Acyclic boron-containing π-ligand complexes: η2- and η3-coordination modes

Cyclic boron-containing π-ligands such as boratabenzenes and borollides are well established, in particular as supporting ligands. By contrast, the chemistry of acyclic boron-containing π-ligands has remained relatively unexplored, presumably in part due to the higher reactivity of acyclic π-ligands relative to cyclic analogues. This perspective is focused on the synthesis, structures and reactivity of isolated transition metal complexes bearing η(n)-coordinated (n = 2 or 3) acyclic boron-containing ligands. Both monometallic and multimetallic compounds are included, and are discussed with an emphasis on metal-ligand and intraligand bonding and parallels with hydrocarbon π-ligand complexes.     

Single-step DNA immobilization on antifouling self-assembled monolayers covalently bound to silicon (111)

Hydrosilylation of alkenes with epoxide-terminated tri(ethylene oxide) moieties on Si-H surfaces yields homogeneous monolayers for the efficient coupling of biomolecules. The wetting properties of the epoxide-functionalized surface allow for the spotting of solutions of biomolecules, making the surface amenable to microarraying. Immobilization of thiolated DNA was achieved in a single step to fabricate biorecognition interfaces showing the hybridization of complementary DNA at low concentrations and negligible binding of noncomplementary DNA.     

Synthesis, structure, and unusual reactivity of beta-halovinyl cobalt porphyrin complexes

The preparation, structures, and reactivity of tetraphenylporphyrin (TPP) cobalt halovinyl complexes are reported. Beta-halovinyl complexes of (TPP)Co(E-CHCHX) (X = Br and I) were prepared from the insertion of acetylene into the cobalt halide bonds of the corresponding halide complexes. The reactivity of these compounds and of the previously reported (TPP)Co(E-CHCHCl), was studied in depth, and it was found that complex reactivity increased with the leaving group ability of the halide. A trans-dichlorovinyl cobalt porphyrin complex, (TPP)Co(Z-CClCHCl), was also prepared through the reaction of (TPP)CoNa and TCE. The structures of (TPP)Co(E-CHCHBr), (TPP)Co(Z-CClCHCl), and (TPP)Co(C(2)H) are reported. The C-C bond length of the vinyl group was found to vary for the beta-halovinyl complexes (TPP)Co(E-CHCHX) from 1.211 A for X = Br to 1.234 A for X = Cl and 1.320 A for (TPP)Co(Z-CClCHCl). A comparison of these structures to many chlorovinyl cobalt complexes shows that trans-2-halo substitution results in a dramatically decreased vinyl C-C bond length. The mechanism of halide substitution for the beta-halovinyl complexes was investigated with kinetic experiments that indicated a dissociative mechanism and supported the intermediacy of a cobalt acetylene complex.     

Evidence for the formation of a cis-dichlorovinyl anion upon reduction of cis-1,2-dichlorovinyl(pyridine)cobaloxime

The reduction of cis-1,2-dichlorovinyl(pyridine)cobaloxime, a model complex for the organometallic intermediate proposed in the dechlorination of trichloroethylene by cobalamin, was studied. Two mechanisms were considered for the Co-C bond cleavage following reduction. In the first, the Co-C bond cleaves to produce Co(I) and a chlorovinyl radical, while the second pathway results in the formation of Co(II) and a chlorovinyl anion. Four reducing agents, cobaltocene, decamethylcobaltocene, cob(I)alamin, and chromium(II), were used in the presence of H atom and proton donor species to identify the presence of chlorovinyl radical or chlorovinyl anion intermediates. Mechanistic conclusions were based on comparisons of the final product ratios of cis-dichloroethylene (cDCE) and chloroacetylene, which were found to have a direct relationship to the amount of proton donor available, with increased proton donor leading to increased cDCE production. The results support the intermediacy of a cis-1,2-dichlorovinyl anion.     

Singlet oxygen production in the reaction of superoxide with organic peroxides

[reaction: see text] A selective chemiluminescent probe for singlet oxygen has been employed to detect and quantify singlet oxygen in the reactions of superoxide with organic peroxides. The production of singlet oxygen has been quantified in the reaction of superoxide with benzoyl peroxide (BP). No singlet oxygen was detected in the reactions of superoxide with cumyl peroxide, tert-butyl peroxide, or tert-butyl hydroperoxide. On the basis of these results and on the temperature dependence of the reaction, we proposed a mechanism for singlet oxygen formation in the reaction of superoxide with BP.     

Structure of SrTiO3 films on Si

The epitaxial deposition of oxides on silicon opens the possibility of incorporating their diverse properties into silicon-device technology. Deposition of SrTiO(3) on silicon was first reported over a decade ago, but growing the coherent, lattice-matched films that are critical for many applications has been difficult for thicknesses beyond 5 unit cells. Using a combination of density functional calculations and x-ray diffraction measurements, we determine the atomic structure of coherent SrTiO(3) films on silicon, finding that the Sr concentration at the interface varies with the film thickness. The structures with the lowest computed energies best match the x-ray diffraction. During growth, Sr diffuses from the interface to the surface of the film; the increasing difficulty of Sr diffusion with film thickness may cause the disorder seen in thicker films. The identification of this unique thickness-dependent interfacial structure opens the possibility of modifying the interface to improve the thickness and quality of metal oxide films on silicon.