Unlocking reactivity: synthetic, structural and catalytic exploration of ruthenium( ii ) complexes featuring pdc and NHC ligands

Synthesis of chiral N-heterocyclic carbene (NHC) ligand precursors and formation of ruthenium(II) complexes for transfer hydrogenation catalysts

Two dimensional (2D) materials such as graphene and transition metal dichalcogenide (TMDC) like MoS2, WTe2 have brought widespread attention as their novel 2D confined properties and applications in nano devices. Among them, the up to date multilayer PtSe2 has been reported to be high mobility, air stable and possess novel phenomenon like Dirac fermions but to date have little study as the tunable electronic properties via structural control. Here we use the first principle calculations based on density functional theory (DFT) to study the tunable structure and electronic properties of monolayer and multilayer PtSe2 by the method of strain. We find that when apply compress strain on monolayer PtSe2 at -3% or more, the photoluminescence will enhance due to a larger density of states at conductance band minimum (CBM). With the increase of layer number, the band gap become small rapidly. The band gap change from 1.3 eV for monolayer to 0.4 eV for bilayer. With three layers, the band gap becomes 0.1 eV. Begin at four layer, the PtSe2 multilayer become a negative band gap semimetal. The DOS under VBM is small for multilayers due to the large splitting between the first valence band with the second valence band. This indicate the possible low photoluminescence strength for these multilayers. Our results can pave a way for the experiment electronic and optical properties tuning in multilayer PtSe2 and possible in the similar TMDCs.

ConspectusMolecules and materials with easily tunable electronic structures and properties are at the forefront of contemporary research. π-Conjugation is fundamental in organic chemistry and plays a key role in the design of molecular materials. In this Account, we showcase the applicability of N-heterocyclic vinyl (NHV) substituents based on classical N-heterocyclic carbenes (NHCs) for tuning the structure, properties, and stability of main-group species (E) via π-conjugation and/or π-donation.NHVs such as [(NHC)═CR] (R = H or aryl) are monoanionic ligands formally derived by the deprotonation of N-heterocyclic olefins (NHOs), (NHC)═CHR. Further deprotonation of [(NHC)═CR] (R = H) is viable, giving rise to N-heterocyclic vinylidene (NHVD) species such as (NHC)═C. NHVs and NHVDs feature a highly polarizable exocyclic CNHC═C bond because of the presence of adjacent π-donor nitrogen atoms. The nature of the NHC, in particular the π-acceptor property, has a direct consequence on the polarity of the CNHC═C bond and hence on the magnitude of π-conjugation in the derived molecules. Thus, the electronic structure, especially the energy and shape of frontier molecular orbitals, HOMO and LUMO, of derived species can be fine-tuned by a judicious choice of the carbene unit. For instance, the HOMO of classical diphosphenes (RP═PR) (R = alkyl or aryl) is invariably the phosphorus lone-pair orbital, while the P═P π-bond is HOMO - 1 or HOMO - 2. In strong contrast, the HOMO of divinyldiphosphenes (R = NHV) is mainly the P═P π-bond. This is owing to the π-conjugation, resulting in the lowering of the LUMO and raising of the HOMO energy. They have a remarkably small HOMO-LUMO energy gap (4.15-4.50 eV) and readily undergo 1e-oxidations, giving rise to stable radical cations and dications.By employing a similar approach, one can access divinyldiarsenes and the corresponding radical cations and dications as crystalline solids. The use of divinyldiphosphenes and divinyldiarsenes as promising ligands in the stabilization of metalloradicals has been shown. By a logical selection of singlet carbenes, stable 2-phosha-1,3-butadiene and 2-arsa-1,3-butadiene compounds, as well as related radical cations and dications, can be prepared as crystalline solids.The relevance of NHV ligands as potent π-donors has been demonstrated for the stabilization of elusive electrophilic phosphinidene and arsinidene complexes {(NHV)E}Fe(CO)4 (E = P or As). Moreover, stable singlet diradicaloid [(NHC)CP]2 and p-quinodimethane derivatives [(NHC)CP2]2 based on an NHVD framework are accessible as stable solids.In this Account, a special emphasis is given to the contributions from this laboratory. The author hopes that this Account will serve as a useful reference guide for researchers interested in studying and applying NHV and NHVD scaffolds in modern molecular chemistry and materials sciences.

The ability to acquire and transfer hydrogen catalytically is fundamental to the development of clean and sustainable fuels. N-heterocyclic carbene (NHC) based ruthenium complexes were synthesized and studied as catalysts for the transfer hydrogenation of ketones. Variations in the catalyst structure were investigated for their impact on hydrogenation. Catalyst attributes included bis- or mono-NHC ligands, pendant ether groups, and arene ligands of varied bulk and donor strength. Variable temperature ¹H NMR studies indicated that arene lability increases in the order hexamethylbenzene < cymene < benzene, and this lability is directly correlated with catalytic activity. The catalysis appears to be homogeneous, and a mechanism invoking arene loss is proposed. The S enantiomers of ester functionalized NHC ligand precursors were synthesized from L-valine. Coordination of these NHC ligands to Ru(II) resulted in the isolation of a monodentate complex (with only NHC coordination) and a bidentate complex, which has carboxylate coordination (from in situ hydrolysis of the ester) in addition to NHC coordination. The evidence shows that both Ru complexes are racemic, but with alternate synthetic methods perhaps racemization during complexation of ligand precursors to metals could be avoided. These ruthenium complexes also serve as catalysts for ketone transfer hydrogenation. Highly active iridium precatalysts for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands are reported. These ligands can readily be protonated/deprotonated in situ to alter the electronic properties at the metal in a switchable and reversible manner. Comparison of initial rates at pH 3-6 with Ir(dhbp) complexes and complexes with bipyridine ligands that lack protic groups, showed that rate enhancement with dhbp complexes occurs at high pH due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes electronic properties upon deprotonation, although preliminary studies suggest a mechanism involving O⁻ groups shuttling protons may also contribute to some extent. Mechanistic studies show that the rate law is first-order in iridium precatalyst, and all evidence indicates that catalysis is homogeneous. Base-free transfer hydrogenation and the direct hydrogenation of ketones and carbon dioxide were also investigated.

Doping of bulk silicon and III-V materials has paved the foundation of the current semiconductor industry. Controlled doping of 2D semiconductors, which can also be used to tune their bandgap and type of carrier thus changing their electronic, optical, and catalytic properties, remains challenging. Here the substitutional doping of nonlike element dopant (Mn) at the Mo sites of 2D MoS2 is reported to tune its electronic and catalytic properties. The key for the successful incorporation of Mn into the MoS2 lattice stems from the development of a new growth technology called dual-additive chemical vapor deposition. First, the addition of a MnO2 additive to the MoS2 growth process reshapes the morphology and increases lateral size of Mn-doped MoS2 . Second, a NaCl additive helps in promoting the substitutional doping and increases the concentration of Mn dopant to 1.7 at%. Because Mn has more valance electrons than Mo, its doping into MoS2 shifts the Fermi level toward the conduction band, resulting in improved electrical contact in field effect transistors. Mn doping also increases the hydrogen evolution activity of MoS2 electrocatalysts. This work provides a growth method for doping nonlike elements into 2D MoS2 and potentially many other 2D materials to modify their properties.

Carbon-doped boron nitride nanostructures including nanosheets, nanoribbons, and nanotubes have drawn enormous research attention because of their tunable electronic properties and widespread applications. In this work, we explore the electronic and magnetic properties of graphene flake-doped single-walled boron nitride nanotubes (BNNTs) on the basis of first-principles calculations. Theoretical results reveal that the band structures of these doped BNNTs can be effectively engineered by embedding graphene flakes with different sizes and shapes. Moreover, the Lieb theorem works for the triangle graphene flake-doped BNNTs, and the corresponding doped systems are ferromagnetic, originating from the spin-polarized interface states. All BNNTs embedded with the triangular graphene flakes with relatively small sizes are typical bipolar magnetic semiconductors, which can be easily tuned into half-metals by carrier doping, opening the door to their promising applications in spintronic devices.

Ruthenium(II) complexes bearing N-heterocyclic carbene ligands with wingtip groups and their catalytic activity in the transfer hydrogenation of ketones

Palladium(II) and ruthenium(II) complexes of benzotriazole functionalized N-heterocyclic carbenes: Cytotoxicity, antimicrobial, and DNA interaction studies

A combinational effect of nanostructured crystallites and π-bonded interfaces is much attractive in solving the conflict between strength/hardness and toughness to design extrinsically superhard materials with enhanced fracture toughness and/or other properties such as tunable electronic properties. In the present work, taking the experimentally observed π-bonded interfaces in nanostructured diamond as the prototype, we theoretically investigated their stabilities, electronic structures, and mechanical strengths with special consideration of the size effect of nanocrystallites or nanolayers. It is unprecedentedly found that the π-bonded interfaces exhibit tunable electronic semiconducting properties, superior fracture toughness, and anomalously large creep-like plasticity at the cost of minor losses in strength/hardness; such unique combination is uncovered to be attributed to the ductile bridging effect of the sp2 bonds across the π-bonded interface that dominates the localized plastic flow channel. As the length scale of nanocrystallites/nanolayers is lower than a critical value, however, the first failure occurring inside nanocrystallites/nanolayers features softening and embrittling. These findings not only provide a novel insight into the unique strengthening and toughening origin observed in ultrahard nanostructured diamonds consisting of nanotwins, nanocomposites, and nanocrystallites but also highlight a unique pathway by combining the nanostructured crystallites and the strongly bonded interface to design the novel superhard materials with superior toughness.

Mesoporous materials with carbon framework structure can offer distinctive functionalities with tunable electronic or catalytic properties. Many synthetic routes including hard or soft templating approaches are developed for the fabrication of various ordered mesoporous carbon based materials which have demonstrated unique catalytic and energy storage properties. So far, most of these techniques deliver only mesoporous carbon with amorphous wall structures which limit their performance in many applications. Fullerenes exhibit unique structure and significant properties including superconductivity, electrochemical stability, and heat resistance. Herein, for the first time, the preparation of highly ordered mesoporous fullerene C70 materials with tunable porous structure and controlled rod‐shaped morphology through the thermal oligomerization of fullerene C70 molecules inside the mesopore channels of SBA‐15 silica as a hard template with the help of chlorinated aromatics, wherein the solubility of fullerenes is high, is reported. It is demonstrated that these metal‐free mesoporous fullerene C70 framework with a high surface area and bimodal pores with multifunctionality exhibit excellent performance in the oxygen reduction reaction for fuel cells and supercapacitors. This simple strategy can also be extended to other fullerene nanostructures with different carbon atoms which can exhibit interesting physicochemical properties and find applications in catalysis and energy storage.

N-Heterocyclic carbenes (NHCs) are versatile ligands in organometallic chemistry, prized for their strong σ-donating and tunable electronic properties. They are used to stabilize a wide range of motifs, including clusters and nanoparticles, based in particular on coinage metals─Cu, Ag, and Au. Notably, the carbene 13C NMR isotropic chemical shift (δiso) of NHC-coinage metal complexes varies significantly across these elements, reflecting the nuanced interplay of electronic and structural factors. Here, we study the carbene carbon chemical shift in NHC-Au(I)-X complexes (X = H, OH, halides, CN, N3, and neutral ligands such as pyridine and NHC) compared to the Cu and Ag congeners. Density functional theory calculations are used to analyze the chemical shielding tensor components, revealing that stronger σ-donor X-ligands lead to greater deshielding of δiso through enhanced paramagnetic contributions and, for Au, spin-orbit contributions of comparable magnitude. Moreover, a correlation between the spin-orbit contribution to the chemical shift (σso) and the Au-carbene bond distance highlights the critical role of trans-influence in modulating spin-orbit coupling and the overall chemical shift. Analysis of σso shows that stronger σ-donor ligands, associated with a greater trans-influence and elongated Au-carbene bond, lead to a higher-lying NHC-Au σ-bond and lower-lying π*-orbital, ultimately yielding greater deshielding and higher 13C chemical shift. This work provides insight into how structural and electronic factors govern carbene chemical shifts in NHC-based Au complexes and clusters, establishing a direct link between NMR spectroscopic descriptors and electronic structure, thus opening avenues for developing structure-activity relationships in catalysis and materials science.

N-heterocyclic carbene (NHC) based ruthenium complexes were studied as catalysts for the transfer hydrogenation of ketones. Variations in the catalyst structure were investigated for their impact on hydrogenation and catalyst stability. Catalyst attributes included bis- or mono-NHC ligands, pendant ether groups in some cases, and arene ligands of varied bulk and donor strength. Ruthenium complexes were synthesized and fully characterized, including complexes with a monodentate NHC ligand containing a tethered ether N substituent (ImEt,CH2CH2OEtRuCl2(η6-arene); arene = benzene (4), p-cymene (5), hexamethylbenzene (6)), a complex with a monodentate NHC ligand with solely alkyl N substituents (ImEt,PentylRuCl2(η6-p-cymene) (8)), and a complex with a bis-NHC ligand ([RuCl(methylenebis(ImEt)2)(η6-p-cymene)]PF6 (7)) (Im = imidazole-derived NHC; superscripts indicate N substituents). X-ray crystal structures were obtained for 4, 5, 7, and 8. All of the ruthenium complexes were tested and found to be active trans...

Tunable electronic, optical, and spintronic properties in InSe/MTe2 (M = Pd, Pt) van der Waals heterostructures

Free standing centimeter-long 1D nanostructures are highly attractive for electronic and optoelectronic devices due to their unique photophysical and electrical properties. Here a simple, large-scale synthesis of centimeter-long 1D carbon nitride (CN) needles with tunable photophysical, electric, and catalytic properties is reported. Successful growth of ultralong needles is acquired by the utilization of 1D organic crystal precursors comprised of CN monomers as reactants. Upon calcination at high temperatures, the shape of the starting crystal is fully preserved while the CN composition and porosity, and optical and electrical properties can be easily tuned by tailoring the starting elements ratio and final calcination temperature. The facile manipulation and visualization of the CN needles endow their direct electrical measurements by placing them between two conductive probes. Moreover, the CN needles exhibit good photocatalytic activity for hydrogen production owing to their improved light harvesting properties, high surface area, and advantageous energy bands position. The new growth strategy developed here may open opportunities for a rational design of CN and other metal-free materials with controllable directionality and tunable photophysical and electronic properties, toward their utilization in (photo)electronic devices.

Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R(2)C:) such as silylenes (R(2)Si:), germylenes (R(2)Ge:), stannylenes (R(2)Sn:), and plumbylenes (R(2)Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl(2) and SiCl(2) are highly reactive intermediates and play an important role in many chemical transformations. GeCl(2) can be stabilized as a dioxane adduct, whereas SnCl(2) and PbCl(2) are available as stable compounds. In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl(3) at 1150 °C, chemists proposed dichlorosilylene (SiCl(2)) as an intermediate, which further dissociates to Si and SiCl(4). Similarly, base induced disproportionation of HSiCl(3) or Si(2)Cl(6) to SiCl(2) is a known reaction. Trapping these products in situ with organic substrates suggested the mechanism for this reaction. In addition, West and co-workers reported a polymeric trans-chain like perchloropolysilane (SiCl(2))(n). However, the isolation of a stable free monomeric dichlorosilylene remained a challenge. The first successful attempt of taming SiCl(2) was the isolation of monochlorosilylene PhC(NtBu)(2)SiCl supported by an amidinate ligand in 2006. In 2009, we succeeded in isolating N-heterocyclic carbene (NHC) stabilized dichlorosilylene (NHC)SiCl(2) with a three coordinate silicon atom. (The NHC is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes).) Notably, this method allows for the almost quantitative synthesis of (NHC)SiCl(2) without using any hazardous reducing agents. Dehydrochlorination of HSiCl(3) with NHC under mild reaction conditions produces (NHC)SiCl(2). We can separate the insoluble side product (NHC)HCl readily and recycle it to form NHC. The high yield and facile access to dichlorosilylene allow us to explore its chemistry to a greater extent. In this Account, we describe the results using (NHC)SiCl(2) primarily from our laboratory, including findings by other researchers. We emphasize the novel silicon compounds, which supposedly existed only as short-lived species. We also discuss silaoxirane, silaimine with tricoordinate silicon atom, silaisonitrile, and silaformyl chloride. In analogy with N-heterocyclic silylenes (NHSis), oxidative addition reactions of organic substrates with (NHC)SiCl(2) produce Si(IV) compounds. The presence of the chloro-substituents both on (NHC)SiCl(2) and its products allows metathesis reactions to produce novel silicon compounds with new functionality. These substituents also offer the possibility to synthesize interesting compounds with low-valent silicon by further reduction. Coordination of NHC to the silicon increases the acidity of the backbone protons on the imidazole ring, and therefore (NHC)SiCl(2) can functionalize NHC at the C-4 or C-5 position.