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CHEMICAL SCIENCES
Original article
DOI: https://doi.org/10.21285/2227-2925-2022-12-2-180-191
Ruthenium indenylidene complexes bearing bis(N-Alkyl/N'-Mesityl)-sided heterocyclic carbene ligands1
Baoyi Yu*, Fatma B. Hamad**, Kristof Van Hecke***, Francis Verpoort****
*Key Laboratory of Urban Agriculture (North China), Ministry of Agriculture, College of Biosciences and Resources Environment, Beijing University of Agriculture, Beijing, China
**College of Education, Dar es Salaam University, Dar es Salaam, Tanzania ***Department of Chemistry, Ghent University, Ghent, Belgium
****State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology
Corresponding author: Francis Verpoort, Francis@whut.edu.cn
Abstract. We report on the synthesis and characterization of new ruthenium indenylidene complexes bearing two unsymmetrical N-heterocyclic carbene (NHC) ligands denoted as RuCl2(3-phenyl-1-indenyli-dene)bis(1-mesityl-3-R-4,5-dihydroimidazole-2-ylidene) in which R is methyl 7a and cyclohexyl 7b. Complexes 7a and 7b were analyzed using single-crystal X-ray diffraction analysis, elemental analysis, IR, NMR spectroscopy, and HRMS. The catalytic activities of complexes 7a and 7b were evaluated in olefin metathesis reactions: ring-opening metathesis polymerization (ROMP) of cis,cis-1,5-cyclooctadiene (COD) and ring-closing metathesis (RCM) of diethyl diallyl malonate (DEDAM) as well as in the isomerization of allylic alcohols. Complexes 7a and 7b failed to initiate the reactions at room temperature in all tested reactions, which might be due to the high thermal stability and low degree of lability of the Ru-CNHC bonds. At 80 °C, the complex 7a showed the best performance due to an increased initiation and a decreased steric obstruction towards the incoming substrates.
Keywords: homogeneous catalysis, indenylidene, olefin metathesis, ruthenium catalysts
Acknowledgements. Baoyi Yu thanks Research Foundation for Young Researcher (Grant/Award Number: SXQN201701; 5056016013/001) and 2018 Scientific and Technological Innovation Service Foundation -Basic Scientific Research Business Fee (5075227115/020). Fatma B. Hamad would like to acknowledge the State Key Lab of Advanced Technology for Materials Synthesis and Processing for financial support (Wuhan University of Technology). Kristof Van Hecke thanks the Hercules Foundation (project AUGE/11/029 "3D-SPACE: 3D Structural Platform Aiming for Chemical Excellence") and the Special Research Fund (BOF) -UGent (project no. 01N03217) for funding. Fatma B. Hamad would like to acknowledge the Dar es Salaam University College of Education and Schlumberger Foundation for funding.
For citation: Yu B., Hamad F. B., Van Hecke K., Verpoort F. Ruthenium indenylidene complexes bearing bis(N-Alkyl/N'-Mesityl)-sided heterocyclic carbene ligands. Izvestiya Vuzov. Prikladnaya Khimiya i Bio-tekhnologiya = Proceedings of Universities. Applied Chemistry and Biotechnology. 2022;12(2):180-191. https://doi.org/10.21285/2227-2925-2022-12-2-180-191.
''Supplementary material (Appendix A) related to this article can be found, in the online version at doi: https://doi.org/10.21285/2227-2925-2022-12-2-180-191
Дополнительный материал (Приложение А), относящийся к этой статье, можно найти в онлайн-версии по адресу https://doi.org/10.21285/2227-2925-2022-12-2-180-191
© Yu B., Hamad F. B., Van Hecke K., Verpoort F., 2022
ХИМИЧЕСКИЕ НАУКИ
Научная статья УДК 546
Инденилиденовые комплексы рутения, содержащие бис(М-алкил/М'-мезитил) гетероциклические
карбеновые лиганды1
Баойи Юй*, Фатьма Б. Хамад**, Кристоф Ван Хекке***, Фрэнсис Верпоорт****
*Ключевая лаборатория городского сельского хозяйства (Северный Китай), Министерство сельского хозяйства, Колледж биологических наук и природных ресурсов Пекинского сельскохозяйственного университета, г. Пекин, Китай
**Педагогический колледж Дар-эс-Саламского университета, г. Дар-эс-Салам, Танзания ***Гентский университет, г. Гент, Бельгия
****Государственная ключевая лаборатория передовых технологий синтеза и обработки материалов, Уханьский технологический университет, г. Ухань, Китай Автор, ответственный за переписку: Фрэнсис Верпоорт, Francis@whut.edu.cn
Аннотация. В работе сообщается о синтезе и исследовании характеристик новых инденилиде-новых комплексов рутения с двумя несимметричными N-гетероциклическими карбеновыми лиган-дами (NHC), обозначаемыми как RuCh(3-фенил-1-инденилиден)бис(1-мезитил-3-R-4,5-дигидроими-дазол-2-илиден), в котором R представляет собой метил 7a и циклогексил 7b. Комплексы 7a и 7b анализировали методами рентгеноструктурного анализа монокристаллов, элементного анализа, ИК-, ЯМР-спектроскопии и HRMS. Каталитическую активность комплексов 7a и 7b оценивали в реакциях метатезиса олефинов: метатезисной полимеризации с раскрытием цикла (ROMP) цис,цис-1,5-циклооктадиена (COD) и метатезиса с замыканием цикла (RCM) диэтилдиаллилмало-ната (DEDAM), а также при изомеризации аллиловых спиртов. Комплексы 7а и 7б не инициировали реакции при комнатной температуре во всех исследованных реакциях, что может быть связано с высокой термической стабильностью и низкой степенью лабильности связей Ru-Cnhc. При 80 °С комплекс 7а показал наилучшие характеристики благодаря усилению инициирования и уменьшению стерической непроходимости по отношению к поступающим субстратам.
Ключевые слова: гомогенный катализ, инденилиден, метатезис олефинов, рутениевые катализаторы
Благодарности. Баойи Юй благодарит Исследовательский фонд для молодых исследователей (номер гранта/награды: SXQN201701; 5056016013/001) и Фонд научно-технических инноваций 2018 г. -Комиссионный сбор за фундаментальные научные исследования (5075227115/020). Фатьма Б. Хамад выражает признательность Государственной ключевой лаборатории передовых технологий синтеза и обработки материалов за финансовую поддержку (Уханьский технологический университет). Кри-стоф Ван Хекке благодарит Фонд Hercules (проект AUGE/11/029 «3D-SPACE: 3D Structural Platform, нацеленный на химическое совершенство») и Фонд специальных исследований (BOF) - UGent (проект № 01N03217) за финансирование. Фатьма Б. Хамад выражает признательность Педагогическому колледжу Дар-эс-Саламского университета и Фонду Шлюмберже за финансирование.
Для цитирования: Инденилиденовые комплексы рутения, содержащие бис(^алкил/^-мезитил) гетероциклические карбеновые лиганды // Известия вузов. Прикладная химия и биотехнология. 2022. Т. 12. N 2. С. 180-191. (In English). https://doi.org/10.21285/2227-2925-2022-12-2-180-191.
INTRODUCTION
Ruthenium-catalyzed olefin metathesis is among the essential methods for carbon-carbon double bond formation/redistribution, utilized widely in both organic and polymer synthesis [1-10]. Contributions made by R. H. Grubbs in olefin metathesis are highly acknowledged due to the invention of a 16-electrons ruthenium benzylidene complex (e.g., 1a, Fig. 1) [11, 12]. Complex 1a bearing a mono-W-heterocyclic
carbene (NHC) ligand named "Grubbs' second-generation catalyst" shows better catalytic performance in terms of both efficiency and stability relative to its bis-PCy3 analogs [1-10]. The strong 5-donating and n-back donating properties enabled NHC ligands to be firmly bonded to the metal center and stabilizing the formed complexes, leading to wide utilization of NHC ligands in organometallic complexes [13-16].
Yu B., Hamad F. B., Van Hecke K. et al. Ruthenium indenylidene complexes bearing ... Юй Б., Хамад Ф. Б., Ван Хекке К. и др. Инденилиденовые комплексы рутения, содержащие .
гл гл гл
R-N N-Mes Mes-N N-Mes R-N N-Mes
I Y.xCl/^Ph Y^Cl/^Ph
Ru=\ Ru: ~ '
СГ I Ph РСУз
er
сГ
РСУз
PCy3
Mes = 2,4,6-trimethylphenyl
la: R = Mes lb: R = Adamantyl lc: R = Methyl Id: R = Cyclohexyl
3a: R = Methyl 3b: R = Cyclohexyl 3c: R = Octyl
4a: R = Methyl 4b: R = Cyclohexyl 4c: R =
(tetrahydrothiophen-2-yl)methyl
Fig. 1. Selected ruthenium metathesis catalysts Рис. 1. Выбранные рутениевые катализаторы метатезисной полимеризации
The catalysts' reactivity performance can be significantly affected by electronic and steric modification of the NHC ligands [13-16]. The unsymmet-rical transformation is worth noticing since it provides a dual-site configuration in the vicinity of the ruthenium core due to the two different steric environments [17-19]. The dual-site configuration can lead to catalysts with improved selectivity in various metathesis reactions such as E:Z selectivity in cross-metathesis (CM), selectivity in diastereo ring-closing metathesis (RCM), and cis-selectivity in ring-opening metathesis (ROMP).
The better-donating properties of aliphatic groups have attracted much attention. Mol's group [20] reported an W-adamantyl/W-mesityl sided NHC containing ruthenium complex 1b (Fig. 1). However, the anticipated increase in catalytic efficiency and stability due to the increased donation was not found as complex 1b showed poor catalytic activity in olefin metathesis reactions. The low catalytic ability of complex 1b was due to the greater-sized ada-mantyl group caused a steric repulsion towards the incoming substrate during metatheses progress. To gain more insight into the structure-activity relationship, catalysts 1c-d bearing W-alkyl/W-mesityl heterocyclic carbenes with decreased steric bulkiness (Fig. 1) were synthesized and evaluated in metathesis reactions [21, 22]. Complex 1c, coordinated with the smallest alkyl (methyl) group, exhibited activity comparable to the parent catalyst 1a.
In some cases, the isomerization reaction is favored over CM or RCM, applying Grubbs' type complexes. For example, complex 1a efficiently mediated isomerization of a range of allyl and homoallyl ethers to the corresponding enol ethers and subsequent to alcohols upon acid treatment [23]. The isomerization of allylamine and W-allyllactam [24] was instead afforded than CM and RCM reactions, respectively, using complex 1a. Rutjes et al. [25] encountered remarkable isomerization of alle-namides to dienamides catalyzed by complex 1a in the RCM of enamides.
Ruthenium indenylidene complexes are considered another unique class of olefin metathesis catalysts, which combine an easy way of preparation, comparative catalytic activities in the metathesis reactions, and relatively greater stability [1-10]. The reports on the strength of N-alkyl/W-aryl heterocyclic carbenes in influencing the catalytic efficiency of ruthenium indenylidene pre-catalysts are also available [26-30]. The ruthenium indenylidene complexes (3a-c) reported [29], showed faster catalytic initiation than the reference complex 2. The less steri-cally hindered complex 3a performed better than others in terms of both catalytic initiation and efficiency. While the improved initiation was attributed to stronger donating properties, the better catalytic efficiency was associated with steric properties. In other strategy [31], the mixed ligands coordinated complexes bearing NHC and NHCewg were developed, and the precatalysts usually needed higher temperature to initiate the activity. During the catalytic process, the electron deficient NHC functioned as the leaving group to generate the active species.
Some of the alkyl-based unsymmetrical hetero-cyclic carbene ligands have shown an exceptional tendency to form b/s-NHCs coordinated complexes. Complexes (4a-c), for example, were obtained preferentially and exhibited substantial olefin metathesis activity at higher temperatures [22, 32]. Ruthenium indenylidene complexes coordinated with b/s-(N-cyloalkyl/W-mesityl) heterocyclic carbenes where cycloalkyl is cyclopentyl or cyclododecyl were also noted to be formed competitively [26]. Based on the previous report, we explored the utility of ruthenium indenylidene catalysts bearing b/s(N-alkyl/N-mesityl)-heterocyclic carbene ligands.
EXPERIMENTAL SECTION
General procedure for the preparat/on of complexes 7a-b. In an oven-dried Schlenk vessel, KHMDS (4 eq., 6.4 mL, 3.2 mmol) (0.5 M toluene solution) was added to a suspension of imidazolium salt (3 eq., 2.4 mmol) in dry toluene (24 mL) at r.t.
Yu B., HamadF. B., Van Hecke K. et al. Ruthenium indenylidene complexes bearing ... Юй Б., Хамад Ф. Б., Ван Хекке К. и др. Инденилиденовые комплексы рутения, содержащие .
and the resulting solution was vigorously stirred for 30 minutes. The solution was then added to the reactor containing a solution of RuCh(3-phenyl-1-indenylidene)(PCy3)2 (6) (1eq., 0.74 g, 0.8 mmol) in toluene (16 mL) and further stirred overnight. Purification using silica gel chromatography (EtOAc for 7a and hexane:EtOAc = 60:1 for 7b) afforded a brown solid. After that, the solid was rinsed three times with pentane and three times with methanol.
RuCl2bis(1-mesityl-3-methylimidazolidin-2-ylidene) (3-phenyl- 1-indenylidene) (7a):
л CI
,17—18
CI —Rui
N----
/ \ ,
17 ^.«го.Х,
20= 19^.
25
21.4 \
24 — 23 11"'
10„
,12 •'13 /
«14
-19 2 V
21
4 \
23: 22
// •5
N7
Brown powder (0.45 g, Yield: 74%). The X-ray structure of complex 7a was measured using the crystals, which were grown up by evaporation of a complex solution in CH2Ch/EtOAc/hexane. Anal. Calcd. (%) for C41H46Cl2N4Ru: C, 64.22; H, 6.05; N, 7.31. found (%): C, 64.53; H, 6.00; N, 7.15; 1H-NMR (500 MHz, CDCl3, TMS, 20 °C): 5 7.98 (d, 1 H, 3Jh,h = 7.0 Hz, H-7), 7.67 (d, 2 H ,3Jh,h = 7.3 Hz, H-11, H-15), 7.50 (t, 1 H, 3Jh,h = 7.3 Hz, H-13), 7.40 (t, 2 H, 3Jh,h = 7.6 Hz, H-12, H-14), 7.08 (t, 1 H, 3Jh,h = 7.0 Hz, H-5), 7.03 (t, 1 H, 3Jh,h = 7.0 Hz, H-6), 6.79 (d, 1 H, 3Jh,h = 7.3 Hz, H-4), 6.77 (s, 1 H, H-2), 6.21 (s, 1 H, H-21), 5.78 (s, 1 H, H-23), 4.16 (s, 6 H, H-28), 3.87-3.91 (m, 2 H, H-17), 3.51-3.67 (m, 2 H, H-18), 1.98 (s, 6 H, H-26), 1.77 (s, 6 H, H-25), 1.73 (s, 6 H, H-27); 13C{1H}NMR (126 MHz, CDCls, 20 °C): 5 299.2 (C-1), 219.6 (C-16), 142.7 (C-8), 139.6 (C-9), 136.9 (C-10), 136.7 (C-22), 136.6 (C-20), 136.43 (C-24), 136.38 (C-3), 136.3 (C-19), 135.4 (C-2), 128.64 (C-12, C-14), 128.61 (C-21), 128.2 (C-23), 127.3 (C-7), 127.2 (C-13), 127.7 (C-6), 126.6 (C-11, C-15), 126.2 (C-5), 115.0 (C-4), 52.5 (C-17), 52.0 (C-18), 38.2 (C-28), 20.8 (C-27), 18.4 (C-25), 18.2 (C-26); IR (Neat): v = 2946, 2915, 2880, 1503, 1490, 1451, 1376, 1357, 1334, 1292, 1259, 1236, 1175, 1161, 1073, 1037, 1029, 1010, 991, 888, 850, 837, 778, 700, 756, 701, 654; ESI-MS: [M]+ calcd for C41H46ChN4Ru, 766.2143; found: 766.2122; [M-Cl]+ calcd for C41H46ClN4Ru, 731.2454; found: 731.2458.
RuCl2bis(1-cyclohexyl-3-mesitylimidazolidin-2-yli-dene)(3-phenylinden-1-ylidene) (7b): .26
>0 / \
•;i9—w N'
25 | nl „ J!
21 — -2? '23S
С
N \
Ly^'
13 /
Brown powder (0.59 g, Yield: 82%). The X-ray structure of complex 7b was measured using the crystals, which were grown up by evaporation of a complex solution in CH2Ch/EtOAc/hexane. The assignments and identification of resonances for C19, C20, C22, C24 could not be achieved because of the low intensity of the carbons signals in the NHC group on the 13C{1H}NMR spectrum. Anal. Calcd. (%) for C51H62Cl2N4Ru: C, 67.83; H, 6.92; N, 6.20. found (%): C, 67.66; H, 6.85; N, 6.13; 1H-NMR (500 MHz, CDCla, TMS, 20 °C): 5 8.47 (d, 1 H, 3Jh,h = 7.0 Hz, H-7), 7.87 (d, 2 H, 3Jh,h = 7.6 Hz, H-11, H-15), 7.54 (d, 2 H, 3Jh,h = 7.0 Hz, H-13), 7.47 (t, 1 H, 3Jh,h = 7.3 Hz, H-5), 7.42 (t, 2 H, 3Jh,h = 7.3 Hz, H-12, H-14), 7.36 (t, 1 H, 3Jh,h = 7.3 Hz, H-6), 7.20 (d, 1 H, 3Jh,h = 7.3 Hz, H-4), 7.05 (s, 1 H, H-2), 7.01 (s, 2 H, H-21/H-23), 6.97 (s, 2 H, H-21/H-23), 3.31-3.83 (m, 8 H, H-17/H-18), 2.43 (s, 6 H, H-27), 2.83 (s, 3 H, H-25/H-26), 2.74 (s, 3 H, H-25/H-26), 2.68 (t, 2 H, 3Jh,h = 11.0 Hz,H-28),1.67 (s, 2 H, H-Cy),1.52 (s, 6 H, H-25/26),1.13-1.38 (m, 8 H, H-Cy), 1.00-1.11 (m, 2 H, H-Cy), 0.88 (s, 2 H, H-Cy), 0.55-0.77 (m, 6 H, H-Cy); 13C{1H}NMR (126 MHz, CDCla, 20 °C): 5300.4 (C-1), 206.6 (C-16), 204.3 (C-16), 144.2 (C-3), 141.2 (C-9), 140.7 (C-8), 137.7 (C-2), 137.0 (C-10), 129.9 (C-21/C23), 129.7 (C-21/C23), 129.3 (C-5), 129.2 (C-6), 129.1 (C-12/C-14), 129.0 (C-21/C23), 128.9 (C-21/C-23), 128.3 (C-7), 128.1(C-13), 126.2 (C-11, C-15), 117.4 (C-4), 56.1 (C-28), 55.8 (C-21), 51.8,51.4, 44.1, 43.6 (C-17/C18), 34.1, 33.7, 32.0, 31.7, 30.0, 29.7, 25.6, 25.2 (C-Cy), 21.3 (C-27), 20.3, 20.2, 19.1, 19.0 (C-25/C-26); IR (Neat): v = 3052, 3002, 2976, 2922, 2853, 1485, 1470, 1446, 1433, 1402, 1385, 1366, 1352, 1329, 1298, 1282, 1250, 1235, 1177, 1030, 994, 896, 852, 846, 775, 750, 697, 651; ESI-MS: [M-Cl]+ calcd for C51H62Cl1N4Ru, 867.3706; found: 867.3704.
Single-crystal X-ray diffraction analysis. Crystal data for compound 7a. CCDC 1830901, C41H46№CbRu, M = 766.79, triclinic, space group P-1 (No. 2), a = 9.8123(6) A, b = 11.1346(8) A, c = 18.2094(9) A, a = 88.320(5)°, p = 85.219(5)°, y = 65.454(7)°, V = 1803.4(2) A3, Z = 2, T = 100 K, ptalc = 1.412 g cm-3, MCu-Ka) = 5.149 mm-1, F(000) = 796, 20129 reflections measured, 6339 unique (Rint = 0.1159) which were used in all calculations. The final R1 was 0.0587 (I >2ct (I)) and wR2 was 0.1433 (all data).
Crystal data for compound 7b. CCDC 1049432, C51H62N4Cl2Ru, M = 903.02, monoclinic, space group P21/c (No. 14), a = 22.6954(6) A, b = 15.7051(5) A, c = 12.8872(3) A, p = 92.902(2)°, V = 4587.5(2) A3, Z = 4, T = 100 K, pcalc = 1.307 g cm-3, ¿u(Cu-Ka) = = 4.124 mm-1, F(000) = 1896, 37921 reflections measured, 9508 unique (Rint = 0.0876) which were used in all calculations. The final R1 was 0.0472 (I >2ct (I)) and wR2 was 0.1241 (all data).
Catalysts screening. Applied procedure for the ROMP of cis,cis-cycloocta-1,5-diene. For example, 0.05 mol% catalyst loading: 4.07 ^mol of the complex was dissolved in 1 mL toluene-ds. An NMR-
4
tube was charged with c/s,c/s-cycloocta-1,5-diene (0.1 mL, 0.81 mmol), toluene-cfe (0.5 mL) and complex solution (0.1 mL). The NMR tube was then closed and the temperature was then raised to 80 °C. By evaluation of integration of the olefinic 1H-NMR signals of the formed polymer and the consumed monomer, the substrate conversion was plotted.
Applied procedure for the RCM of diethyl diallyl malonate. For example, 0.5 mol% catalyst loading: 2.7 ^mol of the complex in NMR tube was dissolved in 0.3 ml of toluene-d8 and left for 2 minutes before addition of 0.13 mL (0.54 mmol) of diethyl diallyl malonate. The NMR tube was then closed and the temperature was raised to 80 °C. Finally, Integration of the allylic methylene peaks in the 1H-NMR spectrum of the diethyl diallyl malonate and the product was used to count the substrate conversion.
Applied procedure for isomerization of allylic alcohols. An NMR-tube was charged with 2.5 ^mol (5 mol%) of catalyst and dissolved in 0.5 mL CDCl3. After that, to the tube, 0.05 mmol of the substrate and 2.5 ^mol (5 mol%) of KOtBu were added. Afterward, the NMR-tube was sealed and kept at r.t. or
at a temperature of 80 °C. The conversion of the substrate was evaluated by integration of the 1H-NMR signals of the starting alcohol (5.68 ppm) and the formed carbonyl compound (1.86 ppm).
RESULTS AND DISCUSSION
Synthesis of the catalysts. Unsymmetrical NHC lig-ands: 1-mesityl-3-methyl-4,5-dihydroimidazolium chloride (5a) and 1-mesityl-3-cyclohexyl-4,5-dihydroimida-zolium chloride (5b) (l) were prepared according to previously reported procedures [21, 22]. The free / W-heterocyclic carbenes were generated by the deprotonation of NHC precursors using potassium hexamethyldisilazide (KHMDS) in toluene at r.t. A replacement of one of the PCy3 ligands from the first-generation ruthenium indenylidene complex (6) using the generated free carbenes [33-36], complexes 7a-b were obtained. The reaction processes were monitored by TLC. The products were purified by silica gel chromatography and subsequently washed with methanol and pentane to afford air-stable reddish-brown solids (7a and 7b) in moderate yields 74 and 82%, respectively.
Cl
N
Mes
R
,N
=J
(1) KHMDS, Toluene, r.t., 0.5 h -»«
(2) 6, Toluene, r.t.
5a-b
ГЛ
R-N .N-Mes
Cl R-N
PCys
VJ
7a-b
a: R = Methyl
b: R = Cyclohexyl PCys V-
6
General applied strategy for the synthesis of catalysts 7a-b
(1)
The complexes (7a-b) were analyzed by 1H and 13C{1H}NMR spectroscopy after isolation. The obtained signals were further assigned using of hetero-nuclear 1H{13C}HSQC, HMBC, and homo-nuclear 1H{1H}COSY, TOCSY, NOESY NMR spectroscopy [37].
The 1H-NMR spectrum of each complex (Fig. S1, S2) shows peaks characteristic of the indenylidene unit: 7a, doublet (5 = 7.98 ppm) and singlet (5 = 6.77 ppm); 7b, doublet (5 = 8.47 ppm) and singlet (5 = 7.05 ppm) [38]. Besides, the imidazolium ligand peaks are also observed as a multiplet at 3.51 -3.91 ppm and 3.31-3.83 ppm for complexes 7a and 7b, respectively. The 1H-NMR spectrum of each complex strongly suggests the isolation of only one conformer. A single conformer was also detected for ruthenium benzylidene complexes coordinated with W-alkyl/W-mesityl heterocyclic carbenes [20, 21, 39]; however, for ruthenium indenylidene complexes 3a-c, two rotamers were observed [29]. The car-
bene-C (Ru=C) as the new complexes' characteristic exhibits doublet peaks at about 300 ppm in the 13C-NMR spectra.
The NOESY spectrum of complex 7a (Fig. 2, a) shows several correlations between the indenylidene moiety and the mesityl's methyl groups from both NHCs. The spectrum reveals no Noe correlation between the W-methyl group and indenylidene moiety, suggesting that both NHC ligands have the mesityl groups oriented toward the indenylidene side of the ruthenium center. This observation is in agreement with the single-crystal X-ray diffraction analysis (see next section). The indenylidene ligands in the complexes 3a-c were also found to be closer to the mesityl group than to the W-alkyl group [29]. In contrast, the NOESY spectrum of complex 7b (Fig. 2, b) shows several correlations between the indenylidene moiety and the W-cyclohexyl groups from both NHCs, indicating their relative position.
Yu B., Hamad F. B., Van Hecke K. et al. Ruthenium indenylidene complexes bearing ... Юй Б., Хамад Ф. Б., Ван Хекке К. и др. Инденилиденовые комплексы рутения, содержащие .
For the new complexes (7a-b), the orientations of the NHC ligands are different from those of the previously reported ruthenium metathesis catalysts featuring two W-alkyl/W-aryl-heterocyclic carbenes. For example, the aromatic groups of the NHCs in complexes 4a-b have been reported as oriented in different directions relative to the indenylidene fragments [22]. In addition, elemental analysis, mass spectra, and single-crystal X-ray analysis were evaluated to confirm the complexes' purities and configurations.
Single-Crystal X-ray Diffraction Analysis. The crystals of complexes 7a-b are obtained from slow evaporation of their solution (CH2Ch/EtOAc/hexane solution). Thereafter, the crystals were used in X-ray diffraction analysis. The solid-state structures of complexes 7a-b are depicted in Fig. 3, and some
selected bond lengths and angles are listed in Tab. 1. Complex 7a-b crystallized in the triclinic centro-symmetric space group P-1 and P2-i/c, respectively. The asymmetric units of 7a-b accommodate only one ruthenium complex.
In general, complexes 7a and 7b exhibit a similar arrangement of the surrounding ligands around the ruthenium core, while an opposite orientation of the W-mesityl side of the NHC ligand is observed for 7b. In complex 7a, the two mesityl groups lay in a parallel fashion on the five-membered ring of the indenylidene moiety, and n-n interactions were found (3.408(4) and 3.467(4) A between the respective ring centroids). In general, the ligands around the ruthenium core in complex 7a exhibit a similar arrangement to complex 3a [27].
H26
4
(a)
28-N i, =
N4 -1?^ '¡о-
16^ 724-2Г I .,,Cl 2V .,12. Cl — Ru'^,_.2,f 11 13
27
\
9
Г^ 26>C?4 ^
1V 'V 64 \ 17
25^24 2\ 2322
-.14
Fig. 2. NOESY spectra (a) complex 7a, chemical shift of 1H-NMR range from 0.0 to 4.0 ppm (horizontal) and 1H-NMR range from 5.0 to 9.0 ppm (vertical); (b) complex 7b, chemical shift of 1H-NMR range from 6.5 to 8.6 ppm (horizontal) and 1H-NMR
range from -1.5 to 5.0 ppm (vertical)
Рис. 2. Спектры NOESY (а) комплекса 7а, химический сдвиг Ж-ЯМР в диапазоне от 0,0 до 4,0 м.д. (по горизонтали) и ^-ЯМР в диапазоне от 5,0 до 9,0 м.д. (по вертикали); (b) комплекс 7b, химический сдвиг в диапазоне ^-ЯМР от 6,5 до 8,6 м.д. (по горизонтали) и в диапазоне ^-ЯМР от -1,5 до 5,0 м.д. (по вертикали)
Fig. 3. Solid-state structures of complexes 7a and 7b (thermal displacement ellipsoids are shown at the level of 30% probability). The atom-labeling scheme of the heteroatoms is displayed, and Hydrogen atoms are omitted for clarity
Рис. 3. Твердотельные структуры комплексов 7а и 7б (показаны эллипсоиды тепловых смещений на уровне 30% вероятности). Отображается схема маркировки гетероатомов, атомы водорода опущены для ясности
Yu B., Hamad F. B., Van Hecke K. et al. Ruthenium indenylidene complexes bearing ... Юй Б., Хамад Ф. Б., Ван Хекке К. и др. Инденилиденовые комплексы рутения, содержащие .
Table 1. Selected bond lengths (A) and bond angles (°) for complexes 7a and 7b
Таблица 1. Выбранные длины связей (А) и валентные углы (°) для комплексов 7a и 7Ь
7a 7b
Cnhc-Ru-Cnhc 158.4(2) 170.2(2)
CNHC-Ru=CInd 101.2(3)I99.9(3) 93.4(2)I96.0 (2)
Cl-Ru-Cl 159.84(6) 168.67(4)
Ru=CInd 1.862(7) 1.864(4)
Ru-Cnhc 2.101(8)I2.086(7) 2.131(4)I2.134(4)
Ru-Cl 2.414(1 )I2.403(1 ) 2.368(1)I2.380(1)
For 7a, the dihedral angles between the least-squares plane of the mesityl groups and the five-membered ring are 5.2(4)° and 11.0(4)°, respectively. In complex 7b, the two cyclohexyl groups in the NHC ligands orient to the indenylidene moiety, leaving the two mesityl groups at the opposite side of the indenylidene ligand. The dihedral angle between the two least-squares planes of the mesityl groups is 34.8 (2)°.
The least-squares plane angles between the respective two imidazole rings of 7a and 7b differ significantly from each other. The imidazole rings in 7b show a larger dihedral angle (63.6(2)°), which is incomparable with that of imidazole rings in 7a (18.3(4)°).
The Ru-Cl bond lengths for 7a and 7b are in values between 2.368(1) A and 2.414(1) A, and the Ru-CNHC bond lengths are within the range of 2.086(7)A to 2.134(4)A. The Ru=Cind bond length is 1.862(7) A for 7a, and 1.864(4) A for 7b. The Cnhc-Ru-Cnhc, Cl-Ru-Cl and Cnhc-Ru=Cm angles for 7a are 158.4(2)°, 159.84(6)° and 101.2(3)°/99.9(3)°,
RCM of DEDAM. The complexes 7a-b were further investigated using the RCM of DEDAM (9) (3). Since these bis-NHC complexes generally need an elevated temperature to be active, the RCM reaction was performed at 80 °C in toluene at different catalyst loadings. Under these conditions, complex 7a required 180 minutes to complete the reaction with a catalyst loading of 5 mol%, 240 minutes for 95% conversion of the substrate at 1.25 mol% catalyst loading, and 1170 minutes for 81.75% conversion at
respectively, while for 7b these are 170.2(2)°, 168.67(4)° and 93.4(2)°/96.0(2)°, respectively.
Catalytic activity of complexes 7a-b in olefin metathesis reactions. Firstly, the catalytic abilities of complexes 7a and 7b on olefin metathesis reactions were evaluated in ring-closing metathesis polymerization (ROMP) of c/'s,c/'s-1,5-cyclooctadiene (COD) and ring-closing metathesis (RCM) of diethyl diallyl malonate (DEDAM).
ROMP of COD. The complexes 7a-b were involved in the ROMP of COD (8) (2) under varying reaction conditions. At room temperature with a catalyst loading of 1 mol% in CDCl3, initiators 7a-b showed negligible conversion after 24 hours. Similar results were found by using complex 4a-b [22] in which no detectable conversion of COD was found after 24 hours. The poor performance of bis-NHC ruthenium complexes at room temperature might be due to the stronger Ru-Cnhc bond relative to Ru-P bond, and thus high thermal stability and low degree of lability [22].
A significant improvement of the catalytic activity was observed when the reaction temperature was controlled at 80 °C. Generally, all catalysts exhibit almost similar kinetic profiles at an elevated temperature. Under a condition of a catalyst loading of 0.33 mol% in toluene at 80 °C, complex 7a reached full conversion after 90 minutes, while 7b achieved complete COD consumption after 120 minutes (Fig. 4). The better performance of 7a might be due to an increased initiation and decreased steric obstruction for the substrates during reaction [29]. Reducing the catalyst loading, a decrease of catalysts initiation rate and the catalytic activity were observed. Complex 7a needed 3 hours to fully convert the COD with a catalyst loading of 0.1 mol%, and the same time was required for 95% with 0.05 mol% catalyst loading (Fig. 4). Complex 7b, on the other hand, could not reach full conversion even after 3 hours of reaction with neither 0.1 mol% nor 0.05 mol% catalyst loadings (Fig. 4).
(2)
0.5 mol% catalyst loading (Fig. 5). In contrast, complex 7b needs 240 minutes for full conversion of the substrate (at a catalyst loading of 5 mol%), 1170 minutes for 95% conversion (1.25 mol%), and 1170 minutes for 81.75% conversion of the substrate (catalyst loading of 0.5 mol%) (Fig. 5). The ability of these catalysts to remain in solution at high temperature (80 °C) for such a long time (up to 1170 minutes) during catalytic reactions without noticeable decomposition proves their stability.
О
Ru
8
The ROMP of COD (a)
Yu B., Hamad F. B., Van Hecke K. et al. Ruthenium indenylidene complexes bearing ... Юй Б., Хамад Ф. Б., Ван Хекке К. и др. Инденилиденовые комплексы рутения, содержащие ...
EtOOC COOEt
9
The RCM of diethyl diallylmalonate
Ru
EtOOC COOEt
(3)
■ - 7a (0.33 mol%) 7b (0.33 mol%) a- 7a (0.10 mol%) T- 7b (0.10 mol%) 7a (0.05 mol%) 4- 7b (0.05 mol%)
60 80 100 120 Time (minutes)
Fig. 4. ROMP of COD with complexes 7a and 7b (0.05-0.33 mol%) at 80 °C in toluene
Рис. 4. Метатезисная полимеризация с раскрытием цикла цис,цис-1,5-циклооктадиена с комплексами 7а и 7б (0,05-0,33 мол.%) при 80 °С в толуоле
Fig. 5. The RCM of DEDAM with complexes 7a and 7b (0.5-5 mol%) at 80 °C in toluene
Рис. 5. Метатезис с замыканием цикла диэтилдиаллилмалоната с комплексами 7а и 7б (0,5-5 мол.%) при 80 °С в толуоле
Isomerization of Allylic Alcohols. In addition, the synthesized catalysts were investigated for their performance in the isomerization of allylic alcohols (4). O
[Ru]
R ^^ R' -
Isomerization of allylic alcohols
R
R'
(4)
00-
80-
60-
O 40-
20-
0
0
20
40
140 160
80
Our initial efforts focused on the isomerization of penten-3-ol, hepten-3-ol, and 2-cyclohexen-1-ol to their corresponding carbonyl compounds at room temperature. However, by applying 5 mol% of either catalyst at room temperature, no noticeable conversion was observed after 72 hours of reaction for all substrates. From a mechanistic perspective, allylic alcohols can coordinate to the ruthenium center either via olefin or alcoholate functionalities. For the isomerization toward the aldehyde to occur, one of the NHC ligands has to disassociate from the complex before consecutive coordination of the olefin moiety to the metal center can be realized [40]. Therefore, the failure to isomerize the substrate using the catalysts at low temperatures is probably due to the difficulty in de-coordination of the NHCs from the ruthenium core. However, upon addition of 5 mol% of KOiBu, 94 and 92% of the isomerization of hepten-3-ol and penten-3-ol, respectively, was observed after 72 hours, applying complex 7a (Fig. 6). Under these conditions, using catalyst 7b, the conversion of hepten-3-ol and penten-3-ol reached 95 and 90%, respectively (Fig. 6). In these two reac-
tions, complex 7a revealed a higher catalyst initiation rate and a similar activity relative to complex 7b. Nevertheless, even with the addition of 5 mol% of KOiBu at room temperature, the isomerization of 2-cyclohexen-1-ol was not possible applying these catalysts.
By raising the temperature to 80 °C, a significant increase in reaction rate was observed. For the isomerization of hept-3-ol, 98 and 90% conversion were revealed, with complexes 7a and 7b, respectively, after 180 minutes (Fig. 7). In the case of the isomerization of penten-3-ol, the reaction reached 95 and 90% conversion, with catalysts 7a and 7b, respectively, after 240 minutes (Fig. 7).
Using 5 mol% of catalyst without base, no conversions were observed with all substrates after 24 hours of reactions, even at 80 °C. However, by increasing the catalyst loading to 10 mol%, all catalysts isomerized the substrates without adding a base (Tab. 2). The catalysts' performance under these conditions resembles those when 5 mol% of the base was used in addition to 5 mol% of catalyst (Fig. 6). It is worth noting that 5 mol% of the cata-
lysts can be replaced by 5 mol% of the base in this reaction; however, the catalyst cost is not comparable to that of the base.
■ — Ta (hepten-3-oI) • — Tb (hepten-3-oI) a- Ta (penten-3-oI) ▼— Tb (penten-3-oI)
30 40 50
Time (hours)
Fig. 6. Isomerization of hepten-3-ol and penten-3-ol using 5 mol% of complexes 7a and 7b with 5 mol% of KOfBu at room temperature
Рис. 6. Изомеризация гептен-3-ола и пентен-3-ола с использованием 5 мол.% комплексов 7а и 7б при 5 мол.% KOtBu при комнатной температуре
Ta (hepten-3-oI) Tb (hepten-3-oI) Ta (penten-3-oI) Tb (penten-3-oI)
0 50 100 150 200 250
Time (minutes)
Fig. 7. Isomerization of hepten-3-ol and penten-3-ol using 5 mol% of catalysts7a and 7b with 5 mol% of KOfBu at 80 ' The lines are intended as a visual aid
Рис. 7. Изомеризация гептен-3-ола и пентен-3-ола с использованием 5 мол.% катализаторов 7а и 7б с 5 мол.% KOtBu при 80 °С. Линии представлены в качестве наглядного пособия
Table 2. Isomerization of allylic alcohols using 10 mol% catalysts at 80 °C
Таблица 2. Изомеризация аллиловых спиртов с использованием 10 мол.% катализаторов при 80 °С
Substrate Catalyst Time (h) %Conversion
hepten-3-ol 7a 2.5 100
hepten-3-ol 7b 4 98
penten-3-ol 7a 4 95
penten-3-ol 7b 5 90
CONCLUSION
We have successfully synthesized two ruthenium indenylidene initiators bearing bis(W-alkyl/N'-mesityl)-heterocyclic carbene ligands (7a-b), which were fully characterized by NMR spectroscopy, elemental analysis, IR, HRMS, and single-crystal X-ray diffraction analysis. It is interesting to observe that the mesityl groups of 7a and 7b orient in the opposite direction toward the indenylidene moiety. For complexes 7a and 7b, a similar ligand arrangement around the ruthenium center is exhibited, while an opposite direction of the NHC ligands toward the indenylidene moiety was found. For the olefin metathesis reactions, complexes 7a-b produced activities comparable with complexes 4a-b, respectively. For the non-metathesis reaction, the new initiators were evaluated on the isomerization of allylic alcohols. The optimum and cost-effective conditions occurred when the KOtBu base was added as co-initiator. The catalyst's failure to initiate reactions at room temperature was associated with the difficulty in de-coordination of the NHCs from the metal center due to the stronger Ru-Cnhc bond relative to Ru-P bond, and thus high thermal stability and low degree of lability. In all tested reactions, complex 7a performed better. The better performance of 7a was associated with increased initiation and decreased steric obstruction for the substrates during the reaction. The results imply that fine-tuning of the ligands environment can substantially impact both the structure and activity pattern of the resulting catalytic system.
80 -
60 -
Ô 40
20 -
0
0
0
20
60
70
80
100-
80-
60-
O 40-
20-
0
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INFORMATION ABOUT THE AUTHORS Baoyi Yu,
Associate Professor, Key Laboratory of Urban Agriculture (North China), Ministry of Agriculture, College of Biosciences and Resources Environment,
Beijing University of Agriculture, Beinong Road, 102206, Beijing, China, yubaoyi123@hotmail.com https://orcid.org/ 0000-0002-2437-6614
Fatma B. Hamad,
Lecturer,
Department of Chemistry, College of Education,
Dar es Salaam University,
P.O. Box 2329, Dar es Salaam, Tanzania,
hamadfatma@yahoo.com
https://orcid.org/ not available
Kristof Van Hecke,
Associate Professor, Department of Chemistry, Ghent University,
Krijgslaan 281-S3, 9000, Ghent, Belgium,
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ИНФОРМАЦИЯ ОБ АВТОРАХ
Юй Б.,
доцент,
Ключевая лаборатория городского сельского хозяйства (Северный Китай), Министерство сельского хозяйства, Колледж биологических наук и ресурсов окружающей среды, Пекинский сельскохозяйственный университет,
102206, г. Пекин, Бейнонг роад, Китай, yubaoyi123@hotmail.com https://orcid.org/ 0000-0002-2437-6614
Хамад Ф. Б.,
преподаватель,
Химический факультет, Педагогический колледж Дар-эс-Саламского университета, а/я 2329, г. Дар-эс-Салам, Танзания, hamadfatma@yahoo.com https://orcid.org/ not available
Ван Хекке К.,
доцент,
кафедра химии, Гентский университет, 9000, г. Гент, Крийгслаан 281-S3, Бельгия, Kristof.VanHecke@UGent.be
Kristof.VanHecke@UGent.be https://orcid.org/ 0000-0002-2455-8856
Francis Verpoort,
Professor,
State Key Laboratory of Advanced Technology
for Material Synthesis and Processing
Wuhan University of Technology,
Luoshi Lu 122, Wuhan 430070, PR China,
Francis@whut.edu.cn
https://orcid.org/ 0000-0002-5184-5500
Contribution of the authors
The authors contributed equally to this article.
Conflict interests
The authors declare no conflict of interests regarding the publication of this article.
The final manuscript has been read and approved by all the co-authors.
Information about the article
The article was submitted 01.04.2022. Approved after reviewing 16.05.2022. Accepted for publication 30.05.2022.
https://orcid.org/ 0000-0002-2455-8856
Верпоорт Ф.,
профессор,
Государственная ключевая лаборатория
перспективных технологий синтеза
и обработки материалов,
Уханьский технологический университет,
430070, г. Ухань, Люоши Лю, 122, Китай,
francis@whut.edu.cn
https://orcid.org/ 0000-0002-5184-5500
Вклад авторов
Все авторы сделали эквивалентный вклад в подготовку публикации.
Конфликт интересов
Авторы заявляют об отсутствии конфликта интересов.
Все авторы прочитали и одобрили окончательный вариант рукописи.
Информация о статье
Поступила в редакцию 01.04.2022. Одобрена после рецензирования 16.05.2022. Принята к публикации 30.05.2022.
