Kenneth W. Barnett

A carbon-13 NMR study of some metal carbonyl compounds containing one-electron ligands

Carbon-13 NMR spectral data for complexes having the general formula CpM(CO)nX (Cp = η5-C5H5; M = Mo or W, n = 3; M = Fe, n = 2; X = halogen, methyl or acetyl) and their phosphine and isocyanide substitution products are reported. For CpM(CO)3X complexes two carbonyl resonances (1 : 2 ratio) are observed in all cases, consistent with the retention of the “piano-stool” geometries observed in the solid state. Substituted complexes CpM(CO)2(L)X (M = Mo or W; L = PR3 or cyclohexyl isocyanide) are unequivocally assigned cis or trans geometries on the basis of the number of observed carbonyl resonances and values of 2J(PC) for the phosphine substituted derivatives. Spectral data for [M(CO)5X]− (M = Cr, Mo or W; X = Cl, Br or I) and η7-C7H7Mo(CO)2X and the halide derivatives above generally show an increase in the shielding for carbonyls adjacent to the halide ligand in the order Cl < Br < I. Carbonyl resonances are more shielded in isostructural complexes in the order Cr < Mo < W (triad effect).

Tertiary phosphine complexes of cyclopentadienylmolybdenum carbonyl halides. Stereoselective syntheses and isomer separations

Abstract Pure cis and trans isomers of CpMo(CO) 2 (L)X (Cp = η 5 -C 5 H 5 , L = PPh 3 or PBu 3 , X = Br, or I) have been separated by chromatography and characterized by infrared and proton NMR spectroscopy. The reactions of trans -CpMo(CO) 2 (L)CH 3 with HgX 2 (X = Cl, Br, I, SCN) afford cis -CpMo(CO) 2 (L)X in high yield. Both linkage isomers are obtained in the reaction with Hg(SCN) 2 , L = PPh 3 . The mercuric halides react with CpMo(CO) 2 (L)COCH 3 to form the metalmetal bonded derivatives trans -CpMo(CO) 2 (L)HgX. Reactions of CpMo(CO) 2 (L)CH 3 or CpMo(CO) 2 (L)COCH 3 with bromine or iodine yield the halide complexes CpMo(CO) 2 (L)X (X = Br and I, respectively), the product mixtures containing high proportions of the trans isomers.

Molybdenum acetyl complexes: a kinetic study of the decarbonylation reaction☆

Abstract The molybdenum acetyl complexes trans-(h5 -C5H5)Mo(CO)2 (L)COCH3 (L = a tertiary phosphine) are thermally decarbonylated in a variety of solvents to the corresponding methyl derivatives (h5-C5H5)Mo(CO)2(L)CH3. Reaction rates are independent of the solvent employed but depend markedly on the nature of the phosphorus ligand. At 60° in acetonitrile the observed order of relative rates is P(cyclo-C6H11)3 (fastest) > P(C6H5)2 (i-C3H7) > P(C6H5)3 > P(p-CH3C6H4)3 > P(p-CH3OC6H4)3 > P(C6H5)2CH3 > P(i-C4H9)3 > P(C6H5)(CH3 )2 ⋟ P(n-C4H9)3 (slowest). These data demonstrate that the rate of decarbonylation is dominated by the steric bulk of the phosphorus ligand, with σ-donor and π-acceptor properties playing only a minor role. A study of the temperature dependence of the reaction rates of the triphenyl- and tri-n-butylphosphine acetyls yielded activation parameters suggesting that dissociation of a terminal CO is the rate-limiting step.

Photochemical isomerization of cyclopentadienylmetal complexes

Abstract The linkage isomers CpM(CO) n SCN and CpM(CO) n NCS (Cp = η-C 5 H 5 ; M = Fe, n = 2; M = Mo, n = 3) are interconverted by 366 nm irradiation in tetrahydrofuran solution at 30°C. Molybdenum and tungsten halide complexes CpM(CO) 2 -(PPh 3 )X undergo cistrans isomerization and disproportionation to CpM(CO)(PPh 3 ) 2 X and CpM(CO)(PPh 3 ) 2 X under similar conditions (benzene solution).

The crystal and molecular sturcture of cyclopentadienyliron dicorbnyl isothiocyanate

Abstract The crystal structure of (η 5 -C 5 H 5 )Fe(CO) 2 NCS has been determined by single crystal X-ray methods. The title compound has a monoclinic unit cell with a 6.400(6), b 7.907(4), c 9.115(8) A and γ 91.48(8)° containing two formula units. The space group is P 2 1 m and the calculated density is 1.69g cm -3 . The structure was refined by least-squares methods to a conventional R index of 0.10 using multiple-film equi-inclination Weissenberg data. The FeNCS fragment is essentially linear and the FeNCS and FeCO linkages are approximately orthogonal. The shortest non-bonded Fe…S interaction is 4.52(1) A.

Rhodium catalyzed reactions of bicyclic hydrocarbons. products, mechanism, and the nature of the catalyst

Abstract The rhodium complexes Rh(PPh 3 ) 3 Cl and Rh(PPh 3 ) 2 (CO)Cl catalyze the ring opening of endo -6-carbomethoxybicyclo[3.1.0]hex-2-ene (I) forming the five possible isomeric carbomethoxycyclohexadienes II–VI. The rate of the reaction is dramatically accelerated by the presence of O 2 , and the dioxygen complex [Rh(PPh 3 ) 2 (Cl)O 2 ] 2 has been shown to be an active catalyst precursor for the reactions. A mechanism involving theintermediacy of cyclohexadienylrhodium hydride complexes accounts for the products formed and their subsequent interconversions.

Rhodium catalyzed reactions of bicyclic hydrocarbons. : Substituent, stereochemical and ring size effects

Tris(triphenylphosphine)rhodium(I) chloride in the presence of oxygencatalyzes the rearrangement of bicyclo[3.1.O]hex-2-enes, producing cyclohexadienes. Rearrangement rates are increased by introduction of methyl or carboxymethyl substituents at C6, the effects being maximized when the methyl group is exo and/or the carboxymethyl group is endo. Bicyclo[4.1.O]hept-2-enes are less reactive but yield analogous products. Bicyclo[6.1.0]non-2-enes are unreactive under the reaction conditions even after two years.