Copper(II) complexes of symmetric and asymmetric bis(imine) ligands: Tuning the Cu(I)/Cu(II) redox couple

Article abstract of DOI:10.1016/j.ica.2012.08.026

Two new pyridylbis(imine) ligands 2-((2-methyl-2-(pyridin-2-yl)-3-(pyridin- 2-ylmethyleneamino)propylimino)methyl)phenol, HL², and 2-methyl-2-(pyridin-2-yl)-N¹,N³-bis(pyridin-2-ylmethylene) propane-1,3-diamine, L³, were synthesized via the Schiff base condensation of 2-methyl-2-pyridin-2-yl-propane-1,3-diamine and either 2-pyridinecarboxaldehyde or 2-hydroxybenzaldehyde. Complexation of Cu(II) ions by HL² and L³ yielded [Cu(HL²)] ₂(ClO₄)₄ (2), and [Cu(L³)](ClO ₄)₂ (3), respectively. The structures and electrochemistry of 2 and 3 were compared to our previously synthesized Cu(II) complex of the ligand 2,2'-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azan-1-yl-1-ylidene) bis(methan-1-yl-1-ylidene)diphenol (H₂L¹), which coordinates to Cu(II) as a dianion to form [Cu(L¹)(CH₃OH)] (1). Whereas (L¹)^2- and L³ form mononuclear complexes with Cu(II) ions, the asymmetric ligand HL² produces copper dimers in the solid state with the phenolate O atoms bridging between copper ions. Solution magnetic moment measurements and ESI-MS suggest that all three species exist as monomers in solution, although small amounts of dimeric 2 were detected in solution by ESI-MS and EPR spectroscopy. Complexes of all three ligands show similar EPR properties with typical axial spectra. Cyclic voltammetry reveals that the Cu(I)/Cu(II) redox couples of 2 and 3 are shifted to more positive potentials than that of 1, with 3 having the most positive one-electron reduction potential: E_1/2(1) = -1.489 V; E _1/2(2) = -1.099 V; E_1/2(3) = -0.438 V, all versus Fc/Fc⁺.

Full text of DOI:10.1016/j.ica.2012.08.026

Inorganica Chimica Acta 394 (2013) 415–422
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Copper(II) complexes of symmetric and asymmetric bis(imine) ligands: Tuning
the Cu(I)/Cu(II) redox couple
Anna Jozwiuk a, Zhaodong Wang a, Douglas R. Powell a, Robert P. Houser a,b,
⇑
a Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK 73019, United States
b Department of Chemistry and Biochemistry, University of Northern Colorado, Greeley, CO 80639, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2012
Received in revised form 27 August 2012
Accepted 28 August 2012
Available online 15 September 2012
Two new pyridylbis(imine) ligands 2-((2-methyl-2-(pyridin-2-yl)-3-(pyridin-2-ylmethyleneamino)
propylimino)methyl)phenol, HL2, and 2-methyl-2-(pyridin-2-yl)-N1,N3-bis(pyridin-2-ylmethylene)
propane-1,3-diamine, L3, were synthesized via the Schiff base condensation of 2-methyl-2-pyridin-2-
yl-propane-1,3-diamine and either 2-pyridinecarboxaldehyde or 2-hydroxybenzaldehyde. Complexation
of Cu(II) ions by HL2 and L3 yielded [Cu(HL2)]2(ClO4)4 (2), and [Cu(L3)](ClO4)2 (3), respectively. The struc-
tures and electrochemistry of 2 and 3 were compared to our previously synthesized Cu(II) complex of the
ligand 2,2’-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-yli-
dene)diphenol (H2L1), which coordinates to Cu(II) as a dianion to form [Cu(L1)(CH3OH)] (1). Whereas
(L1)2ꢀ and L3 form mononuclear complexes with Cu(II) ions, the asymmetric ligand HL2 produces copper
dimers in the solid state with the phenolate O atoms bridging between copper ions. Solution magnetic
moment measurements and ESI–MS suggest that all three species exist as monomers in solution,
although small amounts of dimeric 2 were detected in solution by ESI–MS and EPR spectroscopy. Com-
plexes of all three ligands show similar EPR properties with typical axial spectra. Cyclic voltammetry
reveals that the Cu(I)/Cu(II) redox couples of 2 and 3 are shifted to more positive potentials than that
of 1, with 3 having the most positive one-electron reduction potential: E1/2(1) = ꢀ1.489 V; E1/2(2)
= ꢀ1.099 V; E1/2(3) = ꢀ0.438 V, all versus Fc/Fc+.
Keywords:
Copper complexes
X-ray crystal structures
Electrochemistry
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Copper complexes with chelating Schiff base ligands have been
thoroughly investigated. Schiff bases are important ligands be-
Copper is one of the most abundant redox active metals and
plays an important role in many biological processes. For example,
blue copper electron transfer proteins use copper as a one electron
relay, shuttling between the cuprous and cupric oxidation states
[1]. The tuning of the CuII/CuI redox couple is a critical component
of electron transfer in blue copper proteins, and is also important
in synthetic complexes where copper plays a role as the redox cen-
ter in catalysis [2,3]. The relationship between redox potential and
structure has therefore received considerable attention for many
years [4–8]. While geometric constraints, e.g. tetrahedral versus
square planar copper in blue copper proteins, are of great impor-
tance, ligand donor atoms are also a key factor in redox potential.
Sigma- and pi-donor ligands push more electron density on the
metal ion, moving the CuII/I redox potential to lower (more nega-
tive) potentials [9].
cause of their simplicity in syntheses and the diversity of possible
ligands owing to the mix-and-match ability of the condensation of
various amines with different aldehydes. In particular, tetradentate
Schiff base ligands, including salen and its derivatives, have been
used for a wide variety of coordination chemistry with copper
and other transition metals [10–17]. Generally, the salen-type
ligands coordinate to copper in a square-planar mode (see Fig. 1).
The syntheses of tetradentate Schiff base ligands are versatile
due to their modular nature. An alkanediamine backbone starting
material can easily be transformed into a tetradentate ligand by
condensation with two equivalents of an aldehyde.
Examples of how different tetradentate Schiff base donors influ-
ence the CuII/CuI redox potential of the corresponding complex
have been reported in the literature [16,18–21]. It has been shown
that the length of the alkane chain of the diamine unit plays an
important role in the redox behavior of the metal ion, as this type
of ligand variation selects for different coordination geometries,
(e.g. tetrahedral versus square planar) [18,19]. In a different study,
variation of the weakly coordinated counterion in the axial posi-
tion of the quadridentate Schiff base copper complex influences
the electrochemistry of the copper couple [20]. Although diamines
⇑
Corresponding author at: Department of Chemistry and Biochemistry, Univer-
sity of Northern Colorado, Greeley, CO 80639, United States. Tel.: +1 970 351 2877;
fax: +1 970 351 2176.
E-mail address: robert.houser@unco.edu (R.P. Houser).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.08.026

416
A. Jozwiuk et al. / Inorganica Chimica Acta 394 (2013) 415–422
dene)bis(methan-1-yl-1-ylidene)diphenol; HL2 = 2-((2-methyl-2-
(pyridin-2-yl)-3-(pyridin-2-ylmethyleneamino)propylimino)
methyl)phenol; L3 = 2-methyl-2-(pyridin-2-yl)-N1,N3-bis(pyridin-
2-ylmethylene)propane-1,3-diamine; 1 = [Cu(L1)(CH3OH)]; 2 = [Cu-
(HL2)]2(ClO4)4; 3 = [Cu(L3)](ClO4)2.
2.2. General procedures
Unless otherwise stated, all reagents were used as received
from commercial sources. 2-Methyl-2-(pyridine-2-yl)propane-
1,3-diamine (ppda) was synthesized according to the published
procedure [23]. H2L1 and 1 were synthesized according to the pub-
lished procedures [22]. Solvents used were doubly purified using
alumina columns in a MBraun solvent purification system (MB-
SPS). Infrared spectra were measured from 4000 to 400 cmꢀ1 as
KBr pellets on a BIO-RAD FTS 155 FTIR spectrometer. 1H NMR spec-
tra were measured using a Varian 300 MHz instrument using sol-
vent (CHCl3) as an internal standard. Mass spectra were
measured on a Q-TOF quadrupole time-of-flight mass spectrome-
ter (Micromass, Manchester, UK) equipped with a Z-spray electro-
spray ionization (ESI) source. Elemental analyses were performed
by Atlantic Microlab, Norcross, GA. UV–Vis spectra were measured
using a Shimadzu UV2401PC spectrophotometer in the range 250–
900 nm on solutions ranging in concentration from 1.0 ꢁ 10ꢀ3 to
1.0 ꢁ 10ꢀ4 M. Cyclic voltammetry experiments were performed
using a BAS 50 W potentiometer and a standard three-electrode
cell with a glassy-carbon working electrode, a Pt-wire auxiliary
electrode, and an Ag/AgCl pseudo-reference electrode under an in-
ert atmosphere at room temperature. X-band EPR spectra of the
complexes were recorded at 77 K using a Bruker EMX spectrome-
ter. Magnetic susceptibilities of the complexes in the solid state
were measured at 295 K using a Johnson Matthey magnetic sus-
ceptibility balance (MSB – AUTO) with a magnetic field strength
of 4.5 kGauss and a measurement range of 1.999 ꢁ 10ꢀ4 to
5 ꢁ 10ꢀ10 cgs. Solution magnetic susceptibilities were measured
at 294 K by the Evans method [24].
Fig. 1. General structure of copper(II) complexes with tetradentate Schiff base
ligands. X, Y = donor group (e.g. pyridyl or phenolate), Z = counterion in axial
position and n = number of –(CH2)– units.
should allow stepwise condensation of two different aldehydes
and therefore more variation, not much has been reported on the
electrochemistry of copper complexes with these ligands. Ghosh
and coworkers have shown that the monocondensed product of
1,3-propanediamine and 1-benzoylacetone can be used as a pre-
cursor for condensation with 2-pyridinecarboxaldehyde or 2-ace-
tylpyridine [20]. These asymmetric copper complexes show
drastic shifts in redox potentials of the copper ion compared to
the symmetric condensation product of 1,3-propanediamine with
two equivalents of 1-benzoylacetone [21]. The asymmetric copper
complex of 1:1:1 condensation of 1,3-propanediamine, 2,4-pen-
tanedione and 2-pyridinecarboxaldehyde showed irreversible
reductions and the copper complexes readily reorganized into its
symmetric complexes in the presence of catalytic amounts of acid
and copper(II) ions [16].
In this paper we contrast the electrochemical properties of
copper(II) complexes of three related tetradentate Schiff base
ligands, H2L1, HL2, and L3. Recently we reported the synthesis
of a symmetric Schiff base ligand H2L1 (Fig. 2), derived from
Caution: Perchlorate salts of metal complexes with the organic
ligands are potentially explosive. Although no difficulty was
encountered during the syntheses described herein, they should
be prepared in small amounts and handled with caution.
2-methyl-2-pyridin-2-yl-propane-1,3-diamine
(ppda)
and
2-hydroxybenzaldehyde [22]. Herein we report the synthesis of
the more electron deficient bis(pyridyl) bis(imine), L3, and the
asymmetric hybrid version containing both a pyridyl group and a
phenol group, namely HL2 (see Fig. 2). The latter ligand allows
access to intermediate electronic properties between the two
strictly symmetric ligands.
2.3. HL2
2-Pyridinecarboxaldehyde (0.127 g, 1.15 mmol) was added
dropwise to a solution of ppda (0.190 g, 1.15 mmol) in CH3OH
(5 mL). The resulting yellow solution was refluxed overnight, caus-
ing a color change to light orange. The solvent was removed in va-
cuo to give L4 as an orange-brown oil (0.290 g). 1H NMR (300 MHz,
DMSO-d6, 293 K) d 1.02–1.44 (m, 3H), 2.89–4.51 (m, 4H), 4.47–4.54
(m, 1H), 7.18–7.90 (m, 6H), 8.27–8.62 (m, 2H). See Supporting
information, Fig. S5. Crude L4 was redissolved in CH3OH (5 mL)
without further purification, and 2-hydroxybenzaldehyde
(0.136 g, 1.11 mmol) was added. While refluxing overnight the
solution turned light green. The solvent was removed in vacuo,
yielding HL2 as a highly viscous orange oil (0.400 g). The oil was
kept under nitrogen and solidified after several months. According
to NMR spectroscopy the composition of the ligand did not change
upon solidification. 1H NMR (300 MHz, CDCl3, 293 K) d 1.54 (m,
3H), 3.95–4.30 (m, 4H), 6.78–6.93 (m, 2H), 7.05–7.43 (m, 5H),
7.55–7.74 (m, 2H), 7.92–7.99 (m, 1H), 8.26–8.37 (m, 2H), 8.56–
8.66 (m, 2H), 13.14–13.34 (m, 1H).). ESI–MS (CH3OH): m/z =
359.2 [HL2 + H]+, 255.2 [L4 + H]+ (minor). Solutions of HL2 decom-
pose within hours, probably due to hydrolysis in the presence of
moisture (see Supporting information, Fig. S2). Due to the highly
viscous, sticky nature of the ligand, no elemental analysis was
2. Experimental
2.1. Abbreviations
ppda = 2-methyl-2-pyridin-2-yl-propane-1,3-diamine; H2L1 = 2,20-
(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azan-1-yl-1-yli-
Fig. 2. Structures of the previously synthesized ligand H2L1 and the two new
ligands HL2 and L3. Ionizable protons are highlighted in red. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)

A. Jozwiuk et al. / Inorganica Chimica Acta 394 (2013) 415–422
417
Table 1
performed. Purification was achieved through complex formation
with copper(II) ions.
Crystallographic data for 2 and 3.
2ꢂ2CH3OH
46H52Cl4Cu2N8O20
1305.84
Monoclinic
C2/c
3
2.4. L3
Formula
fw
Crystal system
Space group
C
C21H21Cl2CuN5O8
605.87
Triclinic
2-Pyridinecarboxaldehyde (0.363 g, 3.39 mmol) was added
dropwise to a solution of ppda (0.280 g, 1.69 mmol) in CH3OH
(5 mL). The resulting yellow solution was refluxed overnight, caus-
ing a color change to light orange. The solvent was removed in va-
cuo to give the ligand as a highly viscous dark orange oil (0.575 g).
1H NMR (300 MHz, CDCl3, 293 K) d 1.55 (s, 3H), 4.12–4.24 (m, 4H),
7.05–7.11 (m, 1H), 7.24–7.30 (m, 2H), 7.37–7.42 (m, 1H), 7.55–7.62
(m, 1H), 7.64–7.72 (m, 2H), 7.92–7.98 (m, 2H), 8.32–8.37 (m, 2H),
8.57–8.63 (m, 3H). ESI–MS (CH3OH): m/z = 344.2 [L3 + H]+, 255.2
[L4 + H]+. Solutions of L3 decompose within hours, probably due
to hydrolysis in the presence of moisture (see Supporting informa-
tion, Fig. S3). Due to the highly viscous, sticky nature of the ligand,
no elemental analysis was performed. Purification was achieved
through complex formation with copper(II) ions.
P–1
Unit cell dimensions
a (Å)
b (Å)
c (Å)
20.587(12)
12.266(7)
21.779(12)
90
110.162(16)
90
5163(5)
4
1.680
9.183(4)
10.365(4)
12.064(5)
88.416(8)
84.424(10)
85.125(12)
1138.5(8)
2
1.767
0.71073
1.70 to 28.42°
0.0553, 0.1359
1.001
a
(deg)
b (deg)
c
(deg)
V (Å3)
Z
qCalc. (mg/m3)
l
(mmꢀ1
)
1.119
h (°)
R1,a wR2 [I > 2
Goodness-of-fit (GOF) on F2
1.97 to 28.52
0.0456, 0.1118
1.001
b
r
(I)]
a
R1 =
R
||Fobs| ꢀ |Fcalc||/
R|Fobs|.
b
2
½
wR2 = {
R
[w(Fobs ꢀ Fcalc2)2]/
R
[w(Fobs2)2]}
.
2.5. [Cu(HL2)]2(ClO4)4 (2)
Cu(ClO4)2ꢂ6H2O (0.220 g, 0.586 mmol) dissolved in methanol
(1 mL) was added to a solution of ligand HL2 (0.210 g, 0.586 mmol)
in CH3OH (10 mL). The resulting dark green solution was stirred
overnight at room temperature to yield an olive-green precipitate
which was isolated by filtration, washed with methanol and dieth-
ylether (0.200 g, 53%). Anal. Calc. for 2ꢂ2H2O, powder, C44H48Cl4Cu2-
N8O20: C, 41.36; H, 3.79; N, 8.77. Found: C, 41.47; H, 3.51; N, 8.83%.
X-ray quality crystals were obtained from Et2O diffusion into a
solution of 2 in methanol/acetonitrile. Anal. Calc. for 2, crystals,
2.7. X-ray crystal structure determination
Intensity data for 2 and 3 were collected using a diffractometer
with a Bruker APEX ccd area detector [25,26]. Data were collected
using graphite-monochromated Mo K
a radiation (k = 0.71073 Å).
The samples were cooled to 100(2) K. Cell parameters were deter-
mined from a non-linear least squares fit of the data. The data were
corrected for absorption by the semi-empirical method [27]. The
structures were solved by direct methods and refined by full-ma-
trix least-squares methods on F2 [28,29]. Hydrogen atom positions
of hydrogens bonded to carbons were initially determined by
geometry and refined by a riding model. Hydrogens bonded to
nitrogens or oxygens were located on a difference map, and their
positions were refined independently. Non-hydrogen atoms were
refined with anisotropic displacement parameters. Hydrogen atom
displacement parameters were set to 1.2 (1.5 for methyl) times the
displacement parameters of the bonded atoms. Crystal data for 2
and 3 are summarized in Table 1. Selected bond lengths and angles
for 2 and 3 are summarized in Table 2. Hydrogen bonding informa-
tion of 2 is represented in Table 3.
C
44H44Cl4Cu2N8O18: C, 42.56; H, 3.57; N, 9.02. Found: C, 42.08; H,
3.86; N, 9.41%. UV/Vis (CH3CN) [kmax, nm (
e
, Mꢀ1 cmꢀ1)]: 243
(18300), 272 (15600), 369 (5,470), 573 (128). EPR (9.468 GHz,
mod. amp. 25.0 G, CH3CN, 77 K): g|| = 2.19, g\ = 2.06, and A|| = 205
G. EPR (9.466 GHz, mod. amp. 25.0 G, CH3OH, 77 K): g||(1) = 2.22,
g\(1) = 2.04, and A||(1) = 190 G; g||(2) = 2.31, g\(2) = 2.04, and
A||(2) = 200 G. FTIR (KBr): 2364, 2343, 1616, 1537, 1468, 1448,
1402, 1328, 1301, 1106, 1089, 1030, 964, 780, 768, 624, 533,
508, 417 cmꢀ1. ESI–MS (CH3CN or CH3OH): m/z = 420.1 [Cu(L2)]+,
941.1 [(Cu(L2))2ClO4]+. Solid state magnetic moment (MSB-Auto,
4.5 kG, 22.0 °C): 4.4 lB. Solution magnetic moment (Evans method,
20.9 °C, 16.3 ꢁ 10ꢀ3 M, acetonitrile-d3): 1.76 lB
.
3. Results and discussion
2.6. [Cu(L3)](ClO4)2. (3)
3.1. Syntheses
2-Pyridinecarboxaldehyde (0.027 g, 0.254 mmol) was added
dropwise to solution of hexahydropyrimidine (0.065 g,
a
3.1.1. Ligands
0.254 mmol) in CH3OH (5 mL) and the reaction mixture refluxed
for 3 h. After cooling to room temperature Cu(ClO4)ꢂ6H2O
(0.094 g, 0.254 mmol) in methanol (1 mL) was added. The resulting
turquoise solution was stirred overnight at room temperature to
yield a light blue precipitate which was isolated by filtration,
washed with methanol, diethylether and pentane (0.120 g, 78%).
X-ray quality crystals were obtained from Et2O diffusion into a
solution of 3 in acetonitrile. Anal. Calc. for C21H21Cl2CuN5O8: C,
41.63; H, 3.49; N, 11.56. Found: C, 41.89; H, 3.54; N, 11.64. UV/
The previously reported ligand H2L1 (Fig. 2) was prepared
through Schiff base condensation of ppda with 2-hydroxybenzal-
dehyde, and isolated in high yields in our laboratory [22]. Using
the same approach, two novel ligands HL2 and L3 (see Fig. 2) were
synthesized. HL2 is a potentially monoanionic ligand with an N4O
donor set, whereas L3 is neutral with an N5 donor set. HL2 and L3
were synthesized via condensation of ppda with one equivalent
each of 2-pyridinecarboxaldehyde and 2-hydroxybenzaldehyde,
or two equivalents of 2-pyridinecarboxaldehyde, respectively.
However, we were not able to obtain HL2 and L3 in high purity.
There are challenges in the synthesis of asymmetric ligands like
HL2, such as the step-wise condensation of different aldehydes
with the diamine starting material. Some examples of asymmetric
bis(imine) and bis(amine) ligands, as well as their metal complexes
can be found in the literature [30–34]. An additional difficulty that
must be taken into account in the step-wise condensation of 1,3-
propanediamine units with two different aldehydes is the ten-
Vis (CH3CN) [kmax, nm (e
, Mꢀ1 cmꢀ1)]: 281 (15900), 661 (117).
EPR (9.441 GHz, mod. amp. 25.0 G, CH3CN, 77 K): g|| = 2.19,
g\ = 2.07 and A|| = 185 G. FTIR (KBr): 2364, 2343, 1653, 1602,
1564, 1475, 1429, 1311, 1267, 1230, 1121, 1090, 1023, 981, 954,
787, 760, 671, 622, 502, 420 cmꢀ1. ESI–MS (CH3CN): m/z = 505
[Cu(L3)ClO4]+. Solid state magnetic moment (MSB-Auto, 4.5 kG,
22.0 °C): 3.5 lB
. Solution magnetic moment (Evans method,
20.9 °C, 16.8 ꢁ 10ꢀ3 M, acetonitrile-d3): 1.75 lB
.

418
A. Jozwiuk et al. / Inorganica Chimica Acta 394 (2013) 415–422
Table 2
the appearance of additional signals in the methyl, methylene
and aromatic regions. 1H NMR spectra of solutions of H2L1, HL2
and L3, taken immediately and taken after four days are shown
in the Supporting information, Figs. S1–S3, respectively. The spec-
trum of H2L1 only shows a growth in the water peak signal while
HL2 and L3 show decomposition.
Selected bond lengths (Å) and angles (deg) for 2 and 3.
Complex 2
Cu1–O1
Cu1–N20
Cu1–O1#1
O1–Cu1–N9
N9–Cu1–N20
N9–Cu1–N27
O1–Cu1–O1#1
N9–Cu1–O1#1
1.922(2)
2.020(2)
2.487(2)
94.22(9)
93.97(9)
169.94(9)
82.52(8)
99.14(8)
Cu1–N9
Cu1–N27
Cu1–O2A
O1–Cu1–N27
N27–Cu1–N20
O1–Cu1–N20
N27–Cu1–O1#1
N20–Cu1–O1#1
1.956(3)
2.010(3)
2.763(3)
91.35(9)
80.94(9)
171.38(9)
89.87(8)
93.55(8)
Our observations on the instability of ligands HL2 and L3 are in
good agreement with recently published work on the formation of
bis(imine)s and hexahydropyrimidines via the condensation of 1,3-
propanediamine with various aldehydes. Locke et al. reported the
preference of the formation of hexahydropyrimidines with elec-
tron deficient aldehydes (e.g. 2-pyridinecarboxaldehyde), and the
tendency to form bis(imine)s with electron rich aldehydes like 2-
hydroxybenzaldehyde [35]. The electron withdrawing nature of
the pyridine ring in 2-pyridinecarboxaldehyde seems to induce
the formation of a hexahydropyrimidine rather than an imine with
the 1,3-propanediamine unit [35]. Also, the potential for the for-
mation of intramolecular hydrogen bonds between the phenol
hydrogen atoms and the imine nitrogen atoms in H2L1 may stabi-
lize the bis(imine) while L3 cannot develop similar hydrogen
bonding.
Complex 3
Cu1–N19
Cu1–N1
Cu1–O1B
N19–Cu1–N8
N8–Cu1–N1
N8–Cu1–N22
N19–Cu1–O1B
N1–Cu1–O1B
1.966(2)
2.018(2)
2.573(2)
91.65(9)
82.63(9)
169.95(9)
106.80(9)
82.89(9)
Cu1–N8
Cu1–N22
Cu1–O1A
N19–Cu1–N1
N19–Cu1–N22
N1–Cu1–N22
N8–Cu1–O1B
1.996(2)
2.050(2)
2.660(2)
168.37(9)
81.35(9)
105.50(9)
85.17(8)
Symmetry information used to generate equivalent positions: (ꢀx + ½, ꢀy + ½, ꢀz).
Table 3
Hydrogen bonds (Å) and angles (deg) for 2.
D–Hꢂ ꢂ ꢂA
d(D–H)
d(Hꢂ ꢂ ꢂA)
2.01(3)
2.24(4)
d(Dꢂ ꢂ ꢂA)
2.764(3)
2.864(3)
<(DHA)
3.1.2. Copper complexes
Despite difficulties in isolating HL2 and L3, we were able to obtain
pure metal complexes of the two ligands upon coordination to
copper(II) salts. A methanolic solution of either crude HL2 or L3
was typically treated with copper(II) perchlorate to yield 2 or 3
as olive-green and light blue precipitates, respectively
(Scheme 1a and e). Complex 3 was also obtained when L3 was gen-
erated in situ from L4 in solution, along with one equivalent of 2-
pyridinecarboxaldehyde followed by subsequent addition of cupric
perchlorate as shown in Scheme 1f. Crystals suitable for X-ray
structural analysis were obtained after recrystallization of the
powders. Complex 2 was found in the solid state to be a dimeric
species of two [Cu(HL2)]2+ subunits where the proton on the phe-
nol group has migrated to the pyridyl N atom, namely [Cu(HL2)]2
(ClO4)4. The ligand in complex 2 is therefore still neutral despite
the deprotonated phenolate due to the protonated pyridinium
group. In contrast to 2, complex 3 was isolated as a monomer
[Cu(L3)](ClO4)2. Solutions of 2 in methanol and acetonitrile seem
to dissociate into monomers as ascertained by the solution mag-
netic moment and mass spectrometry. Low intensity peaks in the
ESI–MS show a small amount of dimer present, as well as mono-
mer with solvent coordinated. Additionally, CV and EPR data indi-
cate that there may be small amounts of dimer in solution (see
Section 3.3). Unfortunately, due to solubility constraints in
N14–H14ꢂ ꢂ ꢂO1S
0.83(3)
0.74(4)
151(3)
144(4)
O1S–H1Sꢂ ꢂ ꢂO1A#2
Symmetry information used to generate equivalent positions: (ꢀx + ½, ꢀy + ½, ꢀz).
dency in formation of hexahydropyrimidines (through cyclization)
with the first equivalent of aldehyde [35]. The advantage of the
cyclization, though, is the directed yield in mono condensed prod-
ucts over a mix of doubly condensed and unreacted species. The
most challenging aspect is the hydrolysis of asymmetric bis(i-
mine)s followed by condensation to their symmetric analogues.
However, this problem can be overcome by metal chelation, which
prevents decomposition and rearrangement.
Our attempts to purify L3 through chromatography (silica,
CH2Cl2:CH3OH, 95%/5%) resulted in its decomposition. The two col-
lected fractions were identified as 2-pyridinecarboxaldehyde and a
mixture of compounds (the majority being most likely the cis and
trans isomers of the hexahydropyrimidine L4: see Scheme 1 and
the 1H NMR spectrum in Figure S4). In addition, we observed
decomposition of both ligands HL2 and L3 in solution over time,
to 2-pyridinecarboxaldehyde and possibly the hexahydropyrimi-
dine species, which was indicated by the growth in the 1H NMR
of the aldehydic proton of 2-pyrdinecarboxaldehyde signal, and
Scheme 1. Synthesis of HL2, L3, 2 and 3. (a) Cu(ClO4)2ꢂ6H2O; (b) 1 eq 2-pyridinecarboxladehyde, 1 eq 2-hydroxybenzaldehyde; (c) 2 eq 2-pyridinecarboxladehyde; (d) column
chromatography on silica, or decomposition in solution; (e) Cu(ClO4)2ꢂ6H2O and (f) 1 eq 2-pyridinecarboxladehyde, Cu(ClO4)2ꢂ6H2O.

A. Jozwiuk et al. / Inorganica Chimica Acta 394 (2013) 415–422
419
non-coordinating solvent we were not able to study the nature of
the dimer in solution.
Attempts to synthesize [CuL2]+ by the deprotonation of HL2
were unsuccessful. The potentially monoanionic ligand HL2 was
treated with base either before or after the addition of copper(II)
ions in order to investigate the formation of complexes with
(L2)ꢀ. Addition of base to the ligand in methanol with subsequent
complexation of copper(II) ions resulted in an accumulation of a
mixture of a green and blue precipitate. According to mass spec-
trometry, complexes 2 and 3 were present in this mixture. Upon
recrystallization of the crude powder, crystals of 3 were obtained.
When complex 2 was treated with two equivalents of base (or
water) in acetonitrile, no ligand rearrangement was observed
according to mass spectrometry. Diffusion of diethyl ether into a
solution of 2 in CH3CN and NEt3 did not form any crystalline mate-
rial. The use of base before complex formation prohibits isolation
of pure complex and leads to ligand rearrangement. Treatment of
2 with base to generate a copper(II) complex with (L2)ꢀ were
unsuccessful, despite HL2 being stable towards decomposition
and/or rearrangement of the arms when coordinated to the metal
ion.
Fig. 4. Representation of the X-ray structure of 1 with H atoms removed for clarity.
Reprinted with permission from Ref. [22].
3.2. X-ray crystal structures
The previously published structure of 1 (Fig. 4) revealed that the
copper ion was in a square pyramidal geometry with an N2O2
donor atom set from the (L1)2ꢀ ligand in the equatorial plane and
an axial O atom from a coordinated water molecule [22]. The
pyridyl group does not coordinate in 1, most likely due to geomet-
ric constraints that the bis(imine) ligand imposes on the complex.
Compared to similar ligands with amine functional groups [36,37],
the more planar geometry imposed to the ligand due to the imine
C–N double bonds in 1 prevent the pyridyl N atom from folding
into position where it can coordinate to the copper. This same
constraint is seen in complexes 2 and 3 (vide infra).
According to the X-ray crystal structure of 2 (Fig. 5), each cop-
per(II) ion in dimeric 2 is coordinated in a square pyramidal geom-
etry with a
set from the ligand occupies the basal plane of the pyramid with
the 2-phenolato O atom from another complex coordinating in
the apical position. The resulting structure contains two copper
ions with bis( 2–O) bridging phenolate groups in a diamond core
fashion. The apical O atom bond donor distance to the copper ion
(Cu–O1#1 = 2.487 Å) is longer than the basal donors (ranging from
1.922–2.020 Å) due to the Jahn–Teller effect. The pyridyl ring from
the ligand backbone is non-coordinating and, surprisingly, proton-
ated at the N atom position. The overall +4 charge is balanced by
four perchlorate counterions (Figure S6), two of which are weakly
interacting with the copper(II) in the apical position trans to the
phenolate O atom of each monomeric unit (Cuꢂ ꢂ ꢂO = 2.763 Å),
s5 parameter [38] of 0.024 (Fig. 5). The N3O donor atom
l
Fig. 5. Representation of the X-ray structure of the cationic portion of 2 with all H
atoms except for the protonated pyridyl NH protons removed for clarity. Perchlo-
rate anions and methanol solvent of crystallization molecules are also removed for
clarity. Symmetry information used to generate equivalent positions: (ꢀx + ½,
ꢀy + ½, ꢀz).
l
while the other two are non-coordinating. Hydrogen bonding be-
tween the pyridinium hydrogen and the non-coordinating metha-
nol oxygen atom (N14–H14ꢂ ꢂ ꢂO1S), as well as between the
perchlorate ion oxygen and the methanol hydrogen atom (O1S–
H1Sꢂ ꢂ ꢂO1A#2), stabilizes the structure in the solid state (Figure S6,
Table 3).
A complex similar to 2 was reported by Lee and coworkers with
the ligand N-(salicylidene)-N0-(2-pyridylaldene)propanediamine
[17]. This ligand differs from HL2 only in the absence of the methyl
and pyridyl groups on the propylene backbone. Complex 2 adopts
the same Cu2(l-phenolato)2 diamond core, and has the same N2O
donor atom set as the complex with N-(salicylidene)-N0-(2-pyridyl-
aldene)propanediamine [17]. Since HL2 and N-(salicylidene)-N0-(2-
pyridylaldene)propanediamine are effectively the same except for
the pyridine group, both ligands coordinate in the same fashion,
and the ligand–copper bond lengths and angles are very similar
[17].
The X-ray structure of 3 consists of a mononuclear copper(II)
complex of L3 and two perchlorate anions (Fig. 6). The coordination
geometry in 3 is axially elongated six-coordinate tetragonal. The L3
Fig. 3. Structures of the copper imine complexes discussed in this paper. The
synthesis and structure of copper complex 1, [Cu(L1)(CH3OH)], was reported
previously [22]. Complex 2, [Cu(HL2)]2(ClO4)4, is a dimer in the solid state but is
monomeric, e.g. [Cu(HL2)]2+, in solution. Complex 3, [Cu(L3)](ClO4)2, is a monomer
in solution.

420
A. Jozwiuk et al. / Inorganica Chimica Acta 394 (2013) 415–422
in the coordination environment around the copper ions between
the monomer and the dimer, we expect to see differences in g val-
ues and therefore two sets of axial EPR signals. To further support
our hypothesis we allowed solutions of 2 to equilibrate in a dry ice/
acetone bath for 2 h before freezing the sample in liquid nitrogen.
The corresponding EPR spectrum (Figure S7, red) only shows one
set of signals which would be expected to see for only one species,
which we believe is dimeric 2.
Frozen solutions of 2 in CH3CN only show one axial EPR signal.
Complex 2 has a much greater solubility in CH3CN than in CH3OH
and due to the solubilty difference it is reasonable to assume that 2
dissociates immediately in CH3CN. The slight shift in g for the
monomers is likely due to solvent coordination (CH3OH versus
CH3CN).
CH3CN solutions of 3 were found to contain monomeric species,
as ascertained by electrospray mass spectrometry (m/z = 505, cor-
responding to [Cu(L3)ClO4]+) and solution magnetic moment mea-
Fig. 6. Representation of the X-ray structure of the cationic portion of 3 with all H
atoms removed for clarity. Perchlorate anions are also removed for clarity.
ligand coordinates in a slightly distorted square planar (s4 = 0.154)
[39] manner while the perchlorate oxygens are weakly coordinating
in the axial positions (Cu1–O1A = 2.660 Å and Cu1–O1B = 2.573 Å).
As observed in other Cu(II) complexes with our bis(imine) family of
ligands, here the ligand backbone pyridine nitrogen atom is
non-coordinating. The geometrical constraints imposed by the
ligand prevent the pyridyl ring from getting close enough to coor-
dinate to the copper ion. As in 2, there is significant bond elonga-
tion between the copper(II) ion and the axial donor atoms versus
the donors in the basal plane.
A complex similar to 3 was reported by Ray and coworkers with
the ligand N,N0-bis(2-pyridylaldene)propane-1,3-diamine [16].
Similar to HL2 and its analog, L3 and N,N0-bis(2-pyridylaldene)pro-
pane-1,3-diamine coordinate via the same N4 donor atom set. Both
complexes show the same N4 coordination mode, with similar li-
gand–copper bond lengths and angles, due to the ligands sharing
same ligand backbone and the fact that the pyridyl group of L3 does
not coordinate the the copper atom.
surements (1.75
spectrum.
lB/Cu). Complex 3 likewise possesses an axial EPR
The redox behavior of 2 and 3 were studied by cyclic voltamme-
try and compared to the previously reported electrochemical
parameters for 1 [22] (Fig. 7). The cyclic voltammograms (CVs)
show reversible one-electron redox couples with E1/2 = –1099 mV
versus Fc/Fc+ and
D
E = 77 mV for 2, and E1/2 = ꢀ438 mV versus
Fc/Fc+ and
D
E = 64 mV for 3. The CV of 2 is complicated by a small
reduction feature immediately before the Epc peak that disappears
on successive scans (this unusual redox behavior is undergoing
further study). The CV in Fig. 7 is therefore the second scan, and
the full first-scan CV of 2 is shown in Figure S8. Additional redox
features in the CV are most likely due to some dimeric species.
The redox couples, which were measured in CH3CN, were assigned
to the Cu(II)/Cu(I) pair. In contrast, the previously synthesized
complex 1 exhibits its redox couple, which was measured in CH2-
H2L1, HL2 and L3 have a common coordination mode to cop-
per(II) ions. While the pyridine nitrogen atom of each ligand back-
bone is non-coordinating, the remaining N and O donors chelate in
the basal plane around the metal ion (see Fig. 3). The pyridyl ring
does not coordinate in 1–3 because of geometric constraints im-
posed by the rigid imine groups in H2L1, HL2 and L3. This stands
in contrast to copper complexes of the more flexible series of
bis(amine) ligands synthesized in our laboratory [36]. The geome-
try of 1 is similar to 2 but features a CH3OH oxygen donor atom
(not shown in Fig. 3) in the axial position rather than a bridging
phenolate. Therefore, 1 is found as a monomeric species. Complex
2 and 1 show bond elongation in the axial positions (2.34 Å for
[Cu(L1)(CH3OH)] and 2.49 Å for 2) compared to the basal donors,
averaging in metal–ligand atom bond distances from 1.92 to
2.02 Å. Although 3 was synthesized in CH3OH and crystallized from
CH3CN, no solvent molecule occupies the axial position. Instead,
perchlorate anions are found in weak association to the metal
ion in the axial positions (2.57 and 2.66 Å).
Cl2, at E1/2 = ꢀ1585 mV and
was measured and the E1/2 was slightly lower than it was in CH2Cl2,
E = 93 mV. The potential needed to re-
DE = 136 mV. The CV for 1 in CH3CN
with E1/2 = ꢀ1489 mV and
D
duce the copper(II) ion is most negative for the bis(phe-
nol)bis(imine), 1, followed by the hybrid bis(imine) 2, and finally
the most positive for the bis(pyridyl)bis(imine) 3, all differing by
about half of a volt. This trend can be explained by the electronic
nature of the ligands. H2L1, being the most electron donating as
the dianionic (L1)2ꢀ, stabilizes the copper ion in the higher oxida-
3.3. Spectroscopic and electrochemical characterization
Electrospray mass spectrometry suggests that CH3OH and CH3-
CN solutions of 2 dissociate mostly into monomeric species. A peak
at m/z = 420.1 corresponding to [Cu(L2)]+, and a peak at m/z = 452.1
corresponding to [Cu(L2)(CH3OH)]+ are indicative of monomeric
species. A peak with very low intensity at m/z = 941.1 correspond-
ing to [(Cu(L2))2ClO4]+ dimeric was also detected. This dissociative
behavior in solution is further supported by a solution magnetic
moment of 1.76
the spin-only value of 1.73
l
B/Cu. This magnetic moment, which is close to
B/Cu, suggests the absence of any
l
magnetic coupling and therefore supports the existence of a mono-
meric species in solution.
Frozen solutions of compound 2 in methanol show EPR spectra
with two sets of peaks, which originate from two typical axial EPR
signals (Figure S7, blue). Those two sets of signals most likely cor-
respond to monomeric and dimeric forms of 2. Due to differences
Fig. 7. Cyclic voltammograms (scan rate = 100 mV sꢀ1; 0.1 M TBAPF6 supporting
electrolyte) of 1.0 mM CH3CN solutions of 1 (blue), 2 (red) and 3 (green). (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)

A. Jozwiuk et al. / Inorganica Chimica Acta 394 (2013) 415–422
421
Table 4
Electrochemical data for complexes 1–3 and literature complexes containing similar ligand sets.a
Complex
E1/2 (V)
D
E (mV)
ipc/ipa
Solvent
References
1
1
ꢀ1.585
ꢀ1.489
ꢀ1.300
ꢀ1.099
ꢀ0.932
ꢀ0.438
ꢀ0.550
ꢀ0.526
136
93
325
77
280
64
0.99
1.19
CH2Cl2
CH3CN
DMF
CH3CN
DMF
CH3CN
DMF
CH3CN
[22]
This work
[40]
This work
[40]
This work
[18]
McMillin [Cu(N2O2)] complex
2
McMillin [Cu(N3O)] complex
3
Nakahara [Cu(N4)] complex
McMillin [Cu(N4)] complex
1.06
150
[40]
a
All potentials referenced to the Fc/Fc+ redox couple. CVs for solutions of 1–3 (1.0 mM) were recorded using a glassy carbon electrode with scan rates of 100 mV sꢀ1, and
with 0.1 M TBAPF6 supporting electrolyte.
tion state and disfavors reduction of the copper(II) ion. On the
other extreme, the neutral L3 ligand is the least electron donating
ligand, which results in favorable acceptance of an electron by
N atoms in the equatorial plane, having weakly coordinated per-
chlorate anions in the axial positions. The ligands in all three com-
plexes do not coordinate through the pyridine nitrogen of the
ligand backbone. The inability of the pyridyl group from the ligand
backbone to coordinate is likely due to steric and geometric con-
straints of the rigid imine skeletons of the ligands. The electro-
chemical properties of 1–3 were probed by cyclic voltammetry,
showing a correlation of the electronic properties of the ligands
to the redox potentials of the cupric/cuprous ion couple. The more
electron rich, dianionic (L1)2ꢀ ligand in 1 with its N2O2 donor atom
set highly favors the +2 oxidation state and has the most negative
redox potential. The neutral L3 ligand with its N4 donor atom set
has a redox potential that is more than a volt more positive, while
the mixed ligand HL2 with its N3O donor atom set is midway be-
tween the other two. The electrochemical trends observed for 1–
3 conform with the electrochemical properties of copper(II) com-
plexes with similar ligands [18,40].
the metal ion to form a cuprous species and a less negative E1/2
.
The same trend with similar absolute redox potentials for the
CuII/I couple was observed on related Cu(II) complexes by McMillin
and coworkers [40]. In their study, copper(II) complexes of the
Schiff bases N,N0-bis(salicylidene)propane-1,3-propanediamine
(N2O2 donor), N-(salicylidene)-N0-(2-pyridylaldene)-1,3-propane-
diamine (N3O donor) and N,N0-bis(2-pyridylaldene)-1,3-propane-
diamine (N4 donor) were prepared and the copper-centered
redox couple determined through cyclic voltammetry. For compar-
ison with our data (see Table 4) these potentials were corrected to
the ferrocene/ferrocenium reference (Fc = 0.312 V versus SSCE)
[41].
The difference in potential between our N2O2-donor complex, 1,
and the copper(II) complex with McMillin’s ligand [40] is 285 ꢀmV.
For the N3O set, despite the fact that our complex hꢀas a ClO4 as
the counterion while McMillin’s complex has a NO3 counterion,
the potentials are less than 100 mV different. Similarly, the redox
potentials for the N4 complexes are very similar. The N4 copper(II)
complex of N,N0-bis(2-pyridylaldene)-1,3-propanediamine was
synthesized and characterized by Nakahara as well [18]. The redox
potential for their complex was measured against SCE, and in order
to compare to our complex it was corrected to be versus ferrocene/
ferrocenium (Fc = 0.470 V versus SCE in DMF, [NBu4][ClO4]) [42].
Nakahara and coworker’s reported electrochemistry matches ni-
cely with McMillin’s, even though it was measured in a different
solvent. The relatively minor differences ranging from 100 to
285 mV between our complexes and ones in the literature may
be due to the differences in substitution of the propanediamine
backbone. In general, it is difficult to compare redox potentials di-
rectly due to errors in conversion to different references/electrodes
and systems, but the overall trends are in good agreement.
Acknowledgements
This work was supported by the National Science Foundation
(NSF CHE-0616941) and the University of Oklahoma. Additionally,
we thank the NSF for the purchase of a CCD-equipped X-ray dif-
fractometer (CHE-0130835).
Appendix A. Supplementary material
1H NMR spectra of ligands H2L1, HL2, L3 and L4 (Figs. S1–S5), X-
ray structure highlighting H-bonding and intermolecular interac-
tions in 2 (Fig. S6), EPR spectra of 2 in CH3CN and CH3OH
(Fig. S7), and cyclic voltammogram of 2 (Fig. S8). CCDC 864524
and 864525 contain the supplementary crystallographic data for
complexes 2 and 3, respectively. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2012.08.026.
4. Conclusions
In summary, two new copper(II) complexes, 2 and 3, were syn-
thesized using tetradentate Schiff base ligands HL2 and L3, respec-
tively. Complexes 2 and 3 were compared with our previously
synthesized copper(II) complex, 1, which contains related tetra-
dentate Schiff base ligand H2L1. Structurally, 1–3 are very similar,
with the ligand coordinating to the square pyramidal copper center
in the equatorial square plane. The ligand in complex 1 (H2L1),
which is deprotonated at the phenol group, coordinates through
the two imine N atoms and two phenolate O atoms. Complex 2 is
coordinated by one pyridyl and two imine N atoms, and one phe-
nolate O atom, also in the equatorial plane of the square pyramid.
While the axial ligand in 1 is a coordinated methanol molecule, a
bridging phenolate O atom sits in the axial position of 2, forming
a dimer. Complex 3 is coordinated by two pyridyl and two imine
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