Transmetalation reactions with nitrogen-containing "pincer"-class ligands on platinum(II) centers

The transmetalation reaction of the aryllithium compound [Li(NCN)](2) (NCN is the monoanionic "pincer" ligand [C6H3(CH2NMe2)(2)-2,6](-)) with the cyclometalated arylplatinum complex [PtCl(NCN)] afforded the bisaryl platinum(II) complex [Pt(NCN)(2)] (3) containing one eta (3)-N,C,N-terdentate and the other eta (1)-C-monodentate-bonded pincer ligand. Spectroscopic analyses on 3 suggest that eta (3) to eta (1) interconversion of the ligand binding mode (or vice versa) is inhibited. Two independent X-ray structure determinations on crystals of 3 revealed the existence of a rare polymorph containing one and three crystallographically independent molecules, respectively, in the unit cell. A similar transmetalation reaction with lithium and platinum complexes containing heteroleptic NCNRR' ligands (NCNRR' is [C6H3(CH2NRR')-2,6](-) with R = R' = Me or R = Me, R' = Et) pointed to the formation of a heterodinuclear cationic bisaryl platinum lithium species as an intermediate of a preequilibrium to the final transmetalation products, involving, rapid transcyclometalation (TCM) reactions. These TCM reactions comprise the exchange of the monoanionic NCNRR' ligands between the platinum(II) and lithium centers. A consequence of the latter properties is that the strong Pt-N bonds in [PtX(NCN)] complexes are considerably weakened by the presence of Li+ cations.