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Langmuir 1991, 7, 1198-1205
Preparation and Characterization of Supported Mononuclear Metal Complexes as Model Catalysts Jeffrey C. Kenvint and Mark G. White* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
Mark B. Mitchell Department of Chemistry, Clark-Atlanta University, Atlanta, Georgia 30314 Received November 19,1990. I n Final Form: January 8, 1991 Metal acetylacetonates (Mn+= Cu, Pt, Pd, Fe, Cr, Co, and Mn) were supported on Cab-O-Si1by batch impregnation. All samples were analyzed for metal, hydrogen, and carbon content. The C/Mn+ stoichiometry and metal content of the supported catalysts depended upon the metal acetylacetonate. High metal contents were observed in those supported samples for which Mn+ = Cu, Mn, and Co, whereas the other samples showed only small amountsof metal ion. Moreover, the C/Mn+stoichiometryof the supported complexes containing Cu and Cr were the same as the parent acetylacetonates, whereas the C/Mn+ stoichiometry was lower than that of the metal acetylacetonates for Mn+ = Co, Mn, and Fe. The samples containing Cu were characterized by diffuse reflectance, Fourier transforminfrared spectroscopy (DRIFTS) to determine the structure of the surface species. Subsequently, the copper samples were decomposed in air by heating in a thermal gravimetric apparatus (TGA) to 400 "C. The kinetics of the weight changes depended upon the loading of the Cu complexes in the sample.
Introduction Many supported metal oxide catalysts are prepared by aqueous impregnation of a support with soluble metal salts. These impregnated supports are heated in controlled atmospheres to activate the catalysts. This technique may not develop a surface with uniform morphology. The supported copper oxide system is one example where a nonuniform surface layer develops as a result of the solubility of the copper hydroxide ion in aqueous solution. The copper hydroxide ion is soluble in acidic (pH < 5 ) and basic (pH > 8)solutions but insoluble in aqueous solutions having a pH of 7. As a consequence of this solubility property, only a small fraction (10-20%) of the cupric ions exchange with the silanol protons on silica after the samples are washed with distilled water.' The copper system illustrates how the metal ion solubility in aqueous solutions prevents a fine dispersion of the metal on the support at high loadings. It appeared prudent to investigate nonaqueous impregnation techniques for applying metal complexes to oxide supports as an alternative to the aqueous impregnation of metal salts. We reported a technique to produce monolayer films of metal complexes on Cab-O-Si1by ion exchange of silanol protons with cationic polynuclear metal complexes in acetonitrile.2 The stable amino alcohol ligands in the complex prevented it from rearranging either in solution or during the ion exchange with the silica. The complex was soluble in the solvent, thus all the cations remained in solution until the ion exchangewith the silica. A uniform dispersion of the complexes was developed as a monolayer film up to the loading of 28.4 wt %. This experimental loading was similar to that predicted from a consideration of the size of the complex and the specific surface area of the support.2 This approach appeared promising; however, the complexes could be synthesized * To whom all correspondence should be sent. Present address: Mobil Research & Development, Paulsboro, N J. 08066. ..... (1) Kohler, M. A.; Lee, J. C.; Trimm, D. L.; Cant, N. W.; Wainwright, M. S. Appl. Catal. 1987,31,309-21. (2) Beckler, R. K.; White, M. G. J. Catal. 1986, 86, 252. t
~
for only a few metals. We sought other complexes which have been synthesized for a large number of metals. Metal acetylacetonates, Mn+(acac),, are simple, metal complexes that have been synthesized for many of the transition metals.3 These complexes are formed by reacting the metal ions with a 8-diketone in the appropriate solvent. The structure of the metal complexes depends upon the charge of the metal ion and its symmetry. For example, the M2+(acac)2(M2+= Cu, Pt, and Pd) complex has square-planar shape (Figure 1, refs 3-6). The M3+(acac)~ (M3+= Cr, Fe, and Mn) develops octahedral shape (Figure 2, ref 7). The Co2+(acac)2can form a tetramer, which shows a pseudooctahedral arrangement of oxygens about the C O ,or~ it may form a dihydrate, which also shows an octahedral arrangement of the oxygens about the Co. The stabilities of metal acetylacetonates vary widely as shown by their thermalstability! For example,chromium(111)and copper(I1) acetylacetonates are stable at 191 OC, whereas Mn+(acac),, (M = Co2+,Fe3+, and Mn3+) compounds decompose rapidly a t this temperature. The thermal stabilities of these compounds mirror their chemical reactivities to ligand substitution reaction^.^ These results suggest that the complexes may interact with a support such as silica by different mechanisms. Bis complexes of platinum and palladium acetylacetonates (3) Fackler, J. P., Jr. Metal beta-Ketoenolate Complexes. Prog. Znorg. Chem. 1966, 7, 361. (4) Piper, T. 5,and Belford, R. Linn, Mol. Phys. 1962,5, 169. (5) Grinberg, A. A.; Chapurskii, I. N. Rusa. J. Znorg. Chem. 1959, 4, (2), 137. (6) Grinberg, A. A.; Simonova, L. K. Zhur. Prikl. Khim. 1953,26,880, english translation in Ruse.J. Appl. Chem. 1953, 26, 801. (7) Morosin, B.; Brathovde, J. R. Acta Crystallogr. 1964,17,705. Roof, Raymond, B., Jr. Acta Crystallogr. 1956,9,781. Cotton, F. A.; Elder, R. C. J. Am. Chem. SOC.1964,86,2294. (8) Charles, Robert G.; Pawlikowski, M. Arlene J. Chem. Phys. 1958, 62, 440. (9) Roundhill, D. M. Platinum. In Comprehensiue Coordination Chemistry; Pergamon Press: New York, 1987. Barnard, C. F. J.
Palladium. In Comprehensiue Coordination Chemistry;Pergamon Press: New York, 1987. Rollison, C. L. Chromium, Molybdenum, & Tungsten In Comprehensiue CoordinationChemstry;PergamonPress: New York, 1987; 390. Gilliard, R. D.; Wilkinson, J. G.J. Chem. SOC. 1963, 5399. Kemmet, R. D. W. Manganese. In Comprehemiue Coordination Chemistry; Pergamon Press: Austria, 1973; p 872.
0743-7463/91/2407-ll98$02.50/00 1991 American Chemical Society
Supported Metal Complexes as Model Catalysts
Langmuir, Vol. 7, No. 6,1991 1199
b
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diagrams for M(acac)z: (a) top view; (b) side view. Hydrogens have been omitted for clarity. Figure 1.
0
C
0
c c
C
ORTEP
C
b
m
Figure 3. Models of support-complexinteractions: (a)top view showing two M(acac)z; (b) side view of top picture which has been rotated 4 5 O to show side of complex,one M(acac)zeliminated for clarity. Dotted lines show the projections of silica unit cell onto the (100)plane.
Figure 2. ORTEP diagram for M(acac)a. Hydrogens have been omitted for clarity.
are chemically inert to the formation of adducts, whereas Cu(acac)zwill accept some ligands to form a five-coordinate Cu(II).s For flat complexes,the axial interactions between the metal and the support may determine its affinity to the support. These data of relative stability and structure led us to study the properties of a few metal acetylacetonates which have different symmetries and chemical stabilities so that we may deduce the usefulness of these neutral complexes as precursors to supported metal oxide catalysts. The usefulness of a precursor is determined in part by ita affinity for the support oxide. One method of attaching the complex to a support such as silica is hydrogen bonding between the silanols and nucleophilic, functional groups of the complex. Hydrogen-bonding interactions have been reported between solvents and selected bis square-planar
complexes and tris complexes.10 However, these results cannot be generalized to all Mn+(acac)ncomplexes. Our earlier work showed that cation complexes were anchored to the silica through an ionic attraction to the siloxides.2 We speculated that the attractive forces were global in nature between the complex and the surface but did not rule out that specific interactions between the siloxides and bridging hydroxide protons. For the case of neutral, metal acetylacetonates, the global, attractive forces may be hydrogen bonding between the surface silanols and the quasi-*-electron system of the acetylacetonates, whereas specific interactions may occur through the coordinately unsaturated site(s) of the metal ion; e.g., Cu(11) accepts axial adducts.11J2 These results lead us to hypothesize on a mechanism for interactionsbetween metal acetylacetonates and a silica surface. For bis square-planar complexes, the silanol protons of low index surfaces (e.g., [loo], [OOl], [110], and [ l l l ] ) may hydrogen bond to the quasi-?r-electron system of the acetylacetonates and/or form a bond between siloxide and the metal ion in its axial position (see Figure 3). Preliminary studies of scale models show that for one configuration, the siloxide can be accepted into the metal axial ligand with the other silanol pointing into the quasi?r-electronsystem. Thus, we propose to test this hypothesis by using three bis complexes which have square-planar shapes: Cu(II), Pt(II),and Pd(I1). If the quasi-?r-electron (10) Davis, T. S.; Fackler, J. P., Jr. Inorg. Chem. 1966,6, (2), 242. (11) Ooi, S.; Fernando, Q. Chem. Commun. 1967, 532. (12) Clarke, F. R.; Stienbach, J. F.; Wagner, W. F. J. Inorg. Nucl. Chem. 1964,26, 1311. Reference 394n in ref 3. (13) Babb, K. H.; White, M. G. J. Catal. 1986,98, 343.
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system with the surface silanols is the dominant force for the interaction, then all three complexes should be retained on the surface of the silica in similar concentrations. However, if the specific interaction between the metal ion and the siloxide dominates, we expect that only the Cu(I1) complex will be retained by the silica in high concentrations since it will accept axial adducts and the Pt"- and PdI1 (acac)2 complexes will not. The stable tris octahedral complexes should not hydrogen bond to the surface as a consequence of steric hindrance. Unstable tris complexes may decompose and bond to the surface by another mechanism (e.g., ligand substitution). The square-planar complexes are assumed to interact with the low index planes better than the octahedral complexes since the interaction between the surface and a stable complex is favored by the "flat" structure of the square-planar complex. The Cr(II1) is used to describe the interaction between a stable octahedral complex and the surface. We expect the Cr(II1) complex will not decompose and should not interact with the silica support. To test the hypothesis of ligand substitution with the surface, we chose the less stable complexes of Mn(III), Fe(III), and Co(I1) complexes. . Unstable complexes should interact with the surface by loss of ligands and show a carbon/metal ratio lower than that expected for the parent complexes.
- , 0
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Experimental Section Chemicals. The metal acetylacetones, purchased from Aldrich Chemicals, were used without further purification. The Cab-0-Si1 (M-5 grade, 200 m2/g) was obtained from the Cabot Corp. Absolute methanol and acetonitrilefrom Fischer Scientific were dried over activated molecular sieves. Catalyst preparation. The catalysts were prepared by combining solvent (250 ems), support (10 g), and 1-10 g of Mn+(acac),,in a plastic polyethylene bottle at room temperature for 24 h with stirring. Someof the Cu(acac)2samples were prepared by using methanol as the solvent,whereas,the remainingsamples were prepared with acetonitrile. The samples were vacuum filtered at room temperature and dried in air at 60 O C for 24 h. Finally, the samples were stored in stoppered vials. Some of the dried samples were washed with five aliquots of fresh acetonitrile (50 cms each), vacuum filtered at room temperature,and dried in air at 60 O C for 24 h. The analyses of these samples (Table I) are enclosed in parentheses. Metal, Carbon Analyses. Metal loadings were determined by Applied Testing Servicesof Marietta, GA, and carbon analyses were performed by Atlantic Microlabs of Norcross, GA. DRIFTS. Diffusereflectancespectra were recorded on a Nicolet BO-SXR. Sampleswere diluted in KBr (0.9 g KBr, 0.1g sample) and placed in a Spectra Tech sample holder. Spectra showing a resolution of 2 cm-l were developed from averaging 1000scans. The reflectance spectrawere converted to Kubelka-Munk units. Gravimetric Analysis of Thermal Decomposition. The thermal decompositionof supported complexesw a studied gravimetrically with a Perkin-Elmer TGS-2 (see refs 14 and 15 for description of system). To observe weight loss vs temperature, fresh samples (ca. 10 mg) were equilibrated for 2 h at 100 "C in either dry nitrogen or air flowing at 200 cm3/minand then heated at a constant rate of 5 'C/min to 400 "C. The final weight was recorded at 100 "C. Dispersive, Double Beam, IR. The apparatus and procedures described in refs 15 and 16 were used in this study. Mass Spectrometry Analysis of Thermal Decomposition.
The apparatus and the operating conditions were described in ~~~
~~
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(14) Beckler, R. K.;White, M. G. J. Cutul. 1988,109, 25. (15) Beckler, Robert K.;White, Mark G. J. Cutal. 1988,112, 157. (16) Vlckova, B., Strauch, B., & Horak, M., Collect. Czech. Chem. Commun. 1986,50,306. Kaplan, R. I. M.S. Thesis, School of Chemistry, Georgia Institute of Technology, 1965.
;
0.00
0.000
0.001
0.002
Moles Copper
Figure 4. Elemental analyses of supported Cu2+(acac),: (a) adsorption isotherm at 25 O C ; (b) carbon and metal analysis.
ref 16. All conditions used here are those reported earlier except for the range of the m / e (40-300,here) and the final temperature (400"e).
Results Adsorption isotherms were developed from the analyses of the Mn+(acac), for Mn+ = Cu, Pd, Pt, Cr, Fe, Co, and Mn as illustrated in Figure 4a for Cu, Cr. Comparisons of affinity were made by considering the metal loading developed at similar initial solvent concentrations. The M2+(acac)2(M = Cu, Pd, Pt) are considered first (Table I). These samples were prepared at similar initial solvent concentrations, [Init.], but show different metal loadings (wt % ). For example at 4 w t % initial loadings, the Cu(I1) complex shows 2.5 wt % Cu retained in the catalyst, whereas the Pd(I1) and Pt(I1) complexes show 0.49,0.46 wt 76. These data may be reexpressed as mol of M2+/g catalyst to give the following: Cu(II), 393 pmol/g; Pd(II), 46.2 pmol/g; Pt(II), 23.5 pmol/g. The retention of the Cu(I1) complex on the silica is 16.7 times that of the Pt(I1) complex and 8.5 times that of the Pd(I1) complex. The data at the higher initial concentrations show similar results. These data suggest that the metal ion of squareplanar, bis complexes has a profound effect on the affinity of the complex for the surface. Next, consider the metal analyses for the other complexes (M = Cr, Fe, Co, and Mn). The Cr(II1) complex shows a metal loading of 0.49 wt 96 at an initial concentration of 4 wt % ,whereas the Fe(III), Co(II), and Mn(II1) complexes show analyses of 0.81,2.00,and 2.40 wt 76 metal
Langmuir, Vol. 7, No. 6, 1991 1201
Supported Metal Complexes as Model Catalysts
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Table 1. Metal and Carbon Analyses of Silica-Supported Complexes of Mn+(acac).. wt % found
Mn+(acac)" [Initlb Cu(I1) 4 Cu(I1) 7 4 Pd(I1) 8 Pd(I1) 4 Pt(I1)C 6 Pt(I1) 8 Pt(I1) 4 Fe(II1) Fe(II1) 8 4 Cr(II1) 8 Cr(II1) Mn(II1) 4 Mn(II1) 8 4 Co(1I) 8 Co(1I)
metal 2.50 (0.36) 5.57 (3.81) 0.49 0.79 0.46 0.54 0.56 0.81 (0.29) 1.20 (0.44) 0.49 (0.053) 0.92 (0.11) 2.40 (1.60) 4.30 (3.10) 2.00 (1.60) 3.00 (2.80)
carbon 4.98 (0.99) 11.56 (7.35) 0.84 1.53 4.04 0.71 0.84 2.03 (0.94) 3.05 (0.74) 1.68 (0.72) 2.92 (0.97) 2.63 (1.55) 3.97 (2.08) 2.67 (1.65) 2.46 (1.92)
CIM, M 10.6 (14.6) 11.0 (10.2) 15.2 17.2 143.0 21.4 24.4 11.7 (15.1) 11.8 (7.83) 14.9 (58.9) 13.8 (38.2) 5.0 (4.5) 4.3 (3.1) 6.6 (5.1) 4.0 (3.4)
a Figures in parentheses are for samples that have been washed by five aliquota of fresh acetonitrile. b [Init] is the initial weight of Mn+(acac)ndivided by weight of Cab-O-Si1 + initial weight of M"+(acac), multiplied by 100. Contacted for 72 h, 4.7 wt % initial concentration.
ion a t the same initial concentration. The moles of metal per gram of catalyst are as follows: Cr(III), 94.2 pmol/g; Fe(III), 145.0 pmol/g; Co(II), 340 pmol/g; Mn(III), 436.8 pmol/g. These data show that the Mn(II1) complex is retained on the silica in amounts that are 1.28 times that of the Co(I1) complexes, 3.01 times that of the Fe(II1) complex, and 4.64 times that of the Cr(II1) complex. Moreover, the retention of Mn(II1) is more than that of the retention of Cu(I1) on silica. For the tris octahedral complexes, the metal ion does effect its retention. Some samples were washed with aliquots of fresh acetonitrile and were analyzed to determine the affinity of the complexes with the surface. The affinity was determined qualitatively by comparing the metal loadings of the washed samples to the unwashed samples (Table I). The sample of the tris Cr(II1) complex (0.49 wt % Cr, 94.2 pmol Cr/g of catalyst) lost about 90% of the Cr(II1) when washed with fresh acetonitrile. The sample of tris Fe(II1) complex lost about 65% of the original loading when washed with fresh solvent (0.81-0.29 wt %). The washed Mn(II1) and Co(I1) samples retained 65%, 80%, respectively, of the original amount present in the unwashed sample. Apparently, the affinity of the tris complexes for the surface varied with the type of tris complex in the following manner: Co(II), Mn(II1) > Fe(II1) 7 Cr(II1). M a n d C Analyses. The carbon and metal analyses of Cu(acac)2prepared in acetonitrile are shown in Figure 4b. The solid line is the fit of these data which shows a slope of 10.12 and an intercept of 1.3 X lo4 mol of carbon. These data show an observed C/M stoichiometry (10.12) similar to that of the predicted carbon stoichiometry; however, most of the data for the samples prepared in methanol (not shown) show more carbon (C/M = 12.3) that what is expected for the Cu(acac)z. The choice of solvent changes the observed carbon/metal ratio for the supported Cu(acac)z. This technique of analyzing the Cu, C analyses was used for studying the silica-supported samples of the other metals. The tris Cr(II1) complex shows a stoichiometry (C/M = 13.8,14.9) similar to the parent complex (Table I, C/M ratio = 15). However the carbon/metal ratios for the tris Fe(II1) complex (11.7), COW)complex (6.6), and the tris Mn(II1) complex (4.3-5.0) are lower than the C/M ratio for the parent c0mp1ex.l~Apparently, the Fe(III), Co(II), and Mn(II1) complexes interact with the support to
Figure 5. DRIFTS of Cu(aca& supported on silica: (a) 8.6 w t % Cu; (b) 5.7 wt % Cu; (c) 3.55 wt % ' Cu; (d) 2.5 wt 76 Cu; (e-g) 3.81, 2.41, 0.36 wt % Cu, washed with acetonitrile.
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surrender (acac) ligands and thus become attached to the silica in a fashion that is stronger than what is possible in the Cr(II1) complex. The Pt(I1) and Pd(I1) complexes show more carbon (C/M = 15-20) that what is expected for these bis complexes (C/M = 10). However, when the carbon content of blank silica was subtracted from that of these samples, the C/M ratio was near 10. We speculate that carbon from another source (solvent?) is present in the samples. DRIFTS. Supported and unsupported Cu(acac)z prepared in acetonitrile were examined by DRIFTS. These spectra (Figure 5) for samples prepared with Cu loadings between 0.90 and 8.60 wt 9% were examined over the range of 4000-200 cm-l and the data were reported as KubelkaMunk units for the region characteristic of the acetylacetonate group (1600-1300 cm-1). All samples showed vibrations at 1576, 1528, and 1390 cm-l. These same vibrations are apparent in the spectrum of the Cu(acac)n powder (Figure 6). Two vibrations, 1552 and 1355 cm-l, are apparent in the supported samples (Figure 5a-c) with weight loadings of copper 23.5 wt % and the Cu(acac)z powder (Figure 6). These vibrations are absent from the spectrum of a sample showing a Cu loading of 2.50 wt % (Figure 5d) and less (not shown). We include the spectra of supported Cu(acac)z (Figure 5e-g), which were washed with fresh acetonitrile to produce a copper loading of 3.81
1202 Langmuir, Vol. 7, No. 6,1991 m
Kenuin et al.
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13.5 wt 7% show multiple layers of the bis complex. The unstable tris complexes apparently change their stoichiometry as they mount the surface. The Fe(I1I) complex shows a stoichiometry of C/M near 10 which suggests that it loses 1(acac), whereas the Co(II), Mn(II1) complexes show a C/M near 5. The DRIFTS spectra of the supported Fe(III), Mn(III), and Cu(I1)are very similar and show vibrations characteristic of the (acac); however, the supported Co(I1)does not show evidence of the (acac). We speculate that the (acac) group is present in the (19) RadhakrishnanNair, T. D.; Sreeman, P.; Thankarajan J. Indian Chem. Soc. 1982,59,415 (1982).
Supported Metal Complexes as Model Catalysts supported Fe(II1) and Mn(II1) as follows: Fe111(acac)2+ and Mnn1(acac)2+.The DRIFTS spectra of the “unstable”, supported Fe(II1) and Mn(II1) complexes are very similar to that for the supported Cu(I1) complex and much different from the tris Fe(II1) and Mn(II1) complexes.The “stable”, supported Cu(I1) and Cr(II1) complexes show DRIFTS spectra only slightly different from the unsupported complexes; i.e., the missing overtone relaxation for the supported Cu(I1) complex and frequency shift 10-15 cm-l up for the supported Cr(II1). These results suggest that the unstable complexes show structures that are different from the parent complexes, but the stable complexes show symmetries not unlike the unsupported complexes.
Conclusions Mononuclear metal acetylacetonates (Mn+(acac),) can be supported on Cab-0-Si1 to produce supported metal oxides when thermally decomposed. Some of the complexes [M = Cu(II), Pd(II), Pt(II),and Cr(III)] mount the silica without any rearrangement or loss of the acetylacetonate ligands, whereas others [Co(II),Fe(III), and Mn(III)] surrender at least one (acac) ligand. One stable complex [Cu(acac)z] sorbed to the surface in high concentrations, whereas one complex showing octahedral shape [Cr(III)] and two square-planar complexes [Pd(II) and Pt(II)] did not sorb to the silica and were easily removed by washing with solvent. The low affinity of the octahedral-shaped Cr(II1) complex for the surface suggests that the stable, fully coordinated ion could not gain access to the surface siloxides to affect a ligand exchange and that the (acac) ligands could not be oriented to affect hydrogen-bonding interactions between the surface and the complex. If shape of the complex alone determined the affinity of the complex for the surface, then we expect that Pt, Pd(acac)2 should be adsorbed to the surface in the same proportions as the Cu(acac)2. However, the results do not support a mechanism where the shape alone determines the affinity of the complex for the surface. The low adsorption of isostructural complexes of Pd”and Pt11(acac)2compared to the high adsorption amounts of the Cu(acac)2 suggest that factor(s) other than shape
Langmuir, Vol. 7, No. 6, 1991 1205 of the complex (such as hydrogen bonding and ability of the metal ion to accept a fifth ligand) may determine the adsorption affinity of the complexes to the surface of the silica. Other data are necessary to evaluate the relative importance of these other factors to the adsorption phenomenon. The decomposition of the bis square-planar Cu(I1) complex proceeds in a stepwise fashion with the elimination of acetylacetone for temperatures below 240 “C and then the elimination of acetylacetone plus species of lower molecular weights. The final residue shows a Cu/O stoichiometry of 1. The decomposition kinetics for the monolayer film is different from that of the overlayer. The overlayer shows decomposition kinetics not unlike the unsupported Cu11(acac)2. The unstable tris complexes of Fe, Mn and the bis complex of Co mount the surface with a carbon/metal molar ratio less than the parent complexes. This ratio is 10,5,and 5 for the Fe(III), Mn(III), and Co(I1) complexes. The DRIFTS spectra show evidence of (acac) rings in the Fe(II1) and Mn(II1) samples, which suggest that the supported Fe(II1) and Mn(II1) complexes have retained at least one (acac) ligand. The supported Co(I1)complexes do not show evidence of (acac) ligands. The loss of these ligands allows the metal ion to bind specifically to the siloxides. We speculate that the Fe species interacts with the surface as the Fe111(acac)2+ion binding to one siloxide, whereas the Mn1n(acac)2+ion binds to two siloxides. With increasing numbers of bonds to the surface, we speculate that a stronger attachment is produced between the metal complex and the surface. We are uncertain regarding the structure of the supported Co(I1) complexes.
Acknowledgment. The authors acknowledgethe support from U.S.EPA through cooperative agreement CR812353-01. We also thank Professor J. Aaron Bertrand, School of Chemistry, Georgia Institute of Technology (Atlanta, GA), for his thoughtful insights. Registry No. Cuz+(acac)z,13395-16-9; Pd2+(acac)z,1402461-4;Pt2*(acac)z,15170-57-7; Fe3+(acac)s,14024-181;CrYacac)a, 21679-31-2;Mns+(acac)3,14284-89-0; Coz+(acac)2,14024-48-7.
