[Pd(Fmes)2(tmeda)]: A Case of Intermittent C?H⋅⋅⋅F?C Hydrogen‐Bond Interaction in Solution

The X‐ray structure of the title compound [Pd(Fmes)₂(tmeda)] (Fmes=2,4,6‐tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′‐tetramethylethylenediamine) shows the existence of uncommon C? H???F? C hydrogen‐bond interactions between methyl groups of the TMEDA ligand and ortho‐CF₃ groups of the Fmes ligand. The ¹⁹F NMR spectra in CD₂Cl₂ at very low temperature (157 K) detect restricted rotation for the two ortho‐CF₃ groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen‐bond energy shows that it is too weak (about 7 kJ mol^?1) to account for the rotational barrier observed (ΔH^≠=26.8 kJ mol^?1), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.     

Oxidative decarbonylation of m-terphenyl isocyanide complexes of molybdenum and tungsten: precursors to low-coordinate isocyanide complexes

Synthetic studies are presented addressing the oxidative decarbonylation of molybdenum and tungsten complexes supported by the encumbering m-terphenyl isocyanide ligand CNAr(Dipp2) (Ar(Dipp2) = 2,6-(2,6-(i-Pr)(2)C(6)H(3))(2)C(6)H(3)). These studies represent an effort to access halide or pseudohalide M/CNAr(Dipp2) species (M = Mo, W) for use as precursors to low-coordinate, low-valent group 6 isocyanide complexes. The synthesis and structural chemistry of the tetra- and tricarbonyl tungsten complexes trans-W(CO)(4)(CNAr(Dipp2))(2) and trans-W(NCMe)(CO)(3)(CNAr(Dipp2))(2) are reported. The acetonitrile adducts trans-M(NCMe)(CO)(3)(CNAr(Dipp2))(2) (M = Mo, W) react with I(2) to form divalent, diiodide complexes in which the extent of decarbonylation differs between Mo and W. In the molybdenum example, the diiodide, dicarbonyl complex MoI(2)(CO)(2)(CNAr(Dipp2))(2) is generated, which has an S = 1 ground state in solution. Paramagnetic group 6 MX(2)L(4) complexes are rare, and the structure of MoI(2)(CO)(2)(CNAr(Dipp2))(2) is discussed in relation to other diamagnetic and C(2v)-distorted MX(2)L(4) complexes. Diiodide MoI(2)(CO)(2)(CNAr(Dipp2))(2) reacts further with I(2) to effect complete decarbonylation, producing the paramagnetic tetraiodide complex trans-MoI(4)(CNAr(Dipp2))(2). The reactivity of the trans-M(NCMe)(CO)(3)(CNAr(Dipp2))(2) (M = Mo, W) complexes toward benzoyl peroxide is also surveyed, and it is shown that dicarboxylate complexes can be obtained by oxidative or salt-elimination routes. The reduction behavior of the tetraiodide complex trans-MoI(4)(CNAr(Dipp2))(2) toward Mg metal and sodium amalgam is studied. In benzene solution under N(2), trans-MoI(4)(CNAr(Dipp2))(2) is reduced by Na/Hg to the η(6)-arene-dinitrogen complex, (η(6)-C(6)H(6))Mo(N(2))(CNAr(Dipp2))(2). The diiodide-η(6)-benzene complex (η(6)-C(6)H(6))MoI(2)(CNAr(Dipp2))(2) is an isolable intermediate in this reduction reaction, and its formation and structure are discussed in context of putative low-coordinate, low-valent molybdenum isocyanide complexes.     

A novel organic-inorganic hybrid based on a dinuclear copper complex supported on a keggin polyoxometalate

The autoassembly process of copper-oxalate dimers and Keggin type polyoxometalates leads to the first example of a new family of organic-inorganic hybrids, K(14)[(Cu(2)(bpy)(2)(mu-ox))(SiW(11)O(39)Cu(H(2)O))](2)[SiW(11)O(39)Cu(H(2)O)]. approximately 55H(2)O. This compound crystallizes in the monoclinic space group C2/m, a = 37.932(6) A, b = 21.303(3) A, c = 12.546(2) A, beta = 106.16(1) degrees, Z = 2. The crystal structure reveals the presence of a polymeric hybrid built up of alternating dimer and polyoxometalate entities.     

Sorbent trapping solid-phase microextraction of fragrance allergens in indoor air

Exposure to fragrance substances is exponentially increasing in our daily life due to the enhanced use of scented products. Some fragrances are known to be important sensitizers, inhalation being an important exposure pathway in indoor environments. A simple and sensitive method based on solid-phase enrichment and solid-phase microextraction (SPME) followed by gas chromatography–mass spectrometry (GC–MS) has been developed for the analysis of 24 volatile fragrance allergens in indoor air. Suspected allergens present in the air (0.2 m³) were adsorbed onto a very small quantity of florisil (25 mg) and then transferred to a SPME fiber in the headspace mode (HS). To the best of our knowledge, this paper describes the first application of SPME for the determination of these compounds in air samples. The experimental parameters affecting the microextraction process have been optimized using a multifactor experimental design strategy. Accuracy, linearity, precision and detection limits (LODs) were evaluated to assess the performance of the proposed method. External calibration, using spiked sorbent standards, and not requiring the complete sampling process (only the SPME step), demonstrated to be suitable for the quantification of all suspected allergens. Recovery studies were performed at three concentration levels (0.04, 1.00 and 50 μg m⁻³), obtaining quantitative recoveries (≥85%) in most cases. LOD values at the low ng m⁻³ level were achieved for all the target compounds. The application of the method to daily home air samples demonstrated the ubiquity of this kind of fragrance ingredients in quotidian indoor environments, finding 18 of the 24 considered compounds in concentrations ranging from 0.01 to 56 μg m⁻³. Benzyl alcohol, linalool, citronellol, ionone and lilial were found in most analyzed samples.     

Formation of trinuclear palladium orthometalated complexes with unprecedented asymmetrical (mu 3-S)(mu 3-X) bridges (X = OH, SR, O2CR) from mu 2-hydroxo dimeric complexes and CS2

Complexes [Pd(3)(mu(3)-S)(mu(3)-X)(L)(3)] (L = orthometalated imine), obtained by an unusual reaction of mu(2)-OH dimeric complexes and CS(2), are an unprecedented type of asymmetrical bridges between metallatriangles, which force an all-cis arrangement of the three orthometalated ligands relative to the metallatriangle.     

Pseudo Jahn-Teller origin of nonplanarity and rectangular-ring structure of tetrafluorocyclobutadiene

It is shown that the pseudo Jahn-Teller effect (PJTE) in combination with ab initio calculations explains the origin of instability of the planar configuration of tetrafluorocyclobutadiene, C(4)F(4), with respect to a puckered structure and square-to-rectangle distortion of the carbon ring, and rationalizes its difference from the planar-rectangular geometry of C(4)H(4) and nonplanar (puckered) structure of Si(4)H(4). The two types of instability and distortion of the high-symmetry D(4h) configuration in these systems emerge from the PJT coupling of the ground B(2g) state with the excited A(1g) term producing instability along the b(2g) coordinate (elongation of the carbon or silicon square ring), and with the excited E(g) term resulting in e(g) (puckering) distortion. A rhombic distortion b(1g) of the ring is also possible due to the coupling between excited A(1g) and B(1g) terms. For C(4)F(4), ab initio calculations of the energy profiles allowed us to evaluate the PJTE constants and to show that the two instabilities, square-to-tetragonal b(2g) and puckering e(g) coexist, thus explaining the origin of the observed geometry of this system in the ground state. The preferred cis-trans (e(g) type) puckering in C(4)F(4) versus trans-trans puckering (b(2u) distortion) in Si(4)H(4) follows from the differences in the energy gaps to their excited electronic E(g) and A(1u) terms causing different PJTE in these two cases.     

Physicochemical characteristics of protein-NP bioconjugates: the role of particle curvature and solution conditions on human serum albumin conformation and fibrillogenesis inhibition

Gold nanoparticles (Au NPs) from 5 to 100 nm in size synthesized with HAuCl(4) and sodium citrate were complexed with the plasma protein human serum albumin (HSA). Size, surface charge, and surface plasmon bands of the Au NPs are largely modified by the formation of a protein corona via electrostatic interactions and hydrogen bonding as revealed by thermodynamic data. Negative values of the entropy of binding suggested a restriction in the biomolecule mobility upon adsorption. The structure of the adsorbed protein molecules is slightly affected by the interaction with the metal surface, but this effect is enhanced as the NP curvature decreases. Also, it is observed that the protein molecules adsorbed onto the NP surface are more resistant to complete thermal denaturation than free protein ones as deduced from the increases in the melting temperature of the adsorbed protein. Differences in the conformations of the adsorbed protein molecules onto small (<40 nm) and large NPs were observed on the basis of ζ-potential data and FTIR spectroscopy, also suggesting a better resistance of adsorbed protein molecules to thermal denaturing conditions. We think this enhanced protein stability is responsible for a reduced formation of HSA amyloid-like fibrils in the presence of small Au NPs under HSA fibrillation conditions.