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Dimeric and polymeric square-pyramidal copper(II) complexes containing equatorial–apical chloride or acetate bridges
www.elsevier.nl/locate/poly Polyhedron 19 (2000) 1593 – 1600
Dimeric and polymeric square-pyramidal copper(II) complexes containing equatorial–apical chloride or acetate bridges Nimma Rajaiah Sangeetha, Samudranil Pal * School of Chemistry, Uni6ersity of Hyderabad, Hyderabad 500 046, India Received 20 March 2000; accepted 15 May 2000
Abstract Syntheses, crystal structures and physical properties of four new copper(II) complexes of deprotonated aroylhydrazones of 2-pyridine-carboxaldehyde are reported. Two of the complexes contain chloride as a coligand and the other two contain acetate as a coligand. In each complex, the planar tridentate monoanionic ligand binds the metal ion via the pyridine-N, the imine-N and the amide-O atoms. The fourth site is satisfied by the chloride or the acetate-O to form a square-plane. In the solid state, both complexes containing the acetate as the coligand exist as centrosymmetric dimeric species with the metal ion in a distorted square-pyramidal coordination sphere and the acetate group acting as a monoatomic equatorial – apical bridge. One of the complexes containing chloride is a similar centrosymmetric dichloro-bridged dinuclear species. However, the other one exists as a polymeric chain species via equatorial–apical chloride bridges. The electronic spectra of the complexes in methanol solutions display a ligand-field band in the visible region (687–713 nm). The complexes are redox active and, in each case, two reduction responses are observed in the potential range −0.06 to −0.29 and − 0.60 to − 0.70 V (vs. Ag/AgCl) in methanol solutions. Cryomagnetic studies revealed that no significant spin– spin interaction is operative between the metal ions in any of these four complexes. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Copper(II) complexes; Dinuclear and polynuclear; Crystal structures; Equatorial – apical bridge; Paramagnetic
1. Introduction Dinuclear copper complexes are of interest mainly due to their structural, magnetic, catalytic and electron transfer properties [1 – 4]. Such complexes are also important as models for copper proteins containing dimetallic active sites [5 – 9]. Tridentate Schiff-bases are very efficient as binucleating ligands for metal ions which prefer square-planar based geometry. Aroylhydrazones of 2-pyridine-carboxaldehyde (HL I, H represents the dissociable amide proton) can coordinate a given metal ion via the pyridine-N, the imine-N and the amide-O centres. In addition, the deprotonated monobasic hydrazones can also form amide-O bridged dinuclear copper complexes. We have been interested in such monomeric and dimeric species with a specific aim
of studying the effect of amide protonation state on the coordination geometry, redox and interaction between the two metal ions [10–14]. During our recent attempts to prepare such species, we have isolated four new copper(II) complexes. Two of them have chloride and the other two have acetate as the coligand. In solid state, one of the chloride containing complexes exists as a chain-like polymer formed via equatorial–apical single chloride bridges. On the other hand, rest of the complexes are dimers formed through equatorial–apical chloro- or rare monoatomic acetato-bridges. Herein, we report the syntheses, X-ray structures, spectroscopic, electrochemical and magnetic properties of these complexes.
* Corresponding author. Tel.: +91-40-301-0500; fax: + 91-40-3010120. E-mail address: [email protected] (S. Pal). 0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 4 3 3 - 2
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2. Experimental The hydrazones, Hpabh and Hpamh were prepared in 90% yields by condensing equimolar quantities of 2-pyridine-carboxaldehyde and the corresponding aroylhydrazine in methanol. All other solvents and chemicals used were of analytical grade available commercially.
2.1. Preparation of [Cu2(m-Cl)2(pabh)2] (1) A methanol solution (5 cm3) of CuCl2·2H2O (170 mg, 1.0 mmol) was added to a solution (15 cm3) of Hpabh (225 mg, 1.0 mmol) and KOH (56 mg, 1.0 mmol) in methanol. The mixture was stirred at room temperature in air for 30 min. The green solid which separated was collected by filtration, washed with water and dried in vacuum over anhydrous CaCl2. Yield 220 mg (68%). Anal. Found: C, 48.0; H, 3.0; N, 12.8. Calc. for C13H10N3OCuCl: C, 48.31; H, 3.12; N, 12.99%. Selected IR bands (cm − 1): 1601s, 1561w,1487s, 1462s, 1425m, 1375s, 1294m, 1215w, 1148s, 1082s, 920m, 775m, 714s, 417w.
2.2. Preparation of [Cu(pamh)Cl]n (2) This was synthesised by the procedure described for 1 from Hpamh, KOH and CuCl2·2H2O in 70% yield. Anal. Found: C, 47.4; H, 3.3; N, 11.8. Calc. for C14H12N3O2CuCl: C, 47.60; H, 3.42; N, 11.89%. Selected IR bands (cm − 1): 1605s, 1557w, 1532w, 1508m, 1483m, 1447m, 1370s, 1252s, 1167s, 1076m, 1020m, 912w, 847m, 762m, 675w, 623m, 505w.
2.3. Preparation of [Cu2(m-O2CCH3)2(pabh)2] (3) Hpabh (225 mg, 1.0 mmol) was added to a methanol solution (15 cm3) of Cu(O2CCH3)2·H2O (200 mg, 1.0 mmol) and the mixture was boiled under reflux for 1 h. The reaction mixture was then cooled to room temperature. The complex, which separated as a dark green crystalline solid, was collected by filtration, washed with ice-cold methanol and dried under vacuum.. Yield 230 mg (66%). Anal. Found: C, 51.8; H, 3.8; N, 12.0. Calc. for C15H13N3O3Cu: C, 51.94; H, 3.78; N, 12.11%. Selected IR bands (cm − 1): 1626s, 1606s, 1557w, 1489s, 1464s, 1425m, 1366s, 1213w, 1145s, 1082s, 920s, 800w, 768s, 716s, 681s, 575w, 521m, 463m, 415m.
2.4. Preparation of [Cu2(m-O2CCH3)2(pamh)2] (4) This complex was prepared by the procedure described for 3 from Hpamh and Cu(O2CCH3)2·H2O in 65% yield. Anal. Found: C, 50.8; H, 4.0; N, 11.1. Calc. for C16H15N3O4Cu: C, 50.99; H, 4.01; N, 11.15%. Selected IR bands (cm − 1): 1625s, 1604s, 1556w, 1512m,
1483m, 1451s, 1362s, 1254s, 1167s, 1080s, 920s, 843m, 764s, 679s, 621m, 509m, 417m.
2.5. Physical measurements A Perkin–Elmer model 240C elemental analyser was used to obtain microanalytical (C, H, N) data. Infrared spectra were collected on a JASCO-5300 FT IR spectrophotometer using KBr pellets. Electronic spectra were recorded on a JASCO-7800 UV–Vis spectrophotometer. A JEOL FE-3X spectrometer was used to obtain the ESR spectra. The spectra were calibrated with the help of DPPH. A Cypress model CS-1090/CS1087 electroanalytical system was used for cyclic voltammetric experiments with methanol solutions of the complexes containing tetrabutylammonium perchlorate as supporting electrolyte. The three electrode measurements were carried out at 298 K under a dinitrogen atmosphere with platinum disk working electrode, a platinum wire auxiliary electrode and a Ag/AgCl reference electrode. The potentials reported in this work are uncorrected for junction contributions. The variable temperature (17–300 K) magnetic susceptibility measurements were performed using the Faraday technique with a set-up comprising a George Associates Lewis coil force magnetometer, a CAHN microbalance and an Air Products cryostat. Hg[Co(NCS)4] was used as the standard. Diamagnetic corrections, calculated from Pascal’s constants [15], were used to obtain the molar paramagnetic susceptibilities.
2.6. Crystallographic studies Single crystals were obtained by slow evaporation of methanol solutions of the complexes. Complex 1 crystallises as a tetrahydrate. In each case, the data were collected on an Enraf–Nonius Mach-3 single crystal diffractometer using graphite monochromated Mo Ka radiation (l= 0.71073 A, ) by v-scan method at 293 K. Unit cell parameters were determined by the leastsquares fit of 25 reflections having 2u values in the range 18–30°. Stability of the crystal was monitored by measuring the intensities of three check reflections after every 1.5 h during the data collection. None of the crystals displays any decay during data collection. The data were corrected for Lorentz-polarisation effects. No absorption correction was applied. The structures were solved by direct methods and refined by full-matrix least-squares on F 2 and Fourier techniques. Due to insufficient data, only Cu, N and O atoms were refined anisotropically for 3. For the other three complexes, all the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at calculated positions by using riding model for structure factor calculation, but not refined. Calculations were done using the programs of XTAL software [16] for data reduction and SHELX-97
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
programs [17] for structure solution and refinement. ORTEX6a package [18] was used for molecular graphics. Significant crystal data for all the complexes are listed in Table 1.
3. Results and discussion
3.1. Synthesis The complexes 1 and 2, of empirical formula CuLCl (L = pabh− for 1; L= pamh− for 2), were synthesised in good yields by reacting one equivalent each of HL, CuCl2·2H2O and KOH in methanol. Other two complexes, 3 and 4, containing acetate as the coligand were obtained as CuL(O2CCH3) (L= pabh− for 3; L= pamh− for 4) in comparable yields from reaction of equimolar quantities of Cu(O2CCH3)2·H2O and HL in methanol. While for the chloro-complexes (1 and 2) KOH was required to deprotonate the amide functionality, in the latter cases (3 and 4), the acetate of the metal salt appears to be sufficient for the deprotonation of the amide functionality. Elemental analysis data for the complexes are consistent with the empirical formulae mentioned above.
3.2. Spectroscopic properties Infrared spectrum of each of the two complexes containing chloride as coligand displays a strong peak with a shoulder near 1603 cm − 1. The origin of a peak in this region might involve the stretching due to the HCNN---C(---O) moiety of the deprotonated ligands (II) [19–21]. In addition to the same peak at
1595
1605 cm − 1, the spectra of the complexes containing acetate show a peak near 1625 cm − 1 possibly due to the nas stretch of the unidentate acetate group [22]. All the complexes display a strong peak in the range 1362– 1375 cm − 1. However, this peak is significantly stronger for acetate coordinated complexes compared to the peak observed for chloride containing complexes. This difference is most likely due to the ns stretch of the acetate in this region [22].
The electronic spectra of the complexes display a weak absorption in the range 687–713 nm (Table 2). Absorptions in this region observed for other squareplanar or square-pyramidal copper(II) complexes of Schiff bases and similar ligands have been assigned to d–d transition [12,13,23,24]. In addition to this weak band, the complexes of pabh− display two strong absorptions and the complexes of pamh− exhibit three intense bands in the range 385–252 nm. These higher energy absorptions are most probably due to the ligand-to-metal charge transfer and intraligand transitions [24]. The room temperature powder ESR spectral profiles of the complexes investigated in this study are very similar. For all the complexes g \ gÞ (Table 2) which is typical of square-planar or square-pyramidal copper(II) complexes [25,26]. Thus the unpaired electron resides in the dx 2 − y 2 orbital.
Table 1 Crystallographic data for complexes 1, 2, 3 and 4 Complex
1
2
3
4
Empirical formula Crystal size (mm) Crystal system Space group a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) V (A, 3) Z Reflections measured Reflections unique Reflections [I\2s(I)] Parameters R1, wR2 [I\2s(I)] R1, WR2 (all data)
Cu2Cl2C26H28N6O6 0.48×0.45×0.45 triclinic P1( 8.605(2) 9.199(3) 9.858(3) 76.30(2) 78.35(2) 75.20(2) 724.6(3) 1 2530 2530 2207 190 0.0282, 0.0773 0.0336, 0.0834
CuClC14H12N3O2 0.48×0.40×0.35 orthorhombic Pca21 6.9561(11) 12.2621(13) 16.3550(19) 90.0 90.0 90.0 1395.0(3) 4 2347 2095 1575 191 0.0364, 0.0788 0.0536, 0.0896
Cu2C30H26N6O6 0.28×0.15×0.12 monoclinic P21/c 10.773(4) 8.766(3) 15.952(5) 90.0 101.60(3) 90.0 1475.7(9) 2 2347 2299 914 125 0.0562, 0.0875 0.1454, 0.1257
Cu2C32H30N6O8 0.40×0.16×0.12 monoclinic P21/n 13.7425(15) 8.6684(11) 14.0568(19) 90.0 105.705(10) 90.0 1612.0(3) 2 3169 2839 1226 219 0.0479, 0.0718 0.1252, 0.1015
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Table 2 Electronic spectral data a, ESR g values b, reduction potentials c and magnetic moments b Complex
lmax (nm) (10−3×o (M−1 cm−1)) d
g
gÞ
E1/2 (V) (DEp (mV))
Epc (V)
meff d/mB
1 2 3 4
707 713 694 687
2.26 2.29 2.32 2.33
2.09 2.11 2.19 2.18
−0.07 (180) −0.06 (200) −0.29 e −0.22 (140)
−0.60 −0.70 −0.66 −0.68
1.90 1.84 1.90 1.92
(0.08), (0.09), (0.09), (0.08),
378 385 376 383
(17.9), (20.5), (17.5), (19.0),
261 295 258 285
(16.0) (11.6), 252 (15.0) (16.1) (12.0), 253 (15.1)
a
In methanol. In powder phase at 300 K. c Epa, anodic peak potential; Epc, cathodic peak potential; E1/2 =(Epa+Epc)/2; DEp =Epa−Epc. d Per monomer. e Epc. b
3.3. Redox potentials In methanol solutions cyclic voltammograms of the complexes display two reduction responses (Table 2). The first response is observed in the potential range − 0.06 to −0.29 V and the second one appears in the range − 0.60 to −0.70 V (vs. Ag/AgCl). In each case, assuming a monomeric species in solution, the current height of the first reduction is comparable with that of the known one-electron redox processes under identical conditions [27]. However, the current height of the second reduction is two to three times larger than that of the first reduction. The potential of the first response for chloride containing complexes is higher than that observed for the complexes containing acetate. There is no such trend in the potentials for the second reduction (Table 2). It has been observed earlier that with the increase of the basicity of the coordinating atom, the metal–ligand s-bond strength increases and as a result the metal centred reduction potential decreases [28]. The basicity of the acetate-O is much higher than that of chloride. Thus the first reduction response most probably involves the metal centre and the second reduction is associated with the coordinated ligand.
chelate rings. In 1 and 2 the fourth site of the squareplane around the metal ion is occupied by a chloride ion. An acetate-O completes the square-plane in 3 and 4. The chelate bite angles in the five-membered rings formed by the amide-O and the imine-N, and the pyridine-N and the imine-N are in the range 78.61(14)– 79.2(2) and 80.04(15)–80.55(19)°, respectively. The NN, CN and CO distances in the NN---C(---O) fragment of the coordinated ligands are in the range 1.353(5)–1.380(7), 1.321(7)–1.331(6) and 1.278(8)– 1.289(6) A, , respectively. These distances are consistent with the deprotonated form of the amide functionality [10–14,29,30]. The CuN(pyridine) distance is significantly longer than the CuN(imine) distance. The former is in the range 2.028(2)–2.045(4) A, , whereas the latter is within 1.915(4)–1.933(3) A, . This difference is possibly due to the rigidity of the tridentate ligand [31] and the better p-backbonding in the CuN(imine) bond than that in the CuN(pyridine) bond [29,30,32]. The CuO(amide) distance (1.971(3)–1.989(5) A, ) is shorter than that observed in copper(II) complexes where the
3.4. Description of structures The molecular structures of 1, 2, 3 and 4 are illustrated in Figs. 1–4, respectively. The corresponding selected bond parameters for the complexes are listed in Tables 3–6. The asymmetric unit of complex 1 includes one Cu(pabh)Cl fragment and two water molecules. The water molecules are hydrogen bonded to the oxygen (O1) and the nitrogen (N3) centres (Fig. 1) of the deprotonated amide functionality. The O1O(water) and N3O(water) distances are 3.069(3) and 2.939(3) A, , respectively. The asymmetric units of the other three complexes contain the CuLX (X = Cl− or CH3CO2− ) moiety. In each of the four complexes the planar ligand coordinates the metal ion via the pyridine-N, the imineN and the amide-O atoms forming two five-membered
Fig. 1. Structure of [Cu2(m-Cl)2(pabh)2] (1) showing the 40% probability thermal ellipsoids and the atom-labelling scheme. Hydrogen atoms are omitted for clarity.
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metal···metal distances are 2.7821(13) and 4.2866(7) A, , respectively. The CuClCu bridge angle is 117.60(5)°. The four coordinating atoms O1, N2, N1 and Cl are nearly coplanar (Fig. 2a). The largest deviation from the mean plane is 0.012 A, . The metal centre is at 0.126 A, from the mean plane towards the apical chloride bridge.
Fig. 4. Structure of [Cu2(m-O2CCH3)2(pamh)2] (4) depicting the 30% probability thermal ellipsoids and the atom-labelling scheme. Hydrogen atoms are omitted for clarity.
Fig. 2. (a) View of the asymmetric unit of [Cu(pamh)Cl]n (2) with the atom-labelling scheme. Atoms are represented by their 40% probability thermal ellipsoids. (b) Single chloride bridged chain structure of 2. For clarity hydrogen atoms are not included.
Table 3 Selected bond distances (A, ) Cl)2(pabh)2]·4H2O (1·4H2O) a
and
angles
(°)
for
[Cu2(m-
CuCl CuN(1) CuCl% N(3)C(7) N(2)C(6)
2.2440(9) 2.028(2) 2.6682(10) 1.324(3) 1.276(3)
CuO(1) CuN(2) O(1)C(7) N(2)N(3)
1.9853(17) 1.9299(19) 1.288(3) 1.369(3)
N(2)CuO(1) O(1)CuN(1) O(1)CuCl N(2)CuCl% N(1)CuCl% CuClCu%
78.93(8) 158.59(8) 98.81(6) 104.60(6) 92.28(6) 86.89(3)
N(2)CuN(1) N(2)CuCl N(1)CuCl O(1)CuCl% ClCuCl%
80.15(8) 162.28(6) 99.72(6) 97.34(6) 93.11(3)
a Symmetry transformation used to generate equivalent atoms: 1−x, 1−y, 1−z.
Table 4 Selected bond distances (A, ) and angles (°) for [Cu(pamh)Cl]n (2) a
Fig. 3. Structure of [Cu2(m-O2CCH3)2(pabh)2] (3) with the atom labelling scheme. All atoms are represented by their 20% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.
metal ion is coordinated to the oxygen of a protonated amide functionality [13,33 – 35]. In solid state, 2 forms a one-dimensional chain of copper(II) ions via equatorial – apical chloride bridges (Fig. 2b). This type of single chloride bridged polymeric species is unique for copper(II) containing a tridentate coligand. The Cu···Cl(apical) and inter-monomer
CuCl CuN(1) CuCl% N(2)N(3) O(1)C(7)
2.2177(11) 2.045(4) 2.7821(13) 1.353(5) 1.279(5)
CuO(1) CuN(2) N(2)C(6) N(3)C(7)
1.978(3) 1.933(3) 1.278(5) 1.331(6)
O(1)CuCl O(1)CuN(2) N(1)CuCl N(1)CuCl% O(1)CuCl% CuCl%Cu%
102.16(10) 78.61(14) 98.31(11) 91.59(11) 92.52(10) 117.60(5)
O(1)CuN(1) N(1)CuN(2) N(2)CuCl N(2)CuCl% ClCuCl%
158.03(15) 80.04(15) 172.37(11) 87.23(11) 100.29(3)
a Symmetry transformation used to generate equivalent atoms: 1/2+x, 2−y, z.
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
1598 Table 5 Selected bond distances O2CCH3)2(pabh)2] (3) a
(A, )
and
angles
(°)
for
[Cu2(m-
CuO(1) CuO(2)% CuN(2) O(3)C(14 N(2)N(3) O(1)C(7)
1.989(5) 2.286(6) 1.924(6) 1.224(9) 1.380(7) 1.278(8)
CuO(2) CuN(1) O(2)C(14) N(2)C(6) N(3)C(7)
1.935(5) 2.033(6) 1.266(9) 1.246(9) 1.321(9)
N(2)CuO(2) N(2)CuO(1) O(2)CuN(1) N(2)CuO(2)% O(1)CuO(2)% CuO(2)Cu%
170.8(3) 79.2(2) 100.2(2) 110.6(2) 95.3(2) 101.4(2)
O(2)CuO(1) N(2)CuN(1) O(1)CuN(1) N(1)CuO(2)% O(2)CuO(2)%
100.0(2) 80.1(2) 159.3(2) 93.5(2) 78.6(2)
a Symmetry transformation used to generate equivalent atoms: 1−x, −y, −z.
Table 6 Selected bond distances O2CCH3)2(pamh)2] (4) a
(A, )
and
angles
(°)
for
[Cu2(m-
CuO(1) CuO(3)% CuN(2) O(4)C(15) N(3)C(7) N(2)N(3)
1.971(3) 2.282(4) 1.915(4) 1.223(6) 1.321(6) 1.361(5)
CuO(3) CuN(1) O(3)C(15) O(1)C(7) N(2)C(6)
1.922(3) 2.040(4) 1.282(6) 1.289(6) 1.279(6)
N(2)CuO(3) O(3)CuO(1) O(3)CuN(1) N(1)CuO(3)% O(1)CuO(3)% CuO(3)Cu%
171.83(17) 98.90(15) 100.80(17) 95.05(16) 94.42(14) 101.94(15)
N(2)CuO(1) N(2)CuN(1) O(1)CuN(1) N(2)CuO(3)% O(3)CuO(3)%
79.16(17) 80.55(19) 159.52(16) 109.94(15) 78.06(15)
a
Symmetry transformation used to generate equivalent atoms: 1−x, −y, 1−z.
The other three complexes crystallise as centrosymmetric dinuclear species in which each copper(II) is in a distorted square-pyramidal coordination sphere. In 1, the chloride ions act as equatorial – apical bridges (Fig. 1). The acetate ions perform the role of monoatomic bridges in the same fashion in 3 and 4 (Figs. 3 and 4). The metal···metal distance in the diacetato-bridged complexes, 3 (3.273(2) A, ) and 4 (3.273(1) A, ) are identical. However the same (3.3918(9) A, ) in 1 is significantly longer. The O1, N2, N1 and Cl atoms form the base (largest deviation from the mean plane, 0.11 A, ) of the square-pyramid in 1. The metal centre is displaced from this base towards the apical bridging Cl% by 0.23 A, . In 3 and 4, the planar N,N,O-coordinating ligand and the acetate-O comprise the square-base of the pyramid. The largest deviations from the NNOO mean planes are 0.06 A, (3) and 0.04 A, (4). In 3, the displacement of the metal ion towards the apical bridging acetate-O is 0.11 A, and that in 4 is 0.10 A, . Such displacement is very common in square-pyramidal copper(II) complexes [12,13,36,37]. A few structurally characterised dicop-
per(II) complexes of planar tridentate ligands containing equatorial–apical bridging chloride [38–50] or acetate [51–60] groups are known. In 1, CuCl and CuCl% distances are 2.2440(9) and 2.6682(10) A, . The CuClCu% bridge angle is 86.89(3)°. In similar dichloro-bridged dicopper(II) complexes, the CuCl(equatorial) and CuCl(apical) distances are in the range 2.218–2.329 and 2.526–3.129 A, , respectively. In these complexes the CuClCu bridge angle varies from 83.291–91.989° [38–50]. In 3 and 4, the bond parameters of the Cu2O2 unit are very similar. The CuO and CuO% distances observed are 1.935(5) and 2.286(6) A, and 1.922(3) and 2.282(2) A, for 3 and 4, respectively. The CuOCu% bridge angles are 101.4(2)° (3) and 101.94(15)° (4). The values of CuO(equatorial) and CuO(apical) distances and the CuOCu bridge angles reported for similar diacetato-bridged complexes are in the range 1.951–1.987, 2.427–2.572 and 95.338– 106.681°, respectively [51–60]. An interesting observation in the structures of the two diacetato-bridged complexes is the distances between the uncoordinated acetate oxygen atoms and the azomethine (HCN) carbon atoms of the adjacent dimers (3.162(10) A, for 3 and 3.199(6) A, for 4). Thus a one dimensional assembly of the dimer is formed by involvement of the molecule in two pairs of reciprocal CH···O interactions with its two neighbours on both sides.
3.5. Magnetic properties Magnetic susceptibility measurements for powdered samples of the four complexes were carried out in the temperature range 17–300 K at a constant magnetic field of 5 kG. The room temperature effective magnetic moments (meff/monomer) are in the range 1.84– 1.92 mB (Table 2). On cooling there is no significant change in the magnetic moments. At 17 K the magnetic moments are within 2.07–2.19 mB. That each complex behaves essentially as a Curie paramagnet is readily apparent from the linear plots of inverse magnetic susceptibility values (per monomer) against temperature. The data were fit using the expression for Curie–Weiss law. The Curie (C) and the Weiss constants (u in K) are 0.45 and − 3.57, 0.42 and 1.98, 0.45 and − 1.41, and 0.46 and − 1.04 for 1, 2, 3, and 4, respectively. Magnetostructural studies on equatorial–apical dichloro-bridged copper(II) dimers have been reported [41,61]. The exchange coupling constant (J) in such complexes cover a wide range ( + 6 to − 41.3 cm − 1) [41–43,46,61]. An empirical relationship has been observed between the extent of the Cu···Cu interaction and the ratio f/R, where f is the CuClCu bridge angle and R is the longer CuCl distance [41,61]. With the increase of the ratio f/R the value of −2J decreases and reaches a minimum and then again in-
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
creases. The magnitude of J is very small for most of the complexes in which f/R :33. In 1 this ratio is 32.6. The paramagnetic nature of 1 is consistent with this ratio. Magnetic properties of very few single chloride bridged polymeric copper(II) complexes analogous to 2 are reported [62]. In all of them the intrachain magnetic interaction is very weak (J = −6.1 – 1.58 cm − 1). However a trend in the CuClCu bridge angle (f) and the value of J has been observed [62]. The coupling constant (J) decreases with the increase of f. For large f (144.6°), the J is negative (−6.1 cm − 1). Weak ferromagnetic interaction is observed for species with smaller CuClCu angle (f = 113.58(5)°, J = 1.58 cm − 1; f = 128.1°, J = 0.48 cm − 1). Although the small positive Weiss constant (1.98 K) observed for 2 indicates the near paramagnetic nature, it is interesting that f (117.60(5)°) in 2 is intermediate of the above two values. Compared to the dichloro-bridged complexes magnetic data for diacetato-bridged dimeric species similar to 3 and 4 are scarce. The coupling constants J reported for the previously studied complexes are very small and within the range − 1.54 to + 0.63 cm − 1 [51,53,54,56]. The small values of u observed for 3 and 4 suggest a similar situation where no significant spin – spin interaction exists between the metal ions.
4. Conclusions One polymeric and three dimeric copper(II) complexes with N,N,O-donor ligands have been synthesised and structurally characterised by X-ray crystallography. Metal ions are in distorted square-pyramidal N2OCl2 or N2O3 coordination sphere in these complexes. The polymeric chain complex is formed via single chloride bridges. In the dimers, chloride or acetate ions act as the monoatomic bridging units. In all the complexes the bridging units are in equatorial – apical fashion. The spectral properties (UV – Vis and ESR) are similar to those of other complexes containing square-planar or square-pyramidal copper(II). The complexes are redox active in methanol solutions. Variable temperature magnetic susceptibility measurements revealed the paramagnetic nature of the complexes.
5. Supplementary material Crystallographic data are available from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk) on request, quoting the deposition Nos. CCDC 141222 – 141225.
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Acknowledgements Financial assistance for this work was provided by the Council of Scientific and Industrial Research, New Delhi (Grant no. 01(1422)/96/EMR-II). N.R.S thanks the University Grants Commission, New Delhi for a research fellowship. We are grateful to Professor A.R. Chakravarty for providing the variable temperature magnetic susceptibility data and to Professor P.S. Zacharias for allowing the use of the electroanalytical system. The X-ray crystallographic studies were performed at the National Single Crystal Diffractometer Facility, School of Chemistry, University of Hyderabad. We thank the Department of Science and Technology, New Delhi, for setting up this facility.
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Dimeric and polymeric square-pyramidal copper(II) complexes containing equatorial–apical chloride or acetate bridges Nimma Rajaiah Sangeetha, Samudranil Pal * School of Chemistry, Uni6ersity of Hyderabad, Hyderabad 500 046, India Received 20 March 2000; accepted 15 May 2000
Abstract Syntheses, crystal structures and physical properties of four new copper(II) complexes of deprotonated aroylhydrazones of 2-pyridine-carboxaldehyde are reported. Two of the complexes contain chloride as a coligand and the other two contain acetate as a coligand. In each complex, the planar tridentate monoanionic ligand binds the metal ion via the pyridine-N, the imine-N and the amide-O atoms. The fourth site is satisfied by the chloride or the acetate-O to form a square-plane. In the solid state, both complexes containing the acetate as the coligand exist as centrosymmetric dimeric species with the metal ion in a distorted square-pyramidal coordination sphere and the acetate group acting as a monoatomic equatorial – apical bridge. One of the complexes containing chloride is a similar centrosymmetric dichloro-bridged dinuclear species. However, the other one exists as a polymeric chain species via equatorial–apical chloride bridges. The electronic spectra of the complexes in methanol solutions display a ligand-field band in the visible region (687–713 nm). The complexes are redox active and, in each case, two reduction responses are observed in the potential range −0.06 to −0.29 and − 0.60 to − 0.70 V (vs. Ag/AgCl) in methanol solutions. Cryomagnetic studies revealed that no significant spin– spin interaction is operative between the metal ions in any of these four complexes. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Copper(II) complexes; Dinuclear and polynuclear; Crystal structures; Equatorial – apical bridge; Paramagnetic
1. Introduction Dinuclear copper complexes are of interest mainly due to their structural, magnetic, catalytic and electron transfer properties [1 – 4]. Such complexes are also important as models for copper proteins containing dimetallic active sites [5 – 9]. Tridentate Schiff-bases are very efficient as binucleating ligands for metal ions which prefer square-planar based geometry. Aroylhydrazones of 2-pyridine-carboxaldehyde (HL I, H represents the dissociable amide proton) can coordinate a given metal ion via the pyridine-N, the imine-N and the amide-O centres. In addition, the deprotonated monobasic hydrazones can also form amide-O bridged dinuclear copper complexes. We have been interested in such monomeric and dimeric species with a specific aim
of studying the effect of amide protonation state on the coordination geometry, redox and interaction between the two metal ions [10–14]. During our recent attempts to prepare such species, we have isolated four new copper(II) complexes. Two of them have chloride and the other two have acetate as the coligand. In solid state, one of the chloride containing complexes exists as a chain-like polymer formed via equatorial–apical single chloride bridges. On the other hand, rest of the complexes are dimers formed through equatorial–apical chloro- or rare monoatomic acetato-bridges. Herein, we report the syntheses, X-ray structures, spectroscopic, electrochemical and magnetic properties of these complexes.
* Corresponding author. Tel.: +91-40-301-0500; fax: + 91-40-3010120. E-mail address: [email protected] (S. Pal). 0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 4 3 3 - 2
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2. Experimental The hydrazones, Hpabh and Hpamh were prepared in 90% yields by condensing equimolar quantities of 2-pyridine-carboxaldehyde and the corresponding aroylhydrazine in methanol. All other solvents and chemicals used were of analytical grade available commercially.
2.1. Preparation of [Cu2(m-Cl)2(pabh)2] (1) A methanol solution (5 cm3) of CuCl2·2H2O (170 mg, 1.0 mmol) was added to a solution (15 cm3) of Hpabh (225 mg, 1.0 mmol) and KOH (56 mg, 1.0 mmol) in methanol. The mixture was stirred at room temperature in air for 30 min. The green solid which separated was collected by filtration, washed with water and dried in vacuum over anhydrous CaCl2. Yield 220 mg (68%). Anal. Found: C, 48.0; H, 3.0; N, 12.8. Calc. for C13H10N3OCuCl: C, 48.31; H, 3.12; N, 12.99%. Selected IR bands (cm − 1): 1601s, 1561w,1487s, 1462s, 1425m, 1375s, 1294m, 1215w, 1148s, 1082s, 920m, 775m, 714s, 417w.
2.2. Preparation of [Cu(pamh)Cl]n (2) This was synthesised by the procedure described for 1 from Hpamh, KOH and CuCl2·2H2O in 70% yield. Anal. Found: C, 47.4; H, 3.3; N, 11.8. Calc. for C14H12N3O2CuCl: C, 47.60; H, 3.42; N, 11.89%. Selected IR bands (cm − 1): 1605s, 1557w, 1532w, 1508m, 1483m, 1447m, 1370s, 1252s, 1167s, 1076m, 1020m, 912w, 847m, 762m, 675w, 623m, 505w.
2.3. Preparation of [Cu2(m-O2CCH3)2(pabh)2] (3) Hpabh (225 mg, 1.0 mmol) was added to a methanol solution (15 cm3) of Cu(O2CCH3)2·H2O (200 mg, 1.0 mmol) and the mixture was boiled under reflux for 1 h. The reaction mixture was then cooled to room temperature. The complex, which separated as a dark green crystalline solid, was collected by filtration, washed with ice-cold methanol and dried under vacuum.. Yield 230 mg (66%). Anal. Found: C, 51.8; H, 3.8; N, 12.0. Calc. for C15H13N3O3Cu: C, 51.94; H, 3.78; N, 12.11%. Selected IR bands (cm − 1): 1626s, 1606s, 1557w, 1489s, 1464s, 1425m, 1366s, 1213w, 1145s, 1082s, 920s, 800w, 768s, 716s, 681s, 575w, 521m, 463m, 415m.
2.4. Preparation of [Cu2(m-O2CCH3)2(pamh)2] (4) This complex was prepared by the procedure described for 3 from Hpamh and Cu(O2CCH3)2·H2O in 65% yield. Anal. Found: C, 50.8; H, 4.0; N, 11.1. Calc. for C16H15N3O4Cu: C, 50.99; H, 4.01; N, 11.15%. Selected IR bands (cm − 1): 1625s, 1604s, 1556w, 1512m,
1483m, 1451s, 1362s, 1254s, 1167s, 1080s, 920s, 843m, 764s, 679s, 621m, 509m, 417m.
2.5. Physical measurements A Perkin–Elmer model 240C elemental analyser was used to obtain microanalytical (C, H, N) data. Infrared spectra were collected on a JASCO-5300 FT IR spectrophotometer using KBr pellets. Electronic spectra were recorded on a JASCO-7800 UV–Vis spectrophotometer. A JEOL FE-3X spectrometer was used to obtain the ESR spectra. The spectra were calibrated with the help of DPPH. A Cypress model CS-1090/CS1087 electroanalytical system was used for cyclic voltammetric experiments with methanol solutions of the complexes containing tetrabutylammonium perchlorate as supporting electrolyte. The three electrode measurements were carried out at 298 K under a dinitrogen atmosphere with platinum disk working electrode, a platinum wire auxiliary electrode and a Ag/AgCl reference electrode. The potentials reported in this work are uncorrected for junction contributions. The variable temperature (17–300 K) magnetic susceptibility measurements were performed using the Faraday technique with a set-up comprising a George Associates Lewis coil force magnetometer, a CAHN microbalance and an Air Products cryostat. Hg[Co(NCS)4] was used as the standard. Diamagnetic corrections, calculated from Pascal’s constants [15], were used to obtain the molar paramagnetic susceptibilities.
2.6. Crystallographic studies Single crystals were obtained by slow evaporation of methanol solutions of the complexes. Complex 1 crystallises as a tetrahydrate. In each case, the data were collected on an Enraf–Nonius Mach-3 single crystal diffractometer using graphite monochromated Mo Ka radiation (l= 0.71073 A, ) by v-scan method at 293 K. Unit cell parameters were determined by the leastsquares fit of 25 reflections having 2u values in the range 18–30°. Stability of the crystal was monitored by measuring the intensities of three check reflections after every 1.5 h during the data collection. None of the crystals displays any decay during data collection. The data were corrected for Lorentz-polarisation effects. No absorption correction was applied. The structures were solved by direct methods and refined by full-matrix least-squares on F 2 and Fourier techniques. Due to insufficient data, only Cu, N and O atoms were refined anisotropically for 3. For the other three complexes, all the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at calculated positions by using riding model for structure factor calculation, but not refined. Calculations were done using the programs of XTAL software [16] for data reduction and SHELX-97
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
programs [17] for structure solution and refinement. ORTEX6a package [18] was used for molecular graphics. Significant crystal data for all the complexes are listed in Table 1.
3. Results and discussion
3.1. Synthesis The complexes 1 and 2, of empirical formula CuLCl (L = pabh− for 1; L= pamh− for 2), were synthesised in good yields by reacting one equivalent each of HL, CuCl2·2H2O and KOH in methanol. Other two complexes, 3 and 4, containing acetate as the coligand were obtained as CuL(O2CCH3) (L= pabh− for 3; L= pamh− for 4) in comparable yields from reaction of equimolar quantities of Cu(O2CCH3)2·H2O and HL in methanol. While for the chloro-complexes (1 and 2) KOH was required to deprotonate the amide functionality, in the latter cases (3 and 4), the acetate of the metal salt appears to be sufficient for the deprotonation of the amide functionality. Elemental analysis data for the complexes are consistent with the empirical formulae mentioned above.
3.2. Spectroscopic properties Infrared spectrum of each of the two complexes containing chloride as coligand displays a strong peak with a shoulder near 1603 cm − 1. The origin of a peak in this region might involve the stretching due to the HCNN---C(---O) moiety of the deprotonated ligands (II) [19–21]. In addition to the same peak at
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1605 cm − 1, the spectra of the complexes containing acetate show a peak near 1625 cm − 1 possibly due to the nas stretch of the unidentate acetate group [22]. All the complexes display a strong peak in the range 1362– 1375 cm − 1. However, this peak is significantly stronger for acetate coordinated complexes compared to the peak observed for chloride containing complexes. This difference is most likely due to the ns stretch of the acetate in this region [22].
The electronic spectra of the complexes display a weak absorption in the range 687–713 nm (Table 2). Absorptions in this region observed for other squareplanar or square-pyramidal copper(II) complexes of Schiff bases and similar ligands have been assigned to d–d transition [12,13,23,24]. In addition to this weak band, the complexes of pabh− display two strong absorptions and the complexes of pamh− exhibit three intense bands in the range 385–252 nm. These higher energy absorptions are most probably due to the ligand-to-metal charge transfer and intraligand transitions [24]. The room temperature powder ESR spectral profiles of the complexes investigated in this study are very similar. For all the complexes g \ gÞ (Table 2) which is typical of square-planar or square-pyramidal copper(II) complexes [25,26]. Thus the unpaired electron resides in the dx 2 − y 2 orbital.
Table 1 Crystallographic data for complexes 1, 2, 3 and 4 Complex
1
2
3
4
Empirical formula Crystal size (mm) Crystal system Space group a (A, ) b (A, ) c (A, ) a (°) b (°) g (°) V (A, 3) Z Reflections measured Reflections unique Reflections [I\2s(I)] Parameters R1, wR2 [I\2s(I)] R1, WR2 (all data)
Cu2Cl2C26H28N6O6 0.48×0.45×0.45 triclinic P1( 8.605(2) 9.199(3) 9.858(3) 76.30(2) 78.35(2) 75.20(2) 724.6(3) 1 2530 2530 2207 190 0.0282, 0.0773 0.0336, 0.0834
CuClC14H12N3O2 0.48×0.40×0.35 orthorhombic Pca21 6.9561(11) 12.2621(13) 16.3550(19) 90.0 90.0 90.0 1395.0(3) 4 2347 2095 1575 191 0.0364, 0.0788 0.0536, 0.0896
Cu2C30H26N6O6 0.28×0.15×0.12 monoclinic P21/c 10.773(4) 8.766(3) 15.952(5) 90.0 101.60(3) 90.0 1475.7(9) 2 2347 2299 914 125 0.0562, 0.0875 0.1454, 0.1257
Cu2C32H30N6O8 0.40×0.16×0.12 monoclinic P21/n 13.7425(15) 8.6684(11) 14.0568(19) 90.0 105.705(10) 90.0 1612.0(3) 2 3169 2839 1226 219 0.0479, 0.0718 0.1252, 0.1015
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Table 2 Electronic spectral data a, ESR g values b, reduction potentials c and magnetic moments b Complex
lmax (nm) (10−3×o (M−1 cm−1)) d
g
gÞ
E1/2 (V) (DEp (mV))
Epc (V)
meff d/mB
1 2 3 4
707 713 694 687
2.26 2.29 2.32 2.33
2.09 2.11 2.19 2.18
−0.07 (180) −0.06 (200) −0.29 e −0.22 (140)
−0.60 −0.70 −0.66 −0.68
1.90 1.84 1.90 1.92
(0.08), (0.09), (0.09), (0.08),
378 385 376 383
(17.9), (20.5), (17.5), (19.0),
261 295 258 285
(16.0) (11.6), 252 (15.0) (16.1) (12.0), 253 (15.1)
a
In methanol. In powder phase at 300 K. c Epa, anodic peak potential; Epc, cathodic peak potential; E1/2 =(Epa+Epc)/2; DEp =Epa−Epc. d Per monomer. e Epc. b
3.3. Redox potentials In methanol solutions cyclic voltammograms of the complexes display two reduction responses (Table 2). The first response is observed in the potential range − 0.06 to −0.29 V and the second one appears in the range − 0.60 to −0.70 V (vs. Ag/AgCl). In each case, assuming a monomeric species in solution, the current height of the first reduction is comparable with that of the known one-electron redox processes under identical conditions [27]. However, the current height of the second reduction is two to three times larger than that of the first reduction. The potential of the first response for chloride containing complexes is higher than that observed for the complexes containing acetate. There is no such trend in the potentials for the second reduction (Table 2). It has been observed earlier that with the increase of the basicity of the coordinating atom, the metal–ligand s-bond strength increases and as a result the metal centred reduction potential decreases [28]. The basicity of the acetate-O is much higher than that of chloride. Thus the first reduction response most probably involves the metal centre and the second reduction is associated with the coordinated ligand.
chelate rings. In 1 and 2 the fourth site of the squareplane around the metal ion is occupied by a chloride ion. An acetate-O completes the square-plane in 3 and 4. The chelate bite angles in the five-membered rings formed by the amide-O and the imine-N, and the pyridine-N and the imine-N are in the range 78.61(14)– 79.2(2) and 80.04(15)–80.55(19)°, respectively. The NN, CN and CO distances in the NN---C(---O) fragment of the coordinated ligands are in the range 1.353(5)–1.380(7), 1.321(7)–1.331(6) and 1.278(8)– 1.289(6) A, , respectively. These distances are consistent with the deprotonated form of the amide functionality [10–14,29,30]. The CuN(pyridine) distance is significantly longer than the CuN(imine) distance. The former is in the range 2.028(2)–2.045(4) A, , whereas the latter is within 1.915(4)–1.933(3) A, . This difference is possibly due to the rigidity of the tridentate ligand [31] and the better p-backbonding in the CuN(imine) bond than that in the CuN(pyridine) bond [29,30,32]. The CuO(amide) distance (1.971(3)–1.989(5) A, ) is shorter than that observed in copper(II) complexes where the
3.4. Description of structures The molecular structures of 1, 2, 3 and 4 are illustrated in Figs. 1–4, respectively. The corresponding selected bond parameters for the complexes are listed in Tables 3–6. The asymmetric unit of complex 1 includes one Cu(pabh)Cl fragment and two water molecules. The water molecules are hydrogen bonded to the oxygen (O1) and the nitrogen (N3) centres (Fig. 1) of the deprotonated amide functionality. The O1O(water) and N3O(water) distances are 3.069(3) and 2.939(3) A, , respectively. The asymmetric units of the other three complexes contain the CuLX (X = Cl− or CH3CO2− ) moiety. In each of the four complexes the planar ligand coordinates the metal ion via the pyridine-N, the imineN and the amide-O atoms forming two five-membered
Fig. 1. Structure of [Cu2(m-Cl)2(pabh)2] (1) showing the 40% probability thermal ellipsoids and the atom-labelling scheme. Hydrogen atoms are omitted for clarity.
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
1597
metal···metal distances are 2.7821(13) and 4.2866(7) A, , respectively. The CuClCu bridge angle is 117.60(5)°. The four coordinating atoms O1, N2, N1 and Cl are nearly coplanar (Fig. 2a). The largest deviation from the mean plane is 0.012 A, . The metal centre is at 0.126 A, from the mean plane towards the apical chloride bridge.
Fig. 4. Structure of [Cu2(m-O2CCH3)2(pamh)2] (4) depicting the 30% probability thermal ellipsoids and the atom-labelling scheme. Hydrogen atoms are omitted for clarity.
Fig. 2. (a) View of the asymmetric unit of [Cu(pamh)Cl]n (2) with the atom-labelling scheme. Atoms are represented by their 40% probability thermal ellipsoids. (b) Single chloride bridged chain structure of 2. For clarity hydrogen atoms are not included.
Table 3 Selected bond distances (A, ) Cl)2(pabh)2]·4H2O (1·4H2O) a
and
angles
(°)
for
[Cu2(m-
CuCl CuN(1) CuCl% N(3)C(7) N(2)C(6)
2.2440(9) 2.028(2) 2.6682(10) 1.324(3) 1.276(3)
CuO(1) CuN(2) O(1)C(7) N(2)N(3)
1.9853(17) 1.9299(19) 1.288(3) 1.369(3)
N(2)CuO(1) O(1)CuN(1) O(1)CuCl N(2)CuCl% N(1)CuCl% CuClCu%
78.93(8) 158.59(8) 98.81(6) 104.60(6) 92.28(6) 86.89(3)
N(2)CuN(1) N(2)CuCl N(1)CuCl O(1)CuCl% ClCuCl%
80.15(8) 162.28(6) 99.72(6) 97.34(6) 93.11(3)
a Symmetry transformation used to generate equivalent atoms: 1−x, 1−y, 1−z.
Table 4 Selected bond distances (A, ) and angles (°) for [Cu(pamh)Cl]n (2) a
Fig. 3. Structure of [Cu2(m-O2CCH3)2(pabh)2] (3) with the atom labelling scheme. All atoms are represented by their 20% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.
metal ion is coordinated to the oxygen of a protonated amide functionality [13,33 – 35]. In solid state, 2 forms a one-dimensional chain of copper(II) ions via equatorial – apical chloride bridges (Fig. 2b). This type of single chloride bridged polymeric species is unique for copper(II) containing a tridentate coligand. The Cu···Cl(apical) and inter-monomer
CuCl CuN(1) CuCl% N(2)N(3) O(1)C(7)
2.2177(11) 2.045(4) 2.7821(13) 1.353(5) 1.279(5)
CuO(1) CuN(2) N(2)C(6) N(3)C(7)
1.978(3) 1.933(3) 1.278(5) 1.331(6)
O(1)CuCl O(1)CuN(2) N(1)CuCl N(1)CuCl% O(1)CuCl% CuCl%Cu%
102.16(10) 78.61(14) 98.31(11) 91.59(11) 92.52(10) 117.60(5)
O(1)CuN(1) N(1)CuN(2) N(2)CuCl N(2)CuCl% ClCuCl%
158.03(15) 80.04(15) 172.37(11) 87.23(11) 100.29(3)
a Symmetry transformation used to generate equivalent atoms: 1/2+x, 2−y, z.
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
1598 Table 5 Selected bond distances O2CCH3)2(pabh)2] (3) a
(A, )
and
angles
(°)
for
[Cu2(m-
CuO(1) CuO(2)% CuN(2) O(3)C(14 N(2)N(3) O(1)C(7)
1.989(5) 2.286(6) 1.924(6) 1.224(9) 1.380(7) 1.278(8)
CuO(2) CuN(1) O(2)C(14) N(2)C(6) N(3)C(7)
1.935(5) 2.033(6) 1.266(9) 1.246(9) 1.321(9)
N(2)CuO(2) N(2)CuO(1) O(2)CuN(1) N(2)CuO(2)% O(1)CuO(2)% CuO(2)Cu%
170.8(3) 79.2(2) 100.2(2) 110.6(2) 95.3(2) 101.4(2)
O(2)CuO(1) N(2)CuN(1) O(1)CuN(1) N(1)CuO(2)% O(2)CuO(2)%
100.0(2) 80.1(2) 159.3(2) 93.5(2) 78.6(2)
a Symmetry transformation used to generate equivalent atoms: 1−x, −y, −z.
Table 6 Selected bond distances O2CCH3)2(pamh)2] (4) a
(A, )
and
angles
(°)
for
[Cu2(m-
CuO(1) CuO(3)% CuN(2) O(4)C(15) N(3)C(7) N(2)N(3)
1.971(3) 2.282(4) 1.915(4) 1.223(6) 1.321(6) 1.361(5)
CuO(3) CuN(1) O(3)C(15) O(1)C(7) N(2)C(6)
1.922(3) 2.040(4) 1.282(6) 1.289(6) 1.279(6)
N(2)CuO(3) O(3)CuO(1) O(3)CuN(1) N(1)CuO(3)% O(1)CuO(3)% CuO(3)Cu%
171.83(17) 98.90(15) 100.80(17) 95.05(16) 94.42(14) 101.94(15)
N(2)CuO(1) N(2)CuN(1) O(1)CuN(1) N(2)CuO(3)% O(3)CuO(3)%
79.16(17) 80.55(19) 159.52(16) 109.94(15) 78.06(15)
a
Symmetry transformation used to generate equivalent atoms: 1−x, −y, 1−z.
The other three complexes crystallise as centrosymmetric dinuclear species in which each copper(II) is in a distorted square-pyramidal coordination sphere. In 1, the chloride ions act as equatorial – apical bridges (Fig. 1). The acetate ions perform the role of monoatomic bridges in the same fashion in 3 and 4 (Figs. 3 and 4). The metal···metal distance in the diacetato-bridged complexes, 3 (3.273(2) A, ) and 4 (3.273(1) A, ) are identical. However the same (3.3918(9) A, ) in 1 is significantly longer. The O1, N2, N1 and Cl atoms form the base (largest deviation from the mean plane, 0.11 A, ) of the square-pyramid in 1. The metal centre is displaced from this base towards the apical bridging Cl% by 0.23 A, . In 3 and 4, the planar N,N,O-coordinating ligand and the acetate-O comprise the square-base of the pyramid. The largest deviations from the NNOO mean planes are 0.06 A, (3) and 0.04 A, (4). In 3, the displacement of the metal ion towards the apical bridging acetate-O is 0.11 A, and that in 4 is 0.10 A, . Such displacement is very common in square-pyramidal copper(II) complexes [12,13,36,37]. A few structurally characterised dicop-
per(II) complexes of planar tridentate ligands containing equatorial–apical bridging chloride [38–50] or acetate [51–60] groups are known. In 1, CuCl and CuCl% distances are 2.2440(9) and 2.6682(10) A, . The CuClCu% bridge angle is 86.89(3)°. In similar dichloro-bridged dicopper(II) complexes, the CuCl(equatorial) and CuCl(apical) distances are in the range 2.218–2.329 and 2.526–3.129 A, , respectively. In these complexes the CuClCu bridge angle varies from 83.291–91.989° [38–50]. In 3 and 4, the bond parameters of the Cu2O2 unit are very similar. The CuO and CuO% distances observed are 1.935(5) and 2.286(6) A, and 1.922(3) and 2.282(2) A, for 3 and 4, respectively. The CuOCu% bridge angles are 101.4(2)° (3) and 101.94(15)° (4). The values of CuO(equatorial) and CuO(apical) distances and the CuOCu bridge angles reported for similar diacetato-bridged complexes are in the range 1.951–1.987, 2.427–2.572 and 95.338– 106.681°, respectively [51–60]. An interesting observation in the structures of the two diacetato-bridged complexes is the distances between the uncoordinated acetate oxygen atoms and the azomethine (HCN) carbon atoms of the adjacent dimers (3.162(10) A, for 3 and 3.199(6) A, for 4). Thus a one dimensional assembly of the dimer is formed by involvement of the molecule in two pairs of reciprocal CH···O interactions with its two neighbours on both sides.
3.5. Magnetic properties Magnetic susceptibility measurements for powdered samples of the four complexes were carried out in the temperature range 17–300 K at a constant magnetic field of 5 kG. The room temperature effective magnetic moments (meff/monomer) are in the range 1.84– 1.92 mB (Table 2). On cooling there is no significant change in the magnetic moments. At 17 K the magnetic moments are within 2.07–2.19 mB. That each complex behaves essentially as a Curie paramagnet is readily apparent from the linear plots of inverse magnetic susceptibility values (per monomer) against temperature. The data were fit using the expression for Curie–Weiss law. The Curie (C) and the Weiss constants (u in K) are 0.45 and − 3.57, 0.42 and 1.98, 0.45 and − 1.41, and 0.46 and − 1.04 for 1, 2, 3, and 4, respectively. Magnetostructural studies on equatorial–apical dichloro-bridged copper(II) dimers have been reported [41,61]. The exchange coupling constant (J) in such complexes cover a wide range ( + 6 to − 41.3 cm − 1) [41–43,46,61]. An empirical relationship has been observed between the extent of the Cu···Cu interaction and the ratio f/R, where f is the CuClCu bridge angle and R is the longer CuCl distance [41,61]. With the increase of the ratio f/R the value of −2J decreases and reaches a minimum and then again in-
N.R. Sangeetha, S. Pal / Polyhedron 19 (2000) 1593–1600
creases. The magnitude of J is very small for most of the complexes in which f/R :33. In 1 this ratio is 32.6. The paramagnetic nature of 1 is consistent with this ratio. Magnetic properties of very few single chloride bridged polymeric copper(II) complexes analogous to 2 are reported [62]. In all of them the intrachain magnetic interaction is very weak (J = −6.1 – 1.58 cm − 1). However a trend in the CuClCu bridge angle (f) and the value of J has been observed [62]. The coupling constant (J) decreases with the increase of f. For large f (144.6°), the J is negative (−6.1 cm − 1). Weak ferromagnetic interaction is observed for species with smaller CuClCu angle (f = 113.58(5)°, J = 1.58 cm − 1; f = 128.1°, J = 0.48 cm − 1). Although the small positive Weiss constant (1.98 K) observed for 2 indicates the near paramagnetic nature, it is interesting that f (117.60(5)°) in 2 is intermediate of the above two values. Compared to the dichloro-bridged complexes magnetic data for diacetato-bridged dimeric species similar to 3 and 4 are scarce. The coupling constants J reported for the previously studied complexes are very small and within the range − 1.54 to + 0.63 cm − 1 [51,53,54,56]. The small values of u observed for 3 and 4 suggest a similar situation where no significant spin – spin interaction exists between the metal ions.
4. Conclusions One polymeric and three dimeric copper(II) complexes with N,N,O-donor ligands have been synthesised and structurally characterised by X-ray crystallography. Metal ions are in distorted square-pyramidal N2OCl2 or N2O3 coordination sphere in these complexes. The polymeric chain complex is formed via single chloride bridges. In the dimers, chloride or acetate ions act as the monoatomic bridging units. In all the complexes the bridging units are in equatorial – apical fashion. The spectral properties (UV – Vis and ESR) are similar to those of other complexes containing square-planar or square-pyramidal copper(II). The complexes are redox active in methanol solutions. Variable temperature magnetic susceptibility measurements revealed the paramagnetic nature of the complexes.
5. Supplementary material Crystallographic data are available from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk) on request, quoting the deposition Nos. CCDC 141222 – 141225.
1599
Acknowledgements Financial assistance for this work was provided by the Council of Scientific and Industrial Research, New Delhi (Grant no. 01(1422)/96/EMR-II). N.R.S thanks the University Grants Commission, New Delhi for a research fellowship. We are grateful to Professor A.R. Chakravarty for providing the variable temperature magnetic susceptibility data and to Professor P.S. Zacharias for allowing the use of the electroanalytical system. The X-ray crystallographic studies were performed at the National Single Crystal Diffractometer Facility, School of Chemistry, University of Hyderabad. We thank the Department of Science and Technology, New Delhi, for setting up this facility.
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