Open Access Dissertation
Date of Award
This dissertation focuses on the design and reactions of novel late transition metal carbene complexes featuring alkoxide ligand environments. The high-valent cobalt carbene Co(OR)2(=CPh2) (OR = OCtBu2Ph), featuring short Co=C bond of 1.773(3) Å, was previously reported from the reaction of Co(OR)2(THF)2 with diphenyldiazoalkane. Magnetic and spectroscopic (EPR) studies demonstrated Co(OR)2(=CPh2) to be a low-spin S = ½ complex. Computational studies, in agreement with experimental data, suggested that the electronic structure of Co(OR)2(=CPh2) lies between intermediate spin Co(III) anti-ferromagnetically coupled to a carbene radical and a Co(IV) alkylidene. This dissertation began with investigation of this complex in carbene transfer reactivity. Stoichiometric ketenimine formation occurs upon reaction with various isocyanides CNR′ (CNR′ =2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide, 2-chloro-6-methylphenyl isocyanide, adamantyl isocyanide). The reaction is accompanied by the formation of a cobalt bis(alkoxide) bis(isocyanide) complexes Co(OR)2(CNR)2, which were independently synthesized and characterized. Excess isocyanide was required to form ketenimine due to formation of the bis(isocyanide) complex. DFT calculations suggest the mechanism proceeds through isocyanide binding to cobalt, in contrast to nucleophilic attack at the carbene carbon. This is followed by intramolecular insertion into the Co-carbene bond to form the ketenimine complex. Dissociation of free ketenimine from cobalt then leads to the bis(isocyanide) complex. Catalytic formation of ketenimines was investigated at room temperature by exposing the mixtures of the carbene precursors and isocyanides to Co(OR)2(THF)2. The carbene precursors investigated included both diazoalkane (diphenyldiazomethane) and diazoesters (methyl phenyldiazoacetate, and ethyl diazoacetate). Both aryl isocyanides (2,6-dimethylphenyl isocyanide, 4-methoxyphenyl isocyanide) and alkyl isocyanides (adamantyl isocyanide, cyclohexyl isocyanide, and benzyl isocyanide) differing in their steric bulk were investigated. While no catalytic reactivity was observed for diphenyldiazoalkane, the use of diazoester demonstrated catalytic turnover. The highest yields were obtained with the bulkier aliphatic substituent on the isocyanide: adamantyl and cyclohexyl isocyanide. Mechanistic studies suggested that the lack of catalytic reactivity involving diphenyldiazomethane results from the inability of Co(OR)2(CNR)2 to undergo carbene formation upon reaction with N2CPh2. In contrast, facile reaction is observed between Co(OR)2(CNR)2 and diazoesters, enabling the overall catalytic reactivity.
The Co(OR)2(=CPh2) species demonstrated several well-defined quasi-reversible reduction events in cyclic voltammetry experiments. Therefore, its chemical reduction was explored by exposing Co(OR)2(=CPh2) to chemical reductants. Reduction with decamethylcobaltocene or potassium graphite resulted in the anionic carbene products [Co(OR)2(CPh2)](CoCp*2) and [Co(OR)2(CPh2)](K(18-crown-6)), respectively. In contrast to the low-spin electronic structure of the neutral carbene, [Co(OR)2(CPh2)]- was demonstrated by magnetic and computational experiments to be a high-spin Co(II) species. The S = 1 state with a high-spin Co(II) antiferromagnetically coupled to a carbene radical was found to be 1 kcal/mol more stable than the S = 2 high-spin Co(II) ferromagnetically coupled state. The low-spin Co(II) state was found to be significantly higher in energy. Reactivity with isocyanides was explored to see if ketenimine formation was feasible with the anionic species. Treating [Co(OR)2(CPh2)](CoCp*2) with 2,6-dimethylisocyanide formed two new C-C bonds resulting in the imine-Cp*-Co(I) fulvene complex. The formation of the complex likely took places via the initial deprotonation of the decamethylcobaltacene counterion, which then attacked transient ketenimine. Un-derivatized ketenimine was formed by treating [Co(OR)2(CPh2)](CoCp*2) with adamantyl isocyanide or [Co(OR)2(CPh2)](K(18-crown-6)) with 2,6-dimethylisocyanide.
Following the investigation of the chemistry of Co(OR)2(THF)2 with diazoalkanes, the analogous chemistry was investigated with the iron analogue, Fe(OR)2(THF)2. While treatment of Fe(OR)2(THF)2 with diphenyldiazoalkane resulted in azine formation, utilizing diazoesters as the carbene precursor led to unprecedented reductive coupling of a transition-metal through the terminal nitrogens. The resulting reductively-coupled products were characterized by X-ray diffraction, elemental analysis, UV-Vis, IR, and Mössbauer spectroscopies. The complexes were formulated as high-spin Fe(III) with the tetrazene-bridged bis(diazenylacetate) serving as a novel dinucleating ligand. In contrast to facile azine formation with diphenyldiazoalkane, azine formation from the isolated reductively-coupled complexes was sluggish. DFT calculations were performed to explore why carbene chemistry is observed with cobalt whereas reductive coupling of diazoalkanes is observed with iron. The calculations suggest that the reductive coupling takes place via a κ2 diazoester intermediate which induces significant radical character on the terminal nitrogen and consists of a high-spin M(III) center. Reductive coupling of two of these species is essentially barrierless for iron. However, this intermediate is less stable than the high-spin M(II) κ1 diazoester intermediate with cobalt. Thus, carbene formation occurs from the κ1 intermediate upon N2 extrusion, explaining the carbene chemistry observed with cobalt.
Grass, Amanda, "Reactions Of First-Row Transition Metal Complexes In Bis(alkoxide) Ligand Environments With Diazoalkanes: Formation Of Carbenes Versus Reductive Coupling To Form Bridging Tetrazenes" (2020). Wayne State University Dissertations. 2449.