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Vol. 74 No. 3 March 1997 Journal of Chemical Education 325

In the Laboratory

Coordination Chemistry of Pd(II) Complexes with P-Donor Ligands

An Introduction to Synthesis and Structural Characterization in CoordinationChemistry

Esteban P. Urriolabeitia

Departamento de Qumica Inorgnica, Instituto de Ciencia de Materiales de Aragn, Universidad de Zaragoza-CSIC,50009 Zaragoza, Spain

reactions illustrated in Figure 1 summarize the practicalwork and introduce to the student some common conceptsand skills in coordination (and in organometallic) chemistry:

substitution of weakly coordinated ligands

generation of coordinatively unsaturated species symmetrical cleavage of halogeno-bridged dimers chelate effect exclusion of light in reactions involving silver-contain-

ing reactants

Experimental Procedures

CAUTION: THT (tetrahydrothiophene) is toxic andhas an unpleasant odor; it should be handled withcare and reactions involving it must be carried outin an efficient ventilation hood; Tl(acac) (acac =acetylacetonate) is also toxic and should be handledwith care; the use of gloves and protective glasses ishighly recommended.

General

Molar conductance (M) measurements were performedon a Philips PW9509 digital conductivity meter in acetonesolutions (ca. 5 104 M). IR spectra (4000200 cm1) wererecorded on a Perkin-Elmer 883 spectrophotometer, usinga Nujol mull between polyethylene sheets. 1H and 31P{1H}NMR spectra were recorded on a Varian Unity-300 using

CDCl3 as solvent and referenced to TMS and H3PO4 (85%),respectively. Tl(acac) (14 ) and DPPM [Ph2PCH2PPh2,bis(diphenylphosphino)methane] (15) were prepared as pre-viously described, with slight modifications. AgBF4, THT,and PPh3 (Fluka), and PdCl2 were used as received. Sol-vents used do not require a high degree of purity and wereused as received [Probus (R. A.) and Carlo Erba (R.)].

Preparation of Compounds15

Preparation of [Pd(-Cl)(PPh3)2]2(BF4)2 (1)

Synthesis of the starting compound (1) involves twopreliminary steps:

a. Synthesis of trans-PdCl2(THT)2. To a suspension offinely ground PdCl2 (0.500 g, 2.82 mmol) in 30 mL of CH2Cl2,THT (0.50 mL, 5.67 mmol) was added. The mixture was

stirred at room temperature for 3 h giving a red solution, andthen filtered to remove traces of metallic palladium. The re-sulting solution was evaporated to dryness and the residuewas washed with n-hexane (50 mL) giving the complex trans-PdCl2(THT)2 as an orange-red solid, which was filtered, gen-tly washed with n-hexane (3 20 mL) to remove the freeTHT, and air dried. Obtained: 0.85 g (85% yield).

b. Synthesis of trans-PdCl2(PPh3)2. The PdCl2(THT)2obtained (0.85 g, 2.40 mmol) was dissolved in 30 mL ofCH2Cl2, and PPh3 (1.26 g, 4.80 mmol) was added. A yellowsolid precipitated immediately. The suspension was stirredfor an additional 30 min at room temperature, then filtered,washed three times with 10-mL portions of Et 2O, and airdried, givingtrans-PdCl2(PPh3)2 as a deep yellow solid. Ob-tained: 1.60 g (95% yield).

Figure 1. Scheme of the reactions performed for the synthesis of

complexes 15.

Ph3P

Pd

Cl

ClPh3P

Pd

PPh3

PPh3

Ph3P

Pd

Cl

PPh3Ph3P

Ph3P

Pd

PPh2

PPh2Cl

Ph3P

Pd

O

OPh3P

Ph3P

Pd

PPh2

PPh2Ph3P

CH2

Me

Me

CH2

AgCl

PPh3 TlCl

AgCl

dppm Tl(acac)

dppm

AgBF4

PPh3 AgBF4

2 PPh3

2 tht

(1)

(5)

(BF4)2

(4)

(BF4)

(3)

(2)

(BF4)

1/2

(BF4) PdCl2(PPh3)2

PdCl2(tht)2

PdCl2

(BF4)2

2 tht

The phosphorous-containing ligands, such as phos-phines (PR3) or diphosphines [R2P(CH2)n-PR2] (R = alkylor aryl; n = 1, 2, 3, ) are one of the most widely usedgroups of ligands in palladium chemistry (1). Complexescontaining the PdPR3 or Pd[R2P(CH2)nPR2P,P] unitsare known for all the most common oxidation states of thePd atom (2). They have shown important and useful appli-cations as starting compounds in coordination and organo-metallic chemistry (3, 4) and as catalysts in organic syn-thesis (4, 5). For instance, complexes such as PdX2(PR3)2 orPdX2(diphosphine) are used in the synthesis of several Pd(I)

and Pd(0) derivatives, clusters of mixed valence, etc. (3, 4,6). Moreover, these PdX2L2 (L = phosphine) complexes arerecognized catalysts in a very wide range of organic reac-tions, such as molecular rearrangements (7), substitutionprocesses at allylic carbons (8), coupling of alkene andalkyne groups (9), hydrogenation of alkynes, dienes, andalkenes (10), and carbonylation and oxidation of severalsubstrates (11). Complexes such as Pd(PR3)n (n = 2, 3, 4)have had wide use in the catalysis of hydrosilylation reac-tions (12, 13), dimerizations, and oligomerizations.

We propose here the synthesis and complete character-ization of a family of Pd(II) complexes containing two ormore PdP bonds as a laboratory experiment complemen-tary to a classroom course on coordination chemistry. Thesynthesis section has the advantage that all of the com-

pounds prepared are air-stable, both in the solid state andin solution (avoiding the use of special apparatus), and theycan be prepared under mild conditions in short reactiontimes. Characterization includes the use of several instru-mental techniques (molar conductances in solution, IR spec-tra, 1H and 31P{1H} NMR measurements; see details in sec-tion on experimental procedures) that allow students to be-come familiar with the more usual structural methods andwith the interpretation of the resulting data.

This laboratory work can be done individually. We rec-ommend five periods of four hours period to complete it. The

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c. Synthesis of [Pd(-Cl)(PPh3)2]2(BF4)2. A suspensionof trans-PdCl2(PPh3)2 (1.50 g, 2.13 mmol) in 130 mL ofCH2Cl2 was treated with AgBF4 (0.416 g, 2.13 mmol). Thismixture was stirred at room temperature for two hours withexclusion of light. The initial yellow suspension graduallychanged color, giving a white precipitate of AgCl and an or-ange solution. After removal of the AgCl the orange solu-tion was evaporated to dryness and the residue treated with40 mL of n-hexane, giving the complex [Pd( -Cl)(PPh3)2]2(BF4)2 (1) as a lemon-yellow powder, which wasfiltered, washed with n-hexane (20 mL), and air dried. Ob-tained: 1.500 g (93% yield).

Preparation of [PdCl(PPh3)3](BF4) (2)

Complex 1 (0.225 g, 0.149 mmol) was dissolved in 25mL of CH2Cl2 and to the resulting solution PPh3 (0.078 g,0.298 mmol) was added. The initial lemon-yellow solutionimmediately became deep yellow. The solution was refluxedfor 1 hour. After cooling, the solvent was evaporated to dry-ness and the residue treated with Et2O (25 mL) giving2 asa yellow solid, which was collected and air dried. Obtained:0.276 g (91% yield).

Preparation of [PdCl(PPh2CH2PPh2-P,P)(PPh3)](BF4) (3)

Complex 1 (0.225 g, 0.149 mmol) was dissolved in 25mL of CH2Cl2, and DPPM (0.115 g, 0.298 mmol) was addedto the solution. An orange solution was obtained, which wasrefluxed for 1 h. After cooling, the solvent was evaporated

to dryness and Et2O (20 mL) was added to the residue. Con-tinuous stirring gave 3 as an orange solid, which was fil-tered, washed twice with 10-mL portions of Et2O, and airdried. Obtained: 0.238 g (91% yield).

Preparation of [Pd(acac-O,O)(PPh3)2](BF4) (4)

Complex 1 (0.225 g, 0.149 mmol) was dissolved in 25mL of CH2Cl2 and Tl(acac) (0.091 g, 0.298 mmol) was addedto the solution. A white precipitate of TlCl was formed in-stantaneously. The suspension was stirred at room tempera-ture for 30 min and then filtered. The resulting yellow so-lution was evaporated to dryness and n-hexane (20 mL) wasadded to the oily residue. Continuous stirring gave 4 as ayellow solid, which was filtered and air dried. Obtained:0.161 g (66% yield).

Preparation of [Pd(PPh2CH2PPh2-P,P)(PPh3)2](BF4)2 (5)

Complex 1 (0.250 g, 0.166 mmol) was suspended in 35mL of acetone, and AgBF4 (0.065 g, 0.332 mmol) was added.The mixture was stirred for 1.5 h at room temperature withexclusion of light and then filtered to remove the AgCl pre-cipitate. The freshly obtained solution of[Pd(PPh3)2(acetone)2](BF4)2 was treated with DPPM (0.128g, 0.332 mmol) and stirred for another 45 min. During thistime, a white solid (5) precipitated, which was collected,washed with acetone (3 3 mL), and air dried. Obtained:0.168 g (43 % yield).

Results and Discussion

The reaction between PdCl2 and THT (1:2 molar ratio)leads to the formation of the complex trans-PdCl2(THT)2.By reaction of this complex with PPh3 (1:2 molar ratio), theweakly coordinated ligand THT is easily displaced from thestarting compound by the PPh3 ligand, and the complextrans-PdCl2(PPh3)2 is obtained in good yield. Further reac-tion of the complex trans-PdCl2(PPh3)2 with AgBF4 (1:1 mo-lar ratio) produces the dinuclear complex 1. Such complexes,with -halogen bridges, are quite reactive and are conve-nient precursors in the synthesis of other Pd(II)-complexes.We then perform two types of synthetic exercises:

1. Cleavage of the (-X)2 bridging unit with P-donorligands. Cleavage with monodentate ligands such as PPh3gives the mononuclear complex (2) and cleavage with

bidentate ligands such as DPPM gives the mononuclearcomplex (3). For complex 3, the chelate effect of the DPPMligand promotes the displacement of a PPh3 ligand and thebidentate-P,P coordination of the DPPM, instead of themonodentate-P coordination derived from the simple break-age of the (-X)2 unit.

2. Displacement of the -X ligands by their precipita-tion as insoluble salts and generation of the coordinativelyunsaturated species cis-[Pd(PPh3)2]

2+. In this way, the reac-tion between 1 and Tl(acac) produces the precipitation ofTlCl and the coordination of the acac- ligand to the cis-[Pd(PPh3)2]

2+ group, giving complex 4. The reaction of1 withAgBF4 in acetone produces the precipitation of AgCl and thesolvated complex [Pd(PPh3)2(acetone)2]

2+, which subse-quently reacts with DPPM affording complex 5.

51sexelpmoCrofataD.1elbaT

xelpmoCRI

( mc, 1)

1 RMNH( ;mpp, J )zH,

13 {P 1 RMN}H( ;mpp, J )zH,

ecnatcudnocraloM(

M, 1 mc 2 lom 1)

1 FB{16014

,315,325,835,} 13.83=)P( 352

hPP{9943

{903,} ts

)lC-dP(rb}

2 FB{45014

} P(a

66.92=)d, 921

,)hs(005,315,125,935 P(x

94.23=)t,

hPP{394 3 {613,} ts )lC-dP( t}2

J P( aPx 44.51=)

3 FB{85014

,115,225,545,} P(a

,22.22=)d, 2J P(aP

x564=) 301

hPP{774,184,5943

}MPPD+ P(m

=)d, ,27.83 2J P(mP

x18=)

{613 ts

)lC-dP(t} P(

x=)dd, 17.25

4 {6151,2251,4651 ,}caca,)O=C( 14.5=)s,caca-HC( 68.53=)P( 431

FB{85014

{947,} ,}caca,)H-C( HC(3

94.1=)s,caca-

hPP{894,615,525,5453

} C(6H

57=)m, 8

5 FB{36014

,594,605,125,235,} P(a

62.42=) yltneiciffusnI

hPP+MPPD{164,2743

} P(x

=) 56.62 elbulos

2J P(aP

x;063=) 2J P(

aP-

'x=) 22

I2J P(aP

'a+) 2J P(

xP

'x)| 77=

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In the Laboratory

The stereochemistry of complexes 15 can be inferredfrom the analysis of their molar conductance values (ac-etone solutions) and IR and NMR spectra (Table 1). Thus,for complex 1, the absorption st(Pd-Cl) at 309 cm

1 suggestsa bridging coordination of the chlorine ligands (16) and fourabsorptions in the 550500 cm1 region indicate the pres-ence of PPh3 ligands (17). A single resonance in the

31P{1H}NMR spectrum is observed. These data, together with theobserved molar conductance in acetone solution (1:2 elec-

trolyte, see Table 1) (18) supply the information required todeduce the stereochemistry of complex 1 and to understandits further reactivity.

Complex 2 shows a very complicated pattern of absorp-tions in the 550500 cm1 region (presence of phosphines)and a strong absorption at 316 cm1 [st(Pd-Cl)]. This shiftto higher energies suggests a change in the coordinationmode of the Cl ligand. Moreover, the analysis of the 31P{1H}NMR spectrum provides additional valuable information:the observation of an A2X spin system points out the exist-ence of a Pd(PPh3)3 unit. These data, together with themeasured molar conductance (typical for 1:1 electrolytes)(18), allow an unequivocal determination of the structureof2 as that depicted in Figure 1. For complex 3, the mean-ing of the observed changes in the IR spectrum and the

analysis of the 31P{1H} NMR spectrum (AMX spin system,Fig. 2) can be done by the students as an exercise, takinginto account the comments above for complex 2.

For complex 4, two strong absorptions at 1564 and 1522cm1 in the IR spectrum are due to the chelate acac ligandO,O-coordinated (19) and four absorptions in the 550500cm1 region show the presence of phosphine ligands (17).These data suggest the structure depicted in Figure 1 forcomplex 4. In agreement with this structure, the 1H NMRspectrum shows a sharp singlet at 5.41 ppm [C

H-(acac)]

and another singlet (relative intensity 1:6) at 1.49 ppm[CH3-acac], showing the chemical equivalence of the twoCH3 groups, and the

31P{1H} NMR spectrum shows a singleresonance, resulting from the chemical equivalence of thetwo PPh3 groups.

Finally, the unequivocal elucidation of the molecularstructure of5 comes from the analysis of the 31P{1H} NMRspectrum, which is a clear example of a deceptively simpleAAXX spin system (Fig. 3) (20), thus giving proof of itsmononuclear nature.

Acknowledgments

I thank J. Fornis and R. Navarro for helpful discus-sions and DGICYT (Spain) for financial support (ProjectPB92-0364).

Literature Cited

1. Hartley, F. R.The Chemistry of Platinum and Palladium ; Applied

Science: London, 1973.2. McAuliffe, C. A.; Levanson, W.Phosphine, Arsine and Stibine Com-plexes of the Transition Elements; Elsevier: Amsterdam, 1979.

3. Albeniz, A. C.; Espinet, P. Encyclopedia of Inorganic Chemistry;King, R. B., Ed.; John Wiley & Sons: West Sussex, England, 1994;

Vol. 6, pp 3010 and following.4. Suggs, J. W.Encyclopedia of Inorganic Chemistry; King, R. B., Ed.;

John Wiley & Sons: West Sussex, England, 1994; Vol. 6, pp 3023and following.

5. Heck, R. F.Palladium Reagents in Organic Synthesis; Academic:London, 1985.

6. Barnard, C. F. J.; Russell, M. J. H. Comprehensive CoordinationChemistry; Wilkinson, G.; Gillard, R. D.; McCleverty, J. A., Eds.;Pergamon: Oxford, 1987. Vol. 5, pp 1099 and following.

7. Trost, B. M.Acc. Chem. Res.1990,23, 34.8. Yamaguchi, M.; Shima, T.; Yamagishi, T.; Hida, M. Tetrahedron Lett.

1990,31, 5049.9. Trost, B. M.; Shi, Y.J. Am. Chem. Soc.1992,114, 791.

10. Stern, E. W.; Maples, R. K.J. Catalysis1972,27, 120; ibid., p 134.

Figure 2. 31P{1H} NMR spectrum of complex 3.

Figure 3. 31P{1H} NMR spectrum of complex 5.

11. Spencer, A. Comprehensive Coordination Chemistry; Wilkinson, G.;Gillard, R. D.; McCleverty, J. A., Eds.; Pergamon: Oxford, 1987; Vol.6, pp 229 and following.

12. Tsuji, J.; Hara, M.; Ohno, K. Tetrahedron1974,30, 2143.13. Mitchell, T. N.; Amamria, A.; Killing, H.; Rutschow, D.J. Organomet.

Chem.1983,241, C45.14. Taylor, E. C.; Hawks, G. H., III; McKillop, A.J. Am. Chem. Soc.

1968,90, 2421.15. Issleib, K.; Muller, D. W. Chem. Ber.1959,92, 3175.16. Nakamoto, K.Infrared and Raman Spectra of Inorganic and Co-

ordination Compounds; Wiley: New York, 1986; pp 328331.17. Mastin, S. H.Inorg. Chem.1974,13, 1003 and refs cited therein.18. Geary, G. Coord. Chem. Rev.1971,7, 81.19. Nakamoto, K.Infrared and Raman Spectra of Inorganic and Co-

ordination Compounds; Wiley: New York, 1986; pp 259263.20. Gnter, H.Angew. Chem. Int. Ed. Engl.1972,11, 861.