Electronically-coupled MM quadruply-bonded complexes of molybdenum and tungsten

The reaction of M2(O2CBu(t))4 (M = Mo, W) with a dicarboxylic acid in toluene yields compounds of general formula [M2]-O2C-X-CO2-[M2] ([M2] = M2(O2CBu(t))3; X = conjugated spacer). The M2 units are electronically coupled via interactions between the M2 delta and dicarboxylate pi* orbitals, and the magnitude of this coupling is revealed by electronic structure calculations and spectroscopic data. These compounds show intense metal to ligand charge transfer (MLCT) absorptions in the visible region of the electronic spectrum that are temperature and solvent dependent. Evidence of electronic coupling is seen in their cyclic voltammograms, which show two successive one-electron oxidations. The extent of electronic coupling in the mixed valence radical cations [M2]-O2C-X-CO2-[M2]+, generated by oxidation with one equivalent of AgPF6 or FeCp2PF6, is evaluated by EPR and UV-vis-NIR spectroscopic data, and delocalized behavior is observed in compounds with W2 units separated by up to 13.6 angstroms. The simplicity of the frontier M2 orbital interactions with the bridge pi orbitals provides a convenient system with which to study electron transfer in mixed valence systems, as compared to the extensively studied, but more complicated, dinuclear t(2g)6/t(2g)5 mixed valence compounds. Oligomeric and polymeric compounds incorporating M2 units have also been synthesized, having general formula [M2(O2CR)2(O2C-Thio-CO2)]n (Thio = n-hexyl substituted ter- and quinque-thiophenes). They can be deposited as thin films by spin coating, and show photoluminescence and electroluminescence. These metallo-polythiophenes show potential for application in electronic materials. (     

Dimolybdenum bis((S,S,S)-triisopropanolaminate(3-)): a blue compound with an unusual Mo-Mo triple bond

Mo2(OtBu)6 and Mo2(NMe2)6 each react with (S,S,S)-triisopropanolamine (2 equiv) in benzene to yield dimolybdenum bis((S,S,S)-isopropanolaminate(3-)), Mo2[(OC-(S)-HMeCH2)3N]2 (M identical to M), as a blue crystalline solid. Cell parameters at -160 degrees C: a = 17.389(6) A, b = 10.843(3) A, c = 10.463(3) A, beta = 125.28(1) degrees, Z = 2 in space group C2. The molecular structure involves an Mo2 unit inside an O6N2 distorted cubic box. The Mo2 axis is disordered about three positions with occupancy factors of ca. 45%, 45%, and 10%. Despite this disorder, the molecular structure is shown to contain a central Mo identical to Mo unit of distance 2.15(3) A coordinated to two triolate ligands which each have two chelating arms and one that spans the Mo identical to Mo bond. The local Mo2O6N2 moiety has approximate C2h symmetry, and the Mo-N distances are long, 2.4 A. The 1H and 13C(1H) NMR spectra recorded in benzene-d6 are consistent with the geometry found in the solid-state structure. The blue color arises from weak absorptions, epsilon approximately 150 dm3 mol-1 cm-1, at 580 and 450 nm in the visible region of the electronic absorption spectrum. Raman spectra recorded in KCl reveal pronounced resonance effects with excitation wavelengths of 488.0, 514.5, and 568.2 nm, particularly for the 322 cm-1 band, which can probably be assigned to nu(Mo identical to Mo). The electronic structure of this compound is investigated by B3LYP DFT calculations, and a comparison is made with the more typical ethane-like (D3d) Mo2(OR)6 compounds is presented. The distortion imposed on the molecule by the triisopropanolaminate(3-) ligands removes the degeneracy of the M-M pi molecular orbitals. The HOMO and SHOMO are both M-M pi and M-O sigma* in character, while the LUMO is M-M pi* and the SLUMO is predominantly M-O sigma* with metal sp character. The calculated singlet-singlet transition energies are compared with those implicit in the observed electronic spectrum.     

Total synthesis of (+)-amphidinolide A. Structure elucidation and completion of the synthesis

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of NMR chemical shift analysis and total synthesis. The 20-membered ring of amphidinolide A was formed by a ruthenium-catalyzed alkene-alkyne coupling to forge the C15-C16 bond. Using the reported structure 1 as a starting point, a number of diastereomers of amphidinolide A were prepared. Deviations of the chemical shift of key protons in each isomer relative to the natural material were used as a guide to determine the locations of the errors in the relative stereochemistry. The spectroscopic data for the synthetic and natural material are in excellent agreement.     

On R(4) symmetries in atomic structure

By defining a new rotation group in four dimensions we show that previous phase discrepancies can be accounted for. Furthermore we demonstrate that the new R(4) group is the one to be used in the study of atomic correlation.     

Development of combinatorial chemistry methods for coatings: high-throughput adhesion evaluation and scale-up of combinatorial leads

Coupling of combinatorial chemistry methods with high-throughput (HT) performance testing and measurements of resulting properties has provided a powerful set of tools for the 10-fold accelerated discovery of new high-performance coating materials for automotive applications. Our approach replaces labor-intensive steps with automated systems for evaluation of adhesion of 8 x 6 arrays of coating elements that are discretely deposited on a single 9 x 12 cm plastic substrate. Performance of coatings is evaluated with respect to their resistance to adhesion loss, because this parameter is one of the primary considerations in end-use automotive applications. Our HT adhesion evaluation provides previously unavailable capabilities of high speed and reproducibility of testing by using a robotic automation, an expanded range of types of tested coatings by using the coating tagging strategy, and an improved quantitation by using high signal-to-noise automatic imaging. Upon testing, the coatings undergo changes that are impossible to quantitatively predict using existing knowledge. Using our HT methodology, we have developed several coatings leads. These HT screening results for the best coating compositions have been validated on the traditional scales of coating formulation and adhesion loss testing. These validation results have confirmed the superb performance of combinatorially developed coatings over conventional coatings on the traditional scale.     

Intermolecular recognition and crystal packing in molybdenum and tungsten coordination polymers as deduced from powder X-ray diffraction data

The molecular conformations and packing of [(tBuCO2)3M2(mu-X)M2(O2CtBu)3], where M = Mo and W, and X = oxalate and perfluoroterephthalate, determined in the solid-state from powder X-ray diffraction analysis, reveal one-dimensional coordination polymers involving pivalate-oxygen to metal interactions (X = perfluoroterephthalate), and oxalate--as well as pivalate-oxygen to metal bonds (X = oxalate), and allows explanation of the unusual state-dependent chromic properties of these compounds.