title: Understanding Pd-Pd bond length variation in (PNP)Pd-Pd(PNP) dimers.
abstract: 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.
authors: Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77842, United States.
authors: Justin R
authors: JR
authors: Walensky
authors: Claudia M
authors: CM
authors: Fafard
authors: Chengyun
authors: C
authors: Guo
authors: Christina M
authors: CM
authors: Brammell
authors: Bruce M
authors: BM
authors: Foxman
authors: Michael B
authors: MB
authors: Hall
authors: Oleg V
authors: OV
authors: Ozerov
mesh_terms: D000577:Amides / Q000737:chemistry*; D018360:Crystallography, X-Ray; D019281:Dimerization; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*; D010720:Phosphines / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D000577:Amides; D008024:Ligands; D009942:Organometallic Compounds; D010720:Phosphines; D010165:Palladium