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

The behavior of the potassium alumanyl, [{SiNDipp}AlK]2 ({SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3), 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 [η2-C=N-Al] alumina azacyclopropane unit. In the cases of o- and m-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. In contrast, the increased steric profile of alkyl nitriles imposes a marked influence on the nature of the products formed. Consistent with the proposed sequential pathway, reaction of [{SiNDipp}AlK]2 with t-BuCN provides an isolable alumina cyclopropane species that is kinetically resistant to onward reaction with a further nitrile equivalent. While reduction in the alkyl nitrile steric demands by use of i-PrCN again facilitates C-C bond formation, the crowding of the Al center by the resultant alumina-diazabutadienediide moiety appears to be beyond the limit of kinetic viability, resulting in an unusual 2-fold C-H to N-H isomerization from one of the C-iso-propyl substituents and the isolation of a 1-alumina-2,5-diazabutadiene structure.

A series of neutral alkaline earth metal hydride complexes supported by a bulky, unsymmetrical ß-diketiminate ligand, [{(Dip/TCHPNacnac)M(µ-H)}2] (M = Mg, Ca, Sr or Ba).

A bulky, unsymmetrical ß-diketiminate ligand, [HC{MeCN(Dip)}{MeCN(TCHP)}]- (Dip/TCHPNacnac; Dip = 2,6-diisopropylphenyl, TCHP = 2,4,6-tricyclohexylphenyl), has been utilised in the preparation of a series of magnesium alkyl and calcium, strontium and barium amide complexes. Reaction of these with PhSiH3 afforded the first complete series of ß-diketiminato heavier group 2 metal hydride complexes, [{(Dip/TCHPNacnac)M(µ-H)}2] (M = Mg, Ca, Sr or Ba). The unsymmetrical nature of the ß-diketiminate ligand seemingly promotes stabilising interactions of ligand Dip groups with the metal centres in the Ca, Sr and Ba hydride complexes.

Pathway to a molecular calcium methyl.

The dimeric ß-diketiminato calcium hydride, [(BDI)CaH]2 (BDI = HC{(Me)CN-2,6-iPr2C6H3}2), reacts with ZnMe2 to afford the bimetallic calcium zincate complex, [(BDI)Ca(µ-CH3)2Zn(µ-H)]2, which subsequently undergoes an intramolecular reaction to effect the formation of [(BDI)CaMe]2, a notable omission from the homologous series of ß-diketiminato alkylcalcium derivatives.

Alkali metal reduction of crown ether encapsulated alkali metal cations.

[{SiNDipp}BeClM]2 ({SiNDipp} = {CH2SiMe2N(Dipp)}2; 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 [{SiNDipp}BeCl]-[M(12-cr-4)2]+ (M = Na, K), the resultant sodium species is resistant to reduction by potassium. These observations are rationalised by a hybrid experimental/theoretical analysis.

Heteroallene Insertions into Tin(II) Alkoxide Bonds.

A series of iso-carbamate complexes have been synthesized by the reaction of [SnII(OiPr)2] or [SnII(OtBu)2] with either aryl or alkyl isocyanates, ONC-R (R = 2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (Dipp), isopropyl (iPr), cyclohexyl (Cy) and tert-butyl (tBu)). In the case of aryl isocyanates, mono-insertion occurs to form structurally characterized complexes [Sn{κ2-N,O-R-NC(OiPr)O}(µ-OiPr)]2 (1: R = Mes, 2: R = Dipp) and [Sn{κ2-N,O-R-NC(OtBu)O}(µ-OtBu)]2 (3: R = Mes, 4: R = Dipp). The complicated solution-state chemistry of these species has been explored using 1H DOSY experiments. In contrast, reactions of tin(II) alkoxides with alkyl isocyanates result in the formation of bis-insertion products [Sn{κ2-N,O-R-NC(OiPr)O}2] (5: R = iPr, and 6: R = Cy) and [Sn{κ2-N,O-R-NC(OtBu)O}2] (7: R = iPr, 8: R = Cy), of which complexes 6-8 have also been structurally characterized. 1H NMR studies show that the reaction of tBu-NCO with either [Sn(OiPr)2] or [Sn(OtBu)2] results in a reversible mono-insertion. Variable-temperature 2D 1H-1H exchange spectroscopy (VT-2D-EXSY) was used to determine the rate of exchange between free tBu-NCO and the coordinated tBu-iso-carbamate ligand for the {OiPr} alkoxide complex, as well as the activation energy (Ea = 92.2 ± 0.8 kJ mol-1), enthalpy (ΔH‡ = 89.4 ± 0.8 kJ mol-1), and entropy (ΔS‡ = 12.6 ± 2.9 J mol-1 K-1) for the process [Sn(OiPr)2] + tBu-NCO ↔ [Sn{κ2-N,O-tBu-NC(OiPr)O}(OiPr)]. Attempts to form Sn(II) alkyl carbonates by the insertion of CO2 into either [Sn(OiPr)2] or [Sn(OtBu)2] proved unsuccessful. However, 119Sn{1H} NMR spectroscopy of the reaction of excess CO2 with [Sn(OiPr)2] reveals the presence of a new Sn(II) species, i.e., [(iPrO)Sn(O2COiPr)], VT-2D-EXSY (1H) of which confirms the reversible alkyl carbonate formation (Ea = 70.3 ± 13.0 kJ mol-1; ΔH‡ = 68.0 ± 1.3 kJ mol-1 and ΔS‡ = -8.07 ± 2.8 J mol-1 K-1).

The borylamino-diborata-allyl anion.

Reactions of ß-diketiminato alkaline earth alkyldiboranate derivatives [(BDI)Ae{pinBB(R)pin}] (BDI = HC{(Me)CNDipp}2; Dipp = 2,6-i-Pr2C6H3; Ae = Mg, R = n-Bu or Ae = Ca, R = n-hexyl) with t-BuNC provide access to the respective group 2 derivatives of unprecedented diborata-allyl, {(pinB)2CNBpin(t-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. DFT calculations, thus, support a reaction pathway predicated on initial RNC attack at a peripheral boron centre and the intermediacy of such dibora-alkyl intermediates.

[{SiNDipp}MgNa]2: A Potent Molecular Reducing Agent.

The bimetallic species, [{SiNDipp}MgNa]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2; (Dipp = 2,6-i-Pr2C6H3)], 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 [{SiNDipp}Mg(OCPh2)2], however, illustrate that treatment with such reducible, but O-basic, species can also result in reactivity in which the metals provide independent reaction products. The notable E-stereochemistry of the diphenylacetylene reduction product prompted a computational investigation of the PhC≡CPh addition. This analysis invokes a series of elementary steps that necessitate ring-opening via Mg+ → Na+ amido group migration of the SiNDipp ligand, providing insight into the previously observed lability of the bidentate dianion and its consequent proclivity toward macrocyclization.

From alkaline earth to coinage metal carboranyls.

The ß-diketiminato calcium and magnesium complexes, [(BDI)MgnBu] and [(BDI)CaH]2 (BDI = HC{C(Me)NDipp}2; Dipp = 2,6-di-isopropylphenyl), react with ortho-carborane (o-C2B10H12) to provide the respective [(BDI)Ae(o-C2B10H11)] (Ae = Mg or Ca) complexes. While the lighter group 2 species is a monomer with magnesium in a distorted trigonal planar environment, the heavier analogue displays a puckered geometry at calcium in the solid state due to Ca⋯H-B intermolecular interactions. These secondary contacts are, however, readily disrupted upon addition of THF to provide the 4-coordinate monomer, [(BDI)Ca(THF)(o-C2B10H11)]. [(BDI)Mg(o-C2B10H11)] was reacted with [NHCIPrMCl] (NHCIPr = 1,3-bis(isopropyl)imidazol-2-ylidene; M = Cu, Ag, Au) to provide [NHCIPrM(o-C2B10H11)], rare C-bonded examples of coinage metal derivatives of unsubstituted (o-C2B10H11)- and confirming the alkaline earth compounds as viable reagents for the transmetalation of the carboranyl anion.

Cesium Reduction of a Lithium Diamidochloroberyllate.

Room temperature reaction of elemental cesium with the dimeric lithium chloroberyllate [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2, where Dipp = 2,6-di-isopropylphenyl, in C6D6 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 sp3 C-H bond and the isolation of the polymeric cesium benzylberyllate, [Cs({SiNDipp}BeCH2C6H5)]∞.

Alkali metal reduction of alkali metal cations.

Counter to synthetic convention and expectation provided by the relevant standard reduction potentials, the chloroberyllate, [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3)], reacts with the group 1 elements (M = Na, K, Rb, Cs) to provide the respective heavier alkali metal analogues, [{SiNDipp}BeClM]2, through selective reduction of the Li+ cation. Whereas only [{SiNDipp}BeClRb]2 is amenable to reduction by potassium to its nearest lighter congener, these species may also be sequentially interconverted by treatment of [{SiNDipp}BeClM]2 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.

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 [{SiNDipp}AlI] ({SiNDipp} = {CH2SiMe2NDipp}2) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiNDipp}AlM]2. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiNDipp}Na2] 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. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C-H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M+ cation, which becomes increasingly favorable as group 1 is descended.

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

The potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2), reacts with the terminal B-O bonds of pinacolato boron esters, ROBpin (R = Me, i-Pr), and B(OMe)3 to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiNDipp)(OR)(Bpin)}]n (R = Me, n = 2; R = i-Pr, n = ∞) and [K{Al(SiNDipp)(OMe)(B(OMe)2)}]∞, comprising Al-B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, N,N'-diisopropylcarbodiimide and CO2, highlights the kinetic inaccessibility of their Al-B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO2 preferentially inserts into the Al-O bond of [K{Al(SiNDipp)(OMe)(Bpin)}]2 to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal N2Al-BO2 coordination environments at both boron and aluminum.

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

The dimeric ß-diketiminato calcium hydride, [(Dipp BDI)CaH]2 (Dipp BDI = HC{(Me)CN-2,6-i-Pr2 C6 H3 }2 ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [(Dipp BDI)2 Ca2 (µ-H)(µ-o-,m-,p-tolyl)], in which dimer propagation occurs either via µ2 -η1 -η1 or µ2 -η1 -η6 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, [(Dipp BDI)Ca(m-tolyl)]2 and [(Dipp BDI)Ca(p-tolyl)]2 , can be prepared and are stable, the ortho-tolyl isomer is prone to isomerization to a calcium benzyl analog. Computational analysis of this latter process with density functional theory (DFT) highlights an unusual mechanism invoking the generation of an intermediate dicalcium species in which the group 2 centers are bridged by a toluene dianion formed by the formal attachment of the original hydride anion to the initially generated ortho-tolyl substituent. Use of a more sterically encumbered aryl substituent, {3,5-t-Bu2 C6 H3 }, facilitates the selective formation of [(Dipp BDI)Ca(µ-H)(µ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)], which can be converted into the unsymmetrically-substituted σ-aryl calcium complexes, [(Dipp BDI)Ca(µ-Ph)(µ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] and [(Dipp BDI)Ca(µ-p-tolyl)(µ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] by reaction with the appropriate mercuric diaryl. Conversion of [(Dipp BDI)Ca(H)(Ph)Ca(Dipp BDI)] to afford [{{(Dipp BDI)Ca}2 (µ2 -Cl)}2 (C6 H5 -C6 H5 )], comprising a biphenyl dianion, is also reported. Although this latter transformation is serendipitous, AIM analysis highlights that, in a related manner to the ortho-tolyl to benzyl isomerization, the requisite C-C coupling may be facilitated in an "across dimer" fashion by the experimentally-observed polyhapto engagement of the aryl substituents with each calcium.

Cooperative dihydrogen activation at a Na(I)2/Mg(I)2 ensemble.

[{SiNDipp}MgNa]2 ({SiNDipp} = {CH2SiMe2N(Dipp)}2; Dipp = 2,6-i-Pr2C6H3) reacts directly with H2 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 H2 and the tetrametallic core of [{SiNDipp}MgNa]2.

Structural snapshots of an Al-Cu bond-mediated transformation of terminal acetylenes.

The copper(i) alumanyl derivative, [{SiNDipp}Al-Cu(NHCiPr)] (SiNDipp = {CH2SiMe2NDipp}2; Dipp = 2,6-di-isopropylphenyl; NHCiPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), reacts in a stepwise fashion with up to three equivalents of various terminal alkynes. This reactivity results in the sequential formation of cuprous (hydrido)(alkynyl)aluminate, (alkenyl)(alkynyl)aluminate and bis(alkynyl)aluminate derivatives, examples of which have been fully characterised. The process of alkene liberation resulting from the latter reaction step constitutes a unique case of alkyne transfer semi-hydrogenation in which the C-H acidic alkyne itself acts as a source of proton, with the Cu-Al bond providing the requisite electrons to effect reduction. This reaction sequence is validated by DFT calculations, which rationalise the variable stability of the initially formed heterobimetallic hydrides.

Beryllium-centred C-H activation of benzene.

Reaction of BeCl2 with the dilithium diamide, [{SiNDipp}Li2] ({SiNDipp} = {CH2SiMe2NDipp}2), provides the dimeric chloroberyllate, [{SiNDippBeCl}Li]2, en route to the 2-coordinate beryllium amide, [SiNDippBe]. Lithium or sodium reduction of [SiNDippBe] in benzene, provides the relevant organoberyllate products, [{SiNDippBePh}M] (M = Li or Na), via the presumed intermediacy of transient Be(I) radicals.

Reduction of Na+ within a {Mg2 Na2 } Assembly.

Ionic compounds containing sodium cations are notable for their stability and resistance to redox reactivity unless highly reducing electrical potentials are applied. Here we report that treatment of a low oxidation state {Mg2 Na2 } species with non-reducible organic bases induces the spontaneous and completely selective extrusion of sodium metal and oxidation of the MgI centers to the more conventional MgII state. Although these processes are also characterized by a structural reorganisation of the initially chelated diamide spectator ligand, computational quantum chemical studies indicate that intramolecular electron transfer is abetted by the frontier molecular orbitals (HOMO/LUMO) of the {Mg2 Na2 } ensemble, which arise exclusively from the 3s valence atomic orbitals of the constituent sodium and magnesium atoms.

Diverse reactivity of a magnesium silanide toward ketones.

The reactivity of a molecular magnesium silanide toward ketones displays a significant variability of outcome, resulting in adduct formation, deprotonation, dearomatisation or deoxygenation, that is dependent on the structure and electronic character of the carbonyl-containing reagent.

Diverse reactivity of an Al(I)-centred anion towards ketones.

The reactivity of a seven-membered cyclic potassium diamidoalumanyl toward a variety of ketone small molecules has been assessed. Whilst acetophenone generates an aluminium pinacolate derivative, reductive C-C coupling is induced between the ketyl and ortho-carbon centres of two equivalents of benzophenone. In contrast, whereas oxidative addition of an enolisable proton is observed with 2,4-dimethyl-3-pentanone, 2,2,4,4-tetramethyl-3-pentanone undergoes an unprecedented hydroalumination process, where the reducing hydride may be traced to intramolecular oxidative addition of a (sp3)C-H bond.

On the reactivity of Al-group 11 (Cu, Ag, Au) bonds.

Reactions of the seven-membered heterocyclic potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2; Dipp = 2,6-di-isopropylphenyl), with a variety of Cu(I), Ag(I) and Au(I) chloride N-heterocyclic carbene (NHC) adducts are described. The resultant group 11-Al bonded derivatives have been characterised in solution by NMR spectroscopy and, in the case of [{SiNDipp}Al-Au(NHCiPr)] (NHCiPr = N,N'-di-isopropyl-4,5-dimethyl-2-ylidene), by single crystal X-ray diffraction. Although similar reactions of LAgCl and LAuCl, where L is a more basic cyclic alkyl amino carbene (CAAC), generally resulted in reduction of the group 11 cations to the base metals, X-ray analysis of [(CyCAAC)AgAl(SiNDipp)] (CyCAAC = 2-[2,6-bis(1-methylethyl)phenyl]-3,3-dimethyl-2-azaspiro[4.5]dec-1-ylidene) provides the first solid-state authentication of an Ag-Al σ bond. The reactivity of the NHC-supported Cu, Ag and Au alumanyl derivatives was assayed with the isoelectronic unsaturated small molecules, N,N'-di-isopropylcarbodiimide and CO2. While these reactions generally provided products consistent with nucleophilic attack of the group 11 atom at the electrophilic heteroallene carbon centre, treatment of the NHC-supported copper and silver alumanyls with N,N'-di-isopropylcarbodiimide yielded less symmetric Cu-C and Ag-C-bonded isomers. In contrast to the previously described copper and silver alumanyl derivatives, [(NON)Al(O2C)M(Pt-Bu3)] (M = Cu or Ag; NON = 4,5-bis(2,6-di-isopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), which were prone to facile CO extrusion and formation of carbonate derivatives, the NHC-supported dioxocarbene species, [(NHCiPr)M(CO2)Al(SiNDipp)] (M = Cu, Ag, Au), are all stable at room and moderately elevated temperatures. The stabilising role of the NHC co-ligand was, thus, assessed by preparation of the t-Bu3P adducted copper-alumanyl, [(t-Bu3P)CuAl(SiNDipp)]. Treatment of this latter compound, which was also structurally characterised by X-ray analysis, with both N,N'-di-isopropylcarbodiimide and CO2 again provided smooth heteroallene insertion and formation of the relevant Cu-C-bonded products. Although both compounds were quite stable at room temperature, heating of [(t-Bu3P)Cu(CO2)Al(SiNDipp)] at 60 °C induced elimination of CO and formation of the analogous carbonate, [(t-Bu3P)Cu(OCO2)Al(SiNDipp)], which was identified by 13C and 31P NMR spectroscopy. Reflective of the more reliable nucleophilic behaviour of the gold centres in these group 11 alumanyls, computational (QTAIM and NBO) analysis highlighted a lower level of covalency of the Al-Au linkage in comparison to the analogous Al-Cu and Al-Ag interactions. Although substitution of the co-ligand significantly perturbs the charge distribution across the Cu-Al bond of [LCuAl(SiNDipp)] (L = NHCiPr or t-Bu3P), only a negligible difference is observed between the phosphine-coordinated copper systems derived from either the [SiNDipp]- or (NON)-based alumanyl ligands. Computational mapping of the reaction profiles arising from treatment of the various group 11 alumanyls with N,N'-di-isopropylcarbodiimide indicates that the observed formation of the Cu-N and Ag-N bound isomers do not provide the thermodynamic reaction outcome. In contrast, examination of the CO2-derived reactions, and their potential toward CO extrusion and subsequent carbonate formation, implies that the identity of the co-ligand exerts a greater influence on this aspect of reactivity than the architecture of the diamidoalumanyl anion.