Characterization of pyridinylimine and pyridinylmethylamine derivatives and their corresponding metal complexes

Article abstract of DOI:10.1016/j.ica.2011.08.056

Bidentate pyridinylimine (L1-a and L1-b) and pyridinylmethylamine (L2-a and L2-b) derivatives have recently received attention due to their potential use as chemical reagants that can target metal-associated amyloid-β (Aβ) species and modulate metal-induced Aβ aggregation and neurotoxicity in vitro and in living cells. Herein, we report the characterization of these bidentate ligands and their corresponding metal complexes by X-ray crystallography and spectroscopic methods, including UV-Vis spectroscopy (UV-Vis), nuclear magnetic resonance spectroscopy (NMR), and Fourier transform infrared spectroscopy (FT-IR). Our studies presented how the different structural moieties of ligand frameworks (L1-a, L1-b, L2-a, and L2-b), in addition to effects of metal binding, contribute to variations in structure and chemical environments of the ligands and their corresponding metal complexes.

Full text of DOI:10.1016/j.ica.2011.08.056

Inorganica Chimica Acta 380 (2012) 261–268
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Inorganica Chimica Acta
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Characterization of pyridinylimine and pyridinylmethylamine derivatives
and their corresponding metal complexes
Joseph J. Braymer a, Nathan M. Merrill a, Mi Hee Lim a,b,
⇑
a Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA
b Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 12 September 2011
Bidentate pyridinylimine (L1-a and L1-b) and pyridinylmethylamine (L2-a and L2-b) derivatives have
recently received attention due to their potential use as chemical reagants that can target metal-associ-
ated amyloid-b (Ab) species and modulate metal-induced Ab aggregation and neurotoxicity in vitro and in
living cells. Herein, we report the characterization of these bidentate ligands and their corresponding
metal complexes by X-ray crystallography and spectroscopic methods, including UV–Vis spectroscopy
(UV–Vis), nuclear magnetic resonance spectroscopy (NMR), and Fourier transform infrared spectroscopy
(FT-IR). Our studies presented how the different structural moieties of ligand frameworks (L1-a, L1-b, L2-
a, and L2-b), in addition to effects of metal binding, contribute to variations in structure and chemical
environments of the ligands and their corresponding metal complexes.
Young Investigator Award Special Issue
Keywords:
Bioinorganic chemistry
Metal complexes of pyridinylimine and
pyridinylmethylamine derivatives
Spectroscopic studies
Ó 2011 Elsevier B.V. All rights reserved.
X-ray crystal structure determination
1. Introduction
elucidate the roles of these potentially toxic metal-peptide species
and modulate metal-Ab interaction [1–3,6,17–25]. With rational
The development of metal chelators for the attenuation of
metal-involved neurodegeneration in Alzheimer’s disease (AD)
and other neurodegenerative diseases has been of interest [1–7].
It is proposed that an imbalance in metal ion homeostasis contrib-
utes to misregulation of the normal functioning of the central ner-
vous system in diseased brains [1,3,7–9]. In particular, metals such
as Fe, Cu, and Zn have been observed in elevated concentrations
and colocalized with amyloid-b (Ab) deposits in the AD brain, sug-
gesting a simultaneous neurotoxic assault by aberrantly controlled
peptides and metal ions through aggregation processes and pro-
duction of reactive oxygen species (ROS) [1,7,8,10–14]. Compounds
that can capture and/or redistribute metal ions, such as clioquinol
and PBT2, have demonstrated utility towards alleviating metal-in-
duced cell stresses in vitro and in vivo [1–3,6,7,15,16]. The metal
ion chelation approach, however, has yet to answer the mechanis-
tic role of metal-associated Ab species (metal-Ab) in correlation
with neurotoxicity and the progression of AD. Bifunctional small
molecules that specifically target metal ions associated with Ab
peptides could be utilized as chemical tools in order to help
structured-based design approaches, better selectivity for and
interaction with metal-Ab species may provide chemical tools
more effective for unraveling AD pathogenesis and may potentially
offer valuable therapeutic options.
We have recently reported the design of bifunctional small mol-
ecules that are capable of targeting and modulating metal ions
surrounded by Ab [1,2,6,7,17,20–23]. Through the incorporation
of a metal binding site directly into a structural framework known
to interact with Ab aggregates (i.e., p-I-stilbene, Fig. 1), we have
constructed a family of stilbene derived compounds (pyridinyli-
mine derivatives (L1-a and L1-b; Fig. 1) as well as pyridinylmeth-
ylamine derivatives (L2-a and L2-b; Fig. 1)) that exhibited an
ability to inhibit the formation of metal-induced Ab aggregates
and disaggregate pre-formed metal-Ab aggregates, and regulate
neurotoxicity including ROS generation in vitro [6,7,17,20,21,23].
From the studies employing L1-a, L1-b, L2-a, and L2-b, an impor-
tant structure-interaction-reactivity relationship has been identi-
fied. In these molecules, the presence of
a dimethylamino
functionality improved regulation, compared to its absence, of me-
tal-Ab interaction and metal-triggered Ab aggregation [21,23]. The
dimethylamino group has been suggested to be important for bet-
ter interaction with Ab aggregates [26–28]. Here, we report the
observation of how this moiety and other structural modifications
(i.e., imine versus amine) could affect chemical properties of the li-
gands as well as their corresponding metal (Cu(II) or Zn(II)) com-
plexes (Fig. 1). Through X-ray crystal structure determination
and spectroscopic investigations (UV–Vis, NMR, and FT-IR), the
Abbreviations: AD, Alzheimer’s disease; ROS, reactive oxygen species; Metal-Ab,
metal-associated Ab species; p-I-stilbene, (E)-4-(4-iodostyryl)-N,N-dimethylaniline;
L1-a, N-(pyridin-2-ylmethylene)aniline; L1-b, N1,N1-dimethyl-N4-(pyridin-2-
ylmethylene)benzene-1,4-diamine; L2-a, N-(pyridin-2-ylmethyl)aniline; L2-b,
N1,N1-dimethyl-N4-(pyridin-2-ylmethyl)benzene-1,4-diamine.
⇑
Corresponding author at: Life Sciences Institute, University of Michigan, Ann
Arbor, MI 48109, USA.
E-mail address: mhlim@umich.edu (M.H. Lim).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.08.056

262
J.J. Braymer et al. / Inorganica Chimica Acta 380 (2012) 261–268
Fig. 1. Chemical structures of p-I-Stilbene, (a) ligands and (b) their corresponding metal complexes. The general labeling of the pyridyl and phenyl protons for the ligands is
shown in the imine framework. Abbreviations: p-l-stilbene = (E)-4-(4-iodostyryl)-N,N-dimethylaniline; L1-a = N-(pyridin-2-ylmethylene)aniline; L1-b = N1,N1-dimethyl-N4-
pyridin-2-ylmethylene)benzene-1,4-diamine; L2-a = N-(pyridin-2-ylmethyl)aniline; L2-b = N1,N1-dimethyl-N4-pyridin-2-ylmethy)benzene-1,4-diamine.
structural and electronic variations of these four compounds and
their corresponding metal complexes were observed.
conducted with the compound in the presence of CuCl2 ([CuCl2]/
[L2-a] = 1:2). At least 30 spectra were acquired from pH ranges
of 2.0–7.2. Acidity and stability constants were calculated using
the HypSpec program (Protonic Software, UK) [30]. Speciation dia-
grams of compounds and their corresponding metal complexes
were modeled using the HySS2009 program (Protonic Software)
[31].
2. Experimental
2.1. Materials and methods
All reagents were purchased from commercial suppliers and
used as received unless stated otherwise. The compounds L1-b,
L2-a, and L2-b were synthesized by previously reported methods
and characterization agreed with prior literature [20,21,29]. The
preparation and characterization of the Zn(II) complexes [Zn(L1-
a)Cl2] and [Zn(L1-b)Cl2] have been recently reported [23]. For
NMR studies, [Zn(L2-a)Cl2] was prepared in situ by the addition
of 1.0 equivalent of ZnCl2 to a 4.0 mM acetonitrile-d3 (CD3CN) solu-
tion of L2-a followed by 1 h incubation. UV–Vis studies were con-
ducted on an HP 8453 UV–Vis spectrophotometer. NMR and FT-IR
spectra of the ligands and their corresponding metal complexes
were recorded on a 400 MHz Varian NMR spectrometer and on a
Perkin-Elmer Spectrum BX FT-IR instrument, respectively. Mass
spectrometric measurements were performed using a Micromass
LCT Electrospray Time-of-Flight mass spectrometer.
2.3. X-ray crystallographic studies
Single crystals suitable for data collection were mounted on a
Bruker SMART APEX CCD-based X-ray diffractometer equipped
with a low temperature device and fine focus Mo-target X-ray tube
(k = 0.71073 Å) operated at 1500 W power (50 kV, 30 mA). The X-
ray intensities were measured at 85(2) K. Empirical absorption cor-
rections were calculated with the SADABS or TWINABS program
[32]. Structures were solved and refined with the SAINTPLUS and
SHELXTL software packages [33,34]. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were assigned in idealized
positions and each was given a thermal parameter equivalent to
1.2 times the thermal parameter of the atom to which it was at-
tached. The structure solution was checked for higher symmetry
with PLATON [35].
2.2. UV–Vis studies
2.3.1. [Cu(L2-a)Cl2]
Spectrophotometric pH titrations were conducted to determine
acidity constants (Ka) of L2-a and stability constants (logb) of
Cu(II)ꢀL2-a following the previously established methods
[21,22]. The pKa value (pKa = ꢀlog[Ka]) was measured at room tem-
perature in a solution of 100 mM NaCl and 10 mM NaOH starting
from pH 12.5. At least 30 spectra were acquired from pH 2–10 after
adding small aliquots of HCl to the solution containing compound
Green needle crystals were grown by vapor diffusion of diethyl
ether (Et2O) into a methanol (CH3OH) solution of L2-a (5.0 mg,
27
night. The crystals were isolated, washed with Et2O three times,
and dried in vacuo (6.2 mg, 19
mol, 72%). FTIR (KBr, cmꢀ1):
lmol) and CuCl2 (3.7 mg, 27 lmol) at room temperature over-
l
3445 (w, br), 3189 (s), 3113 (vw, sh), 3073 (w), 3043 (vw, sh),
2956 (vw), 2921 (w), 2893 (vw, sh), 2832 (vw), 1612 (m), 1600
(w, sh), 1573 (w), 1490 (s), 1482 (s), 1447 (m), 1419 (s), 1384
([L2-a] = 500 lM). Under similar conditions, titrations were

J.J. Braymer et al. / Inorganica Chimica Acta 380 (2012) 261–268
263
Table 1
(w), 1365 (w), 1312 (vw), 1283 (m), 1264 (w), 1222 (vw), 1202 (m),
Summary of X-ray crystallographic data.
1180 (w), 1156 (m), 1107 (w), 1090 (w), 1061 (w), 1049 (w, sh),
1028 (m), 1016 (m), 996 (m), 984 (w), 967 (vw), 956 (vw), 891
(vw), 820 (w), 787 (s), 765 (vs), 719 (w), 694 (s), 652 (m), 614
(vw), 572 (w), 525 (m), 464 (w), 418 (w). ESI(+)MS (m/z): [MꢀCl]+
Calcd for C12H12ClCuN2, 282.0, found 282.0. Anal. Calc. for
[Cu(L2-a)Cl2]
12H12Cl2N2Cu
318.69
P21/c
[Zn(L2-b)Cl2]
Formula
Formula weight
Space group
a (Å)
b (Å)
c (Å)
C
C14H17Cl2N3Zn
363.60
P21/c
10.624(3)
16.242(4)
7.5734(19)
–
18.0852(11)
7.4481(5)
11.6044(7)
–
C
12H12Cl2CuN2: C, 45.23; H, 3.80; N, 8.79. Found: C, 45.27; H,
3.74; N, 8.74%. UV–Vis [CH3CN; k nm (
(3.3), 400 (1.8).
e
ꢁ 103, Mꢀ1 cmꢀ1)]: 310
a
(°)
b (°)
104.961(4)
–
1262.5(6)
4
1.677
85(2)
104.783(1)
–
1511.38(16)
4
1.598
85(2)
c
(°)
2.3.2. [Zn(L2-b)Cl2]
V (Å3)
Pink crystals were grown by vapor diffusion of Et2O into a
MeOH solution of L2-b (5.0 mg, 22 mol) and ZnCl2 (3.0 mg,
22 mol) at room temperature overnight. The crystals were iso-
lated, washed with Et2O three times, and dried in vacuo (6.3 mg,
17
mol, 79%). FTIR (KBr, cmꢀ1): 3440 (w, br), 3224 (s), 3087 (w),
Z
qcalc (gcmꢀ3
T (K)
)
l
l
l
(Mo K
a
) (mmꢀ1
)
2.129
1.971
h Limits (°)
2.35–27.31
52832
2798
158
1.055
0.0280
0.0683
0.385, ꢀ0.271
2.33–29.61
72714
4252
187
1.076
0.0245
0.0675
0.990, ꢀ0.416
l
Total number of data
Number of unique data
Number of parameters
Goodness-of-fit (GOF)a
Rb
3066 (w), 3038 (vw, sh), 2996 (vw), 2891 (m), 2860 (vw, sh),
2818 (w), 1866 (w), 1760 (vw), 1737 (vw), 1680 (vw), 1627 (m,
sh), 1612 (s), 1572 (m), 1533 (vs), 1484 (s), 1450 (s), 1442
(s), 1403 (m), 1369 (s), 1290 (m), 1253 (w, sh), 1244 (m), 1217
(s), 1191 (s), 1175 (w, sh), 1157 (w), 1124 (w), 1109 (m),
1091 (s), 1059 (s), 1030 (s), 1008 (w, sh), 995 (s), 976 (w), 949
(m), 926 (vw), 898 (vw), 863 (w), 807 (s), 776 (s), 729 (w), 710
(vw), 651 (w), 615 (vw), 539 (m), 526 (w, sh), 510 (vw, sh), 480
(w), 454 (vw), 442 (vw), 426 (w), 417 (vw, sh). 1H NMR
(400 MHz, CD3CN): d = 2.88 (s, 6H, N(CH3)2), 4.51 (s, 2H, CH2),
5.64 (s, 1H, NH), 6.71 (d, 2H, Ph-H), 7.04 (d, J = 12 Hz, 2H, Ph-H),
7.62 (d, J = 4 Hz, 1H, Py-H), 7.63 (t, J = 4 Hz, 1H, Py-H), 8.11 (t,
J = 8, 4 Hz, 1H, Py-H), 8.58 (d, J = 4 Hz, 1H, Py-H). ESI(+)MS (m/z):
[MꢀCl]+ Calcd for C14H15ClN3Zn, 326.0. Found 326.0. Anal. Calc.
for C14H17Cl2N3Zn: C, 46.25; H, 4.71; N, 11.56. Found: C, 46.14;
c
wR2
Maximum, minimum peaks (eÅꢀ3
)
ꢄ
ꢂ
ꢃ
ꢅ
1=2
ꢀ
ꢁ
2
GOFðgoodness of fit based on F2Þ ¼
of reflections, n = number of parameters refined).
R
w
F20 ꢀ F2c
=ðm ꢀ nÞ
(m = number
a
b
R =
R
||Fo| ꢀ |Fc||/
R|Fo|.
ꢄ
ꢂ
ꢃ
ꢂ
ꢃꢅ
1=2
ꢀ
ꢁ
ꢀ
ꢁ
2
2
wR2
¼
R
w
F20 ꢀ F2c
=R
w
F02
.
c
Table 2
Selected bond lengths (A) and angles (°).a
H, 4.62; N, 11.41%. UV–Vis [CH3CN; k nm (
310 (2.9), 473 (0.3).
e
ꢁ 103, Mꢀ1 cmꢀ1)]:
[Cu(L2-a)Cl2]
[Zn(L2-b)Cl2]
Zn1/Cu1–N1
Zn1/Cu1–N2
Zn1/Cu1–CI1
Zn1/Cu1–CI2
N1–Zn1/Cu1–N2
N1–Zn1/Cu1–CI1
N1–Zn1/Cu1–CI2
N2–Zn1/Cu1–CI1
N2–Zn1/Cu1–CI2
CI1–Zn1/Cu1–CI2
2.0052(18)
2.0744(18)
2.2701(7)
2.2442(7)
82.11(7)
2.0451(10)
2.0922(10)
2.2366(3)
2.2070(3)
81.79(4)
108.34(3)
112.21(3)
116.25(3)
114.30(3)
117.979(14)
3. Results and discussion
94.77(6)
3.1. Preparation of the ligands and their corresponding metal
complexes
168.44(5)
176.28(6)
89.53(6)
The small molecules (L1-b, L2-a, and L2-b, Fig. 1a) except for
the commercially available L1-a were synthesized based on previ-
ous procedures [20,21,29]. X-ray crystal quality metal complexes
of L2-a and L2-b ([Cu(L2-a)Cl2] and [Zn(L2-b)Cl2]; Fig. 1b) were
isolated by slow vapor diffusion with Et2O into a MeOH solution
of 1:1 ligand to metal chloride salt (CuCl2 or ZnCl2) and character-
ized by X-ray crystallography (vide infra). Lastly, to further pursue
spectroscopic investigations of [Zn(L1-a)Cl2] and [Zn(L1-b)Cl2]
(Fig. 1b), these crystalline complexes were prepared following
the previously reported methods [23].
93.27(3)
a
Numbers in parentheses are estimated standard deviations in the last signifi-
cant figures. Atoms are labeled as indicated in Fig. 2.
as compared to L2-a and L2-b may cause the deprotonation of
the secondary amino group upon metal chelation.
For [Cu(L2-a)Cl2], nearly square planar geometry was observed
showing a dihedral angle (H) of 7.4°, measured between the planes
of the three- and five-membered rings on the metal center
(Fig. 2a). Bond lengths between Cu(II) and the N donor atoms of
the pyridyl and secondary amino group were 2.0744(18) Å and
2.0052(18) Å (Table 2), respectively, (N1) and (N2), were compara-
ble to similar tridentate ligands containing the aryl substituted 2-
(methylamino)pyridine moiety bound to a CuCl2 center (C–N,
1.984–2.334) [38–40]. Relating the molecular structure of
[Cu(L2-a)Cl2] to that of [Zn(L2-b)Cl2], a similar twisting of the
amine-based ligand framework with respect to the aromatic rings
was observed, with the aromatic planes intersecting at 80° and 73°
for the two complexes, respectively (Fig. 2).
3.2. Description of the crystal structures
Crystallographic data of [Cu(L2-a)Cl2] and [Zn(L2-b)Cl2] and se-
lected bond lengths and angles are summarized in Tables 1 and 2,
respectively. As depicted in Fig. 2, the structures presented that 1:1
metal to ligand complexes were formed (both metal complexes
crystallized in the monoclinic space group, P21/c, Z = 4). In both
structures, the secondary amino group of L2-a an L2-b remained
protonated when CuCl2 or ZnCl2 was bound to the ligand, which
was also found in the ruthenium(II) complex [Ru(
g
6-C6H4MeCH-
The structure of [Zn(L2-b)Cl2] exhibited
a
bite angle
Me2)(L2-a)Cl] [36]. Different from this observation, some Zn(II)
complexes containing napthalimide-based fluorescent chemo sen-
sors composed of the pyridinylmethylamine moiety showed the
deprotonation of the secondary amine group [37]. The relatively
stronger electron withdrawing ability of the napthalimide group
(81.79(4)°) of the two N donor atoms (N1 and N2) to the Zn(II)
center and a distorted tetrahedral geometry on the metal center
(Fig. 2b and Table 2). The observed Zn–N and Zn–Cl bond
lengths for [Zn(L2-b)Cl2] were comparable with the reported
values from the structures of Zn(II) complexes of similar ligand

264
J.J. Braymer et al. / Inorganica Chimica Acta 380 (2012) 261–268
Fig. 2. ORTEP diagrams of metal complexes, (a) [Cu(L2-a)Cl2] and (b) [Zn(L2-b)Cl2], showing 50% probability thermal ellipsoids. Summary of X-ray data are shown in Table 1.
frameworks in the literature [41]. Comparing to the previously
reported Zn(II) complex [Zn(L1-b)Cl2] [23], bond lengths of
[Zn(L2-b)Cl2] do not vary significantly. On the other hand, bond
angles showed slightly more variation for [Zn(L2-b)Cl2] as
compared to the imine complexes both with and without the
dimethylamino group (i.e., [Zn(L1-a)Cl2], [Zn(L2-b)Cl2], and [Zn
(L1-b)Cl2]; N2–Zn1–Cl2, 117.10(6)°, 114.30(3)°, and 120.03(6)°,
respectively) [23]. These results suggest subtle changes in Zn–
N and Zn–Cl bond lengths and angles depending on the C–N
hybridization and presence of the dimethylamino group. Overall,
the X-ray crystal structures of the metal complexes demonstrate
metal chelation by our compounds (L2-a and L2-b) displaying
different ligand conformations of the metal complexes associated
with C–N bond order.
considered in the calculations) [43,44]. For L2-a, a first species con-
version was shown with an isosbestic point at 295 nm correspond-
ing to the equilibrium Cu þ L ¢ CuL (logb1 = 5.857(2), Fig. 3b). A
second transformation with four isosbestic points at 279, 316,
337, and 362 nm was assigned to Cu þ 2L ¢ CuL2 (logb2 =
10.151(5)). From these stability constants, the distribution of metal
species as a function of pH was plotted and indicated that at phys-
iological pH, both CuL and CuL2 species existed (ca. 35:65; Fig. 3b,
right). According to the solution speciation diagram, free Cu(II)
remained in solution up to pH 6.0. Using the solution speciation re-
sults, concentrations of unbound Cu(II) at a specific pH can be cal-
culated (pCu = ꢀlog[Cu], pCu = 5.86 and 5.87 at pH 6.6 and 7.0,
respectively). The pCu values reported here for L2-a and the previ-
ously reported numbers for L2-b reflect approximate dissociation
constants (Kd) of the pyridinylmethylamine ligands (ranging from
high picomolar to low micromolar) [21].
3.3. UV–Vis studies
Previous UV–Vis absorption studies indicated that our com-
pounds can bind both Cu(II) and Zn(II) in solution and in particular,
L2-b was shown to be relatively selective for Cu(II) over other diva-
lent metal ions [20,21,23]. To understand and compare the solution
speciation of L2-a over L2-b in the absence and presence of a metal
chloride salt, UV–Vis pH-variable titration experiments were per-
formed following the previously reported methods [21,22]. First,
the acid dissociation constant (Ka) of L2-a was determined by
titrating the compound with small aliquots of HCl (room tempera-
ture, I = 0.10 M NaCl). Upon lowering the pH, spectral changes
were observed with an isosbestic point at 314 nm indicating the
formation of a protonated species (Fig. 3a, left). A pKa value of
5.037(2) was determined, which was assigned to protonation of
the pyridyl N atom based on comparison with similar nitrogen-
containing molecules in the literature [42]. Using the titration data,
a solution speciation diagram was generated showing that at pH
7.4, the neutral form of L2-a was predominant in aqueous solution
(Fig. 3a, ca. 100%, right).
3.4. NMR investigations
In order to elucidate differences in structural and electronic
environments of the four ligands (Fig. 1), analysis of proton reso-
nances by 1H NMR spectroscopy was executed. For all of the li-
gands (L1-a, L1-b, L2-a, and L2-b), pyridyl protons were
downfield with respect to phenyl protons, which indicated less
shielding of the CH bonds in the heterocycle. Changes in chemical
shifts of aromatic protons due to the imine versus amine function-
ality were identified (Fig. 4). With reduction of the Schiff base to
the secondary amine, upfield shifting of aromatic 1H signals was
observed, which is expected due to increased shielding from the
addition of electron density to the framework. The proximity of
Hd to the C–N bond (Fig. 1) also depended on whether a single or
double bond was present with
the L1 and L2 series, respectively (Fig. 4). Further differentiating
the imine and amine functionalities was the upfield shifting of
the phenyl protons (Dd (He, L1-a versus L2-a) = 0.69 ppm; Dd
Dd shifts of 0.83 and 0.78 ppm for
Secondly, a similar spectrophotometric titration was carried out
in the presence of CuCl2 to identify metal species that exist in the
same aqueous condition. Due to no significant change in the optical
spectra, the solution speciation investigation of L2-a with ZnCl2
was not conducted. The incubation of compound with CuCl2 for
(He, L1-b versus L2-b) = 0.68 ppm), a result of the electron donat-
ing nature of the secondary amine.
In addition to the C–N functionality, the presence of the dimeth-
ylamino group affected the proton chemical shifts of the ligands.
Comparing L1-a to L1-b or L2-a to L2-b (without and with the
dimethylamino group), an increase in shielding throughout the
aromatic rings was observed (Fig. 4). Based on the addition of this
electron-donating moiety, the greatest shift of the Hf protons
10 min at ca. pH 7.2 ([CuCl2]/[L] = 1:2, [L2-a] = 500 lM, room tem-
perature, I = 0.10 M NaCl), resulted in an absorption band at
388 nm (Fig. 3b, left). With the addition of small aliquots of HCl,
noticeable shifts in the spectra were exhibited which allowed for
the determination of stability constants of metal complexes (logb,
pKa values of the ligand and hydrolysis of free metal ions were
(Fig. 1) to lower frequencies with
Dd of 0.66 and 0.53 ppm for
L1-a versus L1-b and L2-a versus L2-b, respectively, was shown.
The presence of the tertiary amino group did not significantly

J.J. Braymer et al. / Inorganica Chimica Acta 380 (2012) 261–268
265
Fig. 3. UV–Vis variable-pH titrations of L2-a and its corresponding copper complexes, (a) UV–Vis spectra (left) and speciation diagram (right) from pH 10 to 2 (500 lM).
⁄Indicates isosbestic point at 314 nm. (b) Spectral traces upon the addition of small aliquots of HCI to a solution of CuCl2 and L2-a (250
lM and 500 lM, respectively, left; pH
7.2 to pH 2.0) and the calculated speciation diagram. ⁄Show isosbestic points for two different species conversions, one at 365 nm (CuL2 to CuL; L = L2-a) and another at
295 nm (CuL to free Cu and L). Titrations were carried out at room temperature and I = 0.10 M NaCI. Charges are omitted for clarity.
cause changes of chemical shifts of the methine or methylene pro-
tons, although a more observable influence on the secondary
amine proton was indicated with an upfield shift of 0.39 ppm
(Fig. 4c and d). Overall, the environment of the protons throughout
the scaffold showed sensitivity to both C–N hybridization as well
as the presence of the dimethylamino group.
corresponding Zn(II) complexes indicated that the electronic
environment of the compounds was affected by Zn(II) coordina-
tion in solution and that N–H bonds remained intact, suggesting
availability for hydrogen bonding in the Zn(II) complexes of the
pyridinylmethylamine derivatives.
Further verification of Zn(II) binding to the four ligands in
solution was confirmed by 1H NMR analysis of the isolated Zn(II)
complexes, [Zn(L1-a)Cl2], [Zn(L1-b)Cl2], [Zn(L2-a)Cl2] (generated
in situ), and [Zn(L2-b)Cl2], by comparison with the free ligands
(Fig. 4). Generally, downfield shifting for all protons was ob-
served upon Zn(II) binding to the ligands, indicating that Zn(II)
withdraws electron density from the ligand framework. For Zn(II)
binding, the chemical shifts of Hb and Hc for the pyridyl rings
3.5. IR studies
To investigate how structural alterations can affect individual
bond vibrations, FT-IR spectroscopy of the free ligands was first
carried out (Fig. 5a). Reduction from the imine to amine affected
the C–N, C@N, and C@C stretching frequencies between 1100 and
1700 cmꢀ1 as well as the N–H bending mode
q
(NꢀH) in this re-
gion. The Schiff base stretching bands at 1580 cmꢀ1 (L1-a) and
1572 cmꢀ1 (L1-b) were absent in both L2-a and L2-b, as expected
with reduction of the C@N bond. Formation of the secondary amine
(Fig. 1) were moved downfield the most (Dd > 0.40 ppm) while
proton density at Ha and Hd was mainly retained, consistent with
previous studies [45]. Furthermore, the signals at 5.99 and
5.64 ppm for the secondary amine proton of [Zn(L2-a)Cl2] and
[Zn(L2-b)Cl2], respectively, indicated that the ligands were not
deprotonated upon Zn(II) binding (Fig. 4b and d), in agreement
with the characterization in the solid state (X-ray structural
determination, vida supra and FT-IR, infra vide) and in previous
literature for metal polyamine complexes [46]. The downfield
shifting along with the sharpening of the N–H signal due to an
increased exchange between the proton and solvent suggested
that Zn(II) was coordinated to the secondary amine of both li-
gands [47]. Therefore, 1H NMR studies of the ligands and their
was evident due to asymmetric and symmetric
m
(N–H) stretching
bands located between 3300 to 3400 cmꢀ1 as well as the
q
(N–H)
bending mode near 1600 cmꢀ1. The appearance of the C–N stretch-
ing frequency at 1570 and 1528 cmꢀ1 for L1-b and L2-b, respec-
tively, was indicative of the sp2 character of the dimethylamino
moiety, which showed an effect mainly in the fingerprint region
of the spectra (Fig. 5a). In addition, the broad m(N–H) stretching
peak of 3299 cmꢀ1 for L2-a was found to resolve into two peaks
and shift to 3369 and 3394 cmꢀ1 for L2-b, suggesting a stronger
N–H bond in the presence of the dimethylamino group in the
structure, consistent with NMR solution studies.

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J.J. Braymer et al. / Inorganica Chimica Acta 380 (2012) 261–268
Fig. 4. 1H NMR spectra of (a) L1-a and [Zn(L1-a)Cl2], (b) L1-b and [Zn(L1-b)Cl2], (c) L2-a and [Zn(L2-a)Cl2] (generated in situ by reacting L2-a with 1 equivalent of ZnCl2) and
(d) L2-b and [Zn(L2-b)Cl2] in CD3CN at room temperature.
Additional confirmation of metal binding in the solid state was
obtained by FT-IR of the isolated metal complexes ([Zn(L1-a)Cl2],
[Zn(L1-b)Cl2], [Cu(L2-a)Cl2], and [Zn(L2-b)Cl2], Fig. 1) and effects
on bond frequency patterns were compared with the free ligands
(Fig. 5b). Shifting of bond frequencies throughout the spectrum,
including changes in the region 1100 to 1700 cmꢀ1, suggested me-
tal binding to afford Zn(II)/Cu(II) complexes (Fig. 5). In the metal
bound form, the imine (C@N) stretching frequency shifted to high-
er wavenumbers, from 1580 (L1-a) to 1594 cmꢀ1 and 1572 (L1-b)
to 1598 cmꢀ1 for [Zn(L1-a)Cl2] and [Zn(L1-b)Cl2], respectively. A
similar shift was also observed for previously reported Zn(II)–
imine complexes and was attributed to the reorganization of the
s character of the nitrogen lone pair [48,49]. In the IR spectrum
of [Cu(L2-a)Cl2], the C–N stretching frequency seen from that of
the free ligand was not observed at 1529 cmꢀ1, suggesting that me-
tal binding changed the character of the C–N bond of the dimeth-
experiment (Fig. 4). The appearance of mainly one peak for m(N–
H) vibrations suggested the preference of one vibrational mode
due to metal binding. In summary, changes in the bond frequencies
indicative of metal chelation presented an overall effect on the
ligand frameworks in the metal bound form.
4. Conclusion
Four small molecules and their corresponding metal complexes
have been characterized by X-ray crystallography and/or spectro-
scopic methods in solution or the solid state. Depending on ligand
functionality (i.e., imine versus amine and with or without the
dimethylamino group), different structural and electronic proper-
ties of the ligands were exhibited. The binding of Cu(II) and/or
Zn(II) was also found to have an influence on these structural moi-
eties and over the molecular framework. The observation that the
neutral pyridinylmethylamine derivatives existed predominantly
at physiological pH and were not deprotonated upon metal coordi-
nation suggest that the compounds/complexes may be suitable for
hydrogen bonding. Changes in structure shown with these modi-
fied stilbene-derived compounds indicate the value of understand-
ing chemical properties of small molecules and their
corresponding metal complexes that may contribute to elucidating
a structure-interaction-reactivity relationship of small molecules
ylamino group. The clear appearance of both m(N–H) and q(N–H)
bands for both [Cu(L2-a)Cl2] and [Zn(L2-b)Cl2] excluded the
deprotonation process in the metal complexes as expected from
the crystal structural and 1H NMR studies. For both complexes,
the
m
(N–H) bands of the free ligands (3299/3394 cmꢀ1 and
3369 cmꢀ1 for L2-a and L2-b, respectively) were found to shift to
a lower wavenumber (3189 cmꢀ1 and 3223 cmꢀ1 for [Cu(L2-
a)Cl2] and [Zn(L2-b)Cl2], respectively) corresponding to weakening
of the N–H bond (Fig. 5), in agreement with the 1H NMR

J.J. Braymer et al. / Inorganica Chimica Acta 380 (2012) 261–268
267
Fig. 5. FT-IR spectra of (a) L1-a, L1-b, L2-a, and L2-b; (b) [Zn(L1-a)Cl2], [Zn(L1-b)Cl2], [Cu(L2-a)Cl2], and [Zn(L2-b)Cl2] in KBr.
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Acknowledgments
This work was supported by start-up funding from the University
of Michigan, the Alzheimer’s Art Quilt Initiative (AAQI), as well as the
Alzheimer’s Association (NIRG-10-172326) (to M.H.L.). We
acknowledge funding from NSF Grant CHE-0840456 for X-ray
instrumentation. We thank Dr. Jung-Suk Choi for the speciation
measurementofL2-awithandwithoutCuCl2 aswellasDr. Xiaoming
He and Alaina DeToma for fruitful discussion.
[22] J.-S. Choi, J.J. Braymer, S.K. Park, S. Mustafa, J. Chae, M.H. Lim, Metallomics 3
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