Synthesis and structural characterization of a novel dipalladium complex with an unprecedented PdCN bonding motif

Article abstract of DOI:10.1021/om030235u

The reactivity of palladium-imidoyl complexes with imines was studied. Investigations showed the formation of a novel dipalladium complex, with one of the palladium centers being η²-coordinated to an imidoyl group. The mechanism of the process

Full text of DOI:10.1021/om030235u

Organometallics 2003, 22, 3025-3027
3025
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of a Novel
Dip a lla d iu m Com p lex w ith a n Un p r eced en ted P d CN
Bon d in g Motif
Gareth R. Owen, Ramo´n Vilar,* Andrew J . P. White, and David J . Williams
Department of Chemistry, Imperial College London,
South Kensington, London SW7 2AZ, U.K.
Received April 1, 2003
Sch em e 1. P ossible Coor d in a tion Mod es of
Meta l-Im id oyl (a ) a n d Meta l-Im in e (b) Moieties
Summary: Removal of the chloride atom from Pd{C-
(dNXy)CH3}(Cl)(But bipy) initiates a series of reactions
2
that lead to the formation of the novel dipalladium
complex [Pd(But bipy){η2-[Pd(But bipy){N(Xy)dC(CH3)-
2
2
C(CH3)dNXy}]}][BF4]2. In this process, two imidoyl
groups undergo a C-C coupling reaction generating a
bis-imino ligand that coordinates to two palladium cen-
ters, one of which is bound to a C-N bond of the ligand
giving an unprecedented three-membered PdCN ring.
complexes,4 with the softer late transition metals
η1-coordination is usually observed. In group 10, for ex-
ample, only nickel complexes have been reported to
display η2-coordination to the imidoyl group.5 The first
fully characterized example of this type of complexs
with a nickel(II) centerswas reported in 1990 by Car-
mona et al.5a Similarly, imines tend to form η1 (σ-co-
ordinated) complexes with the late transition metals
rather than η2-species. In group 10, although several
nickel(0) complexes with η2-coordination of imines have
been reported,6 very few are known with palladium7 or
platinum.8
Insertion of small molecules such as CO, C2H4, and
CNR into metal-carbon bonds has proven to be an
efficient route for the synthesis of novel organic species
and polymeric materials.1,2 The migratory insertion of
isocyanides (which are generally regarded as better
σ-donors and weaker π-acceptors than CO)3 into metal-
carbon bonds yields imidoyl groups, which, in principle,
can be coordinated to the metal in either an η1- or
η2-fashion (see Scheme 1). While the lanthanides, ac-
tinides, and early transition metals tend to form η2-
* Corresponding author. Fax: (+44) 20-7594-5880. E-mail: r.vilar@
imperial.ac.uk.
As part of a systematic investigation into metal-
mediated C-C and C-N bond formation,9 we have
recently studied the reactivity of palladium-imidoyl
complexes with a range of unsaturated species (such as
olefins and isocyanates). By analogy with the work
recently reported by Sen10 and Arndtsen,11 we investi-
gated the possibility of inserting imines into the pal-
(1) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987. (b) Tsuji, J . Pal-
ladium Reagents: Innovations in Organic Synthesis; Wiley: Chiches-
ter, 1995. (c) Yamamoto, A. Organotransition Metal Chemistry; Wiley-
Interscience: New York, 1986.
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Sen, A. Acc. Chem. Res. 1993, 26, 303. (c) Baar, C. R.; J ennings, M.
C.; Puddephatt, R. J . Organometallics 2001, 20, 3459. (d) Carfagna,
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(e) Brookhart, M.; Rix, F. C.; DeSimone, J . M.; Barborak, J . C. J . Am.
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(b) Yamamoto, Y. Coord. Chem. Rev. 1980, 32, 193. (c) Singleton, E.;
Oosthuizen, H. E. Adv. Organomet. Chem. 1983, 22, 209. (d) Becker,
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Bombieri, G. Organometallics 1986, 5, 1369. (f) Maitlis, P. N.; Espinet,
P.; Russel, M. J . H. In Comprehensive Organometallic Chemistry I;
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Oxford, U.K., 1982; Vol. 6, p 284. (g) Dixon, K. R.; Dixon, A. C. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone F. G.
A., Wilkinson, G. Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 9, p
208. (h) Yamamoto, Y.; Yamazaki, H. Bull. Chem. Soc. J pn. 1970, 43,
2653. (i) Yamamoto, Y.; Yamazaki, H. Bull. Chem. Soc. J pn. 1970, 43,
3634. (j) Yamamoto, Y.; Yamazaki, H. Bull. Chem. Soc. J pn. 1971, 44,
1873. (k) Yamamoto, Y.; Yamazaki, H. Inorg. Chem. 1975, 13, 438. (l)
Kim, Y.-J .; Song, S.-W.; Lee, S.-C.; Lee, S.-W.; Osakada, K.; Yamamoto,
T. J . Chem. Soc., Dalton Trans. 1998, 1775. (m) Crociani, B.; Bandoli,
G.; Clemente, D. A. J . Organomet. Chem. 1980, 184, 269. (n) Crociani,
B.; Sala, M.; Polo, A.; Bombieri, G. Organometallics 1986, 5, 1369. (o)
Crociani, B.; Granozzi, G. Inorg. Chim. Acta 1987, 132, 197. (p)
Crociani, B.; Nicolini, M.; Richards, R. L. J . Chem. Soc., Dalton Trans.
1978, 1479. (q) Vicente, J .; Abad, J .-A.; Frankland, A. D.; Lo´pez-
Serrano, J .; Ram´ırez de Arellano, M. C.; J ones, P. G. Organometallics
2002, 21, 272.
ladium-imidoyl bond of Pd{C(dNXy)CH3}(Cl)(But bipy)
2
(2) (where But bipy ) 4,4′-di-tert-butyl-2,2′-dipyridyl; Xy
2
) 2,6-dimethylphenyl isocyanide). In the course of these
(4) (a) Durfee, L. D.; McMullen, A. K.; Rothwell, I. P. J . Am. Chem.
Soc. 1988, 110, 1463. (b) Durfee, L. D.; Hill, J . E.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1990, 9, 75. (c) Vivanco, M.; Ruiz, J .;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 1802.
(5) (a) Carmona, E.; Palma, P.; Paneque, M.; Poveda, M. L. Orga-
nometallics 1990, 9, 583. (b) Ca´mpora, J .; Carmona, E.; Gutie´rrez, E.;
Palma, P.; Poveda, M. L.; Ruiz, C. Organometallics 1992, 11, 11. (c)
Ca´mpora, J .; Gutie´rrez, E.; Monge, A.; Poveda, M. L.; Ruiz, C.;
Carmona, E. Organometallics 1993, 12, 4025.
(6) (a) Karsch, H. H.; Leithe, A. W.; Reisky, M.; Witt, E. Organo-
metallics 1999, 18, 90. (b) Hoberg, H.; Go¨tz, V.; Kru¨ger, C.; Tsay, Y.-
H. J . Organomet. Chem. 1979, 169, 209. (c) Gabor, B.; Kru¨ger, C.;
Marczinke, B.; Mynott, R.; Wilke, G. Angew. Chem., Int. Ed. Engl.
1991, 30, 1666.
(7) (a) Butler, G.; Eaborn, C.; Pidcock, A. J . Organomet. Chem. 1980,
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Dalton Trans. 1973, 1620.
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metallics 2002, 21, 4799.
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10.1021/om030235u CCC: $25.00 © 2003 American Chemical Society
Publication on Web 06/25/2003

3026 Organometallics, Vol. 22, No. 15, 2003
Communications
Sch em e 2. Rea ction Sch em e Sh ow in g th e Tw o
Rou tes for th e Syn th esis of 4
spectrum of 3 showed the characteristic stretching
frequencies associated with the iminoacyl group and
the σ-coordinated imine (intense and broad bands
1
between 1617 and 1587 cm-1). The H NMR spectrum
shows all the expected resonances associated with the
imine, the iminoyl groups, and the But bipy chelating
2
ligand. Particularly indicative of the presence of the
coordinated imine are the singlets at 8.65 and 3.91 ppm,
which correspond to the CH and methyl protons in
PhCHdNCH3, respectively. The formulation of 3 was
further confirmed by FAB(+) mass spectrometry (which
showed the molecular peak at 641 amu corresponding
to 3 - BF4) and by elemental analyses. The spectro-
scopic characterization of this compound unambiguously
demonstrated the coordination of the imine; however it
did not show any evidence of insertion.
With the aim of obtaining single crystals of 3 for
structural characterization, the crude solid was redis-
solved in CH2Cl2 and a layer of diethyl ether was added.
The solution was left to stand for 2 days, after which
time yellow-orange crystals were formed. Surprisingly,
these crystals proved to have a different formulation
than the original compound 3. A single-crystal X-ray
study of this material14 revealed it to be the novel
dipalladium species [Pd(But bipy){η2-[Pd(But bipy)-
2
2
{N(Xy)dC(CH3)C(CH3)dNXy}]}][BF4]2 (4). The unusual
bimetallic structure of 4 comprises two “Pd(But bipy)”
2
units linked by an organic fragment formed by the
coupling of two iminoacyl units from the starting
material (see Scheme 2 and Figure 1). The geometry at
Pd(1) is distorted square planar with Pd-N bonds in
the range 2.038(7)-2.059 Å, each of the two five-
membered chelate rings being planar to within 0.038
Å. The coordination at Pd(2) comprises a conventional
studies the formation of the unprecedented dipalladium
species [Pd(But bipy){η2-[Pd(But bipy){N(Xy)dC(CH3)-
2
2
C(CH3)dNXy}]}][BF4]2 (4) was observed and is herein
reported (see Scheme 2). In this complex one of the
palladium centers is η2-coordinated to an imidoyl group,
representing the first structurally characterized ex-
ample of such a coordination mode for palladium.
The palladium-iminoacyl complex Pd{C(dNXy)CH3}-
chelation to a But bipy ligand and an unprecedented
2
coordination to a C-N bond from one of the coupled
“iminoacyl” groups to give a three-membered PdCN
ring, the nitrogen atom bridging the two palladium
(Cl)(But bipy) (2) was prepared by reacting Pd(CH3)-
2
(Cl)(But bipy) (1) with CNXy (following the method
2
reported by Vrieze12). The IR spectrum of 2 shows an
intense band at 1616 cm-1 associated with the charac-
teristic ν(CdN) of inserted isocyanides. The 1H NMR
spectrum shows the expected resonances for the protons
present in 2; particularly indicative of the insertion
process is a singlet at 2.27 ppm corresponding to the
methyl protons of the -C(dNXy)CH3 fragment (the
same protons in the starting material appear at 0.82
ppm). The 13C NMR spectrum also shows the expected
signals for the proposed formulation. Two singlets at
29.1 and 163.5 ppmsassigned to the -CH3 and CdNXy
carbon atoms of the -C(dNXy)CH3 moietysindicate
that the insertion of CNXy has taken place. The
formulation was further confirmed by FAB(+) mass
spectrometry (which showed the molecular peak at 556
amu corresponding to 2) and elemental analyses.
With the aim of inserting an imine into the pal-
ladium-iminoacyl bond, complex 2 was reacted with 1
equiv of PhCHdNCH3 in the presence of 1 equiv of
AgBF4. This reaction produced the new palladium
centers. The Pd(2)-N bonds to the But bipy ligand
2
[2.109(7) Å to N(5) and 2.129(8) Å to N(6)] are both ca.
0.06 Å longer than their counterparts to Pd(1). The bond
to N(6) is slightly longer than that to N(5), reflecting
the different trans influences of carbon versus nitrogen.
Within the three-membered ring the Pd(2)-N(4) and
Pd(2)-C(1) distances of 2.092(8) and 2.054(10) Å are
unexceptional, but the C(1)-N(4) distance of 1.431(10)
(13) Synthesis of [Pd{C(dNXy)CH3}(But2bipy){N(CH3)dCHPh}]-
[BF4] (3): To a solution of Pd{C(dNXy)CH3}(Cl)(But2bipy) (0.25 g, 0.45
mmol) in CH2Cl2 (25 mL) were added PhCHdNCH3 (52 µL, 0.45 mmol)
and AgBF4 (0.09 g, 0.45 mmol). The reaction mixture was stirred for
30 min, during which time a fine precipitate (AgCl) was formed. The
mixture was filtered to remove the silver salt and then concentrated
under reduced pressure to ca. 10 mL. Upon addition of hexane (20 mL)
a bright yellow solid precipitated and was isolated by filtration. Yield:
0.25 g, 0.34 mmol, 76%. Anal. Found: C, 59.55; H, 6.29; N, 7.69 (calcd
for C36H45N4PdBF4: C, 59.48; H, 6.24; N, 7.71). IR (KBr): ν 1617 br,
t
1587 (CdN), 1058 (BF4). 1H (CDCl3): δ 1.40 (s, 9H, Bu), 1.46 (s, 9H,
tBu), 1.99 [br s, 9H, {6H, C6H3(CH3)2} + {3H, Pd(CdNR)CH3}], 3.91
[br, s, 3H, N(CH3)dCPhH], 6.72 [m, 1H, C6H3(CH3)2], 6.86 [m, 2H,
C6H3(CH3)2], 7.48 [m, 5H, {3,3′-bipy} + {3,4,5-NdC(C6H5)H}], 7.91 [d,
3
1H, {2-bipy}, J HH ) 5 Hz], 8.16 [s, 1H, 5-bipy], 8.20 [s, 1H, 5′-bipy],
8.49 [d, 1H, {2′-bipy}, 3J HH ) 5.5 Hz], 8.65 [s, 1H, NdCPhH], 8.75 [m,
2H, {2,6-NdC(C6H5)H}]. MS (FAB)+ m/z (rel int): 641 (3) [M - BF4]+,
520 (100) [M - BF4 - imine]+.
complex [Pd{C(dNXy)CH3}(But bipy){N(CH3)dCHPh}]-
2
[BF4] (3), where the imine is σ-coordinated to the vacant
(14) Crystal data for 4: [C56H72N6Pd2][BF4]2‚1.5C6H13, M ) 1343.4,
monoclinic, C2/c (no. 15), a ) 34.094(3) Å, b ) 16.575(2) Å, c )
site left upon removal of the chloride atom.13 The IR
24.868(2) Å, â ) 113.15(1)°, V ) 12922(2) Å3, Z ) 8, Dc ) 1.381 g cm-3
,
(11) Arndtsen, B. A.; Dghaym, R. D.; Yaccato, K. J . Organometallics
1998, 17, 4.
(12) Delis, J . G. P.; Aubel, P. G.; Vrieze, K.; van Leeuwen, P. W. N.
M.; Veldman, R.; Spek, A. L. Organometallics 1997, 16, 2948.
µ(Cu KR) ) 5.04 mm-1, T ) 203 K, yellow-orange needles; 8899
independent measured reflections, F2 refinement, R1 ) 0.071, wR2
)
0.163, 6368 independent observed absorption corrected reflections [|Fo|
> 4σ(|Fo|), 2θmax ) 120°], 680 parameters. CCDC 211066.

Communications
Organometallics, Vol. 22, No. 15, 2003 3027
the But bipy ligand and with the Xy and C(CH3) methyl
2
groups of the two different -C(CH3)dNXy moieties. The
integration of these signals and those present in the
aromatic region is consistent with the formulation of 4
(which was also confirmed by elemental analyses).
The conversion of 3 to the novel binuclear complex 4
implies the decoordination of the imine from the original
compound and the carbon-carbon coupling of two
iminoacyl fragments (which come from two different
molecules of 3). Consequently, 4 should also be formed
by “dimerization” of the cationic species [Pd{C(dNXy)-
CH3}(But bipy)]+ (A in Scheme 2). Thus, it was of
2
interest to study the stability of 2 in solution upon
removal of its chloride ligand. When 2 was dissolved in
CH2Cl2 and 1 equiv of AgBF4 was added to remove the
chloride, a yellow-orange solution was obtained. After
30 min the solution was filtered to remove the AgCl,
and upon addition of hexane a yellow-orange solid was
obtained and characterized as 4 on the basis of 1H NMR
and IR spectroscopy, elemental analyses, and FAB(+)
mass spectrometry (all of which gave the same data as
the product obtained from 3 in CH2Cl2).18
F igu r e 1. Molecular structure of 4.
Å emphasizes its single-bond character. The three-
membered ring is inclined approximately orthogonally
(ca. 86°) to the ring formed by Pd(1) and the coupled
“iminoacyl” moiety. The organic unit formed by the
coupling of two iminoacyl groups can be regarded as a
bis-imino ligand which chelates to Pd(1) and on coor-
dination to Pd(2) loses the double-bond character of one
of its original CdN linkages [C(1)-N(4)], while the other
[C(2)dN(3) 1.306(11) Å] remains as an imino group. We
believe that this is the first structurally characterized
example of a three-membered PdCN ring. Indeed we
have identified only one structurally characterized
example of an analogous ring system occurring for
platinum.15 Examples involving nickel are more com-
monplace, but the C-N bond length in these systems
is usually appreciably shorter and indicative of a partial
π-coordination.16,17
The details of the pathway by which [Pd{C(dNXy)-
CH3}(But bipy)]+ is converted into the novel dipalladium
2
species 4 are not yet known. However, a mechanism for
this process is likely to involve, first, the transformation
of the η1-imidoyl complex A into the η2-imidoyl species
B (see Scheme 2). The latter can then undergo a Pd-C
bond cleavage followed by dimerization of two of these
intermediate species via formation of new C-C and
Pd-N bonds. The exact mechanism by which these
events occur is not obvious, and further investigations
will be carried out to elucidate it.
Ack n ow led gm en t. We are very grateful to EPSRC
for financial support (G.R.O.) and J ohnson Matthey PLC
for a loan of palladium.
The spectroscopic and analytical characterization of
4 is consistent with the structure revealed by X-ray
crystallography. The IR spectrum shows the presence
of intense (and broad) CdN stretching frequencies in
the region between 1630 and 1549. These can be
associated with the two different CdN(Xy) bonds and
Su p p or tin g In for m a tion Ava ila ble: Details about the
syntheses of compounds 1-4 and the X-ray crystal structure,
including ORTEP diagrams, and tables of crystal data and
structure refinement, atomic coordinates, bond lengths and
angles, and anisotropic displacement parameters for 4, are
available free of charge via the Internet at http://pubs.acs.org.
with the CdN bonds present in the But bipy ligand. The
2
relatively low ν(CdN) value associated with the η2-
coordinated imidoyl group (in comparison to the value
of around 1700 cm-1 for other metal η2-imidoyl com-
plexes)5 is probably due to the coordination of this group,
in 4, to a second palladium center. The FAB(+) mass
spectrum shows the molecular peak at 1127 amu, which
corresponds to [4 - BF4]+. The 1H NMR spectrum shows
several broad singlets in the region between 1.34 and
2.75 ppm. These singlets correspond to the several
methyl groups present in 4, i.e., those associated with
OM030235U
(18) Synthesis of [Pd(But2bipy){η2-[Pd(But2bipy){N(Xy)dC(CH3)-
C(CH3)dNXy}]}][BF4]2 (4): To a solution of Pd{C(dNXy)CH3}(Cl)-
(But2bipy) (0.16 g, 0.29 mmol) in CH2Cl2 (20 mL) was added AgBF4
(0.06 g, 0.30 mmol). The resulting mixture was stirred for 30 min,
during which time a fine precipitate (AgCl) was formed. The reaction
mixture was filtered to remove the silver salt, and the yellow-orange
filtrate was concentrated (to ca. 10 mL) under reduced pressure. Upon
addition of hexane (15 mL), a yellow-orange solid was obtained. Yield:
0.12 g, 0.194 mmol, 68%. Anal. Found: C, 55.28; H, 5.96; N, 6.86 (calcd
for C56H72N6Pd2B2F8: C, 55.33; H, 5.97; N, 6.91). IR (KBr): ν ) 1617
(CdN), 1549 (CdN), 1059 (BF4). 1H NMR (CD2Cl2): δ 1.34-1.45 [m,
42H, {4 × 9H, C(CH3)3} + {2 × 3H, N-C(CH3)}], 2.45 [s, 3H, {C6H3-
(CH3)2}], 2.53 [s, 3H, {C6H3(CH3)2}], 2.59 [s, 3H, {C6H3(CH3)2}, 2.75
[s, 3H, {C6H3(CH3)], 7.34-8.44 [m, 18H, [2 × 3H, {C6H3(CH3)2}] + [4
× 3H, {2,3,5-pyridine-H}]. MS (FAB)+ m/z (rel int): 1127 (15) [M -
BF4]+, 1041 (4) [M - 2BF4]+, 896 (20) [M - 2BF4 - {C(CH3)dNXy}]+,
665 (26) [(But2bipy)Pd{C(CH3)dNXy}2]+, 520 (78) [(But2bipy)Pd-
{C(CH3)dNXy}]+, 374 (100) [(But2bipy)Pd]+.
(15) Oliver, J . D.; Davis, R. E. J . Organomet. Chem. 1977, 137, 373.
(16) Karsch, H. H.; Leithe, A. W.; Reisky, M.; Witt, E. Organome-
tallics 1999, 18, 90.
(17) Sepelak, D. J .; Pierpont, C. G.; Barefield, E. K.; Budz, J . T.;
Poffenberger, C. A. J . Am. Chem. Soc. 1976, 98, 6178.