Synthetic and redox chemistry of phosphine and polyhydride complexes of dirhenium

Dawn Renee Root, Purdue University

Abstract

The dirhenium complexes Re$\sb2$X$\sb4$(PMe$\sb3)\sb4$ (X = Cl or Br) have been synthesized. These complexes are volatile, and the gas-phase photoelectron spectrum of Re$\sb2$Cl$\sb4$(PMe$\sb3)\sb4$ provides the first data of this type for an electron-rich triply bonded ($\sigma\sp2\pi\sp4\delta\sp2\delta\sp{\* 2}$) compound. Reactions of these complexes with NOPF$\sb6$ yielded the oxidation products (Re$\sb2$X$\sb4$(PMe$\sb3)\sb4$) PF$\sb6$. While the reaction of Re$\sb2$Cl$\sb4$(PMe$\sb3)\sb4$ with dppe gave the expected phosphine substituted product Re$\sb2$Cl$\sb4$(dppe)$\sb2$, reactions with dppm or dppa (LL) led only to the partially substituted complexes Re$\sb2$Cl$\sb4$(PMe$\sb3)\sb2$(LL). Replacement of the remaining PMe$\sb3$ ligands was achieved, however, by further reaction of Re$\sb2$Cl$\sb4$(PMe$\sb3)\sb2$(LL) with the bidentate phosphines dppe or arphos (LL$\sp\prime$). The resulting mixed bidentate phosphine complexes Re$\sb2$Cl$\sb4$(LL)(LL$\sp\prime$) contain bridging phosphine ligands in a transoid disposition, as confirmed by the X-ray structure analysis of Re$\sb2$Cl$\sb{\rm 4}$(dppm)(dppe). The complexes Re$\sb2$X$\sb4$(PR$\sb3$) react with LiAlH$\sb4$ to yield the dirhenium octahydride complexes Re$\sb2$H$\sb8$(PR$\sb3)\sb4$. This route is the first general synthetic procedure adaptable for use with a wide range of phosphine ligands, both monodentate (PR$\sb3$ = PMe$\sb3$, PEt$\sb3$, PPr$\sbsp{3}{\rm n}$, PMe$\sb2$Ph, PEt$\sb2$Ph, or PMePh$\sb2$) and bidentate (PR$\sb3$ = 1/2 (dppm) or 1/2(dppe)). The electrochemical and NMR spectral properties, of these octahydride complexes have been thoroughly examined. Although the monodentate phosphine derivatives are all believed to have the expected Re$\sb2$($\mu$-H)$\sb4$H$\sb4$(PR$\sb3)\sb4$ structure, somewhat different structures are proposed for the dppm and dppe derivatives. An X-ray crystal structure determination of the dppm complex has shown the presence of bridging phosphines and only two bridging hydride ligands, i.e. Re$\sb2(\mu$-H)$\sb2$H$\sb6(\mu$-dppm)$\sb2$. Further reactivity studies of Re$\sb2$H$\sb8$(PMe$\sb3$)$\sb4$ and Re$\sb2$H$\sb8$(dppe)$\sb2$ have been carried out. The PMe$\sb3$ derivative can be protonated upon treatment with HBF$\sb4$ to give (Re$\sb2$H$\sb9$(PMe$\sb3$)$\sb4$) BF$\sb4$, which can, in turn, be reversibly deprotonated using NEt$\sb3$. When Re$\sb2$H$\sb8$(PMe$\sb3$)$\sb4$ is reacted with excess LiAlH$\sb4$, the dirhenium unit is cleaved to give ReH$\sb7$(PMe$\sb3$)$\sb2$. Two products, (Re$\sb2$H$\sb5$(PMe$\sb3$)$\sb6$) PF$\sb6$ and (Re$\sb2$H$\sb5$(PMe$\sb3$)$\sb6$) (PF$\sb6$)$\sb2$, have been obtained from the reaction of Re$\sb2$H$\sb8$(PMe$\sb3$)$\sb4$ with PMe$\sb3$. However, when Re$\sb2$H$\sb8$(dppe)$\sb2$ was reacted with dppe, Re$\sb2$H$\sb4$(dppe)$\sb3$ was the only identifiable product. While Re$\sb2$H$\sb4$(dppe)$\sb3$ could be protonated using HBF$\sb4$ to give (Re$\sb2$H$\sb5$(dppe)$\sb3$) BF$\sb4$, this reaction was found to be irreversible.

Degree

Ph.D.

Advisors

Walton, Purdue University.

Subject Area

Chemistry

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