3600
Organometallics 1995, 14, 3600-3602
lo3Rh NMR Shielding Correlation with Rate Data for Aryl Migration in RhCp*I(CO)(p-XCa): Mechanistic Implicationst Vanda Tedesco and Wolfgang von Philipsborn" Organisch-chemisches Institut, Universitat Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Received January 23, 1995@ Summary: The rhodium-103 NMR chemical shifts of the complexes RhCp*I[CWp,XCfid]PPh3 (Cp* = CSMes; X = H, Me, Cl, CHO, CN, N0.d exhibit a linear correlation (R2 = 0,991) with ln(k,J, where k,l is the relative rate constant for the PPhs-assisted migration of a coordinated aryl group to a coordinated CO in the complexes RhCp*I(CO)(p-XCdYdreported by Bassetti et al. This is the first example of a correlation of the transition-metal chemical shifts of the product with reaction rates and leads to the postulate that such reactions are characterized by a product-like (late) transition state. Rhodium-103 NMR spectroscopy has provided an elegant method for the study of reactivity and mechanism in stoichiometric2 and catalytic3reactions. Several papers have described successful attempts to correlate transition-metal NMR chemical shifts with various rate data,2t4catalytic a ~ t i v i t ysteric , ~ parameters," electronic parameter^,^ stability constants8 and phosphorus-31 NMR data.g Although there are many examples of rhodium-103 NMR data in the literature10-12there is surprisingly little information concerning complexes containing the pentamethylcyclopentadienyl ligand, complexes which are widely studied for their potential catalytic activity and C-H bond activation properties.13-15 The syntheses of RhCp*I[CO(p-XCsH4)1PPh3(3: Cp* = CsMes; X = H, Me, C1, CHO, CN, NO21 have been described,16J7and their synthetic route is shown in Figure l.16-19 The kinetics of the migration step16J7to + Transition Metal NMR Spectroscopy. 25. Part 24: reference 1. @Abstractpublished in Advance ACS Abstracts, J u n e 1, 1995. (1) Torocheshnikov, V.; Rentsch, D.; von Philipsborn, W. Magn. Reson. Chem. 1994,32,348. (2) Koller, M.; von Philipsborn, W. Organometallics 1992,11, 467. (3) Bender, B. R.; Koller, M.; Nanz, D.; von Philipsborn, W. J. Am. Chem. SOC.1993,115,5889. (4) DeShong, P.; Sidler, D. R.; Rybczynski, P. J.; Ogilvie, A. A.; von Philipsborn, W. J. Org. Chem. 1989,54, 5432. ( 5 ) Bonnemann, H.; Brijoux, W.; Brinkmann, R.; Meurers, W.; Mynott, R.; von Philipsborn, W.; Egolf, T. J. Organomet. Chem. 1984,
272,231. (6)Tavagnacco, C.; Balducci, G.; Costa, G.; Taschler, K.; von Philipsborn, W. Zhdv. Chim. Acta 1990,73,1469. (7) Graham, P. B.; Rausch, M. D.; Taschler, K.; von Philipsborn, W. Organometallics 1991,10,3049. (8)Akermark, B.; Blomberg, M. R. A.; Glaser, J.; Ohrstrom, L.; Wahlberg, S.; Warnmark, K.; Zetterberg, K. J.Am. Chem. Soc. 1994,
116,3405. (9)Ernsting, J. M.; Elsevier, C. J.; de Lange, W. G. J.;Timmer, K. Magn. Reson. Chem. 1991,29,5118. (10)Goodfellow, R. J . In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York, 1987. (11)Mann, B. E. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.; Academic Press: New York, 1983. (12) Mann, B. E. I n Transition Metal Nuclear Magnetic Resonance; Pregosin, P. S., Ed.; Elsevier: Amsterdam, The Netherlands, 1991. (13) Maitlis, P. M. Acc. Chem. Res. 1978,11, 301. (14) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989,22,91. (15)Periana, R. A.; Bergman, R. G. J.Am. Chem. Soc. 1986,108, 7332.
--Fk-*& 4YR MeMe /?\Me0
*-* MeCO 1
/ bcoR
J\PP% COR
2
3
Figure 1. Reaction scheme (for R see Table 1).
give complexes 3 have been reported; it was found that the mechanism was associative and solvent independent,16J7and it is also possible that ring slippage is not involved.20t21The following discussion will present the results of a rhodium-103 NMR study of the complexes shown in Figure 1 in order to gain further insight into the mechanism of the migration reaction. The lo3Rhresonances were measured by direct detection for complexes 2, with the exception of the methyl derivative, for which 2D inverse detection via the methyl protons was applied. The same 2D (lH, lo3Rh)experiment was used to detect the rhodium signals of complexes 1, whereas a triple-resonance 2D (31P,103Rh){1H} inverse pulse sequence was utilized for complexes 3. Figure 2 illustrates for RhCp*MeCO(p-CNCsH4)the successful use of 2J(Rh,H)for inverse detection of the lo3Rhresonance. Because of the large sensitivity enhancement factors, ( y $ y ~ h ) ~=/ 5689 ~ and ( y p / y ~ h )=~ / ~ 593, respectively, for the inverse versus direct detection experiments, they constitute the method of choice for organometallic substrates. Table 1 lists the rhodium-103 NMR chemical shifts for complexes 1-3. The starting material RhCp*Me2(MezSO)has a rhodium-103 chemical shift at -148 ppm. It was also found16J7that in complexes 2 when X = CN and NO2, substitution of CO for PPh3 occurs in competi(16) Bassetti, M.; Sunley, G. J.;Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1988,1012. (17) Bassetti, M.; Sunley, G. J.;Fanizzi, F. P.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1990, 1799. (18) de Miguel, A. V.; m m e z , M.; Isobe, K.; Taylor, B. F.; Mann, B. E.; Maitlis. P. M. Orpanometallics 1983.2 . 1724. ( 1 9 ) m m e z , M.; Gsenyi, J. M.; Sunley, G. J.; Maitlis, P. M. J . Organomet. Chem. 1986,296,197. (20)Bassetti, M.; Mannina, L.; Monti, D. Organometallics 1994,13, 3293. (21) Monti, D.;Bassetti, M. J. Am. Chem. SOC.1993,115,4658.
0276-733319512314-3600$09.0OlO 0 1995 American Chemical Society
Organometallics, Vol. 14, No. 7, 1995 3601
Notes
M
&'W
'
0.;5
I
0.50 ppm
Figure 3. Plot of ln(kredversus Wo3Rh)for complexes 3.
Figure 2. 2D('H,lo3Rh)NMR spectrum of RhCp*MeCO@-CNC6H4)('J(Rh,H) = 2.5 Hz). Table 1. Rhodium-103N M R Chemical Shifts of the Complexes Shown in Figure 1 and Relative Rates for the Reaction 2 3
-
6P03Rh)(ppm) R Me p-MeC&
Ph p-ClCsH4 p-CHOC& p-CNCsH4 ~-NO~CL&
1 -829 -584 -578 -582 -566 -567 -565
2
-366 -106 -102 -122 -121 -127 -134
3 849 813 804 784 765 748 742
*
&(103Rh)[ppm] for complexes 3
relative ratea k,l 0.26 2.8 1 0.31 0.034 0.008 0.007
+
PPh3
-
X 2
Reference 17.
tion with aryl migration to give RhCp*I(PPh3)(pCNCsH4) (d(lo3Rh) 505 ppm) and RhCp*I(PPh3)(pN02CsH4) (d(lo3Rh) 507 ppm), respectively, together with the expected complexes 3. The rhodium nuclei in complexes 1 are, in general, deshielded when the substituent in the para position of the phenyl group is electron withdrawing (X = C1, CHO, CN, NO2) and shielded when the substituent is electron donating (X = Me). The shielding effects for complexes 2 are reverse to those observed for complexes 1; that is, electron-withdrawing substituents on the phenyl ring shield the rhodium nucleus. The difference between complexes 1 and 2 is that an electron-donating Me group has been substituted for an electron-withdrawing iodide ligand. The presence of the soft iodide ligand could cause some cooperative effect with the donor/acceptor properties of the X substituent in the aryl group that causes the shielding/deshieldingeffect on the rhodium nucleus to invert. It was found16J7that the rate of migration (Table 1) for complexes 2 was significantly slowed for those compounds containing electron-withdrawing substituents in the para position of the aryl group. The rates decrease with increasing shielding of the rhodium nucleus as for the COPPh3 displacement reaction in ($C 5 1 ) R h ( C O hcomplexes;2however, the range of chemical shifts is small (Ad = 32 ppm) and the correlation between the two parameters ln(krel)and d(Rh) is insignificant (R2= 0.813). In complexes 3, electron-withdrawing substituents also shield the rhodium nucleus, while electron-donating ones deshield it. A plot of ln(k,l) versus d(103Rh)(Figure 3) for the aryl derivatives is linear with a high correlation coefficient; R2 = 0.991, This is the first example of
3
Figure 4. Proposed mechanism for the PPh-assisted aryl migration reaction of 2 to give 3. a correlation between transition-metal chemical shifts of the product with rate data. A plot of up (Hammett electronic parameter22) against d('03Rh) is also linear with the correlation coefficient R2 = 0.991. A suggested mechanism for the PPha-assisted aryl migration reaction 2 3 is shown in Figure 4. It is most likely that aryl migration to CO occurs rather than CO insertion into the Rh-C(ary1) bond, as this would require the breaking of two Rh-C o-bonds. There are many examples of CO insertion23into a metal-methyl bond, but these normally occur when CO is the incoming ligand. When both the CO and Me are coordinated to the metal center, then the migration mechanism is favored. It was suggested" that the mechanism of the aryl migration 2 3 is associative and that the transition state [RhCp*(aryl)(CO)(PPh3)1]* is involved. It is also possible that a molecular complex of the type [RhCp*(aryl)(CO)I,PPhal is initially formed in a rapid preequilibrium before the rate-determining aryl migration similar to that proposed in the reaction of Fe($-
-
23924
-
(22)Hansch, C.;Leo, A.; Taft, R. W. Chem. Rev. 1991,91, 165. (23)Collman, J. P.;Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotmnsition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (24)Koga, N.; Morokuma, K. J. Am. Chem. Soc. 1988,108,6136.
3602 Organometallics, Vol. 14, No. 7, 1995
Notes
indenyl)I(CO)Me (indenyl is CgH,) with PR3,20,21 but there is no evidence for this. A proposed transition state is shown in Figure 4. The linear relationship between ln&d and the rhodium103 NMR chemical shifts of the product rather than of the reactant leads to the postulate that the aryl migration proceeds via a late transition state, which is structurally more similar t o the product, complex 3, than the reactant, complex 2. This implies that in the transition state the C(aryl)-C(O) and Rh-PPh bonds are already formed to an appreciable extent with simultaneous breaking of the Rh-C(ary1) bond. Finally, complete breakage of the Rh-C(ary1) bond and formation of the Rh-P bond gives the final product, complex 3.
20 mg of sample dissolved in 0.5 cm3 of CDCl3 in 5 mm NMR tubes was used, while for phosphorus observation 30-40 mg of sample under the same conditions was employed. An inverse broad-band probehead was employed for the 2D ('H,lo3Rh)experiments. For the 2D (31P,103Rh){ lH) experiments on the AM400, a triple-resonance probehead (outer coil tunable to a low-frequencynucleus (15Nto la70s)and an inner coil tunable to lock, proton, and phosphorus) was utilized together with a second PTS160 synthesizer and a BSV3 hetereodecoupling unit for pulsing of the rhodium frequency. Coupling constants of the orders 2J(Rh,H) = 2.5 Hz and lJ(Rh,P) = 160 Hz were used for the polarization transfer. Typically 128 increments, each of 4 (proton) or 8 (phosphorus) scans, were accumulated in 1-2 h spectral width in the rhodium dimension 2000-5000 Hz, spectral width for proton or phosphorus 500-1000 Hz, 90" pulse length for rhodium 125 ps (-600) or 40 ps (AM400),90"pulse length for proton 11 ps, 90" pulse length for phosphorus 14ps, and relaxation delay Experimental Details 3.5 s. Before Fourier transformation, the data were zero-filled All compounds were prepared as previously d e s ~ r i b e d . ~ ~ - ~in ~ *the ~ rhodium dimension and multiplied by sine window Direct detection of the rhodium-103 resonances was perfunctions in both dimensions. formed on a Bruker -600 NMR spectrometer operating at All samples were measured at 300 K. Rhodium chemical 18.9 MHz (Bo = 14.1 T), without proton decoupling, using 20shifts are reported relative to a reference frequency of 3.16 30 mg of sample dissolved in 0.5 cm3of CDC13 in 5 mm NMR MHz in a field where the protons of TMS resonate with exactly tubes. Typically 1000-2000 scans were accumulated in 1-2 100 MHz.l0 All shifts to high frequency are positive. The h using a spectral width of 5000 Hz, a pulse width of 20 ps estimated error in the chemical shifts is &1ppm. (correspondingto a flip angle of about 607, an acquisition time of 1.4 s, and a relaxation delay of 2.5 s. Inverse 2D NMR experiments, (*H,lo3Rh)and (31P,103Rh)Acknowledgment. We thank the Swiss National { lH}, were carried out on a Bruker -600 or AM400 NMR Science Foundation for financial support. spectrometer operating at 18.9 MHz (BO= 14.1TI or 12.6 MHz OM950055N (Bo = 9.4T), respectively, using the pulse sequences reported by Bax et and Benn et al.27 For proton observation about (26) Bax, A.; Griffey, R. H.; Hawkins, B. L. J. Magn. Reson. 1983,
(25)White, C.; Yates, A.; Maitlie, P. M.; Heinekey, D. M. Znorg. Synth. 1992,29, 228.
55, 301.
(27) Benn, R.; Brevard, C. J. Am. Chem. SOC.1986,108,5622.
