Acceptor Pincer Chemistry of Ruthenium: Catalytic Alkane

Jul 13, 2011 - The synthesis and reactivity of a series of Ru(II) complexes based on the strongly π-accepting pincer ligand 1,3-C6H3(CH2P(CF3)2)2 (CF...
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Acceptor Pincer Chemistry of Ruthenium: Catalytic Alkane Dehydrogenation by (CF3PCP)Ru(cod)(H) Brian C. Gruver, Jeramie J. Adams, Seth J. Warner, Navamoney Arulsamy, and Dean M. Roddick* Department of Chemistry, The University of Wyoming, Department 3838, 1000 E. University Avenue, Laramie, Wyoming 82071, United States

bS Supporting Information ABSTRACT: The synthesis and reactivity of a series of Ru(II) complexes based on the strongly π-accepting pincer ligand 1,3C6H3(CH2P(CF3)2)2 (CF3PCP) is reported. Thermolysis of [Ru(cod)(η3-2-methylallyl)2] with CF3PCPH under H2 affords a mixture of the three complexes (μ-CF3PCPH)Ru(H)(μ-H)(μ-η6,k3 -CF3PCP)Ru(H), [(CF3PCP)Ru(H)]2(μ-CF3PCPH)2, and (CF3PCP)Ru(cod)H, which were structurally characterized and individually prepared in moderate yields. (CF3PCP)Ru(cod)H reacts with (C2F5)2PCH2CH2P(C2F5)2 (dfepe) to give (CF3PCP)Ru(dfepe)H. (CF3PCP)Ru(cod)H is moderately active as an alkane dehydrogenation catalyst. Thermolysis in 1:1 mixtures of cyclooctane and tert-butylethylene at 150 and 200 °C resulted in initial rates of 180 and 1000 turnovers h 1 of cyclooctene, respectively. Acceptorless dehydrogenation of cyclooctane also occurs, with an initial rate of 14 turnovers h 1. The decrease of catalyst activity over time was found to be due to thermal catalyst decomposition rather than product inhibition by cyclooctene.

’ INTRODUCTION Selective dehydrogenation of alkanes to useful olefin compounds is a process with tremendous potential value in the production of chemical feedstocks.1 Iridium pincer catalysts have been well studied and are known to be suitable catalysts for dehydrogenation of alkanes at relatively low temperatures (150 200 °C)2,3 in comparison to heterogeneous catalysts report of rhodium- and iridium(over 400 °C).1c In an initial t based pincer systems, ( BuPCP)Ir was shown to have much higher catalytic activity than the rhodium analogue for the dehydrogenation of t cyclooctane in the presence of tertbutylethylene.4 The ( BuPCP)Ir system was also shown to be capable of acceptorless dehydrogenation.5 In 2004 Brookhart t Bu reported related POCOP systems based on the resorcinol backbone which are an order of magnitude more active than tBu PCP systems.6 Pincer systems with metallocenes incorporated into the pincer backbone similarly exhibit enhanced activity.7 Steric effects of phosphine substituent groups have recently been examined,8 and the PCP-based catalysts have also been supported on alumina in an effort to increase stability.9 Together, these findings demonstrate that even relatively modest changes in steric and electronic properties can have a significant impact on reactivity. Although good progress has been made in developing iridium pincer catalysts, an increase in the efficiency and durability of alkane dehydrogenation systems is still desirable. Acceptor-promoted alkane dehydrogenation is a special case of the more general process of transfer hydrogenation. Catalytic transfer hydrogenation is an attractive method for reducing ketones and imines, and a range of ruthenium and iridium systems have been developed.1a,10 Ruthenium PCP complexes are well-known as ketone alcohol hydrogen transfer catalysts.11 r 2011 American Chemical Society

Despite the considerable recent interest in group 9 alkane dehydrogenation chemistry, to our knowledge no corresponding activity for homogeneous group 8 metal systems has been reported. Accordingly, we are interested in extending pincer hydrogen transfer chemistry to d6 ruthenium and osmium systems, with a particular focus on alkane/alkene hydrogen transfer. A comparison of the generally accepted catalytic scheme for iridium-mediated acceptorless alkane dehydrogenation with a potential cycle based on group 8 metals is shown in parts A and B of Scheme 1. While the key intermediate in iridium alkane activation is believed to be a d8 14-electron 3-coordinate Ir(I) complex,12,13 the corresponding active species for ruthenium and osmium pincer systems would be a 4-coordinate d6 M(II) hydride fragment. Theoretical studies on ligand and C H addition have shown marked differences between 3- and 4-coordinate intermediates and the contrasting electronic requirements for C H coordination and C H bond addition.14 In Scheme 1B facile H2 elimination is expected subsequent to alkane C H bond oxidative addition. This is in contrast with group 9 dehydrogenation catalysts, where β elimination from an alkyl intermediate occurs prior to H2 loss. We have recently reported the synthesis of Ir(III) and Ir(I) systems incorporating the strong acceptor PCP pincer ligand CF3 PCP.15 (CF3PCP)Ir(I) complexes exhibit unusual 5-coordinate tdynamics and H2 addition chemistry15b and, like their RPCP and BuPOCOP analogues, catalyze alkane dehydrogenation.16 Ru(cod)(η3-2-methylallyl)2 has recently been shown to be a useful precursor to PCP and PNP ruthenium dihydride pincer Received: April 26, 2011 Published: July 13, 2011 5133

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Scheme 1

Scheme 2

complexes, Ru(dtbpmp)(η2-H2)(H)2 (dtbpmp = 2,6-bis[(ditert-butylphosphino)methyl]pyridine), Ru(dtbpmb)(η2-H2)2H, and Ru(dtbpmb)(η2-H2)H (dtbpmb = 1,3-bis((di-tert-butylphosphino)methyl)benzyl), which are capable of C H bond activation and H/D exchange.17 In view of these results, we have examined the reaction of Ru(cod)(η3-2-methylallyl)2 with CF3 PCPH as a route to (CF3PCP)Ru(L)(H)x hydride products for study. Under carefully controlled reaction conditions (CF3PCP)Ru(cod)(H) has been prepared and its reactivity surveyed. (CF3PCP)Ru(cod)(H) serves as a convenient catalyst precursor and exhibits moderate acceptor (tbe) and acceptorless catalytic cycloalkane dehydrogenation activity.

’ RESULTS AND DISCUSSION Synthesis of Ru(II) Complexes from Ru(cod)(η3-2methylallyl)2. The reaction of Ru(cod)(η3-2-methylallyl)2 with

donor pincer ligands in the presence of H2 leads directly to (pincer)Ru(H)x products in reasonable yield.17 In contrast, the course of reaction of Ru(cod)(η3-2-methylallyl)2 with CF3PCPH in the presence of H2 is very sensitive to reaction conditions and three distinct products are observed (Scheme 2). Thermolysis of a 1:1 mixture of the Ru(cod)(η3-2-methylallyl)2 and CF3PCPH under 3 atm H2 for 10 min at 130 °C produced a product mixture with four hydride resonances at δ 9.9, 11.2, 12.0, and 14.2, 5134

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Figure 1. Molecular structure of (μ-CF3PCPH)Ru(H)(μ-H)(μ-η6,k3-CF3PCP)Ru(H) (1) (one of two independent molecules in the asymmetric unit). The hydride hydrogen atoms are shown; the remaining hydrogen atoms and selected fluorine atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected metrical data (distances in Å and angles in deg): Ru(1) C(1) = 2.110(2), Ru(1) P(1) = 2.2383(6), Ru(1) P(2) = 2.2756(6), Ru(1) P(3) = 2.2917(6), Ru(1) H(1) = 1.45(3), Ru(1) H(1B) = 1.96(3) Å, Ru(1) Ru(2) = 2.9773(2), Ru(2) H(2) = 1.59(3), Ru(2) H(1B) = 1.71(3), Ru(2) P(4) = 2.2210(6), Ru(2) C(1) = 2.139(2), Ru(2) C(2) = 2.198(2), Ru(2) C(3) = 2.233(2), Ru(2) C(4) = 2.257(2), Ru(2) C(5) = 2.305(2), Ru(2) C(6) = 2.277(2); P(1) Ru(1) P(2) = 149.39(2), P(3) Ru(1) C(1) = 168.53(6), H(1) Ru(1) H(1B) = 174.5(15), H(2) Ru(2) H(1B) = 80.8(16), H(2) Ru(2) P(4) = 76.8(13).

whose relative integrations corresponded to a 4:1:5 mixture of products 1 3. Variation of the [Ru] to CF3PCPH ratio as well as the reaction time and temperature allowed for the separation and identification of each product. Treatment of Ru(cod)(η3-2methylallyl)2 with 0.7 equiv of CF3PCPH in benzene under 3 atm of H2 gave a ∼2:1 ratio of 1 to 3, as judged by 31P NMR. Crystals suitable for X-ray diffraction were grown from slow evaporation of toluene, and the major species was identified as the red dimeric complex (μ-CF3PCPH)Ru(H)(μ-H)(μ-η6,k3-CF3PCP)Ru(H) (1) with an unusual asymmetrical bridging CF3 PCP group η6 bound to one metal center and k3 bound to the other (Figure 1). 1H NMR spectra for 1 exhibit two hydride resonances, a pseudoquartet at δ 11.54 and a doublet at 14.13 in a 1:2 ratio, not the three resonances predicted from the solid-state structure. Additionally, 19F NMR spectra show four CF3 resonances in a 1:1:1:1 ratio, which is also inconsistent with a static structure. These results indicate a fluxional process involving the rapid exchange of bridging and terminal hydride ligands on the η6-arene coordinated ruthenium center. When the reaction between Ru(cod)(η3-2-methylallyl)2, 1 equiv of CF3PCPH, and H2 was stopped after 5 min at 130 °C, product 2 was the major (90%) species observed. X-ray diffraction revealed this species to be the monomeric cod complex (CF3PCP)Ru(cod)H (2) (Figure 2). We surmised that 2 is derived from the incomplete hydrogenolysis of Ru(cod)(η3-2methylallyl)2 and should therefore be favored in the presence of

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Figure 2. Molecular structure of (CF3PCP)Ru(cod)H (2). With the exception of the hydride hydrogen atom, all remaining hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 50% probability. Selected metrical data (distances in Å and angles in deg): Ru(1) C(1) = 2.154(2), Ru(1) P(1) = 2.2360(6), Ru(1) P(2) = 2.2392(6), Ru(1) C(13) = 2.282(2), Ru(1) C(14) = 2.276(2), Ru(1) C(17) = 2.305(3), Ru(1) C(18) = 2.314(3), Ru(1) H(1) = 1.53(4); P(1) Ru(1) P(2) = 132.93(2), H(1) Ru(1) C(1) = 109.2(15).

Figure 3. Molecular structure of [(CF3PCP)Ru(H)]2(μ-CF3PCPH)2 (3). With the exception of the hydride hydrogen atoms, all remaining hydrogen atoms and all CF3 groups are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Symmetric equivalents are unlabeled. Selected metrical data (distances in Å and angles in deg): Ru(1) C(1) = 2.188(3), Ru(1) P(1) = 2.2776(9), Ru(1) P(2) = 2.2708(9), Ru(1) P(3) = 2.3940(8), Ru(1) P(4) = 2.3309(8), Ru(1) H(1) = 1.47(4), Ru(1) Ru(1)#1 = 9.9827(5); P(1) Ru(1) P(2) = 141.10(3), P(4) Ru(1) C(1) = 171.55(8), P(3) Ru(1) H(1) = 164.7(15).

excess cod. Accordingly, 2 was formed in moderate yield from the reaction of Ru(cod)(η3-2-methylallyl)2, CF3PCPH, and 1.5 equiv of 1,5-cyclooctadiene under 3 atm of H2 for 30 min at 130 °C. Reaction of Ru(cod)(η3-2-methylallyl)2 with 2 equiv of CF3 PCPH in benzene under 3 atm of H2 at 130 °C resulted in the clean formation of 3, which was identified by X-ray diffraction and NMR as the doubly bridged compound [(CF3PCP) Ru(H)]2(μ-CF3PCPH)2 (3) (Figure 3). Key NMR features for 5135

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rather than any polyhydride product (eq 2). This result is consistent with the preliminary Ru(cod)(methylallyl)2 pincer trapping studies under hydrogen and suggests that 1 is a thermodynamic sink for the unsaturated 14e (CF3PCP)Ru(H) fragment.

CF3

Figure 4. Molecular structure of ( PCP)Ru(dfepe)H (4). The hydride hydrogen atom is shown, whereas the rest of the hydrogen atoms are omitted. The thermal ellipsoids are drawn at the 50% probability level. Selected metrical data (distances in Å and angles in deg): Ru(1) C(1) = 2.1802(14), Ru(1) P(1) = 2.2725(4), Ru(1) P(2) = 2.2778(4), Ru(1) P(3) = 2.3055(4), Ru(1) P(4) = 2.3666(4), Ru(1) H(1) = 1.53(3); P(1) Ru(1) P(2) = 139.078(15), H(1) Ru(1) C(1) = 95.0(10), P(3) Ru(1) P(4) = 82.422 (14).

3 are the presence of three 31P resonances at 88.1, 72.0, and 59.7 which give an integral ratio of 2:1:1, and a broad hydride multiplet at δ 9.80. Significantly, complexes 1 3 can readily be interconverted (130 °C, 30 min): 2 can be prepared by the reaction of 1 or 3 with excess cod, and 3 can be obtained by thermolysis of 2 with excess CF3 PCPH. In the absence of trapping ligands, 2 thermally decomposes to 1 (see later). 1 may be converted to 3 by reaction with excess CF3PCPH; however, no direct thermal conversion of 3 into 1 after several days at 130 °C was observed. (CF3PCP)Ru(cod)H Reactivity. The cod ligand in 2 is readily displaced to form (CF3PCP)Ru(L)2H products. As an extension of our prior work with the bidentate ligand (C2F5)2PCH2CH2P(C2F5)2 (dfepe), cod displacement from 2 by 1 equiv of dfepe occurred after 30 min at 130 °C to give (CF3PCP)Ru(dfepe)H (4) (eq 1, Figure 4). Key NMR features for 4 are the presence of three 31P resonances at δ 101.7, 93.6, and 86.3, which give an integral ratio of 1:2:1, and a single complex hydride resonance at δ 8.85. Attempts to prepare 4 directly from Ru(cod)(η3-2methylallyl)2 in the presence of excess (1.1 equiv) dfepe resulted in a 1:3 mixture of 4 and the previously reported complex cisRu(dfepe)2(H)2.18 Attempts to displace cod with the monodentate perfluoroalkylphosphine (C2F5)2MeP (dfmp) under similar conditions resulted in the hydrogenation of cod to cyclooctane and formation of a complex mixture of products.

The reaction of 2 with H2 in the absence of trapping ligands was examined in an effort to generate (CF3PCP)Ru(H)x products. Free cyclooctane was observed upon warming a benzene solution of 2 to 80 °C for 24 h under 3 atm of H2. 31P and 19F NMR spectra indicated the formation of dimeric 1 (∼80%)

Crystallographic Studies. Crystallographic data obtained for (CF3PCP)Ru(II) complexes 1 4 allow us to compare structural features with those of known ruthenium pincer systems. The average Ru P bond length found for metalated (CF3PCP)Ru(II) systems is 2.255 Å. As previously observed in platinum and iridium pincer chemistry, electron-withdrawing phosphine CF3 groups result in significantly reduced M P bond distances. For comparison, the average Ru P bond length range for donor phosphine RPCP pincer systems (R = tBu, Ph, C6F5) is 2.29 2.45 Å (2.36 Å average).11b,19 Terminal hydride bonds in complexes 1 4 average 1.50 Å, which is similar to distances previously reported for (PCP)RuH compounds.19d,e π-Arene coordination of the k3 pincer ligand in 1 has been reported previously in heterobimetallic complexes by van Koten.20 In these systems metal arene coordination induces pincer steric and electronic effects but does not directly interact with the k3 center. For [PdCl(NCN)Ru(C5Me5)]+ and [PtCl(NCN)Ru(C5Me5)]+ the metal metal distances are 3.9150(2) and 3.9320(3) Å, respectively, while the ruthenium centers in 1 are supported by a single hydride bridging ligand and are 2.9773(2) Å apart. Single-hydride-bridged Ru dimers in the literature show an average Ru Ru distance of 3.22 Å.21 Several structurally characterized compounds with unmetalated bridging CF3PCPH ligands have been previously reported.22 The average Pt P bond length for cis-[(CF3PCPH)PtMe2]2 is 2.264 Å. The average bond length for Ru P(μ-PCPH) in 3 is much longer at 2.362 Å, which can be attributed to the difference in the covalent radii (Pt, 136 pm; Ru, 146 pm).23 The Ru P(3) bond trans to the strong trans influence hydride ligand is 2.3940(8) Å, while the Ru P bond trans to carbon from the PCP backbone, Ru P(4), is slightly shorter (2.3309(8) Å). Alkane Dehydrogenation Catalysis. In the absence of an isolable hydride precatalyst such as (CF3PCP)Ru(H)(H2)2, we have examined the cyclooctane (coa) dehydrogenation activity of (CF3PCP)Ru(cod)H (2). Thermolysis of 12.5 mM 2 (∼3.32  10 3 mol %) in coa afforded ca. 3 equiv of cyclooctene after 10 min at 165 °C. After the mixture was cooled to 20 °C, a redbrown precipitate was observed and the complete conversion of 2 to dimeric 1 was indicated by 31P and 19F NMR. Thermolysis of 6.25 mM 2 in a 1:1 mixture of coa and tert-butylethylene produced 18 equiv of coe after 10 min. 31P and 19F NMR showed decomposition of 2 to a mixture of 1 (∼30%) and other unidentified products. Since 1 can be reconverted into 2 in the presence of excess cod, we examined the effect of added cod on the dehydrogenation activity of 2. Under identical reaction conditions in the presence of 2 equiv of cod, only 4 equiv of coe was produced after 10 min; this corresponds to activity inhibition by cod of ∼78%. Significantly, the major metal species observed in solution at this point was 2 (90%), not 1. 5136

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Figure 5. Catalytic turnover plots for (CF3PCP)Ru(cod)H (2; 1.65  10 4 mol %) with 1/1 tbutylethylene/cyclooctane: (b, in red) 150 °C; (9, in blue) 200 °C.

Previous catalytic dehydrogenation studies were performed at lower catalyst loadings, and under these conditions we anticipated that the bimolecular deactivation pathway to form 1 would be disfavored. Thermolyses of 1.25 mM 3 (1.65  10 4 mol %) in 1:1 mixtures of coa and tbe at 150 and 200 °C under 1 atm of N2 were monitored by 1H NMR (Figure 5).24 Initial rates of coe production were taken from 1H NMR integration after 10 min and correspond to 180 and 1000 turnovers h 1 at 150 and 200 °C, respectively. The catalytic activity of the (CF3PCP)Ru(cod)H system is limited under these conditions: the activity of 2 at 150 °C levels off after 3 h, and at 200 °C the activity ceases after only 30 min. A total of 164 (150 °C) and 186 turnovers (200 °C) were observed. The initial turnover rate and total turnovers are considerably less than those reported for iridium phosphinite t complexes such as (p-ArFPCP Bu)IrH2 (8715 turnovers h 1, 2200 TON at 200 °C)6 and the ferrocene-bridged pincers tBu ( PCPFe)IrHt 2 (3,300 TON at 180 °C)7 but are comparable to those of ( BuPCP)IrH2 (1170 turnovers h 1, 230 TON at than those previously 200 °C).6b They are also much higher t reported for the rhodium analogue ( BuPCP)RhH2, which showed rates of only 0.8 and 1.8 TON h 1 at 150 and 200 °C, respectively.4 Strong catalytic rate inhibition by alkene product buildup is observed for all reported iridium alkane dehydrogenation catalyst systems. Inhibition for the (CF3PCP)Ru(cod)H system was examined in the presence of 0 800 equiv of coe (Figure 6). The catalytic activity of 2 is inhibited by the presence of coe, although significant dehydrogenation activity (∼20%) is still observed in the presence of 800 equiv of coe. These inhibition studies indicate that the limited turnovers found for 2 are not solely due to product inhibition. To test catalyst stability, thermolysis of 2 in 1:1 coa:tbe was carried out for 1 h at 200 °C. All volatiles were removed, and the residue was taken back up in fresh coa/tbe. After 1 h at 200 °C, no further production of coe was observed, confirming that thermal decomposition of 2 is the primary contributing factor to the short-lived catalyst activity of (CF3PCP)Ru(cod)H. 19F NMR spectra of catalyst solutions after 30 min at 200 °C show a major (∼60%) product resonance at 82.6 ppm, which is 20 ppm upfield of the normal range for CF3 in CF3PCP systems (∼ 54 to 64 ppm), and no significant amount of free CF3PCPH. The major catalyst decomposition product has not yet been identified.

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Figure 6. Catalytic activity plot for (CF3PCP)Ru(cod)H (2; 1.65  10 4 mol %), with 1/1 tert-butylethylene/cyclooctane, at 200 °C as a function of added equivalents of cyclooctene.

Figure 7. Acceptorless dehydrogenation activity for (CF3PCP)Ru(cod)H (2; 2.5 mmol) in cyclooctane under reflux conditions (∼140 °C, 590 Torr of N2 pressure).

Acceptorless dehydrogenation by 2 was also examined. Refluxing 2.5 mM 2 in cyclooctane (∼140 °C at 590 Torr ambient pressure) for 1 h produced 10 turnovers of cyclooctene (Figure 7) with an initial rate of 14 turnovers h 1. 19F NMR shows complete conversion of 2 to 1 after 1 h, after which not additional cyclooctene was produced. For comparison, 2 mM ( BuPCP)IrH2 gave 190 turnovers after 120 h, with an initialt rate of 11 turnovers h 1; less than 1 turnover was reported for ( BuPCP)RhH2.5 The catalytic activity for (CF3PCP)Ru(dfepe)H was also surveyed. Thermolysis of 1.25 mM (CF3PCP)Ru(dfepe)H in 1/1 coa/tbe produced 7 equiv of cyclooctene after 1 h at 200 °C. Less than 0.5 equiv of cyclooctene was observed after refluxing (CF3PCP)Ru(dfepe)H under N2 in cyclooctane for 24 h. 19F NMR spectra at this point show only resonances due to free CF3 PCPH and dfepe, indicating that complete decomplexation of both perfluorophosphine ligands has occurred. In previous alkane dehydrogenation studies, the catalytic activity for iridium PCP pincer compounds was investigated under argon due to strong inhibition by dinitrogen, owing to formation of bridged Ir2(μ-N2) products.4,25,6a A series of 5137

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Organometallics catalyst runs (1.25 mM of 2 in 1/1 coa/tbe, 30 min at 200 °C) were carried out under vacuum, 1 atm of argon, 1 atm of oxygen, 1 atm of N2, and 1 atm of N2 with 100 equiv of added water. No significant change in initial rate, total turnovers, or decomposition products were observed. A control run in the presence of added mercury metal showed no difference in activity or catalyst lifetime, supporting the homogeneity of the cyclooctane dehydrogenation process. Since C H bond activation is a critical step in both alkane dehydrogenation and aldehyde decarbonylation, the decarbonylation activity of (CF3PCP)Ru(cod)H was also surveyed: the reaction of (CF3PCP)Ru(cod)H with 20 equiv of 2-napthaldehyde in diglyme was carried out at 200 °C. After 10 min ca. 2 equiv of naphthalene was observed in the 1H NMR as well as conversion to primarily (> 85%) (CF3PCP)Ru(CO)2H,26 indicating that decarbonylation by this system is essentially stoichiometric.

’ SUMMARY In our previous work with (CF3PCP)Ir systems we have observed a general preference for coordinative saturation relative to donor pincer analogues.15 In keeping with this trend, the reaction of [Ru(cod)(η3-2-methylallyl)2] with CF3PCPH results in a mixture of (μ-CF3PCPH)Ru(H)(μ-H)(μ-η6,k3-CF3PCP)Ru(H), [(CF3PCP)Ru(H)]2(μ-CF3PCPH)2, and (CF3PCP)Ru(cod)H complexes which incorporate additional modes of bonding to electrophilic ruthenium centers in order to satisfy the ruthenium valency. Careful manipulation of reaction conditions allowed the three complexes to be prepared individually in moderate yields. To our knowledge, the bimolecular coordination of one pincer arene ring to satisfy another pincer complex’s valency is unprecedented and represents a previously unrecognized decomposition pathway for arene-bridged pincer ligand systems. The addition of competing ligands such as cod and CF3PCPH reverses arene coordination in (CF3PCP)Ru systems, but added Ht 2 does not produce monomeric products analogous to ( BuPCP)Ru(H)(H2)x.17b While the electron-withdrawing nature of CF3PCP may play a significant role in favoring coordinatively saturated complex products,27 it is also likely that pincer sterics play an important role in the observed chemistry. Pendant P(CF3)2 groups (θ((CF3)3P) = 137°) are significantly smaller than commonly studied pincers with P(tBu)2 or P(iPr)2 donor phosphine groups (θ(tBu3P) = 182°, (θ(iPr3P) = 160°).28 Recent work by Goldman has examined the effectt of successive replacement of tBu groups by methyl groups in ( BuPCP)Ir dehydrogenation chemistry and noted an increased tendency toward dinuclear cluster formation with increased methyl substitution.29 Thus, the formation of dinuclear (CF3PCP)Ru complexes, particularly complex 1, may be at least in part due to increased steric access to the metal center. Future extensions of RfPCP pincer chemistry to ligands with larger Rf groups (C2F5, CF(CF3)2, etc.) will address this important issue. activity for alkane dehy(CF3PCP)Ru(cod)H shows catalytic t drogenation comparable to that of ( BuPCP)IrH2 under similar conditions, though the catalyst lifetimet of the ruthenium system is very limited. The rhodium system ( BuPCP)RhH2 reportedly shows significant decomposition after at 24 h.4 We have not yet identified the metal-containing products of catalyst deactivation but note that it is not simply due to demetalation (reductive elimination of the pincer arene and terminal hydride ligands) and formation of free CF3PCPH.

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An interesting feature of the observed catalyst initial activity rates is that they are quite insensitive to added N2, H2O, or even O2. This suggests the very real possibility of coupling alkane dehydrogenation and oxidation in either a combined or tandem catalytic process in order to thermodynamically drive alkane functionalization to useful products.12e,30,31 Investigations are underway to explore this concept in ruthenium, osmium, iridium, and related alkane dehydrogenation systems.

’ EXPERIMENTAL SECTION General Procedures. All manipulations were conducted using high-vacuum-line, Schlenk, and/or N2 glovebox techniques. All ambient-pressure chemistry was carried out under a pressure of approximately 590 Torr (elevation ∼2195 m). Dry, oxygen-free solvents were prepared using standard procedures. Aprotic deuterated solvents used in  molecular sieves. All NMR experiments were dried over activated 3 Å elemental analyses were performed by Columbia Analytical Services. NMR spectra were obtained with a Bruker DRX-400 instrument using 5 mm NMR tubes fitted with Teflon valves (Chemglass CG-512 or New Era CAV-VBP). 31P spectra were referenced to an 85% H3PO4 external standard. 19F spectra were referenced to CF3CO2CH2CH3 and CF3C6H5 external standards. Ru(cod)(η3-2-methylallyl)2 and CF3PCPH were prepared by literature methods.32,33 (μ-CF3PCPH)Ru(H)(μ-H)(μ-η6,j3-CF3PCP)Ru(H) (1). While 1 may be obtained in variable amounts as a mixture with 2 and 3 in the direct thermolysis of Ru(cod)(η3-2-methylallyl)2 with CF3PCPH and H2, cleaner samples of 1 may be obtained from the hydrogenolysis of 2: a 5 mm NMR tube fitted with a Teflon valve was charged with 15 mg of 2, 0.5 mL of benzene-d6, and 3 atm of H2. Warming to 80 °C for 24 h resulted in a small amount of red-brown precipitate and a red solution; the supernatant was estimated to be ∼80% 1 by NMR. Crystals of 1 suitable for X-ray diffraction were grown by slow evaporation of a crude mixture of 1 3 from toluene. 1H NMR (C6D6, 400.13 MHz, 20 °C): δ 7.82 (br s, 1H; C6H4(CH2P(CF3)2)2), 6.82 (br s, 2H; C6H4(CH2P(CF3)2)2), 6.67 (br s, 1H; C6H4(CH2P(CF3)2)2), 4.75 (br s, 3H; μη6,k3-C6H3(CH2P(CF3)2)2), 3.36 (d, 2JHP = 7 Hz, 2H; C6H4(CH2P(CF3)2)2), 3.21 (br d, 2JHH = 15 Hz, 2H; μ-η6,k3-C6H3(CH2P(CF3)2)2), 3.13 (d, 2JHP = 10 Hz, 2H; C6H4(CH2P(CF3)2)2), 2.48 (br d, 2JHH = 15 Hz, 2H; μ-η6,k3-C6H3(CH2P(CF3)2)2), 11.54 (ps q, 2 JHP = 28 Hz, 1H; RuH), 14.13 (d, 2JHP = 40 Hz, 2H; exchanging Ru(μ-H), RuH). 31P{1H} NMR (C6D6, 161.97 MHz, 20 °C): δ 82.9 (m, 3P; overlapping C6H3(CH2P(CF3)2)2 (2P) and μ-C6H4(CH2P(CF3)2)2 (1P)), 77.5 (m, 1P; μ-C6H4(CH2P(CF3)2)2). 19F NMR (C6D6, 376.5 MHz, 20 °C): δ 57.6 (br s, 6F; PCF3), 61.1 (d, 3 JFF = 63 Hz, 6F; μ-PCF3), 62.9 (d, 3JFF = 65 Hz, 6F; PCF3), 63.6 (ps. t, JFP = 37 Hz, 6F; PCF3). (CF3PCP)Ru(cod)H (2). Ru(cod)(η3-2-methylallyl)2 (0.300 g, 0.940 mmol), CF3PCPH (263 uL, 0.940 mmol), 1,5-cyclooctadiene (174 uL, 1.41 mmol), and 8 mL of toluene were added to a medium-walled Pyrex reaction tube (ca. 30 mL) fitted with a Teflon valve and placed under 3 atm of H2. After it was heated for 30 min at 130 °C, the orange solution was transferred to a round-bottom flask and the volatiles were removed. The yellow residue was triturated with petroleum ether and stirred for 10 min at room temperature to give a yellow solution. The volume was reduced to 5 10 mL and then cooled to 78 °C and cold filtered to give an off-white powder (0.342 g, 56% yield). Crystals suitable for X-ray diffraction were grown by slow evaporation from toluene. Anal. Calcd for RuP2F12C20H20: C, 36.88; H, 3.09. Found: C, 36.48; H, 2.89. 1H NMR (C6D6, 400.13 MHz, 20 °C): δ 6.72 (s, 3H; C6H3(CH2P(CF3)2)2), 4.24 (s, 2H; vinylic cod), 3.61 (m, 2H; C6H3(CH2P(CF3)2)2), 3.40 (d, 2JHH = 17 Hz, 2H; C6H3(CH2P(CF3)2)2), 3.07 (s, 2H; vinylic cod), 2.27 (m, 4H; aliphatic cod), 1.73 (m, 4H; aliphatic cod), 11.94 (t, 2JHP = 24 Hz, 1H; RuH). 31P{1H} NMR (C6D6, 161.97 MHz, 20 °C): δ 102.7 (m). 5138

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Organometallics F NMR (C6D6, 376.50 MHz, 20 °C): δ 55.8 (m, 6F; PCF3), 63.0 (t, JFP = 31 Hz, 6F; PCF3). [(CF3PCP)Ru(H)]2(μ-CF3PCPH)2 (3). Ru(cod)(η3-2-methylallyl)2 (0.300 g, 0.940 mmol), CF3PCPH (526 uL, 1.88 mmol) and 8 mL of toluene were added to a medium-walled Pyrex reaction tube fitted with a Teflon valve and placed under 3 atm of H2. After the mixture was warmed to 130 °C for 1 h, the resulting orange solution turned yellow upon cooling to room temperature. The solution was transferred to a round-bottom flask, and the volatiles were removed. The resulting yellow residue was triturated with petroleum ether and filtered to give a white powder (660 mg, 36% yield). Complex 3 is only slightly soluble in benzene, methylene chloride, methanol, acetone, or dimethyl sulfoxide at room temperature. Dilute NMR spectra were obtained by dissolving 3 in hot C6D6 then cooling to ambient temperature. Crystals suitable for X-ray analysis were obtained from warm benzene. Anal. Calcd for Ru2P8F48C48H30: C, 29.29; H, 1.53. Found: C, 29.50; H, 1.95. 1 H NMR (C6D6, 400.13 MHz, 20 °C): δ 7.40 6.60 (m, 14H; overlapping C6H3(CH2P(CF3)2)2 and C6H4(CH2P(CF3)2)2), 3.90 3.20 (m, 16H; (CH2P(CF3)2)2), 9.80 (m, 2H; RuH). 31P{1H} NMR (C6D6, 161.97 MHz, 20 °C): δ 88.1 (m, 4P; C6H3(CH2P(CF3)2)2), 72.0 (m, 2P; μ-C6H4(CH2P(CF3)2)2), 59.7 (m, 2P; μ-C6H4(CH2P(CF3)2)2). 19F NMR (C6D6, 376.50 MHz, 20 °C): δ 53.4 (m, 12F; PCF3), 55.2 (m, 12F; PCF3), 56.0 (s, 12F; PCF3), 64.2 (s, 12F; PCF3). (CF3PCP)Ru(dfepe)H (4). (CF3PCP)Ru(cod)H (0.300 g, 0.460 mmol), dfepe (158 uL, 0.460 mmol), and 8 mL of toluene were added to a medium-walled Pyrex reaction tube fitted with a Teflon valve and placed under 3 atm of H2. After the mixture was warmed to 130 °C for 30 min, white crystals fell from solution upon cooling. After removal of volatiles and trituration of the residue with petroleum ether, cooling to 78 °C and cold filtration afforded a white powder (0.260 g, 51%). Crystals suitable for X-ray diffraction were grown by slow evaporation from benzene. Anal. Calcd for RuP4F32C22H12: C, 23.82; H, 1.09. Found: C, 23.36; H, 1.10. 1H NMR (C6D6, 400.13 MHz, 20 °C): δ 6.78 (m, 3H; C6H3(CH2P(CF3)2)2), 3.70 (m, 4H; C6H3(CH2P(CF3)2)2), 1.81 (m, 4H; ((C2F5)PCH2)2), 8.85 (dm, 2JHP = 105 Hz, 1H; RuH). 31 1 P{ H} NMR (C6D6, 161.97 MHz, 20 °C): δ 101.7 (m, 1P; (C2F5)2PCH2CH2P(C2F5)2), 93.6 (m, 2P; C6H3(CH2P(CF3)2)2), 86.3 (m, 1P; (C2F5)2PCH2CH2P(C2F5)2). 19F NMR (C6D6, 376.50 MHz, 20 °C): δ 52.8 (br s, 6F; PCF3), 64.6 (t, 3JFF = 34 Hz, 6F; PCF3), 76.0 (s, 6F; CF2CF3), 76.1 (s, 6F; CF2CF3), 102 to 108 (overlapping ABX multiplets, 8F; CF2CF3). Transfer Dehydrogeation Studies. A stock catalyst solution was prepared from 5 mg of (CF3PCP)Ru(cod)H (7.7 μmol) and 3.12 mL of cyclooctadiene (3030 equiv). A 250 μL portion of stock solution and 240 μL of tert-butylethylene (3030 equiv) were added to a 5 mm NMR tube fitted with a Teflon valve with an acetone-d6 capillary external lock, and the mixture was then heated to the desired temperature in an oil bath. NMR tubes were removed from the oil bath periodically and cooled to room temperature before collecting spectra. The amounts of cyclooctene produced were calculated by integration of the vinylic coe resonance at 4.96 ppm against the tbe resonance at 5.18 ppm. Acceptorless Dehydrogenation Studies. A solution was prepared from 5 mg of (CF3PCP)Ru(cod)H (7.7 μmol) and 3.12 mL of cyclooctadiene (3030 equiv), and mesitylene (5.3 μL, 5 equiv) was added as an internal standard. The solution was refluxed under a flow of N2 for the desired time before cooling to room temperature and withdrawing an aliquot for NMR analysis. The amounts of cyclooctene produced were calculated by integration of the product vinylic resonance at 4.96 ppm against the mesitylene aromatic resonance at 5.90 ppm. X-ray Crystallography. The X-ray diffraction data for all complexes were measured at 150 K on a Bruker SMART APEX II CCD area detector system equipped with a graphite monochromator and a Mo KR fine-focus sealed tube operated at 1.5 kW power (50 kV, 30 mA). 19

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Crystals were attached to glass fibers using Paratone N oil. Collection and refinement details are included in the Supporting Information.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text and tables giving X-ray diffraction data collection and refinement details and CIF files giving crystallographic data for complexes 1 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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