Spin c osso e Fe
II
complexes as empla es o bime allic
oxala e-based 3D magne s
Eugenio
Co onado
a,
*
,
Jos
e
´
R.
Gal
a
´
n
Masca o
´
s
a,
*
,
Ma i
Ca men
Gim
e
´nez-L
o
´
pez
a
,
Manuel
Almeida
b
,
Jo
a
˜
o
C
.
Wae enbo gh
b
a Ins i u o de Ciencia Molecula , Uni e sidad de Valencia, Polıgono de la Coma, s/n, E-46980 Pa e na, Spain
b
Dep . Quimica,
ITN/CFMC-
UL,
P-2686-95
3
Saca
e
´
m,
Po ugal
Abs ac
We p esen he syn hesis and s uc u al cha ac e iza ion o he sal [Fe(bpp)2][MnC (ox)3]2
·
bpp
·
CH3OH. I c ys allizes in he mono-
clinic space g oup. This ma e ial con ains an anionic [MnC (ox)
3
]
—
3D 10-gon e omagne ic ne wo k, ha o de s below 3.0
K.
The
channels c ea ed by his a chi ec u e a e filled by he spin c osso e ca ions [Fe(bpp)
2
]
2+
(bpp
=
2,6(bispy azol-3-yl)py idine), ee ligand
and sol en molecules. No spin ansi ion has been obse ed a ambien p essu e.
Keywo ds:
Coo dina
ion chemis y;
Molecula
ma e ials;
Spin
c osso e ; Magne ic p ope ie
s;
M
o
¨
ssbaue spec oscopy
1. In oduc ion
The amilies o molecule-based magne s buil om poly-
me ic bime allic oxala e complexes ep esen one o he
mos explo ed se ies in he sea ch o molecule-based mag-
ne s. One o he key easons is he syn he ic con ol eached
o e he la ice dimensionali y. Indeed, bime allic dime s
[1], ime s [2], e ame s [3], 1D chains [4], and ex ended
2D [5,6] and 3D la ices [7,8] ha e been epo ed om
essen ially he same building blocks, whe e he coun e ion
and syn he ic condi ions dic a e he o ma ion o a gi en
oxala e-b idged s uc u e. Magne ic o de ing has been
obse ed in dimensionali ies 1 o highe .
Fe omagne ic bime allic anionic chains [K(18-c own-6)]-
[Mn(H
2
O)
2
C (ox)
3
] a e o med in he p esence o fla mon-
oca ionic complexes o alkali me als wi h c own-e he
ligands [4]. Fe omagne ic in e chain in e ac ions yield e -
omagne ic o de ing below 3.5 K, due o he sho p oxim-
* Co esponding au ho . Tel.: +34 963544420; ax: +34 963543273.
E-mail add esses: eugenio.co onado@u .es (E. Co onado), jose. .
galan@u
.es
(J.R.
Gal
a
´
n
M
asca o
´
s).
i y be ween hese chains in he solid s a e, whe e hey
appea ela ed by hyd ogen bonding. These same ca ions,
in diffe en syn he ic condi ions, can also yield anionic 2D
ne wo ks [K(18-c own-6)]
3
[Mn
3
(H
2
O)
4
{C (ox)
3
}
3
] whe e he
p esence o wa e molecules as ligands induces he
o ma ion o a dis o ed honeycomb ne wo k [9]. The onse
o e omagne ic o de ing is obse ed below 4 K.
The o ma ion o anionic 2D hexagonal ne wo ks o
o mula
[M
II
M
III
(ox)
3
]
—
(M
II
=
Mn,
Fe,
Co,
Ni,
Cu;
M
III
=
C , Fe, Ru, Rh) [5,10,11] is p omo ed by he use
o bulky o ganic monoca ions o he e aalkylammonium
ype. In hese laye s, whe e only oxala e ligands a e p esen
o achie e maximum magne ic connec i i y, he me al cen-
e s show al e na ing chi ali y. Fe o- o e imagne ic
o de ing wi h c i ical empe a u es up o 44 K and coe ci e
fields o e 2 T, ha e been epo ed in his se ies.
Chi al oc ahed al ischela ed complexes, on he o he hand,
p omo e he o ma ion o homochi al 3D anionic ne wo ks o
he same o mula, wi h c i ical empe a u es much lowe
han hose o hei 2D coun e pa s [12]. This has been
ela ed o he weake magne ic in e ac ions p esen in his
case p omo ed by a diffe en ela i e o ien a ion
Þ
Scheme 1.
o he magne ic o bi als and o la ge me al o me al dis-
ances. Mos o hese compounds show Tc’s below 3 K, wi h
ew excep ions eaching up o 6 K.
The use o ‘‘elec oac i e’’ ca ions, ins ead o he elec-
onically ‘‘innocen ’’ ones in he o ma ion o such anionic
ne wo ks has been one o he mos success ul app oaches
o mul i unc ional ma e ials, whe e unp eceden ed mul i-
unc ional magne s ha e been ob ained, such as e omag-
ne ic molecula me als [13,14], chemically-buil magne ic
mul ilaye s [15], pho oac i e magne s [16], o chi al mag- ne s
[7,8,17]. Mos o hese examples a e buil om he hex-
agona
l
2
D
honeycomb
[M
II
M
III
(ox)
3
]
—
ne wo ks.
The
3D
analogs ha e been much less explo ed on his ega d, p ob-
ably due o he mo e syn he ically s ic condi ions needed.
Spin c osso e complexes [18] and o he ca ions wi h in e -
es ing pho ophysical p ope ies [19] ha e been embedded
in o hese 3D sys ems, bu including diamagne ic alkali me als
in he ne wo k, p e en ing he appea ance o mag- ne ic
o de ing.
We ha e been in es iga ing o se e al yea s he possi-
bili y o use Fe
II
complexes as empla es o oxala e-based
magne s. Un il now, all ou a emp s yielded he highly
insol
uble sal s
[A
I
][Fe(
L)
3
][M
III
(ox)
3
]
x
H
2
O
(
A
=
Li,
Na,
K, NH
4þ
[20]. These compounds exhibi indeed spin c oss-
o e beha io , bu no magne ic o de ing is obse ed, since
he pa amagne ic cen e s a e isola ed om one ano he in
he s uc u e. He e we epo how he complex [Fe(bpp)
2
]
2+
(Scheme 1); bpp = 2,6(bis(py azol-3-yl)py idine) ha
exhibi s spin c osso e beha io in o he simple sal s [21]
including he
LIEST
effec [22], is able o be used as ca -
ionic gues o bime allic oxala e-based 3D magne s.
2.
Expe imen al
2.1.
Syn hesis
All eagen s we e comme cially a ailable and used wi h-
ou u he pu ifica ion The p ecu so Ag
3
C (ox)
3
3H
2
O
was p epa ed by me a hesis in wa e om K
3
C (ox)
3
·
3H
2
O
·
Ag
3
C (ox)
3
·
3H
2
O (0.098 g, 0.142 mmol) was sus-
pended in 7.5 mL me hanol, and hen a solu ion o MnCl
2
·
4H
2
O (0.042 g, 0.213 mmol) in 7.5 mL me hanol was
added d opwise. The whi e AgCl p ecipi a e o med
solu ion o Fe(ClO
4
)
2
·
H
2
O (0.090 g, 0.355 mmol) and
bpp (0.149 g, 0.71 mmol) in 15
mL
o ho MeOH. This
solu ion was efluxed o 2 h. The yellowish p ecipi a e
o med was ho fil e ed, and washed wi h me hanol and
ace one. This powde was dissol ed in
DMF
and c ys als
o [Fe(bpp)
2
][MnC (ox)
3
]
2
·
bpp
·
CH
3
OH (1) we e g own
om laye ing his solu ion wi h ace one. Yield: 30%.
IR
(cm
—1
): 3427, w; 3124, w; 2929, w; 1654, s; 1652, s; 1628,
m; 1629, m; 1462, m; 1384, m; 1277, m; 906, w; 811, w;
669, s; 541, m; 478, m; 414, m. Mn
2
N
15
O
25
C
2
FeC
46
H
31
,
M
w
= 1463.59. Elemen al Anal. Calc.: C, 37.75; H, 2.13;
N 14.36. Found: C, 37.12; H, 2.45; N, 15.20%. Desol a ion
o he sample was ca ied ou by hea ing he c ys als in ai
a 125
°
C o 6 h.
2.2.
S uc u al cha ac e iza ion
A eddish p isma ic single c ys al o 1 was fixed on a
glass fibe and moun ed on a Kappa CCD diff ac ome e
equipped wi h g aphi e-monoch oma ed Mo K
a
adia ion
(k
=
0.71073
A
˚
).
Cell
efin
emen s
and da a educ
ion
we
e
pe o med a 150
K
using he
DENZO
and
SCALEPACK
p o-
g ams [23]. The s uc u e was sol ed by di ec me hods
using he
SIR
97 p og am [24] and efined on F
2
wi h he
SHELXL
-97
p og am [25].
All
a oms belonging o he anionic
ne wo k we e loca ed by successi e Fou ie ans o m ou-
ines, and efined aniso opically. The ca ions and sol en
molecules, occupying he wholes le in he s uc u e, could
no be loca ed due o he p esence o se e e c ys allog aphic
diso de . Two c ys allog aphic independen posi ions o
he Fe cen e s we e ound, wi h pa ial occupancy. Only he
fi s coo dina ion sphe e o
N
a oms could be loca ed. No
good model o hese complexes and sol en molecules,
including he in e s i ial bpp moie y could be ound due o
hea y diso de . C ys al, da a collec ion, and efinemen
pa ame e s a e summa ized in Table 1.
was fil e ed, and he g een solu ion was added o a ed
—
Table 2
Es ima ed pa ame e
s
o
low-spin and high-spin
Fe
II
om
he
M
o
¨
ssbaue
spec a aken a diffe en empe a u es o ec ys allized [Fe(bpp)2]
[MnC (ox)3]2 be o e and a e hea ea men
T (K) C ys als
Desol a ed sample
IS QS C
I
(%) IS QS C
I
(%)
297
Fe L.S.
0.39
0.73
0.57
73
0.36
0.78
0.54
80
Fe H.S.
1.01
2.53
0.39
27
0.97
2.34
0.36
20
120 Fe
L.S.
0.49
0.78
0.49
64
0.45
0.77
0.46
73
Fe H.S.
1.10
3.11
0.33
36
1.11
2.63
0.48
27
4 Fe
L.S.
0.50
0.78
0.62
58
0.49
0.74
0.50
67
Fe H.S.
1.15
3.25
0.35
42
1.20
2.80
0.50
33
IS (mm/s) isome shi ela i e o me allic
a
-Fe a 295 K; QS (mm/s)
quad upole spli ing; C (mm/s) line-wid h; I ela i e a ea. Es ima ed e o s
6
0.02 mm/s o IS, QS, C, and < 2% o I.
2.3. Magne ic cha ac e iza ion
Bulk magne iza ion measu emen s we e ca ied ou on
g ained polyc ys alline samples wi h a Quan um Design
magne ome e wi h an applied field o 1000 G (0.1 T) in he
empe a u e ange 2–300 K, wi h cooling and wa ming
cycles. AC da a was collec ed in he 2–10 K ange wi h an
al e na ing field o 3.95 G (3.95
·
10
—4
T) wi h equencies
be ween 1 and 1000 Hz. Hys e esis loops a 2 K we e col-
lec ed be ween 5 and 5 T. Da a we e co ec ed o dia-
magne ic con ibu ions calcula ed using he Pascal
cons an s.
2.4.
Mössbaue spec oscopy
M
o
¨
ssbaue
spec
a
we e
collec ed a
diffe en
empe
a-
u es in ansmission mode using a con en ional con-
s an -accele a ion spec ome e and a 25 mCi
57
Co sou ce
in a Rh ma ix. The eloci y scale was calib a ed using
a
-
Fe oil. Isome shi s (IS, Table 2) a e gi en ela i e o
me allic
a
-Fe a oom empe a u e. Low- empe a u e spec-
a we e collec ed using a JANIS ba h c yos a . The spec a
we e fi ed o Lo en zian lines using a non-linea leas -
squa es me hod [26]. The ela i e a eas and wid hs o bo h
peaks in a quad upole double we e kep equal du ing
efinemen .
3. Resul s and discussion
3.1. Syn hesis and s uc u e
The inse ion o he spin c osso e ca ion [Fe(bpp)
2
]
2+
as gues in o oxala e-based bime allic ne wo ks had o s a
by o e coming he p ecipi a ion o he kine ic p oduc
[A
I
][Fe(bpp)
2
][M
III
(ox)
3
]
x
H
2
O.
This
was done
oll
owing
he p ocedu e used in he p epa a ion o o he oxala e
bime allic complexes a oiding he p esence o any o he
ca ions in solu ion bu he ansi ion me al hemsel es.
Howe e , when a solu ion o Mn
3
[C (ox)
3
]
2
is ea ed wi h
any sal o [Fe(bpp)
2
]
2+
, again, an undesi ed p ecipi a e is
ob ained. This ma e ial could no be s uc u ally cha ac e -
ized. The 2:2:1 a io ound o Fe:C :Mn by analysis and
he pa amagne ic beha io sugges ha maybe i is o med
by oxala e-b idged ime s [2]. I he same eac ion is ca -
ied ou unde eflux condi ions o se e al hou s his
ini ial p ecipi a e slowly edisol es o yield a second p od-
uc , whe e no aces o his fi s compound can be ound.
The main componen o his p ecipi a e can be isola ed
by laye ing a DMF solu ion wi h ace one, o yield a c op
o single c ys als o [Fe(bpp)
2
][MnC (ox)
3
]
2
·
bpp
·
CH
3
OH
(1).
The s uc u e o 1 is buil om an anionic 3D polyme ic
oxala e-b idged bime allic s uc u e. As in he well-known
chi al 3D ne wo ks, his anionic s uc u e is o med by bis-
chela ing oxala e ligands ha connec each di alen me al
o h ee i alen me als, and ice e sa, building en-
membe ed ings in a (10, 3) opology (Fig. 1). The big di -
e ence wi h he epo ed chi al 3D ne wo ks is ha , in his
case, he compound is achi al. Ac ually, i belongs o he
cen osymme ic monoclinic P2
1
/n space g oup, wi h me al
cen e s o bo h chi ali ies p esen . This is somehow su p is-
ing because i was discussed in he pas ha only homochi-
al ne wo ks would be able o adop such a 3D s uc u e.
In his case, Mn(II) and C (III) occupy c ys allog aphi-
cally diffe en posi ions, since hey a e non-equi alen , and
can be clea ly dis inguished a ending o he M–O dis-
ances. The me al posi ion wi h longe dis ances is assigned
o
Mn
II
–O
(2.05(3)–2.28(3)
A
˚
),
and
he
o he o
C
III
–O
(1.94(3)–2.04(3)
A
˚
).
The
e
a e
wo c ys
allog aphically
independen Mn and C posi ions in he asymme ic uni ,
each one belonging o a diffe en chi ali y, ha a e b idged
oge he h ough he e ochi al connec ions. Thus, i we con-
Fig. 1. Two 10-membe ed ings in he s uc u e o [Fe(bpp)
2
]-
[MnC (ox)3]2 bpp CH3OH (1) o opposi e chi ali y, connec ed along
he c
di ec ion.
side he ings pe pendicula o he a and b axis, hese a e
o med by fi e adjacen uni s o one chi ali y and opposi e
o ha o he o he fi e. The p ojec ions o his achi al 3D
s uc u e on he ac and bc planes a e almos iden ical o
hose o he well-known chi al 3D analogs (Fig. 2), whe e all
h ee p ojec ions (ab, bc, and ac) a e iden ical. On he
con a y, he p ojec ion on he ab plane o compound 1
now shows an eclipsed disposi ion o he ings (Fig. 3). This
can be unde s ood as a ela i e mo emen o he ings on
his plane o allow o he he e ochi al junc ions be ween
homochi al ings.
The elec on densi y inside his 3D ne wo k could no be
assigned o a good chemical model, due o he p esence o
impo an c ys allog aphic diso de . One eason o his,
when compa ed wi h he chi al 3D analogs, is he much
highe p esen o in e s i ial sol en molecules, since only
Fig. 2. P ojec ions o he c ys al s uc u e o [Fe(bpp)2][MnC (ox)3]2
·
bpp
·
CH3OH ( op) and [Ru(bpy)2][MnC (ox)3]2
·
bpp
·
CH3OH [7] (bo om)
on he bc plane.
Fig. 3. P ojec ions o he c ys al s uc u e o [Fe(bpp)2][MnC (ox)3]2
·
bpp
CH
3
OH
on he ab plane.
one Fe complex is p esen pe MnC pai , and he e o e
no ex a anions a e needed as in hose. The ca ionic com-
plexes occupy analogous posi ions in he c ys al s uc u e,
bu now wi h hal occupancy, since his posi ions a e sha ed
wi h diso de ed sol en and ee bpp moie ies. Wi h he
da a ob ained, only he posi ion o he Fe a oms, including
he fi s coo dina ion sphe e o six N a oms could be
loca ed.
3.2. Magne ic p ope ies
Wi h excep ion o he achi al na u e o his ne wo k, he
connec i i y be ween he me als is iden ical o ha o he
chi al 3D bime allic magne s desc ibed be o e and hus, he
magne ic p ope ies we e expec ed o be analogous, wi h
he addi ion o he magne ic beha io o he embed- ded
Fe
II
complexes. The he mal dependence o he
m
T
p oduc (Fig. 4) shows a small bu con inuous dec ease
when cooling down he sample. Since he in e ac ions
Fig. 4. The mal dependence o he T p oduc o [Fe(bpp)2]- [MnC (ox)3]2
·
bpp
·
CH3OH.
be ween
Mn
II
and
C
III
cen e s p omo ed by he oxala e
b idge a e expec ed o be e omagne ic, his beha io sug-
ges s a slow and non-coope a i e spin ansi ion o he Fe
cen e s, be ween high spin (HS) and low spin (LS)
configu a ions. The high empe a u e egime can be fi ed
o a Cu ie Weiss law wi h a Cu ie cons an o 15.49 emu
K mol
—
1
, in pe ec ag eemen wi h he spin only
expec ed alue o a magne ically dilu ed sample. A
300
K
he
m
T is 15.05 emu
K
mol
—
1
. This sugges s ha
a ound 15% o he Fe cen e s a e LS a his empe a u e.
The
m
T p oduc eaches a minimum a 35
K
(14.03
emu K mol
—1
) and s a s o inc ease apidly, due o he
e omagne ic in e ac ion in he ne wo k, and sug- ges ing
he p esence o magne ic o de ing a e y low
empe a u es.
AC
suscep ibili y magne ic da a (Fig. 5) confi m he
onse o e omagne ic o de ing below 3.0
K,
wi h an
appea ance o a equency independen ou -o -phase sig-
nal, accompanying a maximum in he in-phase signal. This
empe a u e is in good ag eemen wi h ha ound o o he
3D [MnC (ox)
3
]
—
ne wo ks, whe e e omagne ic o de ing
is achie ed up o 3.9
K
[7]. The appea ance o e omag-
ne ic o de ing was also confi med by ze o field cooled and
field cooled
DC
measu emen s a e y low applied
magne ic fields ha de ia e om one ano he a his e y
same empe a u e.
The field dependence o he magne iza ion in he
o de ed s a e a 2
K
(Fig. 6) shows a e y as inc ease a
low fields and hen ends o sa u a ion, al hough i is no
eached
e en
a
5
T
,
wi h a alue
o
15.06
l
B
.
This
is
e y
close o
he
16
l
B
expec e
d
o
e omagne
ic
alignmen
o
he Mn and C cen e s in he ne wo k, and confi ms ha
he magne ic o de ing is indeed o e omagne ic na u e.
No clea con ibu ion o he Fe cen e s is ound, wha sug-
ges s ha a his empe a u e mos o he Fe cen e s will be
in hei
LS
configu a ion.
16
14
12
10
8
6
4
2
0
0 1 2 3 4 5
H (T)
Fig. 6. Field dependence o he magne iza ion a 2
K
o [Fe(bpp)
2
]-
[MnC (ox)
3
]
2
·
bpp
·
CH
3
OH a 333 Hz.
Since desol a ion usually affec s he spin c osso e
beha io , we also pe o med DC magne ic measu emen s
on desol a ed samples. We ound ha he beha io is
essen ially iden ical, while he
m
T p oduc is sligh ly
lowe , indica ing ha desol a ion inc eases he popula ion
o LS Fe
II
, bu i does no affec quali a i ely he he mal
magne ic beha io o he sample.
3.3.
Mössbaue spec oscopy
All
he
M
o
¨
ssbaue spec a
(
Fig.
7
)
may
be
fi ed
wi h wo
quad upole double s. The es ima ed pa ame e s (Table
2) o
he double wi h he la ges quad upole spli ing, QS, a e
ypical o HS
Fe
II
wi h S = 2 [27]. The significan ly lowe QS
and IS as well as he empe a u e independen
Fig. 5. The mal dependence o he AC in-phase (
0
, ull ci cles) and
ou -
o -phase ( 00, emp y ci cles) magne ic suscep ibili ies o [Fe(bpp)2]-
[MnC (ox)
3
]
2
·
bpp
·
CH
3
OH a 333 Hz.
Fig.
7.
M
o
¨
ssbaue spec a
o
[Fe(bpp)
2
][MnC (ox)
3
]
2
·
bpp
·
CH
3
OH
a
diffe en empe a u es.
297 K
120 K
4 K
M (
µB)
QS o he o he double a e consis en wi h
LS Fe
II
, S =
0.
A e hea ea men , QS and he peak wid hs es ima ed o
HS
Fe
II
change significan ly (Table 2). Fu he mo e es i-
ma ed ela i e a eas indica e a s abiliza ion o he LS ela-
i e o he HS s a e a e desol a ion.
I
he
ecoil
- ee
ac ions
o
bo h
HS
and
L
S
F
e
II
a e
simila in he measu ed empe a u e ange a much unex-
pec ed conclusion is de i ed: he ac ion o
Fe
II
in he
HS
s a e significan ly dec eases wi h inc easing empe a u e
o bo h un ea ed and hea - ea ed samples. As a as we
know, such a esul has ne e been epo ed.
A
mo e
de ailed in es iga ion is howe e necessa y as he obse ed
diffe ence in he empe a u e dependencies o he HS and
LS Fe
II
ela i e a eas may be due o much s onge chemical
bonds es ablished by
HS Fe
II
han by
LS Fe
II
.
This would
imply a s onge dec ease wi h inc easing empe a u e o
he ecoilless ac o o he
LS Fe
II
as compa ed o
HS Fe
II
.
4. Conclusions
We ha e shown how i is possible o ob ain molecule-
based magne s based on bime allic oxala e complexes using
as empla e he spin c osso e complex [Fe(bpp)
2
]
2+
. The
compound [Fe(bpp)
2
][MnC (ox)
3
]
2
·
bpp
·
CH
3
OH shows
e omagne ic o de ing below 3.0
K.
F om he s uc u al
poin o iew, his is he fi s achi al polymo ph o he well-
known chi al 3D cubic s uc u e [Z(L)
3
][ClO
4
]-
[MnC
(ox)
3
]
(
Z
=
Ru,
Fe;
L
=
2,2
0
-dipy idyl).
I
was
dis- cussed in he
pas ha he ex a pe chlo a e anions we e needed o
ob ain his ype o sal s. Now we ha e also dem- ons a ed
ha his is no ue, and hese magne s can also be
ob ained by subs i u ion o hal o he ca ions by sol en
molecules. This lowe occupancy o ca ions and anions
inside he anionic ne wo k could be esponsible o losing
he enan iopu e cha ac e o hese sys ems.
The [Fe(bpp)
2
]
2+
uni s embedded in his oxala e ne -
wo k a e no showing a clea spin ansi ion. Appa en ly,
LS
and HS cen e s co-exis a all empe a u es. Magne ic
measu emen s sugges ha he e is a small dec ease in he
HS
popula ion when he empe a u e is dec eased, ha would
be esponsible o he lowe ing o he magne ic
momen
.
O
n
he
o he ha
nd,
M
o
¨
ssbaue spec a do no
suppo
his hypo hesis. They sugges qui e he opposi e, i ecoil-
ee ac ions o
LS
and HS
Fe
II
a e assumed o be simila .
A his momen he e is no sa is ac o y explana- ion o
hese appa en ly con adic o y da a. In any case, al hough
magne ic o de ing is achie ed, he lack o coope - a i e spin
ansi ion in his compound p ecludes he s udy o possible
syne gy be ween bo h p ocesses. O he spin c osso e
complexes, able o be e fill he holes in such s uc u al
mo i would p obably be be e candida es in he sea ch o
such a ma e ial.
Acknowledgemen s
This wo k was suppo ed by he Eu opean Union
(Ne wo k o Excellence: MAGMANET). Financial sup-
po
om
he
Minis
e io
de
Educaci
o
´
n
y
Ciencia
(P ojec s MAT2004-3849 and BQU2002-01091), and om
he Gene ali a Valenciana (GV04A/77) is also
acknowledged.
Appendix A. Supplemen a y ma e ial
CCDC 618990 con ains he supplemen a y c ys allo-
g aphic da a o his pape . These da a can be ob ained ee
o cha ge ia h p://www.ccdc.cam.ac.uk/con s/ e ie ing.
h ml, o om he Camb idge C ys allog aphic Da a Cen- e,
12 Union Road, Camb idge CB2 1EZ, UK; ax: (+44)
1223-336-033; o e-mail: [email p o ec ed].
Supplemen a y da a associa ed wi h his a icle can be
ound, in he online e sion, a doi:10.1016/j.poly.2006.
09.064.
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