RESUMEN

The intricate σ and π-bonding of N-heterocyclic carbenes (NHCs) to metals and the need to quantify their electronic properties to rationalize reactivity of complexes have resulted in the creation of numerous methodologies to understand the NHC-metal interaction which are, as we now show, flawed. Our search for a unified, easily accessible system to gauge these fundamental properties has resulted in the discovery of two systems that highlight the flaws present in existing systems and provide a more accurate measure of the NHC ligand electronic properties.

RESUMEN

A process for the catalytic reduction of nitrous oxide using NHC-ligated copper(I) tert-butoxide precatalysts and B2pin2 as the reductant is reported. The reaction proceeds under mild conditions via copper(I)-boryl intermediates which react with N2O by facile O-atom insertion into the Cu-B bond and liberate N2. Turnover numbers > 800 can be achieved at 80 °C under 1 bar N2O.

RESUMEN

Liquid assisted ball milling of [NHC]HBr (NHC = N-heterocyclic carbene) salts with copper(I) chloride, and a range of alkali metal complexes was shown to efficiently produce (NHC)CuX (NHC = normal or RE-NHC, X = halide, alkoxide, amide, alkyl, aryl; RE-NHC = ring-expanded NHC).

RESUMEN

Deprotonation of triphenyl germane with NHC-supported copper alkoxides afforded four novel (NHC)CuGePh3 complexes. Of these, (IPr)CuGePh3 (IPr = :C{N(2,6-iPr2C6H3)CH}2) was selected for further investigation. Analysis by EDA-NOCV indicates it to be a germyl nucleophile and its σ-bond metathesis reaction with a range of p-block halides confirmed it to be a convenient source of [Ph3Ge]-. The Cu-Ge bond of (IPr)CuGePh3 underwent π-bond insertions with tBuNCS, CS2, and PhNCO to furnish a series of germyl substituted carboxylate derivatives, (IPr)CuXC(Y)GePh3 (X = S, NPh; Y = S, NtBu, O), which were structurally characterised. (IPr)CuGePh3 inserted phenyl acetylene, providing both the Markovnikov and anti-Markovnikov products. The (NHC)CuGePh3 compounds were validated as catalytic intermediates; addition of 10 mol% of NHC-copper(i) alkoxide to a mixture of triphenyl germane and a tin(iv) alkoxide resulted in a tin/germanium cross coupling with concomitant formation of alcohol. Moreover, a catalytic hydrogermylation of Michael acceptors was developed with Ph3GeH adding to 7 activated alkenes in good conversions and yields in the presence of 10 mol% of NHC-copper(i) alkoxide. In all cases, this reaction provided the ß-germylated substrate implicating nucleophilicity at germanium.

RESUMEN

Reaction of (6-Dipp)CuOtBu (6-Dipp=C{NDippCH2 }2 CH2 , Dipp=2,6-iPr2 C6 H3 ) with B2 (OMe)4 provided access to (6-Dipp)CuB(OMe)2 via σ-bond metathesis. (6-Dipp)CuB(OMe)2 was characterised by NMR spectroscopy and X-ray crystallography and shown to be a monomeric acyclic boryl of copper. (6-Dipp)CuB(OMe)2 reacted with ethylene and diphenylacetylene to provide insertion compounds into the Cu-B bond which were characterised by NMR spectroscopy in both cases and X-ray crystallography in the latter. It was also competent in the rapid catalytic deoxygenation of CO2 in the presence of excess B2 (OMe)4 . Alongside π-insertion, (6-Dipp)CuB(OMe)2 reacted with LiNMe2 to provide a salt metathesis reaction at boron, giving (6-Dipp)CuB(OMe)NMe2 , a second monomeric acyclic boryl, which also cuproborated diphenylacetylene. Computational interrogation validated these acyclic boryl species to be electronically similar to (6-Dipp)CuBpin.

RESUMEN

The synthesis, isolation and full characterisation of a [Cu(IPr)(OC(H)(CF3)2)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) complex are reported. This new Cu(I) complex is a versatile synthon and can activate numerous X-H bonds including C-H, N-H and S-H bonds. [Cu(IPr)(OC(H)(CF3)2)] was investigated as a pre-catalyst in several catalytic reactions.

RESUMEN

The potassium aluminyl complex K2[Al{N(Dipp)SiMe3}2]2 was synthesised via reduction of [AlI{N(Dipp)SiMe3}2] (Dipp = 2,6-i-Pr2C6H3). This represents the first example of an aluminyl anion supported by an acyclic ligand framework. Attempts to yield the same structure with a larger ligand framework, {N(Dipp)Si(i-Pr)3}, led to C-H cleavage. K2[Al{N(Dipp)SiMe3}2]2 behaves as a nucleophilic source of aluminium; reaction with an electrophilic ß-diketiminate supported magnesium(II) iodide forms a monomeric, acyclic magnesium aluminyl complex.

RESUMEN

The (IPr)CuOtBu catalysed reduction of 11 aryl and alkyl isocyanates with pinacolborane gave only the boraformamides, pinBN(R)C(O)H, in most cases. Overreduction, which hampers almost all isocyanate hydroborations, was restricted to electron poor aryl isocyanates (4-NC-C6H4NCO, 4-F3C-C6H4NCO, 3-O2N-C6H4NCO). Computational analysis showed stability of [(IPr)CuH]2, which was proposed to be the catalyst resting state, drives selectivity, suggesting an approach to prevent overreduction in future work. In the case of iPrNCO, formation of this species renders overreduction kinetically inaccessible. For 4-NC-C6H4NCO, however, the barrier height for the first step of over-reduction is much lower, even relative to [(IPr)CuH]2, resulting in unselective reduction.

Asunto(s)

RESUMEN

Thermolysis of a 1,3-dioxa-2-phospholane supported by the terphenyl ligand AriPr4 (AriPr4 =[C6 H3 -2,6-(C6 H3 -2,6-iPr2 )]) at 150 °C gives [AriPr4 PO2 ]2 via loss of ethene. [AriPr4 PO2 ]2 was characterised by X-ray crystallography and NMR spectroscopy; it contains a 4-membered P-O-P-O ring and is the isostructural oxygen analogue of Lawesson's and Woollins' reagents. The dimeric structure of [AriPr4 PO2 ]2 was found to persist in solution through VT NMR spectroscopy and DOSY, supported by DFT calculations. The addition of DMAP to the 1,3-dioxa-2-phospholane facilitates the loss of ethene to give AriPr4 (DMAP)PO2 after days at room temperature, with this product also characterised by X-ray crystallography and NMR spectroscopy. Replacement of the DMAP with pyridine induces ethene loss from the 1,3-dioxa-2-phospholane to provide gram-scale samples of [AriPr4 PO2 ]2 in 75 % yield in 2 days at only 100 °C.

RESUMEN

Three ring-expanded N-heterocyclic carbene-supported copper(I) triphenylstannyls have been synthesised by the reaction of (RE-NHC)CuOtBu with triphenylstannane (RE-NHC = 6-Mes, 6-Dipp, 7-Dipp). The compounds were characterised by NMR spectroscopy and X-ray crystallography. Reaction of (6-Mes)CuSnPh3 with di-p-tolyl carbodiimide, phenyl isocyanate and phenylisothiocyanate gives access to a copper(I) benzamidinate, benzamide and benzothiamide respectively via phenyl transfer from the triphenylstannyl anion with concomitant formation of (Ph2Sn)n. Attempts to exploit this reactivity under a catalytic regime were hindered by rapid copper(I)-catalysed dismutation of Ph3SnH to Ph4Sn, various perphenylated tin oligomers, H2 and a metallic material thought to be Sn(0). Mechanistic insight was provided by reaction monitoring via NMR spectroscopy and mass spectrometry.

RESUMEN

Reaction of bis(pinacolato)diboron with (6-Dipp)CuOtBu generates a ring-expanded N-heterocyclic carbene supported copper(I) boryl, (6-Dipp)CuBpin. This compound showed remarkable stability and was characterised by NMR spectroscopy and X-ray crystallography. (6-Dipp)CuBpin readily dechalcogenated a range of heterocumulenes such as CO2, isocyanates and isothiocyanates to yield (6-Dipp)CuXBpin (X = O, S). In the case of CO2 catalytic reduction to CO is viable in the presence of excess bis(pinacolato)diboron. In contrast, in the case of iso(thio)cyanates, the isocyanide byproduct of dechalcogenation reacted with (6-Dipp)CuBpin to generate a copper(I) borylimidinate, (6-Dipp)CuC(NR)Bpin, which went on to react with heterocumulenes. This off-cycle reactivity gives selective access to a range of novel boron-containing heterocycles bonded to copper, but precludes catalytic reactivity.

RESUMEN

A range of N-heterocyclic carbene-supported copper diphenylphosphides (NHC = IPr, 6-Dipp, SIMes and 6-Mes) were synthesised. These include the first reports of ring-expanded NHC-copper(i) phosphides. The compounds were characterised by NMR spectroscopy and X-ray crystallography. Reaction of (6-Dipp)CuPPh2 with isocyanates, isothiocyanates and carbon disulfide results in the insertion of the heterocumulene into the Cu-P bond. The NHC-copper phosphides were found to be the most selective catalysts yet reported for the hydrophosphination of isocyanates. They provide access to a broad range of phosphinocarboxamides in excellent conversion and good yield.

RESUMEN

We demonstrate that trimethylamine borane can exhibit desirable piezoelectric and pyroelectric properties. The material was shown to be able operate as a flexible film for both thermal sensing, thermal energy conversion and mechanical sensing with high open circuit voltages (>10 V). A piezoelectric coefficient of d33 ≈10-16 pC N-1 , and pyroelectric coefficient of p≈25.8 µC m-2 K-1 were achieved after poling, with high pyroelectric figure of merits for sensing and harvesting, along with a relative permittivity of ϵ 33 σ ≈ 6.3.

RESUMEN

The reaction of diphenyltin dihydride with LiAlH4 gives access to a set of charged tin cages as their lithium salts. Variation in the ratio of reactants provides a perstannabicyclooctane dianion and a perstannanorbornane as the di- and monoanions. These compounds can be synthesised selectively by careful stoichiometric control and have been characterised by single crystal X-ray diffractometry, NMR and UV-vis spectroscopy. Computational exploration of the electronic structures of these compounds was undertaken and, in agreement with structural and spectroscopic features, indicated significant σ-delocalisation in the tin skeletons.

RESUMEN

The facile heterodehydrocoupling of a range of primary or secondary amines and even ammonia with pinacolborane (HBPin) was accomplished using {ArMe6Sn(µ-OMe)}2 (1, ArMe6 = C6H3-2,6-(C6H2-2,4,6-Me3)2) as pre-catalysts for a catalytically active tin(ii) hydride. The more sterically hindered pre-catalyst 2, {AriPr4Sn(µ-OMe)}2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) facilitated the dehydrocoupling only of primary amines with HBPin, and at an increased rate relative to the less crowded {ArMe6Sn(µ-OMe)}2. Also presented is {ArMe6Sn(µ-NEt2)}2 (3), which can be converted into the structurally characterizable {ArMe6Sn(µ-NEt2)(µ-H)SnArMe6} (4) via the addition of pinacol borane. This, alongside stoichiometric studies, give insight into the mechanism of the catalysis.

RESUMEN

The transition metal tetra- and trinorbornyl bromide complexes, M(nor)4 (M=Fe, Co, Ni) and Ni(nor)3 Br (nor=1-bicyclo[2.2.1]hept-1-yl) and their homolytic fragmentations were studied computationally using hybrid density functional theory (DFT) at the B3PW91 and B3PW91-D3 dispersion-corrected levels. Experimental structures were well replicated; the dispersion correction resulted in shortened M-C bond lengths for the stable complexes, and it was found that Fe(nor)4 receives a remarkable 45.9 kcal mol-1 stabilization from the dispersion effects whereas the tetragonalized Co(nor)4 shows stabilization of 38.3 kcal mol-1 . Ni(nor)4 was calculated to be highly tetragonalized with long Ni-C bonds, providing a rationale for its current synthetic inaccessibility. Isodesmic exchange evaluation for Fe(nor)4 confirmed that dispersion force attraction between norbornyl substituents is fundamental to the stability of these species.

RESUMEN

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

RESUMEN

A ß-diketiminato n-butylmagnesium complex is presented as a selective precatalyst for the reductive hydroboration of organic nitriles with pinacolborane (HBpin). Stoichiometric reactivity studies indicate that catalytic turnover ensues through the generation of magnesium aldimido, aldimidoborate and borylamido intermediates, which are formed in a sequence of intramolecular nitrile insertion and inter- and intramolecular B-H metathesis events. Kinetic studies highlight variations in mechanism for the catalytic dihydroboration of alkyl nitriles, aryl nitriles bearing electron withdrawing (Ar(EWG)CN) and aryl nitriles bearing electron donating (Ar(EDG)CN) substitution patterns. Kinetic isotope effects (KIEs) for catalysis performed with DBpin indicate that B-H bond breaking and C-H bond forming reactions are involved in the rate determining processes during the dihydroboration of alkyl nitriles and Ar(EDG)CN substrates, which display divergent first and second order rate dependences on [HBpin] respectively. In contrast, the hydroboration of Ar(EWG)CN substrates provides no KIE and HBpin is not implicated in the rate determining process during catalysis. Irrespective of these differences, a common mechanism is proposed in which the rate determining steps are deduced to vary through the establishment of several pre-equilibria, the relative positions of which are determined by the respective stabilities of the dimeric and monomeric magnesium aldimide and magnesium aldimidoborate intermediates as a result of adjustments to the basicity of the nitrile substrate. More generally, these observations indicate that homogeneous processes performed under heavier alkaline earth catalysis are likely to demonstrate previously unappreciated mechanistic diversity.

RESUMEN

The alkaline-earth element bis(trimethylsilyl)amides, [Ae{N(SiMe3 )2 }2 (thf)2 ] [Ae=Mg, Ca, Sr], are effective precatalysts for boron-nitrogen bond formation through the desilacoupling of amines, RR'NH (R=alkyl, aryl; R'=H, alkyl, aryl), and pinBSiMe2 Ph. This reactivity also yields a stoichiometric quantity of Me2 PhSiH and provides the first example of a catalytic main-group element-element coupling that is not dependent on the concurrent elimination of H2 .