Terminal Phosphido Complexes of the Ru(η5-Cp*) Fragment

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Terminal Phosphido Complexes of the Ru(η5‑Cp*) Fragment Jin Yang,† Sophie Langis-Barsetti,† Hayley C. Parkin,† Robert McDonald,‡ and Lisa Rosenberg*,† †

Department of Chemistry, University of Victoria, P.O. Box 1700 STN CSC, Victoria, British Columbia V8W 2Y2, Canada X-ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

‡

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S Supporting Information *

ABSTRACT: In situ generation of the five-coordinate complex Ru(η5Cp*)(PR2)(PPh3) (2), via dehydrohalogenation of Ru(η5-Cp*)Cl(PR2H)(PPh3), has allowed its reactivity toward a range of small molecules to be compared with that of its well-studied analogue Ru(η5-indenyl)(PR2)(PPh3) (1), in a study designed to assess the likelihood of variable hapticity in the chemistry of complex 1. Reactions of 2 with hydrogen, carbon monoxide, phenylacetylene, ethylene, acrylonitrile, and 1-hexene demonstrate enhanced nucleophilicity/basicity of the terminal phosphido ligand in 2 relative to that in complex 1. Complex 2 also exhibits greater lability of the PPh3 ligand, leading to substitutional product mixtures that were not observed for 1. Both of these features are consistent with the more electron-rich and sterically imposing nature of the Cp* ligand in 2 relative to the indenyl ligand in 1. Nevertheless, the fundamental transformations of the phosphido ligand are comparable for the two complexes. This suggests that variable hapticity does not play a role in reactions of indenyl complex 1, since η5−η3 shifts are unlikely to occur for Cp* complex 2. The implications of these reactivity studies for the design of highly active, yet stable, ruthenium half-sandwich catalysts for hydrophosphination are discussed.

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INTRODUCTION Metal complexes with terminal phosphido ligands have been implicated as intermediates in catalytic hydrophosphination (addition) and phosphination (cross-coupling) reactions.1 Their participation in both of these processes is often proposed to rely on the established nucleophilicity of the M-PR2 fragment, for example in outer-sphere nucleophilic attack of the phosphido ligand at activated substrates (e.g. RX or Michael-type alkenes) or inner-sphere migratory insertion processes.1e Our studies of phosphido complexes of a ruthenium indenyl half-sandwich fragment have demonstrated both inner- and outer-sphere reactivities,2 and we have reported the activity of this system for the catalytic hydrophosphination of activated alkenes, which apparently relies on outer-sphere P−C bond formation.3 Two key features of the low-coordinate phosphido complex Ru(η5-indenyl)(PR2)(PPh3) (1) drive its reactivity (e.g. Scheme 1). One is Lewis acidity at Ru; adducts with L-donors such as CO form readily (a), which disrupts the Ru = P double bond, changing the geometry at phosphorus from planar to pyramidal.2a,d,4 The other is nucleophilicity/basicity at P; for example, polar addenda such as MeI or HCl react to place electropositive Me+ or H+ at P (b).2a,d,4 The strength of this Pbasicity is illustrated by the addition of acetonitrile to 1 when R = Cy or Pri: instead of an N-bound nitrile adduct, the metalated complex Ru(η5-indenyl)(CH2CN)(PR2H)(PPh3) results (c).2a These two features appear to work in concert in the addition of H2 and in the insertion of unactivated © XXXX American Chemical Society

Scheme 1. Reactivity of Indenyl Phosphido Complex 1

alkenes such as ethylene and 1-hexene into the Ru−P bond in 1 (d).2d,e We have been asked whether variable (η5 → η3) hapticity of the indenyl ligand is important in the reactions described above. We are reasonably certain that it is not, since we have found no evidence for ring-slippage in our studies so far, and because the change of phosphido geometry from planar to pyramidal apparently provides ready access to a vacant coordination site at Ru, for example in the insertion of ethylene into the Ru-PR2 bond.2e Also, kinetic studies of phosphine substitution at the parent complex Ru(η5-indenyl)(Cl)(PPh3)2 and a cationic derivative [Ru(η5-indenyl)(NCPh)(PPh3)2]+ point to a dissociative mechanism, rather than the associative mechanism that would belie the Received: April 24, 2019

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DOI: 10.1021/acs.organomet.9b00266 Organometallics XXXX, XXX, XXX−XXX

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Organometallics participation of variable hapticity.5 Nevertheless, we initiated study of the analogous Cp* system to address this issue, since pathways relying on variable hapticity should be disfavored for this η5-ligand.6,7 Further incentive to take a closer look at the behavior of phosphido ligands at the Ru(η5-Cp*) fragment relative to Ru(η5-indenyl) comes from the report of Morris and coworkers of the activity of Ru(η5-Cp*)(PPh2)(PP) complexes, where “PP” is 1,2-bis(diphenylphosphino)ethene or -benzene, in the catalytic hydrophosphination of acrylonitrile.8 These complexes show initial activities an order of magnitude greater than we observe for our indenyl system, for which the structurally similar Ru(η5-indenyl)(PPh2)(PPh2H)(PPh3) has been implicated as an active species.3 However, unlike the indenyl system, these Cp* catalysts deactivate rapidly, via an unknown mechanism, so the reaction does not go to completion. Here we report in situ generation of the highly reactive planar phosphido complex Ru(η5-Cp*)(PR2)(PPh3) (2) and an examination of its reactivity with a variety of small molecules. These studies show sufficient similarities to the established chemistry of the indenyl complex 1 to suggest that ring-slippage does not play an important role in these halfsandwich systems. However, they also highlight some striking differences in reactivity that can be attributed to increased steric crowding at the Ru(η5-Cp*) fragment and enhanced Pbasicity of its phosphido complexes. The implications of these results for harnessing the Cp*Ru core in catalytic hydrophosphination are discussed.

The 1H and 13C NMR spectra for complexes 3a−d are very similar to those for their η5-indenyl analogues, including significant broadening of the 1H signals for PPh3 in 3a,c,d, which is attributed to slowed rotation around the Ru-PPh3 bond on the NMR time scale for these sterically congested complexes.10 31P{1H} NMR spectra show comparable shifts for signals due to the PPh3 ligand (δ 45−50 ppm), but signals due to the secondary phosphines, which for the indenyl complexes appear at lower field than the PPh3 signals (δ 50−68 ppm), all appear at higher field (δ 34−48 ppm) for the Cp* complexes. This corresponds to a smaller Δδ 31P relative to the chemical shifts for the free secondary phosphines. These 31P{1H} signals for 3a−d also exhibit slightly lower 2JPP values (40−43 Hz, compared to 42−49 Hz for the indenyl complexes). We presume that the greater substitutional lability leading to oversubstitution in these complexes derives primarily from increased steric hindrance associated with the bulky Cp* ligand. Substitution and oversubstitution is driven by the loss of relatively bulky PPh3. Some evidence for the importance of this steric hindrance in solution comes from the observation of ligand redistribution when analytically pure, crystalline 3a (R = Cy) is dissolved in C6D6; 31P{1H} NMR shows trace (1.3%), equimolar amounts of both Ru(η5-Cp*)Cl(PPh3)2 and 4a in solution, in addition to 3a. PCy2H is about the same size as PPh3 (cone angles 148° and 145°, respectively); no such redistribution is seen for the η5-indenyl analogue of 3a (3ai),10 or for complexes 3b−d, which contain smaller phosphines. Solid state structures of 3a,b (Figures 1, 2) also give some indication of the impact of the steric bulk of the Cp* ligand.

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RESULTS AND DISCUSSION Synthesis and Characterization of Secondary Phosphine Precursors. We prepared secondary phosphine complexes 3a−d from Ru(η5-Cp*)Cl(PPh3)2 (Scheme 2). Scheme 2. Synthesis of Complex 3

These substitution reactions occur rapidly at RT in toluene, and, unlike the analogous indenyl chemistry, inevitably give small to significant amounts of the disubstituted products 4a− d. Lubian and Paz-Sandoval previously reported the synthesis of the diphenylphosphine complex 3c; they too describe the ease of formation of 4c in this reaction.9 For the indenyl system, the issue of unintended oversubstitution arose only for smallest of the secondary phosphines examined, PEt2H.10 In that case the Cl− was substituted to give [Ru(η5-indenyl)(PEt2H)2(PPh3)]Cl as the major product; we see no evidence for analogous substitution of the chloride ligand in these reactions of the Ru(η5-Cp*) fragment. The high solubility of these Cp* complexes in even nonpolar hydrocarbons posed challenges in the isolation and purification of 3. However, the slightly higher solubility of 4 allowed the fractional crystallization of 3 from toluene layered with pentane in all cases except for 3c (R = Ph), which typically contained about 4% 4c. (The literature synthesis of 3c also leads to an impurity of 5% 4c in the isolated complex.9)

Figure 1. Molecular structure of Ru(η5-Cp*)Cl(PCy2H)(PPh3) (3a) showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 30% probability level. The hydrogen atom attached to P1 is shown with arbitrarily small thermal parameters.

While the Ru−C5(centroid) and Ru−PPh3 distances are close to those previously observed for the indenyl analogue of 3a (see Supporting Information), the Ru-PR2H distances are significantly longer for the two Cp* complexes: 2.33 Å (R = Cy) and 2.29 Å (R = Et), compared with 2.26 Å for Ru(η5indenyl)Cl(PCy2H)(PPh3).10 These longer Ru−P bonds, along with the solution 31P{1H} chemical shift and 2JPP data, are consistent with weaker coordinate bonding of the secondary phosphines in 3 relative to their indenyl analogues. Dehydrohalogenation Reactions of 3a−d. Complexes 3a−d react with ∼1.2 equiv of KOBut in C6D6 to give a color change from clear orange to dark red (3a,b) or dark blue (3c,d), consistent with the formation of the five-coordinate B

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Organometallics

Figure 2. Molecular structure of Ru(η5-Cp*)Cl(PEt2H)(PPh3) (3b) showing the atom-labeling scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids at the 30% probability level. The hydrogen atom attached to P1 is shown with arbitrarily small thermal parameters.

complexes Ru(η5-Cp*)(PR2)(PPh3) (2a−d) (Scheme 3). While NMR analysis of these mixtures confirms the presence Figure 3. Time-dependent speciation plots for the reactions of two dialkylphosphine complexes, 3ai and 3b, with KOBut (300.27 MHz 1 H NMR, C6D6).

Scheme 3. Dehydrohalogenation of 3 Using KOBut

Ru(η5-Cp*)H(PCy2H)(PPh3) (6a, vide infra), as determined by 1H and 31P{1H} NMR, apparently resulting from β-H elimination from an intermediate butyl complex (Scheme 4; Scheme 4. Dehydrohalogenation of 3a Using n-BuLi

of 2 (diagnostic 31P{1H} signals for the planar PR2 ligand appear as doublets at low field (δ 156−211 ppm) with relatively large coupling to PPh3 (δ 56−63 ppm, 2JPP 67−72)), during the time taken for consumption of 3 the reactions give significant amounts of a second product, 5, resulting from irreversible orthometalation of the PPh3 ligand in 2 (e.g., Scheme 3). Complexes 5a−d show upfield 31P{1H} NMR signals for the κ2-(o-C6H4)PPh2 ligand at −10 to −19 ppm, similar to the shifts we observe for their η5-indenyl analogues.2d,4 We have been unable to isolate pure 2a−d free of orthometalated 5a−d due to the high solubility of both complexes and the steady conversion from 2 to 5 during workup. Longer reaction times give the orthometalated complexes 5a−d as the major products of all these dehydrohalogenation reactions, and 5b−d can be isolated. For 3a (R = Cy) the presence of other products in the reaction mixture (Figure S15, Scheme S2) has precluded isolation of pure 5a. Monitoring the progress of these dehydrohalogenation reactions provided insight into the steric and electronic characteristics of this Ru(η5-Cp*) system. Time-dependent speciation (Figure 3a) shows that indenyl complex 3ai (R = Cy) is consumed quite rapidly over 1 h in its reaction with KOBut, and that subsequent decomposition to orthometalated 5ai is very slow (the reaction takes days at RT). The analogous Cp* complex 3a reacts much more slowly, showing just 40% consumption over 4 h (Figure S15). The particular steric congestion at 3a must play some role in the slow rate of deprotonation, since the less hindered but comparably electron-rich diethylphosphine complex 3b is consumed in this reaction within 2 h (Figure 3b). However, attempted dehydrohalogenation of 3a using the less sterically hindered base n-BuLi gave butene and the ruthenium hydride complex

transient 31P{1H} signals due to the putative intermediate were observed (Figure S19)). This selectivity for chloride substitution and β-H elimination, rather than dehydrohalogenation, is consistent with the previously observed reaction of 3ai with the comparably small base NaOMe, which gives the hydride complex Ru(η5-indenyl)H(PCy2H)(PPh3).10 Progress of the dehydrohalogenation of 3a is complicated by the competing ligand redistribution chemistry described above, but complex 3b (Figure 3b) reacts cleanly, and serves to highlight the apparent impact of greater electron density in the Cp* system, since the steric impact is probably balanced by the small size of the PEt2H ligand. The consumption of 3b takes approximately twice as long as 3ai; this is consistent with ratelimiting proton transfer that is slowed by less facile polarization of the P−H bond in the more electron-rich Cp* complex. This impact of the electron-rich Cp* ligand is notably offset by the more acidic diarylphosphines in complexes 3c,d (1H δPH ∼ 6.9 ppm, relative to ∼4.1 ppm for the dialkyl complexes 3a,b) during dehydrohalogenation; these complexes are consumed in under 30 min (Figures S17−S18), even faster than 3ai is consumed under identical conditions. For all four phosphido products 2a−d, orthometalation occurs much faster than for the indenyl phosphido complex 1, giving 60−80% 5a−d over 4 h (Figures 3, S15−S18). This result is consistent both with higher P-basicities of 2 relative to C

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Organometallics 1, which favors this intramolecular deprotonation reaction (accordingly, the diaryl complexes 2c,d orthometalate slightly more slowly than the dialkyl complexes 2a,b), and with steric compression of the non-Cp* ligands in 2 relative to 1, which places the ortho-C−H bonds of PPh3 in 2 closer to the phosphido P. Reactions of Phosphido Complexes 2 with Small Molecules. To limit the rapid conversion of phosphido complexes 2 to orthometalated 5 during NMR-scale reactions of 2 with a variety of different reagents, we generated 2 in situ under trapping conditions, adding the reagents of interest to complex 3 in the presence of base. As described below, this worked well for the reactions of complexes 3b−d, giving relatively clean product mixtures, for which the major products have been completely spectroscopically characterized via 1H, 13 C, and 31P NMR (see below and Supporting Information). More complex mixtures were observed for the reactions of complex 3a (R = Cy), due partly to the presence of equilibrium amounts of phosphine redistribution products (vide supra) and partly to the extreme steric crowding in this complex; the latter slows the rate of intermolecular reactions relative to intramolecular orthometalation to give complex 5a, and also allows the formation of some unusual low coordinate species and substitution products. Hydrogen. Similar to their indenyl analogues,2d,4 complexes 2b−d react rapidly with dihydrogen to give yelloworange solutions in which the major products are Ru(η5Cp*)H(PR2H)(PPh3), 6b−d, resulting from 1,2-addition across the Ru−P double bond (Scheme 5). These complexes

Scheme 6. Product Distribution after 3 h for Reaction of Dihydrogen with 2a Generated in Situ

Figure 4. Partial 202.51 MHz 31P{1H} NMR showing product mixture resulting from addition of H2(g) to in situ-generated 2a in C7D8. Inset: hydride region of 500.27 MHz 1H NMR. [Ru] = Ru(η5Cp*).

phosphine analogue Ru(η5-Cp*)H3(PCy2H). These doublets (2JHP ∼ 20 Hz) are consistent with the trigonal bipyramidal geometry shown in Figure 4, in which all three hydride ligands are in equivalent equatorial sites.12 However, adjacent to each of the 31P{1H} singlets for these complexes is an additional singlet of lower intensity, which we attribute to the presence of square-based pyramidal isomers.13 Variable, low temperature 1 H NMR confirms the presence of additional hydride signals due to these isomers and points to fluxionality of these less symmetric complexes on the NMR time scale (see Supporting Information). These trihydride complexes probably form via the substitution of PPh3 by η2-H2 (and subsequent oxidative addition), for example during dehydrohalogenation of the halide precursors Ru(η5-Cp*)Cl(PPh3)2 and 3a. (We see signal due to free PPh3 in the 31P{1H} NMR spectra of this mixture but do not see any free PCy2H.) We do not observe this oxidative addition chemistry during the 1,2-addition of hydrogen to the analogous indenyl complex 1,4 probably because the PPh3 in that complex is not labile toward substitution by H2, and possibly also because 1 is less electronrich than 2a. Carbon Monoxide. Complexes 2a−d react readily with CO in sealed NMR tube experiments to give bright orange Ru(η5-Cp*)(PR2)(CO)(PPh3), 7a−d (Scheme 7). As for their indenyl analogues,2d,4 31P{1H} NMR signals for these carbonyl adducts show very small (negligible, in this case) 2JPP coupling between the pyramidal PR2 ligand and PPh3. For 7b−d,

Scheme 5. Reaction of Dihydrogen with 2b−d Generated in Situ

show triplets (2JHP ∼ 35 Hz) in the hydride region of their 1H NMR spectra (−12 to −13 ppm). For the diaryl phosphine complexes these 1,2-addition reactions are complete within 30 min, while for the diethylphosphine complex, which is generated more slowly via dehydrohalogenation, the reaction is complete within 3 h at room temperature. Small amounts of complex 5b−d also form in these reactions, and complex 6c (R = Ph) is also accompanied by ∼2% of Ru(η5-Cp*)H(PR2H)2, which forms from the small amount of 4c present in the starting material. Complexes 6b−d are stable in the absence of H2, and can be isolated as yellow powders. Although the orthometalated complex 5a (R = Cy) dominates the reaction mixture after 3 h when hydrogen is added to in situ-generated 2a, 31P{1H} and 1H NMR signals for a small amount of the expected hydride complex 6a are also observed (Scheme 6, Figure 4). In addition, the 1H NMR spectrum shows signals due to three other hydride-containing products, each of which correlates with a singlet in the 62−72 ppm region of the 31P{1H} NMR spectrum (1H/31P HMBC). One hydride signal is a triplet that corresponds to Ru(η5Cp*)H(PCy2H)2, which forms from the small equilibrium amount of 4a present in the starting material. The two other hydride signals are doublets that correspond to the known trihydride complex Ru(η5-Cp*)H3(PPh3)11 and its secondary

Scheme 7. Reaction of Carbon Monoxide with 2 Generated in Situ

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Organometallics slightly broadened singlets (ω1/2 ∼ 15 Hz) are observed for both ligands (δPR2 15−30 ppm, δPPh3 54−61 ppm). For 7a the Ru-PPh3 signal at 57 ppm is also a singlet, but the Ru-PCy2 is significantly broadened (ω1/2 58 Hz) and shifted downfield to 63 ppm. This probably indicates a solution equilibrium between the planar phosphido complex 2a (δPCy2 211 ppm) and the pyramidal CO adduct 7a. The relative instability of 7a with respect to 2a again reflects the amplification of steric congestion at this Cp*Ru core with PCy2 or PCy2H ligands. We previously observed a similar equilibrium for the pyridine adduct of the indenyl analogue 2ai, but not for the adduct of the smaller CO ligand.2a Although complexes 7b−d can be isolated as bright yellow (7b) or orange (7c,d) powders, they show some decomposition to as-yet unidentified products during workup, as determined by IR (for 7c, which showed the most decomposition, ∼30%) and 31P{1H} NMR (Figures S51−S52). This data suggests the new complexes still contain terminal CO and phosphido ligands, but further work is required to characterize these products. The Ru-PPh3 ligand in carbonyl adducts 7a−d can be substituted by excess CO in the sealed NMR samples to give the yellow bis-carbonyl adducts Ru(η5-Cp*)(PR2)(CO)2, 8a− d (Scheme 7). This reaction occurs for the indenyl analogues only when samples are left under an atmosphere of CO for 15 d or longer,14 but for complexes 7b−d we see impurities of 5− 13% 8b−d, respectively, after 3 h. The added steric congestion in 7a facilitates this further substitution; 7a is the major product observed after 30 min, but it is completely consumed within 3 h to give 85% 8a, along with 8% 5a and some other, unidentified products. Phenylacetylene. The relatively sterically unhindered 2b (R = Et) undergoes insertion of phenylacetylene to give a phosphametallacyclobutene complex 9b as the major product (80%, Scheme 8). The metallacyclic PEt2 group in 9b shows a

diethylphosphido ligand at the Cp*Ru core in 2b, relative to the dicyclohexylphosphido ligand in 1. The diarylphosphido complexes 2c,d are slightly less P-basic than 2b, based on their slower decomposition via orthometalation (vide supra). However, the reactions of 2c,d with phenylacetylene gave the alkynyl products 10c,d as the major products (82− 85%),15,16 accompanied by 15−18% of the insertion products 9c,d. Since the diarylphosphido ligands in 2c,d are bulkier than the diethylphosphido ligand in 2b, these results may represent a balance point in terms of the ability of the phenylacetylene to approach closely enough to Cp*Ru to allow insertion (an inner-sphere process) and the basicity and availability of the lone pair at the phosphido P.17 In this context, we might expect the reaction of phenylacetylene with the very P-basic and sterically hindered 2a to strongly favor formation of the alkynyl complex 10a over the insertion product 9a. This selectivity is indeed observed, in that we see some (∼20%) 10a and no 9a at all, but the reaction is once again dominated by intramolecular decomposition to the orthometalated complex 5a (57%), and small amounts of some other products are observed (Figure S56). Reduced steric congestion in the PEt2-containing complex 2b also permits its low catalytic activity for alkyne dimerization (Scheme 9). When ∼10 equiv phenylacetylene is added to 2b Scheme 9. Dimerization of Phenylacetylene Catalyzed by 2b

Scheme 8. Reaction of Phenylacetylene with 2b−d Generated in Situ

we observe signals due to the E and Z isomers of the dimer 1,4diphenyl-1-buten-3-yne, as determined by 1H COSY NMR (Figure S65), in addition to signals due to 9b and 10b. Dimerization of terminal alkynes is known to be catalyzed by a variety of half-sandwich Ru complexes.18 In this system, the reaction probably involves substitution of PPh3 in 10b by PhCCH to give an intermediate η2-alkyne complex, which subsequently rearranges to two conformers of a vinylidene/ alkynyl complex, followed by insertion of the carbene portion of the vinylidene ligand into the Ru−Cα bond of the alkynyl ligand (Scheme 9).19 We do not observe this dimerization chemistry for the other, more sterically crowded phosphido complexes 2a,c,d, or for the (less labile) indenyl system. Ethylene. Complexes 2a−d react with ethylene to give mixtures of two insertion products (Scheme 10). One of these

diagnostic upfield 31P{1H} NMR signal at −41.9 ppm. The regiochemistry of phenylacetylene insertion in 9b is confirmed by correlation of the β-H signal at 7.45 ppm with the PEt2 phosphorus in the 1H/31P HMBC spectrum, and with the β-C signal at 125.0 ppm in the 1H/13C HSQC spectrum. This 13C signal shows larger coupling to PEt2 (1JCP = 43 Hz) than to PPh3 (3JCP = 2 Hz). The quaternary α-C signal in 9b could not be detected directly, but is observed indirectly in the 1H/13C HMBC spectrum at ∼193 ppm, through its correlation with signals due to the ortho protons on the adjacent phenyl group. 31 1 P{ H} signals for a minor product (20%) of this reaction include a doublet (2JPP = 38 Hz) due to PEt2H at 41.5 ppm, which shows 1JPH = 343 Hz in the 31P spectrum; we presume this product is the terminal alkynyl complex 10b that results from deprotonation of the alkyne by RuPEt2. The reaction of indenyl complex 1 with phenylacetylene gave analogous products to 9b and 10b,2b but the product distribution leaned more heavily toward insertion (95%) than deprotonation (5%). This points to higher basicity of the

Scheme 10. Reaction of Ethylene with 2 Generated in Situ

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Organometallics is the metallacyclic complex 11, analogous to the ethylene insertion product obtained for the reaction of indenyl complex 1a with ethylene.2c,e For 2b−d, a second metallacyclic product, 12, results from the replacement of PPh3 in 11 by an η2ethylene ligand. Complexes 2b−d are consumed within 2 h in these reactions with ethylene and give small amounts of the orthometalation byproducts 5b−d (1−7%), while complex 2a reacts more slowly (3−4 h) to give 5a as the major product, along with just 14% 11a, 5% 12a, and other minor, unidentified products (Figure S66). Complex 11b dominates the product mixture for R = Et (82%), while the η2-ethylene adducts 12c,d are the major products (74−83%) for the reactions of diaryl phosphido complexes 2c,d with ethylene. We did not observe substitution of PPh3 by ethylene for the indenyl analogue of 11. Complexes 11 and 12 show diagnostic high field 31P{1H} NMR signals for the PR2 moiety incorporated into the phosphametallacyclobutane (e.g. −13.0 ppm and −15.3 ppm for the PCy2 group in 11a and 12a, respectively). Relatively high field 13C{1H} shifts for the metallacycle carbon α- to Ru (Figure 5) are also diagnostic of these structures (e.g. −16.1

Figure 6. 202.51 MHz 31P{1H} NMR of a sample of 2b in C7D8 that was cooled to 0 °C in an ice bath while being sealed under an atmosphere of ethylene. The bottom spectrum was run immediately after the sample was sealed; the top spectrum was obtained 3 h later, after the sample had warmed to RT.

occurred via an η2-ethylene adduct (e.g. Figure 6), and were able to identify this intermediate both visually (it is a deep red color, while 1 is deep blue) and spectroscopically, by monitoring the reaction at −80 °C by NMR.2e In the current studies we have been able to detect the formation of a comparable intermediate by monitoring the reactions of ethylene with samples of 2b−d that have been cooled to 0 °C. As for the indenyl system, an intense red color is associated with formation of the adduct, which then changes to yelloworange as the mixture is warmed to RT. 31P{1H} NMR signals attributable to this intermediate in formation of the PEt2 metallacycle 11b are marked with red dots in the bottom spectrum in Figure 6. The relatively sharp signal at 53 ppm is assigned to PPh3 and a broad singlet at −1 ppm is assigned to the pyramidal PEt2 ligand. As for the analogous indenyl complex, the breadth and relatively high field shift of the PEt2 signal probably result from an equilibrium between the η2adduct and the final 1,2-insertion product, 11b. Acrylonitrile and 1-Hexene. Since the indenyl phosphido complex 1 reacts with a wide range of alkenes to give substituted metallacyclic products analogous to 11, we screened two representative terminal alkenes for their reactivity with 2, electron-deficient acrylonitrile and the simple, electronrich 1-hexene. Unfortunately, in the presence of the KOBut used to generate 2 in situ, the activated acrylonitrile apparently polymerizes;20 these attempted reactions gave pale yellow precipitates, and 31P{1H} NMR indicated only unreacted 3 was present in solution. The most striking difference in reactivity that we observe for the Cp*Ru phosphido complex 2 relative to the indenyl complex 1 occurs for the addition of 1-hexene to 2. We anticipated alkene insertion comparable to the ethylene insertion reactions described above, to yield n-butylsubstituted phosphametallacylobutanes. Instead, though, the diaryl complexes 2c,d are consumed within 3 h in the presence of this electron-rich, terminal alkene to give mixtures of the η3allyl complexes 13c,d and the orthometalated complexes 5c,d (Scheme 11). 31P NMR signals for 13c,d show large 1JPH (324−325 Hz) for the secondary phosphine, while 1H spectra show diagnostic downfield multiplets due to the central proton

Figure 5. Positional labeling scheme for metallacycles 11 and 12.

ppm for the PEt2 complex 11b), while the 13C signal for the βcarbon is typically shifted downfield (e.g. 36.3 ppm for 11b). The metallacycle protons HA‑D show distinct 1H NMR multiplets that were assigned through multinuclear twodimensional correlation and NOE experiments (see Supporting Information); the signal for HD is shifted downfield relative to the other protons on the ring, typically appearing between 3 and 4 ppm. Two broad signals of equal intensity due to the η2ethylene ligand in complexes 12c,d appear between 1.6 and 2.0 ppm in the 1H NMR. These show NOE interactions with HC on the metallacycle, correlations in the 1H/31P HMBC spectra with the metallacycle PAr2 phosphorus, and correlations with each other in the 2D 1H/1H TOCSY spectra. Cooling a sample of 12c to −70 °C causes decoalescence of the two ethylene 1H signals to four multiplets of equal intensity, consistent with slowed rotation around the ethylene-Ru bond (Figure S72). However, we see no signal due to the η2-ethylene carbons in the 13C{1H} NMR spectra of these complexes, even at −70 °C. Likewise, in 1H and 13C{1H} spectra of the reaction mixture resulting from addition of ethylene to 2b (R = Et), we were unable to find signals for the η2-ethylene ligand in 12b, even when the sample was cooled to −70 °C. Since this species comprises just 11% of the product mixture we presume these signals are lost in the baseline or (for the 1H spectrum) overlap with other signals in the busy alkyl region. Nevertheless, we are confident of the structural assignment for complex 12b, based on the 1:1 intensity of the 31P{1H} signals for 12b and free PPh3 in the reaction mixture (consistent with substitution of PPh3 by ethylene (Figure 6, top), and our ability to identify the metallacycle protons by in 12b by 1H/31P HMBC (Figure S69). In our studies of the reaction of ethylene with the indenyl complex 1 we established computationally that 1,2-insertion F

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Organometallics

catalysts, in which the PPh3 ligand in 2 is replaced with substrate phosphine PR2H (Ru(η5-Cp*)(PR2)(PR2H)): deprotonation of the coordinated secondary phosphine in this putative complex would simply regenerate the reactive phosphido ligand PR2−. We are currently exploring synthetic routes to this complex and the related, coordinatively saturated Ru(η5-Cp*)(PR2)(PR2H)2, in order to assess their stability with respect to orthometalation and their activity in the hydrophosphination of activated alkenes.

Scheme 11. Reaction of 1-Hexene with 2c−d Generated in Situ

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in the allyl group (e.g. δ 3.14−3.05 ppm for 13c), and signals for all protons of allyl moiety correlate with the 31P signal in the 1H/31P HMBC spectra. 13C signals for both terminal carbons of the allyl moiety correlate with 1H P−H signals from the secondary phosphine (2JCP = 4−5 Hz) in the 1H/13C HMBC spectra; no correlation with the central carbon is observed but a 2JCP of 2 Hz is observed in the 13C NMR. Although complexes 13c,d are stable and can be prepared on a larger scale, they could not be isolated free of 5c,d and PPh3, due to their high solubility even in cold pentane. Under the same conditions (Scheme 11), dialkyl phosphido complexes 2a,b do not react at all with 1-hexene, giving only the orthometalation product 5a,b.21 Thus, for these more Pbasic complexes, intramolecular deprotonation leading to orthometalation is faster than intermolecular reaction with 1hexene to give 13. Formation of the η3-allyl group in 13 requires substitution of the PPh3 ligand in 2 by the terminal alkene moiety of 1-hexene, in addition to deprotonation of the allylic C−H bond. We note that the reactions of 2a−d with ethylene shown above (Scheme 10) suggest this substitution occurs more readily for the diaryl phosphido complexes 2c,d than for 2a,b, which probably contributes to the observed reactivity differences with 1-hexene.

EXPERIMENTAL SECTION

General Details. Unless otherwise noted, all procedures were carried out under nitrogen in a glovebox or using conventional Schlenk techniques. Solvents (except methanol and ethanol, see Supporting Information) were degassed by sparging with nitrogen for 25 min and dried by passing through columns of activated alumina in a solvent purification system. Deuterated solvents (C7D8, C6D6, Sigma-Aldrich) were dried over Na/benzophenone, degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use. Unless otherwise specified, reagents were purchased from SigmaAldrich Canada or Praxair Canada and used as received or dried and degassed using established procedures. Ru(η5-Cp*)Cl(PPh3)2 was prepared by a modified literature procedure (see Supporting Information) and Ru(η5-indenyl)Cl(PCy2H)PPh3 (3ai)10 was prepared using the literature procedure. NMR spectra were recorded at ambient temperature, unless otherwise noted, on a Bruker AVANCE 500 spectrometer (500.27 MHz for 1H, 125.79 MHz for 13C, and 202.51 MHz for 31P), on a Bruker AVANCE 360 spectrometer (360.13 MHz for 1H and 145.78 MHz for 31P) or on a Bruker AVANCE 300 spectrometer (300.27 MHz for 1H and 121.55 MHz for 31P). Chemical shifts are reported in ppm and referenced to residual protonated solvent peaks: 7.26 (CHCl3), 7.16 ppm (C6D5H), 2.08 ppm (C7D7H) for 1H; 128.39 ppm (C6D6), 20.43 ppm (C7D8) for 13C. All 1H and 13C chemical shifts are reported relative to tetramethylsilane (TMS), and 31P shifts are relative to 85% H3PO4(aq). 1H, 13C, and 31P NMR assignments for all identified products are included in the Supporting Information. 31 P NMR data is also included below for isolated complexes. Melting temperatures were recorded using a Gallenkamp apparatus for capillary samples. IR spectra were recorded on PerkinElmer FTIR Spectrum One spectrophotometer using KBr pellets under a nitrogen atmosphere. Microanalysis was performed by Canadian Microanalytical Service Ltd. (Delta, BC, Canada). General Method for Synthesis of Ru(η5-Cp*)Cl(PR2H)(PPh3) (3a−d). This method is adapted slightly from a procedure previously reported for the synthesis of 3c (R = Ph).9 Toluene (20 mL) was added to a flask containing Ru(η5-Cp*)Cl(PPh3)2 (0.61−0.65 mmol) and a solution of PR2H in hexanes (0.76−1.6 mmol). (In some cases, excess phosphine was added to offset equilibria including starting material 3 and the bis(phosphine) complex 4.) The orange suspension was stirred at RT until it became a homogeneous orange solution (1−2.5 h). The solvent was removed under vacuum. The residue was dissolved in a minimum volume of toluene (∼2−3 mL), layered with pentane (∼10−15 mL), and left to stand overnight. The supernatant was decanted from the resulting solid product, which was washed with pentane (at least 3 × 10 mL) and dried under vacuum. Ru(η5-Cp*)Cl(PCy2H)(PPh3) (3a). Ru(η5-Cp*)Cl(PPh3)2 (0.52 g, 0.65 mmol) and PCy2H in hexanes (0.33 M, 2.3 mL, 0.78 mmol) were used. The mixture was stirred for 2.5 h. An orange-red powder was obtained (0.24 g, 0.33 mmol, 50% yield). Melting point: 199− 201 °C. IR (KBr, cm−1) 2329 (w, νPH). Anal. found (calcd for C40H53P2ClRu) C, 65.65 (65.60); H, 7.72 (7.30). 31P{1H} NMR (202.51 MHz, C6D6) δ 49.6 (d, 40, PPh3), 48.2 (d, PCy2H). Ru(η5-Cp*)Cl(PEt2H)(PPh3) (3b). Ru(η5-Cp*)Cl(PPh3)2 (0.52g, 0.65 mmol) and PEt2H in hexane (0.75 M, 2.0 mL, 1.5 mmol) were used. The suspension was stirred for 2 h. An orange-red microcrystalline solid was obtained (0.11 g, 0.18 mmol, 27% yield). Melting point: 196−198 °C. IR (KBr, cm−1) 2327 (w, νPH). Anal.

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CONCLUSIONS The Ru(η5-Cp*) phosphido complex 2 participates in a rich variety of chemistry with small molecules that mostly parallels that of the analogous indenyl complex 1. Given the similarity of most of the addition, insertion, and deprotonation products we observe for this rigidly η5-system to those observed for 1, we feel confident that variable hapticity is not critical to the chemistry of the indenyl system. At the same time we are intrigued by the implications of the higher phosphido P-basicity in this electron-rich system and the substitutional lability of the PPh3 ligands, in the context of possible catalytic hydrophosphination activity. For example, the phosphido ligand in 2 should participate very readily in conjugate addition at activated alkenes, the proposed P−C bond-forming step in hydrophosphination catalyzed by both our Ru indenyl system and the Ru Cp* system reported by Morris.8 However, the presence of PPh3 in 2 is a liability, since rapid orthometalation precludes isolation of this complex and limits further reactivity, even when 2 is generated in situ. Indeed, Morris comments on the challenges in identifying both ancillary ligands and unsaturated hydrophosphination substrates that will not be susceptible to deprotonation in the presence of the potent Ru(η5-Cp*) phosphido base or the basic carbanion generated during a conjugate addition step in catalysis. We suspect that orthometalation or some other ligand aryl group deprotonation may be responsible for the deactivation observed for the Morris system during catalysis. In this context, the high substitutional lability of the PPh3 ligand in our Ru(η5-Cp*) complexes presents an alternative strategy for the generation of highly active hydrophosphination G

DOI: 10.1021/acs.organomet.9b00266 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

yellow powder (0.030 g, 0.042 mmol, 32% yield). Dec. point: 178− 180 °C. IR (KBr, cm−1) 2304 (w, νPH). 31P{1H} NMR (202.51 MHz, C6D6) δ 46.9 (d, 33, PTolp2H), −13.3 (d, -PPh2-). NMR-Scale Reactions of Complex 2. These trapping reactions were carried out for complex 2 generated in situ from the combination of complexes 3a−d with base. General Procedure for Gaseous Reagents. C7D8 (1 mL) was added to a mixture of KOBut (2 mg, 0.02 mmol) and metal complex (3a: 12 mg, 0.016 mmol; 3b: 10 mg, 0.016 mmol; 3c: 12 mg, 0.017 mmol; 3d: 12 mg, 0.016 mmol) in a flame-sealable NMR tube. The sample was degassed by three freeze−pump−thaw cycles before ∼0.9 atm of gas (hydrogen, carbon monoxide or ethylene) was introduced. The tube was sealed and cooled to 0 °C in an ice bath. The sealed orange sample with a thin dark red (for 3a,b) or blue (for 3c,d) bottom layer was inverted to mix the reagents and then quickly placed in NMR spectrometer. The samples were also shaken to mix reagents between NMR experiments. Hydrogen. The solutions became yellow/orange after the samples were inverted. Carbon Monoxide. The solutions became red after the samples were inverted, and eventually turned yellow. Ethylene. The solutions became orange after the samples were inverted. General Procedure for Liquid Reagents. C6D6 (1 mL) was added to KOBut (2 mg, 0.02 mmol) in a small vial, which was allowed to dissolve completely (24 h). The metal complex (3a: 12 mg, 0.016 mmol; 3b: 10 mg, 0.016 mmol; 3c: 10 mg, 0.016 mmol; 3d: 12 mg, 0.016 mmol) and liquid reagent (phenylacetylene, acrylonitrile and 1hexene) were added to the base solution in this vial. The sample was swirled for 30 s before being transferred to an NMR tube for analysis. Phenylacetylene. The solutions became yellow-orange immediately after adding phenylacetylene (3 μL, 0.03 mmol). Acrylonitrile. The solutions stayed orange and a pale yellow precipitate formed immediately upon addition of acrylonitrile (2 μL, 0.03 mmol). Only 3 was observed in 31P{1H} spectra. 1-Hexene. The dark red (for 3a,b) or blue (for 3c,d) solution slowly changed to yellow-orange after addition of 1-hexene (10 μL, 0.08 mmol). General Method for Synthesis of Ru(η5-Cp*)H(PR2H)(PPh3) (6b−d). Toluene (10 mL) was added to a Schlenk flask containing complex 3b−d and KOBut (∼1.3 equiv). The solution was degassed by three freeze−pump−thaw cycles before H2 gas (∼0.9 atm) was introduced. The solution was stirred overnight. The resulting yellow solution was filtered through Celite to remove solid (KCl, excess KOBut) and gelatinous HOBut. Solvent was removed under vacuum from the filtrate. The resulting yellow residue was dissolved in 3 mL of pentane and stored at −22 °C to afford yellow solid 6b−d, which was washed with cold pentane (3 × 1 mL). The high solubility of 6b−d even in cold pentane precluded washing to give analytically pure product, so no microanalysis was obtained. NMR spectra show small amounts of solvent, grease, and/or unidentified products (see Supporting Information). Ru(η5-Cp*)H(PEt2H)(PPh3) (6b). Ru(η5-Cp*)Cl(PEt2H)(PPh3) (3b, 0.15 g, 0.24 mmol) and KOBut (0.035 g, 0.31 mmol) were used. A yellow powder was obtained (0.052 g, 0.088 mmol, 37%). Melting point: 152−155 °C. IR (KBr, cm−1) 2309 (m, νPH), 1877 (m, νRuH). 31P{1H} NMR (202.51 MHz, C6D6) δ 74.7 (d, 33, PPh3), 43.9 (d, PEt2H). Ru(η5-Cp*)H(PPh2H)(PPh3) (6c). Ru(η5-Cp*)Cl(PPh2H)(PPh3) (3c, 0.20 g, 0.28 mmol) and KOBut (0.040 g, 0.36 mmol) were used. A yellow powder was obtained (0.075 g, 0.11 mmol, 39%). Melting point: 130−134 °C. IR (KBr, cm−1) 2299 (m, νPH), 1908 (m, νRuH). 31P{1H} NMR (202.51 MHz, C6D6) δ 74.5 (d, 31, PPh3), 45.0 (d, PPh2H). Ru(η5-Cp*)H(PTolp2H)(PPh3) (6d). Ru(η5-Cp*)Cl(PTolp2H)(PPh3) (3d, 0.15 g, 0.20 mmol) and KOBut (0.029 g, 0.26 mmol) were used. A yellow powder was obtained (0.044 g, 0.062 mmol, 31%). Melting point: 158−160 °C. IR (KBr, cm−1) 2242 (m, νPH), 1925 (m, νRuH). 31 1 P{ H} NMR (202.51 MHz, C6D6) δ 74.6 (d, 33, PPh3), 43.0 (d, PTolp2H).

found (calcd for C32H41P2ClRu) C, 61.26 (61.58); H, 6.70 (6.62). 31 1 P{ H} NMR (202.51 MHz, C6D6) δ 50.5 (d, 43, PPh3), 38.7 (d, PEt2H). Ru(η5-Cp*)Cl(PPh2H)(PPh3) (3c). Ru(η5-Cp*)Cl(PPh3)2 (0.50 g, 0.63 mmol) and PPh2H in hexanes (0.54 M, 1.4 mL, 0.76 mmol) were used. The mixture was stirred for 1 h. Orange microcrystals were obtained (0.31 g, 0.43 mmol, 68% yield). The product contained an impurity of ∼4% of the disubstituted product 4c. Melting point: 217− 219 °C. IR (KBr, cm−1) 2343 (w, νPH). 31P{1H} NMR (202.51 MHz, C6D6) δ 45.3 (d, 42, PPh3), 36.5 (d, PPh2H). Ru(η5-Cp*)Cl(PTolp2H)(PPh3) (3d). Ru(η5-Cp*)Cl(PPh3)2 (0.61 g, 0.77 mmol) and PTolp2H in hexanes (0.45 M, 3.5 mL, 1.6 mmol) were used. The solution was stirred for 1 h. Orange-red microcrystals were obtained (0.38 g, 0.51 mmol, 66% yield). Melting point: 218− 220 °C. IR (KBr, cm−1) 2339 (w, νPH). Anal. found (calcd for C42H45P2ClRu) C, 67.12 (67.41); H, 6.07 (6.06). 31P{1H} NMR (202.51 MHz, C6D6) δ 45.5 (d, 42, PPh3), 34.6 (d, PTolp2H). X-ray Diffraction Studies of Complexes 3a,b. Crystals of 3a,b were grown by layering pentane onto a toluene solution of the compound. Suitable crystals were coated with a thin layer of ParatoneN. Dehydrohalogenation Reactions Monitored by NMR. Reactions with KOBut. Solid KOBut (2 mg, 0.02 mmol) was dissolved in 1 mL of C6D6 in a small vial; this happened slowly, so the mixture was left to stand for 24 h. The metal complex (3ai: 10 mg, 0.014 mmol; 3a: 12 mg, 0.016 mmol; 3b: 10 mg, 0.016 mmol; 3c: 10 mg, 0.016 mmol; 3d: 12 mg, 0.016 mmol) was added to this solution and the mixture was transferred to an NMR tube. The reaction was monitored by 1H and 31P{1H} NMR spectroscopy. The tube was shaken between each measurement. Relative amounts of phosphoruscontaining species in solution were estimated from 31P{1H} NMR. Reaction of 3a with n-BuLi. Solid 3a (12 mg, 0.016 mmol) was dissolved in 1 mL of C6D6 in a J. Young NMR tube to give an orange solution. A slight excess of n-BuLi (8 μL, 0.020 mmol, 2.5 M in hexanes) was added by syringe, which caused a layer of dark red to appear at the top of the sample. When the tube was inverted to mix prior to placing the sample in the spectrometer, the entire solution turned yellow. The reaction was monitored by 1H and 31P{1H} NMR spectroscopy. The tube was inverted between each measurement. In the initial spectra, recorded after 0.5 h, signals due to hydride complex 6a dominate (Figure S19). Spectra recorded after 24 h showed the same product distribution. General Method for Synthesis of Ru(η5-Cp*){κ2-(o-C6H4)PPh2}(PR2H) (5b−d). Benzene or toluene (10 mL) was added to a Schlenk flask containing complex 3b−d and KOBut (∼1.1 equiv). The solution was stirred for 24 h to ensure complete conversion to the orthometalated complex. The resulting cloudy orange-brown mixture was filtered through Celite to remove solid KCl and gelatinous HOBut. Solvent was removed under vacuum from the filtrate, and the resulting paste was dissolved in pentane (10 mL) and worked up as described below. The high solubility of 5b−d in pentane precluded washing to give analytically pure product, so no microanalysis was obtained. The compounds appear pure by 31 1 P{ H} NMR but show some solvent and grease impurities by 1H NMR (see Supporting Information). Ru(η5-Cp*){κ2-(o-C6H4)PPh2}(PEt2H) (5b). Ru(η5-Cp*)Cl(PEt2H)(PPh3) (3b, 0.060 g, 0.096 mmol) and KOBut (0.012 g, 0.11 mmol) were used. Pentane was removed under vacuum to give an orange paste. The product was triturated with pentane (8 × 10 mL), but remained a paste (0.036 g, 0.061 mmol, 64% yield). 31P{1H} NMR (202.51 MHz, C6D6) δ 43.6 (d, 34, PEt2H), −10.0 (d, -PPh2-). Ru(η5-Cp*){κ2-(o-C6H4)PPh2}(PPh2H) (5c). Ru(η5-Cp*)Cl(PPh2H)(PPh3) (3c, 0.10 g, 0.14 mmol) and KOBut (0.018 g, 0.16 mmol) were used. Pentane was removed under vacuum to give a brownish yellow powder (0.028 g, 0.041 mmol, 29% yield). Dec. point: 169− 172 °C. IR (KBr, cm−1) 2302 (w, νPH). 31P{1H} NMR (202.51 MHz, C6D6) δ 48.6 (d, 31, PPh2H), −13.6 (d, -PPh2-). Ru(η 5 -Cp*){κ 2 -(o-C 6 H 4 )PPh 2 }(PTol p 2 H) (5d). Ru(η 5 -Cp*)Cl(PTolp2H)(PPh3) (3d, 0.10 g, 0.13 mmol) and KOBut (0.018 g, 0.16 mmol) were used. Pentane was removed under vacuum to give a H

DOI: 10.1021/acs.organomet.9b00266 Organometallics XXXX, XXX, XXX−XXX

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Organometallics General Method for Synthesis of Ru(η5-Cp*) (PR2)(CO)(PPh3) (7b−d). Toluene (10 mL) was added to a Schlenk flask containing complex 3b−d and KOBut (∼1.4−1.5 equiv). The solution was degassed by three freeze−pump−thaw cycles before CO gas (∼0.9 atm) was introduced. The solution was stirred overnight, at which point NMR analysis showed the product mixture was comparable to that described for the NMR-scale reactions above. The resulting intense orange (for 3b) or red (for 3c,d) solution was filtered through Celite to remove solid (KCl, excess KOBut) and gelatinous HOBut. Solvent was removed under vacuum from the filtrate, and the resulting bright orange or red paste was dissolved in 10 mL of hexanes. The solvent was removed under vacuum to give a yellow or orange powder, which was washed with cold hexanes (3 × 1 mL). Isolated 7b−d contained impurities of as-yet unidentified products that formed during workup; for solution NMR samples we also noted the slow growth of signal due to free PPh3 (see Supporting Information). Ru(η5-Cp*)(PEt2)(CO)(PPh3) (7b). Ru(η5-Cp*)Cl(PEt2H)(PPh3) (3b, 0.15 g, 0.24 mmol) and KOBut (0.040 g, 0.36 mmol) were used. A bright yellow powder was obtained (0.045 g, 0.073 mmol, 30%) in 97% purity by 31P{1H} NMR. Dec. point: 158−162 °C. IR (KBr, cm−1) 1899 (s, νCO). 31P{1H} NMR (121.55 MHz, C6D6) δ 61.5 (d, 5, PPh3), 29.7 (d, PEt2). Ru(η5-Cp*)(PPh2)(CO)(PPh3) (7c). Ru(η5-Cp*)Cl(PPh2H)(PPh3) (3c, 0.20 g, 0.28 mmol) and KOBut (0.046 g, 0.40 mmol) were used. A bright orange powder was obtained (0.051 g), which contained 7c (∼70%) and unidentified product (∼30%), as determined by 31P{1H} NMR. Dec. point: 190−196 °C. IR (KBr, cm−1) 1929 (s, νCO), 1902 (s, νCO). The extra CO IR stretch band is attributed to the unidentified product. 31P{1H} NMR (121.55 MHz, C6D6) δ 53.8 (d, 4, PPh3), 14.8 (d, PPh2). Ru(η5-Cp*)(PTolp2)(CO)(PPh3) (7d). Ru(η5-Cp*)Cl(PTolp2H)(PPh3) (3d, 0.15 g, 0.20 mmol) and KOBut (0.033 g, 0.29 mmol) were used. A bright orange powder was obtained (0.062 g, 0.084 mmol, 42%) in 96% purity by 31P{1H} NMR. Dec. point: 160−164 °C. IR (KBr, cm−1) 1909 (s, νCO). 31P{1H} NMR (121.55 MHz, C6D6) δ 54.3 (d, 4, PPh3), 17.9 (d, PTolp2). Attempted Isolation of Ru(η 5 -Cp*)(η 3 -hexenyl)(PR 2 H) (13c,d). Toluene (10 mL) was added to a Schlenk flask containing metal complex, KOBut (∼1.1−1.2 equiv) and 1-hexene (5 equiv). The solution was stirred overnight to ensure complete conversion. The resulting yellow mixture was filtered through Celite to remove solid KCl and gelatinous HOBut. The filtrate was removed under vacuum, and the resulting bright yellow paste was triturated with pentane (5 × 10 mL). The pentane was removed under vacuum to give an oily yellow paste. 31P{1H} NMR indicates this paste contains the same products (13, 5, PPh3) as those observed in the NMR scale reactions. The high solubility of this paste in pentane precluded isolating pure product 13c,d. Ru(η5-Cp*)(η3-hexenyl)(PPh2H) (13c). Ru(η5-Cp*)Cl(PPh2H)(PPh3) (3c, 0.11 g, 0.15 mmol), KOBut (0.019 g, 0.17 mmol) and 1-hexene (83 μL, 0.67 mmol) were used. A bright yellow paste was obtained (0.020 g), which contains 13c (62%), 5c (33%), free PPh3 and an unidentified Ru product (5%). Ru(η5-Cp*)(η3-hexenyl)(PTolp2H) (13d). Ru(η5-Cp*)Cl(PTolp2H)(PPh3) (3d, 0.10 g, 0.13 mmol), KOBut (0.017 g, 0.16 mmol) and 1hexene (89 μL, 0.71 mmol) were used. A bright yellow paste was obtained (0.1 g), which contained 13d (66%), 5d (31%), free PPh3 and an unidentified Ru product (3%).

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Accession Codes

CCDC 1911831−1911832 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jin Yang: 0000-0001-8004-4668 Sophie Langis-Barsetti: 0000-0001-5455-666X Hayley C. Parkin: 0000-0002-3218-1411 Robert McDonald: 0000-0002-4065-6181 Lisa Rosenberg: 0000-0003-4917-184X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank NSERC of Canada for financial support (Discovery Grant to LR). REFERENCES

(1) Leading references for metal-catalyzed hydrophosphination and phosphination that describe the participation of M-PR2 intermediates include (a) Glueck, D. S. Recent advances in metal-catalyzed C-P bond formation. Top. Organomet. Chem. 2010, 31, 65−100. (b) Tappe, F. M. J.; Trepohl, V. T.; Oestreich, M. Transitionmetal-catalyzed C-P cross-coupling reactions. Synthesis 2010, 2010, 3037−3062. (c) Beletskaya, I. P.; Ananikov, V. P.; Khemchyan, L. L. Synthesis of phosphorus compounds via metal-catalyzed addition of P-H bond to unsaturated organic molecules. Catal. Met. Complexes 2011, 37, 213−264. (d) Li, Y. M.; Yang, S. D. New strategies for transition-metal-catalyzed C-P bond formation. Synlett 2013, 24, 1739−1744. (e) Rosenberg, L. Mechanisms of metal-catalyzed hydrophosphination of alkenes and alkynes. ACS Catal. 2013, 3, 2845−2855. (f) Koshti, V.; Gaikwad, S.; Chikkali, S. H. Contemporary avenues in catalytic P-H bond addition reaction: A case study of hydrophosphination. Coord. Chem. Rev. 2014, 265, 52−73. (g) Trifonov, A. A.; Basalov, I. V.; Kissel, A. A. Use of organolanthanides in the catalytic intermolecular hydrophosphination and hydroamination of multiple C-C bonds. Dalton Trans 2016, 45, 19172−19193. (h) Gibbons, S. K.; Xu, Z.; Hughes, R. P.; Glueck, D. S.; Rheingold, A. L. Chiral bis(phospholane) PCP pincer complexes: synthesis, structure, and nickel-catalyzed asymmetric phosphine alkylation. Organometallics 2018, 37, 2159−2166. (i) Wang, G.; Gibbons, S. K.; Glueck, D. S.; Sibbald, C.; Fleming, J. T.; Higham, L. J.; Rheingold, A. L. Copper-phosphido intermediates in Cu(iPr)catalyzed synthesis of 1-phosphapyracenes via tandem alkylation/ arylation of primary phosphines. Organometallics 2018, 37, 1760− 1772. (2) (a) Derrah, E. J.; Giesbrecht, K. E.; McDonald, R.; Rosenberg, L. Ruthenated acetonitrile: unusual Bronsted acidity of a polar “aprotic” solvent. Organometallics 2008, 27, 5025−5032. (b) Derrah, E. J.; McDonald, R.; Rosenberg, L. The 2 + 2 cycloaddition of alkynes at a Ru-P π-bond. Chem. Commun. 2010, 46, 4592−4594. (c) Derrah, E. J.; Pantazis, D. A.; McDonald, R.; Rosenberg, L. Concerted 2 + 2 cycloaddition of alkenes to a ruthenium-phosphorus double bond. Angew. Chem., Int. Ed. 2010, 49, 3367−3370. (d) Hoyle, M. A. M.; Pantazis, D. A.; Burton, H. M.; McDonald, R.; Rosenberg, L. Benzonitrile adducts of terminal diarylphosphido complexes: preparative sources Of “Ru = PR2. Organometallics 2011, 30, 6458− 6465. (e) Burton, K. M. E.; Pantazis, D. A.; Belli, R. G.; McDonald,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00266. Modified procedure for the synthesis of Ru(η5-Cp*)Cl(PPh3)2, NMR spectroscopic data and representative spectra for new complexes and reaction mixtures, X-ray crystallographic data (PDF) I

DOI: 10.1021/acs.organomet.9b00266 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

triphosphine, Ru(H)(η2-BH4)(ttp). J. Am. Chem. Soc. 1982, 104, 3898−3905. (13) Davies, S. G.; Moon, S. D.; Simpson, S. J. (η5-C5H5)Ru(PPh3)H3 - a stable ruthenium(IV) trihydride. J. Chem. Soc., Chem. Commun. 1983, 1278−1279. (14) Derrah, E. J. Ph.D. Thesis, University of Victoria, 2009. (15) Diagnostic NMR data for alkynyl complexes 10c,d include large 1 JHP (∼380 Hz) for Ru-PR2H observed in their 1H and 31P NMR spectra. 13C signals for the alkynyl carbons C to Ru were not observed, but those for the carbons (δ 112.3 ppm) showed 1H/13C HMBC correlations with the relatively sharp 1H signals for the ortho protons on the alkynyl phenyl group for both complexes. This shift is downfield from the corresponding signal in free phenylacetylene (δ 83.8 ppm, CDCl3, ref 16a), and is close to those reported for the similar complexes Ru(η5-indenyl)(CCPh)(PCy2H)(PPh3) (δ 113.3 ppm, C6D6, ref 2b) and Ru(η5-Cp)(CCPh)(PPh3)2 (δ 114.6 ppm, CD2Cl2, ref 16b). (16) (a) Aitken, R. A.; Atherton, J. I. Flash vacuum pyrolysis of stabilized phosphorus ylides. 1. Preparation of aliphatic and terminal alkynes. J. Chem. Soc., Perkin Trans. 1 1994, 1, 1281−1284. (b) Milner, L. M.; Hall, L. M.; Pridmore, N. E.; Skeats, M. K.; Whitwood, A. C.; Lynam, J. M.; Slattery, J. M. Access to novel fluorovinylidene ligands via exploitation of outer-sphere electrophilic fluorination: new insights into C-F bond formation and activation. Dalton Trans 2016, 45, 1717−1726. (17) Given our “one pot” reaction conditions, we cannot rule out the possibility that KOBut in the reaction mixtures is involved in generating reactive alkynyl anions. (18) (a) Slugovc, C.; Mereiter, K.; Zobetz, E.; Schmid, R.; Kirchner, K. Ruthenium-catalyzed dimerization of terminal alkynes initiated by a neutral vinylidene complex. Organometallics 1996, 15, 5275−5277. (b) Yi, C. S.; Liu, N. H. Homogeneous catalytic dimerization of terminal alkynes by C5Me5Ru(L)H3 (L = PPh3, PCy3, PMe3). Organometallics 1996, 15, 3968−3971. (c) Bassetti, M.; Marini, S.; Tortorella, F.; Cadierno, V.; Diez, J.; Gamasa, M. P.; Gimeno, J. Dimerization of terminal alkynes catalyzed by indenyl ruthenium(II) complexes. J. Organomet. Chem. 2000, 593, 292−298. (d) Kirss, R. U.; Ernst, R. D.; Arif, A. M. Chloro(η 5 -dihydropentalenyl)bis(triphenylphosphine)ruthenium(II): synthesis, structural characterization and catalytic activity in the dimerization of phenylacetylene. J. Organomet. Chem. 2004, 689, 419−428. (e) Daniels, M.; Kirss, R. U. Dimerization of terminal alkynes catalyzed by chloro(η5-pentadienyl)bis(triphenylphosphine)ruthenium(II) and kinetics of phosphine substitution. J. Organomet. Chem. 2007, 692, 1716−1725. (19) See ref 18b and references therein. (20) The polymerization of acrylonitrile catalyzed by metal alkoxides is known. Zilkha, A.; Feit, B. A.; Frankel, M. Anionic homogeneous polymerization of acrylonitrile and methacrylonitrile by quaternary ammonium hydroxides. J. Polym. Sci. 1961, 49, 231−240. (21) In the reaction of 2b with 1-hexene, we do observe a trace amount (1%) of 13b by 31P{1H} NMR after 24 h.

R.; Rosenberg, L. Alkene insertions into a Ru-PR2 bond. Organometallics 2016, 35, 3970−3980. (3) Belli, R. G.; Burton, K. M. E.; Rufh, S. A.; McDonald, R.; Rosenberg, L. Inner- and outer-sphere roles of ruthenium phosphido complexes in the hydrophosphination of alkenes. Organometallics 2015, 34, 5637−5646. (4) Derrah, E. J.; Pantazis, D. A.; McDonald, R.; Rosenberg, L. A highly reactive ruthenium phosphido complex exhibiting Ru-P πbonding. Organometallics 2007, 26, 1473−1482. (5) (a) Gamasa, M. P.; Gimeno, J.; GonzalezBernardo, C.; MartinVaca, B. M.; Monti, D.; Bassetti, M. Phosphine substitution in indenyl- and cyclopentadienylruthenium complexes. Effect of the η5 ligand in a dissociative pathway. Organometallics 1996, 15, 302−308. (b) Belli, R. G.; Wu, Y.; Ji, H.; Joshi, A.; Yunker, L. P. E.; McIndoe, J. S.; Rosenberg, L. Competitive ligand exchange and dissociation in Ru indenyl complexes. Inorg. Chem. 2019, 58, 747−755. (6) Very few η3-Cp* transition metal complexes have been reported. Examples supported by crystallographic or kinetic analyses include (a) Glueck, D. S.; Bergman, R. G. Mechanism of ligand substitution in an iridium amide complex. Organometallics 1991, 10, 1479−1486. (b) Jones, W. D.; Kuykendall, V. L.; Selmeczy, A. D. Ring migration reactions of (C5Me5)Rh(PMe3)H2 - evidence for η3 slippage and metal-to-ring hydride migration. Organometallics 1991, 10, 1577− 1586. (c) Redshaw, C.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse, M. B. Synthesis and X-ray crystal-structure of the organotungsten(V) o-phenylene diamido complex, (η-C5Me5)WCl2(1,2-(HN)2C6H4). Polyhedron 1993, 12, 2417−2420. (d) Redshaw, C.; Gibson, V. C.; Clegg, W.; Edwards, A. J.; Miles, B. Pentamethylcyclopentadienyl tungsten complexes containing imido, hydrazido and amino acid derived N-O chelate ligands. J. Chem. Soc., Dalton Trans. 1997, 3343− 3347. (e) Shin, J. H.; Bridgewater, B. M.; Churchill, D. G.; Baik, M. H.; Friesner, R. A.; Parkin, G. An experimental and computational analysis of the formation of the terminal nitrido complex (η3Cp*)2Mo(N)(N3) by elimination of N2 from Cp*Mo2(N3)2: The barrier to elimination is strongly influenced by the exo versus endo configuration of the azide ligand. J. Am. Chem. Soc. 2001, 123, 10111−10112. (7) The observation of very slow rates for Cp* complexes, relative to indenyl and Cp analogues, in reactions of half-sandwich complexes known to proceed via bimolecular, associative mechanisms (e.g., ref 7a) has been attributed to higher barriers to distortion from η5hapticity for the Cp* ligand. This has been rationalized by the destabilizing influence of the electron-donating methyl substituents on a more localized negative charge in η3-complexes (ref 7b). (a) Rerek, M. E.; Basolo, F. Kinetics and mechanism of the substitution-reactions of η5-pentamethylcyclopentadienyl)dicarbonylrhodium(I) and η5pentamethylcyclopentadienyl)dicarbonylcobalt(I). Organometallics 1983, 2, 372−376. (b) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis; University Science Books, 2010; pp 250−252. (8) Sues, P. E.; Lough, A. J.; Morris, R. H. Reactivity of ruthenium phosphido species generated through the deprotonation of a tripodal phosphine ligand and implications for hydrophosphination. J. Am. Chem. Soc. 2014, 136, 4746−4760. (9) Lubian, R. T.; PazSandoval, M. A. Synthesis, properties and crystal structures of pentamethylcyclopentadienyl- and cyclopentadienyl-ruthenium(II) diphenylphosphine complexes. J. Organomet. Chem. 1997, 532, 17−29. (10) Derrah, E. J.; Marlinga, J. C.; Mitra, D.; Friesen, D. M.; Hall, S. A.; McDonald, R.; Rosenberg, L. Electronic control of conformation in mixed-phosphine complexes of the ruthenium η5-indenyl fragment. Organometallics 2005, 24, 5817−5827. (11) Suzuki, H.; Lee, D. H.; Oshima, N.; Morooka, Y. Hydride and borohydride derivatives of (pentamethylcyclopentadienyl)(tertiary phosphine)ruthenium. Organometallics 1987, 6, 1569−1575. (12) Letts, J. B.; Mazanec, T. J.; Meek, D. W. The synthesis, characterization, and reactivity of an unusual, amphoteric (tetrahydroborato)ruthenium hydride complex of a chelating J

DOI: 10.1021/acs.organomet.9b00266 Organometallics XXXX, XXX, XXX−XXX