Ting
Zhang
,
Wei
Guan
,
Tengying
Ma
,
Yan
Lin
,
Likai
Yan
* and
Zhongmin
Su
*
Institute of Functional Material Chemistry, Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China. E-mail: yanlk924@nenu.edu.cn; zmsu@nenu.edu.cn; Fax: +86-431-5684009
First published on 8th April 2015
The density functional theory (DFT) method and natural bond orbital (NBO) analysis have been employed to investigate the electronic structures and second-order nonlinear optical (NLO) properties for tricarbonyl metal derivatives of a Lindqvist-type polyoxometalate, [Nb2W4O19M(CO)3]3−/2− (M = MnI, TcI, ReI, FeII, RuII and OsII). The position of the tricarbonyl metal ligand and the substitution of different metal atoms M have an influence on the electronic absorption spectra and NLO responses of all complexes. Among the three isomers of [Nb2W4O19Mn(CO)3]3−, the β0 value of isomer 1a is the largest, which is 1.4 times and 2.3 times larger than those of isomers 1b and 1c, respectively. The β0 value also depends on the tricarbonyl metal M, the β0 value decreases in the order M = Ru > Re > Fe > Os > Mn > Tc. In addition, the analysis of the main transition orbitals shows that the tricarbonyl metal ligand acts as an electron donor in [Nb2W4O19M(CO)3]3−/2− (M = MnI, TcI, ReI, RuII and OsII), while the [Fe(CO)3]2+ ligand acts as an electron acceptor in [Nb2W4O19Fe(CO)3]2−.
In 1980, Besecker and co-workers synthesized [Nb2W4O19M(CO)3]3− (M = Mn, Re) complexes in which [Nb2W4O19]4− is coordinated to a tricarbonyl Mn/Re unit by a triangle of three adjacent bridging oxygens (Ob).1,2 Interestingly, 17O NMR spectroscopy provides conclusive evidence for the presence of all three [Nb2W4O19M(CO)3]3− isomers. The studies of POM-supported metal carbonyl derivatives have been in progress all the time.3–10 Recently, Hill and co-workers have synthesized [P4W35O124{Re(CO)3}]16− and [X2W20O70{M(CO)3}2]12− (X = Sb, Bi; M = Mn, Re) complexes, and density functional theory (DFT) calculations show that these complexes exhibit an intense Re-to-POM charge-transfer (CT) transition, an example of a potentially general transition, called metal-to-POM (MP)CT.9,10 The chromophoric and catalytic properties, and photoactivities of these CT complexes are under investigation.
Nonlinear optical (NLO) materials have received tremendous attention owing to their unique applications in optical fibers, data storage, optical limiting, computing and switching. Among these NLO materials, the donor–acceptor (D–A) model has been widely used. POM complexes with the electron storage ability can be designed as a good electron-acceptor group.11–13 So the POM-based complexes with good thermal and photochemical stability as well as redox switching ability are potential NLO materials. Theoretical studies would be helpful in rationalization of the observed properties and design of novel POM-based hybrid materials with excellent properties. Previous studies on related POMs show that DFT calculations give a satisfactory description of the geometrical structures, bonding character, redox properties, charge-transfer and NLO properties.14–17
Obviously, POM-supported metal carbonyl derivatives are potential NLO materials, while studies on second-order NLO properties and electronic structures of tricarbonyl metal polyanions are rare. In the present work, DFT calculations were performed to investigate the electronic structures and nonlinear optical properties of tricarbonyl metal derivatives of the Lindqvist-type POM. The calculation models of complexes are shown in Fig. 1. In cis-[Nb2W4O19]4−, there are three different types of bridging oxygen atoms, which bond to two niobiums (Ob1), one niobium and one tungsten (Ob2), or two tungstens (Ob3). The experimental spectra2 provide conclusive evidence for the presence of all three isomers. In isomer a, the M(CO)3 binding site is formed by one ONb2 oxygen and two ONbW oxygens. In isomer b, two ONbW oxygens and one OW2 oxygen are involved, and in isomer c, three OW2 oxygens form the binding site. The complex [Nb2W4O19Mn(CO)3]3− was taken as an example to investigate the difference in geometrical and electronic structures among the three isomers. In order to investigate the effect of the tricarbonyl center metal M on NLO properties of [Nb2W4O19M(CO)3]3−/2−, Mn or Re was substituted by other d6 metals (TcI, FeII, RuII, and OsII).
β0 = (βx2 + βy2 + βz2)1/2 |
In order to obtain a more intuitive description of the second-order NLO behavior of the studied complexes, time-dependent DFT (TDDFT) methods were used to simulate the molecular electronic spectra at the PBE1PBE/6-31+G(d) level (LANL2DZ basis set for metal atoms). PBE1PBE, also known as PBE0, is obtained by casting the function and correlation of Perdew, Burke and Erzenrhof in the hybrid HF/DFT scheme with a fixed 1/4 ratio.22 This functional has been shown to improve the accuracy of excitation energies and CT bands in metal complexes for solvation calculations.
All the calculations were performed in acetonitrile solvent. The solvent effect was considered using the polarizable continuum model (PCM)23 using the integral equation formalism variant. The Wiberg bond index (WBI) was computed by natural bond orbital (NBO) calculations at the same level with the geometrical optimization.
Bond length | |||
---|---|---|---|
C–O | M–C | M–Ob | |
1a | 1.173 | 1.780 | 2.087 |
1b | 1.173 | 1.781 | 2.079 |
1c | 1.174 | 1.781 | 2.071 |
Exp | 1.144 | 1.843 | 2.001 |
2 | 1.175 | 1.909 | 2.213 |
3 | 1.180 | 1.908 | 2.200 |
4 | 1.156 | 1.800 | 1.987 |
5 | 1.158 | 1.923 | 2.121 |
6 | 1.163 | 1.913 | 2.122 |
Among 1a, 1b and 1c, the calculated average bond length of C–O is 1.17 Å, which is in good agreement with the experimental results 1.14 Å. It suggests that the functional and basis sets that we adopted are reliable for the studied complexes. In 1a, 1b and 1c, the difference in Mn–C and C–O bond lengths is about 0.001 Å, illustrating that the structure of the tricarbonyl metal ligand in the three isomers is basically consistent. The Mn–Ob bond length is 2.087 Å for 1a, 2.079 Å for 1b, and 2.071 Å for 1c, showing that the position of the tricarbonyl metal ligand slightly affects the Mn–Ob bond length.
The experimental results confirmed that isomer b predominates in solution. For 2–6, the isomer b was investigated. The M–Ob bond length increases in the order 1b (2.079) < 2 (2.213) ≈ 3 (2.200); 4 (1.987) < 5 (2.121) ≈ 6 (2.122), which is consistent with the radii of the metal atom. The M–Ob bond length decreases in the order: 4 < 1b, 5 < 2, 6<3, indicating that the interactions between the metal M (M = Fe, Ru and Os) and Ob are stronger than those of Mn, Tc and Re.
Bond | 1a (M = Mn) | 1b (M = Mn) | 1c (M = Mn) | 2 (M = Fe) |
---|---|---|---|---|
M–C1 | 1.169 | 1.182 | 1.180 | 1.290 |
M–Ob1 | 0.386 | 0.384 | 0.389 | 0.379 |
M–Ob2 | 0.386 | 0.399 | 0.389 | 0.390 |
M–Ob3 | 0.399 | 0.393 | 0.404 | 0.383 |
Bond | 3 (M = Re) | 4 (M = Fe) | 5 (M = Ru) | 6 (M = Os) |
---|---|---|---|---|
M–C1 | 1.365 | 0.962 | 1.059 | 1.195 |
M–Ob1 | 0.398 | 0.483 | 0.466 | 0.490 |
M–Ob2 | 0.410 | 0.498 | 0.482 | 0.505 |
M–Ob3 | 0.401 | 0.494 | 0.473 | 0.494 |
The Mn–C bonds in three isomers (1a–1c) are very similar with obvious single-bond nature as indicated by the calculated WBI of 1.169–1.182. However, with different metals, M = Mn (1), Tc (2), Re (3), Fe (4), Ru (5), Os (6), the interaction between M and C is slightly different, 3 (1.365) > 6 (1.195); 2 (1.290) > 5 (1.059); 1b (1.182) > 4 (0.962), which is consistent with the M–C bond length, 3 (1.908) < 6 (1.913); 2 (1.909) < 5 (1.923); 1b (1.781) < 4 (1.800). It indicates that the interactions between M and C in [Nb2W4O19M(CO)3]3− (M = Mn, Tc and Re) are stronger than those of [Nb2W4O19M(CO)3]2− (M = Fe, Ru and Os). The WBI of Mn–Ob for the three isomers (0.384–0.404) shows weak interactions between Mn and bridging oxygen Ob. Compared with 1 and 3, the M–Ob1 bond orders of 4, 5 and 6 are larger, 6 (0.490) > 4 (0.483) > 5 (0.466) > 3 (0.398) > 1b (0.384) > 2 (0.379), which are consistent with the M–Ob bond lengths.
Fig. 2 (a) The absorption UV-Vis spectra of three isomers 1a–1c; (b) the absorption UV-Vis spectra of systems 1b–6. |
λ | ΔE | f 0 | Major contribution | |
---|---|---|---|---|
1a | 276.3 | 4.49 | 0.0400 | H−3 → L (19%) |
275.1 | 4.51 | 0.0342 | H−4 → L (40%) | |
H−2 → L+3 (20%) | ||||
268.8 | 4.61 | 0.0954 | H−5 → L+1 (33%) | |
H−6 → L+1 (17%) | ||||
1b | 265.0 | 4.68 | 0.0392 | H−2 → L+6 (17%) |
H−6 → L+1 (13%) | ||||
264.7 | 4.69 | 0.0194 | H−2 → L+6 (16%) | |
H−11 → L (12%) | ||||
262.7 | 4.72 | 0.0199 | H−1 → L+5 (17%) | |
H−2 → L+6 (14%) | ||||
1c | 263.0 | 4.71 | 0.0212 | H−2 → L+5 (42%) |
260.7 | 4.76 | 0.0760 | H−5 → L+1 (34%) | |
H−7 → L+1 (22%) | ||||
254.3 | 4.88 | 0.0151 | H−6 → L+2 (58%) | |
2 | 267.0 | 4.64 | 0.0152 | H−5 → L+1 (33%) |
264.6 | 4.69 | 0.0195 | H−6 → L+1 (11%) | |
263.9 | 4.70 | 0.0661 | H−5 → L+1 (17%) | |
3 | 263.7 | 4.70 | 0.0283 | H−6 → L+2 (11%) |
262.6 | 4.72 | 0.0426 | H−8 → L (14%) | |
261.0 | 4.76 | 0.0215 | H−2 → L+9 (16%) | |
4 | 268.8 | 4.61 | 0.0525 | H−6 → L+1 (39%) |
253.3 | 4.89 | 0.0158 | H−8 → L+2 (7%) | |
H → L+4 (6%) | ||||
251.9 | 4.92 | 0.0140 | H → L+4 (14%) | |
5 | 261.4 | 4.74 | 0.0264 | H−9 → L (8%) |
261.0 | 4.75 | 0.0429 | H−4 → L+2 (19%) | |
H−7 → L+1 (18%) | ||||
253.8 | 4.88 | 0.0199 | H−5 → L+2 (16%) | |
6 | 262.9 | 4.71 | 0.0130 | H−4 → L+2 (16%) |
257.0 | 4.82 | 0.0222 | H−7 → L+1 (21%) | |
252.3 | 4.91 | 0.0156 | H−9 → L (16%) |
As shown in Fig. 2(b), 1b, 2 and 3 show an intense absorption at around 265 nm, 4–6 show a broad band at 240–260 nm. The spectra of 2 and 3 have similar transition features to 1b. The blue shifts of the absorption are observed in 4–6 compared with 1b, 2 and 3. It indicates that the more positive charge of [M(CO)3] (FeII, RuII and OsII) units causes the blue shifted absorption peak. In order to concisely describe the transition character, the electron density differences between the ground state and the dominant excited state were also plotted using GaussSum2.2 (Fig. 4). For 2, 3, 5 and 6, the decreasing electron density mainly localizes on the tricarbonyl segment and oxygen atoms of POM, while the increasing electron density mainly localizes on the Nb/W atoms. The tricarbonyl ligand acts as an electron acceptor and the POM acts as an electron donor, which is consistent with MPCT. It can be seen that the increasing electron density in 4 mainly localizes on the tricarbonyl segment and oxygen atoms of POM, while the decreasing electron density mainly localizes on the Nb/W atoms. We infer that the large electronegativity of FeII (4) has an important influence, which leads to the tricarbonyl ligand acting as an electron acceptor.
Fig. 4 The electron density difference maps (EDDM) of systems 2–6 between the ground state and the dominant electronic transitions with an isodensity surface of 0.001 au. |
Δμ | Function | β x | β y | β z | β 0 | |
---|---|---|---|---|---|---|
1a | 3.27 | MPWPW91* | 3.28 | 8.22 | −0.16 | 8.85 |
CAM-B3LYP | 3.80 | 8.25 | −0.16 | 9.08 | ||
1b | 7.05 | MPWPW91* | −0.19 | 2.97 | 5.54 | 6.29 |
CAM-B3LYP | −0.19 | 2.41 | 6.33 | 6.77 | ||
1c | 9.12 | MPWPW91* | −3.52 | −1.73 | −0.00 | 3.92 |
CAM-B3LYP | −3.98 | −2.81 | 0.00 | 4.87 | ||
2 | 7.23 | MPWPW91* | −0.41 | −0.46 | 5.42 | 5.46 |
CAM-B3LYP | −0.38 | −0.91 | 6.24 | 6.32 | ||
3 | 6.43 | MPWPW91* | −0.41 | −9.60 | 5.39 | 11.02 |
CAM-B3LYP | −0.38 | −9.45 | 6.23 | 11.33 | ||
4 | 15.92 | MPWPW91* | −0.76 | 9.74 | 4.62 | 10.81 |
CAM-B3LYP | −0.80 | 8.95 | 5.39 | 10.48 | ||
5 | 17.06 | MPWPW91* | −0.36 | 11.285 | 5.26 | 12.46 |
CAM-B3LYP | −0.38 | 9.46 | 6.03 | 11.22 | ||
6 | 14.22 | MPWPW91* | −0.49 | 4.88 | 4.92 | 6.95 |
CAM-B3LYP | −0.52 | 3.43 | 5.76 | 6.72 |
When M is Mn, Tc and Re, the β0 values present the order: β0 (3) > β0 (1b) > β0 (2); when M is Fe, Ru and Os, the β0 values present the order: β0 (5) > β0 (4) > β0 (6). Among all complexes, 5 shows the largest β0 value, 12.456 × 10−30 esu according to the MPWPW91* calculations. Compared with those of 1b and 2 (6.287 and 5.455 × 10−30 esu), 4 and 5 have larger β0 values, 10.807 and 12.456 × 10−30 esu, respectively. Furthermore, for all complexes except for 2βy tensor components are small; the large βy tensor components of complexes indicate that the charge transfer in the y direction plays an important role.
In order to elucidate the origin of second-order nonlinear responses of all complexes, we took a further step to analyse the main excited states that contribute to the β value. From the two-level model
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