Impact of Constitutional Isomers of (BMes2)phenylpyridine on Structure, Stability, Phosphorescenc, and Lewis Acidity of Mononuclear and Dinuclear Pt(II) Complexes.

The impact of two constitutional isomers, 2-(4-BMes2-Ph)-pyridine (p-B-ppy, 1) and 5-Mes2-2-ph0pyridine (p-ppy-B, 2), as N,C-chelate ligands on the structures, stabilities, electronic na dphtophysical properties, and Lewis acidities of Pt(II) complexes has been investigated. SIx Pt(II) complexes, Pt(p-B-ppy)Ph(DMSO) (1a), Pt(p-B-ppy)Ph(py)Ph(py) (1b), [Pt(p-B-ppy)Ph]2(4,4'-bipy) (1c), Pt(p-ppy-B)Ph(DMSO) (2a), Pt(p-ppy-B)Ph(py) (2b), and [Pt(p-ppy-B)Ph]2(4,4'-bipy) (2c), have been synthesized and fully characterized. The structures of 1a, 1c, 2a, and 2c were established by sing;e-crystal X-ray diffraction analysis. All complexes adopt a cis geometry with the phenyk ligand being cis to the phenyl ring of the chelate. The dinuclear complexes 2a and 2c were found to exist in two isomeric forms in solution, syn and anti, with respect to the relative orientation of the two BMes2 groups in the molecule. While all complexes are stable in solution under ambient air, acompound 2a was found to react wtih H2O slowly in solution and form complex 2a-OH, where one of the mesityl groups on the boron center was replaced by an OH group. This instability of 2a is attributed to an internal dimethylsulfoxide-directed hydrolysis process via hydrogen bonds. The electron-accepting ability of the free ligands and the complexes were examined by cyclic voltammetry, establishing that, for p-ppy-B, Pt(II) chelation enhances the elctron-accepting ability while, for p-B-ppy, Pt(II) chelation has little impact. All Pt(II) complexes display oxygen-sensitive phosphorescence in solution at ambient temperature, dominated by B-ppy or ppy-B centered π→π* transitions. The Lewis acidity of the complexes was examined by fluoride titration experiments using UV—vis, phosphorescence, and NMR spectroscopic methods, establishing that the p-ppy-B complexes have similar and strong binding constants while the p-B-ppy complexes have a much lower affinity toward F—, compared to the tree ligands. In the dinuclear complexes, weak electronic communication between the two Pt(II) units is evident in 1c but absent in 2c, attributable to the different steric interactions in the two molecules. [ABSTRACT FROM AUTHOR]