Functionalized Triazines and Tetrazines: Synthesis and Applications

The molecules possessing triazine and tetrazine moieties belong to a special class of heterocyclic compounds. Both triazines and tetrazines are building blocks and have provided a new dimension to the design of biologically important organic molecules. Several of their derivatives with fine-tuned electronic properties have been identified as multifunctional, adaptable, switchable, remarkably antifungal, anticancer, antiviral, antitumor, cardiotonic, anti-HIV, analgesic, anti-protozoal, etc. The objective of this review is to comprehensively describe the recent developments in synthesis, coordination properties, and various applications of triazine and tetrazine molecules. The rich literature demonstrates various synthetic routes for a variety of triazines and tetrazines through microwave-assisted, solid-phase, metal-based, [4+2] cycloaddition, and multicomponent one-pot reactions. Synthetic approaches contain linear, angular, and fused triazine and tetrazine heterocycles through a combinatorial method. Notably, the triazines and tetrazines undergo a variety of organic transformations, including electrophilic addition, coupling, nucleophilic displacement, and intramolecular cyclization. The mechanistic aspects of these heterocycles are discussed in a detailed way. The bioorthogonal application of these polyazines with various strained alkenes and alkynes provides a new prospect for investigations in chemical biology. This review systematically encapsulates the recent developments and challenges in the synthesis and possible potential applications of various triazine and tetrazine systems.

MLCT Excited-State Behavior of Trinuclear Ruthenium(II) 2,2'-Bipyridine Complexes

Four trinuclear ruthenium(II) polypyridyl complexes were synthesized, and a detailed investigation of their excited-state properties was performed. The tritopic sexi-pyridine bridging ligands were obtained via para or meta substitution of a central 2,2'-bipyridine fragment. A para connection between the 2,2'-bipyridine chelating moieties of the bridging ligand led to a red-shifted MLCT absorption band in the visible part of the spectra, whereas the meta connection induced a broadening of the LC transitions in the UV region. A convergent energy transfer from the two peripheral metal centers to the central Ru(II) moiety was observed for all trinuclear complexes. These complexes were in thermal equilibrium with an upper-lying ³MLCT excited state over the investigated range of temperatures. For all complexes, deactivation via the ³MC excited state was absent at room temperature. Importantly, the connection in the para position for both central and peripheral 2,2'-bipyridines of the bridging ligand resulted in a trinuclear complex (T_pp) that absorbed more visible light, had a longer-lived excited state, and had a higher photoluminescence quantum yield than the parent [Ru(bpy)₃]²⁺, despite its red-shifted photoluminescence. This behavior was attributed to the presence of a highly delocalized excited state for T_pp.

Crystal structures of three 4-substituted-2,2'-bipyridines synthesized by Sonogashira and Suzuki-Miyaura cross-coupling reactions

Facile synthetic routes for three 4-substituted 2,2'-bi-pyridine derivatives, 4-[2-(4-methyl-phenyl)-ethyn-yl]-2,2'-bi-pyridine, C19H14N2, (I), 4-[2-(pyridin-3-yl)ethyn-yl]-2,2'-bi-pyridine, C17H11N3, (II), and 4-(indol-4-yl)-2,2'-bi-pyridine, C18H13N3, (III), via Sonogashira and Suzuki-Miyaura cross-coupling reactions, respect-ively, are described. As indicated by X-ray analysis, the 2,2'-bi-pyridine core, the ethyl-ene linkage and the substituents of (I) and (II) are almost planar [dihedral angles between the two ring systems: 8.98 (5) and 9.90 (6)° for the two mol-ecules of (I) in the asymmetric unit and 2.66 (14)° for (II)], allowing π-conjugation. On the contrary, in (III), the indole substituent ring is rotated significantly out of the bi-pyridine plane [dihedral angle = 55.82 (3)°], due to steric hindrance. The crystal packings of (I) and (II) are dominated by π-π inter-actions, resulting in layers of mol-ecules parallel to (30-2) in (I) and columns of mol-ecules along the a axis in (II). The packing of (III) exhibits zigzag chains of mol-ecules along the c axis inter-acting through N-H⋯N hydrogen bonds and π-π inter-actions. The contributions of unknown disordered solvent mol-ecules to the diffraction intensities in (II) were removed with the SQUEEZE [Spek (2015 ▸). Acta Cryst. C71, 9-18] algorithm of PLATON. The given chemical formula and other crystal data do not take into account these solvent mol-ecules.

Selective Sensing of Phosphates by a New Bis-heteroleptic RuII Complex through Halogen Bonding: A Superior Sensor over Its Hydrogen-Bonding Analogue

The selective phosphate-sensing property of a bis-heteroleptic Ru^II complex, 1[PF₆ ]₂ , which has a halogen-bonding iodotriazole unit, is demonstrated and is shown to be superior to its hydrogen-bonding analogue, 2[PF₆ ]₂ . Complex 1[PF₆ ]₂ , exploiting halogen-bonding interactions, shows enhanced phosphate recognition in both acetonitrile and aqueous acetonitrile compared with its hydrogen-bonding analogue, owing to considerable amplification of the Ru^II -center-based metal-to-ligand charge transfer (MLCT) emission response and luminescence lifetime. Detailed solution-state studies reveal a higher association constant, lower limit of detection, and greater change in lifetime for complex 1 in the presence of phosphates compared with its hydrogen-bonding analogue, complex 2. The ¹ H NMR titration study with H₂ PO₄^- ascertains that the binding of H₂ PO₄^- occurs exclusively through halogen-bonding or hydrogen-bonding interactions in complexes 1[PF₆ ]₂ and 2[PF₆ ]₂ , respectively. Importantly, the single-crystal X-ray structure confirms the first ever report on metal-assisted second-sphere recognition of H₂ PO₄^- and H₂ P₂ O₇^2- with 1 through a solitary C-I⋅⋅⋅anion halogen-bonding interaction.

A heptanuclear light-harvesting metal-based antenna dendrimer with six Ru(ii)-based chromophores directly powering a single Os(ii)-based energy trap

Quantitative Dexter energy transfer occurs in a novel metal dendrimer 1, from the peripheral Ru(ii) centres to the Os(ii) core, at a rate constant of 9.1 × 10¹⁰ s⁻¹.

Ruthenium bistridentate complexes with non-symmetrical hexahydro-pyrimidopyrimidine ligands: a structural and theoretical investigation of their optical and electrochemical properties

The optical and electronic properties of six Ru complexes with non-symmetrical tridentate ligands have been investigated and, as corroborated by electrochemical data, the presence of the hpp ligand strongly affects the oxidation potential of the metal ion.

A Cyanuric Acid Platform Based Tripodal Bis-heteroleptic Ru(II) Complex of Click Generated Ligand for Selective Sensing of Phosphates via C-H···Anion Interaction

A new bis-heteroleptic trinuclear Ru(II) complex (1[PF6]6) has been synthesized from electron deficient cyanuric acid platform based copper-catalyzed azide-alkyne cycloaddition, i.e., CuAAC click generated ligand, 1,3,5-tris [(2-aminoethyl-1H-1,2,3-triazol-4-yl)-pyridine]-1,3,5-triazinane-2,4,6-trione (L1). Complex 1[PF6]6 displays weak luminescence (ϕf = 0.002) at room temperature with a short lifetime of ∼5 ns in acetonitrile. It shows selective sensing of hydrogen pyrophosphate (HP2O7(3-)) through 20-fold enhanced emission intensity (ϕf = 0.039) with a 15 nm red shift in emission maxima even in the presence of a large excess of various competitive anions like F(-), Cl(-), AcO(-), BzO(-), NO3(-), HCO3(-), HSO4(-), HO(-), and H2PO4(-) in acetonitrile. Selective change in the decay profile as well as in the lifetime of 1[PF6]6 in the presence of HP2O7(3-) (108 ns) further supports its selectivity toward HP2O7(3-). UV-vis and photoluminescence titration profiles and corresponding Job's plot analyses suggest 1:3 host-guest stoichiometric binding between 1[PF6]6 and HP2O7(3-). High emission enhancement of 1[PF6]6 in the presence of HP2O7(3-) has resulted in the detection limit of the anion being as low as 0.02 μM. However, 1[PF6]6 shows selectivity toward higher analogues of phosphates (e.g., ATP, ADP, and AMP) over HP2O7(3-)/H2PO4(-) in 10% Tris HCl buffer (10 mM)/acetonitrile medium. Downfield shifting of the triazole C-H in a (1)H NMR titration study confirms that the binding of HP2O7(3-)/H2PO4(-) is occurring via C-H···anion interaction. The single crystal X-ray structure of complex 1 having NO3(-) counteranion, 1[NO3]6 shows binding of NO3(-) with complex 1 via C-H···NO3(-) interactions.