Homoleptic Rare-Earth-Metal Sandwiches with Dibenzo[a,e]cyclooctatetraene Dianions.

A family of rare-earth complexes [RE(III) = Y, La, Gd, Tb, Dy, and Er] with doubly reduced dibenzo[a,e]cyclooctatetraene (DBCOT) has been synthesized and structurally characterized. X-ray diffraction analysis confirms that all products of the [RE(DBCOT)(THF)4][RE(DBCOT)2] composition consist of the anionic sandwich [RE(DBCOT)2]- and the cationic counterpart [RE(DBCOT)(THF)4]+. Within the sandwich, two elongated π decks are slightly bent toward the metal center (avg. 7.3°) with a rotation angle of 35.9-37.6°. The RE(III) ion is entrapped between the central eight-membered rings of DBCOT2- in a η8 fashion. The trends in the RE-COT bond lengths are consistent with the variations of the ionic radii of RE(III) centers. The 1H NMR spectra of the diamagnetic Y(III) and La(III) analogues illustrate the distinct solution behavior for the cationic and anionic parts in this series. Magnetic measurements for the Dy analogue reveal single-molecule magnetism, which was rationalized by considering the effect of crystal-field splitting for both building units analyzed by electronic structure calculations.

Doubly-Reduced Pentacene in Different Coordination Environments: X-ray Crystallographic and Theoretical Insights into Structural and Electronic Changes.

Chemical reduction of pentacene (C22 H14 , 1) with Group 1 metals ranging from Li to Cs revealed that 1 readily undergoes a two-fold reduction to afford a doubly-reduced 12- anion in THF. With the help of 18-crown-6 ether used as a secondary coordinating agent, five π-complexes of 12- with different alkali metal counterions have been isolated and fully characterized. This series of complexes enables the first evaluation of alkali-metal ion binding patterns and structural changes of the 12- dianion based on the crystallographically confirmed examples. The difference in coordination of the smallest Li+ ion vs. heavier Group 1 congeners has been demonstrated. In addition, the use of benzo-15-crown-5 in the reaction of 1 with Na metal allowed the isolation of the unique solvent-separated ion product with a "naked" dianion, 12- . The detailed structural analyses of the series revealed the C-C bond alteration and core deformation of pentacene upon two-fold reduction and complexation. The negative charge localization at the central six-membered ring of 12- identified by theoretical calculations corroborates with the X-ray crystallographic results. Subsequent in-depth theoretical analysis provided a detailed description of changes in the electronic structure and aromaticity of pentacene upon reduction.

Stepwise Generation of Mono-, Di-, and Triply-Reduced Warped Nanographenes: Charge-Dependent Aromaticity, Surface Nonequivalence, Swing Distortion, and Metal Binding Sites.

The stepwise chemical reduction of a molecular warped nanographene (WNG) having a negatively curved π-surface and defined C80 H30 composition with Cs metal used as the reducing and complexing agent allowed the isolation of three different reduced states with one, two, and three electrons added to its π-conjugated system. This provided a unique series of nanosized carbanions with increasing negative charge for in-depth structural analysis of consequences of controlled electron charging of non-planar nanographenes, using X-ray crystallographic and computational tools. The 3D molecular electrostatic potential (MEP) maps identified the negative charge localization at the central part of the WNG surface where selective coordination of Cs+ ions is confirmed crystallographically. In-depth theoretical investigation revealed a complex response of the WNG to the stepwise electron acquisition. The extended and contorted π-surface of the WNG undergoes subtle swinging distortions that are accompanied by notable changes in the electronic structure and site-dependent aromaticity of the resulting carbanions.

Generation and Reactivity of a NiIII2(µ-1,2-peroxo) Complex.

High-valent transition metal-oxo, -peroxo, and -superoxo complexes are crucial intermediates in both biological and synthetic oxidation of organic substrates, water oxidation, and oxygen reduction. While high-valent oxygenated complexes of Mn, Fe, Co, and Cu are increasingly well-known, high-valent oxygenated Ni complexes are comparatively rarer. Herein we report the isolation of such an unusual high-valent species in a thermally unstable NiIII2(µ-1,2-peroxo) complex, which has been characterized using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV-vis spectroscopies. Reactivity studies show that this complex is stable toward dissociation of oxygen but reacts with simple nucleophiles and electrophiles.

Homolytic Versus Heterolytic Bond Breaking in Functionalized [R-C20 H10 ]+ Systems.

The comprehensive theoretical investigation of stability of functionalized corannulene cations [R-C20 H10 ]+ with respect to two alternative bond-breaking mechanisms, namely, homolytic or radical ([R-C20 H10 ]+ → R• + C20 H10 +• ) and heterolytic or cationic ([R-C20 H10 ]+ → R+ + C20 H10 ), was accomplished. The special focus was on the influence of the nature of R-group on the energetics of the bond cleavage. Detailed study of energetics of both mechanisms has revealed that the systems with small alkyl groups such as methyl tend to undergo bond breaking in accordance with homolytic mechanism. Subsequent elongation of the chain of the R-group resulted in shifting the paradigm, making heterolytic path more energetically favorable. Subsequent analysis of different components of the bonding between R-group and corannulene polyaromatic core helped to shed light on trends observed. In both mechanisms, the covalent contribution was found to be dominating, whereas ionic part contributes ~25-27%. Two leading components of ΔEorb , C20 H10 → R and R → C20 H10 , were identified with NOCV-EDA approach. While the homolytic pathway is best described as R → C20 H10 process, the heterolytic mechanism shows domination of the C20 H10 → R term. Surprisingly, the preparation energy (ΔEprep ) was identified as a key player in stability tendencies found. In other words, the relative stability of corresponding molecular fragments (here R-groups as the corannulene fragment remains the same for all systems) in their cationic or radical forms determine the preference given to a specific bond breaking path and, as consequence, the total stability of target functionalized cations. These conclusions were further confirmed by extending a set of R-groups to conjugated (allyl, phenyl), bulky (iPr, tBu), ß-silyl (CH2 SiH3 , CH2 SiMe3 ), and benzyl (CH2 Ph) groups. © 2019 Wiley Periodicals, Inc.

Coupling of two curved polyaromatic radical-anions: stabilization of dimers by counterions.

In this study, a comprehensive theoretical investigation of both kinetic and thermodynamic stabilities was performed for dimeric dianionic systems (C20H10)22- and (C28H14)22-, neutralized by two alkali metal cations. The influence of the counterions was of primary interest. The impact of the additional/spectator ligand(s) was elucidated by considering adducts with four molecules of diglyme or two molecules of 18-crown-6 ether. Importantly, both types of systems - in the form of contact-ion pair (CIP) and solvent-separated ion pair (SSIP) - were considered. The SSIP set was augmented by the adduct, in which the dimeric dianionic species were neutralized with purely organic cations N(CH3)4+ and P(CH3)4+. Detailed analysis of the bonding revealed that the presence of the counterions made these systems thermodynamically stable. This finding is in sharp contrast with results obtained for isolated (PAH)22- systems, which were previously found to be thermodynamically unstable, but kinetically persistent. The introduction of the alkali metal cations to the system significantly increases the ionic term (ΔEelstat), whereas the repulsive ΔEPauli one was found to be substantially reduced. Considering that the orbital component (ΔEorb) exhibited only a moderate decrease and the preparation energy (ΔEprep) showed no changes, the above-mentioned changes in ΔEelstat and ΔEPauli provided a clear explanation for the increase of the thermodynamic stability of the target species. Importantly, a clear correlation between the size of the alkali metal cation and stability of the target dimeric product was established. Thermodynamic stability of the system rises with a decrease in the size of M+ due to enlargement of the ΔEorb. Evaluated energy barriers (as spin-crossing points between singlet and triplet energy surfaces) were found to be equal to +15.85 kcal mol-1 and +18.5 kcal mol-1 for [(Cs+)2{(C20H10)22-}] and [(Cs+)2{(C28H14)22-}], respectively, which is substantially higher than those calculated for isolated (PAH)22- systems (+10.00 kcal mol-1 for (C20H10)22- and +12.35 kcal mol-1 for (C28H14)22-). Thus, this study identified the presence of counterions as the key factor, which have a dramatic influence on the thermodynamic and kinetic stabilities of the aimed dianionic dimeric systems, which are formed by two curved polyaromatic monoanion-radicals.

Heterotrimetallic Mixed-Valent Molecular Precursors Containing Periodic Table Neighbors: Assignment of Metal Positions and Oxidation States.

A known trinuclear structure was used to design the heterobimetallic mixed-valent, mixed-ligand molecule [CoII (hfac)3 -Na-CoIII (acac)3 ] (1). This was used as a template structure to develop heterotrimetallic molecules [CoII (hfac)3 -Na-FeIII (acac)3 ] (2) and [NiII (hfac)3 -Na-CoIII (acac)3 ] (3) via isovalent site-specific substitution at either of the cobalt positions. Diffraction methods, synchrotron resonant diffraction, and multiple-wavelength anomalous diffraction were applied beyond simple structural investigation to provide an unambiguous assignment of the positions and oxidation states for the periodic table neighbors in the heterometallic assemblies. Molecules of 2 and 3 are true heterotrimetallic rather than a statistical mixture of two heterobimetallic counterparts. Trinuclear platform 1 exhibits flexibility in accommodating a variety of di- and trivalent metals, which can be further utilized in the design of molecular precursors for the NaMM'O4 functional oxide materials.

Record Alkali Metal Intercalation by Highly Charged Corannulene.

The need for advanced energy storage technologies demands the development of new functional materials. Novel carbon-rich and carbon-based materials of different structural topologies attract significant attention in this regard. Attractive systems include a unique class of bowl-shaped polycyclic aromatic hydrocarbons that map onto fullerene surfaces and are thus often referred to as fullerene fragments, buckybowls, or π-bowls. Importantly, carbon bowls are able to acquire multiple electrons in stepwise reduction reactions producing sets of successively reduced carbanions. The resulting negatively charged π-bowls exhibit unique supramolecular assembly and metal intercalation patterns that only recently have begun to be uncovered. First, we have resolved the long-standing mystery behind the supramolecular structure formed by a highly reduced fullerene fragment called corannulene (C20H104-) with multiple lithium ions, using X-ray crystallography coupled with NMR spectroscopy and theoretical calculations. This work provided a new paradigm for lithium ion intercalation between the curved carbon π-surfaces and facilitated understanding of the lithium ion storage mechanism in carbonaceous matrices. Next, we have initiated a new research direction, an investigation of the mixed alkali metal reduction reactions using bowl-shaped corannulene as a remarkable multielectron reservoir and unique ligand with open convex and concave π-surfaces. As a result, we have revealed the cooperative effect of lithium with heavier Group 1 metals in reduction and self-assembly processes of corannulene. Moreover, we have discovered a new class of organometallic supramolecules having heterometallic cores with high nuclearity and charge such as Li3M36+ and LiM56+ (M = K, Rb, and Cs) sandwiched between two tetrareduced corannulene decks. The resulting triple-decker supramolecular assemblies, fully characterized by X-ray diffraction and spectroscopic methods, were found to exhibit a record ability of the highly charged corannulene π-surfaces to be fully engaged in intercalation of multiple metal ions. Based on this unique ability, curved π-ligands with extended carbon frameworks are expected to show remarkable potential for alkali metal storage compared to flat polycyclic arenes. Notably, a previously unseen mode of internal lithium binding revealed in the heterobimetallic sandwiches is accompanied by unprecedented negative shifts (up to -25 ppm) in 7Li NMR spectra. Based on in-depth analysis of NMR data, augmented by DFT calculations, we have rationalized the observed experimental trends and proposed the mechanism of stepwise alkali metal substitution reactions. Furthermore, we have correlated the origin of the record 7Li NMR shifts with unique electronic structures of these novel supramolecular aggregates. Herein we present comprehensive analysis of unusual structural and electronic features of remarkable heterometallic self-assemblies formed by tetrareduced corannulene, using a wealth of our recent experimental and computational results. This work uncovers unique potential of highly negatively charged bowl-shaped π-ligands for new supramolecular chemistry and materials chemistry applications.

Mono-reduced Corannulene: To Couple and Not to Couple in One Crystal.

One-electron reduction of corannulene, C20 H10 , with Li metal in diglyme resulted in crystallization of [{Li+ (diglyme)2 }4 (C20 H10 .- )2 (C20 H10 -C20 H10 )2- ] (1), as revealed by single-crystal X-ray diffraction. This hybrid product contains two corannulene monoanion-radicals along with a dianionic dimer, crystallized with four Li+ ions wrapped by diglyme molecules. The dimeric (C20 H10 -C20 H10 )2- anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C20 H10 .- . The C-C bond length between the two C20 H10 .- bowls of 1.588(5) Šis consistent with the single σ-bond character of the linker. The trans-disposition of two bowls in the centrosymmetric (C20 H10 -C20 H10 )2- dimer is observed with the torsion angle around the central C-C bond of 180°. Comprehensive theoretical analysis of formation/decomposition processes of the dimeric dianion has been carried out in order to evaluate the nature of bonding and energetics of the C20 H10 .- coupling. It is found that such σ-bonded dimers are thermodynamically unstable due to large preparation energy and repulsive Pauli component of the bonding, but kinetically persistent due to a high energy barrier provided by the existing spin-crossing point.

Modulating stability of functionalized fullerene cations [R-C60 ]+ with the nature of R-group.

In this study, the first comprehensive theoretical investigation of the stability of functionalized fullerene-based cations [R-C60 ]+ and its relationship with the nature of the attached R-group was performed. C60 -Fullerene core was functionalized with an alkyl group of different length (R = (CH2 )n CH3 , where n = 0-9). This set was further complemented by bulky isopropyl and tert-butyl and conjugated phenyl groups. A detailed study of the relative stability of target cationic species was accompanied by in-depth investigation of their electronic structure and aromaticity using a large set of descriptors of different nature. The stability of target species was considered with respect to two alternative and competing mechanisms of bond breaking, namely, heterolytic ([R-C60 ]+ → R+ + C60 ) and homolytic ([R-C60 ]+ → R• + C60 +• ) ones. The transformation of strained sp2 -carbon atom in unperturbed C60 -fullerene to nonconstrained tetrahedral sp3 -type in functionalized derivatives was found to be the driving force for the formation of its functionalized cations. In spite of the fact that all systems under consideration were found to be corresponding to local minima on corresponding potential energy surfaces, the functionalization of C60 -core with the smallest and simplest methyl group resulted in most stable compound, as evaluated by bonding energy between R+ and fullerene fragment (in the light of both mechanisms). Subsequent elongation of the alkyl chain or introducing bulky groups led to notable decrease of the bonding energy and, as consequence, of the stability within the framework of heterolytic bond cleavage, whereas homolytic pathway assumes opposite-slight increase of stability along with lengthening of the R-group. The orbital interaction (ΔEorb ) was identified as the main driving force for these trends. In general, the homolytic path was found to be dominating for small-length R-groups such as those with n = 0 and 1. At n = 2, heterolytic and homolytic pathways are equally probable (the difference in corresponding bonding energies is about 1 kcal/mol). However, when the alkyl chain becomes longer (n = 3-9), the cationic bond cleavage appears as the most energetically favorable. © 2018 Wiley Periodicals, Inc.

Fusing a Planar Group to a π-Bowl: Electronic and Molecular Structure, Aromaticity and Solid-State Packing of Naphthocorannulene and its Anions.

Molecular and electronic structure, reduction electron transfer and coordination abilities of a polycyclic aromatic hydrocarbon (PAH) having a planar naphtho-group fused to the corannulene bowl have been investigated for the first time using a combination of theoretical and experimental tools. A direct comparison of naphtho[2,3-a]corannulene (C28 H14 , 1) with parent corannulene (C20 H10 , 2) revealed the effect of framework topology change on electronic properties and aromaticity of 1. The presence of two reduction steps for 1 was predicted theoretically and confirmed experimentally. Two reversible one-electron reduction processes with the formal reduction potentials at -2.30 and -2.77 V versus Fc+/0 were detected by cyclic voltammetry (CV) measurements, demonstrating accessibility of the corresponding mono- and dianionic states of 1. The products of the singly and doubly reduced napththocorannulene were prepared using chemical reduction with Group 1 metals and isolated as sodium and rubidium salts. Their X-ray diffraction study revealed the formation of "naked" mono- and dianions crystallized as solvent-separated ion products with one or two sodium cations as [Na+ (18-crown-6)(THF)2 ][C28 H14- ] and [Na+ (18-crown-6)(THF)2 ]2 [C28 H142- ] (3⋅THF and 4⋅THF, respectively). The dianion of 1 was also isolated as a contact-ion complex with two rubidium countercations, [{Rb+ (18-crown-6)}2 (C28 H142- )] (5⋅THF). The structural consequences of adding one and two electrons to the carbon framework of 1 are compared for 3, 4 and 5. Changes in aromaticity and charge distribution stemming from the stepwise electron acquisition are discussed based on DFT computational study.

Comprehensive Theoretical Study of Interactions between Ag+ and Polycyclic Aromatic Hydrocarbons.

The first comprehensive and systematic theoretical exploration of the bonding nature and energetics of the interactions between Ag(I) cation and a wide set of π-ligands was accomplished. This set ranges from simple ethylene and aromatic benzene to planar and curved polyaromatic molecules and to closed-cage C60 -fullerene. Simultaneous application of two energy decomposition schemes based on different ideas, namely, NBO-NEDA and EDA-NOCV, allowed shedding light on the nature of the bonding and its energetics. Importantly, our results unambiguously indicate that reliable results can be obtained only if using more than one theoretical approach. All methods clearly revealed the importance and even domination of the ionic contribution of the bonding in all adducts, except for those of C60 -fullerene, in which the covalent component was found to be the largest. Subsequent decomposition of the orbital term onto components showed that it consists of two major parts: (i) ligand-to-metal (π(C=C)→s(Ag), L→M) and (ii) metal-to-ligand (M→L) terms, with significant domination of the former. Interestingly, while the L→M component is essentially the same for all systems considered, the nature of the M→L one depends on the coordination site of the polycyclic aromatic hydrocarbons (PAH). In most of adducts, the M→L can be described as dxy (Ag)→π* (C=C) donation, whereas for systems [Ag-spoke-C12 H8 ]+ and [Ag-spoke-C20 H10 ]+ it corresponds to the dz2 (Ag)→π* (C=C) type of interaction. As a result, the coordination mode in such complexes is switched from η2 -type to η1 . Thus, the nature of the bonding, its energetics and even coordination mode in adducts of unsaturated hydrocarbons with late transition metal cations should be considered as a function of many components, which primarily includes the topology and aromaticity of the (poly)aromatic molecules.

Power of Three: Incremental Increase in the Ligand Field Strength of N-Alkylated 2,2'-Biimidazoles Leads to Spin Crossover in Homoleptic Tris-Chelated Fe(II) Complexes.

Homoleptic complexes [Fe(L n)]X2 (L1 = 1,1'-(α,α'- o-xylyl)-2,2'-biimidazole, L2 = 1,1'-(α,α'-3,4-dibromo- o-xylyl)-2,2'-biimidazole, L3 = 1,1'-(α,α'-2,5-dimethoxy- o-xylyl)-2,2'-biimidazole; X = BF4- or ClO4-) were synthesized by direct reactions of the Fe(II) precursor salts and bidentate ligands L1, L2, or L3. All mononuclear complexes undergo gradual temperature-driven spin-crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Complexes with ligands L1 and L2 synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand L3, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe(L3)3](BF4)2 and ∼1:1 for [Fe(L3)3](ClO4)2. Complexes with L1 can also be recrystallized from ethanol/dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with L3. The differences in magnetic behavior have been traced back to peculiarities of molecular packing observed in the corresponding crystal structures. Density-functional theoretical calculations provide justification to the SCO behavior of these complexes, as compared to the HS-only behavior observed for the parent [Fe(bim)3]2+ complex with nonalkylated 2,2'-biimidazole (bim).

Site-Directed Dimerization of Bowl-Shaped Radical Anions to Form a σ-Bonded Dibenzocorannulene Dimer.

Designed site-directed dimerization of the monoanion radicals of a π-bowl in the solid state is reported. Dibenzo[a,g]corannulene (C28 H14 ) was selected based on the asymmetry of the charge/spin localization in the C28 H14.- anion. Controlled one-electron reduction of C28 H14 with Cs metal in diglyme resulted in crystallization of a new dimer, [{Cs+ (diglyme)}2 (C28 H14 -C28 H14 )2- ] (1), as revealed by single crystal X-ray diffraction study performed in a broad range of temperatures. The C-C bond length between two C28 H14.- bowls (1.560(8) Å) measured at -143 °C does not significantly change upon heating of the crystal to +67 °C. The single σ-bond character of the C-C linker is confirmed by calculations. The trans-disposition of two bowls in 1 is observed with the torsion angles around the central C-C bond of 172.3(5)° and 173.5(5)°. A systematic theoretical evaluation of dimerization pathways of C28 H14.- radicals confirmed that the trans-isomer found in 1 is energetically favored.

A Naphtho-p-quinodimethane Exhibiting Baird's (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence.

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T1 state with a phosphorescence 3(7a)* lifetime of τP = 395 µs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

Position Assignment and Oxidation State Recognition of Fe and Co Centers in Heterometallic Mixed-Valent Molecular Precursors for the Low-Temperature Preparation of Target Spinel Oxide Materials.

A series of mixed-valent, heterometallic (mixed-transition metal) diketonates that can be utilized as prospective volatile single-source precursors for the low-temperature preparation of MxM'3-xO4 spinel oxide materials is reported. Three iron-cobalt complexes with Fe/Co ratios of 1:1, 1:2, and 2:1 were synthesized by several methods using both solid-state and solution reactions. On the basis of nearly quantitative reaction yields, elemental analyses, and comparison of metal-oxygen bonds with those in homometallic analogues, heterometallic compounds were formulated as [FeIII(acac)3][CoII(hfac)2] (1), [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2), and [FeII(hfac)2][FeIII(acac)3][CoII(hfac)2] (3). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated CoII/FeII centers chelated by two hexafluoroacetylacetonate (hfac) ligands maintain bridging interactions with oxygen atoms of acetylacetonate (acac) groups that chelate the neighboring FeIII metal ion. Preliminary assignment of Fe and Co positions/oxidation states in 1-3 drawn from X-ray structural investigation was corroborated by a number of complementary techniques. Single-crystal resonant synchrotron diffraction and neutron diffraction experiments unambiguously confirmed the location of Fe and Co sites in the molecules of dinuclear (1) and trinuclear (2) complexes, respectively. Direct analysis in real time mass spectrometry revealed the presence of FeIII- and CoII-based fragments in the gas phase upon evaporation of precursors 1 and 2 as well as of FeIII, FeII, and CoII species for complex 3. Theoretical investigation of two possible "valent isomers", [FeIII(acac)3][CoII(hfac)2] (1) and [CoIII(acac)3][FeII(hfac)2] (1'), provided an additional support for the metal site/oxidation state assignment giving a preference of 6.48 kcal/mol for the experimentally observed molecule 1. Magnetic susceptibility measurements data are in agreement with the presence of high-spin FeIII and CoII magnetic centers with weak anti-ferromagnetic coupling between those in molecules of 1 and 2. Highly volatile heterometallic complexes 1-3 were found to act as effective single-source precursors for the low-temperature preparation of iron-cobalt spinel oxides FexCo3-xO4 known as important materials for diverse energy-related applications.

Stepwise deprotonation of sumanene: electronic structures, energetics and aromaticity alterations.

The first comprehensive theoretical investigation of structural, energetic, and electronic changes in a sumanene skeleton, C21H12, upon a step-wise deprotonation process is performed. This study is complemented by a detailed consideration of aromaticity in target bowl-shaped systems, including neutral sumanene and its three deprotonated anions, namely C21H111-, C21H102-, and C21H93-. In order to obtain the most reliable and method-independent characteristics, a set of aromatic descriptors of different nature has been applied. It included structure-based HOMA, topological descriptors PDI and FLU, as well as magnetic NICS and ACID. The calculation results reveal that the neutral sumanene can be best described as mechanically bent triphenylene, in which π-conjugation is mostly localized over three peripheral 6-membered rings. Sequential deprotonation changed the system from the localized mono-anionic to semi-localized di-anionic, and eventually to the fully delocalized tri-anionic sumanenyl species. Structural changes, namely, bond equalization upon the deprotonation process, are in excellent agreement with alterations observed in electronic structures and aromaticity. Deprotonation results in a significant reduction of the barrier for a bowl-to-bowl transition only in the tri-anionic sumanenyl system, whereas the first and the second deprotonation steps show no notable effect. This clearly indicates that only complete aromatization of the sumanene core in C21H93- leads to a substantial increase of bowl flexibility.

Double Concave Cesium Encapsulation by Two Charged Sumanenyl Bowls.

The controlled reaction of Na and Cs, two alkali metals of different ionic sizes and binding abilities, with sumanene (C21 H12 ) affords a novel type of organometallic sandwich [Cs(C21 H11- )2 ]- , which crystallized as a solvent-separated ion pair with a [Na(18-crown-6)(THF)2 ]+ cation (where THF=tetrahydrofuran). The unprecedented double concave encapsulation of a metal ion by two bowl-shaped sumanenyl anions in [Cs(C21 H11- )2 ]- was revealed crystallographically. Evaluation of bonding and energetics of the remarkable product was accomplished computationally (B2PLYP-D/TZVP/ZORA), providing insights into its formation.

Aromatic stabilization of functionalized corannulene cations.

The first comprehensive theoretical investigation of aromaticity in functionalized corannulene cations of general formula [CH3-C20H10](+) was accomplished. The experimentally known system [CH3-hub-C20H10](+) was augmented by two other possible isomers, namely, rim- and spoke-ones. Changes in aromaticity, when going from neutral corannulene to its functionalized cations, were monitored with the help of descriptors of different nature such as structure-based HOMA, topological PDI and FLU, and magnetic NICS. A highly efficient tool for analysis and visualization of delocalization and conjugation named ACID was also utilized. In the final step, a complete set of (1)H and (13)C chemical shifts was calculated and compared with the available experimental data. Conservation of aromaticity of 6-membered rings along with vanishing anti-aromatic character of central 5-membered rings was found to be the main reason for the exceptional stability of the hub-isomer. At the same time, functionalization of the corannulene moiety at the rim- or spoke-site resulted in dramatic elimination of aromaticity of 6-membered rings, whereas anti-aromatic character of the central ring remained. Altogether, it led to much lower stability of these isomers in comparison with that of the hub-one.

Oligomerization of N-Heterocyclic Silylene into Zwitterionic Silenes.

N-Heterocyclic carbenes (NHC's) are known to serve as efficient substrates for the stabilization of various transient species possessing low-valent Group 14 elements and for the generation of double E=C bonds. Herein, we report that the thermal tri- and tetramerizations of pyridoannulated silylene 1 lead to the formation of remarkably stable silenes 2 and 3 featuring zwitterionic distribution of electron density. Co-oligomerization of 1 and its germanium analogue gives a related tetrameric product 4 containing low-valent germanium atom stabilized by binding with the partial carbene-character C atom. Bonding situations in 2-4 are best described as silene or germene with the significant zwitterionic distribution of electron density. The singlet diradical electronic state of 2 is 10 kcal mol-1 higher than the ground state configuration.