C(sp3)‐H and O−H bond breaking steps in the oxidation of 1,4‐cyclohexadiene and phenol by a Au(III)‐OH complex were studied computationally. The analysis reveals that for both types of bonds the ...initial X−H cleavage step proceeds via concerted proton coupled electron transfer (cPCET), reflecting electron transfer from the substrate directly to the Au(III) centre and proton transfer to the Au‐bound oxygen. This mechanistic picture is distinct from the analogous formal Cu(III)‐OH complexes studied by the Tolman group (J. Am. Chem. Soc. 2019, 141, 17236–17244), which proceed via hydrogen atom transfer (HAT) for C−H bonds and cPCET for O−H bonds. Hence, care should be taken when transferring concepts between Cu−OH and Au−OH species. Furthermore, the ability of Au−OH complexes to perform cPCET suggests further possibilities for one‐electron chemistry at the Au centre, for which only limited examples exist.
The mechanism of homolytic X‐H bond cleavage by a Au(III)‐OH complex was investigated computationally. Both C−H and O−H bonds are cleaved via a concerted proton coupled electron transfer mechanism (cPCET) rather than a hydrogen atom transfer mechanism (HAT). The results suggest that one‐electron events at gold may be more accessible than anticipated.
The base metal complex Bu4NFe(CO)3(NO) (TBAFe) catalyzes the rearrangement of vinyl and arylcyclopropanes both under thermal or photochemical conditions to give the corresponding vinyl or ...aryldihydrofurans in good to excellent yields. Under photochemical conditions the reaction is performed at room temperature. Spectroscopic investigations show that the metal carbonyl catalyst is not decarbonylated. The best performance was observed at a wavelength of 415 nm. icMRCI+Q analysis of the excited singlet and triplet states of the Fe(CO)3(NO) anion was performed and used to calculate the vertical excitation energies which are in good agreement with the experimental data. CASSCF analysis indicates that the Fe center in all excited states of the ferrate becomes more electrophilic while adopting a distorted tetrahedral configuration. Both aspects have a positive synergistic effect on the formation of the initial pi -complex with the incoming organic substrate.
A quantum chemical investigation of the Bu4NFe(CO)3(NO)‐catalyzed Cloke–Wilson rearrangement of vinyl cyclopropanes is reported. It was found that allylic CC bond activation can proceed through a ...SN2′ or SN2‐type mechanism. The application of the recently reported intrinsic bond orbital (IBO) method for all structures indicated that one FeN π bond is directly involved. Further analysis showed that during the reaction oxidation occurs at the NO ligand exclusively.
Iron works: The Bu4NFe(CO)3(NO)‐catalyzed Cloke–Wilson rearrangement of activated vinyl cyclopropanes occurs readily at 45 °C in the absence of added ligands. Quantum chemical calculations showed that the initial electron transfer originates from one FeN π bond resulting in formal oxidation of the NO ligand (see scheme).
By using a scale framework, we examine how cross-scale interactions influence the implementation of climate adaptation and mitigation actions in different urban sectors. Based on stakeholder ...interviews and content analysis of strategies and projects relevant to climate adaptation and mitigation in the cities of Copenhagen and Helsinki, we present empirical examples of synergies, conflicts and trade-offs between adaptation and mitigation that are driven by the cross-scale interactions. These examples show that jurisdictional and institutional scales shape the implementation of adaptation and mitigation strategies, projects and tasks at the management scale, creating benefits of integrated solutions, but also challenges. Investigating the linkages between adaptation and mitigation through a scale framework provides new knowledge for urban climate change planning and decision-making. The results increase the understanding of why adaptation and mitigation are sometimes handled as two separate policy areas and also why attempts to integrate the two policies may fail.
Using computational approaches, we qualitatively and quantitatively assess the bonding components of a series of experimentally characterized Au(I) diarylallenylidene complexes (N.Kim, ...R.A.Widenhoefer, Angew. Chem. Int. Ed. 2018, 57, 4722–4726). Our results clearly demonstrate that Au(I) engages only weakly in π‐backbonding, which is, however, a tunable bonding component. Computationally identified trends in bonding are clearly correlated with the substitution patterns of the aryl substituents in the Au(I) diarylallenylidene complexes and good agreement is found with the previously reported experimental data, such as IR spectra, 13C NMR chemical shifts and rates of decomposition together with their corresponding barrier heights, further substantiating the computational findings. The description of the bonding patterns in these complexes allow predictions of their spectroscopic features, their reactivity and stability.
Bonding disentangled: The bonding components of a series of experimentally characterized Au(I) diarylallenylidene complexes employing computational approaches. Trends found clearly correlate with the substitution patterns of the aryl substituents in the Au(I) diarylallenylidene complexes in agreement with reported experimental data.
Nonheme mononuclear hydroxoiron(III) species are important intermediates in biological oxidations, but well-characterized examples of synthetic complexes are scarce due to their instability or ...tendency to form μ-oxodiiron(III) complexes, which are the thermodynamic sink for such chemistry. Herein, we report the successful stabilization and characterization of a mononuclear hydroxoiron(III) complex, FeIII(OH)(TMC-py)2+ (3; TMC-py = 1-(pyridyl-2′-methyl)-4,8,11-trimethyl-1,4,8,11-tetrazacyclotetradecane), which is directly generated from the reaction of FeIV(O)(TMC-py)2+ (2) with 1,4-cyclohexadiene at −40 °C by H-atom abstraction. Complex 3 exhibits a UV spectrum with a λmax at 335 nm (ε ≈ 3500 M–1 cm–1) and a molecular ion in its electrospray ionization mass spectrum at m/z 555 with an isotope distribution pattern consistent with its formulation. Electron paramagnetic resonance and Mössbauer spectroscopy show 3 to be a high-spin Fe(III) center that is formed in 85% yield. Extended X-ray absorption fine structure analysis reveals an Fe–OH bond distance of 1.84 Å, which is also found in (TMC-py)FeIII–O–CrIII(OTf)3+ (4) obtained from the reaction of 2 with Cr(OTf)2. The S = 5/2 spin ground state and the 1.84 Å Fe–OH bond distance are supported computationally. Complex 3 reacts with 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) at −40 °C with a second-order rate constant of 7.1 M–1 s–1 and an OH/OD kinetic isotope effect value of 6. On the basis of density functional theory calculations, the reaction between 3 and TEMPOH is classified as a proton-coupled electron transfer as opposed to a hydrogen-atom transfer.
Ceric ammonium nitrate (CAN) or CeIV(NH4)2(NO3)6 is often used in artificial water oxidation and generally considered to be an outer‐sphere oxidant. Herein we report the spectroscopic and ...crystallographic characterization of (N4Py)FeIII‐O‐CeIV(OH2)(NO3)4+ (3), a complex obtained from the reaction of (N4Py)FeII(NCMe)2+ with 2 equiv CAN or (N4Py)FeIV=O2+ (2) with CeIII(NO3)3 in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the FeIV and CeIV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S=1 FeIV in 2 to S=5/2 in 3, which is found to be facile despite the formal spin‐forbidden nature of this process. This observation suggests that FeIV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.
Cerium caught in the redox act: CeIII reacts with FeIV(O) complexes in acetonitrile to form inner‐sphere FeIII‐O‐CeIV adducts. The transformation is easily reversed by adding water. The facile equilibrium is tuned by the ligands on the Ce center, and its existence shows that the CeIV/III and FeIV/III potentials must fall within a range of 0.9–1.1 V measured against a saturated calomel electrode.
The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pKa ...for the bound H2O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex FeIV(O)(TMC)(OH2)2+ (1‐OH2; TMC=1,4,8,11‐tetramethylcyclam) by determining the pKa of the H2O ligand. We establish that 1‐OH2 has a pKa of 6.9±0.5, a value that falls in between those found for FeIII(OH2)63+ and FeII(OH2)62+. This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1‐OH2, anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1‐OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.
Can FeIV(O)s swim? The pKa of an aqua ligand bound trans to an FeIV(O) unit has been determined and sheds light on the Lewis acidity of high‐valent iron‐oxo centers. For the conjugate base, a mixture of syn and anti tautomers is observed.
Computationally obtaining structural parameters along a reaction coordinate is commonly performed with Kohn‐Sham density functional theory which generally provides a good balance between speed and ...accuracy. However, CPU times still range from inconvenient to prohibitive, depending on the size of the system under study. Herein, the tight binding GFN2‐xTB method C. Bannwarth, S. Ehlert, S. Grimme, J. Chem. Theory Comput. 2019, 15, 1652 is investigated as an alternative to produce reasonable geometries along a reaction path, that is, reactant, product and transition state structures for a series of transformations involving gold complexes. A small mean error (1 kcal/mol) was found, with respect to an efficient composite hybrid‐GGA exchange‐correlation functional (PBEh‐3c) paired with a double‐ζ basis set, which is 2–3 orders of magnitude slower. The outlined protocol may serve as a rapid tool to probe the viability of proposed mechanistic pathways in the field of gold catalysis.
Rapid tool: An efficient computational protocol for the rapid evaluation of the viability of proposed mechanistic pathways in the field of gold catalysis is described. The combination of the tight binding GFN2‐xTB method for obtaining geometrical parameters and DFT (e. g. PW6B95‐D3(BJ)) for evaluating electronic energies is recommended.
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