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Synthesis, Structures, and Ethylene Dimerization Reactivity of Palladium Alkyl Complexes That Contain a Chelating Phosphine–Trifluoroborate Ligand

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journal contribution
posted on 22.08.2011, 00:00 by Youngmin Kim, Richard F. Jordan
The chemistry of palladium alkyl complexes that incorporate the phosphine–trifluoroborate ligand ortho-(Ph2P)C6H4(BF3) (PF) is described. The reaction of the pinacol borane ortho-(Ph2P)C6H4(Bpin) with K[HF2] yields ortho-(Ph2P)C6H4(BF3K) (K[PF], 1). Crystallization of 1 from Et2O/THF in the presence of 18-crown-6 yields [K-(18-crown-6)][PF]·0.5THF (2·0.5THF). In the solid state, the phosphine–borate anion of 2 is ion-paired with the [K-(18-crown-6)] cation through weak contacts with the phosphorus and two fluorine atoms. 1 reacts with (COD)PdMeCl in the presence of 18-crown-6 to form [K-(18-crown-6)][(PF)PdMeCl] (3) and with (COD)PdMeCl and 2,4,6-collidine (col) to yield (PF)PdMe(col) (4). The PF ligands in 3 and 4 are bound to Pd in a κ2 mode through the phosphine and one fluorine of the −ArBF3 unit. The other two fluorines are weakly bound to the K(18-crown-6)+ cation in 3. NMR studies show that the Pd–F interactions in 3 and 4 are maintained in solution and that, for 4, the three fluorine atoms undergo fast site exchange on the NMR time scale. 4 reacts with excess pyridine to yield (κ1-P-PF)PdMe(py)2 (6), in which the −ArBF3 unit has been completely displaced by pyridine. 4 slowly dimerizes ethylene to 1-butene (36 t.o./h, 23 °C, CH2Cl2, 400 psi ethylene). The catalyst resting state is (PF)PdEt(col) (7). Addition of [H(OEt2)2][B(3,5-(CF3)2-C6H3)4] traps the collidine as [collidinium] [B(3,5-(CF3)2-C6H3)4] and results in a 10-fold increase in the ethylene dimerization rate (385 t.o./h, 23 °C, CD2Cl2, 150 psi ethylene).