Unprecedented chemical transformation of semicarbazones mediated by Wilkinson's catalyst.

Abstract

para-Nitrobenzaldehyde semicarbazone undergoes an unusual chemical transformation upon reaction with [Rh(PPh(3))(3)Cl] in the presence of trialkyl and dialkylamines (NR(2)R'; R = Et,(i)Pr, (n)Bu; R' = H or R' = R) via dissociation of the C-NH(2) bond and formation of a new C-NR(2) bond (where the NR(2) fragment is provided by the amine). The transformed semicarbazone ligand binds to rhodium as a dianionic C,N,O-donor to afford complexes of type [Rh(PPh(3))(2)(CNO-NR(2))Cl] (CNO-NR(2) = the coordinated semicarbazone ligand). Another group of semicarbazones (viz. salicylaldehyde semicarbazone, 2-hydroxyacetophenone semicarbazone, and 2-hydroxynaphthaldehyde semicarbazone) has also been observed to undergo a similar chemical transformation upon reaction with [Rh(PPh(3))(3)Cl] under similar experimental conditions as before, and these transformed semicarbazones bind to rhodium as dianionic O,N,O-donors affording complexes of the type [Rh(PPh(3))(2)(ONO(n)-NR(2))Cl] (ONO(n)-NR(2) = the coordinated semicarbazone ligand; n = 1-3). The structure of the [Rh(PPh(3))(2)(CNO-NEt(2))Cl] and [Rh(PPh(3))(2)(ONO(2)-NR(2))Cl] complexes has been determined. All the complexes show characteristic (1)H NMR signals. They also show intense absorptions in the visible and ultraviolet region. Cyclic voltammetry on the complexes shows an oxidative response within 0.52-0.97 V versus SCE and a reductive response within -1.00 to -1.27 V versus SCE, where both the responses are believed to be centered on the semicarbazone ligand.