Verhalten linearer 3d-Metall(I)-Komplexe gegenüber C/N-Mehrfachbindungen und aromatischen Systemen

Im Rahmen dieser Doktorarbeit wurde das Verhalten von linearen 3d-Metall(I)-Silylamiden (M = Cr – Co) gegenüber C/N-Mehrfachbindungen und aromatischen Sys-temen untersucht. Im ersten Teil der Arbeit konnten die reduzierten Metall(II)-(bipy)-Komplexe (M = Cr – Co, Zn) in guten Ausbeuten erhalten wer...

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Bibliographic Details
Main Author: Müller, Igor
Contributors: Werncke, C. Gunnar (Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2021
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he scope of this doctoral thesis is the chemical behavior of linear 3d-metal(I) silylamides of the type [K{18-c-6}][MI(hmds)2] (M = Cr – Co) towards C/N multiple bonds and aromatic systems. In the first part of this work it could be shown that the reduced metal(I) complexes reacted readily with 2,2'-bipyridine under substrate reduction and formation of metal(II) complexes bearing a bipyridyl radical anion. Alternatively the metal(II) complexes [MII(hmds)2] (M = Cr – Co, Zn) could be reacted first with 2,2'-bipyridine and subsequently reduced by KC8 in the presence of {18-c-6}, allowing for the isolation of the zinc derivative. As 2,2´-bipyridine can be reduced to a radical monoanion, and a diamagnetic dianion by ac-cepting one or two electrons respectively. Using various spectroscopic methods and single crystal structure analysis, it shows that for all compounds the description as MnII(bipy*-) is valid. This is remarkable, as the reduction potential needed for 2,2'-bipyridine reduction is lower by up to 1 V that provided by the metal(I) complexes. In order to expand this approach to even more challenging to needed substrates, metal(I) silylamides (M = Cr – Co) were reacted with pyridine to possible create metal bound pyridyl radical anions. However, binuclear dianions were found by solid-state analysis. Two metal ions are bridged by a dihydrobipyridyl, originating from the reductive dimeriza-tion of pyridine in the para-position. The bond metrics of the bipyridyl ligand shows a loss of aromaticity with single and double bonds now being localized. When the 4-position of pyridine was blocked by a nitrogen- or carbon-based substituent (NMe2 or tBu), no dimerization or coordination was detected. For cobalt, no interaction with pyridine or with pyridine derivatives could be observed in any of these cases. The reaction between pyridine and metal(I) complexes indicates the necessity of substrate reduction before or during the coordination to the metal center. In order to rule out simple steric effects, 3,5-lutidine was reacted with the metal(I) silylamides. However, dimerization was also observed. In order to possibly stabilize the radical character via a larger aromatic system, 2-phenylpyridine was tested, too, yielding the same out-come for manganese and iron. In addition, blue crystals could be obtained by reacting 2-phenylpyridine with the manga-nese(I) complex as a side product. Single crystal structure analysis revealed the presence of a 2-phenylpyridyl unit, which is sandwiched between two [K{18-c-6}]+ cations with [Mn(hmds)3]– acting as counter ion. The counter ion indicates the presence of an unprece-dented pyridyl radical anion, which was confirmed using bond metrics as well as UV/vis spectroscopy. Its more selective formation was achieved by reacting 2-phenylpyridine with KC8 in the presence of 18-c-6 and [K{18-c-6}][MnII(hmds)3]. The latter is needed to provide the additional stabilizing [K{18-c-6}] cation. dditionally to the just discussed reductive coupling of pyridines, electron-deficient fluoro-pyridines, bearing variable degrees of fluorination, were used to facilitate the reduction, especially for the so far unreactive cobalt(I) complex [Co(hmds)2]–. Although reactions took place readily in all instances (except for 2-fluoropyridine with the cobalt(I) complex) no reductive coupling to a dihydropyridyl anion but C–F bond cleavage was ob-served. This gave very rare three-coordinate metal(II) fluoride of the type [K{18-c-6}][MII(F)(hmds)2] (M = Mn – Co). n addition, totally degradation of higher fluorinated pyridines is observed, that act as elec-tron scavenger. Only for the substrate pentafluoropyridine, the by-product could be identi-fied spectroscopically as perfluorobipyridine. In the reaction with 2-fluoropyridine, the pyridyl complex could be identified as a by-product by means of single-crystal structure analysis. In addition to the reactivity of metal(I) complexes towards 2,2´-bipyridine and pyri-dine, this work could also show the synthesis of low-coordinated 3d-transition metal alkyne complexes resulting from the reaction of quasi-linear metal(I) silylamides [K{18-c-6}][MX2] (M = Cr – Co ; X = –N(SiMe3)2), –N(Dipp)SiMe3; Dipp = 2,6-di-iso-propylphenyl) with aliphatic and aromatic internal alkynes. While only weak and reversible alkyne coordination is observed for cobalt, the formation of anionic side-on alkyne complexes of the type [M(hmds)2(η2-RCCR)]– for iron occurs easily. In case of manganese, first examples of low-coordinated manganese alkyne complexes and, depending on the substrate, unique examples of the manganese-mediate a reduction of the alkyne to a dianionic structure or even alkyne trimerization were observed. An alkyne coordination or reduction to the alkene dianion could be also observed for chromium. A quantum chemical analysis of the [M(hmds)2(η2-PhCCPh)]– complexes (M = Cr – Co) using DFT and CASSCF methods was performed, showed that the electronic situation of all these complexes can be described best as formal metal(II) bound alkynyl radical anions. In case of chromium, evidence of further contributions of a metal(III) cyclopropene resonance struc-ture was found. The computational analysis rationalizes the bimetallic reduction to bis-metalated alkene dianions, a rarely observed phenomenon. Reactions between terminal alkynes and quasi-linear iron(I)-silylamides [K{18-c-6}][Fe(hmds)2] showed a very similar coordination pattern. In both cases, a side-on complex was formed with the particularity of the substrate 3-phenyl-1-propyne, where a shift of the triple bond was additionally observed. After observing this unusual triple bond rearrangement, we attempted the bond isomerisa-tion on a catalytic scale for this and other substrates using different iron complexes. Indeed, 1mol% of [K{18-c-6}][Fe(hmds)2] mediated the instantaneous rearrangement of 3-Phenyl-1-propin to the internal alkyne. K{18-c-6}][Fe(N(Dipp(SiMe3)2] is less active but leads to the exclusive formation of phenyl allene. It also rapidly showed, that the substrate scope is fairly limited as only 4-phenyl butyne could also be transformed to the allene using [K{18-c-6}][Fe(hmds)2]. The mechanism of this transformation is unresolved but participation of a metal hydride is unlikely for these systems. In addition [K{18-c-6}][Fe(hmds)3], K(hmds) und Li(hmds) were employed as catalysts (10 mol%). Thereby it showed, that [K{18-c-6}][Fe(hmds)3] mediated the (slow) transformation of 3-Phenyl-1-propin to phenyl allene. Intriguingly, the simple K(hmds) is highly active which yields the internal alkyne, whereas Li(hmds) is slower and by that reveals the stepwise allene and internal alkyne formation. In addition to the reactivity of metal(I) complexes toward alkynes, this work demon-strated that a number of rare low-coordination 3d transition metal-alkene complexes can result from the reaction of linear metal(I) silylamides [K{18-c-6}[MX2] (18-c-6, X = N(SiMe3)2), hmds, (M = Cr – Co) with terminal internal alkenes and diene. Only weak and reversible alkene coordination could be observed for cobalt. For the terminal alkenes, a side-on alkene complex of the type [Fe(hmds)2(η2-PhCHCH)]– could be shown in the case of iron. In contrast, for chromium and manganese, reductive coupling of the substituents was observed. The interaction of internal alkenes with linear metal(I) complexes as a function of both the metal and the alkene substituent was also studied. The formation of side-on alkene com-plexes of the type [M(hmds)2(η2-PhCHCHR)]– readily takes place. As such, the first low-coordinated manganese-alkene complexes could be obtained. In addition, Z/E isomerization of the Z-substrates (stilbene and -methylstyrene) could be observed. The simple Z/E isomerization of the substrates is tentatively attributed to the formulation of these compounds as metal(II) stabilized radical anions. The presence of such a species was authenticated by X-Ray diffractometer analysis, however not 3d-metal bound. Finally, the interaction of a diene (2,3-dimethylbutadiene) with linear metal(I) complexes was studied as a function of metal. 2,3-Dimethylbutadiene showed a versatility of different activation modes towards the met-al(I) complexes (M = Cr – Fe). For chromium, a side-on diene complex could be presented, with the two double bonds delocalized and distributed over the substrate. In the case of manganese, a ligand exchange took place and the "naked" manganese is coordinated by two 2,3-dimethylbutadienes, with manganese having the oxidation state of -I. For iron, the sub-strate 2,3-dimethylbutadiene was twofold reduced and is coordinated by two [FeII(hmds)2] complexes. For cobalt, no reactivity towards 2,3-dimethylbutadiene was observed. Overall, the first comprehensive study on the chemical behaviour of anionic, linear 3d-metall(I) complex was presented revealing unusual results. In addition, the catalytic use of such compounds in some substrate conversions could also be revealed, which will be part of future endeavours.