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Khalil A Abboud

Publications and source records attributed to Khalil A Abboud.

At least 37 records · Page 2Linked to original sources
PubMed2005

[Mn12O12(OMe)2(O2CPh)16(H2O)2]2- single-molecule magnets and other manganese compounds from a reductive aggregation procedure.

A new synthetic procedure has been developed in Mn cluster chemistry involving reductive aggregation of permanganate (MnO4-) ions in MeOH in the presence of benzoic acid, and the first products from its use are described. The reductive aggregation of NBu(n)4MnO4 in MeOH/benzoic acid gave the new 4Mn(IV), 8Mn(III) anion [Mn12O12(OMe)2(O2CPh)16(H2O)2]2-, which was isolated as a mixture of two crystal forms (NBu(n)4)2[Mn12O12(OMe)2(O2CPh)16(H2O)2].2H2O.4CH2Cl2 (1a) and (NBu(n)4)2[Mn12O12(OMe)2(O2CPh)16(H2O)2].2H2O.CH2Cl2 (1b). The anion of 1 contains a central [Mn(IV)4(mu3-O)2(mu-O)2(mu-OMe)2]6+ unit surrounded by a nonplanar ring of eight Mn(III) atoms that are connected to the central Mn4 unit by eight bridging mu3-O2- ions. This compound is very similar to the well-known [Mn12O12(O2CR)16(H2O)4] complexes (hereafter called "normal Mn12"), with the main difference being the structure of the central cores. Longer reaction times (approximately 2 weeks) led to isolation of polymeric [Mn(OMe)(O2CPh)2]n2, which contains a linear chain of repeating [Mn(III)(mu-O2CPh)2(mu-OMe)Mn(III)] units. The chains are parallel to each other and interact weakly through pi-stacking between the benzoate rings. When KMnO4 was used instead of NBu(n)4MnO4, two types of compounds were obtained, [Mn12O12(O2CPh)16(H2O)4] (3), a normal Mn12 complex, and [Mn4O2(O2CPh)8(MeOH)4].2MeOH (4.2MeOH), a new member of the Mn4 butterfly family. The cyclic voltammogram of 1 exhibits three irreversible processes, two reductions and one oxidation. One-electron reduction of 1 by treatment with 1 equiv of I- in CH2Cl2 gave (NBu(n)4[Mn12O12(O2CPh)16(H2O)3].6CH2Cl2 (5.6CH2Cl2), a normal Mn12 complex in a one-electron reduced state. The variable-temperature magnetic properties of 1, 2, and 5 were studied by both direct current (dc) and alternating current (ac) magnetic susceptibility measurements. Variable-temperature dc magnetic susceptibility studies revealed that (i) complex 1 possesses an S = 6 ground state, (ii) complex 2 contains antiferromagnetically coupled chains, and (iii) complex 5 is a typical [Mn12]- cluster with an S = 19/2 ground state. Variable-temperature ac susceptibility measurements suggested that 5 and both isomeric forms of 1 (1a,b) are single-molecule magnets (SMMs). This was confirmed by the observation of hysteresis loops in magnetization vs dc field scans. In addition, 1a,b, like normal Mn12 clusters, display both faster and slower relaxing magnetization dynamics that are assigned to the presence of Jahn-Teller isomerism.

Journal Article↗
PubMed2005

Single-molecule magnets: structural characterization, magnetic properties, and (19)F NMR spectroscopy of a Mn(12) family spanning three oxidation levels.

The syntheses, crystal structures, and magnetic properties of [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (2), (NMe(4))[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (3), and (NMe(4))(2)[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (4) are reported. Complex 2 displays quasi-reversible redox couples when examined by cyclic voltammetry in CH(2)Cl(2): one-electron reductions are observed at 0.64 and 0.30 V vs ferrocene. The reaction of complex 2 with 1 and 2 equiv of NMe(4)I yields the one- and two-electron reduced analogues, 3 and 4, respectively. Complexes 2.3CH(2)Cl(2), 3.4.5CH(2)Cl(2).(1)/(2)H(2)O, and 4.6C(7)H(8) crystallize in the triclinic P, monoclinic P2/c, and monoclinic C2/c space groups, respectively. The molecular structures are all very similar, consisting of a central [Mn(IV)O(4)] cubane surrounded by a nonplanar alternating ring of eight Mn and eight mu(3)-O(2)(-) ions. Peripheral ligation is provided by 16 bridging C(6)F(5)CO(2)(-) and 4 H(2)O ligands. Bond valence sum calculations establish that the added electrons in 3 and 4 are localized on former Mn(III) ions giving trapped-valence Mn(IV)(4)Mn(III)(7)Mn(II) and Mn(IV)(4)Mn(III)(6)Mn(II)(2) anions, respectively. (19)F NMR spectroscopy in CD(2)Cl(2) shows retention of the solid-state structure upon dissolution and detrapping of the added electrons in 3 and 4 among the outer ring of Mn ions on the (19)F NMR time scale. DC studies on dried microcrystalline samples of 2, 3, and 4.2.5C(7)H(8) restrained in eicosane in the 1.80-10.0 K and 1-70 kG ranges were fit to give S = 10, D = -0.40 cm(-)(1), g = 1.87, D/g = 0.21 cm(-)(1) for 2, S = 19/2, D = -0.34 cm(-)(1), g = 2.04, D/g = 0.17 cm(-)(1) for 3, and S = 10, D = -0.29 cm(-)(1), g = 2.05, D/g = 0.14 cm(-)(1) for 4, where D is the axial zero-field splitting parameter. The clusters exhibit out-of-phase AC susceptibility signals (chi(M)' ') indicative of slow magnetization relaxation in the 6-8 K range for 2, 4-6 K range for 3, and 2-4 K range for 4; the shift to lower temperatures reflects the decreasing magnetic anisotropy upon successive reduction and, hence, the decreasing energy barrier to magnetization relaxation. Relaxation rate vs T data obtained from chi(M)' ' vs AC oscillation frequency studies down to 1.8 K were combined with rate vs T data from DC magnetization decay vs time measurements at lower temperatures to generate an Arrhenius plot from which the effective barrier (U(eff)) to magnetization reversal was obtained; the U(eff) values are 59 K for 2, 49 and 21 K for the slower- and faster-relaxing species of 3, respectively, and 25 K for 4. Hysteresis loops obtained from single-crystal magnetization vs DC field scans are typical of single-molecule magnets with the coercivities increasing with decreasing T and increasing field sweep rate and containing steps caused by the quantum tunneling of magnetization (QTM). The step separations gave D/g values of 0.22 cm(-)(1) for 2, 0.15 and 0.042 cm(-)(1) for the slower- and faster-relaxing species of 3, and 0.15 cm(-)(1) for 4.

Crystallography, X-Ray↗
PubMed2005

New polynuclear manganese clusters from the use of the hydrophobic carboxylate ligand 2,2-dimethylbutyrate.

The syntheses, structures, and magnetic properties are reported of [Mn12O12(O2CPe(t))16(MeOH)4] (4), [Mn6O2(O2CH2)(O2CPe(t))11(HO2CPe(t))2(O2CMe)] (5), [Mn9O6(OH)(CO3)(O2CPe(t))12(H2O)2] (6), and [Mn4O2(O2CPe(t))6(bpy)2] (7, bpy = 2,2'-bipyridine), where Pe(t) = tert-pentyl (Pe(t)CO2H = 2,2-dimethylbutyric acid). These complexes were all prepared from reactions of [Mn12O12(O2CPe(t))16(H2O)4] (3) in CH2Cl2. Complex 4 x 2MeCN crystallizes in the triclinic space group P1 and contains a central [Mn(IV)4O4] cubane core that is surrounded by a nonplanar ring of eight alternating Mn(III) and eight mu3-O(2-) ions. This is only the third Mn12 complex in which the four bound water molecules have been replaced by other ligands, in this case MeOH. Complex 5 x (1/2)CH2Cl2 crystallizes in the monoclinic space group P2(1)/c and contains two [Mn3(mu3-O)]7+ units linked at two of their apexes by two Pe(t)CO2(-) ligands and one mu4-CH2O2(2-) bridge. The complex is a new structural type in Mn chemistry, and also contains only the third example of a gem-diolate unit bridging four metal ions. Complex 6 x H2O x Pe(t)CO2H crystallizes in the orthorhombic space group Cmc2(1) and possesses a [Mn(III)9(mu3-O)6(mu-OH)(mu3-CO3)]12+ core. The molecule contains a mu3-CO3(2-) ion, the first example in a discrete Mn complex. Complex 7 x 2H2O crystallizes in the monoclinic space group P2(1)/c and contains a known [Mn(III)2Mn(II)2(mu3-O)2]6+ core that can be considered as two edge-sharing, triangular [Mn3O] units. Additionally, the synthesis and magnetic properties of a new enneanuclear cluster of formula [Mn9O7(O2CCH2Bu(t))13(THF)2] (8, THF = tetrahydrofuran) are reported. The molecule was obtained by the reaction of [Mn12O12(O2CCH2Bu(t))16(H2O)4] (2) with THF. Complexes 2 and 4 display quasireversible redox couples when examined by cyclic voltammetry in CH2Cl2; oxidations are observed at -0.07 V (2) and -0.21 V (4) vs ferrocene. The magnetic properties of complexes 4-8 have been studied by direct current (DC) and alternating current (AC) magnetic susceptibility techniques. The ground-state spin of 4 was established by magnetization measurements in the 1.80-4.00 K and 0.5-7 T ranges. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating a full powder average and including only axial anisotropy, gave S = 10, g = 2.0(1), and D = -0.39(10) cm(-1). The complex exhibits two frequency-dependent out-of-phase AC susceptibility signals (chi(M)'') indicative of slow magnetization relaxation. An Arrhenius plot obtained from chi(M)'' vs T data gave an effective energy barrier to relaxation (U(eff)) of 62 and 35 K for the slower and faster relaxing species, respectively. These studies suggest that complex 4 is a single-molecule magnet (SMM). DC susceptibility studies on complexes 5-8 display overall antiferromagnetic behavior and indicate ground-state spin values of S < or = 2. AC susceptibility studies at < 10 K confirm these small values and indicate the population of low-lying excited states even at these low temperatures. This supports the small ground-state spin values to be due to spin frustration effects.

Journal Article↗
PubMed2005

Tungsten allylimido complexes Cl4(RCN)W(NC3H5) as single-source CVD precursors for WNxCy thin films. Correlation of precursor fragmentation to film properties.

A mixture of the tungsten allylimido complexes Cl(4)(RCN)W(NC(3)H(5)) (3a, R = CH(3) and 3b, R = Ph) was tested as a single-source precursor for growth of tungsten nitride (WN(x)) or carbonitride (WN(x)C(y)) thin films. Films deposited from 3a,b below 550 degrees C contained amorphous beta-WN(x)C(y), while those deposited at higher temperatures were polycrystalline. Film growth rates from 3a,b ranged from 5 to 10 A/min over a temperature range of 450-650 degrees C, and the apparent activation energy for film growth was 0.15 eV. A plot of the E(a) values for deposition from Cl(4)(RCN)W(NR') [R' = Ph, (i)Pr, allyl] against the N-C imido bond strengths for the analogous amines R'NH(2) is linear, implicating cleavage of the N-C bond as the rate-determining step in film growth. The correlation of mass spectral fragmentation patterns for Cl(4)(RCN)W(NR') with film properties such as nitrogen content supports the significance of facile N-C bond cleavage in film growth.

Journal Article↗
PubMed2005

A platinum acetylide polymer with sterically demanding substituents: effect of aggregation on the triplet excited state.

The effect of interchain interaction on the triplet excited state is explored in two Pt-acetylide polymers of the type [-trans-Pt(PBu(3))(2)-C triple bond C-Ar-C triple bond C-](n), where Ar is either 1,4-phenylene or is based on the pentiptycene unit (polymers 2 and 3, respectively). To explore the effect of interchain interaction in Pt-acetylide materials, the optical properties of parent polymer 2 are compared with those of polymer 3 in which interchain interaction is precluded by the sterically bulky pentiptycene moiety. Insight into the effect of the pentiptycene unit on packing in the solid state comes from the X-ray structure of monomer 1b, Ph-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ar-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ph. Spectroscopic studies indicate that weak phosphorescence emission from an interchain aggregate is observed from parent polymer 2, both in solution and in the solid state. By contrast, the photophysics of 3 is dominated by the intrachain triplet exciton. Interestingly, the phosphorescence emission of polymer 3 in the solid state is nearly superimposable with that of a single crystal of monomer 1b, suggesting that the solid polymer experiences an environment that is similar to that of the monomer in the crystal.

Journal Article↗
PubMed2005

Highly active, regioselective, and enantioselective hydroformylation with Rh catalysts ligated by Bis-3,4-diazaphospholanes.

Azines made by the reaction of hydrazine with ortho-formylbenzoic acid react with 1,2-diphosphinobenzene and either succinyl chloride or phthaloyl chloride in ca. 30% yield to give rac-bis-3,4-diazaphospholanes bearing benzoic acid groups in the 2 and 5 positions. Condensation of the benzoic acid functionalities with enantiomerically pure amines affords diastereomeric benzoamides which can be separated by flash chromatography. Application of the resolved bis-3,4-diazaphosholanes to Rh-catalyzed enantioselective hydroformylation of styrene, allyl cyanide, and vinyl acetate under mild pressures (20-500 psig of CO/H2) and temperatures (40-120 degrees C) reveals high activities and selectivities for all three substrates. At 60 degrees C and 500 psig syn gas, the best ligand provides outstanding regio- and enantioselectivities (styrene, 89% ee, b:l = 30:1; allyl cyanide, 87% ee, b:l = 4.8:1; vinyl acetate, 95% ee, b:l = 40:1) while achieving turnover frequencies of ca. 3000 h-1.

Journal Article↗
PubMed2005

Dramatic variation of magnetic exchange through double end-on azide bridges in a series of ladder-like copper(II) coordination polymers.

Three ladder-like coordination polymers, [Cu2(phprpy)2-mu-(N3)2(N3)2], 1; [Cu2(terpy)2-mu-(N3)4Cu2-mu-(N3)2(N3)2], 2; and[Cu2(terpy)2-mu-(N3)2(N3)2Cu3-mu-(N3)4(N3)2], 3, consisting of Cu2+ ions with double end-on azide bridges were synthesized, their crystal structures and magnetic properties were determined, and spin dimer analysis was performed to explain the signs and strengths of their strong spin exchange interactions [phprpy is 4-(3-phenylpropyl)pyridine and terpy is 2,2':6,2''-terpyridine]. Although these compounds have ladder-like arrangements of Cu2+ ions, their magnetic structures are described as isolated dimers for 1 and 2 and as isolated trimers for 3. The predominant spin exchange paths in 1-3 have double end-on azide bridges linking adjacent Cu2+ ions, and the geometrical parameters of these bridging structures are similar. However, the spin dimer of 1 exhibits a strong ferromagnetic coupling; that of 2, a strong antiferromagnetic coupling; and that of 3, a weak ferromagnetic coupling. These findings are well explained by the present spin dimer analysis and show that the nature and geometry of the nonbridging ligands can have a strong influence on the sign and strength of the spin exchange interaction between Cu2+ ions connected by double end-on azide bridges.

Journal Article↗
PubMed2004

Donor-mediated band gap reduction in a homologous series of conjugated polymers.

A family of six donor-acceptor-donor monomers was synthesized using combinations of thiophene, 3,4-ethylenedioxythiophene and 3,4-ethylenedioxypyrrole as donor moieties, and cyanovinylene as the acceptor moiety, to understand the effects of modified donor ability on the optoelectronic and redox properties of the resulting electropolymerized materials. Spectroelectrochemistry, differential pulse voltammetry, and cyclic voltammetry results indicate band gaps ranging from 1.1 to 1.6 eV and suggest that these polymers can be both p-type and n-type doped at accessible potentials. In situ conductivity results indicate that the n-type conductivity magnitude is modest, and the conductivity profile indicates a redox conductivity mechanism as opposed to a delocalized electronic band mechanism as observed for p-type doping.

Journal Article↗
PubMed2004

Initial observation of magnetization hysteresis and quantum tunneling in mixed manganese-lanthanide single-molecule magnets.

The preparation of a new family of mixed transition metal/lanthanide clusters is reported. The reaction of [Mn3O(O2CPh)6(py)2(H2O)] with Ln(NO3)3 (Ln = Nd, Gd, Dy, Ho, and Eu) in a 1:2 molar ratio in MeOH/MeCN (1:20 v/v) leads to dark crystals in 55-60% isolated yield of complexes all containing the [Mn11Ln4]45+ core. The Dy compound has been found to give out-of-phase AC susceptibility signals, suggesting it might be a single-molecule magnet (SMM). This was confirmed by the observation of magnetization hysteresis loops. An Arrhenius plot constructed from magnetization decay data gave a barrier to relaxation of 9.3 K and showed the temperature-independent relaxation at very low temperatures indicative of quantum tunneling of magnetization. This is the initial demonstration of hysteresis and quantum behavior in a mixed 3d/4f SMM.

Journal Article↗
PubMed2004

A family of Mn16 single-molecule magnets from a reductive aggregation route.

The synthesis and magnetic properties of three isostructural hexadecametallic manganese clusters [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Ph)(16)(MeOH)(6)] (1), [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Cl)(16)(MeOH)(6)] (2), and [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Br)(16)(MeOH)(6)] (3) are reported. The complexes were prepared by a reductive aggregation reaction involving phenylacetic acid, chloroacetic acid or bromoacetic acid, and NBu(n)()(4)MnO(4) in MeOH. Complex 1 crystallizes in the monoclinic space group C2/c and consists of 6 Mn(IV) and 10 Mn(III) ions held together by 14 mu(3)-O(2)(-), 2 mu-O(2)(-), 4 mu-MeO(-), and 2 mu-O(2)CCH(2)Ph(-) groups. The remaining 14 mu-O(2)CCH(2)Ph(-) ligands, 2 mu-MeO(-) groups, and 6 terminal MeOH molecules constitute the peripheral ligation in the complex. Variable-temperature, solid-state dc magnetic susceptibility measurements on 1-3 in the temperature range 5.0-300 K reveal that all three complexes are dominated by intramolecular antiferromagnetic exchange interactions. Low-lying excited states preclude an exact determination of the spin ground state for 1-3 by magnetization measurements. Alternating current susceptibility measurements at zero dc field in the temperature range 1.8-10 K and a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display, at temperatures below 3 K, a nonzero, frequency-dependent chi(M)"signal for complexes 1-3, with the peak maxima lying at temperatures less than 1.8 K. Single-crystal magnetization versus dc field scans down to 0.04 K for complex 1 show hysteresis behavior at <1 K, establishing 1 as a new member of the SMM family. No clear steps characteristic of quantum tunneling of magnetization (QTM) were observed in the hysteresis loops.

Journal Article↗
PubMed2004

New chiral building blocks from tetrabromocyclopropene and furan.

The cyclocondensation of tetrabromocyclopropene and furan leads directly to a halogenated oxabicyclo[3.2.1]octadiene derivative. Over the past several years, we have utilized these compounds as intermediates for natural product synthesis. Herein, we describe the preparation of nonracemic dibromoenone building blocks from the racemic cycloadduct. Conversion of the adduct to a mixture of tartrate-derived ketals followed by separation of the diastereomers and hydrolysis allows for the formation of novel chiral synthons with either absolute configuration.

Journal Article↗
PubMed2004

Mixed-valence MnIIIMnIV clusters [Mn7O8(O2SePh)8(O2CMe)(H2O)] and [Mn7O8(O2SePh)9(H2O)]: single-chain magnets exhibiting quantum tunneling of magnetization.

The syntheses, structures, and magnetic properties of two new Mn7 complexes containing phenylseleninate ligands are reported. [Mn7O8(O2SePh)8(O2CMe)(H2O)] (1) and [Mn7O8(O2SePh)9(H2O)] (2) were both prepared by the reaction of 18 equiv of benzeneseleninic acid (PhSeO2H) with [Mn12O12(O2CMe)16(H2O)4] in MeCN. Complex 1 x 6MeCN crystallizes in the triclinic space group P, and complex 2 x 2CH2Cl2 crystallizes in the monoclinic space group P2(1)/m. Both compounds possess an unprecedented [Mn7O8]9+ core comprising a central [MnIII3(micro3-O)4]+ unit attached to [MnIV2(micro-O)2]4+ and [MnIV2(micro-O)(micro3-O)]4+ units on either side. In each cluster, the PhSeO2- groups function as bridging ligands between adjacent Mn centers. The structure reveals strong Se.O intermolecular contacts between Mn7 units to give a one-dimensional chain structure, with weak interchain interactions. Solid-state DC magnetic susceptibility measurements of complexes 1 and 2 reveal that they have very similar properties, and detailed studies on 1 by AC susceptibility measurements confirm an S = 2 ground-state spin value. In addition, out-of-phase AC signals are observed, suggesting slow magnetization relaxation. Magnetization versus DC field sweeps down to 0.04 K reveals hysteresis loops, but the temperature dependence of the coercivity is not what is expected of a single-molecule magnet. Instead, the behavior is due to single-chain magnetism, albeit with weak antiferromagnetic interactions between the chains, with the barrier to relaxation arising from a combination of molecular anisotropy and ferromagnetic intermolecular exchange interactions mediated by the Se...O contacts. An Arrhenius plot was constructed from the magnetization versus time decay data. The thermally activated region at > 0.5 K gave an effective relaxation barrier (Ueff) of 14.2 K. Below approximately 0.1 K, the relaxation is independent of temperature, which is characteristic of magnetization quantum tunneling through the anisotropy barrier. These Mn7 compounds are thus the first single-chain magnets to comprise polynuclear metal clusters and also the first for which the temperature-independent relaxation characteristic of tunneling has been identified. The work also emphasizes that out-of-phase AC signals for ostensibly molecular compounds are not sufficient proof by themselves of a single-molecule magnet.

Journal Article↗