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[M(dipicH2)(H2O)3]2+, M = Ni, Cu, Zn (dipicH2 = dipicolinic acid) – A combined crystallographic, spectroscopic and computational study
Polyhedron 26 (2007) 1364–1372 www.elsevier.com/locate/poly
[M(dipicH2)(H2O)3]2+, M = Ni, Cu, Zn (dipicH2 = dipicolinic acid) – A combined crystallographic, spectroscopic and computational study T.K. Prasad, M.V. Rajasekharan
*
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad, AP 500 046, India Received 19 October 2006; accepted 11 November 2006 Available online 23 November 2006
Abstract Cationic metal complexes of dipicolinic acid (dipicH2) are stabilized by [Ce(dipic)3]2 ions in the three isomorphous crystals [M(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (M = Ni, 1; Cu, 2; Zn, 3). Magnetic dilution provided by the bulky anions leads to well-resolved EPR spectra in polycrystalline samples of 2. The cations have 4+2 coordination, the carbonyl atom of the carboxylic acid groups coordinating weakly from trans positions. In the case of 2 this steric distortion is augmented by Jahn–Teller distortion. All the three structures are satisfactorily modelled by calculations based on density functional theory (DFT). The switch of the Jahn–Teller axis upon deprotonation of the complex, leading to the neutral species Cu(dipic)(H2O)3, is also reproduced by DFT. Electronic transition energies as well as the g-tensor component of the d9 complex obtained are in good agreement with experiment. However, the calculated hyperfine coupling constants are in error. DFT also fails to satisfactorily account for the electronic transition in the d8 ion in 1. 2006 Elsevier Ltd. All rights reserved. Keywords: Crystal structures; Ce(IV) complexes; Ni(II), Cu(II), Zn(II) complexes; Heterometallic coordination compounds; EPR; DFT; TDDFT
1. Introduction
2. Experimental
Heterometallic coordination compounds of lanthanide ions in combination with other metal ions are of interest due to their potentially useful solid-state properties [1]. We have earlier exploited the multifunctionality of dipicolinic acid (dipicH2) to generate several coordination networks containing Ce4+ and alkaline earth ions [2]. This work is now extended to transition metal ions (Cu2+, Ni2+) and Zn2+. However, contrary to alkaline earth ions, no coordination networks are obtained. Instead, three isomorphous ionic compounds, viz., [M(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O, M = Ni, Cu, Zn, are obtained. The structures, spectral and thermal properties of these complexes are reported in this paper. The structural changes in the coordination polyhedron accompanying the deprotonation of dipicH2 is modelled using density functional theory (DFT) calculations and compared with the experimental results.
2.1. Materials
*
Corresponding author. Tel.: +91 40 23134857; fax: +91 40 23012460. E-mail address: [email protected] (M.V. Rajasekharan).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.11.029
NiCl2 Æ 6H2O, Cu(NO3)2 Æ 3H2O, Zn(NO3)2 Æ 6H2O and (NH4)2Ce(NO3)6 were purchased from Merck Chemicals. DipicH2 was purchased from Lancaster Chemicals. 2.1.1. Preparation of [Ni(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (1) To a 10 ml aqueous solution of (NH4)2Ce(NO3)6 (0.551 g, 1.00 mmol) and NiCl2 Æ 6H2O (0.241 g, 1.01 mmol), 30 ml methanolic solution of dipicH2 (0.670 g, 4.01 mmol) was added and stirred for 2 min to get a clear solution. It was filtered and kept for evaporation at RT. After about 4–5 days, pale green crystalline blocks formed, which were removed by filtration and dried (0.801 g, 82%). Recrystallization from hot water yielded pale green single crystals of 1 suitable for X-ray data collection. Anal. Calc. for C28H26CeN4NiO22: C, 34.69; H, 2.70; N, 5.78. Found: C, 34.76; H, 2.35; N, 5.84%. mmax(KBr)/cm1 3414, 1647, 1429, 1396, 1271,
T.K. Prasad, M.V. Rajasekharan / Polyhedron 26 (2007) 1364–1372
1182, 1024, 918 and 727. Magnetic moment at 298 K, 3.29 B.M. 2.1.2. Preparation of [Cu(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (2) This compound was prepared by similar method to that of 1, with (NH4)2Ce(NO3)6 (0.552 g, 1.01 mmol), Cu(NO3)2 Æ 3H2O (0.241 g, 0.998 mmol) and dipicH2 (0.670 g, 4.01 mmol). Pale blue blocks were formed in 4–5 days (0.740 g, 76%). During the preparation a small amount of blue coloured crystals, identified as Cu(dipic)(dipicH2) Æ 3H2O was also formed. The pale blue blocks of 2 were separated and recrystallized from hot water. Anal. Calc. for C28H26CeCuN4O22: C, 34.52; H, 2.69; N, 5.75. Found: C, 34.56; H, 2.50; N, 5.76%. mmax(KBr)/ cm1 3084, 1645, 1477, 1431, 1361, 1273, 1176, 1072, 1022, 923, 765 and 733. Magnetic moment at 298 K, 1.94 B.M. 2.1.3. Preparation of [Zn(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (3) The preparation was similar to that of 1 and 2 with (NH4)2Ce(NO3)6 (0.550 g, 1.00 mmol), Zn(NO3)2 Æ 6H2O (0.310 g, 1.04 mmol) and dipicH2 (0.672 g, 4.02 mmol). Pale yellow blocks were formed in 4–5 days (0.770 g, 78%). These blocks were recrystallized from hot water. Anal. Calc. for C28H26CeN4O22Zn: C, 34.45; H, 2.68; N, 5.74. Found: C, 34.14; H, 2.41; N, 5.02%. mmax(KBr)/ cm1 3406, 1647, 1427, 1383, 1178, 1078, 1024, 916 and 729.
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2.2. X-ray crystallography X-ray data were collected for compounds 1 and 2 on an Enraf Nonius CAD-4 diffractometer and for 3 on a Bruker SMART APEX CCD X-ray diffractometer, using graphite˚ ). Data monochromated Mo Ka radiation (k = 0.71073 A of 1 and 2 were reduced using XCAD4 [3] and psi-scan [4] absorption corrections were applied. For 3 the data was reduced using SAINTPLUS [5] and a multiscan absorption correction using SADABS [6] was performed. The structure was solved using SHELXS-97 and refined using SHELXL-97 [7]. All ring hydrogens were assigned on the basis of geometrical considerations and were allowed to ride upon the respective carbon atoms. Drawings were made using ORTEP3 for Windows [8] and Mercury [9]. In compound 3, one lattice water molecule was disordered in two positions. The crystal data for the three compounds are presented in Table 1 and selected bond lengths and angles in Tables 2–4. Powder diffractograms were measured using a PW3710 model Philips Analytical X-ray diffractometer. 2.3. Physical measurements EPR spectra were measured on a JEOL JES – FA 200 spectrometer. Powder simulations were done using a previously described computer program [10]. Electronic Reflectance spectra were measured by using a Shimadzu UV-3100 spectrometer equipped with an ISR-3100 integrating sphere attachment. Thermogravimetric analysis was per-
Table 1 Crystal data and structure refinement for 1, 2 and 3
Formula Formula weight Crystal system ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Space group T (K) Crystal size (mm) l (mm1) Reflections collected Unique reflections, Rint Completeness to 2h = 25.0 (%) Refinement method RðF 2o Þa [I > 2r(I)] Rw ðF 2o Þb [I > 2r(I)] P P a R ¼ kF o j j F c k= j F o j. hP i1=2 P b Rw ¼ wðF 2o F 2c Þ2 . ðwF 4o Þ
1
2
3
C28H26CeN4NiO22 969.36 triclinic 10.5250(8) 11.6839(14) 14.851(3) 68.362(16) 88.539(10) 87.983(8) 1696.4(5) 2 P 1 298(2) 0.56 · 0.24 · 0.24 1.981 7904 7757, 0.0018 99.7 full-matrix least squares on F2 0.0279 0.0759
C28H26CeCuN4O22 974.19 triclinic 10.5553(14) 11.561(3) 14.923(3) 68.221(16) 88.552(13) 88.23(2) 1690.1(6) 2 P 1 298(2) 0.60 · 0.56 · 0.52 2.060 7842 7704, 0.003 99.5 full-matrix least squares on F2 0.0224 0.0619
C28H26CeN4O22Zn 976.02 triclinic 10.5178(5) 11.4883(5) 14.9564(7) 67.9900(10) 88.9330(10) 88.5350(10) 1674.87(13) 2 P 1 100(2) 0.45 · 0.39 · 0.29 2.160 19 574 7841, 0.0161 99.6 full-matrix least squares on F2 0.0199 0.0524
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Table 2 ˚ ) and angles () for 1 Selected bond lengths (A Ce–O(11) Ce–O(31) Ce–N(21) Ni–N(41) Ni–O(51)
2.323(3) 2.346(3) 2.498(3) 1.994(3) 2.049(3)
N(41)–Ni–O(52) O(52)–Ni–O(53) O(52)–Ni–O(51) N(41)–Ni–O(43) O(53)–Ni–O(43) O(51)–Ni–O(43) O(53)–Ni–O(41) O(43)–Ni–O(41)
Ce–O(21) Ce–O(33) Ce–N(11) Ni–O(52) Ni–O(43) 177.22(13) 84.85(12) 84.74(13) 77.11(12) 89.92(12) 90.92(12) 94.06(12) 153.40(11)
2.339(3) 2.366(3) 2.520(3) 2.030(3) 2.172(3)
Ce–O(13) Ce–O(23) Ce–N(31) Ni–O(53) Ni–O(41)
N(41)–Ni–O(53) N(41)–Ni–O(51) O(53)–Ni–O(51) O(52)–Ni–O(43) O(52)–Ni–O(41) N(41)–Ni–O(41) O(51)–Ni–O(41)
2.345(3) 2.368(3) 2.525(3) 2.030(3) 2.178(3) 94.74(13) 95.76(13) 169.39(12) 105.63(12) 100.92(12) 76.35(12) 89.92(13)
2.3308(18) 2.3524(17) 2.4996(19) 1.9595(19) 2.014(2)
O(52)–Cu–O(53) O(53)–Cu–O(51) O(53)–Cu–N(41) O(52)–Cu–O(43) O(51)–Cu–O(43) O(52)–Cu–O(41) O(51)–Cu–O(41) O(43)–Cu–O(41)
Ce–O(21) Ce–O(23) Ce–N(31) Cu–O(53) Cu–O(43) 86.62(8) 172.26(8) 93.36(8) 108.52(8) 89.96(8) 101.50(8) 88.88(8) 149.81(7)
2.3331(18) 2.3768(17) 2.523(2) 1.9683(19) 2.338(2)
Ce–O(33) Ce–O(11) Ce–N(21) Cu–O(51) Cu–O(41)
O(52)–Cu–O(51) O(52)–Cu–N(41) O(51)–Cu–N(41) O(53)–Cu–O(43) N(41)–Cu–O(43) O(53)–Cu–O(41) N(41)–Cu–O(41)
2.3337(18) 2.3835(17) 2.5258(19) 1.9967(19) 2.350(2) 86.26(8) 176.31(8) 93.94(8) 89.43(8) 75.17(8) 95.51(8) 74.82(8)
Table 4 ˚ ) and angles () for 3 Selected bond lengths (A Ce–O(31) Ce–O(11) Ce–N(11) Zn–O(52) Zn–O(53)
2.3306(12) 2.3426(12) 2.5011(14) 2.0341(13) 2.0831(13)
O(52)–Zn–O(51) O(51)–Zn–N(41) O(51)–Zn–O(53) O(52)–Zn–O(43) N(41)–Zn–O(43) O(52)–Zn–O(41) N(41)–Zn–O(41) O(43)–Zn–O(41)
Ce–O(33) Ce–O(13) Ce–N(31) Zn–O(51) Zn–O(43) 85.76(5) 96.04(5) 169.14(5) 111.97(5) 75.74(5) 98.13(5) 74.11(5) 149.81(5)
3. Results and discussion 3.1. Synthesis
Table 3 ˚ ) and angles () for 2 Selected bond lengths (A Ce–O(31) Ce–O(13) Ce–N(11) Cu–O(52) Cu–N(41)
calculations, triple-f basis sets [13] were used for Ni, Cu and Zn atoms and double-f basis sets [14] were used for all other atoms. Other basis sets in the Gaussian-03 suite were also used for comparison. Ligand field theory calculations based on the angular overlap model (AOM) [15] were done using the AOMX program [16]. EPR g and A tensors were calculated using the program package ORCA [17] at the B3LYP level of DFT. A triple-f basis set [13] was used for all atoms.
2.3321(12) 2.3648(12) 2.5236(14) 2.0465(13) 2.2100(13)
Ce–O(21) Ce–O(23) Ce–N(21) Zn–N(41) Zn–O(41)
O(52)–Zn–N(41) O(52)–Zn–O(53) N(41)–Zn–O(53) O(51)–Zn–O(43) O(53)–Zn–O(43) O(51)–Zn–O(41) O(53)–Zn–O(41)
2.3402(12) 2.3692(12) 2.5259(14) 2.0659(15) 2.2635(13) 172.16(5) 83.96(5) 94.62(5) 89.02(5) 91.56(5) 95.60(5) 89.33(5)
formed using a NETZSCH STA 409 PC/PG instrument. Room temperature magnetic susceptibilities of the powdered samples were measured using a Sherwood Scientific magnetic susceptibility balance. 2.4. Computational details DFT along with geometry optimizations and time dependent DFT (TDDFT) have been performed using the B3LYP exchange correlation functional [11] as implemented in the Gaussian-03 [12]. The spin-unrestricted version was employed for open shell ions. For both types of
All three compounds are obtained in good yield by mixing the reagents in the appropriate molar ratio. The initial crystalline blocks obtained from aqueous methanolic solution and the crystals formed from hot water were found to be identical by comparing their X-ray powder patterns. The powder patterns also matched with those calculated based on the crystal structure. DipicH2 can coordinate in the neutral form as well as in monoanionic (dipcH) and dianionic (dipic2) forms. The following complex species have been previously reported: M(dipicH)2 M = Ni, Zn [18], MðdipicÞ2 2 M = Ni, Cu, Zn [19], Cu(dipic)(dipicH2) [18b,20], Cu(dipic)(OH2)2 [21] and Cu(dipic)(OH2)3 [21b,22]. In the preparation of 2, a small amount of Cu(dipic)(dipicH2) Æ 3H2O was formed along with the main compound. Besides this, no other side products were formed in the present synthesis. The presence of CeðdipicÞ3 2 appears to ensure the crystallization of cationic complexes containing the neutral form of the ligand in all cases. 3.2. Structures All the three compounds 1–3 are obtained as isomorphous ionic crystals containing [M(dipicH2)(OH2)3]2+ and [Ce(dipic)3]2 ions (Figs. 1–3). The tri-capped trigonal prismatic Ce(IV) chelate has also been observed in Ce(IV)-alkaline earth-dipic complexes [2]. However, in all these cases some of the free oxygen atoms of the chelate coordinate to M(II) ions forming chains and networks. In the present cases there is no coordination link between the anions and cations. Instead, the free oxygen atoms form H-bonds with carboxylic groups and coordinated water molecules of the cation. All the three cations, [M(dipicH2)(OH2)3]2+, have very similar 4+2 coordination polyhedra. The equatorial plane contains the nitrogen atom of dipicH2 and the O atoms of the three water molecules, while the axial positions are occupied by the carbonyl oxygen atoms of dipicH2. The major difference is that the axial elongation is significantly more in the case of the copper complex. The average equa˚ ) are 2.026(3), 2.175(3) (Ni); torial and axial distances (A 1.984(2), 2.344(2) (Cu); 2.057(1), 2.236(1) (Zn). It would
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Fig. 1. ORTEP view of compound 1 showing the atom labeling. Thermal ellipsoids are drawn at the 50% probability level and ring hydrogen atoms and lattice water molecules have been omitted for clarity.
Fig. 2. ORTEP view of compound 2, showing the atom labeling. Thermal ellipsoids are drawn at the 50% probability level and ring hydrogen atoms and lattice water molecules have been omitted for clarity.
appear that the expected vibronic effects are superimposed on the steric requirements of the ligand to produce the observed static structure in the case of the copper complex. The average off-axis deviation () of the axial ligand atoms are 13.5 (Ni) and 15.1 (Cu, Zn). The equatorial atoms including the metal atoms are very nearly coplanar in all ˚ ) from the cases with a largest deviation of the atoms (A mean plane 0.038(3) (Ni), 0.062(2) (Cu) and 0.111(2) (Zn). The deviations amount to a very slight tetrahedral distortion with the following inter planar angles (): 3.3(2) (Ni); 4.6(1) (Cu); 8.2(1) (Zn).
Hydrogen bonding interactions between cations and anions generate a one-dimensional chain in all three crystals (Fig. 4). Further H-bonding involving lattice water molecules generates a three-dimensional network. 3.3. Electronic spectra Assuming a tetragonal symmetry for the d8 complex ion, the broad features in the solid state spectrum of 1 in the range 8000–20 000 cm1 may be assigned to transitions arising from the ground state (3A2) to the split components
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Fig. 3. ORTEP view of compound 3 showing the atom labeling. Thermal ellipsoids are drawn at the 50% probability level and ring hydrogen atoms and lattice water molecules have been omitted for clarity.
Fig. 4. 1D chain formed by H-bond interactions of cation and anion units.
of 3T2 and 3T1. The band positions and assignments are shown in Table 5. In the case of the copper complex 2, two broad bands are seen which are assigned to excitations from d2z and dxy to dx2 y 2 . Higher energy transitions could not be measured for these compounds due to the presence of strongly absorbing [Ce(dipic)3]2 ions. The assignments are in general agreement with the calculations based on the AOM model as well as TDDFT theory. As indicated in Table 5, the axial perturbation in the case of the Cu(II) complex is only about one-fourth of the corresponding interaction in the Ni(II) complex. The excitation energies obtained from TDDFT calculations for the Cu(II) complex are in satisfactory agreement with the AOM calculations. The agreement is less than satisfactory in the case of the Ni(II) complex. It has been reported that transitions involving double excitations are not well represented in the TDDFT model [23]. In a low-symmetry six-coordinate d8 complex, transitions to the split components of 3T2 and
3
T1(F) are essentially single electron excitations. The six lowest energy excitations quoted in Table 5 correspond to these. It is seen that the energies are overestimated by about 2000–3000 cm1. All the eight different basis sets used gave similar results. 3.4. EPR Spectrum of 2 The packing of the Cu(II) complex cations alternating with the bulky diamagnetic anions in the crystals lead to magnetic dilution. The EPR spectrum of the polycrystalline sample is shown in Fig. 5, in which the hyperfine splitting is fully resolved in the parallel region. The EPR parameters derived by computer simulation of the experimental spectra are given in Table 6. The parameters are consistent with the tetragonally distorted Cu(II) complex. Included in the table are the EPR parameters derived using DFT calculations with the experimental geometry. The calculated g-values
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Table 5 Electronic spectral data Band maxima/1000 cm1
Compound
Experimental [Cu(dipicH2)(OH2)3]
2+
8.8 13.0
[Ni(dipicH2)(OH2)3]
2+
7.6 13.6 15.2
AOMa
TDDFTb
Assignmentc
8.89 13.44 14.21, 14.36
7.99 13.70 15.75, 15.87
1
A1 B2 2 E
B1 B1 2 B1
7.49, 7.95 12.32 13.57 16.78, 16.85 25.12 26.78, 26.91
10.30, 10.79 13.98 16.84 20.28, 23.43
3
3
2
E B2 3 A2 3 E 3 A2 3 E 3
2
2
B1 3 B1 3 B1 3 B1 3 B1 3 B1
The bonding parameters (cm1) used: Cu(II) complex, er(N) = 4900, ep(N) = 800, er(O) = 4800, er(Oaxial) = 400. Ni(II) complex, er(N) = 4900, ep(N) = 600, er(O) = 4200, er(Oaxial) = 1700, B = 850, C = 3400. b The values quoted are for the basis set B3LYP/TZV+DZ – see text. Other basis sets gave similar results. c The labels correspond to D4h symmetry. The pattern of calculated splittings shows that D4h is a good approximation to the molecular symmetry, which in fact, is not higher than Cs. a
Fig. 5. EPR spectrum (X band, m = 9.158 GHz) of a powder sample of 2 at room temperature.
Table 6 EPR parameters of 2a
gx gy gz Ax Ay Az AF AD x AD y AD z Aso x Aso y Aso x
Experimental
DFT
LFTb
2.050 2.100 2.383 620 620 140
2.056 2.099 2.237 37 76 159 72.4 124.4 64.4
102 99 99
189.0 24.3 44.9 102.0
198 23 23 161
A-values in units of 104 cm1 (sign of A not determined). Spin–orbit parameters (k) = 828 cm1, Ær3æ = 7.64 a.u. See text for covalency factors. a
b
are within 1% (gx, gy) and 6% (gz) of the experimental value. The parallel component (gz) is the one most influenced by spin–orbit perturbation. To a first approximation, its deviation from the free spin value is directly proportional to the square of the coefficient of the metal d-orbital in the SOMO and inversely proportional to the energy gap between the molecular orbitals based on dx2 y 2 (SOMO) and dxy. Simple ligand field analysis [24] assuming axial symmetry and using the proposed electronic spectral assignments (vide infra) leads to the following values for the d-orbital coefficients in the molecular orbitals: 0.98 ðdx2 y 2 Þ, 0.94 (dxy) and 0.89 (dxz, dyz). The values of the different contributions to the metal hyperfine splitting viz. D D Fermi contact (AF), Spin dipolar ðAD z ; Ax ; Ay Þ and spin– so so so orbit ðAz ; Ax ; Ay Þ based on this analysis are included in Table 6. It is seen that the agreement of the hyperfine splitting components calculated by DFT with the ligand field
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analysis is poor. The deviations in the calculated values are as much as 200% for the in-plane component and 13% for the axial component. It is seen that the better agreement in the case of the axial component in mainly due to the cancellation of errors in AF and AD z having a negative sign and Aso z , which makes a positive contribution. A similar observation has been made earlier with regard to hyperfine splitting calculations using the program ORCA [25]. In spite of the poor agreement of the magnitude of the calculated hyperfine tensor components with the experiment, the orientation of the g- and A-tensor are collinear with each other (within 1) and the in-plane components are along the Cu–N and Cu–O bonds (within 3). While this orientation is reasonable for the observed structure (all in plane bond angles being 90 ± 2), the powder spectrum does not afford the orientation of the g and A principal directions. 3.5. Thermal analysis The thermogravimetric analyses of all compounds show the same trend. The weight loss begins at 50 C and the loss of five water molecules is complete at 160 C (wt. loss calculated 9.28%, found 9.39% for 1; 9.23% and 9.59% for 2;
9.22% and 10.28% for 3). The loss of the last coordinated water takes place gradually and the structures were found to completely break down above 270 C. This indicates that the four-coordinate structures are thermally stable relative to the six-coordinate ones at temperatures higher than about 160 C. 3.6. Geometry optimization The geometry of the cationic part of 1, 2 and 3 were optimized without any constraints. The optimized structures of the isolated cations are superimposed on the crystallographic structures in Fig. 6. The bond distances in the ˚ of the crystallooptimized geometry are within 0.06 A graphic values in all cases. It is noteworthy that the DFT calculation captures the essential feature of the static Jahn–Teller distortion in the case of the Cu(II) complex, viz., elongation of the axial bonds [26]. In this context, it is worth investigating the structural changes associated with the deprotonation of dipicH2 in these complexes. The results are shown in Fig. 7, wherein the calculated minimum energy structures of M(dipic)(OH2)3 are superimposed on the corresponding [M(dipicH2)(OH2)3]2+ structures. In the case of Ni(II), there was only a small
Fig. 6. Optimized structures of the isolated cations, superimposed on the crystallographic structures.
Fig. 7. Optimized structures of M(dipic)(OH2)3 and [M(dipicH2)(OH2)3]2+ (see text for description).
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reduction in the two axial bond distances, by 0.05 and ˚ . In the case of Cu(II), there is a switch of the elonga0.1 A tion axis. The carboxylate oxygen atoms close in on copper with a simultaneous weakening of the two trans bonds to water molecules. The final structure is square planar, the two water molecules which are no longer in the coordination sphere of Cu(II) are involved in H-bonding on the periphery of the complex. The Cu–Ocarboxylate bonds are ˚ . The crystal structure of Cu(dipic)shortened by 0.3 A (OH2)3 [22] differs from the computed gas phase structure. It consists of six-coordinated molecules with weak axial bonds to water molecules. Nevertheless, the present result shows that the calculation is able to model the switch of the Jahn–Teller axis accompanying the deprotonation of the ligand. The Zn(II) complex transforms to a fivecoordinate structure upon deprotonation in which the ˚. Zn–Ocarboxylate bonds are shortened by 0.2 A 4. Conclusion The cations [M(dipicH2)(OH2)3]2+, containing strongly acidic hydrogen atoms, are stabilized in crystals by bulky [Ce(dipic)3]2 anions. Even though the three crystals with M = Ni, Cu, Zn are isomorphous, the cations maintain the structural features characteristic of their electronic configurations. This is especially prominent for the Cu(II) complex in which a vibronic distortion is superimposed on the steric distortion. This complex also affords wellresolved EPR spectra due to magnetic isolation of the cations provided by the bulky anions. It is shown that TDDFT calculations are able to reproduce the electronic transition energies with reasonable accuracy for the d9 complex. The agreement is not satisfactory for the d8 complex. Geometry optimizations using DFT are able to model static Jahn–Teller elongation of the Cu(II) complex ion as well as changes accompanying the deprotonation of the coordinated dipicH2. While DFT is able to model the structure, electronic transition and g-anisotropy of the d9 complex, the agreement of the hyperfine splitting constants is poor. Various contributions to hyperfine coupling are not satisfactorily modelled at the present level of theory. Acknowledgements The X-ray data were collected at the diffractometer facility at the University of Hyderabad, established by the Department of Science and Technology, Government of India. Computations were performed at CMSD. Infrastructure support from UGC (UPE program) is also acknowledged. This work was supported by CSIR, India. T.K.P. thanks the CSIR for the award of a research fellowship. Appendix A. Supplementary material CCDC 622453, 622454 and 622455 contain the supplementary crystallographic data for this paper. These data
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can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2006.11.029. References [1] (a) X.-P. Yang, R.A. Jones, W.-K. Wong, V. Lynch, M.M. Oye, A.L. Holmes, Chem. Commun. (2006) 1836; (b) J.-P. Costes, F. Dahan, W. Wernsdorfer, Inorg. Chem. 45 (2006) 5; (c) J.-C.G. Bunzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048; (d) C.M. Zaleski, E.C. Depperman, J.W. Kampf, M.L. Kirk, V.L. Pecoraro, Angew. Chem., Int. Ed. 43 (2004) 3912. [2] (a) T.K. Prasad, M.V. Rajasekharan, Inorg. Chem. Commun. 8 (2005) 1116; (b) T.K. Prasad, S. Sailaja, M.V. Rajasekharan, Polyhedron 24 (2005) 1487; (c) S. Sailaja, M.V. Rajasekharan, Acta Crystallogr., Sect. E 57 (2001) m341; (d) G. Swarnabala, M.V. Rajasekharan, Inorg. Chem. 37 (1998) 1483. [3] K. Harms, S. Wocadlo, XCAD4-CAD4 Data Reduction, University of Marburg, Marburg, Germany, 1995. [4] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr., Sect. A 24 (1968) 351. [5] SAINTPLUS, Bruker AXS Inc., Madison, WI, USA, 2003. [6] G.M. Sheldrick, SADABS Program for Empirical Absorption Correction, University of Gottingen, Germany, 1996. [7] G.M. Sheldrick, SHELXS-97, SHELXL-97, University of Gottingen, Gottingen, Germany, 1997. [8] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565. [9] C.F. Macrae, P.R. Edgington, P. McCabe, E. Pidcock, G.P. Shields, R. Taylor, M. Towler, J. van de Streek, J. Appl. Crystallogr. 39 (2006) 453. [10] G. Swarnabala, M.V. Rajasekharan, Inorg. Chem. 28 (1989) 662. [11] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. AlLaham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision B.05, Gaussian, Inc., Pittsburgh PA, 2003. [13] A. Schaefer, C. Huber, R. Ahlrichs, J. Chem. Phys. 100 (1994) 5829. [14] A. Schaefer, H. Horn, R. Ahlrichs, J. Chem. Phys. 97 (1992) 2571. [15] B.N. Figgis, M.A. Hitchman, Ligand Field Theory and its Applications, Wiley-VCH, New York, 2000. [16] H. Adamsky, AOMX, 09-Jun-96 Version, an Angular Overlap Model Computer Program, Department of Theoretical Chemistry, HeinrichHeine-Universitat, Dusseldorf, Germany. [17] F. Neese, ORCA – An Ab initio, Density Functional and SemiEmpirical Program Package Version 2.4, Max-Planck-Insitut fu¨r Bioanorganische Chemie, Mu¨lheim, der Ruhr, 2004. [18] (a) L.C. Nathan, T.D. Mai, J. Chem. Crystallogr. 30 (2000) 509; (b) N. Okabe, N. Oya, Acta Crystallogr., Sect. C 56 (2000) 305;
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(c) K. Hakansson, M. Lindahl, G. Svensson, J. Albertsson, Acta Chem. Scand. 47 (1993) 449; (d) P. Quaglieri, H. Loiseleur, G. Thomas, Acta Crystallogr., Sect. B 28 (1972) 2583. [19] (a) S.K. Ghosh, J. Ribas, P.K. Bharadwaj, Cryst. Growth Des. 5 (2005) 623; (b) S.K. Ghosh, P.K. Bharadwaj, Inorg. Chem. 44 (2005) 5553; (c) A.-Y. Fu, D.-Q. Wang, D.-Z. Sun, Acta Crystallogr., Sect. E 61 (2005) m401; (d) N.W. Alcock, G.J. Clarkson, G.A. Lawrance, P. Moore, Aust. J. Chem. 57 (2004) 565; (e) J.C. MacDonald, T.-J.M. Luo, G.T.R. Palmore, Cryst. Growth Des. 4 (2004) 1203; (f) A. Moghimi, M. Ranjbar, H. Aghabozorg, F. Jalali, M. Shamsipur, R.K. Chadah, J. Chem. Res. 477 (2002) 1047; (g) M. Ranjbar, M. Taghavipur, H. Aghabozorg, A. Moghimi, F. Jalali, M. Shamispur, Pol. J. Chem. 76 (2002) 785; (h) J.C. MacDonald, P.C. Dorrestein, M.M. Pilley, M.M. Foote, J.L. Lundburg, R.W. Henning, A.J. Schultz, J.L. Manson, J. Am. Chem. Soc. 122 (2000) 11692; (i) J.G.H.du. Preez, H.E. Rohwer, B.J. van Brecht, M.R. Caira, J. Chem. Soc., Dalton Trans. (1984) 975. [20] E.E. Sileo, M.A. Blesa, G. Rigotti, B.E. Rivero, E.E. Castellano, Polyhedron 15 (1996) 4531. [21] (a) M. Koman, J. Moncol, D. Hudecova, B. Dudova, M. Melnik, M. Korabik, J. Mrozinski, Pol. J. Chem. 75 (2001) 957;
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[23] [24] [25] [26]
(b) E.E. Sileo, G. Rigotti, B.E. Rivero, M.A. Blesa, J. Phys. Chem. Solids 58 (1997) 1127. (a) L. Mao, Y. Wang, Y. Qi, M. Cao, C. Hu, J. Mol. Struct. 668 (2004) 197; (b) A.J. Blake, M. Felloni, P. Hubberstey, C. Wilson, M. Schroder, Acta Crystallogr., Sect. E 58 (2002) 43. P.W. Thulstrup, L. Broge, E. Larsen, J. Springborg, J. Chem. Soc., Dalton Trans. (2003) 3199. B.A. Goodman, J.B. Raynor, Adv. Inorg. Chem. Radiochem. 13 (1970) 135. P. Comba, Y.D. Lampeka, A.I. Prikhodko, G. Rajaraman, Inorg. Chem. 45 (2006) 3632. (a) As a check on this procedure, we optimized the structure of [Cu(OH2)6]2+ starting with a regular octahedral geometry having a ˚ . The water molecules were constrained with O– bond distance 2.12 A ˚ and H–O–H = 105. The optimized structure is of tetragH = 0.96 A ˚, onal symmetry with axial and equatorial distances of 2.20 and 1.99 A respectively. The structure is stabilized to the extent of 23.4 kJ mol1 with respect to the regular octahedral structure. Similar optimizations in the case of [Ni(OH2)6]2+ and [Zn(OH2)6]2+ results in regular ˚ (Ni2+) and 2.07 A ˚ octahedral geometries with bond distances 2.04 A (Zn2+). The optimized structure of [Cu(OH2)6]2+ compares well with ˚ the previous ab initio SCF calculations [26b]: bond distances, 2.24 A ˚ (equatorial), stabilization energy = 7.0 kJ mol1; (axial), 2.06 A (b) R. Akesson, L.G.M. Pettersson, M. Sandstrom, U. Wahlgren, J. Phys. Chem. 96 (1992) 150.
[M(dipicH2)(H2O)3]2+, M = Ni, Cu, Zn (dipicH2 = dipicolinic acid) – A combined crystallographic, spectroscopic and computational study T.K. Prasad, M.V. Rajasekharan
*
School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad, AP 500 046, India Received 19 October 2006; accepted 11 November 2006 Available online 23 November 2006
Abstract Cationic metal complexes of dipicolinic acid (dipicH2) are stabilized by [Ce(dipic)3]2 ions in the three isomorphous crystals [M(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (M = Ni, 1; Cu, 2; Zn, 3). Magnetic dilution provided by the bulky anions leads to well-resolved EPR spectra in polycrystalline samples of 2. The cations have 4+2 coordination, the carbonyl atom of the carboxylic acid groups coordinating weakly from trans positions. In the case of 2 this steric distortion is augmented by Jahn–Teller distortion. All the three structures are satisfactorily modelled by calculations based on density functional theory (DFT). The switch of the Jahn–Teller axis upon deprotonation of the complex, leading to the neutral species Cu(dipic)(H2O)3, is also reproduced by DFT. Electronic transition energies as well as the g-tensor component of the d9 complex obtained are in good agreement with experiment. However, the calculated hyperfine coupling constants are in error. DFT also fails to satisfactorily account for the electronic transition in the d8 ion in 1. 2006 Elsevier Ltd. All rights reserved. Keywords: Crystal structures; Ce(IV) complexes; Ni(II), Cu(II), Zn(II) complexes; Heterometallic coordination compounds; EPR; DFT; TDDFT
1. Introduction
2. Experimental
Heterometallic coordination compounds of lanthanide ions in combination with other metal ions are of interest due to their potentially useful solid-state properties [1]. We have earlier exploited the multifunctionality of dipicolinic acid (dipicH2) to generate several coordination networks containing Ce4+ and alkaline earth ions [2]. This work is now extended to transition metal ions (Cu2+, Ni2+) and Zn2+. However, contrary to alkaline earth ions, no coordination networks are obtained. Instead, three isomorphous ionic compounds, viz., [M(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O, M = Ni, Cu, Zn, are obtained. The structures, spectral and thermal properties of these complexes are reported in this paper. The structural changes in the coordination polyhedron accompanying the deprotonation of dipicH2 is modelled using density functional theory (DFT) calculations and compared with the experimental results.
2.1. Materials
*
Corresponding author. Tel.: +91 40 23134857; fax: +91 40 23012460. E-mail address: [email protected] (M.V. Rajasekharan).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.11.029
NiCl2 Æ 6H2O, Cu(NO3)2 Æ 3H2O, Zn(NO3)2 Æ 6H2O and (NH4)2Ce(NO3)6 were purchased from Merck Chemicals. DipicH2 was purchased from Lancaster Chemicals. 2.1.1. Preparation of [Ni(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (1) To a 10 ml aqueous solution of (NH4)2Ce(NO3)6 (0.551 g, 1.00 mmol) and NiCl2 Æ 6H2O (0.241 g, 1.01 mmol), 30 ml methanolic solution of dipicH2 (0.670 g, 4.01 mmol) was added and stirred for 2 min to get a clear solution. It was filtered and kept for evaporation at RT. After about 4–5 days, pale green crystalline blocks formed, which were removed by filtration and dried (0.801 g, 82%). Recrystallization from hot water yielded pale green single crystals of 1 suitable for X-ray data collection. Anal. Calc. for C28H26CeN4NiO22: C, 34.69; H, 2.70; N, 5.78. Found: C, 34.76; H, 2.35; N, 5.84%. mmax(KBr)/cm1 3414, 1647, 1429, 1396, 1271,
T.K. Prasad, M.V. Rajasekharan / Polyhedron 26 (2007) 1364–1372
1182, 1024, 918 and 727. Magnetic moment at 298 K, 3.29 B.M. 2.1.2. Preparation of [Cu(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (2) This compound was prepared by similar method to that of 1, with (NH4)2Ce(NO3)6 (0.552 g, 1.01 mmol), Cu(NO3)2 Æ 3H2O (0.241 g, 0.998 mmol) and dipicH2 (0.670 g, 4.01 mmol). Pale blue blocks were formed in 4–5 days (0.740 g, 76%). During the preparation a small amount of blue coloured crystals, identified as Cu(dipic)(dipicH2) Æ 3H2O was also formed. The pale blue blocks of 2 were separated and recrystallized from hot water. Anal. Calc. for C28H26CeCuN4O22: C, 34.52; H, 2.69; N, 5.75. Found: C, 34.56; H, 2.50; N, 5.76%. mmax(KBr)/ cm1 3084, 1645, 1477, 1431, 1361, 1273, 1176, 1072, 1022, 923, 765 and 733. Magnetic moment at 298 K, 1.94 B.M. 2.1.3. Preparation of [Zn(dipicH2)(OH2)3][Ce(dipic)3] Æ 3H2O (3) The preparation was similar to that of 1 and 2 with (NH4)2Ce(NO3)6 (0.550 g, 1.00 mmol), Zn(NO3)2 Æ 6H2O (0.310 g, 1.04 mmol) and dipicH2 (0.672 g, 4.02 mmol). Pale yellow blocks were formed in 4–5 days (0.770 g, 78%). These blocks were recrystallized from hot water. Anal. Calc. for C28H26CeN4O22Zn: C, 34.45; H, 2.68; N, 5.74. Found: C, 34.14; H, 2.41; N, 5.02%. mmax(KBr)/ cm1 3406, 1647, 1427, 1383, 1178, 1078, 1024, 916 and 729.
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2.2. X-ray crystallography X-ray data were collected for compounds 1 and 2 on an Enraf Nonius CAD-4 diffractometer and for 3 on a Bruker SMART APEX CCD X-ray diffractometer, using graphite˚ ). Data monochromated Mo Ka radiation (k = 0.71073 A of 1 and 2 were reduced using XCAD4 [3] and psi-scan [4] absorption corrections were applied. For 3 the data was reduced using SAINTPLUS [5] and a multiscan absorption correction using SADABS [6] was performed. The structure was solved using SHELXS-97 and refined using SHELXL-97 [7]. All ring hydrogens were assigned on the basis of geometrical considerations and were allowed to ride upon the respective carbon atoms. Drawings were made using ORTEP3 for Windows [8] and Mercury [9]. In compound 3, one lattice water molecule was disordered in two positions. The crystal data for the three compounds are presented in Table 1 and selected bond lengths and angles in Tables 2–4. Powder diffractograms were measured using a PW3710 model Philips Analytical X-ray diffractometer. 2.3. Physical measurements EPR spectra were measured on a JEOL JES – FA 200 spectrometer. Powder simulations were done using a previously described computer program [10]. Electronic Reflectance spectra were measured by using a Shimadzu UV-3100 spectrometer equipped with an ISR-3100 integrating sphere attachment. Thermogravimetric analysis was per-
Table 1 Crystal data and structure refinement for 1, 2 and 3
Formula Formula weight Crystal system ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Space group T (K) Crystal size (mm) l (mm1) Reflections collected Unique reflections, Rint Completeness to 2h = 25.0 (%) Refinement method RðF 2o Þa [I > 2r(I)] Rw ðF 2o Þb [I > 2r(I)] P P a R ¼ kF o j j F c k= j F o j. hP i1=2 P b Rw ¼ wðF 2o F 2c Þ2 . ðwF 4o Þ
1
2
3
C28H26CeN4NiO22 969.36 triclinic 10.5250(8) 11.6839(14) 14.851(3) 68.362(16) 88.539(10) 87.983(8) 1696.4(5) 2 P 1 298(2) 0.56 · 0.24 · 0.24 1.981 7904 7757, 0.0018 99.7 full-matrix least squares on F2 0.0279 0.0759
C28H26CeCuN4O22 974.19 triclinic 10.5553(14) 11.561(3) 14.923(3) 68.221(16) 88.552(13) 88.23(2) 1690.1(6) 2 P 1 298(2) 0.60 · 0.56 · 0.52 2.060 7842 7704, 0.003 99.5 full-matrix least squares on F2 0.0224 0.0619
C28H26CeN4O22Zn 976.02 triclinic 10.5178(5) 11.4883(5) 14.9564(7) 67.9900(10) 88.9330(10) 88.5350(10) 1674.87(13) 2 P 1 100(2) 0.45 · 0.39 · 0.29 2.160 19 574 7841, 0.0161 99.6 full-matrix least squares on F2 0.0199 0.0524
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Table 2 ˚ ) and angles () for 1 Selected bond lengths (A Ce–O(11) Ce–O(31) Ce–N(21) Ni–N(41) Ni–O(51)
2.323(3) 2.346(3) 2.498(3) 1.994(3) 2.049(3)
N(41)–Ni–O(52) O(52)–Ni–O(53) O(52)–Ni–O(51) N(41)–Ni–O(43) O(53)–Ni–O(43) O(51)–Ni–O(43) O(53)–Ni–O(41) O(43)–Ni–O(41)
Ce–O(21) Ce–O(33) Ce–N(11) Ni–O(52) Ni–O(43) 177.22(13) 84.85(12) 84.74(13) 77.11(12) 89.92(12) 90.92(12) 94.06(12) 153.40(11)
2.339(3) 2.366(3) 2.520(3) 2.030(3) 2.172(3)
Ce–O(13) Ce–O(23) Ce–N(31) Ni–O(53) Ni–O(41)
N(41)–Ni–O(53) N(41)–Ni–O(51) O(53)–Ni–O(51) O(52)–Ni–O(43) O(52)–Ni–O(41) N(41)–Ni–O(41) O(51)–Ni–O(41)
2.345(3) 2.368(3) 2.525(3) 2.030(3) 2.178(3) 94.74(13) 95.76(13) 169.39(12) 105.63(12) 100.92(12) 76.35(12) 89.92(13)
2.3308(18) 2.3524(17) 2.4996(19) 1.9595(19) 2.014(2)
O(52)–Cu–O(53) O(53)–Cu–O(51) O(53)–Cu–N(41) O(52)–Cu–O(43) O(51)–Cu–O(43) O(52)–Cu–O(41) O(51)–Cu–O(41) O(43)–Cu–O(41)
Ce–O(21) Ce–O(23) Ce–N(31) Cu–O(53) Cu–O(43) 86.62(8) 172.26(8) 93.36(8) 108.52(8) 89.96(8) 101.50(8) 88.88(8) 149.81(7)
2.3331(18) 2.3768(17) 2.523(2) 1.9683(19) 2.338(2)
Ce–O(33) Ce–O(11) Ce–N(21) Cu–O(51) Cu–O(41)
O(52)–Cu–O(51) O(52)–Cu–N(41) O(51)–Cu–N(41) O(53)–Cu–O(43) N(41)–Cu–O(43) O(53)–Cu–O(41) N(41)–Cu–O(41)
2.3337(18) 2.3835(17) 2.5258(19) 1.9967(19) 2.350(2) 86.26(8) 176.31(8) 93.94(8) 89.43(8) 75.17(8) 95.51(8) 74.82(8)
Table 4 ˚ ) and angles () for 3 Selected bond lengths (A Ce–O(31) Ce–O(11) Ce–N(11) Zn–O(52) Zn–O(53)
2.3306(12) 2.3426(12) 2.5011(14) 2.0341(13) 2.0831(13)
O(52)–Zn–O(51) O(51)–Zn–N(41) O(51)–Zn–O(53) O(52)–Zn–O(43) N(41)–Zn–O(43) O(52)–Zn–O(41) N(41)–Zn–O(41) O(43)–Zn–O(41)
Ce–O(33) Ce–O(13) Ce–N(31) Zn–O(51) Zn–O(43) 85.76(5) 96.04(5) 169.14(5) 111.97(5) 75.74(5) 98.13(5) 74.11(5) 149.81(5)
3. Results and discussion 3.1. Synthesis
Table 3 ˚ ) and angles () for 2 Selected bond lengths (A Ce–O(31) Ce–O(13) Ce–N(11) Cu–O(52) Cu–N(41)
calculations, triple-f basis sets [13] were used for Ni, Cu and Zn atoms and double-f basis sets [14] were used for all other atoms. Other basis sets in the Gaussian-03 suite were also used for comparison. Ligand field theory calculations based on the angular overlap model (AOM) [15] were done using the AOMX program [16]. EPR g and A tensors were calculated using the program package ORCA [17] at the B3LYP level of DFT. A triple-f basis set [13] was used for all atoms.
2.3321(12) 2.3648(12) 2.5236(14) 2.0465(13) 2.2100(13)
Ce–O(21) Ce–O(23) Ce–N(21) Zn–N(41) Zn–O(41)
O(52)–Zn–N(41) O(52)–Zn–O(53) N(41)–Zn–O(53) O(51)–Zn–O(43) O(53)–Zn–O(43) O(51)–Zn–O(41) O(53)–Zn–O(41)
2.3402(12) 2.3692(12) 2.5259(14) 2.0659(15) 2.2635(13) 172.16(5) 83.96(5) 94.62(5) 89.02(5) 91.56(5) 95.60(5) 89.33(5)
formed using a NETZSCH STA 409 PC/PG instrument. Room temperature magnetic susceptibilities of the powdered samples were measured using a Sherwood Scientific magnetic susceptibility balance. 2.4. Computational details DFT along with geometry optimizations and time dependent DFT (TDDFT) have been performed using the B3LYP exchange correlation functional [11] as implemented in the Gaussian-03 [12]. The spin-unrestricted version was employed for open shell ions. For both types of
All three compounds are obtained in good yield by mixing the reagents in the appropriate molar ratio. The initial crystalline blocks obtained from aqueous methanolic solution and the crystals formed from hot water were found to be identical by comparing their X-ray powder patterns. The powder patterns also matched with those calculated based on the crystal structure. DipicH2 can coordinate in the neutral form as well as in monoanionic (dipcH) and dianionic (dipic2) forms. The following complex species have been previously reported: M(dipicH)2 M = Ni, Zn [18], MðdipicÞ2 2 M = Ni, Cu, Zn [19], Cu(dipic)(dipicH2) [18b,20], Cu(dipic)(OH2)2 [21] and Cu(dipic)(OH2)3 [21b,22]. In the preparation of 2, a small amount of Cu(dipic)(dipicH2) Æ 3H2O was formed along with the main compound. Besides this, no other side products were formed in the present synthesis. The presence of CeðdipicÞ3 2 appears to ensure the crystallization of cationic complexes containing the neutral form of the ligand in all cases. 3.2. Structures All the three compounds 1–3 are obtained as isomorphous ionic crystals containing [M(dipicH2)(OH2)3]2+ and [Ce(dipic)3]2 ions (Figs. 1–3). The tri-capped trigonal prismatic Ce(IV) chelate has also been observed in Ce(IV)-alkaline earth-dipic complexes [2]. However, in all these cases some of the free oxygen atoms of the chelate coordinate to M(II) ions forming chains and networks. In the present cases there is no coordination link between the anions and cations. Instead, the free oxygen atoms form H-bonds with carboxylic groups and coordinated water molecules of the cation. All the three cations, [M(dipicH2)(OH2)3]2+, have very similar 4+2 coordination polyhedra. The equatorial plane contains the nitrogen atom of dipicH2 and the O atoms of the three water molecules, while the axial positions are occupied by the carbonyl oxygen atoms of dipicH2. The major difference is that the axial elongation is significantly more in the case of the copper complex. The average equa˚ ) are 2.026(3), 2.175(3) (Ni); torial and axial distances (A 1.984(2), 2.344(2) (Cu); 2.057(1), 2.236(1) (Zn). It would
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Fig. 1. ORTEP view of compound 1 showing the atom labeling. Thermal ellipsoids are drawn at the 50% probability level and ring hydrogen atoms and lattice water molecules have been omitted for clarity.
Fig. 2. ORTEP view of compound 2, showing the atom labeling. Thermal ellipsoids are drawn at the 50% probability level and ring hydrogen atoms and lattice water molecules have been omitted for clarity.
appear that the expected vibronic effects are superimposed on the steric requirements of the ligand to produce the observed static structure in the case of the copper complex. The average off-axis deviation () of the axial ligand atoms are 13.5 (Ni) and 15.1 (Cu, Zn). The equatorial atoms including the metal atoms are very nearly coplanar in all ˚ ) from the cases with a largest deviation of the atoms (A mean plane 0.038(3) (Ni), 0.062(2) (Cu) and 0.111(2) (Zn). The deviations amount to a very slight tetrahedral distortion with the following inter planar angles (): 3.3(2) (Ni); 4.6(1) (Cu); 8.2(1) (Zn).
Hydrogen bonding interactions between cations and anions generate a one-dimensional chain in all three crystals (Fig. 4). Further H-bonding involving lattice water molecules generates a three-dimensional network. 3.3. Electronic spectra Assuming a tetragonal symmetry for the d8 complex ion, the broad features in the solid state spectrum of 1 in the range 8000–20 000 cm1 may be assigned to transitions arising from the ground state (3A2) to the split components
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Fig. 3. ORTEP view of compound 3 showing the atom labeling. Thermal ellipsoids are drawn at the 50% probability level and ring hydrogen atoms and lattice water molecules have been omitted for clarity.
Fig. 4. 1D chain formed by H-bond interactions of cation and anion units.
of 3T2 and 3T1. The band positions and assignments are shown in Table 5. In the case of the copper complex 2, two broad bands are seen which are assigned to excitations from d2z and dxy to dx2 y 2 . Higher energy transitions could not be measured for these compounds due to the presence of strongly absorbing [Ce(dipic)3]2 ions. The assignments are in general agreement with the calculations based on the AOM model as well as TDDFT theory. As indicated in Table 5, the axial perturbation in the case of the Cu(II) complex is only about one-fourth of the corresponding interaction in the Ni(II) complex. The excitation energies obtained from TDDFT calculations for the Cu(II) complex are in satisfactory agreement with the AOM calculations. The agreement is less than satisfactory in the case of the Ni(II) complex. It has been reported that transitions involving double excitations are not well represented in the TDDFT model [23]. In a low-symmetry six-coordinate d8 complex, transitions to the split components of 3T2 and
3
T1(F) are essentially single electron excitations. The six lowest energy excitations quoted in Table 5 correspond to these. It is seen that the energies are overestimated by about 2000–3000 cm1. All the eight different basis sets used gave similar results. 3.4. EPR Spectrum of 2 The packing of the Cu(II) complex cations alternating with the bulky diamagnetic anions in the crystals lead to magnetic dilution. The EPR spectrum of the polycrystalline sample is shown in Fig. 5, in which the hyperfine splitting is fully resolved in the parallel region. The EPR parameters derived by computer simulation of the experimental spectra are given in Table 6. The parameters are consistent with the tetragonally distorted Cu(II) complex. Included in the table are the EPR parameters derived using DFT calculations with the experimental geometry. The calculated g-values
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Table 5 Electronic spectral data Band maxima/1000 cm1
Compound
Experimental [Cu(dipicH2)(OH2)3]
2+
8.8 13.0
[Ni(dipicH2)(OH2)3]
2+
7.6 13.6 15.2
AOMa
TDDFTb
Assignmentc
8.89 13.44 14.21, 14.36
7.99 13.70 15.75, 15.87
1
A1 B2 2 E
B1 B1 2 B1
7.49, 7.95 12.32 13.57 16.78, 16.85 25.12 26.78, 26.91
10.30, 10.79 13.98 16.84 20.28, 23.43
3
3
2
E B2 3 A2 3 E 3 A2 3 E 3
2
2
B1 3 B1 3 B1 3 B1 3 B1 3 B1
The bonding parameters (cm1) used: Cu(II) complex, er(N) = 4900, ep(N) = 800, er(O) = 4800, er(Oaxial) = 400. Ni(II) complex, er(N) = 4900, ep(N) = 600, er(O) = 4200, er(Oaxial) = 1700, B = 850, C = 3400. b The values quoted are for the basis set B3LYP/TZV+DZ – see text. Other basis sets gave similar results. c The labels correspond to D4h symmetry. The pattern of calculated splittings shows that D4h is a good approximation to the molecular symmetry, which in fact, is not higher than Cs. a
Fig. 5. EPR spectrum (X band, m = 9.158 GHz) of a powder sample of 2 at room temperature.
Table 6 EPR parameters of 2a
gx gy gz Ax Ay Az AF AD x AD y AD z Aso x Aso y Aso x
Experimental
DFT
LFTb
2.050 2.100 2.383 620 620 140
2.056 2.099 2.237 37 76 159 72.4 124.4 64.4
102 99 99
189.0 24.3 44.9 102.0
198 23 23 161
A-values in units of 104 cm1 (sign of A not determined). Spin–orbit parameters (k) = 828 cm1, Ær3æ = 7.64 a.u. See text for covalency factors. a
b
are within 1% (gx, gy) and 6% (gz) of the experimental value. The parallel component (gz) is the one most influenced by spin–orbit perturbation. To a first approximation, its deviation from the free spin value is directly proportional to the square of the coefficient of the metal d-orbital in the SOMO and inversely proportional to the energy gap between the molecular orbitals based on dx2 y 2 (SOMO) and dxy. Simple ligand field analysis [24] assuming axial symmetry and using the proposed electronic spectral assignments (vide infra) leads to the following values for the d-orbital coefficients in the molecular orbitals: 0.98 ðdx2 y 2 Þ, 0.94 (dxy) and 0.89 (dxz, dyz). The values of the different contributions to the metal hyperfine splitting viz. D D Fermi contact (AF), Spin dipolar ðAD z ; Ax ; Ay Þ and spin– so so so orbit ðAz ; Ax ; Ay Þ based on this analysis are included in Table 6. It is seen that the agreement of the hyperfine splitting components calculated by DFT with the ligand field
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analysis is poor. The deviations in the calculated values are as much as 200% for the in-plane component and 13% for the axial component. It is seen that the better agreement in the case of the axial component in mainly due to the cancellation of errors in AF and AD z having a negative sign and Aso z , which makes a positive contribution. A similar observation has been made earlier with regard to hyperfine splitting calculations using the program ORCA [25]. In spite of the poor agreement of the magnitude of the calculated hyperfine tensor components with the experiment, the orientation of the g- and A-tensor are collinear with each other (within 1) and the in-plane components are along the Cu–N and Cu–O bonds (within 3). While this orientation is reasonable for the observed structure (all in plane bond angles being 90 ± 2), the powder spectrum does not afford the orientation of the g and A principal directions. 3.5. Thermal analysis The thermogravimetric analyses of all compounds show the same trend. The weight loss begins at 50 C and the loss of five water molecules is complete at 160 C (wt. loss calculated 9.28%, found 9.39% for 1; 9.23% and 9.59% for 2;
9.22% and 10.28% for 3). The loss of the last coordinated water takes place gradually and the structures were found to completely break down above 270 C. This indicates that the four-coordinate structures are thermally stable relative to the six-coordinate ones at temperatures higher than about 160 C. 3.6. Geometry optimization The geometry of the cationic part of 1, 2 and 3 were optimized without any constraints. The optimized structures of the isolated cations are superimposed on the crystallographic structures in Fig. 6. The bond distances in the ˚ of the crystallooptimized geometry are within 0.06 A graphic values in all cases. It is noteworthy that the DFT calculation captures the essential feature of the static Jahn–Teller distortion in the case of the Cu(II) complex, viz., elongation of the axial bonds [26]. In this context, it is worth investigating the structural changes associated with the deprotonation of dipicH2 in these complexes. The results are shown in Fig. 7, wherein the calculated minimum energy structures of M(dipic)(OH2)3 are superimposed on the corresponding [M(dipicH2)(OH2)3]2+ structures. In the case of Ni(II), there was only a small
Fig. 6. Optimized structures of the isolated cations, superimposed on the crystallographic structures.
Fig. 7. Optimized structures of M(dipic)(OH2)3 and [M(dipicH2)(OH2)3]2+ (see text for description).
T.K. Prasad, M.V. Rajasekharan / Polyhedron 26 (2007) 1364–1372
reduction in the two axial bond distances, by 0.05 and ˚ . In the case of Cu(II), there is a switch of the elonga0.1 A tion axis. The carboxylate oxygen atoms close in on copper with a simultaneous weakening of the two trans bonds to water molecules. The final structure is square planar, the two water molecules which are no longer in the coordination sphere of Cu(II) are involved in H-bonding on the periphery of the complex. The Cu–Ocarboxylate bonds are ˚ . The crystal structure of Cu(dipic)shortened by 0.3 A (OH2)3 [22] differs from the computed gas phase structure. It consists of six-coordinated molecules with weak axial bonds to water molecules. Nevertheless, the present result shows that the calculation is able to model the switch of the Jahn–Teller axis accompanying the deprotonation of the ligand. The Zn(II) complex transforms to a fivecoordinate structure upon deprotonation in which the ˚. Zn–Ocarboxylate bonds are shortened by 0.2 A 4. Conclusion The cations [M(dipicH2)(OH2)3]2+, containing strongly acidic hydrogen atoms, are stabilized in crystals by bulky [Ce(dipic)3]2 anions. Even though the three crystals with M = Ni, Cu, Zn are isomorphous, the cations maintain the structural features characteristic of their electronic configurations. This is especially prominent for the Cu(II) complex in which a vibronic distortion is superimposed on the steric distortion. This complex also affords wellresolved EPR spectra due to magnetic isolation of the cations provided by the bulky anions. It is shown that TDDFT calculations are able to reproduce the electronic transition energies with reasonable accuracy for the d9 complex. The agreement is not satisfactory for the d8 complex. Geometry optimizations using DFT are able to model static Jahn–Teller elongation of the Cu(II) complex ion as well as changes accompanying the deprotonation of the coordinated dipicH2. While DFT is able to model the structure, electronic transition and g-anisotropy of the d9 complex, the agreement of the hyperfine splitting constants is poor. Various contributions to hyperfine coupling are not satisfactorily modelled at the present level of theory. Acknowledgements The X-ray data were collected at the diffractometer facility at the University of Hyderabad, established by the Department of Science and Technology, Government of India. Computations were performed at CMSD. Infrastructure support from UGC (UPE program) is also acknowledged. This work was supported by CSIR, India. T.K.P. thanks the CSIR for the award of a research fellowship. Appendix A. Supplementary material CCDC 622453, 622454 and 622455 contain the supplementary crystallographic data for this paper. These data
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(b) E.E. Sileo, G. Rigotti, B.E. Rivero, M.A. Blesa, J. Phys. Chem. Solids 58 (1997) 1127. (a) L. Mao, Y. Wang, Y. Qi, M. Cao, C. Hu, J. Mol. Struct. 668 (2004) 197; (b) A.J. Blake, M. Felloni, P. Hubberstey, C. Wilson, M. Schroder, Acta Crystallogr., Sect. E 58 (2002) 43. P.W. Thulstrup, L. Broge, E. Larsen, J. Springborg, J. Chem. Soc., Dalton Trans. (2003) 3199. B.A. Goodman, J.B. Raynor, Adv. Inorg. Chem. Radiochem. 13 (1970) 135. P. Comba, Y.D. Lampeka, A.I. Prikhodko, G. Rajaraman, Inorg. Chem. 45 (2006) 3632. (a) As a check on this procedure, we optimized the structure of [Cu(OH2)6]2+ starting with a regular octahedral geometry having a ˚ . The water molecules were constrained with O– bond distance 2.12 A ˚ and H–O–H = 105. The optimized structure is of tetragH = 0.96 A ˚, onal symmetry with axial and equatorial distances of 2.20 and 1.99 A respectively. The structure is stabilized to the extent of 23.4 kJ mol1 with respect to the regular octahedral structure. Similar optimizations in the case of [Ni(OH2)6]2+ and [Zn(OH2)6]2+ results in regular ˚ (Ni2+) and 2.07 A ˚ octahedral geometries with bond distances 2.04 A (Zn2+). The optimized structure of [Cu(OH2)6]2+ compares well with ˚ the previous ab initio SCF calculations [26b]: bond distances, 2.24 A ˚ (equatorial), stabilization energy = 7.0 kJ mol1; (axial), 2.06 A (b) R. Akesson, L.G.M. Pettersson, M. Sandstrom, U. Wahlgren, J. Phys. Chem. 96 (1992) 150.