Aqua and Diethyl Ether PNNP Complexes of Ruthenium(II): Structure

ARTICLE pubs.acs.org/Organometallics

Aqua and Diethyl Ether PNNP Complexes of Ruthenium(II): Structure and Solution Behavior Christoph Schotes, Marco Ranocchiari, and Antonio Mezzetti* Department of Chemistry and Applied Biosciences, ETH Z€urich, CH-8093 Z€urich, Switzerland

bS Supporting Information ABSTRACT: Addition of water to CD2Cl2 solutions of the five-coordinate complex [RuCl(PNNP)]PF6 (2) (formed by treating [RuCl2(PNNP)] (1) with TlPF6 (1 equiv) in dichloromethane) yields the aqua complexes cis-β-[RuCl(OH2)(PNNP)]PF6 (3) and trans-[RuCl(OH2)(PNNP)]PF6 (4) in a 6:1 ratio (PNNP is the chiral ligand (1S,2S)-N, N0 -bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine). An excess of H2O (10 equiv) is needed for quantitative conversion of 2. The cis-β isomer 3 is more labile than the trans analogue 4 and is in slow-regime chemical exchange with five-coordinate 2 in CD2Cl2. The trans aqua complex 4 does not dissociate in CDCl3 over 6 h, but undergoes rapid equilibration with 2 and 3 upon dissolution in CD2Cl2. The X-ray structure of (Λ)-cis-β-3 shows an intramolecular hydrogen bond between the aqua and the chloro ligands. The elusive Et2O adduct [RuCl(OEt2)(PNNP)]PF6 (5), obtained by treating 1 with (Et3O)PF6 (1 equiv) in dichloromethane, is formed as the cis isomer only. At room temperature, 5 exists in equilibrium with five-coordinate 2 and free Et2O. The equilibrium is fast on the NMR time scale and is shifted toward 5 at low temperature.

’ INTRODUCTION The affinity for oxygen-containing ligands (oxophilicity) is a key property of early transition metals, which progressively declines on going toward the late transition elements.1 Oxophilicity is a manifestation of the hard nature of these metal ions, as expressed by the HSAB principle.2 However, according to the principle of symbiosis, the ligands influence the hard/soft character of the metal in a complex.3 Therefore, the whole coordination sphere must be considered when discussing the affinity of a metal for a particular ligand class. For example, hard co-ligands increase the affinity of ruthenium for oxygen donors, the most striking and elegant example being [Ru(OH2)6]2þ.4 In combination with soft or borderline ligands, such as phosphines and chloro, the affinity of ruthenium(II) for oxygen donors is low, and only a few aqua complexes are known when P and Cl donors are simultaneously present in its coordination sphere.5 In the last years, we have developed highly enantioselective cationic ruthenium catalysts6 containing chiral tetradentate ligands with a P2N2 donor set such as (1S,2S)-N,N0 -bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine (PNNP). Single chloride abstraction from [RuCl2(PNNP)] (1)7 with Tl(I)8 or Ag(I)9 salts gives the monocationic, five-coordinate complex [RuCl(PNNP)]þ (2) (Scheme 1), which catalyzes enantioselective atom-transfer reactions such as epoxidation,8 cyclopropanation,9,10 and aziridination.11 Complex 2, and in general the ruthenium PNNP fragment, is unusually oxophilic for a late transition metal.6 For instance, we have previously isolated the aqua complex [RuCl(OH2)(PNNP)]þ as a mixture of isomers, whose structural characterization is the topic of this report.8 Also, we have reported that chloride abstraction from 1 r 2011 American Chemical Society

with (Et3O)PF6 (Meerwein’s salt) produces the elusive cationic Et2O adduct [RuCl(OEt2)(PNNP)]PF6 (5), in which the Et2O molecule formed in the alkylation of chloride binds to the ruthenium PNNP fragment to give a rare example of an ether complex of ruthenium(II).12 Double chloride abstraction from 1 with (Et3O)PF6 (2 equiv) produces a particularly oxophilic dicationic Ru/PNNP fragment that binds β-ketoesters in a chelating fashion to give rare examples of complexes containing 1,3-dicarbonyl compounds in their non-enolized form.13 These complexes catalyze the Michael addition13 and electrophilic functionalization of β-ketoesters.14 Recently, we have reported [2þ4] and [2þ2] cycloaddition reactions with the corresponding unsaturated β-ketoesters.15 Chloride scavengers based on Ag(I) salts abstract both chloro ligands from complexes of type 1 containing differently substituted PNNP ligands and give aqua complexes of the type cis-β-[Ru(OH2 )2 (PNNP)]2þ upon addition of water.16 Although the monocationic complex [RuCl(PNNP)]þ (2) is considerably less oxophilic than the dicationic species just mentioned, the coordination of oxygen donors (such as water, diethyl ether, THF, or acetone) plays an important role, as it modifies the catalytic activity and selectivity of the Ru/PNNP complexes. In epoxidation and cyclopropanation, the presence of water or other oxygen donors reduces the activity and enantioselectivity of the catalyst.810,12 In contrast, we have recently found that the monocationic six-coordinate adducts [RuCl(L)(PNNP)]þ Received: April 4, 2011 Published: June 14, 2011 3596

dx.doi.org/10.1021/om200289x | Organometallics 2011, 30, 3596–3602

Organometallics Scheme 1

Figure 1. (31P,31P)-EXSY NMR spectrum of the reaction solution of 2 with H2O (1 equiv) (CD2Cl2, 202 MHz, 25 °C).

(L = OH2 or OEt2) are the most suitable ruthenium PNNP complexes for the aziridination of imines with ethyl diazoacetate.11 In particular, the temperature effects observed in the imine aziridination reaction catalyzed by these aqua and Et2O adducts called for a better understanding of the dissociation and cistrans equilibria involving the Et2O and aqua adducts.11 Therefore, we reinvestigated the aqua complex cis-β-[RuCl(OH2)(PNNP)]PF6 (3), its trans analogue 4, and the elusive Et2O adduct [RuCl(OEt2)(PNNP)]PF6 (5).12 We describe herein the structural features and dissociation equilibria of these complexes.

’ RESULTS AND DISCUSSION Addition of Water to [RuCl(PNNP)]PF6 (2). The five-coordinate complex [RuCl(PNNP)]PF6 (2) was prepared by chloride abstraction from [RuCl2(PNNP)] (1) with TlPF6 in CD2Cl2 according to the usual procedure (Scheme 1).8 Upon addition of water (1 equiv), the water droplet dissolved into CD2Cl2 within 1 min, and broad signals of the cis-β aqua complex [RuCl(OH2)(PNNP)]PF6 (3) (63%) appeared in the roomtemperature inverse-gated decoupled 31P NMR spectrum of the reaction solution. The other species present were unreacted 2 (29%) and trans-[RuCl(OH2)(PNNP)]PF6 (4) (8%). The room-temperature (31P,31P)-EXSY spectrum (202 MHz, mixing time = 50 ms) shows that five-coordinate 2 and the cis-β aqua complex 3 are in mutual exchange (Figure 1). The observation of

ARTICLE

Scheme 2

individual, nonaveraged signals for 2 and 3 indicates that the system is in the slow-exchange regime. No detectable exchange process involves the trans isomer 4, which implies that its dissociation is slow on the NMR time scale at room temperature.17 The equilibria involving the five-coordinate complex 2 and the aqua derivatives 3 and 4 are summarized in Scheme 2. The (31P,31P)-EXSY NMR spectrum at 10 °C (202 MHz, mixing time = 40 ms, 2 þ 1 equiv H2O) shows that 2 and 3 are still slowly exchanging, whereas no exchange is detected at 15 °C. A series of (31P,31P)-EXSY experiments at room temperature with different mixing times gave a value of 47(9) s1 for the exchange rate constant k. Upon increasing the overall amount of water from 1 to 5 equiv versus ruthenium, all 31P NMR signals sharpened, and the overall conversion of the fivecoordinate species 2 to the aqua complexes 3 and 4 increased from 71% to 93% (82% and 11% for 3 and 4, respectively). The composition of the solution (81% 3, 13% 4, 6% 2) and the chemical shifts of 3 and 4 did not change upon increasing the excess of water to a total of 20 equiv (Table 1). In the range between 1 and 20 equiv of H2O, the 3:4 cis/trans ratio remained essentially constant at ca. 6:1. The stereochemistry of the isomeric aqua complexes 3 and 4, prepared by adding water (20 equiv) to five-coordinate 2 in CD2Cl2, was inferred from the 202 MHz (31P,1H)-HMQC spectrum, which was recorded at 60 °C to prevent possible cross-peaks from chemical exchange. In the major isomer 3, the deshielded 31P NMR signal at δ 62.8 does not correlate to either imine proton, whereas the low-frequency phosphine signal shows a cross-peak to the imine signal at δ 8.85 with a resolved P,H coupling across the PRuNdCH moiety (Figure 2). The 4JP,H coupling constant of 9.6 Hz is diagnostic of a mutual trans arrangement of the phosphine and imine. In fact, its magnitude depends on the same electronic effects that influence the value of the 2JP,P’ coupling constant, which is known to be larger in trans complexes than in cis ones.18 Accordingly, the other imine proton gives a slightly broadened singlet at δ 8.73 in the 1H NMR spectrum with no correlation to phosphorus. In fact, when the phosphino and imino ligands are mutually cis, the 4 JP,H coupling is too small to be resolved (20

67 62.7

45.9 45.5

br 31.6

CD2Cl2

25

20

50.6

44.1

27.1

CD2Cl2

25

5

50.6

44.1

27.1

CD2Cl2

25

1

50.7

43.8

27.1

CD2Cl2

20

1

51.1

43.8

26.9

CDCl3

25

0

52.2

44.2

26.6

CDCl3

25

20

50.7

43.5

27.3

CD2Cl2 CD2Cl2

25 25

0 >20

52 50.6

45 44.1

27.3 26.5

41

b

c

55.5

36.9

29.6

CD2Cl2

25

CD2Cl2

78

The signal at δ 64.0 is broad and exhibits a reduced 2JP,P' constant of 25 Hz. b Not observed. c The signal at δ 41 is broad and featureless.

The above assignment is supported by the observation that one of the phosphines in 3 resonates at a much higher frequency (ca. δ 63) than the other one (ca. δ 45). This indicates that these P donors are trans to ligands with a largely different trans influence,19 such as aqua and imine, and is hence diagnostic of a cis-β configuration. The same pattern has been observed for the 31 P NMR chemical shifts in [Ru(OH2)2(PNNP)]2þ.16 In the trans isomer 4, the P atoms resonate at similar and significantly lower frequency (δ 50.6 and 44.1, Table 1), as they are trans to the same donor type, that is, imine. On the basis of these results, we correct herewith our previous tentative assignment of the signals of 4 to a cis-β complex.8 For comparison, we measured a (31P,1H)-HMQC NMR spectrum of the five-coordinate complex [RuCl(PNNP)]PF6 (2) at 60 °C (Figure 3). As usually observed despite all precautions (see Experimental Section), the aqua complexes 3 and (traces of) 4 are present as impurities. An interesting feature is that the 1H NMR signals of the imine protons of 2 overlap with those of the cis-β aqua complex 3. This suggests that the PNNP ligand assumes a cis-β configuration in both complexes and lends further support6 to our assignment of the pseudo-trigonal-bipyramidal geometry20 to the five-coordinate complex 2 (Scheme 2). Isolated [RuCl(OH2)(PNNP)]PF6. The aqua complexes 3 and 4 were prepared and isolated as previously reported8 by adding a dichloromethane solution of five-coordinate 2 to a water/2propanol solution (1:1 v/v), followed by evaporation of dichloromethane. The 31P NMR spectrum of the yellow-orange solid in CDCl3 showed the signals of 3 and 4 in an approximate 7:3 ratio as previously observed.8 The signals of the trans isomer 4 appear as a sharp AX pattern (27% of the total intensity) at δ 52.2 and 44.2, which indicates slow or no dissociation of the aqua ligand, and the diastereomeric composition remains constant over 6 h. In contrast, the cis-β isomer 3 (64%) gives broad doublets at δ 69.2 and 46.0, and low-intensity, featureless signals at 60.4

Figure 2. Section of the (31P,1H)-HMQC NMR spectrum of the reaction solution of 2 with H2O (20 equiv) (CD2Cl2, 500 MHz, 60 °C).

and 51.1 are assigned to five-coordinate 2 (9%). The observation that the cis-β aqua complex 3 dissociates to a small extent in carefully dried CDCl3 contrasts with our original report, in which 3 was found to be stable in CDCl3. We conclude that the solvents used in our earlier study8 contained a substantially larger amount of adventitious water than in the present case. Accordingly, the signals of the cis-β isomer 3 sharpen upon addition of water (10 equiv) to the CDCl3 solution (Table 1) and, to some extent, in aged samples (24 h). 3598

dx.doi.org/10.1021/om200289x |Organometallics 2011, 30, 3596–3602

Organometallics

ARTICLE

Figure 4. ORTEP plot of (Λ)-cis-β-3 (ellipsoids at 30% probability).

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 3

Figure 3. Section of the (31P,1H)-HMQC NMR spectrum of [RuCl(PNNP)]PF6 (2) (CD2Cl2, 700 MHz, 60 °C). Minor amounts of the aqua complexes 3 and 4 are present.

For comparison with the detailed solution studies described above, the isomer mixture of 3 and 4 was then dissolved in carefully dried CD2Cl2, and a 31P NMR spectrum was recorded immediately. The products observed under these conditions are the cis-β isomer 3 (61%), five-coordinate 2 (31%), and the trans aqua complex 4 (8%). The signals of 3 and 2 are broad, as these species are in chemical exchange in CD2Cl2 at room temperature (see above). Five-coordinate 2, as well as some minor, unknown impurities, were converted to the aqua complexes 3 (87%) and 4 (13%) upon addition of water (10 equiv). The equilibration of the 7:3 isomeric mixture of 3 and 4 is a very interesting result, as it shows that the trans aqua complex 4 undergoes dissociation in CD2Cl2, too. However, the process is slow on the NMR time scale, as indicated by the absence of exchange involving 4 under these conditions (Figure 1). Overall, the 31P NMR data show that the cis aqua complex 3 is more labile than the trans isomer 4 (Table 1), in agreement with the higher trans influence of phosphine as compared to the chloro ligand. Analogously, we have previously observed that the aqua ligand trans to the phosphine in cis-β-[Ru(OH2)2(PNNP)]2þ is much more labile than the one trans to the imine.16 Furthermore, the solvent affects both the rate of water dissociation and the position of the equilibria between 2, 3, and 4. Thus, we find now that the trans isomer 4 is more inert in CDCl3 than in CD2Cl2, which we did not notice previously, as the NMR spectra were recorded in CDCl3 only.8 Also, the equilibria in Scheme 2 are less shifted toward 2 in CDCl3 than in CD2Cl2. These observations suggest that dichloromethane plays a role in the stabilization of the five-coordinate complex, its higher polarity with respect to chloroform being the most straightforward explanation. An intriguing issue concerning the nature of the chlorideabstracted species [RuCl(PNNP)]PF6 (2) emerges from the bulk of the 31P NMR data. As Table 1 shows, the chemical shift of the deshielded phosphine PA, which is trans to the aqua ligand in the cis-β complex 3, approaches its low-exchange δ value of 62.8

RuCl(1)

2.4285(6)

RuO(1)

2.215(2)

RuP(1)

2.2959(6)

RuN(1)

2.049(2)

RuP(2)

2.2437(6)

RuN(2)

2.086(2)

O(1) 3 3 O(1) 3 3

3 F(1)

2.806(4)

O(1) 3 3 3 F(2)

3.089(5)

3 Cl(1)

3.013(2)

Cl(1)RuP(1)

94.23(2)

Cl(1)RuP(2)

90.14(2)

Cl(1)RuN(1)

168.50(5)

Cl(1)RuN(2)

92.32(5) 103.09(2)

Cl(1)RuO(1)

80.78(5)

P(1)RuP(2)

P(1)RuN(1)

92.38(5)

P(2)RuN(1)

97.51(5)

P(1)RuN(2)

169.41(6)

P(2)RuN(2)

85.19(6) 164.31(5)

P(1)RuO(1)

90.40(5)

P(2)RuO(1)

N(1)RuO(1) N(1)RuN(2)

89.79(7) 79.87(7)

N(2)RuO(1) O(1)H 3 3 3 Cl(1)

O(1)H 3 3 3 F(1)

161(3)

O(1)H 3 3 3 F(2)

82.44(7) 105(2) 127(2)

from higher frequencies, which strongly suggests that the exchange partner is a species with a higher δ value. This is inconsistent with the corresponding chemical shift of PA in 2, which is lower (δ 60.5). Therefore, we suspect that a further species is present in solution, which is either in fast exchange or in low concentration and hence remains undetected. Future investigations will address the existence and nature of such a species. X-ray Structure of (Λ)-cis-β-3. Crystals of the cis-β isomer 3 were obtained serendipitously during attempts to crystallize [RuCl(PNNP)]PF6 (2) from CD2Cl2/hexane at 25 °C. The red crystals contain discrete 3 cations and PF6 anions with normal nonbonded distances, as well as two disordered dichloromethane molecules. The distorted octahedral coordination of ruthenium features the PNNP ligand in the (Λ)-cis-β configuration (Figure 4). The mutually cis aqua and chloro ligands are involved in a hydrogen bond involving one of the H atoms (O(1) 3 3 3 Cl(1) = 3.013(2) Å). The second hydrogen atom interacts with the hexafluorophosphate anion, as indicated by the short O(1) 3 3 3 F(1) separation of 2.806(4) Å. The phosphine donor P(2), which is trans to the aqua ligand, binds more strongly to ruthenium than P(1) (Table 2), in agreement with the higher trans influence of imine as compared to the aqua ligand. Accordingly, the RuO(1) distance of 2.215(2) Å is similar to that found for the aqua ligand trans to P in the related diaqua complex [Ru(OH2)2(PNNP)]2þ (2.229(5) Å) and much longer 3599

dx.doi.org/10.1021/om200289x |Organometallics 2011, 30, 3596–3602

Organometallics than those trans to imine (2.180(5) Å) in the same complex (PNNP is here bis(3,5-bis(trifluoromethyl)phenyl)phosphino)benzylidene]cyclohexane-1,2-diamine).16 Although long RuO bonds (>2.2 Å) are often observed when the aqua ligand is trans to a P donor5b,e,21 or to another ligand with strong trans influence,5d a perusal of the literature indicates that the overall trends do not depend on trans influence only. We suggest that other factors may stabilize aqua complexes of the scarcely oxophilic ruthenium(II) ion and affect the metaloxygen bond length in such species. For instance, many aqua complexes of Ru(II) (i) are positively charged (mostly dicationic),22 (ii) contain hard ligands such as N donors23 or coordinated oxo anions, such as a sulfonate21a or triflate,21b or (iii) contain strong π-accepting ligands, such as CO24 or carbene.25 To outweigh the low oxophilicity of ruthenium(II), a combination of two or more such features is usually observed. We have previously suggested in the context of fluoro complexes of late transition metals that the features mentioned above enhance the hardness (and thus the oxophilicity) of the soft ruthenium(II).26 Obviously, this qualitative analysis can be extended to other hard ligands such as aqua or ethers. In addition to the above features, hydrogen bonding often contributes to stabilize aqua complexes in either an inter- or an intramolecular fashion. The negatively charged oxygen atom of an oxo anion is the most common H-bond acceptor,4a,21b,27 but even the oxygen atom of a carbonyl ligand can act as such.23b Efficient acceptors are dangling nitrogen donors23a and the F atoms of fluoroanions, such as hexafluoroantimonate.16 Intramolecular hydrogen bonding to a chloro ligand, as observed in 3, is well documented.5a,b,f For instance, all-cis-[RuCl2(MeCN)(OH2)(DPEphos,kP,P)] (DPEphos is bis(2-(diphenylphosphino)phenyl) ether), in which H2O is trans to phosphorus, features a relatively short RuO distance of 2.160(3) Å.5c Interestingly, a closer inspection of the structure reveals that the aqua ligand is involved in a very short H bond to one of the chloro ligands (O 3 3 3 Cl = 2.62 Å). Finally, steric effects play a pivotal role in the stabilization of aqua complex 3. As discussed elsewhere,28 sp2 N donors such as imine are less sterically demanding than the PPh2 group, which implies that the five-coordinate complexes [RuCl(PNNP)]þ are less crowded than the diphosphine (PP) analogues [RuCl(PP)2]þ. As steric crowding contributes to the stabilization of coordinatively unsaturated, 16-electron complexes such as 2, PNNP complexes are intrinsically prone to form adducts with weakly coordinating molecules, such as water in the present case. We conclude that the formation of the aqua complex 3 is promoted by its positive charge, the hardness of the two nitrogen donors, the hydrogen bonds to the chloro ligand and to [PF6], and the modest steric crowding of the PNNP ligand. cis-β-[RuCl(OEt2)(PNNP)]PF6 (5). We have previously reported the reaction of dichloro complex 1 with (Et3O)PF6 (1 equiv) in dichloromethane to give a complex that we formulated as the ether adduct [RuCl(OEt2)(PNNP)]PF6 (5).12 Prompted by recent studies on imine aziridination,11 we have reinvestigated this reaction by NMR spectroscopy. After stirring 1 and (Et3O)PF6 (1 equiv) in CD2Cl2 overnight, the room-temperature 31P NMR spectrum of the reaction solution shows a broad signal at δ 41 (along with those of minor impurities), which decoalesces into two broad signals at δ ca. 37 and 43 upon lowering the temperature to 0 °C (Figure 5). At this temperature, a broad signal appears at about δ 55.5, which we assign to PA, the P donor trans to Et2O, by analogy with the cis-β

ARTICLE

aqua complex 4. We suspect that, at room temperature, this signal is too broad to be observed. At 40 °C, the dominant feature of the 31P NMR spectrum species is a sharp AX pattern at δ 55.5 and 36.9 (Table 1), which we assign to the Et2O adduct 5.29 As only the shielded phosphine couples to the imine proton (Figure 6), the Et2O complex 5 is in a cis-β configuration. The chemical shift differences between the inequivalent P donors follow the trends discussed for the cis-β and trans isomers of the aqua complexes 3 and 4 (Table 1). The chemical analogy with the cis-β aqua complex 3 suggests that the dynamic process involved is the equilibrium between five-coordinate 2 and the Et2O adduct 5 (Scheme 2). As we did not observe resolved signals for the five-coordinate complex 2 over the whole temperature range, this equilibrium must be fast on the NMR time scale and is shifted toward the six-coordinate adduct 5 at low temperature, as expected for a dissociation equilibrium.30 Support for this interpretation comes from the position of the chemical shift of the broad, averaged PX signal at room temperature (δ 41), which is intermediate between those of PX in 2 and 5 (ca. 51 and 37, respectively). The rate of the exchange process can be roughly estimated from the frequency difference between the corresponding signals of the main exchanging species 2 and 5. At 202 MHz, a difference of 1054 Hz is estimated for the high-frequency PA signals and 2800 Hz for the low-frequency PX resonances (on the basis of the sharp signals of the 202 MHz 31P NMR spectrum of 5 at 78 °C). As both signal pairs are close to their coalescence points at room temperature (Figure 6), the rate constant has to be close to, or in between, these values, that is, around 2  103 s1.31 Hence, as the exchange between five-coordinate 2 and the cis-β aqua complex 3 has a rate constant of 47(9) s1 at room temperature, we estimate that the corresponding process of the ether complex is faster by 12 orders of magnitude. Final Remarks. In general, ether complexes are less stable than aqua complexes and, to the best of our knowledge, ethers form stable ruthenium phosphine complexes only if the oxygen donor is incorporated in a chelate ring.32 The five-coordinate complex [RuCl(PNNP)]PF6 (2), however, exhibits similar affinity for water and Et2O. In fact, both 3 and 5 undergo dissociation to a significant extent when the oxygen donor L (L = H2O or Et2O) is present in a 1:1 ratio to ruthenium. The most evident difference between the aqua and Et2O complexes 3 and 5 is the kinetic behavior, the Et2O adduct 5 being significantly more labile. Factors that may account for this difference are the OH 3 3 3 Cl hydrogen bonding, which is only possible in the aqua complex, and the larger steric bulk of Et2O as compared to water. These conclusions are highly relevant to the recently reported aziridination of imines with ethyl diazoacetate, which is catalyzed by five-coordinate 2 and its aqua and Et2O derivatives 3 and 5. In particular, the six-coordinate adducts 3 and 5 show similar catalytic behavior, provided that a temperature gradient between 78 and 25 °C is used with the Et2O adduct 5 as catalyst.11,33 The NMR spectroscopic studies discussed above suggest that the temperature gradient is required to inhibit (thermodynamically and/or kinetically) the dissociation of 5. Therefore, the present results offer a handle to fine-tune the reactivity of catalysts based on the 16-electron complex [RuCl(PNNP)]þ (2) in a rational fashion. A detailed study of the mechanistic origin of these effects is ongoing and will be reported in due course. 3600

dx.doi.org/10.1021/om200289x |Organometallics 2011, 30, 3596–3602

Organometallics

ARTICLE

’ EXPERIMENTAL SECTION

’ ASSOCIATED CONTENT

General Procedures. Reactions with air- or moisture-sensitive materials were carried out under an argon atmosphere using Schlenk techniques or in a glovebox under purified nitrogen. All solvents were distilled from an appropriate drying agent under argon prior to use (CH2Cl2 and CD2Cl2 from CaH2, and hexane from Na/ benzophenone). CDCl3 was dried over activated molecular sieves (4 Å). Complex 1 was prepared by a literature procedure.7 1H, 13C, and 31P NMR spectra as well as multidimensional NMR were recorded on Bruker AVANCE spectrometers DPX 400, DPX 500, and DPX 700. 1 H and 13C positive chemical shifts in ppm are downfield from tetramethylsilane. 31P NMR spectra are referenced to external 85% H3PO4. The rate of the chemical exchange between complexes 2 and 3 was determined from a series of 2D (31P,31P)-EXSY experiments34 at room temperature with different mixing times (see Supporting Information).35 [RuCl(PNNP)]PF6 (2) þ H2O. TlPF6 (8.4 mg, 0.024 mmol) was added to an NMR tube under argon atmosphere. A CD2Cl2 solution (0.5 mL) of [RuCl2(PNNP)] (20 mg, 0.024 mmol, 1 equiv) was added thereto, and the slurry was shaken at room temperature overnight. Water was added in portions (0.43, 2.2, and 8.7 μL; 1, 5, and 20 equiv, respectively) with a microsyringe, and the solution was shaken for 10 min before analyzing it by NMR spectroscopy. 1H NMR (500 MHz, CD2Cl2, 20 equiv H2O, 298 K): δ 9.26 (d, 4JP,H = 9.0 Hz, 1H, HCdN, 4), 8.98 (d, 4JP,H = 9.4 Hz, 1H, HCdN, 4), 8.85 (d, 4JP,H = 9.6 Hz, 1H, HCdN, 3), 8.73 (br s, 1H, HCdN, 3), 7.966.52 (m, 54H, arom., 3 þ 4), 6.07 (t, J = 8.9 Hz, 1H, arom., 3), 5.86 (t, J = 8.5 Hz, 1H, arom., 4), 4.80 (br, 1H, 3), 4.43 (t, J = 9.1 Hz, 1H, 3), 3.99 (t, J = 10.2 Hz, 1H, 4), 3.58 (t, J = 11.2 Hz, 1H, 4), 2.94 (d, J = 10.0 Hz, 1H, 4), 2.67 (d, J = 11.0 Hz, 1H, 4), 2.642.04 (m, 2H, 3), 2.010.86 (m, 12H, 3 þ 4 þ H2O). 31 P NMR (202 MHz, CD2Cl2, 20 equiv H2O, 298 K): δ 62.8 (d, 2JP,P’ = 32.0 Hz, 1P, 3), 50.6 (d, 2JP,P’ = 27.1 Hz, 1P, 4), 45.4 (d, 2JP,P’ = 32.0 Hz, 1P, 3), 44.1 (d, 2JP,P’ = 27.1 Hz, 1P, 4), 144.4 (septet, JP.F = 707 Hz, PF6). X-ray Structure of 3. Red crystals of (rac)-3 were grown from a CD2Cl2 solution of the five-coordinate complex 2, which was covered with a layer of hexane under an argon atmosphere and stored at 25 °C for several weeks. Crystal data: triclinic, P1, 0.46  0.17  0.17 mm, a = 11.6344(11) Å, b = 12.6473(12) Å, c = 17.2954(17) Å, R = 71.278(2)°, β = 112.163(2)°, γ = 82.365(2)°, V = 2372.4(4) Å3, Z = 2, F(000) = 1140, Dcalcd = 1.576 Mg cm3, μ = 0.775 mm1. Data were collected at 200 K on a Bruker AXS SMART APEX platform in the θ range 1.7137.18°. The structure was solved with SHELXTL using direct methods. The asymmetric unit contains one 3 cation, one [PF6], and two dichloromethane molecules, one of which disordered over different orientations. The hydrogen atoms of the aqua ligand were detected on a difference Fourier map, and their position was refined. Of the 94 268 measured (19 e h e 19, 20 e k e 20, 28 e l e 28), 22 583 unique reflections were used in the refinement (full-matrix leastsquares on F2 with anisotropic displacement parameters). R1 = 0.0559 (16 679 data with Fo > 2σ(Fo)), wR2 = 0.1578 (all data). Max. and min. difference peaks were þ1.983 and 1.402 e Å3. cis-β-[RuCl(OEt2)(PNNP)]PF6 (5). [RuCl2(PNNP)] (1) (24.3 mg, 0.029 mmol) and Et3OPF6 (7.3 mg, 0.029 mmol) were placed in a NMR tube under a N2 atmosphere. The tube was evacuated, and CD2Cl2 (0.5 mL) was added under argon at room temperature. The solution was shaken overnight and then analyzed by NMR spectroscopy. 1 H NMR (500 MHz, CD2Cl2, 195 K): δ 9.31 (d, 1H, 4JP,H = 10.1 Hz, NdCH), 8.21 (m, 2H, arom.), 8.04 (s, 1H, NdCH), 7.726.18 (m, 26H, arom.), 5.31 (m, 1H, NCH), 2.99 (m, 1H, NCH), 2.84 (m, 1H, CH2), 1.92 (m, 3H, CH2), 1.67 (m, 1H, CH2), 1.39 (m, 1H, CH2). 31P NMR (202 MHz, CD2Cl2, 195 K): δ 55.5 (d, 2JP,P0 = 29.5 Hz), 36.9 (d, 2 JP,P0 = 29.5 Hz).

bS

Supporting Information. Details of the calculation of the rate constant k for the chemical exchange between 2 and 3 are available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. Rene Verel (ETH Z€urich) for his kind support with NMR spectroscopy and Dr. Pietro Butti (ETH Z€urich) for the X-ray structure of 3. ’ REFERENCES (1) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 5th ed.; Wiley: Hoboken, 2009; p 73. (2) Pearson, R. G. Chemical Hardness; Wiley-VCH: Weinheim, 1997; Chapter 1. (3) Jørgensen, C. K. Inorg. Chem. 1964, 3, 1201. (4) (a) Bernhard, P.; Burgi, H.-B.; Hauser, J.; Lehmann, H.; Ludi, A. Inorg. Chem. 1982, 21, 3936. For a carboxylato complex with a bridging aqua ligand, see:(b) Albers, M. O.; Liles, D. C.; Singleton, E.; Yates, J. E. J. Organomet. Chem. 1984, 272, C62. (5) Selected examples: (a) Sun, Y.; Taylor, N. J.; Carty, A. J. Inorg. Chem. 1993, 32, 4457. (b) Russo, L.; Figueira, J.; Rodrigues, J.; Rissanen, K. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62, m699. (c) Venkateswaran, R.; Mague, J. T.; Balakrishna, M. S. Inorg. Chem. 2007, 46, 809. (d) Shaffer, E. A.; Chen, C.-L.; Beatty, A. M.; Valente, E. J.; Schanz, H.-J. J. Organomet. Chem. 2007, 692, 5221. (e) Vertlib, V.; Figueira, J.; Mesquita, J.; Rodrigues, J.; Nattinen, K.; Rissanen, K. Eur. J. Inorg. Chem. 2007, 1920. (f) Hayashi, A.; Yoshitomi, T.; Umeda, K.; Okazaki, M.; Ozawa, F. Organometallics 2008, 27, 2321. (6) Account articles: (a) Mezzetti, A. Dalton Trans. 2010, 7851. (b) Bonaccorsi, C.; Mezzetti, A. Curr. Org. Chem. 2006, 10, 225. (7) For a seminal paper on [RuCl2(PNNP)] (1), see: Gao, J. X.; Ikariya, T.; Noyori, R. Organometallics 1996, 15, 1087. (8) Stoop, R. M.; Bachmann, S.; Valentini, M.; Mezzetti, A. Organometallics 2000, 19, 4117. (9) Bonaccorsi, C.; Mezzetti, A. Organometallics 2005, 24, 4953. (10) Bachmann, S.; Furler, M.; Mezzetti, A. Organometallics 2001, 20, 2102. (11) Ranocchiari, M.; Mezzetti, A. Organometallics 2009, 28, 3611. (12) Bonaccorsi, C.; Bachmann, S.; Mezzetti, A. Tetrahedron: Asymmetry 2003, 14, 845. (13) (a) Althaus, M.; Bonaccorsi, C.; Mezzetti, A.; Santoro, F. Organometallics 2006, 25, 3108. (b) Santoro, F.; Althaus, M.; Bonaccorsi, C.; Gischig, S.; Mezzetti, A. Organometallics 2008, 27, 3866. (14) (a) Althaus, M.; Becker, C.; Togni, A.; Mezzetti, A. Organometallics 2007, 26, 5902. (b) Bonaccorsi, C.; Althaus, M.; Becker, C.; Togni, A.; Mezzetti, A. Pure Appl. Chem. 2006, 78, 391. (c) Toullec, P. Y.; Bonaccorsi, C.; Mezzetti, A.; Togni, A. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 5810. (15) (a) Schotes, C.; Mezzetti, A. J. Am. Chem. Soc. 2010, 132, 3652. (b) Schotes, C.; Mezzetti, A. Angew. Chem., Int. Ed. 2011, 50, 3072. (16) Bonaccorsi, C.; Santoro, F.; Gischig, S.; Mezzetti, A. Organometallics 2006, 25, 2002. (17) We shall see below that the cis and trans isomers 3 and 4 equilibrate in CD2Cl2, which implies that 4 does dissociate in CD2Cl2, but the process is too slow on the NMR time scale to be detected. (18) Pregosin, P. S.; Kunz, R. W. 31P and 13C NMR of Transition Metal Phosphine Complexes. In NMR Basic Principles and Progress; Diehl, P.; Fluck, E.; Kosfeld, R., Eds.; Springer: Berlin, 1979; p 28. 3601

dx.doi.org/10.1021/om200289x |Organometallics 2011, 30, 3596–3602

Organometallics

ARTICLE

(19) See ref 18, p 52. (20) (a) Low-spin five-coordinate complexes of the type [MXL4]nþ (M = d6 metal ion, here Ru(II); X = π-donor, here Cl) assume a distorted trigonal-bipyramidal geometry for electronic reasons. See: (b) Riehl, J. F.; Jean, Y.; Eisenstein, O.; Pelissier, M. Organometallics 1992, 11, 729. (21) (a) Harding, P. A.; Robinson, S. D.; Henrick, K. J. Chem. Soc., Dalton Trans. 1988, 415. (b) Mahon, M. F.; Whittlesey, M. K.; Wood, P. T. Organometallics 1999, 18, 4068. (c) Albertin, G.; Antoniutti, S.; Bacchi, A.; Boato, M.; Pelizzi, G. Dalton Trans. 2002, 3313. (d) Tsunawaki, F.; Ura, Y.; Kondo, T.; Iwasa, T.; Harumashi, T.; Mitsudo, T. Organometallics 2006, 25, 2547. (e) Martin, M.; Horvath, H.; Sola, E.; Katho, A.; Joo, F. Organometallics 2009, 28, 561. (22) For selected examples of dicationic Ru(II) aqua complexes, see refs 4a, 16, 21c, 21e, 23a, and 23c. (23) Recent examples: (a) Zong, R.; Thummel, R. P. J. Am. Chem. Soc. 2005, 127, 12802. (b) Che, C. M.; Zhang, J. L.; Huang, J. S.; Lai, T. S.; Tsui, W. M.; Zhou, Z. Y.; Zhu, N.; Chang, C. K. Chem.—Eur. J. 2005, 11, 7040. For a seminal paper, see:(c) Durham, B.; Wilson, S. R.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 600. (24) Recent examples: (a) Goicoechea, J. M.; Mahon, M. F.; Whittlesey, M. K.; Kumar, P. G. A.; Pregosin, P. S. Dalton Trans. 2005, 588. (b) Haukka, M.; Jakonen, M.; Nivajarvi, T.; Kallinen, M. Dalton Trans. 2006, 3212. (25) Szadkowska, A.; Makal, A.; Wozniak, K.; Kadyrov, R.; Grela, K Organometallics 2009, 28, 2693. For an allenylidene aqua complex, see ref 5d. (26) (a) Becker, C.; Kieltsch, I.; Broggini, D.; Mezzetti, A. Inorg. Chem. 2003, 42, 8417. (b) Becker, C.; Mezzetti, A. Helv. Chim. Acta 2002, 85, 2686. (27) Recent examples: (a) Kuznetsov, V. F.; Yap, G. P. A.; Alper, H. Organometallics 2001, 20, 1300. (b) Volland, M. A. O.; Hansen, S. M.; Rominger, F.; Hofmann, P. Organometallics 2003, 23, 800. (28) For a discussion of steric and electronic factors stabilizing 16electron ruthenium complexes, see: Barthazy, P.; Broggini, D.; Mezzetti, A. Can. J. Chem. 2001, 79, 904. (29) We have previously reported that the broad 31P NMR signal observed for 5 at room temperature resolves into an AX pattern with doublets at δ 66.8 and 45.9, 2JP,P0 = 30.6 Hz in the presence of a large excess of Et2O (32 equiv) in CD2Cl2.12 As these 31P NMR data are similar to those of cis-β-[RuCl(OH2)(PNNP)]PF6 (3), the species previously observed under such conditions was probably the cis-aqua complex β-3, possibly due to the presence of adventitious water. (30) Additionally, the 31P NMR spectrum of 5 at 78 °C shows weak signals (less than 3% of the total integrated intensity) that we assign to the cis-β aqua complex 3. (31) If the rate constant were considerably smaller than 1054 s1, the shielded PX (Δv = 2800 Hz between 2 and 5) would still give resolved, individual signals around room temperature. At a larger rate constant, the PA signals (Δv = 1054 Hz) would have passed their coalescence point and would form a sharp averaged signal at room temperature. (32) (a) See, for instance, ref 5c. Further recent examples: (b) F€urstner, A.; Davies, P. W.; Lehmann, C. W. Organometallics 2005, 24, 4065 and references therein. (c) Matsugi, M.; Curran, D. P. J. Org. Chem. 2005, 70, 1636. (33) See ref 11, Table 1, entries 37. (34) Jeener, J; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. (35) Perrin, C. L.; Dwyer, J. T. Chem. Rev. 1990, 90, 935.

3602

dx.doi.org/10.1021/om200289x |Organometallics 2011, 30, 3596–3602