Alkyne-Induced Facile C–C Bond Formation of Two η²‑Alkynes on Dinuclear Tantalum Bis(alkyne) Complexes To Give Dinuclear Tantalacyclopentadienes

The dinuclear tantalum–alkyne complexes [(η²-RCCR)­TaCl₂]₂(μ-OMe)₂(μ-thf) (<b>2a</b>, R = Et; <b>2b</b>, R = ^<i>n</i>Pr) were synthesized by treating the mononuclear tantalum–alkyne complexes (η²-RCCR)­TaCl₃(dme) (<b>1a</b>, R = Et; <b>1b</b>, R = ^<i>n</i>Pr) with 1 equiv of NaOMe in THF. We found that adding a catalytic amount (20 mol %) of 3-hexyne to <b>2a</b> induced the spontaneous formation of Ta₂Cl₄(OMe)₂(μ-C₄Et₄)­(thf) (<b>4a</b>). Similarly, Ta₂Cl₄(OMe)₂(μ-C₄^<i>n</i>Pr₄)­(thf) (<b>4b</b>) was obtained by treatment of <b>2b</b> with a catalytic amount (20 mol %) of 4-octyne. Reaction of <b>4a</b>,<b>b</b> with 4-dimethylaminopyridine gave 4-dimethylaminopyridine-coordinated complexes <b>6a</b>,<b>b</b>, whose structures were elucidated by the X-ray structure of <b>6a</b>. We conducted a control experiment in which 10 equiv of 4-octyne was added to <b>2a</b> to give Ta₂Cl₄(OMe)₂(μ-C₄-2,3-^<i>n</i>Pr₂-4,5-Et₂)­(thf) (<b>7</b>) in 90% yield, indicating that free 4-octyne reacted with the tantalacyclopropene moiety of <b>2a</b> to form a dissymmetric tantalacyclopentadiene, followed by the release of 3-hexyne. The catalytic activity of <b>4a</b>–<b>6a</b> for [2 + 2 + 2] cyclotrimerization of 3-hexyne was examined, and we found that their activities were in the order <b>5a</b> > <b>4a</b> ≫ <b>6a</b>

Synthesis and Reactions of DitantalumAllyl Complexes Derived from Intramolecular C–H Bond Activation of the Methylene of the Ethyl Group Bound to Ditantallacyclopentadiene

Reaction of a dinuclear tantallacyclopentadiene complex, Ta₂Cl₆(μ-C₄Et₄) (<b>1</b>), with PhSiH₃ quantitatively afforded a polymeric dinuclear tantalum η³-allyl complex, {Ta₂Cl₅[μ-C₄Et₃(CHMe)]}_<i>n</i> (<b>2</b>), whose η³-allyl moiety was derived from selective C–H bond activation of the methylene moiety of the ethyl group bound to the tantallacyclopentadiene fragment. Lewis bases, such as THF and PMe₂Ph, coordinated to <b>2</b> to give Ta₂Cl₅(L)₂[μ-C₄Et₃(CHMe)] (<b>3</b>: L = thf; <b>4</b>: L = PMe₂Ph). An insertion reaction of diphenylacetylene into the η³-allyl moiety of <b>3</b> afforded the diphenylacetylene-incorporated complex <b>5</b>. Similarly, unsaturated organic substrates, such as trimethylsilylacetylene, 2-vinylpyridine, and benzaldehyde, inserted into the η³-allyl moiety of <b>3</b> to afford the corresponding complexes <b>6</b>–<b>8</b>

Electronic Structure–Reactivity Relationship on Ruthenium Step-Edge Sites from Carbonyl ¹³C Chemical Shift Analysis

Ru nanoparticles are highly active catalysts for the Fischer–Tropsch and the Haber–Bosch processes. They show various types of surface sites upon CO adsorption according to NMR spectroscopy. Compared to terminal and bridging η¹ adsorption modes on terraces or edges, little is known about side-on η² CO species coordinated to B₅ or B₆ step-edges, the proposed active sites for CO and N₂ cleavage. By using solid-state NMR and DFT calculations, we analyze ¹³C chemical shift tensors (CSTs) of carbonyl ligands on the molecular cluster model for Ru nanoparticles, Ru₆(η²-μ₄-CO)₂(CO)₁₃(η⁶-C₆Me₆), and show that, contrary to η¹ carbonyls, the CST principal components parallel to the C–O bond are extremely deshielded in the η² species due to the population of the C–O π* antibonding orbital, which weakens the bond prior to dissociation. The carbonyl CST is thus an indicator of the reactivity of both Ru clusters and Ru nanoparticles step-edge sites toward C–O bond cleavage

Preparation and Structure of Iminopyrrolyl and Amidopyrrolyl Complexes of Group 2 Metals

Reactions of <i>N</i>-aryliminopyrrolyl ligand <b>1a</b>, 2-(2,6-^<i>i</i>Pr₂C₆H₃NCH)-C₄H₃NH (Imp^Dipp-H), with dibenzylcalcium gave two types of pyrrolylcalcium complexes, bis­(iminopyrrolyl)calcium (<b>2a</b>) and (amidopyrrolyl)­calcium (<b>3a</b>), via alkane elimination and ligand alkylation reaction, respectively. Preparation of a mono­(iminopyrrolyl) complex, (iminopyrrolyl)­Ca­[N­(SiMe₃)₂]­(THF)₂ (<b>4a</b>), was accomplished by the addition of 1 equiv of <b>1a</b> to Ca­[N­(SiMe₃)₂]₂(THF)₂. A series of group 2 metal bis­(iminopyrrolyl) complexes, [(Imp^Dipp)₂M­(THF)₃] (M = Sr (<b>5a</b>), Ba, (<b>6a</b>)) and [(Imp^Me)₂Ca­(THF)₂] (<b>2b</b>) (2-(4-MeC₆H₄NCCH₃)-C₄H₃NH (Imp^Me-H)), was selectively prepared via amine elimination reactions, and their molecular structures were clarified by X-ray diffraction studies

Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors

Metallacyclobutanes are an important class of organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M–C_α/α′ and M···C_β distances, long C_α/α′–C_β bond length, and isotropic ¹³C chemical shifts for both early d⁰ and late d⁴ transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have ¹³C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d⁰ systems, with upfield shifts to ca. −30 ppm for the α-carbon of metallacycles with higher d^<i>n</i> electron counts (<i>n</i> = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (δ₁₁ ≥ δ₂₂ ≥ δ₃₃) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local π*­(M–C_α/α′) character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates