Palladium β-diiminate chemistry: Reactivity towards monodentate ligands and arylboronic acids

Article abstract of DOI:10.1016/j.ica.2011.09.044

Here we report the synthesis and characterization of a series of palladium β-diiminate complexes. Chloro-bridged dimers [Pd(Ar₂nacnac) (μ-Cl)]₂ (where [Ar₂nacnac]^- is [(ArNC(CH₃))₂CH]^-

Full text of DOI:10.1016/j.ica.2011.09.044

Inorganica Chimica Acta 380 (2012) 308–321
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Palladium b-diiminate chemistry: Reactivity towards monodentate ligands
and arylboronic acids
⇑
Vincent T. Annibale, Runyu Tan, John Janetzko, Liisa M. Lund, Datong Song
Davenport Chemical Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 2 October 2011
Here we report the synthesis and characterization of a series of palladium b-diiminate complexes.
Chloro-bridged dimers [Pd(Ar2nacnac)(l
-Cl)]2 (where [Ar2nacnac]ꢀ is [(ArNC(CH3))2CH]ꢀ with Ar being
aryl substituents) can serve as versatile starting materials, and can be cleaved by neutral monodentate
ligands (L) to form mononuclear complexes [Pd(Ar2nacnac)(L)Cl], or engage in unusual transmetallation
reactions with boronic acids to form tetrapallada-macrocycles. The reactivity of the tetrapallada-macro-
cyclic complexes towards L-type ligands, as well as the reactivity of Pd(Ar2nacnac)(L)Cl] complexes
towards boronic acids was also also explored.
Young Investigator Award Special Issue
Keywords:
b-Diiminate ligands
Palladium
Ó 2011 Elsevier B.V. All rights reserved.
Reactivity
Monodentate ligands
Boronic acids
Nacnac
1. Introduction
Our initial discovery in 2008 of a Pd nacnac chloro-bridged di-
mer, complex 1a [6], has led to an expansion of the chemistry by
Aryl-substituted b-diiminate ligands (commonly referred to as
‘‘nacnac’’ ligands since they are bidentate, monoanionic N-donor li-
gands derived from acetylacetonate (acac)) are used extensively in
inorganic and organometallic chemistry [1]. Nacnac complexes
have been reported for elements across the periodic table, and
interest continues since the nacnac ligands have been found to
stabilize metals in unusual oxidation states and coordination envi-
ronments, and find use in active catalytic systems [1]. Varying the
‘‘Ar’’ groups of the [Ar2nacnac]ꢀ ligands, see Fig. 1B, allows for the
sterics and electronics of the ligand environment to be easily
tuned.
allowing easy access to a host of mono-, and multi-nuclear Pd nac-
nac species [6,7]. Cleavage of 1a with suitable monodentate ligands
(L) allows easy access to mononuclear species of the general form
[Pd(Ph2nacnac)(Cl)(L)] [6]. Reactions of 1a with 4-tertbutylaniline
can generate bimetallic and trimetallic complexes with chloro-
amido double bridges over time [7].
In our recent investigations into palladium nacnac chemistry,
we have explored the reactivity of [Pd(Ph2nacnac)(Cl)(L)] towards
various transmetallation reagents, aiming to generate olefin poly-
merization pre-catalysts, [Pd(Ph2nacnac)(R)(L)], where R is an al-
kyl or aryl group. Alternatively we have proposed that that if a
suitable transmetallation reagent could replace the chloride li-
gands from 1a itself, the resulting coordinatively unsaturated
organopalladium species ‘‘[Pd(Ph2nacnac)(R)]’’ or its oligomers
might be used directly as activator-free olefin polymerization cat-
alysts. Our interest in the reactivity of 1a towards transmetalla-
tion reagents led to the discovery of an unusual self-assembly
when boronic acids were used. We reported the formation of a
tetrapallada-macrocycle induced by an unusual transmetallation,
in which an anionic bidentate chelating nacnac ligand is replaced
by a phenyl ligand from phenylboronic acid, leaving the chloride
ligands intact [8]. The reactivity of these tetrapallada-macrocy-
cles towards L-type ligands is presented in this paper. In addition,
we detail the syntheses and characterization of a series of new Pd
nacnac complexes where we expand upon the reactivity of Pd
Palladium nacnac chemistry prior to our initial work in the area
waslimitedtoa few wellcharacterized examples. Theseearlierexam-
ples include the fully characterized complexes [Pd(iPr2nacnac)2] [2],
[Pd(
(Ar2nacnac)] where (Ar) = 2,6-diisopropylphenyl) that isomerizes
into [Pd(acac)(
2-C,N-Ar2nacnac)] [2], [Pd(Ar2nacnac)ClMe] [3], and
the dinuclear complex [(CH3CN)3Pd-{ -CH(C(Me)NAr)2}Pd(CH3CN)2]
g
3-C3H5)(iPr2nacnac)] [2], the unstable complex [Pd(acac)
j
l
(BF4)3 [4]. Monomethylpalladium nacnac complexes where benzene
C–H activation and migratory insertion of olefins are competitive
have also been studied [5].
⇑
Corresponding author. Tel.: +1 416 978 7014; fax: +1 416 978 7013.
E-mail address: dsong@chem.utoronto.ca (D. Song).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.044

V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
2.3. Preparation of complexes
309
2.3.1. Synthesis of [Pd(Dipph2nacnac)(l-Cl)]2 (1b)
H[Dipph2nacnac] (48 mg, 0.11 mmol) and Pd(PhCN)2Cl2 (38 mg,
0.099 mmol) were dissolved in 5 mL of methanol, KOtBu (11 mg,
0.099 mmol) was then added. The mixture was stirred overnight
and the precipitate changed colour from red to brown and finally
to green. The precipitated product was filtered off and washed with
a small amount of methanol. [Pd(Dipph2nacnac)Cl]2 (1b) was ob-
tained in 58% yield (38 mg). The crystals suitable for X-ray crystal-
lographic analysis were obtained by layering a CH2Cl2 solution
(approximately 8 mg of 1b in 0.1 mL of CH2Cl2) with methanol
(ꢂ3.5 mL) at ꢀ30 °C.
Fig. 1. Generic structure of [Ar2nacnac]ꢀ ligands where the b-C is noted. We refer to
the central carbon of the [Ar2nacnac]ꢀ ligand as the b-C since it is b to the hetero-
atom in the free ligand, we extend this convention to metal complexes as well in
order to avoid confusion.
1H NMR (400 MHz, CD2Cl2): d 7.01 (m, 4H), 6.83 (m, 8H), 4.68 (s,
2H, nacnac b-CH), 3.09 (septet, 8H, –CHMe2), 1.44 (s, 12H, nacnac-
CH3), 1.29 (d, 24H, J = 6.9 Hz, –CH(CH3)2), 1.05 (d, 24H, J = 6.9 Hz,
–CH(CH3)2). 13C NMR (75 MHz, CD2Cl2): d 155.93, 145.93, 142.59,
126.69, 123.92, 94.17, 28.48, 24.46, 24.06, 23.53. Anal. Calc. for
(C58H82Cl2N4Pd2)ꢁ(CH3OH): C, 61.56; H, 7.53; N, 4.87. Found: C,
61.46; H, 7.06; N, 4.88%.
nacnac complexes towards monodentate ligands and boronic
acids.
2. Experimental
2.3.2. Synthesis of [Pd(ArF2nacnac)( -Cl)]2 (1c) and [Pd(ArF2nacnac)2]
l
(2)
2.1. Materials and methods
A milky white suspension of H[ArF2nacnac] (1.412 g, 2.7 mmol)
and KOtBu (281 mg, 2.58 mmol) in methanol (9 mL) was added
dropwise to a stirred solution of Na2PdCl4 (788 mg, 2.67 mmol)
in methanol (4 mL). The mixture was stirred overnight resulting
in a dark brown precipitate which was collected by filtration and
washed with water, followed by methanol. The crude brown pre-
cipitate is a 2:5 mixture of [Pd(ArF2nacnac)2] (2) to [Pd(ArF2nac-
All experimental procedures were performed in air unless other-
wise stated. Air or moisture sensitive reactions were performed
under dry nitrogen or argon with conventional glovebox or Schlenk
techniques. [Pd(Ph2nacnac)(l-Cl)]2 (1a) [6], H[Ph2nacnac] [9], H[Dip-
ph2nacnac] (where Dipph = diisopropylphenyl) [4,10], H[ArF nac-
2
nac] (where ArF = 3,5-bis(trifluoromethyl)phenyl) [11], Pd(PhCN)2
Cl2 [12], trans-[Pd(THT)2(C6F5)2] (where THT = tetrahydrothiophene)
[13], [(Ph)Pd(l-Cl)2Pd(Ph2nacnac)]2 (7a) [8], pentafluorophenylbo-
nac)(l
-Cl)]2 (1c) by 1H, and 19F NMR spectroscopy. The crude
brown solid (ꢂ1.3 g) was dissolved in a minimal amount of diethyl
ether (ꢂ4 mL) in a vial, and hexanes (ꢂ15 mL) was layered on top,
after ꢂ1 h the majority of the red crystals of 2 began to form at the
bottom of vial resulting in a greenish brown supernatant which
was transferred off and chilled to ꢀ30 °C. After 20 min at ꢀ30 °C
a second crop of red crystals had formed leaving an emerald green
supernatant containing primarily 1c. The red crystals from the two
crops were redissolved in a minimal amount of CHCl3 (ꢂ1 mL) and
layered with hexanes (ꢂ10 mL) and chilled to ꢀ30 °C, this resulted
in crystals suitable for X-ray crystallographic analysis, and elemen-
tal analysis after drying under vacuum. The yield of recrystallized 2
was 6% (172 mg). The solvent was removed from the emerald
green supernatant, the residue was re-crystallized from a minimal
amount of CHCl3 (ꢂ2 mL), layered with hexanes (ꢂ15 mL), and
chilled to ꢀ30 °C. Green crystals suitable for X-ray crystallographic
analysis were obtained from slow evapouration of a CH2Cl2 solu-
tion of 1c. The yield of recrystallized 1c was 25% (867 mg).
ronic acid [14], [Pd(Ph2nacnac)(Py)Cl] (4a) [8] were prepared accord-
ing to literature procedures. Other reagents were obtained from
commercial sources and used as received. THF and benzene-d6 was
dried over Na/benzophenone and vacuum transferred before use.
Dry and degassed pyridine and CH2Cl2 for experiments where their
use is specified was dried over CaH2 and vacuum transferred before
use. Toluene was dried after passing through a Pure Solv Innovative
Technology Grubbs’-type solvent purification system, and degassed
through three consecutive freeze–pump–thaw cycles. 1H, 11B, 13C,
19F NMR spectra were recorded on a Varian 400 MHz, or 300 MHz
NMR instrument. All chemical shifts are reported in parts per million
(d) and the residual protio-solvent peaks were used as a reference (1H
NMR, d, ppm: dichloromethane-d2, 5.32; chloroform-d, 7.26; ben-
zene-d6, 7.16, acetone-d6, 2.05. 13C NMR, d, ppm: dichloromethane-
d2, 53.8; chloroform-d, 77.16; benzene-d6, 175.82; acetone-d6,
29.84, 206.26). 11B NMR spectra were referenced using an external
standard of KBF4 in D2O (0.29 M) flame sealed in a glass capillary
Characterization of Pd(ArF2nacnac)( -Cl)]2 (1c): 1H NMR
l
(
11B NMR, d, ppm: 0.00). Elemental analysis was performed by ANA-
(400 MHz, acetone-d6): d 7.65 (s, 4H), 7.56 (d, 8H, J = 1.4 Hz),
LEST at the University of Toronto.
5.00 (s, 2H, nacnac b-CH), 1.62 (s, 12H, nacnac-CH3). 19F NMR
2.2. X-ray crystallography
(376 MHz, acetone-d6):
d
ꢀ64.26 (s, 24F, –CF3). 13C NMR
(100 MHz, acetone-d6): d 158.38, 152.19, 131.91, 125.36, 122.66,
119.96, 99.82, 23.8. Anal. Calc. for C42H26N2F24Cl2Pd2: C, 38.03; H,
1.98; N, 4.22. Found: C, 38.18; H, 1.95; N, 4.30%.
The X-ray diffraction data for structures 1b, 1cꢁ(CH2Cl2), 2, 4b,
5ꢁ(CH2Cl2), 6 were collected [15] on a Nonius Kappa-CCD diffrac-
tometer and processed with the DENZOSMN package [16]. The
X-ray diffraction data for structures 3, 4cꢁ0.5(CH2Cl2), 7b, trans-
[Pd(Py)2(Ph)Cl], 8, 9 were collected on a Bruker Kappa Apex II
diffractometer, and processed with the Bruker Apex 2 software
package [17]. All data were collected with graphite monochromat-
Characterization of [Pd(ArF2nacnac)2] (2): 1H NMR (300 MHz,
acetone-d6): d 7.74 (d, 8H, J = 1.3 Hz), 7.64 (s, 4H), 5.61 (s, 2H, nac-
nac b-CH), 1.86 (s, 12H, nacnac-CH3). 19F NMR (376 MHz, acetone-
d6): d ꢀ64.55 (s, 24F, –CF3). 13C NMR (75 MHz, acetone-d6): d
163.60, 150.93, 132.04, 131.60, 125.82, 112.89, 22.58. Anal. Calc.
for C42H26N2F24Pd: C, 43.90; H, 2.28; N, 4.88. Found: C, 43.63; H,
2.29; N, 4.94%.
ed Mo Ka radiation (k = 0.71073 Å), at 150 K controlled by an Ox-
ford Cryostream 700 series low temperature system. All
structures were solved by the direct methods or Patterson method
and refined using SHELXTL V6.10 [18]. Disordered CF3 groups and
CH2Cl2 solvent molecules were modelled successfully. The crystal-
lographic data are summarized in Table 1.
2.3.3. Synthesis of [Pd((PhN=C(CH3))2CH2)(C6F5)2] (3)
In a Schlenk flask under argon trans-Pd(THT)2(C6F5)2 (303 mg,
0.49 mmol) and [Ph2nacnac]H (127 mg, 0.50 mmol) were dissolved

310
V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
Table 1
Crystallographic data.
1b
1c ꢁ (CH2Cl2)
2
3
Formula
Formula weight
T (K)
Space group
a (Å)
C
58H82Cl2N4Pd2
C44H26Cl6F24N4 Pd2
1492.19
150(2)
C42H26F24N4Pd
1149.07
150(2)
C29H18F10N2Pd
690.85
150(2)
1118.98
150(2)
P21/n
9.1465(18)
14.233(3)
21.023(4)
90
P21/n
P21/n
P21/n
11.428(2)
15.760(3)
15.404(3)
90
14.571(3)
8.6434(17)
17.805(4)
90
9.2861(2)
20.7555(4)
14.2909(3)
90
b (Å)
c (Å)
a
(°)
b (°)
92.36(3)
90
2734.5(9)
2
1.359
0.795
103.99(3)
90
2692.3(9)
2
1.841
1.086
108.52(3)
90
2126.3(7)
2
1.795
0.581
96.7330(10)
90°
2735.40(10)
c
(°)
V (Å3)
Z
4
Dcalc (g cmꢀ3
)
1.678
0.769
l
(mmꢀ1
)
Number of reflections collected
Number of independent reflections
Goodness-of-fit (GOF) on F2
19863
6203
1.005
10366
4454
1.276
13916
3733
1.013
20988
4791
1.034
R [I > 2
r
(I)]
R1 = 0.0395
wR2 = 0.0987
R1 = 0.0628
wR2 = 0.1143
R1 = 0.0556
wR2 = 0.1614
R1 = 0.0679
wR2 = 0.1705
R1 = 0.0607
wR2 = 0.1370
R1 = 0.1162
wR2 = 0.1656
R1 = 0.0213
wR2 = 0.0526
R1 = 0.0244
wR2 = 0.0545
R (all data)
4b
2(4c) ꢁ (CH2Cl2)
5 ꢁ (CH2Cl2)
6
Formula
Formula weight
T (K)
C
34H46ClN3Pd
C53H36Cl4F24N6Pd2
1567.48
150(2)
C34H50Cl3N4O2Pd
759.53
C49H45BF4N6Pd
911.12
150(2)
638.59
150(2)
P1
150(2)
ꢀ
ꢀ
Space group
P21/n
P21/c
P1
a (Å)
b (Å)
c (Å)
8.7150(17)
12.045(2)
16.923(3)
75.78(3)
83.29(3)
68.89(3)
1605.7(6)
2
14.9527(4)
16.7010(4)
24.4803(6)
90
101.9270(10)
90
5981.4(3)
4
1.741
12.746(3)
12.972(3)
13.036(3)
85.68(3)
87.44(3)
61.35(3)
1886.1(6)
2
9.6654(19
17.208(3)
26.098(5)
90
96.32(3)
90
4314.3(15)
4
1.403
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmꢀ3
)
1.321
0.687
1.336
0.738
l
(mmꢀ1
)
0.897
0.490
Number of reflections collected
Number of independent reflections
Goodness-of-fit (GOF) on F2
12163
5360
1.793
46968
10534
1.015
20933
8527
1.043
38581
9824
1.030
R [I > 2
r
(I)]
R1 = 0.1262
wR2 = 0.3007
R1 = 0.1449
wR2 = 0.3093
R1 = 0.0444
wR2 = 0.1035
R1 = 0.0608
wR2 = 0.1145
R1 = 0.0372
wR2 = 0.0862
R1 = 0.0491
wR2 = 0.0931
R1 = 0.0526
wR2 = 0.1228
R1 = 0.1089
wR2 = 0.1516
R (all data)
7b
Trans-[Pd(Py)2(Ph)Cl]
8
9
Formula
Formula weight
T (K)
C
46H42Cl4F2N4Pd4
C16H15ClN2Pd
377.15
150(2)
C23H18ClF5N2Pd
559.24
150(2)
C114H102Cl8F10N12Pd8
2964.88
150(2)
1256.24
150(2)
Space group
a (Å)
b (Å)
C2/c
C2/c
P212121
12.1558(5)
12.9993(5)
13.5993(6)
90
P21/n
21.3987(4)
14.0182(3)
18.7361(6)
90
22.9651(8)
9.3409(3)
16.0016(6)
90
12.7442(4)
34.4264(10)
13.9746(4)
90
c (Å)
a
(°)
b (°)
124.7160(10)
90
4619.8(2)
4
1.806
1.809
118.9200(10)
90
3004.52(18)
8
1.668
1.403
90
90
115.3620(10)
90
5540.2(3)
2
1.777
1.532
c
(°)
V (Å3)
2148.92(15)
4
1.729
Z
Dcalc (g cmꢀ3
)
l
(mmꢀ1
)
1.044
Number of reflections collected
Number of in dependent reflections
Goodness-on fit (GOF) on F2
17232
4067
1.028
13346
3446
0.958
16125
3784
1.019
36981
9756
1.003
R [I > 2
r
(I)]
R1 = 0.0250
wR2 = 0.0548
R1 = 0.0375
wR2 = 0.0597
R1 = 0.0445
wR2 = 0.0734
R1 = 0.0917
wR2 = 0.0852
R1 = 0.0319
wR2 = 0.0561
R1 = 0.0417
wR2 = 0.0589
R1 = 0.0420
wR2 = 0.0932
R1 = 0.0666
wR2 = 0.1036
R (all data)
in 20 mL of dry and degassed toluene and heated to reflux over-
night. After cooling the workup was performed in air, the reaction
mixture was filtered through Celite and the filtrate was concen-
trated under vacuum. Hexanes (ꢂ100 mL) was added to the filtrate
precipitate was collected by filtration and washed with hexanes,
and dried under vacuum. The yield of 3 was 50% (175 mg). Crystals
suitable for X-ray crystallographic analysis were grown by layering
a concentrated CH2Cl2 solution of 3 (approximately 8 mg of 3 in
0.1 mL of CH2Cl2) with methanol (ꢂ3.5 mL) at ꢀ30 °C. 1H NMR
to cause
a white microcrystalline precipitate to form, the

V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
311
(400 MHz, acetone-d6): d 7.26–7.23 (m, 4H), 7.16–7.12 (m, 2H),
2.3.7. Synthesis of [Pd(Ph2nacnac)(L2)2]BF4 (6)
[Pd(Ph2nacnac)( -Cl)]2 (1a) (29.9 mg, 0.0382 mmol) and N-
6.76–6.74 (m, 4H), 4.46 (s, 2H, b-CH2), 2.30 (s, 6H, b-diimine
CH3). 19F NMR (376 MHz, acetone-d6): ꢀ116.54 (dd, 4F, J = 5.5 Hz,
J = 27.4 Hz), ꢀ165.11 (t, 2F, J = 19.6 Hz), ꢀ168.8 to ꢀ168.95 (m,
4F). 13C NMR (100 MHz, acetone-d6): d 177.54, 149.08, 129.43,
126.80, 122.21, 51.71, 24.75. Anal. Calc. for C29H18F10N2Pd: C,
50.42; H, 2.63; N, 4.05. Found: C, 50.33; H, 2.64; N, 4.21%.
l
methyl-4,5-diphenylimidazole (L2; 34.9 mg, 0.149 mmol) were
added to a Schlenk flask, which was then purged with argon. Dry
THF (10 mL) was added, and the solution turned from dark green
to orange within a minute. Sodium tetrafluoroborate (8.6 mg,
0.0783 mmol) was added to the reaction mixture after 30 min,
and the solution was stirred overnight. The solution was gravity fil-
tered, and the solvent was removed from the filtrate under vacuum
to leave an orange residue. The residue was further washed with
water and hexanes and dried under vacuum to give an orange
powder. Yield 81% (56.4 mg). Crystals suitable for X-ray crystallo-
graphic analysis were grown by slow vapour diffusion of hexanes
into a concentrated THF solution of [Pd(Ph2nacnac)(L2)2]BF4 (6)
(ꢂ10 mg of 6 in 0.1 mL of THF). 1H NMR (400 MHz, CD2Cl2): d
7.69–6.89 (m, 30H, aromatic C–H), 5.39 (s, 2H, imidazole C–H),
5.09 (s, 1H, nacnac b-H), 2.85 (s, 6H, imidazole CH3), 1.87 (s, 6H,
nacnac CH3). 13C NMR (100 MHz, CD2Cl2): d 24.3, 30.6, 33.5, 98.4,
125.6, 125.9, 127.3, 127.8, 128.6, 128.7, 128.8, 128.9, 129.5,
129.7, 130.7, 132.7, 136.2, 139.4, 151.2, 159.2. Anal. Calc. for
2.3.4. Synthesis of [Pd(Dipph2nacnac)(Py)Cl] (4b)
In air, pyridine (0.10 mL, 1.2 mmol) was added to a dark green
solution of 1b (50 mg, 0.045 mmol) in 10 mL of CHCl3. The initially
green solution was heated at 70 °C, gradually the colour changed to
brown and finally to bright red after 3 h of heating. The solvent and
volatiles were removed under vacuum and the residue was washed
with hexanes, and the residue was recrystallized from a CH2Cl2/
pentane solvent mixture. The orange-red crystals of [Pd(Dip-
ph2nacnac)(Py)Cl] (4b) were dried under vacuum; the yield was
70% (40 mg). Crystals suitable for X-ray crystallographic analysis
were grown by slow evapouration of a CH2Cl2 solution of 4b. 1H
NMR (400 MHz, CDCl3): d 7.89 (dd, 2H, J = 1.5 Hz, J = 6.5 Hz), 7.36
(tt, 1H, J = 1.4 Hz, J = 7.7 Hz), 7.14 (dd, 1H, J = 6.7 Hz, J = 8.4 Hz),
7.07–7.00 (m, 3H), 6.88 (d, 2H, J = 7.7 Hz), 6.85 (dd, 2H, J = 6.6 Hz,
J = 7.5 Hz), 4.90 (s, 1H, nacnac b-CH), 3.61 (septet, 2H,-CHMe2),
3.42 (septet, 2H, –CHMe2), 1.70 (s, 3H, nacnac-CH3), 1.66 (s, 3H,
nacnac-CH3), 1.51 (d, 6H, J = 6.8 Hz, -CH(CH3)2), 1.22 (d, 6H,
J = 6.9 Hz, -CH(CH3)2), 1.16 (dd, 12H, J = 1.5 Hz, J = 6.8 Hz,
–CH(CH3)2). 13C NMR (100 MHz, CDCl3): d 158.32, 157.40, 152.27,
148.81, 146.42, 143.49, 142.58, 136.47, 126.38, 125.73, 123.74,
123.65, 122.87, 95.50, 28.48, 28.05, 24.81, 24.60, 24.39, 24.20.
Anal. Calcd for (C34H46ClN3Pd)ꢃ0.5(CH2Cl2): C, 60.84; H, 6.96; N,
6.17. Found: C, 60.40; H, 6.41; N, 6.16.
C49H45BF4N6Pd: C, 64.60; H, 4.98; N, 9.23. Found: C, 64.43; H,
5.44; N, 8.77%.
2.3.8. Synthesis of [(4-FPh)Pd(l-Cl)2Pd(Ph2nacnac)]2 (7b)
Following a procedure similar to that described previously for
the synthesis of 7a [8], in air and with non-anhydrous solvents
and reagents directly from commercial sources, 1a (50 mg,
0.064 mmol) was dissolved in 1 mL of a benzene:acetone solvent
mixture (9:1 v/v) and transferred to a 4 mL vial containing 4-fluor-
ophenylboronic acid (11.5 mg, 0.082 mmol). After mixing the vial
was sealed and gradually heated to 75 °C. After heating overnight,
rhombus-shaped, orange, X-ray quality single crystals of [(4-
FPh)Pd(l-Cl)2Pd(Ph2nacnac)]2 (7b) deposited on the insides of
2.3.5. Synthesis of [Pd(ArF2nacnac)(Py)Cl] (4c)
the vial. The mother-liquor was aspirated off and the crystals of
7b were washed with benzene, followed by hexanes and dried in
vacuum. The yield was 47% (19.7 mg). Because of poor solubility
not all 13C resonances are visible even after overnight scans.
1H NMR (400 MHz, CD2Cl2): d 7.88–7.96 (m, 2 H), 7.45–7.52 (m,
4 H), 7.30–7.41 (m, 10 H), 7.19–7.28 (m, 4 H), 7.05–7.11 (m, 4 H),
6.90–6.98 (m, 2 H), 6.73–6.82 (m, 2 H), 3.89 (s, 2 H), 2.07 (s, 6H),
0.95 ppm (s, 6H). 19F NMR (376 MHz, CD2Cl2): ꢀ122.25 to
ꢀ122.36 (m, 2F).
In air, pyridine (73 lL, 0.90 mmol) of was added to a dark green
solution of 1c (187 mg, 0.14 mmol) in 30 mL of CH2Cl2, which
turned orange within a minute. The reaction mixture was stirred
for 15 min and the solvent and volatiles were removed under vac-
uum. The residue was washed with hexanes, and the material was
recrystallized from a CH2Cl2/pentane solvent mixture. The orange-
red crystals of 4c were dried under vacuum; the yield was 88%
(185 mg). Crystals suitable for X-ray crystallographic analysis were
grown by slow evapouration of a CH2Cl2 solution of 4c. 1H NMR
(400 MHz, CDCl3): d 8.60 (m, 1H), 8.24 (m, 2H), 7.63 (m, 2H), 7.51
(m, 2H), 7.43 (m, 2H), 7.30 (m, 2H), 6.92 (m, 2H), 4.86 (s, 1H, nacnac
b-CH), 1.73 (s, 3H, nacnac-CH3), 1.72 (s, 3H, nacnac-CH3). 19F NMR
(376 MHz, CDCl3): d ꢀ63.03 (s, 6F), ꢀ63.49 (s, 6F). 13C NMR
13C NMR (100 MHz, CD2Cl2): d 186.19, 129.36, 128.76, 127.15,
125.84, 124.76, 23.92, 23.35.
Anal. Calcd for C46H42Cl4F2N4Pd4: C, 43.98; H, 3.37; N, 4.46.
Found: C, 44.58; H, 3.32; N, 4.51.
(100 MHz, CDCl3):
d 159.53, 157.34, 152.67, 152.17, 151.93,
2.3.9. Synthesis of [Pd((PhN=C(CH3))2CH2)Cl(C6F5)] (8) and trace
octanuclear complex 9
149.83, 137.58, 135.87, 131.89, 131.56, 131.02, 130.69, 127.89,
126.89, 124.95, 118.71, 118.18, 98.43, 24.83, 23.78. Anal. Calc. for
In a nitrogen-filled glovebox 1a (64 mg, 0.082 mmol) and
(C6F5)B(OH)2 (22.1 mg, 0.10 mmol) were dissolved in 1.3 mL of
dry, degassed, toluene. The reaction mixture was placed in a 7 mL
vial, sealed, and heated in a 75 °C oil bath overnight. The yellow X-
ray diffraction quality crystals of [Pd((PhN = C(CH3))2CH2)Cl(C6F5)]
(8) were collected by filtration, washed with toluene, pentane, and
dried in vacuum. The yield was 54% (49 mg). The X-ray crystal struc-
ture of octanuclear Pd complex (9) was obtained from a trace
amount of red–orange crystals also produced in this reaction. Poor
solubility prevented 13C NMR data from being obtained for 8. 1H
NMR (400 MHz, CDCl3): d 7.50–7.46 (m, 2H), 7.33–7.31(m, 1H),
7.23–7.21(m, 2H), 7.18–7.16 (m, 1H), 7.13–7.11 (m, 2H), 6.70–6.68
(m, 2H), 4.60 (s, 2H, b-CH2), 2.24 (s, 3H, b-diimine CH3), 2.23(s, 3H,
b-diimine CH3). 19F NMR (376 MHz, CDCl3): d ꢀ120.99 (m, 2F),
ꢀ161.34 (t, 1F, J = 19.9 Hz), ꢀ163.97 (m, 2F). Anal. Calc. for
C
26H18N3F12ClPd: C, 42.07; H, 2.44; N, 5.66. Found: C, 42.08; H,
2.58; N, 5.91%.
2.3.6. Synthesis of O2-activation product [Pd(L1)Cl(Me-Im)] (5), (poor
reproducibility)
In air, 1b (22 mg, 0.02 mmol) and 1-methylimidazole (Me-Im,
3.25 lL, 0.04 mmol) were dissolved in 10 mL of CH2Cl2, after
2 months at R.T. red X-ray diffraction quality crystals of
[Pd(L1)Cl(Me-Im)] (6) were obtained and the 1H NMR spectrum
was recorded. 1H NMR (400 MHz, CD2Cl2): d 7.93 (s, 1H), 7.29–
7.06 (m, 6H), 6.82 (m, 1H), 5.02 (s, 1H), 3.66 (s, 3H), 3.60 (septet,
1H, J = 13.7, 6.8 Hz), 3.48 (septet, 1H, J = 13.7, 6.8 Hz), 3.25 (septet,
1H, J = 13.7, 6.8 Hz), 2.79 (septet, 1H, J = 13.7, 6.8 Hz, 1H), 2.04 (s,
3H), 1.81 (s, 2H), 1.51 (d, 3H, J = 6.7 Hz), 1.29 (d, 3H, J = 6.8 Hz),
1.25 (d, 3H, J = 6.9 Hz), 1.21 (d, 3H, J = 6.7 Hz), 1.11 (d, 3H, 6.9 Hz),
1.04 (dd, 6H, J = 6.7, 4.7 Hz).
C
23H18ClF5N2Pd: C, 49.39; H, 3.24; N, 5.01. Found: C, 49.16; H,
3.58; N, 4.83%.

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2.4. Cleavage of 7a with pyridine yielding 4a, and trans-
[Pd(Py)2(Ph)Cl]
0.098 M), and solutions of pyridine in C6D6 where combined such
that the final volume of solution is 500 L. The solutions were al-
l
lowed to equilibrate for at least 1 h at R. T. before being subjected
to 1H NMR spectroscopy. Afterwards a flame-sealed capillary con-
taining a 0.29 M solution of KBF4 in D2O was inserted and 11B NMR
spectroscopy was performed. See Supporting Information for NMR
spectra.
Performed in a nitrogen-filled glovebox with CH2Cl2 and pyridine
that had been dried over calcium hydride and vacuum-transferred
prior to use. In a Pyrex bomb with a Teflon valve, 7a (10 mg,
0.0082 mmol) was suspended in 5 mL of CH2Cl2 and the mixture
was sonicated for 1 h in order to dissolve the orange starting material.
The bomb was brought back into the glovebox and pyridine (4 lL,
3. Results and discussion
0.049 mmol) was added, after 10 min the solvent and volatiles were
removed under vacuum, and NMR spectroscopy was performed. 1H
NMR spectroscopy of the mixture indicates that only 4a and trans-
[Pd(Py)2(Ph)Cl] form when compared with 1H NMR spectral data
reported for 4a [8], and trans-[Pd(Py)2(Ph)Cl] [19]. Recrystallization
of the mixture (ꢂ10 mg) from a CH2Cl2 solution (ꢂ0.1 mL) layered
with hexanes (ꢂ5 mL) at ꢀ30 °C gave X-ray diffraction quality crys-
tals of both orange 4a, and colourless trans-[Pd(Py)2(Ph)Cl].
3.1. Synthesis and characterization of [Pd(Ar2nacnac)(
complexes
l-Cl)]2
An entry point into palladium nacnac chemistry is the synthesis
of chloro-bridged dimers of the general form [Pd(Ar2nacnac)(
l-
Cl)]2, these versatile starting materials allow simple access to other
palladium nacnac complexes. Varying the Ar-groups of the
[Ar2nacnac]ꢀ ligand allows for a comparison of the reactivities of
these compounds. The reaction of H[Ar2nacnac] (where Ar = Ph,
or, 2,6-diisopropylphenyl), KOtBu, and Pd(PhCN)2Cl2 at a 1:1:1 M
ratio in methanol produces the previously synthesized [Pd(Ph2nac-
2.5. Reaction of 4a with 1.2 equiv. phenylboronic acid
In a nitrogen-filled glovebox, in a J. Young NMR tube 19.0 mg
(0.0403 mmol) of orange-red 4a and 6.0 mg (0.0492 mmol) of
phenylboronic acid were dissolved in 0.5 mL of dry and degassed
C6D6 and left at R.T. overnight giving a brown solution. 1H NMR
was performed, then a flame-sealed capillary containing a 0.29 M
solution of KBF4 in D2O was inserted and 11B NMR was performed.
See Supporting Information for NMR spectra.
nac)(l-Cl)]2 (1a), and newly characterized [Pd(Dipph2nacnac)(l-
Cl)]2 (1b) as green precipitates in moderate yields of 65%, and
58%, respectively (Scheme 1). An alternative palladium source that
can be used is Na2PdCl4 which gives approximately the same yield
in the synthesis of 1a, but a lower yield of 35% for the synthesis of
1b.
1H NMR of 4a (400 MHz, C6D6): d 7.89–7.86 (m, 2H), 7.44–7.40
(m, 2H), 7.27–7.21 (m, 2H), 7.06–7.01 (m, 1H), 6.54–6.30 (m, 5H),
6.36–6.31 (m, 1H), 5.93–5.89 (m, 2H), 4.90 (s, 1H, nacnac b-CH),
1.77 (s, 3H, nacnac-CH3), 1.67 (s, 3H, nacnac-CH3).
When the synthesis of [Pd(ArF2nacnac)(
l-Cl)]2 (1c) (where
ArF = 3,5-bis(trifluoromethyl)phenyl) was carried out by mixing
H[ArF2nacnac], KOtBu, and Na2PdCl4 at a 1:1:1 M ratio in methanol,
a dark brown precipitate resulted after overnight. 1H, and 19F NMR
spectra of the crude brown precipitate revealed the presence of
two symmetrical species in a 2:5 ratio; this was indicated in the
1H NMR spectrum by the presence of two singlets at 5.61 and
5.00 ppm which were tentatively assigned as the b-CH, and two
singlets at 1.86 and 1.62 ppm which were tentatively assigned as
the nacnac backbone methyl signals, and two singlets in the 19F
NMR spectrum at ꢀ64.55 and ꢀ64.26 ppm. Leaving a CH2Cl2 solu-
tion of the brown crude to slowly evapourate lead to the observa-
tion of two distinct types of crystals which were green and red. The
1H NMR of 1a (400 MHz, C6D6): d 6.99–6.94 (m, 8H), 6.87–6.82
(m, 4H), 6.79–6.76(m, 8H), 4.57 (s, 2H, nacnac b-CH), 1.45 (s, 12H,
nacnac-CH3).
2.6. Control experiments to determine stoichiometry of Lewis acid–
base adduct from reaction of 4a with phenylboronic acid
In a nitrogen-filled glovebox in a J. Young tube, solutions of
phenylboronic acid in dry/degassed C6D6 (final concentration
Scheme 1. Synthesis of [Pd(Ph2nacnac)(l–Cl)]2 (1a) previously reported [6] and [Pd(Dipph2nacnac)(l-Cl)]2 (1b), where Dipph = 2,6-diisopropylphenyl.
Scheme 2. Synthesis of [Pd(ArF2nacnac)( -Cl)]2 (1c) and [Pd(ArF2nacnac)2] (2), where ArF = 3,5-bis(trifluoromethyl)phenyl.
l

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313
Fig. 2. Molecular structures of 1b (a) and 1c (b) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) for 1b: Pd1–
N1, 2.013(3); Pd1–N2, 2.023(2); Pd1–Cl1, 2.3538(11); N1–Pd1–N2, 91.77(10); N1–Pd1–Cl1, 94.29(7), N2–Pd1–Cl1A: 93.68(7). Selected bond lengths (Å) and angles (°) for 1c:
Pd1–N2, 1.989(4); Pd1–N1, 2.005(4); Pd1–Cl1, 2.3498(14); N2–Pd1–N1, 90.95(17); N1–Pd1–Cl1, 94.20(13); N2–Pd1-Cl1A, 92.41(12).
red crystals seemed to also form faster than the green ones, and
also more near the bottom of the recrystallization vial. After a
series of recrystallizations (see Section 2 for details) the green,
and red crystals could be cleanly separated; X-ray crystallography
adopts a typical square-planar coordination geometry, with two N
and two bridging Cl atoms occupying the four coordination sites
and displaying typical bond lengths and angles. The six-membered
chelate rings in all three derivatives adopt a boat conformation,
folding along N1ꢁ ꢁ ꢁN2 and C2ꢁ ꢁ ꢁC4 axes. The dihedral angle be-
tween the planes defined by N1, Pd, N2 and C2, C3, C4 is 27° for
1a [6], 19° for 1b, and 26° for 1c.
revealed that the green crystals were of [Pd(ArF2nacnac)(
l-Cl)]2
(1c), and the red crystals were of [Pd(ArF2nacnac)2] (2); satisfactory
elemental analysis results were obtained. The yields of 1c and 2
were 25% and 6%, respectively (Scheme 2).
The 1H NMR spectra of 1b and 1c reveal the symmetrical struc-
tures of these complexes in solution giving rise to a singlet for the
b-CH, and a singlet for the nacnac backbone methyl protons with
an integration of 1:3. In 1H NMR spectrum for 1b there are two sets
of doublets corresponding to methyl protons of the iPr groups sug-
gesting hindered rotations on the NMR timescale.
All three [Pd(Ar2nacnac)(l-Cl)]2 complexes 1a [6], 1b, and 1c
are green and air- and moisture-stable both in the solid state,
and in solution as monitored by NMR spectroscopy over a few
days. They tend to be very soluble in most common organic sol-
vents such as chloroform, dichloromethane, THF, diethyl ether,
benzene, and toluene, somewhat soluble in acetone, but poorly sol-
uble in methanol, water or hexanes and pentane.
The X-ray crystal structures of 1b and 1c are shown in Fig. 2. All
three 1a [6], 1b, 1c crystallize in the monoclinic space group P21/n
with a crystallographically imposed inversion centre in the middle
of the molecule. In the structure of 1c the trifluoromethyl groups
are disordered over two positions and there is also a disordered
CH2Cl2 solvent molecule. The Pd(II) centres in all three derivatives
The X-ray crystal structure of 2 is shown in Fig. 3. Structurally
similar bis-nacnac Pd complexes characterized by X-ray crystallog-
raphy in the literature are [Pd(iPr2nacnac)2] [2], and [Pd(Ph2nac-
nac)2] [20]. Complex 2 crystallizes in the monoclinic space group
P21/n, with a crystallographically imposed inversion centre at
Pd1. The six-membered chelate rings adopt a puckered conforma-
tion, folding along N1. . .N2 axis. The binding mode of [ArF2nac-
nac]ꢀ, seen clearly in Fig. 2b demonstrates that Pd is not in the
Fig. 3. Molecular structure of 2 (a) and an alternate side-view of 2 (b) displaying the binding mode of the two [ArF2nacnac]ꢀ ligands more clearly. Both views of the structure
are shown with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. In b), only the ipso-carbons of the 3,5-bis(trifluoromethyl)phenyl groups are shown for
clarity. Selected bond lengths (Å) and angles (°) for 2: Pd1–N1, 2.027(4); Pd1–N2, 2.034(4); N1–Pd1–N2, 87.63(18).

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V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
Scheme 3. Synthesis of 3.
ligand plane. The dihedral angle between the planes defined by N1,
Pd1, N2 (the metal coordination plane) and N1, C2, C3, C4, N2 (the
[ArF2nacnac]ꢀ ligand plane) is 45° for 2. There is a change in the
dihedral angle between the metal coordination plane and the nac-
nac ligand plane upon changing the N,N0-substituents from iPr, to
Ph, to ArF, likely due to sterics. In [Pd(iPr2nacnac)2] the aforemen-
tioned dihedral angle is 54° [2], and in [Pd(Ph2nacnac)2] the angle
is 50° [20].
The X-ray crystal structure of 3 is shown in Fig. 4, where the Pd
centre displays a typical square planar geometry with two cis-N
and two cis-C donor atoms occupying the four coordination sites
and displaying typical bond lengths and angles. Complex 3 crystal-
lizes in monoclinic space group P21/n. The six-membered chelate
ring adopts a boat conformation, folding along N1ꢁ ꢁ ꢁN2 and
C2ꢁ ꢁ ꢁC4 axes. The dihedral angle between the planes defined by
N1, Pd, N2 and C2, C3, C4 is 86.43°.
Usually cis-diarylpalladium complexes are difficult to isolate
because of facile reductive elimination of biaryl compounds from
the complex. Complex 3 is quite thermally stable, also it is air-
and moisture-stable in the solid state and in solution, likely
because the Pd-C(sp2) bonds are strengthened by fluorine substitu-
tion. This also has implications for palladium catalyzed C–C cou-
pling reactions which typically involve [(L)Pd(Ar)(Ar0)]
intermediates, if both Ar and Ar0 are electron-withdrawing the
reductive elimination is usually extremely slow/infrequent result-
ing in low yields of biaryl product. Several stable cis-[Pd(C6F5)2(L0)]
(where L0 is a neutral bidentate ligand) complexes have been char-
acterized [13,21–23].
Yamamoto and coworkers found that reductive elimination
could be promoted from complexes of the form cis-[Pd(C6F5)2(L0)]
to form (C6F5)-(C6F5) upon the addition of acid, where the trend in
efficiency of this process seemed to be dependent on a few struc-
tural parameters such as Pd–Cipso bond length, Cipso–Cipso distance,
the Cipso–Pd–Cipso angle, and dihedral angle between the two planes
of the C6F5 rings [23]. Generally, the longer Pd–Cipso bond length, the
shorter the Cipso–Cipso distance, the narrower the Cipso–Pd–Cipso an-
gle, and the more acute the dihedral angle between the two C6F5
planes, the more facile the reductive elimination to form (C6F5)–
(C6F5) was. The values for the aforementioned structural parameters
in complex 3 are as follows (distances in Å, angles in °); Pd1-C18:
2.0050(19), Pd1-C24: 2.0037(19), C18-C24: 2.762, C18-Pd1-C24:
87.12(8), plane(C6F5)-plane(C6F5): 84.56. The Pd–Cipso bonds in 3
are relatively short as compared with either [Pd(C6F5)2(cod)]
(2.032(4)), [Pd(C6F5)2(bpy)] (2.009(6)(av.)), [Pd(C6F5)2(dppe)]
(2.087(2)(av.)), [Pd(C6F5)2(dppb)] (2.062(4)) [23]. The Cipso–Cipso
distance is relatively short as compared with either [Pd(C6F5)2(cod)]
(2.842(6)), [Pd(C6F5)2(bpy)] (2.747(8)), [Pd(C6F5)2(dppe)] (3.026
(3)), [Pd(C6F5)2(dppb)] (2.829(5)) [23]. The Cipso–Pd–Cipso angle in
3 is intermediate when compared with either [Pd(C6F5)2(cod)]
(88.75(19)), [Pd(C6F5)2(bpy)] (86.3(2)), [Pd(C6F5)2(dppe)] (92.90
(8)), [Pd(C6F5)2(dppb)] (86.65(16)) [23]. Finally the dihedral angle
between the two C6F5 planes in 3 is relatively acute as compared
with either [Pd(C6F5)2(cod)] (88.9), [Pd(C6F5)2(bpy)] (89.9),
[Pd(C6F5)2(dppe)] (104.1), [Pd(C6F5)2(dppb)] (86.6) [23]. Yamamoto
and co-workers found that reductive elimination from
[Pd(C6F5)2(cod)] and[Pd(C6F5)2(dppb)] upon the addition of HNO3
selectively gave reductive elimination of (C6F5)–(C6F5), while
3.2. Synthesis of [Pd((PhN=C(CH3))2CH2)(C6F5)2] (3)
In an attempt to synthesize [Pd(Ph2nacnac)(C6F5)(THT)] (where
THT = tetrahydrothiophene), which would provide a 19F NMR spec-
troscopic handle to study insertion/polymerization reactivity we
isolated a palladium(II) b-diimine complex 3. Refluxing trans-
Pd(THT)2(C6F5)2 with H[Ph2nacnac] in toluene (Scheme 3) gave a
pale yellow solution with some Pd black. After filtration to remove
the Pd black and concentration of the filtrate a white microcrystal-
line powder of 3 can be precipitated out upon the addition of hex-
anes in 50% yield. X-ray diffraction quality crystals can be grown
from a CH2Cl2–MeOH solvent mixture at ꢀ30 °C (Fig. 4). The 1H
NMR spectrum reflects the symmetrical nature of 3 giving rise to
a singlet at 4.46 ppm corresponding to the b-CH2, and a singlet at
2.30 ppm corresponding to the methyl protons of the b-diimine
backbone, the 19F NMR spectrum has three sets of peaks corre-
sponding to the ortho-, meta- and para- fluorines.
Fig. 4. Molecular structure of 3 with 30% probability ellipsoids. Hydrogen atoms are
shown as spheres of arbitrary radius. Selected bond lengths (Å) and angles (°) for 3:
Pd1–N1, 2.0941(16); Pd1–N2, 2.0872(16); Pd1–C18, 2.0050(19); Pd1–C24,
2.0037(19); N2–Pd1–N1, 86.59(6); C24–Pd1–C18, 87.12(8); C24–Pd1–N2,
90.23(7); C18–Pd1–N1, 95.94(7).

V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
315
Scheme 4. Cleavage of [Pd(Ar2nacnac)(l-Cl)]2 (1) by pyridine (Py).
[Pd(C6F5)2(bpy)] gave only a very low yield of biaryl after extended
period of time, and [Pd(C6F5)2(dppe)] gave the protonolysis product
HC6F5 [23].
The six-membered chelate ring in complex 4a was reported as
having a puckered conformation where dihedral angle between
the N1, Pd, N2 and C2, C3, C4 planes was 19.78° [8]. The six-mem-
bered chelate ring in complex 4b adopts an almost planar confor-
mation, but does fold slightly along N1ꢁ ꢁ ꢁN2 axis, and the dihedral
angle between the N1, Pd, N2 and C2, C3, C4 planes is only 13.94°.
The six-membered chelate ring in complex 4b adopts a puckered
conformation, folding along N1ꢁ ꢁ ꢁN2 axis, the dihedral angle
between the N1, Pd, N2 and C2, C3, C4 planes is 37.39°.
3.3. Reactivity of [Pd(Ar2nacnac)(l-Cl)]2 towards neutral
monodentate ligands
[Pd(Ph2nacnac)(l-Cl)]2 (1a) has been shown previously to be an
excellent starting material for the preparation of mononuclear spe-
cies of the form [Pd(Ph2nacnac)(L)Cl]. When neutral monodentate
ligands (L) such as CO [6], N-methyl-4,5-diphenylimidazole (L2)
[6], 4-t-butylaniline [6,7], and pyridine [8] are added the chloride
bridge breaks. Complex 1a reacts with CO to form [Pd(Ph2nac-
nac)(CO)Cl] which is only stable under an atmosphere of CO; even
in the solid-state wet crystals [Pd(Ph2nacnac)(CO)Cl] can lose CO to
reform complex 1a [6]. When CO is bubbled through a CDCl3 solu-
tion of 1b there is no apparent change by NMR spectroscopy and
the solution remains a dark green colour. Apparently the bulky
Dipph groups of 1b seem to inhibit dimer cleavage by CO.
Complexes 1a and 1c react rapidly upon mixing at room tem-
perature with pyridine in CH2Cl2 to form the previously character-
ized complex 4a [8], and 4c, respectively which can be isolated as
red–orange crystals (Scheme 4). However simply reacting 1b with
excess pyridine at room temperature does not give any reaction
even after 1 h. In order to cleave the chloride bridges in complex
1b excess pyridine and elevated temperatures of 70 °C for 3 h are
required to form 4b (Scheme 4).
3.4. Metal–ligand co-operative dioxygen activation in a Pd nacnac
complex
When a CH2Cl2 solution of 1b and 2 equivalents of 1-methylim-
idazole (Me-Im) was left in air for 2 months red crystals were ob-
tained of complex [Pd(L1)Cl(Me-Im)] (5) (Scheme 5). In the process
of generating complex 5, the chloride bridge has been cleaved, and
the backbone b-C activated dioxygen to form an N,O-chelate che-
lating ligand (L1ꢀ) which forms a 5-membered chelate ring with
Pd. The synthesis of complex 5 is an example of the non-innocent
and reactive nature of the b-C in nacnac ligands, a reoccurring
theme in the chemistry of this ligand set. Examples for the reactiv-
ity of the b-C towards dioxygen in transition metal nacnac com-
plexes are known [24–26]. Itoh and co-workers reported Cu(II)
and Zn(II) complexes of [Mes2nacnac]ꢀ (where Mes is mesityl) li-
gand that easily underwent an oxidative degradation to afford a
ketone diimine where the backbone b-C had been oxidized to a car-
bonyl [24]. An N,O-chelate ligand very similar to L1ꢀ has been seen
on Cu(II) where the aryl groups on N are either Mes [24], or Dipph
[25], where the main difference is that the dioxygen-derived func-
tional group is a hemiacetal. Itoh and coworkers had suggested
[24], and Goldberg and co-workers have demonstrated [26] that
a ‘‘metal–O–O–C’’ peroxo intermediate might be involved in the
oxidation of nacnac to a ketone diimine. Goldberg and co-workers
have isolated a Pt(IV) complex with a ligand similar to L1ꢀ gener-
ated from dioxygen activation where the ligand acts as a tridentate
N,O,N-donor ligand to {Pt(Me)3}+ cation [26].
1H NMR spectra for either 4b or 4c reflects the fact that the two
nacnac backbone methyl groups are inequivalent; for complex 4b
these resonances appear at 1.70 and 1.66 ppm, and for complex
4c these appear at 1.73 and 1.72 ppm. Fig. 5 depicts the X-ray crys-
tal structures of 4b and 4c, in both of the complexes the Pd centre
adopts a typical square planar geometry and is coordinated to two
N-donors from the nacnac ligand in a cis-fashion, one terminal Cl,
and an N-donor atom from pyridine to occupy the four coordina-
ꢀ
tion sites. Complex 4b crystallizes in the triclinic space group P1,
and complex 4c crystallizes in the monoclinic space group P21/n
with half a molecule of disordered CH2Cl2 per each molecule of
4c. The trifluoromethyl groups in complex 4c are disordered over
two positions.
The crystal structure of complex 5 is shown in Fig. 6. The Pd(II)
centre adopts a typical square-planar coordination geometry, with
two cis-coordination sites occupied by the N,O chelate L1, a Cl

316
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Fig. 5. Molecular structures of 4b (a) and 4c (b) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) for 4b: Pd1–
N1, 2.031(11); Pd1–N2, 2.014(10); Pd1–N3, 2.078(10); Pd1–Cl1, 2.314(4); N2–Pd1–N1, 91.7(4); N3–Pd1–Cl1, 82.0(3); N1–Pd1–N3, 94.3(4); N2–Pd1–Cl1, 92.0(3). Selected
bond lengths (Å) and angles (°) for 4c: Pd1–N1, 2.027(3); Pd1–N2, 2.005(4); Pd1–N3, 2.040(4); Pd1–Cl1, 2.3165(11); N2–Pd1–N1, 89.71(14); N3–Pd1–Cl1, 86.96(11); N1–
Pd1–N3, 91.43(14); N2–Pd1–Cl1, 91.98(10).
atom, and N-atom from Me-Im occupying the four coordination
sites, and displaying typical bond lengths and angles. The 1H
NMR spectrum shows that all four –CH(Me)2 are inequivalent, four
septets were observed at 3.60, 3.48, 3.25, and 2.79 ppm.
Unfortunately the synthesis of complex 5 has poor reproducibil-
ity and only an X-ray crystal structure, and 1H NMR spectrum was
obtained.
bond distances are 2.005(3) Å and 1.998(3) Å, respectively, and
the Pd-Ph2nacnac ring adopts a boat conformation, with a dihedral
angle between the N1, Pd1, N2 and C2, C3, C4 planes of ꢂ49°. The
conformation that the ring adopts (planar or boat) seems to de-
pend on the nature of the ligands around the Pd centre. Boat con-
formations are generally seen in [Pd(Ar2nacnac)(l-Cl)]2 and in
complexes where there is a bond or interaction at the b-C of the
nacnac backbone [4,6–8], whereas a planar conformation was seen
in [Pd(Ph2nacnac)Cl(L2)] [6]. The imidazole rings in 6 have their
methyl groups pointing in opposite directions. The imidazole
methyl protons (5.39 ppm) are shifted significantly upfield from
that of [Pd(Ph2nacnac)Cl(L2)] (7.26 ppm), most likely due to their
positioning with respect to the phenyl groups on imidazole, result-
ing in a ring current effect.
3.5. Synthesis and characterization of [Pd(Ph2nacnac)L2]+ cationic
complexes
The reaction of 1a with neutral and monodentate L-type ligands
can be further expanded to produce complexes of the form
[Pd(Ph2nacnac)L2]BF4 (Scheme 6). Reaction of 1a under argon with
four equivalents of a L-type ligand, such as N-methyl-4,5-diphenyl-
imidazole (L2), and two equivalents of NaBF4 results in chloride
abstraction and the formation of [Pd(Ph2nacnac)(L2)2]BF4 (6).
Complex 6 crystallized in the monoclinic space group P21/c
(Fig. 7). The Pd(II) centre is square-planar with all four coordina-
tion sites occupied by nitrogen atoms. The Pd1–N1 and Pd1–N2
3.6. Unusual transmetallation-induced formation of a C2-symmetric
tetrapallada-macrocycles and reactivity
Recently we discovered
a rather surprising example of
transmetallation when phenylboronic acid is used as the
Scheme 5. Synthesis of 5.

V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
317
to other literature values for Pd-(b-C) bond lengths [4,7]. The ipso-
carbon of the phenyl ligand and the b-carbon donor of the [Ph2nac-
nac]ꢀ ligand are mutually cis- and the coordination environment
around each Pd(II) center is square planar. The two methyl groups
of each [Ph2nacnac]ꢀ ligand are no longer equivalent: the methyl
group containing C5 (Fig. 8) is situated in the shielding region of a
phenyl ligand, while the methyl group containing C1 has no phenyl
ligand nearby. Accordingly, the two singlets at 2.07, and 0.95 ppm in
the 1H NMR spectrum of 7b can be assigned to the two methyl
groups containing C1 and C5, respectively.
We decided to probe the reactivity of 7a towards L-type ligands
in an attempt to force the reductive coupling of the mutually cis-
phenyl ligand and [Ph2nacnac]ꢀ ligand. Similar to complex 1a,
complex 7a possesses bridging chlorides. When complex 7a is trea-
ted with 6 equiv. of pyridine the chloride bridges are cleaved, in
addition the Pd–(b-C) bond is cleaved and in the process two
equivalents of 4a and two equivalents of trans-[Pd(Py)2(Ph)Cl]
form as monitored by 1H NMR spectroscopy (Scheme 8). The
NMR data of both 4a [8], and trans-[Pd(Py)2(Ph)Cl] [19] are re-
ported previously. Furthermore, recrystallization of reaction mix-
ture from CH2Cl2–hexanes gave both orange crystals of 4a, and
colourless crystals of trans-[Pd(Py)2(Ph)Cl]. The synthesis and
NMR characterization of trans-[Pd(Py)2(Ph)Cl] has been reported
in the literature [19], here we report the X-ray crystal structure
of this simple coordination complex that crystallized in the mono-
clinic space group C2/c (see Fig. 9). The addition of 1 equiv. of pyr-
idine at a time in an attempt to pinpoint which bond is cleaved first
only showed incomplete conversion to 4a and trans-
[Pd(Py)2(Ph)Cl].
Fig. 6. Molecular structure of O2–activation product
5 with 30% probability
ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°) for 5: Pd1–O1, 2.0031(16); Pd1–N1, 2.021(2); Pd1–N3, 2.022(2); Pd1–Cl1,
2.2814(9); O1-Pd1–N1, 82.15(8); N3–Pd1–Cl1, 91.83(7); O1–Pd1–N3, 90.11(8); N1-
Pd1–Cl1, 95.91(6).
transmetallation reagent to synthesize the C2-symmetric tetrapal-
lada-macrocycle complex 7a [8]. This transmetallation reaction is
unusual since an anionic bidentate chelate ligand is replaced by a
phenyl ligand from phenylboronic acid, leaving the chloride li-
gands intact. As shown in Scheme 7, when a mixture of 1a and
1.3 equiv. of an arylboronic acid (where Ar = Ph, or 4-fluorophenyl)
in a wet benzene/acetone solvent mixture is heated at 75 °C, with-
in hours orange X-ray diffraction quality single crystals of analyti-
cally pure 7a, and 7b can be isolated in ꢂ50% yield. Both 7a and 7b
are slightly soluble in dichloromethane and 1,2-dichloroethane,
but insoluble in benzene, acetone, hexanes, diethyl ether, THF,
methanol, water, chloroform and DMSO. Interestingly 1b and 1c
do not react at all with arylboronic acids.
3.7. Pyridine abstraction from 4a by boronic acids
The unusual transmetallation reaction observed between 1a
and boronic acids to form either complex 7a or 7b prompted us
to investigate whether the fact that 1a possesses bridging chlorides
as opposed to terminal chlorides might be the reason. Previously
we showed that when complex 4a, which possesses a terminal
chloride, is treated with MeLi the conventional [27] transmetalla-
tion product [Pd(Ph2nacnac)(Py)(Me)] was isolated and character-
ized [8]. When complex 4a is treated with 1.2 equiv. of
phenylboronic acid in dry C6D6, an equilibrium is established be-
tween the starting material 4a, and [Pd(Ph2nacnac)Cl]2 (1a) the
product of pyridine abstraction by phenylboronic acid (Scheme
9). The addition of an excess of phenylboronic acid to 4a pushes
the equilibrium towards the formation [Pd(Ph2nacnac)Cl]2 (see
Supporting Information for NMR spectra where 2.5 and 5 equiv.
of phenylboronic acid was added). Pyridine abstraction from a
transition metal complex by 3-coordinate Lewis acidic boranes,
such as B(C6F5)3, has been employed several times in the past,
and in the process of generating catalytically active species for ole-
fin polymerization [28,29]. To the best of our knowledge this
The X-ray crystal structure of 7b is shown in Fig. 8, and is very
similar to that of 7a [8], both crystallize in the monoclinic space
group C2/c with a C2 axis running down the centre of the molecule.
The structure can be viewed as two {(4-FPh)Pd(l-Cl)2Pd(Ph2nac-
nac)} units assembled via two Pd–C (the central backbone b-carbon
of [Ph2nacnac]ꢀ ligand) bonds. Each [Ph2nacnac]ꢀ ligand in 7b uses
two N-donors to chelate to a Pd center in one {(4-FPh)Pd(l-
Cl)2Pd(Ph2nacnac)]} unit, while the sp3 hybridized b-carbon coordi-
nates to a Pd center in the other unit. The six-membered chelate ring
comprised of the {(Ph2nacnac)Pd} moiety adopts a boat conforma-
tion. A similar coordination mode for [Ar2nacnac]ꢀ has also been re-
ported by Feldman and co-workers in a dinuclear tricationic Pd(II)
complex [4], and our group in an unusual trinuclear Pd(II)–Ph2nac-
nac complex with amido-chloro double-bridges [7]. The Pd2–C3A
bond length in 7b is 2.103(3) Å, similar to 7a (2.099(3) Å) [8], and
Scheme 6. Synthesis of 6.

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V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
Fig. 8. Molecular structure of 7b with 30% probability ellipsoids. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles (°) for 7b: Pd1–N1,
2.014(3); Pd1–N2, 1.999(3); Pd1-Cl1, 2.3363(9); Pd1––Cl2, 2.3414(9); Pd2–Cl1,
2.3911(9); Pd2–Cl2, 2.4966(9); Pd2–C18, 1.991(3); Pd2-C3, 2.103(3); N2–Pd1–N1,
85.90(12); N2–Pd1–Cl1, 93.49(9); N1–Pd1–Cl2, 94.07(8); Cl1-Pd1–Cl2, 86.53(3);
C18–Pd2–C3, 87.41(14); C18–Pd2–Cl1, 90.86(10); C3–Pd2–Cl2, 99.83(10); Cl1–Pd2-
Cl2, 81.95(3); Pd1–Cl2–Pd2, 85.16(3); Pd1–Cl1-Pd2, 87.71(3).
Fig. 7. Molecular structure of 6 with 30% probability ellipsoids. The counteranion
and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°)
for 6: Pd1–N1, 2.005(3); Pd1–N2, 1.998(3); Pd1-N3, 2.032(3); Pd1–N5, 2.058(3);
N2–Pd1–N1, 88.27(12); N3–Pd1–N5, 88.21(12); N1–Pd1–N3, 89.60(12); N2–Pd1–
N5, 93.78(12).
represents the first example of pyridine abstraction from a transi-
tion metal complex with a boronic acid. In the 11B NMR spectrum
there are two species (referenced against KBF4 dissolved in D2O in
a flame-sealed capillary), a major species given by a broad singlet
at 24.67 ppm, and a minor species given by a sharp singlet at
3.60 ppm.
3.8. Reactivity of pentafluorophenylboronic acid with 1a
After it was observed that 4-fluorophenylboronic acid reacts
with 1a to form 7b, it was investigated whether penta-
fluorophenylboronic acid could be used to form a similar tetrapal-
ladacycle. Using similar reaction conditions as shown in Scheme 7
in a wet benzene:acetone solvent mixture the major product that
was isolated out of the reaction mixture were yellow crystals of
[Pd((PhN = C(CH3))2CH2)Cl(C6F5)] (8). A C6F5-group was transferred
onto Pd and the backbone b-C of [Ph2nacnac]ꢀ was protonated. The
1H NMR spectrum shows that the two b-diimine methyl groups
were inequivalent resonating at 2.24 and 2.23 ppm, and the b-
CH2 peak at 4.60 ppm integrated for 2H relative to other peaks.
The X-ray crystal structure for complex 8 is shown in Fig. 10. Com-
pound 8 crystallized in the orthorhombic space group P212121.
Similar to complex 3 which also has the same b-diimine ligand
and a –C6F5 group, the six-membered chelate ring adopts a boat
conformation, folding along N1ꢁ ꢁ ꢁN2 and C2ꢁ ꢁ ꢁC4 axes. The dihedral
angle between the planes defined by N1, Pd, N2 and C2, C3, C4 is
86.4°. The Pd–N1 bond length is 2.029(3) Å and is shorter than
the Pd–N2 bond length which is 2.119(3) Å, this observation is in
agreement with the higher trans-influence of the C6F5 group being
The minor boron-containing species at 3.60 ppm is likely a 4-
coordinate borate species with [Ph2nacnac]ꢀ, or chloride coordi-
nated to boron which could not be characterized, the result of a
small amount of Pd black which formed in the reaction. The major
species at 24.67 ppm is likely a Lewis acid–base adduct with
approximately 1:3 pyridine:boronic acid stoichiometry, in line
with what had been investigated by Yabroff and coworkers by ele-
mental analysis, and Snyder and coworkers by IR [30–32]. 11B NMR
spectroscopy (externally referenced with KBF4) was conducted on
mixtures of pyridine and phenylboronic acid at different stoichi-
ometric ratios in C6D6 under dry/inert atmosphere. It was deter-
mined that the somewhere between 0.25 equiv. of Py per
phenylboronic acid (11B NMR, 25.67 ppm) and 0.33 equiv. of Py
(
11B NMR, 22.7 ppm) lies the stoichiometry of the resulting Lewis
acid–base complex. The broadness of the peak at 24.67 ppm im-
plies that there is dynamic solution behaviour in this complex.
Scheme 7. Synthesis of [(Ph)Pd(l-Cl)2Pd(Ph2nacnac)]2 (7a) which was previously reported,[8] and the synthesis of [(4-FPh)Pd(l-Cl)2Pd(Ph2nacnac)]2 (7b).

V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
319
Scheme 8. Cleavage of chloride bridges of 7a with pyridine yielding 4a, and trans-[Pd(Py)2(Ph)Cl].
a better
r-donor character compared to the Cl ligand. Similar
observations about the two Pd–N distances being different has been
noted in other complexes of [Pd(L0)(C6F5)(X)] where L0 is a neutral
bidentate ligand and X is either a halide or hydroxide [33,34]. Since
the backbone b-C of [Ph2nacnac]ꢀ was protonated in order to form
complex 8 from 1a it was thought this was perhaps because the
reaction was carried out in solvent which had not been dried. When
the reaction of 1a with pentafluorophenylboronic acid was carried
out in dry toluene (Scheme 10) the major product isolated in 54%
yield as yellow X-ray diffraction quality crystals was still complex
8. Surprisingly, in the same reaction mixture a trace amount of
red crystals had also formed at the gas-solution interface, these
red crystals were of octanuclear S-shaped complex 9 (see Fig. 11).
Complex 9 crystallizes in the monoclinic space group P21/n with
a crystallographically imposed inversion centre in the middle of
the molecule. The structure can be thought of as three {[Pd(Ph2nac-
nac)(
tween, the two outer {[Pd(Ph2nacnac)(
one {Pd(C6F5)Cl} unit each through one of its b-carbons, while
the central {[Pd(Ph2nacnac)( -Cl)]2} unit is bound to both
l-Cl)]2} units with two {Pd(C6F5)Cl} units sandwiched in be-
l-Cl)]2} units are bound to
l
{Pd(C6F5)Cl} units through both of its b-carbons. The –C6F5 groups
Scheme 9. Pyridine abstraction from [Pd(Ph2nacnac)(Py)Cl] (4a) to form 1a with
PhB(OH)2.
all point inward. Each Pd centre is square planar, where similar to
1a in the {[Pd(Ph2nacnac)(l-Cl)]2} fragments there are two N and
two bridging Cl atoms occupying the four coordination sites, while
the in the {Pd(C6F5)Cl} fragment, the C-donor from C6F5 and
Fig. 10. Molecular structure of 8 with 30% probability ellipsoids. Hydrogen atoms
are shown as spheres of arbitrary radius. Selected bond lengths (Å) and angles (°) for
8: Pd1–N1, 2.029(3); Pd1–N2, 2.119(3); Pd1–C18, 2.002(4); Pd1–Cl1, 2.2896(10);
N1–Pd1–N2, 86.74(13); C18–Pd1–Cl1, 89.91(12); N2–Pd1–Cl1, 94.32(9); C18–Pd1–
N1, 89.15(15).
Fig. 9. Molecular structure of trans-[Pd(Py)2(Ph)Cl] with 30% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) for
trans-[Pd(Py)2(Ph)Cl]: Pd1–C6, 1.990(5); Pd1–N1, 2.049(4); Pd1–N2, 2.036(4); Pd1–
Cl1, 2.4188(11); C6–Pd1–N1, 88.86(16); C6–Pd1–N2, 89.07(16); N1–Pd1–Cl1,
91.17(11); N2–Pd1–Cl1, 91.11(11).
chloride are trans to each other, and two b-C-donors from [Ph2nac-
nac]ꢀ are trans to each other. The Pd3–C20 and Pd3–C43 bond
lengths are 2.179(6) and 2.167(5) Å, respectively, which are longer
than other Pd-(b-C) bond lengths such as in 7a, 7b and other

320
V.T. Annibale et al. / Inorganica Chimica Acta 380 (2012) 308–321
Scheme 10. Synthesis of 8, and trace amounts of 9.
Fig. 11. Molecular structure of 9 with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) for 9: Pd1–N1, 2.000(5);
Pd1–N2, 1.984(5); Pd1–Cl1, 2.3508(15); Pd1–Cl2, 2.3321(16); Pd2–N3, 2.019(5); Pd2–N4, 2.008(5); Pd2–Cl1, 2.3263(14); Pd2–Cl2, 2.3175(16); Pd3–C35, 1.995(6); Pd3–C43,
2.167(5); Pd3-C20, 2.179(6); Pd3–Cl3, 2.3870(15); Pd4–N5, 2.000(5); Pd4–N6, 2.007(5); Pd4–Cl4, 2.3409(14); N2–Pd1–N1, 91.0(2); Cl2–Pd1–Cl1, 81.63(5); N1–Pd1–Cl2,
93.05(15); N2–Pd1–Cl1, 94.27(15); Pd2–Cl1–Pd1, 96.94(5); Pd2–Cl2–Pd1, 97.70(6); N4–Pd2–N3, 91.8(2); Cl2–Pd2–Cl1, 82.47(5); N3–Pd2–Cl2, 92.73(15); N4–Pd2–Cl1,
93.07(14); C35–Pd3-C43, 89.7(2); C35–Pd3–C20, 93.4(2); C43–Pd3–Cl3, 88.69(15); C20–Pd3–Cl3, 88.23(16); N5–Pd4–N6, 89.39(18); N5–Pd4–Cl4, 94.49(14); N6–Pd4–Cl4,
93.81(13).
examples mentioned previously [4,7,8]. Compound 9 is intriguing,
not only due to its intrinsic beauty but also because it possesses
Pd(II) centres with three carbon donors which is a rare feature,
there are some examples such as complexes with bis-carbenes
[35], or CCC-pincers [36], or bis-isocyanide ligands [37].
These chloro-bridged dimers can be used to generate mononuclear
complexes when the chloride bridge is cleaved by monodentate li-
gands such as pyridine to form complexes 4a–4c. Phenylboronic
acid can also act as pyridine abstraction reagent from complex 4a
to form complex 1a. Also these chloro-bridged dimers engage in
unusual transmetallation reactivity with arylboronic acids to form
tetrapallada-macrocycles 7a and 7b. Divergent transmetallation
reactivity was encountered when 1a was reacted with penta-
fluorophenylboronic acid, complex 8 and a trace amount of
4. Conclusions
In conclusion we have demonstrated the syntheses and charac-
terization of several [Pd(Ar2nacnac)(l-Cl)]2 complexes (1a–1c).
complex
9 formed selectively. We also demonstrated the

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Acknowledgements
We thank the Natural Science and Engineering Research Council
of Canada (NSERC), the Canadian Foundation for Innovation (CFI),
the Ontario Ministry of Research and Innovation and the University
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CCDC 849170–849181 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
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