The Re(I) coordination chemistry of a series of pyrido[2,3-b]pyrazine-derived ligands: Syntheses,characterisation and crystal structures

Reaction of 2,3-diaminopyridine with one equivalent of a functionalised vicinal diketone, in ethanol, yields a series of ligands based upon the pyrido[2,3-b]pyrazine core. The ligands were characterised by ¹H, ¹³C–{¹H} NMR, MS and UV–Vis spectroscopy. Reaction of the ligands with one equivalent of {ReBr(CO)₅} gave a series of Re-Lⁿ complexes based upon the general formula fac-{ReBr(CO)₃(L)} (where L = pyrido[2,3-b]pyrazine-derived ligands, L¹–L⁶). Solution IR studies confirmed the retention of the facially capped, tri-carbonyl coordination geometry at rhenium, and ¹H NMR studies confirmed coordination of the ligand to Re(I). EI HR MS data were obtained for each complex confirming the proposed formulation and stoichiometry. Single crystal X-ray structures were obtained for three of the complexes (Re-L¹, Re-L², Re-L⁶), with each demonstrating that the ligands coordinate to Re(I) in a bidentate manner, via a four-membered chelate ring, which was unsymmetrical in the former two cases. The electronic absorption spectra of the complexes showed absorption into the visible region ca. 375–500 nm, (the complexes are orange-red in appearance). Following irradiation at 350–450 nm, the complexes display a solid-state broad emission peaking between 600–700 nm. The complexes were not sufficiently luminescent in solution to allow further investigation into the origin of this emission band, although with reference to related 1,8-naphthyridine complexes of Re(I) it is likely to incorporate significant ³MLCT character.     

Synthesis,spectroscopic and structural characterisation of vanadium(IV) and oxovanadium(IV) complexes with arsenic donor ligands

The reaction of o-C₆H₄(AsMe₂)₂ with VCl₄ in anhydrous CCl₄ produces orange eight-coordinate [VCl₄{o-C₆H₄(AsMe₂)₂}₂], whilst in CH₂Cl₂ the product is the brown, six-coordinate [VCl₄{o-C₆H₄(AsMe₂)₂}]. In dilute CH₂Cl₂ solution slow decomposition occurs to form the V^III complex [V₂Cl₆{o-C₆H₄(AsMe₂)₂}₂]. Six-coordination is also found in [VCl₄{MeC(CH₂AsMe₂)₃}] and [VCl₄{Et₃As)₂]. Hydrolysis of these complexes occurs readily to form vanadyl (VO²⁺) species, pure samples of which are obtained by reaction of [VOCl₂(thf)₂(H₂O)] with the arsines to form green [VOCl₂{o-C₆H₄(AsMe₂)₂}], [VOCl₂{MeC(CH₂AsMe₂)₃}(H₂O)] and [VOCl₂(Et₃As)₂]. Green [VOCl₂(o-C₆H₄(PMe₂)₂}] is formed from [VOCl₂(thf)₂(H₂O)] and the ligand. The [VOCl₂{o-C₆H₄(PMe₂)₂}] decomposes in thf solution open to air to form the diphosphine dioxide complex [VO{o-C₆H₄(P(O)Me₂)₂}₂(H₂O)]Cl₂, but in contrast, the products formed from similar treatment of [VCl₄{o-C₆H₄(AsMe₂)₂}_x] or [VOCl₂{o-C₆H₄(AsMe₂)₂}] contain the novel arsenic(V) cation [o-C₆H₄(AsMe₂Cl)(μ-O)(AsMe₂)]⁺. X-ray crystal structures are reported for [V₂Cl₆{o-C₆H₄(AsMe₂)₂}₂], [VO(H₂O){o-C₆H₄(P(O)Me₂)₂}₂]Cl₂, [o-C₆H₄(AsMe₂Cl)(μ-O)(AsMe₂)]Cl·[VO(H₂O)₃Cl₂] and powder neutron diffraction data for [VCl₄{o-C₆H₄(AsMe₂)₂}₂].     

Calculation and Measurement of Terahertz Active Normal Modes in Crystalline PETN

The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover 1 with the COMPASS 2 force field, CASTEP^[3] and PWscf. 4 The calculations are compared to each other and to terahertz spectra (0.3–3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.     

Exciplex Formation and Excited State Deactivation of Difluoroborondipyrromethene (Bodipy) Dyads

Two series of geometrically‐related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups. These units are anchored directly, or via a phenylene spacer, to the Bodipy core at the meso position in one series ( BD‐MHQ , BD‐MQ , BD‐MPHQ , BD‐MPQ ), but for the second series the attachment site is the 2‐position ( BD‐SHQ , BD‐SQ , BD‐SPHQ , BD‐SPQ ). The compounds show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern. In non‐polar solvents such as toluene, diethyl ether and dichlorobenzene, the S₁ state deactivation of the Bodipy unit in BD‐SPQ and BD‐MPQ is dominated by ^{1, 3}exciplex formation, which has not been reported for Bodipy derivatives so far. In the latter molecule, the decay of the exciplex is divided between population of the Bodipy triplet state (13 %–21 %) and ground state reformation. This partitioning is not seen for the side‐on substituted derivative, BD‐SPQ , and only ground state reformation is observed following decay of the exciplex. This difference in behavior is explained by the radical‐pair inter‐system‐crossing mechanism, which more effectively operates in BD‐MPQ because of the orthogonality of the donor‐acceptor units. In the more polar solvent CH₃CN all the quinone derivatives show fast formation of the charge‐separated state (k_CS) followed by slower charge recombination (k_CR). The ratio k_CS/k_CR≤80.     

Ambipolar hexa-peri-hexabenzocoronene-fullerene hybrid materials

A new class of self-assembling hexa-peri-hexbenzocoronene (HBC)-fullerene hybrid materials has been synthesized and characterized. Photoluminescence experiments indicate that energy transfer processes can be tuned in these donor-acceptor systems by varying the length and nature of the linker group. In preliminary device testing, ambipolar charge transport behavior is observed in organic field effect transistors, while single active component organic photovoltaic devices consisting of these materials achieved a maximum external quantum efficiency of 30%.     

Heronapyrroles A-C: farnesylated 2-nitropyrroles from an Australian marine-derived Streptomyces sp

Chemical analysis of a marine-derived Streptomyces sp. (CMB-M0423) isolated from beach sand off Heron Island, Australia, yielded three new members of the rare pyrroloterpene biosynthetic structure class. Identified by detailed spectroscopic analysis as the first reported examples of naturally occurring 2-nitropyrroles, heronapyrroles A-C (1-3) displayed promising biological activity-with low to submicromolar IC(50) activity against Gram-positive bacteria but no cytotoxicity toward mammalian cell lines.     

Di- and trisubstituted γ-lactams via Rh(II)-carbenoid reaction of N-Cα-branched, N-bis(trimethylsilyl)methyl α-diazoamides. Synthesis of (±)-α-allokainic acid

Acyclic N-C(α)-branched, N-bis(trimethylsilyl)methyl (N-BTMSM) diazoamides undergo regio-, chemo-, and diastereoselective Rh(II)-carbenoid C-H insertion to give 4,5-disubstituted and 3,4,5-trisubstituted γ-lactams. The conformational influence of the N-BTMSM group and the electronic effect of the O-pivaloyl moiety of the C(α)-oxymethylene unit are essential for the observed regioselectivity. The synthesis of α-allokainic acid demonstrates the utility of the method.     

Synthesis and characterization of aryl substituted bis(2-pyridyl)amines and their copper olefin complexes: investigation of remote steric control over olefin binding

The aryl-functionalized pyridylamine 2-(i)PrC(6)H(4)N(H)py (1) and bis(2-pyridyl)amines of the type ArN(py)(2) for Ar = Mes (2), 2,6-Et(2)C(6)H(3) (3), 2-(i)PrC(6)H(4) (4), 2,6-(i)Pr(2)C(6)H(3) (5), and 1-naph (6), have been prepared by the palladium-catalyzed cross-coupling of substituted anilines with 2-bromopyridine, and have been characterized by (1)H and (13)C NMR NMR, FTIR, MS, and TGA. Complexes of these new N-aryl bis(2-pyridyl)amines have been prepared for the acid salts [H{ArN(py)(2)}]BF(4) where Ar = Mes (7) and 2-(i)PrC(6)H(4) (8), and the dimeric bridged complexes [Cu{ArN(py)(2)}(μ-X)(Y)](2) where X/Y = Cl(-) and Ar = Ph (9), 2-(i)PrC(6)H(4) (10), and 1-naph (11), in addition to X = OH(-), Y = H(2)O and Ar = Mes (12). The olefin complexes [Cu(Ar-dpa)(styrene)]BF(4) for Ar = Ph (13), Mes (14), 2-(i)PrC(6)H(4) (15), and 1-naph (16), in addition to the norborylene complexes of Ar = Mes (17) and 2-(i)PrC(6)H(4) (18) have been prepared and characterized by (1)H and (13)C NMR, FTIR, and TGA. The crystal structures have been determined for compounds 1-17. Secondary amine 1 crystallizes in hydrogen-bonded head-to-tail dimers, while the N-aryl bis(2-pyridyl)amines 2-6 crystallize in a three-bladed propellar conformation, having nearly planar geometries about the amine nitrogen. The geometry about copper centers in the dimeric complexes 9-12 is distorted trigonal bypyramidal, with the axial positions occupied by one of the two pyridyl nitrogens and one of the bridging ligands (i.e., Cl or OH). The copper atoms in each of the olefin complexes 13-17 are coordinated to the two pyridine nitrogen atoms and the appropriate olefin; consistent with a pseudo three-coordinate Cu(I) cation. Distortion of pyridyl ring geometries about the copper centers, and concomitant bending of the aryl groups away from the CuN(amine) vectors were found to correlate with the steric bulk of the aryl group present in both dimeric and olefin complexes. Such distortion is also observed to a lesser extent in the acid salts as well. The (1)H and (13)C NMR spectra of [Cu(Ar-dpa)(olefin)]BF(4) exhibit an upfield shift in the olefin signal as compared to free olefin. A good correlation exists between the (1)H and (13)C NMR Δδ values and olefin dissociation temperatures, confirming that the shift of the olefin NMR resonances upon coordination is associated with the binding strength of the complex.     

Computational design of a self-assembling β-peptide oligomer

The first computationally designed self-assembling oligomer consisting of exclusively β-amino acids (βAAs) is presented. The packing of a β-3(14) helix into coiled-coils of varying stoichiometries as a function of amino acid sequence is examined. β-Peptides with hVal repeating every third residue in the sequence appeared to have a strong propensity to pack into hexameric bundles. The designed sequence was synthesized and characterized with CD spectroscopy, NMR, and analytical ultracentrifugation, suggesting that the peptide adopts a well-folded hexameric structure.