Atmospheric CO2 fixation

Atmospheric CO2 fixation by dinuclear Ni(II) complex, [TPANi(II)((j,-0H)2Ni(II)TPA](C104)2 (TPA = Tris(pyridylmethyl)amine)... 

The atmospheric CO2 fixation is of special interest from the point of view of enviromnen-tal chemistry [1], The increase of atmospheric CO2 will cause serious environmental problem in the near fijture. Thus, the development of effective chemical method to eliminate CO2 is an urgent matter. 

Many CO2 insertion reactions to the metal-anion ligand bonds are known [2]. However most of the complexes having such reactivity are dioxygen reactive, therefore they can not be the appropriate means for atmospheric CO2 fixation. Here we will describe the atmospheric CO2 fixation by using Ni(II) complex. Some reports have presented the reaction of the metal-hydroxy complex with CO2 affording metal carbonato or bicarbonato complex as shown in scheme 1 [3]. 

Our research goal is to develop effective system for atmospheric CO2 fixation by using such metal hydroxy complex. To design the suitable ligand for the metal hydroxy complex, it is necessary to investigate the reactivity of the complex toward CO2 systematically by using various ligands. 

The final cluster obtained from stoichiometric control resulted from variation in the reaction conditions used to form cluster 15, and has a general formula of [Mn 4Ln 4(bis-C[4])2(p3-OH)4(p-C03)2(dmf)8(H20)4] (17, Ln = Gd, Tb or Dy, Fig. 25.5h) [41]. The metallic skeleton is a distorted square of Ln ions that is doubly-capped on distal edges by [Mn 2(bis-C[4])]" moieties (Fig. 25.6h), a structiwal feature observed in the Fe 5Ln 4 cluster described above (14, Fig. 25.5e). Each half of the assembly can be considered a distorted Mn 2Ln 2 butterfly-Uke cluster, with these being connected by p-carbonates that arise from atmospheric CO2 fixation, a relatively commMi phenomenon in Ln coordinatimi cluster chemistiy and one that we have also seen with C[4]-supported systems. 

S. Minakata, 1. Sasaki, T. Ide, Atmospheric CO2 fixation by unsaturated alcohols using fBuOI under neutral conditions, Angew. Chem. Int. Ed. 49 (2010) 1309-1311. 

Y. Takeda, S. Okumura, S. Tone, 1. Sasaki, S. Minakata, CycUzative atmospheric CO2 fixation by unsaturated amines with f-BuOl leading to cyclic carbamates, Org. Lett. 14 (2012) 4874r-4877. 

Atmospheric CO2 first moves through the stomata, dissolves into leaf water and enters the outer layer of photosynthetic cells, the mesophyll cell. Mesophyll CO2 is directly converted by the enzyme ribulose biphosphate carboxylase/oxygenase ( Rubisco ) to a six carbon molecule that is then cleaved into two molecules of phosphoglycerate (PGA), each with three carbon atoms (plants using this photosynthetic pathway are therefore called C3 plants). Most PGA is recycled to make ribulose biphosphate, but some is used to make carbohydrates. Free exchange between external and mesophyll CO2 makes the carbon fixation process less efficient, which causes the observed large C-depletions of C3 plants. 

Although hydrogenase linked H2 production does not require ATP utilization, normal aerobic fixation of atmospheric CO2 does. As will be discussed below, when CO2 fixation does not occur (as is the case under anaerobic, sulfur-deprived condi tions), the accumulation of ATP molecules in the stroma inhibits ATPase function. This results in the non dissipation of the proton gradient and causes the build-up of the proton motive force. It has been shown that, under these conditions, photosynthetic electron transport is down regulated917 and consequently reductant is not available for efficiently producing H2.140... 

Natrajan, L., Pecaut, J., and Mazzanti, M. (2006) Fixation of atmospheric CO2 by a dimeric lanthanum hydroxide complex assembly of an unusual hexameric carbonate. Dalton Transactions, 1002—1005. 

Example 4.8. pH Change Resulting from Photosynthetic CO2 Assimilation As a result of photosynthesis with NOs assimilation, a surface water with an alkalinity of 8.5 X 10 eq liter" showed within a 3-h period a pH variation from 9.0 to 9.5. What is the rate of net CO2 fixation [Assume a closed aqueous system, that is, no exchange of CO2 with the atmosphere and no deposition of CaC03 (25°C), pATi = 6.3, pK2 = 10.2.]... 

The ratio of the specificity in the carboxylase reaction to that of the oxygenase reaction is called "relative specificity (Sr)" [7]. This is a constant determined by the enzymatic properties of RuBisCO. RuBisCO from a photosynthetic bacterium, Rhodospirillum rubrum, has the Sr value of 10 the oxygenase reaction proceeds two-times faster than the carboxylase reaction in the ordinary atmosphere. The upper most in the Sr value has been thought to be possessed by RuBisCO from higher plants. The value reaches 90 to 95 [8] the carboxylase reaction is three to four times faster than the oxygenase reaction. The Sr value is a determinant of the CO2 compensation point, where CO2 fixation by RuBisCO and CO2 release from photorespiration are balanced and no net CO2 fixation is observed. 

Increasing of CO2 in atmosphere is so serious problem for mankind that many R D are implemented to reduce CO2 discharge. One is a biological CO2 fixation using the photosynthetic function of micro-algae. 

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