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

Abstract

The reaction of Cu(ClO₄)·6H₂O, Cu turnings, and 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) in Me₂CO under C₂H₄ afforded brownish-red needle crystals of {[Cu₂(bptz˙⁻)(C₂H₄)₂](ClO₄)}₂ (1) as a minor product and brown plate crystals of [Cu₂(2,5-H₂bptz)(C₂H₄)₂](ClO₄)₂·2Me₂CO (2). The X-ray crystallographic study showed that complex 1 is composed of two independent [Cu₂(bptz˙⁻)(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˙⁻ and the CC bond of C₂H₄ in the trigonal-planar geometry to form a unique transoid dinuclear Cu(I)–bptz˙⁻/C₂H₄ adduct bearing a metal-stabilized tetrazine anion radical, bptz˙⁻, which was produced by the one-electron reduction of bptz. In contrast, complex 2 consists of one [Cu₂(2,5-H₂bptz)(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-H₂bptz and the CC bond of C₂H₄ in the trigonal-planar geometry to afford a transoid dinuclear Cu(I)–C₂H₄ adduct with 2,5-H₂bptz, which was produced by the two-electron reduction of bptz. Additionally, the similar reaction of Cu(ClO₄)·6H₂O, Cu turnings, and bptz in Me₂CO or MeEtCO under C₂H₄ in the presence of ferrocene afforded orange brick crystals of [Cu₂(bpdpyz)(C₂H₄)₂](ClO₄)₂·Me₂CO (3) and [Cu₂(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 cycloaddition of bptz with C₂H₄via 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 CC 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 [Cu₄(4,5-H₂bptz)₄](ClO₄)₄·2Me₂CO (5) together with complex 1. Complex 5 is the first [4 × 4] grid-like tetranuclear Cu(I) complex constructed by the combination of Cu(I) ion and 4,5-H₂bptz. The four Cu(I) atoms are spanned by four 4,5-H₂bptz ligands, with two lying above the Cu₄ 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 {[Cu₂(2,5-H₂bptz)(C₂H₄)₂](ClO₄)₂}₂·Me₂CO (6) together with complexes 1 and 5. Complex 6 is composed of two independent [Cu₂(2,5-H₂bptz)(C₂H₄)₂]²⁺ cation moieties, four ClO₄⁻ anions, and a solvated Me₂CO molecule. It is interesting that the structures of the two [Cu₂(2,5-H₂bptz)(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.