Andrew G. Tennyson

Arrested Catalysis: Controlling Kumada Coupling Activity via a Redox-Active N-Heterocyclic Carbene

Optimized syntheses for 1,3-dimesitylnaphthoquinimidazolium chloride [1H][Cl] and the corresponding silver-NHC complex [AgCl(1)] (2) were developed, enabling access to this versatile reagent in near-quantitative yield. Transmetalation from 2 to [NiCl(2)(PPh(3))(2)], trans-[PdCl(2)(PhCN)(2)], or trans-[PtCl(2)(PhCN)(2)] afforded the Group 10 complexes trans-[MCl(2)(1)(2)] (3a-c, M = Ni, Pd, and Pt, respectively) in excellent overall yield (>95%) after three steps from commercially available starting materials. Electrochemical measurements indicated that the E(1/2) and DeltaE(1/2) values for the quinone reduction couples were independent of the identity of the bridging transition metal in these complexes. Whereas attempts to isolate the reduced complexes were unsuccessful, UV/vis spectroelectrochemical analysis confirmed that electrochemical reduction of 3a-c in situ afforded optical difference spectra consistent with the formation of the expected reduced species. Complex 3a was found to catalyze the Kumada cross-coupling reaction between PhMgCl and a range of bromoarenes at room temperature. Addition of 2 equiv of cobaltocene (with respect to 3a) to the coupling reaction with bromotoluene caused a decrease in catalytic activity (from 4.7 x 10(-5) to 2.7 x 10(-6) s(-1)), which was attributed to the conversion of 3a to an arrested state. Subsequent introduction of ferrocenium tetrafluoroborate (2 equiv with respect to 3a) restored a significant degree of catalytic activity (k(obs) = 1.2 x 10(-5) s(-1)). Redox-switching experiments performed over different time scales revealed that the catalyst was stable in the reduced/inactive state and that extended durations in this state did not impede catalytic reactivation upon subsequent oxidation.

Bimetallic N-Heterocyclic Carbene−Iridium Complexes: Investigating Metal−Metal and Metal−Ligand Communication via Electrochemistry and Phosphorescence Spectroscopy

Bimetallic [Ir(COD)Cl] and [Ir(ppy)(2)] (COD = 1,5-cyclooctadiene; ppy = 2-phenylpyridyl) complexes bridged by 1,7-dimethyl-3,5-diphenylbenzobis(imidazolylidene) (1), in addition to their monometallic analogues supported by 1-methyl-3-phenylbenzimidazolylidene (2), were synthesized and studied. Electrochemical analyses indicated that 1 facilitated moderate electronic coupling between [Ir(COD)Cl] units (DeltaE = approximately 60 mV), but not [Ir(ppy)(2)]. The metal-based oxidation potentials for the bimetallic complexes were within 20 mV of those for their monometallic analogues. Furthermore, spectroscopic analyses of the [Ir(ppy)(2)] bimetallic and monometallic complexes revealed nearly identical phosphorescence profiles, indicating that carbene coordination does not affect the energy of the emissive states. Collectively, these results suggest that N-heterocyclic carbenes (NHCs) such as 1 could link together two emissive fragments without altering their fundamental phosphorescence profiles. Ultimately, employing multitopic NHCs as non-interfering molecular connectors could facilitate the rational design of new phosphorescent materials as well as second-generation phosphor dopants.

Mechanical activation of catalysts for C-C bond forming and anionic polymerization reactions from a single macromolecular reagent.

Coupling of pyridine-capped poly(methyl acrylate)s, PyP(M) (where M corresponds to the number average molecular weight in kDa), to the SCS-cyclometalated dipalladium complex [(1)(CH(3)CN)(2)] afforded organometallic polymers [(1)(PyP(M))(2)] with a concomitant doubling in molecular weight. Ultrasonication of solutions containing [(1)(PyP(M))(2)] effected the mechanical scission of a palladium-pyridine bond, where the liberated PyP(M) was trapped with excess HBF(4) as the corresponding pyridinium salt, harnessed to effect the stoichiometric deprotonation of a colorimetric indicator, or used to catalyze the anionic polymerization of α-trifluoromethyl-2,2,2-trifluoroethyl acrylate. The mechanically induced chain scission also unmasked a catalytically active palladium species which was used to facilitate carbon-carbon bond formation between benzyl cyanide and N-tosyl imines. Spectroscopic and macromolecular analyses as well as a series of control experiments demonstrated that the aforementioned structural changes were derived from mechanical forces that originated from ultrasound-induced dissociation of the polymer chains connected to the aforementioned Pd complexes.

Synthesis and Characterization of {Ni(NO)}10 and {Co(NO)2}10 Complexes Supported by Thiolate Ligands

Nitric oxide is an important molecule in biology and modulates a variety of physiological and pathophysiological processes. Some of its regulatory functions are exerted through interactions with redox-active elements, including iron, nickel, cobalt, and sulfur. Metalloenzymes containing [ nFe- nS] ( n = 2 or 4) clusters can be activated or inactivated by reaction with NO, affording dinitrosyl iron complexes. Studies of the NO chemistry of small-molecule iron thiolate complexes have provided insight into these biological processes and suggested probable intermediates. To explore this chemistry from a different perspective, we prepared nickel and cobalt thiolate complexes and investigated their reactions with NO and related compounds. We report here the first examples of anionic complexes containing {Ni(NO)} (10) and {Co(NO) 2} (10) units, the reactivity of which suggests possible intermediates in the interconversion of iron thiolate nitrosyl compounds. Our results demonstrate new chemistry involving NO and simple complexes of nickel and cobalt supported by thiolates, which have been known for more than 30 years. The use of mass balance methodology was key to their discovery. Among the novel complexes reported are (Et 4N) 2[Ni(NO)(SPh) 3] ( 2), from (Et 4N) 2[Ni(SPh) 4] ( 1) and NO, (Et 4N) 2[Ni 2(NO) 2(mu-SPh) 2(SPh) 2] ( 3), from 1 and NO (+) or 2 and Me 3O (+), (Et 4N)[Co(NO) 2(SPh) 2] ( 5), from (Et 4N) 2[Co(SPh) 4] ( 4) and NO, and [Co 3(NO) 6(mu-SPh) 3] ( 6), from 5 and Me 3O (+). In the syntheses of 2 and 5, NO could be replaced by the convenient solid Ph 3CSNO.

Redox-Active Ligands: An Advanced Tool To Modulate Polyethylene Microstructure.

The ability to control catalytic activity and selectivity via in situ changes in catalyst oxidation-state represents an intriguing tool for enhanced polymerization control. Herein, we report foundational evidence that catalysts bearing redox-active moieties may be used to synthesize high molecular weight polyethylene with tailored microstructure. The ability to modulate branching density and identity is facilitated by ligand-based redox chemistry, and is realized via the addition of chemical reductants into the polymerization reactor. Detailed GPC and NMR analyses demonstrate that branching density may be altered by up to ∼ 30% as a function of in situ added reductant.

Bipyridyl-modified phosphonium polyelectrolytes: synthesis, photophysics, metal ion coordination and layer-by-layer assembly with anionic conjugated polymers

A novel class of chromophoric polyelectrolytes bearing phosphonium moieties in their backbones has been prepared to incorporate metal-binding 2,2′-bipyridyl (bipy) units. The effect of metal ion binding on the photophysical properties of the polyelectrolyte was probed. By binding metal ions to the polycation, the relative charge loading could be progressively increased as a strategy to control its supramolecular assembly with polyanions. The metallated and metal-free polyelectrolytes were utilized in layer-by-layer self-assembly with poly(acrylic acid) or anionic conjugated polymers including a poly(p-phenylene vinylene) and polythiophene derivative. The photophysical properties, nanomorphology, and substrate dependence of the resultant multilayer films were examined, and indicate that layer density, composition, and morphology can be tuned by metallation and ion pairing effects.

Catalytic Radical Reduction in Aqueous Solution by a Ruthenium Hydride Intermediate.

Some manganese complexes can catalyze both antioxidant and pro-oxidant reactions, whereby the disparate reactivity modes are determined by the catalyst environment and afford distinct therapeutic effects. We recently reported the reduction of radicals in buffered aqueous solution catalyzed by a ruthenium complex with biologically relevant non-tertiary alcohols as terminal reductants. Mechanistic evidence is presented, indicating that this catalytic radical reduction is achieved by a Ru-hydride intermediate formed by β-hydride elimination from a Ru-alkoxide species. A similar mechanism and Ru-hydride intermediate was previously reported to kill cancer cells with catalytic pro-oxidant effects. Therefore, our demonstration of catalytic antioxidant effects by the same type of intermediate reveals new potential therapeutic strategies and applications for catalytic systems that form Ru-hydride intermediates.

Synthesis, photophysical and electrochemical properties of conjugated polymers incorporating 9,9-dialkyl-1,4-fluorenylene units with thiophene, carbazole and triarylamine comonomers

Three new poly(p-phenylenevinylene) derivatives were synthesized via the Horner–Wittig condensation polymerization. The new polymers (P3–P5) incorporate recently developed 9,9-dihexyl-1,4-fluorenylene repeat units copolymerized with three different functional groups that are of interest for optoelectronic applications of organic semiconducting polymers (triphenylamine (P3), thiophene (P4) and carbazole (P5). The electrochemical properties were probed via cyclic voltammetry and differential pulse voltammetry for new polymers P3–P5, as well as previously reported but not electrochemically characterized poly(p-phenylene vinylene) derivatives P1 and P2. The electronic absorption and photoluminescence properties of all five 1,4-fluorenylene-containing copolymers were characterized in solution and were compared to other poly(p-phenylenevinylene) copolymers with similar functional groups.

Conjugated polymers with m-pyridine linkages: synthesis, photophysics, solution structure and film morphology

The synthesis of two π-conjugated polymers whose structures include m-pyridyl linkages that create regularly spaced bends and disrupt π-conjugation along the polymer backbones is described. The π-conjugated segments in the two polymers were comprised of p-(2,5-dialkoxyphenylene)vinylene, p-phenylenevinylene and 2,7-carbazole subunits. Additionally, each m-pyridylene unit is enshrouded within a steric cleft provided by 2,6-diaryl substitution of the pyridyl ring, thus minimizing inter-chain interactions of these sites. The concentration-dependence of photophysical characteristics along with small angle neutron scattering (SANS) was employed to resolve solution aggregation properties and atomic force microscopy was employed to assess the surface morphology of polymer films.