Palladium Complexes with the Tridentate Dianionic Ligand Pyridine-2,6-dicarboxylate, dipic. Crystal Structure of [Pd(dipic)(PBu3)]2

Article abstract of DOI:10.1021/ic951024n

The reactions of [Pd(acac)₂] or [Pd(OAc)₂]₃ with pyridine-2,6-dicarboxylic acid (H₂dipic) in acetonitrile afford [Pd(dipic)(NCMe)] in high yield. This complex has been used as starting material in the preparatio

Full text of DOI:10.1021/ic951024n

Inorg. Chem. 1996, 35, 2287-2291
2287
Palladium Complexes with the Tridentate Dianionic Ligand Pyridine-2,6-dicarboxylate,
dipic. Crystal Structure of [Pd(dipic)(PBu3)]2
Pablo Espinet* and Jesu´s A. Miguel
Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
47005 Valladolid, Spain
Santiago Garc´ıa-Granda and Daniel Miguel
Departamento de Qu´ımica F´ısica y Anal´ıtica and Instituto Universitario de Qu´ımica Organometa´lica
“Enrique Moles”-Unidad asociada del CSIC, Universidad de Oviedo, 33071 Oviedo, Spain
ReceiVed August 4, 1995X
The reactions of [Pd(acac)2] or [Pd(OAc)2]3 with pyridine-2,6-dicarboxylic acid (H2dipic) in acetonitrile afford
[Pd(dipic)(NCMe)] in high yield. This complex has been used as starting material in the preparation of a variety
of neutral an anionic complexes. The dipicolinate anion behaves as a tridentate ligand in all cases, but two
modes of coordination are found, depending on the ligand: as a pincer ligand O,N,O-bonded to the same palladium,
giving mononuclear complexes, and as an O,N-chelate N,O′-bridging ligand in dinuclear complexes. An X-ray
determination of the structure of a dimer, [Pd(dipic)(PBu3)]2 (monoclinic, space group P21/n, a ) 18.144(4) Å,
b ) 13.191(2) Å, c ) 19.571(3) Å, â ) 113.45(2)°, Z ) 4, R ) 0.050, Rw ) 0.054) shows that the ligand is
coordinated to one palladium in a η2-N,O chelate fashion and one oxygen atom of the other carboxylate group
makes a bridge to the other palladium atom, in a novel bonding mode for the dipic ligand.
Introduction
This exploration of palladium dipicolinate complexes has been
motivated by our interest in palladium chemistry and in metal-
containing liquid crystals.1 The rigid tridentate coordination
of this flat chelate ligand (Figure 1a), found for many bivalent
or trivalent transition metals,2,3 can provide a metal containing
rigid core which, conveniently modified, should eventually
produce palladium-containing liquid crystals with a net dipolar
moment along the Pd-N bond. Moreover, it has been reported
that K[Pt(dipic)Cl] forms gel in water, what already suggests
that the anisotropic shape of the molecule indeed induces
uncommon physical properties.2 It is not difficult to envisage
the potentiality of tuning of these molecular designs for
* To whom correspondence should be addressed. E-mail espinet@
cpd.uva.es.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) Reviews: Giroud-Godquin, A. M.; Maitlis, P. M. Angew. Chem. 1991,
103, 370; Angew. Chem. Int. Ed. Engl. 1991, 30, 375. Espinet, P.;
Esteruelas, M. A.; Oro, L. A.; Serrano, J. L.; Sola, E. Coord. Chem.
ReV. 1992, 117, 215. Inorganic Materials; Bruce, D., O’Hare, D., Eds.;
John Wiley and Sons: New York, 1992. Hudson, S. A.; Maitlis, P.
M. Chem. ReV. 1993, 93, 861.
Figure 1. Coordination modes for the dipicolinate dianion.
(2) Zhou, X. Y.; Kostic, N. M. Inorg. Chem. 1988, 27, 4402.
(3) See for example (a) Drew, M. G. B.; Fowles, G. W. A.; Matthews, R.
W.; Walton, R. A. J. Am. Chem. Soc. 1969, 91, 7769. (b) Drew, M.
G. B.; Matthews, R. W.; Walton, R. A. J. Chem Soc. A 1970, 1405.
(c) Gaw, H.; Robinson, W. R.; Walton, R. A. Inorg. Nucl. Chem. Lett.
1971, 7, 695. (d) Quaglieri, P. P.; Loiseleur, H.; Thomas, G. Acta
Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, B28,
2583. (e) Fu¨rst, W.; Gouzerh; P.; Jeannin, Y. J. Coord. Chem. 1979,
8, 237. (f) du Preez, J. G. H.; Rohwer, H. E.; van Brecht, B. J. A. M.;
Caira, M. R. Inorg. Chim. Acta 1983, 73, 67. (g) Bresciani-Pahor, N.;
Nardin, G.; Bonomo R. P.; Purello, R. Transition Met. Chem. 1985,
10, 316. (h) du Preez, J. G. H.; van Brecht, B. J. A. M. J. Chem. Soc.,
Dalton Trans. 1989, 253. (i) Ang, H. G.; Kwick, W. L.; Hanson, G.
R.; Crowther, J. A.; McPartlin, M.; Choi, N. J. Chem. Soc., Dalton
Trans. 1991, 3193. (j) Casellato, U.; Graziani, R.; Bonomo, R. P.; Di
Bilio, A. J. J. Chem. Soc., Dalton Trans. 1991, 23. (k) Chessa, G.;
Marangoni, G.; Pitteri, B.; Bertolassi, V.; Gilli, G.; Ferreti, V. Inorg.
Chim. Acta 1991, 185, 201.
materials, not only as liquid crystals but also in the field of
nonlinear optics.
There are however a few examples of other coordination
modes of this ligand such as bridging of two metals atoms
(Figure 1b),4 polymerization of chelate complexes by coordina-
tion of the carbonyl atom (structural assignments based only
upon electronic and infrared spectral data, Figure 1c),5 and
bidentate N-O coordination (Figure 1d).2,3k,6 For this reason
(4) Nardin, G.; Randaccio, L.; Bonomo, R. P.; Rizzarelli, E. J. Chem.
Soc., Dalton Trans. 1980, 369.
(5) Sengupta, S. K.; Shani, S. K.; Kapoor, R. N. Polyhedron 1983, 2,
317.
(6) Herring, A. M.; Henling, L.; Labinger, J. A.; Bercaw, J. E. Inorg.
Chem. 1991, 30, 851.
0020-1669/96/1335-2287$12.00/0 © 1996 American Chemical Society

2288 Inorganic Chemistry, Vol. 35, No. 8, 1996
Espinet et al.
Scheme 1
it seemed convenient to explore first the coordination behavior
on Pd with the simple and easily available unmodified dipic
ligand prior to attempting modifications on the ligand, some-
times difficult to achieve, in order to introduce the desired
physical properties.
We have previously reported palladium complexes containing
the sulfur analog dinegative anion of 2,6-bis(thiocarboxylic) acid
(H2pdtc).7 The tridentate ligand dipic incorporates harder donor
atoms (N,O,O) than pdtc (N,S,S). The differences in the
coordination behavior of the two ligands, including a new
bonding mode for the dipicolinic ligand, are reported and
discussed here.
complex 2, which has been characterized by analytical and
spectroscopic methods. Its IR spectrum in Nujol shows a strong
absorption at 910 cm-1 for the SdO stretching mode, with
O-coordination of the DMSO ligand.10a It is interesting to note
that the hard dipic ligand induces the palladium atom to select
the hard donor atom of the DMSO ligand. The 1H NMR spectra
of 1 and 2 in DMSO solution are identical, and consist of two
sets of signals of two different products. The major isomer
(ca. 83%) shows the symmetric pattern expected for the pyridine
protons in the structure depicted in Scheme 1. The minor isomer
(ca. 17%) exhibits three distinct pyridinic proton signals showing
the nonequivalence of the three pyridinic hydrogens, which
requires an asymmetric bonding mode for the chelating dipic
ligand. Although a detailed assignment of the structure is
precluded by the absence of other data, the results found for
the phosphine complexes 9-14 (Vide infra) point to a dimeric
nature of this minor isomer, but we cannot be sure whether the
DMSO is still O-coordinated.10b
A general synthetic route to neutral derivatives is shown in
Scheme 1. N- and P-donor ligands also displace easily the
weakly coordinated acetonitrile to give the corresponding neutral
complexes [Pd(dipic)L], 3-13, in high yield.
(a) N-Donor Ligands. The insolubility of the complexes
with pyridine (3), pyrazine (4), or p-toluidine (5), precluded
the acquisition of informative 1H NMR spectra; therefore, these
complexes have been characterized only by IR and elemental
analysis. In order to obtain more soluble compounds with
N-donors, the acetonitrile was replaced by 3,5-lutidine, 2,6-
lutidine or di-n-octylamine, to afford compounds 6, 7, and 8.
The 1H NMR spectra of 6-8 in CDCl3 show AB2 spin systems
which are in agreement with the mononuclear structure shown
in Scheme 1. The compound with dioctylamine (8) is soluble
Results and Discussion
The syntheses and structures of the new complexes are
summarized in Scheme 1.
Neutral dipic Complexes. [Pd(dipic)(NCCH3)] (1) has been
synthesized in very good yield by reacting a palladium salt of
a weak acid ([Pd(acac)2] or [Pd(AcO)2]3) with dipicolinic acid
(pKa1 ) 2.83, pKa2 ) 3.66),8 in acetonitrile.
The pale yellow complex 1, which contains the four
coordination positions of the palladium atom bonded to hard
donor atoms, is insoluble in common organic solvents. The
elemental analysis (see Experimental Section) reveals a 1:1:1
(Pd, dipic, NCMe) stoichiometry. The most prominent features
of its IR spectrum are a broad intense band at ca. 1688 cm-1
,
which is assigned to the ν(CO) vibrations of the coordinated
-CO2- groups, and two absorptions at 2334 and 2306 cm-1 in
the ν(C≡N) region (ν(C≡N), δ(CH3) + ν(CsC)) which are
1
similar to those of free NCMe (2254, 2290 cm-1).9 The H
NMR spectrum could only be obtained in deuterated DMSO,
revealing the displacement of acetonitrile (the spectrum contains
a resonance at 2.06 ppm corresponding to free NCMe). This
has been further confirmed by the isolation of the DMSO
(10) (a) Annibale, G.; Cattalini, L.; Bertolasi, V.; Ferretti, V.; Gilli, G.;
Tobe, M. L. J. Chem. Soc., Dalton Trans. 1989, 1265 and references
therein. (b) π-bound DMSO intermediates have been proposed to
explain its high trans effect; see: Collman, J. P.; Hegedus, L. S.;
Norton, J. R.; Finke, R. G. Principles and Applications of Organ-
otransition Metal Chemistry; University Science Books: Mill Valley,
CA, 1987; p 243.
(7) Espinet, P.; Lorenzo, C.; Miguel, J. A.; Bois, C.; Jeannin, Y. Inorg.
Chem. 1994, 33, 2052.
(8) Hildebrand, U.; Hu¨bner, J.; Budzikiewicz, H. Tetrahedron 1986, 42,
5969.
(9) Storhoff, B. N.; Lewis, H. C. Coord. Chem. ReV. 1977, 1, 23.

Pd-dipic Complexes
Inorganic Chemistry, Vol. 35, No. 8, 1996 2289
enough in benzene, where it exhibits an AX2 spin system for
the pyridinic protons with notable displacement of the signals
with respect to the CDCl3 solution, suggesting strong interactions
between the pyridyl ligand and the solvent.
(b) P-Donor Ligands. In contrast to the simple spectra
observed for complexes with N-donor ligands and for the related
1
complex [Pd(pdtc)PPh3],7 the H NMR spectrum of the tri-
methylphosphine derivative 9 in CDCl3 solution shows three
inequivalent pyridinic hydrogens: One triplet at 8.11 ppm (3JHH
) 7.8 Hz) assigned to H4; and two doublets of triplets at 7.99
5
and 7.66 ppm (4JHH ≈ JHP ≈ 1.5 Hz) for H3 and H5. This
spectrum reveals an asymmetric bonding mode for the dipic
ligand and is similar to the spectra reported for complexes trans-
[M(dipic)2]2- (M ) Pd, Pt) containing dipic as a bidentate N,O
ligand.3k,6,12 The 31P{1H} NMR spectrum of 9 in CDCl3
solution exhibits a single resonance at δ ) 6.1 ppm.
These data, together with the results of molecular weight
measurements (680 in CHCl3), strongly suggest the dinuclear
structure proposed for 9 in Scheme 1, which is actually found
by X-ray diffaction for the related complex 10 (see below). The
dipic ligand is coordinated to one palladium in a η2-N,O chelate
fashion, and the second carboxylic group serves as a bridge to
the other palladium atom, in an asymmetric bonding mode
previously unknown for the dipic ligand. It is remarkable,
however, that the difference of bonding between the two
carboxylate groups is not reflected in the IR spectra of the
phosphine complexes which, in the region of νasym(COO-), are
similar to those recorded for the monomeric complexes 3-8.
Thus the infrared spectra are useless in this case for elucidation
of the stereochemistry.
Figure 2. View of the structure of [Pd(dipic)(PBu3)]2 10 showing the
atom numbering scheme; only the first carbons of the PBu3 are drawn
for clarity.
Table 1. Crystallographic Data for [Pd(dipic)(PBu3)]2 (10)
formula
fw
cryst system monoclinic
C38H60N2O8P2Pd2
947.65
Z
T, K
4
293
F
calc, g cm-3 1.47
space group P21/n (No. 14)
F(000)
1952
a, Å
b, Å
c, Å
18.114(4)
13.191(2)
19.571(3)
113.45(2)
4290(1)
λ(Mo KR), Å 0.710 73 (graphite
monochromator)
µ, cm-1
9.47
â, deg
cryst size,
mm; color
0.2 × 0.16 × 0.13;
V, Å3
yellow
The reaction of 1 with PBu3 or PCy3 afford compounds 10
2
a R ) ∑(||Fo| - |Fc||)/∑|Fo|, Rw ) [∑(w(||Fo| - |Fc||)2/∑w|Fo| ]1/2
.
1
and 11, respectively. Their H NMR spectra in CDCl3 show
the coexistence in solution of the dinuclear (still very dominant,
virtually exclusive for 10 and a majority for 11) and the
monomeric isomers. This dimer-monomer equilibrium is
somewhat influenced by the solvent and the 1H NMR in actone
or benzene spectra exhibit a higher dimer-monomer ratio.
With triarylphosphines the solubility of the corresponding
complexes are decreased. Nevertheless, the complexes 12-14
are soluble enough in CDCl3, where their 31P NMR spectra
exhibit two signals suggesting again a dimer-monomer mixture.
1
by H NMR for the equilibrium dimer h 2 monomer for 13
are ∆H ) 27 ( 2 kJ mol-1 and ∆S ) 48 ( 5 J K-1 mol-1
(from the changing ratios of the integrated intensities of the
pyridinic protons, over the range 265-315 K, for a solution of
0.007 mmol of 13 in 0.5 mL of CDCl3).
The values of the equilibrium constants (K ) [monomer]2/
[dimer], in mol L-1) determined for the different neutral
complexes by NMR integration are as follows: 0 (only dimer
is observed) for complex 9; very small for complex 10; ca. 5.5
× 10-3 for complexes 11-13; 2.5 × 10-2 for 14; and infinite
(only monomer observed) for complexes 6-8. These values
suggest that the monomer is favored by the ligands with the
lower trans influence or with the higher steric requirement. Thus,
all the N-donor ligands give only monomer, whereas phosphines
tend to give mainly dimer. Alkylphosphines give almost
exclusively dimer except for the bulky PCy3 (cone angle 170°),12
and arylphosphines show the highest proportion of monomer
for the bulkiest P(o-Tol)3 (cone angle 194°).12
1
Their H NMR spectra are complex and show an overlap of
resonances in the aromatic region, yet the signals corresponding
to the monomer and the dimer can be recognized for the
pyridinic protons. The signals of the AB2 spin system attribut-
able to the mononuclear compound (minor component) are
distinctly observed. For the dimer, H5 is observed as doublet
of triplets at low field; H3 appears as a broad signal at a rather
high field (δ 6.2-6.8 ppm), due to anisotropic shielding by the
phenyl rings of the phosphine; finally H4 is overlapped by
signals of the aryl protons of the phosphines, but its presence
in the range 7.4-7.7 ppm can be deduced from integration and
by COSY experiments.
This behavior can be understood considering the fact that
the pincer structure of the ligand adopted in the monomers is
quite strained and imposes short N-Pd distances (more compat-
ible with L ligands with low trans influence), and low O-Pd-O
angles (more favored by bulky L ligands.
Further evidence for a solution equilibrium is obtained from
1
variable-temperature H or 31P NMR studies, where a change
of the monomer/dimer ratio with temperature is observed. Upon
increase of the temperature to 315 K, the resonances of the
monomer increase in intensity, while those of the dimer
decrease. The opposite behavior is observed with decreasing
temperature. These data are consistent with an equilibrium
between dimer and monomer where the monomers are entropi-
cally favored, as found recently for palladium complexes with
2-pyridinethiolate.11 The thermodynamic parameters determined
Structure of [Pd(dipic)PBu3]2 (10). The X-ray structure and
numbering scheme for complex 10 are shown in Figure 2.
Positional and thermal parameters for the relevant atoms are
given in Table 1, and selected bond lengths and angles in Table
2. The X-ray determination confirms the structure proposed
on the basis of spectroscopic data. The molecule is a dimer in
which each dipic ligand is coordinated to one palladium in a
chelate manner through one carboxylate oxygen atom and the
pyridyl nitrogen, while the other carboxylate group, twisted out
(11) Gupta, M.; Cramer, R. E.; Ho, K.; Pettersen, C.; Mishina, Staffany;
Belli, J.; Jensen, C. M. Inorg. Chem. 1995, 34, 60.
(12) Tolman, C. A. Chem. ReV. 1977, 77, 313.

2290 Inorganic Chemistry, Vol. 35, No. 8, 1996
Espinet et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[Pd(dipic)(PBu3)]2 (10)
The tendency of the dipicolinate complexes to dimerization
is absent in the related complex [Pd(pdtc)PPh3], as determined
1
by H NMR spectroscopy.7 A plausible explanation for this
Pd(1)-P(1)
Pd(1)-O(1)
Pd(2)-N(2)
Pd(2)-O(20)
N(1)-C(12)
O(1)-C(11)
N(2)-C(22)
O(2)-C(21)
C(10)-O(12)
C(12)-C(11)
C(11)-O(11)
C(20)-O(22)
2.234(2)
2.000(5)
2.139(6)
2.020(5)
1.361(8)
1.299(9)
1.364(9)
1.27(1)
1.223(9)
1.51(1)
1.213(8)
1.20(1)
Pd(1)-N(1)
Pd(1)-O(10)
Pd(2)-O(2)
Pd(2)-P(2)
N(1)-C(16)
O(10)-C(10)
N(2)-C(26)
O(20)-C(20)
C(10)-C(26)
C(16)-C(20)
O(21)-C(21)
C(21)-C(22)
2.140(5)
2.000(5)
1.990(6)
2.219(3)
1.351(9)
1.282(9)
1.349(9)
1.292(9)
1.52(1)
different behavior is suggested by the structural features of both
families. For [Pd(pdtc)Br]- the chelating cycle is relatively free
of strain (N-Pd-S ) 86.44(6)°), whereas for 10 the chelating
carboxylate groups have a smaller bite angle (N(1)-Pd(1)-
O(1) ) 81.0(2)°; N(2)-Pd(2)-O(2) ) 81.2(3)°). This is related
to the shorter C-O distance (1.299(9) and 1.27(1) Å) compared
to the C-S distance (1.711(9) Å, as well as by the longer Pd-N
distance in 10. Consequently the chelating cycles in the
dipicolinate derivatives are more strained and have a higher
tendency to relieve this strain by forming dimers (with only
one strained cycle per palladium) rather than monomers (with
two strained cycles per palladium), specially when the ancillary
ligand induces a long Pd-N bond.
1.49(1)
1.24(1)
1.54(1)
N(1)-Pd(1)-P(1)
O(1)-Pd(1)-N(1)
O(10)-Pd(1)-N(1)
O(2)-Pd(2)-N(2)
O(20)-Pd(2)-O(2)
P(2)-Pd(2)-O(2)
C(31)-P(1)-Pd(1)
C(41)-P(1)-C(31)
C(51)-P(1)-C(31)
C(12)-N(1)-Pd(1)
C(16)-N(1)-C(12)
169.8(2) O(1)-Pd(1)-P(1)
81.0(2) O(10)-Pd(1)-P(1)
101.4(2) O(10)-Pd(1)-O(1)
81.2(3) O(20)-Pd(2)-N(2)
176.0(3) P(2)-Pd(2)-N(2)
90.8(2) P(2)-Pd(2)-O(20)
114.9(3) C(41)-P(1)-Pd(1)
106.2(4) C(51)-P(1)-Pd(1)
107.8(4) C(51)-P(1)-C(41)
108.3(4) C(16)-N(1)-Pd(1)
118.2(6) C(11)-O(1)-Pd(1)
89.8(2)
87.8(2)
177.4(2)
102.6(2)
171.7(2)
85.4(2)
109.5(3)
115.7(3)
101.5(4)
131.9(5)
115.7(4)
109.7(5)
117.0(7)
Ionic Dipic Complexes. A fast reaction takes place when
[NBun ]Br is added to a suspension of 1 in acetone with
4
formation of [NBun ][Pd(dipic)Br] (16). Alternatively
4
[NBun ][Pd(dipic)Cl] (15) was made by reaction of K2PdCl4
4
with (NH4)2dipic in water, followed by cation exchange with
[NBun ]Cl, in a procedure similar to that reported by Zhou and
4
C(10)-O(10)-Pd(1) 119.1(4) C(22)-N(2)-Pd(2)
Kostic for the platinum complex [NBun ][Pt(dipic)Cl].2 Using
4
C(26)-N(2)-Pd(2)
C(21)-O(2)-Pd(2)
132.8(5) C(26)-N(2)-C(22)
115.9(6) C(20)-O(20)-Pd(2) 118.5(5)
this alternative method, the formation of a gellike substance
was observed for K[Pd(dipic)Cl] in water, as reported for the
platinum complex, although their behavior was not further
O(12)-C(10)-O(10) 127.4(7) C(26)-C(10)-O(10) 112.9(6)
C(26)-C(10)-O(12) 119.4(7) C(20)-C(16)-N(1)
119.8(6)
124.8(7)
studied. These [NBun ]+ salts are soluble in polar organic
C(12)-C(11)-O(1)
116.0(6) O(11)-C(11)-O(1)
4
O(11)-C(11)-C(12) 119.2(7) C(16)-C(20)-O(20) 112.6(7)
O(22)-C(20)-O(20) 126.0(8) O(22)-C(20)-C(16) 121.3(8)
1
solvents. The H NMR spectra of the yellow solutions of 15
and 16 in CDCl3 show AX2 systems for the pyridinic protons
in agreement with a mononuclear structure.
O(21)-C(21)-O(2)
C(22)-C(21)-O(21) 117(1)
C(10)-C(26)-N(2) 117.8(7)
125(1)
C(22)-C(21)-O(2)
C(21)-C(22)-N(2)
118.3(8)
113.3(8)
The reaction of 15 with AgAcO in acetone produces a yellow
solution from which, after filtration to eliminate the precipitate
AgCl, yellow crystals of 17 were isolated. The IR spectrum of
this compound affords little structural information since it
exhibits overlapped absorptions in the carboxylate stretching
region. The acetato group is formulated as monodentate ligand
on the basis of the observation of an AX2 pattern for the
of the pyridine plane, is bonded to the second palladium atom
of the dimer. The two bridges are arranged in a complementary
head-to-tail manner. The resulting coordination around each
palladium is distorted square planar with the remaining fourth
coordination site filled by the phosphorus atom of the tribu-
tylphosphine.
1
pyridinic protons in the H NMR spectrum.
In related complexes the pyridine ring and the chelating
carboxylate are nearly coplanar with the coordination plane.2,3k,13
However, in 10 the pyridine ring is twisted by 20.0(2)° (for
that on Pd(1)), or 12.2(2)° (for that on Pd(2)), with respect to
the coordination planes. The deviation from coplanarity of the
chelated carboxylate groups with respect to the pyridine ring is
reflected in the torsion angles O(1)-(11)-C(12)-N(1) )
-3(1)°, and O(2)-C(21)-C(22)-N(2) ) -7(1)°. The bridging
carboxylate groups are rotated with respect to the pyridine ring
by 36.4(4) and 42.1(4)°. The C-O distances and angles for
the chelate and bridged carboxylate groups are very similar since
both carboxylates are coordinated. The dipic ligands are
oriented in such a way that their aromatic rings are very nearly
perpendicular to one another (dihedral angle 84.4(2)°), whereas
the dihedral angle between the coordination planes of the two
palladium atoms is 69.76(8)°.
The phosphine seems to exert a marked influence on the trans
Pd-N distances (2.140(5) and 2.139(6) Å). They are noticeably
longer than the distances found in [Pd(pdtc)Br]- (2.034(8) Å)7
or in dipicolinate complexes of platinum (1.88-2.03 Å)2 and
are similar to those found in the series of dimers [Pd(µ-η2-pyS-
N,S)Cl(PR3)]2 (PR3 ) PMe3, PMe2Ph, and PMePh2),14 all of
them having the PR3 ligand in position trans to the N atom,
which are in the range of 2.124(6)-2.137(9) Å.
Other attempts to obtain complexes that had been easily
prepared for the homologous dptc series failed in the dipic
derivatives. Thus, several attempts at obtaining cyano com-
plexes by treating halide complexes 15 or 16 with AgCN, or
the acetonitrile derivative 1 with [NBun ]CN, gave mixtures of
4
compounds containing terminal (ν(CN) 2141 cm-1) and bridging
(ν(CN) 2181 cm-1) cyanide ligands, which could not be
separated as pure samples.
Conclusions
The dipicolinate anion behaves as a tridentate ligand toward
palladium, but its coordination as a pincer ligand produces a
somewhat strained situation. For this reason some monomeric
complexes show a tendency to dimerization in what is a novel
coordination mode for this ligand, with relief of strain. These
cases look not propitious to produce palladium liquid crystals.
The strain in the coordination of dipic as pincer ligand for
Pd seems to arise mainly from the short C-O distances,
compared to the longer Pd-S distances in the related pdtc
derivatives (pdtc is very stable as pincer ligand). Consequently
the use of modified dipic or pdtc ligands in order to produce
metal-containing liquid crystals where these ligands act as
tridentate pincers should be oriented along two possible lines:
(i) O,N,O-pincers might be used for the smaller (first row)
transition metals where the shorter M-N distances will reduce
the strain in the cycle or with ancillary ligands which favor
monomeric species. (ii) S,N,S-pincers should be used preferably
(13) Annibale, G.; Cattalini, L.; Canovese, L.; Pitteri, B.; Tiripicchio, A.;
Tiripicchio Camellini, M.; Tobe, M. L. J. Chem. Soc., Dalton Trans.
1986, 1101.
(14) Yamamoto, J. H.; Yoshida, W.; Jensen, C. M. Inorg. Chem. 1991,
30, 1353.

Pd-dipic Complexes
Inorganic Chemistry, Vol. 35, No. 8, 1996 2291
Preparation of [NBun ][Pd(dipic)Br] (16). Complex 1 (0.1 g, 0.32
for heavier transition metals. Research along these lines with
adequately modified dipic and pdtc ligands is underway.
4
mmol) and (Bun N)Br (0.103 g, 0.32 mmol) were stirred in acetone
4
(10 mL). After 10 min a solution had been formed which was filtered
through Celite and concentrated in Vacuo to ca. 5 mL. Addition of
diethyl ether produced orange crystals of 16. Yield: 0.159 g (84%).
IR (Nujol, cm-1): ν(CdO) 1665, ν(PdsBr) 258. 1H NMR (CDCl3):
δ 8.11 (t, 1, JHH ) 7.7 Hz, H4 dipic), 7.82 (d, 2, JHH ) 7.7 Hz, H3,5
dipic), 3.28 (m, 8, NCH2), 1.69 (m, 8, NCH2CH2), 1.48 (m, 8, CH2-
CH3), 1.02 (t, 12, -CH3).
Experimental Section
Literature methods were used to prepare [Pd(acac)2],15 [Pd(OAc)2]3,16
and (NH4)2dipic.2 Instrumentation was described previously.7 All the
compounds gave satisfactory elemental analyses (Table S9, Supporting
Information). Only representative NMR data are given here.
complete listing of NMR data is available as Supporting Information
(Table S10).
A
Preparation of [NBun ][Pd(dipic)(OAc)] (17). Silver acetate (0.073
4
Preparation of [Pd(dipic)(NCCH3)] (1). Method a. Palladium
acetate (0.5 g, 2.23 mmol) and pyridine-2,6-dicarboxylic acid (0.372
g, 2.23 mmol) were stirred at room temperature in acetonitrile (10 mL)
for 6 h. A pale yellow precipitate of 1 was formed which was collected
on a frit, washed with acetone (3 × 3 mL) and air-dried. Yield: 0.668
g (96%). IR (Nujol, cm-1): ν(CO) 1688, ν(MeCN)) 2334, 2306.
Method b. [Pd(acac)2] (1.00 g, 3.28 mmol), and pyridine-2,6-
dicarboxylic acid (0.5485 g, 3.28 mmol) were stirred in acetonitrile
(10 mL) for 24 h. The precipitate was collected on a frit, washed with
chloroform (3 × 5 mL) and acetone (3 × 5 mL) and air-dried. Yield:
0.872 g (85%).
Preparation of [Pd(dipic)(DMSO)] (2). Complex 1 (0.1 g, 0.32
mmol) was dissolved in 5 mL of hot DMSO. When the solution was
clear, the solvent was removed in vacuo. The resulting oily residue
was triturated in chloroform to give a solid which was collected on a
frit and air-dried. Yield: 0.102 g (91%). IR (Nujol, cm-1): ν(CdO)
1660; ν(SdO) 910. 1H NMR (DMSO-d6): monomer δ 8.33 (t, 1, JHH
g, 0.437 mmol) and 15 (0.2 g, 0.364 mmol) in acetone (20 mL) were
stirred in the dark overnight. The AgCl precipitate was filtered off,
and the yellow solution was concentrated. Addition of diethyl ether
gave orange crystals which were recrystallized from acetone-diethyl
ether by cooling at -20 °C. Yield: 0.18 g (86%). IR (Nujol, cm-1):
ν(CdO) 1660, 1638. 1H NMR (CDCl3): δ 8.05 (t, 1, JHH ) 7.7 Hz,
H4 dipic), 7.73 (d, 2, JHH ) 7.7 Hz, H3,5 dipic), 3.28 (m, 8, NCH2),
1.99 (s, 3, CH3CO2-), 1.69 (m, 8, NCH2CH2), 1.48 (m, 8, CH2CH3),
1.02 (t, 12, -CH3).
X-ray Structure of [Pd(dipic)(PBu3)]2. Crystals suitable for X-ray
determination were grown by slow diffusion of hexane into a
concentrated solution of 10 in CH2Cl2. Relevant crystallographic details
are given in Table 1. Unit cell parameters were determined from the
least-squares refinement of a set of 25 centered reflections. Three
reflections were measured every 1 h as orientation and intensity control.
Significant decay was not observed. Heavy atoms were located from
a Patterson synthesis, and the remaining non-hydrogen atoms by
DIRDIF.17 Full-matrix least-squares refinements were made with
SHELX76.18 After isotropic refinement, an absorption correction was
applied with DIFABS.19 The carbon atoms of two butyl groups [C(71)
to C(74), and C(81) to C(84)] of one of the PBu3 ligands showed some
degree of thermal disorder which, despite repeated attempts, could not
be modeled satisfactorily. Therefore, the two butyl groups were refined
as rigid groups, affixed to the same phosphorus atom, P(2), with
individual isotropic temperature factors. The remaining non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were geo-
metrically positioned (except for the carbon atoms involved in the
disorder), and were given a common isotropic temperature factor which
was refined. Torsion angles and least-squares planes were calculated
with PARST.20 The drawing was made with PLATON.21
) 7.8 Hz, H4), 7.82 (d, 2, JHH ) 7.8 Hz, H3,5); dimer 8.32 (t, 2, JHH
)
7.8 Hz, H4), 7.92, 7.51 (dd, 2, 2, JHH ) 7.9, JHH ) 1.3 Hz, H3, H5).
Preparation of [Pd(dipic)py] (3). To a suspension of 1 (0.1 g,
0.32 mmol) in acetone was added pyridine (0.028 mL, 0.35 mmol),
and the mixture was stirred overnight. The new pale yellow precipitate
was collected on a frit, washed with acetone (3 × 3 mL), and dried in
vacuo. Yield: 0.162 g (86%). IR (Nujol, cm-1): ν(CdO) 1667.
Compounds 4-14 were made similarly, for complexes 6-8 a
solution was obtained, and the solvent was removed in a rotatory
evaporator. The residue was washed in ether and recrystallized in
chloroform/diethyl ether to yield yellow crystals, which were filtered
and washed with diethyl ether.
3
4
6. Yield: 86%. IR (Nujol, cm-1): ν(CdO) 1667. 1H NMR
(CDCl3): δ 8.21, 7.93 (AB2 spin system, JHH ) 7.8 Hz, H4, H3,5 dipic),
8.10 (s, 2, H2,6 py), 7.57 (s, 1, H4 py), 2.36 (s, 6, Me).
11. Yield: 83%. IR (Nujol, cm-1): ν(CdO) 1674. 1H NMR
(CDCl3): dimer δ 7.97 (t, 2, JHH ) 7.8 Hz, H4 dipic), 8.14, 7.82 (dt,
Acknowledgment. We gratefully acknowledge financial
support by the Comisio´n Interministerial de Ciencia y Tecno-
log´ıa (Project MAT93-0329) and the Direccio´n General de
Investigacio´n Cient´ıfica y Te´cnica (Projects PB93-0222 and
PB93-0330).
5
2, 3JHH ) 7.8 Hz, 4JHH ≈ JHP ≈ 1.4 Hz, H3, H5 dipic), 2.2-1.1 (m, 66,
C6H11); monomer: 8.21, 8.02 (AB2 spin system, H4, H3,5 dipic). K25°C
) 5.1 × 10-3 mol L-1
.
31P{1H} NMR (CDCl3): dimer 44.7 s;
monomer 41.1 s.
13. Yield: 94%. IR (Nujol, cm-1): ν(CdO) 1676. 1H NMR
3
4
5
(CDCl3): dimer, δ 7.95 (dt, 2, JHH ) 7.8 Hz, JHH ≈ JHP ≈ 1.4 Hz,
H5 dipic), 7.60 (t, 2, JHH ) 7.8 Hz, H4 dipic), 6.24 (b, 2, H3 dipic),
7.7-7.1 (m, C6H4), 2.32 (s, 18, Me); monomer, δ 8.21, 8.02 (AB2 spin
system, JHH ) 7.8 Hz, H4, H3,5 dipic), 2.39 (s, 9, Me). K25°C ) 5.5 ×
Supporting Information Available: Tables of atomic coordinates
(Table S1), anisotropic thermal parameters for non-hydrogen atoms
(Table S2), atomic parameters for hydrogen atoms (Table S3), bond
distances (Table S4) and angles (Table S5), torsion angles (Table S6),
least-squares planes (Table S7), crystallographic details including text
with experimental details (Table S8), microanalyses, yield, and IR data
(Table S9), and NMR data (Table S10) (15 pages). Ordering
information is given on any current masthead page.
.
10-3 mol L-1 31P{1H} NMR (CDCl3): dimer 22.2 s, monomer 19.4
s.
Preparation of [NBun ][Pd(dipic)Cl] (15). To a solution of K2-
4
[PdCl4] obtained by mixing in water (20 mL) PdCl2 (0.5 g, 2.82 mmol)
and KCl (0.42 g, 5.64 mmol) was added (NH4)2dipic (0.57 mmol, 2.84
mmol), and the solution was stirred for 4 h at 75 °C. Then a solution
of (Bun N)NO3 (prepared by mixing concentrated solutions of AgNO3
4
IC951024N
(0.575 g, 3.38 mmol) and (Bun N)Br (1.091 g, 3.38 mmol) in water)
4
was added. A yellow precipitate was formed which was filtered,
washed with cold water, and dried in vacuo. The solid was recrystal-
lized from dichloromethane-diethyl ether. Yield: 1.212 g (78%). IR
(Nujol, cm-1): ν(CdO) 1663, ν(PdsCl) 315. 1H NMR (CDCl3): δ
8.11 (t, 1, JHH ) 7.7 Hz, H4 dipic), 7.81 (d, 2, JHH ) 7.7 Hz, H3,5
dipic), 3.28 (m, 8, NCH2), 1.69 (m, 8, NCH2CH2), 1.48 (m, 8, CH2-
CH3), 1.02 (t, 12, -CH3).
(17) Beurskens, P. T.; Admiraal, G.; Bosman, W. P.; Beurskens, G.;
Doesburg, H. M.; Garc´ıa-Granda, S.; Gould, R. O.; Smits, J. M. M.;
Smikalla, C. The DIRDIF Program System, Technical Report of the
Crystallography Laboratory. University of Nijmegen, The Netherlands,
1982.
(18) Sheldrick, G. M. SHELX76, Program for Crystal Structure Determina-
tions. University of Cambridge, 1976. Local version: Van der Maelen,
F. J. Ph.D. Thesis, University of Oviedo, Oviedo, Spain, 1991.
(19) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 168.
(20) Nardelli, M. Comput. Chem. 1983, 7, 95.
(15) Grinberg, A. A.; Simonova, L. K. Zhur. Priklad. Khim. 1953, 6, 880;
Chem. Abstr. 1953, 47, 11060g.
(16) Stephenson, T. A.; Morehouse, S. M.; Powell, A. R.; Heffer, J. P.;
Wilkinson, G. J. Chem. Soc. 1965, 3632.
(21) Spek, A. L. The EUCLID Package. In Computational Crystallography;
Sayre, E., Ed.; Clarendon Press: Oxford, England, 1982; p 528.