J. Phys. Chem. 1992,96, 7219-7225
7219
Isotoplc Ligand Exchange In the HigkNuciearity Platinum Clusters [Pt24(CO)M]nand [Pt28(CO)a2]n( n = 0 to -6) in Dichioromethane: Substitution Klnetlcs, Carbonyl Dipole Coupling, and Comparisons with Pt( 111) Electrodes Joseph D.Roth,t Gregory J. Lewis,* Xudong Jiang: Lawrence F. Daw*and Michael J. Weaver*-f Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, and Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: December 30, 1991)
The effect of substituting I2COby I3COon the infrared spectra of the electroactive [PtW(C0),,]"and [Pt26(CO)3Jnseries (n = 0 to -6) in dichloromethanehas been studied in order to explore the nature of the ligand substitution kinetics and dynamic dipoltdipole coupling for these high-nuclearity metal clusters in relation to metal surfaces. The chargeable nature of the platinum clusters, as scrutinized by FTIR spectroelectrochemical tactics, offers interesting comparisons with the potentialdependent dipole coupling observed for mixed 13CO/'%0 adlayers at the Pt( 11l)-dichloromethane interface, also reported here. Unlike the electrochemical surface, CO ligand exchange on [PtZ4(CO)&- and [F'tzs(CO)3z]z-is slow and nonrandom, incomplete '2CO/13C0substitution occurring even on long (more than several hours) time scales. Substantial deviations from first-order substitution kinetics are also observed, especially for [PtW(CO)30]2where only 65% of the CO ligands are observed to undergo exchange. The results suggest the occurrence of a 'merry-go-round" ligand-migrationmechanism from active substitution centers within the monocrystalline Pt facets that make up the cluster surfaces. Consistent with this, the formation of high local I3COcoverages even early in the substitution process is indicated from the strong I 3 C O dipole coupling, deduced from the terminal C-O stretching frequencies, ufco, together with the nonsymmetrical nature of the I3u&- and 12ufco-isotopemole fraction plots. Even though only small CO islands can be formed on the clusters, the u& upshifts due approach those observed, 30-35 an-',for the Pt(1 1 1)/CO interface. The magnitude to dynamic dipoltdipole coupling, bD, of the dipoltdipole coupling is also dependent on the surface electronic charge for both the clusters and the electrode surface, yet in different ways. For the letter, AuD diminishes markedly as the electrode charge becomes more negative.
There is considerable interest in exploring the physical and chemical properties of high-nuclearity metal clusters, especially in relation to the behavior of related metal surfaces.' Notable membera of such systems, studied in detail by the Wisconsin group, are the class of platinum carbonyl anions represented by [F't24(CO),]2- and [Ptz6(co)3z]2-. These clusters feature cubic closapacked (ccp)and hexagonal close-packed (hcp) metal ma, respectively, with carbonyl ligands in well-defmed terminal and 2-fold bridging coordination geometries as established by X-ray cry&hgmphy.14 Intensting properties exhibited by such clusters arc that they are soluble in a range of nonaqueous media and they undergo a sequence of reversible one (and two-) electron-transfer s t e p to form [Pfa(C0)30]nand [Ptz(CO)3$ series, where n varies between 0 and at least -8.5~6 The solution-phase clusters are therefore chargeable in a manner closely analogous to metalsolution interfaces, offering the intriguing prospect of exploring the *ionizable metal cluster-electrode surface" analogy? Furthermore, we have demonstrated recently that infrared spectroelectrochemistry provides a powerful means of examining the chargedependent structural properties of such metal carbonyl clusters, given the sensitivity of the C-O stretching frequency, uco, to the carbonyl coordination geometry?" Such measurements show that the vc0 frequencies are sensitive to the cluster charge in a similar fashion to the potential-dependent (and hence charge-dependent) properties of CO adlayers on platinum electrodes. A detailed assessment along these lines has uncovered interesting differences as well as similarities in the potential-dependent spcctdcctrochemical behavior of corresponding platinum carbonyl clusters and CO-covered platinum electrodes. These differences are attributable to geometric as well as electronic factors.6 Such fiidings also raise the issue of the manner and extent to which the carbonyl ligand environments may differ in the cluster and interfacial systems. In the latter case, it is well-known that large changes in the uco frequency are usually induced by increasing the CO surface coverage, These local adsorbateadsorbate interaction effects can be separated into dynamic diUniversity. *University of Wimnsin-Madison.
+ hrdue
pole-dipole and "static chemical" contributions by acquiring infrared spectra for differing isotopic mixtures, such as 1K!O/13C0, The former effect, in particular, is seen as a function of ecOs7 to provide a major component of the large vco frequency upshifts commonly observed as ec0increa~es.~ Although most reported data of this type refer to metal-ultrahigh vacuum (uhv) interfa=, the Purdue group has recently utilized such tactics in exploring environmental effects upon the CO adlayer structure at ordered low-index platinum and rhodium electrodes in aqueous media.81~ Analogous ligand-interaction effects can also be envisaged in metal carbonyl clusters. However, while isotopic substitution tactics have been utilized in small metal clusters, primarily to elucidate ligand-substitution pathways,lo essentially no information of this type is available for the higher-nuclearity clusters which are more likely to provide viable models of metal surfaces. We report here infrared spectra for [Pt24(CO)M]nand [Ptz(CO)3Jn ( n = 0 to -6) in dichloromethane, where 13CO/12C0isotopic exchange is induced to occur by exposing the I2CO clusters to near-saturated concentrationsof "CO. In contrast to saturated CO adlayers on Pt surfaces, 13CO/12C0ligand exchange is observed to be sluggish and, in the case of [Ptu(CO)30]n,incomplete even on long (ca. 1-day) time scales. The observed ligand s u b stitution kinetics, along with the observed infrared spectra for different '3CO/12C0ligand ratios, are utilized to deduce the manner of carbonyl exchange and the nature of dipoldipole coupling for these large metal clusters. Comparisons are made with dipole coupling for saturated CO adlayers at the Pt(1 1l)-dichloromethane interface as extracted from surface infrared spectra for 13CO/'2C0 mixtures.
Experimental Section Much of the experimental detail, including the cluster dianion syntheses and the spectroelectrochemical procedures, is outlined elsewhere? The reflectance-type spectroelectrochemical cell employed for the cluster infrared studies is described in detail in ref 11. It features a micrometer adjustment of the thin-layer thickness (defined by the electrode-surface CaFz window separation) and the provision of forced hydrodynamic flow. The latter enables small solution samples, as used here, to efficiently be injected into the thin-layer cavity via a Pt tube entry through the
0022-3654/92/2096-7219$03.00/0Q 1992 American Chemical Society
7220 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 Septa
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Figure 1. Diagram of the reactor used to expose the Pt clusters to I3CO in dichloromethane.
optical window. The electrode was a 0.9cm gold disk; this metal was chosen so as to avoid spectral interferences from adsorbed
co.
The 13CO/'2C0ligand exchange for the metal clusters was undertaken in the reactor illustrated in Figure 1. The clustercontaining solution [in dichloromethane (Fisher) with 0.15 M tetrabutylammoniumperchlorate (TBAP)] was injected into the nitrogen-flushed reactor through the top septum. The solution was subjected to two freeze-pumpthaw cycles and then frozen at liquid nitrogen temperatures. Any noncondensible gases were removed with a vacuum p p . The reactor was then isolated from the pump by closing valve C. The "CO (99%, Aldrich) was transferred via a metal tube through the top septum, and valve A then closed to prevent leakage through the perforated septum. Since CO is a liquid at 77 K,this allowed for a near-quantitative transfer. The 500-mL gas bulb initially contained 4.8 mmol of I3CO and about 30 mg of cluster (as the tetraphenylarsonium dianion salt). This resulted in approximately a 150-fold excess of I3CO in the reactor. Thus, the back-reaction, involving an exchange of '2Co for I3CO, le0 for I3COis negligible. Aliquots of the solution were transferred into a small reservoir for ensuing spectroelectrochemical examination at various times by opening valve B slightly. After valve B was closed, the solution was drawn off with a syringe and placed in a small vial which had previously been purged with N2. The solution in the arm of the reactor after valve B was opened was then returned to the flask by introducing a small positive pressure of N2in the sampling chamber and opening B slightly. Great care was taken during this procedure to prevent N2from being admitted into the reaction chamber. The aliquots were subjected immediately to two freeze-pump-thaw cycles in order to remove solution CO, thereby halting the carbonyl isotopic-exchange process, before spectroelectrochemicalinvestigation. The Pt(ll1) crystal (9" diameter, 4 mm thick) was purchased from the Material Preparation Facility at Come11 University. It was oriented within *lo, as verified by X-ray diffracton. The surface pretreatment procedure, including hydrogenair flame annealing, oooling in the presence of iodine vapor, and replacement of the adsorbed iodine with CO in 0.1 M HC104, has been described previ~usly.'~The crystal was then thoroughly rinsed with dichlmethane and immediately tralisferred to the infrared cell, filled with dichloromethane containing 0.15 M TBAP and a near-saturated 12CO/13C0mixture having the desired isotopic ratio. The latter was prepared by mixing I3CO and %O-containing dichloromethane in the appropriate ratio. The I3CO solution was prepared similarly to above and the 12C0solution by bubbling in natural-abundance CO gas (Matheson). The electrode potentials were measured and are quoted versus the ferrocenium-fmocene (Fc+I0)couple in the same solvent; this involved the w of an equimolar Fc+/Fcmixture in contact with a Pt wire, contained in a separate compartment. All measurements were made at room temperature, 23 k 1 OC. ReSdtS Mixed l%O/lwO Adlayers on Pt(ll1). While the primary focus here is on the infrared spectral examination of '3CO/12C0
Roth et al.
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F i p e 2. Potential-dependent infrared spectra for saturated mixed 13CO/12C0adlayers on an ordered Pt( 1 1 1) electrode in dichloromethane containing 0.15 M TBAP for (A) 49% I2CO and (B) 26% l2CO. Potentials indicated are vs ferrocenium-ferrocene (Fc+/O) reference electrode. See text for procedural details.
ligand mixtures in the Pt clusters, as noted above it is of interest to compare this behavior with that for mixed 13CO/'2C0adlayers at a closely related platinum electrode interface, Pt(ll1) in dichloromethane. The latter results will be described fmt since they provide a basis for assessing the less conventional properties of the Pt clusters. Typical potential-dependent infrared spectra in the terminal uc0 frequency region for a saturated (eco= 0.7) mixed I3CO/ lZCOadlayer on Pt(ll1) in dichloromethane containing 0.15 M TBAP are shown in Figure 2A and 2B, referring to I2C0 percentages of 49% and 26% respectively. [Although a second vm band at lower frequencies, ca. 178&1860 cm-I, is also obtained," it is markedly weaker and does not yield well-defined spectral features for 12CO/13C0mixtures (Figure 2).*'] These spectra were obtained by aquiring sets of 100 interferometer scans (each consuming ca. 60 s) during a staircase potential ramp from -2.0 V to sequentially more positive potentials, as indicated alongside each spectrum. The solvent and 0 t h spectral ~ interferences were removed, as usual,8 by subtractinga reference spectrum recorded subsequently at a sufficiently positive potential, 1.0 V,so that the CO was removed electrooxidatively. (Although CO electrooxidation to form C02clearly will not occur in rigorously anhydrous dichloromethane, the small amounts of water purposefully present in the solvent enabled this reaction to occur satisfactorily by about 1.OV.) The proportion of ' T O and I3COadsorbed on the surface (and present in solution) can be determined readily from the ratio of the band intensities appearing in such potential-difference infrared spectra at 2343 and 2275 an-',associated with the 'eo2 and I3COzproducts, respectively, trapped in the spectral thin layer.8aJ4 The spectra in Figure 2 display the usual hallmarks of 1 3 C O / W 0 dipole coupling, especially at higher electrode potentials. Thus, substantial "intensity transfer" is observed under these conditions, so that the higher-frequency-band partner is markedly more intenserelative to the lower-frequency component than is commensurate with the ' T O mole fraction. In addition, the frequenciesof both band partners downshift significantly as the 12C0fraction is decreased. This latter feature arises from a progressive diminution in the extent of dipole coupling involving nearby W O adsorbate molecules upon dilution with 13C0. The extent of dipoledipole coupling within the saturated ' T O adlayer at each potential can be quantified readily by plotting,
Isotopic Ligand Exchange in Pt Carbonyl Clusters I
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The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7221
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as the frequencies of the higher- and lower-frequencyband partners, 12vco and l3vcO (associated with '2CO and I3CO, respectively), as a function of the isotopic composition. Figures 3 and 4 show examples of such plots of l2vc0 (filled circles) and l3vcO (filled squares) vs l2C0 at electrode potentials of 0.2 and -1.2 V, respectively. The % l2C0 ranges over which the l2vc0 and l3vcOvalues are given, 220% and 57596, respectively, were limited by the ability to discern the peaks in the mixed-isotope spectra. The frequency shift due to dynamic dipole-dipole coupling within the pure I2CO (or I3CO) adlayer, AvD, can be found from the difference between the pure-isotope frequency, l2vc0 (or l3ucO), and the l2vCo (or l3vcO) value extrapolated to infinite dilution of that isotope at the same total ec0. Since the use of 12CO/13C0mixtures for adlayer dosing necessarily creates a random distribution of isotopes on the surface, we expect that' A12vco(12x1I2x2) = A13vco(13xI 13x2) (1) where A'*vcois the shift of the high-frequency peak caused by altering the mole fraction of I2CO from xI to x2and Ai3vc0 is the low-frequency peak shift caused by the same alteration of the
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Figure 3. Plot of frequencies of higher- and lower-frequency-bandcomponents, 1 2 u (filled ~ ~ circles) and l3ucO (filled squares), vs % I2CO saturated mixed '%O/I3CO adlayer on Pt( 1 1 1) in dichloromethane containing 0.15 M TBAP. Electrode potential is 0.2 V vs Fc+/O. Open I%O data by using eq triangles are I3ua values estimated from '*va-% 1 as outlined in the text.
0
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Figure 5. Shift in higher-frequency-band partner, A12ua, between 27% and 100% IT0 for saturated 13CO/12C0adlayer on Pt(ll1) in dichloromethane vs the electrode potential (vs Fc+/O).
mole fraction in "CO. In other words, the 12vm-12xand l3um-I3x plots should be mirror images of each other. As an illustration, the open triangles in Figures 3 and 4 are estimates of the 13vm-13x plot obtained from the corresponding 12vco-12x data by using eq 1 and assuming that the vko difference for the pure W O and I3CO These adlayers is determined by the reduced massa (ca.45 ax1). estimated 13vco-13x points are seen to be close to the actual 13vm-13x data (squares); the residual discrepancies are due partly to slight deviations in 12vm - 13vm from the frequency shifts based on reduced m a w . " The use of eq 1 to examine the nonrandom distribution of 12CO/13C0isotopes within dipole-coupled ligand arrays for the Pt clusters is pursued below. The present dipole-coupling data for Pt(1 1 1)/CO in dichloromethane are similar to those reported earlier for saturated CO adlayers at the Pt( 11l)-aqueous interface." The latter refer to the more restricted potential range from -0.25 to 0.2 V vs the saturated calomel electrode, SCE (equivalent to ca. -0.45 to 0 V vs Fc'IO). Sequential, rather than concurrent, surface dosing with 13C0and l2C0 yielded near-complete isotopic replacement within ca. 10 min, indicating that the ligand-exchange process is rapid in the presence of solution CO. Mixed 12CO/13C0adlayers formed by such sequential dosing yielded esentially identical dipole coupling features to those formed by concurrent dosing, indicating that randomly distributed isotopic mixtures are formed under these conditionsasb One other, more unexpected, feature of the present results for Pt( 111)/CO is that the extent of dipole-dipole coupling depends noticeably on the electrode potential, the coupling tending to diminish toward more negative potentials. This can be discerned most easily from the spectra in Figure 2, in that the extent of intensity transfer toward the higher-frequency-band partner decreases markedly as the potential becomes more negative. More pointedly, the frequency upshift due to dipole coupling, AvD, also diminishes under these conditions. As a semiquantitative illustration, Figure 5 is a plot of the shift in the higher-frequency-band partner, A12~co,between 27% and 100% l2CO mixtures as a function of the electrodepotential, E. While the plot exhibits some scatter, A12vco clearly decreases monotonically for potentials negative of ca. -0.5 to 0 V. Interestingly, the lZCO adlayer structure on pt( 111) is known to undergo a structural change at about -0.3 V vs Fc+/O (ca. 0 V vs SCE), as discerned from a change in the vLo-E slope and a marked alteration in the frapency of the bridging vm feature." The terminal band intensity for the pure l2CO adlayer remains essentially constant Over the potential ~~ this indicates range -0.5 to -2.0 V, whereas A 1 3 vdiminishes; that the potential-dependent dipole coupling is unlikely to be due to variations in CO coverage.
7222 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992
Roth et al.
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FIgm 6. First-order plots of percentage IT0 (logarithmic scale) re(filled circles) and (B) [pt26(CO)32]2maining on (A) [Pt24(CO)H)]2(filled triangles) vs time of exposure (minutes) to I3CO in dichloromethane. The open circles in A are the 9% I2CO values minus 35%, i.e., with the slow-exchangingportion subtracted out (see text for details).
'W/" Exdlmge in [ptu(co)drad pt&D)ny? since the metal clusters examined here were synthesized as naturalabundant l2C0 carbonyls, in order to examine dipole coupling effects, it is necessary to partially replace the IT0 ligands with by isotopic exchange. As noted in the Experimentalsaction, a ca. 0.4 mM solution of each cluster in dichloromethane was exposed to a 150-fold excess of dissolved "CO, aliquots being removed for spectrotlectrochemical examination at various time intervals. Since the I3CO is in large excess, virtually complete replacement of l2C0 ligands by I3CO ligands should occur. However, the kinetics of ligand exchange for both [PtU(CO),l2and [ptzs(CO)32]2-turn out to be sufficimtly sluggish so that only partial isotopic exchange oocurs wen on long (more than several hours) time scales. The fraction of ligandrCmaining on each cluster after a @wn exposure time,t13, was determined from the ratio of the W02-td3C02 band intensities (at 2343 and 2275 cm-I, reepectiVay), obtained by elcctrooxidizing entirely the cluster CO ligancts at the gold ekctrodc at an appropriately higb potential, ca. 1.0 V. [This tactic is similar to that used to assay mixed 13CO/1T0adlayers on Pt(l11) as noted above. One difference, however, is that uncoordinated (solution-phase) CO is absent during the cluster spectrotlectrochemistry, so that the ligand composition should faithfully be determined.] These experiments enable the rate of ' T O ligand substitution by 13C0 to be examined as a function of the isotopic carbonyl compition. In the simplest (albeit unexpected) case w h m each of the carbonyl coordination sites in the cluster is equivalent, the isotopic substitution kinetics will be fmt order with respect to % IQO, in which case one will obtain a linear plot of log (9% ' T O ) vs 113, w h m % T O is the percentage of l2C0ligands surviving in the clusters at a given reaction time, t13. Figure 6 shows such first-order plots (filled circles) for [Pt24(CO)3,]2- (A) and [Pt26(co)# (B). The former plot is clearly nonlinear, the isotopic subtitution essentially ceasing (over ca. 11/3 days) when 96 ' T O declines to 35% (Figure 6A). N e v e r t b h , the substitution kinetics prior to this point are approximately first order, as can be seen by replotting only the "replaceable" portion of I-, Le., log (96 ' T O 35%) vs 113 (open circles in Figure 6A). The slope of this initial region yields a half-life, f1/2r of ca. 90 min. Somewhat different behavior was observed for [Pt26(c0)32l2(Figure 6B). While the first ca. 15%of the ' T O replacement occurred very rapidly (S10 min), the remaining substitution yielded roughly uniform fmt-order kinetics, with tl12 250 min. Given these marked deviations from pure fust-order kinetics, the substitution proem is expected to yield nonrandom '2co/l3Co
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Figure 7. Infrared spectra of [Pt&O)30]n in dichloromethane containing 0.15 M TBAP as a function of the charge, n, as indicated, for a cluster'tontaining (A) 48% and (B) 35% 'K!O. The mixed 1K!O/13C0 clusters were prepared as noted in the text, involving prior exposure of [qt2,(CO)M]2-to I3COin dichloromethane for (A) 162 min and (B) 1855 min.
mixing on the cluster surfaces. Evidence that this is indeed the case is obtained by examining the cluster vm infrared spectra as a function of % I2CO. Figure 7 shows illustrative spectra for (n = -2 to -6, as indicated) for a pair of % I2C0 [Pt24(C0)30]n values, 50% (A) and 35% (B). The spectra for the consecutive reduced states (-n > 2) were obtained by sweeping the electrode potential negative at 5 mV s-l so as to sequentially reduce [Pt24(CO)30]2-in one-electron steps. The bottom spectrum in Figure 7 was obtained after reoxidizing the cluster to the dianion form. Two significant features of these spectra are immediately evident. First, the form of the mixed isotope spectra in the terminal vco region (ca. 19W2050 cm-I) changes substantially as the cluster is reduced. For -n I4, a pair of distinct vco bands are obtained that appear roughly similar to those observed for CO adlayers (Figure 2). The ca. 15-cm-I vco frequency downshifts seen here upon each sequential one-electron reduction are close to those observed for the single terminal band in the pure lZCO spectra (see Figure 2 in ref 5). For the more highly reduced clusters (n = -5, -6), however, the band becomes broader and the higher-frequency (I2CO) partner merges into the lower-frequency component. This findmg is indicative of a chargeinduced alteration in the carbonyl environment. The changes are largely reversed, however, upon reoxidizing the cluster from n = 1-6 to n = -2 (Figure 7). Second, for -n I4 there is a substantial difference in the relative intensities of the higher- and lowerfrequency-band partners between parts A and B of Figure 7, wen though % ' T O drops only from 50% to 35%. While the former spectra indicate the presence of marked intensity transfer, such an effect is virtually absent in the latter spectra in that the relative intensities of the high- and low-frequency-band partners (- 1:2) match closely the 12CO:13C0ligand ratio. A nonrandom nature of the progressive 'TO/13C0substitution on [Pt24(CO)M]nis indicated further from plots of I2um (circles) and l3ucO (squares) vs % W O for [Pt24(C0)30]n; examples are given for n = -1 and -3 in Figures 8 and 9, respectively. As for the analogous data for 1TO/13C0adlayers on Pt( 1 1 1) (Figures 3 and 4), included in Figures 8 and 9 are I3u,-o-% W O points (triangles, dashed trace) estimated from the corresponding 12ua-% I2CO data by means of eq 1 as outlined above. Unlike the Pt(1 ll)/CO interfaces these observed and estimated 13va-% W O plots for the Pt carbonyl clusters are distinctly divergent, especially toward higher % I2CO (Figures 8 and 9). Most notably, while I2vc0downshifts quite markedly as % I2COdecreases, especially
Isotopic Ligand Exchange in Pt Carbonyl Clusters
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7223
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Figwe 8. As in Figure 3 but for [Pt,(CO),]containing various % IT0 following progreaPive replacement of ' T O in [Ptu(CO)30jgby exposure to "CO in dichloromethane. Data refer to m24(CO)30]-in dichloromethane containing 0.15 M TBAP, formed by electrooxidation of [Pt24(CO)30]2in a spectroelectrochemical cell following removal of aliquots from a 13C0reactor (see text for further details).
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Figure 10. Infrared spectra of [Pt26(C0))2]" in dichloromethane containing 0.15 M TBAP as a function of the charge, n, as indicated, for a cluster containing (A) 64%and (B) 49%IQO. The mixed %O/13C0 clusters were prepared as noted in the text, involving prior exposure of [Pt26(CO)32]2to "CO in dichloromethane for (A) 105 min and (B) 275
min.
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v;o,m - 1 Figure 9. As for Figure 8 but for [Pt24(CO)~]3-.
for % IT0 S 5096, l3vcO exhibits much smaller changes than anticipated from eq 1 on this basis. These findings suggest that while the extent of dipole coupling experienced by the W O ligands diminishes markedly as 96 'TO dweasea, that for the incoming 13C0ligands remains more invariant as isotopic substitution proceeds. Moreover, even at high 5% I2CO values, Z80%, the frequency differences (12v~~-13vc0) in Figures 8 and 9 are comparable to those, ca. 45 cm-I, expected in the absence of dipole coupling (Le., that arising merely from the difference in the ' T O and I3CO reduced masses) and markedly smaller than those anticipated from eq 1. Provided that significant dipole coupling is indeed present in the pure W O ligated and 13CO-ligated)clusters, this result indicates that the 13C0ligands experience a substantial fraction of the coupling men at early stages of the substitution process. Roughly comparable results were also obtained for [Pt26(cO),,ln. Displayed in parts A and B of Figure 10 are infrared spectra from n = 0 to -6 for 64% and 49% l2C0, respectively. are inac(Note that the n = -3 and -5 states for [Pt26(C0)32]n cessible in dichloromethane, direct two-electron reductions from
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Figure 11. As in Figure 3 but for [Pt2s(C0)32]-containing various 8 IZCOfollowing progressive replacement of I2CO in [Pt2s(CO)32]zby exposure to "CO in dichloromethane. Data refer to [Pt26(CO)3J-in dichloromethanecontaining 0.15 M TBAP, formed by electroxidationof [Pt26(co)32]2-in a spectroelectrochemicl cell following removal of aliquots from a I3COreactor (see text for further details).
the n = -2 and -4 species being observed insteadq6)Plots of I2vc0 (circles) and I3vc0 (squares) vs % l2CO for the n = -1 and -4 forms of [Pf26(CO)32]" are given in Figures 11 and 12, respectively. the 12vc0 downshifts Similarly to Figures 8 and 9 for [Pt24(C0)30]nr induced as 96 'TOdecreases are not mirrored by complementary changes in 13vco. Indeed, l3vcO tends to downshut slightly as 95 IT0 decream, rather than increase in the manner predicted by eq 1 (as before, plotted as triangles in Figures 11 and 12). Again, the '2vc0 - l3vcO differences are relatively small, ca. 40-45 cm-I, even at high % l2COvalues (k70%),indicating that comparable amounts of dipole coupling are experienced by I3CO even in the initial stages of isotopic replacement, where % "CO
