cr(btmp)₂]³⁺ (btmp = 2,6-bis(4-phenyl-1,2,3-triazol-1-yl-methyl)pyridine) (1³⁺), which displays near-infrared (NIR) luminescence at 760 nm (τ = 13.7 μs, ϕ = 0.1%) in fluid solution. The excited-state properties of 1³⁺ are probed in detail through a combination of ultrafast transient absorption (TA) and femtosecond-to-picosecond FLUPS. Although TA spectroscopy allows us to observe the evolution of phosphorescent excited states within the doublet manifold, more significantly and for the first time for a complex of Cr(III), we utilize FLUPS to capture the short-lived fluorescence from initially populated quartet excited states immediately prior to the intersystem crossing process. The decay of fluorescence from the low-lying ⁴MC state therefore allows us to assign a value of (823 fs)^-1 to the rate of intersystem crossing. Importantly, the sensitivity of FLUPS to only luminescent states allows us to disentangle the rate of intersystem crossing from other closely associated excited-state events, something which has not been possible in the spectroscopic studies previously reported for luminescent Cr(III) systems.

Sensitizer-controlled photochemical reactivity via upconversion of red light

By combining the energy input from two red photons, chemical reactions that would normally require blue or ultraviolet irradiation become accessible. Key advantages of this biphotonic excitation strategy are that red light usually penetrates deeper into complex reaction mixtures and causes less photo-damage than direct illumination in the blue or ultraviolet. Here, we demonstrate that the primary light-absorber of a dual photocatalytic system comprised of a transition metal-based photosensitizer and an organic co-catalyst can completely alter the reaction outcome. Photochemical reductions are achieved with a copper(i) complex in the presence of a sacrificial electron donor, whereas oxidative substrate activation occurs with an osmium(ii) photosensitizer. Based on time-resolved laser spectroscopy, this changeover in photochemical reactivity is due to different underlying biphotonic mechanisms. Following triplet energy transfer from the osmium(ii) photosensitizer to 9,10-dicyanoanthracene (DCA) and subsequent triplet-triplet annihilation upconversion, the fluorescent singlet excited state of DCA triggers oxidative substrate activation, which initiates the cis to trans isomerization of an olefin, a [2 + 2] cycloaddition, an aryl ether to ester rearrangement, and a Newman-Kwart rearrangement. This oxidative substrate activation stands in contrast to the reactivity with a copper(i) photosensitizer, where photoinduced electron transfer generates the DCA radical anion, which upon further excitation triggers reductive dehalogenations and detosylations. Our study provides the proof-of-concept for controlling the outcome of a red-light driven biphotonic reaction by altering the photosensitizer, and this seems relevant in the greater context of tailoring photochemical reactivities.

Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)₂(N^∧N)]²⁺ (bpy = 2,2'-bipyridyl; N^∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N^∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N^∧N ligand. Complexes 2 to 6 all release the ligand N^∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)₂(NCMe)₂]²⁺, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N^∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (³MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of ³MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a ³MC_cis state but promotes the population of ³MC_trans states which rapidly deactivate ³MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of ³MC_cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Regioselective Synthesis and Molecular Docking Studies of 1,5-Disubstituted 1,2,3-Triazole Derivatives of Pyrimidine Nucleobases

1,2,3-triazoles are versatile building blocks with growing interest in medicinal chemistry. For this reason, organic chemistry focuses on the development of new synthetic pathways to obtain 1,2,3-triazole derivatives, especially with pyridine moieties. In this work, a novel series of 1,5-disubstituted-1,2,3-triazoles functionalized with pyrimidine nucleobases were prepared via 1,3-dipolar cycloaddition reaction in a regioselective manner for the first time. The N1-propargyl nucleobases, used as an alkyne intermediate, were obtained in high yields (87-92%) with a new two-step procedure that selectively led to the monoalkylated compounds. Then, FeCl₃ was employed as an efficient Lewis acid catalyst for 1,3-dipolar cycloaddition between different aryl and benzyl azides and the N1-propargyl nucleobases previously synthesized. This new protocol allows the synthesis of a series of new 1,2,3-triazole derivatives with good to excellent yields (82-92%). The ADME (Absorption, Distribution, Metabolism, and Excretion) analysis showed good pharmacokinetic properties and no violations of Lipinsky's rules, suggesting an appropriate drug likeness for these new compounds. Molecular docking simulations, conducted on different targets, revealed that two of these new hybrids could be potential ligands for viral and bacterial protein receptors such as human norovirus capsid protein, SARS-CoV-2 NSP13 helicase, and metallo-β-lactamase.

Post-Functionalization of Organometallic Complexes via Click-Reaction

CuAAC (Cu catalyzed azide-alkyne cycloaddition) click-reaction is a simple and powerful method for the post-synthetic modification of organometallic complexes of transition metals. This approach allows the selective introduction of additional donor sites or functional groups to the periphery of the ligand environment. This is especially important if a metalloligand with free donor sites, which are of the same nature as the primary site for the coordination of the primary metal, has to be created. The concept of post-synthetic modification of organometallic complexes by click-reaction is relatively recent and the currently available experimental material does not yet allow us to identify trends and formulate recommendations to address specific problems. In the present study, we have applied the CuAAC reaction for the post-synthetic modification of diimine mononuclear complexes Re(I), Pt(II) and Ir(III) with C≡C bonds at the periphery of the ligand environment and demonstrated that click-chemistry is a powerful tool for the tunable chemical post-synthetic modification of coordination compounds.

Synthesis methods of 1,2,3-/1,2,4-triazoles: A review

Triazole, comprising three nitrogen atoms and two carbon atoms, is divided into two isomers 1,2,3-triazole and 1,2,4-triazole. Compounds containing a triazole are one of the significant heterocycles that exhibit broad biological activities, such as antimicrobial, analgesic, anti-inflammatory, anticonvulsant, antineoplastic, antimalarial, antiviral, antiproliferative, and anticancer activities. A great quantity of drugs with a triazole structure has been developed and proved, for example, ketoconazole and fluconazole. Given the importance of the triazole scaffold, its synthesis has attracted much attention. This review summarizes the synthetic methods of triazole compounds from various nitrogen sources in the past 20 years.

Photophysical Properties and DNA Binding of Two Intercalating Osmium Polypyridyl Complexes Showing Light-Switch Effects

The synthesis and photophysical characterization of two osmium(II) polypyridyl complexes, [Os(TAP)₂dppz]²⁺ (1) and [Os(TAP)₂dppp2]²⁺ (2) containing dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppp2 (pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline) intercalating ligands and TAP (1,4,5,8-tetraazaphenanthrene) ancillary ligands, are reported. The complexes exhibit complex electrochemistry with five distinct reductive redox couples, the first of which is assigned to a TAP-based process. The complexes emit in the near-IR (1 at 761 nm and 2 at 740 nm) with lifetimes of >35 ns with a low quantum yield of luminescence in aqueous solution (∼0.25%). The Δ and Λ enantiomers of 1 and 2 are found to bind to natural DNA and with AT and GC oligodeoxynucleotides with high affinities. In the presence of natural DNA, the visible absorption spectra are found to display significant hypochromic shifts, which is strongly evident for the ligand-centered π–π* dppp2 transition at 355 nm, which undergoes 46% hypochromism. The emission of both complexes increases upon DNA binding, which is observed to be sensitive to the Δ or Λ enantiomer and the DNA composition. A striking result is the sensitivity of Λ-2 to the presence of AT DNA, where a 6-fold enhancement of luminescence is observed and reflects the nature of the binding for the enantiomer and the protection from solution. Thermal denaturation studies show that both complexes are found to stabilize natural DNA. Finally, cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Osmium(II) polypyridyl complexes comprising extended dipyrido[3,2-a:2′,3′-c]phenazine (1) and pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (2) intercalating ligands are shown to be effective DNA binders accompanied by enhanced near-IR emission. The emission response to B-DNA is found to be sensitive to the enantiomer and the composition of DNA, with greater emission observed for AT-rich sequences. Thermal denaturation studies show that both complexes stabilize natural DNA. Cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Synthesis and characterisation of group 8 tris(1-benzyl-1,2,3-triazol-4-yl)-p-anisolylmethane complexes

The tris(1,2,3-triazol-4-yl)methane framework offers a highly versatile architecture for ligand design, yet the coordination chemistry of this class of ligand remains largely unexplored. We report here the synthesis and characterisation of the homoleptic complexes [M(ttzm)₂](PF₆)₂ (ttzm = tris(1-benzyl-1,2,3-triazol-4-yl)-p-anisolylmethane; M = Fe (Fe), Ru (Ru), Os (Os)). Initial attempts to prepare Ru by reaction of [Ru(p-cymene)Cl₂]₂ and ttzm also led to the isolation of the heteroleptic complex [Ru(p-cymene)(ttzm)](PF₆)₂. The structures of [Ru(p-cymene)(ttzm)](PF₆)₂, [Fe(ttzm)₂]²⁺ (as its BPh₄^- salt) and Os were solved by X-ray diffraction. The homoleptic Fe(II) and Os(II) containing cations adopt distorted octahedral geometries due to the steric interactions between the ansiole and triazole rings of the ttzm ligands. The homoleptic complexes all adopt a low-spin d⁶ configuration and exhibit reversible M(II)/M(III) processes (+0.35 to +0.72 V vs. Fc/Fc⁺). These oxidation processes are cathodically shifted relative to those of related hexatriazole donor based complexes with density functional theory (DFT) calculations showing the metal d-orbitals are destabilised through a π-donor contribution from the triazole rings. The complexes all show prominent UV-visible absorption bands between 350 and 450 nm assigned to transitions of ¹MLCT character. Whilst none of the homoleptic complexes are emissive in room temperature fluid solutions, Os is emissive at 77 K in an EtOH/MeOH glass (λ_max 472 nm).

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

Photoredox catalysis typically relies on the use of single chromophores, whereas strategies, in which two different light absorbers are combined, are rare. In photosystems I and II of green plants, the two separate chromophores P₆₈₀ and P₇₀₀ both absorb light independently of one another, and then their excitation energy is combined in the so-called Z-scheme, to drive an overall reaction that is thermodynamically very demanding. Here, we adapt this concept to perform photoredox reactions on organic substrates with the combined energy input of two red photons instead of blue or UV light. Specifically, a Cu^I bis(α-diimine) complex in combination with in situ formed 9,10-dicyanoanthracenyl radical anion in the presence of excess diisopropylethylamine catalyzes ca. 50 dehalogenation and detosylation reactions. This dual photoredox approach seems useful because red light is less damaging and has a greater penetration depth than blue or UV radiation. UV-vis transient absorption spectroscopy reveals that the subtle change in solvent from acetonitrile to acetone induces a changeover in the reaction mechanism, involving either a dominant photoinduced electron transfer or a dominant triplet-triplet energy transfer pathway. Our study illustrates the mechanistic complexity in systems operating under multiphotonic excitation conditions, and it provides insights into how the competition between desirable and unwanted reaction steps can become more controllable.

4-Phenyl-1,2,3-triazoles as Versatile Ligands for Cationic Cyclometalated Iridium(III) Complexes

Five cationic iridium(III) complexes (1–5) were synthesized exploiting two triazole-based cyclometalating ligands, namely, 1-methyl-4-phenyl-1H-1,2,3-triazole (A) and the corresponding mesoionic carbene 1,3-dimethyl-4-phenyl-1H-1,2,3-triazol-5-ylidene (B). From the combination of these two ligands and the ancillary one, i.e., 4,4′-di-tert-butyl-2,2′-bipyridine (for 1–3) or tert-butyl isocyanide (for 4 and 5), not only the typical bis-heteroleptic complexes but also the much less explored tris-heteroleptic analogues (2 and 5) could be synthesized. The redox and emission properties of all of the complexes are effectively fine-tuned by the different ligands: (i) cyclometalating ligand A induces a stronger highest occupied molecular orbital (HOMO) stabilization compared to B and leads to complexes with progressively narrower HOMO–lowest unoccupied molecular orbital (LUMO) and redox gaps, and lower emission energy; (ii) complexes 1–3, equipped with the bipyridine ancillary ligand, display fully reversible redox processes and emit from predominantly metal-to-ligand charge transfer (MLCT) states with high emission quantum yields, up to 60% in polymeric matrix; (iii) complexes 4 and 5, equipped with high-field isocyanide ligands, display irreversible redox processes and high-energy emission from strongly ligand-centered triplets with long emission lifetimes but relatively low quantum yields (below 6%, both in room-temperature solution and in solid state). This work demonstrates the versatility of phenyl-triazole derivatives as cyclometalating ligands with different chelation modes (i.e., C^∧N and C^∧C:) for the synthesis of photoactive iridium(III) complexes with highly tunable properties.

Triazole-based cyclometalating ligands serving as both standard C^∧N chelators (as in 4-phenyl-triazole) and C^∧C: carbene ones (as in 4-phenyl-triazolylidene) have been used to synthesize a series of cationic iridium(III) complexes. Accordingly, unusual tris-heteroleptic complexes could be obtained by simultaneously exploiting both chelation modes. These ligands allow fine tuning of the properties of the related complexes, which exhibit luminescence all the way from blue to red and quantum yields up to 60% in a diluted polymeric matrix at 298 K.

Radical borylation of vinyl azides with NHC-boranes: divergent synthesis of α-boryl ketones and borylated triazoles

A divergent radical borylation of vinyl azides with N-heterocyclic carbene (NHC) boranes in the presence of ^tBuSH is described. The protocol enables the divergent synthesis of α-boryl ketones and borylated triazoles with excellent functional group tolerance and a broad substrate scope. Remarkably, this work shows that vinyl azides can serve as unprecedented five-atom synthons for the construction of 1,2,3-triazoles without N₂ extrusion.

Cerocene and Lanthanocene Chalcogenides: Synthesis, Structure, and Luminescence

A series of lanthanide chalcogenides {[η⁵-1,3-(Me₃C)₂C₅H₃]₂Ln}₂(μ-η²:η²-Te₂) (Ln = Ce 1, La 2), {[η⁵-1,3-(Me₃C)₂C₅H₃]₂Ln(THF)}₂(μ-Se) (Ln = Ce 3, La 4), and {[η⁵-1,3-(Me₃C)₂C₅H₃]₂Ln(THF)}₂(μ-Te) (Ln = Ce 5, La 6) can be readily obtained by the reaction of the alkyl complexes [η⁵-1,3-(Me₃C)₂C₅H₃]₂Ln(CH₂C₆H₄-o-NMe₂) with elemental selenium or tellurium in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN). The reaction of the alkyl complexes [η⁵-1,3-(Me₃C)₂C₅H₃]₂Ln(CH₂C₆H₄-o-NMe₂) with 9-BBN in 1:2 molar ratio afforded the lanthanide (cyclooctane-1,5-diyl)dihydroborate complexes [η⁵-1,3-(Me₃C)₂C₅H₃]₂Ln[(μ-H)₂BC₈H₁₄] (Ln = Ce 7, La 8) concomitant with the (Me₂N-o-C₆H₄CH₂)BC₈H₁₄ release, indicating that [η⁵-1,3-(Me₃C)₂C₅H₃]₂LnH may be the reactive species for the synthesis of lanthanide chalcogenides. All the new compounds were characterized by various spectroscopic methods, and their solid-state structures were further confirmed by single-crystal X-ray diffraction analyses. Luminescence spectroscopy was also employed to characterize complexes 1-6. The Ce(III) complexes 3 and 5 display distinct luminescence properties at room temperature, as compared to the corresponding La(III) complexes 4 and 6. The complex {[η⁵-1,3-(Me₃C)₂C₅H₃]₂Ce(THF)}₂(μ-Te) (5) exhibits unexpectedly red emission in solution which is found to depend strongly on the excitation wavelength.

Luminescent First-Row Transition Metal Complexes

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

Can 2-Pyridyl-1,2,3-triazole "Click" Ligands be Used to Develop Cu(I)/Cu(II) Molecular Switches?

Molecular switching processes are important in a range of areas including the development of molecular machines. While there are numerous organic switching systems available, there are far less examples that exploit inorganic materials. The most common inorganic switching system remains the copper(I)/copper(II) switch developed by Sauvage and co-workers over 20 years ago. Herein, we examine if bidentate 2-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine (pytri) and tridentate 2,6-bis[(4-phenyl-1H-1,2,3-triazol-1-yl)methyl]pyridine (tripy) moieties can be used to replace the more commonly exploited polypyridyl ligands 2,2'-bypyridine (bpy)/1,10-phenanthroline (phen) and 2,2';6',2″-terpyridine (terpy) in a copper(I)/(II) switching system. Two new ditopic ligands that feature bidentate (pytri, L1 or bpytri, L2) and tridentate tripy metal binding pockets were synthesized and used to generate a family of heteroleptic copper(I) and copper(II) 6,6'-dimesityl-2,2'-bipyridine (diMesbpy) complexes. Additionally, we synthesized a series of model copper(I) and copper(II) diMesbpy complexes. A combination of techniques including nuclear magnetic resonance (NMR) and UV-vis spectroscopies, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography was used to examine the behavior of the compounds. It was found that L1 and L2 formed [(diMesbpy)Cu(L1 or L2)]²⁺ complexes where the copper(II) diMesbpy unit was coordinated exclusively in the tridenate tripy binding site. However, when the ligands (L1 and L2) were complexed with copper(I) diMesbpy units, a complex mixture was obtained. NMR and MS data indicated that a 1:1 stoichiometry of [Cu(diMesbpy)]⁺ and either L1 or L2 generated three complexes in solution, the dimetallic [(diMesbpy)₂Cu₂(L1 or L2)]²⁺ and the monometallic [(diMesbpy)Cu(L1 or L2)]⁺ isomers where the [Cu(diMesbpy)]⁺ unit is coordinated to either the bidentate or tridentate tripy binding sites of the ditopic ligands. The dimetallic [(diMesbpy)₂Cu₂(L1 or L2)](PF₆)₂ complexes were structurally characterized using X-ray crystallography. Both complexes feature a [Cu(diMesbpy)]⁺ coordinated to the bidentate (pytri or bpytri) pocket of the ditopic ligands (L1 or L2), as expected. They also feature a second [Cu(diMesbpy)]⁺ coordinated to the nominally tridentate tripy binding site in a four-coordinate hypodentate κ²-fashion. Competition experiments with model complexes showed that the binding strength of the bidentate pytri is similar to that of the κ²-tripy ligand, leading to the lack of selectivity. The results suggest that the pytri/tripy and bpytri/tripy ligand pairs cannot be used as replacements for the more common bpy/phen-terpy partners due to the lack of selectivity in the copper(I) state.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Metal-free syntheses of N-functionalized and NH-1,2,3-triazoles: an update on recent developments

An overview of the latest developments in the metal-free synthesis of non-benzo-fused N-functionalized and NH-1,2,3-triazoles is provided in this feature article. Synthetic studies that appeared from 2016 until August 2020 are organized according to a wide-ranging classification, comprising oxidative and eliminative azide-dipolarophile cycloadditions, diazo transfer reactions and N-tosylhydrazone-mediated syntheses. The newly developed methods constitute a significant contribution to the field of 1,2,3-triazole synthesis in terms of structural variation via either the exploration of novel reactions, or the exploitation of existing methodologies.

Triazole-based, optically-pure metallosupramolecules; highly potent and selective anticancer compounds

Functionalised triazole aldehydes are used in the highly selective self-assembly of water-compatible, optically pure, low symmetry Fe(ii)- and Zn(ii)-based metallohelices. Sub-micromolar antiproliferative activity is observed against various cancerous cell lines, accompanied by excellent selectivity versus non-cancerous cells and potential for synergistic combinatorial therapy with cisplatin.

Quenching of the phosphorescence of thermally reversible photochromic naphthopyran Re(i) complexes initiated by either visible or ultraviolet radiation

Re(i) complexes bearing thermally reversible photochromic naphthopyran axial ligands undergo highly efficient, reversible phosphorescence quenching actuated by either visible or UV irradiation. The photoinduced quenching of the triplet metal-to-ligand charge-transfer (³MLCT) emission is interpreted based on changes in the relative energies of the excited states.

First cycloruthenation of 2-alkenylpyridines: synthesis, characterization and properties

Several cyclometalated ruthenium complexes 1-5 with 2-alkenylpyridines as C,N-chelating ligands were synthesized and then characterized by NMR, MS, IR and UV-Vis spectra. According to the single crystal of complex 2, it is evident that carbon from vinyl group is successfully bonded to Ru(ii) center. Moreover, the Ru-N bond trans to the Ru-C bond is elongated (2.127(5) Å), which is consistent with the strong trans effect of the carbon atom compared to that of the nitrogen atom. With different electron-donating groups linked to vinyl, these complexes exhibited regular changes in MLCT absorption bands, which were identified by UV-Vis and CV spectra in combination with DFT and TD-DFT. Interestingly, protonated intermediate species of these complexes in acidic solutions were tracked by the absorption changes and MS spectra, which displayed a possible protonation process of these complexes with the cleavage of Ru-C σ bonds.

Triazolization of Enolizable Ketones with Primary Amines: A General Strategy toward Multifunctional 1,2,3-Triazoles

The development of metal-free syntheses toward 1,2,3-triazoles has been a burgeoning research area throughout the past decade. Despite the numerous advances, the scarceness of methods for the preparation of 1,5-disubstituted 1,2,3-triazoles from readily available substrates remained a challenge that was addressed by our group in 2016. A metal-free three-component reaction, which we have dubbed the triazolization reaction, was established for the rapid synthesis of 1,5-disubstituted, fully functionalized and NH-1,2,3-triazoles. This novel approach stands out because it utilizes widely available starting materials, namely primary amines and enolizable ketones. Furthermore, the broad substrate scope is a major advantage, and was further expanded by the number of modified protocols that have been reported. Triazolization products have successfully found utility as intermediates in various synthetic transformations, and were the subject of a few interesting biological activity studies.

Copper-Mediated Synthesis of (E)-1-Azido and (Z)-1,2-Diazido Alkenes from 1-Alkene-1,2-diboronic Esters: An Approach to Mono- and 1,2-Di-(1,2,3-Triazolyl)-Alkenes and Fused Bis-(1,2,3-Triazolo)-Pyrazines

A stereoselective and convenient route has been demonstrated to access (Z)-1,2-diazido alkenes from the corresponding 1,2-diboronic esters via a copper-mediated reaction with sodium azide. Alternately, mono-functionalization was regioselectively carried out with trimethylsilyl azide as an azidation reactant. The in situ conversion of bis-azides to the corresponding bis-triazoles can be readily achieved in the presence of copper sulfate and sodium ascorbate, while the modification of the catalytic system opened a new convenient route to bis-triazolo-pyrazines, a new class of fused heterocycles.

Photophysical and Electrocatalytic Properties of Rhenium(I) Triazole-Based Complexes

A series of [Re(N^N)(CO)3(Cl)] (N^N = diimine) complexes based on 4-(pyrid-2-yl)-1,2,3-triazole (1), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (2), and 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (3) diimine ligands were prepared and their photophysical and electrochemical properties were characterized. The ligand-based reduction wave is shown to be highly sensitive to the nature of the triazole-based ligand, with the peak potential shifting by up to 600 mV toward more positive potential from 1 to 3. All three complexes are phosphorescent in solution at room temperature with λmax ranging from 540 nm (1) to 638 nm (3). Interestingly, the complexes appear to show inverted energy-gap law behaviour (τ = 43 ns for 1 versus 92 ns for 3), which is tentatively interpreted as reduced thermal accessibility of metal-centred (3MC) states from photoexcited metal to ligand charge transfer (3MLCT) states upon stabilisation of the N^N-centred lowest unoccupied molecular orbital (LUMO). The photophysical characterisation, supported by computational data, demonstrated a progressive stabilization of the LUMO from complex 1 to 3, which results in a narrowing of the HOMO–LUMO energy gap (HOMO = highest occupied molecular orbital) across the series and, correspondingly, red-shifted electronic absorption and photoluminescence spectra. The two complexes bearing pyridyl (1) and pyrimidyl (2) moieties, respectively, showed a modest ability to catalyse the electroreduction of CO2, with a peak potential at ca. −2.3 V versus Fc/Fc+. The catalytic wave that is observed in the cyclic voltammograms is slightly enhanced by the addition of water as a proton source.

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

The crystal structure of 1-benzyl-4-(2-(phenylethynyl)phenyl)-1H-1,2,3-triazole, C23H17N3

C23H17N3, monoclinic, P21 (no. 4), a = 11.7817(14) Å, b = 5.3165(6) Å, c = 14.1747(16) Å, β = 95.731(2)°, V = 883.43(18) Å3, Z = 2, R gt(F) = 0.0663, wR ref(F 2) = 1567, T = 298(2) K.

Toward an understanding of electronic excitation energies beyond the molecular orbital picture

Can we gain an intuitive understanding of excitation energies beyond the molecular picture?

Ru(II) coordination compounds of N-N bidentate chelators with 1,2,3 triazole and isoquinoline subunits: Synthesis, spectroscopy and antimicrobial properties

Bidentate chelators 1-(1-benzyl-1,2,3-triazol-4-yl)isoquinoline and 3-(1-benzyl-1,2,3-triazol-4-yl)isoquinoline were prepared from benzyl bromide and trimethylsilylethynylisoquinoline precursors using a tandem deprotection/substitution/CuAAC synthetic approach. Each chelator is capable of forming a stable 3:1 Ru(II) coordination compound, which forms as a geometric isomer mixture. These Ru(II) complexes possess unique MLCT absorbance signatures at 450/472 nm (1-isomer) and 367 nm (3-isomer) relative to their constituent chelating units. Minimum inhibitory concentration values as low as 0.4 μM are observed for Ru(II) complexes against representative Gram-positive bacteria Bacillus subtilis and Staphylococcus epidermidis. Comparing the MIC values of these isoquinoline compounds with analogous 2-(1-benzyl-1,2,3-triazol-4-yl)pyridine compounds shows a 2.5- to 40-fold improvement in potency. This study establishes that increased hydrophobicity introduced at the central chelating units of Ru(II) coordination compounds can be a useful means by which to optimize antimicrobial activity that is complimentary to the variation of peripheral substituent identity at the chelator's N1 triazole position.

Novel Re(I) tricarbonyl coordination compounds based on 2-pyridyl-1,2,3-triazole derivatives bearing a 4-amino-substituted benzenesulfonamide arm: synthesis, crystal structure, computational studies and inhibitory activity against carbonic anhydrase I, II, and IX isoforms†

In this work, two bidentate 2-pyridyl-1,2,3-triazole ligands (3a and 3b) containing a 4-substituted benzenesulfonamide pharmacophore prepared by classical click chemistry procedures, as well as their corresponding rhenium complexes, 4a and 4b of general formula [ReCl(CO)3(L)] (L = 3a or 3b) were prepared and fully characterised by spectroscopic methods (IR, NMR, MS, UV-Vis), elemental analysis, X-ray diffraction, and theoretical studies using DFT and TD-DFT methods. In particular, we showed that, in the solid state, the pyridine and the triazole rings of 3b adopted an uncommon cis configuration which stems from intermolecular hydrogen bonds. Preliminary assays demonstrated a promising nanomolar inhibitory activity against carbonic anhydrase isoform IX for both ligands and complexes with a strong affinity Ki of 2.8 nM for ligand 3a. More interestingly, complex 4b exhibited a pronounced selectivity against hCA IX over the off-targets hCA I and hCA II which makes this compound a promising potential anticancer drug candidate.

A conceptual framework for the development of iridium(iii) complex-based electrogenerated chemiluminescence labels

Translation of the highly promising electrogenerated chemiluminescence (ECL) properties of Ir(iii) complexes (with tri-n-propylamine (TPrA) as a co-reactant) into a new generation of ECL labels for ligand binding assays necessitates the introduction of functionality suitable for bioconjugation. Modification of the ligands, however, can affect not only the photophysical and electrochemical properties of the complex, but also the reaction pathways available to generate light. Through a combined theoretical and experimental study, we reveal the limitations of conventional approaches to the design of electrochemiluminophores and introduce a new class of ECL label, [Ir(C^N)2(pt-TOxT-Sq)]+ (where C^N is a range of possible cyclometalating ligands, and pt-TOxT-Sq is a pyridyltriazole ligand with trioxatridecane chain and squarate amide ethyl ester), which outperformed commercial Ir(iii) complex labels in two commonly used assay formats. Predicted limits on the redox potentials and emission wavelengths of Ir(iii) complexes capable of generating ECL via the dominant pathway applicable in microbead supported ECL assays were experimentally verified by measuring the ECL intensities of the parent luminophores at different applied potentials, and comparing the ECL responses for the corresponding labels under assay conditions. This study provides a framework to tailor ECL labels for specific assay conditions and a fundamental understanding of the ECL pathways that will underpin exploration of new luminophores and co-reactants.

A triazole-based fluorescence probe for detecting Hg2+ ions and its biological application

A new compound, ethyl 5-phenyl-2-(p-tolyl)-2H-1,2,3-triazole-4-carboxylate was successfully introduced and synthesized as a novel rhodamine B derivative named REPPC, and characterized by ¹ H nuclear magnetic resonance (NMR), ¹³ C NMR, and high resolution mass spectrometry (HRMS). It showed an obvious fluorescence and UV-visible light absorption enhancement towards Hg²⁺ ion without interference from common metal ions in N,N-dimethylformamide-H₂ O (pH 7.4). The spirolactam ring moiety of rhodamine in REPPC was converted to the open-ring form generating a 1:1 complex with the intervention of a mercury ion, verified by electrospray ionization-mass spectroscopy testing and density functional theory calculation. REPPC was used to visualize the level of mercury ions in living HeLa cells with encouraging results.