Structural effects on electrochemical properties of the carboxamide complexes [NiII(Mebpb)] and [NiII(Mebqb)]

Inorganica Chimica Acta 362 (2009) 3934–3940

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Structural effects on electrochemical properties of the carboxamide complexes [NiII(Mebpb)] and [NiII(Mebqb)] Ahmad Amiri a, Mehdi Amirnasr a,*, Soraia Meghdadi a, Kurt Mereiter b, Vahid Ghodsi a, Akram Gholami a a b

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Faculty of Chemistry, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060 Vienna, Austria

a r t i c l e

i n f o

Article history: Received 23 January 2009 Received in revised form 1 May 2009 Accepted 9 May 2009 Available online 18 May 2009 Keywords: Ni(II) bispyridylamide complexes N4-amido ligand Crystal structure Cyclic voltammetry

a b s t r a c t Two Ni(II) complexes of the dianionic ligands, Mebpb2, [H2Mebpb = N,N0 -bis(pyridine-2-carboxamido)4-methylbenzene] and Mebqb2, [H2Mebqb = N,N0 -bis(quinoline-2-carboxamido)-4-methylbenzene] have been synthesized and characterized by elemental analyses, IR, and UV–Vis spectroscopy. The crystal and molecular structures of [Ni(Mebpb)], (1), and [Ni(Mebqb)], (2), were determined by X-ray crystallography. Both complexes exhibit distorted square–planar NiN4 coordination figures with two short and two long Ni–N bonds (Ni–N 1.84 and 1.95 Å, respectively). The electrochemical behavior of these complexes with the goal of evaluating the structural effects on the redox properties has been studied. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction There has been continued interest in the design of new peptide ligands containing the pyridine-2-carboxamide functionality and development of coordination chemistry of such ligands, toward transition metal ions [1–16]. In recent years, chemists have paid special attention to the synthesis of nickel carboxamido complexes of biological importance. Nickel(II) complexes with several oligopeptide ligands cause DNA cleavage in the presence of oxidants, in some instances with sequence selectivity [17,18]. It has been postulated that Ni(III) and/or Ni(II) complexes are among the species responsible for this DNA cleavage. Due to the involvement of Ni(I) in catalytic reactions and in enzymatic processes, studies of monovalent nickel complexes are considered to be of interest [1]. The ability of such amide-containing ligands to stabilize the high oxidation states of nickel(III, IV) [2–4], and the presence of this metal(III) ion in active sites of native enzymes lent a further impetus to the study of these compounds [4,5]. In addition, the design of Pt(II) and Pd(II) complexes with novel structural features is of great relevance to the self-assembly with the amidopyridine ligands [6] and the search for new anticancer agents with innovative chemical and biological properties [7]. The non-covalent interactions of biological molecules provide the flexibility and specificity required in most important biological processes [19]. The interactions are of fundamental importance in many supramolecular complexes [20,21]. Among them the

* Corresponding author. Tel.: +98 311 391 2351; fax: +98 311 391 2350. E-mail address: [email protected] (M. Amirnasr). 0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2009.05.020

interactions between p-systems are stimulating for their relevance to the base stacking in DNA. At the same time, much higher structural variability is inherent of these ligands; both structural parameters and electronic characteristics can be varied. Therefore, the study of physicochemical properties of the complexes formed by this class of ligands allows one to gain deeper insight into the question of how the metal ion and the ligand are interacting in oligopeptide complexes [8]. These developments have subsequently promoted us to synthesize and investigate the properties of the nickel(II) complexes of H2Mebpb and H2Mebqb, and herein we report the syntheses and characterization (structural, elemental analyses, IR, UV–Vis and redox properties) of two new complexes [NiII(Mebpb)] (1) and [NiII(Mebqb)] (2). The results obtained in this work give us an opportunity to compare the effect of the methyl substituent and the fused benzene ring on the spectral and electrochemical properties of these complexes in going from bpb2 to Mebpb2 and Mebqb2. The relation between the structural changes and the redox potential of nickel(II) complexes are also discussed and compared with those already reported in the literature [9–14]. 2. Experimental 2.1. Materials and general methods All solvents and chemicals were of commercial reagent grade and used as received from Aldrich and Merck. Elemental analyses were performed by using a Perkin–Elmer 2400II CHNS-O elemental analyzer. UV–Vis spectra were recorded on a JASCO V-570

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spectrophotometer. Infrared spectra (KBr pellets) were obtained on a FT-IR JASCO 680 plus spectrophotometer. Cyclic voltammograms were recorded by using a SAMA 500 Research Analyzer. Three electrodes were utilized in this system, a glassy carbon working electrode, a platinum disk auxiliary electrode and Ag wire as reference electrode. The glassy carbon working electrode (Metrohm 6.1204.110) with 2.0 ± 0.1 mm diameter was manually cleaned with 1 lm alumina polish prior to each scan. Tetrabutylammonium hexafluorophosphate (TBAH) was used as supporting electrolyte. Acetonitrile was dried over CaH2. These solutions were deoxygenated by purging with Ar for 5 min. All electrochemical potentials were calibrated versus internal Fc+/0 couple under the same conditions [22]. 2.2. Synthesis 2.2.1. Synthesis of the ligands The H2Mebpb ligand was synthesized by a procedure reported in the literature [23]. C19H16N4O2: Yield (70%). m.p. 168–169 °C. FT-IR (KBr, cm1) mmax: 3302 (s, N–H), 1689, 1659 (s, C@O), 1589 (m, C@C), 1520 (m, C–N). UV–Vis: kmax (nm) (e, L mol1 cm1) (CH3CN): 300 (9427), 267 (15 662), 219 (26 563). The H2Mebqb ligand was prepared according to the method described in the literature with some modification [24]. C27H20N4O2: Yield (76%). m.p. 196 °C. FT-IR (KBr, cm1): mmax: 3291 (s, N–H), 1683 (s, C@O), 1590 (m, C@C), 1546 (m, C–N). UV–Vis (chloroform): kmax (nm) (e, L mol1 cm1): 328 (12 740), 318 (14 921), 286 (18 596), 241 (88 573). 2.2.2. Synthesis of [Ni(Mebpb)] (1) To a stirring solution of Ni(OCOCH3)24H2O (24.8 mg, 0.1 mmol) in ethanol (30 mL) was added a solution of H2Mebpb (33.2 mg, 0.1 mmol) in acetone (10 mL), resulting in a light orange solution. The solution was stirred for additional 5 min. Slow evaporation of the solvent afforded an orange crystalline precipitate, which was filtered off and washed with ethanol, and dried in vacuum. Crystals of (1) suitable for X-ray crystallography were obtained by recrystallization from chloroform–ethanol (1:1 v/v). Yield: 80%. Anal. Calc. for C19H14N4O2Ni: C, 58.66; H, 3.63; N, 14.40. Found: C, 58.27; H, 3.51; N, 14.35%. FT-IR (KBr, cm1) mmax: 1644 (s, C@O), 1604 (s, C@C), 1566 (s, C–N). UV–Vis: kmax (nm) (e, L mol1 cm1) (CHCl3): 446 (3267), 336 (16 693), 274 (14 232). 2.2.3. Synthesis of [Ni(Mebqb)] (2) To a solution of Ni(OCOCH3)24H2O (49.8 mg, 0.2 mmol) in ethanol (20 mL) was added slowly a solution of H2Mebqb (86.4 mg, 0.2 mmol) in chloroform (20 mL). Dark brown crystals were obtained by slow evaporation of the solvent. The crystals were isolated by filtration and washed with ethanol–chloroform (10:1 v/ v), and dried in vacuum. Yield: 88%. Anal. Calc. for C27H18N4O2Ni (489.16): C, 66.30; H, 3.71; N, 11.45. Found: C, 66.31; H, 3.65; N, 11.43%. FT-IR (KBr, cm1): mmax: 1630 (s, C@O), 1591 (m, C@C), 1559 (m, C–N). UV–Vis (chloroform): kmax (nm) (e, L mol1 cm1): 520 (2362), 422 (5838), 354 (15 263), 321 (26 540).

Table 1 Crystal data and structure refinement for 1 and 2. Compound

1

2

Chemical formula Formula weight T (K) Crystal system space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z, Calculated density (g/cm3) Crystal size (mm) l (mm1) F(0 0 0) h ranges (°) Index ranges

C19H14N4NiO2 389.05 100(2) Orthorhombic Pbcn 18.7630(8) 12.6709(5) 6.8525(3) 90 90 90 1629.14(12) 4, 1.586

C27H18N4NiO2 489.16 100(2) Monoclinic Cc 17.0923(5) 13.7911(4) 9.6920(3) 90 115.1830(10) 90 2067.47(11) 4, 1.572

0.52  0.18  0.15 1.213 800 3.22–30.07 26 6 h 6 26, 17 6 k 6 17, 9 6 l 6 8 16 223 2379 (0.0374)

0.36  0.25  0.20 0.974 1008 2.59–29.97 24 6 h624, 18 6 k 6 19 13 6 l 6 13 15 907 5526 (0.0166)

Multi scan

Multi scan

0.65, 0.83

0.72, 0.82

2379/0/124

5526/169/213

1.075 R1 = 0.0461, wR2 = 0.1113

1.024 R1 = 0.0595, wR2 = 0.1226

R1 = 0.0632, wR2 = 0.1252 1.29 and 0.52

R1 = 0.0619, wR2 = 0.1244 1.45 and 0.92

Reflections collected Independent reflections (Rint) Absorption correction Min. and max. transmission Data/restraints/ parameters Goodness-of-fit on F2 Final R indices [F2 > 2r(F)] R indices (all data) Max./min. Dq (e Å3)

Both structures showed pseudosymmetry problems arising from the methyl group that is asymmetrically attached to the C2-symmetric complex cores. In 1 the methyl group is disordered and attached with 50% occupancy to both sides of the crystallographically C2-symmetric complex (space group Pbcn). The structure of 2, refined in space group Cc, is ordered but is very close to space group symmetry C2/c and chemically equivalent 1–2 and 1–3 distances were restrained to be similar. The displacement ellipsoids of this structure model were superior to one with space group symmetry C2/c (the structure model in space group C2/c had R1 = 0.062/0.063 (cf. Table 1) and generated extremely anisotropic equivalents of C22, C23, C24, and C25 (U3 up to 0.35 Å2), whereas the terminal methyl carbon C27 was nearly isotropic and displayed C–C–C bond angles of 103° and 136°; these features demonstrated the phenyl ring C21 through C26 to violate significantly C2-symmetry and led to the preference of space group Cc; with this model also the C–C–C angles involving C27 became acceptable). The crystallographic and refinement data are summarized in Table 1. 3. Results and discussion

2.3. X-ray crystallography

3.1. Synthesis

X-ray data of 1 and 2 were collected at T = 100 K on a Bruker Smart APEX CCD diffractometer with graphite monochromated Mo Ka (k = 0.71073 Å) radiation and 0.3° x-scan frames. Cell refinement and data reduction were performed with the help of program SAINT [25]. Corrections for absorption were carried out with the multi-scan method and program SADABS [25]. The structures were solved with direct methods using program SHELXS97 and structure refinement on F2 was carried out with program SHELXL97 [26]. All non-hydrogen atoms were refined anisotropically.

3.1.1. Description of structures of [Ni(Mebpb)] (1) and [Ni(Mebqb)] (2) The structures of 1 and 2 were determined by low-temperature single crystal X-ray diffraction. Crystal data, together with other relevant information on structure determination, are listed in Table 1. Table 2 gives significant bond lengths and angles, and Figs. 1 and 2, show the ORTEP diagrams for 1 and 2, respectively. Packing diagrams of the molecules in the unit cell are shown in Figs. 3 and 4. The carboxamide ligand coordinates to the metal ion upon double deprotonation to generate a 5-,5-,5-membered chelate ring system.

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Table 2 Selected bond lengths (Å) and angles (°) for (1) and (2). Complex (1)

Complex (2)

Bond lengths Ni(1)–N(2) Ni(1)–N(2A) Ni(1)–N(1) Ni(1)–N(1A) O(1)–C(6) N(2)–C(6) N(2)–C(7) N(1)–C(5) N(1)–C(1)

1.844(2) 1.844(2) 1.935(2) 1.935(2) 1.235(3) 1.348(3) 1.409(3) 1.359(3) 1.344(3)

Ni(1)–N(2) Ni(1)–N(4) Ni(1)–N(1) Ni(1)–N(3) O(2)–C(11) N(4)–C(11) N(4)–C(26) N(3)–C(12) N(3)–C(20)

1.837(2) 1.843(2) 1.962(2) 1.955(2) 1.239(3) 1.347(4) 1.419(4) 1.349(3) 1.377(3)

Bond angles N(2)–Ni(1)–N(2A) N(1A)–Ni(1)–N(2A) N(2)–Ni(1)–N(1A) N(1)–Ni(1)–N(2A) N(1)–Ni(1)–N(2) N(1)–Ni(1)–N(1A) C(6)–N(2)–C(7) C(6)–N(2)–Ni(1) C(7)–N(2)–Ni(1) C(1)–N(1)–C(5) C(5)–N(1)–Ni(1) C(1)–N(1)–Ni(1) C(7A)–C(7)–N(2) O(1)–C(6)–N(2) O(1)–C(6)–C(5) N(2)–C(6)–C(5) N(1)–C(5)–C(6)

84.10(12) 84.11(9) 167.96(9) 167.96(9) 84.11(9) 107.76(13) 125.7(2) 118.2(2) 115.7(2) 117.1(2) 111.6(2) 131.3(2) 112.2(1) 128.5(2) 121.6(2) 109.9(2) 115.3(2)

N(2)–Ni(1)–N(4) N(2)–Ni(1)–N(1) N(4)–Ni(1)–N(1) N(2)–Ni(1)–N(3) N(4)–Ni(1)–N(3) N(3)–Ni(1)–N(1) C(11)–N(4)–C(26) C(11)–N(4)–Ni(1) C(26)–N(4)–Ni(1) C(12)–N(3)–C(20) C(12)–N(3)–Ni(1) C(20)–N(3)–Ni(1) C(21)–C(26)–N(4) O(2)–C(11)–N(4) O(2)–C(11)–C(12) N(4)–C(11)–C(12) N(3)–C(12)–C(11)

84.51(17) 82.47(13) 162.9(2) 164.6(2) 84.47(13) 110.27(11) 128.3(3) 116.6(2) 114.8(2) 119.5(3) 109.7(2) 130.8(2) 112.3(3) 129.6(4) 119.9(4) 110.4(3) 115.7(3)

Fig. 2. ORTEP diagram of complex 2 with the atom labeling scheme. View inclined to mean plane of the complex.

Note: symmetry transformations used to generate equivalent atoms: #1 x + 1,y,z + 1/2.

Fig. 3. Packing diagram of 1 viewed down b-axis showing the p–p stacked columns with intermolecular Nam–Ni–Nam interactions (green broken lines) and C(3)– (H)O(1) interactions (red broken lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 1. ORTEP diagram of complex 1 with the atom labeling scheme.

The nickel ion is in a distinctly distorted square planar environment in both 1 and 2. This distortion is the combined result of (i) the chemical inequivalence of the amidate and the pyridine nitrogen atoms, (ii) the bite properties of the tetradentate ligand, and (iii) the steric demand and mutual intramolecular interference of the terminal pyridine/quinoline moieties. As expected, the average Ni–N(amidate) bond distances, 1.844(2) in 1 and 1.840(2) Å in 2, are notably smaller than the average Ni–N(pyridine/quinoline) bond distances, 1.935(2) in 1 and 1.958(2) Å in 2, respectively, and agree well with the corresponding values in related nickel(II) complexes [9–14]. This is in accord with the fact that the deproto-

nated amide nitrogen is a very strong r donor. The cis angles in the two complexes span a wide range, 84.1(1)–107.8(1)° in 1 and 82.5(1)–110.3(1)° in 2, which is usual for complexes of bpb and bqb ligands. The tetrahedral twist, defined by the dihedral angle between the Ni/Nam/Nam0 and Ni/Npy/Npy0 planes at the nickel atom [13] is 3.0° for 1 and 13.3° for 2. The dihedral angles between the two pyridine rings are 15.5° for 1 and 37.6° for 2. Thus, the intramolecular steric interference of the quinoline moieties in 2 leads, as expected, to a larger tetrahedral distortion of the nickel atom than interference of the pyridines in 1. This in turn has some influence on the electronic spectra and the redox potentials of the two complexes (vide infra). The outlined geometrical properties of the two complexes in solid state comply essentially with the tendencies observed for the parent complexes [Ni(bpb)] [12,27] and [Ni(bqb)] [14] and their relatives [Co(bpb)] [28], [Cu(bpb)] [29], and [Cu(Mebpb)] [30]. In the solid state both Ni complexes, 1 and 2, are stabilized by significant inter-ligand p–p-stacking interactions supplemented by Ni–Nam, Ni–O and C–HO interactions, as shown in Figs. 3 and 4. In 1 the rather flat Ni-complexes are oriented parallel to  and form columnar p–p stacks that are extending (1 0 1) or (1 0 1) along the c-axis of the orthorhombic unit cell and are passing through x,y = 0,0 and x,y = 1/2,1/2 (Fig. 3). Within one column the complexes are stacked in a head-to-tail fashion. The p–p stacking distance is d(1 0 1) = 3.24 Å, the shortest corresponding contact distances are C5–O1 = 3.12 Å, N2–C6 = 3.28 Å, C1–C8 = 3.29 Å. The NiNi distance between adjacent complexes in 1 is 3.745 Å. Complex 1 is isostructural with the copper compound [Cu(Mebpb)] [30] (orthorhombic, space group Pbcn, a = 18.777(5), b = 12.749(3), c = 6.8213(17) Å at T = 125 K). The parent complex [Ni(bpb)] [27]

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C-centered lattices of similar unit cell dimensions and largely identical architecture ([Ni(bqb)]: a = 17.4253(2), b = 12.9676(1), c = 9.7265(1); b = 115.275(1)° at T = 183 K). Due to the fact that the complex [Ni(bqb)] is C2-symmetric, it crystallizes in the centrosymmetric space group C2/c with Ni located on a lattice C2 axes. The methyl group in [Ni(Mebqb)], 2, violates molecular C2-symmetry and causes the solid to adopt an analogous structure with Ccentering and c-glide planes retained, but without the lattice C2 axes. The resulting space group symmetry of solid 2 is Cc, but the structure exhibits naturally a significant C2/c pseudosymmetry. Additional columnar stabilization in crystal packing of 1 and 2 comes from stacking interactions between the Ni atom with Nam and OC@O of adjacent molecules, respectively. The distances of NamNiNam in 1 are 2  3.27 Å, and the distances of OCONiOCO in 2 are 3.29–3.32 Å. Another factor that stabilized the chelate molecules of 1 and 2, is a modest intermolecular hydrogen bonding, as reported by Table 3. 3.2. Spectral studies

Fig. 4. Packing diagram of 2 viewed along b-axis showing p–p stacking, the NiOCO intermolecular interactions (red broken lines) and C–(H)O interactions (orange lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and their relatives [Co(bpb)] [28] and [Cu(bpb)] [29], which represent an isostructural series of monoclinic symmetry and space group P21/c, form in the solid state columns of p–p-stacked complexes nearly identical with those of 1, but due to the missing methyl group, the packing of their columns is denser than in orthorhombic [Ni(Mebpb)], 1. It should be added that [Ni(bpb)] can crystallize also as a hydrate [Ni(bpb)]H2O with water not bonded to Ni [12]. This solid is also based on p–p-stacked molecular columns that are separated by water molecules hydrogen bonded to the oxygen atoms of bpb. However, in [Ni(bpb)]H2O the columns contain direct NiNi interactions of 3.26 Å [12] which are lacking in 1. In 2 the Ni-complexes are much more twisted than in 1 and its Co/Cu analogs, and therefore p–p stacking interactions are segmented: the bis-carboxybenzenediamidate parts are mutually p–p stacked on both of their sides with considerable mutual slipping whereas the quinoline moieties show only one-sided p–p-contacts forming compact p–p-stacked pairs of quinoline. The contact distances within these stacks vary and start with C–C and C–O distances of ca. 3.3 Å, e.g. C1–C26 = 3.28 Å or C7–O2 = 3.30 Å. In contrast to 1, p–p–stacking in 2 leads to a framework-type structural edifice with a larger separation of the Ni-atoms, the shortest NiNi distance being 5.676 Å. It is interesting to note, that the parent compound of 2, namely [Ni(bqb)] [14] is practically isostructural with 2 in solid state. Both solids crystallize in monoclinic

Table 3 Hydrogen bond lengths (Å) and angles (°) for (1) and (2). Compound

D–HA

D–H (Å)

HA (Å)

DA (Å)

1

C(3B)–H(3B)O1

0.95

2.26

3.101

D–HA (°) 147

2 2

C(16A)–H(16A)O1 C(6B)–H(6B)O2

0.95 0.95

2.30 2.46

3.223 3.352

163 155

The FT-IR data of the complexes are listed in Section 2. Meaningful information regarding the bonding sites of the ligand molecules can be obtained by comparing the IR spectra of nickel complexes with the uncomplexed ligands. The IR spectra of the H2Mebpb and H2Mebqb ligands exhibit a band at 3291– 3302 cm1 due to NH group. The absence of m(N–H) in the IR spectra of the two complexes confirms that the ligands are coordinated in their deprotonated form [31]. The two sharp C@O stretching vibration bands at 1689 and 1659 cm1 for H2Mebpb and 1683 cm1 for H2Mebqb are shifted to lower frequencies upon coordination of the deprotonated amide, which is in agreement with the data reported for the related complexes [14,23]. The C– N stretching vibration of medium intensity appearing at 1520 cm1 for H2Mebpb and at 1546 cm1 for H2Mebqb undergoes a sizable shift to higher frequencies in the corresponding complexes. This displacement is to be expected because of the resonance enhancement in the deprotonated amide which in turn leads to the strengthening of the C–N bond [31]. The UV–Vis data are presented in Section 2. The electronic spectra of the H2Mebpb ligand shows a shoulder at 300 nm (e = 9427 L mol1 cm1) and two bands in the higher energy region at 267 nm (e = 15 662 L mol1 cm1) and 219 nm (e = 26 563 L mol1 cm1). The ligand centered transitions are red shifted by 55–69 nm in the spectrum of Ni(Mebpb) complex compared to the free ligand presumably due to the increased conjugation after coordination with the metal ion. The appearance of an additional new shoulder at 446 nm (e = 3267 L mol1 cm1) in the spectrum of Ni(Mebpb) complex is attributed to a LF transition with CT admixture, and is consistent with the square planar structure of the complex [32] having slight tetrahedral distortion (vide supra). The UV–Vis spectrum of the H2Mebqb ligand consists of a shoulder at 328 nm (e = 12 740 L mol1 cm1), two overlapping bands at 318 nm (e = 14 921 L mol1 cm1) and 286 nm (e = 18 596 L mol1 cm1) and a well resolved band at 241 nm (e = 88 573 L mol1 cm1). The ligand centered transitions are also observed in the spectrum of Ni(Mebqb) complex with a red shift of 68–104 nm in the peak positions relative to the free ligand. The two broad bands at 422 nm (e = 5838 L mol1 cm1) and 520 nm (e = 2362 L mol1 cm1) are assigned to the LF transitions with CT admixture. It is reasonable to assume that the larger tetrahedral distortion in (2) relative to (1) would indeed lead to the splitting of the electronic states and the appearance of additional bands. Interestingly, the first transition in the electronic absorption spectrum of (1) appears at a shorter wavelength (446 nm) relative to that of (2) (520 nm) by 74 nm, presumably due to a more

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Fig. 6. Cyclic voltammogram of [NiII(Mebpb)], c = 2.2  103 M in acetonitrile at 298 K. Scan rate, 100 mV s1. Fig. 5. Cyclic voltammogram of H2Mebpb ligand in acetonitrile solution at 293 K. Scan rate: 100 mV s1, c = 2.8  103 M.

efficient overlap between the Mebpb2 donor orbitals and the d-orbitals of the central nickel. This observation is in accord with the shorter Ni–N(pyridine) bonds in the Ni(Mebpb) complex (1.934 Å) as compared to Ni(Mebqb) complex (1.958 Å). 3.3. Electrochemistry Cyclic voltammograms of the complexes in acetonitrile, dichloromethane and DMF solutions with 0.1 M [N(n-Bu)4]PF6 as the supporting electrolyte were recorded at a glassy carbon working electrode. The approximate concentrations of the compounds were 1  103106 M. 3.3.1. Electrochemistry in acetonitrile solvent As reported in our previous work, the H2Mebpb ligand in acetonitrile is electroactive in the potential range of 1.5 to 2.2 V [23]. Three redox couples are observed in the cyclic voltammogram of H2Mebpb. Similar to other pyridine amide ligands [33], the first electrochemically irreversible oxidation wave at +1.33 V, is attributed to 1,2-diaminobenzene ring (Fig. 5). As has elegantly been demonstrated by Wieghardt et al., using spectroelectrochemistry and EPR spectroscopy [33], the p-system in the 1,2-diaminobenzene ring is the expected site of oxidation, generating a cation radical. Upon the coordination of the deprotonated ligand this process becomes quasi-reversible and occurs at the less anodic potentials. The two additional ligand centered reductions, observed at 1.89 and 2.2 V (Fig. 5), are attributed to the pyridyl rings and are shifted to the more positive values in the corresponding complexes (Table 4).

The reduction potentials for the two Ni(II) complexes are summarized in Table 4. The cyclic voltammogram of the [NiII(Mebpb)] (1) was measured in acetonitrile solvent and the representative voltammogram is shown in Fig. 6. The first reversible oxidative response at E1/2 = 1 V (Fig. 6 and Table 4), is suggested to be mainly ligand-centered. The coordination of the deprotonated ligand to Ni(II) would help to stabilize the oxidized form of the ligand due to the increased delocalization of electrons in the more planer conformation, acquired by the coordinated ligand. The observation of a similar redox process in the CV’s of the [CoIII(Mebpb)(amine)2]+ complexes [23] gives further support to this assignment. The second nearly reversible oxidation process at E1/2 = 0.864 V (Fig. 6, Table 4), is suggested to be metal centered and corresponds to the NiII–NiIII redox process, which is in agreement with similar Ni(II) complexes [15]. The final electrochemically reversible reduction couples observed at E1/2 of 1.54 and 2 V, are attributed to the consecutive reductions of the two pyridyl rings in [NiII(Mebpb)] complex. Similar results have been reported for Ni(bpb) and Ni(bpen) complexes by Fenton et al. [15], and for [CoIII(Mebpb)(amine)2]+ complexes [23]. An interesting observation is the shift to the more positive potentials of the two pyridyl rings in the cyclic voltammogram of the [NiII(Mebpb)] complex as compared to those of [CoIII(Mebpb)(amine)2]+ complexes, establishing the fact that the electron density removed from the Mebpb2 ligand by the Ni(II) ion is more than that by the isoelectronic Co(I), produced during the final reduction processes [23]. 3.3.2. Electrochemistry in dichloromethane solvent The limitation caused by the insolubility of [Ni(Mebqb)] (2) in acetonitrile led us to the investigation of the electrochemical behavior of both [Ni(Mebpb)] (1) and [Ni(Mebqb)] (2) complexes

Table 4 Redox potentials of H2Mebpb and H2Mebqb ligand and corresponding complexes.a Compound

Solvent

Epc1

Epa1

Epc2

Epa2

Epc3

Epa3

H2Mebpb H2Mebqb H2Mebpb NiII(Mebpb) NiII(Mebpb) NiII(Mebqb) NiII(Mebpb)

Acetonitrile Dichloromethane Dichloromethane Acetonitrile Dichloromethane Dichloromethane DMF

0.39 0.304 0.22 0.961 1.158 1.306

1.89 1.695

0.11 0.293

2.20 2.109

2.09 1.740

1.575 1.635 1.366 1.56

1.513

2.042

1.953

1.48

1.764 2.02

1.504 1.93

NiII(Mebqb)

DMF

1.33 1.523 1.42 1.036 1.259 1.502 0.87 0.97 0.75 0.84

1.58

1.49

a b

1.2

Potentials are vs. Fc+/0 in 0.1 M TBAH, T = 293 K. Scan rate 100 mV s1. Approximate concentrations: 1  103–106 M. At the san rate of 200 mV s1.

Epc NiIII/II

Epa NiII/III

0.818 0.838 0.803 0.4

0.909 0.94 1.002 0.302b 1.23

A. Amiri et al. / Inorganica Chimica Acta 362 (2009) 3934–3940

in dichloromethane solution. The cyclic voltammograms of the two ligands in dichloromethane solution show features similar to those observed for H2Mebpb in acetonitrile solution, with a slight difference in the negative region due to the limitation in the potential scanning in dichloromethane solvent. The electrochemical data are presented in Table 4 and the representative voltammogram of [Ni(Mebpb)] (1) in dichloromethane solution is shown in Fig. 7. The two electrochemically quasi-reversible oxidation processes at E1/2 = 1.21 and 0.889 V (Fig. 7 and Table 4), corresponding to the ligand centered and NiII–NiIII redox process, are shifted to more positive potentials in comparison with those obtained in acetonitrile. It is evident that, in general, a marked shift of E1/2 to more negative potentials takes place in acetonitrile, demonstrating that the oxidized cationic species are formed easier in acetonitrile solvent. These results can be explained considering the donor–acceptor concept proposed by Gutmann [34] for solvent–solute interactions. According to Guttmann’s theory [34], the acceptor capacity of the oxidized cationic species is expected to be greater in comparison with that of the neutral [Ni(Mebpb)] complex. Consequently, the oxidized species are strongly affected by the interaction with the solvent component of the highest donor number DN (thus the highest Lewis basicity) than the reduced neutral [Ni(Mebpb)] species. Hence, the formation of oxidized species are expected to be more effortless in acetonitrile solution which is the component with the highest DN (DN = 59.0 kJ mol1) [34] and less effortless in dichloromethane solution, which is the component with negligible DN (DN = 4.18 kJ mol1) [34]. The final electrochemically irreversible reduction process observed at Epc of 1.63 V, is attributed to the one step reduction of the pyridyl rings. The second step reduction of the pyridyl rings is obscured because of the solvent limitations. The cyclic voltammogram of the [NiII(Mebqb)] complex (2) was measured in dichloromethane solution and the representative voltammogram is displayed in Fig. 8. In general, the redox couples of [NiII(Mebqb)] (2) complex are shifted to more positive values relative to those of the corresponding [NiII(Mebpb)] (1) complex demonstrating the electron-withdrawing effect of the benzene rings of the quinoline moiety in [NiII(Mebqb)] complex. Moreover, the two reduction steps corresponding to the quinoline moiety are well resolved. The fact that the deviation from square planar geometry is more pronounced in (2) relative to (1) renders the Ni atom more electron deficient in (2), and in turn the reduction processes are expected to occur at more positive potentials in this complex. These

Fig. 7. Cyclic voltammogram of [NiII(Mebpb)], c = 5.6  105 M in dichloromethane at 298 K. Scan rate, 100 mV s1.

3939

experimental observations indicate that the electronic and the structural effects are operating synergistically in modifying the redox properties of the nickel complexes reported in this paper. 3.3.3. Electrochemistry in DMF solution The electrochemical properties of the two complexes were also studied in DMF. The electrochemical data are presented in Table 4 and the representative voltammograms of [Ni(Mebpb)] (1) and Ni(Mebqb) (2) in DMF solution are shown in Figs. 9 and 10, respectively. The electrochemical behavior of Ni(Mebpb) in DMF is more involved, since the voltammogram is dependent on scan rate or used potential window. For the Ni(Mebpb) complex (1) the NiIII/II oxidation process is irreversible at the scan rate of 100 mV s1 or when the measurements is carried out in the potential range of 1.3 to 2.2 V. However, with increasing the scan rate to 200 mV s1 or decreasing the scanning potential window to 1.3– 0.7 V, this process becomes reversible (Fig. 9). Contrary to the electrochemical behavior observed for Ni(Mebpb), the NiIII/II oxidation process for Ni(Mebqb) complex (2) (Fig. 10) is irreversible, even at the high scan rates or low potential ranges and the NiII/III oxidation peak appear at 1.23 V. It seems that the Mebqb ligand can not stabilize NiIII in DMF solvent. The destabilization of the trivalent oxidation state in DMF can be attributed to the lower donor capacity of Mebqb relative to Mebpb, resulting from the electron-withdrawing effect of the additional benzene rings of the quinoline moiety and more delocalization of the negative charge on the deprotonated Mebqb amide ligand in [NiII(Mebqb)] complex.

Fig. 8. Cyclic voltammogram of [NiII(Mebqb)], c = 5.6  105 M in dichloromethane at 298 K. Scan rate, 100 mV s1.

Fig. 9. Cyclic voltammogram of [NiII(Mebpb)], c = 5.1  104 M in DMF at 298 K. Scan rate, 100 mV s1.

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A. Amiri et al. / Inorganica Chimica Acta 362 (2009) 3934–3940

Acknowledgement Partial support of this work by the Isfahan University of Technology Research Council is gratefully acknowledged. Appendix A. Supplementary data CCDC 698173 and 698174 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2009.05.020. References

Fig. 10. Cyclic voltammogram of [NiII(Mebqb)], c = 4.6  103 M in DMF at 298 K. Scan rate, 100 mV s1.

Several ligand centered redox processes are also observed. The first ligand centered oxidation process for both (1) and (2) is irreversible in DMF solvent. Similar result have been reported for Ni(II) amide complexes by Gupta et al. [35]. The two nearly reversible reduction process observed at E1/2 of 1.52 and 1.98 V in the voltammogram of Ni(Mebpb) are attributed to the consecutive reductions of the two pyridyl rings. These reductions are also observed in the voltammogram of Ni(Mebqb) complex, however the first reduction process is irreversible and resembles that observed for the free ligand (9-S1, supplementary material). These observations hint to the fact that the Mebqb complex is less stable in DMF relative to Mebpb complex and some modification of this complex during the redox processes in DMF can be speculated. 4. Conclusion In this investigation, we have reported the synthesis and characterization of two Ni(II) complexes with H2Mebpb and H2Mebqb amide ligands. The coordination geometry around Ni(II) center in these complexes is a distorted square planar. In solid state the two Ni complexes are stabilized by inter-ligand p-stacking interactions. The electrochemical studies of these complexes show that the redox couples of [NiII(Mebqb)] complex are shifted to more positive values relative to those of the corresponding [NiII(Mebpb)] complex due to the (i) electronic effects (electron-withdrawing character of the benzene ring of quinoline moiety in [NiII(Mebpb)] complex) and (ii) structural parameters (deviation from square planar geometry in [NiII(Mebqb)] complex). These two important elements renders the Ni atom more electron deficient in [NiII(Mebqb)], and in turn the reduction processes occur at more positive potentials in this complex.

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