Kinetics of phosphine substitution in CpRu(PPh₃)₂X (X = Cl, Br, I, N₃, and NCO)

Abstract

The kinetics of phosphine substitution in CpRu(PPh₃)₂X (X = Br, 1b, X = I, 1c, X = N₃, 1d, and X = NCO, 1e) have been measured under pseudo-first order conditions in THF solution and compared with data for CpRu(PPh₃)₂X (1a). The relative rate of substitution is found to be 1a > 1d > 1b > 1e > 1c. Substitution rates decrease in the presence of added PPh₃ and are independent of added X consistent with a dissociative process. Activation parameters for 1a–1c (ΔH^† = 113–135 kJ mol⁻¹, ΔS^† = 21–102 J mol⁻¹ K⁻¹) and DFT calculations support a dissociative or dissociative interchange pathway even though negative activation entropies (ΔS^† = −48 ± 16 to −105 ± 5 J mol⁻¹ K⁻¹) are observed for 1d–e. Differences in Ru–ligand bond angles in 1d–e point to different π-acceptor properties of the pseudohalide ligands, contributing to the faster rate of substitution for the azide complexes, 1d relative to the cyanate derivative 1e. Substitution is not observed when X = F, 1f, X = H, 1g, X = SnF₃, 1h, or X = SnCl₃, 1i. Compounds 1b–1e also react with chloroform to yield 1a. The rates of halide exchange are comparable to phosphine substitution for 1c and 1d. The latter reaction is inhibited by excess triphenylphosphine and is unaffected by both radical inhibitors and radical traps suggesting that a radical mechanism is unlikely.

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Introduction

Cyclopentadienyl ruthenium bis(triphenylphosphine) chloride, CpRu(PPh₃)₂Cl (1a), is a versatile catalyst for a range of useful transformations.¹ Changing the halide ligand in CpRu(PPh₃)₂Cl for other halides or pseudohalides affects both the reactivity and selectivity in these processes.² For example, CpRu(PPh₃)₂I (generated in situ) is reported to be more effective than CpRu(PPh₃)₂Cl in catalyzing the cycloaddition of norbornene and norbornadiene.³ A mechanism based on faster phosphine dissociation is proposed as the explanation for the increased catalytic activity of CpRu(PPh₃)₂I. On the other hand, CpRu(PPh₃)₂X catalyzed conversion of cyclohexylamine and methanol to CyNMe₂ is nearly quantitative after 6 hours at 100 °C for X = Cl while only 40% conversion to 2.4 : 2.8 : 1 ratio of cyclohexylimine, methylcyclohexylamine and CyNMe₂ is observed for X = I.⁴ In this case, the ionization of the Ru–X bond is proposed as the key step the reaction mechanism. The conversion rate of methanol to methyl acetate in the presence of catalytic amounts of CpRu(PPh₃)₂X (X = F, Cl, Br, SnF₃, SnCl₃ and SnBr₃) follows the order: X = SnF₃ > SnCl₃ ≈ SnBr₃ > F > Cl ≈ Br.⁵ In this case, dissociation of chloride is thought to be counterproductive to efficient catalysis with the greater activity of CpRu(PPh₃)₂SnF₃ attributed to phosphine dissociation. The kinetics of phosphine substitution in CpRu(PAr₃)₂Cl^6,7 and the rate of solvolysis of the halide in CpRu(PR₂R′)₂X (R = Ph, Me, X = Cl, Br, I)⁸ have both been measured but the effect of X on the rate of phosphine substitution (eqn (1)) has not been extensively explored. Only for the related Cp*Ru(PMe₃)₂X⁹ has the effect of the ancillary X ligand on the rate of phosphine substitution been systematically investigated. The data for the latter were consistent with a dissociative mechanism with a marked increase in rate for better π-donor X ligands. In the present study we report on the phosphine substitution in CpRu(PPh₃)₂X (eqn (1), 1b–i where X = Br, I, N₃, NCO, H, F, SnCl₃, and SnF₃) in THF as well as on the unexpected halide exchange reaction between 1b–e and CDCl₃. The results provide some insight into the relative importance of Ru–P dissociation in catalytic reactions involving 1a–i.

Experimental

All compounds described in this work were handled using Schlenk techniques or a M. I. Braun glove box under purified nitrogen atmospheres.¹⁰ RuCl₃·xH₂O was purchased from Pressure Chemical, Inc. Tertiary phosphines, PMePh₂ and PPh₃, were obtained from Strem Chemical, Inc. and used as received. Solvents were purified by refluxing over Na/benzophenone (toluene, tetrahydrofuran, benzene, hexane, pentane), P₂O₅ (dichloromethane) or MgSO₄ (ethanol) and distilled prior to use. Chloroform-d¹ and benzene-d⁶ (Cambridge Isotope Laboratories) were purified by distillation from CaH₂ and Na/benzophenone, respectively. Ruthenium(II) compounds CpRu(PPh₃)₂Cl (1a),¹¹ CpRu(PPh₃)₂Br (1b),¹² CpRu(PPh₃)₂I (1c),¹² CpRu(PPh₃)₂N₃ (1d),¹³ CpRu(PPh₃)₂NCO (1e),¹² CpRu(PPh₃)₂H (1f),¹² CpRu(PPh₃)₂F (1g),¹⁴ CpRu(PPh₃)₂SnF₃ (1h),¹³ CpRu(PPh₃)₂SnCl₃ (1i),¹³ and CpRu(PPh₃)(PPh₂Me)Cl (2a),¹⁵ were prepared by literature procedures. Melting points were determined in capillary tubes using an Electrothermal 9110 melting point apparatus and are uncorrected. Elemental analyses (C, H) were performed by Columbia Analytical Services, Inc. Tucson, AZ.

NMR spectra were recorded at 400 MHz for ¹H and 162 MHz for ³¹P{¹H} on a Mercury XL300 spectrometer. Proton chemical shifts are reported relative to residual protons in the solvent (CD₂HCl at δ 7.24 ppm relative to TMS at 0.00 ppm). Phosphorus chemical shifts are reported relative to 85% H₃PO₄ at 0.0 ppm.

Electrochemical measurements were made under nitrogen on a BAS 100 B/W electrochemical workstation at 22 °C using 1 × 10⁻³ M solutions in dry CH₂Cl₂, 0.1 M ⁿBu₄NPF₆ as supporting electrolyte at a scan rate of 100 mV s⁻¹. The working electrode was a 3 mm Pt disk with a Pt wire as auxiliary electrode. A silver wire was used as a pseudo-reference electrode with ferrocene added as an internal standard. All potentials for 1a–e, h and i (Table 1) are referenced to ferrocene (E_1/2 = 0.00 V).

Table 1 Electrochemical potentials for selected CpRu(PPh₃)₂X complexes^a

Compound	E° (mV)	Compound	E° (mV)
a 1 × 10^−3 M solutions in dry CH2Cl2, 0.1 M ^nBu4NPF6 as supporting electrolyte at a scan rate of 100 mV s^−1 at 22 °C vs. Fc/Fc^+ at 0.00 mV.b 1h is not sufficiently soluble for the experiment.
X = Cl, 1a	136	X = NCO, 1e	168
X = Br, 1b	138	X = F, 1g	790
X = I, 1c	182	X = SnF3, 1h	^b
X = N3, 1d	20	X = SnCl3, 1i	730

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Synthesis of CpRu(PPh₃)(PMePh₂)X (X = Br, I, NCO, N₃, SCN, and SnCl₃)

General procedure. A slurry of CpRu(PPh₃)(PMePh₂) Cl (2a) and a 5–10 fold excess of KX (X = Br, I, N₃, NCO, SCN) was refluxed in 25 mL absolute ethanol for 16–18 h under nitrogen. Solvent was evaporated under vacuum and the product extracted with 2 × 25 mL CH₂Cl₂. After filtration to remove the potassium salts, the filtrate was evaporated to dryness and the crude product crystallized from CH₂Cl₂/hexane to yield CpRu(PPh₃)₂X (1b–f). Chromatography on neutral alumina with dichloromethane served as an additional purification method.
CpRu(PPh₃)(PMePh₂)Br (2b). Yellow-orange solid, 75% yield. Mp turns dark brown without melting above 160 °C.

Calculated for C₃₆H₃₃P₂RuBr·CH₂Cl₂: 56.01% C, 4.45% H; found: 56.53% C, 5.35% H.

¹H (CDCl₃) δ 1.19 d (J = 8.8 Hz, 3H, PCH₃), 4.20 s (5H, Cp), 5.29 s (2H, CH₂Cl₂), 7.0–7.8 m (25 H, aryl).

³¹P (CDCl₃) δ 42.9 d (J_PP = 43 Hz), 29.9 d (J_PP = 43 Hz).

CpRu(PPh₃)(PMePh₂)I (2c). Yellow-orange solid, 51% yield. Mp turns dark brown without melting above 140 °C.

Calculated for C₃₆H₃₃P₂RuI·CH₂Cl₂: 52.87% C, 4.20% H; found: 53.08% C, 4.67% H.

¹H (CDCl₃) δ 1.31 d (J = 8.8 Hz, 3H, PCH₃), 4.27 s (5H, Cp), 5.24 s (2H, CH₂Cl₂), 7.0–7.8 m (25 H, aryl).

³¹P (CDCl₃) δ 42.9 d (J_PP = 43 Hz), 30.0 d (J_PP = 43 Hz).

CpRu(PPh₃)(PMePh₂)N₃ (2d). Yellow-orange solid, 15% yield. Mp turns dark brown without melting above 163 °C.

Calculated for C₃₆H₃₃N₃P₂Ru: 64.47% C, 4.96% H; found: 63.93% C, 5.31% H.

¹H (CDCl₃) δ 1.17 d (J = 8.8 Hz, 3H, PCH₃), 4.23 s (5H, Cp), 7.21–7.46 m (25 H, aryl).

³¹P (CDCl₃) δ 41.3 d (J_PP = 43 Hz), 30.3 d (J_PP = 43 Hz).

CpRu(PPh₃)(PMePh₂)NCO (2e). Yellow-orange solid, 74% yield. Mp turns black without melting above 160 °C.

Calculated for C₃₇H₃₃NOP₂Ru: 66.26% C, 4.96% H; found: 66.45% C, 5.28% H.

¹H (CDCl₃) δ 1.06 d (J = 8.8 Hz, 3H, PCH₃), 4.15 s (5H, Cp), 7.18–7.3 m (25 H, aryl).

³¹P (CDCl₃) δ 39.5 d (J_PP = 43 Hz), 30.7 d (J_PP = 42 Hz).

CpRu(PPh₃)(PMePh₂)SnCl₃ (2i). A solution of 172 mg (0.26 mmol) 2a and 54 mg (0.28 mmol) SnCl₂ in 50 mL absolute ethanol was refluxed for 90 minutes. The resulting precipitate was isolated by filtration, washed 2 × 5 mL methanol and dried under vacuum. Compound 2i was isolated in 68% yield as an orange solid. Mp. turns dark brown without melting 151–153 °C.

Calculated for C₃₆H₃₃P₂RuSnCl₃: 50.65% C, 3.90% H; found: 50.83% C, 4.54% H.

¹H (CDCl₃) δ 1.19 d (J = 8.8 Hz, 3H, PCH₃), 4.19 s (5H, Cp), 6.9–7.7 m (28 H, aryl).

³¹P (CDCl₃) δ 43.4 d (J_PP = 44 Hz), 30.4 d (J_PP = 44 Hz).

Kinetic measurements

Reactions of 1b–e with PMePh₂. The collection of kinetic data for reactions between 1b–e with PMePh₂ followed procedures described for reactions between CpRu(PAr₃)₂Cl and PMePh₂.⁶ Stock solutions of 1b–e (10.0 mL) were prepared in volumetric flasks by dissolving an appropriate amount of 1b–e and a 10–15 fold excess of PMePh₂ in CDCl₃ or THF containing 10% C₆D₆. Samples for the kinetic experiments were prepared by transferring 600 μL of the stock solution to 5 mm NMR tubes attached to 14/20 ground glass joints. The tubes were flame-sealed sealed under vacuum. Samples were stored at −20 °C until needed and then heated in thermostated block heaters. The rate of substitution of PPh₃ by PMe₂Ph was measured by monitoring the decrease in the singlet for CpRu (PPh₃)₂X (1b–e) over time relative to the doublets for CpRu(PPh₃)(PMePh₂)X (2b–e). Three independent measurements of the substitution rate were made at each temperature to determine the rate constants for the reaction.

To assess the effect of excess PPh₃ and X⁻, additional experiments were carried out by adding 600 μL of the stock solution to weighed amounts of PPh₃ (3–10 equivalents) or ⁿBu₄NX (≈10 equivalents). The resulting solutions were transferred to NMR tubes and sealed as described above. These experiments were typically limited to a single measurement of the substitution rate at one temperature.

Activation parameters were determined using the Eyring equation by plotting ln(k_obs/T) vs. 1/T where the slope = −ΔH^‡/R and the intercept = ΔS^‡/R + ln k_B/h as described in our prior work.⁶ The activation entropies and enthalpies were also calculated from the slope and intercept of a plot of T ln(k/T) vs. T, respectively.¹⁶ The same values for ΔH^‡ and ΔS^‡ were obtained using each method within error. Errors in ΔS^‡ and ΔH^‡ were calculated using the statistical packages in Excel and by procedures described in standard analytical chemistry texts.¹⁷

Reactions of 1c–d with CDCl₃. Flame sealed tubes containing 10–15 mM solutions of 1c–d were prepared as described for the reactions with PMePh₂. The rate of the halide exchange reaction was determined by integration of the singlets assigned to 1a and 1c–d in the ³¹P NMR spectra. Additional tubes containing PPh₃ (6–21 eq.), 9,10-dihydroanthraene (3–16 eq.) and duroquinone (2–24 eq.) were prepared by adding 600 μL of the stock solution to weighed amounts of these reagents.

Computational methods

All calculations were conducted using density functional theory (DFT) as implemented in the Gaussian09 Revision B.01 suite of ab initio quantum chemistry programs as described for phosphine substitution in 1a and related CpRu(PAr₃)₂Cl complexes.⁶

Results

Kinetics of phosphine exchange

The substitution of one PPh₃ in 1b–e by PPh₂Me (10–15 equivalents, pseudo first order conditions) was followed by ³¹P NMR in both CDCl₃ and THF/10% C₆D₆ (v/v) solution between 25 and 60 °C. The singlet resonance for the starting material is replaced by a pair of doublets assigned to the mono-substituted products, CpRu(PPh₃)(PPh₂Me)X (2b–e) with concurrent appearance of resonances for PPh₃ (δ − 4.4 ppm in CDCl₃, −4.6 ppm in THF/10% C₆D₆). The ³¹P chemical shifts of the products were verified by comparison with independently synthesized and characterized samples of 2b–f. Formation of CpRu(PPh₂Me)₂X (i.e. di-substitution) is not observed during the reaction period even in the presence of ≈10 equivalents of PMePh₂. Formation of 1b–c from reactions between 2b–c and PPh₃ is not observed. Qualitatively, the rate of reaction at 40 °C is found to be 1a > 1d > 1b ≈ 1e > 1c.

Reactions between CpRu(PPh₃)₂X and PMePh₂ in THF solution follow first order kinetics over several half-lives. Rate constants, half-lives and activation parameters for reactions in THF/C₆D₆ mixtures are summarized in Fig. 1 and Table 2. The reaction rates are largely independent of the [PMePh₂], up to 60 equivalents (Fig. 2 and Table S3†). By comparison, the reaction rate decreases dramatically in the presence of added PPh₃. In addition, the reaction rates are unaffected by the addition of excess ⁿBu₄NX in all four cases.¹⁸ The rates of phosphine substitution in 1a in both CDCl₃ and in THF are known.^6,7 The remaining complexes, 1f–g and 1i fail to react with excess PMePh₂ in THF/C₆D₆, dioxane/C₆D₆ or other solvent mixtures up to the boiling point of the solvents even after 30 days or more. Compound 1h has minimal solubility in THF and dioxane hampering comparable studies, however, phosphine substitution was not observed.

Fig. 1 Eyring plots of ln(k_obs/T) vs. 1/T for 1b–e in THF containing 10% v/v C₆D₆.

Table 2 Rate constants, half-lives, and activation parameters for the substitution of PPh₃ by PMePh₂ in 1a–e in THF containing 10% (v/v) C₆D₆^a

X	k30,THF (×10^6 s^−1)	t1/2 (h)	ΔH^‡ (kJ mol^−1)	ΔS^‡ (J mol^−1 K^−1)	ΔG^‡ (kJ mol^−1)
a Concentrations of 1b–e ranged from 8 to 17 mM with a ≈ 10–15 fold excess of PMePh2. Benzene-d^6 is added to lock and shim the spectrometer.b From ref. 7.
1a, Cl^−	29 ± 2^b	0.66	121 ± 4^b	71 ± 8^b	100
1b, Br^−	7.89 ± 0.79	24	135 ± 7	102 ± 23	105
1c, I^−	2.49 ± 0.3	77	113 ± 4	21 ± 12	107
1d, N3^−	24.6 ± 1.5	7.8	86 ± 5	−48 ± 16	100
1e, NCO^−	16.1 ± 3.6	12	70 ± 7	−105 ± 23	101

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Fig. 2 Plots of k_obs as a function of [PMePh₂] and [PPh₃] for the reaction between CpRu(PPh₃)₂X (1b–e) and excess PMePh₂ in THF. The data are for reactions at 30 °C except for X = N₃ (1d) which was collected at 35 °C.

The activation parameters reveal different trends for the halide complexes 1a–c and the pseudohalide complexes 1d–e. Activation enthalpies for the former are generally larger and the activation entropies are positive. The activation entropies for 1d and 1e, however, are negative. The free energies of activation (ΔG^‡) calculated at 25 °C (298 K) for 1a–e are similar to those reported for Cp*Ru(PMe₃)₂X: 109 kJ mol⁻¹, 106 kJ mol⁻¹ and 113 kJ mol⁻¹ for X = Cl, Br, and I, respectively.⁹ Pseudohalide derivatives in the Cp*Ru(PMe₃)₂X series were not studied.

Reactions between 1b–e and PMePh₂ were also investigated in CDCl₃ but were complicated by the appearance of 1a (δ 39.9 ppm) and 2a as the reaction progressed. The formation of 1a is the result of reaction between the starting materials and the solvent since the starting materials were pure by ³¹P NMR at the outset of the reaction. Thus the final reaction mixtures in CDCl₃ contain 2b–e and 2a. Nevertheless, the rate of reaction between excess PMePh₂ (10–15 equivalents, i.e. pseudo first order conditions in PMePh₂) and 1b–e at early reactions times could be measured by integration of the ³¹P resonances for reactant and product before halide exchange led to measurable quantities of 1a. Qualitatively, the order of the rates for the reaction of 1b–e with PMePh₂ in CDCl₃ is the same as in THF: 1a > 1d > 1b > 1e > 1c. Reasonable estimates of first order rate constants (k_subs,CDCl3) for the substitution reactions in CDCl₃ at early reaction times, when less than 5% of 1a (and no 2a) is observed in the solution, are summarized in Table 3. The substitution is slowed by the addition of excess PPh₃ and the formation of 1a in these reactions is suppressed in the presence of added ⁿBu₄NX. The rate of substitution, however, remains unaffected by the presence of excess X⁻. Comparison of the values for k_subs,THF with k_subs,CDCl3 for 1a–e indicate that reactions are between 1.5 and 5 times faster in THF solution.

Table 3 Estimated first order rate constants for substitution of PPh₃ by PMePh₂ in 1a–e in CDCl₃^a and first order rate constants for the reaction of 1c–d with CDCl₃

X	k30,susb,CDCl3 (×10^6 s^−1)	k30,THF/k30,susb,CDCl3	k30,CDCl3 (×10^6 s^−1)	k30,susb,CDCl3/k30,CDCl3
a Concentrations of 1a–e ranged from 12 to 18 mM in CDCl3 with a ≈ 10 fold excess of PMePh2.b From data in ref. 6 and 7.
1a, Cl	13^b	2.2^b	—	—
1b, Br	5.0 ± 0.3	1.6		—
1c, I	1.8 ± 0.2	1.4	0.54 ± 0.2	3
1d, N3	6.1 ± 0.1	4.0	6.6 ± 0.4	1
1e, NCO	3.5 ± 0.5	4.6	—	—

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Kinetics of halide exchange between 1c–d and CDCl₃

The rates of the halide exchange reactions between 1c and 1d with CDCl₃ were measured independently by integration of the ³¹P resonances for reactants (1c–d) and product (1a) in CDCl₃ at 30 °C. Linear plots of ln[CpRu(PPh₃)₂X] vs. time are observed for both compounds, with first order rate constants for the reaction (k_CDCl3) being listed in Table 3. The rate of reaction with CDCl₃ reflects the same order observed for phosphine substitution: 1d > 1c. The reaction rates of 1c–d in CDCl₃ were further investigated in the presence of excess PPh₃, (6–21 eq.), a radical initiator, 9,10-dihydroanthracene, (3–16 eq.), and a radical trap, duroquinone, (2–24 eq.). Fig. 3 reveals that the reaction rates are essentially independent of radical initiators and traps but are slowed significantly by the presence of PPh₃. The k_subs,CDCl3/k_CDCl3 ratio in Table 3 reveals that the rate of reaction with CDCl₃ is competitive with the rate of phosphine substitution for 1c–d.

Fig. 3 Plot of ln[CpRu(PPh₃)₂I] vs. time (s) for halide exchange in CDCl₃ solution at 30 °C. (a) 1c in the presence of 9,10-dihydroanthracene (DHA, 3 eq.), duroquinone (DQ, 7 eq.) and PPh₃ (21 eq.) and (b) 1d in the presence of 9,10-dihydroanthracene (DHA, 2 eq.), duroquinone (DQ, 3 eq.) and PPh₃ (6 eq.).

Computational studies

DFT calculations were initially used to optimize the structures of 1a–e (Table 4). The calculated values for bond distances and bond angles for 1a–b and 1d compare favorably with the published structures determined by X-ray crystallography: the calculated bond distances are only slightly longer than the observed values.¹⁹

Table 4 Calculated^a and observed^b bond distances and bond angles for 1a–e

Compound	dRu–X (Å)	dRu–P1 (Å)	dRu–P2 (Å)	dRu–Cp,centroid (Å)	∠Ru–X (°)
a The isocyanate ligand is treated as N bonded. Calculations use the B3LYP functional and the DGDZVP basis set on the Gaussian 09 suite. Normal convergence conditions were applied and geometries were determined to be of a minimal through a frequency calculation.b From X-ray crystallography see ref. 19. This value seems abnormally short for a Cp–Ru bond.
1a	2.513	2.401	2.396	2.27	—
	2.448^b	2.323^b	2.329^b	2.20^b	—
1b	2.648	2.406	2.411	2.27	—
	2.568^b	2.323^b	2.329^b	2.214^b	—
1c	2.842	2.416	2.413	2.275	—
1d	2.196	2.401	2.400	2.275	118.5
	2.135^b	2.329^b	2.330^b	1.843^b	124.5^b
1e	2.136	2.400	2.3999	2.27	153.5

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Computational chemistry was then applied to the calculation of the relative energies of potential intermediates in a dissociation of PPh₃ in 1a–e. The free energies for the 16-electron intermediate that results from PPh₃ dissociation from 1a–e (second column in Table 5) are quite similar to each other and lower than the energies for intermediates resulting from halide dissociation and coordination of THF (third column in Table 5). The calculated free energy changes for the overall conversion of 1a–e to 2a–e are listed in the fourth column of Table 5 indicating a fairly narrow range of value for ΔG of about 12 kJ mol⁻¹.

Table 5 Calculated Gibbs free energies (kJ mol⁻¹) for PPh₃ dissociation, halide dissociation and the overall phosphine substitution reactions of 1a–e^a

CpRu(PPh3)2X	ΔG (kJ mol^−1)	ΔG (kJ mol^−1)	ΔG (kJ mol^−1)
	CpRu(PPh3)2X ⇒ CpRu(PPh3)X + PPh3	CpRu(PPh3)2X + THF ⇒ CpRu(PPh3)2(THF)^+ + X^−	CpRu(PPh3)2X + PMePh2 ⇒ CpRu(PMePh2)(PPh3)X + PPh3
a Geometry optimizations were optimized in the gas phase using the B3LYP exchange–correlation functional and DGDZVP basis set followed by a single point energy calculation using a polarizable continuum model (PCM) for THF solvation.
1a, X = Cl	43.5	70.9	−35.8
1b, X = Br	40.7	59.0	−40.8
1c, X = I	43.6	47.2	−45.2
1d, X = N3	47.2	105.1	−32.6
1e, X = NCO	43.9	108.8	−31.9

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The energies of the transition states for the two steps in eqn (2) were also calculated (Table 6). The data indicate that the activation energy for the dissociation of PPh₃ is greater than for the reaction of the 16 e⁻ intermediate, CpRu(PPh₃)X, with PMePh₂, consistent with the kinetic measurements. The calculated values of ΔG for the transitions states of 1a–e are also quite close in energy, covering a range of <4 kJ mol⁻¹ for the rate-determining step and about 8–12 kJ mol⁻¹ less than the values of ΔG^‡ from experiment.

Table 6 Calculated Gibbs free energies (kJ mol⁻¹) for transition states for PPh₃ dissociation and the subsequent phosphine substitution reactions of 1a–e^a

CpRu(PPh3)2X	ΔG^‡TS1 (kJ mol^−1)	ΔG^‡TS2 (kJ mol^−1)
	CpRu(PPh3)2X ⇒ [CpRu(PPh3)X⋯PPh3]^‡	CpRu(PPh3)2X + PMePh2 ⇒ [CpRu(PMePh2)(PPh3)X]^‡
a The transition state optimization was performed using the synchronous transit and quasi-Newton methods (STQN). The guess structure used was the maximum of a relaxed PES scan along the Ru–P bond. They were confirmed as first order saddle points by harmonic frequency analysis.
1a, X = Cl	92.1	73.7
1b, X = Br	93.2	75.7
1c, X = I	91.1	80.0
1d, X = N3	89.6	79.9
1e, X = NCO	91.1	79.5

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Discussion

The effect of the X group on phosphine substitution rates in 1a–e is qualitatively similar to those reported previously for Cp*Ru(PMe₃)₂X for the same set of X ligands. An increase in the rate of substitution in Cp*Ru(PMe₃)₂X is observed for X ligands with lone pairs of electrons on the donor atom, e.g. X = Cl, Br, I, NPh₂, NHPh, OPh, OH, and SH relative to such σ-donor ligands such as H, CH₃, CH₂Ph, Ph and CH₂SiMe₃.⁹ Kinetic data for phosphine exchange between Cp*Ru(PMe₃)₂X and PMe₃ in aromatic hydrocarbon solution are consistent with a dissociative process through 16-electron Cp*Ru(PMe₃)X intermediates.⁹ The relative rates of substitution in Cp*Ru(PMe₃)₂X were judged to reflect both ground state and transition state effects of X.⁹ The observation that 1g–i (X = H, SnF₃, and SnCl₃) do not react at all with PMePh₂ under the reaction conditions is consistent with the observations for Cp*Ru(PMe₃)₂X: good σ-donors lead to slower reaction. The corresponding indenyl complex, (η⁵-C₉H₇)Ru(PPh₃)₂H, is also known to be inert toward phosphine substitution.²⁰ The effect of σ-donor, π-donor, and possibly π-acceptor properties of the ligands on both ground state and transition state energies are likely to be relevant to interpretations of the rate data for 1a–e.

We start by considering the halide derivatives 1a–c. The observed order of substitution rates in 1a–c are the same as for Cp*Ru(PMe₃)₂X: Cl > Br > I. The substitution rates in 1a–c span a relatively small range; k_obs for 1a (X = Cl) is ≈ 50 times greater than for 1c (X = I) in THF, a slightly broader range of k_obs values for 1a–c than for Cp*Ru(PMe₃)₂X for the same X ligands. A dissociative mechanism for phosphine substitution has been suggested for reactions of 1a with PMePh₂ in both THF and CDCl₃.^6,7 The kinetic data for substitution in 1b and 1c in Table 1 in THF are also consistent with a dissociative or dissociative interchange mechanism with the loss of PPh₃ as the rate-determining step.⁶ This conclusion is supported by the observed decrease in rate in the presence of added PPh₃, the independence of the rate on PMePh₂ concentration and the observed positive activation entropies. Closer examination of the effect of added PPh₃ on the substitution rate reveals that the effect is not the same across the series 1b–e.

Ionization of Ru–X bonds in CpRu(PR₂R′)₂X (R = Ph, Me, X = Cl, Br, I) systems in Lewis basic solvents such as alcohols, acetonitrile, or dimethylsulfoxide is well established but does not seem to play a significant role in the substitution reactions in THF.²¹ The absence of any significant effect of added X⁻ on the rate suggests that formation of [CpRu(PPh₃)₂(THF)]⁺ and X⁻ ions in THF solution is unlikely to be the rate determining step; one would expect a decrease in rate if dissociation of X⁻ was the rate determining step. With the exception of 1c calculations of the relative energies of CpRu(PPh₃)X and [CpRu(PPh₃)₂(THF)]⁺[X]⁻ confirm that the latter is significantly higher in energy than the former. Even in the case of CpRu(PPh₃)₂I (1c), the 16 e⁻ intermediate is 3–4 kJ mol⁻¹ lower in energy than [CpRu(PPh₃)₂(THF)]⁺[I]⁻ (in the gas phase).

The absence of significant differences in the Ru–P or Ru–Cp bond distances in 1a–c in either the crystal structures or in the calculated structures (Table 4) suggests that only small differences exist in the ground state energies of 1a–c. Despite a significantly larger ionic radius and a longer Ru–X bond distance, the iodide (1c), reacts slower than the chloride (1a). Increasing the size of X (X = I > Br > Cl) does not increase the rate of the reaction suggesting that transition state effects also contribute to the order of substitution rates for 1a–c.^9,22 The electrochemical potentials of CpRu(PPh₃)₂X (Table 1) reveal surprisingly similar E° values for 1a–c. The E° values for 1a–c are essentially indistinguishable: 136 vs. 138 mV vs. Fc/Fc⁺ for 1a and 1b, respectively and less than a 50 mV difference in E° between the chloride and iodide complexes. Although 1c does react slower than 1a–b, the small difference in E° values remains consistent with minimal contribution from ground state effects to the substitution reaction. Further support for small ground state effects of chloride, bromide and iodide is seen in the ν_CO for CpRu(CO)₂X (ν_CO X = Cl > Br > I) which differ by only 11 cm⁻¹.²³

Interestingly CpRu(PPh₃)₂F (1f) has a significantly larger positive E°, 790 mV, which may help explain the lack of reactivity toward PMePh₂. Fluoride is a weaker σ-donor and a stronger π-donor than Cl⁻, Br⁻ and I⁻.² One not on might expect greater π-donation to accelerate the substitution rate but the opposite is observed. The much greater electronegativity of fluoride as reflected by E°, suggests that the Ru–PPh₃ bond is significantly stronger in 1f than in 1a–c contributing to the failure of CpRu(PPh₃)₂F (1f) to react with PMePh₂ under the conditions of the experiment. No data is available for Cp*Ru(PMe₃)₂F for CpRu(CO)₂F making further comparisons difficult.

The calculated free energies of the 16-electron CpRu(PPh₃)X fragments span a narrow range, about 10 kJ mol⁻¹ (Table 5). It was previously shown that PPh₃ dissociation from 1a yields a lower energy intermediate than dissociation of Cl⁻ to form CpRu(PPh₃)₂⁺, the common intermediate from halide dissociation from 1a–c.⁶ The computational results for the free energies of the CpRu(PPh₃)X intermediate must be treated with caution when comparing calculations in the gas phase to the kinetic measurements in solution. As expected, the calculated free energy changes for substitution of one PPh₃ by PMePh₂ for the halide compounds are exergonic (ΔG < 0, Table 5) and differ by <15 kJ mol⁻¹ as a function of the halide ligand.

Support for the role of transition state effects on the reactivity of 1a–c comes from decades-old studies of carbonyl substitution reactions of M(CO)₅X (M = Re, Mn) and M(CO)₅X⁻ (M = Cr, Mo, where X = Cl, Br and I).²⁴ Substitution cis to the X group is observed in all cases and kinetic data for these reactions are consistent with a dissociative pathway. The rate of substitution in the chloride complexes is between 15 and 250 times the rate of substitution in the corresponding iodides. This effect was attributed to stabilization of the 16-electron intermediate or transition state by the stronger σ-donation from the halide ligand: Cl > Br > I.²⁴ There are strong parallels between the substitution rates in these mononuclear metal carbonyl halides and 1a–c. The observed order of rates, Cl > Br > I, is the same and substitution in 1a–c also occurs cis to the X group if one considers the Cp ligand to occupy a fac geometry in a pseudo-octahedral geometry. A stabilizing role for π-donation from X is less likely because the order of π-donation, I > Br > Cl, does not match the relative rates of phosphine substitution.^1,22 The kinetics of carbonyl substitution in CpRu(CO)₂X provide an even better comparison with the reactions of 1a–c.²⁵ In xylene, the rate of substitution in CpRu(CO)₂Cl with P(OPh)₃ is faster than for the bromide and iodide. A dissociative process is proposed for all three CpRu(CO)₂X compounds.

Finally, the calculated transition state energies (ΔG^‡) for the reactions of 1a–c with PMePh₂ support the interpretation of the experimental data. The first step, dissociation of PPh₃, is the rate determining step with subsequent reaction of the coordinatively unsaturated CpRu(PPh₃)X intermediate with PMePh₂: ΔG^‡_TS1 > ΔG^‡_TS2. The difference between ΔG^‡_TS1 and ΔG^‡ (Table 2) is small. The range of values for ΔG^‡_TS1 is quite narrow and mirrors the trend for ΔG^‡ in Table 2 suggesting that only small differences in the transition state contribute to the observed order of reaction rates: 1a > 1b > 1c. For 1c, the similar energies for two intermediates, CpRu(PPh₃)I and [CpRu(PPh₃)₂(THF)]⁺[I]⁻ in Table 5 may account for the greater difference between ΔG^‡_TS1 and ΔG^‡.

The compounds with pseudohalide ligands (N₃⁻ and NCO⁻), 1d and 1e, introduce ligands with both π-donating and π-accepting properties. Compounds 1d and e react with PMePh₂ as fast, or even faster, than 1b. Unlike 1a–c, the activation entropies for 1d and 1e are negative: ΔS^‡ = −48 ± 16 and −105 ± 23 J mol⁻¹ K⁻¹, respectively. This raises the possibility of a change in mechanism from a dissociative interchange to an associative interchange pathway. Nevertheless, the observation that the substitution rate in both 1d and 1e decreases in the presence of excess PPh₃ and is unchanged when excess pseudohalide is added to the solution argues for a dissociative or dissociative interchange mechanism for 1a–e. The greatest effect of added PPh₃ on rate is seen for 1d, the compound that reacts the fastest and the smallest effect is seen for 1c, which exhibits the slowest rate of phosphine substitution. One possible explanation is that the halide complexes, 1b–c react by a dissociative interchange mechanism while substitution in 1d–e follows a more dissociative pathway.

If ionization of the pseudohalide ligand in 1d–e represents the rate determining step, then one expects a decrease in rate when excess N₃⁻ or NCO⁻ is added to the reaction mixture, yet the rate is unchanged. Calculated values of ΔG for product of substitution of N₃⁻ or NCO⁻ by THF, [CpRu(PPh₃)₂(THF)]⁺[X]⁻, are more than double the ΔG for CpRu(PPh₃)X, suggesting that dissociation of X⁻ also does not play a role in the reaction with PMePh₂. Large negative values for ΔS^‡ were also reported for phosphine substitution in (η⁵-pentadienyl)Ru(PPh₃)₂Cl in what appears to be a dissociative mechanism and have been observed in halide exchange reactions of CpRu(prophos)Cl.²⁶ The large positive ΔS^‡ values for substitution in Cp*Ru(PMe₃)₂X were attributed to a late or product like transition state⁹ so one possible explanation for the differences in ΔS^‡ values between 1a–c and 1d–e is an earlier, more ordered transition state in 1d–e than in 1a–c. For comparison, the activation entropy for substitution in Re(CO)₅NCO, ΔS^‡ = +8 J mol⁻¹ K⁻¹, is less positive than ΔS^‡ = +73 and +44 J mol⁻¹ K⁻¹ for substitution in Re(CO)₅Cl and Re(CO)₅Br, respectively.²⁷ The rate of substitution in the rhenium(I) series reveals that Re(CO)₅NCO reacts slightly slower than Re(CO)₅Cl but faster than the bromide derivative similar to our observations for 1a–b and 1e.²⁷ Detailed calculations of the structure of the transition state for 1a–e are in progress but the data for ΔG^‡_TS1 indicate a lower activation energy for 1d and correlate well with the values for ΔG^‡ in Table 2, as observed for 1a–c.

The Ru–P bond distances in the solid state structure of 1d^19c and the results of DFT calculations (Tables 4 and 5) for 1d–1e do not reveal any striking structural anomalies. The electrochemical potential for 1e is again indistinguishable from the values for 1a–1c suggesting similar ground state energies. The electrochemistry of 1d, however, indicates that it is much easier to oxidize than 1a or 1b by about 160 mV. The significance of this E° value on the relative value of k_obs is not entirely clear but may indicate a slightly higher energy for the ground state in 1d.

Crystallography confirms that the azide ligand in 1d is bent with a Ru–N–N bond angle of 124.5°.^19c DFT calculations are consistent with this geometry yielding a calculated bond angle, ∠_Ru–N–N = 118.5°. The calculated Ru–N–C bond angle in 1e (153.5°) reveals that the NCO ligand is more linear in 1e, consistent with a greater contribution of resonance forms C and D in Fig. 4, while structures A and B are likely to be the major contributors to the bonding of N₃⁻ in 1d. The importance of structures C and D may make the linear NCO ligand a better π-acceptor than the bent N₃ ligand.

Fig. 4 Resonance forms for the pseudohalide ligands in 1d–e.

Transition state stabilization and increased substitution rates for square planar complexes bearing ancillary π-acceptor ligands is well established but the effect of π-acceptor ligands on substitution rates in octahedral complexes is less documented.²² Seminal studies on dissociative substitution reactions of group 6 and group 7 carbonyls suggest that 16 e⁻ transition states are stabilized by electron donors and destabilized by acceptor ligands.^22,24,27 If this is true, than the bent N₃ ligand in 1d stabilizes the transition state and accounts for the faster reaction of 1d compared to 1e. Conversely, the better π-acceptor, linear NCO ligand may destabilize (raise the energy of) the transition state decreasing the reaction rate. The linear π-accepting phenylacetylide ligand in Cp*Ru(PMe₃)₂CCPh increases the Ru–PMe₃ bond energy by about 38 kJ mol⁻¹ and reduces the rate of phosphine dissociation.⁹ Significantly slower phosphine substitution was also observed in reactions of (η⁵-C₉H₇)Ru(PPh₃)₂CCPh compared to (η⁵-C₉H₇)Ru(PPh₃)₂Cl.²⁰

In addition to 1f, phosphine substitution was also not observed in 1g–i all of which contain good σ-donors: hydride and trihalotin (SnX₃⁻, X = Cl, F) ligands. To understand the lack of reaction, we turn to the studies of phosphine substitution that include Cp*Ru(PMe₃)₂Cl, Cp*Ru(PMe₃)₂H, and Cp*Ru(PMe₃)₂CH₃.⁹ The data for the latter three compounds suggests that the activation enthalpy, ΔH^‡, for the reaction closely approximates the Ru–PMe₃ bond energies, leading to the conclusion that the Ru–PMe₃ bonds in Cp*Ru(PMe₃)₂H and Cp*Ru(PMe₃)₂CH₃ are 29–59 kcal mol⁻¹ greater than for Cp*Ru(PMe₃)₂Cl. The lack of phosphine substitution in 1g–i is therefore, most likely the result of a small, strong σ-donor hydride ligands that substantially greater Ru–P bond strength.

The observation of halide exchange reactions between CpRu(PPh₃)₂X and CDCl₃ has not been previously reported²⁸ for 1b–e although reaction between 1a and acetyl halides, CH₃COX where X = Br and I, was recently reported to yield 1b–c.²⁹ An increase in the rate of halide exchange was observed in the presence of 9,10-dihydroanthracene (radical initiator) and a concomitant decrease in conversion when TEMPO (radical trap) is added to the reaction mixture supporting a radical mechanism. Computational chemistry suggested a pathway where phosphine dissociation is followed by halogen atom abstraction from CH₃COX and formation of a radical pair.²⁹ Further support for radical intermediates in the chemistry of 1 is found in the catalytic activity of CpRu(PPh₃)(PMe₃)Cl in the atom transfer radical addition (ATRA) reactions of CCl₄ and styrene.³⁰ There are also two reports of the reaction between 1a and excess iodomethane yielding 1c in situ and as a synthetic method but the mechanism of the reaction was not explored.³

The reactions between 1c–d and CDCl₃, however, are inconsistent with radical mechanisms given the absence of any noticeable effect of 1–16 equivalents of 9,10-dihydroanthracene or duroquinone (Fig. 3).^25a The addition of PPh₃ significantly reduces the rate of the halide exchange reaction. The latter observation argues for phosphine substitution as the potential rate-limiting step in the halide exchange reaction. The relative rates of halide exchange for 1c and 1d mimic the trend for the phosphine substitution rates in these two compounds. Both the oxidative addition of C-halide bonds and concerted mechanisms (Fig. 5) must be considered for the conversion of 1c–d to 1a.

Fig. 5 Possible mechanisms for halide exchange in 1c–d.

Limited evidence for both mechanisms can be found in the literature. Oxidative addition of allyl chloride to CpRu(PPh₃)₂Cl yields CpRu(C₃H₅)Cl₂ (ref. 31) while a halocarbon complex, [CpRu(PPh₃)₂(CH₃I)][PF₆] is isolated from reaction of 1a with Ag⁺ and methyl iodide.³² A further mechanistic proposal for the halide exchange reaction is the formation of quaternary phosphonium salts by reaction between the dissociated PPh₃ and CDCl₃ followed by dissociation of Cl⁻ and subsequent halide exchange with 1b–e. The latter pathway was proposed for the catalytic halogen exchange between MeI and CH₂Cl₂ catalyzed by a broad range of group 9 transition metal complexes.³³ Although no new resonances are observed in the ³¹P NMR spectrum of PPh₃ in CDCl₃, the possibility of halide exchange in 1b–e by this mechanism cannot be excluded at this time.

The rate data for phosphine substitution in 1a–i provide some insight into reaction mechanisms where 1a–i show catalytic activity. The assertion that faster phosphine dissociation accounts for higher yields in the cycloaddition of norbornene and norbornadiene³ when 1c is used in place of 1a is inconsistent with the relative rates of phosphine substitution reported herein. In fact, our data suggest that any catalytic process that relies on phosphine dissociation from 1a–e should proceed fastest for X = Cl with X = N₃ as the next most active catalyst precursor. The effect of 1b–e on the rate and selectivity of ruthenium-catalyzed dimerization of alkynes³⁴ and the 1,3-dipolar addition of azides to alkynes³⁵ represent potential future studies of the effect of the X ligand on catalytic properties. Phosphine substitution in trihalotin ligands in 1h–i are clearly slow and consistent with the high temperatures required for converting methanol to methylacetate⁵ in their presence.

Conclusions

The results of the kinetic study of phosphine substitution in CpRu(PPh₃)₂X for five halide and pseudohalide derivatives in THF and CDCl₃ solution reveals a likely dissociative or dissociative interchange process. These data suggest that dissociative substitution mechanisms reported for CpRu(PAr₃)₂Cl⁶ and Cp*Ru(PMe₃)₂X complexes⁹ are a general reaction pathway for 18-electron, cyclopentadienyl ruthenium(II) derivatives. Differences in the rate of substitution in 1a–e are likely a combination of ground state and transition state effects. Dissociation of phosphine in 1b–e is a likely step in the exchange of Ru–X bonds for Ru–Cl bonds when CpRu(PPh₃)₂X is dissolved in CDCl₃, however, further mechanistic studies are needed to identify the likely mechanism.

For reactions where Ru–X bond ionization is important, the data on phosphine substitution in 1a–e offer more limited insight. Compounds 1a, 1c–d, 1g and 1i all catalyze the N-methylation of cyclohexylamines⁴ to varying degrees in methanol solution. An order of relative rates, 1a > 1g > 1c ≥ 1d ≫ 1i (no reaction), can be inferred from the observed product ratios of CyNMe₂ : CyNHMe : CyNH₂. Among these, 1a is by far the best catalyst but the position of the hydride complex, 1g, is anomalous suggesting that more work is needed to understand the effect of different ligand environments on the reactivity of cyclopentadienyl ruthenium(II) complexes in carbon–carbon and carbon–nitrogen bond forming processes.

References

1. Ruthenium in Organic Synthesis, ed. S.-I. Murahashi, Wiley-VCH, 2004.
2. For a review of halide effect in transition metal catalyzed reactions see K. Fagnou and M. Lautens, Angew. Chem., Int. Ed., 2002, 41, 26–47.
3. (a) A. Tenaglia and L. Giordano, Synlett, 2003, 2333–2336; (b) A. Tenaglia and M. Sylvain, J. Org. Chem., 2008, 73, 1397–1402; (c) A. Tenaglia, L. Giordano, M. Sylvain and I. De Riggi, Angew. Chem. Int. Ed., 2011, 50, 9062–9065.
4. A. Del Zotto, W. Baratta, M. Sandri, G. Verardo and P. Rigo, Eur. J. Inorg. Chem., 2004, 524–529.
5. P. A. Robles-Dutenhefner, E. M. Mora, G. J. Gama, H. G. L. Siebald and E. V. Gusevskaya, J. Mol. Catal. A: Chem., 2000, 164, 39–47.
6. (a) M. J. Verschoor-Kirss, O. Hendricks, L. Renna, D. Hill and R. U. Kirss, Dalton Trans., 2014, 43, 15221–15227; (b) C. Peng and H. B. Schlegel, Isr. J. Chem., 1993, 33, 449.
7. M. P. Gamasa, J. Gimeno, C. Gonzalz-Bernardo, B. M. Martin-Vaca, D. Monti and M. Bassetti, Organometallics, 1996, 15, 302–308.
8. (a) R. J. Haines and A. L. DuPreez, J. Organomet. Chem., 1975, 84, 357–367; (b) P. M. Treichel and P. J. Vincenti, Inorg. Chem., 1985, 24, 228–230.
9. H. E. Bryndza, P. J. Domaille, R. A. Paciello and J. E. Bercaw, Organometallics, 1989, 8, 379–385.
10. D. F. Shriver, Manipulation of Air Sensitive Compounds, McGraw Hill, 1969.
11. M. I. Bruce, C. Hameister, A. G. Swincer and R. C. Wallis, Inorg. Synth., 1990, 28, 270.
12. T. Wilczewski, M. Bochenska and J. F. Biernat, J. Organomet. Chem., 1981, 215, 87–96.
13. E. M. Moura, H. G. L. Siebald and G. M. de Lima, Polyhedron, 2002, 21, 2323–2331.
14. (a) M. I. Bruce, C. Hameister, A. G. Swincer and R. C. Wallis, Inorg. Synth., 1982, 21, 78–84; (b) M. I. Bruce, R. C. F. Gardner and F. G. A. Stone, J. Chem. Soc., Dalton Trans., 1976, 81–89.
15. P. M. Treichel and D. A. Komar, Synth. React. Inorg. Met.-Org. Chem., 1980, 10, 205–218.
16. G. Lente, I. Fabian and A. J. Poe, New J. Chem., 2005, 29, 759–760.
17. D. A. Skoog, D. M. West and J. Holler, Analytical Chemistry: An Introduction, Saunders, 7th edn, 2000.
18. See ESI† for plots of ln[Ru] vs. time, complete tables of rate constants and Eyring plots for reactions of 1b–e with PMePh₂.
19. (a) M. I. Bruce, F. S. Wong, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1981, 1398–1405; (b) M. I. Bruce, P. J. Low, B. W. Skelton, E. R. T. Tietink, A. Werth and A. H. White, Aust. J. Chem., 1995, 48, 1887–1892; (c) M. M. T. Khan, M. M. Bhadbhade, M. R. H. Siddiqui and K. Venkatasubramanian, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1994, 50, 502–504.
20. M. Bassetti, S. Marini, F. Tortorella, V. Cadierno, J. Diez, M. P. Gamasa and J. Gimeno, J. Organomet. Chem., 2000, 593–594, 292–298.
21. Rate constants for solvation for CpRu(PPh₂OMe)₂X and CpRu(PPh₂Me)₂X in CD₃CN comparable to the substitution rates for CpRu(PPh₃)₂X at 30 °C require more than doubling the reaction temperature to 67 °C. Compound 1a–c do not dissolve sufficiently in CD₃CN for a direct comparison. See ref. 8.
22. J. M. Atwood, Inorganic and Organometallic Reaction Mechanisms, Wiley-VCH, 2nd edn, 1997.
23. R. J. Haines and A. L. duPreez, J. Chem. Soc., Dalton Trans., 1972, 944–948.
24. J. D. Atwood and T. L. Brown, J. Am. Chem. Soc., 1976, 98, 3160–3166.
25. (a) K. Tabataiana and K. White, Inorg. Chem., 1981, 20, 2020–2022; (b) D. A. Brown, H. J. Lyons and R. T. Sane, Inorg. Chim. Acta, 1970, 4, 621–625.
26. (a) M. Daniels and R. U. Kirss, J. Organomet. Chem., 2007, 692, 1716–1725; (b) H. Brunner, M. Muschiol, T. Tsuno, T. Takahashi and M. Zabel, Organometallics, 2008, 27, 3514–3525.
27. R. J. Angelici and G. C. Faber, Inorg. Chem., 1971, 10, 514–517.
28. Halide exchange is also observed between 1c and benzylbromide but not with bromobenzene.
29. H. Kuniyasu, A. Sanagawa, T. Nakajima, T. Iwasaki, N. Kambe, K. Bobuatong and M. Ehara, J. Organomet. Chem., 2014, 769, 34–39.
30. R. P. Nair, T. H. Kim and B. J. Frost, Organometallics, 2009, 28, 4681–4688.
31. H. Nagashima, K. Mukai, Y. Shiota, K. Yamaguchi, K.-I. Ara, T. Fukahori, H. Suzuki, M. Akita, Y. Moro-oka and K. Itoh, Organometallics, 1990, 9, 799–807.
32. (a) R. J. Kulawiec, J. W. Faller and R. H. Crabtree, Organometallics, 1990, 9, 745–755; (b) A related complex, [CpRu(PPh₃)(CN^tBu)(CH₃I)]⁺ is also known: F. M. Conroy-Lewis, A. D. Redhouse and S. J. Simpson, J. Organomet. Chem., 1989, 366, 357–367.
33. D. Forster, J. Chem. Soc., Chem. Commun., 1975, 917–918.
34. M. Daniels and R. U. Kirss, J. Organomet. Chem., 2007, 692, 1716–1725.
35. L. Zhang, X. Cheng, P. Xue, H. H. Y. Sun, I. D. Williams, K. B. Sharpless, V. V. Fokin and G. Jia, J. Am. Chem. Soc., 2005, 127, 15998–15999.

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Footnote

† Electronic supplementary information (ESI) available: Representative plots ln[CpRu(PPh₃)₂X] vs. t for phosphine substitution and halide exchange, Eyring plots, and coordinates for the optimized geometries for 1a–e. See DOI: 10.1039/c7ra02793a

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