Hydrazine Formation via Coupling of a Nickel(III)-NH2 Radical

Regulating Access to Active Sites via Hydrogen Bonding and Cation-Dipole Interactions: A Dual Cofactor Approach to Switchable Catalysis

Hydrogen bonding networks are ubiquitous in biological systems and play a key role in controlling the conformational dynamics and allosteric interactions of enzymes. Yet in small organometallic catalysts, hydrogen bonding rarely controls ligand binding to the metal center. In this work, a hydrogen bonding network within a well-defined organometallic catalyst works in concert with cation-dipole interactions to gate substrate access to the active site. An ammine ligand acts as one cofactor, templating a hydrogen bonding network within a pendent crown ether and preventing the binding of strong donor ligands, such as nitriles, to the nickel center. Sodium ions are the second cofactor, disrupting hydrogen bonding to enable switchable ligand substitution reactions. Thermodynamic analyses provide insight into the energetic requirements of the different supramolecular interactions that enable substrate gating. The dual cofactor approach enables switchable catalytic hydroamination of crotononitrile. Systematic comparisons of catalysts with varying structural features provide support for the critical role of the dual cofactors in achieving on/off catalysis with substrates containing strongly donating functional groups that might otherwise interfere with switchable catalysts.

Oxidation-induced ambiphilicity triggers N-N bond formation and dinitrogen release in octahedral terminal molybdenum(v) nitrido complexes

Coupling of octahedral, terminal d1 molybdenum(v) nitrido complexes supported by a dianionic pentadentate ligand via N-N bond formation to give μ-dinitrogen complexes was found to be thermodynamically feasible but faces significant kinetic barriers. However, upon oxidation, a kinetically favored nucleophilic/electrophilic N-N bond forming mechanism was enabled to give monocationic μ-dinitrogen dimers. Computational and experimental evidence for this "oxidation-induced ambiphilic nitrido coupling" mechanism is presented. The factors influencing release of dinitrogen from the resulting μ-dinitrogen dimers were also probed and it was found that further oxidation to a dicationic species is required to induce (very rapid) loss of dinitrogen. The mechanistic path discovered for N-N bond formation and dinitrogen release follows an ECECC sequence (E = "electrochemical step"; C = "chemical step"). Experimental evidence for the intermediacy of a highly electrophilic, cationic d0 molybdenum(vi) nitrido in the N-N bond forming mechanism via trapping with an isonitrile reagent is also discussed. Together these results are relevant to the development of molecular catalysts capable of mediating ammonia oxidation to dihydrogen and dinitrogen.

Divergent Bimetallic Mechanisms in Copper(II)-Mediated C-C, N-N, and O-O Oxidative Coupling Reactions

Copper-catalyzed aerobic oxidative coupling of diaryl imines provides a route for conversion of ammonia to hydrazine. The present study uses experimental and density functional theory computational methods to investigate the mechanism of N-N bond formation, and the data support a mechanism involving bimolecular coupling of Cu-coordinated iminyl radicals. Computational analysis is extended to CuII-mediated C-C, N-N, and O-O coupling reactions involved in the formation of cyanogen (NC-CN) from HCN, 1,3-butadiyne from ethyne (i.e., Glaser coupling), hydrazine from ammonia, and hydrogen peroxide from water. The results reveal two different mechanistic pathways. Heteroatom ligands with an uncoordinated lone pair (iminyl, NH2, OH) undergo charge transfer to CuII, generating ligand-centered radicals that undergo facile bimolecular radical-radical coupling. Ligands lacking a lone pair (CN and CCH) form bridged binuclear diamond-core structures that undergo C-C coupling. This mechanistic bifurcation is rationalized by analysis of spin densities in key intermediates and transition states, as well as multiconfigurational calculations. Radical-radical coupling is especially favorable for N-N coupling owing to energetically favorable charge transfer in the intermediate and thermodynamically favorable product formation.

Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex

We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6-tri-tert-butylphenoxyl (^t Bu₃ ArO⋅) as a H atom acceptor to cleave the N-H bond of a coordinated NH₃ ligand up to 56 equiv of N₂ per Ni center can be generated. Employing the N-oxyl radical 2,2,6,6-(tetramethylpiperidin-1-yl)oxyl (TEMPO⋅) as the H-atom acceptor, up to 15 equiv of N₂ per Ni center are formed. A bridging Ni-hydrazine product identified by isotopic nitrogen (¹⁵ N) studies and supported by computational models indicates the N-N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]-NH₂ fragments. Ni-mediated hydrazine disproportionation to N₂ and NH₃ completes the catalytic cycle.

Hydrogen Atom Abstraction from an OsII(NH3)2 Complex Generates an OsIV(NH2)2 Complex: Experimental and Computational Analysis of the N-H Bond Dissociation Free Energies and Reactivity

Double hydrogen atom abstraction from (TMP)Os^II(NH₃)₂ (TMP = tetramesitylporphyrin) with phenoxyl or nitroxyl radicals leads to (TMP)Os^IV(NH₂)₂. This unusual bis(amide) complex is diamagnetic and displays an N-H resonance at 12.0 ppm in its ¹H NMR spectrum. ¹H-¹⁵N correlation experiments identified a ¹⁵N NMR spectroscopic resonance signal at -267 ppm. Experimental reactivity studies and density functional theory calculations support relatively weak N-H bonds of 73.3 kcal/mol for (TMP)Os^II(NH₃)₂ and 74.2 kcal/mol for (TMP)Os^III(NH₃)(NH₂). Cyclic voltammetry experiments provide an estimate of the pK_a of [(TMP)Os^III(NH₃)₂]⁺. In the presence of Barton's base, a current enhancement is observed at the Os(III/II) couple, consistent with an ECE event. Spectroscopic experiments confirmed (TMP)Os^IV(NH₂)₂ as the product of bulk electrolysis. Double hydrogen atom abstraction is influenced by π donation from the amides of (TMP)Os^IV(NH₂)₂ into the d orbitals of the Os center, favoring the formation of (TMP)Os^IV(NH₂)₂ over N-N coupling. This π donation leads to a Jahn-Teller distortion that splits the energy levels of the d_xz and d_yz orbitals of Os, results in a low-spin electron configuration, and leads to minimal aminyl character on the N atoms, rendering (TMP)Os^IV(NH₂)₂ unreactive toward amide-amide coupling.

Weakening the N-H Bonds of NH3 Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride

Weakening and cleaving N-H bonds is crucial for improving molecular ammonia (NH₃) oxidation catalysts. We report the synthesis and H-atom-abstraction reaction of bis(ammonia)chromium porphyrin complexes Cr(TPP)(NH₃)₂ and Cr(TMP)(NH₃)₂ (TPP = 5,10,15,20-tetraphenyl-meso-porphyrin and TMP = 5,10,15,20-tetramesityl-meso-porphyrin) using bulky aryloxyl radicals. The triple H-atom-abstraction reaction results in the formation of Cr^V(por)(≡N), with the nitride derived from NH₃, as indicated by UV-vis and IR and single-crystal structural determination of Cr(TPP)(≡N). Subsequent oxidation of this chromium(V) nitrido complex results in the formation of Cr^III(por), with scission of the Cr≡N bond. Computational analysis illustrates the progression from Cr^II to Cr^V and evaluates the energetics of abstracting H atoms from Cr^II-NH₃ to generate Cr^V≡N. The formation and isolation of Cr^V(por)(≡N) illustrates the stability of these species and the need to chemically activate the nitride ligand for atom transfer or N-N coupling reactivity.

Photoinduced, Copper-Catalyzed Enantioconvergent Alkylations of Anilines by Racemic Tertiary Electrophiles: Synthesis and Mechanism

Transition-metal catalysis of substitution reactions of alkyl electrophiles by nitrogen nucleophiles is beginning to emerge as a powerful strategy for synthesizing higher-order amines, as well as controlling their stereochemistry. Herein, we report that a readily accessible chiral copper catalyst (commercially available components) can achieve the photoinduced, enantioconvergent coupling of a variety of racemic tertiary alkyl electrophiles with aniline nucleophiles to generate a new C-N bond with good ee at the fully substituted stereocenter of the product; whereas this photoinduced, copper-catalyzed coupling proceeds at -78 °C, in the absence of light and catalyst, virtually no C-N bond formation is observed even upon heating to 80 °C. The mechanism of this new catalytic enantioconvergent substitution process has been interrogated with the aid of a wide array of tools, including the independent synthesis of proposed intermediates and reactivity studies, spectroscopic investigations featuring photophysical and EPR data, and DFT calculations. These studies led to the identification of three copper-based intermediates in the proposed catalytic cycle, including a chiral three-coordinate formally copper(II)-anilido (DFT analysis points to its formulation as a copper(I)-anilidyl radical) complex that serves as a persistent radical that couples with a tertiary organic radical to generate the desired C-N bond with good enantioselectivity.

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺ ([1-NH₃]⁺), which processes electrocatalytic ammonia oxidation to N₂ and H₂ at a mild potential. Through establishing elementary e^-/H⁺ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH₂]⁺, together with its conjugated Lewis acid, [1-NH₃]²⁺, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH₂]⁺ induces the reaction of the parent amido unit with NH₃. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

Copper(II) Perfluorinated Carboxylate Complexes with Small Aliphatic Amines as Universal Precursors for Nanomaterial Fabrication

Copper(II) carboxylate compounds with ethylamine and isopropylamine of the general formula [Cu2(RNH2)2(µ-O2CRf)4], where R = Et, iPr, and Rf = CnF2n+1, n = 1-6, were characterised in the condensed and gas phases by electron impact mass spectrometry (EI MS), IR spectroscopy, and thermal analysis. A mass spectra analysis confirmed the presence of metallated species in the gas phase. Among the observed fragments, the pseudomolecular ions [Cu2(RNH2)2(µ-O2CRf)3]+ were found, which suggests the dimeric structure of the studied complexes with axially N-coordinated ethyl- or isopropylamine molecules and bridging perfluorinated carboxylates. TGA studies demonstrated that copper transfer to the gas phase occurs even under atmospheric pressure. The temperature range of the [Cu2(RNH2)2(µ-O2CRf)4] and other copper carriers detection, observed in variable temperature infrared spectra, depends on the type of amine. The possible mechanisms of the decomposition of the tested compounds are proposed. The copper films were produced without additional reducing agents despite using Cu(II) CVD precursors in the chemical vapor deposition experiments. The layers of the gel-like complexes were fabricated in both spin- and dip-coating experiments, resulting in copper or copper oxide materials when heated. Dinuclear copper(II) carboxylate complexes with ethyl- and isopropylamine [Cu2(RNH2)2(µ-O2CRf)4] can be applied for the formation of metal or metal oxide materials, also in the nanoscale, by vapour and 'wet' deposition methods.

Uncovering Redox Non-innocent Hydrogen-Bonding in Cu(I)-Diazene Complexes

The life-sustaining reduction of N₂ to NH₃ is thermoneutral yet kinetically challenged by high-energy intermediates such as N₂H₂. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [^xHetTpCu]₂(μ-N₂H₂) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (^xHet) such as pyridine and a bridging trans-N₂H₂ ligand at copper(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [^pyMeTpCu]₂(μ-N₂H₂) while low-temperature ¹H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the ^xHet pendant negligibly influences the electronic structure of ^xHetTpCu^I centers in ^xHetTpCu(CNAr^2,6-Me₂) complexes that lack H-bonding as judged by nearly indistinguishable ν(CN) frequencies (2113-2117 cm^-1). Nonetheless, H-bonding in the corresponding [^xHetTpCu]₂(μ-N₂H₂) complexes results in marked changes in ν(NN) (1398-1419 cm^-1) revealed through resonance Raman studies. Due to the closely matched N-H BDEs of N₂H₂ and the pyH⁰ cation radical, the aromatic N-heterocyclic pendants may encourage partial H-atom transfer (HAT) from N₂H₂ to ^xHet through redox-non-innocent H-bonding in [^xHetTpCu]₂(μ-N₂H₂). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N₂H₂ to the ^xHet pendants to generate tautomeric [^xHetHTpCu]₂(μ-N₂) complexes, identifying metal-assisted concerted dual HAT as a thermodynamically favorable pathway for N₂/N₂H₂ interconversion.

Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron Complex Featuring Cis Coordination Sites

The goal of using ammonia as a solar fuel motivates the development of selective ammonia oxidation (AO) catalysts for fuel cell applications. Herein, we describe Fe-mediated AO electrocatalysis with [(bpyPy₂Me)Fe(MeCN)₂]²⁺, exhibiting the highest turnover number (TON) reported to date for a molecular system. To improve on our recent report of a related iron AO electrocatalyst, [(TPA)Fe(MeCN)₂]²⁺ (TON of 16), the present [(bpyPy₂Me)Fe(MeCN)₂]²⁺ system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains cis-labile sites and a dominant low-spin population at the Fe(II) state. The latter is posited to mitigate demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the catalytic conditions. Additionally, the [(bpyPy₂Me)Fe(MeCN)₂]²⁺ system exhibits a substantially faster AO rate (ca. 50×) at significantly lower (∼250 mV) applied bias compared to [(TPA)Fe(MeCN)₂]²⁺. Electrochemical data are consistent with an initial E₁ net H-atom abstraction step that furnishes the cis amide/ammine complex [(bpyPy₂Me)Fe(NH₂)(NH₃)]²⁺, followed by the onset of catalysis at E₂. Theoretical calculations suggest the possibility of N-N bond formation via multiple thermodynamically plausible pathways, including both reductive elimination and ammonia nucleophilic attack. In sum, this study underscores that Fe, an earth-abundant metal, is a promising metal for further development in metal-mediated AO catalysis by molecular systems.