f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C_5–8), cyclobutadienyl (C₄) complexes remain exceedingly rare. Here, we report that reaction of [Li₂{C₄(SiMe₃)₄}(THF)₂] ( 1 ) with [U(BH₄)₃(THF)₂] ( 2 ) gives the pianostool complex [U{C₄(SiMe₃)₄}(BH₄)₃][Li(THF)₄] ( 3 ), where use of a borohydride and preformed C₄-unit circumvents difficulties in product isolation and closing a C₄-ring at uranium. Complex 3 is an unprecedented example of an f-element half-sandwich cyclobutadienyl complex, and it is only the second example of an actinide-cyclobutadienyl complex, the other being an inverse-sandwich. The U−C distances are short (av. 2.513 Å), reflecting the formal 2− charge of the C₄-unit, and the SiMe₃ groups are displaced from the C₄-plane, which we propose maximises U−C₄ orbital overlap. DFT calculations identify two quasi-degenerate U−C₄ π-bonds utilising the ψ₂ and ψ₃ molecular orbitals of the C₄-unit, but the potential δ-bond using the ψ₄ orbital is vacant.     

f‐Element Half‐Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

Despite there being numerous examples of f‐element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C_5–8), cyclobutadienyl (C₄) complexes remain exceedingly rare. Here, we report that reaction of [Li₂{C₄(SiMe₃)₄}(THF)₂] ( 1 ) with [U(BH₄)₃(THF)₂] ( 2 ) gives the pianostool complex [U{C₄(SiMe₃)₄}(BH₄)₃][Li(THF)₄] ( 3 ), where use of a borohydride and preformed C₄‐unit circumvents difficulties in product isolation and closing a C₄‐ring at uranium. Complex 3 is an unprecedented example of an f‐element half‐sandwich cyclobutadienyl complex, and it is only the second example of an actinide‐cyclobutadienyl complex, the other being an inverse‐sandwich. The U?C distances are short (av. 2.513 Å), reflecting the formal 2? charge of the C₄‐unit, and the SiMe₃ groups are displaced from the C₄‐plane, which we propose maximises U?C₄ orbital overlap. DFT calculations identify two quasi‐degenerate U?C₄ π‐bonds utilising the ψ₂ and ψ₃ molecular orbitals of the C₄‐unit, but the potential δ‐bond using the ψ₄ orbital is vacant.     

Digital pulse interval modulation for optical communications

This article presents a study of digital pulse interval modulation (DPIM) for optical wireless communications using intensity modulation with direct detection. The DPIM code properties are discussed, and expressions for the transmission capacity, power spectrum, and error probability are presented. We show that for a given number of bits per symbol, DPIM has a higher transmission capacity, a similar spectral profile, and only a marginally inferior error probability performance compared with pulse position modulation, and is rather less complex to implement. Finally, problems associated with the nonuniform symbol length characteristics of DPIM together with possible solutions are discussed     

A Series of Rare-Earth Mesoionic Carbene Complexes

We report the synthesis and characterisation of a series of rare-earth mesoionic carbene complexes, [RE{N(SiMe₃)₂}₃{CN(Me)C(Me)N(Me)CH}] ( 3RE , RE=Sc, Ce, Pr, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), greatly expanding the limited library of f-block mesoionic carbene complexes. These complexes were prepared by treatment of the parent RE-triamides with an N-heterocyclic olefin (NHO), where an NHO backbone proton undergoes a formal 1,4-proton migration to the NHO-methylene group. For all RE(III) metals, as expected, quantum chemical calculations suggest only a σ-component to the metal−carbene bonding, in contrast to a previously reported uranium(III) congener where the 5f³ metal engages in a weak π-back-bond to the MIC. All complexes were characterised by static variable-temperature magnetic measurements, and dynamic magnetic measurements reveal that 3Dy and 3Er are field-induced single-molecule magnets (SMMs), with U_eff energy barriers of 35 and 128 K, respectively. Complex 3Dy is, as expected, a poorly performing SMM, but conversely 3Er performs unexpectedly well.     

New bent core mesogens with exceptionally high clearing points

A homologous series of novel two-ring alkoxythiophene-containing liquid crystals has been synthesized using a recently developed cyclization methodology to construct the heterocyclic core. The new heterocyclic unit is expected to confer high negative dielectric anisotropy and high dielectric biaxiality when incorporated into suitable mesogenic structures. The materials reported here possess nematic phases, with the longer chain homologues displaying smectic mesomorphism. In all cases the clearing points are significantly enhanced when compared with other two-ring thiophene-based mesogens. The synthesis, mesomorphic behaviour and stability are discussed, and the mesomorphic behaviour is compared with the analogous 4-alkoxy-4'-cyanobiphenyl derivatives synthesized by Gray.     

SINGLET OXYGEN INDUCED MUTAGENESIS OF BENZO[α]PYRENE DERIVATIVES

Singlet oxygen activates the mutagenicity of several benzo[a]pyrene (BP) derivatives in the absence of mammalian metabolic action. This has been demonstrated using a separated-surface-sensitizer system for generating chemically pure singlet oxygen, eliminating most of the complications that arise with singlet oxygen generation by conventional photosensitization. Salmonella typhimurium bacteria were exposed to singlet oxygen in the presence of certain BP derivatives and the mutation frequency determined with an azaguanine forward mutation assay. The mutation frequency was increased by exposure to singlet oxygen compared to light-only controls for those BP derivatives that were saturated at either the 7,8 or 9,10 positions but not both. The increase in mutation frequency depends on both the concentration of BP derivative and on the dose of singlet oxygen. Mutation frequency was also significantly increased when bacteria were treated with a solution of trans-7,8-dihydrodiol-BP that had been separately exposed to singlet oxygen, unequivocally demonstrating that the mutagenicity is due to the formation of a product of BP derivative oxidation by singlet oxygen and that this product has a lifetime at least on the order of minutes in acetonitrile. The requirement for singlet oxygen rather than some other form of reactive oxygen was confirmed by determination of the gas phase lifetime of the intermediate responsible for activating mutagenicity. This was performed by measuring the dependence of the mutation frequency on the distance separating the sensitizer from the target. This gives a value of 88 +/- 35 ms, which is in excellent agreement with the mean value of 89 ms calculated from previous independent determinations of the gas phase lifetime of singlet oxygen reported in the literature.