Investigation into the reactivity of 16-electron complexes Cp^#Co(S₂C₂B₁₀H₁₀) (Cp^# = Cp, Cp*) towards methyl diazoacetate and toluenesulphonyl azide

Abstract

A three-component reaction of the 16-electron half-sandwich complex Cp*Co(S₂C₂B₁₀H₁₀) (2) with methyl diazoacetate (MDA) and toluenesulphonyl azide (TsN₃) gave di-inserted products Cp*Co(S₂C₂B₁₀H₁₀)(C–CO₂Me)(CHCO₂Me)(NHTs) (3) and Cp*Co(S₂C₂B₁₀H₁₀)(CHCO₂Me)(CHCO₂Me)(N₃Ts) (4), where MDA and TsN₃ in a molar ratio of 2 : 1 were inserted into the Co–S bond to form a five-membered metallacyclic ring with different coordination modes. Its two-component reaction with MDA affording complexes Cp*Co(S₂C₂B₉H₉)(CH₂CO₂Me)(CHCO₂Me) (5), Cp*Co(S₂C₂B₁₀H₁₀)(CH₂CO₂) (6) and Cp*Co(S₂C₂B₉H₉) (CH₂CO₂) (7) proved that the abovementioned three-component reaction is not a simple stepwise reaction. For another 16-electron half-sandwich complex CpCo(S₂C₂B₁₀H₁₀) (1), two stepwise reaction routes were designed to achieve its incorporation with MDA and TsN₃. The reaction occurred via the alkylidene-bridged adduct CpCo(S₂C₂B₁₀H₁₀)(CHCO₂Me) (8) or the imido-bridged adduct CpCo(S₂C₂B₁₀H₁₀)(NTs) (9), and complex CpCo(S₂C₂B₁₀H₁₀)(CHCO₂Me)(NTs) (10) was isolated as the sole product in moderate yield. In each reaction route, one molecule of MDA and one molecule of TsN₃ insert into the two Co–S bonds of complex 1, respectively, with the loss of N₂. All the new complexes were characterized using NMR spectroscopy, mass spectrometry, IR spectroscopy, elemental analysis and X-ray structural analysis.

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Introduction

The interaction between metal complexes and organic small molecules has been studied extensively for the reason that it is the source for developing new catalytic reactions and catalysts.¹ Most metal-catalyzed reactions occur via direct or indirect interactions between the metal centre and the substrate. In particular, some of these catalytic reactions involve the key step of inserting a M–X (X = C, N, S, O, etc.) bond into unsaturated organic substrates, such as alkenes,² alkynes,³ allenes,⁴ diazo compounds⁵ and azides.⁶ On the other hand, the successful isolation and characterization of some unstable metal–ligand complexes provides evidence for the existence of some intermediates in metal-induced catalytic cycles and helps researchers to better understand the catalytic mechanism.⁷

As useful synthons, diazo compounds^5,8 and azides⁹ have been widely used in both organic and organometallic synthesis based on their various reactivities. For example, diazo compounds can be readily decomposed by transition-metal complexes to generate Fischer-type metal carbene intermediates, which can subsequently undergo various chemical transformations such as X–H (X = C, O, S, N, etc.) insertion,¹⁰ cyclopropanation¹¹ and ylide formation.¹² Azides can be easily transformed into amines, isocyanates and other functional molecules and have more recently received increasing interest as valuable and versatile reagents within the concept of “Click Chemistry”.^9b,9c,13 Furthermore, reactions of azides with transition metal complexes afford various products including metal–organo azide complexes,¹⁴ metal–imide complexes,¹⁵ tetraazabutadiene complexes,¹⁶ isocyanate derivatives^9e,15b and triazenide complexes.¹⁷

For over 10 years, we have been working continually to study the reaction chemistry of the unsaturated 16-electron half-sandwich complexes [CpM(E₂C₂B₁₀H₁₀)] (Cp = cyclopentadienyl; M = Co; E = S, Se), [Cp*M(E₂C₂B₁₀H₁₀)] (Cp* = pentamethylcyclopentadienyl; M = Co, Rh, Ir; E = S, Se) and [(p-cymene)M(S₂C₂B₁₀H₁₀)] (M = Ru, Os) with organic small molecules.¹⁸ In our previous work, we reported the reactivity of the 16-electron complex CpCo(S₂C₂B₁₀H₁₀) (1) with ethyl diazoacetate which afforded a series of products with Co–B bond formation that were not predicted.^18k Very recently, we have systematically investigated the reaction of 16-electron complex 1 with various azides.^18m On the basis of these results, we assume that the three-component reactions of such 16-electron species with diazo compounds and azides would lead to various chemical transformations. As part of our intensive studies in this field, herein we report our investigation of the three-component reactions of Cp^#Co(S₂C₂B₁₀H₁₀) (Cp^# = Cp, 1; Cp^# = Cp*, 2) with both methyl diazoacetate (MDA) and toluenesulphonyl azide (TsN₃) in one-pot.

Results and discussion

Screening of substrates

Our previous reports suggest that diazo compounds are very reactive towards the 16-electron half-sandwich complex 1,^18k while azides are relatively inert and often performs in harsh conditions such as at reflux temperature.^18m For the purpose of the three-component reaction of 16-electron species with both diazo compounds and azides, suitable substrates need to be carefully screened. Considering the steric effect of the Cp* ligand, which has been proved to decrease the reaction activity of its complexes towards alkynes,^18r the 16-electron complex Cp*CoS₂C₂B₁₀H₁₀ (2) may be a good candidate to achieve this goal. Furthermore, we reduce the disparity of reactivity between diazo compounds and azides by choosing a relatively inert diazo compound (methyl diazoacetate, MDA) and a relatively reactive azide (toluenesulphonyl azide, TsN₃) (Scheme 1).

Scheme 1 Substrates for investigation.

The three-component reaction of complex 2 with MDA and TsN₃

The three-component reaction has been achieved by mixing 16-electron complex 2, TsN₃ and MDA sequentially at ambient temperature, affording products 3 and 4 in the yields of 52% and 22%, respectively, as shown in Scheme 2. It is worthwhile to note that the order of adding TsN₃ and MDA is very important for the formation of products 3 and 4. TsN₃ should be added before MDA, otherwise many by-products derived from the reaction of complex 2 with MDA can be isolated. This is in agreement with the different reactivities of azides^18m,s and diazo compounds^18k towards 16-electron species.

Scheme 2 The three-component reaction of complex 2 with MDA and TsN₃ at ambient temperature.

Complexes 3 and 4 were characterized using elemental analysis, IR, NMR (¹H, ¹¹B, ¹³C) and mass spectrometry. The most representative signals in the ¹H NMR spectrum of complex 3 are those that can be attributed to Ts–NH and CO₂Me–CH groups, which appear as two doublets (J = 5.0 Hz) at δ = 4.39 and 5.79 ppm, respectively. For complex 4, the signals attributed to S–CH and N–CH groups also appear as two doublets (J = 11.0 Hz) at δ = 4.23 and 4.78 ppm, respectively. The ¹³C NMR spectra show the corresponding signals attributed to the N–C carbon atoms (δ = 56–59 ppm), and the ones assigned to S–C carbon atoms (δ = 71–78 ppm).

The absolute configurations of complexes 3 and 4 were further confirmed using X-ray crystallographic analysis. ORTEP representations of 3 and 4 with selected bond distances and angles are shown in Fig. 1 and 2, respectively. The molecular structure of 3 reveals that incorporation of one TsN₃ and two MDA ligands happen to construct a five-membered metallacyclic ring Co–O–C–C–S. In this process, both the TsN₃ and the two MDA ligands lose N₂ and then couple together to form a new “SSO” tridentate ligand. One feature that should be mentioned is that the coupling process makes the complex 3 unsymmetrical and the sp³ centre (C16) becomes a chiral centre, though complex 3 crystallizes in the achiral space group P2(1)/n as the R enantiomer. The distance of O(1)–C(14) (1.282(3) Å) is between that expected for carbon–oxygen single and double bonds. The distance of C(14)–C(15) (1.384(4) Å), which is derived from the same MDA ligand, is also shorter than the usual C–C bond but longer than the normal C=C bond. This fact indicates that O1–C14–C15 is a conjugated ⁴₃π system, similar to the cases we previously reported.^18m The distances of C(15)–C(16) (1.508(3) Å) and C(16)–N(1) (1.454(3) Å) are slightly shorter than the normal C–C and C–N single bonds, respectively. Different from complex 3, the azido group of TsN₃ still exists in complex 4, together with two molecules of MDA losing N₂ to construct the other new “SSN” tridentate ligand, which features two chiral carbon atoms (C13 and C16). It is interesting to note that only one of the two possible diastereoisomers is formed. The average distance of N–N bonds of the –N₃ group (1.298(3) Å) is slightly longer than the normal one. All other bond distances are in the expected range.

Fig. 1 Molecular structure of complex 3. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.2336(11), Co(1)–S(2) = 2.2454(9), Co(1)–O(1) = 1.969(2), S(1)–C(1) = 1.776(3), S(2)–C(2) = 1.824(3), S(2)–C(15) = 1.713(3), O(1)–C(14) = 1.282(3), C(1)–C(2) = 1.659(4), C(14)–C(15) = 1.384(4), C(15)–C(16) = 1.508(3), C(16)–N(1) = 1.454(3); S(1)–Co(1)–S(2) = 95.19(3), S(1)–Co(1)–O(1) = 94.52(6), S(2)–Co(1)–O(1) = 85.09(6), Co(1)–S(1)–C(1) = 105.71(11), Co(1)–S(2)–C(2) = 104.59(9), Co(1)–S(2)–C(15) = 96.84(10), C(2)–S(2)–C(15) = 107.02(13), Co(1)–O(1)–C(14) = 115.02(18), S(1)–C(1)–C(2) = 117.9(2), S(2)–C(2)–C(1) = 116.61(19), O(1)–C(14)–C(15) = 124.2(2), S(2)–C(15)–C(14) = 114.37(19).

Fig. 2 Molecular structure of complex 4. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.2640(8), Co(1)–S(2) = 2.2090(8), Co(1)–N(3) = 1.937(2) S(1)–C(1) = 1.770(3), S(2)–C(2) = 1.800(3), S(2)–C(13) = 1.845(3), N(1)–N(2) = 1.298(3), N(2)–N(3) = 1.288(3), N(3)–C(16) = 1.478(3), C(1)–C(2) = 1.685(4), C(13)–C(16) = 1.520(4); S(1)–Co(1)–S(2) = 93.99(3), S(1)–Co(1)–N(3) = 93.10(6), S(1)–Co(1)–C(3) = 127.83(7), S(2)–Co(1)–N(3) = 84.86(6), Co(1)–S(1)–C(1) = 104.60(9), Co(1)–S(2)–C(2) = 107.28(10), Co(1)–S(2)–C(13) = 101.71(9), C(2)–S(2)–C(13) = 103.37(13), S(3)–N(1)–N(2) = 110.09(17), N(1)–N(2)–N(3) = 114.0(2), Co(1)–N(3)–N(2) = 129.51(17), Co(1)–N(3)–C(16) = 118.63(16), N(2)–N(3)–C(16) = 111.8(2), S(1)–C(1)–C(2) = 117.83(18), S(2)–C(2)–C(1) = 114.44(18), S(2)–C(13)–C(16) = 110.72(18), N(3)–C(16)–C(13) = 108.4(2).

The two-component reaction of complex 2 with MDA

To gain insight into the mechanism of the formation of complexes 3 and 4, a two-component reaction of complex 2 with MDA was also investigated. The results showed that the above three-component reaction is a concerted reaction rather than a simple stepwise one. As shown in Scheme 3, the 16-electron complex 2 reacting with MDA led to the formation of complexes 5–7 in the yield of 9%, 53% and 7%, respectively. It has to be noted that complexes 5 and 7 are obtained in low yields. This is due to the generation of a mixture of complexes in the reaction, and so the reaction products were purified by column chromatography, with the concomitant purification loss. It is totally different from the analogous reaction of complex 1 with ethyl diazoacetate (EDA), which gives interesting products featuring stable Co–B bonds or selective B–H activation at the carborane.^18k In this new case, neither B–H activation nor Co–B bond formation happened. Such a striking difference can possibly be attributed to the steric effect of the Cp* ligand in complex 2.

Scheme 3 The two-component reaction of complex 2 with MDA at ambient temperature.

Complexes 5–7 have been fully characterized using IR, NMR, mass spectrometry and elemental analysis. The most significant doublets shown in the ¹H NMR spectra of these complexes are due to the diastereotopic protons at the methylene group of the –SCH₂ branch with chemical shifts in the range of 3.0–4.5 ppm (J = 15.0–17.5 Hz). Also for complexes 5 and 7, other signals shown in the ¹H NMR spectra representative of the complexes are those attributed to the B–H–B hydride at the open face of the nido-C₂B₉ cluster, which appear as singlets at δ = −2.20 and −2.96 ppm, respectively. These observations agree well with their solid state structures.

The molecular structures of complexes 5 and 7 (Fig. 3 and 5) confirm that one of the apex BH close to the two carbon atoms of o-carborane has been lost, resulting in a nido-C₂B₉ cage instead of a closo-C₂B₁₀ unit. This phenomenon has been observed in many previous reports and the basic reaction environment or alcoholic solvent would be responsible for the loss of an apex BH in the o-carborane cage.¹⁹ In our case, the purification process by chromatography using silica may be the reason for such a transformation. However, in complex 6, such a transformation did not happen (Fig. 4). In fact, the reaction of complex 6 with silica for several hours at room temperature led to the formation of complex 7 in good yield. Interestingly, both complexes 6 and 7 represent cases where demethylation of the ester branch has occurred, leading to the formation of a carboxylate to coordinate the cobalt centre. A reasonable explanation for this may be that there was a very small amount of water present in the purification or work-up process, similar to what was recently described by Peris and co-workers for Rh^III complexes.²⁰

Fig. 3 Molecular structure of complex 5. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.2093(7), Co(1)–S(2) = 2.1757(8), Co(1)–C(16) = 1.976(3), C(16)–S(2) = 1.765(3), C(16)–C(17) = 1.468(4), C(2)–S(2) = 1.790(3), C(1)–C(2) = 1.552(4), C(1)–S(1) = 1.797(3), C(13)–S(1) = 1.814(3), C(13)–C(14) = 1.567(5); S(1)–Co(1)–S(2) = 93.37(3), C(16)–Co(1)–S(2) = 50.04(9), Co(1)–S(2)–C(16) = 59.09(9), Co(1)–S(2)–C(2) = 106.44(9), C(1)–C(2)–S(2) = 117.4(2), C(2)–C(1)–S(1) = 116.97(19), Co(1)–S(1)–C(1) = 105.52(9), Co(1)–C(16)–C(17) = 125.70(19), S(2)–C(16)–C(17) = 125.0(2), Co(1)–S(1)–C(13) = 113.23(10), C(1)–S(1)–C(13) = 102.44(13), S(1)–C(13)–C(14) = 108.39(2).

Fig. 4 Molecular structure of complex 6. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.248(2), Co(1)–S(2) = 2.234(2), Co(1)–O(1) = 1.960(4), S(1)–C(1) = 1.775(7), S(2)–C(2) = 1.786(7), S(2)–C(13) = 1.805(5), O(1)–C(14) = 1.278(8), O(2)–C(14) = 1.230(7), C(1)–C(2) = 1.658(9), C(13)–C(14) = 1.512(9); S(1)–Co(1)–S(2) = 93.46(7), Co(1)–S(1)–C(1) = 105.81(19), S(1)–Co(1)–O(1) = 94.99(12), Co(1)–S(2)–C(2) = 106.3(2), Co(1)–S(2)–C(13) = 95.5(2), C(2)–S(2)–C(13) = 103.1(3), Co(1)–O(1)–C(14) = 121.1(4), S(1)–C(1)–C(2) = 117.4(4), S(2)–Co(1)–O(1) = 86.78(13), S(2)–C(13)–C(14) = 113.0(4), O(1)–C(14)–O(2) = 124.5(6), O(1)–C(14)–C(13) = 117.4(5), O(2)–C(14)–C(13) = 118.1(6).

Fig. 5 Molecular structure of complex 7. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.2363(13), Co(1)–S(2) = 2.2530(14), Co(1)–O(1) = 1.901(4), S(1)–C(1) = 1.774(5), S(1)–C(13) = 1.809(5), S(2)–C(2) = 1.761(5), O(1)–C(14) = 1.316(6), O(2)–C(14) = 1.237(6), C(1)–C(2) = 1.591(7), C(13)–C(14) = 1.581(7); S(1)–Co(1)–S(2) = 92.21(5), S(1)–Co(1)–O(1) = 88.44(12), S(2)–Co(1)–O(1) = 90.65(13), Co(1)–S(1)–C(1) = 105.61(15), Co(1)–S(1)–C(13) = 99.24(17), C(1)–S(1)–C(13) = 103.0(2), Co(1)–S(2)–C(2) = 106.48(16), Co(1)–O(1)–C(14) = 121.4(3), S(1)–C(1)–C(2) = 118.6(3), S(2)–C(2)–C(1) = 116.8(3), S(1)–C(13)–C(14) = 109.6(3), O(1)–C(14)–C(13) = 119.5(4).

The stepwise reaction of complex 1 with MDA and TsN₃

As we discussed above, the 16-electron complex 1 was shown to be so highly reactive with diazo compounds that the three-component reaction of 1 with MDA and TsN₃ could not be achieved. Bearing an alternative approach in mind, the stepwise reaction of 1 with MDA and TsN₃ would aim to incorporate the three moieties into the final product.

As shown in Scheme 4, there are two parallel routes to synthesize the target three-component adduct 10. One is derived from the alkylidene-bridged adduct 8, which was prepared according to our recent report.^18k But in this new case, to get complex 8 in high yield and to avoid further reaction with excess MDA, the reaction conditions were optimized as a 1 : 1 molar ratio of raw materials and 5 minutes of reaction time. Another route involves an imido-bridged adduct 9, whose formation and characterization has been reported in our previous study.^18m Further reaction of complex 8 with TsN₃ at 90 °C or complex 9 with MDA at room temperature led to the formation of product 10 in moderate yield. While previous studies indicated that the alkylidene-bridged adduct and the imido-bridged adduct show comparative reactivity towards small organic molecules,^18k,m the different reactivity presented here may be due to the nature of the TsN₃ and MDA ligands.

Scheme 4 Two routes for stepwise reactions of complex 1 with MDA and TsN₃.

Good analytical data were obtained for complexes 8–10, and were further supported by the X-ray crystal structure determination (complex 9 was reported elsewhere^18m). The ORTEP plots and selected bond parameters of complexes 8 and 10 are shown in Fig. 6 and 7, respectively. Obviously, similarly to other alkylidene-bridged and imido-bridged complexes,^18k,m,21 MDA inserts into one Co–S bond of complex 1 with the loss of N₂ to form a three-membered metallacyclic ring Co1–S2–C8. And in complex 10, one TsN₃ and one MDA insert into the two Co–S bonds of complex 1, respectively, to generate metallacyclic rings Co1–S1–C8 and Co1–S1–C1–C2–S2–N1. It is well-known that two geometrical isomers of alkylidene-adducts exist based on the position of R in the –CHR group with respect to the iridadithiolene ring,²² namely the endo and exo forms. In our cases, both complexes 8 and 10 are assigned to the exo form according to their ¹H NMR spectra. To our surprise, the second insertion reaction did not happen to either the Co–C bond in complex 8 or the Co–N bond in complex 9, which are proven active sites for further insertion.^18k,m We speculate that the high stability of complex 10 and the nature of the carbene and nitrene derived from MDA and TsN₃ are responsible for such an insertion mode.

Fig. 6 Molecular structure of complex 8. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.265(10), Co(1)–S(2) = 2.174(10), Co(1)–C(8) = 1.970(3), C(8)–S(2) = 1.775(3), C(2)–S(2) = 1.803(3), C(1)–C(2) = 1.653(4), C(1)–S(1) = 1.771(3), C(8)–C(9) = 1.472(5); S(1)–Co(1)–S(2) = 95.15(3), C(8)–Co(1)–S(2) = 50.44(10), Co(1)–S(2)–C(8) = 58.84(11), Co(1)–S(2)–C(2) = 105.36(10), C(1)–C(2)–S(2) = 116.1(2), C(2)–C(1)–S(1) = 117.72(18), Co(1)–S(1)–C(1) = 102.84(10).

Fig. 7 Molecular structure of complex 10. The thermal ellipsoids are depicted at 30% probability. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Co(1)–S(1) = 2.1950(7), Co(1)–N(1) = 1.9661(18), Co(1)–C(8) = 1.970(2), S(1)–C(1) = 1.803(2), S(1)–C(8) = 1.768(2), S(2)–N(1) = 1.6923(18), S(2)–C(2) = 1.787(2), S(3)–N(1) = 1.6324(17), C(1)–C(2) = 1.690(3); S(1)–Co(1)–N(1) = 94.20(5), N(1)–Co(1)–C(8) = 89.89(8), S(1)–Co(1)–C(8) = 49.87(6), Co(1)–S(1)–C(1) = 116.42(7), Co(1)–S(1)–C(8) = 58.44(8), C(1)–S(1)–C(8) = 109.50(10), N(1)–S(2)–C(2) = 100.89(10), S(1)–C(1)–C(2) = 121.77(15), S(2)–C(2)–C(1) = 118.49(15), Co(1)–C(8)–S(1) = 71.69(9), Co(1)–N(1)–S(2) = 112.81(9), Co(1)–N(1)–S(3) = 122.20(10), S(2)–N(1)–S(3) = 113.60(10).

Mechanistic implications

According to our recent report,^18k the alkylidene-bridged adduct IT may act as a key intermediate in these serial transformations. Due to the steric effect of the Cp* ligand, the intermediate IT is not stable even at a low temperature. The same phenomenon happened in the imido-bridged adduct, which features nucleophilic addition with H₂O to break the Co–N bond.^18s,23 Any attempt to isolate the intermediate IT failed, but the evidence for its formation could be captured using an in situ proton NMR experiment. As shown in Fig. 8, mixing a 1,2-dichloroethane solution of MDA with complex 2 in the molar ratio of 1 : 1 in deuterated chloroform for 10 minutes presented three new proton signals with a ratio of 15 : 3 : 1 at δ = 1.57, 3.80 and 4.10 ppm (δ = 3.73 ppm is assigned to 1,2-dichloroethane), which fit the chemical formula of IT well. Furthermore, similar to other analogous alkylidene-bridged adducts featuring the characteristic proton NMR peak of a methenyl group,^18k,24 the peak at δ = 4.10 ppm appeared while the peak at δ = 4.75 ppm attributed to the –CHN₂ group of MDA almost disappeared after conducting the reaction for 10 minutes (Fig. 9). Based on the formation of the intermediate IT, the formation of complexes 3–7 was illustrated in Scheme 5. For complexes 3–5, the insertion of a ligand (MDA or both MDA and TsN₃) together with a coupling reaction happened in these transformations. The presence of silica or water resulted in the loss of a BH vertex at the carborane and the hydrolysis of an ester branch, which was responsible for the formation of complexes 5–7.

Fig. 8 ¹H NMR spectra of IT.

Fig. 9 A comparison of characteristic peaks of MDA and IT.

Scheme 5 The proposed mechanism for the formation of complexes 3–7.

Conclusions

In summary, a three-component reaction of a 16-electron half-sandwich complex towards a diazo compound and an azide could be achieved by screening for suitable reactants. That is, complex 2 reacting with MDA and TsN₃, affording two novel products 3 and 4 with incorporation of these three components. For the mechanistic studies, a two-component reaction of complex 2 and MDA have been also investigated, which gave the formation of complexes 5–7. The significant disparities between these two reactions proved that the abovementioned three-component reaction is not a simple stepwise one. However, for the analogous complex 1, a three-component reaction could only be conducted step by step rather than in one-pot. Product 10 di-inserted with the MDA and TsN₃ moieties could be derived from the alkylidene-bridged adduct 8 or the imido-bridged adduct 9. These diverse transformations presented in this work may be helpful for the design of catalysts in carbene and nitrene chemistry.

Experimental

General procedures

CpCoS₂C₂B₁₀H₁₀,²⁵ Cp*CoS₂C₂B₁₀H₁₀,²⁵ methyl diazoacetate²⁶ and toluenesulphonyl azide²⁷ were prepared according to the literature procedures. Note: methyl diazoacetate is so reactive that it is stored as a 1,2-dichloroethane solution at a low temperature (2.76 M). All reactions were carried out under argon using standard Schlenk techniques. All solvents were dried and deoxygenated prior to use. Diethyl ether, tetrahydrofuran, and petroleum ether were refluxed and distilled over sodium/benzophenone under nitrogen. CH₂Cl₂ was distilled over CaH₂ under nitrogen. The NMR measurements were performed on a Bruker DRX 500 spectrometer. The chemical shifts were given with respect to CHCl₃/CDCl₃ (δ ¹H = 7.26 ppm; δ ¹³C = 77.0 ppm) and external Et₂O·BF₃ (δ ¹¹B = 0 ppm). The IR spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr pellets in the region of 4000–400 cm⁻¹. The C, H and N microanalyses were carried out with an Elementar Vario EL III elemental analyzer. The mass data were determined using the LCQ (ESI-MS, Thermo Finnigan) mass spectrometer.

Synthesis of complexes 3 and 4

The solution of 2 (120.0 mg, 0.30 mmol) in DCM (20 mL) was added to toluenesulphonyl azide (88.7 mg, 0.45 mmol) and methyl diazoacetate (2.76 M in a solution of 1,2-dichloroethane, 41.4 mg in 0.15 mL). The colour changed from dark yellow to blue quickly after mixing, then the colour slowly changed to dark green. The mixture was stirred for 5 hours at ambient temperature. After removal of all solvent under vacuum, the residue was chromatographically separated on silica, and elution with petroleum ether–DCM gave 3 (130.0 mg, 52%) and 4 (55.0 mg, 22%).

3: green solid. Mp = 153 °C (dec.). ¹H NMR (CDCl₃): δ 1.39 (s, 15H, Cp*), 2.45 (s, 3H, Ph–CH₃), 3.59 (s, 3H, O–CH₃), 3.70 (s, 3H, O–CH₃), 4.39 (d, 1H, J = 5.0 Hz, N–H), 5.79 (d, 1H, J = 5.0 Hz, CH–CO₂Me), 7.35 (d, 2H, J = 7.0 Hz, Ph), 7.88 (d, 2H, J = 7.5 Hz, Ph), ¹¹B NMR (CDCl₃): δ −6.57 (3B), −5.31 (3B), −2.88 (4B), ¹³C NMR (CDCl₃): δ 9.39 (Cp*–CH₃), 21.57 (Ph–CH₃), 52.60 (O–CH₃), 53.70 (O–CH₃), 58.11 (N–C), 71.44 (S–C=), 94.99 (Cp*), 95.17 (carborane), 103.46 (carborane), 136.19 (Ph), 143.78 (2 × Ph), 127.39 (Ph), 129.78 (2 × Ph), 170.77 (O=C), 175.28 (O=C). ESI-MS (70 eV): m/z 736.17 ([M + MeOH]⁺, 95%). IR (KBr): ν (cm⁻¹) 1744 (C=O), 2582 (B–H). Anal. calcd (%) for C₂₅H₄₀B₁₀CoNO₆S₃: C, 42.06; H, 5.65; N, 1.96. Found: C, 42.16; H, 5.92; N, 2.12.

4: green solid. Mp = 168 °C (dec.). ¹H NMR (CDCl₃): δ 1.54 (s, 15H, Cp*), 2.39 (s, 3H, Ph–CH₃), 3.84 (s, 3H, O–CH₃), 3.94 (s, 3H, O–CH₃), 4.23 (d, 1H, J = 11.0 Hz, S–CH), 4.78 (d, 1H, J = 11.0 Hz, N–CH), 7.22 (d, 2H, J = 8.0 Hz, Ph), 7.64 (d, 2H, J = 8.5 Hz, Ph), ¹¹B NMR (CDCl₃): δ −8.96 (4B), −7.06 (3B), −5.18 (2B), −1.18 (1B), ¹³C NMR (CDCl₃): δ 10.12 (Cp*–CH₃), 21.50 (Ph–CH₃), 53.53 (O–CH₃), 54.35 (O–CH₃), 56.53 (N–C), 77.79 (S–C), 92.94 (carborane), 98.49 (Cp*), 127.19 (2 × Ph), 128.94 (2 × Ph), 138.50 (Ph), 142.00 (Ph), 166.30 (O=C), 167.92 (O=C). ESI-MS (70 eV): m/z 731.33 (M⁺, 95%). IR (KBr): ν (cm⁻¹) 1745 (C=O), 2588 (B–H). Anal. calcd (%) for C₂₅H₄₀B₁₀CoN₃O₆S₃: C, 40.48; H, 5.43; N, 5.66. Found: C, 40.33; H, 5.59; N, 5.39.

Synthesis of complexes 5–7

The solution of 2 (120.0 mg, 0.30 mmol) in DCM (20 mL) was added to methyl diazoacetate (2.76 M in a solution of 1,2-dichloroethane, 41.4 mg in 0.15 mL). The colour changed from dark yellow to blue quickly after mixing, then the colour slowly changed to dark green. The mixture was stirred for 10 hours at ambient temperature. After removal of all solvent under vacuum, the residue was chromatographically separated on silica, and elution with petroleum ether–DCM gave 5 (15.2 mg, 9%), 6 (85.5 mg, 53%) and 7 (8.1 mg, 5%).

5. Red solid. Mp = 169 °C (dec.). ¹H NMR (CDCl₃): δ 1.66 (s, 15H, Cp*), 3.00 (d, 1H, J = 16.0 Hz, S–CH₂), 3.36 (s, 1H, S–CH), 3.81 (s, 3H, O–CH₃), 3.83 (s, 3H, O–CH₃), 4.12 (d, 1H, J = 16.0 Hz, S–CH₂), −2.20 (s, 1H, B–H–B), ¹¹B NMR (CDCl₃): δ −31.09 (2B), −26.40 (2B), −14.42 (1B), −12.30 (2B), −5.97 (2B), ESI-MS (70 eV): m/z 558.25 ([M + Na]⁺, 25%). IR (KBr): ν (cm⁻¹) 1631 (C=O), 2547 (B–H). Anal. calcd (%) for C₁₈H₃₄B₉CoO₄S₂: C, 40.42; H, 6.41. Found: C, 40.30; H, 6.50.

6. Green solid. Mp = 186 °C (dec.). ¹H NMR (CDCl₃): δ 1.45 (s, 15H, Cp*), 3.70 (d, 1H, J = 17.5 Hz, S–CH₂), 3.82 (d, 1H, J = 17.5 Hz, S–CH₂), ¹¹B NMR (CDCl₃): δ −8.44 (1B), −6.80 (2B), −5.78 (1B), −3.60 (4B), −2.22 (2B), ¹³C NMR (CDCl₃): δ 9.61 (Cp*–CH₃), 29.28 (S–CH₂), 43.51 (S–CH₂), 90.87 (carborane), 95.77 (Cp*), 97.32 (carborane), 174.21 (O=C), ESI-MS (70 eV): m/z 490.75 ([M + MeOH]⁺, 15%). IR (KBr): ν (cm⁻¹) 1639 (C=O), 2574 (B–H). Anal. calcd (%) for C₁₄H₂₇B₁₀CoO₂S₂·H₂O: C, 35.28; H, 6.13. Found: C, 35.50; H, 6.19.

7. Red solid. Mp = 189 °C (dec.). ¹H NMR (CDCl₃): δ 1.56 (s, 15H, Cp*), 3.50 (d, 1H, J = 15.0 Hz, S–CH₂), 4.47 (d, 1H, J = 15.0 Hz, S–CH₂), −2.96 (s, 1H, B–H–B), ¹¹B NMR (CDCl₃): δ −31.05 (1B), −25.46 (1B), −14.20 (2B), −12.06 (3B), −6.72 (2B), ESI-MS (70 eV): m/z 444.92 ([M]⁻, 40%). IR (KBr): ν (cm⁻¹) 1645 (C=O), 2545 (B–H). Anal. calcd (%) for C₁₄H₂₇B₉CoO₂S₂: C, 37.56; H, 6.08. Found: C, 37.70; H, 6.25.

Synthesis of complex 8

The solution of 1 (99.0 mg, 0.30 mmol) in DCM (20 mL) was added to methyl diazoacetate (2.76 M in a solution of 1,2-dichloroethane, 27.6 mg in 0.1 mL) in a molar ratio of about 1 : 1. The colour changed from red to blue rapidly after mixing, and the mixture was stirred for 5 minutes at ambient temperature. After removal of all solvent under vacuum, the residue was chromatographically separated on silica, and elution with petroleum ether–DCM gave 8 (88.6 mg, 69%).

8. Blue solid. Mp = 158 °C. ¹H NMR (CDCl₃): δ 3.82 (s, 3H, O–CH₃), 4.37 (s, 1H, S–CH), 5.02 (s, 5H, Cp), ¹¹B NMR (CDCl₃): δ −7.93 (2B), −6.55 (2B), −4.89 (1B), −2.20 (3B), −1.17 (2B), ¹³C NMR (CDCl₃): δ 45.03 (S–CH), 52.80 (O–CH₃), 85.01 (Cp), 88.49 (carborane), 92.81 (carborane), 175.19 (O=C). ESI-MS (70 eV): m/z 392.33 ([M]⁻, 60%). IR (KBr): ν (cm⁻¹) 1697(C=O), 2596 (B–H). Anal. calcd (%) for C₁₀H₁₉B₁₀CoO₂S₂: C, 29.85; H, 4.76. Found: C, 29.99; H, 4.95.

Synthesis of complex 10

Method 1. Toluenesulphonyl azide (59.1 mg, 0.3 mmol) was added to the blue solution of 8 (80.6 mg, 0.2 mmol) in toluene (20 mL). The mixture was heated at 90 °C for 6 h. The colour gradually turned from blue to purple. After removal of the solvent, the residue was chromatographically separated on silica, and elution with petroleum ether–DCM gave 10 (67 mg, 57%).

Method 2. Complex 9 was prepared according to our recent work.^18m The solution of 9 (105.4 mg, 0.2 mmol) in DCM (20 mL) was added to methyl diazoacetate (2.76 M in solution of 1,2-dichloroethane, 41.4 mg in 0.15 mL). The mixture was stirred for 6 h at ambient temperature. The colour gradually turned from green to purple. After removal of the solvent, the residue was chromatographically separated on silica, and elution with petroleum ether–DCM gave 10 (69 mg, 59%).

10: Purple solid. Mp = 117 °C. ¹H NMR (CDCl₃): δ 2.43 (s, 3H, Ph–CH₃), 3.91 (s, 3H, O–CH₃), 4.05 (s, 1H, S–CH), 5.14 (s, 5H, Cp), 7.29 (d, 2H, J = 8.0 Hz, Ph), 7.76 (d, 2H, J = 9.0 Hz, Ph), ¹¹B NMR (CDCl₃): δ −12.25 (1B), −10.27 (1B), −9.01 (2B), −7.84 (1B), −6.07 (1B), −2.96 (1B), 0.87 (1B), 2.81 (2B), ¹³C NMR (CDCl₃): δ 21.48 (Ph–CH₃), 43.27 (S–CH), 53.03 (O–CH₃), 57.38 (carborane), 85.17 (Cp), 86.06 (carborane), 127.84 (2 × Ph), 129.32 (2 × Ph), 137.69 (Ph), 143.01 (Ph), 175.59 (O=C). ESI-MS (70 eV): m/z 573 ([M + H]⁺, 60%), 595 ([M + Na]⁺, 100%). IR (KBr): ν (cm⁻¹) 1707 (C=O), 2579 (B–H). Anal. calcd (%) for C₁₇H₂₆B₁₀CoNO₄S₃: C, 35.72; H, 4.58; N, 2.45. Found: C, 35.88; H, 4.86; N, 2.59.

X-ray structure determinations

Crystals suitable for X-ray analysis were obtained by the slow evaporation of the solutions containing the compounds in petroleum ether–dichloromethane. The diffraction data were collected on a Bruker SMART Apex II CCD diffractometer by means of graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation. During the collection of the intensity data, no significant decay was observed. The intensities were corrected for Lorentz polarization effects and empirical absorption using the SADABS program.²⁸ The structures were solved using direct methods with the SHELXS-97 program²⁹ and were refined on F² using SHELXTL (version 6.14).³⁰ All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in calculated positions and were refined using a riding model. A summary of the crystal data, data collection parameters, and structure refinement details is given in Tables 1 and 2.†

Table 1 Crystallographic data for complexes 3–6

Compound	3	4	5	6
Chemical formula	C25H40B10CoNO6S3	C25H40B10CoN3O6S3	C18H34B9CoO4S2	C14H27B10CoO2S2·H2O
Formula weight	713.79	741.81	534.79	476.52
Crystal size (mm)	0.15 × 0.13 × 0.12	0.18 × 0.14 × 0.11	0.25 × 0.22 × 0.18	0.20 × 0.16 × 0.12
Temperature (K)	296(2)	296(2)	296(2)	296(2)
Radiation	Mo-Kα (0.71073 Å)	Mo-Kα (0.71073 Å)	Mo-Kα (0.71073 Å)	Mo-Kα (0.71073 Å)
Crystal system	Monoclinic	Orthorhombic	Triclinic	Triclinic
Space group	P2(1)/n	Pbca	P1̄	P1̄
a (Å)	14.9811(14)	15.4506(10)	9.4239(11)	7.504(4)
b (Å)	12.0288(11)	13.6578(9)	9.8236(11)	10.477(5)
c (Å)	19.4267(18)	34.644(2)	15.0407(17)	16.018(8)
α (°)	90.00	90.00	82.461(2)	103.281(8)
β (°)	97.9710(10)	90.00	89.994(2)	96.540(8)
γ (°)	90.00	90.00	73.199(2)	107.695(8)
V (Å^3)	3467.0(6)	7310.7(8)	1320.4(3)	1144.6(10)
Z	4	8	2	2
ρ(calc) (g cm^−3)	1.368	1.348	1.345	1.383
F (000)	1480	3072	556	492
Absorption coefficient (mm^−1)	0.715	0.683	0.832	0.947
θ range (deg)	1.61 to 25.50	1.18 to 26.00	2.19 to 26.00	2.12 to 25.00
Reflns collected	19970 (Rint = 0.0502)	41674 (Rint = 0.1417)	10038 (Rint = 0.0212)	6910 (Rint = 0.0528)
Indep. reflns	6407	7187	5163	3839
Refns obs. [I > 2σ(I)]	4341	3015	4429	2496
Data/restr./paras.	6407/1/423	7187/86/449	5163/0/357	3839/0/276
GOF	1.012	0.986	1.036	0.996
R1/wR2 [I > 2σ(I)]	0.0473/0.1117	0.0489/0.0736	0.0436/0.1328	0.0658/0.1515
R1/wR2 (all data)	0.0770/0.1216	0.1490/0.0820	0.0506/0.1385	0.1132/0.1654
Large peak and hole (e Å^−3)	0.382/−0.381	0.450/−0.371	0.810/−0.443	0.992/−0.553

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Table 2 Crystallographic data for complexes 7, 8 and 10

Compound	7	8	10
Chemical formula	C14H27B9CoO2S2	C10H19B10CoO2S2	C17H26B10CoNO4S3
Formula weight	447.70	402.40	571.60
Crystal size (mm)	0.28 × 0.24 × 0.22	0.30 × 0.23 × 0.18	0.27 × 0.24 × 0.20
Temperature (K)	291(2)	296(2)	296(2)
Radiation	Mo-Kα (0.71073 Å)	Mo-Kα (0.71073 Å)	Mo-Kα (0.71073 Å)
Crystal system	Orthorhombic	Monoclinic	Triclinic
Space group	P2(1)2(1)2(1)	P2(1)/c	P1̄
a (Å)	10.3728(8)	6.5496(8)	7.6763(6)
b (Å)	13.1268(10)	12.0999(15)	10.9331(8)
c (Å)	16.7553(11)	22.472(3)	16.1145(12)
α (°)	90.00	90.00	94.3150(10)
β (°)	90.00	94.648(2)	92.6730(10)
γ (°)	90.00	90.00	95.6580(10)
V (Å^3)	2281.4(3)	1775.1(4)	1340.03(17)
Z	4	4	2
ρ(calc) (g cm^−3)	1.303	1.506	1.417
F (000)	924	816	584
Absorption coefficient (mm^−1)	0.943	1.202	0.900
θ range (deg)	1.97 to 26.00	1.82 to 26.00	1.88 to 26.00
Reflns collected	10510 (Rint = 0.0492)	11610 (Rint = 0.0863)	7376 (Rint = 0.0398)
Indep. reflns	4482	3461	5174
Refns obs. [I > 2σ(I)]	3394	2561	4087
Data/restr./paras.	4482/0/258	3461/0/230	5174/0/367
GOF	1.085	0.945	1.038
R1/wR2 [I > 2σ(I)]	0.0564/0.1121	0.0511/0.1063	0.0355/0.0808
R1/wR2 (all data)	0.0686/0.1153	0.0657/0.1109	0.0447/0.0837
Large peak and hole (e Å^−3)	0.624/−0.381	0.832/−0.580	0.285/−0.234

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Acknowledgements

This work was supported by the National Basic Research Program of China (2013CB922101) and the Natural Science Foundation of China (21271102 and 21301071).

Notes and references

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Footnote

† CCDC 1009704–1009710. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra13017k

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