Magnesium(I) Reduction of Aluminum(III) Hydride Complexes: Generation of Mixed Valence Aluminum (AlI /Al0 ) Hydride Cluster Compounds, [Al6 H8 (NR3 )2 {Mg(ß-diketiminate)}4 ].

Reduction of a range of amido- and aryloxy-aluminum dihydride complexes, e.g. [AlH2 (NR3 ){N(SiMe3 )2 }] (NR3 =NMe3 or N-methylpiperidine (NMP)), with ß-diketiminato dimagnesium(I) reagents, [{(Ar Nacnac)Mg}2 ] (Ar Nacnac=[HC(MeCNAr)2 ]- , Ar=mesityl (Mes) or 2,6-xylyl (Xyl)), have afforded deep red mixed valence aluminum hydride cluster compounds, [Al6 H8 (NR3 )2 {Mg(Ar Nacnac)}4 ], which have an average Al oxidation state of +0.66, the lowest for any well-defined aluminum hydride compound. In the solid-state, the clusters are shown to have distorted octahedral Al6 cores, having zero-valent Al axial sites and mono-valent AlH2 - equatorial units. Several novel by-products were isolated from the reactions that gave the clusters, including the Mg-Al bonded magnesio-aluminate complexes, [(Ar Nacnac)(Me3 N)Mg-Al(µ-H)3 [{Mg(Ar Nacnac)}2 (µ-H)]]. Computational analyses of one aluminum hydride cluster revealed its Al6 core to be electronically delocalized, and to possess one unoccupied, and six occupied, skeletal molecular orbitals.

Facile activation of inert small molecules using a 1,2-disilylene.

Reactions of the known amidinate stabilised 1,2-disilylene, [{ArC(NDip)2}Si]21 (Dip = 2,6-diisopropylphenyl, Ar = 4-C6H4But) with a series of inert, unsaturated small molecule substrates have been carried out. Compound 1 reduces ButNC: to give the singlet biradicaloid 1,3-disilacyclobutanediyl [{ArC(NDip)2}Si(µ-CNBut)]23, which can be oxidised by 1,2-dibromoethane to give [{ArC(NDip)2}(Br)Si(µ-CNBut)]24. Disilylene 1 reduces two molecules of ethylene to give an unprecedented disilabicyclo[2.2.0]hexane, [{ArC(NDip)2}Si(µ-C2H4)]25. In contrast, only one molecule of ethylene inserts in the Ge-Ge bond of the digermylene analogue of 1, viz. [{ArC(NDip)2}Ge]26, leading to the formation of the bis(germylene), [{ArC(NDip)2}Ge]2(µ-C2H4) 7. Compound 1 reduces CO2, generating CO, and the oxo/carbonate-bridged disilicon(IV) system, {ArC(NDip)2}Si(µ-CO3)2(µ-O)Si{(NDip)2CAr} 10, while its reaction with N2O proceeds via generation of N2, and a hydrogen abstraction process, to give the oxo/hydroxy disilicon(IV) species, [{ArC(NDip)2}(HO)Si(µ-O)]211. This study highlights new small molecule activation chemistry for 1,2-disilylenes, which could lead to further adoption of compound 1 as a potent reducing reagent for the transformation of inert unsaturated molecules into value added products.

Activation of CO Using a 1,2-Disilylene: Facile Synthesis of an Abnormal N-Heterocyclic Silylene.

Reaction of the 1,2-disilylene, [{ArC(NDip)2 }Si]2 1 (Dip=2,6-diisopropylphenyl, Ar=4-C6 H4 But ), with CO proceeds via insertion of CO into one Si-N bond, and Si-Si bond cleavage, to cleanly give the bis(silylene), {ArC(NDip)2 }Si(:)O C S i ( : ) ( N D i p )​ 2 C ‾ Ar 2, under ambient conditions. The reaction can be partially reversed when solutions of 2 are subjected to UV irradiation. The five-membered heterocyclic fragment of 2 represents the first silicon analogue of an "abnormal" N-heterocyclic carbene (aNHC), a view which is substantiated by a computational analysis of the compound. Reaction of 2 with [Mo(CO)6 ] under UV light affords the chelate complex, [Mo(CO)4 (κ2 -Si,Si-2)] 3, while reaction with [Fe(CO)5 ] gives the unusual silyleneyl bridged complex, [{Fe2 (CO)6 }{µ-Si[(NDip)2 CAr]}2 ] 4. The same coordination complexes can be accessed by reaction of 1 with [Mo(CO)6 ] or [Fe(CO)5 ] under UV light. As is the case for aNHCs, d-block metal complexes of bis(silylene) 2 could prove useful as bespoke catalysts for organic transformations.

An NHC-Mediated Metal-Free Approach towards an NHC-Coordinated Endocyclic Disilene.

A convenient metal-free approach towards an N-heterocyclic carbene (NHC)-coordinated disilene 2 is described. Compound 2, featuring the disilene incorporated in cyclopolysilane framework, was obtained in good yield and characterized using NMR spectroscopy and X-ray crystallography. Density functional theory (DFT) calculations of the reaction mechanism provide a rationale for the observed reactivity and give detailed information on the bonding situation of the base-stabilized disilene. Compound 2 undergoes thermal or light- induced (λ=456 nm) NHC loss, and a dimerization process to give a corresponding dimer with a Si10 skeleton. In order to shed light on the dimerization mechanism, DFT calculations were performed. Moreover, the reactivity of 2 was examined with selected examples of transition metal carbonyl compounds.

Bulky arene-bridged bis(amide) and bis(amidinate) complexes of germanium(II) and tin(II).

Several new, very bulky arene-bridged bis(amine) (viz. 1,3- and 1,4-{N(H)(SiPri3)}2(µ-C6H4), L1H2 and L2H2, respectively) and bis(amidine) pro-ligands (viz. 4,6-{[Dip(H)N](DipN)C}2(µ-DBF), DBF = dibenzofurandiyl, L3H2; and 1,3-{Ar†N(H)C(But)N}2(µ-C6H4), Ar† = C6H2{C(H)Ph2}2Pri-2,6,4, L4H2) have been developed. All can be doubly deprotonated with LiBun. The resultant dilithium salts react with either GeCl2·(dioxane) or SnBr2 to yield a series of amidotetrelylenecyclophanes (:E(µ-L1)2E: and :E(µ-L2)2E:, E = Ge or Sn) and bis(halotetrelylene) complexes (:E(X)(µ-L3)(X)E:, E = Ge or Sn, X = Cl or Br; and :Ge(Cl)(µ-L4)(Cl)Ge:). Reduction of :Ge(Cl)(µ-L4)(Cl)Ge: with KC8 afforded the crystallographically characterised bis(amido/amidinatogermylene) compound, :Ge(µ-L4)2Ge:, which is believed to have formed via a disproportionation process.

Synthesis and Characterization of a Magnesium Boryl and a Beryllium-Substituted Diazaborole.

Reaction of a lithium boryl, [(THF)2 Li{B(DAB)}] (DAB=[(DipNCH)2 ]2- , Dip=2,6-diisopropylphenyl), with a dinuclear magnesium(I) compound [{(Mes Nacnac)Mg}2 ] (Mes Nacnac=[HC(MeCNMes)2 ]- , Mes=mesityl) unexpectedly afforded a rare example of a terminal magnesium boryl species, [(Mes Nacnac)(THF)Mg{B(DAB)}]. Attempts to prepare the magnesium boryl via a salt metathesis reaction between the lithium boryl and a ß-diketiminato magnesium iodide compound, instead led to an intractable mixture of products. Similarly, reaction of the lithium boryl with a ß-diketiminato beryllium bromide precursor, [(Dep Nacnac)BeBr] (Dep=2,6-diethylphenyl) did not give a beryllium boryl, but instead afforded an unprecedented example of a beryllium substituted diazaborole heterocycle, [{(Dep Nacnac)Be(4-DAB-H )}BBr]. For sake of comparison, the same group 2 halide precursor compounds were treated with a potassium gallyl analogue of the lithium boryl, viz. [(tmeda)K{:Ga(DAB)}] (tmeda=N,N,N',N'-tetramethylethylenediamine), but no reactions were observed.

Acyclic 1,2-dimagnesioethanes/-ethene derived from magnesium(i) compounds: multipurpose reagents for organometallic synthesis.

Reactions of three magnesium(i) dimers, [{(ArNacnac)Mg-}2] (ArNacnac = [(ArNCMe)2CH]-; Ar = xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), α-methylstyrene (MS), trans-stilbene (TS) or diphenylacetylene (DPA) led to the 1,2-addition of the Mg-Mg bond across the substrate, giving rise to the 1,2-dimagnesioethanes, [{(XylNacnac)Mg}2(µ-DPE)], [{(DepNacnac)Mg}2(µ-MS)], [{(ArNacnac)Mg}2(µ-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{(MesNacnac)Mg}2(µ-DPA)]. The reactions involving the 1,1-substituted alkenes are shown to be readily redox reversible, in that the reaction products are in equilibrium with a significant proportion of the starting materials at room temperature. Variable temperature NMR spectroscopy and a van't Hoff analysis point to low kinetic barriers to these weakly exergonic reactions. [{(MesNacnac)Mg}2(µ-DPE)] and [{(MesNacnac)Mg}2(µ-DPA)] behave as 1,2-di-Grignard reagents in their reactions with very bulky amido-zinc bromides, yielding the first examples of a 1,2-dizincioethane, [(L*Zn)2(µ-DPE)] (L* = -N(Ar*)(SiPri 3); Ar* = C6H2Me{C(H)Ph2}2-4,2,6), and a 1,2-dizincioethene, [(TBoLZn)2(µ-DPA)] (TBoL = -N(SiMe3){B(DipNCH)2}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{(MesNacnac)Mg}2(µ-DPE)], which behaves as a two-electron reducing agent when treated with amido-cadmium and amido-magnesium halide precursors, yielding the cadmium(i) and magnesium(i) dimers, [PhBoLCdCdPhBoL] (PhBoL = -N(SiPh3){B(DipNCH)2}) and [L†MgMgL†] (L† = -N(Ar†)(SiMe3); Ar† = C6H2Pri{C(H)Ph2}2-4,2,6), respectively. A further class of reactivity for [{(MesNacnac)Mg}2(µ-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCl, which gives a magnesio-germane, presumably via intramolecular C-H activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg(MesNacnac)}]. [{(MesNacnac)Mg}2(µ-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in this reaction.

Redox transmetallation approaches to the synthesis of extremely bulky amido-lanthanoid(ii) and -calcium(ii) complexes.

Redox transmetallation protolysis reactions between HgPh2, elemental metals, M = Ca, Eu, or Yb, and the bulkyl arylsilylamine PhL†H (HN(SiPh3)(Ar†), Ar† = C6H2Pri{C(H)Ph2}2-4,2,6), or the borylsilylamine PhLBoH (HN(SiPh3){B(DipNCH)2}, Dip = C6H3Pri2-2,6) pro-ligands yielded complexes incorporating doubly deprotonated, N,C-chelating amido/organyl ligands, viz. [M(L-H) (thf)x]n, L-H = PhL†-H (1-3) or PhLBo-H (4-5); M = Ca (1, 4), Eu (2) or Yb (3, 5); x = 0 (5) or 2 (1-4); n = 1 (1-4) or 2 (5). Structural differences between 4 and 5 represent a rare divergence in the chemistry of divalent calcium and ytterbium. Utilisation of a less hindered bis(aryl) amine, DLMH (HN(Dip)(Mes), Mes = C6H2Me3-2,4,6) in a similar reaction yielded a three-coordinate, trigonal planar ytterbium complex [Yb(DLM)2(thf)] (6). Direct redox transmetallation reactions between an amido-mercurial iodide [(MeL†)HgI] (7, MeL† = -N(SiMe3)(Ar†)) and ytterbium or europium metal afforded homoleptic [Yb(MeL†)2] (8) and heteroleptic [{Eu(MeL†)(µ-I)(thf)}2] (9) respectively. This study highlights the versatility of redox transmetallation pathways to amido-lanthanoid complexes, especially where such compounds are difficult to access using conventional salt metathesis pathways.

Boryl substituted group 13 metallylenes: complexes with an iron carbonyl fragment.

The first examples of boryl substituted aluminylene and gallylene complexes, [(DAB)B(THF)Al{Fe(CO)3(µ-CO)}]2 and [(DAB)BGa{µ-Fe(CO)4}]2 (DAB = {(C6H3Pri2-2,6)NCH}2) have been prepared by reduction of MX2(THF){B(DAB)} (M = Al or Ga, X = Cl or Br) with K2[Fe(CO)4]. Spectroscopic and crystallographic analyses of the compounds show them to be structurally distinct dimers, the latter of which possesses a close GaGa separation that computational analyses reveal has negligible bonding character.

Two-coordinate terminal zinc hydride complexes: synthesis, structure and preliminary reactivity studies.

The first examples of essentially two-coordinate, monomeric zinc hydride complexes, LZnH (L = -N(Ar)(SiR3)) (Ar = C6H2{C(H)Ph2}2R'-2,6,4; R = Me, R' = Pr(i) (L'); R = Pr(i), R' = Me (L*); R = Pr(i), R' = Pr(i) (L(†))) have been prepared and shown by crystallographic studies to have near linear N-Zn-H fragments. The results of computational studies imply that any PhZn interactions in the compounds are weak at best. Preliminary reactivity studies reveal the compounds to be effective for the stoichiometric hydrozincation and catalytic hydrosilylation of carbonyl compounds.

Two-Coordinate Magnesium(I) Dimers Stabilized by Super Bulky Amido Ligands.

A variety of very bulky amido magnesium iodide complexes, LMgI(solvent)0/1 and [LMg(µ-I)(solvent)0/1 ]2 (L=-N(Ar)(SiR3 ); Ar=C6 H2 {C(H)Ph2 }2 R'-2,6,4; R=Me, Pr(i) , Ph, or OBu(t) ; R'=Pr(i) or Me) have been prepared by three synthetic routes. Structurally characterized examples of these materials include the first unsolvated amido magnesium halide complexes, such as [LMg(µ-I)]2 (R=Me, R'=Pr(i) ). Reductions of several such complexes with KC8 in the absence of coordinating solvents have afforded the first two-coordinate magnesium(I) dimers, LMg-MgL (R=Me, Pr(i) or Ph; R'=Pr(i) , or Me), in low to good yields. Reductions of two of the precursor complexes in the presence of THF have given the related THF adduct complexes, L(THF)Mg-Mg(THF)L (R=Me; R'=Pr(i) ) and LMg-Mg(THF)L (R=Pr(i) ; R'=Me) in trace yields. The X-ray crystal structures of all magnesium(I) complexes were obtained. DFT calculations on the unsolvated examples reveal their Mg-Mg bonds to be covalent and of high s-character, while Ph⋅⋅⋅Mg bonding interactions in the compounds were found to be weak at best.

Chemical bonding and electronic localization in a Ga(I) amide.

The electron density in a one-coordinate [Ga(I) N(SiMe3 )R] complex has been determined from ab initio calculations and multipole modeling of 90 K X-ray data. The topologies of the Laplacian distribution and the ELI-D match a situation having an sp(3) -hybridized nitrogen with a tetrahedral arrangement of two single σ-bonds (to carbon and silicon) and two lone pairs pointing towards gallium in a scissor-grasping fashion. The analysis of the Laplacian distribution furthermore reveals a ligand-induced charge concentration (LICC) in the outer core of gallium oriented directly towards the nitrogen atom, and thus in between the two lone pairs. These observations might suggest that the trigonal planar nitrogen geometry result from a dative GaN bond, in which the roles of the metal and the ligand have been reversed with respect to a "standard" metal-ligand interaction, that is, the metal is here electron-donating. The ELI-D reveals a diffuse and directional lone pair on gallium, suggesting that this complex could serve as a σ-donor.

Utilisation of a lithium boryl as a reducing agent in low oxidation state group 15 chemistry: synthesis and characterisation of an amido-distibene and a boryl-dibismuthene.

The first examples of an amido-distibene, L(†)Sb[double bond, length as m-dash]SbL(†) (L(†) = -N(Ar(†))(SiPr(i)3), Ar(†) = C6H2{C(H)Ph2}2Pr(i)-2,6,4), and a boryl dibismuthene, {(DAB)B}Bi[double bond, length as m-dash]Bi{B(DAB)} (DAB = {(C6H3Pr(i)2-2,6)NCH}2, have been prepared by reaction of a lithium boryl complex, (THF)2LiB(DAB), with extremely bulky amido-group 15 dihalide precursor compounds. In these reactions, the lithium boryl acts as a boryl transfer reagent and/or a strong reducing agent.

Circumventing redox chemistry: synthesis of transition metal boryl complexes from a boryl nucleophile by decarbonylation.

The very strong reducing capabilities of the boryllithium nucleophile (THF)2Li{B(NDippCH)2} (1, Dipp = 2,6-iPr2C6H3) render impractical its use for the direct introduction of the {B(NDippCH)2} ligand via metathesis chemistry into the immediate coordination sphere of transition metals (d(n), with n ≠ 0 or 10). Instead, 1 typically reacts with metal halide, amide and hydrocarbyl electrophiles either via electron transfer or halide abstraction. Evidence for the formation of M-B bonds is obtained only in the case of the d(5) system [{(HCDippN)2B}Mn(THF)(µ-Br)]2. Lower oxidation state metal carbonyl complexes such as Fe(CO)5 and Cr(CO)6 react with 1 via nucleophilic attack at the carbonyl carbon atom to give boryl-functionalized Fischer carbene complexes Fe(CO)4{C(OLi(THF)3)B(NDippCH)2} and Cr(CO)5{C(OLi(THF)2)B(NDippCH)2}. Although C-to-M boryl transfer does not occur for these formally anionic systems, more labile charge neutral bora-acyl derivatives of the type LnM{C(O)B(NDippCH)2} [LnM = Mn(CO)5, Re(CO)5, CpFe(CO)2] can be synthesized, which cleanly lose CO to generate M-B bonds. From a mechanistic standpoint, an archetypal organometallic mode of reactivity, carbonyl extrusion, has thus been shown to be applicable to the boryl ligand class, with (13)C isotopic labeling studies confirming a dissociation/migration pathway. These proof-of-methodology synthetic studies can be extended beyond boryl complexes of the group 7 and 8 metals (for which a number of versatile synthetic routes already exist) to provide access to complexes of cobalt, which have hitherto proven only sporadically accessible.

Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies.

Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH2}2CH2, Dip = 2,6-diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl3] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl3] with KC8 yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)2(µ-P4)]Cl2, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl3. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH2}2CH2](+), Mes = mesityl) with KC8, leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)2. An X-ray crystallographic analysis of (6-MesH)2 shows the compound to have a long central C-C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.

Oxidative bond formation and reductive bond cleavage at main group metal centers: reactivity of five-valence-electron MX2 radicals.

Monomeric five-valence-electron bis(boryl) complexes of gallium, indium, and thallium undergo oxidative M-C bond formation with 2,3-dimethylbutadiene, in a manner consistent with both the redox properties expected for M(II) species and with metal-centered radical character. The weaker nature of the M-C bond for the heavier two elements leads to the observation of reversibility in M-C bond formation (for indium) and to the isolation of products resulting from subsequent B-C reductive elimination (for both indium and thallium).

Stable GaX2, InX2 and TlX2 radicals.

The chemistry of the Group 13 metals is dominated by the +1 and +3 oxidation states, and simple monomeric M(II) species are typically short-lived, highly reactive species. Here we report the first thermally robust monomeric MX2 radicals of gallium, indium and thallium. By making use of sterically demanding boryl substituents, compounds of the type M(II)(boryl)2 (M = Ga, In, Tl) can be synthesized. These decompose above 130 °C and are amenable to structural characterization in the solid state by X-ray crystallography. Electron paramagnetic resonance and computational studies reveal a dominant metal-centred character for all three radicals (>70% spin density at the metal). M(II) species have been invoked as key short-lived intermediates in well-known electron-transfer processes; consistently, the chemical behaviour of these novel isolated species reveals facile one-electron shuttling processes at the metal centre.

Heavy metal boryl chemistry: complexes of cadmium, mercury and lead.

Synthetic routes to the first boryl complexes of cadmium and mercury are reported via transmetallation from boryllithium; the syntheses of related group 14 systems highlight the additional factors associated with extension to more redox-active post-transition elements.

Monomeric group 13 metal(I) amides: enforcing one-coordination through extreme ligand steric bulk.

Reactions of the extremely bulky amido alkali metal complexes, [KL'(η(6)-toluene)], or in situ generated [LiL'] or [LiL″] {L'/ L″ = N(Ar*)(SiR(3)), where Ar* = C(6)H(2){C(H)Ph(2)}(2)Me-2,6,4 and R = Me (L') or Ph (L″)} with group 13 metal(I) halides have yielded a series of monomeric metal(I) amide complexes, [ML'] (M = Ga, In, or Tl) and [ML″] (M = Ga or Tl), all but one of which have been crystallographically characterized. The results of the crystallographic studies, in combination with computational analyses, reveal that the metal centers in these compounds are one coordinate and do not exhibit any significant intra- or intermolecular interactions, other than their N-M linkages. One of the complexes, [InL'], represents the first example of a one-coordinate indium(I) amide. Attempts to extend this study to the preparation of the analogous aluminum(I) amide, [AlL'], were not successful. Despite this, a range of novel and potentially synthetically useful aluminum(III) halide and hydride complexes were prepared en route to [AlL'], the majority of which were crystallographically characterized. These include the alkali metal aluminate complexes, [L'AlH(2)(µ-H)Li(OEt(2))(2)(THF)] and [{L'Al(µ-H)(3)K}(2)], the neutral amido-aluminum hydride complex, [{L'AlH(µ-H)}(2)], and the aluminum halide complexes, [L'AlBr(2)(THF)] and [L'AlI(2)]. Reaction of the latter two systems with a variety of reducing agents led only to intractable product mixtures.