Structural diversity among copper(I) ethylene adducts of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine

The reaction of Cu(ClO4)center dot 6H(2)O, Cu turnings, and 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) in Me2CO under C2H4 afforded brownish-red needle crystals of {[Cu-2(bptz(center dot-))(C2H4)(2)](ClO4)}(2) (1) as a minor product and brown plate crystals of [Cu-2(2,5-H(2)bptz)(C2H4)(2)](ClO4)(2)center dot 2Me(2)CO (2). The X-ray crystallographic study showed that complex 1 is composed of two independent [Cu-2(bptz(center dot-))(C2H4)(2)](+) cation moieties and two ClO4- anions. In both of the cation moieties, each Cu(I) atom is coordinated by two chelate N atoms of bptz(center dot-) and the C=C bond of C2H4 in the trigonal-planar geometry to form a unique transoid dinuclear Cu(I)-bptz(center dot-)/C2H4 adduct bearing a metal-stabilized tetrazine anion radical, bptz(center dot-), which was produced by the one-electron reduction of bptz. In contrast, complex 2 consists of one [Cu-2(2,5-H(2)bptz)(C2H4)(2)](2+) cation moiety, two ClO4- anions, and two solvated Me2CO molecules. Each Cu(I) atom is coordinated by two chelate N atoms of 2,5-H(2)bptz and the C=C bond of C2H4 in the trigonal-planar geometry to afford a transoid dinuclear Cu(I)-C2H4 adduct with 2,5-H(2)bptz, which was produced by the two-electron reduction of bptz. Additionally, the similar reaction of Cu(ClO4)center dot 6H(2)O, Cu turnings, and bptz in Me2CO or MeEtCO under C2H4 in the presence of ferrocene afforded orange brick crystals of [Cu-2(bpdpyz)(C2H4)(2)]- (ClO4)(2)center dot Me2CO (3) and [Cu-2(bpdpyz)(C2H4)(2)](ClO4)(2) (4), respectively, together with complex 1 in higher yield. This result shows that 3,6-bis(2-pyridyl)-4,5-dihydropyridazine (bpdpyz) was produced by the cyclo-addition of bptz with C2H4 via a Diels-Alder addition and retro-Diels-Alder elimination of dinitrogen. In complexes 3 and 4, each Cu(I) atom is coordinated by two chelate N atoms of bpdpyz and the C=C bond of C2H4 to afford an unusual cisoid dinuclear Cu(I)-C2H4 adduct. Further attempts to react excess [Cu(C2H4)(n)]ClO4 with bptz in Me2CO under C2H4 afforded black brick crystals of [Cu-4(4,5-H(2)bptz)(4)](ClO4)(4)center dot 2Me(2)CO (5) together with complex 1. Complex 5 is the first [4 x 4] grid-like tetranuclear Cu(I) complex constructed by the combination of Cu(I) ion and 4,5-H(2)bptz. The four Cu(I) atoms are spanned by four 4,5-H(2)bptz ligands, with two lying above the Cu-4 mean plane and two lying below. In contrast, the similar reaction of excess [Cu(C2H4)(n)]ClO4 with bptz in Me2CO under C2H4 in the presence of ferrocene afforded reddish-brown brick crystals of {[Cu-2(2,5-H(2)bptz)(C2H4)(2)](ClO4)(2)}(2)center dot Me2CO (6) together with complexes 1 and 5. Complex 6 is composed of two independent [Cu-2(2,5-H(2)bptz)(C2H4)(2)](2+) cation moieties, four ClO4- anions, and a solvated Me2CO molecule. It is interesting that the structures of the two [Cu-2(2,5-H(2)bptz)(C2H4)(2)](2+) cation moieties are different: one has a nearly planar structure and the other possesses a boat conformation owing to the bent central dihydrotetrazine ring.