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Published February 15, 2010 | Supplemental Material
Journal Article Open

Electronic Structures of Pd^(II) Dimers


The Pd^(II) dimers [(2-phenylpyridine)Pd(μ-X)]_2 and [(2-p-tolylpyridine)Pd(μ-X)]_2 (X = OAc or TFA) do not exhibit the expected planar geometry (of approximate D_(2h) symmetry) but instead resemble an open "clamshell" in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2-arylpyridine ligands, with the Pd atoms brought quite close to one another (approximate distance 2.85 Å). The molecules adopt this unusual geometry in part because of a d^8−d^8 bonding interaction between the two Pd centers. The Pd−Pd dimers exhibit two successive one-electron oxidations: Pd^(II)−Pd^(II) to Pd^(II)−Pd^(III) to Pd^(III)−Pd^(III). Photophysical measurements reveal clear differences in the UV−visible and low-temperature fluorescence spectra between the clamshell dimers and related planar dimeric [(2-phenylpyridine)Pd(μ-Cl)]_2 and monomeric [(2-phenylpyridine)Pd(en)][Cl] (en = ethylenediamine) complexes that do not have any close Pd−Pd contacts. Density functional theory and atoms in molecules analyses confirm the presence of a Pd−Pd bonding interaction in [(2-phenylpyridine)Pd(μ-X)]_2 and show that the highest occupied molecular orbital is a d_(z2) σ* Pd−Pd antibonding orbital, while the lowest unoccupied molecular orbital and close-lying empty orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other Pd^(II)−Pd^(II) dimers that have short Pd−Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(μ-X)]_2, but quite different from that found for d^8−d^8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd and may explain why Pd d^8−d^8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.

Additional Information

© 2010 American Chemical Society. Received November 5, 2009. Publication Date (Web): January 21, 2010 We thank George Rossman and Elizabeth Miura Boyd for assistance with single-crystal Raman spectroscopy and Larry Henling and Michael Day for assistance with X-ray crystallography. This work was supported by BP through the MC2 program, the NSF Center for Chemical Innovation (Powering the Planet, CHE-0802907 and CHE- 0947829), and CCSER (Gordon and Betty Moore Foundation). The Bruker KAPPA APEXII X-ray diffractometer used in this work was purchased via an NSF CRIF:MU CHE-0639094 award to the California Institute of Technology.

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