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《有机化学》课程教学资源（参考书籍）Activation of Small Molecules, Organometallic and Bioinorganic Perspectives, Edited by William B. Tolman

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61Carbon Dioxide Reduction and Uses as Chemical Feedstock the formation of inorganic carbonates from CO and oxides(see Table 1.2).are characterized by a high kinetic barrier that makes them proceed slowly.For ex ample,the natural weathering of silicates [19]that converts silicates into carbo- nates (Eq.1)and free silica is a very slow process that requires activation to oc- cur in solution: M2Si04 +2CO22MCO3 Sioz (1) In general,the reactions of CO2 can be classified into two categories,according to their energetics: Reactions that do not require an external energy input,such as those that in- corporate into a chemical the whole CO2 moiety,or,more generally,those in which the carbon atom maintains the formal +4 oxidation state.Such reac tions produce carboxylates and lactones(RCOOR),carbamates (RR'NCOOR"). ureas (RR'NCONRR),isocyanates(RNCO),and carbonates [ROC(O)OR']. Reactions that generate reduced forms of COz,such as:HCOO(formates). [C(O)O(oxalates).HCO (formaldehyde),CO.CHOH and CH The latte that i ided as heat (th esses),elec Th ot de a pr 02 doe pen d on its enc or e xergo nicity. a m atter of fact,seve oday on- eam are strongly endo C,co sume significant amou ts energy and produce large quantities of waste.Therefore,the convenience of developing a process based on CO2 for substituting an existing one must be evaluated by comparing the two processes by applying the LCA methodology-the use of COz will be convenient if it minimizes the material and energy consumption and the CO2 emission. Any one of the above reactions will require a catalyst.which often is a metal system.After the discovery of the first transition metal complex of CO2 20]. emphasis has been on the study of the coordination chemistry of CO2 with the aim of discovering new catalysts for COz chemical utilization. 1.3 CO2 Coordination to Metal Centers and Reactivity of Coordinated CO2 1.3.1 Modes of Coordination The modes of coordination of CO2 to a metal center(s)are classified in Table 1.3.While the Co [201 and C [22a.bl co modes have been komocahe th no mode was nk. onstrated [231 ing from theto the ond order decr ward to say e CO2 molecu Whether the co

the formation of inorganic carbonates from CO2 and oxides (see Table 1.2), are characterized by a high kinetic barrier that makes them proceed slowly. For example, the natural weathering of silicates [19] that converts silicates into carbonates (Eq. 1) and free silica is a very slow process that requires activation to occur in solution: M2SiO4 2 CO2 2 MCO3 SiO2 1 In general, the reactions of CO2 can be classified into two categories, according to their energetics: Reactions that do not require an external energy input, such as those that incorporate into a chemical the whole CO2 moiety, or, more generally, those in which the carbon atom maintains the formal +4 oxidation state. Such reactions produce carboxylates and lactones (RCOOR), carbamates (RRNCOOR), ureas (RRNCONRR), isocyanates (RNCO), and carbonates [ROC(O)OR]. Reactions that generate reduced forms of CO2, such as: HCOO– (formates), [C(O)O]2 2– (oxalates), H2CO (formaldehyde), CO, CH3OH and CH4. The latter require energy that is provided as heat (thermal processes), electrons (electrochemical processes) or irradiation (photochemical processes). The convenience of developing a process based on CO2 does not depend on its endoor exergonicity. As a matter of fact, several processes are today on-stream that are strongly endothermic, consume significant amounts of energy and produce large quantities of waste. Therefore, the convenience of developing a process based on CO2 for substituting an existing one must be evaluated by comparing the two processes by applying the LCA methodology – the use of CO2 will be convenient if it minimizes the material and energy consumption and the CO2 emission. Any one of the above reactions will require a catalyst, which often is a metal system. After the discovery of the first transition metal complex of CO2 [20], emphasis has been on the study of the coordination chemistry of CO2 with the aim of discovering new catalysts for CO2 chemical utilization. 1.3 CO2 Coordination to Metal Centers and Reactivity of Coordinated CO2 1.3.1 Modes of Coordination The modes of coordination of CO2 to a metal center(s) are classified in Table 1.3. While the 2 -C,O [20] and 1 -C [22 a, b] coordination modes have been known for a long time, the 1 -O mode was only recently demonstrated [23]. Moving from the 1 to the 4-5 mode [40], the C–O bond order decreases and the length of the bond increases. Nevertheless, it is not straightforward to say that coordination increases the “reactivity” of the CO2 molecule. Whether the co- 6 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock

1Carbon Dioxide Reduction and Usess Chemical Feedstock A consequent question is whether or not the coordination of CO to a metal center is a prerequisite for its conversion into other chemicals.In fact,it de- pends on the kind of reaction CO2 has to undergo.The data reported in Section 1.3.3 seem to suggest that the coordination to a metal center is necessary only if the reduction of COz to CO is considered.In coupling reactions (C-C or C-E bond formation).CO2 may react directly with nucleophiles produced in the reac- tion medium by the metal catalyst (e.g.activated olefins):pre-coordination to metal centers may not be necessary. 1.3.2 eraction of COz with Metal Atoms at Low Temperature:Stability of the Adducts The interaction of with main groupnd transition metal en studie ansform IR theory (DFT) [43].The latter has played a key role in determining the mode of coordination of the cumulene in the complex,predicting the equilibrium properties of the identified species and describing the bonding.Different behavior has been dem- onstrated for late transition metal atoms [Fe,Co.Ni,Ag and Cu form 1:1 M(CO2)complexes]compared to early transition elements (Ti,V and Cr insert spontaneously into one of the C-O bonds yielding oxo-carbonyl species)[43]. Isotopic experiments with CO2 and C1O2 have permitted the spectroscopic identification of the bonding modes in organometallic species.Interestingly,the coordination of CO,to a metal atom is influenced by the gas matrix.For exam- ple.it has been shown that the cumulene binds in side-on fashion to nickel to ersely,if N2 is dded to the argon diluted ma NiNz"with a b ng e experimenta ons fo avior of such systems and ning been shown that Ti inserts with no "energy barrier"into one of the Co bonds of CO2,to afford a OTico species,which is more stable than any of the possi- ble Ti(CO2)complexes [43]. 1.3.3 Reactivity of CO2 Coordinated to Transition Metal Systems While several examples demonstrate that coordinated CO2 undergoes electrophilic attack by protons or other similar reagents at the 2-bonded oxy there is little evidence [15.25b,41]that coordination promotes the formatio ofa C-C bond. C, e,it is atively.a thre e documented my operate (1.1. reactions of coordinated

A consequent question is whether or not the coordination of CO2 to a metal center is a prerequisite for its conversion into other chemicals. In fact, it depends on the kind of reaction CO2 has to undergo. The data reported in Section 1.3.3 seem to suggest that the coordination to a metal center is necessary only if the reduction of CO2 to CO is considered. In coupling reactions (C–C or C–E bond formation), CO2 may react directly with nucleophiles produced in the reaction medium by the metal catalyst (e.g. activated olefins); pre-coordination to metal centers may not be necessary. 1.3.2 Interaction of CO2 with Metal Atoms at Low Temperature: Stability of the Adducts The interaction of the CO2 molecule with main group and transition metal atoms has been studied using matrix isolation Fourier transform IR spectroscopy [42], which recently has been coupled with density functional theory (DFT) [43]. The latter has played a key role in determining the mode of coordination of the cumulene in the complex, predicting the equilibrium properties of the identified species and describing the bonding. Different behavior has been demonstrated for late transition metal atoms [Fe, Co, Ni, Ag and Cu form 1 : 1 M(CO2) complexes] compared to early transition elements (Ti, V and Cr insert spontaneously into one of the C=O bonds yielding oxo-carbonyl species) [43]. Isotopic experiments with 13CO2 and C18O2 have permitted the spectroscopic identification of the bonding modes in organometallic species. Interestingly, the coordination of CO2 to a metal atom is influenced by the gas matrix. For example, it has been shown that the cumulene binds in side-on fashion to nickel to afford a 1 : 1 complex with a binding energy equal to 75 kJ mol–1 in a pure CO2 matrix, while in argon diluted matrices, no reaction occurs. Conversely, if N2 is added to the rare gas matrix, the coordination of CO2 occurs to preformed “NiN2” with a binding energy equal to 133.8 kJ mol–1 [43]. The combination of experimental studies with DFT calculations has been fruitful for explaining the behavior of such systems and the role of N2. Additionally, using DFT it has been shown that Ti inserts with no “energy barrier” into one of the CO bonds of CO2, to afford a OTiCO species, which is more stable than any of the possible Ti(CO2) complexes [43]. 1.3.3 Reactivity of CO2 Coordinated to Transition Metal Systems While several examples demonstrate that coordinated CO2 undergoes electrophilic attack by protons or other similar reagents at the 2 -bonded oxygen, there is little evidence [15, 25 b, 41] that coordination promotes the formation of a C–C bond, e.g. between CO2 and an olefin. In the latter case, it is more likely that CO2 interacts with a M(olefin)-adduct. Alternatively, a three-molecular mechanism involving the metal center, the olefin and CO2 may operate (see Section 1.4.1.1). Scheme 1.2 gives an overview of the documented reactions of coordinated CO2. 8 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock

101 Carbon Dioxide Reduction and Uses as a Chemical Feedstock substituted [12]Conversely,if CO is reduced to other C1 molecules,an energy input may be necessary.The real benefit in this case can be evaluated by com- paring the CO2 reduction to synthesis gas(syngas)production,which represents the current route to any reduced form of carbon-based products. 141 Carboxylation Reactions The incorporation of COz into an organic substrate to afford C-COOH, C-COOC,E-COO-C (E=N,O)or C-OC(O)O-C moieties is of great importance from the industrial point of view as it would allow the implementation of direct methodologies in place of those on-stream that do not respond to the energy- or atom-economy principles.The formation of a terminal "carboxylic moiety C-CO2 is today achieved through quite lengthy and waste-producing procedures. Thus,the oxidation of an organic moiety (i.e.CH,or benzene skeleton)or the hydration of CN groups are typically used,via multistep procedures,with pro- duction of waste and loss of carbon [521.Alternatively.Grignard reagents can be mole of vlate produced.Eve e used for the sis of cyclic ds c a "CO"moiety The catalytic strate would be of great n thi repre ep sustai carboxylic acids cesses. mong th the synt or ctones,the ca amines and carbonates s are particularly important due to the large market for the product (of the order of several megatons per year).Special attention will be dedicated to such compounds in the following sections. 1.4.1.1 C-C Bond Formation The carboxylation of an organic substrate such as a saturated hydrocarbon (Eq.2)or benzene (Eq.3)can be considered as a formal insertion of CO2 into the C-H bond.The enthalpy of such a reaction is in general favorable,although dependent on the reagents.In fact,the carboxylation of methane or benzene is characterized by a negative change of enthalpy.Nevertheless,it must be empha- sized that the dependence on entropy is quite different in the two cases.so the free energy change may be quite different: CH4(g)+CO2(g)-CH3COOH(1) (2) △H=-16.6kmol-1 △G28=+71.17kmol-1 C6H6①)+COz(g)一C6H5C0OH(s) 3 △HE -40.7mol-1

substituted [12]. Conversely, if CO2 is reduced to other C1 molecules, an energy input may be necessary. The real benefit in this case can be evaluated by comparing the CO2 reduction to synthesis gas (syngas) production, which represents the current route to any reduced form of carbon-based products. 1.4.1 Carboxylation Reactions The incorporation of CO2 into an organic substrate to afford C-COOH, C-COOC, E-COO-C (E = N, O) or C-OC(O)O-C moieties is of great importance from the industrial point of view as it would allow the implementation of direct methodologies in place of those on-stream that do not respond to the energyor atom-economy principles. The formation of a terminal “carboxylic moiety” C-CO2 is today achieved through quite lengthy and waste-producing procedures. Thus, the oxidation of an organic moiety (i.e. CH3 or benzene skeleton) or the hydration of CN groups are typically used, via multistep procedures, with production of waste and loss of carbon [52]. Alternatively, Grignard reagents can be reacted with CO2 with loss of 1 mol Mg per mole of carboxylate produced. Even more complex routes are used for the synthesis of cyclic compounds containing a “CO2” moiety. The catalytic carboxylation of olefins, or other organic substrates, would be of great value in this case and would represent a step forward towards sustainable processes. Among the carboxylation reactions, the synthesis of carboxylic acids or lactones, the carbamation of amines and the synthesis of carbonates are particularly important due to the large market for the products (of the order of several megatons per year). Special attention will be dedicated to such compounds in the following sections. 1.4.1.1 C–C Bond Formation The carboxylation of an organic substrate such as a saturated hydrocarbon (Eq. 2) or benzene (Eq. 3) can be considered as a formal insertion of CO2 into the C–H bond. The enthalpy of such a reaction is in general favorable, although dependent on the reagents. In fact, the carboxylation of methane or benzene is characterized by a negative change of enthalpy. Nevertheless, it must be emphasized that the dependence on entropy is quite different in the two cases, so the free energy change may be quite different: CH4g CO2g CH3COOHl 2 H 166 kJ mol1 G298 7117 kJ mol1 C6H6l CO2g C6H5COOHs 3 H 407 kJ mol1 10 1 Carbon Dioxide Reduction and Uses as a Chemical Feedstock

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