Sustainable Synthesis of Silicon Precursors Coupled with Hydrogen Delivery Based on Circular Economy via Molecular Cobalt-Based Catalysts

The development of a circular economy is a key target to reduce our dependence on fossil fuels and create more sustainable processes. Concerning hydrogen as an energy vector, the use of liquid organic hydrogen carriers is a promising strategy, but most of them present limitations for hydrogen release, such as harsh reaction conditions, poor recyclability, and low-value byproducts. Herein, we present a novel sustainable methodology to produce value-added silicon precursors and concomitant hydrogen via dehydrogenative coupling by using an air- and water-stable cobalt-based catalyst synthesized from cheap and commercially available starting materials. This methodology is applied to the one-pot synthesis of a wide range of alkoxy-substituted silanes using different hydrosilanes and terminal alkenes as reactants in alcohols as green solvents under mild reaction conditions (room temperature and 0.1 mol % cobalt loading). We also demonstrate that the selectivity toward hydrosilylation/hydroalkoxysilylation can be fully controlled by varying the alcohol/water ratio. This implies the development of a circular approach for hydrosilylation/hydroalkoxysilylation reactions, which is unprecedented in this research field up to date. Kinetic and in situ spectroscopic studies (electron paramagnetic resonance, nuclear magnetic resonance, and electrospray ionization mass spectrometry), together with density functional theory simulations, further provide a detailed mechanistic picture of the dehydrogenative coupling and subsequent hydrosilylation. Finally, we illustrate the application of our catalytic system in the synthesis of an industrially relevant polymer precursor coupled with the production of green hydrogen on demand

[Ln(BH₄)₂(THF)₂] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies

The divalent lanthanide borohydrides [Ln­(BH₄)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(BD₄)₂(<i>d</i>₈-THF)₂], which showed a quantum yield of 93%. The monocationic species [Ln­(BH₄)­(THF)₅]­[BPh₄] and the bis­(phosphinimino)­methanides [{(Me₃SiNPPh₂)₂CH}­Ln­(BH₄)­(THF)₂] have been prepared from [Ln­(BH₄)₂(THF)₂]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me₃SiNPPh₂)₂CH}­Yb­(BH₄)­(THF)₂], we performed a 2D ³¹P/¹⁷¹Yb HMQC experiment

[Ln(BH₄)₂(THF)₂] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies

The divalent lanthanide borohydrides [Ln­(BH₄)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(BD₄)₂(<i>d</i>₈-THF)₂], which showed a quantum yield of 93%. The monocationic species [Ln­(BH₄)­(THF)₅]­[BPh₄] and the bis­(phosphinimino)­methanides [{(Me₃SiNPPh₂)₂CH}­Ln­(BH₄)­(THF)₂] have been prepared from [Ln­(BH₄)₂(THF)₂]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me₃SiNPPh₂)₂CH}­Yb­(BH₄)­(THF)₂], we performed a 2D ³¹P/¹⁷¹Yb HMQC experiment

[Ln(BH₄)₂(THF)₂] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies

The divalent lanthanide borohydrides [Ln­(BH₄)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(BD₄)₂(<i>d</i>₈-THF)₂], which showed a quantum yield of 93%. The monocationic species [Ln­(BH₄)­(THF)₅]­[BPh₄] and the bis­(phosphinimino)­methanides [{(Me₃SiNPPh₂)₂CH}­Ln­(BH₄)­(THF)₂] have been prepared from [Ln­(BH₄)₂(THF)₂]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me₃SiNPPh₂)₂CH}­Yb­(BH₄)­(THF)₂], we performed a 2D ³¹P/¹⁷¹Yb HMQC experiment

[Ln(BH₄)₂(THF)₂] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies

The divalent lanthanide borohydrides [Ln­(BH₄)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(BD₄)₂(<i>d</i>₈-THF)₂], which showed a quantum yield of 93%. The monocationic species [Ln­(BH₄)­(THF)₅]­[BPh₄] and the bis­(phosphinimino)­methanides [{(Me₃SiNPPh₂)₂CH}­Ln­(BH₄)­(THF)₂] have been prepared from [Ln­(BH₄)₂(THF)₂]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me₃SiNPPh₂)₂CH}­Yb­(BH₄)­(THF)₂], we performed a 2D ³¹P/¹⁷¹Yb HMQC experiment

[Ln(BH₄)₂(THF)₂] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies

The divalent lanthanide borohydrides [Ln­(BH₄)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(BD₄)₂(<i>d</i>₈-THF)₂], which showed a quantum yield of 93%. The monocationic species [Ln­(BH₄)­(THF)₅]­[BPh₄] and the bis­(phosphinimino)­methanides [{(Me₃SiNPPh₂)₂CH}­Ln­(BH₄)­(THF)₂] have been prepared from [Ln­(BH₄)₂(THF)₂]. They show significantly lower or no luminescence. Using the diamagnetic compound [{(Me₃SiNPPh₂)₂CH}­Yb­(BH₄)­(THF)₂], we performed a 2D ³¹P/¹⁷¹Yb HMQC experiment

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

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

Chiral Rare Earth Borohydride Complexes Supported by Amidinate Ligands: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of <i>rac</i>-Lactide

The monoamidinato bisborohydride rare earth complexes [Ln­{(<i>S</i>)-PEBA}­(BH₄)₂(THF)₂] (Ln = Sc (<b>1</b>), La (<b>2</b>), Nd (<b>3</b>), Sm (<b>4</b>), Yb (<b>5</b>), Lu (<b>6</b>)) were isolated as crystalline materials upon treatment of potassium <i>N</i>,<i>N</i>′-bis­((<i>S</i>)-1-phenylethyl)­benzamidinate ((<i>S</i>)-KPEBA) with the homoleptic trisborohydrides [Sc­(BH₄)₃(THF)₂] and [Ln­(BH₄)₃(THF)₃] (Ln = La, Nd, Sm, Yb, Lu), respectively. Compounds <b>1</b>–<b>6</b> are unique examples of enantiopure borohydride complexes of the rare earth metals. Different ionic radii of the metal centers were selected to cover the whole range of these elements with respect to the extent of the coordination sphere. All new complexes were thoroughly characterized by ¹H, ¹³C­{¹H}, ¹¹B, and ¹⁵N NMR and IR spectroscopies, also including single-crystal X-ray diffraction structure determination of each compound. The scandium, lanthanum, samarium, and lutetium complexes <b>1</b>, <b>2</b>, <b>4</b>, and <b>6</b> were found active in the ring-opening polymerization of <i>rac</i>-lactide under mild operating conditions, providing atactic α,ω-dihydroxytelechelic poly­(lactic acid) (PLA; <i>M</i>_n,SEC up to 18 800 g·mol^–1). Most of the polymerizations proceed with a certain degree of control that is directed by molar mass values and relatively narrow dispersities (1.10 < <i>Đ</i>_M < 1.34), within a moderate monomer-to-initiator ratio

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

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

Mechanistic Investigations of Water Oxidation by a Molecular Cobalt Oxide Analogue: Evidence for a Highly Oxidized Intermediate and Exclusive Terminal Oxo Participation

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