Michael G. Gardiner

Polymer-Supported Carbene Complexes of Palladium: Well-Defined, Air-Stable, Recyclable Catalysts for the Heck Reaction

N-Heterocyclic dicarbene chelate complexes of formula [cis-CH2N(H)C=C(H)N(R)C2PdX2] (X = Br, I; R = (CH2)nOH; n = 2, 3) have been prepared and structurally characterized (for X= I, n = 2). The complexes were immobilized on a functionalized polystyrene support (Wang resin) through one of the oxygen centres. The complexes efficiently catalyze the Heck reaction of activated and non-activated arylbromides, are recyclable under aerobic conditions and exhibit hardly any leaching, which is in line with our theoretical investigations on ligand dissociation energies related to Pd0 and PdII centres.

Advances in the chemistry of Lewis base adducts of alane and gallane

Abstract Tertiary amine adducts of alane show a diverse range of structures based on four and five coordinate species, hydride bridging dimeric and polymeric species, and ionic species, whereas those of gallane are usually restricted to four coordinate species. Phosphine adducts of alane are four or five coordinate, and four coordinate for gallane. Stable, volatile adducts of both alane and gallane are available. Mixed donor adducts of alane are accessible, such as those based on N -alkylmorpholine, and these have relevance in the binding of [H 3 A1NMe 3 ] to oxidised surfaces. Alane metallates secondary amines, bulky amines excepted which can yield stable amine adducts; secondary amines based on N , N ′-disubstituted ethylenediamine give a range of products depending on the nature of the alane, the use of [H 3 A1NMe 3 ] is complicated by its tendency to form less reactive [H 3 A1(NMe 3 ) 2 ]. The tricyclohexylphosphine adduct of monochlorogallane is a useful precursor for forming the phosphidogallium species [{H 2 Ga(µ-PCy 2 )} 3 ] via reaction with [Li(PCy 2 )(THF) n ]. Reaction of [H 3 AINMe 3 ] with p -Bu 1 -calix[4]arene and its dimethyl ether afford respectively a divergent receptor bis-calixarene species and a monocalixarene species with a residual hydride either endo- or exo- to the calixarene cavity. Lewis base adducts of alane and gallane show promise in functional group chemistry with gallane behaving as a milder, more selective reducing agent. Reactions of (ER) 2 , E=Se or Te, R=alkyl oraryl, with [H 3 MNMe 3 ], M=A1 or Ga, afford trimethylamine adducts of the tris(seIenolato- or tellurolato-)nictal(III) species, [Me 3 NM(ER) 3 ], and reactions of elemental Se or Te with [H 3 AINMe 3 ] afford the mixed chalcogenidc/hydride tran-[{Me 3 N(H)Al(µ-E)} 2 ].

Valence‐Shell Charge Concentrations and Electron Delocalization in Alkyllithium Complexes: Negative Hyperconjugation and Agostic Bonding

In this paper we present the results of density functional theory (DFT) calculations on the ethyl ligand and some related organic moieties; we then proceed to consider a range of alkyllithium complexes studied by DFT calculations and high-resolution X-ray and neutron diffraction. Topological analysis of the charge density is used to follow changes in the electronic structure of the organic fragment. The charge concentrations (CCs) in the valence shell at the alpha and beta atoms reveal faithfully the delocalization of the lone pair at the Calpha atom or of the Li-C bonding electrons. Negative hyperconjugation is thus shown to arise from delocalization of the lone pair or the Li-C bonding electrons over the alkyl fragment, with depletion of the metal-directed charge concentration at Calpha, and characteristic ellipticity profiles for the bonds involved in hyperconjugative delocalization. In the case of so-called lithium agostic complexes, we show that close Li.H contacts are a consequence of this delocalization and further secondary interactions, with Li.H-C agostic interactions, playing only a minor role. The ellipticity profiles and the magnitude of the CCs at Calpha provide a quantitative measure of the extent of delocalization, and show excellent agreement between experiment and theory.

First x-ray structure of a N -naphthaloyl tethered chiral dirhodium(II) complex: structural basis for tether substitution improving asymmetric control in olefin cyclopropanation

Good-ee! The first one-pot olefin cyclopropanation that gives high ee is reported. The optimized catalyst is the 4-Br-substituted chiral paddle-wheel tetracarboxylatodirhodium(II) complex based on N-naphthaloyl-(S)-tert-leucinate. X-ray structural analysis of the parent catalyst reveals a square chiral crown cavity shrouding the axial coordination site (see figure).

Switchable π-coordination and C–H metallation in small-cavity macrocyclic uranium and thorium complexes

New, conformationally restricted ThIV and UIV complexes, [ThCl2(L)] and [UI2(L)], of the small-cavity, dipyrrolide, dianionic macrocycle trans-calix[2]benzene[2]pyrrolide (L)2− are reported and are shown to have unusual κ5:κ5 binding in a bent metallocene-type structure. Single-electron reduction of [UI2(L)] affords [UI(THF)(L)] and results in a switch in ligand binding from κ5-pyrrolide to η6-arene sandwich coordination, demonstrating the preference for arene binding by the electron-rich UIII ion. Facile loss of THF from [UI(THF)(L)] further increases the amount of U–arene back donation. [UI(L)] can incorporate a further UIII equivalent, UI3, to form the very unusual dinuclear complex [U2I4(L)] in which the single macrocycle adopts both κ5:κ5 and η6:κ1:η6:κ1 binding modes in the same complex. Hybrid density functional theory calculations carried out to compare the electronic structures and bonding of [UIIII(L)] and [UIII2I4(L)] indicate increased contributions to the covalent bonding in [U2I4(L)] than in [UI(L)], and similar U–arene interactions in both. MO analysis and QTAIM calculations find minimal U–U interaction in [U2I4(L)]. In contrast to the reducible U complex, treatment of [ThCl2(L)] with either a reductant or non-nucleophilic base results in metallation of the aryl rings of the macrocycle to form the (L−2H)4− tetraanion and two new and robust Th–C bonds in the –ate complexes [K(THF)2ThIV(μ-Cl)(L−2H)]2 and K[ThIV{N(SiMe3)2}(L−2H)].

Reduction of a Chelating Bis(NHC) Palladium(II) Complex to [{μ‐bis(NHC)}2Pd2H]+: A Terminal Hydride in a Binuclear Palladium(I) Species Formed under Catalytically Relevant Conditions

Catalytic processes featuring N-heterocyclic carbene (NHC) ligands have been extensively studied following the isolation of free imidazol-2-ylidenes An important reaction class is the base-assisted palladium-mediated C–C/N coupling, such as the Sonogashira, Buchwald–Hartwig, and Mizoroki–Heck reactions

New Chemistry from an Old Reagent: Mono- and Dinuclear Macrocyclic Uranium(III) Complexes from (U(BH4)3(THF)2)**

A new robust and high-yielding synthesis of the valuable U(III) synthon [U(BH4)3(THF)2] is reported. Reactivity in ligand exchange reactions is found to contrast significantly to that of uranium triiodide. This is exemplified by the synthesis and characterization of azamacrocyclic U(III) complexes, including mononuclear [U(BH4)(L)] and dinuclear [Li(THF)4][{U(BH4)}2(μ-BH4)(L(Me))] and [Na(THF)4][{U(BH4)}2(μ-BH4)(L(A))(THF)2]. The structures of all complexes have been determined by single-crystal X-ray diffraction and display two new U(III)2(BH4)3 motifs.

Characterization of tetra-aryl benzene isomers by using preparative gas chromatography with mass spectrometry, nuclear magnetic resonance spectroscopy, and x-ray crystallographic methods

The present study describes a preparative gas chromatographic (GC) approach employed to study a series of products arising from reaction of phenylacetylene with para-substituted aryl-iodides under Sonogashira catalysis conditions. GC analysis resolves the isomers from each reaction; however, it cannot provide structural details (their MS data are virtually identical). Since classical liquid chromatography cannot resolve the isomers, preparative-scale GC is the only practical approach to provide further spectroscopic characterization of the isomers. The products are well separated by GC so a single thick-film capillary column is adequate for this case, with operation up to approximately +300 degrees C. By collection of 50+ repeat injections, sufficient material could be isolated for (1)H NMR spectral analysis of the isomers, and for one isomer (isomer I) of a number of analogous related catalytic reaction mixtures, X-ray crystal structure determination enabled complete structural elucidation (absolute configuration) of the substitution pattern of the structure of this isomer. This confirmed isomer I to be the 2-para-aryl-substituted 1,3,5-triphenylbenzene product in all cases. (1)H NMR spectra of isomer I products generally had similar patterns but differed markedly from the second major isomer product (isomer II).

Structural aspects of tertiary amine adducts of alane and gallane

Abstract Structures of N-bound alane (AlH3) and gallane (GaH3) are reported: the dimethylbenzylamine adduct of gallane, [(PhCH2(Me)2NGaH3], which is monomeric with four-coordinate Ga, and the first chlorine containing amino complex of alane, [ClCH2CH2CH2(Me)2NAIH3]2, revealing that hydride bridging is favoured over AlCl interaction in Al obtaining a five-coordination environment. Also presented is a reinvestigation of the previously poorly defined structural determination of the trimethylamine adduct of gallane, [Me3NGaH3].

Organometallic neptunium(III) complexes

Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chemistry of uranium, which has focused strongly on U(III) and has seen some spectacular advances, that of the transuranics is significantly technically more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(IV). Here we report the synthesis of three new Np(III) organometallic compounds and the characterization of their molecular and electronic structures. These studies suggest that Np(III) complexes could act as single-molecule magnets, and that the lower oxidation state of Np(II) is chemically accessible. In comparison with lanthanide analogues, significant d- and f-electron contributions to key Np(III) orbitals are observed, which shows that fundamental neptunium organometallic chemistry can provide new insights into the behaviour of f-elements.