Synthesis and Reactivity of an Olefin Metathesis-Catalyzing Ruthenium Complex with a Selenoether Moiety in the Benzylidene Ligand

Abstract

A Hoveyda–Grubbs (HG)-type olefin metathesis complex with a selenoether moiety at the terminus of phenoxy moiety was synthesized. The complex showed direct selenium-atom coordination to the ruthenium center, resulting in higher thermodynamic stability compared with the parent HG catalyst. The selenium atom binding enhanced the tolerance to protic solvent molecules in ring-closing metathesis of N-tosyldiallylamide and diethyl diallylmalonate, and also in the cross metathesis between 3-butenylbenzoate and methyl acrylate. The large activation enthalpy observed in the reaction with n-butyl vinyl ether indicated an increased contribution of the “dissociative mechanism” during the initiation of the catalytic cycle. Introducing coordinative atoms or functional groups at the terminal of the phenoxy moiety is a useful strategy to regulate the thermostability of HG-type olefin metathesis catalysts.

Introduction

Olefin metathesis (OM) catalyzed by transition metal complexes is a useful method for constructing new olefin moieties through the rearrangement of carbon–carbon double bonds in both laboratory and industrial scale syntheses because OM reactions are highly selective in olefins among various functional groups.¹⁻⁴ Hoveyda–Grubbs second generation complex (HG-II (1);⁵Chart 1) is the most widely used OM catalyst due to its higher thermostability than other OM transition metal catalysts and ease of functionalization through structural modifications of the N-heterocyclic carbenoid (NHC) and benzylidene ligand moieties. These characteristics have led researchers to investigate the applicability of HG-II (1) and its derivatives to reactions involving biomolecules in aqueous media.,^2,6,7 Furthermore, several studies have shown that the reactivity of HG-II (1) can be modulated by introducing functional groups at the terminal of phenoxy moiety in the complex.⁸⁻¹¹ Coordinative functional groups, such as imide,^12,13 carbonyl,^14,15 and sulfonamide¹⁶ can directly influence the reactivity of the metal center. Additionally, complex 2 regulates the reactivities of the metal center through second coordination sphere effect (i.e., amide-NH ••• Cl interaction).¹⁷ Replacing the amide group with an ether moiety (complex 3) led to higher initial velocities in OM reactions compared to those in HG-II (1). This is due to electrostatic repulsion between the oxygen and chlorido ligand, which accelerated the benzylidene ligand dissociation.¹⁸ However, the complex stability was found to be low, as indicated by the solution color change from green to brown during OM reactions. On the other hand, the sulfur atom in complex 4 directly coordinates to the metal center with an apparent 18-electron coordination fashion.¹⁸ Despite this, the complex achieves final yields in OM reactions similar to those observed in complexes 1 and 3. Unlike the case for O-coordinating complex 3, S-coordinating complex 4 showed no change in solution color during OM reactions. Furthermore, it was found that complex 4 has a higher tolerance to protic solvents compared to complexes 1 and 3. These observations highlighted different modes of reactivity regulation by chalcogen atoms (O and S) at the terminus of the phenoxy ether moiety. Expecting that heavier chalcogen atoms might reveal new modes of reactivity regulation for HG-II-type complexes, we investigated the effects of selenium (Se) atoms on the reactivities of HG-II-type complexes using a ruthenium complex containing a selenium atom at the terminus of the phenoxy ether moiety (complex 5).¹⁹

Chart 1. Structures of ruthenium complexes (1–5).

Results and Discussion

Synthesis and Characterization of Selenoether-Containing Complex 5

The synthesis of complex 5 was performed according to the scheme shown in Scheme 1. Styrenyl selenoether ligand precursor 6 was obtained from salicylaldehyde in 3 steps: One bromide side of 1,2-dibromoethane was subjected to Williamson ether synthesis to yield compound 7, which was subjected to a Wittig reaction to prepare compound 8. Thereafter, a selenium atom was incorporated into the bromide moiety in compound 8, followed by in situ methylation to prepare styrenyl selenoether 6. Complex 5 was obtained through the reaction of compound 6 with Grubbs second-generation complex (G-II) in the presence of CuCl as a phosphine scavenger^5,20 and characterized using nuclear magnetic resonance (NMR) spectroscopy (Figures S1–S3) and mass analysis.

Scheme 1. Synthesis of Complex 5 Starting from 1,2-Dibromoethane and Salicylaldehyde.

The ¹H NMR spectrum of complex 5 (Figure S1) showed the benzylidene proton signal at 16.3 ppm and the Se-methyl proton signal at 1.63 ppm. The Se-methyl proton signal was shifted upfield by 0.45 ppm compared to the Se-methyl protons in compound 6. These spectral features are similar to those observed in S-containing complex 4,¹⁸ suggesting that complex 5 has a trans-configuration in the geometry of chlorido ligands, with Se-coordination to the metal center, akin to complex 4. The Se-coordination was also suggested by the ⁷⁷Se-NMR spectrum with a signal at 114.2 ppm (Figure S3), which was shifted downfield from the signal observed in compound 6 (79.5 ppm). The X-ray crystallographic structure of complex 5 (Figure 1 and Table S1) confirmed Se-coordination with a Ru(1)–Se(1) bond distance of 2.7875(5) Å. The bond length is larger than the typical Ru–Se distances (∼ 2.45 Å) recorded in the Cambridge Crystallographic Data Center (CCDC) database, indicating a relatively weak Se-coordination in complex 5. The trans-configuration observed in complex 5 sharply contrasts with a previously reported Se-coordinating Hoveyda-Grubbs-type complex, where the substitution of the phenolic oxygen in HG-II (1) with a selenium resulted in the cis-configuration.^11,19

Figure 1.

X-ray crystallographic structure of complex 5. Thermal ellipsoids are drawn at the 50% possibility level. Selected bond lengths: Ru(1)–Se(1) 2.7875(5) Å; Ru(1)–C(1) 1.866(3) Å; Ru(1)–O(1) 2.236(1) Å.

Ligand Exchange Reactions between HG-II (1) and Selenoether Ligand 6

Next, we investigated the effect of Se coordination on the relative thermodynamic stability of complex 5 against HG-II (1) using the stoichiometric ligand exchange reactions between HG-II (1) and ligand 6 (Scheme 2).^17,18 Yields of over 50% for complex 5 are equivalent to the higher thermodynamic stability of the complex than the parent HG-II (1). Monitoring the reaction using ¹H NMR over 48 h (Figures S4 and S5 ,Table S2) showed the formation of complex 5 in 95% yield along with the production of styrenyl ligand 9. This fact indicates that Se coordination occurs in solutions as well as in the solid state, leading to the higher thermodynamic stability of complex 5 than HG-II (1).

Scheme 2. Ligand Exchange between HG-II (1) and Styrenyl Selenoether 6 to form Complex 5 and Styrenyl Ligand 9.

OM Catalytic Activities

Our previous report showed that sulfur-coordinated complex 4 exhibits the catalytic reactivity of olefin metathesis despite a saturated coordination structure around the metal center.¹⁸ Furthermore, its catalytic activity in the presence of protic solvents is higher than that of HG-II (1). Accordingly, we investigated the OM catalytic activity of complex 5 using the ring-closing metathesis (RCM) of N-tosyldiallylamide (TDA, 10), a commonly used benchmark reaction for evaluating the reactivities of OM catalysts (Tables 1 and S3). The RCM reaction in 1,2-dichloroethane (1,2-DCE) was monitored by ¹H NMR spectroscopy (Figures S6 and S7). To examine the tolerance of complex 5 to protic media, the reactions were also conducted in the presence of methanol (Figures S8 and S9).

Table 1. RCM Reactions of TDA (10) Mediated by Ruthenium Catalysts^a.

			yield(%)
entry	catalyst	MeOH(%(v/v))	11	12
1	1	0	93 (0.3 h)b, > 99 (6 h)b,c	N.D.d
2	3	0	89 (0.3 h)b, 99 (6 h)b,c	N.D.d
3	4	0	53 (0.3 h)b, 86 (6 h)b,c	N.D.d
4	5	0	12 (0.3 h), 95 (48 h)c	N.D.d
5	1	10	38 (12 h)b,c	trace (12 h)
6	3	10	19 (12 h)b,c	trace (12 h)
7	4	10	59 (48 h)b,c	7 (48 h)
8	5	10	47 (48 h)c	10 (48 h)

^areaction conditions: [10] = 42 mM and [complex] = 0.042 mM (0.1 mol % catalyst load) in 1,2-dichloroethane (1,2-DCE) at 40 °C.

^bquoted from ref.¹⁸

^cyield when a reaction plateaued.

^dnot detected.

In the absence of methanol, the complex 5-mediated RCM reaction proceeded with a slower rate compared to other complexes (as seen by the yields at 0.3 h in entry 4 of Table 1), although the RCM product 11 ultimately reached a yield of ca. 90% at 48 h without forming cycloisomerized byproduct 12. All investigated catalysts displayed a final RCM product yield of over 90% (entries 1–4 in Table 1). Additionally, in the presence of methanol, the 5-catalyzed RCM reaction was slower than those mediated by other complexes (entry 8 vs entries 5–7). However, the final RCM product yield was higher than that in 1- or 3-catalyzed reactions (47% for 5 (entry 8); 38% for 1 (entry 5); and 19% for 3 (entry 6). The higher tolerance of complex 5 to methanol compared to complexes 1 and 3 was attributed to the Se-coordination similar to S-coordinated complex 4. The coordinative selenoether ligand can protect the reactive intermediate from catalyst deactivation during the catalytic cycle as a release–return ligand (i.e., so-called “boomerang effect”).²¹ On the other hand, Se-coordinated complex 5 showed a lower RCM product yield compared to S-coordinated complex 4. The byproduct yield increased slightly (7% for 4 and 10% for 5).

Apart from the “boomerang effect” during the catalytic cycle, another possible reason for the higher tolerance of complex 5 to methanol is its intrinsic stability in MeOH-containing media. Previously, we have compared intrinsic stabilities of HG-II (1), O-coordinating complex 3, and S-coordinating complex 4 in MeOH-containing media monitoring the changes in the ultraviolet–visible (UV–vis) spectra of these complexes without olefinic substrate molecules,¹⁸ where the decrease in the metal-to-ligand charge transfer (MLCT) absorption band was observed. Accordingly, we monitored the UV–vis spectral changes of the complexes in MeOH-containing solution (Figure S10a– d) and compared the absorbance change at the maximum wavelengths of the MLCT bands (Figure S10e). Complex 5 exhibited an MLCT band at 390 nm. The wavelength is close to that of complex 4 (386 nm) and red-shifted from those of complexes 1 and 3 (1: 374 nm, 3: 370 nm) because of Se coordination. As shown in Figure S10e, all of the complexes showed a decrease in the absorbance of the MLCT band in MeOH-containing solutions over 24 h; however, the extent of the absorbance changes was similar among the four complexes. This fact indicates that these complexes have similar intrinsic stabilities in MeOH-containing solution. Namely, the increased tolerance of complex 5 in MeOH-containing solutions is mainly due to participation of the released Se-containing styrenyl ligand in the catalytic cycle.

Moreover, we also examined the RCM of diethyl diallylmalonate (DDM (13)) and cross metathesis (CM) between 3-butenylbenzoate (14) and methyl acrylate (15) to investigate whether the reactivity tendency presented above is observed for other substrates and types of olefin metathesis.

In the RCM of DDM (Tables 2 and S4 Figure S11), all the catalysts produced RCM product 16 with more than 90% yields in the absence of methanol, where no isomerized product 17 was observed (Entries 1–4 in Table 2). Complexes 4 and 5 showed slower reaction velocities compared to those of complexes 1 and 3, which is similar to that observed in the RCM reactions of TDA (10). In the presence of methanol, complexes 1 and 3 showed the decreased yields of the RCM product (>99% → 41% for 1; > 99% → 22% for 3), along with the formation of considerable amounts of isomerized compound 17. Particularly, O-coordinated complex 2 produced more amount of isomerized compound 17 than RCM product 16. In contrast, complexes 4 (S-coordination) and 5 (Se-coordination) mainly afforded the RCM product, even in the presence of methanol. S-coordinating complex 4 showed the moderate decrease in the yields in MeOH-containing solution (>97% → 71%; entries 3 vs. 7 in Table 2), resulting in the highest yields among the studied catalysts. This fact reflects the complex tolerance to methanol, as observed in the RCM of TDA (10). Complex 5 also showed a higher yield of RCM product 16 than O-coordinated complex 2 because of the increased tolerance to methanol provided by Se coordination.

Table 2. RCM Reactions of DDM (13) Mediated by Ruthenium Catalysts^a.

			yield(%)
entry	catalyst	MeOH (%(v/v))	16	17
1	1	0	97 (6 h)b	N.D.c
2	3	0	>99 (1 h)b	N.D.c
3	4	0	97 (12 h)b	N.D.c
4	5	0	98 (12 h)b	N.D.c
5	1	10	39 (9 h)b	20 (9 h)b
6	3	10	22 (9 h)b	33 (9 h)b
7	4	10	71 (36 h)b	7 (36 h)
8	5	10	31 (30 h)b	8 (30 h)

^areaction conditions: [13] = 45 mM and [complex] = 0.225 mM (0.5 mol % catalyst load) in 1,2-dichloroethane (1,2-DCE) at 40 °C.

^byield when a reaction plateaued.

^cnot detected.

In the CM reactions of 3-butenylbenzoate (14) with excess methyl acrylate (15) (14: 15 = 1:2; Tables 3 and S5 Figure S12), the reactions in the absence of methanol (Entries 1–4 in Table 3) showed similar final product yields among the complexes and slow reaction rates of 4- and 5-catalyzed reactions, as observed in the above-presented OM reactions. In MeOH-containing media, complexes 4 and 5 afforded higher product yields compared to complexes 1 and 2. This is ascribed to the enhanced tolerance to methanol owing to the coordinative characteristic of the sulfur and selenium atoms in complexes 4 and 5.

Table 3. CM Reactions between 3-Butenylbenzoate (14) and Methyl Acrylate (15) and Mediated by Ruthenium Catalysts^a.

			yield(%)
entry	catalyst	MeOH (%(v/v))	18	19
1	1	0	79 (12 h)b,c	4 (12 h)b,d
2	3	0	71 (24 h)b,c	7 (24 h)b,d
3	4	0	81 (36 h)b,c	6 (36 h)b,d
4	5	0	79 (36 h)b,c	6 (36 h)b,d
5	1	10	22 (24 h)b,c	3 (24 h)b,d
6	3	10	16 (24 h)b,c	4 (24 h)b,d
7	4	10	36 (36 h)b,c	7 (36 h)b,d
8	5	10	34 (48 h)b,c	7 (48 h)b,d

^areaction conditions: [14] = 42 mM and in [15] = 84 mM in 1,2-DCE; 1 mol % catalyst load; at 40 °C.

^byield when a reaction plateaued.

^cE/Z > 20/1.

^dE/Z > 20/1.

In whole, complex 4 tends to afford higher OM product yields than complex 5 in the presence of methanol. In other words, the reactivities of chalcogen-atom-containing catalysts (i.e., complexes 3 (O-containing), 4 (S-containing), and 5 (Se-containing)) do not simply follow the order of the periodic table. However, the enhanced tolerance of complexes 4 and 5 to methanol demonstrates the utility of coordinative atoms at the terminus of the phenoxy ether moiety for increasing the final yields of RCM products in the presence of protic solvents.

Kinetic Analyses for the Initial Phase in Catalytic Cycles

The chalcogen atom coordination to the metal center in complexes 4 and 5 may influence the initial phase of the OM catalytic cycle, as the dissociation of the chalcogen atom is involved in the activation of the precatalyst form. To evaluate the effects of chalcogen atom coordination on the initiation of OM catalytic cycles, we performed kinetic analyses of the reaction of OM catalysts with n-butyl vinyl ether (BuVE),²²⁻²⁴ where the reaction produces a Fischer-type complex 20, a catalytically inactive form and enables us to analyze the initial step of the catalytic cycle. The transient spectra observed in the presence of excess BuVE and pseudo-first-order kinetic plots are shown in Figures S13 and S14, respectively. The pseudo-first-order rate constants (k_init) at 25 °C for the reaction of Ru complexes with BuVE (Table 4) indicated smaller values for complexes 4 and 5 compared to those for complexes 1 and 3. This is because the decoordination of the sulfur or selenium atom in the ligand is necessary for BuVE to access the metal center. The activation parameters of the reaction of Ru complexes with BuVE were determined by using Eyring plots shown in Figure S15. Complexes 1 and 3 exhibited more negative activation entropy (ΔS^‡) values compared to those of complexes 4 and 5. HG-II (1) is known to mainly initiate OM catalytic cycles via “interchange mechanism” (i.e., substrate-binding along with the dissociation of the benzylidene ligand in the transition state).²³⁻²⁵ Since complex 3 which contains an oxygen in the ligand moiety but no O-metal coordination), showed similar activation parameters to HG-II (1), the “inter-change mechanism” is the predominant pathway in the initial phase of 3-catalyzed OM reactions. In contrast, complexes 4 and 5 displayed larger ΔH^‡ and ΔS^‡ values, reflecting the increased contribution of ligand dissociation in the initial step of the OM catalytic cycles. This fact indicates that the introduction of a sulfur or selenium atom at the terminus of the phenoxy ether moiety shifts the favorable mechanism in this step from the “interchange” pathway to a “dissociative” pathway.

Table 4. Rate Constants (k_init) at 25 °C and Activation Parameters for the Reaction of n-Butyl Vinyl Ether (BuVE) with Ruthenium Complexes^a.

complex	kinitialb (M–1 s–1)	ΔH‡ (kJmol–1)	ΔS‡ (Jmol–1)
1	(1.6 ± 0.1) × 10–2	64 ± 4c (59 ± 5d)	–90 ± 10c (−78 ± 20d)
3	(4.6 ± 0.3) × 10–2	59 ± 6c	–96 ± 15c
4	(3.2 ± 0.2) × 10–4	79 ± 3c	–51 ± 9c
5	(1.8 ± 0.1) × 10–4	80 ± 8c	–50 ± 9c

^areaction conditions: [Ru complex] = 100 μM; in 1,2-dichloroethane (DCE).

^brate constants obtained by pseudo-first-order kinetic analysis under excess BuVE ([BuVE] > 500 × [Ru complex]) at 25 °C.

^caverage of three repeating experiments.

^dvalues in CH₂Cl₂ (quoted from ref;²³ converted into joule unit).

Conclusion

In conclusion, the HG-II-type complex with a selenoether moiety in the benzylidene ligand exhibits the Se-coordination in its precatalyst form. The structural features result in the enhanced thermodynamic stability of the complex and greater tolerance to protic solvents compared to HG-II (1). A series of experimental results indicate that introducing coordinative atoms at the phenoxy ether moiety allows us to optimize OM reactivity, particularly in protic media, and to effectively regulate the reaction pathway in catalytic cycles. Since the solubilities of HG-type complexes in protic solvents can be increased by introducing ionic groups into the NHC ligand,^8,9,26,27 the findings of this study indicate the potential of HG-type complexes with a coordinative group in the phenoxy moiety as experimental tools for chemical modification of biomolecules in protic media.

Methods

Materials

All chemicals were obtained from conventional commercial sources and used as received, unless otherwise noted. Hoveyda-Grubbs second generation catalyst (HG-II, 1) and Grubbs second generation catalyst (G-II) were purchased from Sigma-Aldrich. Ruthenium complexes 3 and 4 were synthesized by previously reported methods.¹⁸ Complex 5 and styrenyl selenoether ligand precursor 6 were synthesized according to the method described in the Supporting Information. N-Tosyldiallylamide (TDA, 10)²⁸ and 3-butenylbezoate²⁹ were prepared according to the reported method. For silica gel chromatography, Wakosil C-200 (60–200 mesh) or Shodex purif-pack SI50 series were used.

Instruments

¹H NMR (400 MHz), ¹³C NMR (100 MHz), and ⁷⁷Se-NMR (76.2 MHz) spectra were collected by using a JEOL JNM-ECP 400 NMR spectrometer. ¹H NMR and ¹³C NMR chemical shifts are reported in ppm relative to tetramethylsilane (TMS). ⁷⁷Se-NMR chemical shifts were determined based on diphenyl diselenide (Ph–Se–Se-Ph. δ_Se = 480 ppm). ESI-MS analyses were carried out using a JEOL JMS-T100LC mass spectrometer. EI- and CI-MS analyses were conducted by using a JEOL JMS-700 MStation spectrophotometer. UV–vis spectra were measured using a Shimadzu UV-2550 spectrophotometer equipped with a thermostated folder. X-ray crystallographic data were collected on a Rigaku AXIS-RAPID Imaging Plate diffractometer and analyzed by using SHELXL-2019/2.

X-ray Crystallographic Analysis of Complex 5

A single crystal of complex 5 was prepared by vapor diffusion of hexane into a CH₂Cl₂ solution at 5 °C. X-ray diffraction was collected at 125 ± 0.1 K. The crystal data, diffraction collection details, and structure refinement are given in Table S1. The crystallographic data were deposited in Cambridge Crystallographic Data Center (CCDC).

Ligand Exchange Reactions between HG-II (1) and Ligand 6

Sample preparation was conducted in a N₂-filled glovebox. HG-II (1), ligand 6, and hexamethyldisiloxane (HMDSO, internal standard) were dissolved in 600 μL of dried/degassed CD₂Cl₂ ([1] = [ligand] = 50 mM; [HMDSO] = 5 mM in final). The solution was transferred into an NMR tube with a J. Young screw cap, and the reaction at 25 °C in the dark was monitored by ¹H NMR measurements with presaturation technique for the undeuterated protons in solvent. The reactants and products were quantified using eq 1 shown below:

1

, where

C_rxn: concentration of a reactant or product

I_rxn: NMR signal intensity of the monitored peak for a reactant or product (see below)

I_IS: NMR signal intensity of the peak from an internal standard

H_IS: the number of protons in an internal standard

H_rxn: the number of protons in the monitored peak for a reactant or product

C_IS: concentration of an internal standard.

In this case, H_IS = 18 and C_IS = 5 mM. The peaks observed for reactants are as follows: HG-II (1) (4.87 ppm, 1H, (CH₃)₂CHO−); complex 5 (1.63 ppm, 3H, CH₃SeCH₂CH₂O−); ligand 6 (2.92 ppm, 2H, CH₃SeCH₂CH₂O−); ligand 9 (4.55 ppm, 1H, (CH₃)₂CHO−). The material balance was checked throughout the reaction by monitoring the sum of [HG-II (1)] + [product complex 5] and [reacting ligand 6] + [product ligand 9]. The experimental data are presented based on the average of experiments repeated three times.

Ring-Closing Metathesis (RCM) of TDA (10)

In a 20 mL Schlenk tube, TDA (10) (42 mM) was dissolved in dry/degassed 1,2-dichloroethane (1,2-DCE) or methanol-containing 1,2-DCE (10% MeOH(v/v)). After a Ru catalyst was added (0.1 mol % load), the RCM reaction was conducted at 40 °C in the dark. At defined times, an aliquot of solution (100 μL) was sampled, and ethyl vinyl ether was added to quench the reaction. Thereafter, the solvent was evaporated, and 600 μL of dimethylsulfone (DMS; an internal standard; δ_H = 2.99 ppm) in CDCl₃ (3 mM) was added. The solution was subjected to ¹H NMR measurements to quantify the reaction components based on eq 1. In this case, H_IS = 6 and C_IS = 3 mM. The monitored peaks for the products are as follows: RCM product 11 (4.12 ppm, 4H, -NCH₂-CH = ), and isomerized product 12 (1.04 ppm, 3H, > CH-CH₃).³⁰ The experimental data are presented based on the average of experiments repeated three times.

Ring-Closing Metathesis (RCM) Reactions of DDM (13)

Sample preparation and reactions were conducted in the same manner as described above except for the use of 0.5 mol % catalyst load. After sampling an aliquot of solution (100 μL) at defined time, the solution was diluted with 500 μL of CDCl₃ that contains HMDSO (internal standard, 0.234 mM in final), which was subjected to ¹H NMR measurements with the presaturation technique for signals derived from 1,2-DCE (3.74 ppm). The quantification of DDM and products was conducted based on eq 1, where H_IS = 18 and C_IS = 0.2 mM. The monitored peaks for products are as follows: RCM product 16 (3.00 ppm, 4H, (EtO₂C)₂-C-(CH₂-CH = )₂); isomerized product 17 (1.08 ppm, 3H, > CH-CH₃).³⁰

Cross Metathesis (CM) Reactions between 3-Butenylbenzoate (14) and Methyl Acrylate (15)

Sample preparation, reactions, and quantification of reaction components were conducted in the same manner as for the RCM reactions of DDM (13). The concentration of a Ru catalyst was set to 1 mol % of DDM concentration. The monitored peaks for products are as follows: hetero-CM product 18 (4.44 ppm, 2H, -OCH₂-CH₂–CH = ), and self-CM product (derived from 15) 19 (4.32 ppm, 4H, -OCH₂-CH₂–CH = ).^31,32

Kinetic Measurements of Initial Phases in Olefin Metathesis reactions²²⁻²⁴

Sample preparation was conducted in a N₂-filled glovebox. A solution of a ruthenium catalyst in 1,2-DCE was charged into a quartz cell equipped with a septum cap and incubated at the desired temperatures. After a solution of n-butyl vinyl ether (BuVE) was added to the Ru complex solution with a syringe, the absorbance in the metal-to-ligand charge transfer (MLCT) band (378 nm for HG-II (1); 372 nm for 3; 391 nm for 4; 395 nm for 5) was monitored. The final concentrations of the reactants were set as follows: [Ru complex] = 2 × 10^–4 M and [BuVE] = 0.005 M–0.3 M (for HG-II (1) and 3), 0.05 M 0.3 M (for 4), or 0.05 M–0.6 M (for 5), where pseudo-first-order kinetic conditions were kept. The changes in the absorbance were analyzed by eq 2:

2

where k_obs is an apparent pseudo-first-order rate constant and k_initial is the rate constant of the initial phase in the catalytic cycle.

At the lowest concentrations of BuVE, the transient spectra in the range from 250 to 700 nm were also recorded to confirm UV–vis spectral changes with isosbestic points. To obtain activation parameters, pseudo-first-order kinetic constants at constant BuVE concentrations were plotted against the reciprocal of temperatures (Eyring plot) and analyzed using eq 3:

3

where T is temperature; ΔH^‡is the activation enthalpy; R is the gas constant; k_B is Boltzmann constant; h is Planck constant; ΔS^‡ is the activation entropy. The experimental data are presented based on the average of experiments repeated three times.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c09137.

• Crystallographic data for complex 5(CIF)

• Synthetic procedures, X-ray crystallographic data, NMR spectra of complex 5, NMR spectral changes during ligand exchange, quantification of ligand exchange and olefin metathesis reactions, UV–vis spectra, and kinetic analyses (PDF)

Author Contributions

T.K., S.M., and N.O.: Investigation; Resources; Visualization; Writing–review and editing; T.M.: Conceptualization; Funding acquisition; Investigation; Project administration; Resources; Supervision; Visualization; Writing–review and editing.

The authors declare no competing financial interest.