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N,N,2,4,6-Pentamethylanilinetricarbonylchromium (0): an interesting case of restricted rotation in the arenetricarbonylchromium series
ELSEVIER
Inorganica Chimica Acta 244 (1996) 265-267
Note
N,N,2,4,6-Pentamethylanilinetricarbonylchromium(0)" an interesting case of restricted rotation in the arenetricarbonylchromium series J. Hamilton, C.A.L. Mahaffy *, J. Rawlings Department of Chemistry, Auburn Universityat Montgomery, Montgomery, AL 36117-3598, USA Received 2 May 1995; revised 11 July 1995
Abstract
Restricted rotation of the dialkylamino groups in N,N,2,4,6-pentamethylanilinetricarbonylchromium(0) (1) and N,N-diethyl-2,4,6-trimethylanilinetricarbonylchromium(0) (2) has been investigated using variable temperature 1H and ~3C NMR. Below 266 K the ~H NMR spectrum of 1 shows a sharp doublet for the signal of the NMe2 group which gradually broadens as the temperature is increased. Above room temperature the signal width decreases, becoming a sharp singlet at > 310 K. Analogous changes are seen for the NEt2 group in 2. The barrier to rotation of the NMe2 group in 1 was found to be 13.7 kcal mol- 1. Keywords: Chromium complexes; Carbonyl complexes; Aniline complexes
1. Introduction
2. Results and discussion
The best known examples of restricted rotation in arenetricarbonylchromium complexes involve slowed tripodal rotation of the tricarbonylchromium group. This phenomenon has recently been reviewed [ 1 ]. In addition to the above conformational preferences, restricted rotation of a substituent has been observed in a number of arenetricarbonylchromium complexes including those with alkyl [2], alkoxy [3], benzoyl [2a], benzyl [4] and silyl [5] groups. We would now like to report an interesting example of restricted rotation of dialkylamino groups attached to 2,6-substituted arenetricarbonylchromium complexes. The dialkylamino group has been seen to undergo restricted rotation in a number of other systems [ 5 ].
Fig. 1 shows the variable temperature ~H NMR spectrum of N,N,2,4,6-pentamethylanilinetricarbonylchromium(0) (1). At temperatures below 266 K the signal for the NMe2 group is a sharp doublet, which as the temperature is increased, gradually broadens. Above room temperature this signal again sharpens, becoming a sharp singlet. In the L3C NMR spectrum at room temperature (Fig. 2) there are no observed signals for this group, but at low temperature, two clearly resolved signals are seen (Fig. 3) Analysis of the temperature dependence of the ~H NMR spectrum of 1 clearly shows the coalescence of the two signals for this group at 282 K, corresponding to a free energy of activation of 13.7 kcal mol -t. This value is comparable to other values for restricted rotation in the NMe2 group in the literature [5a,b]. We have also examined the room temperature ~H and 13C NMR spectra of N,N-diethyl-2,4,6-trimethylanilinetricarbonylchromium(0) (2) and found that the signals for the methylene protons of the ethyl group show the doubling that is expected if rotation is restricted. The methylene quartets are centered at 8 3.1 (50.9 Hz separation of diastereomeric resonances). Broadened triplets for the methyl signals centered at 81.1 (74.3 Hz separation) are also found. Likewise the ~3C NMR spectrum shows the analogous doubling. The methylene signals occur at 6 52.4 and 45.8 (673 Hz separation) and the methyl signals of the N-ethyl groups occur at 8 15.2 and 14.1 ( 110 Hz separation) respec-
Me
Me~
NR2
I Mo Cr(CO)3 1 R=Me 2 R=Et
* Corresponding author. 0020-1693/96/$15.00 © 1996 Elsevier Science S,A. All rights reserved
SSD10020- 1 6 9 3 ( 9 5 ) 0 4 7 6 8 - 5
266
J. Hamilton et al. / Inorganica Chimica Acta 244 (1996) 265-267
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J~
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~ L. . . .
.
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Fig. 1. Variable temperature 400 MHz tH NMR spectrum of N,N,2,4,6-pentametbylanilinetricarbonylchromium showing the splitting of the signal for the NMe2 groups just below room temperature.
2'~0 . . . . . . 200 . . . . . . . i60 . . . . . . . . 120. . . . .
80. . . . . . . . . 4'0. . . . . . . . . . . 0 ~ PPM
Fig. 3. 100 MHz 13C NMR spectrum of I at 232 K.
240
200
160
120
80
40
0 PPM
1H and 13C NMR spectra were recorded in CDC13 by Spectral Data Services using 360 and 100 MHz instruments, respectively. The arenes used to prepare complexes 2-4 were prepared by standard techniques.
Fig. 2. 100 MHz 13C NMR spectrum of 1 at room temperature.
tively. At the highest possible experimental temperature (323 K) coalescence of these signals was not observed. The ~H and ~3C NMR spectra for the tricarbonylchromium complexes of both N-ethyl-2,4,6-trimethylaniline (3) and N-methyl-2,6-diethylaniline (4) were examined down to 230 K and showed no doubling of any signal. It therefore appears necessary to have a dialkylamino system before this effect is seen.
Acknowledgements We thank Dr James T. Kenny and the Auburn University at Montgomery Grant-in-Aid program, together with the Chemistry Department for support.
References 3. Experimental The preparation of 1 has been previously reported [6]. Other complexes were synthesized using the same method.
[ 1] M.J. McGlinchey, Adv. Organomet. Chem., 34 (1992) 285. [2] (a) W.S. Trahanovsky, D.J. Kowalski and M.J. Avery, J. Am. Chem. Soc., 96 (1974) 1502; (b) D.J. Iverson and K. Mislow, Organometallics, 1 (1982) 3.
J. Hamilton et al. / Inorganica Chimica Acta 244 (1996) 265-267 [3] (a) E.M. Campi, B.M.K: Gatehouse, W.R. Jackson, I.D. Rae and M.G. Wong, Can. Z Chem., 62 (1984) 2566; (b) E.M. Campi, B.M.K. Gatehouse, W.R. Jackson, I.D. Rae and M.G. Wong, J. Chem. Soc., Chem. Commun., (1984) 175. [4] (a) S. Top, G. Jaouen, DLG. Sayer and M.J. McGlinchey, J. Am. Chem. Soc., 105 (1983) 6426; (b) I.I. Schuster, W. Weissensteiner and K. Mislow, J. Am. Chem. Soc., 108 (1986) 6661. [5] (a) L. Lunazzi, D. Maceiantelli, D. Spinelli and G. Consiglio, J. Org. Chem., 47 (1982) 3759; (b) T.B. Grindley, A.R. Katritzky and R.D. Topsom, J. Chem. Soc., Perkin Trans. 2, (1974) 289; (c) J.M. Bernassau, M. Fetizon and E.R. Maia, J. Phys. Chem., 90 (1986) 6129; (d) F.S. Kamounah, I.S. A1-Sheibani, N.L. Shibaldain and S.R. Salman, Magn. Reson. Chem., 23 (1985) 521; (e) T.H. Siddall and C.A. Prohaska, J. Am. Chem. Soc., 88 (1966) 1172; (f) T. Yokoyama, I. Hanazome, M. Mishima, M.J. Kamlet and R.W. Taft, J. Org. Chem.,
267
52 (1987) 163; (g) G. Bott, L.D. Field and S. Sternhell, J. Am. Chem. Soc., 102 (1980) 5618; (h) Y. Shvo, E.C. Taylor, K. Mislow and M. Raban, J. Am. Chem. Soc,, 89 (1967) 4910; (i) G. Gilli, V. Bertolasi, F. Bellucci and V. Ferretti, J. Am. Chem. Soc., 108 (1986) 2420; (j) J. von Jouanne and J. Heidberg, J. Am. Chem. Soc., 95 (1973) 487; (k) B.M. Fung, R.V. Sigh and M.M. Alcock, Z Am. Chem. Soc., 106 (1984) 7301; (1) M.D. Wunderlich, L.K. Leung, J.A. Sandberg, K.D. Meyer and C.H. Yoder, J. Am. Chem. Soc., 100 (1978) 1500; (m) A.F. Lindmark and R.C. Fay, lnorg. Chem., 22 (1983) 2000; (n) W. Rettig and R. Gleiter, J. Phys. Chem., 89 ( 1985 ) 4676; (o) W.C. Harris, L.B. Knight, Jr., R.W. MacNamee and J.R. Durig, lnorg. Chem., 13 (1974) 2297; (p) A. Dondoni, L. Lunazzi, P. Giorgianni and D. MaccianteUi, J. Org. Chem., 46 (1981) 4813. [61 C.A.L. Mahaffy and J. Hamilton, Synth. React. Inorg. Met.-Org. Chem., 17 (1987) 849.
Inorganica Chimica Acta 244 (1996) 265-267
Note
N,N,2,4,6-Pentamethylanilinetricarbonylchromium(0)" an interesting case of restricted rotation in the arenetricarbonylchromium series J. Hamilton, C.A.L. Mahaffy *, J. Rawlings Department of Chemistry, Auburn Universityat Montgomery, Montgomery, AL 36117-3598, USA Received 2 May 1995; revised 11 July 1995
Abstract
Restricted rotation of the dialkylamino groups in N,N,2,4,6-pentamethylanilinetricarbonylchromium(0) (1) and N,N-diethyl-2,4,6-trimethylanilinetricarbonylchromium(0) (2) has been investigated using variable temperature 1H and ~3C NMR. Below 266 K the ~H NMR spectrum of 1 shows a sharp doublet for the signal of the NMe2 group which gradually broadens as the temperature is increased. Above room temperature the signal width decreases, becoming a sharp singlet at > 310 K. Analogous changes are seen for the NEt2 group in 2. The barrier to rotation of the NMe2 group in 1 was found to be 13.7 kcal mol- 1. Keywords: Chromium complexes; Carbonyl complexes; Aniline complexes
1. Introduction
2. Results and discussion
The best known examples of restricted rotation in arenetricarbonylchromium complexes involve slowed tripodal rotation of the tricarbonylchromium group. This phenomenon has recently been reviewed [ 1 ]. In addition to the above conformational preferences, restricted rotation of a substituent has been observed in a number of arenetricarbonylchromium complexes including those with alkyl [2], alkoxy [3], benzoyl [2a], benzyl [4] and silyl [5] groups. We would now like to report an interesting example of restricted rotation of dialkylamino groups attached to 2,6-substituted arenetricarbonylchromium complexes. The dialkylamino group has been seen to undergo restricted rotation in a number of other systems [ 5 ].
Fig. 1 shows the variable temperature ~H NMR spectrum of N,N,2,4,6-pentamethylanilinetricarbonylchromium(0) (1). At temperatures below 266 K the signal for the NMe2 group is a sharp doublet, which as the temperature is increased, gradually broadens. Above room temperature this signal again sharpens, becoming a sharp singlet. In the L3C NMR spectrum at room temperature (Fig. 2) there are no observed signals for this group, but at low temperature, two clearly resolved signals are seen (Fig. 3) Analysis of the temperature dependence of the ~H NMR spectrum of 1 clearly shows the coalescence of the two signals for this group at 282 K, corresponding to a free energy of activation of 13.7 kcal mol -t. This value is comparable to other values for restricted rotation in the NMe2 group in the literature [5a,b]. We have also examined the room temperature ~H and 13C NMR spectra of N,N-diethyl-2,4,6-trimethylanilinetricarbonylchromium(0) (2) and found that the signals for the methylene protons of the ethyl group show the doubling that is expected if rotation is restricted. The methylene quartets are centered at 8 3.1 (50.9 Hz separation of diastereomeric resonances). Broadened triplets for the methyl signals centered at 81.1 (74.3 Hz separation) are also found. Likewise the ~3C NMR spectrum shows the analogous doubling. The methylene signals occur at 6 52.4 and 45.8 (673 Hz separation) and the methyl signals of the N-ethyl groups occur at 8 15.2 and 14.1 ( 110 Hz separation) respec-
Me
Me~
NR2
I Mo Cr(CO)3 1 R=Me 2 R=Et
* Corresponding author. 0020-1693/96/$15.00 © 1996 Elsevier Science S,A. All rights reserved
SSD10020- 1 6 9 3 ( 9 5 ) 0 4 7 6 8 - 5
266
J. Hamilton et al. / Inorganica Chimica Acta 244 (1996) 265-267
....
-I -I --t
,-I--ql-it -H-I-I :° -i[
/ J[
/lie
J~
JvL__J'L____ £0 £s 2~o
do
~ L. . . .
.
g_at3K
1.,~
2la~K
o;o
PPM
Fig. 1. Variable temperature 400 MHz tH NMR spectrum of N,N,2,4,6-pentametbylanilinetricarbonylchromium showing the splitting of the signal for the NMe2 groups just below room temperature.
2'~0 . . . . . . 200 . . . . . . . i60 . . . . . . . . 120. . . . .
80. . . . . . . . . 4'0. . . . . . . . . . . 0 ~ PPM
Fig. 3. 100 MHz 13C NMR spectrum of I at 232 K.
240
200
160
120
80
40
0 PPM
1H and 13C NMR spectra were recorded in CDC13 by Spectral Data Services using 360 and 100 MHz instruments, respectively. The arenes used to prepare complexes 2-4 were prepared by standard techniques.
Fig. 2. 100 MHz 13C NMR spectrum of 1 at room temperature.
tively. At the highest possible experimental temperature (323 K) coalescence of these signals was not observed. The ~H and ~3C NMR spectra for the tricarbonylchromium complexes of both N-ethyl-2,4,6-trimethylaniline (3) and N-methyl-2,6-diethylaniline (4) were examined down to 230 K and showed no doubling of any signal. It therefore appears necessary to have a dialkylamino system before this effect is seen.
Acknowledgements We thank Dr James T. Kenny and the Auburn University at Montgomery Grant-in-Aid program, together with the Chemistry Department for support.
References 3. Experimental The preparation of 1 has been previously reported [6]. Other complexes were synthesized using the same method.
[ 1] M.J. McGlinchey, Adv. Organomet. Chem., 34 (1992) 285. [2] (a) W.S. Trahanovsky, D.J. Kowalski and M.J. Avery, J. Am. Chem. Soc., 96 (1974) 1502; (b) D.J. Iverson and K. Mislow, Organometallics, 1 (1982) 3.
J. Hamilton et al. / Inorganica Chimica Acta 244 (1996) 265-267 [3] (a) E.M. Campi, B.M.K: Gatehouse, W.R. Jackson, I.D. Rae and M.G. Wong, Can. Z Chem., 62 (1984) 2566; (b) E.M. Campi, B.M.K. Gatehouse, W.R. Jackson, I.D. Rae and M.G. Wong, J. Chem. Soc., Chem. Commun., (1984) 175. [4] (a) S. Top, G. Jaouen, DLG. Sayer and M.J. McGlinchey, J. Am. Chem. Soc., 105 (1983) 6426; (b) I.I. Schuster, W. Weissensteiner and K. Mislow, J. Am. Chem. Soc., 108 (1986) 6661. [5] (a) L. Lunazzi, D. Maceiantelli, D. Spinelli and G. Consiglio, J. Org. Chem., 47 (1982) 3759; (b) T.B. Grindley, A.R. Katritzky and R.D. Topsom, J. Chem. Soc., Perkin Trans. 2, (1974) 289; (c) J.M. Bernassau, M. Fetizon and E.R. Maia, J. Phys. Chem., 90 (1986) 6129; (d) F.S. Kamounah, I.S. A1-Sheibani, N.L. Shibaldain and S.R. Salman, Magn. Reson. Chem., 23 (1985) 521; (e) T.H. Siddall and C.A. Prohaska, J. Am. Chem. Soc., 88 (1966) 1172; (f) T. Yokoyama, I. Hanazome, M. Mishima, M.J. Kamlet and R.W. Taft, J. Org. Chem.,
267
52 (1987) 163; (g) G. Bott, L.D. Field and S. Sternhell, J. Am. Chem. Soc., 102 (1980) 5618; (h) Y. Shvo, E.C. Taylor, K. Mislow and M. Raban, J. Am. Chem. Soc,, 89 (1967) 4910; (i) G. Gilli, V. Bertolasi, F. Bellucci and V. Ferretti, J. Am. Chem. Soc., 108 (1986) 2420; (j) J. von Jouanne and J. Heidberg, J. Am. Chem. Soc., 95 (1973) 487; (k) B.M. Fung, R.V. Sigh and M.M. Alcock, Z Am. Chem. Soc., 106 (1984) 7301; (1) M.D. Wunderlich, L.K. Leung, J.A. Sandberg, K.D. Meyer and C.H. Yoder, J. Am. Chem. Soc., 100 (1978) 1500; (m) A.F. Lindmark and R.C. Fay, lnorg. Chem., 22 (1983) 2000; (n) W. Rettig and R. Gleiter, J. Phys. Chem., 89 ( 1985 ) 4676; (o) W.C. Harris, L.B. Knight, Jr., R.W. MacNamee and J.R. Durig, lnorg. Chem., 13 (1974) 2297; (p) A. Dondoni, L. Lunazzi, P. Giorgianni and D. MaccianteUi, J. Org. Chem., 46 (1981) 4813. [61 C.A.L. Mahaffy and J. Hamilton, Synth. React. Inorg. Met.-Org. Chem., 17 (1987) 849.