Open Access Dissertation
Date of Award
Cooperative reactivity of bimettalics can be is observed in many different areas of chemistry and have been increasingly investigated because of the advantageous reactivity when compared to the corresponding mononuclear systems. The focus of my dissertation is on (1) investigation of the homobimetallic cooperativity in lactide polymerization catalysis; (2) investigation of the heterobimetallic cooperativity in the biomimetic studies of Mo-Cu carbon monoxide dehydrogenase (CODH) enzyme in order to make a functional model of its active site.
Three new main group bis(alkoxide) complexes Mg(OR)2(THF)2, Zn(Cl)(μ2-OR)2Li(THF) and In(OR)2(μ2-Cl)2Li(THF)2 featuring bulky alkoxide [OCtBu2Ph] were synthesized serve as metal alkoxide precursors for bimetallic lactide polymerization catalyst. A new, potentially dinucleating xanthene-bridged bis(iminophenolate) ligand L1 (L1 = 6,6′-((1E,1′E)-((2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(azanylylidene))bis(methanylylidene))bis(2,4-di-tert-butylphenol)) has been synthesized in order to study bimetallic cooperativity in lactide polymerization. The coordination chemistry of L1 with zinc precursors featuring alkoxide, chloride, and ethyl leaving groups has been investigated. The reaction of a zinc precursor bearing two bulky alkoxides, Zn(Cl)(μ2-OR)2Li(THF) (OR = di-tert-butyl-phenylmethoxide), formed a mononuclear complex Zn(L1) that was isolated as an H-bond adduct with HOR, Zn(L1)·HOR. In contrast, the reaction of L1 (or its lithium salt) with diethylzinc (or zinc chloride) led to the formation of the corresponding dinuclear complexes Zn2(L1)(Et)2 and Zn2(L1)(μ2-Cl)4Li2(OEt2)2. X-ray crystallography revealed syn-parallel geometry for Zn2(L1)(Et)2 (Zn⋯Zn distance of 4.5 Å) and anti-parallel geometry for Zn2(L1)(μ2-Cl)4Li2(OEt2)2 (Zn⋯Zn distance of 6.7 Å). Zn2(L1)(Et)2 was found to be somewhat unstable, demonstrating decomposition into Zn(L1) and ZnEt2; this decomposition can be reversed by the addition of excess ZnEt2. Treatment of Zn2(L1)(Et)2 with benzyl alcohol (BnOH) in deuterated benzene, toluene, or dichloromethane resulted in the formation of Zn2(L1)(OBn)2, which was characterized by 1H and 13C NMR spectroscopy. Zn2(L1)(OBn)2 was found to be active in the ring-opening polymerization of rac-lactide to afford heterotactically inclined PLA.
The synthesis of a heterodinucleating ligand L3 (L3 = (E)-3-(((2,7-di-tert-butyl-9,9-dimethyl-5-((pyridin-2-ylmethylene)amino)-9H-xanthen-4-yl)amino)methyl)benzene-1,2-diol) was undertaken toward a functional model of the bimetallic active site found in Mo–Cu carbon monoxide dehydrogenase (Mo–Cu CODH), and to understand the origins of heterobimetallic cooperativity exhibited by the enzyme. L3 features a hard potentially dianionic catechol chelate for binding Mo(VI) and a soft iminopyridine chelate for binding Cu(I). Treatment of L3 with either Cu(I) or M(VI) (M = Mo, W) sources leads to the anticipated site-selective incorporation of the respective metals. While both [CuI(L3)]+ and [MVIO3(L3)]2− complexes are stable in the solid state, [MVIO3(L3)]2− complexes disproportionate in solution to give [MVIO2(L3)2](NEt4)2 complexes, with [MVIO4]2− as the by-product. The incorporation of BOTH Mo(VI) and Cu(I) into L3 forms a highly reactive heterobimetallic complex [MoVIO3CuI(L3)](NEt4)2, whose formation and reactivity was interrogated via1H NMR/UV-vis spectroscopy and DFT calculations. These studies reveal that the combination of the two metals triggers oxidation reactivity, in which a nucleophilic Mo(VI) trioxo attacks Cu(I)-bound imine. The major product of the reaction is a crystallographically characterized molybdenum(VI) complex [Mo(L4)O2](NEt4) coordinated by a modified ligand L4 that contains a new C–O bond in place of the imine functionality. This observed hydroxylation reactivity is consistent with the postulated first step of Mo–Cu CODH (nucleophilic attack of the Mo(VI)–oxo on the Cu(I)-bound electrophilic CO) and xanthine oxidoreductase (nucleophilic attack of Mo(VI)–oxo on the electrophilic xanthine carbon). This is the first example of a functional model for both molybdoenzymes.
Hollingsworth, Thilini Samangi, "Design, Synthesis, And Reactivity Of Homo- And Heterobimetallic Complexes Bridged By A Xanthene Linker" (2018). Wayne State University Dissertations. 2105.