1-s2.0-S0277538707002537-main

r3,

B

r th

ersi

007Available online 27 April 2007

Keywords: Dipyridoquinoxaline; 3d Metal complexes; Crystal structure; Paramagnetic 1H NMR; Electronic spectra; Electrochemistry

periodic table, have led to a wide variety of studies dealing these ligands have been studied for a variety of reasons,including intense interest in their catalytic properties andbiomimetic behaviour. These types of copper complexesare also studied for their relevance to the active-site struc-tures of metalloproteins [14]. The redox chemistry of copperpolypyridines is of particular interest as these complexes

* Corresponding authors. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805(P. Biswas).

E-mail addresses: [email protected] (M. Ghosh), [email protected](P. Biswas).

Polyhedron 26 (2007) 31. Introduction

For a long time, 2,2 0-bipyridine (bpy) and 1,10-phenan-throline (phen) have been extensively used as chelatingligands in both analytical and preparative coordinationchemistry [1]. More recently, systematic studies of substi-tuted bipyridines, phenanthrolines and other a-diiminederivatives have been undertaken [29]. Complexes of theseligands, with transition metal ions spanning much of the

with structural, spectroscopic, photoredox, catalytic andbiomimetic properties [10]. A key feature of these six-membered heterocyclic rings is their p-electron deciency,which make them excellent p-acceptor ligands. Conse-quently, they have been used to stabilize various metal com-plexes in lower oxidation states [11,12]. There has been alsoconsiderable interest in the DNA binding and cleavageproperties by redox and photoactive transition metal poly-pyridine complexes [13]. In particular, copper complexes ofAbstract

The chemistry of rst row transition metal complexes obtained from the ligand dipyrido[3,2-f:2 0,3 0-h]-quinoxaline (dpq) have beenreported. The reaction betweenCu(ClO4)2 6H2Owith dpqunder dierent reaction conditions led to the isolation of three polymorphic cop-per(II) complexes [Cu(dpq)2(H2O)](ClO4)2 H2O (2), [Cu(dpq)2(ClO4)](ClO4) (3) and [{Cu(dpq)2(H2O)}{Cu(dpq)2(ClO4)}](ClO4)3 (4). Thebluish-green compound 2, obtained by reactingCu(ClO4)2 6H2Owith dpq inmethanol, has a distorted trigonal bipyramidal structure withs = 0.55. The reaction between Cu(ClO4)2 6H2O and dpq in dry acetonitrile produced the blue compound 3 in which the copper(II) centrehas a distorted square planar geometry. When the condensation reaction between 1,10-phenanthroline-5,6-dione and 1,2-diaminoethanewas carried out in the presence of Cu(ClO4)2 6H2O in methanol, the green copper(II) complex 4 was isolated along with 1. The structuredeterminationof 4has established the presence of twodierent complex cations in the asymmetric unit and they are considered as co-crystals.In the zinc(II) compound [Zn(dpq)2(ClO4)2] (5), the two perchlorates are unidentately coordinated to themetal centre, providing a distortedoctahedral geometry. The quinoxaline ring in 5 is involved in intermolecular pp interactions, leading to the generation of a sinusoidal chain.The protonNMR spectra, especially those of the paramagnetic complexes [Ni(dpq)3](ClO4)2 (6) and [Co(dpq)3](ClO4)2 (7), have been stud-ied in detail. The electronic absorption spectra and the redox behaviour of the copper(I), copper(II), cobalt(II) and cobalt(III) complexeshave been studied. The three copper(II) compounds 24 show identical absorption spectra and redox properties when measured in aceto-nitrile, although in nitromethane they show small but denite dierences in their spectral and redox features. 2007 Elsevier Ltd. All rights reserved.Structural, spectroscopic andmetal complexes of dipyrido[

Meenakshi Ghosh a,*, Papua Department of Inorganic Chemistry, Indian Association fo

b Anorganische und Analytische Chemie, Univ

Received 14 February 20277-5387/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.poly.2007.04.014edox properties of transition2-f:2 0,3 0-h]-quinoxaline (dpq)

iswas a,*, Ulrich Florke b

e Cultivation of Science, Jadavpur, Kolkata 700 032, India

tat Paderborn, D-33098 Paderborn, Germany

; accepted 5 April 2007

www.elsevier.com/locate/poly

7503762

droundergo signicant changes in coordination geometry dur-ing the redox process. The ability to deliberately controlthe microstructure of a coordination polymer by using mac-roscopic principles, through dissociation/binding of ligandsfrom/to metal centres by an applied potential can havenumerous material applications [15]. Copper(I) polypyridinecomplexes also display interesting photoluminescent proper-ties. There has been considerable interest to harness thephotophysical properties of copper(I) polypyridines formany practical applications, such as light harvesters, molec-ular sensors and switches [16]. The excellent photophysicalproperties of copper(I) polypyridines have opened a newarea of study involving their intercalation with biologicalsystems, in particular DNA intercalation and cleavage[17].

Phenanthroline based ligands, such as dipyrido[3,2-f:2 0,3 0-h]-quinoxaline (dpq), dipyridophenazine (dppz) and7,8-dimethylpyridophenazine (dppx) with an extended pla-nar quinoxaline moiety, can act as good bidentate ligandsas well as good binders to DNA. Several ruthenium(II)complexes, such as [RuL2(dpq)]

2+ (L = bpy or phen), havebeen synthesized because they bind to DNA strongly byintercalating the aromatic dpq ligand between the basepair. Moreover, on binding to DNA they strongly lumi-nesce, acting as light switches [18]. More recently, thesynthesis, structure and DNA binding of a copper(I) com-plex of dpq has been reported [19]. It has also been shownthat copper(II) complexes having photoactive and DNAbinding ligands cleave DNA on irradiation with red light[20]. Ternary copper(II) complexes derived from tridentateONO and ONS ligands and the DNA binding ligand dpqcleaves DNA on exposure to red light [21]. In view of thegrowing interest in the use of the ligand dpq, which besidesthe studies made above has not been used extensively, wehave been interested to explore the coordination chemistryof the dpq transition metal complexes in greater detail.Herein we report the synthesis, X-ray crystal structures,spectroscopic properties and redox activities of zinc(II),copper(I), copper(II), nickel(II), cobalt(II), cobalt(III),iron(II) and manganese(II) complexes of dpq.

N

N N

N

dpq

2. Experimental

2.1. Materials

All chemicals were obtained from commercial sources

M. Ghosh et al. / Polyheand were used as received. Solvents were dried and puriedaccording to standard methods [22]. 1,2-Diaminoethanewas dried over potassium hydroxide before vacuum distil-lation. 1,10-Phenanthroline-5,6-dione was prepared accordingto the reported method [23].

Caution: The perchlorate salts reported here are poten-tially explosive and, therefore, should be handled withcare.

2.2. Preparation of dipyrido[3,2-f:2 0,3 0-h]-quinoxaline(dpq)

The ligand was prepared by the condensation of 1,10-phenanthroline-5,6-dione and 1,2-diaminoethane in a mod-ied method [24]. To an acetonitrile solution (20 mL) of1,10-phenanthroline-5,6-dione (1.05 g, 5 mmol), an aceto-nitrile solution (10 mL) of 1,2-diaminoethane (0.30 g,5 mmol) was added. The mixture was reuxed for 3 h afterwhich it was cooled to room temperature. A black resinousprecipitate was removed by ltration and the ltrate wasdried on a rotary evaporator. The crude product thusobtained was recrystallized from acetonitrile to get thewhite crystalline pure compound. Yield: 2.5 g (70%). Anal.Calc. for C14H8N4: C, 72.40; H, 3.47; N, 24.12. Found: C,72.40; H, 3.48; N, 24.08%. IR (KBr): 3446br, 1635w,1577m, 1519w, 1467m, 1392s, 1207w, 1116w, 1080m,870w, 1212m, 808s, 740s, 686w, 621m. 1H NMR (CDCl3,300 MHz): d 9.45 (d, 2H), 9.23 (d, 2H), 8.93 (s, 2H), 7.74(m, 2H).

2.3. Preparation of the metal complexes

2.3.1. [CuI(dpq)2](ClO4) (1)To a solution of dpq (0.46 g, 2 mmol) in dry methanol

(10 mL) was added a solution of [Cu(CH3CN)4](ClO4)(0.33 g, 1 mmol) in dry methanol (10 mL) under a nitrogenatmosphere. The resulting dark red solution was reuxedfor 0.5 h, during which time a dark maroon microcrystal-line compound was deposited. The product was collectedby ltration and washed with methanol. Yield: 0.50 g(80%). Anal. Calc. for C28H16N8O4ClCu: C, 53.59; H,2.57; N, 17.86. Found: C, 53.50; H, 2.60; N, 17.78%.ESI-MS (positive, in acetonitrile): m/z = 527.08 (100%)[Cu(dpq)2]

+. IR (KBr): 3434br, 3085w, 1627w, 1576m,1470m, 1434w, 1420s, 1384s, 1275w, 1085s, 839w, 734m,630m cm1. 1H NMR (DMSO-d6, 300 MHz): d 9.65 (d,2H), 9.31 (s, 2H), 9.14 (d, 2H), 8.18 (dd, 2H). UVVisNIR (kmax/nm (e/M

1 cm1)): 215 (62000), 255 (100000),300 (34000), 445 (4700).

2.3.2. [CuII(dpq)2(H2O)](ClO4)2 H2O (2)To a solution of dpq (0.46 g, 2 mmol) in methanol

(15 mL) was added a methanol solution (10 mL) ofCu(ClO4)2 6H2O (0.37 g, 1.0 mmol). The resulting bluesolution was stirred at room temperature for 0.5 h. On con-centration of the solution to ca. 10 mL on a hot plate, acrystalline product was deposited. This was ltered and

n 26 (2007) 37503762 3751recrystallized from 1:1 acetonitrileethanol to obtain singlecrystals of a bluish-green colour. Yield: 0.60 g (75%). Anal.

droCalc. for C28H20N8O10Cl2Cu: C, 44.08; H, 2.64; N, 14.69.Found: C, 44.18; H, 2.62; N, 14.65%. ESI-MS (positive,in acetonitrile): m/z = 626.03 (15%) [Cu(dpq)2(ClO4)]

+;264.55 (100%) [Cu(dpq)2]

2+. IR (KBr): 3403br, 3085w,1626w, 1582m, 1532m, 1488s, 1428s, 1388s, 1282w, 1087s,818m, 731s, 622m cm1. UVVisNIR (kmax/nm (e/M1 cm1)): 215 (63000), 260 (100000), 295 (35000), 700(100), 900 (115).

2.3.3. [CuII(dpq)2](ClO4)2 (3)A dry acetonitrile solution (15 mL) of Cu(ClO4)2 6H2O

(0.37 g, 1.0 mmol) and a solution of dpq (0.46 g, 2 mmol)in the same solvent were mixed together. The resultingsolution was reuxed for 0.5 h, during which period a clearblue solution was obtained. The solution was ltered andthe ltrate was concentrated on a water bath to ca. 5 mL.On slow evaporation of the solvent at room temperature,a blue crystalline product was deposited. This was lteredand washed with 1:1 acetonitrilediethyl ether. Yield:0.58 g (80%). Anal. Calc. for C28H16N8O8Cl2Cu: C,46.26; H, 2.22; N, 15.41. Found: C, 46.18; H, 2.20; N,15.36%. ESI-MS (positive, in acetonitrile): m/z = 626.03(20%) [Cu(dpq)2(ClO4)]

+; 264.55 (100%) [Cu(dpq)2]2+. IR

(KBr): 3078w, 1619w, 1582m, 1531w, 1485m, 1407s,1387m, 1308m, 1213w, 1088s, 818m, 731s, 625m cm1.UVVisNIR (kmax/nm (e/M

1 cm1)): 215 (63000), 260(100000), 295 (35000), 700 (50), 900 (55).

2.3.4. [{CuII(dpq)2(H2O)}{CuII(dpq)2(ClO4)}](ClO4)3

(4)To a dry acetonitrile solution (15 mL) of compound

1,10-phenanthroline-5,6-dione (0.42 g, 2 mmol) was addedan acetonitrile solution (10 mL) of Cu(ClO4)2 6H2O(0.37 g, 1.0 mmol) and the resulting green solution wasset to reux. A solution of 1,2-diaminoethane (0.12 g,2 mmol) in 10 mL of dry acetonitrile was gradually addedto the reaction mixture under reuxing conditions. Thesolution became reddish-brown and the reux was contin-ued for 2 h during which time a dark maroon microcrystal-line compound was deposited. The product was collectedby ltration and was identied to be copper(I) compound1. The green ltrate on standing at room temperature pro-duced bright green single crystals of compound 4. Thesewere ltered and washed with 1:1 acetonitrilediethyl ether.Yield: 0.45 g (60%). Anal. Calc. for C56H34N16O17Cl4Cu2:C, 45.70; H, 2.33; N, 15.23. Found: C, 45.65; H, 2.30; N,15.17%. ESI-MS (positive, in acetonitrile): m/z = 626.03(15%) [Cu(dpq)2(ClO4)]

+; 264.55 (100%) [Cu(dpq)2]2+.

IR (KBr): 3407br, 3081w, 1615w, 1581m, 1531w, 1486m,1407s, 1388m, 1308w, 1214w, 1088s, 818m, 732m, 625mcm1. UVVisNIR (kmax/nm (e/M

1 cm1)): 215(63000), 260 (100000), 295 (35000), 700 (190), 900 (200).

2.3.5. [ZnII(dpq)2(ClO4)2] (5)A solution of Zn(ClO4)2 6H2O (0.34 g, 1 mmol) in

3752 M. Ghosh et al. / Polyhe10 mL of dry acetonitrile was added to a solution of dpq(0.46 g, 2 mmol) in acetonitrile (10 mL) and the mixturewas reuxed for 1 h. The solution was ltered and the l-trate was concentrated to ca. 5 mL on a hot plate. On slowevaporation of the solvent at the room temperature, lightyellow crystals deposited. The product was ltered andwashed with 1:1 acetonitrilediethyl ether. Yield: 0.55 g(75%). Anal. Calc. for C28H16N8O8Cl2Zn: C, 46.14; H,2.21; N, 15.38. Found: C, 46.11; H, 2.19; N, 15.41%.ESI-MS (positive, in acetonitrile): m/z = 627.03 (20%)[Zn(dpq)2(ClO4)]

+; 265.05 (100%) [Zn(dpq)2]2+. IR (KBr):

3423br, 3052w, 1613w, 1582m, 1529w, 1481m, 1446w,1403s, 1388s, 1338w, 1262w, 1212w, 1084s, 925w, 873w,827m, 737m, 630s cm1. 1H NMR (DMSO-d6,300 MHz): d 9.74 (d, 2H), 9.17 (s, 2H), 8.38 (d, 2H), 7.95(dd, 2H). UVVisNIR (kmax/nm (e/M

1 cm1)): 225(46600), 255 (92000), 300 (28600).

2.3.6. [NiII(dpq)3](ClO4)2 (6)To a methanol solution of dpq (0.35 g, 1.5 mmol) was

added a solution (10 mL) of Ni(ClO4)2 6H2O (0.16 g,0.5 mmol) in methanol. The pink coloured solution wasreuxed for 0.5 h and the microcrystalline product thatdeposited was ltered and recrystallized from 1:1 acetoni-trileethanol. Yield: 0.38 g (85%). Anal. Calc. forC42H24N12O8Cl2Ni: C, 52.86; H, 2.53; N, 17.70. Found:C, 52.76; H, 2.50; N, 17.61%. ESI-MS (positive, in acetoni-trile): m/z = 378.09 (100%) [Ni(dpq)3]

2+. IR (KBr): 3419br,3091w, 1613w, 1581m, 1530w, 1481m, 1446w, 1388s,1338w, 1304w, 1262w, 1212m, 1084s, 925w, 873w, 827m,737m, 630m cm1. 1H NMR (DMSO-d6, 300 MHz): d50.30 (2H), 17.93 (2H), 10.14 (2H). UVVisNIR (kmax/nm (e/M1 cm1)): 215 (60000), 260 (110000), 300(4000), 785 (11), 855 (9).

2.3.7. [CoII(dpq)3](ClO4)2 (7)This compound was prepared in the same way as

described for 6, using Co(ClO4)2 6H2O instead ofNi(ClO4)2 6H2O. Yield: 85%. Anal. Calc. forC42H24N12O8Cl2Co: C, 52.85; H, 2.53; N, 17.61. Found:C, 52.79; H, 2.51; N, 17.55%. ESI-MS (positive, in acetoni-trile): m/z = 378.59 (100%) [Co(dpq)3]

2+. IR (KBr):3415br, 3083w, 1611w, 1581m, 1530m, 1481m, 1404s,1389s, 1314w, 1282w, 1213m, 1088s, 816m, 735m, 623mcm1. 1H NMR (DMSO-d6, 300 MHz): d 99.88 (2H),49.84 (2H), 18.14 (2H), 13.63 (2H). UVVisNIR (kmax/nm (e/M1 cm1)): 210 (58000), 250 (107000), 290(52000), 910 (10).

2.3.8. [CoIII(dpq)3](ClO4)3 (8)Compound 7 (0.48 g, 0.5 mmol) was dissolved in 15 mL

of dry methanol and a mixture of (C4H9)4NBr (0.16 g,0.5 mmol) and Br2 (80 mg, 0.5 mmol) in methanol (2 mL)was added to it over a period of 0.5 h. A pink colouredcompound of the composition [CoIII(dpq)3]Br3 that depos-ited was collected by ltration and washed thoroughly withmethanol. The product was suspended in methanol

n 26 (2007) 37503762(20 mL) and a methanol (5 mL) solution of AgClO4(3 mmol) was added to it. The mixture was stirred for 1 h

droand the precipitated AgBr was removed by ltration. Theltrate on concentration produced bright pink crystals.Yield: 85%. Anal. Calc. for C42H24N12O12Cl3Co: C,47.86; H, 2.29; N, 15.95. Found: C, 47.78; H, 2.25; N,15.89%. ESI-MS (positive, in acetonitrile): m/z = 426.06(80%) [Co(dpq)3(ClO4)]

2+; 252.73 (100%) [Co(dpq)3]3+.

IR (KBr): 3415br, 3078br, 1619w, 1582m, 1531w, 1485m,1407s, 1387m, 1213w, 1088s, 818m, 731s, 625m cm1. 1HNMR (DMSO-d6, 300 MHz): d 9.86 (d, 2H), 9.50 (s,2H), 8.14 (dd, 2H), 7.88 (d, 2H). UVVisNIR (kmax/nm(e/M1 cm1)): 210 (66000), 250 (95000), 290 (sh, 34000),460 (210).

2.3.9. [Fe(dpq)3](ClO4)2 (9)To a methanol solution of dpq (0.35 g, 1.5 mmol) was

added solid Fe(ClO4)2 6H2O (0.16 g, 0.5 mmol) undernitrogen. The solution was heated under reux for 0.5 h,during which time a deep red crystalline compound wasdeposited. This was collected by ltration and recrystal-lized from 1:1 ethanolacetonitrile. Yield: 0.35 g, 85%.Anal. Calc. for C42H24N12O8Cl2Fe: C, 53.02; H, 2.54; N,17.66. Found: C, 52.97; H, 2.48; N, 17.60%. ESI-MS (posi-tive, in acetonitrile): m/z = 378.59 (100%) [Fe(dpq)3]

2+. IR(KBr): 3415br, 3083w, 1632m, 1580w, 1483m, 1406s, 1385s,1260w, 1210w, 1107s, 1088s, 813m, 733m, 627m cm1. 1HNMR (DMSO-d6, 300 MHz): d 9.55 (d, 2H), 9.37 (s,2H), 7.94 (d, 2H), 7.88 (dd, 2H). UVVisNIR (kmax/nm(e/M1 cm1)): 255 (144000), 295 (54000), 440 (7100),520 (10600), 850 (5).

2.3.10. [MnII(dpq)3](ClO4)2 (10)This compound was prepared in the same way as

described for 6, using Mn(ClO4)2 6H2O instead ofNi(ClO4)2 6H2O. Yield: 0.38 g (80%). Anal. Calc. forC42H24N12O8Cl2Mn: C, 53.07; H, 2.54; N, 17.68. Found:C, 53.00; H, 2.50; N, 17.68%. ESI-MS (positive, in acetoni-trile): m/z = 376.59 (100%) [Mn(dpq)3]

2+. IR (KBr):3421br, 3098w, 1646w, 1581m, 1529w, 1478m, 1436w,1389s, 1307w, 1213w, 1085s, 878w, 737m, 627m cm1.UVVisNIR (kmax/nm (e/M

1 cm1)): 225 (58300), 255(115000), 295 (35000).

2.4. Physical measurements

The C, H and N analyses were performed on a PerkinElmer 2400 II elemental analyzer. IR spectra were recordedusing KBr disks on a Shimadzu FTIR 8400S spectrometer.The electronic absorption spectra were obtained with aPerkinElmer 950 UV/Vis/NIR spectrophotometer. Theelectrospray ionization mass spectra (ESI-MS) were mea-sured on a Micromass Qtof YA 263 mass spectrometer.The electrochemical measurements were carried out witha BAS 100 B electrochemistry system using a three-elec-trode assembly comprising a glassy carbon working elec-trode, Pt auxiliary electrode and an aqueous Ag/AgCl

M. Ghosh et al. / Polyhereference electrode. The cyclic voltammetric (CV) andsquare wave voltammetric (SWV) measurements were car-ried out at 25 C under a nitrogen atmosphere at roomtemperature. The solutions were 1 mmol dm3 in com-plexes and 0.1 mol dm3 in tetraethylammonium perchlo-rate (TEAP) as the supporting electrolyte. The referenceelectrode was separated from the bulk electrolyte by a saltbridge containing the supporting electrolyte in nitrometh-ane solutions. IR compensation was made automaticallyduring each run. Under the experimental conditions theE1/2 values of the ferrocene/ferrocenium couple wereobserved at 435 and 410 mV in acetonitrile and nitrometh-ane, respectively.

2.5. Crystallography

Crystals suitable for structure determinations of[CuII(dpq)2(H2O)](ClO4)2 H2O (2), [Cu

II(dpq)2](ClO4)2(3), [{CuII(dpq)2(H2O)}{Cu

II(dpq)2(ClO4)}](ClO4)3 (4)and [ZnII (dpq)2(ClO4)2] (5) were obtained by slow evapo-ration of their acetonitrileethanol solutions. The crystalswere mounted on glass bres using peruoropolyetheroil. Intensity data were collected on a Bruker AXSSMART APEX diractometer at 153(2) K for 2, 4 and 5and at 223(2) K for 3 using graphite-monochromated MoKa radiation (k = 0.71073 A). The data were processedwith SAINT [25] and absorption corrections were made withSADABS [25] The structures were solved by direct and Fou-rier methods and rened by full-matrix least-squares meth-ods based on F2 using SHELX-97 [26]. For the structuresolutions and renements the SHELX-TL software package[27] was used. The non-hydrogen atoms were rened aniso-tropically, while the hydrogen atoms were placed at geo-metrically calculated positions with xed thermalparameters. Crystal data and details of data collectionare listed in Table 1.

3. Results and discussion

3.1. Synthesis and characterization

The ligand dipyrido[3,2-f:2 0,3 0-h] quinoxaline, abbrevi-ated as dpq, has been obtained by [1+1] cyclocondensationbetween 1,10-phenanthroline-5,6-dione and 1,2-diaminoe-thane. The ligand dpq readily reacts with [Cu(CH3CN)4](ClO4) in acetonitrile under nitrogen to produce the cop-per(I) complex [CuI(dpq)2](ClO4) (1). The reaction withCu(ClO4)2 6H2O under slightly dierent conditions, how-ever, gives rise to three dierent products 24, all of whichhave been structurally characterized. For example, thereaction between the ligand and the metal salt in methanolproduces the bluish-green compound [CuII(dpq)2(H2O)](ClO4)2 H2O (2), while the reaction carried out in dry ace-tonitrile produces blue crystals of composition[CuII(dpq)2](ClO4)2 (3). In contrast, if the condensationreaction between 1,10-phenanthroline-5,6-dione and 1,2-diaminoethane is carried out in acetonitrile in the presence

n 26 (2007) 37503762 3753of Cu(ClO4)2 6H2O, initially the copper(I) complex 1 isdeposited and from the ltrate a green coloured co-crystal-

dpq)2(H2O)](ClO4)2 H2O (2), [CuII(dpq)2](ClO4)2 (3), [{Cu

II(dpq)2(H2O)}

4 5

8O8Cu C56H34Cl4N16O17Cu2 C28H16Cl2N8O8Zn1471.87 728.76153(2) 153(2)triclinic monoclinic

0.3

, wR, wR

dron 26 (2007) 37503762lized compound of composition [{CuII(dpq)2(H2O)}-{CuII(dpq)2 (ClO4)}](ClO4)3 (4) is isolated. The reactionbetween dpq and Zn(ClO4)2 6H2O in a 2:1 ratio in meth-

Table 1Crystallographic data and structure renement parameters for [CuII({CuII(dpq)2(ClO4)}](ClO4)3 (4) and [Zn

II(dpq)2(ClO4)2] (5)

Compound 2 3

Empirical formula C28H20Cl2N8O10Cu C28H16Cl2NFormula weight 762.96 726.9Temperature (K) 153(2) 223(2)Crystal system triclinic monoclinicSpace group P1 C2/cUnit cell dimensions

a (A) 8.5250(5) 29.9863(14)b (A) 12.2436(8) 13.7129(7)c (A) 14.1431(9) 14.4062(7)a () 94.583(1) 90b () 91.130(1) 110.202(1)c () 100.944(1) 90

Volume (A3) 1443.81(16) 5559.4(5)Z 2 8Dcalc (Mg/m

3) 1.755 1.737F(000) 774 2936Crystal size (mm) 0.30 0.20 0.20 0.40 0.35 Reections collected 9210 17184Data/restraints/parameters 6390/8/454 6306/51/451Goodness-of-t on F2 0.892 1.034R indices (all data) R1

a = 0.0384, wR2b = 0.0938 R1 = 0.0371

Final R indices [I > 2r(I)] R1 = 0.0501, wR2 = 0.1000 R1 = 0.0455a R1 =

PiFoj jFci/

PjFoj.b wR2(F

2)[Pw(Fo

2 Fc2)2/Pw(Fo

2)2]1/2.

3754 M. Ghosh et al. / Polyheanol leads to the formation of [ZnII(dpq)2(ClO4)2] (5). Thetris-dpq complexes [MII(dpq)3](ClO4)2 (M = Ni (6), Co (7),Fe (9) and Mn (10)) are obtained by the direct reactionbetween the ligand and the metal salt in a 1:3 ratio. Thecobalt(II) complex on oxidation with bromine producesthe corresponding cobalt(III) complex as the bromide salt,which on treatment with a stoichiometric amount of silverperchlorate aords the cobalt(III) compound [CoIII(dpq)3](ClO4)3 (8).

In all the perchlorate compounds, characteristic ClO4

vibrations for ionic perchlorate are observed at about10851090 and 625630 cm1.

Compounds 110 have been characterized by their ESI(positive) mass spectra measurements in acetonitrile. Com-plex [CuI(dpq)2](ClO4) (1) exhibits a single peak at 527.08due to the unipositive cation [CuI(dpq)2]

+. For complexes25, similar spectral features have been observed. Theyare characterized by the observation of both unipositivelycharged [MII(dpq)2(ClO4)]

+ and doubly-positively charged[MII(dpq)2]

2+ (M = Cu and Zn) species. For the tris-dpqcompounds 6, 7, 9 and 10 only one peak is observed dueto the dication [MII(dpq)3]

2+ (M = Ni, Co, Fe and Mn).The mass spectra of the cobalt(III) complex [Co(dpq)3]-(ClO4)3 (8) exhibits the features expected for the doubly-positively charged [Co(dpq)3(ClO4)]

2+ and triply-positivelycharged [Co(dpq)3]

3+ species. As examples, the observedand the simulated spectra of compounds 8 and 10 areshown in Fig. S1a and b.3.2. 1H NMR spectra

1H NMR spectroscopic studies were carried out for

P1 C2/c

13.6639(11) 8.4231(6)14.3677(11) 13.8767(6)14.8581(11) 23.9103(13)98.110(1) 90100.885(1) 96.110(1)93.983(1) 902821.9(4) 2778.9(3)2 41.732 1.7421488 1472

2 0.40 0.36 0.20 0.35 0.35 0.3017664 870312471/0/856 3260/21/2130.866 1.045

2 = 0.0976 R1 = 0.0643, wR2 = 0.1775 R1 = 0.0464, wR2 = 0.1249

2 = 0.1025 R1 = 0.1033, wR2 = 0.1936 R1 = 0.0493, wR2 = 0.1275complexes 1, 5, 6, 7, 8 and 9 in (CD3)2SO. The observedchemical shifts along with the spectral assignments aregiven in Section 2.

For the diamagnetic complexes 1, 5, 8 and 9 the ligandresonances are expected to be observed as a doubletH(1), doublet of doublet H(2), doublet H(3) and a singletH(4) (the numbering scheme for the protons is shown inScheme 1). For complex 1, the two doublets are observedat 9.65 (J = 8.3 Hz) and 9.14 (J = 5.2 Hz) ppm for H(1)and H(3), respectively, while the singlet and the doubletof doublet appear at 9.31 and 8.18 (J = 5.1, 2.9 Hz) ppm.The spectral features exhibited by 5 and 9 are quite similar.The H(4) proton is observed as a sharp singlet atd = 9.17 ppm for 5 and at 9.37 ppm for 9. Complex 5exhibits two doublets for H(1) and H(3) at d = 9.73(J = 8.8 Hz) and 8.37 (J = 4.8 Hz) ppm, respectively; how-ever, for 9 these two peaks are observed at d = 9.55(J = 8.2 Hz) and 7.95 (J = 5.2 Hz) ppm. The H(2) proton

N

N N

NH(1)

H(2)H(3)

H(4)

Scheme 1.

appears as a doublet of doublet at d = 7.94 ppm (J = 4.9,3.3 Hz) in 5 and at 7.87 ppm (J = 5.2,2.8 Hz) in 9. Thespectrum of 9 is shown in Fig. S2a. Finally, [CoIII(dpq)3]-(ClO4)3 (8) exhibits a doublet at 9.88 ppm (J = 8.2 Hz),the singlet at 9.49 ppm, the doublet of doublet at8.13 ppm (J = 5.8, 2.4 Hz) and a doublet at 7.87 ppm(J = 5.6 Hz), which are due to the H(1), H(4), H(2) andH(3) protons, respectively (Fig. S2b).

Studies on the paramagnetic 1H NMR spectroscopicbehaviour of the two complexes 6 and 7 have been carried

M. Ghosh et al. / Polyhedroout in (CD3)2SO solution. In paramagnetic compounds,hyperne-shifted resonances are observed due to interac-tions between nuclear spin and unpaired electron spins,in addition to the normal diamagnetic nuclear spin interac-tions. Electron and nuclear spin interactions occurringthrough bonds give rise to the Fermi contact shift, whilethose interactions occurring through space give rise to adipolar or pseudocontact shift. The dipolar shift dependson the magnetic anisotropy of the system and the positionin space of a given proton. The contribution of contactshift decreases rapidly with the increase of the number ofbonds connecting the proton with the paramagnetic centre,and vanishes after three or four bond separations. How-ever, when the unpaired electron spin is delocalized theinteraction remains signicant for protons many bondsaway from the metal centre. For assignment of signals inparamagnetic compounds, measurement of longitudinalrelaxation times (T1) and transverse relaxation times (T2)are particularly important. T1 correlates with the proximityof proton to paramagnetic centre, while T2 = 1/s(fwh),where fwh is the full width of a signal at its half-height, cor-relates the line width to the proximity of a proton to theparamagnetic site. Closer proximity of a proton to themetal centre gives rise to a shorter T1 and broader linewidth.

Fig. S2c shows the spectrum observed for the high-spincobalt(II) complex 7. Of the four observed resonances, thethree signals that are observed at 49.84, 18.14 and13.63 ppm are sharp, while the one observed at99.90 ppm is broader. The assignments made for these sig-nals from T1 values, line widths and integration of protonsare given in Table 2. The spectrum of the high-spin nicke-l(II) complex (6), however, exhibits (Fig. S2d) only threesignals at 50.30 H(2), 17.90 H(3) and 10.15 H(4) ppm, allhaving the same areas of integration. The assignment of

Table 2Chemical shifts, T1 values, line widths and spectral assignments for[CoII(dpq)3](ClO4)2 (7) in DMSO-d6

d (ppm) Relative area T1 (ms) D1/2 (Hz)a Assignmentb

99.89 1

dro3756 M. Ghosh et al. / Polyheexpected, the inuence of temperature becomes more con-spicuous in the relatively weaker hydrogen bridged dis-tances. Since with the lowering of temperature thethermal vibration gets reduced, accordingly O O andO N distances at 153 K are found to be shorter by0.030.09 A relative to those reported at 293 K.

3.3.2. [Cu(dpq)2](ClO4)2 (3)In complex 3, one of the perchlorate anions is disor-

dered, whose three oxygen atoms O(22), O(23) and O(24)have double occupancy (0.55:0.55), while O(21) and Cl(2)have unique occupancy. The ORTEP projection of the cat-ion [Cu(dpq)2]

2+ is shown in Fig. 2. The four-coordinatedCuN4 core, in terms of a square planar description, doesnot turn out to be satisfactory because the donor atomsinvolved are alternatively displaced above and below theleast-squares plane by 0.40 A, albeit the copper centre islying exactly on the mean plane. The alternative descrip-tion in terms of a tetrahedral conguration is also unsatis-factory because the dihedral angle h between the opposing

Fig. 1. (a). An ORTEP representation of the molecule [CuII(dpq)2(H2O)](ClOstructures formed by the intermolecular hydrogen bonding network inv[CuII(dpq)2(H2O)](ClO4)2 H2O (2).n 26 (2007) 37503762CuN2 planes is 37.9 (h = 0 for square planar and 90 fortetrahedral). In 3, the distances CuN(1) [2.022(2) A] andCuN(2) [1.992(2) A] are nearly equal, as are the compli-mentary distances CuN(3) [2.026(2) A] and CuN(4)[1.987(2) A]. The transoid angles N(1)CuN(3)[146.40(7)] and N(2)CuN(4) [167.54(8)] deviate consid-erably from the values expected for a square planar geom-etry. The cisoid angles N(1)CuN(2) and N(3)CuN(4)are equal [82.20(7)], while the two other opposing anglesN(1)CuN(4) and N(2)CuN(3) are 100.14(7) and102.70(7), respectively. Clearly, the tetrahedral descriptionis worse relative to the square planar description.

A consideration of the proximities of the oxygen atomsof one of the perchlorate anions to the copper centrereveals that the disordered oxygen O(22) is 2.427(4) A awayfrom the metal atom. If this distance is reckoned as a bond,Fig. S3 provides another ORTEP representation of the[Cu(dpq)2(ClO4)]

+ cation. In the ve-coordinated environ-ment [CuN4O], the geometry of the copper centre may beeither trigonal bipyramidal (tbp) or square pyramidal

4)2 H2O (2) showing 50% probability displacement ellipsoids. (b) Dimericolving the perchlorate ion and solvated water molecule in complex

18)18)17)18)

droTable 3Selective bond distances (A) and angles () for compounds 25

[CuII(dpq)2(H2O)](ClO4)2 H2O (2) [CuII(dpq)2](ClO4)2 (3)

Cu(1)N(1) 2.1047(19) Cu(1)N(1) 2.0225(Cu(1)N(2) 1.9660(19) Cu(1)N(2) 1.9918(Cu(1)N(3) 2.0288(19) Cu(1)N(3) 2.0265(Cu(1)N(4) 1.9715(15) Cu(1)N(4) 1.9875(

M. Ghosh et al. / Polyhe(sp). To consider the tbp geometry, the atoms N(1), N(3)and O(221) form the trigonal plane and the atoms N(2)and N(4) are trans-axially disposed. However, the anglesN(1)CuN(3) [146.40(7)], N(1)CuO(221) [95.34(16)]and N(3)CuO(221) [117.56(16)] all deviate from theideal angle of 120. The trans angles N(2)CuN(4)[167.54(8)] also deviates from 180. Moreover, the valueof s = 0.35 falls short of that to be a tbp candidate.

Cu(1)WA(1) 2.1236(18) Cu(1)O(221A) 2.427(4)

N(1)Cu(1)N(2) 81.40(8) N(1)Cu(1)N(2) 82.19(7)N(3)Cu(1)N(4) 82.39(8) N(3)Cu(1)N(4) 82.21(7)N(1)Cu(1)N(3) 125.55(8) N(1)Cu(1)N(3) 146.40(7)N(1)Cu(1)N(4) 105.04(8) N(1)Cu(1)N(4) 100.14(7)N(2)Cu(1)N(3) 97.52(8) N(2)Cu(1)N(3) 102.70(7)N(2)Cu(1)N(4) 172.21(8) N(2)Cu(1)N(4) 167.45(8)N(1)Cu(1)WA(1) 95.24(8) N(1)Cu(1)O(221A) 95.34(16)N(2)Cu(1)WA(1) 87.59(7) N(2)Cu(1)O(221A) 90.81(11)N(3)Cu(1)WA(1) 139.21(7) N(3)Cu(1)O(221A) 117.56(16)N(4)Cu(1)WA(1) 87.45(8) N(4)Cu(1)O(221A) 76.82(11)

[ZnII(dpq)2(ClO4)2] (5)a

Zn(1)N(1) 2.127(2) N(1)Zn(1)N(2)Zn(1)N(1A) 2.127(2) N(1)Zn(1)N(2A)Zn(1)N(2) 2.120(2) N(2)Zn(1)N(2A)Zn(1)N(2A) 2.120(2) N(2)Zn(1)N(1A)Zn(1)O(14) 2.1899(19) N(2A)Zn(1)N(1A)Zn(1)O(14A) 2.1899(19) N(1)Zn(1)N(1A)

N(1)Zn(1)O(14)N(2)Zn(1)O(14)

a For 5 A indicates: x, y, z + 1/2.

Fig. 2. A four-coordinated ORTEP projection of the molecule [CuII(d[{CuII(dpq)2(H2O)}{CuII(dpq)2(ClO4)}](ClO4)3 (4)

Cu(1)N(11) 2.042(4) Cu(2)N(21) 1.982(4)Cu(1)N(12) 1.982(4) Cu(2)N(22) 2.046(4)Cu(1)N(13) 2.043(4) Cu(2)N(23) 2.041(4)Cu(1)N(14) 1.981(3) Cu(2)N(24) 1.977(4)

n 26 (2007) 37503762 3757In favor or against the alternative sp geometry with aCuO(221) apical distance of 2.427(4) A, the same argu-ments made for the square planar geometry apply.

3.3.3. [{CuII(dpq)2(H2O)}{CuII(dpq)2(ClO4)}](ClO4)3

(4)Complex 4 is a co-crystallized compound in which the

asymmetric unit contains two cationic species viz.

Cu(1)O(1) 2.427(4) Cu(2)O(42) 2.540(4)Cu(2)O(44) 2.510(5)

N(11)Cu(1)N(12) 81.71(15) N(21)Cu(2)N(22) 82.01(17)N(13)Cu(1)N(14) 82.00(15) N(23)Cu(2)N(24) 83.01(14)N(11)Cu(1)N(13) 135.76(14) N(21)Cu(2)N(23) 98.18(16)N(11)Cu(1)N(14) 100.11(15) N(21)Cu(2)N(24) 171.00(16)N(12)Cu(1)N(13) 101.24(15) N(22)Cu(2)N(23) 137.32(15)N(12)Cu(1)N(14) 173.32(15) N(22)Cu(2)N(24) 103.21(15)N(11)Cu(1)O(1) 115.29(18) O(42)Cu(2)O(44) 52.02(15)N(12)Cu(1)O(1) 85.33(14) N(21)Cu(2)O(42) 89.53(17)N(13)Cu(1)O(1) 108.93(18) N(22)Cu(2)O(42) 82.85(14)N(14)Cu(1)O(1) 88.09(14) N(23)Cu(2)O(42) 139.70(13)

N(24)Cu(2)O(42) 83.92(17)N(21)Cu(2)O(44) 83.95(18)N(22)Cu(2)O(44) 132.68(13)N(23)Cu(2)O(44) 89.35(14)N(24)Cu(2)O(44) 87.15(16)

77.92(8) N(1)Zn(1)O(14A) 89.54(9)95.45(8) N(2)Zn(1)O(14A) 161.42(8)102.21(11) N(1A)Zn(1)O(14) 89.54(9)95.45(8) N(2A)Zn(1)O(14) 161.42(8)77.92(8) N(1A)Zn(1)O(14A) 98.72(8)169.57(11) N(2A)Zn(1)O(14A) 92.49(8)98.72(8) O(14)Zn(1)O(14A) 75.75(12)92.49(8)

pq)2](ClO4)2 (3) showing 50% probability displacement ellipsoids.

2(H2O)}{CuII(dpq)2(ClO4)}](ClO4)3 (4) showing 50% probability displacement

dron 26 (2007) 37503762{Cu(dpq)2(H2O)}2+ and {Cu(dpq)2(ClO4)}

+, and threeperchlorate anions. An ORTEP projection of the cationsin 4 is shown in Fig. 3. It may be noted that 4 can be con-sidered as the 1:1 co-crystals of 2 and 3. However, it will beevident that notwithstanding the same composition, thecationic species of 4 have subtle structural dierences from2 and 3.

In 4, the ve-coordinated copper(II) centre in[Cu(dpq)2(H2O)]

2+ cation obtains a tbp geometry withN(11), N(13) and O(1) forming the trigonal plane andN(12) and N(14) occupying the axial positions. Unlike 3,in which the two equatorial CuN distances are signi-cantly dierent, in 6 the two equatorial CuN(11) andCuN(13) distances are equal [2.043(4) A]. The third equa-torial CuO(1) bond length, however, is long [2.144(3) A].The trans axially disposed CuN(12) and CuN(14) bonddistances are also equal [1.982(4) A]. The NCuN andNCuO bond angles in the trigonal plane lie in the range109136. The trans N(12)CuN(14) angle is 173.32(14).

Fig. 3. An ORTEP representation of the cation of the complex [{CuII(dpq)ellipsoids.

3758 M. Ghosh et al. / PolyheThe value of s in this case (0.63) is greater than that of 3(0.55), indicating lesser distortion in the present case.

The ORTEP projection of the cation [Cu(dpq)2(ClO4)]+

in 4 (Fig. 4) shows that the two perchlorate oxygens O(42)and O(44) are bound to the copper(II) centre in a bidentatefashion. For a better view of the six-coordinated metal core[CuN4O2] another ORTEP projection is shown in Fig. S4.It may be noted that in terms of an octahedral description,the metal centre is located in the basal plane formed by theatoms N(22), N(23), O(44) and O(42). However, theseatoms are displaced from the least-squares plane by0.215(5) A for the oxygen atoms and 0.123(4) A forthe nitrogen atoms. Since the CuN(22)/N(23) distances[2.046(4) and 2.041(4) A] are much shorter as comparedto the CuO(42)/O(44) distances [2.540(4) and2.510(5) A], the metal atom in the N2O2 plane is locatedcloser to the nitrogen atoms. The trans-axial CuN(21)/N(24) distances are 1.982(4) and 1.977(4) A, and are evenshorter than the equatorial distances, indicating a distortedcompressed geometry.3.3.4. [Zn(dpq)2(ClO4)2] (5)The molecular structure of the zinc(II) complex

[Zn(dpq)2(ClO4)2] (5) is shown in Fig. 4. A twofold axispasses mid-way through the metal-bound perchlorate oxy-gens O(14) and O(14A). The coordination environmentaround the six-coordinated metal centre [ZnN4O2] maybe considered as distorted octahedral. The atoms N(2),N(2A), O(14A) and O(14) form the best plane, althoughthe atoms N(2)/N(2A) are displaced from the mean planeby 0.214(2) A and O(14)/O(14A) by 0.265(2) A. Themetal atom lies exactly on the mean plane. The nitrogenatoms N(1) and N(1A) are trans-axially disposed to themetal centre. The equatorial and axial ZnN distances[2.120(2) and 2.127(2) A] are almost equal, while theequatorial ZnO distances [2.190(2) A] are slightly longer.The trans-axial N(1)ZnN(1A) angle is 169.57(11), whilethe trans-equatorial N(2)ZnO(14A) angle is 161.42(8).The trigonal planes N(1)O(14A)N(2A) and N(1A)O(14)-N(2) are inclined to each other by 50.0, again indicating

a distorted octahedral geometry. The relevant bond dis-tances and bond angles are listed in Table 3.

Fig. 4. An ORTEP projection of the complex [ZnII(dpq)2(ClO4)2] (5)showing 50% probability displacement ellipsoids. A indicates thesymmetry operation (1 x, y, z + 1/2).

q)2

dron 26 (2007) 37503762 3759Another interesting structural feature of this compoundis the presence of a pp interaction between the pyrazinering C(5)N(6)C(7)C(8)C(9)C(10) of the adjacent molecule,forming a sinusoidal pattern, as shown in Fig. 5. The dis-tance between the centroid of the pyrazine rings is 3.542 A.

3.4. Electronic spectra

The absorption spectroscopic behaviour of compounds110 have been studied in acetonitrile. In addition, thespectra of compounds 24 have been obtained in nitro-methane. The UVVisNIR spectral data for theses com-pounds in acetonitrile are given in Section 2.

All of the complexes exhibit three very strong absorp-tion bands at 215225 nm, 250260 nm and 290300 nm,due to the ligand centred pp* transitions. In the case ofthe copper(I) complex 1, a strong band observed at415 nm (e = 4700 M1 cm1) is due to a metal-to-ligandcharge transfer (MLCT) transition. This charge transfertransition occurs from the lled metal orbital (d10) to theempty p* antibonding orbital of the bipyridine ligand.Although the solid state structures of the three copper(II)complexes [CuII(dpq)2(H2O)](ClO4)2 (2), [Cu

II(dpq)2]

Fig. 5. An 1-D growth of the molecule [ZnII(dp

M. Ghosh et al. / Polyhe(ClO4)2 (3) and [{CuII(dpq)2(H2O)}{Cu

II(dpq)2(ClO4)}](ClO4)3 (4) are dierent, in acetonitrile solution they exhibitsimilar spectral features in the visible and near IR region.Two broad absorption bands are observed at 700 and900 nm. It is documented, especially for copper(II) com-plexes, that a single dd band with a high energy shoulderis indicative of a trigonal bipyramidal stereochemistryaround the metal centre, while an absorption envelop witha low energy shoulder is characteristic of a square pyrami-dal geometry. Some penta-coordinated copper(II) com-plexes with two dd bands of equal intensity appear topossess an intermediate distorted geometry [29]. Thebroadness of these bands are indicative of the presence ofmore than two transitions at lower energies as expectedfor copper(II) in a distorted trigonal bipyramidal environ-ment. Indeed, deconvolution of the absorption spectrum of2 in acetonitrile by Gaussian line-shape analysis gives riseto three peaks (shown in Fig. S5), which are due to thedx2y2 ! dz2 , dxy ! dz2 and dxz, dyz ! dz2 transitions inascending order of energies. It appears that the identicalspectra observed for complexes 24 in acetonitrile is duethe presence of the same penta-coordinated species,[CuII(dpq)2(CH3CN)]

2+, in all three cases. However, innitromethane, which is a non-coordinating solvent, somevariation in positions and intensities of these bands in thethree cases becomes evident. This is illustrated inFig. S6ac by deconvoluting the absorption spectraobserved for 24 in nitromethane. The spectral data andthe deconvoluted peak positions of 24 in nitromethaneare listed in Table 4.

The absorption spectrum of the nickel(II) complex[Ni(dpq)3](ClO4)2 (6) shows the presence of bands at 215(e = 60000 M1 cm1), 260 (e = 110000 M1 cm1),300 nm (e = 40000 M1 cm1), 785 (e = 11 M1 cm1) and855 nm (e = 9 M1 cm1). In octahedral nickel(II) com-plexes three spin-allowed transitions 3A2g ! 3T2g, 3T1g,3T1g(P) are normally observed. Because the separation ofthe energy between the 3A2g ! 3T2g and 3A2g ! 3T1g transi-tions in a wide variety of octahedral nickel(II) complexes liesin the range 50008000 cm1, it is highly unlikely that thetwo absorption peaks observed as a doublet at 855 nm

(ClO4)2] (5) through pp stacking interactions.(11700 cm1) and 785 nm (12750 cm1), which dier onlyby 1000 cm1, could be assigned due to these transitions.On the other hand, when Dq/B approaches unity the 1Egstate lies close to 3T1g, and due to their extensive mixingthe spin forbidden 3A2g ! 1Eg transitions steals intensityfrom the spin-allowed 3A2g ! 3T1g transition, beingobserved as a doublet.

Table 4UVVis and deconvoluted absorption peaks for complexes 24 innitromethane

Compound kmax/nm(e/M1 cm1)

Deconvolutedpeaks (nm)

[CuII(dpq)2(H2O)](ClO4)2 H2O (2) 740 (140),870 (115)

695, 865, 935

[CuII(dpq)2](ClO4)2 (3) 710 (150),840 (120)

685, 835, 940

[{CuII(dpq)2(H2O)}{CuII(dpq)2(ClO4)}]

(ClO4)3 (4)725 (150),870 (115)

695, 880, 950

The tris-dpq cobalt(II) complex 7 exhibits a lone ddbandat 910 nm (e = 10 M1 cm1) in addition to the internalligand transitions observed at 210 (e = 58000 M1 cm1),250 (e = 107000 M1 cm1) and 290 nm (e = 52000M1 cm1). The observed dd band is due to the 4T1g ! 4T2gtransition. A second band, which is expected to be observedat ca. 500 nm in octahedral cobalt(II) complexes due to the4T1g ! 4T1g (P) transition, is not seen in the present case.In the cobalt(III) complex [Co(dpq)3]

3+ (93+), aside fromthe bands observed at 210 (e = 66000 M1 cm1), 250(e = 95000 M1 cm1) and 290 nm (e = 34000 M1 cm1),a band due to the 1A1g ! 1T1g transition is observed at460 nm (e = 210 M1 cm1). The higher energy 1A1g ! 1T2gtransition is apparentlymasked by the intense bandobserved

3760 M. Ghosh et al. / Polyhedroat 290 nm. Since the energy of the 1T1g state is 10Dq C(C 4B 2000 cm1) [9], the value of 10Dq is estimatedto be 24200 cm1.

The low-spin iron(II) complex 10 exhibits MLCTabsorptions at 440 and 520 nm and a dd transition at850 nm.

3.5. Electrochemistry

The electrochemical characteristics of complexes 1, 2, 3,4, 7 and 8 have been examined by CV and SWV and theobserved redox potentials are given in Table 5.

The copper(I) complex [Cu(dpq)2](ClO4) (1) undergoes aone electron quasireversible (DEp = 120 mV) oxidation atE1/2 = 110 mV in acetonitrile. As mentioned earlier, theabsorption spectra of the copper(II) complexes (24) havebeen examined in acetonitrile and nitromethane, hence,the electrochemical behaviour of these compounds havealso been studied in those two solvents. In acetonitrile, sim-ilar to the absorption spectral behaviour, all the three com-pounds (24) exhibit identical electrochemical responseswith E1/2 = 120 mV and DEp = 150 mV. Cyclic voltammo-grams of these compounds, however, dier signicantlyin nitromethane, as observed in their electronic spectra.In nitromethane, the E1/2 value of [Cu(dpq)2(H2O)]-(ClO4)2 H2O (2) is 325 mV (DEp = 70 mV) (Fig. S7a), thatof [Cu(dpq)2](ClO4)2 (3) is 355 mV (DEp = 70 mV)

Table 5Electrochemical dataa for complexes 14, 7 and 8

Compound Acetonitrile Nitromethane

E1/2b (mV) DEp

c E1/2 (mV) DEp

[CuI(dpq)2](ClO4) (1) 110 120 330 70[CuII(dpq)2(H2O)](ClO4)2 (2) 110 130 325 70[CuII(dpq)2](ClO4)2 (3) 105 120 355 70[{CuII(dpq)2(H2O)}

{CuII(dpq)2(ClO4)}](ClO4)3 (4)100 120 350 70

[CoII(dpq)3](ClO4)2 (7) 535 70[CoIII(dpq)3](ClO4)3 (8) 540 70

a All the potentials are against Ag/AgCl reference electrode.b E1/2 values are the average of those obtained from cyclic voltammetricand square wave voltammetric measurements, which are within 5 mV.c DEp refers to the peak to peak separation at a scan rate of 100 mV s

1.(Fig. S7b) and nally for [{CuII(dpq)2(H2O)}{CuII(dpq)2-

(ClO4)}](ClO4)3 (4) (Fig. S7c) the E1/2 and DEp values are350 mV and 70 mV. As expected, the redox behaviour ofthe cobalt(II) complex 7 in the oxidative mode and thecobalt(III) complex 8 in the reductive mode are identical.The E1/2 value of the fully reversible (DEp = 70 mV)CoII/CoIII couple is 540 mV (shown in Fig. S8).

4. Conclusion

Copper(I), copper(II), zinc(II), nickel(II), cobalt(II),cobalt(III), manganese(II) and iron(II) complexes (110)of dipyrido[3,2-f:2 0,3 0-h]-quinoxaline (dpq) have been syn-thesized and structurally characterized. Various spectro-scopic and electrochemical properties of these compoundshave been studied.

The most signicant aspect of the study is the isolationand structural characterization of three copper(II) com-plexes of dpq. The compound of composition [CuII(dpq)2(H2O)](ClO4)2 (1) has been obtained by reactingCu(ClO4)2 6H2O with dpq in methanol. The structuredetermination of this bluish-green compound has revealedthat copper(II) has a highly deformed penta-coordinatedgeometry and can be best described as a distorted trigonalbipyramid with a s value of 0.55. When the reactionbetween Cu(ClO4)2 6H2O and dpq is carried out in dryacetonitrile, the blue coloured product isolated has thecomposition [Cu(dpq)2](ClO4)2 (3). This compound hasan irregular geometry. As such, the [Cu(dpq)2]

2+ cationdeviates from a square planar conguration to a tetrahe-dral geometry, although the extent of tetrahedral distortionis rather less (dihedral angle h = 38). The close proximityof an oxygen atom of a distorted perchlorate (CuOClO3 = 2.43 A) confers a ve-coordinate geometry,which again is neither square planar nor trigonal bipyrami-dal. In the third method of preparation, the condensationreaction between 1,10-phenanthroline-5,6-dione and 1,2-diaminoethane has been carried out in the presence ofCu(ClO4)2 6H2O in methanol. Initially, a dark colouredcopper(I) complex of composition [Cu(dpq)2](ClO4) (1)gets deposited and from the solution a second green col-oured complex of copper(II) having the composition[{Cu(dpq)2(H2O)}{Cu(dpq)2(OClO3)](ClO4)3 (4) has beenisolated. The structure determination of this compoundhas established the presence of the two dierent complexcations in the asymmetric unit. It may therefore be consid-ered as a co-crystal. The structure of the [Cu(dpq)2(H2O)]

2+ cation is not exactly the same as the earlierdescribed compound. The tbp geometry is more regular(s = 0.63) in this case. In the cation [Cu(dpq)2(ClO4)]

+,the perchlorate is bound to the metal centre in a bidentatefashion [CuO = 2.51 and 2.54 A], thus conferring a six-coordinate geometry to the copper. Similar to the otherstructures, in this case the metal centre also obtains ahighly irregular geometry which is intermediate between

n 26 (2007) 37503762octahedral and trigonal prismatic (the angle between thetwo trigonal faces is 32).

droThe zinc compound [Zn(dpq)2(OClO3)2] (5) also showsan interesting structure. Because the two perchlorates areunidentately coordinated to the metal centre, the zinc(II)centre obtains a distorted octahedral geometry. In the crys-tal lattice, the quinoxaline ring of the complex unit isinvolved in an intermolecular pp interaction which leadsto the generation of a sinusoidal chain.

The proton NMR spectra of the complexes, includingthose of the paramagnetic complexes [NiII(dpq)3](ClO4)2(6) and [CoII(dpq)3](ClO4)2 (7) have been studied. Theassignment of protons for complex 7 has been made onthe basis of their T1 and T2 values. The redox behaviourof copper(II) complexes 24 in acetonitrile is identical butin nitromethane they exhibit dierent behaviour from eachother. The cobalt(II) and cobalt(III) complexes 7 and 8undergo a reversible electron transfer reaction and the E1/2values of the CoII/CoIII couple in acetonitrile is 540 mV.

Acknowledgements

We gratefully acknowledge Prof. K. Nag for his invalu-able help and suggestions during the course of this workand in the preparation of the manuscript. M.G. and P.B.are also thankful to CSIR, India for the award of a re-search fellowship.

Appendix A. Supplementary material

CCDC 637011, 637012, 637013 and 637014 contain thesupplementary crystallographic data for 2, 3, 4 and 5.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from theCambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. ESI-MS (positive) for com-plexes 8 and 10 (Fig. S1a and b), 1H NMR spectra of 6, 7,8, 9 (Fig. S2ad), ORTEP representation of cation[Cu(dpq)2(ClO4)]

+ in complex 3 (Fig. S3), six-coordinatedmetal core [CuN4O2] in one of the units of the complex 4(Fig. S4), UVVisNIR spectra of 2 in acetonitrile(Fig. S5) and 24 in nitromethane (Fig. S6ac) with decon-volution, cyclic voltammogram of complexes 24 andferrocene in nitromethane (Fig. S7ac) and cyclic voltam-mogram of complexes 7 and 8 (Fig. S8). Supplementarydata associated with this article can be found, in the onlineversion, at doi:10.1016/j.poly.2007.04.014.

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3762 M. Ghosh et al. / Polyhedron 26 (2007) 37503762

Structural, spectroscopic and redox properties of transition metal complexes of dipyrido[3,2-f:2 prime ,3 prime -h]-quinoxaline (dpq)IntroductionExperimentalMaterialsPreparation of dipyrido[3,2-f:2 prime ,3 prime -h]-quinoxaline (dpq)Preparation of the metal complexes[CuI(dpq)2](ClO4) (1)[CuII(dpq)2(H2O)](ClO4)2 middot H2O (2)[CuII(dpq)2](ClO4)2 (3)[{CuII(dpq)2(H2O)}{CuII(dpq)2(ClO4)}](ClO4)3 (4)[ZnII(dpq)2(ClO4)2] (5)[NiII(dpq)3](ClO4)2 (6)[CoII(dpq)3](ClO4)2 (7)[CoIII(dpq)3](ClO4)3 (8)[Fe(dpq)3](ClO4)2 (9)[MnII(dpq)3](ClO4)2 (10)

Physical measurementsCrystallography

Results and discussionSynthesis and characterization1H NMR spectraCrystal structures[CuII(dpq)2(H2O)](ClO4)2 middot H2O (2)[Cu(dpq)2](ClO4)2 (3)[{CuII(dpq)2(H2O)}{CuII(dpq)2(ClO4)}](ClO4)3 (4)[Zn(dpq)2(ClO4)2] (5)

Electronic spectraElectrochemistry

ConclusionAcknowledgementsSupplementary materialReferences