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Cleavage of an Ether Bond via -Elimination of Ethanol from [(H2O)5CrCH2CH2OC2H5]2+ in Aqueous Solutions; A Pulse Radiolysis Study.

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temperature range 100-298 K were readily fitted using an
isotropic Heisenberg model, H ' = 2 J S , .Sz ( S , =Sz = 2) and
g = 2.24. The spin exchange coupling constant J of - 14(2)
cm - I," 'I which is in excellent agreement with the value derived for deoxyhemerythrin,[61 indicates weak antiferromagnetic behavior. 1 is thus a very good model for the
bonding of the Fe" centers in deoxyhemerythrin and confirms the structure proposed by Solomon et a1.161and Stenkamp et al.["l
- h(nm]
- X[nm]
Fig. 2. Lett. Electronic spectrum of 1 in methanol at 293 K ; right: electronic
spectrum of 2 in methanol at 293 K (a further absorption was observed at
1031n m ( 1 : = 7 c m - ' m o l - ' L).
Oxidation of 1 by air yields IL,Fez(p-O)(p-CH3COZ)z]
(CIO& 2 , which has already been prepared previously via
an alternative route.'"] The two Fe"' centers are linked via
a pz-oxo bridge and two acetato bridges (Fig. 3). The Fe-N
and Fe-0 bond lengths correspond to those of other
model compounds for the diiron(ir1) centers in metazidohemerythrin.I3] The electronic spectrum of 2 in methanol
(Fig. 2, right) resembles that of metazidohemerythrin. The
magnetic susceptibility data for 2 (100-298 K) yielded a J
value of - I15 cm- ', which is very similar to the value of
- 134 cm I for metazidohemerythrin, and indicates strong
antiferromagnetic coupling.
c 11c
[I] a) R. E. Stenkamp, L. C . Sieker, L. H. Jensen, J. Sanders-Loehr, Narure
/London) 291 (1981) 263; b) W. T. Elam, E. A. Stern, J. D. McCallum, J.
Sanders-Loehr, J . Am. Chem. Soc. 104 (1982) 6369; c) R. E. Stenkamp,
L. C. Sieker, L. H. Jensen, Acta Crys/allogr. B39 (1983) 697: J. Inorg.
Biochem. 19 (1983) 247: J . Am. Chem Soc. 106 (1984) 618.
[2] J. W. Dawson, H. B. Gray, H. E. Hoenig, G. R. Rossman, J. M. Schredder, R. H. Wang, Biochemistry 1 I (1972) 46 I
[3] a) W. H. Armstrong, S. J. Lippard. J. Am. Chem. Soc. 105 (1983) 4837; b)
K. Wieghardt, K. Pohl, W. Gebert, Angew. Chem. 9S (1983) 739: Angew.
Chem. In/. Ed. Engl. 22 (1983) 727: c) W. H. Armstrong, A. Spool, G. C.
Papaefthymiou, R. B. Fraenkel, S. J. Lippard, J. Am. Chem. Soc. 106
(1984) 3653; d) A. Spool, I . D. Williams, S. J. Lippard, Inury. Chem 24
(1985) 2156.
[4] R. H . Holm, J. A. Ibers, Science 209 (1980) 223.
IS] J. Sanders-Loehr, T. M. Loehr, A. G. Mauk, H. B. Gray, J . Am. Chem
Soc. 102 (1980) 6992.
[6] R. C . Reem, E. I. Solomon, J. Am. Chem. Sor. 106 (1984) 8323.
[7] Procedure: 1: All operations were carried out under argon. A stirred solution of N . N'.N"-trimethyl-l,4,7-tr1azacyclononane (L) [ lo] (0.51 g,
3 mmol) in anhydrous methanol (40 mL) was treated with 0.36g
( I mmol) of Fe(CI0.&.6H20. After 2.5 hours' stirring at room temperature, anhydrous sodium acetate (0.2 g, 2.4 mmol) was added to the solution and stirring was continued for a further 30 min. After addition of
0.4 g NaCIO,.H,O and reduction of the volume of the reaction mixture
to 15 mL, 150 rng of green-yellow crystals of 1 precipitated, which were
filtered off and dried in an argon atmosphere.-2: When air was passed
through the above reaction solution, the color changed from greenishyellow to reddish-brown, and, after allowing to stand for 48 h in the
presence of air, 470 mg of reddish-brown dichroic crystals of 2 precipitated out.
[8] X-ray structure analysis of 1 and 2 (data for 2 in brackets): P4,2,2
(Amam): u = 1092(1) (1307.8(2)), b = 1092(1) (1773.2(3)), c=2785(3)
(1529.0(3)) p m ; Z=4(4): p'.,,'= 1.416 (1.513) g c m - ' : R s 0 . 0 8 2 (0.079)
for 1264 (2980) independent reflections ( / > I.So(!)); Mar,, radiation:
Siemens AED 11 diffractometer. Further details of the crystal structure
investigations are available o n requesr from the Fachinformationszentrum Energie Physik Mathematik, D-7514 Eggenstein-Leopoldshafen 2,
on quoting the depository number CSD 51 466. the names of the authors, and full citation of the journal.
[9] K. Wieghardt, K . Pohl, D. Ventur, Angew Chem. 97 (1985) 415: Angrw,.
Chem. I n t . Ed. Engl. 24 (1985) 392.
[lo] K. Wieghardt, P. Chaudhuri. B. Nuber, J . Weiss, Inorg. Chem. 21 (1982)
[II ] Protonation of the 0x0-bridge in [L'(Fe(g-0)(g-CH,C02),FeL'] to give
the cation [L'Fe(g'-OH)(g-CHICO,).FeL']' also leads to a conslderable
reduction in the antiferromagnetic coupling of the Fe"' centers: W. H.
Armstrong, S. J. Lippard, J. Am. Chem. Soc. 106 (1984) 4632.
[I21 R. E. Stenkamp, L. C. Sieker, L. H. Jensen, J. D. McCallum, J. SandersLoehr, Proc. N o / / . Acad. Sci. USA 82 (1985) 713.
Cleavage of an Ether Bond via B-Elimination of
Ethanol from I(H20)5CrCH2CH20C2H5]2+
Aqueous Solutions; A Pulse Radiolysis Study**
By Haim Cohen and Dan Meyerstein*
Dedicated to Professor Schulte-Frohlinde on the occasion
of his 60th birthday
Recently it has been shown that pentaaqua(p-hydroxyalkyl)chromium(iii) complexes decompose via B-elimination
of the hydroxy radical to yield [ C T ( H , O ) ~ ] ~
@ the corresponding alkene.['-41 The ~-hydroxyalkylchrornium(iii)
complexes therefore have a considerably shorter lifetime
[*] Prof. Dr. D. Meyerstein
Chemistry Department, Ben-Gurion University of the Negev
Beer-Sheva (Israel)
Fig. 3 . Structure of the cation [LZFei"(0)(C'H,COI)~]~+
in the crystal of 2.
Important bond lengths [pml and angles ["I: Fe-Ol 180.0(3), Fe-02 203.4(3),
Fe-NI 219.8(4), Fe-N2 226.8(6), C 5 - 0 2 124.6(4), CS-C6 146( I), Fe-FelB
3 12( I): 0 1 - F e - 0 2 97.2(2), 0 1 - F e - 0 2 A 97.2(3), 02-Fe-O2A 96.5(2), Fe-01FelB 119.7(1).
Dr. Haim Cohen
Nuclear Research Centre Negev and Coal Research Center,
Ben Gurion University of the Negev
Beer-Sheva (Israel)
Received: June 3, 1985;
revised: July 10, 1985 [Z 1327 IE]
German version: Angew. Chem. 97 (1985) 774
Angew. Clirm. f n t . Ed. Engl. 24 (198s) No. 9
This research was supported in part by a grant from the Israel-US. Rinational Science Foundation (Jerusalem). We wish to thank Prof. J . H .
Espenson and Prof. H . Pines for helpful discussions and Mr. D. Carmr
for technical assistance.
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than the analogous alkyl- or u-hydroxyalkyIchromium(iir)
complexes.1' 'I The p-elimination reaction has been shown
to be a relatively fast acid-catalyzed process,['-41resulting
in the formation of an unstable alkenechromium(iir) complex." 41 The products of the reaction between
[Cr( H20)6]2aand a,@-dihaloalkanes are also alkenes, indicating that pentaaqua(p-haloalkyl)chromium(iri) complexes decompose via fi-elimination of halides.[51It seemed
of interest to check whether ether bonds, which are considered to be kinetically considerably more stable, also decompose via [%elimination. We report here that the complex 1 (with Cloy as counterion) indeed decomposes in an
acid-catalyzed process to yield [Cr(H20)6]3e,ethylene, and
The relatively stable complex Z1".'] is usually prepared
via Reactions (a)-(c).
[Cr(HZ0),l'@ + H202+ H 3 0 e
k(a)=7.1 x lo4 M - ' s - ' [8]
+ (C2HS),0
k(b)= 1 . 4 10'
' [9]
+ OOH + 2 H z 0
+ 'CH(CH3)0C2HS
H ~ O
Se z , OOH, H, H,, H ~ O ~
(the relative yields of the primary products are
2.65, 2.65, 0.60, 0.45, and 0.75, respectively [ I I])
e ~+
t N,O
This synthetic route is based on the observation that the
major product of reaction of hydroxy radicals with diethyl
ether is the u-radical [Reaction (b)].I"] However, from the
relative rates of hydrogen abstraction at the u- and p-carbons of diethyl ether by hydroxy radicals,['] one would expect that ca. 20% of the free radicals should be formed via
Reaction (d). Thus one would expect that ca. 20% of the
product should be 1 [Reaction (e)].
+ (C2H5)20 oCH2CHzOC,H, + HzO
[Cr(H20),l2@+ 'CHzCHzOCZHs
+ HIOa
+ 'OH + H,O
+ HzO
k(i)=2.3x 10'" M - ' s - ' [I21
(the relative yields of Reactions (h) and (i) depend on the pH)
+ H,O'
k(h)=8.7x 10'
[Cr(H20),l2@+ 'CH(CH3)OCZHs
k(c)=3.4X 10'
gas chromatography (T.C.D. Varian 1440) for possible formation of ethylene within 20 min from the start of the reaction (i.e., long before 2 had decomposed). In all the reaction mixtures ethylene was detected; the yield of ethylene
was 2 10Y0 of the total yield of free radicals. Since the relative rates of Reactions (b) and (d) (formation of a- and /3radicals, respectively) are not known exactly, no effort was
made to determine exactly the yield of ethylene.
The formation of ethylene in these systems clearly
proves that Reaction (f) indeed occurs. In order to measure
the rate of Reaction (f), we used the pulse radiolysis technique. The experimental setup was identical to that described earlier.""] N,O-saturated solutions containing 0.5
to 0.8 M (C,H,),O and 1 x
to 1 x lo-' M [Cr(HzO),]"
in the pH range 1.0 to 4.0 were irradiated. Under these
conditions the following processes occur in addition to
Reactions (b) and (d):
+ (C,H,),O
+ (CZHS)?O
+ H1
+ H2
~ ~ ) = 4 . 7 x 1 o 7 ~ ~ ' s ~ ~ [ ~ 3 1
(the relative yields of a - and P-abstraction by H atoms are larger than those
by O H radicals since the latter are more reactive)
Under the experimental conditions, Reactions (8) to 6 )
as well as (b) and (d) are over within the time duration of
the pulse (ca. 1 w s ) .
These reactions are followed by Reactions (c), (e), (a),
(k), (I), and probably (0.
However, the decomposition of 2 obeys a purely firstorder rate
suggesting that only one species is decomposing. Accordingly, 1 decomposes prior to the start of the
measurement of the kinetics of decomposition of 2. Since
the hydrolysis of [(H20)5CrCH3]2eis not fast, a plausible
mechanism for the fast decomposition of 1 could be pelimination [Reaction (f)].
In order to check this mechanism, the complex 2 was
prepared via two routes. In the first procedure, solutions
containing 0.5 M (C,H,),O and 0.05 M [Cr(Hzo),]*@ at pH
1.0 or 3.5 were mixed with aqueous H 2 0 zso that the final
amount of H z 0 2 was 1 . 0 lo-'
M . In the second procedure, He-saturated solutions at pH 1.0 or N20-saturated
solutions at pH 3.5 containing 0.5 M (C2H5)20 and
5 x lo-' M [Cr(H,O),]'" were irradiated using a ,'Co ysource (4500 rad/min, total dose 67500 rad, i.e., ca.
mol free radicals formed). The reaction mixtures
were analyzed by FT-IR spectroscopy (Nicolet MXs) and
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We have observed three kinetic processes (Fig. 1). The
first process is attributed to Reactions (c) and (e), which
are expected to have similar rate constants and might hide
a small contribution from Reaction (k), which is considerably faster.['41The second process is attributed to Reaction
(a) followed by reactions (b) to (e), since in this process the
absorption increases somewhat due to the alkylchromium
complex and since the rate is pseudo first order in
[Cr(H,O),]'@ and agrees with the reported rate constant of
Reaction (a).[']
The third process can be due either to Reaction (I) or to
Reaction (f). We attribute it to the latter for the following
reasons. First, the process obeys a pseudo-first-order rate
law in H 3 0 0 . The pH dependence of the reaction is plotted
in Figure 2 , from which k(f)=(4.6*0.8)~ lo3 M - ' s - ' is
calculated. This rate constant differs significantly from
k(l). Second, the percentage of decrease in the absorption
caused by this process at pH 3 to 4 is nearly independent
of the ratio of the concentration of ( C 2 H & 0 to that of
[Cr(H,0),12@ although the relative yields of Reactions (j)
and (k) are linearly dependent on this ratio. Third, the percentage of decrease in the absorption caused by this pro-
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Angew. Chem. Int. Ed. Engl. 24 (1988) No. 9
pound^,'^.'^ thus suggesting a similar transition state for the
two processes. It is tempting to speculate that the fast decomposition of complex 3 is not a hydrolysis reaction as
suggested[” but rather a @-eliminationreaction [Reaction
352 mV
Received: December 17, 1984;
revised: May 10. 1985 [Z 1 1 13 IE]
German version: Angew. Chem. Y7 (1985) 785
[ I ] H. Cohen, D. Meyerstein, Inorg. Chem. 13 (1974) 2434.
121 D. A. Ryan, J. H. Espenson, Inorg. Chem. 21 (1982) 527.
[3] H . Cohen, D. Meyerstein, A. Shusterman, M. Weiss, J . Am. Chem. Sac.
I06 (1984) 1876.
[4] D. Meyerstein, reported in part at the 23rd Int. Conf. Coord. Chem.,
Boulder, CO, August 1984, and unpublished results.
[5] C. E. Castro, W. C. Kray, Jr., J. Am. Chem. Soc. 85 (1963) 2768; W. C .
Kray, Jr., C. E. Castro, ibid. 86 (1964) 4603; C . E. Castro. W. C. Kray,
Jr., ibid. 88 (1966) 4447.
[6] W. Schmidt, J. H. Swinehart, H. Taube, J . Am. Chem. Soc. Y3 (1971)
[7] J. H. Espenson, Adu. Inorg. Bioinorg. Mech. I (1982).
[XI A. Bakac, J. H. Espenson, Inorg. Chem. 22 (1983) 779.
[9] M. Anbar, D. Meyerstein, P. Neta, J . Chem. Soc. B 1966. 742.
[lo] Y . Sorek, H. Cohen, W. A. Mulac, K. H. Schmidt, D. Meyerstein, Inorg.
Chem. 22 (1983) 3040.
[ l l ] M. Matheson, L. M. Dorfman: Pulse Radiolysis. M.I.T. Press, Cambridge, MA 1969.
[I21 M. Anbar, M. Banbenek, A. B. Ross, Natl. Stand. Ref Dutu Ser. ( U . S .
Natl. Bur. Stand.) NSRDS-NBS 43 (1973).
[13] M. Anbar, Farhataziz, A. B. Ross, Narl. Stand. R e / . Daru Srr. ( U . S .
Nal!. Bur. S f a n d . ) N S R D S - N B S 51 (1975).
[14] H. Cohen, D. Meyerstein, J . Chem. Soc. Dalton Trans. 1977. 1056.
- a
n n
9x 9x
o n
0 0
c [Sl
Fig. I . Kinetic traces of absorption changes upon irradiation at 3 10 n m of
N20-saturated aqueous solutions containing 0.5 M (C2HI)20 and 0.05 M
[Cr(H,O),,]’“ at pH 3.5. a) Formation of 2 and 1 . b ) Formation of Cr-C
compounds by the reaction of H 2 0 2with (C2HS)20and (Cr(H20)6]2e.c ) Decomposition of 1. d) First-order decrease in the concentration of 1.
--1,,=755 mV.
Selectivity and Mechanism of
Diene Cyclodimerization on Iron(0) Complexes**
By Heindirk tom D i c k * and Jorg Dietrich
Dedicated to Professor Gunther Wilke on the occasion
of his 60th birthday
c (HCLOb)
The cyclodimerization of 1,3-dienes on “nickel-ligand”
catalysts”] is one of the most investigated homogeneous
catalytic reactions, but the mechanism of the product release is still unknown. To clarify this we have carried out
investigations on the diazadiene-iron system.[31
Reaction of 1,4-diaza-1,3-dienes (dad), 1, and iron(11)
chloride leads to formation of the complexes 2 , which, for
the butadiene cyclodimerization, can be reductively activated by Grignard compounds or butadienemagnesium
Mg.C4H6.2THF to
Mg.2C,H6.2THF14’ to give 4, stabilized by solvent). In
the presence of an excess of butadiene, the reducing agent
has no influence on the ratio 6 : 7 (Table I). On complete
conversion, u p to 98% of 7 is formed (with l a ) and u p to
80% 6 (with 10a). When dad is too bulky ( l c , Id), the catalytic reaction is suppressed at room temperature, or (e.g.
with le) a disproportionation to Fe(dad)2, 8, and “Fe”, 9,
is observed. Diene-stabilized iron atoms 9 (without diene,
iron particles are formed!) appear to be responsible for the
comparatively slow formation of linear dimers (see Table
1) and higher oligomers; 8 is only catalytically active at
higher temperature^.^^^
Ligands, which-due to steric requirements-do not
form (dad)2M complexes or, if so, only slowly, such as If
Fig. 2 . Dependence of the rate of decomposition of I on pH, measured in
N?O-saturated aqueous solutions containing 0.5 M (C2H5)*Oand I x l o - ’ to
I x lo-’ M [ C F ( H ~ O )at~ 315
] ~ ~nm. lonicstrength was not taken into consideration.
cess is nearly pH independent in the p H range 2 to 4 although the relative yields of Reactions (b) and (h) depend
linearly on the pH.
The large residual absorbance remaining after the third
process is that due to complex 2 .
We thus conclude that the third process responds to
concentration variables as expected and, since ethylene is
detected as a product, this process is indeed the p-elimination of ethanol [Reaction (f)] and k ( f ) = (4.6 k 0.8) x lo’
This is the first reported rate constant for the cleavage of
an ether bond via e-elimination. The rate constant is similar to that reported for @-eliminations of the hydroxyl
group from [(H20)5CrCH2C(CH1)20H]’@
and related com-
[ C I - ( H ~ O ) ~ ] ’+
,k(m)=0.48 11
Prof. Dr. H. tom Dieck, Dr. J. Dietrich
lnstitut fur Anorganische und Angewandte Chemie der Universitat
Martin-Luther-King-Platz 6, D-2000 Hamburg 13 (FRG)
I**]Diazadienes as Control Ligands in Homogeneous Catalysis, Part 11.
Anyew. Cliem. lnr Ed. Engl. 24 (lY85J No. 9
This work was supported by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, and Hoechst AG.-Part 10: [I].
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radiolysis, bond, 5crch2ch2oc2h5, cleavage, h2o, pulse, elimination, solutions, ethanol, ethers, stud, aqueous, via
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