Fully localized mixed-valence oxidation products of molecules containing two linked dimolybdenum units: an effective structural criterion

Two previously reported compounds [Mo(2)](CH(3)O)(2)M(CH(3)O)(2)[Mo(2)] (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 4619), in which [Mo(2)] is an abbreviation for the quadruply bonded Mo(2)(4+) unit embraced by three (p-anisyl)NC(H)N(p-anisyl) anions and M = Zn (1) or Co (2), have been chemically oxidized. One-electron oxidation products [Mo(2)](CH(3)O)(2)M(CH(3)O)(2)[Mo(2)](PF(6)) (3, M = Zn; 4, M = Co) and the two-electron oxidation product [Mo(2)](CH(3)O)(2)Zn(CH(3)O)(OH)[Mo(2)](PF(6))(2) (5) have been isolated and structurally characterized. As expected, oxidations occur at the dimolybdenum units. The mono-charged cations in 3 and 4 have asymmetric molecular structures with two distinct [Mo(2)] units. In each case, one of the [Mo(2)] units has a lengthened Mo-Mo bond distance of 2.151[1] A, as expected for one-electron oxidation, whereas the other remains unchanged at 2.115[1] A. These correspond to bond orders of 3.5 (sigma(2)pi(4)delta(1)) and 4.0 (sigma(2)pi(4)delta(2)), respectively. The crystallographic results thus show unambiguously that in the crystalline state, the mixed-valence compounds (3 and 4) are electronically localized and the unpaired electron is trapped on one [Mo(2)] unit. These results are supported by the EPR spectra. The doubly oxidized compound 5 has two equivalent [Mo(2)] units, both with a Mo-Mo bond distance of 2.149[1] A. EPR and magnetic susceptibility measurements for 5 indicate that there is no significant ferromagnetic or antiferromagnetic spin coupling and the species is valence-trapped.     

Asymmetric trimethylsilylcyanation of aldehydes catalyzed by chiral salen TiIV complexes with theC 1 symmetry

Asymmetric trimethylsilylcyanation of a number of aromatic and aliphatic aldehydes catalyzed by chiral Ti^IV complexes preparedin situ from Ti(OPrⁱ)₄ and (1S)-[N,N′-bis(2′-hydroxy-3′-tert-butylbenzylidene)]-1,2-diaminoalkanes gives products with (S)-absolute configurations. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2040–2042, November, 1997.     

Influence of the metal and chiral diamine on metal(II)salen catalysed, asymmetric synthesis of α-methyl α-amino acids

The influence of the metal ion and chiral diamine used to form a metal(salen) complex on the catalytic activity of the complex in the asymmetric benzylation of an alanine enolate was investigated. Only metal ions which could form square-planar complexes gave catalytically active complexes, and best results were obtained with metal ions from the first row of transition metals, particularly copper(II) and cobalt(II). Salen ligands derived from acyclic, chiral 1,2-diamines were found to generate poor catalysts, an effect which seems to correlate with the ability of the substituents within the diamine to adopt a conformation in which they are anti to one another. Complexes derived from a variety of 5- and 6-membered cyclic 1,2-diamines did form active catalysts, but the enantioselectivity was always far lower than that of the parent cyclohexane-1,2-diamine derived complex.     

Alkene and enyne metathesis reactions on allylic and propargylic amines

Treatment of cis-4,5-diaminocyclohexene derivatives bearing allyl or propargyl groups on the nitrogen atoms, with first or second generation Grubbs metathesis catalysts, results in initiation of metathesis cascades which include ring-opening of the unstrained cyclohexene ring. This contrasts with the previous work on the analogous cyclohexene ethers where metathesis reactions occurred exclusively between the side-chains and no ring-opening of the cyclohexene unit was observed.     

Asymmetric cyanohydrin synthesis using heterobimetallic catalysts obtained from titanium and vanadium complexes of chiral and achiral salen ligands

Titanium(IV)(salen) and vanadium(V)(salen) complexes are both known to form catalysts for asymmetric cyanohydrin synthesis. When a mixture of titanium and vanadium complexes derived from the same or different salen ligands is used for the asymmetric addition of trimethylsilyl cyanide to benzaldehyde, the absolute configuration of the product and level of asymmetric induction can only be explained by in situ formation of a catalytically active heterobimetallic complex, and is not consistent with two monometallic species acting cooperatively. Combined use of complexes containing chiral and achiral salen ligands demonstrates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to the vanadium rather than the titanium ion. The titanium complexes also catalyse the asymmetric addition of ethyl cyanoformate to aldehydes, a reaction in which vanadium(V)(salen) complexes are not active. For this reaction, use of a mixture of titanium and vanadium(salen) complexes results in a complete loss of catalytic activity, a result which again can only be explained by in situ formation of a heterometallic complex. Both the titanium and vanadium based catalysts also induce the asymmetric addition of potassium cyanide/acetic anhydride to aldehydes. For this reaction, combined use of chiral and achiral complexes indicates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to titanium rather than vanadium, a result which contrasts with the observed results when trimethylsilyl cyanide is used as the cyanide source.     

Small-angle X-ray scattering studies of a-Si:C:H

Small-angle X-ray scattering studies have been performed on a series of four a-Si:C:H alloys, prepared by rf glow discharge decomposition of varying proportions of propane and silane, in an attempt to elucidate their mesoscopic structure. The observed broad scattering peak has been interpreted as originating from irregular, elongated voids with a repeat distance of about 20 Å and correlation length of about 25 Å. The implications of this result in explaining the photo-oxidation properties of the material are also discussed.     

A direct stereospecific synthesis of 9,11-etheno- and 9,11-ethano-PGH1 derivatives

The reaction of methyl 10-oxodec-S-ynoate with cyclopentadiene is reported as the key step in the synthesis of 9,11-etheno- and 9,11-ethano-PGH₁ derivatives.     

Diffractographic torque sensor

Described is a novel torque sensor utilizing changes in the diffraction pattern of a slit aperture whose edges move relative to each other in proportion to the applied torque. In common with other electro-optical torque sensors, the device is ‘noncontacting’ and, therefore, requires no slip rings and may be read visually. In this form, it is a very simple and inexpensive device, exhibiting no zero shift and capable of operation in extreme environments. The principal disadvantage of the device is its inability to sense torque continuously through the rotational cycle.     

Synthesis of Pt(dpk)Cl(4) and the reversible hydration to Pt(dpk-O-OH)Cl(3).H-phenCl: X-ray, spectroscopic, and electrochemical characterization

We report on the synthesis of a platinum(IV) compound containing a di-2-pyridyl ketone (dpk) ligand that is stable both in its anhydrous form [Pt(dpk)Cl(4)] (1) and in its hydrated form [Pt(dpk-O-OH)Cl(3)].H-phenCl (2). The crystal structure of the hydrated form shows that one of the hydroxide groups from the resulting gem-diol has undergone a cyclometalation/condensation reaction resulting in an oxygen atom directly coordinated to the Pt(IV) center and the formation of H-phenCl. We correlate our physical data with predictions made by molecular modeling, and we propose an explanation for the unusual activity found for this dpk ketone. Spectroscopic and solubility studies are presented here, as well. Electrochemical studies of 1 indicate that it undergoes a highly irreversible reduction at a potential of about -0.45 V vs Ag(+)/Ag in CH(3)CN and that the irreversibility is likely due to an EC mechanism, the nature of which is currently under further investigation. Another distinct redox pair, apparently reversible, appears at a potential of about -1.1 V vs Ag(+)/Ag.     

Synthesis of Cyclic Carbonates Catalysed by Chromium and Aluminium Salphen Complexes

Chromium and aluminium salphen complexes have been found to display remarkable catalytic activity in the synthesis of cyclic carbonates from a range of epoxides and carbon dioxide. The Al(salphen) complex is more reactive towards terminal epoxides at ambient temperature and pressure, whereas the Cr(salphen) complex exhibits higher catalytic activity towards more challenging internal epoxides at elevated temperature and pressure.