Khalil A. Abboud

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

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 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 [Mn3CaO4] 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 MnIV species, including the OEC S2 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.

Self-Assembled Dinuclear Cobalt(II)-Salen Catalyst Through Hydrogen-Bonding and Its Application to Enantioselective Nitro-Aldol (Henry) Reaction

Novel chiral dinuclear Co(II)-salen catalysts self-assembled through the pyridone/aminopyridine hydrogen-bonding pair have been developed. The self-assembled dinuclear Co(II)-Salen catalyst results in significant rate acceleration (48 times faster rate) as well as excellent enantioselectivity (96% ee vs 55% ee) in a nitro-aldol reaction, compared to the corresponding monomeric catalyst. A bimetallic mechanism is suggested by the kinetic experiment showing that the rate law is second order in [catalyst]. The self-assembly through hydrogen bonding was confirmed by X-ray structure and by the 1H experiments revealing that the dimerization constant of the novel hydrogen-bonding capable salen ligand in 25% v/v CD3NO2/CDCl3 is 53 ± 21 M−1. This result proves the validity of self-assembly based approaches toward the efficient construction of chiral bimetallic catalyst system.

Mn21Dy Cluster with a Record Magnetization Reversal Barrier for a Mixed 3d/4f Single-Molecule Magnet

A high-oxidation-state Mn(III,IV)21Dy(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.

Discrete Photopatternable π-Conjugated Oligomers for Electrochromic Devices

Three discrete oligomeric systems including an all-thiophene ( T6) system, a thiophene/phenylene ( TPTTPT) system, and a thiophene/EDOT/phenylene ( TPEEPT) system have been constructed and characterized with emphasis on structural, optical, electrochemical, and spectroelectrochemical properties. For all three chromophores, the radical cation, the dication, and the pi-dimer have been identified and characterized. EPR spectroscopy reveals that the radical cations of TPTTPT and TPEEPT have g values of 2.008-2.012 and peak-to-peak widths in the range 4.2-5.3 G. Formation of the radical cation takes place at a lower potential for TPEEPT than for TPTTPT and T6, whereas subsequent oxidation to the dication occurs more easily for TPTTPT than for TPEEPT and T6. We ascribe this observation to more localized charges in the oxidized species of TPEEPT, which is supported by our finding that the radical cation of TPEEPT is less prone to undergo pi-dimerization than the radical cations of TPTTPT and T6. All the oxidized species are sufficiently stable to allow for optical characterization, and the relative positions of all absorption bands are found to be in agreement with the electrochemical data. For further solid-state modifications of these materials, we have effectively modified the synthetic design and grafted terminal functionalities (e.s. acrylates) onto the discrete oligomers. Of these novel materials, TPEEPT proves to be the most promising anodically coloring material for electrochromics, and it undergoes reversible switching between two different colored states (bright yellow and clear blue) and one almost transparent and color neutral state. Contrast ratios, measured as Delta% T at lambda max, are as high as 62.5%, and switching times are in the range 2-5 s for the coloration process, though significantly longer for the bleaching process. As a proof of concept, we have successfully constructed a simple photopatterned electrochromic device by exploiting the terminal acrylate functionalities of the oligomers in a UV-initiated cross-linking process. To the best of our knowledge, this is the first oligomer-based photopatterned electrochromic device reported in the literature.

Covalently Linked Dimers of Clusters: Loop‐ and Dumbbell‐Shaped Mn24 and Mn26 Single‐Molecule Magnets

Molecular clusters of paramagnetic 3d transition metals continue to be a major research area because of their fascinating physical properties and their complex structures. In particular, they often have high-spin ground states and easy-axis-type magnetic anisotropy, giving a significant energy barrier to reversal of the magnetization vector. Thus, at sufficiently low temperatures they function as nanoscale magnetic particles. Such single-molecule magnets (SMMs) also straddle the classical/quantum interface by displaying not just classical magnetization hysteresis but also quantum tunneling of magnetization (QTM) and quantum phase interference. SMMs represent a molecular, or “bottom-up”, route to nanoscale magnetic materials, with potential applications in information storage and spintronics at the molecular level and use as quantum bits (qubits) in quantum computation. The upper limit to the barrier (U) is given by S 2 jD j or (S 2 1/4) jD j for integer and halfinteger spins (S), respectively; in practice, QTM through upper regions of the barrier makes the true or effective barrier (Ueff) less than U. Manganese carboxylate chemistry has been the main source of new SMMs, and we are therefore developing new synthetic methods to Mn clusters of various types. The N3 ion bridging in the 1,1-fashion (end-on) is a strong ferromagnetic mediator for a wide range of M-N-M angles, and thus it opens an attractive route to new high-spin Mn clusters and SMMs. In past work, we have shown that azide and the bidentate N,O chelate hmp (the anion of 2-(hydroxymethyl)pyridine) or the tridentate N,O,O chelates pdmH /pdm (the anions of 2,6-pyridinedimethanol) yield [Mn10O4(N3)4(hmp)12] 2+ with S= 22 and [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6] 2+ with S= 51/2, respectively. Both clusters are high-spin molecules, but with smallUeff values, and include coordinated azide groups as ancillary ligands. In the present work, we have explored reactions of Mn precursors with azide and the potentially tetradentate N,N,O,O gem-diolate of di-2-pyridylketone, (py)2C(O)2 2 (dpkd ), formed in situ from dpk, which has previously been a useful route to non-azido metal clusters. We considered dpkd particularly attractive because it can be

It Takes More Than an Imine: The Role of the Central Atom on the Electron-Accepting Ability of Benzotriazole and Benzothiadiazole Oligomers

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

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.

V. Molecular structures of acetate-bridged dimers of a 2-arylpyridine and a 2,6-diarylpyridine cyclometallated by palladium(II)

Abstract The crystal and molecular structures of dimeric cyclopalladated acetato(OAc)-bridged complexes of a 2-arylpyridine ( 1 ) and a 2,6-diarylpyridine ( 2 ), [Pd( 1 )OAc] 2 ( 3 ) and [Pd( 2 )OAc] 2 ( 4 ) have been determined by X-ray diffraction. In each. two planar acetates, nearly perpendicular to each other, cis -bridge two Pd II atoms, while the other two square planar sites on each metal are occupied by the cyclometallated aryl-substituted pyridine. The two metal coordination planes are in near parallel registry with one another, giving a “boat” molecule shape of exact ( 3 or near ( 4 ) two-fold symmetry. The non-bonding Pd⋯Pd distance (δ) strongly correlates with the angle (α) subtended by the metal coordination planes, which in turn seems related to the steric requirements of the non-bridging ligands. In 3 , with the planar nitrophenylpyridine ligand, α 25.7° and δ 2.822(6) A, the shortest Pd⋯Pd distance yet observed in dimers of this type. The PdC lengths, 1.94 A in 3 and 1.944 A in 4 , are significantly shorter than the calculated length, 2.05 A, suggesting partial multiple bond character in these shortest PdC (aryl) bonds so far observed. PdN bond lengths are near expected single bond lengths. Failure of trans -dimetallation to occur in 4 , analogous to that recently observed with certain 2,6-dialkylpyridines or to the tridentate behavior of terpyridine in [Pd terpy X] + compounds, appears due to a combination of the shortened PdC bond in the first-formed metallated ring and the larger constraining angles (near 120° for sp 2 carbons) in the remaining (unmetallated) ring.

Unusual structural types in nickel cluster chemistry from the use of pyridyl oximes: Ni5, Ni12Na2, and Ni14 clusters.

Syntheses, crystal structures, and magnetochemical characterization are reported for the three new nickel(II) clusters [Ni(14)(OH)(4)(N(3))(8)(pao)(14)(paoH)(2)(H(2)O)(2)](ClO(4))(2) (1), [Ni(12)Na(2)(OH)(4)(N(3))(8)(pao)(12)(H(2)O)(10)](OH)(2) (2), and [Ni(5)(ppko)(5)(H(2)O)(7)](NO(3))(5) (3) (paoH = pyridine-2-carbaldehyde oxime, ppkoH = di-2-pyridyl ketone oxime). The reaction of Ni(ClO(4))(2).6H(2)O with paoH and NBu(n)(4)N(3) in H(2)O/MeCN in the presence of NEt(3), gave 1 in 65% yield. Complex 2 was obtained in 60% yield from the reaction of NiCl(2).6H(2)O with paoH and NaN(3) in H(2)O/MeCN in the presence of NaOH. The reaction of Ni(NO(3))(2).6H(2)O with ppkoH in EtOH in the presence of LiOH afforded complex 3 in 75% yield. The complexes all contain novel core topologies. The core of 1 comprises a central Ni(4) rhombus between two Ni(5). Complex 1 is the largest metal/oxime cluster discovered to date, as well as the first Ni(II)(14) coordination complex and the largest Ni(II)/N(3)(-) cluster. Complex 2 has a Ni(12)Na(2) topology that is very similar to that of 1, but with two central Ni(II) atoms of 1 replaced with Na(I) atoms. The core of 3 consists of four Ni(II) atoms forming a highly distorted tetrahedron, with the fifth Ni(II) atom lying almost on one of the edges. Variable-temperature, solid-state dc susceptibility and magnetization studies were carried out on complexes 1-3, and these were complemented with ac susceptibility data for 1 and 2. Fitting of the obtained M/(Nmu(B)) vs H/T data by matrix diagonalization and including axial zero-field splitting (D) gave ground-state spin (S) and D values of S = 6, D = -0.12(3) cm(-1) for 1 and S = 3, D = -0.20(5) cm(-1) for each of the two essentially noninteracting S = 3 Ni(6) subunits of 2. The data for 3 indicate antiferromagnetic exchange interactions and an S = 1 ground state. A simple 2-J model was found to be adequate to describe the variable-temperature dc susceptibility data. The combined work demonstrates the ligating flexibility of pao(-) and ppko(-), and their usefulness in the synthesis of polynuclear Ni(x) clusters with or without the presence of ancillary ligands.

Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts.

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.