Electronic Structures of Pd^II Dimers

Abstract

The PdII 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 d8−d8 bonding interaction between the two Pd centers. The Pd−Pd dimers exhibit two successive one-electron oxidations: PdII−PdII to PdII−PdIII to PdIII−PdIII. 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 z 2 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 PdII−PdII 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 d8−d8 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 d8−d8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.

Redox-Induced Aromatic C–H Bond Functionalization in Metal Complex Catalysis from the Electrochemical Point of View

“…Nitrogen-containing directing groups were found to promote the insertion of palladium into an ortho-C-H bond, which led to the formation of stable and isolable cyclopalladated complexes from Pd(II) starting material (Scheme 6) [25,[31][32][33][34][35][36][37][38][39][40]. Despite the large literature devoted to the synthesis of various palladacycles and studies of their reactivity, their redox properties, which are necessary to describe and understand the mechanisms of catalytic reactions involving them, are elucidated only in few works [11,15,24,25,[41][42][43][44]. However, the electrochemical properties of cyclometalated Pd(II) complexes (reduction/oxidation potentials) determine the mechanism of the reactions.…”

“…However, the electrochemical properties of cyclometalated Pd(II) complexes (reduction/oxidation potentials) determine the mechanism of the reactions. Despite the large literature devoted to the synthesis of various palladacycles and studies of their reactivity, their redox properties, which are necessary to describe and understand the mechanisms of catalytic reactions involving them, are elucidated only in few works [11,15,24,25,[41][42][43][44]. However, …”

Crystal structure of di(μ2-chlorido)bis[2-(2-pyridyl)phenyl-κ2 N,C 1]dipalladium(II), C22H16Cl2N2Pd2

“…unexpectedly obtained from the reaction of Na 2 PdCl4, 2,6-pyridinedicarboxylic acid and ppyH, and has a trans chloridobridged dimeric structure, whereas the cis analogue was prepared by the reaction of 2-phenylpyridine with Na 2 PdCl4 or Li 2 PdCl4, and its crystal structure was reported previously [3][4][5]. Both cis-and trans-complexes crystallize in the monoclinic space group P21/c, but the latter has an inversion center in the middle of the molecule, and therefore the asymmetric unit contains one half of the complex.…”

“Directed” Assembly of Metallacalix[n]arenes with Pyrimidine Nucleobase Ligands of Low Symmetry: Interchanging Metals in Mixed‐Metal Metallacalix[4]arenes and Incorporating Additional Metals at the Exocyclic Groups

“…These Pd-Pd distances are virtually identical to those found in the corresponding Pd 8 complex. [7] Of the known dinuclear Pd II complexes, with [19] or without bpy ligands, [20] these Pd-Pd distances are amongst the shortest. In particular, they are also significantly shorter than, for example, those in the closely related head-to-tail dimer…”