Hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands: Imidazole and oxazole ring Lewis basicity, Ni(II)/Pd(II) complex structures and spectra

Article abstract of DOI:10.1016/j.poly.2010.02.039

Fourteen new organic molecules A1-A4, B1-B5, C1-C4 and D and a series of transition metal(II) complexes (Ni1-Ni9 and Pd1-Pd2b) were synthesized and studied in order to characterize the hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridi

Full text of DOI:10.1016/j.poly.2010.02.039

Polyhedron 29 (2010) 1891–1901
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine
ligands: Imidazole and oxazole ring Lewis basicity, Ni(II)/Pd(II) complex
structures and spectra
c
a
b
b,
**
Abiodun O. Eseola a,b, , Wen Li , Olalere G. Adeyemi , Nelson O. Obi-Egbedi , Joseph A.O. Woods
*
a Chemical Sciences Department, Redeemer’s University, Redemption City, Km. 46 Lagos–Ibadan Expressway, Nigeria
b Inorganic and Physical Chemistry Units, Department of Chemistry, University of Ibadan, Ibadan, Nigeria
c Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Fourteen new organic molecules A1–A4, B1–B5, C1–C4 and D and a series of transition metal(II) com-
plexes (Ni1–Ni9 and Pd1–Pd2b) were synthesized and studied in order to characterize the hemilability
of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands (A1–A4 = 2-R2-6-(4,5-diphenyl-1R1-
Received 19 January 2010
Accepted 26 February 2010
Available online 12 March 2010
imidazol-2-yl)pyridines,
R
1 = H or CH3, R2 = H or CH3; B1–B5 = 1-R2-2-(pyridin-2-yl)-1R1-phenan-
thro[9,10-d]imidazoles/oxazoles, R1 = H or CH3, R2 = H or CH3; C1–C4 = 2-(6-R2-pyridin-2-yl)-1H-imi-
dazo/oxazo[4,5-f][1,10]phenanthrolines, R2 = H or CH3; D = 2-mesityl-1H-imidazo[4,5-f][1,10]phenan-
throline). They were also used to study the substituent effects on the donor strengths as well as the coor-
dination chemistries of the imidazole/oxazole fragments of the hemilabile ligands.
Keywords:
Hemilability
Lewis basicity
Protonation–deprotonation
Self-assembly
All the observed protonation–deprotonation processes found within pH 1–14 media pertain to the
imidazole or oxazole rings rather than the pyridyl Lewis bases. The donor characteristics of the imidaz-
ole/oxazole ring can be estimated by spectroscopic methods regardless of the presence of other strong N
donor fragments. The oxazoles possessed notably lower donor strengths than the imidazoles. The elec-
tron-withdrawing influence and capacity to hinder the azole base donor strength of 4,5-azole substitu-
ents were found to be in the order phenanthrenyl (B series) > 4,5-diphenyl (A series) > phenanthrolinyl
(C series). An X-ray structure of Ni5b gave evidence for solvent induced ligand reconstitution while
the structure of Pd2b provided evidence for solvent induced metal–ligand bond disconnection.
Interestingly, alkylation of 1H-imidazoles did not necessarily produce the anticipated push of electron
density to the donor nitrogen. Furthermore, substituents on the 4,5-carbons of the azole ring were more
important for tuning donor strength of the azole base. DFT calculations were employed to investigate the
observed trends. It is believed that the information provided on substituent effects and trends in this fam-
ily of ligands will be useful in the rational design and synthesis of desired azole-containing chelate
ligands, tuning of donor properties and application of this family of ligands in inorganic architectural
designs, template-directed coordination polymer preparations, mixed-ligand inorganic self-assemblies,
etc.
Substituent effects
DFT calculations
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
mechanisms of complicated systems [4,5]. Imidazole and oxazole
compounds have drawn considerable attention for their desirable
Research efforts on the science and application of intermolecular
or intramolecular hydrogen bonding interactions are still receiving
attention [1–3]. Investigations on hydrogen-bonded species of rel-
atively simple molecules is routinely applied towards understand-
ing protonation–deprotonation and proton/electron transfer
properties in the fields of coordination chemistry, photolumines-
cence/pH sensing studies, biochemistry of metalloenzymes, medic-
inal chemistry, etc. [6–12]. Moieties such as adenine, guanine, and
histidine, contain the imidazole ring and are involved in intramo-
lecular and/or intermolecular chemistries of biological macromole-
cules under pH specific conditions [1,13–15]. The imidazole
fragment of histidine is recognized to be an interesting ligand in
the bioinorganic chemistry of most hemoproteins and studies on
the basicity and steric effects using synthetic imidazole compounds
provide an understanding of the roles played by such donor frag-
ments in biochemical reaction mechanisms [10,14,15].
* Corresponding author at: P.M.B. 3005, Redemption City, Ogun, Nigeria. Tel.:
+230 (0) 7062192104.
** Corresponding author. Tel.: +234 (0) 8060156565.
E-mail addresses: bioduneseola@run.edu.ng (A.O. Eseola), jao.woods@mail.ui.
edu.ng, jaowoods@yahoo.com (J.A.O. Woods).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.02.039

1892
A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
Ligand chemical recognition is an important phenomenon in
electronic properties by subjecting a series of related organic bases
to the same media conditions. Aqueous-organic mixtures are often
employed at various pH for ionization constant determinations
[44]. Computational works, largely restricted to 2-(1H-
benzo[d]imidazol-2-yl)phenols, are known and the B3LYP/6-
311+G basis set has been successfully employed as a reliable
method in DFT calculations of imidazole-containing organic sys-
tems [47–49].
Herein, we present our results on the syntheses and compara-
tive studies of substituent effects on imidazole/oxazole Lewis base
donor characteristics for a series of new, hemilabile 2-(1H-imida-
zol-2-yl)pyridines/2-(oxazol-2-yl)pyridines (Scheme 1). For com-
parison, 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (D)
was also prepared and studied. Spectroscopic properties of the di-
halo Ni(II) complexes (Ni1–Ni9) and dichloro Pd(II) complexes
(Pd1 and Pd2a), resulting from coordination of the respective
M(II) halides with members of the A and B ligand series, were
examined for clues on ligand donor characteristics. He X-ray struc-
tures of Ni5b and Pd2b are also presented as further evidence for
the observed trends. The calculated properties, based on optimized
geometries of the ligand molecules, and experimental 1H NMR and
FTIR data of the imidazole active protons were obtained and dis-
cussed in an attempt to rationalize the experimentally observed li-
gand properties.
the science of inorganic coordination polymers and supramolecu-
lar self-assembly complex architectures, which are rapidly expand-
ing structural chemistry fields, and it is possible to control 3-D
structures by rational ligand framework modifications as well as
by careful selection of the metal ion [16–21]. Ligand exchange pro-
cesses, which are dependent on relative ligand field strengths and/
or steric requirements of leaving and incoming ligands, are very
important in the synthetic chemistry of coordination complexes
[22–24]. Therefore, knowledge of lability is important towards
the successful execution of template-directed coordination poly-
mer preparations or ligand exchange reactions [25–30]. The gen-
eral conclusions from isolated cases report that the N donors of
azoles possess pKa values ꢀ 6–9, which are within the pH scale
[31–34]. However, on account of substituent effects, a variation
of pKa in the range 1.1–6.4 was obtained in our recent studies on
a series of new 2-(1H-imidazol-2-yl)phenols in which distant phe-
nol ring substituents were observed to be more effective in tuning
the basicity of the imidazole fragment [35]. Among the azole li-
gands, imidazoles are more attractive due to the anticipated oppor-
tunities of directly modifying the electronic nature of the imidazole
N donor through substitution of the active imidazole proton [36].
For instance, an attempt to modifying electrophilicity of transition
metal active sites towards olefin homogeneous polymerization
through imidazole proton alkylation was recently reported [37].
Furthermore, imidazole ligands could also find application in sur-
face chemistry because they can be immobilized on solid supports
through the imidazole proton [38].
*
2. Experimental
2.1. General considerations
Works concerning azole donor chelate ligands have been largely
restricted to benzo[d]imidazoles, benzo[d]oxazoles and benzo
[d]thiazoles [11,33,34,37]. Despite the considerable interest in
azole ligands, a systematic investigation of substituent effects on
the azole ring donor characteristics is still lacking. The concept of
combining hard and soft donors such as N–P, into the same chela-
tor is of current interest [39], but azole based chelators are yet to
be characterized or exploited as hemilabile ligands. Primarily, the
interest in this work resulted from observed lability of metal com-
plexes of some bidentate 2-(1H-imidazol/oxazol-2-yl)phenols and
2-(1H-imidazol/oxazol-2-yl)pyridines during attempts to prepare
their Zn(II) and Ni(II) complexes, respectively, despite expected
chelate effects [40,41]. For some of the ligands, coordination was
difficult to attain or solvent molecules, such as methanol, readily
and completely detached the weakly chelating imidazole/oxazole
ligands.
Spectroscopic determination of ionization constants of weak
acids or bases is of widespread application and considered suitable
even under extremes of pH, poor solute solubilities and for species
prone to conformational photo-tautomerism [42,43]. For instance,
the pKa of a drug is one of the most important parameters em-
ployed to explain its physicochemical activities, acid–base proper-
ties and design of pharmaceutical pre-formulations [44]. Ionization
constants of organic materials are strongly solvent dependent
[45,46]. Our aim in this work was to study the trends in inherent
All manipulations of oxidation-prone reactions were performed
under a nitrogen atmosphere using standard Schlenk techniques.
All starting materials were obtained commercially as reagent grade
and were used without further purification. Compounds A1–A4,
B1, B2, B4 and Ni1–Ni9 were prepared according to recently pub-
lished procedures [41]. 1,10-Phenanthroline-5,6-dione was syn-
thesized according to a known method [50]. The synthesized
organic compounds were purified on a silica gel column to exclude
impurities. Elemental analyses were performed on a Flash EA 1112
microanalyzer, CE Instruments. 1H and 13C NMR spectra were re-
corded on a Bruker ARX-400 MHz instrument using deuterated
chloroform as the solvent and TMS as an internal standard. IR spec-
tra were recorded on a Nicolet 6700 FTIR spectrometer or Shima-
dzu 8740 FTIR spectrometer as KBr discs in the range 4000–
400 cmꢁ1. UV–Vis measurements were recorded on Shimadzu
1600 spectrophotometer. The pKa determination experiments were
carried out using ꢀ 10ꢁ5 M solutions of the compounds in an eth-
anol–water mixture which consisted of 70% absolute EtOH and
30% H2O. A calculated amount of potassium chloride was used to
achieve the 0.1 M concentration of the salt in order to maintain a
fairly constant ionic strength. The adjustment of the pH of solu-
tions was achieved through the addition of potassium hydroxide
or hydrochloric acid solutions and monitored with the aid of a cal-
ibrated pH meter. The pH measurements were made on an Omega
R2
B series R2
R2
R2
A series R1 R2
R1
N
N
N
Y
N
Y
Y
NH
O
N
C series R2
Y
HN
N
N
B1
B2
B3
B4
B5
H
H
H
N
N
D
A1
H
CH3
H
H
H
CH3
N
C1
C2
C3
C4
H
H
NH
O
NCH3
N
A2
A3
A4
N
CH3 NH
CH3
CH3 NH
CH3
O
CH3 CH3
O
Scheme 1. Ligand frameworks investigated in this work.

A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
1893
PHB-212 Microprocessor pH Meter using buffers of pH 4.0 and 9.2
as calibrants. Optimized geometries and calculated properties
were obtained by DFT calculations conducted at the B3LYP/6-
tates were filtered, washed with small amount of ethanol and dried
to afford isolated products of C1 (2.60 g, 36.4%) and C2 (0.67 g,
9.4%) as white powders. Characterization data are as follows:
C1: M.p. >300 °C. Selected IR peaks (KBr disc, cmꢁ1):
m 3008 m,
*
311+G level using the GAUSSIAN 98 package of programs [51]. Input
files were created based on simulated dielectric constants for a 70%
ethanol–water mixture.
2944 m, 1590s, 1562s, 1431vs, 1070 m, 738vs. 1H NMR (400 MHz,
TMS, CDCl3) (d, ppm): 11.60 (br, s, 1H); 9.17 (d, J = 4.0 Hz, 2H); 9.05
(d, J = 8.0 Hz, 1H); 8.67 (d, J = 4.4 Hz, 1H); 8.52 (d, 7.6 Hz, 1H); 8.45
(d, J = 8.0 Hz, 1H); 7.92 (dd, J = 8 Hz, 1H); 7.74 (dd, J = 4.4, 8.0 Hz,
1H); 7.68 (dd, J = 4.4, 8.0 Hz, 1H); 7.39 (dd, J = 7.6 Hz, 1H). 13C
NMR (100 MHz, CDCl3) (d, ppm): 149.82, 149.17 148.84, 148.78,
148.05, 144.95, 144.78, 137.48, 137.35, 130.34, 128.85, 125.79,
124.40, 123.40, 122.84, 121.26, 118.95. Anal. Calc. for
C18H11N5ꢂ½H2O: C, 70.58; H, 3.95; N, 22.86. Found: C, 70.96; H,
3.66; N, 22.93%.
2.2. Preparation of imidazoles/oxazoles
2.2.1. 1-Methyl-2-(pyridin-2-yl)-1H-phenanthro[9,10-d]imidazole
(B3)
2-(Pyridin-2-yl)-1H-phenanthro[9,10-d]imidazole (0.50 g, 1.693
mmol) and K2CO3 (0.47 g, 3.39 mmol) were refluxed in acetonitrile
for 10 min followed by addition of iodomethane (0.2 mL,
ꢀ1.5 equiv.). Refluxing was continued for 10 h. The reaction flask
was cooled and the solvent removed under vacuum. The residue
was extracted with 100 mL dichloromethane and the organic sol-
vent was removed under vacuum. The crude product was purified
over silica gel with 15:5:1 petroleum ether/ethyl acetate/acetone
as eluent to obtain the white micro-crystalline compound B3
(0.21 g, 40%). M.p. 204–206 °C. Selected IR peaks (KBr disc, cmꢁ1):
C2: M.p. 269–271 °C. Selected IR peaks (KBr disc, cmꢁ1):
m
3053s, 2982m, 1584m, 1560m, 1456vs, 1446vs, 1059s, 741vs. 1H
NMR (400 MHz, TMS, CDCl3) (d, ppm): 9.26 (d, J = 3.8 Hz, 2H);
9.03 (dd, J = 1.7, 8.1 Hz, 1H); 8.91 (d, J = 4.5 Hz, 1H); 8.82 (dd,
J = 1.7, 8.1 Hz, 1H); 8.47 (d, J = 7.9 Hz, 1H); 7.97 (dd, J = 7.9 Hz,
1H); 7.97 (m, 2H); 7.51 (dd, J = 4.8, 7.5 Hz, 1H). 13C NMR
(100 MHz, CDCl3) (d, ppm): 162.09, 150.47, 150.02, 149.82,
145.73, 145.19, 145.07, 144.04, 137.22, 134.77, 130.86, 129.01,
125.53, 123.64, 123.38, 123.25, 122.94, 117.97. Anal. Calc. for
C18H10N4Oꢂ½H2O: C, 70.35; H, 3.61; N, 18.23. Found: C, 70.52; H,
3.65; 18.20%.
m
3041m, 2947m, 1607m, 1583s, 1454vs, 1237m, 799s, 752vs. 1H
NMR (d, ppm): 8.84 (d, J = 7.97 Hz, 1H); 8.79 (d, J = 7.88 Hz, 1H);
8.74 (d, J = 4.71 Hz, 1H); 8.70 (d, J = 8.26 Hz, 1H); 8.57 (d, J =
8.11 Hz, 1H); 8.44 (d, J = 7.924 Hz, 1H); 7.91 (dd, J = 7.77 Hz, 1H);
7.66 (m, 4H); 7.37 (dd, J = 4.9 Hz, 1H); 4.75 (s, 3H). Anal. Calc. for
C21H15N3: C, 81.53; H, 4.89; N, 13.58. Found: C, 81.29; H, 4.92; N,
13.50%.
2.2.4. 2-(6-Methylpyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline
(C3) and 2-(6-methylpyridin-2-yl)oxazolo[5,4-f][1,10]phenanthroline
(C4)
2.2.2. 2-(6-Methylpyridin-2-yl)phenanthro[9,10-d]oxazole (B5)
Phenanthrenequinone (1.00 g, 4.803 mmol) and 6-methylpyri-
dine-2-carboxaldehyde (0.64 g, 5.283 mmol) were refluxed in a
chloroform (5 mL)/ethanol (5 mL) mixture for 3 h in the presence
of ammonium acetate (7.40 g, 96.006 mmol) and glacial acetic acid
(0.5 mL) as a catalyst. The reaction solution was cooled and stirred
with a few drops of concentrated aqueous ammonia at room tem-
perature to neutralize residual acid. The mixture was extracted
twice with dichloromethane (60 mL and 20 mL) and the combined
organic extract was concentrated under vacuum. The crude mix-
ture was purified by column chromatography on a silica gel col-
umn using petroleum ether/dichloromethane (1:4) as the eluent.
The eluent portion containing the product was collected and con-
centrated under vacuum. Addition of petroleum ether precipitated
B3 as microcrystalline needles which were filtered washed with
petroleum ether and dried under vacuum at 60 °C (0.43 g, 28.3%).
1,10-Phenanthroline-5,6-dione (3.00 g, 0.014 mol), ammonium
acetate (22 g, 0.29 mol) and 6-methylpyridine-2-carboxaldehyde
(2.1818 g, 1.25 equiv.) were reacted according to the procedure
for C1 and C2 above to obtain C3 (1.10 g, 24.5%) and C4 (0.37 g,
8.2%) as light yellow and purple micro-crystals, respectively.
C3: Mp. >300 °C. Selected IR peaks (KBr disc, cm-1):
m 3057s,
3016s, 1590m, 1562s, 1440s, 1068s, 739vs. 1H NMR (400 MHz,
TMS, CDCl3 (d, ppm): 11.60 (br, s, 1H); 9.17 (d, J = 4.1 Hz, 2H);
9.05 (d, J = 8.2 Hz, 1H); 8.49 (d, J = 7.7 Hz, 1H); 8.52 (d, 7.6 Hz,
1H); 8.30 (d, J = 8.0 Hz, 1H); 7.80 (dd, J = 7.7 Hz, 1H); 7.74 (dd,
J = 4.4, 8.0 Hz, 1H); 7.68 (dd, J = 4.4, 8.0 Hz, 1H); 7.24 (d, partially
overlapped with a CDCl3 residual peak, 1H); 2.66(s, 3H). Anal. Calc.
for C19H13N5: C, 72.25; H, 4.31; N, 22.17. Found: C, 72.47; H, 4.26;
N, 22.05%.
C4: M.p. 266–268 °C. Selected IR peaks (KBr disc, cmꢁ1):
m
3016s, 1587s, 1558s, 1459vs, 1062s, 739vs. 1H NMR (400 MHz,
TMS, CDCl3 (d, ppm): 9.27 (dd, J = 2.0, 5.7 Hz, 2H); 9.05 (dd,
J = 2.1, 10.8 Hz, 1H); 8.02 (dd, J = 2.1, 10.9 Hz, 1H); 8.28 (d,
J = 10.3 Hz, 1H); 7.80 (m, 3H); 7.38 (d, J = 10.3 Hz, 1H); 2.79 (s,
3H). 13C NMR (100 MHz, CDCl3) (d, ppm): 162.50, 159.85, 150.03,
149.75, 145.24, 145.09, 144.07, 137.40, 134.87, 131.20, 129.21,
125.54, 123.65, 123.40, 123.07, 120.62, 118.09, 24.79. Anal. Calc.
for C19H12N4O: C, 73.07; H, 3.87; N, 17.94. Found: C, 72.98; H,
3.85; N, 17.87%.
M.p. 199–201 °C. Selected IR peaks (KBr disc, cmꢁ1):
m 3061s,
1616s, 1591s, 1569vs, 1459vs, 1429vs, 1233vs, 1080vs, 798vs,
761vs. 1H NMR (400 MHz, TMS, CDCl3) (d, ppm): 8.75 (m, 3H, phen-
anthrene); 8.47 (d, J = 8.03 Hz, 1H, phenanthrene); 8.28 (d,
J = 7.78 Hz, 1H, py); 7.83 (dd, J = 7.77 Hz, 1H, py); 7.73 (m, 4H,
phenanthrene); 7.33 (d, J = 7.72 Hz, 1H, py); 2.79 (s, 3H, Me). 13C
NMR (100 MHz, CDCl3) (d, ppm): 160.96, 159.58, 150.02, 145.72,
145.57, 137.22, 135.59, 129.77, 129.11, 127.48, 127.27, 126,82,
126.36, 126.18, 124.97, 123.71, 123.40, 123.29, 121.47, 121.03,
120.33, 24.77. Anal. Calc. for C21H14N2Oꢂ(4/5)H2O: C, 77.66; H,
4.84; N, 8.63. Found: C, 77.48; H, 4.85; N, 8.65%.
2.2.5. 2-Mesityl-1H-imidazo[4,5-f][1,10]phenanthroline (D)
1,10-Phenanthroline-5,6-dione (0.50 g, 2.37 mmol), mesitalde-
hyde (0.35 g, 2.37 mmol), ammonium acetate (3.65 g, 47.40 mmol)
and glacial acetic acid (10 mL) were heated under reflux conditions
for 2 h, followed by cooling, dilution in ꢀ20 mL water and neutral-
ization with concentrated aqueous ammonia solution. The crude
product was filtered off as a yellow precipitate which was recrys-
tallized from ethanol to obtain compound D as microcrystals
(0.75 g, 93%). M.p./Dec. >300 °C. Selected IR peaks (KBr disc,
2.2.3. 2-(Pyridin-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (C1)
and 2-(pyridin-2-yl)oxazolo[5,4-f][1,10]phenanthroline (C2)
1,10-Phenanthroline-5,6-dione (5.00 g, 0.024 mol), ammonium
acetate (37 g, 0.480 mol), glacial acetic acid as a catalyst (1 mL)
and 2-pyridinecarboxaldehyde (2.86 mL, 0.030 mol) were reacted
according to the procedure for B5 above, except that the crude
mixture was purified on a silica gel column using ethanol/ethylace-
tate/petroleum ether (1:10:10) as the eluent. The portions that
contained the pure products were concentrated and the precipi-
cmꢁ1):
m
3272vs, 3061s, 1610m, 1542s, 1454s, 741s. 1H NMR
(400 MHz, TMS, CDCl3) (d, ppm): 8.85 (d, 4H); 7.46 (br, s, 2H);
6.65 (s, 2H); 2.19 (s, 3H); 1.93 (s, 6H). 13C NMR (TMS, CDCl3) (d,

1894
A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
ppm): 151.24, 147.69, 143.87, 139.12, 137.83, 130.54, 127.97,
127.56, 123.03, 21.15, 19.90. Anal. Calc. for C22H18N4ꢂ2H2O: C,
70.57; H, 5.92; N, 14.96. Found: C, 70.74; H, 5.74; N, 14.59%.
2.4. X-ray experiments
Suitable crystals of Ni5b were obtained by slow diffusion of
diethyl ether into a methanol solution of Ni5a. Single crystals of
Pd2b were grown by dissolution of Pd2a in DMSO followed by lay-
ering with MeOH. X-ray diffraction measurements for complexes
Ni5b and Pd2b were done on a Rigaku R-AXIS Rapid IP diffractom-
2.3. Preparation of Pd(II) complexes
2.3.1. Dichloro{2-(pyridin-2-yl)-1H-phenanthro[9,10-d]imidazole}
palladium(II) (Pd1)
eter with graphite monochromated Mo
Ka
radiation
(k = 0.71073 Å). Intensities were corrected for Lorentz and polari-
zation effects and empirical absorption. The direct method was ap-
plied in solving the structural data and refinement was done by
full-matrix least squares on F2. All non-hydrogen atoms were re-
fined anisotropically. All hydrogen atoms were placed in calculated
positions. Structure solution and refinement were performed using
the SHELXL-97 package [52].
Using freshly distilled dichloromethane as the solvent, a solu-
tion of B1 (0.15 g, 0.51 mmol) in 20 mL solvent was added drop-
wise into a solution of PdCl2ꢂ2CNCH3 (0.13 g, 0.51 mmol) in
10 mL of solvent, and the resulting reaction mixture was stirred
at room temperature for 12 h. The crude product of Pd1 was fil-
tered as an orange powder, washed with diethyl ether and dried
under vacuum at 60 °C. Selected IR peaks (KBr, cmꢁ1):
m 3489,
3396, 3076, 2968, 1611, 1529, 1473, 1458, 1448, 1159, 774, 758,
721. Anal. Calc. for C20H15Cl2N3OPd (as the monohydrate): C,
48.96; H, 3.08; N, 8.56. Found: C, 48.77; H, 3.22; N, 8.22%.
3. Results and discussion
3.1. Absorption properties and protonation states
2.3.2. Dichloro[2-(pyridin-2-yl)phenanthro[9,10-d]oxazole]
palladium(II) (Pd2a, Pd2b)
Table 1 contains the spectral and pKa data for the investigated
ligands. The different protonation states observed within pH 0–
14 (three for 1H-imidazole ligands and two for the other ligands)
afforded species with different absorption band positions and/or
shapes (Fig. 1). Compounds that lacked the 1H-imidazole active
proton (N-methyl substituted ligands: A2, A4, B3; oxazoles: B2,
B5, C2 and C4; Table 1) gave no change in absorption properties
in the pH 6–14 range. Generally, absorption bands were red-
shifted in going from acidic through neutral to basic media. Use
of spectroscopic methods in quantifying the concentrations of
B2 (0.11 g, 0.39 mmol.) and PdCl2ꢂ2CNCH3 (0.10 g, 0.39 mmol)
were reacted in the same manner as for the preparation of Pd1
above to obtain Pd2a as yellow micro-needles in 83.1% yield.
Pd2b was obtained as blocks of brown crystals by standing Pd2a
in dimethylsulfoxide (DMSO) for a few days. Selected IR peaks
(KBr, cmꢁ1):
m 3079, 3052, 1638, 1617, 1546, 1515, 1478, 1434,
1389, 1264, 1097, 771, 721. Anal. Calc. for C20H12Cl2N2OPdꢂ½H2O:
C, 49.77; H, 2.71; N, 5.80. Found: C, 49.77; H, 2.63; N, 6.11%.
Table 1
Experimental FTIR, 1H NMR, UV absorption and pKa data of the studied imidazole molecules.
Compounds
FTIR and NMR signals
kmax (nm) [
e
(dm3/mol cm)]a
Neutral
Ionization constant
pKa,N: Se
mN–H (cmꢁ1
)
1H dN–H (ppm)c
Cation
Anion
pKa,NH Se
b
A1
A2
A3
A4
B1
B2
B3
B4
B5
C1
C2
C3
C4
D
3422m
(br)
NA
10.56
[br]
NA
311
[22 500]
284
[28 510]
314
[23 960]
296
323
[24 180]
305
[27 490]
326
[24 120]
306
[25 520]
362
[34 360]
359
[18 080]
356
[10 260]
363
[23 420]
359
[22 980]
323
[33 780]
323
[33 500]
324
[27 450]
325
[34 210]
285
336
3.71 0.05
>13.90
ed
3.33 0.02
3.75 0.07
3.63 0.15
2.89 0.02
<0.83 0.16
2.71 0.10
2.97 0.07
<1.26 0.08
4.36 0.10
2.92 0.05
4.31 0.17
2.94 0.05
4.17 0.05
3429m
(br)
NA
10.39
(br)
NA
334
>14.00
ed
[25 570]
353
3433vs
(sh)
NA
11.15
(sh)
NA
382
[25 120]
13.26 0.02
[27 560]
390
ed
NAe
NAe
345
[12 570]
350
3436vs
(sh)
NA
11.17
(br)
NA
382
[15 410]
14.08 0.61
10.06 0.13
10.73 0.16
12.60 0.19
[18 520]
390
ed
3120m
[br]
NA
11.48
[br]
NA
303
[54 810]
290
[52160]
304
[41670]
289
[50980]
298
[28760]
338
[35 910]
3120m
[br]
NA
11.39
[br]
NA
337
[29 220]
3272vs
(br)
NO
305
[27 620]
[26 550]
NA = Not applicable; NO = Not observe; sh = sharp; br = broad; s = strong; vs = very strong.
a
kmax = the longest wavelength absorption peaks at the various protonation states.
Solid state in KBr disc.
In deuterated chloroform.
b
c
d
e
could not be estimated due to lack of total conversion at these extremes of pH.
e
S represents Standard error.

A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
1895
equilibrium species in a solution is known to render reliable results
and the ionization constants can be obtained as pKa values on the
basis of Henderson-Hasselbalch theory [53]. The pKa values were
obtained from sigmoidal fittings on plots of absorbance values at
selected wavelengths against pH (Fig. 1, inset) with R2 values in
the range 0.98–0.99. Some of the equilibria occurred at extremes
of pH and only gave the onset of the sigmoidal plot as illustrated
by Fig. 1(b) for ligand A3 at the pH 14 end. Although the pKa values
for such equilibria could not be firmly estimated by spectroscopic
methods, such plots show that the ionization constants are within
the vicinity of the respective pH regions.
methyl group at the o-position to the pyridyl N heteroatom did
not yield any significant effects in the three series. This observation
is supported by a similarity in the 1H NMR and FTIR data of the 1H-
imidazole active protons for the pairs A1 and A3, B1 and B4, and C1
and C3 (Table 1). However, substitution of the 1H-imidazole pro-
ton by a methyl group or the replacement of the imidazole ring
by an oxazole ring increased the energies of the longest wave-
length absorption bands in the A and B ligand series. Therefore,
it could be concluded that the imidazole/oxazole ring possesses
some influence on the electronic characteristics of the ligands in
the A and B series. A similar comparison between imidazoles and
oxazoles in the C series did not produce any notable changes, an
indication that the electronic properties may depend less on the
Comparing the longest wavelength absorption bands of the var-
ious ligands under similar protonation states, substitution of the
pH 9.29
pH 1.63
CH3
(a)
H3
C
(b)
N
N
HN
N
pH 10.47
pH 11.78
pH 12.50
pH 13.00
pH 13.60
pH 13.70
pH 2.15
pH 2.88
pH 3.72
pH 4.87
pH 7.51
pH 8.17
N
N
A2
A3
0.2
0.1
0.0
0.2
0.28
0.1
0.0
0.15
0.10
0.26
0.24
8
10
12
14
0
2
4
6
8
pH
pH
300
Wavelength (nm)
350
250
300
350
Wavelength (nm)
pH 1.30
pH 1.94
pH 2.30
pH 2.86
pH 3.77
pH 4.99
pH 7.30
pH 7.88
pH 8.02
pH 6.76
pH 7.99
pH 8.14
pH 11.84
pH 13.05
pH 13.87
pH 14.10
pH 14.35
(d)
(c)
N
N
HN
B1
N
H3C
N
N
0.2
0.1
B3
0.2
0.10
0.08
0.1
0.0
6
8
10
12
14
0
2
4
6
8
pH
pH
0.0
0.0
300
350
300
350
Wavelength (nm)
400
Wavelength (nm)
pH 1.16
pH 1.43
pH 1.61
pH 2.06
pH 2.77
pH 3.29
pH 3.71
pH 4.25
pH 6.83
pH 7.98
pH 8.32
(e)
(f)
pH 7.90
O
N
pH 10.50
pH 11.80
pH 12.41
pH 12.91
pH 13.12
pH 13.40
pH 13.75
HN
N
N
N
C2
N
D
0.4
0.2
0.0
2
1
0
0.5
0.4
0.3
2
4
6
8
pH
250
300
350
200
250
300
350
Wavelength (nm)
Wavelength (nm)
Fig. 1. Overlays of absorption spectra at different pH values; (a) A2, (b) A3, (c) B1, (d) B3, (e) C2 and (f) D.

1896
A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
imidazole/oxazole heterocycle, but more on the phenanthroline
heterocycles. All the ligands series A (305–326 nm), B (359–
363 nm) and C (323–325 nm) gave longer absorption wavelengths
than compound D (285 nm), which suggests the existence of great-
er conjugation from the imidazole/oxazole rings to the pyridyl ring
as well as co-planarity of the imidazole/oxazole ring with pyridyl
ring in the series A, B and C ligands. Absorption kmax values indi-
cate that the extent of such an extension of conjugation generally
followed the order ligand B series > ligand A series > ligand C serie-
s > ligand D. It is noteworthy that slight changes in pH of the media
between pH 12–14 resulted in ꢀ10-fold absorbance jump for the
absorption band around 210 nm whilst having an insignificant ef-
fect on the longer wavelength bands (Fig. 1(f)).
in-plane orientation for the N(3) donor lone pair is better posi-
tioned for donation, unlike the two lone pairs on the oxo-atom
which would point at an angle to the aromatic oxazole plane. Re-
ports on the acid dissociation constants of the Ru(II) complex of
2-(4-benzoxazolyl)phenylimidazo[4,5-f][1,10]phenanthroline li-
gand, which has a pendant benzoxazole unit, also concluded that
the oxo-atom of the oxazole was not involved in any of the three
reported protonation–deprotonation equilibria (ꢀpKa1 1.70, pKa2
5.23 and pKa3 8.22) [56]. This view is further supported by scarcity
of literature (if any exists) presenting M–O coordinated metal–oxa-
zole compounds or intermolecular/intramolecular interactions be-
tween the oxo-atom of the oxazole ring and lone pair acceptors. In
contrast, X-ray structures of free and even metal-coordinated 1H-
imidazoles are commonly found with hydrogen-bonded networks
involving the N(1) proton and the N(3) base [40,57,11].
3.2. Hemilability of imidazole-pyridyl/oxazole-pyridyl ligands from
pKa values
3.3. Effect of substituents on imidazole/oxazole donor strengths
Scheme 2(a) and (b) shows a general sketch of the imidazole/
oxazole skeleton at the various protonation states while Scheme
2(c) presents a descriptive numbering of atoms for which Mulliken
charges were calculated and presented in Table 4. All imidazole li-
gands bearing the active N–H proton (Table 1: ligands A1, A3, B1,
B4, C1, C3, D) exhibited two equilibria which consist of the proton-
ation equilibrium of the N(3) base (Scheme 2(c)) in lower pH media
(i.e. higher [H+], ꢀpKa 1–5) and the deprotonation reaction of the
N(1) proton in higher pH environments (i.e. lower [H+], ꢀpKa 10–
14). However, the oxazoles (Table 1: ligands B2, B5, C2, C4) as well
as imidazoles in which the active proton has been substituted by a
methyl group (Table 1: ligands A2, A4, B3) gave only one proton-
ation–deprotonation equilibrium which corresponds to N(3) pro-
tonation in the lower pH 0–4 environments. This difference
supports the assignment of pKa 10–14 values in unsubstituted
imidazole compounds to 1H-imidazole deprotonation events at
higher alkalinities. The broad 1H NMR and FTIR peaks that are com-
monly observed for 1H-imidazole protons are typical of ionizable
protons (Table 1). Previous conclusions of 1H-imidazole deproto-
nation equilibria are also known for imidazo[4,5-f][1,10]phen-
anthrolines bound to Ru2+ (ꢀpKa 8.0–9.0) [54,55]. Therefore, we
concluded that the two types of equilibria observed in the ligand
series involve the imidazole/oxazole rings rather than the pyridyl
heterocycles and that the imidazole/oxazole moieties are substan-
tially weaker Lewis bases relative to the pyridyl fragments. The
requirement of larger [H+] (i.e. lower pH environments) for ligand
protonation is an indication that the imidazole/oxazole rings pos-
sess poor donor strengths. Experimental data in this work suggest
that protonation–deprotonation equilibria concerning the pyridyl
bases (either at 2-imidazole carbon or on the phenanthrolinyl moi-
eties), which are stronger Lewis bases, would occur substantially
above pKa 14 in 70% ethanol.
The strength of lone pair donation in the series of ligands could
be compared on the basis of pKa,N: values (Table 1). Substituents
with electron-withdrawing character on the basic nitrogen atom
of the imidazole/oxazole ring would be expected to impart a reduc-
tion in the Lewis basicity, and vice versa. It was anticipated that
conjugation between phenyl, phenanthrene or phenanthroline
substituent groups and the imidazole/oxazole aromaticity would
produce varying extents of charge density shift in the direction
from the C(2) atom of the imidazole ring towards the C(4)–C(5)
carbons on which the substituents are borne. Of all the investi-
gated ligands, the B series ligands (phenanthrene substituted
azoles) possessed the weakest imidazole/oxazole base donor char-
acteristics. The general trend in donor abilities according to the
pKa,N: of the ligand groupings is in the order B series (ꢀ0–2.9) < A
series (ꢀ3.3–3.7) < C series (ꢀ2.9–4.3). A notable observation is
the fact that the observed trend suggests that the phenanthrenyl
moiety is more electron withdrawing on the imidazole/oxazole
ring than the phenanthrolinyl substituent, which is supported by
sharper and stronger FTIR vibrational frequencies for B1 (3433vs
cmꢁ1) and B4 (3436vs cmꢁ1) among the investigated ligand
frameworks (Fig. 2(d); Table 1). This is the converse to the ex-
pected electronegativity influence of the N heteroatoms on the
phenanthroline C series.
The substitution of 1H-imidazole positions by a methyl group,
as in A2, A4 and B3, was expected to produce an electron releasing
influence towards the N(3) nitrogen base from the N(1) and conse-
quently higher pKas. However, it lead to slightly lower pKa,N: values
compared to the unsubstituted imidazoles A1, A3 and B1, respec-
tively (Table 1). This suggests that methyl substitution facilitated
a shift in electron density of the imidazole ring from the C(2) end
to the C(4)–C(5) carbons on which the electron-withdrawing sub-
stituents are borne, leading to further reduction of charge density
on the donor atom. Therefore, 1H-imidazole alkylation may not
necessarily impart an increased donor strength on the N(3) hetero-
Participation of the oxo-atom on the oxazole ring in proton-
ation–deprotonation equilibria was ruled out since it is considered
to be a relatively poorer lone pair donor than the N(3) donor. The
pKa,N:
R2
R2
pKa,N:
pKa,NH
R2
R2
H
R2
C(2)
N
N
-H+
+H+
N
N
N
N
N
N
N
(1)X
N(3)
-H+
+H+
-H+
+H+
H
H
(5)C
C(4)
H
O
N
O
N
N
N
X = N(1)-H or O(1)
(a) 2-(1H-imidazol-2-yl)pyridines
(c) Numbering for Table 4
(b) 2-(oxazol-2-yl)pyridines
Scheme 2. Protonation–deprotonation equilibria for (a) imidazole rings, (b) oxazole rings within pH 0–14 environments and (c) descriptive atom numbering of the azole ring
for reported charge densities in Table 4.

A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
1897
(a)
(b)
A3
0.3
0.2
0.1
A1
Ni1
Ni2
Ni5a(green)
Ni5b(Yellow)
0.2
0.1
0.2
0.1
0.2
1600
1500
1600
1500
-1
-1
Wavenumbers (cm
)
Wavenumbers (cm
)
N
N
HN
HN
N
N
3500
3000
Wavenumbers (cm-1)
2500
3500
3000
Wavenumber (cm-1)
2500
(c)
(d)
Pd2a
Pd2b
B2
B1
#
Sharp N-H peaks
for B1 and B4
B2
B3
B4
B5
#
0.4
0.6
0.4
0.2
∗
#
∗
0.2
∗
1600
1500
1400
3500
3000
Wavenumber (cm -1)
2500
Wavenumber (cm -1)
Fig. 2. Solid-state FTIR spectra (as KBr discs) of (a) nickel complexes with A1, (b) nickel complexes with A3, (c) palladium complexes with B2 and (d) the B series ligands.
atom and electron density shifts across C(2) to the C(4)–C(5) axis is
probably more important for tuning donor attributes of the imid-
azole base, as has been proposed for the 2-(1H-imidazol-2-yl)phe-
nols which are prone to Excited State Intramolecular Proton-
electron Transfer (ESIPT) [35].
However, replacement of the NH groups by an oxo-atom, as in
the oxazoles B2, B5, C2 and C4, yielded a notable reduction in
the pKa,N: values of the respective imidazoles B1, B4, C1 and C3
understandably through the higher electronegativity of the oxo-
atom (Table 1). According to the pKa data, deprotonation of the C
series imidazoles is easier than for the B series analogues, which
supports the observed trends of phenanthrenyl substituent effects
relative to the phenanthrolinyl analogue.
3.4. Supports from spectroscopic data of Ni(II) and Pd(II) complexes
One of the initial motivations for this work is the observation
that oxazole B5 formed a yellow complex with NiCl2ꢂ6H2O, but
the free ligand was readily precipitated in the presence of metha-
nol. A summary of the preparation of the Ni(II) and Pd(II) com-
plexes is presented in Scheme 3. The non-bridging 2-(imidazol/
oxazol-2-yl)pyridine ligands (A and B series except ligand B5) re-
acted with dihalonickel(II) salts to give Ni1–N9, which have been
previously characterized and applied to homogeneous olefin poly-
merization catalysis [41]. Ligands A1 and A2 only selectively
formed the blue, bis-ligated nickel complexes, even when deliber-
ate use of excess Ni(II) salt over 1:1 ratio was employed (Scheme 3;
.
(DME)NiBr2
or NiCl2 6H2O
NiCl2 6H2O
.
.
or PdCl2 (CH3CN)2
(Ligand)nNiX2
A series
B series
(Ligand)MX2
Solvent
Solvent
Ni1 Ni2 Ni3 Ni4 Ni5a Ni6
Ni7 Ni8 Ni9 Pd1 Pd2a
R1
R2
X
H
H
H
H
CH3
H
H
H
CH3
R2
Y
H
NH
H
O
CH3
NH NH
H
H
O
CH3 CH3 CH3
Cl Br Cl Cl Br Cl
n
2
2
2
1
1
1
Scheme 3. Summary of the reaction of Ni(II) and Pd(II) compounds with members of the A and B ligand series.

1898
A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
Ni1–Ni3). However, ligands A3 and A4, having R2 = CH3, and the
Table 2
Colour, FTIR (
compounds.
mN@C, C@C) and melting/decomposition point data of the Ni(II) and Pd(II)
phenanthrenyl ligands B1–B4 formed the yellow 1:1 complexes
(Scheme 3; Ni4–Ni9). These observations were established by X-
ray structures for Ni1, Ni3, Ni4 and Ni9 in addition to elemental
analysis of all the complexes [41]. Complexes of the C series li-
gands were not studied because all attempts to obtain a definite
metal–organic architecture proved futile, with only polymeric
products resulting. The appearance of two 1H-imidazole peaks
around 3450 cmꢁ1 and 3350 cmꢁ1 for the bis-ligand complexes
(Ni1 and Ni2) suggests that the coordinated ligands exists in
slightly different electronic environments, probably due to differ-
ent hydrogen bonding environments in the solid state (Fig. 2(a);
Table 2).
A notable observation is that coordination of the A and B ligand
series to Ni(II) or Pd(II) metal centers lead to an increase in the
vibrational frequencies assigned to the C@N/C@C double bonds
for the coordinated ligands relative to the free ligands (Table 2;
Fig. 2(a) and (b) inset for nickel; Fig. 2(c) for palladium). This phe-
nomenon is contrary to the usual shift to lower wavenumbers in
similar N-donor ligand frameworks, which is usually reported as
a proof for ligand coordination to transition metals and is often
attributed to increased electron delocalization that results from
lone pair donation [58,59]. Consequently, the observation was con-
sidered to indicate a somewhat less favourable coordination for the
imidazo-pyridine/oxazo-pyridine ligands compared with other
well known bidentate ligands such as bipyridine and phenanthro-
line. It also suggests that this family of ligands may be useful for
rational ligand exchange reactions and self-assembly protocols in
Compounds
Colour
mN@C,
(cmꢁ1
)
mN-H
Mp./Dp. (°C)
C@C
A1
White
Blue
Blue
White
Blue
1598, 1566
1616, 1586
1612, 1581
1585, 1564
1605, 1573
1597, 1574
1616, 1570
1616, 1574
3422
3443, 3285
3451, 3324
174–176d
>302d,f
Ni1
Ni2
A2
Ni3
A3
Ni4
Ni5aa
Ni5bb
A4
Ni6
B1
Ni7
Pd1
B2
Ni8
Pd2aa
Pd2bc
B4
>312d,f
121–123d
>394d,f
White
Yellow
Green
Yellow
White
Yellow
White
Yellow
Orange
White
Green
Orange
Brown
White
Yellow
White
3429
3380
3431
3447, 3155
198–200d
>394d,f
242–244e
>300e
1612, 1572
1590, 1574
1604, 1573
120–122d
>214d,f
1612m, 1592vs
1637w, 1609s
1631m, 1612vs
1617m, 1586vs
1640vs, 1615s
1639s, 1617vs
1636m, 1604s
1616m, 1597s
1604vs
3433vs(sh)
3394m(br)
3490vs, 3395s
200–202d
>420d,f
>300
199–201d
>422d,f
295–297e
>300e
3436s(sh)
3355s(br)
218–220d
>390d,f
Ni9
B5
1594m, 1569s
199–201d
s = strong; vs = very strong; m = medium; w = weak; sh = sharp.
a
Crude product filtered from preparative reaction.
Crystallized from MeOH layered with diethyl ether.
Crystallized from DMSO.
Data according to Ref. [41].
Melting with decomposition.
Decomposition.
b
c
d
e
f
(a)
(b)
NiBr2(A3)2
Yellow blocks
MeOH / C2H5OC2H5
NiBr2(A3)
NiBr2(A3)
Green blocks
Deposited
NiBr2
(c)
Fig. 3. (a) Ortep plot of complex Ni5b with thermal ellipsoids drawn at the 50% probability level, (b) proposed scheme for disproportionation0 and (c) the 1-dimensional
hydrogen bonding network of the Ni5b cation and Brꢁ. Some atomic labels and protons have been omitted for clarity. Selected bond lengths (AÅ): Br(1)–Ni(1) = 2.4586(6);
Ni(1)–N(5) = 2.023(2); Ni(1)–N(2) = 2.032(2); Ni(1)–N(1) = 2.097(2); Ni(1)–N(4) = 2.119(2). Selected bond angles (°): N(5)–Ni(1)–N(2) = 102.68(8); N(5)–Ni(1)–
N(1) = 101.63(8); N(2)–Ni(1)–N(1) = 79.95(9); N(5)–Ni(1)–N(4) = 79.97(8); N(2)–Ni(1)–N(4) = 98.88(8); N(1)–Ni(1)–N(4) = 178.17(8); N(5)–Ni(1)–Br(1) = 130.66(6); N(2)–
0
Ni(1)–Br(1) = 126.65(6); N(1)–Ni(1)–Br(1) = 88.33(6); N(4)–Ni(1)–Br(1) = 91.27(6). Hydrogen bonds parameters (AÅ and °): D–Hꢂ ꢂ ꢂA = N(6BA)–H(6 N)ꢂ ꢂ ꢂBr(2B); d(D–
H) = 0.87(3); d(Hꢂ ꢂ ꢂA) = 2.42(3); d(Dꢂ ꢂ ꢂA) = 3.285(2); <(DHA) = 170(3); D–Hꢂ ꢂ ꢂA = N(3BB)–H(3 N)ꢂ ꢂ ꢂBr(2B); d(D–H) = 0.87(3); d(Hꢂ ꢂ ꢂA) = 2.45(3); d(Dꢂ ꢂ ꢂA) = 3.320(2);
<(DHA) = 173(3).

A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
1899
the presence of common interacting solvents or other donors with
comparable or higher ligand field strengths.
brown and large block of crystal was observed to have formed after
about 2 weeks. However, it was observed that standing the satu-
rated DMSO solution without layered solvent and without solvent
evaporation also produced the same rectangular blocks of single
crystals within a few days. Therefore, we conclude that Pd2a dis-
solves in and reacts with DMSO, leading to detachment of the oxa-
zo-Pd bond and DMSO then coordinates to the palladium metal
instead. The first significant difference between Pd2a and Pd2b
is that Pd2b is insoluble in DMSO while the Pd2a is soluble. Hem-
ilability of the ligand B2 is believed to be confirmed from the struc-
ture of Pd2b, for which the Ortep plot is shown in Fig. 4.
Comparative overlay of IR data for Pd2a, Pd2b and B2 is presented
3.5. X-ray structures of the Ni(II) and Pd(II) complexes
Further evidence for the hemilability of the studied imidazo-
pyridine/oxazo-pyridine ligands came from observed behaviours
in solution of some of the complexes. It is noteworthy that all
the yellow mono-ligated Ni(II) complexes dissolve in methanol to
give dark green solutions from which green crystals typically pre-
cipitated in the presence of diethyl ether. The colour change from
yellow to green in solution may be considered to be as a result of
methanol coordination to vacant sites on the complexes, as ob-
served in our published structure for Ni4. During attempts to ob-
tain single crystals for Ni5a by dissolution of the complex in
methanol and layering with diethyl ether via slow diffusion, a
few sets of yellow crystal blocks of Ni5b were observed to grow
as the diethyl ether solvate in the diethyl ether layer, in addition
to the green blocks of Ni5a in the lower methanol layer, after a
few days. Ni5b is believed to result from solvent mediated ligand
disproportionation (Fig. 3(b)). Comparing the solid state FTIR data
for Ni5a and Ni5b showed two peaks for active 1H-imidazole pro-
tons on Ni5b while only one peak was observed for Ni5a (Table 2,
Fig. 2(b)). The observed difference is due to the mono-ligand struc-
ture of Ni5a compared to the bis-ligand structure for Ni5b, as
shown in its Ortep plot (Fig. 3(a)). Selected bond lengths and angles
around the metal center as well as hydrogen bonding parameters
are presented along with the label of the figure. Ni5b shows a tri-
gonal bipyramidal geometry around the nickel center. Br(1), N(2)
and N(5) atoms form the trigonal plane, while N(1) and N(2) atoms
occupied the axial positions (Fig. 3(a)). In comparison with other
mono- and bis-ligand Ni(II) complexes of the A series ligands, the
difference in melting/decomposition point data for Ni5a (242–
244 °C) and Ni5b (>300 °C) is in line with their mono-ligand and
bis-ligand structural identities, respectively (Table 2).
in Fig. 2(c). The two mN@C, C@C bands for B2, 1617 and 1586 cmꢁ1
,
were shifted to higher wavenumbers by 23 and 29 cmꢁ1, respec-
tively after coordination in Pd2a, and the detachment of the oxa-
zo-palladium bond reversed the shift to lower wavenumbers
after by 3 and 13 cmꢁ1, respectively. It appears that the 1586 cm-
1
band pertains to vibrations on the 5-membered oxazole ring
since it was more affected (13 cmꢁ1) after detachment of the oxa-
zole-palladium coordination bond. Crystal data and processing
parameters for Ni5b and Pd2b are presented in Table 3.
3.6. DFT calculation on the ligands
In an effort to further explore the experimental behaviours ob-
served on account of ligand substituent effects, optimized geome-
tries of all the organic ligands were calculated using GAUSSIAN 98 and
properties such as the frontier orbitals as well as atomic charge
densities were analyzed. The optimized structures revealed co-pla-
narity between the imidazole/oxazole rings and the pyridyl rings
attached to C(2), and loss of planarity for the D molecule (Fig. 5).
The B series ligands generally possessed the lowest energy separa-
Table 3
Crystal data processing parameters for Ni5b and Pd2b.
The compound Pd2b was also obtained during attempts to
grow a suitable single crystal of Pd2a. Pd2a was dissolved in DMSO
and layered with MeOH, in which the complex is insoluble. A single
Ni5bꢂC2H5OC2H5
C46H44Br2N6NiO
915.40
173(2)
Pd2bꢂDMSO
C22H18Cl2N2O2PdS
551.74
173(2)
Formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.71073
monoclinic
P2(1)/n
16.842(3)
14.284(3)
17.359(4)
90
0.71073
monoclinic
C2/c
29.247(6)
10.217(2)
14.507(3)
90
a
(°)
b (°)
95.71(3)
90
4155.3(15)
4
1.463
0.685
103.04(3)
90
4223.0(14)
8
1.736
1.253
c
(°)
V (Å3)
Z
DCalc (g cmꢁ3
)
l
(mmꢁ1
)
F(0 0 0)
1872
2208
Crystal size (mm)
h Range (°)
Limiting indicates
0.27 ꢃ 0.23 ꢃ 0.15 0.37 ꢃ 0.27 ꢃ 0.19
3.27–27.48
ꢁ21 6 h 6 21
ꢁ18 6 k 6 14
ꢁ22 6 l 6 14
24 650
1.43–27.47
ꢁ37 6 h 6 37
ꢁ13 6 k 6 13
ꢁ18 6 l 6 18
9116
Number of reflections collected
Number of unique reflections
R(int)
9479
4813
0.0293
0.0276
Completeness to h (%)
Number of parameters
Goodness-of-fit (GOF) on F2
99.5% (h = 27.48)
517
1.093
99.6% (h = 27.47)
273
1.193
Final R indices (I > 2
r
(I))
R1 = 0.0439
wR2 = 0.0835
R1 = 0.0522
wR2 = 0.0873
R1 = 0.0852
wR2 = 0.1947
R1 = 0.0997
wR2 = 0.2032
0.550, ꢁ0.897
Fig. 4. Ortep plot of complex Pd2b with thermal ellipsoids drawn at the 50%
probability level. Some atomic labels and protons have been omitted for clarity.
0
R indices (all data)
Selected bond lengths (AÅ): Pd(1)–N(1) = 2.060(7); Pd(1)–S(1) = 2.246(2); Pd(1)–
Cl(2) = 2.285(2); Pd(1)–Cl(1) = 2.318(2). Selected bond angles (°): N(1)–Pd(1)–
S(1) = 91.24(19); N(1)–Pd(1)–Cl(2) = 179.2(2); S(1)–Pd(1)–Cl(2) = 89.59(8); N(1)–
Pd(1)–Cl(1) = 88.05(18); S(1)–Pd(1)–Cl(1) = 175.20(8); Cl(2)–Pd(1)–Cl(1) = 91.12(8).
Largest difference in peak and hole 0.515, ꢁ0.490
(e Åꢁ3
)

1900
A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
Fig. 5. Optimized structures of representative members of the various ligand groups showing the frontier molecular orbitals at the LUMO, HOMO and HOMOꢁ1 levels.
Hydrogen atoms are omitted for clarity.
Table 4
series (Fig. 5(a)–(d)). Dominance of the phenanthroline fragment
on the electronic character of the HOMO levels in the C series is
also supported by a concentration of the frontier orbitals on the
phenanthroline end of the ligands (Fig. 5(e) and (f)).
Ligand dipole moments, LUMO-HOMO separations and Mulliken charge densities of
selected atoms determined at the B3LYP/6-311+G Level.
*
Ligands
D
E(LUMO)–(HOMO) (eV)a Mulliken charge densities (Q)
N(3)
N(1)/O(1) N(3)–N(1) C(2)
A1
A2
A3
A4
B1
B2
B3
B4
B5
C1
C2
C3
C4
D
7.591
7.285
7.497
7.173
5.894
5.912
5.760
5.758
6.020
6.233
5.587
6.239
5.600
6.378
ꢁ0.401 ꢁ0.468
ꢁ0.424 ꢁ0.764
ꢁ0.401 ꢁ0.468
ꢁ0.412 ꢁ0.763
ꢁ0.381 ꢁ0.516
ꢁ0.374 ꢁ0.476
ꢁ0.404 ꢁ0.814
ꢁ0.380 ꢁ0.516
ꢁ0.373 ꢁ0.476
ꢁ0.379 ꢁ0.512
ꢁ0.373 ꢁ0.473
ꢁ0.378 ꢁ0.513
ꢁ0.372 ꢁ0.473
ꢁ0.365 ꢁ0.503
0.067
0.340
0.067
0.342
0.135
0.102
0.410
0.136
0.103
0.133
0.103
0.135
0.101
0.138
0.403
0.423
0.377
0.407
0.391
0.330
0.140
0.117
0.082
0.090
0.332
0.371
0.304
0.194
4. Conclusion
Fourteen new organic molecules (A1–A4, B1–B5, C1–C4 and D)
and a series of transition metal(II) complexes (Ni1–Ni9 and Pd1–
Pd2b) were synthesized and studied in order to characterize the
hemilability of 2-(1H-imidazol-2-yl)pyridines and 2-(oxazol-2-
yl)pyridines and to explore trends caused by substituent effects
on the donor strengths as well as coordination chemistries of imid-
azole/oxazole fragments of the hemilabile ligands. Protonation of
imidazole/oxazole N donor atoms gave equilibria with pKa values
in the range 0–5, while deprotonation of imidazole active protons
were characterized by pKa values in the 10–14 range. The experi-
mental results lead to the conclusion that all observed proton-
ation–deprotonation processes observed within pH 1–14 media
pertain to the 5-membered rings rather than the 6-membered pyr-
idyl Lewis bases, and also that donor characteristics of the imidaz-
ole/oxazole ring can be estimated by spectroscopic methods
regardless of the presence of other strong N donor fragments.
The low pKa values demonstrate weak Lewis basicity of the 5-
membered rings and hemilability of the chelate ligands as a whole.
The donor strengths of the oxazoles are notably lower than those of
imidazoles. The phenanthrenyl analogues (B series ligands) also
possessed weaker donor strengths than their 4,5-diphenyl-substi-
tuted counterparts (A series ligands). It is noteworthy that, in dis-
agreement with the expected greater electron-withdrawing
influence of the phenanthroline heterocycle on the azole ring, ba-
sicity studies revealed stronger imidazole N donor qualities for
the phenanthrolinyl ligands (C series) than for the phenanthrenyl
analogues (B series), which indicates a greater electron-withdraw-
ing influence for the phenanthrene substituent. It is also notewor-
thy that FTIR data for the complexes produced stronger mN@C, C@C
vibrations for all the ligands after coordination to M(II) ions con-
trary to the usually reported shifts to lower wavenumbers after li-
gand coordination. The single crystal structures for Ni5b and Pd2b
provided further evidence of ligand hemilability.
a
LUMO = Lowest Unoccupied Molecular Orbital, HOMO = Highest Occupied
Molecular Orbital (HOMO), eV = electron Volts. Selected atomic labels correspond
to notations according to Fig. 4(b).
tions between the Highest Occupied Molecular Orbital (HOMO)
and the Lowest Unoccupied Molecular Orbital (LUMO) (DE(LUMO)–
(HOMO), Table 4) which is in agreement with significantly lower
energies of their longest wavelength bands (359–363 nm) com-
pared with other ligands in this study (Table 1). Mulliken charge
densities (Q) on atoms generally showed lower electron densities
on the C(2) atoms as well as on N(3) donor atoms for the B series
ligands compared to the other ligands. This tends to support the
experimental suggestions on the stronger electron-withdrawing
role of the phenanthrenyl substituent.
Finally, the frontier orbitals in all the molecules show the least
activities for the imidazole/oxazole N(3) atoms compared to the
considerably larger concentration of frontier orbitals on the pyridyl
N donors (whether or not pyridyl of phenanthrolinyl; Fig. 5).
Therefore, the computational data is in support of hemilability of
2-(1H-imidazol-2-yl)pyridines and 2-(oxazol-2-yl)pyridines. Con-
sidering the HOMO and HOMOꢁ1 levels of the studied ligands,
the calculations also support the importance of the imidazole/oxa-
zole ring in the electronic energies of members of A and B ligand

A.O. Eseola et al. / Polyhedron 29 (2010) 1891–1901
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of Sciences and the Academy of Sciences for the Developing World
(Italy) for granting a postgraduate fellowship (CAS-TWAS) to A.O.E.
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