η⁵–η¹ Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR ¹<i>J</i>_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, LEt; triphenyl, LPh; tricyclohexyl, LCy) to yield the original complexes Cp*2YbLEt (1-LEt), Cp*2YbLPh (1-LPh), Tmp2YbLEt (2-LEt), Tmp2YbLPh (2-LPh), and Tmp2YbLCy (2-LCy), while no reaction occurs between 1 and LCy. 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 LR differs depending on the nature of the fragments 1 and 2 and on the steric bulk of LR. The crystal structure of the divalent Tmp2Yb­(py)2 (3) is also reported for structural and spectroscopic comparisons. Interestingly, in both 2-LEt and 2-LCy, 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 1JYb–P coupling is used to track the coordination mode of the Tmp ligand and a large difference in the 1JYb–P coupling constant allows the distinction between an η5 coordination mode and a dynamic η5–η1 switch.