Negative Ion Photoelectron Spectroscopy Reveals Remarkable

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Negative Ion Photoelectron Spectroscopy Reveals Remarkable Noninnocence of Ligands in Nickel Bis(dithiolene) Complexes [Ni(dddt)] and [Ni(edo)] 2

–

2

–

Xing Liu, Gao-Lei Hou, Xuefeng Wang, and Xue-Bin Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02711 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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The Journal of Physical Chemistry

Negative Ion Photoelectron Spectroscopy Reveals Remarkable Noninnocence of Ligands in Nickel Bis(dithiolene) Complexes [Ni(dddt)2]– and [Ni(edo)2]–

Xing Liu,a,c,† Gao-Lei Hou,b,† Xuefeng Wang,c,* and Xue-Bin Wangb,* a

College of Chemistry and Chemical Engineering, Southwest University, Chongqing

400715, China b

Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999,

MS K8-88, Richland, Washington 99352, United States c

Department of Chemistry, Tongji University, Shanghai 200092, China

†

These authors contributed equally to this work.

Email Address: [email protected] (X.-B.W.); [email protected] (X.-F.W.)

1 ACS Paragon Plus Environment


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ABSTRACT [Ni(dddt)2]– (dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) and [Ni(edo)2]– (edo = 5,6-dihydro-1,4-dioxine-2,3-dithiolate) are two donor-type nickel bis(dithiolene) complexes, with the tendency of donating low binding energy electrons.

These two

structurally similar complexes differ only with respect to the outer atoms in the ligand framework where the former has four S atoms while the latter has four O atoms. Herein, we report a negative ion photoelectron spectroscopy (NIPES) study on these two complexes to probe electronic structures of the anions and their corresponding neutrals. The NIPE spectra exhibit the adiabatic electron detachment energy (ADE) or, equivalently, the electron affinity (EA) of the neutral [Ni(L)2]0 to be relatively low for this type complexes, 2.780 and 2.375 eV for L = dddt and edo, respectively. The 0.4 eV difference in ADEs shows significant substitution effect for sulfur in dddt by oxygen in edo, i.e., noninnocence of the ligands, which has decreased the electronic stability of [Ni(edo)2]– by lowering its electron binding energy by ~0.4 eV. The observed substitution effect on gas-phase EA values correlates well with the measured redox potentials for [Ni(dddt)2]–/0 and [Ni(edo)2]–/0 in solutions. The singlet-triplet splitting (∆EST) of [Ni(dddt)2]0 and [Ni(edo)2]0 is also determined from the spectra to be 0.57 and 0.53 eV, respectively. Accompanying DFT calculations and molecular orbital (MO) composition analyses show significant ligand contributions to the redox MOs and allow the components of the orbitals involved in each electronic transition and spectral assignments to be identified.


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INTRODUCTION Nickel bis(dithiolene) complexes have been extensively studied as a result of their interesting optical,1-3 magnetic,4,5 and conducting properties,6 unique electrochemical reactivity,7,8 relevance to important bioinorganic centers,9-11 and as building blocks for novel organic topological insulator nanosheets.12-14 The intrinsic electronic structures and bonding characters of these complexes are the key molecular properties to defining and controlling their functionalities. A variety of techniques, including cyclic voltammetry,2 X-ray crystallography,6 X-ray photoelectron spectroscopy,14 EPR15,16 ENDOR/ESEEM spectroscopies,17 sulfur K-edge X-ray absorption spectroscopy,18 infrared multiple photon dissociation spectroscopy (IRMPD),19,20 and negative ion photoelectron spectroscopy (NIPES)21-25

have been

employed to investigate their geometrical structures, electronic properties, and redox potentials. These studies indicate appreciable dependence (noninnocence) of dithiolene ligands in the complexes for their electronic structures, redox chemistry, and frontier orbitals due to the significant degree of mixing between metal and ligand based orbitals.26

Figure 1. The molecular structures of Ni(dddt)2, Ni(edo)2 and Ni(mnt)2, and their comparison with BEDT-TTF and BEDO-TTF.



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Ni(dddt)2 and Ni(edo)2 (dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate and edo = 5,6-dihydro-1,4-dioxine-2,3-dithiolate) (Figure metal-dithiolene complexes. well-known

27-30

molecular

tetrathiafulvalene),

27-29

1) are two novel donor-type

The former is a coordination analogy of the conductor

BEDT-TTF

(bis(ethylenedithio)

in which the central C=C double bond of BEDT-TTF is

substituted by nickel; the latter, which is similar to that of BEDO-TTF (bis(ethylenedioxy)tetrathiafulvalene)30 can be regarded as a derivative of Ni(dddt)2, in which the four outer sulfur atoms are substituted by the oxygen atoms. Electrochemical studies indicate that Ni(edo)2 ranks first as a donor-type complex, being 0.27 and 1.27 V easier to donate electrons than Ni(dddt)2 and Ni(mnt)2 (mnt = 1,2-S2C2(CN)2, Figure 1), respectively,30 showing significant noninnocence of dithiolene ligands in determining their redox chemistry. It is particularly remarkable to notice that the 0.27 V difference in E1/2(0/−) results from the substitution of S in Ni(dddt)2 by O in Ni(edo)2, and that the substituted atoms reside at the third layer of coordination from the core Ni atom after the inner S and C atoms. NIPES has been demonstrated as a powerful spectroscopic method to study intrinsic electronic structures of isolated metal complexes free from bulk environments.21-24,31,32 A number of gas phase studies using NIPES to probe the electronic structures of [MoOS4]−25 and transition metal bis(dithiolene) centers [M(mnt)2]− 21,22 have been reported, revealing interesting and subtle interplay between ligand- and metal-based redox chemistry. In addition, the obtained gas-phase electron affinities (EAs) can be compared with the the solution-phase measured E1/2(0/−) reduction potentials, providing an excellent opportunity to tease apart intrinsic electronic part and solvation effect in the solution redox measurements. For example, we recently found that the change of the EAs among the triad of CpM(CO)3• (Cp =

η5-C5H5; M = Cr, Mo, W) radicals tracks the variation of their measured E1/2(0/−) values in acetonitrile quite well,32 suggesting similar solvation effects for this series of complexes upon electron transfer in solutions. However, in another recent study of the electronic and electrochemical properties of a series of seven structurally-similar organoflurine-buckyball

1,7-C60(RF)2

using

NIPES

and

cyclic/square-wave

voltammetry, we found that the EA variation among different RF groups is nearly counter-balanced by the changes of solvation energies, resulting in virtually identical E1/2(0/−) reduction potentials.33 4 ACS Paragon Plus Environment

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We herein report joint NIPES study and quantum chemical calculations on [Ni(dddt)2]−/0 and [Ni(edo)2]−/0 to investigate their intrinsic electronic structures and to obtain accurate gas-phase EA values with a focus on the critical (noninnocent) roles of dithiolene ligands. Well-resolved and rich spectral features are obtained for each species, yielding detailed electronic structure information which is assigned with the aid of electronic structure calculations. The obtained gas-phase EA values are compared to the solution-phase measured reduction potentials as well. EXPERIMENTAL AND THEORETICAL METHODS Negative Ion Photoelectron Spectroscopy The

NIPES

experiments

were

performed

using

a

low-temperature,

magnetic-bottle time-of-flight (TOF) photoelectron spectrometer, coupled with an electrospray ionization (ESI) source and a temperature-controllable cryogenic ion-trap.34 [Ni(dddt)2]– anions were generated by spraying freshly prepared ~0.1 mM acetonitrilic aqueous solution of (TBA)Ni(dddt)2 into the vacuum; [Ni(edo)2]– was produced by spraying the CH3CN/H2O Ni(edo)2 solution, which was titrated by adding diluted acetonitrilic tetrakis(dimethylamino)ethylene (TDAE) dropwise until the solution color changes from blue to reddish. The produced anions were transported by RF-only ion guides into the 3D cryogenic ion trap (set at 20 K) to be accumulated and collisionally cooled with cold buffer gas (20% H2 balanced in helium) for 20-100 ms, eliminating possible extra features due to vibrational hot bands in photoelectron spectra. The cold ions were then transferred into the extraction zone of a TOF mass spectrometer for mass and charge analyses. During each NIPES experiment, the desired [Ni(L)2]– anions were mass-selected and decelerated before being photodetached by a laser beam in the photodetachment zone. Three photon energies were employed in the current study, i. e., 355 nm (3.496 eV) and 266 nm (4.661 eV) from a Nd:YAG laser, and 193 nm (6.424 eV) from a ArF excimer laser. All lasers were operated at a 20 Hz repetition rate with the ion beam off at alternating laser shots to enable shot-by-shot

background subtraction.

Photoelectrons were collected at nearly 100% efficiency by the magnetic-bottle and analyzed in a 5.2 m long calibrated electron flight tube. The TOF photoelectron spectra were converted into electron kinetic energy spectra, and the electron binding energies (EBEs), given in the spectra in Figures 2 and 3, were obtained by subtracting 5 ACS Paragon Plus Environment


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the electron kinetic energies from the detachment photon energies. The energy resolution (∆E/E) was approximately 2%, i.e., ~ 20 meV full width at half maximum for 1 eV kinetic energy electrons.

Theoretical Methods Density functional theory (DFT) calculations with the BP86 functional35,36 were carried out using the Gaussian 09 program.37 This functional was chosen because it has previously shown to perform quite well for [Ni(mnt)2]–/0 complexes.21,22 The 6-311+G(d) basis set was used for H, C, O, and S atoms.38 The Stuttgart quasi-relativistic effective core potentials and basis sets augmented with two f-type and one g-type polarization functions39-41 obtained from the EMSL basis set exchange42 was used for Ni. The optimization of [Ni(L)2]–/0 (L = dddt and edo) was done without any symmetry constraint. Harmonic vibrational analysis was carried out to verify that each structure is the real minimum. The theoretical adiabatic detachment energy (ADE) was calculated as the energy difference between the neutral and the anion at their respective optimized structures, including zero-point energy (ZPE) corrections. The first vertical detachment energy (VDE) was calculated by the ∆SCF method, i.e., the energy difference between the neutral and the corresponding anion both at the anionic geometry. The VDEs for higher EBE detachment features were calculated from the energy differences between the excited states of the neutral complex and the ground state of the anion. These calculations were conducted using time-dependent density functional theory (TD–DFT).43-45 In order to simulate the vibrational structure of the first band (feature X) shown in the NIPE spectra of [Ni(L)2]–, the FC Classes program embedded into Gaussian09 was employed to calculate the Franck-Condon factors (FCFs) for vibrational transitions from the lowest electronic state of [Ni(L)2]– to the ground state of neutral [Ni(L)2]0. In addition, orbital composition calculations (based on natural population analysis)46 were performed for the top eight occupied molecular orbitals (MOs) to obtain quantitative metal and ligand contributions for each MO, and to provide insightful molecular orbital description for understanding the noninnocent roles of ligands.


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EXPERIMENTAL RESULTS NIPE Spectra of [Ni(L)2]– (L = dddt and edo)

Figure 2. The 20 K NIPE spectra of [Ni(dddt)2]– at a) 355, b) 266, and c) 193 nm, respectively. The calculated VDEs are indicated (blue bars for singlets and green bars for triplets). The calculated 1st VDE is shifted to align with the X band peak and the rest of the calculated VDEs are adjusted by the same amount accordingly.



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Figure 3. The 20 K NIPE spectra of [Ni(edo)2]– at a) 355, b) 266, and c) 193 nm, respectively. The calculated VDEs are indicated (blue bars for singlets and green bars for triplets). The calculated 1st VDE is shifted to align with the X band peak and the rest of the calculated VDEs are adjusted by the same amount accordingly. Figure 2 shows the 20 K NIPE spectra of [Ni(dddt)2]– recorded with 355, 266, and 193 nm, revealing rich and multiple spectral bands. The measured spectral features are labled X, and A to I in the order of increasing EBE. It can be seen that consistent spectral bands are observed at three wavelengths,

but as the intrinsic

energy resolution of the instrument improves with the decrease of detachment photon energy, the measured spectral features become narrower and the spectra are better resolved. For instance, among the first three bands (X, A, and B), only X is partially resolved at 193 nm, but at 266 nm all three bands show fine structures, and the X band is further resolved in the 355 nm spectrum showing one vibrational progression with a 8 ACS Paragon Plus Environment

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frequency of 1360 ± 40 cm-1. The ADE of [Ni(dddt)2]–, or equally the electron affinity (EA) of neutral Ni(dddt)2, is determined from the first resolved 0–0 transition to be 2.780 ± 0.010 eV. The 20 K NIPE spectra of [Ni(edo)2]– measured with 355, 266, and 193 nm are presented in Figure 3. The spectra of [Ni(edo)2]– are overall similar to that of [Ni(dddt)2]– with ten resolved spectral bands (X, A to I) due to transitions from the ground state of the anion to the ground and excited states of the neutral conterpart. One vibrational progression with a frequency of 1455 ± 40 cm-1 is identified in the 355 nm spectrum. The ADE of [Ni(edo)2]– or the EA of Ni(edo)2 is measured to be 2.375 ± 0.010 eV, ~0.4 eV lower than that of [Ni(dddt)2]–. This exhibits that the subsitution of S by O even at the outer layer of dithiolene ligands has a significant effect on the electronic stability of nickel bis(dithiolene) complexes. THEORETICAL RESULTS Optimized Structures

Figure 4. Selected bond lengths (Å) and bond angles (o) of the optimized structures of [Ni(L)2]– (L = dddt and edo) anions (black) and corresponding neutrals (red). The hydrogen atoms have been omitted for clarity. The optimized structures for the [Ni(L)2]– anions and corresponding [Ni(L)2] neutrals are shown in Figure 4. The calculated bond lengths and angles are consistent 9 ACS Paragon Plus Environment


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with available solid-state structures.29,31,32 Comparison of the anionic and neutral structures reveals minor geometric changes upon the electron detachment. It shows that the Ni–S, S–C, and C–S(O) bonds all shorten by ~0.02-0.03 Å while the C=C bond increases by 0.027 Å; this accords with previous studies of similar nickel bis(dithiolene) systems.5,21,22,47,48 The good agreement between our calculated structures and those determined in the solid state, and the consistent anion-to-neutral structural changes from our calculations comparing with previous calculations, lends consideral credence in validating the theoretical method employed in this work. Interestingly, oxygen substitution at the outer coordination layer induces appreciable bond length variation in the [Ni(S2C2)2] core, i.e., the S–C bond contracts by ~0.014 Å while the Ni–S bond increases by about 0.015 Å. This bond length changes may be presumably due to the higher electronegativity of O than S that results in an increase of the positive charge on the neighbor double bond C atoms, which reinforces the Columb attraction between the S and C atoms while weakens the attraction between S and Ni atoms. The natural charge distribution based on the Natural Population Analysis (NPA) shown in Table S1 of the Supporting Information (SI) supports the above inference. Molecular Orbital and Chemical Bonding Analyses Figure 5 illustrates the lowest unoccupied molecular orbital (LUMO), singly occupied MO (SOMO) and the top seven highest occupied MOs of [Ni(L)2]– (L = dddt (left) and edo (right)). It can be seen from the left panel that the major consitutents of the [Ni(dddt)2]– MOs are from Ni 3d, S 3p, and C 2p atomic orbitals. Based on the components of each MOs these orbitals can be classified into three categories: metal-based orbitals (HOMO-3 and HOMO-7), ligand-based orbitals (SOMO, HOMO-1, HOMO-5, and HOMO-6), and metal & ligand mixed orbitals (HOMO-2 and HOMO-4). This classification is in good agreement with the MO composition analyses (Table 1). The percentage of the metal consitituent accounts for


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Figure 5. Molecular orbital contour plots (contour cutoff = 0.02) and energy levels for LUMO, SOMO and the top seven occupied orbitals of [Ni(L)2]– (L = dddt and edo). Only the α- sets are shown and the orbital symmetries are indicated.

more than 80% for HOMO-7 and approaches 94% for HOMO-3. However, the contribution of metal decreases to 50% or less for HOMO-2 and HOMO-4, and it becomes less than 10% for all of the other orbitals. Both HOMO-1 and HOMO-6 are bonding orbitals, while HOMO-2 and HOMO-4 are anti-bonding orbitals, and the rest are non-bonding in nature. The MOs of [Ni(edo)2]– as shown in the right panel of Figure 5 are similar to those of [Ni(dddt)2]– and therefore similar conclusions can be drawn, except that the order of the last two MOs, i.e., HOMO-6 and HOMO-7, is reversed. We also noted that the metal contributions to the MOs are slightly larger in 11 ACS Paragon Plus Environment


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[Ni(edo)2]– than [Ni(dddt)2]–, except for the SOMOs and HOMO-7 for which the contributions from metal are almost the same, and slightly larger in [Ni(dddt)2]–, respectively. It is also noticeable that the contributions of S and C increase upon substitution of sulfur by oxygen from [Ni(dddt)2]– to [Ni(edo)2]– in their respective HOMO-5 (Table 1). Since HOMO-5 is a S–C bonding orbital, the increased contributions of the constituent atoms are expected to result in a shorter S–C bond, which is consistent with the optimized structures of the complexes. However, if the same argument is applied to HOMO-1, it would result in a shortened Ni–S bond for [Ni(edo)2]– upon the O substitition of S, but this expectation is overly conter-balanced by the opposite effect contributed from HOMO-2. The Ni, S, and C compositions of other MOs barely change from L = dddt to edo and their contributions to the Ni–S and S–C bond length variations can be ignored. Table 1. DFT Calculated Molecular Orbital Compositions for [Ni(L)2]– (L = dddt and edo) Anions.a

a

MO

E (eV)

SOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4 HOMO-5 HOMO-6 HOMO-7

-0.81(-0.48) -1.55(-1.40) -1.72(-1.54) -1.91(-1.82) -2.37(-1.97) -2.48(-2.33) -2.96(-2.89) -3.10(-2.95)

SOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4 HOMO-5 HOMO-6 HOMO-7

-0.34(-0.03) -1.13(-0.95) -1.32(-1.15) -1.45(-1.37) -2.11(-1.62) -2.35(-2.16) -2.64(-2.51) -2.69(-2.64)

MO pop. analysis: % M characterb Ni S C S/Oc – [Ni(dddt)2] 9.79(18.77) 57.48(56.52) 21.8(17.28) 9.08(5.64) 4.20(4.90) 29.12(31.24) 34.36(34.92) 29.64(26.40) 55.87(55.79) 29.68(29.60) 4.44(5.20) 8.44(7.76) 93.95(94.59) 5.12(4.48) 0.04(0.04) 0.32(0.28) 35.85(50.45) 7.76(0.64) 10.56(16.68) 42.76(30.00) 0.28(0.25) 62.16(64.96) 2.88(4.12) 32.00(28.20) 7.38(8.08) 86.64(83.04) 2.84(1.96) 0.92(4.52) 80.54(80.53) 12.8(12.92) 5.12(5.08) 0.52(0.44) – [Ni(edo)2] 9.63(16.00) 57.32(56.20) 24.24(20.68) 6.96(5.24) 4.79(5.40) 35.20(35.76) 40.60(40.68) 17.08(15.88) 64.32(62.98) 26.88(27.72) 4.60(5.04) 2.92(2.92) 95.00(95.44) 4.20(3.72) 0.04(0.08) 0.16(0.12) 65.76(67.22) 1.00(0.48) 17.24(19.32) 14.28(11.56) 0.80(0.46) 79.12(79.60) 6.92(7.76) 11.04(10.20) 81.86(82.15) 11.40(11.24) 4.72(4.68) 0.96(0.92) 3.94(3.63) 92.60(92.84) 2.00(1.96) 0.76(0.84)

The singly occupied molecular orbital (SOMO) and the top seven highest occupied

MOs of both the α- and β- sets (in parentheses) are given. b Minor compositions of the terminal alkyl groups are ignored. c Four outermost sulfur atoms of [Ni(dddt)2]– and corresponding oxygen atoms of [Ni(edo)2]–. 12 ACS Paragon Plus Environment

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Calculated EBEs and Vibrational Frequencies The ADEs of [Ni(L)2]– (or the EAs of [Ni(L)2]) are calculated to be 2.550 and 2.214 eV for L = dddt and edo, respectively, both in good agreement with the experimental measurements of 2.780 and 2.375 eV. The calculated ADE difference of 0.336 eV also compares well with the measured difference of 0.405 eV. Under the single particle framework (Koopmans’ approximation), detaching one electron from each SOMO of [Ni(L)2]– (L = dddt and edo) in their respective ground doublet state allows formation of the singlet ground state of [Ni(L)2]0, shown as band X in their NIPE spectra. Electron detachment from the deep occupied MOs (HOMO-1, HOMO-2, etc.) results in formation of excited states, in either singlets or triplets of the neutral product [Ni(L)2]0, corresponding to the spins of two unpaired electrons antiparallel or parallel, respectively. The formation of these excited states is shown as spectral bands at higher EBE in the measured NIPE spectra. TD-DFT was used to calculate the energies accessing the relevant excited states, and the calculated results are summarized in Table 2. They are also presented in Figures 2 and 3 as vertical bars for direct comparison with the experimental spectra. It can be seen that the calculated EBEs for the ground and excited states overall match well with the experimental spectral band positions. Table 2. Experimental and Theoretical Vertical Detachment Energies (VDEs, in eV) of [Ni(L)2]– (L = dddt and edo) Anions.

Feature X A B C D E F G H I

[Ni(dddt)2]– Exptl. Calc. 2.780 2.585 3.350 3.063 3.505 3.335 3.830 3.653 4.000 3.748 4.18 3.893 4.31 3.990 4.54 4.471 5.21 4.994 5.477 5.77

[Ni(edo)2]– Exptl. 2.375 2.905 3.085 3.600 3.750 3.92 4.12 4.41 5.08 5.75


Calc. 2.304 2.807 3.161 3.410 3.612 3.633 3.936 4.099 5.050 5.686


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Supplementary calculations for the neutral species indicate that the Ni–L(ligand) breathing mode, S–Ni–S bending mode, and C=C stretching mode mainly contribute to the fine structures of the X band in each [Ni(L)2]– spectrum. The vibrational frequencies calculated for Ni(dddt)2 are νNi-L: 136.6 cm-1, δS-Ni-S: 367.0 cm-1, and νC=C: 1355.0 cm-1. Similar predictions are obtained for Ni(edo)2 complex (νNi-L: 157.4 cm-1, δs-Ni-S: 342.2 cm-1, and νC=C: 1456.9 cm-1). Thus, we can assign the experimentally measured frequencies of 1360 and 1455 cm-1 (Figure 2 and 3) to the C=C stretching in each [Ni(L)2]0 complex; the calculated C=C stretching mode frequencies are also consistent with typical frequencies for C=C vibration.31,49 Furthermore, the calculated frequency difference from L = dddt to edo compares very well with the experimental measurement (102 vs 95 cm-1). DISCUSSION Spectral Assignment for Anions [Ni(L)2]– (L = dddt and edo) As described above, the spectral features (X-I) in the NIPE spectra (Figures 2 and 3) of [Ni(L)2]– correspond to the transitions from the ground state of the anion to the ground and excited states of the neutral complex [Ni(L)2]0. With the aid of TD-DFT calculations, correlations between the calculated and experimental VDEs can be identified. The X feature of [Ni(dddt)2]– originates from electron detachment of SOMO, which has Ni–S, S–C, and C–S anti-bonding character and C=C bonding character. Hence, ejection of electron from this orbital is expected to give rise to contraction of Ni–S, S–C, and C–S bonds and elongation of C=C double bond, consistent with the structural changes between [Ni(dddt)2]– and [Ni(dddt)2]0 obtained from the DFT optimizations (Figure 4).

Features A and B correspond to the

formation of the first and second excited states of [Ni(dddt)2]0. TD-DFT calculations predict two low lying, triplet excited states of [Ni(dddt)2]0 arising from one-electron transition, i.e., HOMO-1(b1) → SOMO(b2) and HOMO-2(b3) → SOMO(b2) to be higher in energy by 0.478 and 0.750 eV relative to the ground state, respectively. These values reproduce well the experimental measurements of VDE difference between feature A and X (0.57 eV), and between feature B and X (0.73 eV), respectively, thus allowing assignment of spectral band A and B accordingly. The transition of HOMO-3(a) → SOMO(b2) is predicted to be 1.068 eV, in good agreement with the observed position of feature C, which is measured to be 1.05 eV


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higher than the X feature. The other states predicted from the DFT calculations are also in good agreement with the experiments (Figure 2 and Table 2). As already noted above, the VDE of the X feature of [Ni(edo)2]– is 2.375 eV, ~0.4 eV lower than that of [Ni(dddt)2]–, showing a significant subsitution effect of sulfur by oxygen. This difference can be tracked back to the nature of the SOMO which accommodates an unpaired electron. From Table 1, it can be seen that the metal center only contributes ~10 % for the SOMOs of [Ni(L)2]– and that detaching electrons from these orbitals will be substantially influenced by the “noninnocent” ligand backbone;5,48,50 the electron affinity of O is much lower by ~0.6 eV than that of S.51 Theoretical calculation predicts the VDE of feature X in [Ni(edo)2]– to be 2.304 eV, only ~0.07 eV underestimated compared to the experimental value. The structural changes due to electron detachment from [Ni(edo)2]– to [Ni(edo)2]0 are found similar to those for [Ni(dddt)2]–/0 species, which can be rationalized using the similar reasoning described above. We can also make analogous assignments to the spectral features in [Ni(edo)2]– (See Figure 3 and Table 2 for the details of the assignments). Simulation of Band X in NIPE Spectra of [Ni(L)2]– (L = dddt and edo)

Figure 6. Simulated NIPE spectra of [Ni(L)2]– (L = dddt and edo) in their X feature regions using Gaussian line shapes with the default full widths at half maxima 15 ACS Paragon Plus Environment


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(FWHM) of 33 meV using the Gaussian program. The simulated spectra are superimposed on the experimental 266 nm spectra. The origins of the calculated 0−0 bands have been adjusted to match the experimentally measured values of 2.780 and 2.375 eV, respectively. Band X in each of the NIPE spectra of [Ni(L)2]– is formed by removal of one excess electron from the respective SOMO of the anion. Because SOMO possesses a bonding character in the C=C bond, detaching one electron from it is expected to lengthen its bond and activate the C=C bond stretching mode.

The observed

vibrational progression of band X can be assigned to the C=C stretching vibration for each one of the neutral complex Ni(L)2. We simulated in Figure 6 the NIPE spectra of both [Ni(dddt)2]– and [Ni(edo)2]– in their X feature regions based on the calculated frequencies and FCFs. It can be seen that the simulated spectra match the experiments very well. Noninnocent Dithiolene Ligands L = dddt vs. edo, and vs. mnt in [Ni(L)2]–/0 [Ni(dddt)2] differs from [Ni(edo)2] only on the outer atoms in the ligand framework where the former has four S atoms while the latter has four O atoms. Yet, our NIPES study shows an appreciable EA difference with EA of [Ni(dddt)2] ~0.4 eV larger than that of [Ni(edo)2], illustrating noninnocence of the ligands influencing electron binding energy. The nature of this ligand noninnocence can be seen clearly in Figure 5 and Table 1, both suggesting that the frontier MOs contain significant contributions from the ligands, in particular, SOMO is dominant from the ligand contributions. Comparison of EAs of [Ni(edo)2] and [Ni(dddt)2] with that of [Ni(mnt)2] is even striking;

the EAs of [Ni(edo)2] and [Ni(dddt)2] are 2.2 eV, and 1.8 eV

smaller than that of [Ni(mnt)2] (Table 3),21,22 validating that the former two complexes studied in this work are donor-type complexes.29-32 It should be pointed out that the noninnocence of dithiolene ligands mainly refers to electron acceptance or donation abilities for their metal complexes. Ligand influences on excitation energy are much smaller. For instance, the singlet-triplet splitting (∆EST) determined from the EBE difference of features X and A are 0.57 and 0.53 eV, for [Ni(dddt)2]0 and [Ni(edo)2]0, respectively, and both are not very different from that of [Ni(mnt)2]0 (0.79 eV).21 The

∆EST is an important parameter of electronic structures of [Ni(L)2]0 neutrals and can be useful to be compared with the optic absorption bands. 16 ACS Paragon Plus Environment

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Table 3. Experimental and Calculated Adiabatic Detachment Energies (ADEs) for [Ni(L)2]– (L = dddt, edo, and mnt) Anions and Their Comparison with Electrochemical Potentials in Solutions. Bun4-NClO4 / benzonitrile solution

Gas phase L

Exptl. ADE (eV)a

Calc. ADE (eV)b

E1/2 (V vs Ag/AgNO3)c

edo

2.375±0.008 (0.00)

2.21 (0.00)

-0.86 (0.00)

dddt

2.780±0.008 (0.41)

2.55 (0.34)

-0.59 (0.27)

mnt

4.56 ± 0.04 (2.19)d

4.57 (2.26)d

+0.41(1.27)

a

Equivalent to the electron affinity (EA) of the neutrals [Ni(L)2]0. zero-point energy corrections. c From Ref 30. d Ref. 21.

b

Including

Since one-electron oxidation in solution and the electron detachment in the gas phase are both one-electron loss process, the reduction potentials of [Ni(L)2]–/0 in solutions are expected to be closely related to the gas phase EAs of [Ni(L)2]0.24,52 In Table 3, we compared the gas-phase EAs with the solution-phase measured E1/2([Ni(L)2]–/0) reduction potentials (measured in Bun4-NClO4/benzonitrile by cyclic voltammetry). Within the experimental error, it can be seen that the change of the EAs between [Ni(dddt)2]0 and [Ni(edo)2]0 and between the currently studied two complexes with the previously reported [Ni(mnt)2]0 tracks the variation of E1/2(–/0) well, although the absolute values are quite different as expected. The lower EA of [Ni(edo)2]0 also explains why [Ni(edo)2]– ranks first as a donor-type complex among the nickel bis(dithiolene) complexes (including the mixed-ligand complexes).15,16,26 CONCLUSIONS We report the NIPE spectra of two donor-type nickel bis(dithiolene) complexes [Ni(L)2]– (L = dddt and edo). From the spectra, the electron affinities of [Ni(L)2]0 are determined to be 2.780 and 2.375 eV for L = dddt and edo, respectively. We also obtained the singlet-triplet splittings (∆EST) of [Ni(dddt)2]0 and [Ni(edo)2]0 to be 0.57 and 0.53 eV, respectively. With the aid of DFT calculations and molecular orbital composition analyses, the components of the orbitals that are involved in each transition are clarified. It is found that the top two occupied MOs are mainly ligand-based and contribute to the first two spectral bands (X & A) while the band C is predominantly associated with the metal center. Comparison between [Ni(dddt)2]–/0 17 ACS Paragon Plus Environment


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and [Ni(edo)2]–/0 reveals that substitution of sulfur by oxygen has induced a significant effect on the electronic stability of [Ni(edo)2]–/0, lowering its EA by ~0.4 eV. Interestingly, this value correlates well with the difference of the reported redox potentials for [Ni(dddt)2]–/0 and [Ni(edo)2]–/0 measured in solution. The current NIPES data provides valuable experimental information on the ground and excited states of neutral [Ni(L)2]0, and shows convincingly the noninnonence of dithiolene ligands in determining their redox chemistry. ASSOCIATED CONTENT Supporting Information Top and side views of the optimized structures for [Ni(L)2]– (L = dddt and edo) anions and neutrals; natural charges calculated for [Ni(L)2]–/0 (L = dddt and edo) species based on NPA analysis; cartesian coordinates of the optimized structures for [Ni(L)2]– (L = dddt and edo) anions and neutrals. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Xue-Bin Wang: tel, +1 509-371-6132; e-mail, [email protected] * Xuefeng Wang: tel, +86 2165980301; e-mail, [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Fundamental Research Funds for the Central Universities (Grant No. XDJK2016C030 and SWU115072) and by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. The experimental work was performed using EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle Memorial Institute for the DOE. The theoretical calculations were conducted on Rockscluster of Tongji University.


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