Carbon dioxide cleavage by a Ni_2 complex supported by a binucleating bis(N-heterocyclic carbene) framework

A binucleating bis(N-heterocyclic carbene) ligand was designed as a means to coordinate and proximally constrain two transition metal centers. Using an imidazopyridine-based NHC afforded a framework structurally related to previously reported para-terphenyl diphosphines. Bimetallic copper, cobalt, and nickel complexes supported by this framework were synthesized and structurally characterized. Strong interactions between the metal centers and the central arene were observed in all nickel complexes. Dinickel(0) complexes of this ligand framework were found to react with CO_2 to form a dicarbonyl-bridged dinickel(0) product, demonstrating facile CO_2 reduction

Synthetic Cluster Models of Biological and Heterogeneous Manganese Catalysts for O_2 Evolution

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O_2 remains a significant challenge in this context. A mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This Award Article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex of photosystem II. These structural motifs are also related to heterogeneous mixed-metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low-oxidation-state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure−reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pK_a value of the redox-inactive metal−aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction potential and acidity; each pK_a unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed

Reduction potentials of heterometallic manganese-oxido cubane complexes modulated by redox-inactive metals

Understanding the effect of redox-inactive metals on the properties of biological and heterogeneous water oxidation catalysts is important both fundamentally and for improvement of future catalyst designs. In this work, heterometallic manganese–oxido cubane clusters [MMn_3O_4] (M = Sr^(2+), Zn^(2+), Sc^(3+), Y^(3+)) structurally relevant to the oxygen-evolving complex (OEC) of photosystem II were prepared and characterized. The reduction potentials of these clusters and other related mixed metal manganese–tetraoxido complexes are correlated with the Lewis acidity of the apical redox-inactive metal in a manner similar to a related series of heterometallic manganese–dioxido clusters. The redox potentials of the [SrMn_3O_4] and [CaMn_3O_4] clusters are close, which is consistent with the observation that the OEC is functional only with one of these two metals. Considering our previous studies of [MMn_3O_2] moieties, the present results with more structurally accurate models of the OEC ([MMn_3O_4]) suggest a general relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidities of incorporated cations that applies to diverse structural motifs. These findings support proposals that one function of calcium in the OEC is to modulate the reduction potential of the cluster to allow electron transfer

Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn_4CaO_5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions

Trinucleating Copper: Synthesis and Magnetostructural Characterization of Complexes Supported by a Hexapyridyl 1,3,5-Triarylbenzene Ligand

Copper threesome: A hexapyridyl ligand based upon a 1,3,5-triphenylbenzene framework coordinates three metal centers in a constrained environment (see picture). The tricopper(I) complex reduces dioxygen to form a tricopper(II) cluster. The capping anions affect the magnetism and EPR spectra of these species and reveal a linear dependence between the antiferromagnetic exchange parameter and the Cu-O-Cu angles

A CaMn_4O_2 model of the biological oxygen evolving complex: synthesis via cluster expansion on a low symmetry ligand

Using a new multinucleating ligand featuring two dipyridyl alkoxide moieties and a carboxylate moiety, low symmetry tetranuclear complexes 1-M (M = Mn, Fe, and Co) have been synthesized. Complex 1-Mn was used as a precursor for the synthesis of a pentanuclear CaMn_4O_2 cluster (3) with the same metal stoichiometry as the biological OEC

Redox-inactive metals modulate the reduction potential in heterometallic manganese–oxido clusters

Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but, at present, their roles in catalysis are not well understood. Here, we report a series of high-oxidation-state tetranuclear-dioxido clusters comprising three manganese centres and a redox-inactive metal (M). Crystallographic studies show an unprecedented Mn_3M(µ_4-O)(µ_2-O) core that remains intact on changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV and are dependent on the Lewis acidity of the second metal. With the pK_a of the redox-inactive metal–aqua complex as a measure of Lewis acidity, these compounds demonstrate a linear dependence between reduction potential and acidity with a slope of ∼100 mV per pK_a unit. The Sr^(2+) and Ca^(2+) compounds show similar potentials, an observation that correlates with the behaviour of the oxygen-evolving complex of photosystem II, which is active only if one of these two metals is present

Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn_(3)MO_n Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

The oxygen-evolving complex (OEC) of photosystem II contains a Mn_(4)CaO_n catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn_(3)MO_n complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ_(3)-oxido moieties. We found that Mn^(IV)_(3)CaO_4 and Mn^(IV)_(3)ScO_4 were unreactive toward trimethylphosphine (PMe_3). In contrast, our Mn^(III)_(2)Mn^(IV)_(2)O_4 cubane reacts with this phosphine within minutes to generate a novel Mn^(III)_(4)O_3 partial cubane plus Me_(3)PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn^(III)_(2)Mn^(IV)_(2)O_4 and Mn^(IV)_(3)CaO_4. We found that the most favorable reaction path leads to partial detachment of the CH_(3)COO^– ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn^(III)_(2)Mn^(IV)_(2)O_4. These results indicate that even with a strong oxygen atom acceptor, such as PMe_3, the oxygen atom transfer chemistry from Mn_(3)MO_4 cubanes is controlled by ligand lability, with the Mn^(IV)_(3)CaO_4 OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, Mn^(III)_(4)O_3, was observed experimentally upon treatment with water, base, and oxidizing equivalents. ^(18)O-labeling experiments provided mechanistic insight into the position of incorporation in the partial cubane structure, consistent with mechanisms involving migration of oxide moieties within the cluster but not consistent with selective incorporation at the site available in the starting species. These results support recent proposals for the mechanism of the OEC, involving oxido migration between distinct positions within the cluster

Effect of the Mn Oxidation State on Single-Molecule-Magnet Properties: Mn^(III) vs Mn^(IV) in Biologically Inspired DyMn_3O_4 Cubanes

Inspired by the ferromagnetic coupling in the cubane model CaMn^(IV)_3O_4 of the oxygen-evolving complex of photosystem II, 3d–4f mixed-metal DyMn_3O_4 clusters were prepared for investigation of the magnetic properties. For comparison, YMn^(IV)_3O_4 and YMn^(IV)_2Mn^(III)O_4 clusters were investigated as well and showed ferromagnetic interactions, like the calcium analogue. DyMn^(IV)_3O_4 displays single-molecule-magnet properties, while the one-electron-reduced species (DyMn^(IV)_2Mn^(III)O_4) does not, despite the presence of a Mn^(III) center with higher spin and single-ion anisotropy