Searching for Precursors to Metal−Metal Bonded Dipalladium Species:  A Study of Pd₂⁴⁺ Complexes

Reactions of Pd₃(OAc)₆ with lithium salts of mononegative bidentate N,N-ligands, L, of various types, such as formamidinates, benzamidinates, triazinates, and guanidinates, were investigated in a search for ways to obtain Pd₂⁴⁺ compounds that could serve as precursors to paddlewheel complexes with a metal−metal bond and a Pd₂⁵⁺ core. It was found that the reactions are complex and that either square planar mononuclear or dinuclear species may be formed depending on the reaction conditions or the method of isolation. For Pd₂L₄ compounds, α and β isomers were sometimes obtainable. In the α isomer, all N,N-ligands serve as bridges, whereas in the β isomer, two ligands bridge the Pd₂⁴⁺ unit and each of the other two chelate to a metal atom. Electrochemistry shows that the paddlewheel compounds Pd₂(TolNC(H)NTol)₄, Pd₂(PhNC(Ph)NPh)₄, and Pd₂(PhNC(Ph)NPh)₃(OAc) and the orthometalated complex cis-Pd₂[η²-C₆H₄NC(Ph)N(H)Ph]₂(μ-OAc)₂ have reversible oxidation waves between 0.70 and 0.92 V vs Ag/AgCl, which makes them good candidates for chemical oxidation.