π-Extended 9b-Boraphenalenes: Synthesis, Structure, and Physical Properties

Boraphenalenes, compounds in which one carbon atom in the phenalenyl skeleton is replaced with a boron atom, have attracted attention for their solid-state and electronic structures; however, the construction of boraphenalene skeletons remains challenging because of the lack of suitable methods. Through this study, we showed that the tandem borylative cyclization of C3-symmetric dehydrobenzo[12]annulenes produces a new class of fully fused boron-atom-embedded polycyclic hydrocarbons possessing a 9b-boraphenalene skeleton. The obtained compounds exhibited high electron-accepting characteristics, and their two-step redox process was reversible in the reductive region, involving interconversion of 9b-boraphenalene between Hückel aromaticity and antiaromaticity. Notably, the benzo[b]fluorene-fused derivative exhibited a stepwise single-crystal-to-single-crystal (SCSC) phase transition triggered by thermal annealing. Intermolecular electron coupling calculation of the crystal structures suggested a significant improvement of charge transporting ability associated with the SCSC phase transition. Moreover, adequate photoconductivity was observed for the single crystals before and after the SCSC phase transition through flash photolysis-time-resolved microwave conductivity.

Synthesis, Structures, and Reactivities of BCN-Heterocyclic B-N Chains

The BCN-heterocyclic B-N chain compounds, the sodium and potassium salts of 3 and 4 anions (Na3, Na4, and K4), were synthesized by reactions of ethane 1,2-diamineborane (BH3NH2CH2CH2NH2BH3, 1) and propane 1,2-diamineborane (BH3NH2CH2CH2CH2NH2BH3, 2) with MH (M = Na and K). Then, the neutral B-N chain compounds 5 and 6 were prepared with dehydrogenation of [NH4]3 and [NH4]4, formed by metathesis reactions of Na3 and Na4 with NH4Cl or NH4SCN, respectively. Compounds 7 and 8, analog 5, were also prepared using pyridine and 4-methoxypyridine instead of NH3 in 5. These synthesized compounds were characterized spectroscopically, and the singe-crystal structures of the Na3·18-crown-6 and K4·18-crown-6 adducts were determined. Furthermore, the reactions of Na3 and Na4 with cationic B-N chain compounds, [NH3BH2NH3]Cl and [NH3BH2NH2BH2NH3]Cl, could not form longer BCN-heterocyclic B-N chain. The solubility of metal hydrides, the ability for proton abstracting, the basicity of Lewis bases, and the chelate effect may influence these reactions even though the reaction mechanism is not fully understood.

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

Dimeric aminoboranes, [H₂BNR₂]₂ (R = Me or CH₂CH₂) containing B₂N₂ cores, can be activated by I₂, HNTf₂ (NTf₂ = [N(SO₂CF₃)₂]), or [Ph₃C][B(C₆F₅)₄] to form isolable H₂B(μ-NR₂)₂BHX (for X = I or NTf₂). For X = [B(C₆F₅)₄]⁻ further reactivity, presumably between [H₂B(μ-NMe₂)₂BH][B(C₆F₅)₄] and aminoborane, forms a B₃N₃-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H₂B(μ-NMe₂)₂BH(I) and [(μ-NMe₂)BH(NTf₂)]₂ indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf₂ binding. While the iodide congener reacted very slowly with alkynes, the NTf₂ analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)₂BNR₂ species and R₂NBH(NTf₂) as the major products. Further studies indicated that the B₂N₂ core is maintained during the first hydroboration, and that it is during subsequent steps that B₂N₂ dissociation occurs. In the mono-boron systems, for example, ⁱPr₂NBH(NTf₂), NTf₂ is N-bound; thus, they have less steric crowding around boron relative to the B₂N₂ systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B₂N₂-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B₂N₂ electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H₂B(μ-N₂C₃H₃)]₂ analogues, making them less useful for the directed diborylation of a single substrate.

Reactions of a Four‐Membered Borete with Carbon, Silicon, and Gallium Donor Ligands: Fused and Spiro‐Type Boracycles

Donor‐acceptor cyclopropanes or cyclobutanes are dipolar reagents, which are widely used in the synthesis of complex organic (hetero)cycles in ring expansion reactions. Applying this concept to boron containing heterocycles, the four‐membered borete cyclo‐iPr₂N‐BC₁₀H₆ reacted with the carbon donor ligands 2,6‐xylylisonitrile and the carbene IMes :C(NMesCH)₂ with ring expansion and ring fusion, respectively. In particular, the tetracyclic structure formed with IMes displays zwitterionic character and absorption in the visible region. In contrast to the carbene IMes, the heavier carbenoids :Si(NDippCH)₂ and :Ga(AmIm) with a two‐coordinate donor atom afford spiro‐type bicyclic compounds, which display four‐coordinate geometry at silicon or gallium. (TD‐)DFT calculations provide deeper insight into the mechanism of formation and the absorption properties of these new compounds.

BNB-doped phenalenyls - aromaticity switch upon one-electron reduction

Fully aromatic, luminescent, and highly robust BNB-doped phenalenyls have been prepared, which are isoelectronic to the phenalenyl cation. B-Fluoromesityl-substitution leads to fluorescence in an extremely narrow range and significant increase in the reduction potential. Detailed theoretical investigations revealed an intramolecular aromaticity switch upon one-electron reduction.

BNB-Doped Phenalenyls: Modular Synthesis, Optoelectronic Properties, and One-Electron Reduction

A highly modular synthesis of BNB- and BOB-doped phenalenyls is presented. Treatment of the 1,8-naphthalenediyl-bridged boronic acid anhydride 1 with LiAlH₄/Me₃SiCl afforded the corresponding 1,8-naphthalenediyl-supported diborane(6) 2, which served as the starting material for all subsequent transformations. Upon addition of MesMgBr/Me₃SiCl, 2 was readily converted to the tetraorganyl diborane(6) 5. The further heteroatoms were finally introduced through the reaction of 2 with (Me₃Si)₂NR' or 5 with H₂NR' or H₂O (R' = H, Me, p-Tol). A helically twisted, fully BNB-embedded PAH 11 was prepared by combining 2 with a dibrominated m-terphenylamine, followed by a Grignard-mediated double ring-closure reaction. All compounds devoid of B-H bonds show favorable optoelectronic properties, such as luminescence and reversible reduction behavior. In the case of the BNB-phenalenyl 7 (BMes, NMe), the radical-anion salt K[7^•] was generated through chemical reduction with K metal and characterized by EPR spectroscopy. K[7^•] is not long-term stable in a THF/c-hexane solution, but abstracts an H atom with formation of the diamagnetic BNB-doped 1H-phenalene K[7H].

Tetraaryldimethoxybenziporphyrins. At the Edge of Carbaporphyrinoid Aromaticity

A series of eight dimethoxybenziporphyrins were prepared in three steps from 1,3-dimethoxybenzene or 2,6-dimethoxytoluene. Dibromination, followed by lithium-halogen exchange and reaction with benzaldehyde gave dicarbinol intermediates. These reacted with pyrrole and aryl aldehydes in the presence of BF3.Et2O in chloroform, followed by oxidation with DDQ, to give the benziporphyrins in 15-25% yield. These compounds readily gave nickel(II) and palladium(II) organometallic derivatives and could be selectively reduced with sodium borohydride to give unstable benziphlorins. Regioselective oxidation with silver acetate afforded the related 22-acetoxybenziporphyrins in 52-64% yield. The dimethoxybenziporphyrins showed chemical shifts by proton NMR spectroscopy that were consistent with weakly diatropic macrocycles. However, addition of TFA gave dications that showed far more significant shifts that are attributed to the presence of a more substantial diatropic ring current. The internal CH for 11H2(2+) was observed at 3.5 ppm, but this effect was diminished for the 3-methylbenziporphyrins 12H2(2+) where this resonance appears at 4.7 ppm. Even in the absence of the methoxy substituents, the dication derived from tetraphenylbenziporphyrin 8H2(2+) shows an upfield shift for this resonance to 5.5 ppm. The dications of the 22-acetoxybenziporphyrins also show similar effects despite the presence of an internal ester moiety. These results demonstrate that a spectrum of diatropic character can manifest even in highly crowded benziporphyrin derivatives.

Core-Modified, meso-Alkylidenyl Porphyrins and Their Expanded Analogues: A New Family of Nonplanar Porphyrinoids

New core-modified, meso-alkylidenyl porphyrinoids bearing multiple exocyclic double bonds were synthesized and characterized. The synthesis was accomplished using a typical "3 + 1"-type condensation approach. Stable exocyclic tautomers bearing double bonds at the meso positions, as well the corresponding endocyclic tautomers, were isolated in the case of both thiabenziporphyrin and thiapyriporphyrin products prepared in the course of this study. On the other hand, only the exocyclic tautomer was isolated in the case of the congeneric oxapyriporphyrin and oxabenziporphyrin. Expanded analogues of the exocyclic forms of oxabenziporphyrin and thiabenziporphyrin were also isolated as minor products. A single-crystal X-ray diffraction analysis of the expanded thiabenziporphyrin (20) revealed that all four pyrrole rings displayed an inverted geometry, presumably reflecting the strong hydrogen-bonding extant between the pyrrole N-H proton and the carbonyl group of the malonate moiety in the solid state. On the other hand, the expanded oxabenziporphyrin (14) was found to possess a severely distorted geometry with only one pyrrole ring being inverted. Careful analysis of the structure revealed that the solid-state geometry of the expanded macrocycles correlates well with the internal angle defined by the 2- and 5 substituents and the centers of the furan (14) or thiophene (20) subunits.

Molecular and Electronic Structure of 1,8-Peribridged Naphthalenes

The crystal and molecular structure of 1,8-thianaphthalene has been determined and compared with other single-atom peribridged naphthalenes (SAPN). The measured CSC angle is 73.06 degrees, which is the smallest bridging angle yet recorded for a SAPN derivative. The ab initio calculations using G3(MP2)//B3LYP method were performed for peribridged naphthalenes in order to determine how the strain of the four-membered ring is influenced by the type of bridge linking 1,8 positions. The electronic structure of 1,8-thia- and 1,8-sulfonenaphthalenes has been studied by UV photoelectron spectroscopy. We have tried to identify and distinguish the strain effect on the electronic structure of the naphthalene moiety.

Structural and Electrochemical Investigations of the High Fluoride Affinity of Sterically Hindered 1,8-Bis(boryl)naphthalenes

The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10'-bora-9'-oxaanthryl)naphthalene (2). This diborane reacts with [Me3SiF2][S(NMe2)3)] to afford the anionic complex [2-mu2-F]-, which has been isolated as a [S(NMe2)3]+ salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10'-bora-9'-thiaanthryl)naphthalene (1) exhibit two reversible reductions at E(1/2) = -2.200 and -2.566 V (vs FcH/FcH+) for 1 and E(1/2) = -2.248 and -2.620 V (vs FcH/FcH+) for 2 corresponding to the sequential reduction of the two boron centers. These two waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes [1-mu2-F]- and [2-mu2-F]-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures of 2 and [2-mu2-F]- have been studied experimentally and computationally. The crystallographic studies show that the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring boryl moieties. By contrast, the structure of the anionic complex [2-mu2-F]- is much more sterically relaxed than that of 2, as indicated by a reduction of the B-B distance from 3.279(4) A in 2 to 2.922(7) A in [2-mu2-F]-. The structural results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved in the B-F-B bridge of [2-mu2-F]- has been studied using density functional theory calculations and Atoms-In-Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B-F bonds in [2-mu2-F]- only amounts to a fraction of the energy of a terminal B-F bond. These calculations also suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride affinity of these types of molecules.

Porphyrins Bearing Stable meso-Alkylidenyl Double Bonds. A New Family of Nonplanar Porphyrinoids

[Structure: see text] Unique core-modified porphyrinoids, such as oxabenziporphyrins, oxapyriporphyrins, and thiapyriporphyrins, bearing exocyclic C-C double bonds at meso-positions, have been synthesized and characterized. The synthesis was accomplished by utilizing typical "3+1"-type condensation. Two different stable tautomeric forms were isolated, and the two tautomeric forms can be interconvertible upon treatment with base. In contrast, only the structure bearing an exocyclic double bond was isolated in the case of oxapyriporphyrin and oxabenziporphyrin.

Dithiadiazuliporphyrin: Facile Generation of Carbaporphyrinoid Cation Radical and Dication

Dithiadiazuliporphyrin is a nonaromatic porphyrinoid, readily and reversibly oxidizable to its cation radical and to the unprecedented aromatic carbaporphyrinoid dication, which can be viewed as a 21,23-dicarba-22,24-dithiaporphyrin with two fused tropylium rings.

Benziporphyrins: exploring arene chemistry in a macrocyclic environment

Benziporphyrins are synthetic porphyrin analogues, in which one of the pyrroles is replaced with a benzenoid ring. The arene can be incorporated into the macrocycle in several ways leading to molecules with distinct physical and chemical properties. By appropriately changing the structure of benziporphyrins, it is possible to affect their aromaticity, tautomeric equilibria, and reactivity. Benziporphyrins are versatile ligands, which offer a means to study the metal-arene couple in a macrocyclic environment. The coordination brings the metal ion into the vicinity of the arene fragment leading to activation of C-H bonds or weak interactions, which can be observed spectroscopically.

New Riches in Carbaporphyrin Chemistry: Silver and Gold Organometallic Complexes of Benzocarbaporphyrins

The NH-N-NH-N core of the porphyrin system represents one of the best studied and most versatile platforms for coordination chemistry. However, the replacement of one or more of the interior nitrogens with carbon atoms would be expected to diminish the ability of these systems to form metallo derivatives considerably. Despite this expectation, carbaporphyrinoid systems have been shown to form stable organometallic derivatives. Although azuliporphyrins and benziporphyrins act as dianionic ligands, benzocarbaporphyrins are trianionic ligands. Treatment of five different meso unsubstituted benzocarbaporphyrins and two different meso tetraarylbenzocarbaporphyrins with excess silver(I) acetate afforded 65-97% yields of the corresponding silver(III) organometallic derivatives. The insertion of silver metal was confirmed by mass spectrometry and X-ray crystallography. The UV-vis spectra showed a strong Soret band at wavelengths between 437 and 451 nm, together with a series of Q-type bands at longer wavelengths. The new metallo carbaporphyrins demonstrate the presence of a strong diatropic ring current in their proton NMR spectra, and carbon-13 NMR spectroscopy indicates that the derivatives retain a plane of symmetry. The reaction of meso tetraaryl carbaporphyrins with gold(III) acetate afforded the related gold(III) complexes, and these also showed strongly porphyrin-like aromatic characteristics. The UV-vis spectra for the gold complexes again showed a strong Soret band between 437-439 nm, but a secondary band near 400 nm is somewhat intensified for the gold species compared to the spectra for the related silver(III) meso tetrasubstituted carbaporphyrins. The ring currents observed for the gold(III) complexes by proton NMR spectroscopy were comparable to those of the silver(III) derivatives, implying that both series have similar macrocyclic conformations. Cyclic voltammetry was performed on two different carbaporphyrins, their silver(III) derivatives, and a gold(III) complex. The silver complexes display a reversible cathodic wave that is assigned to the Ag(III/II) couple. However, the gold porphyrinoid gave a value for the reductive wave that could be due to a gold(III/II) couple or a ligand-based process.

Reactivity of the dimesityl-1,8-naphthalenediylborate anion: isolation of the borataalkene isomer and synthesis of 1,8-diborylnaphthalenes

The anionic boron peri-bridged naphthalene derivative, namely dimesityl-1,8-naphthalenediylborate (1), undergoes a hydrolysis reaction to afford dimesityl-1-naphthylborane (2) whose structure has been determined. Upon standing at room temperature in toluene for an extended period of time, 1 undergoes a ring expansion reaction to afford 8,10,11a-trimethyl-7-mesityl-11aH-7-boratabenzo[de]anthracene (3). As shown by its crystal structure, compound 3 constitutes a rare example of a borataalkene and features a carbon-boron double bond of 1.475(6) Angstroms incorporated in a conjugated hexa-1-boratatriene system. The reaction of 1 with 9-chloro-9-borafluorene and 5-bromo-10,11-dihydrodibenzo[b,f]borepin results in the formation of diboranes 4 and 5 which bear two different boryl moieties at the peri-positions of naphthalene. These diboranes have been characterized by multinuclear NMR spectroscopy and X-ray single crystal analysis. The boron center of the borafluorenyl moiety is pi-coordinated to the ipso-carbon of a mesityl group with which it forms a contact of 2.730(3) Angstroms. The cyclic voltammogram of 2 in THF shows a quasi-reversible reduction wave at E(1/2)-2.41 V (vs. Fc/Fc+) corresponding to the formation of the radical anion. In the case of diboranes 4, 5 and 1-(dimesitylboryl)-8-(diphenylboryl)naphthalene (6), two distinct waves are observed at E(1/2)-2.14 and -2.56 V for 4, E(1/2)-2.26 and -2.78 V for 5, and E(1/2)-2.41 and -2.84 V for 6. The first reduction wave most likely indicates the formation of a radical anion in which the unpaired electron is sigma-delocalized over the two boron centers.

Synthesis of heteronuclear bifunctional Lewis acids by transmetalation of 1,8-bis(trimethylstannyl)naphthalene with BCl3

Reaction of 1,8-bis(trimethylstannyl)naphthalene (1) with an excess of BCl3 at –78°, followed by warming to 0°C, results in the exclusive formation of the novel bifunctional Lewis acid 1-(chlorodimethylstannyl)-8-(dichloroboryl)naphthalene (2), a compound in which a boryl and a stannyl moiety coexist at the peri-positions of a naphthalene core. At elevated temperature compound 2 undergoes a chloride–methyl exchange, which affords 1-(dichloromethylstannyl)-8-(chloromethylboryl)naphthalene (3). Compounds 2 and 3 have been characterized by multi-nuclear NMR spectroscopy. The single crystal X-ray analysis for compound 3 reveals a sterically crowded structure with an essentially trigonal planar boron center, and a tin center pentacoordinated in a [4+1]-fashion. Upon exposure to traces of water, compound 3 is converted into the borinic acid derivative 1-(dichloromethylstannyl)-8-(hydroxymethylboryl)naphthalene (4), which has been characterized by multi-nuclear NMR spectroscopy and single crystal X-ray analysis.Key words: multidentate Lewis acids, organoboranes, organostannanes.