The temperature dependence of the magnetic susceptibility, optical reflectivity and electrical conductivity of [Fe(HB(pz) 3 ) 2 ] (pz = pyrazolyl) revealed irreversible changes in the material during the low-spin to high-spin transition when the ''as-prepared'' sample was heated above B400 K for the first time. During this first heating sequence, the initially fine powder sample became coarse, and its crystal structure changed from tetragonal to monoclinic. Single-crystals of the monoclinic form suitable for X-ray analysis could be isolated after the first thermal cycle, and their structure was resolved in the P2 1 /n (Z = 4) space group. Successive cooling and heating cycles did not lead to further modification of the crystal structure, and the temperature dependence of the physical properties remained invariable. Remarkably, the electrical conductivity of the sample measured at 293 K dropped from 6.1 Â 10 À8 to 2.1 Â 10 À11 S m À1 following the first thermal cycle-suggesting possible applications of this material in read-only memory devices (ROM).
We report the synthesis of a niobium cyclopropyl complex, Tp(Me2)NbMe(c-C(3)H(5))(MeCCMe), and show that thermal loss of methane from this compound generates an intermediate that is capable of activating both aliphatic and aromatic C-H bonds. Isotopic labeling, trapping studies, a detailed kinetic analysis, and density functional theory all suggest that the active intermediate is an η(2)-cyclopropene complex formed via β-hydrogen abstraction rather than an isomeric cyclopropylidene species. C-H activation chemistry of this type represents a rather unusual reactivity pattern for η(2)-alkene complexes but is favored in this case by the strain in the C(3) ring which prevents the decomposition of the key intermediate via loss of cyclopropene.
Despite the growing interest in iron catalysis and hydroamination reactions, iron-catalyzed hydroamination of unprotected primary aliphatic amines and unactivated alkenes has not been reported to date. Herein, a novel well-defined four-coordinate β-diketiminatoiron(II) alkyl complex is shown to be an excellent precatalyst for the highly selective cyclohydroamination of primary aliphatic alkenylamines at mild temperatures (70-90 °C). Both empirical kinetic analyses and the reactivity of an isolated iron(II) amidoalkene dimer, [LFe(NHCH2 CPh2 CH2 CHCH2 )]2 favor a stepwise σ-insertive mechanism that entails migratory insertion of the pendant alkene into an iron-amido bond associated with a rate-determining aminolysis step.
The structures of three NH-indazoles (3-methyl, 3-trifluoromethyl and 3-trifluoromethyl-4,5,6,7tetrafluoroindazoles) have been determined by X-ray crystallography. These three compounds, together with 3-methyl-4,5,6,7-tetrafluoroindazole, whose X-ray structure could not be determined, have been studied using multinuclear magnetic resonance spectroscopy, including solid-state CPMAS. They all are 1H-tautomers. In the crystal, 3-methyl-1H-indazole forms hydrogen bonded dimers, whereas 3-trifluoromethyl-1H-indazole and 3-trifluoromethyl-4,5,6,7tetrafluoro-1H-indazole crystallize as catemers. These catemers are chiral space group P3 2. They are the first examples of indazoles crystallizing in the form of helices of threefold screw axis. Attempts at rationalizing this behavior on the basis of supramolecular interactions (hydrogen bonds and aromatic interactions) and GIAO calculations are discussed. The present paper focuses on the four related 1H-indazoles 6-9 depicted in Scheme 2. Scheme 1 Indazoles and a 3-trifluoromethylpyrazole of biological relevance.
[IndH(2)(Ph(2)P=X)(2)] derivatives (1, X = NMes; 4, X = S) react with [Zr(NMe(2))(4)] and [PdCl(2)(cod)] to afford the complexes {[Ind(Ph(2)PNMes)(2)]Zr(NMe(2))(2)} (3), {[Ind(Ph(2)PS)(2)]Zr(NMe(2))(2)} (5), and {[Ind(Ph(2)PS)(2)]Pd(HNc-Hex(2))} (7). The ability of the phosphazene and thiophosphinoyl side arms to support the coordination of the indenyl ring as 2-indenylidene was evidenced by NMR spectroscopy and X-ray diffraction studies. Analysis of the bonding situation by density functional theory calculations revealed a strong sigma interaction but a negligible (if any) pi interaction between C2 and the metal.
The intermolecular C-H bond activation of benzene occurs under very mild conditions (room temperature) via a rare stereospecific 1,3-H addition on an unsaturated eta2-cyclopropene intermediate generated by a beta-H abstraction of CH4 from TpMe2NbMe(c-C3H5)(MeCCMe) to give TpMe2NbPh(c-C3H5)(MeCCMe).
Each step of the conversion of a series of 9-alkynyl-9H-fluorenes into the corresponding fluorenylidene-allenes that dimerize and proceed sequentially via head-to-tail and tail-to-tail dialkylidene-cyclobutanes, en route to electroluminescent tetracenes, has been characterized X-ray crystallographically. Allenes possessing substituents of very different electronic and steric character, such as aryl, halogeno, silyl, phosphino, and ferrocenyl, exhibit novel and unexpected reactivity patterns. The silyl-allenes dimerize to yield 1,2-bis(fluorenylidene)cyclobutanes of intrinsic C 2 symmetry as a result of the overlapping fluorenylidenes with their large wingspans. Thermal rearrangement of a bis(fluorenyl)bis(trimethylsilyl)-diallene generates the tetrabenzo-quatercyclopentadiene, C 60 H 36 , which represents 60 % of the C 60 framework. An attempt to isolate a "push-pull" allene, whose central carbon possesses carbene character, was made by incorporating a cation-stabilizing substituent (ferrocenyl) and an aromatic anionic moiety (fluorenide) at the termini. However, the allene underwent facile dimerization to the very heavily congested 3,4-di(spirofluorenyl)-1,2-bis(ferrocenyl-chloromethylene)cyclobutane that exhibits a very long (1.65 Å) C(3)-C(4) bond. Extension of this chemistry to dibenzosuberenylidene-allenes led to a straightforward route to the hitherto difficultly available dibenz[c,d,h]azulene system. Moreover, the reaction of 5-phenylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol with dicobalt octacarbonyl yielded, surprisingly, the first isolated example of a (µ-alkyne)Co 2 (CO) 5 (η 2 -alkene)complex, the long-sought first intermediate in the proposed mechanism of the Pauson-Khand reaction (PKR).
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