Experimental and Theoretical Investigations of New Dinuclear Palladium Complexes as Precatalysts for the Amination of Aryl Chlorides

A series of new palladium dinuclear species with general formula [Pd2X(μ-X){μ- PtBu2(Bph-R)}]
(X = Cl, Br; Bph = biphenyl; R = H, Me, NMe2) have been prepared. The two palladium centers in these
species are bridged by one of the aromatic rings of the biphenyl group present in the corresponding
phosphine. The X-ray crystal structure of one of these complexes has been obtained, providing a clear
picture of the bonding pattern. The stability of these dimers in solution is shown to be highly dependent on
the nature of the phosphine R group and also on the bridging halide. When R = NMe2, the dimers dissociate,
yielding the palladium(II) compounds PdX2{PtBu2(BPh-NMe2)} (X = Cl, Br), and the X-ray crystal structure
of one of them (X = Br) has shown that the biphenyl group from the phosphine interacts directly with the
metal center. This interaction seems to play an important role in stabilizing the otherwise coordinatively
unsaturated palladium(II) complex. In contrast, when R = H or Me, the analogous monomeric palladium(II)
complexes are unstable and undergo cyclometalation to generate a palladium(II) dinuclear species in which
each of the two phosphines cyclometalates with the palladium centers forming a strained four-membered
ring. In addition to their unusual structures, these aryl-bridged dimers have also proven to be excellent
precatalysts for the amination of aryl chlorides. To rationalize some of the experimental results, a detailed
DFT computational study has been carried out and is presented herein.