Syntheses, structures, and reactivities of novel palladium β-diiminato-acetate complexes

Article abstract of DOI:10.1021/ic801557c

The reaction between (Ar₂nacnac)H (Ar = C₆H ₅, ligand a; 2,6-(ⁱPr)₂C₆H ₃, ligand b) and [Pd(OAc)₂] produces red complexes [Pd(Ar₂nacnac)(OAc)] (1a: Ar = C

Full text of DOI:10.1021/ic801557c

Inorg. Chem. 2008, 47, 12010-12017
Syntheses, Structures, and Reactivities of Novel Palladium
ꢀ-Diiminato-Acetate Complexes
Alen Hadzovic and Datong Song*
DaVenport Chemical Research Laboratories, Department of Chemistry, UniVersity of Toronto,
80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
Received August 14, 2008
The reaction between (Ar2nacnac)H (Ar ) C6H5, ligand a; 2,6-(iPr)2C6H3, ligand b) and [Pd(OAc)2] produces red
complexes [Pd(Ar2nacnac)(OAc)] (1a: Ar ) C6H5; 1b: Ar ) 2,6-(iPr)2C6H3). Complex 1a has a dimeric structure in
the solid state with two bridging acetates, while a monomer-dimer equilibrium establishes in solution. Complex 1b
is monomeric in both the solid state and solution. Both are air- and moisture-stable compounds, unlike
[Pd(Ph2nacnac)2] (2), which easily hydrolyzes in the presence of moisture to give [Pd(Phnacac)2] (3) (Phnacac )
{CH3C(NPh)CHC(O)CH3}-). Compound 1a reacts with metal acetates to produce heterotrimetallic complexes of a
general formula [Pd(Ph2nacnac)]2-µ-[M(OAc)4] (4, M ) Cu; 5, M ) Zn). Treating 1a with KOH in THF, or alternatively
[Pd(Ph2nacnac)Cl]2 with KOtBu in wet THF, produces [Pd(Ph2nacnac)(OH)]2 (6).
Introduction
sandwich complexes of V(I)10 and Cr(I),11 a Cr(II) hydride,12
low-coordinate Ni(I) compounds,13,14 and a Zn(I) dimer.15
The transition metal nacnac chemistry has received a boost
after the discoveries of highly active Zn-nacnac catalysts for
the copolymerization of CO2 and epoxides and the copo-
lymerization of epoxides and cyclic anhydrides,16-19 Fe-
nacnac dinitrogen complexes,20,21 and Cu-nacnac dioxygen
complexes.22-24
The ꢀ-diiminate ligands include a large variety of versatile
bidentate, monoanionic N-donor ligands. Among these
ligands, the ones derived from acetylacetone (acacH),
nicknamed nacnac, have been a center of lively activities
since the pioneering work done by Holm and Parks and by
McGeachin.1,2 The straightforward synthesis, broad tunability
of both steric and electronic properties at the N donors, as
well as the carbon backbone render the nacnac ligands
popular in coordination chemistry.3 They have been exten-
sively used to stabilize metal centers of uncommon oxidation
states and geometries. Some examples in main group
chemistry include rare Mg(I),4 Al(I),5,6 Ge(I),7 Ge(II),6 and
In(I).8,9 Examples from transition metals include inverted
(10) Tsai, Y.-C.; Wang, P.-Y.; Lin, K.-M.; Chen, S.-A.; Chen, J.-M. Chem.
Commun. 2008, 205.
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R. B.; Schleyer, P. v. R.; Schaefer, H. F., III; Robinson, G. H. J. Am.
Chem. Soc. 2005, 127, 11944.
* Author to whom correspondence should be addressed. E-mail:
dsong@chem.utoronto.ca.
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12010 Inorganic Chemistry, Vol. 47, No. 24, 2008
10.1021/ic801557c CCC: $40.75  2008 American Chemical Society
Published on Web 11/12/2008

NoWel Palladium ꢀ-Diiminato-Acetate Complexes
Scheme 1. Synthesis of Complex 1
Despite the interest in transition metal nacnac complexes,
relatively little work has been done with heavy transition
metals. Among these, some interest has been paid to
platinum25-27 and iridium28,29 complexes, all related to the
C-H bond activation. The interest in Pd-nacnac chemistry
stems directly from the Brookhart et al.’s work published in
the mid-1990s. Brookhart and co-workers have developed a
family of catalysts for olefin polymerization in which Pd is
supported by R-diimine ligands.30,31 These catalysts are
particularly useful for the copolymerization of olefinic esters
and nonpolar olefins for which the traditional early transition
metal catalysts give unsatisfactory performance owing to their
high oxophilicity. The ꢀ-diiminate analogues of these
catalysts are yet to be developed and evaluated. Following
previous reports on a series of well-studied Pd-nacnac
complexes,34-36 we have recently shown that [Pd(Ph2-
nacnac)Cl]2 represents a versatile starting material for Pd-
nacnac chemistry.32,33 Herein, we report the syntheses,
characterizations, and reactivities of novel [Pd(Ar2-
nacnac)(OAc)] (Ar ) C6H5 or 2,6-(iPr)2C6H3) complexes.
Figure 1. The molecular structure of 1a with thermal ellipsoids drawn at
50% probability. Hydrogen atoms and phenyl rings (except for the ipso-
carbons) are omitted for clarity. Selected bond lengths (Å): Pd1-N1,
1.989(3); Pd1-N2, 1.989(3); Pd1-O1, 2.062(2); Pd1-O3, 2.054(2);
Pd2-N3, 1.994(3); Pd2-N4, 1.985(3); Pd2-O2, 2.054(2); Pd2-O4,
2.061(2); N1-C2, 1.325(4); N2-C4, 1.323(4); N3-C19, 1.328(4); N4-C21,
1.326(4). Selected bond angles (deg): N2-Pd1-N1, 90.25(11); N4-Pd2-N3,
91.55(11); O3-Pd1-O1, 87.74(9); O2-Pd2-O4, 88.39(9).
CH3CN). They dissolve well in nonpolar solvents (hexanes
and ether), but their solubility in polar solvents (alcohols
and water) is low.
The molecular structure of 1a is shown in Figure 1, while
selected crystallographic data are presented in Table 1. The
dimeric complex crystallized in the monoclinic space group
P21/n. Two Pd(II) centers, bridged by two acetates, adopt
the typical square-planar coordination geometry with two N
and two O atoms taking the four coordination sites. The
overall dimeric structure is similar to the structures of
recently reported37,38 cationic species [Pd(dmp)(OAc)]2+
(dmp ) 2,9-dimethyl-1,10-phenanthroline) and [Pd(bis-
carbene)(OAc)]2+ as well as the neutral compounds
[Pd(Ph2nacCl)(OAc)]2 (Ph2nacCl ) {CH3(PhN)C}2CCl)32
and palladacycle acetate dimer.39
Unlike 1a, complex 1b is a monomer (Figure 2) in the
solid state. It crystallized in the monoclinic space group P21/
n, with two independent molecules per asymmetric unit. Each
square-planar Pd(II) center is chelated by a nacnac and an
acetate ligand. The overall molecular geometry is similar to
that of the previously reported [Pd(acac)(Ar2nacnac)] (Ar )
2,6-iPr2C6H3).34
Results and Discussion
The addition of a [Ar2nacnac]H (a: Ar ) Ph; b: Ar )
2,6-iPr2C6H3) solution to a Pd(OAc)2 solution produces
complexes 1a and 1b according to Scheme 1 in good yields
(75-80%). Occasionally, and particularly in a high concen-
tration regime (>0.01 M), a light pink precipitate forms
within 15 min. While its formation lowers the yield, it does
not change the identity of the final product. This pink material
has extremely low solubility in common solvents, and its
identity is still unknown.
The deep red complexes 1a and 1b are air- and
moisture-stable. They have high solubility in common
organic solvents (CH2Cl2, CHCl3, acetone, THF, and
In general, ꢀ-diiminato-acetate complexes are relatively
scarce in the literature. Coates et al.’s work on
[Zn(Ar2nacnac)(OAc)]2 (Ar ) bulky aryl substituent) led to
the development of highly active catalysts for the copolym-
erization of CO2 and epoxides16–18 and epoxides and cyclic
anhydrides.19 As confirmed by X-ray crystallographic analy-
sis, these Zn complexes are all dimers in the solid state
regardless of the steric bulk from the aryl substituents, likely
because of the preferred tetrahedral geometry at the Zn(II)
ion. The same has been observed for dimeric
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Inorganic Chemistry, Vol. 47, No. 24, 2008 12011

Hadzovic and Song
Table 1. Crystallographic Data for 1-6
1a
1b
2
3
4 ·2CH2Cl2
5·THF
6·2C6H6
formula
fw
T (K)
space group
a (Å)
b (Å)
C38H40N4O4Pd2 C31H44N2O2Pd C34H34N4Pd
C22H24N2O2Pd C44H50Cl4CuN4O8Pd2 C46H54N4O9Pd2Zn C46H48N4O2Pd2
829.54
150(2)
P21/n
583.08
296(2)
P21/n
605.05
150(2)
P21/c
454.83
150(2)
C2/c
1181.02
150(2)
P1
1085.16
150(2)
Pbcn
901.72
150(2)
P21/c
j
17.0440(5)
11.5241(4)
18.4938(4)
90
17.692(6)
16.787(6)
21.497(7)
90
10.8762(4)
25.4132(9)
15.3475(4)
90
16.9433(16)
6.7425(7)
18.0146(17)
90
108.697(3)
90
8.4111(4)
11.6681(3)
13.2324(5)
84.756(2)
73.2241(18)
71.043(2)
1175.94(8)
1
28.9299(8)
17.8215(5)
17.8255(5)
90
90
90
13.7156(7)
12.5422(7)
11.6662(4)
90
93.826(3)
90
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
97.9268(18)
90
96.165(12)
90
97.4760(10)
90
3597.78(18)
4
6348(4)
8
4206.0(2)
6
1949.4(3)
4
9190.4(4)
8
2002.39(17)
2
Dcalcd (g·cm-3
)
1.531
1.220
1.433
1.550
1.668
1.568
1.495
µ (mm-1
)
1.044
0.611
0.692
0.971
1.487
1.350
0.941
no. of reflns collected 24340
46538
11094
0.999
36941
9594
1.011
R1 ) 0.0342
5956
12361
43705
11836
no. of ind. reflns
8169
1.086
R1 ) 0.0392
wR2 ) 0.0891
R1 ) 0.0650
wR2 ) 0.1017
2222
5301
7951
3514
GOF on F2
0.989
1.044
1.038
1.006
a
R [I > 2σ(I)]
R1 ) 0.0535
R1 ) 0.0262
R1 ) 0.0434
R1 ) 0.0511
wR2 ) 0.1152
R1 ) 0.0911
wR2 ) 0.1396
R1 ) 0.0482
wR2 ) 0.1104
R1 ) 0.0835
wR2 ) 0.1309
b
wR2 ) 0.0943 wR2 ) 0.0711 wR2 ) 0.0642 wR2 ) 0.0957
R (all data)
R1 ) 0.1617
R1 ) 0.0578
R1 ) 0.0553
R1 ) 0.0622
wR2 ) 0.1296 wR2 ) 0.0796 wR2 ) 0.0760 wR2 ) 0.1080
a R1 ) ∑(Fo - Fc)/∑Fo. b wR2 ) [∑[w(Fo - Fc )2]/∑w(Fo )2]1/2
.
2
2
2
Scheme 2. Monomer/Dimer Equilibrium in a Solution of 1a
Figure 2. The molecular structure of complex 1b with thermal ellipsoids
drawn at 50% probability. All hydrogen atoms are omitted, and the
disordered isopropyl groups are shown in only one orientation for clarity.
Selected bond lengths (Å): Pd1-N1, 1.964(5); Pd1-N2, 1.971(5); Pd1-O1,
2.089(4); Pd1-O2, 2.118(4); N1-C2, 1.328(8); N2-C4, 1.309(9). Selected
bond angles (deg): N1-Pd-N2, 92.1(2); O1-Pd1-O2, 61.94(16);
N1-Pd1-O1, 102.43(17); N2-Pd1-O2, 103.58(19).
via a, so far unobserved, κ-O-OAc intermediate with acetate
as a monodentate ligand and a vacant coordination site. The
previously mentioned [Zn(Ar2nacnac)(OAc)]2 complexes also
exist in a monomer-dimer equilibrium in their solutions with
experimental evidence indicating involvement of a bimetallic
complex in epoxide enchainment.43
[Mn(Ar2nacnac)(OAc)]240 and [Mg(Ar2nacnac)(OAc)]2 com-
plexes (Ar ) 2,6-(iPr)2C6H3).41 The only fully characterized
monomeric nacnac-OAc complex other than 1b is
[Cu(Ar2nacnac)(OAc)] (Ar ) 2,4,6-Me2C6H2).42
On the other hand, no equilibrium has been observed for
the monomeric complex 1b in solution. Thus, the 1H NMR
spectrum shows two sets of resonances for diastereotopic
isopropyl CH3 groups at 1.24 and 1.44 ppm and singlets at
1.63 and 4.91 ppm for CH3 and CH groups of the nacnac
backbone. The acetate CH3 group shows as a singlet at 1.48
ppm comparable to the resonance observed for the 1a
monomer (1.55 ppm). It is likely that the dimerization of 1b
On the basis of 1H and 13C NMR spectra, 1a is in
equilibrium with, presumably, its monomer in solution
(Scheme 2). The ratio of the two species is concentration-
dependent, with higher concentrations favoring the dimeric
form. Measurements of relative intensities of the resonances
at 1.74 ppm (monomer) and 1.87 ppm (dimer) for nacnac
CH3 groups as a function of concentration (measured in the
concentration range of 1.2∼6.5 × 10-5 M) provided the
equilibrium constant of ∼4 mM-1 at 25 °C in CD2Cl2,
indicating that the equilibrium lies on the dimer side. This
is comparable to the value of 5 mM-1 for the related
[Pd(dmp)(OAc)]2(OTf)2 (dmp ) 2,9-dimethyl-1,10-phenan-
throline).36 The formation of the dimer probably proceeds
i
is suppressed as a result of the steric effects of Pr groups
on phenyl rings in 1b.
An alternative explanation for the observed equilibrium
in a solution of 1a could be ligand “scrambling”. In this
scenario, the 1a dimer would be in equilibrium with
[Pd(Ph2nacnac)2] and [Pd(OAc)2] rather than with its mono-
mer. To examine this possibility, we have prepared
1
[Pd(Ph2nacnac)2], complex 2 (Scheme 3). The H NMR
(41) Dove, A. P.; Gibson, V. C.; Hormnirun, P.; Marshall, E. L.; Segal,
J. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 3088.
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Chem. Soc. 2003, 125, 11911.
spectrum of 2 shows two singlets at 1.66 and 5.04 ppm from
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B. M.; Simonovic, S. Dalton Trans. 2004, 3528.
12012 Inorganic Chemistry, Vol. 47, No. 24, 2008

NoWel Palladium ꢀ-Diiminato-Acetate Complexes
Figure 4. The molecular structure of 3. All hydrogen atoms omitted for
clarity. Thermal ellipsoids are shown at 50% probability. Selected bond
lengths (Å): Pd1-O1, 1.996(9); Pd1-N1, 2.042(10); O1-C2, 1.294(15);
N1-C4, 1.328(16). Selected bond angles (deg): O1-Pd1-N1, 91.4(4).
Figure 3. The molecular structure of complex 2 with thermal ellipsoids
drawn at 50% probability. All hydrogen atoms omitted for clarity. Selected
bond lengths (Å): Pd1-N1, 2.020(2); Pd1-N2, 2.0294(19); Pd1-N3,
2.044(2); Pd1-N4, 2.0417(19); N1-C2, 1.316(3); N2-C4, 1.333(3);
N3-C19, 1.311(3); N4-C21, 1.353(3). Selected bond angles (deg):
N1-Pd1-N2, 86.77(8); N3-Pd1-N4, 86.35(8); N1-Pd1-N4, 93.74(8);
N2-Pd1-N3, 93.13(8).
Scheme 4. Syntheses of Trimetallic Complexes 4 and 5
Scheme 3. Synthesis of Complex 2
However, to the best of our knowledge, this is the first
example of partial hydrolysis of a nacnac ligand in a metal
coordination sphere.
Complex 1a reacts with metal acetates, M(OAc)2 (M )
Cu, Zn) to produce heterotrimetallic complexes of the general
formula [Pd(Ph2nacnac)]2-µ-[M(OAc)4] (M ) Cu, 4; M )
Zn, 5; Scheme 4). The reactions proceed smoothly at ambient
temperature in THF, although the starting copper and zinc
acetates are insoluble. Complexes 4 and 5 can be isolated in
80% and 70% yields, respectively.
the CH3 groups and H-C(ꢀ) on the ligand backbone,
respectively. These resonances have not been observed in
the NMR spectra of 1a, and thus the ligand scrambling
scenario can be ruled out. The X-ray structural analysis
confirmed the structure of 2 as a bis-nacnac complex (Figure
3 and Table 1). Structurally, the closest relative of 2 is the
previously reported complex [Pd(iPr2nacnac)2].34 However,
the other possible explanation for the solution behavior of
1a, a higher-nuclearity species in equilibrium with the 1a
dimer, cannot be presently excluded.
Unlike complexes 1a and 1b, [Pd(Ph2nacnac)2] hydrolyzes
easily in the presence of moisture. Thus, if the ether solution
of 2 is left in air, within hours pale orange crystals of
[Pd(Phnacac)2] (3), (Phnacac ) {CH3C(O)CHC(NPh)CH3}-)
start to form. Within a couple of days, the hydrolysis reaches
∼60% yield. The hydrolysis of 2 produces 3 and free aniline
simultaneously. The presence of aniline in the mother liquor
has been confirmed by 1H NMR spectroscopy. The structure
of 3 has been confirmed by both NMR spectroscopy (1H
and 13C) and single-crystal X-ray crystallography. Thus, the
NMR spectra of 3 show resonances for two inequivalent CH3
groups of the ligand backbone: 1.34 and 1.64 ppm in the 1H
and 23.76 and 24.19 ppm in the 13C NMR spectra. The
molecular structure of 3 is shown in Figure 4, and selected
crystallographic data are given in Table 1. The compound
crystallizes in the monoclinic space group C2/c with the
palladium atom located on the center of inversion. The
tendency of imine ligands to hydrolyze in the palladium
coordination sphere has been well documented. The reactions
took place with the aid of an acid44-46 or water.47-49
A single-crystal X-ray diffraction analysis confirmed the
structures for 4 and 5. Their molecular structures are shown
in Figures 5 and 6, respectively, while the crystallographic
data have been summarized in Table 1. Complex 4 has an
“S”-shaped structure with two [Pd(Ph2nacnac)]+ moieties
bridged with a [Cu(OAc)4]2- core. The Cu(II) center resides
on a crystallographically imposed center of inversion. The
Pd(II) and Cu(II) centers adopt the typical square-planar
coordination geometry, with typical bond lengths and angles.
In the case of complex 5, significant distortion from an ideal
tetrahedral geometry at the Zn(II) center occurs. The
O-Zn-O angles range from ∼89° (O2-Zn1-O4) to ∼132°
1
(O5-Zn1-O8). Judging from H and 13C NMR spectra,
complex 5 does not retain its solid-state structure in solution
since, along the resonances attributable to 5, signals from
other species have been observed, even in the case of
analytically pure samples (E.A.). The other species present
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Inorganic Chemistry, Vol. 47, No. 24, 2008 12013

Hadzovic and Song
Figure 5. Two views of the molecular structure of complex 4 with thermal ellipsoids drawn at 50% probability. All hydrogen atoms are omitted, and the
phenyl rings in the right view have been reduced to the ipso-carbons for clarity. Selected bond lengths (Å): Pd1-N1, 1.990(3); Pd1-N2, 1.989(3); Pd1-O1,
2.044(3); Pd1-O3, 2.053(3); N1-C2, 1.336(5); N2-C4, 1.326(5); Cu1-O4, 1.941(3); Cu1-O2, 1.966(3). Selected bond angles (deg): N1-Pd1-N2, 92.39(13);
N1-Pd1-O1, 91.09(12); N2-Pd1-O3, 91.52(12); O1-Pd1-O3, 85.00(11); O2-Cu1-O4, 90.43(12).
Scheme 5. Syntheses of [Pd(Ph2nacnac)(OH)]2 (6)
Figure 6. The molecular structure of complex 5 with thermal ellipsoids
also in close contact with two fluorine atoms from a
neighboring BArF4 (ArF ) 3,5-(CF3)2C6H3) counterion (dNa-F
) ∼2.5 Å) in the crystal lattice.38
drawn at 50% probability. All hydrogen atoms omitted for clarity. Selected
bond lengths (Å): N1-Pd1, 1.996(4); N2-Pd1, 1.997(5); O7-Pd1,
2.049(4); O3-Pd1, 2.060(4); N4-Pd2, 1.995(4); N3-Pd2, 1.998(4);
O1-Pd2, 2.041(4); O6-Pd2, 2.053(4); Zn1-O2, 1.980(4); Zn1-O4,
1.971(4); Zn1-O5, 1.941(4); Zn1-O8, 1.935(4). Selected bond angles
(deg): N1-Pd1-N2, 92.7(2); O7-Pd1-O3, 85.92(15); N3-Pd2-N4,
93.00(19); O1-Pd2-O6, 86.06(15); O2-Zn1-O4, 88.94(17); O2-Zn1-O5,
111.44(17);O2-Zn1-O8,104.53(16);O4-Zn1-O5,101.15(17);O4-Zn1-O8,
110.51(18), O5-Zn1-O8, 131.93(17).
When 1a is treated with 2 equiv (considering 1a as a
dimer) of KOH in THF, the acetate ligands are easily
replaced by two bridging hydroxo ligands to produce
[Pd(Ph2nacnac)(OH)]2 (6). Alternatively, complex 6 can be
prepared from [Pd(Ph2nacnac)Cl]2 and KOtBu in wet THF
in a comparable yield (80% starting from 1a vs 75% starting
from [Pd(Ph2nacnac)Cl]2) (Scheme 5).
in the solution have not been identified yet but are probably
lower nuclearity complexes.
The structure of complex 6 has been confirmed by single-
crystal X-ray diffraction analysis (Figure 7). It crystallizes
from concentrated benzene solutions as a solvate, 6 · 2C6H6,
in the monoclinic space group P21/c. There is a crystallo-
graphically imposed center of inversion at the geometric
center of each molecule of 6. The palladium(II) center adopts
a typical square-planar geometry with two N and two
bridging O donor atoms occupying four coordination sites.
Among a few crystallographically characterized [M(nacna-
c)(OH)]2 complexes, 6 is most similar to the paramagnetic
[Cu(Ar2nacnac)(OH)]2 (Ar ) 2,6-dimethylphenyl) with two
square-planar Cu(II) centers.58 In the Ni(II) analogue
[Ni(Ar2nacnacCF3)(OH)]2 (Ar ) 2,6-dimethylphenyl; nac-
nacCF3 ) nacnac derived from 1,1,1,5,5,5-hexafluopentane-
2,4-dione), the square-planar geometry at the Ni(II) centers
is tetrahedrally distorted.59
Complexes 4 and 5 are related to several previously
reported [Pd(L2)]-µ-[M(OAc)4] complexes (L2 ) a bidentate
or two monodentate ligands and M ) Pd).50-54 However,
complexes 4 and 5 are the first rationally synthesized
heterotrimetallic complexes in the family of [Pd(L2)]-µ-
[M(OAc)4] where M is a transition metal. Only recently, a
related complex with a main group metal (M ) Na) has been
reported as well. The Na+ center in this case, however, is
(51) Chitanda, J. M.; Prokopchuk, D. E.; Quail, J. W.; Foley, S. R.
Organometallics 2008, 27, 2337.
(52) Friedlein, F. K.; Hampel, F.; Gladysz, J. A. Organometallics 2005,
24, 4103.
(53) Friedlein, F. K.; Kromm, K.; Hampel, F.; Gladysz, J. A. Chem.sEur.
J. 2006, 12, 5267.
(54) Giri, R.; Liang, J.; Lei, J.; Li, J.; Wang, D.; Chen, X.; Naggar, I. C.;
Guo, C.; Foxman, B. M.; Yu, J. Angew. Chem., Int. Ed. 2005, 44,
7420.
The hydroxyl protons of 6 give rise to a resonance at
(55) Diez, L.; Espinet, P.; Miguel, J. A. J. Chem. Soc., Dalton Trans. 2001,
1189.
1
-5.24 ppm in the H NMR spectrum, significantly more
12014 Inorganic Chemistry, Vol. 47, No. 24, 2008

NoWel Palladium ꢀ-Diiminato-Acetate Complexes
produce the desired product. Rather 1a was recovered along
with unidentified species after 24∼42 h. Only small amount
of [Pd(Ph2nacnac)Cl]2 was isolated (∼10% yield after
recrystallization) by treating 1a with NaCl in a CH2Cl2/water
biphasic system in the presence of a phase-transfer catalyst
nBu4NCl.
Summary
We have prepared and fully characterized two novel Pd-
nacnac acetate complexes, 1a and 1b. These can be prepared
in good yields from [Pd(OAc)2] and the corresponding
nacnacH ligands. The reactions do not require an external
base, dry solvents, or low temperatures. The complexes are
air- and moisture-stable solids. Complex 1a is a dimer in
the solid state, while in solution a monomer-dimer equi-
librium is established. Complex 1b exists as a monomer in
the solid state and in solution, presumably because of the
steric effect of the 2-propyl groups. Complex 1a, similar to
the previously reported [Pd(Ph2nacnac)Cl]2 dimer, is a
suitable starting material for the preparation of Pd-nacnac
complexes. Thus, it reacts with M(OAc)2 to produce novel
heterotrimetallic species of a general formula [Pd(Ph2nacnac)]2-
µ-[M(OAc)4] (M ) Cu, Zn). The acetate ligands can be
replaced with bridging hydroxyl groups to afford 6. However,
the substitution of acetates with chloride ligands produced
dimeric [Pd(Ph2nacnac)Cl]2 in very low yield (∼10%).
Unlike 1, the bis-nacnac complex [Pd(Ph2nacnac)]2 (2) is
prone to hydrolysis, and the partially hydrolyzed species
[Pd(Phnacac)]2 (3) can be isolated in good yield from a wet
ether solution of 2.
Figure 7. The molecular structure of complex 6. All hydrogen atoms, except
the hydroxyl hydrogen atoms, are omitted for clarity. Selected bond lengths
(Å): Pd1-O1, 2.039(4); Pd1-N1, 1.984(4); N1-C2, 1.336(7); N2-C4,
1.337(7). Selected bond angles (deg): O1-Pd1-N2, 94.08(16); N1-Pd1-N2,
92.81(16).
upfield compared to the usual range of -0.25 to -3.5 ppm
documented for both neutral and cationic Pd2(µ-OH)2 spe-
cies.55-57 This upfield resonance can be explained by the
shielding effect of phenyl rings on the Ph2nacnac- backbone
in whose pockets the OH groups lie. Tian et al. have observed
similar shielding of acac methyl groups by aromatic rings
on the nacnac ligand in [Pd(Ar2nacnac)(acac)] (Ar ) 2,6-
iPr2C6H3) compared to [Pd(iPr2nacnac)(acac)].34
Unlike its chloro-bridged analogue [Pd(Ph2nacnac)Cl]2, the
hydroxo-bridged dimer is a rather unreactive species. While
[Pd(Ph2nacnac)Cl]2 reacts cleanly and rapidly with many
monodentate, charge-neutral ligands,32 complex 6 remains
intact under forcing conditions. Thus, for example, no
reaction has been observed between 6 and 2 equiv of
N-methyl-4,5-diphenylimidazole at ambient temperature after
7 days or at elevated temperatures (∼50 °C) in C6D6 after
several hours. The same low reactivity has been observed
toward aniline and p-tBu-aniline. Both aniline and p-tBu-
aniline readily react with [Pd(Ph2nacnac)Cl]2 and are known
to react easily with µ-hydroxo Pd dimers to produce hydroxo-
amido or bis-amido bridged Pd dimers.55,60,61
Experimental Section
General Methods and Materials. [Pd(OAc)2], Cu(OAc)2 ·H2O
(98%), Zn(OAc)2 ·2H2O, LiOtBu, and KOtBu were obtained from
commercial sources and used without further purification. The
ligands a66 and b35 and [PdCl2(PhCN)2]67 were prepared according
to literature methods. The NMR spectra were recorded on a Varian
1
400 spectrometer working at 400 MHz for H and 100 MHz for
13C. Both 1H and 13C spectra were recorded relative to the solvent
residual signals but are reported relative to Me4Si. Elemental
analyses were performed at our department using a PE 2400 C/H/
N/S analyzer.
X-Ray Crystallographic Analysis. Single crystals suitable for
X-ray crystallographic analysis were obtained as described for each
complex below. The single-crystal X-ray diffraction data for
complexes 1a, 4, 5, and 6 were collected at 150 K on a Bruker-
Nonius Kappa-CCD diffractometer68 with Mo KR radiation (λ )
0.71073 Å). The data were processed with the DENZO-SMN
package.69 The diffraction data for complexes 1b, 2, and 3 were
collected at the same temperature (except for 1b, 296 K) on a Bruker
Kappa-APEX II diffractometer. These data were processed using
the Bruker Apex 2 software package.70 All structures were solved
Our attempts to prepare [Pd(Ph2nacnac)Cl]2 from 1a via
OAc-/Cl- exchange were unsatisfactory. Thus, the usual
approaches for this transformation (e.g., treating 1a with MCl
(M ) Li, Na) in water, acetone, or mixed solvent systems
(water/acetone, water/MeOH, or water/EtOH)62-65 did not
(56) Getty, A. D.; Goldberg, K. I. Organometallics 2001, 20, 2545.
(57) Nama, D.; Schott, D.; Pregosin, P. S.; Veiros, L. F.; Calhorda, M. J.
Organometallics 2006, 25, 4596.
(58) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998.
(59) Li, Y.; Jiang, L.; Wang, L.; Gao, H.; Zhu, F.; Wu, Q. Appl. Organomet.
Chem. 2006, 20, 181.
(60) Driver, M. S.; Hartwig, J. F. Organometallics 1997, 16, 5706.
(61) Kannan, S.; James, A. J.; Sharp, P. R. Inorg. Chim. Acta 2003, 345,
8.
(62) Vicente, J.; Abad, J.-A.; Clemente, R.; Lo´pez-Serrano, J.; Ramirez
de Arellano, M. C.; Jones, P. G.; Bautista, D. Organometallics 2003,
22, 4248.
(63) Vila, J. M.; Alberdi, G.; Pereira, M. T.; Marin˜o, M.; Ferna´ndez, A.;
Lo´pez-Torres, M.; Ares, R. Polyhedron 2003, 22, 241.
(64) Vila, J.; Gayoso, M.; Pereira, M. T.; Lo´pez Torres, M.; Ferna´ndez,
J. J.; Ferna´ndez, A.; Ortigueira, J. M. J. Organomet. Chem. 1996,
506, 165.
(65) Vila, J. M.; Gayoso, M.; Pereira, M. T.; Lo´pez, M.; Alonso, G.;
Ferna´ndez, J. J. J. Organomet. Chem. 1993, 445, 287.
(66) Tang, L. M.; Duan, Y. Q.; Li, X. F.; Li, Y. S. J. Organomet. Chem.
2006, 691, 2023.
(67) Anderson, G. K.; Lin, M.; Sen, A.; Gretz, E. Inorg. Synth. 1990, 28,
61.
(68) B. V. Nonius, Delft, The Netherlands 1997-2002.
(69) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(70) Bruker AXS Inc., Madison, WI, 2008, 2008.3.
(71) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
Inorganic Chemistry, Vol. 47, No. 24, 2008 12015

Hadzovic and Song
by the direct methods and refined with SHELXTL V6.10.71 All
non-hydrogen atoms were refined anisotropically, except in the case
of 3 (see below). The positions of hydrogen atoms were calculated
with the riding model, and their contributions were included in the
structure factor calculations. The crystallographic data are sum-
marized in Table 1.
was extracted with dry pentane. The resulting solution was filtered
over a pad of Celite, and solvents from the filtrate were then
removed to yield 0.250 g (83%) of 2. The crystals suitable for X-ray
crystallographic analysis were obtained from slow evaporation of
the pentane solution. 1H NMR (CD2Cl2): δ 1.66 (s, 12H, CH3
nacnac backbone), 5.04 (s, 2H, CH nacnac backbone), 6.91-7.10
(m, 20H, Ph). 13C NMR (CD2Cl2): δ 22.37 (CH3 nacnac backbone),
108.8 (CH nacnac backbone), 122.9, 126.5, 127.7 (phenyl), 149.9,
160.5 (ipso-CPh-N and CdN nacnac). Anal. calcd for C34H34N4Pd:
C, 67.49%; H, 5.66; N, 9.26%. Found: C, 67.27%; H, 5.49%; N,
9.15%.
In the crystal structure of 1b, the disordering observed for the
isopropyl groups was successfully modeled. During the modeling
process, a few C-C bond lengths were fixed with DFIX restraints.
In the crystal structure of 3, the whole molecule except for the Pd
center is disordered over two positions, which has been modeled
successfully. All hetero atoms in 3 were refined anisotropically,
while carbon and hydrogen atoms were refined isotropically.
Synthesis of [Pd(Ph2nacnac)(OAc)] (1a). A solution of
Ph2nacnacH (0.500 g, 2 mmol) in acetone (50 mL) was slowly
added to a solution of [Pd(OAc)2] (0.450 g, 2 mmol) in acetone
(100 mL) with vigorous stirring. During the addition, the reaction
mixture turns deep red. Occasionally, and especially if less solvent
is used, a small amount of insoluble light pink material precipitates
out, which can be removed by gravity filtration. After 1.5 h, the
solvents were removed to dryness in vacuo, leaving a deep red
solid product. Yield: 0.92 g (88.3%). Single crystals suitable for
the X-ray analysis were obtained from the slow evaporation of a
Hydrolysis of 2: Formation of Phnacac Complex 3. Ap-
proximately 80 mg of 2 was dissolved in 3 mL of regular (nondried)
ether to produce a deep red solution. Pale orange crystals of 3 started
to form within hours from the solution kept at ambient temperature
in a sealed vial. After the solution was left overnight, the crystals
were collected and washed with a small amount of cold ether to
give crystalline 3 in ∼65% yield (0.045 g). The collected crystals
were suitable for the X-ray crystallographic analysis. 1H NMR
(CD2Cl2): δ 1.34 (s, 6H, CH3), 1.64 (s, 6H, CH3 nacac backbone),
3
4.84 (s, 2H, CH nacac backbone), 6.93 (dd, 4H, Ph, JHH ) 8.4
Hz, 4JHH ) 0.8 Hz), 7.14 (m, 2H, Ph), 7.26 (m, 4H, Ph). 13C NMR
(CD2Cl2): δ 23.76, 24.19 (CH3 nacac backbone), 98.12 (CH nacac
backbone), 125.1, 126.6, 128.3 (phenyl), 149.2, 163.4, 176.6 (ipso-
CPh-N, CdN and CdO nacac). Anal. calcd for C22H24N2O2Pd: C,
58.09%; H, 5.32%, N, 6.16%. Found: C, 58.25%; H, 5.21%; N,
6.08%. The solvents were removed from the mother liquor, and
the NMR spectrum of a viscous residue revealed the presence of
aniline.
1
CHCl3/hexanes (∼1:2 vol) solution over several days. H NMR
(CD2Cl2, monomer): δ 1.53 (s, 3H, CH3 acetate), 1.74 (s, 6H, CH3
1
nacnac backbone), 4.85 (s, 1H, CH nacnac backbone). H NMR
(CD2Cl2, dimer): δ 0.93 (s, 3H, CH3 acetate), 1.88 (s, 6H, CH3
nacnac backbone), 4.88 (s, 1H, CH nacnac backbone). The aromatic
1
region of the H NMR spectrum shows multiplets in the range of
7.11-7.31 ppm, for which the contributions from the monomer
and dimer cannot be assigned clearly. 13C NMR (CD2Cl2, mono-
mer): δ 22.96 (CH3 acetate), 23.53 (CH3 nacnac backbone), 98.13
(CH nacnac backbone), 125.0, 128.20, 129.5 (phenyl), 150.2, 157.8
(ipso-CPh-N and CdN), 172.1 (CO, acetate). 13C NMR (CD2Cl2,
dimer): 20.58 (CH3 acetate), 23.02 (CH3 nacnac backbone), 100.5
(CH nacnac backbone), 125.5, 128.5, 128.7 (phenyl), 150.6, 157.8
(ipso-CPh-N and CdN), 182.4 (CO acetate). Anal. calcd for
C19H20N2O2Pd: C, 55.02%; H, 4.86%; N, 6.75%. Found: C, 55.20%;
H, 4.85%; N, 6.60%.
Synthesis of [Pd(Ph2nacnac)]2-µ-[Cu(OAc)4] (4). To a solution
of 1a (0.045 g, 0.1 mmol) in 4 mL of THF was added solid
Cu(OAc)2 · H2O (0.01 g, 0.05 mmol) with stirring. Solid
Cu(OAc)2 ·H2O gradually dissolved, and the reaction mixture turned
to brown red. After 4 h, the solution was filtered through a small
pad of Celite, and solvents of the filtrate were removed in vacuo.
The brown residue was redissolved in DCM (∼2 mL), and this
solution was top-layered with hexanes (∼2 mL). Brown crystals
of 4·2CH2Cl2 formed after several days. The crystals were collected
and air-dried to give 0.047 g (79.5%) of 4·2CH2Cl2. The crystals
were suitable for the X-ray crystallographic analysis. Anal. calcd
for C44H50Cl4N4O8CuPd2: C, 44.74%; H, 4.27%; N, 4.74%. Found:
C, 44.60%; H, 4.23%; N, 4.99%.
Synthesis of [Pd(DIPPh2nacnac)(OAc)] (DIPPh
)
2,6-(iPr)2C6H3) (1b). This complex was prepared following the
same procedure as described above for 1a. The reaction between
DIPPh2nacnacH (0.420 g, 1 mmol) and [Pd(OAc)2] (0.225 g, 1
mmol) gave 0.470 g (81%) of red product. Single crystals suitable
for X-ray analysis were obtained by the slow evaporation of
Synthesis of [Pd(Ph2nacnac)]2-µ-[Zn(OAc)4] (5). This complex
was prepared as described for 4 but using Zn(OAc)2 ·2H2O (0.011
g, 0.05 mmol). The light red complex crystallized from a THF/
1
saturated Et2O solution. H NMR (CD2Cl2): δ 1.21 (d, 12H, CH3
3
iPr, JHH ) 6.8 Hz), 1.45 (s, 3H, CH3 acetate), 1.59 (s, 6H, CH3
nacnac backbone), 1.73 (d, 12 H, CH3 iPr, JHH ) 6.8 Hz), 3.73
3
1
hexanes mixture to give crystalline 5. Yield: 0.035 g (70%). H
i
3
(sept, 1H, CH Pr, JHH ) 6.8 Hz), 4.84 (s, 1H, CH nacnac
NMR (CD2Cl2): δ 1.28 (s, 12H, CH3 acetate), 1.70 (s, 12H, CH3
nacnac backbone), 4.99 (s, 2H, CH nacnac backbone), 7.10-7.30
(m, 20H, phenyl). 13C NMR (CD2Cl2): δ 22.51 (CH3 acetate), 24.42
(CH3 nacnac backbone), 96.61 (CH, nacnac backbone), 125.7,
128.6, 129.1 (phenyl), 151.1, 156.9 (ipso-CPh-N and CdN), 181.2
(COO, acetate). Anal. calcd for C42H46N4O8Pd2Zn: C, 49.81%; H,
4.54%; N, 5.56%. Found: C, 49.94%; H, 4.93%; N, 5.43%.
Synthesis of [Pd(Ph2nacnac)(OH)]2 (6). From 1a. Finely
ground solid KOH (0.056 g, 1 mmol) was added in one portion to
a solution of 1a (0.410 g, 0.5 mmol calculated as a dimer) in 20
mL of THF with stirring. After 2.5 h of stirring, the reaction mixture
was filtered through a small pad of Celite, and solvents were then
removed in vacuo from the filtrate. The red residue was extracted
with benzene, and the resulting solution was filtered through Celite
3
3
backbone), 7.09 (d, 4H, Ph, JHH ) 7.6 Hz), 7.22 (t, 2H, Ph, JHH
) 7.6 Hz). 13C NMR (CD2Cl2): δ 22.44 (CH3 nacnac backbone),
i
22.94 (CH3 acetate), 23.77, 24.40 (CH3 Pr), 28.84 (CH, iPr), 95.78
(CH nacnac backbone), 123.8, 127.3, 144.8 (phenyl), 142.6, 155.9
(ipso-CPh-N and CdN), 189.4 (CdO acetate). Anal. calcd for
C31H44N2O2Pd: C, 63.85%; H, 7.61%; N, 4.80%. Found: C, 63.92%;
H, 7.45%; N, 4.72%.
Synthesis of [Pd(Ph2nacnac)2] (2).
A
solution of
[PdCl2(PhCN)2] (0.180 g, 0.5 mmol) in dry THF (4 mL) was added
dropwise to a freshly prepared mixture of Ph2nacnacH (0.250 g, 1
mmol) and LiOtBu (0.085 g, 1 mmol) in dry THF (12 mL). During
the addition, the orange-red reaction mixture obtained a deep red
color. After 2.5 h, the reaction mixture was filtered through a pad
of Celite, and the filtrate was dried under vacuum. The red residue
12016 Inorganic Chemistry, Vol. 47, No. 24, 2008

NoWel Palladium ꢀ-Diiminato-Acetate Complexes
again. The red crystals of 6·2C6H6 (suitable for X-ray crystal-
lographic analysis) were deposited overnight. Yield: 0.362 g (80%).
From [Pd(Ph2nacnac)Cl]2. KOtBu (0.112 g, 1 mmol) was added
to a solution of [Pd(Ph2nacnac)Cl]2 (0.390 g, 0.5 mmol) in THF (4
mL) followed by a drop of distilled water with stirring. The color
of the reaction changed gradually from deep green to red. After
4 h, the reaction was filtered through Celite, and the solvent
evaporated to dryness. The red residue was extracted with benzene
and filtered through Celite. Slow evaporation of benzene from the
filtrate produced red crystals of 6·2C6H6. Yield: 0.340 g (75%).
1H NMR (CD2Cl2): δ -5.24 (s, 2H, OH), 1.37 (s, 12H, CH3),
4.53 (s, 2H, CH), 6.58 (m, 8H, Ph), 6.88 (m, 4H, Ph), 6.94 (m,
8H, Ph), 7.23 (s, 6H, C6H6). 13C NMR (CD2Cl2): δ 23.38 (CH3),
97.39 (CH), 126.0, 127.1, 129.2 (Ph), 149.9, 156.4 (ipso-CPh-N
and CdN). Anal. calcd for C34H36N4O2Pd2: C, 54.78%; H, 4.83%;
N, 7.52%. Found: C, 54.56%; H, 4.95%; N, 7.41%.
Acknowledgment. This research was supported by grants
to D.S. from the Natural Science and Engineering Research
Council (NSERC) of Canada, the Canadian Foundation for
Innovation, the Ontario Research Fund (ORF), and the
University of Toronto (Connaught Foundation).
Supporting Information Available: 1H NMR spectra of mono-
mer/dimer equilibrium for complex 1a at 25 °C and crystallographic
data for 1-6 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC801557C
Inorganic Chemistry, Vol. 47, No. 24, 2008 12017