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The cis-[lcub]PdII(Me)(alkene)[rcub] fragment: stabilization and reactivity
E LSEVI E R
InorganicaChimicaActa258 (1997) 105-108
Note
The cis- {Pd ~ (Me) (alkene) } fragment: stabilization and reactivity A u g u s t o D e R e n z i * , I d a Orabona, F r a n c e s c o R u f f o Dipartimento di Chimica, Universit~di Napoli 'Federico I1". via Mezzocannone4.1-80134 Naples. Italy ReceivedI July 1996;accepted4 October 1996
Abstract The reaction of trimethyloxonium tetrafluomborate, Me3OBF4, with [Pd(dmf)(N,N-chelate)] precursors (N,N-chelateffi1,10-phenanthroline (phen) or 2,9-Me2-1,10-phenanthroline (dmphen); dmffdimethylfumarate) has been examined. When phen is present in the coordination sphere, the cyclometaUated square-planar complex [Pd{C~H(CO2Me)CH(Me)COOMe)}(phen) ]BF,, forms. Conversely, the more crowded dmphen stabilizes the five-coordinate adduct [Pd(Me)(dmf)(dmphen)(H20)]BF, t. This species reacts with e[eoa~qt-ric~ olefins RCH=CH2 (R = H, Me, Ph) leading to the corresponding [Pd(Me)(RCHfCH2)(dmphen)(H20)]BEt compounds, which ale stable in solution and afford within a few hours propene, 2-methylpropene and E-/~methyistyrene, respectively. Keywords: Oxidativeaddition;Palladiumcomplexes;Five-coordination;Alkene;Insertion
1. Introduction A recent report [ 1] by some of us described a facile synthesis of a new class of platinum(II)-alkene-methyl compounds, namely [Pt(Me) (alkene) (N,N-chelate) ] +, which were obtained by reacting the strong electrophile aimethyIoxonium tetrafluoroborate with a suitable three-coordinate [Pt(alkene) (N,N-chelate) ] substrate. These cationic complexes have been fully characterized and are inert towards the migratory insertion process, in spite of the cis arrangement of the alkene and Me group. Since recent theoretical calculations [2] on c/s[M(Me) (alkene) ] (M = I'd, Pt) fragments have shown that the activation energy of the migratory insertion reaction reduces by approximately a half on going from platinum to palladium, the synthesis of [Pd(Me)(alkene)(N,N-chelate) ] + complexes has been attempted, aiming to obtain further information about the stability and/or reactivity of this class of compounds.
2. Experimental 2.1. Reagents and methods
NMR spectra were recorded in CD3NO2 at 270 or 200 MHz on a Bruker AC-270 or a Varian XL-200 spectrometer, * Cotrespoodingauthor. 0020*16931971517.00 © 1997ElsevierScienceS.A.All rightsreserved PlI S0020-1693 (96) 05526-0
respectively, tH and ~3C ~@~IRchemical shifts are reported in 8 (ppm) relative to the solvent (CHD2NO2, 4.33 ppm; ~3CHD2NO2, 62.81 ppm). The following abbreviations are used in description of NMR multiplicities: s, singlet; d, doublet: t, triplet, q, quartet; m, multiplet; br, broad; qt, quaternary. IR spectra were recorded on a Perkin-Elmer 457 spectrophotometer in Nujol mulls. [Pd(dmf)(phen)] and [Pd(dmf)(dmphen)] (dmf=dimethylfinnarate, phen= 1,10-phenanthroline, dmphen=2,9-Me2-1,10-phenanth~ line) were prepared according to literature methods [3]. Trimethyloxonium tetrafluoroborate is commercially available and it was used as received. 2.2. Preparationof[Pdl~H(CO2Me)CH(Me)COOMe)J(phen)lBF,
A solution of Me3OBF4 (0.037 g, 0.25 mmol) in 1.5 ml of nitromethane was added under nitrogen to solid [Pd(dmf)(phen)] (0.108 g, 0.25 retool). Toluene (5 ml) was added to the resulting dark orange solution to produce a brown precipitate. The mixture was filtered through a Pasteur pipet containing a 1 em bed of Celite. The resulting light yellow solution was allowed to stand at 253 K for 24 h before the yellow product was isolated, washed with toluene (1 × 3 m l ) , n-pentane ( 2 × 5 m l ) and dried under vacumn (yield: 0.050 g, 40%). NMR (at 298 K), IR and analytical data are as follows: tH: 9.22 (d, lH), 8.93 (d, lH), 8.84 (d, lH), 8.82 (d, lH), 8.18 (ABq,2H), 8.08 (t, lH), 8.05 (t, lH), 4.26 (s,3H), 3.66 (s,3H), 3.6-3.5 (m,2I-I), 1.30
106
A. De Renziet al. / lnorganicaChiraicaActa258 (1997)105-108
(d,3H) ppm. '3C: 195.1 (CO), 177.3 (CO), 154.6and 150.7 (C2- and C9-phen), 148.9 and 145.5 (2 qt CN-phen), 141.5 and 141.0 (C4-and C7-phen), 132.1 and 131.6 (2 qt phen), 128.9 tC5- and C6-phen), 127.5 aud 12,7.3 (C3- and C8phen), 57.5 (OMe), 52.3 (OMe), 45.9 (CH), 37.3 (CH), 14.1 (CliMe) ppm. v(CO) = 1680, 1590 c m - J. Anal. Cale. for CIgH,gBF4N204Pd: C, 42.85; H, 3.60; N, 5.26. Found: C, 42.68; H, 3.71; N, 5.39.
2.3. Preparation of [Pd(Me)(dmf)(dmphen)(H20)]BF4 A solution of Me.aOBF4 (0.037 g, 0.25 mmol) in 1.5 ml of nitromethane was added to solid [Pd(dmf)(dmphen)] (0.115 g, 0.25 mmol). The resulting light brown suspension was filtered through a Pasteur pipet containing a I cm bed of Celite, and the product was crystallized by adding dry toluene (5--6 ml) to the resulting clear solution. The beige microcrystals were washed with toluene (1 × 3 ml), n-pentane (2 × 5 ml) and dried under vacuum (yield: 0.090 g, 63%). ~H NMR (at 298 K) and analytical data are as follows: 8.67 (d,2H), 8.11 (s,2H), 8.04 (d,2H), 5.20 (d, IH,CH=), 4.70 (d,IH,CH=), 3.89 (s,3H,OMe), 3.86 (s,3H,OMe), 3.33 (s,3H,Me--dmphen), 3.23 (s,3H,Me--dmphen), 2.70 (s,2H,H20), !.63 (s,3H,Pd-Me) ppm. Anal. Calc. for C21H25BF4N2OsPd: C, 43.59; H, 4.35; N, 4.84. Found: C, 43.16; H, 4.10; N, 5.03.
2.4. Preparation of [Pd(Me)(CHz=CHz)(dmphen)(H20)]BF, [Pd(Me)(dmf)(dmphen)(H20)]BF4 (0.058 g, 0.10 mmol) was dissolved in 1 ml of nitromethane at 273 K under ethylene atmosphere. Toluene ( 5 ml) was added dropwise to afford the product as an off-white solid. This was washed with toluene (1 × 3 mi), n-pentane ( 2 × 5 mi) and dried under vacuum (yield: 0.037 g, 80%). IH NMR (at 253 K) and analytical data are as follows: 8.60 (d,2H), 8.06 (s,2H), 7.98 (d,2H), 4.08 (d,2H,CH2=), 3.64 (d,2H,CH2=), 3.36 (s,6H,Me-dmphen), 0.86 (s,3H,Pd-Me) ppm. Anal. Calc. for CtTH2jBF4N2OPd: C, 44.14; H, 4.58; N, 6.06. Found: C, 44.23; H, 4.32; N, 6.38.
2.5. Preparation of[Pd(Me)(MeCH=CHz)(dmphen)(H20)IBF~ [Pd(Me)(dmf)(dmphen)(HzO)]BF4 (0.058 g, 0.10 mmol) was dissolved in 1 mi of nitromethane at 273 K under a propene atmosphere. Toluene (5 ml) and then n-pentane (3 ml) were added dropwise to afford the product as an offwhite solid. This was washed with n-pentane (2 × 5 ml) and dried under vacuum (yield: 0.033 g, 70%). ~H NMR (at 253 K) and analytical data are as follows: 8.60 (br d,2H), 8.10 (br s,2H), 8.0 (br d,2H), 3.6-4.2 (br m,3H,CH2=CH-), 3.42 (br s,3H,Me-dmphen), 3.38 (br s,3H,Me-dmphen), 1.70 (br d,3H,MeCH=), 0.81 (br s, 3H, Pd-Me) ppm. Anal.
Calc. for CIsH23BF4N2OPd: C, 45.36; H, 4.86; N, 5.88. Found: C, 45.08; H, 4.74; N, 6.01.
2.6. Monitoring of the reaction between [Pd(Me)(dmfl(dmphen )(HzO)]( BF4) and Ph CH=CH2 To a solution of [Pd(Me) (dmf) (dmphen) (I-120) ] (BF4) (0.011 g, 0.02 retool) in 1 ml of deuteronitromethane was added styrene (0.002 g, 0.02 retool). The immediate formation of the Pd(II)-styrene complex was detected, which afforded quantitatively E-/~l-methylstyrene within a few hours. Selected 'H NMR (at 298 K) are as follows: 6.2 (br m, lH), 5.3 (br m, lH), 4.6 (br m,lH), 3.35 (br s,6H,Medmphen), 1.10 (br s,3H,Pd-Me) ppm.
3. Results and discussion A simple synthetic procedure possibly affording [Pd(Me)(aikene)(N,N-ehelate)] + species, i.e. ligand exchange on suitable [Pd(Me) (L) (N,N-chelate) ] + (L = neutral ligand) complexes, turned out to be unsatisfactory owing to the presence of the L ligand. In fact, either L does not leave the coordination sphere (a five-coordinate adduct can be thus isolated [4] ), or the released L promotes an insertion process [5]. Instead, the introduction oftbe Me group onto a [ Pd(alkene) (N,N-chelate) ] substrate could be performed through an oxidative addition similar to that successfully achieved in platinum chemistry [ 1]. In a first attempt, when [Pd(dmf) (pben) ] was allowed to react with trimethyloxonium tetrafluoroborate in nitromethane solution i, a yellow complex was isolated by recrystallization. It is a 1:1 electrolyte in nitromethane [6], and its analytical and spectroscopic characterization data [IR: Vco = 1680 and 1590 cm - I; iH NMR: 8oMc= 4.27 and 3.66, ~ = 1.30 (d) ppm; '3C NMR: #co = 195.1 and 177.3 ppm] suggest [ 7] the formation of a metallated five-membered ring as depicted in Fig. 1. When the reaction was monitored by IH NMR spectroscopy, the above-reported complex was immediately observed with no evidence of the presence of a [Pd(Me)(dmf)(phen) ] + species. Since the migratory ' tion reaction requires a coplanar arrangement of the IV vector and of the olefin double bond, a low rotation barn~..round the M alkene bond could account for the lack of isolation of the hypothesized intermediate. Therefore further attempts were made by using a N,N-ehelate with encumbering "in plane" substituents, i.e. dmpben. By adding a nitromethane solution of trimethyloxonium tetrafluoroborate to [Pd(dmf)(dmphen)], a cationic palladium complex was isolated in good yield. This product was i The reactionrequiresHPLCgradenitromethane,sincea less pure solvent was foundto containtracesof acetonitrileor propionitrilewhichlead to the formation of the corresponding square-planar c~mplexes [Pd(Mu)(RCN) (phen) ] +.
A. De Renzi et al. I lnorganica Chimica Acta 258 fl997) 105-108
Fig. 1. Possiblestructureof the yellowcomplex.
stable in the solid state, while in solution a slow decomposition process took place. The tH NMR spectrum shows a clear resonance signal attributable to the Pd-bound methyl ( ~ c = 1.63 ppm) and the non-equivalence of the chemically corresponding dmf protons (=CH and CO2Me), as well as of the two halves of the dmphen. These data suggest a restricted rotation of dmf around the Pd-alkene bond and the absence of fast ligand exchange processes, at least on the tH NMR time-scale. It seems unreliable that the product is a square-planar Pd(II) complex. In fact, its stability, although moderate, is not consistent with the presence of the electrondeficient dmf in a cationic four-coordinate environment. Furthermore, the NMR spectrum in dry deuteronitromethane showed a broad signal at 8=2.7 ppm integrating for two protons. This signal disappeared when the solution was shaken with D20. Thus, a five-coordinate adduct with a water molecule in axial position, as shown by Eq. (1), can better account for the ensemble of results.
<.=~..~-rco,,,,
.~o
~_.~:,r~
~,
(~)
Me
When the reaction was carried out in deuteronitromethane under conditions which aimed to avoid as much as possible the presence of water, could we only detect unreacted Me3OBF4 and small amounts of the above five-coordinate product. A yellow insoluble solid was also present, which was extracted with CDCI3 and found to contain unreacted [Pd(dmphen) (dmO ]. These results suggest that the driving force of reaction (1) is the formation of the five-coord/nate aquo-complex, and therefore the auack proceeds only in presence of water. Furthermore, it is worth mentioning that similar five-coordinate water adducts are already known in platinum(H) chemistry [8]. In nitromethane solution dmf was rapidly displaced by neutral donor ligands, such as MeCN, affording the known square-planar species [Pd(Me)(MeCN)(dmphen)] + [5] or by electron-rich olefins (ethylene, propeue). The correspendiug five-coordinate derivatives [Pd(Me)(RCH=CH2)(dmphen) (H20) ] + (R = H, Me) could be isolated by adding toluene and n-pe=~ne to the reaction mixture at 273 K: [Pd(Me) (dmf) (dmphen) (H20) ] + (BF4) + RCH=CH2 [Pd(Me) (RCH=CH2) (dmphen) (H20) ] + (BF4 ) +dmf
(2)
107
Fast olefin exchange is inferred by the tH NMR spectra of these two complexes, which were recorded at room temperature in CD3NO2 immediately after the dissolution. On cooling at 253 K, the olefin exchange rote clearly lowers and the alkeue rotation is restricted. In fact, in the case of the ethylene complex the broad olefin signal observed at 3.95 6 at 298 K resolves into two doublets (3.70 and 4.10 8) at 253 IC In addition, only one of the two possible retainers is detectable for the propene compound at 253 K. When the above complexes were allowed to stand in nitromethane solution, a migratory insertion process took place going to completion within 1 h at room temperature. In particular, a mixture of propene and 2-methylpropene was obtained starting from the ethylene complex, the all-substituted alkeue deriving from a competing o[efin exchaage process involving the formed propeue and the ethylene compm: ad. Only 2-methylpropene was instead detected when the propene complex was used as starting materiaL Styrene also substitutes dmf in [Pd(Me)(dmf)( dmphen ) (H20) ] + species. No attempts were made to isolate the correspomling complex in the solid state, and the subsequent migratory insertion process qua~tatively afforded E-fl-methylstyrene. The above reported data constitute the expefimemal couw terpart of the cited theoretical calculations [2] on the activation energy of the migratory insertion process. Its low value for Pd(II) versus Pt(II) species ma~es prohibitive the detection at room temperature of four-coordinate [Pd(Me)(alkene)(N,N-chelate)] + complexes 2. The alaempt to increase the barrier, by acting on the contribution related to the rotation of the olefin around the Pd-allcene bond llaeugh the introduction of steric constraints on the comdination plane, is actually effective in lowering the reactionmte. However, the same factor strongly favours the competing fornmtion e r a five-coordinate adduct, which is achieved by a~lition of Lewis bases present in the reaction system.
Acknowledgements We thank the Consiglio Nazionale delle Riceg:he and the Ministerodell'Universit~e dellaRicercaScientificaforfian~ cial supl~n and the Centre Interdipmtimcatalo di Metodologie Chimico-Fisiche, Universit~ di Napoli ' ~ lI', for NMR facilities.
References [I] I. Orabona,A. Paaun~iand F. Ruffo,J. Orgas~net. C/~m.,in pt~ss~ [2] P.M.Siegb~m,K. ~tterbe~. S..~xOml~g.Musa~. M. Svensseaand K. Morokunm,to t~ publishe~ [3] M.E. Cuccinlito,A. De Renzi, F. Ginrdano aad F. Ruffo, ~etallics. 14 (1995) 54.10. =T~ squmc-p~nm- com~x [Pa(Mc)(C2H4)(p~m)] ~ has been detectedin solutinn at 163 K [91.
108
A. De Renzi et at/lnorganica Chimica Acta 258 (1997) 105-108
[4] V.G. Albano, C. Ca~tellafi, M.E. Cucciolito, A. Panunzi and A. Vitagliano, OrganometaUics, 9 (1990) 1269. [5] M.E. Cucciolito, V. De Felice, A. De Renzi, F. Ruffo and D. Tesauro, J. Organomet. Chem., 493 (1995) 1. [6] WJ. Geary, CoorR Chem. Rev., 7 ( 1971 ) 81.
[ 7] V. De Felice, A. De Renzi, F. Ruffo and D. Tesauro, lnorg. Chim. Acta, 219(1994) 169. [8] V. De Felice, M.L. Ferrara, I. Orabona and F. Ruffo, J. Organomet. Chem., 519 (1996) 75. [9] F.C. Rix and M. Brookhart, J. Am. Chem. Soc., 117 (1995) 1137.
InorganicaChimicaActa258 (1997) 105-108
Note
The cis- {Pd ~ (Me) (alkene) } fragment: stabilization and reactivity A u g u s t o D e R e n z i * , I d a Orabona, F r a n c e s c o R u f f o Dipartimento di Chimica, Universit~di Napoli 'Federico I1". via Mezzocannone4.1-80134 Naples. Italy ReceivedI July 1996;accepted4 October 1996
Abstract The reaction of trimethyloxonium tetrafluomborate, Me3OBF4, with [Pd(dmf)(N,N-chelate)] precursors (N,N-chelateffi1,10-phenanthroline (phen) or 2,9-Me2-1,10-phenanthroline (dmphen); dmffdimethylfumarate) has been examined. When phen is present in the coordination sphere, the cyclometaUated square-planar complex [Pd{C~H(CO2Me)CH(Me)COOMe)}(phen) ]BF,, forms. Conversely, the more crowded dmphen stabilizes the five-coordinate adduct [Pd(Me)(dmf)(dmphen)(H20)]BF, t. This species reacts with e[eoa~qt-ric~ olefins RCH=CH2 (R = H, Me, Ph) leading to the corresponding [Pd(Me)(RCHfCH2)(dmphen)(H20)]BEt compounds, which ale stable in solution and afford within a few hours propene, 2-methylpropene and E-/~methyistyrene, respectively. Keywords: Oxidativeaddition;Palladiumcomplexes;Five-coordination;Alkene;Insertion
1. Introduction A recent report [ 1] by some of us described a facile synthesis of a new class of platinum(II)-alkene-methyl compounds, namely [Pt(Me) (alkene) (N,N-chelate) ] +, which were obtained by reacting the strong electrophile aimethyIoxonium tetrafluoroborate with a suitable three-coordinate [Pt(alkene) (N,N-chelate) ] substrate. These cationic complexes have been fully characterized and are inert towards the migratory insertion process, in spite of the cis arrangement of the alkene and Me group. Since recent theoretical calculations [2] on c/s[M(Me) (alkene) ] (M = I'd, Pt) fragments have shown that the activation energy of the migratory insertion reaction reduces by approximately a half on going from platinum to palladium, the synthesis of [Pd(Me)(alkene)(N,N-chelate) ] + complexes has been attempted, aiming to obtain further information about the stability and/or reactivity of this class of compounds.
2. Experimental 2.1. Reagents and methods
NMR spectra were recorded in CD3NO2 at 270 or 200 MHz on a Bruker AC-270 or a Varian XL-200 spectrometer, * Cotrespoodingauthor. 0020*16931971517.00 © 1997ElsevierScienceS.A.All rightsreserved PlI S0020-1693 (96) 05526-0
respectively, tH and ~3C ~@~IRchemical shifts are reported in 8 (ppm) relative to the solvent (CHD2NO2, 4.33 ppm; ~3CHD2NO2, 62.81 ppm). The following abbreviations are used in description of NMR multiplicities: s, singlet; d, doublet: t, triplet, q, quartet; m, multiplet; br, broad; qt, quaternary. IR spectra were recorded on a Perkin-Elmer 457 spectrophotometer in Nujol mulls. [Pd(dmf)(phen)] and [Pd(dmf)(dmphen)] (dmf=dimethylfinnarate, phen= 1,10-phenanthroline, dmphen=2,9-Me2-1,10-phenanth~ line) were prepared according to literature methods [3]. Trimethyloxonium tetrafluoroborate is commercially available and it was used as received. 2.2. Preparationof[Pdl~H(CO2Me)CH(Me)COOMe)J(phen)lBF,
A solution of Me3OBF4 (0.037 g, 0.25 mmol) in 1.5 ml of nitromethane was added under nitrogen to solid [Pd(dmf)(phen)] (0.108 g, 0.25 retool). Toluene (5 ml) was added to the resulting dark orange solution to produce a brown precipitate. The mixture was filtered through a Pasteur pipet containing a 1 em bed of Celite. The resulting light yellow solution was allowed to stand at 253 K for 24 h before the yellow product was isolated, washed with toluene (1 × 3 m l ) , n-pentane ( 2 × 5 m l ) and dried under vacumn (yield: 0.050 g, 40%). NMR (at 298 K), IR and analytical data are as follows: tH: 9.22 (d, lH), 8.93 (d, lH), 8.84 (d, lH), 8.82 (d, lH), 8.18 (ABq,2H), 8.08 (t, lH), 8.05 (t, lH), 4.26 (s,3H), 3.66 (s,3H), 3.6-3.5 (m,2I-I), 1.30
106
A. De Renziet al. / lnorganicaChiraicaActa258 (1997)105-108
(d,3H) ppm. '3C: 195.1 (CO), 177.3 (CO), 154.6and 150.7 (C2- and C9-phen), 148.9 and 145.5 (2 qt CN-phen), 141.5 and 141.0 (C4-and C7-phen), 132.1 and 131.6 (2 qt phen), 128.9 tC5- and C6-phen), 127.5 aud 12,7.3 (C3- and C8phen), 57.5 (OMe), 52.3 (OMe), 45.9 (CH), 37.3 (CH), 14.1 (CliMe) ppm. v(CO) = 1680, 1590 c m - J. Anal. Cale. for CIgH,gBF4N204Pd: C, 42.85; H, 3.60; N, 5.26. Found: C, 42.68; H, 3.71; N, 5.39.
2.3. Preparation of [Pd(Me)(dmf)(dmphen)(H20)]BF4 A solution of Me.aOBF4 (0.037 g, 0.25 mmol) in 1.5 ml of nitromethane was added to solid [Pd(dmf)(dmphen)] (0.115 g, 0.25 mmol). The resulting light brown suspension was filtered through a Pasteur pipet containing a I cm bed of Celite, and the product was crystallized by adding dry toluene (5--6 ml) to the resulting clear solution. The beige microcrystals were washed with toluene (1 × 3 ml), n-pentane (2 × 5 ml) and dried under vacuum (yield: 0.090 g, 63%). ~H NMR (at 298 K) and analytical data are as follows: 8.67 (d,2H), 8.11 (s,2H), 8.04 (d,2H), 5.20 (d, IH,CH=), 4.70 (d,IH,CH=), 3.89 (s,3H,OMe), 3.86 (s,3H,OMe), 3.33 (s,3H,Me--dmphen), 3.23 (s,3H,Me--dmphen), 2.70 (s,2H,H20), !.63 (s,3H,Pd-Me) ppm. Anal. Calc. for C21H25BF4N2OsPd: C, 43.59; H, 4.35; N, 4.84. Found: C, 43.16; H, 4.10; N, 5.03.
2.4. Preparation of [Pd(Me)(CHz=CHz)(dmphen)(H20)]BF, [Pd(Me)(dmf)(dmphen)(H20)]BF4 (0.058 g, 0.10 mmol) was dissolved in 1 ml of nitromethane at 273 K under ethylene atmosphere. Toluene ( 5 ml) was added dropwise to afford the product as an off-white solid. This was washed with toluene (1 × 3 mi), n-pentane ( 2 × 5 mi) and dried under vacuum (yield: 0.037 g, 80%). IH NMR (at 253 K) and analytical data are as follows: 8.60 (d,2H), 8.06 (s,2H), 7.98 (d,2H), 4.08 (d,2H,CH2=), 3.64 (d,2H,CH2=), 3.36 (s,6H,Me-dmphen), 0.86 (s,3H,Pd-Me) ppm. Anal. Calc. for CtTH2jBF4N2OPd: C, 44.14; H, 4.58; N, 6.06. Found: C, 44.23; H, 4.32; N, 6.38.
2.5. Preparation of[Pd(Me)(MeCH=CHz)(dmphen)(H20)IBF~ [Pd(Me)(dmf)(dmphen)(HzO)]BF4 (0.058 g, 0.10 mmol) was dissolved in 1 mi of nitromethane at 273 K under a propene atmosphere. Toluene (5 ml) and then n-pentane (3 ml) were added dropwise to afford the product as an offwhite solid. This was washed with n-pentane (2 × 5 ml) and dried under vacuum (yield: 0.033 g, 70%). ~H NMR (at 253 K) and analytical data are as follows: 8.60 (br d,2H), 8.10 (br s,2H), 8.0 (br d,2H), 3.6-4.2 (br m,3H,CH2=CH-), 3.42 (br s,3H,Me-dmphen), 3.38 (br s,3H,Me-dmphen), 1.70 (br d,3H,MeCH=), 0.81 (br s, 3H, Pd-Me) ppm. Anal.
Calc. for CIsH23BF4N2OPd: C, 45.36; H, 4.86; N, 5.88. Found: C, 45.08; H, 4.74; N, 6.01.
2.6. Monitoring of the reaction between [Pd(Me)(dmfl(dmphen )(HzO)]( BF4) and Ph CH=CH2 To a solution of [Pd(Me) (dmf) (dmphen) (I-120) ] (BF4) (0.011 g, 0.02 retool) in 1 ml of deuteronitromethane was added styrene (0.002 g, 0.02 retool). The immediate formation of the Pd(II)-styrene complex was detected, which afforded quantitatively E-/~l-methylstyrene within a few hours. Selected 'H NMR (at 298 K) are as follows: 6.2 (br m, lH), 5.3 (br m, lH), 4.6 (br m,lH), 3.35 (br s,6H,Medmphen), 1.10 (br s,3H,Pd-Me) ppm.
3. Results and discussion A simple synthetic procedure possibly affording [Pd(Me)(aikene)(N,N-ehelate)] + species, i.e. ligand exchange on suitable [Pd(Me) (L) (N,N-chelate) ] + (L = neutral ligand) complexes, turned out to be unsatisfactory owing to the presence of the L ligand. In fact, either L does not leave the coordination sphere (a five-coordinate adduct can be thus isolated [4] ), or the released L promotes an insertion process [5]. Instead, the introduction oftbe Me group onto a [ Pd(alkene) (N,N-chelate) ] substrate could be performed through an oxidative addition similar to that successfully achieved in platinum chemistry [ 1]. In a first attempt, when [Pd(dmf) (pben) ] was allowed to react with trimethyloxonium tetrafluoroborate in nitromethane solution i, a yellow complex was isolated by recrystallization. It is a 1:1 electrolyte in nitromethane [6], and its analytical and spectroscopic characterization data [IR: Vco = 1680 and 1590 cm - I; iH NMR: 8oMc= 4.27 and 3.66, ~ = 1.30 (d) ppm; '3C NMR: #co = 195.1 and 177.3 ppm] suggest [ 7] the formation of a metallated five-membered ring as depicted in Fig. 1. When the reaction was monitored by IH NMR spectroscopy, the above-reported complex was immediately observed with no evidence of the presence of a [Pd(Me)(dmf)(phen) ] + species. Since the migratory ' tion reaction requires a coplanar arrangement of the IV vector and of the olefin double bond, a low rotation barn~..round the M alkene bond could account for the lack of isolation of the hypothesized intermediate. Therefore further attempts were made by using a N,N-ehelate with encumbering "in plane" substituents, i.e. dmpben. By adding a nitromethane solution of trimethyloxonium tetrafluoroborate to [Pd(dmf)(dmphen)], a cationic palladium complex was isolated in good yield. This product was i The reactionrequiresHPLCgradenitromethane,sincea less pure solvent was foundto containtracesof acetonitrileor propionitrilewhichlead to the formation of the corresponding square-planar c~mplexes [Pd(Mu)(RCN) (phen) ] +.
A. De Renzi et al. I lnorganica Chimica Acta 258 fl997) 105-108
Fig. 1. Possiblestructureof the yellowcomplex.
stable in the solid state, while in solution a slow decomposition process took place. The tH NMR spectrum shows a clear resonance signal attributable to the Pd-bound methyl ( ~ c = 1.63 ppm) and the non-equivalence of the chemically corresponding dmf protons (=CH and CO2Me), as well as of the two halves of the dmphen. These data suggest a restricted rotation of dmf around the Pd-alkene bond and the absence of fast ligand exchange processes, at least on the tH NMR time-scale. It seems unreliable that the product is a square-planar Pd(II) complex. In fact, its stability, although moderate, is not consistent with the presence of the electrondeficient dmf in a cationic four-coordinate environment. Furthermore, the NMR spectrum in dry deuteronitromethane showed a broad signal at 8=2.7 ppm integrating for two protons. This signal disappeared when the solution was shaken with D20. Thus, a five-coordinate adduct with a water molecule in axial position, as shown by Eq. (1), can better account for the ensemble of results.
<.=~..~-rco,,,,
.~o
~_.~:,r~
~,
(~)
Me
When the reaction was carried out in deuteronitromethane under conditions which aimed to avoid as much as possible the presence of water, could we only detect unreacted Me3OBF4 and small amounts of the above five-coordinate product. A yellow insoluble solid was also present, which was extracted with CDCI3 and found to contain unreacted [Pd(dmphen) (dmO ]. These results suggest that the driving force of reaction (1) is the formation of the five-coord/nate aquo-complex, and therefore the auack proceeds only in presence of water. Furthermore, it is worth mentioning that similar five-coordinate water adducts are already known in platinum(H) chemistry [8]. In nitromethane solution dmf was rapidly displaced by neutral donor ligands, such as MeCN, affording the known square-planar species [Pd(Me)(MeCN)(dmphen)] + [5] or by electron-rich olefins (ethylene, propeue). The correspendiug five-coordinate derivatives [Pd(Me)(RCH=CH2)(dmphen) (H20) ] + (R = H, Me) could be isolated by adding toluene and n-pe=~ne to the reaction mixture at 273 K: [Pd(Me) (dmf) (dmphen) (H20) ] + (BF4) + RCH=CH2 [Pd(Me) (RCH=CH2) (dmphen) (H20) ] + (BF4 ) +dmf
(2)
107
Fast olefin exchange is inferred by the tH NMR spectra of these two complexes, which were recorded at room temperature in CD3NO2 immediately after the dissolution. On cooling at 253 K, the olefin exchange rote clearly lowers and the alkeue rotation is restricted. In fact, in the case of the ethylene complex the broad olefin signal observed at 3.95 6 at 298 K resolves into two doublets (3.70 and 4.10 8) at 253 IC In addition, only one of the two possible retainers is detectable for the propene compound at 253 K. When the above complexes were allowed to stand in nitromethane solution, a migratory insertion process took place going to completion within 1 h at room temperature. In particular, a mixture of propene and 2-methylpropene was obtained starting from the ethylene complex, the all-substituted alkeue deriving from a competing o[efin exchaage process involving the formed propeue and the ethylene compm: ad. Only 2-methylpropene was instead detected when the propene complex was used as starting materiaL Styrene also substitutes dmf in [Pd(Me)(dmf)( dmphen ) (H20) ] + species. No attempts were made to isolate the correspomling complex in the solid state, and the subsequent migratory insertion process qua~tatively afforded E-fl-methylstyrene. The above reported data constitute the expefimemal couw terpart of the cited theoretical calculations [2] on the activation energy of the migratory insertion process. Its low value for Pd(II) versus Pt(II) species ma~es prohibitive the detection at room temperature of four-coordinate [Pd(Me)(alkene)(N,N-chelate)] + complexes 2. The alaempt to increase the barrier, by acting on the contribution related to the rotation of the olefin around the Pd-allcene bond llaeugh the introduction of steric constraints on the comdination plane, is actually effective in lowering the reactionmte. However, the same factor strongly favours the competing fornmtion e r a five-coordinate adduct, which is achieved by a~lition of Lewis bases present in the reaction system.
Acknowledgements We thank the Consiglio Nazionale delle Riceg:he and the Ministerodell'Universit~e dellaRicercaScientificaforfian~ cial supl~n and the Centre Interdipmtimcatalo di Metodologie Chimico-Fisiche, Universit~ di Napoli ' ~ lI', for NMR facilities.
References [I] I. Orabona,A. Paaun~iand F. Ruffo,J. Orgas~net. C/~m.,in pt~ss~ [2] P.M.Siegb~m,K. ~tterbe~. S..~xOml~g.Musa~. M. Svensseaand K. Morokunm,to t~ publishe~ [3] M.E. Cuccinlito,A. De Renzi, F. Ginrdano aad F. Ruffo, ~etallics. 14 (1995) 54.10. =T~ squmc-p~nm- com~x [Pa(Mc)(C2H4)(p~m)] ~ has been detectedin solutinn at 163 K [91.
108
A. De Renzi et at/lnorganica Chimica Acta 258 (1997) 105-108
[4] V.G. Albano, C. Ca~tellafi, M.E. Cucciolito, A. Panunzi and A. Vitagliano, OrganometaUics, 9 (1990) 1269. [5] M.E. Cucciolito, V. De Felice, A. De Renzi, F. Ruffo and D. Tesauro, J. Organomet. Chem., 493 (1995) 1. [6] WJ. Geary, CoorR Chem. Rev., 7 ( 1971 ) 81.
[ 7] V. De Felice, A. De Renzi, F. Ruffo and D. Tesauro, lnorg. Chim. Acta, 219(1994) 169. [8] V. De Felice, M.L. Ferrara, I. Orabona and F. Ruffo, J. Organomet. Chem., 519 (1996) 75. [9] F.C. Rix and M. Brookhart, J. Am. Chem. Soc., 117 (1995) 1137.