ARTICLE pubs.acs.org/JPCA
Structural and Photochemical Properties of Organosilver Reactive Intermediates MeAg2+ and PhAg2+ C. Brunet,†,‡ R. Antoine,†,‡ M. Broyer,†,‡ P. Dugourd,*,†,‡ A. Kulesza,§ J. Petersen,§ M. I. S. R€ohr,|| R. Mitric,§ V. Bonacic-Koutecky ,*,||,^ and R. A. J. O’Hair*,# †
Universite de Lyon, F-69622 Villeurbanne cedex, France CNRS and Universite Lyon 1, Laboratoire de spectrometrie ionique et moleculaire UMR 5579, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France § Freie Universit€at Berlin, Fachbereich Physik, Arnimallee 14, D-14195 Berlin, Germany Humboldt-Universit€at zu Berlin, Institut f€ur Chemie, Brook-Taylor-Str. 2, D-12489 Berlin, Germany ^ Interdisciplinary Center for Advanced Science and Technology, University of Split, Mestrovicevo Setaliste bb., HR-21000 Split, Croatia # School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, ARC Centre of Excellence in Free Radical Chemistry and Biotechnology, The University of Melbourne, Melbourne, Victoria, 3010, Australia
)
‡
ABSTRACT: Although there is growing interest in silver promoted carboncarbon bond formation, a key challenge in developing robust and reliable organosilver reagents is that thermal and photochemical decomposition reactions can compete with the desired coupling reaction. These undesirable reactions have been poorly understood due to complications arising from factors such as solvent effects and aggregation. Here the unimolecular decomposition reactions of organosilver cations, RAg2+, where R = methyl (Me) and phenyl (Ph), are examined in the gas phase using a combination of mass spectrometry based experiments and theoretical calculations to explore differences between thermal and photochemical decompositions. Under collision-induced dissociation conditions, which mimic thermal decomposition, both PhAg2+ and MeAg2+ fragment via formation of Ag+. The new ionic products, RAg+• and Ag2+•, which arise via bond homolysis, are observed when RAg2+ is subject to photolysis using a UVvis tunable laser OPO. Furthermore, comparisons between the theoretical and experimental UVvis spectra allow us to unambiguously determine the most stable structures of PhAg2+ and MeAg2+ and to identify the central role of the silver part in the optical absorption of these species. The new photoproducts result from fragmentation in electronic excited states. In particular, potential energy surface calculations together with the fragment charges highlight the role of triplet states in these new fragmentation schemes.
’ INTRODUCTION As Nicolaou has noted,1 “The central activity of organic synthesis is the construction of the carboncarbon bond.” Stoichiometric or catalytic metal reagents that promote the formation of carboncarbon bonds have become indispensable tools for the synthetic chemist. Although the Nobel prize spotlight has been on palladium as a metal par excellence in catalyzing carboncarbon bond formation,2 the use of coinage metals in organic synthesis has undergone a renaissance in the past decade.3 While silver has had a reputation for being the “ugly duckling” of the coinage metals in organic synthesis,4 there is growing interest in its ability to promote carboncarbon bond formation.5 One of the major challenges in developing robust and predictable silver-mediated carboncarbon coupling reactions is that the relationship between reactivity and the characteristics of the metal reagent (e.g., structure, cluster size, charge, etc.) and the reaction medium (solvent, counterions, etc.) are often poorly understood. Thus in many synthetic studies (i) ill-defined metal reagents are used and (ii) only the final r 2011 American Chemical Society
organic product(s) are isolated and characterized. To overcome this problem we have been examining the fundamental gas phase reactivity6 of mass selected organometallic ions using a combination of mass spectrometry based approaches and theoretical calculations.7 We have focused our attention on two classes of alkyl silver ions: (i) dialkylargentates, RAgR, the heavy congeners of diakylcuprates or “Gilman” reagents, RCuR, which are widely used in organic synthesis;8,9 (ii) alkyl silver cluster cations, RAgn+, as models for the organometallic intermediates involved in carboncarbon bond formation mediated by silver metal surfaces10 and silver nanoparticles.11,12 For the cationic systems we have shown that both MeAg2+ and AllylAgn+ (n = 2, 4) undergo bond formation with allyliodide, eq 1.13,14 RAg2 þ þ CH2 dCHCH2 I f IAg2 þ þ CH2 dCHCH2 R ð1Þ Received: June 17, 2011 Published: July 29, 2011 9120
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A second, equally important challenge in developing CC bond coupling reactions of organosilver reagents is that thermal15 and photochemical decomposition reactions16 can compete with the desired coupling reaction and these can result in undesired byproducts. The condensed phase decomposition reactions of organosilver reagents are complex and can give rise to a range of products that arise from different pathways including17 β hydride transfer,18 radical reactions,19 and coupling reactions between ligands on adjacent metal centers.20,21 Silver nanoparticles can be formed, which can autocatalytically induce further reactions.12 The mechanistic features of these decompositions are generally not well understood due to complications arising from solvent effects, availability of coligands, and aggregation.22 In order to gain a better understanding of the fragmentation reactions of organosilver ions, we have been examining their unimolecular chemistry under collision-induced dissociation (CID) conditions2123 to mimic thermal activation. With the coupling of UVvis lasers to ion trap mass spectrometers, it is now possible to model thermal and photochemical fragmentation reactions of organosilver species in the same instrument. A recent study on the gas phase unimolecular chemistry of the silver hydride cation, HAg2+, has highlighted that these two different activation techniques can give rise to different types of fragmentation channels. Thus CID gave only one reaction channel (eq 2a), while this and two new bond homolysis reactions (eqs 2b and 2c) were observed under UVvis conditions.24 HAg2 þ f Agþ þ HAg f HAgþ• þ Ag• f Ag2 þ• þ H•
ð2aÞ ð2bÞ ð2cÞ
This has prompted us to examine the relationship between electronic structure and unimolecular reactivity of the cations RAg2+ (where R = Me and Ph) by studying their gas phase CID and UVvis spectroscopy using a combination of experiment and theoretical computation.
’ METHODS Mass Spectrometry and Action Spectroscopy. The experimental setup consists of a mass spectrometer coupled to a UVvis tunable laser OPO.25,26 The mass spectrometer is a quadrupole linear ion trap (LTQ, Thermo Fisher Scientific, San Jose, CA, USA). A quartz window was fitted on the rear of the LTQ chamber to allow the introduction of the laser beam. The laser is a nanosecond frequency-doubled tunable PantherTM EX OPO laser pumped by a SureliteTM II Nd:YAG laser (both from Continuum, Santa Clara, CA, USA). The repetition rate of the laser was 10 Hz. The laser beam passes through two diaphragms (2 mm diameter), lenses, and a mechanical shutter electronically synchronized with the mass spectrometer, after which it is injected on the axis of the linear trap. The maximum laser energy injected in the trap was 3 mJ/pulse. The reflection on the chamber quartz window is used to monitor continuously the laser power. The mechanical shutter is used to synchronize the laser irradiation with the trapping of the ions. To perform laser irradiation for a given number of laser pulses, we add in the ion trap rf sequence an MSn step with an activation amplitude of 0%, during which the shutter located on the laser beam is opened. UV photodissociation (UVPD) mass spectra are recorded after laser
irradiation as a function of the laser wavelength. At each laser wavelength, a laser normalized yield of fragmentation is deduced from the mass spectrum through σ ¼ lnððparent þ
∑fragÞ=parentÞ=ϕ
ð3Þ
where ϕ is the laser fluence, parent is the intensity of the parent peak and ∑frag represents the total intensity of the photofragment peaks. Optical action spectra are obtained by plotting the normalized yield of fragmentation as a function the laser wavelength. Collision induced dissociation (CID) experiments were performed with the same apparatus. This was accomplished using helium gas at a normalized collision energy of 815% for 30 ms. Our experimental setup allows for MSN experiments (multistage MS experiments) and the simultaneous and sequential use of light and collisional excitations. This original feature allows us to combine gas phase ion chemistry and optical spectroscopy.24,26,27 Formation of Organosilver Intermediates. Acetic acid, benzoic acid, and silver nitrate were used as received from Sigma-Aldrich (Saint Quentin Fallavier, France). The methanol was obtained from ROTH (Karlsruhe, Germany). PhAg2+ was “synthesized” via the previously published decarboxylation method13 which involves performing CID on the [M H + 2Ag]+ ion of benzoic acid (eq 4, R = Ph), formed via ESI on a solution of silver nitrate and benzoic acid with concentrations of 0.5 and 1 mM, respectively, in methanol and water (v/v: 50/50). MeAg2+ was obtained via decarboxylation of MeCO2Ag2+ (eq 4, R = Me), formed via ESI on a acidic mixture of silver nitrate and acetic acid with final concentrations of 0.33 and 1 mM, respectively, in methanol and water (v/v: 99.5/0.5) RCO2 Ag2 þ f RAg2 þ þ CO2
ð4Þ
Computational. The density functional theory (DFT) has been used to determine the structural properties of the studied systems employing the hybrid PBE0 functional28 and the relativistic effective core potential (RECP) of the Stuttgart group (MWB) for the silver atoms.29 The TZVPP atomic orbital basis set30 was used for silver as well as for carbon and hydrogen atoms. All structures presented were fully optimized using gradient minimization techniques, and stationary points were characterized as minima by calculating the frequencies. The absorption spectra have been determined in the frame of the time-dependent density functional method (TDDFT) using the same functional, RECP, and basis set. The fragmentation has been theoretically investigated by performing one-dimensional scans in the lowest-lying singlet and triplet states along the distance of the Ag or Ag2 fragment to the rest of the molecule using the CASSCF method.31 We employed an active space of 16 electrons in 12 orbitals for MeAg2+. For PhAg2+, six electrons in six orbitals for Ag loss and four electrons in five orbitals for Ag2 loss have been used. For silver, the Stuttgart MWB RECP and basis set29 were employed, whereas for the light atoms, the aug-cc-pVTZ basis set32 was used in the case of MeAg2+, and the smaller aug-cc-pVDZ32 in the case of PhAg2+. The localization of the positive charge for large distance between the individual fragments in different electronic states was determined using natural bond orbital (NBO) population analysis. Since dynamical correlation effects are not fully accounted for within the CASSCF method, the asymptotic fragmentation is only qualitatively described. 9121
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Figure 1. Two-dimensional cut of the electron localization function (ELF) for the electronic ground state of PhAg2+ (left) and MeAg2+ (right) in the plane containing the silver atoms and the neighboring carbon atom. The side view for PhAg2+ is given as an inset. Yellow and red colors indicate strong localization of electrons. The number of electrons in selected localization basins is given.
Therefore, in order to obtain the quantitatively correct energetics of the fragmentation channels, we have calculated the energies of the individual fragments employing the spin-unrestricted coupled-cluster singles and doubles method (CCSD),33 which very accurately accounts for electron correlation. For this, the same basis sets and RECP as in the CASSCF calculations have been used.
’ RESULTS AND DISCUSSION The lowest energy structures of PhAg2+ as well as of MeAg2+ obtained from DFT are characterized by an Ag2 subunit to which the organic fragment (Ph or Me) is bound via the nonsaturated carbon atom. For PhAg2+, the structure has C2v symmetry with the benzene ring perpendicular to the AgAg bond. In the case of MeAg2+ only one symmetry plane is present as can be seen from Figure 1. The nature of bonding in these organometallic complexes is illustrated by the electron localization function (ELF)34 shown in Figure 1, which provides a measure for the amount of localization of electrons in different parts of a molecule and can be partitioned into spatial regions (basins) according to topological criteria. The nature of these basins can be classified as core, bonding, and lone-pair, allowing the connection of quantum chemical electronic structure calculations with intuitive chemical concepts of bonding.35 Moreover, the integration of the electron density within a given basin can be used to assign the number of electrons contained in it. For PhAg2+ the ELF is characterized by separate core electron contributions centered on the silver atoms, while the bonding between the silver atoms and the nearby carbon can be interpreted as a two-electron three-center bond since 1.93 electrons are contained in the corresponding basin. In the case of MeAg2+ the bonding situation is similar. However, the bonding domain between the silver and carbon atoms contains only 1.33 electrons, indicating a weaker bond. This is also confirmed by the longer AgC distance compared to PhAg2+ (2.25 Å vs 2.16 Å) and by lower fragmentation energies (see also Figure 5 below). The mass spectra of PhAg2+ obtained by collision-induced dissociation (CID) and UV photodissociation (UVPD) at 290 nm are presented in Figure 2 and show the different fragmentation patterns obtained with both approaches. Whereas CID proceeds only via formation of Ag+ (eq 5a), two other homolytic cleavage
Figure 2. Fragment ions observed after isolation and excitation of PhAg2+ (MS3 mass spectrum): (a) collision induced dissociation; (b) UV photodissociation (λ = 290 nm).
pathways become possible for the UVPD, leading to the formation of PhAg+• (eq 5b) and Ag2+• (eq 5c). In addition, as a minor channel also the formation of Ph+ is observed (eq 5d). PhAg2 þ f Agþ þ PhAg f PhAgþ• þ Ag• f Ag2 þ• þ Ph• f Phþ þ Ag2
ð5aÞ ð5bÞ ð5cÞ ð5dÞ
Similarly, for MeAg2+, the CID also proceeds via the loss of Ag+ (eq 6a), while UVPD allows for additional homolytic cleavage reactions (6b, 6c). MeAg2 þ f Agþ þ MeAg f MeAgþ• þ Ag• f Ag2 þ• þ Me•
ð6aÞ ð6bÞ ð6cÞ
These fragmentation pathways are unique to UVPD and lead to the formation of gas phase radical species that were not observed in previous experiments. The optical and chemical 9122
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Figure 3. Comparison between experimental photofragmentation spectrum (red) and calculated absorption spectrum (black) for the lowest isomers of PhAg2+ (a) and MeAg2+ (b). The broadening of theoretical absorption lines has been simulated by a Lorentzian function with a width of 20 nm. fe labels the calculated oscillator strength (black sticks). The structures of the isomers are shown in the inset. The analysis of leading excitations is given on the right panels of the figure. Notice that in the case of PhAg2+ the labels S1 and S2 refer to the lowest-lying bright states, which are analogous to the S1 and S2 states of MeAg2+. Within TDDFT, several dark states with lower energy are present for PhAg2+.
Figure 4. Comparison of calculated absorption spectra for the lowest energy isomer (black) and the second isomer (blue) of RAg2+ (R = Ph, Me) with the experimental photofragmentation spectrum (red). The broadening of theoretical absorption lines has been simulated by a Lorentzian function with a width of 20 nm. fe labels the calculated oscillator strength. The structures of both isomers are also drawn.
properties of these new radical species can be assessed by subsequent optical spectroscopy and/or mass spectrometry investigations. The experimental optical action spectrum of PhAg2+ and the calculated TDDFT electronic absorption spectrum for the lowest energy structure of this species are presented in Figure 3. The experimental spectrum displays a band centered at 290 nm with a shoulder at ∼305 nm. The TDDFT absorption spectrum of PhAg2+ is characterized by a dominant transition at 298 nm (labeled as S2) and by a less intense transition at 307 nm (labeled as S1) which are in a good agreement with the measured features.
As also shown in Figure 3a, these transitions are due to leading excitations mainly from a bonding Ag2 orbital to the antibonding orbitals with nodal planes either perpendicular to the AgAg bond (transition S1) or parallel to the AgAg bond (transition S2). In addition, the transition S2 also exhibits a significant contribution of the charge transfer excitation from the π-system of the phenyl ring into the antibonding AgAg orbital. In the case of MeAg2+ (Figure 3b), the experimental spectrum displays a broad band at 280 nm. The theoretical absorption spectrum of the most stable isomer is dominated by two almost degenerate transitions located around 280 nm (S1, S2). These transitions arise from excitations from the binding orbital of Ag2 to two almost degenerate antibonding orbitals with nodal planes parallel and perpendicular to the AgAg bond. The agreement between the theoretical absorption spectra and experimental photofragmentation measurements is excellent. Moreover, the two spectra of MeAg2+ and PhAg2+ are closely related due to the dominant role of the Ag2 subunit. In both cases the two transitions lie very close in energy and are almost degenerate for MeAg2+. In contrast to the lowest energy isomer of PhAg2+, the second isomer does not contain an intact Ag2 subunit (cf. Figure 4) and the dominant feature of the absorption spectrum is substantially red-shifted. Moreover, this isomer does not show any significant absorption in the range around 290 nm. Thus, we can conclude that the second isomer cannot be responsible for the experimentally observed absorption features. Similarly, in the case of MeAg2+ the second isomer, which lies only 0.06 eV higher than the most stable one, has a linear structure with the methyl group inserted between two silver atoms (cf. Figure 4). This isomer 9123
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Figure 5. Upper part: Experimental branching ratios for UV photodissociation of PhAg2+ (a) and MeAg2+ (b). Lower part: Theoretical fragmentation energies for different fragmentation channels of PhAg2+ (c) and MeAg2+ (d) obtained from CCSD.
gives rise to very strong absorption at 360 nm and almost no absorption in the region at 280 nm. Therefore, we assign the experimental spectrum to the most stable isomer of MeAg2+ with an intact Ag2 subunit. In summary, the comparison between the theoretical and experimental spectra allows us to unambiguously identify the most stable structures of PhAg2+ and MeAg2+ and to identify the central role of the silver part in the optical absorption of these species. In addition to the total photofragmentation yield, experimental UVPD branching ratios for PhAg2+ and MeAg2+ as a function of the excitation wavelength are shown in panels a and b of Figure 5. In the case of PhAg2+, the fragmentation pattern is strongly wavelength-dependent. In the low energy regime around 300320 nm (corresponding to the S1 transition in Figure 3a), the main fragmentation channels involve the loss of a silver cation, whereas for wavelengths below 290 nm (S2) the homolytic fragmentations leading to a neutral silver atom or to Ag2+• become dominant. In contrast, the fragmentation of MeAg2+ is almost independent of the excitation wavelength. It is dominated by the formation of Ag2+• (∼90%), whereas neutral or positively charged silver atoms are only produced in very small amounts. In order to interpret the experimental fragmentation pathways, we first address the energetics of the fragmentation channels and analyze the charge distribution in the parent and fragment ions. The calculated binding energies for the different fragmentation pathways are shown in Figure 5c for PhAg2+ and in Figure 5d for MeAg2+. In both systems, the lowest energy pathway corresponds to the dissociation of the silver dimer followed by the loss of an atomic silver cation Ag+. This is in agreement with the fragments observed in CID experiments (see Figure 2a). The fragmentations of a neutral silver atom Ag0 as well as of the positively charged silver dimer Ag2+• are both about 0.7 eV higher in energy for PhAg2+, whereas for MeAg2+ the channel leading to Ag2+• is only 0.3 eV above the one leading to Ag+, but the channel leading to neutral Ag0 is almost 0.9 eV higher. The formation of the neutral silver dimer accompanied by the organic cations Me+ and Ph+ is strongly energetically disfavored.
The energetic preference for fragmentation channels where the positive charge is located on a fragment containing silver is also supported by the charge distribution in the parent and fragment systems for both PhAg2+ and MeAg2+ which has been analyzed by calculating the molecular electrostatic potential (MEP). This quantity provides a measure for the force exerted by the charge distribution of the molecule on a nearby placed positive charge. The projections of the MEP depicted in Figure 6 clearly show that the largest positive values are exhibited near the silver atoms both in the parent systems and in the fragments. Moreover, for both parent species PhAg2+ and MeAg2+ an NBO population analysis reveals that a positive charge of 1.4 is located on the Ag2 subunits. In the cationic fragments RAg+ (R = Ph, Me), the silver atom carries the positive charge while in the neutral fragments RAg the positive charge on silver is neutralized by the phenyl or methyl group. Furthermore, we investigate the energy of excited singlet and triplet states as a function of the distance between the Ag/Ag2 fragments and the remaining part of the molecule, which offers a qualitative explanation for the observed photoinduced fragmentation channels. For PhAg2+, this is shown in Figure 7, taking into account both neutral and positively charged fragments (Ag0/+ and Ag20/+). In addition, the NBO fragment charges for large distance between the fragments are presented in Table 1. As can be seen from Figure 7a and Table 1, the homolytic fragmentation leading to neutral Ag0 can proceed via the excited S1 state which has dissociative character with respect to the loss of a single silver atom. The lowest triplet state T1 state is only weakly bound and thus can also contribute to the formation of Ag0 provided that the spinorbit coupling to the S1 state is sufficiently large. Regarding the loss of Ag2+ (cf. Figure 7b and Table 1), the excited singlet states S1 and S2 are bound. Even if Ag0 dissociation could occur in the S1 state, its lifetime should still be high enough to sufficiently populate the triplet manifold via intersystem crossing, since due to the presence of the heavy silver atom the spinorbit coupling is enhanced. Whereas the T2 and T3 states are also bound, the lowest triplet state T1 is strongly dissociative for the Ag2+ loss and thus should be involved in the mechanism leading to this fragmentation channel. In summary, Ag+ loss is the major 9124
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Figure 7. Energies of the electronic singlet states S0S2 (black) and triplet states T1T3 (red) of PhAg2+ scanned along the distance between the PhAg+/0 and Ag0/+ fragments (a) as well as between Ph+/0 and Ag20/+ fragments (b), obtained by the CASSCF method. The asymptotic charges of Ag or Ag2 for each state are indicated as 0 or + (cf. Table 1). The fragmentation energies are of qualitative nature; for accurate values cf. the CCSD energies in Figure 5c.
Table 1. NBO Charges for PhAg2+ from CASSCF Scan for ΔR = 9.0 Å between the Fragments charge of fragment
Figure 6. Calculated molecular electrostatic potential map for parent systems RAg2+ (R = Ph, Me) (upper part) and for fragments RAg+ and RAg (R = Ph, Me) (lower part) projected onto the constant electron density surfaces with an isovalue of 0.01. (According to the color bars, red indicates larger and blue lower values of the potential.) The numbers assigned to the subunits refer to the respective NBO charges.
channel in the ground state (cf. Figure 5c and Figure 7a) whereas Ag0 and Ag2+ are formed in excited singlet and triplet states (cf. Figure 7a,b). Formation of neutral Ag20 is only a minor channel due to its high dissociation energy (cf. Figures 5c and 7b). Photodissociation of MeAg2+ leads mainly to the formation of Ag2+• (cf. Figure 5a). In this case, fragmentation in the excited state is totally different from the fragmentation observed after CID. The excited singlet states populated by the laser irradiation are characterized by charge redistribution both parallel and perpendicular to the AgAg bond (cf. Figure 3b), which could result in the formation of Ag as well as of Ag2 fragments. However, the energies of the lowest singlet and triplet states as a function of the distance between the Ag or Ag2 fragments and the rest of the molecule shown in Figure 8, together with the calculated asymptotic NBO charges for the fragments presented in Table 2, clearly indicate that the triplet states are involved in Ag2+ formation. In particular, the T1 state is highly dissociative with respect to the loss of Ag2+ (cf. Figure 8b). This finding is similar to the case of PhAg2+. The minor fragmentation channel involving loss of Ag+ should occur in the S0 state, and the loss of
state
Ag
Ag2
S0
0.994
1.000
S1
0.006
0.000
S2
0.000
0.000
T1 T2
0.000 0.000
1.000 0.000
T3
0.000
0.000
Figure 8. Energies of the electronic singlet states S0S2 (black) and triplet states T1T2 (red) of MeAg2+ scanned along the distance between the MeAg+/0 and Ag0/+ fragments (a) as well as between Me+/0 and Ag20/+ fragments (b), obtained by the CASSCF method. The asymptotic charges of Ag or Ag2 for each state are indicated as 0 or + (cf. Table 2). The fragmentation energies are of qualitative nature; for accurate values cf. the CCSD energies in Figure 5d. 9125
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Table 2. NBO Charges for MeAg2+ from CASSCF Scan for ΔR = 9.0 Å between the Fragments charge of fragment state
Ag
Ag2
S0
0.998
1.000
S1
0.000
1.000
S2 T1
0.999 0.000
1.000 1.000
T2
0.997
1.000
neutral Ag0 should take place in the S1 and T1 states (cf. Figure 8a) as also evidenced by the NBO population analysis given in Table 2. To summarize, similar to the case of PhAg2+ the loss of Ag+ occurs in the ground state (cf. Figures 5d and 8a), whereas Ag0 and Ag2+ are formed in excited singlet and triplet states (cf. panels a and b of Figure 8). The loss of neutral Ag20 does not take place due to its high dissociation energy (cf. Figure 5d).
’ CONCLUSIONS We have performed structural and spectroscopic characterization of organosilver cations RAg2+ (R = Ph,Me) which represent important reactive intermediates involved in carboncarbon coupling reactions. These species have been “synthesized” in the gas phase using electrospray ionization of silver nitrate and carboxylic acids, RCO2H, generating silver carboxylate cations, RCO2Ag2+, which have been subsequently decarboxylated leading to the formation of RAg2+. A modified ion trap has been used to investigate both the thermal and photoinduced fragmentation reactions of RAg2+, and the results have been interpreted by performing high level ab initio quantum chemical calculations. Under collision-induced dissociation conditions, both cations fragment via loss of RAg to form the silver cation. In contrast, UV photodissociation shows very different fragmentation patterns with the formation of additional ionic products corresponding to radical species: MeAg+• and Ag2+• in the case of MeAg2+; PhAg+•, Ag2+• as well as Ph+ in the case of PhAg2+. Although we are unable to directly compare our results with the solution phase photochemistry of these cations, it is worth highlighting that organosilver compounds are susceptible to photochemical decomposition reactions via related radical pathways involving bond homolysis reactions.18 The comparison between experimental photofragmentation and theoretical absorption spectra allowed us to identify the most stable structures of PhAg2+ and MeAg2+ and to characterize their excited electronic states as well as their photofragmentation channels. In particular, we have shown that triplet excited states play an important role in the photochemical fragmentation and formation of radical species. Thus, our study demonstrates that the combination of the multistage mass spectrometry in an ion trap with quantum chemical calculations represents a powerful tool for the identification and characterization of important reactive intermediates. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected];
[email protected].
’ ACKNOWLEDGMENT R.A.J.O. thanks the Australian Research Council for financial support through the CoE program and via Grant DP110103844, The Universite de Lyon for a visiting Professorship, and the School of Chemistry at the University of Melbourne for a short term study leave. V.B.-K. and P.D. thank the Hubert Curien Program for bilateral funding (Procope No. 50068937) as well as the NCBA international laboratory. R.M. and A.K. acknowledge the Deutsche Forschungsgemeinschaft, Emmy Noether program (MI-1236) for financial support. J.P. thanks the Fonds der Chemischen Industrie for financial support. ’ REFERENCES (1) Nicolaou, K. Classics in total synthesis: targets, strategies, methods; VCH: Weinheim, 1996. (2) The 2010 Nobel prize was awarded to Heck, Negishi, and Suzuki for “palladium-catalyzed cross couplings in organic synthesis”, http:// nobelprize.org/nobel_prizes/chemistry/laureates/2010/. (3) Indeed, an entire recent issue of Chemical Reviews (Chem. Rev. 2008, 37, issue 8, Lipshutz, B. H., Yamamoto, Y., Eds.) and much of an issue of Chemical Society Reviews (Chem. Soc. Rev., 2008, 37, issue 9) has been devoted to this theme. (4) Perez, P. J. Angew. Chem., Int. Ed. 2010, 49, 9040. (5) Silver in Organic Chemistry; Harmata, M., Ed.; John Wiley & Sons Inc., 2010. ISBN 978-0470466117. (6) For an excellent review on the gas phase chemistry of coinage metals see: Roithova, J.; Schroeder, D. Coord. Chem. Rev. 2009, 253, 666–677. (7) O’Hair, R. A. J. Chem. Commun. 2006, 1469–1481. (8) For reviews and monographs on organocopper species see: (a) Modern Organocopper Chemistry; Krause, N., Ed.; Wiley-VCH: Weinheim, 2002; pp 315346. (b) Lipshutz, B. H. Organometallics in Synthesis; Schlosser, M., Ed.; Wiley: Chichester, U.K., 1994; pp 283 382. (c) Organocopper Reagents: A Pratical Approach; Taylor, R. J. K., Ed.; Oxford University Press: Oxford, U.K., 1994. (d) The chemistry of organocopper compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, 2009. (9) James, P. F.; O’Hair, R. A. J. Org. Lett. 2004, 6, 2761–2764. (10) For CC bond coupling at silver surfaces, see: (a) Kung, H.; Wu, S. M.; Wu, Y. J.; Yang, Y. W.; Chiang, C. M. J. Am. Chem. Soc. 2008, 130, 10263–10273. (b) Kung, H.; Lee, L. C.; Wu, S. M.; Cheng, H. Y.; Chiang, C. M. J. Am. Chem. Soc. 2007, 129, 1028–1029. (c) Kershen, K.; Celio, H.; Lee, I.; White, J. M. Langmuir 2001, 17, 323–328. (d) Gurevich, A. B.; Teplyakov, A. V.; Yang, M. X.; Bent, B. E. Langmuir 1998, 14, 1419–1427. (11) For CC bond coupling on silver nanoparticles, see: Murugadoss, A.; Goswami, P.; Paul, A.; Chattopadhyay, A. J. Mol. Catal. A: Chem. 2009, 304, 153–158. (12) Tamura, M.; Kochi, J. K. Bull. Chem. Soc. Jpn. 1972, 45, 1120– 1127. (13) Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. Dalton Trans. 2009, 2832–2836. (14) (a) Khairallah, G. N.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2005, 44, 728–731. (b) Khairallah, G. N.; O’Hair, R. A. J. Dalton Trans. 2008, 2956–2965. (c) Wang, F. Q.; Khairallah, G. N.; O’Hair, R. A. J. Int. J. Mass Spectrom. 2009, 283, 17–25. (15) Fast decomposition of n-propyl silver(I) occurs at 60 C: Westmijze, H.; Kleijn, H.; Vermeer, P. J. Organomet. Chem. 1979, 172, 377–383. (16) The earlier literature on the organic photochemistry of silver has been reviewed: Horspool, W. H. The Organic Photochemistry of Silver and Gold. In The Chemistry of organic derivatives of gold and silver; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1999. (17) For reviews and key references on the general mechanistic features of decomposition reactions of organometallics see:(a) Baird, M. C. J. Organomet. Chem. 1974, 64, 289–300. (b) Davidson, P. J.; 9126
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