Topics in Catalysis (2022) 65:433–447 https://doi.org/10.1007/s11244-021-01534-w ORIGINAL PAPER Effect of Lewis Acids on the Catalyst Activity for Alkene Metathesis, Z‑/E‑ Selectivity and Stability of Tungsten Oxo Alkylidenes J. Haydée Merino1 · Jesús Bernad1 · Xavier Solans‑Monfort1 Accepted: 9 November 2021 / Published online: 27 November 2021 © The Author(s) 2021 Abstract Lewis acids increase the catalytic activity of classical heterogeneous catalysts and molecular ­d0 tungsten oxo alkylidenes in a variety of olefin metathesis processes. The formation of labile adducts between the metal complex and the Lewis acid has been observed experimentally and suggested to be involved in the catalyst activity increase. In this contribution, DFT (M06) calculations have been performed to determine the role of Lewis acids on catalyst activity, Z-/E- selectivity and stability by comparing three W(E)(CHR)(2,5-dimethylpyrrolide)(O-2,6-dimesithylphenoxide) (E = oxo, imido or oxo-Lewis acid adduct) alkylidenes. Results show that the formation of the alkylidene—Lewis acid adducts influences the reactivity of tungsten oxo alkylidenes due to both steric and electronic effects. The addition of the Lewis acid on the E group increases its bulkiness and this decreases catalyst Z-selectivity. Moreover, the interaction between the oxo ligand and the Lewis acid decreases the donating ability of the former toward the metal. This is important when the oxo group has either a ligand in trans or in the same plane that is competing for the same metal d orbitals. Therefore, the weakening of oxo donating ability facilitates the cycloaddition and cycloreversion steps and it stabilizes the productive trigonal bipyramid metallacyclobutane isomer. The two factors increase the catalytic activity of the complex. The electron donating tuneability by the coordination of the Lewis acid also applies to catalyst deactivation and particularly the key β-hydride elimination step. In this process, the transition states show a ligand in pseudo trans to the oxo. Therefore, the presence of the Lewis acid decreases the Gibbs energy bar- rier significantly. Overall, the optimization of the E group donating ability in each step of the reaction makes tungsten oxo alkylidenes more reactive and this applies both for the catalytic activity and catalyst deactivation. Keywords Olefin metathesis · Density functional theory · Catalyst deactivation · Metal alkylidenes · Lewis acid 1 Introduction systems enhances their catalytic activity through a process that it is not well understood. Olefin metathesis is a key reaction in organic synthesis that In 1971, Hérrison and Chauvin proposed the today’s implies the exchange of alkylidene substituents between alk- accepted reaction mechanism (Scheme 1) [2, 10]. It states enes. It has been applied to the synthesis of a large variety that metal alkylidenes are the catalytic active species and of molecules including raw materials, polymers, and drugs metallacyclobutanes the key intermediates. Moreover, the [1–8]. The reaction only takes place in presence of a cata- catalytic cycle implies two olefin metathesis processes, lyst. Silica and alumina supported molybdenum and tung- each one composed of a cycloaddition and a cyclorever- sten oxides are within the first catalyst precursors and they sion step. The reaction mechanism proposed by Chauvin have been applied in industrial applications util today [9]. paved the way for the synthesis of metal alkylidenes. Some The active species operating in these systems has not been of the first active well-defined molecular catalysts were characterized in detail, but they are thought to be metal oxo tungsten oxo alkylidenes and oxo alkyl complexes that act alkylidenes. Remarkably, addition of Lewis acids to these as precursors of the metal alkylidene (1–4 in Scheme 2) [11–14]. These complexes are highly active towards ole- * Xavier Solans‑Monfort fin metathesis and their catalytic activity can usually be Xavier.solans@uab.cat increased by adding Lewis acids in the media as in the case of classical heterogeneous catalysts. However, they 1 Departament de Química, Universitat Autònoma de deactivate fast and this, together with the development Barcelona, 08193 Bellaterra, Spain 13 Vol.:(0123456789) 434 Topics in Catalysis (2022) 65:433–447 O tBu O tBu R1 PR3 X W CH tBu CH2 W CH2 R1 R2 X PR3 H2C X + + tBu 1 X = Cl 3 X = Cl 2 X = OtBu 4 X = OtBu R2 iPr iPr iPr iPr N tBu N tBu N M CH RO M CH R2 RO ArO R1 5Mo M = Mo; R = C(CH3)2(CF3) 8Mo M = Mo; Ar = OTPPa 5W M = W; R = C(CH3)2(CF3) 8W M = W; Ar = OTPPa M 6Mo M = Mo; R = C(CH3)(CF3)2 9Mo M = Mo; Ar = HIMTOb 6W M = W; R = C(CH3)(CF3)2 9W M = W; Ar = HIMTOb 7Mo M = Mo; R = C(CF3)3 10Mo M = Mo; Ar = HIPTOc R1 R1 7W M = W; R = C(CF3)3 10W W = W; Ar = HIPTOc M M L1 Cl O tBu N Ru CH W CH R2 L2 Cl Ph ArO L M R1 11 L1 = L2 = P(Cy)3 13 L = PMe2Ph, R1 12 L1 = H2IMes;d Ar = HIMTOb L2 = P(Cy)3 14 L = none, Ar = HIPTOc R2 Mes O tBu N O Me2Ph Scheme 1  Chauvin’s mechanistic proposal ArO W CH N W CH Mes ArO RO 15 Ar = HIMTOb 17 R = C(CH3)(CF3)2 of ­d 0 imido alkylidene complexes (5 Mo -10 Mo and 5 W- 16 Ar = dAdPOe 18 R = 2,6-Ph2(C6H3) 10W) [15–25] as well as ruthenium carbenes (i.e. 11 and 12) [26–34] prevented its further development since last Mes tBu O tBu O tBu N decade. W O Me2Ph W CH N In 2011, Schrock and co-workers reported the synthesis HIMTO O Mes W CH O tBu of a series of tungsten oxo alkylidenes bearing a pyrrolyl Si O Si Si and a large alkoxy ancillary ligand (13 and 14 in Scheme 2) [35]. These complexes are efficient and highly Z-selective 19 20 21 olefin metathesis catalysts particularly on the homocoupling of terminal olefin and the ring opening metathesis polym- O CF3 O tBu Cl erization (ROMP) of substituted norbornenes. In the subse- Mo CF3 O Mo CH O quent years, the Schrock group reported other tungsten oxo HIPTO O L CF3 Si alkylidenes and the role of ancillary ligands on the catalytic activity and product selectivity were deeply discussed (i.e. 22 23 15 and 16) [36–42]. Moreover, Buchmeiser and co-work- ers synthetized a series of very active cationic tungsten Scheme 2  Selected olefin metathesis catalysts and precursors. oxo alkoxy alkylidene bearing an N-heterocyclic carbene a OTPP = 2,3,5,6-Ph4(C6H); bHIMTO = 2,6-Mes2(C6H3); cHIPTO =​ ​ (NHC) as fourth ligand (17–18) [43–45]. Both Schrock and 2,6​-(2​,4,​6-i​Pr​3(​C6​H​2)​)2​(C​6​H3​);dH2IMes = 1,3-Mes2-2-imidazolidi- Buchmeiser tungsten oxo alkylidenes were heterogenized nylidene; edAdPO = 2,5-Ad2-4-CH3(C6H2) by grafting on silica (19–20) [46–48]. Catalyst heterogeni- zation prevented bimolecular deactivation and, as a conse- quence, the resulting complexes show very high catalytic tungsten oxo alkyl complexes that act as alkylidene precur- activities and stabilities overperforming the imido analogues sors (21 in Scheme 2) [49–51]. particularly when reacting with internal olefins. These high The knowledge acquired on tungsten oxo alkylidenes activities and stabilities are also reported for silica grafted allowed the synthesis of molybdenum and vanadium oxo 13 Topics in Catalysis (2022) 65:433–447 435 Olefin metathesis pathway (Scheme 3 for the homocoupling reaction of ethene) E + E E E - E [66, 68, 69]. This implies the inversion of the configu- X X X X W W W W W X ration of the stereogenic metal center [72]. Moreover, Y Y Y Y the square based metallacyclobutane isomer (SBP) is a Y I II III IV V=I resting state of the catalytic process which is involved in unimolecular deactivation through β-hydride elimination, a process taking place trans to the weakest σ-donating E E H + E ligand [60, 68–70]. Since X and Y ligands play a different X X Catalyst deactivation X W W W role, complexes with different X and Y ligands can show Y Y Y higher catalytic activities than species with X = Y. Strong A B C σ-donor X ligands favor alkene coordination and imido E (in comparison with oxo and alkylidyne E ligands) and X W weak σ-donor Y ligands stabilize the metallacyclobutane Y intermediate [60, 68–70]. The overall reactivity is a bal- D ance of these two effects. Catalyst stability with respect β-hydride elimination also depends on the nature of E, X Scheme 3  Olefin metathesis and unimolecular catalyst deactivation and Y. β-hydride elimination is hampered when replacing reaction mechanisms alkyl as X ligand by pyrrolyl and the imido ligand by the stronger donor oxo group [60, 69, 70]. analogues as well as other molybdenum precursors that are In this contribution, we perform DFT calculations to get also active in metathesis (22, 23) [40, 52–58]. The number electronic insights on the role of Lewis acids on the catalytic of existing molybdenum oxo alkylidenes is limited and their activity and Z-/E- selectivity of tungsten oxo alkylidenes as activity for olefin metathesis is usually lower than that of well as their stability toward unimolecular decomposition tungsten species. through β-hydride elimination. For that, we compared the With the aim of bridging the gap between the activity reactivity, selectivity, and stability of three existing com- of classical heterogeneous catalyst and the most recently plexes bearing the same ancillary ligands except the doubly reported well-defined molecular tungsten and molybdenum bonded (E) (9W, 13 and 13·B(C6F5)3 Scheme 2). alkylidene, the effect of Lewis acids on the well-defined molecular complexes has been recently analyzed [35, 59]. Addition of B(C6F5)3 as Lewis acid leads to the formation 2 Computational Details of a labile compound in which the Lewis acid is bonded to the oxo ligand (i.e. 13·B(C6F5)3). Moreover, an increase 2.1 Level of Theory on the catalytic activity both towards terminal olefins and the ROMP of substituted norbornenes is usually observed. All calculations are performed with the Minnesota M06 Remarkably, this activity increase is also commonly associ- hybrid density functional [73, 74]. This functional has been ated with a decrease of the Z-selectivity and more impor- shown to reproduce the experimental trends for systems tantly catalyst stability. Analysis of the formed by-products where weak interactions are important and this includes shows the formation of propene and metallacyclopentene the study of the olefin metathesis reaction [75–77]. Geom- intermediates, suggesting that the deactivation occurs etry optimizations are performed representing main group through β-hydride elimination and metal reduction without elements with the 6-31G(d,p) basis sets [78, 79]. Tungsten the reaction of additional ethene molecules as suggested for is represented with the small core Stuttgart pseudopoten- silica supported rhenium alkyl alkylidene alkylidyne com- tial together with the associated basis set enlarged with plexes [60]. a f polarization function (α = 0.823) [80, 81]. The nature Theoretical studies on ­d0 olefin metathesis have con- of the stationary points (minima or transition states) is tributed to the understanding of the electronic structure verified by vibrational analysis. Moreover, IRC calcula- of metal-alkylidenes and metallacyclobutane intermedi- tions are performed to determine the minima connected ates as well as the factors controlling catalyst activity and through 13·B(C6F5)3-UU-TSII, 13·B(C6F5)3-UU-TSIII, deactivation [60–71]. Today, it is well accepted that ole- 13·B(C6F5)3-TSAB, 13·B(C6F5)3-TSBC and 13·B(C6F5)3- fin coordination occurs trans to the strongest σ-donating TSBD. We assume that for all other systems and orienta- ancillary ligand and the metallacyclobutane involved tions the connected minima are equivalent based on the in the reaction pathway presents a trigonal bipyramid geometrical similarities found within all transition states. structure (TBP) with the doubly bond ligand (E) and The final energetics are obtained by single point calculations the weakest σ-electron donor ligand in apical positions with the larger 6–311 + G(d,p) basis sets [82, 83] for main 13 436 Topics in Catalysis (2022) 65:433–447 group elements and the same representation of tungsten. In 3 Results and Discussion these single point calculations, solvent effects are included using the SMD continuum model [84] and toluene as sol- The aim of the present study is to give atomistic insights vent. Energies reported along the text are based on Gibbs on the effect of Lewis acids on tungsten oxo alkylidenes’ energies (Gsolv = Ggp + ∆Gsolv, where Ggp is the gas phase catalytic activity for olefin metathesis, their Z-/E- selec- free energy and ∆Gsolv stands for the solvation free energies) tivity and their stability. For that, we compare the results at 298.15 K and 1 atm. The thermal corrections are included obtained for 13-I and 9W-I ethylidene complexes as well as at the smallest basis sets. All calculations are performed with the oxo complex interacting with two Lewis acids B(C6F5)3 the Gaussian09 package [85]. (13·B(C6F5)3-I) and ­BF3 (13·BF3-I), see Scheme 2. First, we focus on the structure of the oxo complexes with the 2.2 Model aim of validating the computational approach with existing experimental data [35]. After that, we study the productive The Z-/E- selectivity of tungsten oxo and imido olefin pathways for the propene conversion to Z- or E-2-butene metathesis catalysts is analyzed by considering the 2-butene catalyzed by 13-I, 9W-I, 13·B(C6F5)3-I or 13·BF3-I. Finally, formation by reaction of propene with ethylidene of com- we discuss catalyst stability towards β-hydride elimination plexes 9W, 13 and 13·BX3 (9W-I, 13-I and 13·BX3-I, respec- [59, 60]. The nomenclature is constructed by three parts: the tively). In this context, the Z-/E- selectivity arises from the arabic number that indicates the catalyst precursor according relative feasibilities of the four potential productive path- to Scheme 2, a roman number or a capital letter that indi- ways (Scheme 4). This approach was used before with great cates the reaction intermediate following Scheme 4 (roman success when studying the Z-/E- selectivity of ruthenium number for the productive process and the capital letter for complexes [86]. The tungsten complexes are represented the catalyst deactivation) and a combination of two U or with the full bulk of the ancillary ligands (oxo or N-2,6iPr- D capital letters that indicate the orientation of the methyl C6H3, 2,5-dimethylpyrrolyl and aryloxy ligands). The initial groups of the alkylidene and reacting olefin. U indicates that ethylidene species present two different isomers: syn with the methyl points towards the E group and D that the methyl the methyl of ethylidene towards the oxo and anti with the points towards the aryloxy ligand. The first letter refers to the methyl of the ethylidene away from the oxo and both are substituent of the initial alkylidene while the latter refers to taken into account in the calculations. Moreover, two rep- the methyl group of propene (Scheme 4). resentations of the Lewis acid are considered: the experi- mentally used B(C6F5)3 and B ­ F3 which mainly accounts for 3.1 Methodology Validation electronic effects. The interaction of a Lewis acid with tungsten oxo alky- lidenes is labile according to experiments [35]. Thus, we decided to evaluate the ability of the methodology used to reproduce experimental data of complexes 13 and 14 X E X E E X - E (Scheme 2) and particularly the following three observations W W W W X [35]: (i) the synthesis of complex 13 leads to a pentacoordi- + Y Y Y Y nated with a coordinated phosphine as fifth ligand; (ii) the E II-UU III-UU IV-UU V addition of two equivalents of B(C6F5)3 in a solution con- W X Y taining 13 leads to the formation of the 13·B(C6F5)3 adduct E E X- E syn-I X X W E in which the Lewis acid is labile: and (iii) Substitution of W W W X + Y Y Y 2,6-dimesithylphenoxide (HIMTO) ligand by the bulkier Y II-UD III-UD IV-UD V O-2,6-(2,4,6-iPrC6H2)2C6H3 (HIPTO) ligand allows the iso- lation of tetracoordinated syn-W(O)(CHtBu)(2,5-Me2Pyr) E E E X E X W X - (HIPTO) complex (14). Table 1 compares the computed W W W X + Y Y Y and X-Ray diffraction determined geometry parameters and Y E II-DU III-DU IV-DU V Fig. 1 shows a detailed view of the optimized structures X W computed for 13, 14 and related adducts. Y E E The syn-13 No-PMe2Ph alkylidene presents a distorted X - anti-I E X E X W W W W X tetrahedral coordination around tungsten with a short + Y Y Y Y W=O (1.696 Å). The O=W=C angle is 105.0° and the II-DD III-DD IV-DD V W=C–H angle suggests the presence of an α–CH agostic interaction [65]. The anti isomer presents the same metal Scheme 4  Reaction pathways considered in this work 13 Topics in Catalysis (2022) 65:433–447 437 Table 1  DFT (X-ray) geometry parameters (distances in Å and angles the ethylidene complexes 13-I, 9W-I, 13·B(C6F5)3-I and in degrees) for syn-13, syn-13·B(C6F5)3and syn-14 13·(BF3)3-I. Table 2 summarizes the relative Gibbs energies syn-13 syn-13·B(C6F5)3 syn-14 with respect to separated reactants N-I-U + propene follow- ing the labelling of Schemes 2 and 4 and the relative reaction W=O 1.716(1.717) 1.757(1.759) 1.697(1.695) rates with respect to the UU route. The optimized structures W=Cene 1.894 (1.900) 1.876(1.868) 1.887(1.886) can be found in the Supplementary Information. W–NPyr 2.062(2.074) 1.985(1.968) 2.011(2.001) The propene metathesis occurs through today’s accepted W–OAr 1.984(1.964) 1.870(1.860) 1.893(1.868) reaction mechanism that involves the formation of an olefin W–P 2.667(2.580) complex, a cycloaddition step that leads to a metallacyclobu- O–B 1.536(1.571) tane intermediate with a trigonal pyramid structure and the W=Cene–Cβ 140.7(141.0) 142.3(155.4) 135.4(136.7) reverse cycloreversion and olefin decoordination steps [60, W–OAr–CAr 156.8(159.8) 166.3(150.9) 165.1(166.9) 68–70]. Moreover, the square based metallacyclobutane iso- W–O–B 169.4 mer is a resting state of the reaction. Remarkably, we could P–W–NPyr 153.9 not locate the transition state associated to olefin coordina- tion and olefin decoordination. This is indicative that the barrier likely arises from the entropic term and thus can not coordination without the α–CH agostic interaction and it be located by exploring the potential energy surface. This is less stable by 6.8 kcal ­mol−1. Coordination of P ­ Me2Ph −1 was highlighted in previous contributions [87, 88] and is to syn-13No-PMe2Ph is favorable by -1.5 kcal ­mol and the particularly relevant when dispersion forces are taken into resulting complex (syn-13) presents a trigonal bipyramid account at least partially. coordination around tungsten with apical phosphine and The computed energy barriers with the tungsten oxo eth- pyrrolyl ligands. The computed W-L distance are close to ylidene 13-I show that all steps are easy (within 1.5 and the X-Ray values (Table 1). Remarkably, the anti-13 is less 10.1 kcal ­mol−1) and thus all intermediates and transition stable by 10.6 kcal ­mol−1, thus suggesting that phosphine states lie at energies that are accessible at reaction condi- coordination is less favorable in anti-13No-PMe2Ph. Addition tions. The highest in Gibbs energy structure is 13-TSIII-DD of B(C6F5)3 forms syn-13·B(C6F5)3 in which the Lewis acid and it lies 24.4 kcal ­mol−1 over 13-I-U and propene. As interacts with the oxo group. The B···O distance is 1.532 Å found in model calculations [70], the reactive TBP metal- and this lengthens the W = O bond to 1.759 Å. The syn- lacycle is significantly higher in energy than the SBP isomer 13·B(C6F5)3 is -1.1 kcal ­mol−1 lower in Gibbs energy than (between 7.5 and 12.4 kcal ­mol−1) and this is rationalized syn-13No-PMe2Ph, in agreement with the characterization by the strong trans effect of the oxo ligand, which presents of syn-13·B(C6F5)3 and the labile behavior of B(C6F5)3 in a very short W = O distance (1.71 Å). Overall, the lowest solution. intermediate is separated reactants except for the UD path- Regarding complex 14, calculations suggest that the sub- way, where the SBP metallacycle (13-A-UD) is slightly stitution of the HIMTO ligand by the bulkier HIPTO stabi- lower in energy (-2.2 kcal ­mol−1). The highest in Gibbs lizes the syn- isomer with respect to the anti- one (Fig. 1). energy transition state is in all cases the cycloreversion step Moreover, they indicate that the largest bulkiness of the (13-TSIII), thus the energy span of the reaction is defined as metal complex makes the coordination of ­PMe2Ph unfavora- the energy difference between 13-A-UD and the cyclorever- ble by about + 4.3/ + 4.4 kcal ­mol−1 and this is consistent sion transition state (13-TSIII) and it varies between 17.1 with the isolation of the tetracoordinated complex of syn-14. and 26.7 kcal ­mol−1. Overall, calculations on the precursors show that the The energetics for the same processes catalyzed with methodology used in this work reproduces the structure of tungsten imido alkylidene (9W-I) present several similari- these complexes as well as the associated energetics. Par- ties with the reactivity of 13-I as well as remarkable dif- ticularly, calculations predict: i) the largest stability for the ferences. As for 13-I, the highest in energy transition state syn- isomer; ii) the marginally favorable phosphine coordi- is that associated to the cycloreversion (9W-TSIII), while nation to 13No-PMe2Ph but the unfavorable phosphine coordi- the global minimum is 9W-III-UU. Moreover, the computed nation to 14; and iii) the weak interaction between B(C6F5)3 energy spans are similar to those computed for 13-I albeit and the oxo group. marginally higher (between 18.1 and 25.6 kcal ­mol−1). In contrast, the relative stability between TBP and SBP metal- 3.2 Effect of Lewis Acids on Catalyst Activity lacyclobutane (III vs. A) isomers is inversed. That is, the reactive TBP isomer is more stable than the SBP resting The role of Lewis acids on the catalytic activity for ole- state for the imido complex by 0.8–10.3 kcal ­mol−1. This fin metathesis of tungsten oxo alkylidene complexes is indicates that most of the initial catalyst remains on the pro- discussed by comparing the propene metathesis with ductive pathway, and it does not accumulate on an isomer 13 438 Topics in Catalysis (2022) 65:433–447 that the two complexes should present similar reactivities in absence of catalyst deactivation. The presence of B(C6F5)3 as Lewis acid leads to the for- mation of 13·B(C6F5)3-I in which B(C6F5)3 is weakly inter- acting with the oxo group through a O-B bond of 1.571 Å. The formation of the syn-13·B(C6F5)3-I adduct is exergonic by -1.9 kcal ­mol−1 (-3.7 kcal ­mol−1 for the anti- isomer) and the O-B(C6F5)3 produces a weakening of the W = O bond (W = O distance elongation of about 0.05 Å), as in the case of the catalyst precursor. The reaction of 13·B(C6F5)3-I with propene shows that the formation of 13·B(C6F5)3 adduct is in general marginally exergonic for all intermediates and transition states with respect to the analogous structures of 13 and B(C6F5)3. The main exception is the SBP metalla- cyclobutane intermediate. In this case, although we find the adduct as a minimum of the potential energy surfaces, calcu- lations predict that the Lewis acid–metal complex formation is unfavorable by 2.7–6.4 kcal ­mol−1. The larger stabiliza- tion due to the presence of the Lewis acid is observed for the cycloreversion transition state. Consequently, the energy span defined by the difference between separated reactants (the global minimum of the reaction) and the transition state for either cycloaddition (TSII) or cycloreversion (TSIII) decreases significantly and becomes between 13.3 and 19.7 kcal ­mol−1 and, this agrees with the higher catalytic activity of tungsten oxo alkylidenes in presence of Lewis acids. With the aim of isolating the steric and electronic effects we considered ­B F 3 as a smaller model of Lewis acid. Results for 13·BF3-I are equivalent to those obtained with 13·B(C6F5)3: (i) the formation of the adduct for all interme- diates and transition states involved in the productive pro- cess is exergonic with respect to the analogues structures and separated ­BF3; (ii) The ­BF3-oxo interaction weakens the W=O bond; (iii) the B ­ F3-oxo group interaction is unfavora- ble for the SBP isomer; and (iv) the energy span decreases significantly, thus indicating that the presence of the Lewis acid increases the catalyst activity. Consequently, the effect of the Lewis acid has mainly an electronic contribution that is stronger at the cycloaddition and cycloreversion transition states as well as the at the TBP metallacyclobutane isomer Fig. 1  Detailed view of the optimized structures for syn- and anti- 13 but not on the SBP one. Remarkably, these are the species and 14 alkylidenes and related adducts as well as the relative energies in which the oxo ligand has either a ligand in trans or the ­ ol−1) with respect to the M (in kcal m ­ L4 species syn-13No-PMe2Ph or 14 oxo group is in the same plane of the alkylidene and ary- and either P ­ Me2Ph or B(C6F5)3 loxy ligands. This suggest that the higher catalytic activity of tungsten oxo alkylidenes in presence of a Lewis acid can be not involved in the olefin metathesis process. The stabiliza- mainly attributed to two factors: i) the decrease of the elec- tion of III can be rationalized by the lower trans effect of the tron donating ability of the oxo group, thus the trans influ- imido ligand with respect to the oxo group, while the desta- ence of the oxo ligand and ii) and increase of the electrophi- bilization of A is likely due to steric repulsion between the licity of the metal center when the N·BX3 adduct is formed large aryloxy and imido ligands. Overall, the small differ- as evidenced by the Natural Population Charges of tungsten ence in the energy spans of 9W-I and 13-I and the different in the initial ethylidene species ­(q13 = 1.39, ­q13·B(C6F5)3 = 1.63 relative stabilities of the metallacyclobutane isomers suggest and ­q13·BF3 = 1.51). The higher metal electrophilicity favors 13 Topics in Catalysis (2022) 65:433–447 Table 2  Relative Gibbs energies with respect to syn-N-I and propene (in kcal ­mol−1) of all intermediates and transition states associated with propene homocoupling reaction leading to either E- and Z-2-butene Complex Path I TSI II TSII III TSIV IV TSIV V A kAA/kUUa 13-I UU (Z-)a 0.0 – 8.4 10.5 7.6 14.8 10.9 – 1.2 0.1 1.0 UD (E-)a 0.0 – 12.2 15.0 8.1 18.0 14.0 – 0.3 − 2.3 4.5·10–3 DU (E-)a 3.4 – 12.1 13.9 10.4 20.5 14.4 – 0.3 2.2 6.6·10–6 DD (Z-)a 3.4 – 16.0 17.5 15.4 24.4 11.9 – 1.2 3.0 9.1·10–8 9W-I UU (Z-)a 0.0 – – – − 2.2 16.6 12.7 – 2.3 8.1 1.0 UD (E-)a 0.0 – – – 2.8 18.1 17.3 – 1.4 3.6 6.7·10–2 DU (E-)a 2.2 – – – 7.1 23.4 19.1 – 1.4 9.2 8.7·10–6 DD (Z-)a 2.2 – – – 4.7 23.1 20.5 – 2.3 8.8 1.4·10–5 13·B(C6F5)3-I UU (Z-)a 0.0 (− 1.9) – 10.9 (0.6) 13.3 (0.9) 10.0 (0.4) 11.6 (− 5.2) 7.1 (− 5.7) – 1.0 (− 2.1) 8.5 (6.4) 1.0 UD (E-)a 0.0 (− 1.9) – 9.6 (− 4.5) 15.6 (− 1.3) 0.7 (− 9.3) 11.5 (− 8.4) 10.7 (− 5.2) – 0.1 (− 2.1) 2.3 (2.7) 2.1·10–2 DU (E-)a 1.7 (− 3.7) – 11.9 (− 2.2) 13.4 (− 2.5) 7.7 (− 4.7) 19.7 (− 2.7) 13.8 (− 2.6) – 0.1 (− 2.1) 8.5 (4.4) 2.0·10–5 DD (Z-)a 1.7 (− 3.7) – 10.2(− 7.7) 16.0(− 3.4) 11.1 (− 6.2) 16.7 (− 9.6) 7.1 (− 6.7) – 1.0 (− 2.1) 8.3 (3.4) 3.2·10–3 13·BF3-I UU (Z-)a 0.0 (0.9) – − 0.6 (− 8.1) 3.1 (− 6.5) − 0.7 (− 7.5) 9.7 (− 4.2) 4.8 (− 5.2) – 4.3 (4.0) 3.4 1.0 UD (E-)a 0.0 (0.9) – 4.7 (− 6.6) 4.8 (− 9.3) − 1.0 (− 8.2) 10.5 (− 6.6) 8.0 (− 5.1) – 3.3 (4.0) 0.5 0.26 DU (E-)a 3.4 (0.9) – 6.6 (− 4.6) 9.8 (− 3.2) 1.2 (− 8.3) 10.7 (− 8.9) 8.8(− 4.7) – 3.3 (4.0) 7.6 0.18 DD (Z-)a 3.4 – 8.7 (− 6.4) 10.2 (− 6.4) 4.9 (− 9.6) 14.8 (− 8.6) 11.7 (0.7) – 4.3 (4.0) 7.0 1.8·10–4 [( ) ] − 𝛿GAA − 𝛿GUU ∕ RT a K Relative rate with respect to the UU pathway computed using the energy span model, assuming the global minima as the common most stable intermediate: K AA = e UU 13 439 440 Topics in Catalysis (2022) 65:433–447 the olefin coordination, while the decrease of the trans relative stabilities of the SBP metallacyclobutane species effect of the oxo ligand stabilizes the TBP metallacyclobu- does not have any effect on defining the Z-/E- selectivity, tane intermediate and the cycloaddition and cycloreversion since according to the energy span model this essentially transition states. Overall, the Lewis acid allows adapting the originates from the relative stabilities of the transition state electron donating ability of the oxo ligand in each step of the for cycloreversion. reaction and this leads to a smoother Gibbs energy profile. Overall, the Z-/E- selectivity is defined by the relative Gibbs energies of N-TSIII-UU and N-TSIII-UD, except for 3.3 Effect of Lewis Acids on Catalyst Z‑/E‑Selectivity 13·B(C6F5)3, which is defined by the relative Gibbs energies between 13·B(C6F5)3-TSII-UU and 13·B(C6F5)3-TSII-UD. The analysis of the Z-/E- selectivity of each catalyst implies For the case, of 13-I with a very small oxo ligand as E, the comparing the relative Gibbs energies and rates of the most energy difference is large (3.1 kcal ­mol−1), thus suggesting favorable pathway leading to Z-2-butene and that of the most a > 99% selectivity for the Z-product at least at the initial favorable pathway leading to E-2-butene (Table 2). For that, stages of the reaction (the rate of the UD pathway is 4.5·10–3 we first focus on how the Gibbs energies of each intermedi- times slower than that of the UU route). The Gibbs energy ate and transition state varies as function of the position of difference between the same transition states of the tungsten the methyl groups and then we discuss the energetics, in imido ethylidene analogue (9W-I) is smaller (1.7 kcal ­mol−1) terms of energy span, of the different pathways. suggesting a smaller selectivity that is related to the bulki- The relative stability of the olefin metathesis intermedi- ness increase of the E group (the ratio between kUD and kUU ates involved in the productive process (I, II, III and IV) is 6.7·10–2). Interestingly, the preference for the UU path- and the transition states connecting them (TSII and TSIII) way is also small for 13·BF3-I (∆δG = 0.8 kcal ­mol−1 and for all considered complexes 13-I, 9W-I, 13·B(C6F5)3 and kUD/kUU = 0.26) and 13·B(C6F5)3-I, thus explaining the loss 13·BF3 follow the same general trend with very few excep- in Z-selectivity when adding a Lewis acid in the reaction tions. In general, UU is the most stable isomer and DD mixture. the highest in energy species in each stationary point. The Analysis of TSIII transition state geometries (Fig. 2) give detailed order is UU < UD < DU < DD. The few exceptions further support to the importance of the ligand bulkiness. are mainly associated with those species presenting the bulk- When going from 13 to 13·BF3 and 13·B(C6F5)3, there is ier E ligands (imido or the O–B(C6F5)3). In the UU pathway a general increase of E group size in all directions due to the two substituents point toward the E group and in the tetrahedral environment around boron. This implies an open- case of the less favorable DD route, the substituents points ing of the W···Cole-CCH3 angle in the UU-TSIII pathway toward the aryloxy ligand. Therefore, the relative stability of only (Fig. 2) and a significant variation of the dihedral angle the different routes is mainly controlled by the ligand bulki- defined by the reacting carbon atoms. These two facts indi- ness. That is, since the E group is generally smaller than the cates that the released olefin orientation in the UU route is aryloxy ligand, the preferred relative orientation shows the influenced by the bulk of E in the larger systems, preventing two methyl groups pointing to E, the two pathways present- the achievement of the optimal structure in 9W, 13·BF3 and ing one methyl towards E and the other towards the bulky 13·B(C6F5)3. Moreover, comparison between the UU and aryloxy ligand have intermediate energies and the route with DD pathways as limit cases, indicates that the W–O-Caryloxy the two methyls pointing toward the aryloxy ligand is the angle associated to the Y ligand increases when substituents less favorable one. point toward Y. This suggests that the Y ligand also adapts The relative stabilities of the SBP metallacyclobutane to avoid repulsive interactions with the methyl groups of the resting state do not follow the same order. The SBP metal- releasing olefin. Consequently, TSIII is destabilized in the lacyclebutane stability trend follows: UD > UU ~ DD > DU. UD, DU and DD routes. In summary, despite the reported The puckered form of the metallacyclebutane fragment leads values could be sensitive to the conformational exploration, to the presence of equatorial and axial sites. Thus, the struc- the computed data indicate that the interaction between the ture with the two methyl groups in equatorial (A-UD) is Lewis acid and the oxo group makes the resulting adduct to lower in Gibbs energy than the structures with one methyl have a large E ligand that decreases the preference for the in axial position (A-DD and A-UU), and A-DU, with the two Z- product. In this way, the use of smaller Lewis acids or substituents in axial posions, is usually the less stable one. alternatively small Y ligands that could favor the DD route Overall, the relative stabilities of the SBP isomer are con- may eventually increase the catalyst activity and retain a trolled by the local structure of the metallacyclobutane frag- larger amount of Z- selectivity. ment and not by the bulkiness of the ligands. Consequently, the relative stabilities of the different reactant-alkylidene orientation in the SBP metallacyclobutane isomer are not sensitive to the alkylidene ancillary ligands. However, this 13 Topics in Catalysis (2022) 65:433–447 441 complexes of 13, 9W and 13·B(C6F5)3 catalysts. We consid- ered two different final products: the W(IV) olefin complex (D) and the alkyl allyl complex (C) resulting from ethene addition to the allyl hydride intermediate (B), which has been proposed to be the first step for side product formation and other catalyst deactivation processes [60]. Results for catalyst deactivation are summarized in Fig. 3 (the nomen- clature is based on that used in Schemes 2 and 3) and Table 3 reports the energetics for the degenerate metathesis of ethene for comparison. The Gibbs energies associated to the ethene metathesis with methylidene follows the same trends described before and particularly, the addition of B(C6F5)3 favors the cycload- dition step and stabilizes the TBP metallacyclobutane (III) intermediate with respect to the SBP (A) one. When com- paring with the propene metathesis with the ethylidene complex, one observes that most of the intermediates and transition states are stabilized with respect to separated reac- tants as a consequence of reducing the reactant bulkiness. Therefore, the nature of the olefin and alkylidene substitu- ents tunes the energetics but does not modify the previously described trends. The deactivation of 13 starts with the β-hydride elimina- tion. This step takes place with the hydride being transferred trans to the weakest σ-donating aryloxy ligand. The com- puted Gibbs energy barrier is high (35.0 kcal ­mol−1), thus suggesting a large stability of 13 if bimolecular coupling does not occur. The resulting allyl hydride intermediate (B) is 15.3 kcal ­mol−1 higher in energy than the SBP metal- lacycle (A) thus, it must further evolve to achieve catalyst deactivation. The ethene insertion to the metal hydride presents an even higher in Gibbs energy transition state (42.7 kcal ­mol−1 with respect to methylidene + ethene). This indicates that despite the process is thermodynamically Fig. 2  Detailed view of the metal coordination of the optimized struc- favorable (∆G°298 = − 12.0 kcal ­mol−1) ethene insertion is tures for the UU and DD cycloreversion transition states (TSIII) for a rare event. In contrast, the reductive coupling between the 13, 9W and 13·B(C6F5)3. Distances are in Å and angles in degrees. hydride and the allyl ligands that leads to the formation a The reported dihedral corresponds to that defined by the C ­ ole–Cole– W–Cene atoms W(IV) olefin complex is largely favored thermodynamically (∆G°298 = − 27.4 kcal ­mol−1) and the associated energy bar- rier is low (∆G‡°298 = 4.7 kcal ­mol−1). Indeed, the transition 3.4 Effect of Lewis Acids on Catalyst Deactivation state for the reductive coupling is located 29.4 kcal ­mol−1 lower in Gibbs energy than the transition state for ethene Based on the species formed after olefin metathesis, two insertion, indicating that the most likely deactivation process main catalyst deactivation pathways for d­ 0 alkylidenes have for 13 is the formation of a W(IV) olefin complex and the been reported in the literature: (i) bimolecular coupling of highest in Gibbs energy transition state is that associated two alkylidenes [89] and (ii) β-hydride elimination from with the β-hydride process. This is in contradiction with the unsubstituted SBP metallacyclobutane (A) [59, 60, 90]. the DFT results reported for silica grafted (OSi)Re(CtBu) While the former does not seem to be favored by the pres- (CHtBu)(CH2tBu) [60], but this could be either related to ence of Lewis acids (it may even disfavor it due to the ligand the modeling strategy or to the different reactivity of the two bulkiness increase), the addition of Lewis acid leads to the complex as suggested by the experimentally observed side formation of a W(IV) olefin complex compatible with the products in each case [59, 60]. β-hydride elimination at A.[59] Here, we studied deacti- Substitution of the oxo group by an imido ligand makes vation through β-hydride elimination of the methylidene the β-hydride elimination and the alkene insertion easier. 13 442 Topics in Catalysis (2022) 65:433–447 The Gibbs energies with respect to separated reactants of the associated transition states are 25.4 and 40.2 kcal ­mol−1 for the β-hydride and the ethene insertion respectively. Again, ethene insertion appears as an unlikely process, thus sug- gesting that another process should take place. In this con- text, reductive coupling leading to D is thermodynamically favorable (∆G°298 = − 15.4 kcal ­mol−1) and the associated Gibbs energy barrier lower than that of the ethene insertion (30.5 vs. 40.2 kcal ­mol−1). Noteworthily, the transition state for the reductive coupling is higher in Gibbs energy than that of the β-hydride elimination. Therefore, 9W decomposition through β-hydride elimination leads to also to a W(IV) olefin complex, but, in contrast to 13, the highest in Gibbs energy transition state is that associated with the formation of the olefin complex. Overall, deactivation of 9W is easier than the deactivation of 13. The 13·B(C6F5)3 adducts associated to the deactivation of 13 in presence of a Lewis acid are minima of the poten- tial energy surfaces. The species are especially favorable for the allyl hydride intermediate (B) and the transition states associated to the β-hydride elimination (TSAB) and ethene insertion (TSBC) (values in parenthesis in Fig. 3). Conse- quently, both the β-hydride elimination from 13·B(C6F5)3-A Fig. 3  Gibbs energy profile (in kcal ­mol−1) for the unimolecular deac- and the ethene insertion to 13·B(C6F5)3-B are easier than tivation through β-hydride elimination at the SBP metallacycle. Val- the analogous processes without the presence of the Lewis ues in parenthesis correspond to the interaction energy between the acid. The relative Gibbs energies of 13·B(C6F5)3-TSAB and metal complex and the Lewis acid 13·B(C6F5)3-TSBC with respect to separated 13·B(C6F5)-V and ethene are 31.3 and 36.6 kcal ­mol−1, thus ethene inser- Trends between the three complexes for the relative Gibbs tion is more challenging than the β-hydride. The Gibbs energies of TSAB, TSBC and TSBD can be rationalized in energy barrier for the reductive coupling leading to D is terms of the donating ability of the E ligand. As shown in 20.2 kcal ­mol−1 and, although the process is more challeng- Fig. 4, both the β-hydride elimination and the alkene inser- ing than in absence of the Lewis acid, it is still largely more tion imply the presence of a ligand in (pseudo)-trans to the accessible than the ethene insertion. Therefore, similarly E group. In the case of the β-hydride elimination transition to 9W and 13, the 13·B(C6F5)3 intramolecular deactivation state, one of the ­Cα carbons of the metallacycle is in pseudo leads to a W(IV) olefin complex, and it involves two steps, trans to E (Fig. 4). On the other hand, ethene insertion takes β-hydride elimination and the reductive coupling between place in the vacant site trans to the E and the transition state the allyl and hydride ligands. The transition state for the presents a short W···Cene (between 2.44 and 2.51 Å (Fig. 4). β-hydride is the highest transition state in the catalyst deac- Consequently, in both cases the transition state stability is tivation process, indicating a mechanistic difference between penalized when the E group is a strong donating group (i.e. the oxo complexes (rate determining step is the β-hydride oxo). Moreover, the two processes become easier when the elimination) and the imido alkylidene (the highest in energy donating ability of E becomes weaker, which is achieved step is the reductive coupling). The comparison of the rela- either by replacing the oxo for an imido ligand or by adding tive Gibbs energies of the highest transition states with a Lewis that tunes the electron density of the oxo group. respect to separated species indicate that 13·B(C 6F5)3 is The relative stabilities of the transition states for the more prone to deactivate through a unimolecular process reductive coupling of 13, 9W and 13·B(C6F5)3 are inversed than 13. with respect to TSAB and TSBC and a strong donating E Table 3  Gibbs energies (in kcal Catalyst V TSI II TSII III A ­mol−1) of all intermediates and transition states associated with 13 0.0 – 9.0 10.9 − 0.3 − 6.7 ethene metathesis reaction 13·B(C6F5)3 0.0 – 6.1 9.2 − 3.9 − 3.5 See Schemes 3 and 4 for labelling 13 Topics in Catalysis (2022) 65:433–447 443 Fig. 4  Detailed view of the optimized structures for TSAB, TSBC and TSBD for 13, 9W and 13·B(C6F5)3 complexes. Dis- tances in Å angles in degrees ligand favors the process. In this case, part of the metal- W(E)(CHR)(2,5-dimethylpyrrolide)(2,6-mesithylphenoxide) hydride bond is loss in the TSBD transition state when com- complexes differing only on the nature of the doubly bonded pared with the allyl hydride and thus the presence of strong E group: oxo, imido and an oxo group interacting with a donating ligands better compensate the loss in electron den- Lewis acid. Results show that the formation of a stable oxo- sity on the metal center. This is particularly the case of the Lewis acid adduct occurs mainly when the metal is tetra- tungsten oxo complex 13, which presents a low energy bar- coordinated and more significantly when the E group has rier, while the opposite extreme is represented by the imido another ligand in trans or in the same plane competing for complex 9W. the same empty metal d orbitals. This is particularly the case of the cycloaddition and cycloreversion transition states, the productive TBP metallacycle isomer and the transition state 4 Conclusions for β-hydride elimination, which is the highest in energy transition state associated to the unimolecular deactivation DFT (M06) calculations have been performed to rationalize of tungsten oxo alkylidene complexes. In these adducts, the the effect of Lewis acids on the catalytic activity for olefin resulting E group is bulky due to the presence of the phe- metathesis, Z-/E- selectivity and catalyst stability of tung- nyl rings of the B(C6F5)3 Lewis acid and the ligand steric sten oxo alkylidene complexes. For that, we compared three hindrance is comparable to that of the large aryloxy ligand. 13 444 Topics in Catalysis (2022) 65:433–447 Therefore, the Z-/E- selectivity, defined by the Gibbs energy Declarations difference between the lowest transition state leading to the Z- and E- product, decreases significantly. Moreover, the Conflict of interest Authors declare no conflict of interest. Lewis acid has also an electronic influence. The oxo-Lewis acid interaction decreases the oxo electron donating abil- Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- ity, which decreases its trans influence and makes the metal tion, distribution and reproduction in any medium or format, as long slightly more electrophilic. Consequently, the cycloaddition as you give appropriate credit to the original author(s) and the source, and cycloreversion Gibbs energy barriers with respect to provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are separated reactants are lowered and the TBP metallacyclobu- included in the article's Creative Commons licence, unless indicated tane isomer, which is much higher in Gibbs energy than otherwise in a credit line to the material. If material is not included in the SBP metallacycle resting state for the oxo complex, is the article's Creative Commons licence and your intended use is not stabilized. The two factors lead to an increase of catalyst permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a activity in presence of the Lewis acid, thus rationalizing the copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. experimental observations. 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