Understanding Pd-Pd bond length variation in (PNP)Pd-Pd(PNP) dimers

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Computational Chemistry

Background:

• Palladium-palladium bonding in organometallic complexes is crucial for understanding reactivity.
• Pincer ligands, such as PNP, offer unique coordination environments for metal centers.
• Understanding structural variations in metal-metal bonded complexes is key to catalyst design.

Purpose of the Study:

• To analyze the structural diversity of (PNP)Pd-Pd(PNP) dimers.
• To investigate the influence of ligand substituents on palladium-palladium bond distances.
• To elucidate the interplay between electronic preferences and steric effects in determining molecular geometry.

Main Methods:

• Single-crystal X-ray diffractometry for experimental structure determination.
• Density functional theory (DFT) calculations for theoretical analysis of molecular structures and energetics.
• Comparison of structures with varying substituents on the pincer ligand.

Main Results:

• Two distinct structural motifs were observed for (PNP)Pd-Pd(PNP) dimers: one symmetric with a long Pd-Pd bond, and one distorted with a shorter Pd-Pd bond (approx. 0.14 Å difference).
• DFT calculations confirmed the existence of nearly isoenergetic conformers corresponding to these distinct structures for dimers with bulky isopropyl substituents.
• Dimers with smaller P-substituents (H or Me) exclusively adopted a single, symmetric structure with short Pd-Pd bonds, irrespective of remote ligand modifications.

Conclusions:

• Steric hindrance, particularly from bulky substituents on the phosphine donors, dictates the observed structural motifs in (PNP)Pd-Pd(PNP) dimers.
• The electronically preferred symmetric structure with a short Pd-Pd bond may not be sterically accessible, leading to distortions or longer Pd-Pd bonds.
• Ligand design plays a critical role in controlling the geometry and metal-metal bond characteristics in palladium complexes.