The active site of low-temperature methane hydroxylation in iron-containing zeolites.

An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species-α-Fe(ii) and α-O-are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive 'spectator' iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function-producing what is known in the context of metalloenzymes as an 'entatic' state-might be a useful way to tune the activity of heterogeneous catalysts.

Electronic Structure of Neutral and Anionic Iron-Nitrosyl Corrole. A Multiconfigurational and Density Matrix Renormalization Group Investigation.

An elaborate study with multireference second-order perturbation theory has been performed to elucidate the electronic structure and relative energy of three relevant states of FeNO corroles, namely the S = 0 ground state, the lowest S = 1 state, and the anion S = 1/2 state. On the basis of CASSCF and DMRG calculations with an active space including up to 37 orbitals, the electronic structure of the ground state was analyzed, with special emphasis on the diradical nature of the Fe-corrole and Fe-NO bonds. The results essentially confirm an earlier suggestion from B3LYP of a non-innocent corrole•2- bound to an {FeNO}7 unit, although the contribution of diradical character to the iron-corrole bond is found to be limited to 35%. This limited diradical character explains the high relative energy (16.5 kcal/mol) of the corresponding triplet state, where the corrole•2- is ferromagnetically coupled to the S = 1/2 {FeNO}7 unit. Consistent with experimental findings, reduction is found to take place at the corrole ligand, with a calculated electron affinity of 52.5 kcal/mol. The results obtained from the correlated calculations were also compared to DFT with a broad range of functionals.

Mechanism of the highly effective peptide bond hydrolysis by MOF-808 catalyst under biologically relevant conditions.

Efficient and selective hydrolysis of inert peptide bonds is of paramount importance. MOF-808, a metal-organic framework based on Zr6 nodes, can hydrolyze peptide bonds efficiently under biologically relevant conditions. However, the details of the catalyst structure and of the underlying catalytic reaction mechanism are challenging to establish. By means of DFT calculations we first investigate the speciation of the Zr6 nodes and identify the nature of ligands that bind to the Zr6O8H4-x core in aqueous conditions. The core is predicted to strongly prefer a Zr6O8H4 protonation state and to be predominantly decorated by bridging formate ligands, giving Zr6(µ3-O)4(µ3-OH)4(BTC)2(HCOO)6 and Zr6(µ3-O)4(µ3-OH)4(BTC)2(HCOO)5(OH)(H2O) as the most favorable structures at physiological pH. The GlyGly peptide can bind MOF in several different ways, with the preferred structure involving coordination through the terminal carboxylate analogously to the binding mode of formate ligand. The pre-reactive binding mode in which the amide carbonyl oxygen coordinates the metal core lies 7 kcal higher in free energy. The preferred reaction pathway is predicted to have two close-lying transition states, either of which could be the rate-determining step: nucleophilic attack on the amide carbon atom and C-N bond breaking, with calculated relative free energies of 31 and 32 kcal mol-1, respectively. Replacement of formate by water and hydroxide at the Zr6 node is predicted to be possible, but does not appear to play a role in the hydrolysis mechanism.

Modern quantum chemistry with [Open]Molcas.

MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.

Electronic Structure of N-Bridged High-Valent Diiron-Oxo.

Density functional theory (DFT) and an advanced ab initio technique based on density matrix renormalization group (DMRG-CASPT2) were employed to investigate a reactive N-bridged high-valent diiron-oxo species involved in H-abstraction reactions. We studied in detail two important doublet states, the ground state with two iron(IV) centers and a mixed valence FeV -FeIV excited state. We found that the latter state is low-lying. Furthermore, its electronic structure and spin density imply that it has significantly higher H-abstraction reactivity than the ground state. This low-lying excited state might be the reason behind the high oxidation reactivity of this diiron-oxo species towards methane.

Chemical Mimics of Aspartate-Directed Proteases: Predictive and Strictly Specific Hydrolysis of a Globular Protein at Asp-X Sequence Promoted by Polyoxometalate Complexes Rationalized by a Combined Experimental and Theoretical Approach.

Creating efficient and residue-directed artificial proteases is a challenging task due to the extreme inertness of the peptide bond, combined with the difficulty of achieving specific interactions between the catalysts and the protein side chains. Herein we report strictly site-selective hydrolysis of a multi-subunit globular protein, hemoglobin (Hb) from bovine blood, by a range of ZrIV -substituted polyoxometalates (Zr-POMs) in mildly acidic and physiological pH solutions. Among 570 peptide bonds in Hb, selective cleavage was observed at only eleven sites, each occurring at Asp-X peptide bonds located in the positive patches on the protein surface. The molecular origins of the observed Asp-X selectivity were rationalized by means of molecular docking, DFT-based binding, and mechanistic studies on model peptides. The proposed mechanism of hydrolysis involves coordination of the amide oxygen to ZrIV followed by a direct nucleophilic attack of the side chain carboxylate group on the C-terminal amide carbon atom with formation of a cyclic anhydride, which is further hydrolyzed to give the reaction products. The activation energy for the cleavage of the structurally related Glu-X sequence compared to Asp-X was calculated to be higher by 1.4 kcal mol-1 , which corresponds to a difference of about one order of magnitude in the rates of hydrolysis. The higher activation energy is attributed to the higher strain present in the six-membered ring of glutaric anhydride (Glu-X), as compared to the five-membered ring of the succinic anhydride (Asp-X) intermediate. Similarly, the cleavage at X-Asp and X-Glu bonds are predicted to be kinetically less likely as the corresponding activation energies were 6 kcal mol-1 higher, explaining the experimentally observed selectivity. The synergy between the negatively charged polyoxometalate cluster, which binds at positive patches on protein surfaces, and selective activation of Asp-X peptide bonds located in these regions by ZrIV ions, results in a novel class of artificial proteases with aspartate-directed reactivity, which is very rare among naturally occurring proteases.

Spectroscopic Identification of the α-Fe/α-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H Bond.

The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N2O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology's 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties.

Dinuclear Iron(II) Spin-Crossover Compounds: A Theoretical Study.

The structures and spin-state energetics of two di-iron(II) complexes based on thiadiazole and oxadiazole ligands in different crystals were studied by using density functional theory and second-order perturbation theory based on the density matrix renormalization group approach (DMRG-CASPT2). When taking into account all different contributions to the relative energy, our theoretical approach is capable of providing results that are in excellent agreement with established experimental data. In all cases, we correctly describe the ground state of the complexes as well as predict their spin-crossover behavior. A comparison between the two complexes in the gas phase and in different crystals shows how the structures change by moving from the gas phase to different crystals and reveals a large impact of the crystal stabilization on the relative spin-state energy. This theoretical work also demonstrates the applicability of the DMRG-CASPT2 approach to quantitatively study the spin-state energetics of multinuclear transition-metal complexes.

Protein-Assisted Formation and Stabilization of Catalytically Active Polyoxometalate Species.

The effect of the protein environment on the formation and stabilization of an elusive catalytically active polyoxometalate (POM) species, K6 [Hf(α2 -P2 W17 O61 )] (1), is reported. In the co-crystal of hen egg-white lysozyme (HEWL) with 1, the catalytically active monomeric species is observed, originating from the dimeric 1:2 POM form, while it is intrinsically unstable under physiological pH conditions. The protein-assisted dissociation of the dimeric POM was rationalized by means of DFT calculations. The dissociation process is unfavorable in bulk water, but becomes favorable in the protein-POM complex due to the low dielectric response at the protein surface. The crystal structure shows that the monomeric form is stabilized by electrostatic and water-mediated hydrogen bonding interactions with the protein. It interacts at three distinct sites, close to the aspartate-containing hydrolysis sites, demonstrating high selectivity towards peptide bonds containing this residue.

Reaction Mechanism of [NiFe] Hydrogenase Studied by Computational Methods.

[NiFe] hydrogenases catalyze the reversible conversion of molecular hydrogen to protons and electrons. This seemingly simple reaction has attracted much attention because of the prospective use of H2 as a clean fuel. In this paper, we have studied the full reaction mechanism of this enzyme with various computational methods. Geometries were obtained with combined quantum mechanical and molecular mechanics (QM/MM) calculations. To get more accurate energies and obtain a detailed account of the surroundings, we performed big-QM calculations with 819 atoms in the QM region. Moreover, QM/MM thermodynamic cycle perturbation calculations were performed to obtain free energies. Finally, density matrix renormalisation group complete active space self-consistent field calculations were carried out to study the electronic structures of the various states in the reaction mechanism. Our calculations indicate that the Ni-L state is not involved in the reaction mechanism. Instead, the Ni-C state is reduced by one electron and then the bridging hydride ion is transferred to the sulfur atom of Cys546 as a proton and the two electrons transfer to the Ni ion. This step turned out to be rate-determining with an energy barrier of 58 kJ/mol, which is consistent with the experimental rate of 750 ± 90 s-1 (corresponding to ∼52 kJ/mol). The cleavage of the H-H bond is facile with an energy barrier of 33 kJ/mol, according to our calculations. We also find that the reaction energies are sensitive to the size of the QM system, the basis set, and the density functional theory method, in agreement with previous studies.

Switching the Spin-Crossover Phenomenon by Ligand Design on Imidazole-Diazineiron(II) Complexes.

The iron(II) complexes of two structural isomers of 2-(1 H-imidazol-2-yl)diazine reveal how ligand design can be a successful strategy to control the electronic and magnetic properties of complexes by fine-tuning their ligand field. The two isomers only differ in the position of a single diazinic nitrogen atom, having either a pyrazine (Z) or a pyrimidine (M) moiety. However, [Fe(M)3](ClO4)2 is a spin-crossover complex with a spin transition at 241 K, whereas [Fe(Z)3](ClO4)2 has a stable magnetic behavior between 2 and 300 K. This is corroborated by temperature-dependent Mössbauer spectra showing the presence of a quintet and a singlet state in equilibrium. The temperature-dependent single-crystal X-ray diffraction results relate the spin-crossover observed in [Fe(M)3](ClO4)2 to changes in the bond distances and angles of the coordination sphere of iron(II), hinting at a stronger σ donation of ligand Z in comparison to ligand M. The UV/vis spectra of both complexes are solved by means of the multiconfigurational wave-function-based method CASPT2 and confirm their different spin multiplicities at room temperature, as observed in the Mössbauer spectra. Calculations show larger stabilization of the singlet state in [Fe(Z)3]2+ than in [Fe(M)3]2+, stemming from the slightly stronger ligand field of the former (506 cm-1 in the singlet). This relatively weak effect is indeed capable of changing the spin multiplicity of the complexes and causes the appearance of the spin transition in the M complex.

The dioxygen adducts of iron and manganese porphyrins: electronic structure and binding energy.

In this paper, we present a thorough study of the electronic structures and binding energies of O2 to iron and manganese porphyrins (FeP and MnP), employing a state-of-the-art computational technique known as second-order perturbation theory based on density matrix renormalization group (DMRG-CASPT2). By investigating an extensive list of different binding modes and spin states, we provide a clear and conclusive description of the ground state of MnP-O2, confirming available experimental evidences. Our results show that MnP-O2 favours a side-on quartet structure, with strong charge transfer between MnP and O2. We also calculated the standard binding enthalpies of O2 to different metal porphyrins and showed that an agreement between calculated results and experimental data to within 2 kcal mol-1 can be achieved. Our calculations confirm the experimental observation that the binding of O2 to manganese porphyrin is stronger by around 4-6 kcal mol-1 than to the corresponding ferrous porphyrin.

Identification of α-Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations.

α-Fe is the precursor of the reactive FeIV═O core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe(II) at six-membered-ring cation exchange sites (6MR) in ZSM-5, zeolite beta, and ferrierite was investigated using DFT and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV-vis spectra reported by Snyder et al. From this comparison it is concluded that the 16000 cm-1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high-spin square-planar (SP) Fe(II) located at a 6MR with an Al-Si-Si-Al sequence, where the Al atoms are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10000 cm-1 in the experimental spectra, assigned to spectator Fe(II), are attributed to six-coordinated trigonal-prismatic Fe(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated by making use of the unconstrained Fe(II) model complex FeL2 (with L = [Al(OH)4]-). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral form, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3dz2 orbital with the higher lying 4s.

H2 binding to the active site of [NiFe] hydrogenase studied by multiconfigurational and coupled-cluster methods.

[NiFe] hydrogenases catalyse the reversible conversion of molecular hydrogen to protons and electrons. This seemingly simple reaction has attracted much attention because of the prospective use of H2 as a clean fuel. In this paper, we have studied how H2 binds to the active site of this enzyme. Combined quantum mechanical and molecular mechanics (QM/MM) optimisation was performed to obtain the geometries, using both the TPSS and B3LYP density-functional theory (DFT) methods and considering both the singlet and triplet states of the Ni(ii) ion. To get more accurate energies and obtain a detailed account of the surroundings, we performed calculations with 819 atoms in the QM region. Moreover, coupled-cluster calculations with singles, doubles, and perturbatively treated triples (CCSD(T)) and cumulant-approximated second-order perturbation theory based on the density-matrix renormalisation group (DMRG-CASPT2) were carried out using three models to decide which DFT methods give the most accurate structures and energies. Our calculations show that H2 binding to Ni in the singlet state is the most favourable by at least 47 kJ mol-1. In addition, the TPSS functional gives more accurate energies than B3LYP for this system.

Role of the Imide Axial Ligand in the Spin and Oxidation State of Manganese Corrole and Corrolazine Complexes.

Electronic structure calculations have been performed on four different Mn corrole and corrolazine complexes to clarify the role of the imide axial ligand on the relative stability of the different spin states and the stabilization of the high-valent Mn ion in these complexes. Multiconfigurational perturbation theory energy calculations on the DFT-optimized geometries show that all complexes have a singlet ground state except the complex with the strongest electron-withdrawing substituent on the imide axial ligand, which is found to have a triplet ground state. The analysis of the σ and π interaction between the metal and imide ligand shows that this spin crossover is caused by a subtle interplay of geometrical factors (Mn-N distance and coordination angle) and the electron-withdrawing character of the substituent on the imide, which reduces the electron donation to the metal center. The analysis of the multiconfigurational wave functions reveals that the formally Mn(V) ion is stabilized by an important electron transfer from both the equatorial corrole/corrolazine ligand and the axial imide. The macrocycle donates roughly half an electron, being somewhere between the closed-shell trianionic and the dianionic radical form. The imide ligand transfers 2.5 electrons to the metal center, resulting in an effective d-electron count close to five in all complexes.

Molecular Insight from DFT Computations and Kinetic Measurements into the Steric Factors Influencing Peptide Bond Hydrolysis Catalyzed by a Dimeric Zr(IV)-Substituted Keggin Type Polyoxometalate.

Peptide bond hydrolysis of several peptides with a Gly-X sequence (X = Gly, Ala, Val, Leu, Ile, Phe) catalyzed by a dimeric Zr(IV)-substituted Keggin type polyoxometalate (POM), (Et2NH2)8[{α-PW11O39Zr(µ-OH)(H2O)}2]·7H2O (1), was studied by means of kinetic experiments and (1)H NMR spectroscopy. The observed rate of peptide bond hydrolysis was found to decrease with increase of the side chain bulkiness, from 4.44 × 10(-7) s(-1) for Gly-Gly to 0.81 × 10(-7) s(-1) for Gly-Ile. A thorough DFT investigation was performed to elucidate (a) the nature of the hydrolytically active species in solution, (b) the mechanism of peptide bond hydrolysis, and (c) the influence of the aliphatic residues on the rate of hydrolysis. Formation of substrate-catalyst complexes of the dimeric POM 1 was predicted as thermodynamically unlikely. Instead, the substrates prefer to bind to the monomerization product of 1, [α-PW11O39Zr(OH)(H2O)](4-) (2), which is also present in solution. In the hydrolytically active complex two dipeptide ligands are coordinated to the Zr(IV) center of 2. The first ligand is bidentate-bound through its amino nitrogen and amide oxygen atoms, while the second ligand is monodentate-bound through a carboxylic oxygen atom. The mechanism of hydrolysis involves nucleophilic attack by a solvent water molecule on the amide carbon atom of the bidentate-bound ligand. In this process the uncoordinated carboxylic group of the same ligand acts as a general base to abstract a proton from the attacking water molecule. The decrease of the hydrolysis rate with an increase in the side chain bulkiness is mostly due to the increased ligand conformational strain in the rate-limiting transition state, which elevates the reaction activation energy. The conformational strain increases first upon substitution of Hα in Gly-Gly with the aliphatic α substituent and second with the ß branching of the α substituent.

Spin State Energetics and Oxyl Character of Mn-Oxo Porphyrins by Multiconfigurational ab Initio Calculations: Implications on Reactivity.

Important electromeric states in manganese-oxo porphyrins MnO(P)(+) and MnO(PF4)(+) (porphyrinato or meso-tetrafluoroporphyrinato) have been investigated with correlated ab initio methods (CASPT2, RASPT2), focusing on their possible role in multistate reactivity patterns in oxygen transfer (OAT) reactions. Due to the lack of oxyl character, the Mn(V) singlet ground state is kinetically inert. OAT reactions should therefore rather proceed through thermally accessible triplet and quintet states that have a more pronounced oxyl character. Two states have been identified as possible candidates: a Mn(V) triplet state and a Mn(IV)O(L(•)a2u)(+) quintet state. The latter state is high-lying in MnO(P)(+) but is stabilized by the substitutions of H by F at the meso carbons (where the a2u orbital has a significant amplitude). Oxyl character and Mn-O bond weakening in these two states stems from the fact that the Mn-O π* orbitals become singly (triplet) or doubly occupied (quintet). Moreover, an important role for the reactivity of the triplet state is also likely to be played by the π bond that has an empty π* orbital, because of the manifest diradical character of this π bond, revealed by the CASSCF wave function. Interestingly, the diradical character of this bond increases when the Mn-O bond is stretched, while the singly occupied π* orbital looses its oxygen radical contribution. The RASPT2 results were also used as a benchmark for the description of excited state energetics and Mn-O oxyl character with a wide range of pure and hybrid density functionals. With the latter functionals both the Mn(V) → Mn(IV) promotion energy and the diradical character of the π bond (with empty π*) are found to be extremely dependent on the contribution of exact exchange. For this reason, pure functionals are to be preferred.

Phosphate Ester Bond Hydrolysis Promoted by Lanthanide-Substituted Keggin-type Polyoxometalates Studied by a Combined Experimental and Density Functional Theory Approach.

Hydrolytic cleavage of 4-nitrophenyl phosphate (NPP), a commonly used DNA model substrate, was examined in the presence of series of lanthanide-substituted Keggin-type polyoxometalates (POMs) [Me2NH2]11[CeIII(PW11O39)2], [Me2NH2]10[CeIV(PW11O39)2] (abbreviated as (CeIV(PW11)2), and K4[EuPW11O39] by means of NMR and luminescence spectroscopies and density functional theory (DFT) calculations. Among the examined complexes, the Ce(IV)-substituted Keggin POM (CeIV(PW11)2) showed the highest reactivity, and its aqueous speciation was fully determined under different conditions of pD, temperature, concentration, and ionic strength by means of 31P and 31P diffusion-ordered NMR spectroscopy. The cleavage of the phosphoester bond of NPP in the presence of (CeIV(PW11)2) proceeded with an observed rate constant kobs = (5.31 ± 0.06) × 10-6 s-1 at pD 6.4 and 50 °C. The pD dependence of NPP hydrolysis exhibits a bell-shaped profile, with the fastest rate observed at pD 6.4. The formation constant (Kf = 127 M-1) and catalytic rate constant (kc = 19.41 × 10-5 s-1) for the NPP-Ce(IV)-Keggin POM complex were calculated, and binding between CeIV(PW11)2 and the phosphate group of NPP was also evidenced by the change of the chemical shift of the 31P nucleus in NPP upon addition of the POM complex. DFT calculations revealed that binding of NPP to the parent catalyst CeIV(PW11)2 is thermodynamically unlikely. On the contrary, formation of complexes with the monomeric 1:1 species, CeIVPW11, is considered to be more favorable, and the most stable complex, [CeIVPW11(H2O)2(NPP-κO)2]7-, was found to involve two NPP ligands coordinated to the CeIVcenter of CeIVPW11 in the monodentate fashion. The formation of such species is considered to be responsible for the hydrolytic activity of CeIV(PW11)2 toward phosphomonoesters. On the basis of these findings a principle mechanism for the hydrolysis of NPP by the POM is proposed.

Multinuclear diffusion NMR spectroscopy and DFT modeling: a powerful combination for unraveling the mechanism of phosphoester bond hydrolysis catalyzed by metal-substituted polyoxometalates.

A detailed reaction mechanism is proposed for the hydrolysis of the phosphoester bonds in the DNA model substrate bis(4-nitrophenyl) phosphate (BNPP) in the presence of the Zr(IV)-substituted Keggin type polyoxometalate (Et2NH2)8[{α-PW11O39Zr(µ-OH)(H2O)}2]⋅7 H2O (ZrK 2:2) at pD 6.4. Low-temperature (31)P DOSY spectra at pD 6.4 gave the first experimental evidence for the presence of ZrK 1:1 in fast equilibrium with ZrK 2:2 in purely aqueous solution. Moreover, theoretical calculations identified the ZrK 1:1 form as the potentially active species in solution. The reaction intermediates involved in the hydrolysis were identified by means of (1)H/(31)P NMR studies, including EXSY and DOSY NMR spectroscopy, which were supported by DFT calculations. This experimental/theoretical approach enabled the determination of the structures of four intermediate species in which the starting compound BNPP, nitrophenyl phosphate (NPP), or the end product phosphate (P) is coordinated to ZrK 1:1. In the proposed reaction mechanism, BNPP initially coordinates to ZrK 1:1 in a monodentate fashion, which results in hydrolysis of the first phosphoester bond in BNPP and formation of NPP. EXSY NMR studies showed that the bidentate complex between NPP and ZrK 1:1 is in equilibrium with monobound and free NPP. Subsequently, hydrolysis of NPP results in P, which is in equilibrium with its monobound form.

Reactivity of Dimeric Tetrazirconium(IV) Wells-Dawson Polyoxometalate toward Dipeptide Hydrolysis Studied by a Combined Experimental and Density Functional Theory Approach.

Detailed kinetic studies on the hydrolysis of glycylglycine (Gly-Gly) in the presence of the dimeric tetrazirconium(IV)-substituted Wells-Dawson-type polyoxometalate Na14[Zr4(P2W16O59)2(µ3-O)2(OH)2(H2O)4] · 57H2O (1) were performed by a combination of (1)H, (13)C, and (31)P NMR spectroscopies. The catalyst was shown to be stable under a broad range of reaction conditions. The effect of pD on the hydrolysis of Gly-Gly showed a bell-shaped profile with the fastest hydrolysis observed at pD 7.4. The observed rate constant for the hydrolysis of Gly-Gly at pD 7.4 and 60 °C was 4.67 × 10(-7) s(-1), representing a significant acceleration as compared to the uncatalyzed reaction. (13)C NMR data were indicative for coordination of Gly-Gly to 1 via its amide oxygen and amine nitrogen atoms, resulting in a hydrolytically active complex. Importantly, the effective hydrolysis of a series of Gly-X dipeptides with different X side chain amino acids in the presence of 1 was achieved, and the observed rate constant was shown to be dependent on the volume, chemical nature, and charge of the X amino acid side chain. To give a mechanistic explanation of the observed catalytic hydrolysis of Gly-Gly, a detailed quantum-chemical study was performed. The theoretical results confirmed the nature of the experimentally suggested binding mode in the hydrolytically active complex formed between Gly-Gly and 1. To elucidate the role of 1 in the hydrolytic process, both the uncatalyzed and the polyoxometalate-catalyzed reactions were examined. In the rate-determining step of the uncatalyzed Gly-Gly hydrolysis, a carboxylic oxygen atom abstracts a proton from a solvent water molecule and the nascent OH nucleophile attacks the peptide carbon atom. Analogous general-base activity of the free carboxylic group was found to take place also in the case of polyoxometalate-catalyzed hydrolysis as the main catalytic effect originates from the -C═O···Zr(IV) binding.