Synthesis of multifunctional aza-substituted ruthenocene derivatives displaying charge-transfer transitions And selective Zn(II) ions sensing properties

Article abstract of DOI:10.1021/om700757m

The synthesis, electrochemical, electronic, and cation sensing properties of multinuclear nitrogen-rich [2.2]- and [3.3]-mixed ferrocene and ruthenocene metallocenophanes are presented. Structural features of these new structural motifs are that the two redox organometallics fragments are linked by unsaturated nitrogen functionalities, for example, carbodiimide or aldimine, as well as the nitrogen atom is directly attached to the ruthenocene unit. The key bis(iminophosphorane) 3 is readily prepared by the Staudinger reaction between triphenylphosphine and 1,1′-diazidoruthenocene 2, which has been prepared from 1,1′- dilithioruthenocene and 2,4,6-trisopropylbenzenesulfonyl azide (trisyl azide). Subsequent aza-Wittig reactions of 3 with the appropriate carbonyl or thiocarbonyl compounds provided the opened ruthenocenebased isothiocyanate 4, and the closed carbodiimide 5 and aldimines 6 and 7. Spectroelectrochemical studies of carbodiimide 5 and aldimine 7 revealed the presence of low-energy bands in the near-IR region in the partially oxidized forms, at 1029 and 1481 nm, respectively, which indicate the existence of intramolecular charge transfer between the iron and the ruthenium centers. The experimental data and conclusions are supported by DFT computations. Moreover, the aldimine 7 behaves as a selective colorimetric chemosensor molecules for Zn²⁺ ions. The low-energy (LE) band of the absorption spectrum of this compound is red-shifted by 99 nm, only in the presence of Zn²⁺ ions. This change in the absorption spectrum is accompanied by a dramatic color change, which allows the potential for naked eye detection.

Full text of DOI:10.1021/om700757m

6234
Organometallics 2007, 26, 6234-6242
Synthesis of Multifunctional Aza-Substituted Ruthenocene
Derivatives Displaying Charge-Transfer Transitions and Selective
Zn(II) Ions Sensing Properties
Francisco Oto´n, Arturo Espinosa, Alberto Ta´rraga,* and Pedro Molina*
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad de Murcia,
Campus de Espinardo, E-30100 Murcia, Spain
ReceiVed July 26, 2007
The synthesis, electrochemical, electronic, and cation sensing properties of multinuclear nitrogen-rich
[2.2]- and [3.3]-mixed ferrocene and ruthenocene metallocenophanes are presented. Structural features
of these new structural motifs are that the two redox organometallics fragments are linked by unsaturated
nitrogen functionalities, for example, carbodiimide or aldimine, as well as the nitrogen atom is directly
attached to the ruthenocene unit. The key bis(iminophosphorane) 3 is readily prepared by the Staudinger
reaction between triphenylphosphine and 1,1′-diazidoruthenocene 2, which has been prepared from 1,1′-
dilithioruthenocene and 2,4,6-trisopropylbenzenesulfonyl azide (trisyl azide). Subsequent aza-Wittig
reactions of 3 with the appropriate carbonyl or thiocarbonyl compounds provided the opened ruthenocene-
based isothiocyanate 4, and the closed carbodiimide 5 and aldimines 6 and 7. Spectroelectrochemical
studies of carbodiimide 5 and aldimine 7 revealed the presence of low-energy bands in the near-IR region
in the partially oxidized forms, at 1029 and 1481 nm, respectively, which indicate the existence of
intramolecular charge transfer between the iron and the ruthenium centers. The experimental data and
conclusions are supported by DFT computations. Moreover, the aldimine 7 behaves as a selective
colorimetric chemosensor molecules for Zn2+ ions. The low-energy (LE) band of the absorption spectrum
of this compound is red-shifted by 99 nm, only in the presence of Zn2+ ions. This change in the absorption
spectrum is accompanied by a dramatic color change, which allows the potential for “naked eye” detection.
(trisyl azide).6 Diastereoselective ortho-metalation involving
chiral ferrocenyl derivatives and trapping of the lithiated species
with N,O-bis(trimethylsilyl)hydroxylamine7 or tosyl azide8
represents an efficient route to aminoferrocene derivatives with
planar chirality.
As compared to ferrocene ligands, the ruthenocene ligands
have received much less attention. Although some C-substituted9
and heteroatom-substituted ruthenocene derivatives10 have been
reported, the chemistry of nitrogen-substituted ruthenocenes
remains unexplored.
It is known that the distances between the two cyclopenta-
dienyl rings in ferrocene and ruthenocene are 3.32 and 3.68 Å,
respectively.11 The longer distance by about 10% in ruthenocene
than their ferrocene analogues would be expected to present
different complexation behavior with transition metals. To date,
enhanced responsiveness of a ferrocene reporter group to tran-
sition-metal ions has been achieved only by means of a family
of ligands derived from amino-substituted ferrocenes, in which
at least one nitrogen donor atom of a chelating ligand is directly
linked to the cyclopentadienyl ring of the ferrocene unit.12
Introduction
Metallocenes that have a nitrogen atom directly bonded to
the five-membered ring are not very common because substitu-
tion reactions at the cyclopentadieneyl ring are not straightfor-
ward, neither with nitrogen-based nucleophiles nor with an
electrophilic source of nitrogen, which are strong oxidants or
relatively unstable compounds. Another drawback is the fact
that aminocyclopentadienes of the type C5H5NR2 are thermally
and oxidatively rather sensitive compounds,1 and, consequently,
very few functionalized aminoferrocenes have been reported.
The most general method involves the reaction of metalated
ferrocenes with a nitrogen-based electrophile.2 In this context,
o-benzylhydroxylamine has been used with lithioferrocene to
yield aminoferrocene, albeit in low yield (13%).3 A recent
optimized procedure, which allows the isolation of aminofer-
rocene in 50% yield, involves the use of R-azidostyrene as a
source of nitrogen.4 The synthesis of 1,1′-diazidoferrocene has
been reported by reaction of 1,1′-dilithioferrocene with either
1,1,2,2-tetrabromoethane and subsequent halide displacement
using NaN3/CuCl5 or 2,4,6-triisopropylbenzenesulfonyl azide
(5) Shafir, A.; Power, M. P.; Whitener, G. D.; Arnold, J. Organometallics
2000, 19, 3978-3982.
* To whom correspondence should be addressed. E-mail: pmolina@
um.es (P.M.); atarraga@um.es (A.T.).
(1) Togni, A.; Hayashi, T. Ferrocenes: Homogeneous Catalysis, Organic
Synthesis, Material Science; VCH: Weinheim, 1995.
(2) For a review on electrophilic amination of carbanions, see: Erdick,
E.; Ay, M. Chem. ReV. 1989, 89, 1949.
(3) (a) Knox, R. G.; Pauson, P. L.; Willison, D.; Solcaniova, E.; Toma,
S. Organometallics 1990, 9, 301. (b) El-Shihi, T.; Siglmu¨ler, F.; Herrman,
R.; Calvaho, M. F. N. N.; Pombeiro, A. J. L. J. Organomet. Chem. 1987,
355, 239.
(6) (a) Ta´rraga, A.; Oto´n, F.; Espinosa, A.; Velasco, M. D.; Molina, P.;
Evans, D. J. Chem. Commun. 2004, 458. (b) Oto´n, F.; Espinosa, A.; Ta´rraga,
A.; Ramirez de Arellano, C.; Molina, P. Chem.sEur. J. 2007, 13, 5742.
(7) (a) Casarini, A.; Dembrach, P.; Lazzari, D.; Marini, E.; Reginato,
G.; Ricci, A.; Seconi, G. J. Org. Chem. 1993, 58, 5620. (b) Riant, O.;
Samuel, O.; Flessner, T.; Taudien, S.; Kagan, M. B. J. Org. Chem. 1997,
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Soc., Chem. Commun. 2001, 2026. (b) Priego, J.; Manchen˜o, O. G.; Cabrera,
S.; Carretero, J. C. J. Org. Chem. 2002, 67, 1346.
(4) van Leusen, D.; Hessen, B. Organometallics 2001, 20, 224.
10.1021/om700757m CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/25/2007

Synthesis of Aza-Substituted Ruthenocene DeriVatiVes
Organometallics, Vol. 26, No. 25, 2007 6235
In a continuation of our ongoing studies involving the
synthesis and structural characterization of new families of
nitrogen-rich metallocenophanes with interesting structural,
electrochemical, and optical properties,6,13 we have now focused
our attention on developing convenient synthetic entries to a
new type of 1,1′-diazasubstituted ruthenocenes, diaza[2,2]ruteno-
cenophanes, and an unprecedented tetraaza[3.3]ferrocenorutheno-
cenophane in which the two metallocene units are directly
attached by carbodiimide functions.
Results and Discussion
Figure 1. Calculated (B3LYP/6-31G*/Lanl2DZ-ecp) structure for
the most stable conformer 5C1.
Synthesis. The general strategy used for the synthesis of both
[2,2] and [3,3] metallocenophane derivatives is based on the
aza-Wittig reaction of 1,1′-bis(N-triphenylphosphoranylidene-
amino)ruthenocene derivative with the appropriate carbonyl
component.
triphenylphosphine, which allows the one-flask conversion of
ruthenocene 1 into the bis(iminophosphorane) 3 in an overall
yield of 74%.14
Interestingly, the aza-Wittig reaction of 3 with 1,1′-diisocy-
anatoferrocene15 resulted in the final bis(carbodiimide) 5 (72%),
with an unprecedented tetraza[3,3](1,1′)ferrocenoruthenocenophane
structure. On the other hand, the reaction between the bis-
(iminophosphorane) 3 and carbon disulfide gave the 1,1′-
diisothiocyanatoruthenocene 4 in 41% yield.16 We have found
two conformational minima for this compound 5 by means of
DFT-based theoretical calculations: the absolute minimum in
the Gibbs free energy surface has no symmetry (5C1) (Figure
1) and can be converted into a 1.40 kcal/mol less stable C2-
symmetric conformer (5C2) (see the Supporting Information).
In the most stable conformer 5C1, the intermetallic Ru‚‚‚Fe
distance is 6.525 Å with both metallocenes twisted by 56.0°
from each other (angle between axes connecting Cp ring
centroids in each metallocene), whereas in the 5C1 isomer both
metallocenes are almost orthogonal (twisting angle 78.5°)
bringing closer both metal centers (6.395 Å). Frequency analysis
of 5C1 reveals two characteristic normal vibration modes
corresponding to the asymmetric and quasi-symmetric coupled
The bis(iminophosphorane) 3 is readily prepared by the
Staudinger reaction between triphenylphosphine and 1,1′-diazido-
ruthenocene 2, which has been prepared from ruthenocene by
using 2,4,6-triisopropylbenzenesulfonyl azide (trisylazide) as a
strong azide-transfer reagent. Thus, reaction of 1,1′-dilithioru-
thenocene with trisylazide at 0 °C afforded directly 2, which
could be isolated as a pure but unstable solid in 35% yield.
Reaction of 2 with triphenylphosphine in CH2Cl2 at room
temperature provided the bis(iminophosphorane) 3 in 95%.
Nevertheless, compound 2 could be used, without further
purification, for the next step in the Staudinger reaction with
(9) Ethenes: (a) Sato, M.; Kudo, A.; Kawata, Y.; Saitoh, H. Chem.
Commun. 1996, 25. (b) Sato, M.; Kawata, Y.; Kudo, A.; Saitoh, H.; Ochiai,
S. J. Chem. Soc., Dalton Trans. 1998, 2215. (c) Sato, M.; Nagata, T.;
Tanemura, A.; Fujihara, T.; Kumakura, S.; Untura, K. Chem.sEur. J. 2004,
10, 2166. (d) Obendorf, D.; Schottenberger, H.; Wurst, K.; Schuler, N.;
Laus, G. J. Organomet. Chem. 2005, 690, 811. Ethynes: (e) Sato, M.;
Watanabe, M. Chem. Commun. 2002, 1574. (f) Sato, M.; Kubota, Y.;
Kawata, Y.; Fujihara, T.; Unoura, K.; Oyama, A. Chem.sEur. J. 2006, 12,
2282. Aryl(heteroaryl): (g) Bolm, C.; Hermanns, N.; Kesselgruber, M.;
Hildebrand, J. P. J. Organomet. Chem. 2001, 624, 157. (h) Sato, M.;
Maruyama, G.; Tanemura, A. J. Organomet. Chem. 2002, 655, 23. (i)
Enders, M.; Kohl, G.; Pritzkow, H. Organometallics 2002, 21, 1111. (j)
Sato, M.; Kubota, Y.; Tanemura, A.; Maruyama, G.; Fujihara, T.; Nakayama,
J.; Takayanagi, T.; Takahashi, K.; Unoura, K. Eur. J. Inorg. Chem. 2006,
4577. Carbonyl derivatives: (k) Sanders, R.; Mueller-Westerhoff, U. T. J.
Organomet. Chem. 1996, 512, 219. (l) Sato, M.; Suzuki, M.; Okoshi, M.;
Kurasina, M.; Watanabe, M. J. Organomet. Chem. 2002, 648, 72. (m)
Sanderson, C. T.; Quinian, J. A.; Conover, R. C.; Johnson, M. K.; Murphy,
M.; Dluhy, R. A.; Kutal, C. Inorg. Chem. 2005, 44, 3283. (n) Cooke, M.
W.; Cameron, T. S.; Robertson, K. N.; Swarts, J. C.; Aquino, M. A. S.
Organometallics 2002, 21, 5962.
(10) Borane derivatives: (a) Appel, A.; No¨rth, H.; Schmidt, M. Chem.
Ber. 1995, 128, 621. (b) Wrackmeyer, B.; Do¨rfler, U.; Milius, W.;
Heberhold, M. Polyhedron 1995, 14, 1425. Phosphorous derivatives: (c)
Li, S.; Wei, B.; Low, P. M. N.; Lee, H. K.; Hor, T. S. A.; Xue, F.; Mak,
T. C. W. J. Chem. Soc., Dalton Trans. 1997, 1289. (d) Liu, D.; Xie, F.;
Zhang, W. Tetrahedron Lett. 2007, 48, 585.
(11) (a) Dunitz, J.; Orgel, L.; Rich, A. Acta Crystallogr. 1956, 9, 373.
(b) Hardgrove, G.; Templeton, D. Acta Crystallogr. 1959, 12, 28.
(12) (a) Plenio, H.; Burth, D. Organometallics 1996, 15, 4054. (b) Gibson,
V. C.; Long, N. J.; Marshall, E. L.; Oxford, P. J.; White, A. J. P.; Williams,
D. J. J. Chem. Soc., Dalton Trans. 2001, 1162. (c) Shafir, A.; Fiedler, D.;
Arnold, J. J. Chem. Soc., Dalton Trans. 2002, 555. (d) Shafir, A.; Fiedler,
D.; Arnold, J. Chem. Commun. 2003, 2598. (e) Gibson, V. C.; Halliwell,
C. M.; Long, N. J.; Oxford, P. J.; Smith, A. M.; White, A. J. P.; Williams,
D. J. Dalton Trans. 2003, 918. (f) Weng, Z.; Koh, L. L.; Hor, T. S. A. J.
Organomet. Chem. 2004, 689, 18.
stretching of both carbodiimide moieties (2190 and 2150 cm-1
,
respectively; scaling factor 0.9656), in perfect agreement with
the experimental values (2190 and 2125 cm-1). All attempts to
prepare the [3,3](1,1′)diruthenocenophane analogous to 5, by
the aza-Wittig reaction between the ruthenocene derivatives bis-
(iminophosphorane) 3 and the 1,1′-diisothiocyanatoruthenocene
4, were totally unsuccessful.
The new structural motifs diaza[2,2]homometallocenophane
6 and heterometallocenophane 7 were prepared, in 63% and
42% yield, respectively, by aza-Wittig reaction of the bis-
(iminophosphorane) 3 and the appropriate 1,1′-diformylmetal-
locene as carbonyl partners (Scheme 1). By contrast, the
isomeric heterobimetallic metallocenophane 9 was prepared in
59% yield by aza-Wittig reaction between the bis(iminophos-
phorane) 86 and 1,1′-diformylruthenocene, which was easily
prepared by dilithiation of ruthenocene and subsequent reaction
with dimethylformamide as the electrophile, following a modi-
fication of a previously described procedure.9k For all three
diaza[2,2]dimetallocenophanes 6, 7, and 9, our DFT-based
calculations predict the occurrence of C2-symmetric absolute
minima and slightly less stable quasi-Cs-symmetric conformers
(∆G°C2/qCs 0.57, 0.12, and 1.36 kcal/mol, respectively) (see the
Supporting Information).
(13) (a) Tarraga, A.; Molina, P.; Lopez, J. L.; Velasco, M. D.; Bautista,
D. Organometallics 2002, 21, 2055. (b) Lopez, J. L.; Tarraga, A.; Espinosa
A.; Velasco, M. D.; Molina, P.; Lloveras, V.; Vidal-Gancedo, J.; Rovira,
C.; Veciana, J.; Evans, D. J.; Wurst, K. Chem.-Eur. J. 2004, 10, 1815. (c)
Caballero, A.; Lloveras, V.; Tarraga, A.; Espinosa, A.; Velasco, M. D.;
Vidal-Gancedo, J.; Rovira, C.; Wurst, K.; Molina, P.; Veciana, J. Angew.
Chem., Int. Ed. 2005, 44, 1977. (d) Oto´n, F.; Tarraga, A.; Espinosa, A.;
Velasco, M. D.; Bautista, D.; Molina, P. J. Org. Chem. 2005, 70, 6603. (e)
Caballero, A.; Garcia, R.; Espinosa, A.; Tarraga, A.; Molina, P. J. Org.
Chem. 2007, 72, 1161.
(14) For a review of bis(iminophosphoranes), see: Arques, A.; Molina,
P. Curr. Org. Chem. 2004, 8, 827.
(15) Petrovitch, P. M. Double Liaison 1996, 133, 1093. Petrovitch, P.
M. Chem. Abstr. 1968, 68, 29843s.
(16) For work dealing with the synthesis of isothiocyanates from
iminophosphoranes and carbon disulfide, see: Molina, P.; Alajarin, M.;
Arques, A. Synthesis 1982, 596.

6236 Organometallics, Vol. 26, No. 25, 2007
Oto´n et al.
Scheme 1. Synthesis of 1,1′-Disubstituted Ruthenocene Derivatives 2-4 and 10, and Ruthenocenophanes 5-7 and 9a
a Reagents: (a) (i) n-BuLi, TMEDA, Et2O, rt; (ii) 2,4,6-triisopropylphenylsulfonylazide, 0 °C; (b) PPh3, CH2Cl2, rt; (c) CS2, 45 °C; (d) 1,1′-
bisisocyanatoferrocene, dry THF, rt; (e) 1,1′-diformylruthenocene, toluene, reflux; (f) dichlorobis(acetonitrile)palladium(II), toluene, rt.
The structures of the ruthenocenophane compounds and the
other ruthenocene derivatives prepared were determined by
R- and â-ring protons of the ruthenocene moiety as two broad
singlets downshifted (+1.36 ppm) at δ 5.41 and upshifted
(-0.09 ppm) at δ 3.67 ppm, respectively. Consequently, the
difference in the chemical shift of the monosubstituted cyclo-
pentadienyl ring protons is higher in complex 10 (∆δ ) 1.74
ppm) than in the free ligand 3 (∆δ ) 0.29 ppm). A two-
1
means of standard spectroscopic techniques (IR, H and 13C
NMR), mass spectrometry, and elemental analyses, all data
being in agreement with the proposed structures.
The signals observed in the 1H NMR spectra of the [3,3] and
[2,2] ferrocenoruthenocenophanes are informative for the struc-
tural elucidation of these compounds and were assigned on the
basis of the analogy to that of the corresponding [3,3]-
ferrocenophane previously described6 and by using COSY
experiments. In general, the NMR spectra of these metallo-
cenophanes show four signals, with the appearance of pseudo-
triplets and with a proton ratio of 1:1:1:1, which are in good
agreement with two A2B2 systems typical of monosubstituted
cyclopentadiene rings. The analysis of the 1H NMR correspond-
ing to both [2,2]ferrocenoruthenocenophanes also revealed the
following general facts: (i) HR and Hâ present in the Cp ring
of the metallocene directly linked to the aldimine carbon atom
are deshielded with respect to the corresponding HR′ and Hâ′
present in the Cp ring of the metallocene directly linked to the
nitrogen atom of the imine bridge; (ii) HR and HR′ are
deshielded as compared to Hâ and Hâ′; (iii) HR are deshielded
with respect to HR′; and (iv) Hâ are deshielded with respect to
Hâ′, except in compound 7. It should be underlined that 13C
NMR spectra of diaza[2,2]ruthenocenophane 6 could not be
recorded because of its low solubility. However, its HR-EI mass
spectrum displays an intense isotopic cluster peaking at m/z
512.95658 assignable to the molecular ion. The relative
abundance of the isotopic cluster is in good agreement with
the simulated spectrum.
1
dimensional H-13C correlation experiment (HMQC) verified
the coherence of two 13C signals at 77.9 (Câ) and 72.5 (CR)
ppm with the signals at 5.41 and 3.67 ppm, respectively, in the
1H NMR spectrum. The precise structure of compound 10 is
unknown due to our inability to obtain single crystals for X-ray
analysis. Nevertheless, several structural features can be estab-
lished from the spectral and X-ray data corresponding to several
related ferrocenyl palladium complexes already reported.17 In
such cases, a difference in chemical shifts of the signals
corresponding to the hydrogen atoms of the cyclopentadienyl
rings, of approximately 2 ppm, is indicated that constitutes a
strong evidence of a trans coordination. Additionally, the X-ray
analysis of some of those complexes also revealed that the Pd
center was cationic and nearly square planar with the nitrogen
atoms in a pseudo-trans coordination geometry, which favored
the formation of a dative Fe-Pd bond. On the basis of the
analogy of 10 to the above-mentioned ferrocenyl palladium
complexes, we can then assume the same kind of trans
coordination and the possible presence of a dative Ru-Pd bond
in this new ruthenocenyl palladium complex 10. The presence
of three phenyl rings on each phosphorus atom may restrict the
electron donation to the nitrogen atoms through resonance, thus
leaving the Pd(II) center electronically unsaturated and favoring
the formation of a dative bond Ru-Pd. The FAB+ mass
The behavior of ruthenocenyl bisiminophosphorane 3 as
homobidentate ligand for palladium metal cation was also
studied. Thus, reaction of iminophosphorane 3 with dichlorobis-
(acetonitrile)palladium(II) in toluene gave the corresponding
dichloro palladium(II) complex 10, which was characterized
spectroscopically. Notably, a remarkable downfield shift was
observed in its 31P NMR spectrum (δ 35.8 ppm) when compared
to the free iminophosphorane 3 (δ 7.9 ppm), which indicates
the partial positive character of the phosphorus atom in the
complex. Moreover, the 1H NMR spectrum of 10 afforded the
(17) (a) Mann, G.; Shelby, Q.; Roy, A. H.; Hartwig, J. F. Organometallics
2003, 22, 2775. (b) van Leeuwen, P. W. N. M.; Zuideveld, M. A.;
Swennenhuis, B. H. G.; Freixa, Z.; Kamer, P. C. J.; Goubitz, K.; Fraanje,
J.; Lutz, M.; Spek, A. L. J. Am. Chem. Soc. 2003, 125, 5523. (c) Seyferth,
D.; Hames, B. W.; Rucker, T. G.; Cowie, M.; Dickson, R. S. Organome-
tallics 1983, 2, 472. (d) Fillion, E.; Taylor, N. J. J. Am. Chem. Soc. 2003,
125, 12700. (e) Akabori, S.; Kumagai, T.; Shirahige T.; Sato, S.; Kawazoe,
K.; Tamura, C.; Sato, M. Organometallics 1987, 6, 526. (f) Akabori, S.;
Kumagai, T.; Shirahige T.; Sato, S.; Kawazoe, K.; Tamura, C.; Sato, M.
Organometallics 1987, 6, 2105. (g) Metallinos, C.; Tremblay, D.; Barret,
F. B.; Taylor, N. J. J. Organomet. Chem. 2006, 691, 2044.

Synthesis of Aza-Substituted Ruthenocene DeriVatiVes
Organometallics, Vol. 26, No. 25, 2007 6237
Figure 2. HOMO-4 (left) and HOMO-6 (right) simplified
representation for the calculated (B3LYP/6-31G*/Lanl2DZ-ecp)
structure of model complex 10′.
spectrum of 10 shows a peak at m/e 923, corresponding to M+
-
35Cl. The nature of this bidentate ruthenocene, in which the
coordinating heteroatoms are nitrogen atoms, is also supported
by conductivity measurements18 carried out in acetone solutions
of complex 10 (c ) 2.5 × 10-4 M). The calculated molar
conductivity was ΛM ) 99.28 cm2/Ω·mol, which is within the
range typical for a 1:1 electrolyte. The proposed structural
features for 10 are in agreement with the calculated (B3LYP/
6-31G*/Lanl2DZ-ecp) geometry for the C2-symmetric bis-
(dimethylphenylphosphoranilidene) analogue 10′ (Figure 2) that
shows the Pd atom in a nearly perfect square-planar environment
(angles N-Pd-N 162.3°) with a rather small Ru-Pd distance
(2.799 Å). This suggests some kind of Ru-Pd bond, which is
confirmed by the NBO (Natural Bond Orbital) analysis that
indicates a moderate bonding interaction (WBIRu-Pd 0.193) when
compared to those of both Ru and Pd with their respective
neighbors (WBIRu-C(Cp) 0.285 average; WBIPd-N 0.298; WBIPd-Cl
0.362). This bonding interaction is clearly observed in molecular
orbitals HOMO-4 and HOMO-6 sketched in Figure 2. Also,
using the Bader’s AIM (Atoms-In-Molecules) methodology,19
through a topological analysis of the electronic charge density
F(r) a bond critical point (3,-1) has been found within the
Ru-Pd axis (the so-called bond path) at 1.382 Å from the later,
having F(rc) ) 6.22 × 10-3 e/Å3 and 32F(rc) ) 3.18 × 10-3
e/Å5. Moreover, the gas-phase calculated variation in chemical
Figure 3. Cyclic voltammogram of compound 5 (1 × 10-3 M) in
CH2Cl2 using [(n-Bu)4N]PF6 (0.1 M) as supporting electrolyte,
scanned at 0.2 V s-1 from -0.6 to 0.4 V (solid line) and from
-0.6 to 1.0 V (dashed line). Inset: OSWV of compound 5 (1 ×
10-3 M) in CH2Cl2 using [(n-Bu)4N]PF6 (0.1 M) as supporting
electrolyte, scanned at 0.1 V s-1 from -0.2 to 0.8 V.
ruthenocene and its derivatives, as opposed to their ferrocene
analogues, usually undergo an irreversible one-step two-electron
oxidation process.20 The reasons given for this behavior are
either a rapid dimerization of the ruthenocenium (Cp2Ru+)
species that is generated followed by disproportionation or the
direct disproportionation of (Cp2Ru+), in the presence of weakly
coordinating ligands. A quasi-reversible one-electron oxidation
of ruthenocene derivatives has only been observed in molten
salts,21 in noncoordinating solvent using a noncoordinating
electrolyte,22 or by introducing steric hindrance around the metal
by completely methylating the Cp rings to form Cp*2Ru
species.23
The CV of the tetraza[3,3]ferrocenoruthenocenophane 5, in
CH2Cl2, displays two one-electron oxidation waves (Figure 3).
The first one corresponds to a reversible oxidation process at
0.08 V vs Fc/Fc+ (∆E ) Epc - Epa ) 100 mV) and arises from
the oxidation of the ferrocene unit. The second wave of the
CV appears at Ep ) 0.54 V and is associated with an irreversible
oxidation of the ruthenocene unit. The OSWV voltammogram
also exhibits two oxidation peaks at formal potentials of 0.08
and 0.52 V vs Fc/Fc+, respectively.
The CV of the isomeric diaza[2,2]heterobimethallocenophanes
7 and 9 shows two oxidative processes. In compound 7, a first
one-electron reversible oxidation wave appears at E1/2 ) 0.16
V vs Fc/Fc+ redox couple (∆E ) Epc - Epa ) 84 mV), while
the second one, at Ep ) 0.52 V, is associated with an irreversible
oxidative process (see the Supporting Information). Similarly,
the isomeric metallocenophane 9 also exhibits a reversible
oxidation wave at E1/2 ) -0.08 V vs Fc/Fc+ redox couple (∆E
) Epc - Epa ) 80 mV) and a second irreversible oxidation
process (Ep ) 0.77 V vs Fc/Fc+) (Figure 4). The CV for the
diaza[2,2]ruthenocenophane 6 could not be carried out due to
the very low solubility of this compound.
shift was ∆δP ) +43.7 ppm when moving from the model
calc
1,1′-bis(dimethylphenylphosforanilidenamino)ruthenocene free
ligand 3′ to the corresponding model complex 10′, in reasonable
agreement with the experimentally obtained value (∆δP
+27.9 ppm).
)
exper
Electrochemical and Optical Properties. Electrochemical
studies using cyclic voltammetry (CV) and Osteryoung square
wave voltammetry (OSWV), at room temperature, were per-
formed to evaluate the different redox entities present in the
new metallocene and metallocenophane derivatives prepared.
The CV response of 3 in CH2Cl2, also containing 0.1 M
[n-Bu4N]PF6 as supporting electrolyte, showed an electrochemi-
cally reversible one-electron oxidation process at E1/2 ) -0.58
V versus ferrocene/ferrocenium (Fc/Fc+) redox couple (∆E )
Epc - Epa ) 85 mV) (see the Supporting Information). This
value is higher than that required to oxidize the ferrocene unit
in the related 1,1′-bis(triphenylphosphoranilidenamino)ferrocene
(E1/2 ) -0.84 V).6a In both cases, the large cathodic shifts
observed for their E1/2, when compared to the redox potentials
of other ferrocene and ruthenocene derivatives, are consistent
with the high degree of electron donation from the iminophos-
phorane groups. Likewise, the OSWV voltammogram also
exhibits an oxidation peak at formal potential of -0.58 V vs
Fc/Fc+. The electrochemical behavior of this ruthenocene
derivative is rather unusual because it is well known that
The UV/vis spectra of heterobimetallocenophanes 5, 7, and
9 could be explained on the basis of the individual ferrocene
(20) (a) Kuwana, T.; Bublitz, D. E.; Hoh, G. J. Am. Chem. Soc. 1960,
82, 5811. (b) Gubin, S. P.; Smirnova, L. I.; Denisovich, L. I.; Lubovich, A.
A. J. Organomet. Chem. 1971, 30, 243. (c) Denisovich, L. I.; Zakurin, N.
V.; Bazrukova, A. A.; Gubin, S. P. J. Organomet. Chem. 1974, 81, 207.
(21) Gale, R. J.; Job, R. Inorg. Chem. 1981, 20, 42.
(22) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991, 30,
4687.
(23) (a) Koelle, U.; Salzer, A. J. Organomet. Chem. 1983, 243, C27.
(b) Koelle, U.; Grub, J. J. Organomet. Chem. 1985, 289, 133.
(18) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.
(19) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford
University Press: Oxford, 1990.

6238 Organometallics, Vol. 26, No. 25, 2007
Oto´n et al.
Table 1. UV-Visible/Near-IR Data in CH2Cl2
compd
λ
max [nm] (10-3 ꢀ [M-1 cm-1])
5
260 (sh), 334 (sh), 434 (0.48)
241 (sh), 273 (23.28), 402 (sh),
5•+ a
478 (1.93), 590 (1.83), 1029b (1.47)
277 (sh), 416 (2.59), 589 (sh)
260 (18.10), 351 (5.16), 497 (sh)
262 (18.82), 349 (sh), 406 (2.32),
557 (sh), 1481b (0.95)
52+ a
7
7•+ a
72+ a
265 (13.14), 290 (13.82), 347 (4.34),
421 (3.53)
11
265 (32.70), 338 (sh), 440 (1.46)
274 (48.32), 432 (4.88), 609 (sh),
843 (0.63), 1395b (1.01)
11•+ a
112+ a
406 (11.08), 573 (sh), 859 (4.66)
a Oxidized species obtained electrochemically in CH2Cl2 (5 × 10-4 M)
using [n-Bu4N][PF6] (0.15 M) as supporting electrolyte. b Values obtained
by deconvolution of the experimental spectra.
Figure 4. Cyclic voltammogram of compound 9 (1 × 10-3 M) in
CH2Cl2 using [(n-Bu)4N]PF6 (0.1 M) as supporting electrolyte,
scanned at 0.2 V s-1 from -1.5 to 0.5 V (solid line) and from
-1.5 to 1.5 V (dashed line). Inset: OSWV of compound 9 (1 ×
10-3 M) in CH2Cl2 using [(n-Bu)4N]PF6 (0.1 M) as supporting
electrolyte, scanned at 0.1 V s-1 from -0.4 to 1.0 V.
and ruthenocene components of each dyad (see the Supporting
Information). Thus, the UV/vis spectrum of tetraza[3,3]ferro-
cenoruthenocenophane 5 is characterized by an intense ligand-
centered absorption band at 260 nm along with an absorption
band at 356 nm, which is ascribed to a RuII-to-ligand (MLCT)
transition.24 In addition, another weaker low-energy (LE)
absorption band is visible at 488 nm, which is produced either
by nearly degenerate transitions, an FeII d-d transition,23 or by
a FeII-to-ligand (MLCT) process (dπ-π*).25 Similar UV/vis
spectra are exhibited by the diaza[2,2]ferrocenophanerutheno-
cenophanes 7 and 9 (see the Supporting Information).
Spectroelectrochemistry. To study the electronic interaction
between the two metallocene subunits present in compounds 5,
7, and 9, spectrophotometric studies were carried out on their
electrochemically generated oxidized derivatives. Generation of
the oxidized species derived from 5 was performed by constant
potential electrolysis, 0.12 V above the E1/2 of the ferrocenyl
redox couple (0.20 V), and monitored by absorption spectros-
copy. Interestingly, during the oxidation process, a new weak
and broad band grew in the near-IR region at λmax ) 1029 nm
(ꢀmax ) 1470 M-1 cm-1), reaching its maximum after complete
formation of 5•+ (Table 1) (Figure 5). As in the CV, the first
redox wave (0.08 V) was assigned to the one-electron process
occurring at the Fe center (FeII/III); this new absorption band
(1029 nm) can be attributed to the metal-metal charge transfer
(MMCT) from RuII to FeIII sites. The intensity of this band
subsequently decreases until it disappears when the dication 52+
is completely formed.
Figure 5. Evolution of the UV-vis-NIR spectra during the course
of the oxidation of compound 5 (5 × 10-4 M) in CH2Cl2 with
[(n-Bu)4N]PF6 (0.15 M) as supporting electrolyte when 1 electron
is removed. Arrow indicates absorptions that increase during the
experiment.
Chart 1
monooxidized species derived from this homobimetallic deriva-
tive 11 was performed by constant potential electrolysis at 0.21
V. During the oxidation (0 e n e 1), the band observed in
compound 11 at λ ) 440 nm progressively disappears, and at
the same time a new and weak absorption band appears in the
near-IR region, at λ ) 1395 nm (ꢀ ) 101 M-1 cm-1), whose
intensity continuously increases until one electron is removed,
and which is assigned to an intervalence charge transfer (IVCT)
from the FeII to FeIII sites (Table 1). Along with the changes of
these bands, a defined isosbestic point at λ ) 773 nm is
maintained during the course of the oxidation process. On
removing two electrons (1 e n e 2) at the constant potential
of 0.41 V, the intensity of this band decreases until it disappears
when the dioxidized 112+ species is completely formed (see
the Supporting Information).
To compare the magnitude of the electronic communication
between the two metallocene subunits present in this heterobi-
nuclear system 5 and the previously reported tetraaza[3,3]-
ferrocenophane 116a (Chart 1), we have now carried out the
same spectroelectrochemical study on this derivative.
It was already described6a that compound 11 displays two
reversible one-electron oxidation waves at a formal potential
of +0.09 and +0.29 V vs Fc+/Fc. Thus, the generation of the
(24) (a) Sohn, Y. S.; Hendrickson, D. N.; Gray, M. B. J. Am. Chem.
Soc. 1971, 93, 3603. (b) Sanderson, C. T.; Quinian, J. A.; Conover, R. C.;
Johnson, M. K.; Murphy, M.; Dluhy, R. A.; Kuntal, C. Inorg. Chem. 2005,
44, 3283. (c) Gao, L.-B.; Zhang, L.-Y.; Shi, L.-X.; Cheng, Z.-N. Organo-
metallics 2005, 24, 1678.
(25) Barlow, S.; Bunting, H. E.; Ringham, C.; Green, J. C.; Bublitz, G.
U.; Boxer, S. G.; Perry, J. W.; Marder, S. R. J. Am. Chem. Soc. 1999, 121,
3715.
TD-DFT (time-dependent DFT) calculations performed on
the optimized structures for 5•+ and 11•+ (see the Supporting
Information) showed the expected transitions in the near-IR
region (unscaled values 1194.0 and 1225.3 nm, respectively),
which were attributed to optical excitations from a filled â-MO
mainly located on the reduced metal center (â-HOMO of

Synthesis of Aza-Substituted Ruthenocene DeriVatiVes
Organometallics, Vol. 26, No. 25, 2007 6239
until one electron is removed, and which is also assigned to
the metal-metal charge transfer (MMCT) from the RuII to FeIII
centers. On removing two electrons, at a constant potential of
0.64 V, the intensity of the band at λmax ) 1481 nm decreases
until it disappears when the compound is fully oxidized. This
MMCT band also agrees with our TD-DFT calculations
performed on the optimized structure for 7•+ that predicts the
occurrence of a â-HOMO f â-LUMO transition (mainly of
II
III
2
2
2
2
type Ru -dx -y f Fe -dx -y ) (see the Supporting Information)
at an unscaled wavelength of 1381.9 nm, together with another
near-IR transition, not observed experimentally, at 1097.1 nm.
The application of the two-state Hush26 model to the spectrum
of the mixed-valence compounds 7•+ predicts a half-height
bandwidth [∆ν1/2 ) (2310l)1/2] in the case where the two redox
sites are inequivalent28 of 3721 cm-1, which was larger than
the value determined experimentally (2194 cm-1) by spectral
deconvolution assuming a Gaussian profile (see the Supporting
Information). The origin of this discrepancy is not immediately
clear, although differences between theoretical and experimental
values of ∆ν1/2 are not uncommon. They often arise from system
nonidealities with respect to the model.29 The availability of
Figure 6. Evolution of the UV-vis-NIR spectra during the course
of the oxidation of compound 7 (5 × 10-4 M) in CH2Cl2 with
[(n-Bu)4N]PF6 (0.15 M) as supporting electrolyte when 1 electron
is removed. Arrows indicate absorptions that increase during the
experiment.
E
MMCT in 7•+ also enables the estimation of the Vab ) 406 cm-1
•+
2
2
Ru-dx -y -type in 5 ) to the â-LUMO mainly consisting of a
d
2
2
and the related ground-state delocalization coefficient R ) 0.060.
All attempts to promote a controlled oxidation process in the
[2,2]ferrocenoruthenocenophane 9 failed. Thus, the full oxidized
compound was only obtained, 92+, which, in turn, did not display
any NIR absorption band.
x -y -type AO centered at the oxidized Fe atom.
Previous reports have emphasized that spectral measurements
of the IVCT transitions often serve as a very accurate probe
for the degree of electronic coupling.26aThe spectral parameters
of the bands present in the near-IR, energy (νmax), intensity
(ꢀmax), and half-bandwidth (∆ν1/2), obtained by deconvolution
of the experimental spectra performed on spectral intensity times
wavenumber versus wavenumber assuming Gaussian shapes,
have been used to determine the effective electronic coupling
(Vab) through the two-state classical Marcus-Hush theory.26
According to Robin and Day classification,26b mixed-valence
compounds are classified in three categories: class I, where the
redox centers are completely localized and behave as separate
entities; class II, where an intermediate coupling between the
mixed-valence centers exists; and class III, where the system is
completely delocalized and the redox centers show intermediate
valence states.
Metal Ion Sensing Properties. Previous studies on ferrocene-
based ligands have shown that bands ascribed to the lowest
energy metal-ligand transitions, in the absorption spectra, are
perturbed upon complexation.30 Therefore, the metal recognition
properties of the aza-bridged metallocene dyads 7 and 9 toward
Li+, Na+, K+, Ca2+, Mg2+, Zn2+, Cd2+, Hg2+, Ni2+, and
Pb2+ metal ions (as their perchlorate salts) were evaluated by
UV-vis spectroscopy. Titration experiments for CH3CN/
CH2Cl2 (3/2) solutions of these ligands (c ) 1 × 10-4 M) and
the corresponding ions were performed and analyzed quantita-
tively.31 No changes were observed in the UV-vis spectra upon
addition of Li+, Na+, K+, Ca2+, Mg2+, Hg2+, Ni2+, and Pb2+
even in a large excess to the CH3CN/CH2Cl2 (3/2) solutions of
any of these ligands. However, significant spectrophotometric
changes in the metal-ligand absorption bands were observed
upon addition of increasing amounts of Zn2+ metal ions. In both
cases, the most prominent features observed during the com-
For the class II regime, the ∆ν1/2theo and the electronic
coupling Vab are derived from the Hush equations ∆ν1/2theo
)
(2310νmax max∆ν1/2
)
1/2 and Vab ) (0.0205/dab)(ꢀmax 1/2, where dab
ν
)
is the diabatic metal-metal distance (i.e., the intermetallic
distance in the hypothetical absence of electronic coupling).
Additionally, the delocalization coefficient R, which is a
parameter that quantifies the degree of valence delocalization
in the ground state (i.e., the fraction of valence electronic charge
transferred from the donor to the acceptor metal centers), can
(28) Hush theory indicates that, for a mixed-valence system with two
inequivalent redox centers, the energy of the IVTC transition is given by
ν˜max ) λ + ∆G0, where λmax is the reorganization energy of Marcus theory
and ∆G0 can be estimated from the electrochemical data. If one assumes
that the oxidation potential of one center is unaffected by the oxidation
state of the other, the magnitude of ∆G0 is obtained by converting the
electrochemical value of ∆E1/2 into energy units. For an elegant application,
see: Barlow, S. Inorg. Chem. 2001, 40, 7047-7053.
(29) (a) Elliot, C. M.; Derr, D. L.; Matyushov, D. V.; Newton, M. D. J.
Am. Chem. Soc. 1998, 120, 11714. (b) Curtis, J. C.; Meyer, T. J. Inorg.
Chem. 1982, 21, 1562.
(30) (a) Marder, S. R.; Perry, J. W.; Tiemann, B. G. Organometallics
1991, 10, 1896-1901. (b) Coe, B. J.; Jones, C. J.; McCleverty, J. A.; Bloor,
D.; Cross, G. J. J. Organomet. Chem. 1994, 464, 225-232. (c) Mu¨ller, T.
J.; Netz, A.; Ansorge, M. Organometallics 1999, 18, 5066-5074. (d) Carr,
J. D.; Coles, S. J.; Hassan, M. B.; Hurthouse, M. B.; Malik, K. M. A.;
Tucker, J. H. R. J. Chem. Soc., Dalton Trans. 1999, 57.
(31) Specfit/32 Global Analysis System, 1999-2004 Spectrum Software
Associates (SpecSoft@compuserve.com). The Specfit program was acquired
from Bio-logic, SA (www.bio-logic.info), in January 2005. The equation
to be adjusted by non-linear regression, using the above mentioned software,
was: ∆A/b ) {K11∆ꢀHG[H]tot[G]}/{1 + K11[G]}, where H ) host, G )
guest, HG ) complex, ∆A ) variation in the absorption, b ) cell width,
K11 ) association constant for a 1:1 model, and ∆ꢀHG ) variation of molar
absorptivity.
be calculated by using the equation R ) Vab/νmax.
27 Taking into
account this consideration, the values of Vab ) 493 cm-1 and R
) 0.069 were calculated for compound 11•+, which are quite
similar to those obtained for the heterobimetallic system 5•+:
Vab ) 651 cm-1 and R ) 0.067.
Likewise, generation of the monooxidized species derived
from the [2,2]ferrocenoruthenocenophane 7 was performed by
constant potential electrolysis, 0.12 V above E1/2 of the
ferrocenyl redox couple (Figure 6). During the oxidation process,
a new band, in the near-IR region, appears at λmax ) 1481 nm
(ꢀmax ) 955 M-1 cm-1), whose intensity continuously increases
(26) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Robin, M. B.;
Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247. (c) Creutz, C. Prog.
Inorg. Chem. 1983, 30, 1. (d) Nelsen, S. F. Chem.sEur. J. 2000, 6, 581.
(27) (a) Barlow, S. Inorg. Chem. 2001, 40, 7047. (b) Santi, S.; Orian,
L.; Durante, C.; Bisello, A.; Benetollo, F.; Crociani, L.; Ganis, P.; Ceccon,
A. Chem.-Eur. J. 2007, 13, 1955.

6240 Organometallics, Vol. 26, No. 25, 2007
Oto´n et al.
Figure 9. Evolution of the 1H NMR spectra of 7 in CD2Cl2 when
(a) 0 equiv, (b) 0.4 equiv, and (c) 1 equiv of Zn(ClO4)2 was added.
between the uncomplexed and complexed species occurs. The
new band is red-shifted by ∆λ ) 99 nm and is responsible for
the change of color from pale red (neutral ligand 7) to deep
green (complexed 7‚Zn2+). The resulting titration data suggest
a 2:1 (L:Zn2+) binding model, with the association constant and
detection limit32 being 1.2 × 108 M-2 (error <10%) and 5.8 ×
10-6 M, respectively.
Figure 7. Color changes in CH3CN/CH2Cl2 solutions of com-
pounds 7 and 9 upon addition of several metal cations tested.
Table 2. UV-Vis Data of the Free Ligands 7 and 9 and the
Corresponding Ligand/Zn2+ Complexes
λ
max [nm]
Similarly, when ligand 9 was studied under the above-
mentioned titration conditions, a significant red-shift (∆λ ) 100
nm) of the LE band, appearing at λ ) 488 nm (ꢀ ) 520 M-1
cm-1), was observed (Table 1), which completely disappears
when the complexation was completed (see the Supporting
Information). Three isosbestic points were also found during
the titration process at λ ) 264, 346, and 381 nm. The resulting
titration fitted to a 2:1 (L:Zn2+) binding model, with the
association constant and detection limit being 9.3 × 106 M-2
(error <10%) and 6.0 × 10-6 M, respectively.
compd
(10-3 ꢀ [M-1 cm-1])
7
230 (25.11), 261 (24.50),
350 (7.05), 496 (0.54)
234 (18.78), 263 (22.69),
383 (sh), 595 (1.28)
260 (20.99), 356 (3.80),
488 (0.52)
7‚Zn2+
9
9‚Zn2+
267 (20.87), 362 (sh),
416 (2.73), 588 (1.13)
plexation processes are the following: (i) a progressive red-
shift of the corresponding metal-ligand transition band; (ii)
appearance of well-defined isosbestic points, indicative of the
presence of only two absorbing species in the solution the free
ligand (L) and the complex (L‚Zn2+); and (iii) a neat change of
the color during the complexation event, from pale red to deep
green, which can be used for the “naked eye” detection of this
divalent cation (Figure 7).
To get first-hand information about the coordinating sites of
1
receptors 7 and 9 upon complexation, H NMR experiments
were performed to explore the complexation mechanism with
Zn2+ metal ions. Thus, addition of 1 equiv of Zn(ClO4)2 in
CD3CN to a solution of the ligand 7 in CD2Cl2 gives rise to
significant downfield shifts for the signals corresponding to the
iminic proton (CHdN) at δ ) 10.05 (∆δ ) +0.44) as well as
for the signals of the HR and HR′ (∆δ ) +0.42 and ∆δ )
+0.35 ppm, respectively) and Hâ and Hâ′ (∆δ ) +0.23 and
∆δ ) +0.11 ppm, respectively) protons present in the Cp rings
of the ferrocene and ruthenocene moieties (Figure 9). Similarly,
1H NMR titration studies carried out by addition of 1 equiv of
Zn2+ to a solution of the ligand 9 under the same conditions
also cause downfield shifts analogous to those mentioned for
ligand 7 (see the Supporting Information).
Thus, addition of increasing amounts of a solution of Zn2+
in CH3CN (c ) 2.5 × 10-2 M) to a solution of 7 in CH3CN/
CH2Cl2 (3/2) (c ) 1 × 10-4 M) caused a progressive appearance
of a new more intense band located at λ ) 595 nm (ꢀ ) 1280
M-1 cm-1) as well as the complete disappearance of the initial
LE band at λ ) 496 nm (ꢀ ) 540 M-1 cm-1) (Table 2 and
Figure 8). Two well-defined isosbestic points at λ ) 340 and
375 nm were found, indicating that a neat interconversion
For the reported constants to be taken with confidence, we
have proved the reversibility of the complexation process by
carrying out the following experimental test: 0.5 equiv of
Zn(ClO4)2 was added to a solution of the appropriate ligand (7
or 9) in CH2Cl2 to obtain the complexed L2‚Zn2+ species, whose
DPV voltammogram and UV/vis spectrum were recorded. The
CH2Cl2 solutions of the complexes were washed several times
with water until the color of the solution changed from deep
green to red. The organic layers were dried, the corresponding
optical spectrum, DPV voltammogram, and 1H NMR spectrum
were recorded, and they were found to be the same as those of
the corresponding free receptor L. Afterward, 0.5 equiv of
Zn(ClO4)2 was added to each of these solutions, and the initial
1
UV/vis spectrum, OSWV voltammogram, and H NMR spec-
trum of the complexes L2‚Zn2+ were fully recovered together
with its deep red color. These experiments were carried out over
several cycles, and the optical spectra were recorded after each
Figure 8. Changes in the absorption spectra of compound 7 [c )
10-4 M in CH3CN/CH2Cl2 (3:2, v:v)] upon addition of increasing
amounts of Zn2+. Arrows indicate absorptions that increase during
the experiment.
(32) Shortreed, M.; Kopelman, R.; Kuhn, M.; Hoyland, B. Anal. Chem.
1996, 68, 1414.

Synthesis of Aza-Substituted Ruthenocene DeriVatiVes
Organometallics, Vol. 26, No. 25, 2007 6241
attached to the ruthenocene unit, are presented. Introduction of
the nitrogen functionality into the ruthenocene unit is achieved
by reaction of 1,1′-dilithioruthenocene with 2,4,6-trisopropyl-
benzenesulfonyl azide (trisyl azide), and the resulting 1,1′-
diazidoruthenocene 2 is converted into the key bis(iminophos-
phorane) 3 by Staudinger reaction with triphenylphosphine.
Subsequent aza-Wittig reactions of 3 with carbon disulfide
provided the opened ruthenocene-based isothiocyanate 4, whereas
the reaction with 1,1′-bis(isocyanato)ferrocene afforded the
closed biscarbodiimide 5. When the appropriate 1,1′-diformyl-
metallocene is used, the corresponding bisaldimines 6 and 7
are obtained. These aza-substituted ruthenocene derivatives
comprise remarkable multifunctional molecular systems, because
they could serve as important models for the investigation of
intramolecular charge transfer and for metal recognition pro-
cesses. Mixed ferrocene ruthenocenophanes 5 and 7 form the
mixed-valence species 5•+ and 7•+ by partial oxidation. Interest-
ingly, the spectroelectrochemical study of the mixed metallocene
5 revealed the presence of a LE band in the near-IR region,
which indicates an unprecedented intramolecular charge transfer
between the ferrocene and ruthenocene units through carbodi-
imide bridges. On the other hand, mixed diaza[2.2]ferroceno-
ruthenocenophanes 7 and 9 exhibit interesting cation-sensing
properties, which show high selectivity for Zn2+ ions. The
metal-ligand transition band of the absorption spectra of these
compounds is red-shifted by about 100 nm only in the presence
of Zn2+ ions. This change in the absorption spectra is accomp-
anied by a dramatic color change from pale red to deep green,
which allows the potential for “naked eye” highly selective
detection over some others cations, including the strong com-
petitor Cd2+. Unfortunately, this sensing behavior could not be
studied in water solutions due to the poor solubility of these
ligands in aqueous environments.
Figure 10. Bar diagram showing the results of the cross-selectivity
experiments of 7 in CH3CN/CH2Cl2 (3/2) upon addition of the metal
tested. Blue bars indicate the absorbance increase of ligand 7 after
addition of 1 equiv of the corresponding metal cation. Red bars
indicate the absorbance increase of ligand 7 after addition of 0.5
equiv of Zn2+ to the above-mentioned (ligand+metal cation)
solutions.
step and found to be fully recovered on completion of the step,
thus demonstrating the high degree of reversibility of the
complexation/decomplexation processes.
The interference in the selective response of ligands 7 and 9
in the presence of Zn2+ metal ions, from the other metal cations
tested, was also studied by using cross-selectivity experiments.
Thus, addition of 1 equiv of Li+, Na+, K+, Ca2+, Mg2+, Hg2+
,
Experimental Section
Ni2+, and Pb2+ cations in CH3CN to 1 equiv of the receptors 7
and 9 in CH3CN/CH2Cl2 (3/2) did not give any optical response.
However, further addition of 0.5 equiv of Zn2+ to the above-
mentioned solutions gave optical response identical to that
observed upon addition of 0.5 equiv of Zn2+ to a solution
containing only 1 equiv of the appropriate ligand and which is
free of the other metal cations (see Figure 10 and the Supporting
Information). These results clearly demonstrate that these
receptors have excellent affinity for Zn2+ over those ions.
The demand for sensing Zn2+ metal ions, which is spectro-
scopically silent because of its 3d104s0 electronic configuration,
in competitive media such as Ca2+ and Mg2+ is growing
rapidly.33 A challenge to develop chemosensors that can
discriminate Zn2+ from Cd2+ is still more important. Because
cadmium and zinc are in the same group of the periodic table
and have similar properties, they usually cause similar spectral
changes after interactions with chemosensors. In this sense, a
few colorimetric selective Zn2+ chemosensors have been
designed and synthesized.34
Preparation of 1,1′-Bis(azido)ruthenocene 2. To a solution of
ruthenocene (1 g, 4.32 mmol) in freshly distilled dry diethyl ether
(18 mL) were added n-BuLi (6.7 mL, 10.8 mmol) and N,N,N,N-
tetramethylethylendiamine (TMEDA) (1.63 mL, 10.8 mmol) under
nitrogen. The solution was stirred for 18 h at room temperature,
and then a solution of 2,4,6-triisopropylphenylsulfonylazide (trisyl
azide) (3.9 g, 15.8 mmol) in the same solvent (15 mL) was added
at 0 °C. The mixture was stirred in the dark and at room temperature
for 4 h. Afterward, 30 mL of H2O was added, and the mixture was
extracted with 100 mL of diethyl ether. The organic phase was
dried with anhydrous MgSO4 and chromatographed on a silica gel
column, using n-Hex/AcOEt (19:1) (Rf ) 0.65) as eluent to give
the corresponding azide, which was crystallized from n-Hex at -40
1
°C. Yield: 35%. Mp: 60-63 °C (dec). H NMR (300 MHz): δ
4.83 (st, 4H, CH-Ru), 4.53 (st, 4H, CH-Ru). 13C NMR (75.3
MHz): δ 102.3 (q-Cp-Ru), 68.8 (CH-Ru), 64.9 (CH-Ru). FT-
IR (Nujol): ν 2117, 1290, 1163, 1061, 1026, 935, 813, 737. MS
(FAB+): m/e (%) 314 (M+ + 1, 8). Anal. Calcd for C10H8N6Ru:
C, 38.34; H, 2.57; N, 26.83. Found: C, 38.54; H, 2.85; N, 26.62.
Preparation of 1,1′-Bis(N-triphenylphosphoranilidenamino)-
ruthenocene 3. Method A: To a solution of ruthenocene (1 g, 4.32
mmol) in freshly distilled dry diethyl ether (18 mL) were added
n-BuLi (6.7 mL, 10.8 mmol) and TMEDA (1.63 mL, 10.8 mmol)
under nitrogen. The solution was stirred at room temperature for
18 h, and then a solution of trisyl azide (3.9 g, 15.8 mmol) in the
same solvent (15 mL) was added at 0 °C. The reaction mixture
Conclusion
The synthesis, electrochemical, electronic, and optical proper-
ties of the new structural motifs nitrogen-rich [2.2]- and [3.3]-
mixed ferrocene and ruthenocene metallocenophanes, bearing
a nitrogen functionality (carbodiimide 5; aldimine 6,7) directly
(33) (a) Valeur, B.; Leray, I. Coord. Chem. ReV. 2000, 205, 3. (b) Prodi,
L.; Bolleta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem. ReV. 2000, 205,
59. (c) Jiang, P.; Guo, Z. Coord. Chem. ReV. 2004, 248, 205. (d) Kikuchi,
K.; Komatsu, K.; Nagano, T. Curr. Opin. Chem. Biol. 2004, 8, 182.
(34) (a) Xu, Z.; Qian, X.; Cui, J.; Zhang, R. Tetrahedron 2006, 62, 10117.
(b) Zhang, L.; Dong, S.; Zhu, L. Chem. Commun. 2007, 1891. (c) Zapata,
F.; Caballero, A.; Espinosa, A.; Tarraga, A.; Molina, P. Org. Lett. 2007, 9,
2385.

6242 Organometallics, Vol. 26, No. 25, 2007
Oto´n et al.
was stirred at room temperature for 4 h and protected from light.
Subsequently, H2O (30 mL) was added, and the mixture was
extracted with diethyl ether (100 mL). The organic layers were dried
with anhydrous MgSO4, and the solvent was removed under
vacuum. The resulting solid was dissolved in dry CH2Cl2 (30 mL),
and a solution of triphenylphosphine (2.26 g, 8.64 mmol) in the
same solvent (20 mL) was added. Next, the reaction mixture was
stirred for 5 h at room temperature, and afterward the solvent was
reduced to 5 mL under vacuum. To this solution was added diethyl
ether (30 mL), giving rise to the bisiminophosphorane 3 in 74%
yield, which was used without further purification for the next step
in the aza-Wittig reactions.
metallocene 3 or 8 (0.35 mmol) and the adequate 1,1′-diformyl-
metallocene (0.35 mmol) in dry toluene (20 mL) was heated under
reflux for 10 h under nitrogen. Subsequently, the solution was
concentrated under vacuum until 5 mL, and then 5 mL of diethyl
ether was added to precipitate 6 and 7 as orange solids, which were
washed with CH2Cl2 and crystallized from CH2Cl2/diethyl ether.
Isolation of compound 9 was achieved directly from the reaction
mixture by chromatography using a silica gel column and
CH2Cl2/MeOH (9:1) as solvent (Rf ) 0.45), and further recrystal-
lization from CH2Cl2 yielded the title compound as purple crystals.
1
6. Yield: 63%. Mp: 229-232 °C (dec). H NMR (200 MHz):
δ 9.63 (s, 2H, imine), 5.64 (st, 4H, CH-Ru), 5.38 (st, 4H,
CH-Ru), 4.49 (st, 4H, CH-Ru), 4.28 (st, 4H, CH-Ru). 13C
NMR: good quality spectrum could not be recorded due to the
very low solubility of this compound. FT-IR (Nujol): ν 1613, 1319,
1226, 1194, 1153, 1035, 1020, 954, 923, 856, 804, 723. MS (high-
resolution EI+): m/e (%) 512.95658 (M+, 100). Anal. Calcd for
C22H18N2Ru2: C, 51.55; H, 3.54; N, 5.47. Found: C, 51.79; H,
3.36; N, 5.75.
Method B: To a solution of 1,1′-bis(azido)ruthenocene (0.5 g,
2.16 mmol) in dry THF (30 mL) was added a solution of
triphenylphosphine (1.69 g, 6.48 mmol) in the same solvent (15
mL). The reaction mixture was stirred at room temperature for 5
h, and then the solvent was reduced to 5 mL under vacuum. After
addition of diethyl ether (30 mL), a solid was obtained, which was
crystallized from CH2Cl2/diethyl ether (5:2) at -40 °C. Yield: 95%.
1
1
Mp: 195-198 °C (dec). H NMR (300 MHz): δ 7.69-7.76 (m,
7. Yield: 42%. Mp: 238-241 °C (dec). H NMR (400 MHz):
12H, Ph), 7.39-7.49 (m, 18H, Ph), 4.05 (bs, 4H, CH-Ru), 3.76
(bs, 4H, CH-Ru). 13C NMR (75.3 MHz): δ 132.9 (d, J ) 9.7 Hz,
CHmeta-Ph), 131.4 (d, J ) 2.7 Hz, CHpara-Ph), 130.6 (d, J ) 97.0
Hz, Cipso-Ph), 128.3 (d, J ) 11.8, CHorto-Ph), 77.2 (q-Cp-Ru),
66.8 (CH-Ru), 66.6 (CH-Ru). 31P NMR (162 MHz): δ 7.9.
FT-IR (Nujol): ν 1459, 1285, 1111, 1076, 1001, 822, 787, 753,
738, 693. MS (FAB+): m/e 782 (M+ + 1, 100). Anal. Calcd for
C46H38N2P2Ru: C, 70.67; H, 4.90; N, 3.58. Found: C, 70.42; H,
5.18; N, 3.79.
δ 9.58 (s, 2H, imine), 5.49 (st, 4H, Cp-Fc), 5.38 (st, 4H,
Cp-Ru), 4.42 (st, 4H, CH-Ru), 4.18 (st, 4H, CH-Fc). 13C
NMR (100 MHz): δ 168.5 (CH-imine), 104.3 (q-Cp-Ru), 81.4
(q-Cp-Fc), 70.0 (CH-Fe), 69.7 (CH-Fc), 69.4 (CH-Fc), 67.4
(CH-Fc). FT-IR (Nujol): ν 1614, 1321, 1228, 1195, 1151, 1090,
1022, 925, 873, 790. MS (FAB+): m/e 469 (M+ + 1, 78). Anal.
Calcd for C22H18FeN2Ru: C, 56.54; H, 3.88; N, 5.99. Found: C,
56.32; H, 3.70; N, 5.71.
9. Yield: 59%. Mp: 145-148 °C. 1H NMR (200 MHz): δ 9.31
(s, 2H, imine), 5.68 (st, 4H, CH-Ru), 5.14 (st, 4H, CH-Fc), 4.67
(st, 4H, CH-Ru), 4.09 (st, 4H, CH-Fc). 13C NMR (100 MHz): δ
162.8 (imine), 103.5 (q-Cp-Fc), 84.1 (q-Cp-Ru), 72.9 (CH-Ru),
71.9 (CH-Ru), 67.1 (CH-Fc), 64.7 (CH-Fc). FT-IR (Nujol): ν
1633, 1313, 1294, 1182, 1160, 1106, 995, 875, 748. MS (FAB+):
m/e 469 (M+ + 1, 100). Anal. Calcd for C22H18FeN2Ru: C, 56.54;
H, 3.88; N, 5.99. Found: C, 56.77; H, 3.62; N, 6.27.
Preparation of 1,1′-Bis(isothiocyanate)ruthenocene 4. A solu-
tion of 1,1′-bis(N-triphenylphosphoranylidenamino)ruthenocene 3
(0.2 g, 0.26 mmol) in carbon disulfide (16 mL) was heated at 45
°C under nitrogen for 4 h, and then the solvent was removed under
reduced pressure. The resulting residue was treated with diethyl
ether, giving rise to a precipitate of triphenylphosphine sulfide,
which was separated by filtration. The remaining solution was
chromatographed on a silica gel column, using n-Hex/EtAcO (19:
1) (Rf ) 0.3), and the solid obtained, after removing the solvent,
was crystallized from n-Hex at -40 °C. Yield: 41%. Mp: 112-
[1,1′-Bis(triphenylphosphoranilidenamino)ruthenocene
1
Pd(II)Cl]Cl 10. Yield: 92%. Mp: 249-252 °C (dec). H NMR
(300 MHz): δ 7.69-7.76 (m, 12H, Ph), 7.60-7.65 (m, 6H,
Ph), 7.45-7.51 (m, 12H, Ph), 5.41 (bs, 4H, CH-Ru), 3.67 (bs,
4H, CH-Ru). 13C NMR (75 MHz): δ 133.5 (d, J ) 10.3 Hz,
CHmeta-Ph), 133.3 (s, CHpara-Ph), 128.9 (d, J ) 13 Hz,
CHorto-Ph), 125.9 (d, J ) 101.9 Hz, q-Ph), 96.5 (d, J ) 16.5 Hz,
q-Cp-Ru), 77.9 (s, CHâ-Ru), 72.5 (d, J ) 5.5 Hz, CHR-Ru).
31P NMR (121 MHz): δ 35.84. FT-IR (Nujol): ν 3090, 3048, 1435,
1366, 1310, 1287, 1186, 1158, 1114, 1075, 1058, 1029, 999, 974,
1
113 °C. H NMR (200 MHz): δ 4.94 (st, 4H, CH-Ru), 4.56 (st,
4H, CH-Ru). 13C NMR (50 MHz): δ 89.0 (q-Cp-Ru), 70.4
(CH-Ru), 70.1 (CH-Ru). FT-IR (Nujol): ν 2101, 2060, 1312,
1209, 1163, 1106, 1029, 8.28, 724. MS (FAB+): m/e (%) 346 (M+
+ 1, 7). Anal. Calcd for C12H8N2RuS2: C, 41.73; H, 2.33; N, 8.11.
Found: C, 41.97; H, 2.12; N, 8.28.
Preparation of 1,3,10,12-Tetraaza-[3,3](1,1′)ferrocenoru-
thenocenophane 5. To a solution of 1,1′-bis(isocyanate)ferrocene1
(0.08 g., 0.30 mmol) in dry THF (20 mL) was added a solution of
1,1′-bis(N-triphenilphosphoranylidenamino)ruthenocene 4 (0.155 g.,
0.30 mmol) in the same solvent (40 mL). The reaction mixture
was stirred at room temperature for 1 h, and the solvent was
removed under vacuum. The brown solid obtained was then
crystallized from CH2Cl2 at -40 °C. Yield: 72%. Mp: 190-193
883, 824, 783, 749, 728, 692. MS (FAB+): m/e (%) 923 (M+
-
35Cl, 93), 887 (M+ - 2 35Cl, 57), 816 (M+ - PdCl, 85), 781 (M+
- PdCl2, 51). Anal. Calcd for C46H38Cl2N2P2PdRu: C, 57.60; H,
3.99; N, 2.92. Found: C, 57.77; H, 4.29; N, 3.15.
Acknowledgment. We gratefully acknowledge a grant from
MEC-Spain CTQ2004-02201 and from Fundacio´n Se´neca
(CARM) 02970/PI/05.
1
°C (dec). H NMR (400 MHz): δ 4.85 (st, 4H, CH-Ru), 4.52
(st, 4H, CH-Ru), 4.22 (st, 4H, CH-Fc), 4.16 (st, 4H, CH-Fc).
Supporting Information Available: General comments and
computational details; NMR spectra; electrochemical, UV-vis, and
13C NMR (100 MHz):
δ 134.7 (q, carbodiimide), 97.0
(q-Cp-Ru), 95.4 (q-Cp-Fc), 68.9 (2×CH-Ru), 66.4 (CH-Fc),
66.3 (CH-Fc). FT-IR (Nujol): ν 2190, 2125, 1533, 1349, 1230,
1205, 1026, 917, 812, 728. MS (FAB+): m/e 494 (M+ + 1, 6).
Anal. Calcd for C22H16FeN4Ru: C, 53.56; H, 3.27; N, 11.36.
Found: C, 53.83; H, 3.43; N, 11.09.
General Procedure for the Preparation of Diaza[2,2](1,1′)-
homo- and Heterometallocenophanes 6, 7, and 9. A solution
of the appropriate 1,1′-bis(N-triphenylphosphoranylidenamino)-
1
UV-vis-NIR data; UV-vis and H NMR titration data; cross-
selectivity experiments; calculated structures, Cartesian coordinates,
and energies for compounds 3′, 5, 6, 7, 9, and 10′ and radical-
cations 5•+, 7•+, and 11•+; and MOs involved in MMCT for 5•+
and 7•+. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM700757M