The synthesis and reaction chemistry of dirhenium polyhydride complexes

Karen Elizabeth Meyer, Purdue University

Abstract

The reaction mechanism by which $\rm Re\sb2H\sb8(PMe\sb3)\sb4$ is converted to $\rm\lbrack Re\sb2H\sb5(PMe\sb3)\sb6\rbrack PF\sb6$ in methanol has been elucidated through the use of $\sp1$H NMR spectroscopy. Upon addition of PMe$\sb3$ to the octahydride complex, loss of H$\sb2$ and the sequential formation of two hydride intermediates are observed. These intermediates were isolated and identified as $\rm Re\sb2H\sb6(PMe\sb3)\sb5$ and $\rm\lbrack Re\sb2H\sb7(PMe\sb3)\sb5\rbrack\sp+$. Reaction of $\rm\lbrack Re\sb2H\sb7(PMe\sb3)\sb5\rbrack\sp+$ with PMe$\sb3$ yields the final product $\rm\lbrack Re\sb2H\sb5(PMe\sb3)\sb6\rbrack\sp+$ and H$\sb2$. The polyhydride complex $\rm Re\sb2H\sb4(PMe\sb3)\sb6$ is not isolated even when a non-coordinating solvent such as benzene is used. However, the reaction of $\rm Re\sb2H\sb8(dppe)\sb2$ (dppe = $\rm Ph\sb2PCH\sb2CH\sb2PPh\sb2$) with certain phosphine ligands (PR$\sb3$ = 1/2 $\rm Ph\sb2PCH\sb2PPh\sb2$ (dppm) or PMe$\sb3$) does yield the tetrahydride species $\rm Re\sb2H\sb4(dppe)\sb2(PR\sb3)$ and these are readily protonated to give $\rm\lbrack Re\sb2H\sb5(dppe)\sb2(PR\sb3)\sb2\rbrack\sp+$. Addition of the bidentate phosphine dppe to $\rm Re\sb2H\sb8(PMe\sb3)\sb4$ in benzene yields the tetrahydrido complex $\rm Re\sb2H\sb4(PMe\sb3)\sb4(dppe)$, while reactions with dppe or dppm and KPF$\sb6$ with methanol as the solvent gives the cationic pentahydrides $\rm\lbrack Re\sb2H\sb5(PMe\sb3)\sb4(PR\sb3)\sb2\rbrack PF\sb6$. The polyhydride complex $\rm\lbrack Re\sb2H\sb5(\mu$-dmpm)$\rm\sb3\rbrack PF\sb6$ (dmpm = $\rm Me\sb2PCH\sb2PMe\sb2$) is synthesized from the reaction of $\rm Re\sb2Cl\sb4(dmpm)\sb3$ with LiAlH$\sb4$. It has also been prepared, from the reaction of $\rm Re\sb2H\sb8(\mu$-dmpm)$\sb2$ with dmpm and HBF$\sb4$. Structural characterization of this unusual cationic pentahydride shows the presence of a long Re-Re distance (3.5150(4) A) between electronically unsaturated metal centers. This electron deficient compound reacts with CO and isocyanides at elevated temperatures. The products formed in these reactions have the general stoichiometry $\rm\lbrack Re\sb2H\sb3(\mu$-dmpm)$\rm\sb3(L)\sb2\rbrack PF\sb6$ or $\rm\lbrack Re\sb2H(\mu$-dmpm)$\rm\sb3(L)\sb4\rbrack PF\sb6$ (L = CO, 2,6-xylylisocyanide, or t-BuNC). Representative examples of these complexes have been characterized by X-ray crystallography and found to contain very long Re-Re distances of ca. 3.34 A. The disubstituted xylylisocyanide complex has been shown to possess the unsymmetric structure $\rm\lbrack (xylylNC)\sb2Re(\mu$-H)($\mu$-dmpm)$\rm\sb3ReH\sb2\rbrack\sp+$.

Degree

Ph.D.

Advisors

Walton, Purdue University.

Subject Area

Chemistry

COinS