Photosensitive MetalâOrganic Systems - American Chemical Society

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4 Photocatalytic Behavior of Tungsten, Iron, and Ruthenium Carbonyls on Porous Glass Shu-Ping Xu and Harry D. Gafney Department of Chemistry and Biochemistry, Queens College, City University of New York, Flushing, NY 11367

Although the primary photoprocess of metal carbonyls physisorbed onto porous Vycor glass is similar to that in fluid solution, the secondary chemistry is quite different. UV photolysis of adsorbed W(CO) leads to CH evolution, but photolysis of Ru (CO) yields a surface-grafted oxidative-addition product, (μ-Η)Ru (CO) (μ-OSi). The catalytic activity of these hybrid systems was examined with respect to methane evolution and olefin isomeriza tion. Isotope labeling experiments showed that UV photolysis of W(CO) leads to W0 , and WO photocatalyzes the conversion of CO to CH . Spectroscopic data point to a reactive site com posed of the metal oxide and Lewis acid site in the glass matrix. The addition of 1-pentene to (μ-H)Ru (CO) (μ-OSi) disrupts the multicentered bonds, and photolysis of the adduct leads to cis -and trans-2-pentene. The trans-cis ratio increases during photo lysis but is significantly smaller than the thermodynamic ratio. Deuterating the oxidative addition product yields deuterated ole fins, and the product distribution, 90% as 2-D-2- and 3-D-2pentenes and 10% 1-D-1-pentene, suggests an excited state similar to a pi-allyl complex. 6

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L HE EXCITED STATE differs from the ground state in energy, electron distribution, and nuclear configuration. It is not surprising then that its chemistry can be quite different. Certainly with classical transition metal complexes and metal carbonyls, studies of their spectroscopy, photophysics, and photochemistry over the past 30 years have elucidated a chemis0065-2393/93/0238-0067S06.00/0 © 1993 American Chemical Society

In Photosensitive Metal—Organic Systems; Kutal, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.


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try of the different excited states as well as the intramolecular processes that partition the energy among these states. Nevertheless, it remains a chemistry of high-energy, short-lived intermediates, and developing the means to control their reactivity or promote a specific chemistry is a signi ficant experimental objective. This objective is readily apparent in the catalytic activity of photoactivated metal carbonyls. Although the specific nature of the catalytic intermediate remains controversial, it is generally agreed that light generates a coordinatively unsaturated intermediate, and this species, or a species derived from it, promotes the conversion of the organic substrate. One approach being examined is to bind or adsorb the precursor onto a support (1), that is, to assemble a hybrid catalyst (2). A support provides a unique microenvironment, in which the photochemistry of the precursor and its subsequent catalytic activity reflect the chemical nature and dimensionality of the support surface. Most inorganic oxide supports, including porous glasses, possess hydroxylated surfaces on which the sur face functionality, typically a hydroxyl group, is capable of acting as a scavenging nucleophile. Coordination to a surface functionality stabilizes the primary photoproduct, influences its surface mobility, and changes its optical absorption characteristics. The morphology of the surface, that is, its topology or fractal dimensionality, can also influence reactivity (J). In zeolites, in which cage dimensions approach molecular dimensions, encap sulation of a reagent influences its molecular dynamics (4). Amorphous substrates, such as porous glasses, do not possess the crystalline regularity of zeolites. In fact, porous glasses derived from the base-catalyzed poly merization of alkylorthosilicates (xerogels) frequently exhibit two realms of porosity. A microporosity on the order of tens of angstroms exists within the silicate clusters, and a mesoporosity that can range from tens or hundreds of angstroms to micrometers exists between the clusters (5). Nevertheless, the morphology of these amorphous materials restricts adsorbate mobility and, at least in one case, curtails the fragmentation of a photoactivated cluster (6). As a result, the intermediates generated on a support do not necessarily possess a direct analog among those generated in homogeneous solution and need not exhibit an equivalent photoac tivated catalytic chemistry. Our interest in photoactivated hybrid catalysis stems from studies of the photochemistry of metal carbonyls adsorbed onto Coming's code 7930 porous Vycor glass ( P V G ) (7). In many cases, the intermediates gen erated on this support and their subsequent chemistry differ from that in fluid solution. These changes arise from the participation of the glass in the secondary thermal and photochemical reactions of the adsorbates, but the choice of P V G as a reaction medium does not stem from a specific advantage with respect to catalysis. Rather, P V G and porous glasses in general offer a unique combination of rigidity, transparency, and porosity.


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Behavior of W, Fe, & Ru Carbonyls on Porous Glass

Transparency offers spectroscopic access, and in turn, an amenability to fast reaction techniques. Porosity offers chemical access; access not only in the sense of intercepting a short-lived intermediate, but also in the syn thetic sense of utilizing the chemical nature, rigidity, and morphology of the porous matrix to modify adsorbate chemistry. Certainly in our case, the underlying strategy is to take advantage of the microstructure and microenvironment of the support to impose some control on a photoac tivated reaction system. In this chapter, we summarize the properties of P V G and the photocatalytic activity of W(CO) , Fe(CO) , and R u ( C O ) physisorbed onto this glass.


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Porous Vycor Glass (PVG) P V G (8-11) is a 96% S i 0 , 3% B 0 , and 1% N a 0 and A l O glass (9). When the borosilicate melt is cooled below its phase-transition tempera ture, the silica phase separates from the boron oxide-alkali oxide phase, and acid leaching of the oxide phase yields a random, three-dimensional network of interconnected pores throughout the glass. Pore size and sur face area are determined by the extent of phase separation in the melt and acid leaching. Pore sizes ranging from 20 to 2500 Â are currently avail able, but larger pore sizes reduce optical transparency. The glass used in our experiments has an average pore diameter of 100 ± 10 A and a sur face area of 183 ± 15 m /g (12). Scanning electron microscope (SEM) analyses of calcined (650 °C) samples reveal a nodular surface composed of silicate nodules with inter vening crevices that contain the openings into the interior pore structure (12). The intervening spaces, which in total correspond to a void volume of ~35%, range from 40 to 100 Â (12). This substrate is clearly amor phous, and although the term "amorphous" appears to have a negative connotation with respect to organizing a reaction system, it is a lengthdependent term; its significance depends on the dimensions of the events under consideration. Small-angle X-ray scattering (SAXS) and smallangle neutron scattering (SANS) yield a correlation length, that is, a length of uniform density, of 242 ± 8 À (13, 14). Although it does not possess the geometric regularity of a zeolite, with respect to the dimen sions of an adsorbate and the distance over which its chemistry occurs, this amorphous glass is a relatively uniform substrate. With any heterogeneous medium, the active region is the surface. Diffuse reflectance Fourier transform (DRIFT) infrared spectra of the cal cined glass reveal a surface composed of free 3744-cm" and associated 3655-cm" silanol groups (7, 11). The number of silanol groups depends on the sample's thermal history, but studies of a variety of hydroxylated 2

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silicas yield silanol numbers of 4-7 per 100 Â with the highest density within the pores (15, 16). Trace amounts of chemisorbed water are also present in calcined (650 °C) samples (17). P V G is often likened to silica gel, but the two materials are not chemically equivalent. In addition to the silanol groups, which function as weak Br^nsted acids, P V G also possesses B 0 Lewis acid sites. As a result of its method of manufacture, these sites are dispersed on surfaces throughout the glass matrix. X-ray photoelectron spectroscopic (XPS) analyses indicate that the amount of Β present in the first 50 À of the samples used in our experiments is 2.6 ± 0.1% (12). Impregnation is accomplished by conventional solution adsorption or vapor deposition techniques (17, 18). Regardless of initial loading, how ever, neither technique yields a uniform distribution of the organometallic compound throughout the pore volume. Impregnation of the bulk is lim ited by the narrow, tortuous passes connecting the pores, and typical expo sure times of c ο ο

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Τ (min) Figure 2. Distribution of cis-2-pentene (Ώ) and tmm-2-pentene (+) dur ing 300-nm photolysis of fa-H)Ru (CO) (^OSi) under 400 torr (53.32 kPa) of 1-pentene (m). 3

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Behavior of W, Fe, & Ru Carbonyls on Porous Glass 79

Ru(CO)

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Scheme II. pentenes, and the remainder is present principally as 1-D-l-pentene with trace amounts of l-D-2-pentene. Of the 2-pentenes formed, 2-D-2- and 3-D-2-pentenes are the dom inant products, and the relative peak heights yield a 2-D-2-pentene:3-D2-pentene ratio of 4.8 ± 0.1:1. U V photolysis of 1-pentene physisorbed onto P V G does not result in olefin isomerization. The metal complex is essential to the conversion, but in this case, the reaction appears to be a photoassisted catalytic pro cess in which excitation of III generates an excited state that promotes 1pentene isomerization. The formation of 2-D-2- and 3-D-2-pentenes suggests that the excited state may be similar to a pi-allyl complex, as is shown in Scheme I. In this configuration, the carbons in the 2- and 3-positions are susceptible to deu terium substitution. The displaced olefin hydrogen can be transferred to the metal complex to regenerate the surface grafted cluster, which then reacts with 1-pentene to re-form III (Scheme II). Further experiments are necessary to determine the distribution of deuterium in the cis- and iram^-pentenes. Nevertheless, the trans-cis ratio observed, 2.0 ± 0.1, is considerably smaller than the thermodynamic ratio, 4.82. One possible explanation, of course, is that the topology of the glass surface to which the complex is bound biases the isomer ratio.


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However, if this were the sole determinant of the trans—cis ratio, we would expect a ratio similar to that obtained in the photocatalyzed isomer ization of 1-pentene by Fe(CO) on P V G , that is, 3.7 ± 0.2. In this case, the photochemical reaction generates a thermally activated ground-state catalyst, whereas in the R u - P V G system, photoactivation generates an excited state that promotes olefin isomerization. Consequently, the smaller ratio found with this system may reflect not only the topology of the support, but also the specific catalytic species present on the glass sur face. Downloaded by NORTH CAROLINA STATE UNIV on May 9, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0238.ch004

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Acknowledgments Support of this research by the Research Foundation of the City Univer sity of New York and the National Science Foundation (Grant CHE-8913496) is gratefully acknowledged. H . D. Gafhey thanks Corning Inc. for samples of porous Vycor glass.

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Behavior of W, Fe, & Ru Carbonyls on Porous Glass

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