Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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The Synthesis, Structure, and H/D Exchange Reactions of WaterSoluble Half-Sandwich Ruthenium(II) Hydrides of Indenyl and Dihydropentalenyl Jocelyn P. Lanorio,†,‡ Charles A. Mebi,†,§ and Brian J. Frost*,† †
Department of Chemistry, University of Nevada, Reno, Nevada 89557-0216, United States Department of Chemistry, Illinois College, Jacksonville, Illinois 62650, United States
‡
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ABSTRACT: Two new water-soluble ruthenium hydrides Cp′Ru(PTA)2H, Cp′ = C9H7− (Ind, 1) or C8H9− (Dp, 2), along with IndRu(PTA)2Cl (3) have been synthesized. DpRu(PTA)2H was synthesized by refluxing DpRu(PTA)2Cl with NaCO2H in methanol. IndRu(PTA)2H was unable to be generated in a similar manner due to complications in the synthesis of IndRu(PTA)2Cl. IndRu(PTA)2H was synthesized by conversion of the mixed-phosphine ruthenium complex IndRu(PTA)(PPh3)Cl into IndRu(PTA)(PPh3)H followed by ligand substitution of PPh3 with 1,3,5-triaza-7-phosphaadamantane (PTA). Complexes 1 and 2 are stable and moderately soluble in water (S25°C = 16 mg/mL for 1 and ∼20 mg/mL for 2). The Ru hydrides react with chlorinated solvents, in the case of 1 yielding the challenging synthesize IndRu(PTA)2Cl (3) (S25°C = 17 mg/mL). The synthesis, isolation, and reactivity of 1−3 are described including crystal structures of 1, 2, 3, and [IndRu(PTA)3](Cl). H/D exchange reactions of 1 with D2O were monitored by 31P NMR spectroscopy as a function of temperature: ΔH‡ = 92 ± 3 kJ/mol, ΔS‡ =−22 ± 2 J/mol· K. In addition, H/D exchange reactions of the 1,2-dihydropentalenyl (η5-C8H9−, Dp) analogue, 2, with D2O are also described along with the effects of pH on the 31P NMR spectra.
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INTRODUCTION Transition metal hydrides play a central role in inorganic and organometallic chemistry as key intermediates or catalysts in reactions such as hydrogenation, hydroformylation, olefin polymerization, and the water−gas shift reaction.1−6 Our group has been interested in water-soluble catalysts, including water-soluble ruthenium hydrides, that participate in catalytic reactions in water and/or under biphasic conditions.7−13 To make the metal complexes water-soluble, we have employed the air stable and water-soluble phosphine 1,3,5-triaza-7phosphaadamantane (PTA) and derivatives thereof.14−16 We, and others, have previously reported the synthesis of a series of Cp′RuPTA2X and Cp′RuPTA(PPh3)X complexes, where X = H or Cl and Cp′ = Cp (C5H5−), Cp* (C5Me5−), Dp (C8H9−), or Ind (C9H7−), as catalysts for isomerization,17−21 aqueous or biphasic hydrogenation,8,9,22,23 and highly active atom transfer radical addition (ATRA) catalysts.24,25 Cp′Ru(PPh3)2Cl, due to the lability of PPh3, has proven a useful synthon for the synthesis of Cp′Ru(PTA)2Cl and Cp′Ru(PTA)(PPh3)Cl complexes (Scheme 1). The mixed phosphine complexes Cp′Ru(PTA)(PPh3)Cl can be obtained from Cp′Ru(PPh3)2Cl and one equivalent of PTA when Cp′ = Cp, Cp*, Dp, Tp, or Ind.9,18 The water-soluble bis PTA complexes, Cp′Ru(PTA)2Cl, can be obtained from Cp′Ru(PPh3)2Cl after the © XXXX American Chemical Society
Scheme 1. Synthesis of Cp′Ru(PPh3)(PTA)Cl and Cp′Ru(PTA)2Cl complexes
addition of two equivalents of PTA when Cp′ = Cp, Cp*, or Dp.7,9,16,17 Cp′Ru(PTA)2H or Cp′Ru(PTA)(PPh3)H complexes have previously been obtained from their air-stable chlorides either by reaction with NaOCH3 or NaCO2H.7−9 To this point, neither IndRu(PTA)2Cl nor IndRu(PTA)2H (Ind = η5-C9H7−) have been reported due to complications in the synthesis. Indenyl complexes often behave differently than Received: February 11, 2019
A
DOI: 10.1021/acs.organomet.9b00084 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
substitution with PTA to yield 1, Scheme 2. The pale yellow 1 was obtained in moderate yield (63%) after refluxing the
their Cp relatives due, at least in part, to the ability of the indenyl ligand to undergo facile ring slippage from η5 to η3. Ring slippage can lead to remarkable increases in reaction rate for indenyl complexes relative to the Cp analogues.26 Herein, we report the synthesis and properties of IndRu(PTA)2Cl as well as the water-soluble hydrides IndRu(PTA)2H (1) and DpRu(PTA)2H (2) (Dp = η5-C8H9−).
Scheme 2. Synthesis of IndRu(PTA)2H (1)
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RESULTS AND DISCUSSION Synthesis and Characterization of IndRu(PTA)2H. We have previously reported that the addition of 2 equiv of PTA to IndRu(PPh3)2Cl provides [IndRu(PTA)2(PPh3)]Cl and not the desired IndRu(PTA)2Cl.9 The use of excess PTA provided [IndRu(PTA)3]Cl in 69% yield, presumably via substitution of PPh3 from the previously reported [IndRu(PTA)2(PPh3)]Cl. Derrah and co-workers reported similar observations when attempting the synthesis of IndRu(PEt2H)2Cl.27 There was significant formation of the cationic species [IndRu(PEt2H)2(PPh3)]Cl and [IndRu(PEt2H)3]Cl together with the desired product, IndRu(PEt2H)2Cl. They also obtained [IndRu(PPh2H)3]Cl and [IndRu(PCy2H)3]Cl when excess amounts of the respective secondary phosphines, PPh2H or PCy2H, were added to IndRu(PPh3)2Cl. The 31P{1H} NMR spectrum of the cationic tris-PTA complex, [IndRu(PTA)3]Cl, contains a singlet at −28.2 ppm in water. The solid state structure of the complex, determined by X-ray crystallography, is depicted in Figure 1 along with selected bond lengths and
toluene solution of IndRu(PTA)(PPh3)H and PTA under nitrogen for 24 h. Upon cooling down to room temperature, the reaction mixture was concentrated to around 10 mL before filtering through Celite to remove the excess PTA, which is slightly soluble in toluene. The filtrate was dried under vacuum and the resulting solid washed with diethyl ether to remove residual PPh3, providing 1 as a pale yellow solid. IndRu(PTA)2H is moderately water-soluble (S25°C = 16 mg/mL; 30 mM). The pH of a 13 mM solution of 1 is ca. 8.0, presumably due to the amine functionalities of the PTA ligands. Complex 1 is soluble in common organic solvents such as acetonitrile, acetone, methanol, toluene, and THF and insoluble in hexanes and diethyl ether. Like many metal hydrides, 1 reacts with chlorinated solvents (vide infra). IndRu(PTA)2H was fully characterized by X-ray crystallography, multinuclear NMR spectroscopy (31P{1H}, 1H, and 13C{1H}), ESI+ HRMS, and IR spectroscopy.28 DpRuPTA2H was synthesized in 55% yield in a manner analogous to that of Cp′Ru(PTA)(PR3)H complexes.9 Starting from the previously reported DpRuPTA2Cl,9 the chloride was converted to a hydride by refluxing with NaCO2H in methanol. The solvent was removed and the product extracted with CH2Cl2. DpRuPTA2H was characterized by X-ray crystallography, multinuclear NMR spectroscopy, and IR spectroscopy. Solid State Structure of IndRu(PTA)2H. Single crystal Xray diffraction was used to identify the solid state structure of IndRu(PTA)2H. Yellow crystals suitable for X-ray diffraction were obtained after 2 weeks of slow evaporation of a methanol solution of 1 in a drybox. A thermal ellipsoid view of 1 is depicted in Figure 2, along with the atomic numbering scheme and selected bond distances and bond angles. The geometry about the ruthenium atom can be regarded as distorted octahedral with the η5-indenyl group occupying three facial coordination positions and the PTA ligands and hydride occupying the remaining three positions. The indenyl group displays the asymmetric coordination generally observed with this ligand.29,30 Three of the Ru−C bond lengths, those involving the C(14), C(15), and C(16) atoms, are significantly shorter than the two carbons connected to the phenyl ring,
Figure 1. Thermal ellipsoid representation (50% probability) of [IndRu(PTA)3]+ with the atomic numbering scheme. Hydrogen atoms, solvent, and the chloride anion have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−C19 = 2.361(2); Ru−C20 = 2.225(2); Ru−C21 = 2.215(2); Ru−C22 = 2.248(2); Ru−C23 = 2.378(2); Ru−Cpcent = 1.937; Ru−P1 = 2.2837(5); Ru− P2 = 2.2820(6); Ru−P3 = 2.2406(5); P1−Ru−P2 = 93.98(2); P1− Ru−P3 = 94.098(18); P2−Ru1−P3 = 95.83(2).
angles. [IndRu(PTA)3]Cl exhibits a classic three-legged piano stool geometry. The indenyl group coordinates to the ruthenium center in a slightly asymmetric η5 fashion with the Ru to C20, C21, and C22 distances (2.215−2.248 Å) slightly shorter than the Ru−C19 and Ru−C23 distances (2.361 and 2.378 Å, respectively). The synthesis of IndRu(PTA)2H (1) was ultimately carried out via conversion of IndRu(PTA)(PPh3)Cl to IndRu(PTA)(PPh3)H, as previously reported,9 followed by the ligand B
DOI: 10.1021/acs.organomet.9b00084 Organometallics XXXX, XXX, XXX−XXX
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Figure 3. Thermal ellipsoid representation (50% probability) of DpRu(PTA)2H (2). Hydrogen atoms, except for the hydride, have be omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru− H = 1.62(4); Ru−Cpcent = 1.888; Ru−P1 = 2.2186(13); Ru−P2 = 2.2237(12); Ru−C13 = 2.245(5); Ru−C14 = 2.235(5); Ru−C15 = 2.241(4); Ru−C16 = 2.253(4); Ru−C17 = 2.243(4); P1−Ru−P2 = 96.18(5); P1−Ru−H = 85.5(16); P2−Ru−H = 82.4(16).
Figure 2. Thermal ellipsoid representation (50% probability) of IndRu(PTA)2H (1) with the atomic numbering scheme. Hydrogen atoms, except for the hydride, have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−H = 1.53(4); Ru−Cpcent = 1.926; Ru−P1 = 2.2362(7); Ru−P2 = 2.2343(7); Ru−C13 = 2.347(3); Ru−C14 = 2.238(3); Ru−C15 = 2.219(3); Ru−C16 = 2.237(3); Ru−C17 = 2.340(3); P1−Ru−P2 = 98.51(3); P1−Ru−H = 82.8(14); P2−Ru−H = 79.8(14).
P2, the Cp-ML2 angle, was found to be 62.8° in 2, somewhat more acute than the value of 67.8° reported for CpRu(PTA)2H.7 A comparison of the bond lengths and angles for CpRu(PTA)2H,7 DpRu(PTA)2H, and IndRu(PTA)2H is presented in Table 1. An increase in the P(1)−Ru−P(2)
C(13) and C(17). The indenyl ring of 1 can be best described as η5-distorted or partway between η5 and η3 coordination. The slip distortion is calculated to be 0.112 Å (Δ) and is obtained by the difference in the average Ru to C distances of bridgehead C(13,17) and C(14,15,16). It is well-reported that (η5-indenyl)MLn complexes show significant slip distortions from η5 toward η3 coordination in the ground state.31−35 The PTA ligands are bound through the phosphorus as expected and are positioned away from a third ligand, suggesting the presence of the hydride. The ruthenium-hydride ligand was located in the difference map of 1 and refined to give a Ru−H bond distance of 1.53(4) Å, in the range found for analogous half-sandwich hydride cyclopentadienyl complexes (range 1.427−1.630 Å).36−41 The Ru−H distance is comparable to the reported Ru−H distances of 1.57(3) and 1.59(2) in IndRu(PPh3)2H and IndRu(dppm)H, respectively.42,43 The orientation of the hydride ligand in 1 is trans relative to the benzo ring of the indenyl ligand, conforming with the relative trans influence of the hydride and phosphine ligands.44,45 Bond angles and bond distances for IndRu(PTA)2H obtained by DFT calculations are consistent with the crystallographically obtained structure.46 Solid-State Structure of DpRu(PTA)2H. X-ray quality crystals of DpRu(PTA)2H were grown from a methanol solution of 2 over a period of days. The piano stool complex 2 consists of an η5-dihydropentalenyl (Dp, C8H9−) bound to a ruthenium with two PTA ligands and the hydride, Figure 3. The PTA ligands are bound, as expected, through phosphorus and are tilted away from the hydride ligand, as described by Lemke and Brammer, indicating a third ligand (the hydride).30 The Ru−P bond lengths of ∼2.22 Å are consistent with other published Ru−P distances in similar complexes. The Ru−H bond distance of 1.62(4) Å in 2 agrees well with the previously reported distances in similar complexes. The distance from Ru1 to the Cp centroid is a normal 1.888 Å. The (N)C−N distances of the PTA ligands are 1.469(3) Å, consistent with nonprotonated PTA ligands.47,48 The angle between the plane defined by the Dp ligand and the plane defined by Ru, P1, and
Table 1. Selected Bond Lengths [Å] and Angles [deg] for Cp′Ru(PTA)2H (Cp′ = Cp (C5H5−), Dp (C8H9−), or Ind (C9H7−) Ru−H Ru−P(1) Ru−P(2) Ru−CCp‑Av Ru−Cp′cent Cp′−ML2 P1−Ru−P2 P1−Ru−H P2−Ru−H
CpRu(PTA)2H7
IndRu(PTA)2H (1)
DpRu(PTA)2H(2)
1.68(7) 2.2220(7) 2.2267(7) 2.250(3) 1.846 67.8 95.68(2) 81(2) 73(2)
1.54(4) 2.2362(7) 2.2343(7) 2.2762(3) 1.926 76.33 98.51(3) 82.8(14) 79.8(14)
1.62(4) 2.2186(13) 2.2237(12) 2.2434(4) 1.888 62.8 96.18(5) 85.5(16) 82.4(16)
angle is observed as the Cp′ ligand varied from Cp (95.68°) to Dp (96.18°) to Ind (98.51°). The Ru−P distances varied between 2.2186(13) and 2.2362(7) Å depending on Cp′ with 1 exhibiting the longest Ru−P distances at 2.2362 and 2.2343 Å. The Cp′−ML2 angle increased significantly from 62.8° for DpRu(PTA)2H to 76.3° for IndRu(PTA)2H. The Ru to Cpcent distance of 1.926 Å for IndRu(PTA)2H is similar to other indenyl ruthenium(II) complexes36,37,49 and slightly longer than observed for the Cp (1.846 Å)7 and Dp (1.888 Å) analogues. The Ru−Cp′cent distance of 1.926 Å for 1 is close to the value previously reported for IndRu(PTA)(PPh3)H (1.929 Å) and slightly longer than reported for IndRu(PTA)(PPh3)Cl (1.89 Å) or CpRu(PTA)(PPh3)H (1.894 Å).9 The longer Ru− Cp′cent distance for indenyl complexes may be a result of the greater steric requirements of Ind versus Cp and potentially a weaker indenyl−Ru interaction.50−52 A weaker Ru−Ind interaction, presumably, would result in tighter Ru−H bonding as observed by the shorter Ru−H distance (1.54 Å) in 1 versus C
DOI: 10.1021/acs.organomet.9b00084 Organometallics XXXX, XXX, XXX−XXX
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the peak at 1890 cm−1 and the appearance of a new peak at 1479 cm−1 νRu−D of DpRu(PTA)2D (2D). The IR data provide insight on the strength of the Ru−H bond. Table 2 contains νRu−H data for CpRu(PTA)2H,
CpRu(PTA)2H (1.68 Å) and DpRu(PTA)2H (2; 1.62 Å) and ν(Ru−H) data (vide infra). The formation of IndRu(PTA) 2H from the mixedphosphine hydride was monitored at 80 °C via 1H NMR spectroscopy in toluene-d8. The disappearance of a doublet of doublets (2JPTA‑H = 32.0 Hz and 2JPPh3‑H = 29.2 Hz) at −16.0 ppm was followed along with the appearance of a triplet (2JPTA‑H = 34.4 Hz) at −17.4 ppm.46 The higher field shift of the Ru−H signal for IndRu(PTA)2H relative to IndRu(PTA)(PPh3)H indicates an increase in the shielding of the hydride ligand. The hydride resonance of 1 appears as triplet due to its coupling with two equivalent phosphorus atoms of PTA. A single resonance in the 31P{1H} NMR spectrum of 1 in toluene-d8 is observed at −17.4 ppm consistent with the chemical equivalence of the phosphorus atoms. The 1H NMR spectrum of 1, in d6-DMSO, contains a set of multiplet resonances in the range of δ 7.52−7.48 ppm and δ 6.76−6.74 ppm in an AA′BB′ pattern for the four protons on the six membered ring of the indenyl ligand. The η 5 coordination of the indenyl group in 1 is evident: resonances due to the three protons on the five membered ring of the indenyl ligand appear as a triplet at 5.11 ppm (3JHH = 2.7 Hz) and a doublet at 4.92 ppm (3JHH = 2.7 Hz), respectively, shifted from the aromatic region toward high field. The NCH2N and PCH2N protons of PTA appear as apparent triplets centered at 4.31 ppm (2JHAHB = 13.0 Hz) and 3.54 ppm (2JHAHB = 15.0 Hz), respectively. The hydride resonance is observed as a triplet at −17.4 (2JHP = 34.4 Hz). The 13C{1H} NMR spectrum of 1, in d6-DMSO, contains two Cp(CH)4 resonances at δ 121.6 and 120.8 ppm, and the five membered ring of the indenyl ligand contains the expected three types of carbons, which show up at 105.1, 72.2, and 54.8 ppm. The carbons of the PTA ligand in complex 1 are found in the expected region: 70.9 ppm for NCH2N and 59.5 ppm for PCH2N. The spectroscopic (31P and 1H NMR and IR) data on 2 are consistent with the Cp and Ind analogues. The hydride ligand in DpRu(PTA)2H is found, via 1H NMR spectroscopy, as the expected high-field triplet at δ −14.1 ppm (2JPH = 36.0 Hz, D2O). The NCH2N and PCH2N protons of PTA appear as AB quartets at ∼3.9 ppm for the six PCH2N methylene protons and ∼4.4 ppm for the six NCH2N methylene protons, consistent with other PTA complexes. The three Cp protons are found at 4.6 and 4.3 ppm. The remaining six methylene protons on the Dp ligand are found as a complex multiplet between 1.9 and 2.2 ppm in the 1H NMR spectrum. A single resonance in the 31P{1H} NMR spectrum of 2 in water is observed at −15.6 ppm; in DMSO-d8, the 31P NMR resonance is observed as a doublet −17.8 ppm 2JPH = 36 Hz. IndRu(PTA)2H (1) and DpRu(PTA)2H (2) were also characterized by IR spectroscopy. For both compounds a sharp medium intensity band was observed by IR spectroscopy and identified as νRu−H = 1984 cm−1 for 1 and 1890 cm−1 for 2. DFT calculations were performed on 1 with the observed νRu−H in good agreement with the DFT calculated value of 1980 cm−1. Deuteration of IndRu(PTA)2H results in the disappearance of the absorption band at 1984 cm−1 and the appearance of a new peak at 1465 cm−1 νRu‑D of IndRu(PTA)2D (1D). This shift is consistent with a reduced mass calculation for νRu−D based on νRu−H (a Hooke’s law calculation predicts a νRu−D of 1413 cm−1 for 1D). Similarly, deuteration of DpRu(PTA)2H results in the disappearance of
Table 2. 31P{1H} NMR, 1H NMR, and IR νRu−H Spectroscopic Data for Cp′Ru(PTA)2H (Cp′ = Cp, Ind, Dp) P{1H} NMRa
1
H NMR
IRb
δ
JPH, Hz
δ, Ru−H
νRu−H, cm−1 1927 1907 1890 1984
31
complex
a
2
CpRu(PTA)2H
−12.0
36.6
−14.6 (t)
DpRu(PTA)2H (2) IndRu(PTA)2H (1)
−15.6 −17.4
36.0 34.4
−14.1 (t)c −17.4 (t)c
a
In water. bAs pure solids or KBr pellets. cIn DMSO-d6.
DpRu(PTA)2H, and IndRu(PTA)2H. More electron-donating and sterically hindered ligands appear to result in lower νRu−H. For instance, Dp is a better electron donor and bulkier than Cp, and DpRu(PTA)2H (1890 cm−1) exhibits a lower νRu−H than CpRu(PTA)2H (1927 and 1903 cm−1). Compared with CpRu(PTA)2H and DpRu(PTA)2H, IndRu(PTA)2H exhibits the highest νRu−H of the series at 1984 cm−1. This may suggest IndRu(PTA)2H has the strongest Ru−H among the three bisPTA ruthenium hydride complexes. The observed νRu−H for 1 correlates with the X-ray crystal structure having the longest Ru−Cp′cent (weak indenyl−ruthenium interaction) and shortest Ru−H bond length (strong Ru−H and high νRu−H) among the three Cp′Ru(PTA)2H complexes. The Ru−H force constants for the Cp′Ru(PTA)2H series were calculated using the observed νRu−H of the bis-PTA ruthenium hydride complexes: IndRu(PTA)2H (kRuH of 232 N/m) > CpRu(PTA)2H (kRuH of 219 N/m) > DpRu(PTA)2H (kRuH of 211 N/m). Both IndRu(PTA)2H and DpRu(PTA)2H are extremely airsensitive in solution and the solid state with solutions of 1 or 2 turning brown and ultimately black within ∼30 min upon exposure to air. Compounds 1 (S25°C = 16 mg/mL) and 2 (S25°C = 20 mg/mL) are soluble in aqueous solution and stable, in the absence of oxygen, for weeks in water. As is common with many metal hydrides, 1 and 2 react with chloroalkanes such as CCl4, CHCl3, or CH2Cl2 and undergo H/Cl metathesis. In CHCl3, the yellow 1 is completely converted to the orange IndRu(PTA)2Cl (3) within minutes, while in CH2Cl2 this conversion requires more than a day at room temperature, Scheme 3. In aqueous solution IndRu(PTA)2Cl contains two resonances in the 31P{1H} NMR spectrum with relative intensities that are concentration dependent. The two resonances observed by 31P{1H} NMR spectroscopy in aqueous solution indicate the existence of two species due to Scheme 3. Reaction of IndRu(PTA)2H with CHCl3
D
DOI: 10.1021/acs.organomet.9b00084 Organometallics XXXX, XXX, XXX−XXX
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Organometallics the partial hydrolysis of the Ru−Cl bond and the formation of the corresponding aqua complex as displayed in Figure 4 (S25°C
Figure 5. Thermal ellipsoid representation (50% probability) of IndRu(PTA)2Cl (3) with the atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−Cl = 2.4408(7); Ru−Cpcent = 1.926; Ru−P1 = 2.2362(7); Ru−P2 = 2.2343(7); Ru−C13 = 2.347(3); Ru−C14 = 2.238(3); Ru−C15 = 2.219(3); Ru−C16 = 2.237(3); Ru−C17 = 2.340(3); P1−Ru−P2 = 98.51(3); P1−Ru−H = 82.8(14); P2−Ru− H = 79.8(14).
Figure 4. Hydrolysis of the Ru−Cl bond in IndRu(PTA)2Cl in H2O. (A) 31P{1H} NMR spectrum of IndRu(PTA)2Cl in H2O. (B) 31P{1H} NMR spectrum of IndRu(PTA)2Cl in H2O following addition of ∼10 equiv of KCl.
Table 3. Selected Bond Lengths [Å] and Angles [deg] for the Series of Cp′Ru(PTA)2Cl where Cp′ = Cp, Cp*, Dp, and Ind
= 17 mg/mL). Complex 3 and its hydrolysis product exist in equilibrium, as verified by the addition of Cl−, which shifts the equilibrium resulting in the reduction of the resonance for the aqua complex at −22.3 ppm and growth of IndRu(PTA)2Cl signal at −20.9 ppm, Figure 4. Partial hydrolysis of the Ru−Cl bond has also been reported for Cp′Ru(PTA)2Cl and related complexes.9,53,54 IndRu(PTA)2Cl was characterized by 31P{1H}, 1H, and 13 C{1H} NMR spectroscopies as well as ESI+ HRMS and Xray crystallography. The 13C{1H} NMR spectrum of 3, in CDCl3, contains two Cp(CH)4 resonances at δ 125.1 and 122.6 ppm. The Cp carbons of the indenyl ligand show up at 107.9, 81.5, and 60.8 ppm, while the carbons of the PTA ligand show up at the expected region, 73.6 ppm for NCH2N and 56.8 ppm for PCH2N in the 13C{1H} NMR spectrum of complex 3. The 13C{1H} NMR resonances of IndRu(PTA)2Cl are downfield relative to IndRu(PTA)2H, consistent with the deshielding effect of the chloride. The ESI+ high-resolution mass spectrum, acquired using a methanol solution of IndRu(PTA)2Cl, contained an ion at m/z 567.0902 assigned to C21H32Cl1N6P2Ru1 [M + H]+; consistent with the calculated m/z of 567.0896. Orange needles of IndRu(PTA)2Cl suitable for X-ray diffraction were obtained after 10 days of slow evaporation of a methanol solution of 3. A thermal ellipsoid view of 3 is depicted in Figure 5, along with the atomic numbering scheme and selected bond lengths and angles. The indenyl ligand of 3 is coordinated to the Ru center in a distorted η5 fashion with slip distortion of 0.153(3) Å (Δ). Selected bond distances and bond angles for complex 3 are found in Table 3 along with the data for previously synthesized Cp′Ru(PTA)2Cl. Relative to the other Cp′Ru(PTA)2Cl complexes in Table 3, IndRu(PTA)2Cl exhibits the longest Ru−CCp‑Av and the shortest Ru−P distances. Complex 3 has a slightly smaller P−Ru−P angle in comparison to the Cp and Dp analogues The most electron donating and sterically demanding Cp*
Cp′Ru(PTA)2Cl Ru−Cl Ru−P1 Ru−P2 Ru−CCp‑Av Ru−Cpcent Cp′−ML2 P1−Ru−P2 P1−Ru−Cl P2−Ru−Cl a
Inda
Cp7
Dp9
Cp*,16a
2.4408(7) 2.4526(7) 2.2263(7) 2.2256(7) 2.2667(7) 2.2856(7) 2.251(3) 2.251(3) 1.895 1.893 54.3 56.1 95.56(3) 97.46(3) 89.26(3) 86.70(3) 88.81(3) 85.49(3)
2.445(2)
2.4529(8)
2.258(3)
2.2923(9)
2.247(3)
2.3089(9)
2.197(7)
2.2174(3)
1.846
1.855
2.465(2) 2.468(2) 2.284(1) 2.285(2) 2.285(2) 2.287(2) 2.211(6) 2.208(8) 1.862 1.86 61.3 61.2 93.30(5) 93.37(7) 90.69(6) 90.94(9) 84.38(6) 84.27(8)
55.3
54.2
96.85(5)
97.80(3)
91.61(7)
88.06(3)
86.46(7)
93.57(3)
Two molecules in the asymmetric unit.
ancillary ligand displays the smallest P−Ru−P bond angle among the Cp′Ru(PTA)2Cl series. These observations are consistent with the steric requirements of the Cp′ ligands: Cp* > Ind > Dp > Cp.9 Similar to what was observed in 1 there was a pronounced difference in Ru−C distance between Ru and C14, C15, and C16 (Ru−C = 2.167−2.212 Å) and Ru and C13/C17 (Ru−C = 2.329 and 2.356 Å) in IndRu(PTA)2Cl. This is again indicative of Ru(II) indenyl complexes which can appear to be a combination of benzene and an allyl fragment. H/D Exchange Reactions. The bis-PTA hydrides, CpRu(PTA)2H (20 mg/mL), DpRu(PTA)2H (20 mg/mL), and IndRu(PTA)2H (16 mg/mL), are all stable and soluble in deoxygenated water.7,9 CpRu(PTA)2H in D2O was previously E
DOI: 10.1021/acs.organomet.9b00084 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics reported by us to undergo H/D exchange with D2O.7 With the specific aim of comparing the rates of H/D exchange of the Cp′Ru(PTA)2H series, a similar investigation was performed for IndRu(PTA)2H (Scheme 4) and DpRu(PTA)2H.
obtained from an Eyring plot of the kinetic data, Figure 7. An activation barrier (Ea) of 94 ± 3 kJ/mol is obtained from an
Scheme 4. H/D Exchange Reaction of IndRu(PTA)2H with D2O
Compound 1 undergoes H/D exchange with D2O, t1/2 = 274 min at 25 °C. This reaction was followed by 31P{1H} NMR spectroscopy where, over time, the singlet 31P{1H} NMR resonance of IndRu(PTA)2H at −17.4 ppm disappears, and it is replaced by a triplet at −17.3 ppm (2JPD = 4.9 Hz) indicative of IndRu(PTA)2D (Figure 6). The pseudo-first-
Figure 7. Eyring plot for the H/D exchange between IndRu(PTA)2H and D2O.
Arrhenius plot of the data. The negative value of ΔS‡ and the relatively small positive value of ΔH‡ suggest an associative mechanism for the H/D exchange of IndRu(PTA)2H in D2O. The transition state is presumed to contain significant bond breaking (Ru−H and O−D) and bond making (H−D).46 These results can be compared with our previous findings for CpRu(PTA)2H (ΔH‡ = 68 ± 2 kJ/mol; ΔS‡ = −94 ± 7 J/mol· K; Ea = 71 ± 3 kJ/mol; and t1/2 = 127 min at 25 °C).7 The H/ D exchange reaction of 1 is slower than what was observed for CpRu(PTA)2H, and similarly ΔH‡ and Ea are larger for 1 than found for CpRu(PTA)2H. The slower H/D exchange of IndRu(PTA)2H compared to CpRu(PTA)2H may primarily be due to a stronger Ru−H bond of IndRu(PTA)2H as evidenced in the IR spectrum and X-ray crystal structure. In addition, the sterics imposed by the indenyl ligand may inhibit the approach of D2O to the site of protonation for the exchange reaction to occur. One plausible mechanism is protonation of 1 by water (or D2O) providing a Ru dihyride/dihydrogen intermediate, which is then deprotonated by hydroxide (or −OD). Upon deprotonation of the intermediate Ru(HD)+, either IndRu(PTA)2D is formed (H/D exchanged has occurred) or IndRu(PTA)2H is regenerated. There are numerous studies suggesting the kinetic site of protonation is the hydride, versus the metal center, mainly due to the lack of geometric or electronic rearrangement.55−57 This mechanism is also based on our previous studies involving protonation of CpRu(PTA)2H, resulting in the formation of [CpRu(PTA)(PTAH)H]+ and [CpRu(PTA)2(H)2]+ along with evidence of a dihydrogen intermediate, [CpRu(PTA)2(H2)]+.8 The kinetics of the reaction of IndRu(PTA)2D (1D) with H2O were also followed by 31P{1H} NMR spectroscopy.46 Complex 1D was dissolved in H2O, and the disappearance of the triplet 31P NMR signal at −17.3 ppm was monitored. The observed rate constant for the disappearance of 1D was found to be ∼4.5× larger than that for the disappearance of 1. The pseudo-first-order kinetics of the H/D exchange of 1 by D2O and 1D by H2O at 35 °C are presented in Figure 8. The observation of both forward (Ru−H + D2O) and backward (Ru−D + H2O) reactions at 308 K allows for calculation of the
Figure 6. 31P{1H} NMR spectra for the reaction of IndRu(PTA)2H and D2O at 25 °C. Over time, the disappearance of the singlet at −17.36 ppm due to IndRu(PTA)2H and the appearance of a triplet at −17.27 ppm due to IndRu(PTA)2D are observed.
order kinetics of the H/D exchange reaction of 1 were examined over a range of temperatures (25−45 °C) and concentrations, Table 4. The rate constants for the reaction of 1 with D2O are dependent on temperature but not on the concentration of IndRu(PTA)2H. Activation parameters, ΔH‡= 92 ± 3 kJ/mol and ΔS‡=−22 ± 2 J/mol·K, were Table 4. Observed Pseudo-First Order Rate Constants for the H/D Exchange of IndRu(PTA)2H (1) in D2O as a Function of Temperature temp °C
kobs s−1 (× 105)
25 25 30 30 35 35 40 45
4.20a 4.24b 7.39a 7.41b 12.26a 54.87c 24.95a 45.84a
a
[1] = 0.010 M. b[1] = 0.050 M. cIndRu(PTA)2D in H2O. F
DOI: 10.1021/acs.organomet.9b00084 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 9. 31P{1H} NMR spectra in water of DpRu(PTA)2H (bottom) and after H/D exchange with D2O resulting in DpRu(PTA)2D (top).
Figure 8. Plot of ln([A]/[A∞]) versus t at 35 °C for the H/D exchange of IndRu(PTA)2H with D2O (diamonds) and IndRu(PTA)2D with H2O (circles) based on 31P NMR peak heights.
CpRu(PTA)2H and IndRu(PTA)2H. The fast kinetics may be due to the higher electron donating ability of Dp compared with Ind and Cp and a seemingly weaker Ru−H bond in DpRu(PTA)2H as evidenced by the IR spectroscopy (vide supra). Activation parameters were not obtained as the kinetics were too fast to get good data via 31P{1H} NMR spectroscopy. The 31P NMR spectrum of DpRu(PTA)2H as a function of pH in phosphate buffer (pH 2.1, 6.5, and 9.8) at 25 and 5 °C was explored. At reduced temperature (5 °C), the 31P NMR spectra revealed significant information, Figure 10. The results
primary kinetic isotope effect (KIE) for the H/D exchange reaction described here. The normal kinetic isotope effect (kH/kD) calculated for 1 and 1D is 7.4 at 35 °C, consistent with the 7.9 observed for the Cp analogue.7 The KIE calculation takes into account the frequency that intermediate Ru(HD)+ leads to observable isotopic exchange by utilizing a fractional probability factor, F.51 In this calculation, secondary isotope effects were ignored, leading to k2a and k−1b being equivalent (Scheme 5) and k2b Scheme 5. Kinetic Scheme for the H/D Exchange of Ru−H with D2O
Scheme 6. Kinetic Scheme for the H/D Exchange of Ru-D with H2O
and k−1a being equivalent (Scheme 6).7,51 The rate constants kH and kD are calculated utilizing the F values (FMH = 0.625 for Scheme 5 and FMD = 0.375 for Scheme 6) reported by Norton and co-workers for the protonation of CpW(CO)2(PMe3)H with 4-tert-butyl-N,N-dimethylanilinium.51 The rate constants kH and kD are calculated to be 14.6 × 10−4 s−1 and 1.96 × 10−4 s−1 at 35 °C, respectively (kH/kD = 7.4). H/D Exchange Reaction of DpRu(PTA)2H. The effect of the Cp′ ancillary ligand on the kinetics of H/D exchange was studied in more detail with DpRu(PTA)2H (2). The exchange reaction at room temperature was very fast (
