Carbonyl Bonding on Oxophilic Metal Centers: Infrared

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Carbonyl Bonding on Oxophilic Metal Centers: Infrared Photodissociation Spectroscopy of Mononuclear and Dinuclear Titanium Carbonyl Cation Complexes Xiaojie Zhou, Jieming Cui, Zhen Hua Li, Guanjun Wang, Zhipan Liu,* and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Mononuclear and dinuclear titanium carbonyl cation complexes including Ti(CO)6+, Ti(CO)7+, TiO(CO)5+, Ti2(CO)9+ and Ti2O(CO)9+ are produced via a laser vaporization supersonic cluster source. The ions are mass selected in a tandem time-of-flight mass spectrometer and studied with infrared photodissociation spectroscopy in the CO stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. Only one IR band is observed for the 15-electron Ti(CO)6+ cation, which is characterized to have an octahedral Oh structure. The Ti(CO)7+ cation is determined to be a weakly bound complex involving a Ti(CO)6+ core ion instead of the seventh coordinated ion. The TiO(CO)5+ cation has a completed coordination sphere with a C4v structure. The Ti2(CO)9+ cation is determined to have a doublet Cs structure with two four-electron donor side-on bridging CO groups and one semibridging CO group. The Ti2O(CO)9+ cation has a doublet Cs structure involving a planar cyclic Ti2O(η2-μCO) core with a four electron donor side-on bridging CO. Bonding analysis indicates that the Ti2(CO)9+ and Ti2O(CO)9+ cations each have a Ti−Ti single bond. The results suggest that metal−metal multiple bonding is not favorable, and the oxophilic titanium centers failed to satisfy the 18-electron configuration in these metal carbonyl complexes.

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INTRODUCTION Transition metal carbonyl compounds comprise an important part of transition-metal molecular clusters, which have been studied extensively because of their role in organometallic chemistry and catalysis.1,2 In these transition metal carbonyl complexes, the carbonyl groups can bind to the metal centers in a variety of ways that range from two-electron donor terminal and bridging, to four-electron donor bridging and six-electron donor bridging.3−5 Homoleptic mononuclear and dinuclear metal carbonyls are the simplest representatives of transition metal carbonyl clusters for the study of the nature of metal− carbonyl and metal−metal bonding. The well-known 18electron rule, describing the number of metal and ligand electrons that give the s2p6d10 noble gas configuration, is an important factor governing the coordination and structures of transition metal carbonyls.6,7 The stable binary metal carbonyls of the first row transition metals, including the mononuclear metal carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4, and the dinuclear metal carbonyls Mn2 (CO) 10 , Fe 2(CO) 9, and Co2(CO)8 obeyed this 18-electron rule. For the oxophilic early transition metals with very few valence electrons, high coordination is required to achieve the 18-electron configuration of the metal centers. However, steric effects preclude high coordination for these metals. The synthesized stable neutral binary vanadium carbonyl is not the 18-electron © 2013 American Chemical Society

V2(CO)12, but V(CO)6 with only a 17-electron configuration for the V atom.8 In the case of titanium, no stable homoleptic titanium carbonyl compound was reported except the titanium carbonyl dianion Ti(CO)62−, an 18-electron complex that is isoelectronic with the stable Cr(CO)6 molecule.9 Although the 18-electron Ti(CO)7 molecule was predicted to be stable with the energy of CO loss from Ti(CO)7 to Ti(CO)6 comparable to those of CO loss from Ti(CO)6 and Ti(CO)5,10 only the unsaturated Ti(CO)n (n = 1−6) carbonyl species were observed experimentally in previous matrix isolation studies.11−13 The bond dissociation energies of Ti(CO)n+ (n = 1− 7) measured using guided ion beam mass spectrometry indicate that the 17-electron Ti(CO)7+ is thermodynamically stable with respect to CO loss, but the relatively weak bond energy of 12 kcal/mol suggests that this cation is relatively less stable.14 In this paper, the mononuclear and dinuclear titanium carbonyl cations are studied by infrared photodissociation spectroscopy to elucidate the structure and bonding of CO on the oxophilic cationic titanium metal and metal oxide centers. It has been proven that infrared photodissociation spectroscopy in conjunction with quantum chemical calculations offers one Received: December 6, 2012 Revised: January 18, 2013 Published: January 18, 2013 1514

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frequencies scaled by a factor of 0.976 and a 5 cm−1 full width at half-maximum (fwhm).

of the most direct and generally applicable experimental approaches to structural investigation of mass-selected clusters in the gas phase.15−26 Recently, this technique has been successfully employed in studying later transition-metal carbonyl clusters in the gas phase.27−37

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RESULTS AND DISCUSSION A mass spectrum of titanium carbonyl cluster cations produced by the laser vaporization supersonic cluster source in the m/z range of 100−400 is shown in Figure 1. The mass spectrum is

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EXPERIMENTAL AND COMPUTATIONAL METHODS The infrared photodissociation spectra of the titanium carbonyl cluster cations were measured using a collinear tandem time-offlight mass spectrometer. The experimental apparatus has been described in detail previously.35 The cluster cations were produced in a Smalley-type laser vaporization supersonic cluster source. The 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate and 6 ns pulse width) was used to vaporize a rotating titanium metal target. The titanium carbonyl complexes were produced from the laser vaporization process in expansions of helium gas seeded with 4−6% CO using a pulsed valve (General Valve, Series 9) at 0.4−0.6 MPa backing pressure. After free expansion, the cations were skimmed and analyzed using a Wiley−McLaren time-offlight mass spectrometer. The clusters of interest were each mass selected and decelerated into the extraction region of a second collinear time-of-flight mass spectrometer, where they were dissociated by a tunable IR laser. The fragment and parent cations were reaccelerated, and mass was analyzed by the second time-of-flight mass spectrometer. The infrared source used in this study is generated by an KTP/KTA/AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by a Continuum Powerlite 8000 Nd:YAG laser, which is tunable from 800 to 5000 cm−1, producing about 0.5−1.5 mJ/pulse in the range of 1500−2200 cm−1. The infrared laser is loosely focused by a CaF2 lens. The wavenumber of the OPO laser is calibrated using CO and NH3 absorptions. The IR beam path is purged with nitrogen to minimize absorptions by air. Fragment ions and undissociated parent ions are detected by a dual microchannel plate detector. The ion signal is amplified, collected on a gated integrator, and averaged with a LabView based program. The photodissociation spectrum is obtained by monitoring the yield of the fragment ion of interest as a function of the dissociation IR laser wavelength and normalizing to parent ion signal. Typical spectra were recorded by scanning the dissociation laser in steps of 2 cm−1 and averaging over 300 laser shots at each wavelength. To help the assignment of the experimentally observed vibrational frequencies and to determine the structures of the titanium carbonyl cations, first-principle density functional theory (DFT) calculations were performed with the hybrid B3LYP method in combination with the 6-311+G(d) basis set for the C and O atoms and the Wachters−Hay all-electron basis set for the Ti atom.38,39 Previous studies have shown that B3LYP can give good energetic and vibrational results for transition metal carbonyls.28−37 For each structure, all possible spin states were considered. The stability of the wave functions was checked by the “stable” technique as implemented in the Gaussian 09 program to ensure that the wave functions have no internal instability.40 The integral grid used for all the DFT calculations is a pruned (99 590) grid (the “ultrafine” grid as defined by Gaussian 09). All calculations were performed with the Gaussian 09 suite of quantum chemical software packages.41 Theoretical predicted IR spectra were obtained by applying Lorentzian functions with the theoretical harmonic vibrational

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

dominated by the signals of cationic titanium cluster carbonyls corresponding to the species TiO(CO) 5 + , Ti(CO) 6 + , Ti2(CO)9+ and Ti2O(CO)9+. For each species, the isotopic splitting of titanium can be resolved with their relative intensities matching the natural abundance isotopic distributions. The carbonyl cluster cations of interest are each massselected and subjected to infrared photodissociation. When the laser frequency is resonant with an-IR active vibration of the cation complex, it can absorb photon(s) and subsequently undergo fragmentation via losing CO(s). Infrared photodissociation spectra are obtained by monitoring the fragment ion yield as a function of the dissociation IR laser wavelength. Ti(CO)6+. The Ti(CO)6+ cation dissociates via losing one CO ligand under focused IR laser irradiation with very low efficiency. The parent ions can be depleted by less than 5% at the laser energy of about 0.9 mJ/pulse. The sixth CO binding energy of Ti(CO)6+ was determined experimentally to be 17.8 kcal/mol,14 significantly higher than the energy of IR photons in the terminal CO stretching frequency region. The dissociation detected is apparently due to a multiphoton absorption process. The infrared photodissociation spectrum of Ti(CO)6+ is shown in Figure 2. The spectrum exhibits a single band centered at 2118 cm−1. The single-band spectrum observed suggests high symmetry structure for the Ti(CO)6+ cation. Density functional calculations predicted a 4A1g ground state for Ti(CO)6+ with an octahedral Oh structure. The cation was predicted to have only one IR active CO stretching vibration at 2121 cm−1, which belongs to the triply degenerate antisymmetric CO stretching mode (Table 1). Ti(CO)7+. The Ti(CO)7+ cation is observed to dissociate by losing one CO ligand very efficiently using unfocused laser beam, indicating that the dissociation proceeds via a single photon process and that the seventh CO is loosely bound. The parent ions can be depleted by more than 40%. The infrared 1515

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photodissociation spectrum for the Ti(CO)7+ complex is also shown in Figure 2. The spectrum exhibits a strong band centered at 2116 cm−1 together with a weak band at 2168 cm−1. The experimental observation suggests that no strongly bound seventh coordinated Ti(CO)7+ cation was produced in the experiments, and the observed Ti(CO)7+ cation is due to a weakly bound complex involving a Ti(CO)6+ core ion. The strong 2116 cm−1 band originates from the Ti(CO)6+ core ion, which is only 2 cm−1 red-shifted from that of Ti(CO)6+. The weak 2168 cm−1 band is due to the weakly bound external CO ligand. Previous studies have shown that the weakly bound external CO ligand(s) exhibits weak CO stretching vibration(s) in the range of 2160−2170 cm−1 for transition metal carbonyl cation complexes.30−34 Previous collision-induced dissociation experiments indicate that the seventh coordinated 17-electron Ti(CO)7+ is thermodynamically stable with respect to CO loss.14 Although the bond energy of the Ti(CO)7+ complex is weak (12 kcal/ mol), it is stronger than would be expected for a ligand in the second ligand shell. Present DFT calculations predict that the seventh coordinated Ti(CO)7+ cation has a doublet ground state without symmetry. The seventh CO was predicted to be unbound with respect to the ground state Ti(CO)6+ and CO. As shown in Figure 2e, the seventh coordinated Ti(CO)7+ cation without symmetry has seven IR active modes. Apparently, this structure is not observed in the experiment. The predicted spectrum of the structure with a Ti(CO)6+ core and a weakly bound CO (Figure 2d) matches the experiment. Since the second shell CO ligand is very weakly bound with a predicted Ti−C distance of 4.68 Å, the triply degenerate mode of the Ti(CO)6+ core ion shows negligible split. TiO(CO)5+. The TiO(CO)5+ cation is the most abundant carbonyl cluster cation observed in the mass spectrum, suggesting inherent stability for this complex. The cation can be depleted by about 20% via losing one CO ligand with the focused laser beam. The infrared photodissociation spectrum for the TiO(CO)5+ cluster cation is shown in Figure 3. Two bands centered at 2184 and 2204 cm−1 can be clearly resolved. The TiO(CO)5+ cation complex was predicted to have a 2B1 ground state with C4v symmetry with all the CO ligands

Figure 2. The experimental (in black) and simulated (in blue) vibrational spectra of the Ti(CO)6+ and Ti(CO)7+ cations in the carbonyl stretching frequency region. (a) experimental spectrum of Ti(CO)6+, (b) experimental spectrum of Ti(CO)7+, (c) simulated spectrum of 4A1g state Ti(CO)6+, (d) simulated spectrum of the Ti(CO)6+-CO complex, and (e) simulated spectrum of the seventh coordinated Ti(CO)7+ cation.

Table 1. Comparison of the Band Positions of the TiO(CO)5+, TiO(CO)6+, Ti(CO)6+, Ti(CO)7+, Ti2(CO)9+, and Ti2O(CO)9+ Cluster Cations Measured in the Present Work to Those Computed by DFT at the B3LYP/6311+G(d) Level (IR Intensities in Parentheses in km/mol) complex

exptl.

calcd.a

TiO(CO)5+

2184

2184(696 × 2) 2190(0) 2212(0) 2228(66)

TiO(CO)6+

Ti(CO)6+

Ti(CO)7+ Ti2(CO)9+

Ti2O(CO)9+

2204 2170 2182 2206 2118

2116 2168 1655 1685 1942 2119 2131 2154 2160 2172 2182 1526 2101 2113 2135 2151 2165 2179

2121(1269 × 3) 2135(0 × 2) 2183(0)

1735(492) 1752(599) 1988(575) 2111(682) 2121(1767) 2124(422) 2135(526) 2151(1372) 2172(98) 1604(354) 2087(874) 2101(539) 2137(515), 2138(1191), 2139(741) 2161(242) 2169(988) 2183(27)

a

The calculated harmonic vibrational frequencies are scaled by a factor of 0.976.

Figure 3. The experimental vibrational spectra of (a) TiO(CO)5+ and (b) TiO(CO)6+ and the simulated (in blue, c) vibrational spectrum of 2 B1 ground state TiO(CO)5+ in the carbonyl stretching frequency region. 1516

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terminally bonded to the titanium metal center. The axial Ti− CO bond distance is much longer than that of the four equivalent equatorial Ti−CO bonds. The TiO bond length is predicted to be 1.612 Å, 0.042 Å longer than that of TiO+ calculated at the same level of theory. The 2184 cm−1 band is assigned to the antisymmetric stretching vibration of the equatorial CO ligands, which is doubly degenerate. The 2204 cm−1 band is attributed to the stretching vibration of the axial CO ligand, which was predicted to be much weaker than the equatorial CO vibration (Table 1). The TiO(CO)6+ cation fragments very efficiently by the loss of one CO ligand, producing the infrared spectrum shown in Figure 3b that can be assigned to a weakly bound complex involving a TiO(CO)5+ core ion. The 2182 and 2206 cm−1 bands are originated from the TiO(CO)5+ core ion, while the 2170 cm−1 band is attributed to the weakly bound external CO ligand. The infrared photodissociation spectra of TiO(CO)5+ and TiO(CO)6+ indicate that the TiO+ cation has the first shell coordination number of five. The C4v geometry of TiO(CO)5+ is similar to that observed for the ScO(Ng)5+ ions (Ng = noble gas atoms).42 By contrast, it was found that the neutral TiO molecule could coordinate only four N2 ligands in forming the TiO(NN)4 complex.43 The TiO molecule has a 3Δ ground state with an electron configuration of (core)(σ)1(δ)1. The singly occupied δ orbital is largely the Ti 3d orbital that is mainly nonbonding. The singly occupied σ orbital is primarily a hybrid of the Ti 4s, 4pz, and 3dz2 orbitals that is polarized away from the O atom. Since this σ orbital is singly occupied, the coordination in the axial direction is weak due to σ repulsion. Upon removing this σ electron in forming the 2Δ ground state TiO+ cation, the σ repulsion is reduced, and the fifth CO is able to coordinate to TiO+ in the axial direction. The single CO stretching vibration of Ti(CO)6+ at 2118 cm−1 is red-shifted by 25 cm−1 from the frequency of gas phase carbon monoxide (2143 cm−1),44 while the two bands of TiO(CO)5+ at 2184 and 2204 cm−1 are shifted 41 and 61 cm−1 to the blue from the gas phase CO value. The small red shift observed for the Ti(CO)6+ cation is comparable to those seen for some neutral and charged transition metal carbonyl complexes, which exhibit less back-donation.3,30 The 4A1g ground state Ti(CO)6+ complex has three unpaired electrons, which, according to the DFT calculations, reside in the triply degenerate molecular orbitals that comprise Ti 3d to CO 2π* back-donation. The blue-shifts of the CO stretching vibrations of TiO(CO)5+ is comparable to those observed for nonclassical carbonyls, which have very little or essentially no backdonation.45 The titanium center in TiO+ has only one electron residing in the δ orbital, which can back-donate electrons to the four equatorial CO ligands in TiO(CO)5+, resulting in lower CO stretching frequency of these equatorial CO groups than that of the axial CO ligand. Ti2(CO)9+. The infrared photodissociation spectrum for the Ti2(CO)9+ cation obtained by monitoring the loss of one CO ligand leading to the formation of Ti2(CO)8+ cation is shown in Figure 4. The parent ions can be depleted by about 11% under focused laser beam with an energy of about 0.5 mJ/pulse. The spectrum exhibits six partially resolved bands in the 2100−2200 cm−1 region, which can be attributed to the terminal carbonyl stretching vibrations. Three additional bands centered at 1942, 1655, and 1685 cm−1 were observed in the low frequency region, suggesting that the Ti2(CO)9+ cation involves bridging carbonyl groups.

Figure 4. The experimental and simulated vibrational spectra of the Ti2(CO)9+ cluster cation. The infrared photodissociation spectrum (a) was measured by monitoring the CO fragmentation channel leading to the formation of Ti2(CO)8+. The simulated spectra were obtained from scaled harmonic frequencies and intensities for the two lowestlying structures calculated at the B3LYP/6-311+G(d) level.

To gain further insight into the structure and bonding of the Ti2(CO)9+ cluster cation, quantum chemical calculations were performed using the DFT. The two lowest lying structures for Ti2(CO)9+ are shown in Figure 5. Structure “a” has a doublet

Figure 5. Optimized structures of the two lowest-lying isomers of the Ti2(CO)9+ and Ti2O(CO)9+ cluster cations. The values in parentheses are relative potential energies to the lowest-energy structure in kcal/ mol.

ground state with the quartet state lying more than 20 kcal/mol above the doublet state. Structure “b” has a quartet ground state with the doublet state being 5.6 kcal/mol less stable than the quartet state. Both structures are characterized by two side-on bridging carbonyl groups. Besides the side-on bridging carbonyls, structure “a” involves a semibridging carbonyl with Cs symmetry. In this structure, the two side-on bridging carbonyls are equivalent with a quite long C−O distance of 1.181 Å. The semibridging CO distance of 1.144 Å is 0.037 Å shorter than that of the side-on bridging groups but is about 0.02 Å longer than those of the terminal CO ligands. The sideon bridging carbonyl groups act as both the usual type σ donor to the left-hand titanium atom and π donor to the right-hand titanium center through the CO π bond. Thereby, the sideon bridging CO ligand serves as a four-electron donor. Structure “b” has an asymmetric structure with one Ti bound by four terminal carbonyls and the other Ti atom bound by three terminal carbonyls without symmetry. The two side-on 1517

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The geometries of various possible Ti2O(CO)9+ structures were fully optimized. The two lowest-energy isomers are shown in Figure 5. Both structures have doublet ground states. The quartet spin states lie more than 20 kcal/mol higher in energy than the doublet states. The global minimum structure is characterized by a single four-electron donor side-on bridging CO group with Cs symmetry. The side-on bridging CO unit together with the two titanium atoms and the oxo oxygen atom forms a planar five-membered ring. All the other CO ligands are terminally bonded to the Ti centers with each Ti bound by four. The side-on bridging CO group has a bond distance of 1.220 Å, longer than that of the Ti2(CO)9+ cation calculated at the same level of theory. The next lowest-energy structure of Ti2O(CO)9+ involves a bent TiOTi core without symmetry. All the carbonyl groups in this structure are terminally bonded. This structure lies energetically above the global minimum structure by 6.8 kcal/mol. The calculated infrared spectra of these two structures of Ti2O(CO)9+ are compared to the experimental spectrum in Figure 6. The global minimum structure with a four-electron donor bridging CO group is calculated to have a very low CO stretching frequency at 1604 cm−1, which matches the experiment. Therefore, the experimentally observed Ti2O(CO)9+ cluster cation can be assigned to the calculated global minimum structure involving a planar cyclic Ti2O(η2-μ-CO) core with a four electron donor side-on bridging CO group. Discussion. Present infrared photodissociation spectroscopic study indicates that the Ti(CO)7+ cation is a weakly bound complex involving a Ti(CO)6+ core ion. Consistent with the experimental observation, density functional calculations also predicted that the seventh coordinated Ti(CO)7+ cation is unbound with respect to the ground state Ti(CO)6+ and CO. The 15-electron octahedral Ti(CO)6+ is characterized to be a strongly bound fully coordinated ion. Therefore, the Ti+ cation has a coordination number of 6 instead of 7 toward CO. For the more oxophilic TiO+ center, the TiO(CO)5+ cation is determined to have a completed coordination sphere. The valence electron of Ti can be counted as 17 assuming a Ti−O triple bond. The Ti2(CO)9+ and Ti2O(CO)9+ cations are the most abundant dinuclear carbonyl cluster cations observed in the mass spectrum, indicating that these cluster cations are formed preferentially and suggesting that they have enhanced stability. Both cations are characterized to involve side-on bridging carbonyl group(s) with very low CO stretching frequencies. A similar four-electron donor side-on bridging carbonyl has been observed in a few organometallic complexes.48 The neutral Sc2CO, Ti2(CO)x (x = 1, 2) and Ti3(CO)x (x = 1−3) complexes were characterized to be homoleptic metal carbonyls with similar side-on bridging CO ligands.49−51 These simple carbonyl neutrals exhibit very low CO stretching frequencies in the range of 1100−1400 cm−1. The Ti2(CO)9+ and Ti2O(CO)9+ cation complexes have higher side-on bridging CO stretching frequencies than the above-mentioned neutral complexes, as the Ti2+ and Ti2O+ core cations have less dπ electrons for back-donation to the 2π antibonding orbitals of the CO ligands. The Ti2(CO)9+ cation was predicted to have a quite long Ti−Ti bond with a bond length of 2.850 Å. Natural bond orbital (NBO) analysis indicates that the Ti−Ti bond can roughly be regarded as a single bond with a bond order of 0.66. The NBO analysis also suggests that the Ti2(CO)9+ cation can be viewed as a complex formed via a neutral Ti(CO)6 fragment and a positively charged Ti(CO)3 moiety. The positive charge

bridging CO ligands are inequivalent. The shorter CO distance is 1.184 Å, while the longer CO distance is 1.212 Å. The Ti−Ti distances in structures “a”and “b” are 2.850 and 3.315 Å, respectively, which are much longer than those of Ti2 and Ti2+.46,47 The bond length of the 3Δg ground state Ti2 was experimentally determined to be 1.942 ± 0.008 Å.46 No experimental value is available for Ti2+. Recent theoretical calculations suggest a 2Σg ground state with a bond length of 2.00 Å.47 The two structures are very close in energy. Structure “a” lies energetically above structure “b” by 1.6 kcal/mol. The calculated infrared spectra of these two structures of Ti2(CO)9+ are compared to the experimental spectrum in Figure 4. Only the predicted vibrational spectrum of structure “a” matches the experiment, which supports the assignment of the experimentally observed Ti2(CO)9+ cluster cation to the structure with two equivalent side-on bridging carbonyls and one semibridging carbonyl. The very low CO stretching frequencies at 1655 and 1685 cm−1 are due to the antisymmetric and symmetric stretching vibrations of the two four-electron donor side-on bridging carbonyl groups, which were calculated at 1735 and 1752 cm−1. The 1942 cm−1 band is originated from the semibridging CO ligand, which was predicted to absorb at 1988 cm−1. The structure of Ti2(CO)9+ is quite different from those of binuclear nonacarbonyl cations of other first-row transition metals. The Cr2(CO)9+ cation is characterized to have an unique (OC)5Cr−C−O-Cr(CO)3+ structure with a linear fourelectron donor bridging carbonyl group.37 The Fe2(CO)9+ cation was established to have an asymmetric (OC)5Fe− Fe(CO)4+ structure with only terminally bound CO ligands.34b Ti2O(CO)9+. The Ti2O(CO)9+ cation was depleted by about 23% via losing of one CO ligand with the focused laser beam at an energy of about 0.9 mJ/pulse. The infrared photodissociation spectrum of Ti2O(CO)9+ is shown in Figure 6. Seven bands centered at 1526, 2101, 2113, 2135, 2151, 2165, and 2179 cm−1 are observed. The bands above 2100 cm−1 are apparently due to the stretching vibrations of the terminally bonded CO ligands, while the band at 1526 cm−1 should be attributed to the stretching vibration of the bridging CO ligand.

Figure 6. The experimental and simulated vibrational spectra of the Ti2O(CO)9+ cluster cation. The infrared photodissociation spectrum (a) was measured by monitoring the CO fragmentation channel leading to formation of Ti2O(CO)8+. The simulated spectra were obtained from scaled harmonic frequencies and intensities for the two lowest-lying structures calculated at the B3LYP/6-311+G(d) level. 1518

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and the unpaired electron are located on the Ti(CO)3 moiety. Therefore, the titanium center in the Ti(CO)6 moiety has 17 valence electrons, while the other titanium center has only 14 valence electrons. The Ti2O(CO)9+ cation has a slightly shorter Ti−Ti bond of 2.722 Å. NBO analysis indicates that the Ti−Ti bond is a single bond with a bond order of 0.61. The positive charge and the unpaired electron are located on the right-hand Ti(CO)4 moiety. Thus, both the titanium centers have valence electrons less than 18 (16 electrons for the Ti center in the Ti(CO)5 fragment and 15 electrons for the Ti center in the Ti(CO)4 moiety). These results suggest that metal−metal multiple bonding is not favorable, and the titanium centers failed to satisfy the 18-electron configuration in these oxophilic titanium metal carbonyl complexes.

CONCLUSIONS Carbonyl bonding on oxophilic titanium metal centers is studied. The mononuclear and dinuclear titanium carbonyl cation complexes including Ti(CO)6+, Ti(CO)7+, TiO(CO)5+, Ti2(CO)9+, and Ti2O(CO)9+ are produced via a laser vaporization supersonic cluster source and studied with infrared photodissociation spectroscopy in the CO stretching frequency region. The 15-electron octahedral Ti(CO)6+ is characterized to be a strongly bound fully coordinated ion. The Ti(CO)7+ cation is a weakly bound complex involving a Ti(CO)6+ core ion instead of a seventh coordinated ion, which is predicted to be unbound with respect to the ground state Ti(CO)6+ and CO. For the more oxophilic TiO+ center, the TiO(CO)5+ cation is determined to have a completed coordination sphere with its CO stretching frequencies blue-shifted from the gas phase CO value and higher than that of Ti(CO)6+. The Ti2(CO)9+ and Ti2O(CO)9+ cations are the most abundant dinuclear carbonyl cluster cations observed in the mass spectrum, indicating that these cluster cations are formed preferentially with high stability. Both cations are characterized to involve four-electron donor side-on bridging carbonyl group(s) with very low CO stretching frequencies. Bonding analysis indicates that the Ti2(CO)9+ and Ti2O(CO)9+ cations each has a Ti−Ti single bond with both titanium centers failed to satisfy the 18-electron configuration. ASSOCIATED CONTENT

S Supporting Information *

The calculated geometries, vibrational frequencies and intensities, and complete ref 41. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation (Grant No. 20933030 and 21173053) and the Ministry of Science and Technology of China (2013CB834603, 2010CB732306, and 2012YQ2201133). 1519

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