An anionic beryllium hydride dimer with an exceedingly short Be⋯Be distance.

Heteroleptic hydride complexes of the group 2 metals have seen considerable attention as Earth-abundant synthetic tools, yet anionic derivatives are exceedingly rare. We described the facile synthesis and in-depth characterisation of an anionic beryllium hydride dimer, featuring a dynamic [Be2H3] cluster at its core with a short Be⋯Be distance. Despite this, there is no formal Be-Be bond in this complex, with only hydride bridging interactions leading to this remarkable structural attribute.

On the σ-complex character of bis(gallyl)/digallane transition metal species.

σ-complexes of homoatomic E-E bonds are key intermediates in catalytically relevant oxidative addition reactions, but are as yet unknown for the group 13 elements. Here, stable species best described as σ-complexes of a 1,2-dichlorodigallane derivative with Ni and Pd are reported. They are readily accessed through the combination of a 1,2-dichlorodigallane derivative, which features chelating phosphine functionalities, with Ni0 and Pd0 synthons. In-depth computational analyses of these complexes importantly reveal considerable Ga-Ga bonding interactions in both Ni and Pd complexes, despite the expected elongation of the Ga-Ga bond upon complexation, suggestive of σ-complex character as opposed to more commonly described bis(gallyl) character. Finally, the well-defined disproportion of the Ni complex is described, leading to a unique GaI-nickel complex, with concomitant expulsion of uncomplexed GaIII species.

T-shaped Ni0 Systems Featuring Cationic Tetrylenes: Direct Observation of L/Z-type Ligand Duality, and Alkene Hydrogenation Catalysis.

We report a facile synthetic method for accessing rare T-shaped Ni0 species, stabilised by low-coordinate cationic germylene and stannylene ligands which behave as Z-type ligands toward Ni0 . An in-depth computational analysis indicates significant Nid →Ep donation (E=Ge, Sn), with essentially no E→Ni donation. The tetrylene ligand's Lewis acidity can be modulated in situ through the addition of a donor ligand, which selectively binds at the Lewis acidic tetrylene site. This switches this binding centre from a Z-type to a classical L-type ligand, with a concomitant geometry switch at Ni0 from T-shaped to trigonal planar. Exploring the effects of this geometry switch in catalysis, isolated T-shaped complexes 3 a-c and 4 a-c are capable of the hydrogenation of alkenes under mild conditions, whilst the closely related trigonal planar and tetrahedral Ni0 complexes 5, D, and E, which feature L-type chloro- or cationic-tetrylene ligands, are inactive under these conditions. Further, addition of small amounts of N-bases to the catalytic systems involving T-shaped complexes significantly reduces turnover rates, giving evidence for the in situ modulation of ligand electronics for catalytic switching.

Cationic Tetrylene-Iron(0) Complexes: Access Points for Cooperative, Reversible Bond Activation and Open-Shell Iron(-I) Ferrato-Tetrylenes.

The open-shell cationic stannylene-iron(0) complex 4 (4=[PhiP DippSn⋅Fe⋅IPr]+ ; PhiP Dipp={[Ph2 PCH2 Si(i Pr)2 ](Dipp)N}; Dipp=2,6-i Pr2 C6 H3 ; IPr=[(Dipp)NC(H)]2 C:) cooperatively and reversibly cleaves dihydrogen at the Sn-Fe interface under mild conditions (1.5 bar, 298 K), in forming bridging hydrido-complex 6. The One-electron oreduction of the related GeII -Fe0 complex 3 leads to oxidative addition of one C-P linkage of the PhiP Dipp ligand in an intermediary Fe-I complex, leading to FeI phosphide species 7. One-electron reduction reaction of 4 gives access to the iron(-I) ferrato-stannylene, 8, giving evidence for the transient formation of such a species in the reduction of 3. The covalently bound tin(II)-iron(-I) compound 8 has been characterised through EPR spectroscopy, SQUID magnetometry, and supporting computational analysis, which strongly indicate a high localization of electron spin density at Fe-I in this unique d9 -iron complex.

Accessing cationic tetrylene-nickel(0) systems featuring donor-acceptor E-Ni triple bonds (E = Ge, Sn).

We describe facile synthetic methods for accessing linear cationic tetrylene nickel(0) complexes [SiiPDippE·Ni(PPh3)3]+ (E = Ge (4) and Sn (5); SiiPDipp = [(iPr3Si)(Dipp)N]-), which feature donor-acceptor E-Ni triple bonds. These species are readily accessed in a one-pot protocol, combining the bulky halo-tetrylenes SiiPDippECl (E = Ge (1) and Sn (2)), Ni(cod)2, PPh3, and Na[BArF4]. Given the diamagnetic nature of 4 and 5, they each contain a formal zero-valent Ni centre, making the E-M triple bonds in these complexes unique compared to previously reported metal tetrylidyne complexes, which typically feature covalent/ionic bonding. In-depth computational analyses of these species further support triple bond character in their E-Ni interactions.

Accessing the main-group metal formyl scaffold through CO-activation in beryllium hydride complexes.

Carbon monoxide (CO) is an indispensable C1 building block. For decades this abundant gas has been employed in hydroformylation and Pausen-Khand catalysis, amongst many related chemistries, where a single, non-coupled CO fragment is delivered to an organic molecule. Despite this, organometallic species which react with CO to yield C1 products remain rare, and are elusive for main group metal complexes. Here, we describe a range of amido-beryllium hydride complexes, and demonstrate their reactivity towards CO, in its mono-insertion into the Be-H bonds of these species. The small radius of the Be2+ ion in conjunction with the non-innocent pendant phosphine moiety of the developed ligands leads to a unique beryllium formyl complex with an ylidic P-COC fragment, whereby the carbon centre, remarkably, datively binds Be. This, alongside reactivity toward carbon dioxide, sheds light on the insertion chemistry of the Be-H bond, complimenting the long-known chemistry of the heavier Alkaline Earth hydrides.

Geometrically Constrained Cationic Low-Coordinate Tetrylenes: Highly Lewis Acidic σ-Donor Ligands in Catalytic Systems.

A novel non-innocent ligand class, namely cationic single-centre ambiphiles, is reported in the phosphine-functionalised cationic tetrylene Ni0 complexes, [PhR DippENi(PPh3 )3 ]+ (4 a/b (Ge) and 5 (Sn); PhR Dipp={[Ph2 PCH2 SiR2 ](Dipp)N}- ; R=Ph, i Pr; Dipp=2,6-i Pr2 C6 H3 ). The inherent electronic nature of low-coordinate tetryliumylidenes, combined with the geometrically constrained [N-E-Ni] bending angle enforced by the chelating phosphine arm in these complexes, leads to strongly electrophilic EII centres which readily bind nucleophiles, reversibly in the case of NH3 . Further, the GeII centre in 4 a/b readily abstracts the fluoride ion from [SbF6 ]- to form the fluoro-germylene complex PhR DippGe(F)Ni(PPh3 )3 9, despite this GeII centre simultaneously being a σ-donating ligand towards Ni0 . Alongside the observed catalytic ability of 4 and 5 in the hydrosilylation of alkynes and alkenes, this forms an exciting introduction to a multi-talented ligand class in cationic single-centre ambiphiles.

Reversible metathesis of ammonia in an acyclic germylene-Ni0 complex.

Carbenes, a class of low-valent group 14 ligand, have shifted the paradigm in our understanding of the effects of supporting ligands in transition-metal reactivity and catalysis. We now seek to move towards utilizing the heavier group 14 elements in effective ligand systems, which can potentially surpass carbon in their ability to operate via 'non-innocent' bond activation processes. Herein we describe our initial results towards the development of scalable acyclic chelating germylene ligands (viz. 1a/b), and their utilization in the stabilization of Ni0 complexes (viz. 4a/b), which can readily and reversibly undergo metathesis with ammonia with no net change of oxidation state at the GeII and Ni0 centres, through ammonia bonding at the germylene ligand as opposed to the Ni0 centre. The DFT-derived metathesis mechanism, which surprisingly demonstrates the need for three molecules of ammonia to achieve N-H bond activation, supports reversible ammonia binding at GeII, as well as the observed reversibility in the overall reaction.

Synthesis and Coordination Ability of a Donor-Stabilised Bis-Phosphinidene.

Chelating phosphines have long been a mainstay as efficient directing ligands in transition-metal catalysis. Low-valent derivatives, namely chelating phosphinidenes, are to date unknown, and could lead to chelating complexes containing more than one metal centre due to the intrisic capacity of phosphinidenes to bind two metal fragments at one P-centre. Here we describe the synthesis of the first such chelating bis-phosphinidene ligand, XantP2 (2), generated by the reduction of a diphosphino xanthene derivative, Xant(PH2 )2 (1) with iPr NHC (iPr NHC=[:C{N(iPr)C(H)}2 ]). Initial studies have shown that this novel chelating ligand can act as a bidentate ligand towards element dihalides (i.e. FeCl2 , ZnI2 , GeCl2 , SnBr2 ), forming cationic complexes with the tetryl elements. In contrast, XantP2 demonstrates an ability to bind multiple metal centres in the reaction with CuCl, leading to a cationic Cu3 P3 ring complex, with Cu centres bridged by phosphinidene arms. Density Functional Theory calculations show that 2 indeed holds 4 lone pairs of electrons, shedding further light on the coordination capacity for this novel ligand class through observation of directionality and hybridisation of these electron pairs.

Cycloaddition Chemistry of a Silylene-Nickel Complex toward Organic π-Systems: From Reversibility to C-H Activation.

The versatile cycloaddition chemistry of the Si-Ni multiple bond in the acyclic (amido)(chloro)silylene→Ni0 complex 1, [(TMS L)ClSi→Ni(NHC)2 ] (TMS L=N(SiMe3 )Dipp; Dipp=2,6-iPr2 C6 H4 ; NHC=C[(iPr)NC(Me)]2 ), toward unsaturated organic substrates is reported, which is both reminiscent of and expanding on the reactivity patterns of classical Fischer and Schrock carbene-metal complexes. Thus, 1:1 reaction of 1 with aldehydes, imines, alkynes, and even alkenes proceed to yield [2+2] cycloaddition products, leading to a range of four-membered metallasilacycles. This cycloaddition is in fact reversible for ethylene, whereas addition of an excess of this olefin leads to quantitative sp2 -CH bond activation, via a 1-nickela-4-silacyclohexane intermediate. These results have been supported by DFT calculations giving insights into key mechanistic aspects.

Versatile Tautomerization of EH2-Substituted Silylenes (E = N, P, As) in the Coordination Sphere of Nickel.

The synthesis and tautomerization of a "half-parent" aminosilylene and its heavy P- and As-analogues (TMSLSi-EH2; E = N, P, As; TMSL = N(SiMe3)(2,6- iPr2C6H3)) in the coordination sphere of nickel(0) to give the corresponding side-on η2-RSi(H)═EH and RH2Si-E ("silylpnictinidene") nickel complexes are reported. These complexes can be accessed through salt metathesis reactions of the lithium dihydropnictides LiEH2 with the acyclic chlorosilylene nickel(0) complex 1, [TMSL(Cl)Si → Ni(NHC)2; NHC = :C[( iPr)NC(Me)]2). In addition, we report the facile E-H bond activation reactions of EH3 with 1, which furnished a silyl nickel(II) complex through NH3 activation, but phosphido and arsenido complexes in the activation of PH3 and AsH3, respectively. Notably, reaction of 1 with LiNH2 leads to the acyclic bis(amido)silylene complex [TMSL(H2N)Si → Ni(NHC)2] 5, which does not undergo N-H proton migration to silicon(II) under ambient conditions. The transformation of the P- and As-analogues of 1 furnishes directly the respective side-on Si═E Ni complexes (nickelacycles), [η2-{TMSL(H)Si═E(H)}Ni(NHC)2] (E = P, 6; E = As, 9). These nickelacycles show a vastly different stability in solutions. While 6 is stable for several days at ambient temperature, 9 undergoes further rearrangement processes within minutes of its formation. Given the high acidity of the As-H proton in 9, however, this moiety can be trapped as a highly charge separated metalated-η2-silaarsene nickel complex 12 that is best described as an [AsSiNi] nickelacycle with Si-As multiple bond character. Taken as a whole, these results give, for the first time, insights into the relative stability of the tautomeric forms of side-on silaldimine transition metal complexes. The electronic nature and the rearrangement processes of these compounds were also investigated by quantum chemical calculations.

From As-Zincoarsasilene (LZn-As=SiL') to Arsaethynolato (As≡C-O) and Arsaketenylido (O=C=As) Zinc Complexes.

The reactivity of the As-zincosilaarsene LZn-As=SiL' A (L=[CH(CMeNDipp)2 ]- , Dipp=2,6-i Pr2 C6 H3 , L'=[{C(H)N(2,6-i Pr2 -C6 H3 )}2 ]2- ) towards small molecules was investigated. Due to the pronounced zwitterionic character of the Si=As bond of A, it undergoes addition reactions with H2 O and NH3 , forming LZnAs(H)SiOH(L') 1 and LZnAs(H)SiNH2 (L') 2. Oxygenation of A with N2 O at -60 °C furnishes the deep blue 1,2-disiloxydiarsene, [LZnOSi(L')As]2 4, presumably via dimerization of the arsinidene intermediate LZnOSi(L')As 3. Oxygenation of A with CO2 leads to the monomeric arsaethynolato siloxido zinc complex LZnOSi(L')(OC≡As) 5, essentially trapping the intermediary arsinidene 3 with liberated CO following initial oxidation of the Si=As bond. DFT calculations confirm the ambident coordination mode of the anionic [AsCO] ligand in solution, with the O-arsaethynolato [As≡C-O].- in 5, and the As-arsaketenylido ligand mode [O=C=As]- present in LZnO-Si(L')(-As=C=O) 5' akin to the analogous phosphorus system, [PCO]- .

Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide.

While the transformation of carbon monoxide to multicarbon compounds (fuels and organic bulk chemicals) via reductive scission of the enormously strong CO bond is dominated by transition metals, splitting and deoxygenative reductive coupling of CO under nonmatrix conditions using silicon, the second most abundant nonmetal of the earth's crust, is extremely scarce and mechanistically not well understood. Herein, we report the selective deoxygenative homocoupling of carbon monoxide by divalent silicon utilizing the (LSi:)2Xant 1a [Xant = 9,9-dimethyl-xanthene-4,5-diyl; PhC(N tBu)2] and (LSi:)2Fc 1b (Fc = 1,1'-ferrocenyl) as four-electron reduction reagents under mild reaction conditions (RT, 1 atm), affording the corresponding disilylketenes, Xant(LSi)2(µ-O)(µ-CCO) 2a and Fc(LSi)2(µ-O)(µ-CCO) 2b, respectively. However, the dibenzofuran analogue of 1b, compound 1c, was unreactive toward CO due to the longer distance between the two SiII atoms, which demonstrated the crucial role of the Si···Si distance on cooperative CO binding and activation. This is confirmed by density functional theory (DFT) calculations, and further theoretical investigations on CO homocoupling with 1a and 1b revealed that the initial step of CO binding and scission involved CO acting as a Lewis acid (four-electron acceptor), in sharp contrast to CO activation mediated by transition metals where CO serves as a Lewis base (two-electron donor). This mechanism was strongly reinforced by the reaction of 1a with isocyanide Xyl-NC (Xyl = 2,6-Me2C6H3), isoelectronic with CO. Treatment of 1a with one or two molecules of Xyl-NC furnished the unique (silyl)(imido)silene 3a and the C═C coupled bis(Xyl-NC) product 5, respectively, via the isolable doubly bridged Xant(LSi)2(µ-XylNC)2 intermediate 4. Moreover, compound 3a reacts with 1 molar equivalent of CO to give the disilylketenimine Xant(LSi)2(µ-O)(µ-CCNR) 6, representing, for the first time, a selective heterocoupling product of CO with isoelectronic isocyanide.

Synthesis and Reactivity Studies of Amido-Substituted Germanium(I)/Tin(I) Dimers and Clusters.

Three amide ligands of varying steric bulk and electronic properties were utilized to prepare a series of amido-germanium(II)/tin(II) halide compounds, (LEX)n , (L= -N{B(DipNCH)2 }(SiMe3 ), TBo L; -N{B(DipNCH)2 }(SiPh3 ), PhBo L; -N(Dip)(tBu), DBu L; Dip=C6 H3 iPr2 -2,6; E=Ge or Sn; X=Cl or Br; n=1 or 2). Reductions of these with a magnesium(I) dimer, {(Mes Nacnac)Mg}2 (Mes Nacnac=[(MesNCMe)2 CH]- , Mes=mesityl), afforded singly bonded amido-digermynes (TBo LGe-GeTBo L and PhBo LGe-GePhBo L), and an amido-distannyne (PhBo LSn-SnPhBo L), in addition to several low-valent, amido stabilized tetrel-tetrel bonded cluster compounds, (DBu LGe)4 , (DBu LSn)6 and Sn5 (TBo L)4 . The nature of the products resulting from these reactions was largely dependent on the steric bulk of the amide ligand employed. Cluster (DBu LGe)4 possessed an unusual folded butterfly structure, the bonding and electronic of which were examined using DFT calculations. Reactions of the amido-germanium(I) compounds with H2 were explored, and gave rise to the amido-digermene, TBo L(H)Ge=Ge(H)TBo L and the cyclotetragermane, {DBu L(H)Ge}4 . Reactions of (DBu LGe)4 with a series of unsaturated small molecule substrates yielded DBu LGeOGeDBu L, DBu LGe(µ-C2 H4 )2 GeDBu L and DBu LGe(µ-1,4-C6 H8 )(µ-1,2-C6 H8 )GeDBu L. The latter results imply that (DBu LGe)4 can act as a masked source of the digermyne DBu LGeGeDBu L in these reactions. All further reactivity studies indicated that the germanium(I) compounds exhibit a "transition-metal-like" behavior, which is closely related to that previously described for bulky digermynes and related compounds.

Metal nitrene-like reactivity of a Si[double bond, length as m-dash]N bond towards CO2.

The unusual reactivity of the Si[double bond, length as m-dash]N bond in metallo-iminosilane [DippN = Si(OSiMe3)Ni(Cl)(NHC)2] 1 towards CO2 and cyclohexyl isocyanate (CyNCO) is reported; it reacts with two molar equiv. of CO2 to give the [2+2+2] cycloaddition product 3 with complete scission of the σ- and π-Si-N bonds akin to transition-metal nitrene complexes. Theoretical calculations indicate that this reaction is made viable by the high polarity of the Si-N interaction in the intermediary [2+2] cycloaddition product 2, which could also be crystallographically characterised. In the case of isocyanate, the [2+2] cycloaddition product of the C[double bond, length as m-dash]O and Si[double bond, length as m-dash]N bonds is formed exclusively.

Low-valent group 14 element hydride chemistry: towards catalysis.

The chemistry of group 14 element(ii) hydride complexes has rapidly expanded since the first stable example of such a compound was reported in 2000. Since that time it has become apparent that these systems display remarkable reactivity patterns, in some cases mimicking those of late transition-metal (TM) hydride compounds. This is especially so for the hydroelementation of unsaturated organic substrates. Recently, this aspect of their reactivity has been extended to the use of group 14 element(ii) hydrides as efficient, "TM-like" catalysts in organic synthesis. This review will detail how the chemistry of these hydride compounds has advanced since their early development. Throughout, there is a focus on the importance of ligand effects in these systems, and how ligand design can greatly modify a coordinated complex's electronic structure, reactivity, and catalytic efficiency.

From zinco(ii) arsaketenes to silylene-stabilised zinco arsinidene complexes.

The decarbonylation of the first zinco(ii) arsaketene complexes LZnAsCO (2) and LZn(AsCO)(NHC) (4) (L = {CH(CMeNDipp)2}-, Dipp = 2,6-iPr2C6H3; NHC = [C(Me)N(iPr)]2C:) has been investigated in the presence of the N-heterocyclic silylenes, tBuNHSi (tBuNHSi = [C(H)N(tBu)]2Si:) and DippNHSi (DippNHSi = [C(H)N(2,6-iPr2-C6H3)]2Si:). Depending on the steric demand of the NHSi donor, dimers or monomers of silylene-stabilised arsinidenes are isolated. The bonding situations in all of the novel arsinidene complexes have been elucidated through X-ray diffraction analyses and theoretical calculations.

Synthesis of a Metallo-Iminosilane via a Silanone-Metal π-Complex.

Facile oxygenation of the acyclic amido-chlorosilylene bis(N-heterocyclic carbene) Ni0 complex [{N(Dipp)(SiMe3 )ClSi:→Ni(NHC)2 ] (1; Dipp=2,6-i Pr2 C6 H4 ; N-heterocyclic carbene=C[(i Pr)NC(Me)]2 ) with N2 O furnishes the first Si-metalated iminosilane, [DippN=Si(OSiMe3 )Ni(Cl)(NHC)2 ] (3), in a rearrangement cascade. Markedly, the formation of 3 proceeds via the silanone (Si=O)-Ni π-complex 2 as the initial product, which was predicted by DFT calculations and observed spectroscopically. The Si=O and Si=N moieties in 2 and 3, respectively, show remarkable hydroboration reactivity towards H-B bonds of boranes, in the former case corroborating the proposed formation of a (Si=O)-Ni π-complex at low temperature.

Silylene-Nickel Promoted Cleavage of B-O Bonds: From Catechol Borane to the Hydroborylene Ligand.

The first 16 valence electron [bis(NHC)](silylene)Ni0 complex 1, [(TMS L)ClSi:→Ni(NHC)2 ], bearing the acyclic amido-chlorosilylene (TMS L)ClSi: (TMS L=N(SiMe3 )Dipp; Dipp=2,6-Pri2 C6 H4 ) and two NHC ligands (N-heterocyclic carbene=:C[(Pri )NC(Me)]2 ) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi-NiH complex 2. Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene-coordinated (chloro)(silyl)nickel(II) complex 3, {[cat(TMS L)Si](Cl)Ni←:BH(NHC)2 }, via the cleavage of two B-O bonds and simultaneous formation of two Si-O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:→Ni moiety in the breaking of incredibly strong B-O bonds.

Stabilization of a two-coordinate, acyclic diaminosilylene (ADASi): completion of the series of isolable diaminotetrylenes, :E(NR(2))(2) (E = group 14 element).

An extremely bulky boryl-amide ligand, [N(SiMe3){B(DAB)}](-) (TBoN; DAB = (DipNCH)2, Dip = C6H3Pr(i)2-2,6), has been utilised in the preparation of the first isolable, two-coordinate acyclic diaminosilylene (ADASi), viz. :Si(TBoN)2. This is shown to have a frontier orbital energy separation, and presumed level of reactivity, intermediate between those of the two classes of previously reported isolable two-coordinate, acyclic silylenes.