Coordination-induced reductive elimination from a titanium(IV) complex.

Diamidopyridine-supported titanium dibenzyl complexes undergo coordination-induced C-C reductive elimination upon addition of alkynes and quantitative formation of titanacyclopentadienes. The distinct radical mechanism of this reductive mechanism gives new insights into C-C bond formation with titanium.

Access to 1,2,3-triphospholide ligands by reduction of di--butyldiphosphatetrahedrane.

Di--butyldiphosphatetrahedrane (BuCP) (A) is a reactive tetrahedral molecule which may serve as a source of new phosphaorganic molecules and ligands. However, the redox chemistry of this compound has not yet been investigated. Here, we show that the reduction of A with alkali metals (AM = Li, Na, K, Rb and Cs) affords 1,2,3-triphospholides [AM(crown ether)][1,2,3-PCBu] (1-5, [AM(crown ether)] = [Li([12]crown-4)], [Na([15]crown-5)], [K([18]crown-6)], [Rb([18]crown-6)], and Cs) with 1,3-diphospholides [AM(crown ether)][1,3-PCBu] (6-10) formed as by-products.

Structure and photochemistry of di--butyldiphosphatetrahedrane.

Di--butyldiphosphatetrahedrane (BuCP) (1) is a mixed carbon- and phosphorus-based tetrahedral molecule, isolobal to white phosphorus (P). However, despite the fundamental significance and well-explored reactivity of the latter molecule, the precise structure of the free (BuCP) molecule (1) and a detailed analysis of its electronic properties have remained elusive. Here, single-crystal X-ray structure determination of 1 at low temperature confirms the tetrahedral structure.

Kinetic and thermodynamic control of C(sp)-H activation enables site-selective borylation.

Catalysts that distinguish between electronically distinct carbon-hydrogen (C-H) bonds without relying on steric effects or directing groups are challenging to design. In this work, cobalt precatalysts supported by -alkyl-imidazole-substituted pyridine dicarbene (ACNC) pincer ligands are described that enable undirected, remote borylation of fluoroaromatics and expansion of scope to include electron-rich arenes, pyridines, and tri- and difluoromethoxylated arenes, thereby addressing one of the major limitations of first-row transition metal C-H functionalization catalysts. Mechanistic studies established a kinetic preference for C-H bond activation at the -position despite cobalt-aryl complexes resulting from C-H activation being thermodynamically preferred.

Arene Insertion with Pincer-Supported Molybdenum-Hydrides: Determination of Site Selectivity, Relative Rates, and Arene Complex Formation.

The synthesis of phosphino(oxazoline)pyridine-supported molybdenum(0) cycloocta-1,5-diene complexes is described. Exposure of these complexes to dihydrogen in the presence of an arene resulted in insertion of the substrate into the molybdenum hydride bond and afforded the corresponding molybdenum cyclohexadienyl hydrides. For mono- and disubstituted arenes, the site selectivity for insertion of the most substituted bond increases with increasing size of the substituent from methyl to ethyl, -propyl, and -butyl.

Shining Fresh Light on Complex Photoredox Mechanisms through Isolation of Intermediate Radical Anions.

Photoredox catalysis (PRC) has gained enormous and wide-ranging interest in recent years but has also been subject to significant mechanistic uncertainty, even controversy. To provide a method by which the missing understanding can begin to be filled in, we demonstrate herein that it is possible to isolate as authentic materials the one-electron reduction products of representative PRC catalysts (PCs). Specifically, KC reduction of both 9,10-dicyanoanthracene and a naphthalene monoamide derivative in the presence of a cryptand provides convenient access to the corresponding [K(crypt)][PC] salts as clean materials that can be fully characterized by techniques including EPR and XRD.

Identification of Cyclohexadienyl Hydrides as Intermediates in Molybdenum-Catalyzed Arene Hydrogenation.

Treatment of phosphino(imino)pyridine (PIP) molybdenum cyclooctadiene (COD) complexes [(PIP)Mo(COD)] with dihydrogen in the presence of benzene selectively furnished the molybdenum cyclohexadienyl hydrides [(PIP)MoH(η -C H )], which are precatalysts for the hydrogenation of benzene to cyclohexane. [(PIP)MoH(η -C H )] arises from a rarely observed insertion of benzene into a molybdenum-hydride bond, a key step in the molybdenum-catalyzed homogeneous hydrogenation of arenes. The reaction with toluene afforded a single isomer of the corresponding molybdenum cyclohexadienyl hydride while para-xylene predominantly formed the molybdenum η -arene complex with the insertion product being a minor component.

A homoleptic tris(diphosphacyclobutadiene) tripledecker sandwich complex.

The synthesis and characterisation of [Co(η-PCBu)(μ:η:η-PCBu)] (1) is described, featuring two cobalt atoms and three diphosphacyclobutadiene ligands. This compound is the first triple-decker sandwich complex which exclusively contains cyclobutadiene ligands.

Alcohol Synthesis by Cobalt-Catalyzed Visible-Light-Driven Reductive Hydroformylation.

A cobalt-catalyzed reductive hydroformylation of terminal and 1,1-disubstituted alkenes is described. One-carbon homologated alcohols were synthesized directly from CO and H, affording anti-Markovnikov products (34-87% yield) with exclusive regiocontrol (linear/branch >99:1) for minimally functionalized alkenes. Irradiation of the air-stable cobalt hydride, (dcype)Co(CO)H (dcype = dicyclohexylphosphinoethane) with blue light generated the active catalyst that mediates alkene hydroformylation and subsequent aldehyde hydrogenation.

Diphosphorus Release and Heterocumulene Oligomerisation by Nickel Complexes.

The generation of diphosphorus molecules P under mild conditions in solution is a useful strategy to generate diphosphines [4+2] cycloadditions. We recently described the release of P units from the nickel butterfly complex [{(IMes)Ni(CO)}(μ,η:η-P)] (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) upon addition of CO gas. Herein, we developed an alternative protocol for the same process using heterocumulenes.

Molybdenum-Catalyzed Asymmetric Hydrogenation of Fused Arenes and Heteroarenes.

The synthesis of enantioenriched molybdenum precatalysts for the asymmetric hydrogenation of substituted quinolines and naphthalenes is described. Three classes of pincer ligands with chiral substituents were evaluated as supporting ligands in the molybdenum-catalyzed hydrogenation reactions, where oxazoline imino(pyridine) chelates were identified as optimal. A series of 2,6-disubstituted quinolines was hydrogenated to enantioenriched decahydroquinolines with high diastereo- and enantioselectivities.

Bulking up Cp: A Penta-Terphenyl Cyclopentadienyl Ligand.

The modification of cyclopentadienyl ligands with carefully selected substituents is a widely used strategy for tuning their steric and electronic properties. We describe the synthesis of an extremely bulky penta-terphenyl cyclopentadienyl ligand (Cp) by arylation of cyclopentadiene. Deprotonation reactions with various group 1 metals and bases afforded a complete series of alkali metal salts MCp (M = Li-Cs).

Synthesis of a carborane-substituted bis(phosphanido) cobaltate(i), ligand substitution, and unusual P fragmentation.

Oxidative addition of the P-P single bond of an -carborane-derived 1,2-diphosphetane (1,2-C(PMes)BH) (Mes = 2,4,6-MeCH) to cobalt(-i) and nickel(0) sources affords the first heteroleptic complexes of a carborane-bridged bis(phosphanido) ligand. The complexes also incorporate labile ligands suitable for further functionalisation. Thus, the cobalt(i) complex [K([18]crown-6)][Co{1,2-(PMes)CBH}(cod)] (cod = 1,5-cyclooctadiene) bearing a labile cyclooctadiene ligand undergoes facile ligand exchange reactions with isonitriles and -butyl phosphaalkyne with retention of the bis(phosphanido) ligand.

Di-tert-butyldiphosphatetrahedrane as a Source of 1,2-Diphosphacyclobutadiene Ligands.

Reactions of di-tert-butyldiphosphatetrahedrane (1) with cycloocta-1,5-diene- or anthracene-stabilised metalate anions of iron and cobalt consistently afford complexes of the rarely encountered 1,2-diphosphacyclobutadiene ligand, which have previously been very challenging synthetic targets. The subsequent reactivity of 1,2-diphosphacyclobutadiene cobaltates toward various electrophiles has also been investigated and is compared to reactions of related 1,3-diphosphacyclobutadiene complexes. The results highlight the distinct reactivity of such isomeric species, showing that the 1,2-isomers can act as precursors for previously unknown triphospholium ligands.

Di--butyldiphosphatetrahedrane as a building block for phosphaalkenes and phosphirenes.

The remarkable 'mixed' diphosphatetrahedrane (tBuCP)2 (1) - which is both the elusive dimeric form of the phosphaalkyne tBuCP and an isolobal analogue of the important industrial feedstock P4 - was recently isolated for the first time; however, its chemistry remains unexplored. Herein we report that treatment of 1 with various N-heterocyclic carbenes readily yields unusual, unsaturated organophosphorus motifs. These results demonstrate the significant potential of 1 as a building block for the synthesis of previously unknown organophosphorus compounds.

Activation of Di-tert-butyldiphosphatetrahedrane: Access to (tBuCP) (n=2, 4) Ligand Frameworks by P-C Bond Cleavage.

The first mixed phosphatetrahedranes were reported only recently and their reactivity is virtually unexplored. Herein, we present a reactivity study on di-tert-butyldiphosphatetrahedrane (1), which is the dimer of tert-butylphosphaalkyne. The (tBuCP) tetrahedron is activated selectively by N-heterocyclic carbene (NHC) nickel(I) and nickel(0) complexes, resulting in novel complexes featuring diverse (tBuCP) -frameworks (n=2, 4).

An unusual NiSiP cluster formed by complexation and thermolysis.

[LSi(η-P)] (L = CH[C(Me)N(Dipp)][C(CH)N(Dipp)], Dipp = 2,6-diisopropylphenyl) forms well-defined 1 : 1 and 2 : 1 complexes with N-heterocyclic carbene nickel fragments. The cluster compound [(IDipp)NiP(SiL)] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) is selectively formed by thermolysis of the complex [(IDipp)Ni(μ-η-P)SiL].

Aggregation and Degradation of White Phosphorus Mediated by N-Heterocyclic Carbene Nickel(0) Complexes.

The reaction of zerovalent nickel compounds with white phosphorus (P ) is a barely explored route to binary nickel phosphide clusters. Here, we show that coordinatively and electronically unsaturated N-heterocyclic carbene (NHC) nickel(0) complexes afford unusual cluster compounds with P , P , P and P units. Using [Ni(IMes) ] [IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene], electron-deficient Ni P and Ni P clusters have been isolated, which can be described as superhypercloso and hypercloso clusters according to the Wade-Mingos rules.

Di-tert-butyldiphosphatetrahedrane: Catalytic Synthesis of the Elusive Phosphaalkyne Dimer.

While tetrahedranes as a family are scarce, neutral heteroatomic species are all but unknown, with the only reported example being AsP . Herein, we describe the isolation of a neutral heteroatomic X Y molecular tetrahedron (X, Y=p-block elements), which also is the long-sought-after free phosphaalkyne dimer. Di-tert-butyldiphosphatetrahedrane, (tBuCP) , is formed from the monomer tBuCP in a nickel-catalyzed dimerization reaction using [(NHC)Ni(CO) ] (NHC=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IPr)).

Synthesis and Reactivity of Nickel-Stabilised μ :η ,η -P , As and PAs Units.

The reactivity of two paramagnetic nickel(I) compounds, CpNi(NHC) (where Cp=cyclopentadienyl; NHC=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) or 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr)), towards [Na(dioxane) ][PnCO] (Pn=P, As) is described. These reactions afford symmetric bimetallic compounds (μ :η ,η -Pn ){Ni(NHC)(CO)} . Several novel intermediates en route to such species are identified and characterised, including a compound containing the PCO anion in an unprecedented μ :η ,η -binding mode.

Unexpected migratory insertion reactions of M(alkyl)2 (M=Zn, Cd) and diamidocarbenes.

The electrophilic character of free diamidocarbenes (DACs) allows them to activate inert bonds in small molecules, such as NH3 and P4 . Herein, we report that metal coordinated DACs also exhibit electrophilic reactivity, undergoing attack by Zn and Cd dialkyl precursors to afford the migratory insertion products [(6-MesDAC-R)MR] (M=Zn, Cd; R=Et, Me; Mes=mesityl). These species were formed via the spectroscopically characterised intermediates [(6-MesDAC)MR2 ], exhibiting barriers to migratory insertion which increase in the order MR2 = ZnEt2 < ZnMe2 < CdMe2 .