Catalytic hydrogenation of olefins by a multifunctional molybdenum-sulfur complex.

Exploration of molybdenum complexes as homogeneous hydrogenation catalysts has garnered significant attention, but hydrogenation of unactivated olefins under mild conditions are scarce. Here, we report the synthesis of a molybdenum complex, [Cp*Mo(Ph2PC6H4S-CH = CH2)(Py)]+ (2), which exhibits intriguing reactivity toward C2H2 and H2 under ambient pressure. This vinylthioether complex showcases efficient catalytic activity in the hydrogenation of various aromatic and aliphatic alkenes, demonstrating a broad substrate scope without the need for any additives. The catalytic pathway involves an uncommon oxidative addition of H2 to the cationic Mo(II) center, resulting in a Mo(IV) dihydride intermediate. Moreover, complex 2 also shows catalytic activity toward C2H2, leading to the production of polyacetylene and the extension of the vinylthioether ligand into a pendant triene chain.

[4+1] cyclization of α-diazo esters and mesoionic N-heterocyclic olefins.

Prompted by the recent stepwise mechanistic proposal for the Huisgen [3+2] cycloaddition reaction between enamine and α-diazo ester, where the nucleophilic addition of the enamine carbon onto the terminal nitrogen of diazo ester is crucial, we examined the possible use of N-heterocyclic olefins (NHOs) as highly electron-rich dipolarophiles in these reactions. The mesoionic NHOs derived from 1,2,3-triazoles undergo fast [4+1] cycloaddition to give 3-(triazolium-4-yl)-(3H)-pyrazol-4-olates at room temperature. The reaction mechanism has been explored through experimental and DFT computational studies.

Widespread formation of toxic nitrated bisphenols indoors by heterogeneous reactions with HONO.

With numerous structurally diverse indoor contaminants, indoor transformation chemistry has been largely unexplored. Here, by integrating protein affinity purification and nontargeted mass spectrometry analysis (PUCA), we identified a substantial class of previously unrecognized indoor transformation products formed through gas-surface reactions with nitrous acid (HONO). Through the PUCA, we identified a noncommercial compound, nitrated bisphenol A (BPA), from house dust extracts strongly binding to estrogen-related receptor γ. The compound was detected in 28 of 31 house dust samples with comparable concentrations (ND to 0.30 µg/g) to BPA. Via exposing gaseous HONO to surface-bound BPA, we demonstrated it likely forms via a heterogeneous indoor chemical transformation that is highly selective toward bisphenols with electron-rich aromatic rings. We used 15N-nitrite for in situ labeling and found 110 nitration products formed from indoor contaminants with distinct aromatic moieties. This study demonstrates a previously unidentified class of chemical reactions involving indoor HONO, which should be incorporated into the risk evaluation of indoor contaminants, particularly bisphenols.

Recent advances of mesoionic N-heterocyclic olefins.

This Perspective article highlights the recent development of mesoionic N-heterocyclic olefins (mNHOs), where the exo-cyclic olefinic carbon is not bonded to strongly electron-withdrawing groups. The unquenched basicity and nucleophilicity of the exo-cyclic olefinic carbon make mNHOs strong σ-donors and enable unique reactivity patterns.

Dioxygenation of unprotected mesoionic N-heterocyclic olefins.

We report the dioxygenation of mesoionic N-heterocyclic olefins (mNHOs) using molecular dioxygen. For 1,2,3-triazole-derived mNHOs possessing a vinyl proton and at least one acidic C-H group, they are oxidized into the corresponding triazolium benzoate salts, whereas those without vinyl proton or an acidic C-H group are oxidized into triazolium oxide and ketones/aldehydes.

Constructing fused N-heterocycles from unprotected mesoionic N-heterocyclic olefins and organic azides via diazo transfer.

Mesoionic N-heterocyclic olefins (mNHOs) were first reported last year and their reactivity remains largely unexplored. Herein we report the reaction of unprotected mNHOs and organic azides as a novel synthetic route to a variety of pyrazolo[3,4-d][1,2,3]triazoles, an important structural motif in drug candidates and energetic materials. The only byproduct aniline can be easily recycled and converted back to the starting organic azide, in compliance with the green chemistry principle. The reaction mechanism has been explored through experimental and computational studies.

[2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction.

As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.

Syntheses and characterizations of iron complexes of bulky o-phenylenediamide ligand.

We report the syntheses of a family of tetrahedral iron complexes bearing a bulky redox active o-phenylenediamide ligand. The electronic structures of these complexes have been investigated by Mössbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography.

Iron N-heterocyclic carbene complexes in homogeneous catalysis.

The development of homogeneous iron catalysts has been a prolific research field in recent years. Many catalytic reactions are no longer limited to noble metals. The use of N-heterocyclic carbenes (NHCs) as ancillary ligands has made substantial impact in noble metal catalysis and also started to gain popularity in iron catalysis in recent years. This review article surveys the recent literature on Fe-NHC-catalyzed chemical reactions, including C-C bond formation, reduction, and oxidation reactions.

3D porous metal-organic framework for selective adsorption of methane over dinitrogen under ambient pressure.

We report a highly porous 3D metal-organic framework (MOF) that shows potential for coal mine methane (CMM) capture.

Constructing reactive Fe and Co complexes from isolated picolyl-functionalized N-heterocyclic carbenes.

We report the isolation of free picolyl-functionalized N-heterocyclic carbenes (NHCs), which serve as versatile precursors to access low coordinate iron and cobalt complexes. The reactivities of these new iron and cobalt complexes towards catalytic hydrosilylation of ketones have also been explored. For example, low loadings (0.05-1 mol%) of a four-coordinate iron complex bearing two deprotonated picolyl-NHC ligands can effect the fast catalytic reduction of ketones using the inexpensive industrial byproduct polymethylhydrosiloxane (PMHS) as the reductant at ambient temperature.

A Hydride-Shuttle Mechanism for the Catalytic Hydroboration of CO2.

Herein we report our investigation into the mechanism of CO2 reduction by HBpin catalyzed by [Ru(CO)H(L)(PPh3)2] (2; L is the 4,5-diazafluorenyl ligand with a Bpin functional group at the 9-position) through computational studies using the model complex [Ru(CO)H(L)(PMe3)2] (A1). The reaction consists of four stages: (1) CO2 insertion into the C-B bond of A1 to form A4, (2) the reduction of A4 by HBpin to afford HCOOBpin (P2) and regenerate A1, (3) the reduction of P2 by HBpin to HCHO (P5), and (4) the reduction of P5 to CH3OBpin (P6). We found that Lewis adduct formation plays a key role in all stages of the mechanism, in that it forms more relaxed rings in the key transition states and makes the hydride more hydridic. Oftentimes, the hydride and Bpin moieties can transfer within the Lewis adducts in a concerted manner in our proposed hydride-shuttle mechanism. The energy spans for all stages of our proposed mechanism are within the range of 15.7-22.6 kcal/mol in terms of Gibbs free energy. In contrast, the direct hydroboration and σ-bond metathesis mechanisms proposed in the literature have extremely high energy barriers because of the highly strained four-membered rings in the transition states and the unactivated hydride in HBpin.

Unusual Rearrangement of an N-Donor-Functionalized N-Heterocyclic Carbene Ligand on Group 8 Metals.

We report an unexpected rearrangement of a deprotonated picolyl-functionalized N-heterocyclic carbene (NHC) ligand from N,C-chelate to N,N-chelate in three-legged piano-stool Fe(II) and Ru(II) complexes. The reaction mechanism has been explored for one of the Fe(II) complexes. Experimental and computational studies suggest an unusual intermediate featuring a four-membered chelate ring, where the NHC and the α-carbon of one of the N-substituents coordinate to the Fe(II) center. A possible Fe-alkylidene intermediate has also been predicted by computations.

1,1-Hydroboration and a Borane Adduct of Diphenyldiazomethane: A Potential Prelude to FLP-N2 Chemistry.

Diphenyldiazomethane reacts with HB(C6 F5 )2 and B(C6 F5 )3 , resulting in 1,1-hydroboration and adduct formation, respectively. The hydroboration proceeds via a concerted reaction involving initial formation of the Lewis adduct Ph2 CN2 BH(C6 F5 )2 . The highly sensitive adduct Ph2 CN2 (B(C6 F5 )3 ) liberates N2 and generates Ph2 CB(C6 F5 )3 . DFT computations reveal that formation of Ph2 CN2 B(C6 F5 )3 from carbene, N2 , and borane is thermodynamically favourable, suggesting steric frustration could preclude carbene-borane adduct formation and affect FLP-N2 capture.

Chemical reduction of CO2 facilitated by C-nucleophiles.

The abundance of atmospheric CO2 presents both an opportunity and a challenge for synthetic chemists to transform CO2 into value-added products. A promising strategy involves CO2 reduction driven by the energy stored in chemical bonds and promoted by molecules containing nucleophilic carbon sites. This approach allows the synthesis of new C-C or C-H bonds from CO2-derived carbon. The first part of this Feature article deals with uncatalyzed reductions of CO2 such as insertion into metal-carbon bonds and reactivity towards multidentate actor ligands and metal-free compounds. The second part covers catalytic reduction of CO2 in which a nucleophilic C-site is involved. This review brings together two general approaches in the chemical CO2 reduction field, showing how the discovery of fundamental reactivity of CO2 leads to synthetic applications, and proposes directions for further development.

Reactivity of Fe and Ru Complexes of Picolyl-Substituted N-Heterocyclic Carbene Ligand: Diverse Coordination Modes and Small Molecule Binding.

Complexes [MClCp*(HL)] (1[Fe]/1[Ru]) (where HL = 1-mesityl-3-(pyridin-2-ylmethyl)imidazol-2-ylidene) were synthesized from the reaction of in situ generated HL ligand and [FeClCp*(TMEDA)] (where TMEDA is N,N,N',N'-tetramethylethylenediamine) or [RuClCp*]4, respectively. The deprotonation of 1[Fe]/1[Ru] with 1 equiv of LiHMDS led to cyclometalation through the o-Me of the mesityl group, forming [MCp*(L-κC,κC',κN)] 2[Fe]/2[Ru]. The coordinatively unsaturated compounds [MCp*(HL)]BPh4 3[Fe]/3[Ru] were prepared from 1[Fe]/1[Ru] and halide scavenger NaBPh4. Complex 3[Ru] showed agostic interactions between the o-Me group of the mesityl moiety and the metal center in solution and the solid state. When the vacant coordination site of 3[Fe]/3[Ru] is occupied by CO, the resulting [MCp*(CO)(HL)]BPh4 4[Fe]/4[Ru] can be deprotonated with 1 equiv of KHMDS at the pyridylic position to afford complexes [MCp*L'(CO)] 5[Fe]/5[Ru], where the L'- ligand chelates to the metal center through the nitrogen donor atom of the dearomatized pyridine ring and the carbene carbon. Complex 2[Fe] reacted rapidly with CO to afford the simple ligand substitution product [FeCp*(L-κC,κC')(CO)] 6[Fe], where the L- acts as a bidentate chelating ligand through the carbene carbon and benzylic carbon. Under the same condition, the reaction of 2[Ru] with CO forms [RuCp*L″(CO)] 7[Ru], where the L″- ligand (an isomer of L- and L'-) chelates to the metal center through the carbene carbon and a pyridyl carbon. Complexes 3[Fe]/3[Ru] reversibly bind dinitrogen to form [MCp*(HL)(N2)]BPh4 8[Fe]/8[Ru]. 3[Ru] reversibly binds dihydrogen to give [MCp*(H2)(HL)]BPh4 9[Ru], while no reaction was observed between 3[Fe] and H2. The reaction of 3[Ru] with dioxygen led to the isolation of a stable side-on O2 complex [RuCp*(HL)(O2)]BPh4 10[Ru], while the reaction of 3[Fe] with dioxygen led to an intractable mixture of products.

Homoleptic iron(ii) and cobalt(ii) bis(phosphoranimide) complexes for the selective hydrofunctionalization of unsaturated molecules.

Low-coordinate homoleptic bulky M2(NPtBu3)4 (M = Fe (A), Co (B)) complexes were synthesized and characterized as dimeric structures by crystallographic studies. The iron complex A can catalyze the hydroboration reaction of aldehydes and ketones. The cobalt complex B outperformed its iron counterpart in hydrogenations of several typical alkenes and alkynes under mild conditions. Poisoning experiments indicate that the Co(ii)/HBpin catalytic system could be homogeneous.

Reactivity of heavy carbene analogues towards oxidants: a redox active ligand-enabled isolation of a paramagnetic stannylene.

o-Phenylenediamido Pb(ii) and Sn(ii) complexes react with mesityl azide in analogous C-H activation reactions. The Pb(ii) complex forms a Lewis pair with ONMe3. The Sn(ii) analogue reacts with AgOTf yielding a paramagnetic stannylene; such compounds have been previously observed in situ, but hitherto have not been isolated.

Iron-Catalyzed gem-Specific Dimerization of Terminal Alkynes.

We report a gem-specific homo- and cross-dimerization of terminal alkynes catalyzed by a well-defined iron(II) complex containing Cp* and picolyl N-heterocyclic carbene (NHC) ligands, and featuring a piano-stool structure. This catalytic system requires no additives and is compatible with a broad range of substrates, including those with polar functional groups such as NH and OH.

Zwitterionic indenylammonium with carbon-centred reactivity towards reversible CO2 binding and catalytic reduction.

We report the synthesis and characterization of a zwitterionic indenylammonium compound and its carbon-centred reactivity towards reversible CO2 binding at ambient temperature through its formal insertion into a C-H bond as well as the catalytic hydroboration of CO2 to methanol derivatives.