Person:

Powers, David Charles

Polynuclear transition metal complexes, which are embedded in the active sites of many metalloenzymes, are responsible for effecting a diverse array of oxidation reactions in nature. The range of chemical transformations remains unparalleled in the laboratory. With few noteworthy exceptions, chemists have primarily focused on mononuclear transition metal complexes in developing homogeneous catalysis. Our group is interested in the development of carbon–heteroatom bond-forming reactions, with a particular focus on identifying reactions that can be applied to the synthesis of complex molecules. In this context, we have hypothesized that bimetallic redox chemistry, in which two metals participate synergistically, may lower the activation barriers to redox transformations relevant to catalysis. In this Account, we discuss redox chemistry of binuclear Pd complexes and examine the role of binuclear intermediates in Pd-catalyzed oxidation reactions.

Stoichiometric organometallic studies of the oxidation of binuclear (Pd^{II}) complexes to binuclear (Pd^{III}) complexes and subsequent C–X reductive elimination from the resulting binuclear (Pd^{III}) complexes have confirmed the viability of C–X bond-forming reactions mediated by binuclear (Pd^{III}) complexes. Metal–metal bond formation, which proceeds concurrently with oxidation of binuclear (Pd^{II}) complexes, can lower the activation barrier for oxidation. We also discuss experimental and theoretical work that suggests that C–X reductive elimination is also facilitated by redox cooperation of both metals during reductive elimination. The effect of ligand modification on the structure and reactivity of binuclear (Pd^{III}) complexes will be presented in light of the impact that ligand structure can exert on the structure and reactivity of binuclear (Pd^{III}) complexes.

Historically, oxidation reactions similar to those discussed here have been proposed to proceed via mononuclear (Pd^{IV}) intermediates, and the hypothesis of mononuclear (Pd^{II/IV}) catalysis has guided the successful development of many reactions. Herein we discuss differences between monometallic (Pd^{IV}) and bimetallic (Pd^{III}) redox catalysis. We address whether appreciation of the relevance of bimetallic (Pd^{III}) redox catalysis is of academic interest exclusively, serving to provide a more nuanced description of catalysis, or if the new insight regarding bimetallic (Pd^{III}) chemistry can be a platform to enable future reaction development. To this end, we describe an example in which the hypothesis of bimetallic redox chemistry guided reaction development, leading to the discovery of reactivity distinct from monometallic catalysts.

Cooperative metal–metal (M–M) redox chemistry has the potential to lower activation barriers for redox transformations relevant to catalysis. Pd2(III,III) complexes, generated by oxidation of Pd2(II,II) complexes, have recently been implicated as intermediates in a variety of Pd-catalyzed C–H oxidation reactions. M–M redox synergy, mediated by Pd–Pd bond formation and cleavage, has been proposed to facilitate both oxidation and reductive elimination steps during various Pd-catalyzed directed C–H oxidation reactions. Herein, we report a transition state mimic for the oxidation of Pd2(II,II) complexes which suggests that M–M redox synergy is involved in the oxidation of Pd2(II,II) complexes to Pd2(III,III) complexes.

Photoactivation of M–X bonds is a challenge for photochemical HX splitting, particularly with first-row transition metal complexes because of short intrinsic excited state lifetimes. Herein, we report a tandem H2 photocycle based on combination of a non-basic photoredox phosphine mediator and nickel metal catalyst. Synthetic studies and time-resolved photochemical studies have revealed that phosphines serve as photochemical H-atom donors to Ni(II) trihalide complexes to deliver a Ni(I) centre. The H2 evolution catalytic cycle is closed by sequential disproportionation of Ni(I) to afford Ni(0) and Ni(II) and protolytic H2 evolution from the Ni(0) intermediate. The results of these investigations suggest that H2 photogeneration proceeds by two sequential catalytic cycles: a photoredox cycle catalyzed by phosphines and an H2-evolution cycle catalyzed by Ni complexes to circumvent challenges of photochemistry with first-row transition metal complexes.

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Oxidation of binuclear Pd(II) complexes with PhICl(_2) or PhI(OAc)(_2) has previously been shown to afford binuclear Pd(III) complexes featuring a Pd–Pd bond. In contrast, oxidation of binuclear Pd(II) complexes with electrophilic trifluoromethylating (“CF(_3^+)”) reagents has been reported to afford mononuclear Pd(IV) complexes. Herein, we report experimental and computational studies of the oxidation of a binuclear Pd(II) complex with “CF(_3^+)” reagents. These studies suggest that a mononuclear Pd(IV) complex is generated by an oxidation–fragmentation sequence proceeding via fragmentation of an initially formed, formally binuclear Pd(III), intermediate. The observation that binuclear Pd(III) and mononuclear Pd(IV) complexes are accessible in the same reactions offers an opportunity for understanding the role of nuclearity in both oxidation and subsequent C–X bond-forming reactions.