Inverse stimulation enables ultrasonic binary coding for NDE using a custom linear testing system.

Ultrasonic testing is an established method of non-destructive evaluation. The increasing complexity of material systems requires an extension of conventional methods. In related fields such as radar and medical ultrasound, signal optimisation and coded stimulation are successfully used and offer great potential for optimising state-of-the-art measurements and extending applications. In our work, we highlight the difference between using a coded sequence to stimulate an ultrasonic testing system and the actual performance of the digital code to motivate the exploration of inverse stimulation. In order to study inverse stimulation, a custom-built ultrasonic system was designed. As a first step, the transfer function was obtained by testing pulse and chirp stimulation. In the next step, inverse stimulation was performed based on the linear transfer function to engineer the ultrasonic echoes to have shapes similar to the target code. Finally, the auto-correlation function of the ultrasonic echoes resulting from the inverse stimulation is compared with the function of the original code sequence and the agreement of the recorded ultrasonic echo with the spectrally limited code sequence. With this work we propose an integrated, low-voltage, fully linear ultrasonic testing system where the recording of a linear transfer function allows echo engineering even for a binary coded excitation sequence. We have demonstrated that inverse stimulation enables the generation of binary ultrasonic echoes with performance equal to the digital code.

Diverse ring-opening reactions of rhodium η4-azaborete complexes.

Sequential treatment of [Rh(COE)2Cl]2 (COE = cyclooctene) with PiPr3, alkyne derivatives and t BuN[triple bond, length as m-dash]BMes (Mes = 2,4,6-trimethylphenyl) provided functionalized rhodium η4-1,2-azaborete complexes of the form (η4-azaborete)RhCl(PiPr3). The scope of this reaction was expanded to encompass alkynes with hydrogen, alkyl, aryl, ferrocenyl, alkynyl, azaborinyl and boronate ester substituents. Treatment of these complexes with PMe3 led to insertion of the rhodium atom into the B-C bond of the BNC2 ring, forming 1-rhoda-3,2-azaboroles. Addition of N-heterocyclic carbenes to azaborete complexes led to highly unusual rearrangements to rhodium η2,κ1-allenylborylamino complexes via deprotonation and C-N bond cleavage. Heating and photolysis of an azaborete complex also led to rupture of the C-N bond followed by subsequent rearrangements, yielding an η4-aminoborylallene complex and two isomeric η4-butadiene complexes.

Preparation and Characterization of a π-Conjugated Donor-Acceptor System Containing the Strongly Electron-Accepting Tetraphenylborolyl Unit.

A novel thiophene-bridged donor-acceptor system was synthesized with a carbazole as donor and a borole as acceptor unit. The borole group was successfully installed via the tin-boron exchange reaction of 1,1-dimethyl-2,3,4,5-tetraphenylstannole with 9-(5-(dibromoboryl)thiophen-2-yl)carbazole. The effect of the borole on the optoelectronic properties of the donor-acceptor system was explored by spectroscopic (UV/Vis and fluorescence spectroscopy), electrochemical (cyclic voltammetry) and theoretical (TD-DFT) methods as well as by modifying its structure. The corresponding donor-acceptor compound bearing the widely employed dimesitylboryl acceptor group was also synthesized for comparison.

Spontaneous Metal-Free Transfer Hydrogenation of Iminoboranes with Ammonia Borane and Amine Boranes.

The first spontaneous, metal-free, room-temperature hydrogen transfer from ammonia borane to an iminoborane is reported. Mechanistic studies of the reaction indicate a concerted transfer of H+ and H- from donor to acceptor with an activation energy far below those of comparable concerted transfer hydrogenations. This reaction was employed in the syntheses and isolation of new B,N-disubstituted aminoboranes, a comparatively rare subset within the aminoborane family. This successful transfer hydrogenation to a highly dehydrogenated BN system may serve as a starting point for the design of new systems capable of reversible dehydrogenation/rehydrogenation.

Intriguing migrations in transient iminoborane adducts: two new pathways to aminoboranes.

Two unusual reactions were demonstrated with iminoboranes, both leading to aminoboranes of the type R2B[double bond, length as m-dash]NR2. In one case, a carboboration of di(tert-butyl)iminoborane with B(C6F5)3 led to (tBu)(C6F5)B-N(tBu)(B(C6F5)2). In the other, a transient IDipp (1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene) adduct of MesBNtBu undergoes a shift of one carbene-bound Dipp substituent to the iminoborane nitrogen atom, yielding (1-Dipp-imidazol-2-yl)(Mes)B-N(Dipp)(tBu). Mechanistic DFT studies indicate the intermediacy of a borane·iminoborane adduct in the carboboration reaction.

Release of Isonitrile- and NHC-Stabilized Borylenes from Group†VI Terminal Borylene Complexes.

A family of doubly isonitrile-stabilized terphenyl borylenes could be obtained by addition of three equivalents of isonitrile to the corresponding Cr and W terminal terphenyl-borylene complexes. The mechanism of isonitrile- and carbon-monoxide-induced borylene liberation was investigated computationally and found to be significantly exergonic in both cases. Furthermore, addition of a small N-heterocyclic carbene (NHC) to a terminal Cr borylene complex results in release of an NHC-stabilized borylene carbonyl species, whereas the analogous reaction with bulkier phosphines results in metal-centered substitution.

Increasing the Reactivity of Diborenes: Derivatization of NHC-Supported Dithienyldiborenes with Electron-Donor Groups.

A series of NHC-supported 1,2-dithienyldiborenes was synthesized from the corresponding (dihalo)thienylborane NHC precursors. NMR and UV/Vis spectroscopic data, as well as X-ray crystallographic analyses, were used to assess the electronic and steric influences on the B=B double bond of various NHCs and electron-donating substituents on the thienyl ligands. Crystallographic data showed that the degree of coplanarity of the diborene core and thienyl groups is highly dependent on the sterics of the substituents. Furthermore, any increase in the electron-donating ability of the substituents resulted in the destabilization of the HOMO and greater instability of the resulting diborenes.

Synthesis and Trapping of Iminoboranes by M=B/C=N Bond Metathesis.

Although the metathesis of metal-boron double bonds with elemental chalcogenides is an established process, no similar reactivity has been observed with element-nitrogen bonds. Such a reaction would provide a new route to iminoborane compounds (RB≡NR'), which have recently experienced renewed synthetic interest. Herein, we present the first observation of M=B/C=N metathesis reactions, which led to the isolation of a stable iminoborane in addition to further iminoborane cycloaddition products.

Protonation versus Oxonium Salt Formation: Basicity and Stability Tuning of Cyanoborate Anions.

Anhydrous H[BH2 (CN)2 ] crystallizes from acidic aqueous solutions of the dicyanodihydridoborate anion. The formation of H[BH2 (CN)2 ] is surprising as the protonation of nitriles requires strongly acidic and anhydrous conditions but it can be rationalized based on theoretical data. In contrast, [BX(CN)3 ]- (X=H, F) gives the expected oxonium salts (H3 O)[BX(CN)3 ] while (H3 O)[BF2 (CN)2 ]/H[BF2 (CN)2 ] is unstable. H[BH2 (CN)2 ] forms chains via N-H⋅⋅⋅N bonds in the solid state and melts at 54 °C. Solutions of H[BH2 (CN)2 ] in the room-temperature ionic liquid [EMIm][BH2 (CN)2 ] contain the [(NC)H2 BCN-H⋅⋅⋅NCBH2 (CN)]- anion and are unusually stable, which enabled the study of selected spectroscopic and physical properties. [(NC)H2 BCN-H⋅⋅⋅NCBH2 (CN)]- slowly gives H2 and [(NC)H2 BCN-BH(CN)2 ]- . The latter compound is a source of the free Lewis acid BH(CN)2 , as shown by the generation of [BHF(CN)2 ]- and BH(CN)2 ⋅py.

Isolation of a Reactive Cyclopropane Intermediate via a Unique Catalyst-Free Spontaneous Cyclopropanation Step at 0 °C.

A remarkably mild, catalyst-free cyclopropanation reaction is confirmed by low-temperature isolation of a fused bicyclic cyclopropane intermediate. This intermediate is one of two that were isolated and fully characterised in a three-step temperature-controllable reaction cascade.

Dibora[2]ferrocenophane: A Carbene-Stabilized Diborene in a Strained cis-Configuration.

Unsaturated bridges that link the two cyclopentadienyl ligands together in strained ansa metallocenes are rare and limited to carbon-carbon double bonds. The synthesis and isolation of a strained ferrocenophane containing an unsaturated two-boron bridge, isoelectronic with a C=C double bond, was achieved by reduction of a carbene-stabilized 1,1'-bis(dihaloboryl)ferrocene. A combination of spectroscopic and electrochemical measurements as well as density functional theory (DFT) calculations was used to assess the influence of the unprecedented strained cis configuration on the optical and electrochemical properties of the carbene-stabilized diborene unit. Initial reactivity studies show that the dibora[2]ferrocenophane is prone to boron-boron double bond cleavage reactions.

Elucidating the Reactivity of Vicinal Dicarbenoids: From Lewis Adduct Formation to B-C Bond Activation.

The reactivity of the diaminoacetylene Pip-C≡C-Pip (Pip=piperidyl=NC5 H10 ) towards phenyldichloro- and triphenylborane is presented. In the case of the less Lewis acidic PhBCl2 , the first example of a double Lewis adduct of a vicinal dicarbenoid is reported. For the more Lewis acidic triphenylborane, coordination to the bifunctional carbene leads to a mild B-C bond activation, resulting in a syn-1,2-carboboration. Ensuing cis/trans isomerization yields a novel ethylene-bridged frustrated Lewis pair (FLP). The compounds were characterized using multinuclear NMR spectroscopy, structural analysis, and mass spectrometry. Reactivity studies of both isomers with the N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (IMe) aided in elucidating the proposed isomerization pathway. DFT calculations were carried out to elucidate the reaction mechanism. The rather low free energy of activation is consistent with the observation that the reaction proceeds smoothly at room temperature.

Synthesis of Functionalized 1,4-Azaborinines by the Cyclization of Di-tert-butyliminoborane and Alkynes.

Di-tert-butyliminoborane is found to be a very useful synthon for the synthesis of a variety of functionalized 1,4-azaborinines by the Rh-mediated cyclization of iminoboranes with alkynes. The reactions proceed via [2 + 2] cycloaddition of iminoboranes and alkynes in the presence of [RhCl(PiPr3)2]2, which gives a rhodium η(4)-1,2-azaborete complex that yields 1,4-azaborinines upon reaction with acetylene. This reaction is compatible with substrates containing more than one alkynyl unit, cleanly affording compounds containing multiple 1,4-azaborinines. The substitution of terminal alkynes for acetylene also led to 1,4-azaborinines, enabling ring substitution at a predetermined location. We report the first general synthesis of this new methodology, which provides highly regioselective access to valuable 1,4-azaborinines in moderate yields. A mechanistic rationale for this reaction is supported by DFT calculations, which show the observed regioselectivity to arise from steric effects in the B-C bond coupling en route to the rhodium η(4)-1,2-azaborete complex and the selective oxidative cleavage of the B-N bond of the 1,2-azaborete ligand in its subsequent reaction with acetylene.

Regioselective Catalytic and Stepwise Routes to Bulky, Functional-Group-Appended, and Luminescent 1,2-Azaborinines.

The regioselective syntheses of 1,2-azaborinines is achieved using an unsymmetrical iminoborane through both catalytic and stepwise modular routes. The 1,2-azaborinine ring can be selectively functionalized in the 4- and/or 6-position through control of the stepwise reaction sequence, allowing access to vinyl-functionalized and redox-active, luminescent, donor-functionalized 1,2-azaborinines. The electrochemistry and photochemistry of a tetraarylamine-substituted 1,2-azaborinine are studied. Cyclic voltammetry of this compound, relative to a non-B,N-substituted reference molecule, showed an additional oxidation wave assigned to the oxidation of the azaborinine ring, while emission spectroscopy indicated that the azaborinine was significantly more fluorescent than the reference.

A Binuclear 1,1'-Bis(boratabenzene) Complex: Unprecedented Intramolecular Metal-Metal Communication through a B-B Bond.

We report the synthesis of the first 1,1'-bis(boratabenzene) species by tetrabromodiborane(4)-induced ring-expansion reactions of cobaltocene. Six equivalents of cobaltocene are required as the species plays the dual role of reagent and reductant to yield [{(η(5) -C5 H5 )Co}2 {µ:η(6) ,η(6) -(BC5 H5 )2 }]. The formally dianionic bis(boratabenzene) moiety with a boron-boron single bond can be viewed as a symmetric dimer of the parent boratabenzene anion as well as the first example of a diboron analogue of biphenyl. The solution electrochemistry of the bimetallic complex shows four stepwise redox events, indicating significant intramolecular interaction between the cobalt ions across the 1,1'-bis(boratabenzene) unit. The magnetic properties, as investigated by variable-temperature SQUID magnetometry, reveal weak intramolecular antiferromagnetic interactions. Density functional theory calculations support the experimental results and add insight into the various electronic states of the complex.

The reactivities of iminoboranes with carbenes: BN isosteres of carbene-alkyne adducts.

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N-heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (-45 °C). The adducts show short BN bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di-tert-butyliminoborane was reacted with 1,3-bis(isopropyl)imidazol-2-ylidene (IPr), the initially formed Lewis acid-base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4-position. Warming the iminoborane-CAAC adduct to room temperature resulted in an intramolecular cyclization to give a bicyclic 1,2-azaborilidine compound.

Photoionization and pyrolysis of a 1,4-azaborinine: retro-hydroboration in the cation and identification of novel organoboron ring systems.

The photoionization and dissociative photoionization of 1,4-di-tert-butyl-1,4-azaborinine by means of synchrotron radiation and threshold photoelectron photoion coincidence spectroscopy is reported. The ionization energy of the compound was determined to be 7.89 eV. Several low-lying electronically excited states in the cation were identified. The various pathways for dissociative photoionization were modeled by statistical theory, and appearance energies AE0K were obtained. The loss of isobutene in a retro-hydroboration reaction is the dominant pathway, which proceeds with a reverse barrier. Pyrolysis of the parent compound in a chemical reactor leads to the generation of several yet unobserved boron compounds. The ionization energies of the C4 H6 BN isomers 1,2- and 1,4-dihydro-1,4-azaborinine and the C3 H6 BN isomer 1,2-dihydro-1,3-azaborole were determined from threshold photoelectron spectra.

Direct synthetic route to functionalized 1,2-azaborinines.

A new catalytic synthetic route to functionalized 1,2-azaborinines has been developed by the [2+2]/[2+4] cycloaddition reactions of di-tert-butyliminoboranes and alkynes in presence of a rhodium catalyst. The first examples of ferrocene-functionalized azaborinines have been synthesized using this strategy. Moreover, the regioselectivity of this reaction can be controlled by the formation of an intermediate rhodium 1,2-azaborete complex, which results in the isolation of the first azaborinine boronic ester. The isolation of an NH-containing BN isostere by elimination of isobutene from an N(tBu) group under thermolytic conditions has also been achieved. Theoretical studies give further insight into the formation of 1,2-azaborinines and the elimination of isobutene from the N(tBu) group.

Synthesis and structure of distanna and tristanna ansa half-sandwich complexes of ruthenium and nickel.

The synthesis and structural characterization of the first tin-bridged ansa half-sandwich complexes via a two-step protocol from Na[η(5)-C(5)H(5)Ru(CO)(2)] and in situ generated Na[η(5)-C(5)H(5)Ni(CO)] are presented. Both compounds are characterized by multinuclear NMR spectroscopy and single-crystal diffraction.