Synthesis, Crystal Structures, and Magnetism of Cobalt Coordination Polymers Based on Dicyanamide and Pyrazine-dioxide Derivatives

Three coordination polymers of Co(II) with dicyanamide (dca) were obtained by adding coligands of 2,5-dimethylpyrazine-dioxide (2,5-dmpdo), 2,3,5-trimethylpyrazine-dioxide (2,3,5-tmpdo), or 2,3,5,6-tetramethylpyrazine-dioxide (2,3,5,6-tmpdo) to the binary system of Co-dca. Co2(dca)4(2,5-dmpdo)2 (1) crystallizes in the triclinic space
group P1̄ with a = 7.4962(2), b = 9.0364(2), c = 10.4783(4) Å, α = 72.567(1), β = 72.557(1), γ = 68.814(2)°,
V = 616.61(3) Å3, Z = 1, and R1 = 0.0345. [Co3(dca)6(H2O)4]·2(2,3,5-tmpdo) (2) is in the monoclinic space group
C2/c with a = 29.477(1), b = 7.3735(2), c = 17.4631(7) Å, β = 93.652(1)°, V = 3787.9(2) Å3, Z = 4, and R1
= 0.0532. [Co(dca)2(H2O)2]·2(2,3,5,6-tmpdo) (3) is in the monoclinic space group P21/c with a = 9.4739(3), b =
11.3876(3), c = 12.1778(3) Å, β = 98.967(1)°, V = 1297.74(6) Å3, Z = 4, and R1 = 0.0481. 1 contains an
unusual (4, 4) Co-dca layer, representing a rare example of metal-dca coordination polymers with mixing 1,3-μ2-
and 1,5-μ2-dca bridges. The Co-dca (4, 4) layers are connected by 2,5-dmpdo to give an α-Po-type network,
which displays antiferromagnetic ordering below 10.8 K. 2 is a unique 3D framework composed of 2D twinned
1,5-μ2-dca bridged layers, which are connected by Co−H2O−Co linkages. The noncoordinated 2,3,5-tmpdo units
and the coordination water molecules form hydrogen-bonded chains that thread the framework. 3 has alternating
stacks of usual Co-dca (4, 4) layers through 1,5-μ2-dca bridges and organic layers of noncoordinated 2,3,5,6-tmpdo. Weak antiferromagnetic and weak ferromagnetic coupling was observed in 2 and 3, respectively.