Oxidation of Tris(2-diphenylphosphinoethyl)phosphine An Undergraduate 31P and " ~ eNMR Experiment Dainis Dakternieks, Gail A. Dyson, Jenny L. O'Connell, and Carl H. Schiesser Department of Chemical Sciences, Deakin University, Geelong, Victoria,Australia, 3217
Although hctrronuclear NMR spectroscopy is mentioned in most undergraduate chemistry courses, few laboratory experiments exist that readily demonstrate this technique to students. The experiment described below introduces 31P and NMR spectroscopy hy having students follow the oxidation of tris(2-diphenylphosphinoethy1)phosphine (1)with elemental selenium and sulfur. Students are exposed to concepts such as heteronuclear coupling and nuclear relaxation. Heteronuclear coupling manifests itself in the form of "%e satellites in the 31Pspectra, which is a good starting point for discussions concerning nuclear spin and isotopic abundance. Nuclear relaxation can be discussed in terms of the acquisition time required to accumulate the "Se data. We routinely add tris(acetylacetonato)chromium(III)as a paramagnetic relaxation agent ( 1 ) to our samples for "Se NMR spectroscopy. The Selective Reaction with Selenium
onethird of the terminal phosphorus atoms are oxidized, whereas with three equivalents two-thirds of the terminal phosphorus atoms are oxidized. Heteronuclear Coupling
Compound 2 also shows coupling between the central phosphorus and the '?Se nucleus (8% abundance) as satellites (Jme = 710 HZ)."Se satellites are also clearly evident in the 31PNMR spectrum of 3 (see the figure). The P-Se coupling constants are measured to be 718 and 737 Hz for coupling to the central and terminal phosphorus atoms, respectively. Inspection of the 51.5-MHz NMR spectrum of 3 (see the figure) reveals two doublets (6 = 355.6,-376.2) as expected. The Se-P coupling constants are measured to be 717 and 732 Hz, in close agreement with those obtained from the corresponding 31P spectrum. The 31P and "Se NMR spectra are recorded in parts per million (ppm) downfield from (externally referenced) phosphoric acid (H&) and dimethylselenide (Me2Se), 'YSe rpb respectively.
f
Fch Se
Lack of Selectivity with Sulfur
3 .
The reaction of 1with elemental selenium has been dein some detail by colton and whybe (2), whereas the analogous reaction with sulfur has, to the best of our knowledge, not been reported.
When the entire procedure is repeated using sulfur instead of selenium no selectivity is observed. Addition of one, two, and three equivalents of sulfur appears to produce statistically oxidized phosphorus atoms. This is consistent with other ex~erimentsin
Production of the Monoselenide
As depicted above, 1reacts with one equivalent of selenium selectively at the central phosphorus atom to give the monoselenide (2). The reaction is conveniently monitored by 109.4-MHz 31P NMR spectroscopy. (See the figure.) In its "P N M R spectrum, ligand (1)displays a doublet (6 = -12.4: J = 24 Hz) a quartet (6 = -16.1;J = 24 Hz) The monoselenide (2) displays a doublet quartet (6 (8 = -12.8; 45.7;J = 41 Hz) The shift of the doublet from about -12 ppm, typical of phosphorus(III), to about 46 ppm, typical of phosphorous(V), clearly demonstrates that the selenium has reacted exclusively a t the central phosphorus atom. Production of the Tetraselenide
Reaction with four equivalents of selenium yields the tetraselenide (3), which displays a doublet (6 = 46.8;J = 56 Hz) a quartet (6 = 37.3;J = 56 Hz) Reaction 6 t h two or three equivalents of selenium vields mixtures of comwunds as is evident by 31P NMR spectroscopy. With two kquivalents, all of the Eentral and 168
Journal of Chemical Education
/ J~ -10
-15
-20
50
40
30
L -110
-360
-380
The 3 ' NMR ~ Spectra of tris(2-diphenylphosphinoethyl)phosphine(1) (above left),the tetraselenide (3)(above right),and the correspondina 7 7 ~NMR e soectrum of 3 lbelowh Note the "se satellites in the 3'?' spectrum of3.All chemlci shift; are in ppm (6)
which sulfur appears to react indiscriminately with phosphorus atoms in different chemical environments (2). Experimental .Caution: ~ris(2-diphenylphosphinoethyl)phosphinel is
listed as an irritant and accordingly should be handled in a fume hwd with protective gloves. The experiment is easy to run, requires little setup, and is usually completed in a single 4-h laboratory period. Tris(2-diphenylphosphinoethyl)phosphine (100 mg) is dissolved in dichloromethane (about 3 mL) and transferred to a 10-mm NMR tube. The 31PNMR spectrum is determined. One equivalent of selenium (11.8 mg) is added and the tube vigorously shaken until the selenium is dissolved. 'Tris(2-diphenylphosphinoethyl)phosphine is available from several chemical companies as Tetraphos-2. We purchase our samples from Strem Chemicals.
The 31PNMR spectrum is determined. (We run our spectra on a JEOL JNM-GX270 NMR spectrometer, although lower-field instruments should also suffice.) The sequential addition of further equivalents of selenium is continued until four equivalents have been added, with the 31PNMR speckum being recorded after the addition of each equivalent. Sonication is usually required for complete dissolution of the fourth equivalent of selenium. (To avoid breaking the NMR tube, the solution should be transfered to a test tube during sonication.) Tris(acety1acetonato)chromium(III) (1-2 mg) is added, and the 77Se NMR spectrum is recorded. The entire procedure is repeated using tris(2-diphenylphosphinoethyUphosphine (100 mg) and 4 x 1equivalent of elemental sulfur (4.8 mg), with the exception that the 77Se NMR spectrum is not recorded. Literature Cited 1. Gansow, 0. A,: Burke, A. R.: Vernon, W D.J Am. Chom. Soc. 1972.94.2550 2. Colton,R.;w,,yte.T.Aust. J. Chem 1991,44,5n.
Volume 71 Number 2 February 1994
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