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In this research article, we describe a 4H(+)/4e(−) electron-coupled-proton buffer (ECPB) based on Cu and a redox-active ligand. The protonated/reduced ECPB (complex 1: [Cu(8H(+)/14e(−))](1+)), consisting of Cu(I) with 2 equiv of the ligand ((cat)LH(4): 1,1′-(4,5-dimethoxy-1,2-phenylene)bis(3-(tert-butyl)urea)), reacted with H(+)/e(−) acceptors such as O(2) to generate the deprotonated/oxidized ECPB. The resulting compound, (complex 5: [Cu(4H(+)/10e(−))](1+)), was characterized by X-ray diffraction analysis, nuclear magnetic resonance ((1)H-NMR), and density functional theory, and it is electronically described as a cuprous bis(benzoquinonediimine) species. The stoichiometric 4H(+)/4e(−) reduction of 5 was carried out with H(+)/e(−) donors to generate 1 (Cu(I) and 2 equiv of (cat)LH(4)) and the corresponding oxidation products. The 1/5 ECPB system catalyzed the 4H(+)/4e(−) reduction of O(2) to H(2)O and the dehydrogenation of organic substrates in a decoupled (oxidations and reductions are separated in time and space) and a coupled fashion (oxidations and reductions coincide in time and space). Mechanistic analysis revealed that upon reductive protonation of 5 and oxidative deprotonation of 1, fast disproportionation reactions regenerate complexes 5 and 1 in a stoichiometric fashion to maintain the ECPB equilibrium.

The properties of metal/dioxygen species, which are key intermediates in oxidation catalysis, can be modulated by interaction with redox-inactive Lewis acids, but structural information about these adducts is scarce. Here we demonstrate that even mildly Lewis acidic alkali metal ions, which are typically viewed as innocent "spectators", bind strongly to a reactive cis-peroxo dicopper(II) intermediate. Unprecedented structural insight has now been obtained from X-ray crystallographic characterization of the "bare" CuII2(μ-η1:η1-O2) motif and its Li+, Na+, and K+ complexes. UV-vis, Raman, and electrochemical studies show that the binding persists in MeCN solution, growing stronger in proportion to the cation's Lewis acidity. The affinity for Li+ is surprisingly high (∼70 × 104 M-1), leading to Li+ extraction from its crown ether complex. Computational analysis indicates that the alkali ions influence the entire Cu-OO-Cu core, modulating the degree of charge transfer from copper to dioxygen. This induces significant changes in the electronic, magnetic, and electrochemical signatures of the Cu2O2 species. These findings have far-reaching implications for analyses of transient metal/dioxygen intermediates, which are often studied in situ, and they may be relevant to many (bio)chemical oxidation processes when considering the widespread presence of alkali cations in synthetic and natural environments.

In soluble methane monooxygenase enzymes (sMMO), dioxygen (O2) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate Q that performs the challenging oxidation of methane to meth...

In this research article, we describe the structure, spectroscopy, and reactivity of a family of copper complexes bearing bidentate redox-active ligands that contain H-bonding donor groups. Single-crystal X-ray crystallography shows that these tetracoordinate complexes are stabilized by intramolecular H-bonding interactions between the two ligand scaffolds. Interestingly, the Cu complexes undergo multiple reversible oxidation—reduction processes associated with the metal ion (Cu(I), Cu(II), Cu(III)) and/or the o-phenyldiamido ligand (L(2—), L(●—), L). Moreover, some of the Cu(11) complexes catalyze the aerobic oxidation of alcohols to aldehydes (or ketones) at room temperature. Our extensive mechanistic analysis suggests that the dehydrogenation of alcohols occurs via an unusual reaction pathway for galactose oxidase model systems, in which O(2) reduction occurs concurrently with substrate oxidation.

In soluble methane monooxygenase enzymes (

A reactive high-valent dinuclear nickel(IV) oxido bridged complex is reported that can be formed at room temperature by reaction of [(L) 2 Ni(II) 2 (μ-X) 3 ]X (X = Cl or Br) with NaOCl in methanol or acetonitrile (where L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The unusual Ni(IV) oxido species is stabilized within a dinuclear tris-μ-oxido-bridged structure as [(L) 2 Ni(IV) 2 (μ-O) 3 ] 2+ . Its structure and its reactivity with organic substrates are demonstrated through a combination of UV-vis absorption, resonance Raman, 1 H NMR, EPR, and X-ray absorption (near-edge) spectroscopy, ESI mass spectrometry, and DFT methods. The identification of a Ni(IV)-O species opens opportunities to control the reactivity of NaOCl for selective oxidations The Ubbo Emmius fund of the University of Groningen, the European Research Council (StG, no. 279549, W.R.B.), NWO for a VIDI grant (723.014.010, D.J.M. and M.T.), The Netherlands Ministry of Education, Culture and Science (Gravity program 024.001.035, W.R.B.), MINECO (CTQ2014-59212-P and CTQ2015-70851-ERC, M.S.), Gen- Cat (2014SGR1202, M.S.), FEDER (UNGI10-4E-801, M.S.), and COST action CM1305 “ECOSTBio” (W.R.B., COSTSTSM- CM1305-29045) are acknowledged for financial support

High valent Fe–OH species are often invoked as key intermediates but have only been observed in Compound II of cytochrome P450s. To further address the properties of non-heme FeIV–OH complexes we demonstrate the reversible protonation of a synthetic FeIV–oxo species containing a tris-urea tripodal ligand. The same protonated FeIV–oxo species can be prepared via oxidation, suggesting a putative FeV–oxo species was initially generated. Computational, Mössbauer, XAS, and NRVS studies indicate that protonation of the FeIV–oxo complex most likely occur on the tripodal ligand, which undergoes a structural change that results in the formation of a new intramolecular hydrogen bond with the oxido ligand that aids in stabilizing the protonated adduct. We suggest that similar species for protonated high valent Fe–oxo species may occur in the active sites of proteins. This finding further argues for caution when assigning unverified high valent Fe–OH species to mechanisms.

We have studied the Diels-Alder reaction of 1,3-butadiene with all nonequivalent bonds of Y3N@D3h-C78 at the BP86/TZP//BP86/DZP level of theory. The results obtained are compared with those extracted from a previous study on the free and Sc3N-endohedral C78 fullerene (J. Am. Chem. Soc. 2008, 130, 6206-6214). Our study shows that the most stable regioisomer for the Y3N compound is obtained for the reaction over a corannulene-type [5,6] bond (d), which exhibits the longest bond distance (1.47 A) and a large pyramidalization angle. As far as we know, this is the first case of a cycloaddition reaction where the most stable addition is obtained over one of the longest C-C bonds in the cage. In contrast to Sc3N@D3h-C78, where bonds close to the scandium atoms were destabilized, this bond d has one of the yttrium atoms in close contact. This preference for reacting with those bonds situated close to the yttrium atoms is due to two different factors: first, the D3h cage is extremely deformed, especially in the areas situated close to the yttrium atoms (which contain the most reactive bond d), so the attack reduces the strain energy of the cage; second, in the final adduct, the Y3N cluster gets additional space to adopt a more planar configuration. Since it has been shown (J. Phys. Chem. B 2007, 111, 3363-3369) that the D3h isomer is not the most favorable isomer for endohedral Y3N@C78 (at variance with Sc3N@C78), we also studied the more favorable C2 isomer. The latter contains [5,5] bonds, which are shown to be the most reactive bonds for cycloaddition, in contrast to previous theoretical predictions (J. Org. Chem. 2006, 71, 46-54). This preference for [5,5] bonds is observed for the C2 isomers of both endohedral (Sc3N, Y3N) and free C78 fullerene and is dictated by the fullerene strain energy. We therefore expect that the Diels-Alder reaction on other endohedral metallofullerenes that have already been synthesized (e.g., Tm3N@C2-C78, Dy3N@C2-C78) might lead to the same [5,5] adduct.

We report here for the first time a full comparison of the exohedral reactivity of a given fullerene and its parent trinitride template endohedral metallofullerene. In particular, we study the thermodynamics and kinetics for the Diels-Alder [4 + 2] cycloaddition between 1,3-butadiene and free D3h'-C78 fullerene and between butadiene and the corresponding endohedral D3h-Sc3N@C78 derivative. The reaction is studied for all nonequivalent bonds, in both the free and the endohedral fullerenes, at the BP86/TZP//BP86/DZP level. The change in exohedral reactivity and regioselectivity when a metal cluster is encapsulated inside the cage is profound. Consequently, the Diels-Alder reaction over the free fullerene and the endohedral derivative leads to totally different cycloadducts. This is caused by the metal nitride situated inside the fullerene cage that reduces the reactivity of the free fullerene and favors the reaction over different bonds.

Pentacoordinate phosphorus species play a key role in organic and biological processes. Yet, their nature is still not fully understood, in particular, whether they are stable, intermediate transition complexes (TC) or labile transition states (TS). Through systematic, theoretical analyses of elementary S(N)2@C, S(N)2@Si, and S(N)2@P reactions, we show how increasing the coordination number of the central atom as well as the substituents' steric demand shifts the S(N)2@P mechanism stepwise from a single-well potential (with a stable central TC) that is common for substitution at third-period atoms, via a triple-well potential (featuring a pre- and post-TS before and after the central TC), back to the double-well potential (in which pre- and postbarrier merge into one central TS) that is well-known for substitution reactions at carbon. Our results highlight the steric nature of the S(N)2 barrier, but they also show how electronic effects modulate the barrier height.

Recently, Vakonakis and LiWang (J. Am. Chem. Soc. 2004, 126, 5688) reported experimental evidence for stronger hydrogen bonds in RNA A:U than in DNA A:T base pairs, which was based on differences in NMR shielding for adenine C2. We have analyzed the proposed correlation between NMR shielding and hydrogen-bond strength using density functional theory. Although we agree with the conclusion that A:U is more strongly bound, we find no correlation between the hydrogen-bond strength and the NMR shielding of C2. Our study shows that NMR merely probes the presence/absence of the methyl group in thymine/uracil, without any relation to the strength of the hydrogen bonds involved. In other words, one cannot infer the Watson-Crick hydrogen-bond strength from the NMR shielding constant of adenine C2. Copyright © 2004 American Chemical Society.

Two metastable NiIII complexes, [NiIII(OAc)(L)] and [NiIII(ONO2)(L)] (L = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate, OAc = acetate), were prepared, adding to the previously prepared [NiIII(OCO2H)(L)], with the purpose of probing the properties of terminal late-transition metal oxidants. These high-valent oxidants were prepared by the one-electron oxidation of their NiII precursors ([NiII(OAc)(L)]- and [NiII(ONO2)(L)]-) with tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Fascinatingly, the reaction between any [NiII(X)(L)]- and NaOCl/acetic acid (AcOH) or cerium ammonium nitrate ((NH4)2[CeIV(NO3)6], CAN), yielded [NiIII(OAc)(L)] and [NiIII(ONO2)(L)], respectively. An array of spectroscopic characterizations (electronic absorption, electron paramagnetic resonance, X-ray absorption spectroscopies), electrochemical methods, and computational predictions (density functional theory) have been used to determine the structural, electronic, and magnetic properties of these highly reactive metastable oxidants. The NiIII-oxidants proved competent in the oxidation of phenols (weak O-H bonds) and a series of hydrocarbon substrates (some with strong C-H bonds). Kinetic investigation of the reactions with di-tert-butylphenols showed a 15-fold enhanced reaction rate for [NiIII(ONO2)(L)] compared to [NiIII(OCO2H)(L)] and [NiIII(OAc)(L)], demonstrating the effect of electron-deficiency of the O-ligand on oxidizing power. The oxidation of a series of hydrocarbons by [NiIII(OAc)(L)] was further examined. A linear correlation between the rate constant and the bond dissociation energy of the C-H bonds in the substrates was indicative of a hydrogen atom transfer mechanism. The reaction rate with dihydroanthracene (k2 = 8.1 M-1 s-1) compared favorably with the most reactive high-valent metal-oxidants, and showcases the exceptional reactivity of late transition metal-oxygen adducts This publication has emanated from research supported by the European Union (FP7-333948, ERC-2015-STG-678202). Research in the McDonald lab is supported in part by a research grant from Science Foundation Ireland (SFI/12/RC/2278), and in the Swart lab by the Ministerio de Economia y Competitividad (MINECO, Projects CTQ2014-59212-P and CTQ2015-70851-ERC), the DIUE of the Generalitat de Catalunya (Project 2014SGR1202), and the European Fund for Regional Development (FEDER, UNGI10-4E-801). XAS experiments were conducted at SSRL beamline 2-2 (SLAC National Accelerator Laboratory), with support from the DOE Office of Science (DE-AC02-76SF00515 and DE-SC0012704) and NIH (P30-EB-009998)