Redox-Switchable Hydroelementation of a Cobalt Complex Supported by a Ferrocene-Based Ligand

Activation of robust bonds by carbonyl complexes of Mn, Fe and Co

Metal carbonyl complexes possess among the most storied histories of any compound class in organometallic chemistry. Nonetheless, these old dogs continue to be taught new tricks. In this Feature, we review the historic discoveries and recent advances in cleaving robust bonds (e.g., C-H, C-O, C-F) using carbonyl complexes of three metals: Mn, Fe, and Co. The use of Mn, Fe, and Co carbonyl catalysts in controlling selectivity during hydrofunctionalization reactions is also discussed. The chemistry of these earth-abundant metals in the field of robust bond functionalization is particularly relevant in the context of sustainability. We expect that an up-to-date perspective on these seemingly simple organometallic species will emphasize the wellspring of reactivity that continues to be available for discovery.

NiII and CuII complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere

Herein, we report the synthesis, spectroscopic characterization and electrochemical investigation of the Ni^II and Cu^II complexes of a novel Sal ligand bearing two ferrocene moieties attached at its diimine linker, M(Sal)^Fc. The electronic spectra of M(Sal)^Fc are near identical to its phenyl-substituted counterpart, M(Sal)^Ph, indicating the ferrocene moieties exist in the secondary coordination sphere of M(Sal)^Fc. The cyclic voltammograms of M(Sal)^Fc exhibit an additional two-electron wave in comparison to M(Sal)^Ph, which is assigned to the sequential oxidation of the two ferrocene moieties. The chemical oxidation of M(Sal)^Fc, monitored by low temperature UV-vis spectroscopy, supports the formation of a mixed valent Fe^IIFe^III species followed by a bis(ferrocenium) species upon sequential addition of one and two equivalents of chemical oxidant. The addition of a third equivalent of oxidant to Ni(Sal)^Fc yielded intense near-IR transitions that are indicative of the formation of a fully delocalized Sal-ligand radical (Sal˙), while the same addition to Cu(Sal)^Fc yielded a species that is currently under further spectroscopic investigation. These results suggest the oxidation of the ferrocene moieties of M(Sal)^Fc does not affect the electronic structure of the M(Sal) core, and these are thus in the secondary coordination sphere of the overall complex.

ABC and ABAB Block Copolymers by Electrochemically Controlled Ring-Opening Polymerization

An electrochemically controlled synthesis of multiblock copolymers by alternating the redox states of (salfan)Zr(O^tBu)₂ (salfan = 1,1'-di(2-tert-butyl-6-N-methylmethylenephenoxy)ferrocene) is reported. Aided by electrochemistry with a glassy carbon working electrode, an in situ potential switch alters the catalyst's oxidation state and its subsequent monomer (l-lactide, β-butyrolactone, or cyclohexene oxide) selectivity in one pot. Various multiblock copolymers were prepared, including an ABAB tetrablock copolymer, poly(cyclohexene oxide-b-lactide-b-cyclohexene oxide-b-lactide), and an ABC triblock copolymer, poly(hydroxybutyrate-b-cyclohexene oxide-b-lactide). The polymers produced using this technique are similar to those produced via a chemical redox reagent method, displaying moderately narrow dispersities (1.1-1.5) and molecular weights ranging from 7 to 26 kDa.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

This review summarizes the most noteworthy achievements in the field of C–O and C–N bond formation by hydroalkoxylation and hydroamination reactions on unactivated alkenes (including 1,2- and 1,3-dienes) promoted by earth-abundant 3d transition metal catalysts based on manganese, iron, cobalt, nickel, copper and zinc. The relevant literature from 2012 until early 2021 has been covered.

Highly chemoselective synthesis of hindered amides via cobalt-catalyzed intermolecular oxidative hydroamidation

α-Tertiary amides are of great importance for medicinal chemistry. However, they are often challenging to access through conventional methods due to reactivity and chemoselectivity issues. Here, we report a single-step approach towards such amides via cobalt-catalyzed intermolecular oxidative hydroamidation of unactivated alkenes, using nitriles of either solvent- or reagent-quantities. This protocol is selective for terminal alkenes over groups that rapidly react under known carbocation amidation conditions such as tertiary alcohols, electron-rich alkenes, ketals, weak C−H bonds, and carboxylic acids. Straightforward access to a diverse array of hindered amides is demonstrated, including a rapid synthesis of an aminoadamantane-derived pharmaceutical intermediate.

α-Tertiary amides are common in bioactive natural products and pharmaceuticals, but challenging to access by conventional methods. Here, the authors report a single-step approach toward α-tertiary amides via cobalt-catalyzed intermolecular oxidative hydroamidation of unactivated alkenes.

Redox Neutral Radical-Relay Cobalt Catalysis toward C-H Fluorination and Amination

A redox neutral radical-relay cobalt-catalyzed intramolecular C-H fluorination of N-fluoroamides featuring the in situ formed cobalt fluorides as the latent radical fluorinating agents is reported. Moreover, the reactivity of such a cobalt catalysis could be diverted from C-H fluorination to amination by engineering substrates' conformational flexibility. Preliminary mechanistic studies (UV-vis spectroscopy, cyclic voltammetry studies and DFT calculations, etc.) support the reaction proceeding a redox neutral radical-relay mechanism.

Zirconium complexes supported by a ferrocene-based ligand as redox switches for hydroamination reactions

The synthesis of (thiolfan*)Zr(NEt2)2 (thiolfan* = 1,1'-bis(2,4-di-tert-butyl-6-thiophenoxy)ferrocene) and its catalytic activity for intramolecular hydroamination are reported. In situ oxidation and reduction of the metal complex results in reactivity towards different substrates. The reduced form of (thiolfan*)Zr(NEt2)2 catalyzes hydroamination reactions of primary aminoalkenes, whereas the oxidized form catalyzes hydroamination reactions of secondary aminoalkenes.

Cobalt-Catalyzed Intermolecular Hydrofunctionalization of Alkenes: Evidence for a Bimetallic Pathway

A functional group tolerant cobalt-catalyzed method for the intermolecular hydrofunctionalization of alkenes with oxygen- and nitrogen-based nucleophiles is reported. This protocol features a strategic use of hypervalent iodine(III) reagents that enables a mechanistic shift from conventional cobalt-hydride catalysis. Key evidence was found supporting a unique bimetallic-mediated rate-limiting step involving two distinct cobalt(III) species, from which a new carbon-heteroatom bond is formed.

Redox-Switchable Ring-Opening Polymerization with Ferrocene Derivatives

Switchable catalysts incorporate stimuli-responsive features and allow synthetic tasks that are difficult or impossible to accomplish in other ways. They mimic biological processes in that they can provide both spatial and temporal control, unlike most reactions promoted by human-made catalysts that usually occur according to carefully optimized conditions. In the area of switchable catalysis, redox-switchable ring-opening polymerization (ROP) has attracted much attention, emerging as a powerful strategy for the development of environmentally friendly biodegradable copolymers, especially those containing blocks with complementary properties. Controlling the sequence and regularity of each copolymeric building block can affect the material properties significantly since they are directly related to the respective microstructures. Such control can be exerted with a well-designed redox-switchable catalyst by timing the oxidation and reduction events. In highly selective systems, one form of the catalyst reacts with a monomer until the redox state of the catalyst is altered, at which point the altered state of the catalyst reacts with another monomer. The reaction time may be varied from one cycle to another to generate various designer multiblock copolymers. The first instance of redox-mediated ROP was described by N. Long and co-workers in 2006. This example, as well as many early reported redox-switchable catalysts, could only achieve an on/off switch of activity toward a single monomer or substrate. However, our efforts brought on a general strategy for designing redox-switchable metal complexes that can catalyze different reactions in two oxidation states. In recent years, our contributions to this research field led to the synthesis of several multiblock copolymers prepared from biorenewable resources. This Account provides an overview of reported redox-switchable polymerization catalysts that allow for complementary reactivity in different oxidation states and highlights the potential of this strategy in preparing biodegradable materials. First, we define the field of redox-switchable catalysis and illustrate the design and significance of our ferrocene-chelating ligands, in which the oxidation state of iron in ferrocene can control the reactivity of the resulting metal complexes remotely. Next, we illustrate recent advances in the synthesis of new biodegradable copolymers including (1) how to tune the activity of the ROP catalysts by exploring various metal centers and ferrocene-based ligand combinations; (2) how to synthesize new multiblock copolymers of cyclic esters, epoxides, and carbonates by redox-switchable ROP; and (3) how to understand the mechanism of these reactions by discussing both experimental and theoretical investigations. By the application and development of redox-switchable strategies, various novel materials and reactions can be expected in the future.

Redox- and light-switchable N-heterocyclic carbenes: a “soup-to-nuts” course on contemporary structure–activity relationships

This Feature Article offers in-depth, design-to-application discussions of redox-switchable N-heterocyclic carbenes that have been field tested.

Exploring Oxidation State-Dependent Selectivity in Polymerization of Cyclic Esters and Carbonates with Zinc(II) Complexes

Neutral zinc alkoxide complexes show high activity toward the ring-opening polymerization of cyclic esters and carbonates, to generate biodegradable plastics applicable in several areas. Herein, we use a ferrocene-chelating heteroscorpionate complex in redox-switchable polymerization reactions, and we show that it is a moderately active catalyst for the ring-opening polymerization of L-lactide, ɛ-caprolactone, trimethylene carbonate, and δ-valerolactone. Uniquely for this type of catalyst, the oxidized complex has a similar polymerization activity as the corresponding reduced compound, but displays significantly different rates of reaction in the case of trimethylene carbonate and δ-valerolactone. Investigations of the oxidized compound suggest the presence of an organic radical rather than an Fe(III) complex. Electronic structure and density functional theory (DFT) calculations were performed to support the proposed electronic states of the catalytic complex and to help explain the observed reactivity differences. The catalyst was also compared with a monomeric phenoxide complex to show the influence of the phosphine-zinc interaction on catalytic properties.

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Dimeric and monomeric zinc heteroscorpionate species were prepared

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Monomeric zinc complex gives irreversible chemical redox and is inactive toward ROP

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Dimeric zinc complex gives reversible chemical redox and is inactive toward ROP

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Oxidation of the zinc benzoxide species occurs at the phosphine moiety

Preparation of multiblock copolymers via step-wise addition of l-lactide and trimethylene carbonate

The synthesis of up to pentablock copolymers from various combinations of l-lactide and trimethylene carbonate was accomplished using a dinuclear zinc complex, and the physical, thermal, and mechanical properties of the resulting copolymers evaluated.

Poly(l-lactide) (PLA) is a bioderived and biodegradable polymer that has limited applications due to its hard and brittle nature. Incorporation of 1,3-trimethylene carbonate into PLA, in a block copolymer fashion, improves the mechanical properties, while retaining the biodegradability of the polymer, and broadens its range of applications. However, the preparation of 1,3-trimethylene carbonate (TMC)/l-lactide (LA) copolymers beyond diblock and triblock structures has not been reported, with explanations focusing mostly on thermodynamic reasons that impede the copolymerization of TMC after lactide. We discuss the preparation of multiblock copolymers via the ring opening polymerization (ROP) of LA and TMC, in a step-wise addition, by a ferrocene-chelating heteroscorpionate zinc complex, {[fc(PPh₂)(BH[(3,5-Me)₂pz]₂)]Zn(μ-OCH₂Ph)}₂ ([(fc^P,B)Zn(μ-OCH₂Ph)]₂, fc = 1,1′-ferrocenediyl, pz = pyrazole). The synthesis of up to pentablock copolymers, from various combinations of LA and TMC, was accomplished and the physical, thermal, and mechanical properties of the resulting copolymers evaluated.

First-Row Late Transition Metals for Catalytic Alkene Hydrofunctionalisation: Recent Advances in C-N, C-O and C-P Bond Formation

This review provides an outline of the most noteworthy achievements in the area of C-N, C-O and C-P bond formation by hydroamination, hydroalkoxylation, hydrophosphination, hydrophosphonylation or hydrophosphinylation reaction on unactivated alkenes (including 1,2- and 1,3-dienes) promoted by first-row late transition metal catalytic systems based on manganese, iron, cobalt, nickel, copper and zinc. The relevant literature from 2009 until mid-2017 has been covered.

Transition metal redox switches for reversible "on/off" and "slow/fast" single-molecule magnet behaviour in dysprosium and erbium bis-diamidoferrocene complexes

We present an in-depth experimental study of a new class of heterometallic, redox-switchable single-molecule magnets (SMMs).

Single-molecule magnets (SMMs) are considered viable candidates for next-generation data storage and quantum computing. Systems featuring switchability of their magnetization dynamics are particularly interesting with respect to accessing more complex logic gates and device architectures. Here we show that transition metal based redox events can be exploited to enable reversible switchability of slow magnetic relaxation of magnetically anisotropic lanthanide ions. Specifically, we report anionic homoleptic bis-diamidoferrocene complexes of Dy³⁺ (oblate) and Er³⁺ (prolate) which can be reversibly oxidized by one electron to yield their respective charge neutral redox partners (Dy: [1]^–, 1; Er: [2]^–, 2). Importantly, compounds 1 and 2 are thermally stable which allowed for detailed studies of their magnetization dynamics. We show that the Dy³⁺[1]^–/1 system can function as an “on”/“off” or a “slow”/“fast” redox switchable SMM system in the absence or presence of applied dc fields, respectively. The Er³⁺ based [2]^–/2 system features “on”/“off” switchability of SMM properties in the presence of applied fields. Results from electrochemical investigations, UV-vis-NIR spectroscopy, and ⁵⁷Fe Mössbauer spectroscopy indicate the presence of significant electronic communication between the mixed-valent Fe ions in 1 and 2 in both solution and solid state. This comparative evaluation of redox-switchable magnetization dynamics in low coordinate lanthanide complexes may be used as a potential blueprint toward the development of future switchable magnetic materials.

Investigation of redox switchable titanium and zirconium catalysts for the ring opening polymerization of cyclic esters and epoxides

The synthesis and characterization of (thiolfan*)Zr(O^tBu)₂ (thiolfan* = 1,1′-di(2,4-di-tert-butyl-6-thiophenoxide)ferrocene) is reported, as well as its activity toward the ring-opening polymerizations of l-lactide and ε-caprolactone.

Palladium complexes of ferrocene-based phosphine ligands as redox-switchable catalysts in Buchwald–Hartwig cross-coupling reactions

Redox-switchable catalysis: Palladium(ii) complexes of two differently substituted [1]phosphaferrocenophanes FcPR (R = Mes, biaryl) and of diphenylferrocenyl phosphine Ph₂PFc were applied in redox-switchable Buchwald–Hartwig cross-coupling reactions.