Bis(N-heterocyclic silylene)xanthene in transition-metal catalysis and main group chemistry

Bis(N-heterocyclic silylene)xanthene in Transition-Metal

Catalysis and Main Group Chemistry

vorgelegt von

M. Sc. Chemie

Yuwen Wang

ORCID: 0000-0001-8781-3394

an der Fakultät II – Mathematik und Naturwissenschaften der Technischen Universität Berlin

Institut für Chemie

zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften - Dr. rer. nat. -

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr. Maria Andrea Mroginski (Technische Universität Berlin) Gutachter: Prof. Dr. Matthias Driess (Technische Universität Berlin)

Gutachter: Prof. Dr. Christian Müller (Freie Universität Berlin)

DISSERTATION

by

M. Sc. Chemistry

Yuwen Wang

Die vorliegende Arbeit entstand in der Zeit von Sep. 2019 bis Nov. 2019 unter der Betreuung von Prof. Dr. Matthias Driess am Institut für Chemie der Technischen Universität Berlin.

Von Herzen kommend gilt mein Dank meinem verehrten Lehrer

Herrn Professor Dr. Matthias Driess

für die Aufnahme in seinen Arbeitskreis, für seine engagierte Unterstützung, und für die Forschungsfreiheit.

Acknowledgements

First and foremost, I would like to express my heartfelt and sincere gratitude to my supervisor Prof. Matthias Driess for his continuous support of my Ph.D. study. His immense knowledge, warmly encouragement, and endless support greatly promoted my research. Without his guidance and invaluable input, this research work would not have been possible. I could not have imagined having a better Doktorvater for my Ph.D. study.

I am grateful to Professor Dr. Christian Müller for acting as the external referee for this thesis, and Professor Dr. Maria Andrea Mroginski for accepting the invitation to be the chairman of the doctoral committee.

I would like to express my special gratitude to Dr. Shenglai Yao and Dr. Yun Xiong, who helped me in numerous ways during various stages of my Ph.D. which is not only limited to scientific research but also to life. Their valuable suggestions, insightful discussions, and positive feedback have significantly contributed to the success of my Ph.D. research. Thank you again for being so supportive all the time.

I would also like to take this opportunity to thank Professor Dr. Yitzhak Apeloig, Dr. Miriam Karni (Technion-Israel Institute of Technology, Israel), Dr. Tibor Szilvási (University of Wisconsin−Madison, United State) and Dr. Arseni Kostenko for carrying out the DFT calculations for the research results of this thesis. Their excellent theoretical calculation work has largely deepened the understanding of the silyene chemistry.

The analytic centers of the Institut für Chemie, Technische Universität Berlin are acknowledged for their outstanding service and invaluable discussions. I would like to especially thank Paula Nixdorf for the assistance in the XRD measurement, Dr. Jan Dirk Epping and Dr. Sebastian Kemper for the NMR measurement, Juana Krone for the elemental analysis measurement, and Dr. Maria Schlangen-Ahl and Marc Griffel for the HRMS measurement.

I gratefully acknowledge the China Scholarship Council (CSC) for financial support. I would also like to thank all the members of the Berlin International Graduate School of Natural Sciences and Engineering (BIG-NSE).

I am indebted to all the lab members of the Driess group for their kind help and friendship. These include the current members Dr. Shenglai Yao, Dr. Yun Xiong, Dr. Jan Dirk Epping, Dr. Prashanth Menezes, Dr. Bochao Su, Dr. Arseni Kostenko, Dr. Stephan Kohl, Dr. Biswarup Chakraborty, Andrea Rahmel, Stefan Schutte, Changkai Shan, Jian Xu, Marcel-Philip Lücke, Carsten Walter, Alexander Burchert, André Hermansdorfer, Frank Czerny, Christopher Eberle, Viktoria Forstner, Niklas J. Hausmann, Rodrigo Beltrán Suito, Shweta Kalra, Noah Subat and as well as the former group members Dr. Zhenbo Mo, Dr. Yupeng Zhou, Dr. Xiaohui Zhao, Dr. Ernesto Ballestero, Dr. Terrance Hadlington, Dr. Jan Paulmann, Min Ha Kim and Mandy Prillwitz.

I would like to acknowledge Dr. Shenglai Yao for proof-reading and correcting this thesis. I thank Marcel-Philip Lücke for translating the abstract.

I would like to express my gratitude to Professor Dr. Huadong Wang for his guidance and kind help during my Master study at Fudan University.

Last but not least, I would like to take this opportunity to express my heartfelt gratitude to my parents and family for supporting and encouraging me to pursue my dreams. My special gratitude goes to my sister Yuxiao Wang and brother Qiang Kou who have kept supporting me through years. Finally, I owe thanks to my beloved husband, Xichang for his continued and unfailing love, support and encouragement during my pursuit of Ph.D. degree. I will be forever grateful for your presence in my life.

Publications during doctoral study

1) “Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefin”.

Yuwen Wang, Arseni Kostenko, Shenglai Yao, Matthias Driess*,

J. Am. Chem. Soc., 2017, 139, 13499–13506. (DOI: https://doi.org/10.1021/jacs.7b07167)

2) “Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide”.

Yuwen Wang, Arseni Kostenko, Terrance J. Hadlington, Marcel-Philip Luecke, Shenglai Yao,

Matthias Driess*,

J. Am. Chem. Soc., 2019, 141, 626–634. (DOI: https://doi.org/10.1021/jacs.8b11899)

3) “An Isolable Bis(silylenes)-Stabilized Germylone and Its Reactivity”.

Yuwen Wang, Miriam Karni, Shenglai Yao, Yitzhak Apeloig,* Matthias Driess*, J. Am. Chem. Soc., 2019, 141, 1655−1664. (DOI: https://doi.org/10.1021/jacs.8b11605)

4) “Synthesis of an Isolable Bis(silylenes)-Stabilized Silylone and Its Reactivity Towards Small Gaseous Molecules”.

Yuwen Wang, Miriam Karni, Shenglai Yao, Alexander Kaushansky, Yitzhak Apeloig,* Matthias

Driess*,

J. Am. Chem. Soc., 2019, 141, 12916−12927. (DOI: https://doi.org/10.1021/jacs.9b06603)

5) “Silicon−Mediated Coupling of Carbon Monoxide, Ammonia, and Primary Amines to Form Acetamides”.

Marcel-Philip Luecke, Arseni Kostenko, Yuwen Wang, Shenglai Yao, Matthias Driess*, Angew. Chem. Int. Ed., 2019, 58, 12940–12944. (DOI: https://doi.org/10.1002/anie.201904361)

6) “N-heterocyclic Silylenes as Powerful Steering Ligands in Catalysis”. Saeed Raoufmoghaddam, Yu-Peng Zhou, Yuwen Wang, Matthias Driess*,

Presentations and Posters

1) “The Application of Bis(silylenes) in Main Group Chemistry and the RCO− _{(R = P, As) as Building}

Blocks for Germanium Phosphorus Compounds and Material Precursors”. Yuwen Wang, Matthias Driess*,

Presentation, Oxford Workshop, Berlin, Germany, October 7, 2019.

2) “Bis(N-heterocyclic silylene)xanthene as a Powerful Tool in Transition Metal Catalysis and Main-Group Chemistry”.

Yuwen Wang, Matthias Driess*,

Presentation, 31st China-German Chemical Association Annual Conference, Leipzig, Germany, May 10−11, 2019.

3) “An Isolable Bis(silylenes)-Stabilized Germylone and Its Reactivity”.

Yuwen Wang, Miriam Karni, Shenglai Yao, Yitzhak Apeloig,* Matthias Driess*,

Poster, 2019 Reaxys PhD Prize Symposium, Amsterdam, The Netherlands, October 3−4, 2019.

4) “Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide”.

Yuwen Wang, Arseni Kostenko, Terrance J. Hadlington, Marcel-Philip Luecke, Shenglai Yao,

Matthias Driess*,

Poster, 9th European Silicon Days, Saarbrücken, Germany, September 9−12, 2018.

5) “Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins”.

Yuwen Wang, Arseni Kostenko, Shenglai Yao, Matthias Driess*,

Poster, 18th International Symposium on Silicon Chemistry (ISOS XVIII) in conjunction with

ZUSAMMENFASSUNG

Diese Dissertation ist der Synthese des ersten Xanthen-verbrückten Bis(N-heterocyklischen silylens) [Xant(SiII_L)

2] [Xant = 9,9-Dimethyl-Xanthene-4,5-diyl, L = PhC(NtBu)2] sowie seiner Anwendung in

der Übergangsmetallkatalyse und Hauptgruppenchemie gewidmet.

Zunächst, wurde das neuartige Bis(silylen) [Xant(SiII_L)

2] erfolgreich synthetisiert und als starker σ

Donorligand in der Übergangsmetallchemie eingesetzt. Die Reaktion von [Xant(SiII_L)

2] mit Ni(cod)2

(COD = cycloocta-1,5-dien) führte zur Bildung des Nickel-Olefin-Komplexes [Xant(SiII_L)

2]]Ni(η2

-1,3-cod). Durch einen Ligandenaustausch mit PMe3 gelang die Isolation des Ni-Komplexes

[Xant(SiII_L)

2]Ni(PMe3)2. Beide Ni-Komplexe konnten H2 unter Ausbildung zweier

Nickelhydrid-Komplexe aktivieren. Bemerkenswerterweise ist, der Nickel-Olefin-Komplex [Xant(SiII_L)

2]Ni(η2

-1,3-cod) ein chemoselektiver, effizienter Präkatalysator für die homogene Hydrierung von Olefinen mit einem breiten Substratspektrum bei 1 bar H2-Druck und Raumtemperatur. Berechnungen des

katalytischen Hydrierungsprozesses, basierend auf der Dichtefunktionaltheorie (DFT), ergaben einen neuen Modus der H2-Aktivierung, bei dem die SiII-Atome im [Xant(SiIIL)2] Ligand eine Kooperative

Rolle in den Schritten der H2-Spaltung und Hydrid-Übertragung auf das Olefin besitzen.

Zudem wurde das Bis(silylen) [Xant(SiII_L)

2] eingesetzt, um Kohlenmonoxid zu aktivieren, welches

eine extrem starke C-O-Dreifachbindung enthält. Unerwarteterweise fungierte das Bis(silylen) als Vier-Elektronen-Reduktionsmittel bereits unter milden Reaktionsbedingungen und ermöglichte die selektive desoxygenative Homokupplung von CO zu dem entsprechenden Disilylketen [Xant(SiII_L)

2](µ-O)(µ-CCO). Im Gegensatz dazu war das Dibenzofuran-Analogon von [Xant(SiIIL)2]

mit einem größeren Abstand zwischen den beiden SiII_{-Atomen gegenüber CO-inert. Dies deutet auf}

die entscheidende Rolle des Si∙∙∙Si-Abstand bei der kooperativen CO-Bindung und -Aktivierung hin. Theoretischen Untersuchungen zur CO-Homokupplung mit [Xant(SiII_L)

2] ergaben, dass im ersten

Schritt der CO-Bindung und –Spaltung CO als Lewis-Säure (Vier-Elektronen-Akzeptor) dient. Dies steht im Gegensatz zur CO-Koordination durch Übergangsmetalle, bei der CO als Lewis-Basis (Zwei-Elektronen-Donor) fungiert.

Darüber hinaus war das Bis(silylen) [Xant(SiII_L)

(E = Ge oder Si) erhalten und strukturell, sowie spektroskopisch charakterisiertwerden konnten. Die elektronische Struktur von [Xant(SiII_L)

2]E0 (E = Ge oder Si) wurde durch DFT-Berechnungen

untersucht, die eindeutig zwei freie Elektronenpaare auf dem zentralen E0 _{(E = Ge oder Si) Atom}

zeigten. In seiner Reaktivität zeigt [Xant(SiII_L)

2]E0 (E = Ge oder Si) eine auffällige Chemie gegenüber

kleinen Molekülen. Die [Xant(SiII_L)

2]Ge0 Verbindung konnte ein oder zwei AlBr3-Moleküle

koordinieren, wobei die Lewis-Addukte [Xant(SiII_L)

2]Ge(AlBr3) bzw. [Xant(SiIIL)2]Ge(AlBr3)2

erhalten wurden. Durch Reaktion von [Xant(SiII_L)

2]Ge0 mit 9-Borabicyclo[3.3.1]nonan (9-BBN) als

potentielle Lewis-Säure, gelang die Isolierung des ersten Boryl-Germylen-Komplexes mit einer heteroallylischen Ge∙∙∙Si∙∙∙Si π-Konjugation. Interessanterweise wurde durch Reaktion von [Xant(SiII_L)

2]Ge0 mit Ni(cod)2 der einzigartige {[Xant(SiIIL)2]GeI}2NiII Komplex mit einem

dreigliedrigen Ge2Ni-Ring erhalten. Im Vergleich zu [Xant(SiIIL)2]Ge0, war die [Xant(SiIIL)2]Si0

-Verbindung oxophiler und konnte daher mit dem milden Oxidationsmittel N2O kontrollierbar zur

Reaktion gebracht werden. Durch die Regulierung der eingesetzten Molmenge des zugesetzten N2O

konnten verschiedene Produkte erhalten werden. Darüber hinaus wurde die Spaltung der starken N-H-Bindung in Ammoniak durch [Xant(SiII_L)

2]Si0 ermöglicht, welches das erste Beispiel der NH3

-Aktivierung in der Ylidon-Chemie darstellt. Herausragend ist, dass [Xant(SiII_L)

2]E0 (E = Ge oder Si)

geeignet war, ein frustriertes Lewis-Paar (FLP) mit BPh3 zu bilden, so dass die kooperative Spaltung

von Wasserstoff sowie die Addition von Ethylen gelang. Diese waren die ersten Beispiele und zeigen auf, dass Ylidone in der FLP-Chemie eingesetzt werden können.

Abstract

This dissertation is devoted to the synthesis of the first bis(N-heterocyclic silylene)xanthene compound [Xant(SiII_L)

2] [Xant = 9,9-dimethyl-xanthene-4,5-diyl, L = PhC(NtBu)2] and its application

in transition-metal catalysis and main group chemistry.

At first, the novel bis(silylenes) compound [Xant(SiII_L)

2] was successfully synthesized and utilized

as a strong σ donor ligand in transition-metal chemistry. The reaction of [Xant(SiII_L)

2] with Ni(cod)2

(COD = 1,5-cyclooctadiene) led to the formation of the intriguing Ni-olefin complex [Xant(SiII_L)

2)]Ni(η2-1,3-cod) which could undergo ligand exchange with PMe3 to afford the Ni

complex [Xant(SiII_L)

2]Ni(PMe3)2. Both Ni compounds can activate H2 to generate two hydrido Ni

complexes. Remarkably, [Xant(SiII_L)

2]Ni(η2-1,3-cod) was a strikingly efficient precatalyst for the

homogeneous hydrogenation of olefins with a wide substrate scope at 1 bar H2 pressure and room

temperature. Density Functional Theory (DFT) calculations on the catalytic hydrogenation process revealed a new mode of H2 activation, in which the SiII atoms from the [Xant(SiIIL)2] ligand play a

cooperative role in the steps of H2 cleavage and hydride transfer to the olefin.

Then the bis(silylenes) compound [Xant(SiII_L)

2] was utilized to activate carbon monoxide which

has an extremely strong C≡O bond. Unexpectedly, under mild reaction conditions, the [Xant(SiII_L) 2]

compound acts as a four-electron reduction reagent to facilely achieve the selective deoxygenative homocoupling of CO, affording the corresponding disilylketene [Xant(SiII_L)

2](µ-O)(µ-CCO)

compound. In contrast, the dibenzofuran analogue of [Xant(SiII_L)

2] with a longer SiII∙∙∙SiII distance

was inert towards CO, indicating the crucial role of the Si∙∙∙Si distance in cooperative CO binding and activation. The theoretical investigations on CO homocoupling with [Xant(SiII_L)

2] revealed that the

initial step of CO binding and scission involves CO acting as a Lewis acid (four-electron acceptor), in sharp contrast to CO coordination mediated by transition-metals where CO serves as a Lewis base (two-electron donor).

Moreover, comprising two strong σ donating SiII _{atoms, the bis(silylenes) [Xant(Si}II_L)

2] was capable

to stabilize highly reactive monatomic zero-valent silicon and germanium to give [Xant(SiII_L)

2]E0 (E

unambiguously exhibited two lone-pairs of electrons on the central E0 _{(E = Ge or Si) atom. In its}

reactivity, [Xant(SiII_L)

2]E0 (E = Ge or Si) shows a striking chemistry towards small molecules. The

[Xant(SiII_L)

2]Ge0 compound could coordinate one or two AlBr3 to generate the Lewis adducts

[Xant(SiII_L)

2]Ge(AlBr3) and [Xant(SiIIL)2]Ge(AlBr3)2, respectively. [Xant(SiIIL)2]Ge0 could also

react with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid to furnish the first boryl(silyl)germylene complex possessing a heteroallylic B∙∙∙Ge∙∙∙Si π-conjugation. Interestingly, when [Xant(SiII_L)

2]Ge0 was mixed with Ni(cod)2, the unique {[Xant(SiIIL)2]GeI}2NiII complex with a

three-membered Ge2Ni ring was obtained. Compared with [Xant(SiIIL)2]Ge0, [Xant(SiIIL)2]Si0 is more

oxophilic and therefore could react with the mild oxidant N2O in a controllable way. By changing the

molar amount of the added N2O, distinct products could be obtained. In addition, the cleavage of the

strong N−H bond in ammonia was also accomplished by [Xant(SiII_L)

2]Si0, which was the first case of

NH3 activation in ylidone chemistry. Remarkably, [Xant(SiIIL)2]E0 (E = Ge or Si) was suitable to form

a frustrated Lewis pair (FLP) with BPh3 to cooperatively achieve the heterolytic dihydrogen cleavage

or ethylene addition, representing, for the first time, that a metallylone could be applied in FLP chemistry.

Content

Content

1. INTRODUCTION………..1

1.1 Isolable divalent silicon compounds………...2

1.1.1 Isolable NHSis………...………..4

1.1.1.1 Isolable five-membered NHSis………...4

1.1.1.2 Isolable six-membered NHSis………...5

1.1.1.3 Isolable four-membered NHSis………..6

1.1.2 Isolable bis(NHSis)………...………...7

1.1.2.1 Isolable bis(NHSis) synthesized via reduction of trichlorosilanes………...7

1.1.2.2 Isolable bis(NHSis) synthesized via reduction of bis(chlorosilane)………....8

1.1.2.3 Isolable bis(NHSis) synthesized via introduction of amidinato silylene into chelating scaffold………...9

1.2 Bis(NHSis) as chelating ligands in transition-metal catalysis………..…...…………..10

1.2.1 Bidentate bis(NHSis) ligands in transition-metal catalysis………..…………..11

1.2.2 Tridentate bis(NHSis) ligands in transition-metal catalysis………..……….…13

1.3 Low-valent group 14 element compounds for small molecules activation……..…….………….15

1.3.1 Low-valent group 14 element compounds for carbon monoxide activation……..…….…....15

1.3.1.1 Low-valent carbon compounds for carbon monoxide activation………..….….………...15

1.3.1.2 Low-valent silicon compounds for carbon monoxide activation………..……..…...17

1.3.1.3 Low-valent germanium compounds for carbon monoxide activation…….………..19

1.3.2 Low-valent silicon compounds for carbon dioxide activation…..…..………...….20

1.3.2.1 Divalent silicon compounds for carbon dioxide activation………..…….…….………...20

1.3.2.2 Zero-valent silicon compounds for carbon dioxide activation………...……….………...22

1.4 Isolable zero-valent group 14 element E compounds (E = Si or Ge)…...……….23

1.4.1 Isolable triatomic zero-valent silicon compounds………..23

1.4.2 Isolable diatomic zero-valent group 14 element E compounds (E = Si or Ge)………24

1.4.2.1 Isolable diatomic zero-valent silicon compounds………...24

1.4.2.2 Isolable diatomic zero-valent germinium compounds………...25

Content

1.4.3.1 Cyclic alkyl silylenes supported monatomic zero-valent group 14 element E compounds

(E = Si or Ge)……….………...…………26

1.4.3.2 Stable carbenes supported monatomic zero-valent group 14 element E compounds (E = Si or Ge)….………..…….27

1.4.3.3 Other examples of isolable monatomic zero-valent group 14 element E compounds (E = Si or Ge)….………...………29

2. MOTIVATION AND OBJECTIVES………31

3.RESULTS AND DISCUSSION………33

3.1 Synthesis of bis(NHSis)xanthene and bis(NHSis)dibenzofuran compounds………..……..33

3.1.1 Background………..………..33

3.1.2 Synthesis of bis(NHSis)xanthene compound 1a...………...………..34

3.1.3 Synthesis of bis(NHSis)dibenzofuran compound 1b……….………35

3.1.4 Comparison of 1a and 1b………...36

3.2 Synthesis of bis(NHSis)xanthene-coordinated Ni complexes and their applications in the catalytic hydrogenation of olefins………...39

3.2.1 Background………...……..……...39

3.2.2 Synthesis of bis(NHSis)xanthene-coordinated Ni complexes………..………...…………...41

3.2.3 Stoichiometric H2 activation by the bis(NHSis)xanthene-coordinated Ni complexes 2 and 3………....43

3.2.4 Catalysts screening for the catalytic hydrogenation of olefins………..47

3.2.5 Bis(NHSis)-stabilized Ni complex 2 for catalytic hydrogenation of olefins……..……..…...48

3.2.6 Mechanism of the catalytic hydrogenation process………..……….…….50

3.3 Bis(NHSis)xanthene-mediated small molecules activation………..………55

3.3.1 Bis(NHSis)xanthene-mediated reductive homocoupling of carbon monoxide……...……...55

3.3.1.1 Background...….………....………...55

3.3.1.2 Bis(NHSis)xanthene-mediated reductive homocoupling of carbon monoxide…….……56

3.3.1.3 Mechanism of bis(NHSis)xanthene-mediated homocoupling of carbon monoxide …...58

3.3.1.4 Further functionalization of disilylketene compound 8……….64

Content

3.3.2.2 Bis(NHSis)xanthene-mediated carbon dioxide activation………...……...…66

3.3.2.3 DFT-derived mechanism of bis(NHSis)xanthene-mediated carbon dioxide activation...68

3.4 Synthesis of the first bis(NHSis)-stabilized monatomic zero-valent germanium compound and its reactivity towards small molecules………..……….71

3.4.1 Background...…………..……….71

3.4.2 Synthesis of the first bis(NHSis)-stabilized Ge0 _{compound 15……….………...………….71}

3.4.3 Electron structure of compound 15……….……….…….………...74

3.4.4 Reactivity of compound 15……….……….76

3.4.4.1 Reactivity of compound 15 towards AlBr3………..………...76

3.4.4.2 Reactivity of compound 15 towards 9-BBN………...…...…………78

3.4.4.3 Reactivity of compound 15 towards Ni(cod)2………..…………..…...…81

3.4.4.4 Heterolytic H2 cleavage with compound 15 in the presence of BPh3……….83

3.5 Synthesis of the first bis(NHSis)-stabilized monatomic zero-valent silicon compound and its reactivity towards small gaseous molecules………...….………87

3.5.1 Background………..………..………...….…87

3.5.2 Synthesis of the first bis(NHSis)-stabilized Si0 _{compound 23……...…….………...…….…88}

3.5.3 Electron structure of compound 23……….………..……….92

3.5.4 Reactivity of compound 23………..……..………....93

3.5.4.1 Reactivity of compound 23 towards N2O……….…..………...93

3.5.4.2 Reactivity of compound 23 towards NH3………..………..……..97

3.5.4.3 H2 and ethylene activation by compound 23 with the assistance of BPh3………..99

3.5.4.4 Mechanism of H2 and ethylene activation processes………..……….…….…...102

4. SUMMARY………....107

5. EXPERIMENTAL SECTION………...…….….……..….113

5.1 General consideration……….113

5.2 Analytical methods………...…..113

5.3 Starting materials………115

5.4 Synthesis and characterization of the new compounds………115

Content

5.4.3 Synthesis of complex [Xant(SiII_L)

2]Ni(η2-1,3-cod) 2………117

5.4.4 Synthesis of complex [Xant(SiII_L) 2]Ni(PMe3)2 3………..…118

5.4.5 Synthesis of dinuclear Ni complex 4………..119

5.4.6 Synthesis of dihydrido Ni complex 5……….120

5.4.7 Synthesis of compound [Xant(SiII_L) 2](µ-O)(µ-CCO) 8……….121

5.4.8 Synthesis of 13_{C labeled compound [Xant(Si}II_L) 2](µ-O)(µ-13C13CO) 8……….122

5.4.9 Synthesis of 18_{O labeled compound [Xant(Si}II_L) 2](µ-18O)(µ-CC18O) 8……….123

5.4.10 Synthesis of 13_{C and} 18_{O labeled compounds [Xant(Si}II_L) 2](µ-O)(µ-13C13CO), [Xant(SiIIL)2] (µ-O)(µ-13_CC18_{O), [Xant(Si}II_L) 2](µ-18O)(µ-CC18O) and [Xant(SiIIL)2](µ-18O)(µ-C13CO)] 8………...………...123

5.4.11 Synthesis of compound 12………124

5.4.12 Synthesis of compound 13………...………125

5.4.13 Synthesis of 13_{C labeled compound 13………..………...………126}

5.4.14 Synthesis of compound 14………...………127

5.4.15 Synthesis of compound 15………...………128

5.4.16 One-pot synthesis of compound 15………..129

5.4.17 Synthesis of compound 16………...129

5.4.18 Synthesis of compound 17………...130

5.4.19 Synthesis of compound 18………...131

5.4.20 Synthesis of compound 19………...132

5.4.21 Synthesis of compound 20………...133

5.4.22 Synthesis of deuterated compound 20-d2……….134

5.4.23 Synthesis of compound 21………...135 5.4.24 Synthesis of compound 22………...136 5.4.25 Synthesis of compound 23………...137 5.4.26 Synthesis of compound 24………...138 5.4.27 Synthesis of compound 25...139 5.4.28 Synthesis of compound 26………...140 5.4.29 Synthesis of compound 27………...141

Content

5.4.31 Synthesis of compound 28………...143

5.5 Additional experiments………...144

5.5.1 Additional experiments of section 3.2……….144

5.5.2 Additional experiments of section 3.3……….146

6. REFERENCES………...147

7. APPENDIX………167

7.1 Crystal data and structure refinement………..167

Table 7-1. Crystal data and structure refinement for compounds 1a, 1b and 2………...…...167

Table 7-2. Crystal data and structure refinement compounds 3~5………...………..…..168

Table 7-3. Crystal data and structure refinement compounds 8, 12 and 13……...………...……..169

Table 7-4. Crystal data and structure refinement compounds 14~16………...170

Table 7-5. Crystal data and structure refinement compounds 17~19………...171

Table 7-6. Crystal data and structure refinement compounds 20~22………...172

Table 7-7. Crystal data and structure refinement compounds 23~25………...173

Table 7-8. Crystal data and structure refinement compound 26~28………..……...174

7.2 DFT calculations……….175 7.2.1 DFT calculations of section 3.2………..175 7.2.2 DFT calculations of section 3.3………..175 7.2.3 DFT calculations of section 3.4………..176 7.2.4 DFT calculations of section 3.5………..179 7.3 Abbreviations…….……….181

Introduction

1. INTRODUCTION

It is well-known that transition-metal complexes serve as powerful tools in catalytic transformations and small molecule activation. Recent research has shown that low-valent main group element compounds can behave like transition-metals.1 _{The last three decades have witnessed spectacular}

achievements in the field of low-valent group 14 element chemistry, which is particularly evident by the successful isolation of previously elusive divalent2 _{and zero-valent}3 _{group 14 element compounds.}

Some of the low-valent group 14 element compounds have acted as excellent ligands in transition-metal catalysis,4 _{in view of their strong σ donating and π accepting group 14 element centers.}5

Moreover, the low-valent group 14 element compounds, mimicking transition-metals,1 _{are capable of}

small molecules activation,6 _{e.g., H}

2 activation by the GeI compound ArGeGeAr [Ar =

2,6-(2,6-iPr2C6H3)2C6H3],7 and H2 or NH3 activation by a stable singlet carbene.8

The introduction of my dissertation focuses on the advances of fours subtopics in the low-valent group 14 element chemistry, i.e., the developments of isolable silylenes [especially of N-heterocyclic silylenes (NHSis)], the applications of bis(NHSis) in transition-metal-based catalysis, the utilization of low-valent group 14 element compounds in small molecules activation, and the synthesis and reactivity of isolable zero-valent Ge and Si compounds (Figure 1-1). In the following subchapters, the state-of-the-art knowledge related to these four sections will be discussed in details.

Introduction

1.1 Isolable divalent silicon compounds

The chemistry of divalent silicon compounds is mainly represented by the dicoordinate neutral silylene species, heavy analogues of carbenes. However, in stark contrast to the parent carbene (H2C:)

A (Figure 1-2), in which the carbon atom forms a sp2_{-hybrid orbital and is in a triplet state,}9 _{the parent}

silylene (H2Si:) B (Figure 1-2) possesses singlet ground state adopting a (3s)2(3p)2 valence electron

configuration due to the dramatically different size of the 3s and 3p orbitals.10,11,12 _{Involving one}

lone-pair of electrons and one empty p orbital, silylenes therefore are typically highly reactive species which could only be isolated in cryogenic matrices (≤ 77 K) during the 1980s.13,14 _{Above this temperature,}

the reactive silylenes will immediately undergo dimerization or further polymerization.15

Figure 1-2. The different electronic ground states of the parent carbene A and silylene B.

In order to access the isolable silylenes at room temperature, two marvelous stabilization strategies could be applied: thermodynamic and/or kinetic stabilizations both aiming at an efficient protection of the highly reactive vacant p orbitals of silylenes (Figure 1-3).12

Introduction

The thermodynamic stabilization strategy generally utilizes heteroatom substitutes to partially fill the empty p orbitals of silylenes via the mesomeric effect and the intramolecular coordination of the heteroatoms (Figure 1-3, X). The kinetic stabilization employs extremely bulky substitutes, providing steric hindrance, to prevent silylenes from self oligomerization or attacking by external nucleophiles (Figure 1-3, Y).

By taking advantage of the aforementioned strategies, Jutzi and co-workers reported the first isolable silylene I-1, stabilized by two bulky pentamethylcyclopentadienyls, via the reduction of the corresponding dichlorosilane precursor (Chart 1-1a).16 _{In 1994, the first NHSi was reported by West}

et al.,17 _{which represents a landmark in NHSi chemistry.}

Up to now, remarkable advances have been achieved in the flourishing stable silylene chemistry, which is evident by the successful isolation of various silylenes, e.g., the cyclic alkyl silylenes I-2~3 (Chart 1-1b),18 _{the cyclic (alkyl)(amino)silylene I-4 (Chart 1-1c),}19 _{phosphine-stabilized silylene I-5}

(Chart 1-1d)20 _{and the diverse acyclic silylenes I-6~11 (Chart 1-1e).}21

Chart 1-1. Selected examples of isolable silylenes.

Given the significant role of NHSis in the silylene chemistry, the recent fruitful reactivity will be discussed in detail in the following sections 1.1.1 and 1.1.2.

Introduction

1.1.1 Isolable NHSis

1.1.1.1 Isolable five-membered NHSis

In 1991, Arduengo and coworkers reported the first stable crystalline N-heterocyclic carbene (NHC).22 _{Three years later, the first isolable NHSi I-12a, a heavier analogue of the “Ardueugo type”}

carbene, was synthesized by West and Denk et al. through the reduction of the N-heterocyclic silicon dichloride precursor with potassium (Scheme 1-1).23a _{This seminal work opened the avenue to stable}

NHSis and fueled tremendous interest of chemists for the synthesis of silylene species.

Scheme 1-1. Synthesis of the first isolable NHSi I-12a.

Chart 1-2. Examples of five-membered NHSis.

Recently, a series of unsaturated five-membered NHSis I-12b~f featuring new alkyl23b _{and aryl}23c,d

substitutes on the N atoms were reported (Chart 1-2a). Interestingly, the analogue of I-12a with a saturated backbone (i.e., I-13a) was also reported by West and Denk et al.,24a _{which was more reactive}

than its thermally stable unsaturated analog I-12a and could reversibly oligomerize to disilene in the solid state or in concentrated solutions.24b _{This tendency to oligomerization could be prevented when}

Introduction

Remarkably, the benzo- or pyrido-fused five-membered NHSis 14a~d, including a bis(silylenes)

I-14d, were also isolated (Chart 1-2c).25

1.1.1.2 Isolable six-membered NHSis

The six-membered NHSi chemistry started with the isolation of the modified β-diketiminate (Nacnac) ligand supported NHSi I-18a reported by our group in 2006.26 _{The dibromosilane precursor}

I-17 was obtained by a one-pot synthesis in which the Nacnac scaffold I-15 was firstly lithiated by

n-BuLi to generate I-16 followed by the reaction with SiBr4 in the presence of

N,N,N’,N’-tetramethylethylenediamine (TMEDA) (Scheme 1-2). It is worth noting that, in the absence of TMEDA, the reaction of I-16 with SiBr4 resulted in a mixture of products instead of the compound

I-17. Therefore, TMEDA is considerably significant in both activating SiBr4 by forming a complex and

promoting the dehydrohalogenation to give I-17 instead of the Nacnac-SiBr3. Ultimately, the reduction

of I-17 with potassium graphite led to the desired six-membered NHSi I-18a as yellow crystals (Scheme 1-2).

Scheme 1-2. Synthesis of the six-membered NHSi I-18a26_.

The two resonance structures I-18a and I-18b indicate that there are two possible nucleophilic centers in this NHSi compound: one is at the silicon atom with a lone-pair of electrons and the other is at the exocyclic methylene group of the backbone (Scheme 1-3).26 _{Consequently, NHSi I-18}

exhibited unique reaction modes compared with the aforementioned NHSis. For instance, compound

18 reacted with trimethylsilyl trifluoromethanesulfonate (MeOTf) to give the 1,4-addition product I-19 with the SiMe3 unit bonded at the exocyclic methylene group and the OTf moiety coordinated to

the SiII _{atom (Scheme 1-3).}26 _{Compound I-19 was not stable in solution and isomerized to the}

Introduction

[H(OEt2)2]+[B(C6F5)4]− led to the formation of the silyliumylidene species I-21 stabilized by a planar

aromatic 6π-electron delocalization via the protonation of the nucleophilic exocyclic methylene part (Scheme 1-3).27,28 _{When B(C}

6F5)3 was mixed with I-18, the zwitterionic product I-22 was isolated.27

Interestingly, the exposure of I-18 to water vapor furnished a siloxy silylene I-23 containing both a SiII

and SiIV _{atoms (Scheme 1-3).}29

Scheme 1-3. Resonance structures of I-1826 _{and synthesis of compounds I-19~23}27~29_.

1.1.1.3 Isolable four-membered NHSis

The first isolable four-membered NHSi is the amidinate ligand stabilized chlorosilylene I-25 (Scheme 1-4) synthesized by Roesky et al. in 2006.30 _{The reduction of the trichloride precursor I-24}

with two molar equivalents of potassium resulted in the target silylene I-25 in a mere 10 % isolated yield (Scheme 1-4). Three years later, the same group modified this synthesis method by utilizing NHC (i.e., 1,3-di-tert-butylimidazol-2-ylidene) or LiN(SiMe3)2 as reductive reagents and the dichlorosilane

I-26 as the precursor (Scheme 1-4).31 _{Consequently, the isolated yields were improved to 35% (for the}

Introduction

Interestingly, one of the three chlorine substituents in compound 1-24 could be replaced by NMe2,

OtBu, OiPr or PiPr2 groups leading to the compounds I-27.32 Then the reduction of I-27 by potassium

generated the fournovel heteroleptic NHSis I-28 (Scheme 1-4).

Scheme 1-4. Synthesis of the four-membered NHSis I-2530,31 _{and I-28}32_.

1.1.2 Isolable bis(NHSis)

Recently, bis(NHSis) compounds, containing two NHSi moieties in one molecule, have attracted increasing attentions owing to the enhanced σ-donating property compared with the NHSis involving one silylene site. To date, the strategies to access such isolable bis(NHSis) species could be classified into three categories.

1.1.2.1 Isolable bis(NHSis) synthesized via reduction of trichlorosilanes

The first method represents the reduction of trichlorosilane precursors to generate the bis(NHSis) compounds. For instance, the treatment of the trichloride precursor I-24 with three molar equivalents of KC8 resulted in the isolation of the bis(silylenes) I-29 featuring a SiI−SiI bond (Scheme 1-5).33 The

molecular structure of I-29, determined by an X-ray diffraction analysis, demonstrated that each Si atom adopted a disordered tetrahedral geometry with one lone-pair of electrons occupying the apex.

In 2011, Kato et al. reported the intriguing phosphine-stabilized bis(NHSis) I-31 through an analogous procedure, in which the trichlorosilane precursor I-30 was dechlorinated by three molar equivalents of lithium (Scheme 1-5).20c

Introduction

Scheme 1-5. Synthesis of bis(NHSis) compounds I-2933_{and I-31}20c_.

1.1.2.2 Isolable bis(NHSis) synthesized via reduction of bis(chlorosilanes)

The second strategy to obtain isolable bis(NHSis) is the reduction of bis(chlorosilanes) precursors. In 2005, Gehrhus and Lappert et al. synthesized the first biphenyl-bis(NHSis) I-33 via the reductive dehalogenation of the bis(dichlorosilane) precursor I-14d (Scheme 1-6).25c

Later, an oxygen-bridged bis(NHSis) I-34 was isolated by Driess et al. through the dehydrochlorination of disiloxane I-33 (Scheme 1-6).34 _{Notably, while the two silicon atoms of I-14d}

pointed in opposite directions, the two divalent silicon atoms of I-34 oriented in the same direction providing the possibility to serve as a chelating ligand in transition metal chemistry.

Introduction

1.1.2.3 Isolable bis(NHSis) synthesized via the introduction of amidinato silylene into a chelating scaffold

The chlorosilylene I-2530 _{(Scheme 1-4), stabilized by an amidinate ligand, contains a Si−Cl bond}

and therefore can be facilely attached to the dilithiated chelating backbone via a salt metathesis reaction to access isolable bis(NHSis) compounds.

By taking advantage of this strategy, our group reported the first chelating bis(NHSis) I-37 with a central phenyl group in 2012.35 _{The bis(NHSis) I-37 was synthesized by the initial reaction of}

4,6-di-tert-butylresorcinol I-35 with n-BuLi leading to the dilithiated compound I-36 followed by the salt metathesis reaction with two molar equivalents of chlorosilylene I-25 (Scheme 1-7).

Scheme 1-7. Synthesis of bis(NHSis) I-3735_.

Inspired by the successful isolation of the first chelating bis(NHSis) I-37, the same group utilized ferrocene as a backbone and synthesized the chelating bis(NHSis) I-40 via a similar reaction procedure (Scheme 1-8).36

Introduction

In 2014, the first pyridine-based chelating bis(NHSis) I-42 was synthesized through a one-pot procedure, in which the dilithiated product was generated in-situ and two molar equivalents of chlorosilylene I-25 were subsequently added (Scheme 1-9).37 _{The composition and molecular structure}

of I-42 were unambiguously confirmed by multinuclear NMR spectroscopy and a single-crystal X-ray diffraction analysis.

Recently, employing the similar one-pot synthesis process, Driess et al. also isolated the chelating bis(NHSis) I-44 with a carborane scaffold (Scheme 1-9).38

Scheme 1-9. One–pot synthesis of bis(NHSis) compounds I-4237 _{and I-44}38_.

1.2 Bis(NHSis) as chelating ligands in transition-metal catalysis

The design of novel ligands to steer the reactivity of transition-metal complexes is crucial to the development of organometallic chemistry.39 _{Among the diverse ligand systems, chelating ligands are}

prominent owing to their significant advantages in controlling the electronic and geometric properties of metal complexes.40 _{The well-established transition-metal complexes supported by chelating ligand}

Introduction

Since the first isolation of the NHSi-stabilized iron complex by Welz and Schmid in 1977,42 _NHSi

ligands with strong σ-donating nature5 _{have fueled tremendous interest in organometallic chemistry.}4

Recently, the bis(NHSis) compounds with two strong σ-donating SiII _{atoms have been utilized as}

bidentate (Chart 1-3a) or tridentate (Chart 1-3b) chelating ligands in transition-metal chemistry.4 _Their

transition-metal complexes show excellent catalytic performance in various homogeneous catalytic transformations,4 _{which will be exhibited in detail in the following sections 1.2.1 and 1.2.2.}

Chart 1-3. Examples of chelating bis(NHSis) ligands.

1.2.1 Bidentate bis(NHSis) ligands in transition-metal catalysis

The oxygen bridged bis(NHSis) I-34, serving as a bidentate ligand, reacted with Ni(cod)2 (COD =

1,5-cyclooctadiene) to generate the Ni complex I-45, in which the two SiII _{atoms and one COD were}

coordinated to the Ni center.34 _{Then Enthaler and Inoue et al. showed that this bis(NHSis)-supported}

Ni complex I-45 was an effective precatalyst in the C−C cross-coupling reaction of aryl halides with organometallic zinc or Grignard reagents (Scheme 1-10).43

Introduction

Another intriguing bidentate bis(NHSis) ligand is the one with a ferrocene backbone (i.e., I-40) 36

and its coordination behavior towards the transition-metal cobalt was firstly investigated. Treatment of the ferrocene-based bis(NHSis) I-40 with the CpCoI _{(Cp = Cyclopentadienyl) precursor prepared in}

advance led to the successful isolation of the bis(NHSis) coordinated CpCoI _{complex I-46 (Scheme}

1-11a).36 _{This complex could act as a precatalyst in the [2+2+2] cyclotrimerization reactions of}

phenylacetylene to give two isomers of triphenylbenzene (Scheme 1-11a).36 _{Interestingly, the}

cyclotrimerization of phenylacetylene and acetonitrile catalyzed by I-46 resulted in the substituted pyridines (Scheme 1-11a). In 2017, the ferrocene-based bis(NHSis) ligand I-40 was also utilized to stabilize Fe0 _{complex I-47 with a η}6_{-benzene ring which could achieve the catalytic hydrogenation of}

ketones with a wide substrate scope under 50 bar H2 pressure at 50 oC (Scheme 1-11b).44

Scheme 1-11. a) [2+2+2] cyclotrimerizations catalyzed by bis(NHSis)-CoCp I-46. b) Hydrogenation

reaction catalyzed by bis(NHSis)-Fe(η6_{-benzene) I-47.}

The bis(NHSis) I-44 with a carborane backbone could also act as a bidentate chelating ligand in transition-metal chemistry. Its coordination ability towards NiII _{was studied leading to the isolation of}

Introduction

achieved by bis(NHSis)-NiBr2 I-48 in moderate to good yield (Scheme 1-12).38

Scheme 1-12. Buchwald-Hartwig cross-coupling catalyzed by bis(NHSis)-NiBr2 I-48.

1.2.2 Tridentate bis(NHSis) ligands in transition-metal catalysis

The bis(NHSis) I-37 with a benzene ring can serve as a tridentate ligand and react with various transition-metal precursors to afford pincer-type transition-metal complexes I-49~52 (Chart 1-4).35,45,46 _{Treatment of bis(NHSis) I-37 with Ir and Rh precursors afforded the corresponding}

pincer-type Ir-, Rh-based complexes I-49~51.45 _{Moreover, a Ni pincer complex I-52 could be obtained by the}

reaction of tridentate ligand I-37 with NiBr(dme) (dme = 1,2-dimethoxyethane) in the presence of an excess amount of NEt3.46

Chart 1-4. Bis(NHSis) I-37 stabilized transition-metal complexes I-49~52.

Then the catalytic performances of these typical pincer-type transition-metal complexes I-49~52 were investigated. The in-situ generated complex I-50 successfully catalyzed the C–H borylation of arenes with pinacolborane (Scheme 1-13a).45 _{The complex bis(NHSis)-NiBr I-52 could act as a}

catalyst for the Sonogashira cross-coupling reaction between phenylacetylene and 1-octenyl iodide (Scheme 1-13b).46 _{The mechanism investigations also shed light on the elementary steps in the}

cross-Introduction

coupling process.

Scheme 1-13. a) C-H borylation of benzene catalyzed by in-situ generated bis(NHSis)-IrHCl(coe) I-50 (COE = cyclooctene). b) Sonogashira cross-coupling reaction catalyzed by the bis(NHSis)-NiBr I-52.

Besides the bis(NHSis) I-37 featuring a benzene scaffold, the pyridine-based bis(NHSis) I-42 is also a versatile tridentate ligand in transition-metal chemistry. The reaction of the tridentate ligand

I-42 with Fe(PMe3)4 afforded the pincer-type bis(NHSis)-Fe0(PMe3)2 complex I-53 with the

coordination of the N atom of the pyridine backbone (Scheme 1-14a).37 _{The Fe}0 _{pincer complex I-53}

could also be obtained by the reduction of the bis(NHSis)-FeII_Cl

2 precursor with potassium graphite

in THF in the presence of an excess of PMe3.

The bis(NHSis)-stabilized Fe0 _{complex I-53, with an electron-rich Fe center and two strong σ}

donating Si atoms, served as a good precatalyst in the hydrosilylation of acetophenone and its derivatives (Scheme 1-14a).37

The coordination capability of the bis(NHSis) I-42 towards cobalt complex was also investigated by Cui and our group. The pincer-type bis(NHSis)-CoBr2 complex I-54, which was synthesized by the

reaction of tridentate ligand I-42 with the CoBr2 precursor, could effectively catalyze the regioselective

Introduction

Scheme 1-14. a) Carbonyl hydrosilylation reaction catalyzed by bis(NHSis)-Fe(PMe3)2 I-53. b)

Regioselective borylation reaction catalyzed by bis(NHSis)-CoBr2 I-54.

1.3 Low-valent group 14 element compounds for small molecules activation

Low-valent group 14 element compounds with highly reactive group 14 centers are capable of activating numerous small molecules, such as H2, CO, CO2, NH3, and various organic compounds.5~8

Among these diverse activations, the CO activation is significant and draws tremendous interest as CO can serve as a versatile C1 building block to produce multicarbon compounds (e.g., fuels, solvents, and

organic bulk chemicals) to solve the shortage of fossil fuels.48a _{With the growing environmental issues}

raised by the CO2 emission, the capture48b and transformation48c of CO2 to valuable chemicals has also

been a hot topic nowadays. Therefore, the recent advances regarding COx (x = 1, 2) activation mediated

by the low-valent group 14 element compounds will be discussed in section 1.3.

1.3.1 Low-valent group 14 element compounds for carbon monoxide activation 1.3.1.1 Low-valent carbon compounds for carbon monoxide activation

Introduction

Previous studies have shown that highly transient triplet carbenes could react with carbon monoxide to afford ketene species.49 _{In 2006, Bertrand et al. showed that the stable acyclic and cyclic}

(alkyl)(amino)carbenes (aAAC and cAAC) I-55 and I-57 could also achieve the fixation of CO to generated the corresponding stable ketene compounds I-56 and I-58 (Scheme 1-15a), representing the first examples of the stable singlet aAAC and cAAC for CO activation.50

Scheme 1-15. CO activation by stable carbenes.

Three years later, the cyclic diamidocarbene I-59, which was more electrophilic than classical NHCs, was also shown by Bielawski et al. to react with CO to give the ketene compound I-60 in a reversible fashion (Scheme 1-15b).51 _{Interestingly, the ferrocene-based NHC I-61 reacted with CO to generate}

Introduction

al. reported another example that two molecules of NHC I-64 were added to one molecule of CO to form the oxyallyl derivative I-65. When the temperature was raised to -10 o_{C, compound I-66 was}

obtained via the migration of the Dip (Dip = 2,6-iPr2C6H3) group from the N to the O atom (Scheme

1-15c).53

1.3.1.2 Low-valent silicon compounds for carbon monoxide activation

Elemental silicon and its subvalent compounds may hold a unique position among potentially suitable low-valent group 14 element systems for CO activation, because it has a relatively high reduction ability in both elemental form and sub-valent states (e.g, divalent silicon in silylenes) and is highly abundant with about 28% of the mass of the Earth’s crust. It has been shown that silicon atoms and small clusters react with CO in cryogenic matrices to yield carbonyl complexes which can photochemically rearrange to elusive cyclic four-membered Si2(µ-O)(µ-CSi) and Si2(µ-O)(µ-CCO)

species.54 _{Previous efforts towards divalent silicon (‘silylene’)-mediated CO activation included the}

observation of transient silylene-CO adducts.55

More recently, the reaction of CO with 1, 4-disila (Dewar benzene) I-67 led to the formation of an intriguing cyclic disilyl ketone I-68 (Scheme 1-16a).56 _{In 2013, Sekiguchi and Scheschkewitz et al.}

reported the carbonylation of cyclotrisilene I-69 with CO to afford the unexpected product I-70 (Scheme 1-16b).57

Introduction

It has also been shown that a disilenyl lithium I-71 allows the scission of the CO triple bond and the following C−C coupling to generate the product I-72 (Scheme 1-17).58a _{The ketenyl intermediate I-73}

was proposed but could not be verified experimentally (Scheme 1-17). Interestingly, when disilenide compound I-71 was mixed with M(CO)6 (M=Cr, Mo, W) in benzene, the compound I-74 bearing a

C=C=O ketene moiety was isolated.58a

Scheme 1-17. CO activation by the disilenyl lithium I-71.

Very recently, Apeloig et al. reported the CO activation mediated by silyl lithium and silenyl lithium derivatives.58b _{The reaction of silyl lithium I-75 with CO led to the generation of the intriguing bis(silyl)}

ketyl radical I-76 and tetra(silyl) di(ketyl) biradical I-77. When the silenyl lithium I-78 was exposed to 1 bar CO, the unexpected 1-silaallenolate I-79 was obtained, providing a new strategy to synthesize silaallenes.

Scheme 1-18. CO activation by the silyl lithium and silenyl lithium compounds I-75 and I-78.

In 2019, Aldridge et al. demonstrated that the acyclic silylene I-80 could achieve the reductive coupling of CO to afford the dimeric product I-81 (Scheme 1-19).59 _{The molecular structure of I-81}

Introduction

Scheme 1-19. CO activation by the acyclic silylene I-80.

1.3.1.3 Low-valent germanium compounds for carbon monoxide activation

The first CO activation example by the stable germylene was reported by Power and co-workers in 2009.60 _{Exposure of the diarylgermylene I-82a (R = H) to CO led to the isolation of CO coupling}

product I-83a, in which the coupled (CO)2 moiety was inserted into the Caryl−Ge bond {aryl =

2,6-(2,6-iPr3C6H3)2]C6H3} with a migration of a isopropyl group to form a six-membered ring (Scheme

1-20).

However, when the bulkier germylene I-82b (R = iPr) was exposed to CO, the (CO)2 fragment was

inserted into the bond between the less bulky aryl group {i.e., [2,6-(2,4,6-Me3C6H2)2]C6H3} and Ge

atom with a migration of a methyl group, resulting in the formation of the compound I-83b (Scheme 1-20).

Introduction

1.3.2 Low-valent silicon compounds for carbon dioxide activation 1.3.2.1 Divalent silicon compounds for carbon monoxide activation

In 1996, Jutzi and co-workers reported the CO2 activation by the first isolable silylene I-1,16 i.e.,

decamethylsilicocene (Chart 1-1).61 _{Interestingly, when toluene was utilized as the solvent, compound}

I-84 containing a Si(O2CO2)Si fragment could be obtained (Scheme 1-21). When pyridine was used

as the solvent, compound I-85 featuring an eight-membered ring was generated (Scheme 1-21).

Scheme 1-21. CO2 activation by the decamethylsilicocene I-1.

NHSis also exhibit high reactivity towards CO2. For instance, the siloxy silylene I-23,29 which was

obtained via the reaction of the modified Nacnac ligand supported NHSi I-18a26 _{with water vapor}

(Scheme 1-3), could react with CO2 to give the silanoic silylester I-86 with a Si=O bond.62 The

amidinato silylene I-87 reacted with CO2 affording the silicon carbonate compound I-88 featuring a

Si[O2C=O] fragment.63,64

Introduction

In 2014, Kira and co-workers, utilizing the cyclic alkyl silylene I-2, also achieved CO2 activation.

Exposure of compound I-2 to CO2 led to a color change of the solution, then the addition of MeOH

into the solution gave the bis(silyl) carbonate I-89 as the final product.65

Scheme 1-23. CO2 activation by the cyclic alkyl silylene I-2.

Compound I-91, which was generated through the intermolecular transfer of the transient acyclic silylenes I-92, could behave as a masked acyclic silylene and react with CO2 to afford the silicon

carbonate compound I-93.66 _{The stable acyclic silylene I-6}21a _{could also react with CO}

2 resulting in

the compound I-90 with the elimination of CO.59

Introduction

Kato and Baceiredo et al. reported that one molecule of phosphine-stabilized bis(silylenes) I-31 reacted with four CO2 molecules giving the aminosilicate compound I-94 involving two

pentacoordinate silicon atoms.20c

Notably, for silylene-mediated CO2 activations, silicon carbonate compounds containing a

Si(O2C=O) moiety were typically generated. In the reactions of disilene with CO2, the addition of two

silicon atoms to the C=O bond of CO2 was observed. For example, the disilenes I-95 and I-97 trapped

one molecule of CO2 to give the compounds I-9667 and I-9868, respectively. Interestingly, compound

I-98 could further react with one CO2 molecule to afford the compound I-99 via the insertion of one

O atom from CO2 into the Si−Si bond.

Scheme 1-26. CO2 activation by the disilenes I-95 and I-97.

1.3.2.2 Zero-valent silicon compounds for carbon dioxide activation

Besides the divalent silicon compounds, zero-valent silicon species are also capable of CO2

activation. In 2015, Robinson et al. demonstrated that the NHC-stabilized diatomic zero-valent silicon compound I-100 reacted with CO2 in 1:5 molar ratio to give compound I-101 featuring a six-membered

Introduction

Interestingly, the reaction of one molar equivalent of the bis(NHSis)-stabilized Si0 _{species I-102}

with four molar equivalents of CO2 at -30 oC generated the silicon dicarbonate compound I-103.70

DFT calculations revealed that the CO2 activation process involved a silicon monoxide intermediate

bis(NHC)-Si=O and a dioxide intermediate bis(NHC)-SiO2.

Scheme 1-28. CO2 activation by the monatomic zero-valent silicon compound I-102.

1.4 Isolable zero-valent group 14 element E compounds (E = Si or Ge)

Zero-valent element compounds, although well known in transition-metal chemistry,71 _{are rare in}

main-group chemistry, particularly the highly reactive heavy group 14 elements E0 _{(E = Si, Ge).}2,72~87

Utilizing strongly σ donating and bulky ligands, species bearing heavy zero-valent group 14 elements have been isolated successfully in recent years. They fall into three major categories: triatomic E0

3L3,72

diatomic E0

2L273~76 and monatomic E0L277~87 compounds (E = Si, Ge; L = σ donor ligand), respectively.

Owing to their unique bonding motifs, structures, and reactivities, heavy group 14 E0 _{compounds are}

of interest to both synthetic chemists2,72-87 _{and theoreticians.}88,89 _{In this section, these three categories}

of zero-valent silicon or germanium complex will be discussed in detail.

1.4.1 Isolable triatomic zero-valent silicon compounds

Utilizing cAAC with strong σ donating and enhanced π accepting properties, Roesky et al. successfully synthesized the first cAAC-stabilized triatomic Si0 _{compound I-105 with a Si}

3 cyclic ring

through the reduction of (cAAC)SiCl4 I-104 precursor by potassium graphite (Scheme 1-29).72 The

molecular structure of I-105, ambitiously determined by an X-ray diffraction analysis, exhibited that each silicon atom was three-coordinated and adopted a trigonal pyramidal geometry with one lone-pair of electrons occupying the apex. DFT calculations of the bonding situation in I-105 revealed a

Introduction

partial double bond character of the Si−C bond due to the significant π accepting capability of cAAC.72

Scheme 1-29. Synthesis of triatomic zero-valent silicon compound I-10572_.

1.4.2 Isolable diatomic zero-valent group 14 element E compounds (E = Si or Ge) 1.4.2.1 Isolable diatomic zero-valent silicon compounds

In 2008, Robinson and co-workers successfully reduced the NHC-coordinated SiCl4 compound

I-106 by KC8 to isolate the first NHC-stabilized diatomic zero-valent silicon compound I-100 with a

lone-pair of electrons on each silicon atom, representing a landmark and paving the way for zero-valent silicon chemistry (Scheme 1-30a).73 _{An X-ray structural analysis of species I-100 shows that}

the two C−Si bonds are almost perpendicular to the central Si=Si double bond with the C–Si–Si bond angles of 93.37(5)°.

Introduction

Recently, taking advantage of the similar carbene-stabilization strategy, Roesky et al. reported the cAACs-supported dinuclear Si0 _{species I-108 comprising a Si=Si double bond via the reduction of the}

cAAC-stabilized tetrachlorosilane precursor I-107 (Scheme 1-30b).74 _{The theoretical investigations}

and the experimentally Raman results both demonstrated that the lone-pair of electrons of each silicon atom was polarized towards the C atom of carbene which is analogous to the situation in triatomic Si0

compound I-105.

1.4.2.2 Isolable diatomic zero-valent germanium compounds

Inspired by the isolation of diatomic Si0 _{compound I-100, Jones, Stasch and Frenking et al. reported}

the first germanium analogue of compound I-100. The reduction of the NHC-stabilized GeCl2 I-109

with the Nacnac-supported Mg(I) reagent led to the desired NHC-stabilized diatomic Ge0 _compound

I-110 (Scheme 1-31a).75 _{In 2014, the first NHSi-stabilized dinuclear Ge}0 _{complex I-112 (Scheme}

1-31b) was investigated by So et al..76 _{This species was obtained through the reduction of the}

NHSi-supported dichlorosilane I-111 with KC8 (Scheme 1-31b).76

Introduction

1.4.3 Isolable monatomic zero-valent group 14 element E compounds (E = Si or Ge)

In addition to the triatomic and diatomic Si0 _{species, another newly emerging category of low-valent}

silicon chemistry is the monatomic zero-valent group 14 element compounds L:→E0_{:L. Such}

species with a bent geometry, in which the central E atom, stabilized by donor-acceptor interaction between E and donor ligand L, possesses four valence electrons as two lone-pairs, are termed as ylidones (carbone: E = C; silylone: E = Si; germylone: E = Ge; stannylone: E = Sn; plumbylone: E = Pb) (Figure 1-4).88e _{This section will focus on the currently known silylone and germylone chemistry.}

Figure 1-4. Nomenclature for monatomic zero-valent group 14 element compounds.

1.4.3.1 Cyclic alkyl silylenes supported monatomic zero-valent group 14 element E compounds (E = Si or Ge)

In 2003, Kira and co-workers synthesized the first cyclic alkyl silylene stabilized trisilaallene derivative I-115a (E’ = E = Si) via a two-step synthesis procedure, in which the cyclic alkyl silylene

I-218a _{was firstly inserted into the Si−Cl bond of tetrachlorosilane followed by the reduction of the}

generated chloro(trichlorosilyl)silane precursor I-114a with KC8 (Scheme 1-32).77 Notably, unlike its

light analogue allene, featuring a liner C=C=C structure, the compound I-115a comprised a bent Si=Si=Si unit and two Si−Si bond lengths lied in the typical range of those in disilenes. Employing the heavier congener of I-114a, i.e., cyclic alkyl germylene I-113, the same group reported the synthesis of the stable 1,3-digermasilaallene I-115b (E’ = Ge and E = Si) andtrigermaallene I-115c (E’ = Ge and E = Ge) via a similar synthesis strategy (Scheme 1-32).78 _{Interestingly, the 2-germadisilaallene I-115d}

(E’ = Si and E = Ge) was directly synthesized by mixing the silylene I-2, GeCl2-dioxane complex and

Introduction

Scheme 1-32. Synthesis of the cyclic alkyl silylenes stabilized monatomic zero-valent silicon and

germanium compounds I-115 a~d78~79_.

Frenking et al., based on theoretical studies, argued that compounds I-115 (Scheme 1-32) should rather be classified as ylidones.88 _{Remarkably, the computational investigations by Apeloig et al.}

revealed that the parent trisilaallene H2Si=Si=SiH2 which has a highly acute SiSiSi bond angle of 69.4o

and the trisilacyclopropylidene are bond-stretch isomers.89a,b

1.4.3.2 Stable carbenes supported monatomic zero-valent group 14 element E compounds (E = Si or Ge)

The first cAACs-stabilized zero-valent Si0 _{was reported by Roesky, Frenking and Stalke et. al. in}

2013.80 _{Firstly, the cAAC-stabilized dichlorosilane precursor I-118 was synthesized via the reaction of}

NHC coordinated SiCl2 I-116 with three molar equivalents of cAAC I-117 (Scheme 1-33a). Notably,

the C−Si bonds in compound I-118 were electron sharing bonds instead of the donor-acceptor C→Si bonds and compound I-118 exhibited a diradical character. Then the reduction of I-118 with KC8 led

to the formation of the cAACs-stabilized zero-valent Si0 _{species I-119 (Scheme 1-33a).}80 _{The Natural}

Bond Orbital (NBO) analysis of the bonding situation of the CSiC unit in I-119 revealed a σ lone-pair orbital and a three-center C−Si−C p orbital. The cAACs were then utilized to stabilize the monatomic zero-valent Ge compound. Mixing the cAAC I-117 or I-121, GeCl2-dioxane complex and KC8 in a

1:2:2.1 molar ratio afforded the corresponding Ge0 _{species I-120 or I-122 (Scheme 1-33b).}81 _{The DFT}

calculations demonstrated that compounds I-120 and I-122 are germylones featuring a partially diradicaloid character owing to the strong π accepting nature of the cAACs.

Introduction

Scheme 1-33. Synthesis of the cAACs-stabilized monatomic zero-valent silicon and germanium

compounds I-11980_{, I-120 and I-122}81_.

NHCs, being strong σ donor ligands, were also applied in silylone and germylone chemistry. In 2013, Driess and co-workers, employing the chelating bis(NHC) ligand I-123, successfully synthesized the first bis(NHC)-stabilized zero-valent Si compound I-102 through the reduction of its SiII _precursor

I-124 with sodium naphthalenide (Scheme 1-34).82 _{The bis(NHC)-stabilized chlorosilyliumylidene}

chloride I-124 was obtained via the reaction of bis(NHC) ligand I-123 with NHC-SiCl2 [NHC =

bis(2,6-diisopropyl-phenyl)-imidazol-2-ylidene].

As the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of NHC is larger than that of cAAC, the π accepting property of NHC is weaker than that of cAAC. Consequently, the central silicon atom of I-102 is more electron-rich than I-119 and the C−Si bond lengths in I-102 are longer than those in I-119.82

The bis(NHC)-stabilized Ge0 _{compound I-126 was also isolated by the reaction of its}

chlorogermyliumylidene chloride precursor I-125 with NaC10H8, which is analogous to the synthesis

Introduction

Scheme 1-34. Synthesis of the NHSi-stabilized monatomic zero-valent silicon and germanium

compounds I-10282 _{and I-126}83_.

1.4.3.3 Other examples of isolable monatomic zero-valent group 14 element Ecompounds (E = Si or Ge)

In 2014, a bis(imino)pyridine pincer ligand stabilized germylone I-12784 _{(Scheme 1-35) and an}

imino-NHC-supported germylone I-12885 _{(Scheme 1-35) were reported by Nikonov et al. and et al.,}

respectively. More recently, two germylene coordinated silylone I-12986 _{(Scheme 1-35) and a novel}

Si0 _{species I-130}87 _{(Scheme 1-35) with a two-NHC-stabilized four-membered Si ring were devised}

and synthesized successively.

Scheme 1-35. The selected examples of isolable monatomic zero-valent silicon and germanium

Motivation and Objectives

2. MOTIVATION AND OBJECTIVES

In order to enrich the bis(NHSis) chemistry which is still in its infancy, my dissertation is devoted to the design and synthesis of the novel bis(NHSis) compounds, and their utilization in small molecules activation and transition metal chemistry.

Therefore, the first objective is the synthesis of the bis(NHSis)xanthene [Xant(SiII_L)

2] (xant =

9,9-Dimethyl-Xanthene-4,5-diyl, L = PhC(NtBu)2) and bis(NHSis)dibenzofuran compounds (Scheme

2-1). The distances of the two SiII _{atoms in these bis(NHSis) compounds are expected to be distinct and}

will influence their reactivity.

Scheme 2-1. Synthesis of bis(NHSis) compounds with different Si···Si distances.

Considering the good performance of the xantphos as chelating ligands in transition-metal-mediated catalysis and the limited examples of Ni-mediated homogeneous hydrogenation of olefins, I will then utilize the bis(NHSis)xanthene, an analogue of the xantphos, to stabilize the Ni species and employ their Ni complexes as catalysts in homogeneous hydrogenation of olefins (Scheme 2-2).