Steric effect on construction of extended architectures of Ni(II) complexes directed by intermolecular C-H···F and C-H···O interactions

Article abstract of DOI:10.1007/s10947-010-0140-x

Five new Ni(II) complexes with pyridine carboxamide ligands have been synthesized and the crystal structures of three of the complexes were determined. Strong distortion effects of 6-methyl substitution were observed in the complexes with 6-methyl-substituted pyridyl bpb ligands. The C-H···F and C-H···O hydrogen bond interactions build extended architectures in the crystals studied. This result suggests that the steric effect of 6-methyl substitution plays an important role in the distortion of the structure, and the 6-methyl substitution can facilitate hydrogen bond interactions between methyl hydrogen atoms and O(carbonyl) or F atoms. Twelve Ni(II) complexes, including seven complexes reported previously, show reversible redox behavior, implying that the reduced Ni(I) state of each complex is stable in the time scale of CV measurement. The steric effect of R₁ substituent and the electronic effects of X₁ and X₂ groups were found to be the main factors contributing to the shift of the redox potential of the Ni(II) complexes

Full text of DOI:10.1007/s10947-010-0140-x

Journal of Structural Chemistry. Vol. 51, No. 5, pp. 923-930, 2010
Original Russian Text Copyright © 2010 by H. Kwak, G. H. Eom, S. H. Lee,
H. G. Koo, S. P. Jang, J. Lee, W. Shin, M. S. Lah, C. Kim, S.-J. Kim, and Y. Kim
STERIC EFFECT ON CONSTRUCTION OF EXTENDED
ARCHITECTURES OF Ni(II) COMPLEXES DIRECTED
BY INTERMOLECULAR C–HʞF AND C–HʞO INTERACTIONS
H. Kwak,1 G. H. Eom,1 S. H. Lee,1 H. G. Koo,1
S. P. Jang,1 J. Lee,2 W. Shin,2 M. S. Lah,3 C. Kim,1
S.-J. Kim,4 and Y. Kim4
UDC 541.49:548.736:544.142.4
Five new Ni(II) complexes with pyridine carboxamide ligands have been synthesized and the crystal
structures of three of the complexes were determined. Strong distortion effects of 6-methyl substitution
were observed in the complexes with 6-methyl-substituted pyridyl bpb ligands. The C–HʞF and C–HʞO
hydrogen bond interactions build extended architectures in the crystals studied. This result suggests that the
steric effect of 6-methyl substitution plays an important role in the distortion of the structure, and the 6-
methyl substitution can facilitate hydrogen bond interactions between methyl hydrogen atoms and
O(carbonyl) or F atoms. Twelve Ni(II) complexes, including seven complexes reported previously, show
reversible redox behavior, implying that the reduced Ni(I) state of each complex is stable in the time scale
of CV measurement. The steric effect of R1 substituent and the electronic effects of X1 and X2 groups were
found to be the main factors contributing to the shift of the redox potential of the Ni(II) complexes.
Keywords: Pyridine carboxamide, nickel(II) complex, crystal structure, electronic effect, weak
interactions, hydrogen bond, supramolecular architecture, cyclic voltammetry.
Deprotonation of the carboxamide nitrogen atoms in the pyridine carboxamide type ligand bpb (Scheme 1) makes it
a tetradentate ligand that chelates metal ions to form a planar geometry complex [1-6]. In this planar geometry, hydrogen
atoms in the pyridyl 6ꢀ-position make close intramolecular contacts. If methyl groups are placed in the pyridyl 6ꢀ-position,
they yield highly distorted geometries [7]. The steric effect of 6-methyl substitution in the crystal structures of Cu(II)
complexes have been studied and it was shown that 6-methyl substitution can facilitate the formation of extended structures
due to interactions between Cu(II) ions and carbonyl oxygen atoms of neighboring molecules [7]. Among analogous nickel
complexes containing pyridine carboxamide type ligands, the compound 3 ([Ni(bpb)], see Scheme 1 for the structure) shows
almost planar molecular arrangement, while the compound 9 ([Ni(6-Me2-bpb)]) is a complex that shows the steric effect [8].
These simple Ni(II) complex units can be used as building blocks to form extended structures by the non-classical
C–HʞO or C–HʞF hydrogen bond interactions [7]. It is known that the non-classical interactions can provide good
1Department of Fine Chemistry, and Eco-Product and Materials Education Center, Seoul National University of
2
Technology, Seoul 139-743, Korea; chealkim@snut.ac.kr. Department of Chemistry and Interdisciplinary Program of
Integrated Biotechnology, Sogang University, Seoul 121-742, Korea. 3Department of Applied Chemistry, College of Science,
4
Hanyang University, Ansan-si, Gyeonggi-do 426-791, Korea. Department of Chemistry and Nano Science, Ewha Womans
University, Seoul 120-750, Korea; sjkim@ewha.ac.kr. The text was submitted by the authors in English. Zhurnal Strukturnoi
Khimii, Vol. 51, No. 5, pp. 957-964, September-October, 2010. Original article submitted January 22, 2009.
0022-4766/10/5105-0923 © 2010 Springer Science+Business Media, Inc.
923

X1 = CH3, X2 = CH3, R1 = H
(H2Me2bpb)
(H2Mebpb)
(H2bpb)
1: X1 = CH3, X2 = CH3, R1 = H
(H2Me2bpb)
(H2Mebpb)
(H2bpb)
L1:
L2: X1 = H,
L3: X1 = H,
L4: X1 = H,
L5: X1 = Cl,
L6: X1 = H,
X2 = CH3, R1 = H
2: X1 = H,
3: X1 = H,
4: X1 = H,
5: X1 = Cl,
6: X1 = H,
X2 = CH3, R1 = H
X2 = H,
X2 = F,
X2 = Cl,
R1 = H
R1 = H
R1 = H
X2 = H,
X2 = F,
X2 = Cl,
R1 = H
R1 = H
R1 = H
(H2bpf)
(H2bpf)
(H2bpCl2)
(H2bpn)
(H2bpCl2)
(H2bpn)
X2 = NO2, R1 = H
X2 = NO2, R1 = H
L7: X1 = CH3, X2 = CH3, R2 = CH3 (H26-Me2-Me2bpb) 7: X1 = CH3, X2 = CH3, R2 = CH3 (H26-Me2-Me2bpb)
L8: X1 = H,
L9: X1 = H,
L10: X1 = H,
L11: X1 = Cl,
L12: X1 = H,
X2 = CH3, R2 = CH3 (H26-Me2-Mebpb) 8: X1 = H,
X2 = CH3, R2 = CH3 (H26-Me2-Mebpb)
X2 = H,
X2 = F,
X2 = Cl,
R2 = CH3 (H26-Me2-bpb)
R2 = CH3 (H26-Me2-bpf)
R2 = CH3 (H26-Me2-bpCl2)
9: X1 = H,
10: X1 = H,
11: X1 = Cl,
12: X1 = H,
X2 = H,
X2 = F,
X2 = Cl,
R2 = CH3 (H26-Me2-bpb)
R2 = CH3 (H26-Me2-bpf)
R2 = CH3 (H26-Me2-bpCl2)
X2 = NO2, R2 = CH3 (H26-Me2-bpn)
X2 = NO2, R2 = CH3 (H26-Me2-bpn)
Scheme 1. Syntheses of ligands and their Ni(II) complexes
structural motifs for construction of extended architectures and stabilize the crystal structures [10-18]. We have recently
reported that in Pd(II) complexes containing both C–X (X = F or Cl) and C=O hydrogen bond accepting sites, the C–F and
C–Cl sites can compete with C=O to form hydrogen bond interactions [19].
In order (1) to systematically investigate such a steric effect of a substituent at 6ꢀ-position on construction of Ni(II)
complexes and their voltammetric behaviors, and (2) to study the role of C–HʞF and C–H…O interactions in the formation of
the extended structures, two new ligands containing –CH3 on the pyridyl 6ꢀ-position [H26-Me2-bpCl2 (L11) and H26-Me2-bpn
(L12)] and five new nickel(II) complexes were synthesized. Here we report crystal structures of three new Ni(II) complexes (7,
10 and 12) with pyridine carboxamide ligands (L7, L10 and L12), and discuss the steric effect of 6-methyl substitution for
nickel complexes with/without 6-methyl groups. In addition, the voltammetric studies of the Ni(II) complexes are reported to
exhibit reversible redox behavior which shows that the reduced Ni(I) state of each complex is stable in the timescale of CV
(cyclic voltammetry) measurements. The introduction of CH3 instead of H in R1 position made the reduction of the complex
easier, and the substituent effect of X1 and X2 groups on the potential shift was observed. The steric effect of R1 and the
electronic effects of X1 and X2 are major contributors to the voltammetric behavior of the Ni(II) complexes studied.
EXPERIMENTAL
Ligands. Ligands L1-L10 were available from a previous study [9, 19-24]. Ligands L11 (H26-Me2-bpCl2) and L12
(H26-Me2-bpn) were prepared by the reactions of appropriate diamines (10 mmol) with 6-methyl-2-picolinic acid (20 mmol) in
pyridine in the presence of triphenylphosphite (20 mmol) according to a literature method [19]. For L11, the yield was 2.08 g
(50.0%). 1H NMR (CDCl3, 300 MHz): ꢁ 2.46 (s, 6H, 2CH3), ꢁ 7.32-8.11 (m, 8H, aromatic-H), 10.34 (s, 2H, 2N–H). IR (KBr):
(N–H) 3274 and (C=O) 1688 cm–1. Anal. Calcd for C20H14N4O2Cl2 (413.28): C, 58.12; H, 3.42; N, 13.56. Found: C, 58.23; H,
3.31; N, 13.27. For L12, the yield was 1.76 g (45.2%). 1H NMR (DMSO-d6, 300MHz): ꢁ 2.62 (s, 6H, 2CH3), 6.65-8.80 (m, 9H,
aromatic-H), 10.10 (s, 1H, N–H), 11.05 (s, 1H, N–H). IR (KBr): (N–H) 3253 and 3330 and (C=O) 1669 cm–1 and 1700 cm–1.
Anal. Calcd for C20H15N5O4 (389.4): C, 61.68; H, 3.89; N, 17.99. Found: C, 61.92; H, 4.01; N, 17.57.
Complexes. Complexes 1-6 were available from a previous study [9]. Complexes 7-12 were prepared by the
reaction of a corresponding ligand (0.5 mmol) with nickel(II) acetate (0.5 mmol) in CH3CN (20 ml) in good yields, according
to a literature method [8, 9]. Crystals of the complexes 7, 10 and 12 were grown by slow evaporation of the corresponding
solutions in EtOH/ether for 7, CH3CN/ether for 10, and CHCl3/benzene for 12. For 7, the yield was 0.11 g (48.3%). 1H NMR
924

TABLE 1. Crystallographic Data and Refinement Details for Compounds 7, 10, and 12
Compound
[Ni(6-Me2-Me2bpb)] (7)
[Ni(6-Me2-bpf)] (10)
[Ni(6-Me2-NO2bpb)] (12)
Empirical formula
M
C22H20N4NiO2
431.13
C20H15FN4NiO2
421.07
C23H18N5NiO4
487.13
T, K
293(2)
293(2)
170(2)
Space group
a, b, c, Å
C2/c
P-1
C2/c
11.902(2), 24.955(5),
7.1446(14)
90, 91.17(3), 90
8.012(4), 10.493(5),
26.461(5), 7.0270(12),
11.663(5)
23.628(4)
106.969(7), 97.485(7),
95.006(7)
90, 113.087(3), 90
ꢂ, ꢃ, ꢄ, deg
V, Å3
Z
1977.1(7)
4
1.448
921.7(7)
2
1.517
4041.5(12)
8
d(calc), g/cm3
Absorption coeff., mm–1
Crystal size, mm
1.601
1.004
0.28ꢅ0.28ꢅ0.08
10945
4653 [0.0528]
1.007
0.20ꢅ0.15ꢅ0.15
2016
1.086
0.20ꢅ0.15ꢅ0.15
6451
Reflections collected
Independent reflections
[R(int)]
1918 [0.0173]
3154 [0.0167]
Refined parameters
134
1.092
260
1.068
338
1.070
GOOF on F2
R1 = 0.0456, wR2 = 0.1312
R1 = 0.0571, wR2 = 0.1387
704020
R1 = 0.0595, wR2 = 0.1644
R1 = 0.0620, wR2 = 0.1679
704021
R1 = 0.0641, wR2 = 0.1342
R1 = 0.1156, wR2 = 0.1527
704022
R indices [I > 2ꢆ(I)]
R indices (all data)
CCDC deposition N
(CDCl3, 300 MHz): ꢁ 2.31 (s, 6H, 2CH3), ꢁ 2.74 (s, 6H, 2CH3), ꢁ 7.27-7.85 (m, 8H, aromatic-H). IR (KBr): (C=O) 1633 cm–1.
Anal. Calcd for C22H20N4NiO2 (431.13): C, 61.29; H, 4.69; N, 13.00. Found: C, 60.82; H, 5.10; N, 12.76. For 8, the yield was
1
0.10 g (50.0%). H NMR (CDCl3, 300MHz): ꢁ 2.27 (s, 3H, CH3), ꢁ 2.37 (s, 6H, 2CH3), ꢁ 6.66–8.01 (m, 9H, aromatic-H). IR
(KBr): (C=O) 1634 cm–1. Anal. Calcd for C21H18N4NiO2 (417.12): C, 60.46; H, 4.36; N, 13.43. Found: C, 60.13; H, 4.11; N,
13.72. Complex 9 was synthesized according to a literature method and its crystal structure has been already reported [8]. The
yield was 0.11 g (55.1%). 1H NMR (CDCl3, 300MHz): ꢁ 2.37 (s, 6H, 2CH3), 6.85-8.12 (m, 10H, aromatic-H). IR (KBr): (C=O)
1637 cm–1. Anal. Calcd for C20H16N4NiO2 (403.09): C, 59.59; H, 4.00; N, 13.90. Found: C, 59.50; H, 3.67; N, 14.24. For 10, the
yield was 0.10 g (49.8%). 1H NMR (CDCl3, 300 MHz): ꢁ 2.38 (s, 6H, 2CH3), 6.54-8.07 (m, 9H, aromatic-H). IR (KBr): (C=O)
1634 cm–1. Anal. Calcd for C20H15FN4NiO2 (421.07): C, 57.05; H, 3.60; N, 13.31. Found: C, 57.37; H, 3.55; N, 12.97. For 11, the
yield was 0.16 g (66.5%). 1H NMR (CDCl3, 300 MHz): ꢁ 2.39 (s, 6H, 2CH3), 7.00-8.25 (m, 8H, aromatic-H). IR (KBr): (C=O)
1644 cm–1. Anal. Calcd for C20H14N4NiO2Cl2 (471.97): C, 50.89; H, 3.00; N, 11.87. Found: C, 51.21; H, 3.05; N, 11.52. For 12,
1
the yield was 0.14 g (65.3%). H NMR (DMSO-d6, 300MHz): ꢁ 2.72 (s, 6H, 2CH3), 6.57-8.28 (m, 9H, aromatic-H). IR (KBr):
(C=O) 1650 cm–1. Anal. Calcd for C23H18N5NiO4 (487.13): C, 56.71; H, 3.73; N, 14.38. Found: C, 56.54; H, 3.98; N, 14.27.
Crystallography. The diffraction data for the compound [Ni(6-Me2-Me2bpb)] (7) were collected on an Enraf-
Nonius CAD-4 diffractometer equipped with a monochromated MoKꢂ ( = 0.71073 Å) radiation source. The crystal was
mounted on a glass fiber. The final unit cell parameters were obtained from a least squares refinement of 25 reflections in the
range 9.46 ꢇ ꢈ ꢇ 12.47ꢉ. The crystal structure was determined by direct methods and Fourier technique. All the calculations
were performed on IBM Pentium computer using SHELXS-97 [25] and SHELXL-97 [26]. All hydrogen atoms were placed
in calculated positions. The diffraction data for the compounds [Ni(6-Me2-bpf)] (10) and [Ni(6-Me2-bpn)] (12) were collected
on a Bruker SMART APEX diffractometer with MoKꢂ ( = 0.71073 Å) radiation source and CCD detector. The crystals were
mounted on glass fibers with epoxy glue. The data were integrated and scaled using SAINT; the structures were solved and
refined using SHELXTL [27]. Hydrogen atoms were placed in calculated positions. A summary of crystallographic data for
compounds 7, 10 and 12 is given in Table 1.
925

Full crystallographic information for the compounds 7, 10 and 12 studied in this work was deposited with CCDC
with deposition numbers 704020, 704021 and 704022, respectively. Copies of the data are available free of charge from
http://www.ccdc.cam.ac.uk.
RESULTS AND DISCUSSION
Syntheses. The synthesis of bpb type ligands and their corresponding Ni(II) complexes is summarized in Scheme 1.
Ligands L11 and L12 were prepared in moderate yields (40-60%). The complexes 7-12 were obtained from the reaction of
each ligand with nickel(II) acetate hexahydrate in CH3CN in good yields (50-70%). The complexes 7-12 (with 6-methyl)
were isolated as yellow solids well soluble in DMSO and DMF. Crystals of the complexes 7, 10 and 12 were grown by slow
evaporation of their solutions in appropriate solvent systems as explained in Experimental Section. All the complexes were
stable in air.
Crystal structures. The structures of [Ni(Me2bpb)] (1), [Ni(bpf)] (4) and [Ni(bpn)] (6) reported previously [9] are
similar to that of [Ni(bpb)] (3) and have a planar molecular arrangement [8]. Reported in this paper three new Ni(II)
complexes [Ni(6-Me2-Me2bpb)] (7), [Ni(6-Me2-bpf)] (10) and [Ni(6-Me2-bpn)] (12) contain 6-methyl-substituted pyridyl bpb
ligands (L7, L10 and L12). The X1X2R1-bpb2– ligands coordinate to Ni(II) ions to form monomeric complex units, while C–
HʞF and C–HʞO hydrogen bond interactions extend them to architectures of a higher dimensionality. IR spectroscopy
confirmed the deprotonation of the ligands as ꢊNH bands disappeared, while the ꢊCO bands shifted to lower wave-number
range as compared to those in H2X1X2R1-bpb (see Experimental Section). Selected bond distances and angles characterizing
the coordination are listed in Table 2.
Four N atoms of a corresponding ligand [6-Me2-Me2bpb2– (L7), 6-Me2-bpf2– (L10) or 6-Me2-bpn2– (L12)]
coordinate the Ni(II) ion. The molecular structures of the Ni(II) complexes (7, 10 and 12) are illustrated in Fig. 1. The F atom
in compound 10 is disordered over two positions (C9 and C10). The molecule of compound 12 contains two sets of
disordered NO2-benzene rings (Fig. 1c). The Ni–N(amide) distances of 7, 10 and 12 are shorter than the Ni–N(pyridyl)
distances (Table 2). Two pyridyl rings are considerably tilted, with the dihedral angle of 43.40(9)ꢉ for 7, 42.83(13)ꢉ for 10,
and 37.62(11)ꢉ for 12. The distorting effect of methyl substitution is obvious, and the geometry of all three complexes is
distorted from that expected for square planar complexes. Similar distortion was demonstrated for the analogous copper(II)
complexes [7]; the copper(II) structures showed square pyramidal coordination with interactions between Cu(II) ions and
neighboring carbonyl oxygen atoms [7]. In case of the Ni(II) complexes, there are C–HʞO and C–HʞF interactions to form
extended polymeric motifs.
The C–HʞO or C–HʞF hydrogen bond interactions build extended architectures in the Ni(II) complexes
containing pyridine carboxamide ligands: compounds 1 and 4 without 6-methyl groups in the pyridine rings and compounds
7, 10 and 12 with 6-methyl groups. For both compounds 1 (without 6-methyl) and 7 (with 6-methyl) containing the Me2bpb2–
skeleton, different kinds of C–HʞO hydrogen bonds generate different types of two-dimensional sheets: a staircase 2D sheet
by C(pyridyl)–HʞO interactions for 1, and a 2D sheet by C(methyl)–HʞO interactions for 7 (Fig. 2). The presence of
TABLE 2. Selected Bond Lengths (Å) and Angles (deg) for Compound 7, 10, and 12
Parameter
7
10
12
Ni–N(pyridyl)
Ni–N(amide)
1.931(2)
1.824(2)
1.219(4)
80.8(2)
1.940(3), 1.959(3)
1.835(3), 1.836(3)
1.222(5), 1.235(5)
83.3(1)–84.3(1)
110.4(1)
1.945(4), 1.946(4)
1.815(5), 1.832(4)
1.226(6), 1.242(5)
83.1(2)–84.9(2)
109.8(2)
C=O
N(amide)–Ni–N(amide)
N(pyridyl)–Ni–N(pyridyl)
110.0(1)
926

Fig. 1. Molecular structure of complexes 7 (a), 10 (b), and 12 (c). Displacement ellipsoids are shown at the 30%
probability level.
Fig. 2. Two-dimensional sheets built by C(pyridyl)–H…O hydrogen bonds in compound 1 (a) and
C(methyl)–H…O hydrogen bonds in compound 7 (b). H atoms not involved in intermolecular
interactions have been omitted for clarity. The open dotted lines represent C(methyl)–HʞO
interactions. Hydrogen bond CʞO lengths [C–HʞO angles]: for 1, C3(pyridyl)–H3AʞO1(1.5–x,
–0.5+y, z) 2.341(1) Å [133.32(1)ꢉ]; for 7, C11(methyl)–H11AʞO3(–0.5–x, 1.5–y, –0.5+z)
2.62(3) Å [124.19(1)ꢉ], C11(methyl)–H11BʞO3(1.5–x, 1.5–y, 1–z) 2.61(4) Å [139.47(1)ꢉ].
6-methyl group in the pyridyl 6ꢀ-positions generates hydrogen bonds between methyl hydrogen atoms and carbonyl oxygen
atoms. For both compounds 4 (without 6-methyl) and 10 (with 6-methyl) containing the bpf2– skeleton, the same kinds of
hydrogen bonds generate 2D sheets (Fig. 3). Hydrogen bonds between pyridyl hydrogen atoms and carbonyl oxygen atoms or
fluorine atoms build the same type of 2D sheets in both 4 and 10. The presence of the 6-methyl group in the pyridyl
6ꢀ-positions does not effect hydrogen bonds. In compound 12 with 6-methyl group in the bpn2– ligand, hydrogen bonds
between methyl hydrogen atoms and carbonyl oxygen atoms generate a double chain (Fig. 4). In this case, the presence of
methyl groups in the pyridyl 6ꢀ-positions is important for hydrogen bonds.
927

Fig. 3. Two-dimensional sheets built by C(pyridyl)–HʞO and C(pyridyl)–HʞF hydrogen bonds in compounds 4
(a) and 10 (b). H atoms not involved in intermolecular interactions have been omitted for clarity. The gray dotted
lines represent C(methyl)–HʞO interaction, and the open dotted lines represent C(methyl)–HʞF interactions.
Hydrogen bond CʞO or CʞF lengths [angles]: for 4, C16(pyridyl)–H16AʞF1(1–x, –0.5+y, 0.5–z) 2.50(4) Å
[138.43(1)ꢉ], C17(pyridyl)–H17AʞO1(2–x, –0.5+y, 1.5–z) 2.269(3) Å [166.71(1)ꢉ], C3(pyridyl)–H3AʞO2(1+x,
y, 1+z) 2.36(4) Å [151.06(1)ꢉ]; for 10, C17(pyridyl)–H17ʞF1(1+x, 1+y, z) 2.547(9) Å [156.14(3)ꢉ], C3(pyridyl)–
H3ʞO1(x, y, –1+z) 2.323(6) Å [155.78(3)ꢉ].
Fig. 4. A double chain built by
C(methyl)–HʞO hydrogen bonds in
compound 12. H atoms not involved
in intermolecular interactions have
been omitted for clarity. The open
dotted lines represent C(methyl)–
HʞO interactions. Hydrogen bond
CʞO
lengths
[angles]:
C19(methyl)–H19AʞO2(0.5+x, 0.5–
y, –z) 2.451(2) Å [152.01(1)ꢉ],
C20(methyl)–H20AʞO2(0.5+x, 1.5–
y, –z) 2.506(2) Å [135.13(1)ꢉ].
Voltammetric behavior. All the Ni(II) complexes showed reversible redox behavior with ipa/ipc ꢋ 1 and
ꢌEp ꢋ 60 mV, where ipa is an anodic peak current, ipc is a cathodic peak current, and ꢌEp is a peak separation of the cathodic
and anodic peak potentials. Typical volammograms are shown in Fig. 5 for 3 and 9, the complexes differing only in R1: H for
928

Fig. 5. CVs of 3 and 9 in 0.1 M TBABF4/DMSO at
50 mV/s.
TABLE 3.Formal Potential Data for Ni Complexes in 0.1M TBABF4/DMSO (V vs. Fc/Fc+)
Compound
X1
X2
R1
Eꢉ for Ni(II/I)
1
2
3
4
5
6
7
8
9
CH3
H
H
H
Cl
H
CH3
H
H
H
Cl
H
CH3
CH3
H
F
Cl
NO2
CH3
CH3
H
F
Cl
H
H
H
H
H
–1.85
–1.84
–1.82
–1.77
–1.70
–1.51
–1.72
–1.71
–1.71
–1.64
–1.58
–1.50
H
CH3
CH3
CH3
CH3
CH3
CH3
10
11
12
NO2
3 and CH3 for 9. The reversible behavior implies that the reduced Ni(I) state of each complex is stable in the time scale of CV
measurement. The formal potentials, E 0, of the complexes were measured and calculated from E 0 = (Epc + Epa)/2, where Epc
is a cathodic peak potential and Epa is anodic peak potential. The formal potentials of the Ni complexes measured are listed in
Table 3. The E0 for the complex 3 was –1.82 V, and the E 0 for the complex 9 was –1.70 V. The complex 9, which has methyl
group as R1, has formal potential value 120 mV more positive than 3. It means that the introduction of CH3 instead of H in R1
position made the reduction of the complex easier. A similar behavior was observed for complexes 7 and 8 with easier
reduction as compared to 1 and 2, respectively. The introduction of the methyl group in other positions (X1, X2) caused
almost no change in the potential (Table 3) indicating the electronic effect from the methyl group being insignificant. It is
clear that the substitution effect of methyl group in R1 position is more pronounced than in the positions of X1 or X2. Also, the
electronic effect of the methyl group in R1 position is not important since the potential shift is opposite if only the electron-
donating character of the methyl group is counted. Instead, the distorted crystal structure produced by introducing 6-methyl
group (7, 8 and 9), which stabilize Ni(I) state, corresponds to the voltammetric results with a positive potential shift. It is a
similar behavior to that we reported for the copper complexes [7]. As to the electronic effect caused by the groups X1 and X2,
the potential shifts of the complexes reflect this electronic effect (Table 3). If the complexes 1 through 6 are compared, the
formal potential shifts to more positive values due to increasing electron-withdrawing character of the substituents X1 and X2
when R1 is the same. A similar electronic effect could be observed for the complexes 7 through 12. In summary, the steric
effect of R1 and the electronic effects of X1 and X2 are major contributors to the voltammetric behavior of the Ni complexes.
Financial support from Korea Ministry Environment “ET-Human resource development Project,” the Korean
Science&Engineering Foundation (R01-2008-000-20704-0 and 2009-0074066), the Converging Research Center Program
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through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology
(2009-0082832), and the SRC program of the Korea Science and Engineering Foundation (KOSEF) through the Center for
Intelligent Nano-Bio Materials at Ewha Womans University (grant R11-2005-008-00000-0) is gratefully acknowledged.
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