Highly Selective Nickel-Catalyzed Isomerization-Hydroboration of Alkenes Affords Terminal Functionalization at Remote C-H Position.

We report herein the synthesis and characterization of nickel complexes supported by tridentate and bidentate phosphino(silyl) ancillary ligands, along with the successful application of these complexes as precatalysts for the hydroboration of terminal and internal alkenes using pinacolborane (HBPin). These reactions proceeded with low nickel loadings of 2.5-5 mol % in the absence of co-solvent, and in some cases at room temperature. Isomerization to afford exclusively the terminal hydroboration product was obtained across a range of internal alkenes, including tri- and tetra-substituted examples. This reactivity is unprecedented for nickel and offers a powerful means of achieving functionalization at a C-H position remote from the C=C double bond. Nickel-catalyzed deuteroboration experiments using DBPin support a mechanism involving 1,2-insertion of the alkene and subsequent chain-walking, which results in isotopic scrambling.

BODIPYs with Chalcogenophenes at Boron: Synthesis and Properties.

Reported herein are the synthesis and characterization of BODIPYs bearing heterocycles at boron. To synthesize this series, various chalcogenophenes (furan, thiophene, selenophene, and tellurophene) were lithiated and then used as nucleophiles to attack the boron center of a parent F-BODIPY. Compounds in the series were compared with respect to their photophysical and structural properties, and trends were discussed. By virtue of the "heavy atom effect", as the mass of the heterocycle appended to the BODIPY core increases, compounds exhibit a higher singlet oxygen quantum yield. The BODIPY with tellurophene at boron exhibits the highest quantum yield (ΦΔ = 0.68) in the series and reduced emission (Φf = 0.01).

Synthesis, Characterization, and Single-Crystal X-ray Structures of Refractory Metal Compounds as Precursors for the Single-Source Chemical Vapor Deposition of Metal Nitrides.

The chemical vapor deposition of refractory metal nitrides requires volatile precursors and has previously been achieved by using metal complexes containing a variety of imide ligands. Recently, the 1,4-di-tert-butyl-1,3-diazabutadiene (DAD) adduct of bis(tert-butylimide)dichloridemolybdenum(VI) was shown to be an excellent precursor for the single-source CVD of Mo2N thin films. Leveraging the success of this work, we prepared chromium and tungsten compounds with the same framework. Additionally, the framework has been modified slightly to allow the isolation of mono(tert-butylimide)trichloride complexes of vanadium, niobium, tantalum, and molybdenum(V) to extend the search for new vapor-phase precursors. These compounds were all fully characterized using the standard methods of multinuclear magnetic resonance spectroscopy, combustion analysis, and single-crystal X-ray diffraction. Their thermal properties were determined by using thermogravimetric analysis and differential scanning colorimetry to assess their utility as vapor-phase precursors. Finally, preliminary deposition studies were carried out to investigate their potential as single-source CVD precursors.

Disturbance of intermolecular forces: eutectics as a new tool for the preparation of vapor-phase deposition precursors.

The volatile bis(tert-butylimido)dichloromolybdenum(VI) compounds, (tBuN)2MoCl2·dad (dad = 1,4-di-tert-butyl-1,3-diazabutadiene) (1) and [(tBuN)2MoCl(µ-Cl)·(tBuNH2)]2 (2), form a eutectic, with a two to one composition (χ2 = 0.33). A decrease of 40 °C in the melting temperature has been observed between the eutectic mixture and the pure compounds. We have isolated a co-crystal of (tBuN)2MoCl2·dme (dme = 1,2-dimethoxyethane) (3) and 2, also in a two to one ratio, which serves as a structural model for such mixtures. The lower melting point of carefully chosen eutectic mixtures can offer more consistent precursor delivery in deposition processes.

Synthesis of 5'-Thymidine-Conjugated Formylphenylboronic Acids as Potential Lysine Targeting Iminoboronate Reversible Covalent Enzyme Probes.

The design of reversible-covalent molecules to selectively target the ε-amino functionality of lysine residues in enzymes or proteins is a highly desirable goal. Herein, we describe synthetic methodology used to prepare a series of 5'-thymidine-linked formylphenylboronic acids as probes to interrogate sugar nucleotide processing enzymes that recognize thymidine. The first synthetic strategy mitigated the need for protecting group manipulations of thymidine by capitalizing upon the straightforward preparation, isolation, and reactivity of 5'-azidothymidine. An alkyne cycloaddition partner was installed through either a propargyl or ethynyl phenyl ketone derived boronic acid. The second strategy directly linked formylphenylboronic acids to 5-thymidine through an ether linkage installed using Mitsunobu conditions with 3'-O,3-dibenzoylthymidine. Iminoboronate formation was observed with a selected probe.

Thermal Stability and Decomposition Pathways in Volatile Molybdenum(VI) Bis-imides.

The vapor deposition of many molybdenum-containing films relies on the delivery of volatile compounds with the general bis(tert-butylimido)molybdenum(VI) framework, both in atomic layer deposition and chemical vapor deposition. We have prepared a series of (tBuN)2MoCl2 adducts using neutral N,N'-chelates and investigated their volatility, thermal stability, and decomposition pathways. Volatility has been determined by thermogravimetric analysis, with the 1,4-di-tert-butyl-1,3-diazabutadiene adduct (5) found to be the most volatile (1 Torr of vapor pressure at 135 °C). Thermal stability was measured primarily using differential scanning calorimetry, and the 1,10-phenanthroline adduct (4) was found to be the most stable with an onset of decomposition of 303 °C. We have also investigated molybdenum compounds with other alkyl-substituted imido groups: these compounds all follow a similar decomposition pathway, γ-H activation, with varying reaction barriers. The tert-pentyl, 1-adamantyl, and a cyclic imido (from 2,5-dimethylhexane-2,5-diamine) were systematically studied to probe the kinetics of this pathway. All of these compounds have been fully characterized, including via single-crystal X-ray diffraction, and a total of 19 new structures are reported.

Origin of Decomposition in a Family of Molybdenum Precursor Compounds.

The bis(tert-butylimido)-molybdenum(VI) framework has been used successfully in the design of vapor-phase precursors for molybdenum-containing thin films, so understanding its thermal behavior is important for such applications. Here, we report the thermal decomposition mechanism for a series of volatile bis(alkylimido)-dichloromolybdenum(VI) adducts with neutral N,N'-chelating ligands, to probe the stability and decomposition pathways for these molecules. The alkyl groups explored were tert-butyl, tert-pentyl, 1-adamantyl, and a cyclic imido (from 2,5-dimethylhexane-2,5-diamine). We also report the synthesis of the new tert-octyl imido adducts, (tOctN)2MoCl2·L (L = N,N,N',N'-tetramethylethylenediamine or 2,2'-bipyridine), which have been fully characterized by spectroscopic techniques as well as single-crystal X-ray diffraction and thermal analysis. We found that the decomposition of all compounds follows the same general pathway, proceeding first by the dissociation of the chelating ligand to give the coordinatively unsaturated species (RN)2MoCl2. Subsequent dimerization results in either an imido bridged adduct, [(RN)Mo(µ-NR)Cl2]2, or a chloride bridged adduct, [(RN)2Mo(µ-Cl)Cl]2, depending on the size of the R group. The dimeric species then likely undergoes an intramolecular γ-hydrogen transfer to yield a nitrido-amido adduct, (RHN)MoNCl2, and an alkene. Ultimately, the resulting molybdenum species appears to decompose into free tert-alkylamine and Mo2N or Mo2C. The thermolysis reactions have been monitored using 1H NMR spectroscopy, and the volatile decomposition products were analyzed using gas chromatography-mass spectrometry. A key intermediate has also been detected using electron ionization high-resolution mass spectrometry. Finally, a detailed computational investigation supports the mechanism outlined above and helps explain the relative stabilities of different N,N'-chelated bis(alkylimido)-dichloromolybdenum(VI) adducts.

Mapping Dual-Base-Enabled Nickel-Catalyzed Aryl Amidations: Application in the Synthesis of 4-Quinolones.

The C-N cross-coupling of (hetero)aryl (pseudo)halides with NH substrates employing nickel catalysts and organic amine bases represents an emergent strategy for the sustainable synthesis of (hetero)anilines. However, unlike protocols that rely on photoredox/electrochemical/reductant methods within NiI/III cycles, the reaction steps that comprise a putative Ni0/II C-N cross-coupling cycle for a thermally promoted catalyst system using organic amine base have not been elucidated. Here we disclose an efficient new nickel-catalyzed protocol for the C-N cross-coupling of amides and 2'-(pseudo)halide-substituted acetophenones, for the first time where the (pseudo)halide is chloride or sulfonate, which makes use of the commercial bisphosphine ligand PAd2-DalPhos (L4) in combination with an organic amine base/halide scavenger, leading to 4-quinolones. Room-temperature stoichiometric experiments involving isolated Ni0, I, and II species support a Ni0/II pathway, where the combined action of DBU/NaTFA allows for room-temperature amide cross-couplings.

Short syntheses of 1-substituted dibenzothiophene derivatives.

The 1-substituted dibenzothiophene motif is an unusual substitution pattern that has previously been accessed via a multi-step synthesis. We demonstrate a simple one-pot preparation of 1-lithiodibenzothiophene from commercial materials via a cascade of two benzyne additions and conversion to several derivatives by addition of electrophiles. A chiral amine containing the 1-dibenzothiophene motif was also prepared. This work avoids the use of precious metals or tert-butyllithium and is much shorter and more convenient than existing routes to 1-substituted dibenzothiophenes.

Simple Ion-Gas Mixtures as a Source of Key Molecules Relevant to Prebiotic Chemistry.

Very simple chemistry can result in the rapid and high-yield production of key prebiotic inorganic molecules. The two reactions investigated here involve such simple systems, (a) carbon disulfide (CS2) and acetate (CH3COO¯) and (b) sulfur dioxide (SO2) and formate (HCOO¯). They have been carried out under non-aqueous conditions, either in an organic solvent or with a powdered salt exposed to the requisite gas. Under such dry conditions the first reaction generated the thioacetate anion [CH3COS]¯ while the second produced the radical [SO2·]¯anion. Anhydrous conditions are not rare and may have arisen on the early earth at sites where an interface between different phases (liquid/gas or solid/gas) could be generated. This is one way to rationalize the formation of molecules and ions (such as we have produced) necessary in the prebiotic world. Interpretation of our results provides insight into scenarios consistent with the more prominent theories of abiogenesis.

Heavy Metals Make a Chain: A Catenated Bismuth Compound.

Catenation is common for the light main-group elements whereas it is rare for the heavy elements. Herein, we report the first example of a neutral molecule containing a Bi4 chain. It is prepared in a one-step reaction between bismuth trichloride and bis(diisopropylphosphino)amine in methanol suspension. The same reaction carried out in dichloromethane gives quite different products. All products have been characterized spectroscopically and using single-crystal X-ray analysis.

Synthesis, properties and reactivity of BCl2 aza-BODIPY complexes and salts of the aza-dipyrrinato scaffold.

The synthesis and characterisation of the BCl2-chelated complexes of both archetypal aza-dipyrrin sub-types are presented. A stepwise halogen exchange, leading to a mixed-halide Cl-B-F intermediate, is implicated in the conversion of F-aza-BODIPYs to Cl-aza-BODIPYs upon treatment with BCl3. The utility of the Cl-aza-BODIPY scaffold to facilitate substitutions at boron is demonstrated under mild conditions through treatment with aryl Grignard reagents. Additionally, the lability of the B-Cl bond enables facile removal of the BCl2 group, i.e. deprotection of F-aza-BODIPYs, under aqueous conditions. Three aza-dipyrrin HX salts were also synthesised and characterised. The pKa of the protonated aza-dipyrrin was determined to be 4, thereby providing insight regarding the storage and stability of such species.

Enantioselective Imine Reduction Catalyzed by Phosphenium Ions.

The first use of phosphenium cations in asymmetric catalysis is reported. A diazaphosphenium triflate, prepared in two or three steps on a multigram scale from commercially available materials, catalyzes the hydroboration or hydrosilylation of cyclic imines with enantiomeric ratios of up to 97:3. Catalyst loadings are as low as 0.2 mol %. Twenty-two aryl/heteroaryl pyrrolidines and piperidines were prepared using this method. Imines containing functional groups such as thiophenes or pyridyl rings that can challenge transition-metal catalysts were reduced employing these systems.

Bis-aminocyclopropenylidene carbene borane catalyzed imine hydrogenation.

Certain borenium cations supported by carbenes can function as hydrogenation catalysts for imines. While many carbenes have been explored, variation of the other groups on boron has been less common. We have investigated several carbene-borane adducts in an attempt to understand the ability of a bis-amino cyclopropenylidene (BAC) carbene dicyclohexylborane adduct to hydrogenate relatively sterically unhindered benzyl imines. As an additional variant, a BAC carbene adduct of diphenylborane was prepared. A convenient preparation of diphenylboron fluoride via a potassium fluoroborinate salt was employed in this chemistry. Reaction of diphenylboron fluoride with a BAC carbene afforded a modest yield of a carbene-fluoroborane adduct. Reaction between the fluoroborinate salt and a lithium tetrafluoroborate adduct of the carbene provided the adduct in much improved yield and cleanliness, and the product was structurally characterized. The fluoroborate could be converted to a boron hydride through fluoride-hydride exchange with dimethylchlorosilane. The boron hydride adduct was also structurally characterized. Unlike the BAC carbene dicyclohexylborane adduct, the BAC carbene diphenylborane adduct showed essentially no activity in hydrogenation of imines or enamines.

Synthesis and characterization of pyrrolyldipyrrin F-BODIPYs.

A series of synthetic analogs of the tripyrrolic natural product prodigiosin were complexed with boron trifluoride to generate the corresponding F-BODIPYs. The maximum wavelengths of absorption and emission of the pyrrolyldipyrrin F-BODIPYs were tuned through variation of the substituents about the pyrrolyldipyrrinato core. The limited variation of substituents on the C-ring did not significantly affect absorption and emission. However, variation of substituents on the B-ring and A-ring resulted in a corresponding red-shift in absorption and emission reaching maximum wavelengths of 600 nm. The presence of electron donating substituents on the B-ring caused a decrease in the Stokes shift, while the presence of electron-withdrawing substituents caused an increase, ranging from 3-25 nm. Stokes shifts were solvent-dependant for some compounds. The inclusion of a dimethylamino group resulted in photo-induced electron transfer and thus quenched fluorescence which was restored upon protonation.

Mono- and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis-1,4-([3]Cumulene)-p-carboquinoid System.

An extended π-system containing two [3]cumulene fragments separated by a p-carboquinoid and stabilized by two capping N-heterocyclic carbenes (NHCs) has been prepared. Mono- and bis(imidazolidinium ethynyl) cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4-bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis-1,4-([3]cumulene)-p-carboquinoid as a result of the π-accepting properties of the capping NHCs.

Asymmetric Dipyrrin and F-BODIPYs Conjugated to Terminal Alkynes and Alkenes.

An asymmetric meso-H dipyrrin featuring a conjugated terminal alkyne substituent was converted to its corresponding difluoro boron complex, and the extent of π-conjugation was extended using Sonogashira cross-coupling. Treatment of the alkyne-substituted dipyrrin with BF3·OEt2 and NEt3 revealed the reactivity of the conjugated terminal alkyne toward Lewis-activated electrophilic substitution and led to the isolation of F-BODIPYs bearing terminal bromovinyl and enol substituents.

Comment on "crystallographic snapshot of an arrested intermediate in the biomimetic activation of CO2 ".

Out of focus: A recent Communication published in this journal describes the synthesis of [nBu4 N]HCO3 . The authors performed a single-crystal X-ray study that revealed a putative species described as an incipient hydroxide ion engaging in a long, and presumably weak, interaction with CO2 . Our recent exploration of the coordination chemistry of CO2 with small ions leads us to believe that such an exceptional bonding situation is unlikely. Instead, we argue that the crystal structure is that of [nBu4 N]O2 CCH3 and therefore not representative of the bulk powder from the synthesis.

Bis[2,6-bis-(1H-benzimidazol-2-yl)pyridine]ruthenium(II) bis(hexa-fluorido-phosphate) diethyl ether tris-olvate.

The title compound, [Ru(C19H13N5)2](PF6)2·3C4H10O, was obtained from the reaction of Ru(bimpy)Cl3 [bimpy is 2,6-bis-(1H-benzimidazol-2-yl)pyridine] and bimpy in refluxing ethanol followed by recrystallization from diethyl ether/aceto-nitrile. At 125 K the complex has ortho-rhom-bic (Pca21) symmetry. It is remarkable that the structure is almost centrosymmetric. However, refinement in space group Pbcn leads to disorder and definitely worse results. It is of inter-est with respect to potential catalytic reduction of CO2. The structure displays N-H⋯O, N-H⋯F hydrogen bonding and significant π-π stacking and C-H⋯π stacking inter-actions.