Molecular and crystal structures of ruthenium and osmium arene clusters

The molecular and crystal structures of a number of ruthenium and osmium clusters of nuclearity between three and six containing arene fragments such as C₆H₆, C₆H₃Me₃, C₆H₄Me₂ and C₆H₅Me have been investigated. Attention has been focused on the relationship between the terminal ( ⁶-coordination) and face-capping ( ₃: ²: ²: ²-coordination) bonding modes. Empirical packing potential energy calculations have been employed to investigate the intermolecular organization in the crystal. It has been shown that the arene fragments in mono-arene clusters form ribbons, while in bis-arene clusters graphitic-like interactions throughout the crystal are established. The factors controlling the ease of arene reorientational motion in the solid state has also been investigated in relation to the shape, size and geometry of the molecules and of their interlocking modes.     

Carbon-carbon coupling on tetrahedral iridium clusters: X-ray molecular structures and multinuclear NMR studies of the two isomeric forms of [Ir4(CO)6(μ3-η-HCCPh)(μ2-η-C4H2Ph2)(μ-PPh2)2]

The reactions of [HIr₄(CO)₉(Ph₂PCCPh)(μ-PPh₂)] (1) or [Ir₄(CO)₈(μ₃-η²-HCCPh)(μ-PPh₂)₂] (2) with HCCPh gave two isomeric forms of [Ir₄(CO)₆(μ₃-η²-HCCPh)(μ₂-η⁴-C₄H₂Ph₂)(μ-PPh₂)₂] (3 and 4) in good yields as the only products. These compounds were characterized with analytical and spectroscopic data including ¹H, ¹³C and ³¹P NMR (1 and 2D) spectroscopy and their molecular structures were established by X-ray diffraction studies. Compounds 3 and 4 exhibit the same distorted butterfly metal polyhedral arrangement of metal atoms with two μ-PPh₂ that occupy different positions in the structures of the two isomers. Both molecules contain a HCCPh ligand bonded in a μ₃-η²-// mode to one of the wings of the butterfly and a metallacyclic ring, which resulted from head-to-tail coupling, in the case of [Ir₄(CO)₆(μ₃-η²-HCCPh){μ₂-η⁴-(H)CC(Ph)C(H)C(Ph)}(μ-PPh₂)₂] (3) and tail-to-tail coupling, in that of [Ir₄(CO)₆(μ₃-η²-HCCPh){μ₂-η⁴-(H)CC(Ph)C(Ph)C(H)}(μ-PPh₂)₂] (4), and which is linked to two metal atoms of the second wing of the butterfly.     

Mechanochemical assembly of hydrogen bonded organic-organometallic solid compounds

Solvent-free reactions with molecular systems have been exploited to prepare hybrid organic-organometallic solids: grinding of the complex [Fe(eta 5-C5H4COOH)2] with solid bases B generates quantitatively the corresponding hydrogen bonded salts [Fe(eta 5-C5H4COOH)(eta 5-C5H4COO)][HB] (B = 1,4-diazabicyclo[2.2.2]octane, 1,4-phenylenediamine); gas-solid reactions are also possible with volatile bases.     

Formation and characterization of the hexanuclear platinum cluster [Pt6(mu-PBu(t)2)4(CO)6](CF3SO3)2 through structural, electrochemical, and computational analyses

The reaction between equimolar amounts of Pt(3)(mu-PBu(t)()(2))(3)(H)(CO)(2), Pt(3)()H, and CF(3)SO(3)H under CO atmosphere affords the triangular species [Pt(3)(mu-PBu(t)()(2))(3)(CO)(3)]X, [Pt(3)()(CO)(3)()(+)()]X (X = CF(3)SO(3)(-)), characterized by X-ray crystallography, or in an excess of acid, [Pt(6)(mu-PBu(t)()(2))(4)(CO)(6)]X(2), [Pt(6)()(2+)()]X(2)(). Structural determination shows the latter to be a rare hexanuclear cluster with a Pt(4) tetrahedral core formed by joining the unbridged sides of two orthogonal Pt(3) triangles. The dication Pt(6)()(2+)() features also extensive redox properties as it undergoes two reversible one-electron reductions to the congeners [Pt(6)(mu-PBu(t)()(2))(4)(CO)(6)](+) (Pt(6)()(+)(), E(1/2) = -0.27 V) and Pt(6)(mu-PBu(t)()(2))(4)(CO)(6) (Pt(6)(), E(1/2) = -0.54 V) and a further quasi-reversible two-electron reduction to the unstable dianion Pt(6)()(2)()(-)() (E(1/2) = -1.72 V). The stable radical (Pt(6)()(+)()) and diamagnetic (Pt(6)()) species are also formed via chemical methods by using 1 or 2 equiv of Cp(2)Co, respectively; further reduction of Pt(6)()(2+)() causes fast decomposition. The chloride derivatives [Pt(6)(mu-PBu(t)()(2))(4)(CO)(5)Cl]X, (Pt(6)()Cl(+)())X, and Pt(6)(mu-PBu(t)()(2))(4)(CO)(4)Cl(2), Pt(6)()Cl(2)(), observed as side-products in some electrochemical experiments, were prepared independently. The reaction leading to Pt(3)()(CO)(3)()(+)() has been analyzed with DFT methods, and identification of key intermediates allows outlining the reaction mechanism. Moreover, calculations for the whole series Pt(6)()(2+)() --> Pt(6)()(2)()(-)()( )()afford the otherwise unknown structures of the reduced derivatives. While the primary geometry is maintained by increasing electron population, the system undergoes progressive and concerted out-of-plane rotation of the four phosphido bridges (from D(2)(d)() to D(2) symmetry). The bonding at the central Pt(4) tetrahedron of the hexanuclear clusters (an example of 4c-2e(-) inorganic tetrahedral aromaticity in Pt(6)()(2+)()) is explained in simple MO terms.     

Synthesis, Electrochemistry and Crystal Structure of the [Ni36Pt4(CO)45]6− and [Ni37Pt4(CO)46]6− Hexaanions

The [Ni₃₆Pt₄(CO)₄₅]^6- and [Ni₃₇Pt₄(CO)₄₆]^6- clusters have been obtained in mixture upon reaction in acetonitrile of [Ni₆(CO)₁₂]^2- salts with K₂PtCl₄ in a 2.5:1 molar ratio. The two hexaanions were indistinguishable by spectroscopic techniques. Crystallization of their trimethylbenzylammonium salts led to crystals of composition 0.5[NMe₃CH₂Ph]₆[Ni₃₆Pt₄(CO)₄₅]-0.5[NMe₃CH₂Ph]₆[Ni₃₇Pt₄(CO)₄₆]·C₃H₈O, hexagonal,space group P6₃ (No. 173), a=17.853(9), c=27.127(13) Å, Z=2; final R=0.057. The metal core of the [Ni₃₆Pt₄(CO)₄₅]^6- anion consists of a Pt₄ tetrahedron fully encapsulated in a shell of 36 Ni atoms belonging to a very distorted and incomplete ₅ tetrahedron. The [Ni₃₇Pt₄(CO)₄₆]^6- hexaanion derives from the former by capping the unique triangular face of the metal polyhedron with an additional Ni(CO) fragment. The [Ni₃₆Pt₄(CO)₄₅]^6--[Ni₃₇Pt₄(CO)₄₆]^6- mixture is rapidly degraded to the known [Ni₉Pt₃(CO)₂₁]^4- cluster by exposure to carbon monoxide. Its reaction with protic acids initially affords the corresponding [H_6-nNi₃₆Pt₄(CO)₄₅]^n--[H_6-nNi₃₇Pt₄(CO)₄₆]^n- (n=5, 4) derivatives, and eventually leads to rearrangement to the known [H_6-n Ni₃₈Pt₆(CO)₄₈]^n- species. Both [Ni₃₆Pt₄(CO)₄₅]^6--[Ni₃₇Pt₄(CO)₄₆]^6- and [HNi₃₆Pt₄(CO)₄₅]^5--[HNi₃₇Pt₄(CO)₄₆]^5- mixtures have been chemically and electrochemically reduced to their corresponding [Ni₃₆Pt₄(CO)₄₅]^n--[Ni₃₇Pt₄(CO)₄₆]^n- (n=7–9) and [HNi₃₆Pt₄(CO)₄₅]^n--[HNi₃₇Pt₄(CO)₄₆]^n- (n=6–8) mixtures.     

Origin, nature, and fate of the fluorescent state of the green fluorescent protein chromophore at the CASPT2//CASSCF resolution

Ab initio CASPT2//CASSCF relaxation path computations are employed to determine the intrinsic (e.g., in vacuo) mechanism underlying the rise and decay of the luminescence of the anionic form of the green fluorescent protein (GFP) fluorophore. Production and decay of the fluorescent state occur via a two-mode reaction coordinate. Relaxation along the first (totally symmetric) mode leads to production of the fluorescent state that corresponds to a planar species. The second (out-of-plane) mode controls the fluorescent state decay and mainly corresponds to a barrierless twisting of the fluorophore phenyl moiety. While a "space-saving" hula-twist conical intersection decay channel is found to lie only 5 kcal mol(-1) above the fluorescent state, the direct involvement of a hula-twist deformation in the decay is not supported by our data. The above results indicate that the ultrafast fluorescence decay observed for the GFP chromophore in solution is likely to have an intrinsic origin. The possible effects of the GFP protein cavity on the fluorescence lifetime of the investigated chromophore model are discussed.     

The ring-opening reaction of chromenes: a photochemical mode-dependent transformation

The efficiency of the photochemical ring-opening of chromenes (or benzopyrans) depends on the vibronic transition selected by the chosen excitation wavelength. In the present work, ab initio CASPT2//CASSCF calculations are used to determine the excited-state ring-opening reaction coordinate for 2H-chromene (C) and 2,2-diethyl-2H-chromene (DEC) and provide an explanation for such an unusual mode-dependent behavior. It is shown that excited-state relaxation and decay occur via a multimodal and barrierless (or nearly barrierless) reaction coordinate. In particular, the relaxation out of the Franck-Condon involves a combination of in-plane skeletal stretching and out-of-plane modes, while the second part of the reaction coordinate is dominated exclusively by a different out-of-plane mode. Population of this last mode is shown to be preparatory with respect to both C-O bond breaking and decay via an S(1)/S(0) conical intersection. The observed mode-dependent ring-opening efficiency is explained by showing that the vibrational mode corresponding to the most efficient vibronic transition has the largest projection onto the out-of-plane mode of the reaction coordinate. To support the computationally derived mechanism, we provide experimental evidence that the photochemical ring-opening reaction of 2,2-dimethyl-7,8-benzo(2H)chromene, that similarly to DEC exhibits a mode-dependent photoreaction, has a low ( approximately 1 kcal mol(-1)) activation energy barrier.     

Molecular Size and Electronic Structure Combined Effects on the Electrogenerated Chemiluminescence of Sulfurated Pyrene‐Cored Dendrimers

The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene core have been thoroughly investigated and complemented by theoretical calculations. The redox and luminescence properties of dendrimers are dependent on the generation number. From low to higher generation it is both easier to reduce and oxidize them and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. The analysis of such data evidences that the formation of the singlet excited state by cation–anion annihilation is an energy‐deficient process and, thus, the ECL has been justified through the triplet–triplet annihilation pathway. The study of the dynamics of the ECL emission was achieved both experimentally and theoretically by molecular mechanics and quantum chemical calculations. It has allowed rationalization of a possible mechanism and the experimental dependence of the transient ECL on the dendrimer generation. The theoretically calculated Marcus electron‐transfer rate constant compares very well with that obtained by the finite element simulation of the whole ECL mechanism. This highlights the role played by the thioether dendrons in modulating the redox and photophysical properties, responsible for the occurrence and dynamics of the electron transfer involved in the ECL. Thus, the combination of experimental and computational results allows understanding of the dendrimer size dependence of the ECL transient signal as a result of factors affecting the annihilation electron transfer.     

Mycotoxin profile of Fusarium langsethiae isolated from wheat in Italy: production of type‐A trichothecenes and relevant glucosyl derivatives

Fusarium langsethiae, formally described as a new species over a decade ago, has been identified as the main producer of HT‐2 (HT2) and T‐2 (T2) toxins in Europe in small cereal grains. Mycotoxin contamination caused by this Fusarium species can represent a food safety hazard that deserves further attention. In the present work, the mycotoxin profile in wheat cultures of F. langsethiae is presented with particular reference to the production of major type‐A trichothecenes and their glucosyl derivatives. F. langsethiae isolates, representative of the major Italian wheat cultivation areas, were tested for the production of T2, HT2, diacetoxyscirpenol (DAS) and neosolaniol (NEO), and relevant glucosyl derivatives. Liquid chromatography‐tandem mass spectrometry (LC‐MS/MS) was used for the identification and chemical characterization of these metabolites. F. langsethiae isolates under investigation resulted to be potent producers of T2, HT2 and NEO. Furthermore, a well‐defined set of isolates, all originating from Central Italy, produced also DAS. All isolates were found to be able to produce HT2 glucosyl derivatives, whereas only traces of T2 glucoside were detected in one sample. Furthermore, two mono‐glucosyl derivatives of NEO and one mono‐glucoside derivative of DAS were identified and characterized. The screening for the presence/absence of glucosylated trichothecenes in analyzed fungal extracts revealed a general co‐occurrence of these derivatives with the parent toxin at levels that could be roughly estimated to account up to 37% of the relevant unconjugated toxin. This is the first report of the production of glucosylated trichothecenes by F. langsethiae cultured on small grains. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental design for in-house validation of a screening immunoassay kit. The case of a multiplex dipstick for Fusarium mycotoxins in cereals

The benefits of using rapid qualitative methods to verify compliance of food and feed with legislation requirements include user-friendly format, the possibility of detection without expensive instrumentation, rapid response and affordable price. Prior to their use, however, the methods have to pass validation experiments, in order to assess their performance profile. An experimental protocol for in-house validation of a screening immunoassay has been designed and applied to evaluate performance characteristics of a multiplex dipstick kit for the determination of major Fusarium toxins, namely zearalenone, T-2 and HT-2 toxins, deoxynivalenol and fumonisins in wheat and maize. The test is intended for screening of cereals on the presence/absence of these mycotoxins at maximum permitted levels established by European legislation or target levels. The response of the measurement is determined with a reader device. Samples classified as negative are considered as compliant, whereas positive samples need to be re-analysed with confirmatory methods. The in-house validation design consisted of three steps, namely (1) estimating the precision of the method including "between day" effects and influences from different varieties of the matrices, (2) establishing robust cutoff values for the dipstick response at target mycotoxin levels assuming an acceptable rate of false negative results of 5 % and (3) assessment of the rate of false positive results of blank samples and samples containing the target analytes below the legal limits. The total precision expressed as relative standard deviation and determined individually for each analyte/concentration/matrix combination varied from 9 to 30 % and was considered as acceptable. In 17 out of 28 cases, the repeatability standard deviation was the most important factor. The predominance of the repeatability compared to the other factors (matrix, days) was an indicator for the ruggedness of the assay. The validation study demonstrated that the test was able to differentiate blank samples from samples contaminated at target mycotoxin levels with a false positive rate lower than 6 %. Considering realistic mycotoxin occurrence in European samples, significant economical benefits can be expected when using the test under real-world conditions.