Research experience
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Sep 1963–
presentResearch: University of Oxford
University of Oxford · Department of ChemistryUnited Kingdom · Oxford
Publications (353) View all
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Article: The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds.
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ABSTRACT: Although compounds that feature 3-centre 2-electron (3c-2e) bonds are well known, there has been no previous effort to classify the interactions according to the number of electrons that each atom contributes to the bond, in a manner analogous to the classification of 2-centre 2-electron (2c-2e) bonds as either normal covalent or dative covalent. This article provides an extension to the Covalent Bond Classification (CBC) method by categorizing 3c-2e interactions according to whether (i) the two electrons are provided by one or by two atoms and (ii) the central bridging atom provides two, one, or zero electrons. Class I 3c-2e bonds are defined as those in which two atoms each contribute one electron to the 3-centre orbital, while Class II 3c-2e bonds are defined as systems in which the pair of electrons are provided by a single atom. Class I and Class II 3c-2e interactions can be denoted by structure-bonding representations that employ the "half-arrow" notation, which also provides a convenient means to determine the electron count at a metal centre. In contrast to other methods of electron counting, this approach provides a means to predict metal-metal bond orders that are in accord with theory. For example, compounds that feature symmetrically bridging carbonyl ligands do not necessarily have to be described as "ketone derivatives" because carbon monoxide can also serve as an electron pair donor to two metal centres. This bonding description also provides a simple means to rationalize the theoretical predictions of the absence of M-M bonds in molecules such as Fe(2)(CO)(9) and [CpFe(CO)(2)](2), which are widely misrepresented in textbooks as possessing M-M bonds.Chemical Communications 10/2012; · 6.17 Impact Factor -
Article: Understanding the reactivity of strained sandwich compounds with aluminum or gallium in bridging positions: experiments and DFT calculations.
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ABSTRACT: The aluminum and gallium dichlorides (Mamx)ECl(2)1a (E = Al; 82%) and 1b (E = Ga; 79%) (Mamx = 2,4-di-tert-butyl-6-[(dimethylamino)methyl]phenyl) reacted with dilithioferrocene or dilithioruthenocene to give [1]ferrocenophanes (2a, 2b) and [1]ruthenocenophanes (3a, 3b), respectively. The galla[1]ruthenocenophane 3b could be isolated from the reaction mixture through precipitation into hexane (50%), while 2a, 2b, and 3a underwent ring-opening polymerization under the reaction conditions of their formation reactions to give metallopolymers (M(w) (DLS) between 8.07 and 106 kDa). Monomer 3b was polymerized using Karstedt's catalyst resulting in an M(w) of 28.6(±6.3) kDa. In order to get an indication of the structure of polymers, bis(ferrocenyl) compounds (Mamx)EFc(2) (E = Al (4a), 51%; E = Ga (4b), 49%) were prepared and characterized by single crystal X-ray analysis. DFT calculations shed some light on the unexpected high reactivity of these new strained sandwich species. Optimized geometries of known aluminum and gallium-bridged [1]ferrocenophanes (Al(Pytsi) (6a), Ga(Pytsi) (6b); Pytsi = [dimethyl(2-pyridyl)silyl]bis(trimethylsilyl)methyl) and [1]ruthenocenophanes (Al(Me(2)Ntsi) (7a), Ga(Me(2)Ntsi) (7b); Me(2)Ntsi = [(dimethylamino)dimethylsilyl]bis(trimethylsilyl)methyl) matched very well with experimental molecular structures. Geometries of species 2a, 2b, 3a, and 3b were optimized (BP86/TZ2P) and the structural influence of the tBu group of the Mamx ligand in ortho position was evaluated by optimizing molecular structures of the four unknown species where the ortho-tBu group was replaced by an H atom (2a(H), 2b(H), 3a(H), and 3b(H)). The most pronounced structural effect was seen as a change of the orientation of the bridging moiety with respect to the sandwich unit. As the tBu group was removed, the aromatic ligand moved toward the freed-up space. The energetics (ΔE, ΔH(298K), and ΔG(298K)) accompanied by the structural changes were evaluated by a hydrogenolysis reaction of strained species resulting in Cp(2)M (M = Fe, Ru) and respective aluminum and gallium dihydrides. This nonisodesmic reaction showed that [1]metallocenophanes equipped with the ortho-tBu group were on average 5.5 kcal/mol higher strained (ΔH(298K)) than species where the tBu group was lacking. The investigation of the isodesmic reaction between strained species and Cp(2)M yielding bis(metallocenyl) compounds revealed that the ortho-tBu group sterically interacts with one of the metallocenyl units. The bis(metallocenyl) compounds are model compounds for the respective metallopolymers and one can conclude that even though the ortho-tBu group imposes additional strain on the starting metallocenophanes, this effect cancels out in ROPs because the ortho-tBu group imposes a similar strain on the resulting polymers. The uncovered steric repulsion between the ortho-tBu group and the sandwich moieties probably causes the ortho-tBu to act as an unusually sensitive NMR probe of the tacticity of the polymers.Journal of the American Chemical Society 04/2012; 134(18):7924-36. · 9.91 Impact Factor -
Article: Computational insight into the reductive oligomerisation of CO at uranium(III) mixed-sandwich complexes.
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ABSTRACT: Calculations reveal a multistep pathway towards formation of linear [U](2)-(μ-η(1):η(1)-C(2)O(2)); [U] = U(η-C(8)H(6){SiH(3)-1,4}(2))(η-Cp). However formation of deltate-bridged [U](2)-(μ-η(1):η(2)-C(3)O(3)) requires an alternative mechanism, involving a side-on [U](2)-(μ-η(2):η(2)-CO) complex and whereby the bridging units of [U](2)-(μ-η(2):η(2)-C(n)O(n)) intermediates (n = 1, 2) react directly with free CO.Chemical Communications 03/2012; 48(34):4118-20. · 6.17 Impact Factor -
Article: Mechanistic Studies of the Insertion of CO 2 into Palladium(I) Bridging Allyl Dimers
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ABSTRACT: In contrast to the chemistry of momomeric η 1 -Pd allyls, which act as nucleophiles, and monomeric η 3 -Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demon-strated that Pd I dimers containing two bridging allyl ligands react with one equivalent of CO 2 to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of Pd I bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high trans influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging allyl ligands, whereas for chloride-and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO 2 . Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO 2 because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO 2 with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO 2 . An alternative pathway for CO 2 insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate-and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric η 1 -Pd allyls. ■ INTRODUCTION There has been increasing interest in utilizing CO 2 as a C 1 source for the synthesis of both fine and commodity chemicals because CO 2 is cheap, abundant, nontoxic, and relatively easy to transport. 1 Unfortunately, activating and subsequently con-verting CO 2 into more valuable products under mild conditions is difficult due to the kinetic and thermodynamic stability of CO 2 . Transition metal complexes have been explored extensively to this end, as several transition metal complexes are known to bind CO 2 and weaken its strong C−O double bond. 2 Alternatively, transition metal complexes can promote the formation of C−E bonds (E = H, C, N, or O) through the insertion of CO 2 into M−E bonds, 2a and this elementary step has been utilized in several catalytic cycles for CO 2 con-version. 1f,k−n Our group has studied the reaction of Pd and Ni allyl complexes with CO 2 in detail and developed a catalytic reaction for the coupling of CO 2 with allylstannanes and allyl-boranes. 3 Recently, we discovered that CO 2 cleanly inserts into one of the bridging allyl ligands in Pd I dimers containing two bridging allyl ligands (eq 1), 4 the first examples of a reaction between CO 2 and a bridging allyl ligand on any metal. Despite the numerous examples of Pd I dimers supported by bridging allyl ligands, little has been done to study their reac-tivity and the chemical properties of these species remain unclear. 5 This is in contrast to the well-characterized reactivity of mono-meric η 1 -Pd allyls, which can act as nucleophiles, 6 and monomeric η 3 -Pd allyls, which can act as electrophiles. 7 Figure 1 depicts a general classification scheme for Pd I bridging allyl dimers and related molecules that have been reported. 4a,8−17 Complexes supported by bridging cyclopentadienyl (types IV− VII) and indenyl ligands (types VIII−IX) are included in Figure 1 because spectroscopic, crystallographic, and theoretical evidence indicates that the cyclopentadienyl and indenyl ligandsOrganometallics 01/2012; 31(1):470-485. · 3.96 Impact Factor -
Article: Experimental charge density study into C-C σ-interactions in a Binor-S rhodium complex.
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ABSTRACT: Transition-metal complexes containing (C-C)→M σ-interactions have potential applications in both catalysis and the activation and cleavage of C-C bonds. Fully characterising the bonding and interactions in complexes containing such (C-C)→M σ-interactions is vital to understand their chemical behaviour. As a result a high-resolution experimental X-ray charge density study has been undertaken on [Rh(Binor-S)(PCy(3))][HCB(11)Me(11)] (Binor-S = 1,2,4,5,6,8-dimetheno-s-indacene) which contains a (C-C)→Rh interaction. The data are analysed using Bader's "Atoms in Molecules" (AIM) approach with particular attention paid to the interactions around the rhodium centre. The results provide clear evidence for the σ(C-C)→Rh interaction in the solid-state which is classified as a weak covalent interaction. These results are supported by theoretical calculations.Dalton Transactions 09/2011; 40(40):10708-18. · 3.84 Impact Factor
