J. Appl. Cryst.の発刊に際して

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined:  X = I < Br < Cl and PR3 = PPh3 ≪ PiPr2Ph < PCy2Ph < PiPr3 < PCy3. Additional studies were conducted with the catalyst (PCy3)2Cl2RuCH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways:  the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the addition of CuCl to the reaction.

Well-defined ruthenium olefin metathesis catalysts: Mechanism and activity

Structural chemistry of transition metal complexes of oximes

Chemistry and enzymology of vitamin B12.

The tridentate iminooximes HL2 and HL1 have been prepared by condensation of biacetyl monoxime with 2-(2-aminoethyl)pyridine and 2-(aminomethyl)pyridine, respectively. They reacted with cobalt(II) ion in the presence of oxygen to give the complexes [CoIIIL2]ClO4(L = L1 or L2). The iminooxime may be hydrogenated in the presence of NaBH4 to yield the corresponding aminooximes HL3 and HL4, which, on reaction with CoII, formed the complexes [CoIIIL(HL)][ClO4]2(L = L3 or L4). All these complexes have been structurally characterized by single-crystal X-ray diffraction. The cobalt has an octahedral environment with the iminooxime ligands in mer configuration and the aminooxime ones in fac configuration. The reduction of [CoIIIL22]ClO4 with NaBH4 produces a nucleophilic cobalt(I) species which, on reaction with MeI, gives a stable organocobalt dinuclear complex, previously reported. On the contrary, the reduction of [CoIIIL12]ClO4 involves hydrogenation of the ligand from imino- to amino-oxime, with the formation of a stable cobalt(II) species. Under these conditions, no reduction to CoI was observed. The different reactivity is attributed to the more strained co-ordination in [CoIIIL12]ClO4 with respect to that in [CoIIIL22]ClO4.

Synthesis, structure and reactivity of cobalt(III) complexes with tridentate imino- and amino-oxime ligands

The complex mer-[Co I I I (L 1 Npy) 2 ] + (1') where the L 1 Npy - is the tridentate 3-[(2-pyridyl)methylimino]butan-2-one oximate ligand, gives alkyl-cobalt derivatives after reduction with NaBH 4 /Pd 2 + to the Co 1 and alkylation. The formation of the cobalt-carbon bond is accompanied by the reduction to the amino form of one or both imino ligands (depending on the experimental conditions) initially present in 1'. In one series of experiments, complexes of the type fac-[RCo I I I (L 1 Npy)(H-L 1 NHpy)] + (R = Me, i-Pr, CH 2 Cl, CH 2 Br, CH 2 CF 3 , and Bz) were obtained, in which only one of the two ligands was reduced to the amino form (H-L 1 NHpy). The saturation of one azomethine group causes the products to assume a fac configuration and induces the formation of one asymmetric carbon and one asymmetric nitrogen center in the chelating system. When an excess of reducing agent is used, both azomethine groups may be saturated, causing the introduction of one pair of chiral carbons and one pair of chiral nitrogens. Two isomers of the methyl derivative [Meco I I I (L 1 NHpy)(H-L 1 NHpy)]+ were isolated. The X-ray analysis reveals that these isomers differ from one another in configuration of the C and N chiral centers. Possible reaction mechanisms leading to these different types of complexes are proposed.

New alkyl-cobalt(III) complexes containing chiral centers in the chelating system.

Synthesis, Characterization, and Reactivity of a New Mononuclear Benzyl−Cobalt Complex Containing Mixed Tridentate Imino−Oxime and Diamine Ligands

The kinetics of axial water substitution by cysteine in six different cobaloximes, viz.trans-RCo(Hdmg)(2)H(2)O, where Hdmg = dimethylglyoximate, R = cyclo-C(5)H(9) (c-P), CH(3)CH(2) (Et), CH(3) (Me), C(6)H(5)CH(2) (Bz), C(6)H(5) (Ph) and CF(3)CH(2), were studied as a function of cysteine concentration, temperature and pressure. It was found that cysteine substitutes the coordinated H(2)O molecule trans to the alkyl group with second order rate constants that follow the order of reactivity: c-P > Et > Bz > Me > Ph > CF(3)CH(2). Rate and activation parameters (Deltan H(++), Delta S(++) and Delta V(++)) enable the formulation of a reaction mechanism that can account for the substitution behaviour of the investigated alkylcobaloximes. In particular, a gradual mechanistic changeover from I(d) to I is observed along the series of R groups from c-P to CF(3)CH(2).

Giovanni Tauzher

Papers

Synthesis, structure and reactivity of cobalt(III) complexes with tridentate imino- and amino-oxime ligands

New alkyl-cobalt(III) complexes containing chiral centers in the chelating system.

Synthesis, Characterization, and Reactivity of a New Mononuclear Benzyl−Cobalt Complex Containing Mixed Tridentate Imino−Oxime and Diamine Ligands

Electronically and sterically tuned trans labilization controls the substitution behaviour of cobaloximes

Kinetics and mechanism of ligand exchange reactions of the methylbis(dimethylglyoximato)rhodium(III) complex in non-coordinating solvent