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 κ 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 (κ 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 °C, CH 2 Cl 2, 400 psi ethylene). The catalyst resting state is (PF)PdEt(col) ( 7 ). Addition of [H(OEt 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 °C, CD 2 Cl 2, 150 psi ethylene).