Alloyed Tetranuclear Metal Chains of Pd4-n Ptn (n=0-3) Scaffolded by a New Linear Tetraphosphine Containing a PNP Bridge.

Abstract

A new linear tetraphosphine containing a PNP phosphazane bridge, rac-bis[(diphenylphosphinomethyl)phenylphosphino]phenylamine (rac-dpmppan), was synthesized and utilized to support a series of Pd/Pt mixed metal tetranuclear chains, [Pd4-n Ptn (μ-rac-dpmppan)2 (XylNC)2 ](PF6 )2 (XylNC=xylyl isocyanide; n=0: Pd4 (1), 1: PtPd3 (2), 2: PtPd2 Pt (3), 2: Pt2 Pd2 (4), 3: Pt2 PdPt (5)), in which the number and positions of additional Pt atoms were successfully controlled depending on the respective synthetic procedures using transformations from 1 to 3 through 2 and from 4 to 5 by redox-coupled exchange reactions. The 31 P{1 H} NMR and ESI mass spectra and X-ray diffraction analyses revealed almost identical tetranuclear structures, with slight contraction of metal-metal bonds according to incorporation of Pt atoms. The electronic absorption spectra of 1-5 exhibited characteristic bands at 635-510 nm with an energy propensity depending on the number and positions of Pt centres, which were assigned to HOMO (dσ*σσ*) to LUMO (dσ*σ*σ*) transition by theoretical calculations. The present results demonstrated that the electronic structures of Pd/Pt mixed-metal tetranuclear complexes are finely tuned as orbital-overlapping alloyed metal chains by atomically precise Pt incorporation in the Pd4 chain.