Self-Alignment of Low-Valent Octanuclear Palladium Atoms.

A linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) was used to synthesize linear octapalladium-extended metal atom chains as discrete molecules of [Pd₈(μ-dpmppm)₄](BF₄)₄ ( 1) and [Pd₈(μ-dpmppm)₄L₂](BF₄)₄ (L=2,6-xylyl isocyanide (XylNC; 2), acetonitrile ( 3), and N,N-dimethylformamide (dmf; 4)), which are stable in the solution states and show interesting temperature-dependent photochemical properties in the near IR region. Variable temperature NMR studies demonstrated that at higher temperature T≈140 °C the Pd₈ chains were dissociated into Pd₄ fragments, which were thermodynamically self-aligned to restore the Pd₈ chains at lower temperature T<60 °C. The coldspray ionization mass spectra suggested a possibility for further aggregation of the linear tetrapalladium units. [ABSTRACT FROM AUTHOR]