Joseph A. Heppert

Octahedral Mo6 clusters supported by methoxide ligands

Abstract The structures of the [Mo6(μ3-Cl)8(OMe)6]2− and [Mo6(μ3-OMe)8(OMe)6]2− anions have been determined and reveal interesting comparisons of MM and MO distances with related Mo6(μ3-X)84+ containing compounds and polynuclear alkoxides of molybdenum.

Bisphenol ligands in the stereocontrol of transformations catalyzed by titanium and tungsten

Abstract Bisphenol ligands have a rich coordination chemistry in the early transition elements. 1,1′-Bi-2-naphthols are of particular interest because of their continued utility in controlling the stereochemistry of metal catalyzed processes. In several projects in our laboratory, we have endeavored to examine the potential scope and mechanism by which modifications of bisphenolate ligands can influence the stereoselectivity of electrocyclic reactions. In this paper we discuss two of these studies: an examination of stereochemically modified ROMP precatalysts and a study of well characterized binaphtholate titanium chloride Lewis acid catalysts.

The role of two-component catalysts containing chelating bisaryloxide ligands in controlling the stereochemistry of the metathesis polymerization of norbornene

Abstract The ring-opening metathesis polymerization of norbornene and 5,5-dimethylnorbornene has been studied with a range of tungsten(VI) ring-opening metathesis polymerization procatalysts containing chelating diolate ligands, including catechols, 1,8-dihydroxynaphthalene, 1,1′-bi-2-naphthol, biphenanthrol and bis(2-hydroxyphenyl) methane. One class of these catalysts, W(X)(OArO)Cl 2 (THF) (X = O or NArMe 2 ) was produced via alcoholysis reactions between W(X)Cl 4 and the respective diol. Polymers of norbornene produced with these catalysts showed a reasonable correlation between ring size and cis -selectivity. This effect is most likely steric in origin. Analyses of the tacticity of poly-5,5-dimethylnorbornene showed a correlation betwen high cis -olefin content and syndiotacticity, even when produced with catalysts containing asymmetric chelating diolate ligands.

Tetracarbonyl ferrate derivatives of (η6-arene)Cr(CO)3: new reagents for carbon–carbon bond formation

The reaction of Na2[Fe(CO)4] and (η6-ClC6H5)Cr(CO)3 in tetrahydrofuran/N-methylpyrrolidinone (THF/NMP) produces Na[(η6-{(CO)4Fe}C6H5)Cr(CO)3], while various (η6-o-LiXC6H4)Cr(CO)3 derivatives react with Fe(CO)5 to produce Li[(η6-o-{(CO)4Fe(CO)}XC6H4)Cr(CO)3] complexes.

Attack by allenes on a bridging alkylidyne ligand at a ditungsten centre: formation of a bridging allyl-η3-W1,σ-W2-alkylidene ligand

The alkylidene bridged compounds [(Me3SiCH2)2W(µ-CSiMe3)]2(1), [(Me3SiCH2)(ButO) W(µ-CSiMe3)]2(2) and [(PriO)2W(µ-CSiMe3)]2(3) each react with allenes R1CHCCHR2, where R1= R2= H, Me or R1= Me, Ph and R2= H, to give, by C–C bond formation between the tertiary carbon atom of the allene and the bridging carbon atom of one of the µ-CSiMe3 ligands, a bridging ligand which is a formally a π-allyl to one tungsten atom and a terminal alkylidene to the other; the rate of reactions follows the order (1) > (2) > (3); CH2CCH2 RCHCCH2 > RCHCCHR, and the C–C bond formation is stereoselective.

Hydrogen bonding in tungsten(VI) salicylate free acids

Abstract Hydrogen-bonded free acid dimers of the formula [W(X)Cl3(Hsal–R)]2 (where X=O (1), NC6H3-2,6-Me2 (2), Ph2C2 and Hsal–R=substituted salicylate monoanion) are prepared through reactions between W(X)Cl4 precursors and functionalized salicylic acids (H2sal–R). Download : Download full-size image The free acids are stable at ambient temperature and exist as dimers in solution, although the structure of the dimer is not known. Spectroscopic studies show that the electronic characteristics of the π-donor ligands directly affect the electronic environments of the carboxylate functionality. This influence is observed in weaker binding constants of diethyl ether (KX) as the X ligand becomes more strongly π donating: O⩾NC6H3–2,6-Me2>Ph2C2. In other words, the oxo and aryl imido species are more acidic than the diphenylacetylene compounds in the Bronsted sense owing to the higher Lewis acidity of their tungsten centers. Salicylate adducts of the type W(X)Cl3(Hsal–R)⋯OR2′ (X=O (3), NC6H3–2,6-Me2 (4)) and W(X)Cl3(Hsal–R)⋯NEt3 (X=O (5), NC6H3–2,6-Me2 (6)) have been isolated and characterized. A comparison of two structures, W(NC6H3–2,6-Me2)Cl3(Hsal–3-Me⋯L) where L=OEt2 (4c) and NEt3 (6c), shows that 6c has more charge localization on both the carboxylate group and tungsten center than 4c. The charge separation in amine adducts (i.e. salts) contributes to association of these salts with the free acid (i.e. W(X)Cl3(Hsal–R⋯L)⋯[W(X)Cl3(Hsal–R)]). The strong hydrogen bonding exhibited by the free acids leads to the formation of supramolecular complexes organized around poly(ether) templates, including dimethoxyethane (in 3e) and 18-crown-6.

4,4′-Biphenolate complexes of titanium and zirconium

Abstract 4,4′-Biphenolate ligands (R4BIPOH2) bearing methyl or tert-butyl substituents in the 3,3′,5,5′-positions participate in alcoholysis reactions with metal precursors, including TiCl4, Ti(OiPr)4 and Cp2MCl2. The tetra-tert-butyl derivative shows evidence of an intermediate monotitanium species (Bu4BIPOH)Ti(OiPr)3 during titrations of the ligand with Ti(OiPr)4, while (Bu4BIPO)(TiOiPr)3)2 exists as a simple monomeric molecule in solution. (Me4BIPO)(Ti(OiPr)3)2 shows evidence of aggregation into oligomeric species in solution, although its insolubility hampered the complete characterization of these species. A reaction between the d1 complex TiCl3(THF)3 and 3,3′,5,5′-tBu4-4,4′-biphenoquinone produced (Bu4BIPO)[TiCl3(THF)]2, while dirct reactions between TiCl4 and Bu4BIPOH2 in toluene resulted in the isolation of the dealkylated product, (Bu2BIPO)[TiCl3(Et2O)]2 (where the tBu groups occur in the 3,3′-positions), after treatment with ether. Refluxing Me4BIPOH2 and Cp2MCl2 (M = Ti or Zr) in toluene produced bismetallocene chloride complexes.

Dimetallacyclobutadiene derivatives: Catalysts or catalyst precursors for the ROMP of norbornene?

Abstract Metallacyclobutadiene complexes with the formula (Me3SiCH2)4M2(μ-CSiMe3)2 (M = Ta or W) induce the ring-opening metathesis polymerization (ROMP) of norbornene. The tungsten compound acts as a stand-alone catalyst, polymerizing norbornene with a first-order dependence on both W2 and monomer. In contrast, the tantalum complex requires oxygen as an initiator, and generates an as yet undefined catalyst that shows a zero-order dependence on the monomer concentration.

SUPRAMOLECULAR ASSEMBLIES OF TUNGSTEN COMPLEXES WITH UNUSUAL CHELATING GROUPS

Abstract Metal complexes of ligands with η1-O coordinated carbonyl moieties are well-known throughout the inorganic literature. However, metal complexes containing authentic acid chloride ligands have not previously been isolated. The synthesis and characterization of compounds with the formula W(X)Cl3(OPh-2-COCl) (2) [X  O (2a), NC6H3-2,6-(CH3)2 (2b), and Ph2C2 (2c)] are reported. The crystal structures of 2a and 2c were solved, showing the interaction of the acid chloride carbonyl moiety with the tungsten center. Complex 2c adopts a channelled structure having six-fold symmetric voids in the solid state. As was anticipated, the coordinated acid chloride of 2a reacts readily with nucleophiles to produce the ester WOCl3(OPh-2-COPrn) (3) and the amide WOCl3(OPh-2-CONHBut) (4a). A related amide complex WOCl3(OPh-2-CONH2) (4b), synthesized by the reaction of WOCl4 with salicylamide, displays an infinite-chain structure supported by hydrogen bonding between the amide group and the oxo ligand of an adjacent tungsten complex. The η1-O coordinated nitro compounds W(X)Cl3(OPh-2-NO2) (5) [X  O (5a), NC6H3-2,6-(CH3)2 (5b), and Ph2C2 (5c)] have also been isolated and the crystal structure of 5c is reported.

Chemistry of 1,3-Ditungstacyclobutadienes

Publisher Summary This chapter discusses the chemistry of 1,3-ditungstacyclobutadienes. The metallacycles have enjoyed a pivotal position in recent advances in organometallic chemistry. The investigations by Whitesides and coworkers into the thermal decomposition of L 2 Pt (CH 2 ) n compounds, where n= 4,5, and 6, were among the first to place emphasis on the importance of steric accessibility for the β–hydrogen atom abstraction reaction. X–Ray crystallographic studies have determined M–M bond distances of 2.90, 2.54, and 2.55 A for Nb, W, and Re derivatives, respectively. Addition of alkynes to the tungsten d 1 —d 1 dimers (VI) might then yield, by a coupling of the μ-CsiMe 3 groups, a compound of type X. Allenes have been found to react with W 2 (μ–CSiMe 3 ) 2 -containing compounds to give, by C–C bond formation between the tertiary carbon of the allene and the bridging carbon of one of the μ-CSiMe 3 ligands, a bridging ligand, which is formally a π–allyl to one tungsten atom and a terminal alkylidene to the other tungsten. Isocyanide ligands and carbon monoxide react in hydrocarbon solvents with (Pr i O) 4 W 2 (μ-CSiMe 3 ) 2 to give the compounds of formula (Pr 1 O)(X=)W(μ-CsiMe 3 (μ-η 2 ,η 1 -CCSiMe 3 )W(OPr i ) 3 . The reaction pathway interconverting a dimetallatetrahedane and two alkylidyne metal fragments still remains to be established.