Crystal Structure and C-H Bond-Cleaving Reactivity of a Mononuclear CoIV-Dinitrate Complex

Modelling an Fe-III High-Valent Pincer-type Transition Metal Complex for Dehydrogenation of Ammonia-Borane

Development of efficient and cost-effective catalysts for the dehydrogenation of Ammonia-Borane (AB) has been a challenge which affects the advancement of the hydrogen economy. Over the last decades, pincer-type transition metal complexes have been known to show promising results in catalyzing many chemical reactions ranging from CO2 reduction to C-H bond activation. In this work we investigate the ability of a high-valent Ni-III-Cl complex (complex 1) for the dehydrogenating AB. Our results show that complex 1 can dehydrogenate two equiv. of AB under reaction conditions slightly higher than room temperature. Although the abstraction of H2 from AB can occur at room temperature, higher temperature is required due to relatively higher free-energy barriers for the formation of molecular H2. However, when the Ni-III center is substituted by a Fe-III center (complex 2), AB dehydrogenation can occur at room temperature for one equiv. of AB with a free-energetic span of 21.07 kcal/mol, but this does not remain the same for the second catalytic cycle for complex 2 and the free-energy energetic span increases to 36.1 kcal/mol. Therefore, for the initial cycle of AB dehydrogenation, the Fe-III complex has better functionality and this work exhibits the impact of metal mono-substitution, specifically Fe in activating AB dehydrogenation at room temperature and further paves the way for simple modelling of transition metal-based complexes as catalysts for such reactions.

Synthesis and Reactivity of a Non-Heme μ-Oxodicobalt(IV) Complex

A mononuclear CoIII complex (1) of a bisamide-bisalkoxide donor ligand was synthesized and thoroughly characterized. The reaction of 1 with 0.5 equiv. of m-chloroperbenzoic acid (m-CPBA) in acetonitrile at -40 °C resulted in the formation of a μ-oxodicobalt(IV) complex (2), which was characterized by an array of spectroscopic techniques, including X-ray absorption spectroscopy which revealed a short Co-Ooxo distance of 1.67 Å. Reactivity studies of 2 towards oxidation/oxygenation of hydrocarbon C-H bond and triphenylphosphine or thioanisole derivatives have been examined. UV-vis spectroscopy studies showed the appearance of clear isosbestic points during the oxidation of substrates together with a neat transformation of 2 to 1. Detailed kinetic investigations established that 2 follows a Concerted Proton-Electron Transfer (CPET) mechanism for hydrocarbon oxidation and has a weak electrophilic character. Catalytic behavior of 1 was noted towards the oxygen atom transfer reactions. This study showcases the spectroscopic investigation and reactivity studies of a CoIV(μ-O)CoIV moiety. Although the FeIV analog of such a core has been described before, the study describes the first example with a CoIV center.

N-Oxide Coordination to Mn(III) Chloride

We report on the synthesis and characterization of Mn(III) chloride (MnIIICl3) complexes coordinated with N-oxide ylide ligands, namely trimethyl-N-oxide (Me3NO) and pyridine-N-oxide (PyNO). The compounds are reactive and, while isolable in the solid-state at room temperature, readily decompose into Mn(II). For example, "[MnIIICl3(ONMe3)n]" decomposes into the 2D polymeric network compound complex salt [MnII(µ-Cl)3MnII(µ-ONMe3)]n[MnII(µ-Cl)3]n·(Me3NO·HCl)3n (4). The reaction of MnIIICl3 with PyNO forms varied Mn(III) compounds with PyNO coordination and these react with hexamethylbenzene (HMB) to form the chlorinated organic product 1-cloromethyl-2,3,4,5,6-pentamethylbenzene (8). In contrast to N-oxide coordination to Mn(III), the reaction between [MnIIICl3(OPPh3)2] and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) resulted in electron transfer-forming d5 manganate of the [TEMPO] cation instead of TEMPO-Mn(III) adducts. The reactivity affected by N-oxide coordination is discussed through comparisons with other L-MnIIICl3 complexes within the context of reduction potential.

Mechanistic Insight of High-Valent First-Row Transition Metal Complexes for Dehydrogenation of Ammonia Borane

Designing an efficient and cost-effective catalyst for ammonia borane (AB) dehydrogenation remains a persistent challenge in advancing a hydrogen-based economy. Transition metal complexes, known for their C-H bond activation capabilities, have emerged as promising candidates for AB dehydrogenation. In this study, we investigated two recently synthesized C-H activation catalysts, 1 (CoIV-dinitrate complex) and 2 (NiIV-nitrate complex), and demonstrated their efficacy for AB dehydrogenation. Using density functional theory calculations and a detailed analysis, we elucidated the AB dehydrogenation mechanism of these complexes. Our results revealed that both complexes 1 and 2 can efficiently dehydrogenate AB at room temperature, although the abstraction of molecular H2 from these complexes requires slightly elevated temperatures. We utilized H2 binding free energy calculations to identify potentially active sites and observed that complex 2 can release two equivalents of H2 at a temperature slightly higher than room temperature. Furthermore, we investigated AB dehydrogenation kinetics and thermodynamics in iron (Fe)-substituted systems, complexes 3 and 4. Our results showed that the strategic alteration of the central metal atom, replacing Ni in complex 2 with Fe in complex 4, resulted in enhanced kinetics and thermodynamics for AB dehydrogenation in the initial cycle. These results underscore the potential of high-valent first-row transition metal complexes for facilitating AB dehydrogenation at room temperature. Additionally, our study highlights the beneficial impact of incorporating iron into such mononuclear systems, enhancing their catalytic activity.

Generation, Characterization and Reactivity of a High-Valent Mononuclear Cobalt(IV)-Diazide Complex

High-valent Fe(IV)=O intermediates of metalloenzymes have inspired numerous efforts to generate synthetic analogs to mimic and understand their substrate oxidation reactivities. However, high-valent M(IV) complexes of late transition metals are rare. We have recently reported a novel Co(IV)-dinitrate complex (1-NO3) that activates sp3 C-H bonds up to 87 kcal/mol. In this work, we have shown that the nitrate ligands in 1-NO3 can be replaced by azide, a more basic coordinating base, resulting in the formation of a more potent Co(IV)-diazide species (1-N3) that reacts with substrates (hydrocarbons and phenols) at faster rate constants and activates stronger C-H bonds than the parent complex 1-NO3. We have characterized 1-N3 employing a combination of spectroscopic and computational approaches. Our results clearly show that the coordination of azide leads to the modulation of the Co(IV) electronic structure and the Co(IV/III) redox potential. Together with the higher basicity of azide, these thermodynamic parameters contribute to the higher driving forces of 1-N3 than 1-NO3 for C-H bond activation. Our discoveries are thus insightful for designing more reactive bio-inspired high-valent late transition metal complexes for activating inert aliphatic hydrocarbons.

Synthesis, Characterization, and Reactivity of a Highly Oxidative Mononuclear Manganese(IV)-Bis(Fluoro) Complex

Recently, transition-metal terminal nonoxo complexes have shown a remarkable ability to activate and functionalize C-H bonds via proton-coupled electron transfer (PCET). Here we report the first example of a mononuclear manganese(IV) bis(fluoro) complex bearing a tetradentate pyridinophane ligand, [MnIV(TBDAP)(F)2]2+ (3), with an X-ray single crystal structure and physicochemical characterization. The manganese(IV) bis(fluoro) complex has a very high reduction potential of 1.61 V vs SCE, thereby enabling the four-electron oxidation of mesitylene to 3,5-dimethylbenzaldehyde. Kinetic studies, including the kinetic isotope effect and employment of other toluene derivatives, reveal the electron transfer (ET)-driven PCET in the C-H bond activation of mesitylene by 3. This novel metal halide intermediate would be prominently valuable for expanding transition-metal halide chemistry.

C(sp3)-H cyanation by a formal copper(iii) cyanide complex

High-valent metal oxo complexes are prototypical intermediates for the activation and hydroxylation of alkyl C-H bonds. Substituting the oxo ligand with other functional groups offers the opportunity for additional C-H functionalization beyond C-O bond formation. However, few species aside from metal oxo complexes have been reported to both activate and functionalize alkyl C-H bonds. We herein report the first example of an isolated copper(iii) cyanide complex (LCu^IIICN) and its C-H cyanation reactivity. We found that the redox potential (E _ox) of substrates, instead of C-H bond dissociation energy, is a key determinant of the rate of PCET, suggesting an oxidative asynchronous CPET or ETPT mechanism. Among substrates with the same BDEs, those with low redox potentials transfer H atoms up to a million-fold faster. Capitalizing on this mechanistic insight, we found that LCu^IIICN is highly selective for cyanation of amines, which is predisposed to oxidative asynchronous or stepwise transfer of H⁺/e^-. Our study demonstrates that the asynchronous effect of PCET is an appealing tool for controlling the selectivity of C-H functionalization.

A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [Ni^III(PaPy₃*)]²⁺ (1) in acetone/CH₃CN (v/v 19:1). 1 was synthesized by reacting [Ni^II(PaPy₃*)]⁺ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H₂O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H⁺ to the PaPy₃* ligand of 2; the one-electron reduction potential (E_red) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the E_red value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA^•+ to 2 due to the binding of H⁺ to the PaPy₃* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.

C-H Bond Activation by a Mononuclear Nickel(IV)-Nitrate Complex

The recent focus on developing high-valent non-oxo-metal complexes for late transition metals has proven to be an effective strategy to study the rich chemistry of these high-valent species while bypassing the synthetic challenges of obtaining the oxo-metal counterparts. In our continuing work of exploring late transition metal complexes of unusually high oxidation states, we have obtained in the present study a formal mononuclear Ni(IV)-nitrate complex (2) upon 1-e^- oxidation of its Ni(III) derivatives (1-OH and 1-NO₃). Characterization of these Ni complexes by combined spectroscopic and computational approaches enables deep understanding of their geometric and electronic structures, bonding interactions, and spectroscopic properties, showing that all of them are square planar complexes and exhibit strong π-covalency with the amido N-donors of the N3 ligand. Furthermore, results obtained from X-ray absorption spectroscopy and density functional theory calculations provide strong support for the assignment of the Ni(IV) oxidation state of complex 2, albeit with strong ligand-to-metal charge donation. Notably, 2 is able to oxidize hydrocarbons with C-H bond strength in the range of 76-92 kcal/mol, representing a rare example of high-valent late transition metal complexes capable of activating strong sp³ C-H bonds.

Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands

To elucidate the oxidation behavior of the 2,2′-azobisphenolate (azp) ligand, a series of homoleptic 1:2 AlIII complexes of four azp derivatives (L1) with 5,5′-dichloro-, 5,5′-dimethyl-, 5,5′-di-t-butyl-, 3,3′,5,5′-tetramethyl-substituents and of one imino derivative (L2) were synthesized and obtained as TPP[Al(L)2]·solvent (TPP = tetraphenylphosphonium ion). The X-ray crystal structure analyses showed that the two ONO-tridentate ligands were meridionally coordinated to a central AlIII ion in an almost perpendicular manner to give a homoleptic octahedral coordination structure in all the AlIII complexes. The proton nuclear magnetic resonance spectra suggested that all the AlIII complexes retained the homoleptic coordination structure in solution. From the cyclic voltammetry measurements in dichloromethane solutions, all the AlIII complexes with the azp ligands showed two partially reversible oxidation waves, and an additional reversible or partially reversible reduction wave. The substitution effects on the first oxidation and reduction peak potentials were revealed in the AlIII complexes with the azp ligands. On the other hand, the imino complex showed a partially reversible oxidation wave accompanying a film deposition. The density functional theory (DFT) calculations indicated that the molecular orbital (MO) coefficients of the frontier MOs in the AlIII complexes were present on the ligands and were absent on the AlIII ion. These results confirmed that the azp ligands are susceptible to oxidation and can give a relatively stable oxidation species depending upon substituent effects.

Cobalt-Catalyzed Regioselective para-Amination of Azobenzenes via Nucleophilic Aromatic Substitution of Hydrogen

The metal-catalyzed nucleophilic aromatic substitution of hydrogen (S_NArH) via coordination of the substituent on the aromatic ring to the metal catalyst, in terms of reactivity, substrate type, and reaction selectivity, complements the transition metal-catalyzed C-H functionalization that proceeds via C-H metalation but remains an elusive target. Described herein is the development of an unprecedented cobalt-catalyzed para-selective amination of azobenzenes, which is essentially a metal-promoted S_NArH process as revealed by Hammett analysis, thus illustrating the concept that coordination of the substituent on the arene ring to the metal catalyst may result in electrophilic activation of the arene ring toward S_NArH. This cobalt-catalyzed protocol allows the use of a variety of both aliphatic amines and anilines as aminating reagents, tolerates electronically diverse substituents of azobenzene, and furnishes the corresponding products in good yields with a regiospecific selectivity for para-amination.

Selective oxygenation of C-H and CC bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes

Four cobalt(II)-carboxylate complexes [(6-Me₃-TPA)Co^II(benzoate)](BPh₄) (1), [(6-Me₃-TPA)Co^II(benzilate)](ClO₄) (2), [(6-Me₃-TPA)Co^II(mandelate)](BPh₄) (3), and [(6-Me₃-TPA)Co^II(MPA)](BPh₄) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me₃-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H₂O₂-dependent selective oxygenation of C-H and CC bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C-H bonds and epoxidation of alkenes. An electrophilic cobalt-oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)-acetonitrile complex [(6-Me₃-TPA)Co^II(CH₃CN)₂](ClO₄)₂ (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H₂O₂ followed by the oxidation of substrates by a metal-based oxidant.

Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

A Co/Fe system efficiently catalyses the oxidation of C–H bonds with m-CPBA. The nitric acid promoter hampers the m-CPBA homolysis, suppressing the free radical activity. Experimental and computational data evidence a concerted oxidation mechanism.

Enzyme-Like Hydroxylation of Aliphatic C-H Bonds From an Isolable Co-Oxo Complex

The selective hydroxylation of aliphatic C-H bonds remains a challenging but broadly useful transformation. Nature has evolved systems that excel at this reaction, exemplified by cytochrome P450 enzymes, which use an iron-oxo intermediate to activate aliphatic C-H bonds with k₁ > 1400 s^-1 at 4 °C. Many synthetic catalysts have been inspired by these enzymes and are similarly proposed to use transition metal-oxo intermediates. However, most examples of well-characterized transition metal-oxo species are not capable of reacting with strong, aliphatic C-H bonds, resulting in a lack of understanding of what factors facilitate this reactivity. Here, we report the isolation and characterization of a new terminal Co^III-oxo complex, PhB(^AdIm)₃Co^IIIO. Upon oxidation, a transient Co^IV-oxo intermediate is generated that is capable of hydroxylating aliphatic C-H bonds with an extrapolated k₁ for C-H activation >130 s^-1 at 4 °C, comparable to values observed in cytochrome P450 enzymes. Experimental thermodynamic values and DFT analysis demonstrate that, although the initial C-H activation step in this reaction is endergonic, the overall reaction is driven by an extremely exergonic radical rebound step, similar to what has been proposed in cytochrome P450 enzymes. The rapid C-H hydroxylation reactivity displayed in this well-defined system provides insight into how hydroxylation is accomplished by biological systems and similarly potent synthetic oxidants.

Opening the CoIII,IV2(μ-O)2 Diamond Core by Lewis Bases Leads to Enhanced C-H Bond Cleaving Reactivity

The high-valent diiron(IV) intermediate Q is the key oxidant that cleaves strong C-H bonds of methane in the catalytic cycle of soluble methane monooxygenase (sMMO). sMMO-Q was previously reported as a bis-μ-oxo Fe^IV₂(μ-O)₂ diamond core but was recently described to have an open core with a long Fe···Fe distance. We recently reported a high-valent Co^III,IV₂(μ-O)₂ diamond core complex (1) that is highly reactive with sp³ C-H bonds. In this work, we demonstrated that the C-H bond cleaving reactivity of 1 can be further enhanced by introducing a Lewis base X, affording faster kinetic rate constants and the ability to cleave stronger C-H bonds compared to 1. We proposed that 1 first reacts with X in a fast equilibrium to form an open core species X-Co^III-O-Co^IV-O (1-X). We were able to characterize 1-X using EPR spectroscopy and DFT calculations. 1-X exhibited an S = 1/2 EPR signal distinct from that of the parent complex 1. DFT calculations showed that 1-X has an open core with the spin density heavily delocalized in the Co^IV-O unit. Moreover, 1-X has a more favorable thermodynamic driving force and a smaller activation barrier than 1 to carry out C-H bond activation reactions. Notably, 1-X is at least 4 orders of magnitude more reactive than its diiron open core analogues. Our findings indicate that the diamond core isomerization is likely a practical enzymatic strategy to unmask the strong oxidizing power of sMMO-Q necessary to attack the highly inert C-H bonds of methane.