CAPÍTULO IV RESULTADOS
4.2 Discusión de Resultados
4.2.3 Con la Hipótesis
Figure 4.7; 8^^Se(D20)N.M.R. Spectrum of Na[Fe
4Se
3(NO)y]" (Buffered)
Figure 4.8; 8^^Se(D20)N.M.R. Spectrum of Na[Fe
4Se
3(NO)/]" (Unbuffered)
M echanism Study Intro d u ctio n
I f it is accepted that the core o f n atu rally occuring iro n su lp h u r n itrosyl species comes from the iro n sulphur clusters in the active centres of proteins, then a plausible pathw ay from these b u ild in g blocks to the
methyl ester of Roussin's red salt [Fe2(SMe)2N04] can be accepted.
The pathw ay is described in Chapter One. O nly two o f the seven reaction steps in the pathw ay have n ot yet been established at least as occurring non-biologically. As p art of the early w o rk fo r this thesis, an investigation into one o f these was begun.
A fte r the fo rm a tio n o f the p a ram a g netic iro n species [Fe(SH )2(N O )2l" from the in itia l reaction of the clusters w ith n itrite , one
p ossible ro ute m ay in vo lv e a "m é th y la tio n " re a ctio n to g ive
[Fe(N O )2(SMe)2]% after w hich dim érisation occurs spontaneously. There
are two possible points of substitution w hich are shown in Fig. 4.9 below.
[Fe4S3(NO>7]-
^ [Fe(NO)2(SH>2]-
RSH attacks Fe R X attacks S[Fe2(SMe)2(NO)4]-*-^elüene_ [Fe(NO)2(SMe)2l-
Figure 4.9; Possible Mechanisms O f Conversion for [Fe4S3(N O )/]" to
[Fe2(SMe)2(NO)4].
Substitution m ay occur at the iro n atom, w hich wo u ld be the lik e ly outcome of a reaction using an alkanethiol. A lte rn ative ly, if m éthylation was the result o f a reaction w ith an alkylhalide then substitution w o u ld
obviously be occuring at the sulphur atom. A convincing precedent fo r th io l exchange was described in a paper by Butler e t ^ (7). They showed that dinuclear a lkyl esters o f the type [Fe2(SR)2(N O )4l re a dily underw ent
exchange w ith a range o f thiols, R'SH. The exchange occured faster in p o la r, c o o rd in atin g solvents such as D M F and D M S O w here solvocomplexes of the type [Fe(NO)2(SR)L] and [Fe(NO)2L2l+ were lik e ly to
be formed (7). They proposed a mechanism fo r the exchange in w hich an im po rta n t intermediate was ide ntifie d as [Fe2(N O )2(SR)2]'. A ltho u g h the
reaction o f [Fe4S3(N O )/]" is com plicated by the presence o f a d d itio n a l
mo n o n itros y l apical iro n fragments, the same inte rme d iates were observed fo r this species. W hen NaSH was added to a D M F solution of black salt anion, [Fe2(N O )2(SH)2]’ was identified by E.S.R. spectroscopy.
Since the two clusters have the mononuclear species in com mon as an intermediate, it can be assumed that th io l exchange w ill occur fo r [Fe4Sg(NO)/]- as it has been shown to do for [Fe2(NO )2(SR)2l ' (7).
E xperimental
A n experiment was devised in w hich a solution o f [Fe(NO)2(SH)2]“
was prepared and d ivid e d into two equal volumes. To one a liq u ot was
added an alkanethiol, w h ich w o u ld substitute at the iro n atom and to the other p o rtion was added an a lkyl halide w hich w o u ld attack the su lp h u r atom. W hichever one o f these reactions was successful w o u ld indicate the mechanism of the "m é thyla tio n" step.
Roussin's black anion and a large excess of NaSH were stirred in
D M F under N / overnight at room temperature. The d ark b ro w n solution was transferred in two equal volumes to separate flasks using Schlenk techniques. To one reaction vessel was added a large excess o f bromopentane and to the other was added the same excess of pentanethiol. The expected green colour for the monoiron species failed to
m aterialise even after s tirrin g overnight in an ine rt atmosphere. Toluene was added to dimerise the monoiron species present and both flasks were reduced to dryness by heating and the use of the vacuum pum p. The pe nta neth io l p ro d u ct was a bro w n , o ily residue and th a t o f the bromopentane experiment was an orange solid. The residues were
subm itted fo r infra-red and N.M .R. analysis and T.L.C. o f the products on
silica in TH F solution indicated a large amount of starting m aterial in the
samples.
Results
The F.T.I.R. results, w hich were obtained from a solution in TH F of the residues were unhelpful and showed only large characteristic black salt
peaks but N.M .R. analysis proved to be very interesting.
The iH N.M .R. spectra fo r authentic samples of bromopentane and pentanethiol were firs t obtained and although sim ilar in overall structure, there were differences in the chemical shifts of the peaks (see figs. 4.10 and
4.11) as follows;
8 lH(CDC13) bromopentane 0.9(t),1.4(m), 1.75(m), 2.1 (m), 3.40(t).
81H(CDC13) pentanethiol 0.9(t),1.3(m), 1.60(m), 2.1(s), 2.50(m).
The largest discrepancy occurs between the multiplets furthest dow nfield. The iH N .M .R. spectra of the residues from the corresponding reactions
w ith black salt in D M F solution were almost superim posable however,
such was the strength o f the sim ilarities between them (see figs. 4.12 and
4.13). This suggests that the same product was formed in each reaction.
Further evidence of this coincidence of products is to be obtained by analysis o f the (CDCI3) N.M .R. spectra. Com parison of the five most
intense peaks of the authentic pentanethiol and the pentanethiol product spectra (see figs. 4.14 and 4.15 ) have been made ( see Table 4.3 ).
(CDCI3) pentanethiol (CDCI3) product 14.02 13.96 22.08 22.30 24.66 28.93 30,69 30.70 33.9 39.20 Table 4.3; fo r Pentanethiol and Product.
It can be seen that two carbon peaks have shifted. The peaks at 24.66ppm
and 33.90ppm in the authentic sample have been replaced b y new peaks at
28.93ppm and 39.20ppm in the product spectrum.
SM c bromopentane SU ç product
13.95 13.96
21.20
22.31 28.94 30.46 30.70 32.69 33.77 39.21 Table 4.4; fo r Bromopentane and Product.I f a s im ilar com parison is made between the spectra of
bromopentane and its corresponding product (see Pigs. 4.16 and 4.17) an
interesting coincidence becomes obvious (see Table 4.4).
Figure 4.10; N.M .R. Spectrum of Bromopentane. 148
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