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Problem R-12N. Analyze the 1H decoupled 32.4 MHz 31P NMR spectra of a palladium-phosphine complex shown on the next page (Bartsch, R.; Carmichael, D.; Hitchcock, P. B.; Meidine, M. F.; Nixon, J. F.; Sillett, G. J. D. J. Chem. Soc, Chem. Commun. 1988, 1615).
(a) Identify all signals in the low temperature spectrum (-75 °C), and report approximate coupling constants
using the form: 8 _, xJi_2 -_Hz. Use the numberings shown on the structure. For each signal briefly give
your reasoning for the assignment.
This is the P closest to the two PEt3 groups, so expect triplet splitting.       Me3C 2 P4Et
The dtd (J = 502, 49, 22 Hz) at 8-17 is the only signal that shows a *\r>^* '
p3cy i
triplet, so this must be P1, which should also be coupled to both P2 And I 3O.p    PP c'
P3, as observed. ^( p5F
P3Et,
8 -17, 1JP1„p2 = 502 Hz, 2JP1„p4/5 = 49 Hz, 2JP1_P3 = 20 Hz Me3C
P2 should also show the large 1J to P1, so it must be the dd (J = 500, 45 Hz) at 8 18. P1 and P2 form an ABXY2 system, would need to do an AB quartet calculation to get accurate chemical shifts
C22H28CIP5Pd
8 18, 1JP1_P2 = 500 Hz, 2JP1_P3 = 40 Hz
P3 is coupled to both P1 and P2 (dd, J = 46, 25 Hz), so it has to be the 8 112 signal 8 112, 2JP3_p2 = 40 Hz, 2JP3_P1 = 20 Hz
This is the signal with double area at 8 -122, d, J = 48 Hz p4 p5    8 -122, %m_P1 = 46 Hz
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(b) Identify the process which is responsible for the changes in the NMR spectrum at the higher temperatures (-30 °C and +50 °C). The signal at -122 ppm in the +50 °C spectrum is a triplet. Draw a structure or an equation.
The Pd migrates back and forth between P1 and P2, so their chemical shifts are averaged, and both P3 and P4/P5 become triplets, equally coupled to both. Since the two coupling constants are fairly close (expect the P4/P5 coupling to be (49+0)/2 = 25, and P3 coupling to be (40+20)/2 = 30 Hz in size, the pVp2 signal becomes an approximate quartet.
The exchange is intramolecular, since the coupling between pVp2 and P4/P5 is maintained in the high temperature spectrum
Me3C Me3C
\ \ Pd(PEt3)2CI
/>p _ /Xp
Pd(PEt3)2CI Me3C Me3C
(c) What is the proton frequency (MHz) of the spectrometer which was used for these spectra?
32.4x(100/40.49) = 80 MHz
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Problem R-12N. 32.4 MHz ^P-^H} NMR spectra Solvent toluene-d
(Source: Chem. Commun. 1988, 1615)
+50 °C
ppm