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Cheap metals for noble tasks : low-valent molybdenum and tungsten nitrosyl complexes in homogeneous hydrogenations and hydrosilylations


Chakraborty, Subrata. Cheap metals for noble tasks : low-valent molybdenum and tungsten nitrosyl complexes in homogeneous hydrogenations and hydrosilylations. 2013, University of Zurich, Faculty of Science.

Abstract

The search for alternatives to platinum group metal catalyses became in recent years more and more intense and expected to experience increased growth in the future. The most obvious reason for replacing these platinum group metals is that they are very expensive. The high cost is obviously connected to the low abundance of these metals. Substantial costs are involved in industrial processes when recovery and recycle of the metal is required. In this perspective, the “middle transition elements” which are the “rising stars” in modern homogeneous catalysis field are apt for replacing the platinum group metals. The research presented in this thesis shows the potential of new catalysts that do not require precious metals may ultimately supplant the use of precious metals in Wilkinson and Osborn type hydrogenations, Bullock type ionic hydrogenations, Noyori type bifunctional hydrogenation and Ojima type hydrosilylation reactions. Preparation of a new class of molybdenum and tungsten nitrosyl complexes were realised bearing a tridentate (iPr2PCH2CH2)2PPh, (etpip) ligand. Pentagonal bipyramidal complexes M(NO)Cl3(etpip) (M = Mo, 3a; W, 3b) were isolated as isomeric mixtures starting from the precursors M(NO)Cl3(NCMe)2 (M = Mo, W). 3a and 3b were reduced to 17 e- rare paramagnetic complexes Mo(NO)Cl2(etpip), 4a and W(NO)Cl2(etpip), 4b showing characteristic paramagnetic behavior determined by EPR spectroscopy, as well as by SQUID measurements. Treatment of 1% Na/Hg to the THF solutions of 3a and 3b in the presence of 1 bar carbon monoxide at room temperature led to the formation of low-valent Mo(NO)(CO)Cl(etpip), 6a and W(NO)(CO)Cl(etpip), 6b complexes. Reduction of 3a,b with 1% Na/Hg under 1 bar ethylene furnished the ethylene coordinated complexes Mo(NO)Cl(η2-ethylene)(etpip), 7a and W(NO)Cl(η2-ethylene)(etpip), 7b. The reaction of the 7a with NaHBEt3 at 75°C revealed formation of the hydride complex Mo(NO)H(η2-ethylene)(etpip), 8a. On the other hand LiBH4/Et3N combination was found to be suitable to prepare W(NO)H(η2-ethylene)(etpip), 8b complex from 7b. 8a and 8b were found to be highly efficient for the hydrogenation of several olefins. A maximum TOF of 5253 h-1 was achieved for the hydrogenation of 1-hexene using 8a and Et3SiH/B(C6F5)3 mixture as cocatalyst under 60 bar H2 and 140 °C and for 8b a maximum TOF of 8000 h-1 could be achieved for the hydrogenation of 1-hexene under the similar condition. Two hydride complexes Mo(NO)(CO)H(etpip), 11a and W(NO)(CO)H(etpip), 11b were also prepared from 6a and 6b using LiBH4 in Et3N at elevated temperature.

Abstract

The search for alternatives to platinum group metal catalyses became in recent years more and more intense and expected to experience increased growth in the future. The most obvious reason for replacing these platinum group metals is that they are very expensive. The high cost is obviously connected to the low abundance of these metals. Substantial costs are involved in industrial processes when recovery and recycle of the metal is required. In this perspective, the “middle transition elements” which are the “rising stars” in modern homogeneous catalysis field are apt for replacing the platinum group metals. The research presented in this thesis shows the potential of new catalysts that do not require precious metals may ultimately supplant the use of precious metals in Wilkinson and Osborn type hydrogenations, Bullock type ionic hydrogenations, Noyori type bifunctional hydrogenation and Ojima type hydrosilylation reactions. Preparation of a new class of molybdenum and tungsten nitrosyl complexes were realised bearing a tridentate (iPr2PCH2CH2)2PPh, (etpip) ligand. Pentagonal bipyramidal complexes M(NO)Cl3(etpip) (M = Mo, 3a; W, 3b) were isolated as isomeric mixtures starting from the precursors M(NO)Cl3(NCMe)2 (M = Mo, W). 3a and 3b were reduced to 17 e- rare paramagnetic complexes Mo(NO)Cl2(etpip), 4a and W(NO)Cl2(etpip), 4b showing characteristic paramagnetic behavior determined by EPR spectroscopy, as well as by SQUID measurements. Treatment of 1% Na/Hg to the THF solutions of 3a and 3b in the presence of 1 bar carbon monoxide at room temperature led to the formation of low-valent Mo(NO)(CO)Cl(etpip), 6a and W(NO)(CO)Cl(etpip), 6b complexes. Reduction of 3a,b with 1% Na/Hg under 1 bar ethylene furnished the ethylene coordinated complexes Mo(NO)Cl(η2-ethylene)(etpip), 7a and W(NO)Cl(η2-ethylene)(etpip), 7b. The reaction of the 7a with NaHBEt3 at 75°C revealed formation of the hydride complex Mo(NO)H(η2-ethylene)(etpip), 8a. On the other hand LiBH4/Et3N combination was found to be suitable to prepare W(NO)H(η2-ethylene)(etpip), 8b complex from 7b. 8a and 8b were found to be highly efficient for the hydrogenation of several olefins. A maximum TOF of 5253 h-1 was achieved for the hydrogenation of 1-hexene using 8a and Et3SiH/B(C6F5)3 mixture as cocatalyst under 60 bar H2 and 140 °C and for 8b a maximum TOF of 8000 h-1 could be achieved for the hydrogenation of 1-hexene under the similar condition. Two hydride complexes Mo(NO)(CO)H(etpip), 11a and W(NO)(CO)H(etpip), 11b were also prepared from 6a and 6b using LiBH4 in Et3N at elevated temperature.

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Item Type:Dissertation (monographical)
Referees:Berke Heinz
Communities & Collections:UZH Dissertations
Dewey Decimal Classification:Unspecified
Language:English
Place of Publication:Zürich
Date:2013
Deposited On:10 Apr 2019 12:02
Last Modified:07 Apr 2020 07:17
Number of Pages:202
OA Status:Green
Related URLs:https://www.recherche-portal.ch/primo-explore/fulldisplay?docid=ebi01_prod010035731&context=L&vid=ZAD&search_scope=default_scope&tab=default_tab&lang=de_DE (Library Catalogue)

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