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.

Novel Chelating Agents for Zirconium-89-Positron Emission Tomography (PET) Imaging: Synthesis, DFT Calculation, Radiolabeling, and In Vitro and In Vivo Complex Stability.

We report the synthesis and evaluation of novel chelating agents for zirconium-89 (89Zr) with positron emission tomography (PET) imaging applications. New chelating agents NODHA, NOTHA, and NODHA-PY were constructed on 1,4,7-triazacyclononane (TACN) and possess hydroxamic acid or a pyridine ring as an acyclic binding moiety. The new chelating agents were theoretically studied for complexation with Zr(IV). Structures of Zr(IV)-NODHA, Zr(IV)-NOTHA, and Zr(IV)-NODHA-PY were predicted using density functional methods. NODHA was found to form stronger bonds with Zr(IV) when compared to NOTHA and NODHA-PY. The new chelating agents were evaluated for radiolabeling efficiency in binding 89Zr. The corresponding [89Zr]Zr-labeled chelators were evaluated for complex stability in human serum. All new chelating agents rapidly bound to 89Zr in excellent radiolabeling efficiency at room temperature. Among the new [89Zr]Zr-labeled chelators evaluated, [89Zr]Zr-NODHA showed the highest stability in human serum without losing 89Zr, and [89Zr]Zr-NODHA-PY released a considerable amount of 89Zr in human serum. [89Zr]Zr-NODHA, [89Zr]Zr-NODHA-PY, and [89Zr]Zr-DFO were comparatively evaluated for in vivo complex stability by performing biodistribution studies using normal mice. [89Zr]Zr-DFO had the lowest bone uptake at all time points, while [89Zr]Zr-NODHA-PY showed poor stability in mice as evidenced by high bone accumulation at the 24 h time point. [89Zr]Zr-NODHA exhibited better renal clearance but higher bone uptake than [89Zr]Zr-DFO.

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies.

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)3(THF)3] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P21/c space group with a slight compression of the unit cell from 3396.4(2) Å3 to 3373.2(4) Å3 along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT2- ligands with an end-bound {Ca2+(THF)3} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT2- ligands act in a η8-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)3(THF)3] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)3(THF)3] in contrast to [ErKCa(COT)3(THF)3], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical 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.

Stretching [8]cycloparaphenylene with encapsulated potassium cations: structural and theoretical insights into core perturbation upon four-fold reduction and complexation.

The consequences of four-electron addition to [8]cycloparaphenylene ([8]CPP, 1) have been evaluated crystallographically, revealing a significant core deformation. The structural analysis exposes an elliptical distortion observed upon electron transfer, with the deformation parameter (D.P.) increased by 28% in comparison with neutral [8]CPP. The C-C bond length alteration pattern also indicates a quinoidal structural rearrangement upon four-fold reduction. The large internal cavity of [8]CPP4- allows the encapsulation of two {K+(THF)2} cationic moieties with two additional cations bound externally in the solid-state structure of [{K+(THF)2}4([8]CPP4-)]. The experimental structural data have been used as a benchmark for the comprehensive theoretical description of the geometric changes and electronic properties of the highly-charged [8]CPP4- nanohoop in comparison with its neutral parent. While neutral [8]CPP and the [8]CPP2- anion clearly show aromatic behavior of all six-membered rings, subsequent addition of two more electrons completely reverses their aromatic character to afford the highly-antiaromatic [8]CPP4- anion, as evidenced by structural, topological, and magnetic descriptors. The disentanglement of electron transfer from metal binding effects allowed their contributions to the overall core perturbation of the negatively-charged [8]CPP to be revealed. Consequently, the internal coordination of potassium cations is identified as the main driving force for drastic elliptic distortion of the macrocyclic framework upon reduction.

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.

Incorporation of coinage metal-NHC complexes into heptaphosphide clusters.

Cu(i) and Au(i) ions, capped with an N-heterocyclic carbene (NHC), react with (TMS)3P7 (TMS = trimethyl-silyl) to afford an η4-coordinated anion [NHCDippCu-P7(TMS)]- and a neutral trinuclear complex (NHCDippAu)3P7. Protecting the P7 cage with the TMS groups is instrumental in controlling the course of these reactions.

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.

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.

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.

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.

Three to tango requires a site-specific substitution: heterotrimetallic molecular precursors for high-voltage rechargeable batteries.

Design of heterotrimetallic molecules, especially those containing at least two different metals with close atomic numbers, radii, and the same coordination number/environment is a challenging task. This quest is greatly facilitated by having a heterobimetallic parent molecule that features multiple metal sites with only some of those displaying substitutional flexibility. Recently, a unique heterobimetallic complex LiMn2(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) has been introduced as a single-source precursor for the preparation of a popular spinel cathode material, LiMn2O4. Theoretical calculations convincingly predict that in the above trinuclear molecule only one of the Mn sites is sufficiently flexible to be substituted with another 3d transition metal. Following those predictions, two heterotrimetallic complexes, LiMn2-x Co x (thd)5 (x = 1 (1a) and 0.5 (1b)), that represent full and partial substitution, respectively, of Co for Mn in the parent molecule, have been synthesized. X-ray structural elucidation clearly showed that only one transition metal position in the trinuclear molecule contains Co, while the other site remains fully occupied by Mn. A number of techniques have been employed for deciphering the structure and composition of heterotrimetallic compounds. Synchrotron resonant diffraction experiments unambiguously assigned 3d transition metal positions as well as provided a precise "site-specific Mn/Co elemental analysis" in a single crystal, even in an extremely difficult case of severely disordered structure formed by the superposition of two enantiomers. DART mass spectrometry and magnetic measurements clearly confirmed the presence of heterotrimetallic species LiMnCo(thd)5 rather than a statistical mixture of two heterobimetallic LiMn2(thd)5 and LiCo2(thd)5 molecules. Heterometallic precursors 1a and 1b were found to exhibit a clean decomposition yielding phase-pure LiMnCoO4 and LiMn1.5Co0.5O4 spinels, respectively, at the relatively low temperature of 400 °C. The latter oxide represents an important "5V spinel" cathode material for the lithium ion batteries. Transmission electron microscopy confirmed a homogeneous distribution of transition metals in quaternary oxides obtained by pyrolysis of single-source precursors.

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.

A three body problem: a genuine heterotrimetallic molecule vs. a mixture of two parent heterobimetallic molecules.

This work raises a fundamental question about the "real" structure of molecular compounds containing three different metals: whether they consist of genuine heterotrimetallic species or of a mixture of parent heterobimetallic species. Heterotrimetallic complex Li2CoNi(tbaoac)6 (1, tbaoac = tert-butyl acetoacetate) has been designed based on the model tetranuclear structure featuring two transition metal sites in order to be utilized as a molecular precursor for the low-temperature preparation of the LiCo0.5Ni0.5O2 battery cathode material. An investigation of the structure of 1 appeared to be very challenging, since the Co and Ni atoms have very similar atomic numbers, monoisotopic masses, and radii as well as the same oxidation state and coordination number/environment. Using a statistical analysis of heavily overlaid isotope distribution patterns of the [Li2MM'L5]+ (M/M' = Co2, Ni2, and CoNi) ions in DART mass spectra, it was concluded that the reaction product 1 contains both heterotrimetallic and bimetallic species. A structural analogue approach has been applied to obtain Li2MMg(tbaoac)6 (M = Co (2) and Ni (3)) complexes that contain lighter, diamagnetic magnesium in the place of one of the 3d transition metals. X-ray crystallography, mass spectrometry, and NMR spectroscopy unambiguously confirmed the presence of three types of molecules in the reaction mixture that reaches an equilibrium, Li2M2L6 + Li2Mg2L6 ↔ 2Li2MMgL6, upon prolonged reflux in solution. The equilibrium mixture was shown to have a nearly statistical distribution of the three molecules, and this is fully supported by the results of theoretical calculations revealing that the stabilization energies of heterotrimetallic assemblies fall exactly in between those for the parent heterobimetallic species. The LiCo0.5Ni0.5O2 quaternary oxide has been obtained in its phase-pure form by thermal decomposition of heterometallic precursor 1 at temperatures as low as 450 °C. Its chemical composition, structure, morphology, and transition metal distribution have been studied by X-ray and electron diffraction techniques and compositional energy-dispersive X-ray mapping with nanometer resolution. The work clearly illustrates the advantages of heterometallic single-source precursors over the corresponding multi-source precursors.

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.

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.

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.

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.