Synthesis, characterization, and crystal structure of platinum(II) and palladium(II) chlorides with an acidic α-diimine ligand

Article abstract of DOI:10.1016/S1387-7003(03)00082-0

Reaction of the symmetrical α-diimine ligand L with either PtCl₂ or PdCl₂(C₆H₅CN)₂ affords the square planar d⁸ complexes 1 and 2, respectively. Both complexes were characterized by NMR spectroscopy and elemental analysis. The solid-state structure of 1, which crystallized in the triclinic space group P1 with unit cell parameters a = 8.688(2) A?, b = 13.840(2) A?, c = 14.136(2) A?, α = 111.346(14)°, β = 103.82(2)°, and γ = 102.69(2)°, was established by X-ray crystallography.

Full text of DOI:10.1016/S1387-7003(03)00082-0

Inorganic Chemistry Communications 6 (2003) 680–684
www.elsevier.com/locate/inoche
Synthesis, characterization, and crystal structure of platinum(II)
and palladium(II) chlorides with an acidic a-diimine ligand
*
Brian P. Buffin , Abhijit Kundu
Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008, USA
Received 13 December 2002; accepted 4 March 2003
Abstract
Reaction of the symmetrical a-diimine ligand L with either PtCl2 or PdCl2ðC6H5CNÞ2 affords the square planar d8 complexes 1
and 2, respectively. Both complexes were characterized by NMR spectroscopy and elemental analysis. The solid-state structure of 1,
ꢀ
ꢁ
ꢁ
ꢁ
which crystallized in the triclinic space group P1 with unit cell parameters a ¼ 8:688ð2Þ A, b ¼ 13:840ð2Þ A, c ¼ 14:136ð2Þ A,
a ¼ 111:346ð14Þ°, b ¼ 103:82ð2Þ°, and c ¼ 102:69ð2Þ°, was established by X-ray crystallography.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Diazabutadiene ligands; Platinum(II) complex; Palladium(II) complex
1. Introduction
2. Experimental
Late transition metal complexes of 1,4-diazabutadi-
ene ligands have lately received renewed attention,
primarily due to their utility as homogeneous polymer-
ization catalysts [1]. As a result, the reaction chemistry
of this class of diimine ligand with a variety of transition
metals has become well established [2]. Recently we
communicated our efforts at forming hydrophilic 1,4-
diazabutadiene and related unsymmetrical pyridine-im-
ine ligands for use in aqueous organometallic chemistry
[3]. Coordination of these ligands to low-valent Pt(0)
and Pd(0) metal centers was shown to stabilize the imine
functionality towards hydrolysis. Despite a growing in-
terest in the chemistry of water-soluble organometallic
complexes and their potential application as recoverable
‘‘green’’ catalysts [4,5], the use of diimine complexes in
this area of research has remained relatively unexplored.
Herein, we describe the preparation of divalent platinum
and palladium compounds with an acidic 1,4-diazabut-
adiene ligand and the structure of the Pt(II) species as
determined in the solid-state by X-ray crystallography.
Transition metal starting materials PdCl2ðC6H5CNÞ2
and PtCl2 were obtained from Strem Chemicals, Inc.
(Newburyport, MA) and used without further purifica-
tion. Other reagents were obtained from commercial
sources and used as received. All of the reactions and
manipulations involving transition metal compounds
were performed under N2 using standard inert-atmo-
sphere and Schlenk techniques [6,7]. Solvents were dried
and distilled by standard procedures. H and 13Cf Hg
NMR spectra were recorded at ambient temperature on
a JEOL Eclipse 400 NMR in DMF-d7 at 400 and 100
MHz, respectively. Chemical shifts are reported down-
field in ppm (d) relative to tetramethylsilane by reference
to residual protonated solvent signals with the following
abbreviations: d ¼ doublet, s ¼ singlet, and br ¼ broad
signal. Melting points were recorded in open capillary
tubes. Elemental analyses were performed by Desert
Analytics, Tucson, AZ.
1
1
2.1. Synthesis of 1,4-bis(4-CO2HC6H4)-2,3-dimethyl-1,
4-diazabutadiene, L
4-Aminobenzoic acid (2.88 g, 21 mmol) was dissolved
in dry MeOH (10 ml). Formic acid (90%, 4 drops)
was added, followed by the dropwise addition of 2,
* Corresponding author. Tel.: +1-2693872420; fax: +1-2693872909.
E-mail address: brian.buffin@wmich.edu (B.P. Buffin).
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00082-0

B.P. Buffin, A. Kundu / Inorganic Chemistry Communications 6 (2003) 680–684
681
3-butanedione (0.88 ml, 10 mmol). The solution was
2.3. Synthesis of PdCl2(L), 2
stirred at ambient temperature for 24 h. The pale yellow
precipitate that resulted was isolated by filtration and
washed with cold methanol. Yield 1.1 g (33%). 1H NMR
(DMF-d7): d 13.1 (br, 2H), 8.09 (d, 4H), 7.00 (d, 4H),
A125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PdCl2ðC6H5CNÞ2 (0.100 g, 0.26
mmol) and 0.092 g (0.28 mmol) of the diazabutadiene
ligand L and flushed with nitrogen. THF (30 ml) was
added by syringe, and the resulting solution was sealed
under nitrogen, heated to 50 °C, and stirred for 24 h to
complete transformation to the desired product. The
yellow-orange solid that precipitated from solution
upon cooling to ambient temperature was isolated by
filtration, washed with THF (3 Â 3 ml), and dried under
1
2.15 (s, 6H). 13Cf Hg NMR (DMF-d7): d 168.4, 167.5,
156.0, 130.9, 127.2, 118.8, 15.0.
2.2. Synthesis of PtCl2(L), 1
A125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PtCl2 (0.100 g, 0.38 mmol) and
0.134 g (0.41 mmol) of the diazabutadiene ligand L and
flushed with nitrogen. THF (30 ml) was added by sy-
ringe, and the resulting solution was sealed under ni-
trogen, heated to 50 °C, and stirred for 48 h to complete
transformation to the desired product. The orange solid
that precipitated from solution upon cooling to ambient
temperature was isolated by filtration, washed with
THF, and dried under vacuum. Yield 0.130 g (59%). 1H
NMR (DMF-d7): d 13.2 (br, 2H), 8.19 (d, 4H), 7.37 (d,
1
vacuum. Yield 0.109 g (83%). H NMR (DMF-d7): d
13.2 (br, 2H), 8.14 (d, 4H), 7.30 (d, 4H), 2.33 (s, 6H).
1
13Cf Hg NMR (DMF-d7): d 182.0, 166.9, 149.5, 130.2,
130.1, 123.4, 21.1. Decomposed 308 °C. Anal. calc. for
C18H16Cl2N2O4Pd: C, 43.10%; H, 3.21%; N, 5.58%.
Found: C, 42.84%; H, 3.22%; N, 5.55%.
2.4. Structure determination
1
4H), 2.02 (s, 6H). 13Cf Hg NMR (DMF-d7): d 181.3,
X-ray quality crystals of 1 were grown by slow
evaporation of DMF solvent at ambient temperature.
An orange prism, approximately 0:37 mm  0:37 mm Â
0:25 mm in size, was chosen by size, habit, and polarized
light microscopy and mounted on a glass fiber. Intensity
166.9, 149.5, 130.7, 130.1, 124.1, 20.6. Decomposed 330
°C. Anal. calc. for C18H16Cl2N2O4Pt: C, 36.62%; H,
2.73%; N, 4.75%. Found: C, 36.13%; H, 2.45%; N,
4.83%.
Table 1
Crystallographic data for PtCl2ðLÞ, 1
Formula
C
18H16Cl2N2O4Pt Á C6H14N2O2
Formula weight
Crystal system
Space group
1473.02
Triclinic
ꢀ
P1
ꢁ
a (A)
8.688(2)
13.840(2)
ꢁ
b (A)
ꢁ
c (A)
14.136(2)
111.346(14)
a (°)
b (°)
c (°)
103.82(2)
102.69(2)
1446.32
ꢁ3
V (A Þ
Z
2
1.691
dcalc (g cmÀ3
)
l ðmmÀ1
Þ
5.08
0.71073
ꢁ
k (A)
Crystal dimensions (mm)
T (K)
Scan method
0:37 Â 0:37 Â 0:25
295
x=2h
724 e
F000
Total no. of reflections
No. of reflections [I > 3rðIÞ]
Structural parameters
Refinement method
5553
4533
339
Full-matrix least-squares on F 2
0.0739
a
R1 ½Fo > 4rðFoފ
b
R1a, wR2 (all data)
0.0845, 0.1938
1.049
GOFc
P
P
a R1 ¼ kF j À jFck= jFoj.
P
P o
P
2
2 1=2
b wR2 ¼ ½ ½wðFo2 À Fc2Þ Š= ½wðFo2Þ ŠŠ
.
c Goodness-of-fit ¼ ½ ½wðFo2 À F 2Þ Š=ðNobs À Nparamފ1=2, based on all data.
2
c

682
B.P. Buffin, A. Kundu / Inorganic Chemistry Communications 6 (2003) 680–684
data were collected at ambient temperature on an Enraf-
Nonius CAD-4 diffractometer equipped with graphite
monochromated Mo-Ka radiation. The unit cell was
determined and refined from 25 reflections with 1° <
2h < 25°. Reflection data were corrected for Lorentz-
polarization effects but not for absorption. The structure
was solved by automated Patterson methods and sub-
sequent difference Fourier techniques (DIRDIF-92) [8]
and refined with SHELXL 97 [9]. Non-hydrogen atoms
were refined with anisotropic thermal parameters. Hy-
drogen atoms were placed in idealized positions on
parent atoms in the final refinement. The largest unas-
Reaction of PtCl2 with one equivalent of L in THF at
50 °C resulted in the formation of the Pt(II) coordina-
tion compound 1. For comparison with this Pt dichlo-
ride, the Pd(II) analogue 2 was synthesized by reaction
of L with PdCl2ðC6H5CNÞ2. Diagnostic of chelating li-
gand coordination in these complexes is an overall
1
downfield shift of signals in the H NMR spectra by
about 0.1–0.4 ppm for Pt and 0.05–0.3 ppm for Pd. Not
surprisingly, the aromatic protons ortho- to the imine
nitrogen atoms (d 7.00 in L) are influenced to a greater
extent by metal coordination than the protons in the
meta-positions for both complexes. Similar resonance
shifts have been observed in related compounds [10,12].
Curiously the signals for the –CH3 protons in complex 1
have shifted upfield by 0.13 ppm relative to the free li-
gand whereas the corresponding protons in 2 experience
a downfield shift of 0.18 ppm. The reason for this sole
difference in the NMR data is not fully understood;
however, a similar effect has been observed for the imine
protons in related compounds [10]. Also consistent with
ligand coordination in solution in each of these com-
plexes is a downfield shift of signals in the 13C NMR
spectra by 5.6–6.1 and 12.9–13.6 ppm for the –CH3 and
–CO2H carbons, respectively. Compounds 1 and 2 are
stable in air and readily soluble in polar organic solvents
such as methanol, DMSO, and DMF, but are insoluble
in water. Addition of NaOH to a suspension of either 1
or 2 in H2O results in immediate dissolution of the
transition metal complex, presumably as a result of
anion formation via deprotonation of –CO2H groups.
Unfortunately, neither 1 nor 2 are stable under these
alkaline conditions, and both slowly decompose to un-
traced products.
ꢁ3
signed peak in the final difference map was 3:44 e=A
ꢁ
located 0.91 Afrom the Pt atom. Details of the crystal
parameters, data collection, and structure refinement are
given in Table 1.
3. Results and discussion
The condensation reaction between 2,3-butanedione
and 4-aminobenzoic acid was initially attempted by re-
fluxing a 1:2 mixture of these reagents in ethanol, with
benzene (10%, v/v) added to promote azeotropic re-
moval of water byproduct. All efforts to form the de-
sired diimine product
L by this method were
unsuccessful. The addition of molecular sieves to the
reaction mixture in an attempt to further dry the solu-
tion and promote product formation was also ineffec-
tive. Thus, the catalytic method employed by Eisenberg
and others for the formation of similar diimine ligands
was used [10,11]. Reaction of 2,3-butanedione with two
mole equivalents of 4-aminobenzoic acid in a minimum
amount of dry methanol with a catalytic amount of
formic acid affords the symmetrical 1,4-disubstituted-
2,3-dimethyl-1,4-diazabutadiene ligand L (Fig. 1) in
Single crystals of 1 Á 2ðC3H7NOÞ suitable for X-ray
analysis were grown by slow evaporation of DMF sol-
vent at ambient temperature. The molecular structure of
1 and the associated atom-numbering scheme are de-
picted in Fig. 2 with selected bond lengths and angles
given in Table 2. The geometry around the d8 Pt center
in 1 is that of a distorted square plane, where the rela-
tively small N(1)–Pt(1)–N(2) bond angle of 77.4(4)° is a
result of chelating ligand steric constraints. The bond
angles around the Pt atom, which sum to 359.94(58)°,
1
moderate isolated yield. H and 13C NMR signals are
consistent with formation of the diimine compound.
Specifically, generation of the diimine can be assessed
from the 13C NMR spectrum of L where a singlet res-
onance at 167.5 ppm attributed to the imine carbons is
observed with a corresponding absence of a ketone
carbonyl resonance from the starting 2,3-butanedione.
Fig. 1. Synthetic scheme for the preparation of Pt(II) and Pd(II) dichloride complexes 1 and 2.

B.P. Buffin, A. Kundu / Inorganic Chemistry Communications 6 (2003) 680–684
683
Fig. 2. ORTEP representations of the molecular structure of complex 1 with 50% probability ellipsoids (H atoms and DMF solvent molecules
omitted for clarity).
Table 2
ꢁ
Selected bond distances (A) and angles (°) for PtCl2ðLÞ, 1
ꢁ
Bond distances (A)
Bond angles (°)
Pt(1)–N(1)
Pt(1)–N(2)
Pt(1)–Cl(1)
Pt(1)–Cl(2)
N(1)–C(8)
N(2)–C(10)
N(1)–C(5)
N(2)–C(16)
C(8)–C(10)
C(1)–O(1)
C(1)–O(2)
C(12)–O(3)
C(12)–O(4)
2.017(10)
1.975(10)
2.276(4)
2.275(4)
1.26(2)
1.26(2)
1.44(2)
1.49(2)
1.54(2)
1.33(2)
1.20(2)
1.29(2)
1.26(2)
N(1)–Pt(1)–N(2)
Cl(1)–Pt(1)–Cl(2)
N(1)–Pt(1)–Cl(1)
N(2)–Pt(1)–Cl(2)
C(8)–N(1)–Pt(1)
C(10)–N(2)–Pt(1)
C(8)–C(10)–N(2)
C(10)–C(8)–N(1)
C(5)–N(1)–Pt(1)
C(16)–N(2)–Pt(1)
O(1)–C(1)–O(2)
O(3)–C(12)–O(4)
77.4(4)
89.54(15)
95.8(3)
97.2(3)
116.4(8)
121.4(8)
109.5(11)
115.1(11)
123.5(8)
124.6(8)
120.0(12)
123.4(13)
are consistent with a planar geometry. Amean plane
analysis of the five atoms comprising the transition
metal square plane reveals a maximum deviation from
repulsions, as observed previously [10,13], square planar
Pd and Pt complexes of diazabutadiene ligands with
widely disparate torsion angles have been reported
[14,15]. Generally speaking, 1,4-diaryl diazabutadiene
ligands without sterically demanding groups in the
ortho-positions of the aromatic rings tend to favor a
broader range of plane angles than similar compounds
with greater steric repulsion, but factors such as the
other ligands bound to the metal center also play a
crucial role in determining the overall geometry. The
bond distances and angles for 1 are very similar to those
of other square-planar platinum diimine compounds
[10,16,17]. Single crystals of 2 were also grown by slow
ꢁ
ideality of 0.0184(25) Afor Pt(1). The aryl-ring that
includes C(2)–C(7) is nearly perpendicular to this metal–
ligand plane with a calculated angle between the two
planes of 88.12(42)°. By way of contrast, the ring con-
taining C(13)–C(18) has a torsion angle with the metal–
ligand plane of only 73.82(43)° and is thus tilted relative
to the other aryl-ring. Amean plane angle of 19.27(65) °
has been calculated between the two aryl-rings. While
the aryl-rings on diimine ligands may be expected to lie
perpendicular to the metal–ligand plane based on steric

684
B.P. Buffin, A. Kundu / Inorganic Chemistry Communications 6 (2003) 680–684
evaporation of DMF solvent. Unfortunately, final re-
finement of the X-ray structure data was not fully sat-
isfactory (R1 ¼ 0:1381) due to the presence of solvent
molecules with random occupancy in the crystal lattice.
References
[1] S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000)
1169.
[2] G. van Koten, K. Vrieze, Adv. Organomet. Chem. 21 (1982)
151.
ꢁ
Calculated Pd–Cl (2.2501(31) and 2.2645(29) A ) and
ꢁ
Pd–N (2.0578(60) and 2.0581(90) A) bond distances in 2
[3] A. Kundu, B.P. Buffin, Organometallics 20 (2001) 3635.
[4] B. Cornils, W.A. Herrmann (Eds.), Aqueous-Phase Organome-
tallic Catalysis, Wiley-VCH, Weinheim, 1998.
are nearly identical to those of related compounds
[13,18]. As with the Pt(II) complex 1, the aryl-rings in 2
are rotated 88.12(34)° and 73.23(33)° relative to the
metal–chelate ring. Amean plane angle of 19.48(49) is
calculated between the two aryl-rings in 2. The reaction
chemistry of these metal complexes and related water-
soluble species is currently under investigation.
[5] P.T. Anastas, M.M. Kirchhoff, Acc. Chem. Res. 35 (2002) 686.
[6] D.F. Shriver, The Manipulation of Air-Sensitive Compounds,
McGraw-Hill, New York, 1969.
[7] R.J. Angelici, Synthesis and Technique in Inorganic Chemistry,
second ed., W.B. Saunders, Philadelphia, 1977.
[8] P.T. Beurskens, G. Admiraal, G. Beurskens, W.P. Bosman, S.
Garcia-Granda, R.O. Gould, J.M.M. Smits, C. Smykalla, The
DIRDIF Program System, Technical Report of the Crystallogra-
phy Laboratory, University of Nijmegen, Nijmegen, The Nether-
lands, 1992.
Supplementary materials
[9] G.M. Sheldrick, SHELXL 97, Program for the Refinement of
€
Crystal Structures, University of Gottingen, Germany, 1997.
[10] K. Yang, R.J. Lachicotte, R. Eisenberg, Organometallics 16
(1997) 5234.
Crystallographic data for the structural analysis of 1
in CIF format has been deposited with the Cambridge
Crystallographic Data Center under CCDC no. 199729.
These data can be obtained free of charge via www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk).
[11] H. tom Dieck, M. Svoboda, T. Greiser, Z. Naturforsch. B 36B
(1981) 823.
[12] R. van Asselt, C.J. Elsevier, C. Amatore, A. Jutand, Organomet-
allics 16 (1997) 317.
[13] N.M. Comerlato, G.L. Crossetti, R.A. Howie, P.C.D. Tibultino,
J.L. Wardell, Acta Crystallogr., Sect. E (Struct. Rep. Online) 57
(2001) m295.
[14] L. Johansson, O.B. Ryan, C. Rømming, M. Tilset, Organomet-
allics 17 (1998) 3957.
Acknowledgements
[15] D.J. Tempel, L.K. Johnson, R.L. Huff, P.S. White, M. Brookhart,
J. Am. Chem. Soc. 122 (2000) 6686.
We gratefully thank Dr. Marc W. Perkovic for
solving the X-ray crystal structures. Acknowledgment is
made to the Donors of The Petroleum Research Fund,
administered by the American Chemical Society, for
partial support of this research (PRF# 32973-GB3) and
to Western Michigan University.
[16] H. van der Poel, G. van Koten, M. Kokkes, C.H. Stam, Inorg.
Chem. 20 (1981) 2941.
[17] H. van der Poel, G. van Koten, K. Vrieze, Inorg. Chem. 19 (1980)
1145.
[18] R. van Belzen, R.A. Klein, W.J.J. Smeets, A.L. Spek, R. Benedix,
C.J. Elsevier, Rec. Trav. Chim. Pays-Bas 115 (1996) 275.