Cascade chemistry in azacryptand cages: bridging carbonates and methylcarbonatesElectronic supplementary information (ESI) available: magnetic data. See http://www.rsc.org/suppdata/dt/b1/b110449g/

Abstract

The different tendencies of dinuclear azacryptates of the m-CH 2 C 6 H 4 CH 2 and 2,5-furano-spaced hosts L 1 and L 2 to catalyse CO 2 uptake-reactions within these sterically-protected host cavities are examined. Bridging methylcarbonates are generated catalytically upon exposure of methanol solutions of L 1 , but not L 2 , di-transition cation cryptates to atmospheric CO 2 . X-Ray crystallographic structures of homodinuclear µ-carbonato cryptates of both ligands and µ-methylcarbonato cryptates of L 1 with later first transition series cations are reported. ESI-MS spectra show loss of H 2 CO 3 from µ-carbonato cryptates in collision activation experiments.The oxo-anion, carbonate, currently attracts attention in diverse areas of chemistry. Transformations involving this species, its reaction precursors and products are of considerable biological 1 significance; particularly important here are the carboanhydrases which play an essential role in processes such as photosynthesis, respiration, calcification and pH control. In the light of increasing concern about CO 2 build-up from fossil fuel consumption and potential resulting greenhouse effects, improved understanding of the biological handling of carbonate-related species acquires increasing urgency. Any potential new application of CO 2 as feedstock in chemical processes, will for this reason, attract much interest. In addition, the many and varied coordination modes of the carbonate oxo-anion are of spectroscopic interest as is their capacity to transmit magnetic interaction. 2-4 The carbonate system is also significant in the context of anion coordination chemistry as its behaviour within small molecule hosts may help to elucidate details of transport and location of carbonate or carboxylate anions in enzyme processes.The frequently used strategy of anion coordination via protonated amine, 5,6 or other acidic host, can be problematic in such pH-sensitive systems, so we have adopted an alternative strategy: the oxoanions here are retained via their bridging coordination of cations held within a cryptand cavity. We have already used the "cryptate as host" strategy to coordinate pseudo-halide anions such as azide and cyanate, and studied the spectroscopy and magnetochemistry 7,8 resulting from the consequent (and on first observation unprecedented) colinear M-NXY-M bridging geometry.The secondary coordination of anionic or other bridges between cations themselves coordinated by a cryptand host molecule was quite some time ago termed cascade coordination by Jean-Marie Lehn, 9 in the implicit expectation that the bridging groups might be activated, by reason of their dicoordination, toward further and possibly useful chemical reaction. However, such outcome was not apparent in our pseudo-halide † Electronic supplementary information (ESI) available: magnetic data. See http://www.rsc.org/suppdata/dt/b1/b110449g/

Mechanism of Atmospheric CO2 Fixation in the Cavities of a Dinuclear Cryptate

“…The electronic density was analysed using the natural bond orbital (NBO) technique [39]. To estimate the energetic effects of a methanolic medium, as used experimentally for the fixation process [4,5], solvation effects were calculated at the B3LYP/6-311++ G(2d,2p) level using the universal solvent model, SMD [40].…”

“…Previous work has shown that, in the presence of appropriate ligand environments and electron-rich metal centres, the reduction of CO 2 is possible, producing carbonates, oxalates, for example [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A variety of transition metal complexes with M-OR fragments undergo insertion reactions with CO 2 [4,6,7,18].…”

“…A variety of transition metal complexes with M-OR fragments undergo insertion reactions with CO 2 [4,6,7,18]. If R is an alkyl or aryl group, corresponding alkyl or aryl carbonate species are formed, while if R is a H atom, bicarbonate species are formed.…”

Tethering of Dinuclear Complexes to SBA‐15 and Their Application in CO2 Hydrogenation

“…Early indications of such enhanced reactivity have been manifest by insertion of methanol into the carbonato-bound azacrytates, forming a methyl carbonato bridged complex. [20] This demonstrates activation of the bound carbonate towards nucleophilic attack which we have attempted to exploit in this work by using these complexes as CO 2 reduction catalysts. When these complexes have been used as homogeneous catalysts in the CO 2 + H 2 reaction minor mounts of formic acid and methanol have been noted as products.…”

“…[18,19] Azacryptate complexes containing pairs of metal ions from the first transition metal series have been shown to take up CO 2 spontaneously from the atmosphere. [20,21] This process occurs in a wet solvent system and the CO 2 is fixed as a carbonate bridge between the metal centres.…”