Synthesis and reactivity of cationic palladium phosphine carboxylate complexes

Article abstract of DOI:10.1021/om050356p

Simple routes to two new types of mononuclear cationic palladium phosphine acetate complexes featuring κ¹- or κ ²-carboxylates have been developed, and the resulting materials have been characterized by single-crystal X-ray diffracti

Full text of DOI:10.1021/om050356p

Organometallics 2005, 24, 4099-4102
4099
Communications
Synthesis and Reactivity of Cationic Palladium
Phosphine Carboxylate Complexes
Natesan Thirupathi,†,‡ Dino Amoroso,§ Andrew Bell,§ and John D. Protasiewicz*,†
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, and
Promerus LLC, Brecksville, Ohio 44141
Received May 6, 2005
Summary: Simple routes to two new types of mono-
nuclear cationic palladium phosphine acetate complexes
featuring κ1- or κ2-carboxylates have been developed, and
the resulting materials have been characterized by
single-crystal X-ray diffraction. In acetonitrile the com-
plex [(iPr3P)2Pd(κ2O,O′-OAc)]+ leads not to the indepen-
dently prepared trans-[(iPr3P)2Pd(MeCN)(OAc)]+ but to
reversible cyclometalation to form [(iPr3P)(MeCN)Pd-
(κ2C,P-C(Me)2PiPr2)]+.
Cationic palladium phosphine complexes are under
active study, due to their role in an enormous number
of important synthetic transformations and for the
polymerization of polar and nonpolar olefins.1-5 In
particular, such complexes bearing acetate ligands have
received added attention for catalysis of the copolym-
erization of carbon monoxide and olefins.6-10 Studies of
catalytic systems derived from mixtures of chelating
diphosphines, Pd(OAc)2, and acids have proved more
complicated than was first envisioned. For example, the
autoionization postulated in eq 1 was later revised to
reflect the fact that [(dppe)2Pd]2+ was the kinetic
product of addition of dppe (dppe ) 1,2-bis(diphenyl-
phosphino)ethane) to Pd(OAc)2 in methanol, and a
slower transformation of this material to the thermo-
dynamically favored [(dppe)Pd(OAc)2] (eq 2).11-13 The
† Case Western Reserve University.
‡ Current address: Department of Chemistry, University of Delhi,
Delhi 110 007, India.
§ Promerus LLC.
(1) (a) Negishi, E.; de Meijere, A. Handbook of Organopalladium
Chemistry for Organic Synthesis; Wiley-Interscience: New York, 2002;
Vols. 1 and 2. (b) Tsuji, J. Palladium Reagents and Catalysts:
Innovations in Organic Synthesis; Wiley: New York, 1995. (c) Li, J.
J.; Gribble, G. W. Palladium in Heterocyclic Chemistry: A Guide for
the Synthetic Chemist; New York, 2000.
role of added acid in this system was suggested to
sequester OAc- ions that aided the transformation of
[(dppe)2Pd]2+ to [(dppe)Pd(OAc)2].
(2) (a) Late Transition Metal Polymerization Catalysis; Rieger, B.,
Baugh, L. S., Kacker, S., Striegler, S., Eds.; Wiley-VCH: Weinheim,
Germany, 2003. (b) McLain, S. J.; Johnson, L. K.; Bennett, A. M. A.;
Sweetman, K. J. Polym. Mater. Sci. Eng. 2001, 84, 917-918.
(3) Mecking, S. Coord. Chem. Rev. 2000, 203, 325-351.
(4) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
1169-1204.
On the other hand, studies of the copolymerization
of carbon monoxide and ethylene using the closely
related system [(dppp)Pd(OAc)2] (dppp ) 1,2-bis(diphen-
ylphosphino)propane) have suggested that the spectro-
scopically detected [(dppp)Pd(OAc)]+ (eq 3) is the cata-
lytically important species in methanol/CF3COOH
mixtures.14
While these studies have been ongoing, the utility of
nonchelating phosphines for polymerization catalysis is
far less conspicuous. Our specific interest in cationic
(5) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428-447.
(6) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663-681.
(7) Sen, A. Acc. Chem. Res. 1993, 26, 303-310.
(8) Bianchini, C.; Meli, A. Coord. Chem. Rev. 2002, 225, 35-66.
(9) Bianchini, C.; Meli, A.; Oberhauser, W. Dalton 2003, 2627-2635.
(10) Mul, W. P.; van der Made, A. W.; Smaardijk, A. A.; Drent, E.
Catal. Met. Complexes 2003, 27, 87-140.
(11) Bianchini, C.; A. Meli; Oberhauser, W. Organometallics 2003,
22, 4281-4285.
(12) Marson, A.; Van Ort, A. B.; Mul, W. P. Eur. J. Inorg. Chem.
2002, 11, 3028-3031.
(13) Angulo, I. M.; Bouwman, E.; Lutz, M.; Mul, W. P.; Spek, A. L.
Inorg. Chem. 2001, 40, 2073-2082.
(14) Pivovarov, A. P.; Novikova, E. V.; Belov, G. P. Russ. J. Coord.
Chem. 2000, 26, 38-44.
10.1021/om050356p CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/12/2005

4100 Organometallics, Vol. 24, No. 17, 2005
Communications
Figure 1. X-ray structural diagrams of 1a (left) and 2a (right), with the anions, solvent molecules, and hydrogen atoms
omitted for clarity. Selected bond distances (Å) and angle (deg) for 1a: Pd-P1, 2.376(2); Pd-P2, 2.376(2); Pd-N1, 1.986(5);
Pd-O1, 2.007(4); P1-Pd-P2, 177.46(6). Selected bond distances (Å) and angle (deg) for 2a: Pd-P1, 2.271(1); Pd-P2,
2.257(1); Pd-O1, 2.138(3); Pd-O2, 2.130(3); P1-Pd-P2, 107.14(4).
palladium complexes was initiated during recent efforts
to ascertain how mixtures of trans-[(R3P)2Pd(OAc)2] and
[Li(OEt2)2.5][B(C6F5)4] or [Me2(H)NPh][B(C6F5)4] pro-
mote the polymerization of norbornene.15 Herein we
present the synthesis and structural characterizations
of several cationic palladium phosphine carboxylate
complexes that are derived by the action of lithium salts
or acids on trans-[(R3P)2Pd(OAc)2]. These new mono-
cationic species bridge the previously reported neutral
cis-[P2Pd(OAc)2] and dicationic [P4Pd]2+ complexes.
Furthermore, we show that the nature of these mono-
cationic acetato complexes is very dependent on their
mode of formation and that some of these complexes are
susceptible to facile and reversible cyclometalation of
coordinated triisopropylphosphine ligands.
Reactions of the palladium(II) acetate complexes
trans-[(R3P)2Pd(OAc)2] with [Li(OEt2)2.5][B(C6F5)4] in
acetonitrile led to the selective formation of trans-
[(R3P)2Pd(OAc)(MeCN)][B(C6F5)4] (R ) Cy, 1a; R ) iPr,
1b), as judged by the appearance of new 31P NMR
signals ca. 9-14 ppm downfield from those of their
respective neutral trans-[(R3P)2Pd(OAc)2] precursors.16
1 (left). The geometry of the palladium center in 1a is
square planar and features a palladium-oxygen bond
length of 2.007(4) Å, which is slightly shorter than those
found in structurally characterized trans-[(R3P)2Pd-
(OAc)2]17 but comparable to palladium-oxygen bond
lengths in cationic pyridine palladium acetate com-
plexes.18 Further studies of these reactions showed that
not only were isolated yields dependent upon the
reagent used to abstract the acetate units but also the
identity of the products obtained depended on the choice
of solvent. For example, changing the solvent for such
reactions from acetonitrile to dichloromethane or THF
led to an inseparable mixture of materials. Replacing
[Li(OEt2)2.5][B(C6F5)4] with [Me2(H)NPh][B(C6F5)4] and
performing the reaction in dichloromethane, however,
led to very different types of complexes. Studies of these
reactions by 31P NMR spectroscopy revealed new singlet
resonances at approximately 25-27 ppm downfield from
those signals observed for the corresponding products
1a,b. From the reaction of trans-[(Cy3P)2Pd(OAc)2] with
[Me2(H)NPh][B(C6F5)4], [(Cy3P)2Pd(κ2O,O′-OAc)][B(C6F5)4]
(2a) was thus isolated in 57% yield (Scheme 1).19
A chelating acetate binding mode in 2a was rigorously
identified by a single-crystal X-ray diffraction study
(Figure 1 (right)).20 The P-Pd-P bond angle has nar-
(16) 1a: 31P{1H} NMR (CDCl3) δ 32.7; 1H NMR (CDCl3) δ 1.12-
1.22 (m, 12H), 1.24-1.33 (qt, J ) 12.9 Hz; J ) 3.15 Hz, 6H), 1.62 (q,
J ) 12.45 Hz, 12H), 1.77 (d, J ) 12.6 Hz, 6H), 1.89 (d, J ) 13.8 Hz,
14H), 1.93 (d, J ) 11.4 Hz, 16H), 2.00 (s, 3H), 2.39 (s, 3H); 13C{1H}
2
NMR (CDCl3) δ 3.3, 23.4, 26.3, 27.9 (virtual t, JCP
+
4JCP ) 5.4 Hz),
29.9, 33.7 (virtual t, 1JCP + 3JCP ) 9.45 Hz), 124.5 (br), 127.2, 136.4 (d,
1JCF ) 242.2 Hz), 138.4 (d, JCF ) 244.0 Hz), 148.4 (d, JCF ) 242.9
Hz), 175.5. Anal. Calcd for C64H72NO2P2BF20Pd‚Et2O: C, 53.71; H, 5.44;
N, 0.92. Found: C, 53.85; H, 5.18; N, 0.93. 1b: 31P{1H} NMR (CDCl3)
δ 44.5; 1H NMR (CDCl3) δ 1.37 (m, 36H), 1.92 (s, 3H), 2.22 (m, 6H),
1
1
2.34 (s, 3H); 13C{1H} NMR (CDCl3) δ 4.2, 19.6, 23.2, 24.5 (virtual t,
The pale yellow, air-stable crystalline products are
isolated in near-quantitative yields. The results of the
crystallographic analysis of 1a are displayed in Figure
1JCP
+
3JCP ) 10.1 Hz), 124.4 (br), 128.5, 136.9 (d, JCF ) 247.9 Hz),
1
1
1
138.8 (d, JCF ) 243.0 Hz), 148.7 (d, JCF ) 239.8 Hz), 176.0. Anal.
Calcd for C46H48NO2P2BF20Pd: C, 45.81; H, 4.01; N, 1.16. Found: C,
46.00; H, 3.92; N, 1.18.
(15) (a) Lipian, J.-H.; Rhodes, L. F.; Goodall, B. L.; Bell, A.; Mimna,
R. A.; Fondran, J. C.; Hennis, A. D.; Elia, C. N.; Polley, J. D.; Sen, A.;
Saikumar, J. PCT Int. Appl. WO 2000020472, 2000. (b) Rhodes, L. F.;
Bell, A.; Ravikiran, R.; Fondran, J. C.; Jayaraman, S.; Goodall, B.
Leslie; Mimna, R. A.; Lipian, J.-H. U. S. Pat. Appl. Publ. US
2003181607.
(17) Cambridge Crystallographic Database version 5.25, November
2003.
(18) (a) MacLean, E. J.; Robinson, R. I.; Teat, S. J.; Wilson, C.;
Woodward, S. Dalton 2002, 3518-3524. (b) Canty, A. J.; Minchin, N.
J.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1986,
2205-2210.

Communications
Organometallics, Vol. 24, No. 17, 2005 4101
Scheme 1a
a Legend: (i) [Me2(H)NPh][B(C6F5)4], CH2Cl2; (ii) HOTs‚H2O,
CH2Cl2; (iii) [Li(OEt2)2.5][B(C6F5)4], CH2Cl2.
rowed to 107.14(4)° from the 177.46(6)° seen in 1a.
Notably, the Pd-P bond distances increase by 0.11 Å
on going from 2a to 1a, while the palladium-oxygen
distances in 2a are nearly identical with one another
and are about 0.13 Å longer than the palladium-oxygen
distance found in 1a. Structurally characterized mono-
nuclear palladium complexes possessing chelating car-
boxylates are rare,21 but comparisons to our cationic
derivative can be drawn to the recently reported neutral
[(IPr)Pd(OAc)(κ2O,O′-OAc)] (IPr ) N,N′-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene) complex, which con-
tains both chelated and nonchelated acetate moieties.22
For this complex palladium-oxygen distances of 2.005(4)
Å for the nonchelated acetate and 2.034(4) and 2.163(4)
Å for the chelated acetate group are realized.
Figure 2. X-ray structural diagram of 3b. Anion and
hydrogen atoms omitted for clarity. Selected bond distances
(Å) and angles (deg) for 3b: Pd1-N1, 2.168(4); Pd1-C1,
2.171(5); Pd1-P1, 2.206(1); Pd1-P2, 2.376(1); P1-C1,
1.748(6); P1-C4, 1.846(6); P1-C7, 1.848(7); N1-Pd1-C1,
95.5(2); N1-Pd1-P1, 142.4(1); C1-Pd1-P1, 47.1(2); N1-
Pd1-P2, 99.9(1); C1-Pd1-P2, 164.2(2); P1-Pd1-P2,
117.66(5); P1-C1-Pd1, 67.5(2); C1-P1-Pd1, 65.4(2).
coordination polymer [(py)4Pd][(py)2Pd(OAc)2][B(C6F5)4]2
can be crystallized and structurally characterized.24
While complexes 1 and 2 differ in composition by a
molecule of acetonitrile, addition of acetonitrile to 2 does
not lead to any detectable amounts of 1. The 31P NMR
spectrum of 2b in acetonitrile-d3 after 23 h revealed two
broad peaks (δ 52.1 and 43.6 ppm). Oddly, removal of
acetonitrile-d3 from the reaction product followed by
dissolution of the resulting material in chloroform-d and
analysis by 31P NMR spectroscopy revealed the presence
of a mixture of 2b (ca. 70%) and a new species (3a).
Stirring the acetontrile-d3 solution of 2b in the presence
of excess sodium carbonate resulted in clean formation
of a new species having more resolved NMR signals (δ
In general, samples of pure [(R3P)2Pd(κ2O,O′-OAc)]-
i
[B(C6F5)4] (R ) Cy; 2a; R ) Pr; 2b) were difficult to
isolate in high yields using [Me2(H)NPh][B(C6F5)4];
therefore, a better carboxylate abstraction process was
developed. Reaction of trans-[(R3P)2Pd(OAc)2] with p-
toluenesulfonic acid (HOTs‚H2O) was found to cleanly
generate [(R3P)2Pd(κ2O,O′-OAc)]OTs, having 31P NMR
signals nearly identical with those of 2 (not isolated; 31
P
2
i
51.7 (d), 43.2 (d), JPP ) 30 Hz). Complete purification
NMR δ 59 (R ) Cy) and δ 70 (R ) Pr) ppm), which
upon anion exchange leads to 2a and 2b.19,23 The
formation of 2 can be contrasted to the reaction of trans-
[(py)2Pd(OAc)2] and [Li(OEt2)2.5][B(C6F5)4] (1:1) in CH2-
Cl2, which leads to a dynamic mixture of trans-[(py)2Pd-
(OAc)2], [(py)3Pd(OAc)]+, and [(py)4Pd]2+, from which the
of the complex, unfortunately, was hindered by its waxy
nature, and thus other bases were examined. The 31P
NMR spectrum of a 1:2 mixture of 2b and pyridine in
dichloromethane revealed rapid and selective formation
of a closely related complex showing similar NMR
2
signals (δ 49.1 (d), 37.2 (d), JPP ) 29 Hz). Reducing
the amount of pyridine to 1:1, however, resulted in only
50% conversion of 2b to the new complex. Under the
former conditions the crystalline three-membered pal-
ladacycle 3b (Scheme 2) was isolated in high yields.25
Addition of pyridine to 3a yields 3b quantitatively. The
identity of 3b was established by single-crystal X-ray
diffraction analysis, and the results are shown in Figure
2. The Pd-C and Pd-P bond distances of 2.1751(5) and
(19) 2a: 31P{1H} NMR (CDCl3) δ 59.3; 1H NMR (CDCl3) δ 1.24-
1.34 (m, 20H), 1.66 (q, J ) 11.4 Hz, 12H), 1.80 (br, 6H), 1.90 (br, 12H),
1.96 (d, J ) 13.8 Hz, 12H), 2.00 (d, J ) 12.0 Hz, 4H), 2.04 (s, 3H);
2
13C{1H} NMR (CDCl3) δ 25.1, 25.7, 27.2 (virtual t, JCP
+
4JCP ) 5.5
1
Hz), 30.2, 34.7 (m), 124.2 (br), 136.2 (d, JCF ) 247.8 Hz), 138.1 (d,
1JCF ) 241.8 Hz), 148.2 (d, JCF ) 239.2 Hz), 194.9. Anal. Calcd for
1
C
62H69O2P2BF20Pd: C, 52.99; H, 4.95%. Found, C, 53.29; H, 5.05. 2b:
31P{1H} NMR (CDCl3) δ 69.4; 1H NMR (CDCl3) δ 1.45 (m, 36H), 2.02
(s, 3H), 2.26-2.39 (m, 6H); 13C{1H} NMR (CDCl3) δ 20.1, 25.3, 26.3
(m), 124.4 (br), 136.9 (d, 1JCF ) 241.0 Hz), 138.8 (d, 1JCF ) 243.0 Hz),
1
148.8 (d, JCF ) 237.3 Hz), 196.1. Anal. Calcd for C44H45O2P2PdBF20
:
C, 45.36; H, 3.89. Found: C, 45.37; H, 3.88.
(20) Details for the crystal structures are given in the Supporting
Information in CIF format.
(24) Ma, L.; Smith, R. C.; Protasiewicz, J. D. Inorg. Chim. Acta, in
press.
(21) Such structures have been suggested for supported catalysts:
(a) Churruca, F.; SanMartin, R.; Tellitu, I.; Dominguez, E. Tetrahedron
Lett. 2003, 44, 5925-5929. (b) Colacot, T. J.; Gore, E. S.; Kuber, A.
Organometallics 2002, 21, 3301-3304.
(22) Viciu, M. S.; Stevens, E. D.; Petersen, J. L.; Nolan, S. P.
Organometallics 2004, 23, 3752-3755.
(23) In contrast, reactions of p-toluenesulfonic acid (2 equiv) with
[(Ar3P)2Pd(OAc)2] have been reported to yield the neutral complex
[(Ar3P)2Pd(OTs)2]. See: Seayad, A.; Jayasree, S.; Damodaran, K.;
Toniolo, L.; Chaudhari, R. V. J. Organomet. Chem. 2000, 601, 100-
107.
(25) 3b: 31P{1H} NMR (CDCl3) δ 49.1 (d, nonmetalated phosphorus),
2
37.2 (d, metalated phosphorus), JPP ) 29.3 Hz; 1H NMR (CDCl3) δ
1.14-1.21 (m, 24H, CH(CH3)2, ring-C(CH3)2), 1.41-1.47 (m, 12H,
CH(CH3)2), 2.00 (m, 3H, CH(CH3)2), 2.52 (m, 2H, CH(CH3)2), 7.50 (t,
3JHH3 ) 6.30 Hz, 2H, C5H5N), 7.87 (t, 3JHH ) 7.20 Hz, 1H, C5H5N), 8.51
(d, JHH ) 4.20 Hz, 2H, C5H5N); 13C{1H} NMR (CDCl3) δ 20.1, 20.3,
1
21.8, 22.5, 24.6 (d, 1JCP ) 13.8 Hz), 24.8 (d, JCP ) 26.8 Hz), 40.9 (dd,
2JPC ) 46.0, 28.3 Hz, 1C, ring-C(CH3)2), 124.1 (br), 126.2, 136.4 (d,
1
1
1JCF ) 245.4 Hz), 138.4 (d, JCF ) 244.2 Hz), 138.8, 148.4 (d, JCF
)
237.3 Hz), 151.1. Anal. Calcd for C47H46NP2PdBF20: C, 47.67; H, 3.92;
N, 1.18. Found: C, 47.67; H, 3.63; N, 1.17.

4102 Organometallics, Vol. 24, No. 17, 2005
Communications
Scheme 2
2.206(1) Å are comparable to values found in other
three-membered palladacycles.26,27
this concept, we have found that heating samples of 2b
in acetontrile-d3 with CH3COOD leads to the slow
selective incorporation of deuterium at the isopropyl
methine carbons and formation of [((CH3)2CD)3P)2Pd-
(κ2O,O′-OAc)][B(C6F5)4] (1H NMR). Preliminary studies
of the reaction of 1 with bases such as pyridine are found
to yield the products of substitution of acetonitrile (i.e.,
trans-[(R3P)2Pd(OAc)(L)][B(C6F5)4]), not the products of
metalation. This work thus provides additional evidence
of the inherent complexities associated with deceptively
simple phosphine adducts of palladium acetate. Further
efforts to delineate the generality of these reactions
involving other phosphines and carboxylate ligands are
underway.
Having rigorously identified 3b, the nature of 3a can
be assigned as an analogous palladacyclic complex by
spectroscopic comparisons to 3b. The cis disposition of
phosphines in these compounds is consistent with the
2
small JPP coupling constants (29 and 30 Hz, respec-
tively). While cyclopalladation reactions of palladium
phosphine complexes are well-known,28 it is interesting
to note the mild and selective metalation for a mixture
of acetonitrile and 2b, in comparison to the case for
compound 1a, containing the these same two compo-
nents.
The formation of 3 involves elimination of acetate as
acetic acid and a â-hydrogen elimination from one of
i
the Me2CH groups of Pr3P ligands. The process was
Acknowledgment. We acknowledge Promerus LLC
for support.
found to be reversible, for addition of acetic acid to 3a-
d3 generates 2b quantitatively. Protonation of palladium
metallacycles by acids has been discussed as the rever-
sal of phosphine cyclometalation.29 Also consistent with
Supporting Information Available: Text giving full
experimental details for all compounds and X-ray crystal-
lographic files in CIF format for 1a, 2a, and 3b. This material
is available free of charge via the Internet at http://pubs.acs.org.
(26) Jouaiti, A.; Geoffroy, M.; Bernardinelli, G. J. Chem. Soc., Chem.
Commun. 1996, 437-438.
(27) Sluis, M. v. d.; Beverwijk, V.; Termaten, A.; Bickelhaupt, F.;
Kooijman, H.; Spek, A. L. Organometallics 1999, 18, 1402-1407.
(28) Some reviews: (a) Canty, A. J. In Comprehensive Organome-
tallic Chemistry II; Pergamon: Oxford, New York, 1995; Vol. 9, pp
242-248. (b) Ryabov, A. D. Chem. Rev. 1990, 90, 403-424. (c) Dupont,
J.; Pfeffer, M.; Spencer, J. Eur. J. Inorg. Chem. 2001, 1917-1927.
OM050356P
(29) Campora, J.; Lopez, J. A.; Palma, P.; Valerga, P.; Spillner, E.;
Carmona, E. Angew. Chem., Int. Ed. 1999, 38, 147-151.