Armin J. Mayr

Transition metal heterocyclic chemistry: XI. Manganese cyclopentadienyldicarbonyl complexes of 1,2,3-selena- and thiadiazoles including structural comparison of free and complexed 1,2,3-benzothiadiazole and 4-phenyl-1,2,3-thiadiazole

Reactions of substituted 1,2,3-selena- and thia-diazoles with (η5-C5H4R)Mn(CO)2THF, R  H, Me, lead to isolable blue complexes in which THF is replaced by the diazole ligand. The X-ray structure of the benzothiadiazole complex is reported and shows the intact diazole bonded via N2 to manganese. Ab initio calculations on 1,2,3-thiadiazole generated a structure and protonation energies that confirm N2 as the site of coordination, and also predict significant structural variations upon coordination. However, the structure of the free benzo-1,2,3-thiadiazole was determined and shows no significant structural change upon coordination. Similarly, the structure of 4-phenyl-1,2,3-thiadiazole was deterined and exhibits the same structural parameters as the W(CO)5 substituted ligand.

Transition metal heterocyclic chemistry. IX. An α-selenoketoketene. Trapping subsequent to oxidative release from tricarbonyliron complexes to form vinyl selenides

Treatment of α-selenoketocyclohexylene-Fe2(CO)6 (6) and the related α-selenoketoketene complex (2) with trimethylamine oxide leads to the formation of a transient α-selenoketoketene, a previously unknown species. The transient species may be trapped by alcohols to produce esters of bis(2-carboxy-1-cyclohexenyl)diselenide, examples of vinylselenides. Reductive elimination of selenium from these esters using Raney Nickel leads to the formation of the cyclohexane carboxylate esters.

Decamethylpentasilacycloheptyne·Mo2(CO)4(η5-C5H5)2 and cycloheptyne·Mo2(CO)4(η5-C5H5)2

Abstract A dimolybdenum derivative of decamethylpentasilacycloheptyne (1) was synthesized by direct reaction of the heptyne with Mo2(CO)4(η5-C5H5)2. 1 crystallized in the space group P 1 , a=9.386(2), b=9.866(3), c=20.178(4) A, α=92.17(2), β=97.17(2), γ=115.71(2)°. The acetylenic bond is lengthened from 1.213 A in the free ligand to 1.359(4) A and all the Si–Si bond lengths in 1 are significantly lengthened upon complexation. This is due to relaxation of the ring strain as evidenced by the Si–C–C bond angles in 1 of 132.7 and 140.9° compared to 159.2 and 162.6° in the uncomplexed ring. 29Si-NMR data exhibit significant downfield chemical shifts upon complexation for the Si atoms adjacent to the triple bond, with moderate upfield shifts for the other Si atoms. The related cycloheptyne·Mo2(CO)4(η5-C5H5)2 (2) was synthesized by the reaction of cyclohepteno-1,2,3-selenadiazole with Mo2(CO)4(η5-C5H5)2. 2 crystallized in the space group C21/c, a=30.396(10), b=8.9093(3), c=16.156(4) A, β=115.39(2)°. The acetylenic bond in 2 is 1.345 A, compared with a calculated value (ab initio 3-21 G*) of 1.190 A for the free cycloheptyne.

Electrochemistry and spectroelectrochemistry of a linear triiron cluster

Abstract This work reports electrochemical and spectroelectrochemical studies of a unique linear triiron cluster carbonyl complex, Fe 3 (CO) 7 L 2 , where L is a α-diazothioketone. Oxidation and reduction reactions have been observed in non-aqueous media over the temperature range −40 to 20 °C by differential pulse voltammetry, cyclic voltammetry, thin-layer, UV-Vis spectroelectrochemistry and ESR spectrometry. The sequence of the individual electron-transfer steps comprising the overall redox process is described, and a comparison between the electrochemistry of different non-linear iron carbonyl complexes is discussed. A single one electron reduction produces the radical anion, [Fe 3 (CO) 7 L 2 ]-, which decomposes at temperatures greater than −10 °C to species which are reduced at a more negative potential, an ECE mechanism. A single one-electron oxidation produces the radical cation, [Fe 3 (CO) 7 L 2 ] + , which is unstable, decomposing completely at room temperature, an EC mechanism. Spectroscopic evidence indicates that in non-bonding solvents, the Fe 3 (CO) 7 L 2 framework remains intact at low temperatures for both the anion and cation radical produced electrochemically with radical stability higher than might be expected for a linear structure. Observations indicate only strongly bonding solvents disrupt the structure. Low temperature stability occurs at relatively high temperatures, with the cation radical less than stable and vulnerable to strongly bonding solvents.

Crown ethers as molecular bromine carriers for bromination reactions

Macrocyclic polyethers (crown ethers) form molecular complexes with bromine that may be used as reagents for the bromination of olefins and acetylenes. We have used 12-crown-4,15-crown-5,18-crown-6, polydibenzo-18-crown-6, dibenzo-18-crown-6, and 6-crown-2(dioxan) as carriers for bromine, and have investigated the thermodynamics of complex formation and the kinetics and stereochemistry of bromine addition to cis- and trans-β-methylstyrene. Formation constants for the ethers are similar implying that all interactions are similar to the known 6-crown-2·Br2 polymeric structure, and do not involve encapsulation by the macrocycle. Kinetic results also indicate very little difference between the rates of bromination as the ethers are varied. The stereoselectivity of addition, however, is significantly changed as a function of the ether. The solid dibenzo- and polydibenzo-18-crown-6 ether complexes exhibit very high stereoselectivity for anti-bromination, whilst the other ethers and carriers such as silica gel, alumina, and montmorillonite show enhanced formation of syn-addition, compared with other modes of bromine delivery (e.g. free Br2, pyr·Br2). The use of poly-dibenzo-18-crown-6 as a stationary phase column-packing, enabling stereospecific or stereoselective bromination, is the preferred technique for such brominations.

Crown ether complexes of molecular bromine as stereoselective brominating agents

Isolable crown ether complexes of molecular bromine have been shown to effect highly stereoselective brominations of cis- and trans-β-methylstyrene.