(C6H5)2Ag - American Chemical Society

Letter pubs.acs.org/JPCL

Synthesis and Spectroscopic Characterization of Diphenylargentate, [(C6H5)2Ag]− M. I. S. Röhr,† J. Petersen,‡ C. Brunet,§ R. Antoine,§ M. Broyer,§ P. Dugourd,§ V. Bonačić-Koutecký,†,⊥ R. A. J. O’Hair,# and R. Mitrić*,‡ †

Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany § Université Lyon 1, CNRS, 43 Bd du 11 Novembre 1918, F-69622 Villeurbanne cedex, France ⊥ Interdisciplinary Center for Advanced Science and Technology, University of Split, Meštrovićevo Šetalište 45, 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 ‡

S Supporting Information *

ABSTRACT: We present the structural and optical properties of the isolated diphenylargentate anion, which has been synthesized by multistage mass spectrometry in a quadrupole ion trap. The experimental photodetachment spectrum has been obtained by action spectroscopy. Comparison with quantum chemical calculations of the electronic absorption spectrum allows for a precise characterization of the spectroscopic features, showing that in the low-energy regime, the optical properties of diphenylargentate bear a significant resemblance to those of atomic silver. SECTION: Kinetics and Dynamics

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synthesized and characterized in the gas phase and for their unimolecular and bimolecular reactivity to be probed.9−12 There are a number of classes of organosilver ions that have been the subject of recent studies, such as (i) dialkylargentates, [RAgR]−, the heavy congeners of the Gilman reagents, [RCuR]−, which are widely used in organic synthesis;13−17 (ii) alkyl silver cluster cations, RAgn+18−25 as models for the organometallic intermediates involved in carbon−carbon bond formation mediated by silver metal surfaces26−29 and silver nanoparticles;30,31 or (iii) silver heterocyclic complexes like phenol-Ag+32 and pyridine-Ag+,33,34 in which the influence of heteroatom lone pairs on the bonding situation can be studied. Dialkylargentates represent interesting systems in which many fundamental aspects of metal−carbon chemical bonding

rganometallic reagents, which serve as catalysts in carbon−carbon coupling reactions, play a fundamental role in modern synthetic organic chemistry. Although the main focus of research activities has been on palladium-based catalysts, for which the Nobel prize was awarded in 2010,1 the use of coinage metals in organometallic catalysis has undergone a renaissance in recent years.2,3 In particular, organocopper compounds (“Gilman reagents”), discovered over 50 years ago, have been used in numerous C−C bond coupling reactions involving electrophiles such as alkyl halides.4−7 Due to the fundamental importance of such organometallic species, a variety of experimental and theoretical studies have been performed with the aim of revealing their structural and electronic properties as well as mechanistic details underlying their catalytic efficiency in the C−C coupling reactions.8 In particular, the combination of multistage mass spectrometry experiments, laser spectroscopy, and theoretical calculations has allowed organometallic species to be © 2012 American Chemical Society

Received: March 8, 2012 Accepted: April 19, 2012 Published: April 19, 2012 1197

dx.doi.org/10.1021/jz300280f | J. Phys. Chem. Lett. 2012, 3, 1197−1201

The Journal of Physical Chemistry Letters

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action spectroscopy in the energy range between 3.9 and 5.6 eV. The spectral bands observed using this technique are a signature of resonant one-photon electronic excitation of the precursor ion, followed by autoionization.40,41 In our experiment, a linear dependence of the photodetachment yield on the laser power could be established, clearly indicating a onephoton process (see also Supporting Information, Figure S1). Moreover, as there is no continuous rise of the detachment yield with increasing photon energy, direct detachment to the continuum can be excluded as a significant process. Due to the fact that the vertical electron detachment energy (VDE) of this system is lower than any electronic excitation energy (theoretical value of 3.76 versus 4.77 eV for the lowest-energy transition), all electronic states in the energy range considered here can, in principle, autoionize and are accessible by photodetachment spectroscopy. The experimental photodetachment yield as a function of the energy is presented in Figure 2a and exhibits one weak and broad feature centered at

such as the degree of polarity, ionic character, and their relation to the chemical reactivity can be investigated. In addition to the ground-state properties, the excited states of these systems open new possibilities to control the polarity and electron confinement and thus also the photochemical properties of such organometallic compounds. Therefore, we present here the first experimental and theoretical study of the structural and optical properties of the diphenylargentate anion, [(C6H5)2Ag]−. We will show that the optical properties of this system in the low-energy region have a striking resemblance to the optical absorption of confined neutral silver atoms. Interestingly, this is in stark contrast to previous findings on the structurally similar and isoelectronic dipyridine silver complex, in which the silver is strongly positively polarized and thus can be considered as a cation.35 Moreover, from studies on the monopyridine-silver cation, which have revealed a ligand-to-metal charge-transfer character of the lowest excited state,33,34 it can be inferred that a similar situation could be present in dipyridine-silver. Thus, although isoelectronic, this system exhibits a distinctly different electronic structure. The optimized structure of [(C6H5)2Ag]− is presented in Figure 1 and has D2d symmetry with perpendicular phenyl rings

Figure 1. Optimized structure of [(C6H5)2Ag]− together with the electron localization function (ELF) plotted for the isovalue of 0.75 (violet lobes). Figure 2. (a) Experimental photodepletion yield of [(C6H5)2Ag]− recorded as a function of the excitation energy. (b) Theoretical electronic absorption spectrum obtained from TDDFT (CAMB3LYP/TZVPP).

linked by a C−Ag−C subunit, exhibiting a C−Ag bond length of 2.13 Å. The nature of chemical bonding in this system has been analyzed in terms of the electron localization function (ELF),36 which is also shown in Figure 1 for the cutoff value of 0.75. The ELF allows identification of regions of strong electron localization that correspond to chemical bonds and free electron pairs.37,38 As can be seen from Figure 1, the C−Ag bonding electrons (represented by the small banana-shaped lobes perpendicular to the C−Ag bonds) are strongly shifted toward the carbon atom, reflecting the significant ionic character of the bond. Such strong polarity of the C−Ag bonding has been previously identified in the neutral [(C6H5)2Ag] species by Frenking et al.39 The polar character of the bonding is further confirmed by the natural bonding orbital (NBO) analysis, which gives rise to the silver atomic charge of +0.352, whereas the partial charges of the phenyl groups are −0.676. Furthermore, the natural electronic configuration at the silver atom obtained within the NBO analysis is 4d9.795s0.775p0.09, which is close to the values for an isolated neutral silver atom. Thus, formally, the species can be considered as a slightly positively charged silver atom surrounded by two partially negatively charged phenyl groups. In the experiment, [(C6H5)2Ag]− has been synthesized in the gas phase by multiple stage mass spectrometry starting from the [(C6H5CO2)2Ag]− precursor ion, which is subsequently decarboxylated in two steps.14−17 The optical properties of [(C6H5)2Ag]− have been investigated using photodetachment

4.5 eV and a stronger and sharper peak at 5.4 eV. Comparison with the theoretical absorption spectrum shown in Figure 2b, obtained using time-dependent density functional theory (TDDFT), allows for an analysis of the spectrum in terms of the relevant electronic transitions. The theoretical spectrum exhibits four weak electronic transitions located at 4.77, 5.16, 5.38, and 5.68 eV, as well as a strong one at 5.74 eV. In the following, these transitions are analyzed in terms of the leading Kohn−Sham (KS) molecular orbital excitations, allowing for determination of the character of the corresponding excited electronic states. For this purpose, the energies and spatial shapes of the KS orbitals are presented in Figure 3, and the composition of the intense electronic transitions is given in Table 1. The lowest-energy transition corresponds to the doubly degenerate 1 1E state and is dominated by excitations from the almost degenerate HOMO−1 and HOMO orbitals to the two degenerate LUMO and LUMO+1 orbitals. The HOMO of [(C6H5)2Ag]− is dominantly composed of the Ag 5s atomic orbital with a small contribution of the phenyl σ orbitals, whereas the HOMO−1 has silver 4d contributions as well as phenyl-π character. The doubly degenerate LUMO and LUMO+1 orbitals correspond to p orbitals of the Ag atom with a small contribution of phenyl-π* orbitals. Consequently, the 1198



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involving the phenyl moieties is observed, with the main excitations from HOMO−3/HOMO−2 to LUMO+2/LUMO +3. The intense transition at 5.74 eV has dominant contributions from HOMO−3/HOMO−2 to LUMO/LUMO +1 with mixed d−p and π−π* character as well as from the excitation from HOMO−1 to LUMO+4. Overall, most of the low-energy transitions of [(C6H5)2Ag]− exhibit a strong resemblance to the silver atomic transitions. Therefore, the optical spectrum of [(C6H5)2Ag]− can be interpreted as the spectrum of a silver atom confined between two phenyl rings. In summary, by combining quantum chemical calculations with photodetachment spectroscopy, we have investigated the structural and optical properties of the diphenylargentate anion in the gas phase. The negative charge is mainly distributed over the phenyl rings, whereas the silver atom is only slightly positive. The low-lying electronic transitions bear a strong resemblance to those of neutral atomic silver. Therefore, diphenylargentate can be viewed as a silver atom whose optical properties are only weakly perturbed by the surrounding phenyl moieties.

Figure 3. Schematic representation of energies and characters of the KS molecular orbitals involved in the low-lying transitions of [(C6H5)2Ag]− in the energy range up to 6 eV (cf. Table 1). The orbital energies are given in Hartrees (EH).

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EXPERIMENTAL AND COMPUTATIONAL The experimental setup consists of a quadrupole linear ion trap mass spectrometer (LTQ, Thermo Fisher Scientific, San Jose, CA, U.S.A.) coupled to a UV−vis tunable laser OPO,43,44 which allows for multistage mass spectrometry (MS) experiments and the simultaneous and sequential use of light-induced and collisional excitations. This original feature allows us to combine gas-phase ion chemistry with optical spectroscopy.31,44−46 Collision-induced dissociation (CID) was performed using helium gas at a normalized collision energy of 18% for 30 ms. For photoexcitation, 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, U.S.A.). 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 with a maximum laser energy of 0.3 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 photodetachment mass spectra are recorded after laser irradiation as a function of the laser wavelength. Irradiation leads mainly to electron detachment from the singly negatively charged ion and, therefore, to a decrease in the parent ion signal (see Figure 4). At each laser wavelength, a laser normalized yield of depletion is deduced from the mass spectrum through σ = ln(IOFFt/ION)/φ, where φ is the laser fluence, IOFF is the intensity of the parent peak without laser irradiation, and ION is the intensity of the parent peak after laser irradiation. Optical action spectra are obtained by plotting the normalized yield of fragmentation as a function of the laser wavelength. For the formation of [(C6H5)2Ag]−, acetic acid, benzoic acid, and silver nitrate were used as received from Sigma-Aldrich (Saint Quentin Fallavier, France). The methanol was obtained

Table 1. Characterization of Intense Transitions state 1

1 E

2 1E

3 1E

4 1E

5 1E

Te (eV)a

fe b

excitation

Cic

character

4.77

0.025

4.77

0.025

5.16

0.022

5.16

0.022

5.38

0.022

5.38

0.022

5.68

0.033

5.68

0.033

5.74

0.675

H−1 → L H→L H−1 → L+1 H → L+1 H → L+1 H−1 → L+1 H→L H−1 → L H−3 → L+2 H−-3 → L+3 H−2 → L+2 H−2 → L+3 H−1 → L+5 H → L+5 H−1 → L+6 H → L+6 H−3 → L H−2 → L+1 H−1 → L+4

0.478 0.453 0.478 −0.453 0.463 0.389 0.463 −0.389 0.355 0.354 0.355 −0.354 0.477 0.428 0.477 −0.428 0.374 0.374 0.309

Ag-d → Ag-p Ag-s → Ag-p Ag-d → Ag-p Ag-s → Ag-p Ag-s → Ag-p Ag-d → Ag-p Ag-s → Ag-p Ag-d → Ag-p Ph-π/Ag-d → Ph-π* Ph-π/Ag-d → Ph-π* Ph-π/Ag-d → Ph-π* Ph-π/Ag-d → Ph-π* Ag-d → Ag-p Ag-s → Ag-p Ag-d → Ag-p Ag-s → Ag-p Ph-π/Ag-d → Ag-p Ph-π/Ag-d → Ag-p mixed Ag/Ph

Transition energy. bOscillator strength. cCoefficient of transition (|Ci| ≥ 0.30). a

lowest-energy transition is mainly characterized by silver-like s− p and d−p excitations and can therefore be interpreted as the transition of a silver atom confined between the two phenyl groups. This is confirmed by the theoretical observation that the s−p transition of neutral atomic silver lies at about 3.85 eV. Due to the confinement effects of the phenyl ligands, this value is shifted to higher energies in [(C6H5)2Ag]−. Notice, that a similar shift to higher energies has been experimentally found for silver atoms in rare gas matrices previously.42 By contrast, the lowest-lying transition of the silver cation is of d−s character and is located at 4.85 eV. In the confined system, such a transition would be expected at even higher energies, thus confirming the assignment of the transition to a confined neutral silver atom. The transitions at 5.16 and 5.68 eV are composed similarly as the lowest-lying one. At 5.38 eV, a π−π*-like transition 1199



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position of the atomic s−p transition at 3.9 eV with the experimental value at about 3.8 eV,52 representing a good agreement.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 showing the dependence of the depletion yield on the laser power is provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Mr. A. Kulesza and Mr. L. Gell for their technical assistance. Financial support by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the Emmy Noether program, MI-1236 (R.M.) as well as of the Research Unit 1282 (R.M. and J.P.) is gratefully acknowledged.

Figure 4. Gas-phase formation and laser excitation of [(C6H5)2Ag]−. (a) Negative ion electrospray ionization mass spectrum of a silver nitrate and benzoic acid mixture showing the formation of [(C6H5CO2)2Ag]−. (b) CID of [(C6H5CO2)2Ag]−. A loss of CO2 is observed, leading to the formation of [(C6H5CO2)AgC6H5]−. A second product corresponding to [C6H5CO2]− (m/z 121) is observed at lower m/z values. (c) CID of [(C6H5CO2)AgC6H5]−. A loss of CO2 is observed, leading to the formation of [(C6H5)2Ag]−. Formation of [C6H5CO2]− is also observed. (d) Mass spectrum showing the gasphase isolation of [(C6H5)2Ag]−. (e) Mass spectrum obtained after laser excitation of [(C6H5)2Ag]− at λ = 260 nm (4.77 eV). A decrease in the ion count is observed as compared with (d) due to electron photodetachment. The detachment yield is determined from the ion count recorded with laser Off (part d) and with laser On (part e).

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(1) 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/ (2012). (2) Lipshutz, B. H.; Yamamoto, Y., Eds.; Coinage Metals in Organic Synthesis. Chem. Rev. 2008, 108 (8). (3) Gold  Chemistry, Materials and Catalysis. Chem. Soc. Rev. 2008, 37 (9). (4) Modern Organocopper Chemistry; Krause, N., Ed.; Wiley-VCH: Weinheim, Germany, 2002; pp 315−346. (5) Lipshutz, B. H. Organometallics in Synthesis; Schlosser, M., Ed.; Wiley: Chichester, U. K., 1994; pp 283−382. (6) Organocopper Reagents: A Pratical Approach; Taylor, R. J. K., Ed.; Oxford University Press: Oxford, U.K., 1994. (7) The Chemistry of Organocopper Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester, U.K., 2009. (8) Roithova, J.; Schroeder, D. Theory meets Experiment: Gas-Phase Chemistry of Coinage Metals. Coord. Chem. Rev. 2009, 253, 666−677. (9) Duncan, M. A. Infrared Spectroscopy to Probe Structure and Dynamics in Metal Ion−Molecule Complexes. Int. Rev. Phys. Chem. 2003, 22, 407−435. (10) O’Hair, R. A. J. The 3D Quadrupole Ion Trap Mass Spectrometer as a Complete Chemical Laboratory for Fundamental Gas Phase Studies of Metal Mediated Chemistry. Chem. Commun. 2006, 1469−1481. (11) MacAleese, L.; Maitre, P. Infrared Spectroscopy of Organometallic Ions in the Gas Phase: From Model to Real World Complexes. Mass Spectrom. Rev. 2007, 26, 583. (12) Roithová, J. Characterization of Reaction Intermediates by Ion Spectroscopy. Chem. Soc. Rev. 2012, 41, 547−559. (13) James, P. F.; O’Hair, R. A. J. Dimethyl Cuprate Undergoes C−C Bond Coupling with Methyliodide in the Gas Phase but Dimethyl Argenate Does Not. Org. Lett. 2004, 6, 2761−2764. (14) Rijs, N.; O’Hair, R. A. J. Gas-Phase Synthesis of Organoargenate Anions and Comparisons with Their Organocuprate Analogues. Organometallics 2009, 28, 2684−2692. (15) Rijs, N. J.; O’Hair, R. A. J. Unimolecular Reactions of Organocuprates and Organoargenates. Organometallics 2010, 29, 2282−2291. (16) Rijs, N. J.; Yoshikai, N.; Nakamura, E.; O’Hair, R. A. J. GasPhase Reactivity of Group 11 Dimethylmetallates with Allyl Iodide. J. Am. Chem. Soc. 2012, 134, 2569−2580.

from Roth (Karlsruhe, Germany). [(C6H5)2Ag]− was “synthesized” via the previously published decarboxylation method,13−17 which involves performing two stages of CID on the silver benzoate anion, [(C6H5CO2)2Ag]− (eqs 1 and 2), formed via electrospray ionization (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) with the addition of 0.01% of acetic acid (see Figure 4). [(C6H5CO2 )Ag]− → [(C6H5CO2 )AgC6 H5]− + CO2 −

(1)

−

[(C6H5CO2 )AgC6 H5] → [(C6H5)2 Ag] + CO2

REFERENCES

(2) −

Theoretically, the structural properties of [(C6H5)2Ag] have been determined in the framework of density functional theory (DFT) employing the hybrid B3LYP functional47,48 and the 19 electron relativistic effective core potential (RECP)49 combined with the TZVPP atomic orbital basis set for all involved atoms (def2-tzvpp).50 The bonding situation has been analyzed in terms of the ELF.36−38 All structures were fully optimized using gradient minimization techniques without any symmetry constraints. Subsequently, the stationary points were characterized as minima by calculating the vibrational frequencies. The absorption spectrum has been determined in the frame of TDDFT using the range-separated CAM-B3LYP functional.51 We wish to emphasize that the use of a range-separated functional is mandatory for the description of the optical properties of the system studied herein because commonly used hybrid functionals such as B3LYP give rise to a number of long-range charge-transfer transitions with low intensities and significantly underestimated transition energies. The accuracy of CAM-B3LYP for silver has been validated by comparing the 1200



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