Tantalum Center Research Articles

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand.

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

Selective Reduction of CO2 to a Tantalum Formate Complex and Release of Methyl Formate from the Tantalum(V) Center.

A cyclometalated NHC-coordinated hydrido tantalum alkoxide was found to selectively react with CO2 to afford the genuine tantalum formate (NHC)(HCOO)Ta(ORF)3 with ORF = OC(CF3)2CH3. In the solid state, the presence of a κ2-O,O-formate moiety was established by single-crystal X-ray diffraction and ATR-IR spectroscopy, while NMR experiments and DFT modeling studies suggest that the κ1-O-coordination mode is preferred in solution. Despite the accessibility of the latter κ1-O-formate in solution, no over-reduction to a dinuclear methylene diolate was observed. Upon treatment with MeOTf, the κ1-O-formate was methylated selectively, which led to the formation of a tantalum triflate complex along with methyl formate. This is a rare example in which a value-added oxygen-containing organic product (here HCOOMe) is released from an oxophilic early transition metal (here TaV).

Reductive Hydrogenation under Single-Site Control: Generation and Reactivity of a Transient NHC-Stabilized Tantalum(III) Alkoxide.

One of the most attractive routes for the preparation of reactive tantalum(III) species relies on the efficient salt-free hydrogenolysis of tantalum(V) alkyls or tantalum(V) alkylidenes, a process known as reductive hydrogenation. For silica-crafted tantalum alkyls and alkylidenes, this process necessarily proceeds at well-separated tantalum centers, while related reductive hydrogenations in homogeneous solution commonly involve dimeric complexes. Herein, an NHC scaffold was coordinated to a novel tri(alkoxido)tantalum(V) alkylidene to circumvent the formation of dimers during reductive hydrogenation. Employing this new model system, a key intermediate of the process, namely a hydrido-tantalum alkyl, was isolated for the first time and shown to exhibit a bidirectional reactivity. Upon being heated, the latter complex was found to undergo either an α-elimination or a reductive alkane elimination. In the (overall unproductive) α-elimination step, H2 and the parent alkylidene were regenerated, while the sought-after transient d2-configured tantalum(III) derivative was produced along the reaction coordinate of the reductive alkane elimination. The reactive low-valence metal center was found to rapidly attack one of the NHC substituents via an oxidative C-H activation, which led to the formation of a cyclometalated tantalum(V) hydride. The proposed elemental steps are in line with kinetic data, deuterium labeling experiments, and density functional theory (DFT) modeling studies. DFT calculations also indicated that the S = 0 spin ground state of the Ta(III) center plays a crucial role in the cyclometalation reaction. The cyclometalated Ta(V) hydride was further investigated and reacted with several alkenes and alkynes. In addition to a rich insertion and isomerization chemistry, these studies also revealed that the former hydride may undergo a formal cycloreversion and thus serve as a tantalum(III) synthon, although the original tantalum(III) intermediate is not involved in this process. The latter reactivity was observed upon reaction with internal alkynes and led to the corresponding η2-alkyne derivatives via vinyl intermediates, which rearrange via a remarkable, hitherto unprecedented, hydrogen shift reaction.

Imido Group Interchange in Reactions of Zwitterionic Tantalum(V) Vinylimido Complexes and Nitriles

We report the imido group exchange transformation upon reactions with nitriles with the formation of a new vinylimido group from the incoming nitrile and a free nitrile from the initial imido group. Preliminary studies suggest the mechanism comprises elimination of triethylamine from the amminium group as the initial step, followed by the deprotonation of the incoming nitrile, followed by the rearrangement of its carbanion into an imido group at the tantalum center. The cationic group on the imido ligand can be replaced with another neutral nucleophile, which was demonstrated by NEt3 to PPh3 and NEt3 to DMAP exchange reactions.

Design of Volatile Mixed-Ligand Tantalum(V) Compounds as Precursors to Ta2O5 Films

Synthesis and structural characterization of six monomeric, heteroleptic tantalum(V) complexes of the general formula Ta(OiPr)4(ArTFP), where Ar = pyridine (1), 4,5- dimethyloxazole (2), 4,5-dimethylthiazole (3), benzimidazole (4), benzoxazole (5), benzthiazole (6), and TFP = trifluoropropenol, are described. Introduction of a donor-functionalized β-heteroarylalkenolate in the coordination sphere of Ta in the dimeric Ta2(OiPr)10 increases significantly the stability and volatility of these precursors, simplifying the depositions of Ta2O5. The molecular structures of 1–6 exhibited a distorted octahedral coordination around the tantalum center by four isopropoxide groups and one β-heteroarylalkenolate. Thermal decomposition studies (TG/DTA) and analysis of byproducts by NMR spectroscopy confirmed the decomposition mechanism and gas-phase stability of the heteroleptic compounds necessary for Ta2O5 depositions. Chemical vapor deposition studies with 1 and 2 demonstrated their suitability as efficient precursors for the growth of Ta2O5 thin films, whose properties were compared with Ta2O5 thin films obtained from homoleptic alkoxides.

Tuning the Electronic and Steric Parameters of a Redox-Active Tris(amido) Ligand

A family of tantalum compounds was prepared to probe the electronic effects engendered by the addition of electron-donating or electron-withdrawing groups to the 4/4' positions of the redox-active ligand derived from bis(2-isopropylamino-4-X-phenyl)amine [(X,iPr)(NNN(cat))H3, X = F, H, Me, (t)Bu]). A general synthetic procedure for the (X,iPr)(NNN(cat))H3 ligand family was developed starting from the 4/4' disubstituted diphenylamine derivative. A second ligand modification, incorporation of aromatic substituents at the flanking nitrogen moieties, was achieved via palladium-catalyzed cross-coupling to afford bis(2-3,5-dimethylphenylamino-4-methoxy-phenyl)amine (OMe,DMP)(NNN(cat))H3 (DMP = 3,5-C6H3Me2), allowing a comparative study to the less sterically hindered isopropyl derivative. Treatment of the triamines with 1 equiv of TaMe3Cl2 generated the corresponding dichloro complexes (X,R)(NNN(cat))TaCl2(L) (L = empty or Et2O) in high yields. These neutral dichloride derivatives reacted with [NBnEt3][Cl] to produce the anionic trichloride derivatives [NBnEt3][(X,R)(NNN(cat))TaCl3], whereas the neutral dichloride derivatives reacted with chlorine atom donors to produce the neutral trichloride derivatives (X,R)(NNN(sq))TaCl3, containing the one-electron-oxidized form of the redox-active ligand. Aryl azides reacted with the (X,R)(NNN(cat))TaCl2(L) derivatives, resulting in nitrene transfer to tantalum and two-electron oxidation of the ligand platform to give (X,R)(NNN(q))TaCl2(═NR') (R = iPr; X = OMe, F, H, Me; R' = p-C6H4tBu, p-C6H4CF3; and R = 3,5-C6H3Me2; X = OMe; R' = p-C6H4CH3). Electrochemistry, UV-vis-NIR, IR, and EPR spectroscopies along with X-ray diffraction methods were used to characterize and compare complexes with different redox-active ligand derivatives in each oxidation state. This study demonstrates that while the ligand redox potentials can be adjusted over a 270 mV range through substitutions at the 4/4' ring positions, the coordination chemistry and reactivity patterns at the bound tantalum center remain unchanged, suggesting that such ligand modifications can be used to tune the redox potentials of a complex for a particular substrate of interest.

Synthesis and characterization of tantalum silsesquioxane complexes

Tantalum polyhedral oligosilsesquioxane (POSS) complexes have been synthesised and characterized. X-ray structures of these complexes revealed that the coordination number of the tantalum center greatly affects the cube-like silsesquioxane framework.

Oxygenextrusion from amidate ligands to generate terminal TaO units under reducing conditions. How to successfully use amidate ligands in dinitrogen coordination chemistry

A series of mixed Cp* amidate tantalum complexes Cp*Ta(RNC(O)R')X(3) (where R = Me(2)C(6)H(3), (i)Pr, R' = (t)Bu, Ph, X = Cl, Me) have been prepared via salt metathesis and their fundamental reactivities under reducing conditions have been explored. Reaction of the tantalum chloro precursors with potassium graphite under N(2) or Ar leads to the stereoselective formation of the terminal tantalum oxo species, Cp*Ta=O(η(2)-RN=CR')Cl. This represents the formal extrusion of oxygen from the amidate ligand to the reduced tantalum center and is accompanied by the formation of the iminoacyl fragment bound to Ta(v). Amidate dinitrogen complexes, [Cp*TaCl(RNC(O)(t)Bu)](2)(μ-N(2)) (where R = Me(2)C(6)H(3), (i)Pr) were synthesized via salt metathesis from the known [Cp*TaCl(2)](2)(μ-N(2)) precursor, establishing that amidate ligands can support dinitrogen complexes, but not the reduction process often necessary for their synthesis.

Mono(cycloheptatrienyl) Tantalum Chemistry: Synthesis and Characterization of New Tantalum Halide, Hydride, and Alkyl Species

The new tantalum(II) complex (eta (6)-C 7H 8)TaCl 2(PMe 3) 2 ( 1) was synthesized by the reduction of TaCl 5 with n-butyllithium in the presence of PMe 3 and cycloheptatriene. Compound 1 adopts a four-legged piano stool structure in which the tantalum center is bound to a eta (6)-cycloheptatriene ring in addition to two chlorides and two phosphine ligands in a transoid arrangement. Treatment of 1 with methyllithium results in a loss of the equivalents of HCl and formation of the eta (7)-cycloheptatrienyl complex (eta (7)-C 7H 7)TaCl(PMe 3) 2 ( 2), whereas treatment of 1 with sodium or sodium borohydride affords small amounts of the eta (5)-cycloheptadienyl complex (eta (5)-C 7H 9)TaCl 2(PMe 3) 2 ( 3). Compound 2 adopts a three-legged piano stool structure; the eta (7)-C 7H 7 ring is fully aromatic and planar. The molecular structure of 3 is similar to that of 1, except for the eta (5) binding mode of the seven-membered ring. Treatment of the previously described sandwich compound (C 5Me 5)Ta(C 7H 7) with allyl bromide affords the tantalum(V) product (C 5Me 5)Ta(C 7H 7)Br ( 4), which reacts with LiAlH 4 to give the tantalum(V) hydride (C 5Me 5)Ta(C 7H 7)H ( 5). Compound 4 also reacts with alkylating agents to generate the methyl, allyl, and cyclopropyl complexes (C 5Me 5)Ta(C 7H 7)Me ( 6), (C 5Me 5)Ta(C 7H 7)(eta (1)-CH 2CHCH 2) ( 7), and (C 5Me 5)Ta(C 7H 7)(c-C 3H 5) ( 8). Compounds 4- 8 all adopt bent sandwich structures in which the dihedral angle between the two carbocyclic rings is 34.9 degrees for the bromo compound 4, 26.6 degrees for the hydride 5, 33.1 degrees for the methyl compound 6, 34.2 degrees for the allyl compound 7, and 37.5 degrees for the cyclopropyl compound 8. (1)H and (13)C NMR data are reported for the diamagnetic compounds.

Energetics and Mechanism of Dinitrogen Cleavage at a Mononuclear Surface Tantalum Center: A New Way of Dinitrogen Reduction

Selten wichtig: B3LYP-Rechnungen ergaben, dass die ungewöhnliche side-on-Koordination von N2 an das TaIII-Zentrum in [(SiO)2TaH], der aktiven Spezies für die N2-Reduktion an einem einkernigen Oberflächen-Ta-Zentrum, entscheidend für die anschließenden Hydridübertragungen zur Spaltung von N2 ist. Geschwindigkeitsbestimmend sind der erste (im Bild ist die Potentialenergiefläche gezeigt) und dritte Hydridtransfer vom Metall auf den N2-Liganden.

Energetics and Mechanism of Dinitrogen Cleavage at a Mononuclear Surface Tantalum Center: A New Way of Dinitrogen Reduction

A rare side-on mode of N2 coordination to the TaIII center of [(SiO)2TaH], which was found to be the active species for N2 reduction at a mononuclear surface Ta center, is critical for the subsequent hydride-transfer steps in the cleavage of N2 according to B3LYP calculations. The rate-determining steps are the first (potential energy surface shown in the scheme) and third hydride transfers from the metal to the N2 ligand.

Trichloro- and triisopropoxy-niobium(V) and tantalum(V) bis -( O , O ′-alkylene dithiophosphates): synthesis and characterization

Trichloro- and triisopropoxy-niobium(V) and tantalum(V) alkylene dithiophosphates, (M = Nb(V) or Ta(V); G =–CHMeCHMe–,–CMe2CMe2–,–CH2CMe2CH2–,–CH2CEt2CH2–or–CMe2CH2CHMe–and X = Cl or OPri) have been synthesized by reaction of metal(V) chloride, MCl5, or triisopropoxymetal(V) dichloride, (PriO)3MCl2, with the sodium salts of O,O′-alkylene dithiophosphoric acids, in 1 : 2 molar ratio in THF under anhydrous conditions. These pink-purple or light-yellow compounds are viscous, semi-solid or solid, hydrolyzable and soluble in common organic solvents. These compounds have been characterized by elemental analyses, molecular weight determinations and spectral studies like IR and heteronuclear NMR (1H, 13C and 31P), which indicated a bidentate mode of chelation of dithio ligands, leading to a pentagonal bipyramidal geometry around the niobium(V) or tantalum(V) centers.

Alkyne Exchange Reactions of Silylalkyne Complexes of Tantalum: Mechanistic Investigation and Its Application in the Preparation of New Tantalum Complexes Having Functional Alkynes (PhC⋮CR (R = COOMe, CONMe2))

Silylalkyne complexes of tantalum with the general formula TaCl3(R1C⋮CR2)L2 (1, R1 = R2 = SiMe3, L2 = DME; 2, R1 = SiMe3, R2 = Me, L2 = DME; 6, R1 = R2 = SiMe3, L = py; 7, R1 = SiMe3, R2 = Me, L = py) reacted with an internal alkyne to give corresponding alkyne complexes via an alkyne exchange reaction. These silylalkyne complexes were newly prepared and structurally characterized. Kinetic measurements revealed that the rates of the exchange reactions of the DME complexes 1 and 2 were first-order dependent on the concentration of the complex and the reaction proceeded via dissociative pathway. The exchange reaction rates of the bis(pyridine) complexes 6 and 7 were slower. In contrast to the DME complexes, the exchange reaction of 6 proceeded via an associative interchange pathway, and the exchange mechanism for 7 was an associative pathway. During the exchange, two pyridine ligands coordinated strongly to the tantalum center and the intermediate is proposed to be a bis(alkyne) complex. New tantalum comple...

Dinuclear coordination of a novel four-electron reduced μ-η 1,η 3-allene ligand with β-agostic C–H⋯Ta interaction: Direct addition of allene to (C 5Me 5) 2Ta 2(μ-X) 4 (X = Cl, Br), and molecular structure of (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-Cl)Cl 3

The organoditantalum complexes (η-C 5Me 5) 2Ta 2(μ-X) 4 (X = Cl, Br) react with 1–5 equiv. of allene under mild conditions to afford the fluxional allene adducts (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-X)X 3 as shown by spectroscopic and spectrometric data. Low-temperature-limit proton NMR data at −95 °C for (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-Cl)Cl 3 show a large chemical shift range for the inequivalent allene hydrogens, with a high-field value for one hydrogen at δ −0.36, and inequivalent Cp* groups. The low-temperature-limit 13C NMR resonances for the allene ligand are found at δ 204.4 (central C), δ 70.3 (CH 2, 1 J CH = 154 Hz), and the substantially upfield value of δ 42.9. The latter resonance has remarkably different coupling constants ( 1 J CH = 173, 154 Hz). The allene coordinates in a novel alkylidene-diyl fashion in the solid-state, as shown by X-ray diffractometry on (C 5Me 5) 2Ta 2(μ-η 1,η 3-C 3H 4)(μ-Cl)Cl 3 at 200 K. One tantalum is doubly-bonded to the central allenic carbon (Ta1 C22 distance, 2.011(7) Å) and interacts with an allene hydrogen in a probable β-agostic manner (Ta1–H21A distance of 2.25(10) Å). The second tantalum is coordinated to the allene in a hybrid of η 3-allylic and sigma bonding, with Ta2–C distances to the allene methylenes of 2.206(8) and 2.276(9) Å and a Ta2–C22 bond distance to the central allenic carbon of 2.253(7) Å. The allene hydrogens are oriented above and below the C–C–C plane, consistent with appreciable σ-bonding between the Ta(2) and the allene methylenes. The bent allene ligand has a C–C–C angle of 106.0(6)° and can be viewed as reduced by four electrons, with concomitant oxidation of the two d 2 tantalum centers of the organoditantalum(III) reactant. The long Ta⋯Ta nonbonded distance of 3.3052(8) Å is consistent with two d 0 tantalum centers in an organoditantalum(V) product.

Half-Metallocene 1-Aza-1,3-butadiene Complexes of Tantalum: Auxiliary Ligand Effects on Controlling Coordination Modes of 1-Aza-1,3-butadiene Ligand

Abstract We prepared some half-metallocene complexes of tantalum bearing ortho-substituted aryl derivatives of 1-aza-1,3-butadiene ligand. The coordination mode of the 1-aza-1,3-diene (abbr. AD) ligand highly depended on the substituent(s) on the aryl group of the AD ligand and also on the metal center. The number of methyl substituents on the aryl group of the AD ligand differentiated two coordination modes: one methyl substituent, TaCl2Cp*(η4-supine-o-Tol-AD) (1) (Cp* = pentamethylcyclopentadienyl; o-Tol-AD = 1-(2-methylphenyl)-4-phenyl-1-aza-1,3-butadiene), favored an η4-supine-coordination mode, while two methyl substituents, TaCl2Cp*(η2-C, N-Xyl-AD) (5) (Xy1-AD = 2,6-dimethylphenyl-4-phenyl-1-aza-1,3-butadiene), adopted an η2-C, N-imine coordination fashion. Furthermore, the preferential coordination mode of the AD ligand was delicately affected by the substituent(s) on the tantalum center. Dialkylation of 1 led to the formation of TaR2Cp*(o-Tol-AD) (6: R = Me; 8: R = CH2Ph) in the dimethyl complex 6. The equilibrium between η4-coordination mode and η2-C, N-coordination one was observed and the latter mode was thermally more feasible in solution and as solid state, and the o-Tol-AD ligand predominantly coordinated in an η2-C, N-imine fashin to the tantalum center of 8. In contrast, the monobenzylation of 1 did not change the η4-supine coordination mode, giving a monobenzyl complex Cp*TaCl(CH2Ph)(η4-supine-o-Tol-AD) (9). The dibenzyl complex 8 gradually turned to a benzylidene complex Ta(=CHPh)Cp*(η4-supine-o-Tol-AD) (10) on heating at 70 °C with the change of the coordination mode from η2 to η4. Introduction of a 1,3-butadiene ligand to the metal center gave a mixed-ligand complex TaCp*(η2-C,N-o-Tol-AD)(η4-s-cis-1,3-butadiene) (12), in which the metal center adopted an η2-C,N-fashion.

Synthesis and chemistry of zwitterionic tantala-3-boratacyclopentenes: olefin-like reactivity of a borataalkene ligand.

The compounds, where Cp' = C(5)H(5), a series, and C(5)H(4)Me, b series, are generated via treatment of with HB(C(6)F(5))(2). When allowed to undergo irreversible methane loss in the presence of an excess of the sterically modest alkynes 2-butyne or phenylacetylene, the putative intermediates 1a and 1b are trapped as the tantala-3-boratacyclopentene compounds 2 and 3, respectively. In these complexes, the alkyne and borataalkene ligands have reductively coupled at the d(2) tantalum center. For the unsymmetrical alkyne, a kinetic product resulting from coupling in the opposite regiochemical sense is observed; the thermodynamic products 3-t incorporate the phenyl group in the alpha position of the tantalaboratacyclic ring. Two of these compounds (2b and 3b-t) were characterized crystallographically. For bulkier alkynes (diphenylacetylene, 1-phenyl-1-propyne, and 3-hexyne), intermediates with similar spectroscopic properties to the tantala-3-boratacyclopentenes were observed, but the ultimate products were the vinylalkylidene compounds 5-(R,R'). Compound 5-(Ph,Me) was characterized crystallographically, and it was found that the vinylalkylidene binds to the metal in an eta(1)-bonding mode, with the tantalum center receiving further ligation through a hydridoborate moiety. Mechanistic studies suggest that these products arise via retrocyclization of the tantala-3-boratacyclopentenes formed kinetically. These studies represent the first studies concerning the reactivity of a borataalkene ligand at a transition metal center and show that it can behave in an "olefin-like" manner, despite having a more flexible array of bonding modes available to it than an olefin.

Theoretical study of the bonding capabilities of 1,4-diaza-1,3-butadiene and cis-1,3-butadiene ligands in cyclopentadienyl tantalum(V) complexes

A folded envelope geometry is generally encountered in structurally characterized high valent early transition metal compounds containing a substituted 1,4-diaza-1,3-butadiene ligand. In these cases the diazabutadiene ligand can be regarded as dianionic ene-diamido ligand and the folded five-membered ring can be described as a metallacyclo-2,5-diazapent-3-ene. A comparable metallacyclopent-3-ene description is attributed to η 4-butadiene complexes of early transition metals, in which the ligand exhibits a σ 2, π-character. For this reason, a parallel description was initially adopted by several authors for the ene-diamido ligands and the origin of the folding of the five-membered ring was attributed to the saturation of the metal center through the CC double bond. The aim of this paper is to discriminate the bonding capabilities of the 1,4-diaza-1,3-butadiene and cis-1,3-butadiene ligands on high-valent tantalum complexes, and to establish the origin of the folding of the 1,4-diaza-1,3-butadiene ligand. A Density Functional study (DFT) of the model complex [CpTaCl 2(HNCHCHNH)] ( 1) and compound [CpTaCl 2(η 4-butadiene)] ( 2) has been carried out. The differences in the bonding description of these ligands coordinated to the identical [CpTaCl 2] moiety are discussed through the examination of the HOMOs of 1 and 2 and on the basis of a FMO analysis. In complex 1, there is no evidence of any positive overlap between the tantalum center and the inner carbon atoms of the diazabutadiene ligand, while for 2 the classical σ 2,π-description for the η 4-butadiene ligand is well-founded. Consequently, the generally accepted bonding description of 1,4-diaza-1,3-butadiene ligands coordinated to high valent early transition complexes should be modified. The origin of the folding is the reorientation of the N hybrid orbitals with the purpose of saturating the metal center and no significant donation through the CC bond occurs.

Reaction of [P(2)N(2)]Ta==CH(2)(Me) with ethylene: synthesis of [P(2)N(2)]Ta(C(2)H(4))Et, a neutral species with a beta-agostic ethyl group in equilibrium with an alpha-agostic ethyl group ([P(2)N(2)] = PhP(CH(2)SiMe(2)CH(2))(2)PPh).

The photolysis of [P(2)N(2)]TaMe(3) ([P(2)N(2)] = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) produces [P(2)N(2)]Ta=CH(2)(Me) as the major product. The thermally unstable methylidene complex decomposes in solution in the absence of trapping agents to unidentified products. However, in the presence of ethylene [P(2)N(2)]Ta=CH(2)(Me) is slowly converted to [P(2)N(2)]Ta(C(2)H(4))Et, with [P(2)N(2)]Ta(C(2)H(4))Me observed as a minor product. A mechanistic study suggests that the formation of [P(2)N(2)]Ta(C(2)H(4))Et results from the trapping of [P(2)N(2)]TaEt, formed by the migratory insertion of the methylene moiety into the tantalum-methyl bond. The minor product, [P(2)N(2)]Ta(C(2)H(4))Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene to [P(2)N(2)]Ta=CH(2)(Me) and is accompanied by the production of an equivalent of propylene. Pure [P(2)N(2)]Ta(C(2)H(4))Et can be synthesized by hydrogenation of [P(2)N(2)]TaMe(3) in the presence of PMe(3), followed by the reaction of ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a beta-agostic interaction between the ethyl group and tantalum center in [P(2)N(2)]Ta(C(2)H(4))Et. Partially deuterated analogues of [P(2)N(2)]Ta(C(2)H(4))Et show a large isotopic perturbation of resonance for both the beta-protons and the alpha-protons of the ethyl group, indicative of an equilibrium between a beta-agostic and an alpha-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the (1)H environments of the ethyl and ethylene ligands. The mechanism of this exchange is believed to involve the direct transfer of the beta-agostic hydrogen atom from the ethyl group to the ethylene ligand, via the so-called beta-hydrogen transfer process.

Synthesis and Bonding in the Diamagnetic Dinuclear Tantalum(IV) Hydride Species ([P2N2]Ta)2(μ-H)4and the Paramagnetic Cationic Dinuclear Hydride Species {([P2N2]Ta)2(μ-H)4}+I-([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh): The Reducing Ability of a Metal−Metal Bond

The controlled reaction of the Ta(V) trimethyl species [P2N2]TaMe3, where [P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh, under 0.5 atm of hydrogen gas produces a partially hydrogenated Ta(V) species, ([P2N2]TaMe2(H), of unknown structure. Under 4 atm of hydrogen gas, further hydrogenation does not produce the complex [P2N2]TaH3; instead, reduction of the tantalum center occurs to yield the dinuclear Ta(IV) hydride ([P2N2]Ta)2(μ-H)4. This diamagnetic tetrahydride fails to react with many reagents, including ethylene and carbon monoxide; however, upon addition of iodomethane, {([P2N2]Ta)2(μ-H)4}+I- is produced as a paramagnetic green crystalline solid. The number of hydrides in this reaction product was confirmed by a deuterium labeling study. The results of a variable-temperature magnetic susceptibility study of this tetrahydride cation can be partially modeled with the Curie−Weiss law and a large correction for temperature-independent magnetism. Ab initio calculations using density functional theory were performed in an attempt to further understand the influence of the macrocyclic ligand in the bonding in these complexes.

Control of Stereoselectivity in the Ring-Opening Metathesis Polymerization of Norbornene by the Auxiliary Ligands Butadiene and o-Xylylene in Well-Defined Pentamethylcyclopentadiene Tantalum Carbene Complexes

cis-Dialkyl complexes of tantalum, Ta(CH2Ph)2Cp*(η4-C4H6) (2a) (Cp* = η5-C5Me5), TaMe(CH2SiMe3)Cp*(η4-butadiene) (6), and TaMe(CH2CMe3)Cp*(η4-butadiene) (7), were found to be catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene to give poly(norbornene) with a high cis-vinylene double-bond (97−99%) content, while an o-xylylene complex Ta(CH2Ph)2(η4-o-(CH2)2C6H4)Cp* (9) was also an initiator to give poly(norbornene) with a high trans-vinylene double-bond (92−95%) content. When a Cp−butadiene complex Ta(CH2Ph)2Cp(η4-C4H6) (2b) was used as an initiator, we obtained poly(norbornene) with no selectivity (1:1 mixture of trans- and cis-vinylene bonds). The factors controlling these stereoselectivities have been investigated, and we obtained the following results: (1) We isolated benzylidene complexes and found that the proportion of anti- and syn-rotamers obtained depended on the kind of auxiliary ligands on the tantalum center, i.e., 1,3-butadiene or o-xylylene. Thermolysis of 2a ...