Nathan P. Bowling

Synthesis of Simple Diynals, Diynones, Their Hydrazones, and Diazo Compounds: Precursors to a Family of Dialkynyl Carbenes (R1—C≡C—C̈—C≡C—R2)

A variety of substituted pentadiynols, -diynals, and -diynones have been prepared en route to precursors to dialkynyl carbenes (R(1)-C≡C-C-C≡C-R(2)). In light of the marginal stability associated with these simple systems, several strategies were required to assemble the carbon backbones. The requisite five-carbon skeletons were prepared using 4 + 1, 3 + 2, 2 + 2 + 1, and 2 + 1 + 1 + 1 coupling methodologies. The Dess-Martin periodinane serves as an excellent method for the oxidation of pentadiynols to diynals and diynones, although many of the oxidized products are sufficiently reactive that they were not isolated; rather, they were generated in situ and intercepted with nucleophiles such as tosylhydrazide or trisylhydrazide. The hydrazone derivatives are generally reliable precursors to diazo compounds and carbenes, although cyclization of the hydrazone to afford a pyrazole can be a complicating factor in certain instances.

Spectroscopy and Photochemistry of Triplet Methylpentadiynylidene (Me−C≡C−C̈−C≡C−H)

Triplet carbene methylpentadiynylidene, MeC(5)H (1), was investigated in cryogenic matrices by IR, UV/vis, and EPR spectroscopy. Broadband irradiation (lambda > 497 nm) of the isomeric diazo compounds, 1-diazo-hexa-2,4-diyne (2) or 2-diazo-hexa-3,5-diyne (3), generates triplet carbene 1. EPR spectra yield zero-field splitting parameters (|D/hc| = 0.62 cm(-1), |E/hc| < 0.0006 cm(-1)), which are typical for a triplet carbene with axial symmetry. The electronic spectrum of triplet 1 is characterized by a weak absorption in the near-UV and visible region (350-430 nm) with vibronic progressions corresponding to excitations of the acetylenic stretching and the terminal C[triple bond]C-H bending modes. Chemical trapping of triplet 1 in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at lambda > 399 nm, triplet 1 undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (6).

Intramolecular Halogen Bonding Supported by an Aryldiyne Linker

Intramolecular halogen bonds between aryl halide donors and suitable acceptors, such as carbonyl or quinolinyl groups, held in proximity by 1,2-aryldiyne linkers, provide triangular structures in the solid state. Aryldiyne linkers provide a nearly ideal template for intramolecular halogen bonding as minor deviations from alkyne linearity can accommodate a variety of halogen bonding interactions, including O···Cl, O···Br, O···I, N···Br, and N···I. Halogen bond lengths for these units, observed by single crystal X-ray crystallography, range from 2.75 to 2.97 Å. Internal bond angles of the semirigid bridge between halogen bond donor and acceptor are responsive to changes in the identity of the halogen, the identity of the acceptor, and the electronic environment around the halogen, with the triangles retaining almost perfect co-planarity in even the most strained systems. Consistency between experimental results and structures predicted by M06-2X/6-31G* calculations demonstrates the efficacy of this computational method for modeling halogen-bonded structures of this type.

Evidence of enhanced conjugation in ortho-arylene ethynylenes with transition metal coordination.

The effective conjugation of ortho and ortho-alt-para-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag(I) and Pd(II) cations. Significant bathochromic shifts in the electronic spectra, witnessed upon introduction of these metal bridges, are consistent with enhanced electron delocalization in the unsaturated backbone. Control studies suggest that this electronic behavior is attributable exclusively (in the case of Ag(I)) or partially (in the case of Pd(II)) to conformational restrictions of the conjugated backbones.

Dialkynyl Carbene Derivatives: Generation and Characterization of Triplet tert-Butylpentadiynylidene (t-Bu-C≡C-C̈-C≡C-H) and Dimethylpentadiynylidene (Me-C≡C-C̈-C≡C-Me)

Triplet carbenes t-butylpentadiynylidene (t-BuC(5)H, 1a) and dimethylpentadiynylidene (MeC(5)Me, 1b) have been produced photochemically from their corresponding diazo compound precursors and studied spectroscopically in cryogenic matrices (N(2) or Ar) at 10 K. The infrared, electronic absorption, and electron paramagnetic resonance spectra of these species exhibit numerous similarities to the spectra of pentadiynylidene (HC(5)H) and methylpentadiynylidene (MeC(5)H) recorded previously. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (t-BuC(5)H, 1a: |D/hc| = 0.61 cm(-1), |E/hc| ∼ 0 cm(-1); MeC(5)Me, 1b: |D/hc| = 0.62 cm(-1), |E/hc| ∼ 0 cm(-1)). Electronic spectra are characterized by weak absorptions (T(1) ← T(0)) in the near-UV and visible region (350-430 nm) with extended vibronic progressions. The electronic transitions of several -C(5)- carbenes are compared, and an apparent dependence of the transition wavelength on the level of alkyl substitution of the carbon chain is found. Chemical trapping of triplet 1a in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Both t-BuC(5)H (1a) and MeC(5)Me (1b) undergo photochemical rearrangement upon UV irradiation.

C—I⋯N and C—I⋯π halogen bonding in the structures of 1-benzyl­iodo­imidazole derivatives

Halogen bonding is a well-established and intensively studied intermolecular interaction that has also been used in the preparation of functional materials. While polyfluoroiodo- and polyfluorobromobenzenes have been widely used as aromatic halogen-bond donors, there have been very few studies of iodoimidazoles with regard to halogen bonding. We describe here the X-ray structures of three iodoimidazole derivatives, namely 1-benzyl-2-iodo-1H-imidazole, C10H9IN2, (1), 1-benzyl-4-iodo-1H-imidazole, C10H9IN2, (2), and 1-benzyl-2-iodo-1H-benzimidazole, C14H11IN2, (3), and the halogen bonds that dominate the intermolecular interactions in each of these three structures. The three-dimensional structure of (1) is dominated by a strong C-I...N halogen bond, with an N...I distance of 2.8765 (2) Å, that connects the molecules into one-dimensional zigzag ribbons of molecules. In contrast, the three-dimensional structures of (2) and (3) both feature C-I...π halogen-bonded dimers.

Cooperative halogen bonding and polarized π-stacking in the formation of coloured charge-transfer co-crystals

Red co-crystals are formed between the matched complementary electron rich halogen bond acceptors, isomeric 4-(N,N-dimethylamino)phenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene. The red 1 : 1 cocrystals exhibit strong halogen bonding and strong π-stacking. The N⋯I distances range from 2.80 to 2.85 A and the C–I⋯N angles are between 169.9 and 175.8°. In all four structures the donor and acceptor molecules are alternately π-stacked with the centroid to centroid distances between the dinitrophenyl moiety and the dimethylaminophenyl moiety ranging from 3.61 to 3.73 A. The calculated π–π stacking binding energy is −22.24 kcal mol−1 for the complex between 4-[4-(N,N-dimethylamino)phenylethynyl]pyridine and 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the calculated halogen bond binding energy between the same couple is −7.97 kcal mol−1.

Effects of Halogen and Hydrogen Bonding on the Electronics of a Conjugated Rotor

The electronic properties of a pyrazine-containing arylene ethynylene unit are influenced by hydrogen bond and halogen bond donors that are held in proximity of the pyrazine rotor. These interactions are evident with iodine- and bromine-centered halogen bonds and O-H- and C-H-based hydrogen bonds. Bathochromic shifts of UV-vis and fluorescence signals are the best indicators of this intramolecular attraction. The effects can be attenuated in solvents that are less favorable for intramolecular halogen or hydrogen bonding, such as 2-propanol, and amplified in solvents that are supportive, such as toluene. Intramolecular attractions promote planarity in the pyrazine ethynylene system, likely increasing the effective conjugation of the unsaturated backbone. Additionally, computations at the B3LYP and M062X levels of theory using 6-311++G(2d,p) and aug-cc-pVTZ basis sets suggest that the Lewis acidity of the halogen and hydrogen atoms influences electronic behavior even in the absence of conformational constraints.