Higher Valent Metal Pentadienyl Chemistry: Syntheses, Structures, and Reactions of Zr(6,6-dmch)₂X₂ Complexes (dmch = dimethylcyclohexadienyl; X = Cl, Br, I) and Related Species

The zirconium(II) complex Zr(6,6-dmch)2(PMe3)2 undergoes facile reactions with 1,2-dihaloalkanes, yielding the formal oxidative addition products Zr(6,6-dmch)2X2 (X = Cl, Br, I). A similar reaction with 1-chloro-2-methoxyethane yields Zr(6,6-dmch)2(Cl)(OCH3). Each of the complexes has been characterized analytically, spectroscopically, and structurally. A direct oxidative addition reaction occurs with CH3Br, yielding the thermally unstable Zr(6,6-dmch)2(CH3)(Br), which has been characterized spectroscopically and structurally. As had been observed for related Zr(C5H5)(6,6-dmch)X2 complexes, the Zr−dmch coordination is severely skewed, with quite short Zr−C3 interactions and substantially lengthened interactions with the remaining carbon atoms of the dienyl fragment. This may be attributed to an inability of the contracted Zr(IV) orbitals to interact effectively with all the carbon atoms in the wide, electronically open dienyl ligands

Higher Valent Metal Pentadienyl Chemistry: Syntheses, Structures, and Reactions of Zr(6,6-dmch)₂X₂ Complexes (dmch = dimethylcyclohexadienyl; X = Cl, Br, I) and Related Species

The zirconium(II) complex Zr(6,6-dmch)2(PMe3)2 undergoes facile reactions with 1,2-dihaloalkanes, yielding the formal oxidative addition products Zr(6,6-dmch)2X2 (X = Cl, Br, I). A similar reaction with 1-chloro-2-methoxyethane yields Zr(6,6-dmch)2(Cl)(OCH3). Each of the complexes has been characterized analytically, spectroscopically, and structurally. A direct oxidative addition reaction occurs with CH3Br, yielding the thermally unstable Zr(6,6-dmch)2(CH3)(Br), which has been characterized spectroscopically and structurally. As had been observed for related Zr(C5H5)(6,6-dmch)X2 complexes, the Zr−dmch coordination is severely skewed, with quite short Zr−C3 interactions and substantially lengthened interactions with the remaining carbon atoms of the dienyl fragment. This may be attributed to an inability of the contracted Zr(IV) orbitals to interact effectively with all the carbon atoms in the wide, electronically open dienyl ligands

Higher Valent Metal Pentadienyl Chemistry: Syntheses, Structures, and Reactions of Zr(6,6-dmch)₂X₂ Complexes (dmch = dimethylcyclohexadienyl; X = Cl, Br, I) and Related Species

The zirconium(II) complex Zr(6,6-dmch)2(PMe3)2 undergoes facile reactions with 1,2-dihaloalkanes, yielding the formal oxidative addition products Zr(6,6-dmch)2X2 (X = Cl, Br, I). A similar reaction with 1-chloro-2-methoxyethane yields Zr(6,6-dmch)2(Cl)(OCH3). Each of the complexes has been characterized analytically, spectroscopically, and structurally. A direct oxidative addition reaction occurs with CH3Br, yielding the thermally unstable Zr(6,6-dmch)2(CH3)(Br), which has been characterized spectroscopically and structurally. As had been observed for related Zr(C5H5)(6,6-dmch)X2 complexes, the Zr−dmch coordination is severely skewed, with quite short Zr−C3 interactions and substantially lengthened interactions with the remaining carbon atoms of the dienyl fragment. This may be attributed to an inability of the contracted Zr(IV) orbitals to interact effectively with all the carbon atoms in the wide, electronically open dienyl ligands

Origin of 1,4-Regiochemistry in the Dicouplings of Ketones with 6,6-Dimethylcyclohexadienyl Complexes of Titanium and Zirconium: A Mechanism Arising from Five-Electron Donation by Alkoxide Ligands

In an earlier study of the coupling reactions of Ti(C5H5)(6,6-dmch)(PMe3) (dmch = dimethylcyclohexadienyl) with ketones, the initially formed metal complex could not be isolated, but after hydrolysis an unprecedented 1,4-dicoupling product was isolated. To gain further insight into the origin of this regiochemistry, the reaction of a zirconium analogue, Zr(C5H5)(6,6-dmch)(PMe3)2, with acetone was investigated. A solid dimeric product could be isolated and structurally characterized, which revealed that both acetones had coupled, with the unusual 1,4-regiochemistry. This then necessitated the proposal of a new mechanism for these reactions, in which an alkoxide ligand, generated from a coupling of the initially incorporated ketone, serves as a formal five-electron donor, thereby altering the mode of coordination of the diene fragment. This proposal has been supported by the characterization of a mono(benzophenone) coupling product of Ti(C5H5)(6,6-dmch)(PMe3). This coupling product possessed both a large Ti−O−C angle, indicative of at least a partial contribution of a resonance form in which the alkoxide ligand serves as a five-electron donor, and the expected η2-diene coordination

Reaction of Chromium(III) Chloride with the Cycloheptadienyl Anion: Susceptibility of Edge Bridges To C−H Activation Reactions

The reaction of CrCl3 with 3 equiv of the cycloheptadienyl anion leads to Cr(η7-C7H7)(η4-C7H10) (C7H10 = cycloheptadiene), through a process in which one ligand loses two hydrogen atoms while the other ligand gains one. The same complex had been reported from an entirely different route, involving the reaction of chromium(III) salts with Grignard reagents in the presence of cycloheptadiene and cycloheptatriene. The formulation of the complex has been confirmed by a low-temperature X-ray diffraction study. Related reactions have been found to lead to M(η7-C7H7)(η5-C7H9) (M = Ti, V; C7H9 = cycloheptadienyl) complexes. The preferential formation of one aromatic ηn-CnHn ligand seems to provide the driving force favoring the isolated products and related species having other values of n

Reactions of SF₆ with Organotitanium and Organozirconium Complexes: The “Inert” SF₆ as a Reactive Fluorinating Agent

The normally remarkably inert SF6 has been found to be quite reactive toward low valent organometallic compounds, under conditions in which usually powerful fluorinating agents may be less reactive. Reaction of SF6 with Ti[1,3-C5H3(t-Bu)2](6,6-dmch)(PMe3), for example, leads to {Ti[1,3-C5H3(t-Bu)2]F2}4 (dmch = dimethylcyclohexadienyl), whose structure is based on a cube of fluoride ions with the ligated titanium centers situated above four coplanar face centers

Reactions of SF₆ with Organotitanium and Organozirconium Complexes: The “Inert” SF₆ as a Reactive Fluorinating Agent

The normally remarkably inert SF6 has been found to be quite reactive toward low valent organometallic compounds, under conditions in which usually powerful fluorinating agents may be less reactive. Reaction of SF6 with Ti[1,3-C5H3(t-Bu)2](6,6-dmch)(PMe3), for example, leads to {Ti[1,3-C5H3(t-Bu)2]F2}4 (dmch = dimethylcyclohexadienyl), whose structure is based on a cube of fluoride ions with the ligated titanium centers situated above four coplanar face centers

Reactions of SF₆ with Organotitanium and Organozirconium Complexes: The “Inert” SF₆ as a Reactive Fluorinating Agent

The normally remarkably inert SF6 has been found to be quite reactive toward low valent organometallic compounds, under conditions in which usually powerful fluorinating agents may be less reactive. Reaction of SF6 with Ti[1,3-C5H3(t-Bu)2](6,6-dmch)(PMe3), for example, leads to {Ti[1,3-C5H3(t-Bu)2]F2}4 (dmch = dimethylcyclohexadienyl), whose structure is based on a cube of fluoride ions with the ligated titanium centers situated above four coplanar face centers

Synthesis of Bridged Bicyclic Ethers and Fused Oxetanes from Pyran-4-ones via Tandem Solvent Trapping and Norrish Type II Cyclization¹

Polyalkyl pyran-4-ones 1a−c were irradiated in methanol or ethanol. Although the expected solvent trapping products 3 could be observed, extended irradiation times allowed exclusive formation of secondary photoproducts 4 and 5 in combined yields of 37−64%. These bicyclic compounds are believed to arise from γ-hydrogen abstraction by the excited enone chromophore of 3, followed by closure of the resulting biradical through one of two possible pathways. Moderate stereoselectivity was observed in the radical coupling to produce 4, whereas the analogous closure to 5 was completely diastereoselective. Tautomerization of the enol precursors to 5 also occurred with complete selectivity for protonation from the exo face. Overall, this process converts simple, planar heterocycles and alkanols into complex products in a single transformation

Structural, Spectroscopic, and Electrochemical Studies of Edge-Bridged Open Ferrocenes

The edge-bridged open ferrocenes, Fe(dmch)2 (1), Fe(c-C7H9)2 (2), and Fe(c-C8H11)2 (3, dmch = dimethylcyclohexadienyl; c-C7H9 = cycloheptadienyl; c-C8H11 = cyclooctadienyl) are more readily oxidized than ferrocene by ca. 460−680 mV, the ease of oxidation increasing with ring size. The kinetic stabilities of the 17-electron cations follow a trend that is opposite to their thermodynamic ease of formation. ESR measurements show that the cations are low spin, suggesting a mainly Fe(3dz2) SOMO. The cation Fe(dmch)2+ was isolated as a deep green [Fe(dmch)2][B(C6H5)4] salt. Both the neutral and cationic forms of Fe(dmch)2 have been studied by X-ray diffraction, thereby allowing for structural comparisons of the two species. In particular, the oxidation of the 18-electron Fe(dmch)2 complex leads to the adoption of a nearly staggered conformation, similar to what was previously found only for early metals such as titanium and vanadium. As in the case of ferrocene, oxidation is accompanied by an increase in the average Fe−C bond distance