The detailed reaction pathways for the hydration of carbon dioxide by water and water clusters containing two, three, and four water molecules (CO2 + nH2O → H2CO3 + (n − 1)H2O, n = 1−4) have been investigated in both gas phase and aqueous solution using ab initio molecular orbital (MO) theory up to the quadratic configuration interaction QCISD(T)/6-31G(d,p)//MP2/6-31G(d,p) level, both SCRF and PCM models of continuum theory, and a mixed approach based on MO calculations in conjunction with Monte Carlo and reaction field simulations. It is confirmed that the CO2 hydration constitutes a case of active solvent catalysis where solvent molecules actively participate as a catalyst in the chemical process. In aqueous solution the hydration mechanism is multimolecular, where geometric parameters of the solvent fully intervene in the reaction coordinate. The hydration reaction was found to proceed through an attack of a water oxygen to the CO2 carbon in concert with a proton transfer to a CO2 oxygen. The proton transfer is assisted by a chain of water molecules, which is necessary for a proton relay between different oxygens. Owing to a significantly larger charge separation in the transition structures, nonspecific electrostatic interactions between solute and solvent continuum also play a more important stabilizing role. Regarding the answer to the title question, our calculations suggest that although a water tetramer (n = 4) seems to be necessary for CO2 hydration in the gaseous phase, a reaction channel involving formation of a bridge containing three water molecules (n = 3) is likely to be actively involved in the neutral hydration of CO2 in aqueous solution.

Reaction of YCl3·THF3.5 with 2 equiv of [Me2Si(NCMe3)(OCMe3)]Li produces [Me2Si(NCMe3)(OCMe3)]2Y(μ-Cl)2Li·THF2 (1), which easily loses LiCl to give [Me2Si(NCMe3)(OMe3)]2YCl·THF (2). Salt metathesis of 2 with LiBH4, LiOAr (OAr = O-2,6-(CMe3)2C6H3), NaN(SiMe3)2, and LiCH(SiMe3)2 gives the corresponding yttrium bis((alkoxysilyl)amido) derivatives, [Me2Si(NCMe3)(OCMe3)]2YR (R = BH4·THF (3), OAr (4), N(SiMe3)2 (5), CH(SiMe3)2 (6)). The alkyl compound 6 reacts with H2 in THF to give an unstable hydride {[Me2Si(NCMe3)(OCMe3)]2Y(μ-H)}2 (7), which was identified by 1H NMR as a symmetric dimer in solution. Isolation of the hydride 7 appeared not to be possible; the disproportionation product, [Me2Si(NCMe3)(OCMe3)]3Y (8), was obtained instead. With HC≡CR, 6 undergoes protolysis of both the alkyl and the (alkoxysilyl)amido ligands to yield {Y(μ-C≡CR)3} n for R = SiMe3 (9) and CMe3 (10). In contrast, polymerization to polyphenylacetylene was observed for R = Ph. Compound 6 reacts with N≡CMe with metalation of the methyl group under proton transfer to the alkyl ligand to give CH2(SiMe3)2. Insertion of another N≡CMe into the new Y−C bond and 1,3-H shift produces {[Me2Si(NCMe3)(OCMe3)]2Y(μ(N,N‘)-NHCMeCHC≡N)}2 (11). The molecular structures of 6 and 11 show that the bis(N,O-bis(tert-butyl)(alkoxydimethylsilyl)amido) ligand system is slightly more bulky than the bis(pentamethylcyclopentadienyl) ligand set in compounds Cp*2YR. A ROHF INDO/1 semiempirical molecular orbital study on a stripped and symmetrized model of 6, [H2Si(NH)(OH)]2YCH3, shows that the electronic properties of the bis((alkoxysilyl)amido) ligand system are quite different from those of [C5H5]2YCH3 but compare well with those of the bis(benzamidinato) analogue [HC(NH)2]2YCH3. The (alkoxysilyl)amido ligand binds dominantly through a strong, ionic Y−N bond, while the ether function coordinates only weakly. Like in the bis(benzamidinato)yttrium system, the (alkoxysilyl)amido and the alkyl ligands accumulate negative charge, resulting in essentially ionic compounds. This high ionicity makes the compounds have little tendency to engage in σ-bond metathesis reactions and (catalytic) insertion chemistry. Because of the absence of charge delocalization within the (alkoxysilyl)amido ligands, these behave as strong Brønsted bases and compete successfully with the Y−C bond in C−H bond activation reactions.

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