Synthesis, Characterization, and Theoretical Investigation of Two-Coordinate Palladium(0) and Platinum(0) Complexes Utilizing [pi]-Accepting Carbenes

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π-accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C-Pd-C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange-colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)2Pd/Pt] complexes can be quasi-reversibly oxidized to their corresponding [(cAAC)2Pd/Pt]+ cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations.