Synthesis and crystal structures of 3-alkyl-2,4-pentanedionates and 3-phenyl-2,4-pentanedionate of palladium

Article abstract of DOI:10.1016/j.poly.2012.02.024

A series of 3-substituted pentane-2,4-dionate complexes of palladium have been synthesized and characterized. These complexes are stable at room temperature in air; however, they slowly decompose in solution. The crystal structures of these complexes show that the palladium is square planar with respect to the two pentane-2,4-dionate ligands.

Full text of DOI:10.1016/j.poly.2012.02.024

Polyhedron 38 (2012) 126–130
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and crystal structures of 3-alkyl-2,4-pentanedionates
and 3-phenyl-2,4-pentanedionate of palladium
⇑
Michael A.G. Berg, Melissa K. Ritchie, Joseph S. Merola
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 3-substituted pentane-2,4-dionate complexes of palladium have been synthesized and char-
acterized. These complexes are stable at room temperature in air; however, they slowly decompose in
solution. The crystal structures of these complexes show that the palladium is square planar with respect
to the two pentane-2,4-dionate ligands.
Received 1 December 2011
Accepted 22 February 2012
Available online 3 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Palladium acac
Palladium pentane-2,4-dionate
3-Alkylpentane-2,4-dione
3-Arylpentane-2,4-dione
Crystal packing
1. Introduction
Our interest is in 3-substituted b-diketone complexes of palla-
dium. There is only one example in the Cambridge Crystallographic
The syntheses of metal derivatives of 1,3-diketones have been
well studied [1]. These metal n, n + 2-dionate complexes can be
synthesized using a variety of methods. Many different metals
have been used and various b-dicarbonyl compounds have been
employed to often give very brightly colored metal complexes.
These metal dionate complexes exhibit a wide degree of physical
and chemical properties, often with a lower degree of metal
reactivity.
Database of a 3-substututed pentane-2,4-dionate complex and this
is of the 3-acetyl derivative (CCDC ID: 657358). (In this paper, only
pentane-2,4-dionate or acac complexes are discussed. A future
paper will deal with longer chain n, n + 2 dionates.) These
complexes may be of value as homogeneous catalysts in non-polar
organic solvents. Herein, we report the synthesis and structural
characterization of a variety of palladium complexes with 3-substi-
tuted pentane-2,4-dionate ligands.
While the literature is rife with the use of b-dicarbonyl com-
pounds with various groups attached to the terminal ends of the
alkanedione chains, very few examples of 3-alkyl or 3-aryl substi-
tuted pentane-2,4-diones (acetylacetones or ‘‘acacs’’) are found
[2,3]. One reason for the lack of literature on these types of com-
plexes is due to hydrodeacylation of the 3-substituted b-diketone
during formation of the complexes [2]. Hydrodeacylation is not
observed with the unsubstituted pentane-2,4-dione. Puchberger
investigated the synthesis of 3-ethylpentane-2,4-dione complexes
with Al, Ti and Zr alkoxides. At low ligand to metal ratios (1 or 2
equivalents of 3-ethylpentane-2,4-dione to metal), a complex mix-
ture of diacac complexes and hydrodeacylation products was
formed. More hydrodeacylation products were obtained when
Al(O–iPr)4 was used compared to Ti and Zr alkoxides. At very high
ratios of 3-ethylpentane-2,4-dione (5:1), hydrodeacylation was
completely suppressed.
2. Experimental
2.1. General considerations
All chemicals were reagent grade and used without further
purification. The proton and carbon NMR spectra were obtained
on a Bruker INNOVA 400 MHz instrument. All elemental analyses
were performed by Atlantic Microlab, Inc., Norcross, GA.
2.2. General procedure for the synthesis of palladium 3-alkyl-3-
acetylacetonate
Palladium(II) acetate, [Pd(OAc)2]3 (0.28 g, 1.25 mmol) was dis-
solved in a 5:1 mixture of acetone and water (30 mL). The 3-alkyl-
acetylacetone (6.25 mmol) was dissolved in the acetone water
mixture (10 mL) and slowly added to the orange palladium solu-
tion. The solution was stirred overnight to give a suspension of
the product. The precipitate was filtered, washed with 5:1 acetone
⇑
Corresponding author. Tel.: +1 540 231 4510; fax: +1 540 231 3255.
E-mail address: jmerola@vt.edu (J.S. Merola).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2012.02.024

M.A.G. Berg et al. / Polyhedron 38 (2012) 126–130
127
Table 1
Crystallographic data for compounds 1–5.
Compound
1
2
3
4
5
Formula
C
12H18O4Pd
C14H22O4Pd
360.72
orthorhombic
Pbca
C18H30O4Pd
416.82
C22H38O4Pd
472.96
C22H22O4Pd
456.80
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
332.66
triclinic
P1
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
P1
7.0243(5)
7.528(3)
7.7356(9)
61.08(3)
64.007(11)
67.94(2)
314.76(14)
1
7.993(3)
13.555(2)
13.274(3)
90
90
90
1438.2(7)
4
1.671
4.9291(9)
7.9529(9)
12.381(6)
87.318(19)
85.29(2)
73.943(13)
464.7(2)
1
4.9248(3)
7.9984(6)
14.6764(15)
92.683(7)
92.621(7)
106.892(6)
551.5(8)
1
7.3317(8)
7.7944(6)
17.044(8)
98.044(16)
90.789(18)
96.224(8)
958.3(4)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
q
l
(g cmꢀ3
)
)
1.755
1.474
1.488
1.014
1.424
0.864
1.583
0.993
(Mo K
a
1.297
F(000)
T (K)
168
100
736
100
216
100
248
100
464
100
Reflections (total)
Reflections (unique), Rint
Index ranges
3091
12087
3875
5773
10006
1811, 0.017
ꢀ6 6 h 6 9
ꢀ10 6 k 6 10
ꢀ10 6 l 6 10
98.4
1794, 0.059
ꢀ14 6 h 6 17
ꢀ10 6 k 6 10
ꢀ17 6 l 6 18
99.8
2601, 0.052
ꢀ6 6 h 6 5
ꢀ11 6 k 6 11
ꢀ17 6 l 6 16
95.4
3197, 0.037
ꢀ5 6 h 6 6
ꢀ11 6 k 6 11
ꢀ20 6 l 6 20
98.5
5531, 0.040
ꢀ9 6 h 6 10
ꢀ10 6 k 6 10
ꢀ24 6 l 6 23
98.8
Completeness to 2h = 60° (%)
Goodness-of-fit (GOF)on F2
1.059
1.004
1.017
1.012
0.940
R1, wR2 [I > 2
R1, wR2 (all data)
r
(I)]
0.0190, 0.0478
0.0193, 0.0479
0.0243, 0.0419
0.068, 0.0433
0.0530, 0.1252
0.0619, 0.1368
0.089, 0.0740
0.0422, 0.0726
0.0389, 0.0889
0.0832, 0.0928
Table 2
Selected bond lengths (Å) and angles (°).
1 (R = Me)
2 (R = Et)
3 (R = Bu)
4 (R = Hex)
5 (R = Ph) molecule a
5 (R = Ph) molecule b
Pd–O1
Pd–O2
1.9723(12)
1.9698(12)
1.9635(18)
1.9611(18)
1.973(2)
1.9755(17)
1.984(2)
1.9761(16)
1.981(2)
1.966(2)
1.972(2)
1.964(2)
O1–Pd–O2
C2–C3–C4
C2–C3–C6
87.04(5)
123.07(14)
118.66(15)
92.77(8)
123.2(3)
118.7(3)
93.60(8)
123.1(2)
118.9(2)
93.25(8)
123.92(16)
118.20(16)
92.69(10)
123.4(3)
120.1(3)
93.32(10)
124.7(3)
119.2(3)
Pd–O1–C2–C3
Pd–O2–C4–C3
C2–C3–C6–C7
ꢀ0.9(2)
ꢀ1.1(2)
0.9(4)
ꢀ1.9(4)
1.0(3)
89.1(3)
2.1(2)
ꢀ2.5(5)
5.1(5)
ꢀ87.2(4)
0.1(5)
ꢀ3.0(4)
ꢀ0.4(2)
ꢀ0.2(5)
87.7(3)
ꢀ89.7(2)
91.9(4)
water followed by water. The solid was dried to give a yellow or
greenish yellow powder. All of the compounds appear to be quite
stable in air at room temperature in the solid state. In protic sol-
vents, the acac compounds all decompose rapidly (minutes to
hours). In non-protic solvents, they slowly decompose over a per-
iod of days to weeks.
2.2.4. Synthesis of palladium 3-hexyl-3-acetylacetonate (4)
Yield: (100%). Recrystallized from butanone. 1H NMR (CDCl3): d
0.89 (t, J = 6.4 Hz, 6H), 1.30 (br m, 16H), 2.16 (s, 12H), 2.22 (m, 4H);
13C NMR (CDCl3): d 14.07, 22.70, 25.27, 29,41, 30.68, 31.63, 110.19,
185.87. ESI-MS: m/z 969.3615 [2M+Na+H]+.
2.2.5. Synthesis of palladium 3-phenyl-3-acetylacetonate (5)
Yield: (quantitative). Recrystallized from toluene. 1H NMR
(CDCl3): d 1.83 (s, 12H), 7.14 (d, J = 8 Hz, 4H), 7.36 (m, 6H); 13C
NMR (CDCl3): d 26.92, 115.53, 127.25, 129.01, 131.60, 140.33,
2.2.1. Synthesis of palladium 3-methyl-3-acetylacetonate (1)
Yield: (31%). Recrystallized from acetone. 1H NMR (CDCl3): d
1.87 (s, 6H), 2.14 (s, 12H); 13C NMR (CDCl3): d 16.28, 26.15,
1003.97, 185.52.
186.19. ESI-MS: m/z 937.1089 [2M+Na]+. Anal.
Calc. for
22H22O4Pd: C, 57.84; H, 4.85. Found: C, 58.22; H, 4.87%.
C
2.3. X-ray structural determination
X-ray crystal structures were obtained using Mo
2.2.2. Synthesis of palladium 3-ethyl-3-acetylacetonate (2)
Yield: (54%). Recrystallized from acetone. 1H NMR (CDCl3): d
1.01 (t, J = 7.2 Hz, 6H), 2.16 (s, 12H), 2.30 (q, J = 7.2 Hz, 4H); 13C
NMR (CDCl3): d 14.71, 23.57, 25.00, 111.27, 185.71.
K
a
(k = 0.710713 Å) radiation at 100 K. The chosen crystal was
mounted on a nylon CryoLoop™ (Hampton Research) with KrytoxÒ
Oil (DuPont) and centered on the goniometer of an Oxford Diffrac-
tion Xcalibur™ diffractometer equipped with a Sapphire 3™ CCD
detector. The data collection routine, unit cell refinement, and data
processing were carried out with the program CrysAlisPro [4].
Structure solution and refinement were performed with the graph-
ical user interface WinGX [5]. The structure was solved by either by
SIR92 [6] or SHELXS and refined using SHELX97 [7]. The final refine-
ment model involved anisotropic displacement parameters for
2.2.3. Synthesis of palladium 3-butyl-3-acetylacetonate (3)
Yield: (41%). Recrystallized from acetone. 1H NMR (CDCl3): d
0.93 (t, J = 6.8 Hz, 6H), 1.34 (m, 8H), 2.16 (s, 12H), 2.22 (t,
J = 8 Hz, 4H); 13C NMR (CDCl3): d 13.83, 22.74, 25.21, 30.34,
32.81, 110.12, 185.83. ESI-MS: m/z 857.2287 [2M+Na]+. Anal. Calc.
for C18H30O4Pd: C, 51.86; H, 7.70. Found: C, 51.64; H, 7.09%.

128
M.A.G. Berg et al. / Polyhedron 38 (2012) 126–130
non-hydrogen atoms and a riding model for all hydrogen atoms.
The program package ORTEP-3 was used for molecular graphics gen-
eration [8]. Calculations of least square planes and angles between
planes were accomplished using Mercury CSD 2.0 [9].
O
O
O
O
R
Pd
R
1
2
3
4
5
R = CH3
3. Results and discussion
R = CH2CH3
R = CH2CH2CH2CH3
R = CH2CH2CH2CH2CH2CH3
R = C6H5
3.1. Synthesis and characterization of compounds
A variety of methods for the synthesis of the 3-substituted pen-
tanedionate (3-substituted acac) complexes of palladium was
investigated. Direct reaction of PdCl2 with 3-R-acac and base in a
variety of aqueous and alcoholic solutions were unsuccessful due
to the lack of solubility of the PdCl2, and generally resulted in a
black precipitate, that presumably was palladium or palladium
oxide.
The use of acetone soluble palladium acetate allowed for a reac-
tion to take place with the 3-substituted acetylacetones while also
providing an equivalent of base for the reaction. Furthermore, by
using a 5:1 mixture of acetone to water, the reactions resulted in
precipitates that could be filtered, washed and dried to give the
essentially pure compounds 1–5.
Each of the compounds was recrystallized from various aprotic
solvents, such as acetone, hexane or toluene. However, upon sitting
in these solvents for extended periods of time, a fine black precip-
itate was observed. All attempts to recrystallize complexes 1–5
from protic solvents, such as ethanol or methanol resulted in the
deposition of palladium mirrors on the recrystallization vessel
within 24 h. Chloroform-d solutions used to obtain NMR spectra
resulted in palladium mirrors within 12 h of preparation. This sug-
gests that the complexes are acid sensitive. Acid sensitivity may be
explained by the increase in basicity of the 3-carbon atom in these
3-substituted b-diketones and therefore the greater propensity for
the 3 carbon in the complex to be protonated and dissociate from
the metal. Alkyl substitution on the
a-position of b-diketones has
been shown to increase the pKa of the
a-proton by at least 2 units,
Fig. 1. Thermal ellipsoid plot for the 3-methyl derivative, 1.
a phenomenon showed both experimentally and calculationally
[10–12].
3.2. Crystal and molecular structures
The compounds 1–5 were analyzed by X-ray diffraction (Table
1) and selected bond lengths and angles for those compounds are
reported in Table 2. Thermal ellipsoid plots showing the molecular
structures of 1–5 can be found in Figs. 1–5. In all cases but one, the
6-membered chelate rings in the Pd 3-substituted acetylacetonate
complexes deviate only slightly from ideal planarity. The largest
deviation from planarity in the alkyl complexes is found for
R = ethyl and in that case C3 is 0.071 Å out of the plane defined
by the palladium and the four coordinating oxygen atoms. This is
consistent with that of the unsubstituted Pd(acac)2 which crystal-
lizes in the P21/n space group [13]. Pd(acac)2 was also found to
have a O1–Pd–O2 bond angle of 95.19(4)°. All of the 3-substituted
Fig. 2. Thermal ellipsoid plot for the 3-ethyl derivative, 2.
Fig. 3. Thermal ellipsoid plot of the 3-butyl derivative, 3.

M.A.G. Berg et al. / Polyhedron 38 (2012) 126–130
129
Fig. 4. Thermal ellipsoid plot of the 3-hexyl derivative, 4.
Fig. 7. Space filling models showing a portion of the molecular packing for bis-(3-
methylacetylacetonate)palladium, 2.
compounds reported here have an O1–Pd–O2 bond angle less than
94°, most likely due to steric strain of the substituent on C3.
Closer examination of compounds 1–5 shows that there is a
profound effect of the R group on the crystal packing (and the
space group) that in turn can have a subtle effect on some of the
bond lengths and angles for the complexes. The parent unsubsti-
tuted acetylacetone complex of palladium (CCDC ID: 289749),
crystallizes in P21/n as mentioned above and there are two unique
molecules in the unit cell. Each of the unique molecules pack in
stacks as described in the literature reference but that paper
ignores any steric effects imposed by the second stack of molecules
that places two ends of the independent molecules very close to
each other with an angle of approximately 74°. This potential neg-
ative steric interaction is not discussed by Mazhar and co-workers
[13] but is very well illustrated in Fig. 6. Placement of a methyl
group at the 3-position will put a significant ‘‘spacer’’ in this same
type of packing motif and result in significant crystal void space
were it to be followed. Instead, the 3-methyl derivative crystallizes
Fig. 5. Thermal ellipsoid plot for the 3-phenyl derivative, 5, showing the two
independent molecules in the unit cell. Hydrogen atoms omitted for clarity.
ꢀ
in space group P1 and the molecules stack in such a manner that
each palladium atom sits atop the space between two molecules
beneath it, above the two methyl groups, Fig. 7. The ethyl deriva-
tive is very interesting in terms of packing, but since it crystallizes
in the much higher symmetry space group, Pbca, it is probably best
not to include it in this particular comparison. Now, with the
longer alkyl chains butyl and hexyl, the compounds both crystal-
lize in P-1 but now the packing is characterized by what we dub
‘‘molecular spooning’’ dominated by alkyl side chain van der Waals
interactions (Figs. 8 and 9). Another phenomenon that is most
likely attributable to packing forces is that the C3 alkyl chain is
not perfectly orthogonal to the rest of the molecule, but is slightly
bent from an imaginary Pd–C3 axis by 16° for R = butyl and 12° for
Fig. 6. Space filling model of the two independent molecules of bis-(acetylaceto-
nate)palladium showing the angle and steric interaction between molecules.

130
M.A.G. Berg et al. / Polyhedron 38 (2012) 126–130
molecules, the adherence to planarity of the chelate ring is fol-
lowed with the largest deviation involving C3 being 0.004 Å out
of the plane. However, the second molecule in the unit cell is quite
distorted with C2 and C4 being 0.188 and 0.179 Å out of the PdO4
plane and C3 being 0.356 Å out of that plane. Examination of the
crystal lattice of compound 5 suggests that this deviation comes
from intermolecular interactions such as shown in Fig. 10 where
the phenyl of one of the independent molecules impinges upon
the end of the other molecule causing the observed out of plane
distortion.
4. Conclusions
A series of 3-substituted acetylacetonate complexes of palla-
dium have been synthesized and fully characterized including by
X-ray crystallography. The crystallography of these compounds,
especially in their different packing motifs, show a profound effect
of the side chains on the intermolecular interactions which in turn
have subtle effects on some bond lengths and angles. The impor-
tant message is that these intermolecular interactions must be
examined and analyzed before making any conclusions about
bonding in the molecules themselves. These complexes are quite
stable in the solid state as well as in non-protic organic solvents
so long as they are protected from light. Their sensitivity to protic
solvents may diminish their utility as catalysts, but they may be
excellent catalyst precursors given the easy displacement of the
substituted acac ligands.
Fig. 8. Space filling model showing a portion of the packing for bis(3-butylacetyl-
acetonate)palladium, 3.
Acknowledgments
We thank the NSF (Grant CHE-0131128) for funding the pur-
chase of the Oxford Diffraction Xcalibur2 single crystal
diffractometer.
Fig. 9. Space filling model of
hexylacetylacetonate)palladium, 4.
a portion of the molecular packing for bis-(3-
Appendix A. Supplementary data
CCDC 855244, 855242, 855241, 855245 and 855243 contain the
supplementary crystallographic data for 1, 2, 3, 4, and 5. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
References
[1] W.C. Fernelius, B.E. Bryant, Preparation of metal derivatives of 1,3-diketones,
in: Inorganic Syntheses, John Wiley & Sons, Inc., 1957, pp. 105–113.
[2] M. Puchberger, W. Rupp, U. Bauer, U. Schubert, New J. Chem. 28 (2004) 1289.
p
[3] A. Doehring, R. Goddard, P.W. Jolly, C. Kr °ger, V.R. Polyakov, Inorg. Chem. 36
(1997) 177.
[4] CrysAlisPro, Agilent Technologies XRD, Agilent Technologies, Xcalibur CCD
system, CrysAlisPro Software system, Version 1.171.34., in, 2010.
[5] L. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[6] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr. 26
(1993) 343.
Fig. 10. Space filling model of a portion of the molecular packing for bis-(3-
phenylacetylacetonate)palladium, 5, showing the intermolecular interaction
between unique molecules.
[7] G. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.
[8] L. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[9] C.F. Macrae, I.J. Bruno, J.A. Chisholm, P.R. Edgington, P. McCabe, E. Pidcock, L.
Rodriguez-Monge, R. Taylor, J. van de Streek, P.A. Wood, J. Appl. Crystallogr. 41
(2008) 466.
[10] Q.T.H. Le, S. Umetani, M. Suzuki, M. Matsui, J. Chem. Soc., Dalton Trans. (1997)
643.
[11] M.C. Rezende, Tetrahedron 57 (2001) 5923.
R = hexyl. Each of the compounds 1–4 has a C2–C3–C6 bond angle
less than the expected 120° if C3 is sp2 hybridized. So, examination
of the crystal packing of the butyl and hexyl derivatives 3 and 4
shows that the alkyl chains of neighboring molecules pack together
well. The flexibility of these chains allows for tighter packing and is
probably responsible for the deviation from the Pd–C3 axis.
The biggest anomaly in this regard is found for compound 5
where R = C6H5. In this case, there are two unique molecules per
unit cell as is the case for the parent compound. For one of the
[12] Y. Fu, L. Liu, R.-Q. Li, R. Liu, Q.-X. Guo, J. Am. Chem. Soc. 126 (2003) 814.
[13] M. Hamid, M. Zeller, A.D. Hunter, M. Mazhar, A.A. Tahir, Acta Crystallogr., Sect.
E 61 (2005) m2181.