Showing papers by "Thomas F. Koetzle published in 1994"

Electron distribution in a stable carbene

Abstract: The synthesis and characterization of the stable carbene perdeuterio-1,3,4,5-tetramethylimidazol-2-ylidene (2) are reported. The neutron and X-ray diffraction data from single crystals of this perdeuterocarbene are used to determine the electron distribution in the molecule. Neutron diffraction data were collected at 80 and 173K on 2. The experimentally determined electron density is very closely matched by density functional calculations which show that 2 is a true carbene with negligible ylidic character. 71 refs., 6 figs., 5 tabs.

Single Crystal Neutron Diffraction Study of the Complex [Ru(H.cntdot..cntdot..cntdot.H)(C5Me5)(dppm)]BF4 which Contains an Elongated Dihydrogen Ligand

Abstract: The H-H distance of the dihydrogen ligand in [Ru(H....H)(C{sub 5}Me{sub 5})(dppm)]BF{sub 4} is 1.08(3) A as determined by neutron diffraction on a crystal of volume 3.2 mm{sup 3} at 15 K: monoclinic, space group P2{sub 1}, a = 10.833(3), b = 15.045(4), c = 10.781(2) A, {beta} = 114.47(2){degree}, V = 1599.3(7) A{sup 3}, and D{sub c} = 1.473 g cm{sup -3} for Z = 2; R(F) = 0.071, R(F{sup 2}) = 0.151, R{sub w}(F{sup 2}) = 0.099 for 4198 reflections. The H-H distance, when corrected for the effects of thermal motion, lengthens slightly from 1.08(3) to 1.10(3) A. The complex contains an elongated dihydrogen ligand which is symmetrically, side-on bonded to the ruthenium as one leg of a three-legged piano stool structure. The H{sub 2} ligand lies parallel to the C{sub 5}Me{sub 5} ligand plane, an orientation where {pi}-overlap with an orbital on the metal is optimum. The T{sub 1} NMR method gives an H H distance of 1.10(1) A for the case of restricted rotation. The present structure provides a model for coordinated H{sub 2} at an intermediate stage of oxidative addition. The {sup 1}J(H, D) coupling in the Ru(H-D) isotopomer increases with decreasing temperature: this is interpretedmore » as a slight shortening of the H-D distance. The structural and {sup 1}H NMR data are compared for dihydrogen complexes which have been studied by single crystal neutron diffraction. 34 refs., 3 figs., 4 tabs.« less

OsH5(PMe2Ph)3+: Structure, Reactivity, and Its Use as a Catalyst Precursor for Olefin Hydrogenation and Hydroformylation

Abstract: OsH{sub 5}L{sub 3}{sup +} (as its BF{sub 4}{sup {minus}} salt; L = PMe{sub 2}Ph) is characterized by neutron diffraction as a dodecahedral pentahydride. However, the H/H separations are as short as 1.49(4) {angstrom}. This cation exchanges OsH with D{sub 2} and is converted by CO to OsH(CO){sub 2}L{sub 3}{sup +}. This cation hydrogenated ethylene (<1 h) at 25 {degrees}C to generate cis mer-OsH(C{sub 2}H{sub 4}){sub 2}L{sub 3}{sup +}, which was characterized by variable-temperature {sup 31}P, {sup 1}H, and {sup 13}C NMR spectroscopy and X-ray diffraction. The orientation of the olefins in this compound is explained using extended Hueckel methods, as is the lack of a structural trans influence on Os-C bond lengths and the reversed kinetic trans effect. Studies with Os/D and C{sub 2}D{sub 4} labeling, as well as trapping of transients with CO and with 1,5-cyclooctadiene, serve to define a reaction mechanism for ethylene hydrogenation, and the pentahydride cation also hydrogenates cyclohexene. In spite of the mutual trans stereochemistry of hydride and ethylene in OsH-(CO)(C{sub 2}H{sub 4})L{sub 3}{sup +} and then more slowly to cis,mer-Os[C(O)Et](CO){sub 2}Lp{sub 3}{sup +}. A variety of isotopic labeling studies prove that the first of these reactions does not involve preequilibrium dissociation of CO, PMe{submore » 2}Ph, or C{sub 2}H{sub 4}, nor does it involve bimolecular proton transfer to form the ethyl group. Intramolecular hydrogen migration to bring H cis to C{sub 2}H{sub 4} is consistent with all observations. The carbonyl ligands in OsH(CO){sub 2}L{sub 3}{sup +} are susceptible to nucleophilic attack by solvent in water, to effect the water gas shift reaction. In the presence of water and 1-hexene, Reppe hydroformylation (H{sub 2}O + 2CO + olefin {yields} aldehyde + CO{sub 2}) is catalyzed by OsH(CO)(L{prime})L{sub 3}{sup +} (L{prime} = CO or C{sub 2}H{sub 4}) and by Os(Et)(CO){sub 2}L{sub 3}{sup +}.« less

Determination of the Structure of [Os(.eta.2-H2)en2CH3CO2]PF6 by X-ray and Neutron Diffraction

Abstract: The structure of [Os([eta][sup 2]-H[sub 2])en[sub 2]CH[sub 3]CO[sub 2]]PF[sub 6] (1) has been studied by X-ray diffraction at 120 K and neutron diffraction at 165 K. Compound 1 belongs to a series of dihydrogen complexes of osmium(II) amines, exhibiting an extensive range in the strength of the H-H interaction as judged by the wide variation in observed J[sub H-D] coupling constants. The ethylenediamine ligands in I have a trans arrangement. The H[sub 2] and the acetate ligands occupy positions on opposite sides of the OsN[sub 4] plane, giving overall approximately octahedral coordination. Consistent with the relatively low J[sub H-D] value (9.0 Hz), the H-H distance in 1.134(2) [angstrom] as determined by neutron diffraction, is unusually long and approaches that of 1.357(7) [angstrom] found in the polyhydride ReH[sub 7][P(p-tolyl)[sub 3]][sub 2]. It is presumed that I may represent an intermediate stage in the oxidative addition of dihydrogen at transition-metal centers. The Os-H distances in 1.1.59(1) [angstrom] and 1.60(2) [angstrom], are shorter than those in the classical osmium polyhydride OsH[sub 4](PMe[sub 2]Ph)[sub 3] (mean Os-H 1.659(3) [angstrom]) lending support to the assertion that the Os([eta][sup 2]-H[sub 2]) interaction is relatively strong. Crystal data are provided. 46 refs., 3 figs., 2 tabs.

Regioselective complexation of unprotected carbohydrates by platinum(II). Synthesis, structure, complexation equilibria, and hydrogen-bonding in carbonate-derived bis(phosphine)platinum(II) diolate and alditolate complexes

Abstract: Treatment of bis(phosphine)platinum(II) carbonate complexes (LL)Pt(CO[sub 3]) (e.g., LL = 1,3-bis(diphenylphosphino)propane) with vicinal diols (i.e., HOCR[sup 1]R[sup 2]CR[sup 3]R[sup 4]OH) gives equilibrium conversion to the corresponding diolate complexes (LL)Pt(OCR[sup 1]R[sup 2]CR[sup 3]R[sup 4]O), which are readily isolated in good yield. From competition experiments, relative diol complexation constants were determined as a function of both the diol and the phosphine substituents and found to span a range of over 10[sup 4]. Corresponding triolate and alditolate complexes were similarly prepared, for which very distinct equilibrium isomeric regioselectivities are observed, favoring complexation of [gamma],[delta]-threo diols. An X-ray structure of (dppp)Pt(D-mannitolate) shows that the mannitol is bonded to the platinum as its dianion via the oxygens on C3 and C4 to form a 2,5-dioxaplatinacyclopentane chelate ring and that three different strong intramolecular hydrogen-bonding interactions are present between the hydroxyl hydrogens and the metallacycle oxygens (O-O) (av) = 2.65(2) [angstrom], forming five-, six-, and seven-membered rings. Crystal data for PtP[sub 2]O[sub 6]C[sub 33]H[sub 38]-CH[sub 2]Cl[sub 2]: P2[sub 1]2[sub 1]2[sub 1], Z = 4, T = 20 [degree]C, a = 11.225(2) [angstrom], b = 15.875(3) [angstrom], c = 19.964(4) [angstrom], R(F[sub 0]) = 0.058, R[sub w](F[sub o]) = 0.062. 78 refs., 4 figs., 6 tabs.

Determination of the absolute configuration of (+)-neopentyl-1-d alcohol by neutron and x-ray diffraction analysis.

Abstract: The absolute configuration of (+)-neopentyl-1-d alcohol, prepared by the reduction of 2,2-dimethylpropanal-1-d by actively fermenting yeast, has been determined to be S by neutron diffraction The neutron study was carried out on the phthalate half ester of neopentyl-1-d alcohol, crystallized as its strychnine salt The absolute configuration of the (-)-strychninium cation was first determined by an x-ray anomalous dispersion study of its iodide salt The chiral skeleton of strychnine then served as a reference from which the absolute configuration of the -O-CHD-C(CH3)3 group of neopentyl phthalate was determined Difference Fourier maps calculated from the neutron data showed unambiguously that the -O-CHD-C(CH3)3 groups of both independent molecules in the unit cell had the S configuration This work proves conclusively that the yeast system reduces aldehydes by delivering hydrogen to the re face of the carbonyl group Crystallographic details: (-)-strychninium (+)-neopentyl-1-d phthalate, space group P2(1) (monoclinic), a = 18564(6) A, b = 7713(2) A, c = 23361(8) A, beta = 9418(4) degrees, V = 33360(5) A3, Z = 2 (T = 100 K) Final agreement factors are R(F) = 0073 for 2768 reflections collected at room temperature (x-ray analysis) and R(F) = 0144 for 960 reflections collected at 100 K (neutron analysis)