Multielectron Reactivity Research Articles

Phosphorus-Ligand Redox Cooperative Catalysis: Unraveling Four-Electron Dioxygen Reduction Pathways and Reactive Intermediates.

The reduction of dioxygen to water is crucial in biology and energy technologies, but it is challenging due to the inertness of triplet oxygen and complex mechanisms. Nature leverages high-spin transition metal complexes for this, whereas main-group compounds with their singlet state and limited redox capabilities exhibit subdued reactivity. We present a novel phosphorus complex capable of four-electron dioxygen reduction, facilitated by unique phosphorus-ligand redox cooperativity. Spectroscopic and computational investigations attribute this cooperative reactivity to the unique electronic structure arising from the geometry of the phosphorus complex bestowed by the ligand. Mechanistic study via spectroscopic and kinetic experiments revealed the involvement of elusive phosphorus intermediates resembling those in metalloenzymes. Our result highlights the multielectron reactivity of phosphorus compound emerging from a carefully designed ligand platform with redox cooperativity. We anticipate that the work described expands the strategies in developing main-group catalytic reactions, especially in small molecule fixations demanding multielectron redox processes.

Redox-active ligand promoted multielectron reactivity at earth-abundant transition metal complexes

This review highlights the application of redox-active ligands for achieving substrate activation and reactivity through multielectron transfer at earth-abundant transition metal complexes.

Evolution of Oxygen Ligands upon Large Redox Swings of Li3IrO4

The limits of intercalation electrochemistry continue to be tested in the quest for ever increasing gains in the storage capability of Li-ion cathodes. The subsequent push for multi-electron reactivity has led to the recognition of the extremely versatile role of oxide ligands in charge compensation when there is a large redox swing. Li3IrO4 is a unique model of such activity because it can reversibly cycle between Li1IrO4 and Li4.7IrO4. Here, X-ray spectroscopy, magnetic measurements and computational simulations uncover the evolution of O states in the different steps, compared to the involvement of Ir. While the process between Li1IrO4 and Li3IrO4 is dominated by the unconventional lattice oxygen redox, the process between Li3IrO4 and Li4.7IrO4 involves a conventional change of the formal oxidation state of Ir, which affects O due to the high covalency. The O states of Li3IrO4 exhibit a very high reversibility after the whole 3.7-electron process, completely restoring the pristine state.

The BIAN ligand 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene: An electron sponge or a “normal” α-diimine ligand?

The compounds [fac-Re(CO)3Cl(L)] and [M(CO)4(L)], M=Cr, Mo, W and L=1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene, have been studied electrochemically and spectroelectrochemically (IR, UV–vis-NIR, EPR). The neutral rhenium complex contains an isopropyl-shielded chloride ligand; its reversible oxidation was shown to involve a largely metal-based HOMO. One electron could be added reversibly to the diimine-based LUMO of all complexes, as evident from EPR spectroscopy. However, there was no evidence for the reversible acquisition of more than one electron by L in these complexes, ruling out an electron reservoir functionality with multielectron reactivity. DFT and TD-DFT calculations were performed for the [Re(CO)3Cl(L)]n redox series (n=+, 0, −).

Harnessing redox activity for the formation of uranium tris(imido) compounds.

Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals. Here we show that a redox-flexible pyridine(diimine) ligand can stabilize a series of highly reduced uranium coordination complexes by storing one, two or three electrons in the ligand. These species reduce organoazides easily to form uranium-nitrogen multiple bonds with the release of dinitrogen. The extent of ligand reduction dictates the formation of uranium mono-, bis- and tris(imido) products. Spectroscopic and structural characterization of these compounds supports the idea that electrons are stored in the ligand framework and used in subsequent reactivity. Computational analyses of the uranium imido products probed their molecular and electronic structures, which facilitated a comparison between the bonding in the tris(imido) structure and its tris(oxo) analogue.

Designing Catalysts for Nitrene Transfer Using Early Transition Metals and Redox-Active Ligands

In this Forum Article, we discuss the use of redox-active pincer-type ligands to enable multielectron reactivity, specifically nitrene group transfer, at the electron-poor metals tantalum and zirconium. Two analogous ligand platforms, [ONO] and [NNN], are discussed with a detailed examination of their similarities and differences and the structural and electronic constraints they impose upon coordination to early transition metals. The two-electron redox capabilities of these ligands enable the transfer of organic nitrenes to tantalum(V) and zirconium(IV) metal centers despite formal d(0) electron counts. Under the correct conditions, the resulting metal imido complexes can participate in further multielectron reactions such as imide reduction, nitrene coupling, or formal nitrene transfer to an isocyanide.

Manifestations of Noninnocent Ligand Behavior

The potential of redox-active ligands to behave "noninnocently" in transition-metal coordination compounds is reflected with respect to various aspects and situations. These include the question of establishing "correct" oxidation states, the identification and characterization of differently charged radical ligands, the listing of structural and other consequences of ligand redox reactions, and the distinction between barrierless delocalized "resonance" cases M(n)/L(n) ↔ M(n+1)L(n-1) versus separated valence tautomer equilibrium situations M(n)/L(n) ⇌ M(n+1)L(n-1). Further ambivalence arises for dinuclear systems with radical bridge M(n)(μ-L(•))M(n) versus mixed-valent alternatives M(n+1)(μ-L(-))M(n), for noninnocent ligand-bridged coordination compounds of higher nuclearity such as (μ(3)-L)M(3), (μ(4)-L)M(4), (μ-L)(4)M(4), or coordination polymers. Conversely, the presence of more than one noninnocently behaving ligand at a single transition-metal site in situations such as L(n)-M-L(n-1) or L(•)-M-L(•) may give rise to corresponding ligand-to-ligand interaction phenomena (charge transfer, electron hopping, and spin-spin coupling) and to redox-induced electron transfer with counterintuitive oxidation-state changes. The relationships of noninnocent ligand behavior with excited-state descriptions and perspectives regarding material properties and single-electron or multielectron reactivity are also illustrated briefly.

Chemically Reversible Four-Electron Oxidation and Reduction Utilizing Two Inorganic Functional Groups

Four-electron oxidation of the quadruply bonded W(2)(II,II) compound W(2)(2,2'-dipyridylamide)(4), 1, results in the formation of a novel, diamagnetic ditungsten terminal oxo compound [W(2)O(2,2'-dipyridylamide)(4)](2+), 2. In contrast to the chemical inertness of mononuclear tungsten oxo species, 2 undergoes a four-electron reduction including oxygen-atom transfer in reactions with excess tri-tert-butylphosphine in acetonitrile to recover 1. This unusual chemically reversible multielectron reactivity is ascribed to the cooperation of W-O and W-W multiple bonding.

Synthesis of Heteroleptic Uranium (μ−η6:η6-C6H6)2− Sandwich Complexes via Facile Displacement of (η5-C5Me5)1− by Ligands of Lower Hapticity and Their Conversion to Heteroleptic Bis(imido) Compounds

Reactivity studies on the sterically crowded [(C(5)Me(5))(2)U](2)(mu-eta(6):eta(6)-C(6)H(6)), 1, have revealed that eta(1)-ligands can displace one of the normally inert (eta(5)-C(5)Me(5))(1-) ligands in each metallocene unit to form a series of heteroleptic bimetallic sandwich complexes of nonplanar (C(6)H(6))(2-), namely, [(C(5)Me(5))(X)U](2)(mu-eta(6):eta(6)-C(6)H(6)), where X = N(SiMe(3))(2), OC(6)H(2)(CMe(3))(2)-2,6-Me-4, and CH(SiMe(3))(2). Displacement by an amidinate is also possible, that is, X = (i)PrNC(Me)N(i)Pr. This allows the multielectron reactivity of the (mu-eta(6):eta(6)-C(6)H(6))(2-) sandwich complexes to be studied as a function of ancillary ligands. Specifically, the reaction of 1 with K[N(SiMe(3))(2)], previously found to form {(C(5)Me(5))[(Me(3)Si)(2)N]U}(2)(C(6)H(6)), 2, also occurs with K[OC(6)H(2)(CMe(3))(2)-2,6-Me-4], Li[CH(SiMe(3))(2)], and Li[(i)PrNC(Me)N(i)Pr] to form {(C(5)Me(5))[4-Me-2,6-(Me(3)C)(2)C(6)H(2)O]U}(2)(C(6)H(6)), 3, {(C(5)Me(5))[(Me(3)Si)(2)CH]U}(2)(C(6)H(6)), 4, and {(C(5)Me(5))[(i)PrNC(Me)N(i)Pr]U}(2)(C(6)H(6)), 5, respectively. The reactivity of 2-5 vis-a-vis 1 has been compared with the substrates 1,3,5,7-cyclooctatetraene (C(8)H(8)) and 1-azidoadamantane (AdN(3)). Complex 1 acts as a six electron reductant to convert three equiv of C(8)H(8) to [(C(5)Me(5))(C(8)H(8))U](2)(mu-eta(3)-eta(3)-C(8)H(8)), whereas the sterically less crowded 2-5 provide only four electrons to reduce two equiv of C(8)H(8) generating U(4+) products of formula (C(5)Me(5))(X)U(C(8)H(8)). With AdN(3), complexes 1, 2, and 5 react similarly to form bis(imido) U(6+) complexes, (C(5)Me(5))(X)U(=NAd)(2). Complexes 2 and 5 also form the ligand redistribution product, (C(5)Me(5))(2)U(=NAd)(2). The reaction of 4 with AdN(3) generates at least three imido complexes: (C(5)Me(5))(2)U(=NAd)(2) from reduction and ligand redistribution, (C(5)Me(5))[AdN(3)CH(SiMe(3))(2)-kappa(2)N(1,2)]U(=NAd)(2), from reduction and insertion, and (C(5)Me(5))(eta(5):kappaNu-C(5)Me(4)CH(2)NAd)U(=NAd), from reduction, ligand redistribution, metalation, and insertion.