A novel Cu(II) dimer containing oxime-hydrazone Schiff base ligands with an unusual mode of coordination: Study of magnetic, autoreduction and solution properties

Polyhedron 53 (2013) 48–55

Contents lists available at SciVerse ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

A novel Cu(II) dimer containing oxime-hydrazone Schiff base ligands with an unusual mode of coordination: Study of magnetic, autoreduction and solution properties Manas Sutradhar a,⇑, Tannistha Roy Barman a, Julia Klanke a, Michael G.B. Drew b, Eva Rentschler a a b

Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany School of Chemistry, The University of Reading, PO Box 224 Whiteknights, Reading RG6 6AD, UK

a r t i c l e

i n f o

Article history: Received 2 December 2012 Accepted 27 December 2012 Available online 30 January 2013 Keywords: Cu(II) dimer Oxime-hydrazone Schiff base Autoreduction X-ray diffraction Magnetic properties EPR

a b s t r a c t Synthesis of a new hydrazone based Schiff base ligand, 3-methylpyrazole-5-carbohydrazone of 2,3butanedione monoxime (HL) is reported. The reaction of Cu(ClO4)26H2O with HL in any ratio in ethanol affords the dinuclear complex [CuL(EtOH)]2(ClO4)22H2O (1). An unusual coordination mode of the ligand was observed, in which the ligand forms stable six and five membered chelate rings around the metal centres without enolization of the carbonyl group of the hydrazone moiety. The same coordination behavior of the ligand was observed in its cobalt(III) complex. The reaction of CoCl26H2O in methanol with the ligand HL affords the mononuclear complex [CoL2]Cl (2). Both 1 and 2 were characterized by X-ray crystallography and various physicochemical techniques (elemental analyses, UV–Vis, IR, EPR spectroscopy etc.). In complex 1, the copper centres are bridged through the oxime N–O groups forming a dimer containing a crystallographic centre of symmetry. 1 is found to undergo an autoreduction transformation in solvents such as dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO). The EPR spectrum of 1 in DMF shows the partial dissociation of the dinuclear complex into a mononuclear species. Magnetic studies of 1 show that the two Cu(II) centres are strongly antiferromagnetically coupled via the bridging N–O groups. The structure of 2 is a monomer with crystallographic C2 symmetry in which the metal is bonded to two tridentate ligand anions, L, in mer configurations to give a distorted octahedral geometry. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Metal complexes with hydrazone based Schiff base ligands have been reported in the literature for a long time [1] and the number of reported mononuclear complexes [2–8] is much higher than those with higher nuclearities [9–12]. This is due to the fact that the stable tridentate coordination mode of the majority of ligands prefers to form mononuclear complexes with two ligands in an octahedral coordination. However the use of 2,3 diacetyl monoxime in the synthesis of a hydrazone Schiff base ligand offers the strong possibility of forming complexes of higher nuclearity via oximato bridges [13]. Several metal complexes with extended bridges have been designed in order to obtain detailed information about the magnetic exchange mechanism involved and to identify the factors that influence their magnetic properties. Dinuclear copper complexes are known to be involved in very important biological processes [14]. Oximate ligands have been used in the syntheses of homometallic [15–21] and heterometallic [15,16,19,22–25] polynuclear clusters and coordination polymers ⇑ Corresponding author. E-mail address: [email protected] (M. Sutradhar). 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.12.039

[26] with interesting properties such as single-molecule magnetism [18–20,27] and single-chain magnetism [26,28]. Dimeric complexes of Cu(II) containing oximato bridges are numerous [23,29–42] and their structural and magnetic properties are well known. Two types of geometric models, called A and B, for dimeric oximato bridged Cu(II) complexes have been reported in the literature (Scheme 1). Geometric model A contains 2 five-membered and 3 six-membered rings around the Cu(II) centres, while geometric model B contains 4 five-membered and 1 six-membered ring. The magnetic and other physicochemical properties are significantly different in the two models [42]. In this work a new hydrazone based Schiff base ligand, 3methylpyrazole-5-carbohydrazone of 2,3-butanedione monoxime (HL), and its Cu(II) complex (1) are reported. An interesting and unusual mode of coordination by the ligand was observed in its Cu(II) complex. The ligand forms stable six and five membered chelate rings around the metal centre without enolization of the carbonyl group of the hydrazone moiety. To establish the mode of coordination we also synthesized its cobalt(III) complex (2). Both copper (1) and cobalt (2) complexes were synthesized and characterized by X-ray crystallography and various physicochemical

49

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55

5

X N

N O

X

6 Cu

6

Cu

6

O

N

5

N

5

N

N

Cu

6

Cu

5

O

O

N

5

5 N

X

X

(A)

(B)

Scheme 1. Geometric models, A and B, of oximato bridged dimeric Cu(II) complexes.

techniques (Elemental analyses, UV–Vis, IR, NMR, EPR etc. spectroscopy). It is to be noted that the copper complex is a dimer with a bridging oxime group while the cobalt complex is a monomer. In solution the Cu(II) complex (1) shows partial decomposition and autoreduction properties. Detailed spectral studies of the Cu(II) dimer are reported along with the autoreduction reaction in solution. The autoreduction properties of 1 represent a new finding for Cu(II) dimers with the model A structure.

2. Experimental 2.1. Materials 5-Methyl pyrazole-3-carbohydrazide was prepared as described in the literature [43]. All other chemicals were reagent grade, obtained from commercial sources and used without further purification. Spectroscopic grade solvents were used for the spectral and electrochemical measurements.

2.2. Physical measurements Elemental analyses (C, H and N) were performed with a Foss Heraeus Vario EL elemental analyzer. Molecular weights and formulas were calculated without solvent molecules unless explicitly stated. EPR spectra were measured with a Magnettech Miniscope MS200 benchtop CW EPR spectrometer (m = 9.335 GHz), with a variable-temperature cooling/heating finger around 110 K. The experiments were carried out using a Varian variable-temperature accessory which utilizes an open gas-flow system and uses liquid nitrogen as a coolant. Liquid nitrogen was used to cool the sample. The calibrant was Mn(II) (g = 2.0267). The EPR parameters were obtained from spectral simulations by using the program package Easyspin 3.3.1 [44]. NMR spectra were recorded by using a Bruker DRX 400 spectrometer. Infrared spectra were recorded in the 400– 4000 cm1 range with a Jasco FT/IR-4200 spectrometer. The measurements were carried out by using the pellet technique with KBr as an embedding medium. Electronic spectra of the complexes were recorded on a Jasco V-570 UV–IS–NIR spectrophotometer. Magnetic susceptibility data of 1 were collected with a SQUID magnetometer (MPMS-7 Quantum Design). Experimental susceptibility data were corrected for the underlying diamagnetism using Pascal’s constants. The temperature dependent magnetic contribution of the holder was experimentally determined and subtracted from the measured susceptibility data. The routine JulX was used for spin Hamiltonian simulations of the data [45].

2.3. X-ray crystallographic data collection and refinement of the structures Single crystals of 1 and 2 were coated with perfluoropolyether, attached to a glass fiber, and mounted on a SMART APEX II CCD diffractometer equipped with a nitrogen cold stream operating at 171(2) K. Graphite monochromated Mo Ka radiation (k = 0.71069 Å) from a fine-focus sealed tube was used throughout. Cell constants were obtained from a least-squares fit of the diffraction angles of several thousand strong reflections. Data reduction was carried out with APEX2 v2.0 [46]. The crystals of 1 were twinned and two domains, with some reflections overlapping, were identified and used in subsequent calculations. SIR-97 [47] was used for the structure solutions. All non-hydrogen atoms were refined anisotropically. Hydrogens bonded to carbon and nitrogen were positioned in geometric positions and given thermal parameters equivalent to 1.2 times (or 1.5 times for methyl groups) those of the atoms to which they were bonded. Both structures were refined on F2 with SHELXL-97 [48]. In 1 reflection data for the two domains were refined to a ratio of 57:43. It proved necessary to use isor constraints for some perchlorate oxygen atoms in 1. The relatively high R value for 1 is due to the twinning of the crystal. 2.4. Synthesis 2.4.1. Synthesis of the ligand (HL) The ligand diacetylemonoxime 5-methyl pyrazole-3-carbohydrazone (HL) was synthesized by refluxing an equimolar mixture of diacetyl monooxime and 5-methyl pyrazole-3-carbohydrazide in ethanolic medium following a reported method [5]. Yield: 72%. Anal. Calc. for C9H13N5O2: C, 48.42; H, 5.87; N, 31.37. Found: C, 48.37; H, 5.92; N, 31.28%. IR (KBr) max/cm1: 3315 (OH), 3195, 3118 (NH), 2877 (CH3) 1668 (C@O), 1539 (C@N), 1225 (N–N). 1H NMR H (400 MHz; DMSO d6; Me4Si): 13.06 (1H, s, pyrazole NH), 11.56 (1H, s, hydrazide NH), 10.12 (1H, s, oxime OH), 6.47 (1H, s, pyrazole CH), 2.47 (3H, s, CH3), 2.28 (3H, s, pyrazole CH3), 1.97 (3H, s, CH3). 2.4.2. Synthesis of [CuL(EtOH)]2(ClO4)2.2H2O (1) To an ethanolic suspension (30 ml) of the ligand HL (0.223 g, 1 mmol), an ethanolic solution of copper perchlorate hexahydrate (0.370 g, 1 mmol) was added and the mixture was stirred at room temperature for 5 min. The dark green solution was filtered and kept in a refrigerator, and after 1 day dark green block twin crystals suitable for X-ray diffraction were obtained. The crystals were isolated by filtration, washed with ethanol and were dried in the open air. Yield: 52%. Anal. Calc. for C22H34N10O14Cl2Cu2: C, 30.70; H, 3.98;

50

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55

N, 16.28. Found: C, 30.62; H, 4.03; N, 16.18%. IR (KBr) max/cm1: 3442 (OH), 3198 (NH), 2913 (CH3), 1655 (C@O), 1565, 1506 (C@N), 1274 (N–N). UV–Vis max (DMSO, nm (e, L M1 cm1)): 686 (315), 380 (15,471), 275 (32,136). Caution!!! Perchlorate salts are potentially explosive could detonate upon heating. This complex should be handled with care and only in small amounts.

the positions of the donor atoms in the ligand skeleton, one can certainly envisage the formation of multinuclear metal complexes as shown in Scheme 2. It is well established in the literature that the carbonyl oxygen of the hydradrazone moiety of the ligand usually undergoes enolization in the presence of metal ions in solution [5,6,49,50]. However, in the present case such enolization did not take place; instead an unusual mode of coordination is observed. The carbonyl oxygen O(15) did not take part in complexation and the ligand coordinates via a stable N, N, N coordination around the metal centre. Three possible modes of coordination (I, II and III) of HL are presented in Scheme 2. Common to all three possible modes of coordination is the five membered chelate ring coordinated through N(2) and N(5) atoms. The third donor atom is different in the three modes of coordination. In complex 1, coordination mode I is observed in which N(12) is bonded in contrast to modes II and III where the carbonyl oxygen, O(15) is bonded following enolization. An explanation can be proposed for the unusual mode of coordination based on the highly basic character of the pyrazole N(12) [51]. A likely scenario is that initially a five membered chelate ring around the metal centre is formed using N(2) and N(5) donors, thereafter followed by the third coordination. Now, there will be a competition between O(15) and N(12) atoms to complete the tridentate coordination. Here in complex 1, the copper ions prefer the comparatively more basic N(12) atom for coordination, forming a stable six membered and a five membered chelate ring around the metal centre (Scheme 2).

2.4.3. Synthesis of [CoL2]Cl (2) To a methanolic suspension (30 ml) of the ligand HL (0.223 g, 1 mmol), a methanolic solution of cobalt chloride hexahydrate (0.370 g, 1 mmol) was added and the mixture was stirred at room temperature for 30 min. The dark brown solution was filtered and allowed to evaporate at room temperature. The Co(II) oxidized to Co(III) in solution on reacting with the ligand. After 2–3 days dark brown crystals were obtained which proved to be suitable for Xray diffraction. The crystals were isolated by filtration, washed with methanol and were dried in the open air. Yield: 64%. Anal. Calc. for C18H24N10O4ClCo: C, 40.12; H, 4.49; N, 25.99. Found: C, 40.04; H, 4.54; N, 25.89%. IR (KBr) mmax/cm1: 3443 (OH), 3243 (NH), 2926 (CH3), 1659 (C@O), 1510 (C@N), 1269 (N–N). UV–Vis kmax (DMSO, nm (e, L M1 cm1)): 422 (4943), 289 (32,146), 252 (21,820). 1H NMR dH (400 MHz; D2O): 6.87 (1H, s, pyrazole CH), 2.63 (3H, s, CH3), 2.18 (3H, s, pyrazole CH3), 1.98 (3H, s, CH3). 3. Results and discussion

3.1. Description of crystal structures

Our aim was to synthesize the new ligand, 3-methylpyrazole-5carbohydrazone of 2,3-butanedione monoxime (HL), which would be complexed with metals to form multinuclear complexes via oximato bridges, and then to examine their magnetic properties. From

The structure determinations of 1 and 2 by X-ray crystallography showed that the ligand is present in the complexes as depro-

O N N

HN

NH2

+ O

H

2 N M 12 N HN

N 6

(II)

O1 N

N M

N 5

M

HO

O1

M

OH

EtOH, reflux 2 hrs

M

15 O

N

M

O

- 2H+

- 2H+

N

N

N HN

H

N M

HN

N 12

- H+ M O1

H N

15 O

12 N

N

2

M

N6 H

M

15 O

N 5

(I)

Scheme 2. Possible coordination modes of the ligand HL in the presence of metal ions in solution.

M

N 5 6

(III)

2

51

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55

Fig. 1. The dimeric [Cu2L2(OC2H5)2]2+ cation in 1 with the atom numbering scheme. Ellipsoids at 30% probability.

Table 1 Crystal data and structure determination details for complexes 1 and 2.

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) Data/restraints/param Number of observed reflections [I > 2(I)] R(F), wR(F2) (observed data)a R(F), wR(F2) (all data)b Rint Peaks in final difference map (e Å3) Goodness-of-fit (GOF) on F2 a b

1

2

C11H20N5O8ClCu 449.31 triclinic  P1 8.4071(5) 10.9533(7) 12.8203(9) 111.248(2) 110.54(2) 91.175(2) 1015.79(11) 2 1.469 4426/18/249 3112

C18H22N10O4ClCo 536.84 monoclinic C2/c 9.251(5) 16.441(5) 14.80(5) (90) 103.493(5) (90) 2141.6(15) 4 1.665 2549/0/150 1476

0.0923, 0.2341 0.1291, 0.2622 – 1.443, 0.811 1.016

0.0540, 0.1240 0.1097,0.1392 0.1159 0.619, 0.479 0.908

R = ||Fo|  |Fc||/|Fo|. wR(F2) = [w(|Fo|2  |Fc|2)2/w|Fo|4]1/2.

tonated L and that the coordination mode is type I (Scheme 2) in both Cu(II) and Co(III) complexes, though in dimeric 1 the metal is pentacoordinated and has a distorted square pyramidal structure, while in monomeric 2 the metal is hexacoordinated and has a distorted octahedral structure. In contrast to complex 1, the oxime bridge is not found in 2. 3.1.1. [CuL(EtOH)]2(ClO4)22H2O (1) The blue green complex [CuL(EtOH)]2(ClO4)22H2O(1) crystal The molecular lizes in the triclinic system with the space group P 1. structure and atom labeling scheme in the cationic moiety of 1 is illustrated in Fig. 1. Crystallographic data are summarized in Table 1 and the metric dimensions are presented in Table 2. The structural study reveals that 1 is a dimer with a crystallographic centre of symmetry. The two copper centres are five coordinated,

Table 2 Selected dimensions in complexes 1 and 2 (distances, Å, angles °). 1

2

Bond distances Cu(1)–O(1)$1 Cu(1)–O(17) Cu(1)–N(2) Cu(1)–N(5) Cu(1)–N(12) O(1)–N(2)

1.915(6) 2.235(7) 1.953(7) 2.020(7) 1.974(7) 1.345(8)

Co(1)–N(2) Co(1)–N(5) Co(1)–N(12) O(1)–N(2)

1.903(4) 1.906(3) 1.939(3) 1.262(4)

Bond angles O(1)$1–Cu(1)–O(17) O(1)$1–Cu(1)–N(2) O(1)$1–Cu(1)–N(5) O(1)$1–Cu(1)–N(12) O(17)–Cu(1)–N(2) O(17)–Cu(1)–N(5) O(17)–Cu(1)–N(12) N(2)–Cu(1)–N(5) N(2)–Cu(1)–N(12) N(5)–Cu(1)–N(12) N(6)–C(7)–C(8)

95.3(3) 105.7(3) 172.0(3) 84.6(3) 93.4(3) 90.0(3) 103.2(3) 79.9(3) 159.7(3) 88.3(3) 124.3(7)

N(5)–Co(1)–N(2)$2 N(2)–Co(1)–N(2) N(5)–Co(1)–N(12)$ N(2)–Co(1)–N(12)$2 N(12)–Co(1)–N(12)$ N(5)–Co(1)–N(5)$2

91.5(2) 85.1(2) 2 95.3(1) 92.1(1) 2 91.5(2) 171.2(2)

N(2)–Co(1)–N(5) N(2)–Co(1)–N(12) N(5)–Co(1)–N(12))

81.9(2) 172.2(1) 90.9(2)

Symmetry element $1 1  x, y, z $2 x, y 1/2  z.

the deprotonated ligand L is coordinated to the copper to form a distorted square pyramidal CuN3O2 coordination with the four basal positions occupied by the three donor points N(2) (oxime nitrogen), N(5) (imine nitrogen), N(12) (pyrazolate nitrogen) of the tridentate ligand and the fourth position is occupied by O(1) (oxime oxygen) of the second ligand. The deviation of these four points from the least-squares plane are 0.100(3), 0.110(4), 0.108(4) and 0.097(3) Å, respectively for N(2), N(5), N(12), and O(1) (1  x, y, z), thus showing a small tetrahedral distortion. The axial site is occupied by the oxygen O(17) from the coordinated ethanol molecule. The central Cu atom is shifted 0.186(4) Å towards the axially bound EtOH molecule from the basal plane. The distortion from the square pyramidal geometry for the Cu(II) centres is small, as confirmed by the small value of s = 0.043, [s = (b–a)/60°] where b and a are two largest possible angles around the central atom, with s = 0 and 1 for the perfect square pyramidal and trigonal bipyramidal geometries, respectively [52]. The Cu–O(1) and Cu–N(2) bond distances are 1.915(6) and

52

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55 Table 3 Hydrogen bond distances (Å) and bond angles (°).

1.0

0.32

<(DHA) 159 N(6)–H(6)  O(15)_$3 111 N(11)–H(11)  O(1)$1 133 N(11)–H(11)  O(21)_$4 171 O(17)–H(17)  O(20A) z + 1, $4 = x, y + 1, z

Complex 2 0.86 2.22 3.029(6) 156 0.90 2.51 2.948(4) 164 Symmetry element $5 x  1/2, 1/2  y, 1  z

N(11)–H(11)  Cl(1) N(6)–H(6)  O(15)$5

0.8

Absorbance

D–H H  A D  A 0.88 1.99 2.826(11) 0.88 2.25 2.696(10) 0.88 2.33 2.998(13) 0.99 1.83 2.812(13) Symmetry elements $3 = x + 1, y + 1,

Absorbance

Complex 1

0.6

0.28 0.24 0.20 0

10

0.4

20

30

40

50

Time (min)

0.2 0.0 500

600

700

800

900

1000

Wave length (nm) Fig. 3. The time-dependent UV–Vis spectrum of 1 in DMF at 23 °C. The inset graph shows the exponential decrease of the absorption maxima with time.

The chloride ion acts as an acceptor to a hydrogen bond from N(11) at 3.029(6) Å, while N(6) forms a donor hydrogen bond to O(15) at 2.948(4) Å. Further details are given in Table 3. 3.2. Electronic spectra

Fig. 2. The [CoL2]+ cation in 2 with the atom numbering scheme. Ellipsoids at 30% probability.

1.953(7) Å respectively, which indicates that the two Cu(II) centres are strongly coordinated by the bridging oxime groups. The Cu(1)– O(17) and Cu(1)–N(5) bonds are significantly longer, at 2.235(7) and 2.020(7) Å, respectively. The O(1)–N(2) bond length is 1.345(8) Å, showing single bond character. The Cu–Cu distance is 3.680(2) Å. The metric dimensions of the corresponding bond lengths and bond angles are given in Table 2. Details of the intermolecular hydrogen bonds are given in Table 3. 3.1.2. [CoL2]Cl (2) The mononuclear brown complex 2 crystallizes in the monoclinic system with space group C2/c. The metal atom lies on a twofold axis. The molecular structure and atom labeling scheme of 2 is illustrated in Fig. 2 and metric dimensions are presented in Tables 1 and 2. The central cobalt(III) atom of 2 is hexacoordinated with a distorted octahedral CoN6 coordination. Unlike in 1, here the Co–N bond lengths to N(2) and N(5) are virtually equivalent, at 1.906(3) and 1.903(4) Å, with the bond to N(12) being slightly longer at 1.939(3) Å. The tridentate N3 ligand forms one six membered and one five membered chelate ring at the cobalt(III) acceptor centre, the corresponding bite angles being 90.9(1) and 81.9(2)° respectively. The O(1)–N(2) bond length is 1.262(4) Å, showing double bond character, and it is significantly shorter than the equivalent bond in 1. This is due to the fact that the negative charge on the O(1) atom in 2 is greatly delocalized via the conjugated –N@C– groups, where in 1 the O(1) atom is coordinated to another metal centre.

The electronic spectral data of 1 and 2 are presented in the experimental section. Complex 1 shows a strong absorption band at around 275 nm which is due to an LMCT transition. The band at 380 nm corresponds to an intraligand n–p⁄ transition of the amide group [53]. In the visible region, complex 1 displays a single broad band at 686 nm, which indicates a distorted square pyramidal geometry around the Cu(II) centre. Single broad bands in the region 550–660 nm are quite common for typical Cu(II) complexes having SP or distorted SP geometries, on the other hand TBP complexes of Cu(II) show absorbance maxima at k > 800 nm with a higher energy shoulder [54–56]. Complex 2 is a low spin Co(III) complex and it exhibits three bands at 422, 289 and 252 nm respectively. The band at 422 nm corresponds to an L–Co(dp) LMCT transition and two other absorption bands in the 252–290 nm region correspond to intraligand transitions. More interestingly, the d–d transition band at 686 nm gradually but slowly decreased with time and reached a minimum after 50 min in DMF. The time dependent UV–Vis spectrum at a fixed wavelength of 686 nm is presented in Fig. 3 and this can be explained via first order kinetics (inset diagram of Fig. 3). This is probably due to the decomposition of 1, which undergoes an autoreduction reaction in aprotic solution. Maekawa et al. [42] showed that Cu(II) complexes with the B type geometric model exhibit autoreduction in solution with solvents such as DMF, DMSO and DMA. In our case, a similar property of autoreduction of 1 in solvents such as DMF and DMSO was observed. Other solvents like acetone or methanol were not used due to the insolubility of 1. The d–d band did not completely disappear, which indicates the system reached an equilibrium state after 50 min. This autoreduction reaction is an exceptional observation in Cu(II) oximato bridged dinuclear complexes having the geometric model A. The autoreduction process also depends on the size of the chelate rings and the degree of distortion from planarity of the Cu2 geometric model. In complex 1 the probable mechanism of the autoreduction may be explained via a two step pathway (Scheme 3). The first step is the replacement of the ethanol molecule from the apical position of the Cu(II) centre by the solvent DMF molecule via formation of

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55

53

Scheme 3. Possible mechanism for the autoreduction of 1.

two mononuclear Cu(II) intermediates and the second step is the dissociation of the unstable intermediate into two mononuclear Cu(I) species. When the solvent molecules (DMF) replaced the weakly coordinated ethanol molecule from each of the Cu(II) centres from its apical position, the rigid geometric model would hinder the subsequent coordination of the solvent DMF molecule due its steric bulk and this results in the breaking of the Cu-O bonds, leading to the formation of two mononuclear Cu(I) species. The kinetically slow autoreduction process observed for 1 may be explained as being due to the structural geometric model of type A as well as the ethanol molecules at the apical positions of the two Cu(II) centres, which need to be displaced by the solvent molecules during the autoreduction process. 3.3. IR and 1H NMR spectroscopy IR spectral data for the free HL ligand and its complexes 1 and 2 are reported in the experimental section. In the free ligand and the corresponding complexes 1 and 2, broad bands in the region 3400– 3100 cm1 indicate the presence of hydrogen bonded –OH groups and the N–H group for the amide functionality. It is evident from the spectral data that the carbonyl oxygen does not take part in complexation via enolization. The C@O band is located at 1668 cm1 in the free ligand, 1655 cm1 in complex 1 and 1659 cm1 in complex 2. In other cases this band disappears in the complexes and a new band develops at around 1050 cm1, indicating the coordination of the carbonyl oxygen via enolization [5,6]. The 1H NMR data of the free ligand HL in d6-DMSO and its cobalt(III) complex 2, recorded in D2O, are presented in the experimental section. The spectrum of the ligand indicates the presence of the three methyl groups of the ligand in the 1.97– 2.47 ppm range. The –CH proton of the pyrazole moiety appears as a singlet at 6.76 ppm. The singlet peaks at 13.06 and 11.56 ppm are due to the pyrazole –NH and the hydrazide –NH groups. One more singlet at 10.12 ppm is due to the oxime –OH group. For 2, three individual peaks corresponding to the three methyl groups of the ligand appear as singlets at 2.63, 2.18 and 1.98 ppm, respectively. The singlet peak at 6.87 ppm is due to the –CH proton of the pyrazole moiety. The two –NH peaks are absent in the spectrum, presumably because of the replacement of exchangeable hydrogen by D2O. 3.4. EPR spectroscopy X-band EPR spectra of 1 were recorded in DMF solution (concentration ca. 5  104 mol dm3) at liquid nitrogen temperature. A typical spectrum of 1 is presented in Fig. 4. The presence of both

Fig. 4. EPR spectrum of 1 in DMF at 100 K (after 1 h). Spectrum recorded 2 min after adding DMF (inset diagram).

the mononuclear and dinuclear species at equilibrium in the DMF solution of 1 is established by EPR spectroscopy. A solution was prepared at room temperature and kept for 1 h. The X-band EPR spectrum of the complex at 100 K was recorded after 1 h shows a superposition of two sets of signals. The presence of mononuclear and dinuclear species in solution for Cu(II) are well known [57]. The simulation of the spectrum was carried out considering two non-interacting species, mononuclear and dinuclear, in a molar ratio of 3:2. The dinuclear species in frozen solution exhibits a rhombically distorted g tensor and the mononuclear species an axially distorted g tensor. The best fit parameters for the dinuclear species are: gx = 2.01, gy = 2.12, gz = 2.24 and Ax = 175 MHz, Ay = 100 MHz, Az = 210 MHz. For the mononuclear species: gx = gy = 2.0795, gz = 2.397 and Ax = Ay = 16 MHz, Az = 410 MHz were used to simulate the spectrum. The splittings of 16 and 410 MHz are typical of monomeric Cu(II) which arises due to the dissociation of the dinuclear complex in DMF. The spectrum recorded immediately after adding DMF shows the predominant presence of the dinuclear species, which changes to a mixture of dinuclear and mononuclear species after 1 h thus indicating the dissociation of dinuclear to mononuclear species in DMF solution. 3.5. Magnetochemistry The magnetic data of compound 1 were explored by variable temperature measurements in the temperature range 2–300 K in an applied field of 1 Tesla on a powdered microcrystalline sample. The collected susceptibility data are plotted as vMT versus T in Fig. 5, as well as the simulation obtained by using the following Hamilton operator:

54

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55

0.08 0.06 0.04

M

χ T/cm3mol-1K

0.10

Exp - Sim

0.02

............................................................................................

0.00 0

50

100

150

200

250

PI

300

T/K Fig. 5. Plot of vMT vs. T for a microcrystalline sample of 1 in a 1 Tesla field.

shows an autoreduction reaction in solvents like DMF and DMSO, which follows first order kinetics. This is a new finding for oximato bridged dinuclear Cu(II) complexes having the A geometric model type. The X-band EPR spectrum of 1 in frozen solution indicate the presence of both mononuclear and the dinuclear species with axial and rhombic g tensors. A magnetic study of 1 reveals that the copper(II) centres are strongly antiferromagnetically coupled with an isotropic exchange coupling constant J = 371 cm1 via two bridging N–O groups. Therefore, the ligand used in this work exhibits a new type of coordination mode and its Cu(II) complex exposes some new aspects of oximato bridged dinuclear Cu(II) chemistry. Acknowledgements M.S. is grateful to the Deutscher Akademischer Austausch Dienst (DAAD), Germany for the award of postdoctoral research fellowship. We are thankful to Dr. Dariush Hinderberger for providing the EPR facilities and fruitful discussion and Dr. Luca Carrella for SQUID measurement.

^ ¼ 2J^S1 ^S2 H The value of vMT at room temperature is 0.09 cm3 K mol1, which lies dramatically below the value of 0.75 cm3 K mol1 which is to be expected for two uncoupled spins with S1 = S2 = 1/2. The vMT value rapidly decreases with decreasing temperature, reaching an approximately constant value close to 0 cm3 K mol1 at 100 K, indicating a very strong antiferromagnetic interaction between the two copper centres. The magnetic data can be simulated satisfactorily with J = 371 cm1, g1 = g2 = 2.17 and q = 0.017% for an S = 1/2 species. This type of strong antiferromagnetic interaction is not unusual for copper ions doubly bridged by oximato groups [57–59]. It is also well reported in the literature that the bridging diatomic = N–O group very actively mediates a medium to strong antiferromagnetic interaction which is provided via an orbital exchange pathway of r symmetry. In many cases the given values of the coupling constants, J, are greater than 500 cm1 [58–61]. Thus, for instance, Cu(II) complexes with double oximato bridges usually exhibit complete or nearly complete spin coupling even at room temperature [60–67]. The comparatively longer Cu–Cu distance (3.680(2) Å) found in 1 provides further support for the fact that the superexchange occurs via the bridging atom and not through direct metal–metal exchange [60]. Oximato bridges are found to be effective in mediating strong antiferromagnetic exchange interactions between paramagnetic centres either in syn, anti or O-monoatomic coordination [60–67]. According to Extended-Huckel MO calculations reported in the literature [61] on the Cu–(R@N–O)2–Cu core (R = various substituted groups), planar Cu–(R@N–O)2–Cu ‘‘rings’’ provide the strongest magnetic coupling. Some other factors, such as the electronic properties of the Rsubstituted oximato groups and/or the ligands that complete the coordination sphere of the Cu(II) ions, also play an important role in modulating the magnitude of the coupling [61]. The explanation also relates the magnitude of the antiferromagnetic coupling constant to the deviation of the ring angle at copper from 90°. In fourmembered ring compounds, this angle was considerably less than 90° and in planar six-membered ring compound this angle (105.9 (3)°) is considerably greater than 90°. 4. Conclusions We have successfully synthesized a new Schiff base ligand, 3methylpyrazole-5-carbohydrazone of 2,3-butanedione monoxime (HL), and its dinuclear copper complex (1). A new binding mode of coordination of the ligand was established. Interestingly, 1

Appendix A. Supporting information CCDC 910022 and 910023 contain the supplementary crystallographic data for 1 and 2, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: [email protected]. References [1] J.F. Geldard, F. Lions, Inorg. Chem. 2 (1963) 270. [2] N.R. Sangeetha, S. Pal, Polyhedron 19 (2000) 1593. [3] C.M. Armstrong, P.V. Bernhardt, P. Chin, R. Richardson Eur, J. Inorg. Chem. (2003) 1145. [4] 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. [5] M. Sutradhar, G. Mukherjee, M.G.B. Drew, S. Ghosh, Inorg. Chem. 45 (2006) 5150 (references cited therein). [6] M. Sutradhar, G. Mukherjee, M.G.B. Drew, S. Ghosh, Inorg. Chem. 46 (2007) 5069. [7] D. Matoga, J. Szklarzewicz, K. Stadnicka, M.S. Shongwe, Inorg. Chem. 46 (2007) 9042. [8] P.V. Bernhardt, P. Chin, P.C. Sharpe, D.R. Richardson, Dalton Trans. (2007) 3232. [9] E.W. Ainscough, A.M. Brodie, A.J. Dobbs, J.D. Ranford, J.M. Waters, Inorg. Chim. Acta 267 (1998) 27. [10] N.R. Sangeetha, K. Baradi, R. Gupta, C.K. Pal, V. Manivannan, S. Pal, Polyhedron 18 (1999) 1425. [11] R. Dinda, P. Sengupta, S. Ghosh, T.C.W. Mak, Inorg. Chem. 41 (2002) 1684. [12] Wu La-Mei, Han-Bing Teng, Xi-Chun Feng, Xian-Bing Ke, Qi-Feng Zhu, Su JiangTao, Xu Wen-Jin, Hu Xian-Ming, Cryst. Growth Des. 7 (2007) 1337. [13] M. Sutradhar, L.M. Carrella, E. Rentschler, Eur. J. Inorg. Chem. (2012) 4273. [14] P.A. Vigato, S. Tamburini, D.E. Fenton, Coord. Chem. Rev. 106 (1990) 25. [15] P. Chaudhuri, Coord. Chem. Rev. 243 (2003) 143. [16] A.J.L. Pombeiro, V. Yu Kukushkin, in: J.A. McCleverty, T.J. Meyer (Eds.), Comprehensive Coordination Chemistry II, vol. 1, Elsevier, Amsterdam, 2004, p. 631. [17] R.J. Butcher, C.J. O’Connor, E. Sinn, Inorg. Chem. 20 (1981) 537. [18] C.J. Milios, C.P. Raptopoulou, A. Terzis, F. Lloret, R. Vicente, S.P. Perlepes, A. Escuer, Angew. Chem., Int. Ed. 43 (2004) 210. [19] For a comprehensive review, see C.J. Milios, Th.C. Stamatatos, S.P. Perlepes, Polyhedron 25 (2006) 134 (Polyhedron Report). [20] C.J. Milios, S. Piligkos, E.K. Brechin, Dalton Trans. (2008) 1809 (Dalton Perspective). [21] Th.C. Stamatatos, C. Papatriantafyllopoulou, E. Katsoulakou, C.P. Raptopoulou, S.P. Perlepes, Polyhedron 26 (2007) 1830. [22] P. Chaudhuri, M. Winter, F. Birkelbach, P. Fleischhauer, W. Haase, U. Flörke, H.J. Haupt, Inorg. Chem. 30 (1991) 4291. [23] S. Ross, T. Weyhermüller, E. Bill, K. Wieghardt, P. Chaudhuri, Inorg. Chem. 40 (2001) 6656. [24] C. Lampropoulos, Th.C. Stamatatos, K.A. Abboud, G. Christou, Inorg. Chem. 48 (2009) 429. [25] S. Khanra, T. Weyhermüller, P. Chaudhuri, Dalton Trans. (2007) 4675. [26] R. Clérac, H. Miyasaka, M. Yamashita, C. Coulon, J. Am. Chem. Soc. 124 (2002) 12837.

M. Sutradhar et al. / Polyhedron 53 (2013) 48–55 [27] F. Mori, T. Nyui, T. Ishida, T. Nogami, K.-Y. Choi, H. Nojiri, J. Am. Chem. Soc. 128 (2006) 1440. [28] H. Miyasaka, R. Clérac, Bull. Chem. Soc. Jpn 78 (2005) 1725. [29] A. Escuer, B. Cordero, M. Font-Bardia, Inorg. Chem. 49 (2010) 9752. [30] E. Colacio, J.M. Dominguez-Vera, A. Escuer, R. Kivekaes, A. Romerosa, Inorg. Chem. 33 (1994) 3914. [31] A. Escuer, G. Vlahopoulou, S.P. Perlepes, Inorg. Chem. 50 (2011) 2468. [32] M. Kodera, Y. Tachi, T. Kita, H. Kobushi, Y. Sumi, K. Kano, M. Shiro, M. Koikawa, T. Tokii, M. Ohba, Inorg. Chem. 39 (2000) 226. [33] Y. Jiang, H. Kou, R. Wang, A. Cui, J. Ribas, Inorg. Chem. 44 (2005) 709. [34] J.A. Bertrand, J.H. Smith, D.G. VanDerveer, Inorg. Chem. 16 (1977) 1477. [35] Y. Agnus, R. Louis, B. Metz, C. Boudon, J.P. Gisselbrecht, M. Gross, Inorg. Chem. 30 (1991) 3155. [36] A. Escuer, G. Vlahopoulou, S.P. Perlepes, M. Font-Bardia, Dalton Trans. 40 (2011) 225. [37] B. Cervera, R. Ruiz, F. Lloret, M. Julve, J. Cano, J. Faus, C. Bois, J. Mrozinski, J. Chem. Soc., Dalton Trans. (1997) 395. [38] T. Afrati, C.M. Zaleski, C. Dendrinou-Samara, G. Mezei, J.W. Kampf, V.L. Pecoraro, D.P. Kessissoglou, Dalton Trans. (2007) 2658. [39] R. Ruiz, J. Sanz, F. Lloret, M. Julve, J. Faus, C. Bois, M.C. Muñoz, J. Chem. Soc., Dalton Trans. (1993) 3035. [40] E. Colacio, C. López-Magaña, V. McKee, Antonio Romerosa, J. Chem. Soc., Dalton Trans. (1999) 2923. [41] R. Ruiz, J. Sanz, B. Cervera, F. Lloret, M. Julve, C. Bois, J. Faus, M.C. Muñoz, J. Chem. Soc., Dalton Trans. (1993) 1623. [42] M. Maekawa, S. Kitagawa, Y. Nakao, S. Sakamoto, A. Yatani, W. Mori, S. Kashino, M. Munakata, Inorg. Chim. Acta 293 (1999) 20. [43] N. Saha, K.M. Datta, J. Inorg. Nucl. Chem. 43 (1981) 1405. [44] S. Stoll, A. Schweiger, J. Magn. Reson. 178 (2006) 42. [45] E. Bill, http://ewww.mpi-muelheim.mpg.de/bac/logins/bill/julX_en.php, 2008. [46] Bruker, APEX2 Software Suite version 2.0, 2005. [47] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr. 32 (1999) 115. [48] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997.

55

[49] S. Gupta, A.K. Barik, S. Pal, A. Hazra, S. Roy, R.J. Butcher, S.K. Kar, Polyhedron 26 (2007) 133. [50] S. Roy, T.N. Mandal, A.K. Barik, S. Pal, R.J. Butcher, M.S. El Fallah, J. Tercero, S.K. Kar, Dalton Trans. (2007) 1229. [51] Alan R. Katrizky, Advances in Heterocyclic Chemistry. vol. 41, Elesevier, Academic Press (London Ltd.), UK, 1987. p. 219. [52] A.W. Addison, T.N. Rao, J. Reedjik, J. Van Rijn, G.C. Verschoor, J. Chem. Soc., Dalton Trans. (1984) 1349. [53] S. Banerjee, S. Mondal, W. Chakraborty, S. Sen, R. Gachhui, R.J. Butcher, A.M.Z. Slawin, C. Mandal, S. Mitra, Polyhedron 28 (2009) 2785. [54] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier Science, Amsterdam, 1984. [55] P. Dapporto, M. Formica, V. Fusi, L. Giothi, M. Micheloni, P. Paoli, R. Pontellini, P. Rossi, Inorg. Chem. 40 (2001) 6186. [56] T. Gajda, A. Jancsó, S. Mikkola, H. Lönnberg, H. Sirges, J. Chem. Soc., Dalton Trans. (2002) 1757. [57] S. Tanase, I.A. Koval, E. Bouwman, R. de Gelder, Inorg. Chem. 44 (2005) 7860. [58] J.A. Bertrand, J.H. Smith, P.G. Eller, Inorg. Chem. 13 (1974) 1649. [59] E.S. Koumousi, C.P. Raptopoulou, S.P. Perlepes, A. Escuer, T.C. Stamatatos, Polyhedron 29 (2010) 2004. [60] R. Ruiz, F. Lloret, M. Julve, M.C. Munoz, C. Bois, Inorg. Chim. Acta 219 (1994) 179. [61] J.M. Dominguez-Vera, E. Colacio, A. Escuer, M. Klinga, R. Kivekäs, A. Romerosa, Polyhedron 16 (1997) 281. [62] D. Luneau, H. Oshio, H. Okawa, S. Kida, J. Chem. Soc., Dalton Trans. (1990) 2283. [63] P. Chaudhuri, M. Winter, B.P.C. Della Vedova, E. Bill, A. Trautwein, S. Gehring, P. Fleischauer, B. Nuber, J. Weiss, Inorg. Chem. 30 (1991) 2148. [64] R. Ruiz, J. Sanz, B. Cervera, F. Lloret, M. Julve, C. Bois, J. Faus, M.C. Muñoz, J. Chem. Soc., Dalton Trans. (1993) 1623. [65] R. Ruiz, J. Sanz, F. Lloret, M. Julve, J. Faus, C. Bois, M.C. Munoz, J. Chem. Soc., Dalton Trans. (1993) 3035. [66] E. Colacio, J.M. Dominguez-Vera, A. Escuer, M. Klinga, R. Kivekas, A. Romerosa, J. Chem. Soc., Dalton Trans. (1995) 343. [67] P. Chaudhuri, M. Winter, U. Flörke, H.-J. Haupt, Inorg. Chim. Acta 232 (1995) 125.