Conventional true values compared

According to ISO/CD 18113-1:2005, the conventional true value of a control material can be an assigned value (obtained with a primary or reference measurement procedure), a consensus value or a procedure-defined value. The present study demonstrates that, for some biological quantities, the overall consensus value (“trimmed” mean value”) obtained in a proficiency testing program, which includes all results independently of the method of measurement, is not acceptable as a surrogate for a primary or reference method value. Therefore, for the biological quantities taken into account in this study, overall consensus value is not as good as primary or reference method value to estimate systematic error. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher. Apart from exceptional circumstances, they are not submitted to the usual referee procedure and go essentially unaltered.     

The mechanisms of the Stille reaction

Eighteen years ago in Angewandte Chemie John K. Stille reviewed a novel methodology, which eventually became known by his name, for the coupling of organostannanes with organic electrophiles. Since then that seed has blossomed into a multifaceted methodology full of hidden possibilities to explore, discover, and enjoy. Very recent modifications are making synthetic wishes come true that were only dreamed of a few years ago. Moreover, as important advances are being made in the understanding of the mechanistic details of the process, it is becoming increasingly possible to apply this essential reaction and its new variants in a less empirical way. The purpose of this Review is to give a critical account of this progress.     

Dimeric palladium complexes with bridging aryl groups: when are they stable?

Stable dimeric palladium(II) complexes of general formula [Pd(2)(mu-R)(2)(eta(3)-allyl)(2)] (R=haloaryl, mesityl) have been prepared. Their X-ray crystal structures, determined for some of the complexes, show that the two coordination square planes are usually coplanar. The haloaryl complexes are fluxional in solution, showing exchange between cis and trans isomers (relative to the orientation of the two allyl groups in the dimer) through solvent-assisted associative bridge splitting. A number of other ancillary ligands (O,O, S,S, or C,N donors) failed to stabilize the bridging situation. Also, bridging phenyls were unstable. The reasons for this behavior and the formation of alternative compounds in attempts at synthesizing them are fully analyzed and explained. Stable aryl bridges seem to be favored by a combination of factors: the use of ancillary ligands of small size and lacking electron lone pairs, and the use of aryl ligands reluctant to homo and hetero C--C coupling. These seem to be more important factors in the stabilization of bridging aryl complexes than the strength of the bridges themselves.     

Hydrogen bond vs proton transfer in HZSM5 zeolite. A theoretical study

The interaction of a large set of bases covering a wide range of the basicity scale with HZSM5 medium-size zeolites has been investigated through the use of two model clusters, namely 5T and 7T:63T. The 5T cluster has been treated fully ab initio at the B3LYP level, whereas the 63T cluster has been treated with the ONIOM2 scheme using the B3LYP:MNDO combination for geometry optimizations and B3LYP:HF/3-21G for adsorption energies. The optimized geometries of the different hydrogen bond (HB) and ion pair (IP) complexes obtained with both models are rather similar. However, there are significant dissimilarities as far as the adsorption energies are concerned, in particular when dealing with IP clusters whose intrinsic stability is largely underestimated when the simpler 5T model is used. 5T clusters could be used to obtain reasonable estimates of adsorption energies provided these are scaled by a factor of 1.1 for HB complexes and 1.4 for IP complexes. The zeolite cavity favors the proton transfer process, similarly to that found by third polar partners in gas-phase HB trimers. The intrinsic basicity of the base and its adsorption energy within the zeolite are correlated. From this correlation, is possible to conclude that, in general, bases with proton affinities (PA) larger than 200 kcal mol(-1) should lead to the formation of IPs, whereas bases with PA smaller than this value should form HB complexes.     

Binuclear mesogenic copper(i) isocyanide complexes with an unusual inorganic core formed by two tetrahedra sharing an edge

The preparation of binuclear mesogenic copper(I) isocyanide complexes [CuX(CNR)(2)](2) (X = halogen; R = C(6)H(4)C(6)H(4)OC(10)H(21) (L(A)), C(6)H(4)COOC(6)H(4)OC(n)H(2n+1)((L(B)), C(6)H(2)(3,4,5-OC(n)H(2n+1))(3) (L(C))) with an unusual tetrahedral core is described. The copper complexes with L(A) are not liquid crystals, but the Cu-L(B) complexes show SmA mesophases and the Cu-L(C) derivatives display hexagonal columnar mesophases, some of them at room temperature. The relationship between the molecular structure of the complexes and their thermal behavior is discussed.     

Neutral and anionic pentafluorophenyl complexes of palladium and platinum(II) and their reactivity towards amines

Summary The (n-Bu₄N)[M(C₆F₅)₃(CNR)] complexes (M=Pd or Pt; R=p-Tolyl, Me, Cy ort-Bu, prepared from (n-Bu₄N)[M(C₆F₅)₃(tht)] (tht = tetrahydrothiophene) and the appropriate isocyanide, RNC, prove to be unreactive towards benzylamine or MeOH. Thetrans-[Pd(C₆F₅)₂(CNR)₂] complexes react slowly with benzylamine to give the corresponding carbene complexestrans-[Pd(C₆F₅)₂{C(NHR)(NHBz)}₂], the rate of the reaction decreasing in the order:p-Tolyl > Me > Cy t-Bu (for R=t-Bu the carbene complex cannot be prepared). In the corresponding Pt complexes a marked decrease in reactivity is observed and only the most reactive isonitrile complex (R=p-Tolyl) gives the carbene complextrans-[Pt(C₆F₅)₂{C(NHTolyl-p)(NHBz)}_2}. Thecis-[M(C₆F₅)₂(CNTolyl)₂] complexes do not show any change in reactivity compared to the correspondingtrans-complexes, and givecis[M(C₆F₅)₂{C(NHTolyl-p)(NHBz)}₂].     

Observation of a hidden intermediate in the Stille reaction. Study of the reversal of the transmetalation step

A study of the reaction of cis-[PdRf2(AsPh3)2] (Rf = 3,5-C6Cl2F3) with ISnBu3 (that is the reversal of the natural Stille reaction of [PdRfI(AsPh3)2] with RfSnBu3) allows for the observation of cis-[PdRf2(AsPh3)(ISnBu3)], the expected intermediate from a cyclic transmetalation in the direct Stille reaction, thus providing experimental support to the operation of cyclic transmetalation pathways.     

Gold(I) complexes with hydrogen-bond supported heterocyclic carbenes as active catalysts in reactions of 1,6-enynes

Complexes [AuCl{C(NHR)(NHPy-2)}] (Py-2 ) 2-pyridyl; R ) Me, tBu, nBu, iPr, nheptyl) have been prepared in amodular way from [AuCl(CNPy-2)]. The carbene moiety has a hydrogen-bond supported heterocyclic structure similar to the nitrogen heterocyclic carbenes in the solid state, and in CH2Cl2 or acetone solution, which is open in the presence of MeOH. The compounds are good catalysts for the skeletal rearrangement of enynes, and for the methoxycyclization of enynes. In contrast, the complexes [AuCl{C(NHR)(NHPy-4)}] are scarcely active due to the blocking effect of the coordination position required for the catalysis by the nitrogen of the NHPy-4 group.     

Mononuclear, dinuclear, and hexanuclear goldI complexes with (aza-15-crown-5)dithiocarbamate

The reactions of sodium (aza-15-crown-5)dithiocarbamate with [AuClL] precursors lead to mono-, di-, or hexanuclear derivatives depending on L. The homoleptic hexanuclear gold(I) cluster [Au6(S2CNC10H20O4)6] is formed by displacement of the chloride and isocyanide ligands in [AuCl(CN(2,6-Me2C6H3))]. X-ray diffraction studies show a novel geometry in gold cluster chemistry where the six gold atoms display a cyclohexane-like geometry in a chair conformation with Au-Au-Au angles of 117.028(9) degrees, two short gold-gold distances of 2.9289(5) A, and bidentate bridging dithiocarbamate ligands. The molecular structure shows a crown of gold atoms surrounded by crown ethers. This derivative luminesces at 569 nm at room temperature in the solid state. A dinuclear isomer [Au2(S2CNC10H20O4)2] had been reported previously and was obtained by reaction with [AuCl(SMe2)]. The mechanism to obtain the hexanuclear derivative involves a mononuclear intermediate [Au(S2CNC10H20O4)(CNR)] for which the X-ray structure shows a short gold-gold distance of 3.565 A with the two molecules in an anti configuration. Phosphine gold(I) mononuclear derivatives [Au(S2CNC10H20O4)(PR3)] (R = Me, Ph, both characterized by X-ray diffraction) and dinuclear diphosphine derivatives [{Au(S2CNC10H20O4)}2(mu-P-P)] (P-P = dppm, bis(diphenylphosphinomethane); dppp, 1,3-bis(diphenylphosphinopropane); and dppf, 1,1'-bis(diphenylphosphinoferrocene)) are also reported. In the mononuclear complexes, the molecular structure confirms that the dithiocarbamato ligand is mainly acting as monodentate, with a second longer Au-S distance of 3.197 (PMe3), 2.944(4) (PPh3), and 2.968 A (CNR). Three phosphine complexes are emissive at 562 (PMe3), 528 (PPh3), and 605 nm (dppm), at 77 K. X-ray diffraction studies of the dppm derivative show gold-gold intramolecular contacts of 3.0972(9) A (3.2265(10) A for a second independent molecule) and basically monodentate coordination of the dithiocarbamato ligands. All the complexes extract sodium and potassium salts from aqueous solutions. The diphosphine derivatives are noticeably better extractors than the monophosphino derivatives, mainly for potassium salts.     

Self-organization of dendritic supermolecules, based on isocyanide-gold(I), -copper(I), -palladium(II), and -platinum(II) complexes, into micellar cubic mesophases

First- and second-generation dendrimers with an isocyanide group as the focal functional point (CN-G(n); n: 1,2) and their corresponding organometallic complexes [MCl(CN-G(n))] (M: Au, Cu), [{CuCl(CN-G(n))2}2], and trans-[MI2(CN-G(n))2] (M: Pd, Pt) have been synthesized. The free ligands and the first-generation complexes do not show mesogenic behavior, but all of the second-generation complexes display a thermotropic micellar cubic mesophase, over a large temperature range, and some of them directly at room temperature. The structure of the mesophase consists of the packing of two, discrete polyhedral micellar aggregates in a three-dimensional cubic Im$\bar 3$m lattice.