Search

The structures and the O--O and M--O bonding characters of a series of reported side-on ([[eta].sup.2]) 1:1 metal complexes of [O.sub.2] are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to O--O bond distance, O--O stretching frequency, and O--O and M--O bond orders provides new insights into subtle influences relevant to [O.sub.2] activation processes in biology and catalysis. The degree of charge transfer from the generally electron-rich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal [d.sub.xy] orbital with the in-plane [[pi].sup.*] orbital of O2 and results in increased M--O bond orders and decreased O--O bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the [O.sub.2] bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated [O.sub.2] is identified as a basis for significant underestimation of the O--O distance at high temperature.

A solution of CrII and CrIII complexes, bis(2,5‐bis{[(2,6‐diisopropylphenyl)imino]methyl}pyrrol‐1‐ido)(tetrahydrofuran)chromium(II)–bis(2,5‐bis{[(2,6‐diisopropylphenyl)imino]methyl}pyrrol‐1‐ido)[(trimethylsilyl)methyl]chromium(III) (0.88/0.12), [Cr(C30H38N3)2(C4H8O)]0.88[Cr(C30H38N3)2(C4H11Si)]0.12 or L2CrII(THF)/L2[(trimethylsilyl)methyl]CrIII (L = 2,5‐bis{[(2,6‐diisopropylphenyl)imino]methyl}pyrrol‐1‐ide and THF is tetrahydrofuran), in pentane crystallizes in the monoclinic space group P21/c. The structure obtained shows most of the atoms coincident but with THF disordered with the (trimethylsilyl)methyl ligand. Structures with this disorder, involving more than two or three atoms, seem to appear rarely in the literature; however, in this case, the data set is ideal for the crystallographic instruction of molecular substitution disorder. [ABSTRACT FROM AUTHOR]

The role of calcium and other cations is characterized in the events of the catalytic turnover cycle. Certain 3d-cations bind to the active site of PLA and some are Ks cofactor for substrate or mimic binding, although without measurable kcat. The kcat cofactor role of a cation is seen in terms of a catalytic mechanism, where the water nucleophile in the expanded coordination area of the cation could be in a favorable near-attack conformation.

X-ray crystal structure determination of coordinatively unsaturated [(Tp(superNp)Co)2(mu-N2)] and Tp(superNp)Co(CO) revealed 'bent' molecules, in which the fourth ligand (N2, CO) is bent away from the pseudo-threefold axis of the TpCo-moiety by 27-38 degrees. Magnesium reduction of Tp(supert-Bu,Me)Co(CO) in THF yielded a dark reddish-purple solution containing a high concentration of [Tp(supert-Bu,Me)Co(mu-CO)2]Mg(THH)4, which revealed a linear molecule. Extended Huckel molecular orbital calculations and the density functional theory showed that the actual and theoretical structure determinations of the TpCo-L fragment, where L is a two-electron donor, are in good agreement.

Alterations in spin states have negligible influence on the rate of organometallic transformations. Paramagnetic tris(pyrazolyl)borate (Tp)(super i-Pr,Me)Co(CO) is prepared from diamagnetic Tp(super i-Pr,Me)Co(CO)2 and is characterized by X-ray diffraction. In the presence of CO both the compounds exist in equilibrium. The CO exchange reaction is fast and represents an interconversion between a triplet state and singlet molecule through a organometallic two-electron process.

(eta(super 5)-C(sub 5)R(sub 5) Cr(II)(mu-Cl))2 is used as the catalyst for the synthesis of dinuclear chromium(II) alkyls of the type (eta5-C5Me5CrII(mu-R))2, where R = Me, Et, Bu, Ph and CH2SiMe3. These compounds are characterized by short Cr-Cr distances, low magnetic moments and diminished reactivity associated with strong metal-metal bonding. The compounds participate in Lewis base-induced cleavage of the Cr-Cr bond, aromatic C-H activation and reaction with oxygen forming high-valent chromium oxo alkyls.

An analysis of the levels of degenerate transfer of electrons, protons and hydrogen atoms between metal complexes revealed that the slowest rate constant, estimated k2 (-34 degrees centigrade) equals 9.0 times 10(to the power of -3) M(to the power of -1) S(to the power of -1), proved that an upper limit for the rate of hydrogen atom transfer was slower than proton transfer, which in turn was slower than electron transfer.

An organocatalytic strategy to the activation of alpha, beta-unsaturated ketones using a new chiral amine catalyst are presented. The scope of LUMO-lowering organocatalysis is extended to accomplish the first general enantioselective ketone Diels-Alder reaction.

(Hydrotris(3-isopropyl-5-methylpyrazolyl)borato)iodocobalt(II) is prepared by the reaction of 3-i-Pr-5-Me-Pyrazol, KI and CoI2. The product is a mixture of three isomers and the required product is obtained by the 'inverse recrystallization' technique. This technique works due to differences in the affinity of regioisomers with donor solvents and is used for the separation of structurally-related metal complexes having low coordination numbers. This ligand is a 'tetrahedral enforcer' and flexible.

The synthesis, characterization and electronic structure of chromium coordinated by diimine ligands are described. The coordination chemistry of the diimine ligands when bound to a variety of low-valent chromium fragments is presented.

The study explains the preparation and structure of a dinuclear chromium diazadiene complex with the shortest metal-metal bond. DFT analysis of its electronic structure indicates high-order metal-metal bonding.