Theoretical Insight into the Performance of Mn^II/III-Monosubstituted Heteropolytungstates as Water Oxidation Catalysts

The performance of MnII/III-monosubstituted heteropolytungstates [MnIII(H2O)­GeW11O39]5– ([GT-MnIII-OH2]5–, where GT = GeW11O39) and [MnII(H2O)­GeW11O39]6– ([GT-MnII-OH2]6–) as water oxidation catalysts at pH 9 was explored using density functional theory calculations. The counterion effect was fully considered, in which five and six Na+ ions were included in the calculations for water oxidation catalyzed by [GT-MnIII-OH2]5– and [GT-MnII-OH2]6–, respectively. The process of water oxidation catalysis was divided into three elemental stages: (i) oxidative activation, (ii) O–O bond formation, and (iii) O2 evolution. In the oxidative activation stage, two electrons and two protons are removed from [Na5-GT-MnIII-OH2] and three electrons and two protons are removed from [Na6-GT-MnII-OH2]. Therefore, the MnIV-O• species [Na5-GT-MnIV-O•] is obtained. Two mechanisms, (i) water nucleophilic attack and (ii) oxo–oxo coupling, were demonstrated to be competitive in O–O bond formation triggered from [Na5-GT-MnIV-O•]. In the last stage, the O2 molecule could be readily evolved from the peroxo or dinuclear species and the catalyst returns to the ground state after the coordination of a water molecule(s)

Two-State Reactivity Mechanism of Benzene C–C Activation by Trinuclear Titanium Hydride

The cleavage of inert C–C bonds is a central challenge in modern chemistry. Multinuclear transition metal complexes would be a desirable alternative because of the synergetic effect of multiple metal centers. In this work, carbon–carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride were investigated using density functional theory. The reaction occurs via a novel “two-state reactivity” mechanism. The important elementary steps consist of hydride transfer, benzene coordination, dehydrogenation, oxidative addition, hydride–proton exchange, and reductive elimination. Most importantly, the ground-state potential energy surface switches from nearly degenerate triplet and antiferromagnetic singlet states to a closed-shell singlet state in the dearomatization of benzene, which effectively decreases the activation barrier. Furthermore, the roles of the transition metal centers and hydrides were clarified

Quantum Chemical Studies on High-Valent Metal Nitrido Derivatives of Keggin-Type Polyoxometalates ([PW₁₁O₃₉{M^VIN}]⁴⁻ (M = Ru, Os, Re)): M^VI−N Bonding and Electronic Structures

High-valent MVIN (M = Ru, Os) species are important reagents in nitrogen transfer reactions; the unique withdrawing properties of polyoxometalate (POMs) ligands would possibly modify the reactivity of the MVIN functional group. In the present paper, density functional theory (DFT) and natural bond orbital (NBO) analysis have been employed to calculate electronic structures, MVI−N bonding, and redox properties of high-valent metal nitrido derivatives of Keggin-type POMs, [PW11O39 {MVIN}]4− (M = Ru, Os, Re). Our calculations show that [PW11O39{RuN}]4− possesses stronger antibonding interaction between metal and nitrogen atoms compared with anions [PW11O39{OsN}]4− and [PW11O39{ReN}]4−. A large increase in the Ru−N bond length of anion [PW11O39{RuN}]4− in the excited states has been found; the effective order and composition of the molecular orbital in anion [PW11O39{RuN}]4− is a key factor in determination of the increase of the Ru−N bond length in the excited states. The substitution effects of central tetrahedron heteroatoms (XO4, X = Al, Si, P, As) in anions [XW11O39{RuN}]4− affect the relative energy of the LUMO; the relevant orbital energy increases in the order Al(III) 11O39{RuVIN}]4− shows that the Ru−N bond possesses a covalent feature and displays triple-, double-, and single-bond character when moving along the change of spin state (11 → 31 → 51)

TDDFT Studies on the Electronic Structures and Chiroptical Properties of Mono-Tin-Substituted Wells–Dawson Polyoxotungstates

The UV/CD spectra of tin-bearing acetonyl-substituted Wells–Dawson polyoxotungstates α₁- and α₂-[P₂W₁₇O₆₁{SnCH₂CH₂C­(O)}]^6– were systematically investigated using the time-dependent density functional theory (TDDFT) method. The electronic circular dichroism (ECD) spectra were produced over the range of 3.3–5.8 eV. The calculated ECD spectra of the α₁-R isomer were generally in agreement with the experimental spectra. The CAM-B3LYP hybrid functional was found to predict the excitation energies of tin-containing polyoxotungstates well. The fact that the UV/ECD spectra of α₁-isomers are different from those of α₂-isomers demonstrates the effect of the tin substitution site on the chiroptical properties of the studied isomers. The origins of the ECD bands are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to W atoms, from organic fragments to W atoms, or from the combination of two CT transitions. The results suggest that the organic fragment and polyoxometalate (POM) cage are chiroptical chromophores

Molecular Photocatalysts Based on Quinolinium-Grafted Polyoxometalates for Efficient One-Step Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids

Based on the need for green synthesis, photocatalytic oxidation reactions under ambient conditions are vitally significant. Herein, we developed six new quinolinium-grafted polyoxometalate (Q-POM) structures and explored their photocatalytic ability in the photocatalytic aerobic oxidation of benzyl alcohol-like compounds. Among them, (Bu4N)2+{AlMo6O18[(OCH2)3CCH3][(OCH2)3CNHCOCH2C9H7N]}2– (TBA-Q-Al) can achieve the one-step oxidation of various benzyl alcohols to the corresponding benzoic acid compounds with favorable yields under blue light irradiation at 420–430 nm accompanied by oxygen as the sole oxidant. The photocatalyst combines the ability of quinolinium π–π conjugation to respond to light and the property of the POM backbone to storage-transfer electrons. This work provides guidance for the subsequent development of more efficient quinolinium-grafted POM structures for photocatalytic conversions

Molecular Photocatalysts Based on Quinolinium-Grafted Polyoxometalates for Efficient One-Step Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids

Based on the need for green synthesis, photocatalytic oxidation reactions under ambient conditions are vitally significant. Herein, we developed six new quinolinium-grafted polyoxometalate (Q-POM) structures and explored their photocatalytic ability in the photocatalytic aerobic oxidation of benzyl alcohol-like compounds. Among them, (Bu4N)2+{AlMo6O18[(OCH2)3CCH3][(OCH2)3CNHCOCH2C9H7N]}2– (TBA-Q-Al) can achieve the one-step oxidation of various benzyl alcohols to the corresponding benzoic acid compounds with favorable yields under blue light irradiation at 420–430 nm accompanied by oxygen as the sole oxidant. The photocatalyst combines the ability of quinolinium π–π conjugation to respond to light and the property of the POM backbone to storage-transfer electrons. This work provides guidance for the subsequent development of more efficient quinolinium-grafted POM structures for photocatalytic conversions

TDDFT Studies on the Determination of the Absolute Configurations and Chiroptical Properties of Strandberg-Type Polyoxometalates

The electronic circular dichroism (ECD) and UV–visible absorption (UV–vis) spectra of Strandberg-type polyoxometalates (POMs) (R, R)-[(R*PO₃)₂M₅O₁₅]^2‑ (R* = CH₃CH­(NH₃), (M = Mo, W)) have been explored using the time-dependent density functional theory (TDDFT) method. It demonstrates that the absolute configurations of chiral systems can be determined by chiroptical spectroscopic methods combined with DFT calculations. The calculated ECD spectra of the Strandberg-type molybdate were produced over the range of 3.3–6.5 eV, which are generally in agreement with the experimental spectra. In addition, the ECD spectra of (R, R)-[(R*PO₃)₂W₅O₁₅]^2‑ (R* = CH₃CH­(NH₃)) were produced over the range of 4.5–8.5 eV. The Becke’s half-and-half hybrid exchange-correlation functional (BHandHLYP) with the HF exchange fraction to 55% hybrid functional was found to well predict the excitation energies of studied systems. The origins of the ECD bands of two systems are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to metal atoms in polyanion. The results suggest that the polyanion are chiroptical chromophores. The polyanion plays a role as an optically active chromophore and contribute to the absorptions of ECD spectra. The difference of the UV–vis/ECD spectra between two systems shows that the transition metal atom significantly influences on the chiroptical properties of the studied Strandberg-type POMs

Theoretical Study on the Considerable Second-Order Nonlinear Optical Properties of Naphthylimido-Substituted Hexamolybdates

The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl)imido]hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido]hexamolybdate has considerable large first hyperpolarizability, 6.780 × 10−30 esu, and it is larger than that of [(2,6-dimethylphenyl)arylimido]hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido)hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largest β0 value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to the nitrogen atom, could generate a large static electronic field and give the large contribution to NLO response. The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido)hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido)molybdate derivatives

Assembly of Keggin-/Dawson-type Polyoxotungstate Clusters with Different Metal Units and SeO₃^2– Heteroanion Templates

Using a pH-dependent synthetic approach, the combination of different simple metal salts or metal coordination complexes with SeO32– heteroanion templates was employed to synthesize five distinct assemblies of Keggin-/Dawson-type tungstoselenites: (C2H8N)10KNa­[(α-SeW9O34)­{Zr­(H2O)}­{WO­(H2O)}­(WO2)­(SeO3)­{α-SeW8O31Zr­(H2O)}]2­·14H2O (1) at pH = 1.3; (C2H8N)10KNa5­[(Se2W18O60)2­(μ2-O)4]­·12H2O (2) at pH = 2.5; (C2H8N)4Na4­[Se2W18O62­(H2O)2]­·13H2O (3) at pH = 3.6; (C2H8N)4­K3Na10­[(α-SeW9O33)2­{Ce2(CH3COO)­(H2O)3W3O6}­(α-Se2W14O52)]·26H2O (4) at pH = 4.5; K10Na5­[(α-SeW9O33)2­{Ce2(H2O)4W3O6}­{α-Se2W14O51(OH)}]­·24H2O (5) at pH = 4.5. All five compounds were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric, UV/vis spectroscopy, and ESI-MS. Moreover, their electrochemical properties were investigated. Keggin-type polyoxoanion of 1 remains the first reported Zr-containing tungstoselenites based on {α-SeW9} building blocks. X-ray analysis revealed that the 4d metal Zr centers have seven- and eight-coordinated modes, and SeO32– acts as the templates as well as the linkers. With the increasing of the pH, Dawson-type polyoxoanions of 2 and 3 based on the first reported basic lacunary {α-Se2W14} building blocks are obtained by using 3d-4f metal coordination complexes. Polyoxoanions of 4 and 5 remain similar structures stabilized by the 4f metal Ce centers at pH = 4.5 and that contain the basic Keggin-type {α-SeW9} and Dawson-type {α-Se2W14} building blocks in 1–3 at the same time, presenting the mixed multiple lacunary building blocks being combined into the single polyoxoanion architecture. Furthermore, the density functional theory calculations have been performed on polyoxoanions of 1 and 5 as the representatives to investigate their electronic properties