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A combined computational and experimental study reveals the character of the C60 complexes with piperidine formed under different reaction conditions. The IR and NMR experiments detect the dative bond complex, which according to NMR, is stable in the oxygen-free environment and transforms to the adduct complex in the presence of O2. Computational studies on the character of reaction channels rationalize the experimental observations. They show that the piperidine dimer rather than a single piperidine molecule is required for the complex formation. The calculations reveal significant differences in the dative bond and adduct complexes' character, suggesting a considerable versatility in their electronic properties modulated by the environment. This capability offers new application potential in several fields, such as in energy storage devices.

Organic open-shell compounds are extraordinarily attractive materials for their use in molecular spintronics thanks to their long spin-relaxation times and structural flexibility. Porphyrins (Pors) have widely been used as molecular platforms to craft persistent open-shell structures through solution-based redox chemistry. However, very few examples of inherently open-shell Pors have been reported, which are typically obtained through the fusion of non-Kekulé polyaromatic hydrocarbon moieties to the Por core. The inherent instability and low solubility of these radical species, however, requires the use of bulky substituents and multistep synthetic approaches. On-surface synthesis has emerged as a powerful tool to overcome such limitations, giving access to structures that cannot be obtained through classical methods. Herein, we present a simple and straightforward method for the on-surface synthesis of phenalenyl-fused Pors using readily available molecular precursors. In a systematic study, we examine the structural and electronic properties of three surface-supported Pors, bearing zero, two (PorA2), and four (PorA4) meso-fused phenalenyl moieties. Through atomically resolved real-space imaging by scanning probe microscopy and high-resolution scanning tunneling spectroscopy combined with density functional theory calculations, we unambiguously demonstrate a triplet ground state for PorA2 and a charge-transfer-induced open-shell character for the intrinsically closed-shell PorA4., This work was supported by the Swiss National Science Foundation under Grant No. 200020_182015. Financial support from the Spanish MICINN (CTQ2017-85393-P) andthe ERA-NET/European Commission/MINECO (UNIQUE, SOLAR-ERA.NET Cofund 2 No. 008/PCI2019-111889-2) are acknowledged. IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, Grant SEV2016-0686). R.R. and N.L. are grateful for funding from the EU-FET Open H2020 Mechanics with Molecules project (grant 766864).

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt2−) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt2−. UV-vis-Near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt2− π-manifold from the Fe(II)-centered spin-crossover. The TTFtt2− centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character.

Isolable compounds that contain π bonding without an underlying σ-bond framework (π-type single bonding) remain scarce. Herein we report the synthesis and properties of 1,3-diiodotetrasilabicyclo[1.1.0]butane

The synthesis of polycyclic aromatic hydrocarbons containing various non-benzenoid rings remains a big challenge facing contemporary organic chemistry despite a considerable effort made over the last decades. Herein, we present a novel route, employing on-surface chemistry, to synthesize nonalternant polycyclic aromatic hydrocarbons containing up to four distinct kinds of non-benzenoid rings. We show that the surface-induced mechanical constraints imposed on strained helical reactants play a decisive role leading to the formation of products, energetically unfavorable in solution, with a peculiar ring current stabilizing the aromatic character of the π-conjugated system. Determination of the chemical and electronic structures of the most frequent product reveals its closed-shell character and low band gap. The present study renders a new route for the synthesis of novel nonalternant polycyclic aromatic hydrocarbons or other hydrocarbons driven by internal stress imposed by the surface not available by traditional approaches of organic chemistry in solution.

We report a [6]cyclo-para-phenylmethine ([6]CPPM) macrocycle that shows benzene-like electronic properties. Its mesityl derivative, [6]CPPM-Mes, was isolated in crystalline form. X-ray analysis rev...

Sulfated metal-organic framework-808 (S-MOF-808) exhibits strong Brønsted-acidic character which makes it a potential candidate for the heterogeneous acid catalysis. Here, we report the isomerization and oligomerization reactions of light olefins (C3-C6) over S-MOF-808 at relatively low temperatures and ambient pressure. Different products (dimers, isomers, and heavier oligomers) were obtained for different olefins, and effective C-C coupling was observed between isobutene and isopentene. Among the substrates investigated, facile oligomerization occurred very specifically for the structures with an α-double bond and two substituents at the second carbon atom of the main carbon chain. The possible oligomerization mechanism of light olefins was discussed based on the reactivity and selectivity trends. Moreover, the deactivation and regeneration of S-MOF-808 were investigated. The catalyst deactivates via two mechanisms which predominance depends on the substrate and reaction conditions. Above 110 °C, a loss of acidic sites was observed due to water desorption, and the deactivated catalyst could be regenerated by a simple treatment with water vapor. For C5 substrates and unsaturated ethers, the oligomers with increased molecular weight caused deactivation via blocking of the active sites, which could not be readily reversed. These findings offer the first systematic report on carbocation-mediated olefin coupling within MOFs in which the Brønsted acidity is associated with the secondary building units of the MOF itself and is not related to any guest substance hosted within its pore system.

A nonbenzenoid hydrocarbon, difluoreno[1,9,8- alkj:1',9',8'- gfed]heptalene 1, is synthesized. Experimental and theoretical investigations demonstrate that the planar and symmetric heptalene core within 1 effectively induces the antiaromatic and open-shell character. These properties are not shared by bisanthene 2, a benzenoid isomer of 1.

Principles that predict reactions or properties of materials define the discipline of chemistry. In this work we derive chemical rules, based on atomic distances and chemical bond character, which predict topological materials in compounds that feature the structural motif of a square-net. Using these rules we identify over 300 potential new topological materials. We show that simple chemical heuristics can be a powerful tool to characterize topological matter. In contrast to previous database-driven materials categorization our approach allows us to identify candidates that are alloys, solid-solutions, or compounds with statistical vacancies. While previous material searches relied on density functional theory, our approach is not limited by this method and could also be used to discover magnetic and statistically-disordered topological semimetals., Comment: 30 pages, 8 figures

Mesityl derivatives of the unknown dibenzopentalene isomer dibenzo[a,f]pentalene were synthesized. The molecular geometry and physical properties of dibenzo[a,f]pentalene were investigated. Dibenzo[a,f]pentalene combines a large antiaromatic and appreciable singlet open-shell character, properties not shared by well-known isomer dibenzo[a,e]pentalene.

Butadiene is the simplest polyene and has long served as a model system for many chemical and spectroscopic properties. However, this small molecule has presented significant challenges to theoretical chemistry. The 21Ag state, which is dark but photochemically important, is a prime source of this difficulty. Previous studies attributed the notorious difficulty in treating this state to strong double excitation character of the 21Ag state, which prevents the application of linear response (LR) methods. Therefore, one would require methods with much higher computational cost, especially for the analogues of this state in longer polyenes, and consequently studies of longer polyenes are very limited. In the present work, we argue that the difficulty stems more significantly from the inherently multiconfigurational character of the ground state. In addition, we validate the possibility of employing LR time-dependent density functional theory to investigate such a state with reasonable accuracy.

The nature of the electronic-structure of polyenes, their delocalization features, and potential diradicaloid characters constitute a fundamental problem in chemistry. To address this problem, we used valence bond self-consistent field (VBSCF) calculations and modeling of polyenes, C2nH2n+2 (n = 2–10). The theoretical treatment shows that starting with n = 5, the polyene’s wave function is mainly a shifting 1,4-diradicaloid, a character that increases as the chain length increases, while the contribution of the fundamental Lewis structure with alternating double and single bonds (1) decays quite fast and becomes minor relative to the diradicaloid pack. We show how, nevertheless, it is this wave function that predicts that polyenes will still exhibit alternating short/long CC bonds like the fundamental structure 1. Furthermore, despite the decay of the VB contribution of 1, it remains the single structure with the largest weight among all the individual structures. The mixing of all the 1,4-diradicaloid st...

Understanding the structure-property relationships in antiaromatic molecules is crucial for controlling their electronic properties and designing new organic optoelectronic materials. Dibenzo[ a, f]pentalene, a structural isomer of dibenzopentalene, displays open-shell and antiaromatic character harmonization, which is not shared by the well-known isomer, dibenzo[ a, e]pentalene. The next questions of interest concern the topological effects of the π-extension on the harmonization of the open-shell and antiaromatic character in the dibenzo[ a, f]pentalene π-system. Herein, we describe the synthesis and characterization of the π-extended (bis)annulated analogues, benzo[ a]naphtho[2,1- f]pentalene 4 and dinaphtho[2,1- a, f]pentalene 5. The solid-state structures and the magnetic and optoelectronic properties characterized these π-extended analogues as closed-shell antiaromatic molecules, in sharp contrast with dibenzo[ a, f]pentalene 2. In these π-extended analogues, the open-shell character was annihilated whereas the antiaromatic character was retained. The fusion of additional hexagons into 2 shifted the main 4nπ-conjugated circuit from a global to a local system. Further investigations into magnetic ring currents using gauge-including magnetically induced current (GIMIC) calculations suggested that an enhanced local paratropic ring current appeared in the pentalene core of 5. The preservation of the benzenoid character in the additionally fused hexagons confined the paratropicity to the pentalene subunit, and the inherent presence of an o-quinoidal structure highlighted the 4nπ-electron delocalization on the pentalene unit. The antiaromaticity of 4 and 5 was characterized by their small HOMO-LUMO energy gap. Both experimental and computational results demonstrated that the [ a, f]-type ring fusion of the pentalene core effectively enhanced the antiaromatic character compared with the [ a, e]-type ring fusion in the reported bisannulated[ a, e]pentalenes. The findings of this study could potentially be used for the rational design of optoelectronic devices based on novel antiaromatic molecules.

We report here the remarkable properties of PAd3, a crystalline air-stable solid accessible through a scalable SN1 reaction. Spectroscopic data reveal that PAd3, benefiting from the polarizability inherent to large hydrocarbyl groups, exhibits unexpected electron releasing character that exceeds other alkylphosphines and falls within a range dominated by N-heterocyclic carbenes. Dramatic effects in catalysis are also enabled by PAd3 during Suzuki-Miyaura cross-coupling of chloro(hetero)arenes (40 examples) at low Pd loading, including the late-stage functionalization of commercial drugs. Exceptional space-time yields are demonstrated for the syntheses of industrial precursors to valsartan and boscalid from chloroarenes with ∼2 × 10(4) turnovers in 10 min.

Atomically precise copper nanoclusters (NCs) are of immense interest for a variety of applications, but have remained elusive. Herein, we report the isolation of a copper NC, [Cu25H22(PPh3)12]Cl (1), from the reaction of Cu(OAc) and CuCl with Ph2SiH2, in the presence of PPh3. Complex 1 has been fully characterized, including analysis by X-ray crystallography, XANES, and XPS. In the solid state, complex 1 is constructed around a Cu13 centered-icosahedron and formally features partial Cu(0) character. XANES of 1 reveals a Cu K-edge at 8979.6 eV, intermediate between the edge energies of Cu(0) and Cu(I), confirming our oxidation state assignment. This assignment is further corroborated by determination of the Auger parameter for 1, which also falls between those recorded for Cu(0) and Cu(I).

The activation of the SN2 reaction by π systems is well documented in textbooks. It has been shown previously that this is not primarily due to classical (hyper)conjugative effects. Instead, π-conjugated substituents enhance favorable substrate-nucleophile electrostatic interactions, with electron-withdrawing groups (EWG) on the sp(2) system leading to even stronger activation. Herein we report computational and experimental results which show that this activation by sp(2) EWG-substitution only occurs in a fairly limited number of cases, when the nucleophile involves strong electrostatic interactions (usually strongly basic negatively charged nucleophiles). In other cases, where bond breaking is more advanced than bond making at the transition state, electrophile-nucleophile electrostatic interactions are less important. In such cases, (hyper)conjugative electronic effects determine the reactivity, and EWG-substitution leads to decreased reactivity. The basicity of the nucleophile as well as solvent effects can help to determine which of these two regimes occurs for a given electrophile. ispartof: Journal of the American Chemical Society vol:138 issue:3 pages:734-737 ispartof: location:United States status: published

We describe the regioselective cycloaddition of La(2)@I(h)-C(80) with tetracyanoethylene oxide (TCNEO), which enabled the formation of the corresponding adduct having a tetracyanotetrahydrofuran moiety. X-ray crystallographic analysis revealed that the cycloaddition took place as a [5,6] addition. Along with dynamic swing motion of the metal atoms, the results of this electrochemical study demonstrate that TCNEO addition enhanced the electron-accepting character of La(2)@I(h)-C(80) and that the first reduction potential of the adduct reached -0.21 V versus the ferrocene/ferrocenium couple, which is the lowest value reported for endohedral metallofullerenes and their derivatives to date.

The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types have been investigated systematically through a test family of C,C-chelating ligands containing two moieties of either kind. The overall character of o-C6H4A(a)B(b) ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(eta(2)-C6H4A(a)B(b))Rh(CO)2][TfO]. The test moieties A = NC2H2N(+)(Me)C(-) and B = Ph2P(+)CH2(-) were first considered. While the ligands bearing at least one diaminocarbene end (AA, a = 2 and AB, a = 1) could be generated (and trapped by complexation), the bis-ylide case BB (a = 0) proved to be awkward: treatment of the dication C6H4(P(+)Ph2Me)2 with n-BuLi indeed lead to the Schmidbaur's carbodiphosphorane Ph3PCPPh2Me, through an unprecendented ylido-pentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand type C6H4B2 could however be generated by bridging the phosphonium methyl groups by a methylene link (B2 = (P(+)Ph2CH(-))2CH2), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(eta(2)-C6H4A(a)B(b))Rh(CO)2][TfO] were fully characterized, including by X-ray diffraction analysis and (103)Rh NMR spectroscopy. Comparison of their IR spectra indicated that the A2 type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B2 type bis-ylide ligand. The excellent linear variation of the nu(CO) frequencies vs a (= 0, 1, 2) shows that the coordinating moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-31G**/LANL2DZ*(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.

We investigate the bonding mechanism in ozone (O3) and its sulfur-substituted analogues, SO2, OS2, and S3. By analyzing their ground-state multireference configuration interaction wave functions, we demonstrate that the bonding in these systems can be represented as a mixture of a closed-shell structure with one and a half bonds between the central and terminal atoms and an open-shell structure with a single bond and two lone electrons on each terminal atom (biradical). The biradical character (β) further emerges as a simple measure of the relative contribution of those two classical Lewis structures emanating from the interpretation of the respective wave functions. Our analysis yields a biradical character of 3.5% for OSO, 4.4% for SSO, 11% for S3, 18% for O3, 26% for SOO, and 35% for SOS. The size/electronegativity of the end atoms relative to the central one is the prevalent factor for determining the magnitude of β: smaller and more electronegative central atoms better accommodate a pair of electrons facilitating the localization of the remaining two lone π-electrons on each of the end atoms, therefore increasing the weight of the second picture in the mixed bonding scenario (larger β). The proposed mixture of these two bonding scenarios allows for the definition of the bond order of the covalent bonds being (3-β)/2, and this accounts for the different O-O, S-S, or S-O bond lengths in the triatomic series. The biradical character was furthermore found to be a useful concept for explaining several structural and energetic trends in the series: larger values of β mark a smaller singlet-triplet splitting, closer bond lengths in the ground (1)A' and the first excited (3)A' states, and larger bond dissociation and atomization energies in the ground state. The latter explains the relative energy difference between the OSS/SOS and OOS/OSO isomers due to their different β values.

We report the enantioselective total synthesis of cribrostatin IV (1). Key features of this synthesis involve the convergent coupling of two highly functionalized homochiral components followed by a "lynchpin" Mannich cyclization to establish the pentacyclic core (cf. 19 --> 20).

The reactions of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne 1 with an equivalent amount of trans- and cis-3,3',5,5'-tetramethylazobenzenes produced a Si(2)N(2) four-membered ring biradicaloid [RSi(μ-NAr)(2)SiR] 2 (R = Si(i)Pr[CH(SiMe(3))(2)](2), Ar = 3,5-Me(2)C(6)H(3)), which was isolated as air- and moisture-sensitive dark purple crystals. Compound 2 displays no EPR signal, and the molecular structure of 2 was characterized by NMR spectroscopy and X-ray crystallography, revealing that 2 has a planar centrosymmetric Si(2)N(2) four-membered ring. The Si1-Si1' distance is 2.63380(9) Å, and there is no bond interaction between the Si1 and Si1' atoms of 2. The reactions of 2 with methanol and carbon tetrachloride show that 2 has both closed-shell and radical-type reactivity.

A set of new reactions is described and illustrated in eqs 1-5. Diazenes which customarily participate in diradical processes, compounds 1a and 9 for example, lead to intermediates which are intercepted by water. Diazene 3 is converted to an intermediate which is intercepted both by water and, when the dielectric constant of the medium is sufficiently high, by n-PrOH

Directly linked corrole dimer 1, which has a planar cyclooctatetraene in its core, has been synthesized from 2-borylcorrole in excellent yield. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation of 1 provides its oxidized form 4, which has two NH protons in each corrole core. Dicorrole 4 and its Zn(II) complex 6 exhibit broad electronic absorption spectra, and magnetic measurement has revealed their biradicaloid character.

The first 14-electron disilene palladium complex eta2-[tetrakis(tert-butyldimethylsilyl)disilene](tricyclohexylphosphine)palladium (4) was synthesized. In the solid state, complex 4 has one tricyclohexylphosphine ligand bound unsymmetrically to the palladium center in regard to the eta2-disilene moiety. The elongation of the Si-Si bond length from that of the corresponding free disilene (3.2%) and the bent back angles of the disilene moiety (4.41 degrees and 9.65 degrees ) for 4 were much smaller than those for the corresponding 16-electron eta2-disilene complex (Me3P)2Pd[tetrakis(tert-butyldimethylsilyl)disilene] (4.6% and 27.2 degrees , respectively). Complex 4 is regarded as the complex having the strongest pi-complex character among the known disilene complexes. Highly symmetric NMR spectra were observed for 4; central Si1 and Si2 nuclei were equivalent and appeared as a doublet with 2J(29Si-31P) of 19 Hz, indicating facile flipping of the phosphine ligand in the Pd-Si-Si plane.

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMeOMe, CNCN, CNOMe, HCl, Cl approximately CN, H approximately Me, MeH, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.

Four kinds of novel zigzag boron-carbon nanotubes with quasi-planar tetracoordinate carbons (2m, 0, i) (m = 3-6; i = 1-4) have been constructed, and their structures, stabilities, and bonding properties have been investigated by B3LYP calculations. The results show that the novel nanotubes (m, 0, i) have the character of metal properties with quite small HOM-LUMO gaps. Structurally, there are big windows on the walls of nanotubes and they are promising in design for functional materials.

Experimental and theoretical studies evidence an unconventional biradical character for the titanium enolates derived from α-alkoxy ketones and TiCl4. EPR experiments and ab initio calculations suggest that these titanium enolates have an open shell electronic ground state.

In this contribution we describe a facile method to access a large number of three-dimensional discrete DNA assemblies. The approach involves the use of single-stranded and cyclic 2D DNA building blocks, of predefined geometry, as the faces or sides of the objects to be constructed. Any target three-dimensional discrete object that could be retrosynthetically broken down into a combination of the discrete 2D shapes, could in principle be easily accessed using this method. Using triangle 3, square 4, pentagon 5, and hexamer 6, we constructed triangular, cubic, pentameric, and hexameric prisms, as well as the more complex heteroprism HP and biprism BP assemblies in quantitative yields. The use of single-stranded DNA building blocks inherently allows for dynamic character and addressability. Using a series of rigidifying and eraser strands, we generated a triangular prism capable of structural oscillation between three predefined lengths. The easy access to a large number of complex three-dimensional discret...

The concept of antiaromaticity as applied to 4n-ir electron monocyclic conjugated systems is examined and two sub­ types distinguished. Relative antiaromaticity is the term which describes cyclic systems which are less stable than acyclic con­ jugated analogues. Absolute antiaromaticity refers to cyclic systems which are less stable than even nonconjugated models (e.g., ethylene). The extensive experimental evidence for absolute antiaromaticity (particularly for cyclobutadiene and cyclo- propenide) is critically examined and judged inconclusive in view of the existence of alternate, plausible rationalizations. Theo­ retical analysis also challenges the reality of absolute antiaromaticity, at least in the monocyclic series. Relative antiaromati­ city is affirmed, but only for the three smallest monocyclic systems (the two mentioned above plus cyclopentadienylium). Previous to 1965, nonaromatic cyclic conjugated systems had sometimes been termed "pseudoaromatic". Included in this category, of course, were molecules having monocyclic systems of 4n-7r electrons. In 1965 Breslow concluded that certain 4n monocycles are actually "antiaromatic", these in­ cluding at least cyclopropenide (C3-) and 1,3-cyclobutadiene (C4), as well as possibly cyclopentadienylium (Cs+).1 The term "antiaromatic" denoted and emphasized destabilizing cyclic conjugation. Two reference systems were considered for more explicitly defining the cyclic conjugation energy. When the reference system is an acyclic conjugated analogue (e.g., 1,3-butadiene for 1,3-cyclobutadiene), the ir electron energy difference represents a relative conjugation energy. Alterna­ tively, the reference system can be an unconjugated one (e.g., two ethylene ir units for cyclobutadiene), thereby giving an absolute conjugation energy. Based upon an array of experi­ mental evidence and encouraged by the fact that certain types of MO calculation lead to predictions of negative absolute conjugation energies for C3- and C4, Breslow was impelled to the conclusion that both of these latter are antiaromatic, not only in the relative, but even in the absolute sense. If this con­ clusion is accepted, a new and basic concept has emerged. The present paper presents a critical evaluation of the key aspects of the proof of absolute antiaromaticity. Experimental Proof Cyclobutadiene. The classic experiment for characterizing the antiaromaticity of a cyclobutadienoid system, devised and executed by the Breslow group, involves measuring the energy change (as £1/2) associated with the transformation of a structure having little cyclobutadienoid character to one having much more and comparing this with the corresponding energy change (£"1/2') for an analogous model reaction wherein cy­ clobutadiene character is absent in both reactant and product.2 The two-electron oxidation 1 «=* 2, e.g., has £1/2 = 0.163 V compared to the E, /2 ' value of -0.113 V for 3 ^ 4. The AE1/2 for 1