Unprecedented Chemical Transformation of Semicarbazones Mediated by Wilkinson's Catalyst

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 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.