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

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.