Switchable electrical conductivity in a three-dimensional metal-organic framework via reversible ligand n-doping.

Redox-active metal-organic frameworks (MOFs) are promising materials for a number of next-generation technologies, and recent work has shown that redox manipulation can dramatically enhance electrical conductivity in MOFs. However, ligand-based strategies for controlling conductivity remain under-developed, particularly those that make use of reversible redox processes. Here we report the first use of ligand n-doping to engender electrical conductivity in a porous 3D MOF, leading to tunable conductivity values that span over six orders of magnitude. Moreover, this work represents the first example of redox switching leading to reversible conductivity changes in a 3D MOF.

Red Phosphorescence from Benzo[2,1,3]thiadiazoles at Room Temperature.

We describe the red phosphorescence exhibited by a class of structurally simple benzo[2,1,3]thiadiazoles at room temperature. The photophysical properties of these molecules in deoxygenated cyclohexane, including their absorption spectra, steady-state photoluminescence and excitation spectra, and phosphorescence lifetimes, are presented. Time-dependent density functional theory calculations were carried out to better understand the electronic excited states of these benzo[2,1,3]thiadiazoles and why they are capable of phosphorescence.

Single-walled carbon nanotube/metalloporphyrin composites for the chemiresistive detection of amines and meat spoilage.

Chemiresistive detectors for amine vapors were made from single-walled carbon nanotubes by noncovalent modification with cobalt meso-arylporphyrin complexes. We show that through changes in the oxidation state of the metal, the electron-withdrawing character of the porphyrinato ligand, and the counteranion, the magnitude of the chemiresistive response to ammonia could be improved. The devices exhibited sub-ppm sensitivity and high selectivity toward amines as well as good stability to air, moisture, and time. The application of these chemiresistors in the detection of various biogenic amines (i.e. putrescine, cadaverine) and in the monitoring of spoilage in raw meat and fish samples (chicken, pork, salmon, cod) over several days was also demonstrated.

Multiple, disparate redox pathways exhibited by a tris(pyrrolido)ethane iron complex.

Iron(III) complexes of the tris(pyrrolide)ethane trianion have been synthesized by reaction of one- and two-electron oxidants with [(tpe)Fe(THF)][Li(THF)4] (tpe = tris(5-mesitylpyrrolyl)ethane). X-ray crystallography, (57)Fe Mössbauer, (1)H NMR and EPR spectroscopy, SQUID magnetometry, and density functional theory calculations were employed to rigorously establish the iron 3+ oxidation state. All oxidants employed are proposed to operate via an inner-sphere electron transfer mechanism. Dialkyl peroxides and dibenzyldisulfide served to oxidize iron by one electron, and group transfer of an aryl nitrene unit to the Fe(2+) starting material resulted in formation of Fe(3+) amido species following H-atom abstraction by a presumed nitrenoid intermediate. Single electron transfer to and from diphenyldiazoalkane was also observed to yield a diphenyldiazomethanyl radical anion antiferromagnetically coupled to the S = 5/2 Fe(3+). Isolation of Fe(3+) complexes of tpe, in comparison with previous results wherein the tpe ligand was the redox active moiety, presents an unusual juxtaposition of two noncommunicating redox reservoirs, each accessible via different reaction pathways (namely, inner- and outer-sphere electron transfer).

Co(III) imidos exhibiting spin crossover and C-H bond activation.

The reaction of ((Ar)L)Co(py) with (t)BuN(3) afforded the isolable three-coordinate Co-imido complex ((Ar)L)Co(N(t)Bu), which is paramagnetic at room temperature. Variable-temperature (VT) (1)H NMR spectroscopy, VT crystallography, and magnetic susceptibility measurements revealed that ((Ar)L)Co(N(t)Bu) undergoes a thermally induced spin crossover from an S = 0 ground state to a quintet (S = 2) state. The reaction of ((Ar)L)Co(py) with mesityl azide yielded an isolable S = 1 terminal imido complex that was converted into the metallacycloindoline ((Ar)L)Co(κ(2)-NHC(6)H(2)-2,4-Me(2)-6-CH(2)) via benzylic C-H activation.

Ligand-centered redox activity: redox properties of 3d transition metal ions ligated by the weak-field tris(pyrrolyl)ethane trianion.

First-row transition metal complexes of the tris(pyrrolyl)ethane (tpe) trianion have been prepared. The tpe ligand was found to coordinate in a uniform eta(1),eta(1),eta(1)-coordination mode to the divalent metal series as revealed by X-ray diffraction studies. Magnetic and structural characterization for complexes of the type [(tpe)M(II)(py)][Li(THF)(4)] (M: Mn, Fe, Co, Ni) reveal each divalent ion to be high-spin and have a distorted trigonal-monopyramidal geometry in the solid state. The pyridine ligand binds significantly canted from the molecular C(3) axis due to a stabilizing pi-stacking interaction with a ligand mesityl substituent. Cyclic voltammetry on the [(tpe)M(II)(py)](-) series reveals a common irreversible oxidation pathway that is entirely ligand-based, invariant to the divalent metal bound. This latter observation indicates that fully populated ligand-based orbitals from the tpe construct are energetically most accessible in the electrochemical experiments, akin to their dipyrromethane analogues. Chemical oxidation of [(tpe)Fe(II)(py)](-) yields a product in which the ligand has dissociated one pyrrole (following tpe oxidation and H-atom abstraction) and binds a second equivalent of pyridine to form the neutral, tetrahedral Fe(II) species (kappa(2)-tpe)Fe(py)(2). Similarly, chemical oxidation of the Zn(II) analogue shows evidence for tpe oxidation by electron paramagnetic resonance spectroscopy (77 K, toluene glass) with an isotropic signal for the organic radical at g = 2.002. Density functional theory analysis on this family of complexes reveals that the highest lying molecular orbitals are completely ligand-based, corroborating our proposed electronic structure assignment.

Steric modulation of chiral biaryl diamines via Pd-catalyzed directed C-H arylation.

Palladium-catalyzed directed arylation of 2,2'-diacetamidobiaryls with aryl iodides provides efficient access to chiral ortho-substituted biaryl diamines. Aryl iodides with para- and meta-substituents are tolerated. Deprotection of the acetyl groups under basic conditions furnishes the free diamines, which should find broad utility in asymmetric catalysis.

Synthesis of Pd Complexes Bearing an Enantiomerically-Resolved Seven-Membered N-Heterocyclic Carbene Ligands and Initial Studies of their Use in Asymmetric Wacker-Type Oxidative Cyclization Reactions.

The development of enantiomerically-resolved, axially-chiral seven-membered N-heterocyclic carbene ((7)NHC) ligands for palladium is described. These (7)NHC ligands are derived from enatiomerically pure 2,2'-diamino-6,6'-dimethylbiphenyl, which is transformed via a synthetic sequence consisting of ortho-arylation, N-alkylation, and cyclization to afford seven-membered-ring amidinium salts. Synthesis of the 7-membered amidinium salts benefits from microwave irradiation, and in-situ metalation of the amidinium salts yields (7)NHC-Pd(II) complexes. The chiral (7)NHC-Pd complexes were examined as chiral catalysts under aerobic conditions in two intramolecular oxidative amination reactions of alkenes. In one case, enantioselectivities up to 63% e.e. were obtained, while the other substrate underwent cyclization to afford essentially racemic products. The catalytic data compare favorably to results obtained with a Pd(II) catalyst bearing a chiral five-membered-ring NHC ligand and, thereby, highlight the potential significance of this new class of chiral NHC ligands.