Preparations and structural diversity of copper(I) ethylene adducts with related 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine ligands

Structural diversity of copper(I)-ethylene complexes with 2,4-bis(2-pyridyl)pyrimidine directed by anions

The 3 : 1 reaction of [Cu(C2H4)n]ClO4 with 2,4-bis(2-pyridyl)pyrimidine (bpprd) in Me2CO under C2H4 afforded yellow prism crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(ClO4)2] (1). The 3 : 1 reaction of [Cu(C2H4)n]NO3 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the tetranuclear Cu(I)-C2H4 complex [Cu4(bpprd)2(η2-C2H4)4(μ-NO3)2](NO3)2 (2). The 10 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded yellow plate crystals of the dinuclear Cu(I)-C2H4 complex [Cu2(bpprd)(η2-C2H4)2(BF4)]BF4 (3). The 3 : 1 reaction of [Cu(C2H4)n]BF4 with bpprd in Me2CO under C2H4 afforded red prism crystals of the polymeric Cu(I)-C2H4 complex {[Cu6(bpprd)4(η2-C2H4)2(μ-η2:η2-C2H4)(μ-BF4)2](BF4)4}n (4). The X-ray crystal structures of complexes 1-4 have been determined. The structural diversity of Cu(I)-C2H4 complexes bridged by bpprd with different anions was demonstrated. The 1D Cu(I)-bpprd/C2H4 coordination polymer 4 bridged by unusual μ-η2:η2-C2H4 and the μ-BF4- anion is of particular significance. Complex 1 exhibited relatively well-resolved 1H NMR signals of bpprd and C2H4 (δ = 4.97 ppm) in (CD3)2CO at 23 °C.

Tetrazine metallation boosts rate and regioselectivity of inverse electron demand Diels-Alder (iEDDA) addition of dienophiles

Reported herein is the coordination of rhenium complexes to tetrazine ligand in [ReCl(CO)₃(TzPy)] [1] (TzPy = 3-(2-pyridyl)-1,2,4,5-tetrazine) and the rates of addition of different dienophiles to the tetrazine. Tetrazine coordiation lowers the ΔS^‡ contribution to ΔG^‡ for iEDDA addition.

Tetrazine Assists Reduction of Water by Phosphines: Application in the Mitsunobu Reaction

Reaction of 3,6-disubstituted-1,2,4,5-tetrazines with water and PEt₃ forms the corresponding 1,4-dihydrotetrazine and OPEt₃ . Thus PEt₃ , as a stoichiometric reductant, reduces water, and the resulting two reducing equivalents serve to doubly hydrogenate the tetrazine. A variety of possible initial interactions between electron-deficient tetrazine and electron-rich PR₃ , including a charge transfer complex, were evaluated by density functional calculations which revealed that the energy of all these make them spectroscopically undetectable at equilibrium, but one of these is nevertheless suggested as the intermediate in the observed redox reaction. The relationship of this to the Mitsunobu reaction, which absorbs the components of water evolved in the conversion of alcohol and carboxylic acid to ester, with desirable inversion at the alcohol carbon, is discussed. This enables a modified Mitsunobu reaction, with tetrazine replacing EtO₂ CN=NCO₂ Et (DEAD), which has the advantage that dihydrotetrazine can be recycled to tetrazine by oxidation with O₂ , something impossible with the hydrogenated DEAD. For this tetrazine version, a betaine-like intermediate is undetectable, but its protonated form is characterized, including by X-ray structure and NMR spectroscopy.

Structural, photophysical and magnetic properties of transition metal complexes based on the dipicolylamino-chloro-1,2,4,5-tetrazine ligand

The ligand 3-chloro-6-dipicolylamino-1,2,4,5-tetrazine (Cl-TTZ-dipica) , prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)-amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-dipica-MCl2]2, with M = Zn(II), Cd(II), Mn(II) and Co(II), when reacted with the corresponding metal chlorides. The dinuclear structure of the isostructural complexes was disclosed by single crystal X-ray analysis, clearly indicating the formation of [M(II)-(μ-Cl)2M(II)] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as a substituent on the tetrazine ring, engage respectively in specific anion-π intramolecular and intermolecular interactions with the electron-poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm(-1)) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm(-1)), the spin Hamiltonian being defined as H = -JSA·SB.

Dual Role for 1,2,4,5-Tetrazines in Polymer Networks: Combining Diels-Alder Reactions and Metal Coordination To Generate Functional Supramolecular Gels

The inverse-electron demand Diels-Alder cycloaddition of tetrazines and olefins has emerged as a powerful coupling reaction for the formation of polymer gels with diverse applications. Tetrazines are also excellent ligands for metal atoms. For example, 3,6-bis(2-pyridyl)-1,2,4,5-tetrazines (bptz) have been used to generate discrete supramolecular M_xbptz_y metal clusters and extended 2D grid structures. We reasoned that both the Diels-Alder and the metal-coordination modes of reactivity of bptz derivatives could be leveraged in the context of hydrogel design to yield novel hybrid materials. Here we report on the formation of supramolecular hydrogels via substoichiometric Diels-Alder functionalization of bptz ligands bound to the ends of poly(ethylene glycol) (PEG) chains followed by metal-coordination-induced gelation in the presence of Ni²⁺ and Fe²⁺ salts. Our results show that simple bptz-based polymers are versatile precursors to a diverse range of novel functional materials.