Mehmet Ali Celik

Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines

Carefully chosen carbon substituents stabilize a boron oxidation state that bears an extra electron pair. Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. Here, we report the synthesis of a neutral tricoordinate boron derivative, which acts as a Lewis base and undergoes one-electron oxidation into the corresponding radical cation. These compounds can be regarded as the parent borylene (H-B:) and borinylium (H-B+.), respectively, stabilized by two cyclic (alkyl)(amino)carbenes. Ab initio calculations show that the highest occupied molecular orbital of the borane as well as the singly occupied molecular orbital of the radical cation are essentially a pair and a single electron, respectively, in the p(π) orbital of boron.

A Crystalline Phosphinyl Radical Cation

One-electron oxidation of a readily available phosphaalkene derived from a cyclic (alkyl)(amino)carbene affords a phosphorus-centered radical cation that is indefinitely stable both in solution and in the solid state, allowing a single X-ray diffraction study to be performed. This species can be regarded as a phosphinyl radical bearing a cationic substituent or, alternatively, as a carbene-stabilized phospheniumyl radical (carbene-RP(+*)).

Borylene complexes (BH)L2 and nitrogen cation complexes (N+)L2: isoelectronic homologues of carbones CL2.

Quantum chemical calculations using DFT (BP86, M05-2X) and ab initio methods (CCSD(T), SCS-MP2) have been carried out on the borylene complexes (BH)L(2) and nitrogen cation complexes (N(+))L(2) with the ligands L=CO, N(2), PPh(3), NHC(Me), CAAC, and CAAC(model). The results are compared with those obtained for the isoelectronic carbones CL(2). The geometries and bond dissociation energies of the ligands, the proton affinities, and adducts with the Lewis acids BH(3) and AuCl were calculated. The nature of the bonding has been analyzed with charge and energy partitioning methods. The calculated borylene complexes (BH)L(2) have trigonal planar coordinated boron atoms which possess rather short B-L bonds. The calculated bond dissociation energies (BDEs) of the ligands for complexes where L is a carbene (NHC or CAAC) are very large (D(e) =141.6-177.3 kcal mol(-1)) which suggest that such species might become isolated in a condensed phase. The borylene complexes (BH)(PPh(3))(2) and (BH)(CO)(2) have intermediate bond strengths (D(e) =90.1 and 92.6 kcal mol(-1)). Substituted homologues with bulky groups at boron which protect the boron atom from electrophilic attack might also be stable enough to become isolated. The BDE of (BH)(N(2))(2) is much smaller (D(e) =31.9 kcal mol(-1)), but could become observable in a low-temperature matrix. The proton affinities of the borylene complexes are very large, particularly for the bulky adducts with L=PPh(3), NHC(Me), CAAC(model) and CAAC and thus, they are superbases. All (BH)L(2) molecules bind strongly AuCl either η(1) (L=N(2), PPh(3), NHC(Me), CAAC) or η(2) (L=CO, CAAC(model)). The BDEs of H(3)B-(BH)L(2) adducts which possess a hitherto unknown boron→boron donor-acceptor bond are smaller than for the AuCl complexes. The strongest bonded BH(3) adduct that might be isolable is (BH)(PPh(3))(2)-BH(3) (D(e) =36.2 kcal mol(-1)). The analysis of the bonding situation reveals that (BH)-L(2) bonding comes mainly from the orbital interactions which has three major contributions, that is, the donation from the symmetric (σ) and antisymmetric (π(||)) combination of the ligand lone-pair orbitals into the vacant MOs of BH L→(BH)←L and the L←(BH)→L π backdonation from the boron lone-pair orbital. The nitrogen cation complexes (N(+))L(2) have strongly bent L-N-L geometries, in which the calculated bending angle varies between 113.9° (L=N(2)) and 146.9° (L=CAAC). The BDEs for (N(+))L(2) are much larger than those of the borylene complexes. The carbene ligands NHC and CAAC but also the phosphane ligands PPh(3) bind very strongly between D(e) =358.4 kcal mol(-1) (L=PPh(3)) and D(e) =412.5 kcal mol(-1) (L=CAAC(model)). The proton affinities (PA) of (N(+))L(2) are much smaller and they bind AuCl and BH(3) less strongly compared with (BH)L(2). However, the PAs (N(+))L(2) for complexes with bulky ligands L are still between 139.9 kcal mol(-1) (L=CAAC(model)) and 168.5 kcal mol(-1) (L=CAAC). The analysis of the (N(+))-L(2) bonding situation reveals that the binding interactions come mainly from the L→(N(+))←L donation while L←(N(+) )→L π backdonation is rather weak.

Formation of BN Isosteres of Azo Dyes by Ring Expansion of Boroles with Azides.

Herein, we present the results of our investigations on the effect of ortho substitution of aryl azides on the ring-expansion reaction of boroles, five-membered unsaturated boron heterocycles. These studies led to the isolation of the first 1,2-azaborinine-substituted azo dyes, which are bright yellow solids. One of the derivatives, (E)-2-mesityl-1-(mesityldiazenyl)-3,4,5,6-tetraphenyl-1,2-azaborinine, was found to be unstable in solution and to transform through a Jacobsen-like reaction into an indazole and 1-hydro-1,2-azaborinine. DFT calculations were performed to shed light on possible mechanisms to rationalize the unexpected azo-azaborinine formation and to draw conclusions about the role played by the ortho substituents in the reaction.

Ultrabroadband terahertz spectroscopy of a liquid crystal

Liquid crystals (LCs) are becoming increasingly important for applications in the terahertz frequency range. A detailed understanding of the spectroscopic parameters of these materials over a broad frequency range is crucial in order to design customized LC mixtures for improved performance. We present the frequency dependent index of refraction and the absorption coefficients of the nematic liquid crystal 5CB over a frequency range from 0.3 THz to 15 THz using a dispersion-free THz time-domain spectrometer system based on two-color plasma generation and air biased coherent detection (ABCD). We show that the spectra are dominated by multiple strong spectral features mainly at frequencies above 4 THz, originating from intramolecular vibrational modes of the weakly LC molecules.

Isolable tris(alkyne) and bis(alkyne) complexes of gold(I).

Golden trefoils: Tris(alkyne)gold complex [(coct)(3)Au][SbF(6)] (see picture; 1-SbF(6)) can be synthesized from cyclooctyne (coct) and AuSbF(6) generated in situ. Treatment of AuCl with cyclooctyne led to the bis(alkyne)gold complex [Au(coct)(2)Cl] (2). DFT analysis indicates that the cyclooctyne ligands are net electron donors in 1 but overall electron acceptors in 2. AuSbF(6) is shown to mediate [2+2+2] cycloaddition reactions of alkynes.

Neutral zero-valent s -block complexes with strong multiple bonding

The metals of the s block of the periodic table are well known to be exceptional electron donors, and the vast majority of their molecular complexes therefore contain these metals in their fully oxidized form. Low-valent main-group compounds have recently become desirable synthetic targets owing to their interesting reactivities, sometimes on a par with those of transition-metal complexes. In this work, we used stabilizing cyclic (alkyl)(amino)carbene ligands to isolate and characterize the first neutral compounds that contain a zero-valent s-block metal, beryllium. These brightly coloured complexes display very short beryllium-carbon bond lengths and linear beryllium coordination geometries, indicative of strong multiple Be-C bonding. Structural, spectroscopic and theoretical results show that the complexes adopt a closed-shell singlet configuration with a Be(0) metal centre. The surprising stability of the molecule can be ascribed to an unusually strong three-centre two-electron π bond across the C-Be-C unit.

Uncatalyzed Hydrogenation of First‐Row Main Group Multiple Bonds

Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep=1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a cAAC-supported diboracumulene (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol-1 ) two-step asynchronous H2 addition mechanism proceeding via a bridging hydride.

Generation of Dicoordinate Boron(I) Units by Fragmentation of a Tetra‐Boron(I) Molecular Square

Reduction of carbene-borane adduct [(cAAC)BBr2 (CN)] (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) cleanly yielded the tetra(cyanoborylene) species [(cAAC)B(CN)]4 presenting a 12-membered (BCN)4 ring. The analysis of the Kohn-Sham molecular orbitals showed significant borylene character of the BI atoms. [(cAAC)B(CN)]4 was found to reduce two equivalents of AgCN per boron center to yield [(cAAC)B(CN)3 ] and fragmented into two-coordinate boron(I) units upon reaction with IMeMe (1,3,4,5-tetramethylimidazol-2-ylidene) to yield the corresponding tricoordinate mixed cAAC-NHC cyanoborylene. The analogous cAAC-phosphine cyanoborylene was obtained by reduction of [(cAAC)BBr2 (CN)] in the presence of excess phosphine.

Interactions of Isonitriles with Metal-Boron Bonds: Insertions, Coupling, Ring Formation, and Liberation of Monovalent Boron.

Boryl, borylene, and base-stabilized borylene complexes of manganese and iron undergo a range of different reactions when treated with isonitriles including single, double, and partial isonitrile insertions into metal-boron bonds, ring formation, isonitrile coupling, and the liberation of new monovalent boron species. Two of the resulting cyclic species have also been found to react selectively with anhydrous HCl to form ring-opened products. The diverse isonitrile-promoted reactivity of transition-metal-boron compounds has been explored computationally.