Trinuclear Zinc(II) Complexes and Polymeric Cadmium(II) Complexes with the Ligand 2,5-Bis(2-pyridyl)pyrazine: Synthesis, Spectral Analysis, and Single-Crystal and Powder X-ray Analyses

Three zinc compounds, [ZnCl2(bppz)(dmf)] (1), [Zn3(OAc)6(bppz)2](H2O) (2), and [Zn3(Cl)6(bppz)3](H2O) (3),
and two cadmium complexes, {[Cd(OAc)2(bppz)](H2O)5}n (4) and [Cd(NO3)2(bppz)]n (5), where bppz is 2,5-bis(2-pyridyl)pyrazine, have been synthesized and characterized spectroscopically and crystallographically. The
mononuclear complex 1, C17H17Cl2N5OZn, crystallizes in the monoclinic space group P21/c, with a = 8.654(1)
Å, b = 9.500(1) Å, c = 22.997(1) Å, β = 97.99(1)°, and Z = 4; R1 for 2356 observed reflections [I > 2σ(I)] was
0.058. The zinc atom has a distorted square planar coordination sphere with the ligand bppz connected in a
mono-bidentate manner. The remaining coordination sites are occupied by the chloride counterions and by an
oxygen atom of a solvent molecule. The trinuclear zinc compound 2, C40H38N8O12Zn3·H2O, crystallizes in the
triclinic space group P1̄, with a = 12.238(4) Å, b = 12.986(3) Å, c = 15.470(4) Å, α = 75.65(1)°, β = 97.99(1)°,
γ = 65.98(1)°, and Z = 2; R1 for 4511 observed reflections [I > 2σ(I)] was 0.07. This complex consists of a
linear arrangement of three zinc atoms. The central zinc atom, located on a crystallographic inversion center, is
connected by six bridging acetate groups to two symmetry-related zinc atoms. It has an almost perfect octahedral
coordination environment. The outer symmetry-related zinc atoms are in a square pyramidal environment, and
they coordinate to three acetate groups and to one bppz molecule in a mono-bidentate manner. Compound 3,
C42H30Cl6N12Zn3·1.25H2O, a cyclic zinc(II) trimer, crystallizes in the cubic space group Ia3̄d, with a = 26.311(1)
Å and Z = 16; R1 for 692 observed reflections [I > 2σ(I)] was 0.038. This trinuclear complex has a perfect
triangular arrangement of the zinc atoms. Each zinc atom is connected to the other two by a bppz molecule. The
coordination about the metal is best described as a distorted octahedral with four long distances in the basal
plane, to two chlorines and to two nitrogen atoms, and two short distances in the axial direction, to two nitrogen
atoms. With CdII two polymeric complexes, 4 and 5 were obtained. Compound 4, C18H16N4O4Cd·5H2O, crystallizes
in the triclinic space group P1̄, a = 9.045(1) Å, b = 10.438(1) Å, c = 12.719(1) Å, α = 100.48(1)°, β = 95.05(1)°,
γ = 95.86(1)°, and Z = 2; R1 for 3694 observed reflections [I > 2σ(I)] was 0.029. The analogous Cd(NO3)2
complex with bppz, 5, could only be obtained in microcrystalline form, and its structure was solved by the use
of X-ray powder diffraction methods. Compound 5, C14H10N6O6Cd, crystallizes in the monoclinic space group
C2/c, with a = 11.6601(3) Å, b = 11.9870(3) Å, c = 12.1453(3) Å, β = 103.348(2)°, and Z = 4. In both 4 and
5 the cadmium atoms are bridged by the ligand bppz, so forming uniform one-dimensional coordination polymers.
The cadmium ions exhibit the rare coordination number of 8, with two coordinated ligand molecules and two
chelating acetate (4) or nitrate (5) groups.