Imperial College London

Dr Mark R. Crimmin

Faculty of Natural SciencesDepartment of Chemistry

Reader in Organometallic Chemistry
 
 
 
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Contact

 

+44 (0)20 7594 2846m.crimmin

 
 
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Location

 

Rm. 631 and 831ChemistrySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

89 results found

Coates G, Rekhroukh F, Crimmin MR, 2019, Breaking carbon–fluorine bonds with main group nucleophiles, Synlett, Vol: 30, Pages: A-N, ISSN: 0936-5214

In this Account we describe a series of new reactions that we, and others, have reported that involve the transformation of C–F bonds into C–Mg, C–Al, C–Si, C–Fe and C–Zn bonds. We focus on the use of highly reactive main group nucleophiles and discuss aspects of reaction scope, selectivity and mechanism.

Journal article

Garçon M, George S, Clare B, Alison E, Richard C, White A, Crimmin Met al., 2019, A hexagonal planar transition metal complex, Nature, Vol: 574, Pages: 390-393, ISSN: 0028-0836

Transition metal complexes are widely applied in the physical and biological sciences. They play pivotal roles in aspects of catalysis, synthesis,materials science, photophysics and bioinorganic chemistry.Our understanding of transition metal complexes originates from Alfred Werner’s realisation that their three-dimensional shape influences their properties and reactivity.1The intrinsic link between shape and electronic structure is now firmly underpinned by molecular orbital theory.2-5Despite over a century of advances in this field, transition metal complexes remain limited to a handful of well understood geometries. Archetypal geometriesfor six-coordinate transition metals are octahedral andtrigonal prismatic. Although deviations from idealbond angles and lengths are common,6alternativeparent geometries are staggeringly rare.7Hexagonal planar transition metalsare restricted to those found in condensed metallic phases,8the hexagonal pores of coordination polymers,9orclusters containing more than one transition metal in close proximity.10,11Although [Ni(PtBu)6] could beassignedas a hexagonal planar complex,12,13a molecular orbital analysis ultimately led to the conclusion that it is best described as a 16-electron complex with a trigonal planar geometry.14Here we report the isolation and structural characterisation of the first simple coordination complex in which six ligands form bonds with a central transition metal in a hexagonal planar arrangement.The discovery has the potential to open up new design principles and new ways of thinking about transition metal complexes which could impact multiple fields of science.

Journal article

Hooper TN, Lau S, Chen W, Brown RK, Garçon M, Luong K, Barrow NS, Tatton AS, Sackman GA, Richardson C, White AJP, Cooper RI, Edwards AJ, Casely IJ, Crimmin MRet al., 2019, The partial dehydrogenation of aluminium dihydrides, Chemical Science, Vol: 10, Pages: 8083-8093, ISSN: 2041-6520

The reactions of a series of β-diketiminate stabilised aluminium dihydrides with ruthenium bis(phosphine), palladium bis(phosphine) and palladium cyclopentadienyl complexes is reported. In the case of ruthenium, alane coordination occurs with no evidence for hydrogen loss resulting in the formation of ruthenium complexes with a pseudo–octahedral geometry and cis-relation of phosphine ligands. These new ruthenium complexes have been characterised by multinuclear and variable temperature NMR spectroscopy, IR spectroscopy and single crystal X-ray diffraction. In the case of palladium, a series of structural snapshots of alane dehydrogenation have been isolated and crystallographically characterised. Variation of the palladium precursor and ligand on aluminium allows kinetic control over reactivity and isolation of intermetallic complexes that contain new Pd–Al and Pd–Pd interactions. These complexes differ by the ratio of H : Al (2 : 1, 1.5 : 1 and 1 : 1) with lower hydride content species forming with dihydrogen loss. A combination of X-ray and neutron diffraction studies have been used to interrogate the structures and provide confidence in the assignment of the number and position of hydride ligands. 27Al MAS NMR spectroscopy and calculations (DFT, QTAIM) have been used to gain an understanding of the dehydrogenation processes. The latter provide evidence for dehydrogenation being accompanied by metal–metal bond formation and an increased negative charge on Al due to the covalency of the new metal–metal bonds. To the best of our knowledge, we present the first structural information for intermediate species in alane dehydrogenation including a rare neutron diffraction study of a palladium–aluminium hydride complex. Furthermore, as part of these studies we have obtained the first SS 27Al NMR data on an aluminium(I) complex. Our findings are relevant to hydrogen storage, materials chemistry and catalysis.

Journal article

Phillips NA, O'Hanlon J, Hooper TN, White AJP, Crimmin MRet al., 2019, Dihydridoboranes: Selective Reagents for Hydroboration and Hydrodefluorination, ORGANIC LETTERS, Vol: 21, Pages: 7289-7293, ISSN: 1523-7060

Journal article

Coates G, Tan HY, Kalff C, White AJP, Crimmin MRet al., 2019, Defluorosilylation of Industrially Relevant Fluoroolefins Using Nucleophilic Silicon Reagents, Angewandte Chemie, Vol: 131, Pages: 12644-12648, ISSN: 0044-8249

Journal article

Coates G, Tan HY, Kalff C, White A, Crimmin MRet al., 2019, Defluorosilylation of industrially relevant fluoroolefins using nucleophilic silicon reagents, Angewandte Chemie - International Edition, Vol: 58, Pages: 12514-12518, ISSN: 1433-7851

A number of new magnesium and lithium silyl reagents were prepared and shown to be outstanding nucleophiles in reactions with industrially relevant fluoroolefins. These reactions result in a net transformation of either sp2 or sp3 C-F bonds into C-Si bonds by two modes of nucleophilic attack (SNV or SN2'). The methods are mild, proceeding with high chemo- and regioselectivity. Mechanistic pathways are described that lead to new substitution patterns from HFO-1234yf, HFO-1234ze and HFO-1336mzz, previously inaccessible by transition metal catalysed defluorosilylation routes.

Journal article

Kong RY, Crimmin MR, 2019, Reversible insertion of CO into an aluminium-carbon bond, Chemical Communications, Vol: 55, Pages: 6181-6184, ISSN: 1359-7345

A [2.2.1] aluminium metallobicycle is capable of reversibly insertingCO to form a [2.2.2] metallobicycle at 100 8C. Computational studiesreveal a highly asynchronous, but concerted, transition state for COinsertion. The coordination of CO to aluminium precedes C–C bondformation. The reversible migratory insertion of CO at aluminiumthus mimics well-established transition-metal reactivity.

Journal article

Phillips N, White A, Crimmin M, Selective hydrodefluorination of hexafluoropropene to industrially relevant hydrofluoroolefins, Advanced Synthesis & Catalysis, ISSN: 1615-4150

The selective hydrodefluorination of hexafluoropropene to HFO‐1234ze and HFO‐1234yf can be achieved by reaction with simple group 13 hydrides of the form EH3 ⋅ L (E=B, Al; L=SMe2, NMe3). The chemoselectivity varies depending on the nature of the group 13 element. A combination of experiments and DFT calculations show that competitive nucleophilic vinylic substitution and addition‐elimination mechanisms involving hydroborated intermediates lead to complementary selectivities.

Journal article

Garcon M, Bakewell C, White AJP, Crimmin MRet al., 2019, Unravelling nucleophilic aromatic substitution pathways with bimetallic nucleophiles, Chemical Communications, Vol: 55, Pages: 1805-1808, ISSN: 1359-7345

The reaction of a metal complex containing a polar Fe–Mg bond with 2-(pentafluorophenyl)pyridine leads to selective C–F bond activation. A stepwise SNAr mechanism involving attack of the bimetallic nucleophile on the electron-deficient aromatic ring has been identified by DFT calculations. Despite the long and rich history of metal anions in organic synthesis, this is the first time the SNAr mechanism has been elucidated in detail for metal-based nucleophiles.

Journal article

Bakewell C, White AJP, Crimmin MR, 2019, Reversible alkene binding and allylic C–H activation with an aluminium(i) complex, Chemical Science, Vol: 8, Pages: 2452-2458, ISSN: 2041-6520

The monomeric molecular aluminium(I) complex 1 [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl) reacts with a series of terminal and strained alkenes including ethylene, propylene, allylbenzene and norbornene to form alkene bound products. Remarkably all these reactions are reversible under mild conditions (298–353 K) with alkene binding being disfavoured at higher temperatures due to the positive reaction entropy. Van't Hoff analyses have allowed quantification of the binding events with Image ID:c8sc04865g-t1.gif. Calculations and single crystal X-ray diffraction studies are consistent with the alkene bound species being metallocyclopropane complexes. Alkene binding involves a reversible redox process with changes from the +1 to +3 aluminium oxidation state. Under more forcing conditions the metallocyclopropane complexes undergo non-reversible allylic C–H bond activation to generate aluminium(III) allyl hydride complexes. This represents a rare example of redox-based main group reactivity in which reversible substrate binding is followed by a further productive bond breaking event. Analysis of the mechanism reveals a reaction network in which alkene dissociation and reformation of 1 is required for allylic C–H activation, a realisation that has important implications for the long-term goal of developing redox-based catalytic cycles with main group compounds.

Journal article

Lau S, White A, Casely I, Crimmin Met al., 2018, Tunable binding of dinitrogen to a series of heterobimetallic hydride complexes, Organometallics, Vol: 37, Pages: 4521-4526, ISSN: 0276-7333

The reaction of [Ru(H)2(N2)2(PCy3)2] (1) with β-diketiminate stabilized hydrides of Al, Zn, and Mg generates a series of new heterobimetallic complexes with either H2 or N2 ligated to the ruthenium center. Changing the main-group fragment of the M·Ru-N2 (M = Al, Zn, Mg) complexes can subtly alter the degree of binding, and therefore activation, of the diatomic ligand, as evidenced by the νN≡N absorptions in the infrared data. Experimental and computational data rationalize this tunable binding; decreasing the electronegativity of the main group in the order Al > Zn > Mg infers greater ionic character of these M·Ru-N2 complexes, and this in turn results in greater destabilization of the frontier molecular orbitals of ruthenium and therefore greater Ru(4d) → π*(N2) back-donation.

Journal article

Crimmin MR, Coates G, Bakewell C, Ward B, White Aet al., 2018, Reactions of fluoroalkanes with Mg-Mg bonds: scope, sp3C-F / sp2C-F coupling and mechanism, Chemistry - A European Journal, Vol: 24, Pages: 16282-16286, ISSN: 0947-6539

sp3C-F bonds of fluoroalkanes (7 examples; 1o, 2o and 3o) undergo addition to a low-valent Mg-Mg species generating reactive organomagnesium reagents. Further reactions with a series of electrophiles results in a net C-F to C-B, C-Si, C-Sn or C-C bond transformation (11 examples, diversity). The new reactivity has been exploited in an unprecedented one-pot magnesium-mediated coupling of sp3C-F and sp2C-F bonds. Calculations suggest that the sp3C-F bond activation step occurs by frontside nucleophilic attack of the Mg-Mg reagent on the fluoroalkane.

Journal article

Garçon M, White AJP, Crimmin MR, 2018, Palladium-catalysed magnesiation of benzene, Chemical Communications, Vol: 54, Pages: 12326-12328, ISSN: 1359-7345

In the presence of a catalytic quantity of [Pd(PCy3)2], a reagent containing a Mg-Mg bond effects the C-H functionalisation of benzene resulting in a 100% atom efficient transformation to generate an unprecedented aryl magnesium hydride.

Journal article

Kong RY, Crimmin MR, 2018, Carbon chain growth by sequential reactions of CO and CO2 with [W(CO)6] and an aluminum(I) reductant, Journal of the American Chemical Society, Vol: 140, Pages: 13614-13617, ISSN: 1520-5126

The formation of carbon chains by the coupling of CO x ( x = 1 or 2) units on transition metals is a fundamental step relevant to Fischer-Tropsch catalysis. Fischer-Tropsch catalysis produces energy dense liquid hydrocarbons from synthesis gas (CO and H2) and has been a mainstay of the energy economy since its discovery nearly a century ago. Despite detailed studies aimed at elucidating the steps of catalysis, experimental evidence for chain growth (C n → C n+1; n ≥ 2) from the coupling of CO units on metal complexes is, to the best of our knowledge, unprecedented. In this paper, we show that carbon chains can be grown from sequential reactions of CO or CO2 with a transition metal carbonyl complex. By exploiting the cooperative effect of transition and main group metals, we document the first example of chain propagation from sequential coupling of CO units (C1 → C3 → C4), along with the first example of incorporation of CO2 into the growing carbon chain.

Journal article

Hicken A, White AJP, Crimmin MR, 2018, Preparation and characterisation of heterobimetallic copper-tungsten hydride complexes, Dalton Transactions, Vol: 47, Pages: 10595-10600, ISSN: 1477-9234

The preparation and structural characterisation of three new heterobimetallic hydride complexes containing 3-centre,2-electron W-H-Cu bonds is reported. These complexes have been characterised by single crystal X-ray crystallography and multinuclear NMR spectroscopy. The bonding in these complexes has been analysed by DFT calculations.

Journal article

Hooper TN, Garcon M, White AJP, Crimmin MRet al., 2018, Room temperature catalytic carbon-hydrogen bond alumination of unactivated arenes: mechanism and selectivity, CHEMICAL SCIENCE, Vol: 9, Pages: 5435-5440, ISSN: 2041-6520

We report the first catalytic methods for the transformation of C–H bonds of unactivated arenes into C–Al bonds. The catalytic reactions occur at 25 °C (benzene, toluene and xylenes) with palladium loadings as low as 0.1 mol%. Remarkably, the C–H activation of toluene and xylenes proceeds with ortho- and meta-selectivity. This selectivity is highly unusual and complementary to both Friedel–Crafts and the majority of C–H borylation methods. Through a detailed mechanistic analysis (Eyring analysis, KIE, DFT, QTAIM) we show that unusual Pd–Al intermetallic complexes are on the catalytic cycle and that the selectivity is determined by weak attractive dispersion forces in the transition state for C–H bond breaking.

Journal article

Bakewell C, White AJP, Crimmin MR, 2018, Reactions of Fluoroalkenes with an Aluminium(I) Complex, Angewandte Chemie, Vol: 130, Pages: 6748-6752, ISSN: 0044-8249

Journal article

Bakewell C, White AJP, Crimmin MR, 2018, Reactions of Fluoroalkenes with an Aluminium(I) Complex., Angewandte Chemie International Edition, Vol: 57, Pages: 6638-6642, ISSN: 1521-3757

A series of industrially relevant fluoroalkenes react with a monomeric AlI complex. These reactions break strong sp2 and sp3 C-F bonds, and result in the formation of a diverse array of organoaluminium compounds. Mechanistic studies show that two mechanisms are likely in operation: 1) direct oxidative addition of the C-F bond to AlI occurs with retention of alkene stereochemistry, and 2) stepwise formation and decomposition of a metallocyclopropane intermediate occurs with inversion of alkene stereochemistry. As part of this mechanistic analysis, we have isolated the first aluminium metallocyclopropane complex from oxidative addition of an alkene to AlI . Remarkably this reaction is reversible and reductive elimination of the alkene occurs at higher temperature reforming AlI . Furthermore, in selected cases the organoaluminium products are susceptible toward β-fluoride elimination to yield a double C-F activation pathway.

Journal article

Butler MJ, White AJP, Crimmin MR, 2018, Heterobimetallic rebound: a mechanism for diene-to-alkyne isomerization with M-Zr hydride complexes (M = Al, Zn, and Mg), Organometallics, Vol: 37, Pages: 949-956, ISSN: 0276-7333

The reaction of a series of M·Zr heterobimetallic hydride complexes withdienes and alkynes has been investigated (M = Al, Zn, and Mg). Reaction of M·Zr with1,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene, but for M = Zn alsoresult in an on-metal diene-to-alkyne isomerization. The resulting cyclooctyne fragment istrapped between Zr and Zn metals in a heterobimetallic species that does not form for M= Mg or Al. The scope of diene isomerization and alkyne trapping has been exploredleading to the isolation of three new heterobimetallic slipped metallocyclopropenecomplexes. The mechanism of diene-to-alkyne isomerization was investigated throughkinetics. While the reaction is first-order in Zn·Zr at high diene concentration andproceeds with ΔH‡ = +33.6 ± 0.7 kcal mol−1, ΔS‡ = +23.2 ± 1.7 cal mol−1 K−1, and ΔG⧧298 K = +26.7 ± 1.2 kcal mol−1, the rateis dependent on the nature of the diene. The positive activation entropy is suggestive of involvement of a dissociative step. Onthe basis of DFT calculations, a heterobimetallic rebound mechanism for diene-to-alkyne isomerization has been proposed. Thismechanism explains the origin of heterobimetallic control over selectivity: Mg---Zr complexes are too strongly bound to generatereactive fragments, while Al---Zr complexes are too weakly bound to compensate for the contrathermodynamic isomerizationprocess. Zn---Zr complexes have favorable energetics for both dissociation and trapping steps.

Journal article

Zhurakovskyi O, Türkmen YE, Löffler LE, Moorthie VA, Chen CC, Shaw MA, Crimmin MR, Ferrara M, Ahmad M, Ostovar M, Matlock JV, Aggarwal VKet al., 2018, Enantioselective Synthesis of the Cyclopiazonic Acid Family Using Sulfur Ylides, Angewandte Chemie, Vol: 130, Pages: 1360-1364, ISSN: 0044-8249

Journal article

Bakewell C, Ward BJ, White AJP, Crimmin MRet al., 2018, A combined experimental and computational study on the reaction of fluoroarenes with Mg-Mg, Mg-Zn, Mg-Al and Al-Zn bonds, Chemical Science, Vol: 9, Pages: 2348-2356, ISSN: 2041-6520

Through a combined experimental and computational (DFT) approach, the reaction mechanism of theaddition of fluoroarenes to Mg–Mg bonds has been determined as a concerted SNAr-like pathway inwhich one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determinedGibbs activation energy for the addition of C6F6 to a Mg–Mg bond of a molecular complex,DG‡298 K(experiment) ¼ 21.3 kcal mol1 is modelled by DFT with the uB97X functional, DG‡298 K(DFT) ¼25.7 kcal mol1. The transition state for C–F activation involves a polarisation of the Mg–Mg bond andsignificant negative charge localisation on the fluoroarene moiety. This transition state is augmented bystabilising closed-shell Mg/Fortho interactions that, in combination with the known trends in C–F andC–M bond strengths in fluoroarenes, provide an explanation for the experimentally determinedpreference for C–F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect ofmodification of both the ligand coordination sphere and the nature and polarity of the M–M bond (M ¼Mg, Zn, Al) on C–F activation has been investigated. A series of highly novel b-diketiminate stabilisedcomplexes containing Zn–Mg, Zn–Zn–Zn, Zn–Al and Mg–Al bonds has been prepared, including thefirst crystallographic characterisation of a Mg–Al bond. Reactions of these new M–M containingcomplexes with perfluoroarenes were conducted and modelled by DFT. C–F bond activation is dictatedby the steric accessibility, and not the polarity, of the M–M bond. The more open coordinationcomplexes lead to enhanced Mg/Fortho interactions which in turn lower the energy of the transitionstates for C–F bond activation.

Journal article

Zhurakovskyi O, Tuerkmen YE, Loeffler LE, Moorthie VA, Chen CC, Shaw MA, Crimmin MR, Ferrara M, Ahmad M, Ostovar M, Matlock JV, Aggarwal VKet al., 2018, Enantioselective synthesis of the Cyclopiazonic acid family using sulfur ylides, Angewandte Chemie International Edition, Vol: 57, Pages: 1346-1350, ISSN: 1521-3757

A convergent, nine‐step (LLS), enantioselective synthesis of α‐cyclopiazonic acid and related natural products is reported. The route features a) an enantioselective aziridination of an imine with a chiral sulfur ylide; b) a bioinspired (3+2)‐cycloaddition of the aziridine onto an alkene; and c) installation of the acetyltetramic acid by an unprecedented tandem carbonylative lactamization/N−O cleavage of a bromoisoxazole.

Journal article

Crimmin MR, Hicken A, White AJP, 2017, Selective Reduction of CO2 to a Formate Equivalent with Heterobimetallic Gold---Copper Hydride Complexes., Angewandte Chemie International Edition, Vol: 56, Pages: 15127-15130, ISSN: 1521-3757

A series of heterobimetallic complexes containing 3-center,2-electron Au-H-Cu bonds have been prepared from addition of a parent gold hydride to a bent d10 copper(I) fragment. These highly unusual heterobimetallic complexes represent a missing link in the widely investigated series of neutral and cationic coinage metal hydride complexes containing Cu-H-Cu and M-H-M+ moieties (M = Cu, Ag). The well-defined heterobimetallic hydride complexes act as pre-catalysts for the conversion of CO2 to HCO2Bpin using HBpin as a reductant. The selectivity of the heterobimetallic complexes for the catalytic production of a formate equivalent surpasses that of the parent monomeric group 11 complexes.

Journal article

Chen W, Hooper TN, Ng J, White AJP, Crimmin MRet al., 2017, Palladium-Catalyzed Carbon-Fluorine and Carbon-Hydrogen Bond Alumination of Fluoroarenes and Heteroarenes, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, Vol: 56, Pages: 12687-12691, ISSN: 1433-7851

Through serendipitous discovery, a palladium bis(phosphine) complex was identified as a catalyst for the selective transformation of sp2C−F and sp2C−H bonds of fluoroarenes and heteroarenes to sp2C−Al bonds (19 examples, 1 mol % Pd loading). The carbon–fluorine bond functionalization reaction is highly selective for the formation of organoaluminium products in preference to hydrodefluorination products (selectivity=4.4:1 to 27:1). Evidence is presented for a tandem catalytic process in which hydrodefluorination is followed by sp2C−H alumination.

Journal article

Crimmin MR, Lau S, Ward B, White AJP, Macgregor SA, Casely IJet al., 2017, Mild sp2Carbon-Oxygen Bond Activation by an Isolable Ruthenium(II) bis(Dinitrogen) Complex: Experiment and Theory, Organometallics, Vol: 36, Pages: 3654-3663, ISSN: 0276-7333

The isolable ruthenium(II) bis(dinitrogen) complex [Ru(H)2(N2)2(PCy3)2] (1) reacts with aryl ethers (Ar–OR, R = Me and Ar) containing a ketone directing group to effect sp2C–O bond activation at temperatures below 40 °C. DFT studies support a low-energy Ru(II)/Ru(IV) pathway for C–O bond activation: oxidative addition of the C–O bond to Ru(II) occurs in an asynchronous manner with Ru–C bond formation preceding C–O bond breaking. Alternative pathways based on a Ru(0)/Ru(II) couple are competitive but less accessible due to the high energy of the Ru(0) precursors. Both experimentally and by DFT calculations, sp2C–H bond activation is shown to be more facile than sp2C–O bond activation. The kinetic preference for C–H bond activation over C–O activation is attributed to unfavorable approach of the C–O bond toward the metal in the selectivity determining step of the reaction pathway.

Journal article

Crimmin MR, Chen W, Hooper TN, White AJOet al., 2017, Palladium-Catalysed Carbon-Fluorine and Carbon-Hydrogen Bond Alumination of Fluoroarenes and Heteroarenes, Angewandte Chemie, Vol: 129, Pages: 12861-12865, ISSN: 1521-3757

Through serendipitous discovery, a palladium bis(phosphine) complex was identified as a catalyst for the selective transformation of sp2C−F and sp2C−H bonds of fluoroarenes and heteroarenes to sp2C−Al bonds (19 examples, 1 mol % Pd loading). The carbon–fluorine bond functionalization reaction is highly selective for the formation of organoaluminium products in preference to hydrodefluorination products (selectivity=4.4:1 to 27:1). Evidence is presented for a tandem catalytic process in which hydrodefluorination is followed by sp2C−H alumination.

Journal article

Crimmin MR, hicken A, White AJP, 2017, Reversible Coordination of Boron-, Aluminum-, Zinc-, Magnesium- and Calcium-Hydrogen bonds to bent {CuL2} fragments: Heavy sigma-Complexes of the Lightest Coinage Metal, Inorganic Chemistry, Vol: 56, Pages: 8669-8682, ISSN: 0020-1669

A series of copper(I) complexes bearing electron-deficient β-diketiminate ligands have been prepared. The study includes [{{ArNC(CR3)}2CH}Cu(η2-toluene)n] (Ar = Mes, R = F, n = 0.5, [12·tol]; Ar = C6F5, R = Me, n = 1, [2·tol]; Ar = 2,6-Cl2C6H3, R = H, n = 0.5, [32·tol]). Reactions of [1–3n·tol] with boranes, alanes, a zinc hydride, a magnesium hydride, and a calcium hydride generate the corresponding σ complexes ([1–3·B], [3·B′], [3·Al], [3·Al′], [1–3·Zn], [1·Mg], and [1·Ca]). These species all form reversibly, being in equilibrium with the arene solvates in solution. With the exception of the calcium complex, the complexes have all been characterized by single-crystal X-ray diffraction studies. In solution, the σ-hydride of the aluminum, zinc, magnesium, and calcium derivatives resonates between −0.12 and −1.77 ppm (C6D6 or toluene-d8, 193–298 K). For the σ-borane complexes, the hydrides are observed as a single resonance between 2 and 3.5 ppm (C6D6, 298 K) and bridging and terminal hydrides rapidly exchange on the NMR time scale even at 193 K. Quantification of the solution dynamics by van’t Hoff analysis yields expectedly small values of ΔH° and negative values of ΔS° consistent with weak binding and a reversible process that does not involve aggregation of the copper species. The donor–acceptor complexes can be rationalized in terms of the Dewar–Chatt–Duncanson model. Density functional theory calculations show that the donation of σ-M–H (or E–H) electrons into the 4s-based orbital (LUMO or LUMO+1) of the copper fragment is accompanied by weak back-donation from a dxz-based orbital (HOMO or HOMO–1) into the σ*-M–H (or E–H) orbital.

Journal article

Ekkert O, White AJP, Crimmin MR, 2017, Stereoisomerism of bis(sigma-Zincane) complexes: evidence for an intramolecular pathway, Chemistry-A European Journal, Vol: 23, Pages: 5682-5686, ISSN: 1521-3765

The first bis(σ-zincane) complexes, heterotri- metallic species [M(CO)4(η2-HZnBDI)2], have been prepared (BDI=κ2-{2,6-(iPr)2C6H3NCMe}2CH). For M=Cr, a single stereoisomer is observed in solution and the solid-state. For M=Mo and W, cis and trans isomers were found to reversibly interconvert at 297 K. Despite the huge steric demands of the ligand on zinc, the cis isomer was found to be the most thermodynamically stable in all cases. The activation parameters for the isomerisation when M=Mo are ΔH≠=20.8 kcal mol−1 and ΔS≠=−12.8 cal K−1 mol−1. In combination with DFT calculations, the negative activation entropy suggests an intramolecular rotation mechanism for isomerisation.

Journal article

Pike SD, Crimmin MR, Chaplin AB, 2017, Organometallic chemistry using partially fluorinated benzenes, Chemical Communications, Vol: 53, Pages: 3615-3633, ISSN: 1364-548X

Fluorobenzenes, in particular fluorobenzene (FB) and 1,2-difluorobenzene (1,2-DiFB), are increasingly becoming recognised as versatile solvents for conducting organometallic chemistry and transition-metal-based catalysis. The presence of fluorine substituents reduces the available π-electron density and consequently fluorobenzenes generally bind weakly to metal centres, allowing them to be used as essentially non-coordinating solvents or as readily displaced ligands. In this context, examples of well-defined complexes of fluorobenzenes are discussed, including trends in binding strength with increasing fluorination and different substitution patterns. Compared to more highly fluorinated benzenes, FB and 1,2-DiFB typically demonstrate greater chemical inertness, however, C–H and C–F bond activation reactions can be induced using appropriately reactive transition metal complexes. Such reactions are surveyed, including catalytic examples, not only to provide perspective for the use of FB and 1,2-DiFB as innocent solvent media, but also to highlight opportunities for their exploitation in contemporary organic synthesis.

Journal article

Phanopoulos A, Leung AHM, Yow S, Palomas D, White AJP, Hellgardt K, Horton A, Crimmin MRet al., 2017, Binuclear β-diketiminate complexes of copper(I), Dalton Transactions, Vol: 46, Pages: 2081-2090, ISSN: 1477-9226

The reaction of a series of dinucleating bis(β-diketiminate) pro-ligands with mesitylcopper in the presence and absence of mono and diphosphines has allowed the isolation of a new series of dicopper(I) complexes. Inclusion of trans-1,2-cyclohexyl (1), 2,6-pyridyl (2), and 2,2′-oxydiaryl (3) spacers between the β-diketiminate units has been studied. The isolation of three new copper(I) phosphine complexes [1·Cu2(PPh3)2], [2·Cu2(PPh3)2] and [3·Cu2(PPh3)2] is reported. While these compounds display large Cu⋯Cu separations of 5.4–7.9 Å in the solid state, solution data are consistent with a large degree of conformational freedom. Modification of the monophosphine to a diphosphine, DPPE, allowed the isolation of the novel 11-membered bimetallic macrocycle [2·Cu2(DPPE)] containing both a binucleating nitrogen based ligand and a chelating diphosphine. While acetonitrile adducts of this series could also be generated in situ, under forcing conditions reaction of the 2,6-pyridyl bridged ligand with mesityl copper led to the formation [2·Cu2]2. This latter complex is a dimer of dicopper(I) units in which the bis(β-diketiminate) ligand now binds four copper(I) centers through not only the expected κ2-N,N′-chelation but also κ1- and η2-binding of the central pyridine through orthogonal Cu–N and Cu–arene interactions. Reversible coordination of alkenes, pyridine and quinoline to the copper cluster was identified allowing the isolation and structural characterisation of a further series of dinuclear complexes [2·Cu2(pyridine)2], [2·Cu2(cyclopentene)2] and [2·Cu2(norbornene)2]. Solution studies allow quantification of the reversible binding event through a van't Hoff analysis. Both solution and the solid state data suggest a weak anagostic interaction exists in the latter two alkene complexes of copper(I). The new complexes have been characterized by X-ray dif

Journal article

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