Christian Limberg

The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis

Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential‐determining step. Subsequently, novel cobalt oxide‐based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water‐oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non‐ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox‐potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

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).

Molecular CuII‐O‐CuII Complexes: Still Waters Run Deep

Research on O2 activation at ligated Cu(I) is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu(II) -O-Cu(II) species, which is more special than it appears at first sight: a single oxo ligand between two Cu(II) centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu(II) -O-Cu(II) cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu-grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.

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-

Hydride binding to the active site of [FeFe]-hydrogenase.

[FeFe]-hydrogenase from green algae (HydA1) is the most efficient hydrogen (H2) producing enzyme in nature and of prime interest for (bio)technology. Its active site is a unique six-iron center (H-cluster) composed of a cubane cluster, [4Fe4S]H, cysteine-linked to a diiron unit, [2Fe]H, which carries unusual carbon monoxide (CO) and cyanide ligands and a bridging azadithiolate group. We have probed the molecular and electronic configurations of the H-cluster in functional oxidized, reduced, and super-reduced or CO-inhibited HydA1 protein, in particular searching for intermediates with iron-hydride bonds. Site-selective X-ray absorption and emission spectroscopy were used to distinguish between low- and high-spin iron sites in the two subcomplexes of the H-cluster. The experimental methods and spectral simulations were calibrated using synthetic model complexes with ligand variations and bound hydride species. Distinct X-ray spectroscopic signatures of electronic excitation or decay transitions in [4Fe4S]H and [2Fe]H were obtained, which were quantitatively reproduced by density functional theory calculations, thereby leading to specific H-cluster model structures. We show that iron-hydride bonds are absent in the reduced state, whereas only in the super-reduced state, ligand rotation facilitates hydride binding presumably to the Fe-Fe bridging position at [2Fe]H. These results are in agreement with a catalytic cycle involving three main intermediates and at least two protonation and electron transfer steps prior to the H2 formation chemistry in [FeFe]-hydrogenases.

Low-Molecular-Weight Analogues of the Soluble Methane Monooxygenase (sMMO): From the Structural Mimicking of Resting States and Intermediates to Functional Models

The active centre of sMMO contains a diiron core ligated by histidine and glutamate residues, which is capable of catalysing a remarkable reaction: the oxidation of methane with O(2) yielding methanol. This review describes the results of efforts to prepare low-molecular-weight analogues of this active site directing towards 1) the assignment of the spectroscopic signatures identified for certain intermediates of the sMMO catalytic cycle to structural features and 2) the synthesis of molecular compounds that can mimic the reactivity. The historical development of the model chemistry, which is subdivided into structural and functional mimicking, is illustrated and achievements reached so far are highlighted.

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.

What Does it Really Take to Stabilize Complexes of Late Transition Metals with Terminal Oxo Ligands

Surprisingly stable: Noble-metal complexes with terminal oxo ligands are frequently postulated as intermediates, but they are generally considered elusive, as their d electrons destabilize the M=O units. Until recently, the isolation of such compounds was thought to require strong acceptor ligands, but now a remarkably stable Pt=O complex has been obtained employing a simple pincer ligand.