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Date of Award
Stanislav S. Groysman
An efficient route to synthesize important nitrogen-containing functional groups such as azoarene, carbodiimide, and aziridine is through the transfer of nitrene by means of reactive metal-imido intermediate. The focus of this dissertation is on the design of 3d imido complexes supported by bulky chelating bis(alkoxide) ligands. The major hypothesis of my dissertation is that profoundly weak-field low-coordinate 3d complexes can catalyze nitrene transfer efficiently, including niitrene homocoupling (to form azoarenes), or nitrene transfer to isocyanides (to form carbodiimides) and olefins (to form aziridines).In the first project we report the synthesis of a new chelating bis(alkoxide) ligand H2[OO]Ph ([1,1':4',1''-terphenyl]-2,2''-diylbis(diphenylmethanol)). While H2[OO]Ph exhibited anti conformation in the solid state, it adopted a syn geometry on reaction with iron(II) amide precursor to form Fe[OO]Ph(THF)2. The newly synthesized Fe[OO]Ph(THF)2 exhibits seesaw geometry with an interalkoxide angle (O-Fe-O) of 156o and a C2V symmetry. Solution magnetic moment and DFT calculations suggest the species to be a high spin Fe(II) center. 10 mol% catalyst loading of Fe[OO]Ph(THF)2 at 60oC in C6D6 demonstrated excellent catalytic conversion of unsubstituted phenyl azide to obtain near quantitative yields of the corresponding azoarene. Under the same conditions, complex formed azoarene in good to moderate yields with para and meta-substituted phenyl azides containing different functional groups. No azoarene formation was shown by ortho-substituted azide such as mesityl and 2-methylphenyl azide. This difference in reactivity as compared to the first generation Fe(OR)2(THF)2 was investigated using DFT optimized geometry of the first and third generation meta-imidos Fe(OR)2(NPh), Fe(OR)2(NMes), Fe[OO]Ph(NPh), and Fe[OO]Ph(NMes). In my second project, we focused on the synthesis and reactivity of a Cr(II) complex in a chelating bis(alkoxide) ligand environment. We obtained a chromium complex that can adopt a dimeric (Cr2[OO]Ph2, in CH2Cl2-THF) or a monomeric (Cr[OO]Ph(THF)2, in toluene-THF) form, based on the solvent medium. Excellent to moderate catalytic formation of carbodiimide (50-96% yield) was observed with 2.5 mol% of Cr2[OO]Ph2 for the combination of mesityl azide with four different isocyanides (xylyl, 4-methoxyphenyl, cyclohexyl and 1-adamantyl isocyanide). The catalytic (2.5 mol% Cr2[OO]Ph2) reactivity of slightly less bulky 2-methylphenyl azide with xylyl isocyanide gave good yields of the corresponding carbodiimide (69% yield), whereas the reactivity was poor (7-15% yield) between 2-methylphenyl azide and other isocyanides. No carbodiimide formation was observed with 3,5-dimethylphenyl azide and 4-methylphenyl azide under the same reaction conditions. The product of the reaction of 4-methylphenyl azide and Cr2[OO]Ph2 was isolated and demonstrated by X-ray crystallography to be a Cr(VI) complex Cr[OO]Ph(N(4-MeC6H4))2. The NMR studies indicated a similar Cr(VI) intermediate with 3,5-dimethylphenyl azide and 4-methoxyphenyl azide. Similar studies with larger azides gave paramagnetic products postulated to be a Cr(IV) intermediate. Thus, the catalytic formation of carbodiimide can be linked to the formation of Cr(IV) or Cr(VI) intermediates, which in turn is dependent on the size of azides. Bulky azides led to the formation of a reactive unsaturated Cr(IV) mono(imido) intermediate which can produce carbodiimide catalytically. In contrast, stable and more coordinatively saturated Cr(VI) bis(imido) complex was found to be detrimental to the catalytic cycle. The reaction of Cr2[OO]Ph2 with diphenyldiazomethane (N2CPh2) resulted in a Cr(VI) complex Cr[OO]Ph(NNCPh2)2 instead of a Cr-alkylidene. In my third project, the synthesis and reactivity of Mn[OO]Ph(THF)2 towards aziridination was explored. Mn[OO]Ph(THF)2 was synthesized by the treatment of manganese(II) amide precursors with H2[OO]Ph. Mn[OO]Ph(THF)2 X-ray structure showed a wide inter alkoxide angle (O-Mn-OR) of 150o as compared to first generation Mn(OCtBu2Ph)2(THF)2 complex (~138o). Investigation of Mn[OO]Ph(THF)2 towards aziridination with mesityl, p-tolyl, and phenyl azide and styrene did not produce azoarene or aziridine. Changing the nitrene precursor to hypervalent iodine reagent PhINTs (Ts = p-toluenesulfonyl) formed the corresponding styrene. Carrying out the reaction at room temperature in different solvents (C6D6, CD2Cl2, and CD3CN), with varying catalyst loading (5 and 10 mol%) and PhINTs to styrene ratio (4:1, 2:1, and 1:1) suggested the reaction works well in C6D6, with a 10 mol% catalyst loading and 1:1 ratio of PhINTs:styrene to obtain aziridine in 76% yield. The complex was found to be an effective catalyst in synthesizing aziridines with para-substituted styrene but lacked reactivity with aliphatic alkenes. With α-methyl styrene the complex formed aziridine in 53% yield along with the formation of TsNH2. Sterically less hindered cis β-methyl styrene formed aziridine diastereomers in a combined yield of 46% with an approximate 3:1 ratio of trans:cis. A similar trend was observed with trans β-methyl styrene but with an overall yield of 27%. A 1:1 ratio of cis and trans aziridine was formed with a combined yield close to 19% with stilbenes. The complex also formed TsNH2 in significant quantities with α and β substituted alkenes (13-57%). Spectroscopic studies indicate the presence of Mn-imido radical as the reactive intermediate. In the last project we have used DFT calculations to determine the electronic structure and the oxidations states of formally Mo(0) complexes in iminopyridine N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine (L1) and bis(imino)pyridine (N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine) (L2) ligand environments. Geometry calculations for different oxidation states of the complexes were carried out by changing the intraligand bond length. These different structures converged to the same geometry of Mo(0)/L(0). The optimized geometry of (Mo(0)/L(0)) and crystallographically obtained bidendate ([NN]) complexes of Mo(L1)(CO)3(NCMe), Mo(L1)(CO)4, Mo(L2)(CO)3(NCMe) and Mo(L2)(CO)4 were in good agreement. The frontier orbital calculations and intraligand bond length suggests these complexes to be Mo(0).
Kurup, Sudheer Suresh, "Design, Synthesis, And Reactivity Of First-Row Transition Metal Complexes Ligated By Chelating Bis(alkoxide) Ligands Towards Nitrene Transfer Reactions" (2021). Wayne State University Dissertations. 3576.