This is the accepted version of the journal article: Tejeda, Gabriela; Belov, Dmitry S.; Fenoll, Didac A.; [et al.]. «Vanadium Imido NHC Complexes for Ring-Closing Olefin Metathesis Reactions». Organometallics, Vol. 41, Issue 4 (February 2022), p. 361-365. DOI 10.1021/acs.organomet.2c00013 This version is available at https://ddd.uab.cat/record/280615 under the terms of the license Vanadium Imido NHC Complexes for the Ring-Closing Olefin Me- tathesis Reactions Gabriela Tejeda,1 Dmitry S. Belov,1 Didac A. Fenoll,2 Kelly L. Rue,1 Charlene Tsay,3 Xavier Solans- Monfort,2 and Konstantin V. Bukhryakov1,* 1 Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA 2 Departamento de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain 3 Department of Chemistry, University of California, Riverside, CA 92591, USA ABSTRACT: Vanadium bis-phosphine imido and oxo chloride alkylidenes have been extensively applied in the ring-closing me- tathesis of various acyclic olefins. However, their reactions involving ethylene showed limited productivity due to rapid decomposi- tion. The primary degradation pathway involving V bis-phosphine imido complexes is β-H elimination at unsubstituted metallacy- clobutane. In contrast, β-H elimination is disfavored for V oxo species, but bimolecular decomposition precludes its high productivity. Herein, we present the synthesis of V imido NHC complexes that are the most productive V catalysts toward various terminal olefins in ring-closing metathesis reactions. Experimental and computational studies suggest that β-H elimination and bimolecular decom- position are disfavored for V imido NHC complexes. Ru, Mo, and W-catalyzed olefin metathesis (OM)1-3 have L cat. L E TON been applied to produce commodity chemicals,4 pharmaceuti- 1 PMe3 NC6F5 2.0 Cl cals,5 and advanced materials.6 The shift to more easily acces- V 2 PMe3 N(2,6-Me2C6H3) 6.4 sible and abundant first-row transition metals is desirable to E SiMe3 3 PEt3 NC6F5 12.6 L lower the cost of the obtained materials and decrease the envi- 4 PEt3 O 59.0 ronmental footprint. However, only a few first-row transition cat. metal complexes demonstrate the capacity to complete the OM N Ts N Ts + catalytic cycle.7-8 Among 3d transition metals, OM catalysts 22 °C, open vial based on V are the most developed,9 presumably due to the di- 5, Ts = p-CH3C6H4SO2 6 ethylene agonal relationship between V and Mo.10 V alkylidenes contain- ing imido, alkoxide, and phosphine ligands have been applied Scheme 1. Previously synthesized V alkylidenes, model RCM for ring-opening metathesis polymerization (ROMP) of cyclic reaction, and maximum turnover number (TON) for each cata- olefins showing high activity and selectivity.11-13 However, lyst in the model RCM reaction.16, 18 those complexes exhibit low productivity toward acyclic termi- Increased imido group size and electron-donating properties nal olefins.14-15 improve RCM productivity (1 and 2, Scheme 1). The change of Recently, we reported the facile access to V imido chloride PMe3 to PEt3 leads to an even more pronounced effect (1 and 3, alkylidenes 1-3 (Scheme 1) and showed the first example of Scheme 1). N-heterocyclic carbene (NHC) as a neutral ligand is productive ring-closing metathesis (RCM) utilizing those com- desirable due to high 𝜎-donating abilities and controllable steric plexes.16 The reported equilibrium between polymerization-de- properties,20-21 which can prevent 𝛽-H elimination and bimolec- polymerization of cyclopentene is an example of unproductive ular decomposition. Previously, NHC complexes were utilized RCM.17 Complexes 1-3 can tolerate various common functional to prepare air-stable, highly active, selective, and functional groups. However, those catalysts quickly decompose under eth- group tolerant Mo and W alkylidenes.22 V alkyl alkylidene com- ylene due to 𝛽-hydride (𝛽-H) elimination at unsubstituted plexes of type V(NR)(CHSiMe3)(CH2SiMe3)(NHC), where R metallacyclobutane (MCB). In the following report, we de- is adamantyl23 or 2,6-Y2C6H2 (Y = Me23-24 or Cl24) have been scribed the first catalytically active V oxo alkylidene 4, which reported, but have limited activity in ROMP of norbornene at outperformed 1-3 in reactions involving ethylene (Scheme 1).18 ambient temperature unless C6X5OH (X = F or Cl) is added. We found that V oxo alkylidenes significantly disfavor 𝛽-H Two resulting complexes V[N(2,6-Cl2C6H2)](CHSiMe3) elimination, but the relatively small oxo ligand can bridge two (OC6X5)(IMes) have been isolated in 24% (A, X = F) and 21% metal centers and encourage bimolecular decomposition.19 In (B, X = Cl) yields. Complex A showed a TOF of 208 s-1 in contrast, V imido counterparts are significantly more stable to- ROMP of norbornene, the highest reported activity among V ward bimolecular decomposition and their further development alkylidenes. Unfortunately, complexes A and B have not been to prevent 𝛽-H elimination is highly desirable. structurally characterized by X-ray crystallography.24 Studies of catalytic activity of V imido alkylidenes bearing neutral ligands other than phosphines toward acyclic olefins have not been reported. Here we present the synthesis of V imido NHC chlo- intermediates of the catalytic cycle, such as MCB or new alkyli- ride alkylidenes, their X-ray structures,25 DFT studies, and ac- dene, have not been reported previously. Therefore, the pre- tivity in RCM of various terminal olefins. sented alkylidene exchange is important from a fundamental Complex 7 was synthesized from 1 utilizing ligand ex- standpoint and further supports the Chauvin mechanism in V- change reaction in the presence of IMes in 49% yield (Scheme based OM. In addition, complexes 8 and 9 are the first examples 2). of well-defined catalytically active V alkylidenes that do not contain TMS-alkylidene moiety. From a historical perspective, IMes N N a transient V benzylidene is thought to be the active catalyst in N N early work on V-based ROMP.26 Cl Complexes 8 and 9 were successfully crystallized and stud- 1 F N V Toluene, N2 atmosphere F SiMe3 ied by X-ray crystallography (Figure 2). F PMe3 - PMe3 F F 7, 49% Scheme 2. Synthesis of complex 7. The X-ray structural study showed that complex 7 is syn- isomer (Figure 1) and has a distorted trigonal bipyramidal ge- ometry with IMes and PMe3 in axial positions [V1−C11 2.1828(11) Å, V1−P1 2.4545(3) Å, C11−V1−P1 154.52(3)°]. The V1−C1 distance is 1.8300(11) Å and the large V=C−Si an- gle 145.94(7) is indicative of 𝛼-hydrogen agostic interaction with V center.16 Figure 1. Perspective view of the crystal structure of complex 7 with thermal ellipsoids shown at 30% probability. The direct exchange of one PMe3 to IMes is a suitable Figure 2. Perspective view of the crystal structures of complexes method to synthesize 7 bearing a small imido group (NC6F5). 8 (top) and 9 (bottom) with thermal ellipsoids shown at 30% prob- However, in the case of larger imido group (N(2,6-Me2C6H3), ability. the resulting complex V(N(2,6-Me2C6H3)(CHSiMe3)(IMes)Cl o-MeO-benzylidene moiety was chosen to form 6-coordi- (C) does not contain PMe3 confirmed by NMR spectroscopy. nated complexes. However, the long distance between the V Unfortunately, the isolation of C was challenging due to the and methoxy group (2.524 Å in 8 and 2.517 Å in 9) does not high solubility in common crystallization solvents and slow de- confidently define V1−O1 as a bond. Therefore, we describe the composition of C in solution. To overcome this challenge, we geometry of 8 and 9 as a distorted square pyramid (𝜏27 = 0.38 exchanged TMS-alkylidene for the less sterically demanding o- in 8 and 𝜏 = 0.36 in 9) with the imido group in the apical position MeO-benzylidene. The following replacement of one phos- with a weak interaction between V and methoxy group trans to phine by NHCs led to complexes 8 and 9 in 52% and 55% yield, the imido group. Both 8 and 9 are anti-isomers with neutral lig- respectively, after two steps. (Scheme 3). In both cases, only ands trans to each other and have similar bond distances and one isomer was observed by 1H NMR spectroscopy. angles. Complex 8: V1−C3 2.231(2) Å, V1−P1 2.4939(7) Å, R R C3−V1−P1 172.80(6)°, V1−C30 1.952(2), V1−C30−C31 N N 1. 124.64 (17)°. Complex 9: V1−C3 2.240(5) Å, V1−P1 R' R R R' O Cl V 2.4994(17) Å, C3−V1−P1 172.40(15)°, V1−C32 1.944(7), - CH2=CHTMS O N V1−C32−C33 125.6(5)°. As expected, the V=C distances in 8 2 2. IMes or IEt PMe3 and 9 are longer and V=C−C angles are smaller than analogous - PMe3 8, R = R' = Me, 52% bond and angle in 7 due to the absence of 𝛼-hydrogen agostic 9, R = Et; R' = H, 55% interactions.16, 28 Scheme 3. Synthesis of complexes 8 and 9. We tested complexes 7-9 toward our model substrate 5. Noteworthy, the reactions in open and closed vials showed sim- Although it is assumed that V-based OM involves the ilar conversions for 7-9 suggesting that studied complexes are Chauvin mechanism, the isolation and characterization of key stable toward ethylene. The results and comparison with 1-4 at B(C6F5)3 to remove free PMe3 before adding a new portion of 8 the same conditions are summarized in Table 1. to suppress the formation of D. This strategy resulted in the full Table 1. Model RCM of diallyl-N-tosylamide 5.a conversion of 5 to 6 and required 3 mol% of 8 (entry 13, Table 1). V catalyst To further understand the role of the NHC ligand on catalyst 5 6 + ethylene 22 °C, C6D6, closed vial stability, we performed DFT (B3LYP-D3)29-31 calculations on the unimolecular catalyst deactivation. It is nowadays well ac- # cat. cat., mol% time, h conv., %b TON cepted that OM with d0 alkylidene complexes implies the coor- 1 1 5 5 8 1.6 dination of the olefin trans to the strongest ancillary s-donor 2 2 5 5 29 5.8 ligand to form the trigonal bipyramid MCB (tbp, Scheme 4).32- 33 Unimolecular deactivation starts from the resting-state square 3 3 5 5 42 8.4 pyramid MCB (sp) and occurs through a 𝛽-H elimination trans 4 4 5 5 67 13.4 to the weakest s-donor ligand.34-35 We recently showed that V 5 7 5 5 54 10.8 alkylidenes proceed through the same reaction mechanism.18 6 8 5 5 96 19.2 Therefore, we explored OM between the methylidene derivates of 7 and 8 and ethylene and the subsequent 𝛽-H elimination 7 9 5 5 87 17.4 (Scheme 4). For comparison, we include the Gibbs energies for 8 8 1 0.5 76 76.0 methylidene derivates of 3 and 4 reported previously.18 9 8 0.5 0.5 70 141 metathesis decomposition 10 8 0.1 0.5 14 140 E E E E 11 8 0.1c 0.5 17 170 cat. L V L V L V L V H - L' Cl Cl 12 9 0.1 c 0.5 14 140 - Cl Cl R m tbp sp h 13 8 3c,d 1.3 >99 33.3 TS-h TS-tbp 14 8 3c 0.5 91 30.3 TS-sp 15 8 3c,e 0.5 2 0.7 m tbp sp cat. L E h a 0.12 M 5. bby 1H NMR. c0.24 M 5. d1% increments followed by 3 PEt3 NC6F5 0.0 +8.3 −0.3 +2.7 −0.9 +13.8 −1.1 1% B(C6F5)3. e4 equiv. of PMe3 4 PEt3 O 0.0 +9.7 −0.5 n/ab −0.5 +24.9 −1.8 Complexes 7-9 outperform their bis-phosphine counterparts 7 IMes NC6F5 0.0 +13.7 +1.2 +7.9 +1.3 +22.0 +3.5 1-3 in the model RCM reaction. Complex 8 is the most effective 8 IMes NAra 0.0 +13.1 +2.5 +5.0 0.0 +23.9 +0.9 catalyst among the studied, showing productivity higher than V Scheme 4. Gibbs energies (kcal mol-1) of OM and unimolecular oxo complex 4 (entries 1-7, Table 1). 8 is not observed by 1H decomposition of methylidene derivates of 3, 4, 7, and 8. aAr = NMR spectroscopy after 30 minutes of the reaction in all cases. 2,6-Me2C6H3. bsee ref.18 The decrease of the catalyst’s loading leads to the increase of TON (entries 8-10, Table 1) with the maximum TON of 170 at According to calculations, the energy barrier for the cy- 0.1 mol% of 8 at a higher concentration of 5 (entry 11, Table cloaddition step with 7-m is 5.4 kcal mol-1 higher than that of 1), which is the highest reported TON of V-based OM involving 3-m. Moreover, the tbp and sp MCBs of 7 are less stable with acyclic terminal olefins. respect to separated reactants than those of 3. Both factors sug- Following the catalysis with 8 by 1H NMR spectroscopy gest that 7 is slightly less prone to undertake OM. The lower showed the formation of triplet at 14.1 ppm (3JHP = 8.9 Hz), activity of 7 arises from a weaker alkylidene – olefin interaction which corresponds to alkylidene signal of bis-phosphine com- at the transition state for cycloaddition due to the stronger s- plex V(N(2,6-Me2C6H3)(CHAr')(PMe3)2Cl (D, Ar' = o- donating ability of IMes when compared to PEt3. The substitu- (OMe)C6H4), observed previously during the PMe3→NHC ex- tion of the NC6F5 imido ligand by the more donating N(2,6- change reaction. We explain the formation of D by the reverse Me2C6H3) has a small effect on the energy barrier and MCB NHC→PMe3 exchange reaction in the presence of PMe3, stability. More importantly, the 𝛽-H elimination from the sp for formed during the initiation step. Complex D quickly decom- both 7 and 8 requires overcoming high energy barriers, the tran- poses in the presence of ethylene in analogy to 1 via β-H elimi- sition state lying 22.0 and 23.9 kcal mol-1 above separated reac- nation.16 Thus, the reaction between 8 and ethylene showed the tants, respectively. These values are significantly higher than formation of D and propene (see Supporting Information). those computed for 3 (13.8 kcal mol-1) and comparable to those Based on our observation, the larger IEt ligand dissociates more obtained for the most stable V oxo alkylidene (24.9 kcal mol-1). readily than IMes to form D, explaining the lower productivity This agrees with the experimental results and suggests that the of 9 compared to 8. The rates of two competitive reactions, enhanced stability of 7-9 compared to 4 arises mainly by pre- 8→D and OM, depend on the concentrations of PMe3 and 5 venting the bimolecular decomposition. correspondingly. Therefore, low catalyst loading and increased We expanded the scope to 12 substrates using catalyst 8 and substrate concentration led to higher productivity. The reaction the best conditions obtained for 5 (Scheme 5). in the presence of 4 equiv. of PMe3 showed low conversion sup- porting our hypothesis (entry 15, Table 1). In contrast, the reac- tion in the presence of phosphine scavenger (B(C6F5)3) allowed completing the reaction. The optimal conditions included the addition of 8 in 1% increments followed by 1% increments of 3 mol% of 8a X-ray crystallographic files of 7-9 (CIF). X X + Cartesian Coordinates of the DFT structures (xyz). 22 °C, C6D6, closed vial Accession Codes O O N CCDC 2132696 (7), CCDC 2132697 (8), and CCDC 2132695 (9) N S N S S S N contains the supplementary crystallographic data. These data can O O be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/re- 10, >99% 11, >99% 12, 42% trieving.html, or from the Cambridge Crystallographic Data Cen- tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223- O O 336-033; or by e-mail: deposit@ccdc.cam.ac.uk. N S N S O O O AUTHOR INFORMATION N N 13, 79% 14, 94% 15, 50% Konstantin Bukhryakov - Department of Chemistry and Biochem- istry, Florida International University, Miami, FL 33199, USA. O O kbukhrya@fiu.edu O N S N N S O N Ph Notes O O The authors declare no competing financial interests. 16, >99% 17, 99% 18, 52% CO2Me ACKNOWLEDGMENT O N O Ph We are grateful to the Florida International University and the New S N S N S N Faculty Development Grant provided by U.S. Nuclear Regulatory O O Commission (NRC) for financial support. NRC fellowship grant 19, 94% 20, 50% 21, 36% No. 31310018M0012 awarded to FIU supported KLR. Financial support from MICINN [PID2020-112715GB-I00] and the Gener- Scheme 5. Scope of RCM catalyzed by 8. a1% increments fol- alitat de Catalunya [2017SGR1323] are also gratefully acknowl- lowed by 1% B(C6F5)3. edged. This material is based upon work supported by the National Science Foundation under Award No. 1919677. Organic compounds containing thiophene (10), pyrazole (11), quinoline (13), tertiary aniline (14), isoxazole (16), meth- REFERENCES oxy group (17), and pyridine (19) are easily accessible by using 1. Chauvin, Y., Olefin Metathesis: The Early Days (Nobel 8 with high conversions. Substrates containing a thioether (12), Lecture). Angew. Chem. Int. Ed. 2006, 45 (23), 3740-3747; Angew. an ether (15), N,N-diallylaniline (18), an ester (20), and tertiary Chem. 2006, 118 (23), 3824-3831. amine (21) exhibit moderate conversions, presumably due to 2. Grubbs, R. H., Olefin-Metathesis Catalysts for the Preparation of Molecules and Materials (Nobel Lecture). Angew. competing binding to the catalyst. Chem. Int. Ed. 2006, 45 (23), 3760-3765; Angew. 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Imido Alkylidene d0-Metal Species: How and Why Do They Differ in 25. CCDC 2132696 (7), CCDC 2132697 (8), and CCDC Structure, Activity, and Efficiency in Alkene Metathesis? 2132695 (9) contains the supplementary crystallographic data. These Organometallics 2012, 31 (19), 6812-6822. data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the N N Cl V O N PMe3 X X - ethylene up to 170 TON