Structural Diversity and Giant Birefringence in Cyanates BaCNOX (X = Cl, Br, I, and CNO) Containing Linear π‑Conjugated Units: A Combined Experimental and Theoretical Study

Four novel cyanates BaCNOX (X = Cl, Br, I, and CNO) containing linear π-conjugated anions were obtained by solid-state metathesis reactions for the first time. BaCNOCl crystallizes in a monoclinic space group P2₁/m with a = 5.5757(15) Å, b = 4.4925(10) Å, c = 7.850(2) Å, β = 102.38(3)°, Z = 2, whereas the remaining three compounds belong to the same orthorhombic space group Pnma with a = 8.227–8.713 Å, b = 4.290–4.552 Å, c = 10.986–11.803 Å, Z = 4. The crystal structures of these four compounds are divergent, exhibiting rich structural diversity ranging from simple to complex when X changes from Cl to Br and I, and then to CNO. Their thermal, vibrational, and optical properties were characterized in detail. Besides, the first-principles calculations reveal that all four compounds show giant optical anisotropy of Δn > 0.20 at 800 nm owing to the superior arrangement of linear (CNO)⁻ π-conjugated groups. Successful synthesis of these cyanates not only further explains the influence of microscopic ions on the structures, but also enriches the structure chemistry of cyanates and provides enlightenment for the study of birefringence in CNO-based compounds.