In this work, we report that organocatalyst 1-Bcat-2-PPh2-C6H4 ((1); cat = catechol) acts as an ambiphilic metal-free system for the reduction of carbon dioxide in presence of hydroboranes (HBR2 = HBcat (catecholborane), HBpin (pinacolborane), 9-BBN (9-borabicyclo[3.3.1]nonane), BH3·SMe2 and BH3·THF) to generate CH3OBR2 or (CH3OBO)3, products that can be readily hydrolyzed to methanol. The yields can be as high as 99% with exclusive formation of CH3OBR2 or (CH3OBO)3 with TON (turnover numbers) and TOF (turnover frequencies) reaching >2950 and 853 h(-1), respectively. Furthermore, the catalyst exhibits "living" behavior: once the first loading is consumed, it resumes its activity on adding another loading of reagents.
The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2-C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.
Transition metal complexes are efficient catalysts for the C-H bond functionalization of heteroarenes to generate useful products for the pharmaceutical and agricultural industries. However, the costly need to remove potentially toxic trace metals from the end products has prompted great interest in developing metal-free catalysts that can mimic metallic systems. We demonstrated that the borane (1-TMP-2-BH2-C6H4)2 (TMP, 2,2,6,6-tetramethylpiperidine) can activate the C-H bonds of heteroarenes and catalyze the borylation of furans, pyrroles, and electron-rich thiophenes. The selectivities complement those observed with most transition metal catalysts reported for this transformation.
The FLP species 1-BR2-2-NMe2-C6H4 (R = 2,4,6-Me3C6H2, 2,4,5-Me3C6H2) reacts with H2 in sequential hydrogen activation and protodeborylation reactions to give (1-BH2-2-NMe2-C6H4)2. While reacts with H2/CO2 to give formyl, acetal and methoxy-derivatives, reacts with H2/CO2 to give C6H4(NMe2)(B(2,4,5-Me3C6H2)O)2CH2. The mechanism of CO2 reduction is considered.
A B S T R A C T : N a n o c u b e s a n d n a n o s h e e t s o f [Cu 2 (ndc) 2 (dabco)] n metal−organic framework (ndc = 1,4-naphthalene dicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]-octane) were synthesized by using simultaneously acetic acid and pyridine or only pyridine, respectively, as selective modulators. This approach can tailor crystal growing on different directions for the size-and shape-controlled synthesis of metal−organic framework (MOF) nanocrystals whose structure is composed of two or more types of linkers using selective modulators. These MOF nanocrystals exhibit high crystallinity and higher CO 2 uptakes compared to that of the bulk MOF material or of the [Cu 2 (ndc) 2 (dabco)] n nanorods generated by using only acetic acid as the selective modulator, which may be due to the morphology effect on their gas sorption properties.
The ambiphilic species Al(C 6 H 4 (o-PPh 2 )) 3 (2) was synthesized and fully characterized, notably using X-ray diffraction. Species 2 exhibits pseudo-bipyramidal-trigonal geometry caused by the two Al−P interactions. 2 reacts with CO 2 to generate a CO 2 adduct commonly observed in the activation of CO 2 using frustrated Lewis pairs (FLPs). This ambiphilic species serves as a precatalyst for the reduction of CO 2 in the presence of catecholborane (HBcat) to generate CH 3 OBcat, which can be readily hydrolyzed in methanol. The reaction mixture confirms that, in the presence of HBcat, 2 generates the known CO 2 reduction catalyst 1-Bcat-2-PPh 2 -C 6 H 4 (1) and intractable catecholate aluminum species. It was, however, possible to isolate a single crystal of Al(κ 2 O,O-(MeO) 2 Bcat) 3 (5) supporting this hypothesis. Also, a borane-protected analogue of 2, Al(C 6 H 4 (o-PPh 2 ·BH 3 )) 3 (4), does not react with catecholborane, suggesting the influence of the pendant phosphines in the transformation of 2 into 1.
Novel hybrid sorbents have been designed for the selective extraction of rare earth elements (REEs). The tunning of the ligand bite angle and the grafting of these organic molecules on a silica support allow for selective discrimination of REE ions.
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