Interconversion between Polymeric Orange and Monomeric Green Forms of a Schiff Base-Oxovanadium(IV) Complex

The orange and green forms of the Schiff base-oxovanadium(IV) complex, [VO{sal-(RR)-stien}]·MeOH (1, green), [VO{sal-(RR)-stien}]·2CHC1₃ (2, green), and [VO{sal-(RR)-stien}]·CH₃CN (3, orange) (H₂{sal-(RR)-stien} = N,N′-disalicylidene-(RR)-1,2-diphenyl-1,2-ethanediamine), have been prepared, and their crystal structures were determined by an X-ray method. The green forms, 1 and 2, contain mononuclear square pyramidal molecules of the complex, while the orange form 3 consists of individual complexes stacked to give an infinite linear ⋯V=O⋯V=O⋯ chain. 1, which crystallizes as monoclinic in space group P2₁ with Z=4, has the cell parameters a=16.684(2), b=16.923(4), c=9.249(3) Å, β = 105.76(2)°, and V=2513(1) ų. 2, which crystallizes as triclinic in space group P1 with Z=2, has the cell parameters a = 13.187(3), b = 13.469(3), c = 10.450(3) Å, α = 103.72(2), β = 103.20(3), γ = 63.03(2)°, and V = 1591.2(7) ų. 3 crystallizes as triclinic in space group P1 with Z=2. The cell parameters are a = 13.300(3), b = 13.667(4), c = 7.551(1) Å, α = 90.44(2), β = 94.51(2), γ = 97.36(2)°, and V = 1357(1) ų. The final R indices are R = 0.059 and R_W = 0.062 for 1, R = 0.098 and R_W = 0.090 for 2, and R = 0.053 and R_w = 0.042 for 3. The structural characteristics of these complexes are compared and discussed. 3 shows piezochromism (mechanochromism); its color changes from orange to green upon grinding. 1 and 2 turn orange when heated at 120 °C for a few minutes. The extent of conversion either by grinding or heating was studied by EXAFS (X-ray absorption fine structure) and IR spectroscopy. The orange crystals turn green when exposed to chloroform vapor, and the green crystals turn orange upon exposure to acetonitrile vapor.