The Journal of Physical Chemistry A - ACS Publications - American

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Cation−π and CH−π Interactions in the Coordination and Solvation of Cu+(acetylene)n Complexes Antonio D. Brathwaite,† Timothy B. Ward,‡ Richard S. Walters,‡ and Michael A. Duncan*,‡ †

College of Science and Mathematics, University of the Virgin Islands, St. Thomas, United States Virgin Islands 00802 Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States

‡

S Supporting Information *

ABSTRACT: Copper-acetylene cation complexes of the form Cu(C2H2)n+ (n = 1−8) are produced by laser ablation in a supersonic expansion of acetylene/argon. The ions are mass selected and studied via infrared laser photodissociation spectroscopy in the C−H stretching region (3000−3500 cm−1). The structure and bonding of these complexes are investigated through the number of infrared active bands, their relative intensities and their frequency positions. Density functional theory calculations are carried out in support of the experimental data. The combined data show that cation−π complexes are formed for the n = 1−3 species, resulting in red-shifted C−H stretches on the acetylene ligands. The coordination of the copper cation is completed with three acetylene ligands, forming a “propeller” structure with D3 symmetry. Surprisingly, complexes with even greater numbers of acetylenes than this (4−6) have distinctive infrared band patterns quite different from those of the smaller complexes. Experiment combined with theory establishes that there is a fascinating pattern of second-sphere solvation involving the binding of acetylenes in bifurcated CH−π binding sites at the apex of two core ligands. This binding motif leads to three equivalent sites for second-sphere ligands, which when filled form a highly symmetrical Cu+(C2H2)6 complex. Solvent binding in this complex induces a structural change to planarity in the core, producing an appealing “core−shell” structure with D3h symmetry.

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INTRODUCTION Reactions involving transition metals and unsaturated hydrocarbons play an integral role in many industrial catalytic processes.1−4 Additionally, transition metal complexes with acetylene, ethylene or benzene provide prototypical models for π bonding in organometallic chemistry.5−9 Insight into the structure, stability, and reactivity of transition metal complexes can be obtained from isolated systems in the gas phase, most often in the form of ions studied with mass spectrometry. The spectroscopy of isolated complexes can reveal their intrinsic structure and bonding properties in the absence of solvation or surface binding effects. Such information has the potential to elucidate reaction mechanisms and may eventually guide the design of more efficient catalysts. In this study we employ mass spectrometry, infrared photodissociation spectroscopy and density functional theory (DFT) to investigate the structures and coordination of Cu(C2H2)n+ complexes. Traditionally, the vibrational spectroscopy of organometallic complexes has been conducted in the condensed phase.5−8 However, aggregation, solvation or lattice interactions in such environments can perturb the natural vibrations of these molecules. Gas phase experiments provide isolated molecular ions, free from the perturbations of solvents, surfaces, counterions, or matrices.9 Vibrational spectroscopy in the gas phase, paired with computational predictions of structures and spectra, provides direct probes of structure and bonding. © XXXX American Chemical Society

Information on the reactivity, binding energies, and fragmentation pathways of metal−acetylene ions has been provided previously by mass spectrometry.9−18 Several groups have also investigated the electronic structures and geometries of these complexes with theory.19−27 Electronic spectroscopy has been used to provide details on the bonding configurations and energetics in these systems.25−33 However, these studies were limited to complexes with only one ligand. Infrared spectroscopy is well-suited for studying these complexes, as it can reveal the structure and coordination numbers of multiligand complexes and help to characterize the bonding interactions of inner-sphere versus solvent molecules. Additionally, in some case the products from insertion chemistry can be identified. Our research group has previously studied the infrared spectroscopy of metal-acetylene cation complexes.33−35 These studies have suggested the occurrence of intracluster cyclization reactions33 and provided the first spectroscopic evidence for metallacycle formation in these systems.34 The bonding in such complexes is often discussed using the Dewar−Chatt− Duncanson1,2,5−9 complexation model. According to this model, acetylene ligands donate π electrons from their HOMO into the σ-type, d orbitals on the metal. The metal Received: April 7, 2015 Revised: May 7, 2015

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DOI: 10.1021/acs.jpca.5b03360 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A also donates electrons from its π-symmetry d orbitals into π* antibonding orbitals of C2H2. σ donation and π back-bonding both weaken the acetylene bond and lower its C−C vibrational frequency. This same effect also weakens the C−H bonds, and causes their vibrations to shift to lower frequencies. In our previous study of first-row transition metal-acetylene cations (V, Fe, Co, Ni), a systematic increase in the C−H stretching frequency was observed for these species on going from vanadium to nickel.34 In the present work, we investigate the bonding interactions between acetylene and a d10 cation, where charge donation and transfer are likely to be less efficient. Infrared photodissociation spectra of Cu(C2H2)n+ complexes are measured in the C−H stretching region and compared to the predictions of computational chemistry to explore structures, coordination, and solvation in this unusual system.

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EXPERIMENTAL SECTION Cu(C2H2)n+ ions are produced in a pulsed nozzle laser vaporization source using the third harmonic of a Nd:YAG laser (355 nm; Spectra-Physics INDI). The laser is focused onto a rotating and translating 1/4 in. diameter copper rod mounted with a pulsed nozzle (General Valve Series 9) in the so-called cutaway configuration.36 Ions are produced using a mixture of 10% acetylene in argon. The expansion is skimmed into a differentially pumped chamber where positive ions are extracted into a homemade reflectron time-of-flight mass spectrometer.37 Ions are mass selected by their flight time using pulsed deflection plates located at the end of the first flight tube. Selected ions are excited in the turning region of the reflectron with the tunable output of an infrared OPO laser system (LaserVision) pumped by a Nd:YAG laser (Spectra Physics Pro 230). This OPO provides tunable infrared light in the region of 2000−4000 cm−1 with a line width of about 1 cm−1. Infrared spectra are recorded by monitoring the appearance of one or more fragment ions as a function of the laser wavelength. DFT calculations were carried out to determine the structure and bonding of these complexes using the B3LYP functional38,39 as implemented in the Gaussian 09 computational package.40 The Wachters+f basis set41 was used for copper and the 6-311++G** basis set was employed for carbon, hydrogen and argon. Relative energies are presented throughout the paper without zero point corrections. The computed C−H stretching frequencies were scaled by a factor of 0.96, as discussed previously,35 and given a 5 cm−1 FWHM Lorentzian line shape for comparison to the experimental spectra.

Figure 1. Mass spectrum of Cu(C2H2)n+ complexes.

complexes should be weakly bound, and more efficient ligand elimination is expected for these systems upon IR absorption. The fragmentation patterns of these complexes following excitation in the C−H stretching region (3000−3500 cm−1) are presented in Figure 2. These data are obtained by subtracting a

Figure 2. Infrared photodissociation mass spectra of Cu(C2H2)n+ (n = 4−6) complexes.

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RESULTS AND DISCUSSION Figure 1 shows a mass spectrum of the Cu(C2H2)n+ ions formed in the experiment. All peaks are doubled because of the 63 and 65 amu isotopes of copper. The major peaks correspond to Cu(C2H2)n+ ions, whereas minor features correspond to copper oxide-acetylene ions. The most intense peak is that for Cu(C2H2)3+, suggesting that this complex is formed somewhat preferentially and may have enhanced stability. Cu(C2H2)n+ complexes up to n = 9 are produced. It is very unlikely that the ligands in larger complexes could all be coordinated directly to the central copper cation. Instead, these larger ions are likely to be composed of a strongly bound metal−ligand core, with additional “external” C2H2’s attached via electrostatic and/or van der Waals interactions. The external ligands in larger

mass spectrum obtained with the photodissociation laser “off” from one with it “on.″ The negative peaks indicate depletion of the parent ion, whereas the positive peaks represent the fragment ions produced. Infrared light in the C−H stretching region does not cause photodissociation of the smaller Cu(C2H2)n+ complexes (n = 1−3). This is consistent with the relatively high acetylene binding energies calculated for these species (see below), and with the measured limit on the binding energy of Cu(C2H2)+ (D0 > 30.4 kcal/mol).18 Beginning with n = 4, efficient photodissociation is observed. The larger complexes undergo sequential ligand elimination, terminating at n = 3. This suggests that the Cu(C2H2)3+ species is relatively more stable than larger complexes. Fragmentation behavior like this, in which one metal−ligand complex is B


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The Journal of Physical Chemistry A

shows the relative energies obtained for these complexes, and Table 2 shows the binding energies for the elimination of either an acetylene or argon from these complexes. The acetyleneacetylene binding energies in the larger clusters can be compared to that of the acetylene dimer. This complex is known experimentally to have a T-shaped structure with a CH−π hydrogen bond energy computed at high levels of theory to be about 400 cm−1 (1.14 kcal/mol).44,45 The DFT level employed here finds this bond energy to be 0.85 kcal/mol, and therefore the external ligand binding in the complexes for n > 3 are suggested to be greater than the value for the neutral dimer. This makes sense because of the added attraction of the charge here. In all cases, the triplet species are significantly higher in energy than the corresponding singlets. Additionally, as discussed below and shown in the Supporting Information, the infrared patterns for the triplets do not agree with our experiments. Similar results were obtained previously for the Cu(CO)n+ system, where all stable structures were found to be singlets.43 We therefore omit triplet states from further consideration here. Vibrational spectra for these complexes were obtained by monitoring the fragmentation yield as a function of the infrared laser wavelength. As noted above, the energy of an infrared photon in the C−H stretching region is too small to dissociate the smaller complexes (n = 1−3). For these ions, we employ rare-gas tagging to enhance the dissociation yields.46−52 Cu(C2H2)n+Ar complexes fragment by elimination of the argon atom following photoexcitation of the C−H stretch. Although tagging usually has a small effect on the vibrations of the core ion,34,35 we investigate this with computations on both the tagged and untagged complexes. The spectra for the larger clusters were measured by the elimination of acetylene ligands. This fragmentation channel is expected to be efficient on the basis of the computed acetylene binding energies and confirmed experimentally. The measured C−H stretching vibrations for each cluster size are compared to the predictions of theory in Table 3. Figure 3 shows the infrared spectrum measured for Cu(C2H2)+Ar2. The spectra predicted by theory for the three lowest energy singlet-state Cu(C2H2)+Ar2 isomers, as well as that for the neat Cu(C2H2)+ ion, are also shown. No fragmentation was observed for neat Cu(C2H2)+ or the Cu(C2H2)+Ar complex. This is not surprising because the computed binding energies (Table 2) are relatively high for

resistant to further dissociation, has been observed previously and is often indicative of the ion coordination.42,43 Cu(C2H2)3+ is therefore likely to be the fully coordinated ion, and larger complexes should have external ligands. Computed complex structures, ligand binding energetics, and vibrational spectra test these ideas more directly. To investigate structures and energetics for these complexes, DFT/B3LYP calculations were performed on the singlet and triplet spin states for various Cu(C2H2)n+ and Cu(C2H2)n+Arm ions. The details of these calculations are presented in the Supporting Information. The resulting structures for the n = 1− 3 complexes have copper ions interacting with acetylene ligands via a η2 cation−π configuration, whereas larger complexes have external ligands. The n = 4 complex has a structure with one external ligand (3 + 1 structure); a four-coordinate (4C) tetrahedral isomer is also computed to lie at low energy, but there is no experimental evidence for this structure (see below). Smaller clusters are found to have no stable acetylene-based isomers, but do have isomers involving the attachment sites of argon. When the metal ion is exposed (e.g., for the n=1 complex), argon prefers to bind here, but when acetylene ligands block the metal site, argon binds on the CH or CC positions on acetylene ligands. Various binding configurations for argon were explored, but it should be noted that DFT/ B3LYP theory has well-known difficulties with weak interactions such as those involving argon. The structures and energetics for argon found here are therefore not expected to be highly accurate. The detailed structures for each complex are presented in the Supporting Information; qualitative structures are presented below as insets with predicted spectra. Table 1 Table 1. Structures, Electronic Ground States and Relative Energies (Not Zero Point Corrected) for Cu(C2H2)n+ Complexes Computed Using DFT/B3LYP complex Cu(C2H2)+ Cu(C2H2)2+ Cu(C2H2)3+ Cu(C2H2)4+ (3C+1) Cu(C2H2)4+ (4C)

spin state

symmetry

relative energy (kcal/mol)

singlet triplet singlet triplet singlet triplet singlet singlet

C2v Cs D2d C2v D3 C2v Cs Th

0.0 +77.0 0.0 +76.3 0.0 +74.9 0.0 +2.46

Table 2. . Computed Binding Energies (Not Zero Point Corrected) in kcal/mol for the “Last” Ligand in Cu(C2H2)n+ and Cu(C2H2)nAr+ Complexes complex

E[Cu(C2H2)n‑1+−C2H2]

(C2H2)2 Cu(C2H2)+ Cu(C2H2)Ar+ Cu(C2H2)Ar2+ Cu(C2H2)2+ Cu(C2H2)2Ar+ Cu(C2H2)3+ Cu(C2H2)3Ar+ Cu(C2H2)4+ (3C+1) Cu(C2H2)4+ (4C) Cu(C2H2)5+ Cu(C2H2)6+ Cu(C2H2)7+

0.85 44.22 (Ar on Cu+) (Ar on CH) 40.67 (Ar on CH) 12.15 (Ar on CH) 4.57 2.11 4.31 4.56 2.15

E[Cu(C2H2)n+−Ar]

E[Cu(C2H2)Ar+−Ar]

10.92 2.14 0.40 0.30

C


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Table 3. C−H Stretching Vibrational Frequencies (in cm−1) Computed (Scaled by 0.96) for Singlet States of Cu(C2H2)n+, Cu(C2H2)nAr+, and Cu(C2H2)+Ar2 Complexes, along with Those Measured in the Experimenta species

experiment

Cu(C2H2)+

−

Cu(C2H2)+Ar

−

Cu(C2H2)+Ar2

3194/3204 3275 −

Cu(C2H2)2+ Cu(C2H2)2Ar+ Cu(C2H2)3+ Cu(C2H2)3+Ar Cu(C2H2)4+ (3C + 1)

3199 3278 3226 3304 3182 3189 3208 3228 3263 3286 3308

Cu(C2H2)5+

3169 3190 3263 3290

Cu(C2H2)6+

3172 3258

Cu(C2H2)7+ 3178 3260

theory 3177(252) 3270(16) 3184(234) 3278(21) 3191(207) 3281(27) 3194(434) 3285(60) 3188(291), 3193(216) 3282(53), 3286(15) 3222(124), 3224(360) 3314(85) 3220(194), 3223(152), 3224(180) 3312(50), 3314(43) 3182(75) 3190(546) − 3226(170) 3260(128) 3294(55) 3297(16) 3321(21) 3168(290) 3190(779) 3261(243), 3267(67) 3299(58) 3302(12) 3169(1537) 3259(250), 3261(376) 3154(25) 3167(815), 3169(713) 3228(274) 3257(107), 3259(103), 3261(70) 3263(191), 3272(11), 3273(106)

assignment asym str sym str asym str sym str asym str sym str ip/oop asym str oop sym str ip/oop asym str ip/oop sym str ip/oop asym str oop sym str ip/oop asym str ip/oop sym str oop donor asym str ip donor asym str core ligand asym str ext ligand asym str oop donor sym str core ligand sym str double-donor asym str donor ip asym str ip/oop ext. ligand asym str ip/oop donor sym str double-donor asym str ext ligand asym str double-donor asym str ext ligand asym str ext ligand asym str

Calculated IR intensities (km/mol) are shown in parentheses; only bands with intensities >10 km/mol are listed. “ip” indicates in-phase combination of two ligands; “oop” indicates out-of-phase combination of two ligands.

a

have one intense band between 3150 and 3200 cm−1 and a weaker band around 3275 cm−1. It is therefore possible that the two bands at 3194 and 3204 cm−1 are from two argon isomers, most likely those whose energies are close. Only a single weaker band is detected at higher frequency where the symmetric C− H stretch is expected. It is conceivable that the symmetric stretches for two isomers overlap each other here. Another possibility is that a Fermi resonance splits the asymmetric C−H stretch of a single isomer into two bands. Acetylene itself has such a Fermi resonance involving the overlap of the C−H stretch with the ν2 + ν4 + ν5 combination of the C−C stretch with the trans and cis bending modes (1974 + 612 + 730 cm−1), which leads to a doublet with spacing of 13 cm−1.53 The doublet spacing here is 10 cm−1. If we compare the computed frequencies for these modes in the Cu(C2H2)+Ar2 complex, the appropriate sum is about 100 cm−1 higher than the C−H stretch fundamental, which would not suggest that a Fermi resonance should occur. However, the same sum of computed frequencies also does not predict the known degeneracy in acetylene itself. Apparently, there is considerable uncertainty in either the computations for the low frequency modes or their appropriate scaling. If the asymmetric stretch

these ions (44.2 and 10.9 kcal/mol respectively). However, efficient elimination of argon was observed from the Cu(C2H2)+Ar2 complex. This is expected because the energy of an IR photon in the C−H stretching region (∼9 kcal/mol) is much greater than the calculated binding energy of argon in this complex (2.14 kcal/mol). The Cu(C2H2)+Ar2 spectrum consists of two intense peaks at 3194 and 3204 cm−1 and a weaker feature at 3275 cm−1. The observation of three bands is at first puzzling because only two are expected for an acetylene ligand: a more intense band corresponding to the asymmetric C−H stretch and a weaker band corresponding to the symmetric C−H stretch. The symmetric stretch is of course not IR active in acetylene itself, but it becomes slightly active here because the hydrogen atoms bend away from the metal in the complex structure. Theory may explain the extra peak in the asymmetric stretch position. As shown in the figure, three isomers are identified with singlet spin states corresponding to different attachment sites for the argon atoms. One has both argons attached to the metal ion, a second has one on a CH group and the third has both argons attached to CH hydrogens. The first two of these are predicted to have almost the same energy. All of the calculated spectra D


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Figure 4. Infrared spectrum of Cu(C2H2)2+Ar compared to the spectra predicted by DFT. The dashed vertical lines indicate the positions of the asymmetric and symmetric C−H stretches at 3289 and 3374 cm−1, respectively.

Figure 3. Infrared spectrum of Cu(C2H2)+Ar2 compared to the spectra predicted by DFT. The dashed vertical lines indicate the positions of the asymmetric and symmetric C−H stretches at 3289 and 3374 cm−1, respectively.

is small in this complex, suggesting that efficient dissociation should occur upon absorption of a single photon near 3200 cm−1. Indeed, the argon-tagged complex does dissociate efficiently, yielding a spectrum with an intense band at 3199 cm−1 and a much weaker feature at 3278 cm−1. The positions of the isolated acetylene vibrations are shown again, revealing a red shift in both vibrations (symmetric, −90; asymmetric, −96 cm−1) like those seen for the monoacetylene complex. The structure determined by theory has the two acetylene ligands opposite each other in a D2d arrangement. Both the in-phase and out-of-phase combinations of asymmetric stretches on the two ligands are IR active and occur at about the same frequency; only the out-of-phase combination of the symmetric stretch is IR active. Spectra predicted for the tagged and neat complexes are almost identical, confirming that tagging has essentially no effect on the spectra for the n = 2 species, and these match the experiment almost perfectly. The n = 2 species is therefore assigned as a structure with two π-bonded acetylene ligands coordinated to a copper ion with D2d symmetry. The spectrum of Cu(C2H2)3+Ar is presented in Figure 5, where it is compared to those predicted by theory for the tagged and neat ions. The calculated structure has three acetylene ligands π-bonded to the copper ion in a propeller-like structure with D3 symmetry. As expected from the computed binding energy, these ions undergo efficient fragmentation via argon elimination. Similar to the n = 2 species, two red-shifted bands are observed and predicted by theory. These vibrations correspond to several nearly degenerate in-phase and out-ofphase asymmetric and symmetric stretches, respectively, on the three ligands. All combinations are IR active except the in-phase symmetric stretch. The shifts compared to the vibrations of free-acetylene are less here (−63 and −70 cm−1) than for the smaller complexes. Smaller shifts occur for the multiple ligand complexes because the charge transfer is delocalized over more ligands. Again, tagging has a negligible effect on the spectrum and there is excellent agreement between theory and

doublet is indeed from a Fermi resonance for a single isomer, then it makes sense that we see only a single feature for the higher frequency symmetric stretch. Unfortunately, it is not possible with the present spectra to distinguish between these two interpretations of this spectrum. The bands for the C−H stretches of Cu(C2H2)+Ar2 are shifted to lower frequencies than these vibrations in the isolated acetylene molecule (symmetric stretch, 3374 cm−1; asymmetric stretch, 3289 cm−1),53 whose positions are indicated as dashed vertical lines. The symmetric stretch frequency for acetylene is IR inactive, but measured from Raman spectroscopy. This same kind of shift was documented in our earlier work on first-row transition metal ion−acetylene complexes.33−35 The single symmetric stretch is 99 cm−1 to the red from this frequency in acetylene, while the two asymmetric stretch bands are redshifted by 85 and 95 cm−1. The recommended frequency for the asymmetric stretch is 3289 cm−1, but the Fermi doublet measured experimentally lies at 3295 and 3282 cm−1.53 The slight activation of the IR-forbidden symmetric stretch and the red shift in both C−H stretches are expected for metal ion binding to the π system of acetylene. In the cation−π binding configuration, the metal ion withdraws bonding electron density from the HOMO of acetylene, which extends onto the CH bonds. The reduced bonding density results in weaker bonds and lower vibrational frequencies. The acetylene vibrational shifts here are considerably smaller than they were for the other transition metal ions studied previously (Ni+, Co+, Fe+, V+).34 This makes sense because of the electronic structure of Cu+. Because of its closed-shell configuration, both the acceptance of σ electron density and back-donation into the π* orbitals of acetylene are less effective than for other open-shell metal ions. Figure 4 shows the experimental spectrum of Cu(C2H2)2+Ar along with those predicted by theory for Cu(C2H2)2+Ar and Cu(C2H2)2+. As indicated in Table 2, the argon binding energy E


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Figure 5. Infrared spectrum of Cu(C2H2)3+Ar compared to the spectra predicted by DFT. The dashed vertical lines indicate the positions of the asymmetric and symmetric C−H stretches at 3289 and 3374 cm−1, respectively.

Figure 6. Infrared spectrum of Cu(C2H2)4+ compared to the spectra predicted by DFT for the 3C + 1 (middle/red) and 4C tetrahedral (bottom/blue) isomers, respectively.

experiment. This three-ligand complex represents the fully coordinated copper cation. The n = 4 species is particularly interesting. On the basis of the photofragmentation data, this should be the first complex having an external ligand. Theory finds that the most stable structure is a three coordinate species with one external ligand, indicated as “3C + 1”, although a four-coordinate (4C) tetrahedral species lies only slightly higher in energy. According to the usual behavior of such complexes, if we add a ligand to the stable n = 3 core ion, some of the IR resonances for that complex should be preserved and we should see new bands for the external ligand. The external ligand vibrations may be weaker than those of the core ion. However, if there is instead a tetrahedral four-coordinate species, the high symmetry of its structure should produce a much simpler band pattern. Surprisingly, as shown in Figure 6, the n = 4 spectrum is not at all simple and it also does not look like that of the n = 3 complex. There are seven bands (3182, 3189, 3208, 3228, 3263, 3286, 3308 cm−1), and the most intense of these at 3189 cm−1 is further to the red than any of the bands for the n = 3 complex. Remarkably, theory is able to account for this complex pattern. Figure 6 shows the comparison of the measured spectrum to that predicted by theory for the 3C + 1 and 4C structures. As shown, the spectrum for a particular 3C + 1 structure matches the experimental spectrum, but that for the 4C tetrahedral complex does not. This 3C + 1 structure has a single second-sphere molecule bound in a bifurcated CH−π hydrogen bond site interacting with two inner-sphere ligands. As noted above, a CH−π hydrogen bonding motif is wellknown for the acetylene dimer.44,45 Theory predicts eight IR active bands in the C−H stretch region for this structure. These frequencies and their intensities are presented in Table 3. Unfortunately, some of these features overlap when the spectrum is plotted at the resolution of the experiment and one band is too weak to be observed. The in-phase and out-ofphase symmetric C−H stretches at 3294 and 3296 cm−1 appear as one band at 3294 cm−1, and the in-phase and out-of-phase

asymmetric C−H stretches at 3182 and 3190 cm−1 appear as a lone feature at 3190 cm−1. Theory does not reproduce the band at 3208 cm−1; it is likely a combination band originating from the coupling of low-frequency C−H bending modes. Additionally, the intensity of the symmetric C−H stretch of the external acetylene at 3361 cm−1 is apparently too low to be measured. Therefore, only five bands are predicted at the imposed resolution in the predicted spectrum. In spite of all of this, the band positions and relative intensities of the theoretical spectrum reproduce the experiment amazingly well. This confirms that the three-ligand complex is the fully coordinated ion and that the four-ligand complex has one external ligand. Even though the tetrahedral 4C complex is also predicted to be relatively stable, there is no evidence for its formation. The identities of the bands producing this complicated spectrum are fascinating. The features at 3228 and 3308 cm−1 correspond to those for the n = 3 core ion at 3226 and 3304 cm−1. These confirm that this same core ion is still present. The remaining vibrations result in various ways from the presence of the external ligand. The strongest feature at 3189 cm−1 comes from the two core acetylenes donating CH groups into the π system of the external ligand. Two overlapping bands here are assigned to the in-phase symmetric and in-phase asymmetric C−H stretches of these donor ligands; the in-phase symmetric stretch has the greater IR intensity. An out-of-phase symmetric stretch of these same two donor ligands produces the 3286 cm−1 band. Finally, the 3263 cm−1 band is the asymmetric stretch of the external ligand. Unlike argon, the external acetylene ligand induces a significant perturbation on the C−H vibrations of the core ion, but this applies mainly to the donor ligands. The CH−π hydrogen bond induces a strong red shift on their C−H stretches and they gain significant oscillator strength as they interact with the π system. The intensity of the vibration of the external ligand is also enhanced because of its involvement with the CH−π hydrogen bonds. The external ligand here is much more than a weakly interacting solvent F


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both external ligands. Except for the new kind of double-donor vibration that produces the 3169 cm−1 band, all of these vibrations correspond to similar ones for the n = 4 species. The bands seen for the n = 4 complex that are missing here are the 3228 and 3308 cm−1 features that were assigned to core ligands not interacting with external ligands. It makes sense that these vibrations are no longer present, because there are now no core ligands of this type; all core ligands interact with at least one external ligand. Figure 8 shows the spectrum measured for the Cu(C2H2)6+ complex. It is now simpler again than those for the n = 4 and n

molecule; it is the focal point for charge-transfer interactions. These shifts on the acetylene vibrations are greater than those seen in the neutral acetylene dimer. In its T-shaped structure with a CH−π hydrogen bond, the asymmetric stretch vibration of the acceptor acetylene occurs at about 3280 cm−1 (tunneling multiplet present), and that for the donor occurs at 3272 cm−1.44 The greater shifts here can be attributed to the polarization of acetylene by the charge of the copper and the concerted hydrogen bonding from the two ligands. Figure 7 shows the spectrum measured for the Cu(C2H2)5+ complex. Surprisingly, the pattern is somewhat simpler than

Figure 7. Infrared spectrum of Cu(C2H2)5+ compared to the spectrum predicted by DFT.

Figure 8. Infrared spectrum of Cu(C2H2)6+ compared to the spectrum predicted by DFT.

that of the n = 4 complex, with only four IR bands detected. The computed structure, also shown in the figure, has the same n = 3 core ion, but now with two external acetylene ligands. Both of these are located in the same kind of site, interacting with two core ligands each via bifurcated CH−π hydrogen bonds. This results in one core ligand that acts as a doubledonor, interacting with both external ligands, and the other two are single donors. As in the case of the n = 4 complex, the vibrations with the greatest IR intensities are those involving the π hydrogen bonds. The band observed and predicted at 3169 cm−1 is the asymmetric stretch of the unique doubledonor core ligand, which vibrates into and out of the π clouds on each of the two external ligands. As this vibration is the most highly “solvated”, it is shifted to the red further than any other. The 3190 cm−1 band is analogous to the same feature for the n = 4 complex, arising from both in-phase and out-of-phase combinations of two single-donor core−ligand asymmetric stretches into the π clouds of both external ligands. Likewise, the 3263 cm−1 band here corresponds to the same band for the n = 4 complex, but now corresponding to the in-phase and outof-phase combinations of the asymmetric stretches on the two external ligands. The 3290 cm−1 vibration is analogous to the 3286 cm−1 band seen for the n = 4 complex, again corresponding to both in-phase and out-of-phase combinations of two core-ligand symmetric stretches into the π clouds of

= 5 complexes, having essentially just two main bands at 3172 and 3258 cm−1. The structure predicted by theory has the same three-ligand core seen above, but now there is a third external ligand occupying the final binding site between the CH groups of core ligands, again with bifurcated π hydrogen bonds. Surprisingly, the addition of this last acetylene causes the threeligand core complex to become planar. The overall system now has the appealing D3h structure shown in the figure. Actually, the transition from the propeller structure for the n = 3 complex to the planar structure of the n = 6 occurs gradually. If we define a plane containing the copper ion and the center of mass of the coordinated acetylenes, the out-of-plane angle for the CH π-bond donor ligands changes from 28.8° (n = 3) to 11.9° (n = 4; two single-donors) to 2.7° (n =5; one doubledonor) to 0° (n = 6; three double-donors), Because of the high symmetry of the n = 6 complex, there are fewer kinds of vibrations. All three core ligands are now double-donors, and their in-phase and out-of-phase asymmetric stretches produce the vibration at 3172 cm−1, analogous to the 3169 cm−1 band which first appeared for the n = 5 complex. Likewise, the three external ligands are equivalent, and their in-phase and out-ofphase asymmetric stretches produce the 3258 cm−1 band, analogous to the 3263 cm−1 band seen for the n = 4 and 5 G


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The Journal of Physical Chemistry A complexes. The 3190 and 3290 cm−1 bands for the n = 4 and 5 complexes were attributed to partially solvated core ligands. This kind of ligand is no longer present, and the bands at these frequencies are therefore not seen. Although the experimental spectrum has a hint of weaker features that are not predicted, theory and experiment agree on the main spectral pattern for the n = 6 complex. Perhaps more compelling is the trend for the spectra of the n = 4−6 species. This is shown in Figure 9, where

this size like that shown in Figure 2 for the n = 3 species. Apparently the acetylene binding energy for this fascinating species is not significantly different from those of the larger complexes. The coordination behavior established here for these copperacetylene complexes is unusual. Metal−acetylene complexes are well-known in organometallic chemistry.1 The typical bonding between each ligand and the metal follows the η2 configuration seen here. However, stable multiligand acetylene complexes are rare, and there are not many examples in conventional chemistry to which the present complexes can be compared. We have previously studied the homoleptic carbonyl complexes of Cu+, in which we found a coordination number of four with a tetrahedral structure.43 This complex was noted to be isoelectronic and iso-structural with the corresponding neutral nickel tetracarbonyl system. We have also studied Ni(C2H2)n+ complexes, which form a similar four-coordinate system with a tetrahedral structure.35 The three-coordinate structure for the copper−acetylene cations is therefore somewhat surprising. Acetylene ligands are known to contribute either two or four electrons in bonding schemes for conventional complexes.1 Counting two electrons for each acetylene, a four-coordinate complex would be an 18-electron species, but this does not form. Such a complex should be possible sterically, since this did form for the analogous Ni+ complex, and indeed our computations find such a structure to be relatively stable. A two-coordinate complex would be an 18-electron species counting four electrons for each acetylene, but this also does not form. We assume that the 18-electron configuration for Cu+ is indeed favorable, as illustrated by its carbonyl complex. Therefore, the acetylene electron donation to copper in these complexes must be some admixture of two- and four-electron donation which does not lend itself readily to simple counting schemes. The specific solvation by second-sphere acetylene ligands demonstrated here is also unanticipated. Such effects would likely be obscured in condensed-phase environments, and may only be possible to detect in gas-phase clusters. In other cluster studies, there are many examples of the formation of completed coordination spheres around metal cations,35,42,43 but only a few examples of completed solvation spheres demonstrated spectroscopically. In the highly publicized studies of protonated water clusters, these systems exhibit a single sharp O−H stretching vibrational band for the H+(H2O)21 species believed to have a high-symmetry caged structure.54 Sulfate anions also exhibit a specific IR signature for a solvation shell containing 12 water molecules.55 Our study of protonation in mixed benzene−water clusters found a solvation sphere of three benzenes around the hydronium ion and four benzenes around the Zundel ion.56 In recent work, a much smaller solvation sphere of three water molecules has been reported around a guanidinium ion.57 All of these previous solvation systems involved water hydrogen bonding interactions; the present system is the first to our knowledge involving a ligand like acetylene where the interactions are generally expected to be much weaker. However, the acetylene−acetylene interactions are stronger here because of the unique binding sites in this system. The bifurcated CH−π interactions, enhanced by the charge-induced polarization of the copper cation, are computed to have binding energies close to the hydrogen bond energy of water (∼4−5 kcal/mol). These kind of binding sites may be found in acetylene complexes with other coordination patterns,

Figure 9. Comparison of the spectra for the Cu(C2H2)3−6+ complexes, demonstrating the evolution of different vibrational features as the solvation sphere develops.

the spectra of these three complexes are presented together with that of the n = 3 species. The correspondence of the different vibrational band types (core ligands, donor, double donor, external) for each complex and their evolution with cluster size are illustrated. We have also measured spectra for the Cu(C2 H 2 ) n + complexes for n = 7 and 8, which are shown in the Supporting Information. These larger complexes have the same two main bands seen for the n = 6 complex, but with greater line widths. According to theory (see Supporting Information), these larger clusters have additional acetylene ligands clustering to the outer ligands of the n = 6 complex. The binding of these ligands presumably involve a single CH−π interaction, and the computed binding energies are lower, as expected. The line broadening results from slight shifts in the C−H stretches of the n = 6 system induced by the third-sphere ligands. Apparently, no further specific structures are present in these larger clusters; they represent randomly solvated n = 6 structures. To investigate the relative stability of the n = 6 structure compared to these larger species, we measured fragmentation mass spectra of clusters for n = 7−9. These fragmentation spectra are shown in the Supporting Information. There is no evidence in these mass spectra for a sharp stability maximum for the n = 6 complex or a termination at H


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Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b03360.

but the near-planar 3-fold sites provide by copper cation seem to be particularly favorable for this kind of interaction. π hydrogen bonds are important throughout chemistry and biology, but there are few experiments probing these interactions directly. The present system involving acetylene CH−π interactions can perhaps best be compared to our previous study of protonated water-benzene clusters, which exhibited a variety of OH−π binding configurations with distinctive IR bands.56 In that system, the hydrogen bond energies were computed to be in the range of 6−10 kcal/mol in different clusters at roughly the same level of theory used here, and the vibrational shifts of the O−H stretches were 300−700 cm−1 to the red. Consistent with the expectations for the weaker CH−π interactions, the computed bond energies here are 4−5 kcal/mol and the vibrational shifts of the C−H stretches are only about 100 cm−1. The present interactions are likely enhanced by the polarization from the metal cation, suggesting that nonmetal acetylene systems would have weaker bonding and smaller shifts. The energetics resulting from the DFT calculations used here could of course be improved with more appropriate levels of theory. However, it is noteworthy that these scaled harmonic calculations reproduce the experimental hydrogen bonding frequencies quite nicely in both the water−benzene and copper−acetylene systems. Cluster-ion spectroscopy has the potential to elucidate π hydrogen-bonding interactions in many systems, revealing the subtle details of these noncovalent interactions.

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Corresponding Author

*(M.A.D.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge generous support for this work from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical, Geological, and Biosciences (Grant No. DE-FG02-96ER14658).

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REFERENCES

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CONCLUSION Copper−acetylene cations of the form Cu(C2H2)n+ (n = 1−8), and their argon-tagged analogues, Cu(C2H2)nAr+, were produced and studied with mass-selected infrared photodissociation spectroscopy in the C−H stretching region. The structures and coordination numbers of these complexes are determined by examining the distinctive patterns provided by their infrared spectra, and comparing them to the predictions of theory. All Cu(C2H2)n+ complexes were found to have d10 singlet ground state configurations. All of the small complexes studied have IR-active bands that are shifted to lower frequencies than the C−H vibrations in acetylene. The red shifts observed in this study are much smaller than those observed previously for other transition metal ion−acetylene systems. This is likely due to the inability of the filled d10 orbitals in copper to accept or donate electron density efficiently. The Cu(C2H2)3+ complex with D3 symmetry is the fully coordinated species. The near-planar trigonal structure of this core ion produces highly favorable sites where additional acetylene ligands can bind; three additional molecules complete the first solvation shell. Specific vibrational signatures identify each of the ligand and solvent types as these solvation structures develop. The copper-acetylene system provides fascinating examples of cation−π and CH−π hydrogen bonds like those that are important in many situations throughout chemistry and biology.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

* Supporting Information S

Full details of the DFT computations done in support of the spectroscopy presented here, including the structures, energetics, and vibrational frequencies for each of the complexes considered, and the predicted spectra for singlet versus triplet electronic states and the full citation for ref 40. The Supporting I


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