Coordination of Two N2 Molecules to One Ni+ Site in Ni–ZSM-5: An

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Coordination of Two N2 Molecules to One Ni+ Site in Ni−ZSM-5: An FTIR Spectroscopy Study Videlina Zdravkova, Mihail Mihaylov, and Konstantin Hadjiivanov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria S Supporting Information *

ABSTRACT: Ni+ cations were produced in a Ni−ZSM-5 zeolite by partial reduction with CO, and the ability of the Ni+ and Ni2+ sites to coordinate 14N2, 15N2, and/or CO molecules was studied by FTIR spectroscopy. With Ni+ cations, CO produces mono-, di-, and tricarbonyl species while only mono- and dicarbonyls are formed with Ni2+. Adsorption of 14N2 at 100 K results in formation of Ni2+−14N2 adducts (band at 2343 cm−1) and geminal Ni+(14N2)2 complexes (νs at 2287 and νas at 2270 cm−1). The Ni+(14N2)2 complexes lose their ligands stepwise during evacuation at 100 K, and two kinds of monoligand Ni+−14N2 species are formed (2252 and 2238 cm−1). After 15 N2 adsorption, the Ni+(15N2)2 complexes were observed at 2211 and 2194 cm−1: the two Ni+−15N2 species at 2178 and 2163 cm−1, and the Ni2+−15N2 adducts at 2264 cm−1. To prove the geminal structure, coadsorption of 14N2 and 15N2 was studied. It resulted in formation of Ni+(14N2)(15N2) species that were characterized by 14 N−14N and 15N−15N stretchings at 2277 and 2201 cm−1, respectively. These values are in excellent agreement with those calculated on the basis of the approximate force field model. Experiments on coadsorption of 14N2 and CO have shown formation of Ni+(CO)(14N2) complexes that were characterized by CO and 14N−14N stretchings at 2098 and 2305 cm−1, respectively. The reasons for simultaneous coordination of two 14N2 (or one 14N2 and one CO) molecules to one Ni+ site and the nature of the bonds in the different complexes are discussed.

1. INTRODUCTION The interest in dinitrogen coordination chemistry is mainly associated with the activation of the inert N2 molecule.1 Thus, biological nitrogen fixation is supposed to include a step of coordination of N2 to the metal center in nitrogenase enzymes. On an industrial scale, atmospheric nitrogen is converted, for example, to ammonia, using catalysts where dinitrogen is coordinated. In addition, strong bonding of nitrogen to adsorbates could allow development of processes for the purification of various gases. The ability of dinitrogen to form complexes is the fundamental basis for its use as a probe molecule. Extensive studies of dinitrogen complexes started after 1965, when the first synthesis of such a complex was reported.2 It is well established that N2 is similar to CO as a ligand because both molecules are diatomic with a triple bond.1 CO and N2 usually bind to metal atoms and cations in “end-on” positions. However, while polycarbonyls are well-known species,4 most of the dinitrogen complexes of transition metal cations contain only one N2 ligand. The number of complexes containing two N2 ligands is limited,1−3 and few species with more than two N2 ligands are presently known. The latter mainly concerns matrixisolated complexes of zerovalent atoms, e.g. Ni(N2)4.3 Formation of metal−dinitrogen complexes during adsorption has been studied by many research groups by means of IR spectroscopy.5−14 In particular, it is believed that N2 is a very © 2012 American Chemical Society

convenient IR probe molecule for determination of surface protonic acidity.5 In this case, N2 interacts with the protons electrostatically and the 14N−14N stretching frequency is blueshifted with respect to the gas-phase (Raman peak at 2331 cm−1). The interaction between 14N2 and d0 cations is also electrostatic.6,7 In these cases, dinitrogen has been used to evaluate the number of coordinative vacancies of accessible to adsorption cations.6,7 Forming complexes with transition metal cations of suitable electronic configuration, dinitrogen can be bound by σ- and πbonds which are in synergism. Because the π-bond results in occupation of antibonding orbitals of the dinitrogen molecule, the 14N−14N bond order decreases. As a result, the 14N−14N stretching modes are observed at frequencies lower than vibrations of molecules in the gas phase (2331 cm−1). There are several cases when N2 is reported to interact relatively strongly with the adsorption sites. Many authors have observed stable dinitrogen complexes of Cu+ cations in Cu−ZSM-58 and in other zeolites.9 The 14N−14N stretching frequency of the Cu+−14N2 species is observed around 2300−2285 cm−1 because of back π-donation. Relatively strong adsorption of dinitrogen has been observed for the Cr/SiO2 system10 where Received: April 25, 2012 Revised: May 22, 2012 Published: May 25, 2012 12706

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were registered at room temperature with a Bruker D8 Advance diffractometer using Cu Kα radiation and a SolX detector. Carbon monoxide (>99.997 purity) was supplied by Linde AG. Nitrogen 5.0 (99.999) was purchased from Messer. Labeled nitrogen (15N2, isotopic purity of 98 at. %) was provided by Aldrich. Before adsorption, all gases were additionally purified by passing through a liquid nitrogen trap.

the dinitrogen complexes have been detected in the 2340−2327 cm−1 region. In this case, coordination of two 14N2 molecules to one center has also been proposed. Formation of different complexes with CO and 14N2 ligands was reported for supported Ru11 and Rh12,13 samples, the 14N−14N stretching frequency being observed down to 2173 cm−1. Some years ago we found that reduction of Ni−ZSM-5 with CO or hydrocarbons resulted in formation of Ni+(CO)x (x = 1−3) species15 whereas metal nickel was produced after reduction with hydrogen. This is associated with the stabilization of Ni+ in the carbonyl complexes. Moreover, thermal destruction of these carbonyls leaves bare Ni+ sites on the sample. Serykh and Amiridis14 studied adsorption of 14N2 on CO-reduced Ni−ZSM-5 and reported formation, at ambient temperature, of two kinds of linear complexes: Ni2+−14N2 observed at 2339 cm−1, and Ni+−14N2 at 2250 cm−1. The low 14N−14N stretching frequency in the latter case indicates a significant weakening of the 14N−14N bond and is explained by π-donation from the metal center to the antibonding π* orbital of dinitrogen. It is believed that the reason for coordination of up to three CO molecules to one Ni+ site in ZSM-5 is the low coordination number of the metal cation.15 Therefore, one can expect that more than one 14N2 molecule could be attached to one Ni+ site. Indeed, the existence of 2+ coordinative vacancies allows formation of geminal species even when the adsorption is very weak; a typical example is the coordination of two 14N2 (or two CO) molecules to one Na+ site in NaY or NaEMT zeolites.6 To check the possibility of formation of Ni+(14N2)x (x > 1) species, we revisited the Ni−ZSM-5−dinitrogen system, performing the adsorption experiments at low temperature and thus facilitating adsorption phenomena. As a result, we provide the first evidence of geminal dinitrogen complexes of Ni+ that are formed in Ni−ZSM-5, and the structure was proven by adsorption of 14N2 + 15N2 isotopic mixtures.

3. RESULTS AND DISCUSSION 3.1. Initial Characterization of the Sample. XRD experiments indicated no changes in the zeolite structure and the presence of a small amount of NiO. The FTIR spectrum of activated Ni−ZSM-5 sample exhibits three IR bands in the hydroxyl stretching region: at 3745, 3666, and 3612 cm−1 (Figure S1 in Supporting Information, spectrum a). The band at 3745 cm−1 is attributed to terminal silanol groups.16 The band at 3612 cm−1 corresponds to acidic bridging hydroxyls while the band at 3666 cm−1 is assigned to Al−OH species arising from some extraframework aluminum.16 The FTIR spectrum of CO-reduced Ni−ZSM-5 practically coincides with that of the activated sample. (Figure S1, spectrum b). When the spectra are registered at 100 K, the silanol and bridging OH bands were slightly shifted to 3747 and 3617 cm−1, respectively (Figure S1, spectrum c). 3.2. Coordination State of Ni2+ and Ni+ Ions in Ni− ZSM-5 As Revealed by Probing with CO. Details of CO adsorption on Ni−ZSM-5 have been reported,15 and here we briefly describe only the main features related to the present study. Introduction of CO (200 Pa equilibrium pressure) to the activated sample leads to the formation of different species15 (see Figure S2 in Supporting Information): (i) physically adsorbed CO (bands at 2138 and 2133 cm−1 that disappear quickly during evacuation); (ii) CO attached to the zeolite bridging hydroxyls (band at 2174 cm−1); (iii) Ni2+(CO)2 species (band at 2204 cm−1) easily losing one CO ligand during evacuation and thus being converted into monocarbonyls; (iv) two kinds of Ni2+−CO complexes (bands at 2223 and 2213 cm−1); (v) small amounts of carbonyls formed with Ni+ ions (2140−2090 cm−1 region, for details see below). The Ni2+−CO species are stable at 100 K but disappear during evacuation at higher temperatures. When the sample was reduced by CO, the Ni2+ sites forming carbonyls detected at 2223 cm−1 were practically reduced to Ni+ cations (see Figure S3 in Supporting Information). The latter formed, with CO, tricarbonyls (bands at 2157, 2124, and 2111 cm−1), which lost CO ligands stepwise, during evacuation, and were converted first into dicarbonyls (bands at 2136 and 2092 cm−1) and then to monocarbonyls (band at 2109 cm−1). The monocarbonyls resisted evacuation at ambient temperature but disappeared after outgassing at 573 K (spectra not shown). 3.3. Dinitrogen Complexes of Nickel Ions in Ni−ZSM5: Formation of Ni+(14N2)2. For convenience, the observed IR bands of 14N2 and 15N2 adsorbed on nickel sites and their assignments are summarized in Table 1. Adsorption of 14N2 (1 kPa equilibrium pressure) at 100 K on the activated Ni−ZSM-5 sample leads to the appearance of two main bands in the 14N−14N stretching region, at 2342 and 2331 cm−1 (Figure 1, spectrum a). A weak feature at 2324 cm−1 is also visible. The bands at 2331 and 2324 cm−1 disappear first during evacuation and are attributed to 14N2 interacting with hydroxyl groupss.5 The band at 2343 cm−1 is more stable but decreases in intensity during evacuation even at 100 K (Figure 1, spectra b−k) to finally disappear at higher temperature

2. EXPERIMENTAL SECTION The starting NH4−ZSM-5 material was a commercial zeolite from Zeolits, having a Si-to-Al ratio of 15. The Ni−ZSM-5 sample was prepared by a conventional ion exchange with 2 mol dm−3 solution of Ni(NO3)2. After exchange, the sample was washed with water, dried, and calcined for 1 h at 773 K. The sample thus obtained contained 1.97 wt % of nickel. FTIR spectra were recorded with a Nicolet Avatar 360 spectrometer, accumulating 128 scans at a spectral resolution of 2 cm−1. A self-supporting pellet (ca. 10 mg cm−2) was prepared from the sample powder and treated directly in a purpose-made IR cell, allowing measurements at ambient and low temperature (ca. 100 K). The cell was connected to a vacuum−adsorption apparatus with a residual pressure below 10−3 Pa. Prior to adsorption, the sample was activated at first in oxygen (13.3 kPa) for 1 h at 673 K and then in dynamic vacuum for 1 h at the same temperature. The sample thus treated will be referred to as “activated” samples. To obtain a “reduced” sample, an activated one was heated in the presence of CO (10 kPa) for 10 min at 573 K, followed by 5 min evacuation at the same temperature. All spectra (except those presented in Figure S1, Supporting Information) are background and CO2 gas-phasecorrected. Chemical analysis of the sample was performed with atomic absorption spectrometry using a Solaar M6 atomic absorption spectrometer and air−acetylene flame. Powder XRD patterns 12707

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Table 1. Spectral Characteristics of the Nickel−Dinitrogen Complexes Observed in This Study species 2+

−14N2 2+ 15

and Ni Ni − N2 Ni+−14N2 and Ni+−15N2 Ni+(14N2)2 and Ni+(15N2)2 Ni+(14N2)(15N2) Ni+(CO)(14N2)

14

N−14N bands

2342 cm

−1

15

N−15N bands

note

2264 cm−1

2252, 2237 cm−1

2177, 2163 cm−1

νs at 2288 and νas at 2270 cm−1 2278 cm−1 2305 cm−1

νs at 2212 and νas at 2194 cm−1 2201 cm−1 −

two different linear species converted into linear species CO stretching modes at 2097 cm−1

Figure 2. FTIR spectra of 14N2 adsorbed at 100 K on CO-reduced Ni−ZSM-5. Equilibrium pressure of 100 Pa N2 (a) and evolution of the spectra after short evacuations (b−g) and under dynamic vacuum (h−k).

When the experiments were performed at ambient temperature, the only visible bands were that of Ni2+−14N2 species (observed at 2339 cm−1) and linear Ni+−14N2 complexes (2251 cm−1) (Figure S5 in Supporting Information). Both bands disappeared during evacuation, the band at 2251 cm−1 being shifted to 2248 cm−1 at low coverage. The bands at 2287 and 2270 cm−1 have not been reported so far. The results suggest that they are due to geminal Ni+(14N2)2 complexes that are converted, during coverage decrease, into two different kinds of linear species (2252 and 2238 cm−1). Groppo et al.10 have reported that adsorption of 14N2 on Cr2+ sites on Philips catalyst led to formation of two kinds of linear species: CrA2+−14N2, absorbing at 2327 cm−1, and CrB2+−14N2, detected at 2333 cm−1. With an increase in the equilibrium 14N2 pressure at 77 K, the CrA2+−14N2 species accepted a second 14N2 ligand, thus being converted into CrA2+(14N2)2 geminal complexes. On the basis of literature data concerning bulk compounds, the authors speculated that no coupling between the two 14N2 oscillators was probable and concluded there was no split of the 14N−14N modes into symmetric and antisymmetric ones. Consequently, they proposed that the geminal complexes were characterized by one band only, at 2333 cm−1. In contrast, Miessner et al.11,12 have reported geminal dinitrogen complexes with Rh+/DAY and Ru2+/DAY samples that were characterized by symmetric and anisymmetric modes: νs at 2243 and νas at 2217 cm−1 for Rh+(14N2)2 and νs at 2207 and νas at 2173 cm−1 for Ru2+(14N2)2. Note that these species were produced by a rather compicated procedure consisting of gradual substitution of the CO ligands from polycarbonyl complexes by N2. This arises from the fact that bare Rh+ and Ru2+ cations in zeolites seem to be unstable. The frequencies observed here for Ni+(14N2)2 species (2287 and 2270 cm−1) are somewhat higher as compared to those for Rh+(14N2)2, which indicates a weaker back π-donation. In addition, the split

Figure 1. FTIR spectra of 14N2 adsorbed at 100 K on activated Ni− ZSM-5. Equilibrium pressure of 1 kPa (a), after short evacuations (b− g) and under dynamic vacuum at 100 K (h−k) and at increasing temperatures up to ambient temperature (l−o).

(Figure 1, spectrum o). The maximum at low coverage is shifted to 2341 cm−1. In agreement with literature data14 we assign this band to Ni2+−14N2 species. Although the band could be complex (second derivative suggests weak components at 2340 and 2337 cm−1), no two components are clearly distinguished as in the case of the Ni2+−CO bands. Therefore, we can conclude that 14N2 is less sensitive than CO for fine determination of the acidity of Ni2+ sites. When 14N2 was adsorbed on the CO-reduced sample, the band at 2343 cm−1 appeared with a reduced intensity and two additional intense bands at 2287 and 2270 cm−1 were recorded (Figure 2, spectrum a, and Figure S4 in Supporting Information). A weak feature at 2238 cm−1 was also visible. The bands at 2287 and 2270 cm−1 decreased in concert when the coverage decreased. Simultaneously, the band at 2238 cm−1 gained intensity and then a new band at 2252 cm−1 developed (Figure 2, spectra b−g). The bands at 2252 and 2238 cm−1 vanished after further evacuation and are assigned to two kinds of linear Ni+−14N2 species (Figure 2, spectra h−k).14 A closer inspection of the spectra shows that the band at 2238 cm−1, which is of lower intensity, disappears first during evacuation. 12708

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between the νs and νas modes is smaller for Ni+(14N2)2 (17 cm−1) than for Rh+(14N2)2 (24 cm−1). This indicates a slightly weaker interaction between the two dinitrogen ligands when coordinated to Ni+ sites. 3.4. Proving the Ni+(14N2)2 Geminal Structure by Coadsorption of 14N2 and 15N2. The results of 15N2 adsorption on the sample are consistent with those already obtained with 14N2. All bands were red-shifted by a factor of 1.034 (see Figures S6 and S7 in Supporting Information and Table 1). The different dinitrogen complexes were observed as follows: Ni2+−15N2, at 2265 cm−1; two kinds of Ni+−15N2 species, at 2177 and 2163 cm−1; Ni+(15N2)2, at 2211 and 2194 cm−1. These results confirm the assignment of the bands to dinitrogen species. To prove the geminal structure of the Ni+(14N2)2 species, we have studied coadsoprtion of 14N2 and 15N2. If the bands at 2287 and 2270 cm−1 were due to the symmetric and antisymmetric 14N−14N modes, respectively, of Ni2+(14N2)2 species, adsorption of 14N2 + 15N2 isotopic mixtures should result in formation of Ni+(14N2)(15N2) mixed ligand complexes. Approximate force field model4 predicts that these species should manifest 14N−14N and 15N−15N stretching modes at 2279.4 and 2200.6 cm−1, respectively. The IR spectrum of a 14N2 + 15N2 isotopic mixture (molar ratio of 2:3, 1 kPa total equilibrium pressure) is presented in Figure 3 (spectrum a). It is compared with a simulated spectrum (Figure 3, spectrum b). The latter represents the sum of the spectra of adsorbed 14N2 (multiplied by a factor of 0.4)

and 15N2 (multiplied by a factor of 0.6) to follow the experimental isotopic ratio of 2:3. Thus, the real spectrum should coincide with the simulated one if only linear species were formed. However, there are important differences between the two spectra that are better seen in the difference spectrum (Figure 3, spectrum a−b). The results clearly show that the 14N−14N bands at 2287 and 2270 cm−1 and their 15 N−15N analogues at 2211 and 2194 cm−1 have appeared with reduced intensity, and two new bands, at 2278 and 2201 cm−1, were formed. These bands coincide very well in position with the predicted bands for Ni+(14N2)(15N2) mixed ligand complexes. Therefore, we can unambiguously conclude that the bands at 2287 and 2270 cm−1 (produced after 14N2 adsorption) are due to geminal complexes. Further confirmation of this conclusion could be derived from the spectra of adsorbed 14N2 + 15N2 isotopic mixtures with different molar ratios. Note that these experiments were performed under equilibrium pressures that were high enough to ensure the lack of linear Ni+−N2 complexes. When the 14N2 partial pressure is negligible, the prevailing species should be Ni+(15N2)2 with few Ni+(14N2)(15N2) mixed ligand complexes, while practically no Ni+(14N2)2 species should be observed. With an increase in the partial pressure of 14N2, the concentration of the mixed ligand complexes should rise, reaching a maximum at a molar ratio of 1:1 and then should start to decrease. Simultaneously, a steady increase in concentration of the Ni+(14N2)2 species should be observed. These dependencies are clearly illustrated by the spectra presented in Figure 4. It is seen that addition of 14N2 to the Ni−ZSM-5−15N2 system causes first development and then decrease in intensity of the band at 2278 cm−1 (14N−14N modes of the mixed ligand complexes). The bands at 2287 and 2270 cm−1 (Ni+(14N2)2 species) develop later and continuously

Figure 3. FTIR spectra of 14N2 and 15N2 (molar ratio of 2: 3) coadsorbed at 100 K on CO-reduced Ni−ZSM-5. Experimental spectrum (a) obtained after adsorption of 14N2 + 15N2 mixture (1 kPa equilibrium pressure). Simulated spectrum (b) representing the sum of the spectra of adsorbed 14N2 (dotted line) and spectrum of adsorbed 15 N2 (dash-dotted line) with the corresponding intensity ratio.

Figure 4. FTIR spectra of 14N2 and 15N2 (different molar ratios) coadsorbed on CO-reduced Ni−ZSM-5. Adsorption of 15N2 (1 kPa equilibrium pressure) (a) and successive addition of small doses of 14 N2 (b−e). 12709

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rise in intensity. The band at 2201 cm−1 (15N−15N modes of the mixed ligand complexes) is not resolved because it is masked by the intense band at 2194 cm−1. However, this band is easily observed in the difference spectra (see the inset in Figure 4). Analysis of the spectra indicated an initial increase and then a decrease in the intensity of this band. Usually the bands due to linear species are observed between the νs and νas modes of the respective geminal complexes. Among the many examples, this is well illustrated by the herereported mono- and dicarbonyl species of Ni+: the Ni+−CO band is detected at 2109 cm−1, and the Ni+(CO)2 species manifest bands at 2136 and 2092 cm−1 (see Figure S3). However, the bands due to geminal dinitrogen species are observed at frequencies considerably higher than those of the linear complexes. This indicates a restricted back π-donation. Therefore, we can conclude that the two 14N2 ligands are in a strong competition for electrons which are back-donated. This is in line with earlier findings that the degree of back πdonation in the dinitrogen complexes strongly depends on the existence of other ligands.3 3.5. Mixed Ligand Species: Ni+(CO)(14N2). The next experiments were devoted to check the possibility to form complexes containing 14N2 and CO ligands simultaneously. For that purpose, Ni+−CO species were used as precursors. To produce them, CO was adsorbed at ambient temperature on a reduced sample and then evacuated for 3 min at 373 K. As a result, an intense Ni+−CO band at 2109 cm−1 remained in the spectrum (Figure 5, spectrum a). Because these monocarbonyls

(2252 and 2238 cm−1) declined, and bands at 2287 and 2270 cm−1, already attributed to Ni+(14N2)2, raised in concert (Figure 5A, spectra b−f). Another band at 2305 cm−1, not registered with the CO-free system, was also formed. Consider now the carbonyl stretching region (Figure 5B). Dosage of 14N2 caused a gradual erosion of the Ni+−CO band at 2109 cm−1 and formation of a low-frequency shoulder around 2100 cm−1. Also, a weak-intensity band at 2136 cm−1 developed which indicated formation of dicarbonyl species. The respective antisymmetric modes are detected at 2092 cm−1. However, in the spectra registered after CO adsorption the bands due to mono- and dicarbonyl species were well resolved (see Figure S3), while in the spectra shown in Figure 5B the lower frequency component appeared as a shoulder. This suggests the existence of an additional carbonyl band located between 2109 and 2092 cm−1. Indeed, the second derivative of the spectrum indicates a component at 2096 cm−1. To obtain a more clear picture of this band, we subtracted the spectra of the dicarbonyl species (bands at 2136 and 2092 cm−1), and the resulting spectrum is presented in Figure 6B. It is clearly seen that the residual carbonyl band consists of two components, one at 2109 cm−1 (Ni+−CO species), and one at 2097 cm−1.

Figure 6. Treated FTIR spectra of 14N2 and CO coadsorbed on COreduced Ni−ZSM-5. The bands corresponding to Ni+(14N2)2 (panel A) and Ni+(CO)2 species (panel B) are subtracted from spectrum f presented on Figure 5.

Figure 5. FTIR spectra of 14N2 and CO coadsorbed on CO-reduced Ni−ZSM-5. After reduction, the sample was evacuated for 3 min at 373 K (a), and then small doses of 14N2 were successively added to the system (b−f) until reaching equilibrium pressure of 600 Pa (f).

The spectrum in the 14N−14N region after the subtraction of the bands due to geminal Ni+(N2)2 species is presented in Figure 6A and shows a feature at 2305 cm−1. A careful analysis of the spectra indicates that the band at 2305 cm−1 changes in concert with the carbonyl band at 2097 cm−1. Therefore we assign these two bands to mixed-ligand Ni+(CO)(14N2) species. The carbonyl band appeared at lower frequency as compared to the linear monocarbonyls, which indicates a slightly enhanced π-back-donation and/or reduced σ-bond. In contrast, the 14 N−14N stretchings were found at a significantly higher frequency as compared to that of the linear Ni+−14N2 species. This implies that the back π-donation to the 14N2 molecule in this case is restricted. Evidently, this is due to the stronger electron acceptor properties of the CO molecule as compared to N2. Similar effects were reported with other systems, e.g., for mixed Rh(CO)(14N2)11 and Cu+(CO)(NO) complexes.17

are thermally stable at 373 K, we can stress that practically all of the Ni+ sites are occupied by CO molecules. In contrast, the Ni2+ sites are free. The sample thus treated was cooled to 100 K, and small doses of 14N2 were successively introduced to the system until reaching 14N2 equilibrium pressure of 600 Pa. The first 14N2 dose caused the appearance of a Ni2+−14N2 band 2343 cm−1 and Ni+−14N2 bands at 2252 and 2238 cm−1 (Figure 5A, spectrum b). With an increase in the amount of 14N2 introduced, bands due to OH−14N2 species (2331 and 2324 cm−1) developed, the bands assigned to Ni+−14N2 linear species 12710

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The results obtained demonstrate that addition of 14N2 to reduced Ni−ZSM-5 with preadsorbed CO leads to formation of mixed ligand species. At the same time, however, geminal dinitrogen and geminal carbonyl species are also produced according to the following scheme:

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Bulgarian Science Fund (Grants DCVP 02/2 and DO 02-184).

Ni+−CO + 14 N2 → Ni+(CO)(14 N2) + Ni+(14 N2)2 + Ni+(CO)2

(1)

To the best of our knowledge, similar surface carbonyl− dinitrogen complexes have been reported only with supported Ru11 and Rh12,13 samples. 3.6. Interaction with Other Molecules. Finally, we have checked the possibility of formation of mixed-ligand species with water and the stability of the Ni+−14N2 species in the presence of oxygen. No evidence of mixed aqua−dinitrogen complexes was found by adsorption of 14N2 on samples with different amounts of preadsorbed water (spectra not shown). Oxygen doses were successively added at 100 K to reduced Ni−ZSM-5 with preadsorbed 14N2 (5 kPa equilibrium pressure). As a result, the bands due to geminal Ni+(14N2)2 complexes gradually disappeared (see Figure S8 in Supporting Information). Interestingly, no increase in intensity of the Ni2+−14N2 bands was noted (see the difference spectrum). These results can be rationalized, assuming, as already proposed,15 that Ni+ ions are first oxidized to Ni2+−O2− species where the Ni2+ ions are coordinatively saturated.

4. CONCLUSIONS Ni+ ions in Ni−ZSM-5 are able to coordinate two 14N2 molecules simultaneously, and the resulting Ni +( 14 N 2 ) 2 complexes are characterized by νs at 2288 and νas at 2270 cm−1 (2212 and 2194 cm−1, respectively, for Ni+(15N2)2 complexes). The vibrational interaction between the two 14N2 ligands is additionally proven by coadsorption of 14N2 + 15N2 isotopic mixtures, and the Ni+(14N2)(15N2) mixed ligand species display ν(14N−14N) at 2278 and ν(15N−15N) at 2201 cm−1, in excellent agreement with values calculated on the basis of approximate force field model. The Ni+(14N2)2 complexes lose their ligands stepwise, producing two types of linear Ni+−14N2 species (2252 and 2237 cm−1, respectively). The much lower frequencies of the linear species imply that the two 14 N2 ligands in the geminal complexes are in a strong competition for donated electrons. Ni+ ions in Ni−ZSM-5 are able to form mixed ligand Ni+(CO)(14N2) complexes characterized by ν(C−O) and ν(14N−14N) modes at 2098 and 2305 cm−1, respectively. The high wavenumber of the 14N−14N modes indicates that the back electron donation to the dinitrogen molecule is in this case very restricted because of the competition with the CO ligand having stronger electron acceptor properties.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra (Figures S1−8). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], tel: + 359 2 9793598; fax: + 359 2 8705024. 12711

dx.doi.org/10.1021/jp304001e | J. Phys. Chem. C 2012, 116, 12706−12711