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The use of boron (B) atoms as transition metal mimics opens the door to new research in catalytic chemistry. An emerging class of compounds, bis(Lewis base)borylenes with an electron-rich B(I) center, are potential metal-free catalysts for dinitrogen bonding and reduction. Here, the molecular geometry, electronic structure, and possible reaction mechanism of a series of bis(Lewis base)borylene-dinitrogen compounds corresponding to the nitrogen reduction reaction have been investigated by using density functional theory (DFT) calculations. Our DFT calculations show that these free borylene compounds possess radical features and have the capability to activate N2 molecules via an effective combination of π(B → N2), π(N2 → B), and σ(N2 → B) electron transfer processes. The possible reaction mechanisms for direct conversion of N2 into NH3 for these bis(Lewis base)borylene-dinitrogen compounds have been systematically investigated along distal and alternating paths. The calculated free energy profiles indicate that the limiting potential of a bis(phosphine)borylene-dinitrogen compound is comparable to that of metal-based catalysts, which is the most promising candidate for the reduction of N2 to NH3via the alternating mechanism among all compounds studied here. The electronic structure analysis shows that the B center plays the role of an electron donor and acceptor alternatively in the consecutive six protonation and reduction processes, and thus acts as the electron transfer medium. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu1/PMA (PMA is the phosphomolybdate, Cs3PMo12O40) single-atom catalyst (SAC). Several important aspects of the catalytic system were addressed, including the generation of oxygen vacancies (Ov), formation of N2O2 intermediates, scission of the N–O bond of N2O2 intermediates to form N2O or N2, and decomposition of N2O to form N2. Unlike most previous theoretical studies, which tend to explore the reaction mechanism of polyoxometalate (POM) systems based on the isolated anionic unit, here, we build a model of the catalytic system with neutral species by introduction of counter cations to model the solid structure of the Cu1/PMA SAC. The major findings of our present study are: (1) CO adsorption on Cu sites leads to the formation of cationic Cu carbonyl species; (2) the Oc atom at the surface of the PMA support can easily react with the adsorbed CO to generate a Cu–Ov pair; (3) the Cu–Ov pair embedded on PMA is found to be the active site, not only for the formation of N2O2* by the reaction of two NO molecules via an Eley–Rideal pathway but also for the decomposition of N2O to form N2; (4) the adsorption of a NO molecule on the Cu–Ov pair with a bridging model results in charge transfer from the Cu atom to the π* antibonding orbital of the NO molecule; (5) IR spectroscopy of the key intermediates has been identified based on our DFT calculations; and (6) the Cu atom serves as an electron acceptor in Ov formation steps and an electron donor in N2O2 decomposition steps, and thus represents an electron reservoir. These results suggest that the POM-supported SAC with the cheaper Cu element is an efficient catalyst for the reaction between CO and NO. [ABSTRACT FROM AUTHOR]

The discrete anionic structure of polyoxometalates (POMs) at the interface is more like a separate small “island”, which effectively prevents the diffusion of single atoms and prohibits the agglomeration and generation of metal particles; thus, POMs can enhance the sintering-resistant behavior and increase metal loading on the surface of single-atom catalysts (SACs). To explore the catalytic performance of POM-supported SACs for CO oxidation, we employed density functional theory (DFT) calculations to gain an understanding of some important aspects, including the CO adsorption, the formation of oxygen vacancies, and the activity of the surface oxygen species, of the catalytic system. Compared to previous theoretical studies, in which the catalytic behavior of POMs has been investigated based on the anionic unit with the highest negative charge, herein, we have constructed a model of the POM-supported SACs, which are neutral species. Our DFT calculations indicated that in the series of the SACs studied herein, (1) upon anchoring of a single metal atom on the POM surface, four key surface oxygen atoms were lifted from the POM surface to form a new interface, and thus, the surface oxygen species were activated; (2) CO adsorbed more strongly on the Ir, Os, Rh, Pt, and Ru sites than on the Fe, Mn, and Co sites; (3) it was easy to form an oxygen vacancy on the POM surface in the case of the Pt system when compared with the other systems; (4) the difference in the surface oxygen species for CO oxidation was remarkable, and the Oc atom at the catalyst interface had higher reactivity for CO oxidation as compared to the Ob atom in the Pt system studied herein; and (5) the single Pt atom served as an electron reservoir in the CO oxidation along the reaction pathway. [ABSTRACT FROM AUTHOR]

In the present paper, the electronic structures and bonding features between the ruthenium atom and nitrido ligand of known nitrido ruthenium (VI) porphyrin and hexavalent ruthenium nitrido derivative of Keggin typical polyoxometalate (POM) have been investigated by using density functional theory (DFT) calculations and natural bond orbital (NBO) analysis. The results show that the Keggin POM complex and porphyrin complex have the RuN triple bond, the NBO calculations yield a nitrido ligand charge close to zero in both complexes because of the charge transfer from the rich-electron nitrido ligand to the ruthenium center. The molecular orbitals predict that the ruthenium porphyrin complex would generate oxidized species with no change in the oxidation state of the metal center; however, the ruthenium center of the POM complex would be oxidized in a one-electron-oxidation process because of the different HOMO character, and moreover, the spin unrestricted calculations confirm this prediction made by molecular orbital analysis. On the other hand, both complexes have the same reduced center, the RuN unit, and the one-electron-reduction process weakens the bonding interaction between the ruthenium atom and nitrido ligand for both complexes because the antibonding LUMO is occupied. Compared with the porphyrin complex, the POM complex possesses a higher RuN composition and lower energy π* antibonding orbital according to gas-phase and solvent calculations. [ABSTRACT FROM AUTHOR]

Acetaldehyde (AA) and ethylene oxide (EO) are important fine chemicals, and are also substrates with wide applications for high-value chemical products. Direct electrocatalytic oxidation of ethylene to AA and EO can avoid the untoward effects from harmful byproducts and high energy emissions. The most central intermediate state is the co-adsorption and coupling of ethylene and active oxygen intermediates (*O) at the active site(s), which is restricted by two factors: the stability of the *O intermediate generated during the electrolysis of water on the active site at a certain applied potential and pH range; and the lower kinetic energy barriers of the oxidation process based on the thermo-migration barrier from the *O intermediate to produce AA/EO. The benefit of two adjacent active atoms is more promising, since diverse adsorption and flexible catalytic sites may be provided for elementary reaction steps. Motivated by this strategy, we explored the feasibility of various homonuclear TM 2 N 6 @graphenes with dual-atomic-site catalysts (DASCs) for ethylene electro-oxidation through first-principles calculations via thermodynamic evaluation, analysis of the surface Pourbaix diagram, and kinetic evaluation. Two reaction mechanisms through C-TM versus TM-TM synergism were determined. Between them, a TM-TM mechanism on 4 TM 2 N 6 @graphenes and a C-TM mechanism on 5 TM 2 N 6 @graphenes are built. All 5 TM 2 N 6 @graphenes through the C-TM mechanism exhibit lower kinetic energy barriers for AA and EO generation than the 4 TM 2 N 6 @graphenes through the TM-TM mechanism. In particular, Pd 2 N 6 @graphene exhibits the most excellent catalytic activity, with energy barriers for generating AA and EO of only 0.02 and 0.65 eV at an applied potential of 1.77 V vs. RHE for the generation of an active oxygen intermediate. Electronic structure analysis indicates that the intrinsic C-TM mechanism is more advantageous than the TM-TM mechanism for ethylene electro-oxidation, and this study also provides valuable clues for further experimental exploration., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Density functional theory (DFT) calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu1/PMA (PMA is the phosphomolybdate, Cs3PMo12O40) single-atom catalyst (SAC). Several important aspects of the catalytic system were addressed, including the generation of oxygen vacancies (Ov), formation of N2O2 intermediates, scission of the N-O bond of N2O2 intermediates to form N2O or N2, and decomposition of N2O to form N2. Unlike most previous theoretical studies, which tend to explore the reaction mechanism of polyoxometalate (POM) systems based on the isolated anionic unit, here, we build a model of the catalytic system with neutral species by introduction of counter cations to model the solid structure of the Cu1/PMA SAC. The major findings of our present study are: (1) CO adsorption on Cu sites leads to the formation of cationic Cu carbonyl species; (2) the Oc atom at the surface of the PMA support can easily react with the adsorbed CO to generate a Cu-Ov pair; (3) the Cu-Ov pair embedded on PMA is found to be the active site, not only for the formation of N2O2* by the reaction of two NO molecules via an Eley-Rideal pathway but also for the decomposition of N2O to form N2; (4) the adsorption of a NO molecule on the Cu-Ov pair with a bridging model results in charge transfer from the Cu atom to the π* antibonding orbital of the NO molecule; (5) IR spectroscopy of the key intermediates has been identified based on our DFT calculations; and (6) the Cu atom serves as an electron acceptor in Ov formation steps and an electron donor in N2O2 decomposition steps, and thus represents an electron reservoir. These results suggest that the POM-supported SAC with the cheaper Cu element is an efficient catalyst for the reaction between CO and NO.

The discrete anionic structure of polyoxometalates (POMs) at the interface is more like a separate small "island", which effectively prevents the diffusion of single atoms and prohibits the agglomeration and generation of metal particles; thus, POMs can enhance the sintering-resistant behavior and increase metal loading on the surface of single-atom catalysts (SACs). To explore the catalytic performance of POM-supported SACs for CO oxidation, we employed density functional theory (DFT) calculations to gain an understanding of some important aspects, including the CO adsorption, the formation of oxygen vacancies, and the activity of the surface oxygen species, of the catalytic system. Compared to previous theoretical studies, in which the catalytic behavior of POMs has been investigated based on the anionic unit with the highest negative charge, herein, we have constructed a model of the POM-supported SACs, which are neutral species. Our DFT calculations indicated that in the series of the SACs studied herein, (1) upon anchoring of a single metal atom on the POM surface, four key surface oxygen atoms were lifted from the POM surface to form a new interface, and thus, the surface oxygen species were activated; (2) CO adsorbed more strongly on the Ir, Os, Rh, Pt, and Ru sites than on the Fe, Mn, and Co sites; (3) it was easy to form an oxygen vacancy on the POM surface in the case of the Pt system when compared with the other systems; (4) the difference in the surface oxygen species for CO oxidation was remarkable, and the Oc atom at the catalyst interface had higher reactivity for CO oxidation as compared to the Ob atom in the Pt system studied herein; and (5) the single Pt atom served as an electron reservoir in the CO oxidation along the reaction pathway.

Second-order nonlinear optical properties of a tetrathiafulvalene (TTF) derivative of spiropyran have been studied based on density functional theory (DFT) combined with the finite field (FF) calculations. Our DFT-FF calculations confirm a switching behavior of the static first hyperpolarizability caused by the redox and photochromic reaction. The photochromic reaction generates spiropyran-merocyanine conversion by reversible cleavage of the C-O bond, which is relative to the close- and open-ring forms 1-c and 1-o. The open-ring form 1-o displays the large static first hyperpolarizability relative to its close-ring form 1-c according to our DFT-FF calculations with three functionals. The electronic structure analysis and spin unrestricted calculations show that the redox processes significantly affect the geometrical structure of the TTF unit, and thus enhance the static first hyperpolarizabilities. The one-electron-oxidized species having good planar structure of the TTF unit are ~30 and ~200 times as large as that of the neutral compounds 1-c and 1-o, respectively. But the difference in the static first hyperpolarizability between one- and two-electron-oxidized states of spiropyran species is not substantial according to our DFT-FF calculations, and the spiropyran-merocyanine conversion of two-electron-oxidized species does not largely affect their static first hyperpolarizability. On the basis of the large change in the static first hyperpolarizability, our DFT-FF calculations support a five-state switching of the static first hyperpolarizability based on the redox and photoisomerization., (This journal is © the Owner Societies 2012)

High-valent transition-metal-substituted Keggin-type polyoxometalates (POMs) are active and robust oxidation catalyst. The important oxidized intermediates of these POM complexes are very difficult to be characterized by using the experimental method, and thus no detail information is available on such species. In the present paper, density functional theory (DFT) calculations have been carried out to characterize the electronic structures of a series of mono-ruthenium-substituted Keggin-type POMs. We find that the aquaruthenium(II/III/IV) species possess d(xy)(2)d(xz)(2)d(yz)(2), d(xy)(2)d(xz)(2)d(yz)(1), and d(xy)(2)d(xz)(1)d(yz)(1) electronic configuration, respectively, and hydroxyl/oxoruthenium(IV/V/VI) species possess d(xy)(2)d(xz)(1)π*(yz)(1), d(xy)(2)π*(xz)(1)π*(yz)(1), d(xy)(1)π*(xz)(1)π*(yz)(1), and d(xy)(1)π*(xz)(1)π*(yz)(0) electronic configuration, respectively. Mulliken spin population shows that spin density is localized on the ruthenium center in aquaruthenium(II/III/IV) POM complexes, and the RuO(a) unit in hydroxyl/oxoruthenium(IV/V/VI) POM complexes. The O(a) atom has substantial radical character in oxoruthenium(IV/V) species, and the radical character of the O(a) atom are significantly weakened in the oxoruthenium(VI) species. The relevant energy of the important Ru-O(a)π*-antibonding unoccupied orbitals with high RuO(a) compositions of oxoruthenium(IV/V/VI) POM complexes decrease in the order: oxoruthenium(IV) > oxoruthenium(V) > oxoruthenium(VI). The pH-independent multiple reduction energies for Ru(III/II), Ru(V/IV), and Ru(VI/V) couples are calculated, which is in agreement with the experimental data.

The second-order nonlinear optical (NLO) properties of porphyrin-metal-polyoxometalate (por-metal-POM) sandwich structures [(por)M(PW(11)O(39))](5-) (por = TPP, TPyP, TPPF(20), M = Hf; por = TPP, M = Zr) and [(TPP)Hf(XW(11)O(36))](6-) (X = Si, Ge) are investigated by time-dependent density functional theory (TDDFT). The character of charge-transfer transition indicates that the porphyrin ligand acts as an electron acceptor and the lacunary Keggin-type POM acts as an electron donor. Our results show that this kind of organic-inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability, approximately 100 x 10(-30) esu, and might be an excellent second-order NLO material. Furthermore, the NLO response can be tuned by the element substituents. The computed beta(0) values increase with the auxiliary electron-accepting group on the porphyrin ring (TPPF(20) > TPyP > TPP) and a heavy central heteroatom (Ge > Si > P). The present investigation provides important insight into the NLO properties of this class of por-metal-POM sandwich compound.