Synthesis and Spectroscopic and Spectroelectrochemical Characterization of a New Family of 44e– Tris-Phosphido-Bridged Palladium Triangles

Abstract

Triangular clusters containing a [M3(μ-PR2)3](+) core are very common in platinum chemistry but were virtually unknown for M = Pd. Herein we describe the synthesis and characterization of several palladium derivatives belonging to this class. The trinuclear monohalide clusters {Pd3}(CO)2X [{Pd3} = Pd3(μ-PBu(t)2)3; X = Br, I] were prepared by reacting [(n)Bu4N]X or KX with the dinuclear complex [Pd(PBu(t)2H)(μ-PBu(t)2)]2 and H2O under an atmosphere of CO. The reaction of {Pd3}(CO)2I with CNBu(t) leads to the substitution of all the terminal ligands to afford the symmetrical cluster [{Pd3}(CNBu(t))3]I. The latter reacts with TlPF6 (excess) or AgCF3SO3 (1:1 ratio) to give anion metathesis, whereas the addition of a second equivalent of Ag(+) causes cluster oxidation to the thermally stable paramagnetic 43e(-) dication [{Pd3}(CNBu(t))3](2+). The cationic clusters [{Pd3}(CO)2(L)]PF6 (L = NCCH3, Py or CO) were obtained by reacting {Pd3}(CO)2I with TlPF6, under nitrogen in acetonitrile or in pyridine or under 1 atm of carbon monoxide in THF. Finally, {Pd3}(CO)2Cl was achieved by the reaction of [{Pd3}(CO)3]PF6 with [(PPh3)2N]Cl. All clusters have been obtained in good yields and purity and have been characterized by microanalysis and IR and multinuclear NMR spectroscopy. Single crystal X-ray diffraction studies on {Pd3}(CO)2Br and [{Pd3}(CNBu(t))3]CF3SO3 are also reported. The cyclovoltammetric profile exhibited by the palladium clusters prepared in this work is characterized by the presence of two monoelectronic oxidation processes whose reversibility and potentials depend on the nature of the ligands. Moreover, the UV-vis and IR spectroelectrochemical analysis of {Pd3}(CO)2I, [{Pd3}(CO)3]PF6 and [{Pd3}(CNBu(t))3]PF6 allowed the spectroscopical characterization of some electrogenerated oxidized species and provided some detail for the redox-coupled reactions of metastable products.