Indium(III)-Catalyzed Haloalkynylation Reaction of Alkynes.

Green chemistry strives for sustainability at the molecular level and is gaining increasing relevance in the development of chemical reactions. The haloalkynylation reaction is a highly atom-economical C-C coupling reaction that was previously only achieved using transition metal catalysts. It enables the introduction of an alkyne unit and a halogen atom into the target molecule. Herein, we present a haloalkynylation reaction catalyzed by indium(III) halides. The use of indium(III) bromide as a catalyst leads exclusively to the cis addition products with yields up to 86 %. In addition, iodoacetylenes can be applied for the first time for the haloalkynylation reaction of internal alkynes which is an important step forward in the development of industrially relevant and sustainable catalysts. In contrast to gold catalysis, which proceeds via a similar mechanism, the use of alkyl-substituted haloacetylenes as reagents is also possible. Based on 13C labeling experiments and quantum chemical calculations, we postulate two possible mechanisms for the indium(III)-catalyzed haloalkynylation reactions.

Gallaphosphene L(Cl)GaPGaL: A novel phosphinidene transfer reagent.

Gallaphosphene L(Cl)GaPGaL 1 (L=HC[C(Me)N(Ar)]2; Ar=2,6-iPr2C6H3) reacts with N-heterocyclic carbenes RNHC (RNHC=[CMeN(R)]2C; R=Me, iPr) to RNHC-coordinated phosphinidenes RNHC→PGa(Cl)L (R=Me 2 a, iPr 2 b) and with isonitriles RNC (R=iPr, Cy) to 1,3-phosphaazaallenes L(Cl)GaP=C=N-R (R=iPr 3 a, Cy 3 b), respectively. Quantum chemical calculations reveal that 2 a/2 b possess two localized lone pair of electrons, whereas 3 a/3 b only show one localized lone pair as was reported for gallaphosphene 1. 2 b reacts with 2.5 equivalents of a borane (THF ⋅ BH3) to the NHC-stabilized phosphinidene-borane complex [iPrNHC→P(BH2)]2(BH3)3 4 with concomitant formation of LGa(H)Cl 5. 2-5 are characterized by heteronuclear (1H, 13C{1H}, 31P{1H}) NMR and IR spectroscopy, elemental analysis, and single crystal X-ray diffraction (sc-XRD).

The Origin of Dual-Emission in PLICT Compounds - an Empirical Approach.

Fluorescence spectroscopy proves to be a powerful and versatile tool in scientific research, demanding ongoing advancements of fluorescence probes to cater to a broad range of applications. The particular interest lies in the development of fluorophores with multiplexing capability, emitting from more than one excited state depending on the polarity and viscosity of the surrounding medium. Here, we present a variety of novel dual planarized intramolecular charge transfer (PLICT) emitters based on cyano-1-aminoindole structures. We have chosen a combination of experimental studies, theoretical calculations and the creation of a unique fluorophore cataloging map. This map serves as a useful tool for empirically determining the origin of both fluorescence bands.

From Neutral Diarsenes to Diarsene Radical Ions and Diarsene Dications.

Diarsene [L(MeO)GaAs]2 (L=HC[C(Me)N(Ar)]2, Ar=2,6-iPr2C6H3, 4) reacts with MeOTf and MeNHC (MeNHC=1,3,4,5-tetra-methylimidazol-2-ylidene) to the diarsene [L(TfO)GaAs]2 (5) and the carbene-coordinated diarsene [L(MeO)GaAsAs(MeNHC)Ga(OMe)L] (6). The NHC-coordination results in an inversion of the redox properties of the diarsene 4, which shows only a reversible reduction event at E1/2=-2.06 V vs Fc0/+1, whereas the carbene-coordinated diarsene 6 shows a reversible oxidation event at E1/2=-1.31 V vs Fc0/+1. Single electron transfer reactions of 4 and 6 yielded [K[2.2.2.]cryp][L(MeO)GaAs]2 (8) and [L(MeO)GaAsAs(MeNHC)-Ga(OMe)L][B(C6F5)4] (9) containing the radical anion [L(MeO)GaAs]2⋅- (8⋅-) and the NHC-coordinated radical cation [L(MeO)GaAsAs(MeNHC)Ga(OMe)L]⋅+ (9⋅+), respectively, while the salt-elimination reaction of the triflate-coordinated diarsene 5 with Na[B(C6F5)4] gave [LGaAs]2[B(C6F5)4]2 (11) containing the dication [LGaAs]2 2+ (112+). Compounds 1-11 were characterized by 1H and 13C NMR, EPR (8, 9), IR, and UV-Vis spectroscopy and by single crystal X-ray diffraction (sc-XRD). DFT calculations provided a detailed understanding of the electronic nature of the diarsenes (4, 6) and the radical ions (8⋅-, 9⋅+), respectively.

Multifunctional Organocatalysts - Singly-Linked and Macrocyclic Bisphosphoric Acids for Asymmetric Phase-Transfer and Brønsted-Acid Catalysis.

The linking of phosphoric acids via covalent or mechanical bonds has proven to be a successful strategy for the design of novel organocatalysts. Here, we present the first systematic investigation of singly-linked and macrocyclic bisphosphoric acids, including their synthesis and their application in phase-transfer and Brønsted acid catalysis. We found that the novel bisphosphoric acids show dramatically increased enantioselectivities in comparison to their monophosphoric acid analogues. However, the nature, length and number of linkers has a profound influence on the enantioselectivities. In the asymmetric dearomative fluorination via phase-transfer catalysis, bisphosphoric acids with a single, rigid bisalkyne-linker give the best results with moderate to good enantiomeric excesses. In contrast, bisphosphoric acids with flexible linkers give excellent enantioselectivities in the transfer-hydrogenation of quinolines via cooperative Brønsted acid catalysis. In the latter case, sufficiently long linkers are needed for high stereoselectivities, as found experimentally and supported by DFT calculations.

Chalcogen-Bond-Induced Double Helix Based on Self-Assembly of a Small Planar Building Block.

As a rule, helical structures at the molecular level are formed by non-planar units. This makes the design of helices, starting from planar building blocks via self-assembly, even more fascinating. Until now, however, this has only been achieved in rare cases, where hydrogen and halogen bonds were involved. Here, we show that the carbonyl-tellurium interaction motif is suitable to assemble even small planar units into helical structures in solid phase. We found two different types of helices: both single and double helices, depending on the substitution pattern. In the double helix, the strands are connected by additional Te⋅⋅⋅Te chalcogen bonds. In the case of the single helix, a spontaneous enantiomeric resolution occurs in the crystal. This underlines the potential of the carbonyl-tellurium chalcogen bond to generate complex three-dimensional patterns.

Syntheses and Structures of 5-Membered Heterocycles Featuring 1,2-Diphospha-1,3-Butadiene and Its Radical Anion.

LGa(P2 OC)cAAC 2 features a 1,2-diphospha-1,3-butadiene unit with a delocalized π-type HOMO and a π*-type LUMO according to DFT calculations. [LGa(P2 OC)cAAC][K(DB-18-c-6)] 3[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene radical anion 3⋅- was isolated from the reaction of 2 with KC8 and dibenzo-18-crown-6. 3 reacted with [Fc][B(C6 F5 )4 ] (Fc=ferrocenium) to 2 and with TEMPO to [L-H Ga(P2 OC)cAAC][K(DB-18-c-6)] 4[K(DB-18-c-6] containing the 1,2-diphospha-1,3-butadiene anion 4- . The solid state structures of 2, 3K(DB-18-c-6], and 4[K(DB-18-c-6] were determined by single crystal X-ray diffraction (sc-XRD).

Selective [2+1+1] Fragmentation of P4 by heteroleptic Metallasilylenes.

Small-molecule activation by low-valent main-group element compounds is of general interest. We here report the synthesis and characterization (1 H, 13 C, 29 Si NMR, IR, sc-XRD) of heteroleptic metallasilylenes L1 (Cl)MSiL2 (M=Al 1, Ga 2, L1 =HC[C(Me)NDipp]2 , Dipp=2,6-i Pr2 C6 H3 ; L2 =PhC(Nt Bu)2 ). Their electronic nature was analyzed by quantum chemical computations, while their promising potential in small-molecule activation was demonstrated in reactions with P4 , which occurred with unprecedented [2+1+1] fragmentation of the P4 tetrahedron and formation of L1 (Cl)MPSi(L2 )PPSi(L2 )PM(Cl)L1 (M=Al 3, Ga 4).

Bis-Phosphaketenes LM(PCO)2 (M=Ga, In): A New Class of Reactive Group 13 Metal-Phosphorus Compounds.

Phosphaketenes are versatile reagents in organophosphorus chemistry. We herein report on the synthesis of novel bis-phosphaketenes, LM(PCO)2 (M=Ga 2 a, In 2 b; L=HC[C(Me)N(Ar)]2 ; Ar=2,6-i-Pr2 C6 H3 ) by salt metathesis reactions and their reactions with LGa to metallaphosphenes LGa(OCP)PML (M=Ga 3 a, In 3 b). 3 b represents the first compound with significant In-P π-bonding contribution as was confirmed by DFT calculations. Compounds 3 a and 3 b selectively activate the N-H and O-H bonds of aniline and phenol at the Ga-P bond and both reactions proceed with a rearrangement of the phosphaethynolate group from Ga-OCP to M-PCO bonding. Compounds 2-5 are fully characterized by heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR and IR spectroscopy, elemental analysis, and single crystal X-ray diffraction (sc-XRD).

Switching from Heteronuclear Allyl Cations to Vinyl Cations by Using a Chemical Charge Trap.

Halide abstraction of the carbene-coordinated pnictinidenes (MecAAC)EGa(Cl)L (E = As 1, Sb 2, Bi 3, MecAAC = [H2C(CMe2)2NDipp]C; L = HC[C(Me)NDipp]2; Dipp = 2,6-i-Pr2C6H3) yielded the series of cationic group 15 compounds [(MecAAC)EGaL][Al(ORF)4] (E = As 4, Sb 5; Al(ORF)4 = Al(OC(CF3)3)4) and [(MecAAC)EGaL][B(ArF)4] (E = Sb 6, Bi 7; B(ArF)4 = B[C6H3(CF3)2]4), which were characterized by heteronuclear NMR spectroscopy and sc-XRD. The electronic nature of the cations [(MecAAC)EGaL]+ is controlled by the central pnictogen atom, according to quantum chemical calculations. The calculations furthermore demonstrated that compounds containing the lighter pnictogens (E = N, P) are best described as heteronuclear allyl cations, whereas heavier pnictogen atoms (E = As, Sb, Bi) serve as a trap for the positive charge, resulting in carbene-stabilized heterovinyl-type structures.

Single-Electron Oxidation of Carbene-Coordinated Pnictinidenes-Entry into Heteroleptic Radical Cations and Metalloid Clusters.

Stable heavy main group element radicals are challenging synthetic targets. Although several strategies have been developed to stabilize such odd-electron species, the number of heavier pnictogen-centered radicals is limited. We report on a series of two-coordinated pnictogen-centered radical cations [(MecAAC)EGa(Cl)L][B(C6F5)4] (MecAAC = [H2C(CMe2)2NDipp]C; Dipp = 2,6-i-Pr2C6H3; E = As 1, Sb 2, Bi 3; L = HC[C(Me)NDipp]2) synthesized by one-electron oxidation of L(Cl)Ga-substituted pnictinidenes (MecAAC)EGa(Cl)L (E = As I, Sb II, Bi III). 1-3 were characterized by electron paramagnetic resonance (EPR) spectroscopy and single crystal X-ray diffraction (sc-XRD) (1, 2), while quantum chemical calculations support their description as carbene-coordinated pnictogen-centered radical cations. The low thermal stability of 3 enables access to metalloid bismuth clusters as shown by formation of [{LGa(Cl)}3Bi6][B(C6F5)4] (4).

Sequential Reduction of Borylstibane to an Electronically Nonsymmetric Diboryldistibene Radical Anion.

Understanding the formation of metal-metal bonds and their electronic structures is still a scientific task. We herein report on the stepwise synthesis of boryl-substituted antimony compounds in which the antimony atoms adopt four different oxidation states (+III, +II, +I, +I/0). Sb-C bond homolysis of Cp*[(HCNDip)2B]SbCl (1; Cp* = C5Me5; Dip = 2,6-iPr2C6H3) gave diboryldichlorodistibane [(HCNDip)2BSbCl]2 (2), which reacted with KC8 to form diboryldistibene [(HCNDip)2BSb]2 (3) and traces of cyclotetrastibane [(HCNDip)2B]3Sb4Cl (5). One-electron reduction of 3 yielded the potassium salt of the diboryldistibene radical anion [(HCNDip)2BSb]2̇-, [K(18-c-6)(OEt2)][{(HCNDip)2BSb}2] (4), which exhibits an unprecedented inequivalent spin localization on the Sb-Sb bond and hence an unsymmetric electronic structure. Compounds 1-4 were characterized by heteronuclear nuclear magnetic resonance (NMR) (1H, 13C, 11B), infrared (IR), ultraviolet-visible (UV-vis) spectroscopy (3, 4), and single crystal X-ray diffraction (sc-XRD, 1-5), while the bonding nature of 3 and 4 was analyzed by quantum chemical calculations. EPR spectroscopy resolves the dissimilar Sb hyperfine tensors of 4, reflecting the inequivalent spin distribution, setting 4 uniquely apart from all previously characterized dipnictene radical anions.

Evolution of Artificial Arginine Analogues-Fluorescent Guanidiniocarbonyl-Indoles as Efficient Oxo-Anion Binders.

In this article, we present fluorescent guanidiniocarbonyl-indoles as versatile oxo-anion binders. Herein, the guanidiniocarbonyl-indole (GCI) and methoxy-guanidiniocarbonyl-indole (MGCI) were investigated as ethylamides and compared with the well-known guanidiniocarbonyl-pyrrole (GCP) concerning their photophysical properties as well as their binding behavior towards oxo-anions. Hence, a variety of anionic species, such as carboxylates, phosphonates and sulfonates, have been studied regarding their binding properties with GCP, GCI and MGCI using UV-Vis titrations, in combination with the determination of the complex stoichiometry using the Job method. The emission properties were studied in relation to the pH value using fluorescence spectroscopy as well as the determination of the photoluminescence quantum yields (PLQY). Density functional theory (DFT) calculations were undertaken to obtain a better understanding of the ground-lying electronic properties of the investigated oxo-anion binders. Additionally, X-ray diffraction of GCP and GCI was conducted. We found that GCI and MGCI efficiently bind carboxylates, phosphonates and sulfonates in buffered aqueous solution and in a similar range as GCP (Kass ≈ 1000-18,000 M-1, in bis-tris buffer, pH = 6); thus, they could be regarded as promising emissive oxo-anion binders. They also exhibit a visible fluorescence with a sufficient PLQY. Additionally, the excitation and emission wavelength of MGCI was successfully shifted closer to the visible region of the electromagnetic spectrum by introducing a methoxy-group into the core structure, which makes them interesting for biological applications.

Mapping the Regioisomeric Space and Visible Color Range of Purely Organic Dual Emitters with Ultralong Phosphorescence Components: From Violet to Red Towards Pure White Light.

We mapped the entire visible range of the electromagnetic spectrum and achieved white light emission (CIE: 0.31, 0.34) by combining the intrinsic ns-fluorescence with ultralong ms-phosphorescence from purely organic dual emitters. We realized small molecular materials showing high photoluminescence quantum yields (ΦL ) in the solid state at room temperature, achieved by active exploration of the regioisomeric substitution space. Chromophore stacking-supported stabilization of triplet excitons with assistance from enhanced intersystem crossing channels in the crystalline state played the primary role for the ultra-long phosphorescence. This strategy covers the entire visible spectrum, based on organic phosphorescent emitters with versatile regioisomeric substitution patterns, and provides a single molecular source of white light with long lifetime (up to 163.5 ms) for the phosphorescent component, and high overall photoluminescence quantum yields (up to ΦL =20 %).

Design of Azobenzene beyond Simple On-Off Behavior.

Azobenzenes are without a doubt the most widely used light-induced switching units, and there is a plethora of application examples ranging from supramolecular chemistry to material science and biological chemistry. Here, we present a smart azobenzene, in which the photoswitching capability of the azobenzene moiety can be reversibly switched on and off using a second unit (redox switch). This second switching unit is based on the variation of the strength of a chalcogen bond between the azo group and a Te-Ph unit in ortho position to the azo group. This allows the selective switching of only one azobenzene unit in the presence of other azobenzene switches. The entire double-switch is a very simple, small system that can also be easily synthesized. As a result, this double-switch can be used as a smarter replacement for the established azobenzene system in the future. For example, in contrast to the latter this double-switch could be employed to store state information analogous to a flip-flop in digital electronic systems.

Observation of Discrete Valence Tautomers in Crystalline Cyclopentadienyl Radicals.

Single crystal X-ray (sc XRD) analyses of three symmetrically substituted cyclopentadienyl radicals (1, 2, 5) containing sterically demanding aryl groups showed that they crystallize as discrete valence tautomers (Jahn-Teller distortion) in the solid state with the unpaired electron either located in the b1 orbital (type I, state 2B1), resulting in a localized radical with two adjacent double bonds, or the a2 orbital (type II, state 2A2), leading to an allyl-type radical. Their properties in solution were examined by EPR spectroscopy as well as cyclovoltammetry and UV/vis spectroscopy including two additional cyclopentadienyl radicals (1-5). The electronic nature of 1-5 was further investigated by quantum chemical calculations.

Synthesis and Reactivity of Heteroleptic Ga-P-C Allyl Cation Analogues.

Oxidative addition of cyclic alkyl(amino)carbene-coordinated phosphinidenes (Me cAAC)PX to LGa affords gallium-coordinated phosphinidenes LGa(X)-P(Me cAAC) (L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ; X=Cl 1, Br 2), which react with NaBArF 4 and LiAl(ORF )4 to [LGaP(Me cAAC)][An] (An=B(C6 H3 (CF3 )2 )4 3, B(C6 F5 )4 4, Al(OC(CF3 )3 )4 5). The cations in 3-5 show substantial Ga-P double bond character and represent heteronuclear analogues of allyl cations according to quantum chemical calculations. The reaction of 4 with 4-dimethylaminopyridine (dmap) to adduct 6 confirms the strong electrophilic nature of the gallium center, whereas 5 reacts with ethyl isocyanate with C-C bond formation to the γ-C atom of the ß-diketiminate ligand and formation of compound 7.

Automated Crystal Structure Determination Has its Pitfalls: Correction to the Crystal Structures of Iodine Azide.

Previously published crystal structure determinations of two modifications of iodine azide (IN3 ) are corrected. In the original determinations, the very weak X-ray reflections with odd k Miller indices had been discarded, resulting in too small unit cells and models with misordered, partly occupied atomic positions. Using the original diffraction data, refinements with the correct unit cells yield structures of polymeric (-I-N3 -)n chains that are interlocked to layers. A skilled look at the primary X-ray data is always recommended to overcome the lack of crystallographic expertise of computers at automated structure determinations.

From π-Bonded Gallapnictenes to Nucleophilic, Redox-Active Metal-Coordinated Pnictanides.

A comprehensive reactivity study of gallapnictenes LGaEGa(Cl)L (E=As, Sb; L=HC[C(Me)N(Ar)]2 , Ar=Dip=2,6-i-Pr2 C6 H3 ) proved the nucleophilic character of the pnictogen and the electrophilic nature of the Ga atom. Reactions of LGaEGa(Cl)L with imidazolium chloride [IPrH][Cl] yielded {[LGa(Cl)]2 E- }{IPrH+ } (E=As 1, Sb 2), and those with HCl and MeI gave pnictanes [LGa(Cl)]2 EH (E=As 5, Sb 6) and L(I)GaE(Me)Ga(Cl)L (E=As 7, Sb 8). Pnictanides 1 and 2 also react with [H(OEt2 )2 ][BArF 4 ] (BArF 4 =B(C6 F5 )4 ) to 5 and 6, while reactions with MeI yielded [LGa(Cl)]2 EMe (E=As 9, Sb 10). Single electron oxidation reactions of pnictanides 1 and 2 gave the corresponding radicals [LGa(Cl)]2 E. (E=As, Sb).

Multi-Talented Gallaphosphene for Ga-P-Ga Heteroallyl Cation Generation, CO2 Storage, and C(sp3 )-H Bond Activation.

Gallaphosphene L(Cl)GaPGaL (2; L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ), which is synthesized by reaction of LGa(Cl)PCO (1) with LGa, reacts with [Na(OCP)(dioxane)2.5 ] to LGa(OCP)PGaL (3), whereas chloride abstraction with LiBArF 4 yields [LGaPGaL][BArF 4 ] (4; BArF 4 =B(C6 F5 )4 ). 4 represents a heteronuclear analog of the allyl cation according to quantum chemical calculations. Remarkably, 2 reversibly reacts with CO2 to yield L(Cl)Ga-P[µ-C(O)O]2 GaL (5), while reactions with acetophenone and acetone selectively give compounds 6 and 7 by C(sp3 )-H bond activation.