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Pseudohalide-induced structural variations in hydrazone-based metal complexes: Syntheses, electrochemical studies and structural aspects
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Inorganica Chimica Acta 361 (2008) 2692–2700 www.elsevier.com/locate/ica
Pseudohalide-induced structural variations in hydrazone-based metal complexes: Syntheses, electrochemical studies and structural aspects Sambuddha Banerjee a, Aurkie Ray a, Soma Sen a, Samiran Mitra a,*, David L. Hughes b, Ray J. Butcher c, Stuart R. Batten d, David R. Turner d a
Department of Chemistry, Jadavpur University, Raja S.C. Mallick Road, Kolkata, West Bengal 700 032, India b School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, UK c Department of Chemistry, Howard University, 2400 Sixth Street, NW, Washington, DC 200 59, USA d School of Chemistry, Monash University, 3800, Australia Received 30 August 2007; received in revised form 19 January 2008; accepted 23 January 2008 Available online 1 February 2008
Abstract The effects of pseudohalides on the modes of binding of a hydrazone ligand are investigated and we report here four new hydrazone complexes [Co(L)2] ClO4 (1), [Co(L)2]2[Co(SCN)4] (2), [Mn(L)2] (3) and [Mn(N3)2 (LH)2] H2O (4) [where LH = condensation product of pyridine-2-carboxaldehyde and benzhydrazide]. The hydrazone ligand is quite diverse in its chelating ability and can act as a neutral or mononegative ligand and as a bidentate or tridentate unit. In this paper, we report structures showing different denticities of the ligand having different charges. The complexes are characterized by IR, UV–Vis spectroscopy and cyclic voltammetric studies and the structures are conclusively established by single crystal X-ray diffraction study. Of the complexes, 1 and 3 are purely metal hydrazone complexes whereas 2 and 4 incorporate coordinated pseudohalides in their crystal structures, and 4 shows different coordination modes of the hydrazone ligand. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Hydrazone complexes; Synthesis; X-ray crystal structures; Electrochemical studies; Spectral studies; H-bonded polynuclear structure
1. Introduction Hydrazone ligands have similarities in their donor properties with unsymmetrical salen (condensation product of salicylaldehyde and 1,2-diaminoethane) type ligands. Recently it was reported that such ligands, like non-symmetrical salens, can act as effective catalysts towards alkene epoxidation [1]. Due to the short N–N bond length, the benzoyl hydrazone ligands act mostly as tridentate moieties though they have the potential to act as bridging tetradentate ligands. In analytical chemistry also, hydrazone ligands find wide applications as transition metal binders *
Corresponding author. Tel.: +91 33 2668 2017; fax: +91 33 2414 6414. E-mail address: [email protected] (S. Mitra).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.01.019
[2,3]. Studies have also shown that the azomethine N, which has a lone pair of electrons in an sp2 hybridized orbital, has considerable biological importance [4]. Aroylhydrazone complexes of transition metal ions are known to provide useful models for elucidation of the mechanism of enzyme inhibition by hydrazine derivatives [5] and for their possible pharmacological applications [6]. Additionally, hydrazone complexes have been the subject of studies over many years for their anti-microbial and anti-tumor activities [7–13]. Benzoylhydrazone complexes of copper [14], vanadium [15,16], ruthenium [17] and manganese [18] can be found in the literature. The various modes of coordination of the hydrazone ligand, LH, the condensation product of pyridine-2-carboxaldehyde and benzhydrazide, are shown in Scheme 1.
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Scheme 1. Different modes of coordination of the ligand LH.
In this work, our aim was to exploit the versatility of coordination behaviour of LH by incorporating pseudohalides (azide and thiocyanate) into manganese and cobalt precursor complexes. Herein we report four new hydrazone complexes [Co(L)2] ClO4 (1), [Co(L)2]2[Co(SCN)4] (2), [Mn(L)2] (3) and [Mn(N3)2(LH)2] H2O (4), where the same hydrazone ligand binds, depending upon the metals and pseudohalides used, in tridentate mononegative NNO donor mode for 1, 2 and 3, and bidentate neutral NO and tridentate neutral NNO donor modes in 4. In this paper, we describe the IR, UV–Vis spectra, electrochemical studies and X-ray crystal structures of these four complexes.
were recorded on a Perkin–Elmer RX 1 FT-IR spectrophotometer with a KBr disc. The electronic spectra were recorded, in acetonitrile, on a Perkin–Elmer Lambda 40 (UV–Vis) spectrophotometer. Electrochemical studies of complexes 1–3 were performed using acetonitrile solution on a Versastat-Potentiostat II electrochemical analyser against a standard calomel electrode as reference and platinum as the working electrode. Caution! Perchlorate salts are potentially explosive and should be prepared in small quantities and handled with much care.
2. Experimental
2.3.1. Synthesis of the hydrazone, [C6H5C(O)NHN@C(H)C5H4N] (LH) The pro-ligand was synthesized by condensing benzhydrazide with pyridine-2-carboxaldehyde following a method reported elsewhere [19]. The resulting light yellow liquid was used without purification. Different modes of coordination of the ligand LH is presented in Scheme 1.
2.1. Materials Benzhydrazide, pyridine-2-carboxaldehyde, manganous chloride hexahydrate, cobalt(II) perchlorate hexahydrate, sodium azide, and sodium thiocyanate (Fluka) were used as received without further purification. All the solvents were of reagent grade. 2.2. Physical measurements Elemental analyses were carried out using a Perkin– Elmer 2400 II elemental analyzer. The infrared spectra
2.3. Synthesis
2.3.2. Synthesis of [Co(L)2] ClO4 (1) To a methanolic solution (30 ml) of 1 mmol of Co(II) perchlorate hexahydrate (0.365 g), a solution of the hydrazone (2 mmol) was added and the solution was stirred at room temperature for 45 min. The solution yielded crystals suitable for X-ray diffraction after 3 days. Yield: 78%.
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Anal. Calc. for C26H20CoN6O6Cl: C, 51.45; H, 3.06; N, 17.15. Found: C, 51.41; H, 3.01; N, 17.12%. 2.3.3. Synthesis of [Co(L)2]2[Co(SCN)4] (2) A methanolic solution (30 ml) of 1 (1 mmol, 0.606 g) was made slightly alkaline and refluxed on a water bath for half an hour. The precipitate thus formed was discarded. To the filtrate, 2 mmol (0.160 g) of sodium thiocyanate, dissolved in minimum amount of water, was added with constant stirring. Single crystals suitable for X-ray diffraction were obtained after 3 weeks on keeping the mother liquor in a refrigerator. Yield: 82%. Anal. Calc. for C56H40Co3N16O4S4: C, 51.45; H, 3.06; N, 17.15. Found: C, 51.42; H, 3.03; N, 17.14%.
2.3.4. Synthesis of [Mn(L)2] (3) To a methanolic solution (30 ml) of 1 mmol of manganous chloride (0.197 g), the liquid hydrazone LH (2 mmol) was added with constant stirring. The solution was stirred at room temperature for 30 min and then kept in a refrigerator. Crystals suitable for X-ray diffraction appeared after 7 days. Yield: 80%. Anal. Calc. for C26H20MnN6O2: C, 61.97; H, 3.97; N, 16.68. Found: C, 61.94; H, 3.95; N, 16.66%. 2.3.5. Synthesis of [Mn(N3)2(LH)2] H2O (4) To a methanolic solution (30 ml) of 3 (1 mmol, 0.503 g), sodium azide (2 mmol, 0.130 g), in minimum amount of water, was added. The solution was refluxed for 30 min
Table 1 Crystallographic and refinement data of complexes 1, 2, 3 and 4 Compound
1
2
3
4
Elemental formula
C26H20CoN6O2, ClO4 606.86 monoclinic P21/n (equiv. to No. 14) 10.5363(2) 17.6654(4) 14.0139(4) 90 101.923(3) 90 2552.11(10) 4 1.579 0.833 1240 Bruker SMARTCCD
2(C26H20CoN6O2), C4CoN4S4 1306.07 monoclinic C2/c (No. 15)
C26H20MnN6O2
C26H22MnN12O2, H2O
503.42 monoclinic P21/n (equiv. to No. 14)
607.5 monoclinic P21/c (No. 14)
32.1665(14) 22.3903(13) 16.2540(8) 90 92.926(2) 90 11691.2(10) 8 1.484 1.045 5320 Bruker X8 Apex II CCD
173(2) 0.65 0.35 0.15 4.6–30.7 21 680 7124 (0.036)
123(1) 0.12 0.10 0.03 2.1–0.0 15 423 5432 (0.061)
9.8375(13) 23.9336(16) 10.0407(3) 90 103.736(1) 90 2296.4(3) 4 1.456 0.612 1036 Oxford Diffraction Xcalibur-3 CCD 140(1) 0.68 0.32 0.12 3.7–30.0 35 020 6662 (0.049)
9.3296(6) 33.887(2) 9.7560(3) 90 117.711(4) 90 2729.8(3) 4 1.478 0.537 1252 Oxford Diffraction Xcalibur-3 CCD 140(1) 0.48 0.38 0.12 3.5–27.5 42 166 6245 (0.061)
0.965 R1 = 0.070, wR2 = 0.099
1.068 R1 = 0.114, wR2 = 0.221
1.040 R1 = 0.061, wR2 = 0.075
1.181 R1 = 0.082, wR2 = 0.100
Formula weight Crystal system Space group ˚) a (A ˚ b (A) ˚) c (A a (°) b (°) c (°) ˚ 3) V (A Z D (Mg m3) l (mm1) F(0 0 0) Diffractometer used T (K) Crystal size (mm) h Range for data collection (°) No. of reflections collected No. of independent reflections (Rint) Goodness-of-fit R indices (all data)
Table 2 ˚ ) about the metals in complexes 1, 2, 3 and 4 Selected bond lengths (A Complex 1
Complex 2
Complex 3
Molecule A Co–O(1) Co–N(2) Co–N(22) Co–O(3) Co–N(4) Co–N(42)
1.9052(12) 1.8421(16) 1.9271(15) 1.8981(13) 1.8480(16) 1.9192(15)
Co(1)–O(1) Co(1)–N(2) Co(1)–N(22) Co(1)–O(3) Co(1)–N(4) Co(1)–N(42) Co(3)–N(91) Co(3)–N(92)
Complex 4
Molecule B 1.908(7) 1.864(9) 1.939(9) 1.912(6) 1.840(8) 1.929(8) 2.105(18) 1.951(16)
Co(2)–O(5) Co(2)–N(6) Co(2)–N(62) Co(2)–O(7) Co(2)–N(8) Co(2)–N(82) Co(4)–N(93) Co(4)–N(94)
1.906(7) 1.846(10) 1.939(10) 1.899(7) 1.871(9) 1.924(10) 1.946(13) 1.88(2)
Mn–O(1) Mn–N(2) Mn–N(22) Mn–O(3) Mn–N(4) Mn–N(42)
2.1610(10) 2.1874(11) 2.3577(11) 2.1382(10) 2.1950(11) 2.3191(12)
Mn–O(1) Mn–N(2) Mn–N(22) Mn–O(3) Mn–N(4) Mn–N(51) Mn –N(61)
2.3193(17) 2.303(2) 2.382(2) 2.2706(17) 2.376(2) 2.204(2) 2.210(2)
N(2)–Mn–O(1) N(22)–Mn–O(1) N(2)–Mn–N(22) N(2)–Mn–N(4) O(3)–Mn–N(4) N(51)–Mn–N(61) O(1)–Mn–N(2) O(1)–Mn–N(22) N(2)–Mn–N(22) N(2)–Mn–N(4) O(3)–Mn–N(4) O(3)–Mn–N(42) N(4)–Mn–N(42)
and then kept in a refrigerator. Crystals suitable for X-ray diffraction appeared after seven days. Yield: 65%. Anal. Calc. for C26H24MnN12O3: C, 51.36; H, 3.62; N, 27.65. Found: C, 51.32; H, 3.59; N, 27.61%. 2.4. X-ray crystallography The crystal structure analyses of the complexes 1–4 were performed by several of the authors, in different laboratories; the equipment used for each sample is listed with the crystallographic data and the refinement results in Table 1. The structures of all the complexes were determined by direct method procedures in SHELXS-97, and refined in SHELXL-97 [20]. The hydrogen atoms bonded to water oxygens in compound 4 were freely refined; all other hydrogen atoms were included in idealized positions and set to ride on the parent carbon and nitrogen atoms. 3. Results and discussion 3.1. Description of the crystal structures 3.1.1. Description of the structure of [Co(L)2] ClO4 (1) Relevant bond length and bond angle data for complex 1 are listed, together with corresponding dimensions in the other complexes, in Tables 2 and 3 respectively. Complex 1, shown in Fig. 1, consists of a Co(III) center surrounded by two tridentate hydrazone ligands, with the metal in a distorted octahedral environment [21]. The dimensions in the two ligands are very similar and indicate that the ligands adopt the enolate form (IV in Scheme 1).
N(2)–Co(1)–O(1) O(1)–Co(1)–N(22) N(2)–Co(1)–N(22) N(4)–Co(1)–N(2) N(4)–Co(1)–O(3) O(3)–Co(1)–N(42) N(4)–Co(1)–N(42) N(910 )–Co(3)–N(91) N(92)–Co(3)–N(91) N(920 )–Co(3)–N(91) N(920 )–Co(3)–N(92)
81.1(4) 164.1(4) 83.0(5) 177.7(5) 81.8(3) 164.7(4) 82.9(4) 107.3(8) 110.3(6) 107.4(7) 113.9(11)
N(6)–Co(2)–O(5) O(5)–Co(2)–N(62) N(6)–Co(2)–N(62) N(6)–Co(2)–N(8) N(8)–Co(2)–O(7) O(7)–Co(2)–N(82) N(8)–Co(2)–N(82) N(93)–Co(4)–N(9300 ) N(94)–Co(4)–N(93) N(9400 )–Co(4)–N(93) N(9400 )–Co(4)–N(94)
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N(2)–Co–O(1) O(1)–Co–N(22) N(2)–Co–N(22) N(2)–Co–N(4) N(4)–Co–O(3) O(3)–Co–N(42) N(4)–Co–N(42)
82.36(6) 165.79(6) 83.47(7) 178.20(7) 82.42(6) 165.69(6) 83.55(7)
Molecule A
Complex 2 Complex 1
Table 3 Selected bond angels (°) about the metal atoms in complexes 1, 2, 3 and 4
Molecule B
81.8(4) 164.4(4) 82.7(5) 178.1(5) 81.7(4) 164.4(4) 82.7(5) 102.3(7) 112.9(10) 108.9(7) 111(2)
71.58(4) 142.47(4) 70.89(4) 157.16(4) 71.64(4) 142.13(4) 71.27(4)
Complex 4 Complex 3
66.58(6) 134.83(6) 68.49(7) 142.66(7) 68.72(6) 179.54(9)
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Fig. 1. View of a cation and anion of compound 1, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
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The two central coordinating bonds, Co–N2 and Co–N4, are shortest, providing a z-in type of distortion in the octahedral arrangement. The N–Co–N trans angle is close to the ideal value of 180°, but the other trans angles are constrained within the meridional ligands, and are ca 166°. The intraligand bite angles lie in the 82.36(6)–83.55(6)° range. The angle between the normals to the mean-plane of the O–C–N–N–C–C–N chains in the hydrazone ligands, the ‘interligand angle’, is 86.1°. Each hydrazone ligand acts as a tridentate mononegative unit, and there is one uncoordinated perchlorate anion present to neutralize the 1+ charge on the Co(III) complex cation. The phenyl ring of C(11–16) is overlapped by an almost parallel pyridyl ring C(21–26) of a neighbouring molecule (related by n-glide-plane symmetry), and chains through such overlaps are thus formed through the crystal. Pairs of the other ligands are formed about centres of symmetry, but the overlap here, of the N(3) C(41) section of the central chain, is less pronounced. 3.1.2. Description of the structure of [Co(L)2]2[Co(SCN)4] (2) An ORTEP diagram of 2 is presented in Fig. 2 and bond dimensions are listed in Tables 2 and 3. A complex with similar structure with the same metal but with a completely different ligand system has been reported by our group earlier [22]. In 2, there are four different Co atoms, two independent [Co(III)(L)2]2+ cation complexes, and two separate [Co(II)(SCN)4]2 anions. In the cations, both the Co(III) centers are six-coordinate, having distorted octahedral geometry with two hydrazone ligands disposed in merarrangement, similar to compound 1; the ‘interligand angles’, here, are 89.4° and 89.0°. As in 1, there is overlap
of neighbouring phenyl groups, e.g. the ring of C(11–16) and C(710 –760 ) are close to parallel and overlapping, as are a pair of C(51–56) rings about a centre of symmetry; in these molecules, the pyridinyl rings are not involved in this type of interaction, and there is no extended linking of molecules. The two [Co(SCN)4]2 anions lie on symmetry elements, so that only half of each anion is unique. These anions have Co(II) centres with tetrahedral arrangements of NCS ligands. The NCS groups are almost linear but there is slight bending at the N-atom in each. There is disorder in a pair of the NCS ligands on Co(4), with alternative sites for the C and S atoms. 3.1.3. Description of the structure of [Mn(L)2] (3) An ORTEP diagram of 3 is presented in Fig. 3 with its atom numbering scheme and bond dimensions are listed in Tables 2 and 3. The manganese(II) ion is in an N4O2 coordination sphere [1,18]. Each of the two meridionally spanning ligands coordinates the metal ion as in complexes 1 and 2, with an ‘interligand angle’ of 85.65(3)°. However, the distortion of the MnN4O2 octahedron is rather larger than in 1 and 2, arising from the longer Mn–N and Mn– O distances; also the trans angles are much further from 180° and the intraligand bite angles are correspondingly more acute, lying in the range 70.89(4)–71.64(4)°. The bond lengths and angles in the hydrazone chain conform, as in 1 and 2, to the enolate form of the ligand. The hydrazone ligands of N(1) are linked in chains, as in compound 1, by the overlap of nearly parallel, neighbouring C(11–16) and C(21–26) rings. The pyridinyl ring and three atoms of the bridging chain of the second hydrazone ligand, viz. C(41–46), C(4), N(4) and N(3), overlap the
Fig. 2. View of the two independent cations and one of the anions of compound 2, indicating the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
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Fig. 3. View of a molecule of compound 3, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
group inverted about a centre of symmetry, and thus link molecules in pairs. 3.1.4. Description of the structure of [Mn(N3)2(LH)2] H2O (4) This structural study reveals that 4 consists of a mononuclear unit having a seven-coordinate Mn(II) ion in a pentagonal bipyramidal pattern, Fig. 4 and bond dimensions are listed in Tables 2 and 3. There are two hydrazone ligands in the molecule showing different denticities. The pentagonal plane is occupied by O1, N2, N22 of the first LH ligand
(I in Scheme 1) and O3 and N4 of the second (II in Scheme 1). The two axial positions are occupied by N51 and N61 of the two azido groups. The C1–O1 and C3–O3 bond lengths ˚ ] in the hydrazone ligands indicate that they [both 1.230(3) A coordinate the metal center in the keto form; the locating and refinement of the hydrogen atoms on both N1 and N3 confirm this and show these ligands to be neutral donors. Both the N–H groups are involved in hydrogen bonds, that of N1–H1 linking to a water molecule, whilst N3– H3 bonds intramolecularly to the uncoordinated pyridyl
Fig. 4. View of a molecule of compound 4 with the hydrogen-bonded water molecule, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
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Fig. 5. The hydrogen-bonded polymeric ladder-like structure in compound 4. Thermal ellipsoids are drawn at the 30% probability level.
nitrogen atom, giving the E isomer (with respect to the imine bond) of the hydrazone ligand extra stability. The water molecule is involved in further intermolecular hydrogen-bonding to N53 and N61 of symmetry-related azido ligands, forming a ladder-like structure (Fig. 5 and Table 4). 3.2. IR, UV–Vis and Electrochemical studies 3.2.1. Infrared spectra The IR spectra of all the complexes were analyzed in comparison with that of the free molecule, LH, in the region 4000–250 cm1. The IR spectrum of the hydrazone molecule contains a strong C–O absorption band at 1651– 1659 cm1 and an N–H absorption band at 3187– 3250 cm1. For complexes 1, 2 and 3, both these bands disappear on complexation and a new C–O absorption band appears at 1040–1089 cm1, which indicates that
Table 4 H-bond dimensions for 4 D–H A
D–H
A H
D A
\(D–H A)
N(1)–H(1) O(7) N(3)–H(3) N(42) (7)–H(71) N(610 ) O(7)–H(72) N(5300 )
0.86 0.86 0.82(4) 0.80(3)
1.92 1.96 2.04(4) 2.05(4)
2.742(3) 2.633(4) 2.859(3) 2.843(3)
160 134 176(4) 173(4)
Symmetry operations: 0 : 1 x, 1 y, 1 z; 00 : x, 1 y, z.
the hydrazone ligand has undergone deprotonation on complexation. For complex 4, the N–H absorption band appears at 3010 cm1 whereas the C–O band appears at 1663 cm1; both these bands indicate that in this case the ligand has not undergone the keto–enol tautomerisation and has coordinated the metal in its original keto form. These data provide evidence for the coordination of the ligand to the respective metal ions [Mn(II) and Co(III)] via two nitrogen atoms and one oxygen atom for complexes 1, 2 and 3. The infrared spectra of complexes 1–4, respectively, display IR absorption bands at 1603, 1604, 1593 and 1637 cm1 which can be assigned to the C@N stretching frequencies of the coordinated ligand [23], whereas for the free molecule, the same band is observed at 1663 cm1. The shift of this band on complexation towards lower wave number indicates coordination of the azomethine nitrogen to the metal center [24]. The ligand coordination is substantiated by two bands appearing at 460, 369 cm1 for 1, 480, 372 cm1 for 2, 444, 379 cm1 for 3, and 492, 375 cm1 for 4; these are mainly attributed to mM–N and mM–O, respectively. For 2 there is one strong band at 2065 cm1, which indicates the presence of coordinated SCN in monodentate fashion. For 4 there is one strong band at 2057 cm1 indicating the presence of coordinated azide ligand. The low energy pyridine ring in-plane and out-of-plane vibrations are observed in the spectrum of the LH molecule at 625 and 406 cm1, respectively, whereas the corresponding bands for the complexes are shifted to higher
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frequencies at 638–640 and 419–422 cm1, respectively, which are good indications of the coordination of the heterocyclic nitrogen atom [25]. 3.2.2. UV–Vis spectra UV–Vis spectra of 1, 2 and 3 were recorded at 300 K in HPLC grade acetonitrile solution. For 1, containing distorted octahedral Co(III) ion, the band at 586 nm appeared due to 1A1g ? 1T1g transition. The higher energy band 1 A1g ? 1T2g was obscured by ligand-to-metal charge transfer bands. For 2, two very broad d–d transition bands are observed at 625 and 574 nm, and several CT bands can be observed in the region 442–382 nm. For 3, with distorted octahedral Mn(II) coordination, we obtained two bands in the UV region. The band at 374 nm is considerably stronger than the band at 247 nm and these bands may be assigned to n ? p* (forbidden) and p ? p* transitions in the ligand. Complex 3 does not show any d–d transition [26]. Complex 4 was insoluble in all available common solvents and the UV–Vis spectrum of this compound in Nujol showed no d–d band, but a series of charge transfer bands, as for 3, corresponding to p ? p* transitions of the ligand, was observed.
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sponding to the oxidation of Co(II) to Co(III), is observed at 0.085 V. The separation between the two redox peaks is greater than 50 mV (DEp = 78 mV) and hence the system corresponds to an irreversible oxidation reduction couple. Complex 3 shows (Fig. 6) an oxidative response at 0.51 V versus SCE, which is assigned to the Mn(II) to Mn(III) oxidation, and a reductive response at 0.44 V versus SCE, assigned to Mn(III) to Mn(II) reduction. This oxidation is also reversible, characterized by a peak-to-peak separation (DEp) of 70 mV, which remains unchanged upon varying the scan rate. The anodic peak current (ipa) is again almost equal to the cathodic peak current (ipc), as expected for a reversible electron transfer process. 4. Conclusion In this paper, we have reported four new complexes of a hydrazone ligand with manganese and cobalt metal centers. In complexes 1, 2 and 3, the ligand acts a mononegative, tridentate NNO donor, whereas in 4 there is one neutral tridentate NNO and one neutral bidentate NO donor ligand. Acknowledgements
3.2.3. Cyclic voltammetry study Electrochemical properties of 1, 2 and 3 were studied in HPLC grade acetonitrile medium with tetrabutylammonium perchlorate as supporting electrolyte at a scan rate 50 mV s1. The voltammogram of 1 shows a peak at 0.15 V (versus SCE) which can be attributed to the reduction of Co(III) to Co(II). Another peak, at 0.12 V (versus SCE), was assigned to the oxidation of Co(II) to Co(III). This process is thus reversible, characterized by a peakto-peak separation (DEp) of 30 mV, which remains unchanged upon varying the scan rate. The anodic peak current (ipa) is almost equal to the cathodic peak current (ipc), as expected for a reversible electron transfer process. For 2, the first reductive response, corresponding to the reduction of Co(III) to Co(II), appears at 0.163 V (in the same region as for 1), whereas the second peak, corre-
Fig. 6. Representative cyclic voltammogram for complex 3.
We are grateful to DRDO New Delhi and AICTE New Delhi, India, for funding this research. Appendix A. Supplementary data CCDC 648267, CCDC 642868, CCDC 657006 and CCDC 651331 contain the crystallographic data for 1, 2, 3, 4. 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.2008.01.019. References [1] O. Pouralimardan, A.C. Chamayou, C. Janiak, H.H. Monfared, Inorg. Chim. Acta 360 (2007) 1599. [2] I.H.A. Suez, S.O. Pehkonen, M.R. Hoffmann, Sci. Technol. 28 (1994) 2080. [3] L.H. Terra, A.M.C. Areias, I. Gaubeur, M.E.V. Suez-Iha, Spectrosc. Lett. 32 (1999) 257. [4] S. Patai, The Chemistry of Carbon–Nitrogen Double Bond, Interscience, New York, 1970. [5] (a) J.C. Galiz, J.C. Rub, J. Edger, Nature 34 (1955) 176; (b) M.F. Iskander, S.E. Zayan, M.A. Khalifa, L. Elsayed, J. Inorg. Nucl. Chem. 36 (1974) 556. [6] (a) J.R. Marchent, D.S. Clothia, J. Med. Chem. 13 (1970) 335; (b) J.R. Dilworth, Coord. Chem. Rev. 21 (1976) 29. [7] S. Sridhar, M. Saravanan, A. Ramesh, Eur. J. Med. Chem. 36 (2001) 615. [8] B. Bottari, R. Maccari, F. Monforte, R. Ottana, E. Rotondo, M. Vigorita, Bioorg. Med. Chem. Lett. 10 (2000) 657. [9] B. Kaymakciogul, S. Rollas, Farmaco 57 (2002) 595. [10] E. Ainscough, A. Brodie, J. Ranford, M. Waters, Inorg. Chim. Acta 236 (1995) 83.
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[11] E. Ainscough, A. Brodie, J. Ranford, M. Waters, Inorg. Chim. Acta 267 (1998) 27. [12] J. Easmon, G. Puerstinger, T. Roth, H. Feibig, M. Jenny, W. Jaeger, G. Heinish, J. Hofmann, Int. J. Cancer 94 (2001) 89. [13] S. Ku¨cu¨kgu¨zel, S. Rollas, I. Ku¨cu¨kgu¨zel, M. Kiraz, Eur. J. Med. Chem. 34 (1999) 1093. [14] N.R. Sangeetha, S. Pal, S. Pal, Polyhedron 19 (2000) 2713. [15] W. Plass, A. Pohlmann, H.-P. Yozgatli, J. Inorg. Biochem. 80 (2000) 181. [16] R. Dinda, P. Sengupta, S. Ghosh, T.C.W. Mak, Inorg. Chem. 41 (2002) 1684. [17] R. Raveendran, S. Pal, Polyhedron 24 (2005) 57. [18] S. Naskar, S. Biswas, D. Mishra, B. Adhikary, L.R. Falvello, T. Soler, C.H. Schwalbe, S.K. Chattopadhyay, Inorg. Chim. Acta 357 (2004) 4257. [19] T.E. Khalil, L. Labib, M.F. Iskander, Polyhedron 13 (1994) 2569.
[20] G.M. Sheldrick, SHELX-97 – Programs for Crystal Structure Determination (SHELXS) and Refinement (SHELXL), University of Go¨ttingen, Germany, 1997. [21] B. Samanta, J. Chakraborty, S. Shit, S.R. Batten, P. Jensen, J.D. Masuda, S. Mitra, Inorg. Chim. Acta 360 (2007) 2471. [22] S. Sen, P. Talukder, S.K. Dey, S. Mitra, G. Rosair, D.L. Hughes, G.P.A. Yap, G. Pilet, V. Gramlich, T. Matsushita, Dalton Trans. (2006) 1758. [23] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor, J. Chem. Soc., Dalton Trans. (1984) 1349. [24] R.T. Conley, Infrared Spectroscopy, Allyn & Bacon, Boston, 1966. [25] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed., Wiley, New York, 1997, p. 23. [26] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd ed., Elsevier Science, 1984.
Inorganica Chimica Acta 361 (2008) 2692–2700 www.elsevier.com/locate/ica
Pseudohalide-induced structural variations in hydrazone-based metal complexes: Syntheses, electrochemical studies and structural aspects Sambuddha Banerjee a, Aurkie Ray a, Soma Sen a, Samiran Mitra a,*, David L. Hughes b, Ray J. Butcher c, Stuart R. Batten d, David R. Turner d a
Department of Chemistry, Jadavpur University, Raja S.C. Mallick Road, Kolkata, West Bengal 700 032, India b School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, UK c Department of Chemistry, Howard University, 2400 Sixth Street, NW, Washington, DC 200 59, USA d School of Chemistry, Monash University, 3800, Australia Received 30 August 2007; received in revised form 19 January 2008; accepted 23 January 2008 Available online 1 February 2008
Abstract The effects of pseudohalides on the modes of binding of a hydrazone ligand are investigated and we report here four new hydrazone complexes [Co(L)2] ClO4 (1), [Co(L)2]2[Co(SCN)4] (2), [Mn(L)2] (3) and [Mn(N3)2 (LH)2] H2O (4) [where LH = condensation product of pyridine-2-carboxaldehyde and benzhydrazide]. The hydrazone ligand is quite diverse in its chelating ability and can act as a neutral or mononegative ligand and as a bidentate or tridentate unit. In this paper, we report structures showing different denticities of the ligand having different charges. The complexes are characterized by IR, UV–Vis spectroscopy and cyclic voltammetric studies and the structures are conclusively established by single crystal X-ray diffraction study. Of the complexes, 1 and 3 are purely metal hydrazone complexes whereas 2 and 4 incorporate coordinated pseudohalides in their crystal structures, and 4 shows different coordination modes of the hydrazone ligand. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Hydrazone complexes; Synthesis; X-ray crystal structures; Electrochemical studies; Spectral studies; H-bonded polynuclear structure
1. Introduction Hydrazone ligands have similarities in their donor properties with unsymmetrical salen (condensation product of salicylaldehyde and 1,2-diaminoethane) type ligands. Recently it was reported that such ligands, like non-symmetrical salens, can act as effective catalysts towards alkene epoxidation [1]. Due to the short N–N bond length, the benzoyl hydrazone ligands act mostly as tridentate moieties though they have the potential to act as bridging tetradentate ligands. In analytical chemistry also, hydrazone ligands find wide applications as transition metal binders *
Corresponding author. Tel.: +91 33 2668 2017; fax: +91 33 2414 6414. E-mail address: [email protected] (S. Mitra).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.01.019
[2,3]. Studies have also shown that the azomethine N, which has a lone pair of electrons in an sp2 hybridized orbital, has considerable biological importance [4]. Aroylhydrazone complexes of transition metal ions are known to provide useful models for elucidation of the mechanism of enzyme inhibition by hydrazine derivatives [5] and for their possible pharmacological applications [6]. Additionally, hydrazone complexes have been the subject of studies over many years for their anti-microbial and anti-tumor activities [7–13]. Benzoylhydrazone complexes of copper [14], vanadium [15,16], ruthenium [17] and manganese [18] can be found in the literature. The various modes of coordination of the hydrazone ligand, LH, the condensation product of pyridine-2-carboxaldehyde and benzhydrazide, are shown in Scheme 1.
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Scheme 1. Different modes of coordination of the ligand LH.
In this work, our aim was to exploit the versatility of coordination behaviour of LH by incorporating pseudohalides (azide and thiocyanate) into manganese and cobalt precursor complexes. Herein we report four new hydrazone complexes [Co(L)2] ClO4 (1), [Co(L)2]2[Co(SCN)4] (2), [Mn(L)2] (3) and [Mn(N3)2(LH)2] H2O (4), where the same hydrazone ligand binds, depending upon the metals and pseudohalides used, in tridentate mononegative NNO donor mode for 1, 2 and 3, and bidentate neutral NO and tridentate neutral NNO donor modes in 4. In this paper, we describe the IR, UV–Vis spectra, electrochemical studies and X-ray crystal structures of these four complexes.
were recorded on a Perkin–Elmer RX 1 FT-IR spectrophotometer with a KBr disc. The electronic spectra were recorded, in acetonitrile, on a Perkin–Elmer Lambda 40 (UV–Vis) spectrophotometer. Electrochemical studies of complexes 1–3 were performed using acetonitrile solution on a Versastat-Potentiostat II electrochemical analyser against a standard calomel electrode as reference and platinum as the working electrode. Caution! Perchlorate salts are potentially explosive and should be prepared in small quantities and handled with much care.
2. Experimental
2.3.1. Synthesis of the hydrazone, [C6H5C(O)NHN@C(H)C5H4N] (LH) The pro-ligand was synthesized by condensing benzhydrazide with pyridine-2-carboxaldehyde following a method reported elsewhere [19]. The resulting light yellow liquid was used without purification. Different modes of coordination of the ligand LH is presented in Scheme 1.
2.1. Materials Benzhydrazide, pyridine-2-carboxaldehyde, manganous chloride hexahydrate, cobalt(II) perchlorate hexahydrate, sodium azide, and sodium thiocyanate (Fluka) were used as received without further purification. All the solvents were of reagent grade. 2.2. Physical measurements Elemental analyses were carried out using a Perkin– Elmer 2400 II elemental analyzer. The infrared spectra
2.3. Synthesis
2.3.2. Synthesis of [Co(L)2] ClO4 (1) To a methanolic solution (30 ml) of 1 mmol of Co(II) perchlorate hexahydrate (0.365 g), a solution of the hydrazone (2 mmol) was added and the solution was stirred at room temperature for 45 min. The solution yielded crystals suitable for X-ray diffraction after 3 days. Yield: 78%.
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Anal. Calc. for C26H20CoN6O6Cl: C, 51.45; H, 3.06; N, 17.15. Found: C, 51.41; H, 3.01; N, 17.12%. 2.3.3. Synthesis of [Co(L)2]2[Co(SCN)4] (2) A methanolic solution (30 ml) of 1 (1 mmol, 0.606 g) was made slightly alkaline and refluxed on a water bath for half an hour. The precipitate thus formed was discarded. To the filtrate, 2 mmol (0.160 g) of sodium thiocyanate, dissolved in minimum amount of water, was added with constant stirring. Single crystals suitable for X-ray diffraction were obtained after 3 weeks on keeping the mother liquor in a refrigerator. Yield: 82%. Anal. Calc. for C56H40Co3N16O4S4: C, 51.45; H, 3.06; N, 17.15. Found: C, 51.42; H, 3.03; N, 17.14%.
2.3.4. Synthesis of [Mn(L)2] (3) To a methanolic solution (30 ml) of 1 mmol of manganous chloride (0.197 g), the liquid hydrazone LH (2 mmol) was added with constant stirring. The solution was stirred at room temperature for 30 min and then kept in a refrigerator. Crystals suitable for X-ray diffraction appeared after 7 days. Yield: 80%. Anal. Calc. for C26H20MnN6O2: C, 61.97; H, 3.97; N, 16.68. Found: C, 61.94; H, 3.95; N, 16.66%. 2.3.5. Synthesis of [Mn(N3)2(LH)2] H2O (4) To a methanolic solution (30 ml) of 3 (1 mmol, 0.503 g), sodium azide (2 mmol, 0.130 g), in minimum amount of water, was added. The solution was refluxed for 30 min
Table 1 Crystallographic and refinement data of complexes 1, 2, 3 and 4 Compound
1
2
3
4
Elemental formula
C26H20CoN6O2, ClO4 606.86 monoclinic P21/n (equiv. to No. 14) 10.5363(2) 17.6654(4) 14.0139(4) 90 101.923(3) 90 2552.11(10) 4 1.579 0.833 1240 Bruker SMARTCCD
2(C26H20CoN6O2), C4CoN4S4 1306.07 monoclinic C2/c (No. 15)
C26H20MnN6O2
C26H22MnN12O2, H2O
503.42 monoclinic P21/n (equiv. to No. 14)
607.5 monoclinic P21/c (No. 14)
32.1665(14) 22.3903(13) 16.2540(8) 90 92.926(2) 90 11691.2(10) 8 1.484 1.045 5320 Bruker X8 Apex II CCD
173(2) 0.65 0.35 0.15 4.6–30.7 21 680 7124 (0.036)
123(1) 0.12 0.10 0.03 2.1–0.0 15 423 5432 (0.061)
9.8375(13) 23.9336(16) 10.0407(3) 90 103.736(1) 90 2296.4(3) 4 1.456 0.612 1036 Oxford Diffraction Xcalibur-3 CCD 140(1) 0.68 0.32 0.12 3.7–30.0 35 020 6662 (0.049)
9.3296(6) 33.887(2) 9.7560(3) 90 117.711(4) 90 2729.8(3) 4 1.478 0.537 1252 Oxford Diffraction Xcalibur-3 CCD 140(1) 0.48 0.38 0.12 3.5–27.5 42 166 6245 (0.061)
0.965 R1 = 0.070, wR2 = 0.099
1.068 R1 = 0.114, wR2 = 0.221
1.040 R1 = 0.061, wR2 = 0.075
1.181 R1 = 0.082, wR2 = 0.100
Formula weight Crystal system Space group ˚) a (A ˚ b (A) ˚) c (A a (°) b (°) c (°) ˚ 3) V (A Z D (Mg m3) l (mm1) F(0 0 0) Diffractometer used T (K) Crystal size (mm) h Range for data collection (°) No. of reflections collected No. of independent reflections (Rint) Goodness-of-fit R indices (all data)
Table 2 ˚ ) about the metals in complexes 1, 2, 3 and 4 Selected bond lengths (A Complex 1
Complex 2
Complex 3
Molecule A Co–O(1) Co–N(2) Co–N(22) Co–O(3) Co–N(4) Co–N(42)
1.9052(12) 1.8421(16) 1.9271(15) 1.8981(13) 1.8480(16) 1.9192(15)
Co(1)–O(1) Co(1)–N(2) Co(1)–N(22) Co(1)–O(3) Co(1)–N(4) Co(1)–N(42) Co(3)–N(91) Co(3)–N(92)
Complex 4
Molecule B 1.908(7) 1.864(9) 1.939(9) 1.912(6) 1.840(8) 1.929(8) 2.105(18) 1.951(16)
Co(2)–O(5) Co(2)–N(6) Co(2)–N(62) Co(2)–O(7) Co(2)–N(8) Co(2)–N(82) Co(4)–N(93) Co(4)–N(94)
1.906(7) 1.846(10) 1.939(10) 1.899(7) 1.871(9) 1.924(10) 1.946(13) 1.88(2)
Mn–O(1) Mn–N(2) Mn–N(22) Mn–O(3) Mn–N(4) Mn–N(42)
2.1610(10) 2.1874(11) 2.3577(11) 2.1382(10) 2.1950(11) 2.3191(12)
Mn–O(1) Mn–N(2) Mn–N(22) Mn–O(3) Mn–N(4) Mn–N(51) Mn –N(61)
2.3193(17) 2.303(2) 2.382(2) 2.2706(17) 2.376(2) 2.204(2) 2.210(2)
N(2)–Mn–O(1) N(22)–Mn–O(1) N(2)–Mn–N(22) N(2)–Mn–N(4) O(3)–Mn–N(4) N(51)–Mn–N(61) O(1)–Mn–N(2) O(1)–Mn–N(22) N(2)–Mn–N(22) N(2)–Mn–N(4) O(3)–Mn–N(4) O(3)–Mn–N(42) N(4)–Mn–N(42)
and then kept in a refrigerator. Crystals suitable for X-ray diffraction appeared after seven days. Yield: 65%. Anal. Calc. for C26H24MnN12O3: C, 51.36; H, 3.62; N, 27.65. Found: C, 51.32; H, 3.59; N, 27.61%. 2.4. X-ray crystallography The crystal structure analyses of the complexes 1–4 were performed by several of the authors, in different laboratories; the equipment used for each sample is listed with the crystallographic data and the refinement results in Table 1. The structures of all the complexes were determined by direct method procedures in SHELXS-97, and refined in SHELXL-97 [20]. The hydrogen atoms bonded to water oxygens in compound 4 were freely refined; all other hydrogen atoms were included in idealized positions and set to ride on the parent carbon and nitrogen atoms. 3. Results and discussion 3.1. Description of the crystal structures 3.1.1. Description of the structure of [Co(L)2] ClO4 (1) Relevant bond length and bond angle data for complex 1 are listed, together with corresponding dimensions in the other complexes, in Tables 2 and 3 respectively. Complex 1, shown in Fig. 1, consists of a Co(III) center surrounded by two tridentate hydrazone ligands, with the metal in a distorted octahedral environment [21]. The dimensions in the two ligands are very similar and indicate that the ligands adopt the enolate form (IV in Scheme 1).
N(2)–Co(1)–O(1) O(1)–Co(1)–N(22) N(2)–Co(1)–N(22) N(4)–Co(1)–N(2) N(4)–Co(1)–O(3) O(3)–Co(1)–N(42) N(4)–Co(1)–N(42) N(910 )–Co(3)–N(91) N(92)–Co(3)–N(91) N(920 )–Co(3)–N(91) N(920 )–Co(3)–N(92)
81.1(4) 164.1(4) 83.0(5) 177.7(5) 81.8(3) 164.7(4) 82.9(4) 107.3(8) 110.3(6) 107.4(7) 113.9(11)
N(6)–Co(2)–O(5) O(5)–Co(2)–N(62) N(6)–Co(2)–N(62) N(6)–Co(2)–N(8) N(8)–Co(2)–O(7) O(7)–Co(2)–N(82) N(8)–Co(2)–N(82) N(93)–Co(4)–N(9300 ) N(94)–Co(4)–N(93) N(9400 )–Co(4)–N(93) N(9400 )–Co(4)–N(94)
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N(2)–Co–O(1) O(1)–Co–N(22) N(2)–Co–N(22) N(2)–Co–N(4) N(4)–Co–O(3) O(3)–Co–N(42) N(4)–Co–N(42)
82.36(6) 165.79(6) 83.47(7) 178.20(7) 82.42(6) 165.69(6) 83.55(7)
Molecule A
Complex 2 Complex 1
Table 3 Selected bond angels (°) about the metal atoms in complexes 1, 2, 3 and 4
Molecule B
81.8(4) 164.4(4) 82.7(5) 178.1(5) 81.7(4) 164.4(4) 82.7(5) 102.3(7) 112.9(10) 108.9(7) 111(2)
71.58(4) 142.47(4) 70.89(4) 157.16(4) 71.64(4) 142.13(4) 71.27(4)
Complex 4 Complex 3
66.58(6) 134.83(6) 68.49(7) 142.66(7) 68.72(6) 179.54(9)
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Fig. 1. View of a cation and anion of compound 1, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
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The two central coordinating bonds, Co–N2 and Co–N4, are shortest, providing a z-in type of distortion in the octahedral arrangement. The N–Co–N trans angle is close to the ideal value of 180°, but the other trans angles are constrained within the meridional ligands, and are ca 166°. The intraligand bite angles lie in the 82.36(6)–83.55(6)° range. The angle between the normals to the mean-plane of the O–C–N–N–C–C–N chains in the hydrazone ligands, the ‘interligand angle’, is 86.1°. Each hydrazone ligand acts as a tridentate mononegative unit, and there is one uncoordinated perchlorate anion present to neutralize the 1+ charge on the Co(III) complex cation. The phenyl ring of C(11–16) is overlapped by an almost parallel pyridyl ring C(21–26) of a neighbouring molecule (related by n-glide-plane symmetry), and chains through such overlaps are thus formed through the crystal. Pairs of the other ligands are formed about centres of symmetry, but the overlap here, of the N(3) C(41) section of the central chain, is less pronounced. 3.1.2. Description of the structure of [Co(L)2]2[Co(SCN)4] (2) An ORTEP diagram of 2 is presented in Fig. 2 and bond dimensions are listed in Tables 2 and 3. A complex with similar structure with the same metal but with a completely different ligand system has been reported by our group earlier [22]. In 2, there are four different Co atoms, two independent [Co(III)(L)2]2+ cation complexes, and two separate [Co(II)(SCN)4]2 anions. In the cations, both the Co(III) centers are six-coordinate, having distorted octahedral geometry with two hydrazone ligands disposed in merarrangement, similar to compound 1; the ‘interligand angles’, here, are 89.4° and 89.0°. As in 1, there is overlap
of neighbouring phenyl groups, e.g. the ring of C(11–16) and C(710 –760 ) are close to parallel and overlapping, as are a pair of C(51–56) rings about a centre of symmetry; in these molecules, the pyridinyl rings are not involved in this type of interaction, and there is no extended linking of molecules. The two [Co(SCN)4]2 anions lie on symmetry elements, so that only half of each anion is unique. These anions have Co(II) centres with tetrahedral arrangements of NCS ligands. The NCS groups are almost linear but there is slight bending at the N-atom in each. There is disorder in a pair of the NCS ligands on Co(4), with alternative sites for the C and S atoms. 3.1.3. Description of the structure of [Mn(L)2] (3) An ORTEP diagram of 3 is presented in Fig. 3 with its atom numbering scheme and bond dimensions are listed in Tables 2 and 3. The manganese(II) ion is in an N4O2 coordination sphere [1,18]. Each of the two meridionally spanning ligands coordinates the metal ion as in complexes 1 and 2, with an ‘interligand angle’ of 85.65(3)°. However, the distortion of the MnN4O2 octahedron is rather larger than in 1 and 2, arising from the longer Mn–N and Mn– O distances; also the trans angles are much further from 180° and the intraligand bite angles are correspondingly more acute, lying in the range 70.89(4)–71.64(4)°. The bond lengths and angles in the hydrazone chain conform, as in 1 and 2, to the enolate form of the ligand. The hydrazone ligands of N(1) are linked in chains, as in compound 1, by the overlap of nearly parallel, neighbouring C(11–16) and C(21–26) rings. The pyridinyl ring and three atoms of the bridging chain of the second hydrazone ligand, viz. C(41–46), C(4), N(4) and N(3), overlap the
Fig. 2. View of the two independent cations and one of the anions of compound 2, indicating the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.
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Fig. 3. View of a molecule of compound 3, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
group inverted about a centre of symmetry, and thus link molecules in pairs. 3.1.4. Description of the structure of [Mn(N3)2(LH)2] H2O (4) This structural study reveals that 4 consists of a mononuclear unit having a seven-coordinate Mn(II) ion in a pentagonal bipyramidal pattern, Fig. 4 and bond dimensions are listed in Tables 2 and 3. There are two hydrazone ligands in the molecule showing different denticities. The pentagonal plane is occupied by O1, N2, N22 of the first LH ligand
(I in Scheme 1) and O3 and N4 of the second (II in Scheme 1). The two axial positions are occupied by N51 and N61 of the two azido groups. The C1–O1 and C3–O3 bond lengths ˚ ] in the hydrazone ligands indicate that they [both 1.230(3) A coordinate the metal center in the keto form; the locating and refinement of the hydrogen atoms on both N1 and N3 confirm this and show these ligands to be neutral donors. Both the N–H groups are involved in hydrogen bonds, that of N1–H1 linking to a water molecule, whilst N3– H3 bonds intramolecularly to the uncoordinated pyridyl
Fig. 4. View of a molecule of compound 4 with the hydrogen-bonded water molecule, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
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Fig. 5. The hydrogen-bonded polymeric ladder-like structure in compound 4. Thermal ellipsoids are drawn at the 30% probability level.
nitrogen atom, giving the E isomer (with respect to the imine bond) of the hydrazone ligand extra stability. The water molecule is involved in further intermolecular hydrogen-bonding to N53 and N61 of symmetry-related azido ligands, forming a ladder-like structure (Fig. 5 and Table 4). 3.2. IR, UV–Vis and Electrochemical studies 3.2.1. Infrared spectra The IR spectra of all the complexes were analyzed in comparison with that of the free molecule, LH, in the region 4000–250 cm1. The IR spectrum of the hydrazone molecule contains a strong C–O absorption band at 1651– 1659 cm1 and an N–H absorption band at 3187– 3250 cm1. For complexes 1, 2 and 3, both these bands disappear on complexation and a new C–O absorption band appears at 1040–1089 cm1, which indicates that
Table 4 H-bond dimensions for 4 D–H A
D–H
A H
D A
\(D–H A)
N(1)–H(1) O(7) N(3)–H(3) N(42) (7)–H(71) N(610 ) O(7)–H(72) N(5300 )
0.86 0.86 0.82(4) 0.80(3)
1.92 1.96 2.04(4) 2.05(4)
2.742(3) 2.633(4) 2.859(3) 2.843(3)
160 134 176(4) 173(4)
Symmetry operations: 0 : 1 x, 1 y, 1 z; 00 : x, 1 y, z.
the hydrazone ligand has undergone deprotonation on complexation. For complex 4, the N–H absorption band appears at 3010 cm1 whereas the C–O band appears at 1663 cm1; both these bands indicate that in this case the ligand has not undergone the keto–enol tautomerisation and has coordinated the metal in its original keto form. These data provide evidence for the coordination of the ligand to the respective metal ions [Mn(II) and Co(III)] via two nitrogen atoms and one oxygen atom for complexes 1, 2 and 3. The infrared spectra of complexes 1–4, respectively, display IR absorption bands at 1603, 1604, 1593 and 1637 cm1 which can be assigned to the C@N stretching frequencies of the coordinated ligand [23], whereas for the free molecule, the same band is observed at 1663 cm1. The shift of this band on complexation towards lower wave number indicates coordination of the azomethine nitrogen to the metal center [24]. The ligand coordination is substantiated by two bands appearing at 460, 369 cm1 for 1, 480, 372 cm1 for 2, 444, 379 cm1 for 3, and 492, 375 cm1 for 4; these are mainly attributed to mM–N and mM–O, respectively. For 2 there is one strong band at 2065 cm1, which indicates the presence of coordinated SCN in monodentate fashion. For 4 there is one strong band at 2057 cm1 indicating the presence of coordinated azide ligand. The low energy pyridine ring in-plane and out-of-plane vibrations are observed in the spectrum of the LH molecule at 625 and 406 cm1, respectively, whereas the corresponding bands for the complexes are shifted to higher
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frequencies at 638–640 and 419–422 cm1, respectively, which are good indications of the coordination of the heterocyclic nitrogen atom [25]. 3.2.2. UV–Vis spectra UV–Vis spectra of 1, 2 and 3 were recorded at 300 K in HPLC grade acetonitrile solution. For 1, containing distorted octahedral Co(III) ion, the band at 586 nm appeared due to 1A1g ? 1T1g transition. The higher energy band 1 A1g ? 1T2g was obscured by ligand-to-metal charge transfer bands. For 2, two very broad d–d transition bands are observed at 625 and 574 nm, and several CT bands can be observed in the region 442–382 nm. For 3, with distorted octahedral Mn(II) coordination, we obtained two bands in the UV region. The band at 374 nm is considerably stronger than the band at 247 nm and these bands may be assigned to n ? p* (forbidden) and p ? p* transitions in the ligand. Complex 3 does not show any d–d transition [26]. Complex 4 was insoluble in all available common solvents and the UV–Vis spectrum of this compound in Nujol showed no d–d band, but a series of charge transfer bands, as for 3, corresponding to p ? p* transitions of the ligand, was observed.
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sponding to the oxidation of Co(II) to Co(III), is observed at 0.085 V. The separation between the two redox peaks is greater than 50 mV (DEp = 78 mV) and hence the system corresponds to an irreversible oxidation reduction couple. Complex 3 shows (Fig. 6) an oxidative response at 0.51 V versus SCE, which is assigned to the Mn(II) to Mn(III) oxidation, and a reductive response at 0.44 V versus SCE, assigned to Mn(III) to Mn(II) reduction. This oxidation is also reversible, characterized by a peak-to-peak separation (DEp) of 70 mV, which remains unchanged upon varying the scan rate. The anodic peak current (ipa) is again almost equal to the cathodic peak current (ipc), as expected for a reversible electron transfer process. 4. Conclusion In this paper, we have reported four new complexes of a hydrazone ligand with manganese and cobalt metal centers. In complexes 1, 2 and 3, the ligand acts a mononegative, tridentate NNO donor, whereas in 4 there is one neutral tridentate NNO and one neutral bidentate NO donor ligand. Acknowledgements
3.2.3. Cyclic voltammetry study Electrochemical properties of 1, 2 and 3 were studied in HPLC grade acetonitrile medium with tetrabutylammonium perchlorate as supporting electrolyte at a scan rate 50 mV s1. The voltammogram of 1 shows a peak at 0.15 V (versus SCE) which can be attributed to the reduction of Co(III) to Co(II). Another peak, at 0.12 V (versus SCE), was assigned to the oxidation of Co(II) to Co(III). This process is thus reversible, characterized by a peakto-peak separation (DEp) of 30 mV, which remains unchanged upon varying the scan rate. The anodic peak current (ipa) is almost equal to the cathodic peak current (ipc), as expected for a reversible electron transfer process. For 2, the first reductive response, corresponding to the reduction of Co(III) to Co(II), appears at 0.163 V (in the same region as for 1), whereas the second peak, corre-
Fig. 6. Representative cyclic voltammogram for complex 3.
We are grateful to DRDO New Delhi and AICTE New Delhi, India, for funding this research. Appendix A. Supplementary data CCDC 648267, CCDC 642868, CCDC 657006 and CCDC 651331 contain the crystallographic data for 1, 2, 3, 4. 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.2008.01.019. References [1] O. Pouralimardan, A.C. Chamayou, C. Janiak, H.H. Monfared, Inorg. Chim. Acta 360 (2007) 1599. [2] I.H.A. Suez, S.O. Pehkonen, M.R. Hoffmann, Sci. Technol. 28 (1994) 2080. [3] L.H. Terra, A.M.C. Areias, I. Gaubeur, M.E.V. Suez-Iha, Spectrosc. Lett. 32 (1999) 257. [4] S. Patai, The Chemistry of Carbon–Nitrogen Double Bond, Interscience, New York, 1970. [5] (a) J.C. Galiz, J.C. Rub, J. Edger, Nature 34 (1955) 176; (b) M.F. Iskander, S.E. Zayan, M.A. Khalifa, L. Elsayed, J. Inorg. Nucl. Chem. 36 (1974) 556. [6] (a) J.R. Marchent, D.S. Clothia, J. Med. Chem. 13 (1970) 335; (b) J.R. Dilworth, Coord. Chem. Rev. 21 (1976) 29. [7] S. Sridhar, M. Saravanan, A. Ramesh, Eur. J. Med. Chem. 36 (2001) 615. [8] B. Bottari, R. Maccari, F. Monforte, R. Ottana, E. Rotondo, M. Vigorita, Bioorg. Med. Chem. Lett. 10 (2000) 657. [9] B. Kaymakciogul, S. Rollas, Farmaco 57 (2002) 595. [10] E. Ainscough, A. Brodie, J. Ranford, M. Waters, Inorg. Chim. Acta 236 (1995) 83.
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