Multiple one-electron transfers in bipyridine complexes of bis(phospholyl) thulium

The synthesis of original neutral bis(phospholyl) thulium complexes, Dtp₂Tm(L), where L is tetramethylbiphosphinine (tmbp) and bipyridine (bipy), is reported. The electronic structures of these complexes have been investigated and it appears that, in both cases, an electron transfer occurs from the divalent metal to the ligand, a consequence of the strong reduction potential of the bis(phospholyl) thulium fragment, Dtp₂Tm. When 1 equiv of bipyridine is added to the Dtp₂Tm(tmbp) complex, another electron transfer occurs to form the Dtp₂Tm(bipy) complex along with free tmbp ligand. Astonishingly, despite the apparent trivalent nature of the thulium center, the Dtp₂Tm(bipy) complex is still reactive toward neutral bipyridine to form a new complex in which one phospholyl ligand is replaced by a bipyridine radical anion. An experimental kinetic analysis is reported to rationalize this unprecedented redox reaction with thulium and reveals an associative type of mechanism.