EPR study of the oxidation reaction of nickel(0) phosphine complexes with Lewis and Brønsted acids

The oxidation of Ni(PPh3)4 with BF3 · OEt2, H3CCOOH, and F3CCOOH, and that of (PPh3)2Ni(C2H4) with BF3 · OEt2 is studied by EPR spectroscopy. The reaction of the Ni(0) complexes with BF3 · OEt2 gives Ni(II) complexes with which they react to form Ni(I) compounds with covalent Ni–F and Ni–B bonds that transform with excess BF3 · OEt2 into cationic paramagnetic Ni(I) complexes. Acetic acid also adds oxidatively to Ni(PPh3)4 to form a Ni(II) complex that reacts further to give Ni(I) hydride and carboxylate complexes. The Ni(I) hydride is transformed by the acid into the Ni(I) carboxylate with release of hydrogen, the amount of which depends on the rate of acid addition. The following Ni(I) complexes are identified in the reaction medium: [Ni(PPh3)3]BF4, [(PPh3)2Ni(OEt2)]BF4, [(PPh3)Ni(OEt2)n]BF4, (PPh3)2NiBF2, (PPh3)3NiOOCCH3, and [(PPh3)2Ni(OEt2)P(OEt)3]BF4. Oxidation schemes of Ni(0) complexes by Lewis and Brønsted acids are given.