Synthesis and characterisation of BODIPY radical anions

The redox processes associated with BODIPY analogues are studied by electrochemical and spectroscopic methods revealing a characteristic profile for the persistent BODIPY radical and quenching of fluorescence upon reduction.     

Synthesis, structure, and bonding of stable dialkylsilaketenimines

Stable (N-aryl)- and (N-alkyl)dialkylsilaketenimines R2SiCNR' [R = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl, R' = 2,6-diisopropylphenyl (2a) and 1-adamantyl (2b)] were synthesized as blue and red crystals by the reactions of isolable dialkylsilylene 3 with 2,6-diisopropylphenyl isocyanide and 1-adamantyl isocyanide. X-ray single-crystal analysis disclosed that molecular structures of 2a and 2b were close to each other and were characterized to be allenic rather than zwitterionic or a silylene-isocyanide complex. The bonding characteristics of silaketenimines are found to be affected strongly by the substituents on silicon and nitrogen atoms. Remarkable red-shift of the pi(Si=C) --> pi*(C=N) band of 2a [lambdamax/nm (epsilon) 647(156)] compared with that of 2b [465 nm (109)] is ascribed to lowering of the pi*(C=N) orbital level due to significant interaction between pi*(C=N) and pi*(N-aryl) orbitals.     

Synthesis,structure and bonding of 2-aminopyridinium heptamolybdate trihydrate

Summary 2-aminopyridinium heptamolybdate trihydrate crystallizes in the monoclinic system with space group P2₁/n and Z=4 (R=0.030). The unit cell dimensions area=14.8161(4) Å,b=17.5073(4) Å,c=20.8492(6) Å, =107.503(2)°, V=5157.7(2) ų. The [Mo₇O₂₄]^6– anions in the 2-aminopyridinium, ammonium⁽⁴⁾, guanidium⁽²⁾, propyl- and isopropyl-ammonium⁽¹⁾ molybdates, while similar, show slightly differences in several bond lengths and angles. The distinguishing features of 2-aminopyridinium heptamolybdate trihydrate structure is its extensive hydrogen bonding. The planar cations and the water molecules are positioned so as to be able to form hydrogen bonds with either molybdate oxygen atoms or water oxygen atoms. Four different types of hydrogen bonds have been found-: N-H... O (mono- and bifurcated), N-H... Ow (monofurcated), Ow-Hw... O (mono- and bi-furcated) and Ow-Hw...OW (monofurcated). The closest approach distances associated with 27 of these potential hydrogen bonds vary from 2.67 to 3.24 Å^(7,8)). The proposed strong hydrogen bonding interactions appear to stabilize the structure and explain the way of three water molecules are lost upon heating. Some of these hydrogen bonds can play an important role in the possible photochromism of this compound.     

EPR spectra and structure of the chlorotrifluoroethylene and bromotrifluoroethylene radical anions

EPR spectra attributable to the halogenotrifluoroethylene radical anions have been generated by electron attachment in solid solutions at low temperatures. The spin density distribution in these radicals strongly suggests that the unpaired electron occupies a σ^* orbital, a conclusion which is supported by CNDO/2 calculations.     

1-Phosphinodiazaphosphene: Synthesis,crystal structure,and bonding properties

The iminophosphane, tBu₂P PN NR₂ (RSiMe₃), produced by base-catalyzed elimination of ClSiMe₃ from the corresponding phosphane, possesses an unusually long PN bond (162 pm), which is in accord with quantum chemical calculations.     

Synthesis and radical polymerization of hydrolytically stable dentin bonding agents

Acrylic groups containing phosphonic acids were synthesized by ether formation of ethyl α‐chloromethylacrylate with hydroxyalkyl phosphonates and subsequent hydrolysis to the corresponding phosphonic acid α‐methylsubstituted acrylates. Furthermore, phosphonic acids derived from acrylonitrile and acrylamide were synthesized. The monomers are hydrolytically stable in aqueous ethanol. The radical polymerization of the monofunctional phosphonic acids results in water soluble polymers, whereas in case of a phosphonic acid diacrylate a cross‐linked polymer was formed. The most radical polymerizable phosphonic acids can be used to promote the adhesion to dentin.     

Synthesis,structure, and properties of compounds containing both octahedral rhenium cluster cations and anions

The compound [{Re₆S₈}(3,5-Me₂pzH)₆]₂[{Re₆S₈}Br₆]?9H₂O?4EtOH containing a cluster cation and a cluster anion in a 2: 1 ratio was synthesized by mixing an ethanol solution of the rhenium cluster complex [{Re₆S₈}(3,5-Me₂pzH)₆]Br₂?2(3,5-Me₂pzH) (pzH is pyrazole) with an aqueous solution of Cs₄[{Re₆S₈}Br₆]?2H₂O. The resulting compound was characterized by a combination of physicochemical methods. In particular, luminescence properties were studied and compared with those of the parent cluster complexes. A crystal suitable for single-crystal X-ray diffraction was obtained by layering these solutions. According to single-crystal X-ray diffraction analysis, this compound is composed of a cluster cation and a cluster anion (in a ratio of 1: 1) and also of Zundel cations (H₅O₂ ⁺) and solvate water molecules and is described by the formula (H₅O₂)₂[{Re₆S₈}(3,5-Me₂pzH)₆][{Re₆S₈}Br₆]?4H₂O.     

Photoelectron spectroscopy of radical anions

The photoelectron spectroscopy of a number of radical anions has been investigated. We find the following electron affinities: EA(C₃) =1.981 ±0.020 eV, EA(C₃H) = 1.858 ±0.023 eV, EA(C₃H₂) = 1.794 ± 0.025 eV, EA(C₃O) = 1.34±0.15 eV, EA(C₃O₂) = 0.85±0.15 eV, EA(C₄O)= 2.05±0.15 eV, and EA(CS₂) = 0.895± 0.020 eV. The structure and bonding for each of these ions is discussed.     

Theoretical study of rhenium dinuclear complexes: Re-Re bonding nature and electronic structure

Four dinuclear rhenium complexes, [Re2Cl8](2-) (1), [Re2(mu-Cl)3Cl6](2-) (2a), [Re2(mu-Cl)3Cl6](-) (2b), and [Re2(mu-Cl)2Cl8](2-) (3), were theoretically investigated by the CASSCF, MRMP2, SA-CASSCF, and MCQDPT methods. Interesting differences in electronic structure and Re-Re bonding nature among these complexes are clearly reported here, as follows: In 1, the ground state is the 1A1g state. The approximate stabilization energies by the sigma, pi, and delta bonding interactions are evaluated to be 4.36, 2.89, and 0.52 eV, respectively, by the MRMP2 method. In 2a, the ground state is the 2E" state. The approximate stabilization energy by two degenerate delta bonding interactions is estimated to be 0.36 eV by the MCQDPT method. One delta bonding interaction of 2a is much weaker than that of 1, which is discussed in terms of the Re-Re distance and the Re oxidation state. In 2b, the ground state is the 1A1' state, of which multiconfigurational nature is extremely large unlike that of the 2E" ground state of 2a despite similarities between 2a and 2b. In 3, the sigma, pi, and delta bonding interactions are not effectively formed between two Re centers. As a result, the 1Ag, 3B1u, 5Ag, and 7B1u states are in almost the same energy within 0.03 eV. This result is consistent with the paramagnetism of 3 experimentally reported.     

Synthesis,crystal structure and chemical bonding of the novel compound IrGa2

The crystal structure of the novel constant composition compound IrGa₂ was determined from X-ray single-crystal diffraction data. The compound crystallizes in the space group Cmcm with the lattice parameters a=3.9021(3) Å, b=12.9925(8) Å and c=10.8808(7) Å. Its structure represents a new type with the Pearson symbol oC36. Analysis of the electron localization function (ELF) showed that the d electrons of both iridium atoms in IrGa₂ are involved in the bonding and that the two crystallographically independent iridium atoms have different bonding character.     

Superoxide radical anions on the surface of zirconia and sulfated zirconia: formation mechanisms, properties and structure

In situ ESR spectroscopy has been used for direct comparison of different thermal and light-induced processes leading to generation of superoxide radical anions on the surface of various zirconia and sulfated zirconia materials. For materials of both types the magnetic resonance parameters of the radical anions were found to be practically independent of the generation method, except for oxygen coadsorption with NO that yields radicals with somewhat smaller gz values. The parameters appear to depend mostly on the state of the surface zirconia cations stabilizing the radical anions, so that the g tensor anisotropy is significantly smaller over sulfated zirconia. It is shown that light-induced formation of superoxide radical anions in the presence of coadsorbed hydrocarbons can be initiated with visible light. Original SIET reaction mechanisms are suggested for the formation of superoxide radical anions by coadsorption with hydrocarbons and illumination after such coadsorption to extend the previously known ones to account for the observed phenomena. Cluster model DFT calculations of magnetic resonance parameters of O2- radical anions stabilized on the surface of zirconium dioxide showed that the adsorption complexes have a -shape rather than linear structure. The magnetic resonance parameters obtained by calculations practically match experimental data and adequately describe their changes after the surface modification with sulfates.     

Synthesis,structure, and bonding of the hexagonal form of the oxide carbide ScAlOC

Red single crystals of a hexagonal form of ScAlOC were obtained as a by-product from the synthesis of ScAl₃C₃ by reacting scandium and carbon in an aluminium melt at 1820 °C. The crystal structure (hP8, space group P6₃mc, Z = 2, a = 3.24793(3) Å, c = 10.1739(1) Å, 629 refl., 15 param., R₁(F) = 0.010, wR₂(F²) = 0.023) can directly be derived from the binary nitrides AlN and ScN or the oxide carbides Sc(O,C) and Al(O,C), respectively. ScAlOC-II or h-ScAlOC represents a new structure type with simple closest packing of alternating layers of oxygen and carbon. The stacking sequence is ABAC (=(hc)₂) with oxygen in a cubic and carbon in a hexagonal sequence. According to the difference in size Sc occupies octahedral voids between every second layer leading to layers of edge-sharing ScO₃C₃ octahedra. Aluminium is located in half of the tetrahedral voids. The AlOC₃-tetrahedra are connected to layers by common corners of the carbon atoms. h-ScAlOC continues the row of the rare examples of oxide carbides with ordered anion distribution. Band structure calculations by FP-(L)APW methods revealed that ScAlOC is electron precise with a band gap of 1.2 eV. Calculations of charges by the Bader-method reveal values of Sc^+1.87, Al^+2.33, O^−1.52 and C^−2.67. Together with the charge densities and the values of the Laplacian this stands for a mainly ionic bonding containing significant covalent contributions, too. Despite the close similarity to rhombohedral ScAlOC (r-ScAlOC) there is a striking difference in colour, as r-ScAlOC is black. This is confirmed and explained by the results of the P-DOS, because the lowest states of the conduction band are determined by Sc-d-states. These are significantly lower in r-ScAlOC with ScC₆ and ScO₆ octahedra than in h-ScAlOC leading to a calculated band gap of 0.3 eV.     

Synthesis, structure, and reactivity of a dinuclear metal complex with linear M-H-M bonding

Reaction of two equiv of 1-adamantylzinc bromide with (dippm)NiBr2 (dippm = bis(di-isopropylphosphino)methane) led to a dinuclear metal complex containing a unique linear bridging hydride ligand. The hydride was characterized by neutron diffraction methods, which confirmed a linear bonding mode. Preliminary reactivity studies of this unusual dimer are reported.     

Density functional calculations of electronic g-tensors for semiquinone radical anions. The role of hydrogen bonding and substituent effects

A recently developed density functional approach has been used to carry out a systematic computational study of electronic g-tensors for a series of 1,4-semiquinone radical anions. Good agreement with high-field EPR data in frozen 2-propanol is achieved only after taking into account the significant reduction of g-tensor anisotropy caused by hydrogen bonding to solvent molecules. The comparison of various model systems for the first solvation shell suggests two hydrogen bonds from 2-propanol molecules to each of the carbonyl groups of the radical anions, and one additional hydrogen bond to each of the methoxy groups in ubiquinone systems. 2-Propanol makes stronger hydrogen bonds than water and thus influences g-tensor anisotropy more strongly. Substituent effects at the semiquinone are reproduced quantitatively by the calculations. The g-tensor anisotropy is influenced significantly by the conformations of methyl and methoxy substituents, with opposite contributions. Analyses and interpretations of the interrelations between structure, bonding, and spectroscopic data are provided. The relevance of the computational results for the EPR spectroscopy of semiquinone radical anions in photosynthetic reaction centers is discussed.     

Some aspects of structure and bonding in the perchlorodiphenylaminyl radical revealed by ESCA

The ESCA core level spectra of the perchlorodiphenylaminyl radical have been measured and are interpretated in terms of a significant unpaired spin density on nitrogen. Comparison is drawn with conclusions based on the corresponding ESR data. Satellites are observed to the low kinetic energy side of the direct photoionization peaks and these are interpretated in terms of π → π^* shake up transitions accompany core ionizations.     

Synthesis, structure and bonding of the hypoelectronic cluster compound Sr3Tl5

The Sr3Tl5 phase was prepared by high temperature synthesis techniques through reaction of the high purity elements in the welded Nb tubes.The structure established through X-ray structural analysis shows the compound is a good hypoelectronic trielide example of the Pu3Pd5 structural type in which skeletal electron count is lower than in a traditional Zintl phase(Cmcm,Z = 4;a = 10.604(2) ,b = 8.675(2) ,and c = 10.985(2) ;V = 1010.5(3) 3).The strontium size and the compound’s polarity appear responsible for this thallium phase crystallization in the Pu3Pd5 family type rather than the isoelectronic Sr3In5 version.A first-principle electronic structure calculation(LMTO) demonstrates that the strontium atoms participate substantially in Sr-Tl bonding in the structure.