Bidentate Lewis Base Research Articles

Kinetics of Cyclooctene Epoxidation with tert‐Butyl Hydroperoxide in the Presence of [MoO2X2L]‐Type Catalysts (L = Bidentate Lewis Base)

AbstractMoVI complexes with the general formula [MoO2X2L] [X = Cl, Br; L = 4,4'‐dimethyl‐2,2'‐bipyridine (dmbp) or 4‐hexyl‐4'‐methyl‐2,2'‐bipyridine (hmbp)] have been prepared and characterized by IR and solution NMR (1H, 13C, 17O and 95Mo) spectroscopy. The complexes were applied as catalysts in the homogeneous phase for the epoxidation of cyclooctene, with tert‐butyl hydroperoxide (TBHP) as the oxygen source. The desired epoxide was the only product and turnover frequencies of up to about 9000 h–1 could be reached. The catalytic activities increased in the order [MoO2Br2(dmbp)] < [MoO2Br2(hmbp)] < [MoO2Cl2(dmbp)] < [MoO2Cl2(hmbp)]. A kinetic model was built up for a homogeneous batch reactor based on a simplified mechanism involving three steps: (i) reversible coordination of TBHP to the starting MoVI complex to give a MoVI alkylperoxide; (ii) irreversible oxidation of cyclooctene to cyclooctene oxide by the species formed in step 1, with formation of the starting complex and tert‐butyl alcohol; (iii) reversible coordination of tert‐butyl alcohol to the starting complex. This model is consistent with the observed kinetics. The first step in this reaction mechanism was characterized in more detail by studying the kinetics of the reaction of the starting complexes with TBHP in the absence of any reductant by UV/Vis spectroscopy. Rate constants, equilibrium constants, and activation parameters were determined. All ΔS‡ values were negative and therefore support an associative mechanism in which a seven‐coordinate intermediate is formed. The results also suggest that the first step is not always the rate‐limiting step of cyclooctene epoxidation with these complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Synthesis and catalytic application of octahedral lewis base adducts of dichloro and dialkyl dioxotungsten(VI).

Complexes of the composition W(O)(2)(Cl)(2)L(2) and W(O)(2)(R)(2)L(2) (R = Me, Et; L(2) = bidentate Lewis base ligand) have been prepared and are fully characterized (including an exemplary X-ray crystal structure of W(O)(2)(Cl)(2)(4,4'-di-tert-butyl-2,2'-bipyridine)). This latter compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 8.3198(1) A, b = 13.3224(2) A, c = 18.0415(2) A, and Z = 4. The title complexes are applied as catalysts in olefin epoxidation catalysis with tert-butyl hydroperoxide (TBHP) as the oxidizing agent. The W(VI) complexes display only moderate turnover frequencies but can be reused several times without loss of catalytic activity. The highest activity can be achieved at reaction temperatures of ca. 90 degrees C. Chloro derivatives are somewhat more active than alkyl complexes, and sterically less crowded complexes show also higher activities than their congeners with bulky ligands L(2). Kinetic examinations show that the catalyst formation is the rate determining step and it is observed that tert-butyl alcohol, the byproduct of the epoxidation reaction, acts as a competitor for TBHP, thus lowering the reaction velocity during the course of the reaction but not irreversibly destroying the catalyst.

Bidentate Lewis Base Adducts of Methyltrioxorhenium(VII) and Their Application in Catalytic Epoxidation

Methyltrioxorhenium(VII) (MTO) forms octahedral adducts with bidentate Lewis bases. These complexes were isolated and fully characterized, including X-ray crystallography. The compounds display distorted octahedral geometry in the solid state with a tendency of disorder concerning the Re central atom. At elevated temperatures, they undergo rapid ligand-exchange reactions in solution. The ease of this ligand exchange depends mainly on the Lewis basicity of the ligand. The more Lewis basic the ligand is, the stronger the metal-ligand interaction is, as can be shown by NMR spectroscopy. All examined complexes are temperature stable but quite sensitive to light and moisture. In the presence of H(2)O(2), the complexes form very active and highly selective epoxidation catalysts. Peroxo complexes are generated, and at least one of the Re-N interactions is cleaved during this process. Total ligand dissociation only occurs in the case of very weakly coordinating bidentate ligands. The peroxo complexes of the MTO Lewis base adducts are, in general, more sensitive to water than MTO itself.

Self-Assembled Molecular Architectures on Surfaces: New Strategies Involving Metal−Organic Copolymers

A combination of oligomeric porphyrin copolymers and bidentate Lewis bases has been used to construct 2-dimensional multilayer molecular architectures on siloxane-derivatized glass substrates. The key to this supramolecular chemistry is coordinate covalent bonding between zinc-metalated porphyrins and bipyridyl “spacer” ligands, generating layers in an alternating, iterative fashion. UV−vis, XPS, AFM, and X-ray diffraction data support the formation of ordered thin film arrays. The stepwise deposition of each porphyrin layer is tracked by the linear increase in the absorbance of the porphyrin Soret band. XPS results demonstrate that the pyridinium coupling in the first monolayer is susceptible to X-ray decomposition during prolonged exposure. Extremely weak emission in the 600−800 nm region is attributed to the low chromophore concentration in the multilayer films. These results contrast sharply with those of spin-coated polymer thin films and represent an encouraging first step toward the design of new m...

Four-Coordinate Dimethylgallium Compounds Vary in Stability toward Hydrolysis

Four-coordinate dimethylgallium complexes were prepared by the reaction of dimethylgallium hydroxide with bidentate Lewis bases and evaluated for their stability toward decomplexation in water, a key property in determining their potential usefulness as radiopharmaceuticals, particularly those targeted at specific receptor proteins. Model compounds from six structural classes, containing bidentate ligands with oxygen−oxygen, oxygen−nitrogen, and sulfur−nitrogen donor atoms, were prepared from the ligand and dimethylgallium hydroxide; they were characterized spectroscopically and, in two cases, by X-ray crystallography. The percentage decomplexation of the stable (CH3)2Ga+ unit from the Lewis base when exposed to 1000 equiv of water in acetone-d6, determined by 1H NMR, was used as a measure of the relative hydrolytic stability of each compound. Among the compounds we have investigated, the percent hydrolysis under these conditions ranged from 10% to 100%, the most hydrolytically stable compounds proving to...

Lewis base adducts of halogenorhenium(VII) oxides: 17O NMR spectroscopy, structural aspects and catalysis

Halogenorheium(VII) oxides (XReO 3, 1a-1e, X = F, Cl, Br, CN, SCN) react with the bidentate Lewis base 4,4′-bis(tert-butyl)-2,2′-bipyridine to form soluble complexes of composition XReO 3 · L ( 2a-2e). These complexes were characterized by 17O NMR spectroscopy, mass spectroscoy, IR spectroscopy and elemental analysis. The single crystal X-ray structure of 2c (X = Br) was determined. The ReBr bond distance is comparatively long (265.30(8) pm) and probably partially ionic. In solution the complexes do not exchange the Lewis base ligands or the oxygen ligands rapidly as happens for the closely related organorhenium(VII) trioxides. The 17O NMR spectra of the halogeno complexes do not show temperature dependences. All complexes ( 2a-2e) show considerable catalytic activity in the oxidation of 2,3-dimethylnaphthalene.

Substitution of bi- and mono- dentate Lewis bases in organocobalt(III) complexes holding a tridentate ligand: Routes to novel series of organocobalt compounds

Routes have been designated to prepare new series of hexacoordinate cobalt(III) complexes containing a σ-bonded alkyl group (R), a mixed tridentate ligand and various mono- or bidentate Lewis bases. They consist of one or several reactions of either direct or proton-assisted ligand substitution, as exemplified by a model scheme for introducing two identical neutral monodentate ligands into the complexes with the planar tridentate anion o- −OC 6H 4C(Me) N(CH 2) 2NH 2 (7-Me-salen) −: [ R C o ( 7 - M e - s a l e n ) ( e n ) ] + → L H + [ R C o ( 7 - M e - s a l e n ) L 2 ] + → L ′ [ R C o ( 7 - M e - s a l e n ) L 2 ′ ] + → L ″ H + [ R C o ( 7 - M e - s a l e n ) L 2 ″ ] + Step 1 allows the chelating diamine in starting cation complexes to be replaced by weaker bases. Step 2 is suitable for introducing ligands effectively binding Co(III) ion, viz. typical chelating molecules and anions, or strong uniacidic bases. Step 3 permits the latter monodentate ligands to be exchanged for weaker bases. A variety of cationic, neutral and anionic complexes were thus obtained. Stoichiometry, stability and coordination geometry of these complexes are related to characteristics of newly introduced ligands (basicity, charge, steric requirements, ability to chelate). Their coordination in the trans- R-position was proved much looser and apparently less selective than that in the other ( cis) one. 1H and 13C NMR spectra of complexes under consideration were studied in the course of this work.

Electrochemical oxidation of organometallic complexes. Carbene and Lewis base complexes of chromium, molybdenum, and tungsten carbonyls

Tha oxidative one-electron transfer reactions of a wide variety of Group VI metal carbonyl derivatives have been detected by voltammetry. The compounds studied are of the type M(CO)6 –nLn or M(CO)6 – 2n(LL)n(M = Cr, Mo, W; L = monodentate Lewis base, n= 1 or 2; LL = bidentate Lewis base, n= 1 or 2) and carbene complexes of the type M(CO)5C(X)Y. The value of the potential, E½, is influenced by each of the variables M, L(or LL), n, X, and Y. The order of E½-values follows closely the apparent π-acceptor/σ-donor ratio of the ligand, but comparison with the results of molecular orbital calculations suggests that the influence of L (or of {C(X)Y}) upon the redox orbital is indirect. Steric effects are shown to influence the value of E½, especially where bidentate ligands are present. The oxidation potentials are related to synthetic chemistry in these systems.

Neue Kationische Nickelmonocyclopentadienyl-Komplexe

Abstract [Ni2(C5H5)3]BF4 reacts with monodentate Lewis-bases L (L = P(n-C4H9)3, P(C6H5)3, P(OCH3)3, P(OC2H5)3, P(OC6H5)3, As(C6H5)3, pyridine) as well as with bidentate Lewis-bases L-L (L-L = 1,2-C2H4[P(C6H5)2]2, 2,2′-dipyridyl) to form the compounds [C5H5NiL2]BF4 and [C5H5Ni (L-L)]BF4, respectively. In these reactions, nickelocene is also obtained. The properties of the cationic monocyclopentadienylnickel complexes will be briefly discussed.