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Theoretical studies on the redox stimulated intramolecular isomerization in [Ru(pic)(tpy)(dmso)]+
Computational and Theoretical Chemistry 1050 (2014) 46–50
Contents lists available at ScienceDirect
Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Theoretical studies on the redox stimulated intramolecular isomerization in [Ru(pic)(tpy)(dmso)]+ Huifang Li a,⇑, Nana Wang b, Lisheng Zhang a, Xiaolin Fan a,⇑ a b
Key Laboratory of Organo-Pharmaceutical Chemistry, Gannan Normal University, Jiangxi Province 341000, China Yantai Automobile Engineering Professional College, Shandong Province 265500, China
a r t i c l e
i n f o
Article history: Received 30 August 2014 Received in revised form 15 October 2014 Accepted 16 October 2014 Available online 23 October 2014 Keywords: [Ru(pic)(tpy)(dmso)]+ Redox Intramolecular isomerization Electrochromic mechanism Dispersion corrected density functional theory
a b s t r a c t Redox induced intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group in [Ru(pic)(tpy)(dmso)]+ (pic = 2-picolinate; tpy = 2,20 :60 ,200 -terpyridine; dmso = dimethyl sulfoxide) was explored theoretically. Calculated results show that the energy barrier for the Ru–S–Ru–O linkage isomerization is decreased by one-electron oxidation, while is increased by one-electron reduction. Moreover, compared with that in the Ru(II) and Ru(I) sulfoxide complexes, intramolecular Ru–S ? Ru–O isomerization is more thermodynamically favored in the Ru(III) complexes. Differently, the energy barrier for the reaction path from Ru–O to Ru–S linkage mode is increased by one-electron oxidation, while is decreased by one-electron reduction. Then, it can be concluded that the Ru–S ? Ru–O isomerization is favored by Ru(II) oxidation, while Ru–O ? Ru–S isomerization is easier to be induced by Ru(III) or Ru(II) reduction. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction External signal triggered intramolecular isomerization between two ground states of reversible molecules is central to the operation of molecular information storage and switch devices [1]. Recently, a new class of electrochromic and photochromic ruthenium sulfoxide complexes has been synthesized and developed significantly [2,3]. Previous studies demonstrated that intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of a bound sulfoxide ligand may be triggered either by light or by formal oxidation or reduction [4,5]. The differential bonding characteristics of S- and O-bonded sulfoxide produces distinct electronic and photophysical properties. Such structural alteration from the S-bonded to the O-bonded metastable states of ruthenium sulfoxide complexes is accompanied with pronounced changes of the absorption spectra. As a result, a dramatic change in color can be observed [1–5]. Full description of the ground and excited potential energy surfaces (PESs) of [Ru(bpy)(tpy)dmso]2+ (bpy = 2,20 -bipyridine; tpy = 2,20 :60 ,200 -terpyridine) from theoretical approaches have been reported for providing more detailed information about the phototriggered intramolecular isomerization mechanism of monodentate ruthenium sulfoxide complexes [6]. Recently, photoisomerization
mechanism of photochromic ruthenium complex with a chelating sulfoxide complex, [Ru(bpy)2(OSO)]+ (OSO = 2-methylsulfonylbenzoate), has been explored theoretically by Göttle and his coworkers. They found that triplet metal-centered excited states play a central role in the photoisomerization mechanism of ruthenium sulfoxide complex [7]. As a result, nonadiabatic Ru–S ? Ru–O and Ru–O ? Ru–S isomerization can be understood very well. For better understanding the electrochromic mechanism of such ruthenium sulfoxide complexes, redox stimulated intramolecular isomerization of [Ru(pic)(tpy)(dmso)]+ (shown in Fig. 1) will be explored theoretically. It has been demonstrated that Ru–S ? Ru–O and Ru–O ? Ru–S linkage isomerization of [Ru( pic) (tpy)(dmso)]+ can be triggered through electron transport in electrochemical studies, resulting in dramatic shifts in the absorption maxima (from 421 nm to 527 nm) [8–10]. In this work, potential energy surfaces for the Ru–S ? Ru–O and Ru–O ? Ru–S linkage isomerization of Ru(II), Ru(III) and Ru(I) sulfoxide complexes will be described theoretically to clarify the role of the oxidation and reduction in the electrochromic mechanism of [Ru(pic)(tpy)(dmso)]+.
2. Computational details ⇑ Corresponding authors at: Key Laboratory of Organo-Pharmaceutical Chemistry, Gannan Normal University, Jiangxi Province 341000, China. E-mail addresses: [email protected] (H. Li), [email protected] (X. Fan). http://dx.doi.org/10.1016/j.comptc.2014.10.022 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
Ru–S is a weak interaction and the bond length was overestimated by current density functional theory, as shown in previous report [6]. Göttle and his coworker’s demonstrated that better
H. Li et al. / Computational and Theoretical Chemistry 1050 (2014) 46–50
47
result for Ru–S bond-length can be obtained by MPWB1K functional. We tried to use the same method for our optimization. However, lots of unexpected imaginary frequencies exist, which are hard to be eliminated. Thereby, it was hard for us to proceed. Now, dispersion effects that are missing in DFT functionals have been accounted for according to Grimme’s correction scheme (DFT-D3) [11]. The accuracy and reliability of the DFT-D approach for the description of weak interactions have been proved in recent studies [12]. Thereby, in optimization calculations, the empirical dispersion correction introduced by Grimme [11] was added to the DFT functional with Becke’s and Johnson’s rational damping function [13] and dubbed this variant DFT-D3(BJ). Benchmark calculations results demonstrated that a much better description of the Ru–S bond length can be obtained with PBE0-D3(BJ) functional [14]. Initial geometry of S-linked [Ru(pic)(tpy)(dmso)]+ complex were obtained from experimental X-ray data [10]. We calculated the Ru(II) complexes as closed shell and the Ru(I) and Ru(III) complexes using doublet spins. All molecular geometries of the ruthenium sulfoxide complexes discussed here were fully optimized with PBE0-D3(BJ) method. Basis set is made of SDD for Ru [15], a correlation-consistent polarized double-f basis set (cc-pVDZ) [16] for H atoms, and a correlation-consistent polarized triple-f basis set (cc-pVTZ) [16] for C, N, O and S atoms. In addition, considering solvent may play an important roles in geometry determination, conductor-like polarizable continuum model (CPCM) [17] with solvent acetonitrile (e = 35.688) was also considered for comparison. Harmonic vibrational frequencies were calculated analytically at the same level to confirm that each stationary point was a minimum or a transition state on the potential energy surface. Interaction energy (Ed) between dmso ligand and central ruthenium (II) was determined as: Ed = (Edmso + E½RuðpicÞðtpyÞþ )– E½RuðpicÞðtpyÞðdmsoÞþ . Charge distribution was evaluated with natural population analysis (NPA) [18] for the aim to examine the degree of charge transfer between the central Ru and surrounding ligands. All calculations were performed using the Gaussian 09 package [19].
crystallographic data are also included for comparison. Optimized Ru–S bond in S-[Ru(pic)(tpy)(dmso)]+ is 2.242 Å, in good agreement with experimental X-ray crystallographic data which is 2.215 Å [13]. Moreover, overall satisfactory agreement can be obtained between theoretical and experimental results because all of the computed distances are within the experimental error (0.03 Å). Molecular structure of the S-linked [Ru(pic)(tpy)(dmso)]+ is changed a lot by intramolecular isomerization process, especially for the coordinated dmso ligand. By comparing the S–O bond changes in S-, g2- and O-linked Ru(II) complexes, it is found that the S–O bond of dmso ligand is lengthened from 1.485 Å to 1.509 Å upon Ru(II)–S–Ru(II)–O linkage isomerization. As shown in Table 2, the interaction energy (Ed) between dmso and central Ru using Ru–S linkage is larger than that using Ru–O linkage for about 0.5 eV. Thereby, S-[Ru(pic)(tpy)(dmso)]+ is observed to be more stable than O-[Ru(pic)(tpy)(dmso)]+ in experimental studies. Moreover, it found that the S–O bond-length change upon isomerization in the Ru(II) complexes (0.048 Å) is smaller than that in the Ru(III) complexes (0.081 Å), while is larger than that in the reduced state of the complex (0.040 Å). It means effect of one-electron oxidation on the interaction energy between dmso and central Ru is larger than that of one-electron reduction, as shown in Table 2. There occur considerable geometry changes upon one-electron oxidation or reduction process. Moreover, the variation trend of the bond lengths between Ru and dmso ligand are more pronounced than those between Ru and pic or tpy ligand. As shown in Table 1, when one electron is removed, it is found that Ru–S bond is lengthened by 0.128 Å and S–O bond is shortened by 0.008 Å in S-linked complex, while Ru–O bond is shortened by 0.081 Å and S–O bond is lengthened by 0.025 Å in the O-linked isomer. Oppositely, Ru–S bond is shortened by 0.030 Å and S–O bond is lengthened by 0.006 Å in reduced S-linked complex, while Ru–O bond is lengthened by 0.023 Å and S–O bond is shortened by 0.002 Å in reduced O-linked complex. It means oxidation and reduction may have different effects on the Ru(II)–S or Ru(II)–O bond-strength.
3. Results and discussions
3.2. Electronic properties
Formal oxidation or reduction triggered intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group has been observed experimentally in Ru sulfoxide complexes, as shown in Fig. 2. In this work, electrochromic mechanisms of the monodentate Ru sulfoxide complex, [Ru(pic)(tpy)(dmso)]+, are explored.
Calculated EA (electron affinity) and IP (ionization potential) of S-[Ru(pic)(tpy)(dmso)]+ are 4.44 and 9.49 eV in the gas phase, respectively. As shown in Table 3, IP becomes lower and EA becomes higher upon Ru(II)–S–Ru(II)–O linkage isomerization process. Calculated IP and EA of O-[Ru(pic)(tpy)(dmso)]+ are 8.59 and 4.03 eV, respectively. For better understanding IP and EA changes upon intramolecular isomerization, the frontier orbital contour plots are examined, as shown in Fig. 3. The descriptions of molecular orbital composition are summarized in Table 4. It is found that the HOMO (the highest occupied molecular orbital) electron density is mainly distributed on pic ligand (26.6%) and central Ru atom (58.0%), while the electron density of the LUMO (the lowest unoccupied molecular orbital) is mainly distributed on surrounding tpy ligand (83.3%) in S-linked Ru(II) complex. Due to the thermodynamic ability to gain or lose an electron is determined by the energies of the LUMO (5.41 eV) and HOMO (8.87 eV), the HOMO and LUMO electron density distributions are similar with the spin density distributions, as shown in Fig. 4. As mentioned above, the interaction energy between dmso and central Ru is decreased upon Ru(II)–S–Ru(II)–O linkage isomerization. It is attributed to the larger electron-donating ability of S compared with O. NPA (natural population analysis) charge results from NBO (natural bond orbital) analysis show that charge transfer amount from dmso as well as other ligands to Ru center is decreased by about 0.173 upon Ru(II)–S–Ru(II)–O isomerization
3.1. Molecular structures Geometrical parameters obtained from the structural optimization for the S-, SO(g2)- and O-linked Ru(II), Ru(III) and Ru(I) sulfoxide complexes are listed in Table 1. Experimental X-ray
Fig. 1. Chemical structure of [Ru(pic)(tpy)(dmso)]+.
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H. Li et al. / Computational and Theoretical Chemistry 1050 (2014) 46–50
Fig. 2. Ru–S–Ru–O linkage isomerization of the dmso ligand in [Ru(pic)(tpy)dmso]+ complex.
Table 1 Calculated main bond lengths (angstroms) and bond angles (degrees) for the Ru(II), Ru(III) and Ru(I) sulfoxide complexes obtained in the gas phase. S-Linked
Ru–S Ru–O1 S–O1 Ru–Nt0 Ru–Nt00 Ru–Nt0 00 Ru–Np Ru–O2 a(O1–S–Ru)/a(S–O1–Ru)
O-Linked Exptl.
Ox.
Re.
Ru(II)
Ox.
Re.
Ru(II)
Ox.
Re.
2.242 – 1.485 2.069 1.962 2.065 2.101 2.049 116.7
2.215 – 1.478 2.076 1.959 2.068 2.101 2.085 115.3
2.370 – 1.477 2.073 1.984 2.064 2.110 1.955 112.3
2.212 – 1.491 2.064 1.972 2.066 2.093 2.075 121.1
– 2.163 1.533 2.054 1.942 2.050 2.072 2.024 125.8
– 2.082 1.558 2.080 1.971 2.073 2.081 1.937 123.9
– 2.186 1.531 2.053 1.960 2.045 2.053 2.040 116.2
2.853 2.837 1.509 2.055 1.948 2.058 2.085 1.989 74.0
2.563 2.505 1.516 2.105 1.983 2.068 2.104 1.932 70.5
3.387 3.266 1.503 2.041 1.927 2.040 2.073 2.057 72.5
Table 2 Calculated interaction energies between cyclometalating ligands and Ru center in [Ru(pic)(tpy)(dmso)]+ complexes (in eV). S-Linked
dmso pic tpy
SO-Linked
Ru(II)
Table 4 Main distributions of the frontier MOs as well as spin densities in the Ru complexes. S-Linked
O-Linked
O-Linked
Ru(II)
Ru(II)
Ru(III)
Ru(I)
Ru(II)
Ru(III)
Ru(I)
2.1 4.1 6.0
1.8 5.0 6.9
2.2 3.5 4.3
1.7 4.0 6.2
2.2 5.0 6.5
1.4 3.3 4.3
Ru dmso pic tpy
Ru(III)
Ru(I)
Ru(II)
Ru(III)
Ru(I)
HOMO
LUMO
Spin
Spin
HOMO
LUMO
Spin
Spin
58.0 4.0 26.6 11.5
9.6 5.7 1.4 83.3
81.5 0.0 14.6 5.4
3.5 0.0 0.5 96.3
67.3 4.5 17.4 10.8
10.8 6.0 1.2 82.0
86.7 4.2 10.1 0.0
5.8 1.9 0.0 92.6
Table 3 HOMO and LUMO energies in the ground state of these Ru complexes as well as their corresponding ionization potentials electron affinities (in eV). HOMO
LUMO
IP
EA
S-Linked
8.87 (6.37)a
5.41 (2.59)
9.49 (5.66)
4.44 (3.10)
O-Linked
8.01 (5.57)
4.99 (2.35)
8.59 (4.84)
4.03 (2.85)
a The data in the parentheses are obtained in solution, in which the electron handled for calculation is considered to be 0 in the gas phase.
Fig. 4. The spin density isosurface (in blue) associated with the radical ion states of [Ru(pic)(tpy)dmso]+ complexes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Contour plots of frontier orbitals of the [Ru(pic)(tpy)(dmso)]+ complexes.
(shown in Table 5). As a result, compared with that in the S-linked complex, the HOMO wave-function distributed on tpy ligand and central Ru atom is decreased, while distributed on dmso ligand is increased slightly in O-linked [Ru(pic)(tpy)(dmso)]+. As a result, the HOMO and LUMO energies are destabilized by about 0.86 and 0.42 eV, respectively, when the Ru(II)–S linkage is broken and changes to a Ru(II)–O bonding mode [14]. However, Ru(II)– S–Ru(II)–O linkage isomerization destabilizes the HOMO to a larger extent than LUMO for these two photochromic Ru sulfoxide
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H. Li et al. / Computational and Theoretical Chemistry 1050 (2014) 46–50 Table 5 Natural population charges distributed on the central Ru and surrounding ligands (Q).a S-Linked Ru(II) Ru dmso pic tpy a
–0.131 0.486 0.258 0.903
O-Linked Ru(III) 0.229 0.567 0.005 1.208
Ru(I) 0.157 0.446 0.347 0.058
Ru(II) 0.263 0.313 0.286 0.710
Ru(III) 0.540 0.412 0.051 1.099
Table 6 Free energies of reaction (Er) and reaction barrier (Et) for the intramolecular isomerization of Ru(II), Ru(III) and Ru(I) sulfoxide complexes. These data are given at T = 298.15 K and P = 1 Atm (in eV). Gas Phase
Ru(I) 0.245 0.267 0.388 0.125
Q (ligand) indicates the charge on all the atoms in the ancillary ligand.
complexes, which makes the HOMO–LUMO energy gap decreased from 5.1 eV to 4.7 eV [14]. As a result, red shifted absorption and emission upon Ru(II)–S–Ru(II)–O rearrangement can be observed for such Ru sulfoxide complexes because HOMO–LUMO energy gap is quite reasonable for the description of the first excited singlet transition state.
3.3. PESs for intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization Calculated results demonstrate that the binding energies of pic and tpy ligands with central Ru atom are much larger than that between dmso ligand and central Ru. The same is true for the oxidized and reduced Ru(II) complexes. It means Ru–S or Ru–O bond breaking should be more favorable compared with Ru–N linkage mode breaking. That is why intramolecular isomerization of the sulfoxide group from S-bonding to O-bonding or from O-bonding to S-bonding modes was observed by external stimuli. Calculated potential energy surfaces (PESs) in the gas phase for the intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group in Ru(II), Ru(III) and Ru(I) complexes are shown in Fig. 5. It is found that S-[Ru(pic)(tpy)(dmso)]+ is destabilized by about 0.48 eV upon intramolecular isomerization process. The energy difference between S- and O-linked Ru(II) complexes is changed to 0.42 eV and 0.89 eV, respectively, when one electron is removed or added. It means such intramolecular isomerization process is more thermodynamically favored in the oxidized state of Ru(II) sulfoxide complexes. Moreover, the energy barrier for the intermolecular isomerization process from Ru(II)–S to Ru(II)–O linkage mode is decreased from 1.31 eV to 0.61 eV by one-electron oxidation and increased to 1.35 eV by one-electron reduction. Thereby, oxidation triggered intramolecular isomerization of the sulfoxide from S-bonded to O-bonded linkage mode can be observed for such Ru sulfoxide complexes in experimental studies.
Et
Solution Er
Et
Er
S ? O isomerization Ru(II) 1.31 Ru(III) 0.61 Ru(I) 1.35
0.48 0.42 0.89
1.32 0.60 1.23
0.39 0.43 0.64
O ? S isomerization Ru(II) 0.82 Ru(III) 1.02 Ru(I) 0.46
0.48 0.42 0.89
0.94 1.03 0.59
0.39 0.43 0.64
Different cases are observed for the reaction path from Ru–O to Ru–S linkage mode isomerization. Calculated results indicates that energy barrier for the potential energy surface is increased by about 0.20 eV upon one-electron oxidation, while is decreased by about 0.36 eV upon one-electron reduction. The reaction energy for Ru–O ? Ru–S isomerization is increased from 0.48 eV to 0.42 eV by one-electron oxidation. But it is more thermodynamically favored by one-electron reduction, which makes the reaction energy decreased by about 0.41 eV. Same results can be obtained from the PESs results for the intramolecular isomerization of [Ru(pic)(tpy)(dmso)]+ in acetonitrile solvent (Table 6). Small solvent effects are due to the fact that these are unimolecular processes and not much is happening in terms of changing the charge distribution. In generally, the Ru–S ? Ru–O isomerization is favored by oxidation, while Ru–O ? Ru–S isomerization is much easier to be induced by reduction.
4. Conclusion For the aim to get more information about the electrochromic mechanism of [Ru(pic)(tpy)(dmso)]+, intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group in Ru(II), Ru(III) and Ru(I) sulfoxide complexes was explored theoretically with DFT-D3(BJ) method in this work. It is found that the interaction energy between dmso and Ru center with Ru–S linkage mode is larger than that with Ru–O linkage mode due to the larger electron donating ability of S atom compared with O atom. Then, the charge transfer amount from surrounding ligands to Ru center is decreased by Ru–S ? Ru–O isomerization, which makes the HOMO and LUMO destabilized. Correspondingly, IPs become lower and EAs become higher upon Ru–S ? Ru–O linkage isomerization. Moreover, the HOMO–LUMO energy gap is decreased because HOMO is destabilized to a larger extent than LUMO. PESs results show that the Ru–S ? Ru–O isomerization process is more thermodynamically favored in the Ru(III) sulfoxide complexes. Differently, the Ru–O ? Ru–S linkage mode isomerization is more easier to be induced by reduction of Ru(III) or Ru(II) complexes.
Acknowledgements
Fig. 5. Schematic representation of the potential energy surfaces of the Ru(II) (black curve), Ru(III) (blue curve) or Ru(I) (red curve) complexes. These data are obtained in the gas phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
This work is supported by the National Natural Science Foundation of China (21403037), the Natural Science Foundation of Jiangxi Province (20142BAB213014), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, PR China.
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Theoretical studies on the redox stimulated intramolecular isomerization in [Ru(pic)(tpy)(dmso)]+ Huifang Li a,⇑, Nana Wang b, Lisheng Zhang a, Xiaolin Fan a,⇑ a b
Key Laboratory of Organo-Pharmaceutical Chemistry, Gannan Normal University, Jiangxi Province 341000, China Yantai Automobile Engineering Professional College, Shandong Province 265500, China
a r t i c l e
i n f o
Article history: Received 30 August 2014 Received in revised form 15 October 2014 Accepted 16 October 2014 Available online 23 October 2014 Keywords: [Ru(pic)(tpy)(dmso)]+ Redox Intramolecular isomerization Electrochromic mechanism Dispersion corrected density functional theory
a b s t r a c t Redox induced intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group in [Ru(pic)(tpy)(dmso)]+ (pic = 2-picolinate; tpy = 2,20 :60 ,200 -terpyridine; dmso = dimethyl sulfoxide) was explored theoretically. Calculated results show that the energy barrier for the Ru–S–Ru–O linkage isomerization is decreased by one-electron oxidation, while is increased by one-electron reduction. Moreover, compared with that in the Ru(II) and Ru(I) sulfoxide complexes, intramolecular Ru–S ? Ru–O isomerization is more thermodynamically favored in the Ru(III) complexes. Differently, the energy barrier for the reaction path from Ru–O to Ru–S linkage mode is increased by one-electron oxidation, while is decreased by one-electron reduction. Then, it can be concluded that the Ru–S ? Ru–O isomerization is favored by Ru(II) oxidation, while Ru–O ? Ru–S isomerization is easier to be induced by Ru(III) or Ru(II) reduction. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction External signal triggered intramolecular isomerization between two ground states of reversible molecules is central to the operation of molecular information storage and switch devices [1]. Recently, a new class of electrochromic and photochromic ruthenium sulfoxide complexes has been synthesized and developed significantly [2,3]. Previous studies demonstrated that intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of a bound sulfoxide ligand may be triggered either by light or by formal oxidation or reduction [4,5]. The differential bonding characteristics of S- and O-bonded sulfoxide produces distinct electronic and photophysical properties. Such structural alteration from the S-bonded to the O-bonded metastable states of ruthenium sulfoxide complexes is accompanied with pronounced changes of the absorption spectra. As a result, a dramatic change in color can be observed [1–5]. Full description of the ground and excited potential energy surfaces (PESs) of [Ru(bpy)(tpy)dmso]2+ (bpy = 2,20 -bipyridine; tpy = 2,20 :60 ,200 -terpyridine) from theoretical approaches have been reported for providing more detailed information about the phototriggered intramolecular isomerization mechanism of monodentate ruthenium sulfoxide complexes [6]. Recently, photoisomerization
mechanism of photochromic ruthenium complex with a chelating sulfoxide complex, [Ru(bpy)2(OSO)]+ (OSO = 2-methylsulfonylbenzoate), has been explored theoretically by Göttle and his coworkers. They found that triplet metal-centered excited states play a central role in the photoisomerization mechanism of ruthenium sulfoxide complex [7]. As a result, nonadiabatic Ru–S ? Ru–O and Ru–O ? Ru–S isomerization can be understood very well. For better understanding the electrochromic mechanism of such ruthenium sulfoxide complexes, redox stimulated intramolecular isomerization of [Ru(pic)(tpy)(dmso)]+ (shown in Fig. 1) will be explored theoretically. It has been demonstrated that Ru–S ? Ru–O and Ru–O ? Ru–S linkage isomerization of [Ru( pic) (tpy)(dmso)]+ can be triggered through electron transport in electrochemical studies, resulting in dramatic shifts in the absorption maxima (from 421 nm to 527 nm) [8–10]. In this work, potential energy surfaces for the Ru–S ? Ru–O and Ru–O ? Ru–S linkage isomerization of Ru(II), Ru(III) and Ru(I) sulfoxide complexes will be described theoretically to clarify the role of the oxidation and reduction in the electrochromic mechanism of [Ru(pic)(tpy)(dmso)]+.
2. Computational details ⇑ Corresponding authors at: Key Laboratory of Organo-Pharmaceutical Chemistry, Gannan Normal University, Jiangxi Province 341000, China. E-mail addresses: [email protected] (H. Li), [email protected] (X. Fan). http://dx.doi.org/10.1016/j.comptc.2014.10.022 2210-271X/Ó 2014 Elsevier B.V. All rights reserved.
Ru–S is a weak interaction and the bond length was overestimated by current density functional theory, as shown in previous report [6]. Göttle and his coworker’s demonstrated that better
H. Li et al. / Computational and Theoretical Chemistry 1050 (2014) 46–50
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result for Ru–S bond-length can be obtained by MPWB1K functional. We tried to use the same method for our optimization. However, lots of unexpected imaginary frequencies exist, which are hard to be eliminated. Thereby, it was hard for us to proceed. Now, dispersion effects that are missing in DFT functionals have been accounted for according to Grimme’s correction scheme (DFT-D3) [11]. The accuracy and reliability of the DFT-D approach for the description of weak interactions have been proved in recent studies [12]. Thereby, in optimization calculations, the empirical dispersion correction introduced by Grimme [11] was added to the DFT functional with Becke’s and Johnson’s rational damping function [13] and dubbed this variant DFT-D3(BJ). Benchmark calculations results demonstrated that a much better description of the Ru–S bond length can be obtained with PBE0-D3(BJ) functional [14]. Initial geometry of S-linked [Ru(pic)(tpy)(dmso)]+ complex were obtained from experimental X-ray data [10]. We calculated the Ru(II) complexes as closed shell and the Ru(I) and Ru(III) complexes using doublet spins. All molecular geometries of the ruthenium sulfoxide complexes discussed here were fully optimized with PBE0-D3(BJ) method. Basis set is made of SDD for Ru [15], a correlation-consistent polarized double-f basis set (cc-pVDZ) [16] for H atoms, and a correlation-consistent polarized triple-f basis set (cc-pVTZ) [16] for C, N, O and S atoms. In addition, considering solvent may play an important roles in geometry determination, conductor-like polarizable continuum model (CPCM) [17] with solvent acetonitrile (e = 35.688) was also considered for comparison. Harmonic vibrational frequencies were calculated analytically at the same level to confirm that each stationary point was a minimum or a transition state on the potential energy surface. Interaction energy (Ed) between dmso ligand and central ruthenium (II) was determined as: Ed = (Edmso + E½RuðpicÞðtpyÞþ )– E½RuðpicÞðtpyÞðdmsoÞþ . Charge distribution was evaluated with natural population analysis (NPA) [18] for the aim to examine the degree of charge transfer between the central Ru and surrounding ligands. All calculations were performed using the Gaussian 09 package [19].
crystallographic data are also included for comparison. Optimized Ru–S bond in S-[Ru(pic)(tpy)(dmso)]+ is 2.242 Å, in good agreement with experimental X-ray crystallographic data which is 2.215 Å [13]. Moreover, overall satisfactory agreement can be obtained between theoretical and experimental results because all of the computed distances are within the experimental error (0.03 Å). Molecular structure of the S-linked [Ru(pic)(tpy)(dmso)]+ is changed a lot by intramolecular isomerization process, especially for the coordinated dmso ligand. By comparing the S–O bond changes in S-, g2- and O-linked Ru(II) complexes, it is found that the S–O bond of dmso ligand is lengthened from 1.485 Å to 1.509 Å upon Ru(II)–S–Ru(II)–O linkage isomerization. As shown in Table 2, the interaction energy (Ed) between dmso and central Ru using Ru–S linkage is larger than that using Ru–O linkage for about 0.5 eV. Thereby, S-[Ru(pic)(tpy)(dmso)]+ is observed to be more stable than O-[Ru(pic)(tpy)(dmso)]+ in experimental studies. Moreover, it found that the S–O bond-length change upon isomerization in the Ru(II) complexes (0.048 Å) is smaller than that in the Ru(III) complexes (0.081 Å), while is larger than that in the reduced state of the complex (0.040 Å). It means effect of one-electron oxidation on the interaction energy between dmso and central Ru is larger than that of one-electron reduction, as shown in Table 2. There occur considerable geometry changes upon one-electron oxidation or reduction process. Moreover, the variation trend of the bond lengths between Ru and dmso ligand are more pronounced than those between Ru and pic or tpy ligand. As shown in Table 1, when one electron is removed, it is found that Ru–S bond is lengthened by 0.128 Å and S–O bond is shortened by 0.008 Å in S-linked complex, while Ru–O bond is shortened by 0.081 Å and S–O bond is lengthened by 0.025 Å in the O-linked isomer. Oppositely, Ru–S bond is shortened by 0.030 Å and S–O bond is lengthened by 0.006 Å in reduced S-linked complex, while Ru–O bond is lengthened by 0.023 Å and S–O bond is shortened by 0.002 Å in reduced O-linked complex. It means oxidation and reduction may have different effects on the Ru(II)–S or Ru(II)–O bond-strength.
3. Results and discussions
3.2. Electronic properties
Formal oxidation or reduction triggered intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group has been observed experimentally in Ru sulfoxide complexes, as shown in Fig. 2. In this work, electrochromic mechanisms of the monodentate Ru sulfoxide complex, [Ru(pic)(tpy)(dmso)]+, are explored.
Calculated EA (electron affinity) and IP (ionization potential) of S-[Ru(pic)(tpy)(dmso)]+ are 4.44 and 9.49 eV in the gas phase, respectively. As shown in Table 3, IP becomes lower and EA becomes higher upon Ru(II)–S–Ru(II)–O linkage isomerization process. Calculated IP and EA of O-[Ru(pic)(tpy)(dmso)]+ are 8.59 and 4.03 eV, respectively. For better understanding IP and EA changes upon intramolecular isomerization, the frontier orbital contour plots are examined, as shown in Fig. 3. The descriptions of molecular orbital composition are summarized in Table 4. It is found that the HOMO (the highest occupied molecular orbital) electron density is mainly distributed on pic ligand (26.6%) and central Ru atom (58.0%), while the electron density of the LUMO (the lowest unoccupied molecular orbital) is mainly distributed on surrounding tpy ligand (83.3%) in S-linked Ru(II) complex. Due to the thermodynamic ability to gain or lose an electron is determined by the energies of the LUMO (5.41 eV) and HOMO (8.87 eV), the HOMO and LUMO electron density distributions are similar with the spin density distributions, as shown in Fig. 4. As mentioned above, the interaction energy between dmso and central Ru is decreased upon Ru(II)–S–Ru(II)–O linkage isomerization. It is attributed to the larger electron-donating ability of S compared with O. NPA (natural population analysis) charge results from NBO (natural bond orbital) analysis show that charge transfer amount from dmso as well as other ligands to Ru center is decreased by about 0.173 upon Ru(II)–S–Ru(II)–O isomerization
3.1. Molecular structures Geometrical parameters obtained from the structural optimization for the S-, SO(g2)- and O-linked Ru(II), Ru(III) and Ru(I) sulfoxide complexes are listed in Table 1. Experimental X-ray
Fig. 1. Chemical structure of [Ru(pic)(tpy)(dmso)]+.
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H. Li et al. / Computational and Theoretical Chemistry 1050 (2014) 46–50
Fig. 2. Ru–S–Ru–O linkage isomerization of the dmso ligand in [Ru(pic)(tpy)dmso]+ complex.
Table 1 Calculated main bond lengths (angstroms) and bond angles (degrees) for the Ru(II), Ru(III) and Ru(I) sulfoxide complexes obtained in the gas phase. S-Linked
Ru–S Ru–O1 S–O1 Ru–Nt0 Ru–Nt00 Ru–Nt0 00 Ru–Np Ru–O2 a(O1–S–Ru)/a(S–O1–Ru)
O-Linked Exptl.
Ox.
Re.
Ru(II)
Ox.
Re.
Ru(II)
Ox.
Re.
2.242 – 1.485 2.069 1.962 2.065 2.101 2.049 116.7
2.215 – 1.478 2.076 1.959 2.068 2.101 2.085 115.3
2.370 – 1.477 2.073 1.984 2.064 2.110 1.955 112.3
2.212 – 1.491 2.064 1.972 2.066 2.093 2.075 121.1
– 2.163 1.533 2.054 1.942 2.050 2.072 2.024 125.8
– 2.082 1.558 2.080 1.971 2.073 2.081 1.937 123.9
– 2.186 1.531 2.053 1.960 2.045 2.053 2.040 116.2
2.853 2.837 1.509 2.055 1.948 2.058 2.085 1.989 74.0
2.563 2.505 1.516 2.105 1.983 2.068 2.104 1.932 70.5
3.387 3.266 1.503 2.041 1.927 2.040 2.073 2.057 72.5
Table 2 Calculated interaction energies between cyclometalating ligands and Ru center in [Ru(pic)(tpy)(dmso)]+ complexes (in eV). S-Linked
dmso pic tpy
SO-Linked
Ru(II)
Table 4 Main distributions of the frontier MOs as well as spin densities in the Ru complexes. S-Linked
O-Linked
O-Linked
Ru(II)
Ru(II)
Ru(III)
Ru(I)
Ru(II)
Ru(III)
Ru(I)
2.1 4.1 6.0
1.8 5.0 6.9
2.2 3.5 4.3
1.7 4.0 6.2
2.2 5.0 6.5
1.4 3.3 4.3
Ru dmso pic tpy
Ru(III)
Ru(I)
Ru(II)
Ru(III)
Ru(I)
HOMO
LUMO
Spin
Spin
HOMO
LUMO
Spin
Spin
58.0 4.0 26.6 11.5
9.6 5.7 1.4 83.3
81.5 0.0 14.6 5.4
3.5 0.0 0.5 96.3
67.3 4.5 17.4 10.8
10.8 6.0 1.2 82.0
86.7 4.2 10.1 0.0
5.8 1.9 0.0 92.6
Table 3 HOMO and LUMO energies in the ground state of these Ru complexes as well as their corresponding ionization potentials electron affinities (in eV). HOMO
LUMO
IP
EA
S-Linked
8.87 (6.37)a
5.41 (2.59)
9.49 (5.66)
4.44 (3.10)
O-Linked
8.01 (5.57)
4.99 (2.35)
8.59 (4.84)
4.03 (2.85)
a The data in the parentheses are obtained in solution, in which the electron handled for calculation is considered to be 0 in the gas phase.
Fig. 4. The spin density isosurface (in blue) associated with the radical ion states of [Ru(pic)(tpy)dmso]+ complexes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Contour plots of frontier orbitals of the [Ru(pic)(tpy)(dmso)]+ complexes.
(shown in Table 5). As a result, compared with that in the S-linked complex, the HOMO wave-function distributed on tpy ligand and central Ru atom is decreased, while distributed on dmso ligand is increased slightly in O-linked [Ru(pic)(tpy)(dmso)]+. As a result, the HOMO and LUMO energies are destabilized by about 0.86 and 0.42 eV, respectively, when the Ru(II)–S linkage is broken and changes to a Ru(II)–O bonding mode [14]. However, Ru(II)– S–Ru(II)–O linkage isomerization destabilizes the HOMO to a larger extent than LUMO for these two photochromic Ru sulfoxide
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H. Li et al. / Computational and Theoretical Chemistry 1050 (2014) 46–50 Table 5 Natural population charges distributed on the central Ru and surrounding ligands (Q).a S-Linked Ru(II) Ru dmso pic tpy a
–0.131 0.486 0.258 0.903
O-Linked Ru(III) 0.229 0.567 0.005 1.208
Ru(I) 0.157 0.446 0.347 0.058
Ru(II) 0.263 0.313 0.286 0.710
Ru(III) 0.540 0.412 0.051 1.099
Table 6 Free energies of reaction (Er) and reaction barrier (Et) for the intramolecular isomerization of Ru(II), Ru(III) and Ru(I) sulfoxide complexes. These data are given at T = 298.15 K and P = 1 Atm (in eV). Gas Phase
Ru(I) 0.245 0.267 0.388 0.125
Q (ligand) indicates the charge on all the atoms in the ancillary ligand.
complexes, which makes the HOMO–LUMO energy gap decreased from 5.1 eV to 4.7 eV [14]. As a result, red shifted absorption and emission upon Ru(II)–S–Ru(II)–O rearrangement can be observed for such Ru sulfoxide complexes because HOMO–LUMO energy gap is quite reasonable for the description of the first excited singlet transition state.
3.3. PESs for intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization Calculated results demonstrate that the binding energies of pic and tpy ligands with central Ru atom are much larger than that between dmso ligand and central Ru. The same is true for the oxidized and reduced Ru(II) complexes. It means Ru–S or Ru–O bond breaking should be more favorable compared with Ru–N linkage mode breaking. That is why intramolecular isomerization of the sulfoxide group from S-bonding to O-bonding or from O-bonding to S-bonding modes was observed by external stimuli. Calculated potential energy surfaces (PESs) in the gas phase for the intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group in Ru(II), Ru(III) and Ru(I) complexes are shown in Fig. 5. It is found that S-[Ru(pic)(tpy)(dmso)]+ is destabilized by about 0.48 eV upon intramolecular isomerization process. The energy difference between S- and O-linked Ru(II) complexes is changed to 0.42 eV and 0.89 eV, respectively, when one electron is removed or added. It means such intramolecular isomerization process is more thermodynamically favored in the oxidized state of Ru(II) sulfoxide complexes. Moreover, the energy barrier for the intermolecular isomerization process from Ru(II)–S to Ru(II)–O linkage mode is decreased from 1.31 eV to 0.61 eV by one-electron oxidation and increased to 1.35 eV by one-electron reduction. Thereby, oxidation triggered intramolecular isomerization of the sulfoxide from S-bonded to O-bonded linkage mode can be observed for such Ru sulfoxide complexes in experimental studies.
Et
Solution Er
Et
Er
S ? O isomerization Ru(II) 1.31 Ru(III) 0.61 Ru(I) 1.35
0.48 0.42 0.89
1.32 0.60 1.23
0.39 0.43 0.64
O ? S isomerization Ru(II) 0.82 Ru(III) 1.02 Ru(I) 0.46
0.48 0.42 0.89
0.94 1.03 0.59
0.39 0.43 0.64
Different cases are observed for the reaction path from Ru–O to Ru–S linkage mode isomerization. Calculated results indicates that energy barrier for the potential energy surface is increased by about 0.20 eV upon one-electron oxidation, while is decreased by about 0.36 eV upon one-electron reduction. The reaction energy for Ru–O ? Ru–S isomerization is increased from 0.48 eV to 0.42 eV by one-electron oxidation. But it is more thermodynamically favored by one-electron reduction, which makes the reaction energy decreased by about 0.41 eV. Same results can be obtained from the PESs results for the intramolecular isomerization of [Ru(pic)(tpy)(dmso)]+ in acetonitrile solvent (Table 6). Small solvent effects are due to the fact that these are unimolecular processes and not much is happening in terms of changing the charge distribution. In generally, the Ru–S ? Ru–O isomerization is favored by oxidation, while Ru–O ? Ru–S isomerization is much easier to be induced by reduction.
4. Conclusion For the aim to get more information about the electrochromic mechanism of [Ru(pic)(tpy)(dmso)]+, intramolecular Ru–S ? Ru–O and Ru–O ? Ru–S isomerization of the coordinated sulfoxide group in Ru(II), Ru(III) and Ru(I) sulfoxide complexes was explored theoretically with DFT-D3(BJ) method in this work. It is found that the interaction energy between dmso and Ru center with Ru–S linkage mode is larger than that with Ru–O linkage mode due to the larger electron donating ability of S atom compared with O atom. Then, the charge transfer amount from surrounding ligands to Ru center is decreased by Ru–S ? Ru–O isomerization, which makes the HOMO and LUMO destabilized. Correspondingly, IPs become lower and EAs become higher upon Ru–S ? Ru–O linkage isomerization. Moreover, the HOMO–LUMO energy gap is decreased because HOMO is destabilized to a larger extent than LUMO. PESs results show that the Ru–S ? Ru–O isomerization process is more thermodynamically favored in the Ru(III) sulfoxide complexes. Differently, the Ru–O ? Ru–S linkage mode isomerization is more easier to be induced by reduction of Ru(III) or Ru(II) complexes.
Acknowledgements
Fig. 5. Schematic representation of the potential energy surfaces of the Ru(II) (black curve), Ru(III) (blue curve) or Ru(I) (red curve) complexes. These data are obtained in the gas phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
This work is supported by the National Natural Science Foundation of China (21403037), the Natural Science Foundation of Jiangxi Province (20142BAB213014), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, PR China.
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