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, Pt, and Rh.

This study investigates the coordination chemistry of the tetradentate pyridine-containing 12-membered macrocycles - towards Platinum Group metal ions Pd, Pt, and Rh. The reactions between the chloride salts of these metal ions and the three ligands in MeCN/HO or MeOH/HO (1:1 /) are shown, and the isolated solid compounds are characterized, where possible, by mass spectroscopy and H- and C-NMR spectroscopic measurements. Structural characterization of the 1:1 metal-to-ligand complexes [Pd()Cl][PdCl], Pt()Cl, Rh()Cl, and Rh()Cl·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/Pt, and Rh complexes, respectively. The remaining coordination site(s) are occupied by chlorido ligands. The reaction of with PtCl in MeCN/HO gave by serendipity the complex Pt()(m-1,3-MeCONH)PtCl(MeCN)·HO, 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 - Pt-Pt ground-state nature for this compound, rather than the alleged - Pt-Pt mixed-valence character reported for related dinuclear Pt-complexes.