Palladium(II) complexes bearing 2-(1H-imidazol/oxazol-2-yl)-pyridines: Synthesis, structures and ligand effects in Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1016/j.jorganchem.2013.12.041

A series of dichloropalladium(II) complexes (Pd1-Pd4ox) based on derivatives of the hemilabile 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands were synthesized and characterized. The ligands were designed to introduce a variety of electronic and structural features and crystal structures of Pd1, Pd1Me, Pd3, and Pd3Me were determined. Pd1 and Pd1Me both crystallize in the monoclinic space group C2/c. Pd3 crystallizes in the monoclinic space group P2₁/c while Pd3Me crystallizes in the orthorhombic space group P2₁2₁2₁. All complexes were utilized as pre-catalysts in a comparative study to probe relevant features of the palladium active species during high temperature Suzuki-Miyaura cross-coupling catalysis. Relatively higher catalytic activities were obtained for complexes bearing the bidentate ligands that possess weaker azole donor arms. Results obtained suggest that ligand dissociation steps may be important for generating active species with high efficiency. In particular, the observed ligand effects in the investigated series indicate that higher catalyst efficiencies were observed with complexes for which the coordination of the ligand at the palladium center could readily be converted into a monodentate mode.

Full text of DOI:10.1016/j.jorganchem.2013.12.041

Journal of Organometallic Chemistry 754 (2014) 39e50
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Palladium(II) complexes bearing 2-(1H-imidazol/oxazol-2-yl)-
pyridines: Synthesis, structures and ligand effects in SuzukieMiyaura
cross-coupling
, ,
Abiodun Omokehinde Eseola a 1, Daniel Geibig a, Helmar Görls a, Wen-Hua Sun c,
*
,
Xiang Hao b, Joseph Anthony Orighomisan Woods c, Winfried Plass a
*
a Institut für Anorganische und Analytische Chemie, Lehrstuhl für Anorganische Chemie II, Friedrich-Schiller-Universität Jena, Humboldtstr. 8, 07743 Jena,
Germany
b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
c Inorganic Chemistry Unit, Department of Chemistry, University of Ibadan, Ibadan, Nigeria
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of dichloropalladium(II) complexes (Pd1ePd4ox) based on derivatives of the hemilabile 2-(1H-
imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands were synthesized and characterized. The li-
gands were designed to introduce a variety of electronic and structural features and crystal structures of
Pd1, Pd1Me, Pd3, and Pd3Me were determined. Pd1 and Pd1Me both crystallize in the monoclinic space
group C2/c. Pd3 crystallizes in the monoclinic space group P21/c while Pd3Me crystallizes in the
orthorhombic space group P212121. All complexes were utilized as pre-catalysts in a comparative study to
probe relevant features of the palladium active species during high temperature SuzukieMiyaura cross-
coupling catalysis. Relatively higher catalytic activities were obtained for complexes bearing the
bidentate ligands that possess weaker azole donor arms. Results obtained suggest that ligand dissocia-
tion steps may be important for generating active species with high efficiency. In particular, the observed
ligand effects in the investigated series indicate that higher catalyst efficiencies were observed with
complexes for which the coordination of the ligand at the palladium center could readily be converted
into a monodentate mode.
Received 16 September 2013
Received in revised form
16 December 2013
Accepted 19 December 2013
Keywords:
Palladium
Carbonecarbon cross-coupling
SuzukieMiyaura catalysis
Azole ligands
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
construction of complex supramolecular architectures [5e7]. The
origin of the progress known today in organopalladium mediated
The reactivity of organic molecules with palladium centers has
led to very important applications. Perhaps, the most important
aspect of organopalladium chemistry to synthetic molecular sci-
entists is their reactivity role in carbonecarbon or carbonehet-
eroatom cross-coupling events. Ranging from oxidative addition
over transmetallation to reductive elimination, the reactivity of
catalytic palladium species towards cross-coupling of organic
substrates is unequaled by any other element [1e4]. Metal-
catalyzed cross-coupling protocols are inevitable tools in drug
design, natural product total synthesis, synthetic sciences for next
generation materials, ligand designs in coordination sciences and
cross-coupling reactions goes back to the pioneering works of Heck,
Negishi, and Suzuki [8,9]. Some notably efficient palladium com-
pounds have been designed, fine-tuned and even commercialized;
e.g. PEPPSIÔ [10].
Among the commonly applied palladium-catalyzed CeC
coupling procedures such as Heck, SuzukieMiyaura, Sonogashira,
Stille, Negishi, Kumada, Hiyama, etc., the SuzukieMiyaura coupling
method is perhaps one of the most utilized [11,12]. SuzukieMiyaura
coupling connects aryl halide substrates with boron containing
substrates at the carbon atoms bearing the halide and boronic
functions. Numerous forms of palladium have been studied as pre-
catalysts for these cross-coupling reactions. However, nearly all
reported palladium forms (such as discrete soluble palladium
complexes, solid-supported palladium complexes, supported
palladium nano- and macro-particles, soluble palladium nano-
particles, soluble ligand-free palladium species, and palladium-
exchanged oxides) have been proposed to be the active catalytic
* Corresponding authors. Tel.: þ49 3641 948130; fax: þ49 3641 948132.
E-mail address: sekr.plass@uni-jena.de (W. Plass).
1
Present address: Department of Chemical Sciences, Redeemer’s University,
Redemption City, Ogun State, Nigeria.
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.12.041

40
A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
species [13]. The true nature of the active species (homogeneous or
heterogeneous) has continued to be a subject of discussion [13,14].
In fact, it has been concluded that there is generally more than one
form of active species in palladium-mediated cross-coupling events
and that the actual designation as homogeneous or heterogeneous
depends on the active species dominant in catalysis [15]. Further-
more, there are diverging opinions about the effect of the ligand
environment around palladium active sites on their catalytic effi-
ciency. For instance, reports from the Buchwald group [16] and
others [3,17,18] suggest that unprecedented catalyst performance
would be expected for soluble palladium(0) species bearing mon-
odentate ligands with sterically bulky groups, but there are other
opinions that high activities in palladium-catalyzed cross-coupling
depend on ligandless soluble palladium species derived from
palladium salts such as PdCl2, Pd(AcO)2, Na2PdCl4 and Na2Pd(AcO)4
[19,20]. It is noteworthy that studies reported on the Buchwald li-
gands (i.e. biphenyl-2-yldicyclohexylphosphine derivatives;
Scheme 1), which are known to yield highly active homogeneous
palladium species and are already commercialized, have lead to
conclusions that superior activities result from bulky ligands
bonded in a monodentate mode, which is particularly evident as
the ligands I and II depicted in Scheme 1 are compared [4,16,18,21].
Despite the decades of active research in the field, the above-
mentioned aspects have continued to stimulate investigations
aiming to improve the understanding of the nature of the active
species in cross-coupling catalysis [22e24]. Although new palla-
dium complexes are routinely being tested for their catalytic effi-
ciencies, systematic investigations on catalysis involving complexes
derived from rationally designed ligands, which vary in their
structural and donor strength features or their hemilability, are
scarce [12,24,25].
In recent experiments substantial variations in ligand donor
characteristics were observed for a series of 2-(1H-imidazol/oxa-
zol-2-yl)phenols and 2-(1H-imidazol/oxazol-2-yl)pyridines during
attempts to prepare their zinc(II) [26] and nickel(II) complexes [27],
respectively. Furthermore, experimental and DFT studies showed
that these ligand families are hemilabile and that the electronic
donor strength of the azole bases could vary from very poorly to
fairly strongly donating, depending on the azole ring heteroatom
and/or the substituent variations on the 2-(1H-imidazol/oxazol-2-
yl)phenol or 2-(1H-imidazol/oxazol-2-yl)pyridine basic scaffold
[28e30]. Based on this knowledge the present study on a series of
palladium complexes with 2-(1H-imidazol/oxazol-2-yl)pyridines
was inspired with the intention to shed some light on the nature of
favorable palladium-ligand assembly or disassembly situations in
the active states. Herein the syntheses and structural character-
ization of dichloridopalladium(II) complexes derived from a series
of 2-(1H-imidazol/oxazol-2-yl)pyridine ligands (Scheme 2) is pre-
sented. These complexes were applied as pre-catalysts towards
SuzukieMiyaura CeC coupling reactions under similar conditions
and the results are discussed on the basis of additional theoretical
and spectroscopic data.
unsubstituted 1H-imidazole ligands possessing a free NH proton as
potential hydrogen-bonding donor. This observation is supported
by the presence of dimethylformamide adducts in the crystal
structures of Pd1 and Pd3, containing a free 1H-imidazole proton,
and absence of solvent adduct molecules in the crystal structures of
the methyl-substituted analogs Pd1Me and Pd3Me. Moreover, this
observation is in agreement with previous results obtained from
elemental analyses, spectroscopic results and structural charac-
terizations of related azole compounds [26e30].
2.2. X-ray structures of Pd1, Pd1Me, Pd3, and Pd3Me
The complexes Pd1 and Pd1Me crystallize in the monoclinic
space group C2/c with the crystal structure of Pd1 containing a
hydrogen-bonded DMF molecule. Pd3 crystallizes in the mono-
clinic space group P21/c with two crystallographically independent
complex molecules each bearing a hydrogen-bonded co-crystal-
lized DMF molecule, whereas Pd3Me crystallizes in the ortho-
rhombic space group P212121 without solvent molecules. The
molecular structures of the four complexes are presented in
Figs. 1e4 and selected bond lengths and angles are summarized in
Table 1.
The geometry around the palladium atom is distorted square-
planar in all four crystal structures with nearly the same N1e
Pd1eN2 bite angle for the five-membered chelate rings. Although
the PdeN bond lengths are observed to be within a small range of
ꢀ
2.03e2.06 A, they show an interesting trend as the four complexes
are compared. The imidazole-palladium bond (Pd1eN2) is gener-
ally longer for complexes Pd1, Pd3, and Pd3Me with values around
ꢀ
2.06 A than for the same bond in Pd1Me. Furthermore, for Pd1, Pd3
and Pd3Me the palladiumeimidazole bonds (Pd1eN2) are gener-
ally somewhat longer than the palladiumepyridyl bonds (Pd1e
N1), while for complex Pd1Me these bond lengths are more or less
identical and at the lower end of the observed range. The generally
shorter Pd1eN1 bonds suggest that the pyridyl nitrogen is a
somewhat stronger donor than the imidazole base for the used
ligand frameworks. Since the additional ligands at the palladium
atom are chloride ions in all cases, these differences cannot be
attributed to the influence of trans-effects within the palladium
coordination sphere. Consequently, the variation in bond lengths
might either be related to steric or electronic effects based on the
ligand system and/or packing effects. However, the CeN and CeC
bond lengths of the 2-(imidazol-2-yl)pyridine ligand fragment are
well within the usually observed range and do not show any indi-
cation that electronic effects are operative as the four complexes
are compared. Nevertheless, it is worth noting that the C5eC6
ꢀ
distances of about 1.45 A observed for the four complexes are in
agreement with a C(sp2)eC(sp2) single bond [31], indicating
possible freedom of rotation about the imidazole-pyridine link [32].
Unlike for the obtained structures of Pd1 and Pd1Me, there
exists considerable steric strain within the coordination environ-
ment of the palladium center in complexes Pd3 and Pd3Me, which
bear the rigid phenanthreneyl substituent fused to their 4,5-
imidazole carbons. This is evident by a considerable bending
distortion of the square-plane of the palladium coordination
2. Results and discussion
2.1. Synthesis of palladium complexes
The 2-(1H-imidazol/oxazol-2-yl)pyridine ligands were prepared
and characterized according to previously reported procedures
[27,28]. The preparation of complexes Pd3, Pd3ox, and Pd3oxD has
also been previously reported [28]. All other complexes were ob-
tained in good yields and have been fully characterized. The iso-
lated compounds usually incorporate solvent molecules, which are
sometimes non-stoichiometric as revealed by the elemental ana-
lyses results. This is especially observed for compounds with
Scheme 1. Biphenyl-2-yldicyclohexylphosphine ligands;
metallation with palladium to form a bidentate chelate, while the presence of methyl
groups in II prevents cyclometallation [4,18].
I is capable of cyclo-

A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
41
Scheme 2. Synthetic outline of studied complexes Pd1ePd4ox, and formation of the DMSO adduct Pd3oxD and Pd-Black from PD3ox (circled box).
N1eC5eC6eN2 (Pd1, 5.6ꢀ; Pd1Me, 1.0ꢀ; Pd3, 1.6 and 4.4ꢀ; Pd3Me,
14.5ꢀ) and the dihedral angle between the imidazole and pyridyl
rings (see Table 2). However, the observed distortions within the
complexes may have a considerable contribution from packing ef-
fects which are dominated by
pep stacking, with additional con-
tributions from hydrogen bonding in the case of Pd1 and Pd3. This
view is consistent with the results of the DFT calculations which
show somewhat smaller distortions for optimized geometries of
the complexes (vide infra).
These structural characteristics suggest that, among the com-
plexes coordinated by the imidazole-based ligands, the 4,5-
phenanthreneyl-substituted complexes Pd3, Pd3Me, and Pd4
may be more prone to appropriate bond cleavage as compared to
their 4,5-diphenyl-substituted analogs Pd1, Pd1Me, and Pd2. This
is consistent with previous observations for the oxazole-based
complexes Pd3ox and Pd4ox, for which the hemilabile nature of
the chelate ring is evident from solvent-mediated Pd-oxazole
cleavage during attempts to grow suitable crystals from DMSO
solutions [28]. Instead of the expected bidentate coordination
structure for Pd3ox the formation of the DMSO adduct Pd3oxD was
observed such that the oxazole base was detached from the
palladium center and the vacant coordination site was occupied by
a DMSO molecule (see Fig. 5). Moreover, a similar hemilabile
characteristic has been proposed for palladium complexes with
Fig. 1. Molecular structure of complex Pd1$DMF with thermal ellipsoids drawn at the
50% probability level. Some atomic labels and protons have been omitted for clarity.
Hydrogen bonding distance: N3/O1 267 pm.
environments relative to the 2-(1H-imidazol-2-yl)pyridine plane of
the coordinating ligand as can be seen in Figs. 3(b) and 4(b). This
can be quantified by the corresponding dihedral angles between
the respective mean planes as summarized in Table 2. The largest
deviation from the overall planarity is observed for complex
Pd3Me, while Pd1 possesses the least in this series. As a conse-
quence this also leads to a twist within the five-membered chelate
ring of the ligand system, which is indicated by the torsion angle
Fig. 3. (a) Molecular structures of the two crystallographically independent molecules
of Pd3$DMF with thermal ellipsoids drawn at the 50% probability level. (b) View
approximately along the ligand plane showing the out-of-plane distortion of the
square-planar palladium center. Some atomic labels and protons have been omitted for
clarity. Hydrogen bonding distances: N3/O1 270, N3A/O1A 274 pm.
Fig. 2. Molecular structure of complex Pd1Me with thermal ellipsoids drawn at the
50% probability level. Some atomic labels and protons have been omitted for clarity.

42
A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
considered relevant for their coordination stability and hemilabile
behavior. Therefore the corresponding dihedral angles between the
mean planes around the palladium centers with the pyridine and
azole ring fragments were calculated for all complexes and are
summarized in Table 3.
Over the series of complexes a considerable variation for the
distortion from an overall planarity around the palladium center is
observed. The largest deviation is found for Pd2, Pd4, and Pd4ox,
which all three contain an ortho-methyl substituent at the pyridyl
ring. The methyl substituent leads to coordinative strain that is
somewhat more pronounced than what is observed when the rigid
phenanthreneyl substituent is fused to the 4,5-imidazole ring car-
bons. This fact is obvious by comparison of Pd2 with Pd3, Pd3Me,
and Pd3ox. However, combining both steric contributors as in Pd4
and Pd4ox, leads to the overall most strained situations.
A comparison of the calculated dihedral angels with those
observed in the crystal structures of Pd1, Pd1Me, Pd3, and Pd3Me
(see Tables 2 and 3) clearly shows that, in particular, the molecular
structure of the Pd3Me is governed by additional crystal packing
effects (vide supra). Therefore the molecular steric strain operative
within the investigated series of complexes is better reflected by
the geometries derived from the DFTcalculations. Nevertheless, the
additional distortions due to packing effects are indicative of the
possible flexibility of the molecular systems. Here it is worth
noting, that in case of Pd3Me the additional distortion observed
due to packing effects is mainly related to the orientation of the
imidazole fragment, whereas the calculated and crystallographic
dihedral angle between the pyridyl substituent and the coordina-
tion plane at the palladium center closely resemble each other.
Considering a possible partial detachment of the hemilabile li-
gands, differences in catalytic activities should be anticipated
depending on whether the azole or the pyridyl fragment dissociates
from the palladium center. For monodentate coordination in which
the pyridyl nitrogen is coordinated, as in the structure of Pd3oxD,
the presence of ortho-methyl substituent relative to the pyridyl
nitrogen, as it occurs in Pd2, Pd4 and Pd4ox, might lead to addi-
tional strain in the resulting active species. Interestingly, the com-
plexes Pd1Me and Pd3ox are found at the lower end of the
distortion range together with complex Pd1, for which the smallest
dihedral angles of the series are observed.
Fig. 4. (a) Molecular structure of complex Pd3Me with thermal ellipsoids drawn at the
50% probability level. (b) View approximately along the ligand plane showing the out-
of-plane distortion of the square-planar palladium center. Some atomic labels and
protons have been omitted for clarity.
Table 1
ꢀ
ꢀ
Selected bond lengths (A) and angles ( ) for Pd1, Pd1Me, Pd3, and Pd3Me.
Pd1
2.041(3)
2.057(2)
2.2822(9)
Pd1Me
Pd3a
Pd3Me
PdeN1
PdeN2
PdeCl1
2.034(4)
2.038(4)
2.2851(11)
2.049(5)
2.063(5)
2.291(2)
2.024(5)
2.071(5)
2.033(5)
2.059(5)
2.2852(19) 2.2882(16)
PdeCl2
2.2701(12) 2.2768(12) 2.2993(18) 2.2794(18) 2.2941(16)
N1ePdeN2
N1ePdeCl1
N2ePdeCl2
Cl1ePdeCl2
80.85(9)
93.36(7)
97.20(7)
88.74(3)
80.43(15)
93.23(11)
96.76(11)
89.73(4)
80.50(2)
92.87(16)
98.78(14)
88.10(7)
80.89(19)
92.20(16)
97.95(14)
88.57(7)
80.10(2)
92.53(16)
95.72(14)
90.91(6)
a
Values for both crystallographically independent molecules.
2.4. Determination of reaction parameters for catalysis using
complex Pd1
pyrazole-containing ligands to account for their fluxional behavior
[33].
Three arylboronic acids (4-acetylphenylboronic acid, A1; 3,5-
dimethylphenylboronic acid, A2; 2-phenylphenylboronic acid, A3)
and two aryl bromides ((4-bromophenyl)methanol, H1; 4-bromo-
3,5-dimethoxybenzoic acid, H2) were utilized in various Pd-
catalyzed SuzukieMiyaura coupling reactions (Scheme 3).
Changes in 1H NMR signals resulting from varying composition of
simple solute mixtures in spectral regions devoid of overlap or
interference from other proton functions enables easy tracking of
chemical events in solution and, consequently, have been applied to
cross-coupling catalysis in the past [2,6,9,19,22,34]. Therefore, the
choice of reacting H1 with arylboronic acids was considered
attractive because of the anticipated ease of monitoring the reac-
tion progress via comparison of 1H NMR signals of the methylene
functions in the reactant (H1) and the possible products (A1H1,
A2H1, and A3H1; see Scheme 3(a)e(c)). Furthermore, H1 and A1
possess only para substituents and are thus not sterically
demanding reactants for the CeC coupling reaction. However, H2
containing methoxy groups at both ortho positions of the reactive
carbon atom and A3 bearing an ortho-phenyl substituent in the
vicinity of the reactive carbon atom were selected to study the
2.3. Computational studies on the palladium complexes
DFT calculations were performed for all complexes under
investigation (see Scheme 1). The geometry optimization of the
complexes results in bond lengths and angles similar to what is
observed for the crystal structures. However, the crystallographic
data presented for the palladium complexes indicate that the
distortion of the coordination sphere at the palladium center can be
Table 2
Dihedral angles (ꢀ) between the mean planes of the pyridyl ring (npy: N1, C1, C2, C3,
C4, C5), the azolyl ring (naz: N2, N3, C6, C7, C8), and the coordination plane at the
palladium center (nPd: N1, N2, Cl1, Cl2) for complexes Pd1, Pd1Me, Pd3, and Pd3Me.
:(npy/naz
)
:(npy/nPd
)
:(naz/nPd)
Pd1
5.9
7.7
9.8
4.9
17.7
9.1
12.0
24.8
25.5
23.2
9.5
15.2
21.7
25.1
38.1
Pd1Me
Pd3a
Pd3Me
a
Values for both crystallographically independent molecules.

A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
43
Furthermore, applying the above reaction conditions at room
temperature for five days only produces a yield of 4%.
Fig. 6 shows the spectral stack for aromatic and methylene
proton regions of 1H NMR experiments carried out on a reaction
solution before commencement of reaction and at varying time
duration under the preferred reaction conditions. Aliquots of the
reaction solution were taken for NMR analysis at 0, 15, 30, and
60 min of reaction time. This spectral comparison was done to
clarify and differentiate reactant proton signals from those of the
product. It is important to mention that in the methylene region
tiny signals of species which are neither product nor reactant peaks
could be observed (see Fig. 6(B), peaks marked by asterisk). These
species may be homo-coupling products, reaction intermediates or
some other by-products all originating from the aryl halide reactant
H1. Hence, yields were calculated by comparing the integral of the
product peak with the sum of all observed methylene signals. The
slight shifts observed for the corresponding peaks in the spectral
stacks of Fig. 6 are attributed to small changes in overall solution
properties for individual NMR analysis originating from varying
proportions of solvents (CDCl3, EtOH, H2O) as well as the ratio of
reactants to products in the individual NMR experiments. The
peaks for the purified cross-coupling product are most shifted
relative to their corresponding peaks in the NMR spectra of the
reaction mixtures because only CDCl3 and the product are present
unlike for NMR analyses conducted for catalysis aliquots.
Fig. 5. Ball and stick representation of complex Pd3oxD [28].
efficiency of the catalyst species towards sterically hindered sub-
strates (Scheme 3(d) and (e)).
Since these experiments are mainly aimed to improve the un-
derstanding of what the more efficient palladium active site char-
acteristics are with respect to clues about ligand/ligandless or
homogeneous/heterogeneous phenomena, the catalysis reactions
carried out were not designed to optimize yields. Instead pre-
liminary catalytic experiments were carried out with one of the
complexes for the purpose of determining a suitable reaction
setting such that the reaction conditions should produce an average
catalytic conversion. Thus, changing other variables, like palladium
materials applied as pre-catalysts, catalysis substrates, base addi-
tives, etc., could possibly reveal their respective influences on the
outcome of the catalysis in a measurable manner.
Therefore, in order to determine a desirable reaction setting,
which will be subsequently applied to the series of palladium
compounds under investigation, complex Pd1 was studied under
varying reaction times, pre-catalyst loadings, temperatures, addi-
tive base types, and solvent compositions for the coupling of A1
(1.2 mmol) and H1 (1.0 mmol) (see Scheme 3(a)). A full list of results
is summarized in Table 4. The initial reaction was performed for
60 min under reflux conditions in the presence of 1 mol% of Pd1,
1.5 mmol K2CO3, and 4 mL of 3:1 EtOH/H2O mixture. By comparison
of the 1H NMR peak integrals for the methylene group of the aryl
halide substrate H1 (z4.50 ppm) against that of the cross-coupled
product A1H1 (z4.70 ppm), a yield of 93% was obtained (Table 4,
entry 1; Fig. 6). However, in search of yields around 50%, catalysis
under shorter reaction times (30 min and 15 min) and lower
catalyst loading of Pd1 (0.2 mol%) were carried out (Table 4, entries
2, 3, and 5). Reaction conditions leading to a medium yield were
considered to be suitable for the purpose of comparative studies on
the ligand effects within the series of the palladium complexes
Pd1ePd4ox.
2.5. Catalytic properties of palladium complexes
Catalysis experiments were performed with all synthesized
complexes Pd1ePd4ox as well as for PdCl2 using the same reaction
conditions as employed for entry 3 of Table 4 in order to study the
effect of varying ligand frameworks on the catalytic outcomes. A
summary of the results in presented in Table 5. The comparison of
observed yields for the reactions carried out under air with their
corresponding inert-atmosphere reactions, which were performed
in degassed solvents under nitrogen atmosphere, generally
revealed little or no difference in catalytic performance. However, a
decrease was observed under inert condition for Pd3ox (air, 88%;
N2, 46%), Pd4 (air, 65%; N2,12%) and Pd4ox (air, 49%; N2, 29%), while
the reverse trend was recorded for Pd3oxD (air, 63%; N2, 88%) upon
exclusion of oxygen.
On the other hand, results show that the variation of ligand
structures and properties has a notable influence on the outcome of
the catalytic experiments. In particular, palladium complexes
bearing the weaker azole donors, i.e. the oxazoles Pd3ox and
Pd3oxD or the 1-methyl-substituted imidazoles Pd1Me and
Pd3Me, demonstrated notably greater potential towards gener-
ating catalytic species of higher activities than complexes of their 1-
H-imidazole ligand counterparts (Pd1 and Pd3). The lower per-
formance of the latter may be accounted for by the higher tendency
Table 3
Dihedral angles (ꢀ) between the mean planes of the pyridyl ring (npy), the azolyl ring
(naz) and the coordination plane at the palladium center (nPd) for complexes Pd1e
Pd4ox calculated from DFT optimized geometries.
Consequently, the set of reaction parameters that gave a yield of
51% and the most efficient utilization of the added amount of
catalyst (i.e. turnover frequency of 510, Table 4, entry 3) was
selected for comparing catalyst activity of all complexes in order to
examine the effect of varying the ligand character. These conditions
imply a reaction time of 30 min under reflux conditions with
0.2 mol% pre-catalyst loading, 1.5 mmol K2CO3, and 4 mL of 3:1
EtOHeH2O mixture. Varying the reaction conditions by utilizing
only absolute ethanol as solvent resulted in a yield of 14%.
Comparing catalyst activities in the presence of K2CO3 or Cs2CO3 did
not cause significant differences in yield, whereas modifying the
conditions by not adding base gave only trace amounts of product.
:(npy/naz
)
:(npy/nPd
)
:(naz/nPd)
Pd1
Pd1Me
Pd2
2.6
4.6
13.4
8.5
8.6
5.6
7.9
15.3
14.9
7.1
11.5
26.6
21.0
21.1
12.3
73.5
36.2
30.4
5.9
12.1
29.8
16.8
24.7
9.7
70.0
36.1
32.1
Pd3
Pd3Me
Pd3ox
Pd3oxD
Pd4
Pd4ox

44
A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
Scheme 3. SuzukieMiyaura coupling reactions carried out in this work.
of 1H-imidazole complexes to sustain bidentate ligand coordina-
tion. Another possible cause may be that intermolecular in-
teractions, due to the hydrogen-bonding capability of the 1H-
imidazole function, lead to undesirable consequences for the
palladium active sites, probably by rendering the palladium site
less reactive towards oxidative addition with the aryl bromide
substrate. The observed variations due to ligand effects also indi-
cate the presence of ligands at the active palladium species during
catalysis. Considering yields of about 90% obtained in 30 min under
reflux conditions with a pre-catalyst loading of 0.2 mol%, the cat-
alytic efficiency produced by the complexes Pd1Me, Pd3Me, and
Pd3ox is fairly attractive. In particular, many reported palladium
compounds tested in high-temperature SuzukieMiyaura homoge-
neous catalysis require hours to achieve completion [3e5]. How-
ever, the optimization of the reported catalytic systems is not
within the present focus.
the base and vice versa. The pyridyl rings were observed to possess
significantly higher donor strengths than the azole rings and the
increasing order of donor strength for the azole ring types follows
the trend oxazoles < 1-alkyl-1H-imidazoles < 1H-imidazoles.
Table 5 contains pKa data for the azole fragment of the ligands
attached to the studied palladium complexes. Comparing the cat-
alytic performance of the palladium complexes with the azole pKa
data does not indicate a clear correlation. Nevertheless, the com-
parison within the 4,5-diphenyl analogs (Pd1, Pd1Me and Pd2) or
within the phenanthreneyl analogs (Pd3, Pd3Me, Pd3ox, Pd4 and
Pd4ox) suggests that higher pKa values (i.e. stronger azole donor
strengths) could be associated with lower catalytic yields. The
exception observed for complex Pd4 within the phenanthreneyl
series can be attributed to steric effects, generated by the presence
of a methyl group in ortho position to the pyridyl nitrogen donor.
Complexes bearing oxazole or alkyl-substituted imidazole ligand
fragments (i.e. azole ligands with relatively lower donor strength)
appeared to furnish more efficient catalytic active sites relative to
their corresponding 1H-imidazole analogs [see Table 5; entry 1
(Pd1 51%) vs. entry 4 (Pd1Me 95%); entry 10 (Pd3 24%) vs. entries
13, 16, and 19 (Pd3Me 88%, Pd3ox 88%, and Pd3oxD 63%)]. In
general, these observations may be considered supportive of the
idea that dissociation of the azole donor fragment of the ligands is
an important step towards generating highly efficient active sites.
Such a hemilabile behavior leading to a pyridyl-coordinated
monodentate derivative has been evidenced by the formation of
Pd3oxD from Pd3ox in the presence of DMSO (vide supra, see Fig. 5).
Moreover, for the highly active palladium complexes containing
weak azole donors (Pd1Me, Pd3Me, Pd3ox, Pd3oxD, and Pd4ox)
the formation of small amounts of sparinglysoluble black particles is
observed during the course of reaction, which is independent of
whether an inert atmosphere is used or not. Characterization by X-
ray powder diffraction indicates the presence of palladium metal in
the black particles precipitated during the reactions, which will
further be referred to as Pd-Black (Supplementary material, Fig. S1).
However, this phenomenon is not observed for the 1H-imidazole
analogs (Pd1, Pd1, Pd3, and Pd4), indicating a higher stability of
their coordinating chelates, which is due to the presence of stronger
azole donors in these cases. Consequently, the formation of Pd-
Black particles can be attributed to a stepwise detachment of donor
functions from the palladium center starting with the azole base,
In a recent investigation, the hemilability of 2-(1H-imidazol-2-
yl)pyridine and 2-(oxazol-2-yl)pyridine derivatives was experi-
mentally and theoretically addressed through protonatione
deprotonation equilibrium studies [28,29]. Donor strengths of lone
pairs of the nitrogen base functions were estimated from their pKa
values. The higher the pKa value the stronger the donor strength of
Table 4
Results for the CeC coupling reaction of A1 and H1 catalyzed by Pd1 under aerobic
conditions with varying time, temperature, catalyst loading, base type and solvent
compositions.a
Entry
Time
(min)
Pd
(mol%)
Base
EtOH:H2O
ratio (mL)
Yield
(%)
TOFb
(hꢁ1
)
1
2
3
4
5
6
7
8
60
30
30
30
15
1.0
1.0
0.2
0.2
0.2
0.2
0.2
0.2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
None
3:1
3:1
3:1
4:0
3:1
3:1
3:1
3:1
93
88
51
14
22
04
22
Trace
93
176
510
140
440
0.17
440
Trace
7200c
15
30
a
Reaction conditions: 1.2 mmol of arylboronic acid A1; 1.0 mmol of aryl halide
H1; 1.5 mmol of base; under reflux; amounts of catalyst loading is given as mol% of
aryl halide in the table.
b
TOF (turnover frequency) ¼ amount of product in moles/(amount of catalyst in
moles ꢂ time in hours).
c
Reaction was stirred for 5 days at room temperature.

A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
45
homogeneous with respect to the heterogeneous surface catalysis.
Similar tests performed with ligandless PdCl2 (catalyst loading
0.34 mol%) also showed that catalytic activities of the complex pre-
catalysts are in most cases significantly higher (see Table 5).
Moreover, it should be noted here that the formation of black
particles was not observed during catalysis with the ligandless
PdCl2 catalyst. This is in line with the view that, for the hemilabile
ligand systems under investigation here, initial chelation of ligands
to the palladium ion and their stepwise dissociation events are
necessary for the formation of the black palladium particles. With
the dissociation events most likely resulting from the palladium
center being involved in the catalytic cycle.
2.6. Effect of base stoichiometry on cross-coupling reactions
Deprotonation of 1H-imidazoles with excess K2CO3 as base is
well-known. This is evidenced by the popular use of K2CO3 as base
reagent in alkylation/arylation of the corresponding nitrogen atom
of the 1H-imidazole ring [27]. Therefore, since a deprotonation
reaction is likely to take place at elevated temperature conditions
employed in the catalytic tests, it is important to investigate
possible effects of the variation of base stoichiometry with respect
to the amount of substrates. Consequently, all complexes were also
studied with reduced amount of K2CO3 base.
Table 5
Results for CeC coupling of A1 and H1 (Scheme 3(a)) using Pd1ePd4ox, PdCl2 and
Pd-Black (Scheme 2).a
TOFb
Azole pKa
c
Entry
Complex
K2CO3
(mmol)
Air or N2
atmosphere
Yield
(%)
(hꢁ1
)
1
2
3
4
5
6
7
8
Pd1
Pd1
Pd1
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
1.0
1.5
1.5
Air
N2
Air
Air
N2
Air
Air
N2
Air
Air
N2
Air
Air
N2
Air
Air
N2
Air
Air
N2
Air
Air
N2
51
48
17
95
82
74
23
24
04
24
27
10
88
82
73
88
46
74
63
88
67
65
12
50
49
29
39
36
39
510
480
170
950
820
740
230
240
40
240
270
100
880
820
730
880
460
740
630
880
670
650
120
500
490
290
390
212
166h
3.71 ꢃ 0.05
Pd1Med
Pd1Med
Pd1Med
Pd2
3.33 ꢃ 0.02
3.75 ꢃ 0.07
2.89 ꢃ 0.02
2.71 ꢃ 0.10
<0.83 ꢃ 0.16
<0.83 ꢃ 0.16
2.97 ꢃ 0.07
<1.26 ꢃ 0.08
Pd2
Pd2
Pd3
Pd3
9
Fig. 6. Reaction progress of the CeC coupling reaction of A1 and H1 monitored by 1H
NMR showing (a) aromatic protons and (b) methylene protons; R ¼ reactant (H1),
P ¼ product, by-products are indicated by asterisks; reaction conditions: 0.2 mol% Pd1,
1.5 mmol K2CO3, 4 mL 3:1 EtOH/H2O.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Pd3
Pd3Med
Pd3Med
Pd3Med
Pd3oxd
Pd3oxd
Pd3oxd
Pd3oxDd,e
Pd3oxDd,e
Pd3oxDd,e
Pd4
which is consistent with the presence of an intermediate palladium
species with monodentate coordination of the chelate ligand. In fact,
that ligand detachment processes are taking part in the reaction
sequence is further evidenced by fluorescence spectra measured
after the catalytic reactions (Supplementary material Fig. S2). This is
based on the fact that the investigated palladium complexes are all
non-fluorescent and consequently the observed intensities quali-
tatively correlate with the fluorescence quantum yields of the free
ligands, which have been previously determined and are particu-
larly high for the phenanthro[9,10-d]oxazoles or methyl-substituted
phenanthro[9,10-d]imidazoles and low for the 4,5-diphenyl-
substituted imidazole ligands or those possessing imidazole proton
[35].
To compare the catalytic activity of the Pd-Black particles
generated from Pd3Me (see Scheme 2), with the activity of the
investigated complex pre-catalysts, catalytic reactions were per-
formed utilizing the same conditions (see Table 5, entry 29). The
only difference in the reaction conditions is that a somewhat higher
catalyst loading of 0.47 mol% of the Pd-Black (i.e. 0.5 mg assuming
particle consists of elementary palladium) was used. Nevertheless,
the obtained yields are lower than those observed for reactions
conducted with the ligated pre-catalysts (cf. Table 5). This obser-
vation may be explained by the superior efficiency of the
Pd4
Pd4
Air
Air
N2
Air
Air
Air
Pd4oxd
Pd4oxd
Pd4oxd
f
PdCl2
Pd-Blackg
a
Reaction conditions: 1.2 mmol of arylboronic acid A1; 1.0 mmol of aryl halide
H1; Pd1ePd4ox complex loading, 0.2 mol%; reaction medium, 4 mL of EtOH/H2O
mixture (3:1); under reflux for 30 min.
b
TOF (turnover frequency) ¼ amount of product in moles/(amount of catalyst in
moles ꢂ time in hours).
c
The pKa values were obtained from Ref. [28].
Black Pd particles were observed on the walls of the reaction flask within mi-
d
nutes upon starting reflux.
e
The crystalline material introduced into the reaction flask not fully dissolved.
PdCl2 loading, 0.34 mol%.
Pd-Black loading, 0.5 mg, i.e. equivalent to 0.47 mol% (based on Pd) or 0.45 mol
f
g
% (based on PdO).
h
Calculated for 0.47 mol% catalyst loading, assuming Pd-Black to be Pd.

46
A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
Table 6
dimethylphenylboronic acid (A2) and (4-bromophenyl)methanol
(H1) as depicted in Scheme 3(b). The results presented in Table 6
reveal complex Pd1Me to be the most reactive pre-catalyst in this
series. However, relative to the reactions involving the p-acetyl
substituted boronic acid A1, the lower yields found for A2 are
attributed to deactivation of the boronic acid reagent by its two
methyl substituents.
Further tests employing the overall most active pre-catalyst
Pd1Me were carried out on sterically demanding substrates. The
results summarized in Table 7 show that the presence of a phenyl
substituent in ortho position to the boronic acid group as in
biphenyl-2-ylboronic acid (A3) did not hamper the cross-coupling
process. In fact, complete conversion to the coupling product was
achieved in less than 30 min for the substrate combination A3 and
H1 (see Table 7: entry 1; Fig. 7).
On the contrary, if the aryl halide carries sterically demanding
substituents, as in the case of 4-bromo-3,5-dimethoxybenzoic acid
H2, the anticipated coupling products were not formed in the re-
actions with the boronic acid derivatives A1 and A3 (see Table 7). In
fact, analysis results reveal that the isolated solid compounds are
decomposition products of the arylboronic acids from which the
boronic acid functions have been lost. For instance 1H and 13C NMR
as well as IR and elemental analysis of the isolated product showed
that attempts to couple A3 with H2 produced as much as 80% of
biphenyl (Table 7, entry 3).
Results for the CeC coupling reaction of A2 and H1 (see Scheme 3(b)) catalyzed by
Pd1, Pd1Me, and Pd3Me.a
Entry
Complex
Yield (%)b
1
2
3
Pd1
Pd1Me
Pd3Me
Trace
72
59
a
Reaction conditions: 1.2 mmol of arylboronic acid A2; 1.0 mmol of aryl halide
H1; palladium complex loading, 0.2 mol%; base, K2CO3 (1.2 mmol); reaction me-
dium, 4 mL of 3:1 EtOH/H2O mixture; under air and reflux for 30 min.
b
Product yields are determined by 1H NMR (see text and Fig. 6).
It was observed that using an equimolar amount of base, instead
of the an excess of 1.5 molar equivalents, caused a significant
reduction in yield of about 35% for complex Pd1 which possesses
the acidic 1H-imidazole proton (see Table 5: entry 1 vs. 3). This is
particularly remarkable, since for the corresponding methyl-
substituted derivative Pd1Me this effect is considerably smaller
(see Table 5: entry 4 vs. 6). Although similar effects could also be
observed for the other pre-catalysts with bidentate 2-(azol-2-yl)
pyridine ligands, the obtained differences are somewhat smaller.
This leads to the general conclusion that complexes coordinated by
ligands with methyl-substituted imidazole or oxazole fragments
show less pronounced effects upon reduction of the stoichiometry
of the employed base. Further evidence that the presence of an
acidic 1H-imidazole proton is important for the observed effects
comes from the virtual independence of the corresponding yields
for the monodentate pre-catalyst Pd3oxD (see Table 5: entry 19 vs.
21).
3. Conclusions
The utilization of a series of derivative of the hemilabile 2-(1H-
imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands has led
to the synthesis of nine new dichloropalladium complexes (Pd1e
Pd4ox). These complexes were tested as pre-catalysts in high
temperature SuzukieMiyaura cross-coupling catalysis in order to
elucidate possible ligand effects. The hemilabile nature of the
bidentate 2-(azol-2-yl)pyridine ligands is related to the donor
strength of the azole fragment and can be tuned by changing the
heteroatom or the substitution pattern around the azole ring. A
lower donor strength of the azole fragment leads to a higher cat-
alytic activity in the cross-coupling reaction, i.e. for the palladium
complexes with oxazole and alkyl-substituted imidazole frag-
ments. Furthermore, it is observed that high catalytic activities are
also obtained for complexes with considerable strain within the
coordination environment of the palladium center. In fact, together
with earlier observations that the azole fragment donor can readily
2.7. Substrate effects on SuzukieMiyaura cross-coupling reactions
In an attempt to qualitatively elucidate the substrate profile of
the pre-catalysts based on 2-(azol-2-yl)pyridine ligands, the cata-
lytic activity of selected palladium complexes were investigated
towards substrates with varying electronic or steric properties.
Besides Pd1, which proved to be a catalyst of medium activity
(standard for the setup of reaction conditions, see vide supra), the
two most reactive complexes Pd1Me and Pd3Me as revealed by the
comparative study (see Table 5) have been utilized here.
To probe the reactivity towards variation of the electronic
properties of the substrate the complexes Pd1, Pd1Me, and Pd3Me
have been applied in the cross-coupling reaction of 3,5-
Table 7
Results for the CeC coupling reaction of sterically demanding substrates catalyzed by Pd1Me.a
Entry
Boronic acid
Aryl halide
Time (h)
Product
Yield (%)b
100
1
0.5
2
20
17
e
3
80
a
Reaction conditions: 1.2 mmol of arylboronic acid; 1.0 mmol of aryl halide; Pd1Me complex loading, 0.2 mol%; base, K2CO3 (1.2 mmol); reaction medium, 4 mL of 3:1
EtOH/H2O mixture; under air and reflux for the given time.
b
Based on NMR and analytical data of isolated products.

A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
47
assessment with respect to the presence of dissociated ligands.
Prior to the fluorescence measurements, each reaction mixture was
diluted by addition of an EtOH/H2O mixture (3:1) to a final volume
of 25 mL and subsequently filtered.
4.3. Syntheses of palladium(II) complexes
PdCl2$2CH3CN was dissolved in dichloromethane (30 mL per
1 mmol). To this solution an equimolar amount of the appropriate
ligand from L1 to L4ox dissolved in dichloromethane (60 mL per
1 mmol) was added dropwise with stirring. The resulting reaction
mixture was stirred at room temperature for an additional 12 h and
the resulting precipitate was filtered off, washed with a small
amount of ethanol and dried to obtain the appropriate complex
from Pd1 to Pd4ox as powdered material.
4.3.1. Dichlorido[2-(4,5-diphenyl-1H-imidazol-2-yl)pyridine]
palladium(II) (Pd1)
Fig. 7. Stack plot of 1H NMR spectra showing the progress of palladium catalyzed
cross-coupling reactions of A3 with H1 (cf. Table 7).
PdCl2$2CH3CN (86 mg, 0.33 mmol) and L1 (99 mg, 0.33 mmol):
Orange powder; yield 138 mg (87%). M.p. >300 ꢀC. Anal. calcd. for
C
20H15Cl2N3Pd$H2O: C, 48.76; H, 3.48; N, 8.53%. Found: C, 48.89; H,
dissociate from the palladium atom, this suggests that related
monodentate species are formed during the catalytic turnover.
Finally, it can be summarized that the overall observed ligand ef-
fects within the investigated series of pre-catalysts suggest the
efficient active catalyst to be a monodentate palladium species in
homogeneous phase rather than naked soluble palladium species
or insoluble particles. Besides these general findings, the catalytic
activities observed for the complexes Pd1Me, Pd3Me, and Pd3ox
are attractive in terms of high-temperature SuzukieMiyaura ho-
mogeneous catalysis. Therefore these complexes deserve further
attention with respect to their possible substrate profile and opti-
mized reactions conditions.
3.15; N, 8.33%. Selected IR data (ATR, cmꢁ1): 3130 (m), 3093 (m),
1620 (m),1607 (m),1585 (s),1505 (s),1462 (s),1446 (s),1161 (s), 781
(vs). 1H NMR (400 MHz, DMSO,
d in ppm) 14.59 (s, 1H), 9.02 (d,
J ¼ 5.7 Hz, 1H), 8.33 (d, J ¼ 4.5 Hz, 2H), 7.68 (dd, J ¼ 9.3, 5.4 Hz, 1H),
7.41e7.27 (m, 10H).
4.3.2. Dichlorido[2-(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)
pyridine]palladium(II) (Pd1Me)
PdCl2$2CH3CN (162 mg, 0.62 mmol) and L1Me (194 mg,
0.62 mmol): Orange powder; yield 246 mg (81%). M.p. >300 ꢀC.
Anal. calcd. for C21H17Cl2N3Pd$½H2O: C, 50.68; H, 3.65; N, 8.44%.
Found: C, 50.79; H, 3.27; N, 8.50%. Selected IR data (ATR, cmꢁ1):
3110 (w), 3033 (w), 1604 (s), 1566 (w), 1527 (s), 1443 (vs), 1231 (m),
4. Experimental
781 (vs). 1H NMR (400 MHz, DMSO,
1H), 8.28 (dd, J ¼ 12.7, 7.3 Hz, 2H), 7.71 (d, J ¼ 5.9 Hz, 1H), 7.43 (m,
4H), 7.25e7.13 (m, 6H), 3.89 (s, 3H).
d
in ppm) 9.16 (d, J ¼ 5.4 Hz,
4.1. Materials and methods
The 2-(1H-imidazol/oxazol-2-yl)pyridine ligands [27,28] and
complexes Pd3, Pd3ox, and Pd3oxD [28] (Scheme 1) were pre-
pared according to previously described procedures. PdCl2$2CH3CN
was obtained as yellow crystalline material by refluxing palladium
dichloride in acetonitrile, which was initially purged with argon
[36]. Dichloromethane was purified and distilled prior to use. All
other chemicals were purchased from commercial suppliers and
used without further purification. The EtOH/H2O mixtures used for
catalysis reactions under argon protection were degassed through
freezeevacuumethaw cycles. Reactions requiring exclusion of air
were performed under argon atmosphere using standard Schlenk
techniques. Products of organic reactions were purified on silica gel
column to exclude impurities.
4.3.3. Dichlorido[2-methyl-6-(4,5-diphenyl-1H-imidazol-2-yl)
pyridine]palladium(II) (Pd2)
PdCl.22CH3CN (80 mg, 0.31 mmol) and L2 (96 mg, 0.31 mmol):
Yellow powder; yield 98 mg (65%). M.p. >300 ꢀC. Anal. calcd. for
C
21H17Cl2N3Pd$½H2O: C, 50.68; H, 3.65; N, 8.44%. Found: C, 50.52;
H, 3.55; N, 8.16%. Selected IR data (ATR, cmꢁ1): 3227 (m, br), 1623
(m), 1604 (m), 1564 (m), 1442 (s), 1157 (s), 793 (vs), 772 (vs). 1H
NMR (400 MHz, DMSO,
d
in ppm) 13.80 (s, 1H), 8.20 (d, J ¼ 7.8 Hz,
1H), 8.03 (d, J ¼ 7.8 Hz,1H), 7.99 (d, J ¼ 6.4 Hz,1H), 7.55 (dd, J ¼ 22.0,
7.9 Hz, 4H), 7.40 (m, 6H), 2.90 (s, 3H).
4.3.4. Dichloro{1-methyl-2-(pyridin-2-yl)-1H-phenanthro[9,10-d]
imidazole}palladium(II) (Pd3Me)
4.2. Physical measurements
PdCl2$2CH3CN (58 mg, 0.22 mmol) and L3Me (69 mg,
0.22 mmol): Orange micro-crystals; yield 96 mg (89%). M.p.
>300 ꢀC. Anal. calcd. for C21H15Cl2N3Pd: C, 51.82; H, 3.11; N, 8.63%.
Found: C, 52.07; H, 2.94; N, 8.59%. Selected IR data (ATR, cmꢁ1):
3092 (w), 3075 (w), 1606 (s), 1573 (m), 1516 (s), 1482 (vs), 1453 (vs),
IR spectra in the range between 4000 and 400 cmꢁ1 were
recorded on neat samples utilizing a Bruker IFS55/EQUINOX spec-
trometer equipped with a Specac Golden-Gate ATR unit. 1H NMR,
13C NMR, and 2D COSY NMR experiments were carried out on a
Bruker Avance 400 MHz spectrometer. Elemental analyses were
done on Flash EA 1112 microanalyzer of CE Instruments, Leco
CHNS-932 or El Vario III elemental analyzers. Fluorescence spectra
of all reaction mixtures were recorded after catalysis with a Jasco
1241 (s), 719 (vs). 1H NMR (400 MHz, DMSO,
d in ppm) 8.98 (d,
J ¼ 8.2 Hz, 1H), 8.87 (d, J ¼ 8.3 Hz, 1H), 8.80 (d, J ¼ 4.6 Hz, 1H), 8.69
(d, J ¼ 7.8 Hz, 1H), 8.63 (d, J ¼ 7.8 Hz, 1H), 8.36 (d, J ¼ 7.9 Hz, 1H),
8.07 (t, J ¼ 8.4 Hz,1H), 7.77 (m, 3H), 7.67 (t, J ¼ 7.6 Hz,1H), 7.57e7.53
(m, 1H), 4.72 (s, 3H).
FP-6300 spectrofluorometer in order to make
a qualitative

48
A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
Table 8
Crystal and structural refinement data for complexes Pd1, Pd1Me, Pd3, and Pd3Me.
Pd1$DMF
Pd1Me
Pd3$DMF
Pd3Me
Formula
C23H22Cl2N4OPd
547.75
Monoclinic
C2/c
21.100(4)
13.315(3)
18.409(4)
90
118.17(3)
90
C21H17Cl2N3Pd
488.68
Monoclinic
C2/c
10.665(2)
9.794(2)
22.088(4)
90
97.083(2)
90
C23H20Cl2N4OPd
545.75
Monoclinic
P2(1)/c
8.4913(2)
11.359(2)
12.622(3)
90
125.513(19)
90
C21H15Cl2N3Pd
486.66
Formula weight
Crystal system
Space group
Orthorhombic
P2(1)2(1)2(1)
13.437(3)
9.7111(2)
18.033(4)
90
90
90
1806.0(6)
4
1.790
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
3
ꢀ
Volume (A )
Z
4559.4(16)
8
1.596
3815.1(3)
8
1.702
4235.0(2)
8
1.712
rcalcd (g cmꢁ3
)
m
(mmꢁ1
)
1.071
1.264
1.153
1.335
F(000)
range (ꢀ)
Index ranges
2280
1952
2192
986
q
1.88e27.41
ꢁ27 ꢄ h ꢄ 27
ꢁ17 ꢄ k ꢄ 16
ꢁ23 ꢄ l ꢄ 23
10,011
5184 (0.0350)
290
1.393
2.71e27.50
ꢁ19 ꢄ h ꢄ 19
ꢁ14 ꢄ k ꢄ 13
ꢁ28 ꢄ l ꢄ 29
10,793
4276 (0.0460)
245
1.095
1.03e27.36
ꢁ31 ꢄ h ꢄ 31
ꢁ9 ꢄ k ꢄ 9
ꢁ38 ꢄ l ꢄ 38
18,049
9592 (0.0580)
559
1.306
2.23e27.47
ꢁ14 ꢄ h ꢄ 13
ꢁ15 ꢄ k ꢄ 16
ꢁ17 ꢄ l ꢄ 17
15,445
4117 (0.0520)
245
1.124
Reflections collected
Unique data (Rint
)
No. of parameters
Goodness-of-fit on F2
Final R indices (I > 2
R indices (all data)
Flack parameter
s
(I))
R1 ¼ 0.0377
wR2 ¼ 0.0865
R1 ¼ 0.0452
wR2 ¼ 0.0922
e
R1 ¼ 0.0507
wR2 ¼ 0.0925
R1 ¼ 0.0644
wR2 ¼ 0.0984
e
R1 ¼ 0.0700
wR2 ¼ 0.1116
R1 ¼ 0.1139
wR2 ¼ 0.1383
e
R1 ¼ 0.0454
wR2 ¼ 0.1172
R1 ¼ 0.0534
wR2 ¼ 0.1238
ꢁ0.09(5)
0.839, ꢁ0.106
ꢀꢁ3
Largest diff. peak and hole (e A
)
0.613, ꢁ0.756
0.644, ꢁ0.707
1.088, ꢁ1.480
4.3.5. Dichlorido{2-(6-methylpyridin-2-yl)-1H-phenanthro[9,10-d]
imidazole}palladium(II) (Pd4)
(400 MHz, CDCl3, d in ppm): 197.71 (C]O), 141.00, 139.23, 135.92,
128.93, 127.53, 127.45, 127.15, 64.96 (CH2OH), 26.63 (CH3).
PdCl2$2CH3CN (156 mg, 0.60 mmol) and L4 (186 mg,
0.60 mmol): The initially formed yellow solid subsequently turned
red upon standing for several minutes; red powder; yield 270 mg
(91%). M.p. >300 ꢀC. Anal. calcd. for C21H15Cl2N3Pd: C, 51.82; H,
3.11; N, 8.63%. Found: C, 51.62; H, 3.14; N, 8.34%. Selected IR data
(KBr, cmꢁ1): 3113 (m, br), 3076 (m), 1611 (s), 1566 (m), 1522 (m),
4.4.2. (30,50-Dimethylbiphenyl-4-yl)methanol (A2H1)
1H NMR (400 MHz, TMS, CDCl3,
d
in ppm); 7.58 (d, J ¼ 8.0 Hz, 2H,
p-phenylene), 7.42 (d, J ¼ 8.0 Hz, 2H, p-phenylene), 7.22 (s, 2H,
dimethyl-phenyl), 7.01 (s, 1H, dimethyl-phenyl), 4.74 (s, 2H, CH2),
2.39 (s, 6H, CH3). 13C NMR (400 MHz, CDCl3,
d in ppm): 140.91,
1473 (s), 1427 (s), 1152 (s), 757 (vs). 1H NMR (400 MHz, DMSO,
d in
140.84, 139.69, 138.30, 131.64, 128.98, 128.58, 127.35, 125.03, 65.18
(CH2OH), 21.40 (CH3).
ppm) 14.53 (s, 1H), 9.11e8.92 (m, 3H), 8.86 (d, J ¼ 8.2 Hz, 2H), 8.23
(d, J ¼ 7.7 Hz, 1H), 8.16 (q, J ¼ 7.9 Hz, 1H), 7.91 (t, J ¼ 7.7 Hz, 1H), 7.73
(t, J ¼ 7.4 Hz, 1H), 7.65 (t, J ¼ 7.0 Hz, 2H), 7.38 (d, J ¼ 7.6 Hz, 1H), 7.38
(d, J ¼ 7.6 Hz, 1H), 2.69 (s, 3H).
4.4.3. Biphenyl (formed instead of A3H2)
Anal. calcd. for C12H10: C, 93.46; H, 6.54%. Found: C, 93.06; H,
6.53%. 1H NMR (400 MHz, TMS, CDCl3,
d
in ppm); 7.60 (d, J ¼ 4.8 Hz,
4H, o-protons); 7.45 (dd, J ¼ 4.8 Hz, 4H, m-protons); 7.35 (dd,
4.3.6. Dichlorido{2-(6-methylpyridin-2-yl)phenanthro[9,10-d]
oxazole}palladium(II) (Pd4ox)
J ¼ 5.2 Hz, 2H, p-protons). 13C NMR (400 MHz，CDCl3,
d in ppm):
141.22, 128.74, 127.24, 127.17.
PdCl2$2CNCH3 (118 mg, 0.46 mmol) and L4ox (142 mg,
0.46 mmol): Yellow powder; yield 201 mg (91%). M.p. 295e297 ꢀC.
Anal. calcd. for C21H14Cl2N2OPd$1/3 H2O: C, 51.09; H, 2.99; N, 5.67%.
Found: C, 51.12; H, 2.98; N, 5.68%. Selected IR data (KBr, cmꢁ1): 3089
(vw), 3058 (vw), 1608 (m), 1573 (s), 1552 (m), 1453 (s), 1431 (m),
1248 (m), 1245 (vs), 755 (vs), 722 (vs). 1H NMR (400 MHz, DMSO,
4.5. Typical catalytic procedure
Aryl bromide (1 mmol), arylboronic acid (1.2 mmol), potassium
carbonate (1.5 mmol), and palladium complex (0.2 mol%) were
placed in a 10 mL round-bottom flask equipped with magnetic
stirrer bar. Upon addition of ethanol (3 mL) and deionized water
(1 mL), a clear solution was obtained which was then refluxed for
the desired reaction time. After cooling, an aliquot of the reaction
mixture was taken and washed slowly into an NMR tube over a
short bed of anhydrous sodium sulfate with the aid of deuterated
chloroform in order to minimize residual water from the reaction
mixture. The reactivity of the pre-catalysts was evaluated by
comparison of the NMR signals observed for the methylene protons
of (4-bromophenyl)methanol (H1) and the corresponding product
formed. The yield was calculated from the integral of the product
peak as a percentage of the sum of all observed methylene signals.
d
in ppm) 9.07e8.93 (m, 2H), 8.54 (d, J ¼ 8.9 Hz, 1H), 8.40 (d,
J ¼ 7.6 Hz, 1H), 8.28 (d, J ¼ 7.7 Hz, 1H), 7.99 (s, 1H), 7.84 (s, 4H), 7.52
(d, J ¼ 7.7 Hz, 1H), 2.67 (s, 3H).
4.4. Characterization data for some catalysis products
4.4.1. 1-{40-(Hydroxymethyl)biphenyl-4-yl}ethanone (A1H1)
Anal. calcd. for C15H14O2: C, 79.62; H, 6.24%. Found: C, 79.52; H,
6.31%. 1H NMR (400 MHz, TMS, CDCl3,
d
in ppm); 8.01 (d, J ¼ 8.0 Hz,
2H), 7.66 (d, J ¼ 8.4 Hz, 2H), 7.61 (d, J ¼ 8.4 Hz, 2H), 7.45 (d,
J ¼ 8.0 Hz, 2H), 4.75 (s, 2H, CH2), 2.62 (s, 3H, CH3). 13C NMR

A.O. Eseola et al. / Journal of Organometallic Chemistry 754 (2014) 39e50
49
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graphite monochromated Mo-K
a
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¼ 0.71073 A). Crys-
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Acknowledgments
A.O.E thanks Prof. J. A. Emmanuel for useful ideas and Re-
deemer’s University for granting a study leave. The authors are
thankful for generous grants from the Deutsche For-
schungsgemeinschaft, the Academy of Sciences for the Developing
World (TWAS e Italy), and the Chinese Academy of Sciences for
financial supports (TWAS-DFG and CAS-TWAS).
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Appendix A. Supplementary material
CCDC 803648e803650 and 805529 contain the supplementary
crystallographic data for Pd1, Pd1Me, Pd3, and Pd3Me, respec-
tively. These data can be obtained free of charge from the Cam-
bridge Crystallographic Data Center via: www.ccdc.cam.ac.uk/
data_request/cif.
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