New organometallic Ru(II) and Fe(II) complexes with tetrathia-[7]-helicene derivative ligands

Article abstract of DOI:10.1016/j.poly.2008.12.036

A series of organometallic complexes possessing new tetrathia-[7]-helicene nitrile derivative ligands [TH-7] as chromophores, of general formula [MCp(P-P)(NC{TH-[7]-Y}Z)][PF₆] (M = Ru, Fe, P-P = DPPE, Y = H, NO₂, Z = H, C≡N; M = Ru,

Full text of DOI:10.1016/j.poly.2008.12.036

Polyhedron 28 (2009) 621–629
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Polyhedron
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New organometallic Ru(II) and Fe(II) complexes with tetrathia-[7]-helicene
derivative ligands
a
b
c
Maria Helena Garcia a, , Pedro Florindo , Maria de Fátima M. Piedade , Stefano Maiorana ,
*
Emanuela Licandro c
a Centro de Ciências Moleculares e Materiais, Faculdade de Ciências da Universidade de Lisboa, Ed.C8, Campo Grande, 1749-016 Lisboa, Portugal
b Centro de Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
c Dipartimento di Chimica Organica e Industriale, Università degli Studi di Milano, Via C. Golgi, 19, I-20133 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2008
Accepted 2 December 2008
A series of organometallic complexes possessing new tetrathia-[7]-helicene nitrile derivative ligands
[TH-7] as chromophores, of general formula [MCp(P–P)(NC{TH-[7]-Y}Z)][PF6] (M = Ru, Fe, P–P = DPPE,
Y = H, NO2, Z = H, CꢀN; M = Ru, L–L = 2PPh3, Y = H, Z = H) has been synthesized and fully characterized.
1H NMR, FT-IR and UV–Vis. spectroscopic data were analyzed with in order to evaluate the existence of
electronic delocalization from the metal centre to the coordinated ligand to have some insight on the
potentialities of these new compounds as non-linear optical molecular materials. Slow crystallization
of compound [RuCp(PPh3)2(NC{TH-[7]-H}H)][PF6] 2Ru revealed an interesting isomerization of the heli-
cal ligand with formation of two carbon-carbon bonds between the two terminal thiophenes, leading to
the total closure of the helix (2ꢁRu).
Keywords:
Heterohelicenes
Tetrathia-[7]-helicene
Ruthenium(II)
Iron(II)
Ó 2008 Elsevier Ltd. All rights reserved.
Monocyclopentadienyl
1. Introduction
oxycarbonyl and trifluoromethyl) [13]. The reported optimized
synthesis of tetrathia-[7]-helicene (TH[7]) and the convenient
The search for new organic and organometallic materials with
significant non-linear optical (NLO) properties has been an area
of considerable interest due to their relevance to optical device
technology [1–6]. Nevertheless, the observation of even-order
non-linearity is limited to the asymmetric crystallization of the
bulk materials. Thus, chiral structures appear as ideal systems to
investigate second-order NLO effects due to their intrinsic non-
centrosymmetry, which allows these effects to be observed even
in symmetric media such as chiral isotropic liquids [7]. Large chiral
effects have been demonstrated in surface NLO responses of films
of chiral molecules adsorbed at the air/water interface [8] and of
Langmuir–Blodgett films of chiral polymers [9].
In this context, the intrinsically chiral structures of helicenes
[10] make these compounds very attractive to be investigated,
leading to several theoretical and experimental studies of carboh-
elicenes [11] and heterohelicenes [12]. The easier functionalization
of thiaheterohelicenes, compared to carbohelicenes, allows the
introduction of several different functional groups, either in the
terminal thiophene rings (formyl, alkoxycarbonyl, trifluoromethyl,
amide, trialkylsilyl, cyanoacrylic and acrylates) or in the central
benzene rings (alkyl and electron-withdrawing groups such as alk-
preparation of the formyl derivative 2-CHO-TH[7] [13a] paved
the way for the synthesis of
heterohelicenes.
a
variety of substituted
The present work reports the synthesis of three new TH[7]
derivatives, through the functionalization of one terminal thio-
phene ring with a nitrile group (L1), two terminal thiophene rings
with two nitrile groups (L2) and one terminal thiophene group
with one nitrile group followed by nitration in the benzene rings
(L3). These new TH[7] derivatives were used as ligands for the syn-
thesis of a new family of
g
5-monocyclopentadienyl ruthenium(II)
and iron(II) organometallic complexes represented by the general
formulation [MCp(PP)(NC{TH-[7]-Y} Z)] [PF6] (M = Ru(II), Fe(II),
P–P = DPPE, Y = H, NO2, Z = H, C„N; M = Ru(II), L–L = 2PPh3, Y = H,
Z = H).
The new compounds have been characterized by the usual FT-
IR, UV–Vis, 1H and 31P NMR spectroscopic techniques. Neverthe-
less, the poor solubility of the compounds in common deuterated
solvents did not allow the characterization by 13C NMR. The crys-
tallization of compound [RuCp(PPh3)2(NC{TH-[7]-H}H)] [PF6]
2Ru, by slow diffusion of n-hexane into an acetone solution of
the compound, afforded single crystals suitable for X-ray diffrac-
tion studies. This revealed isomerization of the helical ligand, due
to the formation of two new carbon–carbon bonds between the
terminal thiophene rings, giving the new complex 2ꢁRu.
* Corresponding author. Tel.: +351 21 750 00 75/6; fax: +351 21 750 00 88.
E-mail address: lena.garcia@fc.ul.pt (M.H. Garcia).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.12.036

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M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
acetonitrile-d3 and DMSO-d6) even at high temperatures and, con-
2. Results and discussion
sequently, further characterization by HMQC and HMBC NMR tech-
niques could not be applied to identify the position of the NO2
group; therefore, the structure proposed for L3 in Scheme 1 is sim-
ply one out of six possible structures.
Tetrathia-[7]-helicene-2,13-dicarbonitrile (L2) was synthesized
by the reaction of tetrathia-[7]-helicene (1) with two equivalents
of n-BuLi at ꢂ78 °C in DMF, affording tetrathia-[7]-helicene-2,13-
dicarbaldehyde (3), which was used, without purification, in the
reaction with hydroxylammonium chloride in pyridine, and dehy-
dration in situ with acetic anhydride.
2.1. Synthesis of the tetrathia-[7]-helicene ligands L1, L2 and L3
The synthesis of the TH[7] derived nitrile ligands, tetrathia-[7]-
helicene-2-carbonitrile (L1), tetrathia-[7]-helicene-2,13-dicarboni-
trile (L2) and tetrathia-[7]-helicene-2-carbonitrile (L3), bearing the
nitro group in an unidentified position of the helix, is summarized
in Scheme 1.
Tetrathia-[7]-helicene-2-carbaldehyde (2) was synthesized in
good yield by electrophilic formylation of the
a-anion of tetra-
thia-[7]-helicene, generated with n-BuLi at ꢂ78 °C in DMF, follow-
ing Ref. [13a]. The ligand tetrathia-[7]-helicene-2-carbonitrile (L1)
was synthesized from compound 2 and hydroxyl ammonium chlo-
ride in pyridine, with in situ dehydration by acetic anhydride.
The reaction of L1 with nitric acid in acetic anhydride was car-
ried out with the purpose of synthesizing tetrathia-[7]-helicene-
13-nitro-2-carbonitrile, nitrated at the opposite end of the helix
relatively to the nitrile group, as would be expected following
our previous experience with benzo[1,2-b;4,3-b0]dithiophene
derivative complexes [14]. However the analysis of the 1H NMR
spectrum of the obtained compound showed the presence of two
doublets at 6.65 and 7.57 ppm (JHH = 5.6 Hz), attributed to the thi-
ophene ring protons, revealing that the nitration had occurred in
one of the benzene rings. Nevertheless, substitution of the H on
the two terminal thiophene rings was possible with two nitrile
groups (L2), whilst the introduction of NO2 in position 13 through
an electrophilic substitution failed, thus demonstrating that the
only way to functionalize the helical system in a regioselective
manner is through the generation of anions in the positions 2
and 13 and the reaction with electrophilic reagents. 13C NMR stud-
ies were not carried out due to the low solubility of the organic li-
gands in common deuterated solvents (acetone-d6, chloroform-d,
2.2. Synthesis of complexes [M(
g
5-C5H5)(PP)(L)][PF6]
Complexes of the general formula [M(
(M = Ru and Fe, PP = DPPE, L = L1, L2 and L3; M = Ru(II),
g
5-C5H5)(PP)(L)][PF6]
(PP) = 2PPh3, L = L1) were prepared by halide abstraction with
TlPF6 from the parent neutral complexes [M(g
5-C5H5)(PP)X]
(M = Fe, X = I; M = Ru, X = Cl) in dichloromethane, in the presence
of an adequate excess of the corresponding nitrile (Scheme 2).
The reactions were carried out at reflux, stirring overnight un-
der inert atmosphere. The compounds were recrystallized by slow
diffusion of n-hexane in acetone or dichloromethane, affording
microcrystalline products with colours ranging from orange to
dark red. The compounds were fairly stable in air and moisture,
both in the solid state and in solution, and were obtained in fair
yields of 27–56%. The formulation of the new compounds is sup-
ported by analytical data, FT-IR and 1H, 31P NMR spectroscopic
data, with the exception of compounds L3 and 2Ru, for which
the elemental analysis always gave a significant error. Neverthe-
less, the elemental percentages for L3 containing complexes are
in good agreement with the proposed formulations. An effort is
currently being made at our laboratory to obtain suitable crystals
for X-ray diffraction studies of any of the compounds L3, 4Ru
and 4Fe, so that the structure of L3 can be unequivocally
determined.
The solid state FT-IR spectra (KBr pellets) of the complexes pre-
sented the characteristic bands of the cyclopentadienyl ligand
(ꢃ 3060 cmꢂ1), the PF6 anion (840 and 560 cmꢂ1) and the coordi-
ꢂ
nated nitrile (mCN from 2214 to 2191 cmꢂ1) in all of the studied
complexes. As observed for other ruthenium and iron related com-
pounds,14,15 negative shifts were found for
mCN, compared to the
corresponding values of the uncoordinated nitrile, in particular
for the iron compound containing the coordinated TH-[7] function-
alized with the strong acceptor NO2 group (L3), where a shift of
ꢂ24 cmꢂ1 was found. Negative shifts in the nitrile and acetylide
Scheme 1. Synthesis of ligands L1, L2 and L3. I –THF, ꢂ78 °C, (i) n-BuLi, (ii) DMF
[13a]; II – THF, ꢂ78 °C, (i) n-BuLi, (ii) DMF; III – (i) HONH2 ꢄ HCl, (ii) Ac2O; IV – (i)
HONH2 ꢄ HCl, (ii) Ac2O; V – HNO3/Ac2O.
Scheme 2. Reaction scheme for the synthesis and numbering of the tetrathia-[7]-
helicene Ru/Fe derived complexes.

M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
623
coordinated ligands have been attributed to
p
-back donation, due
phene ring adjacent to the nitrile group coordinated to the metal.
The upfield shift of this proton was more significant for compounds
with DPPE as a coligand, up to ꢂ1.57 ppm for compound 3Fe. The
unexpected higher shielding (ꢅ0.2 ppm) found for compounds 3Ru
and 3Fe (with L2) than for the analogues 4Ru and 4Fe with the bet-
ter acceptor L3 might be explained by the spatial effect of the
neighbouring uncoordinated C„N group of L2.
to bonding between the d orbitals of the metal and the
p
p
* orbital
of the nitrile group, leading to a decreased C„N bond order
[14,15]. The magnitude of the mCN negative shift has been clearly
related to the electron-donor capacity of the organometallic frag-
ment [MCp(P–P)]+ and the electron-attractor capacity of the sub-
stituent group on the chromophore.
1H NMR resonances of the cyclopentadienyl ring are in the char-
acteristic range of monocationic ruthenium(II) and iron(II) com-
plexes. The coordination of TH-[7] derived nitriles leads to a
shielding of most of the protons, especially the one in the thio-
31P NMR spectra of the complexes showed a resonance at
ꢂ144.1 ppm, characteristic of PF6ꢂ, and resonances of the coordi-
nated phosphines. For compounds 2Ru, 4Ru and 4Fe, the phos-
phine resonance signals are singlets, but for compounds 1Ru,
1Fe, 3Ru and 3Fe, the phosphine resonances appear as two dou-
blets with coupling constants ranging from 23.1 to 33.5 Hz, charac-
teristic of phosphorus–phosphorus coupling. This effect may be
due to interactions of the phosphines with the opposite end of
the helical ligand, causing the unequivalency of the phosphorus
atoms. For compounds 4Ru and 4Fe, the higher planarity predicted
for the helical ligand L3, due to the resonant structures originating
from the donor (organometallic moiety)-acceptor (NO2) conjuga-
tion, should minimize the referred interactions, while for com-
pound 2Ru, the less rigid PPh3 coligands, relatively to DPPE, must
allow the phosphorus equivalency.
Table 1
Optical spectral data for the complexes [M(
g
5-C5H5)(PP)(Ln)][PF6] and for the free
TH[7] derived ligands L1, L2 and L3, in CH2Cl2 and MeOH (ca. 10ꢂ4 M) solutions.
Compound
(L1)
kmax/nm (e )
, Mꢂ1 cmꢂ1
CH2Cl2
MeOH
243(27000)
255(sh)
289(16900)
324(13000)
365(sh)
384(13100)
400(13200)
[Ru(g
5-C5H5)(DPPE)(L1)][PF6] (1Ru)
287(sh)
285(sh)
327(13900)
360(sh)
327(22400)
379(sh)
382(sh)
398(13300)
418(sh)
394(21600)
420(sh)
[Fe(g
5-C5H5)(DPPE)(L1)][PF6] (1Fe)
286(sh)
287(sh)
330(15900)
400(13900)
415(13800)
465(sh)
329(20600)
395(17900)
407(17900)
457(sh)
[Ru(g
5-C5H5)(PPh3)2(L1)][PF6] (2Ru)
289(sh)
288(sh)
334(17400)
360(sh)
331(22300)
361(sh)
381(sh)
377(sh)
400(16600)
417(14800)
396(21400)
414(sh)
L2
260(27600)
291(20300)
326(16400)
339(24200)
391(19200)
411(20900)
Fig. 1. Electronic spectra of compounds 3Ru(– – –), 3Fe(—) and of the ligand L2(ꢄ ꢄ ꢄ)
in CH2Cl2.
[Ru(g
5-C5H5)(DPPE)(L2)][PF6] (3Ru)
290(26300)
325(sh)
288(23500)
301(sh)
339(25300)
391(sh)
409(22300)
323(sh)
338(22300)
390(sh)
407(20600)
[Fe(g
5-C5H5)(DPPE)(L2)][PF6] (3Fe)
291(24300)
341(19000)
398(sh)
414(17500)
471(sh)
326(sh)
340(24900)
396(22700)
412(22900)
460(sh)
L3
254(26000)
276(sh)
349(7000)
430(10700)
[Ru(g
5-C5H5)(DPPE)(L3)][PF6] (4Ru)
339(sh)
343(sh)
355(19300)
366(sh)
362(17100)
428(16700)
432(20000)
[Fe(g
5-C5H5)(DPPE)(L3)][PF6] (4Fe)
353(11900)
368(sh)
430(16700)
350(sh)
364(15300)
426(19800)
511(sh)
Fig. 2. Electronic spectra of compounds 3Fe(– – –) and 4Fe(—) in CH2Cl2.

624
M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
Fig. 3. ORTEP of the cation of compound 2ꢁRu.
2.3. Electronic spectra
imposed MLCT band, as was observed before for other compounds
with NO2 substituted nitrile ligands [15e].
Optical absorption spectra of all the complexes [M(
g
5-C5H5)
In the ruthenium compounds there is no evidence for any MLCT
band, although it might be superimposed with the lower energy li-
gand internal transitions bands. Fig. 1 shows the electronic spectra
of compounds [MCp(DPPE)(NC{TH-[7]}CN)][PF6], 3Ru and 3Fe, and
of the free ligand L2. The existence of a CT band at lower energy is
very obvious in compound 3Fe, in accordance with the spectro-
scopic data discussed above.
(P–P)(L)][PF6] were recorded in 10ꢂ4 M dichloromethane and
methanol solutions (Table 1), in order to identify the M ? L charge
transfer and
The electronic spectra of the helical ligands show several bands
in the UV region due to their extensive conjugated systems. As
p–p
* absorption bands expected for these complexes.
p
would be expected, the introduction of acceptor groups, either
CN or NO2, leads to a bathochromic shift of these bands.
Superimposition of the electronic spectra of compounds 3Fe
and 4Fe, pictured in Fig. 2, shows that the ꢅ500 nm CT band found
in 3Fe seems also occur in 4Fe, although in this case it might be ob-
scured by the enlargement of the band at a lower energy.
The electronic spectra of the organometallic compounds are
characterized by an intense absorption in the range 230–280 nm,
attributed to the organometallic fragments [MCp(P–P)]+, and in
the range 280–430 nm, attributed to internal transitions in the
TH-[7] derived ligands, by comparison to the free ligand spectra.
For compounds 1Fe and 3Fe, an additional band is found, as a
shoulder, in the ligands lower energy band, attributed to a metal
2.4. X-ray crystallographic studies
Crystals of compound [RuCp(PPH3)2(NC{TH-[7]})][PF6] ꢄ (CH3)2-
CO were obtained by slow diffusion of n-hexane in an acetone solu-
tion. The results obtained by X-ray diffraction studies showed that
the molecular structure of the cation does not correspond to the
one expected and confirmed by 1H NMR spectroscopy, due to isom-
erization of the coordinated ligand L1. As can be seen in Fig. 3, the
opposite ends of the helical structure of L1 are bonded (C(2)–C(3)
and C(4)–C(23)) in a four-member ring arrangement, with a change
in the hybridization of the carbon atoms involved in these bonds
from sp2 to sp3. A similar phenomenon was reported for another
TH[7] derivative [13a], although only one new bond is formed be-
tween the helix end rings in that case. Selected bond distances and
angles are presented in Table 2. For isomer distinction, the isomer
identified by X-ray diffraction is designated as 2ꢁRu.
to ligand charge transfer, d(MII) ? *(L). For compound 4Fe no
p
additional band was verified, although enlargement of the lower
energy band of the ligand can obscure the existence of any super-
Table 2
Selected bond distances and bond and torsion angles for compound 2ꢁRu.
Bond distances (Å)
Ru(1)–N(1)
2.030(5)
1.8595(6)
2.350(2)
2.353(2)
1.843(6)
1.831(7)
1.817(7)
1.846(7)
P(2)–C(221)
P(2)–C(231)
N(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
1.850(7)
1.829(6)
1.147(8)
1.433(9)
1.598(9)
1.535(9)
1.519(9)
1.565(9)
Ru(1)–Cpa
Ru(1)–P(1)
Ru(1)–P(2)
P(1)–C(111)
P(1)–C(121)
P(1)–C(131)
P(2)–C(211)
The referred isomerization was only detected when X-ray dif-
fraction studies were performed, since the initial 1H NMR spectrum
of compound 2Ru is consistent with the predicted structure, with
the chemical shifts of the thiophene rings protons and the coupling
constant J3–4 (5.6 Hz) in the normal ranges for these structures. The
isomerization must happen during the slow recrystallization and
the 2ꢁRu isomer crystallized preferentially to 2Ru. The 1H NMR
spectrum of 2ꢁRu, described in Section 4, is in accordance with
the structure determined by X-ray, showing a significant shielding
of protons H(4), H(23) and H(3), compared with 2Ru, due to the
hybridization change in the correspondent carbon atoms. More-
over, H(4) appears as a double doublet, due to coupling with
H(3) and H(23), with coupling constants of 6.3 and 8.6 Hz, respec-
tively. The chemical shifts of H(23) and H(3) were attributed based
on the coupling constant magnitudes, since J23–4 is expected to be
higher than J3–4 due to the larger H–C–C–H dihedral angle.
C(4)–C(23)
C(23)–C(2)
Bond angles (°)
N(1)–Ru(1)–Cpa
N(1)–Ru(1)–P(1)
N(1)–Ru(1)–P(2)
P(1)–Ru(1)–P(2)
P(1)–Ru(1)–Cpa
P(2)–Ru(1)–Cpa
Ru(1)–N(1)-C(1)
N(1)–C(1)–C(2)
C(1)–C(2)–C(23)
121.79(14)
87.93(14)
90.83(15)
99.84(6)
124.15(5)
122.98(5)
169.9(5)
175.7(7)
116.4(6)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(2)–C(23)–C(4)
C(3)–C(4)–C(23)
C(3)–C(2)–C(23)
C(5)–C(4)–C(23)
C(4)–C(23)–C(22)
C(2)–C(3)-S(1)
C(3)–C(2)–S(4)
115.3(6)
88.8(5)
90.6(5)
91.2(5)
87.2(5)
106.9(6)
107.7(5)
119.3(5)
116.5(5)
Torsion angles (°)
Ru(1)–N(1)–C(1)–C(2)
N(1)–C(1)–C(2)–C(3)
N(1)–C(1)–C(2)–S(4)
29(11)
N(1)-C(1)–C(2)–C(23)
C(2)–C(3)–C(4)–C(23)
62(9)
-11.0(5)
ꢂ38(9)
ꢂ175(9)
a
Cp ring centroid.

M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
625
Compound 2ꢁRu crystallizes in the centrosymmetric space
center is involved in the isomerization of the helical ligand. The
decoordination of a PPh3 ligand, followed by the sliding of the ni-
trile to side-on (four-electron-donor) coordination allow the inter-
action of the metal centre with H(3). Nucleophilic attack at C(3)
ꢀ
group P. The coordination geometry around the metal centre
shows the typical structure of cyclopentadienyl complexes in pseu-
do-octahedral three-legged piano stool geometry, on the assump-
tion that the cyclopentadienyl ring takes up three coordination
sites, with the two PPh3 phosphorus atoms and the nitrile nitrogen
atom occupying the other three sites. The angles P–Ru–P and
(C„)N–Ru–P, close to 90°, and the angles Cp(centroid)–Ru–
N(„C) and Cp(centroide)–Ru–P (121.79(14)–124.15(5)°), along
with the distances of the coordinated ligands to the metal centre,
Table 3
Interplanar angles, h, between adjacent rings and the terminal rings of the thiophene
rings (°) and the mean-square atom deviations,
compound 2ꢁRu.
D
, from ring least-squares planes, for
g
5-monocyclopentadienylrutheni-
h
D
are within the range found for
um nitrile derivatives with coordinated phosphines [14,15c,16].
The bond lengths of the cycle formed by atoms C(2), C(3), C(4)
and C(23) are in the range 1.519(9)–1.598(9) Å, with bond lengths
C(3)–C(4) (1.535(9) Å) and C(2)–C(23) (1.565(9) Å) clearly superior
to the analogous ones verified in compounds [MCp(L-
L)(NC{BDT}Z)][Y] [14], showing loss of the double bond character.
Angles C(5)–C(4)–C(23) (106.9(6) Å) and C(4)–C(23)–C(22)
(107.7(5) Å) show the almost superposition of the helix ends.
A possible mechanism to explain the isomerization of 2Ru to
2ꢁRu is depicted in Scheme 3. It can be assumed that the metal
Ring(1)–ring(2)
Ring(2)–ring(3)
Ring(3)–ring(4)
Ring(4)–ring(5)
Ring(5)–ring(6)
Ring(6)–ring(7)
Ring(1)–ring(7)
6.33
6.64
4.57
4.86
8.01
6.80
4.43
ring(1)
ring(2)
ring(3)
ring(4)
ring(5)
ring(6)
ring(7)
0.0063
0.0590
0.0250
0.0279
0.0295
0.0489
0.0235
Ring(1): (C(3), C(4), C(5), C(6), S(1)); ring(2): (C(5), C(6), C(7), C(8), C(9), C(10));
ring(3): (C(9), C(10), C(11), S(2)); ring(4): (C(11), C(12), C(13), C(14), C(15), C(16));
ring(5): (C(15), C(16), C(17), C(18), S(3)); ring(6): (C(17), C(18), C(19), C(20), C(21),
C(22)); ring(7): (C(21), C(22), C(23), C(24), S(4)).
Scheme 3. Proposed mechanism for formation of 2ꢁRu.

626
M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
Table 4
Intermolecular contacts for compound 2ꢁRu.
2ꢁRu
C(35)–H(35)ꢄ ꢄ ꢄF(1)
C(124)–H(124)ꢄ ꢄ ꢄF(4)
C(3)–H(3)ꢄ ꢄ ꢄF(4)
2.568
2.647
2.619
2.491
C(13)–H(13)ꢄ ꢄ ꢄO(100)
and subsequent rearrangement results in bonding between the he-
lix ends. Return of the nitrile to end-on coordination and coordina-
tion of PPh3 affords 2ꢁRu. This mechanism is supported by the fact
that isomerization was only detected in one complex, and not in
the free ligand, and this being the complex with coordinated
monodentate phosphines.
The dihedral angles between adjacent rings and terminal rings
of the helicene ligand of compound 2ꢁRu, as well as the mean-
square atom deviations
D from the ring least-squares planes, are
listed in Table 3. The planarity of the aromatic rings in the mole-
cules is apparent from the mean deviations of the atoms from
the least-squares plane of the ring (Table 3). The terminal rings 1
and 7 have an almost flat configuration, but the planarity is poorer
towards the middle thiophene rings and is poorest in the benzene
rings. Also, a decrease of the dihedral angle towards the middle
part of the helix can be noticed, although the deformation of the
rings from planarity does not follow the same correlation. This
behavior is opposite to that observed in free TH[7] [17]. A possible
explanation for this different structural arrangement is the isomer-
ization of the helical ligand that forces a smaller aperture in the
two terminal thiophene rings, as reflected by the dihedral angle be-
tween the rings 1 and 7 (4.43° vs. 45.9° for free TH[7]).
The pitch of the helix in 2ꢁRu compound is very similar to the
free TH[7] ligand (14.30 Å vs. 14.03 Å), since the two thiahelicenes
have the same number of rings [18].
The evaluation of the crystal packing of 2ꢁRu revealed the for-
mation of pseudo dimeric units, with parallel alignment of the
chromophores of distinct cations, via intermolecular interactions
with the PF6 anion and one acetone molecule (Table 4), reinforced
Fig. 5. Supramolecular arrangement of the compound 2ꢁRu, displaying holes along
direction c.
ified for the compound [RuCp(PPh3)2(NC(BDT))][PF6] (BDT =
benzo[1,2-b;4,3-b0]dithiophene) [14], although the holes aperture
is larger in this compound.
3. Conclusion
We have described here the synthesis of the new organometal-
lic complexes [MCp(P–P)(NC{TH-[7]-Y}Z)][PF6] (M = Ru, Fe, P–P =
DPPE, Y = H, NO2, Z = H, C„N; M = Ru, L-L = 2PPh3, Y = H, Z = H),
with coordinated helical ligands derived from tetrathia-[7]-
helicene. Spectroscopic data revealed the electron-donor effect of
the organometallic fragments [MCp(P–P)]+. The magnitude of this
effect depends on the metal and on the presence and/or position
of the acceptor group (C„N, NO2), showing a good conjugation
between the metal centre and the acceptor group on the
chromophore.
The three new substituted tetrathia-[7]-helicene derivatives,
synthesized in this work and used as ligands in the presented orga-
nometallic complexes, may also be interesting as organic materials
for NLO purposes, according to the significant b values predicted by
theoretical studies for other tetrathia-[7]-helicene derivatives [20].
The major interest in this family of compounds is the intrinsic
chirality for NLO purposes, which has not yet been exploited, since
the TH[7] ligands were prepared as a racemic mixture. Neverthe-
less, the synthetic procedure developed in the present work is valid
for the synthesis of the pure isomers complexes, starting from the
respective isomerically pure TH[7], bearing in mind that isomer in-
ter-conversion is not possible.
through p–p interactions of the helical ligand.
This pseudo dimeric arrangement is illustrated in Fig. 4, where
it can also be seen that these units are constituted by two enanti-
omers, (+) and (ꢂ), relative to the helix orientation.
Analysis of the crystal packing disclosed an interesting supra-
molecular array for this compound, which displays holes in the c
direction, as illustrated in Fig. 5, suggesting that this network can
possibly be used for selectively storing guests material [19]. These
holes, that have an almost square shape, have the smaller dimen-
sions between two fluorine atoms of the counter-ion PF6,
4.851 Å, and two of the carbons of the acetone solvent molecule,
4.750 Å. The holes are created by two of the parallel lines produced
via intermolecular interactions involving the helicene–PF6–ace-
tone. A similar array with a significant degree of porosity was ver-
Fig. 4. Dimeric units of compound 2ꢁRu, showing intermolecular
p–p interactions of the helical ligand.

M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
627
a yellow solid. Anal. (%) Calc. for C24H8N2S4: C, 63.69; H, 1.78; N,
4. Experimental
6.19. Found: C, 64.02; H, 1.86; N, 5.99. IR (KBr, cmꢂ1):
m(CN)
2209. 1H NMR (DMSO-d6): 7.21 (s, 2H), 8.46 (s, 2H), 8.49 (d, 2H,
JHH = 8.8), 8.58 (d, 2H, JHH = 8.8).
4.1. General procedures
All experiments were carried out under nitrogen atmosphere
using standard Schlenk techniques. All of the solvents used were
dried using standard methods [21]. The starting materials were
prepared following the methods described in the literature:
4.2.3. Tetrathia-[7]-helicene-?-nitro-2-carbonitrile (L3)
Nitric acid (0.02 mL, 0.35 mmol) was added to a stirred solution
of tetrathia-[7]-helicene-2-carbonitrile (0.11 g, 0.26 mmol) in ace-
tic anhydride (8 mL), cooled to 5 °C. After 1 h, the solution was
poured onto ice and the resultant precipitate was dissolved in
CH2Cl2 (15 mL). The solution was washed with a saturated solution
of NaHCO3, water, dried over MgSO4 and the solvent removed un-
der reduced pressure. The product was purified by flash column
chromatography (eluent:n-hexane/CH2Cl2 2:3), affording 0.05 g
(41%) of tetrathia-[7]-helicene-?-nitro-2-carbonitrile as an orange
[Ru(g g -
5-C5H5)(DPPE)Cl] and [Ru( 5-C5H5)(PPh3)2Cl] [22]; [Fe(g5
C5H5)(DPPE)I] [15c]; tetrathia-[7]-helicene and tetrathia-[7]-heli-
cene-2-carbox aldehyde [16a]. FT-IR spectra were recorded in a
Mattson Satelite FTIR spectrophotometer with KBr; only significant
bands are quoted in the text. 1H and 31P NMR spectra were re-
corded on a Brüker Avance 400 spectrometer at the probe temper-
ature. 1H (DMSO-d6) chemical shifts are reported in parts per
million (ppm) downfield from internal Me4Si and coupling con-
stants are reported in Hertz; 31P (DMSO-d6) NMR spectra are re-
ported in ppm downfield from the external standard, 85% H3PO4.
Elemental analyses were obtained at the Laboratório de Análises,
Instituto Superior Técnico, using a Fisons Instruments EA1108 sys-
tem. Data acquisition, integration and handling were performed
using a PC with software package EAGER-200 (Carlo Erba Instru-
ments). Electronic spectra were recorded at room temperature on
a Jasco V-560 spectrometer in the range 200–900 nm.
solid. IR (KBr, cmꢂ1):
m(CN) 2215, m(NO2) 1507 and 1316, d (NO2)
731. 1H NMR (DMSO-d6): 6.65 (d, 1H, JHH = 5.6), 7.29 (s, 1H), 7.57
(d, 1H, JHH = 5.6), 8.45 (d, 1H, JHH = 8.8), 8.54 (m, 3H), 9.60 (s, 1H).
4.3. Synthesis of the complexes [M(
Y}Z)][PF6]
g
5-C5H5)(P–P)(NC{TH-[7]-
Complexes of the general formula [M(
[7]-Y}Z)][PF6] were prepared by halide abstraction from the parent
neutral complexes [M(
5-C5H5)(LL)X] (1 mmol) with TlPF6
g
5-C5H5)(P–P)(NC{TH-
g
(1 mmol) in dichloromethane, in the presence of a slight excess
(1.1 mmol) of the ligands tetrathia-[7]-helicene-2-carbonitrile
(L1), tetrathia-[7]-helicene-2,20-carbonitrile (L2) or tetrathia-[7]-
helicene-?-nitro-2-carbonitrile (L3), at reflux for 48 h under an in-
ert atmosphere. After cooling to room temperature, filtering and
removing the solvent, the complexes were washed with n-hexane
(3 ꢆ 15 mL) and recrystallized from dichloromethane/n-hexane or
acetone/n-hexane, giving crystalline products.
4.2. Synthesis of the organic ligands
4.2.1. Tetrathia-[7]-helicene-2-carbonitrile (L1)
An H2NOH ꢄ HCl (0.14 g, 2 mmol) solution in pyridine (4 mL)
was added to a stirred solution of tetrathia-[7]-helicene-2-carbal-
dehyde (0.22 g, 0.5 mmol) in pyridine (5 mL), cooled to 5 °C. After
stirring for 1 h at 5 °C, acetic anhydride (1 mL) was added, and
the mixture was refluxed for 2 h. After cooling, the mixture was
poured onto ice and the resulting precipitate was dissolved in
CH2Cl2 (30 mL), washed with a saturated solution of NaHCO3 and
water, and dried over MgSO4. The solvent was removed under re-
duced pressure and the product was purified by flash column chro-
matography (eluent:n-hexane/CH2Cl2 1:1), affording 0.10 g (47%)
of pure tetrathia-[7]-helicene-2-carbonitrile as a yellow solid.
Anal. (%) Calc. for C23H9NS4 ꢄ 0.1C6H14: C, 64.99; H, 2.40; N, 3.21.
4.3.1. [Ru(g
5-C5H5)(DPPE)(NC{TH-[7]})][PF6] (1Ru)
Orange. Yield: 48%. Recrystallized from dichloromethane/n-
hexane. Anal. (%) Calc. for: C54H38NS4P3F6Ru: C, 57.04; H, 3.37; N,
1.23. Found: C, 57.30; H, 3.56; N, 1.60. IR (KBr, cmꢂ1): (C–H, g5
m -
C5H5) 3051,
(m, 4H, –CH2–), 4.91 (s, 5H,
m
(CN) 2210,
m
(PF6ꢂ) 839; 1H NMR ((CD3)2SO): 2.65
5-C5H5), 6.06 (s, 1H), 6.22 (d, 1H,
g
Found: C, 65.72; H, 2.13; N, 3.23. IR (KBr, cmꢂ1): (CN) 2206. 1H
m
JHH = 5.6), 7.04 (d, 1H, JHH = 5.6), 7.24 (m, 4H, C6H5, DPPE), 7.36
(m, 6H, C6H5, DPPE), 7.51 (m, 6H, C6H5, DPPE), 7.78 (m, 4H, C6H5,
DPPE), 8.28 (d, 1H, JHH = 8.8), 8.41 (d, 1H, JHH = 8.4), 8.47 (m, 4H).
NMR (DMSO-d6): 6.49 (d, 1H, JHH = 5.6), 7.23 (s, 1H) 7.40 (d, 1H,
JHH = 5.6), 8.28 (d, 1H, JHH = 8.4), 8.29 (m, 4H), 8.51 (d, 1H, 8.8).
31P NMR ((CD3)2SO): ꢂ144.1 (qt, PF6 JPF = 711.0), 78.3 (d, DPPE,
ꢂ
,
JPP = 24.5), 79.5 (d, DPPE, JPP = 24.5).
4.2.2. Tetrathia-[7]-helicene-2,13-dicarbonitrile (L2)
A n-BuLi 2.0 M pentane solution (0.55 mL, 1.1 mmol) was added
dropwise to a stirred solution of tetrathia-[7]-helicene (0.20 g,
0.5 mmol) in dry THF (20 mL) at ꢂ78 °C. The solution was stirred
for 40 min at ꢂ78 °C, and the resulting dark orange solution was
treated with dry DMF (0.11 mL, 1.2 mmol). After 4 h at ꢂ78 °C,
the solution was warmed up to room temperature and quenched
with a saturated aqueous solution of NH4Cl (5 mL). The THF was
removed under reduced pressure, and the crude was taken up with
CH2Cl2 (20 mL) and washed with water and a saturated solution of
NH4Cl. The solvent was removed under reduced pressure, affording
4.3.2. [Fe(g
5-C5H5)(DPPE)(NC{TH-[7]})][PF6] (1Fe)
Red. Yield: 46%. Recrystallized from dichloromethane/n-hexane.
Anal. (%) Calc. for C54H38NS4P3F6Fe ꢄ 0.3CH2Cl2: C, 58.54; H, 3.48; N,
1.25. Found: C, 58.29; H, 3.78; N, 1.26. IR (KBr, cmꢂ1): (C–H, g5
m -
C5H5) 3052,
m
(CN) 2193,
m
(PF6ꢂ) 840. 1H NMR ((CD3)2SO): 2.46
5-C5H5), 5.97
(m, 2H, –CH2–), 2.62 (m, 2H, –CH2–), 4.53 (s, 5H,
g
(s, 1H), 6.22 (d, 1H, JHH = 5.6), 7.04 (d, 1H, JHH = 5.6), 7.06 (m, 4H,
C6H5, DPPE), 7.56 (m, 12H, C6H5, DPPE), 7.81 (m, 4H, C6H5, DPPE),
8.25 (d, 1H, JHH = 8.8), 8.43 (m, 5H). 31P NMR ((CD3)2SO): ꢂ144.1
(qt, PF6ꢂ, JPF = 710.2), 96.4 (d, DPPE, JPP = 33.2), 96.8 (d, DPPE,
JPP = 33.5).
tetrathia-[7]-heliceno-2,20-dicarbaldehyde, as
a dark oil, used
without further purification. The product was dissolved in pyri-
dine, cooled to 5 °C, and an H2NOH ꢄ HCl (0.14 g, 2.0 mmol) solu-
tion in pyridine (4 mL) was added. After stirring for 5 min at 5 °C,
acetic anhydride (1 mL) was added and the mixture was refluxed
for 2 h. After cooling, the mixture was poured onto ice and the
resulting precipitate was dissolved in CH2Cl2 (20 mL), washed with
water and dried over MgSO4. The product was purified by flash col-
umn chromatography (eluent:n-hexane/CH2Cl2 2:3), affording
0.05 g (25%) of pure tetrathia-[7]-helicene-2,13-dicarbonitrile as
4.3.3. [Ru(
Orange. Yield: 27%. Recrystallized from acetone/n-hexane. IR
(KBr, cmꢂ1): 5-C5H5) 3053, (PF6ꢂ) 840. 1H
(C–H, (CN) 2210,
NMR ((CD3)2SO): 4.61 (s, 5H,
5-C5H5), 6.45 (d, 1H, JHH = 5.6),
g
5-C5H5)(PPh3)2(NC{TH-[7]})][PF6] (2Ru)
m
g
m
m
g
6.90 (s, 1H), 7.21 (m, 12H, C6H5, PPh3), 7.31 (m, 12H, C6H5, PPh3),
7.40 (d, 1H, JHH = 5.6), 7.44 (m, 6H, C6H5, PPh3), 8.14 (d, 1H,
JHH = 8.8), 8.24 (d, 1H, JHH = 8.8), 8.40 (d, 1H, JHH = 8.8), 8.43 (d,

628
M.H. Garcia et al. / Polyhedron 28 (2009) 621–629
1H, JHH = 8.4), 8.47 (d, 1H, JHH = 8.4), 8.55 (d, 1H, JHH = 8.8). 31P NMR
((CD3)2SO): ꢂ144.1 (qt, PF6ꢂ, JPF = 711.1), 41.31 (s, PPh3).
by full-matrix least-squares on F2 with SHELXL97 [26], both included
in the package of programs WINGX-Version 1.70.01 [27].7 Non-
hydrogen atoms were refined with anisotropic thermal parame-
ters, whereas H-atoms were placed in idealised positions and
(2ꢁRu): Orange. 1H NMR ((CD3)2SO): 3.29 (d, 1H, H(3), J3–4
=
6.4), 4.41 (dd, 1H, H(4), J4–3 = 6.3, J4–23 = 8.6), 5.27 (d, 1H, H(23),
J23–4 = 8.8), 6.76 (m, 5H, C6H5, PPh3), 6.96 (m, 6H, C6H5, PPh3),
7.07 (m, 6H, C6H5, PPh3), 7.17 (m, 5H, C6H5, PPh3), 7.26 (m, 3H,
C6H5, PPh3), 7.35 (m, 3H, C6H5, PPh3), 7.48 (m, 2H, C6H5, PPh3),
7.62 (d, 1H, JHH = 8.3), 7.69 (d, 1H, JHH = 8.4), 7.99 (d, 1H,
JHH = 8.3), 8.07 (d, 1H, JHH = 8.3), 8.39 (d, 1H, JHH = 8.6), 8.42 (d,
1H, JHH = 8.4).
allowed to refine riding on the parent
C atom. Graphical
representations were prepared using ORTEP [28] and Mercury
1.1.2 [29]. Crystallographic data: crystal colour orange, crystal
habit plate, crystal dimensions 0.10 ꢆ 0.09 ꢆ 0.02 mm, empirical
formula C67H44F6NO3P3S3Ru, M = 1315.25, crystal system triclinic,
ꢀ
space group P, a = 14.038(3), b = 14.992(3), c = 16.937(3) Å,
a
= 110.185(7), b = 103.719(6),
c
= 102.132(6)°, V = 3079.0(10) Å3,
4.3.4. [Ru(
g
5-C5H5)(DPPE)(NC{TH-[7]}CN)][PF6] (3Ru)
Z = 2, Dcalc. = 1.765 g cmꢂ3
,
T = 150(2) K, reflections collected/
Dark red. Yield: 56%. Recrystallized from dichloromethane/n-
unique 23172/13568, parameters 748, final R indices [I > 2
r(I)]:
hexane. Anal. (%) Calc. for C55H37N2S4P3F6Ru: C, 56.84; H, 3.21; N,
R1 = 0.0687; wR2 = 0.1422.
2.41. Found: C, 56.40; H, 3.38; N, 2.41. IR (KBr, cmꢂ1): (C–H, g5
m -
C5H5) 3051,
m(CN) 2212,
m
(PF6ꢂ) 839. 1H NMR ((CD3)2SO): 2.20
5-C5H5), 5.70
Supplementary data
(m, 2H, –CH2–), 2.79 (m, 2H, –CH2–), 4.89 (s, 5H,
g
(s, 1H), 6.88 (s, 1H), 7.22 (m, 4H, C6H5, DPPE), 7.50 (m, 12H, C6H5,
DPPE), 7.91 (m, 4H, C6H5, DPPE), 8.37 (d, 2H, JHH = 8.8), 8.51 (m,
4H), 8.71 (d, 2H, H6, JHH = 8.8). 31P NMR ((CD3)2SO): ꢂ144.1 (qt,
PF6ꢂ, JPF = 711.4), 78.3 (d, DPPE, JPP = 25.1), 79.7 (d, DPPE, JPP = 23.1).
CCDC 688367 contains the supplementary crystallographic data
for 2ꢁRu. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
4.3.5. [Fe(g
5-C5H5)(DPPE)(NC{TH-[7]}CN)][PF6] (3Fe)
Dark red. Yield: 59%. Recrystallized from dichloromethane/n-
Acknowledgements
hexane. Anal. (%) Calc. for C55H37N2S4P3F6Fe: C, 59.15; H, 3.34; N,
2.51. Found: C, 59.01; H, 3.70; N, 2.14. IR (KBr, cmꢂ1): (C–H, g5
m -
The authors thank Fundação Para a Ciência e Tecnologia (FCT)
for financial support of Project POCTI/QUI/48443/2002. Pedro
Florindo thanks FCT for his Ph.D. Grant (SFRH/BD/12432/2003).
E. Licandro and S. Maiorana acknowledge joint financial support
from the Ministero dell’Istruzione, dell’Università e della Ricerca
Scientifica (MUR), Rome, and the University of Milan (FIRB project,
Bando 2003, Progetto RBNE033KMA, title of the project: ‘‘Molecu-
lar compounds and hybrid nanostructured materials with resonant
and non resonant optical properties for photonic devices” and the
University of Milan for the PRIN 2005 project: ‘‘Nuovi sistemi cata-
litici stereoselettivi e sintesi stereoselettiva di molecole funzionali”.
Centro di Eccellenza CIMAINA and the C.N.R. of Rome is also
acknowledged.
C5H5) 3052,
m
(CN) 2191,
m
(PF6ꢂ) 839. 1H NMR ((CD3)2SO): 2.37
5-C5H5), 5.64
(m, 2H, –CH2–), 2.70 (m, 2H, –CH2–), 4.52 (s, 5H,
g
(s, 1H), 6.85 (s, 1H), 7.26 (m, 4H, C6H5, DPPE), 7.56 (m, 12H,
C6H5, DPPE), 7.93 (m, 4H, C6H5, DPPE), 8.35 (d, 2H, JHH = 8.8), 8.67
(d, 2H, H6, JHH = 8.8). 31P NMR ((CD3)2SO): ꢂ144.1 (qt, PF6
JPF = 710.6), 96.8 (d, DPPE, JPP = 33.4), 97.4 (d, DPPE, JPP = 32.7).
,
ꢂ
4.3.6. [Ru(g
5-C5H5)(DPPE)(NC{TH-[7]-NO2})][PF6] (4Ru)
Dark red. Yield 40%. Recrystallized from dichloromethane/n-
hexane. Anal. (%) Calc. for C54H37N2O2S4P3F6Ru: C, 54.87; H, 3.15;
N, 2.37. Found: C, 54.52; H, 3.56; N, 2.63. IR (KBr, cmꢂ1):
m
(C–H,
(NO2) 1507 and 1314, d(NO2) 730,
(PF6ꢂ) 839. 1H NMR ((CD3)2SO): 2.34 (m, 2H, –CH2–), 2.67 (m,
2H, –CH2–), 4.84 (s, 5H,
5-C5H5), 5.98 (s, 1H), 6.40 (d, 1H,
g
m
5-C5H5) 3054,
m(CN) 2214, m
g
References
JHH = 5.6), 7.29 (d, 1H, JHH = 5.6), 7.35 (m, 6H, C6H5, DPPE), 7.51
(m, 6H, C6H5, DPPE), 7.72 (m, 8H, C6H5, DPPE), 8.33 (d, 1H,
JHH = 8.8), 8.52 (d, 1H, JHH = 8.8), 8.59 (s, 2H), 9.75 (s, 1H). 31P
NMR ((CD3)2SO): ꢂ144.1 (qt, PF6ꢂ, JPF = 712.7), 78.7 (s, DPPE).
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[2] N.J. Long, Angew. Chem., Int. Ed. Engl. 34 (1995) 21.
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5-C5H5)(DPPE)(NC{TH-[7]-NO2})][PF6] (4Fe)
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Dark red. Yield 44%. Recrystallized from dichloromethane/n-
hexane. Anal. (%) Calc. for C54H37N2O2S4P3F6Fe ꢄ 0.3CH2Cl2: C,
56.11; H, 3.26; N, 2.41. Found: C, 55.91; H, 3.39; N, 3.08. IR (KBr,
cmꢂ1):
1313,
(m, 2H, –CH2–), 4.57 (s, 5H,
m
(C–H,
(PF6ꢂ) 839. 1H NMR ((CD3)2SO): 2.12 (m, 2H, –CH2–), 2.35
5-C5H5), 5.90 (s, 1H), 6.39 (d, 1H,
g
5-C5H5) 3052,
m
(CN) 2191,
m
(NO2) 1507 and
m
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g
JHH = 5.6), 7.32 (d, 1H, JHH = 5.6), 7.35 (m, 8H, C6H5, DPPE), 7.55
(m, 8H, C6H5, DPPE), 7.74 (m, 4H, C6H5, DPPE), 8.30 (d, 1H,
JHH = 8.8), 8.49 (d, 1H, JHH = 8.8), 8.57 (s, 2H), 9.72 (s, 1H). 31P
NMR ((CD3)2SO): ꢂ144.1 (qt, PF6ꢂ, JPF = 710.2), 97.0 (s, DPPE).
[9] M. Kauranen, T. Verbiest, E.W. Meijer, E.E. Havinga, M.N. Teerenstra, A.J.
Schouten, R.J.M. Nolte, A. Persoons, Adv. Mater. (Weinheim, Germany) 7 (1995)
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(b) S. Sioncke, S. Van Elshocht, T. Verbiest, A. Persoons, M. Kauranen, K.E.S.
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