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James Lindley Lanchester Polytechnic Coventry, England
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Allyl Palladium Complexes A simple experiment illustrating fluxional behavior
A recent paper in this Journal describes an experiment involving an nmr investigation of the methallylpalladium (11) chloride-triphenylphosphine system ( I ) . A mechanistically similar but experimentally simpler system is allylpalladium (11) chloride-dimethvlsul~hoxide(DMSO), the main advantages are the avoidance of the use of CO, thereby removing the necessity for large areas of hood space, and of the need to engage laboratory-staff in the preparation of starting materials. Di-p-chlorobis(n3-allyl)dipalladium (11) is prepared by a literature method (2),and its 'H NMR spectrum recorded in CDCb and ds-DMSO. The spectrum in CDC13 is of the AMzXz type, (Fig. l a ) which is typical of a +allyl group, the low field multiplet (intensity 1)being assigned to proton H,, the doublet at 4.1 ppm (intensity 2) is assigned to protons H, on the basis of the coupling constant J, = 6.6 Hz which is typical of cis olefinic protons (3),and the doublet at 3.0 ppm being assigned to protons H. (coupling constant J,, = 12 Hz typical of trans protons). The spectrum in DMSO is of the AX4 type, which is typical of a "dynamic" o-allyl (4, 5), showing a quintet at 5.7 ppm (intensity 1) and a doublet at 3.7 ppm (intensity 4) which is assigned to the protons of the now equivalent CH2 groups. This doublet is quite broad in the spectrum recorded at ambient temperature (Fig. lb); however, as the temperature is raised the signal narrows (Fig. lc). These changes in the spectra nicely illustrate the structural chanees that are to be exnected when chaneine" from a n o n coorchating solvent ( ~ ~ 6to1a strongly ~ ) basic coordinating solvent (DMSO) (See Fig. 2). In chloroform the dimeric 73-allyl species (I) is present. However, with the more basic DMSO solvent the haloeen bridee of (I) . . is split . eivine monomer (11) in which the 4-allyl structure is present. Further
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interaction with solvent causes one end of the allyl group to dissociate giving the +allyl species (111). Free rotation about the C-C sinele bond in (111) eives conformer (IV) . and suhsequent lossofsolvent from (IV)gives the ?.I-ally1species (V). Rapid internmversions of this type hetween q"- and ql.allyl speciesdestruys thedistinction hetween thesyn (HJ and anti (HJ vrotons and conseauentlv the twodoublet NMR sienals of these protons (Fig. l a j collapses into a single doublet (Figs. l b , lc) (6, 7). Experimental Synthesis of Di-p-chlorobis(q3-propenyfjdipalladiurn (10 (2) Heat PdCh (0.6 g) in aqueous acetic acid (35em3of 5090~1~) at 100° for 15 min. Filter off any insoluble material. Add ally1 chloride (2 cm3) to the filtrate and heat the solution at 60' for 1hr (temperaturesin excess of this will lead to oxidation of the ally1 chloride to methyl glyoxal).Extract the pale yellow solution with dichloromethane (20 cm" 3) and wash the combined dichloromethane extra& with water (20 cm3 X 5). Dry the dichloromethane extracts over anhydrous magnesium sulfate. Filter off the MgSOl and evaporate the dichloromethane extracts using a rotary evaporator. The material thus obtained is usually sufficiently pure to give reasonable spectra, although the product can be recrystallized from methanol if required. Typical student yields 40-6090. Literature Cned (1) Appleton,T.G.,sndCotton,J.D.,J. CHEM. EDUC.55.131I1978J. ,~~
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(2) Huttel, R.. Krstzer. J., and Bechter, M., Chsm. Ber. 34,766 l19611. (3) Silverstein, R. M.. and B s d e r , G. C., "Spectrometric Identifieation of Organic Campounds," 2nd Ed., John Wiley and Sons, 1967, p. 144. (4) Chcin.J. C. W., and Dehm, H. C., Chem. lnd. (London), 745 (1961). ( 5 ) Stntt0n.G. L..and Ramw, K. C., J Amer. Chem. Soc., 88,1328l1965). ( 6 ) Coafm,G.E..Gleen,M.L. H.. Powell, P.,and Wade. K.,"PrineiplnofOrgsnometallic Chemistry? Methuen and Co. Ltd., London, 1968, p. 192. ('7) C0tton.F. A. and Wi1kinson.G.. "Advanced lnorgsnieChemia~y?3rdEd.,John Wiley and Sons. 1372, p. 763.
PPM fmm TMS
Figure 1.60 MHz spectra of di-p-chlorobi~(?~-sllyIWipalladium (11) (a) in CoCI.
(b) in
ds-OMS0
lc) In dkDMSO at 60'
Figure 2. The reaction scheme. VOlUm.? 57, Number 9,Se~tember1980 1 671
