Maren Pink

Dipole-Promoted and Size-Dependent Cooperativity between Pyridyl-Containing Triazolophanes and Halides Leads to Persistent Sandwich Complexes with Iodide

Triazolophanes that incorporate pyridyl subunits in place of phenylenes show a heightened propensity to form 2:1 sandwich complexes with halides. Persistent iodide-based sandwiches are observed. Binding constants confirm that the inward-facing electron pairs on the pyridyls destabilize the 1:1 complexes with halides. The (1)H NMR spectra verify that the sandwich complexes have two pi-stacked triazolophanes rotated to allow registration between opposite dipoles on the pyridyls (directed inward) and triazoles (directed outward). These dipolar interactions cooperate to lower the pyridyl-based repulsions, therefore, increasing K(2). Modest cooperative effects are observed for the snugly fitting F(-), Cl(-), and Br(-) halides while the too-large I(-) shows highly positive cooperativity.

Conjugated Polymer Sensors Built on π-Extended Borasiloxane Cages

An efficient 2 + 2 cyclocondensation with dihydroxysilane converted simple arylboronic acids to bifunctional borasiloxane cage molecules, which were subsequently electropolymerized to furnish air-stable thin films. The extended [p,pi]-conjugation that defines the rigid backbone of this new conjugated polymer (CP) motif gives rise to longer-wavelength UV-vis transitions upon oxidative doping, the spectral window and intensity of which can be modified by interaction with Lewis basic reagents. Notably, this boron-containing CP undergoes a rapid and reversible color change from green to orange upon exposure to volatile amine samples under ambient conditions. This direct naked-eye detection scheme can best be explained by invoking the reversible B-N dative bond formation that profoundly influences the p-pi* orbital overlap.

Low-Coordinate and Neutral Nitrido Complexes of Vanadium

Two neutral and four-coordinate vanadium(V)-nitrido complexes have been prepared via the thermolysis of metastable vanadium(III)-azido precursors. All complexes have been fully characterized by multinuclear NMR, FT-IR, isotopic labeling, and, in most instances, single crystal X-ray diffraction. On the basis of activation parameters, N(2) extrusion to form the V[triple bond]N moiety is proposed to occur via an ordered and early transition state having three- or four-triazametallacycle frameworks. In addition, we demonstrate the nitrido ligand to undergo incomplete N-atom transfer to CO and CN{2,6-Me(2)-C(6)H(3)) to form the bent V-N=C=X (X = O, N{2,6-Me(2)-C(6)H(3)}) ligands with concomitant 2e(-) reduction at the vanadium center.

A diastereo- and enantioselective synthesis of alpha-substituted anti-alpha,beta-diaminophosphonic acid derivatives.

Highly diastereo- and enantioselective additions of alpha-nitrophosphonates to imines catalyzed by a chiral Brønsted acid are described.

Layer-by-layer synthesis of metal-containing conducting polymers: caged metal centers for interlayer charge transport.

Metal-templated [2 + 3]-type cocondensation of a pi-extended boronic acid and nioxime furnished a series of cage molecules, which were electropolymerized to prepare metal-containing conducting polymers (MCPs). Despite sharing essentially isostructural organic scaffolds, these materials display metal-dependent electrochemical properties as evidenced by different redox windows observed for M = Co, Fe, Ru. Consecutive electropolymerization using two different monomers furnished bilayer MCPs having different metals in each layer. In addition to functioning as heavy atom markers in cross-sectional analysis by FIB and EDX, redox-active metal centers participate in voltage-dependent interlayer electron transport to give rise to cyclic voltammograms that are distinctively different from those of each layer alone or random copolymers. A simple electrochemical technique can thus be used as a straightforward diagnostic tool to investigate the structural ordering of electrically conductive layered materials.

Reduction of a Redox-Active Ligand Drives Switching in a Cu(I) Pseudorotaxane by a Bimolecular Mechanism

The reduction of a redox-active ligand is shown to drive reversible switching of a Cu(I) [2]pseudorotaxane ([2]PR(+)) into the reduced [3]pseudorotaxane ([3]PR(+)) by a bimolecular mechanism. The unreduced pseudorotaxanes [2]PR(+) and [3]PR(2+) are initially self-assembled from the binucleating ligand, 3,6-bis(5-methyl-2-pyridine)-1,2,4,5-tetrazine (Me(2)BPTZ), and a preformed copper-macrocycle moiety (Cu-M(+)) based on 1,10-phenanthroline. X-ray crystallography revealed a syn geometry of the [3]PR(2+). The UV-vis-NIR spectra show low-energy metal-to-ligand charge-transfer transitions that red shift from 808 nm for [2]PR(+) to 1088 nm for [3]PR(2+). Quantitative analysis of the UV-vis-NIR titration shows the stepwise formation constants to be K(1) = 8.9 x 10(8) M(-1) and K(2) = 3.1 x 10(6) M(-1), indicative of negative cooperativity. The cyclic voltammetry (CV) and coulometry of Me(2)BPTZ, [2]PR(+), and [3]PR(2+) shows the one-electron reductions at E(1/2) = -0.96, -0.65, and -0.285 V, respectively, to be stabilized in a stepwise manner by each Cu(+) ion. CVs of [2]PR(+) show changes with scan rate consistent with an EC mechanism of supramolecular disproportionation after reduction: [2]PR(0) + [2]PR(+) = [3]PR(+) + Me(2)BPTZ(0) (K(D)*, k(d)). UV-vis-NIR spectroelectrochemistry was used to confirm the 1:1 product stoichiometry for [3]PR(+):Me(2)BPTZ. The driving force (DeltaG(D)* = -5.1 kcal mol(-1)) for the reaction is based on the enhanced stability of the reduced [3]PR(+) over reduced [2]PR(0) by 365 mV (8.4 kcal mol(-1)). Digital simulations of the CVs are consistent with a bimolecular pathway (k(d) = 12 000 s(-1) M(-1)). Confirmation of the mechanism provides a basis to extend this new switching modality to molecular machines.

Noncovalent Interactions in the Asymmetric Synthesis of Rigid, Conjugated Helical Structures

Tetrakis({beta}-trithiophene) 1 folds into a helical conformation (RRR) that facilitates double ring annelation, with high diastereoselectivity and modest enantioselectivity, to provide bis[7]helicene 2 (MRM). This rigid, helically locked structure has enhanced chiroptical properties similar to the corresponding [15]helicene.

A Spirocyclohexyl Nitroxide Amino Acid Spin Label for Pulsed EPR Spectroscopy Distance Measurements

Site-directed spin labeling and EPR spectroscopy offer accurate, sensitive tools for the characterization of structure and function of macromolecules and their assemblies. A new rigid spin label, spirocyclohexyl nitroxide alpha-amino acid and its N-(9-fluorenylmethoxycarbonyl) derivative, have been synthesized, which exhibit slow enough spin-echo dephasing to permit accurate distance measurements by pulsed EPR spectroscopy at temperatures up to 125 K in 1:1 water/glycerol and at higher temperatures in matrices with higher glass transition temperatures. Distance measurements in the liquid nitrogen temperature range are less expensive than those that require liquid helium, which will greatly facilitate applications of pulsed EPR spectroscopy to the study of structure and conformation of peptides and proteins.

Phosphinidene Complexes of Scandium: Powerful PAr Group-Transfer Vehicles to Organic and Inorganic Substrates

The first phosphinidene complexes of scandium are reported in this contribution. When complex (PNP)Sc(CH(3))Br (1) is treated with 1 equiv of LiPH[Trip] (Trip = 2,4,6-(i)Pr(3)C(6)H(2)), the dinuclear scandium phosphinidene complex [(PNP)Sc(mu(2)-P[Trip])](2) (2) is obtained. However, treating 1 with a bulkier primary phosphide produces the mononuclear scandium ate complex [(PNP)Sc(mu(2)-P[DMP])(mu(2)-Br)Li] (3) (DMP = 2,6-Mes(2)C(6)H(3)). The Li cation in 3 can be partially encapsulated with DME to furnish a phosphinidene salt derivative, (PNP)Sc(mu(2)-P[DMP])(mu(2)-Br)Li(DME)] (4). We also demonstrate that complex 3 can readily deliver the phosphinidene ligand to organic substrates such as OCPh(2) and Cl(2)PMes* as well as inorganic fragments such as Cp(2)ZrCl(2), Cp*(2)TiCl(2), and Cp(2)TiCl(2) in the presence of P(CH(3))(3). Complexes 2-4 have been fully characterized, including single crystal X-ray diffraction and DFT studies.

Intermolecular C-H bond activation of benzene and pyridines by a vanadium(III) alkylidene including a stepwise conversion of benzene to a vanadium-benzyne complex†

Breaking of the carbon–hydrogen bond of benzene and pyridine is observed with (PNP)V(CH2tBu)2 (1), and in the case of benzene, the formation of an intermediate benzyne complex (C) is proposed, and indirect proof of its intermediacy is provided by identification of (PNP)VO(η2-C6H4) in combination with DFT calculations.