Chiral Octapalladium Chains Supported by Enantiopure P-Stereogenic Linear Tetraphosphines, (R,R)- and (S,S)-Ph₂PCH₂P(Ph)CH₂P(Ph)CH₂PPh₂

Abstract

Binuclear Pd(II) and Pt(II) complexes supported by rac-dpmppm (bis[(diphenylphosphinomethyl)phenylphosphino]methane) in a triply-bridged Z-form, [M₂Cl₄(rac-dpmppm)] (M = Pd (3a), Pt (3b)), readily reacted with 2,6-xylyl isocyanide (XylNC) in the presence of NH₄PF₆ to afford [M₂Cl₂(rac-dpmppm)(XylNC)₂](PF₆)₂ (M = Pd (4a), Pt (4b)), in which each metal center accommodates one isocyanide ligand at the trans position to the inner P atom of dpmppm. Similarly, treatment of 3a and 3b with axially chiral (R/S)-1,1'-binaphthyl-2,2'-bisisocyanide (rac-Binac) in the presence of NH₄OTf gave cyclic tetranuclear complexes, [{M₂Cl₂(rac-dpmppm)(rac-Binac)}₂](OTf)₄ (M = Pd (5), Pt (8)), where two {M₂Cl₂(rac-dpmppm)}²⁺ fragments are connected by two rac-Binac ligands through chirality sorting of (R*,R*)-dpmppm and (R*)-Binac. Complex 5 could be transformed into the halide exchanged tetranuclear complexes, [{Pd₂X₂(rac-dpmppm)(rac-Binac)}₂](OTf)₄ (X = Br (6), I (7)), to show that the rectangular arrangement of four Pd(II) ions is elongated by repulsive interaction between halide ligands. By using (R)- and (S)-Binac, enantiopure Pd₄ complexes, [{Pd₂Cl₂((R*,R*)-dpmppm)((R*)-Binac)}₂](OTf)₄ (5_RR/R and 5_SS/S), were successfully isolated as pure crystalline forms, from which enantiopure (R,R)- and (S,S)-dpmppm were obtained by treatment with NaCN aqueous solution. Namely, optical resolution of rac-dpmppm was established through the tetranuclear Pd complexes, which is the first example for methylene-bridged polyphosphines, R₂P(CH₂PR)_nCH₂PR₂ (n > 0). Furthermore, chiral octapalladium chains, [Pd₈((R*,R*)-dpmppm)₄(N≡CCH₃)₂](BF₄)₄ (2_RR and 2_SS), were synthesized by reacting enantiopure P-chiral dpmppm with [Pd₂(CH₃CN)₆](BF₄)₂ and [Pd₂(dba)₃]·C₆H₆ and were characterized by spectroscopic and X-ray crystallographic analyses, to determine the absolute configurational structures. The Pd₈ chains are the longest enantiopure chiral single-metal-atom chains structurally characterized, thus far, and the electronic structures were examined on the basis of DFT calculations of 2_RR.