Synthesis and Characterization of Cyclopropaosmanaphthalenes Containing a Fused σ‐Aromatic Metallacyclopropene Unit

Metalla‐aromatics are important complexes that show unique properties owing to their highly conjugated systems, which show Hückel or Möbius aromaticity. Recently, several metalla‐aromatics showing spiro‐aromaticity or σ‐aromaticity have been reported. Herein, we report the isolation of the first cyclopropametallanaphthalenes, in which the metallacyclopropene ring shows σ‐aromaticity and weak hyperconjugative aromaticity. The reaction of OsCl₂(PPh₃)₃ with o‐ethynylphenyl alkynes in the presence of PPh₃ followed by protonation with HCl yielded the first cyclopropametallanaphthalenes. The reaction mechanism and the aromaticity were also investigated by density functional theory studies.     

Dimorphism in 3′‐amino­cyclo­hexane­spiro‐5′‐hydantoin

The title compound [systematic name: 1′‐amino­cyclo­hexane­spiro‐4′‐imidazole‐2′,5′(3′H,4′H)‐dione], C₈H₁₃N₃O₂, has been synthesized and was found to crystallize in two different structures, both monoclinic and both with the same P2₁/c space group. In the first structure, there are two mol­ecules in the asymmetric unit, one of which uses all of its hydrogen‐bond donors and acceptors and forms undulating layers, while the other forms chains propagating perpendicular to the layers. In the second structure, there is only one independent mol­ecule and the packing is based on a chain structure mediated by hydrogen bonding between the hydantoin moieties and further grouped into hydro­philic layers separated by layers of the hydro­phobic cyclo­hex­yl groups.     

2,2′‐Spiro­bi(1,3‐benzodi­thiole)

The X‐ray structure analysis of 2,2′‐spiro­bi(1,3‐benzodi­thiole), C₁₃H₈S₄, has been performed. The mol­ecule has crystallographic twofold rotation symmetry, the axis passing through the spiro‐C atom. The four S atoms are arranged around the spiro‐C atom in two almost orthogonal CS₂ planes. However, because of large bending of the two five‐membered rings, close contact is present between two connected C atoms on the benzo group of each 1,3‐benzodi­thiole ring and one S atom on the other 1,3‐benzodi­thiole ring.     

A Computational Study on 3‐Azonia‐, 3‐Phosphonia‐, and 3‐Arsoniaspiro[2.2]pentanes and Related Three‐Membered Heterocycles

A theoretical study at the ab initio MP2/6‐311++G(d,p) level of theory is carried out to characterize several heterocyclic spiro[2.2]pentane cations with N, P, and As as spiro atoms. The strain and relative stability of the spiropentanes are obtained through isodesmic reactions. Nucleus‐independent chemical shifts (NICS) and 3D NICS isosurfaces show σ‐aromatic characteristics, similar to those found in cyclopropane. The interaction with the Cl^? anion, which results in four different stationary structures, is studied and characterized by means of the atoms in molecules methodology, and Cl ??? pnicogen, Cl ??? H, and Cl ??? C interactions are found. The most stable structure in all cases corresponds to opening of one of the three‐membered rings, due to the attack of the Cl atom, and C?Cl bond formation. Furthermore, the reaction with the 3‐boranuidaspiro[2.2]pentane anion results in the formation of a new compound through cleavage of one ring of both reactants.     

Two conformers of 10,11‐dihydro‐5H‐dibenzo[a,d]cyclo­heptene spiro‐linked with homobenzoquinone epoxide

The crystal structures of the two thermally equilibrated conformational isomers of the epoxide 1′,5′‐dimethyl­spiro[10,11‐dihydro‐5H‐dibenzo[a,d]cyclo­heptene‐5,8′‐4′‐oxatricyclo[5.1.0.0^3,5]octane]‐2′,6′‐dione, C₂₃H₂₀O₃, have been determined by X‐ray diffraction. In the tricyclic dione skeleton, the oxirane and cyclo­propane rings adopt an anti structure with respect to the conjunct quinone frame. The spiro‐linked 10,11‐dihydro‐5H‐dibenzo[a,d]cyclo­heptene ring of the major isomer has a fairly twisted boat form, folding opposite to the adjoining cyclo­propane methyl substituent, whereas the seven‐membered ring of the minor isomer has an almost ideal twist–boat form, inversely folding to the side of the relevant methyl group. The conformational structures of these isomers have been compared with those of the corresponding isomers of the unepoxidized homobenzoquinone.     

One‐ and two‐dimensional CdII coordination polymers constructed from 2‐(2‐methyl‐1H‐benzimidazol‐1‐yl)acetate ligands

The one‐ and two‐dimensional polymorphic cadmium polycarboxylate coordination polymers, catena‐poly[bis[μ₂‐2‐(2‐methyl‐1H‐benzimidazol‐1‐yl)acetato‐κ³N³:O,O′]cadmium(II)], [Cd(C₁₀H₉N₂O₂)₂]_n, and poly[bis[μ₂‐2‐(2‐methyl‐1H‐benzimidazol‐1‐yl)acetato‐κ³N³:O,O′]cadmium(II)], also [Cd(C₁₀H₉N₂O₂)₂]_n, were prepared under solvothermal conditions. In each structure, each Cd^II atom is coordinated by four O atoms and two N atoms from four different ligands. In the former structure, two crystallographically independent Cd^II atoms are located on twofold symmetry axes and doubly bridged in a μ₂‐N:O,O′‐mode by the ligands into correspondingly independent chains that run in the [100] and [010] directions. Chains containing crystallographically related Cd^II atoms are linked into sheets viaπ–π stacking interactions. Sheets containing one of the distinct types of Cd^II atom are stacked perpendicular to [001] and alternate with sheets containing the other type of Cd^II atom. The second complex is a two‐dimensional homometallic Cd^II (4,4) net structure in which each Cd^II atom is singly bridged to four neighbouring Cd^II atoms by four ligands also acting in a μ₂‐N:O,O′‐mode. A square‐grid network results and the three‐dimensional supramolecular framework is completed by π–π stacking interactions between the aromatic ring systems.     

Two atropisomers of tri­carbonyl­[η6‐7‐chloro‐3‐(3‐chloro‐2‐methyl­phenyl)‐2,4,8‐tri­methyl‐1,2,3,4‐tetra­hydro‐2,4‐dibora‐1,3‐di­aza­naphthalene]­chromium(0)

The structures of two atropisomers of the title compound, [Cr(C₁₆H₁₈B₂Cl₂N₂)(CO)₃], are reported. For both compounds, the Cr(CO)₃ moiety is bound to the C₆ aromatic ring of the mol­ecule; the existence of atropisomers resulting from the non‐equivalence of both faces of the C₆ aromatic ring is a consequence of the 3‐chloro‐2‐methylphenyl ring being nearly perpendicular to the mean plane of the 2,4‐dibora‐1,3‐di­aza­naphthalene ring. The orientation of the Cr(CO)₃ tripod relative to the C₆ aromatic ring is such that it is nearly eclipsed in one isomer (2.4° rotation from being eclipsed with C—N, C—Cl and C—H) and slightly twisted (16.2°) from an eclipsed conformation in the other.     

Can Synthetic All‐Metal Cluster Compound Support Multifold (π and σ) Aromaticity and d‐Orbital Aromaticity?

There are active debates on whether the concept of aromaticity should be extended beyond carbon based organic systems. One argument against such extension is that the proposed new aromatic species are not bottleable. We present herein in‐depth chemical bonding analyses of a synthetic, core‐shell, intermetalloid [Pd₃Sn₈Bi₆]^4‐ cluster. The computational data unravel unprecedented five‐fold (π and σ) aromaticity, including d‐orbital aromaticity. Delocalized electron clouds in this all‐metal system cover the Pd₃ core, trigonal pyramid Sn₄ caps, peripheral Bi₆ ring, and roof‐like Sn₂Bi₂ walls, each following the (4n + 2) Hückel rule. The present finding is beyond imagination, providing a compelling example that all‐metal aromaticity not only exists in bulk compounds, but also can be in multifold π/σ fashion.     

Synthesis and characterization of spirobifluorene‐based polyimides

The synthesis and properties of organosoluble aromatic polyimides, containing spiro‐skeletal units in the polymer backbone on the basis of the spiro‐diamine monomer, 2,2′‐diamino‐9,9′‐spirobifluorene, are described. In the case of the spiro segment, the two fluorene rings are orthogonally arranged and connected through a tetrahedral bonding carbon atom, the spiro center. As a consequence, the polymer chain is periodically zigzagged with a 90° angle at each spiro center. This structural feature minimizes interchain interactions and restricts the close packing of the polymer chains, resulting in amorphous polyimides that have good solubility in organic solvents. Compared with their fluorene‐based cardo analogues, the spirobifluorene‐based polyimides have an improved solubility. Furthermore, the main‐chain rigidity of the polyimide appears to be preserved because of the presence of the spiro structure, which restricts the free segmental mobility. As a result, these polyimides exhibit a high glass‐transition temperature (T_g's) and good thermal stability. The T_g's of these polyimides were in the range of 287–374 °C, and the decomposition temperatures in nitrogen for a 10% weight loss occurred at temperatures above 570 °C. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3615–3621, 2002     

Novel hydrazone derivatives of 7‐hydroxy‐3′,3′‐dimethyl‐3′H‐spiro[chromene‐2,1′‐isobenzofuran]‐8‐carbaldehyde

The structures of new oxaindane spiropyrans derived from 7‐hydroxy‐3′,3′‐dimethyl‐3′H‐spiro[chromene‐2,1′‐isobenzofuran]‐8‐carbaldehyde (SP1), namely N‐benzyl‐2‐[(7‐hydroxy‐3′,3′‐dimethyl‐3′H‐spiro[chromene‐2,1′‐isobenzofuran]‐8‐yl)methylidene]hydrazinecarbothioamide, C₂₇H₂₅N₃O₃S, (I), at 120 (2) K, and N′‐[(7‐hydroxy‐3′,3′‐dimethyl‐3′H‐spiro[chromene‐2,1′‐isobenzofuran]‐8‐yl)methylidene]‐4‐methylbenzohydrazide acetone monosolvate, C₂₇H₂₄N₂O₄·C₃H₆O, (II), at 100 (2) K, are reported. The photochromically active C_spiro—O bond length in (I) is close to that in the parent compound (SP1), and in (II) it is shorter. In (I), centrosymmetric pairs of molecules are bound by two equivalent N—H...S hydrogen bonds, forming an eight‐membered ring with two donors and two acceptors.     

Neue molekulare Indium‐Zinn‐Sauerstoff‐ und Indium‐Zinn‐Natrium‐Sauerstoff‐Chlor‐Cluster

Abstract. The five‐membered heteroelement cluster THF · Cl₂In(O^tBu)₃Sn reacts with the sodium stannate [Na(O^tBu)₃Sn]₂ to produce either the new oxo‐centered alkoxo cluster ClInO[Sn(O^tBu)₂]₃ ( 1 ) (in low yield) or the heteroleptic alkoxo cluster Sn(O^tBu)₃InCl₃Na[Sn(O^tBu)₂]₂ ( 2 ). X‐ray diffraction analyses reveal that in compound 1 the polycyclic entity is made of three tin atoms which together with a central oxygen atom form a trigonal, almost planar triangle, perpendicular to which a further indium atom is connected through the oxygen atom. The metal atoms thus are arranged in a Sn₃In pyramid, the edges of which are all saturated by bridging tert‐butoxy groups. The indium atom has a further chloride ligand. Compound 2 has two trigonal bipyramids as building blocks which are fused together at a six coordinate indium atom. One of the bipyramids is of the type SnO₃In with tert‐butyl groups on the oxygen atoms, while the other has the composition InCl₃Na with chlorine atoms connecting the two metals. The sodium atom in 2 has further contacts to two plus one alkoxide groups which are part of a[Sn(O^tBu)₂]₂ dimer disposing of a Sn₂O₂ central cycle. The hetero element cluster in 2 thus combines three closed entities and its skeleton SnO₃InCl₃NaO₂Sn₂O₂ consists of three different metallic and two different non‐metallic elements.     

High‐Field NMR Spectroscopy Reveals Aromaticity‐Modulated Hydrogen Bonding in Heterocycles

From DNA base pairs to drug–receptor binding, hydrogen (H‐)bonding and aromaticity are common features of heterocycles. Herein, the interplay of these bonding aspects is explored. H‐bond strength modulation due to enhancement or disruption of aromaticity of heterocycles is experimentally revealed by comparing homodimer H‐bond energies of aromatic heterocycles with analogs that have the same H‐bonding moieties but lack cyclic π‐conjugation. NMR studies of dimerization in C₆D₆ find aromaticity‐modulated H‐bonding (AMHB) energy effects of approximately ±30 %, depending on whether they enhance or weaken aromatic delocalization. The attendant ring current perturbations expected from such modulation are confirmed by chemical shift changes in both observed ring C−H and calculated nucleus‐independent sites. In silico modeling confirms that AMHB effects outweigh those of hybridization or dipole–dipole interaction.     

Preference for the Diels–Alder addition of dienes syn to the O atom in cross‐conjugated spiro­cyclic cyclo­hexadienones

In the Diels–Alder reaction, the preferred addition of dienes syn to the O atom in cross‐conjugated cyclo­hexadienones containing an oxa‐­spiro ring system is observed. The two structures reported here, namely rel‐(1R,4aR,9S,9aS,10R)‐4a,9,9a,10‐tetra­hydro‐9,10‐di­phenyl­spiro­[9,10‐epoxy­anthra­cene‐1(4H),2′‐oxiran]‐4‐one, C₂₇H₂₀O₃, and rel‐(1R,4aS,9R,9aS,10S)‐4a,9,9a,10‐tetra­hydro‐9,10‐di­phenyl­spiro­[9,10‐epoxy­anthracene‐1(4H),2′‐oxetane]‐4‐one, C₂₈H₂₂O₃, are the minor and sole products, respectively, of the reactions of di­phenyl­isobenzo­furan with two slightly different cyclo­hexadienones. These structures differ in the size of the oxa‐­spiro ring, by one C atom, and in the relative configuration at the spiro­cyclic ring C atom, leading to some minor conformational differences between the two compounds.     

2,4,6‐Trimethyl‐N‐nitro­aniline

In 2,4,6‐trimethyl‐N‐nitro­aniline (alternatively called mesitylnitramine), C₉H₁₂N₂O₂, the primary nitramino group is planar with a short N—N bond and is nearly perpendicular to the aromatic ring. The methyl group located in the para position is disordered, each H atom having half‐occupancy. The mol­ecules are linked together along the [100] axis by inter­molecular N—H⋯O hydrogen bonds.     

Aromaticity Effects on the Profiles of the Lowest Triplet‐State Potential‐Energy Surfaces for Rotation about the CC Bonds of Olefins with Five‐Membered Ring Substituents: An Example of the Impact of Baird’s Rule

A density functional theory study on olefins with five‐membered monocyclic 4n and 4n+2 π‐electron substituents (C₄H₃X; X=CH⁺, SiH⁺, BH, AlH, CH₂, SiH₂, O, S, NH, and CH^?) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet‐state (T₁) potential‐energy surface (PES) for twisting about olefinic C?C bonds. It exploited both Hückel’s rule on aromaticity in the closed‐shell singlet ground state (S₀) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH₂?CH(C₄H₃X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C₄H₃X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S₀‐antiaromatic/T₁‐aromatic (C₅H₄⁺) to strongly S₀‐aromatic/T₁‐ antiaromatic (C₅H₄^?). Our hypothesis is that the shapes of the T₁ PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T₁ [ΔE(T₁)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T₁) and the (anti)aromaticity changes of the C₄H₃X groups, as measured by the zz‐tensor component of the nucleus‐independent chemical shift ΔNICS(T₁;1)_zz, are found both for sets A and B separately (linear fits; r²=0.949 and 0.851, respectively) and for the two sets combined (linear fit; r²=0.851). For sets A and B combined, strong correlations are also found between ΔE(T₁) and the degree of S₀ (anti)aromaticity as determined by NICS(S₀,1)_zz (sigmoidal fit; r²=0.963), as well as between the T₁ energies of the planar olefins and NICS(S₀,1)_zz (linear fit; r²=0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T₁ PESs suitable for adiabatic Z/E photoisomerization.     

Excited‐State Aromaticity of Gold(III) Hexaphyrins and Metalation Effect Investigated by Time‐Resolved Electronic and Vibrational Spectroscopy

Expanded porphyrins with appropriate metalation provide an excellent opportunity to study excited‐state aromaticity. The coordinated metal allows the excited‐state aromaticity in the triplet state to be detected through the heavy‐atom effect, but other metalation effects on the excited‐state aromaticity were ambiguous. Herein, the excited‐state aromaticity of gold(III) hexaphyrins through the relaxation dynamics was revealed via electronic and vibrational spectroscopy. The S_Q states of gold [26]‐ and [28]‐hexaphyrins showed interconvertible absorption and IR spectra with those of counterparts in the ground‐state, indicating aromaticity reversal. Furthermore, while the T₁ states of gold [28]‐hexaphyrins also exhibited reversed aromaticity according to Baird's rule, the ligand‐to‐metal charge‐transfer state of gold [26]‐hexaphyrins contributed by the gold metal showed non‐aromatic features arising from the odd‐number of π‐electrons.     

Three trifluoro­methyl‐substituted protoporphyrinogen IX oxidase inhibitors

The structures of methyl 5‐[2‐chloro‐4‐(trifluoro­methyl)phenoxy]‐2‐nitro­benzoate, C₁₅H₉ClF₃N₃O₅, (I), methyl 2‐chloro‐5‐[3‐methyl‐2,6‐dioxo‐4‐(trifluoro­methyl)‐1,2,3,6‐tetrahydro­pyrimidin‐1‐yl]benzoate, C₁₄H₁₀ClF₃N₂O₄, (II), and 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]‐4‐(trifluoro­methyl)piperidine‐2,6‐dione, C₁₅H₁₀ClF₄NO₃, (III), are similar in their dihedral angles and in the distances between the farthest two atoms. There are two independent molecules in the structure of (I). The dihedral angles between the two aromatic rings in each molecule in (I), between the benzene and tetrahydro­pyrimidine rings in (II), and between the benzene ring and the five‐atom planar portion of the piperidine‐2,6‐dione ring in (III) are 80.78 (11)/89.75 (11), 89.13 (9) and 87.52 (13)°, respectively. The distances between the farthest two atoms, viz. O⋯F in the two molecules of (I), and Cl⋯F in (II) and (III), are 11.763 (7)/11.953 (6), 10.734 (10) and 10.889 (9) Å, respectively. In all three crystal structures, the molecules are linked to generate sheets of molecules via C—H⋯O interactions.     

An Unusually Small Singlet–Triplet Gap in a Quinoidal 1,6‐Methano[10]annulene Resulting from Baird’s 4n π‐Electron Triplet Stabilization

Within the continuum of π‐extended quinoidal electronic structures exist molecules that by design can support open‐shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open‐shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open‐shell structure is accompanied by structural reorganization from a contorted Möbius aromatic‐like shape in S₀ to a more planar shape in the Hückel aromatic‐like T₁. This stability was attributed to Baird’s Rule which dictates the aromaticity of 4n π‐electron triplet excited states.     

Nickel(I)‐Komplexe mit 1,1′‐Bis(phosphino)ferrocenen als Liganden

Nickel(I) Complexes with 1,1′‐Bis(phosphino)ferrocenes as Ligands The thermically stable monomeric Nickel(I) complexes [(d^tbpf)Ni(acac)] ( 1 ) and [(dⁱppf)NiCl] ( 2 ) were synthesized and characterized by elemental analyses, EPR spectroscopy, and by X‐ray crystal structure analyses of single crystals (d^tbpf: 1,1′‐bis(di‐tertbutylphosphino)ferrocene; dⁱppf: 1,1′‐bis(diisopropylphosphino)ferrocene). 1 is formed by reduction of Ni(acac)₂ with triethylaluminium in the presence of d^tbpf, together with the nickel(0) complex [(d^tbpf)Ni(C₂H₄)]. 1 contains a Ni^I atom surrounded of two O‐ and two P donor atoms in a distorted tetrahedral coordination. 2 was obtained by reduction of [(dⁱppf)NiCl₂] with NaBH₄. In 2 the nickel(I) atom adopts trigonal planar coordination.     

Reactive intermediates in peptide synthesis. ortho‐Nitrophenyl Nα‐para‐toluenesulfonyl‐α‐aminoisobutyrate

The preparation, characterization, and molecular and crystal structures of the title compound [IUPAC name: 2‐nitro­phenyl 2‐methyl‐2‐(para‐toluene­sulfonyl­amino)­propanoate], C₁₇H₁₈­N₂O₆S, are reported. The phenyl group is almost perpendicular to the plane of the adjacent ester moiety. One O atom of the nitro group is wedged between the two ester O atoms. The implications of this peculiar conformation for the chemistry of ortho‐nitro­phenyl esters in peptide synthesis are discussed.