Cite this: RSC Advances, 2013, 3,15139

Synthesis, characterization and electrochemicalproperties of bis(m2-perchlorato)tricopper(II) complexesderived from succinoyldihydrazones3

Received 25th April 2013,Accepted 10th June 2013

DOI: 10.1039/c3ra42048e

www.rsc.org/advances

R. Borthakur,a A. Kumar,b A. Lemtura and R. A. Lal*a

Three new homotrimetallic copper(II) complexes ([Cu3(Ln)(ClO4)2(H2O)m] with H4Ln = H4L1 2 H4L3, m = 0, 3)

have been synthesized from substituted succinoyldihydrazones (H4Ln) in methanol. The composition of the

complexes has been established on the basis of data obtained from analytical and mass spectral studies

and molecular weight determination in DMSO. The structure of the complexes has been discussed in the

light of molar conductance, magnetic moment data and electronic, EPR, IR and FT-IR spectral studies. The

molar conductance values for the complexes in the region of 1.2–1.7 V21 cm2 mol21 in DMSO indicate that

the complexes are nonelectrolytes. The magnetic moment values for the complexes suggest considerable

metal–metal interaction in the structural unit of the complexes. Copper centres have square planar and

square pyramidal stereochemistry. The EPR parameters of complexes 1 and 3 indicate that the copper

centre has a doublet ground state, while for complex 2, the ground state is a mixture of both doublet and

quartet states. Electron transfer reactions of the complexes have been investigated by cyclic voltammetry.

1. Introduction

Copper is an essential element with important biologicalfunctions, and is responsible for numerous catalytic processesin living systems1 where it is often present in differentnuclearities. Copper is present in enzymes in biologicalsystems either alone2 or in combination with other metal ionsto cooperatively realize its biological function. Copper occursalone in the active centre of particulate methane monoox-ygenase (pMMO) coupled sites3 where it is often present in di-or trinuclear assemblies. On the basis of magnetic suscept-ibility and EPR studies, it has been proposed that the coppercentre of pMMO is organized into a trinuclear catalytic orelectron transfer cluster.4 However, a mononuclear activecentre as well has been supported by another interpretation ofEPR spectra from similarly isolated pMMO samples.5

Moreover, a crystallographic study of a membrane preparationof pMMO at 2.3 Å resolution has identified only a mono-nuclear and a dinuclear metal site per enzyme unit.6 Theimportance of multimetallic copper complexes also lies intheir relevance in the development of novel functionalmaterials with molecular ferromagnetism7 and specific cata-lytic properties.8 Homo- and heterotrimetallic copper com-

plexes offer the opportunities to test magnetic exchangemodels on more complicated systems. Copper(II) complexesfind application as catalysts for the oxidation of alcohols toaldehydes and ketones. This makes studies on the hetero-geneous catalytic activities of homo and hetero-metal coppercomplexes attractive and challenging.9 Further, trinuclearcomplexes with bridging oxo and carboxylato motifs attractconsiderable attention due to the isolation and crystal-lographic characterization of trinuclear metal constellationsin the active sites of metalloenzymes, such as P1 nuclease(Zn3), phospholipase C (Zn3) and alkaline phosphatase(Zn2Mg).6c

Polyfunctional ligands serve as means for synthesizingmonometallic, homo- and heterobimetallic, trimetallic andpolymetallic complexes. Hydrazones are a special kind ofpolyfunctional Schiff base ligands derived from condensationof organic acid hydrazines and o-hydroxy aromatic aldehydesand ketones10–13 which have potential for yielding homo- andheterometallic complexes. Dihydrazones derived from con-densation of succinoyldihydrazines and o-hydroxy aromaticaldehydes have been selected in the present study. Thesemultidentate ligands possess as many as eight bonding sitesand can utilize up to six bonding sites simultaneously inbonding to metal ions in polynuclear metal complexes. Insuccinoyldihydrazones, the two hydrazone parts are joinedtogether through two methylene groups. These dihydrazonesare unique in the sense that their succinoyl fraction offersgreater flexibility in three dimensional space because of theirability to freely rotate around the C–C single bond as

aDepartment of Chemistry, North-Eastern Hill University, Shillong-793022,

Meghalaya, India. E-mail: profralal@gmail.combDepartment of Chemistry, Faculty of Science and Agriculture, The University of

West-Indies, St Augustine, Trinidad and Tobago, West-Indies

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra42048e

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compared to those in which the two hydrazone groups arejoined together either directly (oxaloyl) or through phenyl orpyridyl groups. The oxaloyl, phenyl or pyridyl fractions byvirtue of their planar characteristics impose planarity over thetwo hydrazone parts reducing their flexibility and reactivity. Byvirtue of their greater flexibility, succinoyl dihydrazones areexpected to be more reactive ligands than those derived fromoxaloyl, phenyl and pyridoyl dihydrazones, and to yield morestable complexes. Although such dihydrazones might exist inonly one configuration in the free state, in metal complexesthe can exist in a staggered anti-cis or syn-cis configuration.13

The resulting dihydrazones contain salicylaldimine, 2-hydroxy-naphthaldimine and 5-bromo-salicylaldimine in their mole-cular skeleton. Further, as we go from salicylaldiminedihydrazone to naphthaldimine dihydrazone to 5-bromo-salicylaldimine dihydrazone, the bulkiness and electronega-tivity increase in the same order. It would be interesting to seehow the properties of the complexes change as we move fromsalicylaldimine to 2-hydroxy-1-naphthaldimine to 5-bromosa-licylaldimine dihydrazones. Among the methodologies that areused for the construction of polynuclear assemblies ofpredesigned compositions, the most popular method is thestepwise construction of mononuclear transition metal com-plexes of polydentate ligands in which some of the donor sitesare unoccupied, and which often serve as efficient buildingblocks.14 Although some of these compounds have beenprepared by using either well-designed organic ligands ortransition metal complex ligands,15 much remains to belearnt.

A literature survey revealed that although some isolatedstudies are available on metal complexes of succinoyldihy-drazones13 and related dihydrazones,10–13 reports on thesynthesis and characterization of homo- and heterotrinuclearcomplexes of dihydrazones are quite meagre.16 Moreover, asystematic study on trinuclear metal complexes of succinoyl-dihydrazones is absent to the best of our knowledge, in spite oftheir highly flexible nature. In view of the above, theimportance of trinuclear metal complexes in general andcopper complexes in particular, the absence of work onhomotrinuclear complexes of succinoyldihydrazones and thehighly flexible polyfunctional nature of succinoyldihydra-zones, it was in our interest to synthesize the homotrinuclearcopper complexes of the title dihydrazones (Fig. 1), and tocharacterize them by various physicochemical and spectro-scopic studies. The electron transfer reactions of the com-plexes have also been investigated by cyclic voltammetry. Theresults of these investigations are presented in this paper.

2. Results and discussion

The complexes isolated in the present study have beensuggested to have the composition [Cu3(Ln)(ClO4)2(H2O)m](H4Ln = H4L1, H4L2 and H4L3; m = 0, 3) on the basis ofanalytical data, molecular weight and mass spectral studies.The complexes decompose without melting above 300 uC. The

melting points of the complexes are higher than those of thecorresponding parent ligand. This suggests that the metal–ligand bonds in these complexes have a more ionic characterthan those of the corresponding parent ligand. All complexesare insoluble in water and common organic solvents, but aresoluble in highly coordinating solvents such as DMSO andDMF. When heated in an electronic oven for four hours,complexes 1 and 3 do not show any weight loss either at 110 uCor 180 uC. This rules out the possibility of the presence ofwater molecules in either the lattice structure or coordinationsphere. Only complex 2 shows weight loss at 180 uC, whichamounts to the equivalent of three water molecules, andsuggests their presence in the first coordination spherearound the metal centre.

An effort was taken to crystallize the complexes in varioussolvent systems under different experimental conditions.Unfortunately, only amorphous compounds precipitated inall our efforts which prevented analysis of the complexes byX-ray crystallography.

2.1. Mass spectral studies

All of the complexes have been characterized by massspectrometry. The molecular ions along with their experi-mental and theoretical mass, for all of the complexes havebeen given in Table S3, ESI.3 Mass spectra of complexes 1 and2 are shown in Fig. S1 and S2, ESI.3

The mass spectrum of complex 1 has a peak at an m/z valueof 1094.77, which is close to the mass of the molecular ion[Cu3(H5L1)(HClO4)2(DMSO)4(H2O)2]+ (1094.65). Hence, thispeak can be attributed to this molecular ion. This molecular ionarises, most probably, from the coordination of four DMSOmolecules and two water molecules to the metal centres in thecomplex.17 The peak at the m/z value of 1027.63 corresponds to theexistence of the molecular ion [Cu3(L1)(ClO4)2(DMSO)3(H2O)3]+

(1027.65). This molecular ion results from coordination of threeDMSO molecules and three water molecules to the metal centres.Another molecular ion peak is obtained at the m/z value of 950.70,which suggests that it corresponds to the molecular ion[Cu3(HL1)(ClO4)2(DMSO)2(H2O)3]+ (950.65). A most prominent peakat the m/z value of 559.04 is observed which is similar to the mass ofthe molecular ion [Cu3(HL1)(H2O)]+ (559.65). This molecular ion most

Fig. 1 R1 = H, R2 = H, X = H: Disalicylaldehydesuccinoyldihydrazone (H4L1); R1 =H, R2 = 5,6-benzo, X = H: disalicylaldehydesuccinoyldihydrazone (H4L1); R1 = H,R2 = H, X = Br: bis(5-bromosalicyladehyde)succinoyldihydrazone (H4L3).

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probably originates from the molecular ion[Cu3(HL1)(ClO4)2(DMSO)2(H2O)]+ through the loss of two perchlorateions and two DMSO molecules.

The origin of different molecular ion peaks in the massspectra of complexes 2 and 3 can be explained in the sameway. Hence, their further discussion seems redundant. Thedifferent molecular ions presented in Table S3, ESI3originatefrom the loss of either coordinated water molecules orcoordinated perchlorate groups followed by coordination ofDMSO molecules. The mass spectral behavior of the com-plexes suggests that all of them are monomeric in nature.

2.2. Molecular weight

The molecular weights of complexes 1 to 3 have beendetermined in the highly coordinating solvent DMSO by thefreezing point depression method based on the insolubility innon-coordinating solvents. The complexes have molecularweights of 780 ¡ 35, 1060 ¡ 52 and 920 ¡ 45, respectively.The experimental values of the molecular weights of themonomers are very close to the theoretical values of 739.65,893.65 and 897.65, rather than the values of 1479.24, 1687.24and 1795.24 for the dimers. This suggests that the complexesare monomeric in nature. However, the experimental mole-cular weight of complex 2 is 1060 ¡ 52 which is slightly higherthan the theoretical value as calculated on the basis ofmonomer formulation. This is, most probably, owing to thereplacement of H2O molecules in complex 2 by some DMSOmolecules in the solution.

2.3. Molar conductance

The complexes have a molar conductance in the region of 1.2–1.7 V21 cm2 mol21. These values are consistent with theirnonelectrolytic nature in this medium.18

2.4. Magnetic moment

The complexes 1–3 have magnetic moments (meff) of 1.79, 1.87and 2.45 mB, per respective molecular formula, and 1.04, 1.08and 1.42 mB per empirical formula. These values are muchlower than the values of 2.99 mB for trinuclear copper(II)complexes and 1.73 mB for mononuclear copper(II) complexes,in which no metal–metal interactions are present. These meff

values suggest that there is a considerable metal–metalinteraction in the structural unit of complexes 1–3.19

The lowering of the magnetic moment values may be eitherdue to intramolecular superexchange arising from the transferof paramagnetic spin density from one metal ion through theorbital overlap of the diamagnetic bridging oxygen atoms andperchlorate anions to adjacent metal ions, or due to directmetal–metal intramolecular interaction via the overlap ofsuitable metal orbitals.20 The magnitude of the magneticmoment increases from the salicylaldimine to the 2-hydroxy-1-naphthaldimine to the 5-bromo-salicylaldimine complex. Thisshows that the magnetic exchange is stronger in thesalicylaldimine complex than that in the 2-hydroxy-1-naphthal-dimine complex in which it is stronger than that in the5-bromo-salicylaldimine complex. This may be related todecreasing metal–metal interactions with increasing stericcrowding in the ligand as the size of the aromatic ringincreases from the salicylaldimine to the 2-hydroxy-1-

naphthaldimine complex to the 5-bromo-salicylaldimine com-plex.

2.5. Electron paramagnetic resonance spectra

EPR spectra of the complexes in the present study are isotropicat RT. The g-values of 2.038–2.051 (Table S1, ESI3) indicate thatthe adjacent metal ions in the structural unit of the complexespersistently show moderately strong interactions. The spectraof complex 3 are isotropic both at RT and LNT in thepolycrystalline state, and the gav-value is 2.048 for bothconditions. The g-tensor does not change when the tempera-ture is lowered to the LNT, indicating the non-flexionalgeometry of the polyhedra. It appears that the roomtemperature narrowing is of an intramolecular nature due tothermal population of the two doublet and quartet states. Thisis confirmed by the fact that the EPR spectrum at LNT forcomplex 3, in which only the lowest doublets and quartetstates are populated, is narrowed by 60 G due to intramole-cular interactions and not broadened. Further, the polycrystal-line powder X-band spectra of complexes 1 and 3 at LNT showno transition at low field with g-values of around 4. Thissuggests that the complexes exhibit a ground state with a totalspin S = 1/2.21,22 However, the polycrystalline powder X-bandspectrum of complex 2 at LNT shows a transition at low fieldwith a g-value of 4.064. This indicates that the ground state ofcomplex 2 is a mixture of both the doublet and quartet statesat low temperature. Each of the complexes 1 and 2 shows threehyperfine lines in the g| region, while only two hyperfine linesare present in the g) region. The hyperfine coupling constantA| at LNT is 180 in the g| region, while A) in the g) region isequal to 55 G. The g| value is around 2.219 for both thecomplexes while g) values are 2.032 and 2.026, respectively.The magnetic parameters for the complexes fall in the order g|

. g) . 2.0023. This shows that in all complexes, copper has(dx22y2) orbitals as the ground state.

The EPR spectrum of complex 1 (Fig. 2) in frozen DMSOglass exhibits a resolved hyperfine structure in the g| regionwith six discernible lines with an average splitting of 95 G.23

The hyperfine lines at higher field around 3100 G are strongerthan the hyperfine lines at lower field around 2700 G. The g|

values corresponding to the weak and strong hyperfine lines

Fig. 2 EPR spectrum of the complex [Cu3(L1)(ClO4)2] (1) in DMSO glass at LNT (f= 9.1 Mz).

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are 2.399 and 2.153 with the average value for the six signalsbeing 2.281, while the g) values are 2.038 and 1.988 with anaverage value of 2.013. The hyperfine coupling constants A| forthe weak and strong signals are 115 and 75 G, respectively,with an average value of about 95 G. This feature may arisefrom the overlap of two sets of peaks corresponding to two setsof different but similar Cu(II) centres in the tetragonalenvironments, preferably a square pyramidal environmentaround a Cu(II) centre with g| . g) . 2. However, this wouldimply a change in stereochemistry for the copper centre fromtrigonal bipyramidal to square pyramidal.24 The EPR spectraof the trigonal bipyramidal complexes are characterized byaxial symmetry with g) . g| . 2.00. Usually, a hyperfinestructure is seen in the g| region with an A| in the range of 60–100 6 1024 cm21.24 The reverse pattern of g| . g) y 2observed for complex 1 indicates a distorted trigonal bipyr-amidal geometry with square pyramidal coordination of thecopper centre. This suggests that in this complex, the dz2

orbital of the copper atom has a significant contribution in theground state.25 The A| value calculated from the resolvedhyperfine structure of the compound is 89 6 1024 cm21 (1 G =0.9323 6 1024 cm21), and the lowest principal g-value is 2.013.This g-value is appreciably different from the values observedfor structurally analogous compounds where Cu has beenshown to be coordinated in a trigonal bipyramidal fashion byX-ray crystallography.26 This fact clearly suggests that thecoordination geometry around the copper atoms in thecomplexes lies in between the tetragonally distorted squarepyramidal and the distorted trigonal bipyramidal geometries.Alternatively, it may be considered that the EPR signal mayoriginate from a weak exchange interaction between thecopper centres.27 The hyperfine splitting pattern would resultfrom coupling of two sets of Cu nuclei with four lines of theweak set, near 2700 G, (most probably tetragonally distortedsquare pyramidal) overlapping with the first line of the strongset (most probably distorted trigonal bipyramidal) near 3100G. The essential features of the EPR spectra support thetrinuclear interaction of the signal. The EPR spectra of thefrozen solution of complexes 2 and 3 essentially exhibit axialsignals (g| = 2.169, g) = 2.041, A| = 120 G, for complex 2; g| =2.193, g) = 2.048, A| = 140 G, A) = 30 G for complex 3) with athree line hyperfine splitting in complex 2 and a two linehyperfine splitting in complex 3 in the g| region due tocoupling to Cu nuclei. The EPR spectrum of complex 3 alsoshows splitting in two lines in the g) region with A) = 30 G.

2.6. Electronic spectroscopy

The electronic spectra of the complexes have been qualitativelystudied in the solid state and quantitatively in DMSO solution(Table S1, ESI3) due to limited solubility. The spectrum ofdisalicyldehyde succinoyldihydrazone shows bands at 293 nmand 330 nm, while the remaining two dihydrazones have fourand three bands, respectively, in the region of 292–382 nm.The bands around 300 and 330 nm are assigned to the intra-ligand p A p* transition, while the bands around 360 and 380nm are assigned to the n A p* transition.11

In the solid state, the complexes show two to three bands inthe region of 300–805 nm. The ligand bands appearing in theregion of 293–382 nm shift to longer wavelengths by 33–153

nm and appear in the regions of 330–441 and 405–470 nm,respectively. Such a feature associated with the red shift of theligand bands in the solid state provides good evidence for thechelation of the metal centre by dihydrazone. The magnitudeof the red shift of the ligand bands upon complexation in thesolid state suggests strong bonding between metal ion andligands. The band at 470 nm in complex 2 appears to have acontribution due to charge-transfer from the ligand to themetal atom, most probably from the naphtholate oxygen atomto the metal atom. The spectra of complexes 1 and 3 show asingle broad band centred at 640 and 620 nm, respectively.This band is assigned to a d–d transition. The d–d transitionoccurs at around 800 nm in the octahedral copper(II)complexes due to the 2T2g r 2Eg transition. The position ofthe d–d bands shifts to a lower wavelength due to Jahn–Tellerdistortion in the complexes, and in extreme cases, it falls inthe region 600–700 nm, reported for square planar com-plexes.28 The position of the d–d band and its shape suggeststhat the copper centre has a square planar geometry in thecomplexes.28

The spectrum of complex 2 shows an asymmetric broadintense band in the region of 620–800 nm in the solid statewith a maximum intensity at 660 nm. The intensity of theband at lower energy is lower than that at higher energy.Hathaway et al.29 and others30 have reported that fivecoordinated copper(II) complexes have a single rather intenseband in the region of 650–550, and 830 nm for squarepyramidal and trigonal bipyramidal stereochemistry, respec-tively. In view of the broad character of the band in the regionof 620–800 nm and its high intensity at around 660 nm, it issuggested that copper centres in the complex 2 have squarepyramidal stereochemistry.

The solution spectra of the complexes are better resolvedthan those in the solid state. Complex 2 shows only two ligandbands at 330 and 420 nm, while complexes 1 and 3 show threebands each. The ligand band in the region of 292–327 nm inthe uncoordinated ligand shifts to a longer wavelength by 7–28nm and appears in the region of 300–330 nm. This band splitsinto two bands in complex 2. The ligand band in the region330–382 nm shows a considerable red shift of about 38–55 nmupon complexation and appears in the region of 375–420 nm.The feature associated with the ligand band upon complexa-tion in the solution state remains essentially the same as thatin the solid state. This indicates that the ligands remaincoordinated to the metal centre in the DMSO solution.31 Alsoin the solution state, the spectra of complexes 1 and 3 have asingle band at 650 nm due to d–d transition. The molarextinction coefficient of this band falls in the region of 65–77dm3 mol21 cm21. This band is slightly red shifted compared tothat in the solid state. This can safely be attributed to thesolvation of the complexes in DMSO solution. The essentialfeature of the band in DMSO solution suggests that thecomplexes have square planar stereochemistry (Fig. 4).

In DMSO solution, complex 2 shows an asymmetric strongintense band at 520 nm with a molar extinction coefficientequal to 224 V21 cm2 mol21. The complex does not show anyother band in the visible region. Hence, this band may beattributed to a d–d transition. This band is shifted to a higherenergy as compared to that in the solid state. This may be due

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to replacement of water molecules by DMSO molecules, and itsweak interaction with the metal centre compared to that ofcoordinated water. However, this band maintains its separateidentity and does not merge with the strong ligand/charge-transfer band. In view of such a feature of this band in DMSOsolution, it is quite reasonable to suggest a square pyramidalstereochemistry for the complex in solution. The higher valueof the molar extinction coefficient than that observed forsquare pyramidal complexes may be attributed to intensitystealing either from an adjacent ligand or a charge-transferband.32

2.7. Infrared spectra

The present ligands show strong broad bands in the regions of3190–3235 cm21 and 3421–3433 cm21 (Table S2, ESI3) whicharise from the stretching vibration of secondary –NH groupsand naphtholic/phenolic –OH groups,10–13 respectively. All ofthe complexes have a strong broad band in the region of 3491–3433 cm21. The essential feature of this band suggests that itarises from a nOH band of either the lattice or coordinatedwater molecules. The complexes do not show any band in theregion of 3190–3235 cm21 due to the nNH in free ligandsexcept complex 2 which shows a strong band at 3212 cm21.This band in complex 2 may be attributed to the symmetricstretching vibration of coordinated water molecules. Theabsence of a nNH band in the IR spectra suggests that theligand is present in the enol form in all of the complexes.Another important and most characteristic feature of the IRspectra of the complexes is the absence a the band in theregion of 1659–1675 cm21 due to the .CLO group in theuncoordinated dihydrazone. This corroborates the fact that

the ligands are present in the enol form in the complexes. Thepresent ligands show a very sharp singlet or doublet in theregion of 1604–1623 cm21, which is assigned to the stretchingvibration of the .CLN group. This band shifts to a lowerfrequency by 6–19 cm21, which suggests that the .CLN groupis involved in bonding to the metal centre. Another importantfeature of the IR spectra of the complexes is a new intenseband in the region of 1521–1539 cm21. This band ischaracteristic of the presence of the NCO-group.11 Thepresence of this band in the IR spectra of the complexessuggests that the dihydrazone undergoes enolization. Thespectra of the ligands show a medium to intense band in theregion of 1267–1280 cm21. This band may be attributed ton(C–O) (phenolate/naphtholate) vibrations. This band shifts toa higher frequency by 9–21 cm21 and appears in the region of1276–1301 cm21 upon complexation. The shift of this band toa higher frequency by +9–21 cm21 indicates bonding of aphenolic/naphtholic oxygen atom to the metal centre viadeprotonation.11

On examining the spectra of the ligands and theircomplexes below 600 cm21, the weak bands appearing in theranges of 536–548 and 426–458 cm21 are tentatively assignedto the n(M–O) (phenolic/naphtholic) and n(M–O) (enolate)stretching vibrations,40 respectively. A doublet arising in theregion of 329–386 cm21 is attributed to the n(M–N) stretchingvibration from the coordination of the azomethine nitrogenatom to the metal centre.33

The complexes show medium intensity bands in the regionof 626–684 and 484–492 cm21. These bands are not visible inthe IR spectra of the free ligands. Hence, they arise due tobridged metal atoms through oxygen atoms.34 The position ofthese bands in the complexes suggests that they originate fromthe formation of dibridges involving enolate oxygen atoms.The band in the region of 626–684 cm21 is assigned toantisymmetric vibrations while the band in the region of 484–492 cm21 is attributed to symmetric vibrations of the

group,34 respectively.The complexes have four strong bands in the regions of

1143–1175, 1121–1140, 1100–1110 and 1080–1090 cm21. Outof the four fundamental vibrations of the perchlorato group,only n3 and n4 are infrared active. The triply degenerate IRactive n3 mode splits into three bands in the IR spectra of thecomplexes. Moreover, the n1 band which is IR forbidden innon-coordinated perchlorate35 appears in the region of 1080–1090 cm21 as a strong band. The appearance of the n1 bandand frequencies of the n3 mode are in good agreement withthose normally associated with the bidentate bridgingperchlorato groups.35

2.8. Cyclic voltammetry

The assignments of redox couples (Table 1) have been madeexcluding the waves of the free ligands. Complex 1 (Fig. 3)shows three reductive waves while complexes 2 and 3 showonly two reductive waves each (Fig. S5 and S6, ESI3). On theother hand, complexes 1 and 2 show three oxidative waves

Fig. 3 Cyclic voltammetry of the complex [Cu3(L1)(ClO4)2] (1) at 100 mV s21.

Fig. 4 Suggested structure of the copper(II) complexes [Cu3(Ln)(ClO4)2] (H4Ln =H4L1, H4L3).

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each while complex 3 shows only two oxidative waves. Thereductive waves centred in the region of 20.79 to 21.23 V donot have their counterpart in the oxidative scan. Hence, thesewaves are assigned to electron transfer reactions centred onthe ligand. Further, an irreversible oxidative wave in the regionof +0.75 to +1.06 V is observed for the complexes. Thisoxidative wave may be attributed to the oxidation of the .CLNgroup in the ligand.36 As the uncoordinated ligands do notshow any other redox reactions in the region of +2 to 22, theremaining reductive and oxidative waves may be associatedwith the electron transfer reaction centred on the metal ion.The peak potential separation for the first redox couple (Epa =+0.14 V and Epc = 20.17 V for complex 1; Epa = +0.05 V and Epc

= 20.18 V for complex 2; Epa = +0.01 V and Epc = 20.39 V forcomplex 3) is more than 200 mV which is much higher thanthat for an uncomplicated one electron redox process (0.06 V).Hence, this redox couple is suggested to be irreversible. Withthe highly negatively charged dihydrazone ligand bonded tothe metal centre, it is expected to help make the reduction ofthese metal centres unfavourable, leading to high, negative Epc

values.37 The high peak separation most probably originatesfrom a slow heterogeneous electron exchange raterather than from an intervening homogeneous reaction.36

Complexes 1 and 2 show another irreversible oxidativewave at +0.35 and +0.29 V, respectively. This may be attributedto the oxidative couple corresponding to the oxidation ofCuIICuIICuII to CuIICuIICuIII. In view of this, the first redoxcouple may be attributed to the reduction of½(L)CuIICuIICuII�2z

/?ze

{e

½(L)CuIICuIICuI�z. The electron transferreactions centred on the metal ion are proposed to be asfollows: ½(L)CuIICuIICuIII�3z

/?ze

{e

½(L)CuIICuIICuII�2z/?ze

{e

½(L)CuII

CuIICuI�z.

3. Conclusion

In the present study, we have synthesized three homotri-nuclear copper(II) complexes of the composition

[Cu3(Ln)(ClO4)2(H2O)m] (m = 0, 3) in excellent yields. All ofthe complexes are monomers and nonelectrolytes. The ligandsare present in the enol form in all the complexes and functionas tetrabasic hexadentate ligands coordinating the metalcentre through phenolate/naphtholate and enolate oxygenatoms, and azomethine nitrogen atoms. The EPR parametersof the complexes 1 and 3 indicate that the copper centre has adoublet state as the ground state while for complex 2 theground state is a mixture of both the doublet and quartetstates. The electron transfer reactions of the complexes havebeen investigated by cyclic voltammetry. On the basis of theresults and discussion of various physicochemical and spectro-scopic studies, complexes 1 and 3 are suggested to have asquare planar structure, whereas complex 2 has a squarepyramidal structure. (Fig. 4 and 5).

4. Experimental section

4.1. Materials

Copper acetate monohydrate, copper chloride dihydrate,diethyl succinate, hydrazine hydrate, substituted salicylalde-hydes and 2-hydroxy-1-naphthaldehyde were E-Merck,Qualigens, Hi-Media or equivalent grade reagents. Theprecursor complexes [Cu(H2Ln)(H2O)] were synthesized by theliterature method using succinoyldihydrazine instead ofoxaloyldihydrazine and salicylaldehyde and substituted salicy-laldehydes in place of salicylaldehyde.38

4.2. Instrumentation and measurement

The amount of copper was determined by the standardliterature procedure.39 The C, H, and N content wasdetermined by microanalytical methods using a Perkin-Elmer 2400 CHNS/O Analyser II. Perchlorate was reduced tochloride with Ti2(SO4)3 and ammonium chloride in a porcelaincrucible in the presence of little platinum powder, and theresulting chloride was detected as AgCl. The molar conduc-tance of the complexes (1023 mol L21 in DMSO) was measuredon a Systronics Direct Reading Conductance Meter-303 with adip-type conductance cell at room temperature. Room tem-perature magnetic susceptibility measurements were made on

Table 1 Electrochemical parameters for the copper complexes (potential vs. Ag/AgCl)

SI. No. CompoundReductionEc (V)

OxidationEa (V) DE (V)

1 [Cu3(L1)(ClO4)2] 21.23 —20.92 —20.17 +0.14 310— +0.35— +1.04[20.58] [20.52]a

2 [Cu3(L2)(ClO4)2(H2O)3] 20.79 —20.18 +0.05 230— +0.29— +1.06[20.84] [+0.24]a

3 [Cu3)(L3)ClO4)2] 20.88 —20.39 +0.01 400— +0.75[20.88] [+0.47]a

a Waves arising from uncoordinated ligands.

Fig. 5 Suggested structure of the copper(II) complexes [Cu3(L2)(ClO4)2(H2O)3].

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a Sherwood Magnetic Susceptibility Balance MSB-Auto.Diamagnetic corrections were carried out using Pascal’sconstant.40 Electronic spectra of the complexes were recordedin DMSO solution at y1023 M concentration on a Perkin-Elmer Lambda-25 spectrophotometer. Electron paramagneticresonance spectra of the complexes were recorded at theX-band frequency on a Varian E-112 E-Line Century ServiceEPR spectrometer using TCNE (g = 2.0027) as an internal fieldmarker. Variable temperature experiments were carried outwith a Varian Variable Temperature accessory. Infrared spectrawere recorded on a BX-III/FT-IR Perkin-Elmer spectrophot-ometer in the range of 4000–400 cm21 in KBr discs. Far-IRspectra were recorded in the range of 600–50 cm21 in CsIdiscs. Mass losses were determined by heating the complexesat 110 uC and 180 uC in an electronic oven. APCI mass spectraof the complexes were recorded on a Waters ZQ 4000Micromass Spectrometer in DMSO solution. Cyclic voltam-metric measurements of the compounds in DMSO wereconducted using a CH Instruments Electrochemical analyzerunder dinitrogen. The electrolytic cell comprises threeelectrodes, the working electrode was a Pt disk, the referenceand auxiliary electrodes were Ag/AgCl (3 M KCl) separatedfrom the sample solution by a salt bridge, and 0.1 mol L21

TBAP was used as the supporting electrolyte.

4.3. Molecular weight determination

The molecular weight of the complexes was determined inDMSO (Kf = 4.07)41 as the solvent by lowering of the freezingpoint of DMSO (f.p. 18.5 uC) with a Beckmann freezing pointdepression instrument. The instrument consisted of aBeckmann thermometer and a stirrer with a non-conductinghandle of wood fitted into a tube through a rubber stopperfixed at its upper end. This tube was supported by a rubberstopper in a bigger glass tube thoroughly purged with drydinitrogen which ensured slow and uniform cooling of theinner tube. The outer tube was immersed in a 1,2-dimethyl-3-nitrobenzene (f.p. 15 uC) freezing bath contained in a glass jar.The jar was covered with a lid of plastic with three holes, onefor inserting the inner tube, another for a large metal stirrerand a third for inserting the ordinary thermometer. Themolecular weight apparatus was enclosed in a dry box that wasconstantly purged with dry nitrogen to minimize the error dueto the highly hygroscopic nature of DMSO. Different samplesof the solutions were prepared in DMSO with differentamounts of complex. All of the samples were placed in anentry port and purged overnight with dry nitrogen beforebringing them into the dry box. 20 g of DMSO were transferredinto the thoroughly cleaned and dried freezing point tubepurged with dry nitrogen. About 0.6 g of the complex wasaccurately weighed and inserted into the DMSO solution. Thetube was heated to dissolve the compound and then broughtto room temperature. Finally, the upper surface of the liquid inthe tube was purged with dry nitrogen and then adjusted intothe outer tube. The freezing point temperatures of the solventand the solutions were directly measured with this apparatusthrough a magnifying glass fitted into the dry box.

4.4. Synthesis and characterization of the ligands

Succinoyldihydrazine was prepared by reacting diethyl succi-nate (10.0 g, 49.50 mmol) with hydrazine hydrate (5.50 g,110.00 mmol) under reflux for 4 h and recrystallized frommethanol. Dihydrazones were prepared by reacting a solutionof succinoyldihydrazine (10.37 mmol) in methanol withaldehydes and ketones (25.93 mmol) in hot methanol andrefluxing the reaction mixture for 30 min. The precipitateobtained on cooling the solution was thoroughly washed withmethanol and air dried to give H4L1, H4L2 and H4L3,respectively.

H4L1: Colour: white; yield: 89%; m.p.: 250 uC; anal. calc. forC18H18N4O4 (354 g mol21) (%): C, 61.02; H, 5.08; N, 15.82.Found: C, 61.23; H, 5.06; N, 15.69. 1H NMR (400 MHz, DMSO-d6) (d (ppm)):11.72 (d, 1H, J = 7.6 Hz, OH), 11.30 (d, J = 5.2 Hz,1H, OH), 11.16 (s, 1H, NH), 10.15 (s, 1H, NH), 8.33 (s, 1H,CHLN), 8.27 (s, 1H, CHLN), 6.8–7.6 (m, 8H, Ar-H), 2.91 (m,2H), 2.53 (m, 2H); 13C NMR (100 MHz, DMSO-d6) (d (ppm)):173.05, 167.8, 156.31, 131.13, 130.89, 126.68, 118.55, 119.38,28.64.

H4L2: Colour: dark yellow; yield: 86%; m.p.: 245 uC; anal.calc. for C26H22N4O4 (454 g mol21) (%): C, 68.72; H, 4.83; N,12.33. Found: C, 68.99; H, 4.83; N, 12.65. 1H NMR (400 MHz,DMSO-d6) (d (ppm)): 12.66 (d, 1H, J = 14.4 Hz, OH), 11.62 (d,1H, OH), 11.3 (s, 1H, NH), 10.05 (d, 2H, NH), 9.17 (s, 1H,CHLN), 8.98 (s, 1H, CHLN), 7.18–8.44 (m, 12H, Ar-H), 2.59 (m,2H), 2.07 (m, 2H); 3C NMR (100 MHz, DMSO-d6) (d (ppm)):167.29, 157.74, 144.81, 138.31, 132.34, 131.73, 127.69, 124.10,123.33, 121.90, 118.25, 108.33, 30.58.

H4L3: Colour: light yellow; yield: 87%; m.p.: 257 uC; anal.calc. for C18H16N4O4Br2 (512 g mol21) (%): C, 42.19; H, 3.13; N,10.94. Found: C, 42.59; H, 3.11; N, 11.24. 1H NMR (400 MHz,DMSO-d6) (d (ppm)): 11.75 (d, 1H, J = 7.8 Hz, OH), 11.36 (d, 1H,J = 4.8 Hz, OH), 11.19 (s, 1H, NH), 10.2 (s, 1H, NH), 8.30 (s, 1H,CHLN), 8.21 (s, 1H, CHLN), 6.8–7.7 (m, 6H, Ar-H), 2.92 (m,2H), 2.53 (m, 2H); 13C NMR (100 MHz, DMSO-d6) (d (ppm)):173.34, 168.08, 156.21, 138.39, 138.27, 132.99, 121.15, 110.29,26.86.

4.5. Synthesis of [Cu3(Ln)(ClO4)2(H2O)m] (where m = 0, 3; H4Ln

= H4L1(1), H4L2(2), H4L3(3))

[Cu(H2L1)(H2O)] (2.0 g; 4.43 mmol) was suspended in methanol (30mL) and stirred vigorously to get a homogeneous suspension. Thissuspension was added to a copper perchlorate hexahydrate solutionmaintaining a [Cu(H2L1)(H2O)] : Cu(ClO4)2?6H2O (1) molar ratio of1 : 3 and was refluxed for 3 h. A green compound precipitated,which was suction filtered under hot conditions and washed threetimes with methanol (30 mL each time) followed by ether, and finallydried over anhydrous CaCl2.

Complexes 2 and 3 were also prepared by following theabove procedure using [Cu(H2L2)(H2O)] (2.0 g; 3.28 mmol) and[Cu(H2L3)(H2O)] (2 g; 3.63 mmol) respectively, instead of[Cu(H2L1)(H2O)].

[Cu3(L1)(ClO4)2] (1): yield: 2.60 g (78%), m.p.: .300 uC, anal.calc. for C18H14N4O12Cu3Cl2 (%): Cu, 25.78; C, 29.20; H, 1.89;N, 7.57; Cl, 10.00. Found: Cu, 26.05; C, 28.70; H, 1.86; N, 7.42;Cl, 9.87. Mol. wt.: theo.: 739.65, exp.: 780 ¡ 35. Molarconductance (DMSO, V21 cm2 mol21): 1.7.

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[Cu3(L2)(ClO4)2(H2O)3] (2): yield: 2.23 g (72%), m.p.: .300uC; anal. calc. for C26H14N4O15Cu3Cl2 (%): Cu, 21.33; C, 34.91;H, 2.69; N, 6.27; Cl, 7.94. Found: Cu, 21.49; C, 34.72; H, 2.71;N, 6.42; Cl, 7.81. Mol. wt.: theo.: 893.65, exp.: 1060 ¡ 52. Molarconductance (DMSO, V21 cm2 mol21): 1.5.

[Cu3(L3)(ClO4)2] (3): yield: 2.45 g (85%), m.p.: .300 uC; anal.calc. for C18H12N4O12 Br2Cu3Cl2 (%): Cu, 21.24; C, 24.06; H,1.34; N, 5.74; Cl, 7.90. Found: Cu, 21.59; C, 24.38; H, 1.37; N,5.74; Cl, 8.10. Mol. wt. theo.: 897.65, exp.: 920 ¡ 45. Molarconductance (DMSO, V21 cm2 mol21): 1.2.

Acknowledgements

The authors are thankful to the HEAD, SAIF, North EasternHill University, Shillong-793022, Meghalaya, India for record-ing NMR and mass spectra and the Head, SAIF, IIT Bombay,India for recording EPR spectra. The authors express theirheartfelt thanks to Prof. A. T. Khan, Department of Chemistry,IIT Guwahati, Assam, India for helping with recording themagnetic moment data. R.B. thanks the University GrantCommission, New Delhi for the award of the ResearchFellowship in Sciences for Meritorious Students (RFSMS).

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