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CoIII heterometallic complexes
Polyhedron 25 (2006) 2655–2662 www.elsevier.com/locate/poly
Structures and XPS studies of several 3d–4f cyano-bridged LnIII–FeIII/CoIII heterometallic complexes Takashiro Akitsu *, Yasuaki Einaga
*
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan Received 3 August 2005; accepted 27 March 2006 Available online 18 April 2006
Abstract Characterization and crystal structure determination of 3d–4f cyano-bridged bimetallic assemblies, Ln(DMF)4(H2O)3Co(CN)6 Æ H2O (DMF = N,N 0 -dimethylformamide, Ln = Tb, Dy, Ho and Er) have been newly carried out. We have observed their isostructural features and gradual decreasing cell constants and corresponding changes of local bond distances and angles around the cyano-bridges in accord with the ionic radii changes due to lanthanoid contraction. In order to elucidate the reasons that only Nd–Fe and Nd–Co complexes can exhibit photo-induced magnetic functions, we have also investigated the electronic properties for several analogous 3d–4f complexes, Ln(DMF)4(H2O)3M(CN)6 Æ H2O (Ln = Ce, Nd, Sm, Gd, Tb, Dy, Ho and Er; M = Co and Fe), by means of X-ray photoelectron spectroscopy (XPS). We found that the characteristic bent features of Nd–N–C bond angles and the slight shift of binding energies of Ln 4d3/2 peaks in the 3d–4f systems being closed to mixed-valence states might be the reasons for the novel functions. 2006 Elsevier Ltd. All rights reserved. Keywords: XPS; Crystal structures; 3d–4f Complexes; Cyanide-bridged; Heterometallates
1. Introduction In the recent development of photo-functional inorganic materials [1,2], such as photo-induced magnetization of Fe–Co Prussian blue analogues [3] and other cyanobridged 3d–4d or 3d–5d [4–6], and 3d–4f [7–10] metal complexes, both fundamental and applied aspects have been studied widely. At present, it has been proposed that typical photo-induced magnetization of Fe–Co Prussian blue analogues is derived from electron transfer by the excitation of a charge transfer band [11,12]. In view of designing molecule-based magnets, theoretical prediction and designing strategies for Prussian blue analogues or Prussian blue-like complexes [13,14] exhibiting high Tc have been established [15]. In the field of preparative coordination chemistry, hexacyanometallate ions are commonly employed as building blocks, and additional metal ions or complex moieties are selected to obtain desirable hybrid*
Corresponding authors. Tel.: +81 45 566 1790; fax: +81 45 566 1697. E-mail address: [email protected] (T. Akitsu).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.03.026
functional Prussian blue-like complexes by self-assembling bottom-up processes. In order to clarify the magnetostructural correlation of cyano-bridged bimetallic systems, a large number of hybrid Prussian blue-like complexes have been studied so far. For example, magnetic properties of Ln(DMF)4(H2O)3Fe(CN)6 Æ H2O (DMF = N,N 0 dimethylformamide) complexes have been reported [16]; negligible super-exchange interaction for Ln = Pr, Eu, Er, Sm, Yb, antiferromagnetic interaction for Ln = Ce, Nd, Gd, Dy and ferromagnetic interaction for Ln = Tb, Ho, Tm (Table 1). Crystal structures have also been reported for several other analogous complexes [17–23]. Interestingly, photo-induced magnetization has been reported for only a few 3d–4f complexes, such as Nd(DMF)4(H2O)3M(CN)6 Æ H2O (M = Fe [7] and Co [8]) and Nd(HP)2(H2O)3Fe(CN)6 (HP = 4-hydroxypyridine) [10]. Despite numerous examples of other cyano-bridged 3d–4f complexes [24–34], correlations between structures and electronic properties related to photo-induced magnetization have not been understood clearly. Herein, we report the undetermined crystal structures of 3d–4f cyano-bridged
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Table 1 Magnetic and structural data known for Ln(DMF)4(H2O)3M(CN)6 Æ H2O (M = Co and Fe) ˚) Configurations and terms Magnetism [16] Bridging Ln–N (A Ce–Co Nd–Co Sm–Co Gd–Co Tb–Co Dy–Co Ho–Co Er–Co Ce–Fe Nd–Fe Sm–Fe Gd–Fe Tb–Fe Dy–Fe Ho–Fe Er–Fe
2
1
F5/2– A1g I9/2–1A1g 6 H9/2–1A1g 8 S7/2–1A1g 7 F6–1A1g 6 H15/2–1A1g 5 I8–1A1g 4 I15/2–1A1g 2 F5/2–2T2g 4 I9/2–2T2g 6 H9/2–2T2g 8 S7/2–2T2g 7 F6–2T2g 6 H15/–2T2g 5 I8–2T2g 4 I15/2–2T2g 4
antiferro antiferro negligible antiferro ferro antiferro ferro negligible
bimetallic assemblies, Ln(DMF)4(H2O)3Co(CN)6 Æ H2O (DMF = N,N 0 -dimethylformamide, Ln = Tb, Dy, Ho and Er), and X-ray photoelectron spectroscopy (XPS) studies of several analogous 3d–4f complexes, Ln(DMF)4(H2O)3Co(CN)6 Æ H2O (Ln = Ce, Nd, Sm, Gd, Tb, Dy, Ho and Er; M = Co and Fe). We also discuss both structural and electronic factors changed by substitution of metal elements for a series of cyano-bridged 3d–4f systems in order to elucidate the reasons for photo-induced magnetization. 2. Experimental 2.1. Materials and physical measurements Chemicals of the highest commercial grade available (Aldrich and Wako) were used as received without further purification. Elemental analyses (C, H, N) were carried out on an Elementar Vario EL analyser at Keio University. Infrared spectra were recorded as KBr pellets on a JASCO FT-IR 660 plus spectrophotometer in the range of 4000– 400 cm1 at 298 K. Diffuse reflectance electronic spectra were measured on a JASCO V-560 spectrophotometer equipped with an integrating sphere in the range of 850–220 nm at 298 K. Emission spectra were measured with a JASCO FP-6500 spectrofluorometer in the solid state at 298 K. The mass concentration of LnIII and FeIII or CoIII ions was measured with a HORIBA X-ray analytical microscope XGT-2700 using X-ray fluorescence analyses. XPS spectra were recorded with a JEOL JPS-9000MX at 298 K. Powder samples were pressed as pellets and put under UHV to reach the 108 Pa range. The non-monochromatized Mg Ka source was used at 10 kV and 10 mA, as a flood gun to compensate for the non-conductive samples. The binding energy of the spectra was calibrated in relation to the C1s binding energy (284.0 eV), which was applied as an internal standard.
Bridging Ln–N–C ()
Structures reference
2.579(2) 2.543(5) 2.513(3) 2.516(10) 2.448(5) 2.431(6) 2.422(4) 2.410(4) 2.568(9) 2.523(6) 2.505(5) 2.51(1)
163.0(2) 165.1(4) 163.5(5) 167.5(9) 162.6(4) 162.9(4) 163.2(4) 163.7(3) 165.3(9) 162.3(6) 165.0(4) 164(1)
[16] [22] [16] [22] this this this this [17] [7] [18] [18]
2.398(14)
165.2(14)
[20]
work work work work
2.2. Preparations The complexes were prepared according to the literature procedure [18,22]. Characterization was carried out by elemental analyses, IR spectroscopy (Table S1) and X-ray fluorescence analyses (Table S2) for the 16 complexes investigated. 2.3. X-ray crystallography The previously unknown crystal structures of Tb–Co, Dy–Co, Ho–Co and Er–Co have been determined, and their crystallographic data are summarized in Table 2. The single crystals of prismatic morphology suitable for X-ray crystallography were mounted on a glass fiber coated with epoxy glue. The intensity data were collected on a Rigaku AFC-7R diffractometer with graphite mono˚ ) radiation using x–2h chromated Mo Ka (k = 0.7073 A scan techniques at 297 K. The calculations were performed on an SGI O2 workstation with the TEXSAN [35] software package. Empirical absorption corrections were applied based on w scans. No significant decay in the intensity of three standard reflections was observed throughout the data collection. The structures were solved by direct methods using SIR-92 [36] and refined on F2 anisotropically for non-hydrogen atoms by full-matrix least-squares methods with SHELXL-97 [37]. The hydrogen atoms, except for hydrogen atoms connected with O5, O6, O7 and O8 as water molecules, were added at geometrically expected positions with bond distances of ˚ , and they were refined isotropically as riding 0.96 A models. Residual electron density could not found in the difference Fourier maps completely. The refinement with the models containing additional water molecules resulted in an increase of R values. Therefore, only one oxygen atom was assigned as water molecules of crystallization for the refinement carried out.
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Table 2 Crystallographic data for Tb–Co, Dy–Co, Ho–Co and Er–Co Compound
Tb–Co
Formula C18H36CoN10O8Tb Molecular weight 738.40 Crystal system monoclinic Space group P21/a (#14) ˚) a (A 24.826(16) ˚) b (A 8.872(5) ˚) c (A 13.931(6) b () 96.34(4) ˚ 3) V (A 3050(3) Z 4 Dcalc (g cm3) 1.608 F(000) 1480 l (mm1) 2.898 Crystal size (mm) 0.50 · 0.20 · 0.20 Unique reflections 7013 Observed reflections (I > 2r(I)) 5554 Parameters 343 R1a, Rwb, S 0.0399, 0.1189, 0.982 ˚ 3) 3.469 and 1.393 Minimum and maximum residual densities (e/A P P a R1= iFoj jFci/ jFoj. P P b Rw ¼ ð wðjF o j jF c jÞ2 = wjF o j2 Þ1=2 ; w ¼ 1=ðr2 ðF o Þ þ ð0:1P Þ2 Þ where P
3. Results and discussion 3.1. Crystal structures
Dy–Co
Ho–Co
Er–Co
C18H36CoDyN10O8 742.00 monoclinic P21/a (#14) 24.79(4) 8.870(9) 13.92(1) 96.4(1) 3042(6) 4 1.619 1484 3.036 0.30 · 0.30 · 0.30 6977 6138 343 0.0404, 0.1431, 1.253 2.094 and 1.466
C18H36CoHoN10O8 744.41 monoclinic P21/a (#14) 24.770(5) 8.8633(18) 13.918(2) 96.279(15) 3037.3(10) 4 1.628 1488 3.190 0.30 · 0.30 · 0.30 6993 5612 343 0.0364, 0.1121, 0.958 2.783 and 1.812
C18H36CoErN10O8 746.74 monoclinic P21/a (#14) 24.722(4) 8.858(1) 13.893(2) 96.36(1) 3023.7(7) 4 1.640 1492 3.355 0.50 · 0.50 · 0.50 6935 6273 343 0.0340, 0.1290, 1.160 1.242 and 1.240
¼ ðF 2o þ 2jF 2c jÞ=3.
Table 3 ˚ , ) for Tb–Co, Dy–Co, Ho–Co and Selected geometric parameters (A Er–Co Compound
The molecular structures of Tb–Co are depicted in Fig. 1. Selected bond lengths and angles for Tb–Co, Dy– Co, Ho–Co and Er–Co are given in Table 3. The isostructural four heterometallate complexes crystallize in the monoclinic space group P21/a with Z = 4, showing slight differences of cell parameters and geometrical parameters. The LnIII ions are eight-coordinated with a distorted dodecahedral geometry, which consists of seven oxygen
Ln1–O1 Ln1–O2 Ln1–O3 Ln1–O4 Ln1–O5 Ln1–O6 Ln1–O7 Ln1–N6 Co1–C1 Co1–C2 Co1–C3 Co1–C4 Co1–C5 Co1–C6 C„N (mean) C6–N6 Co1–C1–N1 Co1–C2–N2 Co1–C3–N3 Co1–C4–N4 Co1–C5–N5 Co1–C6–N6 Co–C„N (mean) Ln1–N6–C6
Fig. 1. The molecular structure of Tb–Co showing the atomic labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are omitted for clarity.
Tb–Co 2.364(4) 2.358(4) 2.375(5) 2.346(4) 2.398(4) 2.390(4) 2.411(4) 2.448(5) 1.898(6) 1.890(5) 1.912(6) 1.907(6) 1.899(5) 1.887(5) 1.145 1.146(7) 177.0(6) 178.5(5) 177.5(5) 179.3(5) 178.4(5) 177.0(5) 178.0 162.6(4)
Dy–Co 2.355(4) 2.353(5) 2.371(5) 2.338(4) 2.386(4) 2.370(4) 2.406(4) 2.431(6) 1.887(6) 1.890(5) 1.911(6) 1.891(6) 1.891(5) 1.886(5) 1.151 1.150(7) 176.8(6) 178.5(5) 178.2(5) 179.7(5) 178.0(5) 176.8(4) 178.0 162.9(4)
Ho–Co 2.355(4) 2.347(4) 2.358(4) 2.328(4) 2.384(3) 2.368(3) 2.389(3) 2.422(4) 1.890(5) 1.892(5) 1.906(5) 1.896(5) 1.894(5) 1.886(5) 1.148 1.145(6) 176.8(6) 178.5(5) 177.3(5) 179.3(5) 178.3(4) 177.1(4) 178.9 163.2(4)
Er–Co 2.343(3) 2.329(3) 2.352(4) 2.318(3) 2.359(3) 2.351(4) 2.382(3) 2.410(4) 1.894(5) 1.890(4) 1.907(4) 1.885(4) 1.895(4) 1.887(4) 1.150 1.142(5) 176.4(5) 177.3(4) 177.6(4) 179.6(5) 178.9(4) 176.8(4) 177.8 163.7(3)
atoms (four DMF ligands and three coordinated water molecules) and one nitrogen atom of the bridging cyanide ligand. The former Ln–O bond distances are shorter than the latter Ln–N6 ones. The order of the Ln–O bond distances are Tb–Co > Dy–Co > Ho–Co > Er–Co, which is attributed to the typical lanthanoid contraction [38]. The features are also similar to the related Ln–Fe complexes
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[16–20]. The magnitude of the Ln1–C6–N6 bond angles of the bridging cyanide ligands is similar, as follows: 162.6(4), 162.9(4), 163.2(4) and 163.7(3) for Tb–Co, Dy–Co, Ho–Co and Er–Co, respectively. However, it should be noted that the Ln1–C6–N6 bond angles in the Nd–Co and Nd–Fe complexes showed exceptional values, that is Ce–Co (163.0(2)) < Nd–Co (165.1(4)) > Sm–Co (163.5(5)), while Ce–Fe (165.3(9)) > Nd–Fe (162.3(6)) < Sm–Fe (165.0(4)). These characteristic bent-bond features are similar to the proposed photo-excited structures of Nd–M complexes by powder XRD patterns [7]. The [Co(CN)6]3 moieties adopt a slightly distorted octahedral coordination environment and no obvious differences are observed among the four complexes. Because hydrogen atoms could not be introduced completely, we cannot discuss intramolecular or intermolecular hydrogen bonding interactions at present. Intramolecular hydrogen bonds are formed around the DMF ligands (O1–O4) and coordination water (O5–O7) atoms. Intermolecular hydrogen bonds are formed along the crystallographic a axis via solvent water (O8) to create the two-dimensional networks. Possible intermolecular hydrogen bonding distances between the cyanide nitrogen and solvent water O8 N4i ˚ for Tb–Co, are 2.824(7), 2.848(7), 2.851(5) and 2.831(6) A Dy–Co, Ho–Co and Er–Co, respectively [symmetry code: (i) x 3/2, y + 1/2, z]. On the other hand, possible intermolecular hydrogen bonds between the cyanide nitrogen and coordinated water of the adjacent molecules N5 O5ii ˚ for Tb–Co, are 2.712(5), 2.717(5), 2.719(4) and 2.715(5) A Dy–Co, Ho–Co and Er–Co, respectively [symmetry code: (ii) x + 5/2, y + 1/2, z + 2]. 3.2. X-ray photoelectron spectroscopy (XPS) The intensity is plotted by arbitrary scales for XPS in this paper. The C 1s, N 1s and O 1s peaks for 16 Ln–M complexes are summarized in Table 4, and the corresponding spectra for eight Ln–Fe and Ln–Co complexes are Table 4 The XPS C 1s, N 1s and O 1s peaks (eV) for Ln–M complexes Compound
C 1s
N 1s
O 1s
Ce–Co Nd–Co Sm–Co Gd–Co Tb–Co Dy–Co Ho–Co Er–Co Ce–Fe Nd–Fe Sm–Fe Gd–Fe Tb–Fe Dy–Fe Ho–Fe Er–Fe
284 284 284 284 284 284 284 284 284 284 284 284 284 284 284 284
396 396 395 395 397 397 397 397 397 397 397 396 396 396 396 396
529 530 530 530 531 531 531 530 531 530 530 530 530 530 530 530
shown in Figs. S1 and S2, respectively. For example, distinct and sharp peaks of the XP spectra for Tb–Co can be assigned as follows: Tb 4d3/2 around 154 eV, C 1s at 284 eV, N 1s at 397 eV, O 1s at 531 eV, Co 2p3/2 at 781 eV, Co 2p3/2 at 796 eV [39–41]. No significant differences could be observed for the C 1s, N 1s and O 1s peaks of the ligand moieties. The narrow survey XPS of Co 2p3/2 for Ln–Co and Fe 2p3/2 for Ln–Fe complexes are shown in Figs. S3–S6, respectively. The Co 2p3/2 peak binding energies are at 785.1, 784,7, 785.0, 784.8, 785.4, 785.3, 785.4 and 785.1 eV for Ce–Co, Nd–Co, Sm–Co, Gd–Co, Tb–Co, Dy–Co, Ho–Co and Er–Co, respectively. While the Fe 2p3/2 peak binding energies are at 712.7, 712.3, 712.0, 711.7, 712.6, 712.1, 711.9, 712.5 eV and the Fe 2p1/2 peaks are at 726.4, 724.8, 725.3, 724.7, 725.9, 726.9, 726.3, 726.6 eV for Ce–Fe, Nd–Fe, Sm–Fe, Gd–Fe, Tb–Fe, Dy– Fe, Ho–Fe and Er–Fe, respectively. These peaks are ascribed to CoIII (or FeIII) oxidation states. Thus, no remarkable shift of peaks could be observed for FeIII or CoIII ions regardless of the LnIII ions. Furthermore, we could not clearly observe shake-up satellites, typical features of anti-bonding p molecular orbitals (p–p* transitions). Figs. 2–5 show the narrow survey Ce and Nd (Fig. 2), Sm and Gd (Fig. 3), Tb and Dy (Fig. 4), and Ho and Er (Fig. 5) 4d3/2 XPS for 16 Ln–M complexes. The Ce 4d3/2 peak binding energies are at 114.0 and 114.7 eV for Ce–Co and Ce–Fe, Nd 4d3/2 are at 124.7 and 125.4 eV for Nd–Co and Nd–Fe, Sm 4d3/2 are at 134.6 and 135.8 eV for Sm–Co and Sm–Fe, Gd 4d3/2 are at 144.3 and 145.4 eV for Nd–Co and Nd–Fe, Tb 4d3/2 are at 153.7 and 153.1 eV for Tb–Co and Tb–Fe, Dy 4d3/2 are at 159.7 and 159.2 eV for Dy–Co and Dy–Fe, Ho 4d3/2 are at 165.6 and 165.3 eV for Ho–Co and Ho–Fe, Er 4d3/2 are at 172.2 and 171.8 eV for Er–Co and Er–Fe, respectively. These peaks are ascribed to LnIII oxidation states, and the binding energies of Ln–Co complexes are lower than that of Ln–Fe complexes for Ln = Ce, Nd, Sm and Gd (the number of 4f electrons are less than 7). In contrast, the binding energies of Ln–Co complexes are similar or higher than that of Ln–Fe complexes for Ln = Tb, Dy, Ho and Er (the number of 4f electrons are more than 7). Commonly, XPS was used to investigate valence states of metal ions and coordination bonding natures in the field of coordination chemistry [42,43]. For example, it has been reported that XPS binding energies indicate good correlations between substituted groups in Au complexes [44], mixed valence in Au complexes [45], electron transfer in tungsten complexes [46], magnetic properties in copper complexes [47,48] and Moessbauer isomer shift in iron complexes [49]. Recently, inorganic solids containing LnIII ions were also measured by using synchrotron radiation [50]. According to them, the bonds with more electronegative atoms would result in greater positive XPS chemical shifts. The peaks of higher oxidized ions would appear with
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Nd-Co
Nd-Fe Nd 4d3/2
Ce-Co
Ce-Fe Ce 4d3/2 150
145
140
135
130
125
120
115
110
105
100
95
90
Binding energy/eV Fig. 2. Narrow survey Ce or Nd 4d3/2 XPS for Ce–M or Nd–M (M = Co and Fe).
Gd-Co
Gd-Fe Gd 4d3/2
Sm-Co
Sm-Fe Sm 4d3/2 180 175 170 165 160 155 150 145 140 135 130 125 120 115 110 105 100
Binding energy/eV Fig. 3. Narrow survey Sm or Gd 4d3/2 XPS for Sm–M or Gd–M (M = Co and Fe).
Dy-Co
Dy 4d3/2
Dy-Fe
Tb-Co
Tb 4d3/2
Tb-Fe 180
175 170
165 160
155
150 145
140 135
130 125
120
Binding energy/eV Fig. 4. Narrow survey Tb or Dy 4d3/2 XPS for Tb–M or Dy–M (M = Co and Fe).
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Er-Co Er 4d3/2
Er-Fe
Ho 4d3/2
Ho-Co
Ho-Fe 190
185
180
175
170 165 160 Binding energy/eV
155
150
145
140
Fig. 5. Narrow survey Ho or Er 4d3/2 XPS for Ho–M or Er–M (M = Co and Fe).
higher binding energies. Accordingly, two kinds of Ln 4d3/2 XPS peak shifts suggest that the cyano-bridged Ln ions of Ln–Fe complexes for Ln = Ce, Nd, Sm and Gd easily release electrons even under these conditions. In other words, these selected Ln ions are easy to take mixedvalence states.
(Dy3+: 6H15/2 ground state), Ho–M (Ho3+: 5I8 ground state) and Er–M (Er3+: 4I15/2 ground state) (M = Co and Fe). In this way, visible light-sensitivity depends on Ln ions for these complexes. Although we have also tried to measure emission spectra (kex = 250, 300 and 350 nm) in the solid states at 298 K, we could not detect emission from these complexes under these experimental conditions.
3.3. IR and electronic spectra 3.4. Electronic properties of 3d–4f cyanide-bridged systems As listed in Table 1, cyanide stretching bands of the IR spectra of Ln–Co and Ln–Fe complexes are in the range of 2127–2137 cm1 and 2123–2128 cm1, respectively. As for the simple K3[Fe(CN)6] complex in Oh point group, normal vibration modes were assigned as follows: 2315 cm1 (A1g), 2130 cm1 (Eg) and 2118 cm1 (F1u) [51]. Despite being sensitive to valence or spin states, the present values are comparable to analogous 3d–4f bimetallic assemblies. By comparing with the related 3d–3d or 3d–4f complexes [52], spectral shift features, that is substitution of CoIII and FeIII ions, is more effective than substitution of LnIII ions. The spectral features of the IR spectra should also correspond to shifts in the charge transfer absorption bands between the 3d orbitals of MIII ions and the unoccupied p* orbitals of bridging cyanide ligands. The splitting by bridged and uncoordinated cyanide ligands was unclear for the present cases. On the other hand, electronic spectra (Figs. S7–S10) exhibit strong and broad bands in the UV region due to p–p* transitions from ligands of both DMF and low-spin [Co(CN)6]3 or low-spin [Fe(CN)6]3 moieties. The electronic spectra for the Ln–Fe complexes show two peaks around 23 000–32 000 cm1 and 30 000–34 000 cm1, while Ln–Co complexes show a peak around 28 000– 32 000 cm1. The spectral shapes of the UV region depend on the electronic states of the [M(CN)6]3 moieties. In addition, several sharp peaks could be observed in the visible region for Nd–M (Nd3+: 4I9/2 ground state), Dy–M
The effect of interaction between LnIII and MIII ions through cyano-bridges can lead to a different charge distribution, overshadowing the decreasing extent of p-backdonation. As elucidated in the present results, this effect is mixed by both structural factors and electronic factors and appeared for specific LnIII elements remarkable for the ground states of these complexes. As for 3d–3d Prussian blue analogues, the electron configurations corresponding to the strongest ferromagnetic interaction are t32g –t62g e2g , while weak antiferromagnetic interaction are tm2g –tn2g e1g or tm2g –tn2g e2g . Unpaired electrons in the t2g orbitals can be used for superexchange interactions through unoccupied p* orbitals of cyanide bridging ligands. So oxidation states as well as the kind of 3d elements are important for the mechanism of these magnetic interactions. Indeed, it has been elucidated that in cyano-bridged Mo–M systems, containing [Mo(CN)8]n moieties, MoIII ions show larger exchange coupling constants than the equivalent compounds with first transition series elements due to the more diffuse character of the 4d orbitals in comparison with the 3d orbitals [15]. The nature of the coordination bonds of the [Fe(CN)6]3 moiety is p-backdonation, in which five 3d electrons occupying t2g orbitals are provided to unoccupied p* orbitals of the cyanide ligands. Recently, detailed electronic states have been confirmed by means of X-ray absorption spectra [53–56]. In Oh symmetry, the electron
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configuration of the FeIII ion is t52g e0g and irreducible representations of 2p orbitals of the cyanide ligands are t1g, t1u, t2g and t2u. Qualitatively speaking, the formation of a [Fe(CN)6]3 complex results in molecular orbitals which are split into occupied t2g, degenerated t1g, t1u and t2u, and t2g orbitals and unoccupied degenerated t1g, t1u and t2u, eg and t2g orbitals. When the electrons of cyanides are excited to p* orbitals (formed from unoccupied t1g, t1u and t2u orbitals) associated with metal to ligand charge transfer from the t52g orbital in the FeIII ion to unoccupied p* orbitals in cyanide ligands [56], p-backdonation may be weaken because of electronic repulsion of electrons in the unoccupied p* orbitals. 4. Conclusion Consequently, we can point out characteristic structural and electronic features for photo-induced magnetization of Nd–M complexes. Systematic crystal structures revealed that gradually changes of isostructural crystals due to lanthanoid contraction lead to remarkable Ln–O coordination bond distances or a decrease of the cell volume. However, only Nd–Fe and Nd–Co complexes show exceptional bent Ln–N–C bond angles, which may be proposed photoexcited structures of bent-bonds. The patterns of Ln 4d3/2 XPS peak shifts for Ln = Ce, Nd, Sm and Gd differ from that of Ln = Tb, Dy, Ho and Er, which suggests that the cyano-bridged LnIII ions of Ln–Fe complexes for Ln = Ce, Nd, Sm and Gd easily become mixed-valence states. In addition, absorption spectra exhibit that p-backdonation of cyano-bridging ligands uses unoccupied p* orbitals and unoccupied t2g orbitals in [Fe(CN)6]3 moieties. Further studies on the structural and electronic natures of other cyano-bridging heterometallic assemblies are in progress in order to rationalize the design of multi-functional materials which consist of (transition) metal complexes. Acknowledgements This work was supported by a Grant-in-Aid for the 21st Century COE program ‘KEIO Life Conjugate Chemistry’ form the Ministry of Education, Culture, Sports, Science, and Technology, Research Foundation for Opto-Science and Technology, and Japan and Mizuho Foundation for the Promotion of Science. Appendix A. Supplementary data Crystallographic information is available (CCDC Nos. 279742, 279743, 279744 and 279745 for Tb–Co, Dy–Co, Ho–Co and Er–Co, respectively) from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail: [email protected]). Characterization data, XPS and diffuse reflectance electronic spectra have been deposited as supplementary data. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2006.03.026.
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Structures and XPS studies of several 3d–4f cyano-bridged LnIII–FeIII/CoIII heterometallic complexes Takashiro Akitsu *, Yasuaki Einaga
*
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan Received 3 August 2005; accepted 27 March 2006 Available online 18 April 2006
Abstract Characterization and crystal structure determination of 3d–4f cyano-bridged bimetallic assemblies, Ln(DMF)4(H2O)3Co(CN)6 Æ H2O (DMF = N,N 0 -dimethylformamide, Ln = Tb, Dy, Ho and Er) have been newly carried out. We have observed their isostructural features and gradual decreasing cell constants and corresponding changes of local bond distances and angles around the cyano-bridges in accord with the ionic radii changes due to lanthanoid contraction. In order to elucidate the reasons that only Nd–Fe and Nd–Co complexes can exhibit photo-induced magnetic functions, we have also investigated the electronic properties for several analogous 3d–4f complexes, Ln(DMF)4(H2O)3M(CN)6 Æ H2O (Ln = Ce, Nd, Sm, Gd, Tb, Dy, Ho and Er; M = Co and Fe), by means of X-ray photoelectron spectroscopy (XPS). We found that the characteristic bent features of Nd–N–C bond angles and the slight shift of binding energies of Ln 4d3/2 peaks in the 3d–4f systems being closed to mixed-valence states might be the reasons for the novel functions. 2006 Elsevier Ltd. All rights reserved. Keywords: XPS; Crystal structures; 3d–4f Complexes; Cyanide-bridged; Heterometallates
1. Introduction In the recent development of photo-functional inorganic materials [1,2], such as photo-induced magnetization of Fe–Co Prussian blue analogues [3] and other cyanobridged 3d–4d or 3d–5d [4–6], and 3d–4f [7–10] metal complexes, both fundamental and applied aspects have been studied widely. At present, it has been proposed that typical photo-induced magnetization of Fe–Co Prussian blue analogues is derived from electron transfer by the excitation of a charge transfer band [11,12]. In view of designing molecule-based magnets, theoretical prediction and designing strategies for Prussian blue analogues or Prussian blue-like complexes [13,14] exhibiting high Tc have been established [15]. In the field of preparative coordination chemistry, hexacyanometallate ions are commonly employed as building blocks, and additional metal ions or complex moieties are selected to obtain desirable hybrid*
Corresponding authors. Tel.: +81 45 566 1790; fax: +81 45 566 1697. E-mail address: [email protected] (T. Akitsu).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.03.026
functional Prussian blue-like complexes by self-assembling bottom-up processes. In order to clarify the magnetostructural correlation of cyano-bridged bimetallic systems, a large number of hybrid Prussian blue-like complexes have been studied so far. For example, magnetic properties of Ln(DMF)4(H2O)3Fe(CN)6 Æ H2O (DMF = N,N 0 dimethylformamide) complexes have been reported [16]; negligible super-exchange interaction for Ln = Pr, Eu, Er, Sm, Yb, antiferromagnetic interaction for Ln = Ce, Nd, Gd, Dy and ferromagnetic interaction for Ln = Tb, Ho, Tm (Table 1). Crystal structures have also been reported for several other analogous complexes [17–23]. Interestingly, photo-induced magnetization has been reported for only a few 3d–4f complexes, such as Nd(DMF)4(H2O)3M(CN)6 Æ H2O (M = Fe [7] and Co [8]) and Nd(HP)2(H2O)3Fe(CN)6 (HP = 4-hydroxypyridine) [10]. Despite numerous examples of other cyano-bridged 3d–4f complexes [24–34], correlations between structures and electronic properties related to photo-induced magnetization have not been understood clearly. Herein, we report the undetermined crystal structures of 3d–4f cyano-bridged
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Table 1 Magnetic and structural data known for Ln(DMF)4(H2O)3M(CN)6 Æ H2O (M = Co and Fe) ˚) Configurations and terms Magnetism [16] Bridging Ln–N (A Ce–Co Nd–Co Sm–Co Gd–Co Tb–Co Dy–Co Ho–Co Er–Co Ce–Fe Nd–Fe Sm–Fe Gd–Fe Tb–Fe Dy–Fe Ho–Fe Er–Fe
2
1
F5/2– A1g I9/2–1A1g 6 H9/2–1A1g 8 S7/2–1A1g 7 F6–1A1g 6 H15/2–1A1g 5 I8–1A1g 4 I15/2–1A1g 2 F5/2–2T2g 4 I9/2–2T2g 6 H9/2–2T2g 8 S7/2–2T2g 7 F6–2T2g 6 H15/–2T2g 5 I8–2T2g 4 I15/2–2T2g 4
antiferro antiferro negligible antiferro ferro antiferro ferro negligible
bimetallic assemblies, Ln(DMF)4(H2O)3Co(CN)6 Æ H2O (DMF = N,N 0 -dimethylformamide, Ln = Tb, Dy, Ho and Er), and X-ray photoelectron spectroscopy (XPS) studies of several analogous 3d–4f complexes, Ln(DMF)4(H2O)3Co(CN)6 Æ H2O (Ln = Ce, Nd, Sm, Gd, Tb, Dy, Ho and Er; M = Co and Fe). We also discuss both structural and electronic factors changed by substitution of metal elements for a series of cyano-bridged 3d–4f systems in order to elucidate the reasons for photo-induced magnetization. 2. Experimental 2.1. Materials and physical measurements Chemicals of the highest commercial grade available (Aldrich and Wako) were used as received without further purification. Elemental analyses (C, H, N) were carried out on an Elementar Vario EL analyser at Keio University. Infrared spectra were recorded as KBr pellets on a JASCO FT-IR 660 plus spectrophotometer in the range of 4000– 400 cm1 at 298 K. Diffuse reflectance electronic spectra were measured on a JASCO V-560 spectrophotometer equipped with an integrating sphere in the range of 850–220 nm at 298 K. Emission spectra were measured with a JASCO FP-6500 spectrofluorometer in the solid state at 298 K. The mass concentration of LnIII and FeIII or CoIII ions was measured with a HORIBA X-ray analytical microscope XGT-2700 using X-ray fluorescence analyses. XPS spectra were recorded with a JEOL JPS-9000MX at 298 K. Powder samples were pressed as pellets and put under UHV to reach the 108 Pa range. The non-monochromatized Mg Ka source was used at 10 kV and 10 mA, as a flood gun to compensate for the non-conductive samples. The binding energy of the spectra was calibrated in relation to the C1s binding energy (284.0 eV), which was applied as an internal standard.
Bridging Ln–N–C ()
Structures reference
2.579(2) 2.543(5) 2.513(3) 2.516(10) 2.448(5) 2.431(6) 2.422(4) 2.410(4) 2.568(9) 2.523(6) 2.505(5) 2.51(1)
163.0(2) 165.1(4) 163.5(5) 167.5(9) 162.6(4) 162.9(4) 163.2(4) 163.7(3) 165.3(9) 162.3(6) 165.0(4) 164(1)
[16] [22] [16] [22] this this this this [17] [7] [18] [18]
2.398(14)
165.2(14)
[20]
work work work work
2.2. Preparations The complexes were prepared according to the literature procedure [18,22]. Characterization was carried out by elemental analyses, IR spectroscopy (Table S1) and X-ray fluorescence analyses (Table S2) for the 16 complexes investigated. 2.3. X-ray crystallography The previously unknown crystal structures of Tb–Co, Dy–Co, Ho–Co and Er–Co have been determined, and their crystallographic data are summarized in Table 2. The single crystals of prismatic morphology suitable for X-ray crystallography were mounted on a glass fiber coated with epoxy glue. The intensity data were collected on a Rigaku AFC-7R diffractometer with graphite mono˚ ) radiation using x–2h chromated Mo Ka (k = 0.7073 A scan techniques at 297 K. The calculations were performed on an SGI O2 workstation with the TEXSAN [35] software package. Empirical absorption corrections were applied based on w scans. No significant decay in the intensity of three standard reflections was observed throughout the data collection. The structures were solved by direct methods using SIR-92 [36] and refined on F2 anisotropically for non-hydrogen atoms by full-matrix least-squares methods with SHELXL-97 [37]. The hydrogen atoms, except for hydrogen atoms connected with O5, O6, O7 and O8 as water molecules, were added at geometrically expected positions with bond distances of ˚ , and they were refined isotropically as riding 0.96 A models. Residual electron density could not found in the difference Fourier maps completely. The refinement with the models containing additional water molecules resulted in an increase of R values. Therefore, only one oxygen atom was assigned as water molecules of crystallization for the refinement carried out.
T. Akitsu, Y. Einaga / Polyhedron 25 (2006) 2655–2662
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Table 2 Crystallographic data for Tb–Co, Dy–Co, Ho–Co and Er–Co Compound
Tb–Co
Formula C18H36CoN10O8Tb Molecular weight 738.40 Crystal system monoclinic Space group P21/a (#14) ˚) a (A 24.826(16) ˚) b (A 8.872(5) ˚) c (A 13.931(6) b () 96.34(4) ˚ 3) V (A 3050(3) Z 4 Dcalc (g cm3) 1.608 F(000) 1480 l (mm1) 2.898 Crystal size (mm) 0.50 · 0.20 · 0.20 Unique reflections 7013 Observed reflections (I > 2r(I)) 5554 Parameters 343 R1a, Rwb, S 0.0399, 0.1189, 0.982 ˚ 3) 3.469 and 1.393 Minimum and maximum residual densities (e/A P P a R1= iFoj jFci/ jFoj. P P b Rw ¼ ð wðjF o j jF c jÞ2 = wjF o j2 Þ1=2 ; w ¼ 1=ðr2 ðF o Þ þ ð0:1P Þ2 Þ where P
3. Results and discussion 3.1. Crystal structures
Dy–Co
Ho–Co
Er–Co
C18H36CoDyN10O8 742.00 monoclinic P21/a (#14) 24.79(4) 8.870(9) 13.92(1) 96.4(1) 3042(6) 4 1.619 1484 3.036 0.30 · 0.30 · 0.30 6977 6138 343 0.0404, 0.1431, 1.253 2.094 and 1.466
C18H36CoHoN10O8 744.41 monoclinic P21/a (#14) 24.770(5) 8.8633(18) 13.918(2) 96.279(15) 3037.3(10) 4 1.628 1488 3.190 0.30 · 0.30 · 0.30 6993 5612 343 0.0364, 0.1121, 0.958 2.783 and 1.812
C18H36CoErN10O8 746.74 monoclinic P21/a (#14) 24.722(4) 8.858(1) 13.893(2) 96.36(1) 3023.7(7) 4 1.640 1492 3.355 0.50 · 0.50 · 0.50 6935 6273 343 0.0340, 0.1290, 1.160 1.242 and 1.240
¼ ðF 2o þ 2jF 2c jÞ=3.
Table 3 ˚ , ) for Tb–Co, Dy–Co, Ho–Co and Selected geometric parameters (A Er–Co Compound
The molecular structures of Tb–Co are depicted in Fig. 1. Selected bond lengths and angles for Tb–Co, Dy– Co, Ho–Co and Er–Co are given in Table 3. The isostructural four heterometallate complexes crystallize in the monoclinic space group P21/a with Z = 4, showing slight differences of cell parameters and geometrical parameters. The LnIII ions are eight-coordinated with a distorted dodecahedral geometry, which consists of seven oxygen
Ln1–O1 Ln1–O2 Ln1–O3 Ln1–O4 Ln1–O5 Ln1–O6 Ln1–O7 Ln1–N6 Co1–C1 Co1–C2 Co1–C3 Co1–C4 Co1–C5 Co1–C6 C„N (mean) C6–N6 Co1–C1–N1 Co1–C2–N2 Co1–C3–N3 Co1–C4–N4 Co1–C5–N5 Co1–C6–N6 Co–C„N (mean) Ln1–N6–C6
Fig. 1. The molecular structure of Tb–Co showing the atomic labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are omitted for clarity.
Tb–Co 2.364(4) 2.358(4) 2.375(5) 2.346(4) 2.398(4) 2.390(4) 2.411(4) 2.448(5) 1.898(6) 1.890(5) 1.912(6) 1.907(6) 1.899(5) 1.887(5) 1.145 1.146(7) 177.0(6) 178.5(5) 177.5(5) 179.3(5) 178.4(5) 177.0(5) 178.0 162.6(4)
Dy–Co 2.355(4) 2.353(5) 2.371(5) 2.338(4) 2.386(4) 2.370(4) 2.406(4) 2.431(6) 1.887(6) 1.890(5) 1.911(6) 1.891(6) 1.891(5) 1.886(5) 1.151 1.150(7) 176.8(6) 178.5(5) 178.2(5) 179.7(5) 178.0(5) 176.8(4) 178.0 162.9(4)
Ho–Co 2.355(4) 2.347(4) 2.358(4) 2.328(4) 2.384(3) 2.368(3) 2.389(3) 2.422(4) 1.890(5) 1.892(5) 1.906(5) 1.896(5) 1.894(5) 1.886(5) 1.148 1.145(6) 176.8(6) 178.5(5) 177.3(5) 179.3(5) 178.3(4) 177.1(4) 178.9 163.2(4)
Er–Co 2.343(3) 2.329(3) 2.352(4) 2.318(3) 2.359(3) 2.351(4) 2.382(3) 2.410(4) 1.894(5) 1.890(4) 1.907(4) 1.885(4) 1.895(4) 1.887(4) 1.150 1.142(5) 176.4(5) 177.3(4) 177.6(4) 179.6(5) 178.9(4) 176.8(4) 177.8 163.7(3)
atoms (four DMF ligands and three coordinated water molecules) and one nitrogen atom of the bridging cyanide ligand. The former Ln–O bond distances are shorter than the latter Ln–N6 ones. The order of the Ln–O bond distances are Tb–Co > Dy–Co > Ho–Co > Er–Co, which is attributed to the typical lanthanoid contraction [38]. The features are also similar to the related Ln–Fe complexes
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[16–20]. The magnitude of the Ln1–C6–N6 bond angles of the bridging cyanide ligands is similar, as follows: 162.6(4), 162.9(4), 163.2(4) and 163.7(3) for Tb–Co, Dy–Co, Ho–Co and Er–Co, respectively. However, it should be noted that the Ln1–C6–N6 bond angles in the Nd–Co and Nd–Fe complexes showed exceptional values, that is Ce–Co (163.0(2)) < Nd–Co (165.1(4)) > Sm–Co (163.5(5)), while Ce–Fe (165.3(9)) > Nd–Fe (162.3(6)) < Sm–Fe (165.0(4)). These characteristic bent-bond features are similar to the proposed photo-excited structures of Nd–M complexes by powder XRD patterns [7]. The [Co(CN)6]3 moieties adopt a slightly distorted octahedral coordination environment and no obvious differences are observed among the four complexes. Because hydrogen atoms could not be introduced completely, we cannot discuss intramolecular or intermolecular hydrogen bonding interactions at present. Intramolecular hydrogen bonds are formed around the DMF ligands (O1–O4) and coordination water (O5–O7) atoms. Intermolecular hydrogen bonds are formed along the crystallographic a axis via solvent water (O8) to create the two-dimensional networks. Possible intermolecular hydrogen bonding distances between the cyanide nitrogen and solvent water O8 N4i ˚ for Tb–Co, are 2.824(7), 2.848(7), 2.851(5) and 2.831(6) A Dy–Co, Ho–Co and Er–Co, respectively [symmetry code: (i) x 3/2, y + 1/2, z]. On the other hand, possible intermolecular hydrogen bonds between the cyanide nitrogen and coordinated water of the adjacent molecules N5 O5ii ˚ for Tb–Co, are 2.712(5), 2.717(5), 2.719(4) and 2.715(5) A Dy–Co, Ho–Co and Er–Co, respectively [symmetry code: (ii) x + 5/2, y + 1/2, z + 2]. 3.2. X-ray photoelectron spectroscopy (XPS) The intensity is plotted by arbitrary scales for XPS in this paper. The C 1s, N 1s and O 1s peaks for 16 Ln–M complexes are summarized in Table 4, and the corresponding spectra for eight Ln–Fe and Ln–Co complexes are Table 4 The XPS C 1s, N 1s and O 1s peaks (eV) for Ln–M complexes Compound
C 1s
N 1s
O 1s
Ce–Co Nd–Co Sm–Co Gd–Co Tb–Co Dy–Co Ho–Co Er–Co Ce–Fe Nd–Fe Sm–Fe Gd–Fe Tb–Fe Dy–Fe Ho–Fe Er–Fe
284 284 284 284 284 284 284 284 284 284 284 284 284 284 284 284
396 396 395 395 397 397 397 397 397 397 397 396 396 396 396 396
529 530 530 530 531 531 531 530 531 530 530 530 530 530 530 530
shown in Figs. S1 and S2, respectively. For example, distinct and sharp peaks of the XP spectra for Tb–Co can be assigned as follows: Tb 4d3/2 around 154 eV, C 1s at 284 eV, N 1s at 397 eV, O 1s at 531 eV, Co 2p3/2 at 781 eV, Co 2p3/2 at 796 eV [39–41]. No significant differences could be observed for the C 1s, N 1s and O 1s peaks of the ligand moieties. The narrow survey XPS of Co 2p3/2 for Ln–Co and Fe 2p3/2 for Ln–Fe complexes are shown in Figs. S3–S6, respectively. The Co 2p3/2 peak binding energies are at 785.1, 784,7, 785.0, 784.8, 785.4, 785.3, 785.4 and 785.1 eV for Ce–Co, Nd–Co, Sm–Co, Gd–Co, Tb–Co, Dy–Co, Ho–Co and Er–Co, respectively. While the Fe 2p3/2 peak binding energies are at 712.7, 712.3, 712.0, 711.7, 712.6, 712.1, 711.9, 712.5 eV and the Fe 2p1/2 peaks are at 726.4, 724.8, 725.3, 724.7, 725.9, 726.9, 726.3, 726.6 eV for Ce–Fe, Nd–Fe, Sm–Fe, Gd–Fe, Tb–Fe, Dy– Fe, Ho–Fe and Er–Fe, respectively. These peaks are ascribed to CoIII (or FeIII) oxidation states. Thus, no remarkable shift of peaks could be observed for FeIII or CoIII ions regardless of the LnIII ions. Furthermore, we could not clearly observe shake-up satellites, typical features of anti-bonding p molecular orbitals (p–p* transitions). Figs. 2–5 show the narrow survey Ce and Nd (Fig. 2), Sm and Gd (Fig. 3), Tb and Dy (Fig. 4), and Ho and Er (Fig. 5) 4d3/2 XPS for 16 Ln–M complexes. The Ce 4d3/2 peak binding energies are at 114.0 and 114.7 eV for Ce–Co and Ce–Fe, Nd 4d3/2 are at 124.7 and 125.4 eV for Nd–Co and Nd–Fe, Sm 4d3/2 are at 134.6 and 135.8 eV for Sm–Co and Sm–Fe, Gd 4d3/2 are at 144.3 and 145.4 eV for Nd–Co and Nd–Fe, Tb 4d3/2 are at 153.7 and 153.1 eV for Tb–Co and Tb–Fe, Dy 4d3/2 are at 159.7 and 159.2 eV for Dy–Co and Dy–Fe, Ho 4d3/2 are at 165.6 and 165.3 eV for Ho–Co and Ho–Fe, Er 4d3/2 are at 172.2 and 171.8 eV for Er–Co and Er–Fe, respectively. These peaks are ascribed to LnIII oxidation states, and the binding energies of Ln–Co complexes are lower than that of Ln–Fe complexes for Ln = Ce, Nd, Sm and Gd (the number of 4f electrons are less than 7). In contrast, the binding energies of Ln–Co complexes are similar or higher than that of Ln–Fe complexes for Ln = Tb, Dy, Ho and Er (the number of 4f electrons are more than 7). Commonly, XPS was used to investigate valence states of metal ions and coordination bonding natures in the field of coordination chemistry [42,43]. For example, it has been reported that XPS binding energies indicate good correlations between substituted groups in Au complexes [44], mixed valence in Au complexes [45], electron transfer in tungsten complexes [46], magnetic properties in copper complexes [47,48] and Moessbauer isomer shift in iron complexes [49]. Recently, inorganic solids containing LnIII ions were also measured by using synchrotron radiation [50]. According to them, the bonds with more electronegative atoms would result in greater positive XPS chemical shifts. The peaks of higher oxidized ions would appear with
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Nd-Co
Nd-Fe Nd 4d3/2
Ce-Co
Ce-Fe Ce 4d3/2 150
145
140
135
130
125
120
115
110
105
100
95
90
Binding energy/eV Fig. 2. Narrow survey Ce or Nd 4d3/2 XPS for Ce–M or Nd–M (M = Co and Fe).
Gd-Co
Gd-Fe Gd 4d3/2
Sm-Co
Sm-Fe Sm 4d3/2 180 175 170 165 160 155 150 145 140 135 130 125 120 115 110 105 100
Binding energy/eV Fig. 3. Narrow survey Sm or Gd 4d3/2 XPS for Sm–M or Gd–M (M = Co and Fe).
Dy-Co
Dy 4d3/2
Dy-Fe
Tb-Co
Tb 4d3/2
Tb-Fe 180
175 170
165 160
155
150 145
140 135
130 125
120
Binding energy/eV Fig. 4. Narrow survey Tb or Dy 4d3/2 XPS for Tb–M or Dy–M (M = Co and Fe).
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Er-Co Er 4d3/2
Er-Fe
Ho 4d3/2
Ho-Co
Ho-Fe 190
185
180
175
170 165 160 Binding energy/eV
155
150
145
140
Fig. 5. Narrow survey Ho or Er 4d3/2 XPS for Ho–M or Er–M (M = Co and Fe).
higher binding energies. Accordingly, two kinds of Ln 4d3/2 XPS peak shifts suggest that the cyano-bridged Ln ions of Ln–Fe complexes for Ln = Ce, Nd, Sm and Gd easily release electrons even under these conditions. In other words, these selected Ln ions are easy to take mixedvalence states.
(Dy3+: 6H15/2 ground state), Ho–M (Ho3+: 5I8 ground state) and Er–M (Er3+: 4I15/2 ground state) (M = Co and Fe). In this way, visible light-sensitivity depends on Ln ions for these complexes. Although we have also tried to measure emission spectra (kex = 250, 300 and 350 nm) in the solid states at 298 K, we could not detect emission from these complexes under these experimental conditions.
3.3. IR and electronic spectra 3.4. Electronic properties of 3d–4f cyanide-bridged systems As listed in Table 1, cyanide stretching bands of the IR spectra of Ln–Co and Ln–Fe complexes are in the range of 2127–2137 cm1 and 2123–2128 cm1, respectively. As for the simple K3[Fe(CN)6] complex in Oh point group, normal vibration modes were assigned as follows: 2315 cm1 (A1g), 2130 cm1 (Eg) and 2118 cm1 (F1u) [51]. Despite being sensitive to valence or spin states, the present values are comparable to analogous 3d–4f bimetallic assemblies. By comparing with the related 3d–3d or 3d–4f complexes [52], spectral shift features, that is substitution of CoIII and FeIII ions, is more effective than substitution of LnIII ions. The spectral features of the IR spectra should also correspond to shifts in the charge transfer absorption bands between the 3d orbitals of MIII ions and the unoccupied p* orbitals of bridging cyanide ligands. The splitting by bridged and uncoordinated cyanide ligands was unclear for the present cases. On the other hand, electronic spectra (Figs. S7–S10) exhibit strong and broad bands in the UV region due to p–p* transitions from ligands of both DMF and low-spin [Co(CN)6]3 or low-spin [Fe(CN)6]3 moieties. The electronic spectra for the Ln–Fe complexes show two peaks around 23 000–32 000 cm1 and 30 000–34 000 cm1, while Ln–Co complexes show a peak around 28 000– 32 000 cm1. The spectral shapes of the UV region depend on the electronic states of the [M(CN)6]3 moieties. In addition, several sharp peaks could be observed in the visible region for Nd–M (Nd3+: 4I9/2 ground state), Dy–M
The effect of interaction between LnIII and MIII ions through cyano-bridges can lead to a different charge distribution, overshadowing the decreasing extent of p-backdonation. As elucidated in the present results, this effect is mixed by both structural factors and electronic factors and appeared for specific LnIII elements remarkable for the ground states of these complexes. As for 3d–3d Prussian blue analogues, the electron configurations corresponding to the strongest ferromagnetic interaction are t32g –t62g e2g , while weak antiferromagnetic interaction are tm2g –tn2g e1g or tm2g –tn2g e2g . Unpaired electrons in the t2g orbitals can be used for superexchange interactions through unoccupied p* orbitals of cyanide bridging ligands. So oxidation states as well as the kind of 3d elements are important for the mechanism of these magnetic interactions. Indeed, it has been elucidated that in cyano-bridged Mo–M systems, containing [Mo(CN)8]n moieties, MoIII ions show larger exchange coupling constants than the equivalent compounds with first transition series elements due to the more diffuse character of the 4d orbitals in comparison with the 3d orbitals [15]. The nature of the coordination bonds of the [Fe(CN)6]3 moiety is p-backdonation, in which five 3d electrons occupying t2g orbitals are provided to unoccupied p* orbitals of the cyanide ligands. Recently, detailed electronic states have been confirmed by means of X-ray absorption spectra [53–56]. In Oh symmetry, the electron
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configuration of the FeIII ion is t52g e0g and irreducible representations of 2p orbitals of the cyanide ligands are t1g, t1u, t2g and t2u. Qualitatively speaking, the formation of a [Fe(CN)6]3 complex results in molecular orbitals which are split into occupied t2g, degenerated t1g, t1u and t2u, and t2g orbitals and unoccupied degenerated t1g, t1u and t2u, eg and t2g orbitals. When the electrons of cyanides are excited to p* orbitals (formed from unoccupied t1g, t1u and t2u orbitals) associated with metal to ligand charge transfer from the t52g orbital in the FeIII ion to unoccupied p* orbitals in cyanide ligands [56], p-backdonation may be weaken because of electronic repulsion of electrons in the unoccupied p* orbitals. 4. Conclusion Consequently, we can point out characteristic structural and electronic features for photo-induced magnetization of Nd–M complexes. Systematic crystal structures revealed that gradually changes of isostructural crystals due to lanthanoid contraction lead to remarkable Ln–O coordination bond distances or a decrease of the cell volume. However, only Nd–Fe and Nd–Co complexes show exceptional bent Ln–N–C bond angles, which may be proposed photoexcited structures of bent-bonds. The patterns of Ln 4d3/2 XPS peak shifts for Ln = Ce, Nd, Sm and Gd differ from that of Ln = Tb, Dy, Ho and Er, which suggests that the cyano-bridged LnIII ions of Ln–Fe complexes for Ln = Ce, Nd, Sm and Gd easily become mixed-valence states. In addition, absorption spectra exhibit that p-backdonation of cyano-bridging ligands uses unoccupied p* orbitals and unoccupied t2g orbitals in [Fe(CN)6]3 moieties. Further studies on the structural and electronic natures of other cyano-bridging heterometallic assemblies are in progress in order to rationalize the design of multi-functional materials which consist of (transition) metal complexes. Acknowledgements This work was supported by a Grant-in-Aid for the 21st Century COE program ‘KEIO Life Conjugate Chemistry’ form the Ministry of Education, Culture, Sports, Science, and Technology, Research Foundation for Opto-Science and Technology, and Japan and Mizuho Foundation for the Promotion of Science. Appendix A. Supplementary data Crystallographic information is available (CCDC Nos. 279742, 279743, 279744 and 279745 for Tb–Co, Dy–Co, Ho–Co and Er–Co, respectively) from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail: [email protected]). Characterization data, XPS and diffuse reflectance electronic spectra have been deposited as supplementary data. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2006.03.026.
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