Organoantimony and organobismuth complexes for CO₂ fixation

Abstract

The utilization of organoantimony (bismuth) complexes in CO₂ fixation is reviewed in this article. The efficient synthesis of cyclic carbonates from CO₂ and epoxides over an organoantimony(V) catalyst was first reported in 1979. After that, several organoantimony(V) complexes were found to be active for CO₂ fixation. In 2009, an organoantimony(III) complex was reported as an effective CO₂ absorbent and it is the only example of this kind. The study of organobismuth complexes for CO₂ fixation started in 2008, and the achievements are: (i) when organobismuth oxides, hydroxides and methoxides are used as CO₂ absorbents, they are converted to carbonates, and the organobismuth carbonates can be renewed as organobismuth oxides; (ii) organobismuth complexes can be used as catalysts to transform CO₂ into epoxides as cyclic carbonates at room temperature in the presence of a co-catalyst such as LiI and Bu₄NI; (iii) there is the development of bimetallic organobismuth complexes that show cooperative catalytic action; (iv) there is the physical fixation of CO₂ by inorganic–organic bismuth complexes; and (v) CO₂ insertion into the Bi–C bond of an organobismuth complex is disclosed. Most of the above catalytic systems can be considered as “electrophile–nucleophile”. In general, the synthesis of cyclic carbonates from CO₂ and epoxides catalyzed by an organoantimony (bismuth) complex follows the mechanism: the complex first reacts with CO₂ to form an organometallic carbonate, then the carbonate reacts with an epoxide with ring-opening by a base (e.g., n-Bu₄NI).

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Yi Chen

Yi Chen was born in Hunan Province, China. She received her B.Sc. degree from Hunan Normal University in 1996, and received a Master's Degree from Hunan University of Chinese Medicine in 2003. From 1996 to present, she worked as a lecturer and later associate professor (2006) in Hunan University of Chinese Medicine. At present, she is a visiting researcher with Prof. S.F. Yin. Her main research interests include basic research of the cardiovascular and cerebrovascular diseases and organbismuth medicine.

Renhua Qiu

Renhua Qiu was born in Hunan Province, China. He received a B.Sc. degree from Hunan Normal University in 2004, and received a Ph.D. from Hunan University in 2011 under the co-supervision of Professors Shuang-Feng Yin and Xinhua Xu. He was a research assistant (2010) with Prof. C.-T. Au in Hong Kong Baptist University and a visiting researcher (2011) with Dr L.-B. Han in AIST (Japan). From 2009 to present, he worked as an assistant researcher and later assistant professor (2011) in Hunan University. At present, he is visiting Osaka University (Japan) as a JSPS fellow (2012) with Prof. N. Kambe. Dr Qiu's main research interests include organometallic chemistry, Lewis acid catalysis, and C–H bond activation.

Xinhua Xu

Xinhua Xu was born in Hunan Province, China. He received his Master's Degree from Hunan Normal University in 1991, and received a Ph.D. from Zhejiang University with Prof. Xian Huang in 1998. He did postdoctoral research in Nankai University from 1998 to 2000 with Prof. Ruyu Chen. From 2000 to present, he worked as an Associate professor and later full professor (2006) in Hunan University. In 2002, he visited Japan as a visiting researcher with Prof. J. Otera. Prof. Xu's main research interests include organometallic chemistry, heteroatom chemistry, and homogeneous catalysis.

Chak-Tong Au

Chak-Tong Au was born in Hong Kong. In 1981, he received a Ph.D. from the University of Bradford, UK. From 1980 to 1986, he did research at the University College, Cardiff, Wales. In 1986, he joined Xiamen University, China as an associate professor and was promoted to professor in 1987. From 1990 to present, he worked as a lecturer and later full professor in Hong Kong Baptist University. He was awarded a D.Sc. degree by the University of Liverpool in 2003. Prof. Au has published over 400 papers in areas of heterogeneous catalysis and novel materials. At present, he serves as editor of Appl. Catal., A.

Shuang-Feng Yin

Shuang-Feng Yin was born in Hunan Province, China. He received a Ph.D. from Tsinghua University in 2003. He did postdoctoral research with Prof. C.-T. Au from 2002 to 2004 in HKBU. From 2004 to present, he worked as a lecturer and later full professor (2006) in Hunan University. From 2004 to 2006, he visited Japan as a JSPS fellow with Dr S. Shimada. Prof. Yin's main research interests include organometallic chemistry, CO2 chemistry, catalysis, new energy, nanomaterials.

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1. Introduction

1.1 CO₂ challenge and opportunity

Atmospheric CO₂ is at its highest level ever recorded, currently amounting to around 390 ppm by volume (590 ppm by mass).¹ The absolute mass of atmospheric CO₂ amounts to an alarming value of 3 × 10¹⁵ tons. It is estimated that human activities currently produce an additional 1.3 × 10¹⁰ tons of CO₂ per year. The impacts of further increase of atmospheric CO₂ on world climate and oceans could be severe as well as uncontrollable. Nonetheless, despite the notorious image, CO₂ can be the ultimate C1 feedstock. As far as the chemical fixation of CO₂ is concerned, there are significant developments in the past decades. It appears that the capture of CO₂ at source (e.g., at the power plants that use coal as fuel) is a suitable strategy for the mitigation of greenhouse effect.^2–5 For such a goal, effective substrates are needed as absorbents. It is of economic significance if valuable chemicals such as cyclic carbonates can be synthesized in the course of fixing CO₂.^6–14

1.2 Special properties of CO₂ (ref. 9 and 15–17)

The CO₂ molecule is linear, showing C–O distances of 1.16 Å. Although overall nonpolar, CO₂ contains polar bonds due to the difference in electronegativity between C and O. Its electronic structure is best represented as O^−δ–C^+2δ–O^−δ, disclosing susceptibility to nucleophilic attack at the carbon and electrophilic attack at the oxygen atoms. Reactions of CO₂ in biological systems tend to be a combination of nucleophilic and electrophilic interactions. Without the combined interactions, powerful electrophiles or nucleophiles are needed to initiate a CO₂ reaction. With an ionization potential of 13.78 eV, CO₂ is nonbasic and interacts only weakly with Brønsted and Lewis acids. It is considered that the major obstacle preventing efficient conversion of CO₂ into useful chemicals is the lack of appropriate catalysts.^18–27

1.3 CO₂ fixation with organometallic complexes

Recently, there are reports on the use of transition metal (e.g., Ni,^28–31 Rh,³² Ir, Fe, Cu, Re, and Co) complexes,^33–40 for CO₂ transformation. Potentially, the use of them could result in better utilization of CO₂ in chemical industry as well as having certain existing processes replaced by ones that are more energy-benign. In real practice, these complexes are not applicable because under mild conditions they do not perform satisfactorily when CO₂ association or dissociation is a matter of concern. For the activation of CO₂ with homogeneous molecular catalysts, there are three major mechanistic steps: (i) CO₂ coordination with the metal or ligand of catalyst, (ii) CO₂ reaction with neighboring ligands, and (iii) release of product and the restoration of catalyst.^41–45

1.4 Organoantimony and organobismuth complexes for CO₂ fixation

Organobismuth and organoantimony complexes have been investigated for chemical fixation of CO₂ since 1979. Bismuth and antimony are group 15 elements. The former is known to be nontoxic and noncarcinogenic while the latter is a boundary element showing metal as well as non-metal features. The compounds of both metals are studied because of their versatile roles (acting as reaction reagents, oxidizing agents, catalysts, and medicine).^46–57 In the past decade, there have been researches on novel cationic organobismuth and organoantimony complexes. We present herein representative results of using this kind of material as absorbents for CO₂ capture and/or as catalysts for cycloaddition of CO₂ to epoxides.

2. CO₂ fixation using organoantimony complexes

2.1 Organoantimony(V) complexes

The chemical fixation of CO₂ using organoantimony complexes as catalysts was first reported by Matsuda et al. in 1979.⁵⁸ They used pentavalent organoantimony compounds to catalyze the reaction between CO₂ and a number of epoxides (Table 1).⁵⁹ In all cases, the corresponding cyclic carbonates are generated in almost quantitative amounts.

Table 1 Reaction of CO₂ with epoxides catalyzed by pentavalent organoantimony compounds^a,59

Catalysts	Epoxides (R)	Temp. (°C)	Time (min)	Yield^b (%)
a Reaction conditions: epoxides, 0.1 mol; CO2, 0.2 mol (50 kg cm^−2); catalysts, 0.001 mol.b Based on epoxides.c Catalysts were recovered.d Trace of poly(styrene oxide) was obtained besides the carbonate.e Poly(styrene oxide) was obtained in ca. 10% yield besides the carbonate.
Ph5Sb (1a)	H	120	330	82
	CH3	120	130	87
Ph4SbBr (1b)	H	120	12	96
	CH3	120	8	92^c
		100	20	98^c
		60	120	93^c
	CH2Cl	120	24	97
	Ph	120	16	91^d
Ph3SbBr2 (1c)	H	120	38	95
	CH3	120	16	97^c
		100	50	92^c
		60	390	97^c
	CH2Cl	120	80	94
	Ph	120	24	82^e
Ph3SbCl2 (1d)	H	120	180	99
		100	480	96
	CH3	120	150	96^c
Me3SbBr2 (1e)	H	120	15	72
		100	40	79
	CH3	120	60	94^c
		100	150	91^c
Ph3SbO (1f)	CH3	120	120	94
Ph3Sb (1g)	CH3	120	360	7
Ph2SbCl (1h)	CH3	120	360	5
SbCl3 (1i)	CH3	120	360	4

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It was observed that the catalysts with halogen atoms and phenyl substituents are more active than those without halogen atoms and with methyl substituent. The complexes halogenated with bromine are more active than those with chlorine. The catalytic activities of the complexes decrease in the order of 1b > 1d > 1e > 1a, 1c, 1f ≫ the trivalent compounds (1g, 1h, 1i). The catalysts 1b–e can be recovered from the crude products in good yields. It was reported that catalysts 1a and 1f cannot be recovered, and the corresponding crude products are light brown and light yellow in color. Such phenomena could be a result of catalyst instability. As reflected by the time of reaction, the epoxides (R) show reactivity that follows the order of CH₃ > Ph > H > CH₂Cl.

Using tetraphenyistibonium halides (Ph₄SbCI and Ph₄SbBr) as catalysts for the hydration of ethylene oxide in the presence of CO₂ or ethylene carbonate at 120 °C, Matsuda et al. gained better control of product distribution.⁶⁰ With a small excess of water in the reaction system, ethylene glycol is generated as the predominant product. When the molar ratio of water to ethylene oxide is 1 or less than 1, the stibonium halides selectively give diethylene glycol as the major product.

As proposed by Matsuda et al., the catalytic mechanism can be described as shown in Scheme 1. Basically, there is the coordination of epoxide with the Sb atom followed by interaction with CO₂.⁵⁹ In the reaction with styrene oxide, polystyrene oxide is formed as by-product (up to a yield of ca. 10%). It is noted that the catalytic activities of organoantimony compounds are superior to those of organotin compounds despite the Lewis acid strength of Sn being stronger than that of Sb.⁵⁹ In the case of organoantimony complexes, effects such as reaction acceleration due to preheating and polymerization of epoxides as inferred by epoxide–antimony coordination are not observed. In addition, there is no obvious change of IR and ¹H NMR spectra upon the mixing of epoxides with the Sb catalysts (1 mol% of catalyst in epoxide solution). All the facts point to the absence of epoxide–zinc coordination. It is hence reasonable to consider that the activation of CO₂ follows the cyclic mechanism shown in Scheme 1.

Scheme 1 Possible mechanism of the reaction of CO₂ with epoxides catalyzed by pentavalent organoantimony compounds.⁵⁹

It was also reported that Ph₄SbBr is an effective catalyst for the synthesis of 3-phenyloxazolidin-2-one from CO₂ and 1-phenylaziridines. At 80 °C in polar aprotic solvents such as hexamethylphosphoramide (HMPA) and dimethyl sulfoxide (DMSO), the yield is up to 93% and 85%, respectively (Scheme 2).⁶¹

Scheme 2 Reaction of carbon dioxide with 1-phenylaziridine catalyzed by organoantimony compound Ph₄SbBr.

In 1992, Nomura et al. reported an efficient catalytic system that involves the combined use of tetraphosphorus decasulfide (P₄S₁₀) and triphenylstibine oxide (Ph₃SbO, 1f) for the synthesis of urea through amine carbonylation with CO₂.⁶² In the Ph₃SbO/P₄S₁₀ system, 1,3-dialkylurea (RNHCONHR, where R = Bu, i-Bu, s-Bu, t-Bu, allyl, Ph) and tetramethylurea were successfully synthesized from the corresponding amines at 80 °C and 4.9 MPa initial CO₂ pressure. Based on the results of ¹³C NMR monitoring, it was concluded that the course of reaction involves the successive thiolation of carbamic acid to intermediate antimony carbamate species, followed by the aminolysis of carbamothioic acid to the final product (Scheme 3). Similarly, cyclic ureas can be synthesized through the carbonylation of diamines (RNHCH₂CH₂NHR′, where R, R′= H, H; Me, H; Ph, H; HOCH₂CH₂, H; HOCHMeCH₂, H; Me, Me) (Table 2). Using the Ph₃SbO/P₄S₁₀ catalyst system, trisubstituted ureas such as l-butyl-3,3-diethylurea can be generated through the selective cocarbonylation of butylamine and diethylamine.

Scheme 3 Synthesis of urea from amine and carbon dioxide (chemical shifts is δ(¹³C NMR)).⁶²

Table 2 Synthesis of cyclic urea from diamines and carbon dioxide^a,62

R	R′	Temp. (°C)	Time (h)	Yield^b (%)
a Reaction conditions: diamine/Ph3SbO/P4S10 = 20/1.0/2.0 mmol, benzene 20 mL, CO2 4.9 MPa.b Isolated yields.c Absence of the catalyst system.
H	H	150	12	85
		150	24	0^c
		80	24	17
Me	H	120	12	60
Ph	H	100	24	40
HOCH2CH2–	H	60	24	95
HOCH(CH3)CH2–	H	120	24	54
Me	Me	120	24	75

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Lermontov et al. reported that the Ph₃SbF₂ complex is a highly efficient catalyst for the synthesis of cyclic carbonates from epoxides and CO₂ in HMPA at 180 °C, giving 100% yield in 5.5 h.⁶³ They proposed that an efficient catalyst for this kind of reaction should be able to behave as a weak acid (for electrophilic activation of the oxirane ring) as well as a weak base (for nucleophilic activation of CO₂). It is understandable because a strong acid means fast polymerization of oxirane and a strong base means irreversible binding of CO₂. They proposed that β-fluoroalkoxide resulting from electrophilic opening of the oxirane ring (eqn (1)) serves as a base. To check this proposal, they carried out the reaction of propylene oxide with Ph₃SbF₂ in the absence of CO₂, and detected fluoroalkoxide anions by ¹⁹F NMR spectroscopy (eqn (2)). In addition, the fluorocarbonate anion resulting from the reaction of fluoride with CO₂ (eqn (3)) could also serve as a base. However, they did not observe any absorption band of FCOO– in the IR spectrum of a mixture of CsF, HMPA, and CO₂; the result, nonetheless, does not exclude the possible formation of FCOO– under the adopted reaction conditions, viz. 140–180 °C and 70–100 atm CO₂ pressure.

The results of Lermontov et al. showed that the catalytic mechanism of Ph₃SbF₂ is different from that of Ph₄SbBr proposed by Nomura et al.⁶² The former involves CO₂ activation by the organoantimony complex, followed by interaction with epoxide. As for the latter, it is epoxide that is first activated followed by interaction with CO₂.

2.2 Organoantimony(III) complexes

In 2009, Dostál et al. reported for the first time the chemical fixation of CO₂ over a trivalent organoantimony(III) complex, showing clear evidence of CO₂ insertion.⁶⁴ The dimeric organoantimony(III) oxide (ArSbO)₂ ((Ar) NCN chelating ligand C₆H₃-2,6-(CH₂NMe₂)₂) was obtained by reacting ArSbCl₂ with KOH (Scheme 4). The oxide bonds with CO₂ to form air-stable monomeric carbonate ArSbCO₃. In turn, CO₂ can be released by heating the carbonate at 130 °C for (ArSbO)₂ recovery (Scheme 4). The image reproduced from ref. 46, 47 and 91 with permission of Elsevier.

Scheme 4 Preparation of (ArSbO)₂ (2) and reversible fixation of carbon dioxide by 2 to form ArSbCO₃ (3).⁶⁴

The molecular structure of (ArSbO)₂ 3 was unambiguously determined by X-ray analysis (Fig. 1). Single crystals of 3 were obtained through slow evaporation of CH₂Cl₂ from a (ArSbO)₂/CH₂Cl₂ solution. The (ArSbO)₂ compound was obtained as a solvate with two molecules of water. Despite the use of dried solvents and under an atmosphere of pure argon, it was not possible to obtain anhydrous (ArSbO)₂ single crystals. The interesting feature of the molecular structure of 3 is the coordination of the carbonate moiety as a terminal ligand in a chelating fashion.

Fig. 1 Pluton plot showing hydrogen bonding in 3. Selected O⋯H–O contacts (Å) and angles (deg.) characterizing respective hydrogen bonding: O(3)⋯H–O(3w) 2.829(5), 166.8; O(3)⋯H–O(2w) 2.773(5), 123.4; O(2)⋯H–O(1w) 2.969(5), 133.2; O(2w)–H–O(1w) 2.833(6), 175.9.⁶⁴ (The image reproduced from ref. 64 with permission from the ACS.)

3. CO₂ fixation using organobismuth complexes

3.1 Organobismuth complexes as CO₂ absorbent

Compared to the use of organoantimony complexes, the use of organobismuth complexes for chemical fixation of CO₂ is rare. For efficient interaction with CO₂, the metal can either be a Lewis base, showing π-type interaction with one of the C=O bonds of CO₂, or be a Lewis acid (or an oxophile) interacting with an oxygen atom of CO₂. The use of organobismuth complex containing Bi–O bond for CO₂ fixation was only reported recently.

In 2008, Breunig et al. firstly reported that exposure of a diethyl ether or a toluene solution of hypervalent diarylbismuth hydroxide [2-(Me₂NCH₂)C₆H₄]₂BiOH (4) as well as that of the corresponding oxide [(2-(Me₂NCH₂)C₆H₄)₂Bi]₂O (5) to air led to CO₂ adsorption and the formation of carbonate [(2-(Me₂NCH₂)C₆H₄)₂Bi]₂CO₃(6) (Scheme 5).⁶⁵ However, the authors did not mention whether carbonate 6 could be transferred back to hydroxide 4 or oxide 5 or not.

Scheme 5 Chemical fixation of CO₂ with hypervalent organobismuth(III) hydroxide (4) and oxide (5) to carbonate (6) with the pendant arm ligand of 2-(Me₂NCH₂)C₆H₄.⁶⁵

Later in the same year, Shimada and co-workers reported another kind of hypervalent organobismuth complexes with a 5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine framework.⁶⁶ The synthesis of organobismuth oxide (7), hydroxide (8) and alkoxide (9) are shown in Scheme 6.

Scheme 6 Chemical fixation of CO₂ with hypervalent organobismuth(III) oxide (7), hydroxide (8), and alkoxide (9) to carbonate (10) and methyl carbonate (11) with a 5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine framework.⁶⁶

The bismuth oxide 7 can be obtained by treating the bismuth chloride with aqueous sodium hydroxide. Analogous to the result of Breunig et al.,⁶⁵ bismuth hydroxide 8 readily loses a water molecule to form oxide 7. In turn, oxide 7 can be completely converted back to hydroxide 8 by the inclusion of a water molecule. When a toluene solution of 8 is vigorously stirred in air at room temperature (RT) for 4.5 h, there is quantitative conversion of 7 to bismuth carbonate 9 (Scheme 6). Also, when an anhydrous CH₂Cl₂ solution of 7 or 8 is exposed to CO₂ (1 atm), a quantitative amount of 10 is obtained readily. The thermal ellipsoid plot of 10 is shown in Fig. 2. It is noted that carbonate 10 is thermally rather stable, and in CH₂Cl₂ or under vacuum at RT, there is no decomposition of carbonate 10. Partial dissociation of carbonate 10 (ca. 30%) and quantitative regeneration of oxide 7 was observed when 10 was heated under vacuum at 100 °C for 10 h. For effective utilization of compounds 8 and 7, it is critical for them to show association as well as dissociation ability with CO₂. Since it is possible to regenerate compounds 7 and 8 after CO₂ absorption, the two are potential candidates for CO₂ capture. It is observed that bismuth methoxide 9 in methanol, CH₂Cl₂, or toluene readily reacts with dry CO₂ (1 atm) to form bismuth methyl carbonate 11, and the process is reversible (Fig. 3). Because methoxide 9 is sensitive to water, it is subject to hydrolysis and undergoes the subsequent reaction in the presence of water.

Fig. 2 A thermal ellipsoid plot (50% probability level) of 10. Hydrogen atoms are omitted for clarity.⁶⁶ (The image reproduced from ref. 66 with permission from the Wiley.)

Fig. 3 A thermal ellipsoid plot (50% probability level) of 11. Hydrogen atoms are omitted for clarity.⁶⁶ (The image reproduced from ref. 66 with permission from the Wiley.)

3.2 Organobismuth complexes as CO₂-fixation catalysts

Yin et al. investigated the possible application of 7 as a catalyst for 2-oxazolidinone synthesis through the fixing of CO₂ into 2-aminoethanol.⁶⁶ When an N-methylpyrrolidone (NMP) solution of 2-aminoethanol under CO₂ (1 atm) was heated at 170 °C for 16 h in the presence of 5 mol% of 7, approximately 5% (determined by ¹H NMR spectroscopy) of 2-oxazolidinone was formed, whereas there was only a trace (less than 0.5%) of 2-oxazolidinone detected in the absence of 7 under the same conditions. Despite the low activity of 7, it is considered that the presence of 7 has positive effect on the formation of 2-oxazolidinone. After the reaction, there is a small amount of black deposit in the reaction mixture. The ¹H NMR spectrum of the reaction solution after the reaction shows only one kind of bismuth species. The results suggest partial decomposition (less than 14%) of the bismuth compound. On the other hand, there was complete decomposition of bismuth oxide [((2,4,6-Me₃C₆H₂)₂Bi)₂O] under the same reaction conditions, and the bismuth oxide did not show any positive effect on 2-oxazolidinone formation.

Later, Yin and Shimada synthesized two new bismuth compounds (12 and 13) bearing a sulfur-bridged bis(phenolato) ligand. They found that these complexes could be used in the solvent-free synthesis of propylene carbonate from CO₂ and propylene oxide in the presence of LiI.⁶⁷

Fig. 4 and 5 show the molecular structures of 12 and 13, respectively. In the solid state, 12 and 13 form dimers through intermolecular Bi–O and Bi–I interactions. The Bi atom in 12 is five-coordinate 12–Bi–5, and its geometry can be described as highly distorted square pyramidal, while that in 13 is six-coordinate 14–Bi–6 having a tetrahydrofuran (THF) molecule as an additional ligand, and its geometry is distorted octahedral. These structural features clearly show the Lewis-acid nature of the bismuth centers of 12 and 13, and are in contrast to those of the trivalent phosphorus compounds bearing 2,20-thiobis-(phenolato) ligands.

Fig. 4 A thermal ellipsoid plot of compound 12 (50% probability level). Hydrogen atoms on the carbon atoms are omitted for clarity.⁶⁷ (The image reproduced from ref. 67 with permission from the RSC.)

Fig. 5 A thermal ellipsoid plot of compound 13 (50% probability level). Hydrogen atoms on the carbon atoms are omitted for clarity.⁶⁷ (The image reproduced from ref. 67 with permission from the RSC.)

Results of initial attempts of using 7 or 10 as catalyst for the synthesis of propylene carbonate (PC) from propylene oxide (PO) and CO₂ showed that these compounds are catalytically active. Nonetheless, bismuth methoxide 12 (catalyst loading = 0.24 mol%) shows low catalytic activity, giving a GC yield of only 29% in 10.5 h at 120 °C with initial CO₂ pressure of 35 atm.

However, in the presence of an iodide salt as co-catalyst, compound 12 exhibits remarkable catalytic activity even under mild reaction conditions. As shown in Table 3, compound 13 is slightly superior to 12 in catalytic performance. The combined use of 13 and LiI results in nearly quantitative yield within 24 h (Table 3, entry 11, yield 98%). The catalytic activity of 13 is much higher than those (at RT and 1 atm CO₂) reported by Ratzenhofer and Kisch.⁶⁸ With ¹H NMR monitoring, it was found that the reaction of 12 with CO₂ does not proceed with the formation of a bismuth carbonate. In the solution, the monomers and dimers of 12 are at equilibrium, having composition depending on solvent polarity. In the case of 13, the compound exists mostly as monomers even in a C₆D₆ solution. It is deduced that both 12 and 13 act as monomeric Lewis acid catalysts, and their mechanistic action is similar to that of the commonly proposed acid–base or electrophile–nucleophile bifunctional mechanism.²⁰ Hence the high efficiency of the catalyst systems can be attributed to the hypervalent structures of 12 and 13. The intramolecular coordination of the sulfur atom to the bismuth atom is extremely flexible, and the flexibility enables the adjustment of Lewis acidity of the bismuth atom. The outcome is facile exchange of product and reactant.

Table 3 PC synthesis from PO and CO₂ catalyzed by 12 (or 13)/iodide catalyst system^a,67

Entry	Catalyst	Conversion^b (%)	PC yield^b (%)
a The coupling reaction was conducted in a glass flask. The product was analyzed by NMR and GC-MS spectroscopies.b The conversion and the PC yield were estimated by ^1H NMR spectroscopy.
1	12	0	0
2	13	0	0
3	Me3PhPI	0	0
4	13 + Me3PhPI	76	76
5	Bu4NI	0	0
6	13 + Bu4NI	77	77
7	NaI	14	14
8	13 + NaI	91	91
9	LiI	26	26
10	12 + LiI	92	92
11	13 + LiI	98	98

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To achieve high and efficient conversion of CO₂ into valuable chemicals, and to exploit new applications of organobismuth compounds, Yin et al. developed another efficient catalytic system for the synthesis of cyclic carbonate from epoxide and CO₂.⁶⁹ They studied for the first time the cationic organobismuthcomplex (14) with the 5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine framework for the transformation of CO₂ into epoxide as cyclic carbonates, using terminal epoxides as substrates and tetrabutylammonium halide as co-catalyst under mild conditions (Table 4). It is a solvent-free system and the catalyst exhibits high activity and selectivity for the coupling reaction of CO₂ with a wide range of terminal epoxides. The selectivity to propylene carbonates could reach 100%, and the maximum turnover frequency was up to 7800 h⁻¹ at 120 °C and 3 MPa initial CO₂ pressure when tetrabutylammonium iodide (1 : 1) was used as co-catalyst (entry 1). Moreover, the catalyst is environment-benign, resistant to air and water, and can be readily reused and recycled without any loss of activity (Fig. 6). In other words, the catalyst has great potential for industrial application.

Table 4 Coupling of CO₂ with terminal epoxides catalyzed by cationic organobismuth complex (14) + ⁿBu₄NI^a,69

Entry	R	R′	Conv.^d (%)	Sel.^d (%)	TOF^d (h^−1)
a Reaction conditions: epoxide, 120 mmol; Bi compound (14), 0.015 mmol; Bu4NI, 0.015 mmol; initial CO2 pressure, 3.0 MPa; 120 °C.b Reaction time, 80 min.c Reaction time, 3 h.d GC analysis, conv.: epoxide conversion; sel.: cyclic carbonate selectivity; TOF: moles of epoxide converted per mole of Bi per hour.
1	Me	H	97.5	>99.0	7800
2	Et	H	92.3	>99.0	7384
3	Bu	H	86.2	>99.0	6896
4	ClCH2	H	90.7	>99.0	7256
5^b	Ph	H	87.9	>99.0	5274
6^c	–CH2(CH2)2CH2–		90.3	>99.0	2408

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Fig. 6 Catalyst recycling using epoxy chloropropane as substrate. For each cycle: epoxy chloropropane, 120 mmol; Bi compound (14), 0.015 mmol; Bu₄NI, 0.015 mmol; CO₂ pressure, 3 MPa; 120 °C; 1 h (ref. 69) (The image reproduced from ref. 69 with permission from the Springer).

Encouraged by the results, Yin et al. incorporated ⁿBu₄NI into binuclear organobismuth oxide complex 7 to generate a bifunctional Lewis acid–base system that shows cooperative catalytic effect for CO₂ fixation (Scheme 6).⁷⁰ Despite binuclear organobismuth oxide (7) having a framework similar to that of bimetallic Salen (Al) (complexes known to be catalytically highly efficient towards the reaction),⁷¹ complex 7 shows no catalytic activity when 2-methyloxirane is adopted as substrate (Scheme 7, Route A). Nonetheless, when 2-(chloromethyl)oxirane is adopted as substrate, cyclic carbonates are obtained almost quantitatively (Scheme 7, Route B). Furthermore, when Bu₄NI is used as co-catalyst (0.1 mol% based on epoxide), CO₂ insertion into 2-methyloxirane occurs in a quantitative manner (Scheme 7, Route C). In contrast to the results of monomeric organobismuth chlorides, methoxide and methanethiolate, the findings of binuclear organobismuth oxide shows cooperative action that enables high efficiency in the synthesis of cyclic carbonates from epoxides and CO₂.

Scheme 7 Selective insertion of CO₂ into epoxides catalyzed by cat. 7 (0.5 mol%) with or without co-catalyst Bu₄NI (0.1 mol%).⁷⁰

However, the binuclear organobismuth oxides 7, 18, 19 are sensitive to moisture and CO₂, and the rigorous requirements for manipulation limit their utilization. For practical application in organic synthesis, a catalyst is better to be air-stable.⁴⁹ Interestingly when the bridging atom is changed from oxygen to sulfur (Scheme 8), the binuclear organobismuth sulfides LBi–S–BiL (20–22) are air-stable. The compounds could remain as dry crystals or powder (slightly yellow in color) for more than one year in ambient environment. These sulfur-bridged binuclear organobismuth complexes represent a rare class of complexes that are air-stable.

Scheme 8 Synthetic routes of organobismuth complexes 7, 18–24 (ref. 70).

The crystal structures of 20–22 freshly obtained after recrystallization in toluene/hexane (CH₂Cl₂/hexane for 20) were determined by X-ray analysis. As shown in Fig. 7, the 5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine framework of complexes 20–22 is bridged by a sulfur atom that occupies a vacant site of the bismuth centre. In the equatorially distorted trigonal bipyramidal geometry, the nitrogen and sulfur atoms occupy the apical positions whereas the carbon atoms occupy the equatorial positions. With sulfur bridging, the bismuth centre is exposed more efficiently than that of their precursors, making the two bismuth atoms of 20–22 closer to each other in comparison to the case of 7. Nonetheless, the distance between the two bismuth atoms of 20–22 is larger than that of a covalent Bi–Bi single bond (3.092(6)–3.2092(8))⁷² and Bi=Bi double bond (3.0648(2)).⁷³ This can be considered as a favorable parameter for maintaining suitable stability of the bimetallic complexes. In addition, the five linked atoms N(1)–Bi(1)–S(1)–Bi(1A)–N(1A) are almost in the same plane (angle distortion <0.5°) with the plane bisecting the butterfly-shaped tetrahydrodibenz[c,f][1,5]azabismocine framework. One can see that the butterfly-shaped phenyl planes of 20–22 are cis-geometrically located to make the two bismuth atoms available for cooperative action. The N-substituents of 20–22 are positioned close to the bismuth centers, protecting the Bi–C bond and Bi–S–Bi bridge from infringers such as water and CO₂. Hence, by using sulfur atom for bridging and varying the N-substituent, one can regulate the state suitability as well as the ligands of the bismuth centers to mimic those of natural metalloenzymes.

Fig. 7 Thermal ellipsoid plots (50% probability level) of 20 (top), 21 (bottom left), 22 (bottom right). Hydrogen atoms on the carbon atoms are omitted for clarity.⁷⁰ (The images reproduced from ref. 70 with permission from the RSC.)

To examine the cooperative action of the binuclear bismuth centers, complexes 7, 18–22 were examined for the synthesis of cyclic carbonates from epoxides and CO₂ (Table 5), and their performance are compared with those of mononuclear organobismuth complexes 15–17 and 23, 24. One can see that most of them show good catalytic efficiency (entries 1–6, GC yield up to 100%). In terms of catalytic efficiency and air-stability, the N-substituent with flexible and electron-donating group (^tBu, 20; Cy, 21) is superior to organobismuth oxides (7, 18–19) (entries 1–3) as well as to those with rigid and electron-withdrawing group (Ph, 22) (entries 4–6). In contrast to the mononuclear organobismuth complexes (entries 7–11), the binuclear organobismuth complexes show a significant cooperative effect on CO₂ transformation. Without a catalyst, the product yield is only 3.7% (entry 12). The results indicate that by simple modification of the N-substituent and the bridging atom in the main framework of bimetallic organobismuth complexes, one can tune catalytic activity as well as achieving air-stability.

Table 5 Optimization of reaction condition in cyclic carbonate synthesis from 2-(chloromethyl)oxirane and CO₂^a,70

Entry	Cat. (mol%)	Time (h)	Temp. (°C)	Yield (%)	Sel. (%)
a Reaction conditions: 2-(chloromethyl)oxirane, 2 mL (25 mmol); initial CO2 pressure, 3.0 MPa, GC yield.b Blank experiment.
1	7 (0.5)	8	140	99.6	99.8
2	18 (0.5)	8	140	100	100.0
3	19 (0.5)	8	140	97.3	98.1
4	20 (0.5)	8	140	99.4	99.7
5	21 (0.5)	8	140	94.3	98.3
6	22 (0.5)	8	140	65.8	98.0
7	15 (1.0)	8	140	25.9	97.7
8	16 (1.0)	8	140	41.3	98.3
9	17 (1.0)	8	140	5.9	96.9
10	23 (1.0)	8	140	32.5	98.3
11	24 (1.0)	8	140	14.7	92.6
12^b	—	8	140	3.7	98.2

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The catalytic performance of 20 was also investigated under optimized conditions across a diversity of epoxides (Table 6). In most of the cases, including 2-phenyloxirane and 7-oxabicyclo[4.1.0]heptane of large steric hindrance, the catalyst is highly effective (entries 1–6). The co-presence of Bu₄NI is favorable for CO₂ insertion into the non-halide epoxides (entries 2 and 7).

Table 6 Synthesis of cyclic carbonates from CO₂ and epoxides^a,70

Entry	R^1	R^2	Conv. (%)	Yield (%)
a Reaction conditions: 2-(chloromethyl)oxirane, 2 mL; initial CO2 pressure 3.0 MPa, cat. 20, 0.5 mol%, 8 h, 140 °C.b Without co-catalyst Bu4NI.c Catalyst loading of 20 was 1.0 mol%.d 12 h.
1	H	H	100.0	98.2
2	CH3	H	99.7	99.7
3^b	CH2Cl	H	100.0	99.8
4^c	CH3	CH3	76.2	76.0
5^d	Ph	H	99.5	99.5
6^c	–CH2(CH2)2CH2–	62.3	62.1	99.3
7^b	CH3	H	2.3	2.3

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3.3 Bi–Fe bimetallic complexes for CO₂ fixation

In addition to the organobismuthcomplexes with 5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine framework, there are other organobismuth complexes that are catalytically active for CO₂ fixation. It was reported by Wójcik et al. that the reaction of Fe₂(CO)₉ with Bi(OSiMe₂^tBu)₃ would result in the formation of soluble [(CO)₄FeBi(OSiMe₂^tBu)]₂ (25) in moderate yield (Fig. 8). Complexes [(CO)₄FeBi(O^tBu)]_n (26) and [(CO)₃FeBi₃(O^tBu)₄(OCO(O^tBu))]₂ (27) were obtained when Bi(O^tBu)₃ was used as starting material (Fig. 9).⁷⁴ During the reaction of diiron nonacarbonyl with bismuth tert-butoxide, complex 27 was released upon the insertion of CO₂ into a Bi-O^tBu bond. The compounds were characterized by IR and ¹H NMR spectroscopy as well as thermo-gravimetric analysis. Additionally, the molecular structures of compounds 25 and 27 were elucidated by single crystal X-ray diffraction. The core structure of [(CO)₄FeBi(OSiMe₂^tBu)]₂ (25) is a four-membered Bi₂Fe₂ ring. As for 27, the compound is composed of two cores of tetrahedral FeBi₃ clusters that dimerise via the bridging of –OCO(O^tBu) ligands. Analysis of the TGA residues by powder X-ray diffraction revealed that compound 26 is the best precursor for the generation of multi-ferroic BiFeO₃ among the compounds studied, despite the detection of Bi₂₅FeO₃₉ as minor impurity.

Fig. 8 ORTEP diagram (50% probability level) of the molecular structure of [(CO)₄FeBi(OSiMe₂^tBu)]₂ (25), H atoms are omitted for clarity.⁷⁴ (The image reproduced from ref. 74 with permission from the Elsevier.)

Fig. 9 ORTEP diagram (50% probability level) of the molecular structure of [(CO)₃FeBi₃(O^tBu)₄(OCO(O^tBu))]₂ (27), H atoms are omitted for clarity.⁷⁴ (The image reproduced from ref. 74 with permission from the Elsevier.)

3.4 CO₂ insertion into Bi–C bond of an organobismuth complex

Just recently, Kindra et al. demonstrated yet another example of CO₂ insertion into Bi–C bonds of bismuth NCN pincer complex bearing an oxyaryl dianionic ligand [2,6-(Me₂NCH₂)₂C₆H₃]Bi(C₆H₂-^tBu₂O).⁷⁵ The reactivity of the unusual oxyaryl dianionic ligand, (C₆H₂^tBu₂-3,5-O-4)²⁻, in the Bi³⁺ NCN pincer complex Ar′Bi(C₆H₂^tBu₂-3,5-O-4) (28) [Ar′ = 2,6-(Me₂NCH₂)₂C₆H₃], was explored with substrates and electrophiles of small molecules. The first insertion reaction of CO₂ into Bi–C bond was observed over this oxyaryl dianionic ligand complex (Scheme 9). The reactions generate new dianions that have quinoidal character similar to that of the oxyaryl dianionic ligand in 28. The oxyarylcarboxy dianionic ligand in Ar′Bi[O₂C(C₆H₂^tBu₂-3-5-O-4)-κ²O,O′] (29) was identified by X-ray crystallography (Fig. 10).

Scheme 9 Insertion of CO₂ into the Bi–C bond of bismuth NCN pincer complex of oxyaryl dianionic ligand [2,6-(Me₂NCH₂)₂C₆H₃]Bi(C₆H₂-^tBu₂O).⁷⁵

Fig. 10 ORTEP representation of Ar′Bi[O₂C(C₆H₂^tBu₂-3-5-O-4)-κ₂O,O′] (29), from two different perspectives, with thermal ellipsoids drawn at t probability. Hydrogen atoms are omitted for clarity.⁷⁵ (The image reproduced from ref. 75 with permission from the ACS.)

3.5 Physical fixation of CO₂ with bismuth–organic frameworks

Cheetham's group⁷⁶ reported two other bismuth complexes with metal–organic frameworks. They are inorganic–organic bismuth [LiBi(2,6-pdc)₃(H₂O)]·2(dma) (30) and Li₅Bi(2,6-pdc)₄(H₂O)₂ (31) with four Li⁺ cations per formula (where dma = dimethyl ammonium cation, dmf = dimethylformamide, and 2,6-pdc = pyridine 2,6-carboxylate). They were synthesized as efficient absorbents for CO₂ absorption. The amount of CO₂ that can be taken up by compound 31 is up to 140 cm³ g⁻¹ (∼6 mmol g⁻¹).

4. Summary

Organoantimony(bismuth) complexes have been used as absorbents and catalysts for CO₂ fixation, giving organometallic carbonates and cyclic carbonates as products, respectively. The physical fixation of CO₂ by inorganic–organic bismuth complexes and CO₂ insertion into a Bi–C bond are new. Since 1979, efficient catalysts based on organoantimony (bismuth) have been developed, and most of them are electrophile–nucleophile or Lewis acid–Lewis base systems. The plausible catalytic mechanism involves the reaction of CO₂ with a organoantimony(bismuth) catalyst to form an organometallic carbonate. With the opening of epoxide by a base such as ⁿBu₄NI, the organometallic carbonate reacts with the epoxide to give the cyclic carbonate. The organoantimony and organobismuth complexes have bright prospects in the field of CO₂ fixation. Further work with organoantimony(bismuth) complexes as CO₂ absorbent or catalyst should be focused on the following aspects: (i) simple and cheap CO₂ absorbents whose CO₂ association or dissociation is fast and efficient under mild condition, should be developed; (ii) air-stable, thermo-stable, recyclable, high Lewis acidic organoantimony(bismuth) complexes catalyst is still highly desirable; (iii) highly efficient synthetic method for the useful chemicals from CO₂, such as polycyclic carbonate, dimethyl carbonate, and dimethyl ether is still not well established, especially in the industrial scale; (iv) the related catalytic mechanism for the generation of cyclic carbonates is unclear, lacking of direct evidence, such as NMR and X-ray evidence; and (v) the reactivity of the C–M bond with CO₂ or other small molecules will be another challenge.

Acknowledgements

This work was supported by the NSFC (Grant nos 21373003, 21003040), the NSF of Hunan Province (14JJ7027), the program for New Century Excellent Talents in Universities (NCET-10-0371), Science Research Foundation for Higher Universities of Hunan Province (10A092), Research-Based Learning and Innovation Experiment Project for Undergraduates of Hunan Province (2011-483), the Fundamental Research Funds for the Central Universities, and the PCSIRT (IRT1238). The authors thank Dr S. Shimada and Dr L.-B. Han of AIST (Japan), Prof. W.-Y. Wong of Hong Kong Baptist University, Prof. N. Kambe and Dr T. Iwasaki of Osaka University, Profs J. Otera and A. Orita of Okayama University of Science for helpful discussion. C.T. Au thanks the Hunan University for an adjunct professorship.

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