Synthesis and photochemical studies of Cu(I) complex with 1,4-bis(3,5- dimethylpyrazol-1yl)tetrazine ligand

1. INTRODUCTION Nitrogen-rich compounds are unique very reactive substances which have high heats of formation. Pyridine is one of the most well known aromatic cycles where a CH is replaced by a nitrogen atom, but the pyridine ring is only one member of the azine heterocycles, in which one or more CH group of benzene is replaced by π-accepting and σ-donating nitrogen atoms, proceeding from diazines (pyridazines, pyrazine), triazines and up to tetrazines. Tetrazines are typical representatives of nitrogen rich heterocyclic compounds. They have a range of applications such as explosives, 1 propellants and pyrotechnic ingredients, 2 biological agents for recognition of anion 3 and precursors for drug development. Self-assembly of metal cations with nitrogen heterocyclic bridging ligands is a central theme in supramolecular chemistry aimed at developing assemblies of electronically coupled metal centres. 6 These ligands can be used to bridge metal centres in various ways, allowing electron and charge transfer processes in the structures. 1,2,4,5-Tetrazines also have very interesting redox-behaviour, which are similar to quinones. 7 The very low-lying π * orbital localised at the four nitrogen atoms in tetrazines might allow intense low-energy charge valence transfer absorptions, electrical conductivity of coordination polymers, unusual stability of paramagnetic radicals or mixed valence intermediates. The ability of tetrazine π-ligand systems is well documented, 1,11a double coordination of metal fragments to such binucleating ligands are known to cause a particularly strong perturbation of the ligand π system as is evident from pronounced spectroscopic effects. 8 1,2,4,5-tetrazines and its 3,6-disubstituted derivatives exhibit interesting coordination chemistry, characterised by electron and charge transfer and by the ability of these nitrogen-rich ligands to bridge metal centres in various ways. Tetrazines also have a rich organic chemistry involving cycloaddition reactions, that makes them valuable organic reagents. Here we consider some aspects of coordination chemistry of known 3,6-disubstituted tetrazines. The presence of at least four nitrogen donor atoms in the tetrazine derivatives allows for a potentially rich coordination chemistry. Tetrazines have a very low lying π * molecular orbital, replacement of the four CH groups in the 1,2,4,5-positions of benzene by the more electronegative nitrogen atoms results in particular stabilisation of the unoccupied a u orbital, making tetrazines very reducible. 9 Their anion radical and 3,6-dihydro forms are often more stable than the 'aromatic' parent molecule. 1,2,4,5-tetrazines can also be readily reduced to 3,6-dihydro derivatives. 10 There has been only a little work done on the unsubstituted …