Syntheses and reactivities of rhenium polyhydrides and alkylidyne complexes

Robert J Thomas, Purdue University

Abstract

The thermal reactions of the heptahydride complex $\rm ReH\sb7(PPh\sb3)\sb2$ with quinoline (qn), 2-hydroxyquinoline (Hhq) and 2-mercaptoquinoline (Hmq) afford the hydridorhenium(V) complexes $\rm ReH\sb5(PPh\sb3)\sb2(qn),\ ReH\sb4(hq)(PPh\sb3)\sb2$ and $\rm ReH\sb4(mq)(PPH\sb3)\sb2$, respectively, when refluxing tetrahydrofuran or ethanol is used as the reaction solvent, whereas the paramagnetic chlororhenium(III) species $\rm ReCl\sb3(qn)\sb2(PPh\sb3)\ ReCl\sb2(hq)(PPh\sb3)\sb2$ and $\rm ReCl\sb2(mq)(PPh\sb3)\sb2$ are formed in refluxing 1,2-dichloroethane solutions. The reactions of the eight-coordinate rhenium(V) polyhydride complex $\rm ReH\sb4(mq)(PPh\sb3)\sb2$ (mq = the anion of 2-mercaptoquinoline) with terminal alkynes R$\equiv$CH (R = H, Et, n-Pr, i-Pr, n-Bu, Ph, p-tol, CH$\sb2$Ph) in the presence of an electrophile (H$\sp+$ as HPF$\sb6$(aq), or $\rm Ph\sb3CPF\sb6$) and dichloromethane as the reaction solvent provide a facile and high yield route to a new class of alkylidyne complexes of composition (Re($\rm{\equiv}CCH\sb2R)H(mq)(PPh\sb3)\sb2\rbrack PF\sb6$ (1). These complexes are reversibly deprotonated to form their neutral monohydrides $\rm Re({\equiv}CCH\sb2R)H(mq)(PPh\sb3)\sb2$ (2). When these same reactions are carried out with the use of an internal alkyne which is an isomer of one of the aforementioned terminal alkynes, then these same alkylidyne complexes are formed. For example, 1-, 2- and 3-hexyne afford the same complex, $\rm\lbrack Re({\equiv}C(CH\sb2)\sb4CH\sb3)H\sb2(mq)(PPh\sb3)\sb2\rbrack PF\sb6$, with no perceptible difference in reaction time or product yield. The isomerization of internal to terminal alkynes probably occurs via $\eta\sp2$-allene intermediates. When $\rm ReH\sb4(mq)(PPh\sb3)\sb2$ is treated with an electrophile (H$\sp+$ or $\rm Ph\sb3C\sp+$) in the absence of an alkyne, the dirhenium(V) complex $\rm\lbrack Re\sb2H\sb6(\mu$-$\rm mq)\sb2(PPh\sb3)\sb4\rbrack (PF\sb6)\sb2$ is formed. The M$\equiv$C bond in the monohydride alkylidyne complex $\rm ReH({\equiv}CCH\sb2R)H(mq)(PPh\sb3)\sb2$ (2) can be cleanly hydrogenated by dihydrogen at elevated temperature (50-75$\sp\circ$C) and high pressure (1000 psi) while using benzene as the reaction solvent. The products of this reaction are the tetrahydride, $\rm ReH\sb4(mq)(PPh\sb3)\sb2$, and the hydrocarbon, $\rm CH\sb3CH\sb2R.$

Degree

Ph.D.

Advisors

Walton, Purdue University.

Subject Area

Chemistry

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