Hydridostannylene Derivatives of Magnesium and Calcium.

Reactions of a m-terphenylhydridostannylene with β-diketiminato magnesium and calcium hydrides provide bis-μ-hydrido species, the heterobimetallic constitutions of which are maintained after the addition of THF donor solvent. In both cases, reactions with hex-1-ene result in the formation of tetravalent organostannyl alkaline earth derivatives. Whereas the magnesium reagent undergoes facile twofold addition, the calcium-centered process is arrested after a single alkene reduction event.

Nucleophilic Triphenylgermanides of Magnesium and Calcium.

The dimeric calcium and magnesium hydrides, [(BDI)AeH] [BDI=HC{(Me)CNDipp}, Dipp=2,6-i-PrCH; Ae=Mg or Ca] do not react with PhGeH in non-coordinating solvent. Addition of THF, however, induces deprotonation and access to monomeric Ae-germanide complexes, [(BDI)Ae{GePh}(THF)], both of which have been structurally characterized. Although this process is facile when Ae=Ca, the analogous magnesium-based reaction requires heating to temperatures >100 °C, under which conditions germanide formation is complicated by THF ring opening and the generation of an alkaline earth germyl-C-terminated n-butoxide, [(BDI)Mg{μ-O(CH)GePh}].

Isolobal Cationic Iridium Dihydride and Dizinc Complexes: A Dual Role for the ZnR Ligand Enhances H Activation.

The reaction of [Ir(IPr)H][BAr] (; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; BAr = B{CH(3,5-CF)}) with ZnMe proceeds with CH elimination to give [Ir(IPr)(IPr')(ZnMe)H][BAr] (, where (IPr') is a cyclometalated IPr ligand). reacts with H to form tetrahydride [Ir(IPr)(ZnMe)H][BAr], , that loses H under forcing conditions to form [Ir(IPr)(ZnMe)H][BAr], . Crystallization of also results in the formation of its noncyclometalated isomer, [Ir(IPr)(ZnMe)][BAr], , in the solid state.

Alumanyl Reduction, Reductive Coupling and C-H Isomerization of Organic Nitriles.

The behavior of the potassium alumanyl, [{SiN}AlK] ({SiN} = {CHSiMeN(Dipp)}; Dipp = 2,6--PrCH), toward organic nitriles has been investigated. In common with earlier studies of the reactivity of charge neutral Al(I) species with multiply bonded small molecules, it is suggested that the initial step in all the reactions involves [2 + 1] cycloaddition and the generation of an [η-C=N-Al] alumina azacyclopropane unit. In the cases of - and -tolyl-substituted aryl nitriles, this species is too kinetically labile to allow its isolation and undergoes C-C coupling via immediate Al-C/C≡N insertion to yield the alumina diazabutadiene derivatives.

Alkali metal reduction of crown ether encapsulated alkali metal cations.

[{SiN}BeClM] ({SiN} = {CHSiMeN(Dipp)}; M = Li, Na, K, Rb) are converted to ionic species by treatment with a crown ether. Whereas the lithium derivative reacts with Na or K to provide [{SiN}BeCl][M(12-cr-4)] (M = Na, K), the resultant sodium species is resistant to reduction by potassium. These observations are rationalised by a hybrid experimental/theoretical analysis.

The borylamino-diborata-allyl anion.

Reactions of β-diketiminato alkaline earth alkyldiboranate derivatives [(BDI)Ae{pinBB(R)pin}] (BDI = HC{(Me)CNDipp}; Dipp = 2,6-i-PrCH; Ae = Mg, R = -Bu or Ae = Ca, R = -hexyl) with -BuNC provide access to the respective group 2 derivatives of unprecedented diborata-allyl, {(pinB)CNBpin(-Bu)}, anions. Although the necessary mode of B-C bond cleavage implicated in these transformations could not be elucidated, further studies of the reactivity of magnesium triboranates toward isonitriles delivered a more general and rational synthetic access to analogous anionic moieties. Extending this latter reactivity to a less symmetric triboranate variant also provided an isomeric Mg-C-bonded dibora-alkyl species and sufficient experimental insight to prompt theoretical evaluation of this reactivity.

[{SiN}MgNa]: A Potent Molecular Reducing Agent.

The bimetallic species, [{SiN}MgNa] [{SiN} = {CHSiMeN(Dipp)}; (Dipp = 2,6--PrCH)], is shown to be a potent reducing agent, able to effect one- or two-electron reduction of either dioxygen, TEMPO, anthracene, benzophenone, or diphenylacetylene. In most cases, the bimetallic reaction products imply that the dissimilar alkaline metal centers react with a level of cooperativity. EPR analysis of the benzophenone-derived reaction and the concurrent isolation of [{SiN}Mg(OCPh)], however, illustrate that treatment with such reducible, but -basic, species can also result in reactivity in which the metals provide independent reaction products.

Dial-a-base mechanochemical synthesis of N-heterocyclic carbene copper complexes.

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).

Cesium Reduction of a Lithium Diamidochloroberyllate.

Room temperature reaction of elemental cesium with the dimeric lithium chloroberyllate [{SiN}BeClLi] [{SiN} = {CHSiMeN(Dipp)}, where Dipp = 2,6-di-isopropylphenyl, in CD results in activation of the arene solvent. Although, in contrast to earlier observations of lithium and sodium metal reduction, the generation of a mooted cesium phenylberyllate could not be confirmed, this process corroborates a previous hypothesis that such beryllium-centered solvent activation also necessitates the formation of hydridoberyllium species. These observations are further borne out by the study of an analogous reaction performed in toluene, in which case the proposed generation of formally low oxidation state beryllium radical anion intermediates induces activation of a toluene C-H bond and the isolation of the polymeric cesium benzylberyllate, [Cs({SiN}BeCHCH)].

The structures and reactivity of NHC-supported copper(i) triphenylgermyls.

Deprotonation of triphenyl germane with NHC-supported copper alkoxides afforded four novel (NHC)CuGePh complexes. Of these, (IPr)CuGePh (IPr = :C{N(2,6-iPrCH)CH}) 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 [PhGe].

Alkali metal reduction of alkali metal cations.

Counter to synthetic convention and expectation provided by the relevant standard reduction potentials, the chloroberyllate, [{SiN}BeClLi] [{SiN} = {CHSiMeN(Dipp)}; Dipp = 2,6-i-PrCH)], reacts with the group 1 elements (M = Na, K, Rb, Cs) to provide the respective heavier alkali metal analogues, [{SiN}BeClM], through selective reduction of the Li cation. Whereas only [{SiN}BeClRb] is amenable to reduction by potassium to its nearest lighter congener, these species may also be sequentially interconverted by treatment of [{SiN}BeClM] by the successively heavier group 1 metal. A theoretical analysis combining density functional theory (DFT) with elemental thermochemistry is used to rationalise these observations, where consideration of the relevant enthalpies of atomisation of each alkali metal in its bulk metallic form proved crucial in accounting for experimental observations.

Iron(II)-Catalyzed Activation of Si-N and Si-O Bonds Using Hydroboranes.

We report the activation and functionalization of Si-N bonds with pinacol borane catalyzed by a three-coordinate iron(II) β-diketiminate complex. The reactions proceed via the mild activation of silazanes to yield useful hydrosilanes and aminoboranes. The reaction is studied by kinetic analysis, along with a detailed investigation of decomposition pathways using catecholborane as an analogue of the pinacol borane used in catalysis.

Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C-H Activation of Arenes.

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiN}AlI] ({SiN} = {CHSiMeNDipp}) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiN}AlM]. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiN}Na] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C-H oxidative addition of arene C-H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species.

Acyclic Boryl Complexes of Copper(I).

Reaction of (6-Dipp)CuOtBu (6-Dipp=C{NDippCH } CH , Dipp=2,6-iPr C H ) with B (OMe) provided access to (6-Dipp)CuB(OMe) via σ-bond metathesis. (6-Dipp)CuB(OMe) was characterised by NMR spectroscopy and X-ray crystallography and shown to be a monomeric acyclic boryl of copper. (6-Dipp)CuB(OMe) 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.

Variable Ca-C Hapticity and its Consequences in Arylcalcium Dimers.

The dimeric β-diketiminato calcium hydride, [( BDI)CaH] ( BDI = HC{(Me)CN-2,6-i-Pr C H } ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [( BDI) Ca (μ-H)(μ-o-,m-,p-tolyl)], in which dimer propagation occurs either via μ -η -η or μ -η -η bridging between the calcium centers. In each case, the orientation and hapticity of the aryl units is dependent upon the position of the methyl substituent. While wholly organometallic meta- and para-tolyl dimers, [( BDI)Ca(m-tolyl)] and [( BDI)Ca(p-tolyl)] , can be prepared and are stable, the ortho-tolyl isomer is prone to isomerization to a calcium benzyl analog.

Aluminum-Boron Bond Formation by Boron Ester Oxidative Addition at an Alumanyl Anion.

The potassium diamidoalumanyl, [K{Al(SiN)}] (SiN = {CHSiMeNDipp}), reacts with the terminal B-O bonds of pinacolato boron esters, ROBpin (R = Me, -Pr), and B(OMe) to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiN)(OR)(Bpin)}] (R = Me, = 2; R = -Pr, = ∞) and [K{Al(SiN)(OMe)(B(OMe))}], comprising Al-B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, ,'-diisopropylcarbodiimide and CO, highlights the kinetic inaccessibility of their Al-B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO preferentially inserts into the Al-O bond of [K{Al(SiN)(OMe)(Bpin)}] to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal NAl-BO coordination environments at both boron and aluminum.

The Complex Reactivity of [(salen)Fe](μ-O) with HBpin and Its Implications in Catalysis.

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate.

An acyclic aluminyl anion.

The potassium aluminyl complex K[Al{N(Dipp)SiMe}] was synthesised reduction of [AlI{N(Dipp)SiMe}] (Dipp = 2,6-i-PrCH). 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)}, led to C-H cleavage.

Cooperative dihydrogen activation at a Na(I)/Mg(I) ensemble.

[{SiN}MgNa] ({SiN} = {CHSiMeN(Dipp)}; Dipp = 2,6-i-PrCH) reacts directly with H to provide a heterobimetallic hydride. Although the transformation is complicated by the simultaneous disproportionation of magnesium, computational density functional theory (DFT) studies suggest that this reactivity is initiated by orbitally-constrained and interactions between the frontier MOs of both H and the tetrametallic core of [{SiN}MgNa].

Beryllium-centred C-H activation of benzene.

Reaction of BeCl with the dilithium diamide, [{SiN}Li] ({SiN} = {CHSiMeNDipp}), provides the dimeric chloroberyllate, [{SiNBeCl}Li], en route to the 2-coordinate beryllium amide, [SiNBe]. Lithium or sodium reduction of [SiNBe] in benzene, provides the relevant organoberyllate products, [{SiNBePh}M] (M = Li or Na), the presumed intermediacy of transient Be(I) radicals.

Selective hydroboration of electron-rich isocyanates by an NHC-copper(I) alkoxide.

The (IPr)CuOBu 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-CHNCO, 4-FC-CHNCO, 3-ON-CHNCO). 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.