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‘(g6-arene)Ru(bis-NHC)’ complexes for the reduction of CO2 to formate withhydrogen and by transfer hydrogenation with iPrOH

Sergio Sanz, Arturo Azua and Eduardo Peris*

Received 16th February 2010, Accepted 30th April 2010First published as an Advance Article on the web 2nd June 2010DOI: 10.1039/c003220d

A series of bis-NHC-complexes of Ru and Ir have been tested in the reduction of carbon dioxide toformate. The use of one of the ‘(h6-arene)Ru(bis-NHC)’ complexes (labeled as 1 in the text) in thereduction of carbon dioxide with hydrogen affords a maximum TON value of 23000 at 200 ◦C. Thesame complexes were used in the reduction of carbon dioxide with iPrOH by the transfer hydrogenationmethodology. For this reaction, a maximum TON value of 874 was achieved, the highest reported todate for this type of process. The activity of the catalyst is highly dependent on its stability because thecomplex has to stand the severe reaction conditions used in the process (200 ◦C).

Introduction

During the last fifteen years, N-heterocyclic carbenes (NHCs) havebeen applied to the preparation of a large number of catalystsfor a wide variety of organic transformations.1 Among NHCs,those acting as chelating ligands have emerged as an extremelyuseful class of ligands because they allow the preparation of stabletransition metal compounds with fine tuned properties such assteric hindrance, bite angles, chirality and fluxional behavior.2,3

Despite the popularity that NHCs have shown in most catalyticprocesses, just a few examples have been reported describing theiruse in the design of metal catalysts for the transformation of CO2,4

but they already illustrate their great potential for this type ofprocess.

The homogeneously catalyzed hydrogenation of CO2 to formicacid,5,6 is a well known reaction that was first reported in 1976.7

This reaction normally implies the reduction of carbon dioxidewith pressurized hydrogen. On the basis of our experience in thestudy of borrowing-hydrogen processes,8 we recently describedthe preparation of a series of ‘Cp*Ir(NHC)’ for the reduction ofCO2 to formate through a hydrogen transfer process with iPrOH.9

Although our results were moderate in terms of the TON valuesachieved, the report constituted an important innovation thatin the near future may constitute an alternative over the use ofhydrogen.

Some recent reports show that ‘RuII(h6-arene)’ complexesconstitute valuable catalysts for the hydrogenation of CO2 toformic acid.10–12 The incorporation of N-heterocyclic carbenes to‘RuII(h6-arene)’ complexes have provided a family of catalysts thathave been used in a wide set of non-metathetical transformationsapplied to the production of advanced materials.13 However,‘Ru(NHC)’ complexes have never been used in CO2 activationprocesses despite their analogies with well-known catalysts thatnormally incorporate bipyridine-like ligands. Bis-NHCs maytopologically mimic bipyridine ligands, especially since we recentlyobtained a bis-triazolylidene ligand (bitz) with a direct N–N bond

Dpto. de Quımica Inorganica y Organica, Universitat Jaume I. Avda., VicenteSos Baynat s/n, Castellon, E-12071, (Spain). E-mail: eperis@qio.uji.es;Fax: +34 964728214

linking the azole rings that affords a planar chelating mode.14,15

With this in mind, here we report the preparation of a seriesof new ‘RuII(h6-arene)(bis-NHC)’ complexes that have been fullycharacterized and tested in the reduction of CO2 with H2.

Results and discussion

In order to design new Ru complexes with potential catalyticactivity in the reduction of CO2, we decided to prepare the‘(h6-arene)Ru(bis-NHC)’ complexes 1–3. These complexes aretopologically similar to well-known ‘(h6-arene)Ru(bpy)’ (bpy =2,2¢-bipyridine) complexes that have been traditionally used in thehydrogenation of carbon dioxide to formate.6,10,11 Compound 2,with a bitriazolylidene ligand is topologically and electronicallysimilar to 2,2¢-bipyridine.14 On the basis of the theoretical studyby Sakaki and co-workers on the hydrogenation of CO2,16

strongly donating ligands would improve the catalytic efficiency,so the stronger electron-donating bis-imidazolylidene ligands incomplexes 1 and 3, may provide enhanced catalytic properties.The introduction of the alcohol functionalities in complexes 3and 4, aims to improve the solubility of the complexes in thecatalytic reaction medium, typically H2O. The iridium complex4, was obtained in order to compare its catalytic outcome withprevious data recently published by us.9

The ruthenium compounds 1 and 3, and the iridium complex4, were obtained via transmetallation from a pre-formed Ag(I)–NHC complex (Scheme 1a, Scheme 1b), while 2 was betterobtained by deprotonating the bitriazolium salt (Scheme 1c). Allcomplexes 1–4 were obtained in high yields (60–82%). All four newcomplexes were characterized by NMR spectroscopy, electrospraymass spectrometry (ESI-MS) and elemental analysis. The mostrepresentative signals on the 1HNMR of complexes 1 and 3 arethose due to the imidazolylidene rings (d 7.41 and 7.18 in 1,and d 7.61 and 7.51 in 3) and the two doublets assigned to thediasterotopic protons of the methylene bridge, that confirm thelost of symmetry of the ligand upon coordination. The 13C NMRspectra show signals at 178.9 (1) and 179.2 ppm (3) attributed tothe R-Ccarbene carbons. Compound 2 shows the signal due to themetalated Ru-Ccarbene at 182.6 ppm.

This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 6339–6343 | 6339

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Scheme 1

The 1H NMR spectrum of the iridium complex 4 also showsthat the symmetry of the ligand is lost upon coordination, as seenby the two doublets attributed to the diasterotopic protons ofthe methylene linker between the two azole rings. The 13C NMRspectrum shows the signal due to the Ir-Ccarbene at 152.3 ppm.

Catalysts 1–4 and also 5 (Scheme 2) were tested in the catalytichydrogenation of CO2. We initially aimed to determine whichof these compounds afforded the best catalytic outcomes in thisreaction, and also to compare our data with related Ru complexesreported in the literature. The reactions were performed using40 atm of a CO2/H2 mixture (1 : 1), in an aqueous solution ofKOH (1 M) at 80 and 200 ◦C. For the reactions carried out at80 ◦C, we observed that catalyst 1 is the one to show the bestcatalytic outcomes, as can be seen by comparison of the dataobtained with a catalyst loading of 0.02 mM after 20 h of reaction(Table 1). Under these reaction conditions, 1 afforded a TONvalue of 1367 (entry 11), high above than the values achieved by2 (TON = 311, entry 20), 3 (TON = 132, entry 21), 4 (TON =122, entry 22) and 5 (TON = 214, entry 24). This result would bein agreement with the previously reported works suggesting thatstronger electron donor ligands would improve the outcome of thecatalytic process,16 as seen from the comparison between the resultsobtained with catalyst 1 (entry 11) and 5 (entry 24). Compound 2,with a bitriazolylidene (bitz) ligand, provides a similar activity tothat provided by 5, which in fact is in agreement with the similartopological and electronic properties of bitz and 2,2¢-bipyridine.14

Our initial attempt to improve the catalytic activity of the catalystsby introducing hydrophilic -CH2OH groups seemed inefficient, ascan be seen from the data obtained with catalysts 3 and 4. Webelieve that under the reaction conditions used, all catalysts aresoluble in the reaction medium because they are introduced as

Scheme 2

Table 1 Reduction of CO2 with H2

CO + H HCOOK + H O2 2cat.

1M KOH/H2O 2⎯ →⎯⎯⎯⎯

Entry Catalyst Time T/◦C [Cat.]/mM nHCOOK/mmol TONa

1 none 20 200 — 0.7 —2 none 1 200 — 0.08 —3 none 75 200 — 1.08 —4 none 20 80 — 0 —5 1 20 80 0.1 0.85 8506 1 20 80 0.02 0.27 13677 1 1 80 0.02 0.01 708 1 4 80 0.02 0.04 2269 1 7 80 0.02 0.06 31210 1 15 80 0.02 0.11 56411 1 20 80 0.02 0.27 136712 1 24 80 0.02 0.35 178013 1 66 80 0.02 0.47 234414 1 20 200 0.02 1.3 650015 1 20 80 0.002 0.04 186016 1 20 200 0.002 0.27 1350017 1 75 200 0.002 0.46 2300018 1 1 200 0.002 0.05 250019 1 20 80 0.001 0.04 388020 2 20 80 0.02 0.06 31121 3 20 80 0.02 0.03 13222 4 20 80 0.02 0.02 12223 4 20 200 0.02 1.9 950024 5 20 80 0.02 0.04 214

Reactions were carried out at 40 atm. (20CO2:20H2), in 10 mL of 1M KOH.nHCOOK (number of mmols of HCOOK) and TONs corrected accordingdata without catalysts, entries 1–4.a TONs based on the formation offormate, calculated by 1HNMR spectroscopy.

ionic species that should further hydrolyze their M–X (X = Cl,I) under the basic conditions of the reaction, so the introductionof hydrophilic groups in the catalyst has little influence on thereaction outcome unless other factors affecting the reactivity ofthe complex are at play.10

Because chelating-NHC ligands are known to provide a veryhigh thermal stability to their metal complexes,2 we thought that1 could be suitable for the catalytic hydrogenation of CO2 at200 ◦C. Under these reaction conditions potassium formate isformed even in the absence of catalyst,17 and thus backgroundcorrections to the catalyst TONs based on the data withoutcatalyst (entries 1–4) were made. As can be seen from the datashown in Table 1, 1 provided a high TON value of 6500 for thereaction carried out with a catalyst loading of 0.02 mM after 20 h.For the reaction carried out with 0.002 mM of 1, a maximumTON of 23000 was achieved after 75 h. Although these resultsare far from the extraordinary high catalytic outcomes providedby the pincer-iridium catalysts recently reported by Nozaki17 andthe ruthenium complex with an oxyanion ligand reported byHimeda,18 our results lay among the highest reported to date,5

and confirm the applicability of NHC ligands in the design ofcatalysts for the efficient reduction of CO2.

We recently proposed that the reduction of CO2 to formate couldalso be achieved by a transfer hydrogenation procedure using analcohol as the hydrogen source.9 The typical alcohol for transferhydrogenation, iPrOH, is an inexpensive and environmentallybenign reagent, that may overcome the experimental inconve-niences of using H2. Besides, the reduction of CO2 with iPrOHhas the advantage that only one gas has to be pressurized, so the

6340 | Dalton Trans., 2010, 39, 6339–6343 This journal is © The Royal Society of Chemistry 2010

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Table 2 Transfer hydrogenation of CO2 with iPrOH

CO + iPrOH HCOOK + (CH ) CO2cat.

0.5M KOH/H2O 3 2⎯ →⎯⎯⎯⎯

Entry Catalyst Time T/◦C [Cat.]/mM nHCOOK/10-6 mol TONa

1 none 16 200 — 90 —2 none 16 110 — 0 —3 1 16 110 0.018 3.24 94 1 16 200 0.018 67 1865 1 16 200 0.0018 32 8746 2 16 110 0.018 1.44 47 3 16 110 0.018 3.96 118 4 16 110 0.018 11 309 4 16 200 0.018 73 20310 5 16 110 0.018 6.12 1711b 6 16 110 0.018 33 9112 6 16 200 0.018 5 13

Reactions were carried out at 110 ◦C, 50 atm. CO2, presence of 0.5 MKOH in 20 ml of H2O : iPrOH (9 : 1). nHCOOK (number of mols of generatedHCOOK) and TONs corrected according data without catalysts, entries 1–2.a Conversions based on the formation of formate, calculated by 1HNMRspectroscopy. b Data taken from ref. 9.

experimental procedure is simplified compared to the methodsimplying the mixing of two gases. As can be seen from the datashown in Table 2, the reactions carried out at 110 ◦C, afford verylow conversions to potassium formate, with TONs ranging from4–17 (entries 3, 6, 7 and 10) for the Ru complexes and 30 for theiridium complex 4 (entry 8). These values are much lower thanthe outcomes shown by the iridium complex 6, that showed aTON value of 91 under the same reaction conditions (entry 11).9

From the data obtained for the reactions carried out at 200 ◦C,we obtained very valuable information. First, we observed thatcatalyst 1 achieved a TON value of 186, for the reaction with acatalyst loading of 1.8 ¥ 10-2 mM (entry 4), which in fact indicatesthat the catalyst is still active under the severe reaction conditionsused. Under the same reaction conditions, a lower catalyst loadingof 1.8 ¥ 10-3 mM afforded a TON value of 874, the highest reportedso far for this type of reaction. This high value confirms thatthe transfer hydrogenation methodology is a valuable alternativeto the ubiquitous reduction of CO2 with H2. Interestingly, theactivity of the mono-NHC-iridium complex 6, is much lower at200 ◦C than at 110 ◦C (compare entries 11 and 12), indicatingthat its thermal stability is probably lower than that shown by thebis-NHC complexes under study in this work. This low stability isalso supported by the observation that the solution resulting fromthe reaction at 200 ◦C with catalyst 6, shows a black solid residuethat probably corresponds to Ir(0) produced by decomposition ofthe catalyst under the reductive conditions of the process.

Conclusions

In summary, we have prepared a series of Ru and Ir complexeswith several bis-NHC ligands. The different bis-NHC ligands werechosen according to their electronic and topological properties,and also attempting to introduce hydrophilic residues that enhancetheir solubility in water. The results shown indicate that thebis-NHC–Ru complex 1 is the most active one among all thecomplexes under study in the reduction of carbon dioxide withhydrogen, achieving very high TON values at 200 ◦C. Thereduction of carbon dioxide with iPrOH indicates that the activity

of the catalysts is highly dependent on the temperature of thereaction. For the reactions carried out at 110 ◦C, low (almostnegligible) activities were obtained for all the catalysts, but thereactions carried out at 200 ◦C provided a maximum TON valueof 874 for catalyst 1, the highest reported so far for this type ofprocess. The stability of the catalyst is crucial in this reaction,as seen by the lowering of the activity of the mono-NHC–Ircomplex 6 upon raising the temperature from 110 ◦C to 200 ◦C,a clear indication that, in this case, the catalyst decomposes dueto the severe conditions of the reaction carried out at the highertemperature.

The high TON values achieved in the reduction of carbondioxide by the transfer hydrogenation methodology, encouragesus to continue our search for active catalysts that can turn thisreaction into a valuable alternative to the reduction of CO2

with H2.

Experimental

NMR spectra were recorded on Varian Innova 300 and 500 MHzspectrometers, using CD3OD and D2O as solvents. Elementalanalyses were carried out in an EA 1108 CHNS-O Carlo Erbaanalyzer. Electrospray mass spectra (ESI-MS) were recorded ona Micromass Quatro LC instrument, and nitrogen was employedas drying and nebulising gas. Solvents and reagents were usedas received from the commercial suppliers. Methylenebis(N-methylimidazolium)dichloride,19 1,1¢-dimethyl-4,4¢-bi-1,2,4-triazolium diiodide,15 N,N¢-dibenzimidazolylmethane20

[RuCl2(C6Me6)]2,21 and compounds 56 and 6,22 were preparedaccording to literature procedures.

Synthesis of methylenebis[(N-2-hydroxyethyl)imidazolium]diiodide

A mixture of N,N¢-dibenzimidazolylmethane (250 mg, 1.69 mmol)and 2-iodoethanol (585 mg, 3.40 mmol) was stirred in 5 ml iPrOHat 100 ◦C for 12 h. The solvent was evaporated under reducedpressure to dryness. The crude solid was washed with CH2Cl2 toobtain the product as a white powder. (yield: 756.80 mg, 91%).1H NMR (D2O, 300 MHz): 7.79 (s, 2H, Himid), 7.65 (s, 2H, Himid),6.69 (s, 2H, NCH2N), 4.35 (bs, 4H, NCH2CH2OH), 3.91 (bs,4H, NCH2CH2OH). 13C{1H} NMR (D2O, 300 MHz): d 137.43(2C, NCN), 124.42 (2C, Cimid), 124.25 (2C, Cimid), 59.71 (2C,NCH2CH2OH), 59.34 (1C, NCH2N), 52.71(2C, NCH2CH2OH).Electrospray MS (15 V, m/z): 237.2 (34%, [M - H]+), 365 (67%,[M + I]+). C11N4H18O2I2 (mol wt 492.10): C, 26.85; H, 3.69; N,11.39. Found: C, 26.47; H, 4.02; N, 11.52.

Synthesis of 1

Silver oxide (139 mg, 0.6 mmol) was added to a solu-tion methylenebis(N-methylimidazolium) dichloride (49.8 mg,0.2 mmol) in CH3OH, and the mixture was stirred at reflux for2 h. Then [RuCl2(C6Me6)]2 (69 mg, 0.1 mmol) was added. Themixture was refluxed for 2 h. The suspension was filtered throughCelite, and the solvent was evaporated under reduced pressure.The crude solid was purified by column chromatography. Elutionwith 20 mL acetone with 30 mg of KPF6 afforded the separationof a brown band that contained the compound and residualKPF6, which was filtered off. The complex was obtained as a

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brown solid by precipitation from CH3CN–Et2O solution (yield:76.84 mg, 62%). 1H NMR (CD3CN, 500 MHz): d 7.41 (s, 2H,Himid), 7.18 (s, 2H, Himid), 6.31(d, 2JH–H = 13.20 Hz, 1H, NCH2N),5.19 (d, 2JH–H= 13.20 Hz, 1H, NCH2N), 3.50 (s, 6H, CH3N),2.02 (s, 18H, C6Me6). 13C{1H} NMR (CD3CN, 300 MHz): d178.96 (2C, NCNimid), 124.61 (2C, Cimid), 123.04 (2C, Cimid), 100.33(6C, C6Me6), 79.23 (1C, NCH2N), 38.33 (2C, NCH3), 16.53 (6C,C6Me6). Electrospray MS (15 V, m/z): 475.1 (100%, [M]+). Anal.Calcd for C21N4H30RuClPF6 (mol wt 619.98): C, 40.68; H, 4.88;N, 9.04. Found: C, 40.43; H, 5.02; N, 9.17.

Synthesis of 2

A mixture of 1,1¢-dimethyl-4,4¢-bi-1,2,4-triazolium diiodide(205.70 mg, 0.49 mmol), [RuCl2(C6Me6)]2 (165.10 mg, 0.24 mmol),KI (478.1 mg, 2.88 mmol), and NaOAc (236.1 mg, 2.88 mmol)was stirred in CH3CN at reflux temperature for 16 h. Aftercooling, the suspension was filtered through Celite, and thesolvent was evaporated under reduced pressure. Elution with50 ml CH2Cl2–acetone afforded the separation of a red-brownband that contained compound 2. The complex was obtained asa red-brown solid by precipitation from CH2Cl2–Et2O solution(yield: 268.16 mg, 82%). 1H NMR (CDCl3, 300 MHz): d 10.21(s, 2H, Himid), 4.08 (s, 6H, NCH3), 2.35 (s, 18H, C6Me6). 13C{1H}NMR (CDCl3, 300 MHz): d 182.64 (2C, NCRu), 136.37 (2C,NCNimid), 102.80 (6C, C6Me6), 40.85 (2C, NCH3), 18.08 (6C,C6Me6). Electrospray MS (15 V, m/z): 555.1 (100%, [M]+). Anal.Calcd for C18N6H26RuI2 (mol wt 681.32): C, 31.73; H, 3.85; N,12.33. Found: C, 31.32; H, 4.12; N, 12.62.

Synthesis of 3

Silver oxide (139 mg, 0.6 mmol) was added to a solu-tion of methylenebis[(N-2-hydroxyethyl)imidazolium] diiodide(98.38 mg, 0.2 mmol) in CH3OH, and the mixture was stirred atreflux for 2 h. Then [RuCl2(C6Me6)]2 (69 mg, 0.1 mmol) and KCl(82 mg, 1.10 mmol) were added. The mixture was refluxed for 2 h.The suspension was filtered through Celite, and the solvent wasevaporated under reduced pressure. The crude solid was purifiedby column chromatography. Elution with 20 ml MeOH affordedthe separation of a brown band that contained compound 3.The complex was obtained as a brown solid by precipitationfrom CH3OH–iPrOH solution (yield: 87.86 mg, 77%). 1H NMR(CD3OD, 500 MHz): d 7.61 (s, 2H, Himid), 7.51 (s, 2H, Himid),6.29(d, 2JH–H = 13.15 Hz, 1H, NCH2N), 5.38 (d, 2JH–H = 13.15 Hz,1H, NCH2N), 4.61-4.39 (m, 2H, NCH2CH2OH), 4.22-4.11 (m,2H, NCH2CH2OH), 3.87-3.73 (m, 2H, NCH2CH2OH), 2.03 (s,18H, C6Me6). 13C{1H} NMR (CD3OD, 500 MHz): d 179.16(2C, NCNimid), 123.99 (2C, Cimid), 122.97 (2C, Cimid), 100.93 (6C,C6Me6), 63.17 (1C, NCH2N), 53.15 (2C, NCH2CH2OH), 30.66(2C, NCH2CH2OH), 16.35 (6C, C6Me6). Electrospray MS (15 V,m/z): 535.1 (100%, [M]+). Anal. Calcd for C23N4H34RuO2Cl2 (molwt 570.52): C, 48.42; H, 6.01; N, 9.82. Found: C, 48.73; H, 6.42;N, 9.66.

Synthesis of 4

Silver oxide (139 mg, 0.6 mmol) was added to a solu-tion of methylenebis[(N-2-hydroxyethyl)imidazolium] diiodide(98.38 mg, 0.2 mmol) in CH3OH, and the mixture was stirred

at reflux for 2 h. Then [IrCl2Cp*]2 (80 mg, 0.1 mmol) and KCl(82 mg, 1.10 mmol) were added. The mixture was refluxed for 2 h.The suspension was filtered through Celite, and the solvent wasevaporated under reduced pressure. The crude solid was purifiedby column chromatography. Elution with 20 ml MeOH affordedthe separation of a brown band that contained compound 4.The complex was obtained as a brown solid by precipitationfrom CH3OH–iPrOH solution (yield: 87.57 mg, 69%). 1H NMR(CD3OD, 300 MHz): d 7.60 (s, 2H, Himid), 7.52 (s, 2H, Himid), 6.32(d, 2JH–H = 13.18 Hz, 1H, NCH2N), 5.61 (d, 2JH–H = 13.18 Hz, 1H,NCH2N), 4.43-4.32 (m, 2H, NCH2CH2OH), 4.23-4.11 (m, 2H,NCH2CH2OH), 3.93-3.78 (m, 4H, NCH2CH2OH), 1.80 (s, 15H,Cp*). 13C{1H}NMR (CD3OD, 300 MHz): d 152.27 (1C, NCNimid),123.50 (1C, C imid), 122.49 (2C, Cimid), 94.51 (5C, Cp*), 63.02 (1C,NCH2N), 62.96 (2C, NCH2CH2OH), 53.07 (2C, NCH2CH2OH),9.43 (5C, Cp*). Electrospray MS (15 V, m/z): 599.1 (100%, [M]+).C21N4H31IrO2Cl2 (mol wt 634.62): C, 39.74; H, 4.92; N, 8.83.Found: C, 39.41; H, 4.61; N, 8.47.

Catalytic hydrogenation of CO2 with H2

Catalytic reactions were carried out in a Hastelloy Autoclave Mini-Reactor system equipped with a 50 mL cylinder. The catalystwas dissolved in a degassed aqueous KOH solution (10 mL). Thereactor was pressurized with 40 bar of CO2 : H2 (1 : 1) and heatedat 80 ◦C or 200 ◦C for the appropriate time. After reducing thepressure to 1 bar and cooling to room temperature, the solvent wasremoved by evaporation, and the residue was solved in D2O. Theyield of HCOOK was determined by 1H NMR in D2O, usingisonicotinic acid as internal standard.

Catalytic hydrogen transfer of CO2 with 2-propanol

The reactions were carried out in a Hastelloy Autoclave Minireac-tor system equipped with a 100 mL cylinder. The catalyst, KOHwere dissolved in 20 mL of a mixture of H2O–alcohol (9 : 1 v.v).The reactor was pressurized with 50 bar of CO2 and heated at110 ◦C or 200 ◦C under 1100 r.p.m. stirring for the durationof the experiment. After equilibration to atmospheric pressureand cooling to room temperature, the solvent was removed byevaporation, and the residue was solved in D2O. The yield ofHCOOK was determined by 1H NMR in D2O, using isonicotinicacid as internal standard.

Acknowledgements

We gratefully acknowledge financial support from the Ministeriode Ciencia e Innovacion of Spain (CTQ2008-04460 and CTQ2007-31175-E/BQU), and Bancaixa (P1.1B2007-04). The authors aregrateful to the Serveis Centrals d’Instrumentacio Cientıfica (SCIC)of the Universitat Jaume I for providing us with spectroscopicfacilities.

Notes and references

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This journal is © The Royal Society of Chemistry 2010 Dalton Trans., 2010, 39, 6339–6343 | 6343

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