Coordination properties towards palladium(II) of a tridentate dianionic ligand acting as a N- or a N,O-donor

The new palladium(II) complex Pd[C5H3N-2,6-(CONPh)2](η1-NCMe) (1), prepared from N,N′-diphenyl-2,6-pyridinedicarboxamide and Pd(OAc)2 in acetonitrile, has been characterized via IR, 1H NMR and single-crystal X-ray diffraction. In this compound the palladium centre is coordinated to three nitrogen donors of the anionic ligand and to the nitrogen atom of acetonitrile.Moreover, the already known Pd[C5H3N-2,6-(CONCH2CH2Ph)2](η1-NCMe) (2) has been studied by 1H NMR spectrometry and found to readily convert into the macrocyclic tetranuclear species 3, {Pd[C5H3N-2,6-(CONCH2CH2Ph)2]}4 which has been isolated and characterized by IR, 1H and 13C{1H} NMR, 1H–13C HETCOR and mass spectrometry, as well as by single-crystal X-ray diffraction. In 3, of S4 symmetry, each palladium atom is coordinated to the three nitrogen atoms of the anionic ligand, while the fourth coordination position is occupied by the amidato oxygen atom of an adjacent unit. This structure is apparently maintained in CDCl3 solution. The substitution reactions of acetonitrile in 2 with the ligands EEt2 (E = S, Se) afford Pd[C5H3N-2,6-(CONCH2CH2Ph)2](EEt2) (4, E = S; 5, E = Se); these products can also be obtained by the addition of EEt2 to 3, as shown by means of 1H- and, in the case of E = Se, 77Se{1H} NMR spectroscopy in CDCl3 solution. These results show that the Pd–O bonds of the tetranuclear species are readily broken by weakly coordinating ligands such as acetonitrile and diethylchalcogenides. Nevertheless, we are dealing with equilibrium reactions and, in some solvents, 3 can be obtained from 2, 4 or 5 being favoured by its low solubility.

Graphical abstractPd(bphepa)(NCMe), used as precursor to study the relative stability of the derivatives Pd(bphepa)(ER2) (E = S, Se), releases acetonitrile in solution, being in equilibrium with [Pd(bphepa)]4, scarsely soluble in hydrocarbons. The formation of Pd(bphepa)(EEt2) with EEt2 (E = S, Se) involve equilibria: the palladium centre preferring in the order SeEt2, SEt2, NCCH3 and the oxygen atom of another Pd(bphepa) fragment.Figure optionsDownload full-size imageDownload as PowerPoint slide