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A scanning tunneling microscopy (STM) study of an unsaturated long-chain-bridged diferrocene, Fc(CH2)6HCCH(CH2)6Fc (u-diFc, Fc ferrocenyl), on Ag(110) reveals three ordered molecular phases, , , and ‘. It is found that the and the phase are predominant on the ordered structures with the Fc group neighboring to the center of the hydrocarbon chain in the former phase but with the Fc group arranging into a row-like structure in the latter phase. High-resolution STM images also show that the ‘ structure is analogous to the phase but is not well-defined and exhibits the zigzag shape because of the variable distance between two neighboring molecular rows and molecular arrangement shifted along the molecular row. Interestingly, the irreversible phase transition from to ‘ has been observed in the recorded regions by using the real-time STM. On the basis of the experimental results, three molecular models are provided, and the stability of these three structures has been investigated as well.

A series of C1-bridged Cp/amido group 4 metal complexes [(“CpCN”)MX2] was prepared. The starting compound 6-tert-butylfulvene (7) was reacted with LiNHR (R = o-anisyl (a), p-anisyl (b), phenyl (c), or tert-butyl (d)) to yield the Li(C5H4-CH(tBu)-NHR) compounds (8). Subsequent deprotonation with n-butyllithium, followed by transmetalation with Cl2Ti(NMe2)2, yielded the (“CpCN”)Ti(NR2)2 complexes 10 (a−d). A second synthetic series started from 6-(dimethylamino)fulvene (11). The reaction with the LiNHR reagents led to addition followed by HNMe2 elimination to yield the respective imino-substituted cyclopentadienides Li(C5H4-CH&dbd;NR) (13). Addition of methyllithium to 13 gave the (Li⁺)2(C5H4-CHMe-NR)^2-, (“CpCN”)^2-, ligands (14). Transmetalation with Cl2Ti(NMe2)2 subsequently yielded the corresponding μ-CHMe-bridged Cp/amido group 4 complexes 15 (a−c). Treatment with Me2SiCl2 cleanly converted the (“CpCN”)M(NR2)2 complexes to the respective (“CpCN”)MCl2 systems [3a−d, 4a−c (Ti), 5a (Zr)]. These were transformed to the (“CpCN”)M(CH3)2 complexes [μ-CHCMe3:  17a−d (Ti), 19a (Zr), μ-CHMe:  18a−c (Ti)] and the (s-cis-supine-η⁴-butadiene)(“CpCN”)Ti systems 21a−d and 22b,c. With respect to X-ray structure analyses the (CpCN)MX2 systems must be regarded as more “constrained” than their (“CpSiN”)MX2 analogues. The (“CpCN”)MCl2 and (“CpCN”)M(CH3)2 complexes were used as catalysts for ethene homopolymerization and ethene/1-octene copolymerization. All systems gave active catalysts, but these were quite different in their catalytic performance when compared with the usually applied (“CpSiN”)TiX2-derived catalysts. The (“CpCN”)M catalyst activities and selectivities were very dependent on the specific activator components employed.

The zirconacyclocumulenes 1a (R = CH3) and 1b (R = Ph) were prepared by isomerization of bis(propynyl)ZrCp2 or bis(phenylethynyl)ZrCp2, respectively, catalyzed by B(C6F5)3 (6 mol %). The compounds 1 added PCl3, PhPCl2, or Ph2PCl with cleavage of only one zirconium−carbon single bond. Chloride was added to zirconium and the remaining −PCl2, −P(Ph)Cl, or −PPh2 moiety attached at the distal β-carbon atom of the strained ZrC4R2 metallacycle to yield the respective ring-opened Zr/P-substituted enyne systems (5−7). The complexes 7a and 7b (both with −PPh2) and 6b (with −P(Ph)Cl) were characterized by X-ray diffraction.

Ruthenium carbene complexes featuring the bulky tridentate N,N‘-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamido pincer-type ligand and different stabilizing phosphine ligands were synthesized and characterized. Preliminary results for the application to ring-closing metathesis show low activity for the cyclization of 1,7-octadiene with benzylidene(triphenylphosphine)[N,N‘-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamido-κ³O,N,O/O,N,N]ruthenium(II) (4a,b). Benzylidene(tricyclohexylphosphine)[N,N‘-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamido-κ³O,N,O]ruthenium(II) (5) successfully transformed 1,7-octadiene to cyclohexene and ethylene in good yields at room temperature or 80 °C.

The reaction of (C5H4−PPh2)2Zr(CH3)2 (5a) with HRh(CO)(PPh3)3 (6) leads to methane evolution and the formation of the early−late heterobimetallic complex [(μ-C5H4−PPh2)2(H3C−Zr−Rh(CO)PPh3)] (7a), which was characterized by X-ray diffraction (d(Zr−Rh) = 2.863(1) Å). The analogous dialkylphosphino-substituted complexes (C5H4−PR2)2Zr(CH3)2 (5b, R = isopropyl; 5c, R = cyclohexyl) react with the hydrido−rhodium reagent 6 to initially form the analogous bimetallic (Zr−Rh) complexes 7b,c, respectively. However, these complexes are not stable at ambient temperature in toluene solution, due to an equilibration with the products [(μ-C5H4−PR2)2(μ-CH3−Zr−Rh(PPh3))] (11) and [(μ-C5H4−PR2)2(H3C−Zr−Rh(CO)2)] (10). Treatment of the initially formed complexes 7b,c with carbon monoxide rapidly leads to the formation of the H3C(Zr−Rh)−dicarbonyl complexes 10b,c. The product 10c was characterized by X-ray crystal structure analysis. The systems 7b,c are not stable in dichloromethane. They react readily with the solvent by abstraction of two chlorine atoms to yield the compounds [(μ-C5H4−PR2)2(μ-O&dbd;C(CH3))(Cl−Zr−Rh−Cl)] (12b,c). Both products were characterized by X-ray diffraction. A cooperative action of the involved metals is assumed to account for the observed reactions at the intact heterobimetallic Zr−Rh framework.

The Group IV bent metallocenes Cp2M (M = Ti, Zr, Hf) were involved in carbene-related chemistry in various ways. Examples from four different areas are used to illustrate this. In situ generated Cp2M: species exhibit some carbene-like character. They add to olefins, and their addition products can incorporate additional unsaturated organic reagents, e.g., alkyne, to form five-membered metallacycles. The high oxophilicity of the Group IV metals helps the addition of alkene-, aryne-, η2-ketone-, η2-aldehyde-, or butadiene-containing reagents to a great variety of metal carbonyl compounds to form the Fischer-type carbene complexes. The resulting zirconoxycarbene complexes have found some application in organometallic chemistry and in organic synthesis. Reactive [Cp2MIVR]+cations can be stabilized by the addition of the Arduengo carbenes that serve as bulky two-electron donor ligands. First examples were structurally characterized. Dialkylimidazol-2-ylidenes also add to the electrophilic MX4compounds yielding stable trans-bis(imidazol-2-ylidene)MX4systems. Several examples are presented where ruthenium carbene complexes are used for carrying out catalytic olefin metathesis reactions at pendant olefinic substituents at the bent metallocenes. These reactions have led to the formation of novel bimetallic metallocene systems, as well as to new ansa-metallocenes. These catalytic reactions have helped in the current efforts to develop a functional group chemistry at the sensitive Group IV bent metallocene frameworks.

Phosphide [PHR¹]Li (R¹ = cyclohexyl, phenyl) addition to the fulvenes (C5H4)CMe2 (1) and (C5H4)CHCMe3 (2) yields the corresponding phosphinoalkyl-substituted cyclopentadienides. Subsequent deprotonation at phosphorus with LDA followed by transmetalation of the resulting [(C5H4)CR²R³PR¹]Li2 dianion equivalents to Cl2Ti(NMe2)2 or Cl2Zr(NEt2)2(THF)2 yields the corresponding [“CpCPR¹”MX2] “constrained-geometry” systems (eight examples, 11−18). These systems contain a chiral phosphorus center that is characterized by a low inversion barrier (ΔG^&thermod;inv ranging from ca. 7.5 to 10.0 kcal mol^-1). The structural and some spectroscopic features of the (CpCPR)Ti and Zr complexes were investigated by DFT calculations. Treatment with a large excess of methylalumoxane gave ethene/1-octene copolymerization catalysts. The [(C5H4)CMe2PR¹]ZrX2 systems gave the most active catalysts in this series.

Treatment of the dimethylamino-[3]ferrocenophane 1a with B(C6F5)3 results in hydride abstraction at the −CH(NMe2) moiety in α-position to the ferrocene nucleus to yield the corresponding cation 4a. The X-ray crystal structure analysis of the salt 4a·[HO{B(C6F5)3}2^-] has revealed a residual Fe−Cα interaction even in this case of a strongly substituent-stabilized α-ferrocenyl carbenium ion. A DFT calculation of a series of derivatives containing nitrogen substituents of a decreasing ability for carbenium ion stabilization (4a, −NMe2; 4b, −pyrrolyl; 4c, −triazolyl) revealed a markedly increased Fe−C interaction with decreasing C⁺ stabilization by the hetarene substituents.

Treatment of 1,1‘-diacetylferrocene (1) with dimethylamine, catalyzed by titanium tetrachloride, proceeded in a Mannich-type C−C coupling reaction to yield the unsaturated [3]ferrocenophane 2. Subsequent catalytic hydrogenation gave the saturated [3]ferrocenophane derivative [(C5H4CHMeCH2CH(NMe2)C5H4)Fe] (trans-3). Directed Cp metalation of trans-3 was carried out by treatment with n-BuLi, followed by reaction with ClPPh2 to yield the “ortho-phosphorylated” P,N-[3]ferrocenophane chelate ligand 4. Its treatment with HPPh2 in acetic acid resulted in a clean substitution of the −NMe2 group by −PPh2 with overall stereochemical retention to yield the P,P-[3]ferrocenophane ligand 5. The stereochemical pathway of such substitution reactions at the [3]ferrocenophane framework, which proceed cleanly by a double-inversion route with participation of the metal center, was modeled by computational chemistry and its reactive cationic intermediate 14 characterized by a DFT calculation. Reaction of 4 with (cod)PdCl2 gave the chelate complex 6. The analogous reaction of 5 with (cod)PdCl2 furnished the P,P-[3]ferrocenophane−PdCl2 chelate complex 7. Both the complexes 6 and 7 and the ligand 5 were characterized by X-ray diffraction. Treatment of 7 with AgBF4 in dichloromethane or alternatively of 5 with palladium acetate, followed by BF3 addition, gave very active catalyst systems for alternating carbon monoxide/ethene copolymerization.

Treatment of 6-(dimethylamino)-6-methylpentafulvene (2a) with methyllithium in ether at −40 °C leads to CH3 addition to the C6-fulvene carbon atom to yield the [C5H4−CMe2−NMe2]Li reagent 3a, whereas treatment of 2a with MeLi in THF at −78 °C results in deprotonation with formation of [C5H4−C(&dbd;CH2)−NMe2]Li (4a). In contrast, treatment of 6-diethylamino-6-methylfulvene (2b) with methyllithium under any of these conditions leads only to H⁺-abstraction to give [C5H4−C(&dbd;CH2)−NEt2]Li (4b). Treatment of the reagents 4 with CpZrCl3 yields the enamino-Cp-substituted bent metallocenes [η⁵-C5H4−C(&dbd;CH2)−NR2]CpZrCl2 5a (R = CH3) and 5b (R = C2H5), respectively. Analogous treatment of 4b with zirconium tetrachloride gave the doubly functionalized zirconocene complex [η⁵-C5H4−C(&dbd;CH2)−NEt2]2ZrCl2 (6b). The complexes 5a, 5b, and 6b were characterized by X-ray diffraction. Treatment of 3a with CpZrCl3 gave [η⁵-C5H4−CMe2−NMe2]CpZrCl2 (8a); the analogous reaction with ZrCl4 gave [η⁵-C5H4−CMe2−NMe2]2ZrCl2 (8b). Both complexes 8 reacted with (butadiene)magnesium with replacement of both chloride ligands at zirconium. The probably generated (η⁴-butadiene)zirconocene complexes were, however, not stable under the applied reaction conditions, but reacted further by C−H activation at the N−CH3 group with intramolecular hydrogen transfer to the butadiene ligand to yield the cyclometalated products [η⁵-C5H4−CMe2− N(CH3)−CH2−]Zr(L)(CH2CHCHCH3) 13a (L = Cp) and 13b (L = [η⁵-C5H4−CMe2−NMe2]), respectively. Complex 5a reacts analogously with (butadiene)magnesium to yield the C−H activation product [η⁵-C5H4−C(&dbd;CH2)−N(CH3)−CH2−]ZrCp(CH2CHCHCH3) 14. The 1-diethylaminoethenyl-Cp-substituted systems 5b and 6b react differently with (butadiene)magnesium. They form the stable η⁴-butadiene complexes s-trans-15 and s-cis-15A/B (from 5b) and s-trans-17 and s-cis-17 (from 6b), respectively. [N]CH activation was not observed in these cases. Treatment of the cyclometalated products 13a, 14, and 15 with B(C6F5)3 gave active homogeneous Ziegler−Natta catalysts for ethene polymerization, whereas the systems 13b/B(C6F5)3 and 17/B(C6F5)3 were close to inactive under comparable conditions.

Double deprotonation of [(C5Me4H)−SiMe2−PH(cyclohexyl)] (4) was achieved by treatment with n-butyllithium in THF/pentane to afford the dianionic ligand [(C5Me4)-SiMe2−P(cyclohexyl)]^2- as its dilithio salt (5). The reagent 5 was transmetalated by subsequent treatment with [Cl2Ti(NMe2)2] (6a) to yield the dimethylsilanediyl-bridged Cp*/phosphido titanium complex [(C5Me4)−SiMe2−PCy]Ti(NMe2)2 (7a; 67% isolated yield). The analogous reaction of 5 with [Cl2Zr(NEt2)2(THF)2] (6b) gave [(C5Me4)−SiMe2−PCy]Zr(NEt2)2 (7b; isolated in 59% yield). Single crystals of the complexes 7a,b were obtained from pentane. The X-ray crystal structure analyses revealed distorted “constrained geometry” group 4 metal complex structures due to the presence of a chiral, nonplanar coordination geometry at the phosphorus atom. Complexes 7a,b both give active homogeneous Ziegler−Natta catalysts for ethene/1-octene copolymerization upon activation with excess methylalumoxane.

Treatment of [Cp2TiCH3(thf)⁺] (5, with [BPh4^-] anion) with 1,3-diisopropylimidazol-2-ylidene (4) at ambient temperature resulted in a rapid displacement of the thf ligand by the stable heterocyclic carbene to yield the Arduengo carbene methyltitanocene cation complex 6a (>90% isolated). The X-ray crystal structure analysis of 6a showed that the heteroatom-stabilized carbene ligand [d(Ti−C(carbene) = 2.289(2) Å, d(Ti−CH3) = 2.178(3) Å] was bonded to titanium in an orientation where the imidazol-2-ylidene ring lies in the major σ-ligand plane of the bent metallocene moiety. A DFT calculation of 6a and a series of related model compounds has revealed that the Arduengo carbene serves as a pure σ-donor ligand to the titanocene moiety. The observed favored “in-plane” orientation of the ligand is steric in origin. Consequently, complex 6a attains an analogous Cs-symmetric structure in solution, featuring symmetry-equivalent Cp rings and a pair of diastereotopic isopropyl substituents as well as chemically differentiated imidazol-2-ylidene C⁴H&dbd;C⁵H groups. The reaction of the ion pair [(Cp2ZrCH3)⁺(CH3B(C6F5)3^-] (7) with 4 gave the analogous Arduengo carbene zirconocene cation complex 6b (>95% isolated, with [CH3B(C6F5)3^-] anion).

Addition of methyllithium or p-tolyllithium to the C-6 carbon atom of the lithium (N-phenylformimidoyl)cyclopentadienide reagent 5 resulted in the formation of the dianionic sp³-C1-alkylidene-bridged Cp/amido ligand systems [C5H4−CHR−NPh]^2- (6a, R = CH3; 6b, R = p-tolyl; each with Li⁺ cations). Transmetalation of 6a from lithium to zirconium was achieved by treatment with ZrCl4(THF)2 (7a) to yield the metallacyclic sp³-C1-linked spiro constrained geometry complex (η⁵-C5H4−CHMe−NPh-κN)2Zr (2a). Treatment of 6a with Cl2Ti(NMe2)2 (7c) gave the alkylidene-bridged Cp/amido titanium complex (η⁵-C5H4−CHMe−NPh-κN)Zr(NMe2)2 (2b). Analogous treatment of Cl2Zr(NEt2)2(THF)2 (7b) with 6a or 6b furnished the related (CpCN)Zr complexes (C5H4−CHR−NPh)Zr(NEt2)2 (2c, R = CH3; 2d, R = p-tolyl) each in ≥70% yield. The complexes 2a and 2b were both characterized by X-ray crystal structure analyses. The observation that both exhibit Cp(centroid)−M−N angles that are by ca. 10° smaller than those of their respective dimethylsilanediyl-bridged analogues renders the CpCN group metal complexes as more constrained than their conventional (Cp*SiN)M analogues. Lithium 4-methylanilide adds to the C-6 carbon atom of the non-CH-acidic pentafulvene (C5H4)&dbd;CHCMe3 (13a) to yield Li[C5H4−CH(CMe3)(NH-p-tolyl)] (14a), which was subsequently NH-deprotonated by treatment with LDA to yield the dianionic reagent [C5H4−CH(CMe3)(N-p-tolyl)]^2- (6c, as the dilithium compound). Analogous addition of lithium tert-butylamide to 13a, followed by NH-deprotonation by treatment with tert-butyllithium, yielded Li2[C5H4−CH(NCMe3)CMe3] (6d). Lithium tert-butylamide added to 1,2,3,4-tetramethylpentafulvene (13b) to yield Li[C5Me4−CH2−NH(CMe3)] (14c), which was subsequently NH-deprotonated by treatment with tert-butyllithium to yield Li2[C5Me4−CH2N(CMe3)] (6e). Transmetalation of 6c by treatment with Cl2Ti(NMe2)2 (7c) or Cl2Zr(NEt2)2(THF)2 (7b) cleanly gave the respective CpCN group 4 metal complexes [C5H4−CH(CMe3)(N−p-tolyl)]M(NR2)2 (2e, M = Ti; 2f, M = Zr). Similarly, the sp³-C1-bridged “constrained geometry” system [C5H4−CH(CMe3)(NCMe3)]Zr(NEt2)2 (2g) was obtained in good yield (70%) from the reaction of 6d with 7b, and treatment of 6e with 7b furnished the (Cp*CN)Zr complex [C5Me4−CH2−N(CMe3)]Zr(NEt2)2 (2h, 65% isolated). Activation of the (CpCN)M(NR2)2 systems by treatment with a large excess of methylalumoxane in toluene solution gave homogeneous Ziegler−Natta catalysts for the polymerization of ethylene (carried out at 60 °C) and the copolymerization of ethene with 1-octene (at 90 °C).

Treatment of 2-indanone with 2-lithio-5-methylfuran, followed by hydrolysis, dehydration, deprotonation with n-butyllithium, and transmetalation, gave bis[2-(5-methyl-2-furyl)indenyl]zirconium dichloride (6a). The dynamic 600 MHz ¹H NMR spectroscopic analysis in the temperature range between 253 and 128 K revealed the presence of two metallocene conformational isomers in a 60:40 ratio. It is likely that this observed feature just originates from freezing out the indenyl−furyl rotation. The corresponding activation barrier of 6a was determined at ΔG^&thermod;rot = 7.0 ± 0.4 kcal mol^-1 by a line shape analysis. A series of analogous 5-methyl-2-furyl- and 5-methyl-2-thienyl-substituted bis(indenyl)zirconium dibromides and dichlorides show analogous conformational behavior and almost identical rotational activation energies. In the bis[2-(5-methyl-2-furyl)indenyl]ZrR2 series the metallocene rotational barrier can apparently be frozen on the 600 MHz ¹H NMR time scale for R = CH2CMe3 and R = CH2Ph. The di(neopentyl)Zr complex 7b shows a metallocene rotational barrier of ΔG^&thermod;rot(Cp)(213 K) = 9.0 ± 0.3 kcal mol^-1, and for the di(benzyl)Zr complex 7c a ΔG^&thermod;rot(Cp) value of 11.4 ± 0.3 kcal mol^-1 was obtained at 243 K. In the cases of 7b and 7c only chiral rac-type metallocene conformers were found in solution within the limits of the accuracy of the ¹H NMR method. The complexes bis[2-(5-ethyl-2-furyl)indenyl]ZrCl2 (8) and bis[2-(5-methyl-2-furyl)indenyl]Zr(neopentyl)2 (7b) were characterized by X-ray crystal structure analyses. In the solid state both complexes exhibit chiral rac-like metallocene frameworks.

Treatment of the reagent [C5H4B(C6F5)3]Na,Li·Et2O (4) with zirconocene dichloride gave the neutral tris(cyclopentadienyl)Zr−betaine-type complex (η⁵-Cp)2[η⁵-C5H4B(C6F5)3]Zr (1). In the crystal complex 1 contains the three η-cyclopentadienide ligands in a nearly trigonal-planar coordination around zirconium with a pronounced Zr−F−C(aryl) coordination perpendicular to it. The Zr−F bond length is 2.310(3) Å. The Zr−F−C(aryl) coordination is persistent in solution. The activation energy of the reversible cleavage of the Zr−F linkage of 1 was determined as ΔG^&thermod;(Zr-F)diss(253 K) = 10.2 ± 0.2 kcal mol^-1 by dynamic ¹⁹F NMR spectroscopy in toluene-d8.

(Butadiene)zirconocene adds B(C6F5)3 at a terminal diene carbon atom to yield the zirconocene−(μ-hydrocarbyl)−borate betaine Cp2Zr[C4H6-B(C6F5)3] (4). The dipolar complex 4 contains a distorted π-allyl moiety and features an additional stabilizing Zr−F−C(arene) coordination. Under kinetic control, an isomeric betaine system is formed, characterized by an internal Zr⁺···CH2[B]^- ion-pair interaction, that rearranges to 4 upon heating. A great variety of ansa-metallocene(butadiene) complexes and related systems cleanly form analogous metallocene−(μ-conjugated diene)−borate betaines upon treatment with B(C6F5)3 and related Lewis acids. Most of these systems represent very active homogeneous single-component Ziegler−Natta catalysts for α-olefin polymerization and copolymerization. In addition, these betaine catalysts are ideally suited for carrying out mechanistic studies in active Ziegler−Natta catalyst systems. They allow for an experimental observation of the first alkene insertion step at the active single-component catalyst. This feature has been used for studying the mechanism of transfer of the stereochemical information from the bent metallocene backbone and for an experimental characterization of the energy profile of the alkene addition/alkene insertion reaction sequence in active homogeneous Ziegler−Natta systems. The neutral dipolar single-component catalysts (e.g., 4) produce a polyolefin-modified R‘-(CHR-CH2)-C4H6-B(C6F5)3^- counterion at the end of the initiation period upon entering into the repetitive active catalytic cycle.

Treatment of 6-dimethylaminofulvene (1) with lithium anilide (2a) yields lithium [(N-phenylformimidoyl)cyclopentadienide], 3a, by an anilide addition/dimethylamine elimination reaction sequence. Similar treatment of 1 with the reagents Li[HNAr] [Ar = p-tolyl (2b), −2,6-dimethylphenyl (2c), or −2,6-diisopropylphenyl (2d)] gave the corresponding Li[C5H4CH&dbd;NAr] products 3b, 3c, and 3d in good yields (80−90% isolated) as THF adducts. Crystallization from THF gave 3d·3THF, which was characterized by X-ray diffraction. It contains monomeric units in the solid state. The lithium atom is coordinated to the imino nitrogen atom and to three THF molecules in a tetrahedral arrangement. The C5H4 unit shows some residual bond alternation. In crystalline 3d it exhibits no contact to the alkali metal. The structural features of 3d can be described by a resonance hybrid of mesomeric structures of iminium-cyclopentadienide (i.e., C5H4CH&dbd;N(Li·3THF)Ar, 3A) and of fulvenoide character (i.e., C5H4&dbd;CHN(Li·3THF)Ar, 3B). The structure of 3c·3THF in the solid state is analogous. Complex 3a crystallizes with 1 equiv of THF. It exhibits a cyclodimeric structure (3a·THF)2 in the crystal, where each Li atom is η⁵-Cp- and κN-coordinated to two different C5H4CH&dbd;NPh^- ligands. The electrostatic nature of the Li−C and Li−N interactions results in very similar structural features of the C5H4CH&dbd;NAr^- ligands in the complexes 3a, 3c, and 3d. The solution structures of these systems were characterized by temperature-dependent ¹H and ⁷Li NMR spectra. The reagents 3a and 3d were employed in the synthesis of the respective bis[1,1‘-(N-aryl)formimidoyl]ferrocenes 4a and 4d (both isolated in ca. 80% yield). The complexes 4a and 4b were characterized by X-ray crystal structure analyses.

A novel approach to gas boronising has been carried out to systematically test organoboranes as precursors for low pressure gas boronising. The precursors of the group of borane–amine adducts were synthesised and subsequently applied to gas boronising of steel 42CrMo4 (AISI 4140). Characterisation of the boride layers was performed using glow discharge opticalemission spectroscopy, X-ray photoelectron spectroscopy, SEM, and metallography. Ultramicrohardness was also determined and the wear behaviour was simulated. With the precursors borane–triethylamine adduct and borane–diethylamine adduct gas boronising was possible. Single phased iron boride layers of type Fe2B with a thickness of several micrometres were generated. The carbon, nitrogen, and oxygen contamination of the boride layers was low. The boride layers are not completely compact which results in a lower hardness compared with commercial powder pack boronised samples. The wear behaviour is similar to powder pack boronised steels and is significantly better than the untreated samples.

Treatment of 6-dimethylaminofulvene (8) with propynyllithium (9a) results in a clean nucleophilic addition at the electrophilic fulvene carbon atom C6 to yield the substituted lithium cyclopentadienide system [C5H4CH(C&tbd1;CCH3)N(CH3)2]Li (10). Likewise, the addition of p-tolyllithium, n-butyllithium, or phenyllithium resulted in the analogous substituted lithium cyclopentadienide systems [C5H4CH(R)N(CH3)2]Li, 11 (R = p-tolyl), 12 (R = C4H9), or 13 (R = Ph), respectively. These four Li−cyclopentadienide systems were characterized by single crystal X-ray structural analyses. This has revealed a series of gradually different oligomeric [C5H4CH(R)N(CH3)2]Li structural aggregate types in the solid state. Compound 10 crystallizes with 1 equiv of THF that is coordinated to Li. The metal is η⁵-coordinated to the Cp ligand and κN-bonded to the −N(CH3)2 substituent of the next ligand (Li−N 2.181(7) Å), thereby constructing an oligomeric chain of a (THF)Li(η⁵-Cp)(L-κN) type. The compound 11 crystallizes without incorporation of solvent. Here the oligomeric chain is constructed by η⁵-Cp coordination to lithium and again κN-bonding of the −N(CH3)2 donor of the adjacent ligand to the metal center. In the solvent-free system 11 the resulting Li−N linkage (2.032(5) Å) is much shorter than in 10. In 12 a beginning coordination of the lithium atom to both its neighboring Cp rings is observed. A Li cation is η⁵-coordinated to one face of the C5H4 ring system, whereas another lithium atom in the oligomeric chain structure is bonded in a η²-fashion from the other face, assisted by κN-coordination of the attached dimethylamino substituent. Eventually, two very different coordination geometries around lithium are characteristic for the oligomeric chain structure of 13:  atom Li1 is rather symmetrically η⁵-coordinated in a metallocene fashion to two cyclopentadienides, whereas the adjacent Li2 atom is coordinated to two −N(CH3)2 groups, assisted by a weaker η²- to η⁴-Cp coordination to the attached Cp ring systems. Supporting quantum chemical calculations have revealed rather shallow minima and easy conversions between these structural types. The [C5H4CH(R)NR2‘]^- ligand is thus suited to experimentally realize a whole family of novel oligomeric (CpLi)n structural types.

2-Indanone was treated with 2-lithio-5-methylfuran and the addition product dehydrated to yield the 2-(5‘-methyl-2‘-furyl)-substituted indene 2. Deprotonation with n-butyllithium followed by transmetalation to zirconium gave the bis(2-furylindenyl)ZrCl2 complex 4a. Subsequent reaction with methyllithium yielded the corresponding bis(2-furylindenyl)ZrMe2 system 6a. The related bis[2-(5‘-methyl-2‘-furyl)indenyl]hafnium complexes (5a, 7a) and the bis[2-(5‘-methyl-2‘-thienyl)indenyl]zirconium and -hafnium complexes 4b−6b were synthesized analogously. In the crystal the bis(2-furylindenyl)ZrCl2 (4a) and -HfCl2 (5a) complexes exhibit meso-like metallocene conformations, as shown by X-ray diffraction. In contrast, the bis(2-furylindenyl)ZrMe2 system (6a) shows a chiral, close to C2-symmetric rac-like metallocene conformation in the solid state. Hydrolysis of 2-(5‘-methyl-2‘-furyl)indene (2a) followed by Paal−Knorr pyrrole synthesis of the resulting 1,4-diketone 8 with methylamine gave (5‘-methyl-2‘-N-methylpyrrolyl)indene (9), which was deprotonated and transmetalated to yield the substituted bis(2-pyrrolylindenyl)ZrCl2 (11), -HfCl2 (12), and -ZrMe2 (13) complexes. Treatment of the [2-(5‘-methyl-2‘-furyl)indenyl]- and [2-(5‘-methyl-2‘-thienyl)indenyl]ZrCl2 systems 4a,b with excess methylalumoxane gave active homogeneous Ziegler catalysts for the production of elastomeric polypropylene. The catalyst derived from 11 yielded only propene oligomers.

The organometallic Lewis acid B(C6F5)3 adds to the terminal &dbd;CH2 group of the (butadiene)metallocene complexes 5a and 5b to give the ansa-metallocene betaine systems [Me2Si(C5H4)2]Zr[C4H6−B(C6F5)3] (6a) and [Me2Si(3-MeC5H3)2]Zr[C4H6−B(C6F5)3] (6b), respectively, in high yield. Both complexes were characterized by X-ray diffraction. They both contain a substituted η³-allyl ligand F of E configuration, and they show a characteristic (ortho aryl)C−F···Zr interaction that stabilizes the electron-deficient metal center inside the dipolar structure. B(C6F5)3 also adds to one butadiene terminus of (s-cis-η⁴-C4H6)[Me2C(C5H4)(indenyl)]Zr to give a high yield of a single isomer of the respective ansa-metallocene [C4H6−B(C6F5)3] betaine complex 9. The X-ray crystal structure analysis of 9 has revealed that in this case a (Z)-η³-allyl-CH2B(C6F5)3 ligand is formed. This precluded the (aryl)C−F···Zr coordination. Instead, the zirconium center in 9 forms a stabilizing internal ion pair interaction between the negatively polarized [B]−C(4)H2 methylene group and the positive zirconium center. The analogously structured ansa-metallocene [(Z)-C4H6−B(C6F5)3] betaine complex 12 is obtained in high yield from B(C6F5)3 addition to (s-cis-η⁴-butadiene)[Me2C(C5H4)(fluorenyl)]Zr. In solution the complexes 6, 9, and 12 exhibit structures that are analogous to those found in the solid state. However, treatment of (butadiene)[Me2Si(C5H4)2]Zr (5a) with B(C6F5)3 under kinetic control (233 K in toluene-d8) quantitatively yields the [Me2Si(C5H4)2]Zr[(Z)-C4H6−B(C6F5)3] betaine isomer 13, which contains the stabilizing [B]−C(4)H2···Zr internal ion pair interaction. Subsequent thermally induced rearrangement of the kinetic product 13 (ΔG^&thermod;rearr(298 K) = 21.5 ± 0.5 kcal mol^-1) then results in the formation of the eventually observed thermodynamic ansa-metallocene betaine product 6a, that contains the (E)-C4H6−B(C6F5)3 ligand and exhibits internal (aryl)C−F···Zr coordination. A similar reaction sequence was observed during the addition of B(C6F5)3 to the parent (butadiene)zirconocene system 1:  at 213 K the kinetic Cp2Zr[(Z)-(1−3η),κC⁴-C4H6−B(C6F5)3] betaine product 14 is formed, which rapidly rearranges at temperatures above 253 K to yield the previously observed stable Cp2Zr[(E)-C4H6−B(C6F5)3] betaine system 2, which is characterized by an internal C−F···Zr bond. The ansa-metallocene betaines 6, 9, and 12 are all active homogeneous single-component Ziegler catalysts for ethene and propene polymerization. They are similarly effective as the usually employed ansa-metallocene dichloride/methylalumoxane catalyst systems.

(Cp-PPh2)2ZrCl2 (5) reacts with (PhCN)2PdCl2 to yield the early−late heterodimetallic complex [(Cp-PPh2)2ZrCl2/PdCl2], 7, which was characterized by an X-ray crystal structure analysis. The phosphine-substituted bent metallocene moiety in 7 serves as a conformationally flexible chelate phosphine ligand (angle P−Pd−P:  96.46(3)°). Complex 7 is an active catalyst for the cross-coupling of sec-butylmagnesium bromide with bromobenzene, leading to excellent regioselectivity and moderate reactivity. Complex 5 reacts with dicarbonylrhodium chloride dimer (0.5 equiv) to yield the triply bridged complex 12, [ClZr(μ-Cp-PPh2)2(μ-Cl)Rh(CO)Cl], which was also characterized by X-ray diffraction (the angle P−Rh−P is 158° at the distorted square-pyramidal pentacoordinated rhodium center). (Cp-PPh2)2Zr(CH3)2 (13) reacts with [H(CO)Rh(PPh3)3] (14) with loss of two PPh3 ligands and instantaneous liberation of methane to form complex 15, [CH3Zr(μ-Cp-PPh2)2Rh(CO)PPh3](Zr−Rh), which probably contains a metal−metal bond between the early and late transition metal. Complex 15 is a very active 1-hexene hydroformylation catalyst (TOF > 600, n/iso ≈ 3 at 80 °C).

Treatment of 3-phenylcyclopent-2-enone (1) with either morpholine or pyrrolidine in the presence of 3 Å molecular sieves yielded the corresponding cyclopentadienamines 2a and 2b. Subsequent deprotonation with n-butyllithium followed by the reaction with zirconium tetrachloride led to the formation of the amino-substituted bent metallocenes 4a and 4b. Crystallization furnished the isomerically pure complex meso-bis(phenyl-3-pyrrolidinylcyclopentadienyl)zirconium dichloride (meso-4b), which was characterized by X-ray diffraction. Complex meso-4b exhibits a C(Cp)−N bond length of 1.349(3) Å. A rotational barrier of ΔG^&thermod;(CN-rot)(248 K) = 12.1 ± 0.2 kcal mol^-1 was measured for meso-4b in solution by dynamic 600 MHz ¹H NMR spectroscopy. Similar treatment of 2-indanone with morpholine, followed by deprotonation and reaction with ZrCl4, gave bis(2-morpholinoindenyl)ZrCl2 (7a), which was also characterized by an X-ray crystal structure analysis (the indenyl-C2/12−N bond lengths are 1.349(5)/1.381(5) Å). Two rotational processes were subsequently frozen out on the 600 MHz ¹H NMR time scale upon decreasing the temperature from ambient to give a C−N rotational barrier of ΔG^&thermod;(CN-rot)(268 K) = 12.9 ± 0.2 kcal mol^-1 and an activation energy of the rotation about the zirconium−Cp(indenyl) vector of ΔG^&thermod;(Ind-rot)(178 K) = 7.6 ± 0.5 kcal mol^-1. In solution and in the solid state only a single C2-symmetric metallocene rotamer of 7a was detected. The same is found for bis(2-pyrrolidinoindenyl)ZrCl2 (7b). By dynamic ¹H NMR spectroscopy the two rotational activation barriers of complex 7b were determined as ΔG^&thermod;(CN-rot)(338 K) = 16.4 ± 0.2 kcal mol^-1 and ΔG^&thermod;(Ind-rot)(198 K) = 9.1 ± 0.5 kcal mol^-1.

The new precursors M(NMe2)2Cl2(dme) (M = Zr (3), Hf (4)) react with the dilithium derivative of the silyl-bridged bis(1-indenyl) ligand bearing pendant 4-pentenyl groups Li2[3,3‘-⁵R2−SBI] (2) in toluene at −35 °C to provide the corresponding bis(dimethylamido) metallocenes with rac/meso ratios of 6−7:1. Subsequent treatment of the substantially rac-enriched zirconocene mixture with excess TMS−Cl in dichloromethane in the presence of a catalytic amount of anhydrous HCl provides rac-[3,3‘-⁵R2−SBI]ZrCl2 (7) in 40% isolated yield free from its meso isomer. Similar reactions of Li2[SBI]·OEt2 and of Li2[EBI]·OEt2 with 3 and 4 also lead to selective formation of the rac-metallocenes with rac/meso ratios as high as 20:1 for [SBI]Zr(NMe2)2 (10). Sodium amalgam reduction of rac-[3,3‘-⁵R2−SBI]ZrCl2 (7) in THF results in the intramolecular tail-to-tail coupling of the pendant olefins to provide the crystallographically characterized α,α‘-disubstituted zirconacyclopentane 13 in 95% isolated yield. In contrast, reduction of rac-[3,3‘-⁵R2−SBI]HfCl2 (8) (prepared from 2 and HfCl4 in toluene) leads to both the symmetric α,α‘-disubstituted and the unsymmetric α,β‘-substituted hafnacyclopentanes (14 and 15, respectively) in a 1:1 ratio. Zirconacycle 13 reacts with [HNMe2Ph]⁺[OTf]^- to give the monotriflate 17, in which one of the 4-pentenyl arms is released and the other is formally protonated at its terminal carbon atom to give this thermally stable secondary alkyl complex. Formation of 17 is best understood as proceeding through a mono-olefin intermediate 16, which can be trapped with 2-butyne to form the corresponding zirconacyclopentene 18. Reaction of 13 with the organometallic Lewis acid B(C6F5)3 results in the formal abstraction of a hydride from the 3-position of one arm to form [HB(C6F5)3]^- and a THF-bound π-allyl cation 19, in which one pendant alkene is also free.

A variety of donor adducts of tris(pentafluorophenyl)borane were experimentally generated by reaction of a Lewis base with an excess of B(C6F5)3 in pentane. In this way, nitrile complexes (C6F5)3B·NCR (R = CH3 1a, p-CH3−C6H4 1b, p-NO2−C6H4 1c), isonitrile complexes (C6F5)3B·CNR (R = C(CH3)3 3a, C(CH3)2CH2C(CH3)3 3b, 2,6-(CH3)2−C6H3 3c), and the phosphine adduct (C6F5)3B·P(C6H5)3 (6) could be prepared. The compounds were characterized by IR and NMR spectroscopy and by X-ray structure analyses (1a, 1c, 3a, 3b, and 6). Coordination of the nitriles as well as the isonitriles to the neutral Lewis acid leads to a substantial increase in the C&tbd1;N bond strength. This is evident from a marked shift of the &nutilde;C&tbd1;N IR band to higher wavenumbers, and this interpretation is supported by the small but experimentally significant decrease of the C&tbd1;N bond length observed by X-ray diffraction. The experimental work is complemented by a density functional study on the model complexes (C6F5)3B·L, L = CNCH3, NCCH3, PH3, CO. A detailed analysis revealed that the bonding in (C6F5)3B·L complexes is mainly dominated by electrostatic interaction, which in turn is responsible for the observed structural and spectroscopic changes. In the context of this work, the bonding of the neutral B(C6F5)3 Lewis acid is compared to the positively charged organometallic d0-Cp3M⁺ system (M = Zr, Hf). It was found that electrostatic effects are more pronounced for B(C6F5)3 than for the transition metal fragments. The question as to the existence of a nonclassical main group carbonyl complex, (C6F5)3B·CO, is addressed.

The (η²-diaryl ketone)zirconocene dimers 17a and 17b (aryl = phenyl or p-tolyl) react with the group 4 metallocene dihydride complexes [^RCp2MH2] (^RCp2M = Cp2Zr, (MeCp)2Zr, or Cp2Hf) to yield the dimetallic compounds [(μ-η¹-O:η²-C,O-Ar2CO)ZrCp2/H2M^RCp2] 18a−f. These systems contain a dimetallabicyclic framework in which the two metal centers are bridged by the η²-diaryl ketone ligand oxygen atom and a μ-hydride ligand. The remaining hydride ligand is trans-oriented to the μ-H moiety at the M^RCp2 unit. The complexes 18a−c were characterized by X-ray diffraction. Treatment of the complexes 18 with B(C6F5)3 results in the selective abstraction of the terminal hydride ligand to form the dimetallic cation complexes [(μ-η¹-O:η²-C,O-Ar2CO)ZrCp2(μ-H)M^RCp2]⁺ 19a−f (with HB(C6F5)3^- anion). The complexes 19 contain the intact (η²-diaryl ketone)ZrCp2 unit to which a (H−M^RCp2)⁺ cation is formally attached. They exhibit a planar dimetallabicyclic framework, in which the two group 4 metal centers are connected by means of the η²-ketone oxygen atom and the μ-hydride ligand. The complexes 19a,b and e were characterized by X-ray crystal structure analyses. The complexes 19 behave chemically as adducts of the highly reactive (^RCp2M−H)⁺ cations with a metallaoxirane matrix. Ethene is polymerized by the systems 19 with high catalyst activities.

(η⁴-Butadiene)(η⁵-cyclopentadienyl)tantalum dichloride (6), synthesized by treatment of CpTaCl4 (5) with (butadiene)magnesium (7), was treated with 2 molar equiv of sodium cyclopentadienide to yield Cp3Ta(butadiene) (9). The spectroscopic analysis revealed the (η⁵-Cp)2(η¹-Cp)(η²-C4H6)Ta structure of 9. Treatment of 9 with 1 molar equiv of the salt [(Cp2Zr−CH3)⁺(CH3B(C6F5)3)^-] (11) resulted in the clean transfer of a cyclopentadienide ligand from tantalum to zirconium with formation of the neutral zirconium compound Cp3Zr−CH3 (12) and the cation Cp2Ta(butadiene)⁺ (10; with CH3B(C6F5)3^- counteranion). The spectroscopic characterization and an X-ray crystal structure analysis has revealed that 10 is a cationic (s-trans-η⁴-butadiene)bis(η⁵-cyclopentadienyl)tantalum complex. It reacts cleanly with a variety of ketones at 60 °C to form the respective 7,7-disubstituted tetrahydrotantalaoxepines 13. These cationic seven-membered metallacyclic σ-allyl complexes undergo a rapid topomerization process on the NMR time scale (ΔG^&thermod;top ≈ 12 kcal/mol^-1). Organic nitriles add similarly at 60 °C to the (s-trans-η⁴-butadiene)tantalocene cation (10) to yield the respective 7-substituted 3,4-dihydrotantala-6H-azepines (14) that exhibit a similar dynamic NMR behavior due to conformational equilibration. Two examples of the seven-membered σ-allyl metallacyclic cation complexes 14 were characterized by X-ray crystal structure analyses (14a, R = CH3, and 14d, R = CH2-p-C6H4CH3). In contrast, 2-butyne adds to the (s-trans-η⁴-butadiene)tantalocene cation in a 1:1 ratio to yield the corresponding metallacyclic (π-allyl)metallocene cation complex 15a (characterized by X-ray diffraction). The addition of 1-pentyne gave a 60:40 mixture of the respective regioisomers 15b and 15c, exhibiting the n-C3H7 substituent at the ring position C5 and C6, respectively.

Treatment of 6-(dimethylamino)-6-methylfulvene (1) with methyllithium followed by the reaction of the resulting [C5H4−CMe2NMe2]Li reagent with CpZrCl3 gave the complex [(η⁵-C5H4−CMe2NMe2)CpZrCl2] (2). Its treatment with 2 molar equiv of methyllithium furnished [(η⁵-C5H4−CMe2NMe2)CpZr(CH3)2] (3). Complex 3 reacted with B(C6F5)3 by methyl group transfer. The in situ [Zr]⁺−CH3 cation system generated in this manner proved to be unstable under the reaction conditions and instantaneously eliminated methane with formation of 4. In the course of this reaction a N−CH3 hydrogen atom was abstracted. Complex 4 was stabilized by the addition of 1 equiv of the alkyl isocyanide RN&tbd1;C (R = CMe3, n-C4H9, −CMe2CH2CMe3) to yield the respective adducts 5. The complex [(η⁵:η²(C,N)-C5H4−CMe2N(&dbd;CH2)CH3)CpZr(κC-C&tbd1;N−CMe3)]⁺[CH3B(C6F5)3]^- (5a) was characterized by an X-ray crystal structure analysis. It exhibits an η²-coordination of the pendant formaldiminium moiety to zirconium (d(Zr−N) = 2.318(7) Å, d(Zr−C) = 2.272(7) Å). The (η²-R2NCH2)Zr moiety shows a characteristic ¹⁵N NMR chemical shift (δ −376 ppm), deshielded by ca. Δδ ≈ −40 ppm relative to the ¹⁵NMe2 NMR resonance found for 3. Complex 4 reacts with butadiene or isoprene by insertion into the Zr−CH2 bond of the (η²-formaldiminium)Zr moiety to form the metallacyclic (π-allyl)metallocene complexes 6a,b. The X-ray crystal structure analysis of 6b shows a close Zr−N contact at 2.491(4) Å. A corresponding ¹⁵N NMR feature was found at δ −357 ppm. The complex [(η⁵-C5H4−CMe2NMe2)2Zr(CH3)2] reacts analogously:  upon treatment with B(C6F5)3 in a 1:1 ratio, CH3 is transferred from zirconium to boron, and 1 equiv of methane is liberated to give complex 8 (¹⁵N NMR signals at δ −357 and −372 ppm). Complex 8 was also characterized by X-ray diffraction. It shows coordination of both nitrogen atoms to zirconium: i.e., the presence of a (η²-R2NCH2)Zr⁺ three-membered-ring system, formed by C−H activation, and a κN-coordinated intact pendant −CMe2−NMe2 group. The latter is displaced upon the addition of tert-butyl isocyanide to yield the (κC-isonitrile)(η²-formaldiminium)metallocene cation complex 9.

Treatment of (η⁴-s-trans-butadiene)tantalocene cation (3, with methyltris(pentafluorophenyl)borane anion) with tert-butyl isocyanide (4 h, at 45 °C in bromobenzene) yields the [(η²-butadiene)(C&tbd1;NCMe3)TaCp2⁺] complex 4a (70% isolated, &nutilde;(C&tbd1;NR) 2164 cm^-1). Likewise treatment with n-butyl or cyclohexyl isocyanide gives the analogous products 4b and 4c, respectively. The cyclohexyl isocyanide adduct 4c was characterized by X-ray diffraction. It shows the η²-butadiene ligand oriented at the front of the Cp2Ta⁺ bent metallocene wedge with the noncoordinated butadiene vinyl group located in the “vinyl-inside” position. The Ta−C&tbd1;N−R unit is almost linear (angles Ta−C−N = 177.2(5)°, C&tbd1;N−R = 174.0(8)°; d(C&tbd1;N) = 1.152(6) Å). Photolysis of [(butadiene)TaCp2⁺] (3) with excess cyclohexyl isocyanide followed by thermal treatment gave rise to the formation of [bis(κC-cyclohexyl isocyanide)TaCp2⁺] (8, with [CH3B(C6F5)3^-] counteranion; >50% isolated, &nutilde;(C&tbd1;NR) 2134 and 2088 cm^-1). Complex 8 was characterized by an X-ray crystal structure analysis. It shows a pseudotetrahedral [Cp2Ta(C&tbd1;NR)2⁺] cation (R = C6H11). The Ta−C&tbd1;N−R units are both slightly bent at nitrogen (angles C&tbd1;N−R = 169.0(13) and 163.7(8)°; the corresponding Ta−C−N angles are 179.4(9) and 177.4(7)°). The bonding situations of complexes 4 and 8 were analyzed by DFT calculations. The computational study shows that metal to isonitrile back-bonding does not contribute significantly to the Cp2Ta⁺−C&tbd1;NR bonding interaction in these complexes. The bonding between Cp2Ta⁺ and the C&tbd1;NR ligands is dominated by electrostatic effects.

Bis(propynyl)zirconocene (1b) reacts with 0.5 molar equiv of trityl tetraphenylborate by a propynyl group transfer to form Ph3CC&tbd1;CCH3 and Cp2ZrC&tbd1;CCH3⁺. The in-situ-generated (propynyl)zirconocene cation is not stable under the reaction conditions but instantaneously reacts with the neutral starting material 1b to form the [(μ-C&tbd1;CCH3)(μ-CH3C&tbd1;CC&tbd1;CCH3)(ZrCp2)2⁺][BPh4^-] salt, 9. Complex 9 was characterized by an X-ray crystal structure analysis. It contains an unsymmetrically bridging hexadiyne ligand and exhibits a planar-tetracoordinate carbon center that is stabilized by a three-center two-electron interaction with the two adjacent zirconocene moieties. In solution, complex 9 exhibits dynamic NMR spectra that indicate a symmetrizing exchange of the bonding situations between the two ends of the μ-hexadiyne ligand. The Gibbs activation energy of the automerization reaction of 9 is ΔG^&thermod;(190 K) ≈ 9.5 kcal mol^-1.

Treatment of Cp2Zr(Cl)CH3 (6) with sodium cyclopentadienide gives Cp3ZrCH3 (7). Its reaction with dimethylanilinium tetraphenylborate yields the Cp3Zr(THF)⁺ cation. [Cp3Zr⁺CH3B(C6F5)3^-] (9) is generated by treatment of 7 with tris(pentafluorophenyl)borate. Nitriles add to 9 to form the ligand-stabilized tris(η⁵-cyclopentadienyl)zirconium cation systems Cp3Zr(N&tbd1;CR)⁺. With tert-butyl isocyanide, 9 is transformed to yield the donor-ligand-stabilized [Cp3Zr(C&tbd1;NCMe)3⁺CH3B(C6F5)3^-] salt 12. Carbon monoxide adds to 9 to give the cationic metal carbonyl complex [Cp3Zr(CO)⁺CH3B(C6F5)3^-] (13). Both cation complexes 12 and 13 were characterized by X-ray diffraction.

Treatment of the phosphorus ylide Ph3P&dbd;CH2 (2a) with B(C6F5)3 (1) yields the adduct Ph3P⁺−CH2−B(C6F5)3^- (3a), which was characterized by X-ray crystal structure analysis. The ylide Ph3P&dbd;CHPh (2b) reacts analogously with B(C6F5)3 at 0 °C to form Ph3P⁺−CHPh−B(C6F5)3^- (3b), but this adduct formation is reversible. Increasing the temperature leads to the formation of Ph3P⁺−CH2−(p-C6H4)−B(C6F5)3^- (4), which is formed by an electrophilic aromatic substitution reaction of the electron-deficient borane reagent at the ylidic phenyl group. Compound 4 is also cleaved upon prolonged thermolysis to eventually yield Ph3P⁺−CHPh−(p-C6F4)−BF(C6F5)2^- (5), which is the product of thermodynamic control in this series. Compound 5 arises from a nucleophilic aromatic substitution reaction of the nonstabilized phosphorus ylide at a −C6F5 ring of the B(C6F5)3 reagent. Compound 5 was also characterized by an X-ray crystal structure analysis.

The dimethylzirconocene complex [(Cp−CMe2−PAr2)2ZrMe2], 2a (Ar = p-tolyl), was treated with 1 molar equiv of B(C6F5)3 to yield the salt [(Cp−CMe2−PAr2)2ZrMe⁺MeB(C6F5)3^-], 3a. The X-ray crystal structure analysis of 3a shows that both −PAr2 units are intramolecularly coordinated to zirconium in a close to C2-symmetric arrangement with the [Zr]−CH3 group placed in the central position in the bent metallocene σ-ligand plane. Treatment of 2a with 2 equiv of B(C6F5)3 generates the highly reactive dication system [(Cp−CMe2−PAr2)2Zr²⁺] (4 with two MeB(C6F5)3^- anions). The highly electrophilic cation 4 abstracts chloride from, e.g., dichloromethane solvent to yield [(Cp−CMe2−PAr2)2Zr-Cl⁺] (5, with MeB(C6F5)3^- anion). The same cation (5‘, with ClB(C6F5)3^- anion) was obtained from the reaction of [(Cp−CMe2−PAr2)2ZrCl2] (1a) with B(C6F5)3. The C2-symmetric, internally −PAr2-stabilized dication adds acetonitrile or 2,6-dimethylphenyl isocyanide to give the respective C2-symmetric donor-ligand adducts in which the −PAr2 coordination to zirconium is retained. The complexes 3a (ΔG^&thermod;enant (300 K) = 14.0 ± 0.5 kcal/mol) and 5 (ΔG^&thermod;enant (360 K) = 17.5 ± 0.5 kcal/mol) show dynamic NMR spectra due to an intramolecular enantiomerization process proceeding with a rate-determining cleavage of the Zr−P linkages. Complex 5 was also characterized by an X-ray crystal structure analysis.