Concomitant dimorphism in poly[di-μ-azido-(5,5'-dimethyl-2,2'-bipyridine)iron(II)].

The title metal-organic polymer, catena-poly[[(5,5'-dimethyl-2,2'-bipyridine-κ 2 N,N')iron(II)]-di-μ-azido-κ 2 N 1 :N 3 -[(5,5'-dimethyl-2,2'-bipyridine-κ 2 N,N')iron(II)]-di-μ-azido-κ 2 N 1 :N 1 ], [Fe(N 3 ) 2 (C 12 H 12 N 2 )] n , features alternating μ-1,1 (end-on mode of coordination) and μ-1,3 (end-to-end mode of coordination) double azide bridges, forming chains running in the [100] direction. The octahedral coordination geometry around the Fe II centre is completed by a bidentate 5,5'-dimethyl-2,2'-bipyridine ligand. Two polymorphs for this compound were obtained from the crude reaction product, the first in the space group P-1 and the other in P2 1 /c. The molecular and crystal structures are very similar for both forms, the main difference being that the eight-membered Fe(μ-1,3-N 3 ) 2 Fe metallacycle formed with end-to-end azide ligands has a nearly flat conformation in the triclinic form and a chair conformation in the monoclinic form. In spite of this geometric difference, both forms have the same density, the same packing index and similar arrangements of the one-dimensional chains in the crystal. As a consequence, they also share very similar Hirshfeld surfaces and fingerprint plots. However, a density functional theory (DFT) computational study showed that the monoclinic form is more stable than the triclinic form by ca 30.5 kJ mol -1 .