Open Access Dissertation
Date of Award
This dissertation focused on the synthesis of new transition metal complexes in bis(alkoxide) ligand environments and the investigation of their reactivity in nitrene-group transfer catalysis and small molecule activation. Treatment of Cr(N(SiMe3)2)2(THF)2 with two equivalents of HOR (OR = OCtBu2Ph) led to the formation of the chromium(II) alkoxide dimer, Cr2(OR)4. Upon the reaction with bulky aryl and alkyl azides, Cr2(OR)4 led to the stable Cr(IV) mono(imido) complexes, Cr(OR)2(NR), featuring trigonal planar metal centers. In contrast, less bulky aryl azides led to the formation of chromium (VI) bis(imido) complexes Cr(OR)2(NR)2, independent of the amount of azide used. Chromium(IV) mono(imido) species Cr(OR)2(NR) is capable of nitrene transfer to isocyanides to form asymmetric carbodiimides (RNCNR’). When excess isocyanide is added to Cr(OR)2(NR), a new chromium(II) complex, Cr(OR)2(CNR2)4, was identified by X-ray crystallography. This tetrakis(isocyanide) chromium(II) complex is also capable of forming carbodiimide when azide is introduced. Efficient catalytic formation of carbodiimides was obtained using 2.5 mol% of Cr2(OR)4 for the mixtures of bulky organoazides and isocyanides; no catalytic reactivity was observed for the non-bulky aryl azides. DFT calculations suggest that trigonal CrIV(OR)2(NR) intermediate is the key species in the reaction mechanism as, due to it coordinative unsaturation, it allows isocyanide binding to the metal, which enables subsequent C-N bond formation.
A bulkier alkoxide ligand HOR’ (R = CtBu2(3,5-Ph2Ph)) was synthesized by lithium halogen exchange reaction. The protonolysis of the metal complexes, M(N(SiMe3)2(THF)x, with HOR’ enabled an easy isolation of new bis(alkoxide) precursors M(OR’)2(THF)2 (where M = Cr, Co, Fe) featuring cis-divacant octahedral geometry. The chemistry of the iron bis(alkoxide) compound Fe(OR’)2(THF)2 with the variety of aryl azides was investigated. Catalytic formation of various azoarenes from the corresponding azides (mesityl azide, 2,6-diethylphenylazide, azidobenzene, 4-azidotoluene, 3,5-dimethylphenylazide) using 5/10 mol% loading of Fe(OR’)2(THF)2 was observed. Mechanistic investigation revealed that the reaction proceeds via formation of tetrazene complexes Fe(OR’)2 (ArNNNNAr). 57Fe Mössbauer and EPR spectroscopy demonstrate a high spin iron(III) center, antiferromagnetically coupled to the tetrazene ligand
radical anion. Tetrazene complexes are capable of azoarene formation, as demonstrated by NMR spectroscopy and mass spectrometry, thus confirming their role as intermediates in nitrene coupling reaction.
My next project focused on the investigation of the reactivity of Cr(OR’)2(THF)2 with organic carbonyls and CO2. Cr(OR’)2(THF)2 catalyzes the reductive coupling of a variety of aromatic aldehydes (benzaldehyde, 4-anisaldehyde, 4-(trifluoromethyl)benzaldehyde, and 2,4,6-trimethylbenzaldehyde) to form Cr(IV) diolate complexes. In contrast, the reaction of benzophenone led to the formation of the ketone adduct Cr(OR’)2(OCPh2). DFT calculations suggested that the reductive coupling of aldehydes proceeds via the Cr(III) bis-aldehyde intermediate. Partial radical character in the coordinated aldehydes in these species is sufficient for C-C bond formation. The calculation also suggested that that the more significant steric demands of ketones prevented their coupling of ketone. No reductive coupling was observed with CO2. Instead, the reaction of Cr(OR’)2(THF)2 with CO2 led to CO2 insertion into Cr-OR’ bonds to form the diamagnetic dinuclear paddlewheel complex Cr2(O2COR’)4(THF)2.
Oxo- and sulfide-group transfer chemistry was also investigated. The reaction of Fe(OR)2(THF)2 with an oxo-transfer reagent, iodosobenzene (PhIO), forms the dinuclear μ-oxo complex Fe2(O)(OR)4(THF)2. Similarly, the use of triphenylantimony sulfide (Ph3SbS) allowed for the isolation of Fe2(S)(OR)4(THF)2. The complexes were isolated and characterized by X-ray crystallography, IR spectroscopy, and elemental analysis. Both Fe2(O)(OR)4(THF)2 and Fe2(S)(OR)4(THF)2 species were found to be unreactive in the group-transfer chemistry with phosphine and olefins.
Yousif, Maryam, "Synthesis Of Low-Coordinate Transition Metal Bis(alkoxide) Complexes And Their Reactivity Toward Small Molecules" (2017). Wayne State University Dissertations. 1903.