Reactivity Studies of a T-Shaped Yttrium Carbene: C–F and C–O Bond Activation and C═C Bond Formation Promoted by [Y(BIPM)(I)(THF)2] (BIPM = C(PPh2NSiMe3)2)

A binuclear guanidinate yttrium carbyne complex: unique reactivity toward unsaturated C-N, C-O and C-S bonds

A guanidinato-stabilized binuclear yttrium carbyne complex [(PhCH2)2NC(NC6H3iPr2-2,6)2]2Y2(μ2-Me)(AlMe3)2(μ4-CH) (1) was synthesized via C-H bond activation and its versatile reactivities were investigated. Complex 1 underwent σ-bond metathesis with PhSSPh and nucleophilic addition with PhCN to form the corresponding yttrium thiolate complex 3 and aza-allyl complex 4 respectively. Additionally, the rare yttrium carbide complex 5 was also prepared by treatment of complex 1 with S8. Interestingly, in the reaction with PhNCS, the C[double bond, length as m-dash]S double bond was cleaved, followed by C-H bond activation to give the yttrium sulfide complex 7 with a ketenimine dianion ligand. Unexpectedly, the reaction of complex 1 with CO (1 atm) resulted in deoxygenative coupling of CO, to afford mono- or dioxo-yttrium complexes at different temperatures. The mechanism of the possible formation processes of complexes 3 and 9 was elucidated by DFT calculations.

An Unsymmetric Imino-Phosphanamidinate Ligand and its Y(III) Complex: Synthesis, Characterization, and Catalytic Hydroboration of Carbonyl Compounds

An imino-phosphanamide ligand, [NHI^iPr2Me2P(Ph)NH-2,6-ⁱPr₂C₆H₃] (LH), containing two different N-substituents was prepared by the direct reaction of the lithium salt of N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenyl phosphanamine, PhP(Cl)NH-2,6-ⁱPr₂-C₆H₃. Reaction of LH with Y(N(SiMe₃)₂)₃ afforded the heteroleptic complex, [{L}Y(N(SiMe₃)₂)₂] (1), by elimination of HN(SiMe₃)₂. Compound 1 was characterized by multinuclear NMR and X-ray crystallography. In the complex, the Y(III) center was found to be tetracoordinate in a distorted tetrahedral geometry. The ligand, imino-phosphanamidinate, [L]^-, functions in a chelating manner, and its coordination to Y(III) results in a distorted 4-membered YPN₂ ring. As a proof of principle of its activity, 1 was used as a precatalyst for the hydroboration of various aldehydes and ketones using HBpin as the hydrogen source. The hydroboration reaction was rapid and clean even with low catalyst loadings (0.01-0.1 mol %). In addition, a very good functional group tolerance was observed in these reactions.

Yttrium and lanthanum bis(phosphine-oxide)methanides: structurally diverse, dynamic, and reactive

Homoleptic yttrium and lanthanum complexes of bis(phosphineoxide) methanides, RE(HPhL)3 and RE2(HMeL)6, promote the first rare-earth mediated Horner-Wittig and acid-base chemistry consistent with multifunctional reactivity (Lewis-acid/Brønstedbase).

Geminal Dianions Stabilized by Main Group Elements

This review is dedicated to the chemistry of stable and isolable species that bear two lone pairs at the same C center, i.e., geminal dianions, stabilized by main group elements. Three cases can thus be considered: the geminal-dilithio derivative, for which the two substituents at C are neutral, the yldiide derivatives, for which one substituent is neutral while the other is charged, and finally the geminal bisylides, for which the two substituents are positively charged. In this review, the syntheses and electronic structures of the geminal dianions are presented, followed by the studies dedicated to their reactivity toward organic substrates and finally to their coordination chemistry and applications.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Evidence for single metal two electron oxidative addition and reductive elimination at uranium

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

Highly Reactive Scandium Phosphinoalkylidene Complex: C-H and H-H Bonds Activation

The first scandium phosphinoalkylidene complex was synthesized and structurally characterized. The complex has the shortest Sc-C bond lengths reported to date (2.089(3) Å). DFT calculations reveal the presence of a three center π interaction in the complex. This scandium phosphinoalkylidene complex undergoes intermolecular C-H bond activation of pyridine, 4-dimethylamino pyridine and 1,3-dimethylpyrazole at room temperature. Furthermore, the complex rapidly activates H₂ under mild conditions. DFT calculations also demonstrate that the C-H activation of 1,3-dimethylpyrazole is selective for thermodynamic reasons and the relatively slow reaction is due to the need of fully breaking the chelating effect of the phosphino group to undergo the reaction whereas this is not the case for H₂.

Reactivity of the uranium(IV) carbene complex [U(BIPM(TMS))(Cl)(μ-Cl)₂Li(THF)₂] (BIPM(TMS) = {C(PPh₂NSiMe₃)₂}) towards carbonyl and heteroallene substrates: metallo-Wittig, adduct formation, C-F bond activation, and [2 + 2]-cycloaddition reactions

The reactivity of the uranium(IV) carbene complex [U(BIPM(TMS))(Cl)(μ-Cl)2Li(THF)2] (1, BIPM(TMS) = {C(PPh2NSiMe3)2}) towards carbonyl and heteroallene substrates is reported. Reaction of 1 with benzophenone proceeds to give the metallo-Wittig terminal alkene product Ph2C=C(PPh2NSiMe3)2 (2); the likely "UOCl2" byproduct could not be isolated. Addition of the bulky ketone PhCOBu(t) to 1 resulted in loss of LiCl, coordination of the ketone, and dimerisation to give [U(BIPM(TMS))(Cl)(μ-Cl){OC(Ph)(Bu(t))}]2 (3). The reaction of 1 with coumarin resulted in ring opening of the cyclic ester and a metallo-Wittig-type reaction to afford [U{BIPM(TMS)[C(O)(CHCHC6H4O-2)]-κ(3)-N,O,O'}(Cl)2(THF)] (4) where the enolate product remains coordinated to uranium. The reaction of PhCOF with 1 resulted in C-F bond activation and oxidation resulting in isolation of [U(O)2(Cl)2(μ-Cl)2{(μ-LiDME)OC(Ph)=C(PPh2NSiMe3)(PPh2NHSiMe3)}2] (5) along with [U(Cl)2(F)2(py)4] (6). The reactions of 1 with tert-butylisocyanate or dicyclohexylcarbodiimide resulted in the isolation of the [2 + 2]-cycloaddition products [U{BIPM(TMS)[C(NBu(t)){OLi(THF)2(μ-Cl)Li(THF)3}]-κ(4)-C,N,N',N''}(Cl)3] (7) and [U{BIPM(TMS)[C(NCy)2]-κ(4)-C,N,N',N''}(Cl)(μ-Cl)2Li(THF)2] (8). Complexes 2-8 have been variously characterised by single crystal X-ray diffraction, multi-nuclear NMR and FTIR spectroscopies, Evans method solution magnetic moments, variable temperature SQUID magnetometry, and elemental analyses.

Si–H activation by means of metal ligand cooperation in a methandiide derived carbene complex

Si–H bond activation of a series of silanes by means of metal ligand cooperation is reported.

Lutetium-methanediide-alkyl complexes: synthesis and chemistry

The first four-coordinate methanediide/alkyl lutetium complex (BODDI)Lu2 (CH2 SiMe3 )2 (μ2 -CHSiMe3 )(THF)2 (BODDI=ArNC(Me)CHCOCHC(Me)NAr, Ar=2,6-iPr2 C6 H3 ) (1) was synthesized by a thermolysis methodology through α-H abstraction from a Lu-CH2 SiMe3 group. Complex 1 reacted with equimolar 2,6-iPrC6 H3 NH2 and Ph2 C+O to give the corresponding lutetium bridging imido and oxo complexes (BODDI)Lu2 (CH2 SiMe3 )2 (μ2 -N-2,6-iPr2 C6 H3 )(THF)2 (2) and (BODDI)Lu2 (CH2 SiMe3 )2 (μ2 -O)(THF)2 (3). Treatment of 3 with Ph2 C=O (4 equiv) caused a rare insertion of Lu-μ2 -O bond into theC=O group to afford a diphenylmethyl diolate complex 4. Reaction of 1 with PhN=C=O (2 equiv) led to the migration of SiMe3 to the amido nitrogen atom to give complex (BODDI)Lu2 (CH2 SiMe3 )2 -μ-{PhNC(O)CHC(O)NPh(SiMe3 )-κ(3) N,O,O}(THF) (5). Reaction of 1 withtBuN=C formed an unprecedented product (BODDI)Lu2 (CH2 SiMe3 ){μ2 -[η(2) :η(2) -tBuN=C(=CH2 )SiMe2 CHC=NtBu-κ(1) N]}(tBuN=C)2 (6) through a cascade reaction of N=C bond insertion, sequential cyclometalative γ-(sp(3) )-H activation, C=C bond formation, and rearrangement of the newly formed carbene intermediate. The possible mechanistic pathways between 1, PhN=C=O, and tBuN=C were elucidated by DFT calculations.

The ketimide ligand is not just an inert spectator: heteroallene insertion reactivity of an actinide-ketimide linkage in a thorium carbene amide ketimide complex

The ketimide anion R2C=N(-) is an important class of chemically robust ligand that binds strongly to metal ions and is considered ideal for supporting reactive metal fragments due to its inert spectator nature; this contrasts with R2N(-) amides that exhibit a wide range of reactivities. Here, we report the synthesis and characterization of a rare example of an actinide ketimide complex [Th(BIPM(TMS)){N(SiMe3)2}(N=CPh2)] [2, BIPM(TMS)=C(PPh2NSiMe3)2]. Complex 2 contains Th=C(carbene), Th-N(amide) and Th-N(ketimide) linkages, thereby presenting the opportunity to probe the preferential reactivity of these linkages. Importantly, reactivity studies of 2 with unsaturated substrates shows that insertion reactions occur preferentially at the Th-N(ketimide) bond rather than at the Th=C(carbene) or Th-N(amide) bonds. This overturns the established view that metal-ketimide linkages are purely inert spectators.

Substitution effects on the formation of T-shaped palladium carbene and thioketone complexes from Li/Cl carbenoids

The preparation of palladium thioketone and T-shaped carbene complexes by treatment of thiophosphoryl substituted Li/Cl carbenoids with a Pd(0) precursor is reported. Depending on the steric demand, the anion-stabilizing ability of the silyl moiety (by negative hyperconjugation effects) and the remaining negative charge at the carbenic carbon atom, isolation of a three-coordinate, T-shaped palladium carbene complex is possible. In contrast, insufficient charge stabilization results in the transfer of the sulfur of the thiophosphoryl moiety and thus in the formation of a thioketone complex. While the thioketones are stable compounds the carbene complexes are revealed to be highly reactive and decompose under elimination of Pd metal. Computational studies revealed that both complexes are formed by a substitution mechanism. While the ketone turned out to be the thermodynamically favored product, the carbene is kinetically favored and thus preferentially formed at low reaction temperatures.

Synthesis and bonding in carbene complexes of an unsymmetrical dilithio methandiide: a combined experimental and theoretical study

Herein, we report the preparation of a new unsymmetrical, bis(thiophosphinoyl)-substituted dilithio methandiide and its application for the synthesis of zirconium- and palladium-carbene complexes. These complexes were found to exhibit remarkably shielded (13)C NMR shifts, which are much more highfield-shifted than those of "normal" carbene complexes. DFT calculations were performed to determine the origin of these observations and to distinguish the electronic structure of these and related carbene complexes compared with the classical Fischer and Schrock-type complexes. Various methods show that these systems are best described as highly polarized Schrock-type complexes, in which the metal-carbon bond possesses more electrostatic contributions than in the prototype Schrock systems, or even as "masked" methandiides. As such, geminal dianions represent a kind of "extreme" Schrock-type ligands favoring the ionic resonance structure M(+)-CR2(-) as often used in textbooks to explain the nucleophilic nature of Schrock complexes.