Accessing Heteroannular Benzoxazole and Benzimidazole Scaffolds via Carbodiimides Using Azide–Isocyanide Cross-Coupling as Catalyzed by Mesoionic Singlet Palladium Carbene Complexes Derived from a Phenothiazine Moiety

The coupling of aryl and aliphatic azides with isocyanides yielding carbodiimides (8–17) were efficiently catalyzed by well-defined structurally characterized trans-(MIC)PdI2(L) [MIC = 1-CH2Ph-3-Me-4-(CH2N(C6H4)2S)-1,2,3-triazol-5-ylidene, L = NC5H5 (4), MesNC (5)], trans-(MIC)2PdI2 (6), and cis-(MIC)Pd(PPh3)I2 (7) type palladium complexes, which incidentally mark the first instances of the use of mesoionic singlet palladium carbene complexes for the said application. As observed from the product yields, the catalytic activity varied in the order 4 > 5 ∼ 6 > 7 for these complexes. A detailed mechanistic studies indicated that the catalysis proceeded via a palladium(0) (4a–7a) species. Using a representative palladium precatalyst (4), the azide–isocyanide coupling was successfully extended to synthesizing two different bioactive heteroannular benzoxazole (18–22) and benzimidazole (23–27) derivatives, thereby broadening the scope of the catalytic application

Accessing Heteroannular Benzoxazole and Benzimidazole Scaffolds via Carbodiimides Using Azide–Isocyanide Cross-Coupling as Catalyzed by Mesoionic Singlet Palladium Carbene Complexes Derived from a Phenothiazine Moiety

The coupling of aryl and aliphatic azides with isocyanides yielding carbodiimides (8–17) were efficiently catalyzed by well-defined structurally characterized trans-(MIC)PdI2(L) [MIC = 1-CH2Ph-3-Me-4-(CH2N(C6H4)2S)-1,2,3-triazol-5-ylidene, L = NC5H5 (4), MesNC (5)], trans-(MIC)2PdI2 (6), and cis-(MIC)Pd(PPh3)I2 (7) type palladium complexes, which incidentally mark the first instances of the use of mesoionic singlet palladium carbene complexes for the said application. As observed from the product yields, the catalytic activity varied in the order 4 > 5 ∼ 6 > 7 for these complexes. A detailed mechanistic studies indicated that the catalysis proceeded via a palladium(0) (4a–7a) species. Using a representative palladium precatalyst (4), the azide–isocyanide coupling was successfully extended to synthesizing two different bioactive heteroannular benzoxazole (18–22) and benzimidazole (23–27) derivatives, thereby broadening the scope of the catalytic application

Synthesis and Structure of a Magnesium Hydroxide Complex Supported by Tris(pyrazolyl)hydroborato Ligation, {[Tp^Ar,Me]Mg(μ-OH)}₂ (Ar = <i>p</i>-Bu^tC₆H₄)

Synthesis and Structure of a Magnesium Hydroxide Complex Supported by Tris(pyrazolyl)hydroborato Ligation, {[TpAr,Me]Mg(μ-OH)}2 (Ar = p-ButC6H4

Selectivity and Reactivity of Pd-Rich PdGa Surfaces toward Selective Hydrogenation of Acetylene: Interplay of Surface Roughness and Ensemble Effect

Recent experiments have shown that PdGa nanocrystallites act as highly selective and reactive catalyst for selective hydrogenation of acetylene to ethylene. Motivated by these experimental results we have studied the mechanism and energetics of the above reaction on low indexed (100) and (110) PdGa surfaces using first-principles density functional theory based calculations. We find that the energetically favorable (100) surface created by cleaving the crystal in the less dense region shows reasonably good selectivity and high reactivity. The reactivity on this surface is comparable to that observed on Pd(111) surfaces. Since this surface termination is stable over a wide range of Ga chemical potential and hence is likely to occupy a substantial fraction of the surface area of PdGa nanocrystallites, we suggest this termination is responsible for the selectivity and reactivity exhibited by PdGa. In contrast to other surfaces where hydrogen adsorption and dissociation is followed by acetylene adsorption and hydrogenation, on this surface we identify a novel reaction mechanism in which hydrogen dissociation occurs in the presence of acetylene. A careful analysis of the factors determining the selectivity shows that selectivity results due to an interplay between surface roughness and chemical nature of the reactive ensemble

C‑Vacancy Mediated Methane Activation and C–C Coupling on TiC(001) Surfaces: A First-Principles Investigation

Methane, the main component of natural gas, is one of the major greenhouse gases contributing to global warming. Therefore, capturing methane and converting it to other useful products are highly desirable. Methane activation is challenging due to the high energy of the C–H bonds and the nonpolar, nonreactive nature of the molecule. In this work, using density functional theory-based calculations and ab initio thermodynamic analysis, we have studied the role of C-vacancies on a TiC(001) surface toward methane activation and its nonoxidative coupling to form C2 hydrocarbons. Our C-vacancy concentration-dependent study of CH4 activation shows that (i) the first C–H bond cleavage is facile and less sensitive to the concentration of C-vacancy and (ii) the dissociation of the subsequent ones strongly depends on the vacancy concentration and becomes arduous in the presence of fewer vacancies. Among the two vacancy concentrations considered in this study, namely, 12.5 and 25%, we find that on the former though the first C–H bond cleavage is facile, the barriers for the subsequent C–H bonds are high suggesting that this might be a good candidate for further C–C coupling studies. Our C–C coupling studies show that this catalyst will yield acetylene at around 800 K. However, the rate-limiting step is the formation of H2 from the H atoms occupying the C-vacancies, which might block the vacancies, thereby deactivating the catalyst

Cadmium Vacancy Minority Defects as Luminescence Centers in Size and Strain Dependent Photoluminescence Shifts in CdS Nanotubes

We have studied the absorbance and photoluminescence properties of cadmium sulfide nanotubes with overall size beyond the quantum confinement regime. While the absorption spectra are unaffected by the change in size there is an anomalous red-shift in the photoluminescence spectra with increase in size. Using density functional calculations, we have identified that the shift in the emission peak of the photoluminescence spectra is a result of the interplay between Cd vacancies on the surface of these nanotubes and the crystalline strain which was incorporated in these nanotubes during their growth process. Most importantly, our results show that rather than the defect concentration, it is the nature of the defect which plays a crucial role in determining the optical properties of these nanotubes. For this particular case of CdS nanotubes we find that though S interstitials are the most abundant ones, however, it is the Cd vacancies with second lowest formation energies which significantly affect the photoluminesence spectra

Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric {(NHC)MCl}₂ (M = Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand

Synthesis, structure, bonding, and photoluminescence studies of a pair of neutral dimeric silver and gold complexes of a N/O-functionalized N-heterocyclic carbene ligand exhibiting closed-shell d10···d10 argentophilic and aurophilic interactions, are reported. In particular, dimeric complexes of the type {[1-(benzyl)-3-(N-tert-butylacetamido)imidazol-2-ylidene]MCl}2 [M = Ag (2); Au (3)] displayed attractive metallophilic interaction in the form of a close ligand-unsupported metal···metal contact [3.1970(12) Å in 2; 3.2042(2) Å in 3] as observed from X-ray diffraction study and also was further verified by low temperature photoluminescence study at 77 K that showed the characteristic emission [527 nm for 2; 529 nm for 3] owing to the metal···metal interaction. The nature of the metallophilic interaction in these complexes was further probed using computational studies that estimated the metal···metal interaction energy to be 12.8 (2) and 8.6 kcal/mol (3). Notably, the argentophilic interaction was found to be stronger than the aurophilic interaction in this series of neutral dimeric complexes. The complexes 2 and 3 were synthesized sequentially, with the silver 2 complex prepared by the reaction of the 1-(benzyl)-3-(N-tert-butylacetamido)imidazolium chloride with Ag2O in 66% yield, while the gold 3 complex was obtained by the transmetallation reaction of the silver 2 complex with (SMe2)AuCl in 86% yield

Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric {(NHC)MCl}₂ (M = Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand

Synthesis, structure, bonding, and photoluminescence studies of a pair of neutral dimeric silver and gold complexes of a N/O-functionalized N-heterocyclic carbene ligand exhibiting closed-shell d10···d10 argentophilic and aurophilic interactions, are reported. In particular, dimeric complexes of the type {[1-(benzyl)-3-(N-tert-butylacetamido)imidazol-2-ylidene]MCl}2 [M = Ag (2); Au (3)] displayed attractive metallophilic interaction in the form of a close ligand-unsupported metal···metal contact [3.1970(12) Å in 2; 3.2042(2) Å in 3] as observed from X-ray diffraction study and also was further verified by low temperature photoluminescence study at 77 K that showed the characteristic emission [527 nm for 2; 529 nm for 3] owing to the metal···metal interaction. The nature of the metallophilic interaction in these complexes was further probed using computational studies that estimated the metal···metal interaction energy to be 12.8 (2) and 8.6 kcal/mol (3). Notably, the argentophilic interaction was found to be stronger than the aurophilic interaction in this series of neutral dimeric complexes. The complexes 2 and 3 were synthesized sequentially, with the silver 2 complex prepared by the reaction of the 1-(benzyl)-3-(N-tert-butylacetamido)imidazolium chloride with Ag2O in 66% yield, while the gold 3 complex was obtained by the transmetallation reaction of the silver 2 complex with (SMe2)AuCl in 86% yield

Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric {(NHC)MCl}₂ (M = Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand

Synthesis, structure, bonding, and photoluminescence studies of a pair of neutral dimeric silver and gold complexes of a N/O-functionalized N-heterocyclic carbene ligand exhibiting closed-shell d10···d10 argentophilic and aurophilic interactions, are reported. In particular, dimeric complexes of the type {[1-(benzyl)-3-(N-tert-butylacetamido)imidazol-2-ylidene]MCl}2 [M = Ag (2); Au (3)] displayed attractive metallophilic interaction in the form of a close ligand-unsupported metal···metal contact [3.1970(12) Å in 2; 3.2042(2) Å in 3] as observed from X-ray diffraction study and also was further verified by low temperature photoluminescence study at 77 K that showed the characteristic emission [527 nm for 2; 529 nm for 3] owing to the metal···metal interaction. The nature of the metallophilic interaction in these complexes was further probed using computational studies that estimated the metal···metal interaction energy to be 12.8 (2) and 8.6 kcal/mol (3). Notably, the argentophilic interaction was found to be stronger than the aurophilic interaction in this series of neutral dimeric complexes. The complexes 2 and 3 were synthesized sequentially, with the silver 2 complex prepared by the reaction of the 1-(benzyl)-3-(N-tert-butylacetamido)imidazolium chloride with Ag2O in 66% yield, while the gold 3 complex was obtained by the transmetallation reaction of the silver 2 complex with (SMe2)AuCl in 86% yield

Synthesis and Molecular Structure of Bis(pyrazolyl)hydroborato Thallium {[Bp]Tl}₂: A [Bp^RR‘]Tl Complex with an Unbridged Close Tl^I···Tl^I Contact

Synthesis and Molecular Structure of Bis(pyrazolyl)hydroborato Thallium {[Bp]Tl}2:  A [BpRR‘]Tl Complex with an Unbridged Close TlI···TlI Contac