Phosphine-bridged dirhenium(II) complexes that contain carbonyl, isocyanide and nitrile ligands

Andrew Christopher Price, Purdue University

Abstract

Multiply bonded dimetal complexes that contain monodentate phosphines react with carbon monoxide and isocyanides to form mononuclear cleavage products. These reactions are quite facile and are difficult to control. By resorting to the phosphine-bridged complexes Re$\sb2$X$\sb4$(dppm)$\sb2$ (X = Cl or Br; dppm = Ph$\sb2$PCH$\sb2$PPh$\sb2$), cleavage by carbon monoxide, isocyanides and nitriles is prevented, presumably due to the very stable Re-Re-P-C-P rings that are present. Carbon monoxide reacts readily with Re$\sb2$X$\sb4$(dppm)$\sb2$ to give Re$\sb2$X$\sb4$(dppm)$\sb2$(CO)$\sb{\rm m}$ (m = 1 or 2). The dicarbonyls Re$\sb2$X$\sb4$(dppm)$\sb2$(CO)$\sb2$ react further with CO, in the presence of TIPF$\sb6$, to give the tricarbonyls (Re$\sb2$X$\sb3$(dppm)$\sb2$(CO)$\sb3$) PF$\sb6$. These salts may be reduced by one electron, using cobaltocene, to the neutral species Re$\sb2$X$\sb3$(dppm)$\sb2$(CO)$\sb3$ and further to (Cp$\sb2$Co) (Re$\sb2$X$\sb3$(dppm)$\sb2$(CO)$\sb3$). The latter constitute the first examples of complexes that contain the Re$\sb2\sp{2+}$ core. Reactions of Re$\sb2$Cl$\sb4$(dppm)$\sb2$(CO) with one equivalent of an isocyanide (RNC) give Re$\sb2$Cl$\sb4$(dppm)$\sb2$(CO)(CNR). When R = $t$-Bu, both $\pi$-acceptors (CO and RNC) are terminally bound but when R = xylyl, the CO ligand bridges the Re-Re bond. These complexes react further with CO, in the presence of TIPF$\sb6$, to yield (Re$\sb2$Cl$\sb3$(dppm)$\sb2$(CO)$\sb2$(CNR)) PF$\sb6$. The structural characterization of the $t$-Bu derivative shows that it contains an unsymmetric arrangement of the $\pi$-acceptor ligands about the planar (Re$\sb2$Cl$\sb3$(CO)$\sb2$(CN-$t$-Bu)) unit. Thermal isomerization occurs upon heating these salts for 12 h in 1,2-dichloroethane to give complexes of the same stoichiometry which possess an all-cis arrangement of chloride ligands on the same side of the molecule. A similar type of isomerization is found in the case of (Re$\sb2$Cl$\sb3$(dppm)$\sb2$(CO)(CNxylyl)$\sb2$) PF$\sb6$. The structural characterization of (Re$\sb2$Cl$\sb3$(dppm)$\sb2$(CO)(CN-$t$-Bu)$\sb2$) PF$\sb6$ shows that there are no bridging Cl$\sp-$ or CO ligands and it has a Re-Re bond distance that is $\simeq$0.2 A shorter than r(Re-Re) for complexes that contain the Re($\mu$-CO)Re entity. The reactions of nitriles (R$\sp\prime$CN) with Re$\sb2$Cl$\sb4$(dppm)$\sb2$(CO) and Re$\sb2$Cl$\sb4$(dppm)$\sb2$(CO)(CNR) give (Re$\sb2$Cl$\sb3$(dppm)$\sb2$(CO)(NCR$\sp\prime)\sb2$) PF$\sb6$ and (Re$\sb2$Cl$\sb3$(dppm)$\sb2$(CO)(CNR)(NCR$\sp\prime$) PF$\sb6$, respectively. The R$\sp\prime$CN ligands in these complexes are labile and may be displaced by CO and RNC.

Degree

Ph.D.

Advisors

Walton, Purdue University.

Subject Area

Chemistry

COinS