Heterometallic Complexes by Transmetalation of Alkynyl Groups from Copper or Silver to Allyl Palladium Complexes: Demetalation Studies and Alkynyl Homocoupling

The reaction of [Pd­(η3-allyl)­ClL] (L = AsPh3, PPh3) with [M­(CCR)]n (M = Cu, Ag; R = nBu, Ph) leads to transmetalation of the alkynyl group from M to Pd. However, the group 11 metal stays η2-bound to the new Pd–alkynyl fragment and heterometallic Pd–M complexes are formed with different nuclearities depending on M: [{Pd­(η3-allyl)­(alkynyl)­L}­CuCl]2 (3, 4) or [{Pd­(η3-allyl)­(alkynyl)­L}2AgCl] (5, 6). The M-containing fragment can be eliminated to give the actual transmetalation complex [Pd­(η3-allyl)­(alkynyl)­L] by adding an excess of arsine or phosphine, whereas amines do not have this effect. Allyl–alkynyl reductive elimination is a slow process; therefore, complexes 3–6 cleanly decompose by dimerization (homocoupling) of the alkynyl group. In the decomposition process reversible alkynyl transmetalation from Pd to Cu has been observed.