Synthesis, Characterisation, Electrochemical, and Spectroscopic Studies of Cobaloximes: Unique Clues of Cobalt(I) Species in Various Solvents
Creators: name: Celestine, Michael John
Publisher: name: Old Dominion University Libraries
Year: 2015

description: With the dwindling amount of fossil fuels in the world’s reserve is said to run out in the future. The use of alternative fuels such as hydrogen can be produced from renewable sources. One source is the use of first row transition metal complexes that can harness the power of the sun to reduce protons to hydrogen. In this thesis we investigated a well-known hydrogen evolution catalyst in a quest to understand the behavior of different oxidation states that occur during the catalytic cycle.

In an attempt to synthesize a binuclear ruthenium(II) complex, [{Ru(phen)2}2{µmes(1,4-phO-Izphen)3}](PF6)4, as a possible photosensitizer for the production of hydrogen from the reduction of protons in various media. The expected product was not synthesized according to the reported procedure that was followed. The reaction was carried out in refluxing ethylene glycol due to low solubility of 2,4,6-trimethyl-1,3,5-tris(4oxymethyl-1-yl(1H-imidazo-2yl-[4,5-f][1,10]phenanthroline)phenyl)benzene in organic solvents such as acetonitrile, DMSO, and DMF. In the first attempt, two (2) equivalents of complex to one (1) equivalence of ligand was first used, which resulted in the formation of a trinuclear complex, [{Ru(phen)2}3{µ-mes(1,4-phO-Izphen)3}](PF6)6•CH3CN•10H2O, with a yield of 48%. In another procedure with 1.5 equivalences of the ruthenium(II) precursor to 1 equivalence of the ligand, [{Ru(phen)2}3{µ-mes(1,4-phOIzphen)3}](PF6)6•xH2O was isolated with a yield of 58%. The fomula of the trinuclear complex was ascertained by elemental analysis, ESI MS, UV-visible and 1H NMR spectroscopies, along with electrochemical studies.

In another study [Co(dmgBF2)2(H2O)2] (where dmgBF2 = difluoroboryldimethylglyoximato) was reacted with triethylamine in either acetone or acetonitrile in an attempt to identify the products that were formed in situ during photocatalytic production of hydrogen when triethylamine was used as a sacrificial reductant. Techniques such as elemental analysis, ESI MS, and UV-visible spectroscopy were used in an attempt to characterize the resulting products from the respective solvents. Futher characterization of the unknown isolated product will be carried out in the near future. [Co(dmgBF2)2(H2O)2] was used to synthesize [Co(dmgBF2)2(H2O)(py)]•0.5(CH3)2CO in acetone. The formulation of [Co(dmgBF2)2(H2O)(py)]•0.5(CH3)2CO was confirmed by elemental analysis, high resolution ESI MS, and FT IR spectroscopy. Equilibrium studies carried out on [Co(dmgBF2)2(H2O)2] proved the formation of a monopyridine species, with formation constants, log K = 5.5, 5.1, 5.0, 4.4, and 3.1 in 2-butanone, dichloromethane, acetone, 1,2difluorobenzene/acetone (4:1, v/v), and acetonitrile, respectively, at 20 oC. In strongly coordinating solvents, such as acetonitrile, the magnitude of K is observed to be lower, and this phenomenon was also observed in the electrochemical studies and in the other spectroscopic studies as well.

A larger formation constant, log K = 4.6 vs 3.1, as calculated for the pyridine coordinated to a cobalt(I) species relative to the cobalt(II) species in acetonitrile at 20 °C. The electrosynthesis of hydrogen by [Co(dmgBF2)2(H2O)2] and [Co(dmgBF2)2(H2O)(py)] •0.5(CH3)2CO in various solvents demonstrated the dramatic effect of the axial ligand on the turn over number of the catalyst, which eventually will assist in the development of the next generation of H2 producing catalysts.