Density Functional Theory Study of Redox Pairs. 1. Dinuclear Iron Complexes That Undergo Multielectron Redox Reactions Accompanied by a Reversible Structural Change

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions

Though uranium is usually present in its +6 oxidation state (as uranyl ion) in aqueous solutions, its conversion to oxidation states such as +4 or +5 is a challenging task. Electrochemical reduction and axial oxo activation are the preferred methods to get stable unusual oxidation states of uranium in an aqueous medium. In previous studies, dicarboxylic acid has been used to stabilize UO2+ in aqueous alkaline solutions. In the present work, a diphosphonate ligand was chosen due to its higher complexing ability compared to that of the carboxylate ligands. Neptunium complexation studies with 2,6-pyridinediphosphonic acid (PyPOH) indicated the formation of different species at different pH values and the complexation facilitates disproportionation of NpO2+ to Np4+ and NpO22+ at pH 2. Hexavalent actinides form insoluble complexes in aqueous media at pH = 2, as confirmed by UO22+ complexation studies. The in situ complexation-driven precipitation resulted in conversion to pure Np4+ in aqueous media as the Np4+-PyPOH complex. A strong complexing ability of the PyPOH ligand toward the Np4+ ion is also seen for the stabilization of the electrochemically generated U4+ in aqueous medium under aerobic conditions. The U4+-PyPOH complex was found to be stable for 3 months. Raman, UV-vis, fluorescence, and cyclic voltametric studies along with density functional theory (DFT) calculations were done to get structural insights into the PyPOH complexes of actinides in different oxidation states.

A Modular Strategy for Expanding Electron-Sink Capacity in Noncanonical Cluster Assemblies

A modular synthetic strategy is described whereby organometallic complexes exhibiting considerable electron-sink capacity may be assembled by using only a few simple molecular components. The Fe₂(PPh₂)₂(CO)₅ fragment was selected as a common electroactive component and was assembled around aromatic cores bearing one, two, or three isocyanide functional groups, with the resultant complexes possessing electron-sink capacities of two, four, and six electrons, respectively. The latter complex is noteworthy in that its electron-sink capacity was found to rival that of large multinuclear clusters (e.g., [Ni₃₂C₆(CO)₃₆]^6– and [Ni₃₈Pt₆(CO)₄₈]^6–), which are often considered as benchmarks of electron-sink behavior. Moreover, the modular assembly bearing three Fe₂(PPh₂)₂(CO)₅ fragments was observed to undergo reduction to a hexaanionic state over a potential window of about −1.4 to −2.1 V (vs Fc/Fc⁺), the relatively compressed range being attributed to potential inversions operative during the addition of the second, fourth, and sixth electrons. Such complexes may be designated noncanonical clusters because they exhibit redox properties similar to those of large multinuclear clusters yet lack the extensive network of metal–metal bonds and the condensed metallic cores that typify the latter.

By use of a modular synthetic strategy and relatively few molecular components, organometallic complexes exhibiting considerable electron-sink capacity have been characterized. Complexes bearing one, two, or three Fe₂(PPh₂)₂(CO)₅ fragments bound to aromatic isocyanide cores were found to possess electron-sink capacities of two, four, and six electrons, respectively, the latter rivaling the electron-sink capacity of large polynuclear cluster benchmarks.

Electrocatalytic Behavior of Tetrathiafulvalene (TTF) and Extended Tetrathiafulvalene (exTTF) [FeFe] Hydrogenase Mimics

TTF- and exTTF-containing [(μ-S₂)Fe₂(CO)₆] complexes have been prepared by the photochemical reaction of TTF or exTTF and [(μ-S₂)Fe₂(CO)₆]. These complexes are able to interact with PAHs. In the absence of air and in acid media an electrocatalytic dihydrogen evolution reaction (HER) occurs, similarly to analogous [(μ-S₂)Fe₂(CO)₆] complexes. However, in the presence of air, the TTF and exTTF organic moieties strongly influence the electrochemistry of these systems. The reported data may be valuable in the design of [FeFe] hydrogenase mimics able to combine the HER properties of the [FeFe] cores with the unique TTF properties.

Rational Design of Fe2 (μ-PR2 )2 (L)6 Coordination Compounds Featuring Tailored Potential Inversion

It was recently discovered that some redox proteins can thermodynamically and spatially split two incoming electrons towards different pathways, resulting in the one-electron reduction of two different substrates, featuring reduction potential respectively higher and lower than the parent reductant. This energy conversion process, referred to as electron bifurcation, is relevant not only from a biochemical perspective, but also for the ground-breaking applications that electron-bifurcating molecular devices could have in the field of energy conversion. Natural electron-bifurcating systems contain a two-electron redox centre featuring potential inversion (PI), i. e. with second reduction easier than the first. With the aim of revealing key factors to tailor the span between first and second redox potentials, we performed a systematic density functional study of a 26-molecule set of models with the general formula Fe₂ (μ-PR₂ )₂ (L)₆ . It turned out that specific features such as i) a Fe-Fe antibonding character of the LUMO, ii) presence of electron-donor groups and iii) low steric congestion in the Fe's coordination sphere, are key ingredients for PI. In particular, the synergic effects of i)-iii) can lead to a span between first and second redox potentials larger than 700 mV. More generally, the "molecular recipes" herein described are expected to inspire the synthesis of Fe₂ P₂ systems with tailored PI, of primary relevance to the design of electron-bifurcating molecular devices.

Bridgehead isomer effects in bis(phosphido)-bridged diiron hexacarbonyl proton reduction electrocatalysts

The influence of the substitution, orientation and structure of the phosphido bridges in [Fe₂(CO)₆(μ-PR₂)₂] electrocatalysts of proton reduction has been studied. The isomers e,a-[Fe₂(CO)₆{μ-P(Ar)H}₂] (1a(Ar): Ar = Ph, 2'-methoxy-1,1'-binaphthyl (bn')), e,e-[Fe₂(CO)₆{μ-P(Ar)H}₂] (1b(Ar): Ar = Ph, bn') were isolated from reactions of iron pentacarbonyl and the corresponding primary phosphine, syntheses that also afforded the phosphinidene-capped tri-iron clusters, [Fe₃(CO)₉(μ-CO)(μ₃-Pbn')] (2) and [Fe₃(CO)₉(μ₃-PAr)₂] (3(Ar), Ar = Ph, bn'). A ferrocenyl (Fc)-substituted dimer [Fe₂(CO)₆{μ:μ'-1,2-(P(CH₂Fc)CH₂)₂C₆H₄}] (4), in which the two phosphido bridges are linked by an o-xylyl group, was also prepared. The molecular structures of complexes 1a(Ph), 1b(Ph), 1b(bn'), 2 and 4 were established by X-ray crystallography. All complexes have been examined as electrocatalysts for proton reduction using p-toluene sulfonic acid in tetrahydrofuran. Cyclic voltammograms of the dimers with acid exhibit two catalysis waves for proton reduction. The first wave, which appears at the potential of the primary reduction, reaches maximum current (turnover) at moderate acid concentrations and is rapidly overtaken by the second wave, which appears at more negative potential. Both of these reductive waves show an initial first order dependence on acid. The electrochemistry and electrocatalyses of the [Fe₂(CO)₆(μ-PR₂)₂] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms.

Revealing the thermodynamic driving force for ligand-based reductions in quinoids; conceptual rules for designing redox active and non-innocent ligands

Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal-ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or suppress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow rational redox-leveling, i.e. the moderation of redox potentials of ligand-centred electron transfer processes, potentially leading to catalysts with low overpotential in multielectron activation processes.

Understanding intrinsically irreversible, non-Nernstian, two-electron redox processes: a combined experimental and computational study of the electrochemical activation of platinum(IV) antitumor prodrugs

Six-coordinate Pt(IV)-complexes are prominent prodrug candidates for the treatment of various cancers where, upon two-electron reduction and loss of two axial ligands, they form more familiar, pharmacologically active four-coordinate Pt(II) drugs. A series of electrochemical experiments coupled with extensive density functional calculations has been employed to elucidate the mechanism for the two-electron reduction of Pt(IV)(NH3)2Cl2L2 to Pt(II)(NH3)2Cl2 (L = CH3COO(-), 1; L = CHCl2COO(-), 2; L = Cl(-), 3). A reliable estimate for the normal reduction potential E(o) is derived for the electrochemically irreversible Pt(IV) reduction and is compared directly to the quantum chemically calculated reduction potentials. The process of electron transfer and Pt-L bond cleavage is found to occur in a stepwise fashion, suggesting that a metastable six-coordinate Pt(III) intermediate is formed upon addition of a single electron, and the loss of both axial ligands is associated with the second electron transfer. The quantum chemically calculated reduction potentials are in excellent agreement with experimentally determined values that are notably more positive than peak potentials reported previously for 1-3.

The mechanism of guanine alkylation by nitrogen mustards: a computational study

The thermodynamics and kinetics for the monofunctional binding of nitrogen mustard class of anticancer drugs to purine bases of DNA were studied computationally using guanine and adenine as model substrates. Mechlorethamine and melphalan are used as model systems in order to better understand the difference in antitumor activity of aliphatic and aromatic mustards, respectively. In good agreement with experiments that suggested the accumulation of a reactive intermediate in the case of mechlorethamine, our model predicts a significant preference for the formation of corresponding aziridinium ion for mechlorethamine, while the formation of the aziridinium ion is not computed to be preferred when melphalan is used. Two effects are found that contribute to this difference. First, the ground state of the drug shows a highly delocalized lone pair on the amine nitrogen of the melphalan, which makes the subsequent cyclization more difficult. Second, because of the aromatic substituent connected to the amine nitrogen of melphalan, a large energy penalty has to be paid for solvation. A detailed study of energy profiles for the two-step mechanism for alkylation of guanine and adenine was performed. Alkylation of guanine is ∼6 kcal mol(-1) preferred over adenine, and the factors contributing to this preference were explained in our previous study of cisplatin binding to purine bases. A detailed analysis of energy profiles of mechlorethamine and melphalan binding to guanine and adenine are presented to provide an insight into rate limiting step and the difference in reactivity and stability of the intermediate in both nitrogen mustards, respectively.

Solvent decompositions and physical properties of decomposition compounds in Li-ion battery electrolytes studied by DFT calculations and molecular dynamics simulations

The density functional theory (DFT) calculations have been performed for the reduction decompositions of solvents widely used in Li-ion secondary battery electrolytes, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonates (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), including a typical electrolyte additive, vinylene carbonate (VC), at the level of B3LYP/6-311+G(2d,p), both in the gas phase and solution using the polarizable conductor calculation model. In the gas phase, the first electron reduction for the cyclic carbonates and for the linear carbonates is found to be exothermic and endothermic, respectively, while the second electron reduction is endothermic for all the compounds examined. On the contrary, in solution both first and second electron reductions are exothermic for all the compounds. Among the solvents and the additive examined, the likelihood of undergoing the first electron reduction in solution was found in the order of EC > PC > VC > DMC > EMC > DEC with EC being the most likely reduced. VC, on the other hand, is most likely to undergo the second electron reduction among the compounds, in the order of VC > EC > PC. Based on the results, the experimentally demonstrated effectiveness of VC as an excellent electrolyte additive was discussed. The bulk thermodynamic properties of two dilithium alkylene glycol dicarbonates, dilithium ethylene glycol dicarbonate (Li-EDC) and dilithium 1,2-propylene glycol dicarbonate (Li-PDC), as the major component of solid-electrolyte interface (SEI) films were also examined through molecular dynamics (MD) simulations in order to understand the stability of the SEI film. It was found that film produced from a decomposition of EC, modeled by Li-EDC, has a higher density, more cohesive energy, and less solubility to the solvent than the film produced from decomposition of PC, Li-PDC. Further, MD simulations of the interface between the decomposition compound and graphite suggested that Li-EDC has more favorable interactions with the graphite surface than Li-PDC. The difference in the SEI film stability and the behavior of Li-ion battery cycling among the solvents were discussed in terms of the molecular structures.

Electrochemical Oxidation of CoCp(CO)2: Radical−Substrate Reaction of a 17 e-/18 e- Pair and Production of a Unique Dimer Radical

Anodic oxidation of the important half-sandwich compound CoCp(CO)2, 1, has been studied under gentle electrolyte conditions, e.g., chlorinated hydrocarbons with weakly coordinating anion (WCA) supporting electrolyte anions. The 17-electron cation 1+ produced at E(1/2)(1) = 0.37 V vs FeCp2(0/+) undergoes a surprising reaction with neutral 1 to form the dimer radical cation [Co2Cp2(CO)4] +, 2+, which has a metal-metal bond unsupported by bridging ligands. The dimer radical is oxidized at a slightly more positive potential (E(1/2) = 0.47 V) to the corresponding dication 2(2+). Observation of the oxidation of 2+ is without precedent in confirming a radical-substrate (R-S) dimerization process by direct voltammetric detection of the R-S intermediate, K(eq) = 3 x 10(4) M(-1) for [2+]/[1][1+]. The R-S mechanism and the reaction products have been characterized by voltammetry, electrolysis, fiber-optic IR spectroscopy, and ESR measurements. DFT calculations indicate that removal of an electron from 1 results in rehybridization in 1+, thereby opening the metal center for interaction with the neutral compound 1, which has a relatively basic metal center. The LUMO of the dimer dication 2(2+) is metal-metal antibonding, and its half-occupancy in 2+ results in lengthening of the Co-Co bond from 2.64 A to 3.14 A. Inclusion of solvent in the (COSMO) calculations shows that solvation effects are necessary to account for the fact that E(1/2)(2) > E(1/2)(1). These results show the importance of medium effects in probing the fundamental redox chemistry of half-sandwich metal complexes.

Integration of EXAFS, Spectroscopic, and DFT Techniques for Elucidation of the Structure of Reactive Diiron Compounds

Strategies for modelling the EXAFS of a range of compounds with structural features common to the diiron subsite of the [FeFe] hydrogenase H-centre are compared, and this has allowed identification of highly constrained models that still permit expression of the main structural characteristics of the compounds. Despite giving self-consistent values of the iron–scatterer distances, the EXAFS analysis fails to give unambiguous identification of the stereochemistry and composition of the compounds, and this necessitates the integration of results obtained using other spectroscopic and computational approaches. The combination of infrared spectroscopy, EXAFS, and ab initio DFT calculations are shown to provide a particularly potent approach for the study of metal carbonyl compounds of this class. In this case the EXAFS-derived iron–scatterer distances provide the basis of the starting point for DFT geometry optimization calculations, and the final distances together with the calculated infrared spectrum provides a means of validating the computed geometry. The approach is applied both to compounds of known structure and to the examination of the unstable products of chemical or electrochemical reduction.

Dual Electron Uptake by Simultaneous Iron and Ligand Reduction in an N-Heterocyclic Carbene Substituted [FeFe] Hydrogenase Model Compound

An N-heterocyclic carbene containing [FeFe]H(2)ase model complex, whose X-ray structure displays an apical carbene, shows an unexpected two-electron reduction to be involved in its electrocatalytic dihydrogen production. Density functional calculations show, in addition to a one-electron Fe-Fe reduction, that the aryl-substituted N-heterocyclic carbene can accept a second electron more readily than the Fe-Fe manifold. The juxtaposition of these two one-electron reductions resembles the [FeFe]H(2)ase active site with an FeFe di-iron unit joined to the electroactive 4Fe4S cluster.

Electrochemical and theoretical study of the redox properties of transition metal complexes with {Pt2S2} cores

The oxidation processes undergone by the [Pt2(mu-S)2] core in [Pt2(P[intersection]P)2(mu-S)2](P[intersection]P = Ph2P(CH2)nPPh2, n= 2,3) complexes have been analysed on the basis of electrochemical measurements. The experimental results are indicative of two consecutive monoelectronic oxidations after which the [Pt2(mu-S)2] core evolves into [Pt2(mu-S2)]2+, containing a bridging disulfide ligand. However, the instability of the monoxidised [Pt2(P[intersection]P)2(mu-S)2]+ species formed initially, which converts into [Pt3(P[intersection]P)3(mu-S)2]2+, hampered the synthesis and characterisation of the mono and dioxidised species. These drawbacks have been surpassed by means of DFT calculations which have also allowed the elucidation of the structural features of the species obtained from the oxidation of [Pt2(P[intersection]P)2(mu-S)2] compounds. The calculated redox potentials corresponding to the oxidation processes are consistent with the experimental data obtained. In addition, calculations on the thermodynamics of possible processes following the degradation of [Pt2(P[intersection]P)2(mu-S)2]+ are fully consistent with the concomitant formation of monometallic [Pt(P[intersection]P)S2)] and trimetallic [Pt3(P[intersection]P)3(mu-S)2]2+ compounds. Extension of the theoretical study on the [Pt2Te2] core and comparisons with the results obtained for [Pt2S2] have given a more general picture of the behaviour of [Pt2X2](X = chalcogenide) cores subject to oxidation processes.

Two-electron transfer reactions in proteins: bridge-mediated and proton-assisted processes

Nonadiabatic two-electron transfer (TET) reactions through donor-bridge-acceptor (DBA) systems is investigated within the approximation of fast vibrational relaxation. For TET reactions in which the population of bridging states remains small (less than 10(-2)) it is demonstrated that a multiexponential transition process reduces to three-state kinetics. The transfer starts at the state with two excess electrons at the D center (D(2-)BA), goes through the intermediate (transient) state with one electron at the D center and one at the A center (D-BA-), and ends up with the two electrons at the A center (DBA2-). Furthermore, if the population of the intermediate state becomes also small the two-exponential kinetics can be transformed with high accuracy to single-exponential D-A TET kinetics. The related overall transfer rate contains contributions from stepwise and from concerted TET. The latter process is determined by a specific two-electron superexchange coupling incorporating the bridging states (D-B-A and DB-A-) as well as the intermediate state (D-BA-). As an example, the reduction of micothione reductase by nicotinamide adenine dinucleotide phosphate is analyzed. Existing experimental data can be explained if one assumes that the proton-assisted reduction of the enzyme is realized by the concerted TET mechanism.

Density functional calculations on electronic circular dichroism spectra of chiral transition metal complexes

Time-dependent density functional theory (TD-DFT) has for the first time been applied to the computation of circular dichroism (CD) spectra of transition metal complexes, and a detailed comparison with experimental spectra has been made. Absorption spectra are also reported. Various Co(III) complexes as well as [Rh(en)(3)](3+) are studied in this work. The resulting simulated CD spectra are generally in good agreement with experimental spectra after corrections for systematic errors in a few of the lowest excitation energies are applied. This allows for an interpretation and assignment of the spectra for the whole experimentally accessible energy range (UV/vis). Solvent effects on the excitations are estimated via inclusion of a continuum solvent model. This significantly improves the computed excitation energies for charge-transfer bands for complexes of charge +3, but has only a small effect on those for neutral or singly charged complexes. The energies of the weak d-to-d transitions of the Co complexes are systematically overestimated due to deficiencies of the density functionals. These errors are much smaller for the 4d metal complex. Taking these systematic errors and the effect of a solvent into consideration, TD-DFT computations are demonstrated to be a reliable tool in order to assist with the assignment and interpretation of CD spectra of chiral transition metal complexes.

Hydroxylation of methane by non-heme diiron enzymes: molecular orbital analysis of C-H bond activation by reactive intermediate Q

The electronic structures of key species involved in methane hydroxylation performed by the hydroxylase component of soluble methane monooxygenase (sMMO), as proposed previously on the basis of high-level density functional theory, were investigated. The reaction starts with initial approach of methane at one of the bridging oxo atoms in intermediate Q, a di(mu-oxo)diiron(IV) unit. This step is accompanied by a proton-coupled outer-sphere transfer of the first electron from a C-H sigma-bond in methane to one of the metal centers. The second electron transfer, also an outer-sphere electron transfer process, occurs along a two-component reaction pathway. Both redox reactions are strongly coupled to structural distortions of the diiron core. The electronic consequence and driving force of these distortions are intuitively explained by using the computed Kohn-Sham orbitals in the broken-symmetry framework to incorporate the experimentally observed antiferromagnetic coupling of the unpaired electrons at the metal centers. The broken-symmetry orbital scheme is essential for describing the C-H bond activation process in a consistent and complete manner, enabling derivation of both an intuitive and quantitative understanding of the most salient electronic features that govern the details of the hydroxylation.

Metal-metal and carbon-carbon bonds as potential components of molecular batteries

The reductive coupling of [M(salophen)] derivatives, where M is an early transition metal and salophen is N,N'-o-phenylenebis(salicylideneaminato) dianion, led to the formation of dimers linked through C-C and M-M bonds. Both of these bonds can potentially function as electron reservoirs: each bond can be used as a reversible source of a pair of electrons under the condition that it is not chemically transformed by the incoming substrate which functions as an electron acceptor. To explore this potential function as well as the competition in the redox processes between C-C and M-M bonds within the same molecular framework, we investigated the reduction of [(tBu4-salophen)NbCl3] (1) and [(tBu4-salophen)MoCl2] (7) as model compounds. In the former case, the reduction led to [(Nb-Nb)(tBu4-*salophen2*)] (2) which contains both a Nb-Nb bond (2.6528(7) A) and two C-C bonds across two imino groups of the ligand. Complex 2 can be reduced further to a transient compound 5 that contains an Nb=Nb bond. In the second case, the reduction of 7 by two electrons led to [(Mo[triplebond]Mo)(tBu4-salophen)2] (8), which does not contain any C-C linkages between the two salophen units. Complexes 2 and 5 are able to transfer one pair and two pairs of electrons, respectively, to give compounds 3, 4, and 6, with the consequent cleavage of the Nb-Nb and Nb=Nb bonds. In the present case, it is surprising that the C-C bonds do not participate in the reduction of the substrates. A careful theoretical treatment anticipates, both in the case of 1 and 7, the preferential formation of metal-metal bonds upon reduction. This is indeed the case for 7, but not for 1, where the formation of C-C bonds competes with that of M-M bonds, the latter being the first ones, however, to be involved in electron-transfer reactions. The theoretical approach allowed us to investigate the possibility of intramolecular electron transfer from C-C bonds to M-M bonds and vice versa.

Dicopper(I) complexes with reduced states of 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine: crystal structures and spectroscopic properties of the free ligand, a radical species, and a complex of the 1,4-dihydro form

The complexes [(mu-bmtz(*-))[Cu(PPh(3))(2)](2)](BF(4)) (1) and [(mu-H(2)bmtz)[Cu(PPh(3))(2)](2)](BF(4))(2) (2) (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine and H(2)bmtz = 1,4-dihydro-3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine) were obtained as stable materials that could be crystallized for structure determination. 1.2 CH(2)Cl(2): C(84)H(70)BCl(4)Cu(2)F(4)N(8)P(4); monoclinic, C2/c; a = 26.215(7) A, b = 22.122(6) A, c = 18.114(5) A, beta = 133.51(1) degrees; Z = 4. 2.CH(2)Cl(2): C(83)H(70)B(2)Cl(2)Cu(2)F(8)N(8)P(4); triclinic, P1; a = 10.948(2) A, b = 12.067(2) A, c = 30.287(6) A, alpha = 93.82(3) degrees, beta = 94.46(3) degrees, gamma = 101.60(3) degrees; Z = 2. Bmtz itself was also structurally characterized (C(10)H(6)N(8); monoclinic, P2(1)/c; a = 3.8234(8) A, b = 10.147(2) A, c = 13.195(3) A, beta = 94.92(3) degrees; Z = 2). Whereas the radical complex ion contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex of H(2)bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine ring.