Palladium salicylaldimine complexes derived from 2,3-dihydroxybenzaldehyde

Article abstract of DOI:10.1016/j.ica.2011.07.051

Schiff bases derived from condensation of 2,3-dihydroxybenzaldehyde with various primary amines, such as 1-adamantanamine hydrochloride, 2,6-dimethylaniline, 2,6-diethylaniline and 2,6-diisopropylaniline, react with palladium(II) acetate to give the corresponding bis(N-arylsalicylaldiminato) palladium(II) complexes. These complexes have been found to be active catalyst precursors for the Suzuki-Miyaura cross-coupling of aryl bromides and iodides with aryl boronic acids using water as a solvent.

Full text of DOI:10.1016/j.ica.2011.07.051

Inorganica Chimica Acta 377 (2011) 84–90
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Palladium salicylaldimine complexes derived from 2,3-dihydroxybenzaldehyde
a,
Eric G. Bowes a, Graham M. Lee a, Christopher M. Vogels a, Andreas Decken b, , Stephen A. Westcott
⇑⇑
⇑
a Department of Chemistry and Biochemistry, Mount Allison University, Sackville, Canada NB E4L 1G8
b Department of Chemistry, University of New Brunswick, Fredericton, Canada NB E3B 5A3
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 April 2011
Received in revised form 26 July 2011
Accepted 27 July 2011
Available online 4 August 2011
Schiff bases derived from condensation of 2,3-dihydroxybenzaldehyde with various primary amines, such
as 1-adamantanamine hydrochloride, 2,6-dimethylaniline, 2,6-diethylaniline and 2,6-diisopropylaniline,
react with palladium(II) acetate to give the corresponding bis(N-arylsalicylaldiminato)palladium(II) com-
plexes. These complexes have been found to be active catalyst precursors for the Suzuki–Miyaura cross-
coupling of aryl bromides and iodides with aryl boronic acids using water as a solvent.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Cross-coupling
Green chemistry
Palladium
Salicylaldimines
Schiff base
Suzuki–Miyaura
1. Introduction
employed. A considerable amount of research has also focused on
designing water-soluble catalysts to provide a greener and more
The use of palladium complexes to catalyze the coupling of
organoboron compounds with aryl or vinyl halides has become
one of the most widely used tools in organic synthesis. The scope
and utility of the Suzuki–Miyaura reaction has been the subject
of several excellent reviews [1–3]. Indeed, the Suzuki–Miyaura
cross-coupling reaction has become an attractive route to generat-
ing biaryls, compounds with vast applications ranging from
materials science to pharmaceutical chemistry. The use of boronic
acids in this cross-coupling process is especially advantageous,
compared to the analogous organotin species, as these boron
compounds are relatively nontoxic and thermally, air- and
moisture-stable [4].
The use of aryl chlorides as substrates and the ability to conduct
this cross-coupling reaction at low temperatures and at low
catalyst loading have greatly increased the versatility of the
Suzuki–Miyaura cross-coupling reaction. Catalyst development
has also played a significant role in improving this reaction
[5–12] as traditional palladium complexes used to catalyze this
transformation contain air-sensitive phosphine ligands. In recent
years, however, the use of catalysts containing N-heterocyclic
carbenes [13–15], oxazolines [16–18], and ‘ligandless’ systems
[19–21], as well as heterogeneous systems [22–24], have also been
environmentally benign alternative to this reaction [25–30]. In a
recent elegant study by Hong et al., it was found that (N-aryl-
salicylaldiminato)palladium(II) complexes are active catalysts for
the Suzuki–Miyaura cross-coupling reaction of a wide array of aryl
bromides [31]. However, reactions were carried out in organic
solvents (THF and toluene) and high yields were only achieved
after several days. Likewise, trinuclear triphenylphosphine Au(I)
complexes with N,N,O-tridentate unsymmetrical Schiff base li-
gands catalyzed the Suzuki cross-coupling reaction to afford
nonsymmetrical biaryls in good yields, whereas the Au(III) com-
plexes gave only arylboronic homocoupling products [32]. As part
of our ongoing investigation into designing new metal Schiff base
complexes, we have prepared several palladium complexes
derived from the condensation of 2,3-dihydroxybenzaldehyde with
various primary amines, such as 1-adamantanamine hydrochlo-
ride, 2,6-dimethylaniline, 2,6-diethylaniline and 2,6-diisopropyl-
aniline and examined their ability to be used as catalysts for the
green Suzuki–Miyaura cross-coupling reaction.
2. Experimental
2.1. General procedures
Reagents and solvents used were purchased from Aldrich
Chemicals. Palladium(II) acetate was purchased from Precious
Metals Online Ltd. (Melbourne, AU). Schiff base 1c was synthesized
as previously reported [33]. NMR spectra were recorded on a JEOL
⇑
Corresponding author.
Corresponding author regarding X-ray crystallography.
E-mail addresses: adecken@unb.ca (A. Decken), swestcott@mta.ca (S.A. West-
⇑⇑
cott).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.051

E.G. Bowes et al. / Inorganica Chimica Acta 377 (2011) 84–90
85
JNM-GSX270 FT spectrometer. 1H NMR chemical shifts are
reported in ppm and referenced to residual solvent protons in
deuterated solvent at 270 MHz. 13C NMR chemical shifts are
referenced to solvent carbon resonances as internal standards at
68 MHz and are reported in ppm. Multiplicities are reported as
singlet (s), doublet (d), triplet (t), septet (sept), multiplet (m), broad
(br), and overlapping (ov). The infrared spectra were obtained
using a Mattson Genesis FT-IR spectrometer and are reported in
cmÀ1. Melting points were determined using a Mel-Temp appara-
tus and are uncorrected. Microanalyses for C, H, and N were carried
out at Guelph Chemical Laboratories (Guelph, ON). GC/MS analyses
were conducted using a Varian Saturn 2000 GC/MS/MS coupled to
a CP-3800 GC. The GC was equipped with both the 1177 injection
port with a CP-8410 liquid autoinjector connected to an SPB-1
(CDCl3) d: 166.9, 149.7, 147.6, 145.4, 128.8, 128.7, 125.5, 123.1,
119.2, 118.3, 118.0, 18.7. IR (Nujol): 2952 (br, OH), 2871 (br, OH),
2868 (s), 1627 (m, C@N), 1520 (m), 1450 (m), 1376 (m), 1201
(m), 1023 (m), 766 (m).
2.2.5. Schiff base 1e
A
MeOH (15 mL) solution of 2,3-dihydroxybenzaldehyde
(1.00 g, 7.23 mmol) was added dropwise to a stirred MeOH
(10 mL) solution of 2,6-diethylaniline (1.07 g, 7.19 mmol). The
solution was heated at reflux for 3 h at which point the volume
was reduced to 5 mL under vacuum and hexane (20 mL) was added
to the solution. After storing the solution at 5 °C a precipitate
formed and was collected by suction filtration to afford 1e as an
orange solid. Yield: 1.56 g (81%); mp 115–118 °C. 1H NMR (CDCl3)
d: 8.32 (s, 1H, C(H)@N), 7.14–7.06 (ov m, 4H, Ar), 6.94–6.82 (ov m,
2H, Ar), 2.55 (q, J = 7.6 Hz, 4H, CH2CH3), 1.16 (t, J = 7.6 Hz, 6H,
CH2CH3). 13C{1H} NMR (CDCl3) d: 166.9, 149.5, 146.9, 145.4,
134.7, 126.8, 125.8, 123.1, 119.2, 118.3, 118.0, 25.1, 15.0. IR
(Nujol): 3444 (br, OH), 2972 (br, OH), 2953 (s), 2929 (s), 2918 (s),
2868 (s), 1623 (m, C@N), 1583 (m), 1462 (m), 1375 (m), 1103
(m), 1070 (m), 878 (m), 796 (m).
(Supelco) fused silica column (30 m  0.25 mm i.d.  0.25
lm)
and the 1079 solid injector chromatoprobe, attached to a 50 cm
transfer line. The GC/MS spectrometer is controlled by the Saturn
Workstation software, Version 5.51.
2.2. Synthesis of Schiff bases
2.2.1. Schiff base 1a
2,3-Dihydroxybenzaldehyde (0.75 g, 5.46 mmol) dissolved in
MeOH (15 mL) was added dropwise to a stirred MeOH (15 mL)
solution of 1-adamantanamine hydrochoride (1.04 g, 5.56 mmol)
and LiOH (0.14 g, 5.85 mmol). The reaction was allowed to proceed
for 3 h at which point a precipitate was collected by suction filtra-
tion to afford 1a as a yellow solid. Yield: 826 mg (56%); mp 218–
220 °C. 1H NMR (CDCl3) d: 8.15 (s, 1H, C(H)@N), 6.91 (d,
J = 8.0 Hz, 1H, Ar), 6.70 (d, J = 8.0 Hz, 1H, Ar), 6.51 (t, J = 8.0 Hz,
1H, Ar), 1.97–1.93 (ov m, 15H, adamantyl). 13C{1H} (CDCl3) d:
162.6, 161.7, 147.9, 122.1, 115.3 (2C), 114.7, 68.7, 37.6, 37.1,
34.2, 31.9, 27.5, 27.0. IR (Nujol): 3163 (br, OH), 2924 (s), 2854
(s), 1644 (m, C@N), 1544 (m), 1513 (m), 1461 (m), 1394 (m),
1215 (m), 1025 (m), 741 (m).
2.2.6. Schiff base 1f
A
MeOH (15 mL) solution of 2,3-dihydroxybenzaldehyde
(0.79 g, 5.68 mmol) was added dropwise to a stirred MeOH
(10 mL) solution of 2,6-diisopropylaniline (1.01 g, 5.70 mmol).
The reaction mixture was heated at reflux for 4 h at which point
the reaction volume was reduced to 5 mL under vacuum and the
solution stored at 5 °C. A solid precipitated and was collected by
suction filtration to afford 1f as an orange solid. Yield: 1.59 g
(94%); mp 138–141 °C. 1H NMR (CDCl3) d: 8.28 (s, 1H, C(H)@N),
7.20–7.07 (ov m, 4H, Ar), 6.96–6.82 (ov m, 2H, Ar), 2.99 (sept,
J = 7.0 Hz, 2H, CH(CH3)2), 1.19 (d, J = 7.0 Hz, 12H, CH(CH3)2).
13C{1H} d: 167.0, 149.6, 145.7, 145.5, 139.2, 126.1, 123.6, 123.2,
119.3, 118.3, 118.2, 28.4, 23.8, 22.7. IR (Nujol): 3462 (br, OH),
2948 (br, OH), 2917 (s), 2911 (s), 2865 (s), 1622 (m, C@N), 1584
(m), 1469 (m), 1272 (m), 1205 (m), 1101 (m), 1059 (m), 933 (m),
869 (m).
2.2.2. Schiff base 1b
A
MeOH (15 mL) solution of 2,3-dihydroxybenzaldehyde
(0.96 g, 6.94 mmol) was added to a stirred solution of p-aminophe-
nol (0.76 g, 6.93 mmol) in MeOH (10 mL). The reaction mixture
was heated at reflux for 4 h at which point a precipitate formed
and was collected by suction filtration to afford 1b as a purple so-
lid. Yield: 1.32 g (83%); mp 194–196 °C. 1H NMR ((CD3)2CO) d: 8.84
(s, 1H, C(H)@N), 7.35 (d, J = 8.8 Hz, 2H, Ar), 7.07–6.77 (ov m, 5H,
Ar). 13C{1H} ((CD3)2CO) d: 160.9, 157.2, 149.5, 145.9, 140.2, 123.0,
122.7, 119.6, 118.9, 118.0, 116.3. IR (Nujol): 2969 (br, OH), 2932
(br, OH), 2924 (s), 2907 (s), 2874 (s), 1623 (m, C@N), 1458 (m),
1376 (m), 1223 (m), 1169 (m), 831 (m), 733 (m).
2.3. Synthesis of palladium complexes
2.3.1. Complex 2a
Palladium(II) acetate (200 mg, 0.90 mmol) was added to a solu-
tion of (E)-3-((adamantylimino)methyl)benzene-1,2-diol (502 mg,
1.85 mmol) in EtOH (50 mL). The solution was heated at reflux
for 2 h, at which point a brick-red solid was collected by suction fil-
tration and washed with EtOH (3 Â 10 mL) and Et2O (3 Â 10 mL) to
afford 2a. Yield: 478 mg (82%); mp 240–243 °C (decomposition).
Anal. Calc. for C34H40N2O4Pd (647.18): C, 63.10; H, 6.24; N, 4.33.
Found: C, 63.55; H, 6.38; N, 4.44%. 1H NMR (CDCl3) d: 13.02 (br s,
OH, 2H), 12.98 (br s, 2H, OH), 7.99 (s, 2H, N@CH), 7.94 (s, 2H,
N@CH), 6.83 (d, J = 7.4 Hz, 2H, Ar), 6.74 (d, J = 7.4 Hz, 2H, Ar),
6.63 (d, J = 7.4 Hz, 2H, Ar), 6.49–6.39 (ov m, 4H, Ar), 6.29 (d,
J = 7.4 Hz, 2H, Ar), 2.24–2.17 (br ov m, 24H, adamantyl), 1.97–
1.94 (br ov m, 12H, adamantyl), 1.81–1.65 (br ov m, 24H, adaman-
tyl). 13C{1H} NMR (CDCl3) d: 173.1, 166.3, 160.8, 157.5, 151.5,
146.9, 124.2, 121.3, 119.7, 118.1, 117.0, 115.1, 114.9, 112.2, 65.0,
56.5, 43.0, 42.7, 36.2, 35.6, 30.4, 29.1. IR (Nujol): 3426 (br, OH),
2949 (s), 2926 (s), 1590 (m, C@N), 1458 (m), 1256 (m), 1111 (m),
1026 (m), 881 (m), 740 (m).
2.2.3. Schiff base 1c
1H NMR (CDCl3) d: 8.58 (s, 1H, C(H)@N), 7.27 (d, J = 8.2 Hz, 2H,
Ar), 7.05–6.77 (ov m, 5H, Ar), 3.85 (s, 3H, OCH3). 13C{1H} NMR
(CDCl3) d: 160.3, 159.2, 149.9, 145.4, 140.6, 122.9, 122.4, 119.0,
118.7, 117.4, 114.9, 55.8. IR (Nujol): 2973 (br, OH), 2935 (br, OH),
2912 (s), 2869 (s), 1613 (m, C@N), 1462 (m), 1381 (m), 1218 (m),
1170 (m), 837 (m), 735 (m).
2.2.4. Schiff base 1d
A
MeOH (15 mL) solution of 2,3-dihydroxybenzaldehyde
(1.16 g, 8.39 mmol) was added dropwise to a stirred MeOH
(10 mL) solution of 2,6-dimethylaniline (1.01 g, 8.34 mmol). The
reaction mixture was heated at reflux for 3 h. Upon cooling the
solution to 5 °C, large red crystals precipitated and were collected
by suction filtration to afford 1d. Yield: 1.67 g (83%); mp 104–
106 °C. 1H NMR (CDCl3) d: 8.32 (s, 1H, C(H)@N), 7.14–7.03 (ov m,
4H, Ar), 6.95–6.81 (ov m, 2H, Ar), 2.22 (s, 6H, CH3). 13C{1H} NMR
2.3.2. Complex 2b
Palladium(II) acetate (250 mg, 1.12 mmol) was added to a solu-
tion of (E)-3-((4-hydroxyphenylimino)methyl)benzene-1,2-diol
(529 mg, 2.31 mmol) in EtOH (50 mL). The mixture was heated at
reflux for 2 h, at which point a brown solid was collected by

86
E.G. Bowes et al. / Inorganica Chimica Acta 377 (2011) 84–90
suction filtration and washed with EtOH (3 Â 10 mL) and Et2O
(3 Â 5 mL) to afford 2b. Yield: 509 mg (81%); mp 290–294 °C
(decomposition). Anal. Calc. for C26H20N2O6Pd (562.90): C, 55.47;
H, 3.59; N, 4.98. Found: C, 55.65; H, 3.68; N, 5.12%. 1H NMR (CDCl3)
d: 9.74 (s, 2H, OH), 8.06 (s, 2H, N@CH), 7.24 (d, J = 8.6 Hz, 4H, Ar),
6.96 (d, J = 7.6 Hz, 2H, Ar), 6.84 (d, J = 8.6 Hz, 4H, Ar), 6.68 (d,
J = 7.6 Hz, 2H, Ar), 6.42 (t, J = 7.6 Hz, 2H, Ar), 4.97 (s, 2H, OH).
13C{1H} NMR (CDCl3) d: 164.3, 156.7, 152.1, 147.0, 140.8, 125.8,
125.4, 119.1, 115.9, 115.8, 115.5. IR (Nujol): 3369 (br, OH), 3316
(br, OH), 2930 (s), 2895 (s), 1595 (m, C@N), 1452 (m), 1377 (m),
1210 (m), 1097 (m), 898 (m), 739 (m).
2.3.6. Complex 2f
Palladium(II) acetate (250 mg, 1.12 mmol) was added to a solu-
tion of (E)-3-((2,6-diisopropylphenylimino)methyl)benzene-1,2-
diol (687 mg, 2.31 mmol) in EtOH (50 mL). The mixture was heated
at reflux for 2 h, at which point an orange–brown precipitate was
collected by suction filtration. The solid was dissolved in hot
CH2Cl2 (10 mL) and stored at 5 °C for 3 days. The resulting orange
precipitate was collected by suction filtration to afford 2f. Yield:
666 mg (85%); mp 325–328 °C (decomposition). Anal. Calc. for
C
38H44N2O4Pd (699.26): C, 65.27; H, 6.36; N, 4.01. Found: C,
65.41; H, 6.49; N, 4.28%. 1H NMR (CDCl3) d: 7.59 (s, 2H, N@CH),
7.46 (dd, J = 7.9, 7.4 Hz, 2H, Ar), 7.30 (d, J = 7.7 Hz, 4H, Ar), 6.76
(dd, J = 7.4, 1.5 Hz, 2H, Ar), 6.71 (dd, J = 7.9, 1.5 Hz, 2H, Ar), 6.45
(t, J = 7.7 Hz, 2H, Ar), 4.62 (s, 2H, OH), 3.50 (sept, J = 6.9 Hz, 4H,
CH(CH3)2), 1.30 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.18 (d, J = 6.9 Hz,
12H, CH(CH3)2). 13C{1H} NMR (CDCl3) d: 163.1, 153.1, 147.5,
144.0, 142.4, 127.7, 124.3, 123.8, 118.2, 116.1, 115.6, 28.6, 24.3,
23.3. IR (Nujol): 3432 (br, OH), 2939 (s), 2882 (s), 1600 (m,
C@N), 1457 (m), 1377 (m), 1230 (m), 1081 (m), 778 (m), 725 (m).
2.3.3. Complex 2c
Palladium(II) acetate (250 mg, 1.12 mmol) was added to a solu-
tion of (E)-3-((4-methoxyphenylimino)methyl)benzene-1,2-diol
(550 mg, 2.26 mmol) in EtOH (50 mL). The mixture was heated at
reflux for 2 h, at which point an orange solid was collected by suc-
tion filtration and washed with EtOH (3 Â 5 mL) and Et2O
(3 Â 5 mL) to afford 2c. Yield: 525 mg (79%); mp 276–278 °C
(decomposition). Anal. Calc. for C28H24N2O6Pd (590.96): C, 56.90;
H, 4.10; N, 4.74. Found: C, 57.11; H, 4.25; N, 5.03%. 1H NMR (CDCl3)
d: 7.72 (s, 2H, N@CH), 7.29 (d, J = 8.4 Hz, 4H, Ar), 6.99 (d, J = 8.4 Hz,
4H, Ar), 6.76–6.70 (ov m, 4H, Ar), 6.45 (t, J = 7.4 Hz, 2H, Ar), 4.92 (s,
2H, OH), 3.87 (s, 6H, OCH3). IR (Nujol): 3393 (br, OH), 2970 (s),
2881 (s), 1602 (m, C@N), 1547 (m), 1457 (m), 1377 (m), 1246
(m), 1181 (m), 864 (m), 744 (m).
2.4. Suzuki–Miyaura cross-coupling reactions
The appropriate palladium complex (2 mol%) was added to a
mixture of an aryl halide (1 mmol), a boronic acid (1 mmol), and
K2CO3 (2 mmol) in H2O (10 mL). The reaction mixture was heated
at reflux for 4 hrs. The organic phase was extracted with CH2Cl2
(2 Â 10 mL), dried over MgSO4, and the solution concentrated un-
der vacuum. The coupling products were characterized by 1H and
13C NMR spectroscopy and GC–MS.
2.3.4. Complex 2d
Palladium(II) acetate (250 mg, 1.12 mmol) was added to a solu-
tion of (E)-3-((2,6-dimethylphenylimino)methyl)benzene-1,2-diol
(557 mg, 2.31 mmol) in EtOH (50 mL) and the solution was heated
at reflux for 2 h. An orange solid was collected by suction filtration
and washed with EtOH (3 Â 5 mL). The resulting orange powder
was dissolved in hot CH2Cl2 (10 mL) and stored at 5 °C. The result-
ing red-orange precipitate was collected by suction filtration to af-
ford 2d. Yield: 550 mg (84%); mp 316 °C (decomposition). Anal.
Calc. for C30H28N2O4Pd (587.02): C, 61.38; H, 4.82; N, 4.77. Found:
C, 61.56; H, 4.71; N, 4.89%. 1H NMR (CDCl3) d: 7.57 (s, 2H, N@CH),
7.33–7.21 (ov m, 6H, Ar), 6.76 (dd, J = 7.4, 1.5 Hz, 2H, Ar), 6.72 (dd,
J = 8.2, 1.5 Hz, 2H, Ar), 6.46 (t, J = 8.0 Hz, 2H, Ar), 4.69 (s, 2H, OH),
2.40 (s, 12H, CH3). 13C{1H} NMR (CDCl3) d: 163.4, 153.2, 147.5,
146.9, 132.1, 128.4, 127.0, 124.4, 118.7, 116.2, 115.7, 19.0. IR
(Nujol): 3427 (br, OH), 2959 (s), 2901 (s), 1605 (m, C@N), 1550
(m), 1451 (m), 1310 (m), 1235 (m), 1172 (m), 1084 (m), 890 (m),
738 (m).
2.5. X-ray crystallography
Crystals of 1a were grown from a 1:1 mixture of THF and CH2Cl2
at 5 °C, 1b from a saturated acetone solution at 5 °C, 1d from a sat-
urated hexane solution at 5 °C, and 2d from a 1:1 solution of
CH2Cl2 and hexane at 5 °C. Single crystals were coated with Par-
atone-N oil, mounted using a polyimide MicroMount and frozen
in the cold stream of the goniometer. A hemisphere of data were
collected on a Bruker AXS P4/SMART 1000 diffractometer using
x
and h scans with a scan width of 0.3° and an exposure times
of 10 (1b, 1d) and 20 (1a, 2d) s. The detector distances were
5 cm. The data were reduced [34] and corrected for absorption
[35]. The structures were solved by direct methods and refined
by full-matrix least squares on F2 [36]. Data collection for 1d was
carried out at 223 K as lower temperatures resulted in the loss of
crystallinity. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were located in Fourier difference maps and re-
fined isotropically.
2.3.5. Complex 2e
Palladium(II) acetate (250 mg, 1.12 mmol) was added to a solu-
tion of (E)-3-((2,6-diethylphenylimino)methyl)benzene-1,2-diol
(606 mg, 2.25 mmol) in EtOH (50 mL). The mixture was heated at
reflux for 2 h, at which point an orange-brown solid was collected
by suction filtration. The solid was dissolved in hot CH2Cl2 (10 mL)
and stored at 5 °C for 2 days. The resulting orange precipitate was
collected by suction filtration to afford 2e. Yield: 585 mg (81%);
mp 322–325 °C (decomposition). Anal. Calc. for C34H36N2O4Pd
(643.12): C, 63.49; H, 5.65; N, 4.36. Found: C, 63.58; H, 5.78; N,
4.55%. 1H NMR (CDCl3) d: 7.59 (s, 2H, N@CH), 7.41 (dd, J = 7.9,
7.1 Hz, 2H, Ar), 7.26 (d, J = 8.0 Hz, 4H, Ar), 6.76 (dd, J = 7.1, 1.5 Hz,
2H, Ar), 6.71 (dd, J = 7.9, 1.5 Hz, 2H, Ar), 6.45 (t, J = 8.0 Hz, 2H,
Ar), 4.67 (s, 2H, OH), 2.82 (m, 8H, CH2CH3), 1.28 (t, J = 7.7 Hz,
12H, CH2CH3). 13C{1H} NMR (CDCl3) d: 163.6, 153.1, 147.5, 145.7,
137.7, 127.4, 126.4, 124.3, 118.3, 116.1, 115.6, 24.9, 14.4. IR (Nu-
jol): 3447 (br, OH), 2969 (s), 2870 (s), 1605 (m, C@N), 1548 (m),
1377 (m), 1227 (m), 1087 (m), 861 (m).
3. Results and discussion
3.1. Salicylaldimines
Schiff bases are remarkable compounds that have been utilized
extensively in organic syntheses. In this present study, we have
found that condensation of 2,3-dihydroxybenzaldehyde with vari-
ous primary amines, such as 1-adamantanamine hydrochloride,
2,6-dimethylaniline, 2,6-diethylaniline and 2,6-diisopropylaniline
gave the corresponding salicylaldimines in high yields (Scheme 1).
Schiff bases have been characterized by multinuclear NMR
spectroscopy, FT-IR spectroscopy, melting point, and by X-ray
diffraction. Adamantanamine was chosen as a representative alkyl
derivative that would be stable to air and water. The crystal struc-
ture of (E)-3-((4-methoxyphenylimino)methyl)benzene-1,2-diol,
1c, has recently been studied by single-crystal X-ray diffraction

E.G. Bowes et al. / Inorganica Chimica Acta 377 (2011) 84–90
87
Scheme 1. Synthesis of salicylaldmines derived from condensation of 2,3-dihydroxybenzaldehyde with various primary amines.
and solid state NMR experiments, where the detailed structural
analysis reveals the presence of four tautomeric structures
distance of 1.414(3) Å implies a significant contribution from the
enamine (NHCH@C) form in the solid state.
´
[33,37]. Indeed, a recent paper by Wozniak and co-workers has
The aryl derivatives 1b–f all give similar spectroscopic solution
data to that found for 1a, suggesting the imine form is predomi-
nant under these conditions. We have also characterized 1b and
1d by single X-ray diffraction studies and their structures are
shown in Fig. 2. For compound 1d, there are four independent mol-
ecules in the asymmetric unit that differ in the orientation of the
arene rings. An extensive network of hydrogen bonding is also
present in the crystal lattice, as is frequently observed in these
compounds [38]. Bond distances and angles are similar to those
observed in 1a and are consistent with a mixture of the imine
and the enamine tautomers. As expected, considerable hydrogen
bonding is observed in these species.
shown that in solution the predominant form of these salicylaldi-
mines is the imine-enol tautomer form. However, in the solid state
the tautomeric equilibrium is shifted and the NH is present as the
major species [38]. As expected, Schiff base derivatives 1a–f have a
shift for the aldehyde proton from 10 to ca. 8.3 ppm in the 1H NMR
spectra and a resonance at ca. 160 ppm in the 13C NMR spectra for
the N@CH methine carbon, consistent with a traditional imine
form in solution. Compound 1a has also been characterized by a
single crystal X-ray diffraction study, the molecular structure of
which is shown in Fig. 1, along with pertinent bond distances
and angles. Crystallographic data for 1a and other structures are
found in Table 1. The asymmetric unit contains two independent
molecules that differ in the orientation of the adamantyl group.
Not surprisingly, the crystal packing shows extensive intermolecu-
lar hydrogen bonding. The ‘imine’ bond, C(1)–N(1), distance is
1.300(2) Å, which is significantly shorter than the adjacent C(8)–
N(1) bond of 1.467(2) Å, still retains a significant amount of double
bond character, indicative of a true Schiff base and is similar to the
data reported for 1c [33]. However, the two ‘hydroxy’ groups have
noticeably different bond distances of C(3)–O(1) 1.299(2) and
C(4)–O(2) 1.364(2) Å, where the former shows significant C@O
double bond contribution. Also, the relatively short C(1)–C(2) bond
3.2. Metal complexes
We have found that addition of 1a–f to Pd(OAc)2 afforded bis
(N-arylsalicylaldiminato)palladium(II) complexes (Scheme 2) in
high yields (ꢀ80%). Complexes 2a–f have been characterized by a
number of physical methods, including multinuclear NMR spec-
troscopy. A significant upfield shift in the 1H NMR spectra is
observed for the imine methine proton, from ca. 8.3 ppm to ca.
7.7 ppm, upon coordination of the ligand to the d8 metal centre.
The diagnostic C@N stretching band in the FT-IR spectra has also
shifted from ca. 1620 cmÀ1 to ca. 1595 cmÀ1 upon coordination
to the metal centre [39]. Elemental analyses are also consistent
with bis-Schiff base formulation. Complex 2a appears to be a mix-
ture of two isomers, as observed by multinuclear NMR
spectroscopy.
Although we were unable to get single crystals of 2a–c, complex
2d has been characterized by a single crystal X-ray diffraction
study (Fig. 3). The environment about Pd is roughly square planar.
The Pd–O and Pd–N distances of 1.985(2) Å and 2.016(2) Å, respec-
tively, are similar to those seen in related complexes [40–42] and
are expected for a typical Schiff base ligand (containing a short
C@N bond distance of 1.295(3) Å) coordinated to a metal centre,
where the imine form is predominant.
3.3. Catalytic activity
As mentioned previously, Hong et al. found that cross-coupling
reactions of arylbromides and phenyl boronic acids could be
catalyzed by Pd(OAc)2 and 2-[1-(2,4,6-trimethylphenylimi-
no)ethyl]phenol (HL). Reactions were carried out in organic sol-
vents (THF and toluene) at room temperature and high yields of
the desired products were only achieved after long periods
Fig. 1. The molecular structure of one of the independent molecules of 1a with
ellipsoids drawn at the 50% probability level. Hydrogen atoms omitted for clarity.
Selected bond lengths (Å) and angles (°): C(1)–N(1) 1.300(2), C(1)–C(2) 1.414(3),
C(2)–C(3) 1.427(3), C(3)–O(1) 1.299(2), C(3)–C(4) 1.424(3), C(4)–O(2) 1.364(2),
C(8)–N(1) 1.467(2), C(8)–C(9) 1.528(3); N(1)–C(1)–C(2) 122.60(19), C(1)–N(1)–C(8)
123.77(18), O(1)–C(3)–C(4) 120.22(17), O(2)–C(4)–C(5) 119.44(18).

88
E.G. Bowes et al. / Inorganica Chimica Acta 377 (2011) 84–90
Table 1
Crystallographic data collection parameters for 1a, 1b, 1d and 2d.
Complex
1a
1b
1d
2d
Chemical formula
C
17H21NO2
C13H11NO3
229.23
0.60 Â 0.20 Â 0.05
monoclinic
P2(1)/c
C15H15NO2
241.28
0.50 Â 0.45 Â 0.20
monoclinic
C2/c
C30H28N2O4Pd
586.94
0.25 Â 0.15 Â 0.12
monclinic
P2(1)/n
Formula mass
271.35
Crystal dimensions (mm3)
0.30 Â 0.30 Â 0.15
orthorhombic
Pbca
Crystal system
Space group
Z
16
8
32
2
a (Å)
b (Å)
c (Å)
10.5400(15)
12.4581(18)
42.926(6)
90
12.495(4)
8.356(3)
20.097(6)
90
21.462(5)
21.379(5)
23.168(6)
90
9.7635(18)
9.8597(18)
13.427(3)
90
a
(°)
b (°)
90
90
97.627(4)
90
2079.9(11)
1.464
112.370(5)
90
9830(4)
1.304
94.067(2)
90
1289.3(4)
1.512
c
(°)
V (Å3)
5636.5(14)
1.279
198(1)
qcalcd (mg mÀ3
T (K)
)
173(1)
223(1)
173(1)
Radiation
Mo K
a
(k = 0.71073 Å)
Mo K
a
(k = 0.71073 Å)
Mo K
a
(k = 0.71073 Å)
Mo Ka (k = 0.71073 Å)
l
(mmÀ1
)
0.083
36 870
6418
529
0.95–27.50
1.101
0.0464
0.1183
0.256 and À0.192
0.105
13 836
4591
395
2.04–27.50
1.111
0.0412
0.1101
0.273 and À0.193
0.087
32 380
11 004
889
1.46–27.50
1.019
0.0456
0.1396
0.274 and À0.201
0.759
8623
2876
172
2.50–27.50
1.152
0.0342
0.1020
1.721 and À1.182
Total reflections
Unique reflections
No. of variables
h (°)
Goodness-of-fit (GOF) on F2
a
R1 (I > 2
r
(I))
b
wR2 (all data)
Largest diff. Peak and hole (e ÅÀ3
)
P
P
a
R1
=
||Fo| À |Fc||/ |Fo|.
P
P
wR2 = ( [w(Fo À Fc2)2]/ [Fo4])1/2
,
where w = 1/[r r r +
2(Fo2) + (0.0301*P)2 + (3.7324*P)] (1a), = 1/[ 2(Fo2) + (0.0268*P)2 + (1.3172*P)] (1b), = 1/[ 2(Fo2) + (0.0661*P)2
b
2
(0.9072*P)] (1d), and = 1/[
r
2(Fo2) + (0.0499*P)2 + (0.3022*P)] (2d), where P = (max (Fo2, 0) + 2*Fc2)/3.
Fig. 2. (a) The molecular structure of one of the independent molecules of 1b with ellipsoids drawn at the 50% probability level. Hydrogen atoms omitted for clarity. (b) The
molecular structure of one of the independent molecules of 1d with ellipsoids drawn at the 50% probability level. Hydrogen atoms omitted for clarity. Selected bond lengths
(Å) and angles (°) for 1b, C(1)–C(7) 1.428(3), C(2)–O(1) 1.321(2), C(2)–C(3) 1.425(3), C(3)–C(4) 1.369(3), C(3)–O(2) 1.372(2), C(7)–N(1) 1.300(2), C(8)–N(1) 1.422(2); C(2)–
C(1)–C(7) 120.27(17), O(1)–C(2)–C(1) 122.45(17), O(1)–C(2)–C(3) 120.24(18), N(1)–C(7)–C(1) 122.01(19), C(7)–N(1)–C(8) 125.60(17). Selected bond lengths (Å) and angles
(°) for 1d, C(1)–N(1) 1.307(2), C(1)–C(2) 1.410(2), N(1)–C(8) 1.425(2), C(3)–O(1) 1.3028(18), C(4)–O(2) 1.3678(18); N(1)–C(1)–C(2) 122.00(15), C(1)–N(1)–C(8) 127.80(14),
O(1)–C(3)–C(4) 120.19(14), O(1)–C(3)–C(2) 122.40(14).
Scheme 2. Synthesis of Schiff base palladium complexes 2a–f.
of time (days). Interestingly, only one molar equivalent of the N,O-
bidentate Schiff base ligand was used in this study, although the
catalyst resting state appears to be the corresponding bis Schiff
base compound PdL2. Reduced catalytic activity was observed for
reactions using this bis Schiff base compound as the active catalyst
precursor was believed to be the mono Schiff base compound
PdL(OAc)(solvent). It is also possible that palladium nanoparticles,
formed by decomposition of the catalyst precursor during the reac-

E.G. Bowes et al. / Inorganica Chimica Acta 377 (2011) 84–90
89
4. Conclusion
In summary, we have prepared a series of (N-arylsalicylaldimi-
nato)palladium(II) complexes derived from condensation with
2,3-dihydroxybenzaldehyde and various primary amines, such as
1-adamantanamine hydrochloride, 2,6-dimethylaniline, 2,6-dieth-
ylaniline and 2,6-diisopropylaniline and examined their ability to
catalyze the Suzuki–Miyaura cross-coupling reaction. While no
significant activity is observed with aryl chlorides, these complexes
catalyze the coupling of both aryl bromides and aryl iodides with
phenylboronic acid in water in moderate yield. Although these
results are promising, further work is needed to make other water
soluble catalyst precursors that will prove to be more durable and
effective in these catalyzed cross-coupling reactions.
Acknowledgements
Thanks are gratefully extended to the Canada Research Chairs
Programme, Canadian Foundation for Innovation-Atlantic Innova-
tion Fund, Natural Science and Engineering Research Council of
Canada, and Mount Allison University for financial support. We
also thank Shrub Durant for his expert technical assistance and
anonymous reviewers for helpful comments.
Fig. 3. The molecular structure of 2d with ellipsoids drawn at the 50% probability
level. Hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°):
Pd(1)–O(1) 1.985(2), Pd(1)–N(1) 2.016(2), N(1)–C(1) 1.295(3), N(1)–C(8) 1.441(4),
O(1)–C(3) 1.320(3), C(4)–O(2) 1.366(4); O(1)–Pd(1)–N(1) 92.11(9), C(1)–N(1)–C(8)
116.4(2), C(1)–N(1)–Pd(1) 124.30(19), C(8)–N(1)–Pd(1) 119.24(17), C(3)–O(1)–
Pd(1) 124.4(2).
Appendix A. Supplementary material
Table 2
CCDC 807117, 807118, 807119, and 807120 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supple-
mentary data associated with this article can be found, in the
online version, at doi:10.1016/j.ica.2011.07.051.
Suzuki–Miyaura cross-coupling reactions catalyzed by complexes 2a–f.
a
Entry
X
Y
Yield (%)b
References
1
2
3
4
5
I
CH3
CH3
CH3
OMe
F
60
60
0
75
10
Br
Cl
I
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