Planochromism of cyanoxime anions: computational and mechanistic studies in solid state and solutions

The benzoylcyanoxime, NC-C(=N-OH)-C(O)-C₆H₅ (later H(BCO)), represents a new ampolydentate ligand that received attention in the light of useful biological properties of its coordination compounds. Colorless H(BCO), upon deprotonation, gains color that depends on the counter-cation and the system in general. Five derivatives of H(BCO), with colorless organic and inorganic mono-cations - Cs, Tl, Ag, N(CH₃)₄ and As(C₆H₅)₄ - were synthesized and characterized by the X-ray analysis, vibrational and electronic spectroscopy. Compounds exhibited unexpected and significant differences in color, both in solid state and in solutions, that were challenging to explain, thereby warranting detailed investigation including high-level DFT/TDFT computations. It was found that the BCO^- anion demonstrates negative solvatochromic in polar protic ROH (R = H, CH₃, C₂H₅, C₃H₇, C₄H₉) solvents, and appears yellow in color, as tetraalkylammonium or alkali metal salts. In polar aprotic solvents, such as CH₃CN, DMF, and DMSO, solutions of the BCO^- anion are red. The color originates from n→π* transition in the anion. Solid state structures evidenced a considerable dependence on planarity of the BCO^- anion on its color, as well as on the character of bonding in the C-N-O fragment: yellow color is associated with an oxime (bond length C-N is shorter than N-O), while red color is due to the nitroso character (bond length C-N is longer than the N-O). An addition of ethanol to red solutions of the BCO^-containing salts leads to the color change to yellow, which is the result of the formation of an H-bond between the C-N-O fragment of cyanoxime and the solvent, also leading to the flattening of the structure. An explanation for this new color-changing effect was offered, based on experimental evidence and TD/DFT calculations. Glossary: H(BCO) is 1-cyano-1-oximino-acetophenone, or benzoylcyanoxime; BCO^- is anion of the benzoylcyanoxime; AsPh₄ ⁺ is tetraphenylarsonium cation; MMe₄ ⁺ is tetramethyammonium cation; ROH - alcohols; DMF - dimethylformamide; DMSO - dimethylsulfoxide; AN - acetonitrile; DFT -Density Functional Theory; TDDFT - Time-Dependent Density Functional Theory.