Synthesis, characterization and crystal structures of mono-, di- and trinuclear rhenium(I) tricarbonyl complexes with 2,3,5,6-tetra(2-pyridyl)pyrazine

A series of rhenium(I) tricarbonyl complexes with the ligand 2,3,5,6-tetra(2-pyridyl)pyrazine (tppz) were synthesized and characterized crystallographically. Two different coordination modes were found when tppz functions as a monobidentate ligand. Rhenium(I) may be bound to tppz through pyrazine and pyridyl nitrogens completing a 5-membered coordination ring when the reaction was carried out in toluene. However, the same reaction in methanol produced a yellow complex in which rhenium(I) was bound to tppz through two adjacent pyridyl nitrogens with a 7-membered coordination ring. In the presence of an excess amount of [ReBr(CO)5], the dinuclear complex [{ReBr(CO)3}2(μ-tppz)] (3) was isolated, while the trinuclear complex [{ReCl(CO)3}3(μ-tppz)] (4) was obtained in the case of [ReCl(CO)5]. Crystal data for 1, C27H16ClN6O3Re·MeOH: monoclinic, P21/n, a=9.2217(7), b=10.7628(8), c=26.932(2) Å, β=94.130(1)°, V=2666.1(3) Å3, Z=4. 2, C27H16BrN6O3Re: monoclinic, P21/n, a=9.2931(6), b=10.2387(7), c=27.993(2) Å, β=94.615(1)°, V=2654.8(3) Å3, Z=4. 1′, C27H16ClN6O3Re·2H2O: monoclinic, C2/c, a=18.584(4), b=21.693(5), c=15.392(4) Å, β=122.328(3)°, V=5243(2) Å3, Z=8. 3, C30H16Br2N6O6Re2: triclinic, a=7.4668(6), b=11.4902(10), c=19.3736(16) Å, α=73.659(2), β=85.183(2), γ=77.097(1)°, V=1554.3(2) Å3, Z=2. 4, C33H16Cl3N6O9Re3·1/2C6H6: orthorhombic, Pbca, a=18.828(1), b=16.715(1), c=25.366(2) Å, V=7983.2(10) Å3, Z=8.