Chemical Interconversions in the System TP^*Zn/CO₂/Alcohol [Tp^* = Substituted Tris(pyrazolyl)borate]

Abstract

The zinc hydroxide complexes Tp*Zn-OH with TpCUmjMe = tris(3-cumenyl-5-methylpyrazolyl)borate and TptBurMe = tris(3-tert-butyl-5-methylpyrazolyl)borate can be converted to the alkyl carbonate complexes Tp*Zn-OCOOR by reaction with dialkyl dicarbonates or with alcohol and COz. An alternative formation reaction is the treatment of the pyrazolyl borate with zinc perchlorate and potassium carbonate in alcohol. The interconversion between TpcUmrMeZn-OH and TpCUmrMeZn-OCOOMe in methanol-containing solution can be repeatedly performed in both directions by bubbling either C 0 2 or N2 through the solution. The alkyl carbonate complexes show a variable sensitivity towards hydrolytic destruction with reformation of the hydroxide complexes. The complexes Tp'Ru,MeZn -0COOR (R = Me, Et) release C 0 2 under high vacuum to form the alkoxide complexes TptBurMeZn-OR, which could not be obtained pure due to their extreme water sensitivity. Indirect evidence for their existence is also obtained by the reaction between TpCunl,MeZn--OCOOMe and methyl iodide, forming TpC"nl~MeZn-I and dimethyl ether. The zinc hydroxide complexes catalyse the formation of diethyl carbonate from ethanol and C 0 2 .

The Insertion of Heterocumulenes into Zn−H and Zn−OH Bonds of Pyrazolylborate−Zinc Complexes

“…We have previously described reactions of Tp*ZnϪOH complexes with CO 2 in alcohols, [7,8] with CS 2 in alcohols [7] and in nonprotic solvents, [12] and with phenylisothiocyanate. [7] We had two reasons to repeat such reactions with Tp Ph,Me ZnϪOH (2).…”

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

“…Step (3): formation of weak associated precursor complex with CO 2 The second step in the first pathway is the formation of a weakly associated precursor complex (PC) from the reactants at infinite separation. The CO 2 molecule locates on the mirror plane perpendicular to the Zn-OH plane, as depicted in Fig.…”

“…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].…”

“…It has also been reported that zinc hydroxyl complexes can be converted to alkyl carbonate complexes by reaction with alcohol [2]. Meanwhile, in biology, for instance, the ubiquitous Zn(II) enzyme carbonic anhydrase removes CO 2 from tissues in the mammalian respiratory process by inserting it into Zn-OH bonds [3].…”

Evidence for a Trajectory of Hydrolytic Reactions brought about by [L3Zn−OH] Species

“…Our own attempts to make these reactions catalytic have met with little success so far. [52] Although solvent mixtures containing water (e.g., dichloromethane/methanol/water 6:4:1) can be found that will dissolve the complexes [Tp 'ZnÀOH] and that allow the determination of the pK a values of the corresponding Tp'ZnÀOH 2 complexes, [53] water could not yet be made a reagent in their chemistry. The advantage of the encapsulation by Tp' ligands that makes the existence of the Zn À OH function at neutral pH possible is a disadvantage in terms of the formation of charged species (i.e., cationic [Tp'ZnL] complexes or intermediates for proton transfers), and also the neutral [TpZnÀX] complexes (e.g., phosphates or carboxylates) with their hydrophobic exterior are too stable and too inert as to favor catalytic turnover.…”