Reductions of nitriles by multiply-bonded dirhenium(II) centers

David Esjornson, Purdue University

Abstract

The reactions of the dirhenium(II) complexes Re$\sb2$X$\sb4$(LL)$\sb2$ (where X = Cl, Br; LL = bidentate phosphine) with nitriles has been explored. The dirhenium(II) complexes (Re$\sb2$X$\sb3(\mu$-dppm)$\sb2$(NCR)$\sb2$) PF$\sb6$ (where X = Cl, Br; dppm = Ph$\sb2$PCH$\sb2$PPh$\sb2$; R = Me, Et, $i$-Pr, $t$-Bu, CH$\sb2$Ph, Ph) are formed in high yield from the room temperature reaction of Re$\sb2$X$\sb4(\mu$-dppm)$\sb2$ with RCN and KPF$\sb6$ in acetone. These $bis$-nitrile complexes react further with nitriles to give the green, paramagnetic complexes (Re$\sb2$X$\sb3(\mu$-HN$\sb2$C$\sb2$R$\sb2$)($\mu$-dppm)$\sb2$(NCR)) PF$\sb6$ in which the HN$\sb2$C$\sb2$R$\sb2$ ligand has been formed from the reductive coupling of two nitriles. The lability of the nitrile ligands RCN has been demonstrated by the conversion of (Re$\sb2$Cl$\sb3(\mu$-HN$\sb2$C$\sb2$Me$\sb2$)($\mu$-dppm)$\sb2$(NCMe)) PF$\sb6$ into (Re$\sb2$Cl$\sb3(\mu$-HN$\sb2$C$\sb2$Me$\sb2$)($\mu$-dppm)$\sb2$(NCR$\sp\prime$)) PF$\sb6$ upon reaction with R$\sp\prime$CN (R$\sp\prime$ = $i$-Pr, Ph). Experiments in which CD$\sb3$CN is used in place of CH$\sb3$CN showed that the source of hydrogen in the HN$\sb2$C$\sb2$Me$\sb2$ ligand is the acetonitrile ligand itself; this hydrogen is very labile and readily exchanges with deuterium. The complexes (Re$\sb2$X$\sb3(\mu$-HN$\sb2$C$\sb2$R$\sb2$)($\mu$-dppm)$\sb2$-(NCR)) PF$\sb6$ are readily oxidized to the red, diamagnetic salts (Re$\sb2$X$\sb3(\mu$-HN$\sb2$C$\sb2$R$\sb2$)($\mu$-dppm)$\sb2$(NCR)) (PF$\sb6)\sb2$ by (($\eta\sp5$-C$\sb5$H$\sb5)\sb2$Fe) PF$\sb6$/acetone or NOPF$\sb6$/CH$\sb2$Cl$\sb2$; these compounds display well defined $\sp1$H and $\sp{31}$P$\{\sp1$H$\}$ NMR spectra. The structural identity of these complexes was confirmed by X-ray structural analyses of (Re$\sb2$Br$\sb3(\mu$-HN$\sb2$C$\sb2$Me$\sb2$)($\mu$-dppm)$\sb2$(NCMe)) (PF$\sb6)\sb{0.8}$(Br)$\sb{0.2}\cdot$1/2Et$\sb2$O$\cdot$Me$\sb2$CO and (Re$\sb2$Cl$\sb3(\mu$-HN$\sb2$C$\sb2$Me$\{$CHO$\}$($\mu$-dppm)$\sb2$(NC-$i$-Pr)) PF$\sb6\cdot$C$\sb2$H$\sb4$Cl$\sb2\cdot$H$\sb2$O. The latter complex contains an aldehyde group which was formed from the oxidation of one of the methyl groups of the coupled nitrile ligand which occurred during the attempts to grow crystals of (Re$\sb2$Cl$\sb3(\mu$-HN$\sb2$C$\sb2$Me$\sb2$)($\mu$-dppm)$\sb2$(NC-$i$-Pr)) PF$\sb6$. The bond distances and angles associated with the metallacycle derived from the coupling of two acetonitrile ligands indicate that the coupled ligand can best be considered in terms of contributions from deprotonated diimine and ene-diamine ligand forms. The reactions of ($n$-Bu$\sb4$N)$\sb2$Re$\sb2$X$\sb8$(X = Cl, Br) with the bidentate phosphine ligand (dppbe) in hot nitrile solvents RCN (R = Me, Et, $i$-Pr) which contain a few drops of conc. HX, the mononuclear alkylimido-rhenium(V) complexes Re(NCH$\sb2$R)X$\sb3$(dppbe) are usually formed. The only exception is the reaction between ($n$-Bu$\sb4$N)$\sb2$Re$\sb2$Cl$\sb8$ and dppbe in acetonitrile which affords trans-ReCl$\sb2$(dppbe)$\sb2$. The formation of the alkylimidorhenium(V) complex Re(NCH$\sb2$Et)Cl$\sb3$(cppbe) also occurs upon reacting $\alpha$-Re$\sb2$Cl$\sb4$(dppbe)$\sb2$ with propionitrile and conc. HCl. Alkylimido-rhenium(V) complexes have been prepared for a variety of other bidentate phosphines, including 1,2-$bis$(diphenylphosphino)ethane, 1,3-$bis$(diphenylphosphino)ethene. The characterization of the alkylimido complexes has included measurement of their electrochemical properties and their $\sp1$H and $\sp{31}$P$\{\sp1$H$\}$ NMR spectra. The upfield $\sp1$H NMR chemical shifts of the $\alpha$-hydrogens of the imido group are explained in terms of the interactions of these hydrogens with the phenyl rings of the phosphine ligands.

Degree

Ph.D.

Advisors

Walton, Purdue University.

Subject Area

Chemistry

COinS