Controlled interaction of benzaldehyde thiosemicarbazones with palladium: formation of bis-complexes with cis-geometry and organopalladium complexes, and their catalytic application in C–C and C–N coupling

Abstract

Reaction of a 1 : 1 mixture of 4-R-benzaldehyde thiosemicarbazone (R = OCH₃, CH₃, H, Cl and NO₂) and 2-picolinic acid with an equivalent quantity of Na₂[PdCl₄] in ethanolic medium, under relatively mild reflux, affords a group of complexes of type [Pd(NS–R)₂] as the major product and another family of complexes of type [{Pd(CNS–R)}_n] as the minor product (where NS–R and CNS–R denote the N,S- and C,N,S-coordinated thiosemicarbazone, respectively). The [{Pd(CNS–R)}_n] complexes are conveniently obtained in good yields from similar reactions carried out under prolonged reflux. The sulfur-bridge in the [{Pd(CNS–R)}_n] complexes undergoes facile cleavage upon their reaction with triphenylphosphine to yield complexes of type [Pd(CNS–R)(PPh₃)]. Crystal structures of [Pd(NS–OCH₃)₂], [Pd(NS–CH₃)₂], [Pd(NS–Cl)₂], [Pd(CNS–CH₃)(PPh₃)], [Pd(CNS–H)(PPh₃)] and [Pd(CNS–Cl)(PPh₃)] have been determined. In the [Pd(NS–R)₂] complexes the thiosemicarbazones are coordinated to the metal center, via dissociation of the acidic proton, as monoanionic bidentate N,S-donors forming five-membered chelate rings, while in the [Pd(CNS–R)(PPh₃)] complexes they are coordinated to palladium, via dissociation of the acidic proton and activation of an ortho C–H bond, as dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. The [Pd(NS–R)₂] and [Pd(CNS–R)(PPh₃)] complexes show characteristic ¹H NMR signals, and in dichloromethane solution they all display intense absorptions in the visible and ultraviolet regions. The [Pd(NS–R)₂] and [Pd(CNS–R)(PPh₃)] complexes have shown notable efficiency in catalyzing C–C and C–N coupling reactions.