Extended Polymorphism in Copper(II) Imidazolate Polymers: A Spectroscopic and XRPD Structural Study

Copper(II) bisimidazolate affords five different polymorphs; of these, one was structurally characterized 40 years
ago by standard single-crystal X-ray diffraction (Jarvis, J. A. J.; Wells, A. F. Acta Crystallogr. 1960, 13, 1027),
while the remaining four, selectively prepared as pure polycrystalline phases, have been now studied by X-ray
powder diffraction (XRPD) methods. Of the four new (blue, green, olive-green, and pink) phases, three were
solved by the ab initio XRPD technique and refined by the Rietveld method, and the fourth phase (pink) could
not be structurally characterized. Crystal data for [Cu(imidazolate)2]n:  blue phase, a = 27.559(3) Å, c = 5.3870(9) Å, trigonal, R3̄, Z = 54; green phase, a = 21.139(1) Å, b = 19.080(1) Å, c = 9.2842(8) Å, orthorhombic,
Ccca, Z = 20; olive-green phase, a = 11.7556(8) Å, b = 23.422(2) Å, c = 9.0727(9) Å, β = 104.993(5)°,
monoclinic, C2/c, Z = 12. All polymorphs contain four-coordinate CuN4 chromophores and (N,N‘)-exobidentate
imidazolate ligands, but show different spectroscopic and structural properties, the latter ranging from 2D to
different 3D networks of the PtS, sodalite, and moganite archetypes. The intermediacy of the [Cu(imidazole)2CO3]·H2O species in the synthesis of the blue polymorph has been confirmed by spectroscopic and thermal analyses.
FTIR, Raman, and electronic spectra were correlated with the structural features revealed in the present work,
and used to gain insight into the coordination geometry of copper(II) ions of the pink polymorph. In addition, the
correct Raman spectrum for copper(II) bisimidazolate, common for all polymorphs, has been definitely determined.