Correction: π-Facial selectivity in the Diels-Alder reaction of glucosamine-based chiral furans and maleimides.

Correction for 'π-Facial selectivity in the Diels-Alder reaction of glucosamine-based chiral furans and maleimides' by Cornelis H. M. van der Loo et al., Org. Biomol. Chem., 2023, 21, 1888-1894, https://doi.org/10.1039/D2OB02221D.

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.

π-Facial selectivity in the Diels-Alder reaction of glucosamine-based chiral furans and maleimides.

Furans derived from carbohydrate feedstocks are a versatile class of bio-renewable building blocks and have been used extensively to access 7-oxanorbornenes via Diels-Alder reactions. Due to their substitution patterns these furans typically have two different π-faces and therefore furnish racemates in [4 + 2]-cycloadditions. We report the use of an enantiopure glucosamine derived furan that under kinetic conditions predominantly affords the exo-product with a high π-face selectivity of 6.5 : 1. The structure of the product has been resolved unequivocally by X-ray crystallography, and a multi-gram synthesis (2.8 g, 58% yield) confirms the facile accessibility of this multifunctional enantiopure building block.

N═N Bond Cleavage by Tantalum Hydride Complexes: Mechanistic Insights and Reactivity.

The reaction of [TaCpRX4] (CpR = η5-C5Me5, η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) with SiH3Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [(TaCpRX2)2(µ-H)2], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCpRX2(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(η5-C5Me5)X2}2(µ-H)2] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(η5-C5Me5)X2}2(µ-NC6H4C6H4N)] along with the release of molecular hydrogen. When the compounds [(TaCpRX2)2(µ-H)2] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCpRX)2{µ-(η2,η2-NC6H4C6H4N)}] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) as intermediates in the N═N bond cleavage process. DFT calculations provide insights into the N═N cleavage mechanism, in which the ditantalum(IV) fragment can promote two-electron reductions of the N═N bond at two different metal-metal bond splitting stages.

Molecular Design of Cyclopentadienyl Tantalum Sulfide Complexes.

Use of (Me3Si)2S and [Ta(η5-C5Me5)Cl4] (1) in a 4:3 ratio afforded the trimetallic sulfide cluster [Ta3(η5-C5Me5)3Cl3(µ3-Cl)(µ-S)3(µ3-S)] (2) with loss of SiClMe3. A similar reaction between 1, TaCl5, and (Me3Si)2S in a 2:1:4 ratio resulted in the analogous complex [Ta3(η5-C5Me5)2Cl4(µ3-Cl)(µ-S)3(µ3-S)] (3). Single-crystal X-ray diffraction analyses of 2 and 3 showed in all cases trinuclear tantalum sulfide clusters. On the other hand, thermal treatment of 2 with SiH3Ph generated very cleanly the dinuclear tantalum(IV) sulfide complex [Ta2(η5-C5Me5)2Cl2(µ-S)2] (4) in a quantitative way. Likewise, we found that 4 was synthesized more easily by a one-pot reaction of 1, (Me3Si)2S, and SiH3Ph in toluene. Reactions of 4 with a series of alkylating reagents rendered the dinuclear peralkylated sulfide complexes [Ta2(η5-C5Me5)2R2(µ-S)2] (R = Me 5, Et 6, CH2SiMe3 7, C3H5 8, Ph 9). Single-crystal X-ray diffraction analyses of 4, 5, and 9 showed in all cases a trans disposition of the chloro or alkyl substituents. The short Ta-Ta distances (2.918(1)-2.951(1) Å) along with DFT calculations indicate a σ-Ta-Ta interaction. Complexes 5, 6, and 8 undergo trans- cis isomerization, and mechanistic proposals are discussed based on DFT calculations.

The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties.

Following the track of the useful titanocene [Ti(η5-C5H5)2Cl] reagent in organic synthesis, the related half-sandwich titanium(III) derivatives [Ti(η5-C5R5)Cl2] are receiving increasing attention in radical chemistry of many catalyzed transformations. However, the structure of the active titanium(III) species remains unknown in the literature. Herein, we describe the synthesis, crystal structure, and electronic structure of titanium(III) aggregates of composition [{Ti(η5-C5Me5)Cl2} n]. The thermolysis of [Ti(η5-C5Me5)Cl2Me] (1) in benzene or hexane at 180 °C results in the clean formation of [{Ti(η5-C5Me5)Cl(µ-Cl)}2] (2), methane, and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 °C leads to a mixture of 2 and the paramagnetic trimer [{Ti(η5-C5Me5)(µ-Cl)2}3] (3). The X-ray crystal structures of compounds 2 and 3 show Ti-Ti distances of 3.267(1) and 3.219(12) Å, respectively. Computational studies (CASPT2//CASSCF and BS DFT methods) for dimer 2 reveal a singlet ground state and a relatively large singlet-triplet energy gap. Nuclear magnetic resonance spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti(η5-C5Me5)Cl2} n] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti(η5-C5Me5)Cl(µ-Cl)}2] and a paramagnetic tetramer [{Ti(η5-C5Me5)(µ-Cl)2}4] in solution. In contrast, complex 2 readily dissolves in tetrahydrofuran to give a green-blue solution from which blue crystals of the mononuclear adduct [Ti(η5-C5Me5)Cl2(thf)] (4) were grown.

A Bridging bis-Allyl Titanium Complex: Mechanistic Insights into the Electronic Structure and Reactivity.

Treatment of the dinuclear compound [{Ti(η5-C5Me5)Cl2}2(µ-O)] with allylmagnesium chloride provides the formation of the allyltitanium(III) derivative [{Ti(η5-C5Me5)(µ-C3H5)}2(µ-O)] (1), structurally identified by single-crystal X-ray analysis. Density functional theory (DFT) calculations confirm that the electronic structure of 1 is a singlet state, and the molecular orbital analysis, along with the short Ti-Ti distance, reveal the presence of a metal-metal single bond between the two Ti(III) centers. Complex 1 reacts rapidly with organic azides, RN3 (R = Ph, SiMe3), to yield the allyl µ-imido derivatives [{Ti(η5-C5Me5)(CH2CH═CH2)2}2(µ-NR)(µ-O)] [R = Ph(2), SiMe3(3)] along with molecular nitrogen release. Reaction of 2 and 3 with H2 leads to the µ-imido propyl species [{Ti(η5-C5Me5)(CH2CH2CH3)2}2(µ-NR)(µ-O)] [R = Ph(4), SiMe3(5)]. Theoretical calculations were used to gain insight into the hydrogenation mechanism of complex 3 and rationalize the lower reactivity of 2. Initially, the µ-imido bridging group in these complexes activates the H2 molecule via addition to the Ti-N bonds. Subsequently, the titanium hydride intermediates induce a change in hapticity of the allyl ligands, and the nucleophilic attack of the hydride to the allyl groups leads to metallacyclopropane intermediates. Finally, the proton transfer from the amido group to the metallacyclopropane moieties affords the propyl complexes 4 and 5.

An Effective Route to Dinuclear Niobium and Tantalum Imido Complexes.

Thermal treatment of the trichloro complexes [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) (Xyl = 2,6-Me2C6H3) under vacuum affords the dinuclear imido species [MCl2(µ-Cl)(NR)py]2 (R = tBu, Xyl; M = Nb 1, 3; Ta 2, 4) with loss of pyridine. Complexes 1-4 can be easily transformed to the mononuclear starting materials [MCl3(NR)py2] (R = tBu, Xyl; M = Nb, Ta) upon reaction with pyridine. While reactions of compounds 1 and 2 with a series of alkylating reagents render the mononuclear peralkylated imido complexes [MR3(NtBu)] (R = Me, CH2Ph, CH2CMe3, CH2CMePh, CH2SiMe3), the analogous treatment with allylmagnesium chloride results in the formation of the dinuclear niobium(IV) derivative [(NtBu)(η3-C3H5)M(µ-C3H5)(µ-Cl)2M(NtBu)py2] (5). Additionally, the treatment of the starting materials 1 and 2 with the organosilicon reductant 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene yields the pyridyl-bridged dinuclear derivatives [M2Cl2(µ-Cl)2(NtBu)2py2]2(µ-NC4H4N)2 (M = Nb 6, Ta 7). Controlled hydrolysis reaction of 1 and 2 affords the oxo chlorido-bridged products [MCl(µ-Cl)(NtBu)py]2(µ-O) (M = Nb 8, Ta 9) in a quantitative way, while the treatment of these latter with one more equivalent of pyridine led to complexes [MCl2(NtBu)py2]2(µ-O) (M = Nb 10, Ta 11). Structural study of these dinuclear imido derivatives has been also performed by X-ray crystallography.

Group 10 Metal Benzene-1,2-dithiolate Derivatives in the Synthesis of Coordination Polymers Containing Potassium Countercations.

The use of theoretical calculations has allowed us to predict the coordination behavior of dithiolene [M(SC6H4S)2]2- (M = Ni, Pd, Pt) entities, giving rise to the first organometallic polymers {[K2(µ-H2O)2][Ni(SC6H4S)2]}n and {[K2(µ-H2O)2(thf)]2[K2(µ-H2O)2(thf)2][Pd3(SC6H4S)6]}n by one-pot reactions of the corresponding d10 metal salts, 1,2-benzenedithiolene, and KOH. The polymers are based on σ,π interactions between potassium atoms and [M(SC6H4S)2]2- (M = Ni, Pd) entities. In contrast, only σ interactions are observed when the analogous platinum derivative is used instead, yielding the coordination polymer {[K2(µ-thf)2][Pt(SC6H4S)2]}n.

Systematic Approach for the Construction of Niobium and Tantalum Sulfide Clusters.

Treatment of the imido complexes [MCl3(NR)py2] (R = (t)Bu, 2,6-Me2C6H3; M = Nb 1, 3; Ta 2, 4) (Xyl = 2,6-Me2C6H3) with (Me3Si)2S in a 1:1 ratio afforded the new cube-type sulfide clusters [MCl(NR)py(µ3-S)]4 (R = (t)Bu, 2,6-Me2C6H3; M = Nb 5, 7; Ta 6, 8) with loss of Me3SiCl. Reactions of 5 and 6 with cyclopentadienyllithium in 1:4 ratio resulted in the rupture of the coordinative M-S bonds and the replacement of a pyridine molecule and a chlorine atom by an η(5)-cyclopentadienyl group in each metal center, affording the compounds [M(η(5)-C5H5)(N(t)Bu)(µ-S)]4 (M = Nb 9, Ta 10). These processes may develop through formation of the complexes [M4(η(5)-C5H5)2(µ-Cl)(N(t)Bu)4py2(µ3-S)2(µ-S)2](C5H5) (M = Nb 11, Ta 12), also obtained by reaction of 5 and 6 with cyclopentadienyllithium in 1:3 ratio. As further evidence, 11 and 12 led to complexes 9 and 10 by treatment with one more equivalent of the lithium reagent. The structural study of these metal sulfide clusters has been also performed by X-ray crystallography.

Carbon-nitrogen bond construction and carbon-oxygen double bond cleavage on a molecular titanium oxonitride: a combined experimental and computational study.

New carbon-nitrogen bonds were formed on addition of isocyanide and ketone reagents to the oxonitride species [{Ti(η(5)-C5Me5)(µ-O)}3(µ3-N)] (1). Reaction of 1 with XylNC (Xyl = 2,6-Me2C6H3) in a 1:3 molar ratio at room temperature leads to compound [{Ti(η(5)-C5Me5)(µ-O)}3(µ-XylNCCNXyl)(NCNXyl)] (2), after the addition of the nitrido group to one coordinated isocyanide and the carbon-carbon coupling of the other two isocyanide molecules have taken place. Thermolysis of 2 gives [{Ti(η(5)-C5Me5)(µ-O)}3(XylNCNXyl)(CN)] (3) where the heterocumulene [XylNCCNXyl] moiety and the carbodiimido [NCNXyl] fragment in 2 have undergone net transformations. Similarly, tert-butyl isocyanide (tBuNC) reacts with the starting material 1 under mild conditions to give the paramagnetic derivative [{Ti3(η(5)-C5Me5)3(µ-O)3(NCNtBu)}2(µ-CN)2] (4). However, compound 1 provides the oxo ketimide derivatives [{Ti3(η(5)-C5Me5)3(µ-O)4}(NCRPh)] [R = Ph (5), p-Me(C6H4) (6), o-Me(C6H4) (7)] upon reaction with benzophenone, p-methylbenzophenone, and o-methylbenzophenone, respectively. In these reactions, the carbon-oxygen double bond is completely ruptured, leading to the formation of a carbon-nitrogen and two metal-oxygen bonds. The molecular structures of complexes 2-4, 6, and 7 were determined by single-crystal X-ray diffraction analyses. Density functional theory calculations were performed on the incorporation of isocyanides and ketones to the model complex [{Ti(η(5)-C5H5)(µ-O)}3(µ3-N)] (1H). The mechanism involves the coordination of the substrates to one of the titanium metal centers, followed by an isomerization to place those substrates cis with respect to the apical nitrogen of 1H, where carbon-nitrogen bond formation occurs with a low-energy barrier. In the case of aryl isocyanides, the resulting complex incorporates additional isocyanide molecules leading to a carbon-carbon coupling. With ketones, the high oxophilicity of titanium promotes the unusual total cleavage of the carbon-oxygen double bond.

Coordination polymers based on diiron tetrakis(dithiolato) bridged by alkali metals, electrical bistability around room temperature, and strong antiferromagnetic coupling.

Coordination polymer chains have been formed by the direct reaction between HSC6H2Cl2SH and FeCl3·6H2O in the presence of an aqueous solution of the corresponding alkali-metal hydroxide (M = Li, Na, and K) or carbonate (M = Rb and Cs). The structures consist of dimeric [Fe2(SC6H2Cl2S)4](2-) entities bridged by [M2(THF)4] [M = K (1), Rb (2), and Cs (3); THF = tetrahydrofuran] or {[Na2(µ-H2O)2(THF)2] (5 and 5') units. The smaller size of the lithium atom yields an anion/cation ion-pair molecule, [Li(THF)4]2[Fe2(SC6H2Cl2S)4] (4), in which the dianionic moieties are held together by Cl···Cl interactions. Electrical characterization of these compounds shows a general semiconductor behavior in which the conductivity and activation energies are mainly determined by the M-Cl and M-S bond distances. Compounds 1 and 5' are interesting examples of bistability showing reversible transitions centered at ca. 350 and 290 K with very large hysteresis of ca. 60 and 35 K, respectively. All of these compounds exhibit intradimer strong antiferromagnetic Fe···Fe interactions.

Half-sandwich Ni(II) complexes bearing enantiopure bidentate NHC-carboxylate ligands: efficient catalysts for the hydrosilylative reduction of acetophenones.

Chiral nickel complexes containing NHC-carboxylate chelate ligands derived from the (S)-isomeric form of amino acids have been synthesised from the corresponding imidazolium salt and nickelocene. The presence of the carboxylate on the N-side arm of the heterocycle results in the competing formation of mixtures of mono- and bis-NHC complexes (i.e., [Ni(η5-Cp)(κ2-C,O-NHC)] and [Ni(κ2-C,O-NHC)2]), both of which retain the (S)-configuration of the stereogenic center and which can be separated by chromatography. Both the 18e- and 16e- complexes are found to be very stable and cannot be interconverted. The composition of the resulting mixtures depends mainly on the entity of the amino acid residue and, of more practical interest, on the reaction conditions. Thus, microwave heating and MeCN as a solvent favor the formation of the half-sandwich nickel complexes, rather than the bis-NHC compounds. Some of the [Ni(η5-Cp)(κ2-C,O-NHC)] complexes turn out to be among the best nickel catalysts for the hydrosilylative reduction of p-acetophenones described to date, although without chiral induction, in the absence of activating additives and under mild catalytic conditions.

One-pot formation of 1,3,4-oxadiazol-2(3H)-ones and dibenzo[c,e]azepines by concomitant cathodic reduction of diazonium salts and phenanthrenequinones.

The one-pot concomitant electrochemical reduction of phenanthrenequinones (1, 2) and arenediazonium salts (3a-f) led to the formation of 1,3,4-oxadiazol-2(3H)-ones (4a-f, 5a) and dibenzo[c,e]azepines (6a-f) when N-methylformamide was used as the solvent. A new pathway, different from those previously described with other aprotic solvents, is proposed. The experimental data support a radical mechanism for the electrochemical process followed by an internal rearrangement to give the products.

Copper(I) and silver(I) complexes supported by the tridentate [{Ti(η(5)-C5Me5)(µ-NH)}3(µ3-N)] metalloligand.

Copper(I) and silver(I) ionic compounds [(L)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}][O(3)SCF(3)] containing [MTi(3)N(4)] cube-type cations have been prepared by reaction of [(CF(3)SO(2)O)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (M = Cu (2), Ag (3)) with a variety of donor molecules L. Treatment of complexes 2 and 3 with NH(3) in toluene at room temperature gives the ammonia adducts [(H(3)N)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}][O(3)SCF(3)] (M = Cu (4), Ag (5)), whose X-ray crystal structures reveal two cube-type cations associated through hydrogen bonding interactions between the ammine ligands and one oxygen atom of each trifluoromethanesulfonate anion. Analogous treatment of 2 and 3 with 1 equiv of pyridine, 2,6-dimethylphenylisocyanide, tert-butylisocyanide, or triphenylphosphane gives the ionic compounds [(L)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}][O(3)SCF(3)] (L = py, M = Cu (6), Ag (7); L = CNAr, M = Cu (8), Ag (9); L = CNtBu, M = Cu (10), Ag (11); L = PPh(3), M = Cu (12), Ag (13)). The reactions of 2 and 3 with methylenebis(diphenylphosphane) (dppm) in a 1:1 ratio lead to the single-cube complexes [(dppm)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}][O(3)SCF(3)] (M = Cu (14), Ag (15)), whereas in a 2:1 ratio give the bisphosphane-bridged double-cube compounds [{M(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}(2)(µ-dppm)][O(3)SCF(3)](2) (M = Cu (16), Ag (17)). Similarly, treatment of 2 or 3 with a half equivalent of ethane-1,2-diylbis(diphenylphosphane) (dppe) affords double-cube derivatives [{M(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}(2)(µ-dppe)][O(3)SCF(3)](2) (M = Cu (18), Ag (19)). The X-ray crystal structures of 4, 5, 10, 14, 15, and 18 have been determined.

Electrical bistability around room temperature in an unprecedented one-dimensional coordination magnetic polymer.

The synthesis, crystal structure, and physical properties of an unprecedented one-dimensional (1D) coordination polymer containing [Fe2(S2C6H2Cl2)4](2-) entities bridged by dicationic [K2(µ-H2O)2(THF)4](2+) units are described. The magnetic properties show that the title compound presents pairwise Fe-Fe antiferromagnetic interactions that can be well reproduced with a S = 1/2 dimer model with an exchange coupling, J = -23 cm(-1). The electrical conductivity measurements show that the title compound is a semiconductor with an activation energy of about 290 meV and two different transitions, both with large hysteresis of about 60 and 30 K at 260-320 K and 350-380 K, respectively. These two transitions are assumed to be due to slight structural changes in the cation-anion interactions. Differential Scanning Calorimetry confirms the presence of both transitions. This compound represents the first sample of a coordination polymer showing electrical bistability.

Co-complexation of lithium gallates on the titanium molecular oxide {[Ti(η5-C5Me5)(µ-O)}3(µ3-CH)].

Amide and lithium aryloxide gallates [Li(+){RGaPh(3)}(-)] (R = NMe(2), O-2,6-Me(2)C(6)H(3)) react with the µ(3)-alkylidyne oxoderivative ligand [{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ(3)-CH)] (1) to afford the gallium-lithium-titanium cubane complexes [{Ph(3)Ga(µ-R)Li}{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ(3)-CH)] [R = NMe(2) (3), O-2,6-Me(2)C(6)H(3) (4)]. The same complexes can be obtained by treatment of the [Ph(3)Ga(µ(3)-O)(3){Ti(η(5)-C(5)Me(5))}(3)(µ(3)-CH)] (2) adduct with the corresponding lithium amide or aryloxide, respectively. Complex 3 evolves with formation of 5 as a solvent-separated ion pair constituted by the lithium dicubane cationic species [Li{(µ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-CH)}(2)](+) together with the anionic [(GaPh(3))(2)(µ-NMe(2))](-) unit. On the other hand, the reaction of 1 with Li(p-MeC(6)H(4)) and GaPh(3) leads to the complex [Li{(µ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-CH)}(2)][GaLi(p-MeC(6)H(4))(2)Ph(3)] (6). X-ray diffraction studies were performed on 1, 2, 4, and 5, while trials to obtain crystals of 6 led to characterization of [Li{(µ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-CH)}(2)][PhLi(µ-C(6)H(5))(2)Ga(p-MeC(6)H(4))Ph] 6a.

Lithium aluminates on a molecular titanium oxide.

Lithium aluminates Li[Al(O-2,6-Me(2)C(6)H(3))R'(3)] (R' = Et, Ph) react with the µ(3)-alkylidyne oxoderivative ligands [{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ(3)-CR)] [R = H (1), Me (2)] to afford the aluminum-lithium-titanium cubane complexes [{R'(3)Al(µ-O-2,6-Me(2)C(6)H(3))Li}(µ(3)-O)(3){Ti(η(5)-C(5)Me(5))}(3)(µ(3)-CR)] [R = H, R' = Et (5), Ph (7); R = Me, R' = Et (6), Ph (8)]. Complex 7 evolves with the formation of a lithium dicubane species and a Li{Al(µ-O-2,6-Me(2)C(6)H(3))Ph(3)}(2)] unit.

Molecular nitrides with titanium and rare-earth metals.

A series of titanium-group 3/lanthanide metal complexes have been prepared by reaction of [{Ti(η(5)-C(5)Me(5))(µ-NH)}(3)(µ(3)-N)] (1) with halide, triflate, or amido derivatives of the rare-earth metals. Treatment of 1 with metal halide complexes [MCl(3)(thf)(n)] or metal trifluoromethanesulfonate derivatives [M(O(3)SCF(3))(3)] at room temperature affords the cube-type adducts [X(3)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (X = Cl, M = Sc (2), Y (3), La (4), Sm (5), Er (6), Lu (7); X = OTf, M = Y (8), Sm (9), Er (10)). Treatment of yttrium (3) and lanthanum (4) halide complexes with 3 equiv of lithium 2,6-dimethylphenoxido [LiOAr] produces the aryloxido complexes [(ArO)(3)M{(µ(3)-NH)(3)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (M = Y (11), La (12)). Complex 1 reacts with 0.5 equiv of rare-earth bis(trimethylsilyl)amido derivatives [M{N(SiMe(3))(2)}(3)] in toluene at 85-180 °C to afford the corner-shared double-cube nitrido compounds [M(µ(3)-N)(3)(µ(3)-NH)(3){Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}(2)] (M = Sc (13), Y (14), La (15), Sm (16), Eu (17), Er (18), Lu (19)) via NH(SiMe(3))(2) elimination. A single-cube intermediate [{(Me(3)Si)(2)N}Sc{(µ(3)-N)(2)(µ(3)-NH)Ti(3)(η(5)-C(5)Me(5))(3)(µ(3)-N)}] (20) was obtained by the treatment of 1 with 1 equiv of the scandium bis(trimethylsilyl)amido derivative [Sc{N(SiMe(3))(2)}(3)]. The X-ray crystal structures of 2, 7, 11, 14, 15, and 19 have been determined. The thermal decomposition in the solid state of double-cube nitrido complexes 14, 15, and 18 has been investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements, as well as by pyrolysis experiments at 1100 °C under different atmospheres (Ar, H(2)/N(2), NH(3)) for the yttrium complex 14.

Ammonia activation by µ3-alkylidyne fragments supported on a titanium molecular oxide model.

Ammonolysis of the µ(3)-alkylidyne derivatives [{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ(3)-CR)] [R = H (1), Me (2)] produces a trinuclear oxonitride species, [{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ(3)-N)] (3), via methane or ethane elimination, respectively. During the course of the reaction, the intermediates amido µ-alkylidene [{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ-CHR)(NH(2))] [(R = H (4), Me (5)] and µ-imido ethyl species [{Ti(η(5)-C(5)Me(5))(µ-O)}(3)(µ-NH)Et] (6) were characterized and/or isolated. This achievement constitutes an example of characterization of the three steps of successive activation of N-H bonds in ammonia within the same transition-metal molecular system. The N-H σ-bond activation of ammonia by the µ(3)-alkylidyne titanium species has been theoretically investigated by DFT method on [{Ti(η(5)-C(5)H(5))(µ-O)}(3)(µ(3)-CH)] model complex. The calculations complement the characterization of the intermediates, showing the multiple bond character of the terminal amido and the bridging nature of imido ligand. They also indicate that the sequential ammonia N-H bonds activation process goes successively downhill in energy and occurs via direct hydron transfer to the alkylidyne group on organometallic oxides 1 and 2. The mechanism can be divided into three stages: (i) coordination of ammonia to a titanium center, in a trans disposition with respect to the alkylidyne group, and then the isomerization to adopt the cis arrangement, allowing the direct hydron migration to the µ(3)-alkylidyne group to yield the amido µ-alkylidene complexes 4 and 5, (ii) hydron migration from the amido moiety to the alkylidene group, and finally (iii) hydron migration from the µ-imido complex to the alkyl group to afford the oxo µ(3)-nitrido titanium complex 3 with alkane elimination.