Stabilization of a Discrete Lanthanide(II) Hydrido Complex by a Bulky Hydrotris(pyrazolyl)borate Ligand

Synthesis and Reactivity of Bis-tris(pyrazolyl)borate Lanthanide/Aluminum Heterobimetallic Trihydride Complexes

Molecular heterobimetallic hydride complexes of lanthanide (Ln) and main-group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(μ-H)2Al(H)(N″)] [2-Ln; Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; N″ = N(SiMe3)2] have been synthesized by facile insertion of aminoalane [Me3N·AlH3] into the Ln-N amide bonds of [Ln(Tp)2(N″)] (1-Ln). Thus, this is a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, with evidence for the cooperative nature of 2-Ln shown by the catalytic amine-borane dehydrocoupling under ambient conditions in contrast to its monomeric counterparts.

Synthesis and Reactivity of Discrete Europium(II) Hydride Complexes

The bulky β-diketiminate ligand frameworks [BDIDCHP]- and [BDIDipp/Ar]- (BDI=[HC{C(Me)2N-Dipp/Ar}2]- (Dipp=2,6-diisopropylphenyl (Dipp); Ar=2,6-dicyclohexylphyenyl (DCHP) or 2,4,6-tricyclohexylphyenyl (TCHP)) have been developed for the kinetic stabilisation of the first europium (II) hydride complexes, [(BDIDCHP)Eu(μ-H)]2, [(BDIDipp/DCHP)Eu(μ-H)]2 and [(BDIDipp/TCHP)Eu(μ-H)]2, respectively. These complexes represent the first step beyond the current lanthanide(II) hydrides that are all based on ytterbium. Tuning the steric profile of β-diketiminate ligands from a symmetrical to unsymmetrical disposition, enhanced solubility and stability in the solution-state. This provides the first opportunity to study the structure and bonding of these novel Eu(II) hydride complexes crystallographically, spectroscopically and computationally, with their preliminary reactivity investigated.

Ca(II) and Yb(II) complexes featuring M(C≡C)4 structural motif: enforced proximity or genuine η2 -bonding?

Bis(carbazolide) complexes M[3,6-tBu2 -1,8-(RC≡C)2 Carb]2 (THF)n (R=SiMe3 , n=0, M=Ca, Yb; R=Ph, n=1, M=Ca, Yb; n=0, M=Yb) were synthesized through transamination reaction of M[N(SiMe3 )2 ]2 (THF)2 with two molar equivalents of carbazoles. The complexes feature M(η2 -C≡C)4 structural motif composed of M(II) ions encapsulated by four acetylene fragments due to atypical for alkaline- and rare-earth metals η2 -interactions with triple C≡C bond. This interaction is evidenced experimentally by X-ray diffraction, Raman spectroscopy in the solid state and by NMR-spectroscopy in the solution. According to QTAIM analysis there are 4 bond critical points (3;-1) between the metal atom and each of the triple bonds, which are connected by a strongly curved, almost T-shaped bond pathway.

Carbon-Carbon Bond Formation from Carbon Monoxide and Hydride: The Role of Metal Formyl Intermediates

Current examples of carbon chain production from metal formyl intermediates with homogeneous metal complexes are described in this Minireview. Mechanistic aspects of these reactions as well as the challenges and opportunities in using this understanding to develop new reactions of CO and H2 are also discussed.

Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction

Homologation of carbon monoxide is central to the heterogeneous Fischer–Tropsch process for the production of hydrocarbon fuels. C–C bond formation has been modeled by homogeneous systems, with [C_nO_n]^2– fragments (n = 2–6) formed by two-electron reduction being commonly encountered. Here, we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminum(I) (“aluminyl”) compounds to give both topologically linear and branched C₄/C₆ chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C₆O₆]^4– from [C₄O₄]^4– shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C–O bond cleavage, while a comparison of the linear [C₄O₄]^4– system with the [C₄O₄]^6– congener formed under more reducing conditions models the net conversion of C–O bonds to C–C bonds in the presence of additional reductants.

Magnesium-stabilised transition metal formyl complexes: structures, bonding, and ethenediolate formation

Herein we report the first comprehensive series of crystallographically characterised transition metal formyl complexes. In these complexes, the formyl ligand is trapped as part of a chelating structure between a transition metal (Cr, Mn, Fe, Co, Rh, W, and Ir) and a magnesium (Mg) cation. Calculations suggest that this bonding mode results in significant oxycarbene-character of the formyl ligand. Further reaction of a heterometallic Cr–Mg formyl complex results in a rare example of C–C coupling and formation of an ethenediolate complex. DFT calculations support a key role for the formyl-intermediate in ethenediolate formation. These results show that well-defined transition metal formyl complexes are potential intermediates in the homologation of carbon monoxide.

Herein we report a comprehensive series of crystallographically characterised transition metal formyl complexes.

CO, CO2 and CS2 activation by divalent ytterbium hydrido complexes

Treatment of a divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (1) with CO, CO₂ and CS₂ resulted in the formation of a divalent ytterbium ethenediolate complex [(Tp^Ad,iPr)Yb]₂(cis-OCHCHO) (2), a formate complex [(Tp^Ad,iPr)Yb(κ²-O₂CH)(THF)] (3), and a trivalent ytterbium ethenetetrathiolate complex [(Tp^Ad,iPr)Yb^III]₂(C₂S₄) (4), respectively. DFT calculations were carried out to elucidate the reaction profiles of complexes 3 and 4.

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.

Stoichiometric carbon monoxide insertion processes leading to metal-formyl complexes are scarce, even for transition metals. Here, light is shed on the underexplored chemistry of beryllium hydrides leading to a stable example of a main group metal-formyl complex.

Extending Chain Growth Beyond C1 → C4 in CO Homologation: Aluminyl Promoted Formation of the [C5O5]5– Ligand

The TMEDA supported potassium aluminyl (NONDipp)Al–K(TMEDA)2 (NONDipp = [O(SiMe2NDipp)2]2–, Dipp = 2,6-iPr2C6H3), containing a Al–K bond, activates and reductively couples cabon monoxide gas to give a new aluminium species contianing...

A mononuclear divalent ytterbium hydrido complex supported by a super-bulky scorpionate ligand

The first mononuclear divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (2) bearing a terminal hydrido ligand was obtained by hydrogenolysis of the benzyl precursor in hexane. Complex 2 exhibited two different reaction patterns towards allenes: Yb-H addition with cyclohexylallene and deprotonation of 1,1-dimethylallene.

Hydroarylation of olefins catalysed by a dimeric ytterbium(II) alkyl

Although the nucleophilic alkylation of aromatics has recently been achieved with a variety of potent main group reagents, all of this reactivity is limited to a stoichiometric regime. We now report that the ytterbium(II) hydride, [BDIDippYbH]2 (BDIDipp = CH[C(CH3)NDipp]2, Dipp = 2,6-diisopropylphenyl), reacts with ethene and propene to provide the ytterbium(II) n-alkyls, [BDIDippYbR]2 (R = Et or Pr), both of which alkylate benzene at room temperature. Density functional theory (DFT) calculations indicate that this latter process operates through the nucleophilic (SN2) displacement of hydride, while the resultant regeneration of [BDIDippYbH]2 facilitates further reaction with ethene or propene and enables the direct catalytic (anti-Markovnikov) hydroarylation of both alkenes with a benzene C-H bond.

Divalent Ytterbium Hydrido Complex Supported by a β-Diketiminato-Based Tetradentate Ligand: Synthesis, Structure, and Reactivity

While the chemistry of trivalent rare-earth metal hydrido complexes has been well developed in the past 40 years, that of the divalent rare-earth metal hydrido complexes remains in its infancy because of the synthetic challenge of such complexes. In this paper, we report the synthesis and structural characterization of a divalent ytterbium hydrido complex supported by a bulky β-diketiminato-based tetradentate ligand. This hydrido complex is a dimer containing two μ-hydrogen ligands, and it easily undergoes a hydrido shift reaction to form a new divalent ytterbium hydrido complex that contains only one hydrido bridge. Furthermore, this hydrido complex reacts with pyridine and pyridine derivatives, showing versatile reactivity [Yb-H addition to pyridine, hydrido shift to ancillary ligand, and ytterbium(II)-center-induced redox reaction with bipyridine]. This hydrido complex reacts with Ph₃P═O, resulting in a P-C^Ph cleavage of Ph₃P═O and an elimination of C₆H₆; on the other hand, the reaction with Ph₃P═S is a hydrido coupling-based redox reaction. The reactions of this hydrido complex with 1 and 2 equiv of PhSSPh clearly indicate that the hydrido coupling-based redox reaction is prior to the ytterbium(II) oxidation-based redox reaction.

Ln(ii) alkyl complexes: from elusive exotics to catalytic applications

The synthesis, structures and reactivity of isolable Ln^II (Ln = Sm, Eu, Yb) alkyl complexes are discussed. The application of Ln^II alkyl derivatives in a variety of catalytic reactions is considered as well.

Hydrodeoxygenative Cyclotetramerization of Carbon Monoxide by a Trinuclear Titanium Polyhydride Complex

The reductive coupling of carbon monoxide (CO) by metal hydrides is of fundamental interest and practical importance. Herein we report an unprecedented hydrodeoxygenative cyclotetramerization of CO by a trinuclear titanium polyhydride complex [(C₅Me₄SiMe₃)Ti]₃(μ₃-H)(μ₂-H)₆ (1). The reaction of CO with 1 at -78 °C gave an ethen-1,2-diyl species [CH═CH]^2- through the hydrodeoxygenative dimerization of two molecules of CO, which upon cycloaddition to another two molecules of CO afforded a cyclobuten-3,4-diyl-1,2-diolate unit [C₄H₂O₂]^4-. The hydrogenolysis of the [C₄H₂O₂]^4- species with H₂ yielded a tetrahydrocyclobuten-1,2-diolate species [C₄H₄O₂]^2-, which on heating at 100 °C gave a cyclobuten-2-yl-1-olate product [C₄H₄O]^2-. The acidolysis of the [C₄H₂O₂]^4- and [C₄H₄O]^2- species with HCl afforded γ-butyrolactone and cyclobutanone, respectively.

Reactivity of a Molecular Calcium Hydride Cation ([CaH]+) Supported by an NNNN Macrocycle

The hydride ligand in the cationic calcium hydride supported by a NNNN-type macrocycle, [(Me₄TACD)₂Ca₂(μ-H)₂(THF)][BAr₄]₂ (1; Me₄TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane; THF = tetrahydrofuran; BAr₄ = B(C₆H₃-3,5-Me₂)₄), shows, in addition to its Brönsted basicity toward weak acids, a pronounced nucleophilicity resulting in nucleophilic substitution or insertion (addition) at a silicon or sp² carbon center. Terminal acetylenes RC≡CH (R = SiMe₃, cyclopropyl) as well as 1,4-diphenylbutadiene were deprotonated by 1 to give dinuclear complexes [(Me₄TACD)₂Ca₂(μ-C≡CR)₂][BAr₄]₂ (2a, R = SiMe₃; 2b, R = cyclopropyl) and [(Me₄TACD)₂Ca₂(μ₂-η⁴-1,4-Ph₂C₄H₂)][BAr₄]₂ (3) with H₂ evolution. The addition reaction with BH₃(THF) gave a tetrahydridoborate complex, [(Me₄TACD)Ca(BH₄)(THF)₂][BAr₄] (4), with κ₂-H₂BH₂ coordination in the solid state, suggesting a pronounced Lewis acidic calcium center. The behavior resulting from both Lewis acidity and hydricity becomes apparent in the nucleophilic substitution of fluorobenzene by 1 to give benzene and the dimeric fluoride complex [(Me₄TACD)₂Ca₂(μ-F)₂(THF)][BAr₄]₂·2.5THF (5). Analogous nucleophilic substitution reaction is observed for heterofunctionalized organosilanes XSiR₃ [X = I, N(SiHMe₂)₂, N₃; R = Me₃ or HMe₂], which resulted in the formation of calcium complexes [(Me₄TACD)Ca(X)(THF)_n][BAr₄] (6-8) containing an X ligand along with hydrosilane HSiR₃. An insertion reaction by 1 was observed with CO₂ and CO to give dinuclear formato complex [(Me₄TACD)₂Ca₂(μ-OCHO)₂][BAr₄]₂ (9) and cis-enediolato complex [(Me₄TACD)₂Ca₂(μ-OCH═CHO)][BAr₄]₂·3.5THF (10), respectively. The latter is believed to have been formed as a result of the dimerization of an initially generated formyl or oxymethylene complex, [(Me₄TACD)Ca(OCH)]⁺.

Ln(ii) and Ca(ii) NCsp3N pincer type diarylmethanido complexes – promising catalysts for C–C and C–E (E = Si, P, N, S) bond formation

The first examples of Ln(ii) (Ln = Yb, Sm) and Ca [NC_sp3N] pincer type diarylmethanido complexes were synthesized and successfully used as efficient and selective precatalyst for intermolecular C–C and C–E bond formation.

The unimolecular dissociation of magnesium chloride squarate (ClMgC4O4−) and reductive cyclooligomerisation of CO on magnesium

Cyclooligomerisation to squarate is initiated by the coupling of two CO units in MgCl⁻.

Synthesis and characterization of 2-t-butylimino-functionalized indolyl rare-earth metal amido complexes for the catalytic addition of terminal alkynes to carbodiimides: the dimeric complexes with the alkynide species in the μ–η1:η2 bonding modes

Dimeric complexes [(μ–η¹:η²-RCC)REL2]₂ (L = 2-(^tBuNCH)C₈H₅N), with alkynyl in the μ–η¹:η² modes, were isolated in the study of the catalytic reaction.

Molecular hydrides of divalent ytterbium supported by a macrocyclic ligand: synthesis, structure and olefin hydrofunctionalization catalysis

Cationic ytterbium hydrides [(Me₄TACD)₂Yb₂H_(2+n)]⁽²⁻ⁿ⁾⁺ (n = 0, 1) catalyzed the H₂/D₂ isotope exchange as well as hydrogenation and hydrosilylation of 1-hexene.

Isolation, structure and reactivity of a scandium boryl oxycarbene complex

A well-defined scandium boryl oxycarbene complex undergoes coupling with CO, C–H activation of lutidine, and cyclopropanation with ethylene.

The reaction of a half-sandwich scandium boryl complex 1 with CO (1 atm) afforded a novel boryl oxycarbene complex 2. The structure of 2 was characterized by ¹H, ¹³C and ¹¹B NMR, X-ray diffraction, and DFT analysis. Further reaction of 2 with CO (1 atm) yielded a phenylamido- and boryl-substituted enediolate complex 3 through C–C bond formation between CO and the carbene unit in 2 and cleavage and rearrangement of the Si–N bond in the silylene-linked Cp–amido ligand. Upon heating, insertion of the carbene atom into a methine C–H bond in the boryl ligand of 2 took place to give an alkoxide complex 4. The reactions of 2 with pyridine and 2-methylpyridine led to insertion of the carbene atom into an ortho-C–H bond of pyridine and into a methyl C–H bond of 2-methylpyridine, respectively. The reaction of 2 with ethylene yielded a borylcyclopropyloxy complex 7 through cycloaddition of the carbene atom to ethylene.

Alkaline-Earth-Promoted CO Homologation and Reductive Catalysis

Reaction between a β-diketiminato magnesium hydride and carbon monoxide results in the isolation of a dimeric cis-enediolate species through the reductive coupling of two CO molecules. Under catalytic conditions with PhSiH₃, an observable magnesium formyl species may be intercepted for the mild reductive cleavage of the CO triple bond.

A divalent heteroleptic lanthanoid fluoride complex stabilised by the tetraphenylcyclopentadienyl ligand, arising from C–F activation of pentafluorobenzene

The divalent heteroleptic lanthanoid fluoride complex, [Yb(C₅Ph₄H)(μ-F)(thf)₂]₂, as well as [Yb(C₅Ph₄H)₂(thf)] and [Yb(C₅Ph₄H)(C₆F₅)(thf)₂] were obtained from reactions of ytterbium metal with Hg(C₆F₅)₂ and tetraphenyl–cyclopentadiene under different conditions.

Trinuclear alkyl hydrido rare-earth complexes supported by amidopyridinato ligands: synthesis, structures, C–Si bond activation and catalytic activity in ethylene polymerization

[(Ap^9MeLu)₃(μ²-H)₃(μ³-H)₂(CH₂SiMe₃)(thf)₂] was synthesized. For Y and Yb C–Si bond activation occurs affording [(Ap^9MeLu)₃(μ²-H)₃(μ³-H)₂(CH₂SiMe₃)(thf)₂] and [(Ap^9MeLn)₃(μ²-H)₃(μ³-H)₂(CH₂SiH₂Ph)(thf)₂].