Cyclometallation, steric and electronic tendencies in a series of Pd(II) complex pre-catalysts bearing imidazole-phenol ligands and effects on Suzuki-Miyaura catalytic efficiencies

Article abstract of DOI:10.1016/j.molcata.2015.06.001

Abstract A series of new structurally and electronically diversified palladium complexes derived from seven less sterically crowded 2-(4,5-dimethyl-1H-imidazol-2-yl) phenol/2-(4,5-diethyl-1H-imidazol-2-yl) phenol ligands and fourteen 2-(4,5-diphenyl-1H-im

Full text of DOI:10.1016/j.molcata.2015.06.001

Accepted Manuscript
Title: Cyclometallation, Steric and Electronic Tendencies In a
Series of Pd(II) Complex Pre-catalysts Bearing
Imidazole-phenol Ligands and Effects on Suzuki-Miyaura
Catalytic Efficiencies
Author: Abiodun O. Eseola Helmar Gorls Joseph A.O. Woods
Winfried Plass
PII:
S1381-1169(15)00212-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.06.001
MOLCAA 9514
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
19-3-2015
30-5-2015
1-6-2015
Please cite this article as: Abiodun O.Eseola, Helmar Gorls, Joseph A.O.Woods,
Winfried Plass, Cyclometallation, Steric and Electronic Tendencies In a Series of
Pd(II) Complex Pre-catalysts Bearing Imidazole-phenol Ligands and Effects on
Suzuki-Miyaura Catalytic Efficiencies, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.06.001
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Cyclometallation, Steric and Electronic Tendencies In a Series of Pd(II)
Complex Pre-catalysts Bearing Imidazole-phenol Ligands and Effects on
Suzuki-Miyaura Catalytic Efficiencies
§
,‡*
‡
Abiodun O. Eseola bioduneseola@hotmail.com bioduneseola@run.edu.ng, Helmar Gorls ,
П
‡
Joseph A. O. Woods , Winfried Plass *sekr.plass@uni-jena.de
Materials Chemistry Group, Department of Chemical Sciences, Redeemer’s University,
§
Redemption City, Ogun State, Nigeria.
‡
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena,
Humboldtstr. 8, D-07743 Jena, Germany.
П
Inorganic Chemistry Unit, Department of Chemistry, University of Ibadan, Ibadan – Nigeria.
∗
Corresponding author. Tel.: +234(0)7062192104 (Abiodun O. Eseola); Tel.: +49 3641 948130;
Fax: +49 3641948132 (Winfried Plass).
1

Graphical Abstract
2

Highlights
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Structural evidences were obtained for unfavourable cyclometallation of phenyls.
Ligand substituent electronic effects seem to affect cyclometallation tendency.
Results suggest that cyclometallation caused poor catalytic prospects.
Ligands with hemilabile or sterically distorted donors gave good catalytic outcome.
Bulky ligand framework is not a necessity for high coupling efficiency.
Catalyst developers should avoid coordinating carbons in the vicinity of palladium.
Highlights
II
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Structural evidences were obtained for cyclometallation of nearby phenyls to Pd .
Results suggest that cyclometallation largely cut down catalytic prospects.
Electronic effects of ligand substituents seem to affect cyclometallation tendency.
Bidentate ligands with hemilabile or sterically hindered donors are beneficial.
Bulky ligand framework is seen not to be a necessity for high coupling efficiency.
3

Abstract
A series of new structurally and electronically diversified palladium complexes derived
from seven less sterically crowded 2-(4,5-dimethyl-1H-imidazol-2-yl)phenol / 2-(4,5-diethyl-1H-
imidazol-2-yl)phenol ligands and fourteen 2-(4,5-diphenyl-1H-imidazol-2-yl)phenols / 2-(1H-
phenanthro[9,10-d]imidazol-2-yl)phenols bearing varying degrees of higher steric bulk have
been prepared and characterized. While single crystals grown from precipitated products of
complexation reactions confirmed the bis-ligand Pd(N^O) coordination, few crystals obtained
2
from reaction filtrates involving the 2-(4,5-diphenyl-1H-imidazol-2-yl)phenols provided
evidence for formation of N^O^C chelation species achieved by cyclometallation. Results from
structural analyses and catalytic outcomes generally indicate that desirable variables on the
ligand frameworks for obtaining superior catalyst activities either provides hemilabile or
sterically strained chelation characters, which would both favour generation of monodentate
coordination species at the catalysis temperature. In particular, correlation was observed between
tendency for cyclometallation in the palladium complexes and poor Suzuki-Miyaura catalytic
prospects. Based on hopeful activity obtained for the complex bearing 4-bromo-2-(4,5-dimethyl-
1
H-imidazol-2-yl)phenol, it was also concluded that sterically bulky ligand is not a necessity for
high coupling efficiency, while presence of potentially cyclometallating substituent moieties in
the vicinity of the palladium centre may in fact destroy catalytic prospects.
Keywords: Organometallic cyclometallation; palladacycle; Homogeneous catalysis; Structure-
Property Correlation; Suzuki-Miyaura coupling
4

Introduction
In the past few decades, palladium compounds have been keenly investigated and
respected for their versatile catalytic roles in synthetic molecular sciences. They provide cheap
and seamless catalytic access to many C-C coupling reactions, which were either only imagined
1
-8
or too expensive to achieve in terms of time, multi-step reactions involved and resources.
Therefore, industrial and academic research fields concerned with rigorous organic synthetic
manipulations such as in drug design, natural product total synthesis, synthetic materials sciences
and supramolecular architectural designs have come to recognize Pd-catalyzed C-C coupling
9
-18
strategies as invaluable molecular building tools.
Some notably efficient catalytic compounds
1
9
have already been commercialized; e.g. PEPPSI™. Suzuki-Miyaura reaction, which is the
direct cross-coupling of an aryl halide with boron-containing reagents in the presence of
palladium catalyst, is arguably one of the easiest and most widely applied C-C cross-coupling
2
0-26
method.
Numerous forms of palladium species have been studied as pre-catalysts for Suzuki-
Miyaura cross-coupling and nearly all reported forms of palladium materials such as soluble
ligand-free palladium species, discrete soluble palladium complexes, palladium-exchanged
oxides, palladium nano- and macro-particles, etc., have been presented as potentially highly
2
7
active catalyst species. Hence, the true homogeneous / heterogeneous nature of the active
2
7-32
species has continued to be a subject of debate.
For instance, while some recent reports
suggested that unprecedented catalyst performance was encountered for soluble Pd(0) species
5
,31
bearing monodentate ligands with sterically bulky side arms,
activities would be obtained with ligandless palladium species.
investigations on Pd-catalyzed C-C cross-coupling catalysis, research activities have continued
other views believe that high
2
9-30
Hence, despite decades of
5

to generate contributions aimed at improving the understanding of the role of the various forms
of palladium species in C-C coupling catalysis. Recent research outputs have progressively
narrowed down on palladium species that could be considered to possess superior activity.
Homogeneous catalytic examination of rationally structured series of complexes is an
important means towards improving the understanding of the catalyst active site configurations.
Such depth of study is less possible with heterogeneously supported catalysis due to less defined
knowledge of molecular-level characteristics for surface supported materials among other set-
3
3-37
becks.
Moreover, investigations involving reasonable number of palladium complex pre-
catalysts derived from ligands with known and systematically varied electronic donor trends /
2
6,34,38,39,40
steric directionalities is scarce.
In recent studies, experimental donor strength determinations coupled with DFT
calculations revealed that our library of 2-(1H-imidazol/oxazol-2-yl)phenols and 2-(1H-
imidazol/oxazol-2-yl)pyridines are hemilabile such that, while the phenolic oxygen-donor atoms
or pyridyl nitrogen have fairly good donor characteristics, donor strengths of the imidazole /
ozazole arm of the bidentate ligands could be varied from very poorly donating to fairly strongly
donating. Furthermore, the azole donor strengths were observed to be influenced by the nature of
substituents directly bonded to the azole rings as well as on those remotely located on the 2-aryl-
substituent of the azole ring. In particular, oxazole moieties in these ligand families were
4
1-45
observed to be poorer donors compared to imidazoles.
On the basis of knowledge derived
3
9,46
from observed structural results of some members of the 2-(1H-imidazol-2-yl)phenol series
4
2,43,45,47
and donor strength studies
conducted for the ligand family, it was anticipated that a
relatively wide variations in coordination / chelation preference would exist for palladium
complexes of the 2-(1H-imidazol-2-yl)phenols. Furthermore, it was anticipated that, with
6

systematic variation on ligand electronic, steric and chelation characters, some information about
the configuration of active palladium species would be learned.
Consequently, the aims of this study are to rationally synthesize and characterize more of
the 2-(1H-imidazol-2-yl)phenol series, prepare their corresponding palladium(II) complex pre-
catalysts and to study the dependence of their Suzuki-Miyaura coupling efficiencies on the
coordinated ligand frameworks (Scheme 1(a) – (c)). We herein present the results for synthesis,
characterization and catalytic performance of a new series of palladium(II) complexes derived
from structurally and electronically diverse ligand frameworks (Scheme 1). On the basis of X-
ray crystal structures of palladium species that crystallized from the filtrates of some
complexation reactions, the observed cyclometallation tendency of the 4,5-diphenyl-imidazolyl
ligand analogues and its poisoning implication on catalytic efficiency is also discussed.
Experimental
General Information
All starting materials for syntheses as well as substrates for catalytic experiments were
obtained commercially as reagent grade and used as supplied. The compounds p-tBu~dm, p-
M~dm, p-N~dm, de, o-tBu~dp, dp, o-H~dp, o-M~dp, p-M~dp, p-Br~dp, p-N~dp, Naph~dp,
m-M~pt, Bis-Im, Pd(p-N~dm) and Pd(de) were obtained according to recently reported
2
2
3
9,42,45,46
procedures.
In order to exclude impurities, the organic compounds were either purified
on silica gel columns or re-crystallized. Elemental analyses were performed on Leco CHNS-932
1
13
or El Vario III elemental analyzers. H and C NMR spectra were recorded on Bruker ARX 200
or 400 MHz instrument using deuterated solvents and TMS as internal standard. IR spectra were
–
1
recorded on Shimadzu 8740 FT-IR spectrometer as KBr discs in the range of 4000–400 cm .
7

Single crystal X-ray analyses were carried out on a Nonius Kappa CCD diffractometer at 133(2)
K using graphite monochromated Mo-Kα radiation (k = 0.71073 Å). Cell parameters were
obtained by global refinement of the positions for all collected reflections. The structures were
solved by direct methods and refined by full-matrix least-squares on F2. All non-hydrogen atoms
were refined anisotropically. The protons on the imidazole rings were picked on the difference
maps during structure refinement, which confirms the azole rings are not oxazoles. Structure
4
8
solutions and refinements were performed using the SHELX-97 package.
Preparation of 2-(1H-Imidazol-2-yl)phenol Ligands
2
-Tert-butyl-6-(4,5-dimethyl-1H-imidazol-2-yl)phenol (o-tBu~dm): 3-tert-butyl-2-
hydroxybenzaldehyde (1.78 g, 10.00 mmol), butane-2,3-dione (0.86 g, 10.00 mmol) and
ammonium acetate (≈10 g, 129.73 mmol) were weighed in to a round bottomed flask and
refluxed for 3 hours after addition of ≈ 10 mL ethanol and in the presence of ≈ 0.5 mL glacial
acetic acid as catalyst. The resulting reaction solution was cooled, diluted with distilled water (≈
3
0 mL), neutralized by concentrated ammonia solution and the crude product was extracted with
dichloromethane, concentrated and purified on silica gel column using ethyl acetate / n-hexane
1:5) as eluent. The product obtained from chromatography was recrystallyzed from ethanol to
(
o
-1
obtain o-tBu~dm (1.05 g, 47 %). Mp. 112-113 C. Selected IR peaks (KBr, cm ): ν 3462vs(vsh),
1
3
1
6
397s, 2947s, 1614vs, 1595m, 1447vs, 740s. H NMR (200 MHz, TMS d -dmso); δ 13.83 (s,
,
6
ppm
H); 12.27 (s, 1H), 7.60 (d, J = 7.8 Hz, 1H, phenol ring); 7.12 (d, J = 7.6 Hz, 1H, phenol ring);
.78 (dd, J = 7.8 Hz, 1H, phenol ring); 2.20 (s, 3H, methyl); 2.12 (s, 3H, methyl); 1.39 (s, 9H, t-
1
3
Bu). C NMR (50 MHz，d -dmso): 155.22, 143.91, 136.27, 129.43, 125.62, 121.82, 117.73,
6
8

.
1
1
13.28, 34.44, 29.23. Anal. Calc. for C H N O / EtOH: C, 73.24; H, 8.36; N, 11.20 %. Found:
15 20 2 7
C, 73.15; H, 8.36; N, 11.20 %.
2
-Methoxy-6-(4,5-dimethyl-1H-imidazol-2-yl)phenol
(o-M~dm):
2-hydroxy-3-
methoxybenzaldehyde (1.52 g, 10.00 mmol), butane-2,3-dione (0.86 g, 10.00 mmol) and
ammonium acetate (≈ 10 g, 129.73 mmol) were refluxed for 2 hours in ethanol (15 mL) in the
presence of catalytic amount of glacial acetic acid. The resulting reaction solution was cooled,
diluted with water (≈ 30 mL), carefully neutralized with aqueous ammonia and extracted with
dichloromethane (≈ 50 mL twice). The organic extract was concentrated and purified on silica
gel column using ethyl acetate / n-hexane (1:4) as eluent to obtain o-M~dm (0.09 g, 4 %). Mp.
o
-1
1
2
34-244 C. Selected IR peaks (KBr, cm ): ν 3339vs(sh), 3015w, 2922m, 1616s, 735s. H NMR
(
400 MHz, TMS d -dmso); δ 13.20 (s, 1H); 12.29 (s, 1H); 7.33 (d, J = 7.6 Hz, 1H, phenol
,
6
ppm
ring); 6.87 (d, J = 8.0 Hz, 1H, phenol ring); 6.80 (dd, J = 8.0 Hz, 1H, phenol ring); 3.77 (s, 3H,
1
3
methoxy); 2.17 (s, 3H); 2.13 (s, 3H). C NMR (100 MHz，d -dmso): 148.27; 146.32, 143.52,
6
+
1
1
6
18.13, 115.64, 113.44, 111.75, 55.57 (methoxy), 11.7. MS (EI) m/z 218 (M , 100 %): 218, 200,
88, 175, 28. Anal. Calc. for C H N O .: C, 65.81; H, 6.37; N, 11.81 %. Found: C, 66.05; H,
1
2
14
2
2
.44; N, 11.50 %.
4
-Bromo-2-(4,5-dimethyl-1H-imidazol-2-yl)phenol
(p-Br~dm):
5-bromo-2-
hydroxybenzaldehyde, (2.01 g, 10.00 mmol), butane-2,3-dione (0.86 g, 10.00 mmol) and
ammonium acetate (≈10 g, 129.73 mmol) were reacted and purified in similar manner as for
o
preparation of o-tBu~dm above to obtain p-Br~dm (0.60 g, 23 %). Mp. 257-258 C. Selected
-1
1
IR peaks (KBr, cm ): ν 3327s(sh), 2922m, 1618s, 1580s, 805s. H NMR (200 MHz, TMS d -
,
6
9

dmso); δppm 13.10 (s, 1H); 12.57 (s, 1H); 7.96 (d, J = 2.4 Hz, 1H, phenol ring); 7.29 (dd, J = 2.4,
1
3
8
.8 Hz, 1H, phenol ring); 6.85 (d, J = 8.6 Hz, 1H, phenol ring); 2.15 (s, 6H, 4,5-dimethyl). C
NMR (50 MHz，d -/): 155.39, 141.86, 131.17, 125.84, 118.77, 115.35, 109.69. MS (EI) m/z
6
+
2
67 (M , 100 %): 267, 187, 159, 42. Anal. Calc. for C H BrN O: C, 49.46; H, 4.15; N, 10.49
1
1
11
2
%. Found: C, 49.78; H, 3.94; N, 10.46 %.
4
-Tert-butyl-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (p-tBu~dp): A mixture of 5-
tert-butyl-2-hydroxybenzaldehyde (0.88 g, 4.95 mmol), benzil (1.04 g, 4.95 mmol) and
ammonium acetate (7.00 g, 90.81 mmol) were reacted as for the preparation of o-tBu~dp, but
recrystallization was carried out in ethyl acetate to afford p-tBu~dp (1.26 g, 69 %). Mp. 121-122
o
-1
C. Selected IR peaks (KBr, cm ): ν 3336m, 3280m, 3064m, 2968s, 1750s (acetic acid), 1718s
1
(
acetic acid), 1699s (acetic acid), 1645s, 1601m, 1396vs. H NMR (400 MHz, TMS CDCl );
,
3
δppm 9.65 (s, 2H); 7.57 (d, J = 6.8 Hz, 4H); 7.46 (s, 1H, phenol ring); 7.40-7.30 (m, 7H); 7.02 (d,
1
3
J = 8.4 Hz, 1H, phenol ring); 1.34 (s, 9H, t-Bu); 2.23 (acetic acid adduct). C NMR (100
MHz，CDCl ): 176.80 (acetic acid), 155.21, 146.08, 141.63, 131.96, 119.61, 117.29, 111.63,
3
+
3
1
6
1.53, 20.72 (acetic acid), 18.33 (acetic acid). MS (EI) m/z 368 (M , 100 %): 368, 353, 325, 194,
.
65 Anal. Calc. for C H N O 1.5CH COOH: C, 73.34; H, 6.59; N, 6.11 %. Found: C, 73.04; H,
2
5
24
2
3
4
6
.90; N, 6.29 % (see Supplementary Information S1 ).
4
-(4,5-Diphenyl-1H-imidazol-2-yl)benzene-1,3-diol (m-H~dp): A mixture of 2,4-
dihydroxybenzaldehyde (1.31 g, 9.51 mmol), benzil (2.00 g, 9.51 mmol) and ammonium acetate
(14.00 g, 181.62 mmol) were reacted as for the preparation of o-tBu~dp, but purified using 1:4
o
ethyl acetate / n-hexane on silica gel column to afford m-H~dp (2.12 g, 84 %). Mp. 110-111 C.
10

-1
1
Selected IR peaks (KBr, cm ): ν 3271s(br), 3059s, 1703s (ethyl acetate), 1604vs, 764s. H NMR
(
200 MHz, TMS d -dmso); δ 12.88 (s, 1H); 12.72 (s, 1H); 9.73 (s, 1H, phenolic OH); 7.83 (d,
,
6
ppm
J = 9 Hz, 1H); 7.40 (m, 10H, 4,5-diphenyl); 6.39 (d, J = 2.2 Hz, 1H, phenol ring); 6.35 (s, 1H,
1
3
phenol ring); 1.91 (ethyl acetate). C NMR (50 MHz，d -dmso): 159.33, 158.40, 133.81.
6
1
1
2
7
33.48, 130.46, 128.68, 128.61, 128.41, 128.03, 126.84, 1265.68, 126.26, 126.03, 106.96,
+
04.91, 102.91, 21.07 (ethyl acetate), 14.04 (ethyl acetate). MS (EI) m/z 328 (M , 100 %): 299,
.
3
71, 165. Anal. Calc. for C H N O / ethyl acetate: C, 73.72; H, 5.50; N, 7.35 %. Found: C,
2
1
16
2
2
4
3.56; H, 5.33; N, 7.37 %.
2
-(4,5-Diphenyl-1H-imidazol-2-yl)benzene-1,4-diol (p-H~dp): A mixture of 2,5-
dihydroxybenzaldehyde (0.66 g, 4.46 mmol), benzil (1.00 g, 4.46 mmol) and ammonium acetate
(7.00 g, 90.81 mmol) were reacted as for the preparation of o-tBu~dp, but purified using 1:2
o
ethyl acetate / n-hexane on silica gel column to afford p-H~dp (1.80 g, 74 %). Mp. 222-223 C.
-1
1
Selected IR peaks (KBr, cm ): ν 3303s, 3194s, 3062s, 1650m, 1603s, 1586m, 1223vs. H NMR
(
400 MHz, TMS d -dmso); δ
12.91 (s, 1H); 12.22 (s, 1H); 8.92 (s, 1H); 7.54-7.39 (m, 8H,
,
6
ppm
4
,5-diphenyl); 7.34 (dd, J = 7.2 Hz, 2H); 7.26 (dd, J = 7.2 Hz, 1H); 6.78 (d, J = 8.4 Hz, 1H,
1
3
phenol ring); 6.73 (dd, J = 2.8, 8.8 Hz, 1H, phenol ring); ethylacetate. C NMR (100 MHz，d -
6
dmso): 150.07, 149.96, 146.42, 134.76, 129.19, 128.96, 128.69, 127.31, 117.62, 111.41. MS (EI)
+
.
m/z 328 (M , 100 %): 328, 299, 271, 165. Anal. Calc. for C H N O ⅛ethyl acetate: C, 76.09;
2
1
16
2
2
H, 5.05; N, 8.25 %. Found: C, 76.04; H, 5.07; N, 8.12 %.
Preparation of 2-(1H-Imidazol-2-yl)phenolatopalladium(II) Complexes
11

Preparation of the palladium complexes was achieved according to a similar general
procedure and the case for Pd(o-tBu~dm) will be presented as typical example. Pd(AcO) (44
2
2
mg, 0.18 mmol) and o-tBu~dm (86 mg, 0.36 mmol) were weighed into a 10 mL round-bottomed
flask. After addition of about 2 mL absolute ethanol, the mixture was stirred for 12 hours at room
temperature. The resulting orange precipitate was filtered, washed with few millilitres of ethanol
o
and dried overnight at 60 C to obtain Pd(o-tBu~dm) .
2
In some of the preparations, diethyl ether, ethyl acetate or acetone was used as solvent in
consideration of ligand solubility. Poor solubility in deuterated solvents hindered NMR
measurements for some of the complexes. Synthetic and analytical data for the complexes are
presented as follows:
Bis-[2-tert-butyl-6-(4,5-dimethyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(o-
-1
tBu~dm) : (55 mg, 52 %). Mp./Dec. ≈ 290 ºC. Selected IR peaks (KBr, cm ): ν 3412m, 2951s,
2
1
1
628s, 1472vs, 753s. H NMR (400 MHz, d6-dmso) δ 11.13 (s, 1H), 7.07 (d, J = 7.7 Hz, 1H),
.93 (d, J = 7.6 Hz, 1H), 6.37 (t, J = 7.6 Hz, 1H), 2.53 (s, 3H), 2.21 (s, 3H), 1.19 (s, 9H). Anal.
6
.
.
Calc. for C H N O Pd EtAcO ½H O: C, 59.17; H, 6.86; N, 8.12 %. Found: C, 59.50; H, 6.27;
3
0
38
4
2
2
4
9
N, 8.30 % (see Supplementary Information S2) .
Bis-[4-tert-butyl-2-(4,5-dimethyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(p-
tBu~dm) : Pd(AcO) (55 mg, 0.25 mmol) and p-tBu~dm (120 mg, 0.50 mmol) gave Pd(p-
2
2
-1
tBu~dm) as orange powder (138 mg, 93 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ):
2
1
ν 3120m, 2962s, 1628s, 1495vs, 1254vs, 1054m. H-NMR (400 MHz, d6-dmso): δ 12.32 (s, 1H),
7
.76 (d, J = 2.3 Hz, 1H), 7.16 (dd, J = 8.6, 2.3 Hz, 1H), 6.78 (d, J = 8.6 Hz, 1H), 2.07 (s, 3H),
12

.
1
.28 (s, 9H), 1.24 (s, 3H). Anal. Calc. for C H N O Pd C H OC H : C, 61.21; H, 7.25; N, 8.40
30 38 4 2 2 5 2 5
%. Found: C, 61.12; H, 7.42; N, 8.60 %.
Bis-[2-methoxy-6-(4,5-dimethyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(o-
M~dm) : Pd(AcO) (23 mg, 0.10 mmol) and o-M~dm (45 mg, 0.21 mmol) gave Pd(o-M~dm)
2
2
2
-1
as orange powder (47 mg, 84 %). Mp./Dec. ≈ 292 ºC. Selected IR peaks (KBr, cm ): ν 3412m,
1
2
950s, 1627s, 1592m, 1471vs, 1243vs, 753s. H-NMR (400 MHz, d6-dmso): δ 12.23, 7.03 (dd, J
=
8.0, 1.2 Hz), 6.60 (d, J = 6.7 Hz), 6.43 (t, J = 7.8 Hz), 3.56, 2.40, 2.17. Anal. Calc. for
.
1
.2
C H N O Pd / EtOH / H O: Elemental Analysis: C, 52.13; H, 5.20; N, 9.86 %. Found: C,
2
4
26
4
4
3
3
2
5
2.13; H, 5.21; N, 9.89 %.
Bis-[4-methoxy-2-(4,5-dimethyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(p-
M~dm) : Pd(AcO) (51 mg, 0.23 mmol) and p-M~dm (109 mg, 0.46 mmol) gave Pd(p-M~dm)
2
2
2
-1
as orange powder (103 mg, 88 %). Mp./Dec. ≈ 290 ºC. Selected IR peaks (KBr, cm ): ν 3177m,
1
3
1
3
1
125s, 3049s 2931ms, 1625m, 1608m, 1490vs, 1220vs, 1043s. H-NMR (400 MHz, d6-dmso): δ
2.31 (s, 1H), 7.03 (d, J = 3.0 Hz, 1H), 6.61 (dd, J = 9.0, 3.0 Hz, 1H), 6.54 (d, J = 8.9 Hz, 1H),
.68 (s, 3H), 2.28 (s, 3H), 2.18 (s, 3H). Anal. Calc. for C H N O Pd: C, 53.29; H, 4.84; N,
2
4
26
4
4
0.36 %. Found: C, 53.40; H, 4.43; N, 10.12 %.
Bis-[4-bromo-2-(4,5-dimethyl-1H-imidazol-2-yl)phenolato]palladium(II)
–
Pd(p-
Br~dm) : Pd(AcO) (56 mg, 0.25 mmol) and p-Br~dm (134 mg, 0.50 mmol) gave Pd(p-
2
2
-1
Br~dm) as orange powder (137 mg, 86 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ):
2
1
ν 3192s, 3138s, 3050m 2977ms, 1624s, 1588m, 1474vs, 1279vs, 1144m. H-NMR (400 MHz,
13

d6-dmso): δ 12.55, 7.64 (d, J = 2.6 Hz), 7.06 (dd, J = 8.8, 2.5 Hz), 6.58 (d, J = 8.9 Hz), 2.26,
2
.17. Anal. Calc. for C H Br N O Pd: C, 41.37; H, 3.16; N, 8.77 %. Found: C, 41.55; H, 2.94;
22 20 2 4 2
N, 8.84 %.
Bis-[2-tert-butyl-6-(4,5-diphenyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(o-
tBu~dp) : Pd(AcO) (69 mg, 0.31 mmol) and o-tBu~dp (225 mg, 0.61 mmol) gave Pd(o-
2
2
-1
tBu~dp) as orange powder (130 mg, 50 %). Mp./Dec. 231 ºC. Selected IR peaks (KBr, cm ): ν
2
1
3
411s, 3179s, 3057s, 2951vs, 2911vs, 1705vs, 1597vs, 1441vs, 766vs. H-NMR (400 MHz, d6-
dmso): δ 12.91 (s, 1H), 8.58 (d, J = 7.5 Hz, 2H), 7.54 (t, J = 7.7 Hz, 2H), 7.38 (m, 7H), 6.84 (d,
.
J = 6.4 Hz, 1H), 6.40 (t, J = 7.6 Hz, 1H), 0.74 (s, 9H). Anal. Calc. for C H N O Pd EtOH: C,
5
0
46
4
2
7
0.38; H, 5.91; N, 6.31 %. Found: C, 70.71; H, 5.59; N, 5.84 % (see Supplementary Information
S3).
Bis-[4-tert-butyl-2-(4,5-diphenyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(p-
tBu~dp) : Pd(AcO) (82 mg, 0.37 mmol) and p-tBu~dp (270 mg, 0.73 mmol) gave Pd(p-
2
2
-1
tBu~dp) as orange powder (156 mg, 50 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ):
2
.
ν 3611m, 3057s, 2954vs, 1612s, 1588m, 1489vs, 1251vs. Anal. Calc. for C H N O Pd 2H O:
5
0
46
4
2
2
C, 68.45; H, 5.74; N, 6.39 %. Found: C, 68.54; H, 5.82; N, 6.04 %.
Bis-[1-(4,5-diphenyl-1H-imidazol-2-yl)naphthalen-2-olato]palladium(II)
–
Pd(Naph~dp) : Pd(AcO) (31 mg, 0.14 mmol) and Naph~dp (102 mg, 0.28 mmol) gave
2
2
.
Pd(Naph~dp) (100 mg, 87 %). Single crystals of Pd(Naph~dp) 4thf) suitable for X-ray
2
2
analysis was obtained by slow evaporation of solvent from it tetrahydrofuran (thf) solution.
14

-1
Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ): ν 3447s, 3185w, 3056s, 1612s, 1549s,
.
1
7
371vs, 766vs. Anal. Calc. for C H N O Pd 2EtOH: C, 70.39; H, 5.03; N, 6.08. Found: C,
5
0
34
4
2
0.11; H, 4.91; N, 5.89.
Bis-[2-(4,5-diphenyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(dp) : Pd(AcO)
2
2
(
52 mg, 0.23 mmol) and dp (144 mg, 0.46 mmol) gave Pd(dp) as orange powder (87 mg, 51
2
-1
%
). Mp./Dec. 231 ºC. Selected IR peaks (KBr, cm ): ν 3162s, 3057s, 2965s, 1603s, 1557s,
.
.
1
1
474vs, 1142m. Anal. Calc. for C H N O Pd 1½EtOH H O: C, 66.22; H, 5.06; N, 6.86 %. H-
42 30 4 2 2
NMR (400 MHz, d6-dmso): δ 11.65 (s, 1H), 7.93 (d, J = 8.2 Hz, 2H), 7.31 (m, 9H), 6.64 (m,
1
H), 6.27 (m, 1H), 5.86 (d, J = 8.6 Hz, 1H). Found: C, 66.19; H, 4.86; N, 6.64 %. TG analysis:
o
o
1
51 – 180 C, –10.30 % (tightly held solvent or water); 315 – 419 C, –76 % (ligands).
Bis-[3-(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,2-diolato]palladium(II)
–
Pd(o-
H~dp) : Pd(AcO) (43 mg, 0.19 mmol) and o-H~dp (126 mg, 0.35 mmol) gave Pd(o-H~dp) as
2
2
2
-1
orange powder (80 mg, 55 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ): ν 3398s,
1
3
055m, 1617m, 1587m, 1479vs, 1241vs. H-NMR (200 MHz, d6-dmso): δ 11.90 (s, 1H), 10.84
(s, 1H), 8.20 (d, J = 7.0 Hz, 2H), 7.35 (m, 8H), 6.30 (m, 2H), 5.22 (s, 1H). Anal. Calc. for
.
C H N O Pd 2H O: C, 64.75; H, 4.96; N, 6.57 %. Found: C, 64.91; H, 4.92; N, 7.05 %.
4
2
30
4
4
2
Bis-[4-(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,3-diolato]palladium(II) – Pd(m-
H~dp) : Pd(AcO) (63 mg, 0.28 mmol) and m-H~dp (186 mg, 0.57 mmol) gave Pd(m-H~dp)
2
2
2
-1
as orange powder (165 mg, 77 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ): ν
1
3
528vs(sh), 3167s, 2964s, 1618vs, 1558vs, 1464vs, 1149vs. H-NMR (400 MHz, d6-dmso): δ
15

1
2.59 (s, 1H), 8.97 (s, 1H), 7.81 (d, J = 7.4 Hz, 2H), 7.43 – 7.22 (m, 9H), 5.87 (dd, J = 8.6, 2.4
.
Hz, 1H), 5.29 (d, J = 2.3 Hz, 1H).Anal. Calc. for C H N O Pd EtOH: C, 65.47; H, 4.50; N,
4
2
30
4
4
6
.94 %. Found: C, 65.37; H, 4.71; N, 6.51 %.
Bis-[2-(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,4-diolato]palladium(II)
–
Pd(p-
H~dp) : Pd(AcO) (41 mg, 0.18 mmol) and p-H~dp (120 mg, 0.36 mmol) gave Pd(p-H~dp) as
2
2
2
.
orange powder (111 mg, 80 %). Single crystals of Pd(p-H~dp) 4dmf suitable for X-ray analysis
2
was obtained overnight by slow evaporation of solvent from it dmf solution. Mp./Dec. > 300 ºC.
-1
Selected IR peaks (KBr, cm ): ν 3254s, 3058s, 3026m, 1603w, 1587m, 1486vs, 1209vs, 1012m.
1
H-NMR (400 MHz, d6-dmso): δ 12.83 (s, 1H), 8.29 (s, 1H), 7.78 (d, J = 7.3 Hz, 2H), 7.41 –
7
.20 (m, 8H), 6.99 (d, J = 2.9 Hz, 1H), 6.28 (dd, J = 8.8, 2.9 Hz, 1H), 5.63 (d, J = 8.8 Hz, 1H).
.
Anal. Calc. for C H N O Pd ½H O: C, 65.50; H, 4.06; N, 7.27. Found: C, 65.80; H, 3.85; N,
4
2
30
4
4
2
o
o
7
.21. TG analysis: 99 – 273 C, –2.26 % (tightly held water); 310 – 386 C, –84 % (ligands).
Bis-[2-methoxy-6-(4,5-diphenyl-1H-imidazol-2-yl)phenolato]palladium(II) – Pd(o-
M~dp) : Pd(AcO) (112 mg, 0.50 mmol) and o-M~dp (340 mg, 0.99 mmol) reacted in acetone
2
2
to give Pd(o-M~dp) as off-white micro-crystals (318 mg, 81 %). Mp./Dec. ≈ 260 ºC. Selected
2
-1
IR peaks (KBr, cm ): ν 3423vs, 3055m, 2967m, 1604vs, 1557m, 1540m, 1477vs, 1159, 764vs.
1
H-NMR (400 MHz, d6-dmso): δ 11.65 (s, 1H), 8.28 (d, J = 7.8 Hz, 2H), 7.37 – 7.29 (m, 8H),
6
.98 (d, J = 7.8 Hz, 1H), 6.40 (d, J = 7.4 Hz, 1H), 6.24 (t, J = 7.7 Hz, 1H), 3.21 (s, 3H). Anal.
.
.
Calc. for C H N O Pd 2H O CH COCH : C, 63.91; H, 5.02; N, 6.34. Found: C, 64.25; H, 5.01;
4
4
34
4
4
2
3
3
N, 6.23.
16

Bis-[2-(4,5-diphenyl-1H-imidazol-2-yl)-5-methoxyphenolato]palladium(II) – Pd(m-
M~dp) : Pd(AcO) (33 mg, 0.15 mmol) and m-M~dp (100 mg, 0.30 mmol) gave Pd(m-M~dp)
2
2
2
-1
as orange powder (100 mg, 87 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ): ν 3411vs,
1
3
223s(br), 3057s, 2838s, 1656s, 1606vs, 1557m, 1488vs, 1155vs, 769vs. H NMR (400 MHz,
d6-dmso): δ 12.69 (s, 1H), 7.74 (d, J = 7.3 Hz, 2H), 7.52 (d, J = 8.8 Hz, 1H), 7.40 – 7.19 (m,
H), 6.00 (dd, J = 8.7, 2.5 Hz, 1H), 5.22 (d, J = 2.5 Hz, 1H), 3.50 (s, 3H). Anal. Calc. for
8
.
.
C H N O Pd EtOH H O: C, 64.75; H, 4.96; N, 6.57 %. Found: C, 64.34; H, 4.86; N, 6.71 %.
4
4
34
4
4
2
o
o
TG analysis: 115 – 153 C, –10.02 % (tightly held solvent or water); 307 – 397 C, –78 %
(ligands).
Bis-[2-(4,5-diphenyl-1H-imidazol-2-yl)-4-methoxyphenolato]palladium(II) – Pd(p-
M~dp) : Pd(AcO) (30 mg, 0.13 mmol) and p-M~dp (99 mg, 0.26 mmol) gave Pd(p-M~dp) as
2
2
2
-1
orange powder (92 mg, 89 %). Mp./Dec. ≈ 280 ºC. Selected IR peaks (KBr, cm ): ν 3423vs,
1
3
055s, 2968m, 2830m, 1603s, 1588s, 1488vs, 1217vs, 766vs. H NMR (400 MHz, d6-dmso): δ
1
2.87 (s, 1H), 7.77 (d, J = 7.4 Hz, 2H), 7.41 – 7.27 (m, 8H), 7.14 (d, J = 3.0 Hz, 1H), 6.38 (dd, J
=
9.0, 2.9 Hz, 1H), 5.62 (d, J = 9.0 Hz, 1H), 3.62 (s, 3H). Anal. Calc. for
.
.
C H N O Pd EtOH ½H O : C, 65.44; H, 4.89; N, 6.64 %. Found: C, 65.22; H, 4.85; N, 6.35 %.
4
4
34
4
4
2
o
o
TG analysis: 116 – 156 C, –9.76 % (tightly held solvent species); 355 – 453 C, –77 %
(ligands).
Bis-[4-bromo-2-(4,5-diphenyl-1H-imidazol-2-yl)phenolato]palladium(II)
–
Pd(p-
Br~dp) : Pd(AcO) (71 mg, 0.31 mmol) and p-Br~dp (248 mg, 0.63 mmol) reacted in acetone
2
2
to give Pd(p-Br~dp) as off-white micro-crystals (257 mg, 93 %). Mp./Dec. ≈ 285 ºC. Selected
2
17

-1
1
IR peaks (KBr, cm ): ν 3173s, 3049s, 2965s, 2855s, 1600s, 1537s, 1474vs, 1290vs, 1051s. H
NMR (400 MHz, d6-dmso): δ 12.60 (s, 1H), 8.97 (s, 1H), 7.81 (d, J = 7.3 Hz, 2H), 7.46 – 7.23
(m, 8H), 5.88 (dd, J = 8.6, 2.4 Hz, 1H), 5.30 (d, J = 2.4 Hz, 1H). Anal. Calc. for
.
C H Br N O Pd EtOH: C, 56.64; H, 3.67; N, 6.01 %. Found: C, 56.70; H, 3.65; N, 5.68 %.
4
2
28
2
4
2
Bis-[2-(4,5-diphenyl-1H-imidazol-2-yl)-4-nitrophenolato]palladium(II)
N~dp) : Pd(AcO) (63 mg, 0.28 mmol) and p-N~dp (200 mg, 0.56 mmol) gave Pd(p-N~dp)
2
–
Pd(p-
2
2
-1
(
210 mg, 92 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ): ν 3400s, 3057s, 1605s,
1
6
563m, 1303vs, 698s. Anal. Calc. for C H N O Pd: C, 61.58; H, 3.45; N, 10.26. Found: C,
42 28 6 6
1.69; H, 3.34; N, 10.24.
Bis-[5-methoxy-2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenolato]palladium(II)
–
Pd(m-M~pt) : Pd(AcO) (42 mg, 0.18 mmol) and m-M~pt (126 mg, 0.37 mmol) gave Pd(m-
2
2
M~pt) after the product, which is very soluble in ethanol, was precipitated by addition of
2
-1
hexane (75 mg, 52 %). Mp./Dec. ≈ 270 ºC. Selected IR peaks (KBr, cm ): ν 3434s, 3307s,
3
062s, 2965m, 1634vs, 1606vs, 1524vs, 754vs. Anal. Calc. for C H N O Pd.⅔C H : C, 68.42;
44 30 4 4 6 14
H, 4.71; N, 6.65 %. Found: C, 68.88; H, 4.31; N, 6.59 %.
Bis-[4-methyl-2,6-bis(4,5-diphenyl-1H-imidazol-2-yl)phenolato]palladium(II)
Pd(Bis-Im) : Pd(AcO) (56 mg, 0.25 mmol) and Bis-Im (273 mg, 0.50 mmol) gave Pd(Bis-Im)
2
–
2
2
-1
(
219 mg, 75 %). Mp./Dec. > 300 ºC. Selected IR peaks (KBr, cm ): ν 3403vs, 3055s, 2919w,
1
604s, 1481vs, 696vs. Anal. Calc. for C H N O Pd: C, 74.46; H, 4.56; N, 9.39 %. Found: C,
74 54 8 2
7
4.88; H, 4.47; N, 9.30 %.
18

Suzuki-Miyaura Cross-Coupling Experiments
In typical reactions, (4-Bromophenyl)methanol (187 mg, 1.0 mmol) was reacted with
either 4-acetylphenylboronic acid (197 mg, 1.2 mmol), 3,5-dimethylphenylboronic acid (180 mg,
1
.2 mmol) or 2-bromophenylboronic acid (240 mg, 1.2 mmol) in the presence of a given
palladium(II) complex (0.2 mol %) as catalyst precursor, K CO (207 mg, 1.5 mmol) as base
2
3
1
and 3 mL dmf / 1 mL H O mixture as solvent. Yields were calculated by carrying out H-NMR
2
analyses on the reaction mixtures through comparison of peak integration values for the
methanolyl –CH – signal on (4-bromophenyl)methanol (the aryl-bromide substrate, δ ≈ 4.5 ppm)
2
with that of the biphenyl products (δ ≈ 4.7 ppm). Catalysis reaction products [1-(4'-
(hydroxymethyl)biphenyl-4-yl)ethanone (1) and (3',5'-dimethylbiphenyl-4-yl)methanol (2)] were
purified by column chromatography on silica gel and characterized. In the case of 2-bromo-4'-
1
methanolylbiphenyl (3), catalysis only gave trace yield as monitored by TLC and H-NMR.
Analytical data of the coupled products are as follows:
1
1
8
2
-(4'-(hydroxymethyl)biphenyl-4-yl)ethanone (1): H NMR (400 MHz, TMS CDCl ); δ
,
3
ppm
.01 (d, J = 8.0 Hz, 2H); 7.66 (d, J = 8.4 Hz, 2H); 7.61 (d, J = 8.4 Hz, 2H); 7.45 (d, J = 8.0 Hz,
1
3
H); 4.75 (s, 2H, –CH –); 2.62 (s, 3H, acetyl). C NMR (400 MHz，CDCl ): 197.71 (carbonyl),
2
3
1
41.00, 139.23, 135.92, 128.93, 127.53, 127.45, 127.15, 64.96 (methanolyl), 26.63 (methyl). MS
+
(
EI) m/z 226 (M , 98 %): 226, 211, 165, 152. Anal. Calc. for C H O : C, 79.62; H, 6.24 %.
1
5
14
2
Found: C, 79.52; H, 6.31 %.
19

1
(
3',5'-dimethylbiphenyl-4-yl)methanol (2): H NMR (400 MHz, TMS CDCl ); δ
7.58 (d, J
,
3
ppm
=
8.0 Hz, 2H, p-substituted phenyl); 7.42 (d, J = 8.0 Hz, 2H, p-substituted phenyl); 7.22 (s, 2H,
dimethyl-substituted phenyl); 7.01 (s, 1H, dimethyl-substituted phenyl); 4.74 (s, 2H, methylene);
1
3
2
.39 (s, 6H, dimethyl). C NMR (400 MHz，CDCl ): 140.91, 140.84, 139.69, 138.30, 131.64,
3
1
28.98, 128.58, 127.35, 125.03, 65.18 (methanolyl), 21.40 (dimethyl).
Results and discussion
Syntheses and Characterization of 2-(1H-imidazol-2-yl)phenols and their Palladium(II)
Complexes
The electronically and structurally varied 2-(1H-imidazol-2-yl)phenol ligands were
prepared using rationally selected salicylaldehydes and α-dicarbonyls, which were condensed in
the presence of ammonium acetate while acetic acid played the dual role of solvent and catalyst
5
0
for the condensation. The palladium(II) complexes were generally obtained in good yields by
room temperature reaction between Pd(AcO) and individual members of the ligand series in
2
suitable solvent media. Elemental analyses and spectroscopic data gave agreement with the
identity of ligands and palladium complexes. Characterization analyses frequently revealed
presence of solvent adducts in the synthesized azole compounds and the occurrence of hydrogen-
bonded polar solvents like ethanol, ethylacetate, water, dimethylformamide, etc., has been
3
9-45,46,47
evidenced by single crystal and NMR analyses.
There is high propensity for 1H-
imidazole moieties to strongly hold nearby polar functions via intermolecular or intramolecular
1
hydrogen-bonding. Furthermore, according to CHN analyses and H NMR results, the solvent
adducts are often present in non-stoichiometric amounts after samples have been air- or oven-
dried. However, the analyses for crystallized samples submitted without prolonged air-drying
20

attempts usually showed stoichiometric presence of the adduct solvents in accordance with
4
0,41,44,45
crystal structure composition of complex-to-solvent ratios.
Thermogravimetric analysis
(TGA) data obtained for the complexes frequently revealed initial weight loss values ranging
o
from 1 % to 14 % at about 100 – 200 C, which could be attributed to loss of some tightly held
4
6
solvents (see Supplementary Information S4).
X-ray Structural Characterization Results
Crystal structures solved for complexes Pd(de) , Pd(o-tBu~dp) , Pd(Naph~dp) and
2
2
2
Pd(m-M~pt) are presented as representative structural characterization data for the target
2
palladium complex series according to Scheme 1 (Fig. 1 – 4, Table 1). Suitable single crystals
for complexes Pd(de) , Pd(Naph~dp) and Pd(m-M~pt) grow by slow evaporation from
2
2
solution of the synthesized complexes in dimethylformamide, tetrahydrofuran and ethanol
respectively. On the other hand, red single crystal blocks of Pd(o-tBu~dp) grow when
2
palladium acetate and o-tBu~dp are allowed to stand overnight in diethyl ether.
Pd(de) , Pd(Naph~dp) and Pd(m-M~pt) crystallize in the monoclinic (P2(1)/c or
2
2
2
P2(1)/n) space group while Pd(o-tBu~dp) crystallizes in the triclinic P-1 space group. Their
2
experimental processing parameters are presented in Table 1 below. The observed diversity in
the pattern of solvent bonding to the imidazole functions is noteworthy because it suggests
presence of strong solute-solute interaction capabilities at the 1H-imidazole moieties of these
compounds. It also serves as evidence for the observed solvent adducts found in NMR, CHN and
TGA. During experiment for slow solvent evaporation from dmf solution of Pd(de) according
2
4
8
to our recently reported synthesis, two crystal habits were observed; the compound crystallizes
as red-orange hexagonal crystal discs containing discrete dmf adducts of Pd(de) *2dmf (Fig. 1)
2
21

4
8
and light-green needles containing Pd(de) *dmf*H O in which dmf and water molecules aided
2
2
formation of hydrogen-bonded, 1-dimensional networks. The complexe Pd(Naph~dp) (Fig. 2)
2
also crystallizes as discrete, hydrogen-bonded thf adduct units. However, the structural result for
Pd(m-M~pt) revealed a 2-dimentional hydrogen bonding network mediated by EtOH
2
molecules. The inclusion of dmf, thf, water, diethyl ether or ethanol solvent molecules by
varying modes of solvent-solute interactions in the analyzed structures is consistent with
observations in the analytical characterization data.
Structural Evidence of Cyclometallation Tendencies Obtained from Species In Filtrates of Some
Complexation Reactions
Owing to the N^O heteroatoms, ligand frameworks prepared towards this study were
expected to coordinate as bidentate mono-anionic chelators in the ratio of two ligand molecules
to one palladium(II) ion in order to yield neutral bis-ligated complexes. As presented above,
analytical data for precipitate of complexation reactions show agreement with the bis-ligated
coordination. However, some unexpected organometallic coordination species believed to be in
small amounts were encountered / isolated from some filtrates (mother liquors). As much as
possible and as a routine in the course of syntheses of complexes in this work, individual
reaction filtrates were collected in clean tubes or vials and allowed to undergo slow evaporation
in attempts to possibly grow single crystals of the target bis-ligated complexes. While only
crystals of anticipated palladium complexes grew from some of the complexation filtrate
solutions (Fig. 4 and Supplementary Information S5), some members of the 2-(4,5-diphenyl-1H-
imidazol-2-yl)phenol ligand series (Scheme 1(b)) produced mainly microcystals / powders of
target complexes along with few (1-5) blocks of crystals, which were found to contain tridentate
C^N^O cyclometallation species in which the palladium metal centre is coordinated to an ortho-
22

carbon position on the nearby phenyl (Scheme 2(a)). Thus, the second set of structural results
shown in Figures 5 – 7 presents the evidence for the tendency to form of cyclometallated species
by certain ligand frameworks and this may suggest chelate stability as well as consequent
tendency for catalyst inactivity for complexes bearing such ligands. It may be reasonable to
conclude that higher chelate stability from cyclometallating ligands would reduce the number of
coordination sites around the active metal centre especially with palladium(II) that is restricted to
coordination number 4.
cy
cy
cy
Specifically, Pd(p-Br~dp)2, Pd (o-M~dp.AcO) and Pd (p-N~dp) crystallized
3
2
4
4
alongside precipitation of the target complexes Pd(p-Br~dp) , Pd(o-M~dp) and Pd(p-N~dp)
2
2
2
and the structure refinement data for the cyclometallated species are collected in Table 2. It is
cy
noteworthy that the single crystal sample of Pd(p-Br~dp) grew as a single colourless block. It
2
could be reasoned that the influence of electron-withdrawing functions (i.e. p-bromo-, o-
methoxy- or p-nitro-substituents) on the phenolate oxygen-donor limits the donor strength of the
anionic oxygen-donor atom and the consequently higher electrophilicity of the palladium(II)
center probably favours the formation of tridentate cyclometallation species. The concept of
ortho,para-direction may also need to be considered in respect of the relative position of the p-
bromo-, o-methoxy- or p-nitro-substituents to the carbon bearing the phenolate oxygen-donor.
Table 3 contains data for comparison of selected bond lengths and angles. The observed values
are within usual range. The Pd1–O1 bond lengths are all longer for the cyclometallated
complexes (Figures 5 – 7) than for the bis-ligated complexes (Figures 1 – 4) and the reverse is
the case for the Pd1–N1. This may be considered to the conclusion of weaker metal to oxygen-
23

donor bonds on the ligand frameworks experiencing electron-withdrawing effects on the
phenolate ring as described above. Furthermore, the torsion angles (C1-C6-C7-N1), which are
smaller for the cyclometallated complexes, provides information on general co-planarity of the
imidazole-phenol rings. The higher co-planarity of the imidazole-phenol rings in the tridentate
organometallic species (Figures 5 – 7) suggests rigidity and coordination stability.
Study of Catalytic Efficiencies as Consequence of Cyclometallation, Steric and Electronic
Properties
The series of palladium complexes were individually examined for their efficiencies as
catalyst in Suzuki-Miyaura cross-coupling reactions in efforts to understand the influence of the
systematically varied characteristics of coordinated organic frameworks towards obtaining
superior palladium(II) active species. Scheme 3 presents the coupling experiments carried out in
this study. The reason for using (4-bromophenyl)methanol as the arylbromide substrate is
1
because of utility of its methylene (–CH –) H-NMR signals in the 4 – 5 ppm chemical shift
2
region (Fig. 8). The absence of interfering NMR signals enables clear observation of the
respective methylene peaks for the C-C coupled biphenyl products and that of the arylbromide
reactant. Fig. 8 shows NMR spectral stack for a reaction mixture before reflux, at intervals (30
min, 60 min, 120 min) and after purification of the coupling product. Therefore, reliable
estimation of the relative amounts of reactants and products present in a given catalytic reaction
mixture is achieved by comparison of peak integration values.
In the first instance, (4-acetylphenyl)boronic acid was coupled with (4-
bromophenyl)methanol. As presented in Table 4, Pd(de) (0.2 mol %) was utilized under
2
various conditions of temperature, reflux duration, presence of base additive and solvent media
24

in order to establish a suitable set of reaction parameters under which the catalytic behaviours of
the series of complexes would be comparatively evaluated. Results showed that presence of
water as well as base additive is important for the coupling reaction (Table 4; entries 2 and 4)
and that acetone proved to be unfavourable solvent (Table 4; entry 3). Furthermore,
o
temperatures below 100 C were found to be unfavourable for the C – C coupling catalysis
(
Table 4; entry 5) and this suggests that the palladium pre-catalysts require some sort of thermal
decomposition in order to make some coordination sites free around the metal centre. The 16.7
yield for entry 6 of Table 4 was considered to be appropriate because it would enable
%
detection of higher catalyst activities when the various complexes are evaluated. It was therefore
concluded that the conditions for further experiments for evaluation of catalytic behaviour of the
o
series of palladium complexes would be 30 min reflux at 100 C using 4 mL of 3:1 dmf/H O
2
solvent mixture in the presence of potassium carbonate (1.5 mol equivalent relative to substrate)
and 0.2 mol % of the individual palladium compounds.
Catalytic data for all the palladium pre-catalysts were collected under the chosen general
o
condition (100 C, 3:1 dmf/H O mixture, 1.5 mol. equivalent of potassium carbonate, 30 min
2
reflux duration and 0.2 mol % palladium complex loading) and they are collected in Table 5. For
comparison, the same coupling experiment was also conducted in the presence of palladium
acetate alone as catalyst and a yield of 29 % was obtained (Table 5; Entry 1). Furthermore, in
attempts to ascertain trends and preferences especially for complexes having higher catalyst
activities, shorter reflux duration, other solvent media and adjusted temperatures were also
studied. For ease of comparison, the yields and corresponding turnover frequencies for all the
complexes have been presented in a bar chart (Fig. 9 (a) and (b)).
25

Consideration of catalyst activities for the first series of palladium complexes bearing the
methyl- or ethyl-substituted imidazoles and for which ligand based steric bulk is absent, suggest
that presence of sterically bulky organic frameworks around the active palladium centre is not
5
,51
necessarily a condition for high catalyst efficiency contrary to recent conclusions.
complexes bearing electron-releasing-groups generally yielded lower turnover frequencies
TOFs) than obtained for the reaction catalyzed by the ligand-free palladium acetate (Table 5;
The
(
Entry 1 vs Entries 2 – 4) and this is attributable to the stronger N^O bidentate chelation stability.
Conversely, the palladium complexes bearing electron-withdrawing-groups (EWGs; bromo-,
nitro-) on the phenolate ring yielded high catalyst efficiencies (Table 5; Entries 7 – 9; Fig. 9 (a)
and (b)). It could be concluded that the poorer catalytic behaviour for complexes of de, o-
tBu~dm, p-tBu~dm, o-M~dm and p-M~dm is attributable to stronger N^O chelation stabilities
while higher catalyst activities, which is consequent on the presence of the electron-
withdrawing-groups and resultant poorer donor character of the phenolate oxygen-donors of
ligands p-Br~dm and p-N~dm, is attributable to poorer chelate stabilities. These results support
the conclusion that hemilabile ligands on palladium complexes, which possess tendency for easy
conversion from chelation to monodentate coordination mode under catalysis condition, would
5
,46,47
be beneficial to catalytic efficiency.
The monodentate coordination by one of the p-Br~dp
ligands in Fig. 5 lends support to the conclusion that electron withdrawing groups at the para-
position tends to reduce the coordination character of the phenolate oxygen-donor atom and the
same conclusion has been crystallographically evidenced by observed double bond character of a
4
6
phenol C–O bond in the presence of p-nitro substituent. Pd(p-Br~dm) showed the highest
2
-1
activity under the 30 min reflux duration (Table 5; Entry 7ii; TOF = 779 h ) and similar result
was obtained when catalysis was carried out by in situ method of generating active species
26

-1
(
Table 5; Entry 8ii; TOF = 800 h ). However, comparison of catalytic outcome at 30 min reflux
with outcomes under shorter reflux time of 15 min for Pd(p-Br~dm) as pre-catalyst or via
2
‘
Pd(AcO) + p-Br~dm’ in situ catalyst generation method revealed that initial rate is notably
2
higher for the in situ catalyst formation method leading to higher TOF (Table 5; Entry 7i vs
Entry 8i; Fig. 9 (a) and (b) and Fig. 10); i.e. the active species are probably formed faster via the
in situ ‘salt+ligand’ catalyst generation method. In general, results for the methy / diethyl (dm /
de) ligand frameworks suggest that electronic properties of the organic ligands possess very
important influence on catalytic outcomes relative to steric characters.
On the other hand, the complex series ligated by the 4,5-diphenyl-1H-imidazole ligands
only showed high catalyst activities for Pd(o-tBu~dp) and Pd(Naph~dp) . Their ligands
2
2
possess moieties that tend to sterically hamper the anticipated bidentate coordination; i.e. in
addition to the steric demands resulting from the presence of pendant phenyl ring on the
imidazole side of the N^O chelating atoms, there is further provision of steric hindrance towards
coordination by tBu group ortho to the coordinating phenolate oxygen-donor atom in Pd(o-
-1
tBu~dp) (Table 5; Entry 10; TOF = 845 h ) and naphthol moiety, which suffers steric
2
hindrance from the 1H-imidazole proton thereby widening the bite angle via loss of imidazole-
-1
naphthol co-planarity in Pd(Naph~dp) (Table 5; Entry 12ii; TOF = 872 h at 30 min reflux)
2
(
Scheme 2(b)). The highly active Pd(Naph~dp) was also evaluated at 15 minutes and found to
2
show high initial rate as presented in Fig. 9 (a) and (b) and Fig. 10. It is noteworthy that the
crystal structural data revealed higher C1-C6-C7-N1 torsion angles (i.e. measure of loss of co-
nd
rd
planarity between imidazole and phenol / naphthol rings; Table 3; 2 and 3 entry rows) for the
catalytically efficient 4,5-diphenyl-1H-imidazole complexes (Scheme 2(b)). Therefore, their
chelation stabilities for the N^O bidentate coordination mode would be weaker with consequent
27

ease of generating N-bonded monodentate coordination species at catalysis temperature. The
structurally observed tendency for formation of palladacyclic species appeared to portend very
unfavourable consequences for catalytic benefits of the complex derivatives bearing 4,5-
diphenyl-1H-imidazole ligands.
-1
It could be anticipated that, for Pd(p-Br~dp) (Entry 20; TOF = 46 h ; Table 5) and
2
-1
Pd(p-N~dp) (Table 5; Entry 21; TOF = 48 h ), the presence of nitro- and bromo-substitution at
2
para position relative to the phenol hydroxyl position would induce similar high catalyst
activities as a result of poor phenolate oxygen-donor characteristics as observed in the case of
dimethylimidazole / diethylimidazole complexes. However, the opposite influence was obtained
in which the ligands appeared to effectively poison the catalyst potential of the palladium metal
centre. The lowest catalytic efficiencies (yield ≈ 5 %) observed for the bromo- and nitro-
substituted 4,5-diphenyl-1H-imidazole complexes indicate absence of any useful amounts of
active palladium metal centres in the catalysis experiments catalyzed by Pd(p-Br~dp) or Pd(p-
2
N~dp) and, based on structural evidence, this outcome is compatible with the above thinking
2
that formation of tridentate N^O^C palladacycle species via cyclometallation between the
palladium metal and nearby phenyl ring is favoured by the presence of electron withdrawing
groups on the phenol ring. Presence of cyclometallation tendency might be responsible for the
generally poor catalyst efficiencies of the 4,5-diphenyl-1H-imidazole complex derivatives. The
values of TOFs obtained for the ortho-, meta- and para-substituted analogues Pd(o-H~dp)2,
-1
Pd(m-H~dp) and Pd(p-H~dp) (Table 5; Entries 14 – 16; TOF = 478, 167 and 84 h
2
2
respectively) apparently suggest that the electron-withdrawing character of the substituent
hydroxyl group is strongest at the para-position and that the consequently higher
cyclometallation tendency in Pd(p-H~dp) is responsible for its low catalyst relevance.
2
28

Furthermore, the wide difference in catalytic efficiency of the phenanthrenyl complex Pd(m-
-1
M~pt) (Table 5; Entry 22i; TOF = 940 h ; highest yield) and its analogous diphenyl complex
2
-1
Pd(m-M~dp) (Table 5; Entry 18; TOF = 142 h ) probably supports presence of N^O^C
2
chelation effects in Pd(m-M~dp) despite the electron-rich character of their methoxy-
2
substituent. The fused and rigid phenanthrenyl rings of Pd(m-M~pt) would prevent
2
cyclometallation and contribute steric hindrance towards coordination, which may favour easy
formation of N-coordinated monodentate species at catalysis temperature. In summary, results
obtained in this work suggest that catalytic outcomes are mainly dependent on chelation modes
(i.e. whether N, N^O or N^O^C is stable), which could be influenced by steric / electronic
properties of substituent moieties born on the ligands. It was observed that the higher the
tendency to form stable multidentate chelate coordination around a palladium metal centre the
more catalytically inactive such palladium metal centre becomes. Moreover, the catalytically
active complexes appear to possess ligands with poorer chelation properties, which would
readily furnish monodentate coordination species at catalysis temperature and which are not
necessarily sterically bulky. According to Fig. 9 (a), the highest catalytic yields obtained from
complex pre-catalysts in this work pertained to complexes Pd(p-Br~dm) (Table 5; Entry 7ii;
2
7
9
7.9 %), Pd(Naph~dp) (Table 5; Entry 12ii; 87.2 %) and Pd(m-M~pt) (Table 5; Entry 22i;
2 2
4.0 %). However, the TOF data (Fig. 9 (b)) indicated that Pd(Naph~dp) possessed superior
2
catalyst capacity on the basis of reduced time (Fig. 10). Therefore, Pd(Naph~dp) was
2
employed as pre-catalyst towards other arylboronic acids and results suggests that the pre-
catalyst is favourable towards diverse electronically substituted arylboronic acid reagent, but not
favourable towards sterically challenged substrates (Table 6).
29