Ferrocene-based pincer complexes of palladium: Synthesis, structures, and spectroscopic and electrochemical properties

Article abstract of DOI:10.1021/om0497285

Palladium P,C,P pincer complexes based on ferrocene were synthesized for the first time. The reaction of the 1,3-bis((dialkylphosphino)methyl)ferrocenes {1,3-(R₂PCH₂)₂C₅H ₃}Fe(C₅H₅ (2a, R = Prⁱ; 2b, R = Bu^t) with PdCl₂(NCPh)₂ in refluxing 2-methoxyethanol leads to the corresponding pincer complexes PdCl[{2,5-(R ₂PCH₂)₂C₅H₅)] (3a,b). The ferrocene-based binuclear compound 3b reacts with [Cp₂Fe]PF ₆ to form the ferrocenium-based pincer complex {PdCl[{2,5-(Bu ^t₂PCH₂)₂C₅H ₂}Fe(C₅H₅)]}PF₆ (4b), and this is the first example of ferrocene-based phosphine chelate oxidation centered on the iron atom. Treatment of complexes 3a,b with NaBH₄ in refluxing ethanol affords the complexes Pd(BH₄)[{2,5-(R₂-PCH ₂)₂C₅H₂}Fe(C₅H ₅)] (5a, R = Prⁱ; 5b, R = Bu^t), containing a BH₄^- group coordinated to the metal in the rarely occurring unidentate mode. The structures of 3a, b and 5b were confirmed by X-ray analyses. Cyclic voltammetric investigations of complexes 2b and 3a, b are also reported. The structural and spectroscopic features of the palladium ferrocenebased pincer complexes are discussed and compared with those of related compounds.

Full text of DOI:10.1021/om0497285

Organometallics 2004, 23, 4585-4593
4585
F er r ocen e-Ba sed P in cer Com p lexes of P a lla d iu m :
Syn th esis, Str u ctu r es, a n d Sp ectr oscop ic a n d
Electr och em ica l P r op er ties
Avthandil A. Koridze,*,†,‡,§ Sergey A. Kuklin,† Aleksey M. Sheloumov,†
Fedor M. Dolgushin,† Veronika Yu. Lagunova,† Irina I. Petukhova,†
Mariam G. Ezernitskaya,† Alexander S. Peregudov,† Pavel V. Petrovskii,†
Evgenii V. Vorontsov,† Miguel Baya,| and Rinaldo Poli*,|
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 Vavilov Street, 119991 Moscow, Russian Federation, I. J avakhishvili Tbilisi State
University, 1 Chavchavadze Avenue, 380028 Tbilisi, Georgia, and Laboratoire de Chimie de
Coordination, 205 Route de Narbonne, 31077 Toulouse Cedex, France
Received April 13, 2004
Palladium P,C,P pincer complexes based on ferrocene were synthesized for the first time.
The reaction of the 1,3-bis((dialkylphosphino)methyl)ferrocenes {1,3-(R2PCH2)2C5H3}Fe(C5H5)
(2a , R ) Pri; 2b, R ) But) with PdCl2(NCPh)2 in refluxing 2-methoxyethanol leads to the
corresponding pincer complexes PdCl[{2,5-(R2PCH2)2C5H2}Fe(C5H5)] (3a ,b). The ferrocene-
based binuclear compound 3b reacts with [Cp2Fe]PF6 to form the ferrocenium-based pincer
complex {PdCl[{2,5-(But PCH2)2C5H2}Fe(C5H5)]}PF6 (4b), and this is the first example of
2
ferrocene-based phosphine chelate oxidation centered on the iron atom. Treatment of
complexes 3a ,b with NaBH4 in refluxing ethanol affords the complexes Pd(BH4)[{2,5-(R2-
-
PCH2)2C5H2}Fe(C5H5)] (5a , R ) Pri; 5b, R ) But), containing a BH4 group coordinated to
the metal in the rarely occurring unidentate mode. The structures of 3a ,b and 5b were
confirmed by X-ray analyses. Cyclic voltammetric investigations of complexes 2b and 3a ,b
are also reported. The structural and spectroscopic features of the palladium ferrocene-
based pincer complexes are discussed and compared with those of related compounds.
In tr od u ction
ation,5 Suzuki aryl coupling,6 enantioselective aldol type
condensation of methyl isocyanoacetate and aldehydes,7
transfer hydrogenation of ketones,8 polymeriza-
tion of phenylacetylene,9 and alkane dehydrogena-
tion.2d,10 It is remarkable that palladium P,C,P pincer
Transition-metal complexes with tridentate monoan-
ionic ligands, so-called “pincer” ligands,1 of general
formula [2,6-(ECH2)2C6H3]-, where E is a neutral two-
electron donor such as NR2, PR2, AsR2, OR, or SR, show
interesting and rich chemistry.2 The first pincer com-
plexes were synthesized on a base of 1,3-bis((di-tert-
butylphosphino)methyl)benzene by Moulton and Shaw3
as early as 1976, but interest and activity in this area
have rapidly increased, especially during the past
decade. The presence of one metal-carbon σ bond and
two chelating donor atoms are responsible for the high
thermal stability of pincer complexes. The highly pro-
tective environment for the multiply bound metal and
possibilities to tune the properties of the metal center
gave impetus to extensive research on the use of pincer
complexes as homogeneous catalysts,2 as well as crys-
talline switches and sensors.2c
(4) van Koten, G.; J astrzebski, J . T. B. H. J . Mol. Catal. A: Chem.
1999, 146, 317.
(5) (a) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J . Am.
Chem. Soc. 1997, 119, 11687. (b) Beletskaya, I. P.; Chuchurjukin, A.
V.; Dijkstra, H. P.; van Klink, G. P. M.; van Koten, G. Tetrahedron
Lett. 2000, 411075. (c) Morales-Morales, D.; Redon, R.; Yang, C.;
J ensen, C. M. Chem. Commun. 2000, 1619. (d) Morales-Morales, D.;
Grause, C.; Kasaoka, K.; Redon, R.; Cramer, R. E.; J ensen, C. M. Inorg.
Chim. Acta 2000, 300-302, 958. (e) Morales-Morales, D.; Cramer, R.
E.; J ensen, C. M. J . Organomet. Chem. 2002, 654, 44.
(6) Bedford, R. B.; Draper, S. M.; Scully, P. N.; Welch, S. L. New J .
Chem. 2000, 24, 745.
(7) (a) Gorla, F.; Togni, A.; Venanzi, L. Organometallics 1994, 13,
1607. (b) Zhang, X.; Longmire, J . M.; Shang, M. Organometallics 1998,
17, 4373.
(8) Dani, P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G.
Angew. Chem., Int. Ed. 2000, 39, 743.
Pincer complexes catalyze various transformations of
(9) Yao, J .; Wong, W. T.; J ia, G. J . Organomet. Chem. 2000, 598,
228.
organics, such as Kharasch addition,4 Heck olefin aryl-
(10) (a) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.;
Fan, H.-J .; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40, 3596. (b)
Mohammad, H. A. Y.; Grimm, J . C.; Eichele, K.; Mack, H.-G.; Speiser,
B.; Novak, F.; Quintanilla, M. G.; Kaska, W. C.; Mayer, H. A.;
Organometallics 2002, 21, 5775. (c) Krogh-J espersen, K.; Czerw, M.;
Goldman, A. S. J . Mol. Catal. A: Chem. 2002, 189, 95. (d) Krogh-
J espersen, K.; Czerw, M.; Renkema, K. B.; Achord, P. D.; Goldman,
A. S. J . Am. Chem. Soc. 2002, 124, 11404. (e) Liu, F.; Goldman, A. S.
Chem. Commun. 1999, 655. (f) Gupta, M.; Hagen, C.; Kaska, W. C.;
Flesher, R.; J ensen, C. M. J . Chem. Soc., Chem. Commun. 1996, 2083.
(g) Xu, W.-W.; Rosini, G. P.; Gupta, M.; J ensen, C. M.; Kaska, W. C.;
Krogh-J espersen, K.; Goldman, A. S. J . Chem. Soc., Chem. Commun.
1997, 2273.
† Russian Academy of Sciences.
‡ I. J avakhishvili Tbilisi State University.
§ Address correspondence to this author at the Russian Academy of
Sciences.
| Laboratoire de Chimie de Coordination.
(1) van Koten, G. Pure Appl. Chem. 1989, 61, 1681.
(2) For reviews, see: (a) van der Boom, M. E.; Milstein, D. Chem.
Rev. 2003, 103, 1759. (b) Singleton, J . T. Tetrahedron 2003, 59, 1837.
(c) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750.
(d) J ensen, C. M. Chem. Commun. 1999, 2443.
(3) Moulton, C. J .; Shaw, B. L. J . Chem. Soc., Dalton Trans. 1976,
1020.
10.1021/om0497285 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 08/31/2004

4586 Organometallics, Vol. 23, No. 20, 2004
Koridze et al.
Ch a r t 1
into less valuable internal alkenes. It should be also
noted that the iron group metallocenes possess high
thermal stability.
The possibility of designing metallocene-based P,C,P
pincer complexes was not evident, since the P donor
atoms in 1,3-bis((diorganylphosphino)methyl)metallocenes
are more remote from each other as compared to those
in the benzene analogues. Metallocene-based pincer
complexes might be expected to be formed only at the
expense of a deviation of the bond angles at the C(2)
and C(5) centers in pincer complexes from the standard
value of 126° for the cyclopentadienyl ring.
complexes catalyze the Heck reaction with turnover
numbers (TON) as high as 500 000.5a Hydridoiridium
P,C,P pincer complexes proved to be the most active
(TON up to 1200) among all known homogeneous
catalysts for alkane dehydrogenation.10e
Although impressive progress has been achieved in
this area, the majority of pincer complexes have not find
industrial applications for a variety of reasons. For
example, the reaction rates are still insufficient, asym-
metric aldol addition gives only moderate (65%) ee’s of
the product, the rates of alkane dehydrogenation by
iridium complexes are also insufficient, and product
inhibition and isomerization of the primarily formed
1-alkenes into internal isomers take place.
In early 2002, we published11 preliminary results on
the synthesis and X-ray structure of the first represen-
tative of ferrocene-based P,C,P pincer complexes, the
rhodium derivative cis-RhCl2(CO)[{2,5-(Pri2PCH2)2C5H2}-
Fe(C5H5)] (1) (eq 1).
It is obvious that a search for approaches to the design
of novel pincer complexes is a topical problem. One way
of solving this problem is to substantially modify the
pincer ligand, including a change of its electronic and
steric properties.
It is rather remarkable that there have been no
reports until recently on attempts to design pincer
complexes based on aromatic homocyclic systems other
than benzene: viz. the cyclopentadienyl ring of metal-
locenes. At the same time, the design of conceptually
novel bimetallic pincer systems based on metallocene
seems to be very attractive. We believe that metallocene-
based pincer complexes of type B could possess elec-
tronic and steric features which distinguish them from
their benzene analogues A and are attractive from the
viewpoint of their use in catalysis (Chart 1).
These features are as follows.11 First, the central atom
of the metallocene unit in metallocene-based pincer
complexes is located in the vicinity (â-position) of the
catalytic center, the chelated metal atom M; this ar-
rangement can facilitate tuning of electronic effects on
the atom M by varying the metallocene central atom
(Fe, Ru, and Os). Second, the iron atom in ferrocene can
be involved in a rapid and reversible redox reaction,
thus providing an additional possibility to tune the
electron density at the catalytic center. Third, the
sandwich nature of ferrocene allows one to design
planar-chiral molecules (the known benzene-based P,C,P
chiral ligands usually possess elements of central chiral-
ity at the benzylic positions, which are remote from the
catalytic center M), in which the chirality of the P,C,P’
ligand results from the presence of different substitu-
ents at the P donor atoms; the presence of the Fe(C5H5)
fragment provides the additional possibility of tuning
the stereochemistry around the catalytic center. This
may prevent isomerization of 1-alkenes, primary prod-
ucts of the iridium-catalyzed alkane dehydrogenation,
Simultaneously, van Koten, Brown, and co-workers12
published a brief report on the synthesis of the rhodium
pincer complex RhH(Cl)[{2,5-(But PCH2)2C5H2}Fe(C5H5)]
2
(a mixture of endo- and exo-H isomers). Thus, it was
demonstrated that it is possible to synthesize metal-
locene-based pincer complexes. As expected, the X-ray
diffraction study of complex 1 revealed that the forma-
tion of the metallocene-based complex led to a decrease
(by ∼5°) of the bond angles at C(2) and C(5) atoms in
fused metallacycles as compared to the standard value.11
The formation of complex 1 gave impetus to a further
search for procedures for the synthesis of precursors of
metallocene-based pincer ligands such as P,CH,P and
P,CH,N,11 and hydridoiridium P,C,P pincer complexes
based on ferrocene and ruthenocene.13a,b
Our interest in the generation of metallocene-based
pincer complexes was stimulated by the reports of
Kaska, J ensen, and Goldman with co-workers on the
extraordinary catalytic activity of iridium pincer com-
(12) Farrington, E. F.; Martinez-Viviente, E.; Williams, B. S.; van
Koten, G.; Brown, J . M. Chem. Commun. 2002, 308.
(13) (a) Sheloumov, A. M.; Kuklin, S. A.; Lagunova, V. Yu.; Petukho-
va, I. I.; Petrovskii, P. V.; Koridze, A. A. In Modern Trends in
Organometallic and Catalytic Chemistry; Book of Abstracts, Moscow,
May 18-23, 2003; P21. (b) Sheloumov, A. M.; Kuklin, S. A., Lagunova,
V. Yu.; Petukhova, I. I.; Petrovskii, P. V.; Koridze, A. A. XVth FECHEM
Conference on Organometallic Chemistry; Book of Abstracts, Zurich,
August 10-15, 2003; P 0267. (c) Koridze, A. A.; Kuklin, S. A.;
Sheloumov, A. M.; Kondrashov, M. V.; Dolgushin, F. M.; Peregudov,
A. S.; Petrovskii, P. V. Russ. Chem. Bull., Int. Ed. 2003, 52, 2754. (d)
Koridze, A. A.; Kuklin, S. A.; Sheloumov, A. M.; Kondrashov, M. V.;
Dolgushin, F. M.; Ezernitskaya, M. G.; Petrovskii, P. V.; Vorontsov,
E. V. Russ. Chem. Bull., Int. Ed. 2003, 52, 2757.
(11) Koridze, A. A.; Sheloumov, A. M.; Kuklin, S. A.; Lagunova, V.
Yu.; Petukhova, I. I.; Dolgushin, F. M.; Ezernitskaya, M. G.; Petrovskii,
P. V.; Macharashvili, A. A.; Chedia, R. V. Russ. Chem. Bull., Int. Ed.
2002, 51, 1077.

Ferrocene-Based Pincer Complexes of Palladium
Organometallics, Vol. 23, No. 20, 2004 4587
plexes IrH2[2,6-(R2PCH2)2C6H3] (R ) But, Pri) in alkane
dehydrogenation.10f,g It seemed extremely interesting to
produce metallocene-based analogues of these com-
plexes, IrH2[{2,5-(R2PCH2)2C5H2}M′(C5H5)] (IrH2[RP,C,
PM′; M′ ) Fe, Ru),13a,b and to evaluate their properties
as catalysts for the dehydrogenation of alkanes and
organics containing alkyl groups. Since accessibility of
the catalytic center for substrates is one of the most
important factors in catalysis, it was desirable to possess
X-ray structural data on the steric environment of the
Ir atom in metallocene-based hydridoiridium complexes.
Unfortunately, we failed to prepare single crystals of
the complexes IrH2[RP,C,PM′] and IrH4[RP,C,PM′] suit-
able for an X-ray diffraction study. For this reason, we
have initiated the study of the chloropalladium com-
plexes PdCl[{2,5-(R2PCH2)2C5H2}Fe(C5H5)] (3a , R ) Pri;
3b, R ) But) as model compounds. In addition, com-
plexes 3a ,b are of independent interest as potential
catalysts for the Heck and related carbon-carbon
coupling reactions.
gave two virtual triplets at δ 1.28 (3J PH ) 6.5 Hz, 18H)
and 1.55 ppm (3J PH ) 7.0 Hz, 18H), in accord with the
expected structure. For complex 3a the methyl protons
of the isopropyl groups are observed as three pseudo
quadruplets at δ 1.07, 1.33, and 1.53 ppm with relative
intensities of 6H, 12H, and 6H, respectively. A similar
spectral pattern was observed earlier11 for the isopropyl
groups in the spectrum of complex 1. The 13C{1H} NMR
spectrum of 3b is also in accordance with the expected
structure, with the cyclopentadienyl resonances being
observed at δ 114.28 (vt, J PC ) 20.1 Hz, 1C, C(1)), 92.75
(vt, J PC ) 13.9 Hz, 2C, C(2,5)), and 63.76 ppm (vt, J PC
) 81 Hz, 2C, C(3,4)).
For m ation of th e Com plex {P dCl[{2,5-(Bu t P CH2)-
2
C5H2}F e(C5H5)]}P F 6 (4b). Complex 3b reacts with the
equivalent amount of [Cp2Fe]PF6 in methylene chloride
at room temperature to form the green ferrocenium-
based pincer complex {PdCl[{2,5-(But PCH2)2C5H2}Fe-
2
(C5H5)]}PF6 (4b) in ∼80% yield (eq 3). This is the first
example of ferrocene-based phosphine chelate oxidation
centered on the iron atom.
This account represents the first full paper on met-
allocene-based P,C,P pincer complexes of platinum-
group metals and includes details of the synthesis of
ferrocene-based P,CH,P diphosphines, P,C,P pincer
complexes of palladium, their crystal structures, and
spectroscopic and redox properties. Some of these results
have been published as preliminary communications.13c,d
Resu lts a n d Discu ssion
Syn th esis of Ch lor op a lla d iu m F er r ocen e-Ba sed
P ,C,P P in cer Com p lexes. The starting 1,3-bis((di-
alkylphosphino)methyl)ferrocenes {1,3-(R2PCH2)2C5H3}-
Fe(C5H5) (2a , R ) Pri; 2b, R ) But) were obtained by
the reaction of 1,3-bis(hydroxymethyl)ferrocene with the
corresponding dialkylphosphines HPR2 in hot acetic
acid. Cyclometalation of diphosphines 2a and 2b with
PdCl2(NCPh)2 in 2-methoxyethanol at 120 °C gives the
chloropalladium pincer complexes 3a ,b, respectively (eq
2).
Complex 4b is poorly soluble in chloroform but readily
soluble in methylene chloride. The NMR spectral data
of 4b indicate its paramagnetic character. In the 1H
NMR spectrum, the methyl protons of the tert-butyl
groups are observed at δ -10.24 and 7.30 ppm, and the
methylene group protons of CH2PBut appear at δ
2
-115.49 (very broad) and -25.87 ppm (broad). In the
31P{1H} NMR spectrum of 4b, there is a singlet at δ
93.9 ppm (2P, PBut ) and a septet at -150.0 ppm (1P,
2
PF6, J PF ) 709.5 Hz). The ferrocenium nature of
complex 4b follows also from its electrochemical behav-
ior, as compared to the redox characteristics obtained
from an electrochemical study of complex 3b (see below).
F or m a tion of th e Com p lexes P d (BH4)[{2,5-(R2-
P CH2)2C5H2}F e(C5H5)] (5a , R ) P r i; 5b, R ) Bu t).
Our initial attempts to synthesize a hydridopalladium
complex by the reaction of 3b with LiAIH4 were unsuc-
cessful, because the reaction was accompanied by the
destruction of the pincer complex to form the free
diphosphine 2b. However, we found that when NaBH4
was used as a reducing agent, both of the chloro
complexes 3a ,b yield the corresponding tetrahydrobo-
rate derivatives of palladium, complexes Pd(BH4)[{2,5-
(R2PCH2)2C5H2}Fe(C5H5)] (5a , R ) Pri; 5b, R ) But) (eq
4).
Chloropalladium complexes form orange air- and
heat-stable crystalline solids (they remain unchanged
after heating in solution at 150 °C for several hours).
Compounds 3a ,b were characterized by NMR and mass
spectrometry and by elemental analysis. A single reso-
nance was observed for the two equivalent phosphorus
nuclei in the 31P{1H} NMR spectrum, at δ 71.87 (3a )
and 85.78 ppm (3b). In the 1H NMR spectra, the
resonances of the cyclopentadienyl protons of the fer-
rocene unit are revealed as two singlets of relative
intensities of 5H and 2H, indicating the cyclometalation
of the substituted ring. The methylene protons of the
CH2PR2 groups are nonequivalent and are revealed as
a doublet of triplets. In the 1H NMR spectrum of
complex 3b, the methyl protons of the tert-butyl groups

4588 Organometallics, Vol. 23, No. 20, 2004
Koridze et al.
Ch a r t 2
1
The H and 31P{1H} NMR spectral data indicate that
the tridentate P,C,P pincer ligand is retained in com-
plexes 5a ,b. The presence of the BH4 - ligand is evident
from the 1H and 11B NMR spectra. The 1H NMR
spectrum of 5b contains a very broad signal centered
at δ -0.50 ppm. The 11B NMR spectrum consists of a
F igu r e 1. Molecular structure of 3a .
quintet at δ -37.28 ppm, with coupling constant J BH
)
83.1 Hz; the multiplicity of the signal indicates fast
averaging of all four H atoms bound to the B atom. The
IR spectrum of 5b contains the B-H stretching bands
at 2380 (sh), 2369 (vs), and 2293 (s) and an intense
absorption at 1064 cm-1 (BH3 deformation); the last
band is characteristic of the unidentate BH4- ligand.14
-
It is known that the BH4 ion can form transition-
metal complexes, in which it coordinates the metal by
three, two, and one hydrogen atoms.14 While the first
two bonding modes are quite common, the unidentate
coordination mode very rarely occurs. To our knowledge,
palladium complexes with the η1-BH4 ligand have not
been characterized earlier.
X-r a y Diffr a ction Stu d y of Com p lexes 3a ,b a n d
5b. It was of interest to study the structures of fer-
rocene-based P,C,P pincer complexes 3a ,b and to com-
pare their geometric features with those of known
benzene-based analogues and related pincer complexes
of palladium. The X-ray structures of several P,C,P
pincer palladium complexes have been described previ-
ously (Chart 2). These include chloro complexes 6a ,15a
6b,15b and 6c,15c complexes 75e and 815d containing chiral
P,C,P ligands, complexes 916a and 1016b based on 1,5-
bis(di-tert-butylphosphino)pentane and 1,8-bis(diphen-
ylphosphino)anthracene pincer ligands, and the phos-
phinito P,C,P pincer complex 11.16c The structures of
cationic complexes containing ancillary two-electron
donor ligands at the Pd atom, [Pd(OH2){2,6-Ph2-
PCH2)2C6H3}]BF4 (12; two crystallographic isomers),15a
F igu r e 2. Molecular structure of 3b.
[Pd(OH2){(But PCH2CH2)2CH}]BPh4
(13), [Pd(NH2-
16a
2
17a
Ph){2,6-(Ph2PCH2)2C6H3}]BF4 (14), and [Pd(PEt3){2,6-
(Ph2PCH2)2C6H3}]BF4,17b (15), have also been deter-
mined.
The molecular structures of 3a ,b are illustrated in
Figures 1 and 2, respectively, while the selected bond
distances and angles are listed in Table 1. As may be
seen from Figures 1 and 2, the structures of both
molecules are very similar and contain the Pd atom in
a distorted-square-planar geometry, with P(1)-Pd-P(2)
angles in 3a ,b being equal to 160.39(6) and 157.57(4)°,
respectively. These values are markedly smaller than
the corresponding angles in benzene-based pincer com-
plexes 6-10, where they are in the 162.0(1)-166.2(1)°
range. They are also smaller relative to the two forms
of complex 12, 166.26(8) and 167.6(1)°, and similar to
(14) (a) Marks, T. J .; Kolb, J . R. Chem. Rev. 1977, 77, 263. (b)
Bommer, J . C.; Morse, K. W. Inorg. Chem. 1980, 19, 587. (c) Makhaev,
V. D. Russ. Chem. Rev. 2000, 69, 727.
(15) (a) Kemmich, B. F. M.; Marshall, W. J .; Fagan, P. J .; Hauptman,
E.; Bullock, R. M. Inorg. Chim. Acta 2002, 330, 52. (b) Cross, R. J .;
Kennedy, A. K.; Muir, K. W. J . Organomet. Chem. 1995, 487, 227. (c)
Gorla, F.; Venanzi, L. M.; Albinati, A. Organometallics 1994, 13, 43.
(d) Longmire, J . M.; Zhang, X.; Shang, M. Organometallics 1998, 17,
4374.
(16) (a) Seligson, A. L.; Trogler, W. C. Organometallics 1993, 12,
738. (b) Haenel, M. W.; J akubik, D.; Kru¨ger, C.; Betz, P. Chem. Ber.
1991, 124, 333. (c) Morales-Morales, D.; Grause, C.; Kasaoka, K.;
Redo´o´n, R.; Cramer, R. E.; J ensen, C. M. Inorg. Chim. Acta 2000, 300-
302, 958.
(17) (a) Kraatz, H.-B.; Milstein, D. J . Organomet. Chem. 1995, 488,
223. (b) Steffey, B. D.; Miedaner, A.; Maciejewski-Farmer, M. L.;
Bernatis, P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois,
D. L. Organometallics 1994, 13, 4844.

Ferrocene-Based Pincer Complexes of Palladium
Organometallics, Vol. 23, No. 20, 2004 4589
Ta ble 1. Selected Geom etr ica l P a r a m eter s for Com p lexes 3a ,b
3a
3b
Bond Lengths (Å)
3a
3b
Pd(1)-Cl(1)
Pd(1)-C(1)
Pd(1)-P(1)
Pd(1)-P(2)
P(1)-C(11)
P(2)-C(12)
C(1)-C(2)
2.400(2)
1.964(5)
2.318(2)
2.305(2)
1.842(6)
1.835(6)
1.433(7)
2.403(1)
1.961(4)
2.342(1)
2.337(1)
1.857(4)
1.858(4)
1.420(6)
C(1)-C(5)
C(2)-C(3)
C(2)-C(12)
C(3)-C(4)
C(4)-C(5)
C(5)-C(11)
1.417(8)
1.436(8)
1.494(8)
1.430(9)
1.416(8)
1.513(8)
1.425(5)
1.421(5)
1.502(6)
1.430(6)
1.414(6)
1.525(6)
Bond Angles (deg)
C(1)-Pd(1)-Cl(1)
C(1)-Pd(1)-P(1)
C(1)-Pd(1)-P(2)
P(1)-Pd(1)-P(2)
P(1)-Pd(1)-Cl(1)
P(2)-Pd(1)-Cl(1)
C(11)-P(1)-Pd(1)
C(12)-P(2)-Pd(1)
C(2)-C(1)-C(5)
C(2)-C(1)-Pd(1)
C(5)-C(1)-Pd(1)
179.0(2)
80.9(2)
81.1(2)
175.5(1)
80.8(1)
80.7(1)
C(1)-C(2)-C(3)
C(1)-C(2)-C(12)
C(3)-C(2)-C(12)
C(2)-C(3)-C(4)
C(5)-C(4)-C(3)
C(4)-C(5)-C(1)
C(4)-C(5)-C(11)
C(1)-C(5)-C(11)
C(5)-C(11)-P(1)
C(2)-C(12)-P(2)
107.0(5)
120.6(5)
132.5(5)
108.0(5)
108.2(5)
108.3(5)
132.0(5)
119.6(5)
106.5(4)
105.8(4)
108.2(4)
119.1(4)
132.7(4)
108.1(4)
107.4(4)
108.8(4)
130.2(4)
121.0(4)
104.8(3)
106.7(3)
160.39(6)
99.70(6)
98.48(6)
106.4(2)
106.7(2)
108.6(5)
124.8(4)
126.4(4)
157.57(4)
99.77(4)
99.65(4)
106.1(2)
104.7(2)
107.5(4)
126.7(3)
125.7(3)
those in the cationic complexes 14 and 15, 160.8(3) and
158.1(1)°, respectively. (In 9, 10, 11, and 13 the P(1)-
Pd-P(2) angles are 166.2(1), 166.2(1), 160.380(6) and
157.770(7) (for two independent molecules), and 167.1-
(1)°, respectively).
As in the case of rhodium complex 1, the formation
of palladium complexes 3a ,b leads to a decrease in the
bond angles at C(2) and C(5) in the fused metallacycles
(C(1)-C(2)-C(12) ) 120.6(5)°, C(1)-C(5)-C(11) ) 119.6-
(5)° for 3a ; C(1)-C(2)-C(12) ) 119.1(4)°, C(1)-C(5)-
C(11) ) 121.0(4)° for 3b). In complexes 3a ,b the Pd atom
deviates from the plane of the cyclopentadienyl ring and
lies 0.147 and 0.069 Å above the ring in 3a ,b, respec-
tively.
F igu r e 3. Schematic views of P,C,P pincer complexes (left,
benzene-based; right, ferrocene-based) along the Cl-Pd-
C(1) axis. The Cp ring deviation from the Pd-Cl axis is
omitted for clarity.
exo position in the metallocene-based pincer complexes
than in the benzene-based systems. Evidently, this
peculiarity should be kept in mind for the design of
metallocene-based P,C,P pincer complexes as catalysts
for various reactions.
There are some other structural features in complexes
3a ,b. Thus, the Pd-Cl distances (2.4002(17) and 2.4030-
(11) Å, respectively) are longer than for 6c (2.367(3) Å)
and close to those for 6a (2.3969(6) Å) and 6b (2.427(2)
Å), which contain bulky alkyl groups at the P atoms.15a,b
It cannot be ruled out that weakening of the Pd-Cl bond
in 3a ,b is the result of steric congestion by the bulky
isopropyl or tert-butyl groups. It should be noted in this
respect that the angles C(1)-Pd-Cl in complexes 3a ,b
are 178.89(17) and 175.46(12)°, respectively, and this
difference may also originate from steric congestion by
the bulkier tert-butyl groups.
The molecular structure of complex 5b is illustrated
in Figure 4; the main bond lengths and angles are
presented in Table 2. The structural features of the
pincer ligand of molecule 5b are similar to those of
complex 3b; in both complexes the cyclopentadienyl
rings deviate from being parallel by 4.2 and 4.5°,
respectively, and have an eclipsed conformation. The
coordination geometry of Pd in 5b is essentially square
planar with a slight distortion toward tetrahedral, with
a P(1)-Pd-P(2) angle of 160.72(3)°. The palladium atom
coordinates only one of the BH4- group hydrogen atoms,
with a Pd-H(1) distance of 2.00(4) Å. As in other
metallocene-based P,C,P pincer complexes, the chelated
metal atom in 5b deviates from the plane of the
cyclopentadienyl ring, and the Pd atom lies 0.361 Å
The distinguishing features of 3a ,b in comparison
with benzene systems are also an increase of the Pd-P
bond distances. Thus, the Pd-P distance (average
2.3391(12) Å) in 3b is markedly longer than in its
benzene analogue 6a , 2.3025(6) Å. In benzene-based
P,C,P pincer complexes the two chelating five-membered
metallacycles are strained and adopt an envelope con-
formation with the methylene carbon at the flap. As a
result, the plane of the aromatic ring is tilted from the
coordination plane. Thus, in 6c, for example, the
coordination plane defined by Pd, Cl, C(1), P(1), and P(2)
forms an angle of 19.85(5)° with the cyclometalated
benzene ring, and the P(1) and P(2) atoms are situated
at the opposite sides of the benzene plane.15c As a
consequence, at each side of the benzene ring of 6c one
of the phenyl groups at the P atom is pseudoaxial and
the other is pseudoequatorial. In contrast to 6c and
other benzene-based P,C,P pincer complexes, the two
pseudoaxial groups in metallocene-based pincer com-
plexes 3a ,b are located at the opposite sides of the
cyclometalated cyclopentadienyl ring with respect to the
two pseudoequatorial groups (see Figure 3). The phos-
phorus atoms P(1) and P(2) are located on the same side
of the plane containing the substituted cyclopentadienyl
ring, opposite to the iron atom. This deviation is
probably due to a close contact between the hydrogen
atoms of the methyl groups in the P(1) and P(2)
substituents and the hydrogen atoms of the unsubsti-
tuted cyclopentadienyl ring. Thus, the chelated metal
atom M is more shielded against the attack from the

4590 Organometallics, Vol. 23, No. 20, 2004
Koridze et al.
angles were observed for the complexes Ag(η1-BH4)-
22
(PMePh2)3 and V(η1-BH4)2(dmpe)2,23 while the iso-
structural complexes M(η1-BH4)(CO)(NO)(PMe3)3 (M )
Mo, W)24 contain two independent molecules, in which
the M-Hb-B angles are 128.5, 142.4° and 138.5, 146.6°,
respectively.
The Pd‚‚‚B distance of 2.614(7) Å is too long to
suppose Pd-B bonding in complex 5b. For comparison,
in the complex Ti(BH4)3(PMe3)2,25 containing unique
“agostic” or “side-on” bonding of the Ti atom with two
BH4- groups, the Ti-B distance is 2.27(1) Å. A similar
short M-B distance was also observed for complexes
where the metal atom was bound to two hydrogen atoms
-
of the BH4 group: for example 2.29 Å in Co(η2-BH4)-
(triphos).26 However, it may be noted that the Pd‚‚‚B
distance in 5b is markedly shorter in comparison to
other η1-BH4 derivatives of transition metals; for ex-
ample 2.833(4), 2.84, and 2.895(16) Å in the complexes
V(η1-BH4)2(dmpe)4,23 FeH(η1-BH4)(dmpe)2,18 and RuH-
(η1-BH4)(Me8[16]aneS4),19 respectively. This fact as well
as the relatively short distance between the palladium
atom and one of the three terminal hydrogen atoms of
-
the BH4 group, Pd‚‚‚H(2) ) 2.56(5) Å, allows us to
describe the latter contact as an “incipient Pd-H
bonding interaction” as was proposed for the complex
Cu(η1-BH4)(PMePh2)3.20a In the related complex Hf2H3-
(BH4)3[N(SiMe2CH2PMe2)2]2, the bonding of one of the
F igu r e 4. Molecular structure of 5b.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) in Com p lex 5b
BH4 groups is intermediate between the η1 and η2
-
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-C(1)
Pd(1)...B(1)
Pd(1)-H(1)
Pd(1)...H(2)
B(1)-H(1)
B(1)-H(2)
B(1)-H(3)
B(1)-H(4)
2.3526(9)
2.3521(9)
1.980(3)
2.614(7)
2.00(4)
2.56(5)
0.92(4)
1.14(5)
0.94(5)
P(1)-C(11)
P(2)-C(12)
C(1)-C(2)
C(1)-C(5)
C(2)-C(3)
C(2)-C(12)
C(3)-C(4)
C(4)-C(5)
C(5)-C(11)
1.854(3)
1.861(3)
1.433(4)
1.424(4)
1.439(4)
1.499(4)
1.421(5)
1.424(4)
1.491(5)
modes (the Hf-H distances are 1.93 and 2.43 Å).20b
Evidently, equilibration of the hydrogen atoms of the
BH4- group in the solution of 5b should include stepwise
formation of η1 and η2 structures accompanied by
-
rotation of the unidentate BH4 group.
-
It is known that the BH4 ion and electronically
neutral boran-Lewis base adducts BH3‚L are isoelec-
tronic with alkanes and form relatively stable complexes
with transition metals.14a,27 At the same time, the
alkane σ-complexes of transition metals are extremely
unstable and only recently have become available for
observation under specific conditions.28 Shimoi and co-
workers showed that properties of metal complexes with
unidentate borane-Lewis base adducts embodied those
of alkane complexes.27
0.98(6)
C(1)-Pd(1)-P(1)
C(1)-Pd(1)-P(2)
C(1)-Pd(1)-B(1)
P(1)-Pd(1)-P(2)
P(1)-Pd(1)-B(1)
P(2)-Pd(1)-B(1)
Pd(1)-H(1)-B(1)
C(11)-P(1)-Pd(1)
C(12)-P(2)-Pd(1)
C(5)-C(1)-C(2)
C(5)-C(1)-Pd(1)
80.67(9)
80.97(9)
173.6(3)
160.72(3)
99.1(1)
99.9(1)
123(3)
105.2(1)
105.6(1)
108.3(3)
125.7(2)
C(2)-C(1)-Pd(1)
C(1)-C(2)-C(3)
C(1)-C(2)-C(12)
C(3)-C(2)-C(12)
C(4)-C(3)-C(2)
C(3)-C(4)-C(5)
C(4)-C(5)-C(1)
C(4)-C(5)-C(11)
C(1)-C(5)-C(11)
C(5)-C(11)-P(1)
C(2)-C(12)-P(2)
125.2(2)
107.4(3)
120.8(3)
131.8(3)
107.9(3)
108.5(3)
107.9(3)
132.0(3)
120.0(3)
106.9(2)
106.2(2)
Several modes of metal-alkane interactions were
suggested for alkane σ-complexes (Chart 3), intermedi-
ates in alkane oxidative addition reactions.
above the ring. The three atoms C(1), Pd, and B form
an angle of 173.6(3)°, and the Pd-H(1)-B angle is
123.3°. It should be mentioned that in transition-metal
(22) (a) Makhaev, V. D.; Borisov, A. P.; Lobkovskii, E. B.; Seme-
nenko, K. N. Izv. Akad. Nauk SSSR, Ser. Khim. 1980, 2614. (b)
Lobkovskii, E. B.; Antipin, M. Yu.; Borisov, A. P.; Makhaev, V. D.;
Semenenko, K. N.; Struchkov, Yu. T. Koord. Khim. 1981, 7, 307.
(23) J ensen, J . A.; Girolami, G. S. Inorg. Chem. 1989, 28, 2107.
(24) (a) Liang, F.; J akobsen, H.; Schmalle, H. W.; Fox, T.; Berke,
H. Organometallics 2000, 19, 1950. (b) Ho¨ck, J .; J acobsen, H.;
Schmalle, H. W.; Artus, G. R. J .; Fox, T.; Amor, J . I.; Ba¨th, F.; Berke,
H. Organometallics 2001, 20, 1533.
(25) J ensen, J . A.; Wilson, S. R.; Girolami, G. S. J . Am. Chem. Soc.
1988, 110, 4977.
(26) (a) Dapporto, P.; Midollini, S.; Orlandini, A.; Sacconi, L. Inorg.
Chem. 1976, 15, 2768.
-
complexes with the unidentate BH4 ligand, M-Hb-B
angles vary in a broad range. Thus, in FeH(η1-BH4)-
(dmpe)218a,b and trans-RuH(η1-BH4)(Me8[16]aneS4)19 the
M-Hb-B angles are 161.7 and 164(1)°, respectively,
while in Cu(η1-BH4)(PMePh2)3
and Cu(η1-BH4)(tri-
20a
phos)21 those angles are substantially smaller: 121.7-
(4) and 121(3)°, respectively. Intermediate M-Hb-B
(18) (a) Bau, R.; Yuan, H. S. H.; Baker, M. V.; Field, L. D. Inorg.
Chim. Acta 1986, 114, L27. (b) Baker, M. V.; Field, L. D. J . Chem.
Soc., Chem. Commun. 1984, 996.
(19) Yoshida, T.; Adachi, T.; Ueda, T.; Akao, H.; Tanaka, T.; Goto,
F. Inorg. Chim. Acta 1995, 231, 95.
(20) (a) Tagusagawa, F.; Fumagalli, A.; Koetzle, T. F.; Shore, S. G.;
Schmitkons, T.; Fratini, A. V.; Morse, K. W.; Wei, C.-Y.; Bau, R. J .
Am. Chem. Soc. 1981, 103, 5165. (b) Fryzuk, M. D.; Rettig, S. J .;
Westerhaus, A.; Williams, H. D. Inorg. Chem. 1985, 24, 4316.
(21) Ghilardi, C. A.; Midollini, S.; Orlandini, A. Inorg. Chem. 1982,
21, 4096.
(27) (a) Shimoi, M.; Nagai, S.-I.; Ichikawa, M.; Kawano, Y.; Katoh,
K.; Uruichi, M.; Ogino, H. J . Am. Chem. Soc. 1999, 121, 11704. (b)
Kakizawa, T.; Kawano, Y.; Shimoi, M. Organometallics 2001, 20, 3211.
(c) Li, S.; Hall, M. B. Organometallics 2001, 20, 2153.
(28) (a) Sun, X.-Z.; Grills, D. C.; Nikiforov, S. M.; Polyakoff, M.;
George, M. W. J . Am. Chem. Soc. 1997, 119, 7521. (b) Geftakis, S.;
Ball, G. E. J . Am. Chem. Soc. 1998, 120, 9953. (c) Evans, D. R.;
Drovetskaya, T.; Bau, R.; Reed, C. A.; Boyd, P. D. W. J . Am. Chem.
Soc. 1997, 119, 3633. (d) Castro-Rodriguez, I.; Nakai, H.; Gantzel, P.;
Zakharov, L. N.; Rheingold, A. L.; Meyr, K. J . Am. Chem. Soc. 2003,
125, 15734.

Ferrocene-Based Pincer Complexes of Palladium
Organometallics, Vol. 23, No. 20, 2004 4591
Ch a r t 3
Although alkane activation was not observed for Pd(II)
in pincer complexes, it is reasonable to regard the
interaction of the chelated, sterical crowded Pd atom
-
with BH4 in 5b as modeling an earlier stage of alkane
oxidative addition to the Ir atom in Ir[RP,C,PM′], cata-
lytic species for alkane dehydrogenation. Remarkably,
the distances between the Pd atom and two hydrogen
-
atoms of the BH4 group in 5b (Pd-H(1) ) 2.00(4) Å
and Pd-H(2) ) 2.56(5) Å) and corresponding Ir-H bond
distances of 2.017 and 2.554 Å, calculated for the
hypothetical intermediate agostic complex of ethane
with Ir[2,6-(H2PCH2)2C6H3], are very similar.27c
F igu r e 5. Cyclic voltammograms of compounds 2b (thin-
ner line) and 3b (thicker line) in THF solution. The scan
Electr och em ica l Stu d ies of 2b a n d 3a ,b. As men-
tioned earlier,11 one of the peculiarities of the ferrocene-
based pincer complexes is their potential to undergo a
rapid and reversible redox process centered on the iron
atom. We suppose that upon altering the redox state of
the iron atom in ferrocene-based pincer complexes we
can modify the chemical and catalytic reactivity of the
chelated metal atom M, situated at the position â to the
iron atom. There is some resemblance to R-ferrocenyl-
carbenium ions, the enhanced stability of which results
from the direct interaction between the iron atom and
the carbenium ion center, also situated at the position
â to iron. Therefore, by altering the iron oxidation state
within the pincer ligand, we can enhance the rate of
either oxidative addition or reductive elimination at M.
Thus, it is known that an increase in the electron
density at the metal favors oxidative addition, whereas
oxidation of metal complexes containing σ-bonded ligands
leads to enhanced tendency toward reductive elimina-
tion by the loss of σ-bonded ligands.29
rate was 100 mV s-1
.
to more negative values relative to the free ligand is
interesting. It means that the global effect of phosphorus
coordination and metalation makes the Fe center more
electron rich. The donation from the P donors to the Pd
center decreases the electron density at the P atoms,
and the transmission of this electronic change to the
ferrocene moiety through the CH2PR2 should make the
ferrocene less electron rich. This means that the re-
placement of a cyclopentadienyl C-H bond by a C-Pd
bond more than compensates for this effect, transmit-
ting more electron density to the Fe center.
Con clu sion s
We have presented here the synthesis, structures, and
electrochemical properties of palladium ferrocene-based
P,C,P pincer complexes, obtained by cyclometalation of
1,3-bis((dialkylphosphino)methyl)ferrocenes. These in-
clude tetrahydroborato derivatives, the first palladium
The redox properties of the ligand 2b and the com-
plexes 3a ,b were investigated by cyclic voltammetry.
The results (see Figure 5) demonstrate that diphosphine
and both complexes display reversible oxidation pro-
cesses. All waves have a ∆Ep value equal to or smaller
than that measured for an internal ferrocene standard,
in agreement with their nature as one-electron redox
processes. The free ligand 2b is oxidized at E1/2 ) -0.11
V vs ferrocene; i.e., its oxidation is slightly more facile
than that of ferrocene, in line with the expected electron-
-
complexes containing a unidentate BH4 ligand. The
X-ray diffraction study of chloropalladium pincer com-
plexes 3a and 3b reveals some interesting structural
features, including the steric environment of the che-
lated metal atom, which distinguishes them from their
benzene analogues. The structural peculiarity of the
-
BH4 ion bonding with the palladium atom in 5b can
be considered as modeling an earlier stage of alkane
oxidative addition to the metal center in Ir[RP,C,PM′],
catalytic species for alkane dehydrogenation.
donating effect of the two CH2PBut arms on the
2
cyclopentadienyl ring. The palladium complexes 3a ,b
show oxidation potentials very close to each other (E1/2
) -0.21 and -0.22 V vs ferrocene, respectively), slightly
more negative than the corresponding free ligands. The
small shift with respect to the free ligand suggests that
the processes are once again Fe-based. In addition,
square-planar Pd(II) complexes do not show reversible
electrochemistry at potentials close to that of fer-
rocene.30 The observation that the redox potentials shift
Unexpected effects of the cyclometalation process on
the Fe-based redox properties have been observed for
the pincer complexes 3a ,b. At the moment, we are
extending our work by probing the effect of the iron
oxidation state on the reactivity and catalytic properties
of the chelated metal atom in ferrocene-based pincer
complexes.
(30) (a) Maisela, L.; Crouch, A.; Darkwa, J .; Guzei, I. Polyhedron
2001, 20, 3189. (b) Grant, G.; Carter, S.; Russell, A.; Poullaos, I.;
VanDerveer, D. J . Organomet. Chem. 2001, 637, 683. (c) Chen, G.; Ou,
S.; Bai, Z.; Han, G.; Duan, C. Chin. J . Inorg. Chem. 2003, 19, 501.
(29) (a) Diversi, P.; Iacoponi, S.; Ingrosso, G.; Laschi, F.; Lucherini,
A.; Zanello, P. J . Chem. Soc., Dalton Trans. 1993, 351. (b) Pedersen,
A.; Tilset, M. Organometallics 1994, 13, 4887.

4592 Organometallics, Vol. 23, No. 20, 2004
Koridze et al.
vacuum. The residue was crystallized from a n-hexane-CH2-
Cl2 mixture. Yield: 320 mg (47%). 31P{1H} NMR (CDCl3): δ
71.87 (s, 2P). 1H NMR (CDCl3): δ 1.07 (vq, J ) 7.8 Hz, 6H,
CH(CH3)2), 1.33 (vq, J ) 7.4 Hz, 12H, CH(CH3)2), 1.53 (vq, J
) 8.1 Hz, 6H, CH(CH3)2), 2.18 (m, 2H, CH(CH3)2), 2.29 (m,
2H, CHAHBP), 2.53 (m, 2H, CH(CH3)2), 2.76 (m, 2H, CHACH-
BP), 4.01 (s, 5H, C5H5), 4.23 (s, 2H, C5H2). MS: m/z 588 [M+].
Anal. Calcd for C24H39ClFeP2Pd (587.2): C, 49.09; H, 6.69.
Found: C, 49.04; H, 6.71.
Exp er im en ta l Section
All manipulations were carried out under an argon atmo-
sphere. The H, 11B, 13C, and 31P NMR spectra were recorded
1
on
a Bruker AMX-400 spectrometer. FTIR spectra were
recorded on a Nicolet Magna 750 spectrophotometer, and mass
spectra were measured on a Finnigan LCQ instrument. Cyclic
voltammograms were recorded with an EG&G 362 potentiostat
connected to a Macintosh computer through MacLab hardware/
sofware. The electrochemical cell was fitted with an Ag-AgCl
reference electrode, a 1 mm diameter Pd-disk working elec-
trode, and a platinum-wire counter electrode. [Bu4N]PF6 (ca.
0.1 M) was used as the supporting electrolyte in THF. All
potentials are reported relative to the ferrocene/ferrocenium
couple. Ferrocene was added and measured as an internal
standard at the end of each experiment. The compound
1-(ethoxycarbonyl)-3-formylferrocene was prepared as de-
scribed in the literature.31
Syn th esis of {1,3-(HOCH2)2C5H3}F e(C5H5). A solution of
1-(ethoxycarbonyl)-3-formylferrocene (150 mg, 0.524 mmol) in
ether (30 mL) was added dropwise to a suspension of LiAlH4
(60 mg, 15.7 mmol) in ether (30 mL). The solution was stirred
at room temperature for 4.5 h. Then water (50 mL) was added,
and the water layer was separated and extracted with dichlo-
romethane. The combined organic layers were dried over Na 2-
SO4. The solvents were evaporated under vacuum, and the
resulting yellow oil was purified on an alumina column (eluent
methanol). Recrystallization of the residue from methanol gave
orange-yellow crystals of the product. Yield: 100 mg (77.5%).
Mp: 115-116 °C; lit.32 mp 115-116 °C. 1H NMR (acetone-
d6): δ 4.11 (s, 5H, C5H5), 4.13 (d, 2H, J ) 1.1 Hz, HC5H2),
4.25 (t, 1H, J ) 1.1 Hz, HC5H2), 4.29 (s, 4H, CH2OH).
Syn th esis of P dCl[{2,5-(Bu t2P CH2)2C5H2}Fe(C5H5)] (3b).
This compound was obtained by the procedure described above
for 3a . Yield: 65.2%. 31P{1H} NMR (CDCl3): δ 85.78 (s, 2P).
1H NMR (CDCl3): δ 1.28 (vt, J ) 6.5 Hz, 18H, CH3), 1.55 (vt,
J ) 7.0 Hz, 18H, CH3), 2.48 (dt, J HH ) 16.5 Hz, J PH ) 2.0 Hz,
2H, CHACHBP), 2.90 (dt, J HH ) 16.5 Hz, J PH ) 1.1 Hz, 2H,
CHACHBP), 3.94 (s, 5H, C5H5), 4.23 (s, 2H, C5H2). 13C{1H} NMR
(CDCl3): δ 24.99 (vt, J PC ) 10.3 Hz, 2C, CH2P), 29.33 (“t”, 6C,
CH3), 29.41 (“t”, 6C, CH3), 35.06 (v.t, J PC ) 6.7 Hz, 2C, CMe3),
35.37 (vt, J PC ) 5.4 Hz, 2C, CMe3), 63.76 (vt, J PC ) 8.1 Hz,
2C, C(3,4)), 70.62 (s, 5C, C5H5), 92.75 (vt, J PC ) 13.9 Hz, 2C,
C(2,5)), 114.28 (vt, J P,C ) 20.1 Hz, 1C, C(1)). MS: m/z 641 [M+].
Anal. Calcd for C28H47ClFeP2Pd (641.5): C, 52.34; H, 7.33.
Found: C, 52.35; H, 7.28.
Syn th esis of {P d Cl[{2,5-(Bu t2P CH2)2C5H2}F e(C5H5)]}-
P F 6 (4b). [Cp2Fe]PF6 (87.1 mg, 0.263 mmol) was added to a
stirred solution of PdCl[{2,5-(But PCH2)2C5H2}Fe(C5H5)] (3b;
2
169 mg, 0.263 mmol) in dichloromethane (20 mL). The solution
rapidly changed color to green. After the solution was stirred
for 1 h, the solvent was removed under vacuum, and the
residue was extracted with toluene (6 × 10 mL) to remove
ferrocene. The green residue was dried under vacuum. Yield:
ca. 80%. 31P{1H} NMR (CD2Cl2): δ 93.9 (s, 2P), -150.0 (sept.,
1J P,F ) 709.5 Hz, 1, PF6). 1H NMR (CD2Cl2): δ -115.50 (br,
2H, CHAHBP), -25.87 (br, 2H, CHAHBP), -10.24 (s, 18H,
C(CH3)3), 7.03 (s, 18H, C(CH3)3), 24.42 (br, 5H, C5H5), 29.52
(br, 2H, C5H2). Anal. Calcd for C28H47ClF6FeP3Pd‚CH2Cl2
(873): C, 39.86; H, 5.61; F, 13.05. Found: C, 39.93; H, 5.51;
F, 12.91.
Syn th esis of {1,3-(P r i2P CH2)2C5H3}F e(C5H5) (2a ). The
phosphine HPPri2 (602 mg, 5.10 mmol) was added to a solution
of the dicarbinol {1,3-(HOCH2)2C5H3}Fe(C5H5) (502 mg, 2.04
mmol) in acetic acid (40 mL). The solution was stirred at 95
°C for 3 h. The solvent was removed under vacuum, and the
residue was extracted with dichloromethane (60 mL). The
extract was washed with a saturated aqueous Na2CO3 solution
(50 mL). The layers were separated, and the aqueous solution
was extracted with CH2Cl2 (2 × 50 mL). The combined organic
solutions were evaporated, and the residue was dissolved in
anhydrous methanol. Crystallization at -78 °C gave air-
sensitive light yellow crystals (yellow-brown oil at room
temperature). Yield: 409 mg (45%). The product appears to
be analytically pure, as indicated by NMR. 31P{1H} NMR
(CDCl3): δ 9.14 (s, 2P). 1H NMR (CDCl3): δ 0.96-1.04 (m, 24H,
CH(CH3)2), 1.6-1.7 (m, 4H, CH(CH3)2), 2.48 (br s, 4H, CH2P),
4.01 (s, 2H, C5H2H), 4.02 (s, 5H, C5H5), 4.13 (s, 1H, C5H2H).
Syn th esis of {1,3-(Bu t2P CH2)2C5H3}F e(C5H5) (2b). The
phosphination of 1,3-bis(hydroxymethyl)ferrocene (500 mg,
Syn th esis of P d (BH4)[{2,5-(Bu t2P CH2)2C5H2}F e(C5H5)]
(5b). To a solution of PdCl[{2,5-(But PCH2)2C5H2}Fe(C5H5)]
2
(3b; 96 mg, 0.15 mmol) in 40 mL of ethanol was added NaBH4
(350 mg), and the mixture was refluxed for 1 h. After the
yellow-orange solution was cooled, additional NaBH4 (100 mg)
was added and the solution was refluxed for another 1 h. After
the solution was cooled, the solvent was decanted and the
white residue was washed several times with ethanol. The
combined ethanol solutions were evaporated to dryness under
vacuum. The residue was extracted with dichloromethane,
n-hexane was added, and the solution was concentrated to a
small volume (3-4 mL) and cooled to -20 °C for 2 h. The
almost colorless supernatant was removed by pipet, and the
solid was washed with hexane and dried under vacuum, to
yield an orange crystalline powder. Yield: 68.2 mg (73%). 31P-
{1H} NMR (CDCl3): δ 91.39 (s, 2P). 1H NMR (CDCl3): δ -0.50
(v. broad, 4H, BH4), 1.23 (t, J HP ) 6.7 Hz, 18H, CH3), 1.53 (t,
J HP ) 7.2 Hz, 18H, CH3), 2.53 (dt, J HH ) 17.0 Hz, J HP ) 4.5
Hz, 2H, CHAHBP), 3.01 (dt, J HH ) 17.0 Hz, J HP ) 2.5 Hz, 2H,
CHAHBP), 3.95 (s, 5H, C5H5), 4.22 (s, 2H, C5H2). 11B NMR
(CDCl3): δ -37.28 (quint, J BH ) 83.1 Hz). IR (KBr): 2380 (sh),
2369 (vs), 2293 (s), 1064 (s) cm -1. MS: m/z 608 (M+ - BH3).
Satisfactory elemental analysis could not be obtained, despite
several attempts. The product, however, appears to be analyti-
cally pure, as indicated by NMR.
2.03 mmol) with HPBut (600 mg, 4.10 mmol) was done
2
according to the procedure described above for the synthesis
of 2a . Precipitation of the product by addition of methanol to
the concentrated solution in dichloromethane gave air-sensi-
tive light yellow needles. Yield: 613 mg (60%). The product
appears to be analytically pure, as indicated by NMR. 31P{1H}
1
NMR (CDCl3): δ 34.46 (s, 2P). H NMR (CDCl3): δ 1.07 (2d,
36H, J P,H ) 10.7 Hz, C(CH3)3), 2.53 (d, J ) 2.3 Hz, 4H, CH2P),
4.02 (s, 5H, C5H5), 4.11 (s, 2H, C5H2H), 4.31 (s, 1H, C5H2H).
MS: m/z 502 [M+].
Syn th esis of P d Cl[{2,5-(P r i2P CH2)2C5H2}F e(C5H5)] (3a ).
PdCl2(NCPh)2 (440 mg, 1.154 mmol) was added to the suspen-
sion of 1,3-bis((diisopropylphosphino)methyl)ferrocene (2a ; 515
mg, 1.154 mmol) in dry 2-methoxyethanol (50 mL). The
mixture was stirred at 125 °C for 3 h. After it was cooled, the
orange solution was filtered and solvent was removed under
Syn th esis of P d (BH4)[{2,5-(P r i2P CH2)2C5H2}F e(C5H5)]
(5a ). This compound was obtained by the procedure described
above for 5a , starting from 128.6 mg of 3a and 400 mg of
NaBH4 in 40 mL of ethanol. Yield: 59.2 mg, 45%. 31P{1H}
NMR (C6D6): δ 78.05 (s, 2P). 1H NMR (C6D6): δ 0.27 (v. broad,
4H, BH4) (pseudo q, J BH ) 97, J BH ) 55 Hz), 0.89 (vq, J ) 7.6
Hz, 6H, CHCH3), 1.00 (vq, J ) 6.4 Hz, 6H, CHCH3), 1.26 (vq,
J ) 7.8 Hz, 6H, CHCH3), 1.53 (vq, J ) 8.0 Hz, 6H, CHCH3),
(31) Bickert, P.; Hildebrandt, B.; Hafner, K. Organometallics 1984,
3, 653.
(32) Kasahira, A.; Izumi, T.; Yoshida, Y.; Shimuzi, I. Bull. Chem.
Soc. J pn. 1982, 55, 1901.

Ferrocene-Based Pincer Complexes of Palladium
Organometallics, Vol. 23, No. 20, 2004 4593
Ta ble 3. Su m m a r y of Cr ysta l Str u ctu r e Deter m in a tion for Com p lexes 3a ,b a n d 5b
3a
3b
5b
formula
mol wt
C
24H39ClP2FePd
C28H47ClP2FePd
643.30
C28H51BP2FePd
622.69
587.19
cryst color, habit
cryst size, mm
cryst syst
space group
temp, K
a, Å
yellow prism
0.30 × 0.20 × 0.10
orthorhombic
P212121
295(2)
12.018(2)
orange prism
0.20 × 0.18 × 0.10
orthorhombic
Pbca
orange prism
0.55 × 0.40 × 0.20
monoclinic
P21/c
295(2)
295(2)
17.2290(6)
16.4470(5)
21.3131(7)
10.977(1)
18.634(2)
15.632(2)
109.312(2)
3017.5(5)
4
1.371
11.97
SMART 1000 CCD
ω and æ
60
b, Å
c, Å
14.031(4)
15.439(3)
â, deg
V, Å3
2603(1)
4
1.498
14.82
Enraf-Nonius CAD4
θ-5/3θ
56
6039.4(3)
8
1.415
Z
d
calcd, g/cm3
µ, cm-1 (Mo KR, λ ) 0.710 73 Å)
diffractometer
12.85
SMART 1000 CCD
ω and æ
56
scan technique
2θmax, deg
no. of rflns collected
3502
44 813
28 100
no. of indep rflns (Rint
)
3502
2588
0.0362
0.0776
7291 (0.0767)
3538
0.0471
8751 (0.0435)
5266
0.0440
no. of obsd rflns (I > 2σ(I))
R1a
wR2b
0.1026
0.0928
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑[w(Fo - Fc2)2]/∑[w(Fo2)2]}1/2
.
a
b
2
1.73 (m, 2H, CHCH3), 2.04 (dt, J HH ) 16.6 Hz, J HP ) 4.3 Hz,
2H, CHAHBP), 2.49 (m, 2H, CHCH3), 2.70 (dt J HH ) 16.6 Hz,
J HP ≈ 1.2 Hz, 2H, CHAHBP), 4.11 (s, 5H, C5H5), 4.25 (s, 2H,
C5H2). 11B NMR (C6D6): δ -37.65 (quint, J BH ) 81.3 Hz).
Satisfactory elemental analysis could not be obtained. The
product appears to be analytically pure, as indicated by NMR.
proximation. The absolute structure for 3a was defined by the
Flack parameter33 refinement (x ) 0.00(4)). The SHELXTL-
97 program package34 was used throughout the calculations.
Ack n ow led gm en t. We are grateful to the Russian
Foundation for Basic Research (Grant Nos. 03-03-32138
and 03-03-32499), to PowerNova Technologies Corp., to
INTAS (Grant No. 00-00179), to the European Com-
mission’s RTN Program (Grant No. HPRN-CT-2002-
00176), to the Leading Scientific School (Grant No. SS-
1060.2003.03), and to the Council of the President of
Russian Federation on the Grants (Grant No. MK-
2212.2004.3), for support of this work.
X-r a y Diffr a ction Stu d ies. X-ray-quality crystals of 3a ,b
and 5b were grown by slow crystallization from n-hexane/CH2-
Cl2 and CH2Cl2/ethanol, respectively. Details of the data
collection and structure refinement are presented in Table 3.
The data were corrected for Lorentz and polarization effects.
An absorption correction was not deemed necessary, because
the single-crystal samples were carefully chosen, rather small,
well formed, and essentially isometric, and the quality of the
obtained results and the low absorption coefficient values
justified no necessity for absorption corrections. All structures
have been solved by direct methods and refined by the full-
matrix least-squares procedure with anisotropic thermal fac-
tors for all non-hydrogen atoms. The H atoms of the BH4 group
in 5b were located in difference syntheses and refined isotro-
pically; the other H atoms in 5b as well as all H atoms in 3a
and 3b were placed at calculated positions and included in
the structure factor calculation in the riding motion ap-
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic data for 3a ,b and 5b as CIF files and figures giving
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM0497285
(33) Flack, H. D. Acta Crystallogr. 1983, A39, 876.
(34) Sheldrick, G. M. SHELXTL-97, V5.10; Bruker AXS Inc.,
Madison, WI 53719, 1997.