Organometallics 1996, 14,551-554
551
Highly Reactive Platinum(0) Carbene Intermediates in the Reactions of Diazo Compounds. A Fast Atom Bombardment Mass Spectrometric Study Roberta Bertani,? Rino A. Michelin,*?$ Mirto Mozzon,*Piero Traldi,e Roberta Seraglia,s Maria de Fatima Costa Guedes da Silva," and Armando J. L. Pombeiro*JI Centro di Chimica e Tecnologia dei Composti Metallorganici degli Elementi di Transizione del CNR and Istituto di Chimica Industriale, Facolta di Ingegneria, Universita di Padova, Via F. Marzolo 9, 35131 Padova, Italy, Servizio di Spettrometria di Massa, Area della Ricerca, CNR, Corso Stati Uniti 4, 35100 Padova, Italy, and Centro de Quimica Estrutural, Complex0 I, Instituto Superior Tkcnico, Av. Rovisco Pais, 1096 Lisbon codex, Portugal Received March 14, 1994@ Summary: The reaction of [Pt(CH2=CHd(PPhdd (1) with N#HCO$t, carried out directly in the FAB matrix and monitored by MS, leads to molecular ions corresponding to the species [Pt{N2C(H)CO&t}(PPhdd, [Pt{ C(H)CO,&t}(PPhdd, and [Pt{ C(H)CO&t}(PPhdd2, which are involved as intermediates in the formation of the final fumarate Pt(0) derivative [Pt{trans-C(H)C02Et=C(H)CO&)(PPhdJ. Chemical evidence of the intermediacy of a Pt(0) carbene stems from the fact that 1 is active in the cyclopropanation of styrene in the presence of N2CHCOat.
formation of the fumarate complex [Pt{trans-CH(COzEt)=CH(COzEt)}(PPh3)21(2) (eq 1):5
Introduction
Although the formation of the olefin in complex 2 was suggested to proceed through the intermediacy of a F't(0)-carbene (in view of the known coupling reactions of highly reactive carbenes to afford olefins6),no such intermediate was spectroscopically detected. Following some promising results that we have recently obtained on ion-molecule reactions performed by mass ~pectrometry,~ we thought it was of interest to study reaction 1under fast atom bombardment (FAB) conditions since mass spectrometry, coupled with the mass-analyzed ion kinetic energy (MIKE) technique, allows the detection as well as the structural characterization of ionic species having lifetimes less than s. Furthermore, FAB conditions, being milder than those of electron impact (EI),s have been shown to be
While there has been considerable interest in the reactions of diazo compounds with transition metal centers to afford carbenes,l a relatively small number of investigations have been reported for zerovalent group 10 transition metal complexes.2 The products of the latter reactions include diazoalkane complexes2b-e or derived species such as ketenimines2eor azines2flgbut no carbenes have been isolated or detected, in spite of being postulated as intermediates in some of the abovementioned processes as well as in the catalytic cyclopropanation of olefin~.la,~,~ Our interest in the synthesis, chemistry, and electrochemistry of group 10 transition metal carbene complexes* led us recently to investigate the reaction of ethyl diazoacetate, N2CHC02Et, with the Pt(0) complex [Pt(CHz=CHz)(PPh3)2](l),which leads in solution to the + Centro di Chimica e Tecnologia dei Composti Metallorganici degli Elementi di Transizione, CNR, Padova. Istituto di Chimica Industriale, Universith di Padova. 4 Servizio di Spettrometria di Massa, Area della Ricerca, CNR, Padova. 11 Instituto Superior TBcnico, Lisbon. Abstract published in Advance ACS Abstracts, November I, 1994. (1)(a) Sutton, D. Chem. Rev. 1993,93,995. (b)Doyle, M. P. Chem. Rev. 1986,86,919. (c) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1978,17,800. (d) Hillhouse, G. L.; Haymore, B. L. J.A m . Chem. SOC. 1982, 104, 1537 and references therein. (2)(a) Schramm, K. D.; Ibers, J. A. Znorg. Chem. 1980, 19, 2441. (b) Nakamura, A.; Yoshida, T.; Cowie, M.; Otsuka, S.; Ibers, J. A. J. Am. Chem. Soc. 1977,99,2108. (c) Otsuka, S.; Nakamura, A.; Koyama, T.; Tatsumo, Y.; Liebigs Ann. Chem. 1975, 626. (d) Otsuka, S.; Nakamura, A.; Koyama, T.; Tataumo, Y. J. Chem. Soc., Chem. Commun. 1972, 1105. (e) Yarrow, D. J.; Ibers, J. A.; Tatsumo, Y.; 1973,95,8590. (0 Clemens, J.; Green, Otsuka, S.J.Am. Chem. SOC. M.; Stone, F. G. A. J.Chem. Soc.,Dalton Trans. 1973,1620. (g) Cardin, D. J.; Cetinkaya, B.; Cetinkaya, E.; Lappert, M. F. Zbid. 1973, 514. (3) Weiss, K. In Transition Metal Carbene Complexes; Verlag Chemie: Weinheim, 1983; p. 228.
*
@
(4) See, for example: Michelin, R. A.; Zanotto, L.; Braga, D.; Sabatino, P.; hgelici, R. J. Znorg. Chem. 1988, 27, 85. Idem. Zbid. 1988,27, 93. Bertani, R.; Mozzon, M.; Michelin, R. A. Znorg. Chem. 1988,27,2809. Michelin, R. A.; Ros, R.; Guadalupi, G.; Bombieri, G.; Benetollo, F.; Chapuis, G. Znorg. Chem. 1989, 28, 840. Michelin, R. A,; Bertani, R.; Mozzon, M.; Zanotto, L.; Benetollo, F.; Bombieri, G. Organometallics 1990, 9, 1449. Zanotto, L.; Bertani, R.; Michelin, R. A. h o g . Chem. 1990,29,3265. Bertani, R.; Mozzon, M.; Michelin, R. A.; Benetollo, F.; Bombieri, G.; Castilho, T. J.;Pombeiro, A. J. L. Znorg. Chim. Acta 1991,189, 175. Bertani, R.; Mozzon, M.; Michelin, R. A.; Castilho, T. J.; Pombeiro, A. J. L. J.Organomet. Chem. 1992,431,117. Castilho, T. J.;Guedes da Silva, M. F. C.; Pombeiro, A. J. L.; Bertani, R.; Mozzon, M.; Michelin, R. A. In Molecular Electrochemisty of Inorganic, Bioinorganic and Organometallic Compounds; Pombeiro, A. J . L., McCleverty, J. A., Eds.; NATO AS1 Series C386; Kluwer Academic Publishers: Dordrecht, Holland, 1993; p 345. Michelin, R. A,; Bertani, R.; Mozzon, M.; Benetollo, F.; Bombieri, G.; Silva, M. F. C. G.; Pombeiro, A. J. L. Organometallics 1993, 12, 2372. (5) Silva, M. F. C. G.; Silva, J. J. R. F.; Pombeiro, A. J. L.; Bertani, R.; Michelin, R. A,; Mozzon, M.; Benetollo, F.; Bombieri, G.; Znorg. Chim. Acta 1993,214, 85. (6) Seitz, W. J.; Saha, A. K.; Hossain, M. M. Organometallics 1993, 12,2604. (b) Woo, L. IC;Smith, D. A. Organometallics 1992,11,2344. (7) (a) Bertani, R.; Mozzon, M.; Michelin, R. A.; Seraglia, R.; Traldi, P. Org. Mass Spectrom. 1992, 27, 1187. (b) Bertani, R.; Cecchetto, W.: Polloni, R.: Crociani, B.: Seraelia, - R.: Traldi, P. Znorg. - Chim. Acta 1990, 174, 61.
0276-733319512314-0551$09.00/0 0 1995 American Chemical Society
552 Organometallics, Vol. 14, No. 1, 1995
Notes
quite suitable for the study of organometallic and coordination systems, which appear to be stable in the matrix.g We report herein a successful detection by FAB MS of a Pt(0)-carbene derivative as well as of other reactive species, which are likely to be involved in reaction 1. Furthermore, chemical evidence for the intermediacy of a zerovalent Pt-carbene species in reaction 1 is given also from experimental results in which 1 is shown to promote the cyclopropanationlO of styrene in the presence of N2CHCOzEt. 804
Experimental Section [P~(CHZ=CH~)(PP~ (1) ~ )was Z ] prepared by a published method,ll whereas ethyl diazoacetate was used as purchased from Aldrich. Compound 2 was prepared as previously r e p ~ r t e d .*H ~ and 13C NMR spectra were recorded on a AC 200 Bruker spectrometer. In a separate experiment, reaction 1was carried out in a N M R tube by reacting 1 (0.075 g, 0.013 mmol) and NzCHCOOEt (210 pL, 0.199 mmol) in CDzClz (0.8 mL), and it was followed by 'H and 13C NMR at variable temperature in the range 223-278 K (see Results and Discussion). All mass spectrometric measurements were performed on a VG ZAB 2F instrument.12 Positive ion FAB mass spectra were obtained by bombarding 3-nitrobenzyl alcohol solutions of the samples with 8-keV Xe atoms.13 Metastable transitions were detected by MIKE ~pectr0metry.l~~ Collisionally induced decomposition mass-analyzed ion kinetic energy (CID MIKE) spectra were obtained with 8-keV ions colliding with air in the second field free region.14bAccurate mass measurements for all metal-containing fragment ions were obtained with the peak matching technique at 20 000 resolving power (10% valley definition). Nominal molecular masses were calculated using the mass 194 isotope of platinum. Precursor ion scans were obtained by B21E = constant linked scans.15 Reaction 1was performed under FAB conditions by initially mixing 1 (37.3 mg, 0.05 mmol) and NzCHCOzEt (119 pL, 0.100 mmol) with 3-nitrobenzyl alcohol (1.5mL), placing the reaction mixture on the probe tip and then introducing it immediately into the source. The cyclopropanation reaction was carried out according to the following procedure. Compound 1 (0.010 g, 0.013 mmol) was dissolved in 5 mL of CHzClz, and then, under stirring, styrene (0.500 g, 0.46 mmol) was added, followed by ethyl diazoacetate (100 pL, 0.084 mmol). The reaction mixture was stirred at ambient temperature for 24 h, and then the solvent was evaporated under reduced pressure t o give an oil, which was treated with 1mL of CHzClz and analyzed by GCMS (on a QMDlOOO instrument) using a PS 264 column, 25 m, 0.25 pm, from 100 t o 250 "C, lO"/min. The GC/MS spectrum showed the presence of two peaks with retention times of 16.73 and 17.38 min in an abundance ratio of about 1:2 and both showing a molecular ion at mlz 190. They were identified by (8)Sharp, T. R.;White, M. R.; Davis, J. F.; Stang, P. J. Org. Muss Spectrom. 1984,19, 107. (9)Bruce, M.I.; Liddell, M. Appl. Orgunomet. Chem. 1987,1 , 191. (10)(a) Doyle, M. P.; Griffin, J. H.; Bagheri, V.; Dorow, R. L. Organometallics 1984,3, 53. (b) Brookhart, M.; Studabaker, W. B. Chem. Reu. 1987,87,411. (c) Scitz, W.J.; Saha, A. K.; Hossain, M. M. Organometallics 1993,12,2604. (11)Nagel,U.Chem. Ber. 1982,115, 1998. (12)Morgan, R. P.;Beynon, J . H.; Bateman, R. H.; Green, B. N. Int. J.Musss Spectrom. Ion Phys. 1978,28,171. (13)(a) Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. J . Chem. Soc., Chem. Commun. 1979,325. (b) Miller, J . M. Adu. Inorg. Chem. Radiochem. 1984,28, 1. (c) Williams, D. H.; Findeis, A. F.; Naylar, S.; Gibson, B. V. J.Am. Chem. Soc. 1987,109, 1980. (14)(a) Cooks, R. J.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metastable Ions; Elsevier: Amsterdam, 1973. (b) Porter, C. J.; Benjnan, J . H.; Ast, T. Org. Muss. Spectrom. 1981,16, 101. (15)Bruins, A.R.;Jennings, K. R.; Evans, S. Int. J.Muss Specrom. Ion Phys. 1978,26,395.
Figure 1. FAB MS spectrum of reaction 1.
t r r I I I n r I I 1 r 1 I I I I r I I I r I I I I I r
eo0
700
800
'
EIV)
Figure 2. (a) MIKE! spectrum of the molecular ion of 2. (b) MIKE! spectrum of ionic species at mlz 890 formed by running reaction 1 under FAB conditions. comparison with the NBS mass spectra collection as cis- and trans-2-phenyl-1-cyclopropane (ethylcarboxylate esters), respectively. The conversion of ethyl diazoacetate to cyclopropanes was 20%. The lH NMR spectrum of the CDzClz solution confirmed the presence of both cyclopropanes identified by comparison with the known N M R spectral data.16
Results and Discussion Reaction 1 was run under FAB MS conditions (see Experimental Section), and the spectrum of the reaction mixture just introduced is reported in Figure 1. The spectrum shows the formation of ionic species at mfz 890 isobaric with the molecular ion of compound 2. A comparison of the MIKE spectrum of M + of 2 (Figure 2a),17independently prepared according to reaction 1, with that of such ionic species (Figure 2b) reveals that they are identical, since both undergo a retrosynthetic process to yield [Pt(PPh&Y+ (mfz718) and loss of a Ph' radical leading to the ions a t mfz 813. CID MIKE spectra of the ions a t mfz 890 are still identical and do not show the occurrence of further decomposition pathways. These results indicate that reaction 1,performed either under FAB conditions or in condensed phase, gives rise to the same product. The FAB mass spectrum of the reaction mixture (Figure 1)also shows well-detectable ions a t mlz 718, (16)Nakamura, A.;Konishi, A.; Tatsumo, Y.; Otsuka, S. J . Am. Chem. SOC.1978,100,3443. (17)The FAB mass spectrum of 2 shows the molecular ion at mlz 890 (2%), ions at mlz 813 (4%) due to the loss of a Ph' radical, and ions a t mlz 732 (see text) together with ions at mlz 718 (100%) orginated by the loss of the fumarate ligand.
Notes
d z 804
m/z 832
r 718
804
I
700
800
3791
I
E(V 1
Figure 3. (a) MIKE spectrum of ionic species at mlz 832 formed by running reaction 1 under FAB conditions. (b) MIKE spectrum of ionic species at mlz 804 formed by running reaction 1 under FAB conditions. 804, 832, and 1608. The first, the most abundant species, corresponds to [Pt(PPh&?+ and is likely derived mainly from the loss of CHz-CHz from 1. The ionic species at mlz 832 originate from the addition of NzCHCOzEt to [Pt(PPhs)zY+,and the MIKE spectrum (Figure 3a) of this adduct shows that the retros-ynthesis (leading to ions at mlz 718) is the most significant process, which is accompanied by the loss of dinitrogen molecule to yield ionic species a t mlz 804. These latter species, which are also well detectable in the FAB spectrum of the reaction mixture (Figure 11, are assigned to the molecular ion of the carbene intermediate [Pt{C(H)COzEt)(PPh3)21'+. The parent ion scan spectrum of these latter species shows only two precursors, i.e., the ions at mlz 832 and 890 in an abundance ratio of 2:lO. Finally, the ionic species at mlz 1608 are attributed to a dimeric form of the carbenic species. Thus, on the basis of the above-mentioned results, the overall formation of the fumarate product 2 may be suggested to occur according to Scheme 1. Reaction 1, performed under FAB conditions, may occur (at least partially) in the gas phase since, on running the reaction with different amounts of matrix, no changes are observed in the relative abundances of synthons and product ions formed, as would be expected if it occurred in the condensed phase. In particular, the FAB mass spectrum of 1 (Figure 4) carried out on a very dilute solution in 3-nitrobenzyl alcohol shows also the presence of its molecular ion at mlz 746 together with the peak a t mlz 718 ([Pt(PPh3)21'+)and ions at mlz 456, 379, and 302, originated by subsequent losses of PPh3, Ph' radical, and a further Ph' radical, respectively, which are typical in the FAB mass spectra of complexes
Figure 4. FAB MS spectrum of 1. containing the [Pt(PPh&l moiety.18 The B21E = constant linked scan spectrum of the ions at mlz 718 confirms that they derive from those at mlz 746. Thus reaction 1,which likely proceeds in solution by the slow substitution of ethylene by the diazo species, is greatly enhanced in these conditions due to the presence of a large quantity of the coordinativelyunsaturated species [Pt(PPh3>z?+. It is of interest to note that, in the FAB spectrum of reaction 1 (Figure 11, ions a t mlz 732, which are proposed to be the molecular ion of the Pt(0)-carbene species [Pt(CH2)(PPh&.],are very well detectable. Their MIKE spectrum shows the loss of only a Ph' radical to give ionic species a t mlz 655 with no evidence of any retrosynthetic process to yield ions at mlz 718, as It is noteworthy that well-detectotherwise 0b~erved.l~ able ions at mlz 732 are also present in the FAB mass spectrum of 1, and the formation of the Pt=CHz moiety may be considered a retrosynthetic process of the Ptethylene complex; this has been suggested chemically also by the formation of Re-ethylene derivatives starting from Re=CH2.20 Although quite reactive and often unstable, terminal methylene complexes are known for most of the transition metals,21but no evidence as far as we known has been so far reported for Pt-CH2 derivatives. The ionic species at mlz 732 are also (18)Asker, K. A.; Greenway, A. M.; Seddon, K. R.; Shimran, A. A. J. Organomet. Chem. 1988,354,257. (19)Bandini, A. L.; Banditelli, G.; Minghetti, G.; Pelli, B.; Traldi, P. Organometallics 1989,8, 590. (20)(a) Menifield, J. H.; Lin, G. Yu.;Kiel, W. A.; Gladysz, J. A. J. Am. Chem. SOC.1983,105,5811.(b)Ott, K. C.; Grubbs, R. H. J.Am. Chem. SOC.1981,103,5922. (c) Kaminsky, W.;Kopf, J.; Sinn, H.; Vollmer, H.-J. Angew. Chem., Znt. Ed. Engl. 1976,15,629. (21)Roper, W.R. In Advances in Metal Curbene Chemistry; Kluwer: Wilheim, 1988;p 27.
554 Organometallics, Vol. 14,No. 1, 1995
present in the MIKE spectrum of ions at mlz 804 (Figure 3b) together with species originating from the retrosynthetic process at mlz 718 and from the loss of 'OCH2CH3 a t mlz 759. It is worthwhile noting that the last step in the proposed mechanism of Scheme 1, i.e., the conversion of the dimer to the fumarate product, could not be confirmed by the MIKE data study owing t o the low abundance of the ionic species at mlz 1608, while the parent ion scan spectrum of ionic species at mlz 890 shows a weak (signal-to-noise ratio 51) signal corresponding to the ions at mlz 1608. Furthermore, such conversion is in agreement with the formation of olefin complexes from dinuclear carbene intermediates as reported for the synthesis of a Re-ethylene derivative from Re-methylene species.20 Stable carbene dimers are also known: a (rpC5H51-substituted TiCH2TiCH2 metallocycle was isolated from a reaction that generated (rl-C5H5)2Ti(=CH2)20b, and an X-ray structure of (~-C5H5)2[(C2H5)3A1C1lZrCH2CH2Zr[AlCl(C2H5)3l(~C5H5)220cshowed an arrangement of core atoms in which a full CH2-CH2 bond is present, but some bonding of each zirconium to the p CH2 remains. The FAB MS and MIKE results do not provide any evidence of alternative mechanisms such as those involving any of the following intermediates, which were not detected: [Pt(NzCHCOzEt)z(PPh3)21(mlz 9461, [Pt{C(H)COzEt}(N2CHC02Et)(PPh3)21(mlz 9181, and [Pt{C(H)C02Et}z(PPh3)21(mlz 890).22 The formation of the ionic species at mlz 832, assigned to the molecular ion of the monoadduct [Pt(N2CHC02Et)(PPh3)23'+,is supported also by 13C NMR data. In fact, when reaction 1 in CDzClz was monitored a t variable temperature (223-278 K),a resonance at 6 37.78 (triplet, 3Jpc 12.56 Hz, 2Jcpt 195.67 Hz) was detected. The value of 2J~pt, which is close to those reported for nitrile Pt(I1) complexes of the type [PtC12(NCR)2l ( 2 J ~ p tca. 230-280 H z ) , ~suggest ~ an q2coordination mode of the diazo ligand. This type of (22) As suggested by a reviewer, the formation of ionic species at mlz 804 does not necessarily require the initial coordination of the diazo ester to the metal and subsequent Nz loss; it could be also explained by a FAB-induced loss of NZfrom the free diazo compound to give the free carbene [(COZEt)(H)CY+,which then reacts with [Pt(PPh&P+. Such a reaction would be an ion-ion reaction, highly unfavored from the thermodynamic point of view; however, the possible presence of :CHR cannot rule out an ion-neutral reaction with [Pt(PPh++. Furthermore, the FAB mass spectrum of free NzCHCOzEt (in either glycerol or m-nitrobenzyl alcohol as matrix) shows the presence of the molecular ion a t mlz 114 and of the carbene ion a t mlz 86,but not of the olefin COZEtCH=CHCOzEt a t mlz 172. This finding indicates that the final Pt(0)-fumarate derivative is not formed by coordination of the preformed olefin to the [Pt(PPh&P+ species.
Notes coordination has been proposed previously for other zerovalent diazo-Pd and -Rcomplexes2aand has been confirmed by an X-ray analysis for the related Ni(0) However, in species, [Ni(diazofluorene)(CN-t-B~)2].~~ the latter studies as well as in the present investigation, no carbene intermediate could be spectroscopically detected, possibly as a result of its low concentration and/or high reactivity. The formation of the carbene intermediate [Pt{C(H)C02Et}(PPh3)21was also substantiated chemically by observing the formation of cyclopropanes from styrene in the reaction mixture of [Pt(CH2=CH2)(PPh3)21with N2CHC02Et (eq 21, which is believed to involve coupling of the carbene with the olefin, as has been reported for several other transition metal-catalyzed cyclopropanation reactions of olefins:1° PhCH=CH,
+
NZCHCO2Et
[Pt(CH,=CH,)(PPh+J.
- N2
*
CH,CI,, RT.24 h
It is noteworthy that when reaction 2 is performed under FAB conditions, a less abundant formation of the fumarate derivative 2 is observed, accompanied by the well-detectable ionic species at mlz 804 and 832 and by the appearance of new ionic species at mlz 908. These last species correspond to the molecular ion of the carbene-olefin adduct [Pt(C(H)COzEt}(CHz=CHPh)(PPh3)2Y+,which is reported to be a key intermediate in the catalytic cyclopropanation and also the metathesis of olefins and for which a metallocyclobutane structure is p r ~ p o s e d . ~ In conclusion, FAB mass spectrometry appears t o be a technique particularly suitable for the detection of highly reactive species such as those described in the present study, which have short lifetimes and are present in low concentration. We are currently investigating the generality of these Pt(0)-promoted cyclopropanation reactions with diazo compounds as well as the role of the Pt(0)-carbene intermediates.
Acknowledgments.R.A.M. and A.J.L.P. thank CNR and JNICT, respectively, for partial support within their international cooperation program. R.A.M. also thanks CNR and MURST for financial support. OM940193X (23) Fracarollo, D.; Bertani, R.; Mozzon, M.; Belluco, U.; Michelin, R. A. Inorg. Chim. Acta 1992, 201, 15.
