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-switchable hemilabile ligand (RHL), (η⁵-C₅H₅)Fe(η⁵-C₅H₄C₆H₄OCH₂CH₂PPh₂) (1), are reported. This ligand, which incorporates a redox-active ferrocenyl group, exhibits oxidation-state-dependent bonding properties and, hence, affords electrochemical control over the electronic and steric environments of bound transition metal centers. Two equivalents of 1 complex to Rh(I) to yield a bis(phosphine), η⁶-arene complex [(η¹:η⁶-(η⁵-C₅H₅)Fe(η⁵-C₅H₄C₆H₄OCH₂CH₂PPh₂))(η¹-(η⁵ -C₅H₅)Fe(η⁵- C₅H₄C₆H₄OCH₂CH₂PPh₂))Rh]⁺BF₄^- (2). Single-crystal X-ray diffraction studies of 2·1.25CH₂Cl₂, as well as solution spectroscopic data of 2, are consistent with this formulated piano-stool geometry. Foremost, the properties of 2 as a function of bound RHL state-of-charge are extensively investigated. Interestingly, 2D ¹H NMR exchange spectroscopy (EXSY) studies demonstrate significantly faster intramolecular η⁶-arene, free arene exchange rates only upon oxidation of the ligand chelated to the Rh(I) center, as found in 2²⁺. This faster exchange rate was used as a qualitative measure of the increased lability of the η⁶-aryl group upon RHL oxidation. Moreover, activation parameters measured for the arene-arene exchange reaction of 2²⁺ also support a decrease in the Rh(I)-arene interaction only upon oxidation of the ferrocenyl group on the bound η⁶-arene moiety. In addition, 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 η⁶-arene moiety in 2²⁺ 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 changes in the reactivity of 2 were observed upon oxidation of the ligand which contained the ferrocenylarene not bound to the Rh(I) center, as found in 2⁺. This dependence of complex reactivity on RHL oxidation state demonstrates the utility of such novel metal complexes for the reversible, electrochemical control of transition metal center small molecule uptake or catalytic activity via the selective labilization of weakly coordinating groups upon ligand oxidation.