Polyhedron Vol. 8, No. 24, pp. 2925-2931, 1989 Printed in Great Britain

0277-5387/89 $3.00+.00 0 1989 Pergamon Press plc

VARIABLE TEMPERATURE SOLUTION AND CP-MAS 13C NMR OF [Mo(CO),(DIGLYME)]. DISPLACEMENT OF DIGLYME

BY SOLVENTS AT SUBAMBIENT TEMPERATURES

GEORGE W. WAGNER and BRIAN E. HANSON*

Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A.

(Received 13 February 1989 ; accepted 7 August 1989)

Abstract-Variable temperature solution 13C NMR of [Mo(CO),(diglyme)] shows that the chelating diglyme ligand is displaced by the monodentate solvents acetone, methanol and ethanol at low temperature. Free and coordinated solvent are observed at low temperature while at ambient temperature rapid exchange prevents the observation of coordinated solvent.

The [Mo(CO),(diglyme)] complex (1) was initially isolated by Werner and Coflield in 1960. ’ Although the structure of the compound has not been cry- stallographically determined, its IR stretching fre- quencies suggest the structure shown in Fig. 1. The tridentate diglyme ligand was observed to be easily displaced by other ligands and thus 1 was recog- nized as a valuable intermediate in the synthesis of molybdenum tricarbonyl complexes. Ligand sub- stitution reactions involving [Mo(CO),L,] com- plexes in solution have been well characterized and a large body of thermodynamic data now exists for these reactions. 2*3

Although “C NMR spectra have been reported for many metal carbonyl complexes,4 no data is available for [Mo(CO),(diglyme)]. The character- ization of this particular complex by 13C NMR is of interest to us because it is a good model com- pound for b-alumina-supported [Mo(CO),(ads)] which is stabilized by OH- and O*- groups on the b-alumina surface.5,7 The b-alumina-supported molybdenum sub-carbonyls have recently been the subject of many solid state 13C NMR studies.&”

Our initial interest in 1 was to determine its solid- state properties by variable temperature CP-MAS 13C NMR. In the absence of any previous NMR data, variable temperature solution 13C NMR was used to initially characterize the complex. These

*Author to whom correspondence should be addressed.

results indicated that substitution reactions were occurring between the complex and solvent mol- ecules at subambient temperatures. This was sur- prising since thermodynamic data suggest that the solvents should not displace diglyme.

A similar effect has been previously noted for the [Mo(CO)~($‘-C~H~CH,)]/THF system using ‘H NMR.2 In this case, THF solvent molecules readily displace the toluene ligand at ambient temperatures because of a favourable AH for this reaction. However, at elevated temperatures the reaction reverses to yield the toluene complex as a result of the positive AS for this process. This term dominates at high temperatures causing a shift of the equilibrium in favour of the toluene complex.

In ;his study, variable temperature solution 13C NMR results are presented demonstrating the

HW “Y “\ A,/ - I

W’ 0 \I/ 0 \

-4

/“;“\ C C

0’ b C '0

Fig. 1. Schematic representation of [Mo(CO) ,(diglyme)].

2925

2926 G. W. WAGNER and B. E. HANSON

reversible displacement of the diglyme ligand in lMo(CO),(diglyme)] by solvent molecules at subam- bient temperatures when the complex is dissolved in acetone-d,, methanol-d and ethanol-d. Also, variable temperature CP-MAS i3C NMR spectra of solid [Mo(C0)3(diglyme)] are shown which indi- cate that the complex is static in the solid state.

EXPERIMENTAL

The diglyme complex, 1, was synthesized by refluxing [Mo(CO)~($-C~H&H,)] in 25% (v/v) diglyme (Aldrich 99%, anhydrous)/pentane solu- tion, resulting in a light yellow precipitate which was filtered and washed with pentane. The toluene complex, [Mo(CO)~($-C~H&H,)], was synthe- sized by refluxing Mo(CO), in toluene. Both pro- ducts gave satisfactory elemental analyses. Further characterization by IR spectroscopy showed stretch- ing frequencies at 1951 and 1752 cm-’ (br) (KBr) for 1 and 1983 and 1913 cm- ’ (pentane) for [Mo(CO)~($-C~H&H~)].~~ The bands for the diglyme complex differ from those reported by Werner and Coffield [1905 and 1835 cn-- ’ (KBr)]. ’

The ‘CO enriched [Mo(CO),(diglyme)] was prepared from the enriched toluene complex. Enriched [Mo(CO),($-C6H5CH3)] was prepared by stirring the complex under a 13C0 atmosphere for 2 days. The Mo(CO), which was formed during this process was reconverted to the toluene com- plex by additional reflux to yield 13CO-enriched

[M~(CO)~($-CGH&H,)]. Deuterated solvents, acetone-d, (99% D) (Cam-

bridge Isotopes), ethanol-d (99.5% D) and meth- anol-d (99.5% D) (Aldrich Chemical Company) were degassed by purging with dry nitrogen before use.

Solutions for NMR samples were prepared under inert atmospheres using degassed NMR solvents. The solutions were then filtered through a glass frit and syringed into an outgassed IO-mm o.d. NMR tube. This procedure avoided exposure of the solu- tion to air.

Variable temperature 13C NMR spectra were rec- orded on a Bruker WP200 NMR spectrometer. The observation frequency for carbon was 50.323 MHz. The spectrometer was equipped with a Bruker vari- able temperature controller which was capable of maintaining temperatures to f 1°C. Samples were allowed to equilibrate at a given temperature for at least 5 min before data aquisition was initiated.

Variable temperature CP-MAS 13C NMR spec- tra were obtained form a 13CO-enriched sample of the diglyme complex using a Bruker MSL300 spectrometer. The observation frequency for car- bon was 75.5 MHz. The sample was packed in a

standard Bruker aluminium oxide rotor. The hole in the boron nitride endcap was sealed with rubber cement to prevent exposure of the sample to air. Nitrogen gas (from liquid nitrogen boil-o@ was used as the drive and bearing gas. The solid-state spectrometer was equipped with a Bruker variable temperature controller which maintained tem- peratures to f 1°C. The sample was allowed to equilibrate for at least 10 min at each temperature before scanning.

RESULTS

The variable temperature 13C NMR spectra for [Mo(CO),(diglyme)] in acetone-d6 are shown in Fig. 2. The solvent peaks are present at 206 and 29.8 ppm (TMS). The acetone-d6 methyl peak at 29.8 ppm was used as the chemical shift reference. Where indicated, the solvent peaks have been truncated to allow amplification of the complex signals. There

ti 264 K

I:I 244K

AilALL 224K

,aII,... b

1111llll~lllllllll~lllll111((r

200 100 Owm

Fig. 2. Variable temperature 50.3 MHz 13C NMR spectra of [Mo(CO),(diglyme)] in acetone-d,. The asterisked

peak is due to coordinated acetone-d,.

13C NMR of [Mo(CO),(diglyme)] 2927

is only a single resonance in the carbonyl region at 230.7 ppm. Signals can also be observed for the diglyme ligand at 72.6,71.5 and 59.2 ppm, which are barely shifted from the values obtained for the uncomplexed diglyme molecule in acetone-d, (72.7, 71.3 and 58.8). Two of the diglyme ligand res- onances are broadened in this spectrum which seems to be due to a concentration effect, since dilute samples demonstrated less severe broaden- ing. These signals are not broadened in the spectra of the uncomplexed diglyme ligand. The assign- ment of the diglyme ligands are not clear. How- ever, it is reasonable to assume that the peak at 59.2 ppm is due to the pair of methyl carbons and the peaks at 72.7 and 72.3 ppm are due to the two pairs of methylene carbons.

A simple “ball-and-stick” model of the [Mo(CO), (diglyme)] complex using an octahedral molyb- denum centre and tetrahedral angles for all of the carbon atoms in the diglyme ligand indicates that the structure shown in Fig. 1 is less sterically hin- dered than a structure in which the methyl groups are directed towards each other. This structure pos- sesses mirror plane symmetry and, thus a total of three resonances are expected for the coordinated diglyme ligand. The presence of a single carbonyl resonance is not predicted by the structure in Fig. 1 or by any other configuration possible for the diglyme ligand. The single carbonyl peak may imply either an accidental degeneracy of the chemical shifts or a motional process in which the carbonyls are rapidly exchanging positions. Many precedents exist for tricarbonyl group rotation in [(arene)M (CO),] complexes in solution.“-“. However, the bonds between the metal atom and the arene ring are not directional (which may account for the low rotational barriers observed in these complexes), whereas in the diglyme molecule, the directional nature of the molybdenum-oxygen bonds may present a high barrier for rotation of the tricarbonyl group.

The carbonyl groups have a longer T, than the diglyme ligand carbons as evidenced by the relative enhancement of the carbonyl resonance by either increasing the delay time between pulses or by the addition of the relaxation agent, Cr(acac)3. These treatments have little effect on the diglyme ligand signals.

At 284 K, the peaks due to the methyl groups and one pair of methylene groups have sharpened considerably and by 264 K they have attained approximately the same intensity and line width of the signal of the other pair of methylene carbons. This again suggests that the methyl groups adopt positions which allow mirror plane symmetry for the diglyme ligand.

At about 244 K, a new signal appears in the carbonyl region at 220.0 ppm and increases in intensity relative to the carbonyl resonance as the temperature is lowered. However, in the low temperature spectra of i3CO-enriched [Mo(CO), (diglyme)] in acetone-d, shown in Fig. 3, the new peak is very weak compared to the carbonyl ligand signal suggesting that the new peak originates from the acetone-d, solvent rather than the carbonyl groups. Thus, it appears that acetone molecules are able to coordinate to the molybdenum atom in the diglyme complex causing a downfield shift of the acetone carbonyl carbon. The chemical shift of the acetone methyl groups appears not to be greatly affected upon coordinated of the molecule to molyb- denum, since their signal is not resolved from the solvent peak. In the lower temperature spectra, the area of the coordinated acetone-d6 solvent mol- ecules is approximately equal to the area of the carbonyl ligands, which indicates that the acetone molecules have completely displaced the diglyme ligand at the lower temperatures. The peaks due to complexed and uncomplexed diglyme ligand are not resolved in the spectra. This might be expected since the respective chemical shifts differ by less than 0.4 ppm. At 184 K, all of the line widths of the signals begin to broaden slightly. This temperature is near the freezing point of acetone, thus the line broad- ening is most likely due to freezing solvent or crystallization of the tricarbonyl complex.

The subambient temperature 13C NMR spectra of [Mo(CO),(diglyme)] in ethanol-d are shown in Fig. 4. The peaks at 56.8 and 17.2 ppm are due to the solvent and the 56.8 ppm signal was used as the chemical shift reference. The ambient temperature spectrum (top spectrum) is virtually identical to the corresponding spectrum obtained in acetone-d6. The carbonyl resonance is at 228.8 ppm and the signals for the diglyme ligand are at 71.4, 69.8 and 52.7 ppm. Lowering the temperature of the sample again causes the signals at 69.8 and 52.7 ppm to sharpen in the lower temperature spectra. However, at 254 K, a new peak at 62.0 ppm begins to grow and attains the same relative intensity of the other methyl and methylene carbons in the diglyme ligand at 234 K. At the lower temperatures, the area of this new peak approaches a value which is 1.5 times the areas of the individual diglyme ligand signals, which suggest that the peak represents three mol- ecules. This new peak is assigned to the methylene groups of coordinated ethanol-d molecules which displace the diglyme ligand at the lower tem- peratures in a manner analogous to acetone. How- ever, the signals for the methyl groups of the coor- dinated ethanol-d molecules is not resolved from the solvent peak. Again, the signal due to com-

2928 G. W. WAGNER and B. E. HANSON

I I I I 251 K

t I I I 204 K

1111111 111111111~111111111~1

200 100 Owm

Fig. 3. Variable temperature 50.3 MHz 13C NMR spectra of ‘3CO-enriched [Mo(CO),(diglyme)] in acetone-d,. The asterisked peak is due to coordinated acetone-d,.

I 8 L1 252 K

t

I rl,+_&_ 234 K

214 K

194 K

llllllll~lllllll1l~lllllllll~

200 100 0 wm

Fig. 4. Variable temperature 50.3 MHz 13C NMR spectra of [Mo(CO),(diglyme)] in ethanol-d. The asterisked peak is due to coordinated ethanol-d.

“C NMR of [Mo(CO),(diglyme)] 2929

plexed and uncomplexed diglyme ligand are not resolved. As the temperature is lowered further, the resonances due to the carbonyl ligands, diglyme molecule and the methylene signal of the coor- dinated ethanol-d molecule and the methylene sig- nal of the coordinated ethanol-d molecules broaden considerably. It should be noted that the peak due to the methyl groups in the diglyme molecule broad- ens at a slower rate than the other signals. At 154 K, the peaks are so severely broadened that they are barely discernable in the spectrum. However, this temperature is very near the freezing point of ethanol which may account for the broadening. The solvent peaks at this temperature are also clearly broadened. The signal due to the methyl groups of the diglyme molecules would broaden at a slower rate since they would have greater mobility than the other carbons in the sample. Therefore, lower temperatures would have to be achieved in order to determine the dynamics of the carbonyl ligands in the substituted complex. In order to further sub- stantiate the assignment of the peak at 62.0 ppm to coordinated ethanol-d molecules, the variable temperature spectra of the diglyme complex in methanol-d were also recorded.

A series of variable temperature solution ’ 3C NMR spectra of [Mo(CO),(diglyme)] in methan- ol-d is shown in Fig. 5. In the 293 K spectrum, the peaks are at 229.6 ppm (carbonyl ligands), 72.0 and 70.4 ppm (methylene groups of the diglyme ligand) and 58.2 ppm (methyl groups of the diglyme ligand). The methanol-d solvent peak is at 49.0 ppm and was used as the chemical shift reference. At 293 K, the signals at 70.4 and 58.2 ppm are not as severely broadened as in the corresponding spectra in the other solvents, since this sample was very dilute. Again, lowering the temperature of the sample causes the signals of all three pairs of carbon atoms in the diglyme iigand to attain nearly equal inten- sities. In the 253 K spectrum, a new peak begins to appear at 58.7 ppm. In the lower temperature spectra, the ratio of the area of the new peak to the area of the diglyme methyl group approaches a value of 1.5. This indicates that three methanol-d mol- ecules are coordinated per molybdenum complex which causes the observed shift of the coordinated methanol-d methyl groups from 49.0 to 53.8 ppm. Thus, as in the previous solvents, methanol-d is displacing the diglyme ligand in the complex at the lower temperatures. Again, the complexed and

I. I II 26K

1 - I. 195 K

1 I: d--m- I75 K

-rr71- Oppm

Fig. 5. Variable temperature 50.3 MHz 13C NMR spectra of [Mo(CO),(digIyme)] in methanol-d. The asterisked peak is due to coordinated &ethanol-d.

2930 G. W. WAGNER and B. E. HANSON

295K

1 I 220 K

i/_L,,,, 11111111~1111,,111,,,1111111,1

200 100 OPIJ~

Fig. 6. Variable temperature 50.3 MHz ’ 'C NMR spectra of [Mo(CO),(acetone),] generated in situ from [Mo(CO)#-C6H6)]. The asterisked peak is due to coor-

dinated acetone-d,.

uncomplexed diglyme ligand signals are not resolv- able. In the solution i3C NMR spectra of this dilute sample of [Mo(CO),(diglyme)] in methanol-d, no significant broadening of the signals was observed at the lower temperatures.

It should be noted that when [Mo(CO);

($-C,H,)] is dissolved in acetone, the coordinated acetone ligands are again seen only at low tempera- ture. Thus, there must be rapid exchange between free and coordinated acetone in [(acetone)3Mo(CO)3] (Fig. 6).

The variable temperature 75.5 MHz CP-MAS 13C NMR spectras of 13CO-enriched [Mo(CO), (diglyme)] are shown in Fig. 7. In the 299 K spec- trum, a single, sharp carbonyl peak is present at 230.7 ppm and is very close to the solution value. The diglyme ligand resonances are at 74.1,72.8 and 67.3 ppm. The peak due to the diglyme methyl groups at 67.3 ppm is slightly enhanced relative to the diglyme methylene groups. The other peaks present in the spectrum are due to spinning side- bands of the carbonyl peak. The spectrum obtained at 260 K is virtually unchanged except that the sidebands have moved toward the carbonyl peak at 230.5 ppm as a result of the slower spin rate. The spinning rates were about 4600 and 4300 Hz in the 299 and 260 K spectra, respectively.

The presence of only a single, sharp resonance for each pair of methyl and methylene groups indicates that the ligand probably has mirror plane symmetry in the solid state as previously suggested by the model shown in Fig. 1. The presence of a single, sharp resonance for the carbonyl ligands at both temperatures with nearly identical linewidths, sug- gests that the complex is static in the solid state and that the chemical shifts of the carbonyl ligands are degenerate. Furthermore, an inspection’6’17 of the intense array of spinning sidebands indicates a very large chemical shift anisotropy of about 400 + 50 ppm for the carbonyl ligands which is close to the

500 400 300 200 100 0 -100 ppm

Fig. 7. Variable temperature 75.5 MHz 13C CO-MAS spectra of ‘3CO-enriched [Mo(CO),(diglyme)].

13C NMR of [Mo(CO),(diglyme)] 293 1

value expected for a static metal carbonyl. I 8 There- in dilute methylene chloride. Solvation of free fore, the [Mo(C0)3(diglyme)] molecule appears to diglyme may be anticipated to be greater in the be static in the solid state. more polar solvents used in the NMR work.

DISCUSSION

The coordination of acetone, methanol and etha- nol to [Mo(CO),(diglyme)] at low temperature is clearly suggested by the variable temperature NMR data for 1. This is contrary to what is expected based on the thermodynamic data of Hoff and co-workers3 for ligand displacement reactions of [(arene)Mo(CO),]. The data in ref. 3 clearly shows that the bond enthalpy for diglyme coordinated to molybdenum is larger than the enthalpy for acetone to molybdenum. Thus, one predicts that diglyme would always displace acetone. However, the pre- sent data suggest that diglyme is rapidly displaced by weak oxygen donor ligands and that at low tem- peratures, up to three acetone, methanol or ethanol ligands may bond to the molybdenum tricarbonyl group.

Acknowle&ementsWe would like to thank Dr George Marcelin, University of Pittsburgh, for the use of the MSWOO NMR spectrometer. Funding for this work was provided by the NSF (DMR 8518364).

REFERENCES

1.

2. 3.

4.

5.

6.

7.

8.

R. P. M. Werner and T. H. Coffield, Chem. Znd. 1960, 936. C. D. Hoff, J. Organomet. Chem. 1985,282,201. S. P. Nolan, R. L. de la Vega and C. D. Hoff, Organometallics 1986,5, 2529. L. J. Todd and J. R. Wilkinson, J. Organomet. Chem. 1974,77, 1. A. Brenner and R. L. But-well Jr, J. Catal. 1978, 52, 353.

The following equilibrium may be suggested for the Mo(CO), group in acetone or other similar solvents.

[(q3-diglyme)Mo(CO),] + acetone e

[(q2-diglyme)(acetone)Mo(CO)3] 9.

[(q2-diglyme)(acetone)Mo(CO),] + acetone I

[(VI-diglyme)(acetone),Mo(CO)J

[(q ‘-diglyme)(acetone)2Mo(C0)3] 1 10.

[(acetone),Mo(CO),] + diglyme.

Furthermore, exchange of free and coordinated acetone appears fast at room temperature. At low temperatures for acetone, methanol and ethanol, the equilibrium clearly goes in favour of complete displacement of diglyme. At room temperature, however, it is impossible to determine the extent of reaction since 13C NMR cannot distinguish between free and coordinated solvent and free and coordinated diglyme.

The apparent discrepancy between this work and that of Hoff’s for the displacement of diglyme by acetone is most likely due to solvation energies, since the NMR work was done in neat acetone, meth- anol or ethanol and the calorimetric work was done

11.

12.

13.

14.

15.

16.

17.

18.

M. Laniecki and R. L. Burwell Jr, J. Colloid. Inter. Sci. 1980,75, 95. A. Kazusaka and R. F. Howe, J. Molec. Catal. 1980, 9,95. (a) B. E. Hanson, G. W. Wagner, R. J. Davis and E. Motell, Znorg. Chem. 1984, 23, 1635 ; (b) G. W. Wagner and B. E. Hanson, J. Am. Chem. Sot., 1989, 111,2558. (a) W. M. Shirley, B. R. McGarvey, B. Maiti, A. Brenner and A. Clichowlas, J. Molec. Catal. 1985, 29, 259; (b) W. M. Shirley, Z. Phys. Chem. 1987, 152,41. T. H. Walter, A. Thompson, M. Kenny, S. Shinoda, T. L. Brown, H. S. Gutowsky and E. Oldfield, J. Am. Chem. Sot. 1988,110,1065. D. A. Brown and F. J. Hughes, J. Chem. Sot. A 1968, 1519. C. G. Kreiter and M. Lang, J. Organomet. Chem. 1973,55, C27. B. P. Rogues and C. Segard, J. Organomet. Chem. 1974,73, 327. W. R. Jackson, C. F. Pincombe, I. D. Rae and S. Thapebinkarn, Aust. J. Chem. 1975,28, 1535. F. A. Cotton and B. E. Hanson, Israel J. Chem. 1076/77, 16, 165. M. Maricq and J. S. Waugh, Chem. Phys. Lett. 1977, 47, 327. J. Herzfeld and A. E. Berger, J. Chem. Phys. 1980, 73, 6021. H. W. Speiss, R. Grosescu and U. Haeberlen, Chem. Phys. 1974,6,226.