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Khalil A Abboud

Publications and source records attributed to Khalil A Abboud.

At least 55 records · Page 3Linked to original sources
PubMed2004

Synthesis, structure, and magnetic properties of a Mn(21) single-molecule magnet.

The reaction of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) (1; 3Mn(III)) with [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (2; 10Mn(III)) in MeCN affords the new mixed-valent complex [Mn(21)O(14)(OH)(2)(O(2)CMe)(16)(hmp)(8)(pic)(2)(py)(H(2)O)](ClO(4))(4) (3; 3Mn(II)-18Mn(III); hmp(-) is the anion of 2-(hydroxymethyl)pyridine), with an average Mn oxidation state of +2.85. Complex 3.7MeCN crystallizes in the triclinic space group P. The structure consists of a low symmetry [Mn(21)(micro(4)-O)(4)(micro(3)-O)(12)(micro-O)(16)] core, with peripheral ligation provided by 16 MeCO(2)(-), 8 hmp(-), and 2 pic(-) groups and one molecule each of water and pyridine. The magnetic properties of 3 were investigated by both dc and ac magnetic susceptibility measurements. Fitting of dc magnetization data collected in the 0.1-0.8 T and 1.8-4.0 K ranges gave S = (17)/(2), D approximately -0.086 cm(-)(1), and g approximately 1.8, where S is the molecular spin of the Mn(21) complex and D is the axial zero-field splitting parameter. ac susceptibility studies in the 10-997 Hz frequency range reveal the presence of a frequency-dependent out-of-phase ac magnetic susceptibility (chi(M)' ') signal consistent with slow magnetization relaxation rates. Fitting of dc magnetization decay versus time data to the Arrhenius equation gave a value of the effective barrier to relaxation (U(eff)) of 13.2 K. Magnetization versus applied dc field sweeps exhibited hysteresis. Thus, complex 3 is a new member of the small but growing family of single-molecule magnets.

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PubMed2004

Synthesis and application of a new bisphosphite ligand collection for asymmetric hydroformylation of allyl cyanide.

A series of mono- and bidentate phosphites was prepared with (S)-5,5',6,6'-tetramethyl-3,3'-di-tert-butyl-1,1'-biphenyl-2,2'-dioxy [(S)-BIPHEN] as a chiral auxiliary and screened in the asymmetric hydroformylation of allyl cyanide. These hydroformylation results were compared with those of two existing chiral ligands, Chiraphite and BINAPHOS, whose utility in asymmetric hydroformylation has been previously demonstrated. Bisphosphite 11 with a 2,2'-biphenol bridge was found to be the best overall ligand for asymmetric hydroformylation of allyl cyanide with up to 80% ee and regioselectivities (branch-to-linear ratio, b/l) of 20 with turnover frequency of 625 [h(-)(1)] at 35 degrees C. BINAPHOS gave enantioselectivities up to 77% ee when the reaction was conducted in either acetone or neat but with poor regioselectivity (b/l 2.8) and activities 7 times lower than that of 11. The product of allyl cyanide hydroformylation using (R,R)-11 was subsequently transformed into (R)-2-methyl-4-aminobutanol, a useful chiral building block. Single-crystal X-ray structures of (S,S)-11 and its rhodium complex 19 were determined.

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PubMed2004

Single-molecule magnets: a Mn25 complex with a record S = 51/2 spin for a molecular species.

The reaction of MnCl2.4H2O (3 equiv), pyridine-2,6-dimethanol (pdmH2) (10 equiv), and NaN3 (10 equiv) in MeOH/MeCN (1:2 v/v) with NMe4OH (1 equiv) gave [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6](Cl)2.12MeCN (1.12MeCN) in approximately 30% yield. The cation of complex 1 comprises five Mnx layers of three types in an ABCBA arrangement. Fitting of variable-temperature and -field magnetization data establishes that 1 has an S = 51/2 ground state, the largest value for a molecular species. The complex also displays hysteresis loops below 0.6 K in magnetization vs applied field sweeps, establishing it as the largest spin single-molecule magnet to date.

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PubMed2004

Bridged synthons from tetrabromocyclopropene: studies on the rearrangement of the primary Diels-Alder adduct with 2,5-dimethylfuran.

The reaction of tetrabromocyclopropene and furan leads directly to 8-oxabicyclo[3.2.1]octadiene derivatives. It has been proposed that this involves an initial Diels-Alder reaction followed by rearrangement of the primary adduct. We have, for the first time, isolated a primary adduct and established through X-ray crystallographic analysis that the adduct is the product of an exo-selective addition. Kinetic studies suggest the intermediacy of charged intermediates during the rearrangement.

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PubMed2004

Divalent ansa-zirconocenes: stereoselective synthesis and high activity for propylene polymerization.

The reduction of ZrCl4(PR3)2 with Li powder, in the presence of a stoichiometric amount of trans-1,4-diphenyl-1,3-butadiene, affords the Zr(II) diene complexes (1) in 90-93% yields. This reaction consists of a rate-limiting step for the formation of the chloride-bridged Zr(III) dimer (2) and a fast diene-driven disproportionation of 2 to 1 and ZrCl4(PR3)2 that re-enters the reduction cycle. The reaction of 1 with Li2{Me2Si(2-Me-4-Ph-Ind)2} in toluene produces quantitatively the desired racemic, divalent ansa-zirconocene (3) that is a highly active isospecific propylene polymerization catalyst upon activation with common activators.

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PubMed2003

Increased reactivity of the *Cr(CO)3(C5Me5) radical with thiones versus thiols: a theoretical and experimental investigation.

2-pyridinethione (2-mercaptopyridine, H-2mp) undergoes rapid oxidative addition with 2 mol of the 17-electron organometallic radical *Cr(CO)3Cp (where Cp*=C5Me5), yielding hydride H-Cr(CO)3Cp* and thiolate (eta1-2mp)Cr(CO)3Cp*. In a slower secondary reaction, (eta1-2mp)Cr(CO)3Cp* loses CO generating the N,S-chelate complex (eta2-2mp)Cr(CO)2Cp* for which the crystal structure is reported. The rate of 2-pyridine thione oxidative addition with *Cr(CO)3Cp* (abbreviated *Cr) in toluene best fits rate=kobs[H-2mp][*Cr]; kobs(288 K)=22 +/- 4 M(-1) s(-1); DeltaH++=4 +/- 1 kcal/mol; DeltaS++=- 40 +/- 5 cal/mol K. The rate of reaction is the same under CO or Ar, and the reaction of deuterated 2-pyridine thione (D-2mp) shows a negligible (inverse) kinetic isotope effect (kD/kH=1.06 +/- 0.10). The rate of decarbonylation of (eta1-2mp)Cr(CO)3Cp* forming (eta2-2mp)Cr(CO)2Cp* obeys simple first-order kinetics with kobs (288 K)=3.1x10(-4) s(-1), DeltaH++=23 +/- 1 kcal/mol, and DeltaS++=+ 5.0 +/- 2 cal/mol K. Reaction of 4-pyridine thione (4-mercaptopyridine, H-4mp) with *Cr(CO)3Cp* in THF and CH2Cl2 also follows second-order kinetics and is approximately 2-5 times faster than H-2mp in the same solvents. The relatively rapid nature of the thione versus thiol reactions is attributed to differences in the proposed 19-electron intermediate complexes, [*(S=C5H4N-H)Cr(CO)3Cp*] versus [*(H-S-C6H5)Cr(CO)3Cp*]. In comparison, reactions of pyridyl disulfides occur by a mechanism similar to that followed by aryl disulfides involving direct attack of the sulfur-sulfur bond by the metal radical. Calorimetric data indicate Cr-SR bond strengths for aryl and pyridyl derivatives are similar. The experimental conclusions are supported by B3LYP/6-311+G(3df,2p) calculations, which also provide additional insight into the reaction pathways open to the thione/thiol tautomers. For example, the reaction between H* radical and the 2-pyridine thione S atom yielding a thionyl radical is exothermic by approximately 30 kcal/mol. In contrast, the thiuranyl radical formed from the addition of H* to the 2-pyridine thiol S atom is predicted to be unstable, eliminating either H* or HS* without barrier.

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PubMed2003

Heptanuclear and decanuclear manganese complexes with the anion of 2-hydroxymethylpyridine.

The synthesis and magnetic properties are reported of two new clusters [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (1) and [Mn(7)(OH)(3)(hmp)(9)Cl(3)](Cl)(ClO(4)) (2). Complex 1 was prepared by treatment of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) with 2-(hydroxymethyl)pyridine (hmpH) in CH(2)Cl(2), whereas 2 was obtained from the reaction of MnCl(2).4H(2)O, hmpH, and NBu(n)(4)MnO(4) in MeCN followed by recrystallization in the presence of NBu(n)(4)ClO(4). Complex 1.2py.10CH(2)Cl(2).2H(2)O crystallizes in the triclinic space group P1. The cation consists of 10 Mn(III) ions, 8 mu(3)-O(2)(-) ions, 2 mu(3)-OH(-) ions, 8 bridging acetates, and 8 bridging and chelating hmp(-) ligands. The hmp(-) ligands bridge through their O atoms in two ways: two with mu(3)-O atoms and six with mu(2)-O atoms. Complex 2.3CH(2)Cl(2).H(2)O crystallizes in the triclinic space group P1. The cation consists of four Mn(II) and three Mn(III) ions, arranged as a Mn(6) hexagon of alternating Mn(II) and Mn(III) ions surrounding a central Mn(II) ion. The remaining ligation is by three mu(3)-OH(-) ions, three terminal chloride ions, and nine bridging and chelating hmp(-) ligands. Six hmp(-) ligands contain mu(2)-O atoms and three contain mu(3)-O atoms. The Cl(-) anion is hydrogen-bonded to the three mu(3)-OH(-) ions. Variable-temperature direct current (dc) magnetic susceptibility data were collected for complex 1 in the 5.00-300 K range in a 5 kG applied field. The chi(M)T value gradually decreases from 17.87 cm(3) mol(-1) K at 300 K to 1.14 cm(3) mol(-1) K at 5.00 K, indicating an S = 0 ground state. The ground-state spin of complex 2 was established by magnetization measurements in the 0.5-3.0 T and 1.80-4.00 K ranges. Fitting of the data by matrix diagonalization, incorporating only axial anisotropy (DS(z)(2)), gave equally good fits with S = 10, g = 2.13, D = -0.14 cm(-1) and S = 11, g = 1.94, D = -0.11 cm(-1). Magnetization versus dc field scans down to 0.04 K reveal no hysteresis attributable to single-molecule magnetism behavior, only weak intermolecular interactions.

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PubMed2003

Facile preparation and photophysics of near-infrared luminescent lanthanide(III) monoporphyrinate complexes.

The synthesis, characterization, and single crystal X-ray diffraction structures of a series of monoporphyrinate, trivalent lanthanide complexes with the monoanionic ligands hydridotris(1-pyrazolyl)borate (Tp) and (cyclopentadienyl)tris(diethylphosphinito)cobaltate (L(OEt)) having the general formulas M(TPP)(L) (M = Yb, Tm, Er, Ho, Nd, Pr; TPP = 5,10,15,20-tetraphenylporphyrinate; L = Tp, L(OEt)) are described. The photophysical properties of these complexes are also presented including their absorption, emission, and transient absorption properties.

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PubMed2003

4,5-dehydro- and 4,5,15,16-bis(dehydro)octafluoro[2.2]paracyclophanes: facile generation and extraordinary Diels-Alder reactivity.

Dehydroiodination of 4-iodo- and 4,15-diiodo-1,1,2,2,9,9,10,10-octafluoro[2.2]paracyclophane by treatment with KO(t)Bu in the presence of benzene, naphthalene, anthracene, and [2.2]paracyclophane affords each of the corresponding Diels-Alder mono- and bis-cycloadducts derived from the presumed aryne intermediates in high yield. Monoadducts of t-butylbenzene and furan are also obtained in excellent yield. All of the products were characterized by their NMR spectra, with four of them also being confirmed by X-ray crystallography. The extraordinary selectivity/reactivity of the aryne intermediate is discussed.

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PubMed2003

Single-molecule magnets: two-electron reduced version of a Mn12 complex and environmental influences on the magnetization relaxation of (PPh4)(2)[Mn(12)O(12)(O(2)CCHCl2)(16)(H2O)4].

The complex [Mn(12)O(12)(O(2)CCHCl(2))(16)(H(2)O)(4)] (2) in MeCN exhibits three quasi-reversible one-electron reduction processes at significantly higher potentials than [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)] (1). This has allowed the two-electron reduced version of 2 to be generated and isolated. Reaction of 2 with one and two equivalents of PPh(4)I led to isolation of (PPh(4))[Mn(12)O(12)(O(2)CCHCl(2))(16)(H(2)O)(4)] (3) and (PPh(4))(2)[Mn(12)O(12)(O(2)CCHCl(2))(16)(H(2)O)(4)] (4), respectively. The latter represents a new isolated oxidation level of the Mn(12) family of single-molecule magnets (SMMs). Crystallization from CH(2)Cl(2)/hexanes yields a mixture of two crystal forms, 4.4CH(2)Cl(2).H(2)O (4a) and 4.6CH(2)Cl(2) (4b), both of which have been structurally characterized as triclinic and monoclinic, respectively. The molecular structures are very similar, with the added electrons localized on former Mn(III) ions to give a trapped-valence 2Mn(II), 6Mn(III), 4Mn(IV) oxidation state description. Dried solid analyzed as unsolvated 4. (1)H NMR spectral data in CD(2)Cl(2) confirm that 4 retains its solid-state structure in solution. Bulk DC magnetization data for dried 4 in the 1.80-4.00 K and 10-70 kG ranges were fit to give S = 10, D = -0.275 cm(-1), g = 2.00 and |D|/g = 0.14 cm(-1), where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4a and 4b give resolvable frequency-dependent out-of-phase (chi(M)'') signals in AC susceptibility studies resulting from the magnetization relaxation of SMMs. Relaxation rate vs T data to 1.8 K obtained from the chi(M)'' vs temperature studies were supplemented with rate vs T data measured to lower temperatures via magnetization vs time decay data, and these were fit to the Arrhenius equation to give the effective barrier to relaxation (U(eff)). The U(eff) values are 18.5 and 30.3 K for 4a and 4b, respectively. A similar analysis for dried 4 using AC data gave U(eff) = 32 K. Magnetization vs DC field sweeps on single crystals of 4a and 4b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded |D|/g values of 0.087 and 0.14 cm(-1) for 4a and 4b, respectively, suggesting that the differences in U(eff) are primarily caused by changes to D. The combined work demonstrates the sensitivity of the magnetic properties of these new [Mn(12)](2-) SMMs to subtle differences in their environment as determined by the precise packing, solvent molecules, and overall crystal symmetry (space group) and represents an important caveat to workers in the field.

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PubMed2003

Symmetric and asymmetric dinuclear manganese(IV) complexes possessing a [MnIV2(mu-O)2(muO2CMe)]3+ core and terminal Cl- ligands.

The synthesis of new dinuclear manganese(IV) complexes possessing the [Mn(IV)(2)(mu-O)(2)(mu-O(2)CMe)](3+) core and containing halide ions as terminal ligands is reported. [Mn(2)O(2)(O(2)CMe)Cl(2)(bpy)(2)](2)[MnCl(4)] (1; bpy = 2,2'-bipyridine) was prepared by sequential addition of [MnCl(3)(bpy)(H(2)O)] and (NBzEt(3))(2)[MnCl(4)] to a CH(2)Cl(2) solution of [Mn(3)O(4)(O(2)CMe)(4)(bpy)(2)]. The complex [Mn(IV)(2)O(2)(O(2)CMe)Cl(bpy)(2)(H(2)O)](NO(3))(2) (2) was obtained from a water/acetic acid solution of MnCl(2).4H(2)O, bpy, and (NH(4))(2)[Ce(NO(3))(6)], whereas the [Mn(IV)(2)O(2)(O(2)CR)X(bpy)(2)(H(2)O)](ClO(4))(2) [X = Cl(-) and R = Me (3), Et (5), or C(2)H(4)Cl (6); and X = F(-), R = Me (4)] were prepared by a slightly modified procedure that includes the addition of HClO(4). For the preparation of 4, MnF(2) was employed instead of MnCl(2).4H(2)O. [Mn(2)O(2)(O(2)CMe)Cl(2)(bpy)(2)](2)[MnCl(4)].2CH(2)Cl(2) (1.2CH(2)Cl(2)) crystallizes in the monoclinic space group C2/c with a = 21.756(2) A, b = 12.0587(7) A, c = 26.192(2) A, alpha = 90 degrees, beta = 111.443(2) degrees, gamma = 90 degrees, V = 6395.8(6) A(3), and Z = 4. [Mn(2)O(2)(O(2)CMe)Cl(H(2)O)(bpy)(2)](NO(3))(2).H(2)O (2.H(2)O) crystallizes in the triclinic space group Ponemacr; with a = 11.907(2) A, b = 12.376(2) A, c = 10.986(2) A, alpha = 108.24(1) degrees, beta = 105.85(2) degrees, gamma = 106.57(1) degrees, V = 1351.98(2) A(3), and Z = 2. [Mn(2)O(2)(O(2)CMe)Cl(H(2)O)(bpy)(2)](ClO(4))(2).MeCN (3.MeCN) crystallizes in the triclinic space group Ponemacr; with a = 11.7817(7) A, b = 12.2400(7) A, c = 13.1672(7) A, alpha = 65.537(2) degrees, beta = 67.407(2) degrees, gamma = 88.638(2) degrees, V = 1574.9(2) A(3), and Z = 2. The cyclic voltammogram (CV) of 1 exhibits two processes, an irreversible oxidation of the [MnCl(4)](2)(-) at E(1/2) approximately 0.69 V vs ferrocene and a reversible reduction at E(1/2) = 0.30 V assigned to the [Mn(2)O(2)(O(2)CMe)Cl(2)(bpy)(2)](+/0) couple (2Mn(IV) to Mn(IV)Mn(III)). In contrast, the CVs of 2 and 3 show only irreversible reduction features. Solid-state magnetic susceptibility (chi(M)) data were collected for complexes 1.1.5H(2)O, 2.H(2)O, and 3.H(2)O in the temperature range 2.00-300 K. The resulting data were fit to the theoretical chi(M)T vs T expression for a Mn(IV)(2) complex derived by use of the isotropic Heisenberg spin Hamiltonian (H = -2JS(1)S(2)) and the Van Vleck equation. The obtained fit parameters were (in the format J/g) -45.0(4) cm(-)(1)/2.00(2), -36.6(4) cm(-)(1)/1.97(1), and -39.3(4) cm(-)(1)/1.92(1), respectively, where J is the exchange interaction parameter between the two Mn(IV) ions. Thus, all three complexes are antiferromagnetically coupled.

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PubMed2003

Three bis-ortho-diynylarenes (BODA).

The molecule of 3,3',4,4'-tetrakis(phenylethynyl)biphenyl, C(44)H(26), is approximately planar and is located on a crystallographic inversion center. Bis[3,4-bis(phenylethynyl)phenyl] ether, C(44)H(26)O, has molecules located on twofold symmetry axes, whereas the molecule of 2,2-bis[3,4-bis(phenylethynyl)phenyl]propane, C(47)H(32), does not exhibit any molecular symmetry.

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PubMed2003

Iron chelation promoted by desazadesferrithiocin analogs: An enantioselective barrier.

For patients who require lifelong blood transfusions, there is no efficient means, unless chelation therapy is employed, for elimination of excess iron. Alternatives to desferrioxamine, the currently accepted treatment for transfusional iron overload, are being investigated. The current article focuses on an enantiomeric pair of analogs of desferrithiocin, (+)-(S)- and (-)-(R)-2-(2,4-dihydroxyphenyl)-4,5-dihydro-4-methyl-4-thiazolecarboxylic acid (4'-hydroxydesazadesferrithiocin). The crystal structure corroborated the absolute configuration of the two compounds, (+) and (-) for the (S)- and (R)-enantiomers, respectively. Job's plots established the tridentate nature of both analogs and circular dichroism spectra confirmed the ligands' antipodal relationship. (+)-(S)-4'-Hydroxydesazadesferrithiocin is a more efficient deferration agent than is the (-)-(R)-enantiomer in a Cebus apella model of iron overload. Pharmacokinetic analyses and IC(50) measurements in L1210 murine leukemia cells were undertaken in an effort to account for the contrast in efficacy between the two enantiomers. Some differences exist in the plasma pharmacokinetic parameters between the two analogs. However, a more plausible explanation may be the apparent differences in transport across the cell membrane; the IC(50) value in L1210 cells of the (+)-(S)-enantiomer was at least 5-fold lower than that of the (-)-(R)-compound.

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