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Inorganic Chemistry Communications 9 (2006) 1053–1057

First oxidative synthetic route of a novel, linear mixed valenceCo(III)ACo(II)ACo(III) complex with bridging acetate and salen

Shouvik Chattopadhyay a, Gabriele Bocelli b, Amos Musatti b, Ashutosh Ghosh a,*

a Department of Chemistry, University College of Science, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, Indiab IMEM-CNR, Parceo Area delle scienze 37a, I-43100 Fontanini, Parma, Italy

Received 28 April 2006; accepted 23 June 2006Available online 30 June 2006

Abstract

A linear, trinuclear mixed valence complex [CoII{(l-L)(l-OAc)CoIII(NCS)}2] has been prepared and its molecular structure has beenelucidated on the basis of X-ray crystallography [H2L = salen = 1,6-bis(2-hydroxyphenyl)-2,5-diazahexa-1,5-diene]. The complex con-tains a high-spin CoII and two terminal low-spin CoIII, i.e., it is a CoIII(S = 0)ACoII(S = 3/2)ACoIII(S = 0) trimer.� 2006 Elsevier B.V. All rights reserved.

Keywords: Mixed valence; Co(II) and Co(III); Salen; Bridging acetate; Trinuclear; Crystal structure

Multinuclear complexes of transition metal ions attracta lot of attention due to their catalytic activity [1–4], utilityin modeling the multimetal active sites of metalloproteins[2] and potential use in nanoscience [4]. Amongst them,cyclic trinuclear cobalt complexes form an important class[5] as they have been found to show some catalytic activityin the epoxidation of olefins [2a,6] and in the autoxidationof hydrocarbons [7]. The linear, trinuclear mixed valence(CoIIIACoIIACoIII) complexes are known since Wernerreported a series of such complexes obtained by partial oxi-dation of the mixtures of cobalt(II) salts and ethylenedia-mine [8]. The main structural feature of those complexions is a Co2+ ion linked to two Co3+ ions on the wingsthrough double OH bridges. Two other ligand sites of thecentral Co2+ ion are occupied by two H2O moleculeswhereas four other ligand sites of each Co3+ ion by twoethylenediamines. Similar complexes with tetradentateSchiff base ligands replacing the two ethylenediamines arescanty; only one such complex is reported till date [9]. Onthe other hand, cobalt complexes based on quadridentate

1387-7003/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2006.06.017

* Corresponding author.E-mail address: ghosh_59@yahoo.com (A. Ghosh).

bis(salicylidene) Schiff base ligands, preserve substantialinterest relevant to vitamin B12 models [10] as well as bio-mimetic catalysts [11]. The concerned ligands also drawconsiderable attention in view of fascinating bridging abil-ity of the phenoxo oxygen atoms affording high nuclearitycomplexes which could act as the promising candidates tooffer valuable insights into various natural electron transferevents [12].

The well known Schiff base, 1,6-bis(2-hydroxyphenyl)-2,5-diazahexa-1,5-diene (commonly known as salen,general abbreviation H2L, where H atoms are dissociablephenolic hydrogen atoms) has been used as a tetradentatechelating ligand in the present work (Scheme 1a). The reac-tion of H2L with Co(II) acetate in presence of NH4SCNfurnished a novel CoIIIACoIIACoIII mixed valence trinu-clear species (Scheme 1b). In the present paper, we reportthe synthesis, spectroscopic characterizations, crystal struc-ture and redox behavior of the trimer.

The ligand H2L was synthesized by condensation of eth-ylenediamine with salicylaldehyde in dry methanol follow-ing the literature method [13]. All other chemicals were ofreagent grade and used without further purification. Sol-vents were dried and distilled prior to use. A methanolsolution (25 cm3) of Co(OAc)2 Æ 6H2O (3 mmol, 747 mg)

N N

HH

O O

Co

Scheme 2.

O

O

N

NCo Co

O

O

N

N

Co

SCN

NCSOC

CH3

O

O

C

O

CH3

Scheme 1b.

OH HO

H H

N N

Scheme 1a.

1054 S. Chattopadhyay et al. / Inorganic Chemistry Communications 9 (2006) 1053–1057

was added into a yellow solution of H2L (2 mmol, 356 mg)in methanol (25 cm3) and vigorously stirred for 15 minfollowed by the addition of NH4SCN (2 mmol, 152 mg).

Fig. 1. ORTEP3 diagram of the complex with 10% ellipsoid probability. SeCo(1)AO(29) 2.042(4), Co(1)AO(31) 2.169(4), Co(1)AO(50) 2.084(4), Co(1)AO1.906(4), Co(2)AN(12) 1.882(5), Co(2)AN(15) 1.853(5), Co(2)AN(24) 1.905(Co(3)AN(39) 1.854(8), Co(3)AN(42) 1.872(6), Co(3)AN(55) 1.908(8); bond an

A green colored compound was precipitated out and wasrecrystallized from acetonitrile solution to obtain prismaticdark green single crystals suitable for X-ray diffraction [14].Yield: 917 mg (85%, based on metal salt). Calc. forC38H30Co3N6O8S2: C, 48.57; H, 3.22; N, 8.94; Found: C,49.10; H, 3.51; N, 8.63. UV–Vis, kmax (nm), (emax

(dm3 mol�1 cm�1)) (acetonitrile), 561 (715), 365 (4099).Magnetic moment l = 5.16 B.M.

The stoichiometric equivalent of the ingredients for thesynthesis of the complex is presented in the followingequation:

3[CoII(OAc)2] + 2H2L + 2NH4SCN + 1/2O2

! [CoIIf(l-L)(l-OAc)CoIII(NCS)g2� þ 2NH4OAc

+ 2HOAc + H2O ð1Þ

If the reaction is performed under anaerobic condition orunder N2 atmosphere, a red compound is obtained as theproduct and it was identified as a mononuclear complexof salen (Cosalen or salcomine) by IR spectra, elementalanalysis and mass spectra (Scheme 2).

Elemental analyses (carbon, hydrogen and nitrogen)were performed using a Perkin–Elmer 240C elemental ana-lyzer. IR spectra in KBr (4500–500 cm�1) were recordedusing a Perkin–Elmer RXI FT-IR spectrophotometer.Electronic spectra in acetonitrile (1200–350 nm) wererecorded in a Hitachi U-3501 spectrophotometer. Themagnetic susceptibility measurements were done with anEG&PAR vibrating sample magnetometer, model 155 atroom temperature and diamagnetic corrections were made

lected bond distances (A): Co(1)AO(4) 2.154(3), Co(1)AO(23) 2.123(4),(53) 2.060(3), Co(2)AO(4) 1.920(4), Co(2)AO(23) 1.912(3), Co(2)AO(27)

5), Co(3)AO(31) 1.902(4), Co(3)AO(50) 1.908(4), Co(3)AO(51) 1.903(5),gles are given in supplementary Table S1.

Fig. 2. Cyclic voltammogram for complex 1 in acetonitrile: (a) oxidationand (b) reduction.

S. Chattopadhyay et al. / Inorganic Chemistry Communications 9 (2006) 1053–1057 1055

using Pascal’s constants. Electrochemical measurementswere performed under a dry nitrogen atmosphere in athree-electrode configuration using Pt disc working elec-trode, Pt auxiliary electrode and Ag/AgCl reference elec-trode on a PAR Versastat-2 electrochemistry system inthis work and were uncorrected for junction contribution.The value for the ferrocenium–ferrocene couple underour conditions is 0.40 V.

An ORTEP view of compound 1, including the atom-labelling scheme, is shown in Fig. 1. The asymmetric unitconsists of one neutral [CoII{(l-L)(l-OAc)CoIII(NCS)}2]molecule. Discrimination between Co(II) and Co(III) ionsis based on bond length considerations. Cobalt(III) andcobalt(II) centres are linked by one acetate ion and twophenoxo oxygen atoms of L2�, forming an l-acetato(O,O

0)-bis(l-phenoxo) bridged CoIIIACoII motif. Another

CoIII centre is again bonded with the CoIIIACoII motifvia similar bridging segment and generates the CoIIIACoII

ACoIII core.Co(2) has a six-coordinate pseudo-octahedral geometry

in which O(4), O(23), N(12) and N(15) atoms of deproto-nated di-Schiff base (L2�) constitute the equatorial planeand N(24) and O(27) atoms of isothiocyanate and acetateligands, respectively, define the axial directions.

The coordination environment around Co(3) is verysimilar to that of Co(2). Co(3) also sits in a pseudo-octahe-dral environment in which the L2� ligand occupies theequatorial positions {O(31), O(50), N(39) and N(42)atoms} and the two apical positions are coordinated bythe oxygen atom O(51) of bridging acetate and by isothio-cyanato nitrogen N(55). A notable feature in the individualCoN3O3 environments is an uniform facial disposition ofboth N3 and O3 donor sets maximizing the back-bondingeffect.

Central Co(1) atom is coordinated by phenoxo oxygenatoms O(23) and O(31) along axial direction and the equa-torial plane is defined by two acetate {O(29) and O(53)}and two other phenoxo {O(4) and O(50)} oxygen donors.

The Co(3). . .Co(1) and Co(1). . .Co(2) intra-molecularseparations are 3.060 and 3.078 A, respectively. None ofthese distances is sufficiently short to imply any metal–metal bonding or allow intra-metal spin exchange throughmutual interaction [9,21,22a]. The bridging angles,Co(1)AO(4)ACo(2), Co(1)AO(23)ACo(2), Co(1)AO(31)ACo(3) and Co(1)AO(50)ACo(3) are 97.95(15), 99.30(16),97.22(16) and 99.97(16), indicate non-collinear CoIIIAOACoII fragments, not suitable for significant electronicexchange [9,21,22a].

In the present complex, CoIIAO distances vary from2.042(4) to 2.169(4) A which are typical for high-spinCo(II)–oxygen distances. The CoIIIAO and CoIIIAN dis-tances are in the range 1.902(4)–1.920(4) and 1.853(5)–1.908(8) A, respectively, as are expected for low-spinCo(III) [9,21,22b]. The bond lengths about the terminalcobalt ions are significantly shorter than those about thecentral cobalt ion, indicating that the terminal ions aretrivalent while the central one is divalent.

The effective magnetic moment at room temperature oftrinuclear complex is measured to be 5.16 B.M. which cor-responds to the presence of a high-spin Co(II) in the mol-ecule. The complex is thus a low-spin CoIII and high-spinCoII, i.e., CoIII(S = 0)ACoII(S = 3/2)ACoIII(S = 0) trimer.

The trimeric mixed valence complex exhibits several dis-tinguishable resonances consistent with the coordinationgeometry, revealed from structure determination. Monod-entate N-bound isothiocyanate group is characterized withan intense signal at 2125 cm�1. Vibrationally active acetateligands display antisymmetric and symmetric stretchingvibrations at 1602 and 1448 cm�1, respectively. The differ-ence in frequency, D(mas(COO�) � ms(COO�)), is 153 cm�1

(163 cm�1 for free acetate ion), indicating the presence ofdeprotonated carboxylate group coordinated to metal cen-tres in a bridged bidentate fashion [23]. The characteristicimine (C@N) stretch of the chelated ligand appeared at1638 cm�1 and the red shift of this C@N absorption ofabout 15 cm�1 on going from the free H2L ligand to thecomplex suggests weak p-accepting ability of the coordi-nated ligand.

The electronic spectrum of the compound in the acetoni-trile solution exhibits the low-energy absorption (561 nm),attributable to a transition in the visible region of a low-spin

1056 S. Chattopadhyay et al. / Inorganic Chemistry Communications 9 (2006) 1053–1057

cobalt(III) in octahedral geometry, obscuring the transitionsof the divalent metal ions. The high-energy, very intenseband at 365 nm is associated with a charge-transfer transi-tion. Bands from Co(II) are Laporte Forbidden transitionsand are assumed to be too weak to be visible.

The cylic voltammetry experiment was performed inacetonitrile solution within the potential range ±2 V versus

AgAAgCl electrode at ambient temperature (300 K) usinga three-electrode configuration. The complex displayed anirreversible oxidative response at +1.72 V (Fig. 2). Thisirreversible signal could be attributed to the CoII! CoIII

oxidation. A quasi-reversible reductive couple at��1.44 V is ascribed to CoIII! CoII reduction. All theredox signals remain virtually invariant under differentscan rates (0.01–1.0 V s�1) in the temperature range 300–280 K. Solvent dependent shift and change in electrochem-ical reversibility of redox couples are not noteworthy.

The report provides the first synthetic route forCoIIIACoIIACoIII trinuclear complex with salen type Schiffbase ligand via partial aerial oxidation of CoII precursor.The only known complex of this type was prepared byreducing Co(III) salt with SO2 [9]. Another trinuclear salpncomplex is reported in the literature [21], but that containsall Co ions in +2 oxidation state and thus not a mixedvalence trimer. The facile synthesis of complex 1 would,therefore, afford a convenient synthetic route for this typeof mixed valance cobalt complexes.

Acknowledgement

We are thankful to Dr. Shyamal Chattopadhyay,Department of Chemistry, Bengal Engineering and ScienceUniversity, Shibpur, Howrah, India for providing facilitiesfor electrochemical studies.

Appendix A. Supplementary data

Crystallgraphic data for the analysis have been depositedwith the Cambridge Crystallographic data Centre, CCDCNo 605150 Copies of this information may be obtained freeof charge from CCDC, 12 Union Road, Cambridge,CB21EZ, UK (fax: +44-1223-336033; e-mail: depos-it@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.inoche.2006.06.017.

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S. Chattopadhyay et al. / Inorganic Chemistry Communications 9 (2006) 1053–1057 1057

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