Platinum(l1) Chemistry Monitored by NMR Spectroscopy David E. Berry University of Victoria, Victoria, BC, Canada, V8W 3P6
Much of the emphasis of organometallic research is placed on the heavier, late transition metals due to their proven role in many catalytic reactions. Although the chemistry of platinum metals is covered in undergraduate lecture courses ( I ) , use of these expensive metals in the laboratory program is rare (2,3).However, the cost may no longer be prohibitive due to the growing acceptance of microscale reactions in teaching labs (4). The topic of this experiment is the coordination chemistry of platinum(I1). The practical details can be easily modified for presentation a t several levels of sophistication. A three-step synthesis of a complex phosphine platinum(I1)species (see Fig. 1)is described below. The individual steps can be clearly identified using 31PNMR (31P:I = 112, 100% naturally abundant), thus eliminating routine recrystallizations and the concomitant loss of product. Although the same chemistry may be conducted with the cheaper palladium, the loss of the 195Ptlabel detracts from the spectroscopic characterization (lS5Pt:I = 112, 33.8% naturally abundant). The spectrum of the final product introduces non-firstorder behavior and is a fine example for a computer simulated analysis. The steps lead the student through a wide range of NMR concepts and can be tailored to meet the needs of the course curriculum. The scale of this experiment can be reduced successfully, but factors such as student skill and NMR spectrometer time should be considered. Experimental Caution: The entire experiment should he conducted in a well-ventilated fume hood. Gloves should be worn when handling either the platinum salts or the phosphines. Recommended: All solutions and solids that have been in contact with platinum may be collected for reclamation of the metal (5). Procedure Preparation of cis-[PtCldPEtdzl(6) Distilled water (50 mL) and ethanol (95%, 20 mL) were mixed in a Schlenk tube and degassed by three cycles of the freeze-pump-thaw method. When the contents returned to room temperature the Schlenk tube was flooded with nitrogen gas. Solid potassium tetrachloroplatinate(I1) (1.0 g, 2.4 mmol) was added with a small stir bar and stirred until the (0.7 mL, 4.8 mmol) solid had diss~lved.~Triethylphosphine was added under nitrogen by syringe to the reaction vessel.' The contents of the Schlenk tube were stirred vigorously overnight a t about 60". The stirring was sufficiently vigorous to prevent the phosphine from layering on the surface of the reaction mixture. Inadequate heating caused the formation of an orange-pink salt (see below). Complete reaction was indicated by the loss of color in the supernatant solution. The solution was concentrated in vacuo to remove most of the ethanol, and the white precipitate was allowed to settle. The supernatant was decanted under a nitrogen atmosphere, and the remaining solid was dried in vacuo. Once the free phosphine had been removed (no further smell!) the product was totally air-stable even in solution.
Figure 1. The sequence of reactions Atypical yield was 0.768 g corresponding to a 63% yield of ~ i s - [ P t C l ~ ( P E tPurity ~ ) ~ l . and characterization was established by 31PNMR spectroscop (dichloromethane or chloroform were suitable solvents).iAll NMR samples were reclaimed by evaporation of the solvent. The product may contain small amounts of the yellow trans isomer mixed with the desired cis-[PtC1~(PEt~)~l (white).Although such an impurity does not affect the synthesis of the dimer [PtzCUPEt&1, conversion of the trans to the cis can be achieved by suspending the dry solid in 150 mL pentane under nitrogen and adding two drops of triethylphosphine, stirring until the color is lost. The less soluble cis isomer may be filtered and dried under vacuum. The 31P NMR spectrum suggests that the orange-pink solid ~roducedbv inadeouate heatine durine the initial contains [ P t d l ( ~ ~ t & l~+ . k cation s usually is mixed with some cis- and trans-IPtCldPEt&l. Tmicallv the total mass of product is lower (0.433 g),b;lt ti;; speLtrum presents an interesting analytical challenge for the students. First-order principles apply The only supplementar, information that may be required are the values of m t P ) for a phosphorus trans to another phosphorous (about 2400 Hz) and for a phosphorus trans to a chlorine 'Anv remainina solid or metal deoosits should be removed at this stage .~htscan Ge wnvenlent y aaornpl~snedby syr ngmg the s.pernatant solahon Into a secono Scn end [Joe unoer nltrogen S-ostant al decompOs111on and loss of prooLct w I1 resLlt oy conm ng w m meta IC osits present. depThe phosphine will oxidize in air, so handle it under nitrogen, and protect the stock supply from oxidation. Used glassware-may be washed with bleach to oxidize the phosphine. This will reduce the smell. 31f this facility is not available, copies of the spectra may be requested fromthe author.
Volume 71 Number 10 October 1994
899
NMR Spectroscopic Dataa
ci~[PtCIz(PEb)z]
CH31.16doft
JPH 17. JHH 7.5 CM 2.04 d of q JPH -8
CH3 8.60 s JPtC 28 CHz 16.92 d JPC 40
9.75 JPtP 3509
B 21.400 943 MHz or 44.06 ppm
12.98 JPtP 2399
Z 21.412 783 MHz
JPtC 41 or
597.32 ppm CH31.22doft
JPH 18.0 JHH 7.6 CM1.80dofq JPH 11.2
CH3 1.00 d Of t JPH 17.5 JHH 7.4 CM (ethyl)I.71 m CM (dppm)4.76 1 JPH 11 JPtH 62 phenyl 7.3-7.9
s C H 2 14.72 d JPC 41 JPtC 39 CH3 7.68
11.41 JPtP 3834
Z 21.425 866 MHz or
1208.69 ppm
18.32 d JPtP 2278 JPP 19 113 1 t JPiP 3439 F€h 18.2 JRP 2333 JPPt- 395 J P P ~-2 s PPhz trans PEb 4 8 . 7 JPtP 1868 JPPcis65 PPhz transCI 4 9 . 6 JPtP 3043 FFg 4 3 . 6 JPF 713
E 21.405 950 MHz or 278.03 ppm
JPC 34 P-Cphenyl126.59 d JPC 48 and 124.45 d JPC 64 Cohanvl-H -. , 123-1 34 m aMultiplicitiesgiven as s (singlet),d (doublet),etc. to describe the panern of the resonance not coupled to platinum. Literature values are referred to in the text. '~emrdedwith an external CsDsreference against external H3P0,. 'Remrdd with an external CeDsreference. Shins in ppm quoted relative to 21.4 MHz. ~
~~
7
(3500 Hz). Conversion of the mixture to the desired cis[PtClz(PEt&l can he achieved by dissolving the dry residue in ethanol (10 mL) in air and gently boiling off the solvent almost to dryness. The solid should not he heated. A yield of 0.413 g of cis-[PtClz(PEt&l was obtained from 0.433 g of mixture, representing an overall yield of 34%. Prepamtion of [PtzClq(PEt&l(7) All of the cis-[PtCla(PEt& prepared was weighed into a test tube, and platinum chloride (1.1equiv) was added. The solids were vigorously stirred into a sluny with xylene (4 mL) and heated for 1 h a t 120 "C using an oil bath. Stimng was maintained to prevent the formation of unreacted clumps of solid. The reaction mixture was allowed to cool to room temperature before dichloromethane (50 mL) was added. The solution was filtered and then concentrated to a volume of about 5 mL. Hexane (100 mL) was added to cause the precipitation of the desired yellow product. From 0.768 gcis-[PtClz(PEt&l was obtained 0.815 g of product, which corresponds to a 69% yield. The 'H NMR spectrum afforded little information not already seen in the proton spectrum of the cis isomer, but the 31Pspectrum was diagnostic. PF6 Preparation of [PtCl(PEt3)1PfPh)zCHfl(Ph)z-P,PlI Bis(diphenylphosphino)methane4(0.600 g, 1.6 mmol) was added to [Pt&14(PEt3)~1 (0.600 g, 0.8 mmol) slurried in acetone (10 mL). Sodium hexafluorophosphate (0.280 g, 1.6 mmol) was added, and the reaction was stirred a t room temperature for 1h. The reaction mixture was evaporated to dryness in vacuo and extracted with dichloromethane (10 mL). After filtering, hexane was added to crystallize the white product.5 A typical yield after recrystallization was 0.908 g (66%). The addition of the sodium salt was necessary to drive the reaction to completion, and any soluble sodium salt can be used. In the present case the 31PNMR spectrum showed both the cation and the anion.
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Journal of Chemical Education
Interpretation of the NMR Spectra Because '06Pt is only 33.6% naturally abundant, all resonances are both platinum-coupled and uncoupled, giving rise to a symmetrical "triplet" of relative intensities 1:4:1 a s exemplified i n t h e 31P11Hl spectrum for cis[PtC1Z(PEt3)21. The triplet is actually the superposition of the two subspectra of a doublet (platinum-coupled) and a singlet (uncoupled). Essentially the same spectrum is seen because the chloride bridges transmit a for [Pt2C14(PEt3)21 very small, long-range 3J(PtP) and 4J(PP). Use of Phosphorus Coupling Constants Although phosphorus chemical shifts (measured against external phosphoric acid) are useful for comparison with literature data, they are not inherently informative (9,101. The coupling constants are far more revealing. The magnitude of the J(PtP) coupling constant is strongly influenced by the ligand trans to that phosphine. For example, a chloride in the trans position typically causes a J(PtP) of 300% 4000 Hz, whereas a methyl in the trans position will lower that value to about 1800-2500 Hz. Another phosphiue, such a s tmns-[PtC1z(PEts)zlwill bring J(PtP) into the range 2200-2800 Hz. These values are rather broadly described in the general sense of platinum(I1) complexes, but as was seen above for the cis-trans isomers the parameter is quite discriminating. The 'H and 13C spectra of either ~ i s - [ P t C l ~ ( P E tor ~)~l [Pt2C14(PEt3)21show the expected multiplets associated with the triethylphosphine. The values of J(PH), J(HH), and J(PC) can he extracted and confirmed by a theoretical model either on a computer program or by drawing a fam'Th s can De purchased or prepared as a separate exper men1 (81. '0flen the product s clean enough wttnout recrysmll~ranon,but 11 IS important that the "P NMR spectrum nas no extra peaks I the
student is going to attempt a simulation
Figure 2. Tne observed "P('H) NMR spectrum of the cation [RCI(PEt1)(P(Ph),Crl~P(Ph)2-eq]' In dicnloromethane at 101 3 MHZ. ~h'region ibsehing the P h anion has been omitted.
Figure 3. The calculated "P('H) NMR spectrum of the cation [PtCl(PEg)(P(Ph)2CH,P(Ph)2-FIfj]+.
ily tree (stick) diagram to scale (11).The spectroscopic data are summarized in the table. Analysis of the 3
1 Spectra ~
The 31P spectrum of the final product is quite complex. The low-frequency septet of the P& anion is most easily identified, and the value of J(PF) can be correlated with that found in the 19Fspectrum, if available. The remaining resonances should correspond to the three types of phosphorus in the cation. The two associated with the chelating his(dipheny1phosphino)methane ligand are very close in chemical shift and should be given the spectroscopic labels A and B to indicate this. The remaining phosphorus (from the triethylphosphine) is labelled X, and the lg5Ptnucleus is M. Before a simulation can be attempted, the spectrum should be analvzed as far as ~ossihleusina first-order orinciples. This wiil roughly idekify the posi%ons of the resonances because they lie in the center of their respective platinum-coupled patterns. Measuring the J(PtP) values will confirm the trans relationship of the triethylphosphine (X) and one of the ends of the chelate ligand (Aor B). The phosphorus-phosphorus coupling is also distinctive. vpically, J(PP) for trans coupling is 300-500 Hz, dropping to &30 Hz in simple cis relationships. Further analysis is more complicated, and most information can be gained by analyzing the ABMX suhspectrum (i.e., the lines caused by platinum coupling) before tackling the more difficult ABX subspectrum. The X part of this ABMX suhspectrum shows no further coupling. Thus, the cis coupling expected between triethylphosphine and his(dipheny1phosphino)methane can be assumed to be zero. The coupling between A and B is found a t a higher value than suggested above for a simple cis relationship. This is a result of the combination of two possible routes of A-B coupling through both the platinum and the carbon backbone of the chelate (12). Simulation of a 3
' Spectrum ~
Although values for the chemical shifts and coupling constants can he extracted as above, they are rather approximate. Some of the smaller features of the spectrum (particularly on the central peaks of XI are difficult to explain without considering a non-first-order approach. To take the theory much further requires a specialist's thirst for perfection, but a computer match of observed and predicted spectra is well within the grasp of an undergraduate student. The simulation is normally carried out on each of the two suhspectra independently (ABXand ABMX) before
Figure 4. The observed ' 9 5 R t ~ NMR ) spectrum of the cation [PtCI(PEt3){P(Ph)2CH2P(Ph)2-eq]tin dichloromethane at 53.5 MHz. adding the appropriate intensity ratio to compare with the observed spectrum. Various simulation programs are available for use on a personal computer. Many have been designed for simple spectral modelling rather than iterative refinements of the model against the observed spectrum. We have used a locally constructed6 program based on UEAITR (13) and NMRPLOT (14) run on an IBM 3090 mainframe computer, but the same results have been satisfactorily achieved on an IBM PC equipped with a plotter (15). The observed and calculated spectra are shown in Figures 2 and 3. Anumber of lines appear that are not predictable from simple first-order analysis. The iterative refinement gave the chemical shifts and coupling constants quoted in the table. Platinum Spectra
The platinum spectra serve to confirm the characterization of the complexes. Although the coupling constants should agree with those found in the phosphorus spectra, they are generally less accurate. Chemical shifts can be quoted in a variety of ways (161, hut the methods of absolute frequency, 5, and ppm relative to 21.4 MHz are gaining popularity. Figure 4 shows the doublet of doublet of doublets expected for the cation,
[PtCl(PEt~)lP(Ph)zCHzP(Ph)~-~PIl+ %y K. R. Dixon, University of Victoria. Volume 71 Number 10 October 1994
901
Further Experiments
A longer synthetic project may incorporate the synthesis of bis(diphenylphosphin0)methane. This involves the ultrasound enhancement of the lithiation of triphenylphosphine described in ref 8, We have run this as a separate experiment incorporating the general principles of the ultrasound technique (17). The oxidation of bis(dipheny1phosphino)methane with sulfur produces a second ligand of similar form. The subsequent cleavage of the dimer with this latter ligand ereates weaker pt-~ bonds rather than Pt-p bonds. The 31P NMR parameters may furnish useful data for a group project (18). The deprotonation of either chelate may be achieved at the CH2site either before or after coordination (12, 18).
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Journal of Chemical Education
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