Zero-valent palladium complexes with monodentate nitrogen σ-donor ligands

Article abstract of DOI:10.1002/anie.200351189

The facile synthesis and isolation of the stable zero-valent complexes [Pd(L)₂(maleic anhydride)] with σ-donor N ligands (L=ammonia, aniline, diethylamine, and pyridine) is remarkable in light of the fact that according to the HSAB principle, l

Full text of DOI:10.1002/anie.200351189

Angewandte
Chemie
Zero-Valent Palladium Complexes
Pörschke and co-workers described the synthesis and use
of “naked palladium” complexes, which contain coordination-
labile 1,6-diene and other {L–Pd0} fragments as highly
reactive building units.[6] It was shown that the homoleptic
dinuclear [Pd2(1,6-diene)3] and [Pt2(1,6-diene)3] complexes
are very useful starting materials for preparing mononuclear
complexes of the general formula [L–M(1,6-diene)] (L =
phosphane, phosphite, or nitrile). However, by using this
route, the authors did not succeed in preparing similar
complexes based on ligands with strong s-donor character,
such as pyridine or an amine.
We now report the synthesis and characterization of
several unprecedented Pd0 compounds with monodentate
N ligands as well as an X-ray structure of one such compound,
[Pd(py)2(ma)] (py = pyridine, ma = maleic anhydride). For
the synthesis of the new Pd0 compounds [Pd(L)2(ma)], and
[Pd(L’)(ma)] in which L’ is a bidentate N ligand, we used
[Pd(nbd)(ma)] (nbd = norbornadiene)[7] as a versatile pre-
cursor. Dibenzylideneacetone (dba) complexes are not very
suitable in this case, as dba is nonvolatile and difficult to
separate from many Pd0 compounds, especially those that are
labile. [Pd(nbd)(ma)] is one of the few zero-valent palladium
precursors containing volatile, readily dissociating ligands,
that can be stored and used at room temperature (similar to
some of the dinuclear 1,6-diene complexes[6]) and can even be
handled in air for a short time without decomposition. By this
new route one can readily prepare a number of zero-valent
palladium complexes of the generic formula [Pd(L)2(ma)]
(L = pyridine, amine, aniline, or ammonia).
Zero-Valent Palladium Complexes with
Monodentate Nitrogen s-Donor Ligands**
Alexander M. Kluwer, Cornelis J. Elsevier,*
Michael Bühl, Martin Lutz, and Anthony L. Spek
Compared to the large number of known, well-characterized
zero-valent [bis(phosphanyl)palladium(alkene)] complexes
A,[1] the number of [Pd0(h2-alkene)] complexes that contain
nitrogen ligands is very limited.[2] Moreover, only complexes
containing bidentate ligands that display both s-donor and p-
acceptor properties are known. Thus far, zero-valent palla-
dium–alkene complexes B that contain simple monodentate
nitrogen ligands have not been isolated and characterized.
The elusiveness of zero-valent palladium complexes contain-
ing ligands that only behave as s donors has been attributed
to the alleged instability of such compounds, which arises
from the combination of a soft, low-valent metal center and a
hard nitrogen donor atom.[3] In contrast, Ni0 complexes with
one simple N-donor ligand are formed already with ethene as
the coligand (e.g. [(L)Ni(C2H4)2] (L = NH3, pyridine, etc.)[4]
and related bisamine complexes [(L)2Ni(C2H4)], when the
strength of coordination of the N ligands is increased by steric
effects.[5] Zero-valent palladium compounds are generally
either formed in situ by reduction of a suitable palladium(ii)
compound, or by starting from a zero-valent palladium
complex with labile ligands.
We prepared [Pd(nbd)(ma)][7] by a modified literature
procedure (THF was used as the solvent instead of acetone)
and found that decomposition to palladium black was
appreciably reduced. Next, the zero-valent [Pd(L)2(ma)]
complexes 1–4 were prepared in typical yields of 65–70%
by substitution of the norbornadiene ligand in [Pd(nbd)(ma)]
by an excess of the monodentate nitrogen ligand in THF.
Importantly, isolation of the product was carried out quickly
to prevent undesired side reactions such as decomposition to
palladium black. The products 1–4 were isolated as yellow
[*] Prof. Dr. C. J. Elsevier, A. M. Kluwer
Institute of Molecular Chemistry, Universiteit van Amsterdam
Nieuwe Achtergracht 166, 1018 WV Amsterdam (The Netherlands)
Fax: (+31)20-525-6456
E-mail: elsevier@science.uva.nl
Priv.-Doz. Dr. M. Bühl
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Dr. M. Lutz, Prof. Dr. A. L. Spek
Bijvoet Center for Biomolecular Research
Vakgroep Kristal- en Struktuurchemie, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
[**] This work was supported in part (A.M.K., A.L.S., M.L.) by The
Netherlands Foundation for Chemical Sciences (CW) with financial
aid from the Netherlands Organization for Scientific Research
(NWO). M.B. thanks Walter Thiel and the Deutsche Forschungs-
gemeinschaft for continuous support. The authors thank Prof. Dr.
K.-R. Pörschke (Max-Planck-Institut für Kohlenforschung, Mülheim
a/d Ruhr, Germany) for carefully reading the manuscript and for
several useful suggestions.
Angew. Chem. Int. Ed. 2003, 42, 3501 –3504
DOI: 10.1002/anie.200351189
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3501

Communications
crystalline solids, which are stable at room temperature for
days in a closed vessel.
consistent with the limited stability of this compound at
ambient temperature and pressure. Comparing, for instance,
entries 1 and 4 (or 10 and 13), the chelate effect (bipyridine vs.
pyridine) is clearly reflected in the calculated values of
enthalpies (DH298) relative to Gibbs free energies (DG298),
The isolated products were characterized by 1H,
13C NMR, and IR spectroscopy and by exact mass determi-
nation. From the NMR spectra, the coordination-induced
shifts (CIS) of the signals for the protons of maleic anhydride,
Dd(H) = ꢀ3.44 ppm for 1 to ꢀ3.87 ppm for 4 and for the
coordinated carbon atoms Dd(C) = ꢀ92.32 ppm for 1 and
ꢀ74.35 ppm for 4, are indicative of a highly electron-rich Pd0
center.
and gives rise to a further stabilization by up to 40 kJmolꢀ1
Furthermore, the bis(dimethylsulfide) complex
.
[Pd(Me2S)2(ma)] (5) as well as the chelate compounds
[Pd(1,6-heptadiene)(ma)] (6) and [Pd(tmeda)(ma)] (7) were
We also carried out several calculations at the BP86/ECP1
level[8] to assess the stabilities of a number of selected
[Pd0(L)2(alkene)] compounds in terms of the driving force for
their formation from the corresponding tris(alkene) model
complexes [Eq. (1)].
½PdðC2H4Þ2ðalkeneފ þ 2 L ! ½PdðLÞ2ðalkeneފ þ 2 C2H4
ð1Þ
These calculations revealed that when one electron-
deficient alkene is coordinated to Pd, the Gibbs free energy
DG298 of this process is only slightly positive or even negative
(Table 1). Such thermo-neutral Pd0–alkene complexes
(DG298 ꢁ 0) can, in principle, be synthesized if an appropriate
kinetic route is chosen. In such a case, the starting compound
should contain highly labile ligands, which can be readily
displaced in solution to form the desired complex. Calculated
thermodynamic data (BP86/ECP1level) of several relevant
compounds are collected in Table 1. The DG298 values
calculated for the [Pd(L)2(ethene)] complexes are all positive,
suggesting that compounds of this type are likely to be
unstable under standard conditions and should not be formed
from precursors with simple, unactivated olefins. Substitution
of ethene by maleic anhydride as the alkene ligand leads to a
decrease in the DG298 values by approximately 40 kJmolꢀ1.
The [Pd(py)2(ma)] complex (1) still has a positive DG298 value,
prepared following the same synthetic route. The monoden-
tate dialkyl sulfide complex can be isolated, even though the
somewhat lower stability of the resulting complexes could
have been expected based on the calculations (see Table 1,
entry 16). Of course, the calculated DG298 values for com-
pounds with mono- and bidentate phosphane as well as with
bidentate N ligands are negative (see Table 1, entries 14, 15,
and 18); in particular, the stabilization for PMe3 is large.
However, the latter complexes can also be obtained by using
more conventional starting materials.
Crystals of 1 were obtained by crystallization from
pyridine/THF/diethyl ether.[9] The molecular structure of
compound 1 with the adopted atom numbering is presented
in Figure 1. The structure of 1 shows that the palladium centre
Table 1: Calculated (BP86/ECP1 level) energies, enthalpies, and free
energies for the formation of [Pd0(L)2(h2-alkene)] complexes according to
Equation (1).[a]
Entry Alkene (L)2
dE [kJmolꢀ1
]
DH298 [kJmolꢀ1
]
DG298 [kJmolꢀ1
]
1
2
3
4
5
6
7
8
C2H4
(py)2
57.9
50.0
69.1
75.7
79.5
61.4
59.7
14.3
ꢀ31.6
11.9
1.7
16.6
36.1
30.1
37.9
17.5
9.2
56.0
47.8
69.9
67.9
74.9
54.3
57.7
7.9
ꢀ33.3
10.5
4.9
18.8
28.8
26.2
31.0
15.9
2.3
52.7
32.1
61.3
19.8
35.6
8.4
49.5
ꢀ23.1
ꢀ41.0
11.9
(NH3)2
(NHMe2)2
bipy
tmeda
dab
(SMe2)2
C7H12
PMe3
9
10
11
12
13
14
15
16
17
18
ma
(py)2
(NH3)2
(NHMe2)2
bipy
tmeda
dab
(SMe2)2
C7H12
PMe3
ꢀ11.4
19.3
ꢀ14.3
ꢀ10.5
ꢀ11.6
17.4
Figure 1. Displacement ellipsoid plot (50% probability level) of com-
pound 1. Selected distances [Š] and angles [8]: Pd-N1 2.1472(19), Pd-
N2 2.1462(18), Pd-C11 2.068(2), Pd-C14 2.062(2), C11-C14 1.430(3),
C11-Pd-C14 40.51(10), N1-Pd-N2 94.44(7), C11-Pd-N1 151.96(9), C14-
ꢀ26.6
ꢀ78.9
ꢀ79.6
ꢀ77.5
[a] ma=maleic anhydride, bipy=2,2’-bipyridine, tmeda=tetramethyl- Pd-N2 153.81(8), C11-Pd-N2 113.32(9), C14-Pd-N1 111.73(8), Pd-C11-
ethylenediamine, dab=1,4-diazabutadiene.
C12 108.29(17), Pd-C14-C13 105.07(17).
3502
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is trigonal planar, as expected for zero-valent complexes of
the type [M(L)2(alkene)] (M = Pd, Pt). The dihedral angle
between the least-squared planes N1/N2/Pd and C11/C14/Pd
is 4.74(16)8 and thus the complex is slightly distorted from
planarity. The Pd–N interatomic distances observed for
compound 1 are equal within standard deviation and
amount to 2.1472(19) and 2.1462(19) Š, slightly smaller
than the BP86/ECP1-optimized values (2.162 and 2.160 Š).
Keywords: coordination compounds · density functional
calculations · donor–acceptor systems · N ligands · palladium
.
[1] a) D. Choueiry, E.-i. Negishi in Handbook of Organopalladium
Chemistry for Organic Synthesis, Vol. 1 (Ed.: E. I. Negishi),
Wiley, New York, 2002, pp. 47 – 65; b) P. M. Maitlis, The Organic
Chemistry of Palladium, Vol. 1, Academic Press, New York,
1971, pp. 106 – 144.
[2] a) F. Ozawa, T. Ito, Y. Nakamura, A. Yamamoto, J. Organomet.
Chem. 1979, 168, 375 – 391; b) K.J. Cavell, D. J. Stufkens, K.
Vrieze, Inorg. Chim. Acta, 1980, 47, 67; c) C. Borriello, M. L.
Ferrara, I. Orabona, A. Panunzi, F. Ruffo, J. Chem. Soc. Dalton
Trans. 2000, 2545 – 2550; d) L. Canovese, F. Visentin, G. Chessa,
P. Uguagliati, A. Dolmella, J. Organomet. Chem. 2000, 601, 1 –
15.
[3] a) R. G. Pearson, J. Am. Chem. Soc. 1963, 85, 3533; b) R. G.
Pearson, Hard and Soft Acids and Bases, Dowden, Hutchinson &
Ross Inc., Stroudsburg, PA, 1973.
[4] W. Kaschube, K.-R. Pörschke, W. Bonrath, C. Krüger, G. Wilke,
Angew. Chem. 1989, 101, 790 – 791; Angew. Chem. Int. Ed. Engl.
1989, 28, 772 – 773.
¼
The alkene bond length C11 C14 is 1.430(3) Š and is
0.098(3) Š longer than in the free alkene, which is slightly
¼
longer than the C C bond lengthening in the previously
reported [Pd(o,o’-iPr2C6H3-bian)(ma)] (bian = bisiminoace-
naphthene) of 1.408(11) Š and [Pd(R’N-SR)(ma)] (R’N-
SR = 2-methyl(sulfanylmethyl)pyridine)
of
1.413(5) Š.[2d,13,14] The maleic anhydride moiety forms a
typical angle of 78.16(16)8 with the C11/C14/Pd plane. The
other distances in the coordinated maleic anhydride part are
similar to the reported complexes and the free molecule.
In summary, the first zero-valent complexes [Pd(L)2(ma)]
with s-donor N ligands (L = ammonia, aniline, diethylamine,
pyridine) have been synthesized and isolated. These
[Pd(L)2(ma)] complexes are stable at room temperature for
days and are useful as catalyst precursors.[15]
[5] K.-J. Haack, R. Goddard, K.-R. Pörschke, J. Am. Chem. Soc.
1997, 119, 7992 – 7999.
[6] J. Krause, G. Cestaric, K.-J. Haack, K. Seevogel, W. Storm, K.-R.
Pörschke, J. Am. Chem. Soc. 1999, 121, 9807 – 9823.
[7] K. Itoh, F. Ueda, K. Hirai, Y. Ishii, Chem. Lett. 1977, 877 – 880.
[8] Geometries were fully optimized using the gradient-corrected
density functional according to A. D. Becke, Phys. Rev. A 1988,
38, 3098 – 3100, J. P. Perdew, Phys. Rev. B 1986, 33, 8822 – 8824,
J. P. Perdew, Phys. Rev. B 1986, 34, 7406, the Stuttgart – Dresden
relativistic effective core potential for Pd (D. Andrae, U.
Häußermann, M. Dolg, H. Stoll, H. Preuß, Theor. Chim. Acta
1990, 77, 123 – 141), and standard 6-31G* basis for all other
atoms. All optimized structures were characterized as minima by
calculation of the harmonic vibrational frequencies, which were
also used to evaluate zero-point, enthalpic, and entropic
corrections. For a recent review concerning theoretical studies
in palladium and platinum molecular chemistry, see: A. Dedieu,
Chem. Rev. 2000, 100, 543 – 600.
Experimental Section
General procedure: To a mixture of dry THF (10 mL) and the
appropriate N ligand (5 mL) was added solid [(h2,h2-nbd)(h2-ma)Pd0]
(0.15 g, 0.5 mmol). In the case of ammonia, 3–4 mL was condensed at
ꢀ708C, to which cold THF (10 mL) was slowly added, followed by the
Pd0 complex as above. After complete dissolution of the latter, the
solution was filtered through Celite and washed with THF (2 ” 5 mL).
Diethyl ether (15 mL) was then added to the solution, after which the
products were obtained as yellow microcrystalline solids in a typical
yield of 65%. Selected data: 1: 1H NMR (500 MHz, [D6]acetone,
258C, TMS): d = 8.58 (m, 4H; py-2-H), 7.97 (m, 2H; py-4-H), 7.54 (m,
4H; py-3-H), 3.9 ppm (br, 2H; C CH); 13C NMR (125 MHz,
[D6]acetone, 258C, TMS): d = 151.68 (py-C2), 137.70 (py-C4),
¼
[9] C14H12N2O3Pd, Fw = 362.66, yellow needle, 0.48 ” 0.12 ”
0.09 mm3, orthorhombic, Pbca (no. 61), a = 8.1477(1), b =
16.0499(3), c = 20.5190(4) Š, V= 2683.27(8) Š3, Z = 8, 1x =
1.795 gcmꢀ3, m = 1.391 mmꢀ1; 19822 reflections were measured
on a Nonius KappaCCD diffractometer with rotating anode (l =
¼
125.40 (py-C3), 39.80 ppm (CH), (C O not observed); IR (THF):
n˜ = 1780, 1726 cmꢀ1 (C O). 2: 1H NMR (500 MHz, CD2Cl2, 258C,
¼
¼
TMS): d = 3.5 (br, 2H; C CH), 2.7 (br, 8H; CH2), 1.3 ppm (br, 12H;
CH3); C NMR (125 MHz, [D6]acetone, 258C, TMS): d = 171.9 (C
13
¼
0.71073 Š) at a temperature of 150(2) K up to a resolution of
ꢀ1
¼
O), 48.0 (C C), 38.3 (CH2), 15.1 ppm (CH3); IR (THF): n˜ = 1793,
(sinq/l)max = 0.65 Š
; 3073 reflections were unique (Rint =
1
1726 cmꢀ1 (C O). 3: H NMR (500 MHz, [D6]acetone, 258C, TMS):
¼
0.051); an absorption correction based on multiple measured
reflections was applied (0.77–0.91transmission); the structure
was solved with direct methods (SIR-97[10]) and refined with
SHELXL-97[11] against F2 of all reflections; non-hydrogen atoms
were refined freely with anisotropic displacement parameters; H
atoms of the maleic anhydride ligand were refined freely with
isotropic displacement parameters; H atoms of the pyridine
ligands were refined as rigid groups; 189 refined parameters, no
restraints; R values (I > 2s(I)): R1 = 0.0262, wR2 = 0.0579;
d = 7.06 (t, J = 8.0 Hz, 4H; ArH), 6.68 (d, J = 7.5 Hz, 4H; ArH), 6.59
¼
(t, J = 7.5 Hz, 2H; ArH), 4.59 (br, 2H; C CH), 2.81ppm (s, 4H;
NH2); HRMS: calcd for C16H17N2O3Pd: 391.0281; found 391.0233. 4:
1
¼
H NMR (500 MHz, [D6]acetone, 258C, TMS): d = 3.47 (s, 2H; C
CH), 3.57 ppm (s, 6H; NH3); 13C NMR (125 MHz, [D6]acetone, 258C,
1
¼
¼
TMS): d = 177.70 (C O), 62.77 ppm (C C). 5: H NMR (500 MHz,
¼
[D6]acetone, 258C, TMS): d = 4.1(br, 2H; C CH), 2.4 ppm (s, 12H).
1
6: H NMR (500 MHz, [D8]THF, 258C, TMS): d = 4.98 (s, 2H), 4.76
¼
(m, 4H), 3.98 (br, 2H), 3.86 (br, 2H; C CH), 2.02 (br, 2H), 1.75 (s,
R values (all reflections): R1 = 0.0429, wR2 = 0.0634; GoF =
2H), 0.92 ppm (br, 2H); 13C NMR (125 MHz, [D8]THF, 258C, TMS):
ꢀ3
1.025; residual electron density between ꢀ0.48 and 0.63 eŠ
;
1
¼
d = 168.31 (C O), 99.96, 73.91, 60.73, 31.29, 30.89 ppm. 7: H NMR
molecular illustration, structure checking, and calculations were
performed with the PLATON package.[12] CCDC-201446 con-
tains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.-
ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB21EZ, UK;
fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
¼
(500 MHz, CD2Cl2, 258C, TMS): d = 3.55 (s, 2H; C CH), 2.73 (s, 6H;
NMe), 2.72 (s, 6H; NMe), 2.5 ppm (m, 4H; CH2CH2); 13C NMR
¼
¼
(125 MHz, CD2Cl2, 258C, TMS): d = 172.22 (C O), 59.94 (C C),
50.64 (CH3), 50.14(CH3), 39.54 ppm (CH2).
Received: February 17, 2003 [Z51189]
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[10] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.
Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori, R.
Spagna, J. Appl. Crystallogr. 1999, 32, 115 – 119.
[11] G. M. Sheldrick, SHELXL-97. Program for crystal structure
refinement, University of Göttingen, Göttingen (Germany),
1997.
[12] a) A. L. Spek, PLATON, A multipurpose crystallographic tool,
Utrecht University, The Netherlands, 2002; b) A. L. Spek, J.
Appl. Cryst. 2003, 36, 7 – 1 3.
[13] R. van Asselt, C. J. Elsevier, W. J. J. Smeets, A. L. Spek, Inorg.
Chem. 1994, 33, 1521 – 1531.
[14] a) The room-temperature X-ray crystal structure of free maleic
¼
anhydride has led to discussions in the literature about the C C
distance in maleic anhydride: R. E. Marsh, E. Ubell, H. E.
Wilcox, Acta Crystallogr. 1962, 15, 35 – 41; b) a neutron diffrac-
tion study improved the precision of the results, but was also
performed at room temperature: S. F. Parker, C. C. Wilson, J.
Tomkinson, D. A. Keen, K. Shankland, A. J. Ramirez-Cuesta,
P. C. H. Mitchell, A. J. Florence, N. Shankland, J. Phys. Chem. A
2001, 105, 3064 – 3070; c) This prompted us to perform a low-
temperature X-ray diffraction study of the free maleic anhy-
¼
dride, which showed that the C C bond length was 1.3322(9) Š:
M. Lutz, Acta Crystallogr. Sect. E 2001, 57, o1136-o1138.
[15] A. M. Kluwer, J. W. Sprengers, C. J. Elsevier, unpublished
results.
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