VOLTAMMETRY, ELECTRON-MICROSCOPY, AND X-RAY ELECTRON-PROBE MICROANALYSIS AT THE ELECTRODE AQUEOUS-ELECTROLYTE INTERFACE OF SOLID MICROCRYSTALLINE CIS-CR(CO)2(DPE)2 AND TRANS-CR(CO)2(DPE)2 AND TRANS-[CR(CO)2(DPE)2]+ COMPLEXES (DPE = PH2PCH2CH2PPH2) MECHANICALLY ATTACHED TO CARBONELECTRODES
Am. Bond et al., VOLTAMMETRY, ELECTRON-MICROSCOPY, AND X-RAY ELECTRON-PROBE MICROANALYSIS AT THE ELECTRODE AQUEOUS-ELECTROLYTE INTERFACE OF SOLID MICROCRYSTALLINE CIS-CR(CO)2(DPE)2 AND TRANS-CR(CO)2(DPE)2 AND TRANS-[CR(CO)2(DPE)2]+ COMPLEXES (DPE = PH2PCH2CH2PPH2) MECHANICALLY ATTACHED TO CARBONELECTRODES, Journal of the American Chemical Society, 115(21), 1993, pp. 9556-9562
Microcrystalline forms (size range 0.1-10 mum) of cis-Cr(CO)2(dpe)2 (c
is0), trans-Cr(CO)2(dpe)2 (trans0), and trans-[Cr(CO)2(dpe)2]+ (trans) (dpe = Ph2PCH2CH2PPh2) may be mechanically attached to carbon electr
odes. The voltammetry of these water-insoluble materials produces exce
edingly well defined processes over a wide scan rate range when the el
ectrode is placed into aqueous media containing 0.1 M NaClO4 or 0.1 M
KClO4 as the electrolyte. Electron probe microanalysis demonstrates th
at ClO4- partially covers the edges and the surface of the solid after
oxidative electrolysis. This suggests that oxidative voltammetry of t
he uncharged complex occurs at the crystal-electrode-solution interfac
e to form a perchlorate complex. The redox process observed for the ar
rays of microcrystalline carbonyl compounds attached to the electrode
may be summarized by the following reaction schemes: cis-Cr(CO)2(dpe)2
half arrow right over half arrow left cis-Cr(CO)2(dPe)2+ e- and trans
-Cr(CO)2(dpe)2 half arrow right over half arrow left trans-[Cr(CO)2(dp
e)2]+ + e- half arrow right over half arrow left trans-[Cr(CO)2(dpe)2]
+ + e- with cis-[Cr(CO)2(dpe)2]+ slowly isomerizing to trans-[Cr(CO)2(
dpe)2]+. Interestingly,the trans0 complex may be reversibly oxidized t
o trans+ and trans2+ under most conditions, but not as readily reduced
from trans+ back to trans0 if the potential is held for short periods
of time at potentials intermediate between the trans+/0 and trans2+/ processes. This indicates that the presence of a pure trans+ phase hi
nders reduction; however stepping the potential to a value more negati
ve than the reduction potential of the trans+/trans0 couple and then s
canning in the positive potential direction restores the current to it
s original value. The experimental results are in accord with an elect
rochemical process that takes place at the solid-solution interface to
form a layer of oxidized material. Electron transfer is postulated to
occur by electron hopping via self exchange and cross redox reactions
with the rate (apparent diffusion coefficient) being dependent on the
state of the electrode-compound-solution interface and the surface ch
arge.