Reversible intramolecular hydride transfer between tantalum and an aromatic ring: an eta(5)-cyclohexadienyl complex as a masked hydride

Reductive Hydrogenation of Sulfido-Bridged Tantalum Alkyl Complexes: A Mechanistic Insight

Hydrogenolysis of a series of alkyl sulfido-bridged tantalum(IV) dinuclear complexes [Ta(η⁵-C₅Me₅)R(μ-S)]₂ [R = Me, nBu (1), Et, CH₂SiMe₃, C₃H₅, Ph, CH₂Ph (2), p-MeC₆H₄CH₂ (3)] has led quantitatively to the Ta(III) tetrametallic sulfide cluster [Ta(η⁵-C₅Me₅)(μ₃-S)]₄ (4) along with the corresponding alkane. Mechanistic information for the formation of the unique low-valent tetrametallic compound 4 was gathered by hydrogenation of the phenyl-substituted precursor [Ta(η⁵-C₅Me₅)Ph(μ-S)]₂, which proceeds through a stepwise hydrogenation process, disclosing the formation of the intermediate tetranuclear hydride sulfide [Ta₂(η⁵-C₅Me₅)₂(H)Ph(μ-S)(μ₃-S)]₂ (5). Extending our studies toward tantalum alkyl precursors containing functional groups susceptible to hydrogenation, such as the allyl-and benzyl-substituted compounds [Ta(η⁵-C₅Me₅)(η³-C₃H₅)(μ-S)]₂ and [Ta(η⁵-C₅Me₅)(CH₂Ph)(μ-S)]₂ (2), enables alternative reaction pathways en route to the formation of 4. In the former case, the dimetallic system undergoes selective hydrogenation of the unsaturated allyl moiety, forming the asymmetric complex [{Ta(η⁵-C₅Me₅)(η³-C₃H₅)}(μ-S)₂{Ta(η⁵-C₅Me₅)(C₃H₇)}] (6) with only one propyl fragment. Species 2, in addition to the hydrogenation of one benzyl fragment and concomitant toluene release, also undergoes partial hydrogenation and dearomatization of the phenyl ring on the vicinal benzyl unity to give a η⁵-cyclohexadienyl complex [Ta₂(η⁵-C₅Me₅)₂(μ-CH₂C₆H₆)(μ-S)₂] (7). The mechanistic implications of the latter hydrogenation process are discussed by means of DFT calculations.

Identification of Cyclohexadienyl Hydrides as Intermediates in Molybdenum-Catalyzed Arene Hydrogenation

Treatment of phosphino(imino)pyridine (PIP) molybdenum cyclooctadiene (COD) complexes [(PIP)Mo(COD)] with dihydrogen in the presence of benzene selectively furnished the molybdenum cyclohexadienyl hydrides [(PIP)MoH(η⁵ -C₆ H₇ )], which are precatalysts for the hydrogenation of benzene to cyclohexane. [(PIP)MoH(η⁵ -C₆ H₇ )] arises from a rarely observed insertion of benzene into a molybdenum-hydride bond, a key step in the molybdenum-catalyzed homogeneous hydrogenation of arenes. The reaction with toluene afforded a single isomer of the corresponding molybdenum cyclohexadienyl hydride while para-xylene predominantly formed the molybdenum η⁶ -arene complex with the insertion product being a minor component. Addition of carbon monoxide to a cyclohexane-d₁₂ solution of [(PIP)MoH(η⁵ -C₆ H₇ )] liberated cyclohexadiene, providing experimental support for a higher kinetic barrier for the subsequent steps en route to cycloalkanes.

A Novel Approach to Functionalization of Aryl Azides through the Generation and Reaction of Organolithium Species Bearing Masked Azides in Flow Microreactors

A novel straightforward method for aryl azides having functional groups based on generation and reactions of aryllithiums bearing a triazene group from polybromoarenes using flow microreactor systems was achieved. The present approach will serve as a powerful method in organolithium chemistry and open a new possibility in the synthesis of polyfunctional organic azides.

Synthesis of 2,6-Hexa-tert-butylterphenyl Derivatives, 2,6-(2,4,6-t-Bu3C6H2)2C6H3X, where X = I, Li, OH, SH, N3, or NH2

A "double benzyne" reaction between 1,3-dichloro-2-iodobenzene and 2,4,6-t-Bu3C6H2MgBr followed by the addition of iodine led to 2,6-(2,4,6-t-Bu3C6H2)2C6H3I (HTBTI) in 65% yield. Lithiation of HTBTI with Li-t-Bu gave Li(Et2O)2HTBT from which HTBTSH, HTBTN3, HTBTNH2, and HTBTOH were prepared. An X-ray structure of W(OHTBT)2Cl4 shows that the two HTBTO ligands are trans to one another with the t-Bu3C6H2 groups on one HTBTO interdigitated with the t-Bu3C6H2 groups on the other HTBTO.

Extremely bulky amide ligands in main group chemistry

The development of extremely sterically demanding, monodentate amide ligands facilitates the isolation of main group species with new and highly reactive coordination modes. An outstanding feature of these ligands is the ability to tune their steric demands. Reactivity investigations highlight the potential for small molecule activation chemistry and catalysis for these compounds.

Synthesis and characterization of three-coordinate Ni(III)-imide complexes

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

Neutral and Cationic Alkyl Tantalum Imido Complexes: Synthesis and Migratory Insertion Reactions

The synthesis and reactivity of dibenzyl cationic tantalum imido complexes is described. The trialkyl tantalum imido compounds Bn(3)Ta=NCMe(3) (1) and Np(3)Ta=NCMe(3) (2) were synthesized as starting materials for the study of dialkyl cationic tantalum imido complexes. Compound 1 undergoes insertion reactions with diisopropylcarbodiimide and 2,6-dimethylphenylisocyanide to give (bisamidinate)imido complex 5 and (bisimino-acyl)imido complex 6, respectively. Treatment of compound 1 with B(C(6)F(5))(3) gives the zwitterionic tantalum complex [Bn(2)Ta=NCMe(3)][BnB(C(6)F(5))(3)] (7) which is stabilized by eta(6)-coordination of the benzyl triaryl borate anion. Coordination of the aryl anion can be displaced by three equivalents of pyridine to give the Lewis base complex 8. Treatment of compound 1 with [Ph(3)C][B(C(6)F(5))(4)] gives the cationic tantalum imido complex [Bn(2)Ta=NCMe(3)][B(C(6)F(5))(4)] (3). This salt forms insoluble aggregates unless trapped by THF coordination or an insertion reaction with an alkyne or an alkene. Cation 3 undergoes migratory insertion reactions with diphenylacetylene, phenylacetylene, norbornene, and cis-cyclooctene to give the corresponding alkenyl or modified alkyl imido complexes. The characterization of these products and the significance of these insertion reactions with respect to Ziegler-Natta polymerizations and hydroamination reactions are described.