Ruthenium Carbonyl Complexes Bearing Bis(pyrazol-1-yl)carboxylato

Article pubs.acs.org/Organometallics

Ruthenium Carbonyl Complexes Bearing Bis(pyrazol-1yl)carboxylato Ligands Gazi Türkoglu,†,§ Stefan Tampier,†,§ Frank Strinitz,†,§ Frank W. Heinemann,† Eike Hübner,‡ and Nicolai Burzlaff*,† †

Inorganic Chemistry, Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), University of Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany ‡ Organic Chemistry, Technical University Clausthal, Leibnizstraße 6, 38678 Clausthal-Zellerfeld, Germany S Supporting Information *

ABSTRACT: The syntheses of the two dicarbonyl complexes [Ru(bdmpza)Cl(CO)2] (3) and [Ru(2,2-bdmpzp)Cl(CO)2] (4), bearing a bis(3,5-dimethylpyrazol-1-yl)acetato (bdmpza) or a 2,2bis(3,5-dimethylpyrazol-1-yl)propionato (2,2-bdmpzp) scorpionate ligand, are described. Both complexes are obtained by reacting the polymer [RuCl2(CO)2]n with either K[bdmpza] or K[2,2-bdmpzp]. Reaction of the acid Hbdmpza with [Ru3(CO)12] results in the formation of two structural isomers of a hydrido complex, [Ru(bdmpza)H(CO)2] (5a,b). Under aerobic conditions conversion of [Ru(bdmpza)H(CO)2] (5a,b) to form the Ru(I) dimer [Ru(bdmpza)(CO)(μ-CO)]2 (6) seems to be hindered in comparison to the case for the η5-C5H5 (Cp) analogues. Dimer 6 is obtained via a reaction of Hbdmpza with catena-[Ru(OAc)(CO)2]n instead. The molecular structures of 3, 4, and 6 have been obtained by single-crystal X-ray structure determinations. The precatalytic properties of the two dicarbonyl complexes 3 and 4 toward the catalytic oxidation of cyclohexene with different oxidizing agents are discussed as well.

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INTRODUCTION Bis(pyrazol-1-yl)acetic acids are a versatile class of N,N,O scorpionate ligands structurally related to the well-known hydrotris(pyrazol-1-yl)borate (Tp) ligand. Since they have been introduced to coordination chemistry in 1999 by Otero,1 efficient syntheses for various bis(pyrazol-1-yl)acetic acids have been developed over the years.1−3 Quite a few transition-metal complexes bearing these tripodal κ3-N,N,O binding ligands have been reported during the past decade.4,5 Among these are bis(pyrazol-1-yl)acetato manganese and rhenium carbonyl complexes that indicate binding properties as weakly electron donating ligands, similar to the case for the Tp ligands.3 Furthermore, several ruthenium(II) bis(pyrazol-1-yl)carboxylato complexes have been reported by us and others: e.g., with bis(3,5-dimethylpyrazol-1-yl)acetic acid (Hbdmpza) (1) or very recently with 2,2-bis(3,5-dimethylpyrazol-1-yl)propionic acid (2,2-Hbdmpzp) (2).6−16 Some of these ruthenium(II) complexes are good structural models for iron oxygenases that exhibit a facial 2-His-1-carboxylate triad as a ferrous iron binding motif.7,15 The uncatalyzed reaction of organic compounds with atmospheric dioxygen is thermodynamically feasible but is a spin-forbidden process. In nature, iron oxygenases make use of their high-spin ferrous centers to overcome this spin mismatch.17−21 Thus, an analogous enzyme activation of dioxygen with ruthenium(II) complexes is not favorable due to their low-spin character. However, the © 2012 American Chemical Society

necessity of a high-spin center becomes irrelevant in so-called peroxide shunt type reactions. Oxidizing agents such as peroxides and iodosylbenzene are used to directly generate high-valent and reactive RuIVO or RuVI(O)2 species which are able to catalytically epoxidize alkenes, oxidize sulfides, or hydroxylate alkanes.22−29 Che and co-workers, for instance, reported on cationic ruthenium(IV) complexes such as [Ru(Me3tacn)(3,3′-Me2-bpy)(O)]2+ and [Ru(terpy)(tmeda)(O)]2+ that can be used to epoxidize alkenes stoichiometrically (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, 3,3′Me2bpy = 3,3′-dimethyl-2,2′-bipyridine, terpy = 2,2′:6′,2″terpyridine, tmeda = N,N,N′,N′-tetramethylethylenediamine).30−33 Furthermore, the complex [Ru(Me3tacn)(OH2)(O2CCF3)](O2CCF3)2 was shown to be an effective catalyst for homogeneous oxidation of alkenes by tert-butyl hydroperoxide (TBHP) as an oxidant.34 Thus, in previous experiments we tested some of the ruthenium bdmpza complexes mentioned above, such as [Ru(bdmpza)Cl(PPh3)2] and [Ru(bdmpza)(OAc)(PPh3)], for their catalytic activity in similar alkene epoxidations.35 Unfortunately, rather poor catalytic activity with only 2−3 turnovers was observed, due to large quantities of OPPh3 byproduct, which inhibit the catalytic epoxidation. Thus, we decided to focus on phosphine-free complexes for Received: September 30, 2011 Published: March 14, 2012 2166

dx.doi.org/10.1021/om2009155 | Organometallics 2012, 31, 2166−2174

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75%)) as well an additional peak indicating traces of a byproduct of the composition [Ru(bdmpza)(CO)(μ2-CO)]2 (6) (m/z 810, 4%). Crystals of complex 3 suitable for a single-crystal structure determination were grown by evaporation of a chloroform solution of [Ru(bdmpza)Cl(CO)2] (3). The substance crystallizes in the space group C2/c as the compound [Ru(bdmpza)Cl(CO)2]·2CHCl3. A molecular presentation of the compound is illustrated in Figure 1 with selected bond

further studies. Since the pioneering report of Groves and Quinn,36a for more than 25 years ruthenium porphyrin catalyzed alkene epoxidations have been well-known and studied intensively by several groups worldwide.36 In these studies RuII(CO)(porphyrin) complexes are often applied as precatalysts that are activated by oxidation with iodosylbenzene or peroxy acids.36 This aroused our interest in ruthenium carbonyl complexes. Thus, in this paper, we report on the syntheses of various carbonyl complexes bearing bis(pyrazol-1yl)carboxylato ligands and first results regarding their precatalytic properties and catalytic activity in the epoxidation of cyclohexene with different oxidizing agents.

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RESULTS AND DISCUSSION Usually, convenient syntheses of the cyclopentadienyl complex [RuCl(η5-C5H5)(CO)2] or the hydridotris(pyrazolyl)borate complex [RuCl{κ3-HB(pz)3}CO2] start from the dimeric ruthenium carbonyl complex [Ru(CO)3Cl2]2.37,38 Recently, the polymeric ruthenium carbonyl precursor [RuCl2(CO)2]n has been reported by Vos et al. as a quite useful and easily accessible tool for the synthesis of various monomeric ruthenium carbonyl complexes. 39 Thus, starting from [RuCl 2 (CO) 2 ] n seems to be obligatory to obtain a phosphine-free ruthenium carbonyl complex bearing the facially coordinating bdmpza ligand. The synthesis of the intended mononuclear dicarbonyl complex [Ru(bdmpza)Cl(CO)2] (3) was achieved by deprotonation of the according ligand Hbdmpza (1) with KOtBu followed by a reaction with the polymeric precursor [RuCl2(CO)2]n (Scheme 1).

Figure 1. Molecular structure of the dicarbonyl complex [Ru(bdmpza)Cl(CO)2] (3). Thermal ellipsoids are drawn at the 50% probability level, and most hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−C(4) = 1.914(2), Ru−C(5) = 1.872(2), Ru−Cl = 2.3885(7), Ru(1)−O(1) = 2.0863(15), Ru−N(11) = 2.0855(18), Ru−N(21) = 2.1375(17), O(4)−C(4) = 1.109(3), O(5)−C(5) = 1.138(3); O(1)−Ru−N(11) = 85.63(6), O(1)−Ru−N(21) = 85.16(6), N(11)−Ru−N(21) = 85.05(6), Cl−Ru−N(11) = 172.06(5), C(4)−Ru−N(21) = 174.42(8), C(5)−Ru−O(1) = 177.00(8), Cl−Ru−C(4) = 90.28(7), Cl−Ru−C(5) = 89.64(7), C(4)−Ru−C(5) = 89.80(10).

Scheme 1. Synthesis of Dicarbonyl Complexes [Ru(bdmpza)Cl(CO)2] (3) and [Ru(2,2-bdmpzp)Cl(CO)2] (4)

lengths and angles given in the figure caption. The molecular structure depicts the C1 symmetry within the molecule that was already indicated by the NMR spectra. Thus, the cell contains both enantiomers of the chiral complex. The ruthenium(II) center is coordinated in an octahedral geometry with one carbonyl ligand trans to the carboxylato ligand and the other carbonyl ligand trans to one of the pyrazole donors. The bond angles of the bdmpza ligand at the central metal are, at about 85°, slightly smaller than the ideal 90°. This indicates a rather rigid and strained bdmpza clamp and has been observed before for several ruthenium complexes bearing the bdmpza ligand.6−15 The angles between the other ligands and the metal are close to 90°. This implies no further steric hindrance caused by the bdmpza ligand. The ruthenium−carbonyl distances and the carbon−oxygen distances within the carbonyl ligand differ only slightly and agree well with those reported for [Ru{κ3-HB(pz)3}(CO)2(p-tolyl)] (Ru−CO = 1.857(2) and 1.866(2) Å).40 Nevertheless, it is apparent that the bond distance between the metal and the carbonyl ligand trans to the carboxylate (d(Ru−C(5)) = 1.872(2) Å) is shorter than the ruthenium−carbonyl bond length of the carbonyl trans to the pyrazolyl (d(Ru−C(4)) = 1.914(2) Å). This effect is caused by the trans influence of the pyrazolyl donor, which is a σ and π donor as well as a π acceptor, whereas no trans influence is apparent for the carboxylato group, which is a mere σ donor. This trans influence was already discussed in comparative DFT calculations for the dissociation energy of several N,N,O ligands.41 Vice versa, the carbonyl bond shows a contrary behavior: the CO bond of the carbonyl ligand trans to the carboxylato group is longer (d(C(5)-O5)) = 1.138(3) Å) than the corresponding distance in the other carbonyl ligand

The reaction lasted for about 12 h. Workup was accomplished by filtration and recrystallization from dichloromethane with pentane to separate the product from unreacted ligand salt and reactant. The substance is obtained as a yellowish powder, which is quite air-stable but turns green within weeks. The 1H NMR spectra show the formation of a C1-symmetric compound. The 1H NMR signals of methyl groups of the bdmpza ligand occur at 2.34, 2.45, 2.46, and 2.59 ppm. The signal corresponding to the bridging methine group is observed at 6.60 ppm. The two signals at 6.12 ppm are assigned to the pyrazole protons. The 13C NMR signals of the ligand confirm the C1 symmetry as well. The signals for the two carbonyl ligands are observed at 192.6 and 196.0 ppm. The IR absorptions of the carbonyl ligands occur at 2066 and 1996 cm−1 (KBr) or 2074 and 2005 cm−1 (CH2Cl2), respectively. These values agree well with those reported by Bruce for the analogous hydridotris(pyrazol-1-yl)borate complex [RuCl{κ3HB(pz)3}CO2] (2071 and 2011 cm−1).38 The FAB+ mass spectrum of the crude product shows two peaks assigned to the molecular ion of 3 (m/z 440 (M+; 100%) and 441 (MH+; 2167


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structure of 4 exhibits an octahedral geometry with one carbonyl ligand trans to the carboxylato ligand and the other carbonyl ligand trans to one of the pyrazole donors. Due to the space group P1̅ the cell contains both enantiomers of the chiral complex. Similar to the case for complex 3, the bond angles between the bdmpza donors of 4 and the ruthenium(II) center are, at about 85°, slightly smaller than the ideal 90°, due to the rigid and strained bdmpza clamp. The angles between the other ligands and the metal center are not as close to 90° as in the case of 3. This implies more steric hindrance caused by the 2,2bdmpzp ligand. The ruthenium−carbonyl distances as well as the carbon−oxygen distances within the carbonyl ligand differ only slightly. Nevertheless, due to the trans influence of the pyrazolyl donor the bond distance between the metal and the carbonyl ligand trans to the carboxylate (d(Ru−C(5)) = 1.900(2) Å) is shorter than the ruthenium−carbonyl bond length of the carbonyl trans to the pyrazolyl (d(Ru−C(4)) = 1.922(2) Å). No significant differences regarding the CO bond length are observed. As shown in Figure 2, the structure of complex 4 reveals d(Ru−N11), d(Ru−N12) and d(Ru−O1) distances which do not differ significantly from the corresponding distances in the analogous complex [Ru(bdmpza)Cl(CO)2] (3). The carboxylate donor is slightly bent, with an angle ∠(C3,C1,C2,O2) of about 10°. As mentioned above, we observed traces of the byproduct [Ru(bdmpza)(CO)(μ2-CO)]2 (6) during the synthesis of 3, indicated by the molecular ion peak of 6 (m/z 810, 4%) in the FAB+ mass spectrum. Thus, it was not as surprising that, in attempts to crystallize complex 3, crystals of this byproduct [Ru(bdmpza)(CO)(μ2-CO)]2 (6) suitable for an X-ray singlecrystal structure determination were isolated by serendipity. The structure determination revealed its molecular structure as a dinuclear μ2-CO complex. A molecular presentation is depicted in Figure 3. Selected bond lengths and angles are given in the caption of Figure 3. The substance crystallizes as [Ru(bdmpza)(CO)(μ 2 CO)]2·4CDCl3 in space group P21/c, residing on a center of symmetry. One solvent molecule shows disorder over two

(d(C(4)−O(4)) = 1.109(3) Å). This should result in a slightly lower bond order for the carbonyl trans to the pyrazolyl in comparison to that for the other carbonyl. Very recently we reported on ruthenium complexes derived from 2,2-bis(3,5-dimethylpyrazol-1-yl)propionic acid (2,2Hbdmpzp) (2) with increased steric hindrance by the bdmpzp ligand and therefore improved reactivity.16 Thus, we also focused on the synthesis of the mononuclear dicarbonyl complex [Ru(2,2-bdmpzp)Cl(CO)2] (4). Following the procedure described above, the synthesis of 4 was achieved by deprotonation of 2,2-Hbdmpzp with KOtBu and a subsequent reaction with [RuCl2(CO)2]n (Scheme 1). The FD mass spectrum of the crude product shows a molecular ion peak indicating the formation of [Ru(2,2-bdmpzp)Cl(CO)2] (4; m/z 455 (MH+, 100%)). The successful formation of complex 4 was also proven by NMR and IR spectroscopy. Similar to the case for complex 3, two sets of signals for the pyrazolyl moieties of 4 appear in the NMR spectra with singlets at 2.38, 2.55, 2.55, and 2.64 ppm (1H) and 15.4, 16.5, 17.5, and 17.7 ppm (13C), respectively, assigned to the four methyl groups. This indicates again the formation of a racemic mixture of an unsymmetrical and chiral isomer with the chlorido ligand in trans position with regard to one pyrazolyl donor. Two 13C resonances for the CO ligands were observed at δ 194.3 and 197.4 ppm in the case of complex 4, which agree well with those of complex 3 (see above). As mentioned above, the IR spectrum of [Ru(bdmpza)Cl(CO)2] (3) exhibits two ν(CO) vibrations at ν 2066 and 1996 cm−1. Similar values have been found for the second dicarbonyl complex [Ru(2,2-bdmpzp)Cl(CO)2] (4): namely, ν 2062 and 1990 cm−1. Again, these values are in good accord with those found for the analogous complexes [Ru(L)Cl(CO)2] with η5-C5H5 and κ3-HB(pz)3 (Tp) ligand systems L, which have been described elsewhere.38−44 For both complexes 3 and 4 an additional band for the asymmetric carboxylate vibration was observed at ν(asCO2) 1676 cm−1. The molecular structure of the complex 4 was verified by a single-crystal X-ray structure determination (Figure 2). The molecular structure depicts the C1 symmetry of the molecule already indicated by the NMR spectra. The molecular

Figure 3. Molecular structure of [Ru(bdmpza)(CO)(μ2-CO)]2 (6). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−C(3) = 2.035(4), Ru(B)− C(3) = 2.018(4), Ru−C(4) = 1.865(4), Ru−O(1) = 2.118(3), Ru− N(11) = 2.183(3), Ru−N(21) = 2.179(3), Ru−Ru(B) = 2.7186(6), O(3)−C(3) = 1.177(5), O(4)−C(4) = 1.139(5); Ru−C(3)−Ru(B) = 84.25(14), O(1)−Ru−N(11) = 84.91(11), O(1)−Ru−N(21) = 84.60(11), N(11)−Ru−N(21) = 79.80(12), C(3)−Ru−O(1) = 86.84(12), C(3)−Ru(B)−O(1B) = 90.44(12), C(3)−Ru−C(4) = 93.62(15), C(3)−Ru(B)−C(4B) = 90.69(16), C(3)−Ru−N(11) = 92.31(13), C(3)−Ru(B)−N(11B) = 168.50(13), C(4)−Ru−N(11) = 94.46(14), C(3)−Ru−N(21) = 171.00(13), C(3)−Ru(B)−N(21B) = 91.47(13), C(4)−Ru−N(21) = 94.20(14).

Figure 2. Molecular structure of the dicarbonyl complex [Ru(2,2bdmpzp)Cl(CO)2] (4). Thermal ellipsoids are drawn at the 50% probability level, and most hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−C(4) = 1.922(2), Ru−C(5) = 1.900(2), Ru−Cl = 2.3726(5), Ru(1)−O(1) = 2.0783(13), Ru−N(11) = 2.1265(15), Ru−N(21) = 2.0703(15), O(4)−C(4) = 1.098(3), O(5)−C(5) = 1.091(3); O(1)−Ru−N(11) = 84.28(6), O(1)−Ru−N(21) = 84.25(6), N(11)−Ru−N(21) = 85.88(6), Cl−Ru−N(11) = 93.09(4), C(4)−Ru−N(21) = 94.52(7), C(5)−Ru−O(1) = 176.08(6), Cl−Ru−C(4) = 86.13(6), Cl−Ru− C(5) = 90.63(5), C(4)−Ru−C(5) = 91.76(8). 2168


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Scheme 2. Synthesis of Carbonyl Complexes [Ru(bdmpza)H(CO)2] (5a,b) and [Ru(bdmpza)(CO)(μ-CO)]2 (6)

synthesized [Ru(η5-C5H5)(CO)(μ2-CO)]2 via reaction of the ruthenium(II) precursor [Ru(CO)2I2]51 with an excess of sodium cyclopentadienide, Na[C5H5].52 Attempts to adopt this procedure by using potassium bis(3,5-dimethylpyrazol-1-yl)acetate instead of Na[C5H5] failed the first time because of the insolubility of K[bdmpza] in the aliphatic solvent. In further attempts an oxidative addition of bis(3,5-dimethylpyrazol-1yl)acetic acid to [Ru3(CO)12] was tried. This should result in the hydrido complex [Ru(bdmpza)H(CO)2], which might then be oxidized by oxygen and dimerize to [Ru(bdmpza)(CO)(μ2CO)]2 as reported for [Ru(η5-C5H5)(CO)(μ2-CO)]2.53 Indeed, the 1H NMR spectrum of the resulting product indicated formation of two hydrido complexes by two singlet signals at −13.32 and −10.10 ppm (in CDCl3) (Scheme 2). These signals have been assigned to two structural isomers, the symmetrical hydrido complex [Ru(bdmpza)H(CO)2] (5a) and the unsymmetrical hydrido complex [Ru(bdmpza)H(CO)2] (5b). In the Cs-symmetric isomer 5a the hydrido ligand resides trans to the carboxylate and only one set of signals is observed for the two 3,5-dimethylpyrazole donors in the 1H NMR spectrum, whereas the C1 symmetric, chiral, but racemic complex [Ru(bdmpza)H(CO)2] (5b) shows two sets of signals in the 1H NMR spectrum, instead. The solubility of the hydrido complex [Ru(bdmpza)H(CO)2] (5a,b) is rather poor in most solvents apart from chloroform and dichloromethane. Unfortunately, the complex decomposes quickly in CDCl3 by formation of the chlorido complex, a reactivity that was also reported for other hydrido complexes such as [Ru(η5-C5H5)H(CO)(PPh3)].53a In dichloromethane the stability of the complex is slightly better. Thus, only 1H NMR data could be obtained so far. Nevertheless, ESI-MS data and elemental analysis prove also the formation of 5a,b. Surprisingly, so far we have not been successful in isolating 6 from solutions of the hydrido complex [Ru(bdmpza)H(CO)2] (5a,b) that had been exposed to air. Obviously, the hydrido complex 5a,b seems to be quite unreactive regarding oxygen. Even heating under reflux in nonpolar solvents such as n-heptane and applying aerobic conditions did not yield complex 6 but mostly unreacted 5a,b. Thus, another attempt was undertaken by reacting the acetate

positions with a 50:50 distribution. The Ru−Ru distance (2.7186(6) Å) is slightly shorter compared to various analogous Cp compounds such as [Ru(η5-C5H5)(CO)(μ2-CO)]2 (d(Ru− Ru) = 2.7377(5)−2.7412(4) Å),45,46 [Ru(η5-C5Me5)(CO)(μ2CO)]2 (d(Ru−Ru) = 2.752(1) Å),47 and [Ru(η5-indenyl)(CO)(μ2-CO)]2 (d(Ru−Ru) = 2.7412(4) Å).48 It is noteworthy that the Ru−Ru distances of the analogous hydridotris(pyrazol-1-yl)borate complex [Ru{κ 3 -HB(pz) 3 }(CO) 2 ] 2 (d(Ru−Ru) = 2.882(1) Å) and of the hydridotris(1,2,4triazol-1-yl)borate complex [Ru{κ3-HB(tz)3}(CO)2]2 (d(Ru− Ru) = 2.8688(7) Å) are significantly longer.42,49 Both complexes do not exhibit μ2-CO ligands at all. In contrast to these dinuclear ruthenium(I) complexes, which exhibit a staggered conformation with the Ru{κ3-HB(pz)3}(CO)2 or Ru{κ3-HB(tz)3}(CO)2 fragments twisted by about 45°, the new dinuclear ruthenium(I) complex [Ru(bdmpza)(CO)(μ2-CO)]2 exhibits a trans conformation with the four pyrazole donors, the bridging carbonyl ligands, and both ruthenium centers almost in plane. This geometry conforms to most molecular structures deposited in the CCDC database for [Ru(η5-C5H5)(CO)(μ2CO)]2.45,46 The CO bond lengths of the terminal and the bridging carbonyl ligands (d(C(4)−O(4)) = 1.139(5) Å and d(C(3)−O(3)) = 1.177(5) Å]) are comparable to those of the referenced Cp complexes, as well as to that of [Ru(η5-tBuC5H4)(CO)(μ2-CO)]2.50 The Ru−CO distances are 1.863(3) Å for the terminal carbonyl and 2.034(3) and 2.014(3) Å for the bridging carbonyls, respectively, which is also quite comparable to the values for the analogous Cp complexes. The bridging Ru−C−Ru angle is 84.34(10)°, compared to 84.6(3)° in [Ru(η5-C5H5)(CO)(μ2-CO)]2.46b Thus, due to its lower steric hindrance, the bdmpza ligand resembles more a Cp ligand than a Tp ligand in these complexes. On the other hand, large differences between Cp and bdmpza are obvious, due to the N and O donors in bdmpza and the thereby resulting diverse trans influences, as discussed above in the case of 3 and 4. There are several procedures described in the literature for the synthesis of the analogous cyclopentadienyl compound [Ru(η5-C5H5)(CO)(μ2-CO)]2. Thus, attempts were undertaken to rationally synthesize this compound. Fischer et al. 2169


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polymer catena-[Ru(OAc)(CO)2]n with Hbdmpza. The polymer catena-[Ru(OAc)(CO)2]n is readily available but is also easily accessible by reacting [Ru3(CO)12] with acetic acid.54 It has been successfully applied in the syntheses of various dinuclear ruthenium(I) complexes before.42,49 Reaction in THF at reflux for 24 h replaced the acetate of catena-[Ru(OAc)(CO)2]n by bis(3,5-dimethylpyrazol-1-yl)acetic acid and resulted in the target complex [Ru(bdmpza)(CO)(μ2-CO)]2 (6) in a yield of 30%. The constitution of the molecule is confirmed by elemental analysis as well as by ESI MS data in acetonitrile, which show a 100% peak at m/z 405.02 (100) assigned to a [Ru(bdmpza)(CO)2]+ fragment and a small (4%) molecular ion peak at m/z 810.05. Due to the low solubility of 6 in all common deuterated solvents, only 1H NMR data could be obtained so far. As expected for the C2h-symmetric molecule depicted in Figure 3, only one set of signals is observed, with the methyl singlet signals observed at 2.35 (Me3) and 2.62 (Me5) ppm. The pyrazole CH proton is found at 6.04 ppm and the methine proton at 6.31 ppm. In theory at least three isomeric forms of complex 6 might be possible: (I) terminal trans-CO/μ2-CO bridged, (II) terminal cis-CO/μ2-CO bridged, (III) nonbridged. Apparently, according to the NMR data only one of these possible isomeric forms seems to be present in solution. This is in contrast to the case for [Ru(η5C5H5)(CO)(μ2-CO)]2, where an equilibrium of various isomeric forms was reported.50,55 The bdmpza ligand exhibits its typical IR vibrations at 1673 cm−1 (as-CO2−) and 1559 cm−1 (CN) as expected for κ3 coordination. The IR spectrum in solution (CHCl3 solvent) is almost identical with that obtained in a KBr matrix. IR vibrations (CHCl3) at 1978 cm−1 (terminal CO) and 1761 cm−1 (μ2-CO) agree well with those reported for μ2-CO isomers of [Ru(η5-C5H5)(CO)(μ2-CO)]2 (ν(CO) (CHCl3 solvent) 2009 cm−1 (terminal CO) and 1768 cm−1 (μ2-CO); ν(CO) (MeCN solvent) 1995 cm−1 (terminal CO) and 1775 cm−1 (μ2-CO)).55b Thus, owing to the observed very strong μ2-CO vibration one μ2-CO isomer seems to dominate in the solid state as well as in solution. Nevertheless, a very weak shoulder around 2010 cm−1 and a weak signal at 1950 cm−1 might indicate traces of a nonbridged species. Due to the steric hindrance of the bdmpza ligands and in accord with DFT calculations (see below), the μ2-CO isomer cis-[Ru(bdmpza)(CO)(μ2-CO)]2 (isomer II) with cis geometry of the terminal CO ligands seems to be thermodynamically disfavored. Thus, in accordance with the solid-state structure (Figure 3) trans[Ru(bdmpza)(CO)(μ2-CO)]2 (6) (isomer I) is the main isomeric form. To elucidate the spectroscopic properties and the binding situation in [Ru(bdmpza)(CO)(μ2-CO)]2 (6) further, DFT calculations were performed for 6 starting from the X-ray structure determination data. The resulting geometry of the DFT calculations was almost identical with the geometry of the X-ray structure determination. The spin density of the two electrons forming the Ru−Ru bond is mainly located at the metal centers and the bridging carbonyl ligands (Figure 4). Surprisingly, the spin density plot does not resemble the contour plots of two dz2 orbitals but the contour plots of dxy, dxz, or dyz orbitals. This implies that the Ru−Ru bond is better described as a π bond than as a σ bond. In order to verify the IR signals of 6, DFT calculations on 6 were performed. It is wellknown for the chosen B3LYP/6-31G* DFT functional and basis set that calculated vibrational frequencies are typically overestimated in comparison to experimental data. These errors arise from the neglect of anharmonicity effects, incomplete incorporation of electron correlation, and the use of finite basis

Figure 4. Spin density plot regarding the electrons forming the Ru− Ru bond.

sets in the theoretical treatment.56 In order to achieve a correlation with observed spectra, a scaling factor of approximately 0.96 has to be applied.56 Depending on the examined vibration, this factor differs slightly even in the same molecule and is usually greater for lower energies.57 We were especially interested in the two carbonyl vibrations, which were predicted (unscaled) at 2078 cm−1 (terminal CO) and at 1851 cm−1 (μ2-CO). This leads to expected vibrations at 1995 and 1777 cm−1. Both values agree well with the experimental data. In further agreement with the experimental data, the trans geometry of the bridged isomer of 6 was found to be the lowest in energy. The energy difference between the bridged and nonbridged (Figure 5) species was found to be rather small,

Figure 5. Calculated geometry of a nonbridged isomer of 6.

with ΔE = 22 kJ/mol in comparison to an energy difference of ΔE = 45 kJ/mol between the cis and trans geometries. The low energy difference toward the unbridged isomer implies a rather high possibility of finding the nonbridged isomer in solution, which may agree with the data of the IR spectra discussed above. The strong asymmetric IR vibrations of the nonbridged CO were predicted (unscaled) at 2075 and 2047 cm−1, which should result in vibrations around 1992 and 1965 cm−1. Experiments regarding the oxidation of cyclohexene mediated by complexes 3 and 4 were carried out in unstabilized, HPLC grade CH2 Cl2 under a dinitrogen 2170


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PhIO in addition to a stoichiometric amount of H2O2 or DCPNO were carried out. However, the yields obtained in these experiments were as low as 3.3 and 4.6%, respectively, indicating that PhIO is required in a stoichiometric amount. These preliminary studies reveal the promising potential of both complexes 3 and 4 as epoxidation catalysts. In this work, the TON for complex 3 could be improved to up to 20.6 by tuning the reactant/catalyst ratio. Even higher TON values might be accessible by optimization of the reaction conditions: e.g., by using soluble iodosylbenzene derivatives. In upcoming work, we also intend to reveal the mechanistic details of the catalytic cycle.

atmosphere using decane as internal standard. In addition to aqueous hydrogen peroxide, two nucleophilic oxidizing agents, namely iodosylbenzene (PhIO) and 2,6-dichloropyridine Noxide (DCPNO), have been tested in these reactions (Scheme 3). In a preliminary, nonquantitative run, it was found that both Scheme 3. Epoxidation of Cyclohexene with [Ru(bdmpza)Cl(CO)2] (3) or [Ru(2,2-bdmpzp)Cl(CO)2] (4)

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SUMMARY Dicarbonyl complexes [Ru(bdmpza)Cl(CO)2] (3) and [Ru(2,2-bdmpzp)Cl(CO)2] (4), bearing a bis(3,5-dimethylpyrazol1-yl)acetato (bdmpza) or a 2,2-bis(3,5-dimethylpyrazol-1yl)propionato (2,2-bdmpzp) scorpionate ligand, are accessible by reacting the polymer [RuCl2(CO)2]n with either K[bdmpza] or K[2,2-bdmpzp]. Reaction of the acid Hbdmpza with [Ru3(CO)12] results in the formation of the hydrido complex [Ru(bdmpza)H(CO)2] (5a,b) as a mixture of structural isomers. The dimer [Ru(bdmpza)(CO)(μ2-CO)]2 (6) is easily obtained via the reaction of Hbdmpza with catena-[Ru(OAc)(CO)2]n, whereas in contrast to the reported synthesis of [Ru(η5-C5H5)(CO)(μ2-CO)]2 conversion of [Ru(bdmpza)H(CO)2] (5a,b) to 6 was not successful so far. Preliminary studies reveal the promising potential of [Ru(bdmpza)Cl(CO)2] (3) and [Ru(2,2-bdmpzp)Cl(CO)2] (4) as epoxidation catalysts, with a TON for complex 3 up to 20. Even higher TON values might be accessible by optimization of the reaction conditions or by applying soluble iodosylbenzene derivatives as oxidant.

complexes 3 and 4 are highly selective for the formation of cyclohexene oxide. Other oxidation products have only been found in traces (Scheme 2). Table 1 summarizes the results of the epoxidation studies. It is noteworthy that complex 3 with the unsubstiuted bdmpza ligand 1 shows a higher catalytic activity than the analogous complex 4 with the bridge-functionalized 2,2-bdmpzp ligand 2. A direct comparison with 10 mol % catalyst reveals that for complex 3 a yield of 65.1% (6.6 turnovers) can be obtained while complex 4 is only obtained in 46.6% yield (4.7 turnovers) when PhIO is used as the oxygen donor. Actually, the catalytic activity of complex 4 bearing the methyl-substituted N,N,O ligand 2 was expected to be higher than that of complex 3. In a recent report we were able to show that changing steric properties caused by the methyl group introduced in ligand 2 can facilitate the dissociation of coligands,16 resulting in coordinatively unsaturated species that might serve as key intermediates in catalytic cycles. Furthermore, it was found that catalytic activity is only found with iodosylbenzene. When H2O2 or DCPNO was utilized as the oxidizing agent, almost no activity was observed. The initially colorless reaction mixtures turned greenish after 24 h, which is probably due to the decomposition of the catalyst. It is very likely that 3 and 4 are only precatalysts. Recently Sariego et al. reported on similar ruthenium(II) complexes of the type [Ru(L)Cl2(CO)2] with bipyridine, phenanthroline, biquinoline, and pyridine ligands L. It was shown that these compounds moderately catalyze the epoxidation of cyclohexene in the presence of iodosylbenzene.58 It was assumed that a RuIV=O species is formed during the catalytic cycle and that the release of a labile bound CO ligand seems to be the crucial factor for catalytic activity. This could also be true for the dicarbonyl complexes 3 and 4. In order to check whether the presence of PhIO is essential for the generation of a catalytically active highvalent oxo species, two experiments using a catalytic amount of

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EXPERIMENTAL SECTION

All operations were carried out under an N2 atmosphere by applying conventional Schlenk techniques. The yields refer to analytically pure substances. Elemental analyses were determined with a Euro EA 3000 (Euro Vector) and EA 1108 (Carlo Erba) instrument (σ = ±1% of the measured content). IR spectra were recorded with an Excalibur FTS3500 FTIR in CaF2 cuvets (0.2 mm) or as KBr pellets. 1H and 13C NMR spectra were measured with a Bruker AC 250 and a Bruker DPX300 Avance instrument. The δ values are given relative to tetramethylsilane (1H) or the deuterated solvent (13C). Mass spectra were recorded with a Finnigan MAT 312 and a JEOL JMS-700 instrument by using either FD or FAB techniques with 3-nitrobenyzl alcohol (NBOH) as matrix. X-ray structure determinations were carried out on a Bruker-Nonius Kappa-CCD diffractometer. Gas chromatography was performed on a Shimadzu GC17A instrument equipped with a flame ionization detector using a Roticap-5 column (length 60 m, i.d. 0.25 mm, film 0.25 μm). Hbdmpza, 2,2-Hbdmpzp, and [RuCl2(CO)2]n were prepared according to the literature.3,16,39,59

Table 1. Results of Epoxidation of Cyclohexene with Dicarbonyl Complexes 3 and 4 cat. [Ru]

oxidant

t, h

neduct(t0), mmol

ncat., mmol

neduct(t), mmol

nproduct(t), mmol

y, %

TON

TOF, 10−5s−1

3 3 3 3 3 3 3 4

PhIO PhIO PhIO H2O2 DCPNO PhIO/H2O2 PhIO/DCPNO PhIO

24 22.5 24 23 23 24 24 22.5

0.503 0.503 0.750 0.503 0.503 0.503 0.503 0.503

0.050 0.025 0.025 0.050 0.050 0.050 0.050 0.050

0.002 0.001 0.001 0.488 0.472 0.440 0.434 0.092

0.327 0.342 0.515 0.008 0.000 0.017 0.023 0.234

65.1 67.9 68.6 1.6 0.0 3.3 4.6 46.6

6.6 13.7 20.6 0.2 0.0 0.3 0.5 4.7

7.58 16.88 23.83 0.20 0.00 0.39 0.53 5.79

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Organometallics

Article

[M5 + Na+]. Anal. Calcd for C14H16N4O4Ru (405.37 g/mol): C, 41.48; H, 3.98; N, 13.82. Found: C, 41.48; H, 3.84; N, 14.03. Synthesis of [Ru(bdmpza)(CO)(μ2-CO)]2 (6). To a suspension of [Ru2(O2CCH3)(CO)2]n (233 mg, 1.07 mmol) in THF (30 mL) was added Hbdmpza (1; 293 mg, 1.18 mmol). The suspension was heated at reflux for 24 h, forming the dinuclear product [Ru2(bdmpza)(CO)(μ2-CO)]2 (6). The yellow precipitate was filtered off, washed with THF (2 × 20 mL), and dried in vacuo. Yield: 132 mg (0.16 mmol, 30%). 1H NMR (CDCl3, 300.13 MHz): δ 2.35 (s, 6 H, Me3, Me3′), 2.62 (s, 6 H, Me5, Me5′), 6.04 (s, 2 H, H4, H4′), 6.31 (s, 1H, CH) ppm. 1H NMR (CD2Cl2, 300.13 MHz): δ 2.43 (s, 6H, Me3, Me3′), 2.67 (s, 6H, Me5, Me5′), 6.15 (s, 2H, H4, H4′), 6.35 (s, 1H, CH) ppm. IR (KBr): ν 2930 (w, CH), 1982 (s, CO), 1762 (s, μ2-CO), 1675 (s, as-CO2−), 1561 (w, CN) cm−1. IR (CHCl3): ν = 2076 (vw), 2069 (vw), 2010 (vw-sh), 1978 (s, CO), 1950 (w), 1761 (s, μ2-CO), 1673 (s, as-CO2−), 1602 (vw), 1559 (w, CN) cm−1. ESI MS (MeCN): m/z (%) 405.02 (100) [Ru(bdmpza)(CO)2+], 810.05 (4) [M+], 828.06 (42) [M+ + H2O], 881.11 (62) [M+ + 4 H2O], 899.10 (60) [M+ + 5 H2O]. UV/vis (CH2Cl2): λmax (log ε) 287.0 (3.88), 342.9 (3.23), 403.0 nm (3.09). Anal. Calcd for C28H30N8O8Ru2 (808.73 g/mol): C, 41.58; H, 3.74; N, 13.86. Found: C, 41.95; H, 3.72; N, 13.67. Mp: 282−285 °C dec. General Procedure for the Epoxidation Catalysis. An evacuated and nitrogen-flushed Schlenk tube with a Teflon rotaflow stopcock was charged with the catalyst (0.03 − 0.10 equiv), cyclohexene (1.0 equiv), and unstabilized CH2Cl2 (5 mL). After addition of the oxidant (1.0−2.5 equiv) the tube was closed and stirred for 22−24 h at ambient temperature under a dinitrogen atmosphere. The crude mixture was filtered over a short plug of silica gel and analyzed by GC. The quantification was done by using calibration curves which have been recorded with standard samples of known concentration of the reactant and the product cyclohexene oxide. Calculations. All DFT calculations and full geometry optimizations were carried out by using Jaguar 7.7.10760 running on Linux 2.6.18− 238.el5 SMP (x86_64) on two AMD Phenom II X6 1090T processor workstations (Beowulf-cluster) parallelized with OpenMPI 1.3.4. The X-ray structure of 6 was used as the starting geometry. Complete geometry optimizations were carried out on the implemented LACVP* (Hay-Wadt ECP basis on heavy atoms, N31G6* for all other atoms) basis set and the B3LYP density functional. The harmonic vibrational frequencies were calculated by the analytical evaluation of the second dervative of the molecular energy on an identical basis set. All calculated structures were proven to be true minima by the absence of imaginary frequencies. The spin density distribution was calculated on the geometry of the X-ray structure. Plots were obtained using Maestro 9.1.207, the graphical interface of Jaguar. X-ray Structure Determinations. A Bruker-Nonius KappaCCD diffractometer was used for data collection (graphite monochromator, Mo Kα radiation, λ = 0.710 73 Å). Single crystals of 3, 4, and 6 were coated with perfluoropolyether, picked with a glass fiber, and immediately mounted in the nitrogen cold gas stream of the diffractometer. The structures were solved by using direct methods and refined with full-matrix least squares against F2 (Siemens SHELX97).61 A weighting scheme was applied in the last steps of the refinement with w = 1/[σ2(Fo2) + (aP)2 + bP] and P = [2Fc2 + max(Fo2,0)]/3. Hydrogen atoms were included in their calculated positions and refined in a riding model. The asymmetric units of 3 and 6 each contain two chloroform molecules. In case of 6 one of these chloroform molecules is disordered over two positions with occupancies of 50% each. All details and parameters of the measurements are summarized in Table S1 (Supporting Information). CCDC 839082 (3), 839083 (4), and 839084 (6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The structure pictures were prepared with the program Diamond 2.1e.62

Synthesis of [Ru(bdmpza)Cl(CO)2] (3). A solution of the Hbdmpza ligand 1 (1.17 g, 4.70 mmol) in THF (50 mL) was treated with KOtBu (527 mg, 4.70 mmol) and stirred for at least 1 h at ambient temperature. After addition of [RuCl2(CO)2]n (1.07 g, 4.70 mmol), the reaction mixture was heated under reflux and controlled by IR spectroscopy on a regular basis. After completion of the reaction (approximately 12 h), the solvent was removed under reduced pressure. The residue was extracted with CH2Cl2 (15 mL) and the extracts were filtered and treated with n-pentane (350 mL), precipitating complex 3 as a cream white powder. Crystals suitable for single-crystal structure analysis were obtained by slow evaporation of a CDCl3 solution of complex 3. Yield: 570 mg (27%). 1H NMR (CDCl3, 300 MHz): δ 2.34 (s, 3 H, CH3), 2.45 (s, 3 H, CH3), 2.46 (s, 3 H, CH3), 2.59 (s, 3 H, CH3), 6.12 (s, 1 H, Cpz H), 6.12 (s, 1 H, Cpz H), 6.60 (s, 1 H, CH) ppm. 13C NMR (CDCl3, 75.5 MHz): δ 10.9 (CH3), 11.2 (CH3), 14.5 (CH3), 15.3 (CH3), 68.4 (CH), 109.3, 109.4 (C4 and C4′), 142.6, 144.0 (C5 and C5′), 154.9, 155.8 (C3 and C3′), 165.3 (CO2−), 192.6 (CO), 196.0 (CO) ppm. IR (KBr): ν 2066 (s, CO), 1996 (s, CO), 1676 (s, as-CO2−), 1559 (w, CN) cm−1. IR (CH2Cl2): ν 2074 (s, CO), 2005 (s, CO), 1681 (s, as-CO2−), 1563 (w, CN) cm−1. FAB MS (NBOH): m/z (%) 440 (100) [M+], 441 (75) [MH+], 880 (21) [M2]. Anal. Calcd for C14H15ClN4O4Ru (439.82 g/ mol): C, 38.23; H, 3.44; N, 12.74. Found: C, 38.12; H, 3.50; N, 12.42. Synthesis of [Ru(2,2-bdmpzp)Cl(CO)2] (4). A solution of the 2,2-Hbdmpzp ligand (2; 525 mg, 2.00 mmol) in dry THF (40 mL) was treated with KOtBu (224 mg, 2.00 mmol) for 1 h at ambient temperature. After addition of [RuCl2(CO)2]n (456 mg, 2.00 mmol), the reaction mixture was heated under reflux and monitored by IR spectroscopy on a regular basis. After completion of the reaction (approximately 24 h), the solvent was removed under reduced pressure. The residue was washed with degassed water (5 × 10 mL) and diethyl ether (5 × 10 mL) and dried in vacuo to yield dicarbonyl complex 4 as a yellow crystalline powder. Single crystals suitable for an X-ray structure determination were obtained by layering a CH2Cl2 solution of complex 4 with diethyl ether. Yield: 633 mg (70%). 1H NMR (CD2Cl2, 300 MHz): δ 2.38 (s, 3 H, CH3), 2.55 (s, 6 H, 2 × CH3), 2.64 (s, 3 H, CH3), 2.67 (s, 3 H, CH3), 6.07 (s, 1 H, Cpz H), 6.09 (s, 1 H, Cpz H) ppm. 13C NMR (CD2Cl2, 75.5 MHz): δ 15.4 (CH3), 16.5 (CH3), 17.5 (CH3), 17.7 (CH3), 23.9 (CH3), 84.1 (Cbridge), 112.7, 112.9 (C4 and C4′), 144.7, 145.9 (C5 and C5′), 154.3, 155.1 (C3 and C3′), 165.9 (CO2−), 194.3 (CO), 197.4 (CO) ppm. IR (THF): ν 2063 (s, CO), 1994 (s, CO), 1691 (m, as-CO2−) cm−1. IR (KBr): ν 2062 (s, CO), 1990 (s, CO), 1676 (s, as-CO2−) cm−1. FDMS (CH2Cl2): m/z (%) 455 (100) [MH+]. Anal. Calcd for C15H17ClN4O4Ru (453.84 g/mol): C, 39.70; H, 3.78; N, 12.34. Found: C, 39.48; H, 3.75; N, 12.13. Mp: 266 °C dec. Synthesis of [Ru(bdmpza)H(CO)2] (5a,b). To a suspension of [Ru3(CO)12] (522 mg, 0.81 mmol) in toluene (50 mL) was added Hbdmpza (1; 811 mg, 3.26 mmol). The suspension was heated at reflux for 24 h until complete decolorization was achieved. The white residue was filtered off, washed with toluene (2 × 10 mL), and dried in vacuo to yield a mixture of isomers 5a,b in a 1:0.7 ratio. Yield: 887 mg (2.19 mmol, 90%). Data for isomer 5a are as follows. 1H NMR (CDCl3, 300.13 MHz): δ −13.32 (s, 1H, Ru−H), 2.38 (s, 6H, Me3, ́ Me3′), 2.42 (s, 6H, Me5, Me5′), 6.02 (s, 2H, H4, H4), 6.48 (s, 1H, CH) 1 ppm. H NMR (CD2Cl2, 300.13 MHz): δ −13.09 (s, 1H, Ru−H), 2.37 (s, 6H, Me3, Me3′), 2.42 (s, 6H, Me5, Me5′), 6.04 (s, 2H, H4, H4′), 6.42 (s, 1H, CH) ppm. Data for isomer 5b are as follows. 1H NMR (CDCl3, 300.13 MHz): δ −10.10 (s, 1H, Ru−H), 2.38 (s, 3H, Me3), 2.41 (s, 3H, Me3′), 2.42 (s, 3H, Me5), 2.45 (s, 3H, Me5′), 6.05 (s, 1H, H4), 6.07 (s, 1H, H4′), 6.43 (s, 1H, CH) ppm. 1H NMR (CD2Cl2, 300.13 MHz): δ −10.10 (s, 1H, Ru−H), 2.40 (s, 3H, Me3), 2.41 (s, 3H, Me3′), 2.42 (s, 3H, Me5), 2.44 (s, 3H, Me5′), 6.07 (s, 1H, H4), 6.09 (s, 1H, H4′), 6.39 (s, 1H, CH) ppm. IR (KBr): ν̃ 3014 (w, CH), 2037 (s, CO), 2002 (w, Ru−H), 1961 (s, CO), 1664 (s, as-CO2−), 1560 (w, CN) cm−1. ESI-MS (MeCN): m/z (%) 407.03 (32) [M + H+], 812.05 (100) [M2 + H+], 835.03 (27) [M2 + Na+], 851.01 (13) [M2 + K+], 1240.06 (28) [M3 + Na+], 1646.08 (22) [M4 + Na+], 2050.11 (4) 2172


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ASSOCIATED CONTENT

S Supporting Information *

A table and CIF files giving crystallographic data and structure refinement details for 3, 4, and 6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+49) 9131-85-28976. Fax: (+49) 9131-85-27387. Email: [email protected]. Author Contributions §

All three authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (SFB 583).

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REFERENCES

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Organometallics

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