Phosphine–alkene ligand-mediated alkyl–alkyl and alkyl–halide elimination processes from palladium(ii)

Abstract

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Combined DFT and experimental studies of C–C and C–X elimination reactions promoted by a chelating phosphine–alkene ligand: the key role of penta-coordinate PdII

“…Natural Bond Orbital (NBO) analysis of complex 1 highlights a strong stabilizing interaction (E 2 ) between the π C=C orbital of the alkene and the Pd centre of 53.5 kcal mol -1 (ligandmetal donation) and a weaker Pd/π* C=C orbital interaction of 24 kcal mol -1 (metalligand back-donation). In contrast to the expectation that the olefinic component of L1 would be subject to significant Pdπ* C=C back-donation in order to relieve ring strain in the aza-norbornene skeleton, 22 25 As is determined experimentally by X-ray diffraction, the two Pd-Me bond distances of complex 1 are computed to be essentially equivalent 2.097 and 2.098 Å (cf. X-ray: 2.068(3) and 2.074(3) Å), despite being located trans to the phosphine and trans to the olefin moieties, respectively.…”

“…22 At room temperature (r.t.) a toluene solution of complex 1 slowly undergoes spontaneous reductive elimination to afford the very stable palladium(0) bis(phosphine-alkene) complex 8 [Pd(κ 2 -P,C-L1) 2 ] (2) and ethane, together with elemental palladium. Complete consumption of complex 1 is achieved after 120 h at room temperature.…”

“…22 To this end, two reaction pathways involving penta-coordinate species 7 have been examined : a ligand-assisted pathway involving a . 46 In our case, the similarity between the TS for the ligand-assisted elimination reaction pathway with that involved in a classical S N 2-type process is further reinforced by the computed Pd···Me···P(L1) bond angle of 165.0° and by the identification of a planar pseudo-pentacoordinate carbon, which is located approximately equidistant from both the Pd and P atoms (Pd-C and C-P distances of 2.49 and 2.35 Å, respectively).…”

“…22 The ligand-assisted pathway involves a pseudo-reductive elimination through an S N 2-like transition state in which the palladium-bound methyl group is slightly activated towards nucleophilic attack by the incoming phosphine L1, with a slight stabilisation of the acceptor  * Pd-C orbital being computed on going from 6 (-1.8 eV ) to 7 (-1.9 eV). Furthermore, this mechanism is favoured as a result of the stability of the leaving group, namely the bis(phosphine-alkene)Pd 0 complex 2.…”

Five Coordinate Platinum(II) in [Pt(bpy)(cod)(Me)][SbF6]: A Structural and Spectroscopic Study

“…Indeed, most of the structurally-characterized five coordinate Pt(II) complexes exhibit clearly a trigonal bipyramidal geometry [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Interest in five coordinate Pt(II) species and their Pd(II) analogues comes from their involvement as intermediates in important Pt(II) or Pd(II) catalyzed organometallic transformations [3,[21][22][23][24][25][26][27][28].…”