Magnetic exchange interactions and magneto-structural correlations in heterobridged μ-phenoxo-μ(1,1)-azide dinickel(II) compounds: a combined experimental and theoretical exploration

This investigation presents the syntheses, crystal structures, magnetic properties, and density functional theoretical modeling of magnetic behavior of two heterobridged μ-phenoxo-μ(1,1)-azido dinickel(II) compounds [Ni(II)(2)(L(1))(2)(μ(1,1)-N(3))(N(3))(H(2)O)]·CH(3)CH(2)OH (1) and [Ni(II)(2)(L(2))(2)(μ(1,1)-N(3))(CH(3)CN)(H(2)O)](ClO(4))·H(2)O·CH(3)CN (2), where HL(1) and HL(2) are the [1+1] condensation products of 3-methoxysalicylaldehyde and 1-(2-aminoethyl)-piperidine (for HL(1))/4-(2-aminoethyl)-morpholine (for HL(2)), along with density functional theoretical magneto-structural correlations of μ-phenoxo-μ(1,1)-azido dinickel(II) systems. Compounds 1 and 2 crystallize in orthorhombic (space group Pbca) and monoclinic (space group P2(1)/c) systems, respectively. The coordination environments of both metal centers are distorted octahedral. The variable-temperature (2-300 K) magnetic susceptibilities at 0.7 T of both compounds have been measured. The interaction between the metal centers is moderately ferromagnetic; J = 16.6 cm(-1), g = 2.2, and D = -7.3 cm(-1) for 1 and J = 16.92 cm(-1), g = 2.2, and D(Ni1) = D(Ni2) = -6.41 cm(-1) for 2. Broken symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and provide a good numerical estimate of J values (15.8 cm(-1) for 1 and 15.35 cm(-1) for 2) compared to experiments. The role of Ni-N bond length asymmetry on the magnetic coupling has been noted by comparing the structures and J values of complexes 1 and 2 together with previously published dimers 3 (Eur. J. Inorg. Chem. 2009, 4982), 4 (Inorg. Chem. 2004, 43, 2427), and 5 (Dalton Trans. 2008, 6539). Our extensive DFT calculations reveal an important clue to the mechanism of coupling where the orientation of the magnetic orbitals seems to differ with asymmetry in the Ni-N bond lengths. This difference in orientation leads to a large change in the overlap integral between the magnetic orbitals and thus the magnetic coupling. DFT calculations have also been extended to develop several magneto-structural correlations in this type of complexes and the correlation aim to focus on the asymmetry of the Ni-N bond lengths reveal that the asymmetry plays a proactive role in governing the magnitude of the coupling. From a completely symmetric Ni-N bond length, two behaviors have been noted: with a decrease in bond length there is an increase in the ferromagnetic coupling, while an increase in the bond lengths leads to a decrease in ferromagnetic interaction. The later correlation is supported by experiments. The magnetic properties of 1, 2, and three previously reported related compounds have been discussed in light of the structural parameters and also in light of the theoretical correlations determined here.     

Structurally powered synergic 2,2,6,6-tetramethylpiperidine bimetallics: new reflections through lithium-mediated ortho aluminations

Recent times have witnessed many notable advances in metalation chemistry with halide salt supported strategies and alkali-metal mediated metalation being particularly prominent. This article begins with a brief account of both of these avant garde metalation methods focusing on selected recent examples not covered previously in a review. New results in the area of Alkali-Metal Mediated Alumination (AMMAl) are also presented. Thus, the putative lithium aluminate base THF·Li(μ-TMP)(2)Al((i)Bu)(2) (4) is shown to act via TMP basicity to efficiently ortho deprotonate a variety of functionalized aromatic molecules at room temperature, tolerating carboxamide and halide functionalities. These metalated species are electrophilically quenched with elemental iodine. Crystal structure determinations of the metalated intermediates confirm unequivocally that direct alumination of the substrates has occurred. Since the homometallic lithium or aluminum reagents are unable to effect such deprotonations these reactions are synergic in nature and can be considered examples of AMMAl. Drawing together previously published work in the field of AMMAl, together with other pertinent experimental observations and new density functional theory (DFT) computational studies, we propose a potential rationale for the "unusual" reactivity patterns witnessed in this branch of heterometallic synthetic chemistry with respect to other Alkali Metal Mediated Metalations which appear to behave in a more conventional manner.     

Access to antigens related to anthrose using pivotal cyclic sulfite/sulfate intermediates

Anthrose is the upstream terminal unit of the tetrasaccharide side chain from a major glycoprotein of Bacillus anthracis exosporium and is part of important antigenic determinants. A novel entry to anthrose-containing antigens and precursors is described. The synthetic route, starting from D(+)-fucose, makes use of intermediates featuring a cyclic sulfite or sulfate function which serves successively as a protecting and a leaving group.     

cis-Apa: a practical linker for the microwave-assisted preparation of cyclic pseudopeptides via RCM cyclative cleavage

A new linker cis-5-aminopent-3-enoic acid (cis-Apa) was prepared for the synthesis of cyclic pseudopeptides by cyclization-cleavage by using ring-closing methatesis (RCM). We developed a new synthetic pathway for the preparation of the cis-Apa linker that was tested in the cyclization-cleavage process of different RGD peptide sequences. Different macrocyclic peptidomimetics were prepared by using this integrated microwave-assisted method, showing that the readily available cis-Apa amino acid is well adapted as a linker in the cyclization-cleavage process.     

A general copper powder-catalyzed Ullmann-type reaction of 3-halo-4(1H)-quinolones with various nitrogen-containing nucleophiles

3-(N-Substituted) 4(1H)-quinolinones were synthesized using the copper-catalyzed Ullmann C-N bond forming strategy in moderate to quantitative yields. Starting from 3-halo-4(1H)-quinolones, various nucleophiles including amides, lactams, sulfonamides and NH-containing azoles have been used successfully. In all cases, the reactions take place rapidly in toluene and proceed by using copper powder as a catalyst, DMEDA as a ligand and K(2)CO(3) as a base. In addition, other related heterocycles such as 3-bromoquinolin-2(1H)-ones, 3-bromocoumarin, and 3,5-dibromo-2-pyridone show good to very high reactivity with various nucleophiles under our Cu/DMEDA catalyst system.     

Hexaphenylbenzene as a rigid template for the straightforward syntheses of "star-shaped" glycodendrimers

Original glycodendrimers emanating from propargylated hexaphenylbenzene cores and containing up to 54 peripheral sugar ligands have been synthesized by Cu(I)-catalyzed [1,3]-dipolar cycloadditions using both convergent and divergent approaches.     

Two-photon absorption properties of two-dimensional π-conjugated chromophores: combined experimental and theoretical study

The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis.     

Fabrication of polymer nanotubes containing nanoparticles and inside functionalization

A polypyrrole nanotube containing nanoparticles is prepared by a dual template approach, and the inside of the nanotube is selectively functionalized with carboxyl and amine groups.     

Reactivity of Lewis pairs (R2PCH2AlMe2)2 with carbon dioxide

Species R(2)PCH(2)AlMe(2) (R = Me, Ph) are stable Lewis adducts but still react with CO(2) both in solution and in the solid state. The CO(2) adducts undergo a rearrangement unprecedented for ambiphilic molecules to form aluminium carboxylates. A new spirocyclic compound was also obtained by double Lewis pair activation of CO(2).     

Selective supramolecular assembly of multifunctional ligands on a Cu(111) surface: metallacycles, propeller trimers and linear chains

We studied the supramolecular assembly of a multifunctional ligand, cis-bis-terpyridine tetraphenyl ethylene, on a Cu(111) surface by low-temperature scanning tunneling microscopy (STM). Three distinctive supramolecular structures, metallacycles, propeller-shaped clusters and extended linear chains, are formed under specific assembly conditions owing to different inter-molecular binding modes of Cu-coordination, van der Waals interaction and hydrogen bonding, respectively.