Terminal Titanium-Ligand Multiple Bonds. Cleavages of CO and CS Double Bonds with Ti Imido Complexes

Tantalum, easy as Pi: understanding differences in metal-imido bonding towards improving Ta/Nb separations

The separation and purification of niobium and tantalum, which co-occur in natural sources, is difficult due to their similar physical and chemical properties. The current industrial method for separating Ta/Nb mixtures uses an energy-intensive process with caustic and toxic conditions. It is of interest to develop alternative, fundamental methodologies for the purification of these technologically important metals that improve upon their environmental impact. Herein, we introduce new Ta/Nb imido compounds: M( t BuN)(TriNOx) (1-M) bound by the TriNOx3- ligand and demonstrate a fundamental, proof-of-concept Ta/Nb separation based on differences in the imido reactivities. Despite the nearly identical structures of 1-M, density functional theory (DFT)-computed electronic structures of 1-M indicate enhanced basic character of the imido group in 1-Ta as compared to 1-Nb. Accordingly, the rate of CO2 insertion into the M[double bond, length as m-dash]Nimido bond of 1-Ta to form a carbamate complex (2-Ta) was selective compared to the analogous, unobserved reaction with 1-Nb. Differences in solubility between the imido and carbamate complexes allowed for separation of the carbamate complex, and led to an efficient Ta/Nb separation (S Ta/Nb = 404 ± 150) dependent on the kinetic differences in nucleophilicities between the imido moieties in 1-Ta and 1-Nb.

Dinitrogen Cleavage and Functionalization with Carbon Dioxide in a Dititanium Dihydride Framework

The activation and functionalization of dinitrogen (N₂) with carbon dioxide (CO₂) are of great interest and importance but highly challenging. We report here for the first time the reaction of N₂ with CO₂ in a dititanium dihydride framework, which leads to N-C bond formation and N-N and C-O bond cleavage. Exposure of a dinitrogen dititanium hydride complex {[(^acriPNP)Ti]₂(μ₂-η¹:η²-N₂)(μ₂-H)₂} (1) (^acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) to a CO₂ atmosphere at room temperature rapidly yielded a nitrido/N,N-dicarboxylamido complex {[(^acriPNP)Ti]₂(μ₂-N)[μ₂-N(CO₂)₂]} (2, 28%) and a diisocyanato/dioxo complex {[(^acriPNP)Ti]₂(NCO)₂(μ₂-O)₂} (3, 52%) with release of H₂. When the reaction of 1 with CO₂ (1 atm) was carried out at -50 °C, complex 2 was selectively formed in 82% yield within 5 min. Heating 2 at 80 °C under 1 atm CO₂ for 30 min afforded 3 in 67% yield. When 1 was allowed to react with 1.5 equiv of CO₂ at room temperature, an isocyanato/nitrido/oxo complex {[(^acriPNP)Ti]₂(NCO)(μ₂-N)(μ₂-O)} (4) was exclusively formed in 89% yield within 5 min. The reaction of 4 with CO₂ at room temperature almost quantitatively yielded the dioxo/diisocyanato complex 3 within 5 min. The mechanistic details were clarified by the ¹⁵N- and ¹³C-labeled experiments and density functional theory (DFT) calculations, providing unprecedented insights into the reaction of N₂ with CO₂. A titanium-mediated cycle for the synthesis of trimethylsilyl isocyanate Me₃SiNCO from N₂, CO₂, and Me₃SiCl using H₂ as a reducing agent was also established.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

The bis(imido) complexes (BDI)Nb(N t Bu)2 and (BDI)Nb(N t Bu)(NAr) (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; Ar = 2,6-diisopropylphenyl) were shown to engage in 1,2-addition and [2 + 2] cycloaddition reactions with a wide variety of substrates. Reaction of the bis(imido) complexes with dihydrogen, silanes, and boranes yielded hydrido-amido-imido complexes via 1,2-addition across Nb-imido π-bonds; some of these complexes were shown to further react via insertion of carbon dioxide to give formate-amido-imido products. Similarly, reaction of (BDI)Nb(N t Bu)2 with tert-butylacetylene yielded an acetylide-amido-imido complex. In contrast to these results, many related mono(imido) Nb BDI complexes do not exhibit 1,2-addition reactivity, suggesting that π-loading plays an important role in activating the Nb-N π-bonds toward addition. The same bis(imido) complexes were also shown to engage in [2 + 2] cycloaddition reactions with oxygen- and sulfur-containing heteroallenes to give carbamate- and thiocarbamate-imido complexes: some of these complexes readily dimerized to give bis-μ-sulfido, bis-μ-iminodicarboxylate, and bis-μ-carbonate complexes. The mononuclear carbamate imido complex (BDI)Nb(NAr)(N( t Bu)CO2) (12) could be induced to eject tert-butylisocyanate to generate a four-coordinate terminal oxo imido intermediate, which could be trapped as the five-coordinate pyridine or DMAP adduct. The DMAP adducted oxo imido complex (BDI)NbO(NAr)(DMAP) (16) was shown to engage in 1,2-addition of silanes across the Nb-oxo π-bond; this represents a new reaction pathway in group 5 chemistry.

Molecular titanium nitrides: nucleophiles unleashed

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ2-K(OEt2)]2[(PN)2Ti[triple bond, length as m-dash]N]2 (1) (PN- = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pK a range of the NH group in (PN)2Ti[triple bond, length as m-dash]NH to be between 26-36. Complex 1 could be produced by a reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the complex (PN)2Ti[double bond, length as m-dash]N[double bond, length as m-dash]N[double bond, length as m-dash]Ti(PN)2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex (PN)2Ti[triple bond, length as m-dash]O, which was also characterized. A combination of solid state 15N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)2Ti[triple bond, length as m-dash]N], and the discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti[triple bond, length as m-dash]N] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15Nnitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

Facile synthesis of a dimeric titanium(iv) complex with terminal TiO moieties and its application as a catalyst for the cycloaddition reaction of CO2to epoxides

Dimeric terminal oxo-titanium complex1having no TiO→Ti bonds could be easily synthesized. The titanium complexes1and2could act as catalysts for cycloaddition between CO₂and epoxides in the presence of cocatalysts.

Metallo-Wittig chemistry of an alkylidene to form a terminal titanium oxo complex

The synthesis and characterization of a terminal titanium oxo complex generated by alkylidene benzophenone cross-metathesis is reported.

Bis[4-(4-bromophenylimino-κN)pent-2-en-2-olato-κO]copper(II)

In the title compound, [Cu(C(11)H(11)BrNO)(2)], the Cu(II) atom is in a distorted square-planar geometry, with the two bidentate ketimine ligands positioned in a trans geometry. Two inter-molecular C-H⋯O hydrogen bond inter-actions are present which link the mol-ecules in a zigzag manner along the a axis. The mol-ecules pack in layers along the diagonal of the bc plane.

Synthesis of ureas from titanium imido complexes using CO2 as a C-1 reagent at ambient temperature and pressure

The coordinatively unsaturated 12-electron complex dichloro t-butylimido bispyridine titanium(IV) (2a) has been shown to react with CO(2) to give N,N-bis-t-butyl urea. Two analogous sterically hindered coordinatively saturated 14-electron complexes dichloro t-butylimido trispyridine titanium(IV) (10a) and dichloro 2,6-(i-Pr)(2)phenylimido trispyridine titanium(IV) (10b) also gave their corresponding symmetrical ureas upon treatment with CO(2). Further experiments support the intermediary of metallocycles formed from heterocumulene metathesis reactions. The unsymmetrical urea N-benzyl, N-t-butyl urea (11) was produced from treatment of 2,6-(i-Pr)(2)phenylimido trispyridine titanium(IV) (10b) with CO(2) and interception with BnNH(2). Equimolar quantities of N,N-bistrimethylsilybenzylamine or N,N-bistrimethylsilyphenethylamine were shown to promote the reaction between t-butylimido bispyridine titanium(IV) (2a) and CO(2) to give near quantitative yields of symmetrical urea. Other symmetrical ureas could be produced from TiCl(4), amine and CO(2) in moderate to quantitative yields depending on the stoichiometry of amine present.

(Z)-4-(2,5-Di-tert-butyl-anilino)pent-3-en-2-one

In the crystal structure of the title ketoamine, C₁₉H₂₉NO, the bond lengths from the N atom through the alkene group to the ketone O atom show the presence of an extensively delocalized π-system. The dihedral angle between the plane of the phenyl ring and that of the alkene component is 63.45 (7)° due to steric hindrance exerted by the tert-butyl groups. The mol­ecule has a Z-configured alkene function, which is facilitated by an intra­molecular N—H⋯O hydrogen bond between the amine and ketone groups. The mol­ecules are linked into extended chains, which run parallel to the [010] direction, by a very weak C—H⋯O inter­action between the methyl substituent of the alkene group and the ketone O atom of a neighbouring mol­ecule.

(E)-3-(1-Naphthyl-amino)-methyl-ene-(+)-camphor

In the crystal structure of the title ketoamine {systematic name: (E)-1,7,7-trimethyl-3-[(1-naphthyl­amino)­methyl­idene]bicyclo­[2.2.1]heptan-2-one}, C₂₁H₂₃NO, there are two independent mol­ecules in the asymmetric unit. Both mol­ecules have an E configuration about the alkene function. The main conformational difference between the mol­ecules is in the orientation of the plane of the naphthyl rings with respect to the camphor fragment. The torsion angle about the enamine C—N bond is 21.3 (7)° for mol­ecule A, but −24.4 (8)° for mol­ecule B. Inter­molecular N—H⋯O hydrogen bonds between the amino and ketone groups of adjacent independent mol­ecules sustain the crystal, and the resulting extended chains, containing an alternating sequence of the two independent mol­ecules, run parallel to the [001] direction and can be described by a graph-set motif of C₂²(12).

Exploring the effects of reduction or Lewis acid coordination on the U=O bond of the uranyl moiety

Reaction of Li(ArNC(Ph)CHC(Ph)O) ((Ar)acnac; Ar = 2,4,6-Me(3)C(6)H(2)) or Na(ArNC(Ph)CHC(Ph)O) (Ar = 3,5-(t)Bu(2)C(6)H(3)) with 0.5 equiv of UO(2)Cl(2)(THF)(3) results in the formation of UO(2)((Ar)acnac)(2) (Ar = 2,4,6-Me(3)C(6)H(2), 1; 3,5-(t)Bu(2)C(6)H(3), 2), which were isolated as orange crystalline solids in good yields. The structure of 2 has been confirmed by X-ray crystallography, while the solution redox properties of 1 and 2 have been measured by cyclic voltammetry. Complex 1 exhibits a reversible reduction feature at E(1/2) = -1.52 V (vs Fc/Fc(+)), while complex 2 exhibits a reduction feature at -1.35 V (vs Fc/Fc(+)). Complexes 1 and 2 react with Cp*(2)Co to generate [Cp*(2)Co][UO(2)((Ar)acnac)(2)] (Ar = 2,4,6-Me(3)C(6)H(2), 3; 3,5-(t)Bu(2)C(6)H(3), 4), in moderate to good yields. Both 3 and 4 have been fully characterized, while the structure of 4 has also been determined by X-ray crystallography. Reaction of 2 with 2 equiv of B(C(6)F(5))(3) in CH(2)Cl(2) leads to the isolation of UO(OB{C(6)F(5)}(3))((Ar)acnac)(2) (Ar = 3,5-(t)Bu(2)C(6)H(3)) (5). Complex 5, generated in situ, exhibits an irreversible reduction at -0.78 V (vs Fc/Fc(+), 100 mV/s scan rate) which is considerably lower than the reduction potential observed for 2, consistent with the removal of electron density from the uranyl moiety by coordination of B(C(6)F(5))(3).

The progression of synthetic strategies to assemble titanium complexes bearing the terminal imide group

The synthesis of terminal titanium imides is described in this account. The incorporation of this functional group has evolved from more simplistic approaches to more complex or unusual methods. Past and current synthetic strategies to incorporate the terminal imide functionality are explained with particular emphasis on low-coordinate titanium environments bearing this ubiquitous but vital type of motif. More recently, the imide functionality has demonstrated to be a key intermediate for modeling important industrial processes such as the hydrodenitrogenation of crude oil.