Different reductive reactivities of SmCp^x ₂(THF)_n (Cp_x = C₅Me₅ and C₅H₃ ^tBu₂) samarocenes toward P₂Ph₄: THF ring-opening and ligand-exchange pathways

The reduction of tetraphenyldiphosphine with two differently substituted samarocenes(II) proceeds via different pathways. With [SmCp^∗ ₂(THF)₂] (Cp^∗ = η⁵- C₅Me₅), the reaction had been known to result in the THF ring-opening product, [SmCp^∗ ₂(O(CH₂)₄PPh₂)], 3, owing to the instability of phosphido complex [SmCp^∗ ₂(PPh₂)] in the presence of THF. Complex 3 crystallizes from apolar solvents as dimeric or polymeric polymorph with butoxo-phosphine bridging ligands in both cases. In contrast, the phosphide [SmCp″₂(PPh₂)] (Cp″ = η⁵-1,3-C₅H₃ ^tBu₂), 5, is not prone to ring-opening owing to insufficient space in the Sm coordination sphere for a THF ligand. Product 5 is inevitably accompanied by homoleptic complex [SmCp″₃] 6 and dinuclear mixed-valent complex [Sm^IIICp″₂(μ-PPh₂)₂Sm^IICp″] 7 as the further products of redox transformations and ligand exchange. The formation of 5-7 is rationalized by a sequence of initial coordination of one or two {Sm^IICp″₂} fragments by P atoms and reductive elimination of PPh₂· or Cp″·E radicals. Further reaction with another equivalent of [SmCp″₂] results in the trapping the radicals and formation of all three products.