Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

The hydrogenation of double bonds is one of the most fundamental transformations [1] in organic chemistry, and has numerous applications in the commodity chemical, agrochemical, pharmaceutical, polymer, and food industries. [2] Despite significant advances in the last 100 years, efforts to improve metal-based technologies for hydrogenation are still the focus of current research. [3] In parallel to these continuing efforts, metal-free strategies for effecting reductions have also been pursued. While organic reagents such as Hantschs esters [4] and silanes [5] have been used as stoichiometric reducing agents, it was not until 2006 [6] that the first metalfree systems, the so-called frustrated Lewis pairs (FLPs), [7] were shown to reversibly activate dihydrogen. This discovery allowed the development of FLP-based catalysts for the reduction of polar unsaturated bonds such as imines, [8] nitriles, [8a,c] aziridines, [8a,c] enamines, [8b] silylenolethers, [9] and aromatic reductions of anilines.[10] Herein, we report the discovery of FLP systems which, while appearing unreactive at room temperature, in fact are capable of dihydrogen activation at temperatures as low as À80 8C. This finding was then exploited for the catalytic hydrogenation of olefins at temperatures between 25 and 70 8C. Experimental and computational data support a plausible mechanism involving protonation of the olefin with subsequent hydride transfer.These FLPs represent the first metal-free hydrogenation catalysts for the reduction of olefins bearing carbocationstabilizing moieties.It is well known that the reactions of olefins with Brønsted acids in the presence of a nucleophilic halide, leads to addition products according to a protonation/addition mechanism. In considering the potential of such a mechanism for FLP hydrogenation of C=C double bonds, it was recognized that while the generated borohydride would act as the nucleophile, this pathway would require the generation of a countercation which was sufficiently acidic to effect protonation of the olefin. While the majority of FLP activations of dihydrogen have been demonstrated for phosphine/borane combinations, [7b] a variety of other donors including amines, [8a, 11] pyridines, [12] carbenes, [13] and phosphinimines [14] have been shown to be effective when paired with boron or aluminum Lewis acids. However, in all of these cases, the generated cations are only weak Brønsted acids and thus are incapable of protonation of olefinic double bonds.Seeking to enhance the Brønsted acidity of the cation generated by the FLP activation of dihydrogen, we initiated investigations employing (C 6 F 5 ) 3 B (1) in combination with the weakly basic phosphine (C 6 F 5 )Ph 2 P (2). An NMR spectroscopic examination of a 1:1 mixture of 1 and 2 at 25 8C resulted in spectra that did not differ from those of the individual components. Exposure of this FLP to hydrogen (5 bar) did not lead to significant changes in the NMR spectra at room temperature. However, the situation altered when the temperatu...

The divalent lanthanide borohydrides [Ln(BH(4))(2)(THF)(2)] (Ln = Eu, Yb) have been prepared in a straightforward approach. The europium compound shows blue luminescence in the solid state, having a quantum yield of 75%. Nonradiative deactivation of C-H and B-H oscillator groups could be excluded in the perdeuterated complex [Eu(BD(4))(2)(d(8)-THF)(2)], which showed a quantum yield of 93%. The monocationic species [Ln(BH(4))(THF)(5)][BPh(4)] and the bis(phosphinimino)methanides [{(Me(3)SiNPPh(2))(2)CH}Ln(BH(4))(THF)(2)] have been prepared from [Ln(BH(4))(2)(THF)(2)]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me(3)SiNPPh(2))(2)CH}Yb(BH(4))(THF)(2)], we performed a 2D (31)P/(171)Yb HMQC experiment.

The frustrated Lewis pair-mediated reversible hydrogen activation is studied as a function of the electrondonor quality of a series of phosphines. The increasing acidity of the generated phosphonium species leads to a stepwise lowering of the temperature for the highly reversible H 2 -activation and permits concrete classification for the first time. The influence of the acid strength on the metal-free hydrogenation of selected olefins is investigated by kinetic experiments and quantum chemical calculations. Detailed information for the rate-determining steps fully support our mechanistic model of a protonation step prior to hydride transfer. The rate of hydrogenation is strongly dependent on the electronic nature of the phosphine and of the acidity of the corresponding phosphonium cation. A careful balance of these two factors provides highly efficient metal-free hydrogenation catalysts. The provided findings are used to revise the reactivity of Lewis bases in the hydrogenation of imines, one of the most recognized applications of FLPs.

Asymmetric ortho-lithiation of N-dialkyl-P,P-diphenylphosphinamides using [n-BuLi.(-)-sparteine] is described as an efficient method for the synthesis of P-chiral ortho-functionalized derivatives in high yields and ee's from 45 to >99%. The method allows access to new enantiomerically pure P-chiral phosphine and diimine ligands.