Organometallics 1983, 2, 665-668
665
Preparation of Chloride-Bridged Organopalladium( I I ) Dimers and Their Role in the Carbonylation of trans-[ PdCIR'(PR,),] Gordon K. Anderson Department of Chemistry, Universiw of Missouri-St. Louis, St. Louis, Missouri 63 12 1 Received Aught 19, 1982
Complexes of the type [Pd2(p-C1)2R2L2] (R = Ph, C6H4Me-p,or CH2Ph; L = PPh,, PMePh,, or PBu3) are prepared by the reaction of RzHg with [Pd2(p-C1)2C12Lz] in benzene and have been characterized by elemental analysis and NMR spectroscopy. Readion with L gives the corresponding bis(phosphine)palladium complex, while treatment with CO yields [Pd2(p-C1),(COR),L2].Carbonylation of trans-[PdClRL2]or bridge cleavage of [Pd,(p-C1),(COR)2Lz]with L produces trans-[PdC1(COR)L2]. The rates of these processes are considered, and the intermediacy of the dimeric complexes in the carbonylation of trans-[PdCIRLz] is discussed. kin-Elmer 337 spectrophotometer. Introduction Melting points were determined on a Thomas Hoover capillary Halide-bridged complexes of palladium(I1) and platimelting point apparatus and are uncorrected. Microanalytical "(11) are extremely useful as starting materials in the data were from Galbraith Laboratories, Inc., Knoxville, TN. syntheses of organometallic and coordination compounds.' Carbon-13 labeled complexes were prepared by using 90% Complexes of the type [Pt2(p-X)2R2Lz](X = halide, R = enriched 13C0obtained from Prochem. organic group, and L = neutral ligand) have recently been Preparation of [Pd2(p-C1),Ph2(PBu3),1. Diphenylmercury (1.488 g, 4.20 mmol) and [Pdz(p-C1)2C12(PBu3)2] (1.592 g, 2.10 prepared2p3 and have been shown to exhibit some intermmol) were mixed under nitrogen, and benzene (50 mL) was esting chemistry. added. After being stirred for 1h, the dark brown solution was A general route to halide-bridged organopalladium(I1) evaporated to dryness and heated in vacuo (85 "C (0.005 torr)) complexes, however, has not been described to date, alfor 13 h. A small amount of PhHgC1 sublimed out, the remaining though a few isolated examples of such complexes have dark solid was treated with benzene and charcoal, and then the been reported. The complex [Pdz(p-C1)2(CH2Ph)2(PPh3)2] solution was filtered. The volume was reduced, and addition of was somewhat fortuitously obtained upon recrystallization petroleum ether caused precipitation of pale yellow crystals (0.744 of trans- [PdCl(CH,Ph)(PPh,),] from chloroform/hexane,4 g, 42%). and a complex described as "[PdI(COPh)(PPh3)].tluenen, [Pdz(~-C1)2Phz(PPh3)2] was prepared similarly. which is likely to be dimeric, was obtained by reaction of Preparation of [Pd2(p-C1)2(C6H4Me-~ )z(PBu3)21.To a solution of [Pd2(p-Cl)zClz(PBu3)2] (0.842 g, 1.11mmol) in benzene iodobenzene with [Pd,(CO),(PPh,),] in the presence of (5 mL), under nitrogen, was added a benzene suspension of dicarbon m ~ n o x i d e . Pentafluorophenyl ~ complexes [Pd2p-tolylmercury (0.849 g, 2.22 mmol) over 10 min. The mixture (p-cl)z(c6F5)2L2]were obtained by organic group transfer darkened and, after 1.5 h, the precipitated p-MeC6H4HgC1was from thallium(III)6 and Grignard' reagents, and reaction filtered (0.639 g, 88%). Addition of petroleum ether caused of [PdR,L,] (R = CsF5or C6C15)with palladium(I1) chloprecipitation of further p-MeC6H4HgCl,and the filtrate was ride yielded the corresponding arylpalladium(I1) dimer.8 treated with charcoal and filtered to give a clear, yellow solution. This paper describes a general synthetic approach to This was concentrated, and addition of petroleum ether gave pale halide-bridged alkyl- and arylpalladium(I1) dimers, using yellow crystals (0.412 g, 43%). diorganomercurials as the organic group transfer agents, [Pd2(~-C1)2(CH2Ph),(PBu3)2] was prepared analogously. DiPreparation of [Pd2(p-Cl)2(CH2Ph)2(PMePh2)21. and their reactions with carbon monoxide and other neubenzylmercury (1.021g, 2.67 mmol), dissolved in benzene (50 mL), tral ligands. was added to a benzene suspension (50 mL), under nitrogen, of [Pd2(~-Cl)2C12(PMePhz)2] (1.007 g, 1.33 mmol) over 2 h. The Experimental Section solvent was evaporated, and the residue was treated at 65 O C (0.005 The 'H NMR spectra were recorded at 60.0 MHz on a Varian torr) for 3 h. Some decomposition occurred, but little PhCH2HgCl T-60 spectrometer,and the 13C(1H]and 31P(1HJ NMR spectra were sublimed, so the solid was washed with warm ethanol to remove obtained at 25.00 and 40.26 MHz, respectively, on a JEOL FX-100 PhCH,HgCl. The residue was then treated with charcoal and spectrometer operating in the FT mode. Spectra were recorded crystallized from benzene/petroleum ether to give yellow crystals for CDC13solutions at 25 "C; 'H and 13C chemical shifts were (0.475 g, 41%). measured relative to Me4%,and 31Pchemical shifts were measured Preparation of [Pd2(p-Cl)zPhz(PMePh,)21. Diphenylrelative to external H3P04,positive shifts representing deshielding. was added, as a solid, to a suspension mercury (1.907 g, 5.38 "01) Infrared spectra were measured in CHC13 solution by using of [Pd2(pC1)2C12(PMePhz)2] (2.029 g, 2.69 mmol) in benzene (100 NaCl cells of 0.5" path length and were recorded on a PermL), under nitrogen. The mixture became black, and, after 0.5 h, charcoal was added and filtration yielded a yellow solution. This was evaporated to dryness and the grayish residue was (1)Hartley, F. R. Organomet. Chem. Rev., Sect. A 1970, 6 , 119. treated at 80 "C (0.005 torr) for 24 h, giving PhHgCl (0.691g). (2) Eabom, C.; Odell, K. J.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1978, 1288. The residue was dissolved in benzene and again treated with (3)Anderson, G.K.;Cross, R. J. J. Chem. Soc., Dalton Trans. 1979, charcoal,and after filtration the addition of pertroleum ether gave 1246. the product as colorless crystals (0.500 g, 22%). (4) Fitton, P.; McKeon, J. E.; Ream, B. C. J . Chem. SOC.,Chem. The filtrate was evaporated to dryness, and a second crysCommun. 1969, 370. tallization from benzene/petroleum ether yielded colorless crystals (5) Hidai, M.; Hikita, T.; Wada, Y.; Fujikura, Y.; Uchida, Y. Bull. Chem. SOC. Jpn. 1976,48, 2075. of tran~-[PdClPh(PMePh~)~] (0.233 9). Anal. Calcd for (6) Uson, R.; Royo, P.; ForniCs, J.; Martinez, F. J. Organomet. Chem. C3zH31ClPzPd:C, 62.05; H, 5.05. Found: C, 62.39; H, 5.22. 1976,90, 367. Preparation of [Pd2(fi-Cl)2(COPh)2(PBu3)2]. A CH2Clz (7)Uson, R.; ForniCs, J.; Martinez, F. J . Organomet. Chem. 1977,132, solution of [Pd2(p-Cl)2Phz(PBu3)z] (0.247 g) was stirred under 1 429. atm of CO for 24 h. The yellow solution was then evaporated (8) Uson, R.; ForniCs, J.; Navmo, R.; Garcia, M. P. Inorg. Chin. Acta to dryness, and the residue was crystallized from benzene/pe1979, 33, 69. 0276-7333/83/2302-0665$01.50/0
0 1983 American Chemical Society
666 Organometallics, Vol. 2, No. 5, 1983
Anderson
Table I. Characterization Data for the Complexes ~ d , ( p - C l ) z R z L z ] found
L
mp, "C
C
H
C
H
S P )
Ph Ph CH,Ph
PPh, PMePh, PMePhZa
232-233 187 170
59.88 54.25 56.90
4.45 4.49 4.96
59.89 54.43 57.04
4.19 4.33 4.79
16.3 20.9
Ph
PBu, PBu,
149-150 146-147
51.24 52.28
7.41 7.73
51.32 52.43
7.66 7.88
18.9
C,H,Me-p CH,Ph
PBu,
113-114
52.38
7.91
52.43
7.88
22.4
R
a
calcd
'H NMR data 6(CH,) 1.50 (d, 'J(P,H) = 10.5 Hz) 6(CH,) 1.90 (d, 'J(P,H) = 10.0 Hz), 6(CHz) 2.95 (d, ,J(P,H) = 3.0 Hz)
S(CH,) 2.20,6(H) 6.75 (d), 7.15 (d,d, 'J(H,H') = 8.0 Hz, 4J(P,H)= 2.0 Hz) 6(CHz) 2.95 (d, ,J(P,H) = 2.0 Hz)
Contains 0.5 molecule of C,H,/dimeric unit, as indicated by integration of the 'H NMR spectrum,
troleum ether to give the product as yellow crystals (0.171 g), mp 150-151 "C. Anal. Calcd for C&&1202PzPdz: C, 50.80; H, 7.19.
Found: C, 50.86; H, 7.32. Other reactions of the organopalladium complexes with carbon monoxide were carried out by stirring a solution of the complex under a 13C0atmosphere or by passing CO through the solution, and the produde were examined in situ. Bridge cleavage reactions were performed by addition of the neat ligand to a solution of the complex.
Results and Discussion Treatment of the complexes [Pd2(p-C1),Cl2L,] (L = PPh3, PMePh,, or PBu3) with HgRz (R = Ph, C6H,Me-p, or CHzPh) in benzene solution produces the corresponding chloride-bridged organopalladium(I1) species [Pd,(pCl),R2L2]and RHgC1. These new dimeric complexes have been characterized by elemental analysis and, where applicable, 'H NMR spectroscopy (Table I). Their 31P(1H) NMR spectra, at ambient temperature, consist of a single line, indicating that only one isomer exists, the cis and trans isomers have coincidental chemical shifts, or, as is most probable, rapid isomerization occurs by a bimolecular process. Indeed, mixing [Pd2(p-Cl)2Ph2(PBu3)2] and [P~,(~L-C~),(CH,P~)~(PM~P~,),] in CDC1, solution caused formation, after 24 h, of an equimolar mixture of the two reactants and [ (Bu,P)PhPd(p-Cl),Pd(CH,Ph)(PMePh,)] (6(PBu3)19.0, G(PMePh2)20.8). Phosphine exchange does not take place, since no products of phosphine and organic group mixing were detected. The 31P(1HJNMR spectra of the analogous platinum complexes indicate the presence of two isomer^.^^^ The use of diorganomercury compounds as organic group transfer agents in this context allows the. introduction of an alkyl or aryl moiety, whereas the thallium(II1) and Grignard reagents mentioned above6?'are rather more restrictive. Similarly, the RSnMe, compounds used in the preparation of the analogous platinum dimers are only suitable where R = aryl., The reactions are accompanied by some decomposition, however. In some cases (see Experimental Section) it is necessary to resort to vacuum sublimation in order to remove the organomercuric chloride byproduct, and the temperatures required to achieve this can lead to further decomposition. Thus the moderate yields are perhaps due in greater measure to the difficulties of product separation than to any inherent instability of the complex involved or to competing side reactions. The reaction pathways involved in the decomposition of the complexes have not been investigated, but elimination of tertiary phosphine (possibly accompanied by reductive elimination of the organic chloride) seems to be involved, as evidenced by the isolation of trans-[PdClPh(PMePh,),] during the preparation of [Pd,(p-Cl),Ph,(PMePh2)~l. (9)
Anderson, G . K.; Cross, R. J. unpublished results.
The complexes are all colorless or pale yellow crystalline solids, which are stable to air and moisture. They are readily soluble in benzene and halocarbon solvents, with the exception of [Pd2(p-C1)2Ph2(PPh3)2] which is only sparingly soluble, but are insoluble in alcohols and petroleum ether. The complexes [Pd,(p-C1),&L2] rapidly undergo bridge cleavage reactions with L to give trans-[PdClRL,]. The trans geometry is indicated by the equivalence of the phosphorus nuclei, as evidenced by the single resonance observed in their 31P(1H)NMR spectra, the triplet resonance for the benzylic protons in trans-[PdCl(CH,Ph)(PBU,)~],and the virtual coupling in trans-[PdClPh(PMePh,),], which gives rise to a triplet for the phosphine exmethyl group. Only trans-[PdC1(CH2Ph)(PMePh2),] hibits an unexpected 'H NMR spectrum, the very broad benzylic resonance and the broad singlet for the phosphine methyl in CDC1, or toluene-d, solution suggesting that tertiary phosphine exchange occurs at ambient temperature. Such a process has been postulated for trans[PdC1(CHZPh)(PPh3),],4where a singlet benzylic resonance was observed. Here the resonance at 6 2.55 is very broad, suggesting that phosphine exchange is slower for PMePh,, which might be expected in terms of the relative nucleophilicities of the ligands. Indeed, the more basic PBu3 does not undergo exchange in trans-[PdCl(CH,Ph)(PBu,),]. Tertiary phosphine exchange does not occur with trans[PdCLPhL,] (L = PPh,, PMePh,, or PBu,), suggesting that it is the electron-releasing nature of the benzyl group that stabilizes the three-coordinate intermediate that would be formed initially upon ligand dissociation. Consistent with this, the complex trans-[PdCl(COCH,Ph)(PMePh,),] itself (vide infra) shows no sign of exchange, but exchange does occur with excess phosphine, the triplet at 6 2.05 (Table 11) being replaced by a singlet resonance at 6 2.00. A second possible source of the broadening of the 'H NMR signals due to trans-[PdCl(CH,Ph)(PMePh,),]could be the equilibration of this complex with [Pd(q3CH,Ph) (PMePh,),]+Cl-. Complexes of the latter type have been preparedlo but only with noncoordinating anions; indeed, addition of lithium chloride to [Pd(v3-CH2Ph)(PEt3),]+BF4- caused regeneration of trans-[PdCl(CH,Ph)(PEt,),]. Thus, it is not expected that such a rearrangement will occur with a chloride ligand present, and, if it should, the (q3-benzyl)palladiumcation would be best stabilized by the more basic PBu3 ligand. Since exchange does not occur for tr~ns-[PdCl(CH~Ph)(PBu~)~], the evidence seems to point to phosphine exchange as the source of the broadening. Cleavage of [Pd2(p- C1)ZPh2(PMePh2) with triphenylphosphine should, in principle, result in formation of a complex with four different ligands. In fact, within a few (10) Becker, Y.; Stille, J. K. J . Am. Chem. SOC.1978, 100, 845.
Organometallics, Vol. 2, No. 5, 1983 667
Chloride-Bridged Organopalladium(II) Dimers
Table 11. Spectroscopic Data for Complexes of the Types [Pd,(cr-Cl),(COR),L,], trans-[PdClRL,], and trans-[PdCl(COR)L,] 6(I3CO)
2J(P,C)/ Hz
216.5 218.1
10 8
Pd,Cl,(COCH,Ph),(PMePh,),
1675 1665 1685
Pd,Cl,( COPh),(PBu,),
1650
Pd,Cl,(COC,H,Me),(PBu,),
1645,1670
220.4 221.4 219.2
9 10 9
Pd,Cl,( COCH ,Ph),(PBu,), PdClPh(PPh,) , PdClPh( PMePh,) , PdC1( CH,Ph)( PMePh,) , PdClPh(PBu,), PdCl(C,H,Me)(PBu,),
1685
complex Pd,Cl,( COPh),( PPh,), Pd,Cl,( COPh) ,(PMePh,),
u(CO)/cm-'
S(P)
'H NMR data 6(CH,) 1 . 7 5 (,J(P,H) = 9 . 5 Hz) 6(CH,) 1 . 7 0 (,J(P,H) = 1 0 . 0 Hz) 6(CH,) 3 . 8 0 6(CH,) 2.30, 6(H) 7.05 and 8.00 ('J(H,H') = 8.0 Hz) 6(CH,) 4 . 1 5
23.4 7.2
6(CH,) 1.65 (,J(P,H) = 3.0 Hz) S(CH,) 2.15 (br), 6(CH,) 2.55 (br)
4.2 6(CH,I 2.20, 6(H) 6 . 8 0 and 7.10 .(,J(H,H1) = 8.O'Hz) 6(CH,) 2.60 (V(P,H) = 7.0 Hz)
PdC1( CH,Ph)( PBu,), PdCl( COPh)(PPh,), PdCI( COPh)(PMePh,), PdCl( COCH,Ph)( PMePh,),
1640 1635 1670
18.5 3.0
230.6 231.3
3 3
PdCl(COPh)(PBu,), PdCl(COC,H,Me)(PBu,),
1625 1625
2.6
234.0
