Synthesis and characterization of a family of M(2+) complexes supported by a trianionic ONO(3-) pincer-type ligand: towards the stabilization of high-spin square-planar complexes.

High-spin square-planar molecular compounds are rare. In an effort to access this unique combination of geometry and spin state, we report the synthesis of a series of M(II) compounds stabilized by a trianionic pincer-type ligand, highlighting the formation of a high-spin square-planar Co(II) complex. Low-temperature, variable-frequency EPR measurements reveal that the ground electronic state of the Co(II) analogue is a highly anisotropic Kramers doublet (effective g values 7.35, 2.51, 1.48). This doublet can be identified with the lowest doublet of a quartet, S = 3/2 spin state that exhibits a very large ZFS, D ≥ 50 cm(-1). The observation of an effective g value considerably greater than the largest spin-only value 6, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Density Functional Theory (DFT) and time-dependent DFT calculations reveal the electronic configurations of the ground and excited orbital states. A qualitative crystal field description of the geff tensor shows that it originates from the spin-orbit coupling acting on states obtained through the transfer of a ß electron from the doubly occupied xy to the singly-occupied {xz/yz} orbitals.

A high-spin square-planar Fe(ii) complex stabilized by a trianionic pincer-type ligand and conclusive evidence for retention of geometry and spin state in solution.

Square-planar high-spin Fe(ii) molecular compounds are rare and the only three non-macrocyclic or sterically-driven examples reported share a common FeO4 core. Using an easily modifiable pincer-type ligand, the successful synthesis of the first compound of this type that breaks the FeO4 motif was achieved. In addition, we present the first evidence that geometry and spin state persist in solution. Extensive characterization includes the first high-field EPR and variable field/temperature Mössbauer spectra for this class of compounds. Analysis of the spectroscopic data indicates this complex exhibits a large and positive zero-field splitting tensor. Furthermore, the unusually small ΔEQ value determined for this compound is rationalized on the basis of DFT calculations.

Catalyzed Reformatsky reactions with ethyl bromofluoroacetate for the synthesis of alpha-fluoro-beta-hydroxy acids.

The presence of catalytic amounts of CeCl(3) improves yields and simplifies procedure in the Reformatsky reactions of ethyl bromofluoroacetate with aldehydes and ketones to generate diastereomeric mixtures of alpha-fluoro-beta-hydroxy esters, some of which can be separated by crystallization or column flash chromatography. Diastereomerically pure alpha-fluoro-beta-hydroxy acids are obtained by mild alkaline hydrolysis of the resolved alpha-fluoro-beta-hydroxy esters. Detailed NMR data of new alpha-fluoro-beta-hydroxy esters and alpha-fluoro-beta-hydroxy acids are also presented.

[N-[dimethyl(eta5-2,3,4,6-tetramethylindenyl)silyl]cyclobutylamido-kappaN](eta4-1,4-diphenyl-1,3-buta-diene)titanium(II).

The titanium metal center in the title compound, [Ti(C19H27NSi)(C16H14)], is coordinated in a distorted tetrahedral geometry by a eta5-indenyl ligand, a dimethylsilyl-bridged N-cyclobutylamido ligand and an s-cis-eta4-1,4-diphenyl-1,3-butadiene ligand in a 'prone' pi-fashion, revealing a formal divalent Ti center.

Total synthesis of (+/-)-asteriscanolide.

The total synthesis of asteriscanolide (1) has been achieved by taking advantage on an intermolecular Pauson-Khand cycloaddition and a ring-closing metathesis as key bond-forming transformations. The approach incorporates the cyclooctane stereogenic center prior to ring formation. Interestingly, the ring-closing metathesis generates a new eight-membered ring with an "in-out" intrabridgehead relationship.

The first synthesis and characterization of both diastereomers of a di[2.2]paracyclophane: 4,4'-bis(1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane).

The synthesis and characterization of both diastereomers of a system comprised of two [2.2]paracyclophane units linked through a single 4,4' bond are described. Both the meso and d,l diastereomers of 4,4'-bis(octafluoro[2.2]paracyclophane) have been prepared via a palladium-catalyzed reductive homocoupling reaction by copper, producing a 3:2 ratio of meso and d,l diastereomers. A similar reductive homocoupling of pseudo-o-iodotrifluoromethyloctafluoro[2.2]paracyclophane gave only the analogous meso diastereomer. Single-crystal X-ray structures were obtained for all of the diparacyclophane products.

[Ni(terpy)(H2O)]-trans-[Ni-(mu-CN)2-(CN)2]n, a one-dimensional linear tetracyanonickelate chain.

The one-dimensional chain catena-poly[[aqua(2,2':6',2"-terpyridyl-kappa(3)N)nickel(II)]-mu-cyano-kappa(2)N:C-[bis(cyano-kappaC)nickelate(II)]-mu-cyano-kappa(2)C:N], [Ni(terpy)(H(2)O)]-trans-[Ni-mu-(CN)(2)-(CN)(2)](n) or [Ni(2)(CN)(4)(C(15)H(11)N(3))(H(2)O)], consists of infinite linear chains along the crystallographic [10-1] direction. The chains are composed of two distinct types of nickel ions, paramagnetic octahedral [Ni(terpy)(H(2)O)](2+) cations (with twofold crystallographic symmetry) and diamagnetic planar [Ni(CN)(4)](2-) anions (with the Ni atom on an inversion center). The [Ni(CN)(4)](2-) units act as bidentate ligands bridging through two trans cyano groups thus giving rise to a new example of a trans-trans chain among planar tetracyanonickelate complexes. The coordination geometry of the planar nickel unit is typical of slightly distorted octahedral nickel(II) complexes, but for the [Ni(CN)(4)](2-) units, the geometry deviates from a planar configuration due to steric interactions with the terpyridine ligands.

Coordinatively unsaturated W(IV)-bis(pyridine) complexes, a reactive source of W(IV).

Addition of 2 equiv of a sigma-donor ligand (L = pyridine, 4-picoline, or quinoline) to complexes of the type [W(NPh)(eta(4)-arene)(o-(Me3SiN)2C6H4)] (arene = CH3CH2C6H5 (3), CH3CH2CH2C6H5 (4)) gave the W(IV)L2 compounds, [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)2] (5), [W(NPh)(o-(Me3SiN)2C6H4)(p-C6H7N)2] (6), and [W(NPh)(o-(Me3SiN)2C6H4)(C9H7N)2] (7). Synthesis of compounds 5 and 6 by Na degrees reduction of [W(NPh)(o-(Me3SiN)2C6H4)Cl2] in the presence of 3 equiv of L (L = 5, pyridine or 6, 4-picoline) is also presented. Compounds 5, 6, and 7 display hindered rotation of the donor ligands about the W-N bonds, resulting from a steric interaction with the Me3Si groups of the diamide ligand. The coordinative unsaturation of 5 and 6 has also been explored. Compounds 5 and 6 readily react with either CO and PMe3 to generated the six coordinate complexes [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)2(CO)] (8a), [W(NPh)(o-(Me3SiN)2C6H4)(C6H7N)2(CO)] (8b), [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)(PMe3)2] (10a), and [W(NPh)(o-(Me3SiN)2C6H4)(C6H7N)(PMe3)2] (10b), respectively.

Synthesis of L-4,4-difluoroglutamic acid via nucleophilic addition to a chiral aldehyde.

Fluorine-containing derivatives of amino acids are assuming increasing importance as probes of biological function and enzyme mechanism. We now report a new, flexible route to enantiomerically pure L-4,4-difluoroglutamic acid that exploits the addition of difluorinated nucleophiles to configurationally stable alpha-aminoaldehydes. Conversion of the difluorinated adducts to L-4,4-difluoroglutamic acid can be accomplished in three steps by Barton-McCombie dehydroxylation and acid hydrolysis.

Synthesis and structure of W(eta(2)-mp)(2)(CO)(3) (mp = monoanion of 2-mercaptopyridine) and its reactions with 2,2'-pyridine disulfide and/or NO to yield W(eta(2)-mp)(4), W(eta(2)-mp)(2)(NO)(2), and W(eta(2)-mp)(3)(NO).

Oxidative addition of the sulfur-sulfur bond of 2,2'-pyridine disulfide (C(5)H(4)NS-SC(5)H(4)N) with L(3)W(CO)(3) [L = pyridine, (1)/(3)CHPT; CHPT = cycloheptatriene] in methylene chloride solution yields the seven-coordinate W(II) thiolate complex W(eta(2)-mp)(2)(CO)(3) (mp = monoanion of 2-mercaptopyridine). This complex undergoes slow further oxidative addition with additional pyridine disulfide, yielding W(eta(2)- mp)(4). Reaction of W(eta(2)-mp)(2)(CO)(3) with NO results in quantitative formation of the six-coordinate W(0) complex W(eta(2)-mp)(2)(NO)(2). Reaction of W(eta(2)-mp)(2)(CO)(3) with NO in the presence of added pyridine disulfide yields the seven-coordinate W(II) nitrosyl complex W(eta(2)-mp)(3)(NO) as well as W(eta(2)-mp)(2)(NO)(2) and trace amounts of W(eta(2)-mp)(4). The complex W(eta(2)-mp)(3)(NO) is formed during the course of the reaction and not by reaction of W(eta(2)-mp)(4) or W(eta(2)-mp)(2)(NO)(2) with NO under these conditions. The crystal structures of W(eta(2)- mp)(2)(CO)(3), W(eta(2)-mp)(2)(NO)(2), and W(eta(2)-mp)(3)(NO) are reported.

Synthesis and biological evaluation of 17beta-alkoxyestra-1,3, 5(10)-trienes as potential neuroprotectants against oxidative stress.

17beta-O-Alkyl ethers (methyl, ethyl, propyl, butyl, hexyl, and octyl) of estradiol were obtained from 3-O-benzyl-17beta-estradiol with sodium hydride/alkyl halide, followed by the removal of the O-benzyl protecting group via catalytic transfer hydrogenation. An increase compared to estradiol in the protection of neural (HT-22) cells against oxidative stress due to exposure of glutamate was furnished by higher (C-3 to C-8) alkyl ethers, while methyl and ethyl ethers decreased the neuroprotective effect significantly. Lipophilic (butyl and octyl) ethers blocking the phenolic hydroxyl (3-OH) of A-ring were inactive.

Bond valence sums in coordination chemistry. Calculation of the oxidation state of chromium in complexes containing only Cr-O bonds and a redetermination of the crystal structure of potassium tetra(peroxo)chromate(V).

A simple method for calculating the oxidation state of Cr in complexes containing only Cr-O bonds is presented. A total of 242 CrOn fragments with n = 3-6 were retrieved from the Cambridge Structural Database (CSD) and, together with the data for K3CrO8, were analyzed using the bond valence sum method. New R0 values for Cr(II) of 1.739(21) A, Cr(III) of 1.708(7) A, Cr(V) of 1.762(14) A, and Cr(VI) of 1.793(7) A were derived. An average R0 value of 1.724 A for Cr-O reproduces the oxidation state of 96 of the 110 Cr(II), Cr(III), and Cr(IV) CrOn complexes (n = 3-6) and that of K3CrO8 within 0.30 valence units. The crystal structure of K3CrO8 was redetermined at 173 K to provide accurate data for a Cr complex with both high oxidation state and coordination number. Potassium tetraperoxochromate(V), K3CrO8, is tetragonal, Space group I42m, a = b = 6.6940(3) A, c = 7.7536(5) A, Z = 2. The difficulties with fitting the observed valence for Cr(V) and Cr(VI) complexes with coordination numbers 4 and 5 are discussed. The use of bond valence sums in gaining chemical insight into Cr complexes with noninnocent ligands and in establishing oxidation states in Cr clusters is presented. An analysis of the Cr-O bond distances used in the calculations shows a large range of values that can be understood in terms of the bond valence sum calculation.

Absolute configurations of three ent-beyerene derivatives.

(1): 11 beta,17-Bis(4-bromobenzoyloxy)-9 beta-hydroxy-ent-beyerene, C34H38Br2O5, Mr = 686.5, orthorhombic, P2(1)2(1)2(1), a = 6.1686 (12), b = 14.613 (3), c = 32.935 (7) A, V = 2969 (1) A3, Z = 4, Dx = 1.536 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 27.43 cm-1, F(000) = 1408, T = 173 K, R = 0.0347 and wR = 0.0358 for 3064 reflections [Io greater than or equal to 3 sigma(Io)]. (2): 7 beta,17-Bis(4-bromobenzoyloxy)-9 beta-hydroxy-ent-beyerene, C34H38Br2O5, Mr = 686.5, monoclinic, P2(1), a = 12.6424 (15), b = 6.8353 (12), c = 18.496 (2) A, beta = 101.901 (9) degrees, V = 1564.0 (4) A3, Z = 2, D chi = 1.458 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 26.03 cm-1, F(000) = 704, T = 173 A, R = 0.0336 and wR = 0.0361 for 4610 reflections [Io greater than or equal to 3 sigma(Io)]. (3): 17-(4-Bromobenzoyloxy)-9 beta,11 beta-epoxy-7 beta-hydroxy-ent-beyerene, C27H33BrO4, Mr = 501.5, orthorhombic, P2(1)2(1)2(1), a = 6.1315 (7), b = 18.389 (3), c = 21.109 (3) A, V = 2380.0 (6) A3, Z = 4, D chi = 1.399 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 17.38 cm-1, F(000) = 1048, T = 198 K, R = 0.0325 and wR = 0.0344 for 2292 reflections [Io greater than or equal to 3 sigma(Io)].(ABSTRACT TRUNCATED AT 250 WORDS)

Structure of 1,2,3,4,5,6-hexa-O-acetyl-myo-inositol.

C18H24O12, Mr = 432.4, monoclinic, Cc, a = 8.996 (3), b = 20.890 (6), c = 11.872 (4) A, beta = 101.11 (2) degrees, V = 2189 (1) A3, Z = 4, Dx = 1.312 g cm-3, Mo K alpha, lambda = 0.71069 A, mu = 1.05 cm-1, F(000) = 912, T = 163 K, R = 0.041, wR = 0.0375 for 2158 reflections (Fo greater than or equal to 6 sigma magnitude of Fo). The ring is in the chair conformation 4C1 with five equatorial groups and one axial group bonded to C(2) as expected. The carbonyl bonds of the acetate groups at positions 2, 4, 5 and 6 are approximately coplanar with their respective ring C-H bonds. However, those at positions 1 and 3 are rotated towards the H(2) atom.

Structure of (+/-)-encecanescin.

C24H30O4, Mr = 382.5, orthorhombic, P212121, a = 13.091 (2), b = 19.711 (1), c = 8.242 (1) A, V = 2126.7 (5) A3, Z = 4, Dx = 1.195 Mg m-3, lamba (Cu K alpha) = 1.54184 A, mu(Cu K alpha) = 0.56 mm-1, F(000) = 824, T = 295 K. Final R = 0.045 for 2446 unique reflections. The planar A ring is bent relative to the rest of the steroid skeleton. The B ring has a typical chair conformation and the C and D rings assume 13 beta,14 alpha-half-chair and 13 beta-envelope conformations, respectively. The conformation of the progesterone side chain is similar to the conformation observed in other 17 alpha-ester pregnanes: C16-C17-C20-O20 torsion angle -27.4(3) degrees.

Structure of a non-planar flavonol, 5,7,2'-trihydroxy-3,6,8,4',5'-pentamethoxyflavone.

C20H20O10, Mr = 420.37, triclinic, P1, a = 8.771 (3), b = 13.508 (3), c = 15.970 (4) A, alpha = 86.04 (2), beta = 81.77 (2), gamma = 84.17 (2) degrees, V = 1860.0 (8) A3, Z = 4 (two molecules per asymmetric unit), Dm (by flotation in ZnCl2/H2O solution at 298 K) = 1.491, Dx = 1.503 g cm-3, Mo K alpha radiation, lambda = 0.71069 A, mu = 1.14 cm-1, F(000) = 880, T = 163 K. Full-matrix least-squares refinement converged to R = 0.039 and wR = 0.037 using 5198 reflections with I greater than 3 sigma (I). The two molecules in the asymmetric unit differ in the torsion angles involving the methoxy groups and in the dihedral angle between the planes of rings B and C, 33.31 (7) and 40.83 (7) degrees in molecules (I) and (II), respectively. This compound belongs to a class of 3,2'-dioxygenated flavonoids whose ultraviolet (UV), mass spectroscopic (MS) and chromatographic properties indicate that in solution these compounds have reduced planarity (and thus reduced conjugation) of the B-ring chromophore with the A, C-ring chromophore.

The X-ray crystal structure of DL-(1,3,5/2,4)-1,2,3,4-tetraacetoxy-5-(acetoxymethyl)cyclohexane.

The crystal and molecular structure of the pseudo-sugar DL-(1,3,5/2,4)-1,2,3,4-tetraacetoxy-5-(acetoxymethyl)cyclohexane ("pseudo-beta-DL-glucopyranose pentaacetate") has been determined by X-ray diffraction and statistical-phasing procedures. The data were refined to R = 0.049 and Rw = 0.054 over 1543 reflections with I greater than 3 sigma(I). The racemic crystals are monoclinic, space group P2(1)/c, a = 11.580(2), b = 8.276(1), c = 22.031(2) A, beta = 104.33(1) degrees, Dc = 1.26 g/cm3, with four molecules in the unit cell. The ring has a 4C1(D), 1C4(L) conformation, with puckering amplitude of 0.582(4) A, a pseudorotation angle of -168.35 degrees, and a gg orientation about the C-5-C-6 bond.