Doubly Cyclometalated Pyridazines:  Contrasting Behavior with Palladium and Platinum

The cyclometalation of 3,6-diphenylpyridazines with palladium and platinum has been studied. When palladium acetate is used as the metal source, two sequential cyclometalations are observed, with the first cyclometalation deactivating the system toward a second cyclometalation. Both monocyclopalladated and dicyclopalladated species have been isolated and characterized as their acetylacetonate derivatives. When potassium tetrachloroplatinate is used as the metal source, only double cycloplatination is observed:  the first cycloplatination activates the system toward a second cycloplatination through a combination of nitrogen coordination and a chloride bridging two platinums. A doubly cycloplatinated compound has been isolated as its triphenylphosphine derivative, which has been fully characterized:  the single-crystal X-ray structure shows that the two platinums are bridged by a chloride.