Alessandro Bismuto

Central control: A series of bismuth aminotroponiminate cations are discussed and differences between neutral and cationic compounds in terms of coordination chemistry, Lewis acidity, and redox behavior are pinpointed. Specifically, cationization allows switching from ligand‐centered redox events to metal‐centered redox events, facilitating rare examples of quasi‐reversible electron transfer at bismuth.

Abstract

The behavior of the redox‐active aminotroponiminate (ATI) ligand in the coordination sphere of bismuth has been investigated in neutral and cationic compounds, [Bi(ATI)₃] and [Bi(ATI)₂L _n ][A] (L=neutral ligand; n=0, 1; A=counteranion). Their coordination chemistry in solution and in the solid state has been analyzed through (variable‐temperature) NMR spectroscopy, line‐shape analysis, and single‐crystal X‐ray diffraction analyses, and their Lewis acidity has been evaluated by using the Gutmann–Beckett method (and modifications thereof). Cyclic voltammetry, in combination with DFT calculations, indicates that switching between ligand‐ and metal‐centered redox events is possible by altering the charge of the compounds from 0 in neutral species to +1 in cationic compounds. This adds important facets to the rich redox chemistry of ATIs and to the redox chemistry of bismuth compounds, which is, so far, largely unexplored.

The first series of bismuth amides of type [Bi(NAr₂)₃] has been synthesized and characterized. Remarkably, they readily release aminyl radicals (NAr₂)^. and facilitate highly selective radical coupling to give (NAr₂)₂. Bismuth amides mediate the selective dehydrocoupling of HPnR₂ (Pn=N, P, As), including very challenging substrates. Conditions, mechanisms, and substrate scopes are complementary or superior to those of more established procedures.

Abstract

The controlled release of well‐defined radical species under mild conditions for subsequent use in selective reactions is an important and challenging task in synthetic chemistry. We show here that simple bismuth amide species [Bi(NAr₂)₃] readily release aminyl radicals [NAr₂]^. at ambient temperature in solution. These reactions yield the corresponding hydrazines, Ar₂N−NAr₂, as a result of highly selective N−N coupling. The exploitation of facile homolytic Bi−Pn bond cleavage for Pn−Pn bond formation was extended to higher homologues of the pnictogens (Pn=N–As): homoleptic bismuth amides mediate the highly selective dehydrocoupling of HPnR₂ to give R₂Pn−PnR₂. Analyses by NMR and EPR spectroscopy, single‐crystal X‐ray diffraction, and DFT calculations reveal low Bi−N homolytic bond‐dissociation energies, suggest radical coupling in the coordination sphere of bismuth, and reveal electronic and steric parameters as effective tools to control these reactions.

Novel derivatives of a bulky aminocarbazole featuring tetravalent Si, Ge or Sn atoms were prepared. Deprotonation of the aminocarbazole R‐NH₂ or R‐N(SiMe₃)H with benzyl potassium and subsequent metathesis with group 14 halides enables access to a range of molecules featuring the N−N(H)‐EX₃ or N−N(SiMe₃)‐EX₃ structural motif (E=Si, Ge, Sn; X=Cl, Br, I).

Abstract

Stepwise metalation and metathesis reactions with a bulky aminocarbazole were conducted to prepare derivatives of tetravalent group 14 elements. These were regarded as putatively suitable precursors for the formation of doubly bonded group 14/group 15 molecules such as imino species. Starting from an N‐aminocarbazole, deprotonation with benzyl potassium formed the corresponding solvent‐free dimeric amide. Metathesis reactions with EBr₄ (E=Si, Ge, Sn) afforded RN(H)EBr₃. Deprotonation of RN(SiMe₃)H with benzyl potassium afforded the solvent‐free monomeric amide RN(SiMe₃)K which was then treated with SiCl₄, GeBr₄ and SnI₄. Both obtained series of compounds, RN(H)EBr₃ and RN(SiMe₃)EX₃, were characterized by multinuclear NMR spectroscopy and SCXRD studies.

A deceptively simple proton abstraction from nickel‐bound formate not only prepares the resulting CO₂ ²⁻ unit for the release of CO in contact with electrophiles but opens up unique complex reaction schemes including reduction of nickel(II) to nickel(I) and coupling of CO₂ units to give oxalate and mesoxalate. Mesoxalate formation from individual CO₂ units is so far unprecedented.

Abstract

The complexes [L ^tBuNi(OCO‐κ ² O,C)]M₃[N(SiMe₃)₂]₂ (M=Li, Na, K), synthesized by deprotonation of a nickel formate complex [L ^tBuNiOOCH] with the corresponding amides M[N(SiMe₃)₂], feature a Ni^II−CO₂ ²⁻ core surrounded by Lewis‐acidic cations (M⁺) and the influence of the latter on the behavior and reactivity was studied. The results point to a decrease of CO₂ activation within the series Li, Na, and K, which is also reflected in the reactivity with Me₃SiOTf leading to the liberation of CO and formation of a Ni−OSiMe₃ complex. Furthermore, in case of K⁺, the {[K₃[N(SiMe₃)₂]₂}⁺ shell around the Ni−CO₂ ²⁻ entity was shown to have a large impact on its stabilization and behavior. If the number of K[N(SiMe₃)₂] equivalents used in the reaction with [L ^tBuNiOOCH] is decreased from 3 to 0.5, the deprotonated part of the precursor enters a complex reaction sequence with formation of [L ^tBuNi^I(μ‐OOCH)Ni^IL ^tBu]K and [L ^tBuNi(C₂O₄)NiL ^tBu]. The same reaction at higher concentrations additionally led to the formation of a unique hexanuclear Ni^II complex containing both oxalate and mesoxalate ([O₂C‐CO₂‐CO₂]⁴⁻) ligands.

A high photoluminescence quantum yield of up to 79 % with a short delayed fluorescence lifetime of approximately 4 μs in the solid state is possible with a novel series of mononuclear aluminum complexes. Solution‐processed organic light‐emitting devices based on these Al complexes achieve a high external quantum efficiency of 17.5 % at 100 cd m⁻².

Abstract

Light metal complexes, such as lithium (Li), sodium (Na), magnesium (Mg), and aluminum (Al) complexes, are attractive candidates for the fabrication of thermally activated delayed fluorescent (TADF) materials. Nevertheless, mononuclear Al complexes with delayed fluorescence have not been developed so far. In this study, we successfully developed a novel series of highly luminescent Al complexes with two phenylacridine‐modified asymmetric acetylacetonate‐type ligands. These complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 79 % in the solid state with a short delayed fluorescence lifetime of approximately 4 μs. Solution‐processed organic light‐emitting devices (OLEDs) using these Al complexes exhibit excellent performance with an external quantum efficiency of 17.5 % at 100 cd m⁻². This is the best performance in light metal‐based TADF OLEDs reported so far. The results are expected to guide the advancement of the next‐generation solid‐state lighting technology.

The Al(I) compound NacNacAl ( 1 , NacNac = [ArNC(Me)CHC(Me)NAr] ‒ , Ar = 2,6‐ i Pr 2 C 6 H 3 ) serves as a template for the chemoselective coupling between carbonyls (benzophenone, fenchone, isophorone, p ‐tolyl benzoate, N,N ‐dimethyl benzamide, (1‐phenylethylidene)aniline) and pyridine. With the CH‐acidic ketone (1R)‐(+) camphor, the reaction affords a hydrido alkoxide compound of Al, formed as the result of enolization, whereas an enolizable imine, (1‐phenylethylidene)aniline, and the bulky ketone isophorone, still chemoselectively couple with pyridine. In contrast, reaction with the ester p ‐tolyl benzoate results in cleavage of the ester bond together with replacement of the alkoxy group by a hydrogen atom of the pyridine moiety. This study demonstrates that for carbonyl substrates featuring phenyl substituents, the reaction proceeds via intermediate formation of h 2 (C,X)‐coordinated (X =O, N) carbonyl adducts, whereas the reaction of 1 with (R)‐(‒)‐fenchone in the absence of pyridine leads to CH activation in the pendant isopropyl group of the Ar substituent of the NacNac ligand.

Several 1,1’‐diphosphanyl‐substituted metallocenes of magnesium (magnesocenes) were synthesized, structurally characterized, and their reactivity and coordination chemistry were investigated. Transmetalation of these magnesocenes gives access to group 14 metallocenes (tetrelocenes), as well as to stibonocenes. These s‐ and p‐block metallocenes represent a novel class of bis(phosphanyl) ligands, exhibiting Lewis‐amphiphilic character. Their coordination chemistry towards different transition metal and maingroup fragments was investigated and different complexes are presented.

Although of purely atomic origin, spin crossover (SCO) spreads like a cascade of dominoes to all dimensions of a material, which fit together like Russian dolls with not only extremely different effects in nature and in amplitude, but also with a mutual interdependence. The multi‐scale description is the only way to understand the full impact of a SCO on a material, incidentally showing the power of the crystallography approach. This concept could be extended to many other molecular phenomena.

Abstract

The spin crossover (SCO) phenomenon corresponds to a modification that originates at the atomic scale. However, the simple consideration of the transformations that occur following the SCO at this scale or in its close vicinity does not allow anyone to truly understand, anticipate and thus take advantage of what happens at the scale of the material, and even less at the device one. As the fruit of years of work and experience on this phenomenon, we formalize here the concept of the multiscale understanding of SCO. Clearly, the deflagration generated by the initial impressive atomic modification on all the physical scales of the solid must be understood in terms of structure‐properties relationships that fit together, like Russian dolls, and propagate according to a kind of domino effect. Each scale can both give different and independent consequences from those of the other scales but at the same time can influence those of a larger or smaller scale, the whole being imperatively to take into account. The concept appears well illustrated by the volume modification, always the same at the atomic level but drastically different and adaptable, in amplitude and sense, at any other physical scale. This approach results in a much wider range of potential applications than the atomic level alone initially suggests, including one serious path to shape memory materials.

Research suggests that borate could have mediated the prebiotic synthesis of RNA precursors (ribose, ribonucleosides, and ribonucleotides) in reactions relevant for the origin of life. This minireview provides an overview of recent developments in prebiological chemistry related to boron species.

Abstract

Boron(III), as borate (or boric acid), mediates the synthesis of ribose, ribonucleosides, and ribonucleotides. These reactions are carried out under moderate temperatures (typically 70–95 °C) with organic molecules (or their derivatives) detected in interstellar space and inorganic ions found in minerals on Earth (and could occur during early stages of prebiotic evolution). Research in this century suggests that borate was a relevant prebiological reagent, thus reinforcing the RNA world hypothesis as an explanation for the origin of life. Herein, these developments on prebiological chemistry related to boron species are reviewed.

How unusual! A rare hexacoordinated dication of Sb^V has been synthesized (see scheme). This [12–Sb–6]²⁺ species exhibits remarkable stability due to the synergistic kinetic and thermodynamic stabilization provided by donor–acceptor interactions with its 2‐(2‐pyridyl)phenyl ligands. This dicationic species preferentially adopts a meridional form both in solution and in the solid state.

Abstract

The hexacoordinated antimony(V) dication [(ppy)₃Sb]²⁺ ([1]²⁺; ppy=2‐(2‐pyridyl)phenyl), stabilized by three intramolecular donor–acceptor interactions, has been isolated as its hexachloroantimonate salt [1][SbCl₆]₂, prepared by the oxidative addition of chlorine to the neutral stibine [(ppy)₃Sb] (1), followed by the abstraction of chloride. Air‐stable [1][SbCl₆]₂ exhibits remarkable thermal stability and the three ppy ligands on the antimony atom are shown to be magnetically inequivalent in the ¹H and ¹³C NMR spectra. A hexacoordinated, meridional octahedral bonding geometry has been determined for [1][SbCl₆]₂ by X‐ray crystallographic analysis. Theoretical calculations were performed to investigate why the meridional form was generated preferentially over the facial form. In addition, the dynamics of the ppy ligands were investigated by variable‐temperature ¹H NMR spectroscopy. The potential to generate dications by using a single‐electron‐transfer reagent has also been investigated. The dication [1]²⁺ is the first [12–Sb–6]²⁺ chemical species to have been structurally determined.

A series of semi‐planar triarylboranes were obtained by three procedures. The effects of the planarization induced by the methylene bridge on their steric shielding, Lewis acidities and optical properties were investigated by experiments and quantum chemical calculations. These methods can be extended to the preparation of functionalized spirocyclic amine‐boranes and unsymmetrical triarylboranes, which might be used in frustrated Lewis pair chemistry or as pH and photo‐responsive boron Lewis acids.

Abstract

Three synthetic methods towards semi‐planar triarylboranes with two aryl rings connected by a methylene bridge have been developed. The fine‐tuning of their stereoelectronic properties and Lewis acidities was achieved by introducing fluorine, methyl, methoxy, n‐butyl and phenyl groups either at their exocyclic or bridged aryl rings. X‐ray diffraction analysis and quantum‐chemical calculations provided quantitative information on the structural distortion experienced by the near planar hydro‐boraanthracene skeleton during the association with Lewis bases such as NH₃ and F⁻. Though the methylene bridge between the ortho‐positions of two aryl rings of triarylboranes decreased the Gibbs free energies of complexation with small Lewis bases by less than 5 kJ mol⁻¹ relative to the classical Lewis acid BAr₃, the steric shielding of the CH₂ bridge is sufficient to avoid the formation of Lewis adducts with larger Lewis bases such as triarylphosphines. A newly synthesized spirocyclic amino‐borane with a long intramolecular B−N bond that could be dissociated under thermal process, UV‐irradiation, or acidic conditions might be a potential candidate in Lewis pairs catalysis.

Structural constraint represents an attractive tool to modify p ‐block element properties without the need for unusual oxidation or valence states. The recently reported methyl‐calix[4]pyrrolato aluminate established the effect of forcing a tetrahedral aluminum anion into a square‐planar coordination mode. However, the generality of this structural motif and any consequence of ligand modification remained open. Herein, we launch a systematic ligand screening and broaden the class of square‐planar aluminum anions by two derivatives that differ in the meso ‐substitution at the calix[4]pyrrolato ligand. Strikingly, this modification provokes opposing trends in the preference for a Lewis acidic binding mode with σ‐donors versus the aluminum‐ligand cooperative binding mode with carbonyls. Insights into the origin of these counterintuitive experimental observations are provided by computation and bond analysis. Importantly, this rationale might allow to exploit mode‐selective binding for catalytic rate control.

How unusual! A rare hexacoordinated dication of Sb^V has been synthesized (see scheme). This [12–Sb–6]²⁺ species exhibits remarkable stability due to the synergistic kinetic and thermodynamic stabilization provided by donor–acceptor interactions with its 2‐(2‐pyridyl)phenyl ligands. This dicationic species preferentially adopts a meridional form both in solution and in the solid state.

Abstract

The hexacoordinated antimony(V) dication [(ppy)₃Sb]²⁺ ([1]²⁺; ppy=2‐(2‐pyridyl)phenyl), stabilized by three intramolecular donor–acceptor interactions, has been isolated as its hexachloroantimonate salt [1][SbCl₆]₂, prepared by the oxidative addition of chlorine to the neutral stibine [(ppy)₃Sb] (1), followed by the abstraction of chloride. Air‐stable [1][SbCl₆]₂ exhibits remarkable thermal stability and the three ppy ligands on the antimony atom are shown to be magnetically inequivalent in the ¹H and ¹³C NMR spectra. A hexacoordinated, meridional octahedral bonding geometry has been determined for [1][SbCl₆]₂ by X‐ray crystallographic analysis. Theoretical calculations were performed to investigate why the meridional form was generated preferentially over the facial form. In addition, the dynamics of the ppy ligands were investigated by variable‐temperature ¹H NMR spectroscopy. The potential to generate dications by using a single‐electron‐transfer reagent has also been investigated. The dication [1]²⁺ is the first [12–Sb–6]²⁺ chemical species to have been structurally determined.

Organometallic allylic reagents are widely used in the construction of C‐C bond by Barbier‐type reactions. In this communication, we have described a photoredox Barbier allylation of aldehydes mediated by bismuth, in absence of other metals as co‐reductants. Mild reaction conditions, tolerance of oxygen, and use of aqueous solvent make this photoredox methodology attractive for green and sustainable synthesis of homoallylic alcohols.

Qualitatively different structures and electron transfer behavior have been established for platinum and palladium complexes [PtL₂]^0/+ and [Pd₂L₄]^0/+ with a redox‐active amidinato ligand L⁻ /L^..

Abstract

Reaction of [Pt(DMSO)₂Cl₂] or [Pd(MeCN)₂Cl₂] with the electron‐rich LH=N,N’‐bis(4‐dimethylaminophenyl)ethanimidamide yielded mononuclear [PtL₂] (1) but dinuclear [Pd₂L₄] (2), a paddle‐wheel complex. The neutral compounds were characterized through experiments (crystal structures, electrochemistry, UV‐vis‐NIR spectroscopy, magnetic resonance) and TD‐DFT calculations as metal(II) species with noninnocent ligands L⁻. The reversibly accessible cations [PtL₂]⁺ and [Pd₂L₄]⁺ were also studied, the latter as [Pd₂L₄][B{3,5‐(CF₃)₂C₆H₃}₄] single crystals. Experimental and computational investigations were directed at the elucidation of the electronic structures, establishing the correct oxidation states within the alternatives [Pt^II(L⁻)₂] or [Pt^.(L )₂], [Pt^II(L^0.5−)₂]⁺ or [Pt^III(L⁻)₂]⁺, [(Pd^II)₂(μ‐L⁻)₄] or [(Pd^1.5)₂(μ‐L^0.75−)₄], and [(Pd^2.5)₂(μ‐L⁻)₄]⁺ or [(Pd^II)₂(μ‐L^0.75−)₄]⁺. In each case, the first alternative was shown to be most appropriate. Remarkable results include the preference of platinum for mononuclear planar [PtL₂] with an N‐Pt‐N bite angle of 62.8(2)° in contrast to [Pd₂L₄], and the dimetal (Pd₂ ⁴⁺→Pd₂ ⁵⁺) instead of ligand (L⁻→L ) oxidation of the dinuclear palladium compound.

Ionic fifteenth: The diverse synthetic pathways to π‐conjugated systems with quaternary phosphorus, its unique connectivity and tunable electron deficient character, along with the non‐covalent cation‐anion interactions, introduce new photophysical and electrochemical features to ionic chromophores.

Abstract

Tunable electron‐accepting properties of the cationic phosphorus center, its geometry and unique preparative chemistry that allows combining this unit with diversity of π‐conjugated motifs, define the appealing photophysical and electrochemical characteristics of organophosphorus ionic chromophores. This Minireview summarizes the achievements in the synthesis of the π‐extended molecules functionalized with P‐cationic fragments, modulation of their properties by means of structural modification, and emphasizes the important effect of cation‐anion interactions, which can drastically change physical behavior of these two‐component systems.

The direct dehydrogenation of alkanes is among the most efficient ways to access valuable alkene products. Although several catalysts have been designed to promote this transformation, they have unfortunately found limited applications in fine chemical synthesis. Here, we report a conceptually novel strategy for the catalytic, intermolecular dehydrogenation of alkanes using a ruthenium catalyst. The combination of a redox‐active ligand and a sterically hindered aryl radical intermediate has unleashed this novel strategy. Importantly, mechanistic investigations have been performed to provide a conceptual framework for the further development of this new catalytic dehydrogenation system.

Protocols for the synthesis of the bulky polyfluorinated triarylboranes 2,6‐(C 6 F 5 ) 2 C 6 F 3 B(C 6 F 5 ) 2 ( 1 ), 2,6‐(C 6 F 5 ) 2 C 6 F 3 B[C 6 H 3 ‐3,5‐(CF 3 ) 2 ] ( 2 ), 2,4,6‐(C 6 F 5 ) 3 C 6 H 2 B(C 6 F 5 ) 2 ( 3 ), 2,4,6‐(C 6 F 5 ) 3 C 6 H 2 B[C 6 H 3 ‐3,5‐(CF 3 ) 2 ] ( 4 ) were developed. All boranes are water tolerant and according to the Gutmann‐Beckett method, 1 – 3 display Lewis acidities larger than that of the prominent B(C 6 F 5 ) 3 .

The yet underdeveloped coordination chemistry of Ga^I species won a new addition to their family. With an unprecedented bonding situation in main‐group chemistry, the complex salt Ga(COD)₂ ⁺[Al(OR^F)₄]⁻ (COD=1,5‐cyclooctadiene; R^F=C(CF₃)₃) offers novel options. Its properties are compared with respect to those of related transition‐metal species.

Abstract

The earth‐metal olefin complex [Ga ^I (COD)₂]⁺[Al(OR^F)₄]⁻ (COD=1,5‐cyclooctadiene; R^F=C(CF₃)₃) constitutes the first homoleptic olefin complex of any main‐group metal accessible as a bulk compound. It is straight forward to prepare in good yield and constitutes an olefin complex of a main‐group metal that—similar to many transition‐metals—may adopt the +1 and +3 oxidation states opening potential applications. Crystallographic‐, vibrational‐ and computational investigations give an insight to the atypical bonding between an olefin and a main‐group metal. They are compared to classical transition‐metal relatives.

Photochemistry is a tremendous research field offering many synthetic possibilities to the chemists. Breakthroughs in this area have been notably driven by the implementation of new classes of photocatalysts. Within this context, Bodipy (Boron‐dipyrromethene) dyes possess attractive chemical and physical features such as their modularity, strong absorption under visible light irradiation, good thermal and photochemical stabilities and high fluorescence quantum yields. As such, this class of compounds has found widespread applications in functionalized materials, biology, medicine or organic chemistry. From an organic‐synthetic point of view, excited states of Bodipy dyes have been harnessed in electron and energy transfer reactions. This minireview collates the relevant literature on the applications of these catalysts in synthetic photochemistry and provides some perspectives of this research area.