posted on 2021-07-07, 20:06authored byMd. Hafizur Rahman, Abderrahman Atifi, Joel Rosenthal, Michael D. Ryan
The
reduction of [Fe(OEP)(NO)] has been studied in the presence
of aprotic room-temperature ionic liquids (RTIL) and protic (PIL)
ionic liquids dissolved within a molecular solvent (MS). The cyclic
voltammetric results showed the formation of RTIL nanodomains at low
concentrations of the RTIL/PIL solutions. The pKa values of the two PILs studied (i.e., trialkylammonium and
[DBU–H]+-based ionic liquids) differed by four units
in THF. While voltammetry in solutions containing all three RTILs
showed similar potential shifts of the first reduction of [Fe(OEP)(NO)]
to [Fe(OEP)(NO)]− at low concentrations, significant
differences were observed at higher concentrations for the ammonium
PIL. The trialkylammonium cation had previously been shown to protonate
the {FeNO}8 species at room temperature. Visible and infrared
spectroelectrochemistry revealed that the [DBU–H]+-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]− rather than formally protonating it. Despite these differences,
both PILs were able to efficiently reduce the nitrosyl species to
the hydroxylamine complex, which could be further reduced to ammonia.
On the voltammetric time scale and when the switching potential was
positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex
was re-oxidized back to the NO complex via direct oxidation of the
coordinated hydroxylamine at low scan rates or initial oxidation of
the ferrous porphyrin at high scan rates. The results of this work
show that, while [DBU–H]+ does not protonate electrochemically
generated [Fe(OEP)(NO)]−, it still plays an important
role in efficiently reducing the nitroxyl ligand via a series of proton-coupled
electron transfer steps to generate hydroxylamine and eventually ammonia.
The overall reaction rates were independent of the PIL concentration,
consistent with the nanodomain formation being important to the reduction
process.