1P-096
Reaction between CO2 and Zinc Hydride: Interesting Theoretical Insights(Fukui Institute for Fundamental Chemistry, Kyoto University) ○ Milind Deshmukh, Shigeyoshi Sakaki[Introduction] Transition metal complexes are shown by number of studies that they can act aseffective catalyst for variety of conversion of chemical entities into desired products. The CO2 fixation
with reactive substrates such as epoxides, alcohols, amines, and alkynes has been achieved by severaltransition metal complexes. The reaction of CO2 with a hydrosilane (R3SiH) in presence of transition
metal catalyst has been shown to be thermodynamically favorable process to give a silyl formate andbelieved as a promising route for the CO2 reduction. Rh,1(a) Ru,1(b) and Ir1(c) complexes were reported
for the synthesis of silyl formates by hydrosilylation of CO2. However, the catalytic activity of thesecomplexes was reported to be low. Also, these metals are costly.
Recently, Sattler and Perking2reported that a zinc hydride catalyst 1(see Scheme 1) acts as a multifunctional
catalyst and can be effectively used forthe hydrosilylation of aldehydes,
ketones and most notably that of CO2.
Further, the hydrosilylation of CO2 by 1
was reported to be very rapid process.
The regeneration of catalyst 1 startingfrom 2 with silane was also shown to be
rapid process. However, the details of
the mechanism of these processes havenot been discussed and also importantfactors for these rapid reactions are not
Scheme1: Catalytic cycle for the reaction of CO2 with 1. clear in their work.
In this work, we theoretically investigated the reaction mechanism of the CO2 reaction with 1 andproposed a complete catalytic cycle (Scheme 1). Our purposes are to understand why CO2functionalization is rapidly achieved by catalyst 1 and to elucidate the electronic processes involved in
the above reactions.
[Computational Details] Geometries of all the complexes were optimized by DFT with B3PW91
functional. Two kinds of basis set systems, BS-I and BS-II, were used. In BS-I, 6-31G* basis sets
were employed for all atoms. The BS-I was employed for the geometry optimization. The better basis
set system, BS-II was employed for calculations of energy and various properties of these complexes.
In BS-II, a (311111/22111/411) basis set was employed for Zn, where its core electrons were replacedby Stuttgart-Dresden-Born (SDD) ECP. 6-311G(d) basis sets were used for C and N atoms, 6311+G(d) basis sets for H, O, Si, and S atoms. Also one set of polarization functions were added tohydride (H) of ZnH catalyst and H atom of silane, and one set of diffuse functions were added to O
and C atoms of CO2 groups. The MP2, SCS-MP2, MP4, and CCSD(T) levels of theory were used toevaluate energy changes. For these calculations, BS-II was employed.
[Results and Discussion] When going from reactant to transition state TS1, the CO2 molecule
approaches to the Zn-H bond of 1 from the top side in a such a way that the C of CO2 interacts with
the H of 1 and one of the O atoms of CO2 interacts with the Zn-center (Figure 1). The sidewise
approach of CO2 to the Zn-H bond of 1 is energetically unfavorable and leads to a higher TS inenergy by 7 kcal mol-1. Hence this sidewise approach of CO2 was not investigated further. This CO2
insertion occurs with the moderate activation barrier (Ga=12.5 kcal mol-1) and is exothermic process(∆G=-8.33 kcal mol-1). In TS1, considerable large Zn-H1 and short H1-C1 distances suggests that thebonding interactions are well established. The analysis of electronic structures suggests a strong
charge transfer (CT) from the valence s- and p-orbital of Zn to the CO2 moiety. This CT from Zn to
CO2 was attributed to small activation for barrier of this reaction.

1 + CO2TS1
a0.00(G =12.5 kcal mol-1)
Figure 1: Detailed mechanism for the reaction of CO2 with catalyst 1.

2(∆∆G=8.33 kcal mol-1)

2 + HSi(OEt)3TS2 (hyper-valent Si)
1
Figure 2: The reaction mechanism for the regeneration of catalyst 1 from 2 and HSi(OEt)3
The regeneration of catalyst 1 starting from 2 with silane occurs through TS2 (Figure 2). A
close look at TS2 suggests that Si in TS2 takes hypervalant structure, where one of the -OEt groupsand H1 take axial position. This geometry is favorable for the formation of hypervalent silicon center.
A very small activation barrier and a small endothermicity suggest that the regeneration of catalyst 1using 2 and HSi(OEt)3 occurs rapidly. However, the activation barrier increases and reaction becomes
endothermic, when PhSiH3 was employed.

Scheme 2: Model reaction for stabilityof hyper-valent silicon

Table 1: ∆G and Ga for regeneration of 1 using 2 and Silane
Ga for TS2∆Ga∆EStabilizationa, bSilane-1-1(kcal mol ) (kcal mol )(kcal mol-1)
HSi(OEt)3
4.23
0.71-62.84
PhSiH3
16.59
20.97-54.48aMP2 level was employed.
bStabilization energy of hyper-valent silicon as in Scheme 2

These differences in the reactivity of HSi(OEt)3 and PhSiH3 is explained in terms of the stability ofhypervalent silicon structure in the transition state TS2. As seen in Table 1, the smaller activation
barrier for regeneration of 1 with HSi(OEt)3 than that with PhSiH3 is correlated well with the higher
stability of hypervalent silicon in the former case than in the later one.
[References]
1. (a) Itagaki, S.; Yamaguchi, K.; Mizuno, N. J. Mol. Catal. A: Chem. 2013, 366, 347 and references
there in. (b) Deglmann, P.; Ember, E.; Hofmann, P.; Pitter, S. Walter, O. Chem.–Eur. J. 2007, 13,2864 and references there in. (c) Lalrempuia, R.; Iglesias, M.; Polo, V.; Sanz Miguel, P. J.;
Fernández- Alvarez, F. J.; Pérez-Torrente, J. J.; Oro, L. A. Angew. Chem. Int. Ed. 2012, 51,12824 and references therein
2. (a) Sattler, W.; Perkin, G. J. Am. Chem. Soc. 2011, 133, 9708. (b)Sattler, W.; Perkin, G. J. Am.
Chem. Soc. 2012, 134, 17462.