An Electrochemical and Fourier Transform Infrared Spectroscopic Study of the Aqueous Oxidation of Quadricyclane and Nortricyclanol

Abstract

The compound quadricyclo(2.2.1.0 2,6 .0 3,5 )heptane, more commonly referred to as quadricyclane (QC), has enjoyed a rich history of chemical investigation. From its initial discovery in 1936, 1 it has been considered for use as a solar energy storage medium, 2 an optical computer memory system, 3 and most recently, and perhaps most realistically, as a high energy aviation fuel. 4 Although several studies have been conducted concerning the nonaqueous chemistry of QC, including its valence isomerization to norbornadiene, [5][6][7][8][9] little is known about the chemistry that might be expected should QC be released into the (aqueous) environment. 10 The study of QC and its isomers by electrochemical means allows for careful assessment of the relevant redox chemistry in a timely fashion, as these processes occur very slowly under typical environmental conditions. The issue of release into the environment is of major concern given the possibility of its use as an aviation fuel additive. Toward this end, we report here a study of the aqueous redox chemistry of QC and nortricyclanol, a hydrolysis product that is readily formed when QC is present in aqueous solutions.QC is a seven-membered carbon quadricycle containing two cyclopropyl, one cyclobutyl, and two cyclopentyl rings (Scheme 1). The compound is a stable liquid at room temperature with a fairly high vapor pressure under normal conditions (bp 108ЊC at 740 mm Hg). The strain energy has been reported as approximately 340 kJ mol Ϫ1 . 11 The unusual reactivity of the system is due to this ring strain and generally results in cleavage of one or both of the C 2 -C 6 and C 3 -C 5 cyclopropyl bonds. QC readily undergoes photochemically, 12 electrochemically, 13 and chemically 14 initiated conversion to its lower energy valence isomer, norbornadiene (NB).There is a relatively high thermal barrier for the conversion of QC to NB (385 kJ mol Ϫ1 ), 15 which can be overcome with irradiation in the presence of a sensitizer. The conversion of QC to NB is achieved easily through a free-radical chain reaction. The initiation of this chain reaction is a one-electron oxidation to form the QC ϩ• radical cation, which quickly rearranges to the NB ϩ• radical cation. This species forms NB by oxidation of an additional QC molecule, in a step that generates another QC ϩ• radical cation, thus continuing the propagation. 15 Until recently, the structures and number of intermediates in this conversion were unknown. Through the use of chemically induced dynamic nuclear polarization (CIDNP) experiments, 16 Roth and Schilling were able to verify the existence of both the elusive QC ϩ• and the longer-lived NB ϩ• cation radicals. Very rapid isomerization of QC ϩ• to NB ϩ• precluded the direct spectroscopic observation of QC ϩ• until time-resolved electron spin resonance spectroscopy 17 was enlisted in the search. Turro et al. established the lifetime of QC ϩ• to be ϳ10 Ϫ6 s (with a thermal barrier of 38 kJ mol Ϫ1 for conversion to NB ϩ• ) 15 and also used laser flash photolysis and time-resol...

Energy Storage in Strained Organic Molecules: (Spectro)Electrochemical Characterization of Norbornadiene and Quadricyclane

“…Prior to every measurement, the organic film generated on the working electrode by fouling was removed by 100 voltammetric cycles between À2.5 Vand + 3.0 Vv ersus Ag pseudo-reference in a0 .1 m aqueous KCl solution at 1Vs À1 . [11] Electrochemical IR spectroscopy:E lectrochemical IR spectra were acquired with aF TIR spectrometer with evacuated optics (Bruker Vertex 80v). The system features IR optics for electrochemical measurements (Bruker) and ah ome-built polytetrafluoroethylene (PTFE) spectroelectrochemistry cell.…”

“…These observations are in line with the decomposition of the radical cation,w hich leads to electrode fouling, noted previously. [11] We notet hat because of the organic film that appearsu pon fouling (and which is impervious to acids, bases, and strong oxidants), the working electrode must be cleanedb ya ne lectrochemical procedure before each new measurement. Otherwise, the film buildup upon repeated cycling causes an overall current decrease (Figure 2a).…”

“…[7] The reverse reaction releases up to 100 kJ mol À1 thermally upon adequate triggering, which formally makes the NBD/QCf ramework as olarf uel. [11] Conversely, several papers have reported oxidation potentials withouts howing the data. [8a, 9] Research into valence isomers as potentials olar fuels peakedi n the 1980s, when the main limitation of the redox agents as cy-We have investigated the electrochemically triggered cycloreversion of quadricyclane (QC) to norbornadiene( NBD), asystem that holds the potentialtocombineb oth energy storage and conversion in as ingle molecule.…”

“…Unfortunately,only asingle paper was dedicated to the electrochemistry of NBD and QC to date, without conclusive statements as to the oxidation potentials. [11] Conversely, several papers have reported oxidation potentials withouts howing the data. [12] This likely has two causes:( 1) most of the attention was devoted to homogeneous redoxc hemical transformations insteado fh eterogeneous, electrochemical strategies;a nd (2) electrode foulingm ade it difficultt oc ollectc lean signals.…”

Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory

“…Another highly strained hydrocarbon, quadricyclane ( Q ), is of high practical interest because of its possible use in photochemical energy storage applications (equilibrium with norbornadiene). , The I v = 7.86 eV of Q is extremely low, and the corresponding radical cation Q • + isomerizes rapidly in a matrix at 77 K, − under pulse radiolysis in liquid methylcyclohexane at 133 K and under chemical or electrochemical , oxidation to the norbornadiene radical cation which is 11 kcal mol -1 more stable at CCSD(T)/6-31G( d )//MP2/6-31G( d ) . Unrearranged Q • + was observed experimentally only with time-resolved ESR spectroscopy .…”