Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Synthesis, characterization and reactivity of trinuclear Cu(II) complexes derived from disalicylaldehyde malonoyldihydrazone Angira Koch a, Arvind Kumar b, Arjun K. De c, Arnab Phukan a, Ram A. Lal a,⇑ a

Department of Chemistry, North-Eastern Hill University, Shillong 793022, Meghalaya, India Department of Chemistry, Faculty of Science and Agriculture, The University of West-Indies, St. Augustine, Trinidad and Tobago c Department of Science and Humanities, Tripura Institute of Technology, Narsingarh 799009, Tripura, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Homotrimetallic copper(II)

The reaction of disalicylaldehyde malonoydihydrazone in methanol with CuX2nH2O (X = Cl, ClO4) maintaining H4slmh: KOH: CuX2nH2O (X = Cl, NO3 and ClO4) molar ratio at 1:4:3.2 yielded the complexes of the composition [Cu3(slmh)(l-X)2(CH3OH)3]nCH3OH [n = 0.5, X = Cl (1); n = 2, X = ClO4(3)] and [Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH (2).

complexes have been synthesized from disalicylaldehyde malonoyldihydrazone.  The dihydrazone ligand is present in enol form in the complexes.  The ground state for the complexes is a mixture of the quartet state (S = 3/2) and doublet state (S = 1/2) at room temperature.  All of the complexes show a reversible one-electron redox couple in their cyclic voltammogram.  The complexes have catalyzed the H2O2 mediated oxidation of benzyl alcohol to benzaldehyde in 53% yield.

a r t i c l e

i n f o

Article history: Received 16 December 2013 Received in revised form 25 February 2014 Accepted 27 February 2014 Available online 27 March 2014 Keywords: Trinuclear copper(II) complexes Disalicylaldehyde malonoyldihydrazone Magnetic moment Spectroscopic studies and cyclic voltammetry

⇑ Corresponding author. Tel.: +91 0364 2722616. E-mail address: [email protected] (R.A. Lal). http://dx.doi.org/10.1016/j.saa.2014.02.202 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

a b s t r a c t Three new homotrinuclear copper(II) complexes [Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH (1), [Cu3(slmh) (NO3)2(CH3OH)5]1.5CH3OH (2) and [Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH (3) from disalicylaldehyde malonoyldihydrazone have been synthesized and characterized. The composition of the complexes has been established on the basis of data obtained from analytical and thermoanalytical data. The structure of the complexes has been discussed in the light of molar conductance, electronic, FT-IR and far-IR spectral data, magnetic moment and EPR spectral studies. The molar conductance values for the complexes in DMSO solution indicate that all of them are non-electrolyte. The magnetic moment values for the complexes suggest considerable metal–metal intramolecular interaction between metal ions in the structural unit of the complexes. The EPR spectral features reveal that at RT, the ground state for the complexes is a mixture of the quartet state (S = 3/2) and doublet state (S = ½). At lower 2 2 temperature, the ground state for the complexes is d2y x with considerable contribution from dz orbital. Dihydrazone ligand is present in enol form in all of the complexes. The complexes have distorted square pyramidal stereochemistry. The electron transfer reactions of the complexes have been investigated by cyclic voltammetry. Hydrogen peroxide mediated oxidation of benzyl alcohol catalyzed by complex 1 has been studied. Ó 2014 Elsevier B.V. All rights reserved.

104

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

Introduction Polyfunctional hydrazones derived from condensation of acyl-, aroyl-, and pyridoyl-dihydrazines with o-hydroxy-aromatic aldehydes and ketones contain several bonding sites including carbonyl oxygen atoms, azomethine nitrogen atoms and secondary amine nitrogen atoms [1–5]. It has been shown that these ligands can yield complexes having several types of nuclearity including mononuclear [6], homobinuclear [2,7], heterobinuclear [8], homotrinuclear [3,9] and heterotrinuclear [10]. Malonoyl dihydrazones are some of the examples [3c,3e,11] out of several multidentate dihydrazones containing o-hydroxy aromatic aldehydes and ketones in their molecular skeleton [1–8]. These multidentate ligands possesses as many as eight bonding sites and can utilize utmost six bonding sites simultaneously in bonding to the metal ions in polynuclear metal complexes. In malonoyldihydrazones, the two hydrazone functions are joined together through active methylene group flanked by carbonyl groups. These dihydrazones are unique in the sense that their malonoyl fraction offers greater flexibility in three dimensional space because of its capability for free rotation about CAC single bond as compared to those in which the two hydrazone groupings are joined together either directly (oxaloyl), or through phenyl or pyridyl groups, respectively. The oxaloyl-, phenyl-, or pyridyl-fractions, by virtue of their planar characteristics impose planarity over the two hydrazone parts reducing their flexibility and reactivity. Further, another most important characteristic of malonoyldihydrazones is that they have potential to yield multinuclear complexes stabilized by three consecutive sixmembered rings as against the remaining dihydrazones which give rise to two six membered rings and one five or higher membered ring, respectively. Consequently, malonoyldihydrazones have potential to give more stable complexes than other types of dihydrazones. Copper complexes constitute an active area of research in contemporary inorganic chemistry because of their importance in biosystem [12], magnetochemistry [13], and catalysis [14]. Copper is present in enzymes in biological systems either alone or in combination with some other metal ions [15] and discharges its biological function by redox cooperativity. Polynuclear copper centres are widespread in biological systems, occurring in type 3 cuproproteins, such as tyrosinase and hemocyanin [16], laccase, ascorbate oxidase, human ceruloplasmin, fungal laccase, FET3 and phenoxazinase which contain either two or more than two copper atoms [16–19]. All multicopper oxidases utilized at least four copper ions to couple the four electron reduction of O2 to H2O with four sequential one-electron substrate oxidation. This privileged position of copper arises due to specific redox properties originating in the unique interplay between demands of d9 and d10 copper atoms towards coordination geometries and ligand fields. Further, the multimetallic copper complexes have also become important because of their relevance in the development of novel functional materials showing molecular ferromagnetism [13]. The multimetallic copper(II) complexes offer the opportunities to test magnetic exchange models on more complicated systems [20]. These studies offer the opportunities to focus attention on the properties of spinquartet ground states in ferromagnetic exchange coupled systems or more complex behaviour due to spin-frustration. Moreover, copper(II) complexes find application as catalyst for the oxidation of alcohols to aldehydes and ketones [14]. This marks the studies on the homogeneous and heterogeneous catalytic activities of homo and hetero-metal copper complexes attractive and challenging [21]. A survey of literature reveals that although some isolated studies are available on metal complexes of malonoyldihydrazones and related dihydrazones [11,22,23], yet the synthesis and characterization of homo- and hetero-trinuclear complexes of dihydrazones

is quite meager [3a,9,10]. Moreover, a systematic study on trinuclear metal complexes of malonoyldihydrazones is absent to the best of our knowledge inspite of their highly flexible nature and presence of central active methylene group capable to yield more stable complexes. In view of the above importance of trinuclear copper complexes in different fields, absence of work on trinuclear metal complexes of malonoyldihydrazones and their highly flexible and active polyfunctional nature, it was of interest to synthesize the trinuclear copper complexes of the title dihydrazone disalicylaldehyde malonoyldihydrazone (H4slmh) (Fig. 1) and to characterize them by various physico-chemical and spectroscopic studies. The electron transfer reactions of the complexes have also been investigated by cyclic voltammetry followed by investigation of their catalytic properties towards hydrogen peroxide mediated oxidation of alcohols. Experimental Materials The copper salts, diethyl malonate, hydrazine hydrate and salicylaldehyde were E-Merck, Himedia or equivalent grades, and all solvents were used as received. All operations were performed under aerobic conditions. Disalicylaldehyde malonoyldihydrazone was synthesized according to literature method [11]. Physical measurements Copper was determined by standard literature [24] procedure. Chloride and nitrate were also determined following literature procedure [24]. In order to determine Cl in perchlorate, it was first reduced to chloride by Ti2(SO4)3 with ammonium chloride in a porcelain crucible in the presence of a little platinum powder [24], the resulting chloride was determined as AgCl [24]. Elemental analyses for C, H and N were performed on a Perkin–Elmer 2400 CHNS/O Analyser 11. The thermogravimetric analyses of the complexes were carried out on a Perkin–Elmer STA6000 (Simultaneous Thermal Analyzer) model in a ceramic crucible under dynamic dinitrogen atmosphere. The heating rate of the samples was maintained at 20 °C min1. The DTA standard used in the experiment is Pt 10% Rh. Infrared spectra in the range 4000–400 cm1 were recorded with KBr pellets and in the range 700–30 cm1 using CsI pellets on a BX-III/FTIR Perkin Elmer Spectrophotometer. Infrared spectra in solution state were recorded in chloroform solution. The electronic spectral analyses were carried out in solid state as Nujol mulls and DMSO solution at room temperature on a Perkin–Elmer Lambda-25 spectrophotometer in the range 200– 1100 nm. Cyclic voltammograms were recorded on a CH Electrochemical Analyzer using a standard three electrode assembly (glassy-carbon working, Pt-wire auxiliary, SCE reference) and 0.1 M TBAP as supporting electrolyte. Room temperature magnetic data of the powdered samples were measured using Sherwood Scientific Magnetic Susceptibility Balance. The magnetic data were corrected for diamagnetism using Pascal’s constants [25]. The molar conductivities of the metal complexes were measured in

O C

H N

H2C C O

N H H

N H

N

H C O O C H

Fig. 1. Disalicylaldehyde malonoyldihydrazone (H4slmh).

105

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

DMSO solution using Wayne Kerr B 95 Automatic Precision Bridge with a dip-type conductivity cell having a platinised platinum electrode. The cell constant was determined using a standard KCl solution. The X-band EPR spectras at room temperature and at variable temperature were recorded from powdered samples and from DMSO solution on a Varian E-112 ESR Spectrometer using TCNE (g = 2.0027) as standard with 100 kHz modulation frequency and 9.1 GHz microwave frequency. 1H and 13C NMR spectra were recorded on a Bruker Avance II (400 MHz) spectrometer in CDCl3. Gas Chromatography (GC) analysis was performed on a Bruker 430-GC Gas Chromatograph equipped with a 30 m  0.32 mm  0.5 lm HP-Innowax capillary column and a flame ionization detector (FID). The product benzaldehyde was known compound and was identified by comparing of its physical and spectral data with those reported in the literature. Synthesis of the complexes In order to synthesize complex (1), the ligand disalicylaldehyde malonoyldihydrazone (1.00 g, 2.94 mmol) was suspended in methanol (20 mL). To this suspension KOH solution (0.66 g) in methanol was added with constant stirring at around 50–60 °C. To the resulting clear yellow coloured solution, CuCl22H2O (1.60 g, 9.38 mmol) solution in methanol (20 mL) was added dropwise to get a greenish brown coloured homogeneous suspension maintaining the molar ratio H4slmh:KOH:CuCl2 at 1:4:3.5. The reaction mixture was refluxed for about 3 h which precipitated a dark brown coloured compound. The product was isolated by vacuum filtration and washed two to three times with hot methanol containing two drops of HCl and finally with diethyl ether and dried over anhydrous CaCl2. The complexes [Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH (2) and [Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH (3) were also prepared by the above method by using CuNO33H2O (2) and CuClO46H2O, respectively, instead of CuCl22H2O and washing the precipitate with 2 drops of HNO3 and HClO4, to yield green and brownish green compounds, respectively. Results and discussion All of the complexes have been prepared by the same general method carrying out reaction between copper salt (CuX2nH2O)

(X = Cl, NO3 and ClO4), H4slmh and KOH in the molar ratio of 3.5:1:4 in methanol. In order to eliminate the possibility of existence of hydroxo species in the resulting complexes, the precipitates were washed several times with 50 mL of methanol and water mixture (4:1 by volume) containing two drops of HCl, HNO3 and HClO4, in the respective complexes. On the basis of analytical, thermoanalytical data, the complexes have been shown to have the compositions [Cu3(slmh)(l-X)2(CH3OH)3]nCH3OH [n = 0.5, 2; X = Cl (1), ClO4 (3)], [Cu3(slmh)(NO3)2(CH3OH)5] 1.5CH3OH. The complexes are air-stable and melt with decomposition at temperature >300 °C, respectively (Table 1). All of the complexes are insoluble in water and common organic solvents but are soluble in highly coordinating solvents such as DMSO and DMF. An effort was taken up to crystallize the complexes in various solvent systems under different experimental conditions. Unfortunately, in all our efforts, only amorphous compounds precipitated which prevented us from the analysis of the complexes by X-ray crystallography. Thermogravimetric analyses The thermal properties of the complexes were investigated by thermogravimetric analysis (TGA) and important data are summarized in Table 2. It can be seen that the complex [Cu3(slmh) (l-Cl)2(CH3OH)3]0.5CH3OH (1) loses weight equal to 2.74% in the temperature range 56–133 °C. This corresponds to loss of half of methanol molecule (theo: 2.26%). The loss of half of methanol molecule in the temperature range 56–133 °C suggests that it is present in the lattice structure of the complex. The complex shows almost no loss of weight in the temperature range 133–238 °C and remains stable. After 238 °C, another decomposition step commences which continues until 335 °C. The weight loss in this temperature range is 13.55% which corresponds to loss of three methanol molecules (theo: 13.53%). The loss of three methanol molecules at such a high temperature indicates that they are coordinated to the metal centre in the complex [26]. After 335 °C, both the coordinated ligand and chloride decompose. However, the weight of the residue does not become constant even at the highest temperature of decomposition i.e. 798 °C. The complex [Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH (2) shows decomposition behaviour almost similar to that of complex 1 except the temperature ranges. The complex shows weight loss equal

Table 1 Complexes, colour, decomposition point and elemental analysis data for trinuclear copper complexes of disalicylaldehyde malonoyldihydrazone. Sl. No.

1 2 3

Complex and colour

M.P. (°C)

[Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH [1] Dark Brown [Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH [2] Green [Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH [3] Brownish Green

>300 >300 >300

Yield (%)

82 78 81

Elemental analysis: Found(cal) % Cu

C

H

N

27.31 (27.31) 22.75 (22.20) 23.82 (23.40)

35.05 (34.67) 33.17 (32.84) 32.79 (32.40)

3.70 (3.66) 4.47 (4.43) 3.93 (3.96)

8.29 (7.90) 10.25 (9.78) 7.27 (6.87)

Table 2 Thermogravimetric analysis (TGA) data of copper complexes of disalicylaldehyde malonoyldihydrazone. Sl. No.

Complexes

Temperature range (°C)

Mass loss (% found) Anal. (Expt.)

Calc. (Theo.)

Assignment

1

[Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH [1]

(i) 56–133 (ii) 238–335

2.74 13.55

2.26 13.53

Loss of 1.5 water molecules Loss of three methanol molecules

2

[Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH [2]

(i) 66–151 (ii)199–372 (iii)372–800

5.45 28.51 30.44

5.59 29.38 31.68

Loss of one and half methanol molecules Loss of five methanol molecules Loss of the ligand

3

[Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH [3]

(i)60–227 (ii) Above 227

7.14

7.19

Loss of two methanol molecules Whole mass exploded

106

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

to 5.45% in the temperature range 66–151 °C which corresponds to loss of one and half molecules of methanol present in the lattice structure of the complex. The second weight loss step starts at 199 °C and continues till 372 °C. In the temperature range 199– 372 °C, the weight loss is 28.51% which corresponds to decomposition of five methanol molecules and two nitrito group. The loss of five methanol molecules and two nitrito groups (theo: 29.38%), in this temperature range suggests that the methanol molecules and nitrato groups are coordinated to the metal centre [10]. Simultaneously, third mass loss step occurs in the temperature range 372–800 °C. The mass loss in this temperature range is 30.44%. This corresponds to loss of coordinated ligand devoid of four oxygen atoms (theo: 31.68%). The final remnant of decomposition is equal to 32.42%. This corresponds to the presence of three molecules of CuO1.5 (theo: 30.10%). It is imperative to mention that copper is present in +3 oxidation state in the final residue, most probably, being oxidized from +2 to +3 oxidation state by coordinated nitrato groups. The complex [Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH (3) shows loss of weight equal to 7.14% in the temperature range 60–227 °C which corresponds to loss of two methanol molecules (theo: 7.19%). This weight loss indicates that they are present in the lattice structure of the complex. At a temperature of 227 °C, the complex explodes leaving a negligible residue. Hence, its further decomposition behaviour could not be investigated. Molar conductance The molar conductance of the complexes falls in the region 1.3– 1.5 X1 cm2 mol1 indicating that they are all non-electrolyte in DMSO solution [27] (Table 3). Electronic spectroscopy The electronic spectra of the complexes have been qualitatively studied in the solid state and quantitatively in DMSO solution because of the solubility reasons. The electronic spectral data are given in Table 3. The uncoordinated ligand shows two bands at 292 (8200) and 324 (7300) nm, respectively. The band at 292 nm is attributed to the intra-ligand p ? p⁄ transition while the band at 324 is assigned to n ? p⁄ transition. The band at 325 nm is attributed to arise due to salicylaldimine part of the ligand characteristic of hydrazone [10]. In the solid state, the complexes show two bands in the region 250–800 nm. The ligand band at 292 and 324 nm shift to longer wavelength and merge into one another giving rise to a single broad band in the region 280–450 nm. Such a feature, associated with the red shift of the ligand bands in the solid state provides

a good evidence for the chelation of dihydrazone to the metal centre. The essential features of the band in the region 380–450 nm in the electronic spectra of the complexes suggest that it appears to have contribution from charge-transfer transition from ligand to metal atom, most probably, from naphtholate oxygen atoms to the metal atom. The complexes show a single broad band centred in the region 662–760 nm. This band is assigned to d–d transition and is asymmetric in nature which is consistent with the square pyramidal geometry of the complexes [28,29]. The solution spectra in DMSO solution of the complexes are better resolved than those in the solid state. The complex 2 shows two bands while the complexes 1 and 3 show three bands each, in the region 290–400 nm. The ligand band at 292 nm remains unsplit in complex 2 while it splits up into two bands and shifts to longer wavelength by 6–11 and 24–25 nm, respectively, in the complexes 1 and 3 and appear in the regions 298–303 and 317 nm. Further, the band at 324 nm also shifts to lower wavelength by 63–69 nm and appears in the region 387–393 nm. This indicates that the ligand remains coordinated to the metal centre in the DMSO solution [26]. In addition to the ligand bands, the complexes show an additional single band asymmetric in nature in the region 665– 692 nm similar to that in the solid state which is attributed to the d–d transition. The molar extinction coefficient of this band falls in the region 131–181 dm3 mol1 cm1. This suggests that the copper centres in the complexes have square pyramidal stereochemistry in solution state similar to that in the solid state [28]. The d–d band in the complex 1 is blue shifted by 68 nm while either remains unshifted in position as in complex 3 or shifts to longer wavelength by 21 nm. The unshifted or red shifted position of the d–d band in the complexes 2 and 3 may be related to replacement of CH3OH molecules from the coordination sphere by solvent DMSO molecule and that the interaction of the solvent molecules with the metal ions is almost same as that of the CH3OH molecules in the solid (complex 3) or more than that in the solid state (complex 2). On the other hand, the blue shift of d–d band in the complex 1 in DMSO solution as compared to that in the solid state indicates that the interaction of solvent molecules with the metal ions in solution state is weak as compared to interaction of methanol molecules with the metal ions in the solid state. Infra-red spectra Some structurally significant IR bands for uncoordinated dihydrazone and its complexes have been set out in Table 4. The uncoordinated dihydrazone shows very strong bands at 3208 and 3055 cm1. These bands may be attributed to joint contribution from stretching vibrations of secondary ANH groups and phenolic AOH groups [30]. The bands at 3208 and 3055 cm1 disappear on complexation. Instead all complexes show a medium to strong

Table 3 Magnetic moment, molar conductance and electronic spectral data for homotrinuclear copper complexes of disalicylaldehyde malonoyldihydrazone. Sl. No.

Complexes

Cu

C

H

N

Magnetic moment leff (BM)

Solid state kmax (nm)

DMSO solution kmax (emax) nm(dm3 mol1 cm1)

Molar conductance (X1 cm2 mol1)

Per molecular formula

Per empirical formula 1.30

360 760

298(5030), 316(4080), 393(3770), 692(181)

1.5

2.25

1.50

360

299(6850), 390(5140), 691(131)

1.9

303(8200), 317(7220), 387(7480), 665(165)

1.7

[Cu3(slmh)(l-Cl2)(CH3OH)3]0.5CH3OH [1]

27.31 (27.31)

35.05 (34.67)

3.70 (3.66)

8.29 (7.90)

2

[Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH [2]

22.75

33.17

4.47

10.25

(22.20)

(32.84)

(4.43)

(9.78)

2.60

3

[Cu3(slmdh)(l-ClO4)2(CH3OH)3]2CH3OH [3]

23.82 (23.40)

32.79 (32.40)

3.96 (3.93)

7.27 (6.87)

2.42

1

Electronic spectral bands

670 1.40

350 662

107

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113 Table 4 Important IR bands for dihydrazone and its homotrinuclear copper complexes of disalicylaldehyde malonoyldihydrazone. Sl. No. Ligand/complex

mOH+NH

mC@O

mC@N

mNCO

mCAO

mNAN

mMAO

mMAO

Phenolic

Carbonyl

mMAN

mCH

Other vibrations

(Uncoordinated/

enolate

H4slmh

3208 vs 1675 s 1618s – 1268 s 1034 w – – 3055vs 1659 s 3431 s – 1610 s 1545 vs 1279 m 1060 w 540 w 502 w

1

[Cu3(slmh)(l-Cl2)(CH3OH)3]0.5CH3OH [1]

2

[Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH [2]

3

[Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH 3405 m [3]

coordinated)

–

–

341 m 261 m

341 m 3437 vs –

–

1611 s 1541 vs 1279 s

1054 w 560 w 505 w

1609 s 1530 vs 1275 s

–

633 w

2964 w

483 w

2857 w

880 w

2927 w

332 m

868 w

375 w

2853 w

566 w 535 w

342 m 1199 s

670 w

2927 w

486 w

2857 w

342 m

broad band in the region 3000–3500 cm1. This band may be attributed to stretching vibration of OH groups from lattice as well as coordinated methanol molecules. The complexes show a new weak band in the region 2927–2964 cm1 and another weak band in the region 2853–2857 cm1. These bands are attributed to CAH stretching vibrations of methanol molecules [31]. The band in the region 2853–2857 cm1 corresponds to coordinated methanol molecules while the band in the region 2927–2964 cm1 to lattice methanol molecules, respectively. The disappearance of the bands at 3208 and 3055 cm1 indicates that the dihydrazone is coordinated to the metal centre in the enol form. Further, the ligand shows a couple of strong band at 1675 and 1659 cm1. These bands are attributed to stretching vibration of carbonyl group. These bands also disappear on complex formation. This corroborates the fact that the ligand is present in the enol form in the complexes. The strong bands appearing at 1624 and 1611 cm1 due to >C@N group shift to lower frequency by 7–9 cm1 indicating coordination of azomethine nitrogen atoms to the metal centre. Another important feature of the IR spectra of the complexes is the appearance of a very strong band in the region 1530– 1545 cm1. This band is not observable in the IR spectrum of the free ligand. Hence, this band is assigned to arise due to the stretching vibration of newly created NCO group produced as a result of enolization of dihydrazone. A strong band at 1268 cm1 due to m(CAO) (phenolic AOH group) shifts to higher frequency by 7– 11 cm1 and appears as a medium to strong intensity band in the region 1275–1279 cm1. This indicates bonding between phenolate oxygen atom and copper centres. The positive shift of this band suggests that the phenyl electron density flows to the metal centre through phenolate oxygen atoms [32]. The complexes show new weak bands in the region 540–566 and 502–535 cm1. These bands are tentatively assigned to the m(MAO) (phenolate) and m(MAO) (enolate) stretching vibrations, respectively [33]. The non-ligand medium intensity band observed in the region 332–342 cm1 is attributed to m(MAN) stretching vibration arising from the coordination of azomethine nitrogen atom to the metal centre [33]. New medium intensity bands are observed at 633, 483 and 670, 486 in the complexes 1 and 3 while new weak bands at 850 and 375 cm1 in the complex 2 [58]. These bands are not visible in the spectrum of the free ligand. Hence, they are attributed to bridged metal atoms through oxygen atoms [34]. The position of the bands in the complexes is consistent with the involvement of enolate oxygen atoms in the bridge formation. The position of the bands in the complexes 1 and 3 suggest that they originate from the formation of dibridge while that in the complex 2, the bands originate from the formation of a

332 m 1384 vs

–

1180 vs 1108 vs 1090 vs

monobridge [35]. The bands at 633 and 670 cm1 are assigned to the antisymmetric vibration while the bands at 482 and 486 cm1 are assigned to symmetric vibrations of group in the complexes 1 and 3 respectively. In the nitrato complex 2, the bands at 850 and 375 cm1 are attributed to arise due to the antisymmetric and symmetric vibrations of monobridge group, respectively [34]. The complex 1 shows a medium intensity band at 261 cm1. The square planar and monomeric octahedral chloride complexes show terminal metal–chloride stretching vibrations in the regions 253–333 and 225–250 cm1, respectively. The polymeric octahedral complexes of first series transition metal complexes show metal–chloride stretching vibrations due to bridging chloride group in the region 170–195 cm1 [36,37]. Further, the square-planar copperAchlorido complexes show CuACl stretching frequency in the region 282–296 cm1. The metal–chloride band in the present complex at 261 cm1 falls in the region suggested for monomeric octahedral complexes of first transition series, yet these ranges can in no way be taken as absolutely well defined. Hence, in view of the square pyramidal geometry of the complexes as judged from electronic spectroscopy, it is quite reasonable to assume that the metal–chloride band position is indicative of the five-coordinate geometry of the complex and that the chloride group is involved in bridge formation [36]. The complex 2 shows a very strong band at 1384 cm1 and a weak band at 880 cm1 in its IR spectrum. These bands are not observed in the IR spectrum of the ligand. Hence, these bands are attributed to m1 and m4 vibrations of coordinated nitrato groups. The position of these bands is consistent with the presence of monodentately coordinated nitrato groups [36]. The complex 3 shows very strong absorption bands at 1199, 1150, 1108 and 1090 cm1. These bands are not visible in the IR spectrum of the uncoordinated dihydrazone. Hence, these bands are assigned to coordinated perchlorato groups. The bands at 1199, 1150 and 1108 cm1 owe their origin due to m3 vibrations of coordinated perchlorato groups while another strong band at 1090 cm1 (IR forbidden in non-coordinated perchlorato group) [36] is attributed to m1 vibration. The frequencies of m3 mode are in good agreement with those normally associated with the bidentate bridging perchlorato groups [36]. Magnetic moment The leff value for the complexes 1–3 per molecular formula fall in the range 2.25–2.6 BM while for empirical formulation in the range 1.30–1.50 BM (Table 3). The leff value for a system with three unpaired electrons, one electron on each metal centre, is

108

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

2.99 BM and for one unpaired electron per copper(II) atom is 1.73 BM on no metal–metal interaction basis. The experimental value of the magnetic moment is less than the values of 2.99 BM for three copper centres per molecular formula and is less than 1.73 BM for one copper centre per empirical formula when there is no interaction between metal atoms in the structural unit of the complexes. This suggests considerable antiferromagnetic interaction between metal ions in the structural unit of the complexes [38,39]. The magnetic moment value for the complexes may be decreased either due to intramolecular superexchange arising from the transfer of the paramagnetic spin density from one metal ion through the orbital overlap of the diamagnetic bridging oxygen atoms and chloride and perchlorate anion to another adjacent metal or due to direct metal–metal intramolecular interaction via the overlap of the suitable metal orbitals [40]. A minute examination of the magnitude of the magnetic moment values shows that the magnetic exchange is stronger in chlorido and perchlorato complexes than that in the nitrato complexes. On the basis of the magnitude of magnetic moment, it may be concluded that in chlorido and perchlorato complexes, there are both oxido and anion bridges, both of which are operative in magnetic exchange while in nitrato complex, only oxido-bridges participate in magnetic exchange [41]. Although, the extent of magnetic exchange between the metal ions due to oxido-bridges would be expected to be the same, in all of the complexes, the contribution towards magnetic exchange due to chloride and perchlorato groups might be different due to difference in size, which, most probably, might account for the difference in observed leff values of the complexes.

Electron paramagnetic resonance The complex 3 ([(Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH]) shows an isotropic spectrum at LNT in polycrystalline state. The giso value is equal to 2.064. This shows that the adjacent metal ions in the structural unit of the complex show persistently moderate strong interaction. The computed spacing between Cu2+ ions of each pair is about 4.26 Å, which is in the range where considerable exchange interaction might be expected [42]. The DMSO solution spectrum of the complex at RT is anisotropic. The complex shows a signal in g|| (2.259) region and another signal in the g\ region (2.090). The magnetic parameters (Table 5) for the complex fall in the order g|| > g\ > 2.0023, which 2 indicate that the d2y x orbital is the ground state. Further, the intensity of the signal in the g|| region is almost equal to that in the g\ region indicating considerable contribution from d2z -orbital to the ground state. The complex shows three signals in the region around 1660 G comparable in intensity to that around 3100 G. This shows that the spin of the ground state at RT is a mixture of the quartet state (S = 3/2) and doublet state (S = 1/2). In the present study, only

the signals within the ±1/2 (gav = 2.146) and ±3/2 (g = 3.917) Kramers doublets are observed. This suggests the energy gap |2D| between the ±1/2 and ±3/2 Kramers doublets is much greater than hm = 0.31 cm1 (where m is the spectrometer frequency) and is around 3.2 cm1 [43]. Furthermore, the variable temperature studies (LNT) reveal that the intensity of the Ms = ± 1/2 EPR spectrum increases with the decreasing temperature and that at Ms = ± 3/2 decreases. The shape of the spectra in the g = 2 region and moderately strong intensity of the half-filled transition reveal that the zero field splitting within the multiplet is quite appreciable in the complex 3 with respect to the incident quantum (0.31 cm1). The hyperfine splitting constant for the complex 3 in the forbidden region at RT is 100 G. The zero-field splitting D for a copper(II)–copper(II) couple is determined by two main contributions between the unpaired electron spins often taken as magnetic dipolar interaction [44]. Direct magnetic interactions as well as super exchange interaction (anisotropic exchange) [45] both contribute to zero-field splitting in a symmetrical multinuclear copper(II) complex (a c2v symmetry may be roughly assumed for the complex). The direct magnetic interaction which would arise from the dipolar interaction between the two local doublets separated by 4.26 Å is 9.74  103 cm1 which should be detectable. As far as the anisotropic exchange is concerned, it is due to the spin–orbit coupling and is proportional to the interaction between the ground state of an ion and the excited state of the other [46]. For a quasi-planar system, the most efficient ground state-excited state interaction is of 2 the type (dxy)(d2y x ) involving in-plane orbitals [47]. In the title com2 pound, we are dealing with d2y x magnetic orbitals centred on each copper(II) ion. This may be large enough for CuX2Cu entities, X being monoatomic bridge which leads to copper–copper separation close to 4.26 Å. Such an interaction quickly approaches to zero when the metal–metal distance decreases by using polyatomic bridges. At 77 K, the complex again shows an isotropic spectrum in DMSO glass with giso value equal to 2.038. The giso value is very close to the value of 2.064 at LNT in the solid state suggesting that the structure of the complex in DMSO glass at LNT is almost same as that in solid state. The complex shows the forbidden transition at g = 3.694 in DMSO glass. Here a variation of g between LNT (77 K) and 300 K on the order of 0.223 is observed. Banci et al. [48] also found that g varies between 4.2 K and 300 K on the order of Dg  0.03 for trinuclear copper(II) complexes while Haase et al. [49] have detected the same phenomenon with a Dg = 1.7. Similar observation has also been made by Borräs et al. [43] on trinuclear copper complexes of N(2-methylpyridyl)toluene sulphonamide. Variation of the thermal population of different spin levels were assumed to explain this effect. The strong g – shift is due to a temperature dependent contribution of the resonance fields of the spin states to a composed EPR. With the decrease in the temperature, the intensity of the forbidden transition around 1600 G decreases while that at around 3100 G increases. This suggests that the ground state for the complex is the doublet state. It is imperative

Table 5 EPR parameters of the homotrinuclear copper complexes of disalicylaldehyde malonoyldihydrazone. Sl. No.

Complex

Phase

Temp

g3(g||)

g2

g1(g\ )

giso/ gav

A3(G)/ Ak

A1(G)/ A\

Aav

gk/Ak

DM s = ± 2

A(G) For DMs = ±2

1

[Cu3(slmh)(l-Cl2)(CH3OH)3]0.5CH3OH (1)

Solid DMSO Soln Solid

LNT RT LNT LNT

2.339 2.356 2.322 2.417

2.026 – – –

1.901 2.097 2.045 2.058

2.089 2.183 2.137 2.178

– – 120 –

– – 70 –

– – 86.7 –

– – 207.6 –

– 135 – –

– 135 – –

2

[Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH (2)

DMSO Soln Solid

RT LNT LNT

2.306 2.347 2.600

– – –

2.071 2.045 2.064

2.149 2.146 2.243

– 120 –

50 – –

– – –

– 209.8 –

– – –

– – –

3

[Cu3(slmh)(lClO4)2(CH3OH)3]2CH3OH (3)

DMSO Soln

RT LNT

2.257 –

– –

2.090 –

2.146 2.038

– –

– –

– –

– –

100 80

100 80

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

to mention that the hyperfine coupling constant for the forbidden transition also decreases from 100 G at RT to 80 G at LNT. The EPR spectrum of the complex 1 ([(Cu3(slmh)(l-Cl)2 (CH3OH)3]0.5CH3OH]) at LNT in polycrystalline state (Figs. 2 and 3) is split into three signals each in g1, g2 and g3 regions, respectively, without hyperfine splitting. The observed g-values for the complex are g1 = 1.901, g2 = 2.026 and g3 = 2.339. The observance of three g-values in EPR spectrum is consistent with the fivecoordinate structure with considerable contribution from d2z orbital to the ground state [50]. The R-value for complex 1 at LNT in polycrystalline state is 0.40 which is less than one indicating 2 d2y x as the ground state [51]. The DMSO solution spectrum of the complex 1 at RT is also anisotropic similar to that of complex 3 with g|| and g\ values equal to 2.356 and 2.097, respectively. The magnetic parameters 2 for the complex falling in the order g|| > g\ > 2.0023 indicate d2y x orbital as the ground state. Almost similar intensity of the signal in the g|| region to that in the g\ region suggests considerable contribution from the d2z orbital to the ground state. The forbidden signal corresponding to DMs = ±2 transition in the X-band spectrum of the complex appears at 1600 G, which is comparable in intensity to that at 3100 G. This reveals that the spin of the ground state at RT is a mixture of the quartet state (S = 3/2) and a doublet state (S = 1/2) in this complex also similar to that in the complex 3. The appearance of the signals within the ±1/2 and ±3/2 Kramers doublets in this complex again suggests that the energy gap |2D| between the ±1/2 and ±3/2 Kramers doublets is much greater than hm = 0.31 cm1 (where m is the spectrometer frequency) and is around 3.2 cm1 [40]. The variable temperature studies reveal that the intensity of the Ms = ±3/2 signal decreases while that of Ms = ±1/2 signal increases

Fig. 2. EPR spectrum for complex 1 in polycrystalline state at LNT (B) (f = 9.1 GHz).

Fig. 3. EPR spectrum of complex 2 in frozen DMSO solution at LNT (B) (f = 9.1 GHz).

109

and ultimately the intensity of Ms = ±3/2 signal vanishes at LNT. This gives a strong indication that the Ms = ±1/2 levels are the ground state at lower temperature similar to that in complex 3. The shape of the spectra in the g = 2 region and moderately strong intensity of the half-filled transition at RT reveal that the zero-field splitting within the multiplet is quite appreciable in complex 1 with respect to the incident quantum (0.31 cm1). The computed Cu  Cu distance in the complex is around 3.75 Å and the direct magnetic interaction arising from the dipolar interaction between the two local doublets separated by 3.75 Å is around 11.06  103 cm1. Further, the complex shows three signals in the forbidden region with hyperfine splitting constant equal to 135 G. The remaining essential features of the EPR spectrum for the complex 1 at RT are the same as that of the complex 3 and hence their further discussion seems redundant. The EPR spectrum of the complex 1 in solution at LNT exhibits a resolved hyperfine structure in both the g|| and g\ regions, respectively. The complex shows four hyperfine lines in g|| region while two hyperfine lines in g\ region. It is imperative to mention that the hyperfine lines at higher field (around 3000 G) are stronger in intensity than those at lower field (around 2700 G) [52]. The g|| values corresponding to weak and strong hyperfine lines are 2.399 and 2.197 with average value being about 2.322 while the g\ values for weak and strong signals are 2.045 and 1.988, respectively. The hyperfine splitting constant A|| values for weak and strong signals are 110 G and 150 G, respectively, with average value being about 130 G. This feature, most probably, may arise due to two sets of different but similar Cu(II) centres in the tetragonal environments. The essential features of these peaks suggest that, they arise from overlap of one set of signals corresponding to one set of copper atoms with another set of signals corresponding to another set of copper atoms. This, preferably, may be a square-pyramidal environment around Cu(II) centre with g|| > g\ > 2. However, this may also imply a change in stereochemistry for the copper centre from trigonal bipyramidal to square-pyramidal [51]. The EPR spectra of the trigonal bipyramidal complexes are characterized by an axial symmetry with g\ > g||  2. The trigonal bipyramidal complexes have usually hyperfine structure in the g|| region with A|| in the range (60–10)  104 cm1 [54]. The A|| value calculated from the resolved hyperfine structure for the compound is 121.2  104 cm1. The reverse pattern of g|| > g\  2 and average value of metal hyperfine coupling constant of 121.2  104 cm1 observed for the compound indicates a distorted trigonal bipyramidal geometry approaching to square-pyramidal. The lowest principal g-value for the compound is 1.988. This g-value is quite close to the values observed for the structurally analogous Cu compound having trigonal bipyramidal stereochemistry established by X-ray Crystallography. From the above discussions, it may be suggested that the coordination geometry around copper in the complex is somewhere in between the tetragonally distorted square pyramidal and trigonal bipyramidal. The EPR spectrum of the complex 2 ([Cu3(slmh)(NO3)2 (CH3OH)5]1.5CH3OH (2)) shows only two signals in the polycrystalline state at LNT, one in the g|| region and the other in the g\ region with g|| and g\ values equal to 2.417 and 2.058, respectively. The essential features of the signals are indicative of distorted square pyramidal stereochemistry (g|| signal is comparable in intensity to the g\ signal). The DMSO solution spectrum of the complex 2 at LNT is similar to those of the complexes 1 and 3 with g|| and g\ values equal to 2.347 and 2.045, respectively. This reveals that the complex has 2 distorted square pyramidal stereochemistry even with d2y x orbital as the ground state with considerable contribution from the d2z orbital. The forbidden signal corresponding to DMs = 2 transition at 1600 G comparable in intensity to that at 3100 G indicates that

110

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

the ground state of the complex at RT is a mixture of the quartet state (S = 3/2) and a doublet state (S = 1/2) [43]. The increase in the intensity of the Ms = ±1/2 EPR signal with decreasing temperature and the decrease in intensity of the Ms = ±3/2 signal and its absence in DMSO glass reveals that in this complex Ms = ±1/2 levels are the ground state at low temperature. Although the Ms = ±3/2 signal for the complexes 1 and 3 shows hyperfine splitting, such a splitting is absent in complex 2. This arises, most probably, due to absence of anion bridging in complex 2. The DMSO glass spectrum of the complex at LNT shows six signals, although weakly resolved in the g|| region with average hyperfine splitting constant equal to 70 G (65.3  104 cm1). The dissociation of the complex is not expected to be responsible for appearance of extra-hyperfine lines since the electronic spectrum of the complex in DMSO solution reveals that the principal dihydrazone ligand is coordinated to the metal centre. The hyperfine lines at higher field around 3000 G are relatively stronger than those at lower field around 2700 G. This feature may arise from the overlap of two sets of peaks corresponding to two sets of different but similar Cu(II) centres in the tetragonal environments, preferably, a square pyramidal environment around Cu(II) with g|| > g\ > 2.0023. However, this implies a change in stereochemistry to the copper centre from trigonal bipyramidal to square pyramidal [53]. This is also corroborated from the reverse pattern g|| > g\  2.006 and hyperfine coupling constant in the g|| region equal to 65.3  104 cm1 of the complex [54]. The lowest principal g-value is 2.045, very close to the values observed for Cu compounds having trigonal bipyramidal stereochemistry [55,56]. The above facts clearly suggest that the coordination geometry around copper in the complex is intermediate between the tetragonally distorted square pyramidal and trigonal bipyramidal. It may be considered, alternately, EPR may originate from considerable exchange interaction between the copper centres [57]. This is also corroborated from the fact that the forbidden DMs = 2 transition at half-filled appears at around 1570 G in the solution at RT. This transition appears when the coupling is appreciable between copper centres. Cyclic voltammetry The cyclic voltammograms for the complexes show ligand-centred electron transfer reactions in the range 1.2 to 2.0 and +1.2 to +2.0 respectively [10]. Hence, the cyclic voltammograms for the complexes have been recorded on a 2 mM solutions at a scan rate of 100 mV/s and 50 mV/s by cyclic voltammetric methods in DMSO solution because of the solubility reasons in non-coordinating organic solvents (CH3CN and CH2Cl2) with a 0.1 M tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte. The cyclic voltammetric data for the complexes have been given in Table 6. The cyclic voltammograms of complexes 1, 2 and 3 are given in Figs. 4–6. The complexes 2 and 3 invariably show two reductive waves at 0.19, 0.81 V and 0.23, 0.89 V, respectively, while the complex 1 shows only one reductive wave at 0.22 V. With the highly

negatively charged dihydrazone ligand bonded to the metal centre, it is expected to make the reduction of these metal centres, leading to quite negative Epc values [38]. The reductive wave at 0.81 and 0.89 V in complexes 2 and 3 does not have its counterparts in the return scan. This suggests that the species produced corresponding to this reductive wave is not sufficiently stable in DMSO solution and reverts back to the original species. This reductive wave may be attributed to ligand based electron transfer reactions. Further, each of the complexes shows three oxidative waves in the region 0.02 to +1.0 V in all of the complexes. An irreversible oxidative wave is observed in the region 0.90–1.00 V. This suggests that the species due to this oxidation does not survive long and reverts back to the original species. Hence, this is attributed to the oxidation of >C@N group in the ligand [59]. The remaining reductive and oxidative waves may be attributed to ligand-centred electron transfer reactions. The first redox couple (Epc = 0.22 V, Epa = 0.16 V and DE = 60 mV in complex 1), (Epc = 0.19 V, Epa = 0.04 and DE = 150 mV in complex 2) and (Epc = 0.23 V, Epa = 0.17 V and DE = 60 mV in complex 3) is either reversible (complexes 1 and 3) or quasi-reversible (in complex 2) in nature [59]. The peak potential separation for the complexes 1 and 3 is 60 mV while that for complex 2 is 150 mV. The complex 3 shows another oxidative wave at 0.02 V. This oxidative wave does not have its counterpart in the reductive scan. This suggests that the metal-centred species corresponding to this oxidative wave is again unstable and reverts back to its corresponding original species. This wave may be attributed to the species [(L)CuII CuII CuIII]3+. The absence of the wave corresponding to this species in the reductive scan suggests that it is not possible to have the species containing copper(III) in the solution. The reductive and oxidative waves for the redox couples are separated from one another by 150 mV in nitrato complex suggesting that the redox process is quasi-reversible. The high peak separation, most probably, originated from a slow heterogeneous electron exchange rather from intervening heterogeneous reaction [59]. In order to confirm that the electron transfer reaction in complex 2 is a quasi-reversible metal-centred electron transfer reaction and the large separation between reductive and oxidative waves is due to slow heterogeneous electron exchange only, the cyclic voltammogram of the complexes were recorded at slower scan rate of 50 mV/s also. The point of crucial importance is that when the scan rate is decreased from 100 mV/s to 50 mV/s, the position of the waves corresponding to metal-centred electron transfer reaction changes and the separation between them decreases from 150 mV to 60 mV respectively. Further, the cyclic voltammograms for the complexes 1 and 3 were also recorded at a scan rate of 50 mV/s. In these cases also, the separation between reductive and oxidative waves was found to be 60 mV. Such a cyclic voltammetric behaviour of the complexes shows that the metal-centred electron transfer reactions are reversible. The electron transfer reaction corresponding to the redox waves may be shown as below:

Table 6 Electrochemical data for the homotrinuclear copper complexes of disalicylaldehyde malonoyldihydrazone. Sl. No.

Complex

Epc (V)

Epa (V)

1

[Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH [1]

– – 0.22

+1.00 +0.36 0.16

DE (mV) 60

2

[Cu3 (slmh)(NO3)2(CH3OH)5]1.5CH3OH [2]

– 0.19 0.81

+1.00 0.04 –

150

3

[Cu3(slmh)(l-ClO4)2(CH3OH)3]CH3OH [3]

– – 0.23 0.89

+0.91 0.02 0.17 –

60

111

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

centres in the complexes 1 and 3 as compared to nitrato groups which functions as a monodentate ligand in complex 2. On the basis of various physio-chemical and spectroscopic data and their discussions, the complexes may be suggested to have structures shown in Figs. 7 and 8. Reactivity study

Fig. 4. Cyclic voltammogram of [Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH in DMSO solution using TBAP as supporting electrolyte (scan rate 100 mV/s).

Fig. 5. Cyclic voltammogram of [Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH in DMSO solution using TBAP as supporting electrolyte (scan rate 100 mV/s).

The selective oxidation of alcohols to their corresponding carbonyl compounds is a fundamental transformation in organic synthesis [60,61]. In particular, the conversion of primary alcohols to aldehydes is crucial in the synthesis of natural products and fine chemicals such as fragrances and food additives [62]. Keeping in view the fact that copper is the catalytic centre in superoxide dismutase and catalyzes the dismutation of superoxide ion, it was worthwhile to investigate the catalytic activity of the present homotrinuclear complex. Accordingly, the catalytic activity of the complex 1 towards the hydrogen peroxide mediated oxidation of primary alcohol viz, benzyl alcohol was studied and the results postulated. Room temperature hydrogen peroxide was chosen and the initial study was carried out using benzyl alcohol as substrate (4.63 mmol) and 1.91 equivalent of H2O2 (8.83 mmol) at 70 °C in presence of 0.022 mmol of catalyst (Scheme 1). The oxidation reaction was conducted at 70 °C keeping [Cu3 (slmh)(l-Cl)2(CH3OH)3]0.5CH3OH:benzyl alcohol:H2O2 molar ratio at (1:209.5:398.2) for 12 h. The result reveal that this homotrinuclear complex efficiently catalyzes the oxidation of primary alcohol to the aldehyde in 53% yield. Fortunately, the usual undesirable further oxidation to acid was not observed at all under the present reaction condition. In this work, direct selective oxidation of benzyl alcohol to benzaldehyde with 30% H2O2 was performed over [Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH as a catalyst with good benzyl alcohol conversion to benzaldehyde. The experimental results indicated that complex was an efficient catalyst for oxygen transfer from H2O2 to the substrate. The catalyst did not induce the unproductive decomposition of H2O2 to any great extent and enables the economic use of the oxidant.

H C

O

N Cu

CH3OH

H2 C

N

X

C

C

O

O

N

N Cu

Cu

H C

O

X CH3OH

CH3OH Fig. 7. Suggested tentative structures for the complexes [Cu3(slmh)(l-X)2(CH3OH)3]nCH3OH[n = 0.5, X = Cl (1); n = 2, X = ClO4 (3)].

Fig. 6. Cyclic voltammogram of [Cu3(slmh)(l-ClO4)2(CH3OH)3]2CH3OH (C) in DMSO solution using TBAP as supporting electrolyte (scan rate 100 mV/s).

½ðLÞCujj Cujj Cujj 

2þ þe

½ðLÞCujj Cujj Cujj 

1þ

e

The difference in cyclic voltammetric behaviour of the complexes 1 and 3 as compared to that of the complex 2 may be inherent in the nature of the coordinated anions. The chloride and perchlorato groups function as a bridge between the two metal

Fig. 8. Suggested tentative (CH3OH)5]1.5CH3OH.

structure

for

the

complex

[Cu3(slmh)(NO3)2

112

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113

[Cu3(slmh)(µ 2-Cl)2(CH3OH)3].0.5CH3OH CH2OH

30 % H2O2 :Heat

CHO

Scheme 1. Benzyl alcohol was oxidized to benzaldehyde in only 53% yield after 12 h.

It was observed that benzoic acid was not produced in the oxidation of benzyl alcohol. It failed when attempted to directly convert benzyl alcohol to benzoic acid. After a mixture of benzyl alcohol (0.5 g, 4.63 mmol) and 30% H2O2 (1 mL, 8.83 mmol) was vigorously stirred in air at 70 °C for 12 h, there was no consumption of H2O2 and neither benzaldehyde nor benzoic acid was obtained, only benzyl alcohol was recovered after work up of reaction mixture. In this system, the complex catalyst exhibit high selectivity for oxidation of benzyl alcohol to benzaldehyde. During the reaction process, the catalyst did not dissolve in the organic substrate. After addition of 150 mL of dichloromethane, the organic phase was extracted and dried over Na2SO4. The extract was distilled off under vacuum. The catalyst was recovered >95% yield (by weight). Conclusion In the present study, we have synthesized three homotrinuclear copper(II) complexes [Cu3(slmh)(l-Cl)2(CH3OH)3]0.5CH3OH, [Cu3(slmh)(NO3)2(CH3OH)5]1.5CH3OH and [Cu3(slmh)(l-ClO4)2 (CH3OH)3]2CH3OH in excellent yields. All of the complexes are monomeric and non-electrolyte. The dihydrazone is present in enol form in all of the complexes and functions as a tetrabasic hexadentate coordinating to the metal centre through phenolate oxygen atoms, enolate oxygen atoms and azomethine nitrogen atoms. All of the complexes involve considerable metal–metal interactions between metal atoms in the structural unit. At RT, the spin states for the complexes is a mixture of quartet state and doublet state while the ground state at low temperature is a doublet state. The complexes have distorted square pyramidal geometry (Figs. 7 and 8). The electron transfer reactions of the complexes have been investigated which show only one metal-centred electron transfer reaction.

[5]

[6]

[7] [8] [9] [10] [11]

[12]

[13]

[14]

[15] [16]

Acknowledgements Authors are thankful to the Head, SAIF, North Eastern Hill University, Shillong 793022, Meghalaya, India for recording mass spectra, the Head, SAIF, IIT Bombay, India for recording EPR spectra. Angira Koch would like to thank the UGC, New Delhi for award of RFSMS Fellowship through North Eastern Hill University, Shillong 793022.

[17] [18] [19]

[20]

References [1] S. Gupta, M.V. Kirrilova, M.F.C. Guedes da Silva, A.J.L. Pombiero, A.M. Kirrilov, Inorg. Chem. 52 (2013) 8601–8611. [2] (a) R.T. Sedaghat, L. Tahmadi, H. Motamasbi, R.R. Martinez, D.M. Morales, J. Organomet. Chem. 66 (2013) 712–724; (b) N.A. Lalami, H.H. Monfared, H. Noei, P. Meyer, Transition Met. Chem. 36 (2011) 669–677. [3] (a) R. Borthakur, A. Kumar, R.A. Lal, Spectrochim. Acta A 118 (2014) 94–101; (b) R. Borthakur, A. Kumar, A. Lemtur, R.A. Lal, RSC Adv. (2013) 15139–15147; (c) A. Koch, A. Phukan, O.B. Chanu, A. Kumar, R.A. Lal, J. Mol. Struct. 1060 (2014) 119–130; (d) A. Kumar, O.B. Chanu, A. Koch, R.A. Lal, J. Struct. Chem. 54 (2013) 702–712; (e) A. Ahmed, R.A. Lal, J. Mol. Struct. 1048 (2013) 321–330. [4] (a) M. Sutradhar, L.M. Carrella, E. Rentschler, Eur. J. Inorg. Chem. (2013) 4273– 4278; (b) M. Sutradhar, J.R. Barman, J. Klauke, M.G.B. Drew, E. Rentschler, Polyhedron 53 (2013) 48–55; (c) S. Roy, T.N. Mandal, A.K. Barik, S. Pal, R.J. Butcher, M.S.E. Fallah, J. Tercero, S.K. Roy, Daltons Trans. (2007) 1229–1234; (d) S. Gupta, A.K. Barik, S. Pal, A. Hazra, S. Roy, R.J. Butcher, S.K. Kar, Polyhedron

[21] [22]

[23]

[24] [25] [26]

[27] [28] [29]

26 (2007) 133–414; (e) M. Sutradhar, G.N. Mukherjee, M.G.B. Drew, S. Ghosh, Inorg. Chem. 46 (2007) 5069–5075; (f) M. Sutradhar, G.N. Mukherjee, M.G.B. Drew, S. Ghosh, Inorg. Chem. 45 (2006) 5150–5161. L. Zhao, V. Niel, L.K. Thompson, Z. Xu, V.A. Milway, R.G. Harvey, D.O. Miller, C. Wilson, M. Leech, J.A.K. Howard, S.L. Heath, J. Chem. Soc. Daltons Trans. (2004) 1446–1455. (a) R.A. Lal, D. Basumatary, A.K. Dey, A. Kumar, Trans. Met. Chem. 32 (2007) 481–493; (b) R.A. Lal, D. Basumatary, O.B. Chanu, A. Lemtur, M. Asthana, A. Kumar, A.K. De, J. Coord. Chem. 64 (2011) 1729–1742. R. Borthakur, M. Asthana, A. Kumar, A. Koch, R.A. Lal, RSC Adv. 3 (2013) 22957– 22962. A. Kumar, R. Borthakur, A. Koch, O.B. Chanu, A. Lemtur, J. Mol. Struct. 99 (2011) 89–1990. O.B. Chanu, A. Kumar, A. Lemtur, R.A. Lal, Spectrochim. Acta A 96 (2012) 854– 861. O.B. Chanu, A. Kumar, S. Ahmed, R.A. Lal, J. Mol. Struct. 1007 (2012) 257–274. (a) A. Ahmed, R.A. Lal, J. Mol. Struct. 148 (2013) 321–340; (b) R.A. Lal, S. Choudhury, A. Ahmed, R. Borthakur, M. Asthana, A. Kumar, Spectrochim. Acta A 75 (2010) 212–224; (c) R.A. Lal, D. Basumatary, S. Adhikari, A. Kumar, Spectrochim. Acta A 69 (2008) 706–714. (a) A. Siegel, H. Siegel (Eds.), Metal Ions in Biological Systems, Marcel Dekker, New York, 2002, p. 39; (b) G. Bjørklund, Acta Neurobiol. Exp. 73 (2013) 225–236; (c) G. Vashchenko, T.A. MacGillivray, Nutrients 5 (2013) 2289–2313. (a) D. Gatteschi, O. Khan, J.S. Miller, F. Palacio, Magnetic Molecular Materials, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1991. NATO ASI Series 198; (b) O. Kahn, Molecular Magnetism, VCH, Weinheim, Germany, 1993. (a) J.M. Hoover, S.S. Stahl, J. Am. Chem. Soc. 133 (2011) 16901–16910; (b) C. Han, M. Yu, W. Sun, Y. Yao, Synlett (2011) 2363–2368; (c) S.G. Babu, P.A. Priyadarshini, R. Karvembu, Appl. Catal. A 392 (2011) 218– 224; (d) R. Dun, X. Wang, M. Tan, Z. Huang, X. Huang, W. Ding, X. Lu, ACS Catal. 3 (2013) 3063–3066; (e) R. Kumar, K. Mahiya, P. Mathur, Dalton Trans. 42 (2013) 8553–8557. E.I. Solomon, R.K. Szilagyi, S.D. George, L. Basumallick, Chem. Rev. 104 (2004) 419–458. (a) C. F Martens, R.J.M. Klein Gebbink, P.J.A. Kenis, A.P.H.J. Schenning, M.C. Feiters, J.L. Ward, K.D. Karlin, R.J.M. Nolte, in: K.D. Karlin, Z. Tyeklär (Eds.), Bioinorganic Chemistry of Copper, Chapman & Hall, New York, 1993, pp. 374– 381; (b) E.I. Solomon, U.M. Sundaram, T.E. Machonkin, Chem. Rev. 96 (1996) 2563– 2606. M. Casarin, C. Corvaja, C.D. Nicola, D. Falcomer, L. Franco, M. Monari, L. Pandolfo, C. Pettinari, F. Piccinelli, Inorg. Chem. 44 (2005) 6265–6276. H. Siegel, Metal Ions in Biological Systems, vol. 13, Marcel Dekker, New York, 1981. (a) T.E. Machonkin, H.H. Zhang, B. Hedman, K.O. Hodgson, E.I. Solomon, Biochemistry 37 (1998) 9570–9578; (b) C.K. Mukhopadhyay, J.K. Attieh, P.L. Fox, Science 279 (1998) 714–717. (a) J. Sanmartin, M.R. Bermejo, A.M. García-Deibe, O.R. Nascimento, L. Lezama, T. Rojo, J. Chem. Soc., Daltons Trans. (2002) 1030–1034; (b) I. Castro, M.L. Calatayud, F. Lloret, J. Sletten, M. Julve, J. Chem. Soc. Daltons Trans. (2002) 2397–2403. A.M. Kirillov, M.N. Kopylovich, M.V. Kirrillova, M. Haukka, M.F.C.G. de Silva, A.J.L. Pombeiro, Angew. Chem. Int. Ed. 44 (2005) 4345–4349. (a) M.K. Singh, N.K. Kar, R.A. Lal, J. Coord. Chem. 61 (2008) 3158–3171; (b) M.K. Singh, N.K. Kar, R.A. Lal, J. Coord. Chem. 62 (2009) 1677–1689; (c) G.A. Al- Hazmi, A.A. El-Asmy, J. Coord. Chem. 62 (2009) 337–345. (a) N.-Y. Jiu, J. Coord. Chem. 65 (2012) 4013–4022; (b) Y. Liu, K. Zhang, R. Lei, J. Liu, T. Zhou, Z.-Y. Yang, J. Coord. Chem. 65 (2012) 2041–2054. A.I. Vogel, A Textbook of Quantitative Inorganic Analysis Including Elementary Instrumentation Analysis, fourth ed., ELBS and Longman, London, 1978. A. Syamal, R.L. Dutta, Elements of Magnetochemistry, East-West Press, Pvt. Ltd., New Delhi, 1993. (a) A.V. Nikolov, V.A. Logvinenko, L.I. Myachina, Thermal Analysis, vol. 2, Academic Press, New York, 1969; (b) R.A. Lal, S. Das, R.K. Thapa, Inorg. Chim. Acta 132 (1987) 129–136. W.J. Geary, Coord. Chem. Rev. 7 (1971) 81–122. T.D. Smith, J.R. Pilbrow, Coord. Chem. Rev. 13 (1974) 173–278. A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier, Amsterdam, 1984.

A. Koch et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 103–113 [30] J.R. Dyer, Applications of Absorption Spectroscopy of Organic Compounds, Prentice Hall of India Pvt. Limited, New Delhi 110001, 1989 (Seventh printing). [31] R.M. Silverstein, F.X. Webster, Spectrometric Identification of Organic Compounds, sixth ed., John Wiley & Sons, Inc., 1998. [32] A. Syamal, K.S. Kale, Inorg. Chem. 18 (1979) 992–995. [33] G.C. Percy, D.A. Thornton, J. Inorg. Nucl. Chem. 34 (1972) 3369–3372. [34] (a) R. Lozano, A. Doadrio, A.L. Doadrio, Polyhedron 1 (1982) 163–167; (b) W.E. Newton, J.L. Corbin, D.C. Bravard, J.E. Searles, J.W. Mc-Donald, Inorg. Chem. 13 (1974) 1100–1104. [35] J. Sanders-Loehr, W.D. Wheeler, A.K. Shiemke, B.A. Averill, T.M. Loehr, J. Am. Chem. Soc. 111 (1989) 8084–8093. [36] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., John-Wiley & Sons, New York, 1986. [37] J.R. Ferraro, Low frequency Vibrations of Inorganic and Coordination Compounds, Plenum Press, New York, 1971. [38] (a) B.M. Gatehouse, S.E. Livingstone, R.S. Nyholm, J. Chem. Soc. 22 (1957) 4222–4225; (b) B.M. Gatehouse, S.E. Livingstone, R.S. Nyholm, J. Inorg. Nucl. Chem. 8 (1958) 79–86; (c) N.F. Curtis, Y.M. Curtis, Inorg. Chem. 4 (1965) 804–808; (d) S.A. Cameron, S. Brooker, Inorg. Chem. 50 (2011) 3697–3706. [39] S. Mukhopadhyay, D. Mandal, P.B. Chatterjee, Cédric Desplanches, J.-P. Sutter, R.J. Butcher, M. Chaudhury, Inorg. Chem. 43 (2004) 8501–8509. [40] F. Birkelbach, M. Winter, U. Floerke, H.-J. Haupt, C. Butzlaff, M. Lengen, E. Bill, A.X. Trautwein, K. Wieghardt, P. Chaudhuri, Inorg. Chem. 33 (1994) 3990– 4001. [41] (a) O. Kahn, Molecular Magnetism, VCH Publishers, Weinheim, Germany, 1993; (b) E. Colacio, M. Ghazi, R. Kivekäs, M. Klinga, F. Lloret, J.M. Moreno, Inorg. Chem. 39 (2000) 2770–2776. [42] S.K. Mandal, L.K. Thompson, M.J. Newlands, E.L. Gabe, F.L. Lee, Inorg. Chem. 29 (1990) 3556–3561. [43] L. Gutierrez, G. Alzuet, J.A. Real, J. Cano, J. Borräs, A. Castiñeiras, Inorg. Chem. 39 (2000) 3608–3614. [44] L. Banci, A. Bencini, D. Gatesschi, J. Am. Chem. Soc. 105 (1983) 761–764. [45] A. Bencini, A.C. Fabretti, C. Zanchini, P. Zannini, Inorg. Chem. 26 (1987) 1445– 1449. [46] (a) I. Castro, J. Sletten, J. Faus, M. Julve, Y. Journaux, F. Lloret, S. Alvarez, Inorg. Chem. 31 (1992) 1889–1894; (b) A. Bencini, D. Gatteschi, C. Zanchini, Inorg. Chem. 24 (1985) 700–703;

[47] [48] [49]

[50]

[51] [52] [53] [54]

[55] [56]

[57] [58]

[59] [60] [61] [62]

113

(c) A. Bencini, D. Gatteschi, Electron Paramagnetic Resonance of Exchange Coupled Systems, Springer, Berlin, 1990. M.F. Charlot, Y. Journaux, O. Kahn, A. Bencini, D. Gatteschi, C. Zanchini, Inorg. Chem. 25 (1986) 1060–1063. L. Banci, A. Bencini, D. Gatteschi, Inorg. Chem. 22 (1983) 2681–2683. (a) W. Haase, S. Gehring, J. Chem. Soc. Dalton Trans. (1985) 2609–2613; (b) S. Gehring, W. Haase, H. Paulus, Acta Crystallogr. C47 (1991) 1814– 1816; (c) P. Fleischhauer, S. Gehring, W. Haase, Ber. Bunsenges. Phys. Chem. 96 (1992) 1701–1704; (d) S. Gehring, P. Fleischhauer, H. Paulus, W. Haase, Inorg. Chem. 32 (1993) 54–60. (a) M.J. New, R.D. Dudley, R.J. Fereday, B.J. Hathaway, R.C. Slade, J. Chem. Soc. A (1971) 1437–1442; (b) P.J.M.W.L. Birker, H.M.J. Hendriks, J. Reedijk, G.C. Verschoor, Inorg. Chem. 20 (1981) 2408–2414. B.J. Hathaway, J. Chem. Soc. Dalton Trans. (1972) 1196–1199. F.H. Fry, L. Spiccia, P. Jensen, B. Moubaraki, K.S. Murray, E.R.T. Tiekink, Inorg. Chem. 42 (2003) 5594–5603. E. Monzani, L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Radaccio, S. Geremia, C. Nardin, P. Faleschini, G. Tabbi, Inorg. Chem. 37 (1998) 553–562. (a) K.R. Justin Thomas, V. Chandrasekhar, P. Pal, S.R. Scott, R. Hallford, A.W. Cordes, Inorg. Chem. 32 (1993) 606–661; (b) R. Barbucci, A. Bencini, D. Gatteschi, Inorg. Chem. 16 (1977) 2117–2120. F. Huq, A.C. Skapski, J. Chem. Soc. A (1971) 1927–1929. (a) T.R. Felthouse, E.J. Laskowski, D.N. Hendrickson, Inorg. Chem. 16 (1977) 1077–1089; (b) T.R. Felthouse, D.N. Hendrickson, Inorg. Chem. 17 (1978) 444–456; (c) M.S. Haddad, D.N. Hendrickson, Inorg. Chem. 17 (1978) 2622–2630. G.A. McLachlan, G.D. Fallon, R.L. Martin, L. Spiccia, Inorg. Chem. 34 (1995) 254– 261. (a) B.M. Gatehouse, S.E. Livingstone, R.S. Nyholm, J. Inorg. Nucl. Chem. 8 (1958) 75–78; (b) N.F. Curtis, Y.M. Curtis, Inorg. Chem. 4 (1965) 804–809. H. Okawa, M. Koikawa, S. Kida, J. Chem. Soc. Daltons Trans. 641 (1988) 274– 278. G. Tozo, M. Fernandez, Oxidation of Alcohols to Aldehydes and Ketones, Basic Reactions in Organic Synthesis, Springer, New York, 2007. G. Tozo, M. Fernandez, Oxidation of Primary Alcohols to Carboxylic Acids: Basic Reactions in Organic Synthesis, Springer, New York, 2007. A.E.J. De Nooy, A.C. Besemer, H. van Bekkum, Synthesis (1996) 1153–1174.