Michael Cowley

The accessibility of triads with deltahedral Zintl clusters in analogy to fullerene–linker–fullerene triads is another example for the close relationship between fullerenes and Zintl clusters. The compound {[K(2.2.2-crypt)]₄[RGe₉-CHCHCHCH-Ge₉R]}(toluene)₂ (R=(2Z,4E)-7-amino-5-aza-hepta-2,4-dien-2-yl), containing two deltahedral [Ge₉] clusters linked by a conjugated (1Z,3Z)-buta-1,3-dien-1,4-diyl bridge, was synthesized through the reaction of 1,4-bis(trimethylsilyl)butadiyne with K₄Ge₉ in ethylenediamine and crystallized after the addition of 2.2.2-cryptand and toluene. The compound was characterized by single-crystal structure analysis as well asNMR and IR spectroscopy.

Zintl triads: The general possibility to synthesize electronically coupled Zintl clusters using conjugated electron π-systems as linkers in analogy to fullerene–linker–fullerene triads opens a new area of applications for Zintl clusters.

Ansa-aminoborane 1 (ortho-TMPC₆H₄BH₂; TMP=2,2,6,6-tetramethylpiperid-1-yl), a frustrated Lewis pair with the smallest possible Lewis acidic boryl site (BH₂), is prepared. Although it is present in quenched forms in solution, and BH₂ represents an acidic site with reduced hydride affinity, 1 reacts with H₂ under mild conditions producing ansa-ammonium trihydroborate 2. The thermodynamic and kinetic features as well as the mechanism of this reaction are studied by variable-temperature NMR spectroscopy, spin-saturation transfer experiments, and DFT calculations, which provide comprehensive insight into the nature of 1.

As simple as that: An intramolecular frustrated Lewis pair that consists of a very simple acidic boryl site (BH₂) and a bulky amino group is found to split dihydrogen in a fast and reversible process. Spin-saturation transfer NMR techniques were used to measure the reaction rates and to obtain accurate kinetic parameters.

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N-heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (−45 °C). The adducts show short BN bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di-tert-butyliminoborane was reacted with 1,3-bis(isopropyl)imidazol-2-ylidene (IPr), the initially formed Lewis acid–base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4-position. Warming the iminoborane–CAAC adduct to room temperature resulted in an intramolecular cyclization to give a bicyclic 1,2-azaborilidine compound.

A B in C’s clothing: The isolation and characterization of a set of carbene adducts of iminoboranes indicates diverse reactivity patterns. Simple adducts present “boraimine” structures, which mimic the organic imine functionality. Rearrangements of these simple adducts yield backbone-substituted carbenes as well as 1,2-azaborolidines.

The first metal-free catalytic hydroboration of carbonyl derivatives has been developed in which a catalytic amount of 1,3,2-diazaphospholene effectively promotes a hydroboration reaction of aliphatic and aromatic aldehydes and ketones. The reaction mechanism involves the cleavage of both the PO bond of the alkoxyphosphine intermediate and the BH bond of pinacolborane as well as the formation of PH and BO bonds. Thus, the reaction proceeds through a non-metal σ-bond metathesis. Kinetic and computational studies suggest that the σ-bond metathesis occurred in a stepwise but nearly concerted manner.

Leave the metal out: A catalytic amount of diazaphospholene effectively promotes a hydroboration reaction of various aldehydes and ketones under metal-free conditions. Kinetic and computational studies show that the reaction mechanism involves a σ-bond metathesis process occurring in a stepwise but nearly concerted manner.

The reaction of the bulky bis(imidazolin-2-iminato) ligand precursor (1,2-(L^MesNH)₂-C₂H₄)[OTs]₂ (1²⁺ 2[OTs]⁻; L^Mes=1,3-dimesityl imidazolin-2-ylidene, OTs=p-toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2-(L^MesN)₂-C₂H₄)BH₂[OTs] (2⁺ [OTs]⁻)_. The boronium cation 2⁺ [OTs]⁻ reacts with elemental sulfur to give the thioxoborane salt (1,2-(L^MesN)₂-C₂H₄)BS[OTs] (3⁺ [OTs]⁻). The hitherto unknown compounds 1²⁺ 2[OTs]⁻, 2⁺ [OTs]⁻, and 3⁺ [OTs]⁻ were fully characterized by spectroscopic methods and single-crystal X-ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2⁺ and 3⁺. The theoretical, as well as crystallographic studies reveal that 3⁺ is the first example for a stable cationic complex of three-coordinate boron that bears a BS double bond.

A healthy relationship: The first three-coordinate boron cation with a boron–sulfur double bond has the shortest BS distance reported for a molecular complex. In the calculated LUMO, the contribution from the boron center confirms that the compound is a boron-centered cation. The HOMO−1 reveals the π interaction which is the cause of the close contact between B and S.

Water exists as two nuclear-spin isomers, para and ortho, determined by the overall spin of its two hydrogen nuclei. For isolated water molecules, the conversion between these isomers is forbidden and they act as different molecular species. Yet, these species are not readily separated, and no pure para sample has been produced. Accordingly, little is known about their specific physical and chemical properties, conversion mechanisms, or interactions. The production of isolated samples of both spin isomers is demonstrated in pure beams of para and ortho water in their respective absolute ground state. These single-quantum-state samples are ideal targets for unraveling spin-conversion mechanisms, for precision spectroscopy and fundamental symmetry-breaking studies, and for spin-enhanced applications, for example laboratory astrophysics and astrochemistry or hypersensitized NMR experiments.

Keep them separated: The two nuclear-spin isomers para and ortho water can be isolated in their absolute ground state in pure molecular beams. No pure para sample had previously been produced. This method for the separation of quantum states is generally applicable to all polar neutral molecules and allows the spatial separation of single quantum states and nuclear-spin isomers.

Reacting white phosphorus (P₄) with sterically encumbered aryl lithium reagents (aryl=2,6-dimesitylphenyl or 2,4,6-tBu₃C₆H₂) and B(C₆F₅)₃ gives the unique, isolable Lewis acid stabilized bicyclo[1.1.0]tetraphosphabutane anion. Subsequent alkylation of the nucleophilic site of the RP₄ anion gives access to non-symmetrical disubstituted bicyclic tetraphosphorus compounds. This novel method enables PC bond formation in a controlled fashion using white phosphorus as starting material.

Controlling P₄ transformations: Reacting P₄ with congested aryl lithium reagents and B(C₆F₅)₃ gives unique, Lewis acid stabilized bicyclo[1.1.0]tetraphosphabutane anions. Alkylating their nucleophilic site results in a non-symmetrical disubstituted bicyclic tetraphosphorus compound. This method enables the formation of PC bonds in a controlled fashion using white phosphorus as the starting material.

Ferrocenes, which are typically air-stable outer-sphere single-electron transfer reagents, were found to react with dioxygen in the presence of B(C₆F₅)₃, a Lewis acid unreactive to O₂, to generate bis(borane) peroxide. Although several Group 13 peroxides have been reported, boron-supported peroxides are rare, with no structurally characterized examples of the BO₂B moiety. The synthesis of a bis(borane)-supported peroxide anion and its structural and electrochemical characterization are described.

A bis(borane)-supported peroxide anion was synthesized and its structural and electrochemical characterization is described. Ferrocenes (see scheme; R=Me, H), which are typically air-stable outer-sphere single-electron transfer reagents, react with dioxygen in the presence of B(C₆F₅)₃, a Lewis acid unreactive to O₂, to generate bis(borane)peroxide.

The incorporation of heavier Group 14 element heteroles into semiconducting polymers leads to unusual optoelectronic properties. However, polymers containing stannoles have not been accessible to date. We report a synthetic route to a well-defined, stannole-containing polymer, the first example of this class of π-conjugated polymers. This route was made possible by developing difunctionalized stannole monomers and highly tin-selective Stille coupling reactions that leave the tin in the stannole untouched. Compared to poly(3-n-hexylthiophene), the resulting polymer displays a remarkable bathochromic shift in its absorption.

The budding potential of stannoles: A stannole monomer is prepared and employed in a highly tin-selective Stille coupling, giving a well-defined and non-annulated stannole-containing polymer, the first example from this class of π-conjugated polymers. Compared to polythiophenes, a strong bathochromic shift in the absorption spectrum was observed.

Chlorosilicates represent important intermediates in S_N2 reactions of chlorosilanes. They can be stabilized by the introduction of electron-withdrawing substituents. Salts of various (pentafluoroethyl)chlorosilicates have been isolated and structurally characterized.

Pentacoordinate chlorosilicates are known to be reactive intermediates. They can be stabilized by the introduction of at least two electron-withdrawing C₂F₅ groups, which has allowed the characterization of a series of (pentafluoroethyl)chlorosilicates (see example) in solution as well as in the solid state.

A one-electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)–bis(trimethylsilyl)aminodichloroborane adduct leads to a stable aminoboryl radical. A second one-electron reduction gives rise to a CAAC–aminoborylene adduct, which features an allenic structure. However, in manner similar to that of stable electrophilic singlet carbenes, this compound activates small molecules, such as CO and H₂.

Boron can do it! The first carbene that was stable at room temperature had a pseudo allenic structure, but owing to its high flexibility, it featured classical carbene reactivity. Similarly, a stable boron compound, isoelectronic with singlet carbenes, has an allenic structure, and is able to activate CO and H₂.

Attempts to prepare Fe(CO)₅⁺ from Ag[Al(OR^F)₄] (R^F=C(CF₃)₃) and Fe(CO)₅ in CH₂Cl₂ yielded the first complex of a neutral metal carbonyl bound to a simple metal cation. The Ag[Fe(CO)₅]₂⁺ cation consists of two Fe(CO)₅ molecules coordinating Ag⁺ in an almost linear fashion. The ν(CO) modes are blue-shifted compared to Fe(CO)₅, with one band above 2143 cm⁻¹ indicating that back-bonding is heavily decreased in the Ag[Fe(CO)₅]₂⁺ cation.

Oxidation or coordination? A simple reaction between a silver salt with a weakly coordinating anion and Fe(CO)₅ yields the unprecedented Ag[Fe(CO)₅]₂⁺ complex. This nonclassical carbonyl cation shows unusual stability as a crystalline solid (up to 150 °C). In the crystal and in DFT calculations it shows a unique bonding pattern with four carbonyl groups with bonds bent towards Ag, breaking the expected 4-fold symmetry.

The N-heterocyclic carbene–phosphinidene adduct IPrPSiMe₃ is introduced as a synthon for the preparation of terminal carbene–phosphinidyne transition metal complexes of the type [(IPrP)ML_n] (ML_n=(η⁶-p-cymene)RuCl) and (η⁵-C₅Me₅)RhCl). Their spectroscopic and structural characteristics, namely low-field ³¹P NMR chemical shifts and short metal–phosphorus bonds, show their similarity with arylphosphinidene complexes. The formally mononegative IPrP ligand is also capable of bridging two or three metal atoms as demonstrated by the preparation of bi- and trimetallic RuAu, RhAu, Rh₂, and Rh₂Au complexes.

IPrPSiMe₃, an N-heterocyclic carbene–phosphinidene adduct, is used as a synthon for the preparation of terminal carbene–phosphinidyne transition metal complexes of the type [(IPrP)ML_n]. These complexes exhibit spectroscopic and structural characteristics similar to those of arylphosphinidene complexes. The IPrP ligand is also capable of bridging two or three metal atoms, e.g., in bi- and trimetallic RuAu, RhAu, Rh₂, and Rh₂Au complexes.

A bridge between classical organic polycyclic aromatic hydrocarbons (PAH) and closo borohydride clusters is established by showing that they share a common origin regulated by the number of valence electrons in an electronic confined space. Application of the proposed electronic confined space analogy (ECSA) method to archetypal PAHs leads to the conclusion that the 4n+2 Wade–Mingos rule for three-dimensional closo boranes is equivalent to the (4n+2)π Hückel rule for two-dimensional PAHs. More importantly, use of ECSA allows design of new interesting fused closo boranes which can be a source of inspiration for synthetic chemists.

Making the rules: The Wade–Mingos 4n+2 rule for three-dimensional closo boranes is equivalent to the Hückel (4n+2)π rule for two-dimensional polycyclic aromatic hydrocarbons.