Khalil A Abboud

Publications of Khalil A Abboud

• An OCO(3-) Trianionic Pincer Tungsten(VI) Alkylidyne: Rational Design of a Highly Active Alkyne Polymerization Catalyst.

  Authors: Soumya Sarkar, Kevin P McGowan, Subramaniam Kuppuswamy, Ion Ghiviriga, Khalil A Abboud, Adam S Veige

  Journal of the American Chemical Society. 03/2012; 134(10):4509-12.

  Synthesis, characterization, and catalytic alkyne polymerization results for the first trianionic pincer alkylidyne complex, [(t)BuOCO]W≡CC(CH(3))(3)(THF)(2) (6), are described. Complex 6 is a highlySynthesis, characterization, and catalytic alkyne polymerization results for the first trianionic pincer alkylidyne complex, [(t)BuOCO]W≡CC(CH(3))(3)(THF)(2) (6), are described. Complex 6 is a highly active catalyst for the polymerization of acetylenes and exhibits a high turnover number (4371), activity (1.05 × 10(6) g(PPA) mol(cat)(-1) h(-1)),and yield (87%) for the polymerization of 1-ethynyl-4-fluorobenzene.

• Synthetic model of the asymmetric [Mn3CaO4] cubane core of the oxygen-evolving complex of photosystem II.

  Authors: Shreya Mukherjee, Jamie A Stull, Junko Yano, Theocharis C Stamatatos, Konstantina Pringouri, Troy A Stich, Khalil A Abboud, R David Britt, Vittal K Yachandra, George Christou

  Proceedings of the National Academy of Sciences of the United States of America. 02/2012; 109(7):2257-62.

  The laboratory synthesis of the oxygen-evolving complex (OEC) of photosystem II has been the objective of synthetic chemists since the early 1970s. However, the absence of structural information onThe laboratory synthesis of the oxygen-evolving complex (OEC) of photosystem II has been the objective of synthetic chemists since the early 1970s. However, the absence of structural information on the OEC has hampered these efforts. Crystallographic reports on photosystem II that have been appearing at ever-improving resolution over the past ten years have finally provided invaluable structural information on the OEC and show that it comprises a [Mn(3)CaO(4)] distorted cubane, to which is attached a fourth, external Mn atom, and the whole unit attached to polypeptides primarily by aspartate and glutamate carboxylate groups. Such a heterometallic Mn/Ca cubane with an additional metal attached to it has been unknown in the literature. This paper reports the laboratory synthesis of such an asymmetric cubane-containing compound with a bound external metal atom, [(1)]. All peripheral ligands are carboxylate or carboxylic acid groups. Variable-temperature magnetic susceptibility data have established 1 to possess an S = 9/2 ground state. EPR spectroscopy confirms this, and the Davies electron nuclear double resonance data reveal similar hyperfine couplings to those of other Mn(IV) species, including the OEC S(2) state. Comparison of the X-ray absorption data with those for the OEC reveal 1 to possess structural parameters that make it a close structural model of the asymmetric-cubane OEC unit. This geometric and electronic structural correspondence opens up a new front in the multidisciplinary study of the properties and function of this important biological unit.

• It takes more than an imine: the role of the central atom on the electron-accepting ability of benzotriazole and benzothiadiazole oligomers.

  Authors: Dinesh G Dan Patel, Fude Feng, Yu-Ya Ohnishi, Khalil A Abboud, So Hirata, Kirk S Schanze, John R Reynolds

  Journal of the American Chemical Society. 02/2012; 134(5):2599-612.

  We report on the comparison of the electronic and photophysical properties of a series of related donor-acceptor-donor oligomers incorporating the previously known 2H-benzo[d][1,2,3]triazole (BTz)We report on the comparison of the electronic and photophysical properties of a series of related donor-acceptor-donor oligomers incorporating the previously known 2H-benzo[d][1,2,3]triazole (BTz) moiety as the acceptor and the recently reported BTzTD acceptor, a hybrid of BTz and 2,1,3-benzothiadiazole (BTD). Although often implied in the polymer literature that BTz has good acceptor character, we show that this moiety is best described as a weak acceptor. We present electrochemical, computational, and photophysical evidence supporting our assertion that BTzTD is a strong electron acceptor while maintaining the alkylation ability of the BTz moiety. Our results show that the identity of the central atom (N or S) in the benzo-fused heterocyclic ring plays an important role in both the electron-accepting and the electron-donating ability of acceptor moieties with sulfur imparting a greater electron-accepting ability and nitrogen affording greater electron-donating character. We report on the X-ray crystal structure of a BTzTD trimer, which exhibits greater local aromatic character in the region of the triazole ring and contains an electron-deficient sulfur that imparts strong electron-accepting ability. Additionally, we examine the transient absorption spectra of BTzTD and BTz oligomers and report that the BTz core promotes efficient intersystem crossing to the triplet state, while the presence of the thiadiazole moiety in BTzTD leads to a negligible triplet yield. Additionally, while BTz does not function as a good acceptor, oligomers containing this moiety do function as excellent sensitizers for the generation of singlet oxygen.

• A supramolecular aggregate of four exchange-biased single-molecule magnets.

  Authors: Tu N Nguyen, Wolfgang Wernsdorfer, Khalil A Abboud, George Christou

  Journal of the American Chemical Society. 12/2011; 133(51):20688-91.

  The reaction between 3-phenyl-1,5-bis(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH(2)) and triangular [Mn(III)(3)O(O(2)CMe)(py)(3)](ClO(4)) (1) affords [Mn(12)O(4)(O(2)CMe)(12)(pdpd)(6))](ClO(4))(4)The reaction between 3-phenyl-1,5-bis(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH(2)) and triangular [Mn(III)(3)O(O(2)CMe)(py)(3)](ClO(4)) (1) affords [Mn(12)O(4)(O(2)CMe)(12)(pdpd)(6))](ClO(4))(4) (3). Complex 3 has a rectangular shape and consists of four [Mn(III)(3)O](7+) triangular units linked covalently by the dioximate ligands into a supramolecular [Mn(3)](4) tetramer. Solid-state dc and ac magnetic susceptibility measurements revealed that [Mn(3)](4) contains four Mn(3) single-molecule magnets (SMMs), each with an S = 6 ground state. Magnetization versus dc-field sweeps on a single crystal gave hysteresis loops below 1 K that exhibited exchange-biased quantum tunneling of magnetization steps, confirming 3 to be a supramolecular aggregate of four weakly exchange-coupled SMM units.

• The influence of reversible trianionic pincer OCO(3-)μ-oxo Cr(iv) dimer formation ([Cr(iv)](2)(μ-O)) and donor ligands in oxygen-atom-transfer (OAT).

  Authors: Matthew E O'Reilly, Trevor J Del Castillo, Khalil A Abboud, Adam S Veige

  Dalton transactions (Cambridge, England : 2003). 11/2011; 41(8):2237-46.

  The oxygen-atom-transfer (OAT) from [(t)BuOCO]Cr(v)(O)(THF) (2) (where (t)BuOCO = [2,6-C(6)H(3)(6-(t)BuC(6)H(3)O)(2)](3-), THF = tetrahydrofuran) to triphenylphosphine (PPh(3)) in THF producesThe oxygen-atom-transfer (OAT) from [(t)BuOCO]Cr(v)(O)(THF) (2) (where (t)BuOCO = [2,6-C(6)H(3)(6-(t)BuC(6)H(3)O)(2)](3-), THF = tetrahydrofuran) to triphenylphosphine (PPh(3)) in THF produces [(t)BuOCO]Cr(iii)(THF)(3) (1) and triphenylphosphine oxide (OPPh(3)) at a rate of 69.5(±1.9) M(-1)s(-1) (22 °C). Identical rate constants were attained when acetonitrile (MeCN) and dichloromethane/THF (CH(2)Cl(2)/THF) were used as solvents. Electron paramagnetic resonance (EPR) data shows that the six-coordinate complex, [(t)BuOCO]Cr(v)(O)(THF)(2) (2a) forms upon addition of THF to 2, suggesting 2a as the active OAT species in THF. Similarly, addition of OPPh(3) has no influence on the rate of OAT, but the addition of triphenylphosphorus ylide (CH(2)PPh(3)) to form [(t)BuOCO]Cr(v)(O)(CH(2)PPh(3)) (4) prevents OAT to PPh(3). In CH(2)Cl(2), a [Cr(iv)](2)(μ-O) intermediate forms during the OAT from 2 to PPh(3). The OAT from {[(t)BuOCO]Cr(iv)(THF)}(2)(μ-O) (3) to PPh(3) reveals a zero-order dependence in PPh(3) indicating the dimer must first dissociate prior to OAT. The decay of 3versus time does not follow first-order kinetics due to the formation of a [(t)BuOCO]Cr(iii)(THF) species (5) that inhibits the dissociation of 3. The change in concentration of 3versus time during OAT was simulated to obtain approximate rate constants.

• New mixed-valent Mn clusters from the use of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (edteH4): Mn3, Mn4, Mn6, and Mn10.

  Authors: Arpita Saha, Khalil A Abboud, George Christou

  Inorganic chemistry. 11/2011; 50(24):12774-84.

  The syntheses, crystal structures, and magnetochemical characterization are reported for the new mixed-valent Mn clusters [Mn(2)(II)Mn(III)(O(2)CMe)(2)(edteH(2))(2)](ClO(4)) (1),The syntheses, crystal structures, and magnetochemical characterization are reported for the new mixed-valent Mn clusters [Mn(2)(II)Mn(III)(O(2)CMe)(2)(edteH(2))(2)](ClO(4)) (1), [Mn(II)(2)Mn(III)(2)(edteH(2))(2)(hmp)(2)Cl(2)](Mn(II)Cl(4)) (2), [Mn(III)(6)O(2)(O(2)CBu(t))(6)(edteH)(2)(N(3))(2)] (3), [Na(2)Mn(III)(8)Mn(II)(2)O(4)(OMe)(2)(O(2)CEt)(6)(edte)(2)(N(3))(6)] (4), and (NEt(4))(2)[Mn(8)(III)Mn(2)(II)O(4)(OH)(2)-(O(2)CEt)(6)(edte)(2)(N(3))(6)](5), where edteH(4) is N,N,N',N'-tetrakis-(2-hydroxyethyl)ethylenediamine and hmpH is 2-(hydroxymethyl)pyridine. 1-5 resulted from a systematic exploration of the effect of different Mn sources, carboxylates, the presence of azide, and other conditions, on the Mn/edteH(4) reaction system. The core of 1 consists of a linear Mn(II)Mn(III)Mn(II) unit, whereas that of 2 is a planar Mn(4) rhombus within a [Mn(II)(2)Mn(III)(2)(μ(3)-OR)(2)] incomplete-dicubane unit. The core of 3 comprises a central [Mn(III)(4)(OR)(2)] incomplete-dicubane on either side of which is edge-fused a triangular [Mn(III)(3)(μ(3)-O)] unit. The cores of 4 and 5 are similar and consist of a central [Mn(II)(2)Mn(III)(2)(μ(3)-OR)(2)] incomplete-dicubane on either side of which is edge-fused a distorted [Mn(II)Mn(III)(3)(μ(3)-O)(2)(μ(3)-OR)(2)] cubane unit. Variable-temperature, solid-state direct current (dc) and alternating current (ac) magnetization studies were carried out on 1-5 in the 5.0-300 K range, and they established the complexes to have ground state spin values of S = 3 for 1, S = 9 for 2, and S = 4 for 3. The study of 3 provided an interesting caveat of potential pitfalls from particularly low-lying excited states. For 4 and 5, the ground state is in the S = 0-4 range, but its identification is precluded by a high density of low-lying excited states.

• Family of double-cubane Mn4Ln2 (Ln = Gd, Tb, Dy, Ho) and Mn4Y2 complexes: a new Mn4Tb2 single-molecule magnet.

  Authors: Arpita Saha, Michael Thompson, Khalil A Abboud, Wolfgang Wernsdorfer, George Christou

  Inorganic chemistry. 09/2011; 50(20):10476-85.

  The synthesis and characterization of a family of Mn(2)(III)Mn(2)(II)Ln(III)(2) complexes (Ln = Gd (1), Tb (2), Dy (3), and Ho (4)) of formula [Mn(4)Ln(2)O(2)(O(2)CBu(t))(6)(edteH(2))(2)(NO(3))(2)]The synthesis and characterization of a family of Mn(2)(III)Mn(2)(II)Ln(III)(2) complexes (Ln = Gd (1), Tb (2), Dy (3), and Ho (4)) of formula [Mn(4)Ln(2)O(2)(O(2)CBu(t))(6)(edteH(2))(2)(NO(3))(2)] are reported, where edteH(4) is N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine. The analogous Mn(4)Y(2) (5) complex has also been prepared. They were obtained from reaction of Ln(NO(3))(3) or Y(NO(3))(3) with Mn(O(2)CBu(t))(2), edteH(4), and NEt(3) in a 2:3:1:2 molar ratio. The crystal structures of representative 1 and 2 were obtained, and their core consists of a face-fused double-cubane [Mn(4)Ln(2)(μ(4)-O(2-))(2)(μ(3)-OR)(4)] unit. Such double-cubane units are extremely rare in 3d metal chemistry and unprecedented in 3d-4f chemistry. Variable-temperature, solid-state dc and ac magnetic susceptibility studies on 1-5 were carried out. Fitting of dc χ(M)T vs T data for 5 gave J(bb) (Mn(III)···Mn(III)) = -32.6(9) cm(-1), J(wb) (Mn(II)···Mn(III)) = +0.5(2) cm(-1), and g = 1.96(1), indicating a |n, 0, n> (n = 0-5) 6-fold-degenerate ground state. The data for 1 indicate an S = 12 ground state, confirmed by fitting of magnetization data, which gave S = 12, D = 0.00(1) cm(-1), and g = 1.93(1) (D is the axial zero-field splitting parameter). This ground state identifies the Mn(II)···Gd(III) interactions to be ferromagnetic. The ac susceptibility data independently confirmed the conclusions about 1 and 5 and revealed that 2 displays slow relaxation of the magnetization vector for the Mn(4)Tb(2) analogue 2. The latter was confirmed as a single-molecule magnet by observation of hysteresis below 0.9 K in magnetization vs dc field scans on a single crystal of 2·MeCN on a micro-SQUID apparatus. The hysteresis loops also displayed well-resolved quantum tunneling of magnetization steps, only the second 3d-4f SMM to do so.

• Autocatalytic O2 cleavage by an OCO3 trianionic pincer Cr(III) complex: isolation and characterization of the autocatalytic intermediate [Cr(IV)]2(μ-O) dimer.

  Authors: Matthew E O'Reilly, Trevor J Del Castillo, Joseph M Falkowski, Vasanth Ramachandran, Mekhala Pati, Marie C Correia, Khalil A Abboud, Naresh S Dalal, David E Richardson, Adam S Veige

  Journal of the American Chemical Society. 08/2011; 133(34):13661-73.

  Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO =Synthetic and kinetic experiments designed to probe the mechanism of O(2) activation by the trianionic pincer chromium(III) complex [(t)BuOCO]Cr(III)(THF)(3) (1) (where (t)BuOCO = [2,6-((t)BuC(6)H(3)O)(2)C(6)H(3)](3-), THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O(2) activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O(2) activation when dimerized. The product from O(2) activation, [(t)BuOCO]Cr(V)(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[(t)BuOCO]Cr(IV)(THF)}(2)(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a Cr(IV)-O-Cr(IV) dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are g(iso) = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O(2) was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O(2). Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh(3) accelerates O(2) activation by forming the five-coordinate complex trans-[(t)BuOCO]Cr(III)(OPPh(3))(2) (4).

• Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects.

  Authors: Constantina Papatriantafyllopoulou, Khalil A Abboud, George Christou

  Inorganic chemistry. 08/2011; 50(18):8959-66.

  The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family ofThe initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).

• 1,3-Dipolar cycloaddition between a metal-azide (Ph3PAuN3) and a metal-acetylide (Ph3PAuC≡CPh): an inorganic version of a click reaction.

  Authors: Trevor J Del Castillo, Soumya Sarkar, Khalil A Abboud, Adam S Veige

  Dalton transactions (Cambridge, England : 2003). 07/2011; 40(32):8140-4.

  This report describes the synthesis and characterization of 1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (3((1,5))). The synthesis of the dinuclear complex 3((1,5)) is achieved via anThis report describes the synthesis and characterization of 1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (3((1,5))). The synthesis of the dinuclear complex 3((1,5)) is achieved via an unprecedented inorganic click (iClick) reaction between the metal-azide PPh(3)AuN(3) (1) and the metal-acetylide PPh(3)Au-C≡CPh (2). Characterization of 3((1,5)) includes multinuclear NMR spectroscopy, combustion analysis, and single crystal X-ray crystallography. Experimental characterization is complemented with density-functional-theory (DFT) calculations which indicate the 1,4-isomer 3((1,4)) is less stable by 3.3 kcal mol(-1). The energetic difference lies primarily in the ability of the phenyl group in the 4-position of 3((1,5)) to lie coplanar with the triazolate to create a delocalized π-bonding HOMO orbital.

• Iron(III) chemistry with ferrocene-1,1'-dicarboxylic acid (fdcH2): an Fe7 cluster with an oxidized fdc(-) ligand.

  Authors: Antonio Masello, Yiannis Sanakis, Athanassios K Boudalis, Khalil A Abboud, George Christou

  Inorganic chemistry. 06/2011; 50(12):5646-54.

  The synthesis and properties are reported of a new Fe(7) cluster obtained from the reaction of ferrocene-1,1'-dicarboxylic acid (fdcH(2)) with FeCl(2)·4H(2)O in MeOH under ambient light conditions.The synthesis and properties are reported of a new Fe(7) cluster obtained from the reaction of ferrocene-1,1'-dicarboxylic acid (fdcH(2)) with FeCl(2)·4H(2)O in MeOH under ambient light conditions. The compound is the mixed-anion salt [Fe(7)O(3)(OMe)(fdc)(6)(MeOH)(3)][FeCl(4)]Cl(2) (1; 8Fe(III)), containing six (fdc(n-)) groups as peripheral ligands. The cation of 1 has virtual C(3) symmetry and contains a central [Fe(4)(μ(3)-O)(3)(μ(3)-OMe)](5+) cubane unit whose three oxide ions each become μ(4) by attaching to a fourth Fe atom outside the cubane. The resulting [Fe(7)(μ(3)-O(3))(μ(3)-OMe)](14+) core is surrounded by six fdc(n-) (n = 1, 2) groups, which divide into two sets by virtual symmetry. The blue color of the complex suggested that some of these ligands are in their oxidized fdc(-) ferricenium (Fe(III)) state, and various data point to there being one fdc(-) ligand in the compound, the initial example of the group acting as a ligand in inorganic chemistry. Variable-temperature, solid-state DC and AC susceptibility measurements reveal the cation to be antiferromagnetically coupled, as expected for high-spin Fe(III), and to have an S = 2 ground state, consistent with an S = (5)/(2) Fe(7) inner core coupled antiferromagnetically to the one paramagnetic fdc(-) (S = (1)/(2)) ligand. Complex 1 displays multiple reductions and oxidations when investigated by electrochemistry in MeCN. (57)Fe Mössbauer spectroscopy supports the presence of only five fdc(2-) ligands, but cannot resolve the signals from the various Fe(III) sites.

• Synthetic entry into polynuclear bismuth-manganese chemistry: high oxidation state Bi(III)2Mn(IV)6 and Bi(III)Mn(III)10 complexes.

  Authors: Theocharis C Stamatatos, Katie Oliver, Khalil A Abboud, George Christou

  Inorganic chemistry. 06/2011; 50(11):5272-82.

  The first high nuclearity, mixed-metal Bi(III)/Mn(IV) and Bi(III)/Mn(III) complexes are reported. The former complexes are [Bi(2)Mn(IV)(6)O(9)(O(2)CEt)(9)(HO(2)CEt)(NO(3))(3)] (1) andThe first high nuclearity, mixed-metal Bi(III)/Mn(IV) and Bi(III)/Mn(III) complexes are reported. The former complexes are [Bi(2)Mn(IV)(6)O(9)(O(2)CEt)(9)(HO(2)CEt)(NO(3))(3)] (1) and [Bi(2)Mn(IV)(6)O(9)(O(2)CPh)(9)(HO(2)CPh)(NO(3))(3)] (2) and were obtained from the comproportionation reaction between Mn(O(2)CR)(2) and MnO(4)(-) in a 10:3 ratio in the presence of Bi(NO(3))(3) (3 equiv) in either a H(2)O/EtCO(2)H (1) or MeCN/PhCO(2)H (2) solvent medium. The same reaction that gives 2, but with Bi(O(2)CMe)(3) and MeNO(2) in place of Bi(NO(3))(3) and MeCN, gave the lower oxidation state product [BiMn(III)(10)O(8)(O(2)CPh)(17)(HO(2)CPh)(H(2)O)] (3). Complexes 1 and 2 are near-isostructural and possess an unusual and high symmetry core topology consisting of a Mn(IV)(6) wheel with two central Bi(III) atoms capping the wheel on each side. In contrast, the [BiMn(III)(10)O(8)](17+) core of 3 is low symmetry, comprising a [BiMn(3)(μ(3)-O)(2)](8+) butterfly unit, four [BiMn(3)(μ(4)-O)](10+) tetrahedra, and two [BiMn(2)(μ(3)-O)](7+) triangles all fused together by sharing common Mn and Bi vertices. Variable-temperature, solid-state dc and ac magnetization data on 1-3 in the 1.8-300 K range revealed that 1 and 2 possess an S = 0 ground state spin, whereas 3 possesses an S = 2 ground state. The work offers the possibility of access to molecular analogs of the multifunctional Bi/Mn/O solids that are of such great interest in materials science.

• Raising the spin of Fe(III)7 disklike clusters: the power of molecular spin frustration.

  Authors: Shreya Mukherjee, Rashmi Bagai, Khalil A Abboud, George Christou

  Inorganic chemistry. 03/2011; 50(9):3849-51.

  Two clusters with a new type of Fe(III)(7) disklike structure have been prepared; in contrast to other Fe(III)(7) disks, they possess high ground-state spins (S = (15)/(2) and (21)/(2)), which haveTwo clusters with a new type of Fe(III)(7) disklike structure have been prepared; in contrast to other Fe(III)(7) disks, they possess high ground-state spins (S = (15)/(2) and (21)/(2)), which have been rationalized by analysis of the spin-frustration patterns.

• Self-assembly approach toward chiral bimetallic catalysts: bis-urea-functionalized (salen)cobalt complexes for the hydrolytic kinetic resolution of epoxides.

  Authors: Jongwoo Park, Kai Lang, Khalil A Abboud, Sukwon Hong

  Chemistry (Weinheim an der Bergstrasse, Germany). 02/2011; 17(7):2236-45.

  A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-ureaA series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.

• High-nuclearity, mixed-valence Mn₁₇, Mn₁₈ and {Mn₆₂}n complexes from the use of triethanolamine.

  Authors: Theocharis C Stamatatos, Dolos Foguet-Albiol, Wolfgang Wernsdorfer, Khalil A Abboud, George Christou

  Chemical communications (Cambridge, England). 01/2011; 47(1):274-6.

  The use of both azide and triethanolamine, with or without the presence of carboxylate groups, has provided new Mn(17), Mn(18) and {Mn(62)}(n) complexes with aesthetically-pleasing cage, layered, andThe use of both azide and triethanolamine, with or without the presence of carboxylate groups, has provided new Mn(17), Mn(18) and {Mn(62)}(n) complexes with aesthetically-pleasing cage, layered, and linked-chain-type structures; two are also new single-molecule magnets.

• Mn(21)Dy Cluster with a Record Magnetization Reversal Barrier for a Mixed 3d/4f Single-Molecule Magnet.

  Authors: Constantina Papatriantafyllopoulou, Wolfgang Wernsdorfer, Khalil A Abboud, George Christou

  Inorganic chemistry. 12/2010;

  A high-oxidation-state Mn(III,IV)(21)Dy(III) cluster with an unusual structure is reported. It also possesses a record barrier to magnetization reversal for a 3d/4f single-molecule magnet (SMM) andA high-oxidation-state Mn(III,IV)(21)Dy(III) cluster with an unusual structure is reported. It also possesses a record barrier to magnetization reversal for a 3d/4f single-molecule magnet (SMM) and provides insight into how the full benefit of lanthanides to the mixed 3d/4f SMM field might be realized.

• Pitiprolamide, a proline-rich dolastatin 16 analogue from the marine cyanobacterium Lyngbya majuscula from Guam.

  Authors: Rana Montaser, Khalil A Abboud, Valerie J Paul, Hendrik Luesch

  Journal of natural products. 12/2010; 74(1):109-12.

  An unusual cyclic depsipeptide, pitiprolamide (1), was isolated from the marine cyanobacterium Lyngbya majuscula collected at Piti Bomb Holes, Guam. The structure was deduced using NMR, MS, X-rayAn unusual cyclic depsipeptide, pitiprolamide (1), was isolated from the marine cyanobacterium Lyngbya majuscula collected at Piti Bomb Holes, Guam. The structure was deduced using NMR, MS, X-ray crystallography, and enantioselective HPLC-MS techniques. Remarkably, proline represents half of the residues forming pitiprolamide (1). Other distinctive features include a 4-phenylvaline (dolaphenvaline, Dpv) moiety initially found in dolastatin 16 and the rare 2,2-dimethyl-3-hydroxyhexanoic acid (Dmhha) unit condensed in a unique sequence in one single molecule. Pitiprolamide (1) showed weak cytotoxic activity against HCT116 colon and MCF7 breast cancer cell lines, as well as weak antibacterial activities against Mycobacterium tuberculosis and Bacillus cereus.

• Primary carbon-nitrogen bond scission and methyl dehydrogenation across a W-W multiple bond.

  Authors: Soumya Sarkar, Jeffrey A Culver, Andrew J Peloquin, Ion Ghiviriga, Khalil A Abboud, Adam S Veige

  Angewandte Chemie (International ed. in English). 11/2010; 49(50):9711-4.

• Grassypeptolides A-C, cytotoxic bis-thiazoline containing marine cyclodepsipeptides.

  Authors: Jason C Kwan, Ranjala Ratnayake, Khalil A Abboud, Valerie J Paul, Hendrik Luesch

  The Journal of organic chemistry. 11/2010; 75(23):8012-23.

  Grassypeptolides A-C (1-3), a group of closely related bis-thiazoline containing cyclic depsipeptides, have been isolated from extracts of the marine cyanobacterium Lyngbya confervoides. AlthoughGrassypeptolides A-C (1-3), a group of closely related bis-thiazoline containing cyclic depsipeptides, have been isolated from extracts of the marine cyanobacterium Lyngbya confervoides. Although structural differences between the analogues are minimal, comparison of the in vitro cytotoxicity of the series revealed a structure-activity relationship. When the ethyl substituent of 1 is changed to a methyl substituent in 2, activity is only slightly reduced (3-4-fold), whereas inversion of the Phe unit flanking the bis-thiazoline moiety results in 16-23-fold greater potency. We show that both 1 and 3 cause G1 phase cell cycle arrest at lower concentrations, followed at higher concentrations by G2/M phase arrest, and that these compounds bind Cu(2+) and Zn(2+). The three-dimensional structure of 2 was determined by MS, NMR, and X-ray crystallography, and the structure of 3 was established by MS, NMR, and chemical degradation. The structure of 3 was explored by in silico molecular modeling, revealing subtle differences in overall conformation between 1 and 3. Attempts to interconvert 1 and 3 with base were unsuccessful, but enzymatic conversion may be possible and could be a novel form of activation for chemical defense.

• Isoquinoline-based chiral monodentate N-heterocyclic carbenes.

  Authors: Dimitri Hirsch-Weil, Khalil A Abboud, Sukwon Hong

  Chemical communications (Cambridge, England). 10/2010; 46(40):7525-7.

  C(1)-symmetric isoquinoline-based chiral diaminocarbene ligands (MIQ) have been developed to block three quadrants of the metal coordination sphere, complementing C(2)-symmetric biisoquinoline-basedC(1)-symmetric isoquinoline-based chiral diaminocarbene ligands (MIQ) have been developed to block three quadrants of the metal coordination sphere, complementing C(2)-symmetric biisoquinoline-based ligands (BIQ). MIQ-Cu complexes catalyzed conjugate borylation of various α,β-unsaturated amides in good yields (82-99%) and enantioselectivities (75-87% ee).