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

The chemistry of palladium alkyl complexes that incorporate the phosphine-trifluoroborate ligand ortho(Ph(2)P)C(6)H(4)(BF(3)(-)) (PF(-)) is described. The reaction of the pinacol borane ortho-(Ph(2)P)C(6)H(4)(Bpin) with K[HF(2)] yields ortho-(Ph(2)P)C(6)H(4)(BF(3)K) (K[PF], 1). Crystallization of 1 from Et(2)O/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 kappa(2) mode through the phosphine and one fluorine of the -ArBF(3)(-) 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 (kappa(1) -P-PF)PdMe(py)(2) (6), in which the -ArBF(3)(-) unit has been completely displaced by pyridine. 4 slowly dimerizes ethylene to 1-butene (36 t.o./h, 23 degrees C, CH(2)Cl(2), 400 psi ethylene). The catalyst resting state is (PF)PdEt(col) (7). Addition of [H(QEt(2))(2)][B(3,5-(CF(3))(2)-C(6)H(3))(4)] traps the collidine as [collidinium] [B(3,5-(CF(3))(2)-C(6)H(3))(4)] and results in a 10 fold increase in the ethylene dimerization rate (385 t.o./h, 23 degrees C, CD(2)Cl(2), 150 psi ethylene).