η<sup>5</sup>–η<sup>1</sup> Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR <sup>1</sup><i>J</i><sub>Yb–P</sub> Coupling Constants
作者:Thibault Cheisson、Audrey Auffrant、Grégory Nocton
DOI:10.1021/acs.organomet.5b00814
日期:2015.11.23
This paper reports the synthesis of a series of complexes based on the bis(pentamethylcyclopentadienyl)ytterbium(II) (1; Cp*Yb-2) and bis(tetramethylphospholyl)ytterbium(II) (2; Tmp(2)Yb) 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, LPh; tricyclohexyl, L-Cy) to yield the original complexes Cp*2YbLEt (1-L-Et), Cp*2YbLPh (1-LPh), Tmp2YbL(Et) (2-L-Et), Tmp2YbLPh (2-LPh), 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 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-L-Et and 2-L-Cy, one of the two Tmp ligands coordinates in an eta(1) rather than in an eta(5) 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 1JYbP coupling is used to track the coordination mode of the Tmp ligand and a large difference in the 1JYbP coupling constant allows the distinction between an eta(5) coordination mode and a dynamic eta(5)-eta(1) switch.