Selective C–H Bond Activation via NOx-Mediated Generation of Strong H-Abstractors

Multiple Promotional Effects of Vanadium Oxide on Boron Nitride for Oxidative Dehydrogenation of Propane

Featuring high olefin selectivity, hexagonal boron nitride (h-BN) has emerged recently as an attractive catalyst for oxidative dehydrogenation of propane (ODHP). Herein, we report that dispersion of vanadium oxide onto BN facilitates the oxyfunctionalization of BN to generate more BO _x active sites to catalyze ODHP via the Eley-Rideal mechanism and concurrently produce nitric oxide to initiate additional gas-phase radical chemistry and to introduce redox VO _x sites to catalyze ODHP via the Mars-van Krevelen mechanism, all of which promote the catalytic performance of BN for ODHP. As a result, loading 0.5 wt % V onto BN has doubled the yield of light alkene (C₂-C₃) at 540-580 °C, and adding an appropriate concentration of NO in the reactants further enhances the catalytic performance. These results provide a potential strategy for developing efficient h-BN-based catalysts through coupling gas-phase and surface reactions for the ODHP process.

Ligand-Controlled Csp2-H versus Csp3-H Bond Formation in Cycloplatinated Complexes: A Joint Experimental and Theoretical Mechanistic Investigation

The cyclometalated platinum(II) complexes [PtMe(C^∧N)(L)] [1_PS: C^∧N = 2-phenylpyridinate (ppy), L = SMe₂; 1_BS: C^∧N = benzo[h]quinolate (bhq), L = SMe₂; 1_PP: C^∧N = ppy, L = PPh₃; and 1_BP: C^∧N = bhq, L = PPh₃] containing two different cyclometalated ligands and two different ancillary ligands have been investigated in the reaction with CX₃CO₂H (X = F or H). When L = SMe₂, the Pt-Me bond rather than the Pt-C bond of the cycloplatinated complex is cleaved to give the complexes [Pt(C^∧N)(CX₃CO₂)(SMe₂)]. When L = PPh₃, the selectivity of the reaction is reversed. In the reaction of [PtMe(C^∧N)(PPh₃)] with CF₃CO₂H, the Pt-C^∧N bond is cleaved rather than the Pt-Me bond. The latter reaction gave [PtMe(κ¹N-Hppy)(PPh₃)(CF₃CO₂)] as an equilibrium mixture of two isomers. For L = PPh₃, no reaction was observed with CH₃CO₂H. The reasons for this difference in selectivity for complexes 1 are computationally discussed based on the energy barrier needed for the protonolysis of the Pt-C_sp³ bond versus the Pt-C_sp² bond. Two pathways including the direct one-step acid attack at the Pt-C bond (S_E2) and stepwise oxidative-addition on the Pt(II) center followed by reductive elimination [S_E(ox)] are proposed. A detailed density functional theory (DFT) study of these protonations along with experimental UV-vis kinetics suggests that a one-step electrophilic attack (S_E2) at the Pt-C bond is the most likely mechanism for complexes 1, and changing the nature of the ancillary ligand can influence the selectivity in the Pt-C bond cleavage. The effect of the nature of the acid and cyclometalated ligand (C^∧N) is also discussed.

Selective Metal-Ligand Bond-Breaking Driven by Weak Intermolecular Interactions: From Metamagnetic Mn(III)-Monomer to Hexacyanoferrate(II)-Bridged Metamagnetic Mn2Fe Trimer

Metal-ligand coordination interactions are usually much stronger than weak intermolecular interactions. Nevertheless, here, we show experimental evidence and theoretical confirmation of a very rare example where metal-ligand bonds dissociate in an irreversible way, helped by a large number of weak intermolecular interactions that surpass the energy of the metal-ligand bond. Thus, we describe the design and synthesis of trinuclear Mn₂Fe complex {[Mn(L)(H₂O)]₂Fe(CN)₆},^2- starting from a mononuclear Mn(III)-Schiff base complex: [Mn(L)(H₂O)Cl] (1) and [Fe(CN)₆]^4- anions. This reaction implies the dissociation of Mn(III)-Cl coordination bonds and the formation of Mn(III)-NC bonds with the help of several intermolecular interactions. Here, we present the synthesis, crystal structure, and magnetic characterization of the monomeric Mn(III) complex [Mn(L)(H₂O)Cl] (1) and of compound (H₃O)[Mn(L)(H₂O)₂]{[Mn(L)(H₂O)]₂Fe(CN)₆}·4H₂O (2) (H₂L = 2,2'-((1E,1'E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))bis(4-methoxyphenol)). Complex 1 is a monomer where the Schiff base ligand (L) is coordinated to the four equatorial positions of the Mn(III) center with a H₂O molecule and a Cl^- ion at the axial sites and the monomeric units are assembled by π-π and hydrogen-bonding interactions to build supramolecular dimers. The combination of [Fe(CN)₆]^4- with complex 1 leads to the formation of linear Mn-NC-Fe-CN-Mn trimers where two trans cyano groups of the [Fe(CN)₆]^4- anion replace the labile chloride from the coordination sphere of two [Mn(L)(H₂O)Cl] complexes, giving rise to the linear anionic {[Mn(L)(H₂O)]₂Fe(CN)₆}^2- trimer. This Mn₂Fe trimer crystallizes with an oxonium cation and a mononuclear [Mn(L)(H₂O)₂]⁺ cation, closely related to the precursor neutral complex [Mn(L)(H₂O)Cl]. In compound 2, the Mn₂Fe trimers are assembled by several hydrogen-bonding and π-π interactions to frame an extended structure similar to that of complex 1. Density functional theoretical (DFT) calculations at the PBE1PBE-D3/def2-TZVP level show that the bond dissociation energy (-29.3 kcal/mol) for the Mn(III)-Cl bond is smaller than the summation of all the weak intermolecular interactions (-30.1 kcal/mol). Variable-temperature magnetic studies imply the existence of weak intermolecular antiferromagnetic couplings in both compounds, which can be can cancelled with a critical field of ca. 2.0 and 2.5 T at 2 K for compounds 1 and 2, respectively. The magnetic properties of compound 1 have been fit with a simple S = 2 monomer with g = 1.959, a weak zero-field splitting (|D| = 1.23 cm^-1), and a very weak intermolecular interaction (zJ = -0.03 cm^-1). For compound 2, we have used a model with an S = 2 monomer with ZFS plus an S = 2 antiferromagnetically coupled dimer with g = 2.009, |D| = 1.21 cm^-1, and J = -0.42 cm^-1. The metamagnetic behavior of both compounds is attributed to the weak intermolecular π-π and hydrogen-bonding interactions.

Oxidative dehydrogenation of propane on the oxygen adsorbed edges of boron nitride nanoribbons

A N₂O/NOx-type site on the boron nitride nanoribbon edge is proposed as highly selective, active site for the oxidative dehydrogenation of propane.