Actinide 2-metallabiphenylenes that satisfy Hückel's rule.

Aromaticity and antiaromaticity, as defined by Hückel's rule, are key ideas in organic chemistry, and are both exemplified in biphenylene1-3-a molecule that consists of two benzene rings joined by a four-membered ring at its core. Biphenylene analogues in which one of the benzene rings has been replaced by a different (4n + 2) π-electron system have so far been associated only with organic compounds4,5. In addition, efforts to prepare a zirconabiphenylene compound resulted in the isolation of a bis(alkyne) zirconocene complex instead6. Here we report the synthesis and characterization of, to our knowledge, the first 2-metallabiphenylene compounds. Single-crystal X-ray diffraction studies reveal that these complexes have nearly planar, 11-membered metallatricycles with metrical parameters that compare well with those reported for biphenylene. Nuclear magnetic resonance spectroscopy, in addition to nucleus-independent chemical shift calculations, provides evidence that these complexes contain an antiaromatic cyclobutadiene ring and an aromatic benzene ring. Furthermore, spectroscopic evidence, Kohn-Sham molecular orbital compositions and natural bond orbital calculations suggest covalency and delocalization of the uranium f2 electrons with the carbon-containing ligand.

Group I Alkoxides and Amylates as Highly Efficient Silicon-Nitrogen Heterodehydrocoupling Precatalysts for the Synthesis of Aminosilanes.

Group I alkoxides are highly active precatalysts in the heterodehydrocoupling of silanes and amines to afford aminosilane products. The broadly soluble and commercially available KOt Amyl was utilized as the benchmark precatalyst for this transformation. Challenging substrates such as anilines were found to readily couple primary, secondary, and tertiary silanes in high conversions (>90 %) after only 2 h at 40 °C. Traditionally challenging silanes such as Ph3 SiH were also easily coupled to simple primary and secondary amines under mild conditions, with reactivity that rivals many rare earth and transition-metal catalysts for this transformation. Preliminary evidence suggests the formation of hypercoordinated intermediates, but radicals were detected under catalytic conditions, indicating a mechanism that is rare for Si-N bond formation.

Silicon-Nitrogen Bond Formation via Heterodehydrocoupling and Catalytic N-Silylation.

Silicon-nitrogen bond formation is an important subfield in main group chemistry, and catalysis is an attractive route for efficient, selective formation of these bonds. Indeed, heterodehydrocoupling and N-silylation offer facile methods for the synthesis of small molecules through the coupling of primary, secondary, and tertiary silanes with N-containing substrates such as amines, carbazoles, indoles, and pyrroles. However, the reactivity of these catalytic systems is far from uniform, and critical issues are often encountered with product selectivity, conversions, substrate scope, catalyst activation, and in some instances, competing side reactions. Herein, a catalogue of catalysts and their reactivity for Si-N heterodehydrocoupling and N-silylation are reported.

Triamidoamine-Supported Zirconium Compounds in Main Group Bond-Formation Catalysis.

The rationale to pursue long-term study of any system must be sound. Quick discoveries and emergent fields are more than temptations. They remind us to ask what are we gaining through continued study of any system. For triamidoamine-supported zirconium, there has been a great deal gained with yet more ahead. Initial study of the system taught much that is applied to catalysis. Cyclometalation of a trimethylsilyl substituent of the ancillary ligand, abbreviated (N3N) when not metalated for simplicity, via C-H bond activation is facile and highly reversible. It has allowed for the synthesis of a range of Zr-E bonds, which are of fundamental interest. More germane, cyclometalation has emerged as our primary product liberation step in catalysis. Cyclometalation also appears to be a catalyst resting state, despite how cyclometalation is a known deactivation step for many a compound in other circumstances. Catalysis with triamidoamine-supported zirconium has been rich. Rather than summarizing the breadth of reactions, a more detailed report on the dehydrocoupling of phosphines and hydrophosphination is provided. Both reactions demonstrate the outward impact that the study of (N3N)Zr-based catalysis has afforded. Dehydrocoupling catalysis, or bond formation via loss of hydrogen, is particular to 3p and heavier main group elements. The reaction has been important in the formation of E-E and E-E' bonds in the main group for molecular species and materials. While study of this reaction at (N3N)Zr compounds provides key insights into mechanism, discoveries in the area of P-P and Si-Si bond formation with (N3N)Zr derivatives as catalysts have greater reach than merely the synthesis of main group element containing products. For example, that work has informed design principles for the identification of catalysts that transfer low-valent fragments. The successful application of these principles was evident in the discovery of a catalyst that transfers phosphinidene ("PR") to unsaturated substrates. Hydrophosphination exhibits perfect atom economy in the formation of P-C bonds. The reaction can proceed without a catalyst, but the purpose of a catalyst is enhanced reactivity and selectivity. Nevertheless, significant challenges in this reaction remain. In particular, (N3N)Zr compounds have demonstrated high activity in hydrophosphination and readily utilize unactivated unsaturated organic molecules, challenging substrates for any heterofunctionalization reaction. This activity has led to not only impressive metrics in the catalysis but access to previously untouched substrates and formation of unique products. The particular properties of the (N3N)Zr system that engage in this reactivity may influence other heterofunctionalization reactions. The recently discovered photocatalytic hydrophosphination with (N3N)ZrPRR' compounds already appears to be general rather than unique and may drive additional bond formation catalysis among early transition-metal compounds.

Evidence for Iron-Catalyzed α-Phosphinidene Elimination with Phenylphosphine.

The ubiquitous half-sandwich iron complex [CpFe(CO)2 Me] (Cp=η5 -C5 H5 ) appears to be a catalyst for α-phosphinidene elimination from primary phosphines. Dehydrocoupling reactions provided initial insight into this unusual reaction mechanism, and trapping reactions with organic substrates gave products consistent with an α elimination mechanism, including a rare example of a three-component reaction. The substrate scope of this reaction is consistent with generation of a triplet phosphinidene. In all, this study presents catalytic phosphinidene transfer to unsaturated organic substrates.

Challenges in Catalytic Hydrophosphination.

Despite significant advances, metal-catalyzed hydrophosphination has ample room for discovery, growth, and development. Many of the key successes in metal-catalyzed hydrophosphination over the last decade have indicated what is needed and what is yet to come. Reactivity that is absent from the literature also speaks to the challenges in catalytic hydrophosphination. This Concept article discusses and highlights recent developments that address the ongoing challenges, and identifies areas in metal-catalyzed hydrophosphination that are underdeveloped. Advances in product selectivity, catalyst design, and both unsaturated and phosphine substrates illustrate the ongoing development of the field. Like all catalytic transformations, the benefits are realized through catalyst, ligand, and conditions, and consideration of those features are the route to a yet more efficient and broadly applicable reaction.

Metal-Ligand Cooperativity in a Methandiide-Derived Iridium Carbene Complex.

The synthesis, electronic structure, and reactivity of the first Group 9 carbene complex, [Cp*IrL] [L=C(Ph2 PS)(SO2 Ph)] (2), based on a dilithio methandiide are reported. Spectroscopic as well as computational studies have shown that, despite using a late transition-metal precursor, sufficient charge transfer occurred from the methandiide to the metal, resulting in a stable, nucleophilic carbene species with pronounced metal-carbon double-bond character. The potential of this iridium complex in the activation of a series of E-H bonds by means of metal-ligand cooperation has been tested. These studies have revealed distinct differences in the reactivity of 2 compared to a previously reported ruthenium analogue. Whereas attempts to activate the O-H bond in different phenol derivatives resulted in ligand cleavage, H-H and Si-H activation as well as dehydrogenation of isopropanol have been accomplished. These reactions are driven by the transformation of the carbene to an alkyl ligand. Contrary to a previously reported ruthenium carbene system, the dihydrogen activation has been found to proceed by a stepwise mechanism, with the activation first taking place solely at the metal. The activated products further reacted to afford a cyclometalated complex through liberation of the activated substrates. In the case of triphenylsilane, cyclometalation could thus be induced by a substoichiometric (i.e., catalytic) amount of silane.

Intermolecular zirconium-catalyzed hydrophosphination of alkenes and dienes with primary phosphines.

Catalytic hydrophosphination of terminal alkenes and dienes with primary phosphines (RPH2; R = Cy, Ph) under mild conditions has been demonstrated using a zirconium complex, [κ(5)-N,N,N,N,C-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH]Zr (1). Exclusively anti-Markovnikov functionalized products were observed, and the catalysis is selective for either the secondary or tertiary phosphine (i.e., double hydrophosphination) products, depending on reaction conditions. The utility of the secondary phosphine products as substrates for further elaboration was demonstrated with a platinum-catalyzed asymmetric alkylation reaction.

Carbon-hydrogen bond activation, C-N bond coupling, and cycloaddition reactivity of a three-coordinate nickel complex featuring a terminal imido ligand.

The three-coordinate imidos (dtbpe)Ni═NR (dtbpe = (t)Bu2PCH2CH2P(t)Bu2, R = 2,6-(i)Pr2C6H3, 2,4,6-Me3C6H2 (Mes), and 1-adamantyl (Ad)), which contain a legitimate Ni-N double bond as well as basic imido nitrogen based on theoretical analysis, readily deprotonate HC≡CPh to form the amide acetylide species (dtbpe)Ni{NH(Ar)}(C≡CPh). In the case of R = 2,6-(i)Pr2C6H3, reductive carbonylation results in formation of the (dtbpe)Ni(CO)2 along with the N-C coupled product keteneimine PhCH═C═N(2,6- (i)Pr2C6H3). Given the ability of the Ni═N bond to have biradical character as suggested by theoretical analysis, H atom abstraction can also occur in (dtbpe)Ni═N{2,6-(i)Pr2C6H3} when this species is treated with HSn((n)Bu)3. Likewise, the microscopic reverse reaction--conversion of the Ni(I) anilide (dtbpe)Ni{NH(2,6-(i)Pr2C6H3)} to the imido (dtbpe)Ni═N{2,6-(i)Pr2C6H3}--is promoted when using the radical Mes*O(•) (Mes* = 2,4,6-(t)Bu3C6H2). Reactivity studies involving the imido complexes, in particular (dtbpe)Ni═N{2,6-(i)Pr2C6H3}, are also reported with small, unsaturated molecules such as diphenylketene, benzylisocyanate, benzaldehyde, and carbon dioxide, including the formation of C-N and N-N bonds by coupling reactions. In addition to NMR spectroscopic data and combustion analysis, we also report structural studies for all the cycloaddition reactions involving the imido (dtbpe)Ni═N{2,6-(i)Pr2C6H3}.

Mechanisms of metal-catalyzed dehydrocoupling reactions.

This review summarizes selected mechanistic insight garnered from reactions that form bonds between main group elements with liberation of hydrogen, dehydrocoupling or dehydropolymerization reactions. Focus has been made where mechanistic study has been done or provides unique insight as compared to previous work. Thus, a limited number of catalysts and substrates will be discussed, but a broad range of mechanistic features will be noted. A comparative approach considering dehydrocoupling reactions that appear to proceed by certain mechanistic steps is taken here, beginning with steps that are well established and general then moving to those that are newer, at least to this application.

Zirconium-mediated synthesis of arsaalkene compounds from arsines and isocyanides.

An atom-economical synthesis of arsaalkenes via a net coupling of aryl arsines with aryl or alkyl isocyanides at zirconium is presented. Reaction of zirconium arsenido complexes (N3N)ZrAsHAr [N3N = N(CH2CH2NSiMe3)3(3-); Ar = Ph, (2) Mes (3)] with aryl and alkyl isocyanides yields arsaalkene products of the general form (N3N)Zr[NRC(H)═As(Ar)]. Two examples (5: R = Mes, Ar = Ph; 6: R = CH2Ph, Ar = Mes) were structurally characterized. Observation of intermediates in the reaction and structural characterization of the previously reported 1,1-insertion product benzylisocyanide with (N3N)ZrAsPh2 (8), (N3N)Zr[η(2)-C(PPh2)=NCH2Ph] (9), support the mechanistic hypothesis that these reactions occur via 1,1-insertion followed by rearrangement.

Metal complexes (M = Zn, Sn, and Pb) of 2-phosphinobenzenethiolates: insights into ligand folding and hemilability.

The divalent metal complexes M(II){(SC6H4-2-PR2)-κ(2)S,P}2 (3-7, and 9-11) (M = Zn, Sn, or Pb; R = (i)Pr, (t)Bu, or Ph), the Sn(IV) complexes Sn{(SC6H4-2-PR2)-κ(2)-S,P}Ph2Cl (12 and 13) (R = (i)Pr and (t)Bu), and the ionic Sn(IV) complexes [Sn{(SC6H4-2-PR2)-κ(2)-S,P}Ph2][BPh4] (14 and 15) (R = (i)Pr and (t)Bu) have been prepared and characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction when suitable crystals were afforded. The Sn(II) and Pb(II) complexes with R = Ph, (i)Pr, or (t)Bu (5, 6, 9, and 10) demonstrated ligand "folding" hinging on the P,S vector-a behavior driven by the repulsions of the metal/phosphorus and metal/sulfur lone pairs and increased M-S sigma bonding strength. This phenomenon was examined by density functional theory (DFT) calculations for the compounds in both folded and unfolded states. The Sn(IV) compound 13 (R = (t)Bu) crystallized with the phosphine in an axial position of the pseudotrigonal bipyramidal complex and also exhibited hemilability in the Sn-P dative bond, while compound 12 (R = (i)Pr), interestingly, crystallized with phosphine in an equatorial position and did not show hemilability. Finally, the crystal structure of 15 (R = (t)Bu) revealed the presence of an uncommon, 4-coordinate, stable Sn(IV) cation.

Commercially available organolithium compounds as effective, simple precatalysts for silicon-nitrogen heterodehydrocoupling.

A family of commercially available organolithium compounds were found to effectively catalyze the heterodehydrocoupling of silanes and amines under ambient conditions. Ubiquitous nBuLi (1) was utilized as the benchmark catalyst, where an array of primary, secondary, and tertiary arylsilanes were coupled to electron-donating amines, affording aminosilanes in high conversions with short reaction times. Preliminary mechanistic analysis is consistent with a nucleophilic-type system that involves the formation of a hypervalent silicon intermediate. This work underscores the accessibility of Si-N heterodehydrocoupling, with organolithium reagents emerging as some of the most straightforward and cost-effective precatalysts for this transformation.

Unusual nuclear exchange within a germanium-containing aromatic ring that results in germanium atom transfer.

The delivery of single atoms is highly desirable for the straightforward synthesis of complex molecules, however this approach is limited by a lack of suitable atomic transfer reagents. Here, we report a germanium atom transfer reaction employing a germanium analogue of the phenyl anion. The reaction yields a germanium-substituted benzene, along with a germanium atom which can be transferred to other chemical species. The transfer of atomic germanium is demonstrated by the formation of well-defined germanium doped molecules. Furthermore, computational studies reveal that the reaction mechanism proceeds via the first example of an aromatic-to-aromatic nuclear germanium replacement reaction on the germabenzene ring. This unusual reaction pathway was further probed by the reaction of our aromatic germanium anion with a molecular silicon species, which selectively yielded the corresponding silicon-substituted benzene derivative.

Synthesis and Characterization of 2-(((2,7-Dihydroxynaphthalen-1-yl)methylene)amino)-3',6'-bis(ethylamino)-2',7'-dimethylspiro[isoindoline-1,9'-xanthen]-3-one and Colorimetric Detection of Uranium in Water.

2-(((2,7-Dihydroxynaphthalen-1-yl)methylene)amino)-3',6'-bis(ethylamino)-2',7'-dimethyl-spiro[isoindoline-1,9'-xanthen]-3-one was synthesized using Rhodamine 6G hydrazide (prepared using literature methods) and commercially available 2,7-dihydroxynaphthalene-1-carbaldehyde via imine condensation. Structural characterization was performed using FT-IR, 1H-NMR, 13C-NMR, X-ray, and HRMS. This Schiff base shows promise as a ligand for the colorimetric analysis of uranium in water.

Recent advances in catalytic pnictogen bond forming reactions via dehydrocoupling and hydrofunctionalization.

An examination of several catalytic reactions among the group 15 elements is presented. The connections between the chemistry of the pnictogens can sometimes be challenging, but aspects of metal-pnictogen reactivity are the key. The connecting reactivity comes from metal-catalyzed transformations such as dehydrocoupling and hydrofunctionalization. Pivotal mechanistic insights from E-N heterodehydrocoupling have informed the development of highly active catalysts for these reactions. Metal-amido nucleophilicity is often at the core of this reactivity, which diverges from phosphine and arsine dehydrocoupling. Nucleophilicity connects to the earliest understanding of hydrophosphination catalysis, but more recent catalysts are leveraging enhanced insertion activity through photolysis. This photocatalysis extends to hydroarsination, which may also have more metal-arsenido nucleophilicity than anticipated. However, metal-catalyzed arsinidene chemistry foreshadowed related phosphinidene chemistry by years. This examination shows the potential for greater influence of individual discoveries and understanding to leverage new advances between these elements, and it also suggests that the chemistry of heavier elements may have more influence on what is possible with lighter elements.

rac-18-Meth-oxy-coronaridine hydro-chloride.

In the crystal structure of the racemic title compound, C(22)H(29)N(2)O(3) (+)·Cl(-), both NH groups form N-H⋯Cl hydrogen bonds with the chloride counter-ion, forming translational chains along the a axis.

Bis(4-methyl-N-{(2Z,4E)-4-[(4-methyl-phen-yl)imino]-pent-2-en-2-yl}anilinido)zinc.

The title compound, [Zn(C(19)H(21)N(2))(2)], appears to be the first example of a zinc complex supported by two ß-diketiminate (nacnac) ligands. This complex crystallizes with a distorted tetra-hedrally coordinated Zn(II) atom that diposes the two nacnac ligands approximately orthogonally to one another [angle between the two N-Zn-N mean planes is 89.91 (10)°], with average Zn-N bond lengths of 1.992 (4) Å.