High-Resolution Electron Spectroscopy of Gas-Phase MetalâAromatic

PERSPECTIVE pubs.acs.org/JPCL

High-Resolution Electron Spectroscopy of Gas-Phase Metal-Aromatic Complexes Dong-Sheng Yang* Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States

ABSTRACT High-resolution electron spectroscopy combines pulsed field ionization zero-electron kinetic energy (ZEKE) detection with in situ laser-assisted synthesis and supersonic expansion. The technique offers sub-meV spectral resolution for the electron spectra of metal complexes and is a powerful tool to study their bonding and structures. This Perspective presents recent progress in single-photon ZEKE spectroscopy of metal-aromatic complexes and focuses on the determination of the electron spin multiplicities, metal binding sites and modes, rotational conformers, and conformational changes of these critical species in organometallic chemistry.

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lmost all photoelectron spectra of gas-phase metal complexes in the literature are broad without resolved vibrational features.1-7 Lack of the vibrational structures impedes the direct investigation of metal-ligand binding and the precise measurement of adiabatic ionization energies (IEs). This Perspective presents a brief account of our recent efforts in sub-meV high-resolution electron spectroscopy of metal-aromatic complexes. Early high-resolution electron spectroscopic studies on metal complexes and clusters were discussed in previous reviews.8-10 The high-resolution electron spectroscopy combines in situ laser-assisted synthesis,11 supersonic expansion,12 and, more importantly, pulsed field ionization zero-electron kinetic energy (ZEKE) detection.13-16 In situ laser-assisted synthesis opens the possibility for studying short-lived metal radicals, supersonic expansion reduces molecular internal temperatures and thus simplifies spectra, and the ZEKE technique detects near-zero-energy electrons and yields very sharp electron bands. ZEKE detection is different from traditional photoelectron spectroscopy in that ionization is not direct but stepwise. It begins with photoexcitation of neutral molecules from ground or low-energy states to highlying Rydberg levels or ionization continua by a tunable laser. The high-energy Rydberg states lie within a few wavenumbers (cm-1) of the ionization threshold and have a lifetime up to tens of microseconds. The optical excitation is followed by a time delay during which electrons with kinetic energy produced by direct photoionization or autoionization travel out of the detection window. Finally, the long-lived Rydberg molecules are ionized and extracted by a small electric field, and spectra are recorded by measuring the yields of the near-zero-energy Rydberg electrons as a function of the laser wavelength. The spectral resolution of the technique depends on the laser line width and the electric field strength but not on the electron energy analyzer as in conventional photoelectron spectroscopy. The intensity profile of a ZEKE spectrum is similar to what would have been obtained from

r 2010 American Chemical Society

direct photoionization because of the continuity of the ionization oscillator strength across the ionization threshold. The electron intensity is governed by the Franck-Condon (FC) principle, although some exceptions are known as well.17,18 Energy shifts in electron bands induced by the electric field can be corrected by a systematic measurement of the field dependence of the band positions. In this way, ZEKE determines precise and accurate IEs of the neutral complexes and vibrational energy levels of the singly charged cation. Molecular beam ZEKE spectroscopy has particular advantages in studying metal complexes. Because the IE of a metal atom is generally lower than that of an organic molecule, ionization of a metal complex removes a metal-based electron and affects mostly metal-ligand coordinates. Therefore, ZEKE is most sensitive to the metal-ligand stretching and bending vibrations, the most important vibrational modes for studying metal-ligand binding. Because the metal-ligand and other low-frequency modes are easily excited thermally, it is possible to measure the vibrational frequencies of the neutral complexes, in addition to those of cations. The IEs of many metal complexes are in a spectral region easily accessible to frequency-doubled dye lasers, making the application of the single-photon ZEKE technique fairly routine. Because electronic states of the metal complexes are normally short-lived, high-resolution ZEKE spectra achieved without relying on a resonant electronic-state selection is particularly attractive. In the following sections, we will discuss the electronic states, metal binding sites and modes, and molecular conformations of metal-aromatic complexes determined by ZEKE spectroscopy in combination with theoretical calculations. Received Date: November 14, 2010 Accepted Date: December 3, 2010 Published on Web Date: December 08, 2010

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DOI: 10.1021/jz101550d |J. Phys. Chem. Lett. 2011, 2, 25–33


expected to come from the weakly bound dσ orbital. Thus, the neutral ground state should have a spin multiplicity of 4 with the electron configuration of (e2)2(a1)1, and the cation state should have a multiplicity of 3 with the electron configuration of (e2)2. Indeed, the spectral simulation of the 3A1 r 4A1 transition (Figure 2a, bottom) obtained from multidimensional FC factor calculations shows excellent agreement with the measured spectrum. In this and other simulations presented in this Perspective, the theoretical IE was shifted to the experimental value for clear comparison, but the calculated vibrational frequencies were not scaled. The FC factors were calculated from the equilibrium geometries, harmonic vibrational frequencies, and normal modes of neutral and ionic states.22 The geometries and vibrational frequencies were calculated with the B3LYP hybrid functional and 6-311þG(d,p) basis. Normal-mode differences between the two electronic states were considered in the FC calculations. Spectral broadening was simulated by giving each line a Lorentzian line shape with the line width of the experimental spectrum. Boltzmann distributions were assumed to simulate spectra at specific temperatures. In contrast to the monobenzene species, the ZEKE spectrum of Sc(C6H6)2 (Figure 2b, top) exhibits two distinct electronic transitions; one originates at 40883(5) cm-1 with 206 and 302 cm-1 intervals, and the other begins at 42112(5) cm-1 with 201 and 422 cm-1 intervals.21 Both transitions display a longer FC profile than the Sc(C6H6) spectrum. The longer FC profile is indicative of a larger structural difference between the neutral and cation states; the larger structural change is caused by the ejection of a more strongly bound electron upon ionization. For a sandwich complex, the ideal molecular symmetry is D6h in the eclipsed conformation and D6d in the staggered form. The energy difference between the two conformers is small because the interaction between the two well-separated benzene molecules is weak. Assuming that the sandwich species is in the eclipsed D6h form and the metal-based orbitals are in the same energy order as that of the monobenzene complex [e.g., dδ(e2g,dxy,x2-y2) < dσ(a1,sdz2) < dπ(e1g,dxz,yz)], the extended vibrational structures in the ZEKE spectrum of Sc(C6H6)2 suggest that the ejected electron should originate from the dδ bonding orbital in the electron configuration of (e2g)3 and the low-spin electronic state of 2E2g. Why does Sc(C6H6)2 prefer a low-spin state whereas Sc(C6H6) favors a high-spin state? This is because coordination of a second benzene to Sc enlarges the energy separation of the metal-based dδ, dσ, and dπ orbitals, and the large dδ and dσ separation forces the Sc three valence electrons into the low-energy dδ orbital, resulting in a doublet electronic ground state. In the 2E2g state, the complex is prone to Jahn-Teller distortion, and the molecular symmetry is reduced from D6h to D2h. In this case, the doubly degenerate 2E2g state splits into 2B1g and 2Ag states. Transitions from the 2B1g (D2h) neutral state to the 1A1g (D6h) and 3 A1g (D6h) cation states were determined to be responsible for the observed ZEKE spectrum of Sc(C6H6)2. In addition to Sc(C6H6)2, other M(C6H6)2 (M = Ti, V, Cr) complexes also favor low-spin ground states, with electron spin multiplicities of 1, 2, and 1 for M = Ti, V, and Cr, respectively.23,24

Figure 1. An orbital interaction diagram between M (M = Sc, Y, and La) and benzene (a) or cyclo-octatetraene (b). Adapted with permission.44 Copyright 2009, American Institute of Physics.

Monobenzene metal complexes prefer high-spin electronic states, whereas dibenzene species favor low-spin states. Transition-metal benzene complexes are classic examples in organometallic chemistry and have received considerable attention in mass spectrometry and molecular spectroscopy.19 For a monobenzene metal species, the ideal molecular symmetry is C6v. Under this point group, the five metal d orbitals span three irreducible representations and are in the energy order of dδ(e2,dxy,x2-y2) < dσ(a1,sdz2) < dπ(e1,dxz,yz), as shown in Figure 1a. The dδ orbital has bonding character, the dπ orbital is antibonding, and in between is the weakly bonding dσ orbital pointing to the center of the benzene ring.20 The electron multiplicity of a metal-benzene complex depends on the number of metal valence electrons, the separation of the d-based molecular orbitals, and the energy required to pair up the electrons. The first element in the transition series, Sc, has three valence electrons in the ground electron configuration (3d14s2); the spin multiplicity (2S þ 1, where S is the electron spin angular moment) of its monobenzene complex may be a doublet or a quartet in the ground state. Figure 2a (top) presents the experimental ZEKE spectrum of Sc(C6H6).21 It exhibits the strongest band at 41600(5) cm-1 [or 5.1578(6) eV] and vibrational intervals of 324 and 375 cm-1. The 41600 cm-1 position of the strongest band corresponds to the adiabatic IE of the complex, and the 324 and 375 cm-1 intervals relate to the Sc-benzene stretching frequencies in the neutral and cation states, respectively. Because the IE of Sc atom (6.5615 eV) is lower than that of benzene (9.24378 eV), ionization of the complex removes an electron from the Sc-based dδ or dσ orbital. Because the spectrum shows a very short FC profile, the ejected electron is


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Figure 2. (a) Experimental ZEKE spectrum with He carrier (top) and simulation at 300 K (bottom) of the 3A1(C6v) r 4A1(C6v) transition of Sc(C6H6). (b) Experimental ZEKE spectrum with He/Ar carrier (top) and simulation at 100 K (bottom) of the 1A1g(D6h) r 2B1g(D2h) and 3 A1g(D6h) r 2B1g(D2h) transitions of Sc(C6H6)2. Adapted with permission.21 Copyright 2005, American Institute of Physics.

Modification of ligand chemical environments or metal electronic configurations switches metal binding sites and modes of aromatic molecules. Substitution of one or more hydrogen atoms in benzene with a functional group provides competing sites for metal coordination and may yield multiple conformers for a metal complex. Figure 3a shows the ZEKE spectrum of Sc(C6H5CN), which is rather different from those of other Sc(C6H5X) (X = H, F, CH3, and OH) complexes.21,25 The adiabatic IE [46006(5) cm-1] of Sc(C6H5CN) is 3000-5000 cm-1 higher than those of Sc(C6H5X), the vibrational interval of the major progression (716 cm-1) is nearly twice as large as those of the Scþ-C6H5X stretching progressions (370-380 cm-1), and more vibrational modes (ranging from 116 to 1170 cm-1) are active in the spectrum of Sc(C6H5CN). All of these observations point to a different binding mode of benzonitrile from that of the other benzene derivatives. There are at least four possible binding modes between C6H5CN and Sc (see insets in Figure 3). These include a ring-π η6 binding with Sc on top of the ring, a nitrileN terminal mode with Sc binding to N of the nitrile group, a nitrile-η2 mode with Sc bridging between C and N of the nitrile group, and a nitrile-η3 mode with Sc triply bound to C-CtN. Among these modes, the nitrile-η2 binding is preferred, as shown by the comparison of the experimental and theoretical ZEKE spectra in Figure 3. In this mode, four major bonding components contribute to the metalligand binding, ligand to metal σ donation, ligand to metal


Figure 3. Experimental ZEKE spectrum of Sc(C6H5CN) seeded in He (a) and simulations (100K) of the nitrile-η2 (b), ring-π η6 (c), nitrile-η3 (d), and nitrile-N terminal (e) isomers of Sc(C6H5CN). Adapted with permission.25 Copyright 2009, American Institute of Physics.

out-of-plane π^ donation, metal to ligand in-plane π|| backdonation, and metal to ligand δ back-donation. These multiple

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interactions lead to a much larger Sc-C6H5CN binding energy (130 kJ mol-1) than those of the ring-π Sc-C6H5X structures (70-80 kJ mol-1). Metal-ligand binding modes can be tuned not only by modifying the chemical environment of an organic molecule but also by changing the identity of metal elements. This approach has been used to alter metal-ligand binding between σ and π26-28 or between mono- and multidentate modes.29-31 Pyridine (C5H5N) is one of the best-known heterocyclic aromatic ligands and has been extensively studied in coordination and surface chemistry. In coordination chemistry, most metal-pyridine complexes are in a σ binding mode, where pyridine uses the N electron lone pair to bind with metal ions. On metal surfaces, pyridine shows a variety of adsorption modes depending on the structure of the metal surface and the coverage of the pyridine molecules. These modes include vertical adsorption via the N lone pair electrons, tilted adsorption via both the lone pair electrons of the N atom and the π electrons of the aromatic ring, flat adsorption via the π electrons, and edge adsorption via the N and C atoms. In the gas phase, we detected both σ and π binding modes depending on metal atomic electron configurations, for example, Li, [He]2s1; Ca, [Ar]4s2; Sc, [Ar]3d4s2; and Cu, [Ar]3d104s. Although all of these atoms contain ns1 or ns2 configurations, Li, Ca, and Cu bind with pyridine via the N lone pair σ mode, whereas Sc does so via the aromatic ring-π mode.26,27 Compared to Sc-benzene, Sc-pyridine favors a low-spin electronic state 2A1 in the C2v point group. The unique π binding of Sc among the four metal atoms is apparently due to the involvement of the Sc 3d orbitals. To be available for π binding, the metal d orbitals must be incompletely filled; the presence of one d electron is sufficient to switch the metal-pyridine binding from the σ to π mode. The Sc-pyridine binding is 2-4 times stronger than that of the other three metals, and the enhanced Sc binding is due to the 6-fold metal-carbon coordination with the electron donation from pyridine to Sc and the back electron donation from Sc to pyridine. For the σ complexes, the Li binding is stronger than the Ca or Cu binding. The different binding strengths among the three metals may be examined by considering the Pauli repulsion between the metal s and nitrogen lone pair electrons. Li and Ca have s1 and s2 occupations, respectively; as the occupation of the s orbital increases, the Pauli repulsion increases, and metal binding is expected to decrease. Although both Li and Cu have s1 occupation, the Pauli repulsion of the Li 2s1 electron should be smaller than that of the Cu 4s1 electron. This is because the s-p orbital energy difference of Li (1.85 eV) is smaller than that of Cu (3.78 eV)32 and the s-pσ hybridization in Li is thus more effective than that in Cu. Such s-pσ hybridization polarizes the s electron density away from the ligand and to the opposite side of the metal atom and thus reduces the Pauli repulsion. Ionization of these σ complexes removes a metal s electron and creates a positive charge on the metal atom. The removal of the s electron reduces the Pauli repulsion and adds a charge-dipole interaction to the metal-ligand binding. As a result, the binding energies of the σ complexes increase by 4-10 times upon ionization. For the Sc-pyridine π complex, the increased binding energy upon ionization is largely due to the additional


Figure 4. Experimental ZEKE spectrum of Li-adenine seeded in He (a) and simulations (50 K) of bidentate Li-N7/NH2 (b) and Li-N1/NH2 (c) and monodentate Li-N3 (d) and Li-N1 (e) isomers of Li-adenine. Adapted with permission.33 Copyright 2010, American Institute of Physics.

charge-quadrupole interaction and the elimination of the electron repulsion between the Sc 4s1 and pyridine π electrons. However, because the charge-quadrupole interaction is usually weaker than the charge-dipole interaction, the increase in the binding energy of the Sc π complex is less profound compared to the binding enhancement in the Li/Ca/ Cu σ complexes. Unlike benzene, the nucleobases in DNA and RNA possess many possible sites for metal binding and form multiple lowenergy isomers upon metal coordination.33-35 Figure 4a presents the experimental ZEKE spectrum of Li(adenine),33 which originates at 32240(5) cm-1 and exhibits transitions consisting of six vibrational intervals of 302, 378, 412, 702, 870, and 1104 cm-1. The 302, 378, and 412 cm-1 intervals correspond to the frequencies of Liþ-adenine bending and stretching vibrations, whereas the 702, 870, and 1104 cm-1 intervals relate to the frequencies of ligand-based vibrations. Possible sites for Li binding to the canonical form of adenine include (a) π binding above the imidazole or pyrimidine ring, (b) monodentate σ binding with one of the nitrogen atoms in the aromatic rings, (c) monodentate σ binding with the NH2 amino group, and (d) bidentate σ binding with N7/NH2 or N1/ NH2. Among these possibilities, the π binding above the fiveor six-membered ring is less desirable because of the highly perturbed electron distribution arising from the presence of heteroatoms. Metal σ binding with the N9 atom requires sp2 f sp3 rehybridization of nitrogen atomic orbitals, which distorts the planarity of the ligand and reduces the stability of the complex. Similarly, metal binding to NH2 may require the rotation of the amino group out of the adenine plane and the rehybridization of the nitrogen atomic orbitals. Thus, the most

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likely monodentate σ binding sites should be N1, N3, or N7. Figure 4 compares the experimental spectrum with the simulations of four low-energy σ isomers; the comparison shows that the simulation of the Li-N7/NH2 isomer has the best match to the experimental spectrum in both vibrational frequencies and intensities, and those of the other three isomers have a longer FC profile, mismatched vibrational frequencies, or many extra bands compared to the measured spectrum.33 The Li-N7 isomer is not include in the figure because ionization converts the monodentate isomer into the bidentate Li-N7/NH2 and leads to a very long FC profile that has no resemblance at all to the observed spectrum. Thus, the comparison of the experiment and theory shows that Li-N7/ NH2 is the carrier of the ZEKE spectrum, which is also the most stable isomer (with others at 8-14 kJ mol-1 higher in energy). The stability order of the four σ isomers is Li-N7/ NH2 > Li-N1/NH2 > Li-N3 > Li-N1. Li-N7/NH2 and LiN1/NH2 are more stable than Li-N3 and Li-N1 because of the additional Li binding to the amino group. Between the two bidentate isomers, Li-N7/NH2 is more stable than Li-N1/ NH2 because of the smaller steric strain in the five-membered ring of Li-N7/NH2 than that in the four-membered ring of Li-N1/NH2 and the similar binding strength of Li-N7 and Li-N1. In the Li-N7/NH2 isomer, the major structural changes of adenine induced by Li coordination are around the nitrogen atom of NH2, which include the rotation of the NH2 group out of the adenine plane, the decrease of the H-N-H degrees (from 120.4 to 104.4), and the increase of the C6-NH2 bond length (from 1.353 to 1.453 Å). All of these changes suggest a sp2 f sp3 rehybridization of the nitrogen orbitals upon metal coordination. In addition to adenine, the preferred metal (Li and Al) binding sites of uracil and thymine have been determined to be the O4 atom.34,35 Although it causes no conformational change of uracil and thymine, metal coordination significantly reduce the C4-O4 bond strength. By combining the bond energies of the Liþ ion complexes36,37 and ionization energies of Li atom and the neutral complexes, the metal-ligand bond energies of the neutral complexes have been determined to be 92 kJ mol-1 for the bidentate Li-adenine and 97 and 92 kJ mol-1 for the monodentate Li-uracil and Li-thymine.

Figure 5. Experimental ZEKE spectra of Cr(toluene)2 seeded in He (a), 2:1 He/Ar (b), and Ar (c), along with 0, 60, 120, and 180 rotational conformers of the complex. Adapted from ref 38.

binding with the phenyl ring is equal for the six carbon atoms, the two rotational isomers of Sc(C6H5OH) should have the same energies and would not be distinguishable in our experiment. However, because Sc binding to the six ringcarbon atoms is not equal due to the OH substitution, the two Sc(C6H5OH) rotamers are not isoenergetic. The energy difference and barrier between the two rotamers were predicted to be ∼0.1 and ∼20 kJ mol-1 by density functional theory, respectively. Their IE difference was measured to be 80 cm-1 from the ZEKE spectrum.25 Although only one isomer was detected for Sc(toluene), multiple rotamers have been identified for M(toluene)2 (M= Cr, Mo, W).38,39 Phenyl rings in the sandwich complexes are essentially free to rotate in isotropic solutions, but they are locked in place by the surrounding anions in the solid state and adopt the conformation dictated by the energetics of the lattice formation. Thus, both eclipsed and staggered conformations have been identified for the sandwich cations in the solid state. In the gas phase, the eclipsed conformation is more stable and has four minimum-energy rotamers with methyl group dihedral angles of 0, 60, 120, and 180. The energy differences among these rotamers are within 1 kJ mol-1, and the energy barriers for their conversion are about 4 kJ mol-1. Because the energy differences among these rotamers are so small, the theoretical identification of the most stable rotamer is problematic. By combining variabletemperature ZEKE spectroscopy and theoretical calculations, we determined the 0 rotamer to be the most stable one, with the highest adiabatic IE among the four rotamers.38 Figure 5 presents the ZEKE spectra of Cr-bis(toluene) seeded in three different carrier gases (He, 2:1 He/Ar, and Ar). The spectrum with He carrier exhibits the strongest band A at 42744 (5) cm-1 and the second strongest band B at ∼63 cm-1 higher in energy.

Metal coordination yields multiple low-energy rotational conformers. Among the phenyl-ring π complexes of Sc(C6H5X), a single conformer was identified for X = H, F, and CH3, but two were indentified for X = OH.25 Unlike the low-energy barrier of the methyl rotation in toluene (5-20 cm-1), the barrier of the OH rotation in phenol is rather high (1050-1250 cm-1). The minimum-energy rotamers of phenol are planar with CCOH dihedral angles of 0 and 180. Under the supersonic expansion, the OH internal rotation is hindered, and the 0 and 180 rotamers are trapped in separate potential wells. Sc coordination to these two phenol rotamers is expected to form two stable rotational isomers of Sc(C6H5OH) as well. If the Sc


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Following these two strong bands, several weaker bands appear in three groups; the first group consists of bands C-E (42907, 42923, and 42939 cm-1), the second F-I (43003, 43015, 43021, and 43030 cm-1), and the third J (43098 cm-1). When the complex is seeded in a 2:1 He/Ar mixture, band A is resolved into two components A1 (42741 cm-1) and A2 (42747 cm-1) because of the reduced spectral line width; the intensity of band B is increased and becomes stronger than those of A1 and A2. Moreover, intensities of bands I and J are increased compared to those in the He spectrum. With pure Ar, intensities of bands A1, A2, and C-H are further reduced, whereas those of bands B, I, and J become even stronger. By comparing the ZEKE spectra with the three carriers and theoretical calculations, bands B, I, and J are assigned to the transitions between the ground vibronic levels and Crþ-toluene stretching and bending excitations of the most stable 0 rotamer. A1 (A2), C (D, E), and F (G, H) are assigned to the ground vibronic transitions and metal-ligand stretching and bending vibrations of the 180 (120 and 60) rotamers, respectively. Similarly, the most stable rotamers of Mo(C6H5CH3) and W(C6H5CH3) are also with a 0 dihedral angle. The population of the 0 rotamer was determined to be 20% in He and 80% in Ar for Cr(C6H5CH3)2 and about 50% in He and almost 100% in Ar for the Mo and W species. The observation of the most stable rotamer with the 0 dihedral angle seems contradictory to the classic textbook assumption that steric considerations play the leading role in deciding the stability of staggered and eclipsed forms. However, steric effects are not the only factors that drive the conformations; hyperconjugation is often a dominating one.40-42

Figure 6. Experimental ZEKE spectrum (a) and simulations (100 K) of the clamshell (b) and half-sandwich (c) isomers of Sc(biphenyl). Reprinted from ref 43.

representative ZEKE spectrum of Sc(biphenyl). The spectrum exhibits the origin band at 39114(5) cm-1 and vibrational intervals of 284, 336, 378, and 568 cm-1. Compared to Sc(benzene),21 Sc(biphenyl) exhibits a lower adiabatic IE and more extensive vibrational structure in the ZEKE spectrum. These observations suggest that the Sc binding mode with biphenyl should be different from that with benzene. Theoretical calculations on Sc(biphenyl) predict two isomers, a clamshell structure with Sc binding to two phenyl rings and a half-sandwich structure with Sc binding to a single ring. The clamshell structure is slightly more stable than the halfsandwich. The 1A1 r 2B1 simulation of the clamshell structure (Figure 6b) matches well with the experimental spectrum, although it misses the weak peak at 284 cm-1. In contrast, the 3 A r 4A simulation of the half-sandwich structure (Figure 6c) severely underestimates the intensities of all major peaks observed in the experiment and exhibits at least four transitions (marked by asterisks) that do not exist in the measured spectrum. This comparison shows clearly that the observed spectrum originates from the 1A1 r 2B1 transition of the clamshell structure. One can imagine the formation of the clamshell structure as occurring by a mechanism in which the two phenyl rings first rotate to become coplanar and then bend toward the Sc atom as it approaches. The structure is clamped in place when the Sc atom coordinates to both phenyl rings by a 12-fold binding mode. The formation of the clamshell structure is astounding; it requires both rotation and bending of the phenyl rings. The energy cost for the ring motion is, however, overcome by the stabilization of the metal-π interaction. Unlike benzene or biphenyl, 1,3,5,7-cyclooctatetraene (COT) is a nonaromatic molecule and has a tub-like shape in its ground electronic state. In the study of M(COT) (M = Sc, Y, and La) complexes, we discovered that metal coordination converted the tub-shaped COT into a planar and aromatic molecule.44 This conversion was induced by a two-electron

Metal coordination induces conformational changes of aromatic and nonaromatic hydrocarbons. Biphenyl (C12H10) has a planar structure in the crystalline state and is twisted in the gas phase. The dihedral angle of the two phenyl rings is determined by the competition between π-conjugation and steric repulsion; the former favors a coplanar configuration, while the latter favors a nonplanar form. Transition-metal-biphenyl complexes have been studied for many years in condensed-phase organometallic chemistry because they are good models for conducting organometallic polymers. Either ring of biphenyl was suggested to have sixfold binding, and metal coordination with two phenyl rings formed dinuclear complexes. In these dinuclear complexes, two metal atoms generally resided on opposite sides of the biphenyl plane to minimize steric repulsions, although structures with two metal atoms on the same side have been identified as well. On the other hand, metal-biphenyl complexes may form different structures in the gas phase because of the twisted configuration of the ligand and lack of stabilizing solvent and counterion molecules. We detected a clamshell structure of Sc(biphenyl) formed by metal-induced phenyl ring rotation and bending.43 Figure 6a presents a


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transfer from M to COT upon the formation of M(COT), as indicated by the molecular orbital interaction diagram in Figure 1b. Assuming that M(COT) belongs to the C8v point group, five occupied π orbitals of COTare in the energy order of π1(a1) < π2, π3(e1) < π4, π5(e2); the five metal d orbitals are split into dz2(a1), dxy,yz(e1) and dxy,x2-y2(e2). Interactions between the COT π and metal d orbitals yield three bonding (1a1, 1e1, and 1e2) and antibonding (2a1, 2e1, and 2e2) orbitals, respectively. The COT π orbitals are stabilized, whereas the M d orbitals are destabilized by these interactions. The interaction between the COT π1 and M dz2 orbitals is weak because of the large energy difference and spatial mismatch and forms the highest occupied molecular orbital (HOMO) 2a1. The formation of M(COT) results in a twoelectron transfer from M to the COT half-filled e2 orbital, making COT a dianion. Thus, the metal-coordinated COT satisfies the requirement of 4n þ 2 electrons and becomes aromatic. The aromatic indices of the metal-coordinated COT molecule calculated using the structure-based harmonic oscillator model of aromaticity45 are in the range of 0.85-0.90, which are about the same as the aromatic index of the COT2dianion (0.86) and only slightly smaller than that of benzene (1.0). Because group 3 M atoms have three outmost valence electrons, the remaining electron is filled in the metal-based 2a1 orbital, leading to a doublet (2A1) ground state for the C8v complexes. Ionization removes the electron from the HOMO and yields the 1A1 ground state for the cations.

experiment is less sensitive to high-frequency ligand-based vibrations (e.g., X-H and C-X (X = C, N, O) stretches) due to poor FC factors. Measurements of the high-frequency modes are also valuable because they may provide additional information about possible changes in the ligand geometry induced by metal coordination and can be carried out using IR-UV resonant two-photon ionization and ZEKE spectroscopy.34,46 Although electronic excited states of metal complexes are normally short-lived, low-lying vibrational states are generally long-lived, making IR excitation ideal for performing two-photon measurements on these systems. Second, mass selection in the single-photon ZEKE experiment is achieved by correlating the ZEKE signal with the threshold photoionization spectrum measured in the mass-analyzed mode of operation and by velocity slippage present in seeded molecular beams.47 The correlation can be achieved because ionization thresholds and velocities of small metal complexes are well-separated. With increasing size of metal systems, differences in ionization thresholds and travel velocities become smaller; assigning ZEKE signals to a particular molecular carrier becomes less straightforward. This problem can be solved in principle by detecting mass-selected ions (rather than electrons) from the pulsed-field ionization of optically excited Rydberg states, even though the separation of the ions produced by the delayed field ionization from the ions produced by direct photoionization is more challenging. Third, some metal systems are expected to have higher IEs than the photon energies provided by currently available frequency-doubled dye lasers; vacuum UV lasers or synchrotron radiation may be required for these higher IE systems. Vacuum UV ZEKE spectroscopy has been applied to other molecular systems,48,49 but its applications to metal complexes or clusters remain to be explored.

Molecular beam ZEKE spectroscopy is a powerful technique for determining the electronic states, metal binding sites and modes, and molecular conformations of the metalaromatic complexes.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected].

Biographies Dong-Sheng Yang received his B.Sc. degree from Nanchang University, China, and a Ph.D. degree from the University of Western Ontario, Canada. He is a professor of chemistry at the University of Kentucky in Lexington. His current research interests include laser-assisted synthesis, mass spectrometry, molecular spectroscopy, and computation of molecular clusters and ions. For further information, see http://www.as.uky.edu/chemistry.

Molecular beam ZEKE spectroscopy is a powerful technique for determining the electronic states, metal binding sites and modes, and molecular conformations of the metalaromatic complexes. Additional measurements on the metal complexes of polycyclic aromatic hydrocarbons, polyphenyls, and nucleobases would be interesting. Benzene rings in the polycyclic aromatic hydrocarbons are not equivalent, and preferential metal binding over a specific ring is possible. Structures of the polyphenyls are flexible; a conformational change may be induced by metal coordination, as shown in Sc(biphenyl). Although heteroatomic sites in the nucleobases are preferred by Li and Al, aromatic rings in these molecules may be favored by transition metals. Technically, several considerations are useful to expand the capability of the ZEKE technique. First, single-photon UV ZEKE spectroscopy probes the metal-ligand and low-frequency ligand-based vibrations of metal complexes. On the other hand, the single-photon


ACKNOWLEDGMENT Financial support from the National Science Foundation, donors of the Petroleum Research Fund of the American Chemical Society, and the Kentucky Science and Engineering Foundation is gratefully acknowledged.

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