OXIDATION-STATE-DEPENDENT REACTIVITY AND CATALYTIC PROPERTIES OF A RH(I) COMPLEX FORMED FROM A REDOX-SWITCHABLE HEMILABILE LIGAND

The synthesis and characterization of a phosphinoalkylarene, redox-swi tchable hemilabile ligand (RHL), (eta(5)-C5H5)Fe((eta(5)-C5H4C6H4OCH2C H2PPh2) (1), are reported. This ligand, which incorporates a redox-act ive ferrocenyl group, exhibits oxidation-state-dependent bonding prope rties and, hence, affords electrochemical control over the electronic and steric environments of bound transition metal centers. Two equival ents of 1 complex to Rh(I) to yield a bis(phosphine), eta(6)-arene com plex [(eta(1): eta(6) -(eta(5)-C5H5)Fe(eta(5)-C5H4C6H4OCH2CH2PPh2))(et a 1 -(eta 5-C5H4C6H4OCH2CH2PPh2))Rh]+BF4-(2). Single-crystal X-ray dif fraction studies of 2 . 1.25CH(2)Cl(2), as well as solution spectrosco pic data of 2, are consistent with this formulated piano-stool geometr y. Foremost, the properties of 2 as a function of bound RHL state-of-c harge are extensively investigated. Interestingly, 2D H-1 NMR exchange spectroscopy (EXSY) studies demonstrate significantly faster intramol ecular eta(6)-arene, free arene exchange rates only upon oxidation of the ligand chelated to the Rh(I) center, as found in 2(2+). This faste r exchange rate was used as a qualitative measure of the increased lab ility of the eta(6)-aryl group upon RHL oxidation. Moreover, activatio n parameters measured for the arene-arene exchange reaction of 2(2+) a lso support a decrease in the Rh(I)-arene interaction only upon oxidat ion of the ferrocenyl group on the bound eta(6)-arene moiety. In addit ion, changes in the stoichiometric and catalytic reactivity of 2 upon RHL oxidation are consistent with the observed charge-dependent arene- arene exchange behavior. Significantly, labilization of a weakly bound eta(6)-arene moiety in 2(2+) results in substantial increases in both the acetonitrile bonding affinity and allyl ethyl ether isomerization activity of the Rh(I)-RHL complex. In contrast, no significant change s in the reactivity of 2 were observed upon oxidation of the ligand wh ich contained the ferrocenylarene not bound to the Rh(I) center, as fo und in 2(+). This dependence of complex reactivity on RHL oxidation st ate demonstrates the utility of such novel metal complexes for the rev ersible, electrochemical control of transition metal center small mole cule uptake or catalytic activity via the selective labilization of we akly coordinating groups upon ligand oxidation.