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Polycyclic aromatic hydrocarbons (PAHs) have found strong interest for their electronic properties and as model systems for graphene. While PAHs have been studied intensively as single units, here PAHs were constructed in ladder-type arrangements using cross-conjugated fulvalene and dithiafulvalene motifs as connecting units and moving forward a convenient synthetic approach for dimerizing (thio)ketones into olefins by the action of Lawesson's reagent. Some of the PAHs can also be regarded as "super-extended" tetrathiafulvalenes (TTFs) with some of the largest cores ever explored, being multi-redox systems that exhibit both reversible oxidations and reductions. Concomitant absorption redshifts were observed when expanding the ladder-type structures from one to two to three indenofluorene units, and optical and electrochemical HOMO-LUMO gaps were found to correlate linearly. Various conformations (and solid-state packing arrangements) were studied by X-ray crystallography and computations.

The first example of a two-state (neutral and reduced), stable electron-accepting material and its radical anion is presented. FV-PDI, generated from cyclocarbonylation and then a carbonyl coupling reaction, shows a largely degenerate LUMO of -4.38 eV based on the delocalization of π-electrons across the whole molecular skeleton through a fulvalene bridge. The stability and electron affinity allow spontaneous electron transfer to afford a stable radical anion. Spectroscopic characterization and structural elucidation showed that the radical anion [FV-PDI].- has remarkable stability and near-infrared absorptions extending to 1200 nm. Single-crystal X-ray diffraction analyses revealed significant changes in the molecular shape and packing arrangement of the formed radical anion. The central C-C bond linking the two PDI halves is lengthened from approximately 1.33 to 1.43 A, and the alternating arrangement of positively and negatively charged units favor the stable charge-transfer complex.

The highly dipolar fulvalene 1 reacts with pentacarbonyl iron(0) and hexacarbonyl tungsten(0) complexes under irradiation with UV light. The cyclopentadienyl complexes 2 and 3 formed are analysed by means of NMR and IR spectroscopy as well as X-ray analysis. They are the first zwitterionic carbonyl complexes of fulvenes or fulvalenes. In the case of iron, the dinuclear complex 2 consisting of a dicarbonyliron(+I)-tetracarbonylferrat(–I) moiety is formed, whereas in the case of tungsten, the mononuclear tricarbonyl tungsten(0) complex 3 is obtained.

A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu(2) in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on Delta H-storage. Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1',3,3'-tetra-tert-butyl (4), 1,2,2',3'-tetraphenyl (9), diiron (28), diosmium (24), mixed iron-ruthenium (27), dimolybdenum (29), and di-tungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3-SiO2 as a good candidate, although catalyst decomposition remains a challenge.

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.

In the currently intensifying quest to harness solar energy for the powering of our planet, most efforts are centered around photoinduced generic charge separation, such as in photovoltaics, water splitting, other small molecule activation, and biologically inspired photosynthetic systems. In contrast, direct collection of heat from sunlight has received much less diversified attention, its bulk devoted to the development of concentrating solar thermal power plants, in which mirrors are used to focus the sun beam on an appropriate heat transfer material. An attractive alternative strategy would be to trap solar energy in the form of chemical bonds, ideally through the photoconversion of a suitable molecule to a higher energy isomer, which, in turn, would release the stored energy by thermal reversal. Such a system would encompass the essential elements of a rechargeable heat battery, with its inherent advantages of storage, transportability, and use on demand. The underlying concept has been explored extensively with organic molecules (such as the norbornadiene-quadricyclane cycle), often in the context of developing photoswitches. On the other hand, organometallic complexes have remained relatively obscure in this capacity, despite a number of advantages, including expanded structural tunability and generally favorable electronic absorption regimes. A highly promising organometallic system is the previously reported, robust photo-thermal fulvalene (Fv) diruthenium couple 1 {l_reversible} 2 (Scheme 1). However, although reversible and moderately efficient, lack of a full, detailed atom-scale understanding of its key conversion and storage mechanisms have limited our ability to improve on its performance or identify optimal variants, such as substituents on the Fv, ligands other than CO, and alternative metals. Here we present a theoretical investigation, in conjunction with corroborating experiments, of the mechanism for the heat releasing step of 2 {yields} 1 and its Fe (4) and Os (6) relatives. The results of the combined study has enabled a rigorous interpretation of earlier and new experimental measurements and paint a surprising picture. First-principles calculations were employed based on spin unrestricted density functional theory (DFT) with a non-empirical gradient corrected exchange-correlation functional. Ultrasoft pseudopotentials were used to describe the valence-core interactions of electrons, including scalar relativistic effects of the core. Wavefunctions and charge densities were expanded in plane waves with kinetic energies up to 25 and 200 Rydberg, respectively. Reaction pathways were delineated with the string method, as implemented within the Car-Parrinello approach. This method allows for the efficient determination of the minimum energy path (MEP) of atomistic transitions and thus also saddle points (transition states, TSs), which are the energy maxima along the MEP. All geometries were optimized until all forces on the atoms were less than 0.02 eV/{angstrom}. The calculated structures of 1 and 2 were in good agreement with their experimental counterparts.

Recent improvements in experimental methods have made it possible to generate radicals and radical ions derived from simple 1,3-dienes, so that their e.s.r. spectra can be observed in fluid solution. Photolysis of various cyclopentadienylmetallic compounds, or even of pentamethylcyclopentadiene itself, serves as a source of the corresponding cyclopentadienyl radicals, and the former method can be extended to the preparation of the fulvalene and pentalene radical anions. Cyclobutadiene radical cations are formed when the complexes formed between dimerised dialkylalkynes and aluminium chloride are photolysed. Di-t-butyl-ethyne may also undergo a double nucleophilic rearrangement when it is treated with aluminium chloride, resulting in the formation of the hexamethylbutadiene radical cation, which can also be prepared from hexamethylbutadiene itself. Radical anions and cations can be regarded as being the conjugate bases and acids, respectively, of neutral radicals, and this principle has been exploited in the generation of cyclopentadiene radical cations from the photolysis of cyclopentadienes in acid solution.

Appropriate bridging ligands allow for the preparation of bimetallic titanium and zirconium precursors to transition-metal-containing polymers using the formally trianionic P(CH2NArR)3 ligand (where ArR = Ph, and 3,5-Me2C6H3 for a and b, respectively). Lithiation of the known amidophosphane ligand precursors P(CH2NHArR)3 (1a,b) with 3 equiv. of nBuLi cleanly provided the diethyl ether adducts P(CH2NArR)3Li3(OEt2)1.5 (2a,b). The THF adduct P(CH2NPh)3Li3(THF)5 (2a-THF) was characterized by X-ray crystallography. Protonolysis reactions were used to prepare the mononuclear titanium complexes P(CH2NArR)3TiOC6H4tBu (4a,b) and dinuclear species [P(CH2NPh)3TiNMe2]2-μ-4,4′{O[3,3′,5,5′-(C6H2Me2)2]O} (5a), [P(CH2NPh)3Ti]2(μ-O) (6a), and P(CH2N-3,5-Me2C6H3)3Ti-μ-O-Ti(NMe2)3 (7b), from P(CH2NArR)3TiNMe2. The reactions of 1a,b with Zr(NEt2)4 afforded metal complexes of the type P(CH2NArR)3ZrNEt2 (8a,b). The chloride complexes P(CH2NArR)3ZrCl(THF) (9a,b) were prepared by treatment of HNEt3Cl with 1 or 2. The complexes 9a,b reacted with cyclopentadienyllithium (C5H5Li) to produce P(CH2NArR)3ZrCp (10a,b), which were also prepared by the reactions of the lithium salts 2a,b with CpZrCl3. The dilithium salt of the fulvalene dianion (Li2C10H8) reacts with 9a,b to produce the dinuclear complexes trans-[P(CH2NArR)3Zr]2(μ-η5:η5-C10H8) (10a,b), respectively. The facile preparation of transition-metal-containing polymers from the bridged dinuclear complexes was illustrated by the reaction of 6a with half an equivalent of [Rh(CO)2(μ-Cl)]2, which precipitated polymeric Cl(CO)Rh[P(CH2NPh)3Ti]2(μ-O). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

For the first time the unstable titanocene(II) has been directly synthesized by the Wilkinson metallocene approach, namely the interaction of a THF-soluble form of titanium(II) chloride with two equivalents of cyclopentadienylsodium in THF solution at 0°–25 °C. Because of the transient existence of the titanocene(II) thereby obtained, it could only be chemically trapped in high yield as 1,1-bis(cyclopentadienyl)-2,3,4,5-tetraphenyltitanacyclopentadiene by two equivalents of diphenylacetylene, if the acetylene was added at 25 °C, without removal of the by-product LiCl and NaCl. If the addition of the acetylene was delayed, in order to filter off the LiCl and NaCl from the reaction mixture, then no trace of the titanacyclopentadiene derivative was found upon hydrolytic workup. Instead, a significant portion of the acetylene was found to have undergone hydrotitanation. This finding is clear evidence that the titanocene(II) had undergone a precedented rearrangement to a known dimer having the structure of a titanocene(III) hydride with a fulvalene bridge between the titanium centers. We suggest that the LiCl and NaCl present in the unfiltered reaction mixture form a dichloro complex with titanocene(II) and thereby retard its dimerizing rearrangement to the titanocene(III) hydride. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Die Umsetzungen von [Cp2TiCl2] (Cp = C5H5) mit tBu2P—P(Li)—PtBu2 oder mit tBu2P—P(SiMe3)Li fuhren zur Bildung von TiIII-Komplexen wie [Cp2Ti{μ-Cl}2Li(THF)2] (monoklin C2/c) oder [(Cp2Ti{μ-Cl})2] und in Gegenwart von PMe3 auch zu [Cp2TiCl(PMe3)]. Gleichzeitig dimerisieren die Phosphanreste zu (tBu2P)2P—P(PtBu2)2 bzw. (tBu2P—PSiMe3)2. Zusammen mit (tBu2P—PSiMe3)2 bilden sich Produkte, die auf das intermediare Auftreten von tBu2P—P hindeuten. Die Umsetzung von [Cp2TiCl2] bzw. von [Cp(Cp*)TiCl2] (Cp* = C5Me5) mit nBuLi in Gegenwart von tBu2P—P(SiMe3)2 liefert unerwartet die TiIII-Fulvalen-Komplexe [{Ti(η5-C5H5)(μ-Cl)}2{μ-(η5-C5H4-η5-C5H4)}] und [{Ti(η5-C5Me5)(μ-Cl)}2{μ-(η5-C5H4-η5-C5H4)}] (triklin, P1¯). Oxidative Dimerisation of Phosphanes and Indications for the Intermediate Presence of tBu2P—P in Reactions of [(η5-C5H5)2TiCl2] and [(η5-C5H5)(η5-C5Me5)TiCl2] with tBu2P—P(SiMe3)Li and tBu2P—P(Li)—PtBu2. Formation of Titanium(III) Fulvalene Complexes The reactions of [Cp2TiCl2] (Cp = C5H5) with tBu2P—P(Li)—PtBu2 or with tBu2P—P(SiMe3)Li lead to TiIII complexes as [Cp2Ti{μ-Cl}2Li(THF)2] (monoclinic C2/c) or [Cp2Ti{μ-Cl}]2 and in the presence of PMe3 also to [Cp2TiCl(PMe3)]. Simultaneously, the phosphane moieties dimerize yielding (tBu2P)2P—P(PtBu2)2 or (tBu2P—PSiMe3)2, respectively. Together with (tBu2P—PSiMe3)2 some products appear which suggest the intermediate presence of tBu2P—P. The reactions of [Cp2TiCl2] or of [Cp(Cp*)TiCl2] (Cp* = C5Me5) with nBuLi in the presence of tBu2P—P(SiMe3)2 unexpectedly yield the TiIII fulvalene complexes [{Ti(η5-C5H5)(μ-Cl)}2{μ-(η5-C5H4-η5-C5H4)}] and [{Ti(η5-C5Me5)(μ-Cl)}2{μ-(η5-C5H4-η5-C5H4)}] (triclinic, P1¯).

A First Example of a Porphyrinoid Fulvalene: Synthesis, Structure, ESR and Electrochemical Investigations The macrocyclic tetraepoxy-1H-[21]annulen-1-one 4 can by synthesized by the cyclizing Wittig reaction of 5,5′-carbonylbis[furan-2-carboxaldehyde] (11) and the bisphosphonium salt 12, obtained from 2,2′-bifuran-5,5′-dicarboxaldehyde (13). According to an X-ray structure analysis, the annulenone 4 is not planar in the crystal; the 1H-NMR spectra of 4 reveal an averaged planarity with respect to the NMR timescale. The McMurry reaction of 4 yields fulvalene 3 in 43% yield as the most expanded fulvalene hithertoo known. The X-ray structure analyses of 3 surprising establishes a ‘syn’-orientation of the two rings with respect to the central CC bond, thus forming a basket-like molecule. The 1H-NMR spectrum confirms the averaged planarity of both macrocycles in 3. CV and spectroelectrochemical measurements of 3 suggest a reversible two-electron reduction producing dianion 15 with two aromatic, anionic 5a,15a-didehydro-10H-21,22,23,24-tetraoxa-5a,15a-dihomocorrole (= tetraoxa[22]porphyrin(2.1.2.0)) ring systems containing 22π electrons each. The formation of 15 can also be achieved chemically by reaction of 3 with metallic potassium. The dication 16 of 3 may be antiaromatic, but the exact electronic structure is dubious. ESR and ENDOR investigations on the radical cation and the radical anion of 3 indicate that the free electron is delocalized in the entire molecule.

Cyclic voltammetry and spectroelectrochemistry of an electropolymerized film of [Fe2(C10H8)(C10H7CClCHCHO)] (1) were carried out in acetonitrile with Bu4NPF6, Bu4NBF4, Bu4NClO4 or Bu4NHSO4 to determine how the two parameters concerning the stability of the mixed valence state, comproportionation constant, Kc, and mixing coefficient, α, depend on the type of counterions. The difference in electrolyte anions causes slight changes in α exhibiting the magnitude of delocalization of charge between two metal centers. However, significant changes in Kc, suggest that the ion-pairing and electrostatic repulsion are main factors that alter the electrochemical stability of the mixed valence state.

Transition-metal sandwich complexes incorporating bifacially coordinated C 2 B 3 planar carborane rings form a large and varied family of generally air-stable, highly robust compounds that feature multidecker stacking. As a group, these complexes offer a number of advantages that are potentially of interest in the development of new electronic, magnetic and/or optical materials. They are remarkably versatile, accommodating a wide range of metals and organic substituents ; they are soluble in organic solvents and are typically resistant to air and moisture ; they can be reversibly oxidized and reduced ; in many cases they are paramagnetic, exhibiting substantial electron delocalization of the unpaired electrons between metal centers. Moreover, the small carborane starting materials can now be prepared in large (ca 100 g) quantities, making the complexes readily accessible. A brief overview of this area is presented with emphasis on the systematic utilization of metallacarborane sandwich complexes in the construction of large multimetallic systems and studies of their electronic and molecular structures.

Halogen—Carbon—Sulfur Compounds: Syntheses with 2-Methylthio-4,5-bis(trifluoromethyl)-1,3-dithiolium Trifluoromethanesulfonate By reactions of the strongly electrophilic 2-methylthio-4,5-bis-(trifluoromethyl)-1,3-dithiol-2-ylium trifluoromethanesulfonate (6) with nucleophiles like secondary amines, phenoles and active methylene compounds the new 2-amino-1,3-dithiolium salts 11 a—e and 1,3-dithiol-2-ylidene compounds 5–10, 12 were obtained. The 1,3-dithiolium salts 14 were prepared in two different ways. Reaction with PPh3 yields the phosphonium salt 16, from which the ethylidene compound 20 was obtained via the intermediate phosphorane 18. The crystal structures of 7, 16 and of the fulvalene 19 were determined.

The first example of fuluvalene derivative containing two monocyclic large-membered rings, 1-(5,10-dimethyl-6,7,8,9-tetradehydrocyclotridecen-1-ylidene)-7,12-dimethyl-8,9,10,11-tetradehydrocyclopentadecene, has been synthesized. Examination of 1H and 13C NMR spectra indicates that the tridecapentadecafulvalene derivative shows no ring current effect but polyolefinic character.

A series of fulvalene ylides was synthesised. Only weak aromatic effects were found across the ylide bond. Their chemistry includes the first example of a dichotomy in phosphonium salt/ylide hydrolysis, interesting dynamic nmrs of the adducts with dimethyl acetylenedicarboxylate and a new synthesis of azulenes.

A new lithium-induced alkyne cyclization reaction of cyclotriynes leads to the formation of a fulvalene dianion derivative in high yield, as confirmed by X-ray crystallography. This unprecedented reaction may be useful for the synthesis of a variety of fulvalene dianons, and these may be used in the synthesis of extended metallocene complexes

The preparation, X-ray crystal structures, conductivity and optical properties of the salts, [ttf]3[W6O19]1, [ttf]3[Mo6O19]2(ttf = tetrathiafulvalene) and [tmtsf]3[W6O19]·2dmf 3(tmtsf = tetramethyltetraselenafulvalene) are reported. Crystal data: 1, triclinic, space group P, a= 9.965(3), b= 10.503(3), c= 10.634(3)A, α= 71.93(2), β= 78.63(2), γ= 63.38(3), Z= 1, R= 0.020; 2, triclinic, space group P, a= 9.942(3), b= 10.417(3), c= 10.601(3), α= 72.33(2), β= 78.77(2), γ= 63.52(3)°, Z= 1, R= 0.030; 3, monoclinic, space group P21/c, a= 11.589(4), b= 19.385(5), c= 13.681(3), β= 99.53(2), Z= 2 and R= 0.049. In all the salts the organic molecules form trimerized stacks. A classical ring-over-double bond overlap is observed in 3, in contrast with 1 and 2 which present an unusual intra-trimer criss-cross overlap. In compound 3 the intra-trimer Se ⋯ Se (3.73–3.81 A) contacts are shorter than those observed in [tmtsf]2X (X = PF6 or BF4) series. The inter-trimer Se ⋯ Se contacts (3.94–4.12 A) are in the range of the van der Waals separation (4 A). Short O ⋯ Se (3.17–3.30 A) contacts are observed. Both electrical and optical measurements and also electronic band-structure calculations reveal that the three salts are semiconductors.

The synthesis and characterization of 2-(1,3-benzodithiol-2-ylidene)-1,3-benzoxathiole (dibenzo-oxatrithiafulvalene, DBOTTF), the first example of a tetraheterofulvalene containing oxygen within the fulvalene moiety, are described. Redox potentials of DBOTTF are compared with the other known dibenzotetraheterofulvalene derivatives.

Ab initio calculations using conventional (HF/6-31G* and MP2/6-31G*) and density functional theory (B-LYP/6-31G*) methods have been used to determine the structures of the [n]fulvene and [n,m]fulvalene (n, m = 3, 5, 7) series of molecules, with particular emphasis on heptafulvalene (n = m = 7: 12). Calculations have also been performed on the parent cycloalkenes: cyclopropene, cyclopentadiene, and cycloheptatriene (1−3, respectively). All the fulvenes (n = 3, 5, 7: 4−6, respectively) and the smaller fulvalenes (n = 3, m = 3, 5, 7: 7−9, respectively, and n = m = 5: 10) are found to be planar. Pentaheptafulvalene (n = 5, m = 7: 11) adopts a very slightly nonplanar Cs arrangement of the five- and seven-membered rings. Heptafulvalene (12) is predicted to have an anti-folded C2h structure, in accord with the X-ray crystal structure. We propose that the underlying reason for 11 and 12 adopting nonplanar conformations is the proximity of the H2 and H2‘ hydrogen atoms which promotes a distortion of the ring...

An [11][13]fulvalene derivative, 13-(4,9-methanocycloundeca-2,4,6,8,10-pentaenylidene)-4,9-dimethylcyclotrideca-1,3,9,11-tetraene-5,7-diyne, has been synthesized. Examination of 1H and 13C NMR spectra indicates that the compound shows no detectable ring-current effect in nonpolar solvents arising from 10π- and 14π-electron systems, which is verified by X-ray crystallographic structural analysis. In a polar solvent, [2H6]dimethyl sulfoxide, a small contribution of the dipolar resonance structure is suggested.

Oxidation of 2,7-disubstituted tetrabenzo[5.5]fulvalene derivatives 1a−d resulted in the formation of dications which are fluorenyl cations linked by a single bond. These fluorenyl cations exhibit significant paratropicity in the 1H NMR spectrum, which is attributed to an antiaromatic ring current. Interaction of the perpendicular ring systems is evident in the upfield shift of carbons a and a‘, presumably due to σ−p donation. The lack of variation in the upfield shift of carbons a and a‘, compared to previously reported systems, is attributed to the similarities of the geometries of 1a−d2+. Substitution at a remote site affects the antiaromaticity of the unsubstituted fluorenyl cation, but the nature of the effect of the substituent is not understood. Direct substitution on the fluorenyl cation by substituents which increase electron density, either through inductive π polarization or through resonance, cause a paratropic shift in the probe proton.

The X-ray crystallographic structures of crystalline fluorenylidenecycloproparenes 7 and 8 and of dibenzocycloheptatrienylidene 9 are reported. Theoretical studies, using ab initio methods at the HF/6-31G(d,p) and the correlated MP2/6-31G(d,p)) levels, have been used to provide assessments of the structure, charge distribution, dipole moment, and thermodynamic stability of the unknown methylidenecyclopropabenzene 3, the derived parent tria-, penta-, and heptafulvalene derivatives 4−6, and the crystalline derivatives 7, 8, and 9. The hydrocarbons are found to be polar, and the cycloproparenylidene moiety acts as electron donor in all but the cyclopropenylidene 4; this compound is the only fulvalene hydrocarbon in the series calculated to have a negatively polarized cyclopropareneyl unit.

Among the various properties of transition metal organometallic compounds is the ease with which they can withstand redox changes. This property led to the concept of ‘organometallic electron-reservoir’. These organometallic electron-reservoirs are used to perform different types of catalytic reactions. The first mentioned is electron transfer chain (ETC) catalysis also called electrocatalysis, which uses them as initiators in reactions that implicate the electron itself as the real and effective catalyst. Examples will illustrate this principle in organometallic chemistry using Fe sandwich complexes: (i) ligand substitution in [Fe(II)Cp(arene)] + PF 6 − by two-electron donors; (ii) intramolecular disproportionation of dinuclear fulvalene carbonyl complexes and selective discrimination of metal centers; (iii) design of a system in which both ETC and organometallic catalyses are coupled (the first known example concerns the polymerization of terminal alkynes by a W 0 complex); (iv) intramolecular dithiocarbamate ligand chelation in Fe(II) piano stool complexes which can be initiated either by reduction or by oxidation, both chain pathways being explained. A second application of these organometallic complexes in catalysis takes place in redox catalysis: when they are used in catalytic amount they act as electron carriers lowering the overpotential existing between an electrode and a substrate. Two cases were examined: (i) redox catalysis of the reduction of water and (ii) redox catalysis of the reduction of nitrogen derivatives: nitrates and nitrites.

Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.

Monoclinic (α) tetra­thia­fulvalene [systematic name: 2-(1,3-dithiol-2-yl­idene)-1,3-dithiole], C6H4S4, undergoes a reversible second-order phase transition at ca 190 K through a displacive modulation with doubling of the a parameter. The low-temperature phase (γ) contains two crystallographically non-equivalent (but centrosymmetric) mol­ecules of very similar geometry.

Oxidation of 3,6-disubstituted tetrabenzo[5.5]fulvalenes by SbF(5) results in the formation of dications that behave like two antiaromatic fluorenyl cations connected by a single bond. Both fluorenyl systems exhibit the paratropic shifts and nucleus independent chemical shifts (NICS) characteristic of antiaromatic species. Comparison with analogous 2,7-disubstituted tetrabenzo[5.5]fulvalenes reveals that the antiaromaticity of the substituted ring system can be altered substantially by changes in the placement of the substituents, possibly due to changes in the delocalization of charge in the system. Substituents in the 3,6-position decrease the antiaromaticity because of the increase in the benzylic resonance compared to 2,7-substituents.

Publisher Summary This chapter explores fulvalene—compounds containing two cyclic polyenic systems with a central double bond. Depending on the ring size, cyclopropylidenecyclopropene, fulvalene, heptafulvalene, and the unsymmetrical hybrid molecules triapentafulvalene, triaheptafulvalene, and pentaheptafulvalene are members of this class of cyclic cross-conjugated systems. The general principle of heterocyclic chemistry to relate heterocyclic compounds to carbocyclic ones was the driving force for the synthesis and their application to heterofulvalenes. The chapter covers nitrogen-containing fulvalenes that can be obtained by replacement of CH=CH and/or CH. Compounds in which nitrogen atoms are arranged on the periphery of the cross-conjugated system as well as derivatives in which the central double bond contains heteroatoms are not included are presented in the chapter. It also discusses that concerning nomenclature, fulvalene and its related systems are the parent structures of this class of heterocyclic cross-conjugated compounds.

Reaction of 1,2,4-tris(trimethylsilyl)cyclopentadienyllithium with ferric chloride in THF at -95°C leads to the formation of a thermally sensitive iron(III) complex which decomposes to the new dihydrofulvalene derivative 3,7-dihydro-1,3,3,5,7,7-hexakis(trimethylsilyl)fulvalene (1).

The preparation and characterization of the first side-by-side fused biferrocenes are reported. 2,5''-Bis((dimethylamino)-methyl) biferrocene was used to prepare N,N-dimethyl-2,5''-(2-azoniapropane-1,3-diyl) biferrocene iodide. Reaction with I 2 in solution gives the analogous pentaiodide salt N,N-dimethyl-2,5''-(2-azoniapropane-1,3-diyl) biferrocene pentaiodide (17). Reaction with LiBH(sec-Bu) 3 gives the nonsalt fused biferrocene N-methyl-2,5''-(2-azapropane-1,3-diyl) biferrocene. Results from the X-ray structure of 17 show that this I 5 − salt crystallizes in the orthorhombic space group Pbca

One-dimensional heteronuclear NOE difference spectroscopy was used to assign 1H and 13C NMR spectra of dinuclear fulvalene complexes with tungsten and molybdenum. It was found that metal-metal bonded complexes show 1H chemical shifts which are the inverse of those in non-metal—metal bonded complexes, whereas the relative 13C chemical shifts are not affected by the metal—metal bond.

Syntheses and Chemical Properties of New ‘Donor-Acceptor’-stahilized Molecular Systems with a Central Bicyclo[4.4.1] undeca-l(10).3,6,8-tetraene-2,5-diylidenc Group The synthesis of X-methyl-2-(5-oxobicyclo[4.4.1]undeca-l)10(,3,6,8-tetraen-2-yliden)-l,3-benzodithiol (4) is described starting with the keto-enol mixture 2 and 5-methyl-1,3-benzodithiolium perchlorate. Under rearomatization of the central frame protonation of 4 yields the salt 4c. The reaction of 4 with dicyanoketene gives the ‘push-pull-substituted’ 5, and with 9-carbonylfluorene the fulvalene derivative 7, which can be protonated by CF3CO2H at C(9″) to the salt 8. The reaction of 2-methylthio-l,3-dithiolium jodide 9 with the keto-enol mixture 2 yields 10, which, on protonation at the carbonyl group by CF3CO2H, gives the salt 11 under rearomatization. The spectral data of the new compounds 4, 4c, 5, 7, 8, 10 and 11 are reported and discussed.

Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(II) Chloride Starting with a nearly quantitative coupling of cyclononatetraenide 7 to 1, 1′-dihydrononafulvalene 8 by means of AgBF4, a simple general synthetic concept for fulvalenes is outlined (Scheme 2), consisting in an oxidative coupling of ‘Huckel anions’ like 2 and 7 to 1, 1′-dihydrofulvalenes 10 with Ag(I) or Cu(II) salts, followed by deprotonation (11) and oxidation (12); it has been realised in the case of pentafulvalene (1; overall yield 61%; Scheme 3) and 1,2:5,6-dibenzopentafulvalene (18; overall yield 66%, Scheme 4). NMR-spectroscopic investigations show that 1 is a non-aromatic compound with strongly alternating bond-lengths, its π-system being even more localised than that of simple pentafulvenes. In fact, 1 is extremely reactive in concentrated solutions above −50°. Besides of polymerisations, Diels-Alder dimerisation 119 followed by a rearrangement 1920 takes place (Scheme 5).

Bei der Umsetzung von 1,2,3,4-Tetrachlorcyclopentadien (1) mit Azomethinen (z. B. 2) entstehen neben den erwarteten Fulvenen die Bis-Kondensationsprodukte 10–13. Mit N-Cyclohexyliden-n-propylamin (2) bildet 1 zusatzlich die Spiroverbindung 8; auserdem lies sich in diesem Fall eine gemeinsame Zwischenstufe in Form des Ammoniumsalzes 5 abfangen, die in polaren, basischen Losungsmitteln hauptsachlich zum Fulven 6 weiterreagiert, in unpolaren Losungsmitteln nach Zusatz von Natriumhydrid und 1 jedoch ausschlieslich 8 und 10 bildet. Die Dechlorierung von 8 liefert das Fulvalen 14. 1,1-Bis(tetrachlorocyclopentadienyl)cyclohexane and Related Products The bis-condensation products 10–13 are formed besides the expected fulvenes by the reaction of 1,2,3,4-tetrachlorocyclopentadiene (1) with azomethines (e. g. 2). In addition the spiro compound 8 has been obtained from 1 and N-cyclohexylidene-n-propylamine (2). During this reaction a common intermediate could be isolated in form of the ammonium salt 5. This decomposes in polar basic solvents to yield mainly fulvene 6, whereas in unpolar solvents 8 and 10 are formed exclusively after the addition of sodium hydride and 1. Dechlorination of 8 leads to the generation of the fulvalene 14.

The rate of the reaction of cyclopentadiene with iodine has been followed spectrophotometrically over the temperature range 171.7° to 276.5°C. The reaction first proceeds almost to the point of equilibrium with cyclopentadienyl iodide and HI, although the final products are fulvalene and HI. Equilibrium constants obtained are those predicted by bond additivity. A third-law value of δH0f 298 (c-C5H5I,g) = 49 kcal/mole is obtained. Rate studies of the reaction up to the iodide equilibrium, yield values for the rate constant . Uncertainty in the Arrhenius parameters, as well as doubts as to the applicability of the usual assumption that E3 = 1 ± 1 kcal/mole, make difficult an evaluation of total cyclopentadienyl stabilization energy (TSE) from these data. However, the value is probably 15 < TSE < 20.

A mass spectrometric examination of five homoannularly chlorinated ferrocenes of the general formula (C5H(5−n))Fe(C5H5) where n = 1 to 5 (Series 1), and five heteroannularly chlorinated ferrocenes of the general formula (C5H(5−n)Cln)2Fe where n = 1 to 5 (Series 2) has been made. This study affords the first look at the mass spectral results of incremental additions of the same substituent up to the ultimate replacement of all hydrogens in the ferrocene molecule. The molecular ion is the most intense peak in both series. The primary fragmentation processes are, for Series 1: [M – FeCl2], [M – C5H(5−n)Cln], [M – C5H5] and [M – HCl]; and for series 2: [M – FeCl2] and [M – C5H(5−n)Cln]. The [M – FeCl2]+. peak is speculatively attributed to a fulvalene derivative. A heteroannular pathway is proposed for HCl elimination. Significant [Fe]+. and [FeCl]+ peaks occur in all compounds. In all compounds having three or more hydrogens on one ring, a peak due to [C3H3]+ is seen. Metastable studies, precise mass measurements and isotopic abundance ratios of chlorine were used to confirm many of the fragmentation processes.

Heterosesquifulvalenes containing sulfur or oxygen as hetero atom may be prepared, e.g., by condensation of cyclopentadienes with thiopyrones or pyrones in acetic anhydride. Heterosesquifulvalenes containing nitrogen can be obtained from the oxygen compounds by O,N-exchange. The ground state of all these compounds is determined mainly by the nonpolar resonance formula.