Thermal dehydrogenation reacton of [N,N'-bis(3-ethoxysalicylidene)-1,2- diphenyl-1,2-ethanediaminato]nickel(II) with [VO(salen)]NO₃ in the solid state

A dehydrogenation reaction of the red-brown powder of [Ni(3-EtOsal-meso- stien)] (H₂(3-EtOsal-meso-stien) = N,N'-bis(3-ethoxysalicylidene)- (R,S)(S,R)-1,2-diphenyl-1,2-ethanediamine) did not occur by heating solely in the air up to 210 °C. However, the reaction occurred upon heating together with [VO(salen)] (H₂(salen) = N,N'-disalicylidene-1,2-ethane-diamine) in the air at 190 °C, and a red powder of [Ni(3-EtOsalton)] (H₂(3-EtOsalton) =N,N'-bis(3-ethoxysalicylidene)- 1,2-diphenyl-1,2-ethenediamine) was obtained. The dehydrogenation reaction could also be carried out under Ar at 160 °C by mixing with [VO(salen)]NO₃. These facts revealed that the oxidants in solid state dehydrogenation in air are the V(v) species originating from the v(IV)=O complex. The crystal structures of [Ni(3-EtOsal- meso-stien)]-CH₃CN-H₂O, (1); [Ni(3-EtOsal-rac-stien)] · 0.5H₂O, (2); and [Ni(3-EtOsalton)] · CH₃OH · 1.33H₂O, (3) were determined by X-ray analyses. [Ni(3-EtOsalton)] was prepared in a dioxane solution of [Ni(3- EtOsalstien)] using DDQ (2,3-dichloro-5,6-dicyano-1,4- benzoquinone) as an oxidant. The rac complex, [Ni(3-EtOsal-rac-stien)] (H₂(3-EtOsal-rac-stien) = N,N'-bis(3-ethoxy-salicylidene)-(R,R)(S,S)-1,2-diphenyl-1,2-ethanediamine), is less reactive in dehydrogenation than the meso in the solid state as well as in solution. The molecular structures in (1) and (2) indicate that the benzylic H atoms are sterically protected by phenyl groups in the rac complex, but are exposed to the oxidant in the meso complex.