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1H and 13C NMR study of cyclopentadienyl metal carbonyls in the solid state
Specrrochinu'caActa,Vol. 49A, No. 9, pp. 1307-1314,1993
0584-8539/93$6.00+0.00 ~) 1993PergamonPressLtd
Printedin GreatBritain
IH and ]3C NMR study of cyclopentadienyi metal carbonyls in the solid state S. A I M E , * L . CORDERO a n d R . G O B E I T O Dipartimento di Chimica Inorganica Chimica Fisica e Chimica dei Materiali, Universit,~t di Torino, via P. Giuria 7, 10125, Torino, Italy
and G . SZALONTAI University of Veszprem, NMR Laboratory, H-8 200, Veszprem Pf 158, Hungary
(Received 28 April 1992; accepted 11 June 1992) Abstract--In this paper we deal with some structural and dynamic properties of Cp2W2(CO)6 (1) and Cp2Ru2(CO)4 (11) as shown by solid state 13C and ]H NMR experiments. The IR and 13C CPMAS spectra of a polycrystalline sample of I show that this compound possesses the anti rotameric structure found in a previously reported X-ray diffraction study. The analysis of the spinning side-band manifold in the ~3C CPMAS spectrum of I allows us to assess a different semi-bridging character between two CO-groups not seen from the X-ray results. The spectral features of compound H are fully consistent with the X-ray and solution structures previously reported. In both compounds the cyclopentadienyl iigands are involved in fast reorientation motions which modulate the magnetic interactions responsible for the relaxation of t3C resonances. The activation energies (Ej) associated with this reorientation process of the Cp ring along their Cs coordination axis have been determined to be 15.5 and 10.2 kJ tool -~ for I and II respectively on the basis of mH Tt measurements at different temperatures. Furthermore, we show that an empirical relationship relates Ea values and Tmm (the temperature at which proton relaxation is more efficient) in a related series of cyclopentadienyl compounds.
INTRODUCTION
ALTHOUGH the number of papers dealing with the application of high resolution 13C CPMAS (cross polarization-magic angle spinning) NMR spectroscopy to organometallic compounds is large [1], there is still a need for a better understanding of the various features involved in the solid state spectra to fully exploit the powerful potential of such an approach. On going from solution to solid state 13C NMR spectra, the main differences commonly found involve: (i) a higher number of signals and (ii) longer longitudinal relaxation times. The former feature is dependent upon the site symmetry, which may be lower than the molecular symmetry, resulting in different signals. In solution, internal vibrations and collisions with the solvent molecules reduce the number of observed resonances to that expected on the basis of a given molecular symmetry [2]. The relaxation behaviour of the 13C resonances in the solid state are dependent upon the dynamic properties of the system under study, as in solution. However, while in solution we deal mainly with molecular tumbling (internal motions superimposed on the overall molecular motion may be important only in limited cases), in the solid state the modulation of the magnetic interactions is provided in most cases by local motions associated with highly mobile moieties such as methyls, aliphatic chains, cyclopentadienyls (Cp), etc. [2]. The ability of a Cp ring to reorient rapidly about its (25 coordination axis was recognized in several metallocenes by the changes detected in the second moment measurement of tH wide-line spectra at different temperatures [3]. An improvement was then brought about by the measurement of tH longitudinal relaxation time T~ at different temperatures [4, 7], which allows the exploitation of a larger temperature range than does second moment measurement. The extent of rotation of a Cp ring may be quite different from system to system and it is mainly determined by the strength of the * Author to whom correspondence should be addressed. s~A) 1:9-M
1307
1308
Fig. 1. Schematic representation
S.
A~MEet al.
of the structures of (C$H,),W,(CO),
(I) and (CSH5)2R~Z(C0)4 (II).
intermolecular van der Waals interactions in the crystal. It may happen that in the same molecule, crystallographically different Cp ligands rotate with quite different rates, as was shown in the case of the cis isomer of CpzFe,(CO), [S]. In this paper we report the results obtained from 13CCPMAS and ‘H and 13Cwide-line NMR spectra of two dinuclear Cp-containing compounds (I and II; Fig.l), which also allows us to draw some generalizations useful to gain a better understanding of the dynamic properties of related derivatives.
EXPERIMENTAL Compounds I and II were prepared by published methods [16,17] and their purities were checked by IR and ‘H NMR spectroscopy. High resolution solid state NMR spectra were recorded on a JEOL GSE spectrometer using a JEOL NM-GSH27MUIVT solid state unit, where the 13C nuclei resonate at 67.8 MHz under conditions of ‘H-13C cross polarization, high power proton decoupling and magic-angle spinning. Samples (typically lOO-200mg) were packed in zirconia rotors and spun at rates in the range of 3.5-4.5 kHz. Chemical shifts (CTscale, high frequency positive) were referenced to external neat liquid tetramethylsilane (TMS). 13Cspin-lattice relaxation times in solids were measured by Torchia’s method, with a proton 90” pulse of 4.5 ps, carbon 90” pulse of 4.5 ~s and contact time of 3 ms. T, was calculated by means of a two-parameter non-linear least-squares program by using at least 10 different z values. Wide-line proton longitudinal relaxation times T, were measured by the inversion recovery pulse sequence on a JEOL GSE 270 spectrometer operating at 270 MHz.
RESULTS AND DISCUSSION
In line with the observations made on the analogous chromium and molybdenum derivatives, it was reported that compound I occurs in solution as a mixture of anti and gauche rotamers. From proton band-shape analysis it was found that the activation energy for their interconversion is 15.2 kcal mol-’ [9]. The relative amounts of the two rotamers are dependent upon the dielectric constants of the solvent, as shown in a detailed IR study by ADAMS and COTTON [lo] on the analogous molybdenum derivative. In fact, the 13C NMR spectral pattern changes markedly on going from chloroform to DMSO solution (Fig. 2a, b). Since the IR spectra of I in these solvents appear identical to those of the molybdenum derivative, we conclude that the anti rotamer (Ia) is the
NMR spectra of metal carbonyls in the solid state
1309
unique species in chloroform solution, whereas in DMSO both isomers are present; however, the gauche rotamer (Ib) is predominant (Fig. 3). Bearing in mind the small differences induced in the 13Cchemical shifts by the different solvent polarity, we assign the r3C0 resonances of I in DMSO solution as follows: 222.1 and 214.6 ppm to Ia; 223.7, 216.2 and 215.5 ppm to Ib. The “C resonances of the cyclopentadienyl rings appear as separate signals at 91.5 and 91.3 ppm, respectively. Now the 13CCPMAS spectrum of I at room temperature (Fig. 2c) consists of three sharp 13C0 resonances at 223.6, 215.9 and 213.8ppm respectively in addition to the 13C Cp resonance at 91.9 ppm. An aid to assign the structure of I in the polycrystalline sample in the 13Cexperiment arises from the comparison of its IR spectrum recorded in CHC13 and DMSO (Fig. 4) solution. The absence of the band at 2002 cm-’ rules out the occurrence of the Ib form in the solid state, thus confirming the presence of structure Ia as previously found from X-ray structure determination. However, from the X-ray data, one could expect that, within the anti rotameric Is structure, two couples of carbonyl ligands have to show a similar degree of semi-bridging character. Actually, the analysis of the intensities of the
(a)
225
, . - *
220
9
215
I
*.
.
.
210
PPM
225
220
215 PPM
,“‘,“.,‘~.,...,...,...,...,...,...-,...,”.-..,..r
300280260240220
200180
160 140120
100 80
60
40
20
PPM Fig. 2. 13CNMR spectra of compound I: (a) solvent CDCf,; (b) solvent DMSO; (c) solid state CPMAS conditions ( n indicates the isotropic region).
1310
S. AWE et al. Tram
Cm rotamcr
Gauche
CP
C2
rotamcr
CP
CP Ia
Ib
Fig. 3. Schematic representation of the two rotamers for compound I.
spinning side-band manifold [ll] in the 13CCPMAS spectrum indicates that the signal at 223.6 ppm has a CSA (chemical shift anisotropy) value typical of the semi-bridging carbonyl, whereas the resonance at 215.9 ppm shows less pronounced semi-bridging character (Table 1). One may explain the apparent discrepancy in terms of higher sensitivity of the chemical shift tensorial components to assess minor structural differences between the two carbonyls involved in the semi-bridging interaction. On the other hand, we cannot exclude the possibility of polymorphs, i.e. the crystal selected for the diffraction study was different from the bulk of the sample. Compound II (Fig. 5) shows a single resonance for the Cp carbons at 93.4 ppm, a terminal CO resonance at 200.8 ppm and a bridging CO resonance at 250.1 ppm, in good agreement with the values found in the solution spectrum [12]. The chemical shift anisotropy (CSA) and the individual values of the tensor components support (Table 2) a highly symmetric bridging CO, which shows a very small CSA value for a metallocarbonyl 13Cresonance. The single Cp resonance found in the 13CCPMAS spectra of both complexes is an indication that the Cp ring is involved in a fast rearrangement process in the solid state. The nature of this motion is clearly understood from their wideline 13Cspectra: the 13CCp resonances show, in fact, a typical axial CSA pattern (Fig. 6a, b) which results from averaging out ull and 022tensor components. As previously shown cm
2000
-1 1900
Fig. 4. IR spectra of compound I in the carbonyl region: (a) solvent CHCIS (1953s. 1906s, br cm-‘); (b) solvent DMSO (2OOlm. 194&, 1896s, br cm-‘); (c) KBr pellet (1947s, 1915m, sh, 1876s cm-‘).
NMR spectra of metal carbonyls in the solid state
1311
Table 1. CSA, q and tensorial components values calculated for compound I ok0 213.8 215.9 223.6
011
022
033
CSA
q
360.8 365.1 391.9
360.2 334.1 294.4
-80.2 -51.5 - 15.5
440 401 358
0 0.12 0.41
in the case of permethylated ferrocene [13], the dynamic process responsible for the averaging of these components of the 13Cchemical shift tensor corresponds to a rotation of the Cp ring in the plane perpendicular to its C, coordination axis. In order to get a more quantitative picture of the dynamic process we measured the longitudinal relaxation times T, of the l’C resonances of I and II, This was conveniently done by using the pulse sequence suggested some years ago by TORCHIA [14]. The method makes use of the enhancement of the 13Cmagnetization (Mcr’) from the cross polarization sequence (to increase the signal-to-noise ratio) and follows its decay towards its equilibrium value according to: Mcp = 2Mop exp( - t/ TJ as in a normal inversion-recovery experiment. The T, relaxation times of the 13CCp resonances are found to be 0.6 and 18.7 s for I and II, respectively. In principle, one would expect that these relaxation times are determined chiefly by the ‘H- 13Cdipolar contribution and to a much lesser extent by the i3C chemical shift anisotropy term [2]. It is straightforward to note that the magnitude of these magnetic interactions have to be very similar for both complexes. It follows that the ratio ( - 34) between the observed 13Crelaxation rates simply reflect differences in their reorientational correlation times. The differences in the rotation rates of the Cp ligands in I and II may also qualitatively account for the longitudinal relaxation behaviour of the carbonyl resonances. Tl is about 24 f 3 s in I, whereas it appears to be exceedingly long in II (more than 100 s). A better insight into the relationship between relaxation and the reorientational process of the Cp ligands arose from the measurement of ‘H T, at different temperatures
1”‘1”‘,“‘1”‘l’~~~~l~~~i~~~l~~~l’~~l~~~l~~~l”’l~~‘l~~~~‘~~l~~~l’~‘l~“l~~~
420400380360340320300280260240220200180160140120100
80 60 40 20 0
PPM Fig. 5. ‘YI CPMAS spectrum at room temperature of compound II (0 and A indicate the isotropic bridging and terminal 13C0 resonances respectively).
1312
S.
AIMEet
al.
Table2.
CSA, q and tensorial component obtained for compound II
200.9 250.1
346.6 327.4
340.9 241.9
- 85.0 115
429 112
values
0.02 0.80
in the wide-line mode (Figs 7 and 8). The resulting profiles have been interpreted basis of the known expression of KUBO and TOMITA [ 151: l/T, = Ci[rc(l +0*+-l
on the
+ 42~(1+ 402s)-r]
(1) where C, is a constant which contains the proton second-moment parameter, and assuming that the correlation time r, follows a simple Arrhenius-type activation law: r,= r, exp[E,lRT&
(2)
Activation energy, E,, values of 15.5 and 10.2 kJ mol-’ have been obtained from a best fit procedure of the calculated and experimental data for I and II respectively. Furthermore, the observation of T, minima allows a direct evaluation of Ci and rc at these temperatures (oOrc = 0.62) (Table 3). We then compared r, values at 298 K for I and II and found that their ratio (25) approaches the ratio of the 13C relaxation rates previously measured in the high resolution experimental mode. The fast rotation of Cp ligands in both complexes at room temperature (rc = 24.6 X lo-” and 1.00 X lo-” s for I and II, respectively) assures that the extreme narrowing conditions (o,r,<< 1) are met and then, as we observed, the 13C relaxation rates are linearly proportional to the Cp reorientation time r,. The E,, values associated with the rotation of Cp rings in the solid state is the result of intra- and intermolecular contributions whose relative weight cannot be determined on the basis of the NMR experiments carried out in this work. However, being a magnetic interaction, Ci is quite similar to both compounds and a simple relationship must exist between E, and the temperature ‘I’minat which the proton relaxation rate is more
I”‘I”‘I”‘I’“I”‘I”‘l”‘I
2so
200
150
100
so
0
-so
-100
so
0
-so
-100
PPM
,“~,“‘,~“l~‘~,~“l~~~l~~~r
250
200
150
100
PPM Fig. 6. Wide-line room temperature
“C spectra of compounds
I (a) and I1 (b).
NMR spectra of metal carbonyls in the solid state
‘r,
/
1000/T Fig. 7. Profile of cyclopentadienyl
proton spin-lattice relaxation times (as log TJ vs l@/T for compound I.
Fig. 8. Profile of cyclopentadienyl
proton spin-lattice relaxation times (as log T,) vs 1dlT for compound II.
Table3.
I
II
Relaxation
parameters for molecular motions in crystalline 1 and II
5, (IO-l3 s)
C (108 s-*)
E. (M mol-‘)
TImin(“C)
4.56 1.64
7.3 12.65
15.58 10.18
- 10 -115
1313
1314
S. ANE et al.
L
I
100
1
I
200
150
I
250
T min
Fig. 9. Plot of E, versus Z’m, for some Cp-containing organometallic compounds; T,,,, values reported in this diagram are reported at the observation frequency of 6OMHz whenever the experimental frequency was used to measure the T, parameters in the original paper. 1, CpMn(CO)S[61; 2,CpWCO)~ PI;3, CpzFe 171;4, Cp,Ru (71;5, Cp2M02(COh [4]; 6, Cp,Mo& [5]; 7, Cp,RuZ(CO)., (thiswork); 8, CP,W~(CO)~ (this work); 9, cir-Fe&(CO), (81.
efficient. Actually, we extended this comparison to other Cp-containing organometallic compounds whose E, and Tminhad been previously determined from ‘H T, profiles [6]. Interestingly, we found that the equation E.JRTmti = 9 f 1 is valid for all the compounds reported in Fig. 9, provided that Tminare recalculated at the same observation frequency. This result may be particularly useful to treat systems where the Tl profile does not reach a minimum. In fact, from the E, value as determined from the slope of the straight line of log Tl vs 1000/T, z, and Ci may also be computed at this temperature [Eqn (2)]. Then by applying Eqn (I), z, may be evaluated at any temperature at which ‘H T, has been measured.
REFERENCES [l] [2] [3] [4] [5]
A. Yamasaki, Coord. Chem. Rev. 109, 107 (1991). C. A. Fyfe, Solid State NMRfor Chemists. C. F. C. Press, Ontario, Canada (1983). S. Anderson, J. Organomet. Chem. 71, 263 (1974). D. F. R. Gilson and G. Gomez, /. Organomet. Chem. 240, 41 (1982). I. S. Butler, P. J. Fitzpatrick, D. F. R. Gilson, G. Gomez and A. Shaver, Mofec. Cryst. Liq. Cryst. 71,213 (1981). [6] D. F. R. Gilson, G. Gomez, L. S. Butler and P. J. Fitzpatrick, Can. J. Chem. 61, 737 (1983). [7] A. J. Campbell, C. A. Fyfe, D. Harold-Smith and K. R. Jeffrey, Molec. Cryst. Liq. Cryst. 36, 1 (1976). [8] S. Aime, M. Botta, R. Gobetto and A. Orlandi, Magn. Res. Chem. 28, S52 (1990). [9] R. D. Adams, D. M. Collins and F. A. Cotton, Inorg. Chem. 13, 1086 (1974). IO] R. D. Adams and F. A. Cotton, Inorg. Chim. Acta 7, 153 (1973). II] J. Herzfeld and A. E. Berger, J. Chem. Phys. 73, 6021 (1980). 121 0. A. Gansow, A. R. Burtle and W. D. Vernon, J. Am. Chem. Sot. 15.5817 (1976). 131 D. E. Wemmer, D. J. Ruben and A. Pines, J. Am. Chem. Sot. 28.28 (1981). 141 D. A. Torchia, 1. Magn. Res. 30, 613 (1978). 151 R. Kubo and K. Tomita, J. Phys. Sot. 9,888 (1954). 161 E. 0. Fischer and A. Vogler, Z. Naturforschg. 17b, 421 (1962). 171 R. B. King, M. Z. Iqbal and A. D. King, J. Org. Chem. 171,53 (1979).
0584-8539/93$6.00+0.00 ~) 1993PergamonPressLtd
Printedin GreatBritain
IH and ]3C NMR study of cyclopentadienyi metal carbonyls in the solid state S. A I M E , * L . CORDERO a n d R . G O B E I T O Dipartimento di Chimica Inorganica Chimica Fisica e Chimica dei Materiali, Universit,~t di Torino, via P. Giuria 7, 10125, Torino, Italy
and G . SZALONTAI University of Veszprem, NMR Laboratory, H-8 200, Veszprem Pf 158, Hungary
(Received 28 April 1992; accepted 11 June 1992) Abstract--In this paper we deal with some structural and dynamic properties of Cp2W2(CO)6 (1) and Cp2Ru2(CO)4 (11) as shown by solid state 13C and ]H NMR experiments. The IR and 13C CPMAS spectra of a polycrystalline sample of I show that this compound possesses the anti rotameric structure found in a previously reported X-ray diffraction study. The analysis of the spinning side-band manifold in the ~3C CPMAS spectrum of I allows us to assess a different semi-bridging character between two CO-groups not seen from the X-ray results. The spectral features of compound H are fully consistent with the X-ray and solution structures previously reported. In both compounds the cyclopentadienyl iigands are involved in fast reorientation motions which modulate the magnetic interactions responsible for the relaxation of t3C resonances. The activation energies (Ej) associated with this reorientation process of the Cp ring along their Cs coordination axis have been determined to be 15.5 and 10.2 kJ tool -~ for I and II respectively on the basis of mH Tt measurements at different temperatures. Furthermore, we show that an empirical relationship relates Ea values and Tmm (the temperature at which proton relaxation is more efficient) in a related series of cyclopentadienyl compounds.
INTRODUCTION
ALTHOUGH the number of papers dealing with the application of high resolution 13C CPMAS (cross polarization-magic angle spinning) NMR spectroscopy to organometallic compounds is large [1], there is still a need for a better understanding of the various features involved in the solid state spectra to fully exploit the powerful potential of such an approach. On going from solution to solid state 13C NMR spectra, the main differences commonly found involve: (i) a higher number of signals and (ii) longer longitudinal relaxation times. The former feature is dependent upon the site symmetry, which may be lower than the molecular symmetry, resulting in different signals. In solution, internal vibrations and collisions with the solvent molecules reduce the number of observed resonances to that expected on the basis of a given molecular symmetry [2]. The relaxation behaviour of the 13C resonances in the solid state are dependent upon the dynamic properties of the system under study, as in solution. However, while in solution we deal mainly with molecular tumbling (internal motions superimposed on the overall molecular motion may be important only in limited cases), in the solid state the modulation of the magnetic interactions is provided in most cases by local motions associated with highly mobile moieties such as methyls, aliphatic chains, cyclopentadienyls (Cp), etc. [2]. The ability of a Cp ring to reorient rapidly about its (25 coordination axis was recognized in several metallocenes by the changes detected in the second moment measurement of tH wide-line spectra at different temperatures [3]. An improvement was then brought about by the measurement of tH longitudinal relaxation time T~ at different temperatures [4, 7], which allows the exploitation of a larger temperature range than does second moment measurement. The extent of rotation of a Cp ring may be quite different from system to system and it is mainly determined by the strength of the * Author to whom correspondence should be addressed. s~A) 1:9-M
1307
1308
Fig. 1. Schematic representation
S.
A~MEet al.
of the structures of (C$H,),W,(CO),
(I) and (CSH5)2R~Z(C0)4 (II).
intermolecular van der Waals interactions in the crystal. It may happen that in the same molecule, crystallographically different Cp ligands rotate with quite different rates, as was shown in the case of the cis isomer of CpzFe,(CO), [S]. In this paper we report the results obtained from 13CCPMAS and ‘H and 13Cwide-line NMR spectra of two dinuclear Cp-containing compounds (I and II; Fig.l), which also allows us to draw some generalizations useful to gain a better understanding of the dynamic properties of related derivatives.
EXPERIMENTAL Compounds I and II were prepared by published methods [16,17] and their purities were checked by IR and ‘H NMR spectroscopy. High resolution solid state NMR spectra were recorded on a JEOL GSE spectrometer using a JEOL NM-GSH27MUIVT solid state unit, where the 13C nuclei resonate at 67.8 MHz under conditions of ‘H-13C cross polarization, high power proton decoupling and magic-angle spinning. Samples (typically lOO-200mg) were packed in zirconia rotors and spun at rates in the range of 3.5-4.5 kHz. Chemical shifts (CTscale, high frequency positive) were referenced to external neat liquid tetramethylsilane (TMS). 13Cspin-lattice relaxation times in solids were measured by Torchia’s method, with a proton 90” pulse of 4.5 ps, carbon 90” pulse of 4.5 ~s and contact time of 3 ms. T, was calculated by means of a two-parameter non-linear least-squares program by using at least 10 different z values. Wide-line proton longitudinal relaxation times T, were measured by the inversion recovery pulse sequence on a JEOL GSE 270 spectrometer operating at 270 MHz.
RESULTS AND DISCUSSION
In line with the observations made on the analogous chromium and molybdenum derivatives, it was reported that compound I occurs in solution as a mixture of anti and gauche rotamers. From proton band-shape analysis it was found that the activation energy for their interconversion is 15.2 kcal mol-’ [9]. The relative amounts of the two rotamers are dependent upon the dielectric constants of the solvent, as shown in a detailed IR study by ADAMS and COTTON [lo] on the analogous molybdenum derivative. In fact, the 13C NMR spectral pattern changes markedly on going from chloroform to DMSO solution (Fig. 2a, b). Since the IR spectra of I in these solvents appear identical to those of the molybdenum derivative, we conclude that the anti rotamer (Ia) is the
NMR spectra of metal carbonyls in the solid state
1309
unique species in chloroform solution, whereas in DMSO both isomers are present; however, the gauche rotamer (Ib) is predominant (Fig. 3). Bearing in mind the small differences induced in the 13Cchemical shifts by the different solvent polarity, we assign the r3C0 resonances of I in DMSO solution as follows: 222.1 and 214.6 ppm to Ia; 223.7, 216.2 and 215.5 ppm to Ib. The “C resonances of the cyclopentadienyl rings appear as separate signals at 91.5 and 91.3 ppm, respectively. Now the 13CCPMAS spectrum of I at room temperature (Fig. 2c) consists of three sharp 13C0 resonances at 223.6, 215.9 and 213.8ppm respectively in addition to the 13C Cp resonance at 91.9 ppm. An aid to assign the structure of I in the polycrystalline sample in the 13Cexperiment arises from the comparison of its IR spectrum recorded in CHC13 and DMSO (Fig. 4) solution. The absence of the band at 2002 cm-’ rules out the occurrence of the Ib form in the solid state, thus confirming the presence of structure Ia as previously found from X-ray structure determination. However, from the X-ray data, one could expect that, within the anti rotameric Is structure, two couples of carbonyl ligands have to show a similar degree of semi-bridging character. Actually, the analysis of the intensities of the
(a)
225
, . - *
220
9
215
I
*.
.
.
210
PPM
225
220
215 PPM
,“‘,“.,‘~.,...,...,...,...,...,...-,...,”.-..,..r
300280260240220
200180
160 140120
100 80
60
40
20
PPM Fig. 2. 13CNMR spectra of compound I: (a) solvent CDCf,; (b) solvent DMSO; (c) solid state CPMAS conditions ( n indicates the isotropic region).
1310
S. AWE et al. Tram
Cm rotamcr
Gauche
CP
C2
rotamcr
CP
CP Ia
Ib
Fig. 3. Schematic representation of the two rotamers for compound I.
spinning side-band manifold [ll] in the 13CCPMAS spectrum indicates that the signal at 223.6 ppm has a CSA (chemical shift anisotropy) value typical of the semi-bridging carbonyl, whereas the resonance at 215.9 ppm shows less pronounced semi-bridging character (Table 1). One may explain the apparent discrepancy in terms of higher sensitivity of the chemical shift tensorial components to assess minor structural differences between the two carbonyls involved in the semi-bridging interaction. On the other hand, we cannot exclude the possibility of polymorphs, i.e. the crystal selected for the diffraction study was different from the bulk of the sample. Compound II (Fig. 5) shows a single resonance for the Cp carbons at 93.4 ppm, a terminal CO resonance at 200.8 ppm and a bridging CO resonance at 250.1 ppm, in good agreement with the values found in the solution spectrum [12]. The chemical shift anisotropy (CSA) and the individual values of the tensor components support (Table 2) a highly symmetric bridging CO, which shows a very small CSA value for a metallocarbonyl 13Cresonance. The single Cp resonance found in the 13CCPMAS spectra of both complexes is an indication that the Cp ring is involved in a fast rearrangement process in the solid state. The nature of this motion is clearly understood from their wideline 13Cspectra: the 13CCp resonances show, in fact, a typical axial CSA pattern (Fig. 6a, b) which results from averaging out ull and 022tensor components. As previously shown cm
2000
-1 1900
Fig. 4. IR spectra of compound I in the carbonyl region: (a) solvent CHCIS (1953s. 1906s, br cm-‘); (b) solvent DMSO (2OOlm. 194&, 1896s, br cm-‘); (c) KBr pellet (1947s, 1915m, sh, 1876s cm-‘).
NMR spectra of metal carbonyls in the solid state
1311
Table 1. CSA, q and tensorial components values calculated for compound I ok0 213.8 215.9 223.6
011
022
033
CSA
q
360.8 365.1 391.9
360.2 334.1 294.4
-80.2 -51.5 - 15.5
440 401 358
0 0.12 0.41
in the case of permethylated ferrocene [13], the dynamic process responsible for the averaging of these components of the 13Cchemical shift tensor corresponds to a rotation of the Cp ring in the plane perpendicular to its C, coordination axis. In order to get a more quantitative picture of the dynamic process we measured the longitudinal relaxation times T, of the l’C resonances of I and II, This was conveniently done by using the pulse sequence suggested some years ago by TORCHIA [14]. The method makes use of the enhancement of the 13Cmagnetization (Mcr’) from the cross polarization sequence (to increase the signal-to-noise ratio) and follows its decay towards its equilibrium value according to: Mcp = 2Mop exp( - t/ TJ as in a normal inversion-recovery experiment. The T, relaxation times of the 13CCp resonances are found to be 0.6 and 18.7 s for I and II, respectively. In principle, one would expect that these relaxation times are determined chiefly by the ‘H- 13Cdipolar contribution and to a much lesser extent by the i3C chemical shift anisotropy term [2]. It is straightforward to note that the magnitude of these magnetic interactions have to be very similar for both complexes. It follows that the ratio ( - 34) between the observed 13Crelaxation rates simply reflect differences in their reorientational correlation times. The differences in the rotation rates of the Cp ligands in I and II may also qualitatively account for the longitudinal relaxation behaviour of the carbonyl resonances. Tl is about 24 f 3 s in I, whereas it appears to be exceedingly long in II (more than 100 s). A better insight into the relationship between relaxation and the reorientational process of the Cp ligands arose from the measurement of ‘H T, at different temperatures
1”‘1”‘,“‘1”‘l’~~~~l~~~i~~~l~~~l’~~l~~~l~~~l”’l~~‘l~~~~‘~~l~~~l’~‘l~“l~~~
420400380360340320300280260240220200180160140120100
80 60 40 20 0
PPM Fig. 5. ‘YI CPMAS spectrum at room temperature of compound II (0 and A indicate the isotropic bridging and terminal 13C0 resonances respectively).
1312
S.
AIMEet
al.
Table2.
CSA, q and tensorial component obtained for compound II
200.9 250.1
346.6 327.4
340.9 241.9
- 85.0 115
429 112
values
0.02 0.80
in the wide-line mode (Figs 7 and 8). The resulting profiles have been interpreted basis of the known expression of KUBO and TOMITA [ 151: l/T, = Ci[rc(l +0*+-l
on the
+ 42~(1+ 402s)-r]
(1) where C, is a constant which contains the proton second-moment parameter, and assuming that the correlation time r, follows a simple Arrhenius-type activation law: r,= r, exp[E,lRT&
(2)
Activation energy, E,, values of 15.5 and 10.2 kJ mol-’ have been obtained from a best fit procedure of the calculated and experimental data for I and II respectively. Furthermore, the observation of T, minima allows a direct evaluation of Ci and rc at these temperatures (oOrc = 0.62) (Table 3). We then compared r, values at 298 K for I and II and found that their ratio (25) approaches the ratio of the 13C relaxation rates previously measured in the high resolution experimental mode. The fast rotation of Cp ligands in both complexes at room temperature (rc = 24.6 X lo-” and 1.00 X lo-” s for I and II, respectively) assures that the extreme narrowing conditions (o,r,<< 1) are met and then, as we observed, the 13C relaxation rates are linearly proportional to the Cp reorientation time r,. The E,, values associated with the rotation of Cp rings in the solid state is the result of intra- and intermolecular contributions whose relative weight cannot be determined on the basis of the NMR experiments carried out in this work. However, being a magnetic interaction, Ci is quite similar to both compounds and a simple relationship must exist between E, and the temperature ‘I’minat which the proton relaxation rate is more
I”‘I”‘I”‘I’“I”‘I”‘l”‘I
2so
200
150
100
so
0
-so
-100
so
0
-so
-100
PPM
,“~,“‘,~“l~‘~,~“l~~~l~~~r
250
200
150
100
PPM Fig. 6. Wide-line room temperature
“C spectra of compounds
I (a) and I1 (b).
NMR spectra of metal carbonyls in the solid state
‘r,
/
1000/T Fig. 7. Profile of cyclopentadienyl
proton spin-lattice relaxation times (as log TJ vs l@/T for compound I.
Fig. 8. Profile of cyclopentadienyl
proton spin-lattice relaxation times (as log T,) vs 1dlT for compound II.
Table3.
I
II
Relaxation
parameters for molecular motions in crystalline 1 and II
5, (IO-l3 s)
C (108 s-*)
E. (M mol-‘)
TImin(“C)
4.56 1.64
7.3 12.65
15.58 10.18
- 10 -115
1313
1314
S. ANE et al.
L
I
100
1
I
200
150
I
250
T min
Fig. 9. Plot of E, versus Z’m, for some Cp-containing organometallic compounds; T,,,, values reported in this diagram are reported at the observation frequency of 6OMHz whenever the experimental frequency was used to measure the T, parameters in the original paper. 1, CpMn(CO)S[61; 2,CpWCO)~ PI;3, CpzFe 171;4, Cp,Ru (71;5, Cp2M02(COh [4]; 6, Cp,Mo& [5]; 7, Cp,RuZ(CO)., (thiswork); 8, CP,W~(CO)~ (this work); 9, cir-Fe&(CO), (81.
efficient. Actually, we extended this comparison to other Cp-containing organometallic compounds whose E, and Tminhad been previously determined from ‘H T, profiles [6]. Interestingly, we found that the equation E.JRTmti = 9 f 1 is valid for all the compounds reported in Fig. 9, provided that Tminare recalculated at the same observation frequency. This result may be particularly useful to treat systems where the Tl profile does not reach a minimum. In fact, from the E, value as determined from the slope of the straight line of log Tl vs 1000/T, z, and Ci may also be computed at this temperature [Eqn (2)]. Then by applying Eqn (I), z, may be evaluated at any temperature at which ‘H T, has been measured.
REFERENCES [l] [2] [3] [4] [5]
A. Yamasaki, Coord. Chem. Rev. 109, 107 (1991). C. A. Fyfe, Solid State NMRfor Chemists. C. F. C. Press, Ontario, Canada (1983). S. Anderson, J. Organomet. Chem. 71, 263 (1974). D. F. R. Gilson and G. Gomez, /. Organomet. Chem. 240, 41 (1982). I. S. Butler, P. J. Fitzpatrick, D. F. R. Gilson, G. Gomez and A. Shaver, Mofec. Cryst. Liq. Cryst. 71,213 (1981). [6] D. F. R. Gilson, G. Gomez, L. S. Butler and P. J. Fitzpatrick, Can. J. Chem. 61, 737 (1983). [7] A. J. Campbell, C. A. Fyfe, D. Harold-Smith and K. R. Jeffrey, Molec. Cryst. Liq. Cryst. 36, 1 (1976). [8] S. Aime, M. Botta, R. Gobetto and A. Orlandi, Magn. Res. Chem. 28, S52 (1990). [9] R. D. Adams, D. M. Collins and F. A. Cotton, Inorg. Chem. 13, 1086 (1974). IO] R. D. Adams and F. A. Cotton, Inorg. Chim. Acta 7, 153 (1973). II] J. Herzfeld and A. E. Berger, J. Chem. Phys. 73, 6021 (1980). 121 0. A. Gansow, A. R. Burtle and W. D. Vernon, J. Am. Chem. Sot. 15.5817 (1976). 131 D. E. Wemmer, D. J. Ruben and A. Pines, J. Am. Chem. Sot. 28.28 (1981). 141 D. A. Torchia, 1. Magn. Res. 30, 613 (1978). 151 R. Kubo and K. Tomita, J. Phys. Sot. 9,888 (1954). 161 E. 0. Fischer and A. Vogler, Z. Naturforschg. 17b, 421 (1962). 171 R. B. King, M. Z. Iqbal and A. D. King, J. Org. Chem. 171,53 (1979).