Infrared Photodissociation Spectroscopy of Mass-Selected

The dicobalt octacarbonyl Co2(CO)8 and the tetracobalt dodecacarbonyl ... (27, 28) The electronic and geometrical structures as well as the metal–meta...
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Infrared Photodissociation Spectroscopy of Mass-Selected Homoleptic Cobalt Carbonyl Cluster Cations in the Gas Phase Jieming Cui, Xiaojie Zhou, Guanjun Wang, Chaoxian Chi, Zhen Hua Li,* and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Infrared spectra of mass-selected homoleptic cobalt carbonyl cluster cations including dinuclear Co2(CO)8+ and Co2(CO)9+, trinuclear Co3(CO)10+ and Co3(CO)11+, as well as tetranuclear Co4(CO)12+ are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures of these complexes are determined by comparison of the experimental spectra with those calculated by density functional theory. The Co2(CO)8+ cation is characterized to have a Co−Co bonded structure with Cs symmetry involving a bridging CO ligand. The Co2(CO)9+ cation is determined to be a mixture of the CO-tagged Co2(CO)8+−CO complex and the Co(CO)5+− Co(CO)4 ion−molecular complex. The Co3(CO)10+ cation is the coordinationsaturated trinuclear cluster, which is characterized to have a triangle Co3 core with C2 symmetry involving two edge-bridging and eight terminal CO ligands. The Co3(CO)11+ cation is a weakly bound complex involving a Co3(CO)10+ core ion. The Co4(CO)12+ cluster cation is deduced to have a tetrahedral Co4+ core structure with three edge-bridging and nine terminal carbonyls.

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INTRODUCTION Homoleptic transition-metal carbonyls have been studied extensively because of their role in organometallic chemistry and catalysis.1−4 Cobalt carbonyl compounds play important roles in many chemical processes such as industrial hydroformylation5 and Fischer−Tropsch synthesis.6 Mononuclear cobalt carbonyl neutrals, cations, and anions have been produced in the gas phase as well as in solid noble gas matrixes, which were studied with various spectroscopic methods.4,7−9 Sequential bond dissociation energies of Co(CO)n+ and Co(CO)n− were determined by collision-induced dissociation in the gas phase.10,11 Their structures and bonding have also been investigated with theory.4,12 Besides the mononuclear carbonyls, di- and multinuclear cobalt carbonyl clusters have also received considerable attention. The dicobalt octacarbonyl Co2(CO)8 and the tetracobalt dodecacarbonyl Co4(CO)12 are well-known stable homoleptic cobalt carbonyls. Their vibrational spectra, structures, and bonding are wellstudied.13−26 Unsaturated dinuclear cobalt carbonyls have been prepared either via photolysis of saturated dicobalt octacarbonyl or by the reaction of carbon monoxide with Co2 dimer and were identified spectroscopically.27,28 The electronic and geometrical structures as well as the metal−metal and metal− CO bonding of Co2(CO)n (n = 1,5−8) were theoretically examined.29,30 The chemical reactivity of cobalt cluster cations, neutrals, and anions toward CO in the gas phase was investigated in flow tube reactors.31−34 The maximum coordination number of CO molecules bound onto each cluster is determined. The adsorption of a single CO molecule © 2014 American Chemical Society

on cationic, neutral, and anionic cobalt clusters containing between 3 and 37 metal atoms was characterized by infrared multiple photon dissociation (IR-MPD) spectroscopy.35 The size and charge dependence of the C−O stretching frequency were reported. Ab initio calculations were also performed to investigate changes in the structural and magnetic properties of pristine cobalt clusters upon CO chemisorption.36 Although the skeletal structures of some saturated cobalt carbonyl clusters were proposed in accordance with the electron-counting rules,33,34 the electronic and geometric structures of gaseous cobalt carbonyl cluster cations are relatively unknown. In this paper, mass-selected homoleptic multinuclear cobalt carbonyl cluster cations are studied by infrared photodissociation spectroscopy. Infrared photodissociation spectroscopy in conjunction with quantum chemical calculations offers one of the most direct and generally applicable experimental approaches to structural investigation of mass-selected clusters in the gas phase.37−45 Recently, this technique was successfully employed in studying transitionmetal carbonyl clusters in the gas phase.46−67 We will show that the experimental spectra provide distinctive patterns, allowing determination of the geometries and electronic structures of the cobalt carbonyl cluster cation complexes by comparison with the predicted spectra from density functional theory (DFT) computations. Received: November 15, 2013 Revised: February 28, 2014 Published: March 27, 2014 2719

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EXPERIMENTAL AND COMPUTATIONAL METHODS The infrared photodissociation spectra of the homoleptic cobalt carbonyl cluster cations were measured using a collinear tandem time-of-flight mass spectrometer (TOFMS). The experimental apparatus has been described in detail elsewhere.61,68 The cluster cations were prepared in a Smalley-type laser vaporization supersonic cluster source using the fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate). The laser beam was focused onto a rotating cobalt metal target. Helium seeded with 4−6% CO at backing pressure of 0.4−0.6 MPa was used as the expansion gas. The ions after free expansion were skimmed and entered into the extraction stage of a Wiley−McLaren-type TOFMS in a differentially pumped chamber. The cluster ions were massselected by their flight time and decelerated by pulsed electronic fields at the end of the flight tube of the first stage TOFMS. The selected ions with low kinetic energy were irradiated by a tunable infrared laser beam in the extraction region of a second collinear TOFMS. The infrared laser is generated by a KTP/KTA/AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by a Continuum Powerlite 8000 Nd: YAG laser. The laser is tunable in the range of 800−5000 cm−1, having pulse energies of about 0.5−2.0 mJ in the 1600−2200 cm−1 range. The laser beam is loosely focused, and the wavenumber is calibrated using the CO absorptions. When the laser frequency is onresonance with an IR-active vibration of the cluster cation, it absorbs the photon(s) and subsequently undergoes fragmentation via losing one or more CO ligands. The resulting fragment ions and the undissociated parent cations were detected by the second TOFMS. Infrared spectra were obtained by monitoring the yield of one or more fragment ions as a function of the IR laser wavelength and normalizing to the parent ion signal. In general, the spectra were scanned in a step of 2 cm−1 and averaged over 250 laser shots at each step. First-principle DFT calculations were performed to determine the molecular structures and to support the assignment of vibrational frequencies of the observed cobalt carbonyl cations. Geometry optimization and harmonic vibrational frequency analysis were performed with the hybrid B3LYP as well as the meta-GGA M06-L methods in combination with the 631+G(d) basis set.69−71 These two methods have both been assigned to be able to provide reliable predictions on the structures and vibrational frequencies of transition-metalcontaining compounds.72−74 The stability of the wave functions was checked. Geometry optimizations were performed on various possible structures starting from the geometries of the corresponding known neutral clusters as well as those assumed from the observed spectral features. The zero-point energies were derived. For each structure, all possible spin states were considered. The integral grid used for all of the DFT calculation was a pruned (99,590) grid (the “ultrafine” grid as defined by Gaussian 09). All calculations were performed with the Gaussian 09 program.75 In the calculations, we found that the relative energies of different low-lying structural isomers and spin states of the cluster cation complexes are sensitive to the functional used. Although DFT methods have problems in predicting the relative energies, previous studies have found that the infrared spectrum predicted for a given structure and spin state and its resulting bonding configuration are usually reliable.51 A

comparison of the predicted and observed infrared spectra is often able to determine the electronic and geometric structures actually present, independent of the relative DFT energetics for the different configurations. In the present study, we found that the M06L functional provides better results on energetic predictions; in contrast, the B3LYP functional gives better fit to the experiments on frequency calculations. Thus, the frequencies presented in the text are due to the B3LYP functional. The computed harmonic vibrational frequencies were scaled by a factor of 0.9697 and are given a 7 cm−1 full width at half-maximum (fwhm) Lorentzian line shape for comparison with experiment. The scaling factor of 0.9697 was determined by calculating the average value needed to make the experimental and calculated frequencies coincide for the Con(CO)m+ cluster cations studied.

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RESULTS AND DISCUSSION The mass spectrum of cobalt carbonyl cluster cations produced by the laser vaporization supersonic cluster source in the m/z range of 100−650 is shown in Figure 1. The spectrum is

Figure 1. Mass spectrum of the cobalt carbonyl cluster cations produced by pulsed laser vaporization of a cobalt metal target in an expansion of helium seeded by carbon monoxide.

obtained at the experimental conditions that favor the formation of saturated coordinated complexes. The mass spectrum is composed of peaks due to mononuclear cobalt carbonyl cations Co(CO)5+, dinuclear cobalt carbonyl cations Co2(CO)n+ (n = 7−9), trinuclear carbonyl cations Co3(CO)n+ (n = 10, 11), and a tetranuclear carbonyl cluster cation Co4(CO)12+. Weak peaks due to Co2C(CO)7+, Co3C(CO)n+ (n = 9, 10), and Co 4 C(CO) 12 + are also observed. The mononuclear Co(CO)5+ cation has previously been characterized to have a completed coordination sphere, consistent with its expected 18-electron stability. It has the same structure (D3h trigonal bipyramid) as its isoelectronic Fe(CO)5 neutral.9 The multinuclear carbonyl cluster cations of interest are each mass-selected and subjected to infrared photodissociation. The band positions from the resulting infrared photodissociation spectra of the cluster cations are listed in Table 1. Co2(CO)8+. The Co2(CO)8+ cation is the most intense peak in the mass spectrum, indicating that this cation complex is formed preferentially with high stability. The parent ions can be 2720

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The observation of a band below 2000 cm−1 indicates that the cation complex involves bridge-bonded CO ligand(s). Geometry optimizations were performed on various possible structures for Co2(CO)8+. Five closely lying structural isomers were obtained, as shown in Figure 3. Structure (a) has a (OC)4Co−Co(CO)4 geometry with C2 symmetry, with all of the carbonyl groups terminally bonded. The second structure (b) has Cs symmetry involving a bridging CO ligand. At the B3LYP level, the two Co−C bond distances to the bridging CO ligand are 1.861 and 2.233 Å, respectively. The bond length of the bridging CO ligand (about 1.157 Å) is longer than those of the terminal bonded ligands (1.136−1.141 Å). Structure (c) also has a (OC)4Co−Co(CO)4 geometry, with all of the carbonyl groups terminally bonded. This structure has a doublet spin state with C2v symmetry. The fourth structure (d) has an asymmetric (OC)3Co−CO−Co(CO)4 geometry involving a linear bridging carbonyl group bonded to one cobalt atom through its carbon atom and to the other cobalt atom through its oxygen atom. The dinuclear chromium carbonyl cations were determined recently to involve such a kind of linear bridging carbonyl group.64 The fifth structure (e) has two bridging CO ligands. Structure (a) was predicted to be the most stable structure at the B3LYP level. In contrast, structure (b) is the most stable structure at the M06L level. The calculated infrared spectra of these low-lying structures of Co2(CO)8+ are compared to the experimental spectrum in Figure 2. The calculated spectrum of structure (b) agrees well with the observed spectrum, indicating that the experimentally observed Co2(CO)8+ cation can be attributed to have structure (b) with Cs symmetry involving a bridging CO ligand. The observed and calculated band positions are compared in Table 1. The observed 1941 cm−1 band is due to the stretching vibration of the bridging CO ligand. The 2068 and 2131 cm−1 bands originated from the stretching vibrations of the terminally bonded CO ligands. Co2(CO)9+. The Co2(CO)9+ cluster cation with low intensity is observed in the mass spectrum. It dissociates quite efficiently via loss of CO fragments. The parent ions can be depleted by 48% at the laser pulse energy of about 1.1 mJ/pulse. The resulting infrared photodissociation spectrum is shown in Figure 4. The spectrum exhibits five well-resolved bands centered at 1935, 2069, 2107, 2143, and 2161 cm−1 together with two partially resolved bands at 2119 and 2127 cm−1. The 2069 cm−1 band is broader than the other bands, suggesting the involvement of more than one vibrational mode in this band. Similar to the isoelectronic Fe2(CO)9− cluster anion, which is determined to involve an Fe2(CO)8− core anion that is solvated by an external CO molecule,59 the Co2(CO)9+ cluster cation is expected to be a weakly bound complex involving a Co2(CO)8+ core ion. As discussed previously, the external CO ligand results in a weak perturbation on the core ion, causing little or no spectral shift in the vibrational spectrum.51−67 However, the infrared photodissociation spectrum of Co2(CO)9+ cannot be assigned to a weakly bound complex with a Co2(CO)8+ core ion. Therefore, other structures should be considered. Besides the CO-tagged complex structures, two other stationary points that are lower in energy than CO-tagged complex structures were found at the B3LYP level, as shown in Figure 5a and b. Both structures can be regarded as weakly bound ion− molecular complexes formed between a neutral Co(CO)4 fragment and a closed-shell Co(CO)5+ cation fragment. The first structure has a (OC)3CoCO−Co(CO)5+ arrangement without symmetry, with a CoCO−Co distance of 3.588 Å. This

Table 1. Comparison of the Band Positions (in cm−1) of the Com(CO)n+ Cluster Cations Measured to Those Computed by DFT at the B3LYP/6-31+G(d) Levela exptl.

calcd.a

Co2(CO)8+

1941 2068 2131

Co2(CO)9+

1935 2069 2107 2119 2127 2143 2161

Co3(CO)10+

2003 2088 2105 2123 2160 1890 1964 2092 2115

1969 (508) 2089 (766), 2101(194), 2107(7) 2120 (913), 2127 (658), 2135 (1103) 2164 (20) 1943 (2061) 2065 (968), 2068 (647) 2111 (149) 2131 (673) 2137 (610) 2149 (558) 2155 (204) 2195 (1) 1939 (32), 1943 (762) 2088 (77), 2090 (244), 2092 (10) 2101 (813), 2102 (222) 2113 (1625), 2120 (1887) 2152 (32) 1898 (771) 1960 (456), 1976 (340) 2075 (365), 2081 (470), 2090 (467), 2097 (229) 2103(443), 2105 (945), 2112 (1229), 2126 (1321) 2148 (71)

Co4(CO)12+

a

The calculated harmonic vibrational frequencies were scaled by a factor of 0.9697. aIR intensities are listed in parentheses in km/mol.

depleted by about 19% at the laser energy of about 0.7 mJ/ pulse. The CO binding energy of Co2(CO)8+ is calculated to be 4.8 kcal/mol. Photodissociation near 2100 cm−1 is therefore most likely a single-photon process. The resulting infrared photodissociation spectrum is shown in Figure 2. The spectrum exhibits three peaks centered at 1941, 2068, and 2131 cm−1.

Figure 2. The experimental and simulated vibrational spectra of the Co2(CO)8+ cluster cation in the carbonyl stretching frequency region. The experimental spectrum (bottom line) was measured by monitoring the CO fragmentation channel leading to the formation of Co2(CO)7+. The simulated spectra (a−e) were obtained from scaled harmonic vibrational frequencies and intensities for the five lowest energy structures (Figure 3a−e) calculated at the B3LYP/631+G(d) level. 2721

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Figure 3. Optimized structures of the Co2(CO)8+ cluster cations at the B3LYP level. The relative energies are given in kcal/mol.

Figure 4. The experimental and simulated vibrational spectra of the Co2(CO)9+ cluster cation in the carbonyl stretching frequency region. The experimental spectrum (bottom line) was measured by monitoring the CO fragmentation channel leading to the formation of Co2(CO)8+. The simulated spectra (a−d) were obtained from scaled harmonic vibrational frequencies and intensities for the four low-lying energy structures (Figure 5a−d) calculated at the B3LYP/631+G(d) level.

Figure 5. Optimized structures of the Co2(CO)9+ cluster cations at the B3LYP level. The relative energies are given in kcal/mol.

spectra of the first ion−molecular complex (a) and the COtagged complex (c) provides good match to the experiment (see spectrum e), suggesting that the observed Co2(CO)9+ cluster cation is due to a mixture of the CO-tagged complex and the Co(CO)5+−Co(CO)4 ion−molecular complex. Co3(CO)10+. The Co3(CO)10+ cluster cation is the most intense trinuclear carbonyl cluster cation observed in the mass spectrum. This cluster cation is able to lose up to two CO ligands. The parent ions can be depleted by about 21% at the laser energy of 1.8 mJ/pulse, suggesting a multiphoton process. The infrared photodissociation spectrum of Co3(CO)10+ by losing one CO is shown in Figure 6a. The spectrum exhibits two bands peaked at 2003 and 2123 cm−1. The 2123 cm−1 band is broad and asymmetric with several partially resolved bands superimposed on it. The spectrum from the loss of two CO ligands has almost the same pattern. Geometry optimizations were performed on Co3(CO)10+ starting from various possible

structure is 3.3 kcal/mol lower in energy than separated Co(CO)5+ and Co(CO)4. The second structure is quite similar to the first one but with slightly different CO arrangement (Cs symmetry) and a much shorter CoCO−Co distance of 2.585 Å. This structure was predicted to be slightly less stable (0.5 kcal/ mol) than the first structure. At the M06L level, the second structure is a saddle point with imaginary frequencies; geometry optimization without symmetry converged to the first structure. The calculated infrared spectra of the two ion−molecular complexes together with the two lowest energy CO-tagged complex structures are compared to the experimental spectrum in Figure 4. None of these spectra are in excellent agreement with the experimental spectrum. However, the sum of the 2722

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bridging CO ligands, while the other bands are attributed to the stretching vibrational modes of the terminal bonded CO ligands, which cannot be well-resolved due to band overlap. A weak peak due to the Co3(CO)11+ cation is also observed in the mass spectrum. This cluster cation photodissociates via loss of one CO ligand very efficiently with an unfocused IR laser, producing a spectrum similar to that of Co3(CO)10+, as shown in Figure 6b. Besides the bands observed in the Co3(CO)10+ spectrum, an additional band at 2160 cm−1is also observed, which is due to an external CO ligand having a negligible effect on the spectrum of the Co3(CO)10+ core ion. Co4(CO)12+. The Co4(CO)12+ cluster cation is the only homoleptic tetranuclear carbonyl species observed in the mass spectrum shown in Figure 1. The cation is able to lose up to three CO ligands. The parent ions can be depleted by 35% at the laser pulse energy of 2.0 mJ/pulse. The infrared photodissociation spectrum of Co4(CO)12+ consists of four bands centered at 1890, 1964, 2092, and 2115 cm−1 (Figure 8).

Figure 6. The experimental spectra of the (a) Co3(CO)10+ and (b) Co3(CO)11+ cluster cations in the carbonyl stretching frequency region. The simulated spectra of Co3(CO)10+ (c,d) from scaled harmonic vibrational frequencies and intensities for the two low-lying structures (Figure 7a and b) calculated at the B3LYP/6-31+G(d) level are also shown for comparison.

structures. The two lowest-lying structures are shown in Figure 7. The first structure involves a triangle Co3 core with C2

Figure 8. The experimental and simulated vibrational spectra of the Co4(CO)12+ cluster cation in the carbonyl stretching frequency region. The experimental spectrum (bottom line) was measured by monitoring the CO fragmentation channel leading to the formation of Co4(CO)11+. The simulated spectra (a,b) were obtained from scaled harmonic vibrational frequencies and intensities for the two lowest energy structures (Figure 9a and b) calculated at the B3LYP/631+G(d) level.

Figure 7. Optimized structures of the Co3(CO)10+ cluster cations at the B3LYP level. The relative energies are given in kcal/mol.

symmetry. In this structure, the axial cobalt center is coordinated by two terminal CO ligands as well as two edgebridging CO ligands, while the other two cobalt centers are each coordinated by three terminal CO fragments and one edge-bridging CO fragment. The second structure has C2v symmetry with an open-chain (OC)3Co−Co(CO)4−Co(CO)3 arrangement. It was found that DFT calculations encountered serious difficulties in predicting the relative stability of these two structures. At the B3LYP level of theory, the open-chainlike structure was predicted to be 25.5 kcal/mol more stable than the triangle structure. In contrast, the open-chain-like structure lies 2.7 kcal/mol above the triangle structure at the M06L level of theory. Although calculations at two levels give different relative stability between the two structures, the predicted vibrational spectra for the same structure are very similar. The calculated infrared spectra of the two structures of Co3(CO)10+ at the B3LYP level are compared to the experimental spectrum in Figure 6. Obviously, only the calculated spectrum of the first structure matches the experiment. Thus, the experimentally observed Co3(CO)10+ cluster cation is determined to have a structure involving a triangle Co3 core with C2 symmetry. The band centered at 2003 cm−1 is assigned to the stretching vibrational mode of the two

The observation of more than one band below 2000 cm−1 indicates that the cation involves more than one bridging CO ligands. Geometry optimizations were performed on various possible structures for Co4(CO)12+, and the two lowest-lying structures are shown in Figure 9. The most stable structure of Co4(CO)12+ is found to involve a tetrahedral Co4 core without

Figure 9. Optimized structures of the Co4(CO)12+ cluster cations at the B3LYP level. The relative energies are given in kcal/mol. 2723

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is 10, though Co3(CO)11+ is predicted to be the most stable structure based on the polyhedral skeletal electron pair theory.33 The Co3(CO)10+ cluster cation is determined to have C2 symmetry involving a triangle Co3 core with two edgebridging and eight terminal carbonyls. The axial cobalt center is coordinated by two terminal and two bridging carbonyls, while the other two cobalt centers are each coordinated by three terminal and one bridging CO ligands. Thus, each cobalt center is coordinated by four CO ligands. The two Co−Co bonds involving the axial cobalt center have a bond distance of 2.503 Å. The distance between the two cobalt centers each coordinated by three terminal carbonyls is 2.649 Å. NBO population analysis indicates that the positive charge is almost evenly distributed on the two cobalt centers each coordinated by three terminal carbonyls. The Co−Co bonds with a distance of 2.503 Å can be regarded as a single bond, while the Co−Co bond with a bond length of 2.649 Å is roughly a half bond. Therefore, each cobalt center has a 17 formal valence electron configuration. The Co4(CO)12+ cluster cation is the only homoleptic tetranuclear carbonyl cluster cation observed in the mass spectrum shown in Figure 1, suggesting that the saturation limit of CO coordination on Co4+ is 12. The observed Co4(CO)12+ cluster cation is characterized to have no symmetry, with a tetrahedral Co4 core and a 2A spin ground state, which is quite similar to that of the neutral Co4(CO)12 cluster. Previous experimental and theoretical studies indicate that the global minimum structure for Co4(CO)12 neutral has C3v symmetry, with three edge-bridging and nine terminal carbonyls.22−26 Removing an electron from the closed-shell neutral cluster to form the open-shell cation cluster results in a Jahn−Teller distortion to low symmetry. All of the Co−Co bond distances in the Co4(CO)12 neutral have similar values (around 2.50 Å) and are consistent with the single bonds required by the 18 electron rule.26 In the cation cluster, the three Co−Co bonds bridged by CO ligands have bond lengths in the range of 2.53− 2.60 Å, very close to those of the neutral, suggesting that these are Co−Co single bonds. The Co−Co bonds without CO bridges have relatively longer bond lengths (ranging from 2.69 to 2.92 Å), indicative of weaker bond order for these Co−Co bonds relative to the Co−Co bonds bridged by CO ligands. The saturation numbers of the cationic cobalt dimer and tetramer are the same as those of the rhodium dimer and tetramer.76 However, the saturation number of the trimer is different between cobalt and rhodium. The cationic rhodium trimer coordinates only nine CO molecules, one less than that of the cobalt trimer cation. The cationic cobalt carbonyl clusters are characterized to have bridge-bonded structures. In contrast, all CO molecules are terminally bound in the small cationic rhodium carbonyl clusters.

symmetry. In this structure, one cobalt center is coordinated by three terminal-bonded CO ligands, and the other three cobalt centers are each coordinated by two terminal-bonded CO ligands and two edge-bridging CO ligands. This structure is very much like the structure of stable Co4(CO)12 neutral, which was experimentally determined to have a C3v structure with three edge-bridging carbonyl groups and nine terminal carbonyl groups.22−26 The second structure involves a rhombus Co4 core with Cs symmetry, which lies higher in energy than the first structure. The calculated infrared spectra of these two low-lying structures are compared to the experimental spectrum in Figure 8. The calculated vibrational spectrum of the first structure matches the experiment better than the second structure, which supports the assignment of the experimentally observed Co4(CO)12+ cluster cation to structure (a) involving a tetrahedral Co4 core. The 1890 and 1964 cm−1 bands are assigned to the stretching vibrations of the edge-bridging CO ligands, while the other bands all belong to the stretching vibrations of the terminal-bonded CO ligands. Discussion. The features observed in the mass spectrum and infrared photodissociation spectra recorded in the present experiments provide some valuable information on the maximum number of CO molecules (saturation limits) that bind to small cobalt cluster cations. Previous studies indicate that the Co(CO)5+ cation has a completed coordination sphere with a D3h trigonal bipyramid structure similar to that of its neutral isoelectronic analogue Fe(CO)5.9 Thus, the Co+ cation has a coordination number of five toward CO. The Co2(CO)8 neutral is a stable cobalt dimer carbonyl in nature. Hence, a maximum number of 8 CO molecules is expected to coordinate onto Co2+. Previous gas-phase kinetics study of size-selected cobalt cluster cations at thermal energies also found that the maximum coordination number of CO molecules onto Co2+ is 8.33 In the present study, the Co2(CO)8+ cation is found to be the most intense peak in the mass spectrum. Although the Co2(CO)9+ cluster cation is also observed in the mass spectrum, it is characterized to be a weakly bound complex. The present experimental observations support the assignment of Co2(CO)8+ as the coordination-saturated cluster cation. The Co−Co bond distance in Co2(CO)8+ is predicted to be 2.589 Å. It is very close to that in the lowest-energy structure of the neutral Co2(CO)8 molecule, which is suggested to be a formal Co−Co single bond to satisfy the 18 electron rule.29 Natural bond orbital (NBO) analysis also suggests that the Co−Co bond in Co2(CO)8+ is a single bond. The two cobalt centers have 18 and 17 formal valence electron configuration, respectively. The structure of the Co2(CO)8+ cation is different from those of the Co2(CO)8 neutral and the isoelectronic Fe2(CO)8− anion. The most stable Fe2(CO)8− cluster anion is determined to have an unbridged structure with an Fe−Fe single bond.59 The Co2(CO)8 neutral is the simplest stable closed- shell cobalt carbonyl, which is commercially available and has the well-known dibridged crystal structure of C2v symmetry.13−22 Two other structures deduced to be the nonbridged D3d and D2d geometries were also experimentally observed.13−15 Although the Co3(CO)11+ cation is the largest trinuclear cluster complex observed in the mass spectrum, it was characterized to be a weakly bound complex involving a Co3(CO)10+ core ion. Thus, the Co3(CO)10+ cation is confirmed to be the coordination-saturated cluster. Previous gas-phase kinetics study also reported that the observed maximum coordination number of CO molecules onto Co3+

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CONCLUSIONS Homoleptic multinuclear cobalt carbonyl cluster cations including dinuclear Co2(CO)8+ and Co2(CO)9+, trinuclear Co 3 (CO) 10 + and Co 3 (CO)11 + , as well as tetranuclear Co4(CO)12+ are produced in a laser vaporization supersonic cluster source in the gas phase. The cluster cations of interest are each mass-selected and studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures of the complexes are determined with the aid of density functional calculations. The Co2(CO)8+ cation is the most intense peak in the mass spectrum. It is determined to be the coordination-saturated 2724

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cluster for Co2+, which is characterized to have Cs symmetry involving a bridging CO ligand and a Co−Co single bond. A weak Co2(CO)9+ peak is also observed in the mass spectrum. This cation complex is determined to be a mixture of the COtagged Co2(CO)8+−CO complex and the Co(CO)5+−Co(CO)4 ion−molecular complex. Two homoleptic trinuclear carbonyl cluster cations are observed in the mass spectrum. The Co3(CO)10+ cluster cation is determined to be the coordination-saturated cluster. It is characterized to involve a triangle Co3 core with C2 symmetry involving two edge-bridging and eight terminal CO ligands. The Co3(CO)11+ cation is a weakly bound complex involving a Co3(CO)10+ core ion. The Co4(CO)12+ cluster cation is the only homoleptic tetranuclear carbonyl cluster cation observed in the mass spectrum, suggesting that the saturation limit of CO coordination on Co4+ is 12. It is characterized to have a tetrahedral Co4+ core structure with three edge-bridging and nine terminal carbonyls.

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(9) Ricks, A. M.; Bakker, J. M.; Douberly, G. E.; Duncan, M. A. Infrared Spectroscopy and Structures of Cobalt Carbonyl Cations, Co(CO)n+ (n = 1−9). J. Phys. Chem. A 2009, 113, 4701−4708. (10) Sunderlin, L. S.; Wang, D. N.; Squires, R. R. Bond Strengths in First-Row-Metal Carbonyl Anions. J. Am. Chem. Soc. 1993, 115, 12060−12070. (11) Goebel, S.; Haynes, C. L.; Khan, F. A.; Armentrout, P. B. Collision-Induced Dissociation Studies of Co(CO)x+, x = 1−5: Sequential Bond Energies and the Heat of Formation of Co(CO)4. J. Am. Chem. Soc. 1995, 117, 6994−7002. (12) Huo, C. F.; Li, Y. W.; Wu, G. S.; Beller, M.; Jiao, H. Structures and Energies of [Co(CO)n]m (m = 0, 1+, 1−) and HCo(CO)n: Density Functional Studies. J. Phys. Chem. A 2002, 106, 12161−12169. (13) Friedel, R. A.; Wender, I.; Shufler, S. L.; Sternberg, H. W. Spectra and Structures of Cobalt Carbonyls. J. Am. Chem. Soc. 1955, 77, 3951−3958. (14) Sweany, R. L.; Brown, T. L. Infrared Spectra of Matrix-Isolated Dicobalt Octacarbonyl. Evidence for the Third Isomer. Inorg. Chem. 1977, 16, 415−421. (15) Abrahamson, H. B.; Frazier, C. C.; Ginley, D. S.; Gray, H. B.; Lilienthal, J.; Tyler, D. R.; Wrighton, M. S. Electronic Spectra of Dinuclear Cobalt Carbonyl Complexes. Inorg. Chem. 1977, 16, 1554− 1556. (16) Lichtenberger, D. L.; Brown, T. L. Non-Bridged Structures of Dicobalt Octacarbonyl. Inorg. Chem. 1978, 17, 1381−1382. (17) Low, A. A.; Kunze, K. L.; MacDougall, P. J.; Hall, M. B. Nature of Metal−Metal Interactions in Systems with Bridging Ligands. 1. Electronic Structure and Bonding in Octacarbonyldicobalt. Inorg. Chem. 1991, 30, 1079−1086. (18) Folga, E.; Ziegler, T. A Density Functional Study on the Strength of the Metal Bonds in Co2(CO)8 and Mn2(CO)10 and the Metal−Hydrogen and Metal−Carbon Bonds in R-Mn(CO)5 and RCo(CO)4. J. Am. Chem. Soc. 1993, 115, 5169−5176. (19) Barckholtz, T. A.; Bursten, B. E. Density Functional Calculations of Dinuclear Organometallic Carbonyl Complexes. Part I: Metal− Metal and Metal−CO Bond Energies. J. Org. Chem. 2000, 596, 212− 220. (20) Kluge, O.; Finger, M.; Reinhold, J. Orbital Contributions to the Molecular Charge and Energy Density Distributions in Co2(CO)8. Inorg. Chem. 2005, 44, 6494−6496. (21) Ponec, R.; Lendvay, G.; Chaves, J. Structure and Bonding in Binuclear Metal Carbonyls from the Analysis of Domain Averaged Fermi Holes. I. Fe2(CO)9 and Co2(CO)8. J. Comput. Chem. 2008, 29, 1387−1398. (22) Onaka, S.; Shriver, D. F. Application of Raman Spectroscopy to Bridged-Nonbridged Equilibria for Polynuclear Metal Carbonyl Derivatives. Metal−Metal Stretching Frequencies of Co2(CO)8, Fe2(CO)82−, Co4(CO)12, and [(η5-C5H5)Ru(CO)2]2. Inorg. Chem. 1976, 15, 915−918. (23) Wei, C. H.; Dahl, L. F. Molecular Structures of Triiron Dodecacarbonyl and Tetracobalt Dodecacarbonyl. J. Am. Chem. Soc. 1966, 88, 1821−1822. (24) Corradini, P. Structure of Tetracobaltdodecarbonyl. J. Chem. Phys. 1959, 31, 1676−1677. (25) Cohen, M. A.; Kidd, D. R.; Brown, T. L. Structures of Dodecacarbonyltetracobalt and Undecacarbonyl (Trimethyl Phosphite) Tetracobalt in Solution. J. Am. Chem. Soc. 1975, 97, 4408−4409. (26) Xie, Y.; King, R. B.; Schaefer, H. F., III. Vibrational Frequencies of the Homoleptic Cobalt Carbonyls: Co4(CO)12 and Co6(CO)16. Spectrochim. Acta, Part A 2005, 61, 1693−1699. (27) Sweany, R. L.; Brown, T. L. Matrix Isolation Spectra of the Thermal and Photochemical Decomposition Products of Dicobalt Octacarbonyl. Inorg. Chem. 1977, 16, 421−424. (28) Tremblay, B.; Manceron, L.; Gutsev, G. L.; Andrews, L.; Partridge, H., III. Experimental and Theoretical Infrared Spectra of Co2CO. J. Chem. Phys. 2002, 117, 8479−8485. (29) Kenny, J. P.; King, R. B.; Schaefer, H. F., III. Cobalt−Cobalt Multiple Bonds in Homoleptic Carbonyls: Co2(CO)x (x = 5−8)

ASSOCIATED CONTENT

S Supporting Information *

Calculated geometries, vibrational frequencies, and intensities; complete refs 46 and 75. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (+86) 21-6564-3532 (M.Z.). *E-mail: [email protected]. Fax: (+86) 21-6564-3532 (Z.H.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from National Natural Science Foundation (Grants 21173053, 21273042, and 21273045), Ministry of Science and Technology of China (2013CB834603, 2010CB732306, and 2012YQ220113-3) and the Committee of Science and Technology of Shanghai (13XD1400800).

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REFERENCES

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