Electrospray Mass Spectrometric Studies of Two Palladium−Allyl

Organometallics 2010, 29, 3979–3986 DOI: 10.1021/om100591c

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Electrospray Mass Spectrometric Studies of Two Palladium-Allyl Complexes of the Trost Standard Ligand Divya Agrawal,† Detlef Schr€ oder,*,† David A. Sale,‡ and Guy C. Lloyd-Jones*,‡ †

Institute of Organic Chemistry and Biochemistry, Flemingovo n am. 2, 16610 Prague 6, Czech Republic, and ‡ School of Chemistry, Cantock’s Close, University of Bristol, BS8 1TS, United Kingdom Received June 16, 2010

Two allyl-palladium(II) complexes bearing the Trost standard ligand (1), i.e., [(C3H5)Pd(S,S-1)]þCF3SO3- and [(c-C6H9)Pd(S,S-1)]þBArF- (BArF-=[B((3,5-(CF3)2)C6H3)4]-), are investigated using electrospray ionization (ESI) mass spectrometry. Both species form abundant quasi-molecular ions [(C3H5)Pd(S,S-1)]þ and [(c-C6H9)Pd(S,S-1)]þ, respectively. At elevated concentrations of the solutions admitted to the ESI source, significant aggregation to binuclear species is observed, which agrees with recent results from solution chemistry. Consistent with solution-phase studies, the tendency of clustering is much less pronounced for the bulky BArF- counterion compared to the smaller triflate.

Introduction Palladium-catalyzed allylic alkylation represents one of the first and most extensively explored methods for asymmetric carbon-carbon bond formation and has led to the design and implementation of a plethora of chiral ligands that provide enantiocontrol in this benchmark transformation.1 In 1992, Trost and co-workers reported the development and application of ligand (R,R)-1 to provide high levels of enantiocontrol in the alkylation of the previously challenging “slim” cyclic allylic substrates (Scheme 1).2 Following this initial report, the ligand has been successfully applied to a large number and range of asymmetric allylic alkylation reactions, and 1 (commonly referred to as the “Trost standard ligand”, TSL) remains state of the art in providing enantiocontrol for the alkylation of cyclic substrates.3 Detailed investigations have provided key insights into the generally accepted mechanism for Pd-catalyzed allylic alkylation,4 where solution and solid-state structural studies of key catalytic intermediates have elucidated the mode of asymmetric induction for a number of ligand systems. The insights achieved have in certain instances led to the development of “secondgeneration” ligands.5,6 Despite the widespread use of 1, there has until recently7 been a distinct lack of information concerning *To whom correspondence should be addressed. E-mail: schroeder@ uochb.cas.cz; [email protected]. (1) For reviews, see: (a) Trost, B. M.; Crawley, M. L. Chem. Rev. 2003, 103, 2921. (b) Trost, B. M.; van Vranken, D. L. Chem. Rev. 1996, 96, 395. (c) Williams, J. M. J.; Acemoglu, L. Handbook of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-I., Ed.; John Wiley & Sons: New York, 2002; p 1689. (d) Lu, Z.; Ma, S. Angew. Chem., Int. Ed. 2008, 47, 258. (2) (a) Trost, B. M.; van Vranken, D. L. Angew. Chem., Int. Ed. 1992, 31, 228. (b) Trost, B. M.; van Vranken, D. L.; Bingel, C. J. Am. Chem. Soc. 1992, 114, 9327. (3) Trost, B. M. Acc. Chem. Res. 1996, 29, 355. (4) Evans, L. A.; Fey, N.; Harvey, J. N.; Hose, D.; Lloyd-Jones, G. C.; Murray, P.; Orpen, G.; Osborne, R.; Owen-Smith, G. J. J.; Purdie, M. J. Am. Chem. Soc. 2008, 130, 14771. (5) Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33, 336. (6) Evans, D. A.; Campos, K. R.; Tedrow, J. S.; Michael, F. E.; Gagne, M. R. J. Am. Chem. Soc. 2000, 122, 7905. (7) Butts, C. P.; Filali, E.; Lloyd-Jones, G. C.; Norrby, P.-O.; Sale, D. A.; Schramm, Y. J. Am. Chem. Soc. 2009, 131, 9945. r 2010 American Chemical Society

the structure of key intermediates of the type [(1)Pd-allyl]þ due to the complexity of the NMR spectra and difficulties in the growth of crystals suitable for X-ray diffraction.8,9 As a result, the rationalizations of the mode of asymmetric induction engendered by 1 were by necessity limited to an empirically derived C2-symmetric “wall-flap” cartoon model (Figure 1).10 In the Pd-mediated alkylation of cycloalkenyl esters with malonate nucleophiles, memory effects have been found to be operative,11 which were initially explained by an asymmetric ionpair model.12 Here, the term “memory effect” refers to a situation where two isomers of a substrate should in principle react with identical selectivity through a common set of intermediates, but in practice do not do so. However, subsequent mechanistic investigations indicated that the memory effect occurred through an enantiodivergent process where the two substrate enantiomers are processed through different manifolds of a multicatalyst ensemble. Preliminary 31P{1H} NMR investigations of the solution-phase behavior of [(1)Pd-allyl]þ (where allyl is C3H5 or c-C6H9) under chloride-free conditions displayed evidence for the reversible oligomerization of the C1-symmetric monomeric chelate.13 On the basis of these preliminary results, it was postulated that the memory effect originated from the differential turnover of matched and mismatched substrate enantiomers through active monomeric and oligomeric catalyst manifolds.14 Following these investigations, an approach involving computational modeling and solution-phase 1H NMR studies with highly (8) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 1999, 121, 4545. (9) Eastoe, J.; Fairlamb, I. J. S.; Fernandez-Hernandez, J. M.; Filali, E.; Jeffery, J. C.; Lloyd-Jones, G. C.; Martorell, A.; Meadowcroft, A.; Norrby, P.-O.; Riis-Johannessen, T.; Sale, D. A.; Tomlin, P. M. Faraday Discuss. 2010, 145, 27. (10) Trost, B. M.; Machacek, M. R.; Aponick, A. Acc. Chem. Res. 2006, 39, 747. (11) Trost, B. M.; Bunt, R. C. J. Am. Chem. Soc. 1994, 116, 4089. (12) Trost, B. M.; Bunt, R. C. J. Am. Chem. Soc. 1996, 118, 235. (13) Fairlamb, I. J. S.; Lloyd-Jones, G. C. Chem. Commun. 2000, 2447. (14) Lloyd-Jones, G. C.; Stephen, S. C.; Fairlamb, I. J. S.; Martorell, A.; Dominguez, B.; Tomlin, P. M.; Murray, M.; Fernandez, J. M.; Jeffery, J. C.; Riis-Johannessen, T.; Guerziz, T. Pure Appl. Chem. 2004, 76, 589. Published on Web 08/09/2010

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Scheme 1. Trost Standard Ligand 1 and Its Use in Asymmetric Allylation Reactions

Scheme 2. Catalytic Cycle for Kinetic Resolution (kS . kR) of Cyclohexenyl Acetate and Enantioselective Alkylation with MC(H)E2 (E = CO2Me) via an [(c-C6H9)Pd(R,R)-1]þ 3 OAcIntermediatea

Figure 1. “Wall-flap” cartoon model for asymmetric induction that utilizes the steric deactivation of disfavored pathways for nucleophilic attack upon the [(c-C6H9)Pd(R,R-1)]þ intermediate: the nucleophile is blocked by the “walls”, but can attack under the “flaps”.

deuterated ligands was employed to interrogate the structure of the monomeric chelate and thus deduce some clues as to the mode of asymmetric induction.7 Ligand coordination was found to result in a C1-symmetric geometry that placed an amide N-H unit in close proximity to the pro-S allylic terminus in the [(cC6H9)Pd(R,R)-1)]þ 3 OAc- intermediate. Computational determination of the transition structure for formation of the [(cC6H9)Pd(R,R)-1]þ 3 OAc- intermediate from S-cyclohexenyl acetate and [Pd(R,R)-1] showed that the amide N-H unit forms a stabilizing hydrogen bond with the departing acetate nucleofuge. Analogously, an N-H hydrogen bond was found in the transition structure for reaction of [(c-C6H9)Pd(R,R)-1]þ with the enolate ion of dimethylmalonate (Scheme 2), thus explaining the experimental observations of both a powerful substrate kinetic resolution15 and a high asymmetric induction on alkylation.7 To address the potential influence of the oligomeric species in catalysis, solution-phase NMR studies were performed in combination with the computational calculation of several possible oligomeric structures for both [(C3H5)Pd(R,R)-1)]þ and [(c-C6H9)Pd(R,R)-1)]þ, i.e., the ions 2þ and 3þ, in which these cations form aggregates with the counterions.9 Calculations of the free cations showed that the simple η3-C3H5 complex 2þ prefers a linear oligomeric structure, whereas the η3-C6H9 complex 3þ displays a tendency for the formation of cyclic oligomeric structures.9 The cyclic structure of oligomeric 3þ bears the c-C6H9 η3cylohexenyl unit on the exterior of the ring with 1,2-cyclohexadiamine ligand backbones orientated toward the center of the ring. In agreement with this relatively constrained geometry, solution-phase 31P{1H} NMR analysis of scalemic samples of (15) For cyclohexenyl acetate, a selectivity factor s g 45 has been found. The selectivity factor s expresses the relative rates of reaction of the two enantiomers and can be estimated by nonlinear regression of s = ln[(1 - c)(1 - ee)]/ln[(1 - c)(1 þ ee)]; see: Dominguez, B.; Hodnett, N. S.; Lloyd-Jones, G. C. Angew. Chem., Int. Ed. 2001, 40, 4289. However, determination by measurement of the absolute rates of reactions of the enantiomers shows this value to be a substantial underestimate ( Sale, D. A.; Lloyd-Jones, G. C. Unpublished results).

a The key hydrogen-bonding interactions are shown in red. The influence of the hydrogen-bonding interaction in both oxidative addition and nucleophilic attack elementary catalytic steps can, under optimized reaction conditions, account for the proficiency of (R,R)-1 in providing substrate kinetic resolution as well as efficient conversion of both substrate enantiomers to a highly enantioenriched alkylation product.

3þ 3 BArF- revealed a much higher propensity for the formation of homochiral oligomers as compared to 2þ 3 BArF-.7,9 In addition, 31P{1H} NMR analysis of 2þ 3 BArF-, 2þ 3 OTf -, 3þ 3 BArF-, and 3þ 3 OTf- indicates a much smaller range in the oligomer size distribution for the cyclohexenyl complexes 3þ (Figure 2). However, for no complex could the exact oligomer size range or the distribution be determined from solution-phase analyses, and any kind of complementary information would help to more deeply understand the complex mechanistic scenario of the Pd-mediated allylation. Some of us recently started to explore the applicability of electrospray ionization mass spectrometry (ESI-MS)16 as a direct probe of similar equilibria in solution via the use of gas-phase techniques.17-20 Initiated by a Faraday Discussion meeting,21 we accordingly began a cooperation aimed to (16) (a) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451. (b) Fenn, J. B. J. Am. Soc. Mass Spectrom. 1993, 4, 524. (17) (a) Tsierkezos, N. G.; Roithova, J.; Schr€ oder, D.; Molinou, I. E.; Schwarz, H. J. Phys. Chem. B 2008, 112, 4365. (b) Mísek, J.; Tichy, M.; Stara, I. G.; Stary, I.; Schr€oder, D. Collect. Czech. Chem. Commun. 2009, 74, 323. (c) Tsierkezos, N. G.; Buchta, M.; Holy, P.; Schr€oder, D. Rapid Commun. Mass Spectrom. 2009, 23, 1550. (d) Agrawal, D.; Zins, E.-L.; Schr€oder, D. Chem. Asian J. 2010, 5, 1667. (18) Tsierkezos, N. G.; Roithova, J.; Schr€ oder, D.; Oncak, M.; Slavı´ cek, P. Inorg. Chem. 2009, 48, 6287. (19) See also: Koszinowski, K. J. Am. Chem. Soc. 2010, 132, 6032. (20) Reviews: (a) Markert, C.; Pfaltz, A. Angew. Chem., Int. Ed. 2005, 43, 2498. (b) Santos, L. S.; Knaack, L.; Metzger, J. O. Int. J. Mass Spectrom. 2005, 246, 84. (c) Di Marco, V. B.; Bombi, G. G. Mass Spectrom. Rev. 2006, 25, 347. (d) Santos, L. S. Eur. J. Org. Chem. 2008, 235. oder, D.; Marcus, R. A.; Cramer, C. J.; Buurma, (21) Shaik, S.; Schr€ N.; Hillman, R. A.; Tahara, T.; Hinde, P.; Bain, C. D.; Warshel, A.; Umapathy, S.; Greig, I.; Guthrie, J. P.; Page, M. I.; Henchman, R. H.; Michl, J.; Aschi, M.; Williams, I.; Lloyd-Jones, G. C.; Bentley, T. W.; Carpenter, B.; Pantos, G. D.; Coldham, I.; Webb, S. J.; Jayaprakash, D.; Hunter, C.; Hunt, N. T.; Glowacki, D.; Harvey, J. N.; Zipse, H. Faraday Discuss. 2010, 145, 121.

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Figure 2. Oligomeric chain structures of 2þ 3 OTf- and 3þ 3 BArF-.

probe whether ESI-MS can provide further insight into the oligomeric species formed from 2þ 3 OTf- and 3þ 3 BArF-, respectively.22 At the outset, we would like to point out, however, that the transfer of the palladium complexes from the solution to the gas phase involves drastic changes of important parameters such as concentration, temperature, and pressure, which may substantially affect the equilibira between monomers and aggregates. Hence, rather than for a 1:1 comparison of the results from the condensed phase and ESI-MS, we search for correlations between both domains, and the major attention is put on the additional molecular information provided by mass spectrometric means.

Experimental Details Unless otherwise mentioned, 4.5  10-5 M solutions of the title compounds in CH3CN were studied in positive-ion mode with a Finnigan LCQ ion-trap mass spectrometer fitted with an electrospray ionization (ESI) source.23 Nitrogen was used as the nebulizer gas, and the samples were introduced into the ESI source via a needle at a flow rate of 5 μL/min. The operating conditions were adjusted to ensure reasonably soft ionization conditions, i.e., spray voltage 5 kV, capillary voltage ca. -120 V, heated capillary temperature 150-200 °C, tube lens offset ca. -120 V, sheath gas flow rate 10 arbitrary units, auxiliary gas flow rate 10 arbitrary units. Here, the term “soft conditions” refers to the transfer region from the ion source operating at 1 bar to the vacuum manifold of the mass spectrometer, in which the use of low voltage settings in ion transfer leads to no or only little collisional activation of the ions formed in the source, whereas elevated voltages in ion transfer lead to considerable (22) For previous studies of ionic Pd-allyl species in the gas phase, see: (a) Schwarz, J.; Schr€ oder, D.; Hrusak, J.; Schwarz, H. Helv. Chim. Acta 1996, 79, 1110. (b) Chen, Y.-M.; Sievers, M. R.; Armentrout, P. B. Int. J. Mass Spectrom. 1997, 167, 195. (c) Hartinger, C. G.; Nazarov, A. A.; Galanski, M.; Reithofer, M.; Keppler, B. K. J. Organomet. Chem. 2005, 690, 3301. (d) Chevrin, C.; Le Bras, J.; Roglans, A.; Harakat, D.; Muzart, J. New J. Chem. 2007, 31, 121. (e) Thiery, E.; Chevrin, C.; Le Bras, J.; Harakat, D.; Muzart, J. J. Org. Chem. 2007, 72, 1859. (f) Markert, C.; Neuburger, M.; Kulicke, K.; Meuwly, M.; Pfaltz, A. Angew. Chem., Int. Ed. 2007, 46, 5892. (g) Muller, C. A.; Pfaltz, A. Angew. Chem., Int. Ed. 2008, 47, 3363. (23) Tintaru, A.; Roithova, J.; Schr€ oder, D.; Charles, L.; Jusinski, I.; Glasovac, Z.; Eckert-Maksic, M. J. Phys. Chem. 2008, 112, 12097. (24) Cech, N. B.; Enke, C. G. Mass Spectrom. Rev. 2001, 20, 362.

heating of the ions by collisions and hence fragmentation.24-26 Using two octopoles, the ions are guided from the source into a Paul ion trap for ion storage and manipulation27 in the presence of ca. 10-3 mbar helium as a trapping gas. For detection, the ions are ejected from the trap to an electron multiplier. For collision-induced dissociation (CID) spectra, the ions are massselected in the ion trap and then kinetically accelerated within the helium buffer gas present in the ion trap as the collision partner. In these experiments, collision energies were chosen that provide reasonable fragment ion yields while the parent ion is still the most abundant ion. Mass spectra were recorded from m/z 50 to 2000, and 50-100 scans were accumulated to improve the signal-to-noise ratio. For increased mass resolution, the LCQ offers a high-resolution, narrow-range scan (ZoomScan) that provides a better resolution of isotopic envelopes; note, however, that the ion abundances in these independent scans are normalized to the largest signal within the zoomed region. A few additional spectra ranging up to m/z 4000 were taken on a Finnigan TSQ mass spectrometer of multipole configuration;28 except for the higher mass range, the experimental findings were very close to those obtained in the ion-trap mass spectrometer.

Results and Discussion Provided that the conditions of ionization are sufficiently soft,24-26 the ESI mass spectrum of a 4.5  10-5 M solution of 2þ 3 TfO- in CH3CN (Figure 3) is dominated by the quasimolecular ion 2þ at m/z 837, as expected from heterolysis of the palladium(II) salt in a dipolar solvent; note that throughout the paper all masses given refer to the major isotope 106Pd. Much less abundant signals at m/z 731, 747, and 796 can be assigned to some fragmentation processes occurring to a minor extent (see below). Conversely, a small peak at m/z 853 and thus 16 amu heavier than the parent compound indicates some oxidation of the ligand (most probably at phosphorus). Additional signals at m/z 877, 1027, 1559, 1633, 1782, and 1823 are due to aggregation (25) Schr€ oder, D.; Weiske, T.; Schwarz, H. Int. J. Mass Spectrom. 2002, 219, 729. (26) Trage, C.; Diefenbach, M.; Schr€ oder, D.; Schwarz, H. Chem.; Eur. J. 2006, 12, 2454. (27) O’Hair, R. A. J. Chem. Commun. 2006, 1469. (28) (a) Roithova, J.; Schr€ oder, D. Phys. Chem. Chem. Phys. 2007, 9, 731. (b) Roithova, J.; Schr€oder, D.; Mísek, J.; Stara, I. G.; Stary, I. J. Mass Spectrom. 2007, 42, 1233.

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Figure 3. (a) ESI mass spectrum of a 4.5  10-5 M solution of 2þ 3 TfO- in CH3CN. The insets (b and c) show expansions of the regions from m/z 725 to 805 and m/z 1000 to 1850, respectively. (d-f) Independent zoom scans of the signals at m/z 796, 837, and 1027, respectively, with intensity scales in arbitrary units.

as well as clustering (see below). In the following, we briefly address all these signals in order to possibly provide a conceptual understanding of the solution chemistry of the title compounds. To this end, scans at enhanced mass resolution for evaluation of the isotope pattern and collision-induced dissociation spectra of several mass-selected ions (“tandem mass spectrometry”) were recorded in order to elucidate the elemental compositions, signify structural features, and unravel the gasphase behavior of these complexes. The major fragmentation of the quasi-molecular ion 2þ corresponds to the loss of the allyl moiety along with a reduction from PdII to PdI, which is observed upon collisioninduced dissociation of the mass-selected ion as well as in the ESI mass spectra as the signal at m/z 796 (2þ - C3H5, Figure 3). The respective isotope patterns of the parent ion at m/z 837 (Figure 3e) and the fragment ion at m/z 796 (Figure 3d) clearly reveal the presence of palladium. In contrast, the isotopic pattern of palladium is absent in the signal at m/z 731, indicating the loss of palladium from the parent ion with coupling of the allyl unit to the neutral ligand (i.e., [2þ - Pd], Figure 3b). The parallel observation of a Pd-free ion at m/z 747 (Figure 3b) is assigned to a partial oxidation of the ligand, which most likely occurs upon sample preparation and handling or in the ESI process itself. The signal at m/z 877 can be explained by the addition of an allyl group to the parent ion concomitant with loss of a proton (most likely from amide-nitrogen atoms). Thus, the signals in the vicinity of the quasi-molecular ion can be explained by simple fragmentation and degradation complexes of the precursor compound occurring either in solution or in the course of the electrospray process. We note in passing that involvement of the solvent acetonitrile (also 41 amu and thus isobaric to C3H5) in any of the complexes has been ruled out by complementary experiments in CD3CN, which did not cause any mass shift in the ESI spectra. As detailed in the Introduction, our major interest concerns any possible mass spectrometric evidence for the aggregation of the palladium complexes in solution and if these can be probed using ESI-MS. Therefore, we focus on the signals in the mass range well above the quasi-molecular

Figure 4. Collision-induced dissociation (CID) spectrum of the ion at m/z 1559 made by ESI of 2þ 3 TfO- in CH3CN. The inset shows an independent zoom scan of the isotope pattern of the parent ion.

ion at m/z 837 (Figure 3c). The isotopic distributions of the signals at m/z 1027 (Figure 3f) and 1559 show the presence of only one palladium atom (see inset in Figure 4). The formation of the signal at m/z 1027 can be explained by the addition of an allyl group to the parent compound, i.e., [2þ 3 TfO- 3 C3H5þ], which formally corresponds to a triple ion consisting of two cations bound by a counterion.29,30 We note that a loss of CF3SO3H (150 amu) from the triple ion also accounts for the peak at m/z 877 in Figure 3. An MS/MS experiment of the ion at m/z 1559 (Figure 4) shows the formation of a fragment ion at m/z 853, which is assigned to the mono-oxygenated parent ion (i.e., 837 þ 16). Likewise, the resulting neutral loss (Δm = -706 amu) corresponds to the mass of the monooxygenated TSL ligand (i.e., 690 þ 16). Based on the isotopic pattern and the MS/MS studies, the structure shown in Chart 1 therefore is proposed for the ion at m/z 1559. If, for example, both phosphorus atoms in a single TSL ligand would have been oxidized, CID should also give rise to the nonoxygenated species m/z 837 and/or (29) (a) Buchner, R.; H€ olzl, C.; Stauber, J.; Barthel, J. Phys. Chem. Chem. Phys. 2002, 4, 2169. (b) Marcus, Y.; Hefter, G. Chem. Rev. 2006, 106, 4585. (30) Schr€ oder, D.; Duchackova, L.; Jusinski, I.; Eckert-Maksic, M.; Heyda, J.; Tuma, L.; Jungwirth, P. Chem. Phys. Lett. 2010, 490, 14.

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Figure 5. Separate zoom scans of the signals at (a) m/z 1633, (b) m/z 1782, and (c) m/z 1823 generated by ESI of 2þ 3 TfO- in CH3CN. Chart 1. Proposed Structure of the Ion at m/z 1559

Figure 6. CID spectra of the mass-selected ions at (a) m/z 1633 and (b) m/z 1823 made by ESI of 2þ 3 TfO- in CH3CN .

the dioxo derivative at m/z 869, rather than exclusively yielding m/z 853, as is observed in the experiment (Figure 4). While the partially oxidized complexes do not form our main interest in this work, we note in passing that the data indicate the operation of pronounced cooperative effects. Thus, the amount of the oxygenated quasi-molecular ion, [2 þ O]þ at m/z 853, amounts to only a few percent relative to 2þ (Figure 3), whereas the doubly oxygenated complex at m/z 1559 has only a small satellite of the nonoxygenated ion at m/z 1527. Beyond any doubt, the isotope distributions of the signals at m/z 1633, 1782, and 1823 indicate the presence of two palladium atoms (Figure 5). An MS/MS experiment of the ion at m/z 1633 shows the exclusive formation of 2þ as an ionic fragment (Figure 6a). On the basis of the isotopic pattern (Figure 5a) and the MS/MS data, the ion at m/z 1633 is assigned to an aggregation of the quasi-molecular ion 2þ (m/z 837) and the low-valent palladium species (1)Pd to formally yield [(1)2Pd2(C3H5)]þ, for which an allyl-bridged cluster appears quite plausible.22f,31 On the basis of the isotopic pattern (Figure 5c) and the MS/MS studies (Figure 6b), the most abundant signal of the dinuclear species at m/z 1823 can be assigned to an aggregate of two quasi-molecular ions with a triflate counterion, i.e., the triple ion [(2þ)2 3 TfO-], which loses the neutral ion pair 2þ 3 TfO- upon CID to afford 2þ. The isotope pattern of the signal centered at about m/z 1782 (Figure 5b) indicates two overlapping isotope envelopes of Pd2 species with m/z 1781 and 1783, formally connected to the triple ion [(2þ)2 3 TfO-] by losses of C3H6 and C3H4, respectively. Likewise, m/z 1633 is formally connected to m/z 1823 via loss of allyl triflate (190 amu). Thus, all cluster ions observed can be related to initial formation of the triple ion [(2þ)2 3 TfO-]. Hence, we can summarize the ESI patterns of a dilute solution of 2þ 3 TfO- in CH3CN as follows: (i) Under soft ionization conditions, the by far leading signal corresponds to the quasi-molecular ion 2þ. Several signals in the lower percent range indicate (ii) the extrusion of palladium from (31) The Pd0 species (1)Pd has been reported to undergo facile N-H bond insertion to PdII species, see: (a) Amatore, C.; Jutand, A.; Mensah, L.; Ricard, L. J. Organomet. Chem. 2007, 692, 1457. (b) Tsarev, V. N.; Wolters, D.; Gais, H.-J. Chem.;Eur. J. 2010, 16, 2904.

the complexes, (iii) the oxidation of the ligand (most probably to phosphine oxides), and (iv) the aggregation of organic species to mononuclear Pd. While studies of these complexes in the condensed phase have indicated a significant amount of aggregation,7 at the low concentrations used in the ESI experiments, the signals due to polynuclear compounds are quite small; we return to this aspect in the context of the concentration-dependent studies (see below). Next, we consider compound 3þ 3 BArF-, which is closely related to the previous palladium complex, except that the allyl group is replaced by a cyclohexenyl moiety and the triflate counterion by a BArF- anion, one of the most weakly coordinating counterions in solution chemistry.32 The ESI mass spectrum of a 4.5  10-5 M solution of 3þ 3 BArF- in CH3CN shows the quasi-molecular ion 3þ at m/z 877 as the major signal (Figure 7). Very weak signals at m/z 771 (loss of Pd) and 797 (loss of C6H8) are assigned to fragmentations, while the ions observed at m/z 893, 1567, 1583, 1599, and 1673 indicate the occurrence of some kind of aggregation and clustering, as detailed in the following. The lack of the isotopic pattern of palladium in the signal at m/z 771 (3þ - 106) indicates the loss of palladium from the parent ion (m/z 877) in the gas phase. The loss of the cyclohexenyl moiety from the parent ion along with the hydrogen transfer can account for the signal at m/z 797 [3þ - C6H8]. The isotopic patterns of the parent ion at m/z 877 and the fragment ion at m/z 797 shown in Figure 8 are fully consistent with these assignments. Further, the phosphine group of the parent ion experiences some oxidation to afford a signal ascribed to [3 þ O]þ at m/z 893. The isotopic distributions of the signals at m/z 1567, 1583, and 1599 show the presence of only one palladium atom (Figure 8), and they are therefore attributed to the coordination of a second TSL ligand to the palladium center, as already found in the case of compound 2þ. The corresponding MS/MS experiments allow one to draw some qualitative conclusions about the ion structures. Specifically, exclusive formation of the fragment 3þ (m/z 877) from the ion at m/z 1567 (Figure 9a) suggests a quasi-symmetrical coordination of two TSL ligands to a Pd(C6H9)þ core. For the ion at m/z 1583, the fragments 3þ (m/z 877) and [3þ þ O] (m/z 893) are (32) See, for example: Pietrzak, M.; Wehling, J. P.; Kong, S.; Tolstoy, P. M.; Shenderovich, I. G.; L opez, C.; Claramunt, R. M.; Elguero, J.; Denisov, G. S.; Limach, H.-H. Chem.;Eur. J. 2010, 16, 1679.

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Figure 7. (a) ESI mass spectrum of a 4.5  10-7 M solution of 3þ 3 BArF- in CH3CN. (b and c) Expansions showing the regions from m/z 760 to 820 and m/z 1540 to 1700, respectively.

Figure 9. CID spectra of the mass-selected ions at (a) m/z 1567, (b) m/z 1583, (c) m/z 1599, and (d) m/z 1673 obtained upon ESI of a solution of 3þ 3 BArF- in CH3CN.

Figure 8. Separate zoom scans of the signals at (a) m/z 797, (b) m/z 877, (c) m/z 1567, (d) m/z 1583, (e) 1599, and (f) 1673 obtained upon ESI of a solution of 3þ 3 BArF- in CH3CN.

observed (Figure 9b), which is consistent with an oxygenation of one of the ligands to the corresponding phosphine oxide. By analogy, we attribute the ion with m/z 1599 (Figure 9c) mostly to single oxygenation of each of the phosphine ligands. Observation of a weak yet significant signal for 3þ (m/z 877) in Figure 9c suggests, however, that oxidation also leads to complexes in which the one of the TSL ligands is oxidized at both phosphorus atoms. Accordingly, we propose the structures shown in Chart 2 as the leading contributors to these ions. Like 2þ, also the cyclohexenyl complex 3þ shows a striking difference between the amount of oxidized complexes in the quasi-molecular ion (3þ:[3 þ O]þ = ca. 40:1) and the complexes with two TSL groups (i.e., ca. 1:2:1 between nonoxygenated m/z 1567 and the mono- and bisoxidized species at m/z 1583 and 1599; Figure 7c). This observation seems to indicate that ligand exchange between the precursor compound and the partially oxidized complexes is slow with respect to the time scale of the experiments.

The isotopic distribution of the signal at m/z 1673 shows the presence of two palladium atoms (Figure 8f). Upon CID (Figure 9d), the fragment 3þ is formed preferentially, which can be accounted for by an aggregation of 3þ with the neutral palladium(0) species Pd(1). Hence, the ion at m/z 1673 is assigned to the dinuclear cluster [(1)2Pd2(c-C6H9)]þ, which is analogous to the fragment at m/z 1633 observed in the case of 2þ 3 TfO-. In the high mass range, also the triple ion [(3þ)2 3 BArF-] was observed (see below).

Concentration Dependence In previous work, Eastoe et al. reported that coordinating counterions increase the equilibrium population of higher-order species in the simple Pd-allyl complexes of the Trost standard ligand.9 The weak interactions between the [B((3,5-(CF3)2)C6H3)4]- anion (“BArF”) and [Pd-allyl]þ cations allow the preparation of monomeric complexes, relatively free of oligomer, whereas other counterions give more pronounced clustering. Similarly, the amount of aggregation as determined via solution-phase NMR spectroscopy shows a distinct dependence on the solvent used. In order to probe possible effects of the solvent and of a pretreatment of the samples in different solvents, sample solutions in CH2Cl2 were investigated as well as samples that have been recrystallized under different conditions

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Chart 2. Proposed Structures of the Ions at m/z 1567, 1583, and 1599

Figure 10. Dependence of the fractional abundance of the dinuclear palladium cations as a function of concentration of the spraying solutions (cspray in mol L-1) of 2þ 3 TfO- and 3þ 3 BArF-, respectively, in CH3CN. For conversion of the concentrations of the feed solutions to the concentrations in the ESI process, we used the factors fspray = 3500, 950, and 23, respectively, for the three sets of data points with cfeed = 9  10-6 M, 4.5  10-5 M, and 4.5  10-3 M (for details of the conversion see ref 18). The inset shows the corresponding ESI mass spectrum of a 4.5  10-3 M solution of 2þ 3 TfO- in CH3CN with significantly more abundant signals of the dinuclear species (cf. Figure 3).

(the solids are usually amorphous and so polymorphism could also interfere). In the gas-phase experiments reported here, however, such variation in the treatment of the condensed-phase samples does not show any effect on the mass spectra obtained. Accordingly, we conclude that the nature of the solvent has no effect on the (unsolvated) complexes observed in the gas phase and that the dissolution of the aggregated complexes leads to their partial or complete dissociation into monomers with rapid equilibria between the remaining oligomers. In order to probe the effect of concentration in solution on the aggregation of such complexes as monitored in the gas phase, additional solutions of 2þ 3 TfO- and 3þ 3 BArF-, respectively, in CH3CN were probed using ESI-MS under the previously mentioned conditions. In comparison to the solution of 2þ 3 TfO- with c = 4.5  10-5 M, for a more dilute sample with c= 9.0  10-6 M a slight decrease of the aggregated Pd2 species is observed. The opposite effect is found for an increased concentration of c = 4.5  10-3 M, for which a pronounced increase in the relative intensities of the signals at m/z 1027, 1559, 1633, 1782, and 1823 is observed in the case of 2þ 3 TfO- (see inset in Figure 10). These results strongly support the aggregation of these complexes at higher concentrations to form dinuclear clusters. In comparison, the concentration effect observed for 3þ 3 BArF- is much smaller. This observation is consistent with the fact that triflate, as the better coordinating counterion, leads to clustering at higher concentration, whereas BArF, having a low interactivity with Pd-allyl cations, lowers the population of the higher-order species. It is to be stressed, however, that similar aggregates to those observed for 2þ 3 TfO-, e.g., the triple ion [(2þ)2 3 TfO-] (m/z 1823), escape our attention in the case of 3þ 3 BArF-, simply because the corresponding species, e.g., the

analogous triple ion [(3þ)2 3 BArF-] (m/z 2617), exceed the upper mass range of the ion-trap mass spectrometer used in the present experiments (2000 amu). In order to address this aspect, CH3CN solutions of 3þ 3 BArF- were also investigated at a multipole mass spectrometer with a mass range up to m/z 4000.28 While the overall ESI mass spectra of 2þ 3 TfO- and 3þ 3 BArF- are in very good agreement in both types of mass spectrometer, at a concentration of 4.5  10-3 M of 3þ 3 BArFwe indeed observe the triple ion [(3þ)2 3 BArF-] (m/z 2617) in the high mass range. Its abundance amounts to about 40% relative to the other Pd2 species below m/z 2000, which is accordingly acknowledged by scaling in the data shown in Figure 10. For a comparison with the condensed-phase data, it is useful to realize that in the course of the electrospray process the concentration of the sample solution serving as a feed to the ESI source (cfeed) experiences a rapid increase due to evaporation of the solvent with the consequence that the apparent concentration in the moment of ion formation (cspray) may significantly exceed cfeed. Recently, we introduced the phenomenological relation cspray = fspray  cfeed as a pragmatic approach toward a more quantitative description of concentration dependence using electrospray mass spectra.18,33 As a very first approximation, in the plot shown in Figure 10 we simply adopt the fspray values derived from the earlier work, i.e., fspray(9  10-6 M) = 3500, fspray(4.5  10-5 M) = 950, and fspray(4.5  10-3 M) = 23.18 In quantitative terms, this assumption may not be justified, but qualitatively it is still useful because aggregation of the title compounds at concentrations on the order of 10-5 M can certainly be neglected in solution,9 while we observe dimeric species in the ESI mass spectra of feed solutions with this concentration. Hence, it is quite reasonable to assume that cspray largely exceeds cfeed. With the conversion from cfeed into cspray, we conclude that a significant amount of aggregation occurs for about 0.1 M solutions, which is the correct order of magnitude deduced from the condensed-phase studies of 2þ 3 TfO- and 3þ 3 BArFreported in ref 9. (33) See also: (a) Gatlin, C. L.; Turecek, F. Anal. Chem. 1994, 66, 712. (b) Walther, C.; Fuss, M.; B€uchner, S. Radiochim. Acta 2008, 96, 411. (c) Urabe, T.; Tsugoshi, T.; Tanaka, M. J. Mass Spectrom. 2009, 44, 193.

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While it would be desirable to establish the exact values of fspray for the present system, we refrain from doing so for several reasons. First, Eastoe et al. have shown that the aggregation of the title compounds is all but trivial in that not just dinuclear species but also several higher aggregates are involved.9 Second and directly related with the first argument, the ion masses to be expected become exceedingly large for our instrumentation. Last but not least, the feed concentrations for which clustering becomes significant are already quite large, and with respect to the high molecular weight of the title compounds, the actual mass concentrations reach several milligrams per milliliter, which poses experimental problems in the electrospray interface, blockage of the transfer capillary in particular. To a first approximation, however, comparison of the gas-phase data with the results obtained in the condensed phase is quite promising with respect to future applications of electrospray mass spectrometry as a complementary tool for the investigation of aggregation phenomena in solution.

Conclusions The palladium(II) complexes 2þ 3 TfO- and 3þ 3 BArFbearing the Trost standard ligand form abundant molecular ions, i.e., 2þ and 3þ, upon electrospray ionization. Minor signals in the mass spectra indicate the occurrence of oxidation to the corresponding phosphine oxides either upon sample handling or in the course of the electrospray process. By reference to the isotope patterns and the fragmentations

Agrawal et al.

observed for the mass-selected ions, feasible structural assignments for these mononuclear species can be made. The dinuclear cluster cations observed using electrospray ionization can all be related to triple ions consisting of two Pd(II)allyl cations with one counterion, i.e., [(2þ)2 3 TfO-] and [(3þ)2 3 BArF-], respectively. With regard to the aggregation of these prototypical palladium(II) allyl complexes in solution, we demonstrate that electrospray ionization mass spectrometry can be used as a qualitative probe for the nature of the species formed, thereby allowing an assignment of the aggregates to a certain cluster size. Moreover, the abundance of the dinuclear palladium cations in the mass spectra correlates with the concentration of the precursor complexes in solution, lending support to the use of electrospray ionization as an assisting tool for solution-phase chemistry. As expected from previous NMR studies in solution, the gas-phase data demonstrate that the clustering of the palladium-allyl species is much less pronounced for the complexes involving the bulky and poorly coordinating BArF counterion compared to analogous triflate complexes.

Acknowledgment. This work was supported by the Czech Academy of Sciences (Z40550506), the European Research Council (AdG HORIZOMS), and the Grant Agency of the Czech Republic (203/08/1487). G.C.L.-J. is a Royal Society Wolfson Merit Award holder.