The geometry of small rings—IV: Molecular geometry of cyclopropene and its derivatives

Numeric structural data for 34 derivatives of cyclopropene and cyclopropenium ion have been retrieved from the Cambridge Crystallographic Database and analysed in conjunction with available gas-phase results. Geometric data indicate that the vinylic C atoms in cyclopropene use sp^1.19 hybrids in bond formation to substituents and contribute sp^2.68 hybridges to the ring σ-framework. The D_3h-symmetric cyclopropenium ion has a bond length of 1.373(3)Å, which can be related to distances in unstrained systems. Comparison of data for cyclopropenylidenes and 3,3-difluorocyclopropene with analogous cyclopropanes shows that π-donor effects (distal bond lengthening, vicinal bond shortening) are apparent in cyclopropene. Rehybridization and π-donation are largely responsible for cyclopropenylidene geometry, rather than significant contributions from dipolar and pseudo-aromatic resonance forms. Insufficient data exist to quantify the effect of π-acceptor substituents on cyclopropene, but some lengthening of vicinal bonds is apparent. Three major bonding patterns are exhibited by organomental derivatives of cyclopropenium and cyclopropene.     

Quadrupolar Cyclopenta[hi]aceanthrylene‐Based Electron Donor‐Acceptor‐Donor Conjugates: Charge Transfer versus Charge Separation

Cyclopenta[hi]aceanthrylenes (CPAs) have been functionalized at two of the peripheral positions with electronically inert trimethylsilylethynyl ( 1 ), as well as with electron‐donating 4‐ethynyl‐N,N‐dimethylaniline ( 2 ), ethynyl Zn^IIphthalocyanine ( 3 ), and ethynyl Zn^IIporphyrin ( 4 ) units. Consistent with X‐ray crystal structures of 2 and 4 , analyses of absorption and fluorescence of 2 – 4 point to strong electronic communication between the CPA and the peripheral units, affording quadrupolar electron donor‐acceptor‐donor charge‐transfer conjugates. By virtue of their quadrupolar/dipolar charge‐transfer characters in the excited state, 2 – 4 exhibit fluoro‐solvatochromism. Transient absorption spectroscopy confirmed delocalized quadrupolar ground states and formation of weakly solvent stabilized quadrupolar singlet excited states. The latter transform into strongly stabilized dipolar excited states before deactivating to the ground state in 2 and give rise to a fully charge separated state in 3 and 4 .     

Green and efficient synthesis of 1H-indazolo[1,2-b] phthalazine-1,6,11(13H)-triones using ZrO(NO3)2.2H2O as a novel catalyst and theoretical study of synthesized compounds

The one-pot three-component synthesis for the preparation of 1H-indazolo[1,2-b] phthalazine-1,6,11(13H)-triones through condensation of phthalimide, hydrazine monohydrate, dimedone, and aromatic aldehydes in the presence of a novel catalytic amount of ZrO(NO₃)₂.2H₂O at reflux conditions in water has been reported. Quantum theoretical calculations for the three structures of compounds ( 5a , 5b , and 5c ) were performed using the G3MP2, LC-ωPBE, MP2, and B3LYP methods with the 6-311 + G** basis set. After optimizing the structures, geometric parameters were obtained and experimental measurements were compared with the calculated data. The structures of the products were confirmed by IR, ¹H NMR, ¹³C NMR, and elemental analysis. IR spectra data and ¹H NMR and ¹³C NMR chemical shifts computations of the 1H-indazolo[1,2-b]phthalazine-1,6,11(13H)-trione derivatives in the ground state were calculated. Frontier molecular orbitals, total density of states, thermodynamic parameters, and molecular electrostatic potentials of the title compounds were investigated by theoretical calculations. Molecular properties such as the ionization potential (I), electron affinity (A), chemical hardness (η), electronic chemical potential (μ), and electrophilicity (ω) were investigated for the structures. Consequently, there was an excellent agreement between experimental and theoretical results.     

An amino–imino resonance study of 2‐amino‐4‐methylpyridinium nitrate and 2‐amino‐5‐methylpyridinium nitrate

The contributions of the amino and imino resonance forms to the ground‐state structures of 2‐amino‐4‐methylpyridinium nitrate, C₆H₉N₂⁺·NO₃⁻, and the previously reported 2‐amino‐5‐methylpyridinium nitrate [Yan, Fan, Bi, Zuo & Zhang (2012). Acta Cryst. E 68 , o2084], were studied using a combination of IR spectroscopy, X‐ray crystallography and density functional theory (DFT). The results show that the structures of 2‐amino‐4‐methylpyridine and 2‐amino‐5‐methylpyridine obtained upon protonation are best described as existing largely in the imino resonance forms.     

Binding H2, N2, H-, and BH3 to transition-metal sulfur sites: synthesis and properties of [RuL(PR3)(N2Me2S2)] Complexes (L=eta2-H2, H-, BH3; R=Cy, iPr)

The reactions of [Ru(N₂)(PR₃)(‘N₂Me₂S₂’)] [‘N₂Me₂S₂’=1,2‐ethanediamine‐N,N′‐dimethyl‐N,N′‐bis(2‐benzenethiolate)(2?)] [ 1 a (R=iPr), 1 b (R=Cy)] and [μ‐N₂{Ru(N₂)(PiPr₃)(‘N₂Me₂S₂’)}₂] ( 1 c ) with H₂, NaBH₄, and NBu₄BH₄, intended to reduce the N₂ ligands, led to substitution of N₂ and formation of the new complexes [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PR₃)(‘N₂Me₂S₂’)] [ 3 a (R=iPr), 3 b (R=Cy)], and [Ru(H)(PR₃)(‘N₂Me₂S₂’)]^? [ 4 a (R=iPr), 4 b (R=Cy)]. The BH₃ and hydride complexes 3 a , 3 b , 4 a , and 4 b were obtained subsequently by rational synthesis from 1 a or 1 b and BH₃?THF or LiBEt₃H. The primary step in all reactions probably is the dissociation of N₂ from the N₂ complexes to give coordinatively unsaturated [Ru(PR₃)(‘N₂Me₂S₂’)] fragments that add H₂, BH₄^?, BH₃, or H^?. All complexes were completely characterized by elemental analysis and common spectroscopic methods. The molecular structures of [Ru(H₂)(PR₃)(‘N₂Me₂S₂’)] [ 2 a (R=iPr), 2 b (R=Cy)], [Ru(BH₃)(PiPr₃)(‘N₂Me₂S₂’)] ( 3 a ), [Li(THF)₂][Ru(H)(PiPr₃)(‘N₂Me₂S₂’)] ([Li(THF)₂]‐ 4 a ), and NBu₄[Ru(H)(PCy₃)(‘N₂Me₂S₂’)] (NBu₄‐ 4 b ) were determined by X‐ray crystal structure analysis. Measurements of the NMR relaxation time T₁ corroborated the η² bonding mode of the H₂ ligands in 2 a (T₁=35 ms) and 2 b (T₁=21 ms). The H,D coupling constants of the analogous HD complexes HD‐ 2 a (¹J(H,D)=26.0 Hz) and HD‐ 2 b (¹J(H,D)=25.9 Hz) enabled calculation of the H? D distances, which agreed with the values found by X‐ray crystal structure analysis ( 2 a : 92 pm (X‐ray) versus 98 pm (calculated), 2 b : 99 versus 98 pm). The BH₃ entities in 3 a and 3 b bind to one thiolate donor of the [Ru(PR₃)(‘N₂Me₂S₂’)] fragment and through a B‐H‐Ru bond to the Ru center. The hydride complex anions 4 a and 4 b are extremely Brønsted basic and are instantanously protonated to give the η²‐H₂ complexes 2 a and 2 b .     

Variants of Solid-State and Solution Structures of (η3-Allyl)- {2-[2′-(diphenylphosphino)phenyl]-4,5-dihydrooxazole-P,N}palladium(II) hexafluorophosphates and tetraphenylborates

Crystal and solution structures of the enantiomerically pure and the racemic pairs of (η³-allyl) {2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-phenyloxazole}palladium(II) hexafluorophosphates ( 1 , and rac- 1 , resp.) and tetraphenylborates ( 2 , and rac- 2 , resp.) as well as (η³-allyl){2-[2′-(diphenylphosphino)phenyl]-4,5-dihydro-4-isopropyloxazole}palladium(II) tetraphenylborate ( 3 ) were characterized by X-ray crystallography and ¹H-NMR spectroscopy. In the solid state, rac- 1 and rac- 2 proved to be disordered with both diastereoisomeric complexes in the crystal. The complexes 2 and 3 exist only in the ‘exo’ form. The X-ray structures show that the [Pd^II(η³-allyl)] moiety may adopt different configurations between a nearly symmetrical three-electron Pd^II(π³-allyl) system and an asymmetrical allyl group with a η¹- and a η²-bonding to the metal center. The [Pd^II(η³-allyl)] system of rac- 1 and of ‘endo’ rac- 2 is closer to the former, and that of 2 , ‘exo’-rac- 2 , and 3 closer to the later geometry. The ¹H-NMR spectra of the hexafluorophosphates 1 and rac- 1 show two sets of signals of the allylic protons in an ‘exo’/‘endo’ ratio of 2:3. The tetraphenylborates 2, rac- 2 , and 3 give only one set of broad signals of the allylic protons.     

Photoisomerization Mechanism of Ruthenium Sulfoxide Complexes: Role of the Metal‐Centered Excited State in the Bond Rupture and Bond Construction Processes

Phototriggered intramolecular isomerization in a series of ruthenium sulfoxide complexes, [Ru(L)(tpy)(DMSO)]ⁿ⁺ (where tpy=2,2’:6’,2’’‐terpyridine; DMSO=dimethyl sulfoxide; L=2,2’‐bipyridine (bpy), n=2; N,N,N’,N’‐tetramethylethylenediamine (tmen) n=2; picolinate (pic), n=1; acetylacetonate (acac), n=1; oxalate (ox), n=0; malonate (mal), n=0), was investigated theoretically. It is observed that the metal‐centered ligand field (³MC) state plays an important role in the excited state S→O isomerization of the coordinated DMSO ligand. If the population of ³MC_S state is thermally accessible and no ³MC_O can be populated from this state, photoisomerization will be turned off because the ³MC_S excited state is expected to lead to fast radiationless decay back to the original ¹GS_S ground state or photodecomposition along the Ru²⁺?S stretching coordinate. On the contrary, if the population of ³MC_S (or ³MC_O) state is inaccessible, photoinduced S→O isomerization can proceed adiabatically on the potential energy surface of the metal‐to‐ligand charge transfer excited states (³MLCT_S→³MLCT_O). It is hoped that these results can provide valuable information for the excited state isomerization in photochromic d⁶ transition‐metal complexes, which is both experimentally and intellectually challenging as a field of study.     

Two C3‐Symmetric Dy3III Complexes with Triple Di‐μ‐methoxo‐μ‐phenoxo Bridges,Magnetic Ground State,and Single‐Molecule Magnetic Behavior

Two series of isostructural C₃‐symmetric Ln₃ complexes Ln₃ ? [BPh₄] and Ln₃ ? 0.33[Ln(NO₃)₆] (in which Ln^III=Gd and Dy) have been prepared from an amino‐bis(phenol) ligand. X‐ray studies reveal that Ln^III ions are connected by one μ₂‐phenoxo and two μ₃‐methoxo bridges, thus leading to a hexagonal bipyramidal Ln₃O₅ bridging core in which Ln^III ions exhibit a biaugmented trigonal‐prismatic geometry. Magnetic susceptibility studies and ab initio complete active space self‐consistent field (CASSCF) calculations indicate that the magnetic coupling between the Dy^III ions, which possess a high axial anisotropy in the ground state, is very weakly antiferromagnetic and mainly dipolar in nature. To reduce the electronic repulsion from the coordinating oxygen atom with the shortest Dy?O distance, the local magnetic moments are oriented almost perpendicular to the Dy₃ plane, thus leading to a paramagnetic ground state. CASSCF plus restricted active space state interaction (RASSI) calculations also show that the ground and first excited state of the Dy^III ions are separated by approximately 150 and 177 cm^?1, for Dy₃ ? [BPh₄] and Dy₃ ? 0.33[Dy(NO₃)₆], respectively. As expected for these large energy gaps, Dy₃ ? [BPh₄] and Dy₃ ? 0.33[Dy(NO₃)₆] exhibit, under zero direct‐current (dc) field, thermally activated slow relaxation of the magnetization, which overlap with a quantum tunneling relaxation process. Under an applied H_dc field of 1000 Oe, Dy₃ ? [BPh₄] exhibits two thermally activated processes with U_eff values of 34.7 and 19.5 cm^?1, whereas Dy₃ ? 0.33[Dy(NO₃)₆] shows only one activated process with U_eff=19.5 cm^?1.     

Novel Synthesis of 2‐Alkylquinolizinium‐1‐olates and Their 1,3‐Dipolar Cycloaddition Reactions with Acetylenes

Several 2‐alkylquinolizinium‐1‐olates 9 , i.e., heterobetaines, were prepared from ketone 11 , the latter being readily available either from pyridine‐2‐carbaldehyde via a Grignard reaction, followed by oxidation with MnO₂, or from 2‐picolinic acid (=pyridine‐2‐carboxylic acid) via the corresponding Weinreb amide and subsequent Grignard reaction. Mesoionic heterobetaines such as quinolizinium derivatives have the potential to undergo cycloaddition reactions with double and triple bonds, e.g., 1,3‐dipolar cycloadditions or Diels? Alder reactions. We here report on the scope and limitations of cycloaddition reactions of 2‐alkylquinolizinium‐1‐olates 9 with electron‐poor acetylene derivatives. As main products of the reaction, 5‐oxopyrrolo[2,1,5‐de]quinolizines (=‘[2.3.3]cyclazin‐5‐ones’) 19 were formed via a regioselective [2+3] cycloaddition, and cyclohexadienone derivatives, formed via a Diels? Alder reaction, were obtained as side products. The structures of 2‐benzylquinolizinium‐1‐olate ( 9a ) and two ‘[2.3.3]cyclazin‐5‐ones’ 19i and 19l were established by X‐ray crystallography.     

Origin of Ferromagnetism and Magnetic Anisotropy in a Family of Copper(II) Triangles

Previously reported ferromagnetic triangles (NnBu₄)₂[Cu₃(μ₃-Cl)₂(μ-4-NO₂-pz)₃Cl₃] ( 1 ), (PPN)₂[Cu₃(μ₃-Cl)₂(μ-pz)₃Cl₃] ( 2 ), (bmim)₂[Cu₃(μ₃-Cl)₂(μ-pz)₃Cl₃] ( 3 ) and newly reported (PPh₄)₂[Cu₃(μ₃-Cl)₂(μ-4-Ph-pz)₃Cl₃] ( 4 ) were studied by magnetic susceptometry, electron paramagnetic resonance (EPR) spectroscopy and ab initio calculations to assess the origins of their ferromagnetism and of the magnetic anisotropy of their ground S=3/2 state (PPN⁺=bis(triphenylphosphine)iminium, bmim⁺=1-butyl-3-methylbenzimidazolium, pz⁻=pyrazolate). Ab initio studies revealed the d character of the magnetic orbitals of the compressed trigonal bipyramidal copper(II) ions. Ferromagnetic interactions were attributed to weak orbital overlap via the pyrazolate bridges. From the wavefunctions expansions, the ratios of the magnetic couplings were determined, which were indeterminate by magnetic susceptometry. Single-crystal EPR studies of 1 were carried out to extend the spin Hamiltonian with terms which induce zero-field splitting (zfs), namely dipolar interactions, anisotropic exchange and Dzyaloshinskii–Moriya interactions (DMI). The data were treated through both a giant-spin model and through a multispin exchange-coupled model. The latter indicated that ≈62 % of the zfs is due to anisotropic and ≈38 % due to dipolar interactions. The powder EPR data of all complexes were fitted to a simplified form of the multispin model and the anisotropic and dipolar contributions to the ground state zfs were estimated.     

The Photolysis of Tricyclo[4.2.1.02,5]nonadiene: Support of a Doughert ‘type N’ mechanism from photoelectron spectroscopical studies

In earlier work on the photolysis of derivatives of tricyclo[4.2.1.0^2,5]nonadiene (I), the intermediacy of a biallyl-like structure has been postulated which is formed by C₁-C₂ bond cleavage of I in its first excited state. This mechanism is now supported by the results of photoelectron spectroscopical studies on I and its two dihydroderivatives. Further support is gained from the theoretically calculated Ehrenson C₁-C₂-bond indices for the ground and the first excited state of these molecules. While the results for I in principle can be rationalized on the (one-electron) basis of ‘through bond’ interaction between the two π-bonds, the absence of C₁-C₂ photocleavage for a substituted tricyclo[4.2.2.0^2,5]decadiene also requires consideration of an (all-electron) thermochemical driving force.     

Physical and Chemical Interaction of Homoconjugated Dienes with Tetracyanoethylene

EDA-complexes of bicyclo[2,2,n]alkadienes (n = 1, 2, 3, 4) ( 1 (n)-series), 1,4-cyclohexadiene ( 2 ) and various other cyclic monoenes, dienes and trienes as donors and tetracyanoethylene (TCNE) as acceptor were investigated. Spectroscopic and thermodynamic constants of the complexes were determined and correlated with the ionisation potentials (I^D) of the hydrocarbon donors obtained from PE. spectroscopy. The nature of the dominant energy contributions to the ground state and the two lowest CT-states of these weak complexes is discussed and structural conclusions are drawn. The role of the complexes in the addition reaction of the hydrocarbon components and TCNE is discussed. The homo Diels-Alder addition product of 1 (2) and TCNE, 9,9,10,10-tetracyanoquadricyclo[2,2,2,0^2,6,2^3,5]decane, and the ‘ene’-addition product of 2 and TCNE, 5-[1′,1′,2′,2′-tetracyanoethyl]-1,3-cyclohexadiene were prepared and characterized. Preliminary results for the mechanistic scheme governing the dehydrogenation of 2 by TCNE are reported.     

A New Helicopodand: Molecular Recognition of Dicarboxylic Acids with High Diastereoselectivity

The helicopodand (PM)- 2 is prepared following the photocyclodehydrogenation route to helicenes (Scheme). At the ends of a [7]helicene backbone, this acyclic receptor (‘podand’) possesses a H-bonding recognition site shaped by two convergent N-(pyridin-2-yl)carboxamide (CONH(py)) units. In the crystal of diethyl [7]helicene-2,17-dicarboxylate ((PM)- 3 ), a direct synthetic precursor of 2 , molecules of the same chirality form stacks, and two stacks of opposite chirality are interlocked in a pair having average face-to-face aromatic contacts of 3.82 Å between benzene rings of different enantiomers (Fig. 2). In contrast, two conformations are observed in the crystal structure of 2 , one with both CONH(py) residues pointing with their H-bonding centers NH/N away from the binding site (‘out-out’) and a second (‘in-out’) with one of the two CONH(py) residues pointing towards the binding site (‘in’; Fig. 4). While no H-bonding network propagates throughout the crystal, enantiomers of 2 in the different conformations ‘out-out’ and ‘in-out’ form H-bonded pairs that are further stabilized by a H-bond to one molecule of CHCl₃. In the productive ‘in-in’ conformation, 2 forms stable 1:1 complexes with α,ω-dicarboxylic acids in CHCl₃, and a diastereoselectivity in complexation of Δ(ΔG°) = 1.4 kcal mol^?1 is measured for two substrates differing only in the (E)/(Z)-configuration at their double bond (see Table 2). A comprehensive force-field molecular-modeling study suggests that only the (E)-derivative possesses the correct geometry for a ditopic four-fold H-bonding interaction between its two COOH residues and the two CONH(py) groups in 2 (Fig. 5). With N,N′-bis [(benzyloxy)carbonyl]-L -cystine, the formation of diastereoisomeric complexes with (PM)- 2 is observed (Fig. 7).     

Structural and Photophysical Properties of Pseudo-Tricapped Trigonal Prismatic Lanthanide building blocks controlled by zinc(II) in heterodinuclear d?f complexes

An efficient combination of electrospray mass spectrometry (ES-MS), spectrophotometric and ¹H-NMR titrations in solution is used to characterize the assembly of the segmental ligand 2-{6-[1-(3,5-dimethoxybenzyl)-1H-benzimidazol-2-yl]pyridin-2-yl}-1, 1′-dimethyl-5,5′-methylene-2′-(5-methylpyridin-2-yl)bis[1H- benzimidazole] ( L ²) with Zn^II and 4f metal ions, Ln^III. Ligand L ² reacts with Zn(ClO₄)₂ in MeCN to give successively [Zn( L ²)₂]²⁺, where the metal ion is coordinated by the tridentate binding units of the ligands, and the double-helical head-to-head complex [Zn₂( L ²)₂]⁴⁺. When L ² reacts with Ln(ClO₄)₃ (Ln = La, Eu, Lu), La^III only leads to a well-defined cylindrical C₁-symmetrical homodinuclear head-to-tail complex [La₂( L ²)₃]⁶⁺ in solution, while chemical-exchange processes prevent the ¹H-NMR characterization of [Eu₂( L ²)₃]⁶⁺, and Lu^III gives complicated mixtures of complexes. However, stoichiometric amounts of Ln^III (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Y, Lu), Zn^II, and L ² in a 1:1:3 ratio lead to the selective formation of the C₃-symmetrical heterodinuclear complexes [LnZn( L ²)₃]⁵⁺ under thermodynamic control. Detailed NOE studies show that the ligands are wrapped about the C₃ axis defined by the metal ions, and the separation of dipolar and contact contributions to the ¹H-NMR paramagnetic shifts of the axial complexes [LnZn( L ²)₃]⁵⁺ (Ln = Ce, Pr, Nd, Sm, Eu) in MeCN establishes that Zn^II occupies the pseudo-octahedral capping coordination site defined by the three bidentate binding units, while Ln^III lies in the resulting ‘facial’ pseudo-tricapped trigonal prismatic site produced by the three remaining tridentate units. Photophysical measurements show that [LnZn( L ²)₃]⁵⁺ (Ln = Eu, Tb) are only weakly luminescent because of quenching processes associated with the C₃-cylindrical structure of the complexes. The use of 3d metal ions to control and design isomerically pure ‘facial’ tricapped trigonal prismatic lanthanide building blocks is discussed together with the calculation of a new nephelauxetic parameter associated with heterocyclic N-atoms coordinated to Ln^III.     

The aromatic diboracyclopropenyl (diboriranyl) anion; CB2H3 −: An ab initio study

The most stable structure of CB₂H₃ ^–, as established computationally, is the aromatic diboracyclopropenyl (diboriranyl) anion (5), while open-chainC _2v, isomer H₂BCBH (7) is only 3 kcal/mol higher in energy at the QCISD(T)/6-311 +G^**//MP2/6-31+G^*+ZPE (HF/6-31 +G^*). The 47-kcal/mol barrier between cyclic,5, and open-chain,7, structures suggests that both of them may be observed. The aromatic stabilization energy of the diboriranyl anion (18 kcal/mol) is half the value in the isoelectronic cyclopropenium ion, C₃H₃ ⁺. The computed, by IGLO method (5a), and experimental (6a) chemical shifts,(¹³C) and(¹¹B), agree within 4 ppm range. The theoretical vibrational frequencies of the most stable isomers,5 and7, are presented for experimental verification of these species.     

Zur Regioselektivität von Cycloadditionen photochemisch erzeugter Benzonitril-isopropylide. 51. Mitteilung über Photoreaktionen

On the regioselectivity of cycloaddition reactions of photochemically generated benzonitrile-isopropylides This paper deals with the physical and chemical differences of zwitterionic benzo-nitrile-isopropylides, which differ by a p-substituent in the phenyl ring (H, F, OCH₃; Scheme 2). These dipolar species (4–6) are produced by irradiation of the corresponding 2 H-azirines ( 1–3 ; Scheme 1) in a 2-methylpentane glass at ?185°. Their UV. spectra are reproduced in the Figure. The spectra of 4 and 5 are characterized by an ‘aromatic band’ at short wavelength, and a longer wavelength band at approximately 275 nm, which is considered to be characteristic of the nitrile-ylide system. The UV. spectrum of the methoxy derivative 6 , which shows a broad absorption at 260 nm, arises by an addition of the ‘nitrile-ylide band’ and the anisole band. The three dipolar species 4–6 do not show any significant differences in the regioselectivity of the cycloaddition with methyl α-methacrylate even though F and OCH₃ have quite different σ-constants (Scheme 1). The addition according to modus A is very much preferred (B/A = 0,076). – It seems, that the substituents F and OCH₃ do not affect the physical and chemical behaviour of the parent benzonitrile-isopropylide (4) . All three dipolar species 4–6 react regiospecifically according to modus A with methyl trifluoroacetate (Scheme 1). The regioselectivity is reduced in the cycloaddition of 4 with methyl propiolate and ethyl phenylpropiolate (B/A = 0,04 and 0,28, respectively). The reduced regioselectivity in the latter case may be attributed to a reduced polarity of the triple bond in the dipolarophile.     

[3+2] Cycloadditions of Metallo Nitrile Ylides and Metallo Nitrile Imines with Metal Carbonyl,Thiocarbonyl, and Isocyanide Complexes: Heterodimetallic Systems with Bridging Heterocycles [1]

The reactions of [Re(CO)₆]⁺, [FeCp(CO)₂CS]⁺ and [FeCp(CNPh)₃]⁺ with the metallo nitrile ylides [M^–{C⁺=N–C^–(H)CO₂Et}(CO)₅]^– (M = Cr, W) and the chromio nitrile imine [Cr^–{C⁺=N–N^–H}(CO)₅]^– (generated by mono‐α‐deprotonation of the parent isocyanide complexes) to give neutral 5‐metallated 1,3‐oxazolin‐ ( 1 ), 1,3‐thiazolin‐ ( 2 ), imidazolin‐ ( 3 , 4 ), 1,3,4‐oxdiazolin‐ ( 5 ), 1,3,4‐thiadiazolin‐ ( 6 ) and 1,3,4‐triazolin‐2‐ylidene ( 8 ) chromium and tungsten complexes represent the first all‐organometallic versions of Huisgen’s 1,3‐dipolar cycloadditions. The formation of 6 and 8 is accompanied by partial decomposition to (OC)₅Cr–C≡N–FeCpL₂ {L = CO ( 7 ), CNPh ( 9 )}. The structures of 4a and 5 have been characterized by X‐ray diffraction.     

The Structural Systematics of Protonation of Some Important Nitrogen‐base Ligands. II. Some Univalent Anion Salts of Singly Protonated Bis(2‐pyridyl)amine

Syntheses and single crystal X‐ray structure determinations are recorded for a number of normal and ‘acid’ salts of bis(2‐pyridylamine), ‘dpa’, with univalent anions, X, variously hydrated, i.e. [dpaH]X·nH₂O, and [dpaH]X·HX·nH₂O. The ‘normal’ salt arrays so characterized are for X = Br^? (n = 2, isomorphous with the previously described chloride compound) and, I^?, ClO₄^?, ‘tca^?’ (≡ Cl₃CCO₂)^? (all n = 1); acid salt arrays are described for X = NO₃^? and tca (both n = 0). In all cases except those of X = ClO₄^?, NO₃^?, there is one independent formula unit devoid of crystallographic symmetry comprising the asymmetric unit of the structure. In all cases, the proton associated with the cation is ‘chelated’ by the pair of ring nitrogen atoms, disposed ‘endo’; in the tca adducts and the nitrate salt, the total cation is disordered in each case by inversion about a real or putative inversion centre between the rings. In the perchlorate compound, the (ordered) cation lies on a crystallographic 2‐axis, as does the water molecule, and the perchlorate ion, which is disordered about such an axis; in the nitrate compound, the acid hydrogen atom is modelled as disposed on a crystallographic inversion centre between a pair of symmetry‐related nitrate groups, containing, like the Htca adduct, the [XHX]^? moiety rather than a diprotonated cation.     

Internal reactions of ion–neutral complexes from some disubstituted protonated benzaldehydes and acetophenones

Protonated benzaldehydes ‘a’ and protonated acetophenones ‘b’, substituted by a methoxymethyl group, a hydroxy-methyl group and a mercaptomethyl group, respectively, in position 3, in addition to a methoxymethyl side chain at position 5, have been prepared by electron impact induced dissociation from the corresponding benzylic alcohols. The spontaneous fragmentations of metastable ions of ‘a’ and ‘b’ have been investigated with the aid of specifically deuterated derivatives. Large signals arc observed for the loss of methanol induced by a proton migration across the aromatic ring. The competing loss of H₂O and H₂S, respectively, from the second side chain is less abundant, in agreement with the smaller PA's of HO? and HS? groups. The elimination of HCOX and CH₃COX (X = OCH₃, OH, SH), respectively, from ‘a’ and ‘b’ is also observed. The label distributions for these reactions are in agreement with a mechanism corresponding to an internal reaction of [CHO] ⁺ and [CH₃CO] ⁺, respectively, with the functional group of the side chains in an intermediary ion–neutral complex. In addition, fragmentations are observed arising from reactions between the two side chains at positions 3 and 5. The D labelling proves specific reactions without any H/D exchange and thus reaction channels separated from the methanol loss. The results are explained by internal ion-molecule reactions in an intermediary ion-neutral complex of a methoxymethyl cation, a hydroxymethyl cation and a mercaptomethyl cation, respectively, formed by a protolytic bond cleavage of the side chains.     

Ligand Effects on the Mechanisms of Thermal Bond Activation in the Gas‐Phase Reactions NiX+/CH4→Ni(CH3)+/HX (X=H,CH3, OH,F). Short Communication

The thermal ion‐molecule reactions NiX⁺+CH₄→Ni(CH₃)⁺+HX (X=H, CH₃, OH, F) have been studied by mass spectrometric methods, and the experimental data are complemented by density functional theory (DFT)‐based computations. With regard to mechanistic aspects, a rather coherent picture emerges such that, for none of the systems studied, oxidative addition/reductive elimination pathways are involved. Rather, the energetically most favored variant corresponds to a σ‐complex‐assisted metathesis (σ‐CAM). For X=H and CH₃, the ligand exchange follows a ‘two‐state reactivity (TSR)’ scenario such that, in the course of the thermal reaction, a twofold spin inversion, i.e., triplet→singlet→triplet, is involved. This TSR feature bypasses the energetically high‐lying transition state of the adiabatic ground‐state triplet surface. In contrast, for X=F, the exothermic ligand exchange proceeds adiabatically on the triplet ground state, and some arguments are proposed to account for the different behavior of NiX⁺/Ni(CH₃)⁺ (X=H, CH₃) vs. NiF⁺. While the couple Ni(OH)⁺/CH₄ does not undergo a thermal ligand switch, the DFT computations suggest a potential‐energy surface that is mechanistically comparable to the NiF⁺/CH₄ system. Obviously, the ligands X act as a mechanistic distributor to switch between single vs. two‐state reactivity patterns.