Date of Award
Doctor of Philosophy (PhD)
Chemistry and Biochemistry
Alvin A. Holder
Craig A. Bayse
John B. Cooper
The kinetics and mechanism of the oxidation of [Co(dmgBF2)2(OH2)2] (where dmgBF2 = difluoroboryldimethylglyoximato) by sodium hypochlorite (NaOCl), sodium bromate (NaBrO3), and bromine (Br2) was investigated by stopped-flow spectrophotometry at 450 nm over a wide temperature range. The pKa1 value for [Co(dmgBF2)2(OH2)2] was calculated as 5.27 ± 0.14 at I = 0.60 (NaCl). From the variation in pH studies, the activation parameters were derived from the proposed mechanisms for NaOCl, ΔH1*, ΔH2*, ΔS1*, and ΔS2* were calculated as 58 ± 7 kJ mol-1, 47 ± 0.1 kJ mol-1, 29 ± 2 J mol-1 K-1, and -9 ± 0.4 J mol-1 K-1, respectively. The reaction with Br2 also had two sets of activation parameters for the two reacting species in solution which were Br2Cl- and OBr- where, ΔH1*, ΔH2*, ΔS1*, and ΔS2* were calculated as 26 ± 0.1 kJ mol-1, 71 ± 2 kJ mol-1, -52 ± 0.2 J mol-1 K-1, and 105 ± 0.6 J mol-1 K-1, respectively. Finally with NaBrO3 there was only one reacting oxidant which was the bromate which gave the following activation parameters: ΔH* and ΔS* which were calculated as 57 ± 1 kJ mol-1 and 34 ± 4 J mol-1 K-1, respectively.
In the interaction study of Et3N with [Co(dmgBF2)2(OH2)2] the isolated product was characterized as [Co(dmgBF2)2(OH2)(N(CH=CH2)3)]•5.25H2O•0.25C6H9N via NMR and mass spectroscopy and elemental analysis. In the UV-visible studies, various changes were observed with the most dramatic being between 500 and 900 nm with the appearance of two new λmax at 554 and 680 nm. In the HFEPR studies the isolated product anisotropic characteristics was observed to be rhombic in nature with the observed g-values for the gx, gy, and gz were 2.25, 2.15, and 2.01, respectively. However, the reaction mixture produced a spectrum that where the anisotropic characteristic was axial with g-values of g|| = 2.19 and g┴ = 2.00.
The complexes [Co(bpy)2(PCA-(CF3)2)](PF6)2•H2O 3, [Co(phen)2(PCA-(CF3)2)](PF6)2•1.25H2O 4, and [Co(tpy)(phen)Cl](PF6)2•0.25CH3CN 7 (where PCA-(CF3)2 = N-(3,5-bis(trifluoromethyl)phenyl)pyridine-2-carbothioamide, tpy = 2,2';6',2"-terpyridine, bpy = 2,2’-bipyridine and phen = 1,10-phenanthroline) were synthesized, characterized, and tested for their overall effectiveness as a hydrogen evolution catalyst. Complexes 3 and 4 demonstrated catalytic HER in the presence of p-cyanoanilinium tetrafluoroborate in CH3CN, with overpotentials for the HER of 730 and 630 mV for complexes 3 and 4, respectively. The reduction potentials suggest that the HER was most likely facilitated by a homolytic pathway. Under photocatalytic conditions in the presence of [Ru(bpy)3](PF6)2 as a photosensitizer and triethanolamine as a sacrificial reductant in DMF, complexes 3, 4, and 7 attained turnover numbers of 91, 140, and 79, respectively, when compared to the standard complex [Co(dmgH)2(py)Cl] (where dmgH = dimethylglyoximato).
Celestine, Michael J..
"Synthesis, Characterization, and Kinetics Studies of New Cobalt Complexes for the Production of H2 in Acidic Media"
(2020). Doctor of Philosophy (PhD), Dissertation, Chemistry and Biochemistry, Old Dominion University, DOI: 10.25777/wz21-0t61
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