Living Ziegler-Natta polymerization by early transition metals: synthesis and evaluation of cationic zirconium alkyl complexes bearing beta-hydrogens as models for propagating centers

The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing beta-hydrogens of general formula {(eta(5)-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a-f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler-Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler-Natta polymerization based on an early transition metal initiator.     

Reduction of bis

The reduction of symmetric, fully-substituted titanocene dichlorides bearing two pendant omega-alkenyl groups, [TiCl2(eta5-C5Me4R)2], R = CH(Me)CH= CH2 (1a). (CH2)2CH=CH2 (1b) and (CH2)3CH=CH2 (1c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [Ti(IV)[eta1:eta1: eta5:eta5-C5Me4CH(Me)CH(Ti)CH2CH(CH2(Ti))CH(Me)C5Me4]] (2a), [Ti(IV)[eta1:eta1:eta5: eta5-C5Me4(CH2)2CH(Ti)(CH2)2CH(Ti)(CH2)2C5Me4]] (2b) and [Ti(IV)[eta1:eta1:eta5:eta5-C5Me4(CH2)2CH(Ti)CH(Me)CH(Me)CH(Ti)(CH2)2C5Me4]](2c), respectively, as the products of oxidative coupling of the double bonds across a titanocene intermediate. For the case of complex 1c, a product of a double bond isomerisation is obtained owing to a preferred formation of five-membered titanacycles. The reaction of the titanacyclopentanes with PbCl2 recovers starting materials 1a from 2a and 1b from 2b, but complex 2c affords, under the same conditions, an isomer of 1c with a shifted carbon - carbon double bond, [TiCl[eta5-C5Me4(CH2CH2CH=CHMe)]2] (1c'). The titanacycles 2a-c can be opened by HCl to give ansa-titanocene dichlorides ansa-[[eta5:eta5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4]TiCl2] (3a), ansa-[[eta5:eta5-C5Me4(CH2)8C5Me4]TiCl2] (3b), along with a minor product ansa-[[eta5:eta5-C5Me4CH2CH=CH(CH2)5C5Me4]TiCl2] (3b'), and ansa-[[eta5:eta5-CsMe4(CH2)3CH(Me)CH(Me)CH=CHCH2C5Me4]TiCl2] (3c), respectively, with the bridging aliphatic chain consisting of five (3a) and eight (3b, 3b' and 3c) carbon atoms. The course of the acidolysis changes with the nature of the pendant group; while the cyclopentadienyl ring-linking carbon chains in 3a and 3b are fully saturated, compounds 3c and 3b' contain one asymetrically placed carbon-carbon double bond, which evidently arises from the beta-hydrogen elimination that follows the HCl addition.     

Ammonolysis of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives

Ammonolyses of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives [Ti(eta5-C5Me5)X3] (X = NMe2, Me, Cl) have been carried out in solution to give polynuclear nitrido complexes. Reaction of the tris(dimethylamido) derivative [Ti(eta5-C5Me5)(NMe2)3] with excess of ammonia at 80-100 degrees C gives the cubane complex [[Ti(eta5-C5Me5)]4(mu3-N)4] (1). Treatment of the trimethyl derivative [Ti(eta5-C5Me5)Me3] with NH3 at room temperature leads to the trinuclear imido-nitrido complex [[Ti(eta/5-CsMes)(mu-NH)]3(mu3-N)] (2) via the intermediate [[Ti(eta5-C5Me5)Me]2(mu-NH)2] (3). The analogous reaction of [Ti(eta5-C5Me5)Me3] with 2,4,6-trimethylaniline (ArNH2) gives the dinuclear imido complex [[Ti(eta5-C5Me5)Me])2(mu-NAr)2] (4) which reacts with ammonia to afford [[Ti(eta5-C5Me5)(NH2)]2(mu-NAr)2] (5). Complex 2 has been used, by treatments with the tris(dimethylamido) derivatives [Ti(eta5-C5H5-nRn)(NMe2)3], as precursor of the cubane nitrido systems [[Ti4(eta5-C5Me5)3(eta5-C5H5-nRn)](mu3-N)4] [R = Me n = 5 (1), R = H n = 0 (6), R = SiMe3 n = 1 (7), R = Me n = 1 (8)] via dimethylamine elimination. Reaction of [Ti(eta5-C5Me5)Cl3] or [Ti(eta5-C5Me5)(NMe2)Cl2] with excess of ammonia at room temperature gives the dinuclear complex [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) where an intramolecular hydrogen bonding and a nonlineal nitrido ligand bridge the "Ti(eta5-C5Me5)Cl(NH3)" and "Ti(eta5-C5Me5)Cl2" moieties. The molecular structures of [[Ti(eta5-C5Me5)Me]2 (mu-NAr)2] (4) and [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) have been determined by X-ray crystallographic studies. Density functional theory calculations also have been conducted on complex 9 to confirm the existence of an intramolecular N-H...Cl hydrogen bond and to evaluate different aspects of its molecular disposition.     

Alkyl-eta2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl-eta5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations

Group 5 metal complexes [M(eta5-C5H5)[eta5-C5H4SiMe2(CH2-eta]2-CH=CH2)]X] (M = Nb, X = Me, CH2Ph, CH2SiMe3; M = Ta, X = Me, CH2Ph) and [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2-eta2-CH=CH2)]X] (X = Cl, Me, CH2Ph, CH2SiMe3) containing a chelating alkene ligand tethered to a cyclopentadienyl ring have been synthesized in high yields by reduction with Na/Hg (X = Cl) and alkylation with reductive elimination (X = alkyl) of the corresponding metal(iv) dichlorides [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]Cl2] (Cp = C5H5, M = Nb, Ta, Cp = C5Me5, M = Ta). These chloro- and alkyl-alkene coordinated complexes react with CO and isocyanides [CNtBu, CN(2,6-Me2C6H3)] to give the ligand-substituted metal(III) compounds [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]XL] (X = Cl, Me, CH2Ph, CH2SiMe3). Reaction of the chloro-alkene tantalum complex with LiNHtBu results in formation of the imido hydride derivative [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2CH=CH2)]H(NtBu)]. NMR studies for all of the new compounds and DFT calculations for the alkene-coordinated metal complexes are compared with those known for related group 4 metal cations.     

Coordinatively unsaturated semisandwich complexes of ruthenium with phosphinoamine ligands and related species: a complex containing (R,R)-1,2-bis((diisopropylphosphino)amino)cyclohexane in a new coordination form kappa(3)P,P',N-eta(2)-P,N

The syntheses of the chloro complexes [Ru(eta5-C5R5)Cl(L)] (R = H, Me; L = phosphinoamine ligand) (1a-d) have been carried out by reaction of [(eta5-C5H5)RuCl(PPh3)2] or {(eta5-C5Me5)RuCl}4 with the corresponding phosphinoamine (R,R)-1,2-bis((diisopropylphosphino)amino)cyclohexane), R,R-dippach, or 1,2-bis(((diisopropylphosphino)amino)ethane), dippae. The chloride abstraction reactions from these compounds lead to different products depending on the starting chlorocomplex and the reaction conditions. Under argon atmosphere, chloride abstraction from [(eta5-C5Me5)RuCl(R,R-dippach)] with NaBAr'4 yields the compound [(eta5-C5Me5)Ru(kappa3P,P'-(R,R)-dippach)][BAr'4] (2b) which exhibits a three-membered ring Ru-N-P by a new coordination form of this phosphinoamine. However, under the same conditions the reaction starting from [(eta5-C5Me5)RuCl(dippae)] yields the unsaturated 16 electron complex [(eta5-C5Me5)Ru(dippae)][BAr'4] (2d). The bonding modes of R,R-dippach and dippae ligands have been analyzed by DFT calculations. The possibility of tridentate P,N,P-coordination of the phosphinoamide ligand to a fragment [(eta5-C5Me5)Ru]+ is always present, but only the presence of a cyclohexane unit in the ligand framework converts this bonding mode in a more favorable option than the usual P,P-coordination. Dinitrogen [(eta5-C5R5)Ru(N2)(L)][BAr'4] (3a-d) and dioxygen complexes [(eta5-C5H5)Ru(O2)(R,R-dippach)][BPh4] (4a) and [(eta5-C5Me5)Ru(O2)(L)][BPh4] (4b,d) have been prepared by chloride abstraction under dinitrogen or dioxygen atmosphere, respectively. The presence of 16 electron [(eta5-C5H5)Ru(R,R-dippach)]+ species in fluorobenzene solutions of the corresponding dinitrogen or dioxygen complexes in conjunction with the presence of [BAr'4]- gave in some cases a small fraction of [Ru(eta5-C5H5)(eta6-C6H5F)][BAr'4] (5a), which has been isolated and characterized by X-ray diffraction.     

Ditantalum dinitrogen complex: reaction of H2 molecule with "end-on-bridged" [Ta(IV)]2(μ-η(1):η(1)-N2) and Bis(μ-nitrido) [Ta(V)]2(μ-N)2 complexes

To elucidate (i) the physicochemical properties of the {(η(5)-C(5)Me(5))[Ta(IV)](i-Pr)C(Me)N(i-Pr)}(2)(μ-η(1):η(1)-N(2)), I, [Ta(IV)](2)(μ-η(1):η(1)-N(2)), and {(η(5)-C(5)Me(5))[Ta(V)](i-Pr)C(Me)N(i-Pr)}(2)(μ-N)(2), II, [Ta(V)](2)(μ-N)(2), complexes; (ii) the mechanism of the I → II isomerization; and (iii) the reaction mechanism of these complexes with an H(2) molecule, we launched density functional (B3LYP) studies of model systems 1, 2, and 3 where the C(5)Me(5) and (i-Pr)C(Me)N(i-Pr) ligands of I (or II) were replaced by C(5)H(5) and HC(NCH(3))(2), respectively. These calculations show that the lower-lying electronic states of 1, [Ta(IV)](2)(μ-η(1):η(1)-N(2)), are nearly degenerate open-shell singlet and triplet states with two unpaired electrons located on the Ta centers. This finding is in reasonable agreement with experiments [J. Am Chem. Soc. 2007, 129, 9284-9285] showing easy accessibility of paramagnetic and diamagnetic states of I. The ground electronic state of the bis(μ-nitrido) complex 2, [Ta(V)](2)(μ-N)(2), is a closed-shell singlet state in agreement with the experimentally reported diamagnetic feature of II. The 1-to-2 rearrangement is a multistep and highly exothermic process. It occurs with a maximum of 28.7 kcal/mol free energy barrier required for the (μ-η(1):η(1)-N(2)) → (μ-η(2):η(2)-N(2)) transformation step. Reaction of 1 with H(2) leading to the 1,4-addition product 3 proceeds with a maximum of 24.2 kcal/mol free energy barrier associated by the (μ-η(1):η(1)-N(2)) → (μ-η(2):η(1)-N(2)) isomerization step. The overall reaction 1 + H(2) → 3 is exothermic by 20.0 kcal/mol. Thus, the addition of H(2) to 1 is kinetically and thermodynamically feasible and proceeds via the rate-determining (μ-η(1):η(1)-N(2)) → (μ-η(2):η(1)-N(2)) isomerization step. The bis(μ-nitrido) complex 2, [Ta(V)](2)(μ-N)(2), does not react with H(2) because of the large energy barrier (49.5 kcal/mol) and high endothermicity of the reaction. This conclusion is also in excellent agreement with the experimental observation [J. Am Chem. Soc. 2007, 129, 9284-9285].     

Cube-type nitrido complexes containing titanium and zinc/copper

Several azaheterometallocubane complexes containing [MTi3N4] cores have been prepared by the reaction of [{Ti(eta5-C5Me5)(mu-NH)}3(mu3-N)] (1) with zinc(II) and copper(I) derivatives. The treatment of 1 with zinc dichloride in toluene at room temperature produces the adduct [Cl2Zn{(mu3-NH)3Ti3(eta5-C5Me5)3(mu3-N)}] (2). Attempts to crystallize 2 in dichloromethane gave yellow crystals of the ammonia adduct [(H3N)Cl2Zn{(mu3-NH)Ti3(eta5-C5Me5)3(mu-NH)2(mu3-N)}] (3). The analogous reaction of 1 with alkyl, (trimethylsilyl)cyclopentadienyl, or amido zinc complexes [ZnR2] leads to the cube-type derivatives [RZn{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (R = CH2SiMe3 (5), CH2Ph (6), Me (7), C5H4SiMe3 (8), N(SiMe3)2 (9)) via RH elimination. The amido complex 9 decomposes in the presence of ambient light to generate the alkyl derivative [{Me3Si(H)N(Me)2SiCH2}Zn{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (10). The chloride complex 2 reacts with lithium cyclopentadienyl or lithium indenyl reagents to give the cyclopentadienyl or indenyl zinc derivatives [RZn{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (R = C5H5 (11), C9H7 (12)). Treatment of 1 with copper(I) halides in toluene at room temperature leads to the adducts [XCu{(mu3-NH)3Ti3(eta5-C5Me5)3(mu3-N)}] (X = Cl (13), I (14)). Complex 13 reacts with lithium bis(trimethylsilyl)amido in toluene to give the precipitation of [{Cu(mu4-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}2] (15). Complex 15 is prepared in a higher yield through the reaction of 1 with [{CuN(SiMe3)2}4] in toluene at 150 degrees C. The addition of triphenylphosphane to 15 in toluene produces the single-cube compound [(Ph3P)Cu{(mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)}] (16). The X-ray crystal structures of 3, 8, 9, and 15 have been determined.     

Molecular nitrides containing group 4 and 5 metals: single and double azatitanocubanes

Treatment of [[Ti(eta(5)-C(5)Me(5))(micro-NH)](3)(micro(3)-N)] (1) with the imido complexes [Ti(NAr)Cl(2)(py)(3)] (Ar=2,4,6-C(6)H(2)Me(3)) and [Ti(NtBu)Cl(2)(py)(3)] in toluene affords the single azatitanocubanes [[Cl(2)(ArN)Ti]( micro(3)-NH)(3)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]].(C(7)H(8)) (2.C(7)H(8)) and [[Cl(2)Ti](micro(3)-N)(2)(micro(3)-NH)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]] (3), respectively. Similar reactions of complex 1 with the niobium and tantalum imido derivatives [[M(NtBu)(NHtBu)Cl(2)(NH(2)tBu)](2)] (M=Nb, Ta) in toluene give the single azaheterometallocubanes [[Cl(2)(tBuN)M](micro(3)-N)(micro(3)-NH)(2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]] (M=Nb (4), Ta (5)), both complexes react with 2,4,6-trimethylaniline to yield the analogous species [[Cl(2)(ArN)M](micro(3)-N)(micro(3)-NH)(2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]].(C(7)H(8)) (Ar=2,4,6-C(6)H(2)Me(3), M=Nb (6.C(7)H(8)), Ta (7.C(7)H(8))). Also the azaheterodicubanes [M[micro(3)-N)(2)(micro(3)-NH)](2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)](2)].2C(7)H(8) [M=Ti (8.2C(7)H(8)), Zr (9.2C(7)H(8))], and [M[(micro(3)-N)(5)(micro(3)-NH)][Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)](2)].2 C(7)H(8) (Nb (10.2C(7)H(8)), Ta (11.2C(7)H(8))) were prepared from 1 and the homoleptic dimethylamido complex [M(NMe(2))(x)] (x=4, M=Ti, Zr; x=5, M=Nb, Ta) in toluene at 150 degrees C. X-ray crystal structure determinations were performed for 6 and 10, which revealed a cube- and double-cube-type core, respectively. For complexes 2 and 4-7 we observed and studied by DNMR a rotation or trigonal-twist of the organometallic ligands [[Ti(eta(5)-C(5)Me(5))(micro-NH)](3)(micro(3)-N)] (1) and [(micro(3)-N)(micro(3)-NH)(2)[Ti(3)(eta(5)-C(5)Me(5))(3)(micro(3)-N)]](1-). Density functional theory calculations were carried out on model complexes of 2, 3, and 8 to establish and understand their structures.     

Activation of a CNXylyl ancillary ligand in the reaction of electron-deficient alkynes with a phosphido niobocene complex

The niobium phosphido complex [Nb(eta5-C5H4SiMe3)2-(CNXylyl)(PPh2)] (2) undergoes an unusual cycloaddition reaction with electron-deficient alkynes to give the novel five-membered heteroniobacycles [Nb(eta5-C5H4SiMe3)2(kappaC-C(=N(Xylyl))C(CO2Me)=C(R)PPh2-kappaP)] (R = H 3 and R = Me 4).     

Dinitrogen functionalization with terminal alkynes, amines, and hydrazines promoted by [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2): observation of side-on and end-on diazenido complexes in the reduction of N2 to hydrazine

Functionalization of the N2 ligand in the side-on bound dinitrogen complex, [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(eta5-C5Me4H)2Zr(C[triple bond]CR)]2(mu2,eta2,eta2-N2H2) (R = nBu, tBu, Ph). Characterization of [(eta5-C5Me4H)2Zr(C[triple bond]CCMe3)]2(mu2,eta2,eta2-N2H2) by X-ray diffraction revealed a side-on bound diazenido ligand in the solid state, while variable-temperature 1H and 15N NMR studies established rapid interconversion between eta1,eta1 and eta2,eta2 hapticity of the [N2H2]2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(eta5-C5Me4H)2ZrX]2(mu2,eta1,eta1-N2H2) (X = Cl, I, OTf, CH2Ph, CH2SiMe3), afforded eta1,eta1 coordination of the [N2H2]2- fragment both in the solid state and in solution, demonstrating that sterically demanding, in some cases pi-donating, ligands can overcome the electronically preferred side-on bonding mode. Unlike [(eta5-C5Me4H)2ZrH]2(mu2,eta2,eta2-N2H2), the acetylide and alkyl zirconocene diazenido complexes are thermally robust, resisting alpha-migration and N2 cleavage up to temperatures of 115 degrees C. Dinitrogen functionalization with [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2) was also accomplished by addition of proton donors. Weak Br?nsted acids such as water and ethanol yield hydrazine and (eta5-C5Me4H)2Zr(OH)2 and (eta5-C5Me4H)2Zr(OEt)2, respectively. Treatment of [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2) with HNMe2 or H2NNMe2 furnished amido or hydrazido zirconocene diazenido complexes that ultimately produce hydrazine upon protonation with ethanol. These results contrast previous observations with [(eta5-C5Me5)2Zr(eta1-N2)]2(mu2,eta1,eta1-N2) where loss of free dinitrogen is observed upon treatment with weak acids. These studies highlight the importance of cyclopentadienyl substituents on transformations involving coordinated dinitrogen.     

Mono(dinitrogen) and carbon monoxide adducts of bis(cyclopentadienyl) titanium sandwiches

The silyl-substituted titanocene complex, (eta5-C5Me4SiMe2Ph)2Ti, coordinates dinitrogen upon cooling to -35 degrees C to yield an unprecedented example of a mono(dinitrogen) complex of a substituted bis(cyclopentadienyl) titanium compound, (eta5-C5Me4SiMe2Ph)2Ti(N2). Analogous monocarbonyl derivatives, (eta5-C5Me4R)2Ti(CO) (R = SiMe3, SiMe2Ph, CHMe2), have been prepared by mixing the dicarbonyl compounds with the corresponding sandwiches. Both (eta5-C5Me4SiMe2Ph)2Ti(N2) and (eta5-C5Me4SiMe2Ph)2Ti(CO) have been characterized by X-ray diffraction, and mixed N2-CO titanocene complexes have also been observed by in situ IR spectroscopy.     

eta1:eta2-Alkynyl-bridged W-Si complexes: formation, structure, and reaction with acetone

Reactions of (eta5-C5Me4R)(CO)2(MeCN)WMe (R = Me, Et) with HPh2SiCCtBu gave the novel alkynyl-bridged W-Si complexes, (eta5-C5Me4R)(CO)2W(mu-eta1:eta2-CCtBu)(SiPh2) (R = Me, Et), whose alkynyl ligands bridge the tungsten and silicon atoms in an eta1:eta2-coordination mode. The structures of these complexes were fully characterized, including X-ray crystallography. Treatment of (eta5-C5Me5)(CO)2W(mu-eta1:eta2-CCtBu)(SiPh2) with acetone resulted in acetone insertion into the silicon-alkynyl linkage followed by intramolecular C-H activation of the tBu group to give the chelate-type alkyl-alkene complex, (eta5-C5Me5)(CO)2W(eta1:eta2-CH2CMe2C=CHSiPh2OCMe2).     

An eta(6)-dienyne transition-metal complex

The ruthenium complexes, [(eta5-C5R5)Ru(CH3CN)3]PF6 (1-Cp*, R = Me; 1-Cp, R = H), underwent reaction with both 1-(2-chloro-1-methylvinyl)-2-pentynyl-(Z)-cyclopentene (6-Z) and 1-(2-chloro-1-methylvinyl)-2-pentynyl-(E)-cyclopentene (6-E) to give (eta5-C5R5)Ru[eta6-(5-chloro-4-methyl-6-propylindan)]PF6 (7-Cp*, R = Me; 7-Cp, R = H). In a similar fashion, reaction of 1-Cp and 1-Cp* with 1-isopropenyl-2-pent-1-ynylcyclopentene (8) led to the formation of (eta5-C5R5)Ru(eta6-4-methyl-6-propylindan)]PF6 (9-Cp*, R = Me; 9-Cp, R = H). The reaction of 1-Cp* with 8 at -60 degrees C in CDCl3 solution led to observation of the eta6-dienyne complex, (eta5-C5Me5)Ru[eta6-(1-isopropenyl-2-pent-1-ynylcyclopentene)]PF6 (10), by 1H NMR spectroscopy. Complexes 7-Cp and 10 were characterized by X-ray crystallographic analysis.     

Synthetic, reactivity, and structural studies on half-sandwich (eta5-C5Me5)Be and related compounds: halide, alkyl, and iminoacyl derivatives

The half-sandwich compounds [(eta(5)-C(5)Me(5))BeX] (X=Cl, 1 a; Br, 1 b), readily prepared from the reaction of the halides BeX(2) and M[C(5)Me(5)] (M=Na or K), are useful synthons for other (eta(5)-C(5)Me(5))Be organometallic compounds, including the alkyl derivatives [(eta(5)-C(5)Me(5))BeR] (R=Me, 2 a; CMe(3), 2 b; CH(2)CMe(3), 2 c; CH(2)Ph, 2 d). The latter compounds can be obtained by metathetical exchange of the halides 1 with the corresponding lithium reagent and exhibit NMR signals and other properties in accord with the proposed formulation. Attempts to make [(eta(5)-C(5)Me(5))BeH] have proved fruitless, probably due to instability of the hydride toward disproportionation into [Be(C(5)Me(5))(2)] and BeH(2). The half-sandwich iminoacyl [(eta(5)-C(5)Me(5))Be(C(NXyl)Cp')] and [(eta(5)-C(5)Me(4)H)Be(C(NXyl)Cp')]3, 6 where Xyl=C(6)H(3)-2,6-Me(2) and Cp'=C(5)Me(5) or C(5)Me(4)H, are formed when the beryllocenes [Be(C(5)Me(5))(2)], [Be(C(5)Me(4)H)(2)], and [Be(C(5)Me(5))(C(5)Me(4)H)] are allowed to react with CNXyl. Isolation of three different iminoacyl isomers from the reaction of the mixed-ring beryllocene [(eta(5)-C(5)Me(5))Be(eta(1)-C(5)Me(4)H)] and CNXyl, namely compounds 5 a, 5 b, and 6, provides compelling evidence for the existence in solution of different beryllocene isomers, generated in the course of two very facile processes that explain the solution dynamics of these metallocenes, that is the 1,5-sigmatropic shift of the Be(eta(5)-Cp') unit around the periphery of the eta(1)-Cp' ring, and the molecular inversion rearrangement that exchanges the roles of the two rings.     

Amido-bridged double-cube nitrido complexes containing titanium and magnesium/calcium

Treatment of the single cube nitrido complexes [(thf)x((Me3Si)2N)M((mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N))](M = Mg, x= 0; Ca, x= 1) with one equivalent of anilines NH2Ar in toluene affords the arylamido complexes [(ArHN)M((mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N))]n[M = Mg (3), n= 1, Ar = 4-MeC6H4; Ca (4), n= 2, Ar = 2,4,6-Me3C6H2]. The magnesium complex 3 has a single-cube structure whereas the X-ray crystal structure of the analogous calcium derivative 4 shows two cube-type azaheterometallocubane moieties Ca((mu3-N)(mu3-NH)2Ti3(eta5-C5Me5)3(mu3-N)) held together by two mu-2,4,6-trimethylanilido ligands. Complexes 3 and 4 react with chloroform-d1 at room temperature to give the metal halide adducts [Cl2M((mu3-NH)3Ti3(eta5-C5Me5)3(mu3-N))](M = Mg, Ca). A solution of 3 in n-hexane gave complex [(Mg2(mu3-N)(mu3-NH)5[Ti3(eta5-C5Me5)3(mu3-N)]2)(mu-NHAr)3] which shows three mu-4-methylanilido ligands bridging two [MgTi3N4] cube type cores according to an X-ray crystal structure determination.     

Structures and reactivity of Zr(IV) chlorobenzene complexes

The synthesis, structures, and unusual reactivity of (C5R5)2ZrR'(ClPh)+ chlorobenzene complexes are described. The reaction of (C5R5)2ZrR'2 with [Ph3C][B(C6F5)4] in C6D5Cl affords [(C5R5)2ZrR'(ClC6D5)][B(C6F5)4] chlorobenzene complexes (1-d5, R' = CH2Ph and (C5R5)2 = (C5H5)2; 2a-d-d5, R' = Me and (C5R5)2 = rac-(1,2-ethylene(bis)indenyl) (2a), (C5H5)2 (2b), (C5H4Me)2 (2c), (C5Me5)2 (2d, C5Me5 = Cp*)). Complexes 1 and 2b,c are thermally robust but are converted to [{(C5R5)2Zr(mu-Cl)}2][B(C6F5)4]2 (4b,c) by a photochemical process in ClPh solution. In contrast, 2d undergoes facile thermal ortho-C-H activation to yield [Cp*2Zr(eta2-C,Cl-2-Cl-C6H4)][B(C6F5)4] (5), which slowly rearranges to [(eta4,eta1-C5Me5C6H4)Cp*ZrCl][B(C6F5)4] (6) via beta-Cl elimination and benzyne insertion into a Zr-CCp* bond. The higher thermal reactivity of 2d versus that of 1 and 2b,c is attributed to steric crowding associated with the Cp* ligands of 2d, which forces a ClPh ortho-hydrogen close to the Zr-Me group.     

Synthesis of zirconium, hafnium, and tantalum complexes with sterically demanding hydrazide ligands

The bulky hydrazine t-BuN(H)NMe2 was synthesized via hydrazone and t-BuN(H)N(H)Me intermediates as the major component in a 90:5:5 mixture consisting of t-BuN(H)NMe2, t-BuN(Me)N(H)Me, and t-BuN(Me)NMe2. Reacting the mixture with n-BuLi followed by distillation and fractional crystallization led to the isolation of the ligand precursor LiN(t-Bu)NMe2. Lithium hydrazides, LiN(R)NMe2, were reacted with metal chlorides to afford the hydrazide complexes M(N(Et)NMe2)4 (M = Zr or Hf), MCl(N(R)NMe2)3 (M = Zr, R = i-Pr or t-Bu; M = Hf, R = t-Bu), and TaCl3(N(i-Pr)NMe2)2. The X-ray crystal structures of [LiN(i-Pr)NMe2]4, [LiN(t-Bu)NMe2.THF]2, ZrCl(N(R)NMe2)3 (R = i-Pr or t-Bu), and TaCl3(N(i-Pr)NMe2)2 were determined. The structural analyses revealed that the hydrazide ligands in ZrCl(N(R)NMe2)3 (R = i-Pr or t-Bu) and TaCl3(N(i-Pr)NMe2)2 are eta2 coordinated.     

Solution structures and dynamic properties of chelated d(0) metal olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) (R = H, Me): Models for the [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" catalysts.

To model the Ti-olefin interaction in the putative [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(R')(olefin)(+) intermediates in "constrained geometry" Ti-catalyzed olefin polymerization, chelated alkoxide olefin complexes [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))(+) have been investigated. The reaction of [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(2) (1a,b; R = H, Me) with HOCMe(2)CH(2)CH(2)CH=CH(2) yields mixtures of [eta(5)-C(5)R(4)SiMe(2)NH(t)Bu]TiMe(2)(OCMe(2)CH(2)CH(2)CH=CH(2)) (2a,b) and [eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]TiMe(OCMe(2)CH(2)CH(2)CH=CH(2)) (3a,b). The reaction of 2a/3a and 2b/3b mixtures with B(C(6)F(5))(3) yields the chelated olefin complexes [[eta(5): eta(1)-C(5)R(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][MeB(C(6)F(5))(3)] (4a,b; 71 and 89% NMR yield). The reaction of 2b/3b with [Ph(3)C][B(C(6)F(5))(4)] yields [[eta(5): eta(1)-C(5)Me(4)SiMe(2)N(t)Bu]Ti(OCMe(2)CH(2)CH(2)CH=CH(2))][B(C(6)F(5))(4)] (5b, 88% NMR yield). NMR studies establish that 4a,b and 5b exist as mixtures of diastereomers (isomer ratios: 4a/4a', 62/38; 4b/4b', 75/25; 5b/5b', 75/25), which differ in the enantioface of the olefin that is coordinated. NMR data for these d(0) metal olefin complexes show that the olefin coordinates to Ti in an unsymmetrical fashion primarily through C(term) such that the C=C pi bond is polarized with positive charge buildup on C(int). Dynamic NMR studies show that 4b/4b' undergoes olefin face exchange by a dissociative mechanism which is accompanied by fast inversion of configuration at Ti ("O-shift") in the olefin-dissociated intermediate. The activation parameters for the conversion of 4b to 4b' (i.e., 4b/4b' face exchange) are: DeltaH = 17.2(8) kcal/mol; DeltaS = 8(1) eu. 4a/4a' also undergoes olefin face exchange but with a lower barrier (DeltaH = 12.2(9) kcal/mol; DeltaS = -2(3) eu), for the conversion of 4a to 4a'.     

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.     

Reaction of Ta(NMe2)5 with O2: formation of aminoxy and unusual (aminomethyl)amide oxo complexes and theoretical studies of the mechanistic pathways

Reaction of d0 Ta(NMe2)5 (1) with O2 yields two aminoxy complexes (Me2N)(n)Ta(eta2-ONMe2)(5-n) (n = 4, 2; 3, 3) as well as (Me2N)4Ta2[eta2-N(Me)CH2NMe2]2(mu-O)2 (4) and (Me2N)6Ta3[eta2-N(Me)CH2NMe2]2(eta2-ONMe2)(mu-O)3 (5) containing novel chelating (aminomethyl)amide-N(Me)CH2NMe2 ligands. The crystal structures of 2-5 have been determined by X-ray crystallography. (Me2N)4Ta(eta2-ONMe2) (2) converts to (Me2N)3Ta(eta2-ONMe2)2 (3) in its reaction with O2. In addition, the reaction of Ta(NMe2)5 with 3 gives 2 only at elevated temperatures. Density functional theory (DFT) calculations have been used to investigate the mechanistic pathways in the reactions of Ta(NMe2)5 (1) with triplet O2. Monomeric reaction pathways in the formation of 2-5 are proposed. A key step is the oxygen insertion into a Ta-N bond in 1 through an intersystem conversion from triplet to singlet energy surface to give an active peroxide complex (Me2N)4Ta(eta2-O-O-NMe2) (A4). The DFT studies indicate that the peroxide ligand plays an important role, including oxidizing an amide to an imine ligand through the abstraction of a hydride. Insertion of Me-N=CH2 into a Ta-amide bond yields the unusual -N(Me)CH2NMe2 ligands.