Lateral thinking: A diagonal approach to CO2 recycling has been explored for the formation of both functionalized and energetic chemicals featuring a reduced carbon center. The strategy relies on the tandem use of a functionalization reagent and a reductant that can be independently modified to access a wide spectrum of chemicals from CO2. It is exemplified with an organocatalytic process to convert CO2 into formamides (see picture).

Reversible fixation of carbon dioxide, considered as a cheap and green C1 feedstock, and its facile transformation into valuable raw and fine materials is one of the most exciting challenges and important priorities for the scientific community.[1] The activation of the thermodynamically and kinetically stable CO 2 molecule has been achieved using metal-containing catalysts, while organic bases are also capable of promoting reactions with CO 2 . The R 2 NH/CO 2 system, which is relevant to industrial processes, has attracted much attention since the beginning of the last century. Hindered amidines and guanidines were found to be particularly efficient catalysts in the synthesis of organic carbonates and urethanes from the respective reactions of alcohols and amines with CO 2 , thus avoiding the use of the highly toxic phosgene and its derivatives. [2][3][4][5] These nitrogen bases also proved to be useful for the coupling of CO 2 and epoxides to give cyclic or polymeric carbonates.[6] It has been suggested that these reactions involve a zwitterionic adduct between the base and CO 2 for 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), which are the most commonly used amidine and guanidine species in these transformations. [2,[6][7][8] The existence of such base-CO 2 adducts was also invoked in the reversible fixation of CO 2 by polymers bearing DBU moieties or aromatic nitrogen bases.[9] While these base-CO 2 adducts are expected to be more stable for amidines and guanidines than for amines because they are capable of greater charge delocalization in the former cases, it was proposed from kinetic studies that such a zwitterionic adduct R 2 NH-CO 2 would be a first intermediate in the synthesis of alkyl ammonium alkyl carbamates [R 2 NH 2 ]-[R 2 NCO 2 ] from CO 2 and primary or secondary amines.[10]Formation of such adducts would also occur between CO 2 and the free amino groups of proteins.[11] Despite their ubiquity from biology to materials science, all attempts to isolate and characterize a zwitterionic adduct between CO 2 and a nitrogen base, especially an amidine or guanidine molecule such as DBU and TBD, were unsuccessful but led in some cases to the formation of the corresponding bicarbonate salt [baseH]- [HCO 3 ] owing to the presence of adventitious water. [7,8] Herein we present the synthesis and characterization, including the X-ray crystal structure, of TBD-CO 2 ; we also analyze the geometry and electronic structure of this adduct by DFT and MP2 calculations including solid or solvent effects.First, an off-white powder of the bicarbonate salt [TBDH][HCO 3 ] was readily obtained upon diffusion of CO 2 into a THF or MeCN solution of TBD in the presence of ambient moisture; colorless crystals were formed after the suspension was heated under reflux. The structure is shown in Figure 1 together with selected bond lengths and angles. The cation and anion are associated through two hydrogen bonds between the nitrogen and oxygen atoms (N1···O1 2.746(2) , N2···O2 2.836(2...

Reversible fixation of carbon dioxide, considered as a cheap and green C1 feedstock, and its facile transformation into valuable raw and fine materials is one of the most exciting challenges and important priorities for the scientific community. [1] The activation of the thermodynamically and kinetically stable CO 2 molecule has been achieved using metal-containing catalysts, while organic bases are also capable of promoting reactions with CO 2 . The R 2 NH/CO 2 system, which is relevant to industrial processes, has attracted much attention since the beginning of the last century. Hindered amidines and guanidines were found to be particularly efficient catalysts in the synthesis of organic carbonates and urethanes from the respective reactions of alcohols and amines with CO 2 , thus avoiding the use of the highly toxic phosgene and its derivatives. [2][3][4][5] These nitrogen bases also proved to be useful for the coupling of CO 2 and epoxides to give cyclic or polymeric carbonates. [6] It has been suggested that these reactions involve a zwitterionic adduct between the base and CO 2 for 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), which are the most commonly used amidine and guanidine species in these transformations. [2,[6][7][8] The existence of such base-CO 2 adducts was also invoked in the reversible fixation of CO 2 by polymers bearing DBU moieties or aromatic nitrogen bases. [9] While these base-CO 2 adducts are expected to be more stable for amidines and guanidines than for amines because they are capable of greater charge delocalization in the former cases, it was proposed from kinetic studies that such a zwitterionic adduct R 2 NH-CO 2 would be a first intermediate in the synthesis of alkyl ammonium alkyl carbamates [R 2 NH 2 ]-[R 2 NCO 2 ] from CO 2 and primary or secondary amines. [10] Formation of such adducts would also occur between CO 2 and the free amino groups of proteins. [11] Despite their ubiquity from biology to materials science, all attempts to isolate and characterize a zwitterionic adduct between CO 2 and a nitrogen base, especially an amidine or guanidine molecule such as DBU and TBD, were unsuccessful but led in some cases to the formation of the corresponding bicarbonate salt [baseH]-[HCO 3 ] owing to the presence of adventitious water. [7, 8] Herein we present the synthesis and characterization, including the X-ray crystal structure, of TBD-CO 2 ; we also analyze the geometry and electronic structure of this adduct by DFT and MP2 calculations including solid or solvent effects.First, an off-white powder of the bicarbonate salt [TBDH][HCO 3 ] was readily obtained upon diffusion of CO 2 into a THF or MeCN solution of TBD in the presence of ambient moisture; colorless crystals were formed after the suspension was heated under reflux. The structure is shown in Figure 1 together with selected bond lengths and angles. The cation and anion are associated through two hydrogen bonds between the nitrogen and oxygen atoms (N1···O1 2.746(2) , N2···O...

The trinuclear [UO2L]36- and tetranuclear [UO2L]48- metallamacrocycles, obtained by reaction of uranyl nitrate with the rigidly angular ligand (2R,3R,4S,5S)-tetrahydrofurantetracarboxylic acid (H4L) in a basic medium, are in equilibrium in methanol solution. Depending on the counterion, one or the other can be selectively isolated in crystal form. These rare examples of supramolecules incorporating actinide ions confirm the high potential and unique features of uranyl as a building block. Uranyl complexation through both the tri- and bidentate sites of the ligand is at variance with previous assumptions resulting from molecular modeling in nuclear waste reprocessing studies.

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Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3.C4H8S, 4.THF, 5, and 6.THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7.C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U...B with respect to the Ce...B distance, which is accompanied by a lengthening of the U-Hb bonds and an opening of the Hb-B-Hb angle (Hb = bridging hydrogen atom of the eta3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 -->M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M-Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the pi bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.

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