Unprecedented chemical transformation of semicarbazones mediated by wilkinson's catalyst

Abstract

para-Nitrobenzaldehyde semicarbazone undergoes an unusual chemical transformation upon reaction with [Rh(PPh₃)₃Cl] in the presence of trialkyl and dialkylamines (NR₂R‘; R = Et, ⁱPr, ⁿBu; R‘ = H or R‘ = R) via dissociation of the C-NH₂ bond and formation of a new C-NR₂ bond (where the NR₂ 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(PPh₃)₂(CNO-NR₂)Cl] (CNO-NR₂ = 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(PPh3)3Cl] 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(PPh₃)₂(ONOⁿ-NR₂)Cl] (ONOⁿ-NR₂ = the coordinated semicarbazone ligand; n = 1-3). The structure of the [Rh(PPh₃)₂(CNO-NEt₂)Cl] and [Rh(PPh₃)₂(ONO²-NR₂)Cl] complexes has been determined. All the complexes show characteristic ¹H 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 ν 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.