η5–η1 Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR 1JYb–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.

本工作报道了一系列基于双(五甲基环戊二烯基)镱(II)(bis(pentamethylcyclopentadienyl)ytterbium(II), 1; Cp*₂Yb)与双(四甲基膦杂环戊二烯基)镱(II)(bis(tetramethylphospholyl)ytterbium(II), 2; Tmp₂Yb)骨架的配合物的合成：通过在双(甲基亚氨基膦基)吡啶配体(L)的磷位点调控空间位阻（分别引入三乙基、三苯基与三环己基取代基，对应配体LEt、LPh与LCy），成功得到目标配合物Cp*₂YbLEt(1-LEt)、Cp*₂YbLPh(1-LPh)、Tmp₂YbLEt(2-LEt)、Tmp₂YbLPh(2-LPh)与Tmp₂YbLCy(2-LCy)，但配合物1与LCy未发生反应。本文同时报道了上述空间拥挤型配合物的晶体结构以及所有配合物的室温核磁共振(NMR)表征数据，带有不同取代基的配体L在固态中的配位模式取决于骨架1、2的性质以及配体自身的空间位阻程度。本文还报道了二价配合物Tmp₂Yb(py)₂(3)的晶体结构，用于结构与光谱学对比分析。值得注意的是，在2-LEt与2-LCy中，两个Tmp配体中的一个以η¹配位模式而非常规的η⁵模式结合，该配位模式对空间诱导还原反应的相关研究具有重要价值。变温核磁共振实验证实，此类配合物在溶液中的行为随配体的空间位阻与电子效应发生变化，在溶液体系中，通过¹J_Yb–P耦合常数可追踪Tmp配体的配位模式，耦合常数的显著差异可用于区分η⁵配位模式与动态η⁵–η¹转换过程。