On the isomerization of cyclooctyne into cycloocta-1,3-diene: synthesis, characterization and structure of a dinuclear platinum(ii) complex with a μ-η2:η2-1,3-COD ligand

Abstract

The Zeise's salt type cyclooctyne compound [K(18C6)][PtCl₃(COC)] (1; COC = cyclooctyne; 18C6 = 18-crown-6) was found to react in chloroform solution at room temperature within several weeks yielding the dinuclear cyclooctadiene compound [K(18C6)]₂[(PtCl₃)₂(μ-η²:η²-1,3-COD)] (2; 1,3-COD = cycloocta-1,3-diene) and non-coordinated cycloocta-1,3-diene. The identity of 2 was confirmed by microanalysis, NMR spectroscopy (¹H, ¹³C) and electrospray ionization mass spectrometry (ESI-MS). A single-crystal X-ray diffraction analysis of 2 exhibited a bridging μ-η²:η²-cycloocta-1,3-diene ligand with non-conjugated double bonds each coordinated to a PtCl₃ fragment. On the basis of DFT calculations as well as energy decomposition analyses (EDA), charge decomposition analyses (CDA) and natural bond orbital (NBO) analyses the peculiarities of the nature of the Pt–C bonds in the dinuclear complex anion [(PtCl₃)₂(μ-η²:η²-1,3-COD)]²⁻ (2a′) compared with those in mononuclear olefin complexes of Zeise's salt type [PtCl₃L]⁻ (L = η²-1,3-COD, 3a′; cis-but-2-ene, 4a′; COE, 5a′; COE = cyclooctene) are discussed. Furthermore, the driving force for the strongly exergonic reaction with formation of the cyclooctadiene complex 2a′ was found to be a significant release of ring strain of the cyclooctyne ligand in the starting compound 1.