Relevance of cis- and trans-dichloride Ru intermediates in Grubbs-II olefin metathesis catalysis (H(2)IMesCl(2)Ru=CHR)

Using density functional theory with the B3LYP and M06 functionals, we show conclusively that the (H2IMes)(Cl)2Ru olefin metathesis mechanism is bottom-bound with the chlorides remaining trans throughout the reaction, thus attempts to effect diastereo- and enantioselectivity should focus on manipulations that maintain the trans-dichloro Ru geometry

Conformational analysis of olefin-carbene ruthenium metathesis catalysts

We settle a long-standing disagreement of DFT with experiment (both solution and gas phase) for the phosphine dissociation process in Grubbs metathesis catalysis. Our findings with the M06 functional provide further support to gas-phase experimental work, concluding that for the ring-closing metathesis of norbornene, the resting state is the alkylidene−olefin complex and the rate-determining step is the loss of norbornene as a ligand and generation of the 14-electron activated species. Comparing to recent solution NMR data on olefin−carbene Ru complexes relevant to olefin metathesis, we find that the M06 density functional leads to accurate predictions for the stability of conformers, ~0.5 kcal/mol better than is found by B3LYP. Using this methodology, we suggest that Piers and co-workers observed the cis-dichloro “down” isomer exclusively following the ring opening of acenaphthalene

Mechanically bonded macromolecules

Mechanically bonded macromolecules constitute a class of challenging synthetic targets in polymer science. The controllable intramolecular motions of mechanical bonds, in combination with the processability and useful physical and mechanical properties of macromolecules, ultimately ensure their potential for applications in materials science, nanotechnology and medicine. This tutorial review describes the syntheses and properties of a library of diverse mechanically bonded macromolecules, which covers (i) main-chain, side-chain, bridged, and pendant oligo/polycatenanes, (ii) main-chain oligo/polyrotaxanes, (iii) poly[c2]daisy chains, and finally (iv) mechanically interlocked dendrimers. A variety of highly efficient synthetic protocols—including template-directed assembly, step-growth polymerisation, quantitative conjugation, etc.—were employed in the construction of these mechanically interlocked architectures. Some of these structures, i.e., side-chain polycatenanes and poly[c2]daisy chains, undergo controllable molecular switching in a manner similar to their small molecular counterparts. The challenges posed by the syntheses of polycatenanes and polyrotaxanes with high molecular weights are contemplated

An Unusual Hydrogen Migration/C−H Activation Reaction with Group 3 Metals

A novel hydrogen migration from the phenyl ring to the pyridine ring of an yttrium pyridyl complex supported by a 1,1′-ferrocene diamide ligand is reported. Density functional theory calculations were instrumental in probing the mechanism for this transformation

Mechanism of C−F Reductive Elimination from Palladium(IV) Fluorides

The first systematic mechanism study of C−F reductive elimination from a transition metal complex is described. C−F bond formation from three different Pd(IV) fluoride complexes was mechanistically evaluated. The experimental data suggest that reductive elimination occurs from cationic Pd(IV) fluoride complexes via a dissociative mechanism. The ancillary pyridyl-sulfonamide ligand plays a crucial role for C−F reductive elimination, likely due to a κ^3 coordination mode, in which an oxygen atom of the sulfonyl group coordinates to Pd. The pyridyl-sulfonamide can support Pd(IV) and has the appropriate geometry and electronic structure to induce reductive elimination

Two Metals Are Better Than One in the Gold Catalyzed Oxidative Heteroarylation of Alkenes

We present a detailed study of the mechanism for oxidative heteroarylation, based on DFT calculations and experimental observations. We propose binuclear Au(II)–Au(II) complexes to be key intermediates in the mechanism for gold catalyzed oxidative heteroarylation. The reaction is thought to proceed via a gold redox cycle involving initial oxidation of Au(I) to binuclear Au(II)–Au(II) complexes by Selectfluor, followed by heteroauration and reductive elimination. While it is tempting to invoke a transmetalation/reductive elimination mechanism similar to that proposed for other transition metal complexes, experimental and DFT studies suggest that the key C–C bond forming reaction occurs via a bimolecular reductive elimination process (devoid of transmetalation). In addition, the stereochemistry of the elimination step was determined experimentally to proceed with complete retention. Ligand and halide effects played an important role in the development and optimization of the catalyst; our data provides an explanation for the ligand effects observed experimentally, useful for future catalyst development. Cyclic voltammetry data is presented that supports redox synergy of the Au···Au aurophilic interaction. The monometallic reductive elimination from mononuclear Au(III) complexes is also studied from which we can predict a ~ 15 kcal/mol advantage for bimetallic reductive elimination

Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization?

Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated. These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H^+) by the addition of trifluoroacetic acid. The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone C═N double bond, leading to isomerization. Treating Z-H^+ with base (K_(2)CO_3) yields a mixture of E and “metastable” Z isomers. The latter thermally equilibrates to reinstate the initial isomer ratio. The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state). However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (E_T) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism). These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways. The highly negative entropy values obtained for both the forward (E → Z-H^+) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion. Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone–azo tautomerization followed by rotation around a C–N single bond, as opposed to the more common rotation mechanism around the C═N double bond

A Molecular-Rotor Device for Nonvolatile High-Density Memory Applications

A novel memory device based on an electrically driven molecular rotor was fabricated and demonstrated to have bistable switching effects. The device showed an on/off ratio of approximately 10^4, a read window of about 2.5 V, and retention performance of greater than 10^4 s. The analysis of the device I–V characteristics suggests the source of the observed switching effects to be the redox-induced ligand rotation around the copper metal center, which is consistent with the observed temperature dependence of the switching behavior. This organic monolayer device holds a potential for nonvolatile high-density memory applications due to its scalability and reduced cost