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Journal of Alloys and Compounds 443 (2007) 53–60

Spin crossover studies in mixed ligand complexes oftris(N-ethyl, N′-n-butyldithiocarbamato)iron(III) with N N, N O and

O O containing bidentate ligands and their thermal decomposition

Sonal Singhal a,∗, A.N. Garg a,b, Kailash Chandra c

a Chemistry Department, Punjab University, Chandigarh 160014, Indiab Department of Chemistry, Indian Institute of Technology, Roorkee 247667, Uttaranchal, India

c Institute Instrumentation Centre, Indian Institute of Technology, Roorkee 247667, Uttaranchal, India

Received 8 July 2006; received in revised form 30 September 2006; accepted 3 October 2006Available online 20 November 2006

bstract

Mixed ligand complexes of the type Fe(ebtc)2L and Fe(ebtc)L2, where ebtc = N-ethyl, N′-n-butyldithiocarbamate and L = 1,10-phenanthrolinephen), 8-hydroxyquinoline (oxine) and acetylacetone (acac) have been synthesized and characterized by IR, magnetic moment and Mossbauerpectral studies. Metal–ligand vibrations corresponding to Fe S, Fe N and Fe O were observed in the far IR region. Especially for Fe S and Fe Nwo different frequencies corresponding to high spin (HS) and low spin (LS) states were observed. Room temperature Mossbauer spectra of theomplexes exhibit an asymmetric doublet resolved into two doublets corresponding to high and low spin states in equilibrium. Variable temperatureagnetic moment and Mossbauer spectral studies suggest spin crossover (6A1g → 2T2g) for [Fe(ebtc)3], [Fe(ebtc)2(phen)]+ and [Fe(ebtc)(phen)2]2+

omplexes, almost no change for [Fe(ebtc)(oxine) ] complex, spin transition (4T → 2T ) in [Fe(ebtc) (acac)] and [Fe(ebtc)(acac) ] complexes

2 1g 2g 2 2

nd high spin to intermediate spin (6A1g → 4T1g) state in [Fe(ebtc)2(oxine)] complex. Thermogravimetric studies suggest that all the complexesre stable up to ∼200 ◦C and decomposition occurs in two or three stages finally yielding Fe2O3 in most cases though two of the complexes seemo be volatile.

2006 Elsevier B.V. All rights reserved.

compo

L1ssat

eywords: Spin crossover; Magnetic moment; Mossbauer spectra; Thermal de

. Introduction

Mixed ligand complexes have been of immense inter-st because of their characteristic spectroscopic and struc-ural properties. Many solution and solid-state studies haveeen reported in literature [1–4]. Takemoto and Hutchin-on [2] investigated spin crossover in [Fe(bipy)2(NCS)2],

Fe(phen)2(NCS)2] and [Fe(phen)2(NCSe)2]. Lanjewar andarg [5] studied mixed ligand complexes of mono and bis(N,N′-iethyldithiocarbamato)iron(III) with chelating agents contain-ng O, N and S donor atoms.

∗ Corresponding author at: Chemistry Department, Punjab University,handigarh 160014, India. Tel.: +91 1332 261968; fax: +91 1332 273560.

E-mail address: sonal1174@gmail.com (S. Singhal).

ia

tN(htVmd

925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.10.007

sition; Mixed ligand complexes

Sutter et al. [6] studied the spin crossover in [FeL(L′)2] where= 2-(p-N-methylpyridinium)-4,4,5,5-teramethyl-imidazoline-

-oxyl; H2L′ = 1,2-dicyano-1,2-ethylenediol. Sugiyarto et al. [7]tudied electronic and structural properties of the spin crossoverystems, bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II)thiocyanatend selenocyanate. Both the complexes undergo an abruptransition to low spin below room temperature and crystallizen a layer-type array involving edge-to-face and face-to-faceryl–aryl type interactions.

In this paper we report the synthesis, magnetic and spec-ral behaviour of mixed ligand complexes of tris(N-ethyl,′-n-butyldithiocarbamato)iron(III) with 1,10-phenanthroline

phen), 8-hydroxyquinoline (oxine) and acetylacetone (acac)aving N N, N O and O O donor atoms, respectively, to see

he effect of change of ligand on the spin crossover behaviour.ariable temperature magnetic moment and Mossbauer spectraleasurements were carried out down to 77 K. Also their thermal

ecomposition behaviour was studied.

5 ys and Compounds 443 (2007) 53–60

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4 S. Singhal et al. / Journal of Allo

. Experimental

.1. Preparation of the complexes

First the dithiocarbamate ligand of N-ethyl, N′-n-butyl was prepared asodium salt by the reaction of the N-ethyl, N′-n-butyl amine in dried diethylther with carbon disulphide and adding sodium hydroxide in equal moleatio with vigorous stirring for 5–6 h at room temperature. The crude prod-ct was recrystallized from chloroform or tetrahydrofuran. The purity of ligandas checked by elemental analysis, and determining mp and characteristic IRands. After that mixed ligand complexes were prepared by mixing ethano-ic solution of iron(III)nitrate (BDH, England), sodium salt of N-ethyl, N′-i-utyl-dithiocarbamate and the respective ligands L = ‘phen’, ‘oxine’ and acacn 1:2:1 and 1:1:2 mole ratios, respectively, as described earlier [5,8]. Theesultant solution was stirred for 1–2 h at room temperature whence colouredompounds separated out which were filtered, washed with water, absolutethanol and finally with ether. In case of ‘phen’ substituted complexes unlikethers, these will be charged species with possibly NO3

− as neutralizingnions.

.2. Physical measurements

All the infrared spectra in the range 4000–400 cm−1 were recorded in KBrellet on a Perkin-Elmer 1600, FT-IR Spectrophotometer. Further, spectra werelso recorded down to 50 cm−1 using Brucker IFS 66r FT-IR spectrophotome-er at the Regional Sophisticated Instrumentation Centre (RSIC), IIT-Madras.

agnetic moments were measured from room temperature (RT) down to liquid

2 temperature (LNT) using a vibrating sample magnetometer (VSM Model55, Princeton Applied Research, USA). The instrument was calibrated usinghigh purity Ni rod as a standard. Thermograms were recorded on a thermo

ravimetric analyzer system, STA–780 series (Stanton Redcroft, UK) in staticir using Al2O3 as reference material. Mossbauer spectra were recorded using aonstant acceleration transducer driven Mossbauer spectrometer (ECIL, Hyder-bad) in conjunction with 1024 MCA (Canberra). A 25 mCi initial activity 57CoRh) source, procured from Amersham, UK, was used. Its activity at the time ofxperiment was about 10 mCi. The spectrometer was calibrated using a naturalron foil and recrystallized sodium nitroprusside dihydrate (SNP) as standards.he spectral data were least square fitted. Low temperature measurements at 77,00, 150, 200 and 250 K were recorded using an in house fabricated cryostat [9]orking on the cold finger principle.

. Results and discussion

All the complexes are black or dark brown coloured solidsnd are stable under normal atmospheric conditions. The com-lexes were characterized by elemental analysis, which matched

ell with the proposed molecular formulae. Electronic spectra ofithiocarbamate complexes are characterized by intense absorp-ion bands attributed to both internal transitions in the ligand andhe metal to ligand (d�–p�) and ligand–metal charge transfer

rtwt

able 1nfrared spectral data of mixed ligand complex of tris(N-ethyl, N′-n-butyldithiocarba

omplexes ν(N C)(cm−1)

ν(NC2)(cm−1)

ν(C S)(cm−1)

ν(Fe S) (cm−1

HS

Fe(ebtc)3] 1495 1140 980, 1283 260Fe(ebtc)2(phen)]+ 1499 1193 999, 1283 212Fe(ebtc)(phen)2]2+ 1519 1182 967, 1229 263Fe(ebtc)2(oxine)] 1494 1193 994, 1278 227Fe(ebtc)(oxine)2] 1495 1173 1002, 1276 257Fe(ebtc)2(acac)] 1490 1192 991, 1279 224Fe(ebtc)(acac)2] 1495 1194 992, 1279 226

ig. 1. Variation of magnetic moment with temperature of mixed ligand com-lexes of iron(III)dithiocarbamates.

10,11]. Spectral features of dithiocarbamate complexes suggestefinite trends in populations of 6A1g and 2T2g states by modifi-ations in the ligand. Also band intensities change markedly withhange in population of the two states. We had recorded elec-ronic spectra of the complexes in acetone which show an intenseand at ∼28,600 cm−1 and a weak broad band at ∼17,200 cm−1

orresponding to transitions 6A1g → 4T1g and 6A1g→4T2g, 4Eg,espectively, in accordance with the assignments of Ewald et al.10] and Ballhausen [11].

IR data for all the complexes are listed in Table 1. Plots ofariable temperature magnetic moments are shown in Fig. 1.ypical Mossbauer spectra of the complexes [Fe(ebtc)2(phen)]+

nd [Fe(ebtc)(phen)2]2+ at different temperatures are shown inig. 2. The Mossbauer parameters δ and �EQ for the two-pin states at four temperatures along with measured mag-etic moments and those derived from these spectra are listedn Table 2. The TGA, DTA and DTG plots for [Fe(ebtc)2L]nd [Fe(ebtc)L2] type complexes are shown in Figs. 3 and 4,espectively. Various thermogravimetric parameters such as ini-

ial decomposition temperature, decomposition stages, constanteight temperature and final weight percentage as obtained from

he TGA plots are listed in Table 3. Results of various spectral

mato)iron(III) complex

) ν(Fe N)/(Fe/O) (cm−1) Other bands

LS

360 – –296 �(Fe N): 207, 372 –309 ν(Fe N): 220, 392 –309 ν(Fe N): 189; ν(Fe O): 299 ν(C O): 1106306 ν(Fe N): 187; ν(Fe O): 289 ν(C O): 1106356 ν(Fe O): 417; ν(Fe O): 288 ν(C C): 1640; ν(C O): 1490360 ν(Fe O): 437; ν(Fe O): 288 ν(C C): 1650; ν(C O): 1483

S. Singhal et al. / Journal of Alloys and Compounds 443 (2007) 53–60 55

com

aa

3

tccddbw

acibaswba

Fig. 2. Variable temperature Mossbauer spectra of mixed ligand

nd magnetic moment measurements and implication of thesere discussed in following lines.

.1. Infrared spectra

Several workers [12–18] have reported IR spectra of transi-ion metal dithiocarbamates and identified stretching vibrationsorresponding to double bond character of N C, chelatingharacter of C S and Fe–S. IR spectra have been used to

istinguish between monodentate and bidentate nature of theithiocarbamate ligand [12,13]. The presence of only one strongand at ∼980 cm−1 supports the bidentate chelating character,hereas a doublet indicates monodentate bonding [12]. Bradley

cci1

plexes of: (A) [Fe(ebtc)2(phen)]+ and (B) [Fe(ebtc)(phen)2]2+.

nd Gitlitz [12] reported ν(C S) for different metal dithio-arbamates in the region 994–1004 cm−1 suggesting chelat-ng character of the C–S group. We have observed a strongand in the region 967–1002 cm−1 suggesting chelating char-cter of the dithiocarbamate ligand (Table 1). Another C Stretching mode in the region 1229–1289 cm−1 is in accordanceith literature [14]. Bradley and Gitlitz [12] also studied IRands in several transition metal N,N′-dialkyldithiocarbamatesnd reported the thioureide (N C) band at ∼1500 cm−1 as a

haracteristic of the ligand indicating considerable double bondharacter in the S2C NR2 bond. We have observed ν(C N)n the range 1490–1519 cm−1. Another strong band in the region173–1194 cm−1 is attributed to the N C2 bond [13,14].

56 S. Singhal et al. / Journal of Alloys and Compounds 443 (2007) 53–60

Table 2Variable temperature Mossbauer spectral and magnetic moment data for mixed ligand complex of iron(III)dithiocarbamates

Complexes High spin Low spin Estimated percentageof spin states

Magnetic momentb

μeff (B.M.)

δa (mm s−1) �EQ (mm s−1) δa (mm s−1) �EQ (mm s−1) HS LS

[Fe(ebtc)3]R.T. 0.33 0.21 0.35 0.15 80 20 5.10 (5.08)200 K 0.36 0.43 0.45 0.23 55 45 4.12 (4.07)150 K 0.39 0.59 0.49 0.36 30 70 2.95 (2.98)77 K 0.37 0.74 0.49 0.56 10 90 1.92 (2.15)

[Fe(ebtc)2(phen)]+

R.T. 0.55 0.91 0.43 0.66 85 15 5.29 (5.28)200 K 0.50 1.11 0.47 0.83 60 40 4.24 (4.20)150 K 0.51 1.20 0.48 0.94 38 62 3.38 (3.31)77 K 0.56 1.37 0.55 1.07 7 93 1.96 (2.02)

[Fe(ebtc)(phen)2]2+

R.T. 0.60 1.40 – – 100 0 5.73 (5.91)200 K 0.60 1.67 0.52 1.40 62 38 4.39 (4.32)150 K 0.57 1.78 0.60 1.47 45 55 3.64 (3.56)77 K – – 0.60 1.58 – 100 1.76 (1.73)

[Fe(ebtc)2(oxine)]R.T. 0.49 0.44 – – 100 – 5.90 (5.91)200 K 0.50 0.57 0.43 0.41 83 17 5.27 (5.20)150 K 0.54 0.68 0.48 0.48 70 30 4.65 (4.70)77 K 0.57 0.93 0.54 0.62 55 45 4.08 (4.07)

[Fe(ebtc)(oxine)2]R.T. 0.51 1.17 – – 100 – 5.96 (5.91)200 K 0.53 1.38 0.44 0.99 85 15 5.52 (5.32)150 K 0.55 1.44 0.44 1.06 92 8 5.11 (5.15)77 K 0.55 1.55 0.47 1.11 85 15 4.78 (4.86)

[Fe(ebtc)2(acac)]R.T. 0.42 0.56 0.44 0.32 48 52 3.80 (3.73)200 K 0.46 0.65 0.48 0.44 33 67 3.14 (3.11)150 K 0.43 0.73 0.49 0.51 20 80 2.65 (2.60)77 K 0.44 0.87 0.52 0.71 10 90 2.01 (2.10)

[Fe(ebtc)(acac)2]R.T. 0.45 0.49 0.46 0.26 40 60 3.63 (3.65)200 K 0.43 0.60 0.46 0.38 30 70 3.01 (2.94)150 K 0.43 0.66 0.46 0.44 17 83 2.38 (2.44)77 K – – 0.47 0.60 – 100 1.80 (1.73)

h and

dtoccHatbaottIa

Hrmphpeoaaaw

a w. r. to iron as standard, ±0.02 mm s−1.b In parenthesis are μeff calculated on the basis of percent contribution of hig

In the case of mixed ligand complexes, metal–ligand bandsue to ν(Fe S), ν(Fe N) and ν(Fe O) may provide informa-ion about the strength of metal–ligand bond and symmetryf the molecule. In tris(N,N′–dialkyldithiocarbamato)iron(III)omplexes, stretching vibrations due to Fe S gives two bandsorresponding to high (HS) and low spin (LS) states [15,16].utchinson et al. [16] showed that the high spin ν(Fe S) appears

t 205–250 cm−1 and the low spin at 305–350 cm−1, whereashe intermediate spin in both regions. In our IR studies ν(Fe S)and in the region 212–263 cm−1 is attributed to HS state andnother band in the region 296–360 cm−1 to LS state. In casef ‘phen’ substituted complexes, ν(Fe N) may also give rise to

wo bands corresponding to HS and LS states which is essen-ially due to heterocyclic or aromatic ring of the ligand [17].n case of two ‘phen’ substituted complexes [Fe(ebtc)2phen]+

nd [Fe(ebtc)(phen)2]2+, ν(Fe N) stretching frequencies in the

h

tc

low spin states.

S and LS states were observed at 207/220 and 372/392 cm−1,espectively [2,17]. Thus, it may be concluded that ‘phen’ liganday also contribute to the spin crossover behaviour of the com-

lexes. In case of ‘oxine’ complexes, Fe N and Fe O vibrationsave been assigned on the basis of earlier work [18]. It has beenointed out that ν(Fe O) of ‘acac’ complexes are most inter-sting since they provide direct information about the strengthf Fe O bond. Using the metal isotope technique, Nakamoto etl. [18a] assigned the stretching bands for Fe(acac)3 complext 436 and 300 cm−1. On this basis, observed bands at 417/437nd 288 cm−1 may be assigned to ν(Fe O). As such ‘acac’ is aeak field ligand-giving rise to high spin complexes only and

ence only one band is expected.

The ligand ‘acac’ can coordinate either as uninegative biden-ate through its enol form, or as neutral monodentate througharbon atom. In the case of enol form coordination, ν(C O)

S. Singhal et al. / Journal of Alloys and Compounds 443 (2007) 53–60 57

Fi

a[nc

Fi

TT

C

[[[[[[

ig. 3. Typical thermograms of mixed ligand complexes of theron(III)dithiocarbamates Fe(ebtc)2L.

nd ν(C C) were found at 1577 and 1529 cm−1, respectively19,20], whereas the strong band due to ν(C O) is obtainedear 1700 cm−1 if coordination is through keto form [21]. In ourase ν(C O) band was observed at 1640 cm−1 and ν(C C)

atsb

able 3hermogravimetric data of mixed ligand complex of tris(N-ethyl, N′-n-butyldithiocar

omplexes Initial decompositiontemperature (◦C)

DTA peak(◦C)

Decompositionprocess

Fe(ebtc)2(phen)]+ 190 420 Fast and slow, thFe(ebtc)(phen)2]2+ 145 440 Fast and slow, thFe(ebtc)2(oxine)] 210 450 Fast and slow, thFe(ebtc)(oxine)2] 320 430, 310 Fast and slow, twFe(ebtc)2(acac)] 170 220 Fast and slow, twFe(ebtc)(acac)2] 190 220 Fast and slow, tw

a In parentheses are calculated weight percent of Fe2O3.

ig. 4. Typical thermograms of mixed ligand complexes of theron(III)dithiocarbamates Fe(ebtc)L2.

−1

t 1490 cm (Table 1) confirming that ‘acac’ was coordinatedo the Fe as uninegative bidentate ligand. In the case of ‘oxine’ubstituted complexes ν(C O) was observed at 1106 cm−1 inoth cases as reported in literature [8].

bamato)iron(III) complex

Weight loss at the endof each stage (%)

Constant weighttemperature (◦C)

Final weight(%)a

ree stage 40.0, 74.0 650 15.5 (13.9)ree stage 50.0, 64.0 500 14.5 (14.1)ree stage 48.0, 58.0 510 8.5 (14.5)o stage 40.0 470 13.5 (15.4)o stage 64.0 650 19.5 (15.7)o stage 64.0 650 16.5 (18.5)

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8 S. Singhal et al. / Journal of Allo

.2. Magnetic moment

As mentioned in Table 2, μeff for tris(N-ethyl, N′-n-utyldithiocarbamato)iron(III) complex at RT is 5.10 B.M. sug-esting it to be primarily a high spin complex which, how-ver, attains low spin state at 77 K with μeff = 1.92 B.M.ccording to spectrochemical series, ‘phen’ and ‘oxine’ with

N and N O donor atoms are strong field ligands, whereasacac’ with O O donor atoms is essentially a weak fieldigand [11]. It is observed from Table 2 that on substi-ution of ‘ebtc’ with one and two ‘phen’ ligands, μeffncreases to 5.29 and 5.73 B.M., respectively, thus convert-ng these into completely HS. Almost similar observation isor [Fe(ebtc)2oxine] and [Fe(ebtc)(oxine)2] complexes whichhow μeff = 5.90 and 5.96 B.M., respectively. Thus, ‘phen’ andoxine’ being strong field ligands, [Fe(ebtc)3] becomes increas-ngly HS when one or two of ebtc are substituted with ‘phen’nd ‘oxine’, respectively. However, when ‘ebtc’ is substitutedith ‘acac’ the observed μeff values are lowered to 3.80 and.63 B.M., respectively. Therefore, substitution of a ligand withnother ligand certainly affects the spin behaviour or magneticoments significantly as shown in Fig. 1. Further, the mixed

igand complexes [Fe(ebtc)2phen]+ and [Fe(ebtc)(phen)2]2+,Fe(ebtc)(oxine)2] and [Fe(ebtc)2(oxine)] are in HS (6A1g) statehereas [Fe(ebtc)2(acac)] and [Fe(ebtc)(acac)2] are in interme-iate spin state (4T1g). Thus, on the basis of RT measurements,eff values of all the six complexes, Fe(ebtc)2L and Fe(ebtc)L2an be divided into two groups: high spin (6A1g) for ‘phen’nd ‘oxine’ substituted complexes whereas an intermediate spin4T2g) state for ‘acac’ substituted complexes. Wajda et al. [4]lso investigated spin crossover behaviour in [Fe(dtc)2(dtc′)]ype complexes by magnetic and Mossbauer spectra and sug-ested shift in spin equilibrium with change in ligand. Thus, theixed ligand complexes do not exhibit simply the slowing down

f the spin state interconversion rate, but a change of the thermalquilibrium.

The magnetic properties of mixed ligand complexesf the type [Fe(edc)2(AA′)] and Fe(edc)(AA′)2 whereA′ = acac/oxine/glycine (gly) were studied by Kanungo et al.

8]. μeff reported for [Fe(edc)2(oxine)], [Fe(edc)(oxine)2] andFe(edc)2(gly)] were 6.07, 5.93 and 6.07 B.M., respectively,s expected for HS (6A1g) state. For other three compoundsiz. [Fe(edc)2(acac)], [Fe(edc)(acac)2] and [Fe(edc)(gly)2], μeffere reported in the range 3.28–4.09 B.M., intermediate ofS and LS configurations. Lanjewar and Garg [5] reportedeff for mixed ligand complexes, [Fe(edc)2L] and [Fe(edc)L2]here L = ‘acac’, ‘oxine’, ‘gly’ and ‘phen’, etc. in the range.74–5.80 B.M. It was observed that the μeff for [Fe(edc)2L]ere somewhat higher (4.21–5.80 B.M.) compared to those for

Fe(edc)L2] (3.74–5.50 B.M.) except for ‘phen’ ligand.Variable temperature magnetic moment studies show that

he complex, [Fe(ebtc)3] exhibits complete spin crossover6A1g → 2T2g) as shown in Fig. 1. The intermediate mag-

etic moments may be due to spin pairing of one of thelectron resulting in S = 3/2. Mitchell and Parker [22] haveuggested that low magnetic moments cannot be explainedy interaction between iron atoms and most probable expla-

wosa

Compounds 443 (2007) 53–60

ation would be the change in spin state. On the basis ofariable temperature magnetic moment studies, it is observedhat both the ‘phen’ substituted complexes exhibit completepin crossover, 6A1g → 2T2g. On the other hand, out of thewo ‘oxine’ substituted complexes, [Fe(ebtc)2(oxine)] does nothow any significant change on going down to 77 K whereasFe(ebtc)(oxine)2] acquires intermediate spin state. However,acac’ substituted complexes change from intermediate to lowpin state (4T1g → 2T2g) as shown in Fig. 1. From the aboveiscussion it is clear that the spin crossover behaviour is signifi-antly affected by the nature of the second ligand L. The ligandontaining N N donor atoms, i.e. ‘phen’ shows complete spinrossover (6A1g → 2T2g), the one containing, O O donor atomshows spin transition (4T2g → 2T2g) and another one containing

O donor atoms ligand do not show any significant change inhe spin state.

.3. Mossbauer spectra

Variable temperature Mossbauer spectral studies have beenxtensively used for investigating the spin crossover in a vari-ty of mixed ligand iron(III) complexes [4,5]. These complexeshow an unusual temperature dependence of their Mossbauerpectral behaviour. Wajda et al. [4] reported asymmetric dou-lets for the complex [Fe(Et2dtc)(morphdtc)] at 300 and 80 Knd attributed this due to shift in spin equilibrium compared tohat for [Fe(morphdtc)3]. Later Lanjewar and Garg [5] reportedT Mossbauer spectral studies for [Fe(edc)2L] and [Fe(edc) L2]

ype complexes. It has been suggested that large �EQ valuesor the mixed ligand complexes compared to that for [Fe(edc)3]ere essentially due to larger geometrical distortion. Mossbauer

pectral studies present an average picture of both the spin statesn thermal equilibrium but not the change of intensity of bandsriginating from both possible states as in the case of iron(II)rossover complexes.

Variable temperature Mossbauer spectral studies ofFe(ebtc)3] show spin crossover behaviour [23]. Typical

ossbauer spectra of ‘phen’ substituted complexes are shownn Fig. 2A and B. Our studies on mixed ligand complexesave shown that room temperature Mossbauer spectra ofFe(ebtc)(phen)2]2+, [Fe(ebtc)2(oxine)] and [Fe(ebtc)(oxine)2]xhibit almost symmetric doublet, whereas those ofFe(ebtc)2(phen)]+, [Fe(ebtc)2(acac)] and [Fe(ebtc)(acac)2]xhibit asymmetric doublets resolved into two doublets. Thiss essentially due to the fact that the former three complexesre in high spin (6A1g) state whereas later three are equilibriumixtures with two doublets corresponding to high and low spin

tates. Furthermore, as the temperature is lowered down to 77 Ksymmetric nature of the doublets of two ‘phen’ substitutedomplexes become more symmetric as shown in Fig. 2. Thiss specially the case with two ‘phen’ substituted complexesFe(ebtc)2(phen)]+ and Fe(ebtc)(phen)2]2+ and two ‘acac’ubstituted complexes [Fe(ebtc)2(acac)] and [Fe(ebtc)(acac)2]

hich all attain low spin state (2T2g) at 77 K. On the contraryriginal symmetry in the Mossbauer spectra of two ‘oxine’ubstituted complexes [Fe(ebtc)2oxine] and [Fe(ebtc)(oxine)2]t room temperature gets increasingly disturbed and spectra

S. Singhal et al. / Journal of Alloys and Compounds 443 (2007) 53–60 59

d iron

bfcciiMs

twigdowtsattmws(stifa[c

ldtcd

fsp

3

stlun[Hpt[cgtlmw

cs[tpcmpto

Fig. 5. Variation of quadrupole splitting with temperature for mixed ligan

ecome highly asymmetric at 77 K. The Mossbauer parametersor the two complexes in Table 2 at 77 K suggest that theomplexes are equilibrium mixtures of two-spin states. Itlearly suggests that [Fe(ebtc)(oxine)2] shows a little changen spin state whereas [Fe(ebtc)2(oxine)] acquires somewhatntermediate spin state (4T1g). Thus, our variable temperature

ossbauer spectral results fully corroborate magnetic momenttudies.

On the basis of areas of two sets of doublets percent contribu-ions of the two states were estimated and net magnetic momentsere calculated as given in last column of Table 2. A compar-

son of the observed and calculated magnetic moments showsood agreement within <±10%. It is suggested that interme-iate magnetic moments are indicative of equilibrium mixturef the two-spin states. However, ‘acac’ substituted complexesere in intermediate spin state having ∼45% HS state contribu-

ion. It is observed that the isomer shift (δ) values for both thepin states are approximately same, 0.42–0.60 mm s−1 for HSnd 0.43–0.52 mm s−1 for LS state. This is essentially due tohe covalent nature of the complexes in accordance with litera-ure [24]. It is observed that �EQ of the HS state for all the sixixed ligand complexes are in the range of 0.44–1.40 mm s−1

hich is higher than the range 0.26–0.66 mm s−1 for the lowpin state at RT. Similarly at 77 K also these values are higher0.87–1.55 mm s−1) for the HS state compared to that for LState (0.60–1.58 mm s−1). Variations of �EQ with tempera-ure for both the spin states in Fig. 5 show decreasing trendn all the cases. In general, it is observed that �EQ valuesor the two ‘phen’ substituted complexes [Fe(ebtc)2(phen)]+

nd [Fe(ebtc)(phen)2]2+ and an ‘oxine’ substituted complex,Fe(ebtc)(oxine)2] are higher compared to those for other threeomplexes in the both cases.

Therefore, it is clear from the above discussion that the mixedigand dithiocarbamate complexes with a ligand containing N N

onor atoms exhibit complete spin crossover behaviour whereas,hose containing N O donor atoms do not show any significanthange in the spin state. However, the complexes with O Oonor atoms exhibit spin transition. Interestingly, �EQ values

wwb[

(III)dithiocarbamato complexes in: (A) high spin and (B) low spin states.

or the two ‘phen’ and one ‘oxine’ substituted complexes showharp decrease. On the contrary �EQ values for other three com-lexes show very little change with temperature.

.4. Thermal decomposition studies

Many workers [5,25,26] have reported thermal decompo-ition of a variety of mixed ligand complexes. In general,hermal decomposition of mixed ligand complexes is simi-ar to that of iron(III)dithiocarbamates because the end prod-ct is same, i.e. �-Fe2O3 but it essentially depends on theature of ligand L. Thermal decomposition studies of [FeA3],FeA2(A′)] and [FeAA′

2] (where HA′ = gly, oxine and acac) andA = piperidino- and morpholinodithiocarboxylic acid) com-lexes by Garg et al. [25] have suggested single step process withhe formation of Fe2O3 as the end product. Lanjewar and Garg5] reported single or two step decomposition for mixed ligandomplexes, [Fe(edc)2L] and [Fe(edc)L2], where L = acac, phen,ly, etc. None of the mixed ligand complexes have been reportedo be volatile. Zemskova et al. [26] further found that the mixedigand complexes of zinc and cadmium diisobutyldithiocarba-

ates with phen, bipy and 4,4′-bipyridine undergo sublimationith the formation of �-CdS and ZnS.The thermogravimetric data in Table 3 suggest that all the

omplexes are stable up to ∼200 ◦C though [Fe(ebtc)(oxine)2]tarts decomposing at 320 ◦C whereas [Fe(ebtc)(phen)2]2+ andFe(ebtc)2(acac)] decomposition starts at comparatively loweremperatures of 145 and 170 ◦C. In general, it is a two-stagerocess for all the complexes except for two phen-substitutedomplexes where decomposition occurs in three stages. Thisay, possibly due to loss of anion associated with these com-

lexes. From the thermograms in Figs. 3 and 4, it is clearhat decomposition is completed at ∼500 ◦C except in casesf [Fe(ebtc)2(phen)]+, [Fe(ebtc)2(acac)] and [Fe(ebtc)(acac)2]

here it is completed up to 650 ◦C. On the basis of constanteight obtained all the six complexes can be divided into tworoad groups: volatile and nonvolatile. As evident from Table 3,Fe(ebtc)2(oxine)] and [Fe(ebtc)(acac)2] complexes seem to be

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olatile as the difference between expected and final weights sobtained for the end product lies in the range 8.5–10.5% cor-esponding to the expected range of 14.5–18.5%, respectively.ll the decomposition studies were carried out in air atmo-

phere and no evidence could be obtained for the formation ofron sulphide. As evident from the DTA plots in Figs. 3 and 4ecomposition seems to occur by exothermic process in allases except for [Fe(ebtc)(oxine)2] where two exotherms werebserved and for [Fe(ebtc)(acac)2] only a small exotherm wasbserved.

The thermal decomposition behaviour of both the ‘phen’ sub-tituted complexes seems to be most complex. A weight loss ofbout 8 and 4% occurs at ∼100 ◦C corresponding to two and oneO3

−/water molecules, respectively. The first stage decompo-ition temperatures for the mono and bis ‘phen’ complexes are40 and 320 ◦C where ∼40 and ∼50% weight loss occurs. Sincehese weights are transitory and the decomposition process stilloes on probably some intra-molecular rearrangement might beaking place in addition to the loss of one ligand. At the sec-nd stage whereby 74–64% weight loss occurs this correspondso the loss of one ‘ebtc’ and one ‘phen’ ligand in the first casend both the ‘phen’ ligands in second case. Final decompositionemperatures for the two complexes are 650 and 500 ◦C, respec-ively, whereby the residual weight corresponds to the probableormation of Fe2O3.

The decomposition of the two ‘oxine’ complexes seems to beomparatively simple. In case of mono ‘oxine’ complex decom-osition during first stage 50% weight loss corresponds to theoss of two ‘oxine’ ligands and the second stage is very slow.n case of bis ‘oxine’ complex, however, it is a very fast pro-ess with two broad exotherms. The decomposition is completedn both cases at 510 and 470 ◦C, respectively. In the first case iteems to be more like a volatile complex whereas, in second caset seems to have converted into Fe2O3. In the case of two ‘acac’omplexes first stage decomposition is very fast, whereas theecond stage is slow. In both cases a sharp exotherm is observedt ∼215 ◦C and 65% weight loss occurs after the first stage ofecomposition. In the first case it corresponds to the loss ofwo ‘ebtc’ ligands whereas in second case it may correspondo the loss of one ‘ebtc’ and one ‘acac’ ligand. During secondtage it is a very slow process suggesting the probable forma-

ion of Fe2O3 in the first case and slow volatilization in theecond case. It is also possible that some intra-molecular rear-angement with evolution of gaseous products might be takinglace.

Compounds 443 (2007) 53–60

cknowledgement

Grateful thanks are due to the Council of Scientific and Indus-rial Research, New Delhi, for the award of SRF to one of theuthors (S.S.).

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