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

Abstract

A series of new palladium dinuclear species with general formula [Pd2X(mu-X)[mu-P(t)Bu2(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[P(t)Bu2(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.

Electronic and Structural Effects of Low-Hapticity Coordination of Arene Rings to Transition Metals

“…34 The transition states (TS), on the other hand, appear clearly within the  1 region (Figure 7), close to several experimental structures of both independent and anchored group 10 metal complexes. 26,35 Finally, the model complexes in which a phenyl group is attached to -bonded (CH 2 ) n groups appear at different positions depending on the length of the bridging alkyl group, but all within the  2 region, surrounded by many experimental structures of anchored aryl groups, some of them very close to our calculated structures, either in mononuclear, 24,36,37 or as bridging groups in metal-metal bonded complexes. 37,38 It is to be noted that two conformations of minimum energy have been found for [(F 5 C 6 ) 2 (PH 3 )Pt(CH 2 Ph)], which correspond to two alternative  2 coordinations of the arene ring.…”

Biaryl Phosphane Ligands in Palladium‐Catalyzed Amination

“…[120] Another characteristic of the dialkylbiaryl phosphine ligands which is believed to promote catalyst stability and increase electron density at the metal center is the possibility of Pdarene interactions between the metal atom and the lower ring of the ligand. [121] Such interactions have been observed in a number of palladium-biaryl phosphine X-ray crystal structures [27][28][29][122][123][124][125] and even in an oxidative addition complex with methyl triflate. [126] Obtaining experimental information on the importance of these interactions in the reactive, catalytically active species has proven more difficult.…”

Catalyst Activation, Deactivation, and Degradation in Palladium‐Mediated Negishi Cross‐Coupling Reactions

“…As the Pd dimers completely vanished upon addition of the organozinc reagent, their formation apparently is reversible under catalytic conditions and does not easily lead to further aggregation; Pfaltz and co-workers came to a similar result. [43] Likewise, the successful use of Pd dimers as (pre)catalysts [48] implies that these species do not readily further aggregate. Therefore, the formation of Pd dimers apparently does not correspond to a major pathway of catalyst degradation.…”