Stabilization of unique valencies of cobalt, nickel and copper by complexation with the tridentate ligand 2-(2′-pyridyl)-8-hydroxyquinoline Dedicated to Prof. George Christou, on his 60th birthday, an extraordinary and enthusiastic inorganic chemist, and the most supportive mentor i could ever ask for (GM).

Article abstract of DOI:10.1016/j.poly.2013.06.038

The complexes of cobalt, copper and nickel ions with the known tridentate ligand 2-(2′-pyridyl)-8-hydroxyquinoline (HQP) were prepared and characterized. The structures of the complexes with Co(II), Cu(II) and Ni(III) were corroborated by crystal structure analyses. The structures of the complexes with Co(III) and Cu(I) were realized from NMR measurements in solution. It was found that HQP stabilizes high oxidation states of the complexed metal ions. The results also suggest that HQP binds Cu(II) and Cu(I) in two different modes; Cu(II) forms an octahedral complex with the three elements of the ligand, namely, the 8-hydroxyl the two nitrogens of bipyridine, while Cu(I) binds only to two nitrogens of the bipyridine, yielding a tetra-coordinated complex.

Full text of DOI:10.1016/j.poly.2013.06.038

Polyhedron 64 (2013) 365–370
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Stabilization of unique valencies of cobalt, nickel and copper by
complexation with the tridentate ligand 2-(20-pyridyl)-8-
hydroxyquinoline
Galia Maayan a, , Yohai Dayagi a,1, Rina Arad-Yellin a,2, Linda J.W. Shimon b, Abraham Shanzer a
⇑
a Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76000, Israel
b Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76000, Israel
a r t i c l e i n f o
a b s t r a c t
The complexes of cobalt, copper and nickel ions with the known tridentate ligand 2-(20-pyridyl)-8-
hydroxyquinoline (HQP) were prepared and characterized. The structures of the complexes with Co(II),
Cu(II) and Ni(III) were corroborated by crystal structure analyses. The structures of the complexes with
Co(III) and Cu(I) were realized from NMR measurements in solution. It was found that HQP stabilizes high
oxidation states of the complexed metal ions. The results also suggest that HQP binds Cu(II) and Cu(I) in
two different modes; Cu(II) forms an octahedral complex with the three elements of the ligand, namely,
the 8-hydroxyl the two nitrogens of bipyridine, while Cu(I) binds only to two nitrogens of the bipyridine,
yielding a tetra-coordinated complex.
Article history:
Received 1 March 2013
Accepted 24 June 2013
Available online 4 July 2013
Dedicated to Prof. George Christou, on his
60th birthday, an extraordinary and
enthusiastic inorganic chemist, and the
most supportive mentor I could ever ask for
(GM).
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Metal coordination
8-Hydroxyquinoline
Bipyridine
2-(20-Pyridyl)-8-hydroxyquinoline
Tridentate ligand
Copper complex
Cobalt complex
Nickel complex
1. Introduction
try of synthetic metal complexes is therefore an important step to-
wards the ability to mimic the structure and function of
Metal ions are key elements in both the structure and function
of natural biopolymers, being employed in tasks spanning from
structure stabilization to catalysis, light-energy conversion and
recognition. The structure and catalytic activity of many enzymes,
for example, depends upon explicit coordination of metal ions such
as copper, cobalt and nickel. Control over the coordination geome-
biopolymers. It can be achieved via the rational design of unique
metal chelators targeted at the binding of specific metal ions with
explicit oxidation states in desired coordination geometries.
Ligands based on the pyridine skeleton, i.e., bipyridine and ter-
pyridine, as well as quinoline ligands, especially 8-hydroxyquino-
line, are strong metal chelators capable of binding a variety of
metal ions yielding different geometries based on the metal ion
and its oxidation state [1–3]. The bidentate ligand 8-hydroxyquin-
oline (HQ), with nitrogen and oxygen atoms for binding, is consid-
ered a ‘‘medium’’ ligand, which binds ‘‘medium’’ as well as ‘‘hard’’
metal ions in different coordination modes. Thus, two molecules of
HQ form tetra-coordinated planar complexes with Cu(II) ions while
coordination to Cu(I) or to Co(II) ions yields tetrahedral complexes
[4]. The fact that HQ is a bidentate ligand might prove limiting;
Co(III), which is known to form stable six coordinated octahedral
complexes, cannot form stable complexes with two HQ ligands
and needs an external ligand for this purpose [5]. The chelator
terpyridine, a tridentate ligand with three nitrogen atoms for
Abbreviations: HQ, 8-hydroxyquinoline; HQP, 2-(20-pyridyl)-8-hydroxy quino-
line; ESI-MS, electron spray ionization mass spectrum; NMR, nuclear magnetic
resonance spectrum; dd, dt, dq, double doublet, triplet, and quartet, respectively;
bs, broad single; UV–Vis, ultraviolet–visible spectrum; IR, infrared; FW, formula
weight; MLCT, metal-to-ligand charge transfer.
⇑
Corresponding author. Address: Schulich Faculty of Chemistry, Technion, Israel
Institute of Technology, Haifa 32000, Israel.
E-mail address: gm92@techunix.technion.ac.il (G. Maayan).
Current address: Organic Synthesis and Polymerization Laboratory, IKI, Ben-
1
Gurion University of the Negev, Beer-Sheva 84105, Israel.
2
Current address: Semorex Technologies Ltd., Weizmann Scientific Park, Ness
Ziona 74140, Israel.
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.06.038

366
G. Maayan et al. / Polyhedron 64 (2013) 365–370
coordination is a ‘‘soft’’ binding ligand, thus, can bind only a lim-
ited range of metal ions. We therefore anticipated that a ligand
2.3. Synthesis
combining both, pyridine and HQ, in which one
dine forms a ligand that contains more than one pyridine group
coupled to the -donating phenol group, will be a good chelator
with an ability to bind a variety of metal ions and stabilize high
oxidation states of metal ions. Several ligands of this kind were al-
ready described in the literature; some of them were found to sta-
bilize high oxidation state of metal ions [6].
P
-accepting pyri-
Methyl 3-hydroxy 2-nitrobenzoate (1): Dry HCl was bubbled
through the refluxing solution of 3-hydroxy 2-nitro benzoic acid
(10 g, 0.055 mol) in 150 ml MeOH for 8 h. The solution was concen-
trated in high vacuum. Addition of H2O to the concentrated solu-
tion precipitated the methyl 3-hydroxy 2-nitro benzoic ester
(64% yield, 6.9 g, 0.035 mol). 1H NMR d = 7.07 (dd, 1H), 7.26 (m,
1H), 7.58 (d, 1H), 3.94 (CO2CH3, s, 3H).
r
In this work, we chose to study the interactions with various 3d
transition metal ions of the previously reported tridentate ligand 2-
(20-pyridyl)-8-hydroxyquinoline (HQP) [7]. HQP combines binding
units of the two ligands HQ and 2,20-bipyridine into one structure
(Scheme 1). Its interaction with ruthenium ions was previously de-
scribed [8]. The structure of HQP complexes with biologically relevant
metal ions, however, was not realized, and its ability to stabilize high
oxidation states as well as to form stable complexes with different oxi-
dation states of the same element (e.g. ‘‘soft‘‘ vs. ‘‘hard’’) was not ex-
plored. Herein we demonstrate that HQP stabilizes the spontaneous
formation of Co(III) and Ni(III) complexes, and suggest that binding
of Cu(II) and Cu(I) lead to the formation of two different complexes
having two different coordination geometries.
Methyl 3-(benzyloxy)-2-nitrobenzoate (2): A solution of methyl
3-hydroxy-2-nitrobenzoate (6.9 g, 0.033 mol) in dry DMF
(100 ml) was cooled to 0 °C under argon and treated with sodium
hydride (0.96 g, 0.04 mol). The reaction mixture was stirred for
10 min and benzyl bromide (6.27 g, 0.036 mol) was added. The
reaction mixture was allowed to warm to 25 °C, and stirring was
continued for an additional 20 h. A solution of saturated NaCl
(30 ml) was added, and the reaction mixture was further diluted
with H2O (100 ml). The aqueous layer was extracted with Et2O
(4 ꢀ 150 ml). The combined ether extracts were washed with
H2O (100 ml), washed with saturated NaCl (30 ml), dried over Na2-
SO4and the solvent was removed in vacuum. Then CHCl3 was
added (30 ml), and the solution was dried over Na2SO4 and evapo-
rated in vacuum to yield methyl 3-(benzyloxy)-2-nitrobenzoate as
yellow crystals (89% yield, 8.95 g). 1H NMR d = 7.58 (dd, 1H, aro-
matic H), 7.23–7.45 (m, 7H, phenyl and aromatic Hs), 5.19 (s, 2H,
PhCH2), 3.88 (CO2CH3, s, 3H).
2. Experimental
2.1. General
3-(Benzyloxy)-2-nitrobenzyl alcohol (3): A solution of methyl
3-(benzyloxy)-2-nitrobenzoate (8.78 g, 0.03 mol) in THF (150 ml)
was treated with LiBH4 (2 g, 0.03 mol, 3.0 eq.) at 25 °C under argon
atmosphere and stirred for 21 h. Saturated solution of ammonium
chloride (70 ml) was added and the reaction mixture was diluted
with H2O (200 ml) and extracted with EtOAc (2 ꢀ 150 ml). The
organic extract was washed with saturated NaCl, dried (Na2SO4),
filtered, and the solvent was removed in vacuum. Column chroma-
tography (SiO2, 30% EtOAc–hexane eluent) afforded 3-(benzyloxy)-
2-nitrobenzyl alcohol as a yellow oil (5.11 g, 66% yield). 1H NMR
d = 7.43–7.26 (m, 6H, phenyl and aromatic Hs), 7.1(d, 1H, aromatic
H), 7.0 (d, 1H, Aromatic H), 5.19 (s, 2H, PhCH2OAr), 4.67 (s, 2H, CH2-
1H NMR spectra were recorded on Bruker AMX-250 or AMX-400
instruments. Unless otherwise mentioned, CDCl3 was used as sol-
vent. UV–Vis spectra were recorded on a Beckman DU-7500 a
diode array spectrophotometer. Mass spectra were recorded on a
Platform LCZ 4000 Micromass, Manchester, UK. Ionization Mode
was ESI. X-ray diffraction measurements were preformed with
Nonius Kappa CCD diffractometer. The crystals were coated with
paraffin oil.
UV–Vis experiments were carried out in quartz cuvettes. In a
typical measurement, 20 ll of a 10 mM HQP solution (in methanol)
or metal complex was placed in a cuvette that contained 3 mL 4:1
methanol:0.1 M aqueous NH4PF6 solution and the spectrum was
recorded. In case of air-sensitive metal complexes, the solutions
were degassed, purged with argon and the experiments were car-
ried out in special Shlank cuvettes under inert atmosphere.
Data collection and treatment of single crystals suitable for X-
ray crystallography analysis were done with a Nonius KappaCCD
OH). IR (CHCl3)
m
= 1020 cmꢁ1 (CH2OH).
3-(Benzyloxy)-2-nitrobenzaldehyde (4): A solution of 3-(benzyl-
oxy)-2-nitrobenzyl alcohol (200 mg, 0.77 mmol) in CH2Cl2
(15 ml)
was treated with pyridinium dichromate (436 mg, 1.16 mmol) at
25 °C under Ar. The reaction mixture was stirred at 25 °C for
11 h, filtered through celite, washed with 5% aqueous HCl
(1 ꢀ 6 ml) and saturated NaCl (1 ꢀ 6 ml), dried (Na2SO4) and the
solvent was removed in vacuum to afford 3-(benzyloxy)-2-nitro-
benzaldehyde as a yellow solid (0.1 g, 0.4 mmol, 50% yield). 1H
NMR d = 9.95 (s, 1H, CHO), 7.60–7.48 (m, 2H, aromatic Hs), 7.39–
7.32 (m, 6H, phenyl and aromatic Hs), 5.23 (s, 2H, PhCH2OAr). IR
diffractometer with Mo
Ka, graphite monochromator (k =
0.71073 Å). The crystals were coated with paraffin oil. Data were
processed with Denzo-Scalepack. The structure was solved by di-
rect methods (SHELXS-97). Full-matrix least-squares refinement
was based on F2 (SHELXL-97). Idealized hydrogen atoms were placed
and refined in a riding mode.
(CHCl3)
m
= 1709 cmꢁ1 (CHO), 1475 cmꢁ1 (NO2).
2-Amino-3-(benzyloxy)-2–benzaldehyde (5): A solution of 3-
(benzyloxy)-2-nitrobenzaldehyde (2.5 g, 10 mmol) in THF
(180 ml) was treated with a solution of sodium hydrosulfite
(8.47 g, 50 mmol) in H2O (90 ml). The reaction mixture was
warmed at 60 °C for 1 h, poured onto H2O (150 ml), and extracted
with EtOAc (3 ꢀ 150 ml). The EtOAc layer was washed with satu-
rated NaCl (100 ml), dried over Na2SO4and the solvent was re-
moved in vacuum. Column chromatography (SiO2, 15% EtOAc–
hexane eluent) afforded 2-amino-3-(benzyloxy)-2-benzaldehyde
as yellow oil (1.5 g, 65% yield). 1H NMR d = 9.9 (s, 1H, CHO), 7.4
(m, 5H, phenyl Hs), 6.1 (d, 1H), 6.9 (d, 1H), 6.7 (t, 1H), 6.65 (bs,
2.2. Materials
All solvents and chemicals were of analytical grade. Solvents
were freed from stabilizers by passing through activated basic alu-
mina columns. THF was distilled from Na in the presence of benzo-
phenone. Methanol was distilled from magnesium turnings and
iodine. Other chemicals and reagents were purchased from com-
mercial sources and were used without further purification.
2H, NH2), 5.0 (s, 2H, CH2Ph). IR (CHCl3)
m
= 1619 cmꢁ1 (NH2CH2).
8-Benzyloxy-2-(20-pyridyl)-8-hydroxyquinoline (Ph-HQP): 2-Ace-
tylpyridine (265 mg, 2.18 mmol) was added to a solution of 2-ami-
no-3-benzyloxybenzaldehyde (495 mg, 2.18 mmol) in THF (3 ml)
and the solution was cooled tot 0 °C. Triton B (1.46 g, 8.72 mmol)
Scheme 1. Molecular design of HQP.

G. Maayan et al. / Polyhedron 64 (2013) 365–370
367
was added, the mixture was stirred for 1 h, the ice bath was re-
moved, and stirring was continued over night. The solvent was
evaporated and the residue was washed 4 times with 0.5 N citric
acid, extracted with CHCl3, dried over Na2SO4 and the solvent
was removed. The product was obtained as a slightly yellow solid
(600 mg, 88% yield). 1H NMR d = 8.7 (m, 2H), 8.6 (d, 1H), 8.25 (d,
1H), 7.8 (dt, 1H), 7.45 (dd, 2H), 7.4 (m, 5H, phenyl Hs), 7.34 (m,
1H), 7.1 (dd, 1H), 5.4 (s, 3H).
on 6073 reflections, goodness-of-fit (GOF)on F2 = 1.038, largest elec-
tron density = 0.914 e Åꢁ3
.
[Cu(I)(HQP)2](PF6): A solution of HQP (10 mg, 0.045 mmol) in
acetonitrile (0.5 ml) was treated with a solution of Cu(I)(CH3CN)4-
PF6 (8.4 mg, 0.0225 mmol) in acetonitrile (0.5 ml) and the mixture
was stirred under inert atmosphere (N2) for 30 min. Removal of the
solvent under vacuum afforded a dark solid (41 mg, 90% yield). ESI-
MS: m/z = 506.7. 1H NMR (CD3CN) d = 8.78 (bd, 1H), 8.75 (bd,1H),
8.6 (d, 1H), 8.45 (shoulder + d, OH + 1H), 8.2 (bt, 1H), 7.53–7.45
(bm, 3H), 7.2 (bd, 1H).
2-(20-pyridyl)-8-hydroxyquinoline(HQP): 8-Benzyloxy-2-(20-pyri-
dyl)-8-hydroxyquinoline (600 mg, 1.9 mmol) was dissolved in for-
mic acid (90 ml) and treated for 3 h with H2 at atmospheric
pressure using 10% Pd/C (300 mg) as catalyst. The reaction mixture
was filtered, the solvent was removed, and the residue was purified
by column chromatography on silica gel with 9% methanol in chlo-
roform as eluent. The product (385 mg, 90% yield) was obtained as
a slightly yellow solid. 1H NMR d = 8.84 (dd, 1H), 8.74 (dq, 1H), 8.64
(d, 1H), 8.45 (bs, OH), 8.39 (d, 1H), 7.9 (dt, 1H), 7.55-7.45 (m, 3H),
7.18 (dd, 1H). ESI-MS m/z = 222.7.
[Ni(III)(HQP)2](PF6): A solution of HQP (20 mg, 0.09 mmol) in
methanol (2 ml) was treated with Ni(II) acetate (11.2 mg,
0.045 mmol) and was stirred for 30 min at room temperature. An
orange solid precipitated after the addition of NH4PF6 (0.56 ml of
a 2.6 M aqueous solution). It was filtered, dried and collected
(43 mg, 86% yield). Crystals were obtained from methanol solution
at room temperature. ESI-MS: m/z = 500.2.
Crystal data: Cell dimensions of the 0.3 ꢀ 0.1 ꢀ 0.05 mm3 orange
ꢀ
plates, C28H18N4O2Ni + CH4O + PF6 + H2O: triclinic, P1 (No. 2),
a = 8.776(1),
b = 13.642(2),
c = 24.911(3) Å,
a = 75.974(8),
2.4. Preparation of the metal complexes
b = 114.660(6), c
= 80.208(2), from 20° of data, T = 120 K,
V = 2843.3(6) Å3, Z = 4, formula weight = 686.19, Dcalc = 1.603 mg/
[Co(III)(HQP)2](PF6): To a solution of HQP (20 mg, 0.09 mmol) in
methanol (2 ml) was added Co(II) acetate (11.2 mg, 0.045 mmol)
and the solution was stirred for 30 minutes. An aqueous solution
of NH4PF6 (0.56 ml) was added and a red solid precipitated. It
was filtered and dried in the air (38.4 mg, 0.076 mmol, 85% yield).
ESI-MS: m/z = 501.77. 1H NMR (CD3CN) d = 8.95 (d, 1H), 8.66 (d,
1H), 8.4 (dd, 1H), 8.1 (dt, 1H), 7.55 (t, 1H), 7.30 (d, 1H), 7.22-7.18
(m, 2H), 6.76 (d, 1H).
mm3, = 0.820 mmꢁ1 5616 reflections collected, 0 6 h 6 6,
l
ꢁ9 6 k 6 10, ꢁ18 6 l 6 19, frame scan width = 0.5°, scan speed 1°/
120 s, typical peak mosaicity = 0.45°, 2617 independent reflections
(Rint = 0.071). Idealized hydrogen atoms were placed and refined in
a
riding mode. 740 parameters with 230 restraints, final
R1 = 0.0957 (based on F2) for data with I > 2
r(I) and R1 = 0.1076 for
all data based on 2617 reflections, goodness-of-fit (GOF) on
F2 = 1.068, largest electron density = 1.388 e Åꢁ3
.
Co(II)(HQP)2: A solution of HQP (10 mg, 0.045 mmol) in metha-
nol (0.5 ml) was treated with a solution of Co(II) acetate (5.6 mg,
0.0225 mmol) in methanol (0.5 ml) and the mixture was stirred
at rt under inert atmosphere (N2) for 30 min. Removal of the sol-
vent in vacuum afforded a dark red solid (39.7 mg, 0.079 mmol,
88% yield). The complex was crystallized from a methanol solution
at room temperature, under inert atmosphere. ESI-MS: m/z = 501.7.
Crystal data: Cell dimensions of the 0.3 ꢀ 0.1 ꢀ 0.05 mm3 orange
3. Results and discussion
3.1. Synthesis of HQP–metal complexes and their characterization by
ESI-MS and UV–Vis
HQP was used for the preparation of complexes with four metal
ions namely Co(II), Cu(II), Cu(I) and Ni(II). In a typical synthetic
protocol, an equivalent of the metal salt was added to two equiva-
lents of the ligand in methanol or acetonitrile solution and stirred
for half an hour at room temperature in air or under nitrogen. An
immediate color change was observed after mixing which signified
the formation of the complex. Isolation of the complexes from the
colored solution was done in one of the following ways: (i) concen-
plates
C28H18N4O2Co + 3(CH4O): Monoclinic, P291/c (No. 14),
a = 13.899(2), b = 14.095(2), c = 15.907(1) Å, b = 114.660(6), from
20° of data, T = 120 K, V = 2832.1(5) Å3, Z = 4, Formula weight =
597.52, Dcalc = 1.401 Mg/m3,
l
= 0.653 mmꢁ1. 5106 reflections col-
lected, 0 6 h 6 13, 0 6 k 6 13, ꢁ15 6 l 6 13, frame scan width =
2.0°, scan speed 1°/180 s, typical peak mosaicity = 0.45°, 2525 inde-
pendent reflections (Rint = 0.062). Idealized hydrogen atoms were
placed and refined in a riding mode. 376 parameters with 0 re-
trated aqueous solution of NH4+PF6 was added, yielding the pre-
ꢁ
straints, final R1 = 0.0464 (based on F2) for data with I > 2
r
(I) and
R1 = 0.0609 for all data based on 2525 reflections, goodness-of-fit
(GOF) on F2 = 1.068, largest electron density = 0.509 e Åꢁ3
cipitation of the complex, which was filtered and collected, or (ii)
the solvent was removed in vacu to yield the solid complex.
All complexes were initially characterized by ESI-MS. As
depicted in Table 1, the m/z data clearly indicate that HQP forms
stable complexes of the type M(HQP)2 with all the metal ions. In
addition, data analysis of the complexes [Co(III)(HQP)2](PF6),
Co(II)(HQP)2, Cu(II)(HQP)2(PF6) and [Ni(III)(HQP)2](PF6) reflects
binding of each ion to two tridentate ligands (four nitrogens and
two oxygen atoms). An interesting observation is that the m/z of
the complexes [Cu(I)(HQP)2](PF6) and Cu(II)(HQP)2(PF6) differ by
two units, which is attributable to two protons on the two hydro-
xyl group on each of the two ligands. This observation suggests
that Cu(I) binds only to four coordinating atoms – the two nitrogen
atoms of each ligand [9]. A molecular peak of m/z = 144, indicative
of a PF6ꢁ anion, was also observed in this spectrum, indicating that
the Cu(I) ion is coordinated to four nitrogen atoms and do not
interact with the OH groups, and therefore its chꢁarge is neutralized
by an exterior hexafluorophosphate anion. PF6 peaks were also
observed in the spectra of all the other metal complexes, excluding
the spectrum of Co(II)(HQP)2. These results are discussed below
.
Cu(II)(HQP)2(PF6): A solution of HQP (20 mg, 0.09 mmol) in
methanol (2 ml) was treated with Cu(II) acetate (9 mg,
0.045 mmol) and was stirred for 30 min under inert atmosphere
(N2). An orange solid precipitated after the addition of NH4PF6
(0.56 ml of a 2.6 M aqueous solution). It was filtered, dried and col-
lected (40.9 mg, 90% yield). The complex was crystallized from
methylene chloride at room temperature. ESI-MS: m/z = 504.7.
Crystal data: Cell dimensions of the 0.2 ꢀ 0.1 ꢀ 0.05 mm3 orange
plates, 2(C28H18N4O2Cu) + 2(PF6) + 2(CH2Cl2): triclinic, P1 (No. 2),
a = 8.4690(4), b = 14.010(6), c = 25.9100(8) Å,
84.540(2), = 80.350(2), from 10° of data, T = 120 K,
V = 2925.5(2) Å3, Z = 4, formula weight = 735.9, Dcalc = 1.671 mg/
mm3, = 1.059 mmꢁ1
22,611 reflections collected, 0 6 h 6 8,
a = 75.180(2), b =
c
l
.
ꢁ13 6 k 6 13, -25 6 l 6 25, frame scan width = 1.0°, scan speed 1°/
60 s, typical peak mosaicity = 0.7°, 6073 independent reflections
(Rint = 0.043). 811 Parameters with 0 restraints, final R1 = 0.0639
(based on F2) for data with I > 2
r(I) and R1 = 0.0797 for all data based

368
G. Maayan et al. / Polyhedron 64 (2013) 365–370
Table 1
Absorption maxima and ESI-MS data for the metal complexes.
Compound
ESI-MS (m/z), calculateda
ESI-MS (m/z), foundeda
kmax (nm)b
kMLCT (nm)b
HQP
Co(III)(HQP)2(PF6)
Co(II)(HQP)2
Ni(III)(HQP)2(PF6)
Cu(II)(HQP)2(PF6)
Cu(I)(HQP)2(PF6)
221.23
501.40
501.40
501.16
506.01
508.03
222.70
501.77
501.70
500.18
504.67
506.72
265, 285
303
306
307
307
463
451
466
467
410
295, 345
a
m/z data for HQP and its metal complexes as obtained in the positive mode spectra.
kmax is the wavelength at the maximum absorption. kMLCT is the wavelength at the maximum metal-to-ligand charge transfer absorption.
b
(Section 3.2) in the context of 1H NMR and X-ray crystallography
characterization of the obtained metal complexes.
the two absorbance bands of the Cu(I) complex with HQP show a
longer wavelength
p⁄ absorption at about 295 and 345 nm, then
p
–
The formation of the complexes was followed by UV–Vis spec-
troscopy. The results are summarized in Table 1. A typical UV–Vis
titration curve, illustrating the formation of Co(III)(HQP)2(PF6)
complex, is depicted in Fig. 1. The UV–Vis spectrum of HQP shows
two kmax absorption bands at about 265 and 285 nm, attributed to
the free HQP at about 265 and 285 nm.
Similar absorbance bands at about 296 and 343 nm were previ-
ously observed for a Cu(I) complex of 6,60-diphenyl-2,20-bipyri-
dine-4,40-dicarboxylic acid ([Cu(H26)2]Cl) [10]. In this complex,
Cu(I) is bound to the two nitrogen atoms of the 2,20-bipyridine part
of the ligand in a tetrahedral coordination geometry. Overall, these
data provide another indication for unique tetra-coordination of
Cu(I) to HQP, as implied by the MS data of this complex. Based
on the distinct difference between the absorbance bands and the
MS data of the complexes [Cu(I)(HQP)2](PF6) and Cu(II)(HQP)2(PF6)
we can propose that two different coordination compounds are
being formed; Cu(II) with a hexa-coordination geometry and
Cu(I) with a tetra-coordination geometry.
p–
p⁄ transitions of HQP, which originate from its two chromoph-
ores and it is in good agreement with the previous published data
[8]. These absorbance bands can be rationalized by comparing the
UV–Vis spectrum of HQP to the spectra obtained for both the
ligands 2,20-bipyridine and HQ measured in the same solvent sys-
tem. Our measurements produced a UV–Vis spectrum of the ‘‘soft’’
2,20-bipyridine, which shows two kmax absorption bands at about
235 nm and 282 nm, and a UV–Vis spectrum of the ‘‘harder’’ HQ,
which shows two kmax absorption bands at about 257 and
384 nm. It is therefore understood that HQP, being ‘‘harder’’ then
3.2. Characterization by solution 1H NMR and X-ray crystallography
2,20-bipyridine and ‘‘softer’’ then HQ undergoes
p–
p⁄ transitions
leading to absorbance bands at longer wavelength then 2,20-bipyr-
idine and shorter wavelength then HQ.
In order to provide a final conformation for our conclusion, we
made attempts to crystallize the two copper complexes for further
characterization of their structure and coordination geometry by
X-ray analysis. The complex Cu(II)(HQP)2(PF6) was crystallized
from dichloromethane solution at room temperature. Full crystal-
lographic data are described in the Section 2. Its crystal structure
is depicted in Fig. 2. This complex exhibits a non-perfect octahedral
geometry. The unit cell contains two molecules related to each
other by mirror-plane symmetry. It is apparent that Cu(II) ions
In the spectra of the complexes, with the exception of
[Cu(I)(HQP)2](PF6), there is only one kmax absorption band in the
303–345 nm region, implying that the absorption of the two chro-
mophores is coupled. These absorption peaks also are attributed to
p–
p⁄ transitions of HQP. An additional MLCT absorption band,
however, in the 410–467 nm region is observed in the spectra of
all the metal complexes. Notably, the complex [Cu(I)(HQP)2](PF6)
is the only one that exhibits two kmax absorption bands, similarly
to the kmax bands of the free HQP. A possible explanation for this
is that Cu(I) is bound to only one part of HQP (most likely to the
two nitrogen atoms), therefore the absorption is not coupled and
1.0
0.8
Co(III) Complex
Free Ligand
0.6
0.4
0.2
0.0
Fig. 2. Perspective view of the Cu(II)(HQP)2(PF6) complex. Selected bond distances
(Å) and angles (°, errors in digits in parentheses): Cu1–N2 = 1.947(4), Cu1–N4 =
2.004(4), Cu1–O = 2.122(4), Cu1–N1 = 2.142(4), Cu1–N3 = 2.223(5), Cu1–O2 = 2.334
(4); N2–Cu1–N4 = 178.95(19), N2–Cu1–O1 = 79.23(16), N4–Cu1–O1 = 100.78(16),
250
300
350
400
450
500
550
600
Wavelength[nm]
N2–Cu1–N1 = 77.41(17),
N4–Cu1–N1 = 102.69(17),
O1–Cu1–N1 = 155.84(15),
Fig. 1. Formation of Co(III) complex followed by UV spectroscopy. A 5 mM solution
of HQP in MeOH was titrated with 10 mM solution of Co(III) ion in MeOH.
0.1 equivalents of the metal ion were added in each step. Saturation of all binding
sites was obtained after 0.5 equivalents of the metal ions were added.
N–Cu1–N3 = 102.16(17), N4–Cu1–N3 = 76.80(17), O1–Cu1–N3 = 97.02(15), N1–
Cu1–N3 = 93.83(16), N2–Cu1–O2 = 105.77(15), N4–Cu1–O2 = 75.28(16), O1–Cu1–
O2 = 89.94(14), N1–Cu1–O2 = 90.52(15), N3–Cu1–O2 = 152.02(14).

G. Maayan et al. / Polyhedron 64 (2013) 365–370
369
Fig. 3. 1H NMR spectra of HQP (B) and its complexes with Cu(I) (A) and Co(III) (C). The structure of HQP and the numbering system is also depicted.
interact differently with the two oxygen atoms. The bond distance
between Cu(1)–O(1) is shorter than that of Cu(1)–O(21). This
observation and the presence of a PF6 anion in the unit cell for
neutralization of the charge, suggests that the bond Cu(1)–O(21)
is an ionic whereas Cu(1)–O(1) is a coordinative bond.
Although the NMR spectra of both these complexes support our
assumption regarding their binding coordination, they do not pro-
vide a clear proof for these suggested coordination geometries. The
broad peaks observed in the NMR spectrum of the Cu(I) complex,
for example, could result from a small amount of Cu(II) present
in solution. However, the combination between the NMR, ESI-MS
and UV data can provide a good basis for our assumption that (i)
Cu(I) is only bound to the bipyridine part of HQP in a tetra coordi-
nation geometry and is not bound to HQP in an octahedral geom-
etry as the other metal ions are, and (ii) that Co(III) is bound to all
N, N and O atoms of HQP in an hexa-coordinated geometry.
ꢁ
Attempts to obtain high quality crystals of Cu(I)(HQP)2(PF6) for
X-ray analysis were not successful. However, taking into account
that Cu(I)(HQP)2(PF6) is diamagnetic, we sought to gain structural
information of this complex by performing 1H NMR experiment in
solution. The results are shown in Fig. 3A and this spectrum is com-
pared to the spectrum of the free HQP ligand (Fig. 3B) and to the
spectrum of the Co(III) complex [Co(III)(HQP)2](PF6), which is also
diamagnetic. The chemical shift of the proton at position 7 of the
quinoline ring (H1) in the 1H NMR spectrum of the Cu(I) complex
is the same as that of the free ligand and is highly shifted in the
spectrum of the Co(III) complex. The broad peak of the OH at
d = 8.45 ppm (Fig. 1B) is absent in the spectrum of the Co(III) com-
plex and present in the spectrum of the Cu(I) complex (broad
shoulder by the dublet of H4, Fig. 3A). These observations suggest
that the OH group of the quinoline ring is taking part in the com-
plexation of Co(III) but it is not involved in the binding of Cu(I)
ions, and support the ESI-MS and UV–Vis data obtained for the
Cu(I) complex.
The clear 1H NMR spectrum depicted on Fig. 3C indicates that
the complex is diamagnetic. Therefore we assumed that when
the formation of the cobalt complex was carried out in the air,
by mixing a solution of HQP with half an equivalent of Co(II) ace-
tate in methanol, the Co(II) ions were spontaneously oxidized to
Co(III) and the complex [Co(III)(HQP)2](PF6) was obtained. In fact,
the observation that phenolate ions stabilize the Co(III) oxidation
state by about 1 V relative to that of Co(II) complexed to
2,20,60,200-terpyridine was already described [6]. The peaks in the
1H spectrum of this complex are shifted relative to those of the free
ligand (Fig. 3B and Fig. 3C) and the broad peak at d = 8.45 ppm,
which corresponds to the OH group, is absent. This observation, to-
gether with the ESI-MS data confirms the participation of O- in the
structure of the complex and supports hexa-coordinated geometry.
ESI-MS and NMR spectra were recorded on the same sample for
both Cu(I)(HQP)2(PF6) and [Co(III)(HQP)2](PF6) complexes.
Fig. 4. Perspective view of the [Co(III)(HQP)2](PF6) complex. Selected bond distances
(Å) and angles (°, errors in digits in parentheses): Co1–N4 = 2.019(4),
Co1–N1 = 2.024(4), Co1–O2 = 2.124(3), Co1–O1 = 2.134(3), Co1–N3 = 2.175(4),
Co1–N2 = 2.180(4); N4–Co1–N1 = 170.64(16), N4–Co1–O2 = 78.59(14), N1–Co1–
O2 = 93.68(13), N4–Co1–O1 = 97.82(14), N1–Co1–O1 = 78.17(15), O2–Co–O1 =
100.02(13), N4–Co1–N3 = 75.02(16), N1–Co1–N3 = 113.22(15), O2–Co1–N3 =
152.71(14),
O1–Co1–N3 = 90.39(14),
N4–Co1–N2 = 110.79(16),
N1–Co1–
N2 = 74.71(17), O2–Co1–N2 = 93.95(14), O1–Co1–N2 = 150.15(14), N3–Co1–N2 =
89.02(15).

370
G. Maayan et al. / Polyhedron 64 (2013) 365–370
synthesized, and its complexes with several metal ions were
prepared. This system is highly interesting because of its ability
to yield complexes with various coordination geometries; metal
ions that prefer to bind to O and N atoms of the HQ form octahedral
geometry while metal ions with preference to the N, N atoms of the
bipyridine system form complexes with tetra-coordinated geome-
try. We describe complexes of the two geometries – Cu(II)(HQP)2
(PF6) having a octahedral geometry and [Cu(I)(HQP)2](PF6) having
a tetra-coordinated geometry. We have also demonstrated the
ability of this N, N, O tridentate ligand to stabilize high oxidation
states of metal ions, i.e., cobalt(III) and nickel(III) by the spontane-
ous formation of the complexes [Co(III)(HQP)2](PF6) and [Ni(III)
(HQP)2](PF6) from the salts of Co(II) and Ni(II), respectively, in air.
Acknowledgements
Fig. 5. Perspective view of the [Ni(III)(HQP)2](PF6) complex. Selected bond
distances (Å) and angles (°, errors in digits in parentheses): Ni1–N3 = 1.967(11),
Ni1–N8 = 1.979(10), Ni1–O1 = 2.184(8), Ni1–N2 = 2.092(11), Ni1–N1 = 2.118(9),
Ni1–O2 = 2.164(7); N3–Ni1–N8 = 176.2(4), N3–Ni1–N2 = 102.4(4), N8–Ni1–
N2 = 77.9(4), N3–Ni1–N1 = 75.5(4), N8–Ni1–N1 = 100.7(4), N2–Ni1–N1 = 94.9(4),
N3–Ni1–O = 79.2(4), N8–Ni1–O2 = 104.6(3), N2–Ni1–O2 = 92.3(3), N1–Ni1–
O2 = 154.7(4), N3–Ni–O1 = 102.9(3), N8–Ni1–O1 = 76.9(4), N2–Ni1–O1 = 154.7(4),
N1–Ni1–O1 = 92.2(3).
We thank Dr. Mark Botoshansky for preparing the CIF files for
submission. G. Maayan thanks Prof. Gonen Ashkenasy for his ded-
icated guidance and support during the performance of this work.
Appendix A. Supplementary data
CCDC 941955–941957 contain the supplementary crystallo-
graphic data for the complexes described in this paper. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2013.06.038.
Mixing HQP with half an equivalent of Co(II) in methanol under
inert atmosphere has yielded the complex Co(II)(HQP)2. The struc-
ture of this complex was corroborated by X-ray structure analysis
(Fig. 4). The crystallographic data is summarized in the Experimen-
tal section. From Fig. 4 it is clear that each cobalt(II) ion is bound to
two molecules of HQP forming a complex with an octahedral
geometry.
References
Another evidence for the ability of HQP to stabilize high oxida-
tion states was the spontaneous formation of the Ni(III) complex
[Ni(III)(HQP)2](PF6) from a Ni(II) salt in air. Thus, mixing HQP with
half an equivalent of Ni(II) acetate in methanol yields a light orange
solution. The orange precipitation obtained after the addition of
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ꢁ
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ꢁ
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4. Conclusions
The tridentate ligand HQP, which is a hybrid of the two
bidentate ligands 2,20-bipyrine and 8-hydroxyquinoline, was