Aljoša
Bolje
a,
Stephan
Hohloch
b,
Janez
Košmrlj
*a and
Biprajit
Sarkar
*b
aFaculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia
bInstitut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany. E-mail: biprajit.sarkar@fu-berlin.de
First published on 7th July 2016
Pyridine-appended triazolylidene donors have been recently used as ligands in various homogeneous catalytic processes. We present here a new pyrimidine substituted triazolium salt which was prepared and used in the coordination to RuII and OsII. The new triazolium salt and the obtained complexes were characterized by multinuclear NMR spectroscopy. The molecular composition of the mentioned compounds was confirmed by positive ion electrospray ionization (ESI+) mass spectra. The new pyrimidyl containing complexes, as well as the related pyridyl-triazolylidene containing complexes, were applied in transfer hydrogenation reactions of carbonyls, alkenes, imines and nitroarenes. The pyrimidyl containing complexes reveal an over-all better activity in comparison to their pyridine bearing analogues. The studies of electronic effects of the ligands, as well as mechanistic insights for the reduction of nitrobenzene with three selected precatalysts are presented.
In the last two decades N-heterocyclic carbenes (NHCs) have been developed as a powerful class of ligands. Despite being used in nearly every field of modern chemical sciences, without a doubt NHCs have most dramatically affected organometallic chemistry and catalysis. Owing to their strong σ-donation ability, air and moisture stability, high structural variability, as well as low toxicity, they have become viable alternatives to widely used phosphines in many catalytic processes. Today, a vast number of transformations are known to proceed only with NHC-based catalysts.2 1,2,3-Triazol-5-ylidenes are a prominent subclass of NHCs possessing a mesoionic carbene (MIC) structure. Due to a lower heteroatom stabilization and mesoionic character, MICs are among one of the best σ-donors among the NHCs.3 The highly efficient and modular nature of click reaction that is generally used to access 1,2,3-triazol-5-ylidenes makes the design and preparation of diverse libraries of these ligands extremely easy.4 Complexes bearing triazolylidenes are increasingly used in homogeneous metal-based catalysis, as well as for photophysical and biochemical purposes.4d Many catalytic systems of this type have been studied, showing comparable or even better efficiency in comparison to the majority of their NHC analogues.5 Metal complexes of MIC ligands have been used as catalysts among others in olefin metathesis,6 oxidations,7 reductions,8 click reactions,9 and C–C/C–N bond formation reactions.10
Recently, three series of complexes consisting of pyridine appended triazolylidenes as ligands with ruthenium(II), iridium(III) and osmium(II) were reported and they were successfully used in transfer hydrogenation of selected carbonyl substrates (Scheme 1).11 All complexes have shown remarkable activity even at very low catalyst loadings with several different substrates investigated. The ruthenium(II) complexes were also used in oxidation of primary and secondary alcohols.7d
![]() | ||
Scheme 1 The RuII, OsII and IrIII pyridine-appended MIC complexes (up),11 and RuII and OsII pyrimidine-appended MIC complexes (bottom) used in this work. |
Encouraged by these results, we were prompted to test the activity of these complexes also for transfer hydrogenation of substrates bearing other functional groups. In particular, we were interested in different carbonyl, olefin, imine, and nitroarene substrates.
In addition to the pyridine-appended MICs, we decided to design novel MICs that are functionalized with a pyrimidine ring. The starting 1-(2-pyrimidyl)-functionalized triazolium salt was prepared in excellent yields and purity and used to construct the corresponding Ru(II) and Os(II) complexes (Scheme 1). The coordination was achieved by a transmetallation protocol and the corresponding complexes were characterized by 1H, 13C and 15N NMR spectroscopy and high resolution mass spectrometry. Unfortunately, the preparation of the Ir(III) analogue failed. The two pyrimidine-appended MIC complexes of Ru(II) and Os(II) and the previously reported7d,11 pyridine-appended MICs of Ru(II), Ir(III), and Os(II), shown in Scheme 1, were used in transfer hydrogenation of the above mentioned functionalities. This is also the first report on pyrimidine-appended triazolium salts, the corresponding MICs and their transition metal complexes. Some mechanistic insights into the reduction of nitrobenzene with the three selected complexes are presented.
Next, the coordination ability of triazolium salt 2 towards Ru(II), Ir(III) and Os(II) was examined. To this end, we chose the already established transmetalation protocol via the intermediate carbenic silver(I) complex, shown in Scheme 3.15
The triazolium salt 2 was stirred with Ag2O and in dry acetonitrile at room temperature for 4 days (Scheme 3). The formation of complex Ag-2 was suggested by the disappearance of the triazolium proton H-5 from the 1H NMR spectral analysis in DMSO-d6. Due to the general instability of such silver complexes, we were unable to obtain more experimental data for its structure. Subsequently, Ag-2 was subjected to transmetalation without previous isolation employing [Ru(p-cym)Cl2]2 as a ruthenium source (Scheme 3). The reaction progress was monitored by TLC analysis. The pure complex Rupyrim was isolated as a triflate salt after 2 hours by simple three-step filtration–solvent evaporation–crystallization workup in 85% yield (Table 1). For the preparation of Ospyrim, which involved transmetalation with the OsII precursor [Os(p-cym)Cl2]2 the reaction mixture had to be stirred for a prolonged time of 3 days. The complex Ospyrim was isolated in the pure form as a PF6 salt in a moderate yield of 53% (Table 1). Unfortunately, we were unable to prepare the IrIII analogue. After the addition of [IrCp*Cl2]2 to the reaction mixture of the triazolium salt 2 and Ag2O in acetonitrile, no pure product could be isolated. Instead a mixture of products was isolated. The use of a base such as Cs2CO3 or heating of the reaction mixture did not lead to any improvement. Even though we could identify the product in the mass spectrum of the crude reaction mixture (see the ESI‡), attempts to purify it led to its decomposition. Although it is not clear at this stage as to why the product decomposes at the purification stage, we think that it does not survive either the filtration through Celite, or the salt metathesis with KPF6 that is necessary for its purification.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: benzaldehyde (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. c Data collected from ref. 11. | |||||
Precatalyst loading: 0.5 mol% | |||||
Ru1![]() |
>99 |
Ir1![]() |
>99 |
Os1![]() |
>99 |
Ru2![]() |
>99 |
Ir2![]() |
>99 |
Os2![]() |
>99 |
Ru3![]() |
>99 |
Ir3![]() |
>99 |
Os3![]() |
>99 |
Rupyrim | >99 | Ospyrim | >99 | ||
Precatalyst loading: 0.01 mol% | |||||
Ru1![]() |
>99 |
Ir1![]() |
>99 |
Os1![]() |
>99 |
Ru2![]() |
>99 |
Ir2![]() |
>99 |
Os2![]() |
>99 |
Ru3![]() |
>99 |
Ir3![]() |
>99 |
Os3![]() |
>99 |
Rupyrim | >99 | Ospyrim | >99 |
The reduction of benzaldehyde under the applied reaction conditions proceeds smoothly with all tested complexes. Applying 0.5 mol% of the precatalysts, as well as with only 0.01 mol%, the reduction occurred with full conversion of the starting material, reflecting in a TON of about 10000. This is comparable to known ruthenium(II)–NHC complexes16 and in line with the activity of their pyridyl-MIC analogues.11 To investigate further the catalytic performance of the Rupyrim and Ospyrim, acetophenone was used as a substrate. Ketones in general are known to be more challenging substrates than aldehydes for the reduction. The results are outlined in Table 3.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: acetophenone (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. c Data collected from ref. 11. | |||||
Precatalyst loading: 0.5 mol% | |||||
Ru1![]() |
>99 |
Ir1![]() |
88 |
Os1![]() |
76 |
Ru2![]() |
>99 |
Ir2![]() |
71 |
Os2![]() |
88 |
Ru3![]() |
>99 |
Ir3![]() |
92 |
Os3![]() |
65 |
Rupyrim | >99 | Ospyrim | 70 | ||
Precatalyst loading: 0.01 mol% | |||||
Ru1![]() |
80 | ||||
Ru2![]() |
89 | ||||
Ru3![]() |
60 | ||||
Rupyrim | 94 |
Whereas the reduction of acetophenone with the ruthenium complex Rupyrim at the precatalyst loadings of 0.5 mol% gave full conversion of acetophenone to 1-phenylethanol, the osmium complex Ospyrim gave only 70% of the reduced substrate. By using 0.01 mol%, the pyrimidine analogue Rupyrim has shown the best activity amongst all tested precatalysts. With 0.5 mol% of the Ru2 precatalyst, we were able to isolate the product in 85% yield (conversion >99%). Prompted by very good overall results with acetophenone we decided to continue with a more sterically hindered substrate, benzophenone. The results are collected in Table 4.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: benzophenone (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. c Data collected from ref. 11. | |||||
Precatalyst loading: 0.5 mol% | |||||
Ru1![]() |
>99 |
Ir1![]() |
65 |
Os1![]() |
68 |
Ru2![]() |
>99 |
Ir2![]() |
55 |
Os2![]() |
88 |
Ru3![]() |
>99 |
Ir3![]() |
85 |
Os3![]() |
25 |
Rupyrim | >99 | Ospyrim | 72 | ||
Precatalyst loading: 0.1 mol% | |||||
Ru1![]() |
69 | ||||
Ru2![]() |
90 | ||||
Ru3![]() |
59 | ||||
Rupyrim | 93 |
Benzophenone was fully converted into diphenylmethanol with the Rupyrim precatalyst at 0.5 mol%. The same complex gave an excellent 93% conversion also at 0.1 mol% precatalyst loading, which is the best performance for all tested compounds. With 0.1 mol% of the Ru1 precatalyst, we were able to isolate the product in 60% yield (conversion 69%, Table 4). On the other hand Ospyrim gave only 72% conversion by employing 0.5 mol% of the precatalyst. This result is in line with other Os(II) complexes Os1–3. To finish, we conducted the reduction of cyclohexanone as a representative of an aliphatic substrate. The results are summarized in Table 5.
Precatalyst | Conv.c (%) | Precatalyst | Conv.c (%) | Precatalyst | Conv.c (%) |
---|---|---|---|---|---|
a General reaction conditions: cyclohexanone (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Precatalyst (0.05 mol%). c Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. d Data collected from ref. 11. | |||||
Precatalyst loading: 0.5 mol% | |||||
Ru1![]() |
>99 |
Ir1![]() |
86 |
Os1![]() |
>99 |
Ru2![]() |
>99 |
Ir2![]() |
75 |
Os2![]() |
>99 |
Ru3![]() |
>99 |
Ir3![]() |
92 |
Os3![]() |
91 |
Rupyrim | >99 | Ospyrim | >99 | ||
Precatalyst loading: 0.1 mol% | |||||
Ru1![]() |
79 |
Os1![]() |
62 | ||
Ru2![]() |
94 |
Os2![]() |
76 | ||
Ru3![]() |
72 |
Os3![]() |
50 | ||
Rupyrim | >99, 78b | Ospyrim | 94 |
The two pyrimidyl-MIC complexes Rupyrim and Ospyrim have shown the best activity for the transfer hydrogenation of cyclohexanone into cyclohexanol. Employing either 0.5 mol% or 0.1 mol% of the precatalyst resulted in an excellent overall activity. The Rupyrim gave a very good 78% conversion also by using only 0.05 mol% of the precatalyst. Good results of the complexes Rupyrim and Ospyrim in the transfer hydrogenation of the carbonyl group prompted us to test other functional groups. We started with olefin substrates: cyclooctene, methylstyrene and trans-stilbene. The results for the reduction of cyclooctene are presented in Table 6.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: cyclooctene (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 24 h. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. | |||||
Precatalyst loading: 1.0 mol% | |||||
Ru1 | >99 | Ir1 | 73 | Os1 | 84 |
Ru2 | >99 | Ir2 | 60 | Os2 | 93 |
Ru3 | >99 | Ir3 | 80 | Os3 | 59 |
Rupyrim | >99 | Ospyrim | 89 | ||
Precatalyst loading: 0.5 mol% | |||||
Ru1 | 79 | ||||
Ru2 | 94 | ||||
Ru3 | 72 | ||||
Rupyrim | 87 |
For the reduction of cyclooctene, we used similar reaction conditions as mentioned above. A mixture of cyclooctene, the selected precatalyst and KOH in iPrOH was stirred under an inert atmosphere at 100 °C for 24 h. By using 1.0 mol% of ruthenium-based precatalysts we observed full conversions into cyclooctane with all four tested complexes. Also with the iridium and osmium analogues, the conversions were very good. Scaling down to 0.5 mol% of Ru(II) complexes, we could still reach excellent conversions, better than those described for already known similar systems.17 The best performance was seen with Ru2, bearing the 4-methoxyphenyl donor group at the triazole, which is in line with the results for the reduction of carbonyls. Excellent performance in the reduction of cyclooctene encouraged us to try other such substrates. We took trans-β-methylstyrene and applied the same reaction conditions as for cyclooctene. The results are summarized in Table 7.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: trans-β-methylstyrene (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 24 h. b Conversion determined by 1H NMR spectroscopy using 4-bromobenzaldehyde as an internal standard. | |||||
Precatalyst loading: 1.0 mol% | |||||
Ru1 | 65 | Ir1 | 62 | Os1 | 8 |
Ru2 | 75 | Ir2 | 58 | Os2 | 6 |
Ru3 | 56 | Ir3 | 67 | Os3 | 4 |
Rupyrim | 78 | Ospyrim | 0 | ||
Precatalyst loading: 0.5 mol% | |||||
Ru1 | 45 | ||||
Ir1 | 25 | ||||
Os1 | 4 |
In comparison to cyclooctene, trans-β-methylstyrene proved to be a more challenging substrate. With the exception of the Os(II) representatives, the complexes still performed very well. Nonetheless, our catalysts have shown similar or better performance in this reduction than the previously reported examples.17 The ligand effect trends were similar to the previous observations. For the Ru(II) complexes the electron-donating substituents at the triazole ring worked better, whereas the Ir(III) complexes preferred electron withdrawing groups.11 Furthermore, we investigated the transfer hydrogenation of trans-stilbene, a sterically more hindered alkene. The results from Table 8 confirm the reduction of trans-stilbene into 1,2-diphenylethane as more demanding for our catalytic system, which has been also reported for other such systems.17
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: trans-stilbene (0.5 mmol), precatalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 24 h. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. | |||||
Precatalyst loading: 1.0 mol% | |||||
Ru1 | 30 | Ir1 | 9 | Os1 | 15 |
Ru2 | 41 | Ir2 | 11 | Os2 | 16 |
Ru3 | 28 | Ir3 | 13 | Os3 | 4 |
Rupyrim | 13 | Ospyrim | 8 |
To continue we took an imine substrate and selected N-benzylideneaniline.18 We employed both K2CO3 as well as KOH as bases and stirred the reaction mixture for 18 h at 100 °C. Both bases delivered similar conversions. The results are collected in Table 9.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a General reaction conditions: N-benzylideneaniline (0.5 mmol), precatalyst, K2CO3 (0.025 mmol), iPrOH (4 mL), 100 °C for 18 h. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard. | |||||
Precatalyst loading: 1.0 mol% | |||||
Ru1 | >99 | Ir1 | 50 | Os1 | 19 |
Ru2 | >99 | Ir2 | 23 | Os2 | 18 |
Ru3 | >99 | Ir3 | 17 | Os3 | 17 |
Rupyrim | >99 | Ospyrim | 30 | ||
Precatalyst loading: 0.5 mol% | |||||
Ru1 | 71 | ||||
Ru2 | 87 | ||||
Ru3 | 60 | ||||
Rupyrim | 90 |
With 1.0 mol% precatalyst loading the ruthenium-based precatalysts converted the N-benzylideneaniline into N-benzylaniline fully. Also by using 0.5 mol% the same complexes gave an excellent overall conversion. The Rupyrim performed best, but also the Ru2 showed excellent activity, confirming the ligand effects for the Ru(II) complex series. On the other hand the Ir(III) and Os(II) precatalysts were less active for the reduction of this type of substrates. The activity of our complexes is comparable to the previously reported ruthenium catalyst in the transfer hydrogenation of N-benzylideneaniline.18 We also tested the reduction of cyclohexenone as an example of an α,β-unsaturated carbonyl. Whereas conversions were high for all tested precatalysts (Table 10), only Os2 displayed relatively high selectivity towards selective reduction of the CC bond over the C
O bond.
Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) | Precatalyst | Conv.b (%) |
---|---|---|---|---|---|
a Reaction conditions: cyclohexenone (0.5 mmol), catalyst (0.5 mol%), KOH (0.1 mmol), isopropanol (4 mL), 100 °C, 20 h reflux. b Conversion determined by 1H NMR spectroscopy using hexamethylbenzene as an internal standard; >99% conversion of starting meterials: ketone/alcohol product ratio. | |||||
Ru1 | 0/100 | Ir1 | 21/79 | Os1 | 67/33 |
Ru2 | 8/92 | Ir2 | 31/69 | Os2 | 83/17 |
Ru3 | 0/100 | Ir3 | 42/58 | Os3 | 64/36 |
Rupyrim | 0/100 | Ospyrim | 30/70 |
To conclude, we tested the transfer hydrogenation of the nitro group. Recently, an efficient transfer hydrogenation catalytic system for the reduction of nitroarenes into anilines was reported and was based on Ru(II) and Ir(III) MIC precatalysts.8a We carried out an initial screening with nitrobenzene as a substrate, applying 1.5 mol% of the precatalysts and 0.5 equiv. of KOH in iPrOH at 80 °C.8a Azobenzene and azoxybenzene were expected as the side products. The results are presented in Table 11. The screening showed a catalyst dependent conversion, as well as product distribution. In contrast to the previously tested functional groups, the Ir(III) precatalysts were the most active of all the tested complexes, affording aniline as the major product. The Ru(II) complexes gave good conversion into aniline and azoxybenzene with only minute amounts of azobenzene. Once again the pyrimidine analogue Rupyrim showed superior activity. The Os(II) complexes performed similar to the Ru(II) analogues, giving slightly higher conversions. The best performance was seen for Os2, where aniline was the main product.
Precatalyst | Anilineb | Starting nitrob | Azob | Azoxyb |
---|---|---|---|---|
a General reaction conditions: nitrobenzene (0.5 mmol), precatalyst, KOH (0.21 mmol), iPrOH (4 mL), 80 °C for 24 h. b Conversion in % determined by GC using hexadecane as an internal standard. | ||||
Precatalyst loading: 1.5 mol% | ||||
Ru1 | 50 | 10 | 10 | 30 |
Ru2 | 56 | 26 | 4 | 14 |
Ru3 | 32 | 40 | 0 | 28 |
Rupyrim | 71 | 0 | 12 | 17 |
Ir1 | 93 | 0 | 7 | 0 |
Ir2 | 85 | 0 | 11 | 4 |
Ir3 | >99 | 0 | 0 | 0 |
Os1 | 50 | 31 | 0 | 19 |
Os2 | 83 | 0 | 3 | 14 |
Os3 | 50 | 35 | 0 | 15 |
Ospyrim | 80 | 0 | 3 | 17 |
Precatalyst loading: 5.0 mol% | ||||
Ru1 | >99 | 0 | 0 | 0 |
Ru2 | >99 | 0 | 0 | 0 |
Ru3 | >99 | 0 | 0 | 0 |
Rupyrim | >99 | 0 | 0 | 0 |
Ir1 | 97 | 0 | 3 | 0 |
Ir2 | 93 | 0 | 7 | 0 |
Ir3 | >99 | 0 | 0 | 0 |
Os1 | 64 | 36 | 0 | 0 |
Os2 | 87 | 10 | 0 | 3 |
Os3 | 60 | 40 | 0 | 0 |
Ospyrim | >99 | 0 | 0 | 0 |
As was previously observed,8a when employing 5.0 mol% of the precatalysts, we observed a drastic improvement in catalytic activity. The best results were seen with all four Ru(II) complexes, giving full conversion of nitrobenzene into aniline. The Ir(III) analogues performed very similarly. On the other hand, the Os(II) complexes were less active, except for Ospyrim, which yielded aniline quantitatively.
![]() | ||
Scheme 4 The postulated mechanism for the transfer hydrogenation of nitrobenzene.8a,19 |
For the transfer hydrogenation of the nitro functional group we chose four additional substrates: 2-, 3- and 4-nitroanisole, as well as 1-chloro-2-nitrobenzene. We selected Os2 and Ir3 as the precatalysts and conducted the transfer hydrogenation with 1.5 mol% precatalyst loading. Each of the four substrates was stirred with 0.5 equiv. of KOH in iPrOH at 80 °C for 24 h (Table 12). 2-Nitroanisole was converted with both tested complexes into o-anisidine as the only product. The conversions were 65% and 54%, respectively. Both complexes catalysed the reduction of 3-nitroanisole into m-anisidine very well. Ir3 gave full conversion into the amino product. The reduction of 4-nitroanisole into p-anisidine proceeded with both complexes, however, giving lower conversions. By using the complex Ir3, 1-chloro-2-nitrobenzene was fully converted into 2-chloroaniline. In contrast, Os2 gave only 58% of the aniline derivative after 24 h. Overall, the Ir(III) complex Ir3 was more active in the transfer hydrogenation catalysis of the selected nitro substrates. Surprisingly no azo- or azoxy-side products were detected. For the reduction of cyclohexene and for the reduction of nitrobenzene, we performed the Hg-poisoning test by adding Hg during the catalysis. The addition of Hg had no effect on the yield or the rate of formation of the catalytic products. Hence, we believe that catalysis in the present case is likely homogeneous.
Precatalyst | Aminob | Nitrob | Azob | Azoxyb |
---|---|---|---|---|
a General reaction conditions: nitro substrate (0.5 mmol), precatalyst (1.5 mol%), KOH (0.21 mmol), iPrOH (4 mL), 80 °C for 24 h. b Conversion in % determined by GC using hexadecane as an internal standard. | ||||
2-Nitroanisole | ||||
Ir3 | 65 | 35 | 0 | 0 |
Os2 | 54 | 46 | 0 | 0 |
3-Nitroanisole | ||||
Ir3 | 100 | 0 | 0 | 0 |
Os2 | 75 | 25 | 0 | 0 |
4-Nitroanisole | ||||
Ir3 | 54 | 46 | 0 | 0 |
Os2 | 18 | 82 | 0 | 0 |
1-Chloro-2-nitrobenzene | ||||
Ir3 | 100 | 0 | 0 | 0 |
Os2 | 58 | 42 | 0 | 0 |
Footnotes |
† Dedicated to Prof. Bernhard Lippert on the occasion of his 70th birthday. |
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c6dt01324d |
This journal is © The Royal Society of Chemistry 2016 |