Metal Influence on Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Aromaticity and Activation Strain

The factors governing 1,3-dipolar cycloaddition reactions involving C≡P-containing compounds are computationally explored in detail using quantum chemical tools. To this end, the parent process involving tBuN3 and tBuCP is analyzed and compared to the analogous reaction involving organometallic cyaphide complexes (metal=Au, Pt, Ge, Mg), in order to understand the role of the metal fragment in such transformations. It is found that while the metal fragment does not significantly influence the aromaticity of the corresponding concerted transition states or the regioselectivity of the transformation, it may modify the reactivity of the cyaphide complexes (i. e. Ge and Mg cyaphide complexes are comparatively more reactive). The computed reactivity trends and the factors behind the regioselectivity of the cycloaddition reaction are quantitatively analyzed with the help of the activation strain model in combination with the energy decomposition analysis method.

An Inorganic Click Reaction for the Synthesis of Interlocked Molecules

We describe the use of the cyaphide-azide 1,3-dipolar cycloaddition reaction for the synthesis of a new class of inorganic rotaxanes containing gold(I) triazaphosphole stoppers. Electron-deficient bis-azides, which thread perethylated pillar[5]arene in aromatic solvents, readily react with two equivalents of Au(IDipp)(CP) (IDipp=1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene) to afford interlocked molecules via an inorganic click reaction. These transformations proceed in good yields (ca. 65 %) and in the absence of a catalyst. The resulting organometallic rotaxanes are air- and moisture-stable and can be purified by column chromatography under aerobic conditions. The targeted rotaxanes were characterized by multi-element nuclear magnetic resonance (NMR) spectroscopy, mass-spectrometry, and single-crystal X-ray diffraction.

Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Scope and Applicability

Several examples of the cyaphide-azide 1,3-dipolar cycloaddition reaction to afford metallo-triazaphospholes are reported. The gold(I) triazaphospholes Au(IDipp)(CPN3 R) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; R=t Bu, Ad, Dipp), magnesium(II) triazaphospholes, {Mg(Dipp NacNac)(CPN3 R)}2 (Dipp NacNac=CH{C(CH3 )N(Dipp)}2 , Dipp=2,6-diisopropylphenyl; R=t Bu, Bn), and germanium(II) triazaphosphole Ge(Dipp NacNac)-(CPN3 t Bu) can be prepared straightforwardly, under mild conditions and in good yields, in a manner reminiscent of the classic alkyne-azide click reaction (albeit without a catalyst). This reactivity can be extended to compounds with two azide functional groups such as 1,3-diazidobenzene. It is shown that the resulting metallo-triazaphospholes can be used as precursors to carbon-functionalized species, including protio- and iodo-triazaphospholes.

Phosphorus derivatives of mesoionic carbenes: synthesis and characterization of triazaphosphole-5-ylidene → BF3 adducts

Trimethylsilyl-substituted triazaphospholes were synthesized by a [3+2] cycloaddition reaction between organic azides and (CH3)3Si-CP. In an attempt to isolate their N-alkylated products, the formation of BF3 adducts of unprecedented triazaphosphol-5-ylidenes was found. The nature of the carboncarbene-boron bond was investigated within the DFT framework, revealing a strong donation of electrons from the carbene carbon atom to the boron atom combined with weak back-bonding.

Reactions of main group compounds with azides forming organic nitrogen-containing species

Since the seminal discovery of phenyl azide by Grieß in 1864, a variety of organic azides (R-N3) have been developed and extensively studied. The amenability of azides to a number of reactions has expanded their utility as building blocks not only in organic synthesis but also in bioorthogonal chemistry and materials science. Over the decades, it has been demonstrated that the reactions of main group compounds with azides lead to diverse N-containing main group molecules. In view of the pronounced progress in this area, this review summarizes the reactions of main group compounds with azides, emphatically introducing their reaction patterns and mechanisms. The reactions of forming inorganic nitrogen species are not included in this review.

Luminescent 1H-1,3-benzazaphospholes

2-R-1H-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σ²P heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2'-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R' = Me and 3b, R = 2-thiophene). The significant fluorescence displayed by the thiophene derivatives 3b (φ = 0.53) and 3c (φ = 0.12) stands in contrast to the weakly emissive methyl substituted analogue 3a (φ = 0.08). Comparative computational investigations of 3a-c offer insights into the interplay between structure-function relationships affecting excited state relaxation processes.

Synthesis, Structure and Reactivity of a Cyapho(dicyano)methanide Salt

We describe the synthesis of a cyapho(dicyano)methanide salt, [K(18-crown-6)][C(CN)2 (CP)], from reaction of [Na(18-crown-6)][PH2 ] (18-crown-6=1,4,7,10,13,16-hexaoxacyclooctadecane) with 1,1-diethoxy-2,2-dicyanoethylene (EtO)2 C=C(CN)2 . The reaction proceeds through a Michael addition-elimination pathway to afford [Na(18-crown-6)][HP{C(OEt)=C(CN)2 }]. Addition of a strong, non-nucleophilic base (KHMDS) to this intermediate results in the formation of [K(18-crown-6)][C(CN)2 (CP)]. Subsequent reactivity studies reveal that the cyapho(dicyano)methanide ion is susceptible to protonation with strong acids to afford the parent acid HC(CN)2 (CP). The reactivity of the cyaphide moiety in [C(CN)2 (CP)]- was explored through coordination to metal centers and in cycloaddition reactions with azides.

A new access to diazaphospholes via cycloaddition–cycloreversion reactions on triazaphospholes

The [4+2] cycloaddition of a triazaphosphole with hexafluoro-2-butyne, followed by a cycloreversion reaction under elimination of pivaloyl nitrile, affords selectively a novel bis-CF3-substituted diazaphosphole in high yield.

Au(I)-mediated N2-elimination from triazaphospholes: a one-pot synthesis of novel N2P2-heterocycles

Novel tosyl- and mesitylsulfonyl-substituted triazaphospholes were synthesized and structurally characterized. In an attempt to prepare the corresponding Au(I)-complexes with stoichiometric amounts of AuCl·S(CH₃)₂, cyclo-1,3-diphospha(III)-2,4-diazane-AuCl-complexes were obtained instead. Our here presented results offer a new strategy for preparing such coordination compounds selectively in a one-pot approach.

Photochemical C(sp)-C(sp2) Bond Activation in Phosphaalkynes: A New Route to Reactive Terminal Cyaphido Complexes LnM-C≡P

The photochemical activation of the C(sp)-C(sp²) bond in Pt(0)-η²-aryl-phosphaalkyne complexes leads selectively to coordination compounds of the type L_nPt(aryl)(C≡P). The oxidative addition reaction is a novel, clean, and atom-economic route for the synthesis of reactive terminal Pt(II)-cyaphido complexes, which can undergo [3 + 2] cycloaddition reactions with organic azides, yielding the corresponding Pt(II)-triazaphospholato complexes. The C-C bond cleavage reaction is thermodynamically uphill. Upon heating, the reverse and quantitative reductive elimination toward the Pt(0)-phosphaalkyne-π-complex is observed.

Phosphetes via Transition Metal Free Ring Closure - Taking the Proper Turn at a Thermodynamic Crossing

A transition metal free route to phosphetes featuring an exocyclic alkene unit is presented. In this approach phosphanides are added to a variety of diynes generating phosphaallylic intermediates which depending on the reaction conditions transform either to phosphetes or the corresponding phospholes. Investigation of the reaction mechanism by combined quantum chemical and experimental means identifies phosphole formation as thermodynamically controlled reaction path, whereas kinetic control furnishes the corresponding phosphetes. Structural and luminescence properties of the rare class of phosphetes are explored, as well as for selected key intermediates.

A Bis-Chelating O N O ^ / N N ^ Ligand for the Synthesis of Heterobimetallic Platinum(II)/Rhenium(I) Complexes: Tools for the Optimization of a New Class of Platinum(II) Anticancer Agents

The two independent and N N ^ coordination sites of a newly synthesized bis[2-(hydroxyphenyl)-1,2,4-triazole] platform have been exploited to prepare four monometallic neutral ()Pt^II complexes carrying DMSO, pyridine, triphenylphosphine, or N-heterocyclic carbene as the fourth ligand. Then, the second N N ^ coordination site was used to introduce an IR-active rhenium tricarbonyl entity, affording the four corresponding heterobimetallic neutral Pt^II /Re^I complexes, as well as a cationic Pt^II /Re^I derivative. X-ray crystallographic studies showed that distortion of the organic platform occurred to accommodate the coordination geometry of both metal centers. No ligand exchange or transchelation occurred upon incubation of the Pt^II complexes in aqueous environment or in the presence of Fe^III , respectively. The antiproliferative activity of the ligand and complexes was first screened on the triple-negative breast cancer cell line MDA-MB-231. Then, the IC₅₀ values of the most active candidates were determined on a wider panel of human cancer cells (MDA-MB-231, MCF-7, and A2780), as well as on a nontumorigenic cell line (MCF-10A). Low micromolar activities were reached for the complexes carrying a DMSO ligand, making them the first examples of highly active, but hydrolytically stable, Pt^II complexes. Finally, the characteristic mid-IR signature of the {Re(CO)₃ } fragment in the Pt/Re heterobimetallic complexes was used to quantify their uptake in breast cancer cells.

Synthesis, Electronic Properties and OLED Devices of Chromophores Based on λ5 -Phosphinines

A new series of 2,4,6-triaryl-λ5 -phosphinines have been synthesized that contain different substituents both on the carbon backbone and the phosphorus atom of the six-membered heterocycle. Their optical and redox properties were studied in detail, supported by in-depth theoretical calculations. The modularity of the synthetic strategy allowed the establishment of structure-property relationships for this class of compounds and an OLED based on a blue phosphinine emitter could be developed for the first time.

Generation and reactivity of neutral 1,3-benzazaphosphole and anionic 1,3-benzazaphospholide ytterbium(iii) complexes

Treatment of Cp3Ln (Ln = Yb, Y) with 5-R3-6-R1-2-R2-1H-1,3-benzazaphosphole (HBp) (HBp1 (1a): R1 = H, R2 = 2,4,6-Me3C6H2, R3 = Me; HBp2(1b): R1 = Me, R2 = C6H5, R3 = H; HBp3(1c): R1 = R3 = H, R2 = C6H5) at room temperature gives the crystalline 1 : 1 Lewis acid-base adducts [(η1(p)-HBp)LnCp3] (2a-d) [Ln = Yb: Bp = Bp1 (2a), Bp2 (2b), Bp3 (2c); Ln = Y: Bp2 (2d)] with Ln-P donor bonds in good yields. Heating 2a-c in toluene leads to the liberation of one molecule of CpH to afford the corresponding N-bonded complexes [Cp2YbBp] (Bp = Bp1 (3a), Bp2 (3b), Bp3 (3c)). Interestingly, the P atom of complexes 3a-c can also be further coordinated to another Lewis acid such as Cp3Yb and B(C6F5)3 to give the adducts [Cp2Yb(μ-η1(N):η2(C,C):η1(P)-Bp)YbCp3] (Bp = Bp1 (4a), Bp2 (4b), Bp3 (4c)) and [Cp2Yb(μ-η1(N):η2(C,C):η1(P)-Bp)B(C6F5)3] (Bp = Bp1 (5a), Bp2 (5b), Bp3 (5c)), respectively. The molecular structures of complexes 2a, 4b-4c and 5c are confirmed by X-ray diffraction analysis.

Controlling the Emissive Activity in Heterocyclic Systems Bearing C═P Bonds

The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the C═N bonds of benzoxazoles are replaced by C═P bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X-C═P), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries.

Reaction of Pentelidene Complexes with Diazoalkanes: Stabilization of Parent 2,3-Dipnictabutadienes

The reaction of the phosphinidene complex [Cp*P{W(CO)₅ }₂ ] (1 a) with diphenyldiazomethane leads to [{W(CO)₅ }Cp*P=NN{W(CO)₅ }=CPh₂ ] (2). Compound 2 is a rare example of a phosphadiazadiene ligand (R-P=N-N=CR'R'') complex. At temperatures above 0 °C, 2 decomposes into the complex [{W(CO)₅ }PCp*{N(H)N=CPh₂ )₂ ] (3), among other species. The reaction of the pentelidene complexes [Cp*E{W(CO)₅ }₂ ] (E=P, As) with diazomethane (CH₂ NN) proceeds differently. For the arsinidene complex (1 b), only the arsaalkene complex 4 b [{W(CO)₅ }₂ {η^1:2 -(Cp*)As=CH₂ }] is formed. The reaction with the phosphinidene complex (1 a) results in three products, the two phosphaalkene complexes [{W(CO)₅ }₂ {η^1:2 -(R)P=CH₂ }] (4 a: R=Cp*, 5: R=H) and the triazaphosphole derivative [{W(CO)₅ }P(Cp*)-CH₂ -N{W(CO)₅ }=N-N(N=CH₂ )] (6 a). The phosphaalkene complex (4 a) and the arsaalkene complex (4 b) are not stable at room temperature and decompose to the complexes [{W(CO)₅ }₄ (CH₂ =E-E=CH₂ )] (7 a: E=P, 7 b: E=As), which are the first examples of complexes with parent 2,3-diphospha-1,3-butadiene and 2,3-diarsa-1,3-butadiene ligands.

Zwitterionic and biradicaloid heteroatomic cyclopentane derivatives containing different group 15 elements

The formal cyclopentane-1,3-diyl derivatives [E1(μ-NTer)2({E2C} = NDmp)] (Ter = 2,6-dimesityl-phenyl, Dmp = 2,6-dimethylphenyl) were prepared by 1,1-insertion of CNDmp into the N-E2 bond of [E1(μ-NTer)2E2] (E1 = N, P; E2 = P, As). The insertion does not occur for E1 = E2 = As or E2 = Sb. Dependent on the choice of formal radical centres E, either a biradicaloid or a zwitterion was obtained. The biradicaloid features a P and an As radical center and its biradical character was established by computations as well as characteristic reactivity with respect to the formation of a housane derivative and the activation of molecules bearing multiple bonds, which was demonstrated using the example of PC t Bu. In contrast, the formally N,As- and N,P-centered biradicaloids are better regarded as zwitterionic species in accord with computations and diminished reactivity, as neither housane formation nor activation of multiple bonds could be observed.