Fourteen membered hexaaza copper macrocycle: synthesis, characterization, crystal structures and the consequence of anion coordination

Inorganica Chimica Acta 384 (2012) 309–317

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Inorganica Chimica Acta

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Fourteen membered hexaaza copper macrocycle: Synthesis, characterization,crystal structures and the consequence of anion coordination

Ahmad Husain a,1, A. Moheman a, Shahab A.A. Nami b, K.S. Siddiqi a,⇑a Department of Chemistry, Aligarh Muslim University, Aligarh 202002, Indiab Department of Kulliyat, Faculty of Unani Medicine, Aligarh Muslim University, Aligarh 202002, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 October 2010Received in revised form 10 August 2011Accepted 13 December 2011Available online 24 December 2011

Keywords:Crystal structureTrans-III conformationMacrocycleCu(II) complexAxially elongated octahedralSquare-planar

0020-1693/$ - see front matter Crown Copyright � 2doi:10.1016/j.ica.2011.12.019

⇑ Corresponding author. Mobile: +91 9837284930.E-mail addresses: [email protected]

com (K.S. Siddiqi).1

Present address: Centre for Supramolecular ChemisChemistry, University of Cape Town, South Africa.

Fourteen membered hexaaza macrocyclic complexes of the type [CuLX2] (where L = 3,10-bis(ben-zyl)1,3,5,8,10,12-hexaazacyclotetradecane and X ¼ ClO�4 , SCN�) and [CuL(CH3CN)2](PF6)2(CH3CN)2 havebeen synthesized by template assisted condensation reaction. They have been characterized by spectraland X-ray crystallographic techniques. It was of interest to investigate the effect of anions on the grossgeometry and chemical properties of Cu(II) ion. The crystal structure of all the complexes show axiallyelongated octahedral geometry with weakly coordinated anions. The macrocyclic ring adopts thetrans-III configuration with six- and five-membered chelate rings in chair and gauche conformation,respectively. The IR and other spectral properties are consistent with the result of X-ray diffraction. Itwas observed that in the solid state the arrangement of the coordination sphere is distorted octahedralwhereas in solution, a square-planar structure is predominant. The molar conductivity of the complexesindicates that the axially bonded anions are almost entirely dissociated in acetonitrile solution. The EPRspectra of complexes 1–3 are axial and consistent with a dx2�y2 ground state.

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1. Introduction

There is a continuing interest in the chemistry of polyazamacro-cycles because of their sturdy chemical and biochemical application.Structural factors, such as ligand rigidity and the electron-donatingproperties of the nitrogen atoms constituting the polyaza networkand their disposition play a significant role in determining thebinding features towards metal cations [1–7]. Moreover, hexaazamacrocyclic complexes show enhanced thermodynamic and kineticstability as compared to their non-cyclic chelate ligands, and haveapplications in modern chemical techniques such as magneticresonance imaging, imaging with radioisotopes and radiotherapy[8,9]. The diverse synthetic utility of hexaaza macrocycles may beattributed due to the liberty offered by these systems in terms ofcavity size, pendant groups and different metal encapsulationability. The complexation of hexaaza macrocycles is governedmainly by the ring size and ionic radii of the metal ions [10].

Out of the two well known strategies of synthesis of polyaza-macrocyclic systems, metal template procedure is considered asbetter as it often provides selective routes toward products thatare not obtainable in the absence of metal ions. Such processes

011 Published by Elsevier B.V. All

(A. Husain), kssiddiqi@gmail.

try Research, Department of

are simple ‘one pot reactions’, inexpensive and high yielding. Metaltemplate condensation involving amines and formaldehyde havebeen employed in the preparation of various saturated polyazamac-rocyclic complexes containing N–CH2–N linkages [11–14]. Some-times, the pendant-arms of the macrocycles can also play animportant role in altering their stereochemistry and thermody-namic parameters, hence affecting their overall physicochemicalproperties [15]. However, the presence of rigid aromatic systemgives particular coordination properties to the ligands defining dis-tinct preorganized binding sites for the metal ions. Copper-contain-ing macrocycles are of considerable interest as low molecularweight models for biological copper-containing redox proteins [16].

In the present study, complexes of 14-membered hexaaza mac-rocyclic ligand with Cu(II) containing different anions have beencharacterized by crystallographic studies to investigate the effectof the anions on the gross geometry and chemical properties ofCu(II) ion. In order to characterize the [CuLX2] entity structurallyand to assess the consequence of anion coordination, IR, electronicspectra and EPR were also recorded and analyzed in this work.

2. Experimental

2.1. Material and methods

Cu(OOCCH3)2�H2O, HClO4 (Merck India), ethylenediamine (E.Merck), benzylamine and formaldehyde (S.D. Finechem India) were

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310 A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317

used as received. Elemental analysis was done with a FLASH EA1112 SERIES CHNS analyzer. IR spectra (4000–400 cm�1) were re-corded with a Spectrolab Interspec FT/IR-2020 spectrometer asKBr disc. The conductivity measurements were carried out with aCM–82T Elico conductivity bridge in acetonitrile. The electronicspectra were recorded with a Perkin Elmer Lambda–35 UV–Visspectrophotometer in CH3CN. Magnetic susceptibility measure-ments in powdered samples were performed at 25 �C by Faradaymethod with a Sherwood Scientific magnetic susceptibility balance,calibrated against Hg[Co(SCN)4]. EPR spectra were recorded in thesolid state and in frozen CH3CN with a JEOL FE3X EPR spectrometerat X-band microwave frequency. Powder XRD pattern of all sampleswere recorded on a Bruker-Axs, D8 Advance diffractometer usingCu Ka X-radiation at 35 kV and 25 mA. Diffraction patterns werecollected over 2h range of 5–50� at scan rate of 1�/min.

Caution! Although our samples never exploded during handling,perchlorate metal complexes are potentially explosive: they shouldbe handled with care.

2.2. Synthesis of [CuL(ClO4)2] (1) [(L = 3,10-bis(benzyl)1,3,5,8,10,12-hexaazacyclotetradecane)]

To an ethanolic (50 mL) solution of Cu(OOCCH3)2�H2O (5 mM,0.998 g) were slowly added ethylenediamine (10 mM, 0.669 mL),40% formaldehyde (20 mM, 1.600 mL) and benzylamine (10 mM,1.092 mL) with constant stirring. After refluxing this mixture for48 h, the red-violet solution was cooled to room temperature andfiltered to remove any insoluble solid. To this filtrate an excess of70% perchloric acid was added with continuous stirring whichyielded pink colored precipitate (Scheme 1). It was filtered andwashed with ethanol and dried over P2O5. The complex was recrys-tallized from acetonitrile. Red crystals were obtained after 7 days.Yield: �80% (2.58 g). M.p.: 255 �C. Km: 247 X�1 cm2 M�1. Anal.Calc. for C22H34Cl2CuN6O8 (1): C, 40.97; H, 5.31; N, 13.03. Found:C, 40.81; H, 5.41; N, 13.15%. IR (KBr, cm�1): 3243, 3173, 2924,2879, 1449, 1421, 1377, 1335, 1275, 1114, 1073, 997, 830, 896,855, 750, 703, 624, 437.

2.3. Synthesis of [CuL(SCN)2] (2)

To a solution of [CuL(ClO4)2] (1) (1 mM, 0.645 g) in acetonitrile(20 mL) was added an excess of potassium thiocyanate and stirredfor 1 h. It was filtered to remove solid KClO4 and the solvent was evap-orated to dryness to give purple blue powder. It was dissolved in ace-tonitrile-DMF mixture with a few drops of water and left to stand atroom temperature. Violet crystals thus obtained after 1 month werefiltered and washed with ethanol. Yield: �95% (0.612 g). M.p.:228 �C. Km: 254 X�1 cm2 M�1. Anal. Calc. for C24H34CuN8S2 (2): C,51.27; H, 6.10; N, 19.93. Found: C, 50.87; H, 5.99; N, 19.73%. IR (KBr,cm�1): 3194, 2897, 2868, 2049, 1459, 1418, 1269, 1188, 1147, 1060,1012, 951, 856, 742, 698, 640, 592, 480, 418.

Scheme 1. Synthesis o

2.4. Synthesis of [CuL(CH3CN)2](PF6)2(CH3CN)2 (3)

To an acetonitrile (20 mL) solution of [CuL(ClO4)2] (1) (1 mM,0.645 g) was added an excess of potassium hexafluorophosphateand filtered to remove solid KClO4. The red filtrate was allowedto stand for a few days at 4 �C to give red crystals. Yield: �94%(0.606 g). M.p.: 297 �C. Km: 275 X�1 cm2 M�1. Anal. Calc. forC22H34CuF12N6P2 (3): C, 40.03; H, 5.15; N, 15.56. Found: C, 40.23;H, 5.28; N, 15.45% IR (KBr, cm�1): 3269, 3173, 2916, 2885, 2294,2259, 1466, 1435, 1379, 1334, 1273, 1188, 1159, 1080, 1057,999, 823, 738, 702, 638, 557, 445.

2.5. X-ray crystal structure determination and refinements

Single crystals suitable for X-ray crystallographic analysis wereobtained by slow evaporation of the solvent from the acetonitrilesolution of 1 at room temperature and that of 3 at 4 �C. Crystalsof 2 were grown by slow evaporation of a 1:1 mixture of acetonitrileand DMF and a few drops of water. X-ray data for 1 were collectedusing graphite-monochromated Mo Ka radiation (K = 0.71073 Å)on ‘‘Bruker SMART APEX CCD Diffractometer’’ at 100(2) K. The pro-gram SMART [17] was used for collecting frames of data, indexingreflections, and determining lattice parameters for 1. The data inte-gration and reduction were processed with SAINT [17] software. Anempirical absorption correction was applied to the collected reflec-tions with SADABS [18] using XPREP [19]. X-ray data for 2 and 3 werecollected on Oxford Diffraction Gemini CCD equipped diffractome-ter at 298(2) K using graphite-monochromated Mo Ka radiation(K = 0.71073 Å). The strategy for the data collection was evaluatedby the CRYSALISPRO CCD software and collected by the standard‘phi-omega scan’ techniques, scaled and reduced using CRYSALISPRO

RED software [20]. The linear absorption coefficients, scatteringfactors for the atoms and the anomalous dispersion correctionswere taken from the International Tables for X-ray Crystallography[21]. The structures were solved by direct method using SHELXS-97program [22] and were refined on F2 by full-matrix least-squaretechnique using the SHELXL-97 [22] program. Figures were drawnusing ORTEP-3.2 [23] and MERCURY-2.3 [24]. All non-hydrogen atomswere refined with anisotropic displacement parameters. H atomswere refined isotropically. The pertinent crystal data and refine-ment parameters for compounds 1, 2 and 3 are compiled in Table 1.

3. Results and discussion

3.1. Crystal structure description of [CuL(ClO4)2] (1)

The molecular structure and the atomic numbering scheme of 1 isshown in Fig. 1. Crystallographic data, selected bond lengths and bondangles are listed in Tables 1 and 2. It was crystallizes in triclinic systemwith P�1 space group. The asymmetric units comprise of half a mole-cule and the other half of the molecules are generated through a

f [CuL(ClO4)2] (1).

Table 1Crystallographic data and structure refinement parameters for 1, 2 and 3.

Complex [CuL(ClO4)2] (1) [CuL(SCN)2] (2) [CuL(CH3CN)2](PF6)2�2CH3CN (3)

Empirical formula C22H34Cl2CuN6O8 C24H34CuN8S2 C30H46CuF12N10P2

Formula weight 644.99 562.25 900.25T (K) 100(2) 298(2) 298(2)

k (ÅA0

) 0.71073 0.71073 0.71073

Crystal system Triclinic Rhombohedral MonoclinicSpace group P�1 R�3 P2(1)/ca (Å) 10.156(3) 29.766(6) 12.5103(9)b (Å) 11.433(3) 29.766(6) 16.0648(11)c (Å) 11.965(3) 7.9265(11) 20.8753(15)a (�) 86.679(5) 90 90b (�) 75.765(4) 90 99.8510(10)c (�) 87.829(5) 120 90V (Å3) 1344.0(6) 6082.2(19) 4133.6(5)Z, qcalc (g cm�3) 2, 1.594 9, 1.382 4, 1.447Absorption coefficient (mm�1) 1.070 0.991 0.696F(000) 670 2655 1852Crystal size (mm) 0.28 � 0.20 � 0.20 0.40 � 0.20 � 0.10 0.25 � 0.20 � 0.20h Range for data collection (�) 2.07–25.00 2.74–24.99 1.61–25.00Limiting indices �12 6 h 6 10, �12 6 k 6 13,

�14 6 l 6 14�35 6 h 6 27, �23 6 k 6 34,�9 6 l 6 9

�14 6 h 6 14, �19 6 k 6 19,�24 6 l 6 24

Reflections collected/unique 6855/4664 [Rint = 0.0245] 8105/2381 [Rint = 0.0628] 39271/7278 [Rint = 0.0400]Maximum and minimum

transmission0.8145 and 0.7539 0.9074 and 0.6926 0.8734 and 0.8453

Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2

Data/restraints/parameters 4664/4/371 2381/3/168 7278/2/516Goodness-of-fit on F2 1.066 0.957 1.039Final R indices [I > 2r(I)] R1 = 0.0475, wR2 = 0.1336 R1 = 0.0506, wR2 = 0.1226 R1 = 0.0699, wR2 = 0.1953R indices (all data) R1 = 0.0528, wR2 = 0.1396 R1 = 0.0787, wR2 = 0.1308 R1 = 0.0864, wR2 = 0.2098Largest diff. peak and hole (e �3) 1.160 and �0.815 0.573 and �0.360 1.021 and �0.426Completeness to h = 25.00 (%) 98.5 99.9 100.0

Fig. 1. Molecular structure (ORTEP diagram with 50% thermal ellipsoids) of 1, alongwith the atomic numbering scheme. Hydrogen atoms have been omitted for clarity.

Table 2Selected bond lengths (Å) and bond angles (�) for [CuL(ClO4)2] (1).

Bond lengths Bond angles

N(1B)–Cu(1B) 2.002(2) N(1B)–Cu(1B)–N(2B)#1 93.92(10)N(2B)–Cu(1B) 2.015(2) N(1B)–Cu(1B)–N(2B) 86.08(10)N(1A)–Cu(1A) 1.994(3) N(1A)–Cu(1A)–N(2A) 86.11(10)N(2A)–Cu(1A) 2.020(3) N(1A)–Cu(1A)–N(2A)#2 93.89(10)

Symmetry transformations used to generate equivalent atoms: #1 �x + 1, �y + 1,�z + 1 #2 �x + 1, �y, �z.

A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317 311

center of symmetry. There are two metal atoms in the asymmetricunit sitting on inversion centers. The two resulting molecules haveequivalent geometries (Table 2). The coordination environment ofthe Cu(II) ion is (N4O2) coordinated with four secondary amine nitro-gen atoms [Cu(1A)–N(1A = 1.994(3) Å, Cu(1A)–N(2A) = 2.020(3) Å)

and (Cu(1B)–N(1B) = 2.002(2) Å, Cu(1B)–N(2B) = 2.015(2) Å] occupy-ing equatorial plane in an essentially square-planar arrangement andtwo weakly coordinated perchlorate anions at the axial position. Thetwo independent molecules in the asymmetric unit show slightly dif-ferent Cu–O (perchlorate) interactions [Cu(1A)–O(4A) = 2.474 ÅA

0

andCu(1B)–O(1B) = 2.651 Å]. The overall coordination geometry aroundCu(II) may therefore, be described as an elongated octahedron whichis expected from the Jahn–Teller distortion.

The six-membered chelate rings adopt a chair conformation,whereas five-membered rings assume a gauche conformation.The chelate angles were N(1A)–Cu(1A)–N(2A) = 86.11(10)�,N(1A)–Cu(1A)–N(2A)#2 = 93.89(10)� and N(1B)–Cu(1B)–N(2B) =86.08(10)�, N(1B)–Cu(1B)–N(2B)#1 = 93.92(10)� for five- and six-membered rings in unit A and B, respectively. The Cu–N and Cu–Obond lengths are similar to other Cu(II) complexes with cyclam-derived ligands where perchlorate is axially coordinated [25,26].The trans-III configuration of N-donors enables the hexaaza ring toadopt least strained conformation, i.e. an eclipsed pair of gauchefive-membered chelate rings and two chair six-membered rings.

The perchlorate ion indicates a lengthening of the Cl–O bondinvolving O(4A) and O(1B), which were weakly coordinated tothe Cu atom. The Cl(1A)–O(4A) and Cl(1B)–O(1B) bond length were1.452(2) and 1.437(2) Å, which were significantly longer than the

Fig. 2. Depicting intermolecular hydrogen bonded 1D polymeric array along a axis for complex 1. Hydrogen atoms except those involved in H-bonding have been omitted forclarity.

Table 3Intramolecular and intermolecular hydrogen bonds of the N–H� � �O and C–H� � �O typein complex [CuL(ClO4)2] (1).

D–H� � �A d(D� � �A)(Å)

d(H� � �A)(Å)

(D–H� � �A) (�)

N(1A)–H(N1A)� � �O(3B)

2.939 2.187 145.3 (x, y � 1, z)

N(1B)–H(N1B)� � �O(1A)

2.976 2.214 147.07 (�x + 1, �y, �z + 1)

N(1B)–H(N1B)� � �O(1B)

3.131 2.639 117.33 (�x + 1, �y + 1,�z + 1)

N(2A)–H(N2A)� � �O(3A)

3.104 2.293 156.66 (�x + 1, �y, �z)

N(2B)–H(N2B)� � �O(2B)

2.994 2.159 161.01

Table 4Selected bond lengths (Å) and bond angles (�) for [CuL(SCN)2] (2).

Bond lengths Bond angles

Cu(1)–N(1) 2.035(3) N(2)–Cu(1)–N(1) 94.14(11)Cu(1)–N(2) 2.011(3) N(2)#1–Cu(1)–N(1) 85.86(11)Cu(1)–N(4) 2.528(3) C(11)–N(1)–C(9) 113.6(3)S(2)–C(12) 1.562(5) C(8)–N(3)–C(9) 115.1(3)C(12)–N(4) 1.192(5) C(8)–N(3)–C(7) 114.8(3)

C(9)–N(3)–C(7) 115.7(3)N(1)–Cu(1)–N(4) 89.93(12)N(1)–Cu(1)–N(4)#1 90.07(12)N(2)–Cu(1)–N(4) 87.59(13)N(2)–Cu(1)–N(4)#1 92.40(13)

Symmetry transformations used to generate equivalent atoms: #1 �x + 1, �y,�z + 1.

312 A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317

mean of the other three Cl–O bonds [27]. The mononuclear unitsare held together by intermolecular hydrogen bonds to form aninfinite one-dimensional polymeric array along a axis (Fig. 2).The hydrogen bonding parameters are collected in Table 3.

3.2. Crystal structure description of [CuL(SCN)2] (2)

The molecular structure of 2 along with the atomic numberingscheme is shown in Fig. 3. Crystallographic data, selected bond

Fig. 3. Molecular structure (ORTEP diagram with 50% thermal ellipsoids) of complex 2,clarity.

lengths and bond angles are given in Tables 1 and 4. It was crystal-lizes in rhombohedral system with R�3 space group with nine for-mula units per unit cell (Fig. 4). The asymmetric unit is made upof half a molecule, the other half of the molecule is generatedthrough a center of symmetry. The crystallographic asymmetricunit of 2 consists of [CuL]2+ cation and two thiocyanate anions inisothiocyanate binding mode. The thiocyanate ion is known to formboth thiocyanato (M–SCN) and isothiocyanato (M–NCS) complexes,depending on the nature of metal ion [28]. Pearson [29] suggests

along with the atomic numbering scheme. Hydrogen atoms have been omitted for

Fig. 4. Unit cell diagram of complex 2. Hydrogen atoms have been omitted forclarity.

Fig. 5. Depicting intermolecular hydrogen bonded 1D polymeric array along a axis in comclarity.

Fig. 6. Molecular structure (ORTEP diagram with 50% thermal ellipsoids) of complex 3,clarity.

A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317 313

that S in SCN� is soft and will prefer to coordinate with soft acidswhereas N in SCN� is hard and coordinates with hard acids. Thecopper atom is in an axially distorted octahedral (N4 + N2) environ-ment with four secondary amine nitrogen atoms occupying theequatorial plane, similar to 1 and two isothiocyanate groups atthe axial positions. Two cross bond angles N(1)–Cu(1)–N(1)#1and N(2)–Cu(1)–N(2)#1 in equatorial plane show that the Cu(II)ion is located in a square plane without any axial displacement.The large Cu(1)–N(4) bond length of 2.528(3) Å in trans configura-tion, are similar to those found for weakly coordinated axial SCN�

in hexa-coordinate (N4N2) copper complexes such as trans-[Cu(Me2en)2(NCS)2] [30], trans-[Cu(amepy)2(NCS)2] and [Cu(a-L)(NCS)2] [31–33]. The thiocyanate groups show small distortions[N(4)–(C12)–S(2) = 177.74(4)�] from linearity as is usually ob-served [34]. The Cu–N–C angles related to the thiocyanate anionare Cu(1)–N(4)–C(12) [119.9(6)�]. Therefore, the overall geometryof Cu(II) may be described as an elongated octahedron similar to1. The chelate angles N(1)–Cu(1)–N(2), N(1)–Cu(1)–N(2)#1 were94.14(11)�, 85.86(11)� for six-membered and five-membered rings,respectively. Analysis of the crystal packing reveals that mononu-clear units are held together by intermolecular hydrogen bondsinvolving the N(2)–H(2A) protons and S(2) with d(D� � �A) = 3.615Å,

plex 2. Hydrogen atoms except those involved in H-bonding have been omitted for

along with the atomic numbering scheme. Hydrogen atoms have been omitted for

Table 5Selected bond lengths (Å) and bond angles (�) for [CuL(CH3CN)2](PF6)2�2CH3CN (3).

Bond lengths Bond angles

Cu(1)–N(1) 2.006(4) N(2)–Cu(1)–N(1) 86.26(14)Cu(1)–N(2) 2.002(3) N(2)–Cu(1)–N(3) 92.96(13)Cu(1)–N(3) 2.011(3) N(3)–Cu(1)–N(4) 86.57(13)Cu(1)–N(4) 2.010(3) N(1)–Cu(1)–N(4) 94.03(14)Cu(1)–N(7) 2.747(4) N(2)–Cu(1)–N(9) 93.22(15)Cu(1)–N(9) 2.432(4) N(1)–Cu(1)–N(9) 89.96(15)N(7)–C(27) 1.118(7) N(3)–Cu(1)–N(9) 93.77(15)N(8)–C(29) 1.127(8) N(4)–Cu(1)–N(9) 89.59(15)N(9)–C(23) 1.120(5) N(1)–Cu(1)–N(3) 176.23(15)N(10)–C(25) 1.118(7) N(2)–Cu(1)–N(4) 177.18(15)

C(27)–N(7)–Cu(1) 143.13(4)C(23)–N(9)–Cu(1) 158.2(4)N(1)–Cu(1)–N(7) 89.33(17)N(2)–Cu(1)–N(7) 85.93(15)N(3)–Cu(1)–N(7) 86.93(16)N(1)–Cu(1)–N(9) 89.96(14)N(2)–Cu(1)–N(9) 93.23(15)N(3)–Cu(1)–N(9) 93.77(14)N(4)–Cu(1)–N(7) 91.25(15)N(4)–Cu(1)–N(9) 89.60(15)

Fig. 7. Unit cell diagram of complex 3. Hydrogen atoms have been omitted forclarity.

Fig. 8. Solvent-anion interaction for complex 3. Arrangement of PF6� nd two solvent molecules due to F� � �H bonding in unit cell. Hydrogen atoms have been omitted for

clarity.

314 A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317

Table 6Hydrogen bonds of the N–H� � �F and N–H� � �N type between the macrocycle and PF6

�

and CH3CN in complex [CuL(CH3CN)2](PF6)2�2CH3CN (3).

D–H� � �A d(D� � �A)(Å)

d(H� � �A)(Å)

(D–H� � �A) (�)

N(1)–H(1N)� � �N(10)

3.031(7) 2.30(5) 154(5) (x, 1/2 � y, �1/2 + z)

N(2)–H(2N)� � �F(10)

3.210(7) 2.41(5) 153(4) (�1 + x, y, z)

N(3)–H(3N)� � �N(8) 2.999(7) 2.27(5) 147(4) (�1 + x, y, z)C(26)–

H(26A)� � �F(2)3.347(8) 2.52 145

C(28)–H(28B)� � �F(4)

3.310(10) 2.41 157 (�1 + x, y, z)

Fig. 9. (a) EPR spectra of 1, 2 and 3 determined at RT on polycrystalline powdersample. (b) EPR spectra of 1, 2 and 3 determined at 140 K on a frozen CH3CN–C6H5CH3 (1:1, v/v) glass.

A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317 315

d(H� � �A) = 2.882 Å and (D–H� � �A) = 143� to form an infinite one-dimensional polymeric chain along the b axis (Fig. 5).

3.3. Crystal structure description of [CuL(CH3CN)2](PF6)2�2CH3CN (3)

The molecular structure of the complex 3 along with the atomicnumbering scheme is shown in Fig. 6. Crystallographic data, selectedbond lengths and bond angles are given in Tables 1 and 5. It was crys-tallizes in the space group P2(1)/c of the monoclinic system. The unitcell was shown in Fig. 7. The crystal structure of 3 contains a mono-meric [CuL(CH3CN)2]2+ cation, two hexafluorophosphate anions andtwo lattice acetonitrile molecules. The Cu(1) atom is (N4 + N2) coordi-nated in a distorted elongated octahedron with four secondary aminenitrogen atoms [Cu(1)–N(1) = 2.006(4) Å, Cu(1)–N(2) = 2.002(3) Å,Cu(1)–N(3) = 2.011(3) Å and Cu(1)–N(4) = 2.010(3) Å] occupyingthe equatorial plane in an square planar arrangement and coordi-nated by two nitrogen atoms from two acetonitrile at the axial posi-tions (Cu(1)–N(9) = 2.432(4) ÅA

0

and Cu(1)–N(7) = 2.747(4) Å). Thetwo trans bond angles in equatorial plane (N(1)–Cu(1)–N(3) andN(2)–Cu(1)–N(4) deviate from 180� indicating that the Cu(II) ion isslightly out of equatorial plane with some axial displacement. Thecopper ion is 0.057 Å above the basal plane constituted via N(1),N(2), N(3), N(4) atoms, towards the N(9) of CH3CN. The two coordi-nated CH3CN are located at trans positions to each other and are notexactly at the axial sites of Cu(II) as evident from the bond angle(N(7)–Cu(1)–N(9) = 178.93�). While in most transition metal com-plexes of acetonitrile the coordination is approximately linear, itdeviates slightly from linearity. The bond angles N(1)–Cu(1)–N(9),N(4)–Cu(1)–N(9), N(2)–Cu(1)–N(9), and N(3)–Cu(1)–N(9) are89.96(15)�, 89.59(15)�, 93.22(15)� and 93.77(15)�, respectively,suggested that the Cu(1)–N(7) and Cu(1)–N(9) linkages are slightlybent off the perpendicular to the N(1)–Cu(1)–N(4) and N(2)–Cu(1)–N(3) plane. The angle between the planes of the two benzene ringsis 23.32� which is possibly due to intramolecular hydrogen.

The P–F bond length ranges from 1.454(5) to 1.571(5) ÅA0

with an

average of 1.528(5) ÅA0

. [CuL(CH3CN)2]2+ cation is linked by intramo-lecular N–H� � �F hydrogen bonds through uncoordinated PF6

� an-ions and N–H� � �NNCMe through two lattice acetonitrile molecules,which are further linked by C–H� � �F hydrogen bonds. In the crystalpacking four PF6

� anions and two lattices CH3CN were arranged ina square manner by H� � �F bonding forming to a 2D network (Fig. 8).Pertinent structural parameters of the hydrogen-bonded contactsare collated in Table 6.

During the solving crystal structure, it was suggested that someF atoms were disordered and multiple splitting was suggested forsingle F atoms. The fluorine atoms of PF6

� counter ions have a highthermal parameter since data was collected at room temperature.We tried refining those atoms in two positions with reduced occu-pancy but there was no decrease in R value and, therefore, we con-sider that our original refinement is the best.

3.4. IR Spectral studies

IR spectra of 1–3 (Fig. S1, Supporting information) show commonabsorption bands, characteristic of the tetraaza macrocyclic ligandin the region 3269–3194 cm�1, indicating the presence of NH groupsin the complexes. Complex 1 shows two splitted peaks at 1111 and1074 cm�1 (m3-antisymmetric stretching) assigned to perchlorateions [35]. This splitting clearly explains the presence of coordinatedperchlorate ion [36]. The band at 999 cm�1 (m2-symmetric stretch-ing) further supports that perchlorate ions are coordinated in thecomplex [37]. The peak around 621 cm�1 (m4-antisymmetric bend-ing) belongs to a perchlorate bending vibration.

The m(CN) stretching frequency of the thiocyanate group can beused for characterization of the coordination mode to the metal ion[38,39]. The stretching frequency m(CN) of thiocyanate are lower inN-bonded (near 2050 cm�1) than S-bonded complexes (near2100 cm�1). The m(CN) observed at 2050 cm�1 in 2 suggests thatthe pseudo-halide is coordinated to copper through the nitrogenatom (terminal N-bonded) [40].

The spectrum of 3 displays broad and intense bands at 845 and555 cm�1 consistent with the characteristic m3(Flu) IR-active modesof the PF6

� anions [38]. It showed two bands at 2341 and2313 cm�1 due to the asymmetric and symmetricm(C„N) of the coor-dinated acetonitrile [41]. A band at 2250 cm�1 is indicative of uncoor-dinated lattice acetonitrile [42]. When IR spectrum of 3 was takenafter drying at 80 �C for 30 min, (Fig. S1, Supporting information)

Table 7EPR parameters of the complexes 1–3.

Complex Temperature g|| g\ giso G A||

[CuL(ClO4)2] (1) RT 2.180 2.043 2.088 4.366 –LNT 2.095 2.025 2.048 4.083 96

[CuL(SCN)2] (2) RT 2.177 2.045 2.089 4.091 –LNT 2.067 2.019 2.035 3.874 170.6

[CuL(CH3CN)2](PF6)2�2CH3CN (3) RT 2.225 2.056 2.112 4.147 –LNT 2.176 2.050 2.092 3.64 214

316 A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317

acetonitrile bands disappear indicating the complete loss of acetoni-trile molecules.

3.5. Electronic spectra and magnetic moment

The electronic spectra of 1–3 complexes were measured in ace-tonitrile solutions. All complexes exhibit very intense bands in the250–275 nm region assignable to p ? p⁄ intraligand charge-trans-fer bands. The appearance of a single broad d–d band in the visiblespectrum of square-planar or tetragonally distorted octahedralcomplexes containing neutral macrocyclic diimine ligands is com-mon [43]. In the present study, the spectra display one broad d–dband at Kmax 497, 532 and 498 nm for complex 1, 2 and 3, respec-tively, contributed from the CuN4 square-based planar coordinationenvironment which indicates that the M–X (where X = ClO4

�, SCN�

and NCCH3) bonds are entirely dissociated [44]. Similar behaviorhas also been reported by Kang et al. [45]. This can be adequatelyexplained as a d–d transition of Cu(II) ion in the complex with aD4h symmetry [46].

Tetragonal distortions involving elongation of the axial bondsthat remove the degeneracy are very common, especially with r-bonding ligands such as 1,2-diamines [47]. This fact suggests thatin tetragonally distorted octahedral Cu(II) complexes involvingunidentate ligands, the axially coordinated groups are weaklybonded and not positioned by lattice packing factors. This is indic-ative of a reduction of the coordination number of the examinedcomplex to form a square planar CuN4 chromophore after solva-tion. The molar conductivity of perchlorate, thiocyanate and hexa-fluorophosphate salts of Cu(II) complexes measured in acetonitrilesolution was found to be in the range of 283–375 X�1 cm2 M�1

corresponding to a 1:2 electrolyte [48]. The contradiction betweenthe crystal structure of Cu(II) complex and the conductivity dataindicate that the perchlorate and hexafluorophosphate anions arecoordinated to the Cu(II) ion in solid state and are labile in solution.The magnetic moment at room temperature for 1–3 was in therange 1.9–1.8 B.M. and these values are within the usual range.

3.6. EPR studies

The X-band EPR spectra of polycrystalline powder of 1, 2 and 3were recorded at room temperature and in frozen acetonitrile–tol-uene (1:1) mixture (Fig. 9). EPR spectral parameters for complexes1–3 are summarized in Table 7. The powder spectra at RT weretypical for axial-type Cu(II) complexes with two g values andg|| > g\ > 2.03, suggesting a dx2�y2 ground state which is consistentwith a tetragonally distorted octahedral stereochemistry [49].The broadening of the spectrum of 2 and 3 is probably due to spinrelaxation. The absence of any half field signal suggests the pres-ence of only one transition metal ion. The g|| < 2.3 indicate a con-siderable covalent character [50]. The smaller g|| value indicatesincreased delocalization of the unpaired electron away from themetal nucleus and has been often interpreted in terms of increasedcovalency in the metal-ligand bond [51]. The calculated G value

[(g|| � 2.0023)/(g\ � 2.0023)] > 4 (Table 7) thus, the extent of inter-action between Cu(II) centers is negligible, indicating an essentiallymononuclear structure [52].

The frozen solution spectra of the complexes are axial withg|| > g\ > 2.0 and G = 3.6–4.1, confirm the square based geometriesof the complexes, as observed in the X-ray crystal structures of 1–3. Frozen spectrum of 1 is well resolved with a quartet hyperfinestructure (Cu, I = 3/2) while those of 2 and 3 are partially resolved.A square-based CuN4 chromophore is expected to show g|| value ofabout 2.200 and an A|| value of 180–220 G. The nuclear hyperfinecoupling for 2 and 3 is observed in the g|| signal with three of thefour components clearly resolved, the last one being obscured bythe g\ component. Accordingly, the orbital ground state of theCu(II) would be basically dx2 � dy2 and the polyhedra could be sta-tistically distorted [53].

3.7. PXRD studies

The phase purity of all Cu(II) complexes was confirmed byPXRD. The experimental PXRD patterns of 1 and 2 are consistentwith the simulated ones, as determined from the single-crystalX-ray diffraction data (Fig. S2, Supporting information). Differencein intensities and peak positions between the PXRD patterns of thetwo sets of compound 3 is due to the loss of solvent molecules dur-ing grinding. Intensity of the experimental XRD patterns is a littleweak, due to the preferred orientation of the powder samples andthe instrumental limitations.

4. Conclusion

Cu(II) complexes have been synthesized by one-pot templatereaction. It may be proposed that in the solid state, most probablydue to crystal packing effects, the arrangement of the coordinationsphere is distorted octahedral whereas in solution, the square-pla-nar one is prevalent. Further, oxygen from perchlorate moleculeand nitrogen from thiocyanate and acetonitrile is semi-coordi-nated to copper, stabilizing an octahedral geometry in the solidstate. The molar conductivity determination reflects that the axi-ally bonded groups are almost entirely dissociated in acetonitrilesolution.

Acknowledgements

We are indebted to the Department of Chemistry, IIT Kanpurand School of Chemistry, University of Hyderabad, for providingsingle crystal X-ray facilities. Dr. Suneel P. Singh, Department ofChemistry, University of Guelph Ontario, Canada is gratefullyacknowledged for his valuable discussion. We are grateful to theCouncil of Scientific and Industrial Research, New Delhi (ResearchScheme No. 21(0685)/07 EMR-II) for the financial support. One ofus (SAAN) thanks Department of Science and Technology for finan-cial assistance under SERC FAST Scheme, No. SR/FT/CS-031/2008,NewDelhi, India.

A. Husain et al. / Inorganica Chimica Acta 384 (2012) 309–317 317

Appendix A. Supplementary material

CCDC 759610, 791738, and 791739 contains the supplementarycrystallographic data for compounds 1, 2, and 3, respectively.These data can be obtained free of charge from The CambridgeCrystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.ica.2011.12.019.

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