Influence of Molecular Oxide Cages on Metal Carbonyls - The Journal

Dec 8, 2009 - Kent State University at Stark, 6000 Frank Avenue NW, North Canton, Ohio 44720. J. Phys. Chem. A , 2010, 114 (2), pp 987–993. DOI: 10...
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J. Phys. Chem. A 2010, 114, 987–993

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Influence of Molecular Oxide Cages on Metal Carbonyls Clarke W. Earley* Kent State UniVersity at Stark, 6000 Frank AVenue NW, North Canton, Ohio 44720 ReceiVed: September 3, 2009; ReVised Manuscript ReceiVed: NoVember 5, 2009

A series of DFT calculations have been performed on mononuclear d6 metal tricarbonyls supported on molecular oxide cages. The molecular cages were chosen both as models for phosphate, silicate, and aluminosilicate surfaces and because experimental data is available for a few of these molecular complexes. By systematically varying the nature of the oxide surface, qualitative estimates of metal carbonyl geometry, relative metal-CO bond strengths, and predictions of the shifts in CtO stretching frequencies upon changes in oxide basicity have been determined for a range of transition metals. Although most of the calculated trends correlate with expectations, additional insights into some of the bonding characteristics of these systems were obtained. Introduction Transition metal carbonyls are one of the most widely studied classes of inorganic compounds. In addition to their use as catalysts,1,2 the fact that the CtO group has such a strong, sharp, IR-active vibrational stretching band has been exploited to study the nature of metal carbonyls on oxide supports.3,4 As a related example, it has recently been suggested that a rhenium tricarbonyl complex5 has potential as a fluorescent probe for cell imaging. A wide range of theoretical investigations on metal carbonyls have been reported. Hocking and Hambley6 reported results of a database analysis of over 20 000 reported crystal structures containing metal carbonyls. An inverse relationship exists between metal-carbon and carbon-oxygen bond lengths, which is explained on the basis of increased backing-bonding from occupied metal d orbitals into empty CO π* orbitals. This analysis also determined that metal-carbon bond lengths increased with increasing covalent radii of the metals and that the CO bond lengths generally decreased going from left to right across rows of the periodic table, suggesting that earlier transition metals exhibit stronger M f CO back-donation. Increasing electron density on the transition metal is expected to result in a decrease in metal-carbon bond lengths and an increase in carbon-oxygen bond lengths. Whereas the change in charge density can be due to a formal change in oxidation state of the metal, one conclusion of Hocking and Hambley’s analysis “implies that a change of ligands in the coordination sphere can have a similar effect on the CtO bond length to a change in oxidation state”.6 In the present study, a series of transition metal carbonyls bonded to molecular oxide cages were examined to determine how changing the metal oxide support affects metal carbonyl bonding. Representative structures of these molecular oxide cages are shown in Figure 1. The molecular oxide supports used can be envisioned as models of phosphate, silicate, and aluminosilicate surfaces. Numerous examples of this type of bonding are known. For example, it has been experimentally demonstrated that metal carbonyls such as Mo(CO)64 and HRe(CO)57 react with zeolites to form metal tricarbonyls bound to the oxide surfaces. Calculations were performed for 12 * E-mail: [email protected].

Figure 1. Representative structures of metal tricarbonyls supported on molecular oxides.

different d6 transition metals from group 6 (Cr, Mo, W) to group 9 (Co, Rh, Ir) supported on five different molecular oxide supports. Computational Methodology All calculations were performed using the PC GAMESS program8 (based on GAMESS9) on personal computers running under a Linux operating system. The Stevens, Balch, and Krauss (SBK) split-valence effective-core potential basis set10,11 was used for all atoms in each of the calculations performed. Molecular geometries were fully optimized within the indicated symmetry constraints. DFT calculations were performed using the nonlocal, hybrid exchange functional B3LYP.12-14 The numerical grids used 128 radial grid points (NRAD ) 128) and a Lebedev angular grid of order 41 (LMAX ) 41), which used 590 points per radial shell. Force calculations were performed on all structures to characterize the stationary points obtained using the same level of theory and identical basis sets for all optimized geometries. In all cases, the Hessian matrices were positive definite, which verified that these optimizations had converged to local minima on the potential energy surface. The total energies reported do not include any corrections for zero-point vibrational energy. Calculated vibrational frequencies were scaled by a factor of 0.9614.15 Results and Discussion Comparison with Known Phosphate Cage Structures. The first series of molecules is based on the structures of

10.1021/jp908531p  2010 American Chemical Society Published on Web 12/08/2009

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TABLE 1: IR Spectra and Calculated Frequencies (in cm-1) for [{P3O9}M(CO)3]-2 Mn

1+

Re1+

c

modea

exptlb

calcdc

difference

E A1 E A1

1913 2034 1885 2018

1886 1973 1848 1960

-27 -61 -37 -58

a Assuming C3V symmetry. b From Besecker, Day, and Klemperer.17 Scaled by 0.9614.

[{P3O9}M(CO)3]2- (M ) Mn or Re), which were reported by Besecker and Klemperer.16,17 The 31P NMR spectra showed a single peak for both of these compounds, consistent with the proposed C3V symmetry of these molecules, and experimental νCO stretching frequencies were reported. These experimental results are compared with the calculated values obtained from the present work in Table 1. For both of these molecules, the unscaled calculated frequencies are larger than the experimental, condensed phase results. For B3LYP/6-31G(d) calculations, harmonic frequencies are commonly scaled by a factor of 0.9614, as suggested by Scott and Radom.15 On the basis of their study of 31 organometallic complexes using a mixed basis set combining the same core potential for transition metals used in the present work and a 6-31G(d) basis for all ligands, Lu, Srinivas, and Schwartz18 have proposed a scale factor of 0.9550. However, these workers noted that this new value was quite similar to the value proposed by Scott and Radom, and “yields only 0.1% improvement relative to the generic” (Scott-Radom) scale factor. El-Azhary and AlKahtani19 have suggested slightly smaller scale factors ranging from 0.927 to 0.943 based on calculations of four main group heterocyclic compounds containing selenium and tellurium using the same effective core potential basis set used in this work. Given the similarity of the values for all of these scale factors and the fact that the calculated νCO stretches in Table 1 fit the experimental data slightly better, the more widely used Radom-Scott scale factor was selected.20 Crystal structures were not reported for either of these compounds, but the structure of a dimeric compound containing two [{P3O9}Ru(CO)]2- units linked by two bridging CO 21 molecules, [{P3O9}Ru(µ -CO)(CO)]42 , has been reported. For this structure, the Ru-O bonds ranged from 213 to 221 pm, and the bond from the Ru to the carbon of the nonbridging CO group was 183 pm. The experimental structure contains a formally Ru1+ ion, so comparison with the present work is not ideal. The structure of [{P3O9}Ru(NCCH3)3]1-, which contains the Ru2+ ion and has Ru-O bond lengths ranging from 211-213 pm, has also been reported.22 In the present work, the optimized structure of [{P3O9}Ru(CO)3]1- (which contains a formally Ru2+ ion) has calculated bond distances of 211 pm for the Ru-O bonds and 195 pm for the Ru-C bonds. These values compare favorably with the experimental results. The experimental structure of the dimer, which contains Ru1+, is expected to have a higher electron density on the ruthenium atom, which should increase donation into the empty π* orbital of the CtO groups (backdonation), resulting in shorter Ru-C bonds. As expected, the experimental Ru1+-C bond length is significantly shorter than the distance calculated for the Ru2+-C bonds. Increasing the Ru-C bond strength is expected to weaken the trans Ru-O bonds, and the decreased electrostatic attraction between the fromally Ru1+ and the O2- atoms is expected to increase the length of all Ru-O bonds. The Ru-O bonds in the Ru1+-

containing dimer are significantly longer than the Ru-O bonds in both the experimental and calculated structures containing Ru2+ atoms. A crystal structure has been reported for [{P4O12}{Ir(CO)2}2]2-,23 which contains two square-planar Ir1+ atoms with Ir-O bond lengths ranging from 204.2 to 206.5 pm and Ir-C bond lengths ranging from 182 to 183 pm. Calculations on [{P3O9}Ir(CO)3] contain Ir in a 3+ oxidation state and have IR-O bond lengths of 203.2 pm and Ir-C bond lengths of 195.1 pm. As observed for Ru, increasing the oxidation state of the transition metal results in a significant increase in the Ir-C bond lengths. The Ir3+-O bonds are expected to be shorter than the Ir1+-O bonds due to increased electrostatic attraction. However, this predicted decrease appears to be offset by the increase in coordination, resulting in a lengthening of this bond and fortuitous agreement between the experimental and calculated results. Comparison with Known Silicate Cage Structures. The silicate cages are based on the R8Si8O12 cage structure, in which one of the RSi groups has been replaced with a M(CO)3 group. Although we are not aware of any reported examples of [{R7Si7O12}M(CO)3]n- cages, a number of compounds containing a MSi7O12 cage core are known, including {(c-C6 H11)7Si7O12}W(NMe2)3,24 {(C5 H9)7Si7O12}MCp′′ (M ) Ti or Zr, Cp′′ ) 1,3-(SiMe3)C5H3),25 [{((CH3)2CHCH2)7Si7O12}FeCl]-1,26 [{(cC6H11)7Si7O12}Mn(Na(Et2O)]2,27 and [{(c-C6H11)7Si7O12}VO]2.28 The reported structure for {(c-C6H11)7Si7O12}W(NMe2)3 has W-O bond lengths of 194.7 pm, with tungsten in a 6+ oxidation state. In this work, the W-O bonds in [{H7Si7O12}W(CO)3]3-, which contains a tungsten with 0+ oxidation state, were calculated to be 228.2 pm. Stronger electrostatic interactions between the highly oxidized tungsten center and the formally O2- atoms is expected to give shorter W-O bonds, consistent with the calculated results. In the [{((CH3)2CHCH2)7Si7O12}FeCl]1- anion, the iron is formally Fe3+. In the crystal structure of this molecule, Fe-O distances range from 183.8 to 184.6 pm. The calculated Fe2+-O bond lengths in [{H7Si7O12}Fe(CO)3]-1 were calculated to range from 196.6 to 196.9 pm. This increase can be explained as a combination of decreased bond strength between the less oxidized Fe2+ and the increase in coordination number. In [{(c-C6H11)7Si7O12}Mn(Na(Et2 O)]2, the manganese atom is in a 2+ oxidation state and the Mn-O bond lengths range from 198.5 to 214.6 pm, with an average bond length of 205.4 pm. In this structure, the Mn is only four-coordinate. In [{(cC5H9)7Si7O12}SiO]2Mn(tmeda)2,29 the Mn2+ is six-coordinate with Mn-O bond lengths of 204.0 pm. Calculated Mn-O bond lengths in [{H7Si7O12}Mn(CO)3]2-, which contains Mn1+, were very slightly longer at 205.9 pm. In summary, for both the phosphate- and silicate-supported metal tricarbonyls, there do not appear to be any published crystal structures that allow for a direct comparison with the present calculational results. However, comparison of structural data for related compounds and the direct comparison with the reported IR vibrational data for the Mn and Re tricarbonyls on the phosphate indicates that the calculational procedure used in this work provides a realistic description of the molecules included in this study. Calculated Properties of Phosphate Cage Structures. All of the metal tricarbonyls supported on the P3O39 anion included in this study have formally d6 metal centers and approximately C3V symmetry. The structure of the [{P3O9}Ru(CO)3}1- cage is representative and is shown in Figure 2.

Metal Carbonyls on Metal Oxides

J. Phys. Chem. A, Vol. 114, No. 2, 2010 989 TABLE 4: Calculated νCO Stretching Frequencies (scaled, in cm-1) for [{P3O9}M(CO)3]-n Cagesa

E A1 E A1 E A1 Figure 2. Calculated geometry of [{P3O9}Ru(CO)3]1- with selected bond lengths given.

TABLE 2: Calculated CtO Bond Lengths in [{P3O9}M(CO)3]n- Cages group 6, (Cr/Mo/W)0

group 7, (Mn/Tc/Re)1+

group 8, (Fe/Ru/Os)2+

group 9, (Co/Rh/Ir)3+

120.7 121.0 121.5

117.7 118.2 118.7

115.5 115.9 116.4

114.3 114.5 114.7

TABLE 3: Calculated M-C and M-O Bond Lengths (pm) in [{P3O9}M(CO)3]n- Cages

M-C M-O M-C M-O M-C M-O

group 6, (Cr/Mo/W)0

group 7, (Mn/Tc/Re)+1

group 8, (Fe/Ru/Os)+2

group 9, (Co/Rh/Ir)+3

181.7 223.0 194.2 234.8 193.6 232.8

181.3 208.2 191.7 219.9 191.3 219.7

185.7 198.1 195.0 210.8 191.9 211.4

194.7 189.1 200.7 203.8 196.5 206.1

Calculated CtO and both M-C and M-O bond lengths are shown in Table 2 and Table 3, respectively. The CtO bond lengths follow a consistent pattern: Carbonyl groups bonded to the group 6 metals have the longest CO bond lengths, which suggests the strongest interaction between the filled metal d orbitals and the empty CO π* orbitals. The CtO bonds become progressively shorter, and presumably stronger, moving across the Periodic Table. It is well-known that d orbital energies decrease going across periods, resulting in poorer overlap of the occupied metal d orbitals with the empty CO π* orbitals. In addition, for the molecules included in this study, the formal positive charge on the transition metals increased moving across periods, which would also decrease the ability of the metals to donate electrons into the CO π* orbitals. Lower occupation of CO π* orbitals results in stronger and shorter CO bonds. Within each period, the calculated CO bond lengths for the [{P3O9}M(CO)3]n- cages are all very similar (within 1 pm). The calculated trend is that compounds containing heavier elements within each period have slightly longer CO bond lengths. The database analysis of experimental structures6 also showed relatively small variations in CO bond lengths within a period. For groups 6 and 7, the lightest metals (Cr and Mn) had the shortest average CO bond lengths. However, this trend was not seen in groups 8 and 9. Bernhardt and co-workers30 examined the experimental structures for different salts of the isoelectronic series [M(CO)6]2+, M ) Fe, Ru, and Os. For this series, these workers found “experimental internuclear C-O distances are all identical with error limits” and indicated that vibrational data should “provide a much more accurate estimate of C-O bond strength”. Their analysis indicated that the iron and ruthenium CO bond strengths are almost identical, and for osmium, the CO bond

group 6, (Cr/Mo/W)0

group 7, (Mn/Tc/Re)1+

group 8, (Fe/Ru/Os)2+

group 9, (Co/Rh/Ir)3+

1711 1829 1699 1822 1689 1817

1886 1973 1860 1964 1848 1960

2052 2092 2016 2083 1995 2078

2169 2177 2139 2168 2121 2168

a Values for both the assymmetric (E) and symmetric (A1) modes are shown.

was slightly weaker. This is consistent with the calculated CO bond length in the [{P3O9}Os(CO)3]1- cage being the longest within this period. However, the calculated bond lengths in the present structures also indicate differences in CO bonds between the Fe and Ru cages that were not observed in the experimental data for the hexacarbonyls. Although this might suggest a limitation of the present calculations, it could also be a consequence of the influence of the oxide surface on these metal carbonyls. As expected, the calculated transition metal-carbon and transition metal-oxygen bond lengths in the [{P3O9}M(CO)3]ncages show much more variation than the CtO bond lengths. The calculated M-O bond lengths generally parallel trends in atomic radii,31 which show a continuous decrease in size moving across periods and an increase in size for the second- and thirdrow metals within a period. However, the M-C bond lengths do not follow these trends, with the M-C bonds calculated to be the shortest for the group 7 metals. The observed patterns can be explained as a result of competing trends. For the compounds included in this study, moving across periods was accompanied by an increase in the positive charge on the metals (to keep the cages isoelectronic). Although this is expected to decrease the size of the metal atoms resulting in shorter M-C bonds, it is also expected to result in less donation from the occupied metal d orbitals into the empty CO π* orbitals and, thus, longer M-C bonds. Whereas some information about CO bond strengths can be inferred on the basis of CO bond lengths, a more direct measure of these bond strengths can be obtained from analysis of the calculated vibrational stretching frequencies of the CO bonds, shown in Table 4. The trends in calculated frequencies closely parallel the results expected on the basis of calculated CO bond lengths. Moving across periods, the decrease in CO bond lengths is accompanied by a calculated increase in the νCO stretching frequencies, consistent with increased CO bond strength for the later period metals. Although the differences in calculated properties within each period are smaller than the differences between periods, the calculated trends are consistent. Because back-donation increases the M-C bond strength and weakens the CO π bond, increased back-donation is expected to result in an increase in CO bond lengths and a decrease in M-C bond lengths. For all of the [{P3O9}M(CO)3]-n cages, within each period, the heavier elements have both longer CO bonds and lower-energy vibration frequencies, consistent with very slight decreases in CO bond strength. This result is similar to that found by Bernhardt and co-workers30 in the [M(CO)6]2+ (M ) Fe, Ru, and Os) salts. If CO bond strengths in these compounds are determined entirely by back-donation, this indicates that the heavier transition metals are slightly better donors than the first-row transition metals. For the series [M(CO)6]2+, (M ) Fe, Ru, and Os), Bernhardt and co-workers30 found different trends for different types of

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TABLE 5: Calculated P-OM and M-OM Bond Lengths in [{P3O9}M(CO)3]n- Cages group 6, group 7, group 8, group 9, (Cr/Mo/W)0 (Mn/Tc/Re)1+ (Fe/Ru/Os)2+ (Co/Rh/Ir)3+ P-OM bond lengths

151.9 152.1 152.3

153.4 153.6 153.8

155.6 155.8 156.0

158.6 158.6 158.8

M-OM bond lengths

223.0 234.8 232.8

208.2 219.9 219.7

198.1 210.8 211.4

189.1 203.8 206.1

TABLE 6: Calculated νCO Stretching Frequencies (scaled, in cm-1) for [{H7 Si7O12}M(CO)3]n- Cagesa

E A1 E A1 E A1 a

group 6, (Cr/Mo/W)0

group 7, (Mn/Tc/Re)1+

group 8, (Fe/Ru/Os)2+

group 9, (Co/Rh/Ir)3+

1695 1815 1681 1807 1671 1800

1866 1957 1837 1946 1823 1940

2030, 2032 2073 1993, 1995 2066 1967, 1968 2058

2138, 2140 2149 2117, 2119 2150 2095, 2096 2149

Frequency assignments based on ideal C3V symmetry.

CO stretches. For the A1g mode, small increases were found going from Fe to Ru to Os, whereas for the Eg and T1u modes, slight decreases were found. In the present work, it can be seen in Table 4 that the A1 and E modes also behave differently, with the E mode decreasing much more significantly than the A1 mode for the heavier elements of each period. The trends in calculated M-O bond lengths can be explained without requiring any type of π orbital interaction between the transition metals and the oxygen atoms in the P3O93- cages. Within each period, the M-O bond lengths are shortest for the first-row transition metals and longer for the two heavier and larger metals. Proceeding across periods, the M-O bonds decrease with increasing formal charge on the metal, consistent with increasing electrostatic attraction. As seen in Table 5, a rough correlation can be seen between the slight increase in P-OM bond lengths between periods and the more significant decrease in M-OM bond lengths. Calculated Properties of Silicate and Aluminosilicate Cage Structures. Trends for the [{H7Si7O12}M(CO)3]-n cages generally parallel the results discussed above for the [{P3O9}M(CO)3]-n cages. The only significant difference found was that the cages containing group 8 and group 9 metals deviated slightly from ideal C3V symmetry. In all cases, bonds that would be equivalent under C3V symmetry constraints were all within 0.2 pm, and the calculated frequencies for the two νCO bands that correspond to the degenerate E bands in ideal C3V symmetry were all within 2 cm-1. Calculated optimized geometries for all of the cages included in this study are provided in the Supporting Information. Calculated vibrational stretching frequencies for the CO bonds in the [{H7Si7O12}M(CO)3]n- cages are shown in Table 6. Trends parallel those seen for the [{P3O9}M(CO)3]n- cages, with νCO increasing moving across periods and decreasing slightly moving down columns. The more significant result here is that the values are all shifted to slightly lower energies (about 20 cm-1 on average) in the silicate cages relative to the phosphate cages. This implies slightly stronger back-donation (M-C π bonding) in the silicate cages, presumably due to a decrease in the effective positive charge on the transition metals in these cages. The simplest explanation for this is the fact that silicon is less electronegative than phosphorus, making the silicate cages better electron donors to the transition metals than the phosphate

Figure 3. Isomers of [{H7AlSi6O12}M(CO)3]n- cages.

TABLE 7: Calculated Energies (in kJ/mol) for Each of the [{H7 AlSi6O12}M(CO)3]n- Cage Isomers Relative to the “Far” Isomer

near middle far near middle far near middle far

group 6, (Cr/Mo/W)0

group 7, (Mn/Tc/Re)1+

group 8, (Fe/Ru/Os)2+

group 9, (Co/Rh/Ir)3+

-161a +56

+134 +39

+70 +19

+3 +3

+184 +54

+130 +39

+61 +14

-9 -12

+177 +53

+123 +37

+60 +15

-16 -17

a In the [{H7 AlSi6O12}Cr(CO)3]4- cage, the chromium atom is bound to only one oxygen and is four-coordinate.

cages. This result is consistent with the optical basicity scale developed by Duffy.32 To further examine the effects of the oxide cages on the metal-carbonyl bonds, calculations were performed on a series of [{H7AlSi6O12}M(CO)3]n- cages. Substitution of silicon with 3aluminum increases the negative charge on the cage (H7Si7O12 4vs H7AlSi6O12 ), which is expected to increase the electron density on the transition metal centers. This should result in increased metal-to-carbonyl back-bonding, which should lower the νCO stretching frequencies. Changing the oxide support from phosphate to silicate resulted in an approximate 20 cm-1 decrease in νCO stretching frequencies. Changing the support from silicate to the aluminosilicates resulted in a similar decrease in the energy of these stretching bands. There are three different isomers possible for the [{H7AlSi6O12}M(CO)3]n- cages, as shown in Figure 3. The labels indicate whether the aluminum atom is in one of the three “near” positions, one of the three “middle” positions, or the unique “far” position. Calculated relative stabilities of these cages are shown in Table 7. In general, the “far” isomers are slightly more stable than the “middle” isomers and significantly more stable than the “near” isomers. The magnitude of relative stabilization is most significant with the earlier transition metals, which is expected for this series on the basis of simple electrostatic effects. As the formal charge on the transition metals increases and the total negative charge of these structures decreases, the difference in energies between different isomers becomes much less significant. For the rhodium- and iridiumcontaining cages, the “near” and “middle” isomers are calculated to be slightly more stable than the “far” isomer. Table 8 contains the calculated highest-energy νCO stretches for all of the [{H7AlSi6O12}M(CO)3]n- cages. The calculated trends generally parallel the results obtained for the phosphate and silicate cages, with νCO increasing across periods and decreasing slightly down columns. Since the aluminum atom

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TABLE 8: Calculated Highest Energy (fully symmetric) ΝCO Stretching Frequencies (scaled, in cm-1) for [{H7 AlSi6O12}M(CO)3]n- Cages

near middle far near middle far near middle far

group 6, (Cr/Mo/W)0

group 7, (Mn/Tc/Re)1+

group 8, (Fe/Ru/Os)2+

group 9, (Co/Rh/Ir)3+

1816a 1786 1789 1764 1778 1780 1757 1770 1773

1914 1926 1931 1903 1917 1920 1898 1911 1915

2029 2027 2044 2026 2031 2041 2019 2028 2031

2115 2111 2119 2116 2117 2125 2115 2123 2126

a In the [{H7 AlSi6O12}Cr(CO)3]-4 cage, the chromium atom is bound to only one oxygen and is four-coordinate.

in these structures is formally Al3+ and the silicon is Si4+, atoms closer to the aluminum atom are expected to have more negative charge than atoms further away. Thus, placing the aluminum atom in the “near” position is expected to give the transition metal the highest electron density, which should result in increased back-donation and give the lowest energy νCO stretching frequencies. For most of the metals included in this study, this prediction is borne out by the calculations. However, for both iron and cobalt, the “middle” isomers are calculated to have slightly lower energy νCO stretches than the “far” isomers. For both the phosphate and silicate cages, the calculated νCO stretches were calculated to decrease for the heavier elements in each period. However, for the aluminosilicates, the opposite trend is observed for the group 9 metals, for which the νCO stretches in the cobalt-containing cages are at slightly lower energies than both the Ir- and Rh-containing cages. Although this trend is unique, there is very little variation in the calculated energies of these stretches. Analysis of these results is complicated by the fact that introduction of either aluminum atoms or transition metals into the silicate cages causes these cages to become somewhat distorted. This is due to the fact that both the Al3+ atoms and the transition metals are all larger than the Si4+ atoms, so the bond lengths between these atoms and oxygen are all longer than the silicon-oxygen bond lengths. The increased size of the Al atoms in these types of cages has been demonstrated experimentally by Feher, Budzichowski, and Weller,33 who reported the crystal structure of the [R′R7AlSi7O12] (R ) C6 H5, R′ ) OP(C6H5)3) cage. These workers found Si-O bond lengths ranging from 159.3 to 163.1 pm, whereas the Al-O bond lengths within this cage ranged from 171.4 to 171.9 pm. In the present work, the calculated average bond lengths in the [{H7AlSi6O12}M(CO)3]n- cages were 164.5 pm for all Si-O bonds and 178.2 pm for the Al-O bonds. One measure of the amount of distortion in these cages due to the increased sizes of the aluminum and transition metal atoms can be found by comparison of the distances between opposite corners of these cages. For the “far” isomers of the [{H7AlSi6O12}M(CO)3]n- cages, the M-Al distances range from 605.9 to 668.4 pm, whereas the Si-Si distances are all substantially shorter, ranging from 533.2 to 546.6 pm. Movement of the transition metals and aluminum away from the center of these cages should result in larger M-O-Si bond angles. Experimental Si-O-Si bond angles in [R8 Si8O12] cages are typically between 145° and 150°.34-38 Calculated values for the M-O-Si angles in [{H7Si7O12}M(CO)3]n- cages are slightly larger than this and range from 147° to 165°. Similar values are found for the angles around oxygen in the “far” isomers of

Figure 4. Structure of the “near” isomer of the [{H7 AlSi6O12}Os(CO)3]-2 cage.

the [{H7AlSi6O12}M(CO)3]n- cages, with M-O-Si bond angles ranging from 141° to 162°. Placing the aluminum atom “near” the metal atom in these cages causes more significant distortion of these cages. For the “near” cage isomers, the two largest metal atoms are closest together, and the M-O-Al angles are calculated to range from 128° to 178°, which is a significantly larger variation than found for the other cube isomers. Several of the “middle” isomers also showed significant distortion of the cage structures, with M-O-Si bond angles ranging from 128° to 169°. This distortion can be clearly seen by examination of the structures of these compounds. In silicate cubes (for example, H8Si8O1239), the oxygen atoms do not lie directly on the line connecting the silicon atoms on the corners of the cube but, instead, are farther from the center of the cage, which gives Si-O-Si angles closer to the expected tetrahedral arrangement. In all of the “near” isomers of the [{H7AlSi6O12}M(CO)3]n- cages, the oxygen atom bridging the aluminum atom and the transition metal was found to be closer to the center of the cages, as shown in Figure 4. In all cases, the O-Al-O angles are within 9° of the ideal tetrahedral angles. For the “near” isomer of the [{H7AlSi6O12}Cr(CO)3]-4 cage, no local minimum was found with a six-coordinate chromium atom as part of the AlSi6O12 core. The only stable structure found contained the Cr(CO)3 fragment bound through a single bridging oxygen atom to the H7AlSi6O12 cage. This structure was calculated to be much more stable than either the “middle” or “far” isomers (see Table 7), both of which contain sixcoordinate chromium, and resulted in much stronger CO bonds, as shown in Table 8. Comparison of Different Oxide Supports. Table 9 allows direct comparison of some of the most important calculated quantities for the Ru(CO)3 fragment on all of the different oxide supports included in this study. The νCO stretching frequencies show the clearest trend, with the P3O93--supported metal carbonyl having the highest energy νCO stretches and the aluminosilicates, the lowest. This result is obtained for all of the transition metals included in this study and is consistent with the expected electron-donating ability or Lewis basicity of the oxide supports. The P3O93- support is expected to be the least basic, donating less electron density to the transition metal, which is reflected in the decreased back-donation into the CO π* orbitals and the strongest CO bonds. The H7Si7O123- support is more basic, and the aluminosilicates are calculated to be the most basic. For the [{H7AlSi6O12}M(CO)3]n- isomers, the “far” isomers generally have the highest-energy νCO stretches, and the “near” isomers have the lowest energy stretching motions.

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TABLE 9: Comparison of Calculated Results for Ru(CO)3 on Different Oxide Supports νCO (cm-1)

compound 1-

[{P3O9}Ru(CO)3] [{H7Si7O12}Ru(CO)3]1[{H7AlSi6O12}Ru(CO)3]2-, “far” [{H7AlSi6O12}Ru(CO)3]2-, “middle” [{H7AlSi6O12}Ru(CO)3]2-, “near”

2016, 1993, 1966, 1951, 1931,

2083 1995, 2066 2041 1963, 2031 1956, 2026

From these results, and consistent with expectations, the relative electron-donating ability of these supports can be ranked as follows:

P3O9 < (least basic) 3-

H7Si7O123-

< H7AlSi6O12

4-

Ru-C (pm)

115.9 116.2 116.5 116.5-116.8 116.7-117.1

195.0 193.9 193.7 193.8-194.4 192.9-196.5

TABLE 10: Difference between Average M-C and M-O Bonds Going from [{H7 Si7O12}M(CO)3]n- to the “Far” Isomer of [{H7 AlSi6O12}M(CO)3]n-a Cr/Mo/W

(far < middle < near) (most basic)

For all of the compounds included in this study, the calculated CO bonds lengths are relatively insensitive to the oxide support. The results shown in Table 9 for ruthenium are typical: For any single metal, the CO bond lengths vary by less than 1.4 pm among all five supports. For most of the silicates and aluminosilicates, the local symmetry of the M(CO)3 fragment is not C3V, and differences in calculated CO bond lengths of up to 0.5 pm within one compound were found. These differences within each structure make analysis of trends between different compounds of each metal less straightforward. However, looking at average CO bond lengths does show the same trend found for the νCO stretching frequencies, with the shortest CO bonds and presumably strongest CO bonds found in the P3O93-supported structures, followed by the H7Si7O123- and H7AlSi6O124- supports, respectively. For all of the metals except chromium (which has an open structure in the “near” isomer), the average CO bond lengths are longest for the “near” isomers and shortest for the “far” isomers of the aluminosilicate compounds. The metal-carbon bond lengths show the most variation in the data shown in Table 9, but the trends are much less obvious due to competing effects. For any one metal, the M-C bond length is calculated to vary by up to 5 pm, depending on the support used. Increasing the basicity of the oxide support is expected to give rise to an increase in electron density on the transition metal, which in turn should lead to an increase in the ability of the metal to donate charge to the carbonyl ligands, resulting in longer CO bonds with lower-energy νCO stretching frequencies. The calculated results discussed above are consistent with this explanation. In addition, the increase in π-bonding between the metal and the carbonyl groups should also shorten the M-C bonds. However, increasing electron density on the transition metal is also expected to lead to an increase in electron-electron repulsions on the metal, which would increase the size of the metal, resulting in longer M-C bonds. For most of the metals included in the study, changing the support from P3O93- to H7Si7O123- results in a slight decrease in the calculate M-C bond lengths, suggesting that the increase in M-C bond strength is the more significant factor. However, changing the support from H7Si7O123- to H7AlSi6O124- results in a change in charge of the compounds, and the trends in M-C bond length are much less consistent. For the first-row transition metals (Cr0, Mn1+, Fe2+, and Co3+), the M-C bonds in [{H7AlSi6O12}M(CO)3]n- are consistently shorter than the corresponding bonds in the silicate clusters, consistent with the expected increase in M-C bond strength. For the heavier metals in each group, the trends are less clear. Comparison of [{H7Si7O12}M(CO)3]n- with the “far” isomers of [{H7AlSi6O12}M(CO)3]n- provides the most direct comparison of structural changes due to increased electron density. The

CtO (pm)

a

Mn/Tc/Re

Fe/Ru/Os

Co/Rh/Ir

+0.1 -0.2 -0.2

∆ M-C (pm) +0.4 +0.9 +0.0 +0.3 -0.1 -0.2

+1.5 +0.4 +0.3

+0.1 +1.0 +1.1

∆ M-O (pm) +0.2 -0.4 +0.9 +0.1 +1.1 +1.2

-0.8 -0.8 -0.2

Sign is + if the distance is larger in the Si7 compound.

local geometries around the transition metals in these two sets of compounds are expected to be almost identical, with the only significant difference being the increase in negative charge of the compounds upon incorporation of aluminum. Since π bonding is not expected to be significant between the d6 transition metals and the oxide ions, comparison of M-C and M-O bonds for each metal is expected to allow these effects to be separated. Table 10 contains a comparison of the calculated M-C and M-O bond lengths, with the values listed as the difference between the average bond lengths in the [{H7Si7O12}M(CO)3]nstructures minus the bond lengths in the “far” isomers of the [{H7AlSi6O12}M(CO)3]n-′ structures. In all cases, the energy of the highest-energy fully symmetric νCO stretch shifted down by 25-30 cm-1, suggesting similar increases in back-donation for all of the metal tricarbonyls on the more basic oxide. For most metals, including all of the first-row transition metals, the M-C bonds are shorter in the aluminosilicates, consistent with increased back-bonding. However, the trends in M-O bond lengths are somewhat different. For Co3+, Rh3+, Ir3+, and Fe2+, the average M-O bonds are longer in the aluminosilicate compounds, consistent with an increase in size of the transition metal due to increased electron-electron repulsion in the more negative clusters. For the remaining metals, the opposite trend is observed, suggesting that an increase in M-O bond strength due to increased electrostatic attraction predominates. Although the shifts in νCO stretching frequencies are very consistent for all of the metals included in the study, the changes in M-C bond lengths are not. Within each period, the first-row transition metals have the largest decreases in M-C bond length upon changing from the silicate to the more basic aluminosilicate cages. Going across periods, the late transition metals show the largest decreases in size. Since the late transition metals have the lowest amount of back-donation (based on νCO stretching frequencies), a uniform increase in back-donation for all metals will be a larger percentage increase and a more significant change for these compounds. Similar arguments can be applied within each period, with the first-row transition metals having the highest-energy νCO stretching frequencies, implying the lowest amount of backdonation to the carbonyl groups and, thus, the largest percentage change going from silicate to aluminosilicate structures. Although the calculated trends have the correct pattern to fit this simple explanation, the magnitude of the shifts is not consistent, indicating that other factors also need to be considered.

Metal Carbonyls on Metal Oxides By comparing bond length trends within periods, the following conclusions can be drawn. Replacing Si4+ with Al3+ results in an increase of negative charge on the oxide support. Increasing basicity of this support gives rise to two competing interactions with the transition metals. Increasing charge on the support should give in a stronger electrostatic attraction between the transition metal and the oxide, resulting in shorter metal-oxygen bonds. However, increasing basicity of the support will also place more electron density on the transition metal, which could result in an increased size in the metal and longer metal-oxygen bonds. The calculations indicate that incorporation of aluminum results in the largest decrease in M-O bond lengths for the heaviest metals within each period. Examination of trends in ionic radii show the largest changes in size upon changes in formal charge for the lightest metals of a period,40 suggesting that the size increase for the first-row transition metals might be significant. Alternatively, compounds having the largest decreases in metal-carbon bond lengths have the largest increases in metal-oxygen bond lengths, suggesting that a “trans effect” might influence these bond lengths. Conclusions In this paper, it has been shown that calculations on a series of d6 transition-metal tricarbonyls supported on a selection of oxide supports gave results consistent with experimental results on related structures. Calculated νCO stretching frequencies were found to be the most consistent indicator of effective charge on these metal centers. The oxide supports used gave results consistent with basicity trends expected for bulk oxide materials, suggesting that the properties of these molecular compounds may have significant similarity to the bulk solid-state oxides. It is well-known that changes in charge on the transition metals results in significant and easily explained changes to CO stretching frequencies and both M-C and M-O bond lengths. In the present work, it was demonstrated that changes to the basicity or electron-donating ability of the oxide support can also lead to significant changes in νCO stretching frequencies, with the molecular phosphate cages being the weakest electron donors and the aluminosilicate cages being the most strongly basic. However, while the trends in νCO stretching frequencies followed expectations, trends in both metal-carbon and metal-oxygen bond lengths were complicated by a variety of competing factors. These results suggest that metal carbonyls can be effective experimental probes of surface basicity and that measured νCO stretching frequencies, in addition to being easier to measure, should be much better indicators of basicity than either metal-carbon or metal-oxygen bond lengths. Most of the transition metals in this study had an energetic preference for avoiding aluminum in aluminosilicates. However, this was not the case for the higher-valent, later transition metals. This result might have significant implications for determining site preference of metals carbonyls on zeolite supports. Acknowledgment. Support for this project from Kent State University at Stark is gratefully acknowledged. Supporting Information Available: Tables of optimized coordinates for all of the compounds included in this study are available. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dyson, P. J. Coord. Chem. ReV. 2004, 248, 2443–2458.

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