|Abstract:||Since palladium and rhodium complexes have the ability to perform chemical bond transformations, such as C-C coupling, C-Heteroatom coupling, and C-H functionalization reactions, fundamental studies on palladium and rhodium complexes have gained increased attention over the past few decades (2000-2020). The metal center’s oxidation state plays an important role in mediating the process of chemical bond formation. During our research, the common oxidation states of the Pd centers changed between 0, II, and IV, while the Rh centers involved the I and III oxidation states in the transition metal-catalyzed reactions. While Pd(0/II/IV) and Rh(I/III) catalytic cycles are well-developed and used to create a multitude of different chemical bonds, it is crucial to investigate other catalytic intermediates for developing novel transition metal-catalyzed transformations.
To expand new chemical bond transformations for Pd and Rh complexes, we explored the rare oxidation states of Pd(I/III) and Rh(II/IV), which are less typically involved in catalysis. Studying these rare oxidation states not only allows us to develop better catalytic performance (potentially replacing current catalytic cycles), but also to create new types of chemical bond formations involving these oxidation states. In our group, we developed the multidentate ligands N,N’-dialkyl-2,11-diaza[3.3 (2,6) pyridinophane ( R N4) to synthesize and stabilize various Pd(III) and Rh(II) complexes. Furthermore, we can modify the R groups on the axial N arms of RN4 to change the electronic and steric properties of metal complexes, which allows easy access to high-valent or low-valent metal complexes and thus increase their reactivity.
First, we investigated the stabilization and reactivity of Pd(III) complexes with a variety of η3-allyl derivatives supported by the tBuN4 or MeN4 ligands. These (RN4)Pd(II)(η3-allyl) (R= tBu or Me) complexes exhibit a chemically accessible oxidation potential of Pd II/III . Upon oxidation, Pd(III)(allyl) complexes were generated and characterized by electron paramagnetic resonance spectroscopy (EPR). Interestingly, the [(MeN4)Pd(III)(η3-allyl)] 2+ complex undergoes a rearrangement to the [(MeN4)Pd(III)(η1-allyl)] 2+ species at low temperatures. Moreover, fast allylic amination occurred within 15 min at room temperature upon the reaction of [(MeN4)Pd(II)(η3-allyl)] + complexes with NFSI and the C-N bond formation step is proposed to occur at the Pd(IV) oxidation state, likely via a Pd(IV)(η1-allyl) intermediate. Next, we investigated the new type of multidentate ligands, 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB- Me cyclam) and 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (CB- Me cyclen), to stabilize the high-valent Pd(III) complexes. The (CB- Me cyclam)Pd(II)Cl and (CB- Me cyclen)Pd(II)Cl complexes were synthesized. Intriguingly, the structures of these Pd(II) complexes prefer to adopt a square pyramidal geometry instead of a square planar geometry. Further oxidation of (CB-Me cyclam)Pd(II)Cl formed the mononuclear (CB- Me cyclam)Pd(III)Cl(MeCN) complex in acetonitrile, while a binuclear (CB- Me cyclam)Pd(III)Cl complex was formed in the solid-state.
Lastly, we studied the oxidation chemistry of (RN4)Rh(III)Me 2 (R= tBu or iPr) and (tBuN4)Rh(III)(cycloneophyl)H complexes. For the oxidation chemistry of (RN4)Rh(III)Me2, instead of producing ethane through C-C bond formation, methane formation was observed. As a result, oxidative induced homolytic cleavage of Rh(IV)-Me was proposed to generate a methyl radical. To stabilize a high-valent Rh(IV) complex, (tBuN4)Rh(III)(cycloneophyl)H complex was synthesized. Subsequently, due to the lower oxidation potential of Rh(III/IV), the proposed [(tBuN4)Rh(IV)(cycloneophyl)H] + complex was synthesized and characterized by EPR and XPS.
In addition, we have obtained a single crystal of [(tBuN4)Rh(II)(neophyl)] + , which was important to prove a Rh(IV/II) C-H bond reductive elimination is accessible.