Seeking Redox Activity in a Tetrazinyl Pincer Ligand: Installing Zerovalent Cr and Mo

Abstract

Reaction of the readily reduced pincer ligand bis-tetrazinylpyridine, btzp, with the zerovalent metal source M­(CO)3(MeCN)3 yields M­(btzp)2 for M = Cr, Mo. These diamagnetic molecules show intrapincer bond lengths consistent with major charge transfer from metal to ligand, a result which is further supported by X-ray photoelectron spectroscopy. These molecules show up to five reversible outer-sphere electron transfers by cyclic voltammetry. The electronic structure of neutral M­(btzp)2 is analyzed by DFT and CASSCF calculations, which reveal the degree of back-donation from the metal into pincer π* orbitals and also subtle differences in metal–ligand interaction for Mo vs Cr. Near-IR absorptions exhibited by both M­(btzp)2 species originate from charge transfer among differently reduced tetrazine rings, which thus further support pincer reduction in these species.

[(η6-p-Cymene)[3-(pyrid-2-yl)-1,2,4,5-tetrazine]chlororuthenium(II)]+, Redox Noninnocence and Dienophile Addition to Coordinated Tetrazine

“…1,2 The substituents on the noninnocent N-donor ligand can be used to tune the reduction potential because they change the energy of the ligand's lowest unoccupied molecular orbital (LUMO). 3−6 The heterocyclic 3-(pyrid-2-yl)-1,2,4,5-tetrazine (TzPy) ligand, isoelectronic to bipy, 7,8 can be modified after coordination by the addition of different dienophiles (Scheme 1). 9,10 The inverse electron demand [4 + 2] pericyclic addition (Diels−Alder) between 1,2,4,5-tetrazines and electron-rich dienophiles provides a convenient method for generating new C−C bonds and heterocycles, and this technology has found applications in biological labeling.…”

“…Ruthenium half-sandwich complexes coordinated to N-donor ligands, such as 2,2′-bipyridine (bipy), have found uses as precatalysts for several transformations, including hydride transfer reactions or electrocatalysts for CO 2 reduction. , The substituents on the noninnocent N-donor ligand can be used to tune the reduction potential because they change the energy of the ligand’s lowest unoccupied molecular orbital (LUMO). − The heterocyclic 3-(pyrid-2-yl)-1,2,4,5-tetrazine (TzPy) ligand, isoelectronic to bipy, , can be modified after coordination by the addition of different dienophiles (Scheme ). , …”

(Spectro)Electrochemistry of 3‐(Pyrid‐2‐yl)‐s‐Tetrazine‐ and 1,2‐ (Dihydro)pyridazine Tricarbonylrhenium(I)chloride

“…[3][4][5][6][7][8] Changes to the N N ligand in ReCl(CO) 3 (N N) provides a means to better electrocatalytic activity for CO 2 reduction. [9][10][11] The heterocyclic 3-(pyrid-2-yl)-1,2,4,5-tetrazine (TzPy) ligand, isoelectronic to bpy = 2,2'-bipyridine, [12][13] can be modified post ligation by the addition of different dienophiles (olefins). Tetrazines have also found many applications in biorthogonal labeling, [14][15][16][17][18][19][20] as well as in materials and organic synthesis.…”