J. Phys. Chem. 1993,97, 9465-9469
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[Pt6(CO)l2l2- and [Pts(CO)18]*- Supported on MgO Synthesis and Spectroscopic Characterization Zhengtian Xu, Arnold L. Rheingold, and Bruce C. Gates' Center for Catalytic Science and Technology, Department of Chemical Engineering and Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 1971 6 Received: June 24. 1993e
[Pt6(Co)12)]2- and [Ptg(C0)18)l2- were synthesized from [Pt(acac)z] on the surface of MgO powder and characterized by infrared spectroscopy. When the reaction was carried out in the presence of CO at 100 atm, [Ptg(C0)lsl2- was formed in high yield. When the reaction was carried out under these conditions with [Rez(CO)lo] also present on the surface, [Pt6(C0)12]2- was formed in high yield. The platinum carbonyl anions were extracted into solution by cation metathesis and characterized spectroscopically. The carbonyl stretching bands of [Pta(C0)l2l2- on MgO were shifted about 10 cm-l to higher energy relative to those of the cluster in tetrahydrofuran solution, indicating weak ion pairing with the Mg2+ ions of the MgO surface; in contrast, the lack of such shifts characterizing [Ptg(C0)18)l2- on MgO indicates the lack of significant ion pairing of this cluster on the surface. In the presence of traces of air, the platinum carbonyl clusters on MgO were converted into small particles of Pt metal.
Introduction Carbonyl clusters of a number of group 8 metals have been prepared on surfaces of metal oxide supports and used as precursors of supported metal Most organometallic reactionson surfacesgive complicated mixtures, and only in some instances and by careful choice of the reaction conditions and reaction precursors has it been possible to form single species in high yield. For example, the conversion of organometallic and salt precursorson the surfaces of basic MgO powders, even in the absence of solvents, gives high yields of some of carbonyl cluster anions of Os (including [ O S S C ( C O ) ~ and ~ ] ~ [0sl~C(CO)24J2l) and Ir (including [HIr4(CO)11]- and [Ir6(CO)l~)]2-).z.3'6 In a few instances, metal clusters have been prepared in high yields and simply extracted from the surface, with the surface-mediated synthesis then being the most efficient synthesis.' It has also been possible in a few instancesto prepare metal carbonylclusters on metal oxide supports and to decarbonylate them with little change in the nuclearity (number of metal atoms); in this way, MgO- and zeolite-supported tetrairidium clusters have been made.'** Because Pt is the noble metal that finds the greatest application in metal catalysts and because Pt is often used in a highly dispersed form on supports, attempts have been made to use surface-mediatedsynthesis to prepare small and structurally uniform supported Pt clusters. Very small clusters are representativeof the Pt in some industrial catalysts, such as, for example, those used for naphtha reforming9 and n-hexane dehydrocyclization to give benzene.1° Here we report the synthesis and characterization by infrared spectroscopy of [Pt6(Co)12lz- on the surface of MgO; the cluster has been extracted from the surface by cation metathesis (ion exchange) and identified spectroscopically in solution. This work began as an attempt to synthesize supported bimetallic clusters containing Re and Pt; the attempt failed, but the incorporationof Re was found to lead to the first surface-mediated synthesis of [Pt6(Co)12)]2-. The analogous synthesis of [Ptg(Co)l~)]~-, in the absence of [Rez(CO)lo], and its characterization on MgO are also reported. ExperimentalSection
The sample preparation and handling were carried out with exclusion of air and moisture on a double-manifold Schlenk vacuum line, in a Nz-filled Braun MBI 50 glovebox, or in a batch autoclavereactor. The solvents were dried and deoxygenated by *Abstract published in Advance ACS Abstracrs, September 1, 1993.
sparging with N2 prior to use. Tetrahydrofuran (THF) and hexanes were freshly distilled from sodium/benzophenoneketyl. Acetone (reagent grade) was dried over activated 4A molecular sieve particles. Platinum acetylacetonate, [Pt(acac)~],as well as [Re2(CO)lo] and [Re(CO)SBr] (Strem) were used without purification. Bis(triphenylphosphorany1idene)ammonium chloride, [PPN] [Cl] (Aldrich), was dried overnight at 100 "C and stored in the drybox. The MgO powder (MCB) was calcined by heating to 400 "C in flowing 0 2 (Matheson, extra dry grade) for 2 h followed by evacuation (1e 3 - l 0-4 Torr) for 14 h. The surface area of the treated powder was approximately 75 m2/g. The reactor was a Parr autoclave with a 200-atm pressure limit. CO for high-pressure reactions (Matheson, UHP grade) was provided in an aluminum cylinder to prevent contamination by iron carbonyl that forms in steel cylinders. This CO was used without purification. The CO used for treatment of samples in transmission infrared spectroscopyexperiments (Matheson,UHP grade) flowed from a high-pressure steel cylinder through a trap containing activated alumina particles that were heated to a temperature exceeding 250 "C to remove traces of metal carbonyl impurities and through a bed of activated zeolite 4A particles to remove moisture. The Pt-containing precursors were made by bringing hexane solutions of (1) [Pt(acac)2] or (2) [Pt(acac)z] with [Rez(CO)lo] or with [Re(CO)sBr] in contact with MgOpowder. Theamounts were chosen to give 1 wt 5% Pt in each resultant solid sample, and either 0 or 2 wt % Re. Each slurry was stirred for 12 h, and then the solvent was removed by evacuation (lO-3-lV Torr) for 12 h. The resultant solids were light yellow. Each of these samples was loaded into the Parr reactor in the N2-filled drybox. The reactor was purged five times with CO at 34 atm to remove the Nz, and then the CO pressure was increased to the desired value, and the reactor was heated to 60 "C and held for 20-24 h. Then the reactor was cooled to room temperature, the pressure was reduced, and the solid sample was removed in the drybox. Extraction of Surface Species. Extraction of anions from the MgO surface was performed by mixing solid samples with a solution of [PPN] [Cl] in THF. The mixture was stirred for about 10 min, and the resultant color of the solution and the loss of color of the powder (it became white) indicated when the extraction was complete. The supernatant solution was transferred with an airtight syringe to either a sealed infrared or ultraviolet-visible
0022-3654/93/2091-9465$04.00/00 1993 American Chemical Society
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The Journal of Physical Chemistry, Vol. 97, No. 37, 1993
cell, and the spectroscopic characterization was completed within a few minutes to minimize oxidation of the samples. Characterization by Infrared and Ultraviolet-Visible Spectroscopies. Infrared spectra were recorded with a Nicolet 510 or a Nicolet 7199 FTIR spectrometer with a spectral resolution of 4 cm-1. Solution spectra were recorded with a sample in an 0.1-mm NaCl cell equipped with a oneway perfektum spring-clip stopcock (Aldrich) that allowed injection of the sample without air contamination. Spectra of solid samples were recorded in a heatablecell to which the flow of gases of controlled compositions could be metered. This cell had a NaCl crystal-glass O-ring seal. Alternatively, spectra were measured with samples in a diffuse reflectance cell equipped with ultrahigh-vacuum fittings.” The samples were loaded into the cells under N2 in the drybox. Electronicspectra were recorded with a Hewlett-Packard 8 152 ultraviolet-visible spectrometer. Solution spectra were recorded with samples in a cell having a circular opening that was sealed with a septum cap to minimize air contamination. Samples were transferred from a Schlenk flask to the cell with an air-tight syringe, and the cell had been filled with CO before introduction of the liquid sample. The solutions were diluted to give the highest quality spectra. Background subtractions were carried out to remove the spectrum of the THF solvent.
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Wavenumber, cm’ Figure 1. Infrared spectra of (A) sample prepared from [Pt(acac)~J supported on MgO treated under CO at 20 atm and 60 OC for 20 h; (B) solution prepared by extraction of the solid sample with [PPN][Cl] in THF.
Results Samples Prepared from [Pt(acac)J Supported on MgO. A sample was treated under CO at 1 atm and 60 O C for 20 h in an infrared cell, and then the gaseous C O was removed by purging the cell with He for 10 min. The resultant spectrum showed no indication of YCO bands, and the sample color remained light yellow, which is the color of the [Pt(acac)2] precursor. The solid sample was mixed with [PPN][Cl] in THF, and the resultant extract solution was characterized by infrared spectroscopy;there were no YCO bands in the spectrum. There was no evidence of the formation of any platinum carbonyl under these mild conditions. Another sample, prepared from [Pt(acac)2] and MgO, was treated under CO a t 20 atm and 60 OC for 20 h. The resultant sample, removed from the autoclave in the drybox, was greenishpink in color. The infrared spectrum of the solid sample (Figure 1) has four bands in the vco region: 2041 (sh), 2015 (s), 1853 (m), and 1834 (m) cm-*. The infrared spectrum of the extract solution (Figure 1) has YCO bands at 2044 (m), 2027 (s), 1858 (m),and 1843 (m) cm-l. This spectrummatches that of amixture of [Pt12(C0)24]~-and [Ptg(CO)18]” in THF solution.12 Still another sample, prepared in thesameway from [ P t ( a ~ a c ) ~ ] and MgO, was treated in the autoclave under forcing conditions, namely, under CO at 100 atm and 60 OC for 24 h in the absence of solvent. The sample was removed from the autoclave in the drybox; it was bright pinkish-violet. The vco infrared spectrum of the solid sample (Figure 2) is characterized by two broad bands, one a t about 2031 (s) cm-1 and one a t about 1846 (m) cm-l. The spectrum of the solution resulting from extraction of the sample with [PPN] [Cl] in THF (Figure 2) is alsocharacterized by two YCO bands, a t 2027 (s) and 1842 (m) cm-1. The extract solutionwas also characterized by ultraviolet-visible spectroscopy (Figure 3); there are two bands, a t 362 (s) and 556 (w) nm. Both the infrared and ultraviolet-visible spectra are indistinguishable from those reported for [Pts(C0)18l2-.l2-l4 The yield of [ P ~ $ I ( C O ) ~was ~ ] ~approximately 50%, as estimated from the ultraviolet-visible spectrum. Another sample, again prepared in the same way from [Pt(acac)z] and MgO, was treated in the autoclave under equimolar C O + H2 at 100 atm and 60 O C for 24 h. The sample color was pinkish. The infrared spectrum of the solid (Figure 4) includes four peaks in the vco region: 2018 (s), 1985 (m), 1853 (w), and 1812 (w) cm-1. Theextract solution was also characterized by infrared spectroscopy. There are four bands in the YCO region: 2027 (s), 1995 (m), 1844 (w), and 1785
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Figure 3. Ultraviolet-visiblespectrumof solution prepared by extraction with [PPN][Cl]in THFof thesolid prepared from [Pt(acac)l] supported on MgO treated under CO at 100 atm and 60 OC for 24 h. (w) cm-1, indicating a mixture of [Pts(C0)l2J2- and [Pt9(C0),*]*-.’2 Samples Prepared from [Pt(acac)z]and [Re2(CO)1o]Supported on MgO. In attempts to prepare mixed metal clusters, [Re2(CO),,] was used along with [Pt(acac)z]. The conditions were virtually unchanged from those of one of the autoclave experiments described above (60 OC and 100atm CO, 20-24 h reaction time), except that [Re2(CO),o] was included with the organoplatinum precursor in the initial stage of the preparation. There was no evidence of bimetallic clusters. Instead, a different platinum carbonyl formed on the MgO surface; it was orange-red in color. The vco spectrum of the solid sample (Figure 5 ) includes two bands besides those of the [Rez(CO)lo] precursor (Figure 6), a t 2004 (s) and 1807 (w) cm-1. The solid sample was highly air-sensitive; upon removal from the autoclave in the drybox, it turned gray after several hours;
The Journal of Physical Chemistry, Vol. 97,No. 37, 1993 9467
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Wavenumber, cm' Figure 6. Infrared spectra of (A) [Rez(CO)lo]supported on M g O (B) [Rez(CO)lo] in hexane solution. exposure to the atmosphere caused the sample to turn gray within a few minutes. The extraction with [PPN] [Cl] in T H F had to be carried out in the presence of CO with strict air exclusion. Without the CO in the extract, the liquid sample changed color fromorange-red toviolet-red toblue-green and then todarkgreen; when the dark green sample was exposed to air, it turned dark gray * The MgO-supported sample in the drybox was also light sensitive; when it was removed from the autoclave in the light, it turned gray within several hours. In contrast, when the sample
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Wavenumber, cm" Figure 7. Infrared spectrum of the solution prepared by extraction with [PPN][Cl] in THF of the solid prepared from [Pt(acac)~]and [Rez(CO)lo]supported on MgO treated under CO at 100 atm and 60 "C for 24 h.
was stored in the dark in the drybox, it appeared to be unchanged for 2 weeks, but thereafter it slowly turned gray. The infrared spectrum of the solution formed by extraction with [PPN][Cl] in T H F in the presence of CO (Figure 7) is characterized by two bands in the YCO region besides those of the [Re2(CO)lo]precursor: 1995 (s) and 1785 (w) cm-1. These bands identify the extracted metal carbonyl as [Pt6(CO)l2]2-. Attempts to characterize the extract solution by ultraviolet-visible spectroscopy were unsuccessful, as the highly air-sensitive sample was oxidized in the handling step. When [Re(CO)sBr] was used instead of [Re2(CO)lo] in the preparations, the results were unchanged, [Pt6(Co)12]2-was the only platinum carbonyl formed under these conditions, and [Re(CO)sBr] was converted to [Re2(CO)lo]under the high-pressure CO during the reaction, as indicated by infrared spectroscopy. Discussion Solution Chemistry of Platinum Carbonyl Anions. Platinum carbonyl anionsof the family [Pt3(Co)6]n2-, described by Longoni and Chini,l2 have been synthesized from Na2PtCl6 in basic solutionsand investigatedthoroughly. Crystallographicdata show that the metal frames of these cluster anions are twisted prisms; each oligomeric cluster anion consists of stacks of Pt3 triangles with three terminal and three bridging CO ligands15 The family of platinum carbonyl anions includes oligomers with n ranging from 1 to 6,and recently synthesis of new members of this family (n = 7, 8) has also been reported.I6 The syntheses of the [Pt3(Co)6]n2-clusters involve reductive carbonylation of Na2PtCl6 by CO in basic solutions; proper choice of the synthesis conditions leads to high yields of the individual cluster anions. The smaller thecluster in the family, themore basicis the solution needed, and to synthesize the smallest of these clusters, alkali metals are used as reducing agents because these clusters are highly reduced species. [Pt3(Co)6]2- is relatively unstable and has not been isolated in a pure crystalline form; [ptS(CO)12]2- is the smallest of these clusters which has been obtained in the crystalline state. The infrared spectrum of each of these dianionic clusters is characterized by two YCO bands, in agreement with the presence of a [Pt3(C0),] triangular unit periodically repeated in one dimension, with a higher frequency band indicative of terminal carbonyls and a lower frequency band indicative of bridging carbonyls. As the size of the cluster decreases, the decrease in the number of metal atoms per unit of negative charge results in a shift of the YCO bands to lower frequencies, but the YCO values are virtually the same when n > 5 .
9468 The Journal of Physical Chemistry, Vol. 97,No. 37, 1993
TABLE I: Infrared Spectra of [PtJ(C0)6Ls(n = 1-6)in THF Solution and on MeO ~~~
~~~
~
~
cluster [Pt18(co)3612-
~
cm-I 2065(vs), 1900(sh), 1875(s) , 1855(s h), 1840(sh) tPtls(co)3olz2055(vs), 1890(mw), 1870(s), 1840(sh), . . l830(sh) [Ptls(CO)3o]Z-/MgO 2059(s), 1877(s), 1853(s) [Pt12(CO)z4122040(vs), 2030(sh), 1880(mw),1860(s), 1825(mw) [Pt1z(CO)u]~-/Mg02046(s), 1846(m) 2030(vs), 1855(sh), [Pt9(Co)lalz1840(s), 1830(sh), 18lO(m) [Pt9(CO)18]2-/Mg0 2031(s), 1846(s) [pts(co)l2121995(vs), 1818(m), VCO,
color ref olive-green 12 yellow-green 12 yellow-green 18 blue-green
12
blue-green violet-red
12
violet-red orange-red
this work
orange-red pink-red
this work 12
17
12
1795(s\
[Pt6(cO) 1#-/Mg0 [Ptl(c0)61’-
2004(s)~~807(w) 1945(vs), 174O(s)
[Pt9(co)18]2- and [Pt1~(CO)24]2have also been synthesized on surfaces by treatment of MgO-supported [Pt(C3Hs)z] with CO at room temperature.” [Ptls(C0)30]~-was formed in 73% yield by treatment of NazPtCl6 and MgO in a methanol slurry [Pt9(Co)1812- has also been suggested to under CO a t 50 have formed in the supercages of zeolite N a y , although the identification has been questioned, in part because the clusters could not be characterized by simple e x t r a c t i ~ n . ~ ~There - ~ l has been no report of the preparation on a surface of Pt cluster anions smaller than [Pt9(CO)18]2-. Thedifficulty of the surfacemediated synthesis of the smallest Pt carbonyl cluster anions is associated with the need for strong reducing environments and strict air exclusion. [Pts(CO)ls& Supported on MgO. The infrared spectrum of the solid sample prepared in this work from [ P t ( a c a ~ ) ~after ], treament under CO at 100 atm and 60 O C for 20 h, is characterized by two broad YCO bands, at about 2031 (s) and 1846 (m) cm-l. The locations of these bands are almost the same as those of the bands characterizing [Pt9(Co)18]2- in THF solution (Table I) and, combined with the pink-violet color of the sample, they indicate the presence of [Pt9(CO)l8l2-onthe MgO surface. Since this cluster anion could be cleanly extracted from the surface, as indicated by the infrared and ultraviolet-visible spectra, we infer that no complicating chemistry took place during the extraction, which evidently took place by simplecationmetathesiswith [PPNIIC11* The similarity in location of the YCO bands of the supported and soluble [Pt9(Co)18)]2- (Table I) indicates the absence of strong interactions of the cluster anion with the MgO surface. Thus the results indicate that [Pt9(co)18]2- was synthesized from [Pt(acac)z] on the MgO surface. The occurrence of the synthetic reaction is understandable on the basis of a comparison with the chemistry of synthesis of this cluster in solution, as follows: In solution, [Pt9(Co)18]z- has been prepared from NazPtC16 with 9 M NaOH in CHJOH under CO; the reaction time was 24 h.Iz The synthesis in solution involves redox chemistry and is rather well understood. The stoichiometry of formation of Pt, carbonyl clusters, in which C O is a reducing agent, is the following:12 3n[PtCl6I2-+ (12n [Pt,(CO)6],”
+ 1)CO + (12n + 2)OH-18flC1- + (6n + 1 ) C o 2 4- (6n + 1 ) H 2 0
(1) As stronger reducing agents are added to the synthesis solution, the yield of lower nuclearity cluster anions increases; for example, when only 9 M NaOH is used with the reagents stated above, the
Xu et al. product is predominantly [Pt9(Co)18]z-.’2 In contrast, when lithium metal is used as a reducing agent, [Pt6(C0)12l2-is formed in 36% yield.12 The observations in this work are consistent with this pattern in the redox chemistry of platinum carbonyl cluster synthesis. For example, the synthesis of the mixture of [Pt9(Co)18]2- + [Pt12(C0)~4]2-on the MgO surface tookplaceunder a less strongly reducing atmosphere (lower CO partial pressure) than the synthesis of nearly pure [Pt9(Co)18]2-, which, in turn took place under less strongly reducing conditions than the synthesis of the mixture of [Pt6(Co)12]2- + [Pt9(Co)18l2-. [Pb(CO),,]sSupported on MgO. The results observed with the mixture of [Pt(acac)z] and [ R ~ Z ( C O on ) ~MgO ~ ] can also be interpreted on the basis of comparisons with the solution chemistry of platinum carbonyl clusters. After reaction in the autoclave under CO a t 100 atm and 60 OC for 20 h, the MgO-supported sample that initially contained [Pt(acac)~]and [Rez(CO)lo] was orange-red, the color of [pts(CO)1~]~-, and the YCO bands a t 2004 (s)and 1807 (~)cm-~areverysimilartothevcospectrumreported by Longoni and Chini12 for [Pt6(Co)12]2-, which we thus infer had been synthesized on the MgO surface. The vco spectrum of the extract solution confirms the presence of this cluster anion, and the remaining YCO bands, a t 2069 (m), 2014 (s), and 1975 (s) cm-l, are indicative of unreacted [Rez(CO),o] extracted from the surface (Figure 6). In the original work of Longoni and Chini,12 [Pt6(Co)12l2was synthesized in THF solution by treatment of a salt of [Ptl~(C0)24]2with lithium metal under CO; the reaction time was 12 h, and [Pt6(Co)lz]2- was the predominant product in solution, characterized by its orange-red color and vco bands at 1995 (s) and 1795 (w) cm-I. Presuming that the synthesis of [Pt6(Co)12]z- on the MgO surface parallels that in solution, we infer that the surface-mediated synthesis of this cluster required a strong reducing agent. There are several candidate reducing agents: the M g O ~ u r f a c e ,CO, ~ ~ Jand ~ [ R e ~ ( C 0 ) ~or 0 ][Re(CO)5Br] . C O in the absence of rhenium compounds is not a sufficiently strong reducing agent, as shown by the result that the products did not include [Pts(CO)1~]2-. Rhenium complexes in basic solutions are reducing agents24but apparently not strong enough for the observed synthesis. There was no indication in the infrared spectra of rhenium carbonyl species besides these starting compounds; thus we have no evidence of the products that might be presumed to result from oxidation of the rhenium precursor. Longoni and Chinil2 stated that metal carbonyls (specifically, [Fe(CO)5]),could be used as reducing agents to prepare platinum cluster anions, but they were not specific about whether [Pt6(Co)12]2- could be prepared with this reagent; thus we attempted to use it in place of the rhenium precursors, but formation of [Pt6(Co),,]2- was not observed. Since the reaction to give the hexaplatinum carbonyl cluster anion did not take place on the MgO surface in the absence of the [Re2(CO)lo]or [Re(CO),Br], weinferthat therheniumplayed a crucial role, which is unknown. We might speculate that the rhenium reagents slowed the formation of the higher platinum carbonyl oligomers by forming undetected intermediates that inhibited the oligomerization. MgO is also important in the synthesis of the platinumcarbonyl clusters. MgO has basic surface groups, namely, 0 2 - and OHgroups. Organic Brcansted acids have been shown to chemisorb on MgO to form surface-bound carbanions and surface hydroxyl gr0ups.~5Highly dehydroxylated MgO is so strong a base that it is capable of deprotonating very weak Brcansted acids such as NH3, propylene, and acetylene;26 even heterolytic dissociative adsorption of dihydrogen on polycrystalline MgO has been reported.27 On the basis of the evident analogy to the solution chemistry of the synthesis of the platinum carbonyl anions, we recognize that the basic character of MgO is a key to its success in the surface-mediated synthesis.
[Pt6(Co)l2I2- and [Ptg(Co)ia12But it seems likely that reducing sites on the MgO surface also play a role in the chemistry reported here. These sites are apparently defect sites, possibly surface cation vacancies,23 and we invoke them because there is no other good plausible explanation for the reducing agents in the surface-mediated synthesis of [Pt6(Co)1z]2-. However, we reemphasize that the role of rhenium, which is a necessary component for the observed synthesis, is still unexplained. The nature of the platinum precursor also plays a role in the synthesis chemistry. Puga et al.17 reported the synthesis of [Pts(CO)ls]z- on MgO by treatment of MgO-supported [Pt(allyl)z] with CO at 1 atm and room temperature. In contrast, in our experiments,there was no metal carbonyl formation under such conditions when [Pt(acac)2]was used instead as the platinum precursor. The platinum in [Pt(allyl)z] has weak bonds to the allyl ligands,whereas in [ P t ( a ~ a c )platinum ~] has stronger bonds to oxygen in the acac ligand. CO can easily replace allyl ligands in [Pt(allyl)2] to form platinum carbonyl clusters; however, in the conversion of [Pt(acac)z], a high CO partial pressure is necessary to break the bond between platinum and oxygen to form carbonyl clusters. The basic character of the MgO surfacewas evidently necessary to stabilize the negatively charged platinum clusters that were formed. The small shift of the YCO bands of [Pt6(C0)12]” on MgO to higher energy relative to those of the cluster anion in solution is interpreted as an indication of the interaction between the carbonyl ligands of the anion and the MgO surface; the Mg2+ ions exposed at the surface may withdraw electrons from the clusters and help to stabilize them in weak ion pairs. There are numerous examples of such band shifts in ion-paired metal carbonyls,28many of them substantiallylarger than those observed here. Such shifts have also been observed for other metal carbonyl cluster anions supported on Mg0.6,29 In summary, the chemistry of the platinum carbonyls on the surface of MgO is broadly consistent with the known solution chemistry of these anions. The changes in color observed when the surface-bound clusters were brought in contact with air is explained by oxidation, giving larger clusters and, ultimately, particles of platinum metal, indicated by the gray c010r.l~The surface-mediatedsynthesis of platinum carbonyl clusters has now been extended for the first time to [Pt6(c0)12]’-. The present results, combined with thosealreadyreported,17J8indicateefficient surface-mediated syntheses of the whole family of clusters [Pts(Co)6]2-, where n = 2-5.
Conclusions [pta(CO) and [pts(CO)1~]~were synthesized in high yields on MgO by surface-mediated reactions. The syntheses parallel
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9469 those occurring in basic solutions. The syntheses are simple and easy to control, providing efficient routes to the cluster anions in solution and on MgO. The supported platinum carbonyls are potential precursors of supported platinum catalysts with simple structures.
Acknowledgnmt. This research was supportedby the National Science Foundation (Grant CTS-9012910). References and Notes (1) Lamb, H. H.; Fung, A. S.; Tooley, P. A.; Puga, J.; Krause, T. R.; Kelley, M. J.; Gates, B. C. J. Am. Chem. Soc. 1989, 111, 8367. (2) Maloney, S.D.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1990, 5, 161. (3) Maloney, S. D.; Kelley, M. J.; Koningsberger, D. C.; Gates, B. C. J . Phys. Chem. 1991,95,9406. (4) Basset, J. M.;Theolier, A.; Commereuc,D.;Chauvin,Y.J.Organomet. Chem. 1985, 279, 147. (5) Lee, T. J.; Gates, B. C. Catal. Lett. 1991.8, 15. (6) Kawi, S.; Gates, B. C. Inorg. Chem. 1992, 31, 2939. (7) van Zon,F. B. M.; Maloney, S. D.; Gates, B. C., Koningsberger, D. C. J. Am. Chem. SOC.,submitted. (8) Kawi, S.;Gates, B. C. Catal. Lett. 1991, 10, 263. (9) Sterba, M. J.; Haensel, V. Ind. Eng. Chem. Prod. Res. Deo. 1976, 15, 2. (10) (1 1) (12) (13) (14)
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