Flat corannulene: when a transition state becomes a stable molecule.

Flat corannulene has been considered so far only as a transition state of the bowl-to-bowl inversion process. This study was driven by the prediction that substituents with strong steric repulsion could destabilize the bowl-shaped conformation of this molecule to such an extent that the highly unstable planar geometry would become an isolable molecule. To examine the substituents' effect on the corannulene bowl depth, optimized structures for the highly-congested decakis(t-butylsulfido)corannulene were calculated. The computations, performed with both the M06-2X/def2-TZVP and the B3LYP/def2-TZVP methods (the latter with and without Grimme's D3 dispersion correction), predict that this molecule can achieve two minimum structures: a flat carbon framework and a bowl-shaped structure, which are very close in energy. This rather unusual compound was easily synthesized from decachlorocorannulene under mild reaction conditions, and X-ray crystallographic studies gave similar results to the theoretical predictions. This compound crystallized in two different polymorphs, one exhibiting a completely flat corannulene core and the other having a bowl-shaped conformation.

Synthesis and stability of cyclic α-hydrogen nitroxides.

Nitroxides (nitroxyl radicals) hold a unique place in science due to their stable radical nature. We have recently reported the first design concept providing a general solution to the problem of designing and preparing monocyclic α-hydrogen nitroxides. The initial studies were limited to aryl derivatives. We now report a wider study showing that alkyl substituents may be employed as well. In addition, we report several additional examples of aryl substituents and reveal some of the structural limitations with regard to nitroxide stability as a function of the α-carbon substituent.

Activation of Homolytic Si-Zn and Si-Hg Bond Cleavage, Mediated by a Pt(0) Complex, via Novel Pt-Zn and Pt-Hg Compounds.

The thermally stable [(tBuMe2 Si)2 M] (M=Zn, Hg) generate R3 Si(.) radicals in the presence of [(dmpe)Pt(PEt3 )2 ] at 60-80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn (2 a and 2 b), M=Hg (3 a and 3 b)) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt-MSiR3 bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn (5), Hg (6)) whose structures were determined by EPR spectroscopy and DFT calculations.

Coordination chemistry of N-heterocyclic nitrenium-based ligands.

Comprehensive studies on the coordination properties of tridentate nitrenium-based ligands are presented. N-heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh(I) , Rh(III) , Mo(0) , Ru(0) , Ru(II) , Pd(II) , Pt(II) , Pt(IV) , and Ag(I) complexes based on these unusual ligands. Formation of nitrenium-metal bonds is unambiguously confirmed both in solution by selective (15) N-labeling experiments and in the solid state by X-ray crystallography. The generality of N-heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second-row transition and post-transition metals (Y-Cd) in terms of the corresponding bond-dissociation energies.

Design concept for α-hydrogen-substituted nitroxides.

Stable nitroxides (nitroxyl radicals) have many essential and unique applications in chemistry, biology and medicine. However, the factors influencing their stability are still under investigation, and this hinders the design and development of new nitroxides. Nitroxides with tertiary alkyl groups are generally stable but obviously highly encumbered. In contrast, α-hydrogen-substituted nitroxides are generally inherently unstable and rapidly decompose. Herein, a novel, concept for the design of stable cyclic α-hydrogen nitroxides is described, and a proof-of-concept in the form of the facile synthesis and characterization of two diverse series of stable α-hydrogen nitroxides is presented. The stability of these unique α-hydrogen nitroxides is attributed to a combination of steric and stereoelectronic effects by which disproportionation is kinetically precluded. These stabilizing effects are achieved by the use of a nitroxide co-planar substituent in the γ-position of the backbone of the nitroxide. This premise is supported by a computational study, which provides insight into the disproportionation pathways of α-hydrogen nitroxides.

Uranium(IV) imidazolin-2-iminato complexes: a new class of actinide complexes.

Acid-base reactions between imidazolin-2-imines (Im(R)NH) and [U(NMeEt)4] selectively afforded uranium(IV) imidazolin-2-iminato complexes of the type [U(NIm(tBu))4] (1), [U(NIm(Mes))3(NMeEt)] (2), and [U(NIm(Dipp))2(NMeEt)2] (3) depending on the steric demand of the substituents in the 1 and 3 positions of the imidazole heterocycle (R = tBu, Mes, Dipp). This new class of actinide complexes displays short U-N bonds and nearly linear U-N-C bond angles, suggesting a U-N bond order higher than 1, as well as extraordinarily high catalytic activity in the polymerization of ε-caprolactone.

Multifarenes: new modular cavitands.

Multifarenes, a new class of macrocycles, which are constructed of alternating building blocks, are conveniently accessible by three complementary syntheses that provide modularity and scalability. In addition to metal-ion coordination, these cavitands show increased flexibility with increasing ring size, offering opportunities for induced fit to guest molecules.

Polymerization of propylene promoted by zirconium benzamidinates.

New bis(N,N-trimethylsilylarylamidinate) zirconium dichloride complexes with various carbon substituents were prepared, and their solid as well as solution state structures were studied. In the polymerization of propylene, after activation by MAO, these catalysts provided two fractions. Ether soluble polymers were obtained at a low activity as sticky polymers with lower molecular weights, except with the o-OMe substituted complex. The solid fractions were composed of a highly isotactic polymer and a moderately syndiotactic polymer. An interesting linear correlation was found between the rates of the 2,1 and 3,1 insertions for the ether soluble fractions.

Mono(imidazolin-2-iminato) titanium complexes for ethylene polymerization at low amounts of methylaluminoxane.

The polymerization of ethylene with titanium complexes bearing one bulky imidazolin-2-iminato ligand (L) in the presence of MAO and/or TTPB as cocatalysts have been explored. The complex LTiCl3 and its methylated forms were prepared to shed light on the nature of the active polymerization species. With some of these complexes, the best catalytic activity was obtained at an Al:Ti ratio of 8.

Organoactinides in the polymerization of ethylene: is TIBA a better cocatalyst than MAO?

The synthesis of two pyridylamidinate bis(N,N'-bis(trimethylsilyl)-2-pyridylamidinate)An(µ-Cl)2Li(TMEDA) (An = U (1), Th (2)) complexes is presented. For complex 1 the solid state X-ray structures were studied and compared to that of complex 2. The organoactinide complexes were studied as pre-catalysts in the polymerization of ethylene when activated by methylalumoxane (MAO). The catalytic activity was improved using a mixture of trityl tetrakispentafluorophenylborate (TTPB) and a small amount of methylalumoxane (MAO) as cocatalysts, and was amazingly improved, providing the greatest activity, using only triisobutyl aluminum (TIBA). We present a combination of ESR, C60 radical trapping, and MALDI-TOF studies describing the formation of the single-site active species, capturing some unique features of the complexes and shedding light on the polymerization mechanism.

Cobalt corrole catalyst for efficient hydrogen evolution reaction from H2O under ambient conditions: reactivity, spectroscopy, and density functional theory calculations.

The feasibility of a hydrogen-based economy relies very much on the availability of catalysts for the hydrogen evolution reaction (HER) that are not based on Pt or other noble elements. Significant breakthroughs have been achieved with certain first row transition metal complexes in terms of low overpotentials and large turnover rates, but the majority of reported work utilized purified and deoxygenated solvents (most commonly mixtures of organic solvents/acids). Realizing that the design of earth abundant metal catalysts that operate under truly ambient conditions remains an unresolved challenge, we have now developed an electronically tuned Co(III) corrole that can catalyze the HER from aqueous sulfuric acid at as low as -0.3 V vs NHE, with a turnover frequency of 600 s(-1) and ≫10(7) catalytic turnovers. Under aerobic conditions, using H2O from naturally available sources without any pretreatment, the same complex catalyzes the reduction of H(+) with a Faradaic Yield (FY) of 52%. Density functional theory (DFT) calculations indicate that the electron density on a putative hydride species is delocalized off from the H atom into the macrocycle. This makes the protonation of a [Co(III)-H](-) species the rate determining step (rds) for the HER consistent with the experimental data.

Chemisorbed monolayers of corannulene penta-thioethers on gold.

Penta(tert-butylthio)corannulene and penta(4-dimethylaminophenylthio)corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C-S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold.

Forming all-carbon quaternary stereogenic centres in acyclic systems from alkynes.

The formation of all-carbon quaternary stereocentres in acyclic systems is one of the most difficult contemporary challenges in modern synthetic organic chemistry. Particularly challenging is the preparation of all-carbon quaternary stereocentres in aldol adducts; this difficulty is problematic because the aldol reaction represents one of the most valuable chemical transformations in organic synthesis. The main problem that limits the formation of these stereocentres is the absence of an efficient method of preparing stereodefined trisubstituted enolates in acyclic systems. Here we describe a different approach that involves the formation of two new stereogenic centres--including the all-carbon quaternary one--via a combined carbometalation-oxidation reaction of an organocuprate to give a stereodefined trisubstituted enolate. We use this method to generate a series of aldol and Mannich products from ynamides with excellent diastereomeric and enantiomeric ratios and moderate yields.

Synthesis of silenyllithiums Li(R'3Si)Si═C(SiR3)(1-Ad) via transient silyne-silylidene intermediates.

The first two lithium silenides, Li(tBu(2)MeSi)Si═C(SiMetBu(2))(1-Ad) (1) and Li(tBuMe(2)Si)Si═C(SiMetBu(2))(1-Ad) (2) were prepared by THF addition to the corresponding lithium-silenolates, [(tBu(2)MeSi)(2)Si═C(OLi)(1-Ad)]·(R(3)SiLi) (3a: R(3)Si═tBu(2)MeSi, 3b: R(3)Si═tBuMe(2)Si). 1 and 2 were crystallized, and their structures were determined by X-ray crystallography. This process requires the presence of both coaggregated silyllithium (R(3)SiLi) (3a and 3b) and THF. Based on reaction products and DFT calculations, it is suggested that elimination of tBu(2)MeSiOLi from 3a (or 3b) produces first the corresponding silyne intermediate which rearranges to the corresponding silylidene, which is then trapped by R(3)SiLi giving 1 (or 2).

Chlorinated corroles.

Fully ß-pyrrole chlorinated corrole was obtained via reaction of the cobalt(III) complex of 5,10,15-tris(pentafluorophenyl)corrole with chlorine. The structural and electronic effects of chlorination are reported, together with the coordination chemistry of this complex.

Bistable cucurbituril rotaxanes without stoppers.

Bistable rotaxanes are important design elements of molecular devices for a broad range of applications, such as controlled drug release, molecular rotary motors, and chemical sensors. The host-guest complexes of cucurbit[6]uril and 1,4-bis(alkylaminomethyl)benzene were found to exhibit two stable binding modes with an unexpectedly high barrier between them. Their structural and dynamic properties, kinetic and thermodynamic parameters, as well as different chemical reactivity towards the azide-alkyne [3+2] cycloaddition reaction (click chemistry), were discovered by NMR spectroscopy, X-ray crystallography, and isothermal titration microcalorimetry. The highly stable 2:1 complex, which is formed at room temperature, was found to be a kinetic product, which may be converted to the thermodynamic 1:1 complex upon prolonged heating to 100 °C. The latter is a very stable rotaxane despite the fact that it lacks bulky end groups.

Convergent diastereoselective preparation of adjacent quaternary stereocenters in an acyclic system.

The combined carbometalation-zinc homologation-allylation reaction of the resulting stereodefined 3,3-disubstituted allylmetal species with ketones allow the preparation of allylic vicinal diol derivatives in good yields with excellent diastereomeric ratios from commercially available alkynes. Two adjacent quaternary centers are formed with the concomitant formation of three new carbon-carbon bonds in a single-pot operation in an acyclic system. The bulky substituent of the ketone occupies a pseudo-axial position in the Zimmerman-Traxler transition state.

Iodinated aluminum(III) corroles with long-lived triplet excited states.

The first reported iodination of a corrole leads to selective functionalization of the four C-H bonds on one pole of the macrocycle. An aluminum(III) complex of the tetraiodinated corrole, which exhibits red fluorescence, possesses a long-lived triplet excited state.

Nitrenium ions as ligands for transition metals.

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium-metal bonds is unambiguously confirmed, in solution by selective (15)N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor σ-donors and good π-acceptors.