Infrared Photodissociation Spectroscopy of Mass-Selected Silver and

Mar 26, 2015 - The [M(NO)n]+ cation complexes (M = Au and Ag) are studied for exploring the coordination and bonding between nitric oxide and noble me...
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Infrared Photodissociation Spectroscopy of Mass-Selected Silver and Gold Nitrosyl Cation Complexes Yuzhen Li, Lichen Wang, Hui Qu, Guanjun Wang, and Mingfei Zhou* Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: The [M(NO)n]+ cation complexes (M = Au and Ag) are studied for exploring the coordination and bonding between nitric oxide and noble metal cations. These species are produced in a laser vaporization supersonic ion source and probed by infrared photodissociation spectroscopy in the NO stretching frequency region using a collinear tandem time-of-flight mass spectrometer. The geometric and electronic structures of these complexes are determined by comparison of the distinctive experimental spectra with simulated spectra derived from density functional theory calculations. All of these noble metal nitrosyl cation complexes are characterized to have bent NO ligands serving as one-electron donors. The spectrum of [Au(NO)2Ar]+ is consistent with 2-fold coordination with a near linear N−Au−N arrangement for this ion. The [Au(NO)n]+ (n = 3−4) cations are determined to be a mixture of 2-fold coordinated form and 3- or 4-fold coordinated form. In contrast, the spectra of [Ag(NO)n]+ (n = 3−6) provide evidence for the completion of the first coordination shell at n = 5. The high [Au(NO)n]+ and [Ag(NO)n]+ (n ≥ 3 for Au, n ≥ 4 for Ag) complexes each involve one or more (NO)2 dimer ligands, as observed in the copper nitrosyl cation complexes, indicating that ligand−ligand coupling plays an important role in the structure and bonding of noble metal nitrosyl cation complexes.

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INTRODUCTION Nitric oxide is a main pollutant from the fossil fuel combustion process. Its removal involves catalytic reduction on the metal surfaces.1,2 It has been shown that noble metal ion exchanged zeolites have high activities in both selective catalytic reduction and direct decomposition of NO. Thus, the coordination and reduction of NO on noble metal ion exchanged zeolites have been the focus of various experimental and theoretical investigations aimed at understanding the reduction mechanisms.3−19 These investigations suggest that the deNOx processes on noble metal ion exchanged zeolites are redox reactions. Various M−NxOy species were experimentally detected, which are considered to be the intermediates. In particular, the metal−dinitrosyl species have been suggested to play a crucial role in N−N bond formation and direct decomposition of NO to N2 and O2. The reactions of NO with bare metal atoms, ions, and clusters serve as the simplest model systems in understanding the mechanism of NO coordination and reduction on metal centers, which can promote the development of efficient catalysts for NO reduction. Such model systems can be studied under well-defined conditions without the effects from solvent and surface active sites. The reactions of laser-ablated noble metal (M = Au, Ag, Cu) atoms with NO have been examined in solid noble gas matrices.20−25 Metal nitrosyl complexes were formed and characterized by infrared absorption spectroscopy and density functional theory calculations. The reactivity of © 2015 American Chemical Society

noble metal (M = Au, Ag, Cu) cations with NO has been studied in the gas phase.26−28 The dissociation energies of [CuNO]+ and [Cu(NO)2]+ were measured via the collisioninduced dissociation method.29 [CuNO]2+ in strong acids was synthesized through the reaction of Cu0 , Cu I , or Cu II compounds with atmospheric NO at room temperature.30 The structure, stability, and bonding of [AuNO](−1,0,+1) and [CuNO](0,+1) have been studied with theory.31−38 The interactions of NO with small noble metal (M = Au, Ag, Cu) clusters have also been theoretically investigated.39−45 It was found that NO adsorption leads to the weakening and elongation of the N−O bond that is important for NO dissociation. The adsorption energies of NO on the cationic gold clusters are generally greater than those on the neutral and anionic clusters,39 whereas those of NO on the silver clusters depend on the structures of different charge states of the silver clusters.40−43 The copper clusters interact with NO considerably stronger than the silver clusters because of the important role of 3d electrons.42 The degree of activation of adsorbed NO on small gold cluster cations is determined as a function of cluster size by measuring the NO stretching frequency using IR multiphoton dissociation spectroscopy in the gas phase.46 The NO reduction on the Au, Ag, and Cu surfaces is affirmed to go Received: January 23, 2015 Revised: March 10, 2015 Published: March 26, 2015 3577

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The Journal of Physical Chemistry A

instability. For each structure, all possible spin states are considered. All the calculations are performed with the Gaussian 09 suite of quantum chemical software packages.68 Similar to those of the copper nitrosyl cation complexes,57 the relative energies of different structural isomers and spin states of the gold and silver nitrosyl cation complexes are sensitive to the functional used due to strong multireference character of the wave functions of these complexes. In principle, multireference based methods are required to obtain reliable results. However, such calculations are too expensive to be practical. Although single-reference DFT methods have problems in predicting the relative energies, previous studies have shown that the infrared spectrum predicted for a given structure and spin state is usually reliable, and a comparison of the predicted and experimentally observed infrared spectra is often able to determine the electronic and geometric structures actually present, independent of the relative DFT energetics for the different configurations.57,69−78 In the present study, we found that the infrared spectra predicted by the B3LYP function provide the best fit to the experiments. Thus, the results presented are due to the B3LYP calculations. The predicted IR spectra are obtained by applying Lorentzian functions with the theoretical harmonic vibrational frequencies scaled by a factor of 0.96 and given an 8 cm−1 full width at halfmaximum (fwhm). The scale factor of 0.96 is determined to give vibrational frequencies closest to the experimental values of the complexes reported in the present study.

through a (NO)2 dimer state both experimentally and theoretically.47−56 Recently, the geometric and electronic structures of the copper nitrosyl cation complexes [Cu(NO)n]+ (n = 1−5) were studied by infrared photodissociation spectroscopy in the gas phase.57 The results show that the bonding between copper cation and NO is quite flexible and ligand−ligand coupling plays an important role. In this paper, we report a similar infrared photodissociation spectroscopic study on the silver and gold nitrosyl cation complexes [M(NO)n]+ (M = Au, Ag; n = 2−6) to observe the trends for the coordination and bonding between nitric oxide and noble metal cations.

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EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental setup for infrared photodissociation spectroscopy based on a collinear tandem time-of-flight mass spectrometer has been described in detail previously.58,59 The gold and silver nitrosyl cation complexes are produced by a pulsed laser vaporization supersonic ion source employing the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate and 6 ns pulse width) to vaporize a rotating gold or silver metal target. The cation complexes are produced from the laser vaporization process in expansions of helium or helium/argon mixture (He:Ar ratio of 4:1 or 5:1) seeded with 0.5−1.0% NO using a pulsed valve (General Valve, Series 9) at 0.4−0.6 MPa backing pressure. After free expansion, the cation complexes are skimmed and analyzed using a Wiley−McLaren time-of-flight mass spectrometer. The ions of interest are each mass selected and decelerated into the extraction region of the second time-of-flight mass spectrometer, where they are dissociated by a tunable IR laser. The fragment and parent ions are reaccelerated and mass analyzed by the second time-of-flight mass spectrometer. Tunable infrared radiation used in this study is generated by a KTP/KTA/AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by a Continuum Powerlite 8000 Nd:YAG laser, which is tunable from 800 to 5000 cm−1. The laser pulse energy in the range 1600−2200 cm−1 is 0.5−1.5 mJ/pulse. The NH3 absorptions are used for calibrating the absolute frequency of the OPO laser output. Fragment ions and undissociated parent ions are detected by a dual microchannel plate detector. The ion signal is amplified, collected on a gated integrator, and averaged with a LabView based program. The photodissociation spectrum is obtained by monitoring the yield of the fragment ion of interest as a function of the dissociation IR laser wavelength and normalizing to the parent ion signal. Typical spectra are recorded by scanning the dissociation laser in steps of 2 cm−1 and averaging over 250 laser shots at each wavelength. Quantum chemical calculations are performed to determine the molecular structures and to support the assignment of vibrational frequencies of the observed species. Geometry optimization and harmonic vibrational frequency analysis are performed using the B3LYP60 and M0661 functionals with the aug-cc-pVTZ basis set for N, O, and Ar and the aug-cc-pVTZpp pseudopotential and basis set for Au and Ag.62−65 These functionals are among the most widely used functionals that can provide reliable predictions on the structures and vibrational frequencies of transition metal-containing compounds.66,67 The stability of the wave functions is checked by the “stable” technique as implemented in the Gaussian 09 program to ensure that the wave functions have no internal

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RESULTS AND DISCUSSION The typical mass spectra of the silver and gold nitrosyl cation complexes produced by the laser vaporization supersonic ion source in the m/z range 130−350 are shown in Figure 1. The

Figure 1. Mass spectra of the [Au(NO)n] + and [Ag(NO)n] + complexes produced by pulsed laser vaporization of an Au/Ag metal target in an expansion of helium seeded by nitric oxide.

mass spectra are composed of a progression of mass peaks due to mononuclear nitrosyl cation complexes [M(NO)n]+ (M = Au, Ag, n = 1−6). It is found that the small cation complexes (n = 1, 2) do not fragment when excited with infrared light in the NO stretching frequency region 1500−2000 cm−1. The bond dissociation energies of [Au(NO)]+, [Au(NO)2]+, [Ag(NO)]+, and [Ag(NO)2]+ are calculated to be 37.3, 31.8, 19.3, and 17.9 kcal/mol, respectively, at the B3LYP level (Table 1), which are 3578

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The Journal of Physical Chemistry A Table 1. Calculated Bond Dissociation Energies (kcal/mol) of the [M(NO)n]+ and [M(NO)nAr]+ (M = Au, Ag) Complexes at the B3LYP Level (after Zero-Point Energy Corrections) ΔENOa

ΔEArb

complex

Au

Ag

Au

Ag

[M(NO)]+ (Cs) [M(NO)Ar]+ (Cs) [M(NO)2]+ (C2) [M(NO)2]+ (C2v) [M(NO)2Ar]+ (C1) [M(NO)2Ar]+ (Cs) [M(NO)3]+ (C1) [M(NO)3]+ (Cs) [M(NO)3Ar]+ (C1) [M(NO)3Ar]+ (Cs) [M(NO)4]+ (C2) [M(NO)4]+ (C2v) [M(NO)4]+ (Cs) [M(NO)4]+ (C4h) [M(NO)4]+ (D2h (Au), D2 (Ag)) [M(NO)4Ar]+ (C2) [M(NO)4Ar]+ (Cs) [M(NO)4Ar]+ (C1) [M(NO)4Ar]+ (C4) [M(NO)4Ar]+ (C2v(Au), C2(Ag)) [M(NO)5]+ (a, C1) [M(NO)5]+ (b, C1) [M(NO)5]+ (c, C1) [M(NO)5]+ (d, C1) [M(NO)5]+ (e, C1)

37.3 36.8 31.8 24.2 21.5 16.3 11.1 17.0 10.8 14.5 8.5 10.0 9.9 9.8 9.6 8.4 9.9 9.7 9.7 9.5 4.6 4.3 4.3 4.3 4.1

19.3 18.7 17.9

11.0

7.1

0.7 3.0

1.7

0.4 0.5

1.1 1.2

0.3 0.4 0.4 0.4 0.4

0.7 0.7 0.7 0.7

12.5 9.0 8.9 8.4 8.4 5.1 7.5 7.5 7.7 7.5 7.1 7.1 7.2 7.1 4.4 4.3 4.3 4.3

Figure 2. Experimental IR spectrum of [Au(NO)2Ar]+ (a) and the simulated vibrational spectra of [Au(NO)2Ar]+ ((b) C1, (c) Cs) and [Au(NO)2]+ ((d) C2, (e) C2v) in the nitrosyl stretching frequency region. The relative energies are given in kcal/mol.

for the [Au(NO)2]+ cation in a neon matrix (1895.7 cm−1).21 Two stable structures of [Au(NO)2]+ are obtained from our DFT calculations (Figure 2 and Figure S1, Supporting Information). The lowest-energy structure of [Au(NO)2]+ has C2 symmetry, where two NO ligands are coordinated on opposite sides of Au+ with a near linear N−Au−N arrangement. The second structure has C2v symmetry and is predicted to be 7.6 kcal/mol less stable than the first structure. This C2v structure is previously reported to be the ground state of [Au(NO)2]+.21 The corresponding argon-tagged complexes are also calculated and the results show that argon prefers to coordinate to the metal center. There is no significant perturbation in the structures by argon coordination. The calculated IR spectra of both structures with and without argon tagging are compared to the experimental spectrum of [Au(NO)2Ar]+ in Figure 2. The C2v structure is predicted to have two NO stretching vibrations (symmetric and antisymmetric) with comparable IR intensities. In contrast, the more stable C2 structure is calculated to have only one strong stretching vibration, consistent with the single band spectrum measured experimentally. The [Ag(NO)2 ]+ complex is predicted to have a similar singlet ground state with C2 symmetry (Figure S1, Supporting Information). The C2v structure is predicted to be a saddle point. [M(NO)3]+. Both the [Au(NO)3]+ and [Ag(NO)3]+ cation complexes are able to dissociate by losing a NO ligand, but with low efficiency (less than 10% at a focused laser pulse energy of 1.0 mJ/pulse). These complexes are not expected to dissociate because of the high binding energies predicted for these ions (Table 1). The dissociation is likely due to multiphoton absorption or from a fraction of the ions containing residual internal energy. The argon-tagged complexes are expected to dissociate via a single photon process, and these ions indeed dissociate quite efficiently via losing the tagged Ar atom. The resulting infrared photodissociation spectra are shown in Figures 3 and 4, respectively. As shown in Figure 3a, the spectrum of the untagged [Au(NO)3]+ bare cation exhibits five relatively board bands centered at 1659, 1787, 1877, 1894, and 1919 cm−1 (Table 2), indicating that more than one isomer is experimentally observed. The tagged ion spectrum (Figure 3d) is about the same as that of the untagged ion except that the

Reaction energy of [M(NO)nArm]+ → [M(NO)n−1Arm]+ + NO (m = 0, 1) at 0 K. bReaction energy of [M(NO)nAr]+ → [M(NO)n]+ + Ar at 0 K. a

much greater than the energy of infrared photons in the N−O stretching frequency region. As listed in Table 1, the predicted last NO binding energy decreases with the increase of NO ligands. Experimentally, we found that the cation complexes with n ≥ 3 are able to dissociate by losing a NO ligand. [M(NO)]+ and [M(NO)2]+. We are not able to obtain the infrared spectra of the mono- and dinitrosyl cation complexes, as these complexes do not fragment when excited with infrared light. The [Au(NO)]+ and [Ag(NO)]+ cation complexes were previously produced via codeposition of laser-ablated metal atoms and cations with NO in solid noble gas matrices. The NO stretching mode is observed at 1917.8 cm−1 for Au and at 1910.9 cm−1 for Ag in solid neon.21,22 Both cation complexes are predicted to have a 2A′ ground state with bent Cs symmetry (Figure S1, Supporting Information). To examine the infrared spectra of the strongly bound monoand dinitrosyl cation complexes, the method of rare gas tagging is employed to enhance the dissociation yield. We prepare the argon-tagged [M(NO)nAr]+ complexes, which can fragment by eliminating argon after photoexcitation of the ligand vibrations. Unfortunately, the densities of the [M(NO)Ar]+ and [Ag(NO)2Ar]+ ions are too low to achieve an effective spectrum. The IR spectrum of [Au(NO)2Ar]+ obtained by monitoring the loss of Ar is shown in Figure 2a. The spectrum exhibits a single band centered at 1901 cm−1, which is blue-shifted by about 25 cm−1 from the free NO stretching frequency in the gas phase (1876 cm−1). The band position is very close to that measured 3579

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Table 2. Experimental Band Positions of the Gold and Silver Nitrosyl Cation Complexes complex

M = Au

[M(NO)2Ar]+ [M(NO)3]+

1901 1659 1787 1877 1894 1919 1665 1788 1879 1895 1923 1678 1806 1840 1862 1885 1885 1906 1673 1803 1835 1863

[M(NO)3Ar]+

[M(NO)4]+

Figure 3. Experimental and simulated IR spectra of the [Au(NO)3]+ and [Au(NO)3Ar]+ complexes in the nitrosyl stretching frequency region: (a) experimental spectrum of [Au(NO)3]+; (b) and (c) simulated spectra of the C1 and Cs structures of [Au(NO)3]+; (d) experimental spectrum of [Au(NO)3Ar]+; (e) and (f) simulated spectra of the C1 and Cs structures of [Au(NO)3Ar]+. The relative energies are given in kcal/mol.

[M(NO)4Ar]+

[M(NO)5]+

[M(NO)6]+

1885 1885 1905 1679 1807 1832 1865 1895

M = Ag

1903

1859 1907

1862 1901

1860 1905

1862 1902

1740 1866 1888 1902

first shell NO ligand. The third NO can thus be regarded as a second shell ligand. This structure has a doublet ground state without symmetry (C1). The second structure has Cs symmetry with all the NO ligands bound directly to the metal center. This structure can be regarded as being formed by adding a bent NO ligand to the C2v structure of [Au(NO)2]+. The first structure is predicted to be more stable by 1.6 kcal/mol than the second structure. The addition of argon induces little change in the structures and relative stabilities of these two [Au(NO)3]+ isomers. The calculated infrared spectra of both structures of [Au(NO) 3]+ and [Au(NO)3 Ar]+ are compared to the experimental spectra in Figure 3. Both structures are predicted to have three IR active NO stretching vibrations, which suggest that the observed spectrum is originated from both isomers. We assign the experimentally observed 1659, 1877, and 1919 cm−1 bands to the first structure (2-fold coordinated), and the 1787, 1877, and 1894 cm−1 bands to the second structure (3-fold coordinated) of [Au(NO)3]+. Two stable structures are predicted for [Ag(NO)3]+ and the corresponding argon-tagged complexes, which are almost isoenergetic (energy difference within 0.1 kcal/mol). The first structure without symmetry (C1) is similar to the second structure of [Au(NO)3]+ involving a (NO)2 dimer ligand and a

+

Figure 4. Experimental and simulated IR spectra of the [Ag(NO)3] and [Ag(NO)3Ar]+ complexes in the nitrosyl stretching frequency region: (a) experimental spectrum of [Ag(NO)3]+; (b) and (c) simulated spectra of the Cs and C1 structures of [Ag(NO)3]+; (d) experimental spectrum of [Ag(NO)3Ar]+; (e) and (f) simulated spectra of the C1 and Cs structures of [Ag(NO)3Ar]+. The relative energies are given in kcal/mol.

band positions in the tagged spectrum are shifted by about 1−6 cm−1 to the blue from those in the untagged complex. The spectrum of [Ag(NO)3]+ (Figure 4a) has only one broad band centered at 1903 cm−1. In contrast, the tagged spectrum (Figure 4d) exhibits two bands with band positions at 1859 and 1907 cm−1. The observation of different spectral features indicates that there is significant geometric or electronic structure change upon Ar tagging for [Ag(NO)3]+. DFT calculations have been performed on the [M(NO)3]+ (M = Au, Ag) ions and the corresponding argon-tagged complexes. Two minimum-energy structures of [Au(NO)3]+ are obtained (Figure 3 and Figure S2 of Supporting Information). The first structure involves a 2-fold coordinated Au(NO)2+ core ion with the third NO bonded to one of the 3580

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The Journal of Physical Chemistry A bent NO ligand. The second structure with Cs symmetry has three separated bent NO ligands that are directly bound to the metal center. The calculated IR spectra of [Ag(NO)3]+ and [Ag(NO)3Ar]+ complexes are compared with the experimental spectra in Figure 4. The calculated IR spectra for the same structure with and without argon tagging are essentially the same, but the spectra between C1 and Cs structures are quite different. The C1 structure is predicted to have two strong NO stretching modes at 1854 and 1903 cm−1 that are separated by approximately 50 cm−1. The Cs structure is predicted to have two intense modes at 1886 and 1894 cm−1, which cannot be well-resolved at the experimental resolution. Accordingly, the experimental spectrum of the untagged complex with a single broad band centered at 1903 cm−1 can be safely assigned to the Cs structure, and the experimental spectrum of the tagged complex with two well-resolved bands at 1859 and 1907 cm−1 can be confidentially attributed to the C1 structure. The results show that argon tagging induces a structural change of the [Ag(NO)3]+ cation complex. Because two isomers coexist for [Au(NO)3]+, whereas only one structure is observed for [Ag(NO)3]+, the spectra of the [Ag(NO)3]+ and [Au(NO)3]+ ions are quite different even though the predicted structures are very similar. As listed in Table 1, the NO binding energy of [Au(NO)3]+ is larger than that of [Ag(NO)3]+, whereas the Ar binding energy of [Au(NO)3Ar]+ is smaller than that of [Ag(NO)3Ar]+. This explains why argon tagging induces a strong spectral change for [Ag(NO)3]+, but no obvious change is observed for [Au(NO)3]+ upon argon tagging. [M(NO)4]+. The peaks that correspond to the [M(NO)4]+ (M = Au, Ag) cation complexes are the most intense peaks in the mass spectra. Both complexes can dissociate by losing a NO ligand. The infrared photodissociation spectrum of [Au(NO)4]+ shown in Figure 5 exhibits six bands centered at 1678, 1806, 1840, 1862, 1885, and 1906 cm−1. In contrast, the spectrum of [Ag(NO)4]+ has only two bands located at 1862 and 1901 cm−1 (Figure 6). As shown in Figures 5 and 6, the

argon-tagged ions have better resolved spectra with the peak positions slightly shifted from those in the untagged complexes.

Figure 6. Experimental and simulated IR spectra of the [Ag(NO)4]+ (A) and [Ag(NO)4Ar]+ (B) complexes in the nitrosyl stretching frequency region: (a) experimental spectrum of [Ag(NO)4]+; (b)−(f) simulated spectra of the C2, C2v, Cs, C4h, and D2 structures of [Ag(NO)4]+; (a′) experimental spectrum of [Ag(NO)4Ar]+; (b′)−(e′) simulated spectra of the Cs , C 1 , C 4, and C 2 structures of [Ag(NO)4Ar]+. The relative energies are given in kcal/mol.

Geometry optimizations have been performed on various possible structures for [M(NO)4]+. The five low-lying structures are shown in Figure S3 (Supporting Information). All of them are predicted to have a singlet ground state with the triplet state being about 3−7 kcal/mol higher in energy than the single state (Table S1, Supporting Information). The first structure is 2-fold coordinated with C2 symmetry involving two equivalent monodentate (NO)2 dimer ligands with each ligand coordinated to the metal center via the N atom. The other structures are 4-fold coordinated with all the NO ligands directly bound to the metal center. Both the second and third structures (with C2v and Cs symmetry) involve one bidentate (NO)2 dimer ligand and two separated bent NO ligands. The fourth structure has four equivalent bent NO ligands with C4h symmetry. The fifth structure includes two equivalent bidentate (NO)2 dimer ligands with D2h symmetry. In the case of [Au(NO)4]+, these structural isomers are very close in energy (within 0.5 kcal/mol energy difference). By comparison of the experimental spectrum with the simulated spectra (Figure 5), it is clear that the observed complex is a mixture involving both the 2-fold and 4-fold coordinated isomers. The observed 1678 and 1906 cm−1 bands are assigned to the 2-fold coordinated structure. The 1678 cm−1 band is due to the vibrations of the first-shell NO ligands, which are predicted at 1686 and 1694 cm−1 that cannot be resolved experimentally. The 1906 cm−1 band is attributed to the antisymmetric stretching vibration of the two second-shell ligands, which is predicted at 1945 cm−1. The corresponding symmetric stretching mode is predicted to have very low IR intensity and is not observed. The remaining bands observed at 1806, 1840, 1862, and 1885 cm−1 are assigned to the 4-fold coordinated isomers. The 1806 and 1862 cm−1 bands are most likely due to the D2h isomer, whereas the 1840, 1862, and 1885 cm−1 bands can be attributed to the C2v structure.

Figure 5. Experimental and simulated IR spectra of the [Au(NO)4]+ (A) and [Au(NO)4Ar]+ (B) complexes in the nitrosyl stretching frequency region: (a) experimental spectrum of [Au(NO)4]+; (b)−(f) simulated spectra of the C2, C2v, Cs, C4h, and D2h structures of [Au(NO)4]+; (a′) experimental spectrum of [Au(NO)4Ar]+; (b′)−(f′) simulated spectra of the C2, Cs, C1, C4, and C2v structures of [Au(NO)4Ar]+. The relative energies are given in kcal/mol. 3581

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The Journal of Physical Chemistry A In the case of [Ag(NO)4]+, the first structure with a 2-fold coordination has the highest energy (about 4.8 kcal/mol). The other four isomers with 4-fold coordination are almost degenerate (about 0.1 kcal/mol energy difference). The 2fold coordination isomer is predicted to have a band at 1684 cm−1. The absence of any signal below 1800 cm−1 in the experimental spectrum indicates that the 2-fold coordinated isomer can be clearly ruled out. Only the simulated spectrum of the D2 structure matches the experimental spectrum very well, which confirms that the observed [Ag(NO)4]+ cation possesses the D2 structure. The 1862 cm−1 band can be attributed to the doubly degenerate antisymmetric stretching vibration involving one (NO)2 ligand. The 1901 cm−1 band is assigned to the antisymmetric stretching mode with the involvement of both (NO)2 units. [M(NO)5]+. The [Au(NO)5]+ cation complex dissociates very efficiently via losing a NO ligand, implying that the fifth NO is loosely bound. The infrared photodissociation spectrum (Figure 7A) exhibits five bands centered at 1679, 1807, 1832,

bent NO ligand directly to the Ag center. Therefore, the silver center is 5-fold coordinated in these structures. The fifth structure with the fifth NO added in the second shell is predicted to lie about 2.9 kcal/mol higher in energy than the first structure. The simulated spectra of the first three isomers are very similar; each of them matches the experimental spectrum. We prefer to assign the observed species to the first structure, which involves the experimentally observed [Ag(NO)4]+ subunit. [Ag(NO)6]+. The [Ag(NO)6]+ cation complex fragments by losing one NO ligand very efficiently using an unfocused laser beam, indicating that the sixth NO is loosely bound. The infrared photodissociation spectrum of [Ag(NO)6]+ (Figure S5 of the Supporting Information) exhibits four bands at 1740, 1866, 1888, and 1902 cm−1. The 1740 cm−1 band is much lower than those observed for the [Ag(NO)n]+ (n = 1−5) cation complexes with all terminal bonded NO ligands, suggesting that the sixth NO in [Ag(NO)6]+ is located on the second shell. The 1740 cm−1 band is originated from the first-shell NO ligand that is coordinated by an external NO ligand. Discussion. It is well-known that NO can act either as a three-electron donor in a linear or near linear coordination structure or as a one-electron donor in a bent coordination structure. In the linear or near linear coordination mode, the bonding interaction between the metal center and NO involves the σ donation from the lone pair electrons of NO to the vacant orbital of metal and the π donation from the single-electron occupied π* orbital of NO to the vacant orbital of metal, as well as the π back-donation from the filler d orbitals of metal to the empty π* orbital of NO. In the bent structure, the σ donation interaction is reduced due to symmetry mismatch. On the basis of present study as well as our earlier work on the [Cu(NO)n]+ complexes,57 all of the noble metal nitrosyl cation complexes are characterized to have bent NO ligands serving as one-electron donors, which are quite different from the other transition-metal nitrosyl cation complexes.35,79,80 The noble metal monovalent cations (Cu, Ag, and Au) have (n − 1)d10 ns0 np0 electron configurations. Their LUMOs are composed of s orbitals. The energies of occupied 5-fold degenerate d orbitals are close to the LUMO s orbitals. The virtual p orbitals lie very high in energy; thus, they play a little role in the bonding interactions. As shown in Figure 8, the bonding between M+ and NO is dominated by the orbital interaction between the LUMO s orbital of M+ and the in-plane π* orbital of NO. It is this π donation interaction that favors the bent NO coordination over the linear coordination structure for the noble metal nitrosyl cation complexes. The Cu+ and Au+ cations have similar s−d or LUMO−HOMO energy gaps but are much smaller than that of Ag+. There are more s-orbital characteristics in the bonding orbitals of the [Cu(NO)n]+ and [Au(NO)n]+ complexes in comparison with the [Ag(NO)n]+ complexes. Therefore, the [Ag(NO)n]+ complexes have smaller binding energies than [Cu(NO)n]+ and [Au(NO)n]+. Among these three cations, the Au+ cation has the lowest LUMO energy, suggesting that Au+ is the strongest electron acceptor. As can be seen in Table 3, the positive charge on the metal center in the C2 symmetry ground state of [M(NO)2]+ decreases from Cu to Au. The electron transfer from NO to the metal cation reduces the strength of additional coordinated NO. In the [Au(NO)n]+ system, the dicoordinated structures are predicted to have better stabilities though the tri- and tetracoordinated structures are also

Figure 7. Experimental and simulated IR spectra of the [Au(NO)5]+ (A) and [Ag(NO)5]+ (B) complexes in the nitrosyl stretching frequency region: (a) experimental spectrum of [Au(NO)5]+; (b)− (f) simulated spectra of the [Au(NO)5]+ structures shown in Figure S4 (Supporting Information); (a′) experimental spectrum of [Ag(NO)5]+; (b′)−(f′) simulated spectra of the [Ag(NO)5]+ structures shown in Figure S4 (Supporting Information). The relative energies are given in kcal/mol.

1865, and 1895 cm−1. The five lowest-energy isomers of [Au(NO)5]+ are shown in Figure S4 (Supporting Information). These structures are very close in energy and can be considered as originating from the low-lying structures of [Au(NO)4]+ by adding the fifth NO to one of the NO ligand of [Au(NO)4]+. None of the simulated spectra of these isomers matches the experimental spectrum, which indicates that the observed complex is due to a mixture involving more than one isomer. It is quite difficult to assign each observed band to a specific structure for this cation complex. The infrared photodissociation spectrum of [Ag(NO)5]+ (Figure 7B) exhibits only two bands centered at 1862 and 1902 cm−1, which is quite similar to that of [Ag(NO)4]+. Figure S4 (Supporting Information) presents the predicted most stable structures of [Ag(NO)5]+. The first four structures are almost isoenergetic and can be regarded as being derived from the 4-fold coordinated [Ag(NO)4]+ by adding an additional 3582

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The Journal of Physical Chemistry A

The binding energy of the (NO)2 dimer is only about 3.0 kcal mol−1.

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CONCLUSIONS Infrared spectra of mass-selected mononuclear gold and silver nitrosyl cation complexes [M(NO)n]+ (M = Au, Ag) with n = 2−6 and their argon-tagged complexes are measured via infrared photodissociation spectroscopy in the nitrosyl stretching frequency region in the gas phase. Density functional calculations have been performed, and the calculated vibrational spectra are compared with the experimental spectra to identify the gas-phase structures of these [M(NO)n]+ and [M(NO)nAr]+ complexes. The results have been compared with our earlier work covering the mononuclear copper nitrosyl cation complexes [Cu(NO)n]+, and several existent trends can be drawn: (1) The noble metal cations exhibit different coordination numbers in these nitrosyl cation complexes. The Ag+ cation has the largest coordination number of five, whereas Cu+ has a coordination number of four. In contrast, the most favorable coordination number of Au+ is found to be only two. (2) All of these noble metal nitrosyl cation complexes are characterized to have bent NO ligands serving as one-electron donors, in which the bonding between the metal cation and NO is governed by the orbital interaction between the LUMO s orbital of M+ and the singly occupied in-plane π* orbital of NO. Besides this metal−ligand interaction, the ligand−ligand coupling also plays an important role in the bonding of the high complexes, which are determined to have structures involving the (NO)2 dimer structural units with O−O bonding. (3) The bonding between noble metal cation and nitric oxide is quite flexible. It is found that argon atom coordination has a strong influence on the geometric and electronic structures of these [M(NO)n]+ complexes, and distinctively different infrared spectra are observed between argon-tagged and untagged complexes for [Cu(NO)2]+, [Cu(NO)3]+, and [Ag(NO)3]+.

Figure 8. Energy levels (kcal/mol) of the frontier Kohn−Sham valence MOs of MNO+ illustrating the bonding interactions between M+ and NO. The inset shows the 3D contours of the bonding MO between M+ and NO.

experimentally observed. In contrast, the coordination number is determined to be five in the [Ag(NO)n]+ complexes. The π donation strengthens the NO bond and tends to increase the NO stretching vibrational frequency. In contrast, the π back-donation will weaken the NO bond and decrease the NO stretching frequency. Electrostatic interaction also plays a role for the metal nitrosyl cation complexes. Electrostatic polarization will strengthen the NO bond and result in a higher NO stretching frequency. The synergism of the electrostatic polarization, π donation and π back-donation determines the strength of the NO bonds and the NO stretching vibrational frequencies. The π donation interaction is the most important factor that determines the bonding in these noble metal nitrosyl cation complexes; the NO stretching frequencies are higher than the corresponding modes of the free NO and (NO)2 dimer except for the vibration of the inner-shell ligand that is coordinated by an additional second-shell ligand. An interesting structural feature observed for the noble metal nitrosyl cation complexes is ligand−ligand interaction. All of the high [M(NO)n]+ complexes (n ≥ 3) prefer to have structures involving the (NO)2 dimer structural unit(s) with O−O bonding. However, the ligand−ligand interaction is not observed in the strongly bound small complexes. Our previous study also finds that there is no dimer ligand in the Fe(NO)n+ (n = 1−4) complexes, which are predicted to be more strongly bound than the corresponding noble metal nitrosyl cation complexes.79 The size of the complex required for NO dimerization is independent of the coordination number of the metal ion. The metal−NO binding energy seems to play a more important role in NO dimerization. As discussed in detail previously,57 the ligand−ligand interaction is relatively weak.

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ASSOCIATED CONTENT

S Supporting Information *

The calculated geometries, vibrational frequencies and intensities, IR spectra, Cartesian coordinates, and complete ref 68. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. Zhou. Tel: (+86) 21-6564-3532. E-mail: mfzhou@fudan. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (2013CB834603 and 2012YQ220113-3),

Table 3. HOMO−LUMO Gap (ΔEH−L, kcal/mol) and Mulliken Atomic Charges of N, O, and M of the Ground State of [M(NO)n]+ (M = Au, Ag, Cu, n = 1, 2) at the B3LYP/aVTZ Level ΔEH−L N O M

[AuNO]+

[AgNO]+

[CuNO]+

[Au(NO)2]+

[Ag(NO)2]+

[Cu(NO)2]+

73.1 0.60 −0.08 0.48

70.9 0.40 −0.07 0.66

70.7 0.56 −0.11 0.55

68.8 0.47 −0.11 0.28

69.9 0.35 −0.10 0.51

68.5 0.33 −0.12 0.58

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DOI: 10.1021/acs.jpca.5b00747 J. Phys. Chem. A 2015, 119, 3577−3586

Article

The Journal of Physical Chemistry A

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the National Natural Science Foundation of China (grant nos. 21173053 and 21433005), and the Committee of Science and Technology of Shanghai (13XD1400800).

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