Andreas Berkefeld

Tandem Frustrated Lewis Pair/Tris(pentafluorophenyl)borane-Catalyzed Deoxygenative Hydrosilylation of Carbon Dioxide

The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C(6)F(5))(3)] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH](+)[HB(C(6)F(5))(3)](-), which recently was shown to react with CO(2) via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH](+)[HC(O)OB(C(6)F(5))(3)](-). In the presence of extra B(C(6)F(5))(3) (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH](+)[HB(C(6)F(5))(3)](-). The formatosilane in turn is rapidly hydrosilated by the B(C(6)F(5))(3)/Et(3)SiH system to CH(4), with (Et(3)Si)(2)O as the byproduct. At low [Et(3)SiH], intermediate CO(2) reduction products are observed; addition of more CO(2)/Et(3)SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO(2) to CH(4).

Deactivation Pathways of Neutral Ni(II) Polymerization Catalysts

The novel dimethyl sulfoxide (DMSO)-coordinated complex [(N,O)Ni(CH(3))(DMSO)] {1-DMSO; (N,O) = kappa(2)-N,O-(2,6-(3,5-(F(3)C)(2)C(6)H(3))(2)C(6)H(3))-N=CH-(3,5-I(2)-2-OC(6)H(2))} was found to be a well-defined, very reactive precursor that enables direct observation of the activation and deactivation of neutral Ni(II) catalysts. Preparative reaction with ethylene afforded the ethyl complex [(N,O)Ni((alpha)CH(2)(beta)CH(3))(DMSO)] (2-DMSO). 2-DMSO is subject to interconversion of the (alpha)C and (beta)C moieties via an intermediate [(N,O)Ni(II)H(ethylene)] complex (this process is slow on the NMR time scale). Exposure of 1-DMSO to ethylene in DMSO solution at 55 degrees C results in partial reaction to form propylene (pseudo-first-order rate constant k(ins,Me) = 6.8 +/- 0.3 x 10(-4) s(-1) at an ethylene concentration of 0.15 M) and conversion to 2-DMSO, which catalyzes the conversion of ethylene to butenes. A relevant decomposition route of the catalyst precursor is the bimolecular elimination of ethane [DeltaH(double dagger) = (57 +/- 1) kJ mol(-1) and DeltaS(double dagger) = -(129 +/- 2) J mol(-1) K(-1) over the temperature range 55-80 degrees C]. This reaction is specific to the Ni(II)-Me complex; corresponding homocoupling of the higher Ni(II)-alkyls of the propagating species in catalytic C-C linkage of ethylene was not observed, but Ni(II)-Me reacted with Ni(II)-Et to form propane, as concluded from studies with 2-DMSO and its analogue that is perdeuterated in the Ni(II)-Et moiety. Under the reaction conditions of the aforementioned catalytic C-C linkage of ethylene, additional ethane evolves from the reaction of intermediate Ni(II)-Et with Ni(II)-H. This is independently supported by reaction of 2-DMSO with the separately prepared hydride complex [(N,O)NiH(PMe(3))] (3-PMe(3)) to afford ethane. Kinetic studies show this reaction to be bimolecular [DeltaH(double dagger) = (47 +/- 6) kJ mol(-1) and DeltaS(double dagger) = -(117 +/- 15) J mol(-1) K(-1) over the temperature range 6-35 degrees C]. In contrast to these reactions identified as decomposition routes, hydrolysis of Ni(II)-alkyls by added water (D(2)O; H(2)O) occurred only to a minor extent for the Ni(II)-Me catalyst precursor, and no clear evidence of hydrolysis was observed for higher Ni(II)-alkyls. The rate of the aforementioned insertion of ethylene in 1-DMSO and the rate of catalytic ethylene dimerization are not affected by the presence of water, indicating that water also does not compete significantly with the substrate for binding sites.

Carbon Monoxide Activation via O-Bound CO Using Decamethylscandocinium–Hydridoborate Ion Pairs

Ion pairs [Cp*(2)Sc](+)[HB(p-C(6)F(4)R)(3)](-) (R = F, 1-F; R = H, 1-H) were prepared and shown to be unreactive toward D(2) and α-olefins, leading to the conclusion that no back-transfer of hydride from boron to scandium occurs. Nevertheless, reaction with CO is observed to yield two products, both ion pairs of the [Cp*(2)Sc](+) cation with formylborate (2-R) and borataepoxide (3-R) counteranions. DFT calculations show that these products arise from the carbonyl adduct of the [Cp*(2)Sc](+) in which the CO is bonded to scandium through the oxygen atom, not the carbon atom. The formylborate 2-R is formed in a two-step process initiated by an abstraction of the hydride by the carbon end of an O-bound CO, which forms an η(2)-formyl intermediate that adds, in a second step, the borane at the carbon. The borataepoxide 3-R is suggested to result from an isomerization of 2-R. This unprecedented reaction represents a new way to activate CO via a reaction channel emanating from the ephemeral isocarbonyl isomer of the CO adduct.

Redox and Acid–Base Properties of Binuclear 4‐Terphenyldithiophenolate Complexes of Nickel

This work reports on the redox and acid-base properties of binuclear complexes of nickel from 1,4-terphenyldithiophenol ligands. The results provide insight into the cooperative electronic interaction between a dinickel core and its ligand. Donor/acceptor contributions flexibly adjust to stabilize different redox states at the metals, which is relevant for redox reactions like proton reduction. Proton transfer to the [S2 Ni2 ] core and Ni-H bond formation are kinetically favored over the thermodynamically favored yet unproductive proton transfer to ligand.

Controlling Near-Infrared Chromophore Electronic Properties through Metal–Ligand Orbital Alignment

Transition-metal complexes of radical ligands can exhibit low-energy electronic transitions in the near-infrared (NIR) spectral region. NIR band energy and intensity sensitively depend on the degree of electronic coupling of the chromophore. Using the example of open-shell complexes derived from platinum and a 1,4-terphenyldithiophenol, we present a novel approach toward spectroscopically distinct NIR dyes for which the degree of electronic coupling correlates with the relative orientation of radical ligand and metal orbitals. Ligand/metal orbital alignment is modulated by auxiliary phosphine donors and selectively results in electron localized Class II-III or delocalized Class III structures that display distinct NIR transitions at 6500 and 4000 cm-1.

Mechanistic Aspects of Redox-Induced Assembly and Disassembly of S-Bridged [2M-2S] Structures

Sulfur-bridged binuclear structures [2M-2S] play a pivotal role in a variety of chemical processes such as bond breaking and formation and electron transfer. In general, structural persistence is deemed essential to the respective function but owing to the lack of a suitable molecular model system, the current understanding of the factors that control the thermodynamic and kinetic stability of [2M-2S] cores clearly is limited. This work reports a series of binuclear complexes of nickel derived from a 1,4-terphenyldithiophenol ligand platform that is ideally suited for mechanistic work to overcome this limitation. Redox-induced assembly and disassembly of S-bridged [2M-2S] fragments have been investigated at the molecular level. As part of an extended square scheme, metastable binuclear structures that are significant mechanistically have been identified, characterized, and their reactivity studied quantitatively. Electronic properties that are inherent to [2M-2S] structures and determine thermodynamic and kinetic stability are differentiated from steric effects imposed by co-ligands.

Tuning of Thiyl/Thiolate Complex Near-Infrared Chromophores of Platinum through Geometrical Constraints

The chemistry of radical-ligand complexes of the transition metals has developed into a vibrant field of research that spans from fundamental studies on the relationship between the chemical and electronic structures to applications in catalysis and functional materials chemistry. In general, fine-tuning of the relevant properties relies on an increasingly diversifying pool of radical-proligand structures. Surprisingly, the variability of the conformational freedom and the number of distinct bonding modes supported by many radical proligands is limited. This work reports on the angular constraints and relative geometric alignment of metal and ligand orbitals as key parameters that render a series of chemically similar thiyl/thiolate complexes of platinum(II) electronically and spectroscopically distinct. The use of conformational flexible thiophenols as primary ligand scaffolds is essential to establishing a defined radical-ligand [(areneS)2PtII]•+ core whose electronic structure is modulated by a series of auxiliary coligands at platinum.