J. Phys. Chem. B 2000, 104, 3471-3475
3471
Calculations and X-ray Absorption Near-Edge Structure of Stacking Structures of Bis(1,2-dione dioximato)nickel(II) Complexes Shuji Matsuo, Toshio Yamaguchi, and Hisanobu Wakita* Department of Chemistry, Faculty of Science, Fukuoka UniVersity, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ReceiVed: September 14, 1999
Ni K X-ray absorption near-edge structure (XANES) spectra for bis(1,2-dione dioximato)nickel(II) complexes of different stacking structures in crystals and in pyridine solution were investigated by a discrete variational XR (DV-XR) molecular orbital (MO) method. It was found that the peaks and shoulders in the experimental XANES spectra for crystals consist mainly of the 1s f 4p transition of the central Ni(II), and especially, the features around 8335-8350 eV are significantly affected by the interaction between the central Ni(II) and atoms of adjacent molecules in the stacking structures. The structure in the pyridine solution revealed, on the other hand, that the molecules are stacked one above the other, although adjacent molecules in the stacking structures slightly deviate from the c0 axis.
Introduction Single-crystal structures of some of a series of bis(1,2-dione dioximato)nickel(II) complexes [Ni(R,R′-dioxH)2], R and R′ ) H or various alkyl substituents (Figure 1), have been studied,1-5 and recently these results have been proved to be useful as fundamental knowledge to study structures of [Ni(R,R′-dioxH)2] showing piezo-6,7 and thermochromisms.8,9 These chromisms are caused by change of stacking structure of [Ni(R,R′-dioxH)2]. The stacking structures of [Ni(R,R′-dioxH)2] are classified into two types: blind and cross types. The blind type is where molecules are stacked one above the other, although the nearest nonbonded atom to the nickel atom in the molecule is an oxygen. The cross type is where molecules are stacked one above the other, although adjacent molecules in the stacking are rotated by 90° (Figure 2). Yamashita et al.10 prepared some of [Ni(R,R′-dioxH)2] and collected Ni K X-ray absorption near-edge structure (XANES) spectra in crystal and in pyridine solution of their complexes. These results showed that the profiles of the XANES spectra correlate with the stacking structure of [Ni(R,R′-dioxH)2] and that structure of [Ni(R,R′-dioxH)2] in a pyridine solution is distinguished from the two structures of the complexes in crystal in the XANES spectra (Figure 3). Thus, the XANES spectra are sensitive to stereo structure around an atom absorbed (e.g., ref 11), so that detailed analysis of the XANES spectra is needed to obtain information on the stereo and electronic structures of [Ni(R,R′-dioxH)2] from the XANES spectra. The aim of the present study is to theoretically correlate the profiles of the XANES spectra with the stacking structure, i.e., blind and cross types, and to determine their structure in a pyridine solution. The XANES spectra were analyzed by a discrete variational XR (DV-XR) molecular orbital (MO) method in order to distinguish between different structure models around the nickel atom. * Corresponding author. Tel +81-92-871-6631, ext 6218; Fax +81-92865-6030; E-mail
[email protected].
Figure 1. Schematic molecular structure of [Ni(R,R′-dioxH)2] with R and R′ ) H or various alkyl substituents.
Figure 2. Schematic stacking structures for the blind- and the crosstype crystals.
Experimental Section Computations. The computational detail of the DV-XR method based on the self-consistent Hartree-Fock-Slater model used in the present work have been described elsewhere,12-14 and the DV-XR method is useful for studies of coordination chemistry.15 In the DV-XR method, a lth MO, φl(rk), used was constructed as a linear combination of atomic orbitals (AO) in eq 1,
φl(rk) )
∑i Cilχi(rk)
(1)
where χi(rk) denotes the AOs, and rk is one of the sampling points in the DV-XR calculation. The coefficient, Cil, represents the spread of the ith AO at the lth MO. The AOs as the numerical basis functions are obtained by solving the Schro¨d-
10.1021/jp993267x CCC: $19.00 © 2000 American Chemical Society Published on Web 03/21/2000
3472 J. Phys. Chem. B, Vol. 104, No. 15, 2000
Matsuo et al.
Figure 3. Ni K XANES spectra for the [Ni(R,R′-dioxH)2] in crystal and in pyridine solution: (a) blind-type crystal, (b) cross-type crystal, and (c) pyridine solution.
Figure 4. Models calculated for each stacking structure in Figure 2 and for NiN4 as reference.
inger equations for each atomic potential in the molecule. Thus, the DV-XR calculation can be performed in a numerical manner. The exchange-correlation interaction, VXC(r), between electrons is expressed as
VXC(r) ) -3R
(8π3 F(r))
1/3
(2)
where F(r) is the molecular charge density and R is the Slater exchange limit, fixed at 0.7 throughout the present work. The electronic transition probability Iij for the electric dipole transition for photon absorption between states for AOs of a core atom in each MO, i and j, is given by
(3)
Figure 5. Models calculated for the [Ni(R,R′-dioxH)2] in pyridine solution.
where ∆E represents the transition energy. Since i and j values are given by eq 1, Iij is obtained directly by numerical integration of the dipole matrix.16 Model Structures in Crystal. The model structures for the blind and cross type crystals are composed of seven atoms, i.e., one Ni atom in the center with a positive charge 2+, four N atoms at the equatorial positions with a negative charge 3-, and two neutral O atoms for the blind-type crystal or two neutral Ni atoms for cross-type crystal at the axial positions. These are referred to Ni(ce)2+, N3-, O(ax)0, and Ni(ax)0, respectively. The model structures are based on crystal structures reported in the literature1-5 and are modified as can be seen in Figure 4. The Ni(ce)2+-N3- bond lengths are set to 1.88 Å for both models, the Ni(ce)2+-O(ax)0 to 3.44 Å for blind-type model, and the Ni(ce)2+-Ni(ax)0 to 3.24 Å for cross-type model. For both models, the N3--Ni(ce)2+-N3- bond angles with acute angles are set to 80° and the N3--Ni(ce)2+-O(ax)0 or N3--Ni(ce)2+-Ni(ax)0 to 90°. These two models have the C2h symmetry, but the calculations were performed as nonsymmetry. Numerical atomic orbitals 1s-5p for Ni(ce)2+ and Ni(ax)0 and 1s-3p for N3- and O(ax)0 were used as a basis set for the molecular orbital calculations. The sample points used in the numerical integration were taken up to 10 000 for each calculation. Self-consistency within 0.01 electrons was obtained for the final orbital populations. Transition probabilities calculated for each model were convoluted by a Gaussian function with a half-width half-height (hwhh) of 1.0 eV to make transition peak shapes comparable with experimental XANES spectra.
The above procedure was also performed for a planar NiN4 model structure (Figure 4). The planar NiN4 model is composed of one Ni(ce)2+ and four N3-. The result of the calculation was compared with those for the blind- and cross-type models. Model Structures in Pyridine Solution. To reveal the structure of the [Ni(R,R′-dioxH)2] complexes in pyridine solution, two model structures were built up on the basis of the crystal structures of the complexes. Their equatorial plane structures are the same as those of the structures in crystal, but the axial coordination structures are different from each other. One of the model structures is an intermediate between the blindand cross-type models; the other is solvated by the pyridine molecules at the axial positions. Hereafter the former is called layer type, and the latter is called solvation type (see Figure 5). In the layer-type model structure, Ni(ax)0-O(ax)0, Ni(ax)0-N(ax)0, and N(ax)0-O(ax)0 bond lengths in the adjacent molecules are set to 2.52, 1.88, and 1.34 Å, respectively. Then, both bond lengths of the Ni(ce)2+-Ni(ax)0 and Ni(ce)2+-O(ax)0 interactions are 4.32 Å. In the model structure of solvation type, the nitrogen atoms of pyridine molecules are placed in the axial positions to the Ni(ce)2+ with the bond length of 3.00 Å. The numerical atomic orbitals for carbon atoms in the pyridine molecules used were 1s-3p orbitals. The calculation for the layer-type model was made in nonsymmetry with the sample points of 20 000, whereas that for the solvation-type model is calculated in D2h symmetry with 22 000 points. The other conditions in calculations were the same as in the previous section.
Iij ∝ ∆E|〈j|r|i〉|2
Bis(1,2-Dione dioximato)nickel(II) Complexes
J. Phys. Chem. B, Vol. 104, No. 15, 2000 3473
Figure 6. Calculated transition probabilities (vertical bars) and peaks (dashed lines) for the blind-type, cross-type, and NiN4 models. Solid lines are Ni K XANES spectra corresponding to each model. Characters of A to E show the peaks corresponding to the remarkable features in Ni K XANES spectra.
Results and Discussion Results of the DV-Xr Calculations. Transition probabilities and peaks for the NiN4 model, for the blind- and cross-type model structures, and for the layer- and solvation-type model structures are shown in Figures 6 and 7, respectively. Needless to say, the larger models (e.g., [Ni(C,C-dioxH)2] model) than these simple models were calculated. As a result, the important transition probabilities figuring the XANES spectra are not much different from between the large models and the simple models with regard to the peak positions and ratios of peak strength of them. However, the calculated results were hard to understand visually due to the increase of the number of the MOs. Therefore, we discuss the spectral analyses with the results calculated by the simple models to be easy to explain hereafter. The energy scale was calibrated by assigning the calculated 1s f 3d peak to the corresponding preedge peak around 8330 eV in each experimental XANES spectrum. The energy scale for the NiN4 model was calibrated by assigning the calculated 1s f 3d peak to the corresponding preedge peak around 8330 eV in experimental XANES spectrum for the blind-type crystal. The characters of A to E are marked for the transitional peaks corresponding to five remarkable peaks and shoulders in each experimental spectrum. The calculated peaks for the blind-, cross-, and layer-type models reproduce the experimental spectral features well for each experimental spectrum. However, the feature in the region above 8355 eV is not discussed in detail because of insufficient outer atomic orbitals of Ni. Constituent atomic orbitals and proportions of the MOs for each model are shown in Tables 1-3. The proportions were obtained by use of the Mulliken population analysis. It is found that the MO of each peak consists of the following orbitals: peak A consists of the N3- and Ni(ce)2+ 4p orbitals, peak A′ of the N3- and Ni(ce)2+ 3d orbitals, peak B of the Ni(ce)2+ 4p and N3- orbitals, peak C of the Ni(ce)2+ 4p, N3-, and O(ax)0 orbitals, peak C′ of the Ni(ce)2+ 4p, N3-, and Ni(ax)0 orbitals, peak C′′ of the Ni(ce)2+ 4p and orbital of the N atoms in the pyridine, peak D of the Ni(ce)2+ 4p orbital, and peak E of the Ni(ce)2+ 5p orbital. Structure of the [Ni(R,R′-dioxH)2] in Crystal. The experimental XANES spectra for the blind- and cross-type crystals in Figure 6 show a larger preedge peak around 8330 eV, which corresponds to peak A in the calculated transition peaks for the blind- and cross-type models, than that of the solvated hexaaqua Ni(II) complex.17 Several spectra of nickel-oxygen coordinated have also similar large peaks at the same position.18 The preedge
Figure 7. Calculated transition probabilities (vertical bars) and peaks (dashed lines) for the layer- and solvation-type models. The Ni K XANES spectrum (solid line) is shown for comparison. Characters of A to E show the peaks corresponding to the remarkable features in the Ni K XANES spectrum.
peak around 8330 eV is thought to be small peak because the peak is ascribed to the forbidden transition (Ni 1s f 3d transition). And the peak appears normally owing to structure distortion; however, each peak in both the experimental spectra
3474 J. Phys. Chem. B, Vol. 104, No. 15, 2000
Matsuo et al. in the blind-type crystal. The calculated energy level of the highest occupied molecular orbital for the blind-type model is only 2 eV ()193 kJ mol-1) higher than that for the cross-type model due to no contribution from the hydrogen bonds. Thus, it is expected that when there are no hydrogen bonds within a molecule, the cross-type crystal is more stable than the blindtype crystal. For the [Ni(emgH)2], Anex et al.20 described that slow evaporation of the complex gives rise to the precipitation of the blind-type crystal, whereas rapid evaporation yields the cross-type crystal. This phenomenon is explained by the hydrogen bonds mentioned above; the metal-metal interaction in rapid evaporation precludes the possibility of the hydrogen bonds in the stack. The stacking structure overlapped by the metal-metal interactions is made by hydrogen bonds in the stack and by sharing the electrons among metal elements.21 The energy level for Ni 4pz orbital with Ni-Ni interactions is lower than that without Ni-Ni interactions.6 Therefore, it is concluded that the stacking structures begin to be formed from cross-type structures with a rotation of 90° for alternate layers owing to ligand structures. Structure of the [Ni(R,R′-dioxH)2] in Pyridine Solution. In Figure 7, the calculated transitional probabilities and peaks for the layer-type model reproduce the profile well at 83358345 eV in the experimental XANES spectrum, compared with that for the solvation-type model. The MOs of this range for the layer-type model are composed by the Ni(ce)2+-N3-, -O(ax)0, and -Ni(ax)0 molecular orbitals in Table 3, so they look like uniting the MOs of the blind- and cross-type model structures. Therefore, it is concluded that in the stacking structure of the [Ni(R,R′-dioxH)2] in the pyridine solution adjacent molecules slightly deviate from the c0 axis to the Ni(ce)2+ in the molecule
TABLE 1: Constituent Atomic Orbitals and Proportion of the MOs for NiN4 Models Calculated NiN4 model atomic orbitals and proportion/% peaka
energy/eV
A
8330.1
10.27:4p 0.03:3d
B D
8332.1 8348.3
E
8352.8
81.49:4p 127.95:4p -16.61:5p 96.52:5p
a
N3-
Ni(ce)2+
33.45:3p 24.49:3s 32.97:2p 23.68:3p -10.66:3s no orbital
See Figure 6.
a and b, which corresponds to peak A in the calculated transition peaks, is relatively large. As can be seen in Table 2, this large peak is generated not only by structure distortion but also by the hybrid orbitals of Ni(ce)2+ 4p and 3d orbitals. As can be seen in Figure 6, the calculated transition probability for the NiN4 model does not appear at 8335-8345 eV, while those for the blind- and cross-type models appear in the same energy region. The MOs for the blind- and cross-type models in the energy region consist of Ni(ce)2+ 4p-O(ax)0 and Ni(ce)2+ 4p-Ni(ax)0 orbitals, respectively. Thus, it is suggested that the profiles at 8335-8345 eV in the experimental XANES spectra for these Ni(II) complexes are influenced by change in the axial coordination structure of Ni(II). I et al.19 obtained the enthalpy, ∆H ()9.1 kJ mol-1), of transformation from the blind-type to the cross-type crystals of the [Ni(emgH)2] by the differential scanning calorimetry and reported that thermally the blind-type crystal is more stable than the cross-type crystal, because of intramolecular hydrogen bonds
TABLE 2: Constituent Atomic Orbitals and Proportion of the MOs for Blind- and Cross-Type Models Calculated blind-type model
cross-type model
atomic orbitals and proportion/% a
peak
energy/eV
A
8330.1
B C
8334.9 8339.4
6.08:4p 0.36:3d 55.78:4p 11.43:4p
atomic orbitals and proportion/%
N3-
O(ax)0
energy/eV
45.52:3p 24.89:3s 39.34:3p 40.49:3p
15.70:3p
8330.2
less 29.66:3p 18.39:3s
Ni(ce)2+
C′
a
D
8348.6
E
8356.3
134.48:4p -19.29:5p 112.44:5p
Ni(ce)2+
N3-
Ni(ax)0
8334.0
7.00:4p 0.01:3d 22.99:4p
56.25:3s 28.72:2p 37.87:3p
8341.0
31.75:4p
less
123.42:4p -17.24:5p 124.96:5p
no orbital
22.28:4p 27.69:5p 11.70:4d no orbital
no orbital
no orbital
no orbital
no orbital
8347.6
no orbital
no orbital
8359.6
5.04:4p 24.48:4d
See Figure 6.
TABLE 3: Constituent Atomic Orbitals and Proportion of the MOs for Layer- and Solvation-Type Models Calculated layer-type model
solvation-type model
atomic orbitals and proportion/% peaka
energy/eV
A′
8329.8
B C C′
8336.8 8338.0 8342.7
C′′ D
8351.7
E
8357.9
a
See Figure 7.
Ni(ce)2+
N3-
O(ax)0
atomic orbitals and proportion/% Ni(ax)0
energy (eV)
Ni(ce)2+
N3-
N (py)0
6.80:3d 0.05:4p 21.60:4p 20.53:4p 0.67:5p 0.27:4p
59.61:2p
less
9.76:3d
8329.6
0.06:4p
59.78:2p
less
23.06:3p less 17.24:3p
30.23:3p 27.84:3p less
11.95:4d 21.07:4s 20.03:4d
8337.1
8.02:4p
35.36:3p
14.38:3s
121.75:4p -12.20:5p 85.73:5p
no orbital
no orbital
no orbital
8345.3 8351.2
14.37:3s no orbital
10.73:3p no orbital
no orbital
no orbital
no orbital
8361.9
22.26:4p 122.21:4p -12.04:5p 110.87:5p -13.21:4p
no orbital
no orbital
Bis(1,2-Dione dioximato)nickel(II) Complexes due to freedom of each molecule in stacking structure. This result supports that solubility of some of [Ni(R,R′-dioxH)2] depends not only on the metal-metal bond length but also on the nature of the ligand.22 Conclusions The Ni K XANES spectra for the blind- and cross-type crystals of the [Ni(R,R′-dioxH)2] have been assigned by means of the DV-XR calculations. The results indicate that each preedge peak about 8330 eV in both the experimental spectra a and b, which corresponds to peak A in the calculated transition peaks, get rise of mainly 1s f 4p transition of Ni(ce)2+. Furthermore, it is revealed that the unique feature around 8338 eV in the experimental spectrum a, which corresponds to peak C in the calculated transition peaks, is generated by the transition of Ni(ce)2+ 1s orbital to molecular orbital composed of Ni(ce)2+ 4p orbital and O(ax)0 orbital in the axial positions and that the unique feature around 8343 eV in the experimental spectrum b, which corresponds to peak C′ in the calculated transition peaks, is generated by the transition of Ni(ce)2+ 1s orbital to molecular orbital composed of Ni(ce)2+ 4p orbital and Ni(ax)0 orbital in the axial positions. On the other hand, the structures of blind- and cross-type crystals in the pyridine solution are united, and adjacent molecules in the stacking structures slightly deviate from the c0 axis to Ni(ce)2+. Acknowledgment. The present work was supported in part by Grants-in-Aid for Scientific Research (C) (11640618) from Japan Society for the Promotion of Science. The authors are grateful to Professor Hirohiko Adachi of Kyoto University for helpful discussions.
J. Phys. Chem. B, Vol. 104, No. 15, 2000 3475 References and Notes (1) Godycki, L. E.; Rundle, R. E. Acta Crystallogr. 1953, 6, 487. (2) Sharpe, A. G.; Wakefield, D. B. J. Chem. Soc. 1957, 281. (3) Frasson, E.; Panattoni, C. Acta Crystallogr. 1960, 13, 893. (4) Calleri, M.; Ferraris, G. Acta Crystallogr. 1967, 22, 468. (5) Bowers, R. H.; Banks, C. V.; Jacobson, R. A. Acta Crystallogr., Sect. B 1972, 28, 2318. (6) Zahner, J. C.; Drickamer, H. G. J. Chem. Phys. 1960, 33, 1625. (7) Shirotani, I.; Suzuki, K.; Suzuki, T.; Yagi, T.; Tanaka, M. Bull. Chem. Soc. Jpn. 1992, 65, 1078. (8) Ohta, K.; Hasebe, H.; Moriya, M.; Fujimoto, T.; Yamamoto, I. J. Mater. Chem. 1991, 1, 831. (9) Ohta, K.; Ikejima, M.; Moriya, M.; Hasebe, H.; Yamamoto, I. J. Mater. Chem. 1998, 8, 1971. (10) Yamashita, S.; Yanase, Y.; Yamaguchi, T.; Wakita, H. Bull. Chem. Soc. Jpn. 1989, 62, 2902. (11) Valli, M. J.; Matsuo, S.; Wakita, H.; Yamaguchi, T.; Nomura, M. Inorg. Chem. 1996, 35, 5642. (12) Adachi, H.; Tsukada, M.; Satoko, C. J. Phys. Soc. Jpn. 1978, 45, 875. (13) Tanaka, I.; Adachi, H. Phys. ReV. B 1996, 54, 4604. (14) Miao, Q.; Adachi, H.; Tanaka, I.; Xin, X.-Q. J. Phys. Chem. A 1997, 101, 5818. (15) Shibahara, T.; Sakane, G.; Naruse, Y.; Taya, K.; Akashi, H.; Ichimura, A.; Adachi, H. Bull. Chem. Soc. Jpn. 1995, 68, 2769. (16) Adachi, H.; Taniguchi, K. J. Phys. Soc. Jpn. 1980, 49, 1944. (17) Garcia, J.; Benfatto, J. M.; Natoli, C. R.; Bianconi, A.; Fontaine, A.; Tolentino, H. Chem. Phys. 1989, 132, 295. (18) Mansour, A. N.; Melendres, C. A.; Pankuch, M.; Brizzolara, R. A. J. Electrochem. Soc. 1994, 141, L69. (19) I, M.; Wakita, H.; Masuda, I. Bull. Chem. Soc. Jpn. 1983, 56, 1627. (20) Anex, B. G.; Krist, F. K. J. Am. Chem. Soc. 1967, 22, 6114. (21) Yamada, S.; Tsuchida, R. Bull. Chem. Soc. Jpn. 1954, 27, 156. (22) Banks, C. V.; Barnum, D. W. J. Am. Chem. Soc. 1958, 80, 3579.
