Can Serendipity Still Hold Any Surprises in the Coordination Chemistry of Mixed-Donor Macrocyclic Ligands? The Case Study of Pyridine-Containing 12-Membered Macrocycles and Platinum Group Metal Ions Pd^II, Pt^II, and Rh^III

• Main Authors: Alessandra Garau, Giacomo Picci, Massimiliano Arca, Alexander J. Blake, Claudia Caltagirone, Greta De Filippo, Francesco Demartin, Francesco Isaia, Vito Lippolis, Anna Pintus, M. Andrea Scorciapino, M. Carla Aragoni
• Format: Article
• Language: English
• Published: MDPI AG 2021-02-01
• Series: Molecules
• Subjects: macrocyclic ligands; palladium; platinum; Rhodium; DFT-Calculations
• Online Access: https://www.mdpi.com/1420-3049/26/5/1286
• ISSN: 1420-3049

This study investigates the coordination chemistry of the tetradentate pyridine-containing 12-membered macrocycles <b>L¹</b>-<b>L³</b> towards Platinum Group metal ions Pd^II, Pt^II, and Rh^III. The reactions between the chloride salts of these metal ions and the three ligands in MeCN/H₂O or MeOH/H₂O (1:1 <i>v</i>/<i>v</i>) are shown, and the isolated solid compounds are characterized, where possible, by mass spectroscopy and ¹H- and ¹³C-NMR spectroscopic measurements. Structural characterization of the 1:1 metal-to-ligand complexes [Pd(<b>L¹</b>)Cl]₂[Pd₂Cl₆], [Pt(<b>L¹</b>)Cl](BF₄), [Rh(<b>L¹</b>)Cl₂](PF₆), and [Rh(<b>L³</b>)Cl₂](BF₄)·MeCN shows the coordinated macrocyclic ligands adopting a folded conformation, and occupying four coordination sites of a distorted square-based pyramidal and octahedral coordination environment for the Pd^II/Pt^II, and Rh^III complexes, respectively. The remaining coordination site(s) are occupied by chlorido ligands. The reaction of <b>L₃</b> with PtCl₂ in MeCN/H₂O gave by serendipity the complex [Pt(<b>L³</b>)(m-1,3-MeCONH)PtCl(MeCN)](BF₄)₂·H₂O, in which two metal centers are bridged by an amidate ligand at a Pt1-Pt2 distance of 2.5798(3) Å and feature one square-planar and one octahedral coordination environment. Density Functional Theory (DFT) calculations, which utilize the broken symmetry approach (DFT-BS), indicate a singlet <i>d</i>⁸-<i>d</i>⁸ Pt^II-Pt^II ground-state nature for this compound, rather than the alleged <i>d⁹</i>-<i>d⁷</i> Pt^I-Pt^IIImixed-valence character reported for related dinuclear Pt-complexes.