Novel palladacycles containing [C(sp2, ferrocene), N, O] - or [C(sp2, ferrocene), N, O]2- terdentate ligands

Article abstract of DOI:10.1021/om050881o

The synthesis, characterization, and study of the reactivity of the novel cyclopalladated complexes [Pd{[(η⁵-C₅H ₃)CH=N(C₆H₄-2-CH₂OH)]Fe(η5- C₅H₅)}Cl] (2) and Pd{[(η

Full text of DOI:10.1021/om050881o

596
Organometallics 2006, 25, 596-601
Novel Palladacycles Containing [C(sp2, ferrocene), N, O]- or
[C(sp2, ferrocene), N, O]2- Terdentate Ligands
Sonia Pe´rez,† Concepcio´n Lo´pez,*,† Amparo Caubet,† Xavier Solans,‡ Merce` Font-Bard´ıa,‡
Anna Roig,§ and Elies Molins§
Departament de Qu´ımica Inorga`nica, Facultat de Qu´ımica, UniVersitat de Barcelona,
Mart´ı i Franque`s 1-11, 08028-Barcelona, Spain, Departament de Cristal‚lografia, Mineralogia i Dipo`sits
Minerals, Facultat de Geologia, UniVersitat de Barcelona, Mart´ı i Franque`s s/n, 08028-Barcelona. Spain,
and Institut de Cie`ncia de Materials de Barcelona (ICMAB-CSIC), Campus de la UniVersitat Auto`noma
de Barcelona, 08193-Bellaterra, Spain
ReceiVed October 14, 2005
The synthesis, characterization, and study of the reactivity of the novel cyclopalladated complexes
[Pd{[(η5-C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl] (2) and [Pd{[(η5-C5H3)CHdN(C6H4-2-CH2O)]-
Fe(η5-C5H5)}]2 (3) with PPh3 or MeO2CCtCCO2Me are described. Electrochemical and 57Fe Mo¨ssbauer
spectroscopic studies of the new palladacycles are also reported.
Palladium(II) compounds derived from ligands containing
modes than their analogues with an OR substituent.11b,14-17
Palladium(II) or platinum(II) complexes derived from organic
imino alcohols have been reported,14-17 and some of them (i.e.
[Pd{C6H4CHdN(C6H4-2-O)}]4) exhibit unusual structural fea-
tures.16
three donor atoms with different hardnesses1 have attracted great
interest due to their potential hemilability,2 which is especially
relevant in view of their use in homogeneous catalysis.3 Despite
of this and the prochiral nature of ferrocenyl ligands in the
cyclopalladation processes,4,5 palladium(II) compounds contain-
ing terdentate [C(sp2, ferrocene), N, X]q- (X ) N, S, O; q ) 1,
2) ligands are scarce.6-10 It is well-known that organometallic
complexes with a σ(Pd-O) bond are particularly interesting,
due to their applications in organic and/or organometallic
synthesis and in homogeneous catalysis.11,12 However, only two
palladacycles with a [C(sp2, ferrocene), N, O]q- ligand are
known,13 and in both cases, the palladium(II) is bound to the
oxygen of an ether group. Recent contributions have shown that
imino alcohols may exhibit greater versatility in the coordination
On the other hand, ring-chain tautomeric equilibria involving
1,3-O,N-heterocyclic systems have generated great interest in
the last few years,18-20 since this process may affect the
reactivity of the species involved. However, none of the articles
published so far have focused on the study of the effect produced
by transition metals upon the reactivity of the two tautomers.
We have recently reported the synthesis of 2-ferrocenyl-2,4-
dihydro-1H-benzo[d][1,3]oxazine (1a) and the tautomeric equi-
librium between 1a and the imino alcohol [(η5-C5H5)Fe{(η5-
C5H4)CHdN(C6H4-2-CH2OH)}] (1b).20 In view of this, 1b
appeared to be a good candidate to attempt the preparation of
palladacycles with terdentate [C(sp2, ferrocene), N, O]- or
[C(sp2, ferrocene), N, O]2- ligands. Here we describe the study
of the reactivity of 1a with palladium(II).
* To whom correspondence should be addressed. E-mail:
conchi.lopez@qi.ub.es.
† Departament de Qu´ımica Inorga`nica, Universitat de Barcelona.
‡ Departament de Cristal‚lografia, Mineralogia i Dipo`sits Minerals,
Universitat de Barcelona.
§ Campus de la Universitat Auto`noma de Barcelona.
(11) For recent contributions in this area see for instance: (a) Lindsell,
W. E.; Palmer, D. D.; Preston, P. N.; Rosair, G. M.; Jones, R. V. H.;
Whitton, A. J. Organometallics 2005, 24, 1119. (b) Kang, M.; Sen, A.
Organometallics 2005, 24, 3508. (c) Vicente, J.; Abad, J. A.; Fo¨rtsch, W.;
Lo´pez-Saez, M. J.; Jones, P. G. Organometallics 2004, 23, 4414.
(12) Mun˜oz, M. P.; Mart´ın-Matute, B.; Ferna´ndez-Rivas, C.; Ca´rdenas,
D. J.; Echavarren, A. M. AdV. Synth. Catal. 2001, 343, 338 and references
therein.
(13) (a) Zhao, G.; Wang, Q. G.; Mak, T. C. W. Organometallics 1998,
17, 3437. (b) Zhao, G.; Wang, Q. G.; Mak, T. C. W. J. Chem. Soc., Dalton
Trans. 1998, 3785.
(14) Allen, T. H. Acta Crystallogr. 2002, B58, 360.
(15) Lo´pez, C.; Caubet, A.; Pe´rez, S.; Solans, X.; Font-Bard´ıa, M. Chem.
Commun. 2004, 540.
(1) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533.
(2) (a) Jeffrey, J. C.; Rauchfuss, T. B. Inorg. Chem. 1979, 18, 2658. (b)
Bader, A.; Lindner, E. Coord. Chem. ReV. 1991, 108, 27. (c) Braunstein,
P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 480.
(3) (a) Omae, I. Applications of Organometallic Compounds; Wiley:
Chichester, U.K., 1998; Chapter 20. (b) van Leeuwen, P. W. N. M.
Homogeneous Catalysis. Understanding the Art; Kluwer Academic: Dor-
drecht, The Netherlands, 2004.
(4) Togni, A.; Hayashi, T. Ferrocenes. Homogeneous Catalysis. Organic
Synthesis and Materials Science; VCH: Weinheim, Germany, 1995.
(5) Sokolov, V. I. Chirality and Optical ActiVity in Organometallic
Compounds; Gordon and Breach: London, 1990.
(6) (a) Butler, I. R. Organometallics 1992, 11, 74. (b) Isaac, C. J.; Price,
C.; Horrocks, B. R.; Houlton, A.; Elsegood, M. R. J.; Clegg, W. J.
Organomet. Chem. 2000, 598, 248.
(16) Lo´pez, C.; Caubet, A.; Pe´rez, S.; Solans, X.; Font-Bard´ıa, M. J.
Organomet. Chem. 2003, 681, 82.
(7) (a) Caubet, A.; Lo´pez, C.; Bosque, R.; Solans, X.; Font-Bard´ıa, M.
J. Organomet. Chem. 1999, 577, 292. (b) Lo´pez, C.; Pawełczyk, A.; Solans,
X.; Font-Bard´ıa, M. Inorg. Chem. Commun. 2003, 6, 451.
(8) Lo´pez, C.; Caubet, A.; Pe´rez, S.; Solans, X.; Font-Bard´ıa, M. J.
Organomet. Chem. 2002, 651, 105.
(9) (a) Pe´rez, S.; Lo´pez, C.; Caubet, A.; Bosque, R.; Solans, X.; Font-
Bard´ıa, M.; Roig, A.; Molins, E. Organometallics 2004, 23, 224. (b) Lo´pez,
C.; Pe´rez, S.; Solans, X.; Font-Bard´ıa, M. J. Organomet. Chem. 2005, 690,
228.
(17) (a) Cativela, C.; Falvello, L. R.; Gine´s, J. C.; Navarro, R.;
Urriolabeitia, E. P. New J. Chem. 2001, 25, 344. (b) Ferna´ndez, A.; Va´zquez-
Garc´ıa, D.; Ferna´ndez, J. J.; Lo´pez-Torres, M.; Sua´rez, A.; Castro-Juiz, S.;
Vila, J. M. New J. Chem. 2002, 26, 398.
(18) Valters, R.; Flitch, W. Ring-Chain Tautomerism; Plenum Press: New
York, 1985.
(19) For the latest contributions in this field, see: (a) La´zar, L.; Fu¨lo¨p,
F. Eur. J. Org. Chem. 2003, 3025 and references therein. (b) Szatmari, I.;
Martinek, T. A.; La´zar, L.; Koch, A.; Kleinpeter, E.; Neuvonen, K.; Fu¨lo¨p,
F. J. Org. Chem. 2004, 69, 3645.
(20) Pe´rez, S.; Lo´pez, C.; Caubet, A.; Roig, A.; Molins, E. J. Org. Chem.
2005, 70, 4857.
(10) Vila, J. M.; Gayoso, E.; Pereira, M. T.; Ortigueira, J. M.; Alberdi,
G.; Marin˜o, M.; AÄ lvarez, R.; Ferna´ndez, A. Eur. J. Inorg. Chem. 2004,
2937.
10.1021/om050881o CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/06/2006

Palladacycles Containing Terdentate Ligands
Organometallics, Vol. 25, No. 3, 2006 597
Scheme 1a
a R1 ) CO2Me. Legend: (i) in MeOH followed by the addition of NaOH (in the molar ratio Pd(II):NaOH ) 1:1) dissolved in MeOH at room
temperature; (ii) PPh3 (in the molar ratio Pd:PPh3 ) 1) in CH2Cl2 at 298 K; (iii) addition of Tl[BF4] in acetone followed by the removal of the TlCl
formed; (iv) PPh3 in CDCl3; (v) NaOH in methanol; (vi) in CDCl3; (vii) addition of MeO2CCtCCO2Me (in the molar ratio alkyne:2 ) 2) in
refluxing CH2Cl2 for 2 h, followed by SiO2 column chromatography (see text).
The treatment of 1a20 with equimolar amounts of
Na2[PdCl4] and NaOAc‚3H2O in methanol at 298 K for 24 h
produced a deep purple crystalline solid. Its characterization
data agreed with those expected for [Pd{[(η5-C5H3)CHd
N(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl] (2) (Scheme 1), in which
the imine form 1b acts as a [C(sp2, ferrocene), N, O]- terdentate
ligand.
chloride (Cl(1) or Cl(2)), a carbon atom of the C5H3 ring of the
ferrocenyl moiety (C(6) (in I) or C(6A) (in II)), the imine
nitrogen (N(1) or N(2)), and the oxygen atom of the alcohol
group (O(1) (in I) or O(2) (in II)). This confirms that, in 2,
ligand 1b behaves as a [C(sp2, ferrocene), N, O]- terdentate
group. The palladium-donor atom bond lengths are similar to
those reported for related palladacycles with [C(sp2, phenyl),
N, O]- groups,14,16,17 and the bond angles around the palladium-
(II) vary from 83.3(4) to 96.2(2)° (in I) or from 82.5(4) to 94.46-
(17)° (in II).
The crystal structure of 221 consists of two nonequivalent
molecules of [Pd{[(η5-C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-
C5H5)}Cl] (hereafter referred to as I and II). In each one of the
molecules (Figure 1) the palladium(II) atom is in a slightly
distorted square-planar environment, where it is bound to a
Molecules I and II contain a [5.5.6.6] tetracyclic system
formed by the C5H3 ring of the ferrocenyl moiety, a practically
planar five-membered palladacycle, a six-membered chelate ring
(with half-chair conformation), and the phenyl group. The
phenyl rings are planar and form angles of 34.3, 33.1, and 33.0°
(in I) or 32.3, 29.1, and 33.9° (in II) with the corresponding
coordination plane, the metallacycle, and the mean plane of the
C5H3 unit. The main difference between molecules I and II
(21) Crystallographic data: C18H16ClFeNOPd, monoclinic, a ) 10.878-
(1) Å, b ) 20.934(1) Å, c ) 14.569(1) Å, R ) γ ) 90.0°, â ) 90.560(1)°,
space group P21/n, V ) 3308.6(4) Å3, Z ) 8, T ) 293(2) K, λ ) 0.710 69
Å, Dcalcd ) 1.847 g cm-3, µ ) 2.217 mm-1, F(000) ) 1824, 10 297
reflections collected, of which 7667 were assumed as independent (Rint
0.0543). R indices: R1 ) 0.0721 and wR2 ) 0.2345 (I > 2σ(I)); R1 )
0.0994 and wR2 ) 0.2556 (all data).
)

598 Organometallics, Vol. 25, No. 3, 2006
Pe´rez et al.
duplicate and none of them were coincident with those of 1a
or 1b. The analysis of the cross-peaks detected in the {1H-
1H} NOESY spectra of 3 suggested the existence of at least
two nonequivalent units of the ligand. In one of them, one of
the protons of the -CH2- moiety was close to the C5H3 ring
of the other ligand, while in the other nonequivalent unit it was
proximal to the H3 proton of the C5H5 ring. All of these findings
suggested that 3 could have higher nuclearity. Elemental
analyses of 3 did not differ significantly from those expected
for [Pd{[(η5-C5H3)CHdN(C6H4-2-CH2O)]Fe(η5-C5H5)}]n, and
the mass spectra showed an intense peak at m/z 848, which
agreed with the value expected for a dimer. In the view of the
characterization data available, we postulate for 3 the dimeric
structure presented in Scheme 1. The complexity of the 1H and
13C{1H} NMR spectra of 3 could be ascribed to several factors,
such as conformational differences of the nonplanar rings of 3
between the halves and/or the presence of isomeric forms of 3
in solution. Since the cyclopalladation produces 1,2-disubstituted
ferrocenyl moieties with planar chirality, the formation of
diastereomers of 3 could also be expected on principle.
Figure 1. ORTEP diagrams of the two nonequivalent molecules
(I and II) present in the crystal structure of [Pd{[(η5-C5H3)CHd
N(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl] (2). Hydrogen atoms have
been omitted for clarity. Selected bond lengths (in Å) and angles
(in deg): Pd(1)-C(6), 1.940(11); Pd(1)-N(1), 2.059(9); Pd(1)-
O(1), 2.190(8); Pd(1)-Cl(1), 2.289(3); Pd(2)-C(6A), 1.914(10);
Pd(2)-N(2), 1.033(8); Pd(2)-O(2), 2.191(8); Pd(2)-Cl(2), 2.292-
(2); C(6)-Pd(1)-N(1), 83.2(4); N(1)-Pd(1)-O(1), 90.0(3); O(1)-
Pd(1)-Cl(1), 96.5(2); C(6)-Pd(1)-Cl(1), 90.7(4); C(6A)-Pd(2)-
N(2), 82.5(4); N(2)-Pd(2)-O(2), 89.2(3); O(2)-Pd(2)-Cl(2),
94.52(18); C(6A)-Pd(2)-Cl(2), 94.3(3).
Examples of palladium(II) compounds with central “Pd2O2”
cores have been described,11c,12,14,26 and the X-ray crystal
structures reported so far show that when these sorts of
compounds contain (L,L′) bidentate ligands, the two identical
donor atoms of the two (L,L′) ligands are in a trans
arrangement.11c,12,14,26 The self-assembly of the two “[Pd{[(η5-
C5H3)CHdN(C6H4-2-CH2O)]Fe(η5-C5H5)}]” units forming the
central “Pd2O2” ring of 3 is only possible if the metalated
carbons of each one of the moieties are in a trans arrangement.
VT-NMR sudies of 3 (in CDCl3) showed that the relative
intensities of the two sets of signals decreased upon cooling
(from 1.0:1.0 (at 298 K) to 1.0:0.7 (at 223 K)) and the formation
of small amounts of insoluble materials was detected. These
findings could be rationalized by assuming the coexistence of
isomeric forms (in a nearly 1:1 molar ratio at 298 K) with
different solubilities at low temperatures.
arises from the planar chirality of the “(η5-C5H5)Fe(η5-C5H3)”
moieties (Sp in I and Rp in II).
The intramolecular and the minimum intermolecular separa-
tions between the iron(II) and palladium(II) atoms are clearly
greater than the sum of their van der Waals radii.22
Bond lengths and angles of the ferrocenyl moieties agree with
data reported for most ferrocene derivatives;14 the pentagonal
rings are practically parallel (tilt angles 1.3 and 2.6° in I and
II) and they deviate by 13.3° (in I) or 2.6° (in II) from the
ideal eclipsed conformation.
Since the molar ratio 1a:1b is dependent on the solvent,20
we also studied the reactivity of 1a with cis-[PdCl2(PhCN)2]
under different experimental conditions (reaction periods in the
range 3 h to 1 day) using equimolar amounts of 1a and cis-
[PdCl2(PhCN)2] (or this reagent and NaAcO) as reactants and
in refluxing toluene or toluene/methanol (5:1) mixtures. Un-
fortunately, in all these reactions the formation of metallic
palladium was detected, even for the shortest reaction periods.23
This suggests that the reduction of the palladium(II) could be
promoted by the ferrocenyl substrate. In fact, electrochemical
studies20 have demonstrated that 1a is more prone to oxidation
than the ferrocenylimines [(η5-C5H5)Fe{(η5-C5H4)CHdN(R1)}]-
(R1 ) substituted phenyl ring).9a,24
In 3 the planar chiralities of the ferrocenyl moieties could be
different ((Rp and Sp)) or identical ((Rp,Rp) or (Sp,Sp)). The
coexistence of these species in solution would produce two sets
of signals in the NMR spectra, one due to the centrosymmetric
form and the other to the noncentrosymmetric species, respec-
tively. The use of molecular models for these forms revealed
that in both cases the environment of the palladium(II) atoms
is rather crowded. In the centrosymmetric form one of the
-CH2- protons is close to the C5H5 group of the other half of
the molecule (this is reflected in a high-field shift of the
resonance), while for the non-centrosymmetric form it is near
the H3 proton. This arrangement of the groups introduces severe
restrictions on the conformational changes of the six-membered
chelate rings.
Treatment of 2 with PPh3 in CH2Cl2 gave [Pd{[(η5-C5H3)-
CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl (PPh3)] (4; Scheme 1,
step B), where the ligand acts as a [C(sp2, ferrocene), N]- group
and the PPh3 is in a cis arrangement to the metalated carbon
atom, in good agreement with the transphobia effect.27Complex
4 results from the cleavage of the σ(Pd-OH) bond and the
To attempt the synthesis of palladacycles having a dianionic
[C(sp2, ferrocene), N, O]2- ligand, 2 was treated with NaOH in
methanol at 298 K. This led to a deep purple solid (3; Scheme
1, step A). Its IR spectrum did not show the typical band due
to the stretching of the -OH group,25 thus suggesting that
1
deprotonation of the alcohol group had taken place. In the H
and 13C{1H} NMR spectra most of the signals appeared in
(22) (a) Bondi, J. J. Phys. Chem. 1964, 64, 441. (b) Kitaigorodskii, A.
I. Molecular Crystals and Molecules; Academic Press: London, 1973.
(23) Only the signals due to ferrocenecarboxaldehyde and the amine H2-
NC6H4-2-CH2OH could be identified in the spectrum. These two products
are possibly generated by the hydrolysis of 1b.
(24) (a) Bosque, R.; Lo´pez, C.; Sales, J. Inorg. Chim. Acta 1996, 244,
141. (b) Lo´pez, C.; Bosque, R.; Pe´rez, S.; Roig, A.; Molins, E.; Solans, X.;
Font-Bard´ıa. M. J. Organomet. Chem., in press.
(26) (a) Kim, J. S.; Sen, A.; Guzei, I. A.; Liable-Sands, L. M.; Rheingold,
A. L. Dalton Trans. 2002, 4726. (b) Getty, A. D.; Goldberg, K. I.
Organometallics 2001, 20, 2545. (c) Dura-Vila, V.; Mingos, D. M. P.; Vilar,
R.; White, A. J. F.; Williams, D. J. Chem. Commun. 2000, 1525. (d) Mann,
G.; Shelby, Q.; Roy, A. H.; Hartwig, J. F. Organometallics 2003, 22, 2775.
(27) (a) Vicente, J.; Arcas, A.; Bautista, D.; Jones, P. G. Organometallics
1997, 16, 2127. (b) Vicente, J.; Abad, J. A.; Frankland, A. D.; Ram´ırez de
Arellano, M. C. Chem. Eur. J. 1999, 5, 30668. (c) Vicente, J.; Abad, J. A.;
Mart´ınez-Viviente, E.; Jones, P. G. Organometallics 2002, 21, 4454.
(25) Nakamoto, K. IR and Raman Spectra of Inorganic and Coordination
Compounds, 5th ed.; Wiley: New York, 1997.

Palladacycles Containing Terdentate Ligands
Organometallics, Vol. 25, No. 3, 2006 599
binding of the PPh3. This process is fast at ca. 293 K and
involves a change of the coordination mode of the ligand from
[C(sp2, ferrocene), N, O]- (in 2) to [C(sp2, ferrocene), N]-
(in 4).
As a first approach to achieve palladacycles containing
simultaneously a [C(sp2, ferrocene), N, O]- ligand and a PPh3
group, the reactions shown in steps C and D of Scheme 1 were
studied. In both cases, [Pd{[(η5-C5H3)CHdN(C6H4-2-CH2OH)]-
Fe(η5-C5H5)}(PPh3)][BF4] (5) was isolated. The differences
detected in the 31P{1H} NMR spectra of 5 and 4 are similar to
those reported for [Pd{(4,5-MeO)2C6H2CHdN(CH2CH2OH)}-
Cl(PPh3)] and [Pd{(4,5-MeO)2C6H2CHdN(CH2CH2OH)}(PPh3)]-
ClO4, which contain a [C(sp2, phenyl), N]- and a [C(sp2,
phenyl), N, O]- ligand, respectively.17a
The addition of PPh3 to a solution of 3 (molar ratio PPh3:Pd
) 1) in CDCl3 gave [Pd{[(η5-C5H3)CHdN(C6H4-2-CH2O)]Fe-
(η5-C5H5)}(PPh3)] (6; Scheme 1, step E). The reactivity of 3
with PPh3 is similar to that of [Pd{C6H4CHdN(C6H4-2-O)}]4,
which, under identical experimental conditions, gave [Pd{C6H4-
CHdN(C6H4-2-O)}(PPh3)], also containing a terdentate [C(sp2,
phenyl), N, O]2- ligand.16 Comparison of the results obtained
in the reactions of 2 and 3 with PPh3 reveal that the Pd-OH
bond (in 2) is more labile than the Pd-Oalkoxo bond (in 3).
When a CDCl3 solution of 4 was treated with the stoichio-
metric amount of NaOD dissolved in methanol (Scheme 2, step
F), the 1H NMR spectrum of the crude reaction mixture revealed
the coexistence of 6 and 4 in the molar ratio 6:4 ) 0.4.
Furthermore, the 1H NMR spectrum of a freshly prepared
solution of 6 (in CDCl3) changed with time. After several
hours of storage the presence of 4 was also detected (Scheme
2, step F′) and the molar ratio 6:4 decreased slowly with
time,28 thus indicating that 6 was less stable than 4 in CDCl3 at
298 K.
57Fe Mo¨ssbauer spectra of 2-429 consisted of a single
quadrupole doublet, indicating a unique iron site. For 3, the
value of the Γ parameter is similar to those of 2 and 4, thus
suggesting that in the solid state the environments of the two
iron(II) centers of 3 should be very similar at 80 K. Previous
studies on 57Fe Mo¨ssbauer spectroscopy of ferrocene derivatives
have shown that the quadrupole splitting parameter (∆Eq) is
strongly sensitive to the nature of the substituents.30 In particular,
it is widely accepted that electron-withdrawing substituents
produce a decrease of the ∆Eq value, relative to ferrocene, while
for electron-donating groups the effect is opposite.30 The ∆Eq
values obtained for 2-429 are smaller than that of ferrocene
(∆Eq ) 2.37 mm/s (at 298 K) and 2.41 mm/s (at 80 K)).31 This
suggests that the formation of the palladacycles has an electron-
withdrawing effect upon the iron(II). A comparison of the ∆Eq
parameters for 229 and [Pd{[(η5-C5H3)CHdN(C6H4-2-SMe)]Fe-
(η5-C5H5)}Cl]9a reveals that in these cyclopalladated complexes
the replacement of the -CH2OH substituent (in 2) by an -SMe
Figure 2. Cyclic voltammograms of the monomeric compounds
[Pd{[(η5-C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl] (2) and
[Pd{[(η5-C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl(PPh3)] (4)
and of the dinuclear product [Pd{[(η5-C5H3)CHdN(C6H4-2-CH2O)]-
Fe(η5-C5H5)}]2 (3) in CH3CN at 293 K (the potentials are referred
to the ferrocene/ferrocenium couple).
group produces an increase of the electron-pulling character of
the substituents on the “(η5-C5H5)Fe(η5-C5H3)” moiety. Ad-
ditionally, the differences detected in the ∆Eq values of 2-4
confirm that the intensity of this effect is dependent on small
changes on the heteroatoms bound to the palladium(II) that are
at a distance of three bonds from the iron(II).
To elucidate the influence of the mode of binding of the lig-
and in 2-4 upon the proclivity of the iron(II) toward oxidation,
electrochemical studies of 2-4, based on cyclic voltammetry,
were also performed.32 The cyclic voltammogram of 2 (Figure
2) showed an anodic peak directly associated with a cathodic
one in the reverse scan. The wave is shifted to a more anodic
area than for ligand 1a. This indicates, according to the
literature,33 that the substituents bound to the ferrocenyl moiety
in 2 have a greater electron-withdrawing character than in 1a.
For 2, the values of the cathodic and anodic potentials are similar
to those of [Pd{[(η5-C5H3)CHdN(R1)]Fe(η5-C5H5)}Cl(L)] (with
R1 ) phenyl group and L ) neutral ligand), in which the imine
1
acts as a bidentate [C(sp2, ferrocene), N]- group.24a The H
NMR spectrum of 2 (in acetonitrile-d3) suggested that this
solvent induced the cleavage of the Pd-OH bond, giving [Pd-
{[(η5-C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl(acetonitrile-
d3)] (7). This confirms the lability of the terminal σ(Pd-OH)
bond in 2. The cyclic voltammogram of 4 (Figure 2) shifted to
more anodic potentials than for 2, and in this case, the shape of
the wave, together with the larger ∆E value and the lack of a
linear relationship between the Ep,a values and the square-root
of the scan rates (υ1/2), suggested a quasi-reversible electro-
(28) The progress of the transformation of 6 (in CDCl3) into 4 was
monitored by 1H NMR. The molar ratio 6:4 after different periods of storage
at 298 K was 4.2 (24 h), 2.3 (48 h), 1.3 (96 h), and 0.9 (5 days; in this case
the presence of traces of ferrocenecarboxaldehyde and other byproducts
was also detected).
(32) Summary of electrochemical data (in mV), with anodic (Ep,a) and
cathodic (Ep,c) potentials and the separation between peaks (∆E) for 2-4:
for 2, Ep,a ) 118, Ep,c ) 27, and ∆E ) 91; for 3. Ep,a(1) ) 99, Ep,a(2) )
291, Ep,c(1) ) 168, and Ep,c(2) ) 12 (see Figure 2), and ∆E ) 123 and ca.
87; for 4, Ep,a ) 214, Ep,c ) 37, and ∆E ) 177.
(33) (a) Little, W. F.; Reilley, C. N., Johnson, J. D.; Saunders: A. P. J.
Am. Chem. Soc. 1964, 86, 1376. (b) Zanello, P.; Cinquantini, A.; Mangani,
S.; Opromolla, G.; Pardi, L.; Janiak, C.; Rausch, M. D. J. Organomet. Chem.
1994, 471, 171. (c) Benito, A.; Cano, J.; Mart´ınez-Man˜ez, R.; Soto, J.; Paya´,
J.; Lloret, F.; Julve, M.; Faus, J.; Cano, M. D. Inorg. Chem. 1993, 32, 1192.
(d) Louati, A.; Gross, M.; Duce, L.; Matt, D. J. Organomet. Chem. 1993,
463, 169. (e) Bracci, M.; Ercolani, E.; Floris, B.; Bassetti, M.; Chiesi-Villa,
A.; Guastini, C. J. Chem. Soc., Dalton Trans. 1990, 1357.
(29) Summary of 57Fe Mo¨ssbauer hyperfine parameters (isomer shift (IS)-
, quadrupole splitting (∆Eq), and full-width at half-height (Γ, in mm/s) at
80 K): for 2, IS ) 0.475(4), ∆Eq ) 2.122(4), and Γ ) 0.25(1); for 3, IS
) 0.527(2), ∆Eq ) 2.146(3), and Γ ) 0.24(1); for 4, IS ) 0.492(4), ∆Eq
) 2.164(6), and Γ ) 0.233(8).
(30) (a) Houlton, A.; Miller, J. R.; Silver, J.; Jassim, N.; Ahmet, M. J.;
Axon, T. L.; Bloor, D.; Cross, G. H. Inorg. Chim. Acta 1993, 205, 65. (b)
Houlton, A.; Miller, J. R.; Roberts, R. M. G.; Silver, J. J. Chem. Soc., Dalton
Trans. 1991, 467.
(31) Houlton, A., Bishop, P. T.; Roberts, R. M. G.; Silver, J.; Herberhold,
M. J. Organomet. Chem. 1989, 364, 381.

600 Organometallics, Vol. 25, No. 3, 2006
Pe´rez et al.
Chart 1
we also studied the reactions of 2 and 3 with MeO2CCtCCO2-
Me. Treatment of 2 with this alkyne (molar ratio alkyne:Pd )
2) under reflux for 2 h allowed us to isolate two products (9
and 10, in the molar ratio 9:10 ) 3.2) (Scheme 1, step G) which
were identified as [Pd{[(MeO2CCdCCO2Me)2(η5-C5H3)CHd
N(C6H4-2-CH2OH)]Fe(η5-C5H5)Cl] and [Pd{[(MeO2CCdCCO2-
Me)(η5-C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl], re-
spectively. Compound 10, which is formed by insertion of one
molecule of the alkyne into the σ(Pd-C(sp2, ferrocene)) bond
of 2, was isolated in higher yield when the reaction was
performed using equimolar amounts of the palladium(II)
complex and the alkyne and shorter refluxing periods (30 min).
The relative arrangements of the -CO2Me substituents on the
alkene, presented in Scheme 1 for 9 and 10, are the common
ones described for all the palladacycles arising from mono or
bis insertion of alkynes that have been characterized by X-ray
diffraction.14,38-40 A comparison of these results and those
obtained for [Pd{[(η5-C5H3)CHdNCH2(CH2)nNMe2]Fe(η5-
C5H5)}Cl]39 and [Pd{[(η5-C5H3)CHdN(C6H4-2-SMe)]Fe(η5-
C5H5)}Cl]9b suggests that for complexes with a terdentate [C(sp2,
ferrocene), N, X]- (X ) O, S, N) the reactivity of the Pd-
C(sp2, ferrocene) bond increases according to the sequence
chemical process.34 The differences between the Ep,a values of
7 and 4 suggest that 7 is more prone to oxidation than 4.
The cyclic voltammogram of 3 (Figure 2) was more complex
than those of 2 and 4 and showed two anodic peaks, in contrast
with the results reported for the polynuclear cyclopalladated
complex 8, depicted in Chart 1, for which only one wave was
detectable in the cyclic voltammogram under identical experi-
mental conditions.35 The differences detected in the electro-
chemical behaviors of 3 and 8 suggest that the replacement of
the acetato bridges (in 8) by the alkoxo oxygen (in 3) is
important in allowing communication between the two iron (II)
centers.
It is well-known that palladacycles are useful precursors in
the synthesis of organic or organometallic compounds.35-41 Most
of these reactions involve the insertion of small molecules, i.e.
alkynes, alkenes, CO, or isonitriles, into the σ(Pd-C) bond of
complexes containing bidentate [C, N]- ligands.36-38 Previous
studies on the alkyne insertion processes have shown that the
nature of the final product formed depends on a wide variety
of factors, including the nature of the metalated carbon, the
electron-withdrawing/electron-donor nature of the substituents
on the alkyne, the stoichiometry of the reaction, the remaining
ligands bound to the palladium(II), etc. However, parallel studies
on palladacycles with terdentate [C(sp2, ferrocene), N, X]q-
chelating groups are not common.9b,40 To the best of our
knowledge, only the reactions of [Pd{[(η5-C5H3)CHdNCH2-
(CH2)nNMe2]Fe(η5-C5H5)Cl] and [Pd{[(η5-C5H3)CHdN(C6H4-
2-SMe)]Fe(η5-C5H5)}Cl] with the alkynes R1CtCR1 (R1 )
CO2Me, Et, Ph) have been described so far.9b,40 In view of this
and in order to elucidate if the nature of the heteroatom X could
affect the reactivity of the σ(Pd-C(sp2, ferrocene)) bond of
complexes with a terdentate [C(sp2, ferrocene), N, X]q- ligand,
Namine < Sthioether < Oalcohol
.
Furthermore, the results obtained for 2 differ from those
reported for [Pd{[(η5-C5H3)CHdN(C6H4-2-SMe)]Fe(η5-C5H5)}-
Cl], for which the reaction with MeO2CCtCCO2Me (in the
molar ratios Pd(II):alkyne ) 1:1 and 1:2) gave [Pd{[(MeO2-
CCdCCO2Me)(η5-C5H3)CHdN(C6H4-2-SMe)]Fe(η5-C5H5)}-
Cl] (11) exclusively.9b Since 10 and 11 contain a [C(sp2, alkene),
N, X]- (X ) O (in 10), S (in 11)) ligand, these findings indicate
that the σ(Pd-C(sp2, alkene)) bond of 10 is more likely to
undergo the insertion of MeO2CCtCCO2Me than when X is
sulfur.
When 3 was treated with MeO2CCtCCO2Me (in the molar
ratio alkyne:Pd ) 1 or 2) and refluxed for periods varying
between 1 h and 1 day, the starting materials were recovered
unchanged and evidence of the formation of any product arising
from the insertion of the alkyne into the σ(Pd-C(sp2, ferrocene))
bond of 3 was not detected by NMR analyses of the crude
mixture of these reactions. This suggests that 2 is more prone
to react with the alkyne than 3.
To sum up, the results presented here provide conclusive
proof of the relevancy of the tautomeric equilibrium between
1a and 1b upon their reactivity with palladium(II). In solvents
such as benzene, where the heterocyclic form is the major
component,20 the reduction of the palladium(II) takes place. This
process may be induced by heterocyclic form 1a, which is more
prone to oxidation than 1b.20 In contrast with these findings,
when 1a reacted with Na2[PdCl4] and NaOAc‚3H2O in methanol
(in which the molar ratio 1a:1b ) 0.5), the complex [Pd{[(η5-
C5H3)CHdN(C6H4-2-CH2OH)]Fe(η5-C5H5)}Cl] (2, with a [C(sp2,
ferrocene), N, O]- ligand) was isolated. Complex 2 is addition-
ally of interest, due to its high reactivity with NaOH, PPh3, and
the alkyne that allows its use as a precursor in the synthesis of
other organometallic palladium(II) compounds having ferrocenyl
units. In particular, the reaction of 2 with NaOH revealed that
the OH group could be easily deprotonated, giving [Pd{[(η5-
C5H3)CHdN(C6H4-2-CH2O)]Fe(η5-C5H5)}]2 (3), which forms
by the self-assembly of two cyclopalladated units in which the
imino alcohol (1b) behaves as a [C(sp2, ferrocene), N, O]2-
terdentate ligand.
(34) (a) Brown, E. R.; Sandifer, J. R. In Physical Methods in Chemistry.
Electrochemical Methods; Rossiter, B. W., Hamilton, J. H., Eds.; Wiley:
New York, 1986; Vol. 4, Chapter 4. (b) Zanello, P. Inorganic Electro-
chemistry. Theory, Practice and Application; RCS: Northhampton, U.K.,
2003.
(35) Moyano, A.; Rosol, M.; Moreno, R. M.; Lo´pez, C.; Maestro, M.
A. Angew. Chem., Int. Ed. 2005, 44, 1865.
(36) For a general overview on this field: (a) Spencer, J.; Pfeffer, M.
AdV. Inorg. Met.-Org. Chem. 1998, 6, 103. (b) Pfeffer, M. Recl. TraV. Chim.
Pays-Bas 1990, 109, 567. (c) Ryabov, A. D. Synthesis 1985, 233.
(37) Vicente, J.; Saura-Llamas, I.; Gru¨nwald, C.; Alcaraz, C.; Jones, P.
G.; Delia, B. Organometallics 2002, 21, 3587.
(38) For articles focusing on the reactivity of palladacycles with bidentate
[C(sp2), N]- or [C(sp3), N]- ligands, see: (a) Ryabov, A. D.; van Eldik,
R.; Le Borgne, G. Organometallics 1993, 12, 1386. (b) Vicente, J.; Saura-
Llamas, I.; Ram´ırez de Arellano, M. C. J. Chem. Soc., Dalton Trans. 1995,
2529 and references therein. (c) Gu¨l, N.; Nelson, J. H.; Willis, A. C.; Rae,
A. D. Organometallics 2002, 21, 2041.
(39) For recent contributions on the insertion of alkynes into the σ(Pd-
C(sp2, ferrocene)) bond of palladacycles, see for instance: (a) Zhao, G.;
Wang, Q. G.; Mak, T. C. W., J. Organomet. Chem. 1999, 574, 311. (b)
Benito, M.; Lo´pez, C.; Morvan, X.; Solans, X.; Font-Bard´ıa, M., Dalton
Trans. 2000, 4470. (c) Bosque, R.; Benito, M.; Lo´pez, C. New J. Chem.
2001, 25, 827. (d) Lo´pez, C.; Caubet, A.; Solans, X.; Font-Bard´ıa, M. J.
Organomet. Chem. 2000, 598, 87.
(40) Pe´rez, S.; Lo´pez, C.; Caubet, A.; Pawełczyk, A.; Solans, X.; Font-
Bard´ıa, M. Organometallics 2003, 22, 2396.
(41) (a) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. ReV. 2005, 105,
2527. (b) Bedford, R. D. Chem. Commun. 2003, 1787.
On the other hand, the studies presented show that the binding
of the oxygen of the -OH group (in 2) or of its deprotonated
form (in 3) clearly affects the reactivity of this sort of compound.
The reactions of 2 and 3 with PPh3 allow a fine tuning of the

Palladacycles Containing Terdentate Ligands
Organometallics, Vol. 25, No. 3, 2006 601
hapticity and mode of binding of 1b ([C(sp2, ferrocene), N]-
(in 4), [C(sp2, ferrocene), N, O]- (in 5), or [C(sp2, ferrocene),
N, O]2- (in 6)) in the final products by the proper selection of
the starting palladium(II) complex (2 or 3) and the experimental
conditions. Additionally, a comparison of the results obtained
indicated that the Pd-OH bond of 2 is more labile than the
Pd-Oalkoxo bond of 3. A comparative study of the reactivity of
the three types of palladacycles containing a [C(sp2, ferrocene),
N, X]- ligand revealed that the ease with which the insertion
of the alkyne takes place depends on the nature of the
heteroatom X (X ) N, S, O).
It is well-known that palladium(II) compounds with a σ(Pd-
O) bond, cyclopalladated complexes, and products formed by
insertion of alkynes into the σ(Pd-C) bond have a variety of
applications in several fields,11,12,36-42 including organic and
organometallic synthesis36-41 and homogeneous catalysis;11,12
the new products presented here appear to be excellent
candidates to be studied in these fields.
Acknowledgment. This work was supported by the Minis-
terio de Ciencia y Tecnolog´ıa of Spain.
Supporting Information Available: Text giving experimental
1
and spectroscopic details for all the compounds, H NMR spectra
of 3 (Figure S1), 57Fe Mo¨ssbauer spectra of 2-4 (Figure S2), and
full details of crystallographic analyses of 2 in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(42) (a) Klaus, A. J. In Modern Colorants: Synthesis and Structure;
Peters, A. E., Freeman, H. S., Eds.; Blackie Academic: London, 1995; p
1. (b) Thompson, N. J.; Serrano, J. L.; Baena, M. J.; Espinet, P. Chem.
Eur. J. 1996, 2, 214. (c) Torres-Lo´pez, M.; Ferna´ndez, A.; Ferna´ndez, J.
J.; Sua´rez, A.; Castro-Juiz, S.; Vila, J. M.; Pereira, M. T. Organometallics
2001, 20, 1350.
OM050881O