Isolierung eines dreifach koordinierten Borkations mit einer Bor-Schwefel-Doppelbindung

A Zwitterionic Heterobimetallic Gold-Iron Complex Supported by Bis(N-Heterocyclic Imine)Silyliumylidene

The facile synthesis of the first bis-N-heterocyclic imine-stabilized chlorosilyliumylidene 1 is reported. Remarkably, consecutive reaction of 1 with PPh3 AuCl and K2 Fe(CO)4 gives rise to the unique heterobimetallic complex 1,2-(Mes NHI)2 -C2 H4 -ClSiAuFe(CO)4 (4). The overall neutral complex 4 bears an unusual linear Si-Au-Fe structure and a rare anagostic interaction between the d10 -configured gold atom and a CH bond of the mesityl ligand. According to the computational analysis and 57 Fe Mössbauer spectroscopy, the formal Fe-oxidation state remains at -II. Thus, the electronic structure of 4 is best described as an overall neutral-yet zwitterionic-heterobimetallic "Si(II)+ -Au(I)+ -Fe(-II)2- "-silyliumylidene complex, derived from double anion exchange. The computational analysis indicates strong hyperconjugative back donation from the gold(I) atom to the silyliumylidene ligand.

Superseding β-Diketiminato Ligands: An Amido Imidazoline-2-Imine Ligand Stabilizes the Exhaustive Series of B=X Boranes (X=O, S, Se, Te)

Boron reluctantly forms B=X (X=O, S, Se, Te) moieties, which has stimulated the quest for such species in the past few years. Based on the N,N'-chelating β-diketiminato ligand (HNacNac), a new amido imidazoline-2-imine ligand system (HAmIm) is presented, giving rise to the isolation of an exhaustive series of Lewis acid free, monomeric chalcogen B=X boranes with documented π-bond character between boron and the chalcogen. The chalcogenoboranes are isoelectronic and isolobal to the respective ketones. The chemical behavior of the oxoborane (B=O) strongly resembles the classical carbonyl reactivity in C=O bonds. The improved stability provided by HAmIm arises from the formation of more-stable five-membered boron chelates versus the six-membered NacNac analogues and from the imidazoline-2-imine moiety providing enhanced σ- and π-donation. The HAmIm ligand class may supersede the widely employed NacNac system in certain applications.

Cationic Boron Formazanate Dyes*

Incorporation of cationic boron atoms into molecular frameworks is an established strategy for creating chemical species with unusual bonding and reactivity but is rarely thought of as a way of enhancing molecular optoelectronic properties. Using boron formazanate dyes as examples, we demonstrate that the wavelengths, intensities, and type of the first electronic transitions in BN heterocycles can be modulated by varying the charge, coordination number, and supporting ligands at the cationic boron atom. UV-vis absorption spectroscopy measurements and density-functional (DFT) calculations show that these modulations are caused by changes in the geometry and extent of π-conjugation of the boron formazanate ring. These findings suggest a new strategy for designing optoelectronic materials based on π-conjugated heterocycles containing boron and other main-group elements.

Crystalline Boragermenes

Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene-PMe₃ adduct 1 with Cl₂ BN(SiMe₃ )₂ followed by reductive dehalogenation with KC₈ . Addition of a Lewis base (^Me NHC) to 3 leads to the formation of the corresponding adduct 4, which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H₂ concomitant with a complete scission of the Ge=B bond.

An N-Heterocyclic Boryloxy Ligand Isoelectronic with N-Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)₂ BO]^- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)₂ BO]^- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

Successive Protonation of an N-Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion

Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a "stable ion" environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N-heterocyclic imine (NHI) derived carbonyl compound (IDippN)₂ CO, 2, to the monocation [(IDippN)(IDippNH) CO]⁺ , [3]⁺ , and the doubly protonated dication [(IDippNH)₂ CO]²⁺ , [4]²⁺ . [3]⁺ represents a rare example of an N-protonated carbonyl cation and [4]²⁺ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X-ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI-stabilized cation [IDippNCO]⁺ , [5]⁺ . Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]⁺ unit (isoelectronic to CCCO) can be described as an acylium cation "masked" as a cumulene.

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid-centered Lewis acidity and allow for its fine-tuning particularly with regard to preference for "hard" or "soft" Lewis bases (i.e., substrates). Especially the isolation of low-coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.

Bis[(dialkylamino)cyclopropenimine]-Stabilized P(III) - and P(V) -Centered Dications

The treatment of bis[(dialkylamino)cyclopropenimines] with dihalophosphines in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) to form diimine-stabilized P(III) -centered dications is reported. The structures of the new compounds were determined by using X-ray diffraction analysis and their donor abilities as ligands evaluated through electrochemical methods. Despite the two positive charges that they bear, these compounds depict intermediate behavior between that of phosphines and phosphites. The coordination of the [L2 PR](2+) moiety to Au(I) and Ag(I) is also reported. Even more surprisingly, these phosphorus centers can be oxidized to the corresponding P(V) dications in the presence of strong oxidants such as peroxides or XeF2 .