Boratastilbene: Synthesis, Structural Characterization, and Photophysics

Phosphidoboratabenzene-rhodium(i) complexes as precatalysts for the hydrogenation of alkenes at room temperature and atmospheric pressure

The di-tert-butylphosphido-boratabenzene ligand (DTBB) reacts with [(C2H4)2RhCl]2 yielding the dimeric species [(C2H4)Rh(DTBB)]2 (1). This species was fully characterized by multinuclear NMR and X-ray crystallography. Complex 1 readily dissociates ethylene in solution and upon exposure to 1 atm of H2 is capable of carrying out the hydrogenation of ethylene. The characterization of two Rh-H species by multinuclear NMR spectroscopy is provided. The reactivity of 1 towards the catalytic hydrogenation of alkenes and alkynes at room temperature and 1 atm of H2 is reported and compared to the activity of Wilkinson's catalyst under the same reaction conditions.

A combined experimental and theoretical study on the isomers of 2,3,4,5-tetracarba-nido-hexaborane(6) derivatives and their photophysical properties

Building upon our ongoing studies on the high-yield synthetic route to 2,3,4,5-tetracarba-1,6-nido hexaborane derivative 5, we continue to explore the chemistry of this system to isolate a range of perphenylated C4 B2 -type carborane isomers. As a result, the photolysis of 5 for an elongated period yielded 6, which exhibits an intense luminescence upon excitation with a UV lamp (λex =366 nm). In contrast, the conversion of 5 under thermal conditions (microwave heating) generated the isomer 7, which does not show any luminescence. The structural disparity among the three isomers 5-7 is the position of the butadiene moiety with respect to the C4 B2 -carborane cage as deduced from single-crystal X-ray diffraction analyses. In an attempt to generate the classical structural motif of 5, an equimolar amount of KOtBu was added to a THF solution of 5, which immediately provided the boratacyclopentadiene derivative 8 in good yield. The observed rearrangement reactions were further investigated by quantum chemical calculations to suggest a plausible reaction pathway. According to the DFT calculations, the energetically favored reaction mechanism most likely involves tricyclic transition states and classical intermediate structures. In addition, the fluorescence emission properties of 6 were investigated in solution as well as in the solid state.

Triphenylene Analogues with B2N2C2Cores: Synthesis, Structure, Redox Behavior, and Photophysical Properties

A series of alkyl (1-3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B(2)N(2)C(2) cores have been synthesized via chelation of pyridazine derivatives using difunctional Lewis acidic diborabiphenyl precursors. In contrast to triphenylene, NICS(1) calculations on 1 suggested high aromaticities for the central (-11.3 ppm) and outer borabenzene rings (-7.7 ppm), along with nonaromatic behavior for the pyridazine ring (-0.7 ppm). Crystal structure analyses supported this analysis. When the a- and c-faces of the pyridazine moiety were free of substitution (1, 3), planar structures resulted, but upon substitution, a twisted B(2)N(2)C(2) core was observed due to steric repulsion of neighboring hydrogen atoms (e.g., 5). The increase of steric bulk from H (1) to (i)Pr (3) in the planar species was found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular B...N interactions of 3.39 Angstrom. One-electron reduction by Cp(2)Co was found to afford the radical anions of 3 and 5, which were characterized by broad, featureless singlets in the EPR spectra; [3](.)(-)[Cp(2)Co](+) was characterized by X-ray crystallography. While the planar structures (1-4) were observed to possess weak fluorescence (Phi(F) = 0.02-0.08) with either yellow-orange (ca. 555 nm) or green emission (521 nm), the twisted structures (5, 6) were found to be nonfluorescent.

Excitation spectra of dibenzoborole containing pi-electron systems: controlling the electronic spectra by changing the p(pi)-pi* conjugation

We report time-dependent density functional theory calculations of the vertical excitation energies for the singlet states of three-coordinate 5H-dibenzoborole (DBB) derivatives and four-coordinate 5-fluoro-5H-dibenzoborole ion (FDBB) derivatives. These molecules show remarkable hypsochromic (blue) shifts in their fluorescence spectra and bathochromic (red) shifts in their absorption spectra when the bridging boron atoms change their coordination number from three to four. We constructed a series of derivatives of DBB and FDBB and studied how the energies of the electronic excitations change. The states with prominent oscillator strength in all of the DBB and FDBB derivatives show similar shifts of their excitation energies upon coordination. The three-coordinate DBB derivative 5-(2,4,6-triisopropylphenyl)-2,8-dimethoxy-3,7-bis[p-(N,N-diphenylamino)phenyl]-5H-dibenzo[d,b]borole has an intense absorption at 3.25 eV, which shifts in the four-coordinate FDBB derivative 5-fluro-5-(2,4,6-triisopropylphenyl)-2,8-dimethoxy-3,7-bis[p-(N,N-dip henylamino)phenyl]-5H-dibenzo[d,b]borole ion to 3.17 eV. The experimental absorption peaks are 3.43 and 3.31 eV, respectively. In addition, we investigated and analyzed the nature of these electronic excitations using attachment/detachment density plots, with which we characterized the changes in electron density that arose from the excitations.

Spectroscopic and Excited-State Properties of Tri-9-anthrylborane III: Crystal and Spectroscopic Polymorphisms

We found that tri-9-anthrylborane (TAB) recrystallized from benzene produced both red cubic-like (R-form) and orange hexagon-like crystals (O-form). In both crystal forms, six TAB molecules are arranged in a honeycomb structure in the ab plane and benzene molecules are incorporated in the honeycomb structure, whose spatial geometry and the total number of benzene rings in the unit cell are different between the two forms: polymorphs with a different benzene content. In the R-form crystal, furthermore, interlayer stacking between left- and right-handed helical TAB molecules was observed in the ac plane, while each layer composed of stacked TAB molecules along the c axis was separated by benzene molecules in the O-form crystal, giving rise to more dense packing of TAB in the R-form crystal as compared to that in the O-form. Reflecting the crystal structures of the two forms, the charge transfer (CT) absorption and fluorescence spectra of the R-form crystal were shifted to the longer wavelength as compared to those in the O-form (i.e., crystal and spectroscopic polymorphisms) and, therefore, electronic interactions between TAB were stronger in the R-form as compared to those in the O-form. Furthermore, in addition to the main absorption (lambdamaxa= 499 nm) and fluorescence peaks (lambdamaxf = 570 nm), distinct absorption (lambdaa = approximately 470 nm) and fluorescence bands (lambdaf = approximately 600 nm) were observed for the R-form crystal, while the relevant absorption band in the O-form crystal (lambdaa = approximately 460 nm) or in solution (lambda(a) = approximately 435 nm) was ambiguous. The results were discussed in terms of participation of the higher energy second CT transition in TAB.

Spectroscopic and Excited-State Properties of Tri-9-anthrylborane I: Solvent Polarity Effects

Spectroscopic and excited-state properties of tri-9-anthrylborane (TAB), showing unique absorption and fluorescence characteristics originating from p(boron)-pi(anthryl group) orbital interactions, were studied in 12 solvents. Although the absorption maximum energy (nu(a)) of TAB which appeared at around 21 x 10(3) cm(-1) (band I) was almost independent of the solvent polarity parameter, f(X) (f(X) = (D(s) - 1)/(2D(s) + 1) - (n(2) - 1)/(2n(2) + 1) where D(s) and n represent the static dielectric constant and the refractive index of a solvent, respectively), the fluorescence maximum energy (nu(f)) showed a linear correlation with f(X). The f(X) dependence of the value of nu(a) - nu(f) demonstrated that the change in the dipole moment of TAB upon light excitation was approximately 8.0 D, indicating that absorption band I was ascribed to an intramolecular charge-transfer transition in nature. The excited electron of TAB was thus concluded to localize primarily on the p orbital of the boron atom. Furthermore, it was shown that the fluorescence lifetime and quantum yield of TAB varied from 11.8 to 1.1 ns and from 0.41 to 0.02, respectively, with an increase in f(X). The present results indicated that the nonradiative decay rate constant (k(nr)) of TAB was influenced significantly by f(X). Excited-state decay of TAB was understood by intramolecular back-electron (charge) transfer from the p orbital of the boron atom to the pi orbital of the anthryl group, which was discussed in terms of the energy gap dependence of k(nr). Specific solvent interactions of TAB revealed by the present spectroscopic and photophysical studies are also discussed.

Cyclic boronium and borenium cations derived from borabenzene–pyridine complexes

A new class of boronium and borenium cations are prepared by reaction of pyridinium hydrochloride with pyridine stabilized borabenzene derivatives.

Dibenzoborole-containing pi-electron systems: remarkable fluorescence change based on the "on/off" control of the p(pi)-pi* conjugation

A series of dibenzoborole derivatives with various groups such as (N,N-diphenylamino)phenyl, thienyl, and bithienyl groups at the 3,7-positions have been synthesized and their photophysical properties studied. These new pi-electron systems show significant solvatochromism in the fluorescence spectra. Thus, about 100-140 nm blue shifts in the emission maxima and 20-30-fold increments in the quantum yields are observed upon changing the solvent from THF to DMF. Similar fluorescence changes are observed upon the addition of n-Bu4NF to their THF solutions, demonstrating their sensing abilities toward a fluoride ion. These fluorescence changes result from the "on/off" control of the ppi-pi* conjugation in their LUMO by the coordination of donor solvents or fluoride ion to the boron atom in the dibenzoborole skeleton.

Spectral Properties of Fluorophores Combining the Boronic Acid Group with Electron Donor or Withdrawing Groups. Implication in the Development of Fluorescence Probes for Saccharides

We evaluated the spectral properties of four stilbene derivatives containing the boronic acid group [-B(OH)₂]: stilbene-4-boronic acid (STBA), 4'-cyanostilbene-4-boronic acid (CSTBA), 4'-methoxystilbene-4-boronic acid (MSTBA), and 4'-(dimethylamino)stilbene-4-boronic acid (DSTBA). The emission spectrum of DSTBA displays a large solvent-polarity dependence showing the formation of a photoinduced charge transfer state (CT). This state is weakly present in MSTBA and not present for CSTBA and STBA for the neutral form of the boronic acid group. These results show the donor withdrawing property of the neutral form of the boronic acid group. At higher pH, the boronic acid group is present in the anionic form [-B(OH)₃ ^-], resulting in a change of the configuration around the boron atom from the triangular planar (sp² hybridization) to the tetrahedral conformation (sp³ hybridization). This change induced a blue shift of about 50 nm and an increase of intensity in the emission spectrum of DSTBA because of the loss of the electron-withdrawing properties for the anionic form of the boronic acid group, leading to the loss of the CT effect. The same effect is also observed for MSTBA. In contrast, a red shift of about 35 nm and a decrease of intensity are observed for CSTBA from the neutral to the anionic forms of the boronic acid group. These observations lead to the conclusion that the anionic form of the boronic acid group acts as an electron donor group and a photoinduced CT state can be formed when an electron withdrawing group is present on the fluorophore. The usefulness of this effect for the development of saccharide probes is also demonstrated. After addition of sugar, the emission spectra of DSTBA and MSTBA showed a blue shift and an increase of the intensity. On the other hand, a red shift and a decrease of the intensity are observed in the emission spectra of CSTBA after addition of sugar. A change from the neutral to the anionic form of the boronic acid group is used to explain these changes. These results show that the use of the combination of electron donor or withdrawing groups with the boronic acid group is a new and promising way to develop ratiometric fluorescent probes for glucose and other saccharides.

Tridurylboranes extended by three arylethynyl groups as a new family of boron-based pi-electron systems

A series of tris(phenylethynylduryl)boranes (R-C(6)H(4)-C&tbd1;C-duryl)(3)B with various substituents R have been prepared as air-stable solids owing to the steric protection of the boron atom by the three bulky duryl groups. These compounds show unique photophysical properties due to the p(pi)-pi conjugation through the p-orbital on the boron atom. In particular, a push-pull type derivative with R = NMe(2) exhibits a significant solvatochromism of fluorescence from blue to orange colors.

Syntheses and structures of Mg(C5H5BMe)2, Mg(3,5-Me2C5H3BNMe2)2, the 2,2'-bipyridine adduct Mg(C5H5BMe)2(bipy), and the N-bonded aminoboratabenzene species Mg(3,5-Me2C5H3BNMe2)2(THF)2

The donor-free magnesocene analogues bis(1-methylboratabenzene)magnesium (1) and bis[3,5-dimethyl-1- (dimethylamino)boratabenzene]magnesium (2) are synthesized in good yields by the reactions of dimethylmagnesium with (trimethylstannyl)dihydroborinine precursors 3 and 4. In the crystalline state, both 1 and 2 possess sandwich structures with eta 6-coordinated boratabenzene ligands and display crystallographic centrosymmetry. Compound 1 reacts with the nitrogen donor 2,2'-bipyridine to give the Lewis base adduct (1)(bipy) (identical to 5). In the crystal structure of 5, one boratabenzene ligand is eta 6-bonded to the central metal while the other ligand adopts an eta 1-bonding mode. Crystallization of compound 2 from THF produces the solvate (2)(THF)2 (identical to 6), which exhibits a distorted tetrahedral N2O2 coordination environment around the magnesium atom. The average Mg-N bond distance is 2.141(3) A, and the N-Mg-N angle is 148.0(1) degrees. The observation of an aminoboratabenzene that is solely sigma-bonded to a metal is without precedent.