Early transition metal complexes stabilized by bulky aminopyridinato ligands [Elektronische Ressource] / vorgelegt von Awal Noor

Noor

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• Publié par: universitat_bayreuth
• Publié le: 01 janvier 2008
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 4 Mo

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Early Transition Metal Complexes Stabilized by Bulky
Aminopyridinato Ligands




DISSERTATION


Zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften
der Universität Bayreuth






vorgelegt von
M.Phil. Awal Noor
geboren in Orakzai Agency/Pakistan




Bayreuth, 2008




Early Transition Metal Complexes Stabilized by Bulky
Aminopyridinato Ligands




DISSERTATION


Zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften (Dr. rer. nat.)
im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften
der Universität Bayreuth






vorgelegt von
M.Phil. Awal Noor
geboren in Orakzai Agency/Pakistan




Bayreuth, 2008This thesis fulfills the requirements of the doctoral degree of the Faculty of Biology,
Chemistry and Geological Sciences at the University of Bayreuth.










Thesis submitted: 02-04-2008
Date of Scientific Colloquium: 09-07-2008










Examination Committee:

Chairman: Prof. Dr. Rainer Schobert
1. Referee: Prof. Dr. Rhett Kempe
2. Referee: Prof. Dr. Jürgen Senker
Prof. Matthias Ballauf The following work was undertaken during the period January 2004 to April 2008 at the
Lehrstuhl für Anorganische Chemie II der Universität Bayreuth under the supervision of
Prof. Dr. Rhett Kempe.




























To my father, to the patience of my wife and to the innocence of my little son Aahil Table of Contents
1. Summary/Zusammenfassung 1
2. Introduction 5
3. Overview of Thesis-Results 9
3.1. Trialkyltantalum Complexes Stabilized by Aminopyridinato Ligands 9
3.2. Highly Active/Selective and Adjustable Zirconium Polymerization Catalysts 10
Stabilized by Aminopyridinato Ligands
3.3. Zirconium and Hafnium Complexes Stabilized by very Bulky Aminopyridinato 11
Ligands
3.4. Acetylenetitanium Complex Stabilized by Aminopyridinato Ligands 12
23.5. Synthesis and Structure of Low Valent Chromium Complexes Stabilized by η - 13
Coordinated Aminopyridinato Ligands
3.6. Metal-Metal Distances at the Limit: A Coordination Compound with an Ultra 14
Short Cr-Cr Bond
3.7. Individual Contribution to Joint Publications 15
4. Synthesis and Structure of Trialkyltantalum Complexes Stabilized by 18
Aminopyridinato Ligands
4.1. Introduction 19
4.2. Results and Discussion 20
4.3. Conclusions 27
4.4. Experimental Section 28
5. Highly Active/Selective and Adjustable Zirconium Polymerization Catalysts 35
Stabilized by Aminopyridinato Ligands
5.1. Introduction 36
5.2. Results and Discussion 37
5.3. Conclusions 48
5.4. Experimental Section 49
6. Synthesis and Structure of Zirconium and Hafnium Complexes Stabilized by 58
very Bulky Aminopyridinato Ligands
6.1. Introduction 59
6.2. Results and Discussion 60
6.3. Conclusions 72 6.4. Experimental Section 73
7. Acetylenetitanium Complex Stabilized by Aminopyridinato Ligands 84
7.1. Introduction 84
7.2. Results and Discussion 85
7.3. Conclusions 89
7.4. Experimental Section 89
8. Synthesis and Structure of Low Valent Chromium Complexes Stabilized by 95
2η -Coordinated Aminopyridinato Ligands
8.1. Introduction 95
8.2. Results and Discussion 96
8.3. Conclusions 101
8.4. Experimental Section 102
9. Metal-Metal Distances at the Limit: A Coordination Compound with an Ultra 107
Short Cr-Cr-Bond
9.1. Main Text 107
9.2. Supporting Information for the paper 117
10. List of Publications 123
1. Summary/Zusammenfassung
1. Summary
A series of early transition metal complexes stabilized by aminopyridinato ligands have been
synthesized. Many of these complexes have been studied in terms of their structure and have
been evaluated in terms of applications in catalysis. The overall evaluation tells about the
importance of electrophilicity of the metal centre, the steric bulk of the applied ligands, and
the route of syntheses.
Trialkyltantalum complexes were synthesized by salt elimination or toluene elimination by
reacting the corresponding lithiated ligand with trialkyltantalum dichloride or the
corresponding ligand with pentabenzyltantalum, respectively. These trialkyltantalum
complexes are unusually thermally stable towards α-H elimination and form rather unstable
organocations.
Bis(aminopyridinato) complexes of zirconium were prepared using salt elimination route. The
steric bulk of the ligands prevented the redistribution to tris- or tetrakis(aminopyridinato)
zirconium complexes. These zirconium complexes are thermally robust, highly active and
selective ethylene polymerization catalysts. Ethylene is polymerized highly selectively out of
a mixture of ethylene and propylene. Slight changes in the steric demand of the bulky ligand
periphery can be used to tune the nature of the formed polymers by maintaining the selectivity
issue. The Zr alkyl cations of the sterically more demanding version of the ligands are able to
polymerize ethylene in a living fashion at 50 °C.
We also became interested in toluene elimination chemistry and observed that the bulky
aminopyridinates that give selectively bis(aminopyridinato) complexes via salt metathesis
chemistry lead selectively to mono(aminopyridinato) tribenzyl Zr/Hf complexes. In the solid
2 1state, one of the three benzyls is η -coordinated and rest are η -coordinated to the electron
deficient metal centres. One of the three benzyls has been partially abstracted using B(C F ) . 6 5 3
6The phenyl ring of B-bounded benzyl in these complexes shows an η -coordination and
essentially blocks the vacant site of the metal centre, consequently, preventing it to
polymerize ethylene at room temperature. At elevated temperature a moderate single site
polymerization activity with the formation of high molecular weight polyethylene was
observed for these zwitterionic complexes. The attempted abstraction of the second benzyl
group failed when the zwitterionic complexes were reacted with an additional equivalent of
B(C F ) . However using one equivalent of [R (Me)NH][B(C F ) ] (R = C H –C H ) 6 5 3 2 6 5 4 16 33 18 37
instead of B(C F ) give catalysts which show moderate activities in ethylene polymerization. 6 5 3
Treatment of the aminopyridinato metal tribenzyls with [R (Me)NH][B(C F ) ] (R = C H –2 6 5 4 16 33
1 1. Summary/Zusammenfassung
C H ) give active ethylene polymerization catalysts, which produce low molecular weight 18 37
polyethylene for the zirconium complexes and high molecular weight for the hafnium ones.
Propylene polymerization under the same conditions failed, whereas during copolymerization
ethylene-propylene copolymers with separated propene units and alternating sequences were
observed.
The versatility of these ligands was flourished by synthesizing a titanium alkyne complex
stabilized by aminopyridinato ligands which may show a very multifaceted chemistry. The
reactivity of this complex was studied by the insertion of acetone into titanium carbon bond.
The complex is not only quite stable at room temperature but also in solution at high
temperatures under argon atmosphere despite a weakly bonded acetylene ligand.
The chemistry of low valent chromium stabilized by sterically demanding aminopyridinato
2 IIligands has been explored and first non-bridging η -coordinated chromium complexes were
synthesized using such ligands. It was foud that reacting deprotonated ligands of the same
II/IIIsteric bulk with the corresponding salts of the low valent chromium can lead to
II IIImono(aminopyridinato) dimeric chromium or monomeric chromium complexes,
respectively. It is worth to note that gradual decrease in the steric bulk leads to
bis(aminopyridinato) mononuclear chromium complexes.
The ability of aminopyridinato ligands to stabilize transition metals in low oxidation state has
Ibeen highlighted by the synthesis of a dimeric chromium complex. The X-ray crystal
structure analysis revealed an exceptionally short chromium-chromium distance of 1.7488(18)
Å, the shortest metal-metal bond reported so far for a stable compound. The homobimetallic
chromium complex was synthesized by the reduction of aminopyridinato ligand stabilized
II/IIIchromium chloride precursor with KC . Anaylsis of its electronic structure indicates 8
quintuple bonding.
2