Chiral Self-Recognition between Stereogenic Tetrapalladium Units Affording Pd8 Chains Supported by Homochiral Tetraphosphines.

Abstract

By using a chiral tetraphosphine, rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane (rac-dpmppm), linear octapalladium chains were synthesized as discrete molecules of [Pd8 (μ-rac-dpmppm)4 L2 ](BF4 )4 (L=CH3 CN, dmf, XylNC), [Pd8 (μ-rac-dpmppm)4 ](BF4 )4 , and [Pd8 (μ-rac-dpmppm)4 (Cl)2 ](BF4 )2 , which are stable in the solution states and characterized by spectroscopic and crystallographic methods to reveal the octapalladium chains supported by homochiral four tetraphosphines. Variable-temperature NMR studies for a 1:1 mixture of [Pd8 (μ-rac-dpmppm)4 (dmf)2 ](BF4 )4 and [Pd8 (μ-meso-dpmppm)4 (dmf)2 ]-(BF4 )4 in [D7 ]DMF revealed that the Pd8 chains were dissociated at higher temperature (T≈140 °C) into the Pd4 units of {Pd4 (μ-rac-dpmppm)2 }2+ and {Pd4 (μ-meso-dpmppm)2 }2+ , and they were thermodynamically self-aligned to restore the Pd8 chains at lower temperature (T<60 °C), through perfect chiral self-recognition between the stereogenic tetrapalladium units.