Compounds Cp Research Articles

Theoretical, Thermodynamic, Spectroscopic, and Structural Studies of the Consequences of One-Electron Oxidation on the Fe−X Bonds in 17- and 18-Electron Cp*Fe(dppe)X Complexes (X = F, Cl, Br, I, H, CH3)

The compounds Cp*Fe(dppe)X ([Fe]X) and the corresponding cation radicals [Fe*]X*+ are available for the series X = F, Cl, Br, I, H, CH3. This has allowed for a detailed investigation of the dependence of the nature of Fe-X bonding on the identity of X and the oxidation state (charge) of the complex. Cyclic voltammetry demonstrates that the electrode potentials for the [Fe]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "inverse halide order" is seen, in which the most electronegative X leads to the most easily oxidized complex. This suggests that F is the best donor among the halides. The halide trend is also reflected in NMR spectroscopic data. Mössbauer spectroscopy data also suggest that the F ligand is a strong donor (relative to H and CH3) in [Fe*]X*+. DFT calculations on CpFe(dpe)X ([Fe]X) model complexes nicely reproduce the trend in the electrode potentials for the [Fe*]X0/+ couples. Analysis of the theoretical data within the halogen series indicates that the energy of the [Fe]X HOMO does not correlate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, which varies in the order I > Br > Cl > F, but rather depends on the destabilizing electrostatic effect caused by X. This effect varies in the order F > Cl > Br > I. A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*]0/+ electrode potentials was used to investigate the effect of the oxidation state of the complex on the homolytic bond dissociation energy (BDEhom), defined for the processes Fe-X --> Fe* + X* and Fe-X*+ --> Fe*+ + X*. For all X, it was found that a one-electron oxidation leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu(Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxidation. The trends in IR and Fe-Cl bond distances were reproduced in the DFT calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe+ + X- and Fe-X*+ --> Fe2+ + X-. In this case, the oxidation led to bond strengthening in all cases. The computed BDE values have been analyzed within Ziegler's transition state methodology and decomposed into two components, one electrostatic and one covalent, describing the interaction between the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to understand their respective variations upon oxidation.

1-Octene Hydrosilylation Catalyzed by Lanthanide and Yttrium Hydrides and Hydrocarbyls: A Mechanistic Study and the Role of Catalyst Association

Dimeric lanthanide (Tb, Yb, Lu) and yttrium hydrides, {Cp‘2Ln(μ-H)}2, where Cp‘ = t-BuC5H4, and hydrocarbyls {Cp‘2Ln(μ-Me)}2, as well as compounds with different bridging, Cp‘2Ln(μ-H)(μ-Me)LnCp‘2, are efficient and selective catalysts of 1-octene hydrosilylation. Binuclear complexes with Ln(μ-H)2Ln and Ln(μ-H)(μ-Alkyl)Ln bridging fragments were found to be the key intermediates in 1-octene hydrosilylation catalyzed by both the hydrides {Cp‘2Ln(μ-H)}2 (Ln = Y, Tb, Yb, Lu) and the mixed compounds Cp‘2Ln(μ-H)(μ-Me)LnCp‘2 (Ln = Y, Lu) in benzene at 75 °C. Therefore, in this case, the dissociation of the starting dimeric organolanthanide into monomeric species is not required for the catalytic reaction to proceed. Under the conditions employed, the rate law for the hydrosilylation of 1-octene with PhMeSiH2 is VHS ∼ [lanthanide]1[olefin]1[silane]0, suggesting rapid hydride transfer from silane to Ln and the rate-limiting addition of Ln−H to olefin. The hydrosilylation of 1-octene with n-Bu2SiH2 is accompanied by olefin dimerization. In the presence of Ph3SiH, Ph2MeSiH, PhMe2SiH, Et3SiH, and Et3GeH, only the dimerization of 1-octene was observed. Alternatively, the lanthanide and yttrium hydrocarbyls {Cp‘2Ln(μ-Me)}2 (Ln = Y, Tb, Yb, Lu) exhibit higher catalytic activity even at room temperature.The initial rate of the 1-octene hydrosilylation with PhMeSiH2 was found to be V0 ∼ [lanthanide]1/2[olefin]1[silane]0. In this case, the hydrosilylation seems to involve the formation of highly active monomeric species Cp‘2LnH. After several minutes, the hydrosilylation rate sharply decreased because of association of the catalyst.

Methane Loss from Cationic μ-Methyl Dimers Formed via Trityl Borate Activation of Monocyclopentadienyl Ketimide Complexes Cp[(tBu)2CN]Ti(CH3)2 (Cp = C5H5, C5Me5, C5Me4SiMe3)

The reactions of the monocyclopentadienyl titanium dimethyl compounds Cp(L)TiMe2 (L = tBu2CN; Cp = C5H5, 1a; C5Me5, 1b; C5Me4SiMe3, 1c) with the trityl borate activator [Ph3C]+[B(C6F5)4]- are described. Formation of μ-methyl dimers of formula {[Cp(L)TiMe]2(μ-Me)}+[B(C6F5)4]- as a 1:1 mixture of rac and meso diastereomers is observed when 0.5 equiv of [Ph3C]+[B(C6F5)4]- is employed (−25 °C, C6D5Br; Cp = C5H5, rac/meso 2a; C5Me5, rac/meso 2b; C5Me4SiMe3, rac/meso 2c). Dynamic NMR and crossover experiments suggest that the dimers 2 are relatively nonlabile with respect to dissociation, intramolecular methyl group exchange, or diastereomer interconversion. Dimers 2 are observed to undergo methane loss in solution at room temperature, affording the new dimeric compounds 3a−c, {[Cp(L)Ti]2(μ-CH2)(μ-CH3)}+[B(C6F5)4]-. For the less sterically demanding C5H5 ligand, 3a is formed as a mixture of rac/meso diastereomers (7:3), but for the bulkier C5Me5 and C5Me4SiMe3 ligands, the rac isomers of 3b and 3c are formed exclusively. In contrast to μ-methyl dimers 2, in which rac/meso interconversion is not observed, the diastereomers of 3 do undergo interchange, as determined by EXSY spectroscopy, and thus the rac/meso ratios observed are thermodynamic.

Novel cyclopentadienyl silanes and disilanes: synthesis, structure and gas-phase pyrolysis

The new chloro(cyclopentadienyl)silanes Cp′SiH y Cl 3− y (Cp′=Me 4EtC 5, y=1: 1; Cp′=Me 4C 5H, y=1: 2; y=0: 3; Cp′=Me 3C 5H 2, y=1: 4 and pentachloro(cyclopentadienyl)disilanes Cp′Si 2Cl 5 (Cp′=Me 5C 5 5, Me 4EtC 5 6, Me 4C 5H 7, Me 3C 5H 2 8, Me 3SiC 5H 4 9) are synthesized in good yields via metathesis reactions. Treatment of 1– 9 with LiAlH 4 leads under Cl–H exchange to the hydridosilyl compounds Cp′SiH 3 (Cp′=Me 4EtC 5 10, Me 4C 5H 11, Me 3C 5H 2 12) and to the hydridodisilanyl compounds Cp′Si 2H 5 (Cp′=Me 5C 5 13, Me 4EtC 5 14, Me 4C 5H 15, Me 3C 5H 2 16, Me 3SiC 5H 4 17). Complexes 1– 17 are characterized by 1H, 13C, and 29Si-NMR spectroscopy, IR spectroscopy, mass spectrometry and CH-analysis. The structures of 6, 7 and 9 are determined by single-crystal X-ray diffraction analysis. Pyrolysis studies of the cyclopentadienylsilanes 10– 12 and disilanes 13– 17 show their suitability as precursors in the MOCVD process.

Unbridged homo and hetero dinuclear complexes of Group 6 and 8 metals: synthesis, characterization and comparison of X-ray crystallographic data

Several novel compounds of the type Cp/Cp*(CO) 3MM′(CO) n Cp/Cp* (M=Cr, Mo, W; M′=Ru, n=2; M′=Mo, n=3; Cp=C 5H 5, Cp*=C 5Me 5) were synthesized, characterized, their structures compared and preliminary reactivity studies were carried out. The main investigations were focused on the molybdenum and ruthenium containing heterobimetallic compounds Cp(CO) 3MoRu(CO) 2Cp ( 1) as well as Cp(CO) 3MoRu(CO) 2Cp* ( 2) and Cp*(CO) 3MoRu(CO) 2Cp ( 3), the latter two representing a ‘mixed-metal mixed-ligand’ type of complexes with the former ( 1) having identical ligands on both metal cores. For the first time comparison of properties of unbridged bimetallic complexes with molybdenum and ruthenium metal cores was elaborated. The novel Cp(CO) 3MoMo(CO) 3Cp* ( 4) consists of two identical metal cores and of ‘mixed’ ligands. 4 was prepared by treating NaMo(CO) 3Cp with Cp*Mo(CO) 3Br ( 5) after a route for the synthesis of the latter was established. 1 reacted with halides or AlCl 3, respectively, under cleavage of the metal–metal bond to form Cp(CO) n MHal (M=Mo, n=3; M=Ru, n=2; Hal=Br, I, Cl) but was shown to be inert towards substrates such as CO 2, CO, PPh 3 or CS 2 under the chosen conditions. The corresponding anionic species Cp(CO) n M − were obtained when 1 was reacted with alkali metal benzophenyl ketyl.

Synthesis, crystal structure and reactivity of the dihydroborate [Li(Et 2O)][B(C 6F 5) 2(H) 2

The new dihydroborate [Li(Et 2O)][B(C 6F 5) 2(H) 2] ( 1) has been prepared and characterised by single-crystal X-ray diffraction. In the solid state 1 exhibits a one-dimensional chain structure that is a result of intermolecular LiF interactions. Reaction between 1 and one equivalent of Cp 2TiCl or 0.5 equivalents of Cp 2TiCl 2 gives the violet compound Cp 2Ti{(μ-H) 2B(C 6F 5) 2} ( 2), which has also been characterised by single-crystal X-ray diffraction. Addition of two equivalents of PMe 3 to 2 does not result in elimination of a borane:phosphine adduct, but yields the ionic compound [Cp 2Ti(PMe 3) 2][H 2B(C 6F 5) 2] ( 3).

The Mechanism of Methane Elimination in B(C6F5)3-Initiated Monocyclopentadienyl-Ketimide Titanium and Related Olefin Polymerization Catalysts

A new class of monocyclopentadienyl titanium olefin polymerization catalysts and their activation with B(C6F5)3 is reported herein. Dichlorides Cp[tBu(R)CN]TiCl2 {Cp = C5H5, R = tBu (1a); Cp = C5Me5, R = tBu (2a); Cp = C5Me4SiMe3, R = tBu (3a); Cp = C5Me5, R = CH2SiMe3 (4a); Cp = C5Me5, R = Me (5a)} were prepared in 50−92% yield from CpTiCl3 and tBu(R)CNLi. Analogous dimethyl compounds 1b−5b were prepare via methylation of dichlorides a using MeMgBr in 89−92% yield. Dimethyl compound 6b (L = C5Me5, R = CH(SiMe3)2) was prepared directly from Cp*TiMe3 and tBu[(Me3Si)2CH]CNH in 40% yield. Dynamic 1H NMR studies showed that the ketimide ligands in compounds b rotate rapidly about Ti−N on the NMR time scale, with a ΔG‡ of 9.6(6) kcal mol-1 or less. The mixed alkyl compound Cp*[tBu(R)CN]Ti(CH3)CH2SiMe3 {R = tBu (7)} was prepared via alkylation of the corresponding methyl chloride derivative with BrMgCH2SiMe3. When treated with B(C6F5)3, compounds 1b−6b are rapidly converted into the ion pairs {Cp[tBu(R)CN]TiCH3...

Anionic dichlorolanthanocene compounds. X-ray crystal structures of [{(Me 3Si) 2C 5H 3} 2LnCl 2][Li(THF) 4] (Ln=Er, Yb) and {(Me 3Si) 2C 5H 3} 2Yb(μ-Cl) 2Li(THF) 2

Treatment of LnCl 3 with two equivalents of Cp″Li (Cp″=(Me 3Si) 2C 5H 3) in THF gave, after crystallization from a THF– n-hexane solution at −30°C, ionic compounds [Cp″ 2LnCl 2][Li(THF) 4] (Ln=Er ( 1), Yb ( 2)) in good yield. Recrystallization of 2 from a toluene solution afforded an ‘ate’ compound Cp″ 2Yb(μ-Cl) 2Li(THF) 2 ( 3). Interestingly, recrystallization of 3 from a mixed solvent of THF– n-hexane did not give compound 2, and only 3 was isolated, indicating that the dissociation of THF may be an irreversible process or 3 is a thermodynamically more stable compound. All three compounds were fully characterized by various spectroscopic date and single-crystal X-ray analyses. Compounds 1 and 2 are the rare structurally characterized examples of an anionic dichlorolanthanocene stabilized by an alkali metal complex ion.

Unexpected Reactivity of Cp*Ga toward Fe−Cl Bonds: Synthesis and Structure of Cp*(Cp*Ga)2FeGaCl2·THF and of Cp*(CO)2FeGa(η2-Cp*)Cl

The complex Cp*(Cp*Ga)2FeGaCl2·THF (1) was obtained in the reaction of FeCl2 with 3 equiv of η5-pentamethylcyclopentadienylgallium (Cp*Ga) in THF. For the formation of 1, the following unprecedented reaction sequence is proposed: (I) coordination of Cp*Ga to the iron center; (II) insertion of Cp*Ga into the Fe−Cl bond; (III) Cp*/Cl-[1,2]2-exchange; (IV) σ−π rearrangement of the Cp* ligand. A comparable insertion step could be observed in the reaction of Cp*Ga with Cp*(CO)2FeCl where the compound Cp*(CO)2FeGa(η2-Cp*)Cl (2) was formed. 1 and 2 are characterized by NMR, IR, and X-ray crystallographic data and by elemental analysis.

Electrochemical oxidation of Cp*Ru(η5-C6H9O) under argon and oxygen atmospheres. Generation of [Cp*Ru(CH3CN)3]+PF6− and Cp*RuO2(η3-C6H9O) complexes

The anodic oxidation of Cp*Ru(η5-C6H9O) was performed in acetonitrile on a glassy carbon electrode. The electrochemical behavior is strongly dependent on the presence of both argon and oxygen in acetonitrile solutions. Under an argon atmosphere an acetonitrile coordinated compound [Cp*Ru(CH3CN)3]+PF6− was obtained. This compound is generated following a mechanism involving cleavage of the mesityl oxide ligand. In contrast, under an oxygen atmosphere, a novel peroxo compound Cp*Ru(O2)(η3-C6H9O) was obtained in 70% yield by oxidative addition of molecular oxygen. A slow chemical reaction between [Cp*Ru(CH3CN)3]+PF6− and Cp*Ru(O2)(η3-C6H9O) leads to the activation of a Cp* ligand giving an aldehyde derivative (η5-C5Me4CHO)Ru(η5-C6H9O).

Reaction of Decamethylsilicocene with Group 13 Element Halides: Insertions, Rearrangements, and Eliminations

In the reaction of decamethylsilicocene (1; (Me5C5)2Si) with halides or organohalides of trivalent boron, aluminum, gallium, and indium, quite different and sometimes very complex pathways are observed which include adduct formation, 1,2-halide or -alkyl shifts, 1,2-dyotropic rearrangements, and reductive elimination and oxidative addition reactions. Cp*BCl2 (Cp* = pentamethylcyclopentadienyl), BCl3, and BBr3 reacted with 1 to form the pentacarba-nido-hexaboronium salts [Cp*SiCl2BCp*]+[Cp*BCl3]- (3a) and [Cp*SiX2BCp*]+[BX4]- (3b, X = Cl; 4, X = Br). A second product (5b) of composition Cp*3Si2Br4B with an arachno-cluster framework was isolated from the reaction with BBr3. With AlCl3 and AlBr3 metathesis reactions gave the ionic compounds [Cp*2Al]+[AlX4]- (6a,b, X = Cl, Br), respectively. The formation of 6a,b is the result of a Lewis-base-catalyzed dismutation of Cp*AlX2 in which 1 is the base, as proved by separate experiments. The compound Cp*Al(Me)Cl (7) was formed in the reaction of 1 with Me2AlCl. Silicocene 1 functioned as a dehalogenating agent in its reactions with GaCl3, GaBr3, Cp*GaBr2, InCl3, and InBr3, giving the corresponding monovalent, metastable (“GaCl”, “GaBr”) or stable (Cp*Ga, InCl, InBr) species. All new compounds were characterized by NMR (1H, 13C, 11B, 29Si) and mass spectrometry. The solid-state structures of 3a and 5b were determined by X-ray diffraction analysis.

Half-Sandwich Group 4 Metal Siloxy and Silsesquioxane Complexes: Soluble Model Systems for Silica-Grafted Olefin Polymerization Catalysts

The cuboctameric hydroxysilsesquioxane (c-C5H9)7Si8O12(OH) (2), obtained after hydrolysis of (c-C5H9)7Si8O12Cl (1), and triphenylsilanol have been applied as model supports for silica-grafted olefin polymerization catalysts. The ligands were introduced on group 4 metals by either chloride metathesis or protonolysis. Treatment of Cp‘ ‘MCl3 (M = Ti, Zr; Cp‘ ‘ = 1,3-C5H3(SiMe3)2) with silsesquioxane and siloxylithium or -thallium salts, [(c-C5H9)7Si8O13]M‘ (M‘ = Tl (3), Li (4), Li·TMEDA (5)) or Ph3SiOTl gave either the dichloride complexes Cp‘ ‘[(c-C5H9)7Si8O13]MCl2 (M = Ti (6a), Zr (7a)) and Cp‘ ‘[Ph3SiO]TiCl2 (8a) or the monochloride species Cp‘ ‘[(c-C5H9)7Si8O13]2MCl (M = Ti (6b), Zr (7b)) and Cp‘ ‘[Ph3SiO]2MCl (M = Ti (8b), Zr (9)). Similarly, protonolysis of Cp‘ ‘MR3 with the silanols 2 and Ph3SiOH yielded the corresponding silsesquioxane bis(alkyl) complexes Cp‘ ‘[(c-C5H9)7Si8O13]TiR2 (R = CH2Ph (10a), Me (10b)) and triphenylsiloxy bis(alkyl) compounds Cp‘ ‘[Ph3SiO]MR2 (M = Ti, R = CH2Ph (11a), Me (11b...

Bis(cyclopentadienyl)yttrium Complexes of the Ligand [N(QPPh(2))(2)](-) (Q = S, Se): Synthesis, Structure, and NMR Properties of Cp(2)Y[eta(3)-N(QPPh(2))(2)

The compounds Cp(2)Y[eta(3)-N(QPPh(2))(2)] (Q = S (1), Se (2)) have been synthesized in good yield from the protonolysis reaction between Cp(3)Y and HN(QPPh(2))(2) in tetrahydrofuran. In both compounds, the [N(QPPh(2))(2)](-) ligand is bound eta(3) to the Y center which, in 1, represents the first example of that mode of binding for the sulfur-containing ligand. The Y atom is also coordinated to two (C(5)H(5))(-) ligands and so is formally 9-coordinate. Both 1 and 2 are stable in inert environments for prolonged periods of time. Each is soluble in THF and CH(2)Cl(2). (1)H, (31)P, (77)Se, and (89)Y NMR data were collected to lend insight into the solution properties of these molecules. Crystallographic data for 1 (-120 degrees C): C(34)H(30)NP(2)S(2)Y, triclinic, P&onemacr;, a = 9.685(5) Å, b = 12.176(6) Å, c = 13.978(7) Å, alpha = 87.382(9) degrees, beta = 87.358(9) degrees, gamma = 68.689(9) degrees, V = 1533(1) Å(3), Z = 2, and R(1)(F) = 0.047 for the 4023 reflections with I > 2sigma(I). Crystallographic data for 2 (-120 degrees C): C(34)H(30)NP(2)Se(2)Y, triclinic, P&onemacr;, a = 9.745(5) Å, b = 12.222(6) Å, c = 13.930(7) Å, alpha = 88.024(9) degrees, beta = 87.380(9) degrees, gamma = 69.137(9) degrees, V = 1548(1) Å(3), Z = 2, and R(1)(F) = 0.056 for the 4324 reflections with I > 2sigma(I).

Structure and reactivity of four-membered titanacycles—theoretical studies

The equilibrium geometries of a series of four-membered titanacycles with exocyclic methylene group Cl 2 Ti–X–Y–C CH 2 (X: CH 2, CH, O, S, N, P; Y: CH 2, CH, CO, CCH 2, N–CH 3) and two derivatives without exocyclic methylene group Cl 2 Ti–X–CH 2–C H 2 (X: CH 2, O) have been calculated at the HF level of theory by using an effective core potential basis set. The geometries were compared with data from X-ray structures for the corresponding compound Cp* 2 Ti–X–Y–C CH 2. The structures of the planar four-membered titanacycles are well represented by the model complexes. The chloride ligands are good theoretical substitutes for the bent metallocene system with cyclic ligand systems in the intersecting plane between the Cp* ligands. The calculated metal–carbon bond distances are found to be shorter than in the X-ray structures of the real molecules. Total energies are calculated at the MP2 level and are used to predict the reactivity of the compounds. Titanacyclobutanes with an exocyclic methylene group are more stable than titanacyclobutanes without this group. Oxatitanacyclobutanes are subject to cycloreversion under formation of Cl 2TiO and allene. Azatitanacyclobutenes undergo spontaneous ring opening reactions. These statements are further supported by the analysis of the molecular orbitals of some selected derivatives.

The first structurally characterized coordination compound containing direct Al–Cr bonding: Cp*Al–Cr(CO) 5

In order to investigate the bonding of the carbene-like ligand AlCp* towards a transition metal, the compound Cp*AlCr(CO) 5 has been prepared starting from Cr(CO) 5COT and AlCp*. With the help of its crystal structure, some IR spectroscopic data and ab initio calculations, the Al–Cr bonding is discussed. The strong reducing ability of AlCp* leads to a shift in electron density towards the chromium atom and finally to the CO ligands.

Synthesis, X-ray crystal structure, and reactivity of the monomeric dithio- o-carboranyl iridium complex [Ir(η 5-C 5Me 5)(η 2-S 2C 2B 10H 10)

The reaction between the pentamethylcyclopentadienyl complex [Cp*IrCl 2] 2 and the dilithium dithio- o-carborane salt, Li 2[S 2C 2B 10H 10] ( 1), in THF affords the o-carborane-substituted compound Cp*Ir(S 2C 2B 10H 10) ( 2) in good yield. The complex [Cp*Ir(S 2C 2B 10H 10)] ( 2) has been isolated and characterized by X-ray diffraction analysis which confirmed a coordinatively unsaturated mononuclear complex. Subsequent addition reactions of complex 2 with the nucleophiles PMe 3, CNBu t, and CO yield the corresponding complexes [Cp*Ir(S 2C 2B 10H 10)(PMe 3)] ( 3), [Cp*Ir(S 2C 2B 10H 10)(CNBu t)] ( 4), and [Cp*Ir(S 2C 2B 10H 10)(CO)] ( 5). All of these new compounds have been isolated in high yield and characterized by IR and NMR spectroscopy. The solid state structures of 3 and 5 were determined by single-crystal X-ray diffraction analysis, exhibiting a characteristic three-legged ‘piano-stool’ arrangement around the iridium metal center.

Structural and functional characterization of seven spermicidal vanadium(IV) complexes: potentiation of activity by methyl substitution on the cyclopentadienyl rings

In a systematic effort to identify and develop effective vanadocene(IV) complexes as a new class of contraceptive agents, the effect of methyl substitution in the cyclopentadienyl rings of Cp x 2VCl 2 on their spermicidal activity has been examined. The spermicidal activities of compounds Cp x 2VCl 2 [Cp x =Me 5Cp (Cp *) ( 1), Cp ( 3), MeCp (Cp′) ( 5)], as well as two of their corresponding vanadium(V) oxidation products Cp *V(O)Cl 2 ( 2) and CpV(O)Cl 2 ( 4), were examined by computer-assisted sperm analysis (CASA). These analyses have established that penta-substitution of the Cp ring by electron-donating methyl groups augments the spermicidal activity 10-fold. The corresponding V(V) oxo compounds, Cp *V(O)Cl 2 ( 2) and CpV(O)Cl 2 ( 4), tested under identical conditions did not show as effective spermicidal activity even though these complexes have a pseudo-tetrahedral geometry similar to the active vanadocene(IV) dichlorides. Two pseudo-octahedral V(IV) complexes with tris-pyrazolyl borate as ligand, (HBpz 3)V(O)Cl ·DMF ( 6) and (HB(3,5-Me 2pz) 3)V(O)Cl ·DMF ( 7), were also found to exhibit potent spermicidal activity. Although some vanadium(IV) complexes may immobilize sperm due to the generation of · OH radicals, the lack of spermicidal activity of VOSO 4 which generates · OH radicals, and the potent spermicidal activity of [Cp 2V(acac)][O 3SCF 3] ( 8), and [Cp 2V(DeDtc)][O 3SCF 3] ( 9) which do not generate · OH radicals, indicate that · OH radical mediated reactions may not be essential for the spermicidal activity of vanadium(IV) complexes.

Ethylene and propylene polymerization by cationic monocyclopentadienyl titanium catalysts containing the weakly coordinating anion [B(C 6F 5) 4] −

The compounds Cp*TiMe 2E (Cp*=η 5-C 5Me 5; E=Me, C 6F 5, OC 6F 5, Cl) react with trityl tetrakis(perfluorophenyl)borate, [Ph 3C][B(C 6F 5) 4], to form the thermally unstable dititanium complexes [(Cp*TiMeE) 2(μ-Me)][B(C 6F 5) 4], all of which behave as sources of the highly electrophilic species [Cp*TiMeE] +. An investigation of the activities of these [B(C 6F 5) 4] − salts as ethylene and propylene polymerization catalysts shows that they are more active than the analogous compounds Cp*TiMeE(μ-Me)B(C 6F 5) 3, as anticipated since [B(C 6F 5) 4] − is a poorer ligand than is [BMe(C 6F 5) 3] −. However, contrary to current perceived wisdom, substitution of a methyl ligand of [Cp*TiMe 2] + by the more electron withdrawing C 6F 5, OC 6F 5 and Cl ligands in these monocyclopentadienyl systems does not generally result in catalysts exhibiting lower activities and producing lower molecular weight polymers. An EPR study of the Cp*TiMe 3/[Ph 3C][B(C 6F 5) 4] system in chlorobenzene at room temperature indicates that <0.01% of the titanium is present occasionally during polymerization as a complex of titanium(III), suggesting that a contribution to the catalytic processes by titanium(III) species is unlikely.

Reaction of cyclopentadienyl zirconium derivatives with sterically encumbered arylaluminum hydrides: X-ray crystal structure of (η 5-C 5H 5) 2(H)Zr( μ2-H) 2Al(H)C 6H 2-2,4,6-Bu t3

The novel reactions between Cp 2ZrMe 2 (Cp=η 5-C 5H 5) and (Mes*AlH) 2 (Mes*=C 6H 2-2,4,6-Bu t 3) to afford Cp 2(H)Zr(μ 2-H) 2Al(Me)Mes* ( 1) and between Cp 2Zr(Cl)H and [Mes*AlH 3Li(THF) 2] 2 to give Cp 2(H)Zr(μ 2-H) 2Al(H)Mes* ( 2) are described. The X-ray crystal structure and NMR spectroscopy of 1 indicate that, in the crystalline product, a methyl group has been transferred from zirconium to aluminum, and the metals have been connected by two hydrogen bridges. The compounds Cp 2ZrH 2 and Mes*AlMe 2 were also obtained as reaction products in the synthesis of 1. The aluminum center is four-coordinate in contrast to the five- or six-coordination observed in previously published complexes. Compound 2, which is believed to have a very similar structure to that of 1, was obtained in moderate yield as a colorless crystalline material.

Synthesis of Hydride and Alkyl Compounds Containing the Cp*Os(NO) Fragment. Crystal Structure of [Cp*Os(μ-NO)]2

Treatment of the pentamethylcyclopentadienyl (Cp*) compound Cp*Os(NO)Br2 (1) with NaBH4 yields the dihydride Cp*Os(NO)H2. The dihydride loses H2 over several days in solution to form [Cp*Os(μ-NO)]2; this dinuclear compound can also be formed directly by reduction of 1 with zinc powder. The X-ray crystal structure of [Cp*Os(μ-NO)]2 shows that nitrosyl ligands are bridging and that the Os−Os distance is 2.539(1) Å. Compound 1 reacts with MgR2 to form the monoalkylated products Cp*Os(NO)RBr, where R = Me, CH2SiMe3, Ph, or o-Tol. The slow ligand substitution kinetics of this reaction prevent the formation of dialkyls of stoichiometry Cp*Os(NO)R2, even after long reaction times or when a large excess of alkylating agent is used. Dialkyls such as Cp*Os(NO)Me2 can, however, be obtained by treatment of Cp*Os(NO)MeBr with silver trifluoromethanesulfonate followed by addition of dimethylmagnesium.