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Understanding Pd–Pd Bond Length Variation in (PNP)Pd–Pd(PNP) Dimers

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posted on 04.03.2013, 00:00 by Justin R. Walensky, Claudia M. Fafard, Chengyun Guo, Christina M. Brammell, Bruce M. Foxman, Michael B. Hall, Oleg V. Ozerov
Analysis of the structures of three (PNP)­Pd–Pd­(PNP) dimers [where PNP stands for anionic diarylamido/bis­(phosphine) pincer ligands] has been carried out with the help of single-crystal X-ray diffractometry and density functional theory (DFT) calculations on isolated molecules. The three dimers under study possess analogous ancillary ligands; two of them differ only by an F versus Me substituent in a remote (five bonds away from Pd) position of the pincer ligand. Despite these close similarities, X-ray structural determinations revealed two distinct structural motifs: a highly symmetric molecule with a long Pd–Pd bond or a highly distorted molecule with Pd–Pd bonds ca. 0.14 Å shorter. DFT calculations on a series of (PNP)­Pd–Pd­(PNP) dimers (as molecules in the gas phase) confirmed the existence of these distinct minima for dimers carrying large isopropyl substituents on the P-donor atoms (as in the experimental structure). These minima are nearly isoergic conformers. Evidently, the electronically preferred symmetric structure for the dimer (with a square-planar environment about Pd and a linear N–Pd–Pd–N vector) is not sterically possible with the preferred Pd–Pd distance. Thus, the minima correspond to either a symmetric structure with a long Pd–Pd bond distance or a structure with a short Pd–Pd distance but with substantial distortions in the Pd coordination environment to alleviate steric conflict. This notion is supported by finding only a single minimum (symmetric and with short Pd–Pd bonds) for each of the dimers carrying smaller substituents (H or Me) on the P atoms, regardless of the remote substitution.