Flexible, linear, tetranuclear palladium complexes supported by tetraphosphine ligands with electron-withdrawing groups

Abstract

A linearly ordered tetraphosphine containing electron-withdrawing substituent groups on the outer phosphorus atoms, meso-bis[{di(3,5-difluorophenyl)phosphinomethyl}phenylphosphino]methane (dpmppmF₂), was prepared and reacted with [Pd₂(RNC)₆](PF₆)₂ and Pd(dba)₂ to afford tetranuclear palladium complexes, [Pd₄(μ-dpmppmF₂)₂(RNC)₃](PF₆)₂ (R = 2,6-xylyl (Xyl) (1), 2,4,6-mesityl (2), 2,6-diisopropylphenyl (3) and tert-butyl (4)), which involve an asymmetric {(RNC)Pd₄(CNR)₂}²⁺ core supported by two dpmppmF₂ ligands in anti-arrangement. Each terminal of the Pd₄ chain was capped by terminal isocyanide and a semi-bridging RNC is introduced into one terminal Pd site. Mechanistic investigation suggested that the dipalladium(I) complex, [Pd₂(μ-dpmppmF₂)₂(RNC)₂](PF₆)₂ (R = Xyl (6)), was a key intermediate to trap Pd⁰ species by the uncoordinated outer phosphine pendants with electron-withdrawing groups. Variable-temperature UV-vis and ³¹P{¹H}, ¹H NMR spectroscopic studies demonstrated that the tetrapalladium complexes are quite fluxional in the solution state at high temperature (>20 °C) relating to a symmetric structure of [Pd₄(μ-dpmppmF₂)₂(RNC)₂](PF₆)₂, and the asymmetric solid state structures are retained even in the solution at low temperature (<−60 °C). Theoretical calculations with DFT methods on the asymmetric (R = Xyl (1)) and symmetric (R = Xyl (1′)) structures suggested that contribution of Pd⁰→Pd^I–Pd⁰–Pd^I with 60 cluster valence electrons (CVEs) would be dominant in 1, while the symmetric structure of 1′ can be recognized as Pd^I–Pd⁰–Pd⁰–Pd^I with 58 CVEs. The new tetraphosphine dpmppmF₂ was proven very effective in organizing dynamically flexible tetrapalladium chains.