Infrared and 23Na Double Rotation Nuclear Magnetic Resonance

The synthesis of intra-Na56Y zeolite anchoring of [Rh6(CO)12(μ3-CO)4] involves the deposition of [Rh(CO)2Cl]2 vapor into the Na56Y cavities followed ...
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J. Phys. Chem. B 2001, 105, 2336-2342

Infrared and 23Na Double Rotation Nuclear Magnetic Resonance Studies for Intrazeolite Anchoring of Rhodium Carbonyl Clusters James G. C. Shen* Department of Chemistry and the Zettlemoyer Center for Surface Studies, 6 East Packer AVenue, Lehigh UniVersity, Bethlehem, PennsylVania 18015 ReceiVed: August 9, 2000; In Final Form: January 8, 2001

The synthesis of intra-Na56Y zeolite anchoring of [Rh6(CO)12(µ3-CO)4] involves the deposition of [Rh(CO)2Cl]2 vapor into the Na56Y cavities followed by reductive carbonylation under a mixed CO and H2O atmosphere. The physicochemical characterization was based on a multianalytical approach, including Fourier transform infrared and 23Na double rotation (DOR) nuclear magnetic resonance (NMR) spectroscopies and CO gas chemisorption. The decarbonylated species of the intrazeolitic [Rh6(CO)12(µ3-CO)4] is proposed to contain a Rh4 cluster, which reacts with CO at low temperature, generating [Rh4(CO)9(µ2-CO)3] clusters, and with CO and H2O at high temperature to form [Rh6(CO)12(µ3-CO)4] clusters. 23Na DOR NMR signal shifts of the site II Na+ cations in the Na56Y supercages and infrared band shifts of bridging CO ligands in the [Rh6(CO)12(µ3-CO)4]-Na56Y provide complementary evidence for the anchoring of [Rh6(CO)12(µ3-CO)4] inside the supercages. These patterns suggest that a strong interaction occurs between the oxygen end of face-bridging CO ligands and the site II Na+ supercage cations.

Introduction I. View of Intrazeolite Anchoring of Metal Carbonyl Clusters. By taking advantage of the nanometer-sized cavity reaction chambers of zeolite, organometallic compounds in dehydrated zeolite cages have been fabricated.1 Chemical approaches to encapsulate organometallic species inside the void spaces of zeolite involve chemical vapor deposition,2-4 metal ion exchange,5,6a,7 vapor-phase impregnation,6b,8 or metal carbonyl (photo)topotaxy1b,9 followed by reductive carbonylation under a mixed CO and H2 atmosphere. In several successful cases, intrazeolite topotactic synthetic methods produced low population densities of “single-size” quantized nanostructures, locked within the void cavities of zeolite.2-8 For example, intrazeolite-anchored Pt12,2 Co6,3 RuCo3,4 Ru6,5 Rh,6 Pd13,7a Ir6,8 and WO31b,9 carbonyl clusters have been synthesized through “ship-in-a-bottle” techniques. Consequently, the zeolite-mediated synthesis of metal carbonyl clusters provides route to compounds not accessible through conventional solution techniques. It produces a higher cluster yield, as well as a better understanding of the nucleation process in cluster formation. The studies conducted in zeolite media have provided empirical rules, which help us to predict the behavior of molecules in new environments. Although this has been successful in some cases, information on the interface between cluster and medium is still missing. In addition, it is difficult to rationalize the behavior of an enclosed molecule. For example, during CO/CH4 hydrogenation on the zeolite-encapsulated Ru and Co clusters, the catalytic effects might involve acid-base or electron-transfer interactions between the zeolitic medium and the metal clusters.10 None of the investigations available, however, has provided evidence for zeolite participation in dispersion of clusters and the electron transfer. In part, this is because many interactions occurring between the reactants and the enhancing effect of medium are often ignored. Moreover, prediction of the effect of this interaction in chemical reactions is not always straightforward.1 By analogy to aqueous acid* E-mail [email protected].

base chemical equilibrium methodology, the zeolitic extraframework Na+ cations are considered to be in contact with enclosed organometallic species. Our research focuses on the design and synthesis of zeolitic rhodium carbonyl clusters through the vapor deposition of a Rh compound into zeolite followed by reductive carbonylation in a mixed CO and steam matrix. We aim to demonstrate much of the chemistry of the zeolitic organometallic compound through the example of the intrazeolite anchoring of Rh carbonyl clusters. A multianalytical approach, including infrared, powder X-ray diffraction (PXRD), and solid-state DOR NMR spectroscopies, is employed to give insight into the formation process of zeolitic clusters as well as the interaction of the cluster and zeolite. II. Prospects for the Use of 23Na NMR as a Probe of Encapsulated Organometallic Species in Na56Y. Na+ cations, located at specific sites within the zeolite lattice, play an important role in determining adsorption, chemical, and catalytic properties of materials. Consequently, it is of great interest to understand the interaction of the Na+ cations in the zeolitic framework and the guest species and their chemical characteristics. The structure of Na56Y zeolite has been determined by PXRD,11 powder neutron diffraction,12 29Si NMR,13 and farinfrared spectroscopy.14 Figure 2B displays a portion of Na56Y structure with four distinct cation sites,15 II (29.4-32.2), I (7.17.7), I′ (13.4-19.5), and III (0-6.5), where the numbers in parentheses represent the X-ray calculated population range of Na+ cations determined by different authors.11 The 23Na nucleus has spin I ) 3/2, 100% natural abundance, and a large quadrupolar moment (0.1 × 1028/m2).16 Thus, quadrupolar broadening of 23Na resonance is often observed in the 23Na NMR spectra of zeolites.17 Different solid-state NMR techniques including magic-angle spinning (MAS), DOR, and dynamicangle spinning (DAS) have been employed to determine the quadrupolar effect and narrow the resonance of the crystallographically distinct sodium sites.18,19 The conventional solidstate MAS NMR studies of 23Na nuclei in zeolite have been limited by the reduced resolution of the spectra, mainly due to the large quadrupolar broadening present in the NMR spec-

10.1021/jp002879a CCC: $20.00 © 2001 American Chemical Society Published on Web 02/21/2001

Intrazeolite Anchoring of Rhodium Carbonyl Clusters tra.20,21 Frydman et al.22 have developed a multiple-quantum MAS (MQ MAS) method in half-integer quadrupolar nuclei based on earlier work of Vega.23 This method has recently been used to explore sodium sites in zeolite and yields high-resolution 23Na NMR spectra.19d,24 Double rotation (DOR) NMR technique further dramatically alters the situation for quadrupolar nuclei like 23Na (I ) 3/2).18a,19b,25, By removing the broadening contribution of the anisotropic resonance, DOR becomes a highresolution spectral probe of structure, bonding, and dynamical aspects of quadrupolar nuclei in zeolite.19b,26 Kno¨zinger and co-workers27a have investigated the adsorption of CH(D)Cl3 or CHF3 on a series of alkali-exchanged Y-zeolites by 23Na MAS NMR and claimed that interaction occurs between the halogen and the site II cations. Grey and co-workers24c,27b have recently identified the Na+ cations and HFC-binding interaction in zeolite by 2D MQ MAS NMR and 23Na MAS NMR spectral simulation. A large reduction in the quadrupole coupling constant (QCC) of the site II and I′ Na+ cations from dehydrated NaY to fully loaded HFC-134/NaY suggests the efficiency of the MQ MAS method is strongly dependent on the QCC values. In general, the smaller the QCC, the stronger the intensity of the resonance in the MQ MAS NMR spectrum for moderate QCC systems. In the second part of this study, the 23Na DOR NMR technique is employed to study the adsorption of [Rh(CO)2Cl]2 species within the cavities of Na56Y zeolite. Specifically, the interaction of the extraframework Na+ cations and the enclosed synthetic Rh carbonyl molecules was investigated by use of 23Na DOR NMR spectroscopy to provide an understanding for the shifts and the intensity changes of the 23Na resonance signal. Experimental Section I. Sample Preparation: (i) Precursor. Na56Y-supported [Rh(CO)2Cl]2 (9.7 wt % Rh loading or 6 [Rh(CO)2Cl]2/unit cell) was prepared by heating a mixture of [Rh(CO)2Cl]2 with Na56Y powder (HSZ-320NAA, lot D1-9915, Si/Al ) 3.0, surface area ) 910 m2 g-1). The Na56Y powder was dehydrated by evacuation in a Pyrex-glass line (10-5 Torr) at 623 K for 4 h and then mixed mechanically with [Rh(CO)2Cl]2 under N2 atmosphere at room temperature. This phase showed PXRD reflections that are ascribed to [Rh(CO)2Cl]2, as well as peaks for the Na56Y. After 32 h of heating at 333 K, the PXRD reflections for [Rh(CO)2Cl]2 completely disappeared, leaving only the PXRD peaks for Na56Y. This suggests that [Rh(CO)2Cl]2 is highly dispersed in the Na56Y cavities. The resulting sample was denoted [Rh(CO)2Cl]2-Na56Y (precursor). (ii) Sample a. The infrared wafer prepared from [Rh(CO)2Cl]2-Na56Y was exposed to 500 Torr of CO and 20 Torr of H2O in a closed circulation system and heated from 298 to 393 K. After 12 h, the resulting sample changed color from dark red to light pink-gray and showed the new infrared carbonyl vibrational frequencies. This product is denoted sample a. (iii) Sample b. Sample a was decarbonylated through heating at 298-473 K under 400 Torr of H2 and gave a decarbonylated sample, which is denoted sample b. (iv) Samples c and d. Sample b was recarbonylated through reductive carbonylation of sample b under 500 Torr of CO at 298 K, giving the red intermediate species of Rh carbonyl clusters, denoted as sample c. When sample c was heated from 298 to 393 K and kept at 393 K for 5 h, the resulting sample became light pink-gray again, like sample a, and is denoted sample d. II. Characterization of Samples: (i) Infrared Spectroscopy. In situ infrared spectra of the samples were recorded with

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Figure 1. (A) In situ infrared spectra of the reaction of [Rh(CO)2Cl]2Na56Y with 500 Torr of CO and 20 Torr of H2O at 298-393 K: (trace a) 343 K, 30 min; (trace b) 373 K, 1 h; (trace c) 393 K, 3 h; (trace d) 393 K, 5 h; (trace e) 393 K, 8 h; (trace f) 393 K, 10 h; (trace g) 393 K, 12 h. (B) Infrared spectrum obtained for crystalline [Rh(CO)2Cl]2. (C) Infrared spectrum obtained for crystalline [Rh6(CO)12(µ3-CO)4].

a Shimadzu infrared 4200 spectrometer with 30-50 coadded scans at 2 cm-1 resolution. The sample was pressed into a selfsupporting wafer (8 mg cm-2) in a N2 atmosphere box and mounted in a quartz infrared cell with CaF2 windows connected to a Pyrex-glass vacuum line (10-5 Torr). The infrared cell was equipped with an electric heater and a liquid N2 reservoir for high- and low-temperature measurements. (ii) Powder X-ray Diffraction and Gas Chromatography. The PXRD measurement was carried out by use of a MAC Science diffractometer with Cu-KR radiation (λ ) 1.5418 Å). The stoichiometry of the samples was determined by a Shimadzu GC-8APE chromatograph. (iii) 23Na DOR NMR Studies. 23Na DOR NMR experiments were carried out at 11.7 T on a Chemagnetics CMX-500 spectrometer with a home-built DOR probe whose features are described in ref 18a. The spinning speed was 5 kHz for the inner rotor and 600-800 Hz for the outer one. All samples were loaded into the airtight sample spinners in an argon drybox. A total of 1000-3000 acquisitions were accumulated in each experiment with a pulse length of 4 µs (298 K). The spectra were zero-filled to 2K data points, with application of 200 Hz Lorentzian broadening. The external reference was a 0.1 M NaCl solution. Results and Discussion I. Characterization for the Generation Process of Sample a: (i) In Situ Infrared Studies. Figure 1A, trace a, shows in situ infrared spectra in the reaction of [Rh(CO)2Cl]2-Na56Y (precursor) with 500 Torr of CO and 20 Torr of H2O at 343 K.

2338 J. Phys. Chem. B, Vol. 105, No. 12, 2001

Shen

TABLE 1: Carbonyl Stretching Frequencies of Samples a, c, and d and Related Species

samples

terminal CO stretching frequency, cm-1

bridging CO stretching frequency, cm-1

a c d [Rh(CO)2Cl]2 [Rh4(CO)9(µ2-CO)3] [Rh6(CO)12(µ3-CO)4] [Rh6(CO)12(µ3-CO)4]-SiO2 [Rh6(CO)12(µ3-CO)4]-Al2O3

2098vs, 2032vs (sh) 2098vs, 2032vs (sh) 2098vs, 2032vs (sh) 2103vs, 2088vs, 2032vs, 2022m 2076vs, 2045vs (sh) 2070vs, 2025vs (sh) 2083vs, 2051s (sh) 2095vs, 2020vs (sh)

1765s 1835s 1765s

The terminal CO bands appear at 2098 and 2032 cm-1, the bridging CO band appears at 1765 cm-1, and CO and CO2 physisorption bands appear at 217628 and 2360 cm1,29 respectively, which are completely distinguished from the CO infrared bands of crystalline [Rh(CO)2Cl]2 at 2103s, 2088vs, 2032vs, and 2002m cm-1 (Figure 1B and Table 1).30 The intensities of the three bands at 2098, 2032, and 1765 cm-1 successively grow with increasing temperature and time (Figure 1A, traces a-g). After 12 h at 393 K, these bands reached their maximum intensities. The steady-state spectrum of the resulting sample exhibits carbonyl bands at 2098vs, 2032vs (sh), and 1765s cm-1 (Figure 1A, trace g). Table 1 summarizes the CO infrared frequencies of the synthetic sample a and related species. The crystalline [Rh(CO)2Cl]2 shows terminal CO bands at 2103s, 2090vs, 2032vs, and 2002m cm-1 (Figure 1B), and there are no bridging CO bands; therefore, the synthetic sample a cannot be assigned to [Rh(CO)2Cl]2 species. The crystalline [Rh4(CO)9(µ2-CO)3] exhibits νCOt bands at 2076vs and 2045s cm-1 and νCOb band at 1886s cm-1 (Table 1),31 whose terminal CO bands are located at higher frequencies in comparison with that of the synthetic sample a (2045vs f 2032vs cm-1). In addition, the red color of crystalline [Rh4(CO)9(µ2-CO)3] differs from the light pink-gray of sample a. Thus, the [Rh4(CO)9(µ2-CO)3] is not a “guest” located in Na56Y cavities. The crystalline [Rh6(CO)12(µ3-CO)4] from the synthesis in a methanolic KOH solution gave the infrared spectrum (νCOt: 2070vs and 2025vs (sh); νCOb 1798s cm-1) shown in Figure 1C and Table 1.32 This is analogous to the spectrum (νCOt 2098vs and 2032vs (sh); νCOb 1765s) of synthetic sample a. The spectra of [Rh6(CO)12(µ3-CO)4] supported on silica [νCOt 2083vs and 2051s (sh); νCOb 1804s in Table 1]33a and supported on alumina [νCOt 2095vs and 2020vs (sh); νCOb 1810s in Table 1]33b are also similar to that of sample a. Thus, [Rh6(CO)12(µ3-CO)4] appears to be formed in Na56Y zeolite. The CO band shifts of [Rh6(CO)12(µ3-CO)4] on passing to [Rh6(CO)12(µ3-CO)4]-Na56Y [νCOb 1798s f 1765s cm-1; νCOt 2025vs (sh) f 2032vs (sh) cm-1 and 2070vs f 2098vs cm-1] can match those caused from ion-pairing effects, including the interaction for oxygen end of carbonyl ligands and the zeolitic extraframework Na+ cations (see later discussion).2-9 Although the synthetic methodology here is different from [Rh(CO)2(acac)]2 precursor impregnated in Na56Y zeolite6b and different from the Rh3+ ion-exchange Na56Y,6a the resulting infrared spectrum (Figure 1A, trace g) is virtually identical. (ii) 23Na DOR NMR Studies. The 23Na DOR NMR spectrum in Figure 2A yields high resolution. The anisotropic quadrupolar broadening is averaged out in the DOR experiment, and there are three distinct peaks appearing at -4, -29, and -40 ppm, respectively (Figure 2A, trace a). Ozin et al.34 claimed that the 23Na resonance at around -30 ppm is ascribed to the site II Na+ cations within the supercages, while the position of peak at around -42 ppm is assigned to the site I′ Na+ cations inside the sodalite cages, and the Gaussian peak at around -5 ppm is assigned to the site I Na+ cations within the hexagonal prisms

1886s 1798s 1804s 1810s

ref this work this work this work 30 31 32 33a 33b

Figure 2. (A) 23Na DOR NMR spectra at 11.7 T of (a) dehydrated Na56Y; (b) 6[Rh(CO)2Cl]2-Na56Y; (c) 2[Rh6(CO)12(µ3-CO)4]-Na56Y; and (d) 4[Rh6(CO)12(µ3-CO)4]-Na56Y. The asterisks indicate spinning sidebands of the outer rotor. (B) Basic structural unit of Na56Y zeolite, where extraframework cations I, I′, II, and III are indicated. (C) Possible anchoring model for a face-bridging CO ligand of the encapsulated [Rh6(CO)12(µ3-CO)4] bound to the site II Na+ inside the supercage.

of Na56Y zeolite (Figure 2B). These were examined through 23Na DOR spectra studies as various quantities of H O 2 molecules were progressively adsorbed onto dehydrated Na56Y zeolite34 and as various quantities of Tl+ exchanged the Na+ of Na56Y zeolite.35 The 23Na DOR NMR spectrum of the bare Na56Y (Figure 2A, trace a) features a different intensity ratio among the Na+ sites. Although the population of the site I Na+ cations (7.1-7.7%) is quite low compared to the site II (29.432.2%) and I′ Na+ cations (13.4-19.5%),11-15 the DOR resonance associated with the 23Na cations at the site I gave rise to the relatively higher intensity and narrow appearance at around -4 ppm. Such a sharp peak at -4 ppm suggests a small electric field gradient at site I.19,24 The Na+ cations in site I feature six-coordinate, essentially Oh symmetry and the smallest second-order quadrupole line broadening, whereas, the Na+ cations at sites II and I′ have low symmetries (C3v), a relatively large charge asymmetry and large second-order quadrupolar effect.19,24,27 Therefore, the 23Na resonance signal at site II and I′ Na+ cations shows the relatively low intensity and the broad background peaks around -29 and -40 ppm. The loss of the 23Na DOR NMR signal at sites II and I′ is due to their large amount of intensity contained in the spinning sidebands (Figure 2A, trace a). Introducing [Rh(CO)2Cl]2 guests into the cavities of Na56Y causes a downfield shift of the 23Na resonance signal in site II

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Figure 3. (A) Molecular configuration of [Rh4(CO)9(µ3-CO)3]. (B) In situ infrared spectra characterizing the recarbonylation of decarbonylated sample b in the presence of 500 Torr of CO at 298 K: (trace a) 10 min; (trace b) 30 min; (trace c) 1 h; (trace d) 3 h; (trace e) 5 h. (C) Molecular configuration of [Rh6(CO)12(µ3-CO)4]. (D) In situ infrared spectra characterizing the recarbonylation of decarbonylated sample b in the presence of 500 Torr of CO at 343-420 K: (trace a) 343 K, 30 min; (trace b) 373 K, 30 min; (trace c) 393 K, 30 min; (trace d) 393 K, 1 h; (trace e) 393 K, 3 h; (trace f) 393 K, 5 h.

Na+ cations, from -29 ppm for dehydrated Na56Y (Figure 2A, trace a) to around -24 ppm for 6[Rh(CO)2Cl]2-Na56Y (Figure 2A, trace b). Such a downfield shift is ascribed to an incorporation of a minor isotropic chemical shift, δcs, related to the charge density around the site II Na+ nuclei, and a major isotropic second-order quadrupolar shift, δQ,iso, related to the asymmetry parameter η and the quadrupole coupling constant (QCC or e2qQ/h) of the site II Na+ cations.19,24c,27,35 The [Rh6(CO)12(µ3-CO)4] molecule has Oh symmetry with 12 CO ligands bound to six Rh atoms and four face-bridging CO ligands each bound to three Rh atoms (Figure 3C).32 The two [Rh6(CO)12(µ3-CO)4] clusters were synthesized and assembled into the Na56Y cavities. This is proposed to change the chemical environments around the site’s Na+ cations. Consequently, the 23Na signal at site II is shifted downfield to -22 ppm (Figure 2A, trace c), from both the dehydrated Na56Y (-29 ppm in Figure 2A, trace a) and the 6[Rh(CO)2Cl]2-Na56Y precursor (-24 ppm in Figure 2A, trace b). [Rh6(CO)12(µ3-CO)4] attachment to the site II Na+ decreased the quadrupolar coupling; therefore, the site II Na+ resonance signal changed to a Gaussianlike line shape (Figure 2A, trace c).19d,24b,27,35 The downfield shift and the intensity growth of the 23Na NMR peak, together with the infrared band

shift of the bridging CO ligands (νCOb 1798s f 1765s cm-1 in Figure 1A,C), suggest that a strong anchoring reaction occurs between the oxygen end of the face-bridging carbonyl ligands in [Rh6(CO)12(µ3-CO)4] and the site II Na+ cations (Figure 2C). Such an interaction is expected to result in a net electron withdrawal from the clusters. This decreases the back-bonding to the terminal CO ligands and strengthens the carbon-oxygen bond. Thus, the terminal CO infrared bands shifted to higher frequency (νCOt 2070vs f 2098vs cm-1 and 2025m f 2032s cm-1 in Figure 1A,C).2-8 Although the same anchoring interaction occurred in the site II Na+ cations and the oxygen end of carbonyl ligands in the precursor and sample a, the 23Na signal associated with the site II Na+ cations, shifted from a broad peak at -24 ppm for 6[Rh(CO)2Cl]2-Na56Y precursor to a narrow peak at -22 ppm for 2[Rh6(CO)12(µ3-CO)4]-Na56Y sample a. This is due to significant basicity of the oxygen in the face-bridging CO ligands in [Rh6(CO)12(µ3-CO)4] in comparison with that of the terminal CO ligands in [Rh(CO)2Cl]2 (as mentioned in refs 2-8, the basicity of the oxygen of CO ligands in metal carbonyl clusters depends on the CO coordination geometry), which further decreased the QCC value at the site II Na+ nuclei.

2340 J. Phys. Chem. B, Vol. 105, No. 12, 2001 The kinetic diameter of [Rh6(CO)12(µ3-CO)4] is significantly larger than the 2.7 Å wide six-ring window between the sodalite cage and supercage.36 This does not permit incorporation of [Rh6(CO)12(µ3-CO)4] within the sodalite cages. Nevertheless, a downfield shift and broadening of the 23Na DOR resonance associated with the site I′ Na+ cations inside the sodalite cavities are observed, from around -40 ppm for the dehydrated Na56Y zeolite (Figure 2A, trace a) to -37 ppm for 2[Rh6(CO)12(µ3CO)4]-Na56Y (Figure 2A, trace c). These might ascribe to an incorporation of an increase of the asymmetry constant (η) and a reduction of QCC at the site I′ Na+ cations, as well as a portion of the site I′ Na+ cations migrating into the supercages.24c,35 The result is consistent with the inference that an indirect negative cooperation effect is exerted by [Rh6(CO)12(µ3-CO)4] guests to the site I′ Na+ cations inside the adjacent sodalite cages.19d The analyses of 23Na DOR NMR and infrared spectra, together with the steric/volume filling requirement of the encapsulated [Rh6(CO)12(µ3-CO)4] in the supercages, suggest an interaction between the oxygen end of the four face-bridging CO ligands and the four site II Na+ cations [there are four site II Na+ cations per supercage in Na56Y (Figure 2B)].36 One of four ZO-Na---(µ3-CO)Rh3 species (where Z indicates silicon or aluminum framework atoms) is illustrated in Figure 2C. The transformation of “half-naked” ZO-Na+ in the dehydrated Na56Y into coordinated ZO-Na+---OC in n[Rh6(CO)12(µ3-CO)4]-Na56Y increased the symmetry around the 23Na nuclei at site II and/or increased the Na+ motion within the supercage to the extent that the 23Na signal contained in the spinning sidebands is recovered. With increasing loading, anchoring of [Rh6(CO)12(µ3-CO)4] further reduces the electric field gradient at site II Na+ as well, resulting in a decrease in magnitude of second-order quadrupolar effect for the site II Na+.19d,24c Thus, the intensity of the signal ascribed to the site II Na+ cations progressively increased as more [Rh6(CO)12(µ3CO)4] molecules were synthesized within the supercages, as shown in Figure 2A, trace d. It should be noted that the 23Na DOR NMR resonance at around -4 ppm, ascribed to the site I Na+ cations inside the hexagonal prism, is not affected by the guest molecules within the Na56Y supercages (Figure 2A, traces a-d). Na+ cations at the site I are spatially constrained between two six-rings comprising the hexagonal prism (Figure 2B); therefore, interactions with the guest species inside the supercages are limited. This is in agreement with studies of 23Na MQ MAS NMR for C6H6, CHF-134, and CH(D)Cl3 adsorbed on NaY zeolite, where the isotropic chemical shift and the quadrupolar interaction of the site I Na+ are hardly affected by adsorbed molecules in the surpercage.19d,24c,27 II. Decarbonylation of Sample a. Sample a was treated with 400 Torr of H2 at a heating rate of 5 K/min from 298 to 423 K. The 2098vs, 2032vs (sh), and 1765s cm-1 carbonyl bands remain at constant frequencies but with decreasing peak intensities, which is consistent with the uniform nature of the [Rh6(CO)12(µ3-CO)4]-Na56Y species. Upon maintenance of the temperature at 423 K for 1 h, the 1765s cm-1 face-bridging carbonyl band completely disappeared. When the sample was heated at 473 K for 2 h, the 2098vs and 2032vs (sh) cm-1 terminal carbonyl bands fully vanished and gave decarbonylated intrazeolitic Rh clusters, which are denoted [Rhx]dec-Na56Y (sample b). III. Recarbonylation of Sample b. The decarbonylated sample was recarbonylated through treatment with 500 Torr of CO in an infrared cell at 298-393 K. Upon exposure to 500

Shen Torr of CO gas at 298 K, the terminal CO bands appear at 2098 and 2032 cm-1, while the bridging CO band appears at 1835 cm-1 (Figure 3B, trace a). With increasing reaction time, the three infrared CO bands grew in intensity, whereas a H2O infrared band at 1640 cm-1 remained at the initial intensity. After 5 h at 298 K, the peak intensities of 2098vs, 2032vs, and 1835s cm-1 reached maximum values (sample c shown in Figure 3B, trace e). The spectrum is analogous to that of crystalline [Rh4(CO)9(µ2-CO)3] [νCOt 2076vs and 2045vs (sh); νCOb 1886s] synthesized in n-hexane NaHCO3 solution.31 In the crystalline [Rh4(CO)9(µ2-CO)3], 12 terminal CO ligands and four edgebridging CO ligands coordinated in a tetrahedral Rh framework (Figure 3A). When the sample was heated to 343 K (Figure 3D, trace a), a new bridging CO band appeared at 1765 cm-1, while the original bridging CO band at 1835 cm-1 and the H2O band at 1640 cm-1 decrease in intensity. With increasing reaction time and further heating, the 2098vs and 2032vs (sh) cm-1 terminal CO bands remained at constant frequencies and intensities, but the 1765 cm-1 bridging CO band increased. Simultaneously the 1835 and 1640 cm-1 bands exhibit decreasing intensity (Figure 3D, traces a-f). After 5 h at 393 K, the 1765 cm-1 band reached its maximum intensity, while the 1835 cm-1 band fully disappeared (Figure 3D, trace f). In addition, it is noteworthy that the infrared band at 2360 cm-1, belonging to the CO2 physical adsorption,29 grew dramatically with the increase of CO band at 1765 cm-1 (Figure 3D, traces c-f, and see later discussion). The asterisks in Figure 3B,D are an indication of artifacts that accompany the high-intensity infrared bands in the Shimadzu infrared 4200 spectrometer. The steadystate infrared spectrum of the resulting sample d (Figure 3D, trace f) is practically identical to that of [Rh6(CO)12(µ3-CO)4]Na56Y (sample a in Figure 1A, trace g), as seen from a comparison of their CO frequencies and intensities. These spectra suggest the regeneration of the [Rh6(CO)12(µ3-CO)4] clusters via [Rh4(CO)9(µ2-CO)3] species inside the Na56Y supercages. The CO chemisorption on the decarbonylated sample b at 298 K showed bands analogous to those of the crystalline [Rh4(CO)9(µ2-CO)3]. The infrared band of H2O at 1640 cm-1 (Figure 3B) maintained constant intensity, suggesting no interaction between the clusters and H2O in zeolitic cages at 298 K. Weber et al.6b studied a more severe H2 treatment (1 atm of H2 at 473 K) of [Rh6(CO)12(µ3-CO)4]-Na56Y in comparison with present decarbonylation (400 Torr of H2 at 473 K) and determined 3.3 Rh coordination numbers in the decarbonylated [Rh6(CO)12(µ3-CO)4]-Na56Y sample by EXAFS analysis. Taking the EXAFS data together with the present infrared spectrum, it is inferred that the Rh cluster nuclearity of the decarbonylated sample b is approximately four rhodium atoms, assembled into the tetrahedral Rh framework in the Na56Y cages,6b,37 denoted as [Rh4]dec-Na56Y (sample b). In addition, the CO/Rh stoichiometry of about 16 mol of evolved CO/1 mol of zeolitic [Rh6(CO)12(µ3-CO)4] (where the stoichiometry is determined by gas chromatography) provides supplemental evidence of the decarbonylated sample b as [Rh4]decNa56Y. The decarbonylation of sample a is shown in

2[Rh6(CO)12(µ3-CO)4]-Na56Y + Na56Y f 3[Rh4]-Na56Y + 32CO (1) CO/Rh6 molar ratio ) 16

(2)

The infrared spectrum of the recarbonylated species at 298 K is similar to that of the crystalline [Rh4(CO)9(µ2-CO)3] and the spectra reported by Rao et al.6a and Weber et al.,6b suggesting

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that the recarbonylated species at 298 K is [Rh4(CO)9(µ2-CO)3] rather than [Rh6(CO)12(µ2-CO)4].6a The chemisorption of 500 Torr of CO at 298 K on the zeolitic decarbonylated Rh clusters is proposed as

[Rh4]dec -Na56Y + 12CO f [Rh4(CO)9(µ2-CO)3]-Na56Y (3) The recarbonylation from samples b to c in a CO atmosphere at 298 K resulted in an increase of M-CO contributions but has no effect on M-M contributions. Increasing the reaction temperature led to growth of M-M contributions, which reflects in the shift of the bridging CO band from 1835 cm-1 (sample c) to 1765 cm-1 (sample d, Figure 3D). These patterns are consistent with the proposition that the transformation of Rh carbonyl clusters occurred via a reaction of the zeolitic water and [Rh4(CO)9(µ2-CO)3] species. The conversion is represented as

3[Rh4(CO)9(µ2-CO)3]-Na56Y + 4H2O f 2[Rh6(CO)12(µ3-CO)4]-Na56Y + Na56Y + 4CO2 + 4H2 (4) Equation 4 is consistent with the growth of the face-bridging CO band at 1765 cm-1 and CO2 physisorption band at 2360 cm-1 (Figure 3D). Such a growth of M-M and M-CO contributions implies mobility of zerovalent Rh carbonyl fragments in the hydrated Na56Y cages. The quantity of H2O in Na56Y lattices is likely to play a significant role in the transformation of [Rh4(CO)9(µ2-CO)3] to [Rh6(CO)12(µ3-CO)4]. The similar conversion occurred readily during absorption of [Rh4(CO)9(µ2-CO)3] on the hydrated SiO2.31c However, in the original synthesis of sample a, since there is enough water in the Na56Y cavities, [Rh6(CO)12(µ3-CO)4] was rapidly formed without observation of the intermediate [Rh4(CO)9(µ2-CO)3]. The decarbonylation of [Rh6(CO)12(µ3-CO)4], either grafted on silica or locked into the Na56Y cavities by O2 treatment, created mixed Rh clusters involving mononuclear Rh species.6a,31c In contrast, the H2 decarbonylation treatment of [Rh6(CO)12(µ3-CO)4]-Na56Y, produced uniform bare Rh4 clusters (sample b). The analogous H2 decarbonylation treatment for Ir4 and Ir6 carbonyl clusters was illustrated by Kawi et al.8 Several authors6a,c,d investigated the synthesis of [Rh6(CO)12(µ3-CO)4] by the reductive carbonylation of rhodium chloride salts in the Na56Y cavities. The synthesis involved the reduction of RhIII to RhI and then to Rh0 of [Rh6(CO)12(µ3-CO)4]. The intermediate RhI(CO)2, attached in the Na56Y lattice, cannot be reduced to [Rh6(CO)12(µ3-CO)4], and consequently yields the mixture of 85% [Rh6(CO)12(µ3-CO)4] and 15% RhI(CO)2.6a The precursor here is [Rh(CO)2Cl]2, which favors the direct generation of [Rh6(CO)12(µ3-CO)4] by CO reduction. Thus, the RhI(CO)2 species are not included in sample a. The formation mechanism of the zeolitic [Rh6(CO)12(µ3-CO)4] is illustrated in equation v.

3[Rh(CO)2Cl]2-Na56Y + 7CO + 3H2O f [Rh6(CO)12(µ3-CO)4]-Na56Y + 2Na56Y + 3CO2 + 6HCl (5) Conclusions [Rh(CO)2Cl]2 was converted into [Rh6(CO)12(µ3-CO)4] clusters inside the Na56Y supercages by heating under an atmosphere of CO and H2O. The decarbonylated bare Rh4 species can regenerate [Rh6(CO)12(µ3-CO)4] clusters inside Na56Y via [Rh4(CO)9(µ2-CO)3] intermediate species in a mixed CO and H2O

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