η⁵-η¹ Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR ¹J_Yb-P Coupling Constants

This paper reports the synthesis of a series of complexes based on the bis(pentamethylcyclopentadienyl)ytterbium(II) (1; Cp∗₂Yb) and bis(tetramethylphospholyl)ytterbium(II) (2; Tmp₂Yb) fragments bearing an additional neutral bis(methyliminophosphoranyl)pyridine ligand (L) on which the steric demand is modulated at the phosphorus position (triethyl, L^Et; triphenyl, L^Ph; tricyclohexyl, L^Cy) to yield the original complexes Cp∗₂YbL^Et (1-L^Et), Cp∗₂YbL^Ph (1-L^Ph), Tmp₂YbL^Et (2-L^Et), Tmp₂YbL^Ph (2-L^Ph), and Tmp₂YbL^Cy (2-L^Cy), while no reaction occurs between 1 and L^Cy. The crystal structures of these sterically crowded complexes are reported as well as room-temperature NMR data for all the complexes. The solid-state coordination mode of L^R differs depending on the nature of the fragments 1 and 2 and on the steric bulk of L^R. The crystal structure of the divalent Tmp₂Yb(py)₂ (3) is also reported for structural and spectroscopic comparisons. Interestingly, in both 2-L^Et and 2-L^Cy, one of the two Tmp ligands coordinates in an η¹ rather than in an η⁵ fashion, a relevant coordination mode for the study of sterically induced reductions. The behavior of those complexes in solution varies with the sterics and electronics of the ligands, as demonstrated by variable-temperature NMR experiments. In solution, the ¹J_Yb-P coupling is used to track the coordination mode of the Tmp ligand and a large difference in the ¹J_Yb-P coupling constant allows the distinction between an η⁵ coordination mode and a dynamic η⁵-η¹ switch.