Christian Herwig

A Dinuclear Nickel(I) Dinitrogen Complex and its Reduction in Single‐Electron Steps

Electron by electron: Beta-diketiminato nickel(I) complex fragments are capable of activating N(2) through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the N-N bond. The picture shows the structure of the singly reduced complex with mu-eta(1):eta(1)-bound N(2).

Gold– and Platinum–Bismuth Donor–Acceptor Interactions Supported by an Ambiphilic PBiP Pincer Ligand

Noble metals meet a heavyweight: A pincer ligand brings together bismuth with gold and platinum, so that metallophilic interactions are established. According to DFT calculations, these interactions contain dominant metal→bismuth contributions.

A Reduced β-Diketiminato-Ligated Ni3H4 Unit Catalyzing H/D Exchange

An investigation concerning the stepwise reduction of the β-diketiminato nickel(II) hydride dimer [LNi(μ-H)(2)NiL], 1 (L = [HC(CMeNC(6)H(3)(iPr)(2))(2)](-)), has been carried out. While the reaction with one equivalent of potassium graphite, KC(8), led to the mixed valent Ni(I)/Ni(II) complex K[LNi(μ-H)(2)NiL], 3, treatment of 1 with two equivalents of KC(8) surprisingly yielded in the trinuclear complex K(2)[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 4, in good yields. The Ni(3)H(4) core contains one Ni(II) and two Ni(I) centers, which are antiferromagnetically coupled so that a singlet ground state results. 4 represents the first structurally characterized molecular compound with three nickel atoms bridged by hydride ligands, and it shows a very interesting chemical behavior: Single-electron oxidation yields in the Ni(II)(2)Ni(I) compound K[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 5, and treatment with CO leads to the elimination of H(2) with formation of the carbonyl complex K(2)[LNi(CO)](2), 6. Beyond that, it could be shown that 4 undergoes H/D exchange with deuterated solvents and the deuteride-compound 4-D(4) reacts with H(2) to give back 4. The crystal structures of the novel compounds 3-6 have been determined, and their electronic structures have been investigated by EPR and NMR spectroscopy, magnetic measurements, and DFT calculations.

OO Bond Activation in Heterobimetallic Peroxides: Synthesis of the Peroxide [LNi(μ,η2:η2‐O2)K] and its Conversion into a Bis(μ‐Hydroxo) Nickel Zinc Complex

The activation of dioxygen for the subsequent oxygenation or oxidation of hydrocarbons is of importance not only in academia and industrial laboratories but also in nature. Some natural systems employ metalloenzymes containing two redox-active metal centers (e.g., tyrosinase or soluble methane monooxygenase), which cooperate in the activation process. On contact with O2, in a first step the oxidation state of each metal atom is increased by + I and a peroxide unit is formed, which may represent the active species or just an intermediate. In a second step, the O O bond can undergo cleavage with concomitant increase of the metal oxidation states by another unit so that M(m-O)2M cores result, which then perform the oxidation chemistry. Attempts to mimic this kind of reactivity by low-molecular-weight analogues have revealed that whether or not step two occurs can depend on subtle changes of properties of the ligands, counterions, and solvents. 2] Complexes with M(m-O)2M moieties (M = Cu, Fe, Ni, Co) usually exhibit electrophilic character and can abstract hydrogen atoms from ligand substituents or exogenous sources such as solvents or added sacrificial substrates. Heterobimetallic species have attracted great interest because the combination of metals with different individual characteristics may lead to synergistic effects arising from asymmetry. The cyctochrome c oxidase, in which a Cu center cooperates with a (heme)Fe unit for the activation of O2, serves as a natural example. The synthesis of mixed metal complexes LM(m-O)2M’L’, however, is far more difficult than the preparation of homobimetallic analogues. In the past, two strategies were pursued: 1) reaction of heterobimetallic M–M’ precursors with O2, and 2) a two-step process in which a well-defined metal dioxygen complex is treated with a reducing metal compound. These routes have allowed for the preparation of desired heterobimetallic oxo complexes with the metal combinations Cu–Ni, Cu–Pd, Cu–Pt, Cu–Ge, Pt–Mo, Pd–Ge, and Pt–Ge. It is well known that NiO2 complexes can serve as efficient oxidants in organic synthesis. Furthermore, NiO2 intermediates have been inferred as highly active species in several C H bond transformations. The consequences arising from the presence of a heterometal ion for O O bond activation could not be explored owing to the lack of suitable precursors. Recently, some of us reported on the synthesis of the first isolable nickel(II) superoxo complex [LNi(O2)] (1) (L = CH(CMeNR)2, R = 2,6-iPr2C6H3; Scheme 1), [8] which could serve as a suitable precursor for the synthesis of heterobimetallic Ni–M peroxo complexes in accordance with the synthetic strategy (2) mentioned above. Herein, we report the formation of the first heterobimetallic nickel(II) peroxo complex [LNi(m,h:h-O2)K] (2) and a metal-exchange reaction that leads to a heterobimetallic bis(m-hydroxo) complex [LNi(m-

Dinuclear copper complexes based on parallel β-diiminato binding sites and their reactions with O2: evidence for a Cu-O-Cu entity.

Investigations concerning the system β-diketiminato-Cu(I)/O(2) have revealed valuable insights that may be discussed in terms of the behavior of mononuclear oxygenases containing copper. On the other hand nature also employs dinuclear Cu enzymes for the activation of O(2). With this background the ligand system [(Me(2))(C(6)H(3))Xanthdim](2-) containing two parallel β-diiminato binding sites linked by a xanthene backbone with 2,3-dimethylphenyl residues at the diiminato units was investigated with respect to its copper coordination chemistry. The diimine [(Me(2))(C(6)H(3))Xanthdim]H(2) was treated with CuOtBu in the presence of acetonitrile, PPh(3), and PMe(3) to yield the corresponding complexes [(Me(2))(C(6)H(3))Xanthdim](Cu(L))(2) (L = CH(3)CN, 1, PPh(3), 2, and PMe(3), 3) that proved to be stable and were fully characterized. Single crystal X-ray diffraction analyses performed for the three complexes showed that considerable steric crowding within the binding pockets of 2 leads to a very long Cu-Cu distance while the structures of 1 and 3 are relaxed. Compounds 2 and 3 are relatively robust toward air, whereas 1 is very sensitive and quantitatively reacts with O(2) at room temperature (r.t.) within less than 2 min to give intractable compounds. At low temperatures the formation of a green intermediate was observed that was identified as a Cu(II)-O-Cu(II) species spectroscopically and chemically. This finding is of relevance also in the context of the results obtained testing 1 as a catalyst for phenol oxidation using O(2): 1 efficiently catalyzes phenol coupling, while there was no evidence for any oxygenation reactions occurring.

A High‐Valent Heterobimetallic [CuIII(μ‐O)2NiIII]2+ Core with Nucleophilic Oxo Groups

A heterobimetallic CuNi bis(μ-oxo) diamond core is shown to possess nucleophilic oxo groups, and has been demonstrated for the first time as a viable intermediate during the deformylation of fatty aldehydes by cyanobacterial aldehyde decarbonylase.

Analysis of semiconductor cluster beam polarization taking small permanent dipole moments into account

Abstract Deflections of semiconductor cluster beams in an inhomogeneous electric field deliver enhanced polarizabilities for cold isolated GaNAsM clusters with odd number of atoms n=N+M up to n=17. One speculated that the high polarizabilities result from their electronic open shell structure. This was qualitatively confirmed by optical absorption cross section measurements, but the enhancement of polarizability is too large to be explained through a Kramers–Kronig-relation as purely electronic effect. Moreover, quantum chemical calculations gave permanent dipole moments of some tenths of a Debye for certain compositions. The clusters are too heavy to identify these dipole moments by beam broadening effects, but we will show in the present analytic investigation within classical top theory, that a slight reversible-adiabatic alignment of the clusters dipole moment in the electric field increases the average beam deflection and the derived polarizability values. This effect was up to now not taken into account and it offers a consistent explanation of the GaNAsM polarizabilities by small permanent dipole moments of some tenths of a Debye and a reasonable rotational temperature between 2 and 12K.

The Conversion of Nickel‐Bound CO into an Acetyl Thioester: Organometallic Chemistry Relevant to the Acetyl Coenzyme A Synthase Active Site

When three become one: Within one nickel-based model system, the three reactants CO, MeI, and PhSH have been assembled to yield an acetyl thioester. The reactivity is of relevance for the functioning of the acetyl coenzyme A synthase active site and provides insights into possible binding sequences.

Monooxygenase‐Like Reactivity of an Unprecedented Heterobimetallic {FeO2Ni} Moiety

The soluble methane monooxygenase, sMMO, catalyzes the remarkable oxidation of methane to methanol under ambient conditions, as well as the oxygenation of a large variety of other substrates, such as ethers and alkanes and also aromatic, heterocyclic, and chlorinated compounds. The activation of dioxygen occurs at a diiron moiety within the hydroxylase subunit, for which currently a three-stage model is postulated: 1) the initial interaction of dioxygen with the diiron(II) center leading to MMOHperoxo, [3–5] 2) O O bond cleavage under formation of MMOHQ, which contains a high-valent dioxodiiron(IV) unit, 6, 7] and finally 3) the monooxygenation of the substrate by MMOHQ, either in a concerted manner or by a hydrogen-abstraction/oxygen-rebound mechanism (Scheme 1). Two-electron reduction of the resulting MMOHox by NADH then reinitiates the catalytic cycle. [9]

CO oxidation at nickel centres by N2O or O2 to yield a novel hexanuclear carbonate

Reaction of a nickel(0) carbonyl complex, K(2)[L(tBu)NiCO](2), with N(2)O generates a cyclic carbonate compound composed of six [Ni(II)(CO(3))K](+) units. The same product can also be obtained using O(2) as the oxidant in a solid-state/gas reaction. These conversions represent unique examples of a nickel-bound CO oxidation by N(2)O and O(2), respectively.